Carbon and air pollutant emissions from China ' s cement industry 1 1990-2015 : trends , evolution of technologies and drivers 2

China is the largest cement producer and consumer in the world. Cement manufacturing is highly energy-intensive, 12 and is one of the major contributors to carbon dioxide (CO2) and air pollutant emissions, which threatens climate mitigation 13 and air quality improvement. In this study, we investigated the decadal changes of carbon dioxide and air pollutant emissions 14 for the period of 1990-2015, based on intensive unit-based information on activity rates, production capacity, operation status, 15 and control technologies, which improved the accuracy of the cement emissions in China. We found that, from 1990 to 2015, 16 accompanied by a 10.9-fold increase in cement production, CO2, SO2, and NOx emissions from China’s cement industry 17 increased by 626%, 59%, and 658%, whereas CO, PM2.5 and PM10 emissions decreased by 9%, 66%, and 63%, respectively. 18 In the 1990s, driven by the rapid growth of cement production, CO2 and air pollutant emissions increased constantly. Then, 19 the production technology innovation of replacing traditional shaft kilns with the new precalciner kilns in the 2000s markedly 20 reduced SO2, CO and PM emissions from the cement industry. Since 2010, the growing trend of emissions has been further 21 curbed by a combination of measures, including promoting large-scale precalciner production lines and phasing out small ones, 22 upgrading emission standards, installing low-NOx burners (LNB) and selective noncatalytic reduction (SNCR) to reduce NOx 23 emissions, as well as adopting more advanced particulate matter control technologies. Our study highlights the effectiveness 24 of advanced technologies on air pollutant emission control, however, CO2 emissions from China’s cement industry kept 25 growing throughout the period, posing challenges to future carbon emission mitigation in China. 26

emission factors are clinker and cement output-based factors, we did not specifically distinguish the fuel emissions from 185 process emissions of PM in this study. We collected the unbated PM emission factor for clinker production and cement grinding 186 from previous Chinese local studies (Lei et al., 2011a;Hua et al., 2016) and the recommended value compiled by CRAES 187 during China's first pollution census (CRAES, 2011), from which we adopted the median value as the unabated PM emission 188 factors for this study ( Table 3). The mass fractions of PM2.5, PM2.5-10, and PM>10 relative to total particulate matter were derived 189 from our previous study (Lei et al., 2011a). 2016. The abated fugitive PM emission factors used in their study were 0.1~0.4 kg t -1 , 0.7 kg t -1 , and 0.6 kg t -1 for PC, SK,and 195 OR kilns, respectively, and 0.2~0.3 kg t -1 for the cement grinding process. However, these emission factors were not directly 196 applicable to establish the historical emission trend because the details on control efficiencies were missing. In this study, we 197 adopted the median values of unabated fugitive PM emission factors compiled by CRAES for China's first pollution census 198 (CRAES, 2011) and used the mass fraction of PM with different diameters from Wang et al (2018) to derive the size-specific 199 PM emission factors (Table 3). The size distributions of PM2.5, PM2.5-10, and PM>10 in fugitive PM emissions were assumed to 200 be 10%, 20%, and 70% for all the fugitive emission processes (Wang et al., 2018). 201 There are five major types of PM removal technologies in China's cement industry, i.e., cyclone (CYC), wet scrubber (WET), 202 electrostatic precipitator (ESP), high-efficiency electrostatic precipitator (ESP2), and bag filters (BAG). We obtained the PM 203 removal technology application for each production line in 2010 from the MEE database and developed the technology 204 evolution model over the 1990-2015 period following our previous methodology (Lei et al., 2011a). Over the past decades, 205 China has progressively issued four editions of emission standards for air pollutants in the cement industry (GB 4915-1985, 206 GB 4915-1996, GB 4915-2004, and GB 4915-2013 and has successively strengthened the particulate matter concentration 207 limits of flue gas in kilns from 800 mg m -3 to 20 mg m -3 . The fugitive PM emissions limits have also been included in the 208 standards since GB 4915-1996 (Table S1). According to the concentration limits of the four phases of emission standards, we 209 divided the entire study period into four phases, i.e., 1990-1996, 1997-2004, 2005-2013, and 2014-2015 dust by PM removal facilities, reducing the transportation distance of raw materials, increasing the cleaning frequency of road 219 dust, and so on. However, information on the implementation details of these technologies was scarce, which hindered us from 220 establishing the unit-level technology evolution. Therefore, we estimated the average abatement rate of fugitive dust for the 221 entire cement industry. According to the on-site measurements conducted by the China Building Materials Academy in 2009, 222 the typical fugitive dust concentration observed 20 m from the factory boundary in the cement industry was 0.3368~2.56 mg 223 m -3 (Wang et al., 2009). Therefore, we assumed the upper limit of 2.56 mg m -3 as the unabated fugitive dust concentration, 224 estimated the average fugitive PM abatement rates for each phase of emission standards, and interpolated the abatement rates 225 across the entire study period (Fig. S1). 226

