Title: Traditional and Nontraditional Uses of Coal Combustion Products Prepared for Canadian Industries Rec
1Traditional and Non-traditional Uses of Coal
Combustion ProductsPrepared for Canadian
Industries Recycling Coal Ash (CIRCA)by
- Theodore W. Bremner Michael D.A. Thomas
- Department of Civil Engineering,
- University of New Brunswick
21.Carbon Rich Products Future Energy Needs
- During the Carboniferous Era some 300 million
years ago lush tropical plants grew and then
geological changes crushed and compressed the
plants. With time they were converted to carbon
in its many forms which are mainly coal, oil and
natural gas. Continental drift then distributed
this storehouse of solar energy near to where our
various modern civilizations developed.
3Carbon Rich Products and Future Energy Needs
- Reserves of coal are estimated to be 8 X 1012
tonnes with annual consumption of 5 X 109 tonnes.
It is unlikely that we will run out of easily
accessible coal deposits in this century.
Because coal is such a low cost source of energy
it is likely to remain the most effective source
for producing electricity and for refining metals.
42. Coal Combustion Products their Nature and
Products
- Unlike other sources of energy coal is consumed
in large facilities where the combustion products
in part can be collected, processed and marketed
to add another revenue stream thereby reducing
the industrys negative impact on the
environment. The various coal combustion
products will be discussed next.
52 a. Fly Ash
- Fly ash is the finely divided residue that
results from both the combustion of pulverized
coal and the activation of the silt deposited
between the layers of coal. This head activated
silica rich material forms the main beneficial
element when fly ash is added to Portland cement
concrete. Also it is the main material in the
waste recyclable stream both in terms of mass
and monetary value. The fly ash is transported
from the combustion chamber by exhaust gas and
remains in suspension until it is recovered by
electrostatic precipitation, or by other methods.
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72 acontinued
- In concrete fly ash acts as a pozzolan in that it
is a siliceous or a siliceous and aluminous
material that, in the presence of water, will
combine with an activator (lime, Portland cement
or kiln dust) to produce a cementitous material.
ASTM C618 Class F Fly Ash has a content of
SiO2
Al2O3 Fe2O3 ? 70
Class C Fly Ash has SiO2 Al2O3 Fe2O3 ?
50
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102 b. Bottom Ash
- When the pulverized coal (70 lt 75 mm) is
injected into the furnace with preheated air,
rapid combustion occurs and approximately 80 of
the residual rises as fly ash and is carried out
by the combustion gases. The remaining
approximately 20 drops to the bottom of the
furnace and is called bottom ash. The latter is
mainly highly abrasive sand sized particles that
normally are sluiced away by a high pressure
water stream.
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122 c. Fluid Gas Desulfurization (FGD) Products
- Coal contains sulfur, and sulfur dioxide (SO2) is
generated during combustion and if allowed to
escape up the chimney it reacts with water to
form sulfuric acid H2SO4, the prime component in
acid rain. The most effective method of
collecting this gas is to inject a lime solution
into the exiting exhaust gas (after the fly ash
has been recovered) to form calcium sulfate
(CaSO4) which finds a ready market in wallboard.
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142 d. Ash Derived from the Burning of Products
from Oil Refining (PET coke)
152 e. Resource Recovery from Ash, eg. Gallium
- Most coals contain trace amounts of valuable
minerals and some coals that are rich in a
particular mineral generate an enriched
combustion product that might warrant recovery.
Recovery usually involves conventional mining
technology such as foam flotation or high
temperature gravity separation.
163. Sustainable Development and the Environment
- Coal can be one of the main driver of our
economic development in the future provided we
accept our responsibilities in both the short and
long term. This requires providing technological
solutions to problems that could compromise the
ability of future generations to meet their
needs. The conditions and influences of the
place in which an organism lives constitutes the
environment, and although change is inevitable,
it is the responsibility of engineers to see that
the effects of these changes are as benign as
possible.
173continued
- As will be discussed later most wastes from
burning coal can be used profitability for
environmentally safe applications but there are
problems to be solved. The CO2 generation during
the burning of coal is certain to affect climate
change and we must find ways to cope with this
either by reducing energy consumption associated
with CO2 emission or with the sequestering of the
CO2 in some safe medium. Also, geological
deposits of oil, gas and coal will be depleted at
some time in the next millennium and other
sources of energy must be identified.
184. Use in Portland Cement Concrete
- Calcium rich cementitious binders served the
Greeks and Romans well where only strength was
required but when durability was of concern they
found that silica in finely divided form was
essential. Their source of silica was ground
volcanic ash and its effectiveness can be seen in
the harbor at Cosa on the west coast of Italy
which was built about 27 BC with silica from
Mount Vesuvius.
