Combustion of Difficult Biomass
Fuels: from Particle Ignition to Large-Scale Studies
Mário Costa
Instituto Superior Técnico, Lisboa, Portugal
Structure of the presentation
 Overview
(facts, projections, resources, potentials)
 Difficult biomass fuels
(what do I mean?, biomass properties)
 Single particle studies
(early combustion stages: ignition & devolatilization)
 Drop tube studies
(soot and char yields, particulate matter & particle fragmentation)
 Large-scale studies
(ash deposition & role of potassium)
 Concluding remarks
(includes research needs)
2
Overview
3
World energy markets by fuel type
World energy consumption by country
grouping, 2012-40 (quadrillion Btu)
Total world energy consumption by energy
source, 1990-2040 (quadrillion Btu)
Dotted lines for coal and renewables show projected
effects of the U.S. Clean Power Plan
Renewables: world’s fastest-growing energy source (increase 2.6%/year from 2012 to 2040)
Nuclear: world’s second fastest-growing energy source (increase 2.3%/year from 2012 to 2040)
Coal: world’s slowest-growing energy source (increase 0.6%/year from 2012 to 2040)
Fossil fuels: still account for 78% of energy use in 2040
Source: International Energy Outlook, 2016
4
World delivered energy use by sector
World net electricity generation by energy
source, 2012-40 (trillion kilowatthours)
buildings, industrial,
and transportation
sectors
Buildings: projected an increase of
2.1%/year from 2012 to 2040 for nonOECD countries (in OECD countries:
increase of 0.6%/year)
Industrial: accounts for more than
half of total energy use through 2040,
but is the slowest-growing sector
(increase of 1.2%/year – in nonOECD countries: 1.5%/year, in OECD
countries: 0.5%/year)
Transportation: liquid fuels remain dominant
source, but share declines from 96% in 2012 to
88% in 2040.
Source: International Energy Outlook, 2016
5
Renewable resources
World net electricity generation from
renewable power by fuel, 2012-40 (trillion kWh)
Solar is the world’s fastest-growing
form of renewable energy, with net
solar generation increasing 8.3%/year
Other includes biomass, waste, and tide/wave/ocean
Source: International Energy Outlook, 2016
Of the around 6 trillion kWh of new
renewable generation added over the
projection period:
• hydroelectric account for 33%
• wind account for 33%
• solar for 15%
• biomass and waste for 14%
6
World carbon dioxide emissions
World energy-related carbon dioxide emissions by
fuel type, 1990-2040 (billion metric tons)
Liquid fuels: accounted for largest
share (43%) of total CO2 emissions
in 1990, 36% in 2012, and projected
to remain at that level through 2040
Coal: accounted for 39% of total
emissions in 1990, 43% in 2012, but
projected to decline to 38% in 2040
Natural gas: share of CO2
emissions (19% in 1990) increases
to 26% of total fossil fuel emissions
in 2040
Source: International Energy Outlook, 2016
7
Biomass resources
Energy crops
 Woody crops
 Agricultural crops
Waste products






Wood residues
Temperate crop wastes
Tropical crop wastes
Animal wastes
Municipal solid waste
Commercial and industrial
wastes
8
Biomass potentials (1/2)
Forest production
(million tonnes)
Technical potential of
agricultural residues
(million tonnes)
Source: Bentsen & Felby (2010). Technical potentials of biomass for energy services
from current agriculture and forestry in selected countries in Europe, The Americas
and Asia. University of Copenhagen
9
Biomass potentials (2/2)
Energy potential from agricultural residues and current forest production
Source: Bentsen & Felby (2010). Technical potentials of biomass for energy services
from current agriculture and forestry in selected countries in Europe, The Americas
and Asia. University of Copenhagen
10
Difficult biomass
fuels
11
Difficult biomass fuels
Biomass fuels with difficult physical properties
Wet sludges, straw ...
Very wet biomass fuels
Wet sludges, wet chips ...
Biomass fuels with difficult ashes
Animal and poultry wastes
12
Biomass fuels with difficult physical properties
Cereal straws for biomass are harvested,
transported, stored and handled as large bales
This requires special equipment, which is expensive
and not very flexible
Straw is difficult to pulverize down to sizes < 300 mm
due to its fibrous structure…
… torrefaction is a useful pre-treatment because
makes the milling process easier
Temperature: 200-300 ºC
Atmospheric pressure
No oxygen
Residence time: 30-90 min
Heating rate: < 50 ºC/min
13
Very wet biomass fuels
Green wood materials generally have total moisture contents up to
around 55%-60%...
