Comparison of Carbon Monoxide and Particulate Matter Emissions

May 12, 2014 - Comparison of Carbon Monoxide and Particulate Matter Emissions from Residential Burnings of Pelletized Biofuels and Traditional Solid. ...
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Comparison of Carbon Monoxide and Particulate Matter Emissions from Residential Burnings of Pelletized Biofuels and Traditional Solid Fuels Guofeng Shen*,†,‡ and Miao Xue† †

Jiangsu Provincial Key Laboratory of Environmental Engineering, Jiangsu Academy of Environmental Sciences, Nanjing 210036, People’s Republic of China ‡ College of Urban and Environmental Sciences, Peking University, Beijing 100891, People’s Republic of China S Supporting Information *

ABSTRACT: Widespread use of solid fuels affects indoor/outdoor air quality, human health, and climate change significantly. Replacing traditional solid fuels with affordable cleaner fuels is a challenge for most developing countries. In this study, carbon monoxide (CO) and particulate matter (PM) emissions and financial costs of a potential cleaner fuel-pelletized biofuels were compared to those of traditional solid fuels, including coal, crop residue, and wood, and a conventional modern fuel, liquid petroleum gas (LPG), in terms of fuel-mass-based emission factor (EF), delivered-energy-based emission factor (EFE), and delivered-energy-based cost (CE). The combustions of pelletized fuels and LPG had not only relatively higher thermal efficiencies but also lower EFs, leading to much lower EFE of these cleaner fuels. The adoption of pelletized fuels burned in a modern pellet burner could reduce pollutant emissions significantly in comparison to traditional solid fuels. When both EFE and CE are taken into consideration, it could be found that the nearly free ordinary biomass fuels and high-cost coals had much higher pollutant emissions, while LPG was the most expensive, although it would produce the lowest emission. Pelletized fuels appear to be a good alternative in rural households because of not only lower pollutant emissions but also relatively low cost. Future studies, including but not limited to emission measurements, potential reductions in air concentrations and health outcome, systematic cost−benefit analysis, and identification of key enablers and barriers affecting the large-scale uptake, are strongly recommended.

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INTRODUCTION In most households in developing countries and areas, solid fuels, such as coal, crop residues, and woody materials, are the predominant energy sources. It had been reported that, globally, although the proportion of households using solid fuels for cooking decreased continuously from 62% in 1980 to 41% in 2010, the absolute number of people relying on solid fuels for cooking has remained stable at about 2.8 billion because of population growth.1 When the population using solid fuels in household heating is taken into consideration, the population exposed to smoke from household solid fuel combustion would be larger. Residential combustions of these solid fuels are usually low-efficient, emit a variety of incomplete combustion pollutants, such as carbon monoxide (CO), particulate matter (PM), and various gaseous and particlebound toxic organics,2−4 and subsequently, affect human health and local/regional climate change significantly.5−7 The latest report on the Global Burden of Disease estimated that the exposure to household air pollution from inefficient solid fuel combustion was associated with about 3.5 million premature deaths in 2010, which was even higher than those of 3.2 and 0.15 million deaths caused by ambient PM and ozone pollution, respectively.8 In China, similar to other developing counties, coal and biomass fuels are the dominant sources for primary household energy, particularly in rural families. Approximately 46% of the Chinese households relied on solid fuels for daily cooking.1 In rural households, solid fuels comprised up to 90% of the energy © 2014 American Chemical Society

