- Email: [email protected]
Co-combustion performance of coal with rice husks and bamboo
ARTICLE IN PRESS
Atmospheric Environment 41 (2007) 7462–7472 www.elsevier.com/locate/atmosenv
Co-combustion performance of coal with rice husks and bamboo Philip C.W. Kwonga, Christopher Y.H. Chaoa,, J.H. Wanga, C.W. Cheungb, Gail Kendallb a
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b CLP Research Institute, 20/F, Tower 1, Grand Century Place, 193 Prince Edward Road West, Kowloon, Hong Kong Received 31 January 2007; received in revised form 16 May 2007; accepted 23 May 2007
Abstract Biomass has been regarded as an important form of renewable energy due to the reduction of green house gas emission such as carbon dioxide. An experimental study of co-combustion of coal and biomass was performed in a laboratory-scale combustion facility. Rice husks and bamboo were the selected biomass fuels in this study due to their abundance in the Asia-Pacific region. Experimental parameters including the biomass blending ratio in the fuel mixture, relative moisture content and biomass grinding size were investigated. Both energy release data and pollutant emission information were obtained. Due to the decrease in the heating value from adding biomass in the fuel mixture, the combustion temperature and energy output from the co-firing process were reduced compared with coal combustion. On the other hand, gaseous pollutant emissions including carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and sulfur dioxide (SO2) were reduced and minimum energy-based emission factors were found in the range of 10–30% biomass blending ratio. With an increase in the moisture content in the biomass, decreases in combustion temperature, SO2, NOx and CO2 emissions were observed, while an increase in CO emissions was found. It has also been observed that chemical kinetics may play an important role compared to mass diffusion in the co-firing process and the change in biomass grinding size does not have much effect on the fuel burning rate and pollutant emissions under the current experimental conditions. r 2007 Elsevier Ltd. All rights reserved. Keywords: Co-combustion; Coal; Rice husks; Bamboo; Gaseous pollutants
1. Introduction Greenhouse gas (GHG) emissions from conventional coal-fired power plants draw worldwide attention on their environmental effects. Coal contributed to around 23–26% of the total world energy consumption in 1990–2004 (EIA, 2007) and it was one of the major sources of energy-related Corresponding author. Tel.: +852 2358 7210; fax: +852 2358 1543. E-mail address: [email protected] (C.Y.H. Chao).
1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.05.040
GHG emissions. According to the same report, coal combustion emitted 36–39% of the total global carbon dioxide (CO2) emissions in 1990–2004 and the emission trend will last for some time. Nevertheless, it is one of the cheapest energy sources and dependence on coal in power generation is still substantial especially in developing countries. Renewable energy resources from biomass can relieve the intensity of GHG emissions and play an important role in power generation under sustainability considerations. Because plants consume carbon dioxide during the photosynthesis process,
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
the overall CO2 emissions during biomass combustion can be offset. This is a fundamental advantage of extracting energy from biomass through combustion processes from the perspective of sustainability. Burning biomass alone cannot give very high energy output when compared with coal combustion due to the high volatile matter and moisture content. Cocombustion involves the combustion of coal and other fuels in the same boiler. Co-combustion is worthy of exploration to improve energy output (Baxter, 2005). It can be a practical and attractive approach to increasing the use of renewable energy while maintaining an efficient power supply with reduction of GHG at a potentially cheaper cost. This is particularly true when the cost of highquality coal has been surging in recent years. Several potential benefits are driving interest in co-combustion (Kruczek et al., 2006; Gani et al., 2005). Leading the way are the possible environmental advantages of co-combustion, particularly the reduction of acid rain precursors and GHG such as sulfur dioxide (SO2), nitrogen oxides (NOx) and CO2. Information on biomass co-combustion can be found in several articles that summarized the state-of-the-art developments in this field (Demirbas, 2005; Sami et al., 2002). These researchers have indicated that co-combustion of coal and biomass is an emerging new area in pollutant reduction and has a high potential for exploration. Rice husks are the outer shell of rice grain and it is a common agricultural residue in China. Bamboo is a kind of construction debris in Hong Kong. These two biomass fuels have relatively high energy contents (Sami et al., 2002; Scurlock, 2000) and they were used as the biomass fuels in this study. Rice husks from the Guangdong Province, China were used. They are one of the most important agricultural residues in terms of quantity. According to the Ministry of Agriculture of China (2005), China alone generated 51 million tons rice husks in 2003 and the amount increased to 54 million tones in 2004. Most rice husks are burned in an uncontrolled way creating significant environmental problems (Hays et al., 2005; Yonemura and Kawashima, 2006; Yan et al., 2006). Farmers consider rice husks as waste only and they may need to pay disposal fees to deposit the husks in landfills. However, if rice husks are co-fired with coal, rice husks can be supplied to power companies for power generation. An extra economic benefit can be offered to farmers at the same time that environmental problems can be tackled. Bamboo is another biomass fuel used in
7463
this study and it has been rarely studied in the past. Most bamboo species have strong, light and flexible woody stems, which are suitable for application as construction materials in Hong Kong. Most of the bamboo in Hong Kong is imported from Guangdong and Sichuan Provinces, China. They are used as temporary scaffolding poles surrounding highrise buildings for construction purposes. Most of them are disposed to landfills when the strength of the bamboo deteriorates or after the completion of the construction works. Co-combustion of bamboo with coal has several benefits including its relatively high energy content among the various biomass fuels. This does not significantly affect the combustion process and can also help to reduce the loading in landfill sites. Our previous paper considered particle and the associated PAH emissions during co-combustion of rice husks and bamboo with coal (Chao et al., 2007). Results showed that the use of biomass blend with a high excess air ratio (30%) would reduce the particle and the associated PAH emissions. However, elements including K, Na, S, Ca, Mg, Fe, Si are involved in the reactions leading to potential slagging, fouling and clinker formation problems in boiler tubes (Demirbas, 2005). The current paper addressed the combustion performance and gaseous pollutant emissions of cocombustion of these two biomass fuels with coal. 2. Experimental Co-combustion tests of pulverized coal, rice husks and bamboo were conducted in a laboratory-scale, pulverized fuel combustion testing facility (Fig. 1), which was specially designed for combustion and pollutant control studies of coal and biomass fuels under well-controlled conditions. Specialized probes for sampling and measuring gaseous pollutants and particulate matters, in situ flame viewing and a temperature measuring system were equipped. The test furnace had a height of 1700 mm and an inner size of 230 mm 230 mm and was made of Bauxite firebricks and fire clay. Kaowool ceramic fiber refractory materials 9 mm in thickness were coated on the outer wall to minimize the heat losses to the surroundings. Pulverized coal and biomass were mixed and supplied by a 5 L storage bin with a variable speed screw conveyor and a stirrer to provide a stable fuel feeding rate in order to mix the fuel with the primary combustion air. The fuel feeding rate could be varied from 0 to 3 kg h1 by
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
Air-Flow
7464
7
Air-Flow 10
6
5 5b
5a 3
9 2
4 8 1. 2. 3. 4. 5.
LPG burner LPG cylinder Solid fuel feeder Primary and secondary air Burner a. Primary air nozzle b. Secondary air nozzles
1 6. Thermocouples with data logger 7. Water cooled heat exchanger 8. Ash hoppers 9. Multi-gas analy zer (Testo 350X L) 10. Particl e sampling device s
Fig. 1. Schematic diagram of the pulverized coal combustion testing facility.