194continued
- To insure durability stone masons have always
preferred silica rich granite to calcium rich
limestone for harbor walls. Thus it was only
natural for Joseph Aspdin, when he patented
Portland Cement in 1824 to require a silica rich
clay as part of the raw feed even though it would
require a high manufacturing temperature. With
the introduction of the rotary kiln, cement
production increased rapidly and because of its
uniformity was well accepted by builders.
204continued
- The increased pace of the construction industry
in the past five decades coupled with the need to
reduce costs caused cement producers to grind the
cement more finely to achieve a higher early
strength. They also reduced the silica and
increased the more soluble calcium to speed up
the cement hydration rate. Unfortunately these
changes were at the expense of long term strength
gain and also caused a reduction in the
durability of concrete in most aggressive
environments.
214continued
- Fortunately these concessions to modern demands
on the cement industry can be alleviated by using
silica rich finely divided fly ash from electric
generating plants. Because silica rich fly ash
can be added after rather than before the kiln,
the energy demand to operate the kiln is not
increased, a major cost saving as most of the
energy to produce cement is required to turn and
fire the kiln.
224 a. Effect on Fresh Properties
234 a i. Increased Setting Time
- Many factors influence the setting time of
concrete with concrete temperature and the
chemical composition of the cement being the most
important. Increasing fly ash content usually
increases both initial and final set at room
temperature from about 5 min. to 20 min. for each
10 increase of cement replacement with fly ash.
At higher temperature the effect is less and at
lower temperature of the concrete setting times
can be greatly delayed and should be determined
for the materials used.
244 a ii. Reduced Water Content
- Most fly ash particles are solid spheres with
some small amount being hollow cenospheres. The
spheres act as pseudo ball bearings that increase
both the consistency (slump) and workability of
the concrete, their effectiveness increasing in
direct proportion to their surface area.
Normally concrete mixtures with fly ash will
require less water per cubic metre for a given
slump than a mixture without fly ash.
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264 a iii. Increased Workability
- For mixtures with and without fly ash that have
the same cementitious content, the one with fly
ash will generally be noticeably more workable
for a given consistency. Consequently, the
coarse aggregate content usually can be increased
when fly ash is used to replace cement.
274 a iv. Reduced Heat of Hydration
- The exothermic reaction between cement and water
can be reduced by replacing some of the cement
with fly ash. Generally the early-age heat
generation of a cement fly ash mixture is 30
less than that of an equivalent mass of portland
cement. When high volume fly ash concrete is
used (56 fly ash with a superplasticizers) a
substantial reduction in maximum temperature
results frequently enabling large sections to be
cast without exceeding a maximum temperature of
40C. As with all concrete attention must be
paid to the cooling rate so that temperature
differentials between the surface and interior do
not exceed 20 C if surface cracking is to be
avoided.
284 b. Effect on Hardened Properties
294 b i. Increased Long Term Strength
- Although concrete mixtures containing fly ash
tend to gain strength at a slower rate than
concrete without fly ash the long term strength
(90 days and after) is usually higher. The
enhanced workability and increased consistency
achieved when fly ash is used in high cemenitious
mixtures allows a very significant reduction in
water content. This water reduction combined
with a superplasticizer enables very high
strength concrete mixtures to be produced typical
of what is used for high-rise buildings.
304 b ii. Increase Volume Stability
- Concrete achieves its volume stability mainly
from the large quantity of fine and coarse
aggregate used. These aggregate have a modifying
or restraining effect on the volume changes that
cement paste undergoes. Hydrating cement paste
shrinks upon loss of water and tends to crack
under the influence of an applied load.
Fortunately, the much stiffer aggregate minimizes
this volume change. Volume change in concrete
containing fly ash is slightly less than that in
concrete without fly ash when the concretes are
compared at equal strengths.
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324 b iii. Reduced Permeability
- During the hydration process, large spaces
between the cement grains are filled with
hydration products and with time these hydration
products become more dense. With time the
pozzolanic effect of fly ash in the concrete
mixture forms even more impermeable hydration
products. Also the use of a pozzolan tends to
reduce the formation of a weak boundary layer
between the cement paste matrix and the normally
impermeable aggregates.
334 b iiicontinued
- Although initially more permeable than a concrete
made without fly ash, a concrete with 25 fly ash
after several years can have a coefficient of
permeability at least one order of magnitude less
than a concrete without fly ash. This leads to
enhanced durability as aggressive agents cannot
attack the concrete from within but are
restricted to the concrete surface.
