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Thermal effects on arsenic emissions during coal combustion process
Science of the Total Environment 612 (2018) 582–589
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Review
Thermal effects on arsenic emissions during coal combustion process Weiqiang Zhang a,b, Qiang Sun a,b,c,⁎, Xiuyuan Yang d a
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of the Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China c Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, Charoltte, NC 28223, USA d Hydrogeological and Environmental Geological Survey Center of China Geological Survey, Baoding, Hebei Province 071051, PR China b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• We studied the variation of arsenic emission during the combustion process of coal. • The relationship between emission rate of arsenic and several important factors is researched. • The measure to reduce arsenic pollution in the process of coal burning is given.
a r t i c l e
i n f o
Article history: Received 3 July 2017 Received in revised form 26 August 2017 Accepted 26 August 2017 Available online xxxx Editor: D. Barcelo Keywords: Thermal effects Emission rate Arsenic Coal combustion Human health
a b s t r a c t In this study, the rate of emission of arsenic during the burning process of different kinds of coal is examined in order to study the volatile characteristics of arsenic during coal combustion which have negative effects on the ecological environment and human health. The results show that the emission rate of arsenic gradually increases with increased burning temperature, with a threshold of approximately 700 °C to 800 °C in the process of temperature increase. Then, the relationships among the arsenic emission rate and combustion environment, original arsenic content, combustion time, burning temperature, air flow and amount of arsenic fixing agent are discussed, and it is found that except for the original arsenic content, the rest of the factors have a nonlinear relationship with the emission rate of arsenic. That is, up to a certain level, they all contribute to the release of arsenic, and then their impact is minimal. The original arsenic content in coal is proportional to the arsenic emission rate. Therefore, taking into consideration the nonlinear relationships between factors that affect the arsenic emission rate can reduce contamination from arsenic. © 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Corresponding author at: School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China. E-mail addresses: [email protected] (W. Zhang), [email protected] (Q. Sun), [email protected] (X. Yang).
http://dx.doi.org/10.1016/j.scitotenv.2017.08.262 0048-9697/© 2017 Elsevier B.V. All rights reserved.
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4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Coal is the main energy source in China, and not only is a large amount of heat produced during coal combustion, but trace elements are also discharged, which can incur serious damage to the environment and human health (Guo et al., 2004; Kang et al., 2011; Li et al., 2012). Among the emitted trace elements, arsenic (As) has received much attention due to its toxicity and carcinogenicity. Studies have shown that As released from coal can pollute the atmosphere, water and soil in a gaseous or chemical form, thereby affecting human health (Wang et al., 2006). For example, statistics (Liu and Chen, 2015) show that in Guizhou Province in China, more than one hundred thousand people are suffering from potential As poisoning, in which there are over 2000 patients who have been poisoned due to drinking water that contains high levels of As, eating food that are prepared by using coal that is highly arsenic, and breathing (the amount of As in the air exceeds standards). Moreover, hundreds of people suffer or have died from the repercussions of As pollution, such as skin, liver and lung cancers. At present, there is no optimal method to counter As poisoning, so there are some restrictions in place for coal mining in which large amounts of As are emitted, but these can result in the waste of resources. Therefore, a study on the standards imposed onto arsenic emissions and the precipitation mechanism of coal combustion would have great theoretical impacts and practical significance for the prevention and control of As pollution as well as improvements in the utilization of resources. To date, many researchers have studied the precipitation of As in the coal combustion process (Bolanz et al., 2012; Frandsen et al., 1994; Liu and Su, 2014). For example, Yu et al. (2009) found that there is a positive correlation between As content in indoor and outdoor air and that of burning coals. Sun et al. (2004) examined the distribution of As in fly ash with different particle size in coal-fired power plants, and found that the concentration of As in different concentrations of particulate matter is gradually increased with a reduction in the particle size of fly ash. Some academics have conducted statistical analysis on the distribution of As in coal in China, and obtained a distribution range of 0–10 μg/g (with an average value of 5 μg/g), which is similar to the average content of As in coal worldwide (Wang et al., 2013; Yudovich and Ketris, 2005; Zhang et al., 1999). Liu et al. (2016a) studied the volatilization of As during the co-combustion of lignite blends with high As levels. Contreras et al. (2009) examined the effects of different compounds in the distribution and mode of occurrence of As in co-combustion processes based on thermodynamic equilibrium calculations. The mineral association, chemistry, distribution and migration of As in different kinds of coal have also been assessed by using different methods, such as using a low temperature ashing process and density fraction) (Jiao et al., 2013; Low and Zhang, 2013; Zhou et al., 2016). These studies all provide a better understanding of the migration and distribution of As in the coal combustion process and a solid foundation for future work. Since As emissions have continued to pollute air, thus negatively affecting the environment and human health, and studies on the origins and process of As emission are relatively rare, As emission is still an area that needs to be improved and studied in-depth. In this paper, the variations in the emission rate of As during the burning of different types of coals under different combustion environments are examined, and the relationships among the As content, air flow, combustion time, oxygen content and As emission are analyzed. Then, the process of As emission is tested by using a thermogravimetric test. The results can further understanding on the source of As volatilization during the coal combustion process and provide guidance and
