The Geological Availability and Emissions of Sulfur and SO 2 from the Typical Coal of China

This study aimed to assess the natural availability of sulfur and SO2 in coal typical of the Jurassic, Permo-Carboniferous, and Cambrian strata in Shaanxi, China, and their emission rates. A total of 93 samples (39 Binxian Jurassic, 37 PermoCarboniferous, and 17 Langao Cambrian) were collected and analyzed via the Eschka method (GB/T 214-1996). The results show that the average sulfur content was 2.40%, 2.85%, and 0.92% in the Binxian coal gangue, raw coal, and coal slime, respectively; 1.48%, 2.41%, and 1.5% in the Hancheng Permo-Carboniferous coal gangue, raw coal, and coal slime, respectively; and 0.84% and 2.44% in the Langao Cambrian stone-like coal and black shale rock, respectively. The annual sulfur emissions from the Binxian urban and rural areas totaled 1.5 kt and 9.3 kt (Kilotons), respectively, which contributed 1.4% of the overall SO2 emitted into the atmosphere. The sulfur emissions from Hancheng urban and rural areas totaled 1.8 kt and 11.9 kt, respectively, which contributed 1.8% of the overall SO2. The sulfur emissions from Langao urban and rural areas was 0.4 kt and 2.8 kt, respectively, which contributed 0.43% of the overall SO2. Coal-waste consumption from 1991 to 2015 increased by 23% and 10% in urban and rural areas, respectively, in China, ultimately reducing the debris from coal waste. Raw-coal consumption from 1991 to 2015 decreased from 96% to 73% and from 97% to 87% in urban and rural areas, respectively. SO2 emissions since 2006 have decreased due to effective desulfurizing technology. According to the results of this study, China has been continuously reducing the emission of SO2 by adopting a green economy. The study recommends installing desulfurizing equipment in power plants to further reduce the SO2 emissions.


INTRODUCTION
Sulfur is one of the major components/impurities of coal, available from 0.2 to 11% both in organic and inorganic form (Ibarra et al., 1994).During coal and other crude oil combustion, sulfur is emitted to atmosphere in the form of SO 2 , SO 4 , etc., while in the coal waste, sulfur is available in the form of pyrite (FeS 2 ), gypsum (CaSO 4 , 2H 2 O), ferrous sulfate (FeSO 4 ), barite, and sphalerite (ZnS) (Lu et al., 2010).Among all the forms, the most active and major emitted form is SO 2 and H 2 S (Hou et al., 2018).Sulfate aerosol in the atmosphere acting as a cooling inhibitor but their cooling effect will affect though atmospheric CO 2 (Ozkaymak et al., 2017;Samset, 2018).The dominant cooling effect of sulfate aerosol is considered the regional and urban pollutant, which has a wide range of health and environment impacts (Ming et al., 2005).The massive emission of sulfur oxides to the atmosphere may cause air pollution; contribute SO 2 While some previous epidemiologist reported 0.8-3% dual mortality by emitting 50 µm m −3 of SO 2 (Katsouyanni and Pershagen, 1997).
To fulfill the energy demand of the country, the Chinese government still planning to install the coal power plants to overcome the energy shortfall.However, these power plants generate more SO 2 .During 1980-1995, China overall SO 2 emission was 23 million tons (Mt) (Yang and Schreifels, 2003), while in 2000, the China coal industries emitted almost 85% (19.9 Mt) of SO 2 (Yang et al., 2002).Due to extreme energy demand in 2006, Chinese government installed 1056 new coal power plants (which generate 63 million kilowatts [MkW] electricity) to achieve the energy goal of 2005 five-year plan (Chen et al., 2018).However, these newly established power plants generate 11.7% SO 2 to the atmosphere (Su et al., 2011).SO 2 emission control is one of the key tasks to all over the world, but very crucial to the China and the USA energy sectors.The Chinese energy cleaning system was improved, but still, the SO 2 emission is greater than 40% (Steinfeld et al., 2009).Chinese industrial SO 2 emission was about 42-52% from 2001 to 2007 (Wang et al., 2018a).Among this emission, ≈90% is associated with coal burning both in industrial and commercial uses (Lu et al., 2010).
Some previous studies estimate the sulfur and SO 2 emission of the China and global up to 2008, which shows a drastic increase in the SO 2 emission from 1999 to 2006 (Smith et al., 2011).However, a few studies compiled the SO 2 emission but lack of the sulfur availability and emission from the coal (Tørseth et al., 2012;Wang and Hao, 2012;Klimont et al., 2013).While some of the research has been conducted on the toxic elements released by coal (Zhao andLuo, 2017, 2018), very limited studies are available on the sulfur emission.Sulfur emission is mostly associated with the burning of coal and other fossil fuels, which could cause serious environmental problems, including atmospheric gases unbalancing, deteriorate air quality, increasing global warming and indirectly affect the cryosphere, agriculture land, food crops, and human's health.Therefore, the study aimed to probe and assess the sulfur quantity in the coal and coal waste byproduct (CBWs).The study also aimed to deliver a foundation and explore the critical estimation of the SO 2 emission by China since last 25 years.

