Gasification is a thermochemical transformation of solid fuel by partial oxidation into synthesis gas comprising of mainly carbon monoxide and hydrogen. Use of this syn gas as fuel
for gas turbines to generate electricity has enhanced the environmental acceptability of coal Gasification.
Coal gasification basically involves the thermal conversion of coal at 1400/2800 oF (700/1500°С) with a limited supply of air and O2, into synthesis gas– a mixture of carbon monoxide (CO)
and hydrogen (H2). This syngas (heating value is about 250 Btu/scf (1/4 of natural gas)) can be used for the production of hydrogen, methane, transportation fuels like gasoline and diesel by fischer Tropsch process. In addition, it is also a chemical “building blocks” for a wide range of commercial products . Recently, It is also being utilized in the gas turbines of integrated gasification combined cycle (IGCC) for electricity generation.
A typical gasification system consists of a gasifier unit, a purification system and an energy recovery system but the core of the gasification process is a gasifier, where the hydrocarbon feedstock is converted into syngas in a partial oxidation reaction. The heat and pressure break apart the chemical bonds of the feedstock, forming syngas. Gasifiers are basically classified as
- fixed bed
- fluidized bed
- entrained flow bed
In a gasifier, the carbonaceous material undergoes several different processes:
1. The pyrolysis process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal.
2. The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions.
3. The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen.
4. In addition, the reversible gas phase water gas shift reaction ( it is the reaction in which the carbon mono oxide reacts with water to produce H2 and CO2) reaches equilibrium very fast at the temperatures in a gasifier. This balances the concentrations of carbon monoxide, steam, carbon dioxide and hydrogen. Syn gas production efficiency and carbon utilization is used as gasifier performance parameters. The influence of gasifier temperature and coal moisture content on these parameters is studied.