Emission Characteristics of Particulate Matter from Rural Household


Mar 7, 2007 - Field measurements in rural households were conducted in three Chinese ... In this study, it appears that particle emissions can be corr...
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Energy & Fuels 2007, 21, 845-851

845

Emission Characteristics of Particulate Matter from Rural Household Biofuel Combustion in China Xinghua Li, Lei Duan, Shuxiao Wang, Jingchun Duan, Xingming Guo, Honghong Yi, Jingnan Hu, Chao Li, and Jiming Hao* Department of EnVironmental Science and Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed April 6, 2006. ReVised Manuscript ReceiVed October 9, 2006

Field measurements in rural households were conducted in three Chinese municipalities/provinces (Beijing, Chongqing, and Henan) to determine the emission characteristics of particulate matter from biofuel combustion. The selected biofuel types and stove types are representative of local rural areas. Particle number concentration, size distribution, and mass size distribution were determined. Both the particle number and mass of these emissions were dominated by submicrometer particles. The emission factor of PM2.5 from combustion is between 1.80 and 7.00 g/kg of biofuel (dry basis) and 0.84-2.40 g/MJ of delivered energy and is averaged to 4.21 g/kg of biofuel (dry basis) and 1.46 g/MJ delivered energy. In this study, it appears that particle emissions can be correlated with combustion conditions and stove configuration. Particle emissions are the highest during the high power phase. Unfortunately, the more thermally efficient stove has higher per kilogram fuel particulate matter (PM) emissions than the less thermally efficient stoves, that is, the increase in thermal efficiency cannot offset the increase in particle emissions.

Introduction With the rapid development of the economy and society, commercial energy, such as electricity and liquid propane gas (LPG), is becoming more popular in China’s rural households. However, biofuel, such as crop waste and wood, still dominates the rural energy supply in China. In 2000, 288 million tons of agricultural biomass and 141 million tons of firewood were directly burned by rural households for cooking and heating, contributing 57% of total rural household energy use (353 million tons of coal equivalent).1,2 Patterns of energy use in China’s rural households, which are dominated by biofuels, will not change significantly for a long time. Unfortunately, biofuel combustion is mainly carried out in small household stoves under poor combustion conditions and without any emission control. This results in high levels of particulate matter, CO, polycyclic aromatic hydrocarbons (PAHs), and other air pollutant emissions, which cause high levels of indoor air pollution,3 local air pollution,4 and regional and global climate impacts.5,6 The particulate matter contains a large carbonaceous fraction, i.e., organic carbon (OC) and black carbon (BC). BC is thought to absorb solar radiation and contribute to global warming.6 Street et al. estimated that BC emissions from biofuel combus-

tion in China were 512 Gg in 1995, comprising 38% of the total national emissions.7 OC contains a multitude of organic compounds, some of which are carcinogenic and mutagenic, such as polycyclic aromatic hydrocarbons (PAHs).8,9 In view of the adverse effect of particulate matter emitted from biofuel combustion, it is necessary to know the emission characteristics so as to reduce the emissions. A few studies on emissions from household stoves in developing countries have been conducted. For these small combustion devices, three major types of methods are used to determine emissions: the chamber method, hood method, and the carbon balance approach.10 Joshi et al. adopted the chamber method to study CO and total suspended particle (TSP) emissions from burning biofuels in metal cook stoves and found that the more efficient stoves have higher emission factors of both CO and TSP for all three biofuels tested.11 Ballard-Tremeer et al. used the hood method to compare efficiencies and emissions of five rural, wood-burning cooking devices and observed that the average emissions of TSP were lowest for the improved open fire and the two-pot ceramic stove.12 Venkataraman et al. adopted the hood method and a dilution sampler to measure CO, size-resolved aerosols, and PAH emissions from biofuel combustion in India.13,14 However, studies of the emission characteristics of particulate matter from

* Author to whom correspondence should be addressed. Telephone: +86-10-62782195. Fax: +86-10-62773650. E-mail address: [email protected] (1) Ministry of Agriculture. P. R.C. China agriculture statistical report 2000; China Agriculture Press: Beijing, China, 2001. (2) Department of Industry and Transport Statistics, National Bureau of Statistics, P. R. C. and Energy Bureau, National Development and Reform Commission. P. R. C. China energy statistical yearbook 2004. China Statistics Press: Beijing, China, 2005. (3) Bruce, N.; Neufeld, L.; Boy, E.; West, C. Int. J. Epidemiol. 1998, 27, 454-458. (4) Guo, H.; Wang, T.; Simpson, I. J.; Blake, D. R.; Yu, X. M.; Kwok, Y. H.; Li, Y. S. Atmos. EnViron. 2004, 38, 4551-4560. (5) Jacobson, M. Nature 2001, 409, 695-697. (6) Menon, S.; Hansen, J.; Nazarenko, L.; Luo, Y. Science 2002, 297, 2250-2253.