adjusting the speed of the conveying screw. Air to the furnace was divided into primary and secondary air. Flow controllers were installed to control the mixing rate of the fuel and the primary air. Pulverized coal and biomass were transported by the primary air at a fixed flow rate, so that the flow dynamics did not vary with the excess air ratios. Tangential firing method was used in the furnace. The pulverized coal and biomass blend was discharged from the burners positioned at the corners of the furnace towards an imaginary circle located in the center of the furnace. The elevations of the burners were 01, so that the mixture came out in a horizontal direction. The burners were located in a square pattern and the angles of location of the burners were adjusted to 561 with respect to the furnace wall in order to maximize the fuel mixing rate and the flame stability. Four rectangular
primary air nozzles each with a size of 15 mm 10 mm were used in providing good flame stability. Eight secondary air nozzles each with 6 mm diameters were divided in two sets; one was located above and one was below the four primary air nozzles to provide extra oxygen and jet flow to the combustion zone. This configuration helped to assist in the entrainment of primary air into the hot furnace gas. The lower level jet of the secondary air could support the biomass blend within the designed zone for combustion and prevented it from dropping to the bottom ash hopper. The upper level jets of the secondary air could control the height of the flame so that it would not exceed the designed zone for combustion. The furnace was preheated with an LPG burner installed at the bottom of the furnace and would gradually shift to pulverized fuel during the burning process.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
2.1. Fuel characterizations Proximate analyses of the fuels were conducted following standard methods for coal and coke testing. The moisture and ash content of the fuel were measured following the British Standard, BS 1016. The volatile matter (VM) content and the higher heating value (HHV) were measured following the standards from International Standard Organization, ISO 562 and ISO 1928, respectively. The ultimate analysis of C, H, N, O and S followed the American Standard Testing Methods (ASTM) D3176. The oxygen content of the fuel was obtained by the percentage difference to the C, H, N, S and the ash content in the fuel sample. The N2 physisorption analysis (BET) for BET surface area and pore volume of coal, rice husks and bamboo were determined by the nitrogen physics-adsorption apparatus (Coulter, SA-3100). 2.2. Temperature and pollutant measurement instrumentations
7465
from the O2 concentration based on the difference between the ambient O2 concentration (20.94%) and the measured O2 concentration. The algorithm assumed all the consumed O2 was converted into CO2. As the CO levels in the experiments were in the order of a few hundred ppm and thus the error was small. The resolution and response time were 0.1% and 20 s for the O2 measurement, 1 ppm and 40 s for the CO, NO and SO2 measurement, 0.1 ppm and 40 s for the NO2 measurement, respectively. Based on the ultimate analysis of the coal, it was calculated that the stoichiometric (theoretical) air–fuel ratio was 8.4 by mass. A fixed fuel feeding rate of 2.1 kg h1 was used and the corresponding air required for the stoichiometric reaction was 240 L min1. By changing the flow rate of the secondary air into the combustion zone, different excess air ratios were obtained. This paper focused on the results obtained at 30% excess air ratio with the corresponding air flow rate at 312 L min1. 2.3. Experimental parameters
An array of type-K thermocouples with a sheath diameter of 0.5 mm was installed along the center of the furnace. A multiplexer (National instrument, SCXI-1330) for multi-thermocouples data acquisition and a data logging software (Labview) were used for temperature data collection. The temperature of the designed combustion zone was measured by four thermocouples installed at 50, 150, 250 and 400 mm above the primary air nozzles at the center of the furnace. The mean combustion temperature inside the furnace was then calculated by averaging the four temperature values. The remaining thermocouples were used to monitor the temperature after the combustion zone and the heat exchanger. A multi-gas analyzer (Testo, 350XL) was used for the measurement of gaseous contaminants in the flue gas. The measurement was conducted by electrochemical cells for CO, O2, NO, NO2 and SO2. The cross interference for the interference gases was minimized by the calibration of each sensor with single and multi-gas cylinders. The O2 sensor was calibrated in the concentration range of 5–21%. The CO and SO2 sensors were calibrated in the concentration range of 300–800 ppm. The NO sensor was calibrated in the concentration range of 100–150 ppm. The calibration and linearity data were stored in each of the gas sensors for compensations. CO2 concentration was also obtained from the same instrument and it was derived
The effects of the biomass blending ratio (BBR), the relative moisture content (RMC) and the biomass grinding size are reported in this paper. Excess air (EA) was fixed at 30% throughout all experimental conditions. 2.3.1. Biomass blending ratio (BBR) Biomass fuels are commonly characterized by higher content of VM compared with coal. This makes biomass fuels easier to ignite and the burning process can be sustained easier (Werther et al., 2000). However, the heating values of biomass fuels are lower than that of coal. It was expected that the biomass mix in coal can affect the co-combustion process and would subsequently affect the emissions of CO, CO2, SO2, NOx and particles. Five different BBRs, 0%, 20%, 30%, 40% and 50% were used in the experiments. BBR was defined as the ratio of the mass of the biomass to the total mass of the coal and biomass mixture. 2.3.2. Relative moisture content (RMC) A high moisture content in the fuel can lower the combustion temperature which in turn hinders the combustion of the reaction products and consequently affects the quality of combustion. In the experiments, fuels at RMCs of 8% and as-received level were used to investigate the effects of moisture
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
on the combustion performance and pollutant emissions. The as-received level was defined as the RMC level of the fuel that did not go through any additional moisture treatment after collected from the source. Coal and biomass were incubated in a humidity-controlled chamber (40 1C and 5% relative humidity) for 4 and 3 h, respectively. The RMC of the fuel was determined by comparing the fuel weight with a dry fuel that had been placed in the same chamber for 6.5 h after which no substantial weight loss was detected. 2.3.3. Biomass grinding size In the combustion process, some studies indicated that the fuel grinding size could affect the combustion performance (McLean et al., 1981; Du and Annamalai, 1994). Small sizes of fuel may enhance the reaction area for the reactants. This may increase the chemical reaction and accordingly affect the combustion performance. Previous research (Strehler and Stuetzle, 1987) on the co-combustion of straw with bituminous coal suggested that fine-sized straw improved the co-combustion behavior. However, one study on the co-combustion of switchgrass with coal (Aerts et al., 1997) suggested that large sizes of biomass did not adversely affect the combustion performance. Therefore, the effect of the biomass size on the cocombustion performance was explored in our experiments. Coal was ground to 75 mm by an automatic ball mill and was fixed at this size throughout the experiments. After the grinding process, pulverized coal was allowed to pass through three standard test sieves with sizes 106, 90 and 75 mm. Only the pulverized coal in the grinding range of 75–106 mm was used in the experiments. Seventy percent of the coal by mass was in the range of 75–90 mm. The remaining 30% was in the range of 90–106 mm. This ratio was chosen based on the standard practice currently adopted in existing power plants. Rice husks and bamboo were ground by a high-speed rotary mill. The biomass with grinding sizes within the ranges of 106–150, 150–212 and 300–425 mm were used in the experiments and defined as the grinding sizes of 100, 150 and 300 mm diameters, respectively, for simplicity. The emission factors (EF) in the following sections were presented in energy-based (mg kWh1) and weight-based (kg T1) units from the gaseous emission data. The energy-based EFe and the weight-based EFw were obtained from Eqs. (1)
and (2), respectively. EFe ðmg kWh1 Þ ¼
EFw ðkg T1 Þ ¼
C QG 3600 , _ HZ m
(1)
C QG , _ 1000 m
(2)
where C (mg m3) is the flue gas concentration of a specific species, QG (m3 h1) is the flue gas _ (kg h1) is the fuel feeding volumetric flow rate, m 1 rate, H (kg kJ ) is the heating value of the fuel and Z (%) is the combustion efficiency. 3. Results and discussion 3.1. Combustion temperature The change in the combustion temperature under different RMCs in the biomass fuels is shown in Fig. 2. There was a general decrease in temperature with the BBR. With reference to the baseline, the combustion temperature at 8% RMC decreased from 926 to 818 1C with 50% rice husk. The temperature decreased to 842 1C with 50% bamboo. The decrease in the combustion temperature was due to the different heating values of the biomass blend. The HHV of coal (27,463 kJ kg1) from Table 1 was about 40% higher than that of the biomass fuels. This implied that the overall energy would decrease as the biomass in the fuel blend increased. As bamboo had an HHV compared with rice husks, the temperature decrease of bamboo was smaller than that of rice husks. In order to investigate the effect of RMC, the grinding sizes of the coal and biomass were fixed at 75 and 300 mm, respectively. Temperature Furnace mean temperature (°C)
7466
950 900 850 800 Rice husk at 8% RMC Rice husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
750 700 0%
10%
20%
30%
40%
50%
Biomass blending ratio / BBR (%) Fig. 2. Combustion temperatures with different BBRs at various RMC.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
7467
Table 1 Proximate and ultimate analysis of coal, rice husks and bamboo Coal (wt%)
Rice husk (wt%)
Bamboo (wt%)
Proximate analysis Moisture Volatile matter Ash Fixed carbon Higher heating value (kJ kg1)
15.86 28.45 14.63 41.06 27463
10.61 64.44 8.93 16.02 16054
9.13 73.71 2.31 14.85 17296
Ultimate analysis (% dry basis) Carbon Hydrogen Nitrogen Oxygen (by difference) Sulfur
69.35 4.40 1.40 9.21 1.01
46.57 4.61 0.73 39.04 0.12
50.81 5.14 0.77 40.89 0.08
Table 2 Weight-based emission factors (EFw) of coal, rice husk and bamboo EFw at 8% RMC (kg T1)
EFw at as-received RMC (kg T1)
CO2
CO
NOx
SO2
CO2
CO
NOx
SO2
BBR (%) 0% (baseline)
1872
2.9
1.5
11.1
1862
3.1
1.4
10.9
Rice husk 10% 20% 50%
1834 1798 1450
2.7 2.3 1.6
1.4 1.2 1.0
10.3 9.1 7.9
1709 1664 1176
2.8 2.6 1.9
1.2 1.1 0.8
10.0 8.9 7.6
Bamboo 10% 20% 50%
1857 1824 1589
2.8 2.4 1.8
1.4 1.3 1.1
10.3 8.9 7.7
1825 1732 1447
2.9 2.6 2.0
1.3 1.2 0.9
9.9 8.8 7.5
changes of the co-combustion processes were compared when RMC was set to 8% and the as-received level (varying from 12% to 16% after mixing, see Table 1). The mean combustion temperature of the biomass/coal mixture at the as-received RMC level was about 5% lower that that at the 8% RMC level. Higher RMC implies higher free moisture content in the fuel. Part of the heat energy was transferred to the latent heat of vaporization during combustion thus lowering the overall furnace temperature. The change in temperature with different grinding sizes (100, 150 and 300 mm) of the biomass fuels was also studied. The RMC was fixed at 8%. The grinding size of the coal remained at 75–100 mm. There was no obvious change in the combustion temperature with the biomass grinding sizes under the same BBR.