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direction on the prevention and control of As pollution (Tang et al., 2009; Zhao et al., 2017). 2. Data sources Arsenic produced through coal combustion is becoming a more popular research topic because of its toxicity and volatility as well as its potential carcinogenic propensity (Liu et al., 2016b). In China, there is uneven distribution of As in coal in the different areas. In general, the concentration of As in Chinese coal is similar to that of the global average value, which is around 5.0 ppm. However, depending on the geochemical and geological characteristics of the particular coal, As concentrations can vary and change significantly in different regions (Zhao et al., 2008). Coal found in northern and northwest China has much lower concentrations of As than that in the southwest and northeast of China (Zhao et al., 2008), especially in the southwest of Guizhou (Chen, 2013; Liu and Chen, 2015; Yu et al., 2009). For instance, the coal in northern and northwest China such as Northern Shaanxi province typically have a concentration of 4.1 ppm while that in southwest and northeast China such as Guizhou have 7.5 ppm. There have been many coal-forming periods in the Guizhou area, and all sorts of coal is found. Aside from stone coal with a large amount of ash formed in the early Paleozoic, there is also lignite formed in the Tertiary period, which has the greatest amount of bitumite and anthracite which formed in the upper Permian and upper Triassic periods. The coal formed in this period is widely distributed, and the geological circumstances of coal formation are diverse with complex structures, which result in higher amounts of trace elements, especially As. Studies (Liu and Chen, 2015; Zhao et al., 2008) have shown that high concentrations of As in coal are mainly derived from the migration of magmatic fluid after the coal forming period, and the distribution is mainly related to the geological structure, especially the composite zones in tectonic settings, and large and deep faults. In this paper, we have collected data published in the literature (see Table 1) on the As emission from coal in Guizhou, Shanxi, Henan, Yunnan, Hunan and Jiangxi (all Chinese provinces) and the coal has high concentrations of As. A comparative analysis is carried out, and we provide a better understanding of the emission information during the thermal process of coal combustion. 3. Experimental results Previous studies have shown that the level of As emissions varies with the burning temperature in the coal combustion process (Senior et al., 2006; Sia and Abdullah, 2012; Su et al., 2013). Generally speaking, As in coal is mainly in the form of arsenic pentoxide (As2O5), which is a white solid, at a temperature below 477 °C. When the temperature increases from 477 °C to 627 °C, As changes in form to first As2O5 which is a solid phase, and then arsenious oxide (As4O6) and arsenic trioxide (AsO), which are gas phases; that is, from 477 °C to 527 °C, As is mainly in the form of As2O5 which is a solid; from 527 °C to 557 °C, As is mainly in the form of As4O6, which is a gas; and from 557 °C to 627 °C, As is mainly in the form of AsO, which is also a gas. When the temperature is higher than 627 °C, the As in coal is found only in the form of AsO or a gas phase. The rate of released As and total amount of As are therefore primarily related to the combustion environment, As concentration in the coal, combustion temperature, a constant time and the oxidant or inhibitor contents (Song et al., 2014; Zhao and Luo, 2017). Fig. 1 shows the changes in the emission rate of As (the formula is shown in Eq. (1)) in the coal combustion process under natural
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Table 1 Detailed information on acquisition of data used in analysis. Rock (mineral) type or coal rank
Publication
Site
Test method
As content (μg/g)
Bituminous (SJS)
Liu and Chen (2015)
Guizhou, China
4.26
Lignite (GX) Bituminous (CA) Anthracite (A1) Anthracite (A2) Lignite (L1) Lignite (KY) Bituminous (JL) Bituminous (XR) Coal gangue Lignite (YH)
Liu et al. (2016a) Liu et al. (2016a) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Yang (2011) Liu and Chen (2015) Wang et al. (2013)
70.24 2.81 0.36 3.19 4.51 68.35 0.96 50.58 75.17 19.64
Coal (CX) Coal (XR) Coal (PX) Coal (SM1) Coal (SM2) Lignite (AL)
Dai and Li (2005) Dai and Li (2005) Dai and Li (2005) Liu and Su (2014) Liu and Su (2014) Chen and Shuai (2012) Chen (2013) Chen (2013) Chen (2013) Chen (2013) Han et al. (2009) Han et al. (2009)
Xingren, Guizhou, China Datong, Shanxi, China China China China China China Xingren, Guizhou, China Xingren, Guizhou, China Xiaolongtan, Yunnan, China Chenxi, Hunan, China Xingren, Guizhou, China Pingxiang, Jiangxi, China Shenmu, Shaanxi, China Shenmu, Shaanxi, China Anlong, Guizhou, China
Fourier transform infrared spectroscopy (FTIR), Thermodynamic analysis (DTA) Hydride generation atomic fluorescence spectrometry (HGAFS) HGAFS HGAFS HGAFS HGAFS HGAFS HGAFS Atomic fluorescence spectrometry (AFS) AFS, SEM-EDAX – Neutron activation analysis (NAA) NAA NAA – – AFS
46.2 430.6 18.6 2.387 3.353 90.04
AFS AFS AFS AFS AFS AFS
– – – – – –
Lignite (AL) Bituminous (ZJ) Anthracite(XR) Stone coal (JL) Coal (JZ) Coal (LPS)
Anlong, Guizhou, China Zhijin, Guizhou, China Xingren, Guizhou, China Jiaole, Guizhou, China Jiaozuo, Henan, China Liupanshui, Guizhou, China
combustion, in which the curves show an overall trend where the As emission rate gradually increases with increased temperature. F¼
w 1− h 100% wm
ð1Þ
where, F is the emission rate of As, wh is the content of As in ash, wm is the content of As in original coal. Fig. 1a shows the changes in the emission rate of As during the combustion of anthracite, bituminous coal and lignite. Note that the changes in the As emission rate of these three different types of coal are similar in that their rate of emission gradually increases with increased temperature, and they show obvious segmentation characteristics. That is, at a temperature that is less than 800 °C, the As emission rate slowly increases with increases in temperature, so that the average emission rate of the seven samples (lignite from Anlong (Guizhou), anthracite A1, anthracite A2, bituminous coal B1, bituminous coal B2, lignite C1, and lignite C2) is 18.8% at 800 °C which is increased from 10.8% at 600 °C. When the temperature reaches 800 °C, the increase in the rate of the emission of As is rapid, and at 1100 °C, the average change in emission is 54.4%. The increase in the change of the rate of emission at this stage is three times that in the previous stage. A temperature of 800 °C is an inflection point