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Co-combustion of agricultural residues with coal in a fluidised bed combustor
Waste Management 29 (2009) 767–773
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Co-combustion of agricultural residues with coal in a fluidised bed combustor W.A.W.A.K. Ghani a,*, A.B. Alias b, R.M. Savory b, K.R. Cliffe c a b c
University Putra Malaysia, Department of Chemical and Environmental Engineering, 43400 UPM, Serdang, Malaysia Universiti Teknologi MARA, Faculty of Chemical Engineering, 40450 Shah Alam, Selangor, Malaysia University of Sheffield, Department of Chemical and Process Engineering, Mappin Street, S1 3JD Sheffield, United Kingdom
a r t i c l e
i n f o
Article history: Accepted 24 March 2008 Available online 9 July 2008
a b s t r a c t Power generation from biomass is an attractive technology that utilizes agricultural residual waste. In order to explain the behavior of biomass-fired fluidised bed incinerator, biomass sources from agricultural residues (rice husk and palm kernel) were co-fired with coal in a 0.15 m diameter and 2.3 m high fluidised bed combustor. The combustion efficiency and carbon monoxide emissions were studied and compared with those for pure coal combustion. Co-combustion of a mixture of biomass with coal in a fluidised bed combustor designed for coal combustion increased combustion efficiency up to 20% depending upon excess air levels. Observed carbon monoxide levels fluctuated between 200 and 900 ppm with the addition of coal. It is evident from this research that efficient co-firing of biomass with coal can be achieved with minimal modifications to existing coal-fired boilers. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Waste-to-energy is gaining increasing attention as landfill costs and environmental concerns rise in many developed countries, including Malaysia. A recent study shows that biomass in Malaysia contributes about 14% of the energy used every year—approximately 340 million barrels of oil equivalent (boe). Paddies and palm plantations are among the main contributors of waste to the biomass energy utilization in Malaysia. The energy potential from these two resources is 16.4 and 229 picojoules, respectively, based on the production of 2194 thousand tonnes and 60,000 thousand tonnes, respectively, in 2000 (BIOGEN, 2004). This implies that an electric energy potential around 375 MW can be generated. Combustion of agricultural residues is commonly used in industries for energy recovery. However, many researchers have found difficulty in achieving high efficiency with stand-alone biomass firing (Natrajan et al., 1998; Bhattacharya and Wu, 1989). Thus, cofiring biomass with coal in industrial and utility boilers could offer an alternative approach with improved combustion efficiency, lower-cost and reduced-risk technology. Significant co-combustion potential for biomass and waste materials exists in all European Union (EU) countries, and such potential is mirrored on a worldwide basis, creating a significant market for equipment and services. For instance, in Finland, large quantities of biomass from forest industries are used as the main fuel in grate-firing, bubbling fluidised bed combustors (BFBC) or circulating fluidised bed (CFBC) combustors within the range of 5–20 MWth (Saloski, 1999). In
* Corresponding author. Tel.: +60 3 89466287; fax: +60 3 86567120. E-mail addresses: [email protected] (W.A.W.A.K. Ghani), k.cliffe@sheffield.ac.uk (K.R. Cliffe). 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.03.025
Sweden, forest residues, sawdust, demolition wood and other waste wood, fibre and paper sludge are commonly used together with a smaller portion of coal or oil (15–30%) in district heating or combined heat power (CHP) plants using a variety of combustion technologies (grate-firing, BFBC, CFBC and pulverised combustion (PC)) (Kallner, 1999). Furthermore, in Austria, co-combustion is used by small industrial boilers located mainly in the pulp and paper industry, which generally use their own biomass wastes (e.g., black liquor, bark) (Hammerschmidt, 1999). In The Netherlands, waste wood is the main supplementary biomass feedstock used in coal-fired PC power plants. In Germany, sewage sludge is the most important co-fired biomass in lignite or coal-fired PC power plants (Rosch and Kaltschmitt, 1999). Fluidised bed combustion (FBC) has been shown to be a versatile technology capable of burning practically any waste combination with low emissions (Tillman, 2000). The significant advantages of fluidised bed combustors over conventional combustors include their compact furnaces, simple designs, effective combustion of a wide variety of fuels, relatively uniform temperatures, and ability to reduce emissions of nitrogen oxide and sulphur dioxide gases (Saxena and Jotshi, 1994). This research was performed with the objective of determining the combustion efficiency of an existing coal-fired combustor while co-firing with agricultural residue. The combustion efficiency calculation is primarily based on the conversion of carbon into carbon monoxide (CO) and carbon dioxide (CO2) emissions. Other gases, such as nitrogen oxides (NOx) and sulphur dioxides (SO2), are neglected. It is assumed that the presence of NOx and SO2 is dominated by the coal fraction in the mixture. Furthermore, this research aims to demonstrate the technical feasibility of a fluidised bed as a clean technology for burning agricultural residues.
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In addition, the effects of biomass properties (such as particle size, particle density and volatility) and the influences of operating parameters (e.g., the amount of excess air, the effect of fluidising velocity on axial temperature profile, the combustion efficiencies, and the CO emissions) are also investigated. 2. Experimental 2.1. Fuel characterization In this study, British coal, agricultural residues (rice husk and palm kernel shell) originating from Perlis were employed as fuel. These fuels were open-air dried for 2–3 days to remove moisture. The proximate and ultimate analyses performed on coal and rice husk are summarized in Table 1. Table 1 shows that the main characteristics of these dry agricultural residues, in comparison with coal, are low calorific value (14–18 MJ/kg), high-volatile matter content (60–75%), ash content (10–25%), low carbon content (35– 45%) and, most importantly, high oxygen content (27–40%). This is of particular importance since it influences the stoichiometric air requirement for combustion. It is also important to note that the residue samples have a lower particle density and vary in size and shape much more than coal. 2.2. Experimental apparatus and operating procedures Fig. 1 shows a schematic diagram of the Atmospheric Fluidised Bed combustor used in this investigation. The system has a 0.15 m diameter and 2.3 m-high combustion chamber; it allows for bed depths of up to 0.3 m using 850 lm silica sand (Geldart Type A); and it includes a cyclone, a screw feeder and a gas analyzer. The combustor body is constructed from 1 cm-thick 306 stainless steel and covered in Kaowool insulation to prevent excessive heat loss during operation. Fluidising air was introduced at the base of the bed through a nozzle distributor and provided air for both fluidisation and combustion. Startup of the bed was achieved using an inbed technique: propane was introduced directly into the distributor plate by injectors and mixed with air in the nozzles, providing a combustible mixture at the nozzle exit. Bed and freeboard temperatures were measured at eight different heights above the distributor plate by means of sheathed type K Ni/Cr–Ni thermocouples (TC). Fuel was fed pneumatically into the bed surface from a sealed hopper through an inclined feeding pipe, the flowrate through which was controlled by a screw feeder. Normally, some of the less dense biomass should be fed in the bed; however, since one entry port was desirable, the feed should be premixed before being fed into the combustor via the entry port. The main objectives of the research are to identify the biomass fuels that could be co-fired
Fig. 1. A schematic diagram of the laboratory scale fluidised bed combustor (Tc, thermocouple).