... which may have a significant impact on the heat balance across
the mill...
14
Biomass fuels with difficult ashes
Levels of Ca, K and P in biomass residues can be extremely high,
which can increase the ash deposition on boiler surfaces
Risk of high temperature corrosion of boiler components because
of the high Cl content of (some) biomass materials
Risks of low ash fusion temperatures and, thus, increased slag
formation, depending on the ash content and ash chemistry of the
biomass
Risks of increased deposition and other ash-related impacts
associated with biomass firing leads to restrictions in the range of
biomass materials that can be used
15
Biomass fuels properties
16
Context of the presentation
In last decades, the environmental pressure on power production
industry focused on the replacement of coal by neutral CO2 fuels
like biomass
Currently, there is an increasing interest on a new generation of
low cost biomass fuels such as agricultural residues and refuse
derived fuels (RDF)
Research on the various aspects of the combustion of these
renewable fuels is going on at three scales:
 single particle studies
 drop tube studies
 large-scale studies
17
Single particle studies
Sandia National Laboratories
USA
Tsinghua University
China
Probe
High-speed
camera
Optical
McKenna
flat flame
burner
Particle
feeder
(2)
(1)
Rotameter
Flowmeters
(1) - Water in
(2) - Water out
CH 4
Northeastern University
USA
Technische Universität Darmstadt
Germany
N 2/O 2
Instituto Superior Técnico
Portugal
18
Topics covered in this lecture on
single particle studies
Early combustion stages of single biomass particles in air (ignition
delay time, ignition mode)
Combustion of single biomass particles in air and simulated oxy-fuel
conditions
Investigation of ignition and volatile combustion of single particles
using advanced optical techniques
19
Early combustion stages of single biomass
particles in air (1/3)
Probe
High-speed
camera
Optical
McKenna
flat flame
burner
Particle
feeder
(2)
(1)
Rotameter
Flowmeters
(1) - Water in
(2) - Water out
Pine (homogeneous
ignition)
CH 4
Straw (heterogeneous
ignition)
Coal (homogeneous
ignition)
N 2/O 2
• pine bark, sycamore branches,
vine branches, kiwi branches,
wheat straw, almonds shells, olive
residues, grape pomace doped
with K and Ca, Soma lignite,
Tunçbilek lignite, bituminous coal
• 80-90 mm, 212-224 mm, 200-250
mm, 224-250 mm
• Temperature:
1460-1800
K;
oxygen: 3.5%-7.6%
Source: Simões, Magalhães, Rabaçal, Costa, PROCI, 2017; Magalhães, Kazanc, Ferreira,
Rabaçal, Costa, submitted to FUEL, 2017
20
Early combustion stages of single biomass
particles in air (2/3)
Typical statistical convergence of
tig and tvol for a biomass residue
tig: minimum of 50 events for each
biomass to have statistical convergence
tvol: minimum of 30 events for each
biomass to have statistical convergence
tig tends to decrease with increase of atmosphere temperature regardless of the solid fuel
tig of solid fuels with the same particle size converge as temperature increases
Source: Simões, Magalhães, Rabaçal, Costa, PROCI, 2017; Magalhães, Kazanc, Ferreira,
Rabaçal, Costa, submitted to FUEL, 2017
21
Early combustion stages of single biomass
particles in air (3/3)
Both tig and tvol increase with the demineralization process
tig decreases as K increases, but for Ca, tig first increases and then decreases as Ca increases
tvol decreases as K and Ca increase (more evident in case of Ca impregnation)
Impact of K and Ca more significant on tvol than on tig
Source: Carvalho, Rabaçal, Costa, Alzueta, Abián, submitted to FUEL, 2017
22
Combustion of single biomass particles in air
and simulated oxy-fuel conditions (1/2)
• olive residue, sugar cane
bagasse, pine sawdust, torrefied
sawdust
• 75-150 mm
• temperature: 1400 K;
oxygen: 21%-50%
Source: Riaza, Khatami, Levendis, Álvarez, Gil, Pevida, Rubiera, Pis, B&B, 2014
23
Combustion of single biomass particles in air
and simulated oxy-fuel conditions (2/2)