3.2. Carbon dioxide emissions The weight basis emission factor (EFw) of CO2 decreased with biomass content as shown in Table 2. The EFw decreased from 1872 kg T1 at baseline (0% BBR) to 1589 kg T1 with 50% bamboo in the fuel blend (50% BBR). The EFw of rice husk at 50% BBR further decreased to around 1450 kg T1. The carbon content of rice husk from ultimate analysis was lower than bamboo and resulted in the lowered EFw at the same BBR. The CO2 EFw in the experiments was lower than the CO2 emission factor (2405 kg T1) suggested by the Environmental Protection Agency (USEPA) for sub-bituminous coal (USEPA, 1998). The laboratory-scale combustion at relatively low combustion temperature altered the CO2 emissions compared to large-scale systems. With reference to the baseline
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
CO2 emission per energy output (g/kWh)
240 220 200 180 160 Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
140 120 100 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 3. CO2 emissions with different BBRs at various RMC.
under the same energy output (EFe) as shown in Fig. 3, rice husks and bamboo showed a minimum emissions range for CO2 at 20–30% BBR. The EFe decreased from 220 g kWh1 at baseline to around 180 g kWh1 for rice husks and 200 g kWh1 for bamboo at 20% BBR and 8% RMC. The energy output is based on the heating value, the fuel feeding rate and the combustion efficiency of the combustion process. It should be noted that the combustion efficiencies based on post-combustion gas composition data were over 90% at different BBR levels. The experimental CO2 emission per unit energy output was much lower than the conventional emission factor because the thermal efficiency of the power plant was not included in the calculations. The theoretical carbon content per unit energy increased slightly (about 5–6%) with BBR. From ultimate analysis, the theoretical carbon content per unit energy output was about 90.91 g C kWh1 for coal, 95.76g C kWh1 when 50% rice husk and 96.48 g C kWh1 when 50% bamboo is mixed with the coal. CO2 emissions follow this trend if a complete reaction takes place. The carbon content, C, in the ultimate analysis was determined by the amount of CO2 released in the product gas when burning the fuel at a constant temperature with complete combustion. However, the experimental CO2 results did not follow the theoretical values. The combustion temperature was not constant and it decreased with the BBR in the experiments. As a result, not all carbon content in the fuel reacted with oxygen to form CO2; the other carbon content might react to form CO and CH4. However, the majority of the fuel carbon not converted to CO2 was entrained in bottom ash. Moisture content in the fuel mixture seemed to
lower the CO2 emissions. The performance of rice husks at 8% and at the as-received RMC level (12–16%) showed a 15% CO2 emissions difference at the BBR while the minimum emissions occurred and there was about 10% difference in bamboo at the minimum range compared with the baseline. That temperature decreased with RMC and the unburned carbon fraction therefore increased. As more unburned carbon became available, CO2 emissions were reduced. The CO2 emissions during the co-combustion process at different biomass grinding sizes were studied. There was no obvious difference in CO2 emissions at different biomass grinding sizes under each BBR. 3.3. Carbon monoxide emissions Table 2 shows that the CO EFw decreased with biomass. The EFw at 8% RMC decreased from 2.9 kg T1 to around 1.6 kg T1 for rice husks and 1.8 kg T1 for bamboo. The experimental CO EFw fell into the range of the CO EF (0.25–3 kg T1) suggested by the USEPA for sub-bituminous coal (USEPA, 1998). A minimum CO EFe at 10–20% BBR for co-combustion with rice husks was found in Fig. 4. The minimum CO EFe occurred at 20–30% BBR in the case of bamboo. When more biomass was mixed with coal, which implied the fraction of energy generated from char combustion would be lower, there was a decrease in CO emissions. Fig. 4 also shows the effect of RMC on CO emissions. With both cases of rice husks and bamboo, there was a gain in the CO concentration with RMC. Rice husks had a 30% reduction in CO emissions when the RMC levels changed from the
380 CO emission per energy output (mg/kWh)
7468
330 280 230 180 Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
130 80 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 4. CO emissions with different BBRs at various RMC.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
220 NOx emission per energy output (mg/kWh)
as-received level (12–16%) to 8% within the minimum emission range, while bamboo had a 20% decrease. The RMC level lowered the combustion temperature and thus yielded a higher unburned carbon fraction resulting in higher CO emissions. Changes in the biomass grinding size showed no obvious difference in CO emissions. The CO to CO2 emission ratio was found to be 1.52 103 at baseline. With the use of 20% rice husk and bamboo, the ratio decreased to 9.56 104 and 1.11 103, respectively. This implied that biomass enhanced the oxidation efficiency of the fuel. These ratios were close to the range of the CO/CO2 ratio (1 104–1.3 103) suggested by the USEPA (USEPA, 1998).
7469
200 180 160 140
Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
120 100 0%
10%
20%
30%
40%
50%
Biomass blending ratio / BBR (%) Fig. 5. NOx emissions with different BBRs at various RMC.
3.4. Nitrogen oxides emissions During the combustion process, NOx was generated through three formation mechanisms: prompt NOx, thermal NOx and fuel NOx. The amount of prompt NOx was significantly small and it was thus neglected in the discussion. The combustion temperature in the test rig was lower than 1000 1C so the thermal NOx formation was not significant. Fuel NOx was the major contributor to the total NOx emissions. The nitrogen content of coal (1.4%) was about two times higher than that in rice husks (0.73%) and bamboo (0.77%) as shown in Table 1. The nitrogen content in the fuel could directly affect nitrogen oxide generation during the combustion process and thus the use of biomass could reduce the overall NOx emissions. In Table 2, the EFw ranged from 1.5 kg T1 at baseline to 1.0 kg T1 for rice husks and 1.1 kg T1 for bamboo. The experimental EFw was lower than the NOx EF (around 4 kg T1) suggested by the USEPA (USEPA, 1998). This was because the formation of thermal NOx in the experimental combustion system was not significant when compared to the large-scale system and resulted in a lowered NOx emission in all cases. In Fig. 5, the minimum NOx emission range (EFe) is 10–20 and 20–30% BBR for rice husks and bamboo, respectively. The effect of RMC on the NOx concentration is also shown in the same figure. An increase in RMC from 8% to the as-received level (12–16%) caused a 20% decrease in NOx emissions within the minimum emission range for both rice husks and bamboo. This was because the RMC increased the unburned carbon fraction, which implied that the fuel nitrogen could not be effectively transferred to NOx and hence reduced
the overall NOx emissions. In addition, there was no significant change of the NOx emissions with biomass grinding size at each BBR. 3.5. Sulfur dioxide emissions The sulfur content in fuel has a direct effect on the generation of sulfur dioxide during combustion. The SO2 EFw decreased with biomass as shown in Table 2. The SO2 EFw was about 11 kg T1 at baseline and decreased to around 10 kg T1 for both rice husks and bamboo at 50% BBR and 8% RMC. The SO2 EF from USEPA was around 17.5 kg T1 for sub-bituminous coal with 1% sulfur content (USEPA, 1998). The experimental EFw were lower than the EF suggested by the USEPA. Part of the fuel sulfur was emitted as particulate sulfate and also retained in the bottom ash. These resulted in a lowered experimental EFw. The minimum SO2 emission range for EFe was shown to be at 20–30% BBR for both rice husks and bamboo in Fig. 6. The reduction of SO2 emissions could be attributed to a decrease in the overall fuel sulfur content. As shown in Table 1, the coal sulfur content was over ten times more than that in biomass. For rice husks and bamboo at the minimum emissions range, the SO2 EFe were the highest at 8% RMC and the difference was around 20%. The combustion temperature at 8% RMC was higher than that at the as-received RMC level. The fuel sulfur content could react with oxygen effectively at low RMC as relatively complete combustion could be attained at a higher combustion temperature. The effect of the biomass grinding size on the SO2 concentration was also studied.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
7470
SO2 emission per energy output (mg/kWh)
1600 1500 1400 1300 1200 1100 1000
Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
900 800 700 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 6. SO2 emissions with different BBRs at various RMC.
