Page 146 - 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

256 Fuel Chemistry

some handling problems. Volatile matter is the material velocity of the fuel and air mixture jet to keep the flame
driven off when coal is heated to 950 C in the absence of at a distance from the burner tip. However, coals with
◦
air under specified conditions, and is determined practi- lower volatile matter have a delay in ignition and there-
cally by measuring the loss of weight. It consists of a mix- fore cause flame stability problems. These coals require
ture of gases, low-boiling-point organic compounds that additional design measures such as high swirl or a “bluff
condense into oils upon cooling, and tars. Volatile matter body” to promote recirculation of hot gases to stabilize
increases with decreasing rank. In general, coals with high the flame. Combustibility of coal is also characterized by
volatile matter are known to ignite easily and are highly bench-scale laboratory techniques such as drop tube reac-
reactive in combustion applications. The calorific value is tor, entrained flow reactor, or thermogravimetric analyzer
the amount of chemical energy stored in the coal, which to determine the reactivity of fuels. One such method is
is released as thermal energy upon combustion and is di- used to determine a burning profile. This is widely used
rectly related to the rank. The calorific value determines to determine the combustion behavior of an unknown fuel
in part the value of a coal as a fuel for combustion appli- relative to a reference fuel. This method originally was
cations. The ultimate analysis includes elemental analysis developed by Babcock and Wilcox. A burning profile is
for carbon, hydrogen, nitrogen, and sulfur and oxygen on a plot of the rate at which a solid fuel sample loses its
a dry ash free basis. Oxygen content in the fuel is obtained weight as a function of temperature, when heated at con-
by subtracting the sum of the percentages of C, H, N, and stant rate in air. The burning profile is used to determine
S from 100. the temperatures at which the onset of devolatilization and
The grindability of a coal is a measure of its resistance char combustion, peak reation rate, and the completion of
to crushing. The ball-mill and Hardgrove grindability tests char oxidation occur. The temperatures range over which
are the two commonly used methods for assessing grind- these take place also indicates the heat release rates and
ability. The Hardgrove method is often preferred to the zones.
ball-mill test because the former is faster to conduct. The For a better understanding of behavior of coal in
test consists of grinding a specially prepared coal sample a furnace, pilot-scale combustion tests are performed
in a laboratory mill of standard design. The percentage by to characterize ignition and flame stability, combustion
weight of the coal that passes through a 200-mesh sieve efficiency, gaseous and particulate emissions, slagging
(screen with openings of three-thousandths of an inch or and fouling, and the erosion and corrosion aspects of a
74 µm) is used to calculate the Hardgrove Grindability fuel.
Index (HGI). Two factors affecting the HGI are the mois-
ture and ash contents of the coal. A correlation between
the HGI and rank indicates that, in general, lignites and III. COMBUSTION OF COAL
anthracite coals are more resistant to grinding (low in-
dices) than are bituminous coals. The index is used as a Combustion is rapid oxidation of fuels generating heat.
guideline for sizing grinding equipment in the coal pro- Combustion of fuels is a complex process the understand-
cessing industry. ing of which involves chemistry (structural features of the
Inorganic constituents in coal upon oxidation produce fuels), thermodynamics (feasibility and energetics of the
ash. The inorganic constituents are present in the form of reactions), mass transfer (diffusion of fuel and oxidant
discrete mineral particles or bonded to the organic coal molecules for a reaction to take place, and products away
structure or sometimes water associated. The manner in from the surface), reaction kinetics (rates of reactions),
which the inorganic constituents are present in raw coals and the fluid dynamics (bulk movement of the fuel and
determines the intermediate and final form of ash. Lower- combustion gases) of the process.
rank coals (lignites and subbituminous) tend to have more Combustion of solid fuels generally involves three
organically associated inorganics compared to high-rank steps—drying or evaporation of moisture, thermal de-
coals. composition and devolatilization, and oxidation of solid
Combustibility is measured by proximate analysis and residue or char. Since solid fuels differ widely in terms
some empirical laboratory tests. Volatile matter is an in- of physical and chemical properties, a general qualitative
dicator of ease of ignition and the fraction of coal that is discussion on the combustion behavior of a coal parti-
burnt in gas phase. The higher the volatile matter the less is cle is given here. The duration and chemistry of each of
left as char that needs to be burnt. The combustion of char these processes depend on the type of fuel burnt and the
takes a longer time because of the heterogeneous reaction size of the particles, heating rate, furnace temperature, and
between carbon and oxygen. Combustion of high volatile particle density. For example, wood, peat, and lignite fu-
coals therefore is easier to ignite and releases most of the els contain a large percentage of moisture and the drying
heat closer to the burner. This requires an increase in the time is quite long. Also the volatile matter is quite high