Page 215 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 201

7.1.2 Processes
The most common modern coal gasification process uses lump coal in a vertical retort
(Speight, 1994 and references cited therein). In the process, coal is fed at the top with air,
and steam is introduced at the bottom. The air and steam rising up the retort heat the coal in
its downward flow and react with the coal to convert it to gas. Ash is removed at the bottom
of the retort. Using air and steam as reacting gases results in a producer gas; using oxygen
and steam results in a water gas. Increasing operating pressure increases the productivity.
Two other processes currently in commercial use react finely powdered coal with steam
and oxygen. For example, one of these, the Winkler process, uses a fluidized bed in which
the powdered coal is agitated with the reactant gases.
In the Winkler process dried, crushed coal is fed to the fluidized bed gasifier through
a variable-speed screw feeder whereupon the coal is contacted with steam and oxygen
injected near the bottom of the vessel (Howard-Smith and Werner, 1976; Baughman, 1978).
The upward flow of steam and oxygen maintains the bed in a fluidized state at a tem-
perature of 815 to 980°C (1499–1796°F) with a pressure that is marginally higher than the
atmospheric pressure. The high operating temperature reduces the amount of tars and other
heavy hydrocarbons in the product (Nowacki, 1980).
Another example, the Koppers-Totzek process (Baughman, 1978; Michaels and Leonard,
1978; van der Burgt, 1979) is an entrained-solids process which operates at atmospheric
pressure. The reactor is a relatively small, cylindrical, refractory-lined coal “burner” into
which coal, oxygen, and steam are charged through at least two burner heads. The feed coal
for the process is crushed (so that 70 percent will pass through a 200-mesh screen), mixed
with oxygen and low-pressure steam, and injected into the gasifier through a burner head.
The heads are spaced 180° or 90° apart (representing two-headed or four-headed opposed
burner arrangements) and are designed such that steam envelopes the flame and protects
the reactor walls from excessive heat.
The reactor typically operates at an exit temperature of 1480°C (2696°F) and the pres-
sure is maintained just slightly above atmospheric pressure. Approximately 85 to 90 percent
of the total carbon may be gasified in a single pass through the gasifier because carbon con-
version is a function of the reactivity of the coal and approaches 100 percent for lignite.
The heat in the reactor causes the formation of slag from mineral ash and this is removed
from the bottom of the gasifier through a water seal. Gases and vaporized hydrocarbons
produced by the coal at medium temperatures immediately 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.
In addition, the high operating temperature ensures that the gas product contains no
ammonia, tars, phenols, or condensable hydrocarbons. The raw gas 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.
As petroleum supplies decrease, the desirability of producing gas from coal may
increase, especially in those areas where natural gas is in short supply. It is also anticipated
that costs of natural gas will increase, allowing coal gasification to compete as an economi-
cally viable process. Research in progress on a laboratory and pilot-plant scale should lead
to the invention of new process technology by the end of the century, thus accelerating the
industrial use of coal gasification.
Thus, the products of coal gasification consist of carbon monoxide, carbon dioxide,
hydrogen, methane, and some other gases in proportions dependent upon the specific reac-
tants and conditions (temperatures and pressures) employed within the reactors and the
treatment steps which the gases undergo subsequent to leaving the gasifier. Similar chemistry
can also be applied to the gasification of coke derived from petroleum and other sources.