Energy & Fuels 2000, 14, 1133-1138
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Experimental Study on Combustion and Pollutant Control of Biobriquette Guoqing Lu* and Qingyue Wang Center for Research and Development of Environmental Conservation, International Good Neighborhood Association, 1-5-5 Shinbashi, Minato-ku, Tokyo 105-0004, Japan
Kazuhiko Sakamoto Graduate School of Science & Engineering, Saitama University, 255 Shino-ohkubo, Urawa, Saitama 338, Japan
Heejoon Kim, Ichiro Naruse, and Jianwei Yuan Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441-8580, Japan
Toshihiko Maruyama and Mitsushi Kamide Hokkaido Foundation for The Promotion of Scientific & Industrial Technology, Prest 1-7, West 7 Kitaichijou, Chuo-ku, Sapporo 060, Japan
Masayashi Sadakata Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received June 11, 1999
To control pollutant emissions from the combustion of both domestic stoves and small-capacity industrial boilers, an artificial solid fuel called biobriquette has been developed. It is manufactured from a mixture of coal, biomass (sawdust), and desulfurizer under a high compression pressure. In this study, the combustion experiments were performed to elucidate the ignition and combustion characteristics of biobriquette. Comparisons were made between coal briquettes and biobriquettes for their combustion efficiencies and pollutant emissions in existing domestic stoves. Byproduct in the gas welding industry was used as a new desulfurizer in the biobriquette, and its desulfurization characteristics were studied. The experimental result shows that the biobriquette has a lower ignition temperature and a higher combustion efficiency than the coal briquette. The new desulfurizer was found to be more effective in desulfurization than the other two desulfurizers, limestone and scallop shell. It is also found that the biobriquette combustion in domestic stoves gives lower CO2 emission than the normal coal briquette. The developed biobriquette provides a simple, economical, and efficient way for coal utilization and pollutant control in some developing countries.
Introduction Fossil fuels currently dominate the world energy production and will continue to dominate in the foreseeable future. Coal has the highest potential as a future stable energy supply among fossil fuels in the world.1 Although a part of coals is used for power generation in large-scale utility boilers and gasifiers, the amount of coals utilized as fuel in both middle- and small-scale industrial boilers and domestic stoves is still large in some developing countries. Because of the sharp industrialization and the economic development in these countries, the coal consumption by boilers and stoves is being rapidly increased, but at the same time, the * Author to whom correspondence should be addressed. (1) Kawakami, Y. J. Jpn. Energy 1991, 72, 136.
pollution caused by coal utilization becomes serious. The direct combustion of coals, especially low-grade coals with high ash, high sulfur and high nitrogen contents and/or low heating values, in industrial boilers and domestic stoves causes more serious pollution than large-scale utility boilers do. On the other hand, a great amount of biomass (agriculture and forestry wastes) is also being discarded in these countries. The biomass is a renewable and clean fuel, and its utilization can meet the requirements of both energy supply and pollutant control. It is estimated that the biomass, including municipal solid waste, can contribute nearly 43% of the total energy required for developing countries and 26% for some developed countries.2 A new artificial solid fuel called biobriquette has been developed in order to effectively utilize low-grade coals
10.1021/ef990115s CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000
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Table 1: Properties of Tested Coal and Biomass proximate analysis [mass %, dry basis]
ultimate analysis [mass %, daf]
heating value [kJ/kg]
sample
ash
VM
FC
fuel ratio [-]
C
H
N
S
QH
QL
coal sawdust
15.4 3.5
21.8 82.4
62.8 14.2
2.9 0.2
85.3 49.4
3.4 6.1
1.2 5.0
4.6 0.0
23300 16300
20800 14700
Table 2: Properties of Tested Desulfurizers [mass %] limestone scallop
SiO2
Al2O3
Fe2O3
MgO
CaO
SO3
lossa
0.1 0.4
0.16 0.036
0.042 0.024
0.027 0.012
0.52 0.22
55.44 53.07
0.0043 0.53
45.53 45.44
insol.(in HCl)
Cl
SO4
Na
K
Mg
Pb
Cr
Ca(OH)2
Mn
Fe
As
0.1
0.01
0.05
0.05
0.05
1
0.003
0.005
96.0
0.001
0.02
5 × 10-7
Ca(OH)2
new desulfurizer (by-product)
new desulfurizer (by-product) a
moist.