(7) Street, D. G.; Gupta, S.; Waldhoff, S. T.; Wang, M. Q.; Bond, T. C.; Bo, Y. Atmos. EnViron. 2001, 35, 4281-4296. (8) Menzie, C. A.; Potoki, B. B.; Santodonato, J. EnViron. Sci. Technol. 1992, 26, 1278-1284. (9) Pedersen, D. U.; Durant, J. L.; Taghizadeh, K.; Hemond, H. F.; Lafleur, A. L.; Cass, G. R. EnViron. Sci. Technol. 2005, 39, 9547-9560. (10) Mitra, A. P.; Morawska, L.; Sharma, C.; Zhang, J. Chemosphere 2002, 49, 903-922. (11) Joshi, V.; Venkataraman, C.; Ahuja, D. R. EnViron. Manage. 1989, 13, 763-772. (12) Ballard-Tremeer, G.; Jawurek, H. Biomass Bioenerg. 1996, 11, 419430. (13) Venkataraman, C.; Rao, G. U. M. EnViron. Sci. Technol. 2001, 35, 2100-2107. (14) Venkataraman, C.; Negi, G.; Sardar, S. B.; Rastogi, R. J. Aerosol Sci. 2003, 33, 503-518.

10.1021/ef060150g CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007

846 Energy & Fuels, Vol. 21, No. 2, 2007

Li et al. Table 1. Stove Configuration Parameters

location

description

Chongqing

improved stove, one main pot and one auxiliary pot, no grate improved stove, one main pot and one auxiliary pot, bottom grate stove/kang, one pot, no grate

Henan Beijing

combust chamber volume (m3)

grate to pot-bottom distance (m)

grate air-inlet area (m2)

fuel inlet area (m2)

0.060

0.23

no grate

0.045

0.034

0.13

0.016

0.041

0.039

0.20

no grate

0.042

fuel used rice straw, maize residue, bean straw, fuel wood branch wheat residue, kaoliang stalk, cotton stalk branch

Table 2. Tested Fuels Proximate and Ultimate Analysis rice straw

maize residue

bean straw

moisture volatile matter fixed carbon ash

6.24 62.50 16.56 14.70

6.49 70.33 16.87 6.31

10.07 71.35 15.59 2.99

C H N S O (by difference) low heating value (dry basis, MJ/kg)

41.42 5.57 0.94 0.16 39.43 15.67

46.32 6.09 1.62 0.15 39.07 17.54

47.54 6.35 1.15 0.16 41.48 17.86

fuel

fuel wood

cotton stalk

wheat residue

kaoliang stalk

branch (II)

Proximate (as Received, mass %) 7.51 7.90 76.14 73.16 14.41 17.14 1.94 1.80

8.40 70.83 17.54 3.23

5.42 68.74 18.07 7.77

5.47 72.27 17.98 4.28

7.06 75.33 16.71 0.90

Ultimate (Dry Basis, mass %) 50.68 48.96 6.37 6.23 0.29 0.76 0.018 0.053 40.54 42.05 19.11 18.21

