In China, the dominant position of coal-fired power plants in energy production will not change in the short term. The thermal power installed capacity accounts for about 70% of the total installed capacity (1). In 2020, the thermal power generation equipment capacity was 1,245.17 million kilowatts (kW), a cumulative increase of 4.7% and accounting for 56.6% of the total power generation capacity (2). By the end of 2020, the installed power generation capacity of overpower plants rated at 6,000 kW or greater was 1,996.01 million kW, and the installed thermal power generation capacity was 1,225.62 million kW, accounting for 61.4% of the total power generation in China (3). Silica dust and coal dust are the main occupational hazards in coal-fired thermal power plants, which could cause pneumoconiosis, silicosis, lung cancer, tuberculosis, and other adverse health effects. The prevalence of occupational lung diseases such as pneumoconiosis and silicosis and its annual increase rates are still high in China (4). According to the latest report in China, there were 15,898 new cases of pneumoconiosis, accounting for 81.8% of the total cases of occupational diseases reported in 2019 (5). Some studies have estimated that a total number of 1.37 million standardized incident cases of lung cancer will be attributed to occupational exposure in coal-fired power by 2025 (6). The thermal power plants have gradually changed from 300 million watt (MW) units to 600 MW and 1,000 MW units with technological advancements, while dust hazards are still present and difficult to control effectively. This study aimed to investigate occupational exposure levels of respirable dust and evaluated the occupational health risk levels of key dust-exposed position in six coal-fired thermal power plants of different sizes and regions, providing information that can be used by other researchers in the future in solving dust exposure problems in the working environment.
A field survey was carried out in 6 coal-fired thermal power plants, including a total of 12 units which included two 2×300 MW units, two 2×600 MW units, and two 2×1,000 MW units. Considering the regional and climate impacts, the power plants we selected are located in Zhejiang, Shanxi, Hebei, Shandong, and Fujian. Overall, 291 respirable dust samples (195 environmental and 96 personal samples) were measured. Safety and health practitioners in each thermal power plant were interviewed and gave information about the production process, positions, and number of workers exposed to dust, and the inspection routes for each position were collected. This study measured the environmental and personal dust exposure levels based on workplaces that produce dust hazards, such as coal transportation, combustion milling, and ash removal. Methods of sampling and respirable dust measurement were determined according to the relevant guidelines GBZ 159–2004 and GBZ/T 192.2–2007. The peak exposures of respirable dust in workplaces were measured by AKFC-92A type mine dust sampler with a flow rate of 20 L/min. The sampling time was 15 minutes at the personal breathing zone of workers during a normal work shift. Individual dust samples were collected using AKFC-92G type individual dust sampler with a flow rate of 2 L/min. For each work position, 1–2 people were selected for three shifts, and the average value was calculated.
Bulk samples were also collected from each plant site and subjected to free silica analysis. The content of free silica in dust was determined by GBZ/T 192.4–2007, the free SiO2 of coal dust is 5.33%–6.70%, and the free SiO2 of silica dust is 14.44%–27.62%. Considering GBZ 2.1–2019, the time weighted average concentration (CTWA) of respirable coal dust (free SiO2<10%) being greater or equal to 2.5 mg/m3 and the CTWA of respirable silica dust (10%≤free SiO2≤50%) being not less than 0.7 mg/m3 were considered to exceed the personal dust exposure standard.
The occupational hazard risk index method (7) was used to evaluate the occupational health risk levels of the capacity and the selected position by comprehensively considering the exposure level, the severity of the hazard, the number of exposed workers, and protective measures, combined with national health standards to give weights and evaluate grades. The risk index was divided into 5 levels, namely no hazard (0–6), light hazard (7–11), moderate hazard (12–23), high hazard (24–80), and extreme hazard (>80). The risk index = 2Health effect grade × 2Exposure ratio × working condition grade, where the health effect grade was divided according to the content of free silica in the dust, exposure ratio = average measured value/occupational exposure limit, working condition grade = [exposure time value × value of the exposed workers number × value of engineering protection measure × value of personal protective equipment (PPE) usage rate]1/4. The classification standards of the health effect grade and working condition grades were described in Table 1. Relevant information was obtained from occupational health surveys and measurements conducted for each thermal power plant.
In this study, we measured 195 respirable dust samples in workplaces among coal-fired power plants of different sizes (Table 2). Coal transportation workplaces were exposed to coal dust, and the average peak exposure was 2.02±1.45 mg/m3. In combustion milling workplaces, the average peak exposure of coal dust was 1.27±1.32 mg/m3 and that of silica dust was 0.86±0.60 mg/m3. The average peak exposure of silica dust measured in ash removal workplaces was 0.93±0.52 mg/m3. With an increase in capacity, the average peak exposure for each workplace decreases gradually.
