Emission Characteristics of Gas-Fired Boilers in Beijing City，China：Category-Specific Emission Factor，Emission Inventory，and Spatial Characteristics

Xiao Yan，Guangwu Song，Jing Yan，Zhiyun Luo，Xuesong Sun，Rui Zhang，Guohao Li

National Engineering Research Center of Urban Environmental Pollution Control，Beijing Municipal Research Institute of Environmental Protection，Beijing 100037，China

Abstract：Gas-fired boilers are the main stationary sources of NOx in Beijing.However，the understanding of gas-fired boilers is limited.In this study，the emission characteristics of NOx，SO2，and CO from gas-fired boilers in Beijing were established using category-specific emission factors（EFs）from field measurements.To obtain category-specific EFs，boilers were classified through influence analysis.Factors such as combustion mode，boiler type，and installed capacity were considered critical for establishing EFs because they play significant roles in pollutant formation.The EFs for NOx，CO，and SO2 ranged from 1.42～6.86g·m-3，0.05～0.67g·m-3 and 0.03～0.48g·m-3 .The emissions of NOx，SO2，and CO for gas-fired boilers in Beijing were 11121t，468t，and 222t in 2014，respectively.The emissions were spatially allocated into grid cells with a resolution of 1km×1km，and the results indicated that top emission contributors were in central Beijing.The uncertainties were quantified using a Monte Carlo simulation.The results indicated high uncertainties in CO（-157% to 154%）and SO2（-127% to 182%）emissions，and relatively low uncertainties（-34% to 34%）in NOx emission.Furthermore，approximately 61.2% and 96.8% of the monitored chamber combustion boilers（CCBs）met the standard limits for NOx and SO2，respectively.Concerning NOx，low-NOx burners and NOx emission control measures are urgently needed for implementing of stricter standards.Adopting terminal control measures is unnecessary for SO2，although its concentration occasionally exceeds standard limits，because reduction of its concentration can be achieved through control of the sulfur content in natural gas at a stable low level.Furthermore，the atmospheric combustion boilers（ACBs）should be substituted with CCBs，because ACBs have a higher emission despite lower gross installed capacity.The results of this study will enable in understanding and controlling emissions from gas-fired boilers in Beijing.

Key word：Classification of boilers；Uncertainty analysis；GIS-based approach；Operating load；Monte Carlo simulation.

1　INTRODUCTION

Beijing，the capital city of China，experiences severe air pollution，making it rank among the most polluted megacities globally（Gurjar et al.，2008；Wu et al.，2016；Zhang et al.，2016）.A series of air pollution control measures have been implemented to improve its air quality.Among these measures，the adjustment of energy structure was the most crucial and effective.Hence，the demand of natural gas has increased drastically and is likely to increase rapidly in the future because it is an environmental friendly and efficient fuel.According to the planning for natural gas consumption from the Beijing Gas Group Co.Ltd，the demand for natural gas will grow from 7 billion m3 in 2011 to 20 billion m3 in 2020.Among the end-users，the natural-gas-fired boiler has contributed substantially to the growth in demand.

However，the combustion of natural gas produces NOx，which is the major contributor to PM2.5 pollution due to the increase of[/]ratio（1.05）in Beijing according to the source apportionment published by the Beijing Municipal Environmental Protection Bureau（BMEPB）（BMEPB，2014）.In addition，NOx acts as a precursor of tropospheric ozone（O3）and fine particulate matter（PM2.5）in the atmosphere，NOx not only poses a serious threat to humans and the environment but also causes severe local or regional air quality deterioration；therefore，it has become the priority pollutant to be monitored and controlled in Beijing（Jaffe et al.，2004；Pope III，2004；He et al.，2011；Simmons and Seakins，2012）.As stated previously，the gas-fired boiler is the main stationary source of NOx in Beijing.Therefore，the monitoring and controlling of NOx should receive more attention in the future，for improving air quality.However，limited studies are available on the emission characteristics of gas-fired boiler（Pulles et al.，2004）.The emission characteristics of gas-fired boilers in the Netherlands and Asia were established by Pulles et al.（2004）through field measurement and by Kato et al.（1992）through theoretical analysis，respectively.Regarding the emission characteristics of gas-fired boilers in China，studies have focused on the emission of NOx（Yao et al.，2009；Xue et al.，2014）.Yao et al.（2009）established a NOx emission factor based on the field measurements of 64 gas-fired boilers in China.Xue et al.（2014）compiled a NOx emission inventory in Beijing through field measurements of five gas-fired boilers.However，no set of systematic and integrated emission factors（EFs）for various species compiled for different types of gas-fired boiler in Beijing is currently available.In this study，the emission characteristics were obtained by establishing category-specific EFs of NOx，SO2，and CO based on field measurements for gas-fired boilers and the emission inventory，determining spatial characteristics with high resolution，and calculating uncertainty using a Monte Carlo simulation.To obtain the EFs，the gas-fired boilers were classified through influence analysis.