344 c. Production
355. Agricultural and Other Uses
365 a. High Calcium Ash to Counter Acidification
- Fly ash is highly alkaline and can be be spread
on grassland to reduce the effect of acid rain.
However some fly ash are deficient in important
elements necessary for plant growth and have an
excess of others. Before using fly ash as a
top-dressing for agricultural purposes it is
important to check the fly ash composition as
well as the soil under consideration to make sure
that they are compatible.
375 b. Sulfur for Acid Tolerant Plants
- Calcium sulfate from Fluid Gas Desulferization
(FGD) can be used to good advantage on crops such
as potatoes.
38Highway Use (Fly Ash and Bottom Ash)
- Depending on haulage costs, fly ash and bottom
ash can be used for highway embankments provided
optimum moisture is maintained and adequate
compaction is provided. The source can be from
an operating power plant or reclaimed ash can be
from a lagoon or stockpiled ash. The product
from an operating plant can normally be delivered
with close limitations on moisture content.
Moisture content from a lagoon or from a
stockpile can vary considerably depending on its
location within the lagoon or stockpile.
396 a. As A Low Density Road Base
- Class F ash usually is hauled to the construction
site in covered dump trucks with the water
content adjusted by spraying water on the loading
conveyor belt or in the case of lagoon ash by
blending the ash with drier silo ash. The ash is
spread in lifts of 150 to 300 mm thick. The lift
is then compacted to obtain the required in-place
density. Depending on the ash quality, leachate
is normally not a problem with compacted ash that
is appropriately graded to impede infiltration
when a proper seeded soil cover is used to
control erosion.
406 a.continued
- Compacted fly ash has mechanical behavior and
compaction characteristics similar to silt. It
also can share some of the difficulties of silt
such as being susceptible to frost heave as a
result of ice lenses where it can wick water from
a shallow groundwater table. Fly ash should not
be used as a fill without an impermeable fill to
prevent infiltration, below the groundwater table
or where a drainage layer is not used at the
bottom of the fill to prevent water wicking.
416 b. To Enhance Soil Properties When Mixed With
In-Place Materials
- Stabilized road bases can be produced by mixing
fly ash and a calcium rich material with
aggregates. The calcium rich material can be
Portland cement, lime or kiln dust. Class C fly
ash frequently has sufficient calcium to serve
both functions. Then it need only be mixed with
the aggregates and water followed by compaction
to achieve optimum density that will generate the
required in-place strength.
426 bcontinued
- A typical mixture will contain 8 to 14 fly ash
and 3 to 8 lime by weight. Type I Portland
cement can be used to accelerate early strength
gain. The aggregates should have a gradation
such that a dense easily compacted mixture
results that has a low permeability and adequate
strength. The mixture can be mixed in-place
using part or all of the original material.
437. Aggregates Derived from Coal Combustion
Products
447 a. Sintered Products
- The manufacture of sintered fly ash aggregates
involves two main operations. The first is to
pelletize the fly ash by spraying a water-fly ash
slurry onto an inclined rotating pan (pan
pellitization) and then to sinter the pellets on
a traveling grate that travels through an
ignition chamber where the 5 to 8 coal in the
fly ash is ignited. In the burning process a
temperature of from 1050 to 1250C is reached.
These aggregates can be used to produce
structural concrete and a plant has operated
successfully since 1958 in the UK but as of this
date no North American plant uses this technique.
457 b. Hydrothermally Treated Fly Ash
- The manufacture of hydrothermal fly ash
aggregates typically involves using 45 quartz
sand, 47 fly ash, 4.5 lime, 2.0 additives and
1.5 water by weight. All of the mixture is
pelletized and then heated in a high humidity
environment and are treated for 6.5 hours at 200
C to produce lightweight aggregate that finds
its primary use in masonry units. A plant in the
Eastern US and several plants in Europe are in
operation, but production is limited.
467 c. Cold Bonded Fly Ash Aggregates
- Pelletized or extruded aggregates made with fly
ash, Portland cement and water are cured for
several days at 70-80 C and at high humidity to
produce an aggregate that has been used for
making masonry units. Unfortunately, both the
matrix and the aggregate experience drying
shrinkage and as a result large volume change is
to be expected.
477 d. Bottom Ash Aggregate
- In a modern pulverized coal boiler 80 of the ash
exits through the stack as fly ash with the
remaining 20 of the coal ash falling to the
bottom of the furnace. This material is known as
bottom ash. It is sand sized particles that can
be used to replace some or all of the coarser
fractions of the fine aggregate for a concrete
mixture.
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4910. Beneficiation of Coal Combustion Products
- The electric utilities operating coal fired
plants have recently installed low NOX burners to
meet new air emissions requirements.
