Energy Fuels 2010, 24, 2146–2153 Published on Web 03/04/2010
: DOI:10.1021/ef901503e
Co-combustion of Agricultural Residues with Coal: Turning Waste into Energy S. Munir,† W. Nimmo,* and B. M. Gibbs Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, U.K. Received December 9, 2009. Revised Manuscript Received February 10, 2010
Agricultural residues are abundantly available in energy deficient developing countries. Many of the agricultural residues are considered as waste, and their energy potential has not been realized in energy recovery schemes. Coutilization of agricultural residues in existing coal fired power plants can help in producing clean energy, disposing of waste, and increasing the income of the rural population. Sheameal, cotton stalk, sugar cane bagasse, and wood chips were cofired with coal in 5, 10, and 15% thermal fractions in a 20 kW down-fired combustor. The combustion behavior of the blends of biomass-coal was examined. It was found that agricultural residues can be used as a potential substitute fuel and can help to control the emission of NOx and SO2.
the same period, the world average per capita electricity consumption was about 2659 kWh, almost 6 times larger than that of Pakistan.1 In 2008, Pakistan was facing an electricity deficit of over 4500 MW, some 40% of the total demand. This deficit could reach over 8000 MW by 2010.5 Electricity demand in Pakistan will increase in the range of 12-17 million tonnes of equivalent oil (MTOE) by the year 2018, at an average growth rate of about 5-7% and will require installed capacity of about 35-50 GW.6 Only 55% of Pakistan’s population has access to electricity. At present, the population is facing severe load shedding/blackout problems due to shortage of about a 3 GW power supply. Gas and oil have 65% share in conventional electricity generation. Indigenous reserves of oil and gas are limited, and the country heavily depends on imported oil. The oil import bill is a serious strain on the country’s economy.7 Pakistan must develop indigenous, environmentally friendly energy resources to meet its future electricity needs. Total coal reserves of Pakistan are estimated to be around 187 billion tonnes.8 Pakistan can overcome this energy crisis by coutilizing its unused agricultural residues and coal reserves. This strategy can solve the energy crises while producing clean energy, disposing of waste, and increasing income of the rural population. Pakistan’s 68% population live in villages and rely on agriculture for their sustenance. The crop residue has a theoretical energy potential of about 35 MTOE. Projections of energy potential of crop residues in Pakistan are given in Table 1.
1. Introduction The biggest share of electricity (66.9%) is still generated by fossil fuels. Worldwide, about 41% of electricity is produced using coal.1 Fossil fuels in general and coal in particular are major sources of pollutant emissions. Probably the fastest and easiest way to replace large amounts of fossil fuel based electricity by sustainable electricity is to replace the combusted fossil fuels by biomass.2 Agricultural residues are a form of biomass that is renewable but largely not utilized in the energy recovery schemes. They are nonedible plant parts that are left in the field after harvest, and often land-fill is the main disposal route but with ramifications, including CH4 release having 21 times greater global warming potential per molecule than CO2.3 The amount of crop residue produced in the world is estimated at 2802 106 t/year for cereal crops, 3107 106 t/year for 17 cereals and legumes, and 3758 106 t/year for 27 food crops. The fuel value of the total annual residue produced is estimated at 11.3 1015 kcal, about 7.5 billion bbl of diesel for the world.4 Many of the agricultural residues are considered as waste and often burnt in the fields or land-filled. Cofiring of these abundantly available agricultural residues with coal can convert a negative value biomass into a positive fuel along with environmental relief. If only 5% of coal energy could be replaced by biomass in all coal-fired power plants, this would result in an emission reduction of around 300 Mt of CO2/year.2 Many developing countries like Pakistan, India, Ghana, and Nigeria are located in climate regions where large amounts of residues are available. Co-combustion of agricultural residues in energy recovery schemes could significantly increase the income of the people in these countries.2 Pakistan is an energy deficient country. The per capita electricity consumption was 480 kWh in 2007-2008. Over
(5) Asif, M. Renewable Sustainable Energy Rev. 2009, 13 (4), 903–909. (6) Uqaili, M. A.; Harijan, K.; Memon, M. D. Prospects of Renewable Energy for Meeting Growing Electricity Demand in Pakistan. Renewable Energy for Sustainable Development in the Asia Pacific Region, Fremantle, Western Australia, February 4-8, 2007; Jennings, P., Ho, G., Mathew, K., Nayar, C. V., Eds. American Institute of Physics Conference Proceedings, Vol. 941; American Institute of Physics: Melville, NY, 2007; pp 53-61. (7) Harijan, K.; Uqaili, M. A.; Memon, M. D. In Renewable Energy for Managing Energy Crisis in Pakistan, Communications in Computer and Information Science 20, Wirless Networks, Information Processings and Systems, Jamshoro, Pakistan, April 11-12, 2008; Hussain, D. M. A., Ed.; Springer-Verlag: Berlin, Heidelberg, Germany, 2008; pp 449-455. (8) Muneer, T.; Asif, M. Renewable Sustainable Energy Rev. 2007, 11 (4), 654–671.