Tectonic Setting and Sampling
The field visit and sampling were carried in the three major coalfields (typical coal of China) of Shaanxi Province, China.(1) Langao County (Cambrian coal) is situated between the latitude 32°26ʹ13ʹʹN to 32°28ʹ7ʹʹN, and longitude 108°56ʹ25.5ʹʹEto 108°58ʹ49ʹʹE (Hussain and Luo, 2018).Tectonically, the area is related to Qinling Mountains, where the most visible strata's are late Proterozoic and early Paleozoic, however, a few Jurassic strata's are also expended in some of the areas.The stone-like coal and black shale are widely spread in the Lujiaping formation of the late Proterozoic Lower-Cambrian.The coal and black shale carbonaceous rocks are enriched with sulfide ores, i.e., molybdenite, pyrite, and chalcopyrite, etc. (Hanjie and Yuzhuo, 1999;Wang et al., 2017;Hussain and Luo, 2018).
The coal, coal wastes and rocks samples were collected from the above three coalfield areas, while the coal slime samples were collected from the washing plants.A total of 93 samples were collected including 39 from the Binxian Jurassic (23 coal gangue, 10 raw coal, 5 coal slime, and 1 ash sample), 37 from the Hancheng Permo-Carboniferous (18 coal gangue, 7 raw coal, 4 coal slime, and 6 spontaneous burning ash), and 17 samples were collected from the Langao Cambrian county (9 stone coal, and 7 Cambrian black shales rock).All these samples were analyzed for sulfur at the Physical and Chemical Laboratory of the Institute of Geographic Sciences and Natural Resource Research, CAS, China.

Experimental Method
Eschka method (GB/T 214-1996) was used to determine the sulfur contents in coal and rocks (Wang et al., 2018b;Zhao and Luo, 2018).The samples were mixed with Eschka reagent (1:2) and then keep in the oven at 800-900°C for 1-2 hours until all the sample were burnt completely.Then passed the sample through a medium sized filter paper using hot water.Add methyl orange indicator and 2 mL HCl (1:1) and transfer to heat to reduce the total volume up to 200 mL.Add 10 mL BaCl 2 (10%), kept the solution for a night, and then filter the solution using a 0.001% ash size filter paper and finally burnt the filtered materials at 850°C.

Emission Rate
Emission rate for the coal combustion was determined through mass distribution method (Eq. 1) as suggested by Luo et al. (2004) and Zhao and Luo (2018), while the emission rate for the sulfur was determined through Eq. ( 2) and integrated emission rate for different areas, source and time were determined through Eqs.(3) and (4), respectively (Hao et al., 2002): , , , , In the above equations, R is the emission rate of the coal combustion, Q is the emission rate of the sulfur, C coal , and C ash are the sulfur concentration in coal and ash, respectively.C fired is the total ash contents.While C e is an element concentration, CC is the coal consumption (NBSC, 2017), and p is the efficiency of elements absorbed at control measure.Where i, j and t are the areas, source (coal, coal gangue etc.), and time (year), respectively.