49.50 6.28 1.25 0.010 39.43 18.71

44.64 5.85 0.56 0.20 40.53 16.61

47.76 6.12 0.59 0.078 40.92 17.89

49.90 6.36 0.70 0.037 42.03 18.69

rural household biofuel combustion in China are limited. Smith et al. studied greenhouse gas and other airborne pollutions (including TSP) emissions from household stoves in China and other developing countries using the carbon balance approach.15-17 The purpose of this paper is to provide an initial assessment of particulate matter emissions from typical rural household biofuels and stove combinations used in China. Therefore, the objectives of this study were to (a) measure particle number concentration, size distribution, and mass size distribution; (b) quantify PM2.5 emission factors, and (c) relate these to stove configuration, fuel, and combustion conditions. Methods Selection of Biofuels and Stoves to Test. In China’s rural households, crop waste which is used as fuel to burn in the household stoves for cooking and heating include the following: rice straw, wheat residue, maize residue, bean straw, cotton stalk and kaoliang stalk, and so on. Fuel wood and branches are also often used as rural household fuel. Improved stoves have become popular in rural households. These are stoves with an enclosed combustion chamber and a flue. About 190 million improved stoves have been installed, accounting for 80% of rural households.1 In the northern rural areas, stove/kang is used commonly for cooking and heating. Kang is a bed made of brick or clay, with passageways inside it, and flue gas, after leaving the stove, passes through the passageways and transfers heat to the kang. Three Chinese municipalities/provinces (Beijing, Chongqing, and Henan), located in the north, southwest, and the middle of China, respectively, were chosen for field measurement. Tests were carried out in local rural households. Before determining the fuel/stove combination to be tested, investigations were carried out. The selected fuels and stove types are representative of the local rural area. In Chongqing, an improved stove and five typical biofuel (15) Zhang, J.; Smith, K. R.; Ma, Y.; Ye, S.; Jiang, F.; Qi, W.; Liu, P.; Khalil, M. A. K.; Rasmussen, R. A.; Thorneloe, S. A. Atmos. EnViron. 2000, 34, 4537-4549. (16) Smith, K. R.; Khalil, M. A. K.; Rasmussen, R. A.; Thorneloe, S. A.; Manegdeg, F.; Apte, M. Chemosphere 1993, 26, 479-505. (17) Smith, K. R.; Uma, R.; Kishore, V. V. N.; Lata, K.; Joshi, V.; Zhang, J.; Rasmussen, R. A.; Khalil, M. A. K. Greenhouse gases from small-scale combustion deVices in deVeloping countries, phase IIa: household stoVes in India; Office of Research and Development, US EPA: Washington, DC, 1999.

branch (I)

types, rice straw, maize residue, bean straw, fuel wood, and branches, were selected for this study. The stove has two combustion chambers, two fuel inlets, two main pots, one auxiliary pot, and a flue. One combustion chamber burns biofuel, and the other burns coal. The former has no bottom grate. Fuel is burned in the combustion chambers, and heat is transferred to the main pot while the flue gas is drawn through the duct to the flue. The auxiliary pot is located in the duct and uses the remaining heat energy of flue gas. Fuel could be burned in both combustion chambers simultaneously or only in one chamber. When one chamber is used, the duct of the other chamber connecting it to the flue is closed. This study is conducted only with the combustion chamber that burns biofuel. In Henan, three different biofuels typical of the area, wheat residue, kaoliang stalk, and cotton stalk, were chosen to burn in a local typical improved stove. The stove has almost the same configuration as stoves in Chongqing. Both combustion chambers burn biofuel and have a bottom grate. The experiment was carried out in one combustion chamber. In Beijing, branches and a stove/ kang were selected as the fuel/stove combination. The stove has one combustion chamber, one fuel inlet, and one main pot. All the selected stoves were made of brick, and their configuration parameters are shown in Table 1. The proximate and ultimate analyses of the tested fuels are listed in Table 2. According to local cooking habits, the tested crop wastes were wrapped into batches of 20-25 cm in length and woody fuel was cut into pieces about 20 cm long. Definition of the Burning Cycle. Cooking is not a steady process causing emissions from the biofuel combustion to vary during the cooking.16 Therefore, it is necessary to choose a burning cycle similar to the common cooking practice in the field. The most common cooking practices include high power and low power phases. High power phase means heating a quantitative amount of water from the ambient water temperature to the boiling temperature as rapidly as possible. A low power phase would involve the water simmering at the lowest power. The “water-boiling test”18 was adopted with slight modification according to a Chinese standard method19 to define a burning cycle. The water-boiling test is described as follows: (18) VITA (Volunteers in Technical Assistance, Inc.). Testing the efficiency of wood-burning cookstoVes; International Standards: Arlington, VA, 1985. (19) Standardization Administration of the People’s Republic of China (SAC). Testing method for the heat characteristics of firewood stoVes; SAC: Beijing, China, 1984.

Emission Characteristics Household Biofuel

Figure 1. Outline of the sampling system.

• Fill the stove pot (including the main and auxiliary pots) to 2/3 of its capacity with room temperature water. • Weigh out a quantity of biofuel such that its weight is about 1/ -1/ of the water weight in the main pot. 3 2 • Cover the pot with a lid and a thermometer is inserted through the lid into the water. • Start the fire at high power to bring the water in the main pot to boil. • Continue the test at low power using the remaining biofuel. • Terminate the test when the water temperature in the main pot dropped by about 1 °C. • Bail out the water in the pot and weigh it. • Extinguish the ash and any unburned fuel residue by shutting off access to air, and weigh them after they are cool. Prior to the planned sampling for each fuel/stove combination, trial tests were performed to standardize the burning cycle. Preliminary tests were carried out until a satisfactory method precision (relative standard deviation less than 20%) for the main parameters, i.e., time to boil, time of burning cycle, and thermal efficiency, was obtained. The time for completion of the burning cycle ranged from 30-50 min for most of the test. During the whole burning cycle, biofuels were manually fed into the stove in batches. The time, temperature, and weight of water were recorded at the beginning and end of the burning cycle, and the amount of biofuel burned was also recorded. Stove thermal efficiency can be determined according to these data. The sampling period covered the whole burning cycle from the moment of the beginning of the burn (biofuel had been ignited) to the end of the burning process (the water temperature in the main pot dropped by about 1 °C). Sampling Approach. An outline of the sampling system is shown in Figure 1. It includes a dilution sampling system, particle measurement, and flue gas monitoring. The system is described in more detail in the following subsection. Dilution Sampling System. The dilution sampling system simulates the cooling and dilution processes after the hot flue gas leaves the stack and is widely used for characterizing emissions from

