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Abrasion Index  Indicates the abrasiveness of coal by monitoring the loss in weight of four metal blades which mechanically stir a sample of sized coal. The index is the number of milligrams of metal abraded from the metal blades per kg of coal used. Based on a test known as the Yancey, Geer and Price test. Now an ISO standard.

AIR- DRIED  Equivalent to as analysed basis when the analyses have been performed on an air-dried sample. For calculation to different bases.

AIR-DRIED MOISTURE  The moisture in the coal sample after achieving equilibrium with the laboratory atmosphere by exposure to it. Sometimes called residual moisture. No equivalent term in ASTM, although air-dried moisture can refer to a sample dried in an oven with warm air.

ALKALI METALS  Important elements in coal are sodium and potassium. They are undesirable in thermal coals because they can give rise to fouling and slagging problems in boilers, although small additions can improve electrostatic precipitation performance. They are undesirable in coking coals because they tend to increase coke reactivity in the blast furnace (see Coke reactivity)

Anthracite  Anthracite is the highest rank of coal and is characterized by low volatile matter (always less than 10%) and high carbon content. It has a semi-metallic luster and is capable of burning without smoke. Semi-anthracite is coal midway between low volatile bituminous coal and anthracite.

As analysed basis  Analytical data expressed as the moisture content at which the sample was analysed. For calculation to different bases.

As received basis  Analytical data calculated to the as received moisture content. For calculation to different bases.

Ash  The inorganic residue after the incineration of coal  to constant weight under standard conditions. It is less than the mineral matter because of the chemical changes occurring during incineration, with the most important differences being loss of water of hydration, loss of carbon dioxide, and loss of sulphurous gases form sulphides.

Ash analysis  Ash is composed of very complex oxides and the ash analysis expresses this composition in terms of its component oxides.  Generally the ash consists mostly of silica (SiOz) and alumina (Al2O3). The presence of large amounts of the oxides of iron (Fe2O3), calcium (CaO), sodium (Na2O) and /or potassium (K2O) generally indicate an ash with low ash fusion temperatures. The ash analysis differs from the composition of the minerals in the parent coal.

Ash fusion properties  The fusion properties of laboratory prepared coal ash which are demonstrated by the heating of the ash in a mildly reducing or oxidizing atmosphere. The temperature range generally used is 900oC up to 1600oC, and the temperatures which can be recorded are initial deformation temperature, softening temperature, hemisphere temperature and fluid (flow) temperature.
Almost invariably, temperatures recorded under reducing atmosphere are lower or equal to those recorded under oxidizing  atmosphere. Of the characteristic temperatures, the initial deformation and flow are generally the most difficult to reproduce.

Ash viscosity  Ash viscosity is a very difficult and expensive measurement to make. Generally, consideration of it takes the form of estimation of the temperature at which the ash viscosity is 250 poise, i.e. T250P, which can be obtained  by calculation from the ash analysis. A viscosity of 250 poise is approximately the maximum acceptable value for tapping slag from cyclone furnaces.

Audibert-Arnu dilatometer  A caking test most commonly used in Europe which measures the expanding and contracting characteristics of coal. Finely crushed coal is compressed into a pencil, which is heated slowly and as the coal passes through its plastic range, initially gets shorter (contracts) and then gets longer (expands).
Measurement taken are the maximum contraction and maximum dilatation (expansion) both expressed as a percentage of the initial pencil length, such that the maximum contraction is always positive, and the maximum dilatation is positive when the pencil increases in length from its initial length, and negative when the pencil decreases in length. Temperatures of initial softening (first indication of the pencil contraction), maximum contraction and maximum dilatation are also recorded. Results from this test are very sensitive to oxidation of the coal being tested.
The Ruhr dilatometer is similar to the Audibert-Arnu dilatometer, the major difference being in the compaction of the coal pencil. Ruhr dilatometers show less contraction and more dilatation than Audibert-Arnu dilatometers.

Basis Equivalent to as analysed basis when the analyses have been performed on an air-dried sample. For calculation to different bases.

Brasion index Indicates the abrasiveness of coal by monitoring the loss in weight of four metal blades which mechanically stir a sample of sized coal. The index is the number of milligrams of metal abraded from the metal blades per kg of coal used. Based on a test known as the Yancey, Geer and Price test. Now an ISO standard.