*To whom correspondence should be addressed. Telephone: 01133432513. Fax: 01132467310. E-mail:
[email protected]. † On leave from the Institute of Chemical Engineering and Technology, University of the Punjab, Lahore, Pakistan. (1) IEA. Key Word Energy Statistics; International Energy Agency: Paris, France, 2008; p 24, 51, 57. (2) Task 32, IEA. Technical Status of Biomass Co-firing; 2009; p 4, 7, 10. (3) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27 (2), 171–214. (4) Lal, R. Environ. Int. 2005, 31 (4), 575–584. r 2010 American Chemical Society
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: DOI:10.1021/ef901503e
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Table 1. Projection of Energy Potential from Crop Residuesa
a
year
TEP (MTOE)
2005 2010 2015 2020 2025 2030
35.5 38.2 41.1 44.3 47.7 51.5
The source is ref 9.
Similar kinds of scenario exist in African countries located in regions facing energy deficiency and having agricultural based economies. Sugar cane and cotton residue have shares of 29%, 18.7% in residue production. The current paper discusses the combustion performance and gaseous pollutant emissions of co-combustion of a combination of coal and different types of agricultural residues when cofired in a 20 kW down-fired combustor. Biomass-coal blends with a biomass blending ratio (BBR) of 5, 10, and 15% on a thermal basis were used in this study. 2. Experimental Section 2.1. Experimental Setup. The experimental setup consists of a 20 kW down fired combustor, a Rospen coal feeder, a Dowson biomass feeder, air and gas supply systems, calibration setup, gas measuring analytical equipment, gas cylinder manifolds, water cooled sample probes, char sample collection quenching system, thermocouples, data logger, and PC. The overall height of the combustor is 3.5 m (Figure 1). It comprises of nine bolted cylindrical sections of varying lengths. The top two sections of the combustor are lined with ceramic fiber tube grade 1600, and the remaining sections are lined with grade 1400. There are over 40 utility ports available, distributed radially along the length of the furnace. This feature has made the system very gas/solid sampling. Primary air, secondary air, was metered through KDG 2000 rota meters. The combustor was heated up for 3 h with natural gas before shifting to coal for every run. After 1 h of operation with coal firing, steady state temperatures were achieved throughout the combustor. The biomass was fed by a specially designed, metered, and calibrated Dowson programmable feeder on a homogenizer vibratory tray of a Rospen loss-in-weight feeder. The coal and biomass were mixed on the vibratory tray of the Rospen feeder and transported to the burner by the primary combustion air flow. This technique promotes even distribution of fuel in the feed system and helps to give stable burner operation. Both the feeders were calibrated for each sample corresponding to their thermal input mass flow rate required. Gas samples were drawn through stainless steel water-cooled probes from any of the available ports. All the gas samples were dried and filtered before entering individual online analyzers. The SO2 sampling system consists of a fixed sample extraction assembly at the exit with built-in filters and an electrically heated line with temperature controllers at both ends to maintain a temperature of 160 °C from the furnace exit to the analyzers. Instruments were calibrated before each run with certificated BOC special gas mixtures. Oxygen was measured using a Servomex paramagnetic analyzer 570A; CO, CO2 by NDIR analyzers (Analytical Development Company; ABB Easyline IR CO2 analyzer); SO2 by Signal Ltd. series 7000GFC analyzer; and NO and NOx by a chemiluminescence analyzer (Signal Ltd. series 440). Each gas analyzer and R-type sheathed thermocouple is connected with a PC through a data logger which registers the measured value after every 10 s on the Excel sheet supported by the software daq-view.
