Environmental Engineering Reference
In-Depth Information
the image associated with pyrolysis as a technology is often negative [51]. It is
clear that new, clean, and efficient technologies for the conversion of biomass to
fuels and other valuable products are necessary to help us meet the challenges of a
growing population, diminishing resources, and climate change.
3.3.3
Gasification
Gasification is a thermochemical process converting biomass into syngas,
which primarily consists of CO and H
2
, by partial oxidation of biomass in the
presence of an oxidizing agent. The process conditions of gasification are simi-
lar to those of pyrolysis, in which the biomass is converted into liquid (hydro-
carbon oil), gaseous (fuel gas), and solid (charcoal) fractions in the absence of
O
2
. Gasification takes place at higher temperatures ranging from 700 to 1000°C
leading to relatively higher gas yields (up to 85%) compared to pyrolysis,
which is up to 35% [57, 58]. The syngas obtained can be used for heat and
power generation and productions of chemicals such as methanol, dimethyl
ether, alcohol, organic acids, and polyesters [59]. Gasification is able to process
various types of biomass, including agricultural residues, forestry residues,
food waste, and organic municipal wastes [59]. In order to facilitate biomass
gasification, ground biomass with a moisture content lower than 10% is recom-
mended for this process [58]. Upon heating to 350°C in the absence of air or
O
2
, the biomass is thermally decomposed and reduced to solid carbonized bio-
mass by devolatilization. At 700-1000°C, oxidizing agent (air, steam, CO
2
, O
2
,
or a mixture of these) is added to oxidize the solid carbonized biomass.
Reduction reaction also takes place at high temperature (800-1000°C) when
the oxidizing agent is consumed. Table 3.2 summarizes the reactions taking
place during the gasification process using steam as oxidizing agent. As a
result, a gas mixture consisting of CO, CO
2
, CH
4
, H
2
and H
2
O is produced
together with impurities including tars, char, alkali, sulfur compounds, and
nitrogen compounds. As a result, the gas mixture needs to undergo a series of
downstream processing steps such as barrier filtering, hot gas cleaning, and wet
scrubbing to remove the impurities before it can be used as syngas.
Table 3.2
Chemical reactions which occur during process of biomass gasification [58, 59].
Oxidation
Partial oxidation reaction
2C + O
2
→
2CO
Complete oxidation reaction
C + O
2
→
CO
2
Reduction
Water-gas reaction
C + H
2
O
→
CO + H
2
Bounded reaction
C + CO
2
↔
2CO
Shift reaction
CO
2
+ H
2
↔
CO + H
2
O
Methane reaction
C + 2H
2
↔
CH
4
Overall
CH
x
O
y
(biomass) + O
2
(21% of air) + H
2
O (steam)
→
CH
4
+ CO + CO
2
+ H
2
+ H
2
O (unreacted steam) + C (char) + tar