C
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
Cl
K2O
CaO
17.0
0.15
0.72
2.39
6.19
0.08
0.89
0.07
0.88
44.7
TiO2
Cr2O3
MnO2
Fe2O3
NiO
CuO
ZnO
SrO
Rh2O3
BaO
others
5.69
0.20
0.72
1.38
0.01
0.15
0.17
9.81
0.01
4.81
3.98
Mass loss after calcinations.
and to control pollutant emissions in the industrial boilers and the domestic stoves. The biobriquette is manufactured from a mixture of low-grade coal, biomass, and additive under high compression pressure. The additive, such as limestone and scallop shell, is adopted as a desulfurizer. In our previous studies,3-6 the combustion and desulfurization characteristics of the biobriquette were studied experimentally in a laboratory-scale electrically heated furnace. It was found that the combustion process of the biobriquette undergoes two stages: the volatile combustion stage followed by the char combustion stage. The desulfurization happens mainly in the char combustion stage for limestone and scallop shell, whereas in both the volatile combustion stage and the char combustion stage for calcium hydroxide. The continuous study is performed in this paper on the biobriquette combustion and pollutant control. The paper focuses on the biobriquette manufactured from a high sulfur content (around 4.6 mass %) coal by an industrial-scale biobriquette production plant in Chongqing, China. Chongqing is currently the largest-population city in China, and the total coal consumption in the city is around 25 Mton/year, 30% of which is used as the fuel in industrial boilers and domestic stoves. The SO2 and CO2 emissions from the combustion of coals are about 0.9 Mton/year and 6.146 Mton/year, respectively. On the other hand, a great amount of biomass (about 17 Mton/year), such as agriculture and forestry wastes, is being abandoned in the countryside around this city. To improve the coal utilization efficiency and the pollutant emissions, the biobriquette technology has been introduced, and a pilot plant for biobriquette (2) Abbas, T.; Costen, P.; Kandamby, N. H.; Lockwood, F. C.; Ou, J. J. Combust. Flame 1994, 99, 617. (3) Lu, G. Q.; Toyama, T.; Kim, H. J.; Naruse, I.; Ohtake, K. Proc. 3rd Int. Symp. Coal Combust. 1995, 37. (4) Lu, G. Q.; Toyama, T.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu 1997, 23, 404. (5) Lu, G. Q.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu 1997, 23, 954. (6) Lu, G. Q.; Kim, H. J.; Naruse, I.; Yuan, J.; Ohtake, K.; Kamide, M. Energy Fuels 1998, 12, 689.
Table 3: Compression Conditions of Biobriquette particle diameters of tested materials [m]
compression pressure [MPa] Ca/S ratio one sample mass [kg]
coal biomass limestone scallop shell calcium hydroxide new desulfurizer (wasted admixture)
less than 2 × 10-3 less than 2 × 10-3 297-420 × 10-6 297-420 × 10-6 less than 25 × 10-6 less than 1 × 10-3 295 1.5 28 × 10-3
production has already been set up in Chongqing. In this paper, the industrial-scale production process in this pilot plant is described. The ignition and combustion characteristics of the biobriquette produced in this plant are experimentally investigated. A kind of byproduct from the gas welding industry was used as a desulfurizer in the biobriquette, and its desulfurization characteristics is also studied. In the last of the paper, a calculation shows that the biobriquette combustion produces less CO2 emission than the normal coal briquette does. Biobriquette Production Process and Combustion Experiments Table 1 gives the properties of used coal and biomass in this study. The coal is a kind of low-grade coal with high contents of sulfur and ash. The biomass is sawdust with a low ash content and almost zero sulfur content. The dominant desulfurizer in the plant is the wasted byproduct from the gas welding industry, and it contains about 44.7% CaO as shown in Table 2. For the comparison, three other desulfurizers were also used in the tested biobriquettes, and their properties are also listed in Table 2. The tested biobriquettes were produced in the pilot plant in Chongqing, China. The production condition and process of the biobriquette are shown in Table 3 and Figure 1, respectively. The coal and the biomass were dried and ground to the diameter under 2 mm separately before mixing together. Desulfurizers were added in the mixing process of the coal and the biomass. The mixture was then compressed under the pressure of about 295 MPa through a