Huanglong Coalfield
The magnitude of sulfur in the typical coal of Huanglong coalfield (Binxian Jurassic) of Shaanxi Province was interpreted in Table 1.The highest concentration of sulfur in the Binxian Jurassic coal gangue was 6.28%, and lowest was 0.56% with an average of 2.4%, which was higher than those reported by Wang et al. (2016) (1.4%) and Luo et al. (2005) (1.02%).The average concentration of sulfur in the Chinese coal gangue was 1.01% (Ren et al., 2006).Similarly, the average quantity of ash was 71.75%, which was equivalent to Wang et al. (2016) (70%).The average ash ratio is one-third after the combustion (Zhao et al., 2008).In 2013, the total average production of coal gangue was 750 million tons (Mt), which exotically undergoes in spontaneous combustion and affect the environment badly (NDRCC, 2014;Wang et al., 2016).
The concentration of sulfur in Binxian raw coal ranged 0.1-5.89 with an average of 2.85% (Table 1), which was higher than the Indian coal (0.4-0.6%) (Lu et al., 2013), Weibei coalmines (0.94%), Shenfu coalfield (0.7%), Xishan coalfield (1.3%) and Pingyinan coalfield (1.4%) (Wang et al., 2016).The Binxian Jurassic sulfur concentration was also higher than those reported by Zhao and Luo (2017) (1.52%) and Dai et al. (2012) (1.14%).The high consumption of coal released a huge amount of sulfur and other toxic gases, which affect the climatic condition, air quality, human health, land disturbance as well as affect the environmental sustainability (Han et al., 2016;Zhao and Luo, 2017;Samset, 2018;Zhao and Luo, 2018).Similarly, the minimum and maximum concentration of sulfur in the coal slime were 0.61 and 1.45%, respectively, with an average of 0.92% (Table 1).The Binxian coal slime sulfur content was considerably lower than the Shaanxi (2.12%) (Zhao and Luo, 2018).The coal slime is similar to briquettes, which has been recommended by the government to use as a clean coal.These materials are generated during the coal cleaning Table 1.Sulfur and ash quantity in the Huanglong coalfield (Binxian Jurassic coalmines).and washing.The domestic coals are mixed with coal slime, which has been used in the household (Li et al., 2014).Nowadays, the coal slime is separated from the raw coal.The coal slime is using in the household for burning purpose as well as also recommended for the poor agriculture soil.