2.2. Fuel Characterization. Cotton is the lifeline and very important component of Pakistan’s economy. Pakistan’s cotton vision program targets cotton production at 15 million bales by the year 2010.10 The cotton stalk sample (CS) was obtained from an agricultural field of Lodhran, Punjab, Pakistan, cultivated during the May-June season, and handpicked in the November-December season. Cotton stalk (Gossypium) is the stem of the cotton plant which is residual biomass material from
(9) Memon, M. D.; Harijan, K.; Uqaili, M. A.; Mirza, U. K. Potential of Crop Residues as Energy Source in Pakistan. Proceedings World Renewable Energy Congress-IX, Florence, Italy, August 19-25, 2006.
(10) Hanif, M.; Khan, S. A.; Nauman, F. A. Agricultural Perspective Policy; Ministry of Food, Agriculture and Livestock: Islamabad, Pakistan, 2004; pp 18-19.
Figure 1. Schematic of the 20 kW down fired combustor.
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Table 2. Proximate and Ultimate Analysis and HHV of the Fuel Samples (As Received Basis) ultimate analysis
proximate analysis
fuels
C (%)
H (%)
Oa (%)
N (%)
S (%)
ash (%)
FC (%)
VM (%)
H2O (%)
bulk density (kg/m3)
HHV (MJ/kg)
SM CS RC WC SBT SBR
41.70 45.19 60.36 42.2 33.6 42.34
5.0 4.40 4.5 4.94 5.3 5.62
32.32 40.6 8.35 35.48 36.27 37.13
2.47 1.0 1.84 0.28 1.5 0.24
0.09 0.0 0.30 0.10 0.0 0.001
4.29 4.9 14 1.70 11.05 9.56
24.58 18 45.48 11.90 13.86 17.11
57 73.1 29.87 71.1 62.81 68.23
14.13 4.0 10.65 15.3 12.28 5.1
490 310 620 270 160 180
17.70 17.70 27.29 16.39 11.80 17.37
a
By difference.
the cotton crop and often burned in the field as rotting vegetation since they harbor diseases that could affect future crops.11-14 Projected estimate for cotton residue availability in 2010 is 13.71 million tonnes. However, using it as supplementary fuel with coal will not only resolve the disposal problem but also convert this biowaste into a high value fuel. Sugar cane bagasse samples SBT and SBR were collected from known sugar cane fields near Faisalabad and Rahim yar Khan, normally supplied to Tandlianwala sugar mills and JDW sugar mills. Bagasse is the fibrous waste that remains after the recovery of sugar juice via crushing and extraction. Pakistan is the fifth largest sugar cane producer in the world with a production of 54 million tonnes/year. There are 81 sugar mills in the country, and 21.27 million tonnes of bagasse are expected to be available in 2010.15 Shea meal (SM) is the residue from the nut of the shea tree (Vitellaria paradoxa), after the removal of fatty “butter” which is used for cooking and contains the fleshy mesocarp, shell, and husk. This biomass material is currently used as fuel in the U.K. power generating industry.11 The U.K. is importing 5 420 tonnes of shea meal annually from Africa for cofiring for electricity production.16 Shea meal, wood chips (WC), and coal (Russian coal) RC fuels for this study were provided by RWE npower (U.K.). The proximate and ultimate analyses along with bulk densities of the samples are given in Table 2. Proximate analyses and ultimate analyses measurements were conducted using a thermogravimetric analyzer (Shimadzu TGA-50) and CE Instruments Flash EA1112 series, respectively. The calorific values were determined by using a Parr 6200 oxygen bomb calorimeter and given in Table 2. For the determination of surface area, the char samples were analyzed using surface area and pore size analyzer model (Quanta Chrome Nova 2200e). Ash analysis was done using a PANalytical Axios Advanced XRF spectrometer aided with PANalytical IQþ Semiquantitative software. The H/C ratio of SM = 0.12, WC = 0.12, SBR = 0.13 and the O/C of SM = 0.77, WC = 0.84, SBR = 0.88 fall in the overlapping region attributed to biomass and RDFs on a Van Krevelen type diagram whereas CS (H/C = 0.09; O/C = 0.89) and SBT (H/C = 0.16; O/C = 1.1) are located in the region that is typically attributed to biomass (Figure 2). Volatile matter/fixed carbon (VM/FC) of the WC = 5.9, SBT = 4.52, CS = 4, SBR = 3.99, and SM = 2.3 as compared to RC = 0.66 and TC = 0.58 (Table2). This is quite evident from the proximate and ultimate
Figure 2. van Krevelen type diagram.