2　METHODOLOGY

2.1　Field Measurements

To investigate emission characteristics of NOx，SO2，and CO from various types of boilers，128 gas-fired boilers were selected.The following factors affecting NOx，SO2，and CO formation and emission were considered：operating load，combustion mode，boiler type，installed capacity，control measures，installation year，pressure of natural gas and control mode of burner.Combustion mode included atmospheric combustion（AC）and chamber combustion（CC），and the latter was subdivided into horizontal internal-combustion（HIC）and others according to boiler type.The boiler capacity of HIC ranged from 1MW to 16MW，which was divided into two grades，namely<10MW and≥10MW by using cluster analysis.The operating load was controlled in the range 15%～90% in this investigation to clarify its role in the pollutants formation.The range of installation year of the existing gas-fired boilers was 1999～2015.The pressure of natural gas ranged from 101.3 to 137kPa.The control mode of burner in gas-fired boiler included mechanical and electrical control modes.

A flue gas analyzer（testo 350，Testo，USA）was used to analyze the O2 content，flue gas temperature，and gaseous pollutant emissions，including NOx，CO and SO2，at the outlet of the gas-fired boilers.Simultaneously，a pilot tube was applied to monitor flue gas velocity.For quality assurance and quality control，the gas analyzers were calibrated with zero gas and targeted standard gases（NO，NO2，CO，SO2 and O2）prior to the first test of the day.

2.2　Methods for Estimating Emission

Emissions from gas-fired boilers were established using the EF method.The emission from gas-fired boilers in individual categories was calculated separately by combining the category-specific EFs and the corresponding natural gas consumption.Then the aggregate was calculated to obtain emission at the district and province levels.The basic formula can be expressed as follows：

　　（1）

where Ei is the emissions of the specific pollutant i in 2014（t）；EFi is the EFs measured（g·m-3），which is calculated based on Eq.（2）as described in section 2.2.1， A represents the natural gas consumption（m3）；i represents the type of pollutants，namely NOx，SO2，or CO；j represents the combustion mode；k represents the type of boiler；l represents the installed capacity of boiler；and m represents the specific district.In this study，the emission was estimated without considering control efficiency，because of the absence of measures adopted to reduce NOx，SO2，or CO concentration as stated in section 3.1.5.

2.2.1　Determination of EFs

To establish the category-specific EFs，gas-fired boilers were classified through influence analysis.The EF of a specific category was calculated using Eq.（2）.

　　（2）

where Cijkl is the average concentration of pollutant i corresponding to combustion mode j，boiler type k and installed capacity l（mg·Nm-3），S is the feeding rate of natural gas（Nm3·h-1），ν is the velocity of dry flue gas under standard condition（m·s-1），and D is the diameter of chimney（m）.

2.2.2　Activity Level and Data Source

Typically，the emission of boiler was treated as point source.However，obtaining the details of emission from each gas-fired boiler in Beijing was not possible.A database（called database of gas-fired boilers）containing detailed information（e.g.，consumption of natural gas，installed capacity，boiler type，burner patterns，installation year，emission control devices and geographical location）of 10293 gas-fired boilers with a gross capacity of 50738 t·h-1 has been established and the separate emissions from each boiler were calculated in this study.These boilers accounted for more than 65.9% of the total boilers and 67.0% of the gross installed capacities in Beijing，which is assumed to be representative of various boilers.Except for the emissions from the point sources of gas-fired boilers，the other sources are all treated as “regional area source” in each district.Thus，the natural gas consumption of a “regional area source” was calculated by subtracting the natural gas consumption of the point source from that of industry and heating at the district level in Beijing.The natural gas consumption from industry and heating is available in a statistical yearbook（BSB，2015）.

2.3　Methods for Spatial Allocation

In this study，an approach based on a geographic information system was adopted for allocating the emission from gas-fired boilers in Beijing by using administrative districts data as a basal map.The “regional area source” in each district was identified on the basis of previous statement.The allocation factor of “regional area source” for a grid cell is the area percentage of the grid cell in the total area of a district.The spatial distributions of gridded emission were determined by the integrated consequence of point source and regional area source in each grid cell.