Uncertainty analysis 227
Following the methodology demonstrated in our previous studies on the power sector (Liu et al., 2015a;Tong et al., 2018), we 228 performed an uncertainty analysis of the emissions estimated in this study at the national and unit levels with a Monte Carlo 229 approach. The "uncertainty" was estimated by the 95% confidential interval (CI) around the central estimate of the emission 230 from 10000 Monte Carlo simulations with a specific probability distribution of input parameters, such as activity rates, coal 231 intensity, emission factors, abatement efficiency of control technologies, and so on. The probability distributions of the related 232 parameters were based on adequate measurements (e.g., CO2 emission factors), model regressions (e.g., coal intensity), a 233 literature review (Lu et al., 2011;Zhao et al., 2011;Liu et al., 2015a;Wang et al., 2019), and our own judgment. Table S2  234 presents the detailed information on the probability distribution of the parameters used in the uncertainty analysis. 235 For the unit-level uncertainty analysis, the uncertainty level of emission estimates in the 1990-2009 period was regarded as 236 larger than that in the 2010-2015 period because all the unit-level data were directly available from the MEE database for the 237 later period. The uncertainties conveyed by input parameters such as activity rates, emission factors, and control technologies 238 could vary with time. Therefore, we also estimated the uncertainty ranges of one representative clinker production line (a 239 precalciner kiln with a capacity of 4000 t-clinker/day, equipped with LNB, SNCR, and a bag filter in 2015) for 2000 and 2015 240 to demonstrate the change in unit-level uncertainties. The probability distribution of the parameters that are different from the 241 parameters used in the national uncertainty analysis is listed in Table S3. 242 3 Results 243

Historical cement production and evolution of technologies 244
Driven by the economic development and urbanization process, China has experienced rapid growth in cement production and 245 technology evolution in the cement industry. From 1990 to 2014, the production of cement and clinker increased from 0.21 246 and 0.16 billion tons to 2.5 and 1.4 billion tons, i.e., by 10.9 and 8.2 times, respectively ( Fig. 3 and Table 5). The total production started to diminish in 2015 as a consequence of recent clean air actions (Zheng et al., 2018). Cement is a blending 248 mixture of clinker and other additives, such as coal fly ash, plaster, clay, and so on. Typically, replacing clinker with other 249 additives can reduce the energy intensity and CO2 emissions. With raised clinker quality from an increased number of new 250 kilns, less clinker is required to produce a given strength of cement; thus, the clinker-to-cement ratio decreased from 74% in 251 1990 to 57% in 2015. 252 In China, the shaft kilns, precalciner kilns and other rotary kilns are the major kiln types for clinker calcination, representing 253 68%, 7%, and 25%, respectively, of the total clinker production in 1990. Prior to 2004, shaft kilns dominated China's cement 254 industry, accounting for over half of the clinker production; they were gradually replaced by new precalciner kilns from 2005 255 to 2015. Currently, the precalciner kiln is the dominant kiln type in China, and the proportions of the other two types are 256 negligible. In accordance with the transition of kiln types, the share of kilns with different designed capacities also varied 257 during the 1990-2015 period. The small-scale production lines (<2000 t-clinker/day), contributed mostly by shaft kilns, had a 258 dominating role in the 1990-2000 period, with a proportion exceeding 85%, whereas the share of large-scale production lines 259 (≥2000 t-clinker/day), majorly contributed by precalciner kilns, increased sharply afterwards, from 14% in 2000 to 97.5% in 260

