**4. Controlling factors to the stability of gasifier operation**

An understanding of the technical challenges of gasification technology requires a basic understanding of the factors that control the stability of gasifier operation. A typical gasification process includes the following four key steps: drying, pyrolysis, oxidation, and reduction. There are no strict boundaries between these steps; they often overlap and a host of factors including the type of gasifier, feedstock type, and process parameters, such as temperature, determines the output of a gasification process involving the four key steps listed above [4, 5, 8]. The operating temperature of a gasification process is a function of the amount of oxygen fed to the gasification system (gasifier), which induces partial gasification. Temperature response will abruptly change at an equivalence ratio (ER) of about 0.25; depending on the source of oxygen, this change point is typical of gasifier temperatures in the range 600–800° C; some quantities of oil and tar are produced in the pyrolysis stage of the gasification process. These products of the pyrolysis stage are stable for about a second at temperatures lower than 600°C [13].

The fixed bed updraft gasifier operates at temperatures below 600°C and generates considerable amounts of tars that are often emitted with the syngas, while its counterpart, the downdraft gasifier (also of the fixed bed type) is self-regulating and produces far less tar relative to the updraft gasifier; the fluidized bed gasifier also has high tar production rate, in fact, its tar production rate is greater than other types of gasifiers like the fixed bed and the entrained flow gasifiers [13, 14, 17].