Energy & Fuels, Vol. 21, No. 2, 2007 847 stationary combustion sources.20 A compact dilution sampling system was developed for this field study that consists of four main parts: sampling inlet, dilution part, residence chamber, and sampler. In the sampling inlet part, flue gas is withdrawn from the flue, put through a cyclone separator, and then advanced to the first dilution. The cyclone, which removes particles larger than 10 µm, is installed outside the flue because the flue is not large enough to accommodate it. The sampling inlet was heated to about 150 °C to reduce particle thermophoresis losses. Because the flue gas velocity in the flue is unstable, isokinetic sampling cannot be achieved. The sampling error arising from the velocity mismatch can be neglected for particles with diameters less than 2.5 µm.21 The dilution part consists of two stage diluters. The operation principle of the first diluter is based on an ejection type dilution (Dekati Ltd, Finland). The ejector diluter is used to keep the dilution ratio constant at about 10. About 0-50 L/min can be drawn from the outlet of the diluter to the second diluter according to research requirements. The second diluter is a cylindrical enclosure with a perforated plate inside. The sample flow from the first diluter is introduced inside the enclosure, and the dilution air is forced through the apertures of the plate into the enclosure where it mixes with the sample flow. Two vortex flow meters record sample flow from the first diluter and the dilution air flow rate in the second diluter. The second diluter can supply a dilution ratio from 1 to 10. The total range of the dilution ratio is from 10 to 100. An oil-free air compressor and a pump supply the first and second diluters dilution air, respectively; the air must be purified before entering into the dilutor. All of the diluted sampling gas is transferred to the residence chamber. The temperature, relative humidity, and pressure in the chamber are monitored. The sampler is attached to the end of the residence chamber and has eight sampling ports for connecting with particle measurement instruments. In this study, an electrical low-pressure impactor, (ELPI, Dekati Ltd., Finland)22 and three parallel PM2.5 cyclones with filter packs were used to collect particles. The gas in the chamber is under a small positive pressure, and extra gas can be automatically discharged from the unused sampling ports. The whole dilution sampling system has shown considerable stability. The sampler is made entirely from stainless steel, copper, and Teflon. A dilution air ratio of about 20 and an aging time of about 80 s were applied in the study. Particle Measurement. ELPI was used to measure in the realtime particle number concentration and size distribution. Operating at a flow rate of 9.89 L/min, the ELPI has 50% cut-point aerodynamic diameters of 0.028, 0.056, 0.095, 0.157, 0.263, 0.382, 0.613, 0.948, 1.600, 2.390, 4.000, 6.680, and 9.920 µm on stages 1-13. Our focus was on the lower ELPI stages 1-9 which characterized the particle size distribution range of 0.028-2.390 µm, the typical particle distribution range of biofuel combustion.13,23-25 Greased and baked aluminum foils of 25 mm in diameter were used as collection foils. A low pressure impactor (LPIswhen ELPI is used without the electrical charger) was used to determine particle mass size distribution over the range of 0.028-2.390 µm. In the upper four stages, aluminum foils were coated with high vacuum grease to prevent particle bounce. For further size resolved chemical analysis, the lower nine stages were not greased to avoid interference. Particle bounce within the lower impactor stages was not a problem.24 (20) Hidemann, L. M.; Cass, G. R.; Markowski, G. R. Aerosol Sci. Tech. 1989, 10, 193-204. (21) Hinds, W.C. Aerosol technology: properties, behaVior, and measurement of airborne particles; John Wiley & Sons, Inc.: New York, 1999. (22) Keskinen, J.; Pietarinen, K.; Lehtima¨ki, M. J. Aerosol Sci. 1992, 23, 353-360. (23) Purvis, C. R.; McCrillis, R. C.; Kariher, P. H. EnViron. Sci. Technol. 2000, 34, 1653-1658. (24) Kleeman, M. J.; Schauer, J. J.; Cass, G. R. EnViron. Sci. Technol. 1999, 33, 3516-3523. (25) Hays, M. D.; Smith, N. D.; Kinsey, J.; Dong, Y.; Kariher, P. J. Aerosol Sci. 2003, 34, 1061-1084.