Bed moisture  The moisture content of coal as it lies in the seam prior to mining. The moisture holding capacity is often said to be equivalent to the bed moisture. Equivalent to inherent moisture in ASTM. Also sometimes referred to as the insitu moisture.

Beneficiation The upgrading of coal (or other mineral) including crushing, sizing and drying but mainly referring to the reduction of ash forming minerals in coal. Reduction is achieved by a combination of liberation.(e.g. crushing) and cleaning techniques. Most cleaning techniques take advantage of the  density difference between coal and mineral. Also known as coal preparation.

Bituminous coal Bituminous coal is that coal which in rank is between sub-bituminous coal and semi-anthracite. Volatile matter on d.a.f basis ranges from between 10% and 14% to 40% and over. Usually divided into three sub groups - low volatile, medium volatile, and high volatile.

Bottom ash That fraction of the ash content of coal, no matter what the combustion process, which remains within the combustion chamber and eventually goes to ash collection facilities below the combustion level.

Brown coal See Lignite.

Carbon burnout The measure of combustion efficiency based on the completeness of burnout of the carbon in coal, a function of coal combustion reactivity and combustion conditions. Combustibles in fly ash represent a penalty in fly ash collection, disposal and a loss of useful energy not recovered in the furnace. See Unburnt carbon.

Carbonization The carbonisation of coal is carried out commercially for the production of coke for the iron and steel industry blast furnaces. The process involves heating coking coal in the absence of air to a temperature of around 1100�C. This is carried out in a coke oven and the coking time is around 14-20 hours. The gases and liquids evolved during the process are used on site for their energy content or sold as plant products. In a process very little used today, town gas can also be prepared by carbonisation, with the resulting coke being used as a domestic fuel.

Chlorine An element in coal which if present in large enough quantities can cause corrosion and fouling problems in boilers. Less than 0.2% chlorine in coal is considered low, while over 0.5% is considered high. Sodium is often associated with chlorine, and can itself cause slagging and fouling problems during combustion.

Coal preparation Coal preparation covers the physical and mechanical processes which can be applied to coals to make them suitable for the market. Mostly, this is applied to run-of-mine coals to obtain lower ash and more consistent coals suitable for steam raising or coke making. The yield of such a treatment process is the ratio of the mass of product to the mass of starting material. The organic efficiency of the the ratio of the mass of the organic material in the product to the mass of the organic material in the starting material. The organic material is usually defined as the total material, less moisture and mineral matter.

Cogeneration The generation of electrical energy as part of some other process such as the supply of low pressure steam to a chemical plant or the recovery of waste heat and gases from a blast furnace.

Coke The solid, spongy residue from the carbonisation of coal.

Coke reactivity A laboratory test designed to simulate the loss of coke through reaction in the reducing atmosphere, as the coke makes its way down the blast furnace. Coke is heated up to 950�C in an inert atmosphere and held at that temperature in an atmosphere of carbon dioxide. The coke is cooled down under the inert atmosphere and the loss in weight expressed as a percent is the reactivity.

Coke strength after reaction A laboratory test designed to give an indication of the strength of coke after being exposed to the reducing atmosphere of the blast furnace. Coke, after exposure to the high temperature and carbon dioxide atmosphere of the coke reactivity test, is tested in a tumbler device to determine its strength.

Coking coal Coal which, because of its characteristics, is suitable for carbonising to produce blast furnace coke. Important properties required technically to produce good coke are good coking and caking properties (measured as fluidity, dilatation, crucible swelling number, etc.) allied with appropriate rank (indicated by reflectance values around 0.9-1.5%). Other properties which are important for commercial reasons are ash and moisture (which should be as low as possible) and, because of their deleterious effects on pig iron, sulphur and phosphorus.

Combined cycle (power generation) A two-stage electrical generation process. In the first stage, electricity is generated by a gas turbine. The waste heat from this process then passes through a heat recovery boiler which produces steam for additional power generation in a conventional steam turbine. This results in an increase in overall power generation efficiency.

Comminution The science of particle size reduction in crushers and grinding mills. Includes the study of stress, deformation, fracture, energy consumption, size classification, etc. in solids subject to size reduction in mills.

Composition balance index An index calculated from macera l analysis and vitrinoid type reflectance data of coal, which in very broad terms is related to the strength of the intergranular union in the coke resulting from that coal. Used in prediction of the ASTM stability factor.

