Environmental Engineering Reference
In-Depth Information
However, SCWO processes can also produce gases, including H
2
and CH
4
.
Because the temperature is maintained at 500°C or below, no oxidation takes
place and catalysts are generally required for substrate conversion [50].
7.4.5
Combustion
Direct biomass combustion (i.e. the burning of biomass in air) is another way of
converting its chemical energy into heat and electricity. It is the most common
process for biomass-to-energy conversion worldwide, accounting for around 97%
of all biomass energy production. It is an economical technology that is generally
better understood than its alternatives, and is extensively used on a commercial
scale [53]. Combustion is usually performed in furnaces, boilers, steam turbines,
and turbo-generators. The combustion produces hot gases at temperatures of 800-
1,000°C or more. The heat produced must be used immediately for heat and/or
power generation because storage is not viable. Biomass combustion involves two
key steps: devolatilization to char and volatiles, and combustion of the volatiles
and char. Several variables affect the combustion process, including the volatiles,
tar, and ash content of the raw material [54]. It is therefore essential to properly
characterize the various potential biomass feedstocks. Biomass is characterized
with respect to both its chemical and physical properties. Chemical characteristics
of interest include moisture content, ash content, abundance of ash-forming ele-
ments, concentration of other inorganic elements, and ash melting behaviour dur-
ing combustion. These depend on the origin of the biomass and the agricultural
conditions under which it was produced. In some cases, the biomass must be
pre-dried to reduce its moisture content to <50% before combustion. Relevant
physical characteristics of biomass feedstocks include particle size, particle shape,
and bulk density. There are several advantages to upgrading biomass before com-
bustion, as discussed in Section 7.2.
Combustion may be performed on a small scale (for household heat produc-
tion), an intermediate scale (in hospitals, government office buildings, etc) or in
large-scale combustions plants for industrial purposes [54]. In the US, rice husk
conversion has been established as a particularly useful commercial option for
large-scale combustion [55].
The effective combustion of biomass normally yields nothing more than CO
2
and H
2
O. However, if the biomass has been contaminated (treated) with chlorine,
as in the case of PVC-coated wood, dioxins may also be formed during combus-
tion (primarily during the cooling of the flue gas). Dioxins are a group of organic
compounds that may have some carcinogenic properties [3].
Co-firing is a process in which biomass and coal are burned together, either
directly or indirectly. This can increase the cost-effectiveness of the overall process
and increase the consumption of biomass for energy production, while reducing
greenhouse gas emissions. There are three types of co-firing processes depending
on the way in which the coal and biomass are combined. In the first type, biomass
