Chapter 5 Combustion Systems for Solid Fossil Fuels
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Chapter 5 Combustion Systems for Solid Fossil Fuels Coal firing systems are comprised of the sub-systems of fuel supply and preparation, fuel and combustion air transport and distribution, the furnace for releasing the heat from the fuel and flue gas cleaning. The systems used for combusting solid fossil fuels are as follows: • Grate firing • Fluidised bed firing • Pulverised fuel firing (Stultz and Kitto 1992; Strauß 2006; STEAG 1988; Dole zalˇ 1990; G unther¨ 1974; Gumz 1962; G orner¨ 1991) Table 5.1 compares the advantages and disadvantages of different combustion systems. Figure 5.1 gives the characteristic gas and solid fuel flow velocities, pres- sure losses and heat transfer coefficients of each of the combustion systems. In a grate firing system, the solid fuel lies in a bulk bed on a moving grate. The fuel burns with the combustion air which is blown through the grate bars and through the bulk. At low flow velocities, single coarse coal particles with sizes up to 30 mm (approximately the size of a nut) remain in the coal layer on the grate. Notable quantities of solids are not entrained. Because of the limited capacity of this furnace type, coal-fired grates are only used for industrial and thermal power plants of small capacity. Grate firing is the preferred system for ballast-containing fuels such as waste, or for solid industrial wastes, or biomass, because no or minor fuel preparation is required. In fluidised bed firing, the solid fuel is fluidised and burns while in a gas – solid suspension. The fluidising medium also provides the oxygen for the oxidation of the fuel. With the lower flow velocities of the bubbling fluidised bed (BFB), only the fine-grained ash from the fluidised bed is entrained in the gas after burnout and abra- sion of the coal. Coarse-grained ash accumulates in the fluidised bed, from where it is removed. With the higher flow velocities of combustion air and combustion gases of the circulating fluidised bed (CFB), the entire solid flow in the furnace is entrained and circulated. The circulating fluidised bed occupies the entire furnace volume. In both systems, the solids stay in the furnace appreciably longer than the gas flow. H. Spliethoff, Power Generation from Solid Fuels, Power Systems, 221 DOI 10.1007/978-3-642-02856-4 5, °C Springer-Verlag Berlin Heidelberg 2010 222 5 Combustion Systems for Solid Fossil Fuels Table 5.1 Comparison of grate, fluidised bed and pulverised fuel firing systems Bubbling fluidised bed (BFB) and circulating fluidised bed Pulverised fuel firing Grate firing systems (CFB) firing systems systems Advantages Advantages Advantages – Relatively minor fuel – Relatively minor fuel – High process availability preparation requirement preparation requirement – Large capacities – Clear design – Flue gas cleaning consists only – High power density – High process availability of particulate collection – Good burnout – simple operation – Utilisable ash – Low auxiliary power demand Disadvantages of BFB and CFB Disadvantages – Low NO x emissions (e.g. – High limestone demand for – Relatively major fuel bituminous coals sulphur capture preparation requirement < 400mg /m3) – Ash not utilisable without – Flue gas cleaning needed – Partial desulphurisation further preparation for particulates, SO 2 and by limestone addition NO x Disadvantages Advantages of CFB against BFB – High combustion losses – Better burnout of 2–4% unburnt carbon – Lower limestone demand for – High flue gas sulphur capture temperatures due to – Lower emission values limited air preheating – No in-bed heating surfaces at – Unsuitable for risk of erosion fine-grained fuels – Better power control In pulverised fuel firing systems, the coal particles are carried along with the air and combustion gas flow. Because particles are entrained in the gas flow, this firing type is also known as entrained-flow combustion. Pulverised fuel and combustion air are injected into the firing via the burner and mixed in the furnace. With a fine raw coal milling degree and high combustion gas flow velocities, particle and gas residence times are almost equal. The combustion of the pulverised coal/air mixture being a rapid process distributed over the entire furnace makes it possible to achieve higher capacities than grate or fluidised bed firing systems. The choice of the firing system depends on the properties of the fuel and on the steam generating capacity (Strauß 2006). Combustion systems for solid fuels are offered on the market with the capacities shown in Table 5.2: Table 5.2 Output ranges of firing systems Firing system Output range [MW th ] Pulverised fuel firing 40 up to 2,500 Bubbling fluidised bed firing up to 80 Circulating fluidised bed firing 40 up to 750 Grate firing 2.5 up to 175 5.1 Combustion Fundamentals 223 Fig. 5.1 Distinctive features Fixed Fluidised bed Pulverised of firing systems (G orner¨ bed bubbling circulating fuel 1991) Heat transfer