In an entrained flow gasifier, the feed and air move cocurrently and the reactions occur in a dense cloud of very fine particles at high pressures, varying between 19.7 and 69.1 atm and very high temperatures >1000 °C. This type of gasifier has an elevated throughput of syngas (Zhang et al., 2010).

2.1.2 Low/High-Temperature Gasification

High-temperature gasification (typically above 1200 °C) results in a gas, which merely contains H2 and CO as combustible components. At low-temperature however (typically below 1000 °C), also hydrocarbons are present in the gas. A CFB gasifier operated on biomass operated at 900 ° C typically produces a gas containing 50% hydrocarbons (mainly methane, ethylene, and benzene) on energy basis (http://www. biosng. com/experimental-line-up/ gasification-technology/).

2.1.3 Heating Source for Gasification

2.1.3.1 INDIRECT (OR ALLOTHERMAL) GASIFICATION

It is characterized by the separation of the processes of heat production and heat consump­tion. It therefore generally consists of two reactors connected by an energy flow. The biomass is gasified in the first reactor and the remaining solid residue (char) is combusted in the second reactor to produce the heat for the first process. Hot sand is circulated to transport the heat from the combustor to the gasifier. These indirect gasifiers theoretically are operated at an equilibrium based on the temperature dependence of the char yield in the gasifier. This means that at a low temperature, much char is remaining from the gasifier. Since this char is combusted to produce the heat, the temperature will rise until char yield matches the energy demand of the gasification (http://www. biosng. com/experimental-line-up/ gasification-technology/).