2.4　Uncertainty Analysis

Quantitative uncertainty analysis on the total emission of various pollutants was conducted with Monte Carlo simulation using a MATLAB software package.Normal distribution with a coefficient of variation（CV，the standard deviation divided by the mean）of 30% was assumed for natural gas consumption.The variation of emission factors was assumed as normal distribution by using means and standard deviations based on the field measurement.The details of the Monte Carlo simulation are available in previous studies（Cullen and Frey，1999；Frey and Zheng，2002）.

3　RESULTS AND DISCUSSION

3.1　Classification of Gas-Fired Boilers Based on Influence Analysis

The formation of gaseous pollutants by boilers，which is closely associated with the flame temperature and O2 content，are affected by the operating load，combustion mode，boiler type，installed capacity，installation year，pressure of natural gas，and control mode of burner.Furthermore，the control measure was a key factor determining gaseous pollutants emission.

3.1.1　Operating Load

Studies have reported that the operating load plays a significant role in NOx，SO2，and CO formation during combustion（Tanetsakunvatana and Kuprianov，2007；Ling et al.，2014；Ma et al.，2016）.Typically，NOx is produced mainly through three pathways including（a）the reaction of N2 with O2，known as thermal-NOx，（b）the oxidation of fuel containing N，known as fuel-NOx，and（c）prompt-NOx（Flagan and Seinfeld，1988；Li et al.，2011；Korpela et al.，2015；Ma et al.，2016）.Considering that the contribution of prompt-NOx is less than 5% and the fuel-nitrogen included in natural gas is in elementary form（N2），which behaves similarly as N2 in atmospheric，the thermal-NOx is the dominant formation pathway.Therefore，chamber temperature and O2 content are the key factors determining NOx formation during natural gas combustion（Korpela et al.，2015）.However，detecting chamber temperature is difficult.In this article，we assumed that the variation in the trend of flue gas temperature is similar to that of chamber temperature for a specific boiler，and thus，is an indicator of NOx emissions.

To clarify the role of chamber temperature，the relationship between the outlet concentration and operating loads of the gas-fired boiler was investigated with capacities of 10.5MW，14MW，and 16MW，whereas the O2 content was not varied considerably，as shown in Figs.1（a）～1（c）.The NOx concentration increased as the operating load increased for all investigated capacities，which can be attributed to the increase in thermal-NOx formation at higher chamber temperatures.The effects of operating load on CO and SO2 concentrations were negligible because their concentrations are maintained at a relatively low level for the investigated capacities，namely≤2.5mg·m-3.

By contrast，the concentrations of NOx，SO2，and CO were investigated when the O2 content increased and decreased with an increase in the operating load，respectively，in order to clarify the role of O2 content（as shown in Figs.1（d）～1（f））.The results depicted in Fig.1（d）indicate that as the load increases，the chamber temperature and O2 content increase，thereby resulting in a higher NOx concentration.By contrast，the NOx concentration either decreased or remained constant although the chamber temperature increased with increase in operating load，which was attributed to the decrease of O2 content，as illustrated in Figs.1（e）and 1（f）.Thus，it was concluded that the effects of the operating load on NOx formation were the integrated consequences of O2 content and chamber temperature.

Considering that the characteristics of NOx differed obviously among operating loads，operating loads above 70% were used in the following studies because they reflect the actual situation of emission levels of gaseous pollutants.

3.1.2　Combustion Mode

In terms of combustion mode，the boilers were classified into atmospheric combustion boilers（ACBs）and chamber combustion boilers（CCBs），and the latter comprised a majority，accounting for 86.46% of gas-fired boilers from the database of gas-fired boilers.The emissions of SO2，CO and NOx from ACBs and CCBs are depicted in Fig.2.The NOx emissions of two CCBs were significantly higher than those of the other CCBs（143mg·m-3 on average），namely 374mg·m-3 and 304mg·m-3，resulting from the higher excess air ratios during ineffective operation（Li et al.，2011）.In terms of NOx，47 boilers，accounting for 38.8% of the monitored CCBs，exceeded the emission limit of current standards because of ineffective operation or insufficient supervision from the department of environmental protection（BMEPB，2007）.Thus，low-NOx or a control measure for NOx emission is urgently required because of the implementation of considerably stricter emission control standards that recommend a limit of 80mg·m-3 for existing boilers（BMEPB，2015）.By contrast，SO2 concentration in flue gas ranges from 0 to 16.05mg·m-3 with an average value of 2.35mg·m-3.The SO2 concentrations of 120 CCBs，which accounted for 96.8% of the monitored CCBs，met the current standards for boiler emission（50mg·m-3 for existing boilers and 20mg·m-3 for newly-built boilers），（BMEPB，2007）.Adopting terminal control measures for SO2 is unnecessary although its concentration occasionally exceeds standard limits because reduction of its concentration can achieved through control of the sulfur content of natural gas at a stable low-level.In addition，the emission of CO was dependent on the combustion efficiency and the average value was 4.32mg·m-3 within a range of 0～93.29mg·m-3，which completely met the emission limit of 95mg·m-3 in the Safety Technical Regulation for Oil and Gas Burners（GAQSIQPRC，2008）.

