23 Thermochemical Conversion Technologies – Gasification

Prof. A.K Jain Jain

1. Introduction

Thermo-chemical conversion of biomass with atmospheric air or oxygen in presence of heat can be written in accordance with the following reaction.

The product formation in the above reaction depends upon the air quantity used. It is important to define equivalence ratio (ER or ) to discuss the reaction mechanism. The equivalence ratio may be represented by the following relation.

In the above reaction, if is zero or in other words if the reaction is taking place in absence of air with some external heat supply, the end products are solid fuel (char), liquid fuel (tar) and gases. This process is called pyrolysis. In this process the chemical energy of biomass is converted into chemical energy of gas, solid and tar. The quantity of slid, liquid and gaseous fuel depends on the process operating parameters.

If = 1 or above one, fuel is completely burned and the end product is flue gas at high temperature. The process is called combustion. Sensible heat energy of the flue gas is the useful end product. All chemical energy of fuel is converted into heat energy of the flue gas. Solid fuel, tar and combustible gases are almost negligible in the products during combustion. Sensible energy of the flue gas can be used for various thermal applications.

When the equivalence ratio ( ) is between zero and one, combustion takes place up to a point till the reacting oxygen is available. This of course will depend on the quantity of actual air supply in the process. Due to combustion, carbon dioxide and water vapors are generated in the process. Since the reacting air is not adequate (less than the stoichiometric), char, tars and other gases like CO, CxHy are also formed. Due to heat energy, carbon dioxide and water vapors react with carbon to form carbon monoxide and hydrogen. The chemical energy of biomass gets transferred into chemical energy of gaseous product. This process is called gasification. The end product is a gaseous fuel called producer gas. Producer gas is a mixture of carbon monoxide, hydrogen and methane as combustible gas. The gas also contains nitrogen, carbon dioxide and small quantities of higher hydrocarbons. The optimum value of equivalence ratio for gasification process falls between 0.3 and 0.4. the sensible and chemical energy of gaseous products during thermal conversion of biomass as a function of equivalence ratio is depicted in Figure 1. The chemical energy of gas increases with the increasing equivalence ratio and becomes maximum when the equivalence ration is 0.24 and starts decreasing thereafter with the increasing equivalence ratio. The sensible energy of the gas increases continuously with increasing equivalence ratio. The total energy of the gases is the sum of sensible energy and the chemical energy. In gasification process the sensible energy of producer gas for engine application is not of any use since the gas needs to be cooled to remove tar and other condensable before feeding it to engine. In case gas use for thermal application the sensible energy of the gas is used up.

2.Gasification Process

Gasification is a thermo-chemical conversion process where fuel column is ignited at one point and exposed to the air blast (30-40% of stoichiometric air required for oxidation). Different processes and operations occurring in the gasifier reactor are mainly in four reaction zones (i.e., oxidation, reduction, pyrolysis and drying). The sequence of reaction zones in a gasifier depends on the type of gasifier and direction of flow of fuel and air or gas. In the oxidation zone, carbon and hydrogen of the biomass are oxidized to carbon dioxide and water vapor in accordance with the following reactions

Reactions 1 and 2 are the principal oxidation reactions. They are highly exothermic. Almost all the oxygen consumed in gasification process is used up in the oxidation zone. The temperature in the oxidation zone range from 800 to 1300 C, depending upon the type of the gasifier and fuel. Carbon dioxide and water vapour leave oxidation zone at high temperature and move into reduction zone.

In  reduction  zone,  carbon  dioxide  and  water  vapours  are  reduced  to  carbon  monoxide,hydrogen and methane in accordance with the following reactions.

Reactions 3 to 6 are the principle reduction reactions and are reversible. For reaction 3 the equilibrium temperature is about 900 °C. The equilibrium temperature for reaction 4 is about 1100 oC. Optimum temperature for reactions 5 and 6 is 700 to 600 °C. It indicates that the higher reactor temperature during gasification process favours higher concentration of CO and H2 in the producer gas. The low reactor temperature leads to higher methane concentration in producer gas.