Again, the biomass grinding size had no obvious effect on the SO2 concentration under different BBR.
grinding size was reduced from 300 to 100 mm. The ratio of this change was much smaller than that of the grinding size reduction and thus it was assumed to be an independent factor in the oxygen diffusion rate. To explain the effect of the grinding size in this process, the burning rate of single fuel particle could be derived based on mass diffusion (Smith, 1983; Caram and Amundson, 1977): _c ¼ m
Y O2 ;1 Y O2 , Rkin þ Rdiff
(3)
where Y O2 ;1 and Y O2 are the mass fraction of oxygen in the free stream and on the surface, respectively. The kinetic and diffusion parameters in Eq. (3) are defined as follows: Rkin ¼
n1 R u T s , 4pr2s MWmix kc P
(4)
n1 þ Y O2 ;s , 4prs rD
(5)
and 3.6. Influence of fuel grinding size Rdiff ¼ As reported in the previous sections, the biomass grinding size did not seem to have a significant impact to the combustion performance. There was no observable change in the combustion temperature and the various pollutant emissions did not differ much at various fuel grinding sizes. This can perhaps be explored more through a review of the fundamental combustion mechanisms. The combustion of a solid fuel such as that in our system proceeds in two stages: (1) gasification and combustion of the VM (volatile oxidation) and (2) combustion of fixed carbon (FC) (char oxidation). In the first stage, the fuel particles are partially pyrolyzed and, at the same time, the VM is released and the char content is left behind. This VM releasing (gasification) rate is controlled by the kinetics of pyrolysis and is fuel dependent. In Table 1, about 65–74% of the total biomass mass was VM. Reducing the grinding size may not have much effect on the overall combustion process since the time scale for combustion of VM is on the order of only a few ms, which is much quicker than the combustion of fixed carbon, which is on the order of seconds (Sami et al., 2002). In the second stage, char is the remaining product. Oxygen diffuses to the char surface/into the porous structure and reacts to form carbon monoxide and carbon dioxide. The pore surface area of the fuel can influence the oxygen diffusion rate. According to the BET surface analysis of rice husks and bamboo, there was only 5% increase of the pore surface area when the
where n1 is the mass stoichiometric coefficient, rs is the fuel particle radius, r is the solid fuel density, D is the mass diffusivity, Ru is the universal gas constant, Ts is the combustion temperature, MWmix is the molecular weight of the flue gas, kc is the kinetic rate constant and P is the pressure. The kinetic parameter (Rkin) is governed by the surface area of the fuel particle ð4pr2s Þ, the fuel kinetics rate (kc) and the combustion temperature (Ts). The surface area of the biomass is inversely proportional to Rkin. When the grinding size of the biomass changed from 300 to 100 mm, Rkin increased inversely to the square of the fuel radius. It should also be noted that the combustion temperature (Ts) has a substantial effect on the kinetic rate constant, kc ¼ A exp[EA/RuTs], where A is the frequency factor and EA is the activation energy. On the other hand, the diffusion parameter (Rdiff) increases inversely with the size of the fuel particle (rs) and the dependence on the combustion temperature (Ts) is weaker than the chemical kinetics term. After using the experimental data in the equations, we found that the ratio between Rkin and Rdiff (Rkin/Rdiff) was greater than unity, indicating that our experimental conditions might be kinetically controlled. Rdiff was small compared to Rkin and thus the mass diffusion parameter was not important. This was due to the fact that, in our experiments, the combustion temperature was not high (around 837–847 1C for rice husks and
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
897–907 1C for bamboo) and the chemical kinetics (the rate-determining step) could play a more important role. In the case of rice husks at 20% BBR and 30% EA, the ratio increased, respectively, from 2.7 to 8.7 when the biomass fuel grinding size was reduced from 300 to 100 mm. For bamboo, the ratio increased, respectively, from 1.6 to 4.7 under the same size shift. The combustion process tended towards the kinetically controlled zone at smaller _ c , would thus grinding sizes. The burning rate, m increase with the square of the biomass fuel radius, which is supported by fitting in our experimental data (the order of magnitude from 1 1010 to 9 1010 kg s1 when the size increases from 100 to 300 mm). The number of fuel particles in the combustion system should be inversely proportional to r3s and, theoretically, the total burning rate should thus be proportional to 1/rs. However, our experimental data indicated that the burning characteristics remained quite independent of the change in the biomass fuel size and did not follow the 1/rs dependence. The possible reasons could be (3) the biomass only contributed o50% of the total fuel; (4) the VM contributed over 60% of the mass in the biomass fuel; and (5) the high excess air ratio took away the heat generated from the system faster than the small increase in the fuel burning rate. This means that the actual fixed carbon contribution from the biomass fuels to the overall combustion process was o15% and in some cases it was as low as 4–5%. This led to the relatively constant combustion behavior with the change of the biomass grinding size under our experimental conditions. This could help to explain why in previous works by other researchers sometimes the burning was size dependent and sometimes it was not.
7471
minimum pollutant emission factors (EFe) normalized by the energy output. General decreases in temperature, SO2, NOx and CO2 were observed with the RMC, while an increase was observed for CO emissions. The minimum range for each pollutant emission was not exactly the same, but they were all within the range. The effect in changing the grinding size of biomass was not significant as volatile combustion and the fuel burning rate did not change much with the biomass grinding size. A comparison between the mass diffusion and the chemical kinetic parameters showed that, under our experimental conditions, the combustion process seemed to be chemically kinetically controlled. Although the use of biomass seemed to reduce the combustion temperature, increasing the excess air to an optimum level (e.g., 30%) and drying the fuel (lower RMC) could increase the co-combustion temperature. Increasing the co-combustion temperature has several advantages including: (1) the potential for slagging and fouling would be reduced since the difference between the furnace and condensation temperatures of the fly ash could be increased; (2) the thermal efficiency of power plants would be improved and (3) the CO and particulate emissions would be lowered. However, SO2, NOx and CO2 emissions would increase with the combustion temperature under the same BBR. Acknowledgments The authors would like to acknowledge the technical and financial support by the CLP Research Institute. Special thanks must be paid to the CLP general laboratory for the fuel characteristics analysis.
4. Conclusions References A laboratory-scale pulverized fuel combustion testing facility was used to investigate the cocombustion performance of coal with biomass. Rice husks and bamboo were chosen as the biomass fuels in this study. Fuel characteristics and gaseous pollutant emission factors for both weight and energy basis were investigated under different BBR, RMCs and biomass grinding sizes. Co-combustion of biomass with coal decreased the emissions of most pollutants, such as CO2, CO, NOx and SO2, from existing pulverized coal firing power plants. A range of 10–30% BBR was found to be the
Aerts, D.J., Bryden, K.M., Hoerning, J.M., Ragland, K.W., 1997. Co-firing switchgrass in a 50MW pulverized coal boiler. In: Proceedings of the 59th Annual American Power Conference, vol. 59 (issue 2), Chicago, IL, pp. 1180–1185. Baxter, L., 2005. Biomass-coal co-combustion: opportunity for affordable renewable energy. Fuel 84 (10), 1295–1302. Caram, H.S., Amundson, N.R., 1977. Diffusion and reaction in a stagnant boundary layer about a carbon particle. Industrial Engineering Chemistry Fundamentals 16 (2), 171–181. Chao, C.Y.H., Kwong, C.W., Wang, J.H., Cheung, C.W., Kendall, G., 2007. Co-firing coal with rice husk and bamboo and the impact on particulate matters and associated
ARTICLE IN PRESS 7472
P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
polycyclic aromatic hydrocarbon emissions. Bioresource Technology, doi:10.1016/j.biortech.2006.11.051. Demirbas, A., 2005. Potential applications of renewable energy sources, biomass combustion problems in boiler systems and combustion related environmental issues. Progress in Energy and Combustion Science 31, 171–192. Du, X., Annamalai, K., 1994. The transient ignition of isolated coal particles. Combustion and Flame 97, 339–354. Energy Information Administration (EIA), 2007 /http://www. eia.doe.gov/fuelcoal.htmlS. Gani, A., Morishita, K., Nishikawa, K., Naruse, I., 2005. Characteristics of co-combustion of low-rank coal with biomass. Energy & Fuels 19 (4), 1652–1659. Hays, M.D., Fine, P.M., Geron, C.D., Kleeman, M.J., Gullett, B.K., 2005. Open burning of agricultural biomass: physical and chemical properties of particle-phase emissions. Atmospheric Environment 39 (36), 6747–6764. Kruczek, H., Raczka, P., Tatarek, A., 2006. The effect of biomass on pollutant emission and burnout in co-combustion with coal. Combustion Science and Technology 178 (8), 1511–1539. McLean, W.J., Hardesty, D.R., Poul, J.H., 1981. Direct observation of devolatlizing pulverized coal particles in a combustion environment. In: Proceedings of the 18th Symposium (International) on Combustion, pp. 1239–48.
Ministry of Agriculture of the People’s Republic of China, 2005 /http://www.agri.gov.cnS. Sami, M., Annamalai, K., Wooldridge, M., 2002. Co-firing of coal and biomass fuel blends. Progress in Energy and Combustion Science 27, 171–214. Scurlock, J.M.O., 2000. Bamboo: an overlooked biomass resource? Environmental Sciences Division, #4963. Smith, I.W., 1983. The Combustion Rates of Coal Chars: A Review. Nineteenth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, 1045–1065. Strehler, A., Stuetzle, W., 1987. Biomass residues. In: Hall, D.O. (Ed.), Biomass. Wiley, New York, pp. 75–97. US Environmental Protection Agency (USEPA), 1998. External combustion sources—bituminous and subbituminous coal combustion, AP 42, 5(1). Werther, J., Saenger, M., Hartge, E.U., Ogada, T., Siagi, Z., 2000. Combustion of agricultural residues. Progress in Energy and Combustion Science 26, 1–27. Yan, X., Ohara, T., Akimoto, H., 2006. Bottom-up estimate of biomass burning in mainland China. Atmospheric Environment 40 (27), 5262–5273. Yonemura, S., Kawashima, S., 2006. Concentration of carbon gases and oxygen and their emission ratios from the combustion of rice hulls in a wind tunnel. Atmospheric Environment.