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To fulfill the rapidly growing demand for cement products and to achieve ever-stringent clean air targets at the same time, 262 China's cement industry has undergone dramatic transitions in the production technology of cement kilns in recent years since 263 2010. Fig. 4 shows the share of different kiln types in the newly built and retired production lines and the cumulative ratio of 264 newly built and retired production lines by unit capacity. During the 2010-2015 period, there were 688 newly built cement 265 production lines, of which the precalciner kilns shared a dominant proportion of 95%. In contrast, there were 665 retired 266 cement production lines, of which the shaft kilns had a majority proportion of 79%. In response to the energy conservation 267 and emission reduction policies, the number of newly built production lines decreased, and the capacity of these newly built 268 China's cement industry increased from 0.15 Pg in 1990 to 1.18 Pg in 2014, i.e., by 6.8 times (Fig. 5). The growth of CO2 284 emissions was slightly lower than that of clinker production due to the offset effect from improved energy efficiency. From 285 1990 to 2015, the CO2 process emissions increased from 77.7 Tg to 694.2 Tg, i.e., by 7.9 times, which was consistent with the 286 growth of clinker production, whereas the CO2 fuel emissions increased more slowly, from 73.5 Tg to 405.9 Tg, i.e., by 4.5 287 times, because the energy intensity of cement kilns decreased significantly at the same time (Fig. 6). During the 1990-2015 288 period, the energy intensity of precalciner kilns, shaft kilns and the other rotary kilns decreased by 17%, 16% and 27%, 289 respectively. As a result, the proportion of CO2 emissions from coal consumption also decreased from 49% in 1990 to 37% in 290 2015. By 2015, cement and clinker production decreased, and the corresponding CO2 emissions dropped to 1.10 Pg. 291 of 10%, driven by the growth of cement production, which was mainly manufactured in the highly polluting shaft kilns (Fig.  295   7). Then, the SO2 emissions decoupled with the increasing trend of cement production and decreased to 0.66 Tg in 2015. The 296 emission decrease was due to the expanding technology transition from the old and polluting shaft kilns to the new and cleaner 297 precalciner kilns, which resulted in a much lower SO2 emission factor ( Table 2). The CO emissions had a similar trend as the 298 SO2 emissions. 299

Gaseous air pollutant emissions 292
In contrast, the NOx emissions exhibited a longer period of growth than other gaseous pollutants. In the 1990s, the NOx 300 emission gradually increased at an annual growth rate of 6.9% with the increase in cement production, which was mainly 301 manufactured in the shaft kilns and other rotary kilns. Since 2003, the rapid growth of cement production and the wide 302 promotion of precalciner kilns to substitute the shaft kilns have accelerated the growth of NOx emissions from the cement 303 industry because the precalciner kilns have a higher NOx emission factor under a higher operation temperature ( Table 2) consistent with that reported in our previous study (Lei et al., 2011a). From 1990 to 1995, PM emissions increased rapidly, 319 driven by the growth of cement production. The decline of PM emissions after 1996 was due to the implementation of the new 320 emission standards for the cement industry issued in 1996 (GB4915-1996, Table S1 The contribution from different processes to the total PM emissions changed significantly during the 25 years. In 1990, the 328 polluting shaft kilns had the largest contribution to PM emissions, followed by other rotary kilns and the cement grinding 329 process. In 2015, the emission from the precalciner kilns was the largest contributor, followed by fugitive emissions and cement 330 grinding processes. The PM emissions from rotary kilns and shaft kilns in 2015 were negligible. Over the whole study period, 331 the contribution of organized emissions from clinker calcination and the cement grinding process was sharply reduced by the 332 implementation of improved PM control technologies, whereas the contribution of unorganized fugitive emission gradually 333 occupied a larger proportion, from 2% to 17% for PM10 and from 1% to 13% for PM2.5, indicating the necessity of more policy 334 arrangements targeting fugitive emissions in China's cement industry. 335 Fig. 10A further shows the historical PM2.5 emissions from the clinker calcination process by production capacity. Prior to 336 2003, the small-scale capacities (<2000 t-clinker/day) dominated the emissions of China's cement industry, with a contribution 337 of 89%, due to their leading roles in clinker production (Fig. 3) and the inefficiency of PM control technologies. After 2003, 338 driven by the rapid development of new precalciner kilns, the share of small-scale production lines gradually declined (Fig. 3). 339 However, a considerable fraction of PM2.5 emissions were still disproportionately produced by a small fraction of clinker 340 production. Fig. S2 presents the PM control technology penetration in production lines by different clinker production capacities and the proportion of different capacities relative to the number of production lines, clinker production, and PM2.5 342 emissions in 2010 and 2015. In 2010, the small production lines (<500 t-clinker/day) only represented 7% of the clinker 343 production but were responsible for 17% of the PM2.5 emissions because more than 20% of the production lines were still 344 equipped with the outdated cyclone or wet scrubbers to reduce PM emissions (Fig. S2A). In 2013, the emission standard for 345 air pollutants was strengthened to fulfill the targets under the Clean Air Action Plan (GB 4915-2013), which accelerated the 346 phase-out of the small and outdated capacity and the transition of bag filters to meet the latest emission legislation. By 2015, 347 69% of the clinker was produced in the cement kilns with a capacity that exceeded 4000 t-clinker/day, and the overall 348 penetration rate of the bag filters reached 87% (Fig. S2B). Fig. 10B shows the changing routes of PM2.5 emission distribution 349 in production lines sorted by clinker production capacity. Overall, during the 2010-2015 period, the contribution of small 350 capacities to the total PM2.5 emissions decreased significantly, and the proportion of large capacities gradually increased as a 351 result of the rapid evolution of production technology in China's cement industry during recent years. 352 Fig. 11 shows the provincial distribution of the clinker production and emissions of CO2, SO2, CO, NOx, and PM2.5 from 354