of the volatility of As versus temperature. Fig. 1b illustrates the volatile characteristics of coal which originate from different regions of China during combustion. The changes in the five curves can be divided into three stages in accordance with the temperature range of 500 °C to 1200 °C: ① Stage 1 (500 °C to 700 °C): changes in the volatility of As are minimal; ② Stage 2 (700 °C to 1000 °C): when the burning temperature reaches 700 °C, the change in the emission rate of As slowly increases, and the average is 38.3% at 1000 °C as opposed to 14.9% at 700 °C; and ③ Stage 3 (1000 °C to 1200 °C): the As emission rate is the highest (the changes in the rate are seven times that in the previous stage), which shows that higher burning temperatures mean higher rate of emission of As. According to the results shown in Fig. 1a and b, the temperature range of 700 °C to 800 °C is critical because As is emitted at a rapid rate during this stage. When the temperature exceeds the threshold, the rate of emission is significantly increased. As the samples and the combustion environment are different, and taking
into consideration experimental errors, the resulting temperature at the critical point may be different, but basically within this range of 700 °C to 800 °C. Fig. 2 shows the change characteristics of the As emission rate of the coal samples with different concentrations of As and under different conditions. As shown in Fig. 2a, the As emission rate in the combustion process with a higher oxygen content is greater than that in a combustion process with a low oxygen content; for instance, with an oxygen content of 21% and carbon dioxide content of 79%, the emitted As at 900 °C is 46.1% while with an oxygen content of 5% and carbon dioxide content of 95%, the emitted As at 900 °C is 42.1%. That is to say, in the studied range, a higher oxygen content in a combustion environment means greater changes in the As emission rate with burning temperature. When there are no changes in the oxygen content, carbon dioxide (CO2) can inhibit the volatility of As relative to nitrogen. This is because a large volume of CO2 reacts with the carbon particles to form CO gas, thus resulting in a temporary reduction atmosphere on the surface of the carbon particles, and affecting the volatilization and migration of As (Liu and Chen, 2015). So, if the technology for the recovery of CO2 is applied, using CO2 and oxygen as the combustion environment can reduce As emissions and other greenhouse gas emissions. The changes in the rate of the emission of As in the coal combustion process with different As levels in normal air are shown in Fig. 2b, which shows that the rate of emission of As from the coal with high concentrations of As is greater than that of coal with low concentrations. The relationship between the emission rate of As and the original concentration of As in the coal is linear (as shown in Fig. 2c), and the mathematical expressions for the fitting lines at 900 °C and 1100 °C are shown in Eqs. (2) and (3). The slope of the two lines shows that the increase in the rate of emission of As versus the original concentration of As at 1100 °C is about two times that at 900 °C. Therefore, the removal of some of the As through coal washing is necessary for coal with high concentrations of As prior to combustion. μ ¼ 45:21 þ 0:37 λ
ð2Þ
μ ¼ 31:31 þ 0:20 λ
ð3Þ
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Fig. 1. Rate of emission of arsenic in coal burning process (sources: Dai and Li, 2005, Han et al., 2009, Chen, 2013, Liu and Chen, 2015). (a) Different types of coal, (b) Different locations of coal.
where, μ is the rate of emission of As and λ is the original As concentration in the coal. The emission rate of As during coal combustion is not only related to the burning temperature, oxygen content and original As concentration in the coal, but also the retention time. As shown in Fig. 3, the volatility of As initially increases with the retention time but then stabilizes to a certain level. The commencement of the basic stabilization stage is defined as the fundamental stability point. From the experimental results of the lignite from Anlong in Guizhou (see Fig. 3b), the retention time that responds to the fundamental stability point of the curves in which the target temperature is 700 °C, 900 °C and 1100 °C is 25 min, 20 min and 10 min, respectively, which prove that a higher temperature means a higher rate of emission of As. These three curves in Fig. 3b also show that the basic stability stage of the As emission rate is increased with increased temperature. The results suggest that combustion should be maintained for at least 30 min at the target temperature when the design temperature to dispose the As pollutant is less than 1000 °C, and the retention time can be appropriately reduced when the design temperature is higher than 1000 °C.
Fig. 2. Change in characteristic of arsenic emission rate under different conditions. (a) Change in emitted arsenic in different atmospheres (source: Liu and Chen, 2015); (b) Change in emitted arsenic in coal with different amounts of arsenic (source: Liu et al., 2016b); (c) Change in emission rate of arsenic versus original arsenic content in coal.
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Fig. 4 shows the effect of air flow on the As emission rate during the coal combustion process. When the air flow is less than 700 mL/min, the release of As increases with increased air flow (as Stage 1 shows in Fig. 4), which is because the air flow provides sufficient oxidation for the combustion of coal, and accelerates the precipitation of As. The rate of emission of As under an air flow of 700 mL/min is increased 23.3% compared to that under an air flow of 200 mL/min. When the air flow is greater than 700 mL/min, the oxidation required for coal combustion is adequate, and the rate of release of As is basically unchanged with changes in the air flow. In natural conditions, most coals contain calcium (mainly in the form of calcium oxide (CaO)), and experiments showed that when CaO is found in the combustion process, the volatility of As will be obviously inhibited due to the chemical reaction, as shown in Eq. (4). Calciumbased fixing agents of As have also been recognized by the majority of researchers (Chen and Shuai, 2012; Tian et al., 2015), which is applied in the process of fixing As in a power plant. As2 O3 þ 3CaO þ O2 →Ca3 ðAsO4 Þ2
ð4Þ
Fig. 3. Changes in volatility of arsenic versus combustion time.
Fig. 4. Change in volatility arsenic versus air flow.
Fig. 5. Variation in rate of retention of arsenic with different fixing agents. (a) Effect of temperature on fixing As rate; (b) Effect of fixing agent amount on fixing As rate. Source: (Yang et al., 2011, Yang, 2011).
