Environmental Engineering Reference
In-Depth Information
There are several factors that influence the choice of conversion process including
the desired form of bioenergy, the type of and quantity of the available biomass
feedstocks, and environmental and economical constraints. Biomass materials
such as agricultural and forestry residues are often difficult to use directly in ther-
mochemical conversion processes for the production of certain energy products
due to their heterogeneous and bulky nature. They therefore need to undergo some
kind of pretreatment or physical upgrading process prior to use, as discussed in
Section 7.2. Biomass may also has a high water content, which must often be
reduced prior to thermochemical treatment. For all these reasons, biomass is often
crushed, dried, and milled before thermochemical processing. As discussed in
Section 7.2.6, torrefaction is another pretreatment process that can be used to
improve the thermochemical conversion of biomass by increasing gasification
efficiency and reducing smoke production during gasification.
7.4.1
Thermal Processing Equipment
There are four main types of thermal processing equipment that are widely used
for biomass gasification, including: fixed bed gasifiers (and moving bed systems);
fluidized bed reactors (gasifiers); circulating fluidized bed (CFB) reactors; and
entrained flow systems, as highlighted in Figure 7.4. Passing gas over the biomass
at a high velocity in thermal processing systems generally promotes mixing and
enhances combustion efficiency.
(b)
Product
gas
(c)
Biomass
Gas
Oxygen
(a)
Cyclone
Cyclone
Cyclone
Freeboard
Product
gas
Ash
Bubbles
Ash
Cooling
screen
Fluid
bed
Biomass
Biomass
& sand
Heat
Sand
Air
Quench
water
Syngas
Slag
Biomass
feed
Air
Figure 7.4
Different kinds of reactors: (a) fluid bed reactor; (b) CFB; and (c) entrained flow
reactor. Reproduced with permission from Rashmi Kataria, Department of Forest Biomaterials
and Technology at SLU.
