Chapter 3 Gasification 3.1 Gasification theory
Gasification process allows the conversion of a solid or liquid feedstock into a gaseous fuel which can be employed for energy purposes, as introduced in the first chapter. Typical steps involved in a gasification process are [13]: Drying; Pyrolysis; Partial combustion of the released gases an char; Gasification of the products of the previous steps. The drying phase takes place thanks to the heat coming from the oxidation zone downstream. It must be highlighted the fact that, in order to achieve satisfying heating values in the syngas, usually the employed systems are fed with feedstock characterized by moisture content in the range 10-20% [13]. Pyrolysis phase involves the thermal cracking of hydrocarbon molecules of the feedstock producing smaller gaseous molecules which can be condensable or not. Condensable part of this product brings to the formation of tars which could create problems in the plant management, thus the produced gas is treated for their reduction (further considerations concerning tars control will be explained in next pages). Fig.18 shows a generic scheme of a downdraft gasifier.
Figure 18 Generic scheme of a downdraft gasifier and its temperature profile [13]
Partial combustion of char is necessary to sustain most of the reaction of the process which are endothermic. Most important oxidation steps involves the reaction of carbon with the production of CO2 and CO. As will be explained in the next paragraphs concerning the gasifier employed in the laboratory, the control of the secondary air is crucial for the control of the
44