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Fig. 5 shows the variation in the fixing As rate in the coal combustion process in which different amounts of several kinds of calcium-based fixing agents of As are added, including dolomite, limestone and cement. It can also be seen in Fig. 5a that there is a significant effect of the fixation of dolomite and limestone on As, and the fixing As rate (the formula is shown in Eq. (5)) is over 70% at a burning temperature that is less than 1000 °C. When the temperature is over 1000 °C, the fixing As rate is gradually reduced, and at 1250 °C, the fixing As rate of dolomite and limestone is reduced to 39.4% and 8.9%, respectively. The changes in the fixing As rate with amount of fixing agents (dolomite and cement) is shown in Fig. 5b. Note that the fixing As rate increases with increased amount of fixing agent, for example, with 9% of fixing agent (dolomite) increased, the fixing As rate is increased 45.8%. The reason that dolomite and cement both have a fixed effect on As is that their main component is CaO, which can react with As so that it becomes a solid, and consequently, As can be removed. Therefore, a certain amount of calcium can be added to reduce the volatility of As in the coal combustion process so as to mitigate the damage to the environment and human health. η¼
F0− F1 F0
100%
ð5Þ
where, η is the fixing arsenic rate, F0 is the emission rate of As of original coal, F1 is the emission rate of As of coal with As fixing agents. 4. Discussion In the coal combustion process, the activity of elements is enhanced due to high temperatures and strong oxidation, and much of the material structure of the coal is damaged, thus leading to the separation of trace elements from the original molecular structure and their recombination and distribution in different combustion products (Camacho et al., 2011). In particular, the trace elements which are found in coal or in the adsorbed or electrostatic form will migrate, and other trace elements found in other forms in coal will also partially ionize. Arsenic is an important trace element in environmental pollution, and released from coal during the combustion process. After a series of physical and chemical changes, As is released in different forms at different temperatures, which can cause damage not only to the environment but also to human health (Wang, 2006). We conducted a thermogravimetric test and observed that there is a rapid stage of decline in the mass of the coal during the combustion process, which is caused by the combustion of organic matters, and the concentration of As is also rapidly reduced in this stage, as shown in Fig. 6. This indicates the rapid decomposition of organic matters and increase in combustion temperature both contribute to the emission of As. Therefore, this stage is important for the removal of As, as the amount of emitted As should be controlled in the early stages of diffusion, thus avoiding the occurrence of the initial pollution which will then need to be rectified. In determining the most critical stage for the removal of As, several other important factors that affected the As emission rate were fully considered. First, an appropriate fixing agent of As should be selected. Under high temperatures, the fixing agent is mainly applied through physical adsorption and chemical reaction to carry out the fixation of As (Zhang et al., 2000). According to existing research (Chen and Shuai, 2012), native dolomite not only sets off a chemical reaction which forms the solid phase compounds of As, but can also produce CaO and magnesium oxide (MgO) which are porous and absorb the gaseous form of As. Therefore, native dolomite is first recommended as the fixing agent of As. At the same time, the amount of the fixing As agent added to the coal should be reasonable. In theory, more fixing agent means better fixing of As. However, when the fixing agent amount reaches a certain level, the effects of adding more fixing agent are minimal. Moreover, excessive amounts of fixing agent will lead to the
Fig. 6. Plot of thermogravimetry and arsenic content under different temperatures.
increase of the ash amount and the decrease of fixed carbon content and the heat value of the coal. Therefore, the appropriate amount of fixing agent added into the coal during the combustion process will minimize As pollution. However, if there are other reactions between the fixing agent and other substances, they also need to be considered. For example, most coal contains sulfur, and the calcium-based fixing agent of As not only reacts with As, but also with sulfur, as shown in Eq. (6). 2CaO þ 2SO2 þ O2 →2CaSO4
ð6Þ
Eqs. (4) and (6) show that the amount of fixing agent (Ca/As) required in theory for the fixing of As and sulfur is calculated to be 1:1, which can be converted to the amount of Ca/S at 5:2. This result is in agreement with the research results of Yang et al. (2011), whose results showed that the amount of calcium-based fixing agent of As (Ca/S) is 2:1. Based on the calculation results and literature, we determined that the amount of calcium-based fixing agent of As (Ca/As) is more suitable between 17:20 and 1:1. If the amount is inadequate, its removal effect of both As and sulfur cannot be achieved. On the contrary, if the amount is too much, there will be excessive slag production. Then, the optimal burning temperature for the retention of As was found after adding the fixing agent, in which there were variations in the rate of emission of As with burning temperature compared to the coal combustion process. Fig. 7 shows the changes in the fixing As rate with temperature during coal combustion with two calcium-based
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(3) After adding an As fixing agent, the amount of As emission is closely related to the combustion temperature, fixing agent amount and the coal particle size. Among these, the combustion temperature and fixing agent amount should be moderate at 1050 °C and 1:1 (Ca/ As), and a higher temperature and more fixing agent do not necessarily provide better effects. Coal with a small particle size also helps with the fixation of As. Acknowledgments This research is supported by the Fundamental Research Funds for the Central Universities (Grant No. 2017CXNL03) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References
Fig. 7. Effect of burning temperature on fixing As rate. Source: (Chen, 2009).
fixing agents, which are calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3). It can be seen from the figure that the fixing As rate increases gradually with increased temperature up to 1050 °C, and then gradually decreases with continued increase of temperature. This is because when the temperature is excessively high, the compounds of As and calcium will gradually decompose, and the amount of calcium for utilization will also be reduced, thus resulting in a reduced rate of retention. Therefore, the optimal combustion temperature should be selected in accordance with the corresponding fixing agent and in this study, the optimal temperature is about 1050 °C. In addition, the amount of emitted As is also related to the size of the coal particles. Although Song et al. (2014) found that the particle size of coal has no influence on the amount of arsenic in coal, however, when there are large coal particles, the combustion is insufficient, which results in the waste of resources. On the contrary, coal with a smaller particle size means a greater specific surface area for reactions to take place, and a higher uptake of calcium. Therefore, a smaller coal particle size contributes to the fixing of As. In summary, in giving consideration to and identifying the original As content of coal, combustion environment and time, and amount of As fixing agent, the As emission during the combustion process can be well controlled, and the negative effects of As pollution on the environment and human health can be subsequently reduced or prevented. 5. Conclusion In summary, this paper focus on the study of the emission characteristics of As during the combustion process of different kinds of coal in different combustion condition, and highlights the significant impact of oxygen content, As content of coal, burning temperature, air flow, constant temperature time, the types and dosage of fixing agent of As and the particle size of coal on the emission rate of As. The following conclusions are made based on the study findings. (1) The emission rate of As in the coal combustion process is gradually increased as the burning temperature increases, with a critical temperature threshold of 700 °C to 800 °C as the rate of precipitation of As. When the temperature reaches or exceeds this temperature interval, the rate of emission of As will be significantly increased. (2) The level of oxygen content, As content of coal, air flow and a constant temperature time all have significant effects on the amount of As emitted during the coal combustion process. Within a certain range, they can promote the release of arsenic, and then the promote effect is very small.