with coal with overbed feeding and result in high efficiency. A cyclone was fitted to the combustor exit, and the carryover from the bed was collected for analysis. CO and O2 were measured using a Xentra 4904 B1 continuous emissions analyzer, and CO2 was measured using a non-dispersive infrared absorption spectrometry analyser. A fly ash sample was collected from the catch-pot after finishing the combustion run. The fly ash sample was then weighed and analysed to determine the total amount of unburned carbon in the test fuels. The percentage of combustion efficiency was computed using the following relation:
Table 1 Coal and biomass characterization Fuel
British coal
Rice husk
Palm kernel shell
Proximate analysis (% wt, dry basis) Fixed carbon Volatile matter Ash
58.90 38.20 2.90
15.00 60.70 24.30
18.60 72.50 8.90
Ultimate analysis (% wt, dry basis) Carbon Hydrogen Nitrogen Sulphur Oxygen
80.10 5.30 0.90 0.70 13.00
36.20 5.71 0.10 0.00 57.99
49.5 6.74 1.85 0.00 41.91
Calorific value (MJ/kg) Particle size (mm)
31.1 1.4–4.8 mm
13.5 0.8 mm diameter 1.00 mm length (cylindrical shape)
18.0 3 mm wide 6 mm length (rectangle)
Particle density (kg/m3)
1200
98
435
769
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E1 ð%Þ ¼ ðB þ unburned carbon in ashÞ=C 100%
ð1Þ
where B and C are, respectively, the mass fractions of burnt and total carbon in the fuel. This efficiency calculation procedure is based on the premise that the flue gas composition, which is burned carbon in the feed, is completely converted to carbon monoxide and carbon dioxide. As unburned carbon could be elutriated, a more accurate combustion efficiency taking this into account was calculated using Eq. (1). Based on the values of combustion efficiency from experiments where duplicate runs were conducted under almost identical conditions, the combustion efficiency values should be within ±2%. 3. Results and discussion This section describes the combustion of agricultural residue in a fluidised bed combustor. The influences of fuel properties (such as particle size, particle density and volatility) and the influences of operating parameters (e.g., the amount of excess air, the effect of fluidising velocity on axial temperature profile, the combustion efficiencies, and the CO emissions) are discussed. 3.1. Carbon combustion efficiencies The combustion of agricultural residues was evaluated in terms of combustion efficiency, which was calculated based on Eq. (1). The combustion tests were performed using different coal mass fractions (0%, 50% and 100%) corresponding to a heat input of 10 kW under optimum excess air conditions. Fig. 2 shows the effect of the different mixtures of rice husk and palm kernel shell with coal on carbon combustion efficiency with the same heat input. Generally, carbon combustion efficiency for a single agricultural residue (rice husk or palm kernel shell) is low (less than 80%), but it increases with increasing coal fraction. The combustion efficiencies, from Eq. (1), range between 67% and 75% for burning 100% rice husk, 80–83% for burning 100% palm kernel shell, and 83–88% and 86–92% for burning coal equally mixed with rice husk and palm kernel shell, respectively. The improved carbon combustion efficiency by co-combustion of rice husk with coal can be attributed to an increase in bed temperature (Fig. 3), which is caused by the addition of fixed carbon content in the mixture. This fixed carbon from coal burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conver-
sion of carbon to carbon dioxide as the coal fraction increases and fewer volatiles escape combustion because of the reduced biomass concentration (Suksankraisorn et al., 2003). In addition, increasing the fluidising velocity increases the turbulence in the bed, which leads to improved solid mixing and gas– solid contacting and, in turn, to improved rates of carbon combustion. Consequently, a higher carbon burnout yields a higher carbon combustion efficiency. However, when the combustion is stabilized, increasing the fluidising velocity contributes to a particle elutriation rate greater than the carbon to CO conversion rate, which creates an increase in unburned carbon (Suksankraisorn et al., 2003). This phenomenon can be seen in Fig. 2, which shows that the carbon combustion efficiency is lower than expected for 50% rice husk mixtures when the fluidising velocity increases beyond the optimum value. Furthermore, raising the fluidising velocity also increases the turbulence in the bed burn in a faster rate. Apart from improving solid mixing, increasing the fluidising velocity also influences the fuel particle settling time during the combustion process in the FBC. A higher fluidising velocity drives the lighter fuel particles upwards and into the freeboard region, which is indicated by higher freeboard temperatures. Thus, the settling time for the agricultural residues will be greater, and a significant portion of the combustion will be completed before the particles return to the bed. However, this is also a function of fuel particle size and density. Moreover, more elutriated solids were observed as the fluidising velocity increased, an effect caused either by fine particles in the feed or by a size reduction by attrition in the bed. This verified that a slower settling time led to a lower combustion efficiency, provided that the bed temperature is maintained within the range of 800–900° C. These observations were in good agreement with Patumsawad (2000), who demonstrated a lower combustion efficiency with increasing fine particle content in the fuel. 3.2. Temperature profiles Fig. 3 shows a plot of the axial temperature distributions along the FBC height for fuel studied at 50% excess air. As can be seen from the figure, coal combustion creates a higher bed temperature (y = 0–40 cm) but lower freeboard temperature (y = 45–120 cm) in comparison to agricultural residues. Beyond 120 cm above the distributor plate, all of the temperatures begin to fall, indicating that most of the combustion was completed. This significant combus-
Combustion efficiency (%)
100
80
60
40
20
40 100% coal 100% palm kernel shell 50% palm kernel shell/ 50% coal
Excess air (%)
60
80
100% rice husk 50% rice husk/ 50% coal
Fig. 2. Carbon combustion efficiency during co-combustion of coal with rice husk and palm kernel shell as a function of excess air.
