Gasification processes incorporate many reactions that are fairly complex to analyse making their design difficult. In this chapter it is shown that general gasification systems are limited by consideration of mass and energy balances only. Here, a ternary Carbon-Hydrogen-Oxygen diagram is developed to represent gasification processes. The diagram incorporates basic chemistry and thermodynamics to define a region in which gasification occurs. The techniques are further validated from data obtained from pilot or laboratory experiments available in literature. In this chapter we develop graphical representation for sawdust gasification and underground coal gasification (UCG), a clean coal technology. The methods described allow for further analysis without considerations to thermodynamic equilibrium, reactor kinetics, reactor design and operation. This analysis is thus an indispensable tool for flowsheet development using gasification and an excellent tool for practitioners to rapidly understand gasification processes.
Part of the book: Gasification
Traditional coal-to-liquid processes use gasification with excess steam to obtain hydrogen-rich syngas for downstream manufacturing of methanol or Fischer-Tropsch liquids. Such processes are shown to produce very large amounts of CO2 directly by the Water-Gas-Shift (WGS) reaction or, indirectly, by combustion in raising steam. It is shown how any coal gasifier can operate under auto-thermal conditions with methane as source of hydrogen instead of steam. This co-gasification system produces syngas for a poly-generation facility while minimising the formation of process CO2. It is shown that minimal steam is required for the process and a limit on the maximum amount of H2:CO can be obtained. Co-gasification of coal is shown to have a major advantage in that a separate WGS reactor is not required, less CO2 is formed and methane is reformed non-catalytically within the gasification unit. Furthermore, regions of thermally balanced operations were identified that enabled a targeting approach for the design of co-gasification systems. The method will guide gasification practitioners to incorporate fossil fuels and renewable-H2 into coal-to-liquids processes that require syngas with H2:CO ratio of 2. An important result shows that low-grade coals can be co-gasified with methane to obtain CO2-free syngas ideal for power generation.
Part of the book: Clean Energy Technologies