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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Review
Thermal effects on arsenic emissions during coal combustion process Weiqiang Zhang a,b, Qiang Sun a,b,c,⁎, Xiuyuan Yang d a
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of the Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China c Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, Charoltte, NC 28223, USA d Hydrogeological and Environmental Geological Survey Center of China Geological Survey, Baoding, Hebei Province 071051, PR China b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• We studied the variation of arsenic emission during the combustion process of coal. • The relationship between emission rate of arsenic and several important factors is researched. • The measure to reduce arsenic pollution in the process of coal burning is given.
a r t i c l e
i n f o
Article history: Received 3 July 2017 Received in revised form 26 August 2017 Accepted 26 August 2017 Available online xxxx Editor: D. Barcelo Keywords: Thermal effects Emission rate Arsenic Coal combustion Human health
a b s t r a c t In this study, the rate of emission of arsenic during the burning process of different kinds of coal is examined in order to study the volatile characteristics of arsenic during coal combustion which have negative effects on the ecological environment and human health. The results show that the emission rate of arsenic gradually increases with increased burning temperature, with a threshold of approximately 700 °C to 800 °C in the process of temperature increase. Then, the relationships among the arsenic emission rate and combustion environment, original arsenic content, combustion time, burning temperature, air flow and amount of arsenic fixing agent are discussed, and it is found that except for the original arsenic content, the rest of the factors have a nonlinear relationship with the emission rate of arsenic. That is, up to a certain level, they all contribute to the release of arsenic, and then their impact is minimal. The original arsenic content in coal is proportional to the arsenic emission rate. Therefore, taking into consideration the nonlinear relationships between factors that affect the arsenic emission rate can reduce contamination from arsenic. © 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Corresponding author at: School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China. E-mail addresses: [email protected] (W. Zhang), [email protected] (Q. Sun), [email protected] (X. Yang).
http://dx.doi.org/10.1016/j.scitotenv.2017.08.262 0048-9697/© 2017 Elsevier B.V. All rights reserved.
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4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Coal is the main energy source in China, and not only is a large amount of heat produced during coal combustion, but trace elements are also discharged, which can incur serious damage to the environment and human health (Guo et al., 2004; Kang et al., 2011; Li et al., 2012). Among the emitted trace elements, arsenic (As) has received much attention due to its toxicity and carcinogenicity. Studies have shown that As released from coal can pollute the atmosphere, water and soil in a gaseous or chemical form, thereby affecting human health (Wang et al., 2006). For example, statistics (Liu and Chen, 2015) show that in Guizhou Province in China, more than one hundred thousand people are suffering from potential As poisoning, in which there are over 2000 patients who have been poisoned due to drinking water that contains high levels of As, eating food that are prepared by using coal that is highly arsenic, and breathing (the amount of As in the air exceeds standards). Moreover, hundreds of people suffer or have died from the repercussions of As pollution, such as skin, liver and lung cancers. At present, there is no optimal method to counter As poisoning, so there are some restrictions in place for coal mining in which large amounts of As are emitted, but these can result in the waste of resources. Therefore, a study on the standards imposed onto arsenic emissions and the precipitation mechanism of coal combustion would have great theoretical impacts and practical significance for the prevention and control of As pollution as well as improvements in the utilization of resources. To date, many researchers have studied the precipitation of As in the coal combustion process (Bolanz et al., 2012; Frandsen et al., 1994; Liu and Su, 2014). For example, Yu et al. (2009) found that there is a positive correlation between As content in indoor and outdoor air and that of burning coals. Sun et al. (2004) examined the distribution of As in fly ash with different particle size in coal-fired power plants, and found that the concentration of As in different concentrations of particulate matter is gradually increased with a reduction in the particle size of fly ash. Some academics have conducted statistical analysis on the distribution of As in coal in China, and obtained a distribution range of 0–10 μg/g (with an average value of 5 μg/g), which is similar to the average content of As in coal worldwide (Wang et al., 2013; Yudovich and Ketris, 2005; Zhang et al., 1999). Liu et al. (2016a) studied the volatilization of As during the co-combustion of lignite blends with high As levels. Contreras et al. (2009) examined the effects of different compounds in the distribution and mode of occurrence of As in co-combustion processes based on thermodynamic equilibrium calculations. The mineral association, chemistry, distribution and migration of As in different kinds of coal have also been assessed by using different methods, such as using a low temperature ashing process and density fraction) (Jiao et al., 2013; Low and Zhang, 2013; Zhou et al., 2016). These studies all provide a better understanding of the migration and distribution of As in the coal combustion process and a solid foundation for future work. Since As emissions have continued to pollute air, thus negatively affecting the environment and human health, and studies on the origins and process of As emission are relatively rare, As emission is still an area that needs to be improved and studied in-depth. In this paper, the variations in the emission rate of As during the burning of different types of coals under different combustion environments are examined, and the relationships among the As content, air flow, combustion time, oxygen content and As emission are analyzed. Then, the process of As emission is tested by using a thermogravimetric test. The results can further understanding on the source of As volatilization during the coal combustion process and provide guidance and