analysis given in Table 2 and the placement of biomasses in the van Krevelen type diagram in Figure 2. Biomass as a class seems very much different from coals. They have high volatile matter, higher oxygen content, generally low nitrogen content, and little or zero sulfur.3,17 Cellulose and lignin are generally recognized as main components in agricultural residues. The composition analysis of the samples are presented in Figure 3. Cellulose is composed of anhydroglucose units linked with 1 f 4-β-glucosidic bonds. Upon oxidation, functional groups will include carbonyls, ketones, and carboxyls. Hemicelluloses are branched chain polysaccharides. Functional groups associated with the hemicelluloses include carboxyls, methyls, and hydroxyls. There are no aromatic components in the cellulose (hemicelluloses þ cellulose).18 In this figure, the value of cellulose represents total cellulose (hemicelluloses þ cellulose). The lignin does contain aromatic rings. Aromaticity is considered as one of the primary determinants of reactivity. The aromaticity of biomasses is less than coals. Generally, woody biomasses have 20-25% carbon atoms in aromatic rings as opposed to bituminous coal with 60-70% carbon atoms in aromatic rings.18 The weight fraction, except for the cellulose and lignin fractions, corresponds to the fraction of acid-soluble hydrocarbons in the biomass.19 Figure 3 shows that all the biomass samples differ from each other as well. Wood chips contain the highest levels (33.66%) of lignin, and sugar cane bagasse SBR has the highest level of cellulose (61.9%) whereas sheameal contains the highest value of (55.83%) acid soluble hydrocarbons. SM appeared to be a unique kind of biomass as compared to all the biomasses discussed in the literature so far. Cotton stalk and sugar cane bagasses (SBT and SBR) contain higher proportions of cellulose than lignin, whereas woodchips and sheameal contain higher proportions of lignin. The particle sizes of the samples measured by laser diffraction (Malvern MasterSizer-2000) are given in Table 3. The average particle size was expressed as the volume mean diameter [4, 3] whereas d [0.1], d [0.5], and d [0.9] are the percentile diameters determined at the 10th, 50th, and 90th percentile of the undersized particles.
(11) Munir, S.; Daood, S. S.; Nimmo, W.; Cunliffe, A. M.; Gibbs, B. M. Bioresour. Technol. 2009, 100 (3), 1413–1418. (12) Reddy, N.; Yang, Y. Bioresour. Technol. 2009, 100 (14), 3563– 3569. (13) Gemtos, T. A.; Tsiricoglou, T. Biomass Bioenergy 1999, 16 (1), 51–59. (14) Akdeniz, R. C.; Acaroglu, M.; Hepbasli, A. Energy Sources 2004, 26 (1), 65–75. (15) Harijan, K.; Uqaili, M. A.; Memon, M. D. Potential of Bagasse Based Cogeneration in Pakistan. Proceedings of World Renewable Energy Congress-X, Glasgow, Scotland, U.K., July 19-25, 2008; pp 195-200. (16) DEFRA. UK Biomass Stratergy; DEFRA: London, U.K., May 2007; p 21.
(17) Demirbas, A. Prog. Energy Combust. Sci. 2004, 30 (2), 219–230. (18) Tillman, D. A. Biomass Bioenergy 2000, 19 (6), 365–384. (19) Gani, A.; Naruse, I. Renewable Energy 2007, 32 (4), 649–661.
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Figure 3. Cellulose and lignin content in the biomass samples. Table 3. Particle Size Analysis samples
D [4, 3]μm
d [0.1]μm
d [0.5]μm
d [0.9]μm
CS SM WC SBT SBR RC
209.891 150.288 586.251 743.110 586 85.292
19.081 15.470 211.129 2.401 206.85 6.664
141.892 110.605 515.878 953.076 511.902 59.096
507.499 341.317 1070.864 1469.637 1079.548 201.394
3. Results and Discussion 3.1. Combustion Behavior. Figure 4 shows combustion behavior of the blends of sheameal, cotton stalk, wood chips, and bagasse with Russian coal. The oxygen concentration profile shown in parts a and b of Figure 4 and the corresponding temperature profiles (parts and b of Figure 5) reveals the impact of addition of biomasses on the ignition behavior. It is quite evident that, especially in the near burner zone, oxygen is rapidly consumed in the case of SMcoal and CS-coal blends as opposed to coal alone. Similarly, CO and CO2 concentration appears significantly high in the near burner zone in the case of SM-coal and CS-coal blends combustion as compared to coal firing alone (parts a and b of Figure 6).This could be linked to higher volatile content in these biomasses as compared to coal (Table 2) because the main gas species of volatile matter for biomass are CO and low-grade HC.20 Addition of biomass can lead to an enhanced devolatilization rate near the burner region.21 A rapid burn out of volatiles in the near burner area and faster oxygen consumption throughout the combustion course displayed lower oxygen values at the exit (parts a and b of Figure 4). This phenomenon could be a reason for the corresponding higher carbon burnouts as shown in Figure 7. However, other reasons quoted by Campbell et al.22 and van Loo and Koppejan23 include the high porosity of biomass
Figure 4. Mean O2 concentration profiles of preblended biomasscoal co-combustion.
