S.1
Introduction to Gasification
Gasification is thermo-chemical process at high temperature that converts carboncontaining fuels, such as coal and biomass, into a combustible gas containing
mainly carbon monoxide, hydrogen, methane and inert gases, through incomplete
combustion and reduction.
Gasification is appealing because:
the combustible gas can be used in IC engines or gas turbines (enabling high
efficient power generation), burned directly or used in the production of methanol
or hydrogen.
a gaseous fuel needs little excess air in combustion and has low levels of
contaminants.
it enables solid fuels to replace oil, considering the rising oil price.
P1.1 Gasification
A solid fuel is usually composed of
carbon, hydrogen and oxygen. In addition,
there are smaller amounts of nitrogen and
sulfur. Ash content varies with different
fuels.
In exothermic reactions energy is released
as heat, the enthalpy of reaction is ∆HR < 0.
These occur, for example, during
combustion and partial combustion of
gases and char. (Temperature 800-1200ºC).
Moisture
Air
or
Oxygen
Ash
Char
Combustible
gases:
H2, CO, CH4,
(tars)
Volatiles
Solid fuel
Other reactions are endothermic, which
means that they require heat and thus the
enthalpy of reaction ∆HR > 0.
Inert gases:
H20, CO2,
(N2)
Emissions:
Particulates
tars, NH3,
alkali
The reduction of char at high temperature
in absence of oxygen is endothermic (heat
is supplied by the exothermic reactions);
and forms hydrogen and carbon monoxide
for the final gas composition.
These reduction reactions are called
gasification reactions and make up for the
difference between a gasification process
1
and a combustion process
For a gasification process, the oxidizing
agent is supplied in deficit, meanwhile a
combustion process needs the oxidizing
agent to be in excess.
In air gasification, the typical air factor (m)
is m= 0.2 – 0.4, while in combustion; m > 1.
P1.2 Literature
Biomass Technology Group, 2006
http://www.btgworld.com/
Bridgewater, A.V. ; 1995
“The Technical and Economic Feasibility of Biomass Gasification for Power Generation”
Fuel, 1995, vol. 74, pp 631-653, Elsevier Science
Chemrec
www.chemrec.se
Erlich, C.; 2005
“Thermochemical conversion of biomass pellets – A parametric study for application in
downdraft gasification”,
Licentiate Thesis , Department of Energy Technology, Royal Institute of Technology,
Stockholm, Sweden
Erlich C., Björnbom E., Bolado D., Giner M. and Fransson T.; 2006
”Pyrolysis and gasification of pellets from sugar cane bagasse and wood”
Fuel, 2006, vol 85, pp 1535-1540, Elsevier Science
Erlich C., Öhman M., Björnbom E. and Fransson T.; 2005
“Thermochemical characteristics of sugar cane bagasse pellets”
Fuel, 2005, vol 84, pp 569-575, Elsevier Science
FAO (Food and Agricultural Organisation of the United Nations); 1986
"Wood gas as engine fuel"
FAO Forestry Papers- 72, http://www.fao.org/DOCREP/T0512E/T0512E00.HTM
Gengas page
www.gengas.nu
Liliedahl, T. and Sjöström, K; 1997
"Modelling of char-gas reaction kinetics"
Fuel, 1997, vol 76, pp 29-37, Elsevier Science
Lindmark G., Widell T., Bohr E., Nordenswan G.W., Blomquist U., Bergquist N.O.;
1950
“Gengas”
The Royal Swedish Academy of Engineerig Sciences (IVA), Sweden
Rensfelt, E.; 2005
"State of the Art of Biomass Gasification and Pyrolysis Technologies 2005"
2
SYNBIOS, Automotive BioFuels International Conference, May 18-20, Stockholm, Sweden
Published on http://www.nykomb.se/
Salo, K., Horvath, A. and Patel, J.; 1998
“Pressurized Gasification of Biomass”
ASME paper 98-GT-349, American Society of Mechanical Engineers.
Stassen, Hubert E.; 1995
“Small-Scale Biomass Gasifiers for Heat and Power: A Global Review”
World Bank Technical Paper no 296
Turn S.Q et al; 1998
"The fate of inorganic constituents of biomass in fluidized bed gasification"
Fuel, 1998, vol. 77, pp 135-146, Elsevier Science
Twigg, M.V.; 1989
"Catayst handbook"
ISBN: 0723408572
P1.3 Acknowledgements
Author: Catharina Erlich, KTH, 2006; co-author Jesus Revuelta, KTH, 2006
Reviewer: Marianne Salomón, 2006
P1.4 Prerequisites:
At least one year of studies in an engineering program at university level;
Basic Thermodynamics (at least 160 LU = 4 weeks of fulltime studies);
S4B1C1 Introduction to combustion
P1.5 LU and TU
Learning Units: 8
Teaching Units: 3
S.2 Educational Objectives
At the end of the chapter the student should:
Understand gasification process and the throughout description of the thermochemical reactions taking place.
Understand the concept of cold gas efficiency, gas composition and heating value
related to operating conditions.
Be able to describe reactivity of the fuels and its effect on reaction rates.
Be able to describe the different kinds of gasifiers.
3
S.3
Historic background.
The gasification process is a well-known
technology.
Between 1920 and 1940, compact gasifier
systems for automotive applications were
developed in Europe.
In the 1970:s, the oil crisis renewed the
interest in gasification, as a relatively
cheap indigenous alternative for smallscale power generation in developing
countries.
Truck with gasifier.
In the early 1980:s, European
manufacturers offered small-scale wood
and charcoal-fueled power plants up to
250 kW electricity.
Currently the development of gasification
systems is directed to the production of
bio fuels and of electricity and heat in
advanced gas turbine based cogeneration
units.
