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266 Fuel Chemistry

in the products. Coal char is considered to be a mixture TABLE IV Effect of Gasification Medium on Products and
of pure carbon and some inorganic impurities and struc- Calorific Value
tural defects. Certain impurities and structural defects are Gasification Calorific value of the
3
known to be catalytic; the absolute reaction rate depends medium Products products (MJ/m )
on the amount and nature of the impurities and structural Air and steam N 2 , CO, H 2 and CO 2 5.6–11.2
defects and also on physical characteristics such as surface Oxygen and steam CO, H 2 11.2–14.5
area and pore structure. The physical structure controls the Hydrogen CH 4 35–38
accessibility of gasification medium to the interior sur-
face. These physical structural characteristics depend on
the feed coal and the devolatilization conditions (heating the heat of combustion. Table V summarizes some impor-
rate and peak temperature). tant gasification processes, conditions, and product gas
There has been considerable interest in gasifying coals compositions.
with several catalysts in order to minimize secondary re-
actions and produce the desired product gas distribution.
C. Major Gasification Processes
However, the major problem with using catalysts is poi-
soning by various hydrocarbons and other impurities in The most important fixed-bed gasifier available commer-
coal. A major research effort in this area has been di- cially is the Lurgi Gasifier. It is a dry-bottom, fixed-bed
rected toward the gasification of carbon using alkali metals system usually operated between 30 and 35 atmospheres
compounds. Potassium chloride and carbonate have been pressure. Since it is a pressurized system, coarse-sized
found to increase the amount of carbon gasified to CO and coal (25–37 mm) is fed into the gasifier through a lock
H 2 . The same catalysts are found to reduce methane yield. hopper from the top. The steam–oxygen mixture (gasify-
ing medium) is introduced through the grate located in the
bottom of the gasifier. The coal charge and the gasifying
B. Classification of Gasification Processes
medium move in opposite directions (counter-currently).
Gasification processes are primarily classified according The gasifier is operated at about 980 C and the oxygen re-
◦
to the operating temperature, pressure, reactant gas, and acts with coal to form carbon dioxide, thereby producing
the mode of contact between the reactant gases and the heat to sustain the endothermic steam–carbon and carbon
coal/char. The operating temperature of a gasifier, usually dioxide–carbon reactions. The raw product gas consist-
dictates the nature of the ash removal system. Operat- ing mainly of carbon monoxide, hydrogen, and methane
◦
ing temperatures below 1000 C allow dry ash removal, leaves the gasifier for further cleanup. Besides participat-
◦
whereas temperatures between 1000 and 1200 C cause ing in the gasification reactions, steam prevents high tem-
the ash to partially melt, become “sticky,” and form ag- peratures at the bottom of the gasifier so as not to sinter or
◦
glomerates. Temperatures above 1200 C result in melt- melt the ash. Therefore, this gasification system is most
ing of the ash and it is removed mostly in the form of suitable for highly reactive coals. Hot ash is periodically
liquid slag. Gasifiers may operate at either atmospheric removed through a lock hopper at the bottom. Large com-
or elevated pressure. Both temperature and pressure af- mercial gasifiers measuring about 4 m in diameter and
fect the composition of the final product gases. Gasifica- 6.3–8.0 m in height are capable of gasifying about 50 tons
tion processes use one or a combination of three reactant of coal per hour. Improved versions of the Lurgi Gasifier
gases: oxygen, steam, and hydrogen. The heat required have been developed but not yet commercialized.
for the endothermic reactions (heat absorbing) is supplied The Winkler gasifier is a fluidized-bed gasification sys-
by combustion reactions between the coal and oxygen. tem, which operates at atmospheric pressure. In this gasi-
Table IV illustrates the effect of gasification medium on fier, crushed coal is fed using a screw feeder and is
the product species and the calorific value. Methods of fluidized by the gasifying medium (steam–air or steam–
contacting the solid feed and the gaseous reactants in a oxygen mixture depending on the desired calorific value
gasifier are of four main types: fixed bed, fluidized bed, of the product gas) entering through a grate at the bottom.
entrained flow, and molten bath. The operating principle The coal charge and the gasification medium move cocur-
of fixed bed, fluidized bed, and entrained flow systems rently (in the same direction). In addition to the main gasi-
is similar to that discussed for combustion systems (see fication reactions taking place in the bed, some may also
previous section). The molten bath approach is similar to take place in the freeboard above the bed. The temperature
the fluidized bed concept in that reactions take place in of the bed is usually maintained at 980 C (1800 F) and the
◦
◦
a molten medium (either slag or salt) with high thermal productgasconsistsprimarilyofcarbonmonoxideandhy-
inertia and the medium both disperses the coal and acts as drogen. The low operating temperature and pressure limit
a heat sink, with high heat transfer rates, for distributing the throughput of the gasifier. Because of the low operating