Feedstocks
Feedstocks for gasification may vary from natural gas to heavy oil residues and coal. Furthermore, waste streams and biomass may be used. In order to carry out proper calculations it is essential that the elemental composition and the standard heat of formation of the fuels are known.
The standard heat of formation of the fuel may be calculated from the heat of combustion and the elemental composition of the fuel without the ash. Ash in the fuel is generally considered as being inert in the calculations. By subtracting the heat of combustion of the various elements in the fuel (in practice, carbon, hydrogen, and sulfur) from the heat of combustion of the fuel, the standard heat of formation is obtained. Care should be taken to normalize the quantities of the fuel and the relevant elements, as well as to ensure that they are all based on either the lower heating value (LHV) or the higher heating value (HHV) of the fuel.
Where no measured combustion values of the fuel are available, the heat of combustion of the fuel may be calculated from the proximate and ultimate analyses. Gasify. exe includes some suitable empirical correlations. It is always recommended to check whether the measured data (when available) are in agreement with the calculated data. If these deviate by more than 2-3%, it is recommended to use the calculated data for coal and heavy oil fuels. For natural gas fuels it is recommended always to use the calculated combustion values. The most difficult fuels are biomass and waste feedstocks, where the calculated and measured values may easily deviate by over 5%. Careful scrutiny of the measured data is then recommended.
Composition and combustion data for coal are often very confusing, as the composition data may be based on an as-received (ar), moisture-free (mf), ash-free (af), or ash-and-moisture-free (maf) basis. Moreover, the heating value can be given as LHV or HHV, possibly on another basis than the proximate and ultimate analysis. For this reason, gasify. exe includes a module that readily converts all these data into each other. For the ash and maf coal it has been arbitrarily assumed that these components have a molecular mass of 100.