U-haul downdraft wood gasifier.
More
P3.1
More
The gasification process is an old technology, which started in the 17th century,
with the
production of the so-called town gas, used for lightning purposes.
Gasification has undergone ups and downs through history.
The first commercial wood gasifier was an air- blown, updraft type installed in 1839
by Bischof (Germany), yielding what is known as “producer gas”, (town gas,
syngas or blue gas).
Then gasifiers were developed for different fuels such as charcoal and wood, for
industrial power and heat applications until the first decades of the 20th century,
when oil-fueled systems replaced the producer gas.
Between 1920 and 1940, compact gasifier systems for automotive applications
were developed in Europe.
During World War II, 600 000 of these were used in Europe due to the petroleum
fuel shortage. For instance, Sweden had 70,000 cars, tractors and buses using
producer gas, Japan had 100,000, France 110,000 and Germany 350,000.
After the war, inexpensive liquid fuels (diesel and gasoline) became available again
for vehicles and the gasification technology was “forgotten”.
In the 70s, the oil crisis renewed the interest in gasification, as a relatively cheap
indigenous alternative for small- scale power generation in developing countries,
4
which had to pay the high oil prices and in the other hand had sufficient biomass
resources, a renewable fuel option.
In the 1980s, European manufacturers offered small-scale wood and charcoalfueled power plants, up to 250 kW electricity, and at the same time developing
countries such as Brazil and India started their own gasifier implementation
programs, based on locally developed technologies.
As a result many systems were installed through joint projects in a large number
of developing countries.
Large scale coal gasification systems became interesting in Western countries for
heat and power applications, as an alternative to using gas and oil.
Large demonstration projects for production of liquid fuels are carried out, (i.e.
methanol). Both IGCC (Integrated Gasification Combined Cycle) and synfuels from
gasification concepts have been proven.
Currently the development of gasification systems is directed to the production of
automotive fuels and for commercialized heat and power generation.
The rising prices of oil will favor the development of systems that are
environmentally friendly and at the same time economically competitive.
S.4 Gasification sub-processes
The gasification process of a solid
particle can be divided into four steps:
Drying: The water within the fuel is
removed by evaporation.
Pyrolysis: The volatile gases, mainly
CO2, CO, and hydrocarbons are
released from the dry fuel through
thermal degradation, in absence of an
oxidant. The solid remaining is called
char.
Combustion: Total and partial
combustion of gas and char. Provides
the energy required in the other steps.
Simultaneous pyrolysis and gas
combustion of a solid particle
Reduction: Remaining char is reduced
with CO2, H2O and heat to form H2 and
CO.
P4.1
Drying
The water content within a particle is bound in several ways:
5
ƒ It can be enclosed in cavities,
ƒ It can be capillary,
ƒ It can be chemically bound with the particle substance. The chemically bound
water requires more energy to vaporise.
During the drying process, water leaves dry fuel behind through vaporization
The vaporization takes place at almost constant temperature (100ºC at atmospheric
pressure), but some water will leave at temperatures lower than the vaporization
temperature due to the fact that water within the fuel has a higher partial pressure
than the atmosphere.
The drying process is endothermic, i.e. it requires heat. This heat is obtained from
the combustion step.
The higher is the humidity content, the more heat is needed to dry the fuel. When
drying takes place at normal conditions, the vaporization heat is 2256 kJ/ kg of
water (latent heat).
P4.2
Pyrolysis
Pyrolysis is an endothermic process where the particle structure decomposes due
to heating.
The decomposition products come out in gas form and are called volatile gases.
Pyrolysis takes place after the drying has finished.
Pyrolysis takes place in the temperature range of 250 ºC to 900 ºC.
During pyrolysis a particle decreases in volume and mass.
The volatile products are mainly CO, CO2, CH4, CnHm, NH3, some H2 and tars (also
called pyrolysis oils).
In biomass the volatile content is about 70-80 wt % (m.a.f.), while coal has 10-30 wt
% (m.a.f.).
The solid residue left after pyrolysis is called char, which mainly consists of
elemental carbon and ash. Charcoal for barbequing is pyrolysed wood.
To start a particle pyrolysis, an external heat source is needed, as pyrolysis is an
endothermic process.
When pyrolysis gases leave the particle and meet an oxidant (e.g. air), they will
ignite.
After ignition, the pyrolysis is self-sustained with heat from the combustion
(exothermic reactions).
P4.3
Combustion
In the combustion sub-process, pyrolysis gases and char will react with provided
oxygen or air.
6
In a gasifier, the oxidizing agent is supplied in deficit; m = 0.2 – 0.4. For
stoichiometric conditions, m = 1. For common combustion processes, oxidant is
supplied in excess, m>1.
The reactions, in which the reactants and products are in the same phase, are
called homogeneous reactions.
The reactions, in which there are both solid and gas phases of reactants and
products, are called heterogeneous reactions
Combustion of gases (homogeneous reactions):
H2 +
1
O2 → H 2O( g )
2
∆HR = – 241 kJ/mol
1
CO + O2 → CO2
2
∆HR = – 281 kJ/mol
CH4 + 2O2 → 2H2O + CO2
∆HR = – 802 kJ/mol
C 2 H 4 + 3O2 → 2 H 2 O + 2CO2
∆HR = – 1326 kJ/mol
Total and partial combustion of char (heterogeneous reactions):
C + O2 → CO 2
∆HR = – 390 kJ/mol
1
C + O2 → CO
2
∆HR = – 109 kJ/mol
Methane production:
CO + 3H 2 → CH 4 + H 2 O( g )
∆HR = – 205 kJ/mol
C + 2 H 2 → CH 4
∆HR = – 71 kJ/mol
P4.4 Reduction
In the reduction sub-process, char is converted into product gas by endothermic
reactions with the hot combustion gases in absence of oxygen.
