MINERAL MATTER IN COAL AND WOOD REPLICATIONS FOR SOLID FUELED GAS TURBINES
by
Kenneth W. Ragland
University of Wisconsin, Madison, WI
and
Andrew J. Baker
U.S. Forest Products Laboratory, Madison, WI
INTRODUCTION
Advanced industrial and utility power systems typically use
direct fired gas turbine engines.
Using coal or wood to directly
power a gas turbine has yet to be accomplished commercially, primarily
because the ash causes erosion of the blades and deposition on the
blades.
If the combustion products contain a significant fraction of
molten ash particles, deposition on the turbine blades occurs which
blocks the flow path and degrades performance.
If the ash particles
are solid, erosion of the blades occurs which also degrades performance. In addition, mineral matter can cause corrosion of the
blades.
The size distribution, concentration and composition of the
ash, as well as the turbine design, determine the lifetime of the
turbine blades (1,2).
Ash particulates are formed from mineral matter due to three
different mechanisms. Part of the mineral matter (in coal but not
wood) is found in layers or bands separate from the organic matter.
This is called adventitious mineral matter, and it can be partially
separated from the coal after crushing and fine grinding.
The
adventitious mineral matter is transformed directly to ash in the
combustor, and the shape is semi-rounded. Secondly, mineral matter in
coal and wood is contained within the organic matrix in the form of
chemically bound molecules and submicron crystals. Grinding does not
liberate this intrinsic mineral matter.
During char combustion the
intrinsic mineral matter forms ash nodules on the char surface which
then coalesce to form particle sizes in the 1 to 10 micron range.
Thirdly, some of the mineral matter is vaporized during combustion and
condenses on cooler surfaces such as turbine blades.
The ash loading and size distribution depend on the type of solid
fuel, the extent of grinding and fuel cleaning, and the combustion
time-temperature history.
Regardless of the type of combustion
system, much of the ash is in the 1 to 20 micron size range.
Particles down to about 10 microns can be efficiently removed by hot
Particles in the 1 to 10 micron range can be
cyclone collectors.
removed by other advanced methods, however this tends to cause
additional pressure drop and heat loss.
Currently there are no
commercially available fine particle control devices which can operate
at high temperature for long time periods.
MINERAL MATTER IN COAL
Mineral matter includes all elements in coal except C, H, O, N
and S. Mineral matter varies widely, and is present as discrete bands
In: Combustion fundamentals and applications:
1987 spring technical meeting of the central
117 states section of The Combustion Institute;
1987 May 11-12; Argonne, IL. Argonne, IL:
Argonne National Laboratory; 1987: 117-122.
and crystals, and as elements bound to the organic matrix. The major
constituents (say greater than 0.5% of the coal) are of primary
interest with regard to deposition and erosion.
Mineral matter
quartz,
and calcite.
primarily includes clays, shales, pyrite,
Sometimes overburden rock material mixes in with coal during mining,
but this can be removed by cleaning techniques.
According to Harvey and Ruth (3), over 125 different minerals
have been reported in coal. Frequently occurring minerals are listed
Minerals occur as discrete grains, flakes or aggregates
in Table 1.
in one of five physical modes:
(1) as microscopically disseminated inclusions within macerals
(distinct organic designations such as vitrinite, liptinite
and inertinite),
(2) as layers of partings wherein fine-grained clay minerals
usually predominate,
(3) as nodules including lenticular and spherical concretions,
(4) as fissures including cleat and other fracture or void
fillings, and
(5) as rock fragments found within the coal bed as a result of
faulting, slumping or related disturbances.
Mineral matter is considered to have been formed by three
syngenesis, or
different
mechansims detrital
deposition,
epigenesis. Detrital grains were introduced into a coal forming basin
(such as swamp land) by rivers, tidal waves and wind. Syngenetic
minerals were formed during the peat stage of coal formation and
include minerals formed by crystallization of inorganic elements in
plants (intrinsic mineral matter).
Epigenitic minerals are those
found as filling of fissures and voids after the peat was formed.
Palmer and Filby (4) determined the size distribution of major
minerals in Powhatan, Ohio coal. pyrite was concentrated in the 5-20
Clays were in the 0.2-2 and 2-5 micron range
and >20 micron size.
Straszheim et al (5)
while Quartz was in the 0.2-2.0 micron range.
recently reported data on mineral analysis versus mineral particle
For the
size for Illinois No. 6 coal and Pittsburgh No. 8 coal.
Illinois coal (Table 2) more than 90% of the mineral matter consisted
of pyrite, kaolinite, illite or quartz whichwere more or less
Removal of mineral
uniformity distributed among the particle sizes.
matter by sink-float at 1.3 sg ranged from 75% for less than 4 microns
For
to 100% for particles greater than 36 microns (Table 3).
