Page 213 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 199

Gasification is a very efficient method for extracting energy from many different types
of organic materials, and also has applications as a clean waste disposal technique. In the
process, coal or coal char is converted to gaseous products by reaction with steam, oxygen,
air, hydrogen, carbon dioxide, or a mixture of these.
The gasification of coal or a derivative (i.e., char produced from coal) is, essentially, the
conversion of coal (by any one of a variety of processes) to produce combustible gases. With
the rapid increase in the use of coal from the fifteenth century onward (Nef, 1957; Taylor
and Singer, 1957) it is not surprising the concept of using coal to produce a flammable gas,
especially the use of the water and hot coal (van Heek and Muhlen, 1991), became common-
place (Elton, 1958). In fact, the production of gas from coal has been a vastly expanding area
of coal technology, leading to numerous research and development programs. As a result,
the characteristics of rank, mineral matter, particle size, and reaction conditions are all rec-
ognized as having a bearing on the outcome of the process; not only in terms of gas yields
but also on gas properties (Massey, 1974; van Heek and Muhlen, 1991).
The products of coal gasification are varied insofar as the gas composition varies with
the system employed (Fryer and Speight, 1976; Anderson and Tillman, 1979; Probstein and
Hicks, 1990; Speight, 1994 and references cited therein). It is emphasized that the gas prod-
uct must be first freed from any pollutants such as particulate matter and sulfur compounds
before further use, particularly when the intended use is a water gas shift or methanation
(Probstein and Hicks, 1990).
The advantage of gasification is that using the syngas is more efficient than direct combus-
tion of the original fuel; more of the energy contained in the fuel is extracted. Syngas may
be burned directly in internal combustion engines, used to produce methanol and hydrogen,
or converted via the Fischer-Tropsch process into synthetic fuel. Gasification can also begin
with materials that are not otherwise useful fuels, such as biomass or organic waste. In addi-
tion, the high-temperature combustion refines out corrosive ash elements such as chloride and
potassium, allowing clean gas production from otherwise problematic fuels.
Gasification of coal has been, and continues to be, widely used on industrial scales to
generate electricity. However, almost any type of organic carbonaceous material can be
used as the raw material for gasification, such as wood, biomass, or even plastic waste.
Thus, gasification may be an important technology for renewable energy. In particular
biomass gasification is carbon neutral.
Another advantage of gasification-based energy systems is that when oxygen is used in
the gasifier in place of air, the carbon dioxide produced by the process is in a concentrated
gas stream, making it easier and less expensive to separate and capture. Once the carbon
dioxide is captured, it can be sequestered and prevented from escaping to the atmosphere,
where it could otherwise potentially contribute to the greenhouse effect.

7.1.1 Chemistry

Gasification relies on chemical processes at elevated temperatures >700°C, which distin-
guishes it from biologic processes such as anaerobic digestion that produce biogas.
In a gasifier, the carbonaceous material undergoes several different processes: (a) pyrol-
ysis of carbonaceous fuels, (b) combustion, and (c) gasification of the remaining char.
Pyrolysis (devolatilization) is the thermal degradation of an organic substance in the
absence of air to produce char, pyrolysis oil, and syngas, for example, the conversion of
wood to charcoal. The pyrolysis process occurs as the carbonaceous feedstock heats up.
Volatiles are released and char is produced, resulting in up to 70 percent weight loss for
coal. The process is very dependent on the properties of the carbonaceous material and
determines the structure and composition of the char, which will then undergo gasification
reactions.