The reducing atmosphere is obtained since the oxidant was supplied in deficit.
7
Specifically, char reacts with steam forming hydrogen and with carbon dioxide
forming carbon monoxide.
The gasification reactions are very slow (compared to pyrolysis and combustion),
and this sub-process is the “bottleneck” of a gasifier.
The residence time for char is of large importance, as well as zone temperature
and char reactivity (explained later) to obtain high char conversion.
In a downdraft gasifier (explained later), the reduction starts at a temperature of
1000ºC and ends at about 700ºC, meanwhile in a fluidized bed (explained later), the
temperature is kept constant (800 ºC -900ºC depending on gasifier).
The higher the reduction temperature, the faster the gasification reactions.
The product gas is a mix of H2, H2O, CO and CO2 in different proportions.
Bouduard reaction (heterogeneous reaction):
C + CO2 → 2CO
∆HR = 172 kJ/mol
Steam Gasification (heterogeneous reactions):
C + H 2O( g ) → CO + H 2
∆HR = 130 kJ/mol
C + 2 H 2O( g ) → CO2 + 2 H 2
∆HR = 88 kJ/mol
An important reversible homogeneous exothermic reaction to consider is the
production of hydrogen from steam and carbon monoxide, where the equilibrium
is shifted depending on the temperature.
The reaction is called water-gas shift reaction:
CO + H 2O( g ) ↔ CO2 + H 2
∆HR - 41 kJ/mol
This reaction exists both in the combustion and the reduction zones of the gasifier,
but is of largest importance in the reduction zone, for final producer gas
composition.
More Water-gas shift reaction
8
P4.4.1
More water-gas shift reaction
It is constant in volume, i.e. equal molar amounts of reactants and products.
It is reversible, i.e. the reaction goes in both directions (both hydrogen is produced
as well as carbon monoxide)
Generally, for reversible chemical reactions, an equilibrium-constant is introduced,
which is the ratio of the reaction rate in one direction and the reaction rate in the
opposite direction.
The equilibrium constant for the water-gas shift reaction is:
KP =
(CO2 ) ⋅ ( H 2 )
(CO) ⋅ ( H 2O)
where (CO2) is the molar amount of CO2 in the reacting mixture, (H2) the molar amount
of H2 and so forth.
The equilibrium constant for the isolated reaction (without interaction of other
chemical reactions or possible catalysts) is empirically expressed as an
exponential function of the temperature (T is in K) [Twigg, 1989]:
KP = e
z=
0.31688+ 4.1778⋅ z +0.63508⋅ z 2 −0.29353⋅ z 3
1000
−1
T
(a help function)
Equilibrium constant for water-gas shift reaction
3
L O G 10 K p
2
1
0
-1
-2
500
700
900
1100
1300
1500
1700
1900
Temperature (K)
Hydrogen production is favoured by lower temperatures (higher Kp).
The water gas shift reaction is utilized in so-called shift-reactors for production of
hydrogen.
9
A shift-reactor may also be connected to the gasifier when synthesis gas for
methanol production is wanted.
Then the product gas from the gasifier is shifted so that the molar ratio:
(H 2 )
=2
(CO)
Methanol synthesis gives:
CO + 2 H2 → CH3OH
S.5 Gas composition and heating value
The gas produced in the gasifier is a mixture of combustible and non-combustible
components.
The main compounds of the producer gas are: H2, CO, H2O, CO2, N2 (if air is used
as oxidizing agent) and several hydrocarbons.
Depending on the content of N2 in the fuel and on the gasification process,
ammonia (NH3) will as well be part of the product gas. NH3 is a potential source of
NOX emission.
The gas composition and heating value of the product gas depend on which is the
gasification agent, i.e. if air, oxygen or steam have been used.
The gas composition and heating value also depend on the fuel used, type of
gasifier, and operating conditions.
H2
CO
CO2
CH4
N2
45-55
Tars
(amount)
Low
Dust
(amount)
Fair
Downdraft
12-20
15-22
8-15
1-3
Updraft
8-14
20-30
5-10
2-3
45-55
High
Low
Fluidized bed
9-15
9-18
15-19
4-8
46-57
Fair
High
Gas composition, vol- % (dry basis), from atmospheric pressure air gasification of
wood for different gasifier types.
P5.1 NH3 is a potential source of NOX emission
During gasification, the nitrogen bound in the fuel has a tendency to form
ammonia.
The tendency is dependent on the fuel and gasification conditions.
When the product gas will be used for combustion in downstream equipment, the
ammonia may oxidize to nitrogen oxides if not cleaned-out from the gas.
10
Ammonia content in dry gas
(ppmv)
The ammonia content of the product gas can be as high as 4000 ppmv.
1800
1600
1400
1200
1000
800
0
0,1
0,2
0,3
0,4
0,5
Fuel nitrogen concentration (% dry basis)
Ammonia concentration in product gas from a small atmospheric fluidized bed reactor
fuelled on banagrass which was prepared with different nitrogen contents.
Reproduced from Turn S.Q et al, 1998, FUEL, vol 77, no 3, pp 135-146.
The problem of NOx formation from ammonia in the product gas may be significant
in gas turbines as these work with much air in excess.
Applying the producer gas to gas burners in a steam boiler (for example), air
staging can be utilized:
1. Understoichiometric primary air supply will cause ammonia to form molecular
nitrogen.
2. Secondary air will be added to combust the rest of the gas.
The air staging technology is not 100% efficient, but may significantly reduce the
final NOx emission from a power plant.