Pittsburgh coal the pyrite was more coarse, while the other minerals
were more fine-sized, and the fine grained minerals were relatively
For Illinois coal the quartz dropped from
untouched by cleaning.
2.51% of the dry coal to 0.63% by cleaning, whereas for the Pittsburgh
coal quartz decreased from 0.39% to 0.34%.
ASH FROM COAL
In the reducing and oxidizing environment of a combustion
chamber, mineral matter undergoes a variety of transformations (6).
Clays are transformed to aluminosilicates and mullite (Al 6 S i2 O 1 3 ),
calcite goes to calcuim oxide and quartz may remain unchanged.
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Mixtures of Al2O3-SiO 2-FeO-CaO-K 2O occur which are partially molten at
900 c. Illite is the first phase to be partially converted to molten
form (6). Representative analysis of ash from bituminous and lignite
coal is shown in Table 4. Of course, Table 4 does not represent the
actual molecular composition.
Adventitious mineral matter is transformed directly to ash in the
combustion zone. Depending on the temperature-time history, the ash
particles
w i l l b e spherical or semi-rounded (7).
The size
distribution of this ash depends on the size distribution of the
adventitious mineral matter. Intrinsic mineral matter forms tiny ash
nodules in the pores of the char, and as char burnout proceeds, the
ash nodules coalesce on the surface of the char.
Frequently,
The size
cenospheres (hollow glass-like spheres) are formed.
distribution of this ash depends on the temperature-time history in
the combustor.
In PFBC tests three hot cyclones yield an ash size
distribution of 98% less than 10 microns and 80% less than 4 microns
(2).
MINERAL MATTER IN WOOD
The composition of mineral matter in wood depend somewhat on the
soil conditions under which the tree grew, and the location of the
A number of mineral constituents are
sample within the tree.
necessary for plant growth. These and other minerals are transported
The minerals are comprised mainly of
from the soil thru the roots.
salts of calcium, potassium and magnesuim, with other salts in lesser
amounts.
The acid radicals are carbonates, phosphates, silicates,
sulfates and oxalates.
In some species, sub micron, crystals of
calcium oxalate (CaC2O 4) have been observed (8).
For bark, in
addition to the minerals transported from the soil, there are wind
blown minerals and minerals picked up during harvesting.
Relatively
little mineral matter is extractable from wood with water.
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Wood is a desirable fuel because it has low sulfur content
compared to coal.
In spite of the high oxygen and moisture content
compared to coal, wood has an adequate heating value.
The heating
value of dry wood is roughly 60% that of dry coal. The ash content of
wood grown in the temperate zones is 0.1-1.0%, whereas wood grown in
the tropical and subtropical zones contains up to 5% ash (9).
The
higher ash is mainly due to crystals of silica in the wood structure
(which tend to dull saw blades). The ash content of bark is typically
3 - 8 % . Bark tends to be available more for combustion than wood at
many sites because it remains after the wood is utilized.
A whole
tree contains 15 to 20% bark. On a equivalent heat input basis, bark
generates nearly as much ash as some coals.
ASH FROM WOOD
Table 4 presents representative data on the mineral content of
wood and bark ash. Wood ash often contains 40-70% calcium oxide, 1030% potassium oxide, and 5-10% magnesium oxide, as well as oxides of
sodium, iron, silicon and, phosphorous. Wood and bark have significant
levels of sodium and potassium which tend to promote fouling because
they volatilize and recondense on particles and surfaces making them
Bark has 10-20 times the ash content of wood with the
sticky.
greatest increase due to calcium oxide. Hardwood species tend to have
more potassium than softwood. Wood and bark as it is used for fuel
often somewhat higher ash content due to extraneous mineral matter
inadvertently picked up during handling.
Regarding the size distribution of wood ash, the State of Oregon
has found that for wood and bark burning stoker spreader boilers, 70%
of the particulate emissions are less than 10 microns in size (13).
Experiments on our novel gravel bed combustor for a gas turbine which
uses wood chips and operates in a downdraft mode with high excess air
have shown that 90% of the particulate are less than 10 microns and
70% less than 5 microns.
CONCLUSIONS
Wood is a good turbine fuel because the mineral matter is
disseminated in sizes less than 1 micron. Coals with low amounts of
adventitious ash, and little pyrite (which tends to be distributed in
the larger sizes) and quartz (which is particularly abrasive) should
Combustion zone temperatures
be designated for use in gas turbines.
should not greatly exceed turbine inlet temperatures to minimize
agglomeration of ash particles.
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