Crossing point temperature See spontaneous combustion.

Crucible swelling number One of the most common, simple caking tests. Finely crushed coal is heated rapidly in a crucible and the coke button obtained is compared with a series of standard profiles to give a number, which is the crucible swelling number (CSN). Values range from 0 (no caking characteristics and therefore no coking properties at all), to 9 then 9+ (superior coking properties). The results obtained in the test can be affected by oxidation, size distribution and moisture of the sample and care should be taken in interpretation of any results. In the US this number is known as free swelling index (FSI).

Drum tests Used for examining the strength properties of coke. Sized coke is placed in a drum which has lifters on the inside. As the drum is rotated for a set time or number of revolutions, the pieces of coke are lifted and then dropped to the bottom of the drum. The coke is then sized again, and the various drum indices are obtained from the amount of oversize and undersize material.

Dry ash-free basis Analytical data calculated to a condition of zero moisture and ash (i.e. first approximation to 'pure coal') to allow comparison of different coals. This is strictly a hypothetical basis because the ash is only generated on the incineration of the coal, but is used frequently because of convenience. Dry mineral matter free basis is more precise, but less easy to obtain.

Dry basis  Analytical data calculated to a condition of zero moisture.

Dry bottom boiler A boiler from which ash is removed conventionally in solid form. This requires a coal with an initial deformation ash fusion minimum temperature of 1200-12500C.

Dry mineral matter free basis Analytical data calculated to a condition of zero moisture and mineral matter and a better approximation of 'pure coal'. Used for comparing different coals and estimating coal rank. Where mineral matter has not been determined or is difficult to determine, it can be estimated by calculation.

Efficiency The overall energy efficiency of using coal in a plant for power generation is the quotient of the energy in the electricity sent out (e.g. in MWh) divided by the gross specific energy in the coal feed stocks (e.g. in GJ). Efficiency losses are due to limitations of the steam cycle, e.g. latent heat is not recovered and electricity is consumed within the power station.

Electrostatic precipitation A commonly used technique for collecting fine particles, especially fly ash entrained in the flue gas downstream of a coal-fired furnace. The surface of the particles receive an electrical charge due to the potential difference set up between an emitting electrode and the collecting plates in the precipitator. This surface charge causes the particles to migrate to the collecting plate with the clean gas passing through to the exhaust stack.

Energy A measure of the amount of electricity used over a period of time. Units used are gigawatt-hours (GWh), megawatt�hours (MWh) or kilowatt-hours (kWh) depending on the power and time scale involved. The kWh is the familiar unit used to measure customer electricity consumption.
1 GWh = 1000 MWh
1 MWh = 1000 kWh

Equilibrium moisture See Moisture holding capacity.

Excess air Is the extra volume or mass of air which is supplied to the furnace with the coal, over and above the minimum which is theoretically required to combust all of the coal. This minimum, known as the stoichiometric air requirement, may be calculated from the oxygen required to combust all of the organic material in the coal.

Exinite The exinite group comprising sporinite, cutinite and resinite is the most reactive maceral group in coal and contains more hydrogen than does vitrinite. It is derived from plant and cell secretions, cuticles and outer membranes of spores and pollen grains, and is normally the minor maceral component of coal.

Fabric filters An alternative to electrostatic precipitation for the collection of fly ash whereby the flue gas is forced to pass through the interstices in fabric bags set up in an array sometimes referred to as a bag house. The fly ash particles build up as a layer on the surface of the fabric when they are occasionally shaken to dislodge and collect the fly ash.

Fixed carbon A component of the proximate analysis calculated by difference, i.e. 100% less the sum of moisture, ash and volatile matter. Intended to give an indication of char yield. Not to be confused with (elemental) carbon in the ultimate analysis.

Flame stability A measure of the combustion stability of a flame. Not an unequivocal measure but rather a relative measure usually between one coal and another in a common combustion environment. Flame stability is particularly important when the coal feed rate is 'turned down', e.g. to follow reductions in power station load.

Flue gas The gaseous effluent from a furnace comprising the products of combustion, nitrogen and other unconverted species from furnace air supply, and moisture. Raw flue gas also contains fly ash and acid gases (SOx and NOx).

Fluidized bed A bubbling (or circulating) bed of particles, e.g. coal particles during combustion, whereby the particles are suspended (or entrained) in an upward flowing stream of air.

