Gasification thermodynamic process

Liu, S., Wang, Y., Yu, L. and Oakey, J. (2006) Thermodynamic equilibrium study of trace element transformation during underground coal gasification. Fuel Processing Technology, 87(3), 209-15. 

Zhang Q, Wu Y, Dor L, Yang W, Blasiak W. A thermodynamic analysis of solid waste gasification in the plasma gasification melting process. Appl Energy 2013 112 405-413. 

As discussed above, the pyrolysis of biomass at high temperature (>1000 °C) results in the formation of synthesis gas, a valuable mixture of CO and H2. The decomposition of carbohydrate to synthesis gas is an endothermic reaction since the heating value of product is —125% of that of the feedstock (Reaction 1). The reaction becomes nearly thermo-neutral upon burning about 1/4 of the products. Since the thermodynamics favors the combustion of H2 over CO, the gasification reaction resemble the theoretical Reaction (2). Indeed numerous gasification processes feed 02 or air to drive the gasification reaction. 

Details of the thermodynamic basis of availability analysis are dealt with by Moran (Availability Analysis, Prentice-Hall, Englewood Cliffs, NJ, 1982). He applies the method to a cooling tower, heat pump, a cryogenic process, coal gasification, and particularly to the efficient use of fuels. 

The quasi-thermodynamic approach outlined above will obviously remain applicable to future coal conversion technologies as well, because the underlying thermodynamic principles are universal and invariant. Figure 2 is illuminating in this context it shows that by-product waters of liquefaction processes can be differentiated from their gasification counterparts by their redox potential and pH characteristics (13). 

Syngas composition, most importantly the H2/CO ratio, varies as a function of production technology and feedstock. Steam methane reforming yields H2/CO ratios of three to one whereas coal and biomass gasification yields ratios closer to unity or lower. Conversely, the required properties of the syngas are a function of the synthesis process. Fewer moles of product almost always occur when H2 and CO are converted to fuels and chemicals. Consequently, syngas conversion processes are more thermodynamically favorable at higher H2 and CO partial pressures. The optimum pressures depend on the specific synthesis process. 

Reaction R-4.7, the water-gas shift reaction, is an exothermic reaction. The water-gas shift reaction has influence on the CO/H2 ratio in the gasification product, which is very important when the gas is used for synthesis purpose. Therefore, the shift process can be found in almost all the ammonia plants and hydrogen generation process in gas plants. The shift reaction can generally be taken into account using thermodynamic chemical equilibrium, since gas-phase temperatures are high. 

Many researchers have studied the thermodynamic equilibrium of the reactions occurring in the gasification process. The reaction enthalpies and equilibrium constants of some reactions at different gasification temperature are listed in Table 4.5. 

A process that is similar to carbon activation is coal gasification. Here coal, consisting primarily of carbon, is reacted at high temperatures with various mixtures of air, oxygen, and steam to produce a fuel gas. We have applied some of the techniques used to analyze the thermodynamics of coal gasification systems to develop a technical data base. 

The thermodynamic combustion/gasification processes involved can be analyzed on the basis of the First Law of Thermodynamics (Heat in = Heat out Material in = Material out) and chemical... 

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