consumed (79.5% biofuels and 12.8% coal), while the contribution of liquid petroleum gas (LPG) was about 1.4%.9 Extensive use of solid fuels under relatively low-efficient burning conditions produces high amounts of incomplete pollutants, causing serious indoor and outdoor air pollution and adverse health outcomes, including lung cancer and respiratory illness in rural China.10−12 In a field survey on indoor CO and PM pollution based on a random sample of 396 rural households, average kitchen PM4 (PM with a diameter less than 4.0 μm) concentrations were 164 ± 110, 282 ± 286, and 142 ± 83 μg/m3 during the summer time when woody material, crop residues, and coal were combusted, respectively.13 In a rural household in northern China, the daily mean PM and PM2.5 levels were found to be as high as 2400 ± 1200 and 1300 ± 800 μg/m3, respectively, when biomass fuels were used for daily cooking and heating in the cold winter.14 For biomass fuels, in addition to be combusted in the residential sector as the primary household energy, open burning in a field is another commonly found practice in many rural areas, especially after harvest. The field burning is a great waste of resource; moreover, the intense pollution episode formed over a relatively short period during the burning often leads to multiple direct or indirect adverse effects on the air quality (poor visibility and regional haze), the traffic safety (congestion Received: March 21, 2014 Revised: April 30, 2014 Published: May 12, 2014 3933

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modern pellet burner. In this comparative study, coals are further classified into two groups of raw chunk and briquette because distinct emissions were found between these two coal forms. According to the statistics of the International Energy Agency (IEA), coals burned in the residential sector in China are all bituminous.37 Even though this might not be true in practice, it is believed that the fraction of anthracite in household coal use, especially in rural areas, is still small nowadays. Only few tests reported the emissions for anthracite,4,23,29 while bituminous coals are widely found in rural households and measured for the pollutant emissions. The woody materials are classified into two groups of wood log and branch/brushwood because of statistically significant different emissions between them.22,34 It is noted that the straws of different crop species (corn, rice, wheat, cotton, etc.) and wood species (elm, poplar, pine, etc.) also produce different pollutant emissions. Moreover, it is preferable to classify these residential combustion sources into specific fuel−stove combinations. Unfortunately, because of very limited data available, the further subdivisions are not practical at this stage. In the present study, we use fuel-specific emission factors,22 instead of further subdivision-based emission factors. Potential bias and uncertainties exit in this method, and the variations are due to not only different fuel properties but also results from distinct stove performance, fire management, fuel−air mixing status, and pollutant measurement methods. For a target pollutant, fuel-mass-based emission factors (EFs) defined as the mass of pollutant per fuel mass (g/kg) are often reported. Because of the different fuel types and distinct thermal efficiencies among the residential combustion sources, a deliveredenergy-based emission factor (EFE) would be more appropriate in a comparative study. EFE (g/MJ) can be calculated as EFE = EF/H/η, where EF is the fuel-mass-based emission factor (g/kg), H is the fuel low calorific value (MJ/kg), and η is the thermal efficiency (%). Thus, simultaneously measured H and η values are also collected. In some study, if H and/or η were not reported, the average values derived from other studies for the same fuel type are adopted. To discuss the financial cost among different fuel options, per delivered-energy-based family cost (CE) is calculated as CE = C/H/η. The same as those in the calculation of EFE, H and η are the fuel low calorific value and thermal efficiency, respectively, and C is the price of each fuel type (RMB/kg). In rural areas, residents usually burn ordinary biomass fuels (crop residue and woody materials) collected from the surrounding area and nearly free of charge. Therefore, in the present analysis, the ordinary biomass cost was set at zero. The prices of coal chunk and briquette vary in different regions depending upon the abundance of coal resource and quality. The average prices for these two forms in most local markets are 0.50−0.90 and 1.0−1.5 RMB/kg, respectively. The prices of pelletized fuels made from wood and crop residues in Chinese local markets are 700−900 and 400−650 RMB/kg, respectively. The price of LPG used in rural area is about 110−120 RMB per pot, equal to about 7.3−8.0 RMB/kg. It is realized that the prices of these fuels are not constant and sometimes may be out of the range estimated here. This would lead to considerable bias and uncertainties in the present cost analysis, and thus, the generation of the results should be in care under certain circumstances. Data for statistical analysis were conducted using Statistica (version 5.5, StatSoft) with a significance level of 0.05. Monte Carlo simulation run 100 000 times was adopted to address the uncertainty.