Unfortunately, these burners operate at a low
temperature and produce fly ash with a high
carbon content. In the past most plants could
easily meet the ASTM requirement of 6 maximum
loss on ignition for their fly ash but now plants
are producing fly ash that exceeds this limit.
This renders the fly ash unsuitable for use in
concrete as high ash content makes it difficult
to entrain air in the concrete. The entrained air
is required for freeze thaw resistance. The
various methods that can be used to lower the
carbon content will be discussed next.
5010 a. To Reduce Carbon Content
5110 a i. Microwave Burnout
- Microwave burnout utilizes a microwave heating
system to modulate the burning process within a
fluidized bed reactor to burn off the excess
carbon and at the same time recover heat the form
of hot water. A pilot plant producing 50 tonnes
per day was commissioned in 1997 and since then a
full scale 300 tonnes per day design has been
prepared. Pilot runs have demonstrated that fly
ash can be processed to have essentially 0
carbon content.
5210 a ii. Fluidized Bed Burnout
- In 1997 the first, full size carbon burn out
plant in the world was constructed. A 1.2m deep
fluidized bed of fly ash is contained in a
refractory lined vessel. Below bed burners are
used during cold startups and once the bed
reaches the residual carbon auto-ignition
temperature of approximately 510 C it will run
on its own. The temperature is modulated by
recycling the cool processed low carbon fly ash
back into the fluidized bed when the incoming fly
ash has so much carbon that the operating
temperature exceeds 510 C. Excess energy in the
form of hot water is recovered from the process.
5310 a iii. Electrostatic Separation
- In 1997 the first commercial plant employing
electrostatic separation was used to reduce the
carbon content of ash from about 8 to 2.00.3
at a separation rate of 35 tonnes per hour.
Employing the principle of triboelectric charging
and electrostatic separation, the fly ash
receives a negative charge and the carbon
receives a positive charge. The charged
particles are then fed to two large counter
moving horizontal open plastic mesh belt
conveyors. These conveyors are sandwiched
between a top negative and a bottom positive
electrode. The fly ash now with a reduced carbon
content is scraped from the bottom electrodes and
a carbon rich fraction is scraped from the top
electrodes.
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5610 a iv. Electrostatic and Streaming
- A laboratory scale separation system was develop
in 1997 that involved charging carbon and fly ash
to opposite polarity by particle to particle or
by particle to surface contact. By manipulating
the polarity and magnitude of the charge, the
carbon and ash was separated by passing them
through an external electric field. The ash
recovered had a carbon content less than 5 by
weight. Unlike electrostatic separation the
separation occurs in a vertical plane as the raw
fuel falls down between two vertical charged
plates
5710 a v. Air Separation
- Fly ash can be separated from the carbon by using
air separation that combines gravitational,
inertial, centrifugal and aerodynamic forces.
The feed material (high carbon content fly ash)
and primary air enters the top of the classifier
in a downward direction and makes a 120 C change
in direction and exits through vanes carrying the
light carbon with it. The heavy fly ash being
too heavy to make the turn, falls to the bottom.
5810 a vi. Carbon Flotation
- The flotation wet process consists of raw feed
silos, wet conditioning systems, processing and
secondary flotation cells, dewatering equipment,
drying equipment and finished product silos.
Normally a frothing agent is used to coat the
unburnt carbon forming hydrophobic carbon
materials. Air is introduced into the bottom of
the cells and the air carries the hydrophobic
unburnt carbon to the top where it is skimmed
off, dried and can be introduced into the furnace
as fuel. The remaining fly ash is dewatered and
dried ready for shipment to a concrete plant.
5910 b. To Reduce Ammonia
- In the quest to reduce NOx emissions the fly ash
and bottom ash is treated so that they adsorbs
ammonia. During the mixing and placing of the
concrete, ammonia gas is released that can
seriously affect the workforce. Several
beneficiation methods to reduce the level of
ammonia in the ash as well as the current methods
available or under development have been found
lacking in terms of either performance or cost.
The approaches tried include high temperature
treatment as well as ammonia fixation. The
latter involves the ammonia reacting with a
chemical to prevent its ultimate volatization.
6010 c. To Reduce Mercury
- The trace element concentrations in many fly
ashes are similar to those found in naturally
occurring soils. However in unusual cases there
can be restrictions as in the case of mercury.
Unburnt carbon in the fly ash acts as a scavenger
for mercury and when beneficiation of fly ash
takes place the recovered carbon, instead of
being reintroduced into the furnace is land
filled in a secure manner to prevent leaching.
Slides 11 to 16 at a later date.