Combustion and Pollutant Control of Biobriquette
Energy & Fuels, Vol. 14, No. 6, 2000 1135
Figure 3. Effect of biomass content on ignition temperature. Figure 1. Production flow chart of biobriquette. Table 4: Both Conditions and Results of Combustion Experiments in a Domestic Stove biocoal briquette briquette electrically heated furnace
temperature [K]
1073
flow rate of air [m3/min] 1 × 10-2 domestic stove burning time 30 [tested water: 5.7 kg; until boiling [min] fuel: 2 kg] boiling time [min] 90 total burning time [min] 187 steam production [kg] 5.08 heat efficiency [%] 32.1 combustion efficiency [%] 80
1173 1 × 10-2 45 50 120 2.53 21.6 65
Figure 2. Experimental apparatus. couple of pressing rollers. The produced biobriquette is in pillow shape, and each pellet weights about 28 g. The Ca/S ratio in the biobriquette was kept as 1.5. The tests show that the produced biobriquette is strong enough to prevent it breaking in transportation. The plant can produce continuously the biobriquette at about 1.25 ton/h. The combustion experiments were performed in a laboratory-scale electrically heated furnace and a domestic stove, respectively. The condition and results of the combustion experiments in the furnace and the stove are shown in Table 4. The furnace, schematically shown in Figure 2, was employed to investigate the ignition, combustion, and desulfurization characteristics of biobriquettes. As shown in Figure 2, the tested biobriquette was suspended by a wire linked to the digital balance and was positioned in the center of the furnace. The combustion air was forced into the furnace and was preheated through a packed bed of alummina balls. The furnace was preheated to a predetermined temperature, and
then moved upward to heat the sample and to start the combustion experiments. The flue gas composition and the sample mass loss were continuously measured by the gasanalyzing system and the digital balance, respectively. The gas-analyzing system includes the analyzers of SO2 (SOA-7000, Shimadzu Corporation of Japan), NOx (NOA-7000, Shimadzu Corporation of Japan), COx (CGT-7000, Shimadzu Corporation of Japan), and N2O (model 45C high-level gas filter correlation N2O analyzer, Thermo Environmental Instruments Inc. of the U.S.A.). As listed in Table 4, the flow rate of combustion air was 1 × 10-2 [m3/min], which provided enough ambient oxygen for complete combustion. The desulfurization efficiency was defined as follows:
ηSOx ) [1 - R SO2(Ca/S)n)/SO2(Ca/S)0)]
(1)
where SO2(Ca/S)0) is the SO2 emission from the combustion of the biobriquette without any desulfurizer and SO2(Ca/S)n) the SO2 emission with a desulfurizer (Ca/S ) n). The Ca/S represents the molar ratio of calcium to sulfur in the biobriquette. Since the coal briquette without any desulfurizer contains more sulfur than the biobriquette with a certain desulfurizer although they have the same mass, the coefficient R is used to make the two samples in comparison keep the same sulfur mass. The domestic stove used in this study is the most popular one in China. The purpose of the experiments in this stove was to study the practical performance of the biobriquette combustion and to investigate the particulate emission. Comparisons were made between the biobriquette and the normal coal briquette for the total combustion time, the time to boil a certain amount of water, the combustion efficiency and the particulate emission. Moreover, the biobriquettes produced in the plant were also supplied to citizens as family fuel for cooking and heating in domestic stoves in order to further compare the practical use results between the coal briquette and the biobriquette.
Results and Discussion Ignition and Combustion Experiments. In general, coal is heated and after it reaches a certain temperature, volatiles release out, ignite, and burn. The volatile combustion leads to the char combustion. The ignition and combustion characteristics of volatile depend on coal type and have large influence on the ignition and combustion of coal. To reveal the influence of biomass on the ignition and combustion of the biobriquette, the ignition and combustion experiments were conducted. Figure 3 shows the experimental results. It was seen from Figure 3 that the biomass had
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Figure 4. Influence of biomass content on combustion profile in the electrically heated furnace.