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1000 900
100% coal 100% rice husk 100% Palm Kernel Shell 50% rice husk / 50% coal 50% palm kernel shell/ 50% coal
Temperature (°C)
800 700 600 500 400 300 200 100
Fuel feed point
Bed
0 0
20
40
60
80
100
120
140
160
180
200
Height above distributor plate (cm) Fig. 3. Axial temperature profile for co-combustion of coal with rice husk and palm kernel shell combustion in the case of excess air = 50%.
tion behavior can be explained by the devolatilisation process of the fuel (Sami et al., 2001). With high volatility (more than 50%) and low ignition temperature (250–350 °C), agricultural residues (both rice husk and palm kernel shell) will start to devolatilise upon feeding at 45 cm above the FBC height (freeboard region) and will be mostly burned before it reaches the bed region, while coal with a low volatility (about 30%) and a higher ignition temperature (400–600 °C) will travel down to the bed and complete its combustion in the bed region. This was also greatly influenced by the settling velocity of the fuel particles, which is a function of the fuel particle size and fluidising velocity (Suksankraisorn et al., 2003). These considerations explain why palm kernel shell has a higher bed temperature than rice husk despite having similar volatilities. This was due to the fact that a greater particle size contributes to a greater devolatilisation time and settling time. A significant improvement of carbon combustion efficiencies was noted with the addition of coal to agricultural residues (see Fig. 2). The improvement can be attributed to an increase in bed temperature (Fig. 3), which is caused by the addition of fixed carbon content to the mixture. This fixed carbon from coal burns in the bed while the volatile gas burns in the freeboard region. Thus,
there is more opportunity for fuel conversion from carbon to carbon dioxide because the coal fraction increases and because fewer volatiles tend to escape combustion due to the reduced biomass concentration. This can be further explained by the fact that biomass fuels with lower density compared to coal (approximately half) tend to burn in the freeboard and that coal tends to burn in the bed region. Therefore, the addition of coal to the agricultural residues increases the amount of fixed carbon reaching the bed, resulting in higher bed temperatures. This observation agrees with the results of Abelbha et al. (2003) and Suksankraisorn et al. (2003), who investigated, respectively, the co-firing of coal with chicken litter and the co-firing of lignite with municipal solid waste in a fluidised bed combustor. 3.3. Carbon monoxide (CO) reading To enable the comparison of CO data, all tests were converted to CO emitted at 6% flue gas oxygen. As shown in Fig. 4, the trends observed during mono-combustion are not reflected in co-combustion. It is evident that there are significant fluctuations in CO emissions, between 200 and 900 ppm under the same conditions.
CO concentration ( ppm at 6% O2)
1000 900 800 700 600 500 400 300 200 100 0 20
30
40
50
60
70
80
Excess air percentage (%) 100% coal 50 % rice husk/ 50% coal
100% rice husk 50% palm kernel shell/ 50%coal
100% palm kernel shell
Fig. 4. CO emissions as a function of excess air and rice husk fraction combustion at heat input 10 kW.
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The orders of fluctuation were similar to those observed by Abelbha et al. (2003), Boavida et al. (2003) and Sami et al. (2001). The fluctuations in this experiment are caused by slight variations in feed composition, and this effect is reflected in the temperature profiles. It is noted that the addition of coal has no significant influence on CO emissions during all co-combustion cases, except at coal (50%)/rice husk (50%), where emissions tend to be lower than expected in reference to the other rice husk fractions. This phenomenon is due to the synergistic nature of the coal and rice husk mixture, which enhances the fuel reactivity and lowers the CO emissions (Kuprianov and Pemchart, 2003). In contrast, Leckner et al. (2004) have found that the CO emissions for coal combustion falls with an increasing air supply to the furnace, whereas the highvolatile fuels (wood and sludge) always yield a low CO emissions, as would be expected in a well-designed combustor (sufficient oxygen, temperature and time). They claimed that the higher CO emission related to coal is caused by the additional CO production from char combustion. However, in this study, the emission of CO seems in most cases relatively insensitive to changes in excess air and fluidising velocity. This insensitivity is mainly due to an increase in fuel segregation in the combustor between the feed point and the bed. If the combustor receives a batch with a relatively high amount of fuel, then CO2 is produced, since the pellets need to be heated and dried first. While this occurs, oxygen is not consumed, resulting in high CO emissions. The decrease in CO levels at low percentages of excess air, not below 50%, can be attributed to rapid enhancement and ignition of volatiles from rice husk or palm kernel shell due to both the low excess air and relatively high bed temperatures (about 900 °C). 3.4. Analysis of carryover Table 2 presents the ash collection and unburned carbon analyses during the combustion tests. Generally, the mass balance on
the ash particles accounted for over 90% of the ash input from the fuel. The analyses from all tests demonstrate that using agricultural residues as the sole fuel results in the least amount of unburned carbon detected in the recovered ash. However, the unburned carbon content increased as the mass fraction of coal increased, which verified that some fine particles were elutriated with the fluidising gases as discussed earlier. However, the amount of unburned carbon was quite low, corresponding to approximately less than 5% of the total carbon input. Such observations seem to suggest that the large particle size and lower heating value of the biomass fuel did not adversely affect combustor performance, probably due to the higher volatile content of the biomass fuel. The volatile matter burns rapidly, and the higher volatile content of the biomass can also result in a highly porous char, thus also accelerating the char combustion (Leckner et al., 2004). In addition, in all cases, the amount of unburned carbon in the ash increased as the percentages of coal increased, which is due to the low volatility of coal. The initial particle sizes of the agricultural residues appeared to be insignificant. Moreover, the percentages of unburned carbon in the ash increased to be within the range of 3–30% with increases of the coal fraction. This can be explained by the fact that, as the coal fraction increased, more char combustion and less volatile combustion occurred. Volatile combustion of biomass is relatively higher and faster than char oxidation of the coal particles. Thus, even though the combustion of volatiles was completed, the char particles did not have a residence time long enough for complete combustion. The unburned carbon percentages in total carbon feed, however, contribute only a small percentage (about a 3% difference) to the overall carbon combustion efficiency calculation. Thus, it was observed that the combustion efficiency was still high at the higher coal fraction. Fig. 5 clearly illustrates that the elutriated carbon loss increased as fluidising velocity increased. As a result, a lower carbon combustion efficiency was obtained.