P5.2 Heating Value
The heating value of a fuel is a measure of how much heat is released during
complete combustion (measured in MJ/m3n for gaseous fuels).
Heating values are classified as higher heating value (HHV) and lower heating
value (LHV). For further explanations, see chapter S4B1C1 slide 4.
The heating value of the product gas is estimated by the gas composition (volume
percentage of each component) and the heating value of each included
combustible compound.
The table below presents heating values of some typical components in the
product gas.
11
Component LVH (MJ/m3n)
H2
CO
10.8
12.6
CH4
CO2
N2
H2O
35.9
0
0
0
Higher hydrocarbons and tars have not included in the table, but to estimate a
precise heating value, these compounds should as well be taken into account.
P5.3 Air
The most common oxidizer is air because of its availability.
Air is cheap and easy to introduce.
It is not a perfect agent because of its nitrogen content; the inert nitrogen gas will
represent around 50% of the final product gas, giving the gas a low heating value.
The product gas from air gasification of wood has generally a lower heating value
LHV= 3-7 MJ/ m3n, where the higher value is obtained in updraft gasification (see
the section Types of Gasifiers).
P5.4 Oxygen
Oxygen as gasification agent does not give any bulk content to the product gas,
i.e. the product gas do not contain nitrogen gas (except for the nitrogen from the
fuel).
It is, however, very costly to produce the oxygen.
Oxygen is used as oxidizer for synthesis gas production where the synthesis gas
will be further processed, for example to methanol.
The heating value of the product gas from wood gasification is LHV = 10-18 MJ/m3n.
P5.5 Steam
The steam is generated in a separate steam generator and superheated before
provided to the gasifier; i.e. it is not the water content of the fuel that constitutes
the steam in this case.
Steam alone can not be used as a single mean to carry out a gasification process
as steam reacting with fuel is an endothermic process which requires more heat
than available in the steam.
The HEAT SOURCE of the steam gasification process can be:
1. Burning off some of the feedstock with air or oxygen
12
2. External heating system
Steam gasification may generate a medium calorific value gas (LHV = 10-14 MJ/
m3n) and furthermore increases the hydrogen content of the product gas.
Steam gasification is suitable when a gas with high hydrogen content of the
product gas is wanted, for example synthesis gas for methanol production or
hydrogen for fuel cells.
Catalysts for improving the steam gasification rate are under development, so that
hydrogen production is maximized during limited residence time of the fuel in the
gasifier.
P5.6 Tars
"Tar" is a collected name for aromatic condensable hydrocarbons.
There is not one unique definition of what tar is within the gasification, on the
contrary there are several definitions such as "the mixture of chemical compounds
which condense on metal surfaces at room temperature" or "all organic
contaminants with a molecular weight larger than benzene" [Biomass Technology
Group, 2006].
Tar is always produced during the pyrolysis, and the amount is dependent on the
fuel, the pyrolysis conditions as well as the gasification process.
Biomass has a quite high content of volatiles meanwhile charcoal (and black-coal)
has very low, therefore coal gasifier systems seldom encounter operational
problems like biomass gasifiers do.
The main problem with tars is that they tend to foul on the downstream equipment
where the product gas is to be applied. If catalysts are utilized in the downstream
equipment, these can be poisoned by the tars.
To avoid operational problems, tars generally need to be removed from the
product gas.
If the tars are condensed and separated from the product gas, the energy potential
they represent is lost since tars are combustible with heating values in the range
of 20 MJ/kg to 40 MJ/kg.
Tars crack naturally at temperatures higher than 1000ºC; therefore some
gasification technologies give very little amount of tars in the product gas
meanwhile other have higher amount.
Tar cracker as a separate reactor unit after the gasifier and with a catalyst included
to reduce the needed cracking temperature, is under development for fluidized bed
gasification systems.
13
S.6
Gasifier efficiency
The cold gas efficiency (CGE) is a measure of gasifier performance.
It can be defined as the ratio between the flow of energy in the gas and the energy
contained within the fuel.
It is called cold gas efficiency as it does not take into account that the product gas
exiting the gasifier is hot.
The higher the CGE, the better the fuel conversion.
ηCG =
LHVgas ⋅ V&gas
⋅ 100(%)
LHV fuel ⋅ m& fuel
(A)
Parameters (A)
If the gas is to be used for direct combustion in gas burners, the need of gas
cooling is very low. The sensible heat of the gas is in this case taken into account
for the efficiency, which in such case will increase.
η thermal =
LHVgas ⋅ V&gas + cPgas ⋅ V&gas ⋅ ρ gas ⋅ ∆T
⋅ 100(%)
LHV fuel ⋅ m& fuel
(B) Parameters (B)
Depending on type and design of the gasifier, as well as on the characteristics of
the fuel, the cold gas efficiency may vary between 50 % and 85 %. In the case of
thermal applications, the value of ηthernal can be as high as 90 %.
Calculation example
P6.1 Parameters (A)
ηCG = Cold gas efficiency %
LHVgas = lower heating value of the product gas (MJ/ m3n)
Vgas = normal volume flow of gas (m3n/s)
LHVfuel =lower heating value of the gasifier solid fuel (MJ/kg)
mfuel = solid fuel flow (kg/s)
P6.2 Parameters (B)
ηThermal = Gasifier efficiency %
LHVgas = lower heating value of the product gas (MJ/m3n)
Vgas = normal volume flow of gas (m3n/s)
LHVfuel =lower heating value of the gasifier solid fuel (MJ/kg)
mfuel = solid fuel flow (kg/s)
14
∆T = temperature difference between the gas at the burner inlet and the fuel
entering the gasifier (ºC)
ρgas =density of the gas (kg/m3n)
cPgas =specific heat of the gas (kJ/kgK)
P6.3 Calculation example
A biomass gasifier is fed with 200 kg wood per hour. The wood has a LHV of 17 MJ/kg.