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direction on the prevention and control of As pollution (Tang et al., 2009; Zhao et al., 2017). 2. Data sources Arsenic produced through coal combustion is becoming a more popular research topic because of its toxicity and volatility as well as its potential carcinogenic propensity (Liu et al., 2016b). In China, there is uneven distribution of As in coal in the different areas. In general, the concentration of As in Chinese coal is similar to that of the global average value, which is around 5.0 ppm. However, depending on the geochemical and geological characteristics of the particular coal, As concentrations can vary and change significantly in different regions (Zhao et al., 2008). Coal found in northern and northwest China has much lower concentrations of As than that in the southwest and northeast of China (Zhao et al., 2008), especially in the southwest of Guizhou (Chen, 2013; Liu and Chen, 2015; Yu et al., 2009). For instance, the coal in northern and northwest China such as Northern Shaanxi province typically have a concentration of 4.1 ppm while that in southwest and northeast China such as Guizhou have 7.5 ppm. There have been many coal-forming periods in the Guizhou area, and all sorts of coal is found. Aside from stone coal with a large amount of ash formed in the early Paleozoic, there is also lignite formed in the Tertiary period, which has the greatest amount of bitumite and anthracite which formed in the upper Permian and upper Triassic periods. The coal formed in this period is widely distributed, and the geological circumstances of coal formation are diverse with complex structures, which result in higher amounts of trace elements, especially As. Studies (Liu and Chen, 2015; Zhao et al., 2008) have shown that high concentrations of As in coal are mainly derived from the migration of magmatic fluid after the coal forming period, and the distribution is mainly related to the geological structure, especially the composite zones in tectonic settings, and large and deep faults. In this paper, we have collected data published in the literature (see Table 1) on the As emission from coal in Guizhou, Shanxi, Henan, Yunnan, Hunan and Jiangxi (all Chinese provinces) and the coal has high concentrations of As. A comparative analysis is carried out, and we provide a better understanding of the emission information during the thermal process of coal combustion. 3. Experimental results Previous studies have shown that the level of As emissions varies with the burning temperature in the coal combustion process (Senior et al., 2006; Sia and Abdullah, 2012; Su et al., 2013). Generally speaking, As in coal is mainly in the form of arsenic pentoxide (As2O5), which is a white solid, at a temperature below 477 °C. When the temperature increases from 477 °C to 627 °C, As changes in form to first As2O5 which is a solid phase, and then arsenious oxide (As4O6) and arsenic trioxide (AsO), which are gas phases; that is, from 477 °C to 527 °C, As is mainly in the form of As2O5 which is a solid; from 527 °C to 557 °C, As is mainly in the form of As4O6, which is a gas; and from 557 °C to 627 °C, As is mainly in the form of AsO, which is also a gas. When the temperature is higher than 627 °C, the As in coal is found only in the form of AsO or a gas phase. The rate of released As and total amount of As are therefore primarily related to the combustion environment, As concentration in the coal, combustion temperature, a constant time and the oxidant or inhibitor contents (Song et al., 2014; Zhao and Luo, 2017). Fig. 1 shows the changes in the emission rate of As (the formula is shown in Eq. (1)) in the coal combustion process under natural
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Table 1 Detailed information on acquisition of data used in analysis. Rock (mineral) type or coal rank
Publication
Site
Test method
As content (μg/g)
Bituminous (SJS)
Liu and Chen (2015)
Guizhou, China
4.26
Lignite (GX) Bituminous (CA) Anthracite (A1) Anthracite (A2) Lignite (L1) Lignite (KY) Bituminous (JL) Bituminous (XR) Coal gangue Lignite (YH)
Liu et al. (2016a) Liu et al. (2016a) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Liu et al. (2016b) Yang (2011) Liu and Chen (2015) Wang et al. (2013)
70.24 2.81 0.36 3.19 4.51 68.35 0.96 50.58 75.17 19.64
Coal (CX) Coal (XR) Coal (PX) Coal (SM1) Coal (SM2) Lignite (AL)
Dai and Li (2005) Dai and Li (2005) Dai and Li (2005) Liu and Su (2014) Liu and Su (2014) Chen and Shuai (2012) Chen (2013) Chen (2013) Chen (2013) Chen (2013) Han et al. (2009) Han et al. (2009)
Xingren, Guizhou, China Datong, Shanxi, China China China China China China Xingren, Guizhou, China Xingren, Guizhou, China Xiaolongtan, Yunnan, China Chenxi, Hunan, China Xingren, Guizhou, China Pingxiang, Jiangxi, China Shenmu, Shaanxi, China Shenmu, Shaanxi, China Anlong, Guizhou, China
Fourier transform infrared spectroscopy (FTIR), Thermodynamic analysis (DTA) Hydride generation atomic fluorescence spectrometry (HGAFS) HGAFS HGAFS HGAFS HGAFS HGAFS HGAFS Atomic fluorescence spectrometry (AFS) AFS, SEM-EDAX – Neutron activation analysis (NAA) NAA NAA – – AFS
46.2 430.6 18.6 2.387 3.353 90.04
AFS AFS AFS AFS AFS AFS
– – – – – –
Lignite (AL) Bituminous (ZJ) Anthracite(XR) Stone coal (JL) Coal (JZ) Coal (LPS)
Anlong, Guizhou, China Zhijin, Guizhou, China Xingren, Guizhou, China Jiaole, Guizhou, China Jiaozuo, Henan, China Liupanshui, Guizhou, China
combustion, in which the curves show an overall trend where the As emission rate gradually increases with increased temperature. F¼
w 1− h 100% wm
ð1Þ
where, F is the emission rate of As, wh is the content of As in ash, wm is the content of As in original coal. Fig. 1a shows the changes in the emission rate of As during the combustion of anthracite, bituminous coal and lignite. Note that the changes in the As emission rate of these three different types of coal are similar in that their rate of emission gradually increases with increased temperature, and they show obvious segmentation characteristics. That is, at a temperature that is less than 800 °C, the As emission rate slowly increases with increases in temperature, so that the average emission rate of the seven samples (lignite from Anlong (Guizhou), anthracite A1, anthracite A2, bituminous coal B1, bituminous coal B2, lignite C1, and lignite C2) is 18.8% at 800 °C which is increased from 10.8% at 600 °C. When the temperature reaches 800 °C, the increase in the rate of the emission of As is rapid, and at 1100 °C, the average change in emission is 54.4%. The increase in the change of the rate of emission at this stage is three times that in the previous stage. A temperature of 800 °C is an inflection point of the volatility of As versus