Biocoal Briquettes Combusted in a Household Cooking Stove ...

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Biocoal Briquettes Combusted in a Household Cooking Stove: Improved Thermal Efficiencies and Reduced Pollutant Emissions Juan Qi,†,∥ Qing Li,‡,∥ Jianjun Wu,*,†,# Jingkun Jiang,*,‡ Zhenyong Miao,† and Duosong Li§ †

School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, 221116, China National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou, 221116, China ‡ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China § School of Environment and Surveying and Mapping, China University of Mining and Technology, Xuzhou, 221116, China #

S Supporting Information *

ABSTRACT: Clean fuels are urgently needed to reduce household cooking emissions. The thermal efficiencies (ηth) and pollutant emission factors (EFs) of biocoal briquettes (made from a mixture of biomass and coal powder) burned in a typical cooking stove were investigated and compared with those of coal briquettes and biomass briquettes. Biocoal briquette samples were obtained by molding blends of anthracite with 10−30 wt % crop straw of various types (maize straw, wheat straw, or rice straw). The optimum proportions for energy savings and PM2.5 EF reduction were found to be 15−20 wt %. Compared with the ηth of coal briquettes and biomass briquettes, the ηth of biocoal briquettes increased by 81−127% and 88− 179%, respectively, with the optimum addition ratios of crop straw, while the delivered energy-based PM2.5 EFs of the biocoal briquettes were reduced by 61−67% and 99.0−99.5%, respectively. Delivered energy-based EFs of NOX, SO2, and toxic elements (As, Se, and Pb) also showed a significant reduction. These results indicated that biocoal briquettes can serve as a promising substitute for domestic solid fuel to reduce pollutant emissions and save energy.

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INTRODUCTION Nearly half of the world’s population directly uses solid fuels, such as coal, wood, and crop straw, for household activities,1 especially in developing countries, including China, India, and Nepal.2−4 The direct burning of raw solid fuel in traditional stoves produces serious pollutant emission, for example, PM2.5 (particle matter with an aerodynamic diameter less than or equal to 2.5 μm), BC (black carbon), BrC (brown carbon), toxic elements, CO, NOX, and SO2.5−10 These emissions give rise to health risks11−13 and cause tremendous environmental problems.14 Household stoves are not likely to be replaced in many areas in the near future due to limited economic conditions and living habits. The combustion properties of solid fuel have been intensively investigated. The volatile content of coal is widely identified to be the most important factor influencing PM formation.8,15−19 Pollutant emissions increase as the volatile content increases. More recent research has revealed that pollutant EFs are positively correlated with volatile contents ranging from 2.8% to 48.7%.20 Biomass produces more serious pollutant emissions because of the higher volatile inclusion.21 Anthracite has been widely acknowledged as a clean fuel with low PM2.5 EFs. In addition to the anthracite’s low yield and high price, the low burnout ratio and poor ignition performance are significant limitations to its utilization.5,22−25 These poor © 2016 American Chemical Society

performance factors are related to the density and structure of coal.26 Therefore, determining a reasonable method to utilize biomass and anthracite is a serious environmental problem. Better combustion performance may be achieved if the structure of anthracite can be adjusted. Biomass, a renewable energy, is a good candidate to be mixed with anthracite for improving its combustion performance. China is a large agricultural country in which most crop straw is treated as waste and usually burned as open fires. Pollutant emissions are produced from the direct biomass burning. Haze contamination usually becomes more serious during the harvest season. Moreover, powdered coal, whose production increases with the development of mining mechanization, poses a direct threat to the environment and aggravates energy waste, while anthracite is regarded as a scarce resource. If biocoal briquettes, the mixture of biomass and coal, are proven to be feasible for largescale use in household combustion, it will not only solve the problem of direct crop straw burning but also address the issue of powdered anthracite utilization and provide a solution that will benefit the environment, energy production and the Received: Revised: Accepted: Published: 1886

July 7, 2016 December 23, 2016 December 30, 2016 December 30, 2016 DOI: 10.1021/acs.est.6b03411 Environ. Sci. Technol. 2017, 51, 1886−1892

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Environmental Science & Technology

