Environmental Engineering Reference
In-Depth Information
Fixed bed reactors are the simplest type of gasification equipment. They have
operating temperatures of 850-1,400°C [37, 38] and have been used for many
years. They consist of an upright cylindrical container that has an inlet and an
outlet for the gases into which the feedstock is fed from above; ashes are removed
at the base of the container. There are several different fixed bed reactor designs
including the updraft (counter current) gasifier, the downdraft (co-current) gasi-
fier, and the cross-flow (crossdraft) gasifier, as shown in Figure 7.5.
In updraft reactors, the biomass is fed in at the top of the reactor and moves
towards the bottom while gas flows in the opposite direction. The biomass is
passed through drying, distillation, and reduction zones in which it is converted to
gas and ash is removed. Air enters the reactor from the bottom and the syngas
outlet is situated at the top of reactor. This design therefore allows vapours from
liquid compounds and tars to distil (flow) over. Conversely, in downdraft gasifiers,
such materials can only exit via the highest temperature zone, where they are
destroyed. In these designs, the incoming air is introduced in the middle of the
reactor. This minimizes tar formation and may obviate the need for further purifi-
cation of the resulting syngas. In the cross-flow gasifier (reactor) design, the syn-
gas is drawn off from the side opposite to that on which the incoming gas enters,
preventing any contact between the reaction products and the fresh feedstock.
Syngas generated in this way is invariably highly contaminated with tar. There are
also some other gasification reactor designs available such as the entrained bed,
twin fluid bed, bubbling fluid bed, and circulating fluid bed reactors. These are
not discussed in this work, but are described at length by Bridgwater [39]. Several
pilot plants for biomass gasification using different reactor technologies have
been established, including entrained flow facilities in Germany (Freiberg) and
Sweden (Piteå) as well as a circulating fluidized bed facility in Austria (Güssing).
Another type of gasifier is the fluidized bed reactor, which is based on counter-
current flows of gas and a particulate material (the fluidizing medium) which
is usually a kind of quartz sand bed with a controlled particle size (i.e. a fixed
particle size distribution). This method maintains a uniform temperature through-
out the bed, enhancing the overall yield of gasification at a lower temperature than
is required in fixed bed reactors. One drawback of this method is the high loss of
heat in the syngas and its residual fuel and ash content. However, a cyclone can be
used to separate the fly ash and particulate matter from the syngas and there are
several other methods that can be used to overcome these problems. The process
of biomass gasification in a fluidized bed reactor is outlined in the following:
• biomass is fed into the fluidized bed;
• biomass is heated with hot sand (to around 1,000°C), forming syngas and char;
• the syngas and char are removed using a cyclone;
• the syngas is purified further before being used as a raw material for fuel
production; and
• the flue gas (the excess heat gas) may be used in steam production and power
generation.