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Table 3. Particle Number Concentrations and GSD during the Whole Burning Cycle stove fuel

Chongqing rice straw

maize residue

avg number concentration 1.2 × 107 2.9 × 107 (particles/(N cm3)) varied range of number 1.1 × 106 to 1.6 × 106 to concentration 4.6 × 107 2.5 × 108 (particles/(N cm3)) GMD (µm) 0.11 0.14

bean straw 1.6 × 107

Henan fuel wood 1.3 × 107

branch (I)

wheat residue kaoliang stalk

3.3 × 107

4.6 × 107

branch (II)

5.0 × 107

1.0 × 107

1.3 × 106 to 2.5 × 106 to 2.1 × 106 to 3.3 × 106 to 5.4 × 106 to 7.0 × 107 1.5 × 108 4.3 × 108 2.6 × 108 1.0 × 108

1.2 × 107 to 1.3 × 108

1.2 × 106 to 7.1 × 107

0.17

0.11

0.21

0.15

Before and after sample collection, all substrates were conditioned for 24 h at about 40% RH and 25 °C in an air-conditioned room and weighed on a microbalance with a resolution 1 µg. All number and mass concentrations measured by ELPI and LPI were back-calculated out to the dilution air ratio at each measurement and normalized to 3% CO2 dry gas at normal temperature (0 °C) and pressure (101.3 kPa). PM2.5 was also collected by three parallel PM2.5 cyclones with filter packs operated at 16.7 L/min. The first filter pack consisted of a 47 mm Teflon-membrane filter for mass by gravimetric and elemental analysis. The other two filter packs consisted of a 47 mm quartz-fiber filter for carbon, ions, and speciated organic compound analysis. The Teflon-membrane filters were conditioned for 24 h at about 40% RH and 25 °C in an air-conditioned room and weighed on a microbalance with a resolution 10 µg. The chemical speciation results will be presented in a companion paper. Flue Gas Monitoring. The concentrations of gaseous air pollutants (including CO2, CO, SO2, O2, and NOx) and temperature in the flue were continuously monitored by a flue gas analyzer (Model KM9106, Keison). The data were recorded every 10 s by a data logger and transmitted to a notebook PC. The instrument was calibrated before each field study. Determination of Emission Factors. The carbon balance approach was used to calculate the emission factors.15 The carbon balance equation for combustion is based on the total carbon mass burned being equal to the total mass of carbon emitted by both as particles and gases. The carbon content of the tested fuels, ash, unburned fuel residue, and PM2.5 was analyzed. The average concentrations of CO2 and CO over the whole burning cycle in the flue were calculated using the data monitored by the flue gas analyzer. The amount of total hydrocarbon was not measured in this study. Estimation based on previous study15 shows that error caused by neglecting total hydrocarbon is less than 5%stherefore, the emission factors in this study are credible. The emission factors are reported on a dry fuel mass basis (gram per kilogram of fuel) and on an energy basis (gram per megajoule of delivered energy). Dry fuel mass can be converted from fuel mass burned and fuel moisture content. The emission factor based

1.9 × 107

Beijing

cotton stalk

0.12

0.13

0.16

on delivered energy can be derived from the emission factor based on dry fuel along with the stove thermal efficiency and fuel heating value. For the stove-burned woody fuel, the unburned fuel residue, i.e., char, is often used later and produces pollutants. In our study, the pollutants generated from char secondary combustion are not calculated in determining the emission factors. In this paper, thermal efficiency is the ratio of energy absorbed by the water in the pots (including the main pot and the auxiliary pot) to the energy content of the fuel consumed. For the stoveburned woody fuel, when calculating its thermal efficiency, the energy content of the fuel consumed was subtracted by the energy content of char. For stove/kang, thermal efficiency just refers to the stove; the heat delivered to the kang is not considered.

