Coal Combustion Studies in a Fluidised Bed Suthum Patumsawad stt@kmitnb.ac.th * Abstract Fluidized bed is one of the most promising methods for combustion today. Its application to boilers is recognized primarily for its low sensitivity to fuel quality and its capacity to limit air pollution. This technology is being used for co-combustion of coal and waste fuels. The objective of this paper is to evaluate the combustion performance from firing coal in a laboratory-scale fluidized bed combustor for the purpose of establishing optimum combustion conditions as a preamble to investigating co-combustion of solid waste as a supplementary fuel with coal. The results show that between 91-95% carbon utilization efficiency and over 99.8% CO combustion efficiency have been achieved at a design feed rate of 1.2 kg/hr and excess air of 30% to 90%. The effects of operating parameters on coal combustion, such as gas velocity and excess air, are discussed. Keywords : Coal, Fluidized Bed, Co-Combustion 1. Introduction There is now increasing interest in the use of biomass and municipal solid/industrial waste within existing coal-fired. plant. The advantages can be summarized as follows: - Continuous supply of waste is not an issue, since the boiler plant would always have the primary fuel (coal) for 100% utilization. - Reduce the need for land based waste disposal. - Can burn high moisture content wastes. - Displacement of a small proportion of coal by the biomass/waste will help to conserve reserves of fossil fuels. - Reduce emissions in particular SO 2. - Relatively small investment as compared to plant dedicated to biomass processing. Fluidized bed combustion has been shown to be a versatile technology capable of burning practically any fuel combination with low emissions (Anthony, 1995). The significant advantages of fluidized bed combustors over conventional ones include their compact furnace, simple design, effectiveness for a wide variety of fuels, relatively uniform temperature and the ability to reduce nitrogen oxide and sulphur dioxide emissions (Saxena and Jotshi, 1994). Fluidized bed combustors can be designed to combust almost any solid, semi-solid or liquid fuel without the use of supplement fuel, as long as the heating value is sufficient to heat up the fuel, drive off the moisture and preheat the combustion air In addition, with appropriate attention to fuel preparation and blending and to operating procedures, wastes can be co-fired with coal in many existing coalfired fluidised bed combustion boilers. Conversion of existing fluidized bed combustion boilers to co-firing wastes with coal is in many cases more cost-effective and efficient than building a dedicated new unit (McGowin and Howe, 1994). The objective of this work was to report experiments on combustion of coal for the purpose of establishing optimum combustion conditions as a preamble to investigating co-combustion of solid waste as a supplementary fuel with coal. Combustion efficiency and flue gas composition were investigated, with excess air, fluidising velocity and fuel feed rate as variables. 2. Materials aiad Method Fig. 1 shows the layout of the experimental fluidised bed unit. The design of this unit was originally for solid fuels and was operated at temperature of 900 o C with fluidising air velocity of 1-2 m/s. The fluidised bed combustor was designed to generate 10 kw thermal power. The combustor body is made of 1 cm thick 306 stainless steel. The combustor is 0.15 m in diameter and 2.3 m high, allowing bed depths up to 0.3 m with 2 m in freeboard height. The bed material is sand of an average size of 850 micron. The combustor is covered with Kaowool insulation. The freeboard height is an important design parameter since unburnt particles elutriated from the bed and volatiles released during the devolatilisation process will continue burning in the freeboard * Department of Mechanical Engineering, Faculty of Engineering, King Mongkut s Institute of Technology North Bangkok.
region. This requires a residence time of approximately 1-2 sec (Wiley, 1987 and Baeyens and Geldart, 1978). Fluidising air is introduced at the base of the bed through a nozzle distributor and used as both fluidisation and combustion air. The distributor plate is a 10 mm thick stainless steel plate bearing nineteen 6-cm-high capped standpipe, each with twenty seven 1.5-mm-diarneter, holes drilled radially just below the top. This configuration allows for a static layer of sand to insulate the plate from the hot bed, removing the requirement for a separate distributor cooling system. Fuel is fed pneumatically to the bed surface from a sealed hopper through an inclined feeding pipe and flow rate is controlled by a screw-feeder. To prevent the fuel from burning insi0e the feeding pipe before entering the combustor, a water-cooled jacket is fixed around the feeding pipe. A cyclone is fifted to the combustor exit and the carryover from the bed is collected for analysis. Gas analyser Cyclone 200 1000 Feeding system 2900 1200 Distributor plate Air 200 φ150 Fig. 1 A drawing of the laboratory scale fluidised bed reactor Start up of the bed is achieved by using an in-bed technique. Propane is introduced directly into the distributor plate by injectors and mixed with air in the nozzles, providing a combustible mixture at the nozzle exit. The propane gas is used as an auxiliary fuel to raise the bed temperature to a designated level, normally above the ignition temperature of the fuel. Bed and freeboard temperatures are measured at 8 different heights above the distributor plate by means of sheathed Ni/Cr-Ni thermocouples type K. Combustion gas samples are obtained from a sampling port located at the cyclone exit and analysed by on- line gas analysers. Gas analysers are susceptible to dust and water vapour thus the gas sample has to be cleaned and dried. The gas sample was passed through a glass wool filter, a water cooling heat exchanger, and a drier consisting of magnesium oxide granules before entering the on-line gas analysers. CO and O 2 are measured using a Xentra 4904 B1 continuous emissions analyser. The analyser uses the Servomex paramagnetic transducer for measuring O 2 and a gas filter correlation (Gfx) transducer for CO. The measurement ranges of O 2 and CO are 0-25% and 0-3000 ppmv, respectively. CO 2 is measured by using a non-dispersive infrared absorption spectrometry analyser. The measurement range of C O 2 is 0-15%. These gas analysers are calibrated with standard gas samples before use. Air and gas flow rates are measured by calibrated rotameters.