Source: Riaza, Khatami, Levendis, Álvarez, Gil, Pevida, Rubiera, Pis, B&B, 2014
24
Investigation of ignition and volatile combustion of
single coal particles using high-speed OH-PLIF
Mean gas temperature (CARS)
OH-PLIF images at z = 14 mm
Radial OH-profiles
• high volatile (36 wt.%)
bituminous coal
• 90-125 mm
• four
oxygen-enriched
exhaust gas environments
• different transport gas
Temporal information obtained from the high-speed OH-LIF measurements allowed to determine the onset of ignition
(sheet-imaging versus chemiluminescence imaging)
Source: Köser, Becker, Goßmann, Böhm, Dreizler, PROCI, 2017
25
Early combustion stages of single particles:
collaboration with Lund University, Sweden
Probe
High-speed
camera
Optical
McKenna
flat flame
burner
Particle
feeder
(2)
(1)
Rotameter
Flowmeters
(1) - Water in
(2) - Water out
CH 4
Images of spontaneous
emissions from
burning
wheat straw particles with
different filters (size: 3x3 cm;
time delay: 40 ms)
N 2/O 2
Distribution of
excimer-laser induced
photo-fragmentation
fluorescence
Source: Weng, Rabaçal, Costa, Li, Aldén, work-in-progress
26
Drop tube studies
Curtin University
Australia
Instituto Superior Técnico
Portugal
Luleå University of Technology
Sweden
Université Catholique de Louvain
Belgium
Université de Haute-Alsace
France
27
Topics covered in this lecture on
drop tube studies
Relationship between biomass pyrolysis conditions and gas
composition, soot and char yields
Particulate matter emissions from combustion of biomass
Particle fragmentation during last stages of combustion of biomass
28
Relationship between biomass pyrolysis conditions
and gas composition, soot and char yields (1/2)
• pinewood,
beechwood,
Danish
wheat
straw,
alfalfa straw, leached
wheat straw
• 200-425 mm
• 1000, 1250, 1400 ºC
Soot and char yields
Gas composition
For all straws char yield significantly decrease as temp increases
Soot yields of wood higher than those from straw above 1250 °C
Leached wheat straw forms less soot than the other two straws
Source: Trubetskaya, Peter Jensen, Anker Jensen, Llamas, Umeki, Glarborg, FPT, 2016
29
Relationship between biomass pyrolysis conditions
and gas composition, soot and char yields (2/2)
Wheat straw char contains Si, K, Ca
and alfalfa straw char Ca, K, S, Si, P
Original alfalfa straw contains much
K and Ca, and therefore forms mainly
K and Ca rich compounds in char
Ash elemental retention of original alfalfa and wheat straw and
their chars at 1000, 1250 and 1400 ºC
Inorganic elements in alfalfa and wheat
straw soot consist mostly of K, Cl, S
High levels of K and Cl in soot caused
by KCl release under the fast heating
rate in DTF
Ash elemental retention of alfalfa and wheat straw soot and
char at 1400 ºC
Source: Trubetskaya, Peter Jensen, Anker Jensen, Llamas, Umeki, Glarborg, FPT, 2016
30
Particulate matter emissions from
combustion of biomass (1/2)
Biomass components have different contents of
inorganic species (especially Na, K, Mg, Ca and Cl)
PM emission from the combustion of individual
biomass components versus whole-tree biomass
dry biomass basis
• components of mallee
trees (bark, leaf, wood),
mixture
of
these
components (15% bark,
35% leaf, 50% wood)
• 75-150 mm
• 1400 ºC
useful energy basis
ash input basis
Mass-based PSDs of PM10 and yields of PM0.1, PM0.1-1,
PM1, PM1-10, PM2.5, and PM10 from the combustion of
bark, leaf and wood
Source: Gao, Rahim, Chen, Wu, PROCI, 2017
31
Particulate matter emissions from
combustion of biomass (2/2)
Measured and calculated mass-based
PSDs of PM10 and yields of PM0.1,
PM0.1-1, PM1, PM1-10, PM2.5, and
PM10 from the combustion of the
whole-tree biomass
Source: Gao, Rahim, Chen, Wu, PROCI, 2017
32
Particle fragmentation during last stages of
combustion of biomass
wheat straw, 400-600 µm
• wheat straw, rice husk
• < 1000 mm, 100-200 µm,
400-600 µm, 800-1000 µm
• 1100 ºC
Source: Branco & Costa, ECM, 2017
straw
rice husk
33
Large-scale studies