and increased accidents), public health, and even the weather system.15,16 To reduce air pollution and protect human health, the replacement of traditional solid fuels with cleaner and more affordable fuels is a top priority. Among various options, the deployment of pelletized fuels is expected to have great potential because a large volume of biomass fuels is available. Such use of biomass fuels can not only improve fuel-burning efficiencies in households and, subsequently, reduce emissions of various pollutants but also be considered as an alternative to avoid the widespread open burning of crop residues in a field. The use of pelletized fuels has been heavily promoted during the last several years in many countries, such as the United States, Sweden, and also China.17−19 In the Medium- and Long-Term Development Plan for Renewable Energy in China, the deployment of pelletized fuels has been strongly supported and the target goal is 50 million tons by 2020.17 It was reported that, by 2010, there are more than 757 factories manufacturing pelletized fuels in China and the annual production was about 3.46 million tons.9 Emission measurements are important in the evaluation of options for emission reductions and also critical in the development of emission permits, regulations, and pollution control strategies. There are significant differences in the residential fuel−stove combinations and subsequent emission characterizations among different countries.20,21 In the last several years, a series of local measurements on pollutant emissions from residential combustions in rural China has been conducted.22−25 These studies are valuable in filling the data gap. The main objective of this study is to compare the emissions of CO and PM from residential combustions of ordinary crop residues, woody materials, coal, pelletized biomass fuels, and LPG, a relatively cleaner fuel, in terms of their emission factors per fuel mass (EF, g/kg), per deliveredenergy-based emission factors (EFE, g/MJ), and per deliveredenergy-based cost (CE, RMB/GJ). We hope such a comparative study can achieve a better understanding of the pollutant emissions and the potential in fuel adoption among these different fuel types in the residential sector.

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MATERIALS AND METHODS

Literature-reported emission factors of CO and PM from rural household combustions using solid fuels, including coal, crop residues, woody materials, and pelletized biomass fuels, and a relatively cleaner fuel, LPG,22−36 are compiled. Data including reported emission factors, low calorific values, and thermal efficiencies of different fuels as well as measurement methods are listed in Table S1 of the Supporting Information in detail. It is noted that, in most studies, combustion thermal efficiency was measured using the water boiling test (WBT), which usually includes two phases of heating an amount of water from ambient temperature to the boiling temperature as rapidly as possible (high power phase) and keeping the water simmering at the lowest power (low power phase), and the thermal efficiency is calculated as the ratio of water-absorbed heat to the fuel energy.22,24 Because emissions among different countries vary dramatically as a result of the differences in many factors, such as fuel properties, stove designs, and fire management behaviors, in the present study, only local measurement studies in rural China are included. After the National Improved Stove Program during the 1980s, improved or socalled improved stoves are very common in rural China. In most studies, stoves used to burn crop residues and woody materials were reported to be the improved stoves, although some differences in stove performance could be found from the authors’ descriptive characterization of the stove in the literature. Coals were burned in ironmovable stoves, and pelletized fuels were combusted in a typical

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RESULTS AND DISCUSSION Thermal Efficiency and Fuel-Mass-Based Emission Factor. For ordinary biomass, the reported thermal efficiencies of the burning in improved stoves ranged from 9.7 to 24.0%, with means of 15.5, 17.7, and 14.2% for crop residues, wood log, and branch/brushwood, respectively. The differences among these three biofuel types were statistically insignificant (p > 0.05). The efficiency was generally lower than the designed thermal efficiency of 20% for improved stoves, which could be due to stove degradation and random and highly variable management behaviors in actual use. However, in 3934

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Figure 1. Emission factors of CO, PM, and PM2.5 per fuel mass (g/kg) for different fuel−stove combinations and thermal efficiencies in rural China. Data shown are means and standard deviations.

