Christian Schorr, Mika Muinonen & Fiia Nurminen
TORREFACTION OF BIOMASS
Julkaisu 1 / 2012
Publication 1 / 2012
6.3.2012
Miktech Oy | Graanintie 5, 50190 Mikkeli
www.miktech.fi
LAAJENNETTU TIIVISTELMÄ
Taustaa
Tämä julkaisu on selvitys puun torrefioinnista eli paahtamisesta, jolla puubiomassasta
tuotetaan biohiiltä. Biohiili on uusiutuva polttoaine ja tulevaisuudessa sillä tulee olemaan
merkittävä osuus fossiilisen hiilen korvaamisessa energiantuotannossa. Selvitys keskittyy
torrefiointiprosessin perustoimintoihin, eli mekanismeihin jotka ovat läsnä kaikissa torrefioinnin teknisissä sovellutuksissa, sekä pelletointiin.
Torrefiointiprosessi
Torrefiointi tarkoittaa biomassan paahtamista korkeassa lämpötilassa (250 °C) hapettomissa olosuhteissa niin, että veden lisäksi osa haihtuvista yhdisteistä poistuu. Prosessin
aikana biomassan painosta katoaa 30 %, mutta sen sisältämästä energiasta vain 10 %.
Torrefioidulla biomassalla on samankaltaiset käsittelyominaisuudet kuin kivihiilellä. Torrefioinnin tarkoitus on parantaa biomassan palamisominaisuuksia ja lämpöarvoa sekä kehittää sen käsittelyominaisuuksia niin, että sitä voidaan polttaa olemassa olevissa kivihiilivoimaloissa.
Torrefiointiprosessi voidaan jakaa viiteen vaiheeseen, joista ensimmäinen on alkulämmitys (initial heating). Lämmitysvaiheessa biomassa lämmitetään pisteeseen, jossa vesi ei
vielä haihdu. Lämpöä käytetään ainoastaan nostamaan biomassan lämpötilaa, ja vaihe
loppuu kun vesi alkaa haihtua. Seuraava vaihe on esikuivaus (pre-drying), jossa lämpötila
säilyy muuttumattomana, mutta vapaa vesi haihtuu biomassasta. Esikuivatusvaiheen ensimmäisessä vaiheessa biomassan kosteuspitoisuus laskee lineaarisesti, jolloin haihtumisnopeus riippuu ympäristön olosuhteista. Kun massan kosteuspitoisuus laskee kriittiseen
pisteeseen, alkaa toinen vaihe, jossa haihtuvan veden täytyy tunkeutua materiaalin läpi,
mikä tarvitsee enemmän energiaa ja aikaa, koska vahvemmat kapillaarivoimat on voitettava. Kun kaikki massaan sitoutunut vesi on haihtunut, alkaa jälkikuivatus- ja keskitasonlämmitysvaihe (post-drying and intermediate heating). Lämpötila alkaa jälleen nousta, ja
fyysisesti sitoutunut vesi vapautuu hitaasti, kunnes biomassassa ei käytännössä ole enää
kosteutta. Tässä vaiheessa ensimmäiset haihtuvat yhdisteet, kuten terpeenit, voivat haihtua, mikä tarkoittaa että ensimmäiset kiinteät yhdisteet kaasuuntuvat ja massa pienenee.
Tämä vaihe lakkaa, kun lämpötila nousee 200 °C:een, jossa määritelmän mukaan alkaa
torrefiointivaihe. Torrefiointivaiheen ollessa koko prosessin ydin, vaiheeseen kuuluu sekä
lämmitys että jäähdytys (cooling). Välissä on myös vaihe, jolloin lämpötila säilyy muuttumattomana, ja yleensä tämä on koko prosessin korkein lämpötila. Lämpötilan noustessa
jopa 300 °C:een, tapahtuu pyrolyyttistä hajoamista, ja biomassa paahtuu. Torrefiointivaiheessa massasta häviää merkittävä osa. Prosessi on periaatteessa endoterminen, mutta
käytännössä jotkut hiukkaset vastaanottavat enemmän lämpöä ja saavuttavat lämpötilatason, jossa reaktiot ovat eksotermisiä. Määritelmän mukaan torrefiointivaihe loppuu,
kun lämpötila laskee jälleen 200 °C:een. Reaktioaika käsittää ajan, jolloin materiaali on
lämmitetty 200 °C:sta haluttuun lämpötilaan, joka pidetään vakiona halutun ajan. Aika,
joka kuluu materiaalin viilentyessä jälleen 200 °C:een, on jätetty pois reaktioajasta, vaikka
se kuuluu torrefiointivaiheeseen. Tämä siksi, että suurin osa termisesti epävakaista yhdisteistä biomassassa on jo hajonnut, ja lämpötilan alkaessa laskea niitä on enää hyvin vähän
tai ei ollenkaan jäljellä. Reaktion voidaan siis katsoa loppuneen, kun lämpötila alkaa laskea. Viimeinen vaihe on kiinteän aineen jäähdyttäminen, jolloin massa jäähdytetään haluttuun loppulämpötilaan. Tämä tehdään anaerobisissa olosuhteissa johtuen syttymis- tai
jopa räjähdysvaarasta.
Torrefiointiprosessin lopputuotteet riippuvat käytetystä biomassasta. Lopputuotteet voidaan jakaa kiinteisiin ja kaasumaisiin yhdisteisiin. Kaasumaiset yhdisteet voidaan jakaa
edelleen tiivistyviin tai nestemäisiin ja ei-tiivistyviin tai pysyvästi kaasumaisiin aineisiin.
Hemiselluloosan hajoamisesta syntyy pääasiassa hiilimonoksidia ja hiilidioksidia. Karboksyyliryhmien lämmittämisestä syntyy karbonyylejä, kuten metanolia, propioaldehydiä ja
muita hiilivetyjä. Nestemäiseksi tiivistyvät lopputuotteet jakautuvat vedeksi, orgaanisiksi
aineiksi ja rasvoiksi. Nestemäisiä aineita, joita tutkimuksissa on todettu, ovat mm. maitohappo, muurahaishappo, furfuraali, hydroksyyliasetoni ja metanoli, mutta myös fenoleja
on löydetty. Nestemäiset lopputuotteet ovat kuitenkin pääasiassa vettä ja etikkahappoa.
Kiinteät aineet ovat mm. sokereita ja uudelleen muodostuneita tai uusia polymeerejä.
Aromaattisen renkaan muodostuminen on mahdollista, kuten myös kivihiilen tapaisten
hiilirakenteiden ja tuhkan muodostuminen. Pysyvät kaasut ovat pääasiassa hiilimonoksidia ja hiilidioksidia, joiden happi on vapautunut alkuperäisistä biomassan yhdisteistä, sekä
pienissä määrin molekyylivetyä ja metaania. Vaikka prosessissa syntyvä kaasuseos yleensä
poltetaan prosessin lämmittämiseksi ennen kun nesteeksi tiivistyvät aineet ehtivät poistua seoksesta, on kuitenkin tärkeä tuntea kaasun koostumus, jotta sen palamisominaisuuksia voidaan tutkia. Yleisesti biomassamolekyylit voivat lämmön vaikutuksesta reagoida ja muodostaa uusia yhdisteitä, tai pysyä muuttumattomina. Torrefioinnissa tapahtuvat
reaktiot ovat kuitenkin monimutkaisia eikä niitä vielä täysin ymmärretä, joten selvitys
keskittyy vain täysin ymmärrettyihin ja todennettuihin reaktioihin.
Ideaalitilanteessa torrefiointi on autoterminen prosessi, mikä tarkoittaa tasapainoa prosessin aikana vapautuvien haihtuvien yhdisteiden sisältämän kemiallisen energian ja sen
polttoprosessin myötä systeemin palauttaman lämpöenergian välillä. Näin ollen energia
kiertää systeemissä, eikä ulkoisia energialähteitä tarvita. Tämän saavuttamiseksi prosessin
parametrit on optimoitava viipymäajan ja torrefiointilämpötilan suhteen, käytetty biomassa sekä prosessin malli huomioon ottaen. Jos torrefiointikaasun energiasisältö ei ole
riittävä, on sekaan lisättävä toista polttoainetta, esimerkiksi maakaasua. Tämä kuitenkin
nostaa operointikustannuksia. On myös mahdollista polttaa käsittelemätöntä biomassaa
lämmön tuomiseksi systeemiin.
Torrefioinnin taloudellista potentiaalia voidaan kasvattaa tiivistämällä torrefioitua biomassaa, esimerkiksi puristamalla siitä brikettejä tai pellettejä. Koska energiatiiviyden kasvaessa kuljetuskustannukset pienenevät, on biohiilipellettejä mahdollista hyödyntää kauempana tuotantopaikasta.
Kivihiilen osittainen korvaaminen biohiilipelleteillä
Soveltuvimmat käyttökohteet biohiilipelleteille liittyvät sen rinnakkaispolttoon olemassa
olevissa kivihiilivoimalaitoksissa. Maailman kivihiilivoimaloista 90 % on hiilipölykattiloita,
ja suuri osa lopusta 10 %:sta on leijupetikattiloita, ja nämä ovatkin kaksi tärkeintä teknologiaa rinnakkaispolttoon liittyen. Yleisesti ottaen rinnakkaispoltto voidaan jakaa suoraan
ja epäsuoraan rinnakkaispolttoon. Suorassa yhteispoltossa biomassa yhdistetään kivihiilen
syöttöön, jolloin kattilaan syötetään biomassan ja kivihiilen sekoitusta. Biomassaa voidaan
siis polttaa olemassa olevissa hiilikattiloissa, mutta monet sen osat saattavat vaatia suuria
muokkauksia. Epäsuorassa yhteispoltossa biomassa ja biohiili poltetaan erillisissä kattiloissa, ja vasta prosessien höyryt tuodaan yhteen turbiinien pyörittämiseksi. Jälkimmäinen
vaihtoehto vaatii suuremmat investointikustannukset, mutta myös käyttökustannukset
voivat olla korkeammat. Useimmissa tapauksissa suora yhteispoltto on edullisempi vaihtoehto, joten se on yleisesti suositeltavampi.
Biomassan rinnakkaispolton järjestäminen hiilivoimaloissa on helpointa kun toissijainen
polttoaine vastaa ominaisuuksiltaan kivihiiltä. Koska torrefioidulla biomassalla on korkeampi lämpöarvo kuin käsittelemättömällä biomassalla, se voi ratkaista ongelman jossain
määrin. Kun tarkastellaan polttoaineiden ominaisuuksia, bitumisen kivihiilen kanssa tulisi
käyttää vain torrefioitua, kuivaa puuta, kun taas kostea biomassa sopii paremmin poltettavaksi ruskohiilen kanssa.
Biohiilipellettien rinnakkaispoltto kivihiilen kanssa vähentää kivihiilen poltossa syntyviä
päästöjä. Biomassan rikkipitoisuus on luontaisesti matalampi, mutta se myös sitoo rikin
kemiallisesti tuhkaansa, joten rikkidioksidia muodostuu vähemmän. Savukaasukoostumuksen muuttuminen voi vaikuttaa esimerkiksi kattilakorroosioon, varsinkin jos savukaasuissa on liikaa klooria. Korkea klooripitoisuus ei kuitenkaan ole todennäköinen kun käytetään puuta raaka-aineena, koska puun ja kivihiilen klooripitoisuudet ovat samankaltaiset. Tutkimukset ovat osoittaneet, että typenoksidipäästöjä voidaan laskea biomassan
rinnakkaispoltolla. Hiilimonoksidipitoisuuksien ei odoteta nousevan. Päästöt huomioon
ottaen hiilivoimalan päästötasapaino ei heikkene biomassan rinnakkaispolton myötä, joissain tapauksissa biomassan lisääminen voi jopa olla edullista. Biomassan laatu on kuitenkin oleellista, ja puu onkin paras raaka-aine verrattuna olkibiomassaan tai lietteeseen,
joka on kaikista ongelmallisinta.
Biohiilipellettien (TOP-pelletti) rinnakkaispolton vaikutuksia ei ole vielä päästy suuressa
mittakaavassa tutkimaan, mutta tämän hetken tietämyksen mukaan biohiilipellettien
osuus kivihiilen rinnakkaispoltossa voi olla useita kymmeniä prosentteja, jopa puolet, kun
tavallisilla puupelleteillä osuus on noin 15 – 20 %. Myös biohiilipellettien biomassateho on
moninkertainen puupelletteihin nähden. Kun pyritään nostamaan uusiutuvien energialähteiden osuus käytetystä polttoaineesta kivihiilikattiloissa merkittäväksi (30 – 50 %), torrefioitu biomassa on yksi parhaista vaihtoehdoista. Suomalaisillakin energiayhtiöillä on biohiilipelletteihin liittyen meneillään useita projekteja, joissa näitä vaikutuksia pyritään selvittämään.