chars making them more reactive relative to coal. The cotton stalk and sheameal char surface areas were found to be 3.958 and 4.549 m2/g, respectively, as compared to 0.128 m2/g for RC char. The particle size of the biomass has a distinct effect on the process of combustion (ignition, carbon burn out).23 Figures 4c, 6c, and 5c shows the combustion progress course comparison of wood chips-coal blends with 100% coal.
(20) Gani, A.; Morishita, K.; Nishikawa, K.; Naruse, I. Energy Fuels 2005, 19 (4), 1652–1659. (21) Abbas, T.; Costen, P.; Kandamby, N. H.; Lockwood, F. C.; Ou, J. J. Combust. Flame 1994, 99. (22) Campbell, P. A.; Mitchell, R. E.; Ma, L. Proc. Combust. Inst. 2002, 29 (1), 519–526. (23) VanLoo, S.; Koppejan, J. The Handbook of Biomass Combustion & Co-firing; Earthscan: London, U.K., 2008; p 236.
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Figure 7. Effect of BBR on carbon burnout.
the addition of 15% WC. The CO concentration appeared higher until the middle of the combustor as compared to the RC profile (Figure 6c). This could be due to a significantly high particle size of WC as compared to SM and CS as given in Table 3. Larger size particles gradually devolatalize resulting in an extension in the combustion zone.24 The oxygen consumption was found to be lower throughout the combustion course (Figure 4c) causing incomplete combustion, which has decreased the carbon burn out as shown in Figure 7. Burn out deteriorates by increasing the biomass share. These findings are in agreement with the findings of Spliethoff and Hein.25 In the case of SBT co-combustion, a higher concentration of oxygen with a temperature drop of 70 °C (at 15% BBR) was observed in the near burner region (Figures 4d and 5d). A higher concentration of CO was observed throughout the combustion process (Figure 6d). The particle size of SBT was the highest among the samples studied (Table 3). The burnout % decreased with the increase in BBR. This phenomenon was similar to WC, but the decrease in carbon burnout % was smaller than the case of WC-coal co-combustion despite the larger particle size of the SBT. This smaller decrease could be due to the low density of SBT as opposed to WC (Table 2). The char surface area for SBT and WC was found to be 21.712 and 4.832 m2/g. The combustion behavior of SBR appeared different to that of SBT. A faster oxygen consumption and reduction in the CO concentration were recorded during the combustion course (Figures 4d and 6d). There was no appreciable drop in temperature observed in the near burner region. This was similar to SM-coal and CS-coal behavior. An increase in char burnout % was found with an increase in the BBR as shown in Figure 7. This was due to the higher volatile content in biomasses despite their larger particle sizes than coal.25,26 Char burnout was evaluated using the ash tracer method, eq 1:
Figure 5. Co-firing temperature profiles of biomass-coal blends.
"
# 1 - Ashsample =Ashchar 100 burnoutð%Þ ¼ 1 -Ashsample
ð1Þ
Figure 6. Mean CO and CO2 profiles. (24) Ballester, J.; Barroso, j.; Cerecedo, L. M.; Ichaso, R. Combust. Flame 2005, 141, 204–215. (25) Spliethoff, H.; Hein, K. R. G. Fuel Process. Technol. 1998, 54 (1-3), 189–205. (26) Ye, T. H.; Azevedo, J.; Costa, M.; Semiao, V. Combust. Sci. Technol. 2004, 176, 2071–2104.