The gasifier produces 2.50 m3n gas per kg of wood. The gas composition (vol % total)
is measured to:
Component
CO
CO2
H2
CH4
H2O
N2
Vol %
18
13
14
2
6
47
a) What is the LHV heating value of the gas (MJ/m3n)?
b) What is the cold gas efficiency of the gasifier (%)?
Solution
From the popup "heating value" on slide 5 in this chapter, the LHV of each component
in the product gas is found.
Component
CO
CO2
H2
CH4
H2O
N2
LHV (MJ/m3n)
12.6
0
10.8
35.9
0
0
The lower heating value of the gas thus becomes:
LHVgas = 0.18*12.6 + 0.13*0 + 0.14*10.8 + 0.02*35.9 + 0.06*0 + 0.47*0 = 4.50 MJ/m3n
Answer a) LHVgas = 4.50 MJ/m3n
The cold gas efficiency is:
ηCG =
LHVgas ⋅ V&gas
⋅ 100(%)
LHV fuel ⋅ m& fuel
Where Vgas = fuel mass flow · gas production per kg fuel
V&gas = 200 ⋅ 2.50 ⋅ (kg / h ⋅ mn3 / kg ) = 500mn3 / h
The cold gas efficiency can now be calculated as
η CG =
4.50 ⋅ 500
⋅100(%) = 66.2%
17 ⋅ 200
15
Answer b) ηCG = 66.2 %
S.7 Reactivity of the fuel
Any unconverted char will leave with
the ashes and is thus a loss of energy
potential in the process leading to
lower gasification efficiency.
It is therefore sufficient to introduce
char reactivity as an important
parameter to study.
The char reactivity depends on
gasification temperature, heating rate
and temperatures in foregoing
pyrolysis and on the fuel type and size.
1
m/m0,char [% m.a.f.]
In order to get an efficient gasification
process, it is important to design the
process so that the solid fuel to as
large extent as possible is converted
to a combustible gas.
0,9
0,8
0,7
0,6
0,5
0,4
0
500
1000
1500
2000
Time (s)
Wood pellets, 8mm, 850 degC
Wood pellets, 8mm, 800 degC
Steam gasification rate as function of
gasification temperature for chars
obtained from slow pyrolysis of wood
pellets.
P7.1 Char reactivity
Char reactivity is the rate of mass loss during the conversion of char:
r=−
1
m0.char
⋅
d (mchar )
, where mchar is mass of char and t is time. Unit is
dt
-1
[s ].
The negative sign indicates that it is a rate of mass loss.
The reactivity of the char depends on which is the reaction to be studied.
Char reacting with O2 forming CO2 is much faster (furthermore an exothermic
reaction) than gasification of char, for example char reacting with H2O forming CO
and H2, which is an endothermic process.
Measuring the relative mass of a char sample during determined conversion
conditions and plotting it in a diagram as function of time, the char reactivity is
then the leaning of the curve.
16
Mass (m/m0char) ash-free
basis
1
0,9
0,8
0,7
0,6
0,5
0,4
0
1000
2000
3000
4000
Time (s)
Mass of char as function of time during gasification at 800°C (isothermal gasification).
The char was obtained from slow pyrolysis of bagasse pellets.
As seen, often the reactivity of char for 20% conversion is not the same as for 50%
char conversion, therefore the reactivity also have to indicate to which extent the
char was converted at this certain rate.
Generally, the more of the char is converted the lower will be the reactivity of the
remaining char, i.e. the first 50% of the char conversion goes faster than
converting the remaining 50% of the char.
No gasifier has 100% char conversion
P7.2 Gasification temperature
The higher is the gasification temperature, the faster are the gasification reactions.
Thus char conversion rate is higher at higher gasification temperature.
In fixed bed gasifiers, the gasification reactions start at higher temperatures, but
with the progress of the endothermic char conversion, temperature is lowered and
thus, reactions become slower.
Gasification at a temperature lower than around 700 – 750°C is very slow and thus
char conversion can be expected to be small in this temperature region.
In a fluidized bed, the temperature is set to a constant value, normally between
800°C and 900°C, and best char conversion is obtained if the un-reacted char,
leaving as fly ash, is re-circulated to the bed, to increase the total residence time.
The higher the gasification temperature, the shorter residence time is needed for
good char conversion.
The higher the gasification temperature, the larger is the risk for ash-slagging
(depends heavily on the type of feedstock and its ash composition, however).
17
m/m0,char [% m.a.f.]
1
0,9
0,8
0,7
0,6
0,5
0,4
0
500
1000
1500
2000
Time (s)
Wood pellets, 8mm, 850 degC
Wood pellets, 8mm, 800 degC
Steam gasification rate as function of gasification temperature for chars obtained
from slow pyrolysis of wood pellets.
P7.3 Heating rates and temperatures in foregoing pyrolysis
Low heating rate of the fuel during pyrolysis favors tar formation and a high char
yield; meanwhile a high heating rate during the pyrolysis favor low char yield and
higher gas amount.
Furthermore, fast heating to high temperature during pyrolysis favors the reactivity
of the remaining char.
Fast heating also makes the char more porous, however, the pore surface area is
not proportional to the char reactivity; i.e. a char particle with less pore area might
be more reactive during gasification than a particle with a larger pore area!
For packed bed gasifiers, a very porous char might collapse at an early stage
leading to a dust layer to be built up, which in turn causes a high pressure drop
over the bed and an obstacle for gases to pass.