Atmospheric Environment 41 (2007) 7462–7472 www.elsevier.com/locate/atmosenv
Co-combustion performance of coal with rice husks and bamboo Philip C.W. Kwonga, Christopher Y.H. Chaoa,, J.H. Wanga, C.W. Cheungb, Gail Kendallb a
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b CLP Research Institute, 20/F, Tower 1, Grand Century Place, 193 Prince Edward Road West, Kowloon, Hong Kong Received 31 January 2007; received in revised form 16 May 2007; accepted 23 May 2007
Abstract Biomass has been regarded as an important form of renewable energy due to the reduction of green house gas emission such as carbon dioxide. An experimental study of co-combustion of coal and biomass was performed in a laboratory-scale combustion facility. Rice husks and bamboo were the selected biomass fuels in this study due to their abundance in the Asia-Pacific region. Experimental parameters including the biomass blending ratio in the fuel mixture, relative moisture content and biomass grinding size were investigated. Both energy release data and pollutant emission information were obtained. Due to the decrease in the heating value from adding biomass in the fuel mixture, the combustion temperature and energy output from the co-firing process were reduced compared with coal combustion. On the other hand, gaseous pollutant emissions including carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and sulfur dioxide (SO2) were reduced and minimum energy-based emission factors were found in the range of 10–30% biomass blending ratio. With an increase in the moisture content in the biomass, decreases in combustion temperature, SO2, NOx and CO2 emissions were observed, while an increase in CO emissions was found. It has also been observed that chemical kinetics may play an important role compared to mass diffusion in the co-firing process and the change in biomass grinding size does not have much effect on the fuel burning rate and pollutant emissions under the current experimental conditions. r 2007 Elsevier Ltd. All rights reserved. Keywords: Co-combustion; Coal; Rice husks; Bamboo; Gaseous pollutants
1. Introduction Greenhouse gas (GHG) emissions from conventional coal-fired power plants draw worldwide attention on their environmental effects. Coal contributed to around 23–26% of the total world energy consumption in 1990–2004 (EIA, 2007) and it was one of the major sources of energy-related Corresponding author. Tel.: +852 2358 7210; fax: +852 2358 1543. E-mail address: [email protected] (C.Y.H. Chao).
1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.05.040
GHG emissions. According to the same report, coal combustion emitted 36–39% of the total global carbon dioxide (CO2) emissions in 1990–2004 and the emission trend will last for some time. Nevertheless, it is one of the cheapest energy sources and dependence on coal in power generation is still substantial especially in developing countries. Renewable energy resources from biomass can relieve the intensity of GHG emissions and play an important role in power generation under sustainability considerations. Because plants consume carbon dioxide during the photosynthesis process,
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
the overall CO2 emissions during biomass combustion can be offset. This is a fundamental advantage of extracting energy from biomass through combustion processes from the perspective of sustainability. Burning biomass alone cannot give very high energy output when compared with coal combustion due to the high volatile matter and moisture content. Cocombustion involves the combustion of coal and other fuels in the same boiler. Co-combustion is worthy of exploration to improve energy output (Baxter, 2005). It can be a practical and attractive approach to increasing the use of renewable energy while maintaining an efficient power supply with reduction of GHG at a potentially cheaper cost. This is particularly true when the cost of highquality coal has been surging in recent years. Several potential benefits are driving interest in co-combustion (Kruczek et al., 2006; Gani et al., 2005). Leading the way are the possible environmental advantages of co-combustion, particularly the reduction of acid rain precursors and GHG such as sulfur dioxide (SO2), nitrogen oxides (NOx) and CO2. Information on biomass co-combustion can be found in several articles that summarized the state-of-the-art developments in this field (Demirbas, 2005; Sami et al., 2002). These researchers have indicated that co-combustion of coal and biomass is an emerging new area in pollutant reduction and has a high potential for exploration. Rice husks are the outer shell of rice grain and it is a common agricultural residue in China. Bamboo is a kind of construction debris in Hong Kong. These two biomass fuels have relatively high energy contents (Sami et al., 2002; Scurlock, 2000) and they were used as the biomass fuels in this study. Rice husks from the Guangdong Province, China were used. They are one of the most important agricultural residues in terms of quantity. According to the Ministry of Agriculture of China (2005), China alone generated 51 million tons rice husks in 2003 and the amount increased to 54 million tones in 2004. Most rice husks are burned in an uncontrolled way creating significant environmental problems (Hays et al., 2005; Yonemura and Kawashima, 2006; Yan et al., 2006). Farmers consider rice husks as waste only and they may need to pay disposal fees to deposit the husks in landfills. However, if rice husks are co-fired with coal, rice husks can be supplied to power companies for power generation. An extra economic benefit can be offered to farmers at the same time that environmental problems can be tackled. Bamboo is another biomass fuel used in
7463
this study and it has been rarely studied in the past. Most bamboo species have strong, light and flexible woody stems, which are suitable for application as construction materials in Hong Kong. Most of the bamboo in Hong Kong is imported from Guangdong and Sichuan Provinces, China. They are used as temporary scaffolding poles surrounding highrise buildings for construction purposes. Most of them are disposed to landfills when the strength of the bamboo deteriorates or after the completion of the construction works. Co-combustion of bamboo with coal has several benefits including its relatively high energy content among the various biomass fuels. This does not significantly affect the combustion process and can also help to reduce the loading in landfill sites. Our previous paper considered particle and the associated PAH emissions during co-combustion of rice husks and bamboo with coal (Chao et al., 2007). Results showed that the use of biomass blend with a high excess air ratio (30%) would reduce the particle and the associated PAH emissions. However, elements including K, Na, S, Ca, Mg, Fe, Si are involved in the reactions leading to potential slagging, fouling and clinker formation problems in boiler tubes (Demirbas, 2005). The current paper addressed the combustion performance and gaseous pollutant emissions of cocombustion of these two biomass fuels with coal. 2. Experimental Co-combustion tests of pulverized coal, rice husks and bamboo were conducted in a laboratory-scale, pulverized fuel combustion testing facility (Fig. 1), which was specially designed for combustion and pollutant control studies of coal and biomass fuels under well-controlled conditions. Specialized probes for sampling and measuring gaseous pollutants and particulate matters, in situ flame viewing and a temperature measuring system were equipped. The test furnace had a height of 1700 mm and an inner size of 230 mm 230 mm and was made of Bauxite firebricks and fire clay. Kaowool ceramic fiber refractory materials 9 mm in thickness were coated on the outer wall to minimize the heat losses to the surroundings. Pulverized coal and biomass were mixed and supplied by a 5 L storage bin with a variable speed screw conveyor and a stirrer to provide a stable fuel feeding rate in order to mix the fuel with the primary combustion air. The fuel feeding rate could be varied from 0 to 3 kg h1 by
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
Air-Flow
7464
7
Air-Flow 10
6
5 5b
5a 3
9 2
4 8 1. 2. 3. 4. 5.
LPG burner LPG cylinder Solid fuel feeder Primary and secondary air Burner a. Primary air nozzle b. Secondary air nozzles
1 6. Thermocouples with data logger 7. Water cooled heat exchanger 8. Ash hoppers 9. Multi-gas analy zer (Testo 350X L) 10. Particl e sampling device s
Fig. 1. Schematic diagram of the pulverized coal combustion testing facility.