coefficient [bar] p ∆ [kW/(m²K)] α Pressure loss Ig Pressure loss lg uf ut Gas velocity [m/s] Particle velocity Gas velocity Increasing Velocity [m/s] Slip particle load Bed expansion 5.1 Combustion Fundamentals The purpose of the combustion process is to release by oxidation the energy which is chemically bound in the fuel and to convert it into sensible heat. The heterogeneous combustion process of solid fuels is more complex than the homogeneous combustion of gaseous fuels. Solid fuels such as coal are composed of different fractions of organic matter and minerals. As the fuel heats up in the furnace, the pyrolysis of the organic matter starts. In this process, volatile interme- diate products such as hydrocarbons, carbon oxides, hydrogen, sulphur and nitrogen compounds and residual char (as a solid intermediate product) are generated. Igni- tion begins the combustion process. Prerequisite for ignition, besides a sufficiently high temperature, is the forming of a burnable mixture. Under these conditions, the volatile matter and the residual char combust together with the oxygen of the combustion air. Figure 5.2 schematically presents the combustion process of coal in pulverised fuel firing. The combustion of solid fuels evolves in the partial processes of (Dole zalˇ 1990; van Heek and M uhlen¨ 1985) • drying, • pyrolysis, • ignition, 224 5 Combustion Systems for Solid Fossil Fuels Temperature 50 % Burnout 90 % 99 % [°C] Volatile matter combustion 1500 Residual char Air preheating Fly ash 1000 Pyrolysis 0.1–10 µm Minerals 500 Coal dust Near burner zone Burnout zone H2O 10–100 µm 1 10 100 1000 Residence time [ms] Fig. 5.2 Schematic drawing of the combustion process in pulverised fuel firing • combustion of volatile matter and • combustion of the residual char. The first two partial processes are a thermal decomposition as a consequence of the heating up of the fuel. The quantity of heat necessary to heat the fuel up to ignition temperature is transferred mostly by convection. In pulverised fuel firing, for example, hot flue gas is admixed in the near-burner zone, while in a fluidised bed, the heat is transferred by particles of solid matter. In grate firing systems, heating up is carried out by means of refractory-lined hot walls transferring the heat to the fuel by radiation. In the last two partial processes – combustion of volatile matter and combus- tion of residual char – the organic matter is converted chemically. Conversion is divided into homogeneous and heterogeneous reactions. The partial processes do not necessarily run one after the other but, depending on the firing type, may over- lap. Table 5.3 provides an estimate of the necessary time for each of the partial processes. It is evident from the table that the total combustion time of all firing systems is determined by the combustion of the residual char. In the following, the partial processes of solid fuel combustion are discussed in more detail. 5.1.1 Drying Water can adhere both to the particle surface and to the pores inside the coal particle. As the fuel heats up in the furnace, water begins to vaporise (at temperatures above 100 ◦C). At temperatures up to 300 ◦C, the vaporised pore water becomes desorbed or released. Besides water vapour, other gases such as methane, carbon dioxide and 5.1 Combustion Fundamentals 225 Table 5.3 Partial processes of coal combustion in firing systems Drying and Time of volatile Time of residual Particle Heating pyrolysis matter char combustion Firing system diameter [mm] rate [K/s] period [s] combustion [s] [s] Fixed bed firing 100 10 0–10 2 ca. 100 Determined by >1,000 release and mixing with combustion air Fluidised bed 5–10 10 3–10 4 10–50 100–500 firing Pulverised fuel 0.05–0.1 10 4–10 6 <0.1 1–2 firing nitrogen, which have formed during the coalification process, outgas as well (van Heek 1988). Depending on the combustion system, the firing is capable of drying fuels with different moisture contents. Whereas grate or fluidised bed firing systems can be fed with moisture-containing fuels without further treatment, for pulverised fuel firing the fuel is predried in mills in order to ensure a fast combustion process within the available residence time. 5.1.2 Pyrolysis The decomposition of the organic coal substance and the formation of gaseous prod- ucts during the heating of the coal are termed devolatilisation or pyrolysis (van Heek and M uhlen¨ 1985; Zelkowski 2004; R udiger¨ 1997; Klose 1992). Devolatilisation of volatile matter by cracking of compounds of organic coal structures starts at temperatures above 300 ◦C. In a temperature range up to about 600 ◦C, tars (liquids at lower temperatures) and gaseous products are formed. The gases consist of carbon dioxide (CO 2), methane (CH 4) and other, lighter hydrocar- bons such as C 2H6, C2H4 and C 2H2. Tars are complex hydrocarbon compounds, in their organic structure similar to the base fuel, which evaporate from the coal sub- stance at temperatures between about 500 and 600 ◦C (Solomon and Colket 1979).