Page 169 - Synthetic Fuels Handbook

FUELS FROM COAL 155

or oxygen. The coal is gasified at 350 to 450 psi (2.4–3.1 MPa) and devolatilization takes
place in the temperature range 615 to 760°C (1139–1400°F); residence time in the reactor
is approximately 1 hour. Hydrogen is supplied by injected steam and the necessary heat is
supplied by the combustion of a portion of the product char. The revolving grate, located at
the bottom of the gasifier supports the bed of coal, removes the ash, and allows steam and
oxygen (or air) to be introduced.
The Lurgi product gas has high methane content relative to the products from non-
pressurized gasifiers. With oxygen injection, the gas has a heat content of approximately
3
3
450 Btu/ft (16.8 MJ/m ). The crude gas which leaves the gasifier contains tar, oil, phe-
nols, ammonia, coal fines, and ash particles. The steam is first quenched to remove the
tar and oil and, prior to methanation, part of the gas passes through a shift converter
and is then washed to remove naphtha and unsaturated hydrocarbons; a subsequent step
removes the acid gases. The gas is then methanated to produce a high heat-content pipe-
line quality product.
The Wellman Galusha Process. The Wellman Galusha process has been in com-
mercial use for more than 50 years (Howard-Smith and Werner, 1976). These are two
types of gasifiers, the standard type and the agitated type and the rated capacity of an
agitated unit may be 25 percent (or more) higher than that of a standard gasifier of
the same size. In addition, an agitated gasifier is capable of treating volatile caking
bituminous coals.
The gasifier is water-jacketed and, therefore, the inner wall of the vessel does not require
a refractory lining. Agitated units include a varying speed revolving horizontal arm which
also spirals vertically below the surface of the coal bed to minimize channeling and to pro-
vide a uniform bed for gasification. A rotating grate is located at the bottom of the gasifier
to remove the ash from the bed uniformly. Steam and oxygen are injected at the bottom
of the bed through tuyeres. Crushed coal is fed to the gasifier through a lock hopper and
vertical feed pipes. The fuel valves are operated so as to maintain a relatively constant flow
of coal to the gasifier to assist in maintaining the stability of the bed and, therefore, the
quality of the product gas.

Entrained-Bed Processes. The Koppers-Totzek Process (Baughman, 1978; Michaels and
Leonard, 1978; van der Burgt, 1979) is, perhaps, the best known of the entrained-solids
processes and operates at atmospheric pressure. The reactor is a relatively small, cylindri-
cal, refractory-lined vessel into which coal, oxygen, and steam are charged. The reactor
typically operates at an exit temperature of about 1480°C (2696°F) and the pressure is
maintained slightly above atmospheric pressure.
Gases and vaporized hydrocarbons produced by the coal at medium temperatures imme-
diately pass through a zone of very high temperature in which they decompose so rapidly
that coal particles in the plastic stage do not agglomerate, and thus any type of coal can
be gasified irrespective of caking tendencies, ash content, or ash fusion temperature. The
gas product contains no ammonia, tars, phenols, or condensables and can be upgraded to
synthesis gas by reacting all or part of the carbon monoxide content with steam to produce
additional hydrogen plus carbon dioxide.
Molten Salt Processes. Molten salt processes feature the use of a molten bath
[>1550°C (>2822°F)] into which coal, steam, and oxygen are injected (Karnavos et al.,
1973; La Rosa and McGarvey, 1975). The coal devolatilizes with some thermal crack-
ing of the volatile constituents. The product gas, which leaves the gasifier, is cooled,
compressed, and fed to a shift converter where a portion of the carbon monoxide is
reacted with steam to attain a CO/H ratio of 1:3. The carbon dioxide so produced is
2
removed and the gas is again cooled and enters a methanator where carbon monoxide
and hydrogen react to form methane.