Gasification output greatly depends on the properties of the feedstock used, as well as on the operating conditions. Some of the factors are listed below:

1

Ultimate analysis of the feedstock: This determines the chemical composition of the type of fuels. Figure 16.4 shows the C-H-O diagram, which demonstrates that liquid fuel only consists of carbon and hydrogen. In solid fuel, carbon and charcoal have less oxygen and biomass has a higher percentage of oxygen. It

also shows the transition from gasification to combustion. The line H2O-CO2 is the axis line for combustion; beyond this all fuel gets combusted. The H-CO line is the axis line for gasification.

2 Moisture content, volatile matter, and ash content of the feedstock: Through proximate analysis of the fuel one can identify the moisture content, volatile matter, and ash content of the feedstock. Fuel with low moisture content is desirable because higher moisture content requires more energy to evaporate liquid forms of moisture. In other words, for a given heat input, high moisture fuel will result in a lower temperature, which will effect the composition of gas produced, resulting in a lower heating value gas. Quaak et al. (1998) suggested that, for downdraft gasification, the moisture content of the feedstock should be less than 25%.

During pyrolysis, the volatile matter in a feedstock is released. This volatile matter mainly consists of organic compounds, commonly known as tar. Tar is classified as primary, secondary, and tertiary. The change of tar from one form to another depends on the temperature. Higher temperatures result in tertiary tar. Tar generally condenses in the cooler part of the gasifier and poses many operating problems, like choking of the pipes. If tar cannot be cracked well, it will cause a problem when the producer gas is used in an engine. Thus, a lower volatile feedstock is better suited for engine applications. However, proper design of the gasifier could also help in tar reduction; for example, using a two-stage gasification process could potentially reduce tar because of a higher oxidation temperature (Bhattacharya et al., 2001).

Ash is the mineral content in fuel or feedstock remaining after combustion. In the gasification process, the remaining material is not only ash but also unburned carbon. Ash interferes with the gasification process in two ways:

(a) It fuses together to form slag and this clinker stops or inhibits the downward flow of the biomass feed in a moving bed gasifier. Even if it does not fuse together, it could offer mass transfer resistance to fuel particles undergoing gasification.

(b) Some inorganic constituents of the ash have an important catalytic effect on the gasification reaction rate of the char.

High ash content feedstock means the ash must be continuously removed from the gasifier. In addition, it is possible that agglomeration can take place inside the gasifier when using high ash content feedstock. However, the melting point of the ash depends on the mineral compositions in it.

3 Size and size distribution of feedstock: The size and size distribution affects the pressure drop through the bed of the gasifier. Low particle size can increase pressure drop across the gasifier. However, large feedstock particles also need more time for complete gasification. Moreover, obstruction of the feedstock flow can take place in the case of using large feedstock particles in a moving bed gasifier.

4 Bulk density of the feedstock: Fuel with higher bulk density is preferable because it has a higher energy content per unit volume, and is easier to transport and handle. Low bulk density fuel may create a problem with improper flow under gravity, mostly in the case of a fixed bed gasifier. This improper flow may result in low calorific value gaseous fuel, difficulty in transporting it from one place to another, and fuel handling that requires a larger space.

5 Energy content of feedstock: The energy content refers to the heating value that affects the energy output of the gasifier. Considering the energy balance, if an adiabatic gasification process is assumed, the reaction temperature of the process depends on the heating value of the feedstock used. For charcoal downdraft gasification, the temperature in the combustion zone is higher than 1100°C, compared with 970°C in the case of wood gasification using the same reactor (Bui, 1996).

6 Temperature: Temperature governs most of the reactions taking place during gasification and the composition of the output completely depends on it. High temperatures — above 800°C — favors the water shift reaction, resulting in higher carbon monoxide in product gas, while temperatures around 650- 800°C favor water gas shift reactions, resulting in higher hydrogen production for the case of steam gasification.

7 Reactor type: The choice of the types of reactor depends on many factors, one of which could be the type of fuel to be handled. If the fuel is low quality coal, then an entrained bed gasifier can be used. For a wide variety of fuel, a fluidized bed gasifier can be used as it can handle different types of fuel in a wide range of particle sizes. However, if the fuel is of low bulk density and high moisture content and the application small scale, then a fixed bed gasifier will be suitable.