Fluidity See Gieseler plastometer.

Fly ash That fraction of the ash content of a coal, no matter .

Forms of sulphur 
Sulphur in coal is considered to be present in three forms - pyritic sulphur, sulphate sulphur and organic sulphur. Where the sulphur is pyritic or sulphatic it is part of the mineral matter and its content can often be lowered by washing the coal. Organic sulphur is distributed through the carbonaceous part of the coal and cannot be washed out by conventional techniques.
Analytical methods are well known for pyritic and sulphate sulphur, but organic sulphur is determined by difference from the total sulphur value.

The presence of sulphate sulphur often indicates the coal has experienced some oxidation during its life. Pyritic sulphur is often blamed for spontaneous combustion problems.

Fossil fuel Fossilised remains of earlier plant life (coal, oil and gas) and animal life (oil and gas) preserved in geological sediments.

Fouling A problem in coal combustion in boilers where ash adheres to surfaces inside the combustion chamber and flue gas outlets causing efficiency losses and reducing heat transfer. See also Fouling index.

Fouling index A factor which can be calculated from the ash analysis and which gives an indication of the propensity for the coal to cause fouling problems during combustion.

Free moisture The difference between total moisture and air-dried moisture. No equivalent in ASTM.

Free swelling index See Crucible swelling number.

Forth flotation Froth flotation is a technique for cleaning fine coal. Unlike most coal preparation techniques which rely on density differences between coal and mineral matter, flotation is based on the hydrophobic nature of coal, ct. the hydrophilic nature of minerals.

Fuel ratio A term primarily used in Japan and equal to fixed carbon divided by volatile matter.

Gieseler plastometer A coking test commonly used in Japan and the U.S.A. which measures the fluidity characteristics of coal. Fine coal (not pulverised) is heated slowly and as it melts and passes through its plastic range, its fluidity is measured. Results are expressed as maximum fluidity in dial divisions per minute (ddpm). Characteristic temperatures recorded are initial softening temperature, maximum fluidity temperature and re-solidification temperature. The plastic range, which is the temperature range during which the coal is in plastic state, is also important. All coking and caking tests are sensitive to oxidation but the Gieseler test is by far the most sensitive.
There are two types of Gieseler plastometers, automatic and manual. The manual equipment can give results which are significantly different to those obtained from automatic equipment.

Gray-King coke type A simple coking test. Finely crushed coal is heated slowly in a glass retort and the shape, texture and appearance of the coke residue is compared to standards to give a letter, which is the Gray-King coke type. Values range from A (no coking characteristics at all) to G, then G1 to G9 (superior coking properties).

Greenhouse effect Refers to the warming effect of radiative gases in the earth's troposphere which allow ultraviolet solar energy to pass to the earth's surface but then trap the re-radiated infra-red energy. The 'Greenhouse' effect keeps the earth suitably warm to sustain life as we know it, but more recently the term has been associated with predictions of rapid warming, above historical levels, due to increasing CO2, NOx, CH4 and CFCs in the troposphere.

Gross calorific value  The amount of heat liberated during laboratory testing when coal is com busted under standardised conditions, with the temperature of starting materials and products being approximately 25�C. During actual combustion in boilers, the gross value is never achieved because some of the products, most importantly water, are lost in the gaseous state with their associated heat of vaporisation.

The maximum achievable calorific value under these conditions is the net calorific value and shows how to calculate this. Calorific value (CV) is also known as specific energy and the gross and net CV are known as the higher and lower heating value in some countries.

Hardgrove grindability index Indicates the relative grindability or ease of pulverisation of a coal in comparison to coals chosen as standards. High values indicate a coal easy to pulverise and low values indicate a hard coal.

Hazardous waste A waste regarded as deleterious to human beings, the physical environment, fauna or flora. The precise definition of the term 'hazardous' is dependent on procedures adopted in individual countries - no specific, comprehensive definition can be given.

Heat rate A measure of the units of fuel energy employed to produce a unit of electrical output, e.g. KJ/kWh.

Higher heating value See Gross calorific value.

Ignition temperature  See Spontaneous combustion.

In situ reserves The quantity of coal in the ground within geological and economic limits. This can include both mineable and unmineable reserves, for which the term resources is sometimes used.

