Solid Fuels
2.1 Fossil Fuels
2.1.2 Composition and Properties of Solid Fuels
Coal is a mixture of organic material and mineral matter. The organic matter is responsible for the energy content of the fuel, while it is the mineral matter that presents significant challenges in the design and operation of a power plant. Sev- eral types of analysis are performed to evaluate the coal properties that affect the design and operation of power plant components and systems. These analyses are the determination of the heating value, the proximate analysis, the ultimate analysis, the mineral analysis of the ash, the determination of the ash fusion temperature, the analysis of the grindability and the determination of the swelling index. In addi- tion, other physical characteristics of the coal may be determined, such as the bulk density and the particle size distribution. The methods for performing the various tests on fossil fuels have been developed by various standards organisations such
2.1 Fossil Fuels 17
Table2.1Compositionofhardandbrowncoals(Effenberger2000)andAlstomPower CoaltypeDryash-freematterbasisRawcoal OriginSiteofdepositVolatiles (%)C (%)H (%)O (%)N (%)S (%)Higherheatingvalue HHV(MJ/kg)Ash(%)Water(%)Lowerheatingvalue: LHV(MJ/kg) Peat IRLDerrygreenagh69.658.05.634.91.20.323.861.555.07.7–7.9 GRPhilippi68.557.55.433.52.80.823.020–2240–457.3–7.9 Softbrowncoal DRhineland55.068.35.027.50.50.526.385–2050–626.3–9.6 DHelmstedt59.472.65.816.70.44.429.7512–2242–469.2–10.5 DSchwandorf55.063.65.026.11.34.025.336–2050–586.3–7.5 DLausitz55.067.55.225.51.00.825.372–555–608.2–8.5 DLeipzig63.071.66.119.50.72.128.355–750–559.0–11 DHalle-Bitterfeld57.572.05.518.30.83.429.815–752–569.6–10.0 GRPtolemais57.065.35.326.51.60.525.256–2252–603.6–6.7 GRMegalopolis62.060.56.230.61.31.424.4513–1760–642.8–4.0 AUSYallourn51.567.54.826.70.70.325.541–263–725.0–7.5 PL,DPatnow,Lusatia58.473.65.119.70.51.128.566–1552–588.0–8.8 HGy¨ongy¨os/Visonta63.063.84.826.81.13.524.8315–3046–545.0–6.7 TRElbistan67.061.45.129.60.85.123.698–2448–623.3–6.2 Hardbrowncoal DPeissenberg52.074.05.514.51.44.629.2312–208–1219.7–23.0 AFohnsdorf47.072.55.416.31.24.630.358–168–1420.0–22.6 AK¨oflach56.067.75.725.01.20.327.216–1030–3513.0–14.7 SLOTrbovlje53.072.55.617.21.23.528.4730–3520–2410.0–11.7 HRRaˇsa50.475.25.46.91.111.534.126–202–427.6–30.1 CZMost(Br¨ux)48.077.55.814.61.01.232.325–1515–2518.8–22.2 CZFalknov54.573.56.017.91.11.530.94–1425–3515.1–18.4 HTatabanya52.073.05.817.70.92.631.46–1212–1423.0–24.3 TRTuncbilek44.576.45.813.82.51.532.1914.2214–2415.0–18.1
18 2 Solid Fuels
Table2.1(continued) CoaltypeDryash-freematterbasisRawcoal OriginSiteofdepositVolatiles (%)C (%)H (%)O (%)N (%)S (%)Higherheatingvalue HHV(MJ/kg)Ash(%)Water(%)Lowerheatingvalue: LHV(MJ/kg) High-volatilebituminouscoal DRuhrBasin,Aachen33.785.95.56.21.60.833.96–97–1027.6–28.5 DSaarBasin38.282.75.29.41.21.533.05–83–528.0–28.9 GBScotland41.581.45.410.32.10.833.94.613.826.3 Medium-volatilecoal DRuhrBasin33.785.95.56.21.60.835.26–78–1027.6–28.0 CZOstrava-Karvin´a38.083.45.09.51.40.733.7124.527.1 PLUpperSilesia33.284.55.27.21.61.232.48–134–1026.2–27.0 GBYorkshire34.484.35.28.01.70.834.86.8231.7 AUSQueensland35.984.75.47.71.70.534.26.8529.0 Fatcoal DRuhrBasin24.488.75.04.11.66.736.26–97–1028.5–29.3 DSaarBasin32.586.95.25.41.31.135.67–98–1028.0–28.4 USAPennsylvania24.683.25.13.41.41.936.38331.2 ZA28.282.54.59.91.71.334.215825.4 Forgecoal DRuhrBasin12.490.74.12.11.71.436.26–97–1028.5–29.3 DAachen13.889.84.82.81.50.535.86–98–1028.5–29.3 LeanCoal DRuhrBasin10.590.83.82.71.70.836.06–97–1028.7–29.3 FNord-PasdeCalais12.089.83.84.81.00.636.46332.3 Anthracite DRuhrBasin7.791.83.62.61.40.735.94–73–531.0–31.4 UADonets4.094.41.81.40.71.734.155.730.0 Source:AlstomPowerandEffenberger(2000).
