Environmental Engineering Reference
In-Depth Information
2.1.1.4 Coal Gasification
Another important thermal method is based on the gasification process, currently
used on industrial scale essentially to generate electricity.
This technology is also the oldest method for hydrogen production and could
convert any type of organic material, such as coal and other petroleum or biomass-
derived mixtures. The interest towards this approach comes from the practical
possibility of using coal as fuel that is the most world-wide available and relative
cheap fossil fuel [
52
].
The gasification of coal or other carbonaceous substances was largely used in
the past century especially for iron making. The process consists of a series of
chemical reactions finally producing, similar to reforming reactions, carbon
monoxide and hydrogen mixtures, also called 'town gas', which represented in the
past century an important chemical feedstock in North American, Europe and
China for domestic heating and lighting, public street lighting and domestic
fertiliser industry. However, the popularity of town gas decreased significantly by
the 1950s as natural gas became widely available. Gasification takes place at high
pressure (up to 60 bar) and temperature superior to 700C, with a controlled
amount of oxygen and/or steam. Similar to hydrocarbon reforming-derived syn-
thesis gas, the effluent mixture may be used to produce hydrogen or methanol,
burned directly in internal combustion engines, or converted via the Fischer-
Tropsch process into synthetic fuel [
53
].
Coal substances have complex chemical structures and their compositions are
highly variable. For example a carbon/hydrogen composition in bituminous coal
may be represented as about one atom of hydrogen per atom of carbon. For a
generic gasification process based on the above coal feedstock, selected as refer-
ence carbonaceous fuel, the following (not balanced) overall chemical equation
can be written as:
CH
þ
O
2
þ
H
2
OCO
þ
CO
2
þ
H
2
þ
other species
ð
2
:
15
Þ
The carbonaceous particles are heated and volatilized at temperatures ranging
from 1000 to 1500C producing carbon oxides and hydrogen gaseous mixtures and
simultaneously char (pyrolysis).
A limited amount of oxidant (oxygen or air) is introduced into the reactor and is
mixed with crushed/pulverized coal feed (either dry or as slurry) to allow volatile
products and some of the char reacts with oxygen to form carbon dioxide and
carbon monoxide. The basic reactions for the CO and CO
2
formation are the
partial and total combustion of C, respectively
C
þ
1
=
2
O
2
!
CO
DH
¼
110
:
4 kJ/mol
ð
2
:
16
Þ
C
þ
O
2
!
CO
2
DH
¼
393
:
1 kJ/mol
ð
2
:
17
Þ
The exothermicity of the above reactions provides heat for the subsequent
gasification reactions. The char (or other resulting hydrocarbons) reacts with steam