Inertinite The inertinite maceral group is generally considered nonreactive. Micrinite, macrinite, fusinite, sclerotine and inertodetrinite are the most common unreactive members of the inertinite group. Semifusinite is partially reactive, but the extent is probably dependent on rank.

Inerts Refers to those macerals which are not reactive, i.e. the inertinite group with the exception of a portion of the semifusinite.

Inherent moisture Often used to indicate air-dried moisture, particularly in Australia. In ASTM, equivalent to bed moisture or equilibrium moisture.

Lignite  Lignite is a low rank coal containing high moisture. Generally a coal is considered to be a lignite if it contains greater than 20% bed moisture (although classification schemes for lignites are generally based on calorific value). Other characteristics of lignite are low reflectance, high volatile matter and high oxygen and low carbon levels and often the presence of some woody structure. In general, the term is synonymous with brown coal.

Lower heating value See Gross calorific value.

Maceral analysis  Obtained by the microscopic examination of coal and is a volumetric distribution of macerals in a coal sample. Fine coal (not pulverised) is set into a small block of epoxy-type material and one face is polished. A number of different points on the polished face (usually at least 500) are examined by microscope and the maceral species observed are recorded. The maceral analysis is usually presented as percentages.

Maceral analysis is important because vitrinite and exinite are more reactive than the other species in coal in both combustion and coke making.

Macerals Macerals are the microscopically recognisable individual organic constituents of coal. They are recognised on the basis of their reflectance and morphology. A given maceral may differ significantly in composition and properties from one coal to another. For some macerals the variation depends mainly on the rank of the coal. There are three maceral groups - exinite, inertinite and vitrinite.

Maximum contraction See Audibert-Arnu dilatometer.

Maximum dilatation  See Audibert-Arnu dilatometer.

Maximum fluidity  See Gieseler plastometer.

Mineable (extractable) reserves The quantity of coal capable of being mined from in situ reserves within environmental and legal limits. This is only realistic when calculated for an area of proved or measured reserves.

Mineral matter  Mineral matter comprises the inorganic components of coal. When coal is burnt the composition of the mineral matter is altered by evolution of the water of hydration of the minerals, (this is not determined during the total moisture determination) and other components such as carbon dioxide and sulphur oxides. The end product, the ash, thus has a different composition to the original mineral matter.

The mineral matter content of coal is difficult to determine but well known empirical equations are usually employed to estimate it.

Moisture holding capacity The moisture in the coal after achieving equilibrium with an atmosphere of 96% relative humidity and 30�C. Called equilibrium moisture in ASTM.

Natural gas Fuel gas, principally methane with some ethane and traces of other hydrocarbons, obtained from natural underground reservoirs. Often associated with petroleum reservoirs but recently also extracted commercially from coal seams.

Net calorific value See "Gross calorific value.

Nitrogen Part of the organic material in coal. Under certain combustion conditions a portion will be emitted to the atmosphere as oxides of nitrogen unless removed from flue gas.

Peat Peat is the first stage in the conversion of vegetable matter to coal. Bed moistures are high, often greater than 75%, and plant remains are clearly visible.

Petrography The scientific description of the composition.of rocks. Coal petrography classifies coal according to macerals deriving from the organic matter, and minerals.

Phosphorus Should be avoided in coking coal because it accumulates in the hot metal and gives undesirable properties to the resultant steel. Can also create problems during coal combustion by the formation of hard phosphatic deposits Iinside boilers.

Pilot coke ovens Small coke ovens used for test purposes. Two commonly used sizes are 7 kg and 250 kg. The smaller oven is useful for comparative work, whilst the large oven is of sufficient size to allow the scaling up of results to match commercial ovens.

Plastic range See Gieseler plastometer.

Possible (inferred) reserves These are undefined reserves. The terms 'possible' or 'inferred' describe reserves in areas without outcrop, borehole or mining data. Estimates are based on assumed continuity of adjacent proved or probable reserves.

Probable (indicated) reserves This describes poorly defined reserves. Minimum distances between boreholes or other data points are usually in the range 1 000 m to 2 000 m.

Proved (measured) reserves This describes well defined reserves. Minimum distances between boreholes vary by country, but are normally in the range 500 m to 1000 m. An accuracy of quantity and quality values better than +20% is expected.

Proximate analysis The empirical determination of the moisture content, volatile matter, ash and, by difference, fixed carbon of a coal sample determined on an air-dried basis.