DISCUSSIONS
The sustainability of atmospheric air quality is essential, however, the overuse of fuels (crude oil and coal) altered the air quality greatly (Wang et al., 2018a).The primary air pollutants released by fuels/coals are SO x , CO x , NO x , CH 4 , particulate matter, mercury, etc.This massive emission of gases alternatively affect the crops production, water quality, the ozone layer, human health and accelerate the global warming (Saidur et al., 2011).The average emission of sulfur to the atmosphere by some typical coalfield of China were given in Tables 3 and 4.Among the different sources, raw coal is the dominant source of sulfur followed by coal gangue and coal slime.A huge amount of coal and coal waste products are used in urban than in rural areas.In Binxian, the rural areas emitted 0.926 × 10 kilotons (kt) and urban regions emitted 0.15 × 10 kt of sulfur into the atmosphere, which counted about 1.4% of the total SO 2 emission (Fig. 3).Similarly, the emission of sulfur by Hancheng rural was 1.19 × 10 kt and urban 0.18 × 10 kt, respectively, which counted ≈1.83% of the SO 2 emission into the atmosphere (Table 3).The total emission of sulfur to the atmosphere by the Langao regions was 0.32 × 10 kt (urban: 0.04 × 10 kt, rural: 0.28 × 10 kt).The estimated SO 2 released by the Langao Cambrian coal was 0.4% (Table 3).The emission of sulfur and SO 2 may affect  the manufactured material, ecological system and the major cause of acid rain ((Eq.5) explored how the sulfur cause acid rain) (Bhargava and Bhargava, 2013;Vela et al., 2017): The consumption of coal and coal wastes for the commercial and local purpose is increasing day by day.During coal extraction, a huge amount of coal wastes is generated (coal gangue and coal slime), which ultimately affect the agriculture land, forest, accelerated land degradation, generating coal dust, air and soil pollution (Hussain et al., 2018).From 1991-2015, the coal wastes consumption is increasing, while alternatively decreasing the sulfur emission (NBSC, 2014(NBSC, , 2017;;Zhao and Luo, 2018).In 1991, the total China coal waste by-products (CWBs) consumption in urban areas was 4% and in rural areas was 3%, while the raw coal consumption was 96% and 97%, respectively (Fig. 4).In 1997, the State Economic and Trading Commission (SETC) banned the small-scale power generation plants (50-megawatt [MW]), because of emitting a substantial amount of pollutants and generate less electricity.In 2000, SETC was further stringent and banned the 10,000 MW (total capacity) power plants.In this way, the total reduction of coal consumption and SO 2 emission was 10 Mt and 0.4 Mt, respectively (Yang and Schreifels, 2003).Similarly, in 2015 (latest data), the total coal consumption (in China) was reduced to 73% and 87% in urban and rural areas, respectively (Fig. 4).The net reduction of coal consumption from 1991 to 2015 was 23% and 10% in urban and rural China, respectively.However, the coal wastes consumption was increased by 23% in urban and 10% in rural (Fig. 4).The increasing of coal wastes consumption will reduce the gangue debris, air pollution, water contamination, acid rain and SO 2 emission (Xu et al., 2000;Yao et al., 2016).
By adopting desulfurization policy, China reduced the coal-sulfur emission by 1.2% (1990), 1.17% (1991), 1.09% (1995), 1.05% (1999), and 1.0% (2000).The total change was observed up to 17% between 1990 and 2000 (Yang and Schreifels, 2003).In 2001-2005, the 10 th five-year plan set a regulation to reduce the SO 2 emission up to the limit of 23.7 Mt.In 2000, SO 2 emission was 23.7 Mt, while in 2005 the SO 2 emission was 19.5 Mt controlled by potential abatement strategy (Yang and Schreifels, 2003;Lu et al., 2010).In 2005, a significantly low amount of SO 2 was emitted by China, which was considerably lower emission than India and some other Asian countries (Fig. 5).The massive consumption of coal energy and installing the desulfurizing equipment in plants reduced the SO 2 emission up to the significant amount.In 2016, China controlled and reduced the massive emission of SO 2 as compared to 2005 (Fig. 5).The SO 2 reduction in China was mostly observed from 2006, while in other countries SO 2 was increasing, which could affect the atmospheric chemistry and sustainability (Kitayama et al., 2008).By exploring the sulfur and SO 2 emission, the actual sulfur content in the Chinese coal and coal wastes is higher than the majority of international studies (Ren et al., 2006;Ketris and Yudovich, 2009;Zhao and Luo, 2017;Rokni et al., 2018;Wang et al., 2018a;Zhao and Luo, 2018).However, the entire SO 2 emission by China was observed significantly lower than India (Fig. 5) as also reported by NASA (NASA, 2016).The mode of occurrence of sulfur in coal is barite, pyrite, and gypsum (Dinur et al., 1980;Vuthaluru et al., 2000;Ribeiro et al., 2010;Chou, 2012).The high sulfur is associated with pyrite; rosenite, reomerite, jarosite, halotrichite, melanterite, while the low sulfur coal is associated with thenardite, iron sulfate and organic coal (Dinur et al., 1980;Vuthaluru et al., 2000).
By the rapid increase in population and industrialization,  China demands more energy over the last decade.Due to lack of the natural gas and oil resources, China used more coal energy compared to another energy resource.According to NBSC, the total national emission of SO 2 in 1991 was 16.22 Mt;1994, 18.25 Mt;2000, 19.95 Mt; and finally reached to a peak position of 25.88 Mt in 2006 (Yang et al., 2002;NBSC, 2017).By installing effective sulfur removing equipment, a significant amount of sulfur was reduced from 2006 and onward (Smith et al., 2011) (Fig. 6).
Similarly, the consumption of fossil fuels and the emission of SO 2 is increasing in developing countries especially in India (Garg et al., 2002) (Fig. 5).The India coal has a much lower sulfur content but due to ineffective SO 2 control technology, the emission rate is high (Lu et al., 2011).In China, SO 2 emission both historic and projected was observed reducing (Fig. 6).The historic emission from 2007 to 2016 was 24.68 to ≈16.36 Mt, respectively (NBSC, 2017;Zhao and Luo, 2018), while the projection from 2017 to 2028 will be ≈16.26 to ≈8.0 Mt (Fig. 6).The study expects more reduction of SO 2 in the future than the projected because of using effective desulfurization technology in China.Shuhua et al. (2012) reported that if the desulfurization of gases and final emission reached to a 60%, a significantly less amount (0.6 Mt) of sulfur will be released to the atmosphere.In China, the coal consumption is still too more than the other countries.In northeastern China, more than 70% of families using coal, in northwestern more than 68% of families using coal resource for heating, cooking and other purposes (Duan et al., 2014).In most of the western part of China, a renewable and hydel-energy is using; therefore western China SO 2 emission is lower than eastern China (Fig. 7).In 2015, the highest SO 2 emission was observed in the Shandon Province (1.53 Mt) followed by Inner Mongolia (1.23 Mt) and the Henan (0.032 Mt), Tibet (0.0054 Mt) being lowest one (Fig. 7).However, China also using gas energy (methane, liquid petroleum gas [LPG], etc.).In 2015 (latest data), China using man-made coal gas 108,306 10K m 3 , natural gas 2,080,061 10K m 3 , and LPG 5,871,062 tons (Zhao and Luo, 2018).
The study clearly explored that the use of coal and other fossil fuel is increasing continuously, while alternatively release a huge amount of greenhouse gases and SO 2 .Most of the countries thrive on their economy and disturbing the natural environment badly.However, China is one of the most responsible country to boost their economy by following green economy practices.Almost 70% of the natural sulfur is bonded with pyrite coal and some other minerals.By the coal beneficiation, approximately 25% of the total sulfur and 50% pyritic-sulfur can be removed (World Bank, 1998).The historical emission and future estimation of SO 2 are important to improve the fossil fuels quality and consumption.Besides, it could be helpful to search the alternative source of clean energy that might sustain our natural environment, air quality, and human health.(NBSC, 2017;Zhao and Luo, 2018).

