Equations
Forgetting for the moment the presence of other compounds and elements such as sulfur, nitrogen, argon, ash, and so on, the following equations will apply in virtually all gasification processes.
1. Carbon balance.
2. Hydrogen balance.
3. Oxygen balance.
4. Dalton equation, stating that the sum of the mole fractions in the product gas equals unity.
5. Heat balance, stating that the sum of the heat of formation and the sensible heat of the product(s) equals that of the corresponding data in the feed stream(s), provided it is corrected for heat that is indirectly added to or subtracted from the process.
6. Reaction constants of the relevant reactions. In general, 3 for the heterogeneous case where carbon is present and 2 for the homogeneous case.
Note that in selecting the reactions for both heterogeneous and homogeneous reactions, it is essential that the relevant compounds in the set of reactions are present. In the heterogeneous case, the reactions 2-5 and 2-6 could just as well have been replaced by reactions 2-7 and 2-8; whereas in the homogeneous case, reaction 2-8 could be replaced by the C02 reforming reaction:
CH4 + C02 S 2CO + 2H2 + 247 MJ/kmol CH4 (2-14)
When pure carbon is to be gasified with a blast of oxygen and carbon dioxide, then the hydrogen balance and reactions 2-5 and 2-6 fall away.
In practice, fuels such as coal and heavy oils will also contain sulfur, nitrogen, and ash, and the oxygen may contain argon. This implies that the material balances have to be extended with the following equations:
7. Sulfur balance.
8. Nitrogen balance.
9. Ash balance.
10. Argon balance.
Of course, these additional elements will also have to be considered in the heat balance (see point #5 above).