obvious influence on the ignition temperature. The temperature was rapidly decreased with the addition of biomass content under 20 mass %, and the ignition temperature of the biobriquette came nearly to the ignition temperature of biomass when the biomass content was over 20 mass %. This phenomenon could be explained as the reason the biomass volatile had a lower ignition temperature than coal. The increase of biomass content contributes the more volatile of lower ignition temperature to the biobriquette. This makes the biobriquette have a lower ignition temperature than the coal briquette. Furthermore, when the biomass content went over 20 mass %, the volatile amount was saturated to ignite the biobriquette. This is because the biobriquette had the almost same ignition temperature as the biomass. Moreover, the influence of biomass on the combustion characteristics of the biobriquette was also explored by the combustion experiments in the furnace. The time histories of unburned fraction of the biobriquettes with different biomass contents in their combustion in the furnace are shown in Figure 4. Two stages appear in the combustion process: the first stage with a rapid mass loss, and the second with a slower mass loss. These two stages can be considered to correspond to the volatile combustion stage and the char combustion stage. The volatile in the biobriquette was evolved and burnt in gas phase around the biobriquette when the biobriquette was heated to the ignition temperature. The char combustion didn’t occur on the surface of the biobriquette until the volatile combustion was almost finished. As oxygen diffused from the surface toward the inside, the char burned and the flame sheet kept moving toward the inside till the center. The char combustion agrees with the shrinking-core reaction model and is controlled by oxygen diffusion through both gas boundary layer and ash layer.4 It was noted that the larger the amount of biomass was, the shorter the burnout time was. This is because biomass had more volatile content than coal. The devolatilization of the biobriquette enlarged the porosity in char and made the oxygen diffusion easier. Thus, the combustion rate of the biobriquette became faster. Generally, in the domestic stove, the faster-burning of fuel can make the flame stronger, the temperature of combustion higher, and the cooking time shorter. To
Lu et al.
compare the heat efficiency between the biobriquette and the coal briquette, the combustion experiments were conducted in the domestic stove. A kettle with water was put on the stove. The water temperature and the steam production in the kettle were measured in the experiments. The experimental results are illustrated in Table 4. It was found that although the tested water and fuel amounts were the same for the biobriquette and the coal briquette (water: 5.7 kg, fuel: 2 kg), the time from the room temperature of the water to the boiling point was 30 min for the former and 45 min for the later since the biobriquette burned faster than the coal briquette did. Besides, the boiling time of the water lasted 96 min for the biobriquette. The total burning time of the biobriquette was 187 min. On the contrary, for the coal briquette, the boiling time of the water was only 50 min and the total burning time was only 120 min. The combustion efficiency was about 80% for the biobriquette and about 65% for the coal briquette. As mentioned above, the biobriquette had more volatile content and its devolatilization increased the porosity in char. This made the oxygen diffusion easer. For the coal briquette, the oxygen diffusion in the later stage of char combustion became more and more difficult, and the combustion chamber temperature dropped rapidly. The extinguishing occurred before the carbon was burned out completely. The steam production was 5.08 kg for the biobriquette and only 2.53 kg for the coal briquette. In addition, the biobriquettes produced in the plant were also supplied to citizens as family fuel for cooking and heating to further compare the practical use results between the coal briquette and the biobriquette. The results for over half a year indicated that around 30% of family fuel can be saved through changing the present coal briquettes for the biobriquettes in Chongqing city. Particulate Emission Control. The flue gas from the combustion of the biobriquette and the coal briquette in the domestic stove was led, respectively, into a particulate capturer system with filter papers. The mass change of filter papers was measured before and after the combustion experiments, and the particulate emission amount was determined from the mass change. The experiment results showed that the particulate emission from the biobriquette combustion is about 0.11 g/kg-fuel, whereas the particulate emission from the coal combustion in the same domestic stove is about 2.46 g/kg-fuel. The biobriquette were produced under extremely high compression pressure and the consolidation of coalbiomass mixture took place under this pressure, and the volatile in biomass promoted the combustion rate. These reduced unburned-combustible particulate emission. Self-Desulfurization in Biobriquette Combustion. Limestone is commonly used to capture SOx in fossil fuel combustion processes. In our former study,6 a kind of typical and abundant seashell called scallop shell was developed as a desulfurizer and its desulfurization results were confirmed to be much better than limestone in the biobriquette combustion. This is because the lager pore size on scallop shell emerges during calcination and the desulfurization reaction is able to come inside the particle of scallop shell, whereas the desulfurization reaction of limestone only comes in the near surface.7 This makes their desulfurization capabil-
Combustion and Pollutant Control of Biobriquette
Figure 5. Time concentration history of SO2 emission in the flue gas in the electrically heated furnace.