Clausthal University of Technology
Germany
Universität Stuttgart
Germany
Xi’an Jiaotong University
China
University of Leeds,
UK
Instituto Superior Técnico
Portugal
34
Topics covered in this lecture on
large-scale studies
Ash deposition during biomass (co-)combustion
Potassium behavior during biomass combustion
35
1650
Ash deposition during biomass co-combustion (1/3)
Probe
Cooling
Air
Test Section
88888
Data logger
Ts
Deposit Layer
15
• sawdust, pine branches, wheat
straw, olive stones, peach
stones, bituminous coal
• biomass fuels: < 1000 mm; coal:
< 300 mm;
• air-cooled
stainless
steel
deposition probes and uncooled
ceramic deposition probes
• 900-1100 °C
Tg
Tin
Tout
50
Dimensions in mm
Source: Abreu, Casaca, Costa, FUEL, 2010; Wang, Pinto, Costa, FUEL, 2014
36
Ash deposition during biomass co-combustion (2/3)
Deposition rate (g/m2h)
200
180
160
140
Sawdust
Olive stones
Coal
120
100
80
60
40
20
0
10
20
30
50
100
Solid fuel in the blend (thermal fraction %)
Source: Abreu, Casaca, Costa, FUEL, 2010; Wang, Pinto, Costa, FUEL, 2014
37
Ash deposition during biomass co-combustion (3/3)
a)
50
Concentration (% wt)
45
• coal + sawdust co-firing
• high content of Si and Al
Coal
10% sawdust
20% sawdust
30% sawdust
50% sawdust
40
35
30
SiO2 e Al2O3 high melting
temperatures
25
20
15
10
5
0
Si
Al
Fe
K
Cu
Ca
Mg
Ti
S
Na
Element
Concentration (% wt)
b)
50
45
Coal
10% olive stones
20% olive stones
30% olive stones
50% olive stones
40
35
30
25
• coal + olive stones co-firing
• high content of K
K2O and K2SO4 have low
melting temperatures
• high content of S
20
Formation of sulfates
15
10
5
0
Si
Al
Fe
K
Cu
Ca
Mg
Ti
S
Na
P
Element
Source: Abreu, Casaca, Costa, FUEL, 2010; Wang, Pinto, Costa, FUEL, 2014
38
Ash deposition during combustion of biomass
Three times more deposit formed for biomass fuels than for coal
Deposition rate
correlates with
particle
temperature for
biomass fuels
• mixed
wood,
sawdust,
fermentation-process
residues,
grain residues, South African coal
• biomass fuels: 40 wt.% < 300 mm;
coal: 75 wt.% < 75 mm
• uncooled ceramic probes
• 950-1200 °C
Source: Weber, Poyraz, Beckmann, Brinker, PROCI, 2015
Sticking efficiency
of impacting
particles 0.03-0.09
at 970 °C and
0.4 at 1170 °C
39
Potassium behavior during biomass combustion
Condensation of K species is the origin of the slag layer
formation on superheater surfaces during biomass combustion
Condensation mechanisms of K species
• wheat straw, corn stalk
• two K salts (KCl, K2SO4),
mixture of both (1:1)
• biomass fuels and K salts: 150250 μm
• stainless steel condensation
probe
• 1000 °C
Source: Jin, Ye, Deng, Che, E&F, 2017
1. nucleation
2. heterogenous
condensation;
3. thermophoresis
and diffusion
4. Inertial impaction
Initial slagging layer is formed by heterogeneous
condensation of KCl vapor, and thermophoresis and
diffusion of KCl and K2SO4 fine particles
40
Concluding remarks
The intense use of wood derived fuels in domestic and industrial combustion
processes is putting an enormous pressure on the forest
To meet the existing scenarios, we must increase the use of alternative
biomass fuels, in particular herbaceous materials and agricultural
residues
... but such residues can cause a number of problems because of the
presence of alkali metals and chlorine and other ash related impacts as well
as corrosion on the metallic surfaces and particulate matter emissions …
… which may limit the variety of biomass fuels that can actually be used in
combustion processes
41
Research needs
Ignition and combustion of single (and streams) biomass particles
Influence of ash composition, particularly of K and Ca elements, on all
aspects of biomass combustion
Particle/char fragmentation during biomass combustion
Characteristics of pure biomass flames in large-scale combustors
42
Acknowledgements
This work was supported by Fundação para a Ciência e a
Tecnologia (FCT), through IDMEC, under LAETA, project
UID/EMS/50022/2013.
43
IST research team