Tested Stove and Operation. To ensure the repeatability of the sampling process, a stove (see Figure S3) that had been used in a typical rural kitchen for 3 years was chosen for the combustion experiment. The ηth value of the stove was lower than that of a new stove. A kettle was placed on it to test ηth, and the water temperature was recorded with a control display unit at the spout. Fuel (biocoal and coal briquettes fixed at 2.0 kg and biomass briquettes fixed at 1.0 kg according to the capacity of the fuel chamber) was added from the upper part of the stove. The burn-out (not being put out) method was adopted, and ash was cleaned from the bottom. New fuel was not added to the stove until the briquettes burned out and the ash was cleaned. For consistency during the laboratory experiments, the fuel sample was ignited using propane gas with a fixed flow rate of 3 L/min until a stable flame was observed, which took approximately 1−5 min. The ignition time and extinction time were recorded. A new kettle with 4.0 kg of water replaced the previous one as soon as the temperature reached the fixed maximum temperature of 90 °C to prevent the water from reaching the boiling temperature, at which point more water vapor would have been released into the exhaust. Sampling System. This study was conducted on simulated civil combustion test equipment (as shown in Figure S4) in a village of Beijing. The sampling method and the testing stove were introduced in previous publications23,43 but are briefly summarized here. A stove and a kettle were positioned in a stainless steel box. An air blower built into the side pumped filtered ambient air into the box to dilute the smoke, which was then sucked into a pipe by a high-power fan fixed at its downstream end. Sampling ports are located at the dilution tunnel. Each sample was tested at least three times. The flue gas, which was filtered via a quartz filter, flowed continuously into three flue gas analyzers (Thermo Scientific; 48i, 43i, and 42i for CO, NOX, and SO2, respectively), CO2 meter (GC-0012; Gas Sensing Solutions Ltd.), and particle sampler (URG-2000-30 EH; URG Inc.) over the entire combustion process. Simultaneous samples were collected on filters in three branches (PM2.5, PM1.0, and TSP), and each branch had two sampling ports for parallel sampling. Particle samples were collected on quartz-fiber filters (Pall; 2500QAOUP; 47 mm diameter) for two burning cycles. Teflon membranes were used to collect PM in one burning cycle to analyze toxic elements. An X-ray fluorescence (XRF) spectrometer (NAS100; Nayur Technology Co., Ltd.) was applied to measure the concentration of toxic elements (Pb, As and Se) captured on the Teflon filters containing PM2.5 samples in offline mode. An ion chromatograph (Dionex-600, Dionex, Thermo Fisher Scientific, Inc.) was used to measure the concentrations of water-soluble SO42−. Analysis Method. The ηth value for fuel was derived from the temperature increase (ΔT (°C)) of the water in the kettle. It is expressed as ηth (%) = (Mw × Cw × ΔT)/(Mc × Qc), where Mw (kg) represents the water mass in the kettle, Cw (kJ/(k°C)) represents the specific heat capacity of water, Mc (kg) represents the fuel mass for each test, and Qc (kJ/kg) represents the net calorific value of the received fuel. The EFs include mass-based (EFm) and delivered energybased (EFt) factors. They can be expressed in terms of pollutant mass per fuel mass and pollutant mass per unit useful delivered energy (not per unit of fuel energy). EFt was determined using EFm based on the following equation: EFt (mg/kJ) = EFm/(ηth × Qc). EFm can be presented as follows:

economy. Biocoal briquettes may be a valuable and feasible way to use the mixture of biomass and coal in residential combustion. Co-combustion of biomass and coal has been intensively studied in industry.27−29 The blends provide a sufficiently high burnout ratio29,30 and reduce pollutant generation.27 The synergistic effect of biomass and coal helps to enhance the combustion performance of the mixture, and more volatile matter than expected is yielded during the pyrolysis process.31,32 The mixture’s high volatility leads to a highly porous char-accelerated combustion.27,33 The degree of uniformity is also increased by the addition of cellulose.34 These changes are beneficial for heat and mass transformation. The flame temperature drops during cofiring, suggesting a moderate reduction in thermal NOX formation.27,35 NO formed from volatile N can be reduced with the gas−solid heterogeneous reactions of char.36 Metallic oxide, which has a high melting point, inhibits the volatilization of toxic metals.37,38 Nevertheless, the pollutant emission of biocoal briquettes used in household cooking stoves has not been studied. Aiming to seek for a substitute solid fuel to overcome the deficiencies of the low burnout ratio and poor ignition performance of anthracite, this study investigated the emission factors (EFs) and thermal efficiencies (ηth) when burning biocoal briquettes (made from a mixture of biomass and anthracite powder) in a typical household cooking stove. Coal briquettes and biomass briquettes were tested for comparison. Environmental implications were also discussed.