adjusting the speed of the conveying screw. Air to the furnace was divided into primary and secondary air. Flow controllers were installed to control the mixing rate of the fuel and the primary air. Pulverized coal and biomass were transported by the primary air at a fixed flow rate, so that the flow dynamics did not vary with the excess air ratios. Tangential firing method was used in the furnace. The pulverized coal and biomass blend was discharged from the burners positioned at the corners of the furnace towards an imaginary circle located in the center of the furnace. The elevations of the burners were 01, so that the mixture came out in a horizontal direction. The burners were located in a square pattern and the angles of location of the burners were adjusted to 561 with respect to the furnace wall in order to maximize the fuel mixing rate and the flame stability. Four rectangular
primary air nozzles each with a size of 15 mm 10 mm were used in providing good flame stability. Eight secondary air nozzles each with 6 mm diameters were divided in two sets; one was located above and one was below the four primary air nozzles to provide extra oxygen and jet flow to the combustion zone. This configuration helped to assist in the entrainment of primary air into the hot furnace gas. The lower level jet of the secondary air could support the biomass blend within the designed zone for combustion and prevented it from dropping to the bottom ash hopper. The upper level jets of the secondary air could control the height of the flame so that it would not exceed the designed zone for combustion. The furnace was preheated with an LPG burner installed at the bottom of the furnace and would gradually shift to pulverized fuel during the burning process.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
2.1. Fuel characterizations Proximate analyses of the fuels were conducted following standard methods for coal and coke testing. The moisture and ash content of the fuel were measured following the British Standard, BS 1016. The volatile matter (VM) content and the higher heating value (HHV) were measured following the standards from International Standard Organization, ISO 562 and ISO 1928, respectively. The ultimate analysis of C, H, N, O and S followed the American Standard Testing Methods (ASTM) D3176. The oxygen content of the fuel was obtained by the percentage difference to the C, H, N, S and the ash content in the fuel sample. The N2 physisorption analysis (BET) for BET surface area and pore volume of coal, rice husks and bamboo were determined by the nitrogen physics-adsorption apparatus (Coulter, SA-3100). 2.2. Temperature and pollutant measurement instrumentations
7465
from the O2 concentration based on the difference between the ambient O2 concentration (20.94%) and the measured O2 concentration. The algorithm assumed all the consumed O2 was converted into CO2. As the CO levels in the experiments were in the order of a few hundred ppm and thus the error was small. The resolution and response time were 0.1% and 20 s for the O2 measurement, 1 ppm and 40 s for the CO, NO and SO2 measurement, 0.1 ppm and 40 s for the NO2 measurement, respectively. Based on the ultimate analysis of the coal, it was calculated that the stoichiometric (theoretical) air–fuel ratio was 8.4 by mass. A fixed fuel feeding rate of 2.1 kg h1 was used and the corresponding air required for the stoichiometric reaction was 240 L min1. By changing the flow rate of the secondary air into the combustion zone, different excess air ratios were obtained. This paper focused on the results obtained at 30% excess air ratio with the corresponding air flow rate at 312 L min1. 2.3. Experimental parameters
An array of type-K thermocouples with a sheath diameter of 0.5 mm was installed along the center of the furnace. A multiplexer (National instrument, SCXI-1330) for multi-thermocouples data acquisition and a data logging software (Labview) were used for temperature data collection. The temperature of the designed combustion zone was measured by four thermocouples installed at 50, 150, 250 and 400 mm above the primary air nozzles at the center of the furnace. The mean combustion temperature inside the furnace was then calculated by averaging the four temperature values. The remaining thermocouples were used to monitor the temperature after the combustion zone and the heat exchanger. A multi-gas analyzer (Testo, 350XL) was used for the measurement of gaseous contaminants in the flue gas. The measurement was conducted by electrochemical cells for CO, O2, NO, NO2 and SO2. The cross interference for the interference gases was minimized by the calibration of each sensor with single and multi-gas cylinders. The O2 sensor was calibrated in the concentration range of 5–21%. The CO and SO2 sensors were calibrated in the concentration range of 300–800 ppm. The NO sensor was calibrated in the concentration range of 100–150 ppm. The calibration and linearity data were stored in each of the gas sensors for compensations. CO2 concentration was also obtained from the same instrument and it was derived
The effects of the biomass blending ratio (BBR), the relative moisture content (RMC) and the biomass grinding size are reported in this paper. Excess air (EA) was fixed at 30% throughout all experimental conditions. 2.3.1. Biomass blending ratio (BBR) Biomass fuels are commonly characterized by higher content of VM compared with coal. This makes biomass fuels easier to ignite and the burning process can be sustained easier (Werther et al., 2000). However, the heating values of biomass fuels are lower than that of coal. It was expected that the biomass mix in coal can affect the co-combustion process and would subsequently affect the emissions of CO, CO2, SO2, NOx and particles. Five different BBRs, 0%, 20%, 30%, 40% and 50% were used in the experiments. BBR was defined as the ratio of the mass of the biomass to the total mass of the coal and biomass mixture. 2.3.2. Relative moisture content (RMC) A high moisture content in the fuel can lower the combustion temperature which in turn hinders the combustion of the reaction products and consequently affects the quality of combustion. In the experiments, fuels at RMCs of 8% and as-received level were used to investigate the effects of moisture
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
on the combustion performance and pollutant emissions. The as-received level was defined as the RMC level of the fuel that did not go through any additional moisture treatment after collected from the source. Coal and biomass were incubated in a humidity-controlled chamber (40 1C and 5% relative humidity) for 4 and 3 h, respectively. The RMC of the fuel was determined by comparing the fuel weight with a dry fuel that had been placed in the same chamber for 6.5 h after which no substantial weight loss was detected. 2.3.3. Biomass grinding size In the combustion process, some studies indicated that the fuel grinding size could affect the combustion performance (McLean et al., 1981; Du and Annamalai, 1994). Small sizes of fuel may enhance the reaction area for the reactants. This may increase the chemical reaction and accordingly affect the combustion performance. Previous research (Strehler and Stuetzle, 1987) on the co-combustion of straw with bituminous coal suggested that fine-sized straw improved the co-combustion behavior. However, one study on the co-combustion of switchgrass with coal (Aerts et al., 1997) suggested that large sizes of biomass did not adversely affect the combustion performance. Therefore, the effect of the biomass size on the cocombustion performance was explored in our experiments. Coal was ground to 75 mm by an automatic ball mill and was fixed at this size throughout the experiments. After the grinding process, pulverized coal was allowed to pass through three standard test sieves with sizes 106, 90 and 75 mm. Only the pulverized coal in the grinding range of 75–106 mm was used in the experiments. Seventy percent of the coal by mass was in the range of 75–90 mm. The remaining 30% was in the range of 90–106 mm. This ratio was chosen based on the standard practice currently adopted in existing power plants. Rice husks and bamboo were ground by a high-speed rotary mill. The biomass with grinding sizes within the ranges of 106–150, 150–212 and 300–425 mm were used in the experiments and defined as the grinding sizes of 100, 150 and 300 mm diameters, respectively, for simplicity. The emission factors (EF) in the following sections were presented in energy-based (mg kWh1) and weight-based (kg T1) units from the gaseous emission data. The energy-based EFe and the weight-based EFw were obtained from Eqs. (1)
and (2), respectively. EFe ðmg kWh1 Þ ¼
EFw ðkg T1 Þ ¼
C QG 3600 , _ HZ m
(1)
C QG , _ 1000 m
(2)
where C (mg m3) is the flue gas concentration of a specific species, QG (m3 h1) is the flue gas _ (kg h1) is the fuel feeding volumetric flow rate, m 1 rate, H (kg kJ ) is the heating value of the fuel and Z (%) is the combustion efficiency. 3. Results and discussion 3.1. Combustion temperature The change in the combustion temperature under different RMCs in the biomass fuels is shown in Fig. 2. There was a general decrease in temperature with the BBR. With reference to the baseline, the combustion temperature at 8% RMC decreased from 926 to 818 1C with 50% rice husk. The temperature decreased to 842 1C with 50% bamboo. The decrease in the combustion temperature was due to the different heating values of the biomass blend. The HHV of coal (27,463 kJ kg1) from Table 1 was about 40% higher than that of the biomass fuels. This implied that the overall energy would decrease as the biomass in the fuel blend increased. As bamboo had an HHV compared with rice husks, the temperature decrease of bamboo was smaller than that of rice husks. In order to investigate the effect of RMC, the grinding sizes of the coal and biomass were fixed at 75 and 300 mm, respectively. Temperature Furnace mean temperature (°C)
7466
950 900 850 800 Rice husk at 8% RMC Rice husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
750 700 0%
10%
20%
30%
40%
50%
Biomass blending ratio / BBR (%) Fig. 2. Combustion temperatures with different BBRs at various RMC.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
7467
Table 1 Proximate and ultimate analysis of coal, rice husks and bamboo Coal (wt%)
Rice husk (wt%)
Bamboo (wt%)
Proximate analysis Moisture Volatile matter Ash Fixed carbon Higher heating value (kJ kg1)
15.86 28.45 14.63 41.06 27463
10.61 64.44 8.93 16.02 16054
9.13 73.71 2.31 14.85 17296
Ultimate analysis (% dry basis) Carbon Hydrogen Nitrogen Oxygen (by difference) Sulfur
69.35 4.40 1.40 9.21 1.01
46.57 4.61 0.73 39.04 0.12
50.81 5.14 0.77 40.89 0.08
Table 2 Weight-based emission factors (EFw) of coal, rice husk and bamboo EFw at 8% RMC (kg T1)
EFw at as-received RMC (kg T1)
CO2
CO
NOx
SO2
CO2
CO
NOx
SO2
BBR (%) 0% (baseline)
1872
2.9
1.5
11.1
1862
3.1
1.4
10.9
Rice husk 10% 20% 50%
1834 1798 1450
2.7 2.3 1.6
1.4 1.2 1.0
10.3 9.1 7.9
1709 1664 1176
2.8 2.6 1.9
1.2 1.1 0.8
10.0 8.9 7.6
Bamboo 10% 20% 50%
1857 1824 1589
2.8 2.4 1.8
1.4 1.3 1.1
10.3 8.9 7.7
1825 1732 1447
2.9 2.6 2.0
1.3 1.2 0.9
9.9 8.8 7.5
changes of the co-combustion processes were compared when RMC was set to 8% and the as-received level (varying from 12% to 16% after mixing, see Table 1). The mean combustion temperature of the biomass/coal mixture at the as-received RMC level was about 5% lower that that at the 8% RMC level. Higher RMC implies higher free moisture content in the fuel. Part of the heat energy was transferred to the latent heat of vaporization during combustion thus lowering the overall furnace temperature. The change in temperature with different grinding sizes (100, 150 and 300 mm) of the biomass fuels was also studied. The RMC was fixed at 8%. The grinding size of the coal remained at 75–100 mm. There was no obvious change in the combustion temperature with the biomass grinding sizes under the same BBR.
