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2. Influence of Coal Properties
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2.2 Combustion
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In the burner region coal particles are rapidly heated to approximately 1300°C in about 0.1 of a
second and devolatilise. The heat transferred to this region (derived from the hot product gases, by
radiation from the surroundings, and from the energy generated in this early stage of combustion)
plays a very important role in stabilising the flame, especially in low NOXswirl burners.Char burnout takes place in the furnace chamber. Combustion efficiency is determined by the time-
temperature history (i.e. furnace design), the char particle size, and char reactivity. The char
particle size is determined mainly by the fineness of grind of the pulverised fuel. Char reactivity is
dependent on the rank and the maceral composition of the original coal, but may be enhanced by
some minerals. The heat content of the volatile matter is also dependent on the rank, type and
mineral matter content of the coal.
Char burnout for blends, made from coals of similar rank, can be estimated from the burnout
characteristics of the component coals. Burnout can be influenced by the blend composition when
- there is a large difference in the rank of the coals which can result in two completing
phenomena
- Adverse Impact - the lower rank coal can react with the available oxygen faster and
therefore impede the burnout of the higher rank coal. This tends to occur when the rank
of the lower rank coal is sub-bituminous or lower and it is the main coal in the blend.
- Beneficial impact - the fast release of the volatiles from the lower rank coal provides a
hotter thermal environment, without significantly reducing the oxygen, this will aid in the
combustion of the higher rank coal. This tends to occur when the lower rank coal is
bituminous of good combustion performance blended with a higher rank coal of poorer
combustion performance or when the lower rank coal is sub- bituminous but is a minor
component in the blend.
- there is a large difference in the milling performance of the coals resulting in preferential milling
of the softer coal which is normally a higher rank coal of marginal combustion performance.
- Coal qualities impact significantly on operating costs of coal-fired installations. Generally coals
with low total moisture, ash and sulphur, high specific energy and good combustion performance
can minimise total plant costs. The table below summarises some of the more significant coal
properties used in the evaluation of thermal coals.
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Coal Property
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Standard
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Influence on combustion
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Sizing
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Coal having more than 30% of minus 2mm fine particles can cause handling
problems with the frequency increasing as the fines percentage increases.
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Total moisture
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AS 1038.1
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High moisture content increases transportation costs per unit of energy and may
increase handling problems, depending on the clay content of the coal. High
moisture coals require increased energy for drying in the mill.
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Proximate
analysis
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AS 1038.3
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The ratio of fixed carbon to volatile matter (fuel ratio)indicates the ease of ignition
and burnout, but the heat content of the volatile matter is a more reliable guide to
ignition. The volatile matter content influences NOX formation. Generally for the
same burner and constant nitrogen content, the higher the volatile matter the lower
the NOX. Low values of ash are generally sought; however, in stoker firing a
minimum ash level of 5% or more is necessary to protect the grate from
overheating. |
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Ultimate analysis
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AS 1038.6
Parts 1, 2, 3
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This analysis is required for calculating stoichiometric air requirements and the
volume and composition of the products of combustion, with the exception of NOX
which also depends on combustion conditions.
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Forms of sulphur
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AS 1038.11
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These figures give the distribution of the total sulphur between organic, inorganic
and sulphate. Total sulphur can be used to estimate SOX emissions, though some
SO2 is absorbed by calcium in the ash. |
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Specific energy
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AS 1038.5
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The heat derived from the combustion of coal is of prime importance and can be
reduced by high levels of ash and moisture, or poor utilisation efficiency. The net
specific energy is obtained by subtracting the latent heat of water in the
combustion products from the gross specific energy.
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Ash fusion
temperatures
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AS 1038.15
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Low ash fusion temperatures may lead to slagging (deposits within the furnace
chamber) or to fouling (deposits in the convective passes of a boiler). |
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Ash analysis
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AS 1038.14
Parts 1, 2, 3
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The composition of the ash of a coal influences the slagging and fouling behaviour
and also the performance of the fly ash collection plant. Use to calculate
relationship between ash analysis and fouling and slagging indices. |
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