Higher concentration of oxygen in the near burner zone (Figure 4c) and temperature profile (Figure 5c) has made the delay in ignition recognizable. A drop in temperature of 94 °C was recorded in the near burner region (Figure 5c) with 2150
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: DOI:10.1021/ef901503e
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Figure 8. Effect of BBR on NO reduction. Figure 9. Effect of BBR on volatility and NOx.
where, ashsample is calculated from the proximate analyses of the individual fuels in the blend. 3.2. NOx Emissions. NOx reduction (%) was evaluated by using the following standard formula: ðNOÞbaseline at 6% O2 - ðNOÞBBR at 6% O2 NO red % ¼ ðNOÞbaseline at 6% O2 100
cofiring. With addition of 15% BBR thermal, VM/FC became greater than unity causing a slight reduction in NO (Figure 9b). All the samples have much higher volatiles than coal (Table2, CO profiles in Figure 6) and less aromaticity than coal (Figure 3). With the addition of biomass, volatilization and gas-phase combustion become the dominant reactions; biomass fuel nitrogen preferentially forms NH3 on pyrolysis in contrast to coal nitrogen, which tends to form HCN.25,28,29 This fact may have a positive effect on NOx reduction when operated under air and fuel staged conditions. During this unstaged cofiring study, no overfire air was employed. 3.3. SO2 Emissions. Figure 10 revealed a reduction in SO2 % with the increase in BBR for each of the biomasses. Agricultural residues have little sulfur compared with coal (Table 2). An increase in biomass content has the effect of turning the blend into a low sulfur fuel. On cofiring, the blends emit low SO2 emissions (compared to alone coal firing) corresponding to sulfur content in the fuel blend. CaO and MgO contents in the biomass ash also absorbs sulfur, which may be a contributing factor in the lower SO2 emissions reduction.25 3.4. Slagging and Fouling. Slagging and fouling (S&F) causes corrosion and erosion of boiler tubes resulting in a reduction in heat transfer affecting boiler efficiency and ultimately tube life. Inorganic constituents in the fuel are the main contributor to slagging and fouling.3,30 The chemical compositions of ash samples obtained from the cofiring tests reported in this paper are presented in Table 4. There is no correlation available which can determine the S&F for biomasses unequivocally. The major elements including alkali metals (K, Na), alkaline earth metals (Ca, Mg), silicon, chlorine, and sulfur are involved in reactions leading to ash slagging and fouling.30-32 Slagging propensity depends on the thermal behavior of the ash. Ash fusion temperatures: initial deformation temperature (IDT), softening
ð2Þ
The baseline value of NO was taken at the exit without the addition of any biomass. This was then corrected at 6% O2. Similarly, all the NO values used were also corrected at 6% O2 via eq 3 to avoid any dilution effect: 20:9% - 6% ð3Þ NO at 6% O2 ¼ NO 20:9% - %O2 Figure 8 revealed that an increase in the BBR increased the NO reduction in general. SBR and WC have almost the same nitrogen and they displayed similar NO reduction efficiency. CS having 3 to 4 times higher nitrogen content than SBR and WC, respectively, displayed approximately the same level of NOx reduction. SBT with 6 to 7 times higher nitrogen content than WC and SBR achieved significantly higher NO reduction. SM having even higher nitrogen content than coal displayed NOx reduction with increasing BBR when cofired. Thus a correlation between NOx reduction % and biomass nitrogen content is not likely. This is in agreement with the findings of Natarajan and Narayanan27 and Spliethoff and Hain.25 As evident from Figure 8, the coal-SBT blend yielded significantly higher NOx reduction. This could be due to a higher concentration of CO throughout the combustion course as visible in Figure 6d. The co-combustion of biomass with coal is influenced by the operating parameters and volatility of the blended fuel. Figure 9a revealed that the VM/FC of SBT is significantly higher than the other biomasses. A decrease in NO reduction was found when BBR was increased from 10 to 15% (Figure 8). On the basis of various cofired power plant boiler data, Tillman18 concluded a significant NOx reduction with an increase of BBR until the VM/FC of the fuel reached 1 and for the fuel having VM/FC > 1 addition of biomass was not beneficial. The same effect was observed in the case of the SBT-coal blend
(28) Werther, J.; Saenger, M.; Hartge, E.-U.; Ogada, T.; Siagi, Z. Prog. Energy Combust. Sci. 2000, 26 (1), 1–27. (29) Baxter, L. Fuel 2005, 84 (10), 1295–1302 (Special Issue Dedicated to Professor Terry Wall). (30) Jenkins, B. M.; Baxter, L. L.; Miles, T. R.; Miles, T. R. Fuel Process. Technol. 1998, 54 (1-3), 17–46. (31) Vamvuka, D.; Zografos, D. Fuel 2004, 83 (14-15), 2051–2057 (East Meets West on Heavy Oil Technology Symposium). (32) Pronobis, M. Biomass Bioenergy 2005, 28 (4), 375–383.
(27) Narayanan, K. V.; Natarajan, E. Renewable Energy 2007, 32 (15), 2548–2558.