Teknologiset vaihtoehdot
Torrefiointilaitos voidaan sijoittaa kivihiiltä käyttävän voimalaitoksen yhteyteen, jolloin
lopputuote hyödynnetään sähkön ja lämmön tuotannossa. Voimalaitoksen hukkalämpöä
voidaan hyödyntää torrefiontireaktorissa ja torrefiontilaitoksen mahdolliset kaasut voidaan puhdistaa voimalaitoksen savukaasunpuhdistusprosessissa. Yhteistyön toteuttamiseksi olemassa olevat voimalaitokset vaatisivat kuitenkin muutoksia, mikä voi heikentää
sähkötuotannon tehokkuutta. Lisäksi prosessit voivat tulla liian riippuvaisiksi toisistaan,
jolloin ongelmat toisessa prosessissa voivat heijastua toiseen. Suurin torrefioinnin hyöty
jäisi näin kuitenkin saavuttamatta, eli edullisemmat kuljetuskustannukset.
Voimalaitoksesta erillisen torrefiointilaitoksen sijainti vaatii hyvää infrastruktuuria ja suuria biomassalähteitä laitoksen läheisyydessä. Prosessin täytyy tällöin tuottaa itse vaatimansa lämpöenergia, mikä voi tarkoittaa raakabiomassan polttoa ja torrefiointikaasun
hyödyntämistä. Hyötynä voidaan kuitenkin pitää pienempiä kuljetuskustannuksia niin
raakabiomassan kuljetuksessa torrefiointilaitokselle kuin valmiin polttoaineen kuljetuksessa voimalaitoksille johtuen suuremmasta energiatiheydestä. Kolmas vaihtoehto on
siirrettävä torrefiontilaitos, joka on sijoitettu esimerkiksi kuorma-autoon. Etuna on, että
prosessi voidaan toteuttaa missä tahansa, missä on paljon (jäte)biomassaa. Mitä todennäköisimmin tällainen systeemi ei voi saavuttaa kiinteän laitoksen tehokkuutta ja kapasiteetin voidaan olettaa olevan hyvin rajoittunut. Tällaiselle systeemille on kuitenkin kysyntää erityisesti alueilla, joilla on paljon biomassaa mutta ongelmallinen infrastruktuuri.
Toinen torrefiointitekniikkaan liittyvä tekijä on tarvittavan lämmön tuominen prosessiin ja
partikkeleihin pyrolyyttisen hajoamisen mahdollistamiseksi. Reaktorin rakenne määrittelee, minkälainen lämmityssysteemi tulee kysymykseen, mutta pääkategoriat ovat suora ja
epäsuora lämmitys. Suorassa lämmityksessä lämmin torrefiointikaasun polton savukaasut
tai torrefiointikaasu itsessään johdetaan suoraan biomassan sekaan sen kuivatusvaiheessa. Epäsuorassa lämmityksessä sen sijaan lämpö johdetaan kiinteän seinän läpi reaktoriin.
Torrefiointikaasu johdetaan polttokammioon ja savukaasut antavat lämpönsä välittäjäaineelle, joka kiertää torrefiointireaktorin ja lämmönvaihtimen välillä. Epäsuora lämmitys
vaatii enemmän panostusta, mutta se estää lämmönvälityksen kielteiset vaikutukset.
Keskeiset teknologiset erot eri ratkaisuissa ovat torrefiointireaktoreissa, ja mahdollisia
reaktoriteknologioita on useita. Mahdollisia ovat mm. pyörivä rumpureaktori, ruuvikuljetin reaktori, mikroaaltoreaktori, liikkuva peti ja värähtelevä hihnakuljetin. On kuitenkin
myös erityisesti pyrolyyttisiin sovelluksiin, kuten torrefiointiin, kehitettyjä reaktoreja,
esimerkiksi Torbed –reaktori, monikerrosuuni (Multiple Hearth Furnance) ja WyssmontTurbo-Dryer® –reaktori.
Kaupallisen kokoluokan biohiilipelletin tuotantolaitoksia ei juuri ole vielä käynnissä, mutta
demonstrointilaitoksia on jo käynnissä. Hollannissa on meneillään käyttöönottovaihe laitoksesta, jonka tuotantokapasiteetti on 60 000 tonnia vuodessa. Johtavaa tutkimusta
tehdään erityisesti Hollannissa ja Belgiassa sekä Pohjois-Amerikassa, ja laitoksia on suunnitteilla ympäri maailmaa. Tällä hetkellä on ainakin 60 yritystä, joilla on torrefiointiin liittyviä hankkeita kehitysasteella. Suurin osa näistä on pieniä yrityksiä, jotka eivät pysty vastaamaan suuren kokoluokan torrefiointilaitoksen pystyttämisestä, mutta myös osa suuris-
ta insinööritoimistoista on aktiivisia tällä sektorilla, kuten Stamproy Green ja KEMA. Lisäksi monet eurooppalaiset sähköyhtiöt kehittävät torrefiointisysteemejä yhdessä tytär- ja
yhteistyöyritysten kanssa. Merkittävimpiä yrityksiä torrenfiointisektorilla ovat mm. ECN,
FoxCoal, Topell Energy, Stramproy Green ja 4Energy Invest Hollannista ja Belgiasta, ranskalainen Thermya, yhdysvaltalaiset Integro Earth Fuels, Zilkha Biomass, Airex Energy ja
Terra Green Energy sekä kanadalainen Biomass Secure Power.
Torrefiontilaitoksen perustamisessa keskeinen haaste on sopivan menetelmän valinta. Eri
teknologioista ei vielä ole riittävästi tietoa hyötyjen ja puutteiden arvioimiseksi, ja toisaalta lopputuotteen laatu riippuu sekä alkuperäisen biomassan laadusta että sen käsittelystä
reaktorissa. Tästä syystä on tärkeää tietää käytettävän biomassan raaka-ainekoostumus jo
hyvissä ajoin tarkoituksenmukaisen teknologian valitsemiseksi. Varsinkin kuoreen varastoituneiden haitallisten aineiden vapautuminen päästöihin voidaan ehkäistä sopivalla
teknologialla.
Yhteenveto
Varsinkin Euroopassa fossiilisten polttoaineiden käytön lainsäädäntö muuttuu tiukemmaksi ja monimutkaisemmaksi, joten uusiutuvien energialähteiden käyttö voimaloissa voi
johtaa taloudellisiin hyötyihin päästökaupan ja kansallisten tukien myötä. Vihreiden energialähteiden käyttö voi myös hyödyttää markkinointia, kun voidaan osoittaa että ympäristön suojelua on toteutettu.
Kivihiilen korvaamiseksi biomassoilla on eri vaihtoehtoja, tärkeimpänä kaasutus, torrefiointi ja bioöljy. Torrefioidun biomassan keskeisiä etuja on sen sopivuus hiilipölykattiloissa
käytettäväksi sellaisenaan, edut kuljetuksessa ja varastoinnissa sekä käsittelyssä hiilimyllyssä ja tulipesässä. Torrefioidun biomassan arvioidaan korvaavan noin 50 % käytetystä
hiilestä. Biohiili soveltuu käytettäväksi olemassa olevissa CHP-kattiloissa, eikä sen käyttöönotto vaadi suuria muutosinvestointeja. (VTT) Torrefioidusta biomassasta voidaan
myös puristaa biohiilipellettejä (TOP-pelletti), mikä helpottaa tuotteen varastointia ja kuljetusta. TOP-pellettien etuna on suurempi energiatiiviys, mikä johtaa pienempiin kuljetuskustannuksiin ja soveltumiseen kivihiilen rinnakkaispolttoon. Biohiili jauhautuu helposti, mikä pienentää energian kulutusta pelletointivaiheessa ja toisaalta edesauttaa hiilipölykattiloissa polttamista. Hiileen verrattuna biohiilen käyttö alentaa hiilidioksidipäästöjä
merkittävästi, koska materiaali on hiilidioksidineutraalia. Torrefioitu biomassa on hydrofobista, jolloin se ei ime kosteutta säilytyksessäkään, eikä se hajoa tai syty itsestään ja on
näin helpompi kuljettaa ja varastoida. Ominaisuuksiltaan biohiili vastaa kivihiiltä, mikä
helpottaa sen käyttöä olemassa olevissa pölypolttolaitoksissa. Prosessoinnista johtuen
raaka-aineen laatuvaatimukset ovat matalat.
CONTENTS
LAAJENNETTU TIIVISTELMÄ ..............................................................................................
ALKUSANAT .......................................................................................................................
PREFACE ............................................................................................................................
1
2
3
4
INTRODUCTION....................................................................................................... 1
1.1
Historical development of pelletizing and torrefaction ................................ 1
1.2
Problems with the status quo and state of affairs ........................................ 2
TORREFACTION ....................................................................................................... 5
2.1
Introduction................................................................................................... 5
2.2
Basic torrefaction pattern ............................................................................. 7
2.3
Lignocellulose biomass .................................................................................. 8
2.4
Thermo-chemical conversion of lignocellulose biomass ............................ 13
2.5
Fuel Refinement .......................................................................................... 23
ALTERNATIVE PROCESS CONCEPTS AND SPECIFICATIONS ................................... 29
3.1
Autothermal operation ............................................................................... 29
3.2
On-site and off-site...................................................................................... 29
3.3
Comparison of direct- and indirect heating ................................................ 31
3.4
Future trends ............................................................................................... 33
CO-FIRING OF UNTREATED BIOMASS AND TORREFIED WOOD IN COAL POWER
PLANTS ......................................................................................................................... 34
5
4.1
Biomass co-firing in general ........................................................................ 34
4.2
Pretreatment ............................................................................................... 35
4.3
Milling .......................................................................................................... 36
4.4
Expected performance of torrefied wood in coal power plants ................. 37
CONCLUSION ........................................................................................................ 42
PICTURE LIST .....................................................................................................................
REFERENCES ......................................................................................................................
ALKUSANAT
Helmikuussa 2012 Mikkelin seudulla käynnisti toimintansa Biosaimaa -klusteri, joka on
alueen yritysten, tutkimuslaitosten, rahoittajien, viranomaisten ja muiden sidosryhmien
yhteinen toimija. Toiminnan painopisteenä on metsäenergia ja keskeisenä tavoitteena on
bioenergialiiketoiminnan kasvattaminen. Klusterin toimintaa koordinoi Miktech Oy. Klusterin yhtenä kärkihankkeena on Ristiinan biologistiikkakeskus, jonka yhteyteen on suunnitteilla suuren kokoluokan biohiilipellettitehdas. Metsähaketta raaka-aineena käyttävän
laitoksen tuotantokapasiteetti tulee olemaan 200 000 t/a. Tämä teknologiaselvitys on osa
investointihankkeen esiselvitystä.
Selvityksen ohjaajana on toiminut kehityspäällikkö Mika Muinonen Miktech Oy:stä. Selvityksen on laatinut Christian Schorr Bingenin ammattikorkeakoulusta Saksasta. Suomenkielisen tiivistelmän laati insinööriopiskelija Fiia Nurminen Mikkelin ammattikorkeakoulusta.
Selvitystyö on toteutettu osana Osaamiskeskusohjelmaa ja Uusiutuva metsäteollisuus –
osaamisklusteria.
Mikkelissä 6.3.2012 Tekijät
PREFACE
In February 2012, the Biosaimaa-cluster was formed in Mikkeli region in Finland. Biosaimaa is a joint actor of the region’s businesses, research institutions, funding agencies,
authorities and other stakeholders. The focus is on forest energy and the main goal is to
increase the bio-energy business. The cluster is coordinated by Miktech Ltd. One of the
main projects of the cluster is launching the large scale torrefied pellet (TOP-pellet) factory acting in association with the biologistic centre in Ristiina, Southern Savonia. The factory would use the forest chips as a raw material, and its production capacity will be
200 000 t/a. This technology study is part of the Ristiina’s biologistic centre project.
The supervisor of the survey has been the Development Manager Mika Muinonen from
Miktech Ltd. The research is drawn up by Christian Schorr from Bingen University of Applied Sciences in Germany. The Finnish abstract is written by the engineering student Fiia
Nurminen from Mikkeli University of Applied Sciences in Finland..
This survey was conducted as a part of Centre of Expertise-programme and the Forest
Industry Future cluster programme.
Mikkeli, 6.3.2012 Authors
1
1 INTRODUCTION
1.1 Historical development of pelletizing and torrefaction
The increasing demand for renewable fuels – due to the scarcity of resources, the partly
abandonment of nuclear power and environmental consciousness being mainly a consequence of climate change – forces the energy industry to find suitable alternatives for
fossil energy carriers.
Bioenergy has been used by men since the very verifiability of human existence and still is
– in the natural form of raw biomass – the primary energy source in poor countries today.
With the coming of fossil fuels the industrial countries no longer had great needs for biomass as an energy carrier. Not until the occurrence of the oil crisis in 1970s bioenergy
attracted attention again, first in the USA and Canada, where the pelleting of wood – a
rather simple mechanical process – was developed to commercial scale for purposes of
domestic and industrial heating. In contrast to America the European countries entered
the pellet market not until the early 1980s, with Sweden and Denmark leading the way.
Today wood pellets are a common fuel for domestic heating with a steadily growing market especially in the European Union. However, against the background of the energy
turnaround, the industry is looking for new ways to use woody biomass and pellets more
effectively in power generation and heat supply.1
In consequence of the rising complexity of energy supply on the one hand and the increase of power consumption on the other, a variety of processes has been developed for
effective ways of biomass-utilization besides standard combustion, including amongst
others liquefaction, gasification, carbonization and pyrolysis, differing in both temperature and excess air ratio. All of them are thermal-chemical conversion processes, resulting
in the production of enhanced secondary fuels that can be transformed into useable energy under both time-wise and spatial decoupling.
