Letter pubs.acs.org/journal/estlcu
Improving the Energy Efficiency of Stoves To Reduce Pollutant Emissions from Household Solid Fuel Combustion in China Qing Li,† Jingkun Jiang,*,†,‡ Juan Qi,§,⊥ Jianguo Deng,† Deshan Yang,∥ Jianjun Wu,§,⊥ Lei Duan,†,‡ and Jiming Hao†,# †
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China ‡ State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China § National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, China ∥ Beijing Association of New Energy and Renewable Energy, Beijing 100029, China ⊥ School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China # Collaborative Innovation Centre for Regional Environmental Quality, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Emissions of air pollutants from household solid fuel combustion in low-efficiency stoves have serious negative impacts on human health and air quality in China. This study compares the thermal efficiency (TE) and emissions from solid fuel combustion in a newly developed underfire heating stove and a typical traditional over-fire heating stove. The average TEs for burning all tested fuel types (semi-coke, anthracite, briquette, bituminous, lignite, and biomass) were 83 and 42% for the new stove and the traditional stove, respectively. The new stove was effective in reducing CO2 and pollutant emissions per unit energy delivered to a radiator. The average reductions were ∼50% for CO2, 79% for PM2.5, 95% for EC, 85% for benzo[a]pyrene equivalent carcinogenic potency, and 66% for eight selected toxic elements (Pb, Cu, Sb, Cd, As, Ag, Se, and Ni) in PM2.5. Improvements in stove technology are demonstrated as a practical approach for improving TE and reducing emissions of hazardous pollutants and CO2.
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diameter of ≤2.5 μm) is greater than that of many industrial sources, such as coal-fired power plants.14 The TE of solid fuel burned in household stoves is lower than that for fuel burned in industrial boilers. The TE of traditional cookstoves in China was commonly less than 10%.15 Replacement stoves have been mainly designed with chimneys to transport emissions from indoors to outdoors without a significant reduction in the total emission or improvement of the TEs during the 1980s and 1990s.16,17 Rapid development of stove technology has occurred in the past 15 years. The TE for newly developed cookstoves, such as gasifier cookstoves, was increased to approximately 27−35%.18−20 The TE of heating stoves promoted by Chinese local governments is commonly lower than 55% according to our surveys, although their claimed TE can be up to 65−75%. The TE of household stoves is mainly governed by two factors: heat transfer and combustion efficiencies. The heat transfer efficiency is mainly affected by the stove chamber structure. The combustion
INTRODUCTION Coal is the dominant energy source in China and will continue to be for a long time. Coal combustion makes the largest contribution to ambient particulate matter (PM) pollution1 and CO2 emissions2 in China. These emissions negatively impact the global climate and human health.3,4 Considerable effort has been spent to limit their emissions in China.5−9 Air pollution control devices, such as electrostatic precipitators, desulfurization systems, and selective catalytic reduction, have been widely installed on industrial coal boilers.6,9−11 There is another way to think about driving down pollutant and CO2 emissions,11 i.e., improving the efficiency of energy utilization. The designed thermal efficiency (TE) of China’s industrial coal boilers is currently 72−80%, whereas the target set in China’s 13th FiveYear Plan (2016−2020) is to exceed 90%. However, the efficiency of household stoves has attracted less interest from the public and the government even though household solid fuel combustion has been one of the major emission sources of CO212 and air pollutants directly associated with negative impacts on human health.13 More than 4 million people die prematurely from illnesses that can be attributed to household air pollution globally, and the contribution of household stoves to atmospheric PM2.5 (particulate matter with an aerodynamic © XXXX American Chemical Society
Received: August 26, 2016 Revised: September 18, 2016 Accepted: September 19, 2016
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DOI: 10.1021/acs.estlett.6b00324 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
new stove is in pilot studies with some units deployed in the Beijing area. Both stoves are operated by natural draft and manual air controls. The internal space of the new stove can be approximately divided into two chambers: a fuel storage chamber and a secondary combustion chamber. The primary combustion of solid fuel is fired at the junction of the two chambers, i.e., the bottom of the stove. Devolatilization occurs in the lower part of the storage chamber because of the high temperature in the combustion region. The carbonized solid fuel is delivered by gravity feed, while the volatile gases are drawn into the primary combustion region and then the secondary combustion chamber by negative pressure. Additional air is supplied in the secondary combustion chamber to further burn the volatile gases in the fuel gas. The surface area for heat transfer to circulating water surrounds the pathway of the flue gas. The traditional heating stove has only one chamber, with the fuel batch placed in the bottom part of the chamber. Fuel is fed into the chamber from the top to the bottom and fired from the bottom to the top with primary air fed into the bottom. The upper fuel is carbonized and releases the volatile gases when the bottom fuel is burning. The surface area for heat transfer is mainly in the chamber walls and top dust baffle, which also has the function of capturing some dust from the flue gas. Panels a and b of Figure S3 show photographs of the two stoves tested, and panel c of Figure S3 suggests that at the moment of fuel addition, the new stove emits a level of pollutants considerably lower than that of the traditional stove. The operational processes for the two stoves are presented in the caption of Figure S3. Compared to the traditional heating stove, the new stove has a relatively larger water surface area because of its longer flue pathway (from the stove bottom to the chimney). Along the flue pathway, secondary air can be supplied to enhance the burning of devolatilized matter. Tested Fuel Samples. Six different types of solid fuel samples were tested, including a semi-coke chunk (made from