Page 158 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

268 Fuel Chemistry

temperatures, lignites and subbituminous coals that have plants for power generation. Another entrained bed gasifi-
high ash fusion temperatures are ideal feedstocks for this cation process under development is a pressurized version
type of gasifier. Units capable of gasifying 40–45 tons per of the K–T process called the Shell–Koppers system.
hour are commercially available. These developing processes are primarily aimed at ei-
TheKoppers–Totzekgasifierhasbeenthemostsuccess- ther increasing the operating pressure to increase the
ful entrained-flow gasifier. This process uses pulverized throughput and provide pressurized product gas for ad-
coal (usually less than 74 µm) entrained (blown) into the vanced power systems or increasing the operating tem-
gasifier by a mixture of steam and oxygen. The gasifier is peratures to accommodate a variety of fuels, or both.
operated at atmospheric pressure and high temperatures
◦
of about 1600–1900 C. The coal dust and gasification IX. COAL LIQUEFACTION
medium flow cocurrent (in the same direction) in the gasi-
fier and because of the small coal particle size, the particle
Similar to coal gasification, coal liquefaction is the process
residence time is approximately 1 sec. Although this resi-
of converting solid coal into liquid fuels. The main differ-
dence time is relatively short, high temperatures enhance
ence between naturally occurring petroleum fuels and coal
the reaction rates, and therefore almost any coal can be
is the hydrogen to carbon molar ratio. Petroleum has ap-
gasified in this system. Tars and oils are evolved at mod-
proximately a hydrogen to carbon ratio of 2 where as coal
erate temperatures and crack at higher temperatures, and
has about 0.8. Coal also contains higher concentrations
therefore, there is no condensible tarry material in the
of oxygen, nitrogen, and significantly higher amounts of
products and the ash melts and flows as slag in the K–T
mineral matter than petroleum. Therefore, conversion of
gasifier. Gasifiers with two diametrically opposite nozzles
coal into liquid fuels involves hydrogenation (addition
(also called heads) are most commonly used. However,
of hydrogen either directly or indirectly). Direct hydro-
the use of four nozzles doubles the throughput (over the
genation either from gaseous hydrogen or from a hydrogen
two, nozzle design) and such configurations are also in
donor solvent is termed direct liquefaction. If the hydro-
use. The product gas is mainly synthesis gas (a mixture
gen is added indirectly through an intermediate series of
of CO and H 2 ) and is primarily used for ammonia man-
compounds, the process is called indirect liquefaction. In
ufacture. Since no heavy-duty moving parts are involved
direct liquefaction processing, the macromolecular struc-
in this system, maintenance is minimal and availability is
ture of the coal is broken down ensuring that the yield
expected to be high.
of the correct size of molecules is maximized and that the
production of the very small molecules that constitute fuel
D. Advanced Coal Gasification Systems gasesisminimized.Incontrast,indirectliquefactionmeth-
ods break down the coal structure all the way to a synthesis
Many attempts have been made to improve the first-
gas mixture (CO and H 2 ), and these molecules are used
generation commercial gasifiers. British Gas Corporation
to rebuild the desired liquid hydrocarbon molecules. This
has converted the Lurgi gasifier into a slagging type by
can be achieved by a variety of gasification techniques as
increasing the operating temperature and thereby accom-
discussed in the Coal Gasification section.
modating higher-rank coals which require higher temper-
atures for complete gasification. Another version of the
Lurgi gasifier is the Ruhr-100 process with operating pres- A. Liquefaction Reactions
sures about three times higher than that of the basic Lurgi Since coal in a complex substance it is often represented
process. Developmental work on the Winkler gasification by an average composition and the reactions occurring
processhasleadtoapressurizedversioncalledthepressur- during direct and indirect liquefaction can be illustrated
ized Winkler Process with an aim of increasing the yield of as follows.
methane to produce Synthetic Natural Gas (SNG). Other
processes in the developmental stages are the U-gas, Hy- 1. Direct Liquefaction
Gas, Cogas, Westinghouse, and Synthane processes. The
Texaco gasification system appears to be most promising CH 0.8 S 0.2 O 0.1 N 0.01 + H 2 −→ (RCH x )
entrained bed gasification system that has been developed.
+ CO 2 , H 2 S, NH 3 , H 2 O.
In this system coal is fed into the gasifier in the form of
coal–water slurry and the water in the slurry serves as
2. Indirect Liquefaction
both a transport and a gasification medium. This system
◦
operates at 1500 C and the ash is removed as molten slag.
Experience on demonstration units has indicated that the CH 0.8 S 0.2 O 0.1 N 0.01 −→ CO + H 2
process has a potential to be used with combined-cycle + H 2 S, NH 3 , H 2 O (Gasification)