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Munir et al. Table 5. Slagging and Fouling (S&F) Indicesa slagging (Babcock)index
RS = (B/A)Sd Sd = % of S in dry fuel
fouling index
Fu = (B/A)(Na2O þ K2O)
ratio-slag viscosity index
SR = (SiO2 100)/ (SiO2 þ MgO þ CaO þ Fe2O3)
a
RS < 0.6 low slagging inclination RS = 0.6-2.0 medium RS = 2.0-2.6 high RS > 2.6 extremely high Fu e 0.6 low fouling inclination Fu = 0.6-40 high Fu g 40 extremely high SR > 72 low slagging inclination 72 g SR > 65 medium SR e 65 high
Source is ref 33.
Table 6. Calculated Values of the Traditional Ash Deposition Indices index
Figure 10. Effect of BBR on SO2 reduction.
SBT
SBR
CS
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO Fe2O3 NiO CuO ZnO Ga2O3 Rb2O SrO Y2O3 ZrO2 BaO PbO
1.68 1.73 5.97 45.66 1.63 2.99 3.95 11.26 0.93 0.35 21.62
0.67 1.79 5.37 53.28 1.46 3.31 7.72 11.68 0.60 0.42 9.97
1.90 2.50 2.09 8.96 5.61
SM
WC
2.41
3.67 2.60 10.47 7.55 13.94 42.49 6.92 0.27 0.23 7.66
3.64 3.79 14.9 46.29 2.43 1.95 4.79 8.43 2.41 0.68 10.4
34.57 18.15
0.167 0.57
0.167 0.57
0.11 0.17
0.14 0.70
0.076 0.28
0.21
0.11 0.56
0.41 0.19 0.018 0.14
RC 0.63 17.67 47.23 0.71 7.23 1.72 3.11 1.85 0.20 17.77 0.08 0.09 0.09 0.03 0.05 0.81 0.01 0.07 0.58 0.03
Fe2 O3 þ CaO þ MgO þ Na2 O þ K2 O SiO2 þ Al2 O3 þ TiO2
CS
SM
WC
RC
to add P2O5 content to the B group. The new ratio is called B/A(þ P).32,33 The B/A ratio can also be used in simplified form as32,33 RðB=AÞ ¼
Fe2 O3 þ CaO þ MgO SiO2 þ Al2 O3
ð5Þ
When R(B/A) continues to decrease from 0.75, ash hemispherical and flow temperatures increase resulting in a decrease in slagging tendency. The most commonly used traditional S&F indices have been given in Table 5. The values calculated for approximation of propensity for slagging and fouling by using traditional S&F indices are presented in Table 6 for pure fuel firing and Table 7 for 15th% blends. The values obtained from the indices (Table 6) showed dichotomous indications about the propensity of slagging and fouling. The slagging index (RS) values for all the biomasses were found to be less than 0.6 indicating low slagging potential. At the same time, the fouling factor (Fu) values for biomasses were found in the range attributed for high or extremely high fouling tendency. As SBT and CS have no sulfur content, their corresponding RS values were zero. For SBT, R(B/A) = 0.6703 < 0.75 indicating low slagging potential along with SR = 56.68 < 65, attributed to high slagging. As a matter of fact, the above-described ash fusibility correlations are coal specific. The information regarding pure agricultural residues ash deposition is scarce in the literature. However (from Table 6), it seems that combustion of pure SM and CS can cause slagging and fouling problems. In the case of co-combustion of biomass with coal, the chemical composition of the biomass-coal blends ash does not differ significantly from the pure coal ash provided BBR (thermal) remains less than 20%.32 The calculated ash composition and ash deposition indices for 15% (thermal) BBR are shown in Table 7. Ash deposition indices of the blends does not show significant variations from pure coal indices except SBT. This variation could be due to the significantly higher mass fraction of the SBT in the blended fuel (30.79%) than rest of the biomasses mass fractions in the blended fuels
temperature (ST), hemispherical temperature (HT), and flow temperature (FT) play a role in deposition.32 The basic compounds in the ash (Fe2O3 þ CaO þ MgO þ Na2O þ K2O) which lower the melting temperature of the ash are categorized as B group compounds. Rb represents the percentage of basic constituents in the ash. The minimum ash softening temperature occurs with 35% < Rb < 55%.32,33 The compounds with a higher melting temperature (SiO2 þ Al2O3 þ TiO2) are termed as A group compounds. The ratio of basic compounds to acidic compounds is considered an index for slagging behavior.31-34 B=A ¼
SBR
40.25 31.85 59.54 60.75 31.05 23.24 Rb B/A 0.76567 0.538 5.3867 4.553 0.4882 0.348 B/A(þ P) 0.7968 0.5623 5.8943 5.119 0.5264 0.359 0.6703 0.3998 2.087 1.397 0.3696 0.3316 R(B/A) 0.00 0.0054 0.00 0.455 0.059 0.118 RS 4.316 4.517 196.49 193.47 4.116 0.6 Fu 56.88 69.44 27.98 36.44 67.17 68.70 SR
Table 4. Ash Chemical Composition of the Samples components (%)
SBT
ð4Þ
A considerable fraction of P2O5 enhances the development of low-melting point phases in the fly ash which can drop the ash melting temperature because for P2O5 HT = 569 °C.32,33 Although the influence of P2O5 on ash fusibility depends on the form in which it occurs in the fly ash, it seems appropriate (33) Tortosa Masi a, A. A.; Buhre, B. J. P.; Gupta, R. P.; Wall, T. F. Fuel Process. Technol. 2007, 88 (11-12), 1071–1081. (34) Kazagic, A.; Smajevic, I. 4th Dubrovnik Conference on Sustainable Development of Energy, Water, and Environment Systems, Dubrovnik, Croatia, June 4-8, 2007; pp 699-707.