temperature. Fig. 1b illustrates the volatile characteristics of coal which originate from different regions of China during combustion. The changes in the five curves can be divided into three stages in accordance with the temperature range of 500 °C to 1200 °C: ① Stage 1 (500 °C to 700 °C): changes in the volatility of As are minimal; ② Stage 2 (700 °C to 1000 °C): when the burning temperature reaches 700 °C, the change in the emission rate of As slowly increases, and the average is 38.3% at 1000 °C as opposed to 14.9% at 700 °C; and ③ Stage 3 (1000 °C to 1200 °C): the As emission rate is the highest (the changes in the rate are seven times that in the previous stage), which shows that higher burning temperatures mean higher rate of emission of As. According to the results shown in Fig. 1a and b, the temperature range of 700 °C to 800 °C is critical because As is emitted at a rapid rate during this stage. When the temperature exceeds the threshold, the rate of emission is significantly increased. As the samples and the combustion environment are different, and taking
into consideration experimental errors, the resulting temperature at the critical point may be different, but basically within this range of 700 °C to 800 °C. Fig. 2 shows the change characteristics of the As emission rate of the coal samples with different concentrations of As and under different conditions. As shown in Fig. 2a, the As emission rate in the combustion process with a higher oxygen content is greater than that in a combustion process with a low oxygen content; for instance, with an oxygen content of 21% and carbon dioxide content of 79%, the emitted As at 900 °C is 46.1% while with an oxygen content of 5% and carbon dioxide content of 95%, the emitted As at 900 °C is 42.1%. That is to say, in the studied range, a higher oxygen content in a combustion environment means greater changes in the As emission rate with burning temperature. When there are no changes in the oxygen content, carbon dioxide (CO2) can inhibit the volatility of As relative to nitrogen. This is because a large volume of CO2 reacts with the carbon particles to form CO gas, thus resulting in a temporary reduction atmosphere on the surface of the carbon particles, and affecting the volatilization and migration of As (Liu and Chen, 2015). So, if the technology for the recovery of CO2 is applied, using CO2 and oxygen as the combustion environment can reduce As emissions and other greenhouse gas emissions. The changes in the rate of the emission of As in the coal combustion process with different As levels in normal air are shown in Fig. 2b, which shows that the rate of emission of As from the coal with high concentrations of As is greater than that of coal with low concentrations. The relationship between the emission rate of As and the original concentration of As in the coal is linear (as shown in Fig. 2c), and the mathematical expressions for the fitting lines at 900 °C and 1100 °C are shown in Eqs. (2) and (3). The slope of the two lines shows that the increase in the rate of emission of As versus the original concentration of As at 1100 °C is about two times that at 900 °C. Therefore, the removal of some of the As through coal washing is necessary for coal with high concentrations of As prior to combustion. μ ¼ 45:21 þ 0:37 λ
ð2Þ
μ ¼ 31:31 þ 0:20 λ
ð3Þ
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Fig. 1. Rate of emission of arsenic in coal burning process (sources: Dai and Li, 2005, Han et al., 2009, Chen, 2013, Liu and Chen, 2015). (a) Different types of coal, (b) Different locations of coal.
where, μ is the rate of emission of As and λ is the original As concentration in the coal. The emission rate of As during coal combustion is not only related to the burning temperature, oxygen content and original As concentration in the coal, but also the retention time. As shown in Fig. 3, the volatility of As initially increases with the retention time but then stabilizes to a certain level. The commencement of the basic stabilization stage is defined as the fundamental stability point. From the experimental results of the lignite from Anlong in Guizhou (see Fig. 3b), the retention time that responds to the fundamental stability point of the curves in which the target temperature is 700 °C, 900 °C and 1100 °C is 25 min, 20 min and 10 min, respectively, which prove that a higher temperature means a higher rate of emission of As. These three curves in Fig. 3b also show that the basic stability stage of the As emission rate is increased with increased temperature. The results suggest that combustion should be maintained for at least 30 min at the target temperature when the design temperature to dispose the As pollutant is less than 1000 °C, and the retention time can be appropriately reduced when the design temperature is higher than 1000 °C.
Fig. 2. Change in characteristic of arsenic emission rate under different conditions. (a) Change in emitted arsenic in different atmospheres (source: Liu and Chen, 2015); (b) Change in emitted arsenic in coal with different amounts of arsenic (source: Liu et al., 2016b); (c) Change in emission rate of arsenic versus original arsenic content in coal.
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Fig. 4 shows the effect of air flow on the As emission rate during the coal combustion process. When the air flow is less than 700 mL/min, the release of As increases with increased air flow (as Stage 1 shows in Fig. 4), which is because the air flow provides sufficient oxidation for the combustion of coal, and accelerates the precipitation of As. The rate of emission of As under an air flow of 700 mL/min is increased 23.3% compared to that under an air flow of 200 mL/min. When the air flow is greater than 700 mL/min, the oxidation required for coal combustion is adequate, and the rate of release of As is basically unchanged with changes in the air flow. In natural conditions, most coals contain calcium (mainly in the form of calcium oxide (CaO)), and experiments showed that when CaO is found in the combustion process, the volatility of As will be obviously inhibited due to the chemical reaction, as shown in Eq. (4). Calciumbased fixing agents of As have also been recognized by the majority of researchers (Chen and Shuai, 2012; Tian et al., 2015), which is applied in the process of fixing As in a power plant. As2 O3 þ 3CaO þ O2 →Ca3 ðAsO4 Þ2
ð4Þ
Fig. 3. Changes in volatility of arsenic versus combustion time.
Fig. 4. Change in volatility arsenic versus air flow.
Fig. 5. Variation in rate of retention of arsenic with different fixing agents. (a) Effect of temperature on fixing As rate; (b) Effect of fixing agent amount on fixing As rate. Source: (Yang et al., 2011, Yang, 2011).