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MATERIALS AND METHODS Tested Samples. To ensure a good representation of biocoal briquettes, three types of crop straw, which is the byproduct of the main crops in China according to China’s rural energy Yearbook (2009−2013) and the 2014 China agricultural development report,39,40 were used for this study: maize straw, wheat straw, and rice straw. They were collected from rural Xuzhou in Jiangsu province, East Shanxi anthracite was selected for blending. Table S1 presents the characteristics of these raw materials, including proximate analysis, elemental analysis, and the net calorific value of the received fuel. Figure S1 shows a typical thermogravimetric analysis (TGA) of the tested biomass and coal samples using a heating rate of 5 K/ min under argon and air atmospheres, respectively. According to previous work, the ignition point is the temperature at which the combustion curve and the pyrolysis curve deviate from each other.41,42 In this study, the ignition point was 200−300 °C for biomass and 450−550 °C for anthracite. The data sufficiently demonstrate that the ignition point of biomass is much lower than that of anthracite. To investigate the actual mechanism of combustion of biomass mixed with anthracite and to identify the optimum biomass content, biocoal briquettes for each of the above crop straws were prepared with different biomass contents (0, 10, 15, 20, 25, 30, and 100 wt %) in the same process conditions: molding pressure of 25 MPa, particle size less than or equal to 1 mm, cylindrical shape with 30 mm diameter and 20 mm height, and 10% clay soil added as a binder. Coal and biomass were crushed into powder using a crusher. These powders and clay were mixed using an electric blender and then formed into briquettes using the cold-press molding technique. Figure S2 shows a photograph of the finished briquettes, the details of which are shown in Table S2. 1887

DOI: 10.1021/acs.est.6b03411 Environ. Sci. Technol. 2017, 51, 1886−1892

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Environmental Science & Technology EF (mg/g) = Mi × F/Mc, where Mi (mg) is the collected sample mass, F is the ratio of the total diluted flue gas flow rate to the sampling flow rate, and Mc (g) is the fuel mass for each test. This study reported the ηth and pollutant emission factors reduced by modifying biomass content mixture with anthracite. Multiobjective optimization method was employed to evaluate a balance point between the two parameters. The corresponding mathematical model with objective function is described as f1 = max (ηth) and f 2 = min (EFt). For convenience, this function was converted to the general form of multiobjective optimization as f1 = max(ηth) and f 2 = max(1/EFt). The overall optimization objective function was defined as follows: Max f = a × η + b × (1 − c), where f is the objective function, η is normalized ηth, c is normalized PM2.5 EFt, and a and b, the ratio of two criteria. The weights of thermal efficiency and PM2.5 emission factor were considered to be equally important in this analysis, namely a = 0.5 and b = 0.5. The mass-weighted average (EFb−c), which was interpolated between the values for 100% biomass and 100% coal according to mass inclusion of biomass and coal, was calculated as follows: EFb−c = EFb × b% + EFc × c%, where EFb and EFc represent EFs (mass-based or delivered energy-based) of biomass briquettes and coal briquettes, respectively; b% and c% are the mixing proportions of biomass and coal, respectively, in each biocoal briquette. The modified combustion efficiency (MCE) was determined using the following formula:43,44 MCE = ΔCO2/(ΔCO2 + ΔCO), where ΔCO2 and ΔCO are the fire-integrated excess molar mixing ratios of CO2 and CO, respectively, and refer to the EFs of the overall combustion process of CO2 and CO. The burnout ratio can represent fuel’s combustion completeness,23,45 which can be calculated as follows: ηbr = (1 − Abot)/(1 − Ad) × 100%, where Abot is the ratio of bottom ash mass to the fuel mass in a combustion cycle, and Ad represents ash on a dry basis obtained by proximate analysis.