One relatively new process – in terms of bioenergy – is torrefaction, a technology that
was developed from the coffee industry, where the coffee beans get roasted to make
them brittle and to gain their special flavors for the final product. More detailed information of the process itself shall not be given at this point.
A pilot plant for biomass torrefaction was engineered and built in France by the company
Pechiney in the mid-1980s, though the torrefied biomass served the purpose of a reducing agent in an aluminium production process and not for energy reasons. Nevertheless,
the plant with a production capacity of roughly 12,000 t/a worked well in terms of the
technology, but still was demounted due to economic aspects in the early 1990s, for the
energy efficiency of approximately 70 % caused high total productions costs in the range
1
DOLZAN, P. et al.: Global Wood Pellets Markets and Industry – Policy Drivers, Market Status and Raw Material Potential; 2007
2
of 150 to 180 €/t. The investment amounted to nearly 3 Mio. €, resulting in specific costs
of 240 €/t of installed annual capacity, respectively 25 €/t.2
1.2 Problems with the status quo and state of affairs
This survey in hand deals with a relative new process within the frame of energy engineering – named torrefaction – that allows converting raw biomass into a high-efficient
fuel with similar handling qualities to fossil coal. Torrefaction is not yet commercially operated, mainly due to processing challenges, but also caused by the technology’s unclear
sector acceptance and its unassigned position in the supply chain of biomass. The following explication shall reveal the current state of torrefaction activities and the general
problems of the technology’s development on a global scale.
Worldwide there are several projects on torrefaction aiming at an extension of the thermo-chemical process`s capacity, that would allow it to become a serious alternative in the
large-scale industry environment. International Standardization is currently being prepared under the number ISO 17255-1, which will include special regulations for thermally
treated materials and thereby differ from the standard for biofuels ISO TC 238.3
In a global view there is one location where R&D referring to torrefaction is focused. That
is the Netherlands, together with Belgium, where several demonstration plants are either
being planned or already being operated in test procedures. But there are also a few minor projects from small companies in the rest of Europe – mostly in France, Spain and
Scandinavia. In the United States there are also several companies working on the innovation. So far there are about sixty companies recognized to deal with the torrefaction process with all of them having projects still in a pre-commercial state. Unfortunately, for
most cases there is no information available on the size of those groups. The impression
gained by enquiry is that quite a big amount of those known companies has less than
twenty employees and consequently won´t be capable to deliver the full service of engineering work for an industrial torrefaction plant`s construction. But at the same time
there are some big engineering companies with strong reputation and well-known consultant firms active in this sector (Stramproy Green, KEMA and more). Towards bigger
companies from the coal and boiler industries can only be guessed whether they are busying themselves with torrefaction. Besides, major European electric power companies
commissioned some of their subsidiaries – for example RWE Innogy working with the
Dutch company Topell Energy4 and Nuon of Vattenfall cooperating with ECN5to advance
their torrefaction systems.
Several companies applied patents for their own developed torrefaction technologies,
now waiting for clients to contract. For many firms this is the obstacle before they are
2
BERGMAN, P.C.A. et al.: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”;
2005
3
ALAKANGAS, E. et al.: Preliminary results of EUBIONET III industrial pellet questionnaire; 2011
4
http://www.rwe.com/web/cms/de/37110/rwe/presse-news/pressemitteilung/?pmid=4002215;
08.01.2012
5
http://www.nuon.com/press/newsfacts/20100607/; 08.01.2012
3
able to manufacture big-scale torrefaction plants: By doing so, they perhaps would bring
the development of the process into the next big and final phase, possibly the last step to
have reliable and well-engineered torrefaction processes. The Netherlands seems to be
ahead in this case: Most of the torrefaction production units being in planning phase now
are developed in this West European region and so the operators and developers of these
demonstration plants may have significant advantages towards other firms, due to the
fact that they will gain experiences with the operating of torrefaction factory components
in industrial scale much earlier. That lead in know-how could make them hard to overtake
for the rest of the world.
Picture 1 Torrefaction development activities in Europe [red marker: developing company headquarters; blue marker: planned or realized pilot plant]. Created with Google
maps; 2011
But also in the US-state of Georgia the construction of one torrefaction plant will take
place in the near future for sure. Furthermore there are an increasing number of torrefaction developers in the United States –although, in contrast with Europe, widely scattered
over the continent.
4
Picture 2 Torrefaction development activities in the USA and Canada [red marker: developing company headquarters; blue marker: planned or realized pilot plant]. Created
with Google maps; 2011
R&D is also made in Canada where the pellet and energy industry recognized the immense potential of torrefaction and initiate the possible change in the market`s structure.6The research is strongly focused in the region around Vancouver, where most developing companies’ headquarters are.
Torrefaction referring company or institute names you may hear most often these days
are among others ECN, FoxCoal, Topell Energy, Stramproy Green, 4Energy Invest from the
Netherlands and Belgium, Thermya from France and Integro Earth Fuels, Zilkha Biomass,
Airex Energy and Terra Green Energy from the United States and Biomass Secure Power
from Canada. That might be caused by announcements of those companies to build torrefaction demo-plants.7 So it seems that what the torrefaction branch is lacking at the moment is investment, so that developers can overcome the existing difficulties and can get
aware of those that will not occur until large-scale torrefaction has been tested.
For companies planning to work with torrefaction-technology from the developers this
results in another challenge: To choose the right technology for their most likely unique
demands is not an easy task, because firstly there is not much known of the real weaknesses and benefits of the different technologies nowadays and secondly the qualities of
the torrefaction products depend crucially on the input material as well as the specially
therefore applied treatment in the reactor. There are first statements and recommendations about which technologies are most promising, though there still is quite an absence
6
7
LAPOINTE, D. et al.: Overview of Torrefaction Activities in Canada;2011
MEIJER, R.: Overview of European torrefaction landscape; 2011
5
of information to find a well-considered decision8. The companies dealing with torrefaction, especially the ones not having a contracted client and therefore making no progress,
claim their secret own-developed technology to be superior over competitive or promote
torrefaction with arguments that obviously are whitewashing the truth. Due to that, the
current state of the torrefaction technology can hardly be estimated and badly needs an
independent and reliable comparative analysis9.
Against this background the objectives of this report were to explain the torrefaction
technology’s current state of the art, the reactions, phenomenona and mechanisms of its
processing, according to present knowledge and to show the latest trends in its development.
2 TORREFACTION
2.1 Introduction
One synonym for torrefaction is mild pyrolysis, which can be derived from the fact that
torrefaction actually is simply the first step of this thermo-chemical conversion under
modified process parameters and it takes place in a reactor that can be designed in different ways. Nevertheless, as torrefaction still is under development various references
present different values for the process parameters and the product’s properties, that are
dependent on the input biomass and the concrete realization of the respective testing
plant. Thus the data for the process’s temperature range differ slightly – yet in general set
within the scope of 200 to 320 °C - the pressure level is in the neighborhood of atmospheric conditions while the residence time in the reactor is usually significantly higher
than in the original pyrolysis treatment, resulting in low heating rates of < 50 °C/min. The
values for optimal residence times lie in the range of thirty to ninety minutes. Still, a few
numbers of experiments were made with short residence times of only a few minutes but
higher temperatures which won’t be majorly considered in this survey, due to the rarity
of such processing. However the torrefaction takes place, the process always has to be
realized in the absence of oxygen in an inert atmosphere, due to the hazards of ignition
and explosion of the modified material.10
The primary goal is to refine raw biomass to an upgraded solid fuel, including better handling qualities and enhanced combustible properties simile to fossil coal’s, leading to decreased costs, but financial gains. The essential principle in this respect is to increase the
energy density of the biomass, requiring a growth of the ratio between energy and mass.
Consequently the calorific value of the torrefied biomass increments as well, since it is a
specific value reflecting the released energy per mass unit for solid fuels.
At this point the difference between the LHV and the HHV shall be shortly defined so
there are no misunderstandings possible during further reading of this report. As woody
8
CIOLKOSZ, D. et al.: A review of torrefaction for bioenergy feedstock production; 2011
DHUNGANA, A.: Torrefaction of Biomass; 2011
10
BERGMAN, P.C.A.: Combined torrefaction and pelletisation - The TOP Process; 2005
9
6
biomass contains hydrogen, water will be formed as one of the products of combustion.
In contrast to condensation where energy is liberated during the change of phase, the
vaporization of water requires a specific amount of energy defined as the enthalpy of
evaporation. As appropriate it is absolutely crucial for the energy balance of an incineration process – especially for fuels with high hydrogen contents – whether the water released during combustion has the form of steam or condensate.
(2)
m
hfg
ug
uf
mass of water formed by combustion
enthalpy of vaporization of water in kJ/kg
specific internal energy of vapor in kJ/kg
specific internal energy of liquid in kJ/kg
However, since for all practical purposes the water released by combustion escapes in the
form of vapor, in this survey the LHV is applied and meant by the term of calorific value
whenever it occurs. Still, it must be kept in mind that the rate of combustion is not taken
account of by neither LHV nor HHV. A fuel’s calorific value can be measured by chemical
analysis in accord with the Dulong’s formula or with the aid of bomb calorimeters in laboratory.11
During the torrefaction process the input biomass loses about 30 % of its mass, but only
10 % of its energy, due to the degassing of low-energy volatile compounds and the escape
of moisture, eventuating in a higher energy density of the biomass of roughly 30 % more
energy per mass unit. However, there are even more advantages of the torrefied biomass, when compared to the untreated feedstock biomass or conventional wood pellets.
Biochemical torrefaction mechanisms cause the biomass’s changed structure, leading to
new properties that make the handling of the final product much easier and also offers
the possibility to utilize it in existing coal-fired boilers as will be shown in the related
chapter.
To anticipate the benefits shortly: The grindability of the input biomass can be increased
significantly by torrefaction due to the modification of its molecular structure, so that
existing problems arising with untreated biomass in the milling component of a coal power plant are overcome. Also the biomass exchanges its hydrophilic properties to hydrophobicity that allows an effortless storage that goes hand in hand with a greater resistance against biological degradation, self-ignition and physical decomposition in general. However, the risk of biological degradation is not overcome completely, but fungal
growth and microbial activity is reduced, since the torrefied material stays very
dry.12Since the torrefied product already loses a great amount of volatiles during the
thermo-chemical conversion, there are less remaining for the following combustion step.
11
12
RAJPUT, R.K.: Engineering Thermodynamics, 2009
BERGMAN, P.C.A.: Combined torrefaction and pelletisation – The TOP Process; 2005
7
That might lead, maybe even more than for conventional biomass, to lower emissions in
terms of “sulfur dioxide (SO2), nitrogen oxides (NOx) and net greenhouse gas emissions of
carbon dioxide (CO2)”13, but also a diminished level of ash formation, respectively a new
composition of the inorganic residues. As one can expect, each of these issues has a tremendous potential in both ecological and economical views.
However, there still are many unproven torrefaction technologies, whose benefits and
disadvantages can hardly be guessed for practical reality. For example the ideas of direct
or indirect co-firing. Nevertheless, the technical differences are quite obvious. The unit
operations and process arrangements are predominantly the same, although the performance conditions vary strongly. Another feature that differs crucially is the component
design, especially the reactor’s, since the process’s development has not yet progressed
to a point of such knowledge that allows statements based on facts, on which design is
really superior to other ones at industrial scale. The resulting perception is simply that
more research and an independent comparison of the technologies still is needed. 14 Realizing the fact that it is not possible to deal with all the different technology types for every
component or to consider all deviances from the common process arrangement in this
text, the report will show the fundamental torrefaction process, expanded with optional
pelleting, meaning the mechanisms that take place in any of the process’s modification on
one hand and the basic process arrangement that seems to be the most effective one
from a present-day perspective on the other. However, there will be one chapter that
shortly shows the technology trends, the alternative ways on how torrefaction may be
conducted and realized in detail, to fulfill the responsibility of considering the current
state of the art in a fast moving industry.
2.2 Basic torrefaction pattern
13
14
DIETENBERGER et al.: The encyclopedia of materials – science and technology; 2001
MELIN, S.: Torrefied wood – a new emerging energy carrier; 2011
8
Picture 3 Basic exemplary operation arrangement for biomass torrefaction
The figure above shows a possible basic arrangement of operations in a wood refinement
plant via torrefaction. From the storage of feedstock biomass the material firstly gets sifted (and usually screened) for impurities and is subsequently ground to smaller particles of
a required size. Following this, the biomass is actively dried, which is one crucial operation
in the process, since the drying requires remarkable amounts of heat. The torrefaction
afterwards roasts the material as explained above in the absence of oxygen. Usually the
torrefied material must be cooled down, since there may occur high reactive fine matter
that may explode when it gets in contact with oxygen. Optionally, yet probably recommendable, is a downstream densification, to further improve the handling qualities of the
product and to increase the volumetric energy density of the solid product. Afterwards
there is a screening to check the product quality, preparing the biomass for the further
handling and delivery to clients.
2.3 Lignocellulose biomass
One important method in applied natural science is to reduce complex issues to a certain
scale by defining a suitable system that serves the understanding of the specific problem.