The final pyrolysis temperature also influences the reactivity of the char: The
higher the final temperature the higher is the reactivity at the same heating rate.
18
Steam gasification at constant 750°C as function of pyrolysis end temperature for
chars obtained from slow pyrolysis at constant heating rate of wood chips.
P7.4 Fuel type and size
The ability of the char to react during gasification is as well due to the fuel type
and its size.
Comparing biomass and black-coal, char from biomass is usually more reactive
than char from coal.
Comparing different biomasses, wood is more reactive than many agricultural
residues. In the figure below, two sorts of bagasse are compared with wood.
The gasification rate can be limited either by the reaction or by the heat and masstransport i.e. if the rate is limited by the reaction then the reaction itself is slower
than the total transport in to an active pore area and out from the same.
Apparently, larger biomass particles have lower gasification reactivity (transportlimited) compared to smaller ones (which may be reaction limited).
Many researchers also discuss the possibility that some components in the ash
(for example potassium, sodium and zinc) may have catalytic effect on the char
reactivity i.e. the presence of these compounds enhances the reactivity.
19
1
Mass (m/m0char) %m.a.f.
0,9
w chips, 0.3g/each
w pell, 6 mm
0,8
w pell, 8 mm
Cu bag shred
0,7
Cu bag pell, 6 mm
0,6
Br bag pell, 6mm
Br bag pell,12mm
0,5
0,4
0
2000
4000
6000
Time (s)
Relative mass (m/m0char) as a function of time for steam gasification of char at 800ºC.
Br bag: Brazilian bagasse; Cu bag: Cuban bagasse; pell: pellets; w: wood.
S.8
Types of Gasifiers
Depending on the end application of the product gas and plant size, there are different
gasifier designs.
Small, packed-bed gasifiers (updraft, downdraft or crossdraft) may be suitable
for stationary IC-engine operation (with electricity generation) or for gas burner
application.
Fluidized bed gasifiers (bubbling, circulating or pressurized) can be quite large
and are thus applied for larger plants, which for example may involve gas
turbines, steam boilers, methanol synthesis etc.
Pressurized entrained flow gasifiers are commercialized for coal as fuel and
under development for black-liquor gasification to produce bio fuels.
Entrained
flow
PFB
BFB and CFB
Updraft
Downdraft
10 kW
100 kW
1 MW
10 MW
100 MW
1000 MW
Fuel power
Gasifier thermal power range.
20
P8.1
Updraft
The updraft is the simplest type of gasifier,
consisting of a fixed bed of fuel.
Feed
Gas
The gasification agent is added at the
bottom, flowing in counter-current
configuration with the feedstock, which is
introduced in the top of the reactor.
Drying
The fuel passes successively through
drying and pyrolysis where it is
decomposed into volatile gases and solid
char.
Reduction
After pyroysis has finished, the char is
reduced by endothermic gasification
reactions.
Pyrolysis
Combustion
Air
Grate
Ash
Principle of updraft gasification
Combustion of char occurs near the grate
and the hot combustion gases transfer heat
to the rest of the process.
Char conversion is high, as the char reacts
with oxygen as a last sub-process and char
combustion reaction is faster than the char
gasification reactions.
The gasification efficiency is high due to
high char conversion and due to that the
gas exit temperature is relatively low (300400°C).
More about updraft technology
P8.1.1
More updraft
As pyrolysis takes place at rather low temperature, tar and methane production
are significant.
As the pyrolysis gases do not pass a combustion zone, instead leaving with the
product gas, the tar content of the product gas is high.
The gas has relative high heating value compared to other gasification
technologies as for the high tar content in the product gas.
The gas is suitable for direct combustion applications, such as a small steam
boiler or for ceramic industry. Using the gas in an IC-engine requires extensive
gas cleaning.
21
The gasifier construction is robust and relatively easy in operation.
The gasifier can use fuel with moisture content up to 60 % (wet basis). However,
the higher the moisture content, the lower the gasification efficiency.
The gasifier accepts size variations in the feedstock.
The fuel must have high mechanical strength and must be non-caking so that it
will form a permeable bed.
The possibility of channeling in the fuel bed can lead to oxygen break through
and the possibility of explosions.
P8.2 Downdraft
There are two main designs of the
downdraft gasifier: the close constricted
and the open core.
Feed
Air
Both designs work with a downwards
moving packed bed of biomass, which is
fed in the top.
Drying
Flaming
Pyrolysis
Air is drawn into the gasifier, for
example with the suction of an engine or
a blower.
For the closed constricted gasifier, the
air enters at some distance above the
constriction meanwhile air enters in the
open top of the open-core gasifier.
Reduction
Gas
Ash
Principle of an open-core gasifier
The fuel, after being dried and
pyrolysed, passes through the oxidation
zone where pyrolysis gases and part of
the char will burn.
In the open-core type, the gas
combustion takes place in the pyrolysis
zone: flaming pyrolysis. In the closed
constricted gasifier, the pyrolysis takes
place above gas combustion zone.
The heat generated from combustion is
used for the char reduction reactions,
pyrolysis and drying.
Grate
Feed
Packed
fuel bed
Drying
Pyrolysis
Air
Air
Gas
Gas
Char layer
working as
insulation
Combustion
Reduction
Constriction
As the pyrolysis gases passes through a
zone with very high temperature, the
tars produced during pyrolysis will to a
large extent crack to light compounds
such as CO, CO2 and CH4.
Ash
Principle of a closed constricted
gasifier
The closed constricted gasifier
concentrates the heat in the
22
constriction, and gives thus very low tar
content in the gas.
The product gas is suitable for IC-engine
operation, for example powering small
villages or industries.