Table 2 Ash balances for single and co-combustion of coal and agricultural residues (rice husk and palm kernel shell) at varies percentage of excess air Fuel
Feed (kg/h)
Superficial velocity (m/s)
Carbon feed (kg/h)
Ash (kg)
Carbon in ash (%)
Efficiency E1 (%)
Coal (100%) Rice husk (100%) Palm kernel shell (100%) Coal (50%): rice husk (50%)
1.20 2.97 1.97 1.60
0.67 0.56 0.59 0.85
0.900 1.038 0.898 1.159
0.039 0.621 0.028 0.196
23.0 14.5 5.0 28.7
90.25 66.62 80.67 83.24
Coal (50%): palm kernel shell (50%)
1.59
0.65
0.962
0.031
14.9
89.86
0.08
Carbon loss elutriated (kg/h)
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
Fluidising velocity (m/s) Fig. 5. The influence of fluidising velocity on carbon loss elutriated during co-combustion runs for all coal/biomass samples.
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0.08
Carbon loss elutriated (kg/h)
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 790
810
830
850
870
890
910
Bed temperature (C) Fig. 6. The influence of bed temperature on carbon loss elutriated during co-combustion runs for all coal/biomass samples.
Table 3 Ash composition of coal, rice husk and palm kernel shell Compound
Coal
Agricultural residues
SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO Na2O K2O
3.411 1.786 1.318 0.073 0.045 0.385 0.066 0.134 0.108
89.57 1.32 2.95 7.56 1.04 0.77 0.76 1.15 1.65
Cl
0.09
1.30
However, it was found that the bed temperature has no strong influence on carbon loss during the tests. The lower carbon losses were determined at a higher bed temperature. For example, higher unburned carbon occurred in the combustion of coal/rice husk and coal/palm kernel shell, although their bed temperatures were similar, as illustrated in Fig. 6. Again, as explained earlier, this unburned carbon contributed only a small percentage to the overall carbon combustion efficiency. Ash composition for the agricultural residues is fundamentally different from ash composition for coal. As can be seen in Table 3, the fuel alkalinity is significant, as the amount of sodium and potassium content were observed to be highest in the ash from agricultural residues in comparison with the ash from coal. Of equal significance to the alkalinity itself is the reactivity of that alkalinity. Noticeably, a significantly high percentage of silicate content in agricultural residues could pose problems, such as bed agglomeration or ash deposition during fluidised bed combustion (Baxter et al., 1998). However, none of these phenomena occurred during all of the combustion runs. This is primarily due to fact that the bed and freeboard temperatures were lower than the ash fusion temperature. Furthermore, almost no bed ashes have been found during the experiments. This might be suggested by the fact that all the carryover was elutriated to the cyclone and that the char combustion was completed during the runs. 4. Conclusion The combustion of agricultural residues in an existing coal-fired boiler was evaluated in terms of combustion efficiency. Generally,
the experimental results gave combustion efficiencies of 60–80% and 80–83% for the mono-combustion of rice husk and palm kernel shell, respectively. However, addition of a 50% mass fraction of coal could increase the carbon combustion efficiency up to 20% with an acceptable CO emission limit (less than 2500 ppm). Generally, the factors that influence combustion efficiency are (1) the loss of carbon in the elutriated solids, and (2) the loss of carbon as CO due to incomplete combustion. The operating parameters such as percentage of excess air, fluidising velocity and bed temperatures played an important role in the co-combustion process. Moreover, burning agricultural residue was found to be different from burning coal due to the difference in properties (i.e., particle size, nature); the differences are clearly shown in the temperature profiles and their ash content. However, from this study, the existing coal-fired fluidised bed boiler was found to be capable of burning agricultural residues with minimum modifications, such as air requirement and fluidising velocity. This is important in order to achieve high combustion efficiency during the operations. Acknowledgements This work was performed within the Department of Chemical and Process Engineering, University of Sheffield. The research project was financially supported by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia. Special acknowledgement goes to Dr. Khudzir Ismail from the Universiti Teknologi MARA for the supply of the raw materials. References Abelbha, P., Gulyurthu, I., Boavida, D., Seabra Barros, J., Cabrita, I., Leahy, J., Kelleher, B., Leahy, M., 2003. Combustion of poultry litter in fluidised bed combustor. Fuel 82, 687–692. Baxter, L.L., Miles, T.R., Miles Jr., T.R., Jenkins, B.M., Milne, T., Dayton, D., Bryers, R.W., Oden, L.L., 1998. The behavior of inorganic material in biomass-fired power plants boilers:field and laboratory experiences. Fuel processing Technology 54, 47–78. Bhattacharya, S.C., Wu, W., 1989. Fluidised bed combustion of rice husk for disposal and energy recovery. Energy from Biomass and Wastes XII, 591–601. BIOGEN, 2004. BIOGEN information Sheet, November, 2004. Boavida, D., Abelha, P., Gulyurtlu, I., Cabrita, I., 2003. Co-combustion of coal and non-recyclable paper and plastic waste in a fluidized bed reactor. Fuel 82, 1931–1938. Hammerschmidt, A., 1999. Technical constraints of co-combustion – experiences in Sweden. In: Proceedings of the Workshop on Technical Constrains in Austria. Kallner, P., 1999. Technical constraints of co-combustion – experiences in Sweden. In: Proceedings of the Workshop on Technical Constrains in Austria.
W.A.W.A.K. Ghani et al. / Waste Management 29 (2009) 767–773 Kuprianov, V.I., Pemchart, W., 2003. Emissions from a conical FBC fired with a biomass fuel. Applied Energy 74, 383–392. Leckner, B., Amand, L.-E., Lucke, K., Wether, J., 2004. Gaseous emissions from cocombustion of sewage sludge and coal/wood in a fluidized bed. Fuel 83, 477– 486. Natrajan, R., Nordin, A., Rao, A.N., 1998. Overview of combustion and gasification of rice husk in fluidised bed reactors. Biomass and Bioenergy 14, 533–546. Patumsawad, S., 2000. Co-combustion of high moisture content MSW with coal in a fluidized bed combustor. PhD thesis, The University of Sheffield. Rosch, C., Kaltschmitt, M., 1999. Energy from biomass -do non-technical barriers prevent an increased use? Biomass and Bioenergy 16 (5), 347–356.