The percentage of combustion efficiency is computed from the following relation: % CO2 in flue gas 100% η = (1) CE (% CO + % CO) in flue gas 2 This efficiency calculation procedure is based on the knowledge of flue gas composition only and assumes that there are no carbon losses, and that cabon composition presented in the feed is converted completely to carbon monoxide and carbon dioxide. As unburnt carbon could be elutriated, a more accurate combustion efficiency taking this into account is calculated using equation (2) (Saxena and Jotshi, 1994). η CE = (B/C) x 100 (2) where B and C are, respectively, the mass fractiont of burnt and total carbon in the fuel. Knowing flue gas composition, fractional excess air, and the ultimate analyses of fuel, B can be calculated. Based on values of combustion efficiency from experiments where duplicate runs are conducted under almost identical conditions, combustion efficiency values should be within +2%. This methodology is convenient since determining experimentally the unburnt carbon is difficult. The coal property is summarised in Table 1. The coal with particle size in the range of 1.4-4.7 mm and containing 6% moisture content was used. Table 1. Proximate and ultimate analyses of coal Proximate analysis, wt% (dry basis) Volatile 38.15 Fixed carbon 58.87 Ash 2.98 Moisture content, wt% (as received) 5.9 Ultimate analysis, wt% (dry basis) Carbon 80.13 Hydrogen 5.31 Oxygen 9.88 Nitrogen 0.96 Sulphur 0.74 HHV, MJ/kg (dry basis) 33 As mentioned earlier, the design heat input for the combustor was I 0 kw thermal, which is equivalent to a coal feed rate of 1.2 kg/hr. Hence, this coal feed rate operated with excess air between 30% to 90% are used as reference states.
3. Experimental Results and Discussions The results of the tests, while burning bituminous coal, classified according to The American Society for Testing and Materials (ASTM) system, are given in Table 2. The gas fluidising velocities and fluidisation numbers computed at the bed temperature and combustion efficiencies calculated from both carbon balance, Eq. (2), and [CO 2,] and [CO] in flue gas composition, Eq. (1), are also reported. Table 2 Combustion of coal in an experimental fluidised bed Run 1 2 3 4 Fuel feed rate (kg/hr) 1.2 1.2 1.2 1.2 Air flow rate (kg/hr) 16.23 18.38 20.80 23.22 Excess air (%) 34 53 73 93 Bed temperature ( C) 932 936 922 900 Bed surface 934 938 926 903 temperature ( C) Flue gas composition CO at 6%O 2 in flue gas 223 157 220 234 (ppm) CO 2 (%) 12.5 11 10 9 O 2 (%) 6.5 7.8 9.2 10.3 Carbon combustion efficiency Eq. 2 (%) 91.27 91.32 94.15 94.75 CO combustion efficiency Eq. 1 (%) 99.83 99.87 99.83 99.81 Fluidising gas velocity 0.98 1.11 1.22 1.35 (m/s) Fluidisation number 3.68 4.16 4.54 5.00 (Ug/Umf)
3.1 General Combustion Chgaracteristics of Coal Generally, the efficiencies were between 91-95% for carbon utilisation efficiency, Eq. (2), and over 99.8% for CO combustion efficiency, Eq. (1), at a design feed rate of 1.2 kg/hr and excess air of 30% to 90%. The low CO emissions, 150-230 ppm imply that most of the burnt carbon was converted to CO 2 in the combustion process and the major loss of combustion efficiency comes from elutriation loss in flue gas. Bed temperatures are in the range of 900-930 o C. Measurement of the bed temperature (at 10 mm, 20 mm, and 30 mm above the distributor plate) showed no measureable variation. Because of over-bed feeding, burning coal particles could be seen on the surface o the bed and there was also evidence of volatile burning. The temperatures above the bed surface at 30 and 40 mm above the distributor plate were found t be more or less the same as the bed temperature indicating freeboard combustion in this region. Fig. 2 shows the temperature profile along the height of the combustor when burning coal. Height above the distributor plate (cm) 120 100 80 60 Run 1 40 Run 2 Run 3 20 Run 4 0 500 600 700 800 900 1000 Temperature (C) Fig. 2 Temperature profile of coal combustion 3.2 Effect of Excess Air The influence of excess air on the carbon utilisation efficiency is shown in Fig. 3. It can be seen that increasing excess air increases the carbon utilisation efficiency from 91% to 95% when the excess air increased from 30% to 90%. This means the amount of unburnt carbon decreases with increase in excess air. These results show the same trend with those of Gibbs and Headley (I 978). But normally the amount of air flow is related to the air velocity in fluidised bed. The higher the amount of air flow rate, the higher the gas velocity. From Fig. 3, the carbon utilisation efficiencies at a feed rate of 1.3 kg/hr are constant rather than increasing continually with increase in excess air. At a feed rate of 1 kg/hr, the carbon utilisation efficiency increases continually with increase in excess air up to a maximum of 100%. These results show a significant effect of fluidising velocity to the combustion efficiency.