More about downdraft technology
P8.2.1
Suitable for IC-engine operation
Biomass
Start-up
blower
Gasifier
Cooler
Air
Cyclone
IC-Engine
& Electr.Generator
Filter
Ash
Ash
A typical small-scale gasifier-engine power plant consists of a downdraft gasifier,
some cleaning steps in form of a cyclone, a filter and a cooler and finally an ICengine.
The plant-size is typically 5-100 kW electrical power.
A gasoline engine (Otto-cycle) can work completely with gasified biomass, no
other fuel is needed.
Without modifications of the engine, the maximum power of the engine will be
somewhat lower when utilizing gasified biomass instead of gasoline.
A typical total efficiency based on the original biomass energy content for a
gasifier-gasoline engine power plant is around 15-20%, but with heat recovery and
more expensive technical features, efficiencies of 25% can be reached.
A diesel engine has a higher efficiency than the gasoline engine due to its higher
pressure ratio.
Gasified biomass can not totally replace diesel in this engine type; thus dual-fuel
operation is common, i.e. some diesel is mixed in with the gasified biomass.
Commonly, gasified biomass constitutes 60-80% of the fuel and the rest is diesel,
the ratio depends on the load of the engine and on the engine itself.
Gasified biomass can also be used directly in gas engines, which now are on the
market.
23
P8.2.2
More about downdraft technology
Draw backs
It is fuel inflexible; there is no “universal fuel gasifier”.
Need of frequent service and maintenance.
Operational problems can occur due to bridging.
Wood gasifiers have to be installed where wood chips are available limiting the
amount of rural areas that are able to install this technology.
It can not be scaled up to larger size than 1 MW.
Open core gasifier
The open-core or stratified type has been developed in USA and China, (rice-husk
fired open-core gasifier).
Usually the open-core design does not have any constriction in the lower part and
it also has an open top, where both air and biomass are introduced.
As air is introduced with the biomass in the top, it is going to be present in the
pyrolysis step, and thus flaming pyrolysis will take place, (i.e. simultaneously
combustion of pyrolysis gases around the particle.
The flaming pyrolysis is followed by char reduction where the combusted
pyrolysis gases reduce the char at high temperature.
The large advantage with the open-core design is that it is more fuel flexible (size
and shape) than the closed constricted type.
However, it has difficulties to establish stable zones for pyrolysis, combustion and
reduction; it is therefore not suitable for varying load.
During shut-down there is a direct path-way for poisonous gases out.
By not having a constriction the risk for bridging is reduced, at least for heavier
materials such as wood.
By not having the constriction the tar production can be excessive since there is
no guarantee that the gases completely passes a high temperature zone.
Closed constricted gasifier
The constriction in the lower part of the gasifier maintains a high reduction
temperature.
Also the constriction allows ash to be retained forming an insulating layer and
thus preventing heat to be transported away from the reduction zone.
The closed constricted gasifier manages varying engine load.
However, the constriction makes it very fuel inflexible; in order to work satisfactory
the fuel needs a certain shape and size.
As for other downdraft gasifiers, present designs cannot be scaled-up. A larger
bed-diameter will cause less-reacted fuel in the centre, as the centre will not reach
as high temperature as with a smaller bed-diameter. Tars will not be able to crack
properly, thus a high tar-content gas is produced.
24
P8.3 Cross-Draft Gasifiers
A cross-draft gasifier is adapted for the
use of charcoal.
The advantage of using charcoal instead
of wood in gasification is that charcoal
gives virtually no tars.
Very few steps of cleaning equipment
are thus needed; only particulates need
to be removed.
The crossdraft gasifier operates in small
scale; installations are typically of less
than 10 kW electricity.
The technology is mainly used in
developing countries.
The gasifier system is simple, cheap and
easy to operate.
The drawback of using charcoal is that
high combustion temperature is reached
(1500°C), which can lead to local
material problems.
Also, manufacturing charcoal from
wood represents a large loss of energy
potential and the manufacturing process
itself is in many developing countries
done without regards to the health of the
workers.
P8.4 Fluidized bed gasifiers
Product gas
Gas, ash, char and bed material
Product
gas
Freeboard
Fluid
bed
Ash
Fuel
Pressurized
product gas
Fluid
bed
Fuel
Freeboard
Cyclone
Ash, char
and bed
material
Compessed
fuel paste
Fluid
bed
Compressor
Ash
Oxidant
Ash
Oxidant Ash
Oxidant
Ash
25
Bubbling bed
Circulating bed
Pressurized bed
The fuel content is 2-3% of the bed material, the rest is inert particles.
Compared to fixed bed gasifiers, the gasification temperature is relatively low; an
even temperature is selected in the range of 750°C to 900°C.
Compared to fixed bed gasifiers, the heating of the fuel during pyrolysis is faster
and therefore the reactivity of the char is high.
Due to the intense mixing, the different reactions phases (drying, pyrolysis,
oxidation, and reduction) can not be distinguished in separate zones.
Contrary to fixed bed gasifiers, the oxidizer-biomass ratio can be changed, and as
a result the bed temperature can be controlled.
The product gas from a fluidized bed has a higher tar content compared to the
downdraft as for the relative low operation temperature.
As understochiometric conditions are needed in the gasifier; possibly an excess
flow of inert gas may be needed to get the proper fluidisation of the bed and
maintaining the reducing atmosphere.
Low grade coals, wood chips, RDF (refuse derived fuel) and other fuel pellets are
suitable.
An important application of fluidized beds are for use in larger scale power plants
(steam plants or combined gas turbine and steam plants) or for synthesis gas
production.
The working principles of fluidized beds for gasification are comparable to those of
combustion (see s1b5c4 slide 5).