For a better insight of the torrefaction process later, this chapter will show up the composition and properties of wood - as far as it is relevant for its transformation to a solid biofuel via torrefaction - by presenting its most important compounds and structures in this
context.
9
The chemical composition of wood can be described as a mixture of organic polymers,
significantly smaller amounts of minerals and a minority of other inorganic and organic
compounds. These organic polymers can be assigned to three main groups, as they are
hemicellulose, cellulose and lignin, combined together known as the lignocellulose fraction. The distribution of these three components differs from wood type to wood type.
Nevertheless, all wood species being in line for torrefaction in Finland share the same
property of a lignocellulose mass fraction in excess of 90 %. It is important to state that
each of these three main groups represents a vast number of different polymers, but all
having common features which are essential for certain qualities of the wood.
From a biological perspective the plant cell wall, where the lignocellulose substances occur exclusively, is one crucial component of the plant’s structure and co-determines the
plants characteristics as it causes the size and form of the cell and tissue, but is also responsible for the plants statics and stability.
Most important for torrefaction is the fact, that each of these main groups has a specific
temperature range where they show signs of thermal degradation due to their different
molecular structures. Another meaningful aspect is the nature of cellulose to bundle to
fibrils that cause the biomass`s fibrous structure, making it resistant to mechanical forces.
In the following subchapters these three main components will be illustrated in more detail15.
2.3.1 Hemicellulose
As will be explained later the hemicellulose fraction is the most important group for the
torrefaction process. Hemicellulose is one major component in the eukaryotic cell walls more precisely in the primary and in the case of wood also in the secondary and tertiary
cell wall – forming the matrix for cellulose fibrils pervading it. Thus the hemicellulose substances mould the framework of the woody plant cell, providing the biomass with stabilization structures and mechanical strength. From a chemical point of view hemicellulose
belongs to the polysaccharide class, meaning long chains of sugar molecules like galactose, mannose, glucose, and xylose. To be more accurate hemicellulose is an amorphous
mixture polymer in forms of chains built by 500 to 1000 sugar units consisting mainly of
C5-monomers and partly of C6-monomers. A view on one hemicellulose molecule shows
that it is of a ramose structure, with irregular branches.
2.3.2 Cellulose
With about 40 – 50 % on mass basis it is the main component of wood and the most
common organic compound on earth: Approximately one half of the biosphere’s organic
carbon is bound in cellulose. Like hemicellulose it belongs to the group of polysaccharides, although it consists of longer, rigid and linear polymeric chains with basic units of
C6-monomers – alternating α- and β-glucose – joining more than 10,000 monomers together. Due to the alternation of α- and β-glucose the three dimensional structure is
strongly influenced so that – unlike to hemicellulose - no molecular branches can be built.
Cellulose congregates to fibrils inside a matrix of hemicellulose and pectins, whereby the
15
EICHHORN, S.E: Biology of Plants; 2005
10
cellulose’s polar OH- and H+ groups lead to hydrogen bonds to the matrix substances and
by doing so causing the biomass’ fibrous nature. Furthermore cellulose molecules attached to each other - micro- and macro fibrils – form highly ordered areas and providing
the cell wall with crystalline properties.
2.3.3 Lignin
The word lignin can be derived from the Latin word for wood, “lignum”, which can be
explained by the fact that every woody biomass contains this substance in great quantities and it is the origin of woody tissue. Lignin can exist in diverse molecular forms, thus it
is hard to phrase a general description. It also is a polymer, though there is no clear order
of chain link compounds and further it is of a very heterogeneous outer structure with
irregular branches and interchangeable chemical groups. Beside the features of providing
stiffness to the cell wall, acting as glue between the cells, lignin also causes hydrophobicity of the cell wall and protects the wood against biological degradation. In fact, lignin is
the least significant group of the lignocellulose fraction in respect to the active reactions
in the wood during torrefaction. Nevertheless it is absolutely crucial for the principle of
torrefaction due to its nature of a higher resistance to thermal degradation and thus the
quality to keep almost its entire energy content during the process. That goes hand in
hand with the – in contrast to hemicellulose and cellulose - large amount of carbon - on
mass basis around 60 % - bound in the molecule and remaining for oxidation after the
process. Also the product’s beneficial qualities are massively dependent on this substance.16
According to SCHWARZOTT, 1993 the lignocellulose fraction for the two wood types, hardwood and sorftood, differs slightly, as shown by the benchmark values in the table below.17
Table 1 Distribution of lignocellulose fractions for deciduous and coniferous wood,
based on SCHWARZOTT, J.: Stickoxidemissionen bei der Holzverbrennung; 1993
Wood category
Hardwood
Softwood
Cellulose [% of dry matter]
Hemicellulose [% of dry matter]
Lignin [% of dry matter]
40-50
22-40
30-35
40-45
24-37
26-38
Additionally must be stated that these amounts differ again from wood type to wood
type in these ranges and naturally also between individual trees. Furthermore the decomposition intensity of the respective groups may differ between the distinctive molecules of the respective group. In the case of hemicelluloses, for instance, the kinetics of
16
17
www.waechtershaeuser.de; 23.11.2011
SCHWARZOTT, J.: Stickoxidemissionen bei der Holzverbrennung; 1993
11
thermal degradation reactions of xylan and arabinogalactanare expected to be distinguishable.18
2.3.4 Elemental analysis of wood
As indicated above the lignocellulose fraction distribution is one important property of
wood. However, there are many more decisive features that determine the performance
in industrial processing. It is absolutely crucial to have detail knowledge of the biomass
feedstock even before the phase of basic for a wood processing plant hence that knowhow is required for an efficient plant design respectively for the choice of a suitable technology. Roughly90 % of the wood’s dry matter is made up of C and O, 6 % of H and the
residual 4 % are composed of a variety of different elements, which most times are bound
in nutrients. Important elements of those are N, K, Ca, P, Mg, S and Fe. Inconveniently
wood also contains irregular amounts of unwanted heavy metals, often concentrated in
the bark, that are released during combustion.19Above organism-specific concentrations
these group of substances have toxic effects and must be avoided as far as technically
possible, also expressed by strict emission limit values. Furthermore, almost in any biomass Cl, Si and Na is detectable, whereby particularly Cl has negative effects in the incineration process, causing acid emissions and can lead, in cases of incomplete combustion,
to formation of toxic PCDD and PCDF.
2.3.5 Characteristics and associated effects of biomass
According to KALTSCHMITT et al., 2009 there are some other characteristics beside the
chemical composition determining important effects during pre-treatment and combustion of biomass. Some of them influence the fuel quality of biomass and other, being
physical-mechanical properties; have impacts on the processing, handling and economics.
The table below shows the most important properties of biomass related to its energetic
utilization.
Table 2 Quality characteristics and important effects of biomass, based on
KALTSCHMITT et al.: Energie aus Biomasse; 2009
Quality characteristic
Important effects
Chemical composition
carbon
hydrogen
oxygen
nitrogen
calorific value, air demand, particle emissions
calorific value, air demand
calorific value, air demand
NOx- and N2O-emissions
Potassium
18
Ash-bonding-behavior, high-temperature corrosion,
particle emissions
PRINS, M.J. et al.: Torrefaction of wood part 1 – Weight loss kinetics; 2006
GUDERIAN, R.: Terrestrische Ökosysteme – Wirkungen auf Pflanzen, Diagnose und Überwachung, Wirkungen auf Tiere; 2001
19
12
magnesium
Ash-bonding-behavior, bonding of pollutants, ash utilization, particle emissions
calcium
Ash-bonding-behavior, bonding of pollutants, ash utilization, particle emissions
sulphur
SOX-emissions, high-temperature corrosion, particle
emissions
chlorine
emission of HCl and halogenorganic compounds, hightemperature corrosion, particle emissions
heavy metals
ash utilization, heavy metal emissions, catalytic effects, particle emissions
Fuel characteristics
moisture content
calorific value
ash content
Ash-bonding-behavior
Physical-mechanical properties
rubble size
particle size distribution
bridging propensity
bulk density
gross density
abrasion resistance
calorific value, storage life, fuel weight, combustion
temperature
energy content, plant design
particle emission, residue generation
bottom ash generation, residues, operational safety,
maintenance requirements
need for preparation, ignition properties, drying
disturbances in conveyors, pourability, bridging, dust,
explosion hazard
flowability, conveying conditions
storage- and transport costs, power of conveyor elements
bulk density, power of conveyors
fines
2.3.6 Definition of water- and moisture content
For the following explanations it is necessary at this point to state some definitions concerning the terms of wood, respectively fuel moisture content and water content in the
context of bioenergy. The quantity of water that can be removed from the fuel under defined conditions is called water content w. It is the ratio of the mass of water mw to the
investigated material’s mass on wet basis, which consequently implies that the denominator is made up of the sum of the feed’s mass on dry matter and the water in it
(3)
However, there is one other term existing that is often used falsely as a synonym to the
water content w. The fuel moisture content u, from time to time also referred as wood
moisture, is the ratio between the mass of water and the fuel’s mass on dry basis, which
makes it possible to calculate the moisture from the water content.
(4)
13
Hence, moisture content can exceed 100 % and a water content of 50 % corresponds with
a moisture value of 100 %. In the framework of biogenic energy technology the water
content has established, so if values refer to the moisture content, it has to be declared
clearly. The water content’s influence on the calorific value is larger than the biomass
type’s. As a consequence of that, a meaningful comparison of different wood types as
biofuels requires calorific values on dry basis. From time to time even the water-and ashfree value is used. However, there is a linear correlation between the calorific value and
the water content as shown in Picture 4 below.
Picture 4 Linear correlation between water content and calorific value
2.4
Thermo-chemical conversion of lignocellulose biomass
According to KALTSCHMITT et al., 2009 the objective of this kind of treatment may be the
production of easily transportable intermediates, but in most cases it is the supply of bioenergy carriers with well qualified properties for one very specific application. Thermochemical refinement processes transform solid bioenergy carriers under heat influence in
solid, gaseous or liquid secondary fuels, in other words a biogenic solid is chemically
changed by heat input for a later release of thermal energy. In addition to this, plain combustion is also one form of thermochemical conversion. This means that all products of
such processes are eventually brought to an oxidation step that may take place decoupled in both spatial and time-wise respect, meaning that the actual and final use of the
modified fuel can be at another location and also at another time. The products of the
oxidation are exhaust gases and incombustible, inorganic residues in forms of ashes.
Though the products and processes technics differ between the main thermochemical
conversion processes, the basic mechanisms of each agree in principle while the essential
disparity is the air-ratio. This chapter will shortly show and explain the principles of the
14
most important alternatives beside torrefaction, with the goal to enable a better understanding for that process in focus by revealing the technical environment to the reader.
2.4.1 Air-ratio
As mentioned above the air-ratio is one crucial characteristic for each thermochemical
conversion process respectively for any technical incineration. To secure a complete
combustion of a fuel by totally oxidizing all oxidative, organic compounds, it is necessary
to have a stoichiometric excess of oxygen, respectively more air than actually required. A
deficiency of oxygen would lead to a suboptimal energy yield and furthermore to higher
pollutant emission. The level of air access is described by the air-number λ. It is defined as
the ratio between the real quantity of air L added to an oxidation process and the stoichiometrically required quantity of air for complete combustion Lmin, under the widespread
assume that the percentage of oxygen in the gas mixture of air is 21 %.
Vmin can be interpreted in this case as the amount of exhaust gas in addition to the fuel’s
(5)
steam. Consequently λ has to be greater or equal 1 for complete combustion and equals
0 if the reactions take place in the absence of oxygen; such reactions are also known as
pyrolytic decomposition. Thus, an air ratio between 1 and 0 leads to incomplete combustion.
2.4.2 Alternative processes
Combustion
Considering the biofuels’ composition of hydrocarbons exclusively, the products of complete combustion are carbon dioxide and water and the release of thermal energy. The
basic stoichiometric equation for this reaction of complete combustion can be seen below.
Cn H m  (n 
m
m
)O2  n CO2  H 2 O
4
2
(6)
In cases of complete combustion, λ has to be greater or equal 1. If not, the fuel residues
will still contain oxidative compounds like hydrocarbons or carbon monoxide. In fact, as
raw biomass contains also oxygen in its chemical compounds, the added air can be expected to be a little lower than in cases of pure hydrocarbons. More detailed values can
be achieved by combustion calculations.
Pyrolysis
The distinctive feature of pyrolysis is that the characterizing reactions occur only under
the influence of heat and the absence of oxygen.
15
Hence biomass can contain large percentages of oxygen it is consequently possible that
the reactions leading to the decomposition of the biogenic tissue can include oxidation.
For wood, with roughly 44 % of oxygen on mass basis, this is not negligible. The long
chains of lignocellulose polymers are broken due to the strong molecular vibration caused
by the heat transfer, resulting in shorter polymers, the degassing of volatile compounds
and also the escape of liquid substances.
Like in KALTSCHMITT et al., 2009 the term of pyrolytic decomposition is in this report not
used as a synonym for pyrolysis, but for the heat-caused thermo-chemical transformation
mechanisms of organic substances without additional oxygen feed. Pyrolysis on the other
hand labels all technical implementations that apply those pyrolytic decomposition reactions and aim at the production of liquid or solid energy carriers. Pyrolysis forms with
primary solid products are carbonization and torrefaction. Since the pyrolytic decomposition is crucial for torrefaction, it will be explained in more detail in one following subchapter.