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Sami, M., Annamalai, K., Woodridge, M., 2001. Co-combustion of coal and biomass fuel blends. Progress Energy Combustion Science 27, 171–214. Saloski, P., 1999. Technical constraints of co-combustion – experiences in Finland. In: Proceedings of the Workshop on Technical Constrains in Austria. Saxena, S.C., Jotshi, C.K., 1994. Fluidized bed incineration of waste materials. Progress in Energy and Combustion Science 20, 281. Suksankraisorn, K., Patumsawad, S., Fungtammasan, B., 2003. Combustion studies of high moisture content waste in a fluidised bed. Waste Management 23, 433–439. Tillman, D.A., 2000. Biomass cofiring: the technology, the experience, the combustion consequences. Biomass and Bioenergy 19 (6), 365–384.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Co-combustion of agricultural residues with coal in a fluidised bed combustor W.A.W.A.K. Ghani a,*, A.B. Alias b, R.M. Savory b, K.R. Cliffe c a b c
University Putra Malaysia, Department of Chemical and Environmental Engineering, 43400 UPM, Serdang, Malaysia Universiti Teknologi MARA, Faculty of Chemical Engineering, 40450 Shah Alam, Selangor, Malaysia University of Sheffield, Department of Chemical and Process Engineering, Mappin Street, S1 3JD Sheffield, United Kingdom
a r t i c l e
i n f o
Article history: Accepted 24 March 2008 Available online 9 July 2008
a b s t r a c t Power generation from biomass is an attractive technology that utilizes agricultural residual waste. In order to explain the behavior of biomass-fired fluidised bed incinerator, biomass sources from agricultural residues (rice husk and palm kernel) were co-fired with coal in a 0.15 m diameter and 2.3 m high fluidised bed combustor. The combustion efficiency and carbon monoxide emissions were studied and compared with those for pure coal combustion. Co-combustion of a mixture of biomass with coal in a fluidised bed combustor designed for coal combustion increased combustion efficiency up to 20% depending upon excess air levels. Observed carbon monoxide levels fluctuated between 200 and 900 ppm with the addition of coal. It is evident from this research that efficient co-firing of biomass with coal can be achieved with minimal modifications to existing coal-fired boilers. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Waste-to-energy is gaining increasing attention as landfill costs and environmental concerns rise in many developed countries, including Malaysia. A recent study shows that biomass in Malaysia contributes about 14% of the energy used every year—approximately 340 million barrels of oil equivalent (boe). Paddies and palm plantations are among the main contributors of waste to the biomass energy utilization in Malaysia. The energy potential from these two resources is 16.4 and 229 picojoules, respectively, based on the production of 2194 thousand tonnes and 60,000 thousand tonnes, respectively, in 2000 (BIOGEN, 2004). This implies that an electric energy potential around 375 MW can be generated. Combustion of agricultural residues is commonly used in industries for energy recovery. However, many researchers have found difficulty in achieving high efficiency with stand-alone biomass firing (Natrajan et al., 1998; Bhattacharya and Wu, 1989). Thus, cofiring biomass with coal in industrial and utility boilers could offer an alternative approach with improved combustion efficiency, lower-cost and reduced-risk technology. Significant co-combustion potential for biomass and waste materials exists in all European Union (EU) countries, and such potential is mirrored on a worldwide basis, creating a significant market for equipment and services. For instance, in Finland, large quantities of biomass from forest industries are used as the main fuel in grate-firing, bubbling fluidised bed combustors (BFBC) or circulating fluidised bed (CFBC) combustors within the range of 5–20 MWth (Saloski, 1999). In
* Corresponding author. Tel.: +60 3 89466287; fax: +60 3 86567120. E-mail addresses: [email protected] (W.A.W.A.K. Ghani), k.cliffe@sheffield.ac.uk (K.R. Cliffe). 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.03.025
Sweden, forest residues, sawdust, demolition wood and other waste wood, fibre and paper sludge are commonly used together with a smaller portion of coal or oil (15–30%) in district heating or combined heat power (CHP) plants using a variety of combustion technologies (grate-firing, BFBC, CFBC and pulverised combustion (PC)) (Kallner, 1999). Furthermore, in Austria, co-combustion is used by small industrial boilers located mainly in the pulp and paper industry, which generally use their own biomass wastes (e.g., black liquor, bark) (Hammerschmidt, 1999). In The Netherlands, waste wood is the main supplementary biomass feedstock used in coal-fired PC power plants. In Germany, sewage sludge is the most important co-fired biomass in lignite or coal-fired PC power plants (Rosch and Kaltschmitt, 1999). Fluidised bed combustion (FBC) has been shown to be a versatile technology capable of burning practically any waste combination with low emissions (Tillman, 2000). The significant advantages of fluidised bed combustors over conventional combustors include their compact furnaces, simple designs, effective combustion of a wide variety of fuels, relatively uniform temperatures, and ability to reduce emissions of nitrogen oxide and sulphur dioxide gases (Saxena and Jotshi, 1994). This research was performed with the objective of determining the combustion efficiency of an existing coal-fired combustor while co-firing with agricultural residue. The combustion efficiency calculation is primarily based on the conversion of carbon into carbon monoxide (CO) and carbon dioxide (CO2) emissions. Other gases, such as nitrogen oxides (NOx) and sulphur dioxides (SO2), are neglected. It is assumed that the presence of NOx and SO2 is dominated by the coal fraction in the mixture. Furthermore, this research aims to demonstrate the technical feasibility of a fluidised bed as a clean technology for burning agricultural residues.