Some advantages of fluidized bed gasification are:
Compact construction because of high heat exchange and high reaction rates due
to intensive mixing in the bed.
Dolomite can be added to the bed collecting sulfur from the biomass.
Good control of temperatures.
Can be done large scale.
Some disadvantages:
High tar and dust content - Needs extensive gas cleaning.
Incomplete carbon burnout.
High producer gas temperature with alkali metals in vapor state.
Complex operation because of the need to control air and solid fuel.
High internal power consumption.
Not suitable for fuels having the ash melting point at low temperature. Ash melting
can cause bed agglomeration. Straw, for example, has ash melting point at around
600°C.
26
P8.4.1
Bubbling bed
In a bubbling bed the gasification oxidant is supplied at a relative low velocity (<
4m/s), but higher than the minimum fluidization velocity.
The bed suspension takes up only a part of the reactor volume and the space
above the bed before the exit of the reactor is called freeboard.
The bed turbulence is significant lower for the bubbling bed than for the circulating
bed, thus a longer residence time for the fuel is needed.
P8.4.2
Circulating bed
In the circulating bed gasifier, the oxidant is supplied at high velocity (>4m/s).
This causes the whole bed to rise over the whole reactor volume, so a cyclone is
needed to separate the solids from the gas stream.
The ashes, bed material and unreacted char are collected in the cyclone and
returned to the gasifier.
The high turbulence favors fast fuel conversion.
P8.4.3
Pressurized bed
In a pressurised fluidized bed gasifier, oxidizer and solid fuel are mixed in a hot
bed of inert material at a working pressure from 7 to 9 bars.
Pressurised gasification is for integration with gas turbines.
By pressurising the gasifier, a very compact unit in a high power range is obtained.
The large advantage of pressurising the gasifier is that the product gas can be
used for direct application in gas turbines as ideally no gas cooling is needed (gas
already compressed).
If biomass is the fuel, tars will stay in gas form and can be combusted within the
gas turbine.
Practically some gas cooling is needed (for the filter), but not as much as in the
case of the atmospheric gasifier.
Ideally, only a hot gas filter cleaning out particulates is needed for gas
conditioning.
27
P8.5 Pressurized entrained flow gasifier
The entrained flow gasifier is operated with fuel and oxygen (air) in co-current flow.
High reaction intensity is provided by a high pressure (20-30 bar), high
temperature (>1400°C) environment.
The flow is extremely turbulent and the particle residence time short.
Entrained flow gasifiers are commonly used for coal because finer particle sizes
and higher operating temperatures can be achieved (compared to biomass, for
example).
The fuel particles must be much smaller than for other gasifiers, requiring fuel
pulverization, which requires much energy.
High temperatures and pressure mean high throughput, however the efficiency is
low as the gas most be cooled before it can be cleaned with existing technology.
Tar and methane are not present in the product gas. There is a high content of H2
in product gas, about 2 times more than CO.
Ash is removed as slag because the operating temperature is well above ash
fusion temperature.
Single pass carbon conversions are in the range of 95-99%.
Entrained flow gasification is specifically suitable for low-grade coals and high
coal throughput.
However, entrained flow gasifiers are not practical for biomass for several reasons,
for example:
- Biomass ash has temperature limiting properties.
- Impracticality of generating finely ground biomass feedstock.
- Difficulties in making biomass slurry for the feeding system
It is however possible to blend 10-15% biomass in the coal slurry.
Application: synthesis gas for methanol production or power generation (IGCC).
28
For bio fuel production, black liquor gasification is under development and
evaluation.
P8.5.1
Black liquor gasification
Black liquor is a combustible rest product from the paper pulp industry.
The world yearly production is presently 200 million tons (year 2006).
Since black liquor has its origin in biomass, it is regarded as a renewable energy
source.
Typical elemental composition of black liquor on dry basis:
Component
C
O2
H2
S
Na
K
Cl
wt % dry
34
34
3
5
22
1
0.5
The typical moisture content is 30% water per kg of black liquor.
Heating value: HHV = 14 MJ/kg (dry basis).
Presently it is utilized in so-called recovery boilers, which produce heat and power
for the process.
Spent liquors (black liquor) from pulp industry contribute with about 40 TWh/year
to the Swedish energy balance.
The drawback with the recovery boiler is that the overall efficiency is relatively low,
as the liquor has high content of both moisture and ash.
An alternative to the recovery boiler is black
liquor gasification at elevated pressure and
high temperature.
This technology has a much higher overall
efficiency and a potential to produce 60-70%
more power than with modern recovery boilers.
Entrained flow gasifiers are used for black
liquor gasification with temperatures up to 950
ºC.
Experimental research is presently (year 2006)
being done in Sweden at Energy Technology
Center in Piteå.
Entrained flow gasification of
black liquor
www.chemrec.se
29
S.9 Summary
Gasification is a thermal process which converts a solid fuel into a combustible
gas by understoichiometric supply of an oxidant.
The product gas is a mixture of combustible and non-combustible components
and has different heating values depending on which gasification agent has been
used.
The gasification process of a solid fuel can be divided into four phases: drying,
pyrolysis, combustion and char reduction.
Reduction reactions, forming H2 and CO, are endothermic and take place at high
temperature in absence of O2 but in presence of H2O and CO2.
The cold gas efficiency (CGE) is a measure of the gasifier performance and does
not take into account that the gas is hot when exiting the gasifier.
Char reactivity of a fuel is a parameter that can be studied to facilitate a gasifier
design with high fuel conversion efficiency.
The main types of gasifiers include: fixed beds (downdraft and updraft), fluidized
beds (bubbling, circulating, pressurized) and pressurized entrained flow reactors.
30