ity quite different. On the other hand, the production cost of the biobriquette plays the most important role in its popularization in these developing countries. The lower both the purchase prices of desulfurizers and the Ca/S value in the biobriquette are, the less the production cost of the biobriquette is. Therefore, in this study, a new desulfurizer, a byproduct in the gas welding industry, which contains a large amount of CaO was developed to capture the SO2 from the biobriquette combustion. Its desulfurization results were investigated by the combustion experiments in the furnace. The time-concentration histories of the SO2 emission in the flue gas are shown in Figure 5, which were measured from the combustion of both the coal briquette without any desulfurizer and the biobriquette with a certain desulfurizer (Ca/S ) 1.5). In this figure, two different parts appear in SO2 concentration profiles. The two parts correspond to the volatile combustion and the char combustion, respectively. The area under the SO2 concentration profile denotes the total SO2 emission. The experimental results showed obviously that the desulfurization occurred in the two stages of the biobriquette combustion when the new desulfurizer was used. This performance is quite different from the desulfurization behavior of both ordinary limestone and scallop shell in the biobriquette combustion. Figure 6 gives the comparison among the new desulfurizer, limestone, scallop shell, and calcium hydroxide (with about 96% purity) in the biobriquette combustion. It was found that the calcium hydroxide and the new desulfurizer had the same desulfurization efficiency. The desulfurization capability of scallop shell was nearly twice as high as that of limestone when the Ca/S is 1.5. The results was consistent with our previous study.6 The desulfurization efficiency of the new desulfurizer was much higher than both the limestone and the scallop shell. This is because the desulfurization happened mainly in the char combustion stage when limestone (7) Naruse, I.; Nishimura K.; Ohtake, K. Kagaku Kogaku Ronbunshu 1995, 21, 904. (8) Kim, H. J.; Hashimoto, S.; Ona, S.; Mutsui, K.; Sadakata, M. J. Jpn. Energy 1997, 76, 205. (9) Naruse, I.; Kim, H. J.; Lu, G. Q.; Yuan, J. W.; Ohtake, K. TwentySeventh Symposium (International) on Combustion Institute 1998, 2973. (10) Wang, Q. Y.; Lu, G. Q.; Kim, H. J.; Maruyama, T.; Sakamoto, K.; Hatakeyama, S. Proc. Atmos. Sci. Appl. Air Quality 6th Int. Conf., 1998.
Energy & Fuels, Vol. 14, No. 6, 2000 1137
Figure 6. Desulfurization efficiency comparison of various desulfurizers.
and scallop shell are used as desulfurizers, whereas in both the volatile combustion stage and the char combustion stage when the calcium hydroxide and the new desulfurizers are added. It was known that the CaCO3 in limestone and scallop shell was converted into CaO at first during calcination, and then CaO reacts with SO2 to produce CaSO4. The calcination temperatures for limestone and scallop shell are about 1023 and 973 K, respectively.6 When the CaCO3 is used as a desulfurizer, under 973 K, the volatile release in the biobriquette has been started but no CaO to desulfurize is produced. Therefore, it should be quite obvious that the desulfurizer based on CaCO3 seems to be ineffective in desulfurization in the biobriquette devolatilization. On the other hand, the CaO can directly capture SO2 and sulfides in volatile such as H2S, etc., in the biobriquette devolatilization [CaO + H2S ) CaS + H2]. The Ca(OH)2 has a much lower decomposition temperature (only about 673 K6) from the Ca(OH)2 to CaO than the CaCO3. These make the CaO and Ca(OH)2 have better desulfurization results than the CaCO3 during the biobriquette combustion. CO2 Emission Reduction from Biobriquette Combustion. As stated above, biomass is a renewable and clean fuel. Its utilization, as fuel, leads to a short-time benign natural recycle and reduces CO2 emission. On the other hand, biomass has higher volatile content and lower ignition temperature than low-grade coal. It plays a trigger role in ignition and contributes more volatile to the biobriquette. The burnout of more volatile leaves larger pores, so oxygen diffusion becomes easier in char combustion. These make the biobriquette have stronger flame, higher burning rate, and higher heat efficiency in domestic stoves. The combustion experiments of the biobriquette used as family fuel for cooking and heating in domestic stoves over half a year in Changqing city, showed that the biobriquette could reduce family fuel consumption effectively and remove the foul smell from the coal briquette combustion (the biobriquette made the H2S in the flue gas be burned out). It was learned through the combustion experiments that by changing the coal briquettes for the biobriquettes, the saving efficiency of family fuel for cooking and heating in this city could reach 30%, the combustion efficiency could come up from about 65% to 80% on an average. Due to energy loss reduction, the CO2 emission reduction from the combustion of family fuel in domestic stoves in this city can also be estimated by the following equations of
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heat balance.