Figure 1. Delivered energy-based PM2.5 EFs, mass-weighted averages (interpolated between the values for 100% biomass and 100% coal), and ηth for the biocoal briquette samples mixed with different contents of biomass: (a) maize straw, (b) wheat straw, and (c) rice straw.

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for potential industrial production, proving the tested process to be feasible for addressing powdered anthracite and crop straw. The convincing experimental results are presented and analyzed below featuring ηth and PM2.5 EFs. Figure 1 shows that the average ηth of the three biocoal briquettes ranged from 6.33 ± 0.26% to 8.53 ± 0.3 7% (maize straw), 8.29 ± 0.35% to 8.86 ± 0.37% (wheat straw) and 6.06 ± 0.24% to 10.02 ± 0.63% (rice straw), demonstrating a common trend of first increasing and then declining for increased biomass. In contrast, biomass-only and coal-only briquettes exhibited lower ηth, ranging from 3.83 ± 0.35% to 4.70 ± 0.28%. The maximum ηth of biocoal briquettes with maize straw, wheat straw and rice straw increased by 81.4 ± 10.5%, 88.4 ± 10.7%, and 127.3 ± 12.62%, respectively, compared with coal briquettes and by 87.5 ± 14.1%, 98.7 ± 12.8%, and 178.9 ± 18.96%, respectively, compared with biomass briquettes. Measured η th for these stove/fuel combinations are lower than those of other stove/fuel combinations reported in the literature.46−48 In addition to the aging of the used stove tested here, the heating effect of the kettle was also not included. Figure 2 shows that the briquette structures exhibited significant differences, which possibly resulted in the different ηth values among these briquettes. Sufficient oxygen can enter biocoal briquettes via pore penetration, leading to complete combustion of the anthracite component and releasing more energy. To support this assertion, we calculated the burnout

RESULTS AND DISCUSSION Improved Thermal Efficiency. Figure 1 shows that the addition of biomass increased ηth and reduced the PM2.5 EFs. However, the proportions of biomass for the peak ηth values were not completely in agreement with those for the lowest PM2.5 EF values. The former occurred when the biomass content in the biocoal briquettes was 15 wt % (maize straw), 15 wt % (wheat straw), and 20 wt % (rice straw). The biomass ingredient composition values were 15, 20, and 20 wt % when the delivered energy-based EFs of PM2.5 declined to a minimum, as illustrated in Table S3 (the mass-based EFs of PM2.5 are presented in Figure S5). The parameter f was introduced to simplify the analysis and certify the optimum biomass content in biocoal briquettes for achieving the best energy savings and PM2.5 EF reduction (see Table S4). Accordingly, 15 wt % (maize straw), 20 wt % (wheat straw), and 20 wt % (rice straw) were determined to be the optimum compositions for biocoal briquettes in the tested stove. One-way ANOVA analysis indicated that ηth of biocoal briquettes was significantly higher (p = 0.008) than that of coal briquettes and biomass briquettes, while the delivered energybased PM2.5 EFs (p = 0.008) were significantly lower than that of biomass briquettes, as shown in Figure 1. This result implies that biomass plays a positive role in promoting the combustion properties of anthracite. In addition to the ηth increase, a further reduction in the PM EFs was also achieved, which is important 1888


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Figure 3. MCE values and the burnout ratios as a function of various maize straw contents in the biocoal briquettes.