CONCLUSION
The typical coal of Shaanxi, China, emits a large amount of sulfur and SO 2 into the atmosphere.The average sulfur content was higher in the Permo-Carboniferous (Weibei coalfield) raw coal (2.4%) and lower in the coal gangue (1.48%) and coal slime (1.5%) than in coal worldwide (2%), whereas it was higher in the Jurassic (Huanglong coalfield) raw coal (2.85%) and coal gangue (2.4%) and lower in the coal slime (0.92) than in coal worldwide.The sulfur content in the Cambrian stone-like coal and black shale was 0.84% and 2.4%, respectively.The amount of SO 2 released by the Binxian Jurassic, Hancheng Permo-Carboniferous, and Langao Cambrian coal was 1.4%, 1.8%, and 0.4%, respectively, of the total SO 2 emitted into the atmosphere.
The average consumption of coal waste in 1991 was 4% and 3% in urban and rural areas, respectively.In 2015, the consumption of coal waste and coal briquettes was 15% and 12%, respectively, in urban areas (27%), and 8% and 7%, respectively, in rural areas (15%).The high consumption reduced the debris from coal waste in an environmentally friendly way.The highest SO 2 emissions from China were recorded in 2006 (25.88 Mt), after which they continuously declined, resulting in 16.36 Mt of observed emissions in 2016.Hence, we expect to reduce the amount of emitted SO 2 below the projected values (16.26 Mt and ≈8.0 Mt for 2017 and 2028, respectively) due to the prompt installation of desulfurizing equipment.A ~63% reduction in SO 2 has been observed in China between 2006 and now.The adoption of a green economy and commercially feasible technology will further decrease the level of SO 2 and other pollutants.The current study recommend using the desulfurized natural gas or LPG, which is economically viable and socially acceptable, as an alternative to coal.

Fig. 1 .
Fig. 1.Tectonic setting and samples location of the study areas.

Fig. 2 .
Fig. 2. The average concentration of sulfur in different type typical coalmines, Shaanxi.

Fig. 3 .
Fig. 3. Average estimated emission of sulfur and SO 2 to the atmosphere by typical coal of China.

Fig. 4 .
Fig. 4. Consumption of coal and coal wastes in between in 1991 and 2015 in China (data taken from NBSC (2017)).

Table 3 .
Regional emission of sulfur to the environment (10 kt).

Table 4 .
Sulfur contrition in the coal and coal wastes different regions.