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In addition, the effects of biomass properties (such as particle size, particle density and volatility) and the influences of operating parameters (e.g., the amount of excess air, the effect of fluidising velocity on axial temperature profile, the combustion efficiencies, and the CO emissions) are also investigated. 2. Experimental 2.1. Fuel characterization In this study, British coal, agricultural residues (rice husk and palm kernel shell) originating from Perlis were employed as fuel. These fuels were open-air dried for 2–3 days to remove moisture. The proximate and ultimate analyses performed on coal and rice husk are summarized in Table 1. Table 1 shows that the main characteristics of these dry agricultural residues, in comparison with coal, are low calorific value (14–18 MJ/kg), high-volatile matter content (60–75%), ash content (10–25%), low carbon content (35– 45%) and, most importantly, high oxygen content (27–40%). This is of particular importance since it influences the stoichiometric air requirement for combustion. It is also important to note that the residue samples have a lower particle density and vary in size and shape much more than coal. 2.2. Experimental apparatus and operating procedures Fig. 1 shows a schematic diagram of the Atmospheric Fluidised Bed combustor used in this investigation. The system has a 0.15 m diameter and 2.3 m-high combustion chamber; it allows for bed depths of up to 0.3 m using 850 lm silica sand (Geldart Type A); and it includes a cyclone, a screw feeder and a gas analyzer. The combustor body is constructed from 1 cm-thick 306 stainless steel and covered in Kaowool insulation to prevent excessive heat loss during operation. Fluidising air was introduced at the base of the bed through a nozzle distributor and provided air for both fluidisation and combustion. Startup of the bed was achieved using an inbed technique: propane was introduced directly into the distributor plate by injectors and mixed with air in the nozzles, providing a combustible mixture at the nozzle exit. Bed and freeboard temperatures were measured at eight different heights above the distributor plate by means of sheathed type K Ni/Cr–Ni thermocouples (TC). Fuel was fed pneumatically into the bed surface from a sealed hopper through an inclined feeding pipe, the flowrate through which was controlled by a screw feeder. Normally, some of the less dense biomass should be fed in the bed; however, since one entry port was desirable, the feed should be premixed before being fed into the combustor via the entry port. The main objectives of the research are to identify the biomass fuels that could be co-fired
Fig. 1. A schematic diagram of the laboratory scale fluidised bed combustor (Tc, thermocouple).
with coal with overbed feeding and result in high efficiency. A cyclone was fitted to the combustor exit, and the carryover from the bed was collected for analysis. CO and O2 were measured using a Xentra 4904 B1 continuous emissions analyzer, and CO2 was measured using a non-dispersive infrared absorption spectrometry analyser. A fly ash sample was collected from the catch-pot after finishing the combustion run. The fly ash sample was then weighed and analysed to determine the total amount of unburned carbon in the test fuels. The percentage of combustion efficiency was computed using the following relation:
Table 1 Coal and biomass characterization Fuel
British coal
Rice husk
Palm kernel shell
Proximate analysis (% wt, dry basis) Fixed carbon Volatile matter Ash
58.90 38.20 2.90
15.00 60.70 24.30
18.60 72.50 8.90
Ultimate analysis (% wt, dry basis) Carbon Hydrogen Nitrogen Sulphur Oxygen
80.10 5.30 0.90 0.70 13.00
36.20 5.71 0.10 0.00 57.99
49.5 6.74 1.85 0.00 41.91
Calorific value (MJ/kg) Particle size (mm)
31.1 1.4–4.8 mm
13.5 0.8 mm diameter 1.00 mm length (cylindrical shape)
18.0 3 mm wide 6 mm length (rectangle)
Particle density (kg/m3)
1200
98
435
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E1 ð%Þ ¼ ðB þ unburned carbon in ashÞ=C 100%
ð1Þ
where B and C are, respectively, the mass fractions of burnt and total carbon in the fuel. This efficiency calculation procedure is based on the premise that the flue gas composition, which is burned carbon in the feed, is completely converted to carbon monoxide and carbon dioxide. As unburned carbon could be elutriated, a more accurate combustion efficiency taking this into account was calculated using Eq. (1). Based on the values of combustion efficiency from experiments where duplicate runs were conducted under almost identical conditions, the combustion efficiency values should be within ±2%. 3. Results and discussion This section describes the combustion of agricultural residue in a fluidised bed combustor. The influences of fuel properties (such as particle size, particle density and volatility) and the influences of operating parameters (e.g., the amount of excess air, the effect of fluidising velocity on axial temperature profile, the combustion efficiencies, and the CO emissions) are discussed. 3.1. Carbon combustion efficiencies The combustion of agricultural residues was evaluated in terms of combustion efficiency, which was calculated based on Eq. (1). The combustion tests were performed using different coal mass fractions (0%, 50% and 100%) corresponding to a heat input of 10 kW under optimum excess air conditions. Fig. 2 shows the effect of the different mixtures of rice husk and palm kernel shell with coal on carbon combustion efficiency with the same heat input. Generally, carbon combustion efficiency for a single agricultural residue (rice husk or palm kernel shell) is low (less than 80%), but it increases with increasing coal fraction. The combustion efficiencies, from Eq. (1), range between 67% and 75% for burning 100% rice husk, 80–83% for burning 100% palm kernel shell, and 83–88% and 86–92% for burning coal equally mixed with rice husk and palm kernel shell, respectively. The improved carbon combustion efficiency by co-combustion of rice husk with coal can be attributed to an increase in bed temperature (Fig. 3), which is caused by the addition of fixed carbon content in the mixture. This fixed carbon from coal burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conver-
sion of carbon to carbon dioxide as the coal fraction increases and fewer volatiles escape combustion because of the reduced biomass concentration (Suksankraisorn et al., 2003). In addition, increasing the fluidising velocity increases the turbulence in the bed, which leads to improved solid mixing and gas– solid contacting and, in turn, to improved rates of carbon combustion. Consequently, a higher carbon burnout yields a higher carbon combustion efficiency. However, when the combustion is stabilized, increasing the fluidising velocity contributes to a particle elutriation rate greater than the carbon to CO conversion rate, which creates an increase in unburned carbon (Suksankraisorn et al., 2003). This phenomenon can be seen in Fig. 2, which shows that the carbon combustion efficiency is lower than expected for 50% rice husk mixtures when the fluidising velocity increases beyond the optimum value. Furthermore, raising the fluidising velocity also increases the turbulence in the bed burn in a faster rate. Apart from improving solid mixing, increasing the fluidising velocity also influences the fuel particle settling time during the combustion process in the FBC. A higher fluidising velocity drives the lighter fuel particles upwards and into the freeboard region, which is indicated by higher freeboard temperatures. Thus, the settling time for the agricultural residues will be greater, and a significant portion of the combustion will be completed before the particles return to the bed. However, this is also a function of fuel particle size and density. Moreover, more elutriated solids were observed as the fluidising velocity increased, an effect caused either by fine particles in the feed or by a size reduction by attrition in the bed. This verified that a slower settling time led to a lower combustion efficiency, provided that the bed temperature is maintained within the range of 800–900° C. These observations were in good agreement with Patumsawad (2000), who demonstrated a lower combustion efficiency with increasing fine particle content in the fuel. 3.2. Temperature profiles Fig. 3 shows a plot of the axial temperature distributions along the FBC height for fuel studied at 50% excess air. As can be seen from the figure, coal combustion creates a higher bed temperature (y = 0–40 cm) but lower freeboard temperature (y = 45–120 cm) in comparison to agricultural residues. Beyond 120 cm above the distributor plate, all of the temperatures begin to fall, indicating that most of the combustion was completed. This significant combus-
Combustion efficiency (%)
100
80
60
40
20
40 100% coal 100% palm kernel shell 50% palm kernel shell/ 50% coal
Excess air (%)
60
80
100% rice husk 50% rice husk/ 50% coal
Fig. 2. Carbon combustion efficiency during co-combustion of coal with rice husk and palm kernel shell as a function of excess air.