The operators mainly engaged in inspection work, with each inspection time being about 2–3 hours and each workplace being inspected twice per shift. A total of 96 personal breathing zone respirable dust samples were collected. Table 3 listed the CTWA of respirable dust for each position of different capacities and the pass rate. The CTWA of coal transportation operators all met the requirements of the GBZ 2.1–2019 guidelines, and other positions had different degrees of excess. Especially for ash removal operators in 300 MW unit, CTWA exceed the permissible limit seriously.
Value Health effect grade Working condition grade The content of free silica Exposure time (h/shift) No. of exposed workers Engineering protection measure PPE usage rate (%)* 5 NA >12 >50 None ≤20 4 NA –12 26–50 Overall control 21–50 3 ≥70% –8 16–25 Partial control, operation but uncertain effect 51–80 2 40%–70% –5 6–15 Partial control, clear effect 81–90 1 10%–40% –2 –5 Confined facility ≥90 0 ≤10% NA NA NA NA Abberviations: NA=not applicable; PPE=personal protective equipment.
* PPE usage rate: number of workers using protective equipment/number of workers exposed to dust×100%.
Table 1. The classification standard of health effect grade and working condition grades in occupational hazard risk index method.
Unit Type of dust Overall 300 MW 600 MW 1,000 MW No. of samples Mean±SD (mg/m3) No. of samples Mean±SD (mg/m3) No. of samples Mean±SD (mg/m3) No. of samples Mean±SD (mg/m3) Coal transportation Coal 73 2.02±1.45 18 2.36±1.73 28 2.11±1.57 27 1.71±1.07 Combustion milling Coal 37 1.27±1.32 13 1.89±2.0 10 0.85±0.48 14 1.00±0.61 Silicon 48 0.86±0.60 17 1.03±0.91 19 0.86±0.34 12 0.63±0.17 Ash removal Silicon 37 0.93±0.52 11 1.26±0.65 14 0.82±0.30 12 0.76±0.50 Total 195 1.39±1.23 59 1.67±1.51 71 1.34±1.18 65 1.18±0.89
Table 2. The peak exposures of respirable dust from coal-fired thermal power plants of different capacities in China, 2017–2019.
Position Type of dust Overall 300 MW 600 MW 1,000 MW No. of samples No. of passes Pass
Mean±SD (mg/m3) No. of samples No. of passes Pass
Mean±SD (mg/m3) No. of samples No. of passes Pass
Mean±SD (mg/m3) No. of samples No. of passes Pass
Mean±SD (mg/m3) Coal transportation operators Coal 38 38 100 0.59±0.55 19 19 100 0.84±0.58 11 11 100 0.43±0.48 8 8 100 0.22±0.09 Combustion milling operators Silicon 39 32 82 0.49±0.63 13 11 85 0.48±0.47 16 14 88 0.42±0.67 10 7 70 0.61±0.76 Ash removal operators Silicon 19 14 74 0.46±0.39 7 2 29 0.83±0.39 6 6 100 0.23±0.23 6 6 100 0.26±0.10 Total 96 84 88 0.53±0.55 39 32 82 0.72±0.54 33 31 94 0.39±0.55 24 21 88 0.39±0.51
Table 3. CTWA of respirable dust and pass rate for positions from coal-fired thermal power plants of different capacities in China, 2017–2019.
From Table 4, occupational health risk assessment results showed that the overall occupational health risk level was medium, most positions had light or negligible hazards, and the risk level of ash removal and combustion milling operators in 300 MW unit and combustion milling operators in 1,000 MW unit were medium①.
Capacity Position Type of dust Health effect grade Exposure ratio* Working condition grade Grade † Risk index § Risk level Exposed workers Exposure time Engineering control measure PPE usage rate No. Value h/shift Value Value % value 300 MW Coal transportation operators Coal 0 0.34 110 5 4 2 Partial control, operation but uncertain effect 3 60 4 3.31 4 None Combustion milling operators Silicon 1 0.69 99 6 3 3.66 12 Medium Ash removal operators Silicon 1 1.19 69 4 2 3.31 15 Medium 600 MW Coal transportation operators Coal 0 0.17 148 5 6 3 Partial control, operation but uncertain effect 3 60 4 3.66 4 None Combustion milling operators Silicon 1 0.60 134 6 3 3.66 11 Light Ash removal operators Silicon 1 0.31 50 4 2 3.31 8 Light 1,000 MW Coal transportation operators Coal 0 0.08 82 5 6 3 Partial control, clear effect 2 75 4 3.31 4 None Combustion milling operators Silicon 1 0.87 176 6 3 3.31 12 Medium Ash removal operators Silicon 1 0.37 62 6 3 3.31 9 Light Total Silicon 1 0.76 930 5 6 3 Partial control, operation but uncertain effect 3 60 4 3.66 12 Medium Abbreviation: PPE=personal protective equipment.