3.2. Carbon dioxide emissions The weight basis emission factor (EFw) of CO2 decreased with biomass content as shown in Table 2. The EFw decreased from 1872 kg T1 at baseline (0% BBR) to 1589 kg T1 with 50% bamboo in the fuel blend (50% BBR). The EFw of rice husk at 50% BBR further decreased to around 1450 kg T1. The carbon content of rice husk from ultimate analysis was lower than bamboo and resulted in the lowered EFw at the same BBR. The CO2 EFw in the experiments was lower than the CO2 emission factor (2405 kg T1) suggested by the Environmental Protection Agency (USEPA) for sub-bituminous coal (USEPA, 1998). The laboratory-scale combustion at relatively low combustion temperature altered the CO2 emissions compared to large-scale systems. With reference to the baseline
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
CO2 emission per energy output (g/kWh)
240 220 200 180 160 Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
140 120 100 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 3. CO2 emissions with different BBRs at various RMC.
under the same energy output (EFe) as shown in Fig. 3, rice husks and bamboo showed a minimum emissions range for CO2 at 20–30% BBR. The EFe decreased from 220 g kWh1 at baseline to around 180 g kWh1 for rice husks and 200 g kWh1 for bamboo at 20% BBR and 8% RMC. The energy output is based on the heating value, the fuel feeding rate and the combustion efficiency of the combustion process. It should be noted that the combustion efficiencies based on post-combustion gas composition data were over 90% at different BBR levels. The experimental CO2 emission per unit energy output was much lower than the conventional emission factor because the thermal efficiency of the power plant was not included in the calculations. The theoretical carbon content per unit energy increased slightly (about 5–6%) with BBR. From ultimate analysis, the theoretical carbon content per unit energy output was about 90.91 g C kWh1 for coal, 95.76g C kWh1 when 50% rice husk and 96.48 g C kWh1 when 50% bamboo is mixed with the coal. CO2 emissions follow this trend if a complete reaction takes place. The carbon content, C, in the ultimate analysis was determined by the amount of CO2 released in the product gas when burning the fuel at a constant temperature with complete combustion. However, the experimental CO2 results did not follow the theoretical values. The combustion temperature was not constant and it decreased with the BBR in the experiments. As a result, not all carbon content in the fuel reacted with oxygen to form CO2; the other carbon content might react to form CO and CH4. However, the majority of the fuel carbon not converted to CO2 was entrained in bottom ash. Moisture content in the fuel mixture seemed to
lower the CO2 emissions. The performance of rice husks at 8% and at the as-received RMC level (12–16%) showed a 15% CO2 emissions difference at the BBR while the minimum emissions occurred and there was about 10% difference in bamboo at the minimum range compared with the baseline. That temperature decreased with RMC and the unburned carbon fraction therefore increased. As more unburned carbon became available, CO2 emissions were reduced. The CO2 emissions during the co-combustion process at different biomass grinding sizes were studied. There was no obvious difference in CO2 emissions at different biomass grinding sizes under each BBR. 3.3. Carbon monoxide emissions Table 2 shows that the CO EFw decreased with biomass. The EFw at 8% RMC decreased from 2.9 kg T1 to around 1.6 kg T1 for rice husks and 1.8 kg T1 for bamboo. The experimental CO EFw fell into the range of the CO EF (0.25–3 kg T1) suggested by the USEPA for sub-bituminous coal (USEPA, 1998). A minimum CO EFe at 10–20% BBR for co-combustion with rice husks was found in Fig. 4. The minimum CO EFe occurred at 20–30% BBR in the case of bamboo. When more biomass was mixed with coal, which implied the fraction of energy generated from char combustion would be lower, there was a decrease in CO emissions. Fig. 4 also shows the effect of RMC on CO emissions. With both cases of rice husks and bamboo, there was a gain in the CO concentration with RMC. Rice husks had a 30% reduction in CO emissions when the RMC levels changed from the
380 CO emission per energy output (mg/kWh)
7468
330 280 230 180 Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
130 80 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 4. CO emissions with different BBRs at various RMC.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
220 NOx emission per energy output (mg/kWh)
as-received level (12–16%) to 8% within the minimum emission range, while bamboo had a 20% decrease. The RMC level lowered the combustion temperature and thus yielded a higher unburned carbon fraction resulting in higher CO emissions. Changes in the biomass grinding size showed no obvious difference in CO emissions. The CO to CO2 emission ratio was found to be 1.52 103 at baseline. With the use of 20% rice husk and bamboo, the ratio decreased to 9.56 104 and 1.11 103, respectively. This implied that biomass enhanced the oxidation efficiency of the fuel. These ratios were close to the range of the CO/CO2 ratio (1 104–1.3 103) suggested by the USEPA (USEPA, 1998).
7469
200 180 160 140
Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
120 100 0%
10%
20%
30%
40%
50%
Biomass blending ratio / BBR (%) Fig. 5. NOx emissions with different BBRs at various RMC.
3.4. Nitrogen oxides emissions During the combustion process, NOx was generated through three formation mechanisms: prompt NOx, thermal NOx and fuel NOx. The amount of prompt NOx was significantly small and it was thus neglected in the discussion. The combustion temperature in the test rig was lower than 1000 1C so the thermal NOx formation was not significant. Fuel NOx was the major contributor to the total NOx emissions. The nitrogen content of coal (1.4%) was about two times higher than that in rice husks (0.73%) and bamboo (0.77%) as shown in Table 1. The nitrogen content in the fuel could directly affect nitrogen oxide generation during the combustion process and thus the use of biomass could reduce the overall NOx emissions. In Table 2, the EFw ranged from 1.5 kg T1 at baseline to 1.0 kg T1 for rice husks and 1.1 kg T1 for bamboo. The experimental EFw was lower than the NOx EF (around 4 kg T1) suggested by the USEPA (USEPA, 1998). This was because the formation of thermal NOx in the experimental combustion system was not significant when compared to the large-scale system and resulted in a lowered NOx emission in all cases. In Fig. 5, the minimum NOx emission range (EFe) is 10–20 and 20–30% BBR for rice husks and bamboo, respectively. The effect of RMC on the NOx concentration is also shown in the same figure. An increase in RMC from 8% to the as-received level (12–16%) caused a 20% decrease in NOx emissions within the minimum emission range for both rice husks and bamboo. This was because the RMC increased the unburned carbon fraction, which implied that the fuel nitrogen could not be effectively transferred to NOx and hence reduced
the overall NOx emissions. In addition, there was no significant change of the NOx emissions with biomass grinding size at each BBR. 3.5. Sulfur dioxide emissions The sulfur content in fuel has a direct effect on the generation of sulfur dioxide during combustion. The SO2 EFw decreased with biomass as shown in Table 2. The SO2 EFw was about 11 kg T1 at baseline and decreased to around 10 kg T1 for both rice husks and bamboo at 50% BBR and 8% RMC. The SO2 EF from USEPA was around 17.5 kg T1 for sub-bituminous coal with 1% sulfur content (USEPA, 1998). The experimental EFw were lower than the EF suggested by the USEPA. Part of the fuel sulfur was emitted as particulate sulfate and also retained in the bottom ash. These resulted in a lowered experimental EFw. The minimum SO2 emission range for EFe was shown to be at 20–30% BBR for both rice husks and bamboo in Fig. 6. The reduction of SO2 emissions could be attributed to a decrease in the overall fuel sulfur content. As shown in Table 1, the coal sulfur content was over ten times more than that in biomass. For rice husks and bamboo at the minimum emissions range, the SO2 EFe were the highest at 8% RMC and the difference was around 20%. The combustion temperature at 8% RMC was higher than that at the as-received RMC level. The fuel sulfur content could react with oxygen effectively at low RMC as relatively complete combustion could be attained at a higher combustion temperature. The effect of the biomass grinding size on the SO2 concentration was also studied.
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
7470
SO2 emission per energy output (mg/kWh)
1600 1500 1400 1300 1200 1100 1000
Rice Husk at 8% RMC Rice Husk at as received RMC Bamboo at 8% RMC Bamboo at as received RMC Rice Husk Bamboo
900 800 700 0%
10% 20% 30% 40% 50% Biomass blending ratio / BBR (%)
Fig. 6. SO2 emissions with different BBRs at various RMC.