Results and Discussion Particle Number Concentration and Size Distribution. Particle number concentrations for all tested biofuel combustion during the whole burning cycle, measured by the ELPI, together with geometric mean diameters (GMDs) are summarized in Table 3. The average particle number concentrations within the range of 0.03-2.39 µm were between 1.0 × 107-5.0 × 107 particles/(N cm3) and GMDs were between 0.11 and 0.21 µm. The particle number concentration varied 1-2 orders of magnitude in the whole burning cycle. The results of the particle number concentration were somewhat consistent with some previous studies. For example, Hueglin et al.26 reported particle number concentrations from residential wood stoves varying from 7.8 × 106-4.4 × 107 particles/(N cm3), and Johansson et al.27 showed particle number concentrations from domestic heating devices varying between 1.4 × 107-13.4 × 107 particles/(N cm3). In this study, the number concentrations in the Chongqing stove are generally less than those measured in Henan. A possible explanation is the difference in stove configuration. The stove in Chongqing has the largest combustion volume,

Figure 2. Particle average number size distribution during the whole burning cycle. (Note: SC, SH, and SB denote the stoves in Chongqing, Henan, and Beijing, respectively.)

Emission Characteristics Household Biofuel

Figure 3. Particle number concentration during the different phases of the burning cycle.

Energy & Fuels, Vol. 21, No. 2, 2007 849

Figure 5. Reproducibility of test data.

Figure 4. Particle average number size distribution during the different phases of the burning cycle.

which can provide a relatively long time for the combustion of volatiles released from pyrolysis of biofuel and, therefore, decreased particle formation from incomplete combustion. The Beijing stove/kang has the least number concentration and the highest GMD, which may be related to its peculiar configuration. Before being drawn to the flue, flue gas, after leaving the stove, passes through the passageways inside the kang and transfers heat to the kang thereby losing temperature. Flue gas residence time in the kang is over 100 ssenough time for particle coagulation and condensation growth that could decrease particle number concentration and shift the GMD toward a high value. Particle number size distributions are given in Figure 2. They are mainly unimodal during the whole burning cycle, with a peak between 0.12 and 0.32 µm. However, they are bimodal for branches burned in the Chongqing stove and wheat residue and kaoliang stalk in the Henan stove, with a nucleation mode peak less than 0.04 µm and an accumulation mode peak near 0.12 µm. Particle number concentrations and size distributions during the whole burning cycle were strongly related to the phase of the cooking practice. Typical particle number concentrations and size distributions during the different phases of the burning cycle are shown in Figures 3 and 4, respectively. In this study, the lower power phase was divided into two parts: the earlier portion of the lower power phase (lower power phase I) and the later portion (lower power phase II). In lower power phase ΙΙ, no fuel was fed and combustion occurs in the burn-out phase. This causes particle emissions to be significantly different from that of lower power phase I. Particle number concentrations were 4.8 × 107 particles/(N cm3) for the high power phase, 5.8 × 106 particles/(N cm3) for lower power phase Ι, and 3.6 × 106 particles/(N cm3) for lower power phase ΙΙ. Total particle number concentrations decreased significantly

Figure 6. Particle mass size distribution of particles emitted from (a) crop waste and (b) woody fuels combustion. (Note: SC, SH, and SB denote the stoves in Chongqing, Henan, and Beijing , respectively.)

along with the cooking procedure, which is correlated with the rate of fuel burning and combustion condition. From Figure 4, it can be observed that a prominent mode was less than 0.04 µm, other than an accumulation mode peak near 0.20 µm in lower power phase Π, which suggests that particle number emissions were largely in the nucleation mode. This observation may be attributed to the lower particle number concentration during lower power phase ΙΙ, which limited the condensation and coagulation process and, hence, particle growth.28,29 Although particle number concentrations and size distributions varied to a certain extent in the repeated runs, a general trend was found. The results of three repeated measurements of particle emissions from stoves in Beijing were shown in Figure 5, and these indicated that the test data were satisfactorily reproducible. Moreover, the relative standard deviation of the particle average number concentration was less than 11%. (26) Hueglin, Ch.; Gaegauf, Ch.; Ku¨nzel, S.; Burtscher, H. EnViron. Sci. Technol. 1997, 31, 3439-3447. (27) Johansson, L. S.; Tullin, C.; Leckner, B.; Sjo¨vall, P. Biomass Bioenerg. 2003, 25, 435-436. (28) Lipsky, E.; Stanier, C. O.; Pandis, S. P.; Robinson, A. L. Energy Fuels 2000, 16, 302-310. (29) Chang, M. C. O.; Chow, J. C.; Watson, J. G.; Hopke, P. K.; Yi, S. M.; England, G. C. J. Air Waste Manage. Assoc. 2004, 54, 1494-1505.

