The Thermodynamics of Gasification
13
each compound. Note that even where the calculations are used for the low temperature
CO shift reaction, which operates at temperatures of 200–250°C, this approximation
gives sufficiently accurate results for basic designs.
Other Compounds
Most fuels contain additional material beyond the carbon, hydrogen, and oxygen
discussed above. Sulfur in the fuel is converted into H
2
S and COS, and the nitrogen
into elemental nitrogen, NH
3
, and HCN. Generally speaking, the quantities of sulfur
and nitrogen in the fuel are sufficiently small that they have a negligible effect on
the main syngas components of hydrogen and carbon monoxide. Nonetheless it is
necessary to consider the fate of sulfur and nitrogen because of the effect of the
resulting compounds downstream of the gas production, for example, environmental
emissions, catalyst poisons, and so on.
Details concerning the formation of sulfur and nitrogen compounds are included
in Section 6.9. Since the amounts of sulfur and nitrogen converted into the various
product molecules are not large, the distribution of sulfur and nitrogen compounds is
usually estimated in advance (for example, H
2
S/COS =9− 9.5, NH
3
=25%, and
HCN =10% of fuel nitrogen, respectively), and interaction with carbon, hydrogen,
and oxygen is then limited to mass and heat balance calculations.
2.2 THERMODYNAMIC MODELING OF GASIFICATION
Both designers and operators need to have some knowledge about thermodynamic
modeling, although in developing models for gasification, it should be noted that the
requirements of a designer and an operator are different.
The designer has the task of calculating a limited number of design cases and
using these to size the plant equipment. He will be interested in throughputs of the
different feedstocks, gas compositions, heat effects, quench requirements, startup
and shutdown requirements, optimal conditions for the design feedstocks, and
process control requirements.
The operator has his equipment as it is, but will need to optimize operations for
feedstocks, which are seldom identical with the formal design case. He will therefore
be more interested in what he can expect when feeding a specific cocktail of feedstocks,
how to interpret gas compositions, and, for example, the steam make in a syngas
cooler. Once the unit runs stably he will become interested in optimizing the process.
A good model will therefore be so built that both requirements can be readily met without
the user having to perform an undue number of iterative calculations to perform his task.
The purpose of gasification modeling is:
• The calculation of the gas composition.
• The calculation of the relative amounts of oxygen and/or steam and/or heat
required per unit fuel intake.