Figure 2. Comparison of per delivered-energy-based emissions of CO and PM (EFE, g/MJ) for different fuel types commonly used in rural Chinese households. The y axis is in the log scale.

significantly reduced after the pelletizing, as shown in Figure 1. The pollutant emissions for pelletized fuels were also significantly lower than that for the coal briquette (p < 0.05). The highest thermal efficiency was found for the use of LPG (52.1%), which also had the lowest EFCO and EFPM of 2.31 ± 3.28 and 0.524 ± 0.901 g/kg, respectively. The results of analysis of variation (ANOVA) show that the differences in fuel-mass-based EFs of CO and PM among different fuel types were statistically significant (p = 1.0 × 10−12 and 5.6 × 10−8 for EFCO and EFPM, respectively). By plotting the thermal efficiency against fuel-mass-based EFs for various fuels in Figure 1, it appears that the change in fuel form, e.g., raw chunk to briquette and ordinary biomass fuels to pelletized fuels, lead to significantly lower pollutant emissions and increased thermal efficiencies. Although because of limited data, we used fuelspecific data in the present discussion. It is kept in mind that such a change in emission and thermal efficiency is the result because of not only different fuel properties but also distinct stoves used. For example, pelletized fuels were burned in a modern pellet burner, while ordinary biomass fuels are combusted in self-made improved brick stoves in most rural households. Comparison of Delivered-Energy-Based Emissions. Because different fuels are burned in different stoves and the thermal efficiencies varies significantly, delivered-energy-based emission factors are believed to be more appropriate in the comparison of emissions among different combustion activities and to estimate the potential reductions by replacing dirty fuels with relatively cleaner fuels. Figure 2 shows per deliveredenergy-based emissions of CO and PM (EFE‑CO, and EFE‑PM, respectively) for different fuels. The y axis is in the log scale. The highest EFE‑CO and EFE‑PM were found for the raw chunk, which were 33.6 ± 15.7 and 3.42 ± 3.13 g/MJ, respectively.

comparison to those in traditional stoves, which were usually lower than 10%,38,39 the thermal efficiencies of the biofuel burning in improved stoves have been significantly improved. In terms of pollutant emission, the reported fuel-mass-based EFs of CO and PM (EFCO and EFPM, respectively) for crop residues were in the range of 29.3−215 and 1.68−18.0 g/kg, with means and standard deviations of 92.7 ± 47.3 and 7.05 ± 4.32 g/kg, respectively. For woody material, the EFCO and EFPM were 52.9 ± 21.5 g/kg (23.6−137 g/kg as a range) and 2.10 ± 1.39 g/kg (0.712−6.23 g/kg as a range) for the wood logs burned in domestic stoves. As expected, much higher pollutant emissions (62.8 ± 19.4 and 3.69 ± 0.89 g/kg of EFCO and EFPM, respectively) were found for the branch/brushwood in comparison to the wood logs. In the residential coal combustion, the reported thermal efficiency of the coal briquette burning was 32.4% (16.2−46.6% as a range), higher than that of 13.9% (6.82−17.6% as a range) of the raw chunk combustion (p = 0.036). The EFCO and EFPM for coal briquette were 43.6 ± 21.7 and 6.37 ± 6.59 g/kg, which in the expectation were significantly lower than those of 118 ± 47.9 and 12.9 ± 10.6 g/kg for the raw chunk (p = 0.001 and 0.035 for EFCO and EFPM, respectively). Lower fuel-mass-based emissions together with improved thermal efficiency (less fuel used) indicate that the replacement of raw chunk by coal briquette would lead to significant reductions of pollutants from residential coal combustion. In the burning of pelletized biomass fuels, the thermal efficiencies were 30.3−41.0%, significantly higher than those of the ordinary biomass burning. The EFCO and EFPM for crop straw pellet were 21.1 ± 16.2 and 2.62 ± 1.20 g/kg, respectively, and those for wood pellet were 4.38 ± 2.25 and 1.17 ± 0.89 g/kg, respectively. In comparison to the ordinary biomass fuels, the fuel-mass-based emission factors were 3935

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Figure 3. Potential reductions in pollutant emissions by replacing traditional solid fuels, including ordinary uncompressed biomass fuels and coal with the (A) crop residue pellets, (B) wood pellets, and (C) LPG.

