Page 171 - Synthetic Fuels Handbook

FUELS FROM COAL 157

5.5.6 The Future
Fuel in the form of gas has major advantages. Unlike electricity, a gaseous fuel can be
stored until it is required and, unlike liquid fuel, gas is clean and leaves no residue in vessels
used for storage or transport. Unlike solid fuel, a gas can be distributed continuously and
delivered at precise rates to exact locations, at any scale from pilot light to power station.
Operators can control the flow from moment to moment, and measure the amount used
accurately for monitoring and billing. They can vary the composition and characteristics of
a gaseous fuel readily, for instance, by blending natural gas with gas from coal.
Furthermore, a gaseous fuel does not have to be supported on a grate or in a combustion
chamber, nor does it leave a solid residue that operators must remove and dispose.
Burning premium gaseous fuels, predominantly natural gas, in combined cycle gas tur-
bines (CCGTs) has had a substantial impact in recent years on the efficiency, cleanliness
and cost of power generation. Future resource projections and strategic considerations,
however, have prompted renewed interest in advanced coal technologies in addition to
combustion of renewable energy sources such as biomass-derived fuels.
An important component in the future exploitation of coal is its efficient gasification
to a mixture of hydrogen and carbon monoxide, together with an inert ballast of nitrogen,
carbon dioxide, and water. Unlike natural gas (or methane), which has attracted a wide
range of combustion applications and for which an extensive combustion database has been
developed, coal-derived gaseous fuel mixtures have not been widely investigated.
Stable, lean-burning combustion systems present a considerable design challenge, even for
premium fuels. The low levels of oxides of nitrogen (NO ) emitted by current natural gas-
x
fired turbines, typically less than 10 ppm in many applications, have been achieved with highly
refined strategies for fuel-air mixture preparation based on a combination of empiricism and
numeric simulation. The starting point for similar combustor developments in relation to coal-
derived gaseous fuel mixtures is much more poorly defined. Differences in the calorific value of
the fuel, reflecting the level of inflammability and flame stability, introduce changes to combus-
tion characteristics that must be accommodated in any design.
The successful exploitation of coal-derived gasified fuels in power generation using
combined cycle gas turbines will require that the emissions performance and operability of
such plant be broadly comparable with those presently demonstrated with natural gas.

5.6 LIQUID FUELS

One of the early processes for the production of liquid fuels from coal involved the
Bergius process. In the process, lignite or subbituminous coal is finely ground and
mixed with heavy oil recycled from the process. Catalyst is typically added to the mix-
ture and the mixture is pumped into a reactor. The reaction occurs at between 400 to
500°C and 20 to 70 MPa hydrogen pressure. The reaction produces heavy oil, middle
oil, gasoline, and gas:

n[C] coal + (n+1)H → C H 2n+2
2
n
A number of catalysts have been developed over the years, including catalysts contain-
ing tungsten, molybdenum, tin, or nickel.
The different fractions can be sent to a refinery for further processing to yield of syn-
thetic fuel or a fuel-blending stock of the desired quality. It has been reported that as much
as 97 percent of the coal carbon can be converted to synthetic fuel but this very much
depends on the coal type, the reactor configuration, and the process parameters.