* Exposure ratio=average measured value/occupational exposure limit, the coal dust limit=2.5 mg/m3, the silica dust limit=0.7 mg/m3.
† Working condition grade=(exposure time value×exposure number value×value of engineering protection measure×value of PPE usage rate)1/4.
§ Risk index = 2Health effect grade × 2Exposure ratio × working condition grade.
Table 4. Occupational hazard risk index method assessment results from coal-fired thermal power plants of different capacities in China, 2017–2019.
There were about 1,000 coal-fired thermal power stations in China, and the dust hazards of thermal power plants have always been a focus of occupational health. The effective implementation of the “National Occupational Disease Prevention and Control Plan (2016–2020)” and the Pneumoconiosis Prevention and Control Action in 2019 by the National Health Commission of the People’s Republic of China have enabled the dust exposure level of thermal power plants to be controlled to a certain extent. The study showed that the personal dust concentration of coal transportation workplaces of different capacities was lower than national standards, indicating that the dust prevention measures adopted by enterprises for the coal transportation workplaces are feasible and can meet the purpose of protecting the health of workers. However, the pass rate of CTWA in the ash removal workplaces was 74%, mainly due to the CTWA of ash removal workers in 300 MW unit exceeding the allowable limit. This study also found that the environmental and personal respirable dust exposure among coal-fired thermal power plants had decreased with an increase of capacity, and the CTWA of respirable dust in 300 MW unit was higher than those among plants with bigger capacities. This was due to the increasing of the capacity is accompanied by technical improvements. Compared with the thermal power plants of 300 MW and 600 MW units, the 1,000 MW thermal power plants were built in recent years with advanced technology, the levels of containment and automation of machines were higher, and the effect of dust proof equipment on preventing dust from escaping was better. However, the expansion of the unit size will lead to more inspection personnel and prolonged inspection time, which will also increase the risk of dust exposure.
Occupational health risk assessment results showed the risk level of ash removal operators in 300 MW unit and combustion milling operators of different capacities was relatively high. Workers in combustion milling workplaces and ash removal workplaces were mainly exposed to silica dust in thermal power plants. Many studies have shown that there was strong epidemiological evidence for the association between occupational silica dust exposure and several diseases (8). Some research found workers engaged in ash removal suffered the highest health risk (9). The dust generated in the combustion process of the boiler escaped relatively easily, and the concentration of silica dust was likely to exceed standards when the ash removal device was not properly closed, especially in small power plants. This was also shown by a study on three coal-fired power plants of different capacities, which suggested that the silica dust concentration of the ash removal workplaces and combustion milling workplaces among the three power plants exceeded the permissible limit to varying degrees (10). Therefore, the ash removal workplaces and the combustion milling workplaces were the key control points for dust hazards in coal-fired thermal power plants.
During site investigations, the main reasons for dust exceeding in coal-fired thermal power plants were imperfect design and installation of dust protection facilities, untimely maintenance, and unscheduled dust removal onsite. Optimally, work processes should be isolated and enclosed and adequate ventilation should be provided. The plants need to strengthen maintenance and upkeep of dust prevention facilities and encourage personal protective equipment use among workers during possible dust exposure. Even with such measures, the exposure levels still exceeded the guidelines in some areas, especially in areas where dust or ash accumulations were present. Consequently, it was clear that continued efforts are needed to train and supervise workers to promote worker safety in terms of dust exposure and to reduce the adverse impact from dust exposure on the health of workers.
The findings in this report were subject to at least three limitations. First, the survey included only 6 coal-fired thermal power plants, and the generalizability of the findings is limited. Second, the occupational risk index method was subjective in the process of evaluating the various grades of the operation and therefore subject to biases. Third, data only included the environmental and individual exposure levels, and the related health data that could be affected by dust among the workers were not studied. Further studies should expand the sample sizes and study the relationship between dust exposure and related health consequences among coal-fired thermal power stations of different capacities and combine this with occupational health to provide a realistic basis for improving the assessment method.
|①||Operations with no hazard risk can be regarded as acceptable operations. Operations with light hazard risks should be further evaluated. Operations with medium hazard risks should be risk controlled based on further evaluation, such as strengthening protection or reducing exposure time. For operations with high hazard risks, measures must be taken to reduce the risk of occupational hazards. Operations with extreme hazard risks should be stopped, and comprehensive measures such as finding new methods or reforming the process flow or strengthening engineering control should be adopted to reduce the hazard risk.|