The EFE‑CO and EFE‑PM for the coal briquette were 8.22 ± 5.63 and 1.07 ± 1.11 g/MJ, respectively, significantly lower than those for the raw chunk. The differences between the raw chunk and coal briquette in per delivered-energy-based EFs were much larger than those in fuel-mass-based results, because the burning of briquette could achieve not only lower fuelmass-based emission factors but also significantly improved thermal efficiencies. Among three types of ordinary biomass fuels, the crop residues had relatively higher EFE‑CO and EFE‑PM of 37.3 ± 19.9 and 2.90 ± 1.86 g/MJ, followed by the branch/ brushwood (26.5 ± 9.31 and 1.60 ± 0.38 g/MJ for EFE‑CO and EFE‑PM, respectively) and the fuel wood logs (15.6 ± 6.10 and 0.617 ± 0.398 g/MJ for EFE‑CO and EFE‑PM, respectively). In comparison to these traditional solid fuels, the crop straw pellet and wood pellet had much lower EFE‑CO and EFE‑PM. The average EFE‑CO and EFE‑PM for the crop straw were 4.27 ± 3.28 and 0.528 ± 0.243 g/MJ, respectively, which were only 11 and 18% of those for the uncompressed crop residues. For the wood pellet, the EFE‑CO and EFE‑PM were 0.746 ± 0.384 and 0.199 ± 0.152 g/MJ, respectively, which again were significantly lower than those for the ordinary woody materials. As a relatively cleaner fuel, LPG burning has the lowest EFE‑CO and EFE‑PM of 0.0996 ± 0.1404 and 0.0249 ± 0.0428 g/MJ, respectively. The reductions in CO and PM emissions by replacing the traditional solid fuels in the residential sector by pelletized fuels and LPG were significant (Figure 3). In comparison to traditional fuels, including raw chunk, crop residue, wood logs, and branch/brushwood, which are commonly used and usually had higher emissions per delivered energy, the use of crop residue pellets would lead to reductions of 77.0−89.8 and 68.7−80.4% in CO and PM emissions, respectively, and if using the wood pellets, the reductions in CO and PM emissions could be as high as 95.8−98.0 and 70.9−93.9%, respectively. The only one notable is that the PM emission from the fuel wood log burning would be reduced by only 10.5% if it was replaced by the crop residue pellets, because the relatively lower PM emission factor in the wood log combustion was found in comparison to other ordinary biomass fuels. This might be probably due to different fuel−air mixing statuses and flame structures in the stove chamber during the burning processes.

Fuel wood logs were usually burned in an ordered arrangement. It was reported that pollutant emissions for wood piled in a random arrangement were much higher than those from the high flaming combustion with an order crib arrangement of fuel.40 In comparison to the emissions from the coal briquette, CO and PM emissions from the pelletized fuel combustions were also significantly lower, with potential reductions of 30.8− 46.3% (replaced by crop straw pellets) and 77.2−90.2% (replaced by wood pellets). In Figure 3, it is also clearly evident that much more significant reductions would be achieved if LPG was used. LPG is often regarded as an environmentally friendly and convenient modern fuel and became more and more popular in rural households. However, because of its relatively high cost, especially in comparison to free biomass fuels, most residents use the LPG for daily cooking occasionally. Relationship between Delivered-Energy-Based Emission and Cost. It is clear that the adoption of pelletized fuels could achieve significant reductions in CO and PM emissions and, subsequently, benefit the environment and human health obviously. However, there are many factors affecting the adoption and sustainable use of these new fuels. It has been widely believed that LPG is a cleaner fuel, but relatively higher cost in comparison to nearly free biomass fuels often prevent a continuous use of the LPG in the daily lives of residents. A similar situation could be in the adoption of pelletized fuels. With a view to comparing the potential costs in the use of different fuels, delivered-energy-based costs (CE) were calculated following the approach in the calculation of deliveredenergy-based emission factors. The uncertainty was addressed by the Monte Carlo simulation. The ordinary biomass fuels are widely abundant in rural areas and, hence, free of cost for residents in daily use. The CE of LPG was the highest at about 313 RMB/GJ (288−339 RMB/GJ as an interquartile range). The CE of the coal briquette was 207 RMB/GJ (147−297 RMB/GJ as an interquartile range), higher than that of 169 RMB/GJ (114−247 RMB/GJ as an interquartile range) of the raw coal. The pellets made from woody materials [133 RMB/ MJ (115−154 RMB/MJ as an interquartile range)] are usually more expensive than those made from crop residues [107 RMB/MJ (81.1−136 RMB/MJ as an interquartile range)]. 3936

