Page 155 - 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

Fuel Chemistry 265

accomplished in the furnace by splitting the air stream for combustible gases such as synthetic gas mixture (CO and
combustion. A part of the combustion air is introduced H 2 ) or by methanation reaction conversion to synthetic
downstream as over-fire air (OFA), intermediate air (IA), natural gas (SNG). This is usually done by thermal de-
or over-burner air (OBA). The principle of fuel staging or composition, partial combustion with air or oxygen, and
reburning involves reduction of the NO x already formed reaction with steam or hydrogen or carbon dioxide. Coal
in the flame zone by reducing it back to nitrogen during contains impurities such as sulfur, nitrogen, and minerals,
combustion.Thisisusuallyaccomplishedbyinjectingfuel which generate pollutants when burned. Most of these im-
into a second substoichiometric combustion zone in order puritiesandpollutantgasescanberemovedduringthepro-
to let the hydrocarbon radicals from the secondary fuel re- cess of converting the coal into synthetic fuels. Therefore,
duce the NO x produced in the primary zone. To complete gasification of coal is desirable from an environmental
the combustion process of the reburn fuel, additional air viewpoint and to overcome the transportation constraints
is introduced downstream. However, the main drawback associated with solid fuels. While the goal of combustion
of this system is the short residence time available for the is to produce the maximum amount of heat possible by
reburn fuel for complete oxidation. Therefore, this method oxidizing all of the combustible material, the goal of gasi-
uses mostly natural gas or any other highly reactive fuel fication is to convert most of the combustible solids into
so that combustion can be completed within the residence combustible gases (such as CO, H 2 , and CH 4 ) with the
available. desired composition and heating value.
In SNCR, chemicals are injected into the boiler, which
then react with NO x and reduce it to N 2 .
A. Gasification Reactions
Most commonly used chemicals are ammonia or urea.
Other chemicals used in research work include amines, During gasification, coal initially undergoes devolatiliza-
amides and amine salts, and cyanuric acid. Good mixing tion (thermal decomposition) and the residual char under-
is essential for the success of this process and the optimum goes some or all of the reactions listed here.
temperature window at which the reactions take place is
solid–gas
900–1100 C. This happens to be in a region where the
◦
heat transfer surfaces are present. Achieving good mixing 2C + O 2 = 2CO (partial combustion)
is difficult.
C + O 2 = CO 2 (combustion)
When the limits of NO x cannot be met by combustion
control or by SNCR, SCR methods are used. NO x concen- C + CO 2 = 2CO (Boudward reaction)
tration in the flue gas is reduced by injection of ammonia
C + H 2 O = CO + H 2 (water gas)
in the presence of a catalyst. The use of a catalyst reduces
the optimum temperature window. The reaction products C + 2H 2 = CH 4 (Hydrogasification)
are water and nitrogen and this reaction is accomplished at
gas–gas
much lower temperatures than SNCR (between 300 and
◦
400 C). At lower temperatures, the unreacted ammonia CO + H 2 O = CO 2 + H 2 (Shift)
can react with sulfur trioxide in the presence of water
CO + 3H 2 = CH 4 + H 2 O
to form ammonium bisulfate (NH 4 HSO 4 ), a sticky com-
pound, which can cause corrosion, fouling, and blocking For the thermodynamic and kinetic considerations, coal
of downstream equipment. char is normally considered to be 100% carbon. The heats
This temperature is usually high enough to prevent con- of reactions of the combustion or partial combustion reac-
densation of (NH 4 HSO 4 ). This system can reduce NO x tions are exothermic (and fast), whereas most other gasi-
emissions by about 90–95%. fication reactions (boudward, water gas, and hydrogasi-
Most of the electric utilities are installing SCR systems fication) are endothermic and relatively slower. Usually,
to meet the current and future NO x emission regulations. the heat requirement for the endothermic gasification re-
actions is met by partial combustion of some of the coal.
It is also important to note that gasification reactions are
VIII. COAL GASIFICATION sensitive to the temperature and pressure in the system.
Equilibrium considerations for the reversible gasification
Gaseous fuels are easy to handle and use. They produce reactions show that high temperature and low pressure are
less by-products. As naturally occurring gaseous and liq- suitable for the formation of most of the gasification prod-
uid fuel resources deplete, synthetic gases and liquids pro- ucts, except for methane. Methane formation is favored
duced from coal can act as suitable replacements. Coal at low temperature and high pressure. High temperatures
gasification is a process of conversion of solid coal into tend to reduce methane, carbon dioxide, and water vapor