Major reduction reactions (i.e., reactions 3, 4 and 5) are endothermic and absorb heat. Heat liberated in the oxidation zone of the gasifier is used for reduction reactions. The gas temperature drops from around 1000 °C in the oxidation zone to 450 °C at the end of the reduction zone.

Apart from the oxidation and reduction reactions, pyrolysis and drying also takes place in gasifiers. Pyrolysis takes place adjacent to the oxidation or reduction zones depending upon the type of gasifier in a temperature range of 200 to 500 °C.

3. Operating Parameters for Gasifiers

The performance of the gasifiers depends mainly upon the following operating parameters.

1. Fuel moisture content
2. Equivalence ratio
3. Temperature
4. Reactor pressure
5. Shape and size of the fuel

3.1 Moisture content

Moisture in the biomass has a marked effect on the gasifier performance producer gas quality and the gasifier efficiency. High moisture content in the fuel tend to reduce the temperatures in the oxidation and reduction zones of the gasifier resulting poor quality producer gas. The best gas is obtained at a moisture content of about 15%. At lower moisture levels the hydrogen content of the gas is low and the carbon monoxide level of gas decreases with increasing moisture. The practical experience indicates that at moisture level of above 25% in a down draft gasifier with throat, the gas quality is so poor that the self sustaining flame is not produced from the gas. Because of lower carbon monoxide and hydrogen content in the producer gas, the gasifier efficiency also decreases at higher moisture. Apart from this, the higher moisture in the fuel creates the problems in smooth operation of the gasifier. The material tends to form a bridge in the pyrolysis zone of the gasifier and stops flowing to oxidation and reduction zones in a down draft gasifier with throat.

3.2 Equivalence ratio

The equivalence ratio determines the air or oxygen supply to the gasifier and in other words the air/fuel ratio. The air fuel ratio controls the gasifier temperature and intern the gas quality. The equilibrium producer gas composition as mole fraction of different constituents of producer gas during gasification of wood is depicted as a function of equivalence ratio in the Figure 3. As the equivalence ratio increases the carbon monoxide level of producer gas increases and after passing through a maxima at equivalence ratio of 25% starts decreasing and the carbon dioxide starts increasing. The methane content of producer gas decreases with increasing equivalence ratio upto 0.25. The best gasifier performance in terms of gas quality and the gasifier efficiency is obtained at equivalence ratio of 0.25 to 0.30. Practically it is observed that the best gasifier efficiency is obtained when the equivalence ratio is between 0.35 to 0.40/ this may be due to the fact that in actual gasifier reactors the equilibrium is difficult to be attained. In down draft gasifier with throat the gas compositions are more close to equilibrium compared to updraft and other gasifier reactors.

3.3 Temperature

Equilibrium producer gas composition is a strong function of temperature. Higher temperature account for higher carbon monoxide and lower methane in producer gas as discussed in the earlier sections. The hydrogen content of producer gas increases upto a temperature of about 1100 oF and starts decreasing thereafter. But at this temperature the carbon monoxide content is very low. Therefore higher temperature around 1500 oF is considered more suitable for gasification.

3.4 Pressure

Equilibrium producer gas composition during gasification of rice husk were determined at constant temperature moisture and air fuel ratio of 1000 oC, 10% and 1.27 respectively. The producer gas composition values are in the Table below. As the process pressure increases from 1 to 15 atmosphere, there is no significant change in the gas composition. It may be concluded here that the gasifier pressure has no effect on the gas quality, or the gasification efficiency during gasification of biomass. However, the higher pressure is known to increase the gasification rate and the gasifier capacity.