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Fig. 5 shows the variation in the fixing As rate in the coal combustion process in which different amounts of several kinds of calcium-based fixing agents of As are added, including dolomite, limestone and cement. It can also be seen in Fig. 5a that there is a significant effect of the fixation of dolomite and limestone on As, and the fixing As rate (the formula is shown in Eq. (5)) is over 70% at a burning temperature that is less than 1000 °C. When the temperature is over 1000 °C, the fixing As rate is gradually reduced, and at 1250 °C, the fixing As rate of dolomite and limestone is reduced to 39.4% and 8.9%, respectively. The changes in the fixing As rate with amount of fixing agents (dolomite and cement) is shown in Fig. 5b. Note that the fixing As rate increases with increased amount of fixing agent, for example, with 9% of fixing agent (dolomite) increased, the fixing As rate is increased 45.8%. The reason that dolomite and cement both have a fixed effect on As is that their main component is CaO, which can react with As so that it becomes a solid, and consequently, As can be removed. Therefore, a certain amount of calcium can be added to reduce the volatility of As in the coal combustion process so as to mitigate the damage to the environment and human health. η¼
F0− F1 F0
100%
ð5Þ
where, η is the fixing arsenic rate, F0 is the emission rate of As of original coal, F1 is the emission rate of As of coal with As fixing agents. 4. Discussion In the coal combustion process, the activity of elements is enhanced due to high temperatures and strong oxidation, and much of the material structure of the coal is damaged, thus leading to the separation of trace elements from the original molecular structure and their recombination and distribution in different combustion products (Camacho et al., 2011). In particular, the trace elements which are found in coal or in the adsorbed or electrostatic form will migrate, and other trace elements found in other forms in coal will also partially ionize. Arsenic is an important trace element in environmental pollution, and released from coal during the combustion process. After a series of physical and chemical changes, As is released in different forms at different temperatures, which can cause damage not only to the environment but also to human health (Wang, 2006). We conducted a thermogravimetric test and observed that there is a rapid stage of decline in the mass of the coal during the combustion process, which is caused by the combustion of organic matters, and the concentration of As is also rapidly reduced in this stage, as shown in Fig. 6. This indicates the rapid decomposition of organic matters and increase in combustion temperature both contribute to the emission of As. Therefore, this stage is important for the removal of As, as the amount of emitted As should be controlled in the early stages of diffusion, thus avoiding the occurrence of the initial pollution which will then need to be rectified. In determining the most critical stage for the removal of As, several other important factors that affected the As emission rate were fully considered. First, an appropriate fixing agent of As should be selected. Under high temperatures, the fixing agent is mainly applied through physical adsorption and chemical reaction to carry out the fixation of As (Zhang et al., 2000). According to existing research (Chen and Shuai, 2012), native dolomite not only sets off a chemical reaction which forms the solid phase compounds of As, but can also produce CaO and magnesium oxide (MgO) which are porous and absorb the gaseous form of As. Therefore, native dolomite is first recommended as the fixing agent of As. At the same time, the amount of the fixing As agent added to the coal should be reasonable. In theory, more fixing agent means better fixing of As. However, when the fixing agent amount reaches a certain level, the effects of adding more fixing agent are minimal. Moreover, excessive amounts of fixing agent will lead to the
Fig. 6. Plot of thermogravimetry and arsenic content under different temperatures.
increase of the ash amount and the decrease of fixed carbon content and the heat value of the coal. Therefore, the appropriate amount of fixing agent added into the coal during the combustion process will minimize As pollution. However, if there are other reactions between the fixing agent and other substances, they also need to be considered. For example, most coal contains sulfur, and the calcium-based fixing agent of As not only reacts with As, but also with sulfur, as shown in Eq. (6). 2CaO þ 2SO2 þ O2 →2CaSO4
ð6Þ
Eqs. (4) and (6) show that the amount of fixing agent (Ca/As) required in theory for the fixing of As and sulfur is calculated to be 1:1, which can be converted to the amount of Ca/S at 5:2. This result is in agreement with the research results of Yang et al. (2011), whose results showed that the amount of calcium-based fixing agent of As (Ca/S) is 2:1. Based on the calculation results and literature, we determined that the amount of calcium-based fixing agent of As (Ca/As) is more suitable between 17:20 and 1:1. If the amount is inadequate, its removal effect of both As and sulfur cannot be achieved. On the contrary, if the amount is too much, there will be excessive slag production. Then, the optimal burning temperature for the retention of As was found after adding the fixing agent, in which there were variations in the rate of emission of As with burning temperature compared to the coal combustion process. Fig. 7 shows the changes in the fixing As rate with temperature during coal combustion with two calcium-based
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(3) After adding an As fixing agent, the amount of As emission is closely related to the combustion temperature, fixing agent amount and the coal particle size. Among these, the combustion temperature and fixing agent amount should be moderate at 1050 °C and 1:1 (Ca/ As), and a higher temperature and more fixing agent do not necessarily provide better effects. Coal with a small particle size also helps with the fixation of As. Acknowledgments This research is supported by the Fundamental Research Funds for the Central Universities (Grant No. 2017CXNL03) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References
Fig. 7. Effect of burning temperature on fixing As rate. Source: (Chen, 2009).
fixing agents, which are calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3). It can be seen from the figure that the fixing As rate increases gradually with increased temperature up to 1050 °C, and then gradually decreases with continued increase of temperature. This is because when the temperature is excessively high, the compounds of As and calcium will gradually decompose, and the amount of calcium for utilization will also be reduced, thus resulting in a reduced rate of retention. Therefore, the optimal combustion temperature should be selected in accordance with the corresponding fixing agent and in this study, the optimal temperature is about 1050 °C. In addition, the amount of emitted As is also related to the size of the coal particles. Although Song et al. (2014) found that the particle size of coal has no influence on the amount of arsenic in coal, however, when there are large coal particles, the combustion is insufficient, which results in the waste of resources. On the contrary, coal with a smaller particle size means a greater specific surface area for reactions to take place, and a higher uptake of calcium. Therefore, a smaller coal particle size contributes to the fixing of As. In summary, in giving consideration to and identifying the original As content of coal, combustion environment and time, and amount of As fixing agent, the As emission during the combustion process can be well controlled, and the negative effects of As pollution on the environment and human health can be subsequently reduced or prevented. 5. Conclusion In summary, this paper focus on the study of the emission characteristics of As during the combustion process of different kinds of coal in different combustion condition, and highlights the significant impact of oxygen content, As content of coal, burning temperature, air flow, constant temperature time, the types and dosage of fixing agent of As and the particle size of coal on the emission rate of As. The following conclusions are made based on the study findings. (1) The emission rate of As in the coal combustion process is gradually increased as the burning temperature increases, with a critical temperature threshold of 700 °C to 800 °C as the rate of precipitation of As. When the temperature reaches or exceeds this temperature interval, the rate of emission of As will be significantly increased. (2) The level of oxygen content, As content of coal, air flow and a constant temperature time all have significant effects on the amount of As emitted during the coal combustion process. Within a certain range, they can promote the release of arsenic, and then the promote effect is very small.
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