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Figure 4. Relationship between delivered-energy-based pollutant emission (EFE, g/MJ) and per delivered-energy-based cost (CE, RMG/GJ) for different fuels.

rural areas was about 12.8 billion m3, covering 41.7 million households.9 However, the household energy structure varies obviously in different regions depending upon the abundance of resources, the socioeconomic development levels, and the cooking/heating habits. Generally, coal and biomass fuels are and still to be in the years to come the predominant resources consumed in rural households. Replacing traditional solid fuels with cleaner and more affordable fuels is a challenge in China. The use of pelletized fuels would lead to significant reductions in emissions of CO and PM, although requiring some cost in comparison to nearly ordinary free biomass fuels, but it is cheaper than LPG. In comparison to coal, not only lower pollutant emission but also lower cost could be found for pelletized fuels. In addition, the development of pelletized fuel manufacture could effectively use biofuels, especially avoiding the waste of resources in open burning in a field. Also, it could provide jobs for rural residents. Thus, it appears that the adoption of pelletized fuels should be under deeper consideration, and it is fortunate to see that the deployment has been under strong support in the national development plan of renewable energy. It is realized that there are few studies supporting a largescale deployment of pelletized fuels at this stage. To achieve robust conclusions on environmental and socioeconomic benefits, more studies in the future are required. The research needs include but are not limited to characterizations of residential combustion sources, measurement on indoor and outdoor air pollution, identification of key factors affecting the large-scale uptake and sustainable use of pelletized fuels, and potential reductions in emission, pollutant concentrations, and human health outcome. For emission characterization, the fuel−stove combinations in rural China differ significantly from those in other countries and even vary dramatically among different regions. Thus far, there are limited combustion measurements on the residential combustion sources in rural China. A variety of different fuel types (traditional solid fuels and cleaner fuels, such as LPG and biogas) and stove designs should be included in the survey, and it would be preferable to conduct these measurements in a field covering fuel−stove combinations in actual use. For indoor and outdoor air pollution, the relationship between pollutant emission reduction and concentration decrease is not linear and varies among different pollutants. It is necessary to conduct measurements on indoor and outdoor pollution levels before and after the intervention of relatively cleaner fuels. For human health outcome, in the emissions of different fuels, there might be significant differences in not only the total PM mass but also

Generally, delivered-energy-based costs of the pelletized fuels were lower than those of coals mainly because of improved thermal efficiencies. When EFE is plotted against CE, these different fuels can fall into four groups, as shown in Figure 4. Relatively high costs and high pollutant emissions of coals made the fuel not a priority choice in household energy use. The costs of ordinary biomass fuels (crop residues, wood log, and branch/brushwood) were nearly zero, but the burning also produced relatively higher pollutant emissions. LPG, although having the lowest pollutant emissions, was the most expensive. The pelletized fuels produced relatively lower pollutant emissions and, more important, were much cheaper compared to the LPG and even coals. When the LPG, pelletized fuels, and ordinary biomass fuels (because these uncompressed biofuels are the widely used raw materials of pellets) are taken into the consideration, it is interesting to see that the relationship between EFE and CE followed the exponential equation as EFE = k exp(−aCE) (the red line in Figure 4). The R2 values for CO and PM were 0.94 and 0.89, respectively. Moreover, the exponential relationship suggested that the unit CE increase would lead to higher reductions in pollutant emissions at a low CE level; for example, raw biofuels were replaced by pelletized fuels, in comparison to that at a relatively high CE level, for instance, using LPG to replace pelletized fuels. It is noted that the cost analysis here only takes the cost of purchasing fuels into account, while the burden of buying a new stove for the specific fuel is not included. In practice, the cost of replacing brick stoves with modern pellet burners could be an obvious barrier in the adoption of pelletized fuels, especially at the initial deployment. Effective financial incentives are necessary in the beginning of a cleaner fuel/stove deployment program. In fact, in the deployment of pelletized fuels in rural Beijing, a subsidy of 400 RMB was provided to the household that replaced traditional biofuels with pellets. It is interesting that, in a future study, the potential benefits of pollutant emission reduction could be estimated and taken into the systematic cost−benefit analysis with more data available, such as the potential change in pollutant concentrations and evident studies on the relationship among human exposure, health outcome, loss of life, and financial burdens. Implication and Research Need. Under the fast economic development in China, the structure of energy consumption in rural households has been changing over the last 3 decades with increased shares of relatively cleaner commercial energy resources, such as LPG, electricity, and biogas. For instance, the national total production of biogas in 3937

