1

S.1 Introduction to Gasification

Gasification is thermo-chemical process at high temperature that converts carbon-containing 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).

Other reactions are endothermic, which

means that they require heat and thus the enthalpy of reaction ∆HR > 0.

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

Combustiblegases:

H2, CO, CH4,(tars)

Inert gases:H20, CO2,

(N2)

Emissions:Particulates

tars, NH3,alkaliSolid fuel

Airor

Oxygen

Volatiles

Char

Moisture

Ash

2

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"

3

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 thermo-chemical 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.

4

S.3 Historic background.

Truck with gasifier.

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 small- scale power generation in developing countries.

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. More

U-haul downdraft wood gasifier.

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,

5

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 charcoal-

fueled 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.

Reduction: Remaining char is reduced

with CO2, H2O and heat to form H2 and CO.

Simultaneous pyrolysis and gas combustion of a solid particle

P4.1 Drying

The water content within a particle is bound in several ways:

6

It can be enclosed in cavities, It can be capillary, 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.

7

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):

)(222 21

gOHOH →+ ∆HR = – 241 kJ/mol

2221 COOCO →+ ∆HR = – 281 kJ/mol

2224 22 COOHOCH +→+ ∆HR = – 802 kJ/mol

22242 223 COOHOHC +→+ ∆HR = – 1326 kJ/mol

Total and partial combustion of char (heterogeneous reactions):

22 COOC →+ ∆HR = – 390 kJ/mol

COOC →+ 221

∆HR = – 109 kJ/mol

Methane production:

)(2423 gOHCHHCO +→+ ∆HR = – 205 kJ/mol

422 CHHC →+ ∆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.

8

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):

COCOC 22 →+ ∆HR = 172 kJ/mol

Steam Gasification (heterogeneous reactions):

2)(2 HCOOHC g +→+ ∆HR = 130 kJ/mol

22)(2 22 HCOOHC g +→+ ∆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:

22)(2 HCOOHCO g +↔+ ∆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

9

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:

)()()()(

2

22

OHCOHCOKP ⋅

⋅=

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]:

32 29353.063508.01778.431688.0 zzzP eK ⋅−⋅+⋅+=

11000−=

Tz (a help function)

Equilibrium constant for water-gas shift reaction

-2

-1

0

1

2

3

500 700 900 1100 1300 1500 1700 1900

Temperature (K)

LOG

10 K

p

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.

10

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:

2)()( 2 =

COH

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 Tars

(amount) Dust

(amount) Downdraft 12-20 15-22 8-15 1-3 45-55 Low Fair 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.

11

The ammonia content of the product gas can be as high as 4000 ppmv.

800

1000

1200

1400

1600

1800

0 0,1 0,2 0,3 0,4 0,5

Fuel nitrogen concentration (% dry basis)

Am

mon

ia c

onte

nt in

dry

gas

(p

pmv)

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/m3

n 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.

12

Component LVH (MJ/m3n)

H2 10.8 CO 12.6

CH4 35.9 CO2 0 N2 0 H2O 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

13

2. External heating system

Steam gasification may generate a medium calorific value gas (LHV = 10-14 MJ/ m3

n) 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.

14

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.

(%)100⋅⋅⋅

=fuelfuel

gasgasCG mLHV

VLHV&

&η (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.

(%)100⋅⋅

∆⋅⋅⋅+⋅=

fuelfuel

gasgasPgasgasgasthermal mLHV

TVcVLHV&

&& ρη (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/ m3

n) Vgas = normal volume flow of gas (m3

n/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/m3

n) Vgas = normal volume flow of gas (m3

n/s) LHVfuel =lower heating value of the gasifier solid fuel (MJ/kg) mfuel = solid fuel flow (kg/s)

15

∆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 m3

n 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/m3

n Answer a) LHVgas = 4.50 MJ/m3

n The cold gas efficiency is:

(%)100⋅⋅⋅

=fuelfuel

gasgasCG mLHV

VLHV&

&η

Where Vgas = fuel mass flow · gas production per kg fuel hmkgmhkgV nngas /500)//(50.2200 33 =⋅⋅⋅=& The cold gas efficiency can now be calculated as

%2.66(%)1002001750050.4

=⋅⋅⋅

=CGη

16

Answer b) ηCG = 66.2 %

S.7 Reactivity of the fuel

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.

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.

0,4

0,5

0,6

0,7

0,8

0,9

1

0 500 1000 1500 2000

Time (s)

m/m

0,ch

ar [%

m.a

.f.]

Wood pellets, 8mm, 850 degCWood 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:

dtmd

mr char

char

)(1

.0

⋅−= , where mchar is mass of char and t is time. Unit is

[s-1].

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.

17

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000Time (s)

Mas

s (m

/m0c

har)

ash

-free

ba

sis

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).

18

0,4

0,5

0,6

0,7

0,8

0,9

1

0 500 1000 1500 2000

Time (s)

m/m

0,ch

ar [%

m.a

.f.]

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.

19

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 mass-

transport 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 (transport-

limited) 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.

20

0,4

0,5

0,6

0,7

0,8

0,9

1

0 2000 4000 6000Time (s)

Mas

s (m

/m0c

har)

%

m.a

.f.

w chips, 0.3g/each

w pell, 6 mm

w pell, 8 mm

Cu bag shred

Cu bag pell, 6 mm

Br bag pell, 6mm

Br bag pell,12mm

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.

Fuel power10 kW 100 kW 1 MW 10 MW 100 MW 1000 MW

Downdraft

Updraft

BFB and CFB

PFB

Entrainedflow

Gasifier thermal power range.

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P8.1 Updraft

The updraft is the simplest type of gasifier, consisting of a fixed bed of fuel.

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.

The fuel passes successively through

drying and pyrolysis where it is decomposed into volatile gases and solid char.

After pyroysis has finished, the char is

reduced by endothermic gasification reactions.

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 (300-400°C).

More about updraft technology

Ash

Feed Gas

Air

Drying

Pyrolysis

Reduction

Combustion

Grate

Principle of updraft gasification

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.

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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.

Both designs work with a downwards

moving packed bed of biomass, which is fed in the top.

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.

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.

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.

The closed constricted gasifier

concentrates the heat in the

Ash

Feed

Gas

Air

Drying

FlamingPyrolysis

Reduction

Grate

Principle of an open-core gasifier

Ash

Feed

Gas

Air

Drying

Pyrolysis

Reduction

Air

Combustion

Gas

Packedfuel bed

Constriction

Char layerworking asinsulation

Principle of a closed constricted

gasifier

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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

Air

Ash

Gasifier

Cyclone

Filter

Cooler

Start-upblower IC-Engine

& Electr.-Generator

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 IC-engine.

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.

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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.

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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

Freeboard

Fluidbed

Product gas

Ash

Fuel

AshOxidant

Product gas

Ash, charand bedmaterial

Fluidbed

Fuel

Cyclone

Ash

Gas, ash, char and bed material

AshOxidant

Pressurizedproduct gas

Compessedfuel paste

AshOxidant

Freeboard

Fluidbed

Compressor

26

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.

27

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.

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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).

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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

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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.