Provincial distribution of emissions 353
China's cement industry in 2015. Anhui was the leading province with respect to CO2 and air pollutant emissions due to its 355 prominent role in clinker production nationwide. In 2015, the clinker output in Anhui was 135 Tg, accounting for 9.5% of the 356 national total, whereas the cement output in Anhui was only 131 Tg (5.5%). The overall clinker to cement rate in Anhui was emissions include both process emissions from the decomposition of limestone and fuel emissions from the burning of coal. 404 Basically, our estimates of total direct CO2 emissions had a consistent trend with other studies (Fig. 12C), and the variations 405 among different studies mainly originated from the variations in the estimates of CO2 fuel emissions. The CO2 process 406 emissions were directly calculated as the product of clinker output and the process CO2 emission factor, which was highly 407 consistent among different studies (Fig. 12A). However, there were larger discrepancies in the estimates of CO2 fuel emissions 408 because the amount of coal use in China's cement industry was not directly available in the statistics and was derived through 409 the coal intensity value, which resulted in higher variations than the estimates of process emissions (Fig. 12B) CO2 fuel emission factor (499 g CO2 kg -1 coal vs. 1940 g CO2 kg -1 coal in this study), whereas the higher estimates of CO2 414 fuel emissions reported by Zhang et al., (2015) were likely due to the application of a higher CO2 fuel emission factor. 415 As shown in Fig. 13, for SO2 emissions, our study presented consistent trajectories with two other Chinese studies (Hua et al., 416 2016;Lei et al., 2011a), whereas for CO emissions, the estimates by Hua et al., (2016) were slightly lower than the lower 417 boundary of the 95% CI calculated in this study after 2009, which was likely due to the adoption of lower energy intensity in 418 clinker production by Hua et al., (2016). For NOx emissions, all studies exhibited a similar growth trend before 2010 (Lei et 419 al., 2011a;Hua et al., 2016)  For PM emissions, all the studies indicated a similar trend during the 25 years, with two peaks occurring in the 1990s and 423 building and retiring dates for ~3100 clinker production lines and ~4500 cement grinding stations. According to our estimates, 436 SO2, NOx, CO, PM2.5, PM10 and CO2 emissions in China's cement industry were 0.66 Tg, 1.59 Tg, 3.46 Tg, 0.77 Tg, 1.37 Tg, 437 and 1.10 Pg, respectively, in 2015. From 1990 to 2015, the CO2, SO2, and NOx emissions from the cement industry increased 438 by 627%, 56%, and 659%, whereas the CO, PM2.5 and PM10 emissions decreased by 9%, 63%, and 59%, respectively. 439 Significant technology transition has occurred in the past 25 years, resulting in different emission trajectories of different 440 species. The CO2 emissions experienced an overall growth driven by the rapid growth of cement production, whereas the SO2 441 and CO emissions declined since 2003 with rapid technology transition from the old shaft kilns to the new precalciner kilns, 442 while the end-of-pipe emission control measures were the major reasons for the decline in the PM and NOx emissions. 443 In the recent years of 2010 to 2015, significant changes have occurred in China's cement industry, driven by the growing 444 demand for cement products and offset by the strengthened emission control policies. Numerous precalciner kilns with a 445 Shen, L., Gao, T., Zhao, J., Wang, L., Wang, L., Liu, L., Chen, F. and Xue, J.: Factory-level measurements on CO2 emission 551 factors of cement production in China, Renew. Sust. Energ. Rev., 34, 337-349, doi:10.1016/j.rser.2014.03.025, 2014 Calculation and evaluation on 553 carbon emission factor of cement production in China, Chin. Sci. Bull., 61(26), 2926-2938, 2016. 554 Shen, W., Cao, L., Li, Q., Zhang, W., Wang, G. and Li, C.: Quantifying CO2 emissions from China's cement industry, 555 Renew. Sust. Energ. Rev., 50, 1004-1012, doi:10.1016/j.rser.2015.05.031, 2015 Sciences, 20(1), 20-23, 1998. 565 Tan, Q., Wen, Z. and Chen, J.: Goal and technology path of CO2 mitigation in China's cement industry: from the perspective 566 of co-benefit, Journal of Cleaner Production, 114, 299-313, doi:10.1016Production, 114, 299-313, doi:10. /j.jclepro.2015Production, 114, 299-313, doi:10. .06.148, 2016 Tang, Q., Chen, X., Xia, X., Wang, L., Wang, H., Jin, L. and Yan, Z.: Scenario Study on PM emission Reduction in Cement 568 Industry, IOP Conf. Ser.: Earth Environ. Sci., 111(1), 012014, doi:10.1088/1755-1315/111/1/012014, 2018 and He, K.: Targeted

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The gray shading illustrates the 95% confidence interval of the emission estimates in this study.