Gasification or Partial Combustion
In the gasification process the solid feedstock biomass is transformed into a gaseous secondary fuel, when the combustion or oxidation is incomplete. This is achieved by keeping
the air ratio between 0 and 1.
As introduced above, such combustible gases can be converted into useable energy in
special applications, independent from the time when and the place where the gasification takes place. There are also some processes that were designed to generate electricity
from such gases and can reach a higher stage of efficiency than, for example, standard
feedstock combustion.
Liquefaction
The main goal of this kind of process is the production of liquid energy carriers, like methanol or pyrolysis oil. In most cases this is achieved by a combination of other thermochemical treatments like gasification, pyrolysis or combustion. Hence the technics differ
greatly it is impossible to define a universally valid air ratio. One promising technique for
the future could be the Fischer-Tropsch-Synthesis, which is strongly discussed these days.
Carbonization
In the carbonization process, being another representative of pyrolysis, the feedstock
biomass is transformed to charcoal. The temperature of the input material is brought up
to higher levels than those of torrefaction, as a general rule above 500 °C, though the
heat source then is no longer external, since the reactions taking place change from endothermic to exothermic. As a result the energy yield is decreased, since the chemical energy is released in the form of heat. In contrast to torrefaction, the pyrolytic decomposition
reactions in the carbonization process occur extensively exhaustive.
2.4.3 Definitions mass yield and energy yield
To have meaningful and characterizing performance data that describe a torrefaction
process’s efficiency, it is necessary to define certain parameters. BERGMAN et al.; 2005
16
introduced the mass and energy yield that several younger publications applied, respectively referred to.
(
)
(
)
yM = Mass yield
(7)
yE = Energy yield
(8)
These parameters describe the shift of mass and chemical energy from the biomass’s reactive part into the solid torrefied material. The term of reactive part means in this case
that the amounts of inorganic substances remaining as ash and also of the free water are
excluded in the definition. Moreover, the LHV is preferred to the HHV since as a rule a
plant’s design for energetic utilization of solid biogenic fuels is not applied for an efficient
use of the condensation heat, which leads to the fact that the LHV is the much more application-related and informative value.
2.4.4 Phases of torrefaction
BERGMAN et al., 2005 defined furthermore five stages, through which the biomass is processed during torrefaction. The locations for these stages are the upstream drying kiln,
the torrefaction reactor and the downstream cooling unit. The advantage of defining
these phases is caused by the fact that it offers the possibility to understand better what
happens to biomass particles, when going through the process. Furthermore it is necessary to avoid obscurities concerning similar sounding terms and the sectioning the process in these different phases allows exact definitions of the reaction or torrefaction time,
respectively the reactor residence time.
Initial heating
Is defined as the phase in which the biomass is initially heated, lasting as long as no water
is evaporated. In consequence the heat is solely used to increase the temperature, so the
stage’s end is reached when moisture starts to escape.
Pre-drying
The term of pre-drying refers to the drying process that can be divided into two sections
as will be explained later. During this stage the temperature remains unchanged and the
free water evaporates at a constant rate from the biomass. Hence the evaporation of this
water is an endothermic reaction, the input thermal energy entirely performs the function of the vaporization enthalpy. That is the amount of energy that is thermodynamically
required to transform the water isothermally and isobarically from the liquid to the gaseous state. Consequently this is also the reason for the stagnation of the temperature level. The end of this stage is marked by the critical moisture content that can be well explained by illustrating the drying process in Picture 6. As one can see, the moisture content decreases linearly in the pre-drying phase, but at the stage’s end the function passes
through a bending point which marks the critical moisture content. The further trajectory
shows the diminishing of the moisture loss until all water is evaporated. That curve shape
can basically be registered during any drying process. However, the critical moisture content divides two phases of the drying progress: the first intercept presents the time in
17
which the adhesive water evaporates, whereby its evaporation speed depends on the
ambient conditions. During the second phase the water must diffuse through the material’s pores, which needs more energy and time, since stronger capillary forces must be
overcome. Picture 5 illustrates these two.
Picture 5 Illustration of adhesive and capillary water, based on SCHEFFOLD, K.H.: Praktikumsskript Umwelttechnik – Trocknung; University of Applied Sciences Bingen; 2010
Post-drying and intermediate heating
When all adhesive water is evaporated, the temperature begins to increase again. Now all
the physically bound water is released slowly until the biomass is practically free of moisture. At this stage also first volatile compounds like terpenes may escape, meaning the
first solid compounds undergo a phase transition to the gaseous state, affecting the mass
yield. The end of this stage is reached, when the temperature level attains 200 °C, characterizing per definition the beginning of the torrefaction phase.
Torrefaction
Being the core of the entire process, this stage consists of both a heating and a cooling
phase. In between, there is also a period in which the temperature remains constant,
usually this temperature represents also the top temperature level of the whole process.
With increasing temperatures up to 300 °C the pyrolytic decomposition occurs and the
biomass gets in the truest sense of the word torrefied, though the intensity of this step is
strongly dependent on the process parameters. That is accompanied by the significant
reduction of the mass yield, as described in the section dealing with pyrolytic decomposition. Since the pyrolytic decomposition in the applied temperature level is endothermic
in theory, the stop of the devolatilization should be well controllable, for only the heat
supply has to be interrupted or the reactor must be cooled actively. In practice, however,
it happens that some particles receive more heat and reach a temperature level so the
reactions are transited to exothermic ones. Furthermore, exothermic reactions may occur
at lower temperatures for certain types of biomass and the respective decomposition
regimes. In this case, the control of the process may be highly problematic. According to
definition, the torrefaction phase ends when the temperature falls down to 200 °C once
more. The reaction time trea is comprised of the time when the material is heated from
200 °C to the required temperature level Ttor that is kept constant after that for the time
period ttor. Consequently the period that follows afterwards from the sinking from Ttor to
200 °C is excluded from the reaction time, though it belongs to the torrefaction phase.
18
The reason behind is that most of the thermally unstable compounds of the feedstock
biomass has already been decomposed, thus in the time period t tor,c there are none or
only few reactive substances left, so decomposition is expected to stop as soon as the
temperature is decreased. As shown in picture 6 the mass loss is also predominantly negligible.
Cooling of the solids
During this period the further cooling to the desired final temperature is completed. In
any cause this must be executed in the absence of oxygen, due to the hazards of ignition
or even explosion of the high reactive dust that may occur during the process. 20
Picture 6 Phases of torrefaction, based on BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process; 2005
20
BERGMAN, P.C.A. et al.: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”;
2005
19
2.4.5 Phases of thermo-chemical conversion
Depending on the conversion process, there are four phases of thermo-chemical conversion, that can proceed in combination with each other or individually and separated from
each other, in other words independent stages. The differences between these phases
are the altering chemical and physical reactions, but also the distinct temperature levels
that initiate the particular heat-induced process. Another important characteristic is the
air ratio. The basic phases are the drying and heat-up phase and the pyrolytic decomposition phase, representing the periods in the absence of oxygen and at the same time the
relevant steps for torrefaction. Still, the following phases may emerge after torrefaction.
Executing gasification can make sense to further refine the torrefied material for certain
fields of use whereas the oxidation phase always represents the final step in energetic
utilization of a biofuel, that may occur time-wise and spatial decoupled for an effective
transformation of the chemically bound energy to useful energy. The area that is primarily relevant for the torrefaction decomposition mechanisms is marked by the red borderlines in the comprehensive picture 7 below.
Picture 7 Stages of oxidation, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009
Heat-up and drying
The first stage occurs until temperatures up to 200 °C and consists mainly of the evaporation of free water. As a consequence of the enthalpy of evaporation the increase of temperature is strongly inhibited and may stay constant until greater amounts of water have
vaporized. Another result of this is the fact that there is nearly no energy used to modify
the organic matter. The decomposition mechanisms occur not until the temperature level
reaches a sufficient scale, or, since coupled to it, the free water has been lost.
The heating rate of the biomass must not exceed certain values, for the water’s volume
increases dramatically when undergoing phase transition to steam. That may lead to a
20
blasting of the plant cell wall, especially in the case of coniferous wood, since resins may
plug the radial vascular tissue, if heated too intensely. Also extension values of wood can
be different for each direction in space, leading only to tension forces in the wood structure at first, but eventually to cracks.
Picture 8 Exemplary thermal gravimetric analysis of wood, based on KALTSCHMITT et
al.: Energie aus Biomasse; 2009
Picture 8 shows the thermal behavior of wet wood, when heated in the absence of oxygen. Taking the explanations of the water content above into account it gets clear that
the wet fuel mass can exceed 100 %. Thermal gravimetric analysis, which is the principle
for such diagrams, detects the mass loss of a sample over time and temperature. The residual product, charcoal, sometimes also referred to as coke, is what remains after pyrolysis and has huge carbon content.
Pyrolytic decomposition
The next step to complete oxidation of the fuel is the pyrolytic decomposition, the breaking of the treated material’s molecules under heat influence in the absence of oxygen.
Even if oxygen occurs in a fuel particle’s atmosphere during this thermo-chemical conversion, it does normally not affect the process if it is executed until its final stage, since the
decomposition products flow from the particle’s core to the outside.
The principle of torrefaction is based on the property of lignocellulose material, to lose its
three different main components at distinct temperature ranges at varying intensities. As
torrefaction takes place at temperatures between 200 and 300 °C, the hemicellulose is
the major constituent that is decomposed. Due to its ramose molecular structure it is very
fragile when it comes to greater heat transfers. In fact, the decomposition of hemicellulose is well described as a two-step mechanism, as stated by DI BLASI et al., 1997. According to their published knowledge, in the first step the sugar structure is changed and rearranged as a result of depolymerization, whereas the second step consists mainly of decomposition reactions leading to the loss of these decomposition products respectively
the formation of chars, steam, CO and CO2 as well as the purging of light volatiles like carbonyl compounds out of the carbon skeleton. The first step’s reactions occur at temperatures up to 250 °C which furthermore is accompanied by a very low mass loss which is
significantly increased with higher temperatures that consequently initiate the second
step’s decomposition reactions.
21
Picture 9 Thermal decomposition of lignocellulose fractions and wood, based on
KALTSCHMITT et al.: Energie aus Biomasse; 2009
However, there is already a certain mass loss before the second step takes place. According to KALTSCHMITT et al., 2009, solid biofuel materials start slowly to decompose even at
150 °C, when first macro molecules are destroyed irreversibly under heat influence. Still,
the appreciable pyrolytic decomposition begins at a temperature of 200 °C, when the
organic material starts to disband and the formation of water, CO2, CO and methanol begins. The decomposition of cellulose and lignin happens much slower and crucially less
intense when compared to hemicellulose, since in general both substances are of a much
more stable molecular structure. Consequently the mass amount of degraded hemicellulose at roughly 300 °C is several times higher than the cellulose and lignin together. In
contrast to cellulose, the lignin composition and also the hemicellulose molecular structure are different for hardwood and softwood, leading to distinguishable kinetic parameters of decomposition for the two basic wood types. According to BOCKHORN et al., 2003
coniferous lignin is expected to feature a greater thermal stability than deciduous lignin.
In the case of hemicellulose, deciduous wood is essentially more reactive and thus, devolatilization and degassing occurs earlier and significantly more intense than for coniferous
wood.21However, cellulose is generally the most thermally stable wood component of the
three lignocellulose groups, especially in the temperature range of torrefaction.
The exact pyrolytic decomposition reactions for the three main components of wood are
not fully understood yet, though BERGMAN et al. (2005) categorized the main reaction
pathways to five main reaction regimes, shown in the picture below.
21
WAGENFÜHR et al.: Holzatlas, 1974
22
Picture 10 Main physic-chemical phenomena during heating of lignocellulose fractions
at torrefaction relevant temperature range, based on BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP
Process; 2005
Except the glass transition and softening that is exclusively reserved for lignin, all main
components run through the same reactions, though the temperature levels are shifted.
The glass transition of lignin is an advantageous feature when it comes to densification of
the biomass, since the softened lignin may cause the possibility to replace external binders. The depolymerization and recondensation step means that the polymers begin to
break up and the shortened polymeric chains condense within the solid structure. It must
be clarified incidentally that the graphic above can only be a vague illustration to convey
an impression of the different reactions in combination with the distinguishable main
components of woody biomass. The transition to the next reaction regime cannot be defined as sharp as it is visualized in the figure. Especially for lignin and cellulose the transitions occur over a wider temperature range, mainly dependent on the exact biomass
type. Furthermore the terms “limited” and “extensive” as used in the graphic are only
valid and proportional in the framework of each polymer group individually. Furthermore
it must be kept in mind that also the biomass type can have a large influence on the actual position and width of transition. Summarizing all that conclusively, Picture 10 can only
answer the purpose to show an insight towards the coherences of the different reaction
23
regimes and the lignocellulose fractions. For determined cases there are several factors to
be considered when a more accurate result is targeted. Therefore it is necessary not to
exceed the optimal temperature range of torrefaction.
2.5
Fuel Refinement
The following chapters shall show the improvements of the torrefied wood against untreated biomass, when it comes to the energetic utilization as a fuel. Furthermore it shall
present the composition of the process products.
