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Title: Traditional and Nontraditional Uses of Coal Combustion Products Prepared for Canadian Industries Rec


1
Traditional 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

2
1.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.

3
Carbon 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.

4
2. 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.

5
2 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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2 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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2 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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2 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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2 d. Ash Derived from the Burning of Products
from Oil Refining (PET coke)
15
2 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.

16
3. 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.

17
3continued
  • 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.

18
4. 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.

19
4continued
  • 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.

20
4continued
  • 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.

21
4continued
  • 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.

22
4 a. Effect on Fresh Properties
23
4 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.

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4 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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4 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.

27
4 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.

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4 b. Effect on Hardened Properties
29
4 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.

30
4 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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4 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.

33
4 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.

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4 c. Production
35
5. Agricultural and Other Uses
36
5 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.

37
5 b. Sulfur for Acid Tolerant Plants
  • Calcium sulfate from Fluid Gas Desulferization
    (FGD) can be used to good advantage on crops such
    as potatoes.

38
Highway 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.

39
6 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.

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6 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.

41
6 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.

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6 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.

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7. Aggregates Derived from Coal Combustion
Products

44
7 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.

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7 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.

46
7 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.

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7 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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10. 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.

50
10 a. To Reduce Carbon Content
51
10 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.

52
10 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.

53
10 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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10 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

57
10 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.

58
10 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.

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10 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.

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10 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.
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