Carbon combustion efficiency (%) 100 95 90 85 80 75 70 0 20 40 60 80 100 120 140 Excess air (%) 1 kg/hr 1.2 kg/hr 1.3 kg/hr Fig. 3 Effect of excess air to carbon combustion efficiency of coal combustion at various feed rate 3.3 Effect of Fluidising Velocity Air velocity used in a fluidised bed combustor is at least at minimum fluidisation velocity of the combustor. It is convenient to use fluidisation number (Ug/Umf) which is the ratio of fluidising velocity with minimum fluidisation velocity. The effect of fluidising velocity to carbon utilisation efficiency is shown in Fig. 4 in terms of the fluidisation number (Ug/Umf) relationship. During the experimental tests, the fluidisation numbers (Ug/Umf) used were in the range of 3.5-5. Carbon combustion efficiency (%) 100 90 80 70 60 50 2.5 3 3.5 4 4.5 5 5.5 Fluidisation number, Ug/Umf Fig. 4 Effect of Fluidisation number on carbon combustion efficiency of coal combustion
It is expected that the carbon utilisation efficiency could be increased when fluidisation number increases since increasing the fluidisation number (Ug/Umf) also increases the amount of excess air at the same fuel feed rate. However, fluidising velocity has an affect on unbumt combustibles in the elutriated carryover. The higher the fluidising velocity, the higher the unbumt combustible loss in the flue gas, and the lower carbon utilisation efficiency. On the other hand it is to be expected as the hydrodynamic activity in the bed is related to solid mixing and gas-solids contacting and these in turn are directly related to carbon utilisation efficiency (Saxena et al., 1992). They found that in the turbulent regime, the carbon utilisation efficiency was a maximum and a further increase in the fluidisation number (Ug/Umt) had an insignificant influence on the bed hydrodynamics and hence the carbon utilisation efficiency. As it can be seen from Fig. 4, the carbon utilisation effiency increases as fluidisation number (Ug/Umf) increases from 3 to 4 and is fairly constant with increasing fluidisation number (Ug/Umf) from 4 to 5. 3.4 Effect of Fuel Feed Rate The designed combustion rate for this experimental rig was 10 kw which corresponded a coal feed rate of 1.2 kg/hr. To study the effect of feed rates, feed rates of I and 1.3 kg/hr were also tested and compared to feed rate of 1.2 kg/hr as shown in Fig. 3. For a feed rate of 1.2 kg/hr at the same percentage of excess air. The decrease in the carbon combustion efficiency was possible due to the reduction in the bed temperature. The bed temperatures are approximately 870 o C and 920 o C at feed rates of 1 and 1.2 kg/hr. respectively. The higher the bed temperature the higher the carbon combustion efficiency (Gibbs and Headley, 1978). For a feed rate of 1.3 kg/hr, the carbon combustion efficiency is lower than at a feed rate of 1.2 kg/hr with increasing percentage of excess air. The bed temperatures at feed rates of 1.2 and 1.3 kg/hr are approximately the same at 920 o C. There are two possible reasons for the drop of carbon utilisation efficiency. Firstly, when considering at constant excess air, increasing fuel feed rate means increasing the amount of combustion air which relates to air fluidising velocity. The higher the fluidising velocity the higher the amount of unbumt combustibles in the flue gas. Secondly, as pointed out by Saxena et al. (1992) and Artos et al. (1991), the carbon loss associated with elutriation rate is proportional to the carbon load. Increase of carbon loading, i.e. fuel feed rate, enhances the rate of particle attrition resulting in greater elutriation loss. This causes the carbon utilisation efficiency to decrease. 4. Conclusions. Combustion of coal in a laboratory-scale fluidised bed was investigated to evaluate its combustion, characteristics. The results show that high combustion efficiencies could be achieved by choosing appropriate operating conditions. The efficiencies were between 91-95% for carbon utilisation efficiency and over 99.8% for CO combustion efficiency at a design feed rate of 1.2 kg/hr and excess air-of 30% to 90%. The effects of operating parameters on coal combustion, such as gas velocity and excess air, were discussed. An optimum fluidisation number around 4-4.5 is recommended 6y this work. Larger fluidising gas velocity should be avoided because the reduction in residence time will Permit volatiles and unburnt particles to escape before complete combustion is achieved. For co-combustion purposes, coal could be used as an auxiliary fuel to obtain and sustain combustion conditions.
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