Page 47 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

26 Biomass Gasification, Pyrolysis, and Torrefaction

When the gasification product, CO, is burnt subsequently in adequate
oxygen, it releases the remaining 72% (283 MJ) of the heat. Thus, the CO
retains only 72% of the energy of the carbon.
We can also go for complete gasification of a biomass where the energy
recovery is 75 88% due to the presence of hydrogen and other hydrocar-
bons. The producer gas reaction is an example of gasification reaction, which
produces hydrogen and carbon monoxide from carbon. This product gas
mixture is also known as synthesis gas or syngas:

C 1 H 2 O-CO 1 H 2 1 131; 000 kJ=kmol (1.7)
Utilization of heavy oil residues in oil refineries is an important applica-
tion of gasification. Low-hydrogen hydrocarbon residues are gasified into
hydrogen:
(1.8)
C n H m 1 ðn=2ÞO 2 5 nCO 1 ðm=2ÞH 2
This hydrogen can be used for hydrocracking of other heavy oil fractions
into lighter oils.
The reaction between steam and carbon monoxide is also used for maxi-
mization of hydrogen production in the gasification process at the expense
of CO:
CO 1 H 2 O-H 2 1 CO 2 41; 000 kJ=kmol (1.9)

1.6.5 Syngas Production

Syngas is also produced from natural gas (.80% CH 4 )using a
steam methane-reforming reaction, instead of from solid carbonaceous fuel
alone. The reforming reaction is, however, not strictly gasification but a
molecular rearrangement:
CH 4 1 H 2 O ðcatalystÞ-CO 1 3H 2 1 206; 000 kJ=kmol (1.10)

Partial oxidation of natural gas or methane is an alternative route for pro-
duction of syngas. In contrast to the reforming reaction, partial oxidation is
exothermic. Partial oxidation of fuel oil also produces syngas:
CH 4 1 1=2O 2 -CO 1 2H 2 36; 000 kJ=kmol (1.11)

1.6.6 Methanol Synthesis
Syngas provides the feedstock for many chemical reactions, including metha-
nol synthesis (Eq. (1.12)). Methanol (CH 3 OH) is a basic building block of
many products, including gasoline:
CO 1 2H 2 ðcatalystsÞ-CH 3 OH (1.12)