3.5 Fuel size and shape

In fixed bed gasifiers the shape and size of the fuel plays an important role in its smooth operation. The gas/air passes through the fuel bed, therefore too small fuel size will increase the pressure drop and may also cause material flow problems. The bigger fuel size will not pass through the throat in down draft gasifier with throat and may cause flow problem. In bigger size fuel it is also possible that some un-pyrolysed biomass in the interior of biomass big piece may pass into the reduction zone and would result high tar in the producer gas. Fluidized bed gasifiers operates well with uniform sized fuel may be uniform pellets or ground biomass. Throatless gasifier operates well with materials like rice husk, ground net shell etc. It is therefor necessary that the appropriate fuel size should be selected for smooth gasifier operation.

4. Substrates for Gasification

The following can be used as fuel in gasifiers

• Coal,
• Biomass fuels available for gasification include charcoal,wood and wood waste (branches, twigs, roots, bark, woodshavings and sawdust) agricultural residues (maize cobs, coconut shells, coconut husks, cereal straws, rice husks, etc.) and peat.MSW

5. Types of Gasifier Reactors

The gasifiers are usually classified on the basis of direction of fuel and air or gas flow in the reactor. The classification of gasification reactors often referred in the literature are given below.

Up draft

Down draft

Cross draft

Fluidized bed

5.1 Up Draft Gasifier

An up draft gasifier is characterised by a counter current flow of the fuel and air or gas in the reactor. Fuel is fed from the top and air is introduced at the bottom of the reactor. The producer gas exits from the top of the gasifier. Oxidation zone is formed at the bottom of the gasifier. The reduction, pyrolysis and drying zones are formed above the oxidation zone in sequence. The reaction zones in an up draft gasifier are shown in figure 5.

In an up draft gasifier, the producer gas passes through the low temperature pyrolysis and drying zone in the gasifier before exit. The tar released in pyrolysis zone and water vapoursformed in drying zone move along with the producer gas unconverted. Thus the gas generated from an up draft gasifier contains high quantity of tar and moisture. The tar content of the producer gas varies from 1 to even 20 g/Nm3. The gas is therefore considered suitable for thermal applications where removal of tar is not very essential. High mechanical efforts are required for cleaning the gas, if the gas is to be usedfor running an internal combustion engine. Up draft gasifiers can operate on large varieties of fuels provided that the material has uniform size. Up draft gasifiers generally, operate at high efficiency (hot gas basis) because of low gas temperature at the gasifier exit. Current R & D work in the country on up draft gasifiers, is mainly for thermal applications.

5.2 Down Draft Gasifier

Down draft gasifiers are characterized by co-current flow of air or gas and the fuel. In a dawn draft gasifier, normally fuel is introduced from the top while air is fed either from the top of the reactor or from the side, in the oxidation zone. Fuel and air or gas move in the same direction.

In a down draft gasifier, the pyrolysis products (i.e., tar and other condensable components) pass through high temperature oxidation and reduction zones before the gas exits from the gasifier. This is why, most of the tars are either burned or cracked thermally in the gasifier itself. The producer gas generated from a down draft gasifier has minimum quantity of tars and other condensable and is considered the best for engine operation. Down draft gasifiers can be further classified as; 1) down draft gasifier with throat (Imbert type) and 2) constant diameter throatless gasifier.

5.2.1 Down Draft Gasifier with Throat (Imbert type)

This gasifier has a narrow section below the air entrance point in the reactor which is called throat. Due to decrease in the cross-sectional area at the throat, the air or gas velocity increases resulting high and better temperature distribution in the oxidation zone.

In a down draft gasifier, pyrolysis zone is formed above the oxidation zone, whereas the reduction zone is formed below the oxidation zone. The reaction zones sequence in a down draft gasifier are shown in Figure 4.

Reaction zones in a down draft gasifier

This type of gasifier is highly fuel specific and operates only on wood chips, charcoal and selected woody agricultural crop residues. Cold gas efficiency of a down draft gasifier with throat is about 75% at full load operation. These gasifiers are available in the capacity range of 2.5 to 250 kW.