2.1 Fossil Fuels 19
Fig. 2.2 Coal composition
as the American Society for Testing Materials (ASTM), the Deutsches Institut f¨ur Normung (DIN), the British Standards Institution, Australian Standards (AS) and the International Standards Organisation (ISO).
Figure 2.2 shows the general composition of a coal. The raw coal, besides the combustible organic substance, contains inert material, which is made up of mineral matter and water. Since the determination of the mineral content requires rather sophisticated methods, the common practice is to use the ash content instead (JBDT 1976; Gumz 1962; Adrian et al. 1986; Ruhrkohle 1987).
The proximate analysis includes the determination of the total moisture, the air- dried moisture, the volatile matter, the fixed carbon and the ash. It involves heat- ing the sample to various temperatures for different periods of time and noting the weight loss in the sample.
A proximate analysis reports moisture in only two categories: total and air-dried.
Air-dried moisture is also referred to as inherent moisture. The total moisture con- tent is composed of free or surface moisture and inherent moisture. While free moisture adheres to the outside surface of the fuel, inherent moisture is bound in the capillaries inside the grain. Drying at room temperature makes the free mois- ture evaporate; the air-dried sample remains. Further heating to 105◦C makes the remaining, inherent moisture evaporate, and the dry, “moisture-free” coal remains.
Chemically bound water, in the form of hydrates of the mineral matter, such as clay minerals, remains in the coal. These hydrates are not taken into consideration in the conventional moisture content determination at 105◦C (Ruhrkohle 1987).
Heating the dry, moisture-free sample to 900◦C in an inert atmosphere releases the volatile components. In this process, a multitude of vapours and gases escape.
The remaining matter is called char. From the weight loss in this process, the volatile matter content is calculated. It should be taken into consideration when assessing this value that, because of the dissociation and release of carbonates, the volatile matter content may appear higher than it actually is. The combustible fraction of the char is described as fixed carbon (fixed C); the incombustible fraction is termed ash.
20 2 Solid Fuels The content of fixed C is not the same as the C content of the fuel which, besides the fixed carbon, also includes the carbon in the volatile matter.
The volatile matter content determined according to the standards does not cor- respond to the volatile matter released in a real combustion process, because the temperature, heating rate and residence time in an industrial furnace differ from the respective values under laboratory conditions. In industrial firing plants the amount of released volatile matter may be considerably higher.
The ash content of a coal is determined by means of the residue left over after burning a sample with air at 815◦C (German standard DIN 51719). This content is not identical to the mineral matter content, because the ash is only the mineral matter residue from combustion. In combustion engineering it is common, though, to give the ash content as a measure of the mineral substances in the fuel. The procedure for the determination of the mineral matter content is more sophisticated than that for the determination of the ash content. The procedure consists of chemical processes in which the sample becomes demineralised by hydrochloric and hydrofluoric acids (Ruhrkohle 1987). The mineral matter content can include inherent mineral matter spread throughout the coal seam as well as extraneous mineral matter from the roof or floor of the seam. Some of the inherent mineral matter in coal is derived from inorganic compounds associated with plant life. This mineral matter is generally responsible for about 1–2% of the ash in the coal. The extraneous mineral matter comprises the bulk of the ash in the coal (Drbal 1996).
The mineral matter undergoes a chemical conversion in the combustion process.
For hard coals, the conversion and release of the volatile products of decomposition has a weight-reducing effect on the ash. The weight of the ash (the residual matter from combustion) is lower than the weight of the original mineral matter content.
In the process of combustion of hard coal, hydrates and carbonates bound to min- eral components are released, while alkalis volatilise, and pyritic sulphur decom- poses. Mineral components are partly transformed into an oxidic form during com- bustion. However, describing the ash composition only in terms of oxides of the elements found in the ash analysis is inaccurate.