2.5.1 Van-Krevelen Diagram
For a first impression of what happens during torrefaction, the Van-Reveled diagram is
one useful tool that allows an assessment of complex organic mixtures such as oil, coal or
biomass. It shows the relative quantity of the three most important elements of combustibles, as they are C, H and O, in form of the H/C ratio on the ordinate and the O/C ratio
on the abscissa. Taking coal as the target figure, it has the lowest of both H/C- and O/Cratio whereas the untreated woody biomass has much higher values. In BERGMAN et al.,
2005 the data of several experiments were brought together to see how the elemental
composition of wood reacts during torrefaction and to compare it with other fuels, presented in picture 11Virhe. Viitteen lähdettä ei löytynyt.. It turned out that an increase of
temperature brings the two ratios closer to the ones of coal and the same is true for a
raise of the residence time of the material in the torrefaction reactor. The higher the rise
of temperature or residence time, the more intense are the ratios’ movements towards
coal. Another conclusion found was that the composition of torrefied wood lies between
the ones from coal and wood – corresponding with the statement above, the values of
torrefied wood at lower temperatures are closer to the ones of untreated wood and - in
contrast to this – those of torrefied wood at higher temperatures are nearer to coal.
However, the torrefied material still has huge differences when compared to charcoal,
that requires higher temperatures and loses much more mass, but also energy during the
heating if its production. As stated in PRINS, M.J., 2005 the achieved properties of wood,
by means of the torrefaction process, depend strongly – apart from temperature and residence time – from the wood type or, in more general, from the biomass feedstock.
24
Picture 11 Van-Krevelen diagram for coal, charcoal, peat, torrefied wood [TW = temperature that was kept constant for about 30 min.] and untreated wood, based on
BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power
stations – “Biocoal”
As can be seen from the diagram above, the biomass’s relative loss of H and O is significantly higher than of C. The reason for this is the degassing of mainly low calorific compounds such as the two dominantly calcinated substances CO2 and H2O. Nevertheless,
also combustive compounds like CO and CH4 are released during the process.22 One consequence of this is the decrease of volatile components of the torrefied wood, as compared to the untreated wood the volatile content is reduced from 80 % to 60 %.23
Due to the reasons above there are many torrefaction technologies, whose benefits and
disadvantages can hardly be guessed in practical reality. Still, the technical differences are
quite obvious. The unit operations and process arrangements are predominantly the
same, although the performance conditions vary strongly.
2.5.2 Products of torrefaction
The reaction products of torrefaction can be classified according to the phase, they would
have at atmospheric pressure and room temperature. Thus, the first class would be the
remaining solids, whereas the second and third would contain all the volatile compounds,
that are released during the process. The volatiles consist of a condensable or liquid and a
non-condensable or permanently gaseous fraction. The yield depends highly on the process conditions like temperature and residence time.24
GAUR et al.; 1998 stated the hemicellulose decomposition consists mainly of two reaction
steps, the first being the formation of light volatiles and the second being the catalytic
22
DHUNGANA, A.: Torrefaction of Biomass 2011
PRINS, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005
24
PRINS, M.J. et al.: Torrefaction of wood part 2 – Analysis of products; 2006
23
25
degradation, resulting in the formation of predominantly CO and CO2. Carboxyl groups
under heat cleavage can lead to the formation of acids, which in turn can catalyze dehydration and together with other heat influenced reactions this may result in carbonyls like
methanol, propionaldehyde and other short hydrocarbons.25
In general, under heat influence the biomass molecules may react to new molecules or
remain modified or unchanged. For example when hydrogen bonds get broken and the
polar forces are lost with the escape of the water molecules, so that the polymers may
rearrange in new orders. Or when hydroxyl groups react with hydrogen to water that
evaporates afterwards from the solid material and the hydrocarbon chain is left unsaturated. However, the reactions that take place during torrefaction are complex and not
fully understood, so only the well comprehended and validated observations shall be explained here.
Furthermore the wood type is remarkably important, concerning the substances that are
produced during the process, since the hemicellulose composition differs between coniferous and deciduous wood.26
Table 3 Hemicellulose composition, based on WAGENFÜHR et al.: Holzatlas; 1974
Hemicellulose substance
Deciduous
Coniferous
4-O methyl glucuronoxylan
4-O methyl glucuronoarabinoxylan
Glucomannan
Galactoglucomannan
Arabinogalactan
Other galactose polysaccharides
Pectin
80-90
>1
1-5
<1
<1
<1
1-5
5-15
15-30
60-70
1-5
15-30
>1
1-5
To be more accurate, crucial reasons for the difference is the coniferous wood’s low percentage of the most reactive hemicellulose substance xylan and its property of containing
bigger amounts of glucomannan, being a hemicellulose component of reduced reactivity.27Other values than in the table above were stated in the publication by Prins et al.,
2006. According to their results, the hemicellulose composition strongly affects the
weight loss kinetics of biomass during pyrolytic decomposition and so is an important
factor of the torrefaction performance.
Nevertheless, the liquid fraction of torrefaction can be divided into the three subclasses
of reaction water, organic compounds and lipids. Depending on the biomass type, the
distribution of the exact substances varies. The organics are predominantly products of
carbonization and devolatilization, whereas the lipid fraction consists of compounds that
all can be detected in the original biomass, which implies that they are only driven out of
the solid material during torrefaction but not real reaction products. Liquid substances
that are confirmed by several research findings are lactic acid, formic acid, furfural, hy-
25
GAUR, S.; REED, T.B.: Thermal Data for Natural and Synthetic Materials; 1998
PRINS, M.J. et al.: Torrefaction of wood part 2 – Analysis of products; 2006
27
CIOLKOSZ, D. et al.: A review of torrefaction for bioenergy feedstock production; 2011
26
26
droxyl acetone, methanol but also phenols. However, acetic acid and water, meaning the
water formed during the torrefaction transformation reactions in addition to the free
water that remains un-evaporated after the upstream drying, are the substances that can
be expected to be the main liquid products for any torrefied biomass.
The solid phase consists of various compounds and remains with an unordered and multitudinous structure of original sugars and modified or entirely new polymers. The occurrence of aromatic rings is possible, as well as the appearance of coal-like carbon skeletons
and first ash formations.28
The permanent gas, also referred to as torrgas or torrefaction gas, includes mainly CO and
CO2, whereby the oxygen is released from the original biomass compounds, and also traces of molecular hydrogen and methane. As described in BERGMAN et al., 2005, the formation of CO2 may be caused by decarboxylation of the wood’s acid groups, whereas the
creation of CO cannot be a result of decarboxylation nor dehydration. One possible explanation for the large outcome of CO is reported in DIETENBERGER et al., 2001, stating that
the reaction of CO2 and steam in the ambiance of porous char produces CO with increasing temperature. However, the gas fraction includes also carcinogenic, light aromatic
compounds like benzene and toluene. Picture 12 summarizes the distribution of products
and the suggested classification method.
Picture 12 Torrefaction product analysis, taken from BERGMAN: Combined Torrefaction
for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP Process;
2005
2.5.3 Combustibility of the torrefaction gas
28
PRINS, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005
27
It is important to keep in mind, that the liquid fraction, as classified above, is usually contained in the torrgas under the real conditions of the torrefaction process. Since this gas
mixture is supposed to be burned to feed the reactor with process heat and thereby enhance the energy balance of the entire process, it is necessary to know the quantitative
composition of the compounds in the gas mixture and how supportive they are for the
combustion step.
As water hinders any incineration, it is necessary for an efficient combustion to condense
the water upstream to the furnace. Furthermore CO2 is another substance obstructing the
incineration, as it cannot be oxidized any further, for the final oxidation stage is already
reached. From an absolute view on energy bound to the permanent gases, CO contains
the largest amount of energy, since the hydrocarbons in this fraction are only present in
traces. The lipids contain the most energy from all the volatile compounds, followed by
the organics, though the measurements were only done for willow. Picture 13shows the
mass and energy percentages of the three main fractions, although the liquid section is
illustrated with its single substance categories. Furthermore it shows the composition of
the permanent gas.
Picture 13 Torrefaction main fractions and permanent gas composition, taken from
BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power
stations – “Biocoal” - The TOP Process; 2005
Picture 13 shows the detailed composition of the organics, where quantitative measurement results were achieved. Other published research results related to similar results.29
29
Prins, M.J.: Thermodynamic analysis of biomass gasification and torrefaction; 2005
28
Picture 14 Detailed composition of organics, taken from BERGMAN: Combined Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal” - The TOP
Process; 2005
BERGMAN et al., 2005 concluded that, even when completely dry biomass is torrefied, the
wet torrgas still possess roughly 50 % water and 10 % CO2, so both incombustible substances occur in huge amounts. Nevertheless, oxidative compounds exist in the torrgas as
well and the real composition must be considered for each individual case, since the process conditions and the input material play key roles for the gas mixture’s composition.
Also several developers claim to feed the reactor exclusively with the heat of the torrefaction gas combustion.
Another aspect that is to be considered in case of torrgas combustion is the adiabatic
flame temperature. To oxidize even the most difficult but still combustible compounds in
the gas mixture, the adiabatic flame temperature needs to be significantly higher than the
auto-ignition temperature of those substances. A difference of 400 °C is a typical value to
secure a stable combustion. Furthermore the analysis in this work revealed that CO and
phenol are the most challenging compounds for combustion, each with the point of autoignition of approximately 600 °C. As a result, an adiabatic flame temperature of roughly
1000 °C is required to secure the combustion of all oxidative components of the torrefaction gas. In simulations of different torrefaction conditions they further figured out a correlation between reaction time, respectively torrefaction temperature and the adiabatic
flame temperature. This can easily explained with the increasing concentration of combustible compounds that follows a rise of the residence time or torrefaction temperature.
So permanent gases with a higher calorific value are produced, leading to the knowledge
that an adiabatic flame temperature of 1000 °C is achievable in most cases, but the margin may shift, depending on the individual case and its parameters.30
30
BERGMAN et al..: Torrefaction for biomass co-firing in existing coal-fired power stations – “Biocoal”; 2005
29
3 ALTERNATIVE PROCESS CONCEPTS AND SPECIFICATIONS
3.1
Autothermal operation
The ideal case of torrefaction processing is an autothermal operation, which basically
means that there is a balance between the chemical energy that is bound in the released
volatiles during the process and the return of this energy to the reactor and to the drying
kiln in the form of heat after combusting the torrefaction gas. Hence, there is a circulation
of energy flows and no necessity of introducing other energy carriers to the process. To
achieve this, the process parameters must be optimized in terms of residence time and
torrefaction temperature in the context of the input biomass and the process design. In
the case of insufficient energy content in the torrefaction gas, it is necessary to add another fuel like natural gas, which then causes increased operating costs. However, it is
also possible to burn some feedstock biomass to supply the process with heat, though the
water content of the biomass may decrease the adiabatic flame temperature. In contrast
to this, another, yet undesired scenario may occur, when the process is operated above
the point of autothermal operation. This may lead to major energy losses, since the torrefaction gas contains too much energy and the remaining biocoal’s calorific value is decreased too strongly and the product depreciates.
3.2
On-site and off-site
To find the best location for a torrefaction plant there are three basic systems to be differed.
The classical categories are on-site- and off-site torrefaction. The on-site torrefaction refers to the physical connection of the torrefaction system and the power plant, where the
torrefied product is utilized for electricity and possibly additional heat generation. The
integration of the torrefaction system into the power plant can be realized in versatile
ways.
30
Picture 15 On-site torrefaction
To state some possibilities, the power generation process could deliver its waste heat to
the torrefaction reactor and drying chamber. BERGMAN et al., 2005 stated that most likely
all existing power plants need to be modified, and may have remarkable negative effects
on the plants steam capacity and soon its electricity efficiency. Furthermore, both torrefaction and electricity generation processes could become highly dependent to each other and problems with one process might deteriorate the other one’s operation. If the torrefaction process produced exhaust gases that need to be cleaned (for example VOC’s
from the drying step and the torrefaction step itself, or flue gases, when the torrgas or
additional biomasses were combusted for heat supply of the process) the power plant’s
exhaust gas cleaning systems could be used to treat those gas flows. From an emissionwise view, the combustion of torrgas would be easy to handle, as long as the integration
of the torrefaction gases to the power plants flue gas pipes is possible. Still, the piping of
an existing plant needs to be analyzed in the individual case, but always means an additional effort. The biggest disadvantage of this system, however, is the loss of one main
torrefaction benefit: The decreased logistics costs after densification of the torrefied material, leading to a higher specific, volumetric energy content. All in all, the eventual effects of an integration of torrefaction into an existing power station are not expected to
be recommendable.
Locations for off-site torrefaction require a good infrastructure and huge biomass sources
in the surrounding area. The process’s required heat must be produced by the torrefaction system itself and may lead to additional feedstock biomass combustion or torrgas
utilization.
31
Picture 16 Off-site torrefaction
Still, the torrefied product may profit from the decreased transportation costs of feedstock biomass to the off-site location, due to its surrounding forest areas, and of the torrefied pellets to the client power plants, because of the higher, volumetric energy density.
The idea of unproblematic outside storage of torrefied biomass must also be considered
to be cost beneficial.