During the second world war about 6 million down draft gasifiers with throat were in operation in Sweden and other parts of Europe mainly for mobile application. These gasifiers were operated either on charcoal or wood. Operational data of these gasifiers is often used as the base material for the design of down draft gasifiers with throat.

5.2 .2Throatless Gasifier

Down draft gasifier with throat produces best quality producer gas, but it is not suitable for gasification of paddy husk. Since paddy husk is an important agricultural crop residue having potential as an energy source, a throatless down draft gasifier was conceived for gasification of paddy husk. The reactor has a constant diameter without throat. The reaction zones sequence in this gasifier is similar to

that of the down draft gasifier with throat. The reaction zones in a batch operated throatless gasifier are shown in Figure 5.

Figure 5. Reaction zones in a throatless gasifier

In batch operation, the residue/ash left after the gasification accumulates in the gasifier. The reaction zones thus keep on moving up in a throatless gasifier. However, in a continuous reactor where ash removal and material feeding are continuous, the reaction zones remain more or less at the same location. The cold gas efficiency of this gasifier varies from 55 to 65% depending upon the operating conditions. Inspite of slightly higher tar and low efficiency as compared to Imbert type gasifier, the most important aspect is that a throatless gasifier operating on paddy husk gives smooth and satisfactory engine operation. Throatless gasifier has been successfully operated using low density leafy biomass, for engine quality gas production.

Simple design, operation and maintenance of a batch operated throatless gasifier supports its use in remote village environment where high level technical skill and workshop facilities are not available. The gasifier can be used to run IC engine (5 to 12 hp) for farm irrigation or water pumping. It can thus be considered as an ideal source of decentralized power.

5.3 Cross Draft Gasifier

Schematic diagram of a cross draft gasifier is shown in Figure 8. This gasifier can operate with wide variety of fuels compared to an up draft or a down draft gasifier. High gas exit temperature, higher gas velocity at the gas exit, poor carbon dioxide reduction are certain characteristics of this type of gasifier. This type of gasifier have been used for gasification of coal earlier. During the present extensive literature search no reference could be identified giving any information on ongoing research and development activity or commercial application of a cross draft gasifier for the gasification of biomass materials, during the last four decades.

5.4 Fluidized Bed Gasifier

Fluidized bed gasifier is a homogeneous reactor bed of some inert sand material. Usually high alumina refractory sand is used as a bed material and fluidization is carried using air/gas. Usually ground or palletized biomass having uniform size is used in these gasifiers. The fuel is introduced in the inert bed material. Air is introduced through an air distributor at the bottom of the bed in the reactor. Oxidation, reduction pyrolysis processes occurs simultaneously. These gasifiers are available in higher capacity ranges (1 to 5 MWe).

In fluidized bed gasifier a precise control of reactor temperature can be maintained at a desired level by adjusting air fuel ratio. So materials with high ash and low ash slagging temperature can also be gasified in a fluidized bed gasifier. As a matter of fact any type of fuel can be used in this type of gasifier provided that the material is of uniform and small size. This gasifier is characterized by high gas exit temperature, very high solid particulate matter in the gas and relatively low efficiency. The producer gas generated from a fluidized bed gasifier requires extensive cleaning for thermal as well as mechanical applications. Fluidized bed gasification process schematic is shown in Figure 9.

5.5 Entrained flow gasifier

In the entrained flow gasifier, the fine coal particles react with cocurrently flowing steam and oxygen for about 99% conversion and the destruction of tar and oil yields pure syngas due to high operational temperature of gasifier. But, the entrained flow gasifier has a high oxygen demand and also the high ash content in the sub-bituminous coal would increase the oxygen consumption.

6. Advantages and disadvantages of different types of gasifiers

7. Gasifiers in India

In India biomass gasifier systems have been set up for grid and off-grid projects worth equivalent to150 MW. More than 300 rice mills and other industries are using gasifier systems for meeting their captive power and thermal applications. More than 230 villages get electrified from about 70 biomass gasifier systems.