A part of the decomposition products of combustion is taken into account in the determination of the volatile matter content. For example, the mineral matter content of coals from the Ruhr basin, on average, is 9% higher than the ash content (Ruhrkohle 1987). For coals which contain alkaline earths as part of the mineral matter, e.g. brown coals, there may also be an increase in the weight of the ash during incineration as a consequence of the absorption of sulphur oxides.
Table 2.2 shows a compilation of the mineral elements occurring in coals, while Table 2.3 gives the main components of hard and brown coal ashes.
Ultimate analysis determines the contents of carbon, moisture, nitrogen, sulphur and chlorine. The difference in the balance between the sum of the contents deter- mined by the ultimate analysis and the total dry ash-free (d.a.f) weight is commonly assumed to be oxygen. The elemental composition is the basis for the combustion calculations of the stoichiometric oxygen demand, the flue gas quantity and the flue gas composition.
2.1 Fossil Fuels 21 Table 2.2 Coal minerals (Adrian et al. 1986)
Fraction (percentage) by
Mineral Formula weight
Clay minerals Up to 50
Kaolinite Al2O3∗2SiO2∗H2O
Illite K2O∗3(Al,Fe)2O3∗16SiO∗4H2O
Carbonates Up to 20
Calcite CaCO3
Dolomite CaMg(CO3)2
Siderite FeCO3
SiO2group 1–15
Quartz SiO2
Chalcedony Si∗O2
Sulphides Up to 20
Pyrite FeS2
Marcasite FeS2
Accessory minerals
Feldspar (K,Na)AlSi3O3
Apatite Ca5F(PO4)3
Hematite Fe2O3
Rock salt NaCl
Rutile TiO2
Table 2.3 Main components of coal ash (Adrian et al. 1986) Brown Hard coal coal/lignite
Ash component (%) (%)
Silica oxide SiO2 30–50 1–50 (mostly 10)
Aluminium oxide Al2O3 15–30 1–35 (mostly 8)
Iron oxide Fe2O3 2–22 4–25
Calcium oxide CaO 1.5–15 15–60
Magnesium oxide MgO 1–8 1.5–12
Sulphur trioxide SO3 1–5 4–40
Phosphoric acid P2O5 0.2–1.5 0.1–1.8
Potassium and sodium oxides K2O+Na2O 1–5 0.5–2
The calorific or heating values are a measure of the thermal energy released in complete combustion. The reference temperature is 25◦C in accordance with Ger- man standard DIN 51900.
Water is contained in the fuel before combustion (the moisture of the fuel) and is formed during the combustion of the hydrous compounds. The higher heating value (HHV) or gross calorific value (GCV) assumes water to be present in a liquid state after combustion. In contrast, the lower heating value (LHV) or net calorific value (NCV) counts both water fractions as being in a vapour state. The higher heating value is higher than the lower heating value by the heat of evaporation of the fuel moisture and the water formed at 25◦C (2,443.5 kJ/kg). Since the heat of evaporation is normally not used in industrial processes, it is common to apply the
22 2 Solid Fuels lower heating value. The higher heating value is determined by a bomb calorimeter (German standard DIN 51900); the lower heating value is calculated from the HHV minus the latent heat of the water vapour.
Higher and lower heating values can also be determined by correlations between the heating value and analysis values from statistical studies. The values calculated this way, however, are only approximate.
The ash fusion behaviour allows some conclusions about the behaviour of the mineral components and the fouling and slagging behaviour during combustion to be drawn. For investigation purposes, a sample body of ash is heated. Changes of shape occur at specific temperatures, giving information as to the characteristics of the sample. The ambient atmosphere is either air (oxidising) or a mixture of CO and CO2(reducing).
In different countries, the methods to determine the ash fusion behaviour are similar but different shapes of sample bodies are used. According to the Ameri- can ASTM Standard D 1857, the ash is pressed in a triangular pyramid of 19 mm in height and a 6.35 mm triangular base (Stultz and Kitto 1992). The test sample according to German standard DIN 51730 has a cylindrical or cubic shape of 3 mm height and 3 mm diameter/width (see Fig. 2.3). Photographs are taken of the shape of the compacted sample body as it changes, and the temperature at each photograph is recorded. The specific temperatures characterising the fusion behaviour are as follows:
• Initial deformation temperature (ID): when the first signs of a change in form are visible.