respectively. The subsequent reduction trend was largely attributable to the unburnt volatile matter that was released (due to higher biomass content) during the fuel ignition stage and a low burnout ratio.15,23 Due to the short reaction time of volatile matter, incomplete combustion was fostered by increased biomass content, and more energy was lost; this conclusion is supported by the declining trend in the MCE after 20 wt % (see Figure 3, details are in Table S5). Reduced PM2.5 EFs. Figure 1 shows that the delivered energy-based PM2.5 EFs from biocoal briquettes were much lower than the calculated mass-weighted averages (delivered energy-based averages), which were interpolated between the measured values for 100% biomass and 100% coal. The former covered comparatively wide ranges (0.34 mg/kJ to 5.33 mg/kJ for maize straw, 0.36 mg/kJ to 3.55 mg/kJ for wheat straw and 0.30 mg/kJ to 5.19 mg/kJ for rice straw in the biocoal briquettes; see details in Table S3). The corresponding maximum reductions peaked at 93 ± 2%, 96 ± 2%, and 98 ± 1%; these values were obtained at contents of 15, 20, and 20 wt % and were compared with the mass-weighted averages (delivered energy-based averages) with an optimum addition ratio. The values declined by 63 ± 9%, 61 ± 18% and 67 ± 5%, respectively, compared with those of coal briquettes, and decreased by up to 98.8 ± 0.3%, 99.0 ± 0.4%, and 99.5 ± 0.1%, respectively, compared with those of biomass briquettes. The PM2.5 EFs were also closely correlated with the volatile matter content. Unburnt volatile matter acting as a PM precursor can positively contribute to the formation of particles.20 In accordance with the above analysis, combustion completeness increased with the addition of biomass, decreasing the amount of unburnt volatile matter. Therefore, the PM2.5 EF curves decreased over a specific range (see Figure 1; details in Table S3). When the biomass content exceeded a certain point (15 wt % for maize straw, 20 wt % for wheat straw and 20 wt % for rice straw), the PM2.5 EFs exhibited an upward trend due to the increase in unburnt volatiles that occurred as biomass increased (see Figure 1; details in Table S3). This finding also illustrates that almost all of the various PM2.5 EFs from the biomass content exhibited an opposite trend to ηth (see Figure 1; details in Table S3). Lower EFs for NO2, SO2 and Toxic Elements. Lower EFs for NO2 and SO2 were discovered in all three types of biocoal briquettes, each displaying a similar tendency. For example, in the biocoal briquettes with the most obvious trend, wheat straw (see Figure 4; details in Table S6), measured delivered energybased EFs for NO2 and SO2 were significantly lower in

Figure 2. Cross-section SEM images of typical samples: (a) coal briquette, (b) biocoal briquette, and (c) biomass briquette.

ratio from the bottom ash mass and the MCE from the measured EFs of CO2 and CO. All three biocoal briquette types exhibited a similar trend. Taking maize straw-coal briquettes as an example, as shown in Figure 3 (details are in Table S5), both the burnout ratio and the MCE significantly increased when maize straw was added, indicating that biomass can have a major effect on improving the completeness of anthracite combustion. An upward trend was found in both the burnout ratio and the MCE (not obvious) until 20 wt %; a similar trend was also identified for ηth with a turning point at 15 wt %, indicating that increased maize straw was beneficial for enhancing ηth over a specific range. The ηth declined when the biomass exceeded 15, 15, and 20 wt % for maize straw, wheat straw and rice straw inclusion, 1889


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Figure 4. Delivered energy-based EFs for NO2 and SO2 compared with mass-weighted averages (interpolated between the values for 100% biomass and 100% coal) for the biocoal briquette samples mixed with different contents of wheat straw.

