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.
• The optimization of the energy in the form of the heat of combustion of the product
gas or, alternatively, of the synthesis gas production per unit fuel intake.
• To provide set points for process control.
Calculations comprising the gasification proper are based on thermodynamics, mass and energy balances and process conditions, such as temperature, pressure, and the addition or subtraction of indirect heat. In all these calculations it is essential that the elemental composition and the temperature of the feed streams are known. For coal, both the proximate analysis (fixed carbon, volatile matter, moisture, ash) and the ultimate analysis (elemental, apart from ash) must be known.
In gasification, use is made of a variety of reactions of which some are exothermic and some are endothermic, as was shown in Section 2.1. In virtually all cases the desired operating temperature is obtained by judiciously playing with the exothermic and endothermic reactions. The reaction of the fuel with oxygen is always complete and exothermic, whereas the reaction with steam or carbon dioxide is always endothermic and never complete because of thermodynamic limitations.
In gasifiers where both oxygen and steam are used to control the temperature, the role of steam is that of a moderator. Some other methods to moderate the temperature are to add nitrogen or carbon dioxide to the oxygen, or to remove heat indirectly from the gasification reactor.
In all cases the fuel to a gasifier will contain carbon. The blast or gasifying agent is the mixture of oxygen-containing gas and/or steam and/or carbon dioxide. Hence the blast always contains oxygen either as free oxygen or bound in the form of, for example, steam.
The companion website to this book includes a simple gasification model (gasify. exe) that illustrates these principles. The following discussion of specific aspects of gasifier modeling includes both general points, applicable to any model as well as an explanation of the particular approach that we have adopted for our own model.