• Spherical or softening temperature (ST): when the sample has deformed to a spherical shape where the height of the sample is equal to the width at the base (H =W ).
r1 1/3 r1
1/3 r2 2r1
r2 2r2 ASME
DIN
Softening range Fluid/melting range
Original sample
Initial deformation temperature
Spherical/
softening temperature
Hemispherical temperature
Fluid temperature 1
2r1
Fig. 2.3 Characteristic ash fusion temperatures according to DIN and ASME
2.1 Fossil Fuels 23
• Hemispherical temperature (HT): when the sample body has changed to a hemi- spherical shape. Its height equals one half the width of the base (H =1/2 W ).
• Fluid temperature (FT): when the sample body has melted down to a flat layer with a maximum height of about one third of its height at the hemispherical temperature.
The temperature range between the initial deformation and hemispherical tem- perature is defined as the softening range, the range between hemispherical and fluid temperature as the fluid temperature range. When the difference between the hemi- spherical temperature and ash fluid temperature is small, then the slag is referred to as “short”; a large difference occurs when the slag is “long”.
The results of the above-described investigations are transferable to an industrial scale only to a limited extent, because the laboratory conditions do not correspond to the conditions in industrial firing systems, either in the way the samples are pre- pared, or in the procedure of the method.
2.1.2.1 Petrographic Analysis
Petrographic analysis classifies the coal according to its structural constituents – the macerals (Chiche 1970, 1973). This information is used to gain an insight into the process of the coal formation, so as to relate the decayed organic matter to the coal.
Maceral is the term for the smallest structural constituent recognisable by an optical microscope. The macerals can be distinguished from one another by their reflectance. In the analysis of maceral groups of hard coal, three maceral types – vitrinite, exinite and inertinite – are distinguished. Vitrinite comes from wood mat- ter, while exinite mainly consists of products of digested sludge. The third maceral group, inertinite, which requires further analysis before being confirmed as origi- nating from the vegetable matter, is relatively unreactive (Ruhrkohle 1987; Adrian et al. 1986). With brown coal, the maceral groups distinguished are huminite, lipti- nite and inertinite, where huminite and liptinite, as far as their origin is concerned, correspond to the hard coal maceral groups of vitrinite and exinite, but with a lower degree of decomposition (Zelkowski 2004). Table 2.4 gives a general compilation of the maceral groups and macerals of hard and brown coals.
The various maceral groups are distinguished by their contents of volatile matter and their reflectance. In the case of hard coal, exinite has the highest volatile matter content and the lowest level of reflectance, while inertinite has the lowest content of volatile components and the highest reflectance of the maceral groups. With higher coalification degrees, the volatile matter contents of all maceral groups decrease while converging towards each other (see Fig. 2.4) (Ruhrkohle 1987).
Hard coals of the northern and the southern hemispheres differ markedly as to their maceral composition. Coals of the northern hemisphere show a dominance of vitrinite, the content being about 60–80%, with the contents of both exinite and inertinite varying, with a maximum of 30% each. Coals of the southern hemisphere have a significantly higher inertinite content of more than 50%. There is a direct correlation between the volatile matter in a coal and the reflectance of vitrinite
24 2 Solid Fuels Table 2.4 Macerals of brown and hard coals (Zelkowski 2004)
Brown coal Hard coal
Maceral group Maceral Maceral group Maceral
Huminite Textinite, ulminite attrinite, densinite gelinite, corpohuminite
Vitrinite Telinite, collinite vitrodetrinite Liptinite Sporinite, cutinite,
resinite, suberinite, alginate, liptodetrinite chlorophyllinite
Exinite Sporinite, cutinite resinite, alginite liptodetrinite Inertinite Fusinite, semifusinite,
macrinite, sclerotinite inertodetrinite
Inertinite Micrinite, macrinite semifusinite, fusinite inertodetrinite
(see Fig. 2.5). This correlation is used to determine the distribution of the contents of volatile matter. Results serve to infer whether the fuel in question is a pure coal or a blended type. For example, despite having the same volatile matter content, the coal types in Fig. 2.6 exhibit clear differences in the distribution of macerals (Ruhrkohle 1987).
Fig. 2.4 Volatile matter of macerals as a function of the coal type (Ruhrkohle 1987)
2.1 Fossil Fuels 25 Fig. 2.5 Correlation of the
volatile matter content to the reflectance Rmof vitrinite (Ruhrkohle 1987)