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placenta with the risk of neural tube defects. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 12770−12775. (7) Venkataraman, C.; Habib, G.; Eiguren-Fernandez, A.; Miguel, A.; Friedlander, S. Residential biofuels in south Asia: Carbonaceous aerosol emissions and climate impacts. Science 2005, 307, 1454−1456. (8) Lim, S.; Vos, T.; Flaxman, A.; Danaei, G.; Shibuya, K.; AdairRohani, H.; Amann, M.; Anderson, H.; Andrews, K.; Aryee, M.; Atkinson, C.; Bacchus, L.; Bahalim, A.; Balakrishnan, K.; Balmes, J.; Barker-Collo, S.; Baxter, A.; Bell, M.; Blore, J.; Blyth, F.; Bonner, C.; Borges, G.; Bourne, R.; Boussinesq, M.; Brauer, M.; Brooks, P.; Bruce, N.; Brunekreef, B.; Bryan-Hancock, C.; Bucello, C.; Buchbinder, R.; Bull, F.; Burnett, R.; Byers, T.; Calabria, B.; Carapetis, J.; Carnahan, E.; Chafe, Z.; Charlson, F.; Chen, H.; Chen, J.; Cheng, A.; Child, J.; Cohen, A.; Colson, K.; Cowie, B.; Darby, S.; Darling, S.; Davis, A.; Degenhardt, L.; Dentener, F.; Des Jarlais, D.; Devries, K.; Dherani, M.; Ding, E.; Dorsey, E.; Driscoll, T.; Edmond, K.; Ali, S.; Engell, R.; Erwin, P.; Fahimi, S.; Falder, G.; Farzadfar, F.; Ferrari, A.; Finucane, M.; Flaxman, S.; Fowkes, F.; Freedman, G.; Freeman, M.; Gakidou, E.; Ghosh, S.; Giovannucci, E.; Gmel, G.; Graham, K.; Grainger, R.; Grant, B.; Gunnell, D.; Gutierrez, H.; Hall, W.; Hoek, H.; Hogan, A.; Hosgood, H., III; Hoy, D.; Hu, H.; Hubbell, B.; Hutchings, S.; Ibeanusi, S.; Jacklyn, G.; Jasrasaria, R.; Jonas, J.; Kan, H.; Kanis, J.; Kassebaum, N.; Kawakami, N.; Khang, Y.; Khatibzadeh, S.; Khoo, J.; Kok, C.; Laden, F.; Lalloo, R.; Lan, Q.; Lathlean, T.; Leasher, J.; Leigh, J.; Li, Y.; Lin, J.; Lipshultz, S.; London, S.; Lozano, R.; Lu, Y.; Mak, J.; Malekzadeh, R.; Mallinger, L.; Marcenes, W.; March, L.; Marks, R.; Martin, R.; McGale, P.; McGrath, J.; Mehta, S.; Mensah, G.; Merriman, T.; Micha, R.; Michaud, C.; Mishra, V.; Hanafiah, K.; Mokdad, A.; Morawska, K.; Mozaffarian, D.; Murphy, T.; Naghavi, M.; Neal, B.; Nelson, P.; Nolla, J.; Norman, R.; Olives, C.; Omer, S.; Orchard, J.; Osborne, R.; Ostro, B.; Page, A.; Pandey, K.; Parry, C.; Passmore, E.; Patra, J.; Pearce, N.; Pelizzari, P.; Petzold, M.; Phillips, M.; Pope, D.; Pope, C., III; Powles, J.; Rao, M.; Razavi, H.; Rehfuess, E.; Rehm, J.; Ritz, B.; Rivara, F.; Roberts, T.; Robinson, C.; Rodriguez-Portales, J.; Romieu, I.; Room, P.; Rosenfeld, L.; Roy, A.; Rushton, L.; Salomon, J.; Sampson, U.; Sanchez-Riera, L.; Sanman, E.; Sapkota, A.; Seedat, S.; Shi, P.; Shield, K.; Shivakoti, R.; Singh, G.; Sleet, D.; Smith, E.; Smith, K.; Stapelberg, N.; Steenland, K.; Stöckl, H.; Stovner, L.; Straif, K.; Straney, L.; Thurston, G.; Tran, J.; van Dingenen, R.; van Donkelaar, A.; Veerman, J.; Vijayakumar, L.; Weintraub, R.; Weissman, M.; White, R.; Whiteford, H.; Wiersma, S.; Wilkinson, J.; Williams, H.; Williams, W.; Wilson, N.; Woolf, A.; Yip, P.; Zielinski, J.; Lopez, A.; Murray, C.; Ezzati, M. A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990−2010: A systematic analysis for the Global Burden of Disease Study 2010. Lancet 2013, 380, 2224−2260. (9) Tian, Y. Development status and trend of rural energy in China. China Energy 2013, 35, 11−15 (in Chinese). (10) Mumford, J.; He, X.; Chapman, R.; Cao, S.; Harris, D.; Li, X.; Xian, Y.; Jiang, W.; Xu, C.; Chuang, J.; Wilson, W.; Cooke, M. Lung cancer and indoor air pollution in Xuanwei, China. Science 1987, 235, 217−220. (11) Zhang, J.; Smith, K. R. Household air pollution from coal and biomass fuels in China: Measurements, health impacts, and interventions. Environ. Health Perspect. 2007, 115, 848−855. (12) Zhang, Y.; Tao, S.; Shen, H.; Ma, J. Inhalation exposure to ambient polycyclic aromatic hydrocarbons and lung cancer risk of Chinese population. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 21063− 21067. (13) Edwards, R.; Liu, Y.; He, G.; Yin, Z.; Sinton, J.; Peabody, J.; Smith, K. Household CO and PM measured as part of a review of China’s National Improved Stove Program. Indoor Air 2007, 17, 189− 203. (14) Zhong, J.; Ding, J.; Su, Y.; Shen, G.; Yang, Y.; Wang, C.; Simonich, S.; Cao, H.; Zhu, Y.; Tao, S. Carbonaceous particulate matter air pollution and human exposure from indoor biomass burning practices in rural northern China. Environ. Eng. Sci. 2012, 29, 1038− 1045.

the chemical compositions of PM emitted. Thereby, it is interesting to simultaneously investigate the health outcome when people are exposed to distinct levels of air pollution. For identification of key factors, there are many programs in the world to deploy cleaner fuels and improve stoves in rural area. Some achieved important successes, but some did not. It is critical to identify key enablers and barriers affecting the adoption and, more importantly, the sustainable use of cleaner fuels and improved stoves, to improve the intervention effectiveness and achieve desired environmental, socioeconomic, and human health benefits.

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ASSOCIATED CONTENT

S Supporting Information *

Compiled data of prices, low heating values, thermal efficiencies, and fuel-mass-based emission factors of CO and PM for different fuel types collected from the literature (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 0086-25-86525031. E-mail: gfshen12@gmail. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The funding for this study was supported by the National Natural Science Foundation (41301554), the Jiangsu Natural Science Foundation (BK20131031), and the China Postdoctoral Science Foundation (2013M531322). The authors thank Yuanchen Chen and Huizhong Shen from Peking University for the help in data statistical analysis and anonymous reviewers for valuable comments.

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