bituminous coals with dimensions in the range of 0.3−4 cm), anthracite chunk (3−7 cm), anthracite briquette (∼4 cm), bituminous chunk (3−7 cm), lignite chunk (3−7 cm), and biomass briquette (sphere-shaped, ∼4 cm) made from sawdust. Table S1 presents the moisture, ash, volatile matter, fixed carbon, and sulfur contents, as well as net calorific value as received, for the various fuels. Among these samples, semi-coke, anthracite chunk, and anthracite briquette are considered as promising potential clean fuels, whereas biomass was recently recommended as a renewable fuel without net CO2 emissions. Bituminous and lignite coals and raw biomass have been recognized as dirty fuels because of their high volatile matter content, which causes high PM emissions, but residential consumers have preferred them because of their ease of ignition and low price. These six samples were selected to represent six types of solid fuels commonly used for household energy needs in rural China.29 Tested Method and Sampling System. Combustion experiments were conducted in the laboratory (see Figure S4) for measuring emissions and TEs, while measurement of TEs was also conducted in a household that has the traditional stove and the new stove (see Figure S3). Five water-filled radiators (for 150 m2 household heating) were connected in series to the test stoves via a water circulating pump for both household and laboratory testing. The fuel weight for each test was fixed at 10.0 and 60.0 kg for the traditional and new stoves, respectively. Three successful tests of full burning cycles (from ignition to
efficiency in household stoves is commonly low and leads to incomplete combustion. The incomplete combustion results in larger emission factors (EFs) of PM and CO in residential stoves than in industrial boilers.21 An estimated 36% of primary PM2.5, 53% of elemental carbon (EC), and 62% of polycyclic aromatic hydrocarbons (PAHs) in China’s annual emissions are from residential solid fuel combustion.18,22−24 Residential solid fuel combustion has led to serious air pollution with an increase in environmental risk factors.13,25−27 The replacement of raw fuels with processed (or clean) fuel has been intensively investigated as a way to reduce household air pollution. The EFs of PM2.5, EC, organic carbon (OC), and PAHs for anthracite coal and semi-coke briquette were reported to be several dozen times lower than that for bituminous coal because of the low volatile matter content,28−31 whereas pelletized biofuels and coal briquettes were reported to have low pollutant EFs due to the change in their burning form.32−36 However, in some cases, cleaner fuels may not be effective for reducing the emissions of CO2, toxic elements, or even PAHs.37,38 Additionally, the cleaner fuels are typically more expensive, which in turn may result in residential consumers not choosing them. In addition, specific stoves are often required for these fuels, which are also a barrier for the deployment of new fuels as residents may have to change their cooking and/or heating habits. Since 2012, the Beijing government has launched a policy of replacing all low-rank coals with anthracite with the help of financial subsidies, and Hebei province has planned the replacement with 15 million tons of clean coal in 2016. These policies require continuous and large amounts of financial support but have not yet made a considerable impact, as illustrated by photographs in Figure S1, which show emissions from typical residential solid fuel combustion in many areas in China. According to the estimation of reduction scenarios applied to the year 2010, cleaner stoves were proposed to provide better emission reductions, even when free fuels are used, compared to the deployment of cleaner fuels alone.16 However, there is little information about the in-use emission and TE of updated cookstoves,18−20 and there is no knowledge of the performance of updated heating stoves.16 The new household stoves should be based on updated combustion technology and well-defined standards that include TE and emissions of the most important pollutants, i.e., CO and PM2.5. With the aim of improving TE and reducing pollutant EFs, this study evaluates a newly developed heating stove that employs under-fire combustion technology, different from the over-fire technology commonly used in household stoves. The technologies used in the new and traditional stoves are termed under-fire and over-fire technologies according to the fire location in the stove chamber.39,40 Emissions from the combustion of semi-coke, anthracite, coal briquette, bituminous, lignite, and biomass briquette samples in the new stove are characterized and compared with those in a typical traditional stove. The reductions in delivered energy-based (based on useful energy delivered) CO2, PM2.5, OC, EC, PAH, and selected toxic element EFs from the new stove are presented, and the combustion technology affecting emissions and TE is discussed.
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MATERIALS AND METHODS Tested Stoves and Combustion Technologies. Figure S2 shows a schematic of the combustion technologies employed by the two tested stoves. The tested traditional stove is one of the most popular household coal stoves. The B
DOI: 10.1021/acs.estlett.6b00324 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters fire extinction) were conducted for each fuel−stove combination in each laboratory test, while three more successful tests were conducted for each combination in the household with continuous operation for 24 h. Figure S4 shows a schematic of the sampling system for measuring pollutant emissions and TEs in the laboratory. The sampling system and calculation of pollutant EFs have been previously described in detail and are briefly outlined here.30,31,41 The two stoves tested were placed in a sealed room, and their flue gases mixed with dilution air were drawn into a dilution tunnel. Possibly because of a higher negative pressure above the stove chimney in the laboratory testing and the resultant increase in combustion efficiency, TEs obtained under laboratory conditions are higher than those obtained under household conditions (Table S2). However, this difference does not affect the comparison between the new stove and the traditional stove when burning various fuels. All pollutant concentrations were measured in the dilution tunnel. A black carbon meter (Aethalometer, model AE 33, Magee Scientific) was used to monitor the concentrations of ultraviolet (370 nm)-absorbing PM and EC (880 nm) with diameters of
