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Gasification

Gasification is a process in which the transformation of any carbon based material into gaseous fuels is possible without combustion. Instead, a chemical reaction is created by combining waste with oxygen and steam under high pressure, generally at temperatures in excess of 800°C. Gasification is "the continuation of the pyrolysis process, where the residual carbon is oxidized from the glowing embers of the pyrolysis coke” (Bilitewski et al., 1997). The process parts everything in molecules and produces syngas, which contains carbon monoxide, hydrogen and methane. The gas has a net calorific value of 4-10 MJ/Nm3 (Zafar, 2009) and can thus be used to generate electricity. A typical gasification plant diagram can be seen in Figure 1.

Modern Gasification Plant Scheme
Schematic of a MSW Gasification and Power Generation Plant (Energos, 2009)

The gasification and power generation plant is composed of basically four modules: a waste pre-processing unit, the gasification/oxidation chambers, the energy recovery section and finally the flue gas cleaning module. In the pre-processing module the waste is sorted, grinded, shredded, stored and dried with the purpose of obtaining a gasification-friendly feed material, free of metals, glass and plastic bottles.

The second module consists of two chambers. Gasification of the solid waste takes place in the primary chamber, at below the stoichiometric air requirement and at temperatures between 400 and 1000°C the carbon reacts with oxygen and water steam to form syngas, i.e. carbon monoxide, carbon dioxide and hydrogen (Equations 1, 2, 3, 4, 5). The ratio of CO/CO2 occurring in the gasifier is determined by the Boudouard reaction, at temperatures lower than 700°C the predominant product will be carbon dioxide, while at higher temperatures the predominant product will be carbon monoxide. In the high temperature oxidation unit, i.e. the secondary chamber, a staged oxidation of the syngas is facilitated by multiple injections of air and recycled flue-gas (Bilitewski et al., 1997; Energos, 2009; BREDL, 2009). At the end of the gasification grate, the bottom ash is discharged.

Equation 1: C + ½O2 → CO

Equation 2: C + O2 → CO2

Equation 3: C + 2H2O → CO2 + 2H2

Equation 4: C + H2O → CO + H2

Equation 5: C + CO2 → 2CO

The last two modules work under the same principle that a waste incineration plant does. First the generated heat in the secondary chamber is utilized in a boiler to heat up the water pipes and convert water into steam to move a turbine and generate electricity. Finally, the remaining flue gases are cleaned using the dry sorption system. Lime is injected on the flue gas stream to adsorb the acid components, while the activated carbon adsorbs the dioxins, heavy metals and total organic carbon (Energos, 2009).