Again, the biomass grinding size had no obvious effect on the SO2 concentration under different BBR.
grinding size was reduced from 300 to 100 mm. The ratio of this change was much smaller than that of the grinding size reduction and thus it was assumed to be an independent factor in the oxygen diffusion rate. To explain the effect of the grinding size in this process, the burning rate of single fuel particle could be derived based on mass diffusion (Smith, 1983; Caram and Amundson, 1977): _c ¼ m
Y O2 ;1 Y O2 , Rkin þ Rdiff
(3)
where Y O2 ;1 and Y O2 are the mass fraction of oxygen in the free stream and on the surface, respectively. The kinetic and diffusion parameters in Eq. (3) are defined as follows: Rkin ¼
n1 R u T s , 4pr2s MWmix kc P
(4)
n1 þ Y O2 ;s , 4prs rD
(5)
and 3.6. Influence of fuel grinding size Rdiff ¼ As reported in the previous sections, the biomass grinding size did not seem to have a significant impact to the combustion performance. There was no observable change in the combustion temperature and the various pollutant emissions did not differ much at various fuel grinding sizes. This can perhaps be explored more through a review of the fundamental combustion mechanisms. The combustion of a solid fuel such as that in our system proceeds in two stages: (1) gasification and combustion of the VM (volatile oxidation) and (2) combustion of fixed carbon (FC) (char oxidation). In the first stage, the fuel particles are partially pyrolyzed and, at the same time, the VM is released and the char content is left behind. This VM releasing (gasification) rate is controlled by the kinetics of pyrolysis and is fuel dependent. In Table 1, about 65–74% of the total biomass mass was VM. Reducing the grinding size may not have much effect on the overall combustion process since the time scale for combustion of VM is on the order of only a few ms, which is much quicker than the combustion of fixed carbon, which is on the order of seconds (Sami et al., 2002). In the second stage, char is the remaining product. Oxygen diffuses to the char surface/into the porous structure and reacts to form carbon monoxide and carbon dioxide. The pore surface area of the fuel can influence the oxygen diffusion rate. According to the BET surface analysis of rice husks and bamboo, there was only 5% increase of the pore surface area when the
where n1 is the mass stoichiometric coefficient, rs is the fuel particle radius, r is the solid fuel density, D is the mass diffusivity, Ru is the universal gas constant, Ts is the combustion temperature, MWmix is the molecular weight of the flue gas, kc is the kinetic rate constant and P is the pressure. The kinetic parameter (Rkin) is governed by the surface area of the fuel particle ð4pr2s Þ, the fuel kinetics rate (kc) and the combustion temperature (Ts). The surface area of the biomass is inversely proportional to Rkin. When the grinding size of the biomass changed from 300 to 100 mm, Rkin increased inversely to the square of the fuel radius. It should also be noted that the combustion temperature (Ts) has a substantial effect on the kinetic rate constant, kc ¼ A exp[EA/RuTs], where A is the frequency factor and EA is the activation energy. On the other hand, the diffusion parameter (Rdiff) increases inversely with the size of the fuel particle (rs) and the dependence on the combustion temperature (Ts) is weaker than the chemical kinetics term. After using the experimental data in the equations, we found that the ratio between Rkin and Rdiff (Rkin/Rdiff) was greater than unity, indicating that our experimental conditions might be kinetically controlled. Rdiff was small compared to Rkin and thus the mass diffusion parameter was not important. This was due to the fact that, in our experiments, the combustion temperature was not high (around 837–847 1C for rice husks and
ARTICLE IN PRESS P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
897–907 1C for bamboo) and the chemical kinetics (the rate-determining step) could play a more important role. In the case of rice husks at 20% BBR and 30% EA, the ratio increased, respectively, from 2.7 to 8.7 when the biomass fuel grinding size was reduced from 300 to 100 mm. For bamboo, the ratio increased, respectively, from 1.6 to 4.7 under the same size shift. The combustion process tended towards the kinetically controlled zone at smaller _ c , would thus grinding sizes. The burning rate, m increase with the square of the biomass fuel radius, which is supported by fitting in our experimental data (the order of magnitude from 1 1010 to 9 1010 kg s1 when the size increases from 100 to 300 mm). The number of fuel particles in the combustion system should be inversely proportional to r3s and, theoretically, the total burning rate should thus be proportional to 1/rs. However, our experimental data indicated that the burning characteristics remained quite independent of the change in the biomass fuel size and did not follow the 1/rs dependence. The possible reasons could be (3) the biomass only contributed o50% of the total fuel; (4) the VM contributed over 60% of the mass in the biomass fuel; and (5) the high excess air ratio took away the heat generated from the system faster than the small increase in the fuel burning rate. This means that the actual fixed carbon contribution from the biomass fuels to the overall combustion process was o15% and in some cases it was as low as 4–5%. This led to the relatively constant combustion behavior with the change of the biomass grinding size under our experimental conditions. This could help to explain why in previous works by other researchers sometimes the burning was size dependent and sometimes it was not.
7471
minimum pollutant emission factors (EFe) normalized by the energy output. General decreases in temperature, SO2, NOx and CO2 were observed with the RMC, while an increase was observed for CO emissions. The minimum range for each pollutant emission was not exactly the same, but they were all within the range. The effect in changing the grinding size of biomass was not significant as volatile combustion and the fuel burning rate did not change much with the biomass grinding size. A comparison between the mass diffusion and the chemical kinetic parameters showed that, under our experimental conditions, the combustion process seemed to be chemically kinetically controlled. Although the use of biomass seemed to reduce the combustion temperature, increasing the excess air to an optimum level (e.g., 30%) and drying the fuel (lower RMC) could increase the co-combustion temperature. Increasing the co-combustion temperature has several advantages including: (1) the potential for slagging and fouling would be reduced since the difference between the furnace and condensation temperatures of the fly ash could be increased; (2) the thermal efficiency of power plants would be improved and (3) the CO and particulate emissions would be lowered. However, SO2, NOx and CO2 emissions would increase with the combustion temperature under the same BBR. Acknowledgments The authors would like to acknowledge the technical and financial support by the CLP Research Institute. Special thanks must be paid to the CLP general laboratory for the fuel characteristics analysis.
4. Conclusions References A laboratory-scale pulverized fuel combustion testing facility was used to investigate the cocombustion performance of coal with biomass. Rice husks and bamboo were chosen as the biomass fuels in this study. Fuel characteristics and gaseous pollutant emission factors for both weight and energy basis were investigated under different BBR, RMCs and biomass grinding sizes. Co-combustion of biomass with coal decreased the emissions of most pollutants, such as CO2, CO, NOx and SO2, from existing pulverized coal firing power plants. A range of 10–30% BBR was found to be the
Aerts, D.J., Bryden, K.M., Hoerning, J.M., Ragland, K.W., 1997. Co-firing switchgrass in a 50MW pulverized coal boiler. In: Proceedings of the 59th Annual American Power Conference, vol. 59 (issue 2), Chicago, IL, pp. 1180–1185. Baxter, L., 2005. Biomass-coal co-combustion: opportunity for affordable renewable energy. Fuel 84 (10), 1295–1302. Caram, H.S., Amundson, N.R., 1977. Diffusion and reaction in a stagnant boundary layer about a carbon particle. Industrial Engineering Chemistry Fundamentals 16 (2), 171–181. Chao, C.Y.H., Kwong, C.W., Wang, J.H., Cheung, C.W., Kendall, G., 2007. Co-firing coal with rice husk and bamboo and the impact on particulate matters and associated
ARTICLE IN PRESS 7472
P.C.W. Kwong et al. / Atmospheric Environment 41 (2007) 7462–7472
polycyclic aromatic hydrocarbon emissions. Bioresource Technology, doi:10.1016/j.biortech.2006.11.051. Demirbas, A., 2005. Potential applications of renewable energy sources, biomass combustion problems in boiler systems and combustion related environmental issues. Progress in Energy and Combustion Science 31, 171–192. Du, X., Annamalai, K., 1994. The transient ignition of isolated coal particles. Combustion and Flame 97, 339–354. Energy Information Administration (EIA), 2007 /http://www. eia.doe.gov/fuelcoal.htmlS. Gani, A., Morishita, K., Nishikawa, K., Naruse, I., 2005. Characteristics of co-combustion of low-rank coal with biomass. Energy & Fuels 19 (4), 1652–1659. Hays, M.D., Fine, P.M., Geron, C.D., Kleeman, M.J., Gullett, B.K., 2005. Open burning of agricultural biomass: physical and chemical properties of particle-phase emissions. Atmospheric Environment 39 (36), 6747–6764. Kruczek, H., Raczka, P., Tatarek, A., 2006. The effect of biomass on pollutant emission and burnout in co-combustion with coal. Combustion Science and Technology 178 (8), 1511–1539. McLean, W.J., Hardesty, D.R., Poul, J.H., 1981. Direct observation of devolatlizing pulverized coal particles in a combustion environment. In: Proceedings of the 18th Symposium (International) on Combustion, pp. 1239–48.
Ministry of Agriculture of the People’s Republic of China, 2005 /http://www.agri.gov.cnS. Sami, M., Annamalai, K., Wooldridge, M., 2002. Co-firing of coal and biomass fuel blends. Progress in Energy and Combustion Science 27, 171–214. Scurlock, J.M.O., 2000. Bamboo: an overlooked biomass resource? Environmental Sciences Division, #4963. Smith, I.W., 1983. The Combustion Rates of Coal Chars: A Review. Nineteenth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, 1045–1065. Strehler, A., Stuetzle, W., 1987. Biomass residues. In: Hall, D.O. (Ed.), Biomass. Wiley, New York, pp. 75–97. US Environmental Protection Agency (USEPA), 1998. External combustion sources—bituminous and subbituminous coal combustion, AP 42, 5(1). Werther, J., Saenger, M., Hartge, E.U., Ogada, T., Siagi, Z., 2000. Combustion of agricultural residues. Progress in Energy and Combustion Science 26, 1–27. Yan, X., Ohara, T., Akimoto, H., 2006. Bottom-up estimate of biomass burning in mainland China. Atmospheric Environment 40 (27), 5262–5273. Yonemura, S., Kawashima, S., 2006. Concentration of carbon gases and oxygen and their emission ratios from the combustion of rice hulls in a wind tunnel. Atmospheric Environment.