Only four ACBs were found because of their small proportion（13.54% of the entire boilers），as depicted in Fig.2.The results indicated that the NOx and SO2 emissions of ACBs were higher than those of CCBs，at 180～268.36mg·m-3 and 3.24～33.45mg·m-3，respectively.The disparity in NOx emission was attributed to higher excess air and burning temperature of ACBs.The combustion modes of ACBs and CCBs are partially premixed combustion and diffusion combustion，respectively.It has been proved that the excess air and burning temperature of partially premixed combustion are higher than that of diffusion combustion（Tongji University，2000）.SO2 concentration in ACBs was higher than that in CCBs because of high excess air of ACBs.The results indicated that the ACBs，which exceeded the emission standards for NOx and SO2，accounted for 75% and 25%，respectively.Therefore，the ACBs must be replaced by CCBs because of the issue of new standards（BMEPB，2015）.The CO emission of ACBs lies in the range 5.06～41.25mg·m-3 with an average concentration of 19.26mg·m-3，which is higher than that for CCBs despite higher excess air than in the CCSs because of higher premixed temperatures in the ACBs than in the CCBs.

3.1.3　Boiler Type

The emission of gaseous pollutants is affected by boiler type.In this study，the investigation of boiler type was based on CCBs because of the small quantity and variety of ACBs.The gross capacity of HIC boilers comprises 60% of the total capacity for CCBs，thus，the HIC is the priority boiler type to be monitored.The other boilers were classified as “others” because of the absence of detailed information on boiler type or the small numbers of the specific type.Fig.3 depicts the emission profiles of HIC boilers and the other CCBs.The results indicated that the average NOx concentration of the HIC boilers was 152.74mg·m-3，which is higher than that of the others with an average of 123.87mg·m-3.This phenomenon was due to the higher temperature of HIC boilers compared with other CCBs，while the slightly lower in the O2 content of HIC cannot influence the NOx formation，as shown in Fig.3.The SO2 concentration of the HIC boiler and the other CCBs were similar，namely 2.31mg·m-3 and 2.50mg·m-3 on average，respectively.A similar phenomenon was found in the CO concentrations，at 4.28mg·m-3 and 4.48mg·m-3 for the HIC boilers and other CCBs on average，respectively.There was no significant difference in SO2 and CO concentration for HIC and other CCBs，which is attributed to their similar O2 concentration.Thus，the effect of boiler type on SO2 and CO formation is likely insignificant，whereas the boiler type plays a crucial role in NOx formation.

3.1.4　Installed Capacity

The effects of installed capacity on gaseous pollutants were analyzed for HIC boilers.The HIC boilers were classified into two grades according to the installed capacity，namely<10MW and≥10MW by cluster analysis with SPSS software.The<10MW units had a higher NOx concentration（averagely 160.87mg·m-3）than the ≥10MW units（averagely 118.41mg·m-3）（Fig.4（a）），indicating smaller boilers produced more NOx than larger ones do.To explore the influence mechanism of installed capacity，the relationship between the gas flue temperature and O2 content with installed capacity was investigated，as depicted in Fig.4（b）.Because the temperature and O2 content of the<10MW units were higher than those of the≥10MW units，smaller units produced more thermal-NOx than large ones did.The average SO2 concentrations of the<10MW and the≥10MW units were similar at 2.35mg·m-3 and 2.14mg·m-3，respectively，implying that the formation of SO2 is minimally affected by installed capacities.Notably，the average CO concentration of the<10MW units was 3.36mg·m-3，which was lower than that of the≥10MW units（8.12mg·m-3）.This phenomenon could be due to the higher O2 content of the<10MW units，thereby leading to the further oxidization of larger amounts of CO to CO2 in the<10MW units than in the≥10MW units.