mcQLc ) q1c + q2c + q3c + q4c + q5c + q6c
(2)
mbQLb ) q1b + q2b + q3b + q4b + q5b + q6b
(3)
where, mc and mb are the fuel consumptions of both the coal briquette and the biobriquette. QLc and QLb represent the heating values of both the coal briquette and the biobriquette. q1c, q2c, q3c, q4c, q5c, and q6c are available heat, flue gas heat loss (flue gas temperature is above atmosphere temperature), heat loss of combustible gases in flue gas (CO, etc.), ash heat loss (ash temperature is more than atmosphere temperature), external heat loss and unburned carbon loss in ash for the coal briquette combustion. q1b, q2b, q3b, q4b, q5b, and q6b are those for the biobriquette. Supposing the same heat is utilized (q1c ) q1b), it is obtained from the above two equations that 5
mcQLc - mbQLb )
∑2 (qic- qib) + (q6c- q6b)
5
∆mCO2 ) 3.67 Qpc
∑2 (qic - qib)
Conclusions The combustion experiments showed that the biobriquette had a lower ignition temperature, higher combustion efficiency, higher heat efficiency, and lower particulate emission than the normal coal briquette in the combustion of domestic stoves. The byproduct in the gas welding industry had more significant desulfurization capability than both ordinary limestone and scallop shell in the biobriquette combustion. The small amount addition (Ca/S ) 1.5) could make desulfurization efficiency reach about 75%, and the desulfurization reaction happened in both the volatile combustion stage and the char combustion stage. The results from the combustion experiments in domestic stoves confirmed the lower CO2 emission from the biobriquette combustion than that from the coal briquette combustion.
(4)
Thus, CO2 emission reduction could be deduced by the above equations regarding the total heat loss’ compensation from the combustion of pure carbon when the 1 kg of coal briquettes is replaced by biobriquettes and it is as follows:
3.67
Chongqing city is about 3.5 Mton/year. Therefore, the total emission of CO2 may be reduced by about 1 Mton/ year as the raw coal is changed for biobriquettes.
)
Qpcmc [QLc (ηc - β) QLb ηb] [kg-CO2‚kg-1-coal] (5)
where ηC and ηb are the mean combustion efficiencies of both the coal briquette and the biobriquette in the domestic stoves, β is the ratio of mb to mc, Qpc is the heating value of pure carbon (here, the value is 7830 × 4.187 kJ/kg). As mentioned above, they could be determined by the statistic calculation from the experimental results of domestic stoves in Chongqing city as the following:
ηC ) 0.65 ηb ) 0.80 β ) 0.7 When QLc ) 20800 kJ/kg and QLb ) 19580 kJ/kg (biomass ) 20 mass %) were taken from Table 1, ∆mCO2 was calculated to be about 0.29 [kg-CO2‚kg-1-coal]. The raw coal used, as family fuel in domestic stoves of
Nomenclature mbsbiobriquette consumption, kg mcscoal briquette consumption, kg q1bsavailable heat for biobriquette combustion, kJ q2bsflue gas heat loss for biobriquette combustion, kJ q3bscombustible loss of flue gas for biobriquette combustion, kJ q4bsash heat loss for biobriquette combustion, kJ q5bsexternal heat loss for biobriquette combustion, kJ q6bsunburned carbon loss in ash for biobriquette combustion, kJ q1csavailable heat for coal briquette combustion, kJ q2csflue gas heat loss for coal briquette combustion, kJ q3cscombustible loss of flue gas for coal briquette combustion, kJ q4csash heat loss for coal briquette combustion, kJ q5csexternal heat loss for coal briquette combustion, kJ q6csunburned carbon loss in ash for coal briquette combustion, kJ Qpcsheating value of purity carbon (here: 7830 × 4.187 kJ/kg) SO2(Ca/S)0)sSO2 emission without desulfurizer, kg SO2(Ca/S)n)sSO2(Ca/S)n) emission with desulfurizer (Ca/S ) n), kg ηbscombustion efficiency of biobriquette, % ηCscombustion efficiency of coal briquette, % ηSOxsdesulfurization efficiency Rsa coefficient for making the two samples in comparison to keep the same sulfur amount βsratio of mb to mc EF990115S
