Gasification Thermodynamics

Chemical equilibrium dictates the extent of reaction possible for a given temperature and pressure. For single simple reactions, an equilibrium constant approach can be used to determine the equilibrium concentration of gases for a given reaction. At equilibrium, the forward and the reverse reaction rates are equal. The equilibrium constant is calculated from the Gibbs free energy, as follows  

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. 

The effects of pressure on equilibrium mole fractions can be illustrated with Equation 4.9, changed into variables of mole fraction and total pressure  

Note that the value of the equilibrium constant Kp does not change with pressure, since it is calculated from AG xn at 1 bar standard state pressure, but that the mole fractions change with pressure if the number of moles of reactants is different from the number of moles of products. In this case (Eq. 4.10), as pressure rises, less CO and H2 will be produced since Kp is a constant with pressure. 

The equilibrium constant approach works well when single simple reactions occur, but not when there are competing reactions. The formal definition of chemical equilibrium is that the total Gibbs free energy is at a minimum  

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Snoeck, J., Froment, G. and Fowles, M. (1997). Filamentous carbon formation and gasification Thermodynamics, driving force, nucleation, and steady-state growth, J. CataL, 169, pp. 240-249. 

Thermodynamically, the formation of methane is favored at low temperatures. The equilibrium constant is 10 at 300 K and is 10 ° at 1000 K (113). High temperatures and catalysts ate needed to achieve appreciable rates of carbon gasification, however. This reaction was studied in the range 820—1020 K, and it was found that nickel catalysts speed the reaction by three to four orders of magnitude (114). The Hterature for the carbon-hydrogen reaction has been surveyed (115). 

Fig. 7.76 Superimposed simplified thermodynamic stability diagrams for three elements with oxygen and sulphur at 871°C. The shaded rectangle indicates possible activity ranges in coal gasification atmospheres (after Stringer )...

Prins, M. J., Thermodynamic Analysis of Biomass Gasification and Torrefaction. PhD Thesis, Technical University of Eindhoven, Eindhoven, 2005. 

Thermodynamic and equilibrium characteristics of gasification systems, if available, could help to determine conditions under which certain desired products may be... 

The physical, chemical, and thermodynamic characteristics of biomass resources vary widely. This variation can occur among different samples of what would nominally seem to be the same resource. Also, variations could occur from one region to another, especially for waste products. This wide variation sometimes makes it difficult to identify a typical value to use when designing a gasification plant. 

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. 

Yan, Q., Guo, L., Lu, Y. 2006. Thermodynamic analysis of hydrogen production from biomass gasification in supercritical water. Energy Convers Manage 47 1515-1528. 

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. 

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. 

The reactants, coal, water and oxygen are converted to hydrogen, carbon monoxide, carbon dioxide, methane, water vapour and hydrogen sulphide at a given temperature and pressure according to thermodynamic equilibria and the kinetics of gasification. A particular coal composition which is characteristic of a German hard coal was taken as a basis (Table I). 

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. 

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