Pulverized coal injection Refers to the injection of coal as a fuel, i.e. to provide heat, into the tuyeres of a blast furnace for iron making. Pulverised coal injection supplements, but cannot completely replace, coke in the blast furnace.

Pyritic sulphur See Forms of sulphur.

Rank Coals vary in composition and properties in accordance with the extent of alteration, or degree of coalification, of the original plant material from which they are derived. The concept of rank is used to indicate the stage of alteration obtained by a particular coal; the greater the alteration, the higher the rank of the coal. Thus, lignites and sub-bituminous coals are of low-rank, while semi anthracites and anthracites are of high rank.

Reactive Reactives refers to the reactive macerals, i.e. those which burn readily during combustion and those which become plastic during carbonisation in the coke oven. Includes vitrinite and exinite maceral groups and a portion of the semifusinite from the inertinite group.

Reflectance  Obtained by the microscopic examination of macerals. See Maceral analysis. Oil immersion of macerals at the polished face allows the measurement of reflectance using incident light. Vitrinite is by far the most common maceral for which reflectance is measured, although measurements are made for some inertinite species. Vitrinite is bi-reflective which means that with a polariser, the reflectance varies as the microscope stage containing the block and vitrinite particle is rotated in the place perpendicular to the incident light.

Under these conditions, two maxima are observed at 1800 to each other. The 'mean maximum reflectance' (Ro max) is thus the mean of a number of measurements of reflectance of separate vitrinite particles (usually at least 50) where the stage is rotated to give maximum readings. Vitrinite reflectance increases with coal rank, and is a commonly used and a very significant parameter for assessing coals for coke-making.

Reflectogram  See Vitrinite reflectance histogram.

Repeatability Repeatability (within laboratory) of an analytical procedure is the maximum acceptable difference between duplicate determinations carried out at different times in the same laboratory, on the same analysis sample, by the same operator using the same apparatus. This is sometimes called 'within laboratory tolerance'.

Reproducibility Reproducibility (between laboratories) for an analytical procedure is the maximum acceptable difference between the mean of acceptable replicate determinations carried out in two respective laboratories, on representative samples, sometimes called 'between laboratory tolerance'.

Reserves The quantity of coal calculated to lie within given boundaries. Reserves are specified within limits of seam thickness, depth, quality, geological conditions and contemporary economic factors. The accuracy of a reserve estimate is implied by adjectives such as proved (measured), probable (indicated), and possible (inferred).

Residual moisture See Air-dried moisture.

Roga index A coking test whereby finely crushed coal is mixed with anthracite and heated to produce a coke button, which is then submitted to a standardised drum test. The Roga index is then calculated by formula from the amount of minus 1 mm material obtained from the drum test.

Roga index A coking test whereby finely crushed coal is mixed with anthracite and heated to produce a coke button, which is then submitted to a standardised drum test. The Roga index is then calculated by formula from the amount of minus 1 mm material obtained from the drum test.

Ruhr dilatometer See Audibert-Arnu dilatometer.

Run-of-mine reserves (ROM) The quantity of coal potentially available to be delivered to a coal preparation facility or stockpile after mining. This is sometimes used synonymously with mineable reserves but will normally show an increase caused by extra unwanted material inevitably taken in mining and added surface moisture.

Saleable reserves The quantity of coal available for sale after extraction by mining and after the removal of low quality coal and unwanted material by coal preparation.

Size range Indicates the largest and smallest size of lumps in a coal sample or stream. The size range of a sample containing sizes from 50 mm down to zero can be written as 50xO mm, -50+0 mm, Ox50 mm or -50 mm. Some national standards allow for such terms as: nominal upper size (or top size) which is the smallest sieve size such that not more than 5% of material is larger than 40 mm. Imperial units are also used and the ASTM allows use of a numerical numbering system for sieves giving a size range such as 2 in x No.4.

Slagging A problem in coal combustion whereby the ash melts and forms a slag, often making ash removal from the furnace difficult. An initial deformation ash fusion temperature of 1 250oC is generally considered the minimum acceptable to avoid slagging problems. Siagging boilers are designed such that ash removal is by molten slag from the, bottom of the furnace and thus require low ash fusion coals to operate correctly.

Slagging factor A factor which can be calculated from the ash analysis and which gives an indication of the propensity of a coal to cause slagging problems during combustion.