The third one, being a very special system, is mobile torrefaction, in form of a moveable
torrefaction plant, for instance located in a truck. The main idea and positive aspect of
mobile torrefaction is that the process can be operated at any location, which is temporarily rich of (waste) biomass and can afterwards be moved to the next place where it
makes sense to torrefy biomass residues. Most likely the mobile system is not able to perform with such a high efficiency as a static one and furthermore the capacity is expected
to be highly limited. However, the company Renewable Fuel Technology LLC from Nebraska, USA is working on such a torrefaction plant and the worldwide interest seems to
be quite big. Especially in regions with problematic infrastructure but high biomass outcome, mobile torrefaction could be a great opportunity to produce a quality biofuel instead out of waste materials.
3.3
Comparison of direct- and indirect heating
Another basic aspect of torrefaction is the concept of how the required heat is brought
into the reactor to the biomass particles, so that pyrolytic decomposition can take place.
Firstly, the reactor design determines what kind of heating strategy can be applied and
this choice decides also the further process arrangement. Today the differentiation of two
main reactor categories gained acceptance, being direct heating and in-direct heating.
32
3.3.1 Process flow sheet: Direct heating
Picture 17 Exemplary process flow sheet: Direct heating
This concept is characterized by direct contact of the biomass in the reactor and a heat
carrying fluid, in all known cases in the form of flue gas of torrgas combustion or circulating torrefaction gas itself. In the illustration above the torrgas is directed to a combustion
chamber, which is additionally supported by natural gas (it could further be fed with input
biomass). A partial torrgas flow is guided to a heat exchanger before it reaches the combustion chamber. From the incineration step the hot flue gas streams through the heat
exchanger. The flue gas heat is transferred to the partial torrgas flow, which is guided into
the reactor once more. The cooled, yet still warm flue gas can then be brought to the drying kiln, before it is released into the atmosphere or, possibly together with the volatile
compounds from the drying, treated in an exhaust gas cleaning system. Usually, the flue
gas cannot be used as the direct heat carrier, because the combustion is operated under
air excess, so oxygen would be brought into the reactor, disturbing the process operation.
33
3.3.2 Process flow sheet: In-direct heating
Picture 18 Exemplary process flow sheet: In-direct heating
In contrast to direct heating, in this concept the heat is transferred through a solid wall
into the reactor. That means that the torrefaction reactor has some analogous tasks as a
heat exchanger and so its design must also feature some conformities. The torrgas is
completely guided to the combustion chamber and the flue gas gives off its heat to a fluid
that circulates between the torrefaction reactor and the heat exchanger. The transmission medium is normally suggested to be superheated steam or some thermo-oil. A second heating circle may be established for drying purposes. In-direct heating means more
effort than direct-heating – on the other hand, the separation of the heat transfer medium and the treated biomass offers no possibility of negative impacts from the heat supply
on the product.
3.4
Future trends
There are, besides alternating process arrangements, even more possibilities in how the
industrial scale torrefaction could look like, if the detailed options in the reactor design
are considered. Possible reactor technologies are in general:
 Rotary drum reactor
 Screw conveyor reactor
 Microwave reactor
 Compact moving bed
 Oscillating belt conveyor
34
However, there are some special reactors that have been designed especially for pyrolytic
applications, like torrefaction.
 Torbed reactor31
 Multiple Hearth Furnace32
 WyssmontTurbo-Dryer®33
4 CO-FIRING OF UNTREATED BIOMASS AND TORREFIED WOOD IN COAL POWER
PLANTS
4.1
Biomass co-firing in general
As the most eligible field of application for the torrefaction’s product most publications
refer to co-firing of the torrefied material in existing coal power plants. This chapter will
show the decisive aspects of this area of implementation in general and state the benefits
of torrefied material towards conventional biomass for co-firing purposes. Furthermore it
will relate these aspects to the two most important technologies that are intended for cofiring, as they are pulverized coal firing and circulating fluidized bed combustion. More
precisely about 90% of the world’s coal firing power plants mill the fuel to powder and
inject it into the combustion chamber, thus called pulverized coal firing. The predominant
number of the remaining 10% of power plants uses modifications of fluidized bed firing,
which means basically a bulk of coal particles that is flown through from below by combustion air.34
In general there are two ways of co-firing, namely direct and indirect co-firing. In the case
of direct co-firing the biomass is brought into the coal input-flow at a certain point of the
plant system, so the boiler is fed with a biomass-coal mixture. Since the biomass properties can be from such a difference to the coal processing, it is usually necessary to modify
the plant components, as will be described later. Indirect co-firing means that a second
pre-treatment and processing line is constructed, which deals exclusively with the biomass feedstock. The biomass is then combusted separately from the coal, and only the
steam of both processing is brought together afterwards to rotate the turbines, driving
the connected generator. This leads first of all to an immense increase of capital investment costs, but may also result in higher operating costs. In most cases the expenses for
direct co-firing are lower, so this way is generally preferred.35 However, other references
divide the co-firing of biomass into other categories, like EVANS, 2008 did.36
31
http://www.torftech.com/applications/biomass_processing.html, 10.01.2012
http://www.cmigroupe.com/en/p/multiple-hearth-furnace-m-h-f-and-shaft-kiln-s-k, 10.01.2012
33
http://www.wyssmont.com/lib/images/pdf/torrefaction-newsletter.pdf, 10.01.2012
34
LÖSCHEL, A.: Die Zukunft der Kohle in der Stromerzeugung in Deutschland; 2009
35
MACIEJEWESKA et al.: Co-firing of biomass with coal – constraints and role of biomass pre-treatment; 2006
36
EVANS, Dr.G.: Techno-Economic Assessment of Biomass “Densification” Technologies; 2008
32
35
The utilization of biomass in coal fired boilers is limited to a certain percentage beside the
used fossil fuel due to several issues. If greater amounts of biomass are to be used, some
plant components need major modifications that are linked with greater costs. Another
limiting factor is the cost-effective supply of biomass at the individual plant’s location. In
fact, with a plant’s capacity rises also its efficiency, thus it would be the best to use biomass material exclusively in plants with a total output of several 100 MW. However, in
reality co-firing takes also place in smaller power plants since at most locations of power
plants the availability of the required biomass resources in the area of catchment is severely limited. In order to secure the plant ‘s supply at higher co-firing amounts with
feedstock biomass, nonetheless, it would be indispensable to bear the additional huge
costs for logistics, that would turn the total economics in all likelihood for the worse. That
is why, at least for the western European countries, the upper limit for the supply of biomass fuel performance for a single power plant is in the range between 50 and 100 MW.
However, considering the large installed electrical output of high-performance power
plants, even co-firing percentages of only 10 % for such plants, result in immense biomass
amounts. Furthermore, since one benefit of torrefaction is the increase of energy density
and the reduction of transport costs at the same time, it is possible to extend the borders
of a high-performance plant’s catchment area. This enables a higher percentage of biomass input for co-firing purposes and in the case of co-firing in high-capacity power plants
a more efficient utilization of the renewable resource of torrefied wood.
Another positive aspect to be considered is the easy replacement of biomass by coal in
the case of supply shortage due to seasonal or other unavailability. Furthermore torrefaction reduces the chance of such situations for the simple reason that its product is not as
degradable as untreated material and can be produced in advance to store it effortlessly
for later use. Aside from that, torrefied biomass tops conventional biomass feedstock in
terms of modification requirements, since it has properties more similar to coal, allowing
it to be added to the existing coal volume flows without greater challenges.
Especially in Europe the legal framework for the combustion of fossil fuels becomes more
and more complex, strict and expensive. Substituting those by renewable secondary energy carriers can lead to financial gains for power plant operators and make it unnecessary to purchase additional CO2-certificates. Furthermore it can help in terms of marketing and promotion, seeing that environmental protection is effectuated.
4.2
Pretreatment
In general, the pretreatment requirements of co-firing applications are dependent on the
furnace and the type of biomass. Basically it is necessary to liberate the biomass material
from any foreign matters to protect downstream process devices like grinding apparatuses like crusher and mills or dosing units like nozzles. This demand is not applied for torrefied material, since the raw biomass has already been freed from extraneous matter. The
same is true for the drying pretreatment, for the torrefied biomass has a maximum uptake of moisture of 3 %. That allows an improved total energy balance for the entire process. In contrast to strongly polluted materials like sewage sludge or chemically treated
wood, torrefied material ordinarily requires no upstream scrubbing, since no special
emissions are to be expected.
36
4.3
Milling
To achieve a complete combustion, especially in case of pulverized coal fired boilers, it is
necessary to mill the coal and added woody biomass to powder of 2 to 4 mm average
size. Nevertheless, in power stations with higher thermal capacity of several 1,000 MW,
the longer residence times in the combustion chamber allow to feed the incineration with
bigger particles as well. The comminuting can be done by upstream cutting-, bowl- and
hammer mills. Unfortunately, untreated biomass can cause malfunctions of these components, due to its fibrous structure that confers mechanical strength and tenacity. During the process the bundled cellulose fibrils may not be broken, so they wrap around the
milling tools, with a chance of stalling them. In general, the higher the biomass’s water
content, the larger is a mill’s power consumption to grind it. In case of pulverized bed
firing it is for most cases sufficient to feed the combustion with wood chips. The power
consumption of the mills rises also with decreasing particle size. In case of the hammer
mill, being the most energy-saving variant, still about 1% of the biomass’s energy is needed for the grinding operation.
An upstream torrefaction can eliminate this set of problems, since the tenacious nature of
biomass is replaced by brittleness and the absorption and content of water is minimized.
The result referring to the milling process is crucial, as Bergman et al., 2005 found out.
The diagrams below show the outcomes of these investigations.
Picture 19 Left: Size reduction of coal, biomass and various torrefied biomasses. [Coding: biomass type (C = Woodcuttings; D = Demolition wood; W = Willow) (torrefaction
temperature, reaction time)]; Right: Mill capacity correlation to average particle size
[same coding], taken from BERGMAN: Combined torrefaction and palletisation – The
TOP Process; 2005
According to the left diagram there are great differences between woodcuttings with relatively low moisture contents of roughly 15% and torrefied wood. The reduction of the
mill’s power consumption lies in the range of 70% to 90% and depends in detail on the
torrefaction’s process parameters and the feedstock material. Generally, the higher the
37
temperatures, the better the improvement for the milling step. For reasons of comparison there are also data points for bituminous coal entered in the diagram and this fossil
fuel’s curve is similar to the torrefied material. In some cases the torrefied products even
exceed the well positioned values of coal. This means, however, that the input of biomass, when torrefied, is no longer limited in consequence of grinding issues. Torrefied
material can easily be brought to a particle size that can be conveyed and handled like
coal, so there are no needs for modifications. Instead, a mill’s capacity can increase significantly by factors ranging from 7.5 to 15 and dependent on the process conditions.
Thus, the target capacity can be achieved with smaller comminuting tools, leading to decreased investment costs for new plants and, in respect to the enhanced energy efficiency, diminished operational costs in general.
4.4 Expected performance of torrefied wood in coal power plants
By co-firing biomass in coal power plants the mass- and volume flows of the fuel, but also
of the exhaust gas can change significantly. Also the conveying and storage systems need
sufficient capacity. The exhaust gas may consist of a higher water percentage when biomass is added to the combustion, which could make it necessary to change the fuel mixture’s residence time in the incineration chamber on the one hand and could lead to altered heat transfer behavior at the steam generator. The following picture shows a comprehensive view on the possible effects of average biomass co-firing on the components
of a pulverized coal firing power plant.
Picture 20 Possible co-firing induced effects and affected components of a pulverized
coal power plant, based on KALTSCHMITT et al.: Energie aus Biomasse; 2009
38
Bringing biomass into the processing system of a coal fire plant increases the input flows
mainly due to the distinctly lower volumetric energy density. The type of coal primarily
burned, decides over the plant’s design, concerning for instant the conveying- and storage capacity or the combustion air supply system. Considering this, it is clear that co-firing
can much easier be obtained when the secondary fuel is similar to the fossil coal. Since
torrefied material has a higher calorific value than the untreated biomass, it can solve the
problem to a certain scale, for the heightening of mass- and volume flows is not as intense as before.
The diagram below illustrates the change of fuel volume flows when certain amounts of
untreated biomass are co-fired. The alteration by co-firing torrefied wood is most likely
not so strong in the combination with bituminous coal, for their calorific values lie closer
together.
Picture 21 Increase of fuel volume flow at goods inward state [bulk densities: Bituminous coal 870 kg/m3, brown coal 740 kg/m3, wood chips 250 kg/m3 (w = 30 %), straw
150 kg/m3 (w = 15 %)], based on KALTSCHMITT et al.: Energie aus Biomasse; 2009
According to KALTSCHMITT et al.; 2009, it is indispensable to consider the different water
contents of the primary and secondary fuels, since the new exhaust gas moisture can
cause immense volume extensions.
39
Picture 22 Change of humid exhaust gas flow at biomass co-firing, based on
KALTSCHMITT et al.: Energie aus Biomasse; 2009
Naturally this is accompanied with increased flow velocities, modified heat transfer and
raised pressure losses. That is another argument why torrefied and thereby dry wood is
only to be co-fired with bituminous coal, whereas humid biomass would better be combined with raw brown coal.