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1000 900
100% coal 100% rice husk 100% Palm Kernel Shell 50% rice husk / 50% coal 50% palm kernel shell/ 50% coal
Temperature (°C)
800 700 600 500 400 300 200 100
Fuel feed point
Bed
0 0
20
40
60
80
100
120
140
160
180
200
Height above distributor plate (cm) Fig. 3. Axial temperature profile for co-combustion of coal with rice husk and palm kernel shell combustion in the case of excess air = 50%.
tion behavior can be explained by the devolatilisation process of the fuel (Sami et al., 2001). With high volatility (more than 50%) and low ignition temperature (250–350 °C), agricultural residues (both rice husk and palm kernel shell) will start to devolatilise upon feeding at 45 cm above the FBC height (freeboard region) and will be mostly burned before it reaches the bed region, while coal with a low volatility (about 30%) and a higher ignition temperature (400–600 °C) will travel down to the bed and complete its combustion in the bed region. This was also greatly influenced by the settling velocity of the fuel particles, which is a function of the fuel particle size and fluidising velocity (Suksankraisorn et al., 2003). These considerations explain why palm kernel shell has a higher bed temperature than rice husk despite having similar volatilities. This was due to the fact that a greater particle size contributes to a greater devolatilisation time and settling time. A significant improvement of carbon combustion efficiencies was noted with the addition of coal to agricultural residues (see Fig. 2). The improvement can be attributed to an increase in bed temperature (Fig. 3), which is caused by the addition of fixed carbon content to the mixture. This fixed carbon from coal burns in the bed while the volatile gas burns in the freeboard region. Thus,
there is more opportunity for fuel conversion from carbon to carbon dioxide because the coal fraction increases and because fewer volatiles tend to escape combustion due to the reduced biomass concentration. This can be further explained by the fact that biomass fuels with lower density compared to coal (approximately half) tend to burn in the freeboard and that coal tends to burn in the bed region. Therefore, the addition of coal to the agricultural residues increases the amount of fixed carbon reaching the bed, resulting in higher bed temperatures. This observation agrees with the results of Abelbha et al. (2003) and Suksankraisorn et al. (2003), who investigated, respectively, the co-firing of coal with chicken litter and the co-firing of lignite with municipal solid waste in a fluidised bed combustor. 3.3. Carbon monoxide (CO) reading To enable the comparison of CO data, all tests were converted to CO emitted at 6% flue gas oxygen. As shown in Fig. 4, the trends observed during mono-combustion are not reflected in co-combustion. It is evident that there are significant fluctuations in CO emissions, between 200 and 900 ppm under the same conditions.
CO concentration ( ppm at 6% O2)
1000 900 800 700 600 500 400 300 200 100 0 20
30
40
50
60
70
80
Excess air percentage (%) 100% coal 50 % rice husk/ 50% coal
100% rice husk 50% palm kernel shell/ 50%coal
100% palm kernel shell
Fig. 4. CO emissions as a function of excess air and rice husk fraction combustion at heat input 10 kW.
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The orders of fluctuation were similar to those observed by Abelbha et al. (2003), Boavida et al. (2003) and Sami et al. (2001). The fluctuations in this experiment are caused by slight variations in feed composition, and this effect is reflected in the temperature profiles. It is noted that the addition of coal has no significant influence on CO emissions during all co-combustion cases, except at coal (50%)/rice husk (50%), where emissions tend to be lower than expected in reference to the other rice husk fractions. This phenomenon is due to the synergistic nature of the coal and rice husk mixture, which enhances the fuel reactivity and lowers the CO emissions (Kuprianov and Pemchart, 2003). In contrast, Leckner et al. (2004) have found that the CO emissions for coal combustion falls with an increasing air supply to the furnace, whereas the highvolatile fuels (wood and sludge) always yield a low CO emissions, as would be expected in a well-designed combustor (sufficient oxygen, temperature and time). They claimed that the higher CO emission related to coal is caused by the additional CO production from char combustion. However, in this study, the emission of CO seems in most cases relatively insensitive to changes in excess air and fluidising velocity. This insensitivity is mainly due to an increase in fuel segregation in the combustor between the feed point and the bed. If the combustor receives a batch with a relatively high amount of fuel, then CO2 is produced, since the pellets need to be heated and dried first. While this occurs, oxygen is not consumed, resulting in high CO emissions. The decrease in CO levels at low percentages of excess air, not below 50%, can be attributed to rapid enhancement and ignition of volatiles from rice husk or palm kernel shell due to both the low excess air and relatively high bed temperatures (about 900 °C). 3.4. Analysis of carryover Table 2 presents the ash collection and unburned carbon analyses during the combustion tests. Generally, the mass balance on
the ash particles accounted for over 90% of the ash input from the fuel. The analyses from all tests demonstrate that using agricultural residues as the sole fuel results in the least amount of unburned carbon detected in the recovered ash. However, the unburned carbon content increased as the mass fraction of coal increased, which verified that some fine particles were elutriated with the fluidising gases as discussed earlier. However, the amount of unburned carbon was quite low, corresponding to approximately less than 5% of the total carbon input. Such observations seem to suggest that the large particle size and lower heating value of the biomass fuel did not adversely affect combustor performance, probably due to the higher volatile content of the biomass fuel. The volatile matter burns rapidly, and the higher volatile content of the biomass can also result in a highly porous char, thus also accelerating the char combustion (Leckner et al., 2004). In addition, in all cases, the amount of unburned carbon in the ash increased as the percentages of coal increased, which is due to the low volatility of coal. The initial particle sizes of the agricultural residues appeared to be insignificant. Moreover, the percentages of unburned carbon in the ash increased to be within the range of 3–30% with increases of the coal fraction. This can be explained by the fact that, as the coal fraction increased, more char combustion and less volatile combustion occurred. Volatile combustion of biomass is relatively higher and faster than char oxidation of the coal particles. Thus, even though the combustion of volatiles was completed, the char particles did not have a residence time long enough for complete combustion. The unburned carbon percentages in total carbon feed, however, contribute only a small percentage (about a 3% difference) to the overall carbon combustion efficiency calculation. Thus, it was observed that the combustion efficiency was still high at the higher coal fraction. Fig. 5 clearly illustrates that the elutriated carbon loss increased as fluidising velocity increased. As a result, a lower carbon combustion efficiency was obtained.