3.1.5　Other Factors

In gaseous pollutants generation，control measure is a significant factor.According to the database of gas-fired boilers，currently，more than 95% burners used in existing gas-fired boilers in Beijing are provided by European enterprises.Regarding the burners’ NOx grading，more than 80% of the burners adopt the level 1 and level 2 standard products，namely EN676，i.e.，NOx≤119.88mg·m-3，which is adequately meets the NOx emission limits of standard（BMEPB，2007），thus neither low-NOx burners nor NOx emission control measures were used in the existing boilers.In addition，the average NOx concentration of CCBs is 1.19 times higher than that of standard in the EN676，which probably because of the small dimensions of the chambers in Beijing compared with those in Europe.

The characteristics of gaseous pollutants were affected by installation year，pressure of natural gas and control mode of burner，as depicted in Fig.5.Although annual deterioration of burner occurs naturally，the average concentrations of NOx，SO2 and CO remained constant during 1999～2005（Fig.5（a）），implying that the formation of gaseous pollutants was not affected by installation year.Fig.5（b）illustrates the correlations between the concentrations of gaseous pollutants and pressures of natural gas were not significant.Furthermore，the results illustrated in Fig.5（c）indicated that the concentrations of NOx，SO2 and CO in boilers with mechanical control mode are similar to those with electrical control mode；thus，the control mode did not affect the gaseous pollutants formation.

In conclusion，operating load，combustion mode，boiler type，and installed capacity are the priority factors affecting pollutants formation，whereas the other factors，namely control measure，installation year，pressure of natural gas，and control mode of burner，did not affect gaseous pollutants formation.Thus，the boilers were classified on the basis of combustion mode，boiler type，and installed capacity.

3.2　Estimate of EFs

On the basis of the classification，the averages of the EFs of various gas-fired boilers were calculated and summarized in Table 1.The category-specific EFs of NOx，CO，and SO2 range were as follows：1.42～6.86g·m-3，0.05～0.67g·m-3 and 0.03～0.48g·m-3，respectively.The variation characteristics of the EF values are summarized as follows.

First，the ACBs have high EFs，which were more than 3.2 times higher than those of CCBs.This phenomenon indicated that combustion mode is the most crucial factor affecting gaseous pollutant formation.Second，boiler type also significantly affected E and EFCO.The averages of E and EFCO in HIC boilers were higher than those in other CCBs by more than 19% and 33%，respectively.Third，there were differences among various installed capacities for E and EFCO in HIC boilers.When the capacity increased from<10.5MW to≥10.5MW，the E decreased from 2.13g·m-3 to 1.42g·m-3，whereas the EFCO increased from 0.05g·m-3 to 0.10g·m-3.Finally，the E remained constant although E and EFCO vary significantly with boiler types and installed capacities.

There is not a set of systematic and integrated emission factors（EFs）for various species compiled for different types of gas-fired boiler in Beijing available currently.Therefore，EFs of gas-fired boilers from other countries or districts were compared with those of Beijing in this study as illustrated in Table 2（Kato et al.，1992；Pulles et al.，2004；EPA，2006；MEPPRC，2007；Yao et al.，2009；Xue et al.，2014）.To make the comparison more scientific and clear，the EFs of CCBs were selected from our study，because those of ACBs were absent in the AP-42，HPPEI，and previous studies.E in this study is lower than that of uncontrolled emission levels in AP-42（2.24～8.8g·m-3）（EPA，2006），Pulles et al.（2004，2.24g·m-3），Yao et al.（2009，2.78g·m-3）and Xue et al.（2014，2.19g·m-3），whereas it is 0.79～1.18 times as that in HPPEI.The E theoretical value calculated by Kato et al.（1992）was 1.90g·m-3，which was 0.89～1.34 times as that in this study.Thus，classification based on the field measurements was more reasonable and more suitable for Beijing.

The E value in our study is 3.85 times and 3.12 times higher than the E values in the study of Kato et al.（1992）and that published by AP-42，respectively.However，it is 0.04 times lower than that in the HPPEI.

The value of EFCO in our study is lower than the AP-42（in the range of 0.56～0.98g·m-3）and research of Pulles et al.（0.78g·m-3）by approximate one order of magnitude.The reasons for this disparity cannot be identified with our present data；therefore，further work is required to identify the causes of this disparity.

