Gasification is a process of reacting solid fuel with air, oxygen, steam, carbon dioxide or mixtures of these gases which produce gases products that fit to its use both as source of energy and as raw materials of chemical industry. The gasification gas is known as Gas Producer.
Coal gasification means that reacting coal with one or mixture of air, oxygen, steam, and carbon dioxide to produce synthesis gas product. Synthesis gas is a gas that consisting of mixture of hydrogen gas (H2) and carbon monoxide gas (C0). Coal gasification is specific reaction; it means that this reaction can take place only on the condition of limited oxygen stoichiometry (60-70% stoichiometry). If this requirement is not met then the coal gasification will not occur. The reaction is shown in equations below:
Decomposition: coal ↔ C + gas + tar
Reduction : C + CO2 ↔ 2CO2
C + H20 ↔ CO + H2
Shift reaction : CO + H20 ↔ CO2 + H2
Oxidation : C + O2 ↔ CO2 + CO
C + O2 ↔ CO2
Metanasion : 2C + 2H2O ↔ CO2 + CH4
2CO + 2H2 ↔ CH4 + CO2
Before gasification reactions perform the equations above, coal will experience the process of drying and pyrolysis respectively. In addition, if the gasification process is done auto thermal, some coals should be burned to generate heat that is required by reduction reaction, pyrolysis, and drying.
Gasifying agent type will determine the composition of gas producer. When air is used as gasifying agent, gas producer will contain lots of N2. Gas producer of this process is categorized as gas with low energy content (low Btu gas). Gasification with pure oxygen produces gas producer with medium hot combustion (medium Btu gas). Steam is often added to the coal gasification to increase H2 content in the gas producer.
Estimated reactions that may occur depending on the temperature and pressure in the gasifier and can be calculated from thermodynamic modeling which basically consists of mass balance, energy balance and thermodynamic relations. Thermodynamic modeling is used to estimate the composition of gas producer and fuel needs as well as gasifier per Nm3 of gas producers.
|Figure 1:Diagram of Coal Gasification|