Specific energy See Gross calorific value.

Spontaneous combustion The generation of heat in bulk coal, leading to fire if uncontrolled, occurring as a result of coal and/or iron sulphide oxidation. Lower rank coals in general are more susceptible to spontaneous combustion. Laboratory tests and procedures can give a good indication of the propensity to self heat, but the only sure way is to conduct tests on large stockpiles of several thousand tonnes. A common laboratory test is determining the crossing point temperature or ignition temperature, in which coal is heated in an oxygen atmosphere.

Steam cycle The cycle most commonly used to generate electric power from coal involving a boiler raising steam which expands through a generating turbine. Based on the Rankine Cycle.

Strength index An index calculated from maceral analysis and vitrinoid-type reflectance data which is related to the strength of individual granular structures in the eventual coke produced from the coal as analysed. Used in the prediction of ASTM stability factor.

Sub-bituminous coal Sub-bituminous coal is the next highest order in rank after lignite. Typical bed moisture levels are 10-20%.

Sulphate sulphur See Forms of sulphur.

Sulphur Can be part of the carbonaceous material in coal or part of the minerals as sulphates, or sulphides. Sulphur dioxide forms during coal combustion. Most countries have regulations regarding SOx emissions to the atmosphere. Undesirable in coking coal because it partly accumulates in the hot metal affecting the eventual microstructure. Requires desulphurisation if above about 0.02% (in hot metal).

Surface moisture The difference between total moisture and moisture-holding capacity. Called free moisture in ASTM.

Total moisture The moisture in the coal as sampled and removable under standard conditions.

Trace elements The inorganic constituents in coal which occur only in trace amounts, less than about 0.1 % of the coal ash.

Trace metals Also often called heavy metals. Both terms are not strictly correct but generally refer to those elements in coal which are present in very small quantities, and are considered harmful to the environment. Examples are mercury, arsenic, selenium, fluorine, cadmium.

Ultimate analysis The elemental composition of the organic part of the coal, i.e. carbon, hydrogen, oxygen, nitrogen and sulphur.

Ultimate analysis The analysis of a coal expressed in terms of carbon, hydrogen, nitrogen, sulphur and oxygen. The analysis refers to the carbonaceous material only and hence is often expressed on a dry, ash-free basis or dry, mineral matter-free basis. Oxygen is estimated by difference although there are direct methods for its determination.

Unburnt carbon or carbon loss The amount of organic material remaining in the fly ash following collection in the ash hoppers. Normally, levels of up to 5% are tolerated, but ideally 2% is targeted. Low ash coals generally result in higher levels of unburnt carbon due to low residence times in the furnace and high combustion rates. See Carbon burnout.

Vitrinite Vitrinite is the most common and most important maceral group in bituminous coals. Vitrinite is generally subdivided into vitrinite A, which has the normal reflectance value for the coal, and vitrinite B which is of slightly lower reflectance. Vitrinite is considered a reactive maceral, burning readily during combustion and becoming fluid during carbonisation. See Reflectance.

Vitrinite reflectance histogram A histogram presenting frequency of values versus reflectance values obtained during the reflectance determination. The histogram is probably the best means of assessing whether a coal is a blend or not because a single seam coal has a reflectance histogram which is symmetrical and covers generally only 2-3 vitrinoid types. A blend, on the other hand, is likely to cover a greater number of vitrinoid types, and present a non-symmetrical distribution. Also called a reflectogram.

Vitrinoid type These are individual reflectance measurement groupings covering a 0.1 % range, e.g. vitrinoid type V6 includes all individual reflectance measurements from 0.60-0.69% inclusive,
V7 = 0.70-0.79%, V8 = 0.80-0.89%,
V12 = 1.20-1.29% etc.
A vitrinite reflectance histogram (also called a reflectogram) can be drawn by plotting the number of individual measurements in each vitrinoid type versus reflectance.

Washability The relationship between coal ash and relative density of a coal fraction, determined using laboratory float and sink techniques.

Waste An end-product with no present value, managed by disposal (or storage).

Wet bottom boiler A boiler from which ash removal is by molten slag from the bottom of the combustion chamber. A suitable coal should thus have an ash fusion flow temperature maximum of 1 300oC.

Yancey, Geer and Price (YGP) test See Abrasion index
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