Table 2 Ash balances for single and co-combustion of coal and agricultural residues (rice husk and palm kernel shell) at varies percentage of excess air Fuel
Feed (kg/h)
Superficial velocity (m/s)
Carbon feed (kg/h)
Ash (kg)
Carbon in ash (%)
Efficiency E1 (%)
Coal (100%) Rice husk (100%) Palm kernel shell (100%) Coal (50%): rice husk (50%)
1.20 2.97 1.97 1.60
0.67 0.56 0.59 0.85
0.900 1.038 0.898 1.159
0.039 0.621 0.028 0.196
23.0 14.5 5.0 28.7
90.25 66.62 80.67 83.24
Coal (50%): palm kernel shell (50%)
1.59
0.65
0.962
0.031
14.9
89.86
0.08
Carbon loss elutriated (kg/h)
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
Fluidising velocity (m/s) Fig. 5. The influence of fluidising velocity on carbon loss elutriated during co-combustion runs for all coal/biomass samples.
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0.08
Carbon loss elutriated (kg/h)
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 790
810
830
850
870
890
910
Bed temperature (C) Fig. 6. The influence of bed temperature on carbon loss elutriated during co-combustion runs for all coal/biomass samples.
Table 3 Ash composition of coal, rice husk and palm kernel shell Compound
Coal
Agricultural residues
SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO Na2O K2O
3.411 1.786 1.318 0.073 0.045 0.385 0.066 0.134 0.108
89.57 1.32 2.95 7.56 1.04 0.77 0.76 1.15 1.65
Cl
0.09
1.30
However, it was found that the bed temperature has no strong influence on carbon loss during the tests. The lower carbon losses were determined at a higher bed temperature. For example, higher unburned carbon occurred in the combustion of coal/rice husk and coal/palm kernel shell, although their bed temperatures were similar, as illustrated in Fig. 6. Again, as explained earlier, this unburned carbon contributed only a small percentage to the overall carbon combustion efficiency. Ash composition for the agricultural residues is fundamentally different from ash composition for coal. As can be seen in Table 3, the fuel alkalinity is significant, as the amount of sodium and potassium content were observed to be highest in the ash from agricultural residues in comparison with the ash from coal. Of equal significance to the alkalinity itself is the reactivity of that alkalinity. Noticeably, a significantly high percentage of silicate content in agricultural residues could pose problems, such as bed agglomeration or ash deposition during fluidised bed combustion (Baxter et al., 1998). However, none of these phenomena occurred during all of the combustion runs. This is primarily due to fact that the bed and freeboard temperatures were lower than the ash fusion temperature. Furthermore, almost no bed ashes have been found during the experiments. This might be suggested by the fact that all the carryover was elutriated to the cyclone and that the char combustion was completed during the runs. 4. Conclusion The combustion of agricultural residues in an existing coal-fired boiler was evaluated in terms of combustion efficiency. Generally,
the experimental results gave combustion efficiencies of 60–80% and 80–83% for the mono-combustion of rice husk and palm kernel shell, respectively. However, addition of a 50% mass fraction of coal could increase the carbon combustion efficiency up to 20% with an acceptable CO emission limit (less than 2500 ppm). Generally, the factors that influence combustion efficiency are (1) the loss of carbon in the elutriated solids, and (2) the loss of carbon as CO due to incomplete combustion. The operating parameters such as percentage of excess air, fluidising velocity and bed temperatures played an important role in the co-combustion process. Moreover, burning agricultural residue was found to be different from burning coal due to the difference in properties (i.e., particle size, nature); the differences are clearly shown in the temperature profiles and their ash content. However, from this study, the existing coal-fired fluidised bed boiler was found to be capable of burning agricultural residues with minimum modifications, such as air requirement and fluidising velocity. This is important in order to achieve high combustion efficiency during the operations. Acknowledgements This work was performed within the Department of Chemical and Process Engineering, University of Sheffield. The research project was financially supported by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia. Special acknowledgement goes to Dr. Khudzir Ismail from the Universiti Teknologi MARA for the supply of the raw materials. References Abelbha, P., Gulyurthu, I., Boavida, D., Seabra Barros, J., Cabrita, I., Leahy, J., Kelleher, B., Leahy, M., 2003. Combustion of poultry litter in fluidised bed combustor. Fuel 82, 687–692. Baxter, L.L., Miles, T.R., Miles Jr., T.R., Jenkins, B.M., Milne, T., Dayton, D., Bryers, R.W., Oden, L.L., 1998. The behavior of inorganic material in biomass-fired power plants boilers:field and laboratory experiences. Fuel processing Technology 54, 47–78. Bhattacharya, S.C., Wu, W., 1989. Fluidised bed combustion of rice husk for disposal and energy recovery. Energy from Biomass and Wastes XII, 591–601. BIOGEN, 2004. BIOGEN information Sheet, November, 2004. Boavida, D., Abelha, P., Gulyurtlu, I., Cabrita, I., 2003. Co-combustion of coal and non-recyclable paper and plastic waste in a fluidized bed reactor. Fuel 82, 1931–1938. Hammerschmidt, A., 1999. Technical constraints of co-combustion – experiences in Sweden. In: Proceedings of the Workshop on Technical Constrains in Austria. Kallner, P., 1999. Technical constraints of co-combustion – experiences in Sweden. In: Proceedings of the Workshop on Technical Constrains in Austria.
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