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Table 4. Thermal Efficiencies and Emission Factors of Biofuel Combustion PM2.5 emission factors stove

fuel

thermal efficiency (%)

Chongqing

rice straw maize residue bean straw fuel wood branch (I) cotton stalk wheat residue kaoliang stalk branch (II)

13.7 ( 1.1b 13.4 ( 1.0 14.3 ( 0.9 16.1 ( 1.1 13.8 ( 1.1 18.7 ( 0.9 18.0 ( 0.9 22.1 ( 0.7 14.8 ( 2.8d

Henan Beijing

Em

(g/kg)a

1.66-1.94c 2.45-3.85 3.28 ( 0.87 2.21-4.58 2.95-3.97 6.04 ( 0.52 5.61-8.39 6.27-7.19 3.04 ( 0.85

CO emission factors

Ee (g/MJ)

Em (g/kg)

Ee (g/MJ)

0.79-0.90 1.03-1.68 1.30 ( 0.35 0.79-1.53 1.17-1.58 1.75 ( 0.11 1.86-2.94 1.63-2.02 1.11 ( 0.37

108.9-128.6 111.5-156.5 77.0 ( 13.9 47.2-65.8 62.7-93.3 68.4 ( 23.8 62.5-68.7 27.9-30.7 56.4 ( 22.6

51.9-59.5 48.5-66.1 30.5 ( 5.5 16.9-21.9 24.8-37.1 19.7 ( 6.4 21.9-22.7 7.3-8.6 20.4 ( 8.5

Dry fuel mass basis. b Three or more repeated tests, the results are given as means ( standard deviations (x ( s). c Two repeated tests, the results are given as a range. d Just stove thermal efficiency, kang thermal efficiency is not included. a

Particle Mass Size Distribution. Particle mass size distributions for all tested biofuel combustion during the whole burning cycle, measured by LPI, are given in Figure 6. Particle mass size distributions from crop waste combustion show a single mode with the peak at approximately 0.20-0.48 µm. However, woody fuels show a bimodal size distribution; one prominent mode peaks between 0.12 and 0.32 µm, and the other weak mode peaks at 0.76 µm. It is supposed that soot contributes to the mode at 0.76 µm. A possible explanation is connected to the fuel itself. Woody fuel has a higher lignin concentration compared to the crop wastes. It was thought that a high lignin concentration could increase the soot yield.30 Unoxidized soot may have undergone agglomeration and then growth to a relatively large size. This needs to be further investigated. The particle mass median aerodynamic diameter (MMD) is between 0.21 and 0.45 µm. For all the biofuel combustion cases, submicrometer particles (less than 1 µm) contributed to over 90% of the mass of PM in the range of 0.03-2.39 µm. PM2.5 Emission. The emission factors of PM2.5 from rural household biofuel combustion are between 1.80 and 7.00 g/kg of biofuel (dry basis) and 0.84-2.40 g/MJ delivered energy; the average emission factor is 4.21 g/kg of biofuel (dry basis) and 1.46 g/MJ delivery energy for all of the biofuels tested (Table 4). The results are consistent with previous studies.13,31-33 The stove thermal efficiencies range from 13.4% to 22.1%. The data are in accord with those measured from Chinese stoves that burned biofuel in the laboratory.15 Within that range, stoves with different configurations have various thermal efficiencies: 13.4-16.1% for stoves in Chongqing, 18.0-22.1% for stoves in Henan, and 14.8% for stoves in Beijing (Table 4). The thermal efficiencies of the Henan stoves are generally greater than the others. The stove in Henan has the least combustion chamber volume, a grate, and the lowest distance from the grate to the pot-bottom. All of those design features contribute to high thermal efficiencies. However, the Henan configuration can cause high emissions, described in the following sections. The mean emission factor of CO is between 29.3 and 134.0 g/kg of biofuel (dry basis) and 7.9-57.3 g/MJ delivered energy, and the averaged value is 76.0 g/kg of biofuel (dry basis) and (30) Wiinikka, H.; Gebart, R. Combust. Sci. Technol. 2005, 177, 741763. (31) McDonald, J. D.; Zielinska, B.; Fujita, E. M.; Sagebiel, J. C.; Chow, J. C. ; Waston, J. G. EnViron. Sci. Technol. 2000, 34, 2080-2091. (32) Sheesley, R. J.; Schauer, J. J.; Chowdhury, Z.; Cass, G. R.; Simoneit, E. R. T. Characterization of organic aerosols emitted from the combustion of biomass indigenous to South Asia. J. Geophys. Res. 2003, 108 (D9), 4285; doi:10.109/2002JD002981. (33) Fine, P. M.; Cass, G. R.; Simoneit, E. R. T. EnViron. Sci. Technol. 2001, 35, 2665-2675.