comparison to the calculated mass-weighted averages. The values of NO2 fluctuated between 1.12 ± 0.02 mg/kJ and 1.62 ± 0.21 mg/kJ. A downward trend was found for the SO2 EFs for increased wheat straw contents. At the minimum NO2 (1.12 ± 0.02 mg/kJ) and SO2 (0.24 ± 0.10 mg/kJ; see details in Table S6), the maximum margin between the measured values and the mass-weighted averages (delivered energy-based averages) of NO2 and SO2 reached 62 ± 2% and 91 ± 4%, respectively. Biocoal briquettes resulted in 68 ± 2% and 93 ± 3% lower NO2 and SO2 EFs, respectively, than coal briquettes. The results can be explained by the porous structure of biocoal briquettes (see Figure 2), which induces the intermediate product of NO to form N2 instead of NO2 via the disoxidation of C.36,49 Meanwhile, the richness of alkaline-earth metals in biomass contributes to the formation of alkaline earth metal sulfation, leading to more S in the ash and reducing the SO2 EFs. To test this hypothesis, we measured the water-solution ionic SO42− in the ash converted per unit of fuel. Compared with the mass-weighted averages (received mass-based averages), the measured values increased, indicating that more water-soluble sulfate was formed (see Figure S6; details in Table S7). Wheat straw additions to coal of up to 30 wt % were not sufficient to reduce NO2 and SO2 emission levels compared to those of pure wheat straw. EFs of toxic element (As, Se, and Pb) from the three types of biocoal briquettes were also significantly reduced for increased biomass contents according to the experimental results. The trend was also illustrated in an example of wheat straw−coal briquettes (see Figure 5; details in Table S8). A downward trend was found for increased wt % of wheat straw, and the minimum values appeared at 30 wt % biomass input. The maximum decreases were 91 ± 2%, 94 ± 3%, and 96 ± 1% based on the mass-weighted values, respectively. The phenomena are closely linked to the enlarged oxygen inlet through the porous structure (see Figure 2) in the wheat straw−coal briquettes, as analyzed above, possibly leaving more toxic elements in the form of oxides in the ash instead of releasing them into the air via flue smoke.

Figure 5. Delivered energy-based EFs for As, Se, and Pb compared with calculated values (interpolated between the values for 100% biomass and 100% coal) for the coal briquette samples mixed with different contents of wheat straw.

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ENVIRONMENTAL IMPLICATIONS To address the low burnout ratio of anthracite in household stoves, we investigated the influence of adding biomass to anthracite. Compared to coal briquettes and biomass briquettes, biocoal briquettes showed a positive effect of dramatic ηth improvement and pollutant EFs reduction. The biomass content exerted a significant influence on the two important parameters. When the biomass composition remained at 15 wt % (maize straw), 20 wt % (wheat straw) and 20 wt % (rice straw), the briquettes displayed the most desirable performance in terms of both ηth and the PM2.5 EFs. The maximum ηth increase in biocoal briquettes was 81−127% compared with that of coal briquettes and 88−179% compared with that of biomass briquettes. The delivered energy-based PM2.5 EFs decreased sharply to as low as 0.30−0.36 mg/kJ for biocoal briquettes with the optimized ingredient composition. In addition, the PM2.5 EFs in biocoal briquettes decreased by 61−67% compared with those in coal briquettes and by approximately 99% compared with those in biomass briquettes. In addition, the delivered energy-based EFs for NO2, SO2 and toxic elements (As, Se, and Pb) were also considerably reduced. The biocoal briquettes demonstrated the desirable properties of high ηth and low pollutant EFs based on the experimental results, rendering them a promising substitute for conventional fuels. The amounts of PM2.5 from residential biomass (430 Mt) and coal (90 Mt) combustion were 3.16 Mt and 0.78 Mt, respectively.50 For a simplified estimation, the total amount (i.e., 3.94 Mt) can be reduced to approximately 0.07 Mt if the currently consumed biomass and coal are replaced by biocoal briquettes of approximately 97 Mt while providing the same 1890


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amount of energy. If biocoal briquettes are comprehensively adopted in household activities throughout China, not only will the problem of crop straw and anthracite coal powder waste be solved but a striking PM2.5 reduction will also occur. The reduction could be as high as 98% based on simplified estimates according to previously reported inventory results from China’s anthropogenic emission sources,23,51 and approximately 410 Mt biomass and 20 Mt coal would be saved. Biocoal is expected to be highly marketable as a substitute for conventional fuels to curtail air pollution from residential sources and promote life quality and health in China.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03411. Additional information as noted in the text (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(W.J.) Phone: +86-516-83591115; e-mail: [email protected]. *(J.J.) Phone: +86-10-62781512; e-mail: [email protected]. cn. ORCID

Jianjun Wu: 0000-0002-0656-8102 Author Contributions ∥

J.Q. and Q.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Key Basic Research Program of China (No. 2012CB214900 & 2013CB228505) and the National Natural Science Foundation of China (51574239, 41227805, 21422703, and 21521064).

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