After all, one problem of co-firing untreated biomass is expected to be overcome completely: The homogeneity of the supporting bio fuel. Whatever origin the torrefied biomass comes from, after torrefaction the physical and chemical properties between the
different feedstock biomasses are more similar than before. After co-firing tests with torrefied biomass in a power plant (400MWelpulverized coal power plant in Borssele, the
Netherlands) WESTSTEYN, 2004 concluded a potential for higher co-firing ratios than those
applied in the investigation, where the torrefied wood was progressively mixed with coal
up to 9 % content on an energy basis. Another outcome of this experiment were, that the
pulverized coal boiler did not deliver any measurable negative effects, so torrefied wood
was assigned to be possibly a viable option for direct co-firing.37
4.4.1 Slug formation, ash accumulation and pollution
With decreasing ash melting points of the feedstock biomass, the pollutions and slugging
at the convective heating walls increase. The ash composition is solely related to the
feedstock, since the torrefaction process itself does neither change the ash properties nor
the melting points. Alkali metals (Li, Na, K, Rb, Cs and Fr) are the elements that cause the
decrease of the melting points, which certainly is not a problem, as long as wood serves
as the only biomass type to be co-fired, because of its low alkali content.
37
WESTSTEYN A.: First Torrefied Wood Successfully Co-Fired; 2004
40
The ash percentage of wood or stalk-type biomass is significantly lower against coal, and
so unburdens the dedusting systems of the plant, since the total amount is always decreased when such biomass is co-fired. Nevertheless, the ash utilization gets more complicated, whenever coal and solid biofuels are combusted together, for the properties,
especially the chemical composition, differ between coal ash and biomass ash. The mixture ash, that remains when both are burned together, is not utilizable, as it would be
possible for the ashes each solely of coal or biomass. Half the amount of available ashes
from100 % coal combustion are used in the construction industry and underground mining, whereas the other half is utilized for restoration of open cast mines, pits and quarries.
Those ashes, especially fly ashes, can be highly contaminated with ecologically damaging
substances like heavy metals. That is also the reason why parts of the ash must be disposed and is not adapted for further utilization. In contrast to this, the biomass ashes
contain usually significantly fewer pollutants, since the inorganic elements that remain as
ashes from the biomass have originally been extracted from the environment where it
was grown. So restoring those substances to these locations can be sustainable, for this
supports and helps closing the mineral cycle of the soils and the ecosystem. Finland’s legislation allows the recycling of wood ashes in the fields of forestry. If it is not advisable to
return the ash to the natural environment, due to possible contaminations, high pollutant
contents or insufficient combustion, DE NIE, et al., 2005 suggest to use it as a raw material
for fertilizer, as a building material or re-using them as a fuel on condition that the ash is
rich on carbon with a high calorific value, which normally is only applicable to fly ashes
from fluidized bed gasification of biomass.38
When indirect co-firing is applied and the combustion of coal and biomass takes place in
individual chambers, the ashes occur also separately and can be used each for the optimal
purposes as described above. However, since the normal way of co-firing is the direct
one, the remaining ash is a mixture of coal and biomass ashes. The mixture is neither appropriate for the utilization ways of exclusive coal ash nor of exclusive biomass ash, so
new strategies are to be found to use this mixture as best as possible. As those utilization
strategies are strongly dependent on the coal and biomass quality, the optimal treatment
can differ between individual cases, so no universal advice can be given at this point.
Nevertheless it is generally accepted that wood is more suitable for co-firing with coal in
respect to the ash formation. Since the ash composition of wood and torrefied wood is
not changed significantly, there are no notable differences expected from this point of
view.
4.4.2 Corrosion and erosion
Another nature of certain biomass types that prevents unlimited input of biomass in coal
fired boilers is that may promote corrosion. While generally stalk-type biomass like straw
has an increased chlorine-content, when compared to coal, and also contains potassium
chloride (KCl) that intensifies corrosion effects, woody biomass does not have such properties. The effect of high chlorine amounts in the feedstock is an intense high38
MACIEJEWSKA, A. et al.: Co-firing of biomass with coal – constraints and role of biomass pre-treatment; 2006
41
temperature corrosion at the convective heating walls, where the steam and exhaust gas
reach their temperature maxima. Hence, the input of such biomass is limited to low percentages of roughly 10 %, while wood can usually be brought into the boiler at higher
rates without hesitation.
The plant components, which are flown through by dust-laden gases, can be under the
influence of erosion, whenever it comes to friction and collisions between particles and
the component surfaces, resulting in material abrasion. Since, in contrast to coal, biomass
types that are in discussion for co-firing have normally very low amounts of ash, erosion
problems, especially in the exhaust gas pipes, may be mitigated. However, effects on the
corrosion and erosion behavior by torrefaction are not expectable, but as long as wood
serves as feedstock material, there are no limitations for the input amount anyway.
4.4.3 Emissions and exhaust gas cleaning
The co-firing of both woody and stalk biomass results in decreased occurrences of the
most harmful gases in the raw gas and in the exhaust gas cleaning. Biomass has naturally
lower amounts of sulphur but also partly binds the sulphur chemically in its ashes during
combustion, so less sulphur dioxide can be formed. So usually the load of the flue gas
desulphurization plant is reduced by co-firing biomass. Furthermore, different other substances like mercury, arsenic, plumbum and more heavy metals are removed beside sulphur oxides in the flue gas desulphurization plant, which affects the denox performance
as explained below. Nevertheless, in particular cases, the change of the exhaust gas composition and new substances can lead to the burden of such plants, especially when too
much chlorine is contained in the gas flow. As described in the section of corrosion, this is
not likely for wood as feedstock material, since the chlorine content is similar to the coal’s
one.
Also positive effects on the emission behavior of nitrous gases maybe observed, due to
advantageous combustion kinetics of biomass, although the state of knowledge is not yet
clear in respect to those emissions, since the observations are irregular. In some cases
also enhancements in the emission balance of nitrous gases are possible, especially when
wood serves as feedstock biomass. Related to the denox-plants in the case of pulverized
coal firing, the possible effects are significantly different for low-dust, respectively highdust arrangements. For the case of low-dust arrangement, the dedusting system and the
flue gas desulphurization are arranged upstream to the denox plant, which decreases the
risk of negative effects on the denitration system, since substances that are hazardous for
the catalyst are removed to the greatest extent. High-dust arrangement can result in
plugging of the catalyst’s active cells (alkaline metals and alkaline earth metals) or deactivation of the catalyst itself (reaction of the catalyst with e.g. K, Na, P and As), though this
is also dependent on the combination of the coal and biomass input and the applied cofiring technology. That is, however, why in this respect the low-dust arrangement is superior to the high-dust system, even though high-dust process arrangement still can be feasible for particular cases.
Again, wood is superior against straw and other stalk-type biomass, since wood contains
or produces generally much less of the problematic substances. Furthermore, experi-
42
ments have shown that nitrous oxide (N2O) can be reduced by biomass co-firing. Especially for pulverized coal firing, there are no particular requirements on the nitrogen content
of the biomass, since even higher amounts of nitrogen can be controlled by adjusting the
combustion conditions, like temperature, fuel feed and excess air ratio.
The emissions of carbon monoxide are likewise not expected to increase, provided that
the biomass is ground to a sufficient size in the case of pulverized coal firing. Co-firing
straw increases the emission of hydrogen chloride (HCl) extremely, whereas the use of
wood results usually in decreased emission values. The same applies for PCDD/PCDF
emissions.
All this considered, the emission balance of co-firing wood does not deteriorate against
exclusive coal combustion, neither in pulverized coal- nor in fluidized bed firing. In some
points the biomass feeding can even be advantageous. However, the type of biomass is
the crucial parameter, when it comes to the assessment of the emissions and in each
case, wood is the superior feedstock, when compared to stalky biomass or even sewage
sludge, which is the most problematic one.39
Torrefied wood has the potential to substitute huge amounts of coal for electricity generation in existing but also in power plants yet to come. Vogel et al.; 2011 speak of possible
co-firing rates of torrefied pellets of 50 %, but state also that more experiences with the
new fuels in terms of technical and logistical aspects are required. In comparison, according to Flyktman et al. (2011) the co-firing rate of the wood pellets is only 15 – 20 %. All in
all, the direct substitution of coal had a tremendous potential to decrease the world’s
greenhouse problems.
5 CONCLUSION
In 2009 the global economic recession caused an enormous downfall of the demand for
forest industry projects.40 To stop the trend and to protect the industry from such happenings in the future, several projects were started to find new fields of expertise and to
secure the existing business environment.
When it comes to the energetic utilization of wood, the technology of torrefaction has
many advantages against existing processes. Especially in Finland, with its huge outcome
of woody biomass and great amounts of forestry waste residues, the idea of refining the
natural resources can be realized much more easily than in other countries in the world,
but especially in contrast to Central Europe, where there simply are not such forestry areas left. It is quite expectable that the big European energy suppliers have a great interest
in biofuel products, as these substances can be used in existing plants without the need of
greater modification investments. At least in theory torrefied material fulfills all requirements that would allow co-firing it in coal power plants.
39
40
KALTSCHMITT et al.: Energie aus Biomasse; 2009
Aarne et al.: Finnish Statistical Yearbook of Forestry 2010; 2010
43
Furthermore it is generally accepted that biomass is CO2-neutral, so there may occur financial benefits related to emission trading. In contrast to conventional biomass products, torrefied wood has several advantages due to its new gained properties of hydrophobicity, effortless grindability and its remarkable calorific value. Considering that densification is applied additionally, the torrefied product (with its increased volumetric energy
density) has even more great advantages, when it comes to long distance transportation.
In this case, the densification would have additional benefits, since great amounts of
greenhouse gases are emitted by coal and other solid fuels during transportation, independently from the final utilization in a power plant. The general trend is: Long distances
deteriorate the specific emission values of woody biomass.41 Since conventional wood
pellets perform remarkably better than wood chips (due to higher energy density and
decreased dusting behavior), torrefied pellets should be able to enhance those values and
to minimize the problem. This is why secondary energy carriers are preferred for longdistance transportation.42
There are, however, still several open questions about torrefaction. First of all it is the
process handling and the process design. How exactly does residence time, heating rate,
particle size, heating strategy, etc. influence the product quality, the decomposition kinetics and the process conditions itself? What exactly happens from a thermo-chemical view
with the biomass during the process, what reactions take place and how does the biomass’ chemical composition influence these reactions? And one very important issue is
the scale-up of the process. Now that first torrefaction plants come nearer to realization,
it will be interesting how large-scale torrefaction will perform and whether there will be
any process designs that gain a general acceptance and eliminate others from the screen.
And of course the economics of the process, which have not been analyzed at all in this
report, are crucial to the technology’s future success. The logistics and net efficiency of
the process, but also subsidies seem to play key roles in this context.
Nevertheless, the process of torrefaction has the potential to enhance the sustainability
character of the energy industry and to reduce environmental pollutions emitted by coal
power plants. Over the last years, the percentage of biomass derived electricity generation has increased and as the global interest for this new technology is rocketing, the demand for a fuel that process developers claim the torrefied product to be, is growing as
well. Considering this, joining this innovative business field is from an economic view on
the one hand and from an ecological view on the other, a reasonable decision.
41
EDEL, M. et al.: Die Mitverbrennung holzartiger Biomasse in Kohlekraftwerken; 2011
FAAIJ, A.: Export of torrefied and non torrefied biomass, comparison of technical and economic performance; 2011
42
PICTURE LIST
Picture 1 Torrefaction development activities in Europe ...................................................... 3
Picture 2 Torrefaction development activities in the USA and Canada ................................ 4
Picture 3 Basic exemplary operation arrangement for biomass torrefaction....................... 8
Picture 4 Linear correlation between water content and calorific value ............................ 13
Picture 5 Illustration of adhesive and capillary water ......................................................... 17
Picture 6 Phases of torrefaction .......................................................................................... 18
Picture 7 Stages of oxidation ............................................................................................... 19
Picture 8 Exemplary thermal gravimetric analysis of wood ................................................ 20
Picture 9 Thermal decomposition of lignocellulose fractions and wood ............................ 21
Picture 10 Main physic-chemical phenomena during heating of lignocellulose fractions at
torrefaction relevant temperature range ............................................................................ 22
Picture 11 Van-Krevelen diagram for coal, charcoal, peat, torrefied wood and untreated
wood .................................................................................................................................... 24
Picture 12 Torrefaction product analysis............................................................................. 26
Picture 13 Torrefaction main fractions and permanent gas composition .......................... 27
Picture 14 Detailed composition of organics ....................................................................... 28
Picture 15 On-site torrefaction ............................................................................................ 30
Picture 16 Off-site torrefaction............................................................................................ 31
Picture 17 Exemplary process flow sheet: Direct heating ................................................... 32
Picture 18 Exemplary process flow sheet: In-direct heating ............................................... 33
Picture 19 Left: Size reduction of coal, biomass and various torrefied biomasses. Right:
Mill capacity correlation to average particle size ................................................................ 36
Picture 20 Possible co-firing induced effects and affected components of a pulverized coal
power plant .......................................................................................................................... 37
Picture 21 Increase of fuel volume flow at goods inward state .......................................... 38
Picture 22 Change of humid exhaust gas flow at biomass co-firing .................................... 39
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