3.3　Compilation of Emission Inventory

The emissions of gas-fired boilers from Beijing in 2014，based on the EFs in this study and the consumption of natural gas were estimated.The emissions were 11121t，468t，and 222t for NOx，CO，and SO2，respectively.A detailed emission inventory is presented in Table 1.The emissions of NOx，CO，and SO2 from ACBs are 487t，48t，and 34t，contributing 4%，10%，15% of the gross emission，respectively，although their capacities account for only 1.12% of the entire capacities.Thus，the substitution of ACBs by CCBs is urgently required.Additionally，the SO2，NOx， and CO emissions from gas-fired boilers accounted for 0.30%，7.07% and 0.047% of total emission from Beijing in 2014，which was obtained from the emission inventory report released by the BMRIEP（BMRIEP，2015）.

3.4　Spatial Characteristics

The emission inventory was spatially allocated into 1km×1km grid cells，as illustrated in Fig.6.At the district level，the top five unit emitters are Xicheng，Dongcheng，Haidian，Chaoyang and Fengtai，respectively.All five districts are in central Beijing.This phenomenon was consistent with the implementation of the program to substitute coal-fired boilers with gas-fired boilers，and the central of Beijing has been given high priority.Moreover，these five districts are densely populated and are residential areas；therefore，a high number of boilers are needed to produce heat，thereby resulting in high emission.Shijingshan，Changping，Daxing，Tongzhou，and Shunyi are less densely populated and are residential area，resulting in low emission of NOx，SO2，and CO.The other districts have the lowest emission because they have the lowest population density，do not have many residential areas，or have the limitation of gas pipelines.

3.5　Uncertainty Analysis

The estimated means and associated uncertainty ranges for pollutant-based emission are presented in Table 3.The means were calculated by averaging 10000 simulated values form Monte Carlo sampling.The uncertainty in CO and SO2 emissions are relatively high，with the relative errors approximately ranging from -157% to 154% and from -127% to 182%，respectively.Compared with the uncertainties in CO and SO2 emission estimation，the uncertainty in NOx emission estimated is relatively small，with the relative errors from -34% to 34% at 95% confidence intervals.Because the uncertainty of an activity level is fixed for varied pollutants，the EFs are dominant uncertainty sources，thereby leading to higher uncertainty in CO and SO2 estimation because of their low concentrations；thus it is evitable.

4　CONCLUSIONS

Gas-fired boilers were classified through influence analysis；thus，category-specific EFs were established.An emission inventory was compiled through the category-specific EFs and specific consumption of natural gas.Finally，the spatial variable characteristics with high resolution（1km×1km）were proposed.The main findings can be summarized as follows：

（1）Gas-fired boilers were classified on the basis of combustion mode，boiler type，and installed capacity，because these factors play significant roles in NOx，SO2，and CO formation，whereas the installation year，pressure of natural gas，and control mode of burner did not significantly affect the formation of gaseous pollutants.

（2）Category-specific EFs were established through classification，and the values for NOx，CO，and SO2 lie within the following ranges，1.42～6.86g·m-3，0.05～0.67g·m-3，0.03～0.48g·m-3，respectively.

（3）On the basis of category-specific EFs and specific consumption of natural gas，the emissions in 2014 were established at 11121 t，468 t，and 222 t for NOx，CO，and SO2，respectively.Additionally，ACBs should be substituted with CCBs because they have higher emission than CCBs do，although their gross installed capacities are lower than those of CCBs.

（4）At the district level，the top five emission contributors are Xicheng，Dongcheng，Haidian，Chaoyang and Fengtai，respectively.All these districts are in central Beijing.

（5）The uncertainties in NOx emission were lower，ranging from -34% to 34%，and those in CO and SO2 emissions were relatively higher（ranging from -157% to 154 and -127% to 182%），which are evitable.

（6）Considering that stricter standards will be implemented in 2017，low-NOx burner or NOx emission control measures are urgently required.Furthermore，supervision should be increased.Adopting terminal control measures for SO2 is unnecessary although its concentration occasionally exceeds the standard limit because the reduction of its concentration can be achieved through effectively controlling of the sulfur content of natural gas at a stable low-level.

5　ACKNOWLEDGMENTS

This work was supported by the Beijing Municipal Environmental Protection Bureau（BMEPB），Beijing Municipal Science and Technology Commission（No.Z161100000716005；2013BAC17B01；2014BAC 23B02），the National Natural Science Foundation of China（No.41303074）and Beijing Municipal Party Committee Organization.This paper has not been subject to BMEPB review.Therefore，it does not necessarily reflect the views of BMEPB and no official endorsement should be inferred.

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