Figure 7. Relationship between stove thermal efficiency and PM2.5 emission factor on fuel mass basis. (Note: SC, SH, and SB denote the stoves in Chongqing, Henan, and Beijing, respectively.)

29.4 g/MJ delivered energy over all the biofuel types tested (Table 4). The results are also consistent with other research.11-13,15,32,34 PM2.5 emission factors in Henan are higher than those measured in Beijing and Chongqing. This may be attributed to different stove configurations. As mentioned above, the stove in Henan has a small combustion volume, which leads to high thermal efficiency, but the small volume may cause incomplete combustion. Incomplete combustion of emitted organic matter significantly enhances the formation of particles. The air supply through the grate increases the temperature in the fuel bed and, thereby, enhances vaporization of ash that would result in high particle emissions.35,36 Ash entrainment in the fuel bed escapes to the flue gas as air flows through the grate. On the basis of our findings, an increased thermal efficiency stove does not imply reduced PM emissions (Figure 7). Other researchers also reported this phenomenon.11,13 Figure 8 shows the relationship between PM2.5 emission factors on an energy basis and stove thermal efficiency, which implies that the increase in thermal efficiency cannot offset the increase in particle emissions. (34) Purious, C. R.; Mccrillis, R. C.; Kariher, P. H. EnViron. Sci. Technol. 2000, 34, 1653-1658. (35) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (36) Wiinikka, H. High temperature aerosol formation and emission minimisation during combustion of wood pellets. Ph.D. Thesis, Technical University of Denmark, Lyngby, Denmark, 2005.

Emission Characteristics Household Biofuel

Energy & Fuels, Vol. 21, No. 2, 2007 851

Figure 9. Relationship between CO and PM2.5 emission factor.

Figure 8. Relationship between stove thermal efficiency and PM2.5 emission factor on energy basis. (Note: SC, SH, and SB denote the stoves in Chongqing, Henan, and Beijing, respectively.)

CO is a product of incomplete combustion. For low temperature combustion in the household stove, organic matter emitted from incomplete combustion enhances the formation of particles. Gupta et al. found that CO emissions are correlated to those of respirable suspended particulates (RSP).37 However, in our studies, a negative correlation between particle and CO emissions was observed (Figure 9). Venkataraman et al. also found opposing trends in PM and CO emissions in their study.38 They pointed out that CO emissions cannot be used as a surrogate for PM emissions across stoves and fuels. This would indicate that PM formation is complicated and needs to be studied further. Conclusions The total average PM number concentrations for biofuel combustion during the whole burning cycle was between 1.0 × 107 and 5.0 × 107 particles/(N cm3). Particle number size distributions are mainly unimodal, with a peak between 0.12 and 0.32 µm. In some cases, bimodal size distributions are also observed, with a nucleation mode peak less than 0.04 µm and (37) Gupta, S.; Saksena, S.; Shankar, V. R.; Joshi, V. Biomass Bioenerg. 1998, 14, 547-559. (38) Venkataraman, C.; Joshi, P.; Sethi, V.; Kohli, S.; Ravi, M. R. Aerosol Sci. Tech. 2004, 38, 50-61.

an accumulation mode peak near 0.12 µm. Particle mass size distributions from crop waste combustion show an obvious single mode peak at 0.20-0.48 µm; however, woody fuels show a bimodal size distribution: one prominent mode peaks between 0.12 and 0.32 µm, and the other weak mode peaks at 0.76 µm. Both with regard to number and mass, particle emissions from biofuel combustion were dominated by submicrometer particles, which imply adverse health concerns. The emission factor of PM2.5 from biofuel combustion is between 1.80 and 7.00 g/kg of biofuel (dry basis) and 0.842.40 g/MJ delivered energy. The averages were 4.21 g/kg of biofuel (dry basis) and 1.46 g/MJ delivered energy. For household stoves, advanced gas cleaning devices are not an economic option. The feasible way to reduce particle emissions is, therefore, to decrease the formation of particles in the combustion process. In this study, it appears that particle emissions are correlated with combustion conditions and stove configuration. Particle emissions are highest during the high power phase. A possible way to reduce emissions in this phase is by feeding the fuel at a moderate pace. Stove configuration impacts both thermal efficiency and emissions. As noted, a dilemma arises in that the more thermally efficient stove has higher per kilogram fuel PM emissions and the increase in thermal efficiency cannot offset the increase in particle emissions. It is a stimulus for developing a stove with high efficiency and low emissions. Acknowledgment. The authors acknowledge the financial support provided by the National Key Basic Research and Development Program of China (Grant No. 2002CB211600) and the National Natural Science Foundation of China (Grant No. 20521140077). EF060150G