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: DOI:10.1021/ef901503e
Munir et al.
Table 7. Estimated Ash Components and Ash Deposition Indices for 15% BBR Blends RC
15% SM þ 85% RC
15% CS þ 85% RC
15% WC 15% þ 85% RC
15% SBT þ 85% RC
15% SBR þ 85% RC
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Fe2O3
0 0.63 17.67 47.23 0.71 1.72 3.11 1.85 17.77
0 0.86 16.53 44.45 1.22 4.79 3.39 1.73 17.0
0.17 0.79 16.29 43.85 1.141 4.61 4.43 1.69 16.42
0.13 0.74 17.57 47.19 0.77 1.83 3.30 1.87 17.50
0.43 0.91 14.63 46.82 0.94 2.29 5.22 1.61 18.77
0.10 0.82 15.65 48.22 0.83 2.70 4.51 1.64 18.40
Rb SR R(B/A) RS
23.24 68.70 0.3316 0.1184
26 67.64 0.3486 0.1199
23.5 68.65 0.3327 0.1011
27.65 65.27 0.4053 0.1025
26.5 67.02 0.3715 0.1043
23.54
30.79
22.30
components (%)
biomass share (wt %)
21.05
Ash Deposition Indices 26 66.95 0.3598 0.1106 21.65
corresponding to the same thermal input of 15% (Table 7). Because of the difference in density and calorific value among the biomasses, the mass share of each biomass in the biomass-coal blend varies. Moreover, slagging and fouling indices are also linked with the ash content in the biomass along with ash composition. SBT was found to have significantly higher ash content of 11.05% compared to all other biomasses (Table2). The estimated values of slagging indices for 15% thermal BBR shown in Table 7 were found to be greatly deviant from the indices values shown in Table 6 for pure biomass firing and fairly close to the pure coal case. Therefore, co-combustion seems the more feasible option than pure biomass combustion along with other merits like handling, storage and seasonal effects.
It was found that agricultural residues have larger fractions of cellulose and acid cellulose hydrocarbons, which indicate less aromaticity as opposed to coal. It was found that co-combustion of agricultural residues with coal has a positive impact on NOx, SO2 reduction, and carbon burnout. The traditional slagging and fouling indices for coal ash fusibility displayed mixed results when applied to pure agricultural residue ash. Co-combustion of agricultural residues with coal seems more practicable than pure agricultural residues firing due to the potential risk of slagging and fouling. There is clearly a need to develop corelations, specifically to predict ash fusibility behavior of different varieties of agricultural residues. Each of the samples studied displayed a significantly stronger release of volatility matter than pure coal during devolatilization. Biomass fuel nitrogen is known to form NH3 in contrast to coal nitrogen which tends to form HCN. Therefore, it is anticipated that co-combustion of agricultural residues with coal may have larger effects on NOx reduction when operated under air and fuel staging conditions.
Conclusions Combustion behavior of sheameal-coal, cotton stalk/coal, sugar cane bagasse/coal, and wood chip/coal blends were investigated to realize their energy potential thermochemically in a 20 kW pulverized coal fired combustor. Biomass blending ratios of 5, 10, and 15% (thermal) were used in each set of experiments.
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