Behaviors of Chromium in Coal-Fired Power Plants and Associated Atmospheric Emissions in Guizhou, Southwest China

Abstract

Coal burning is a main concern for a range of atmospheric pollutants, including the environmentally sensitive element chromium (Cr). Cr migrates to the environment through stack emissions and can leach out from solid coal-burning byproducts, thereby causing adverse effects on the ecosystem. In this study, atmospheric emissions of Cr from six coal-fired power plants (CFPPs), as well as the distribution of Cr inside these CFPPs in Guizhou Province, Southwest China, were investigated. Among the six CFPPs, one was a circulating fluidized bed boiler and the others were pulverized coal boilers. The results showed that Cr in the feed fuel of these CFPPs ranged from 39.5 to 101.5 mg·kg⁻¹ (average: 68.0 ± 24.8 mg·kg⁻¹) and was approximately four times higher than the national and global average. Cr in the feed fuel correlated significantly with the ash yield, demonstrating that Cr in coal is closely associated with ash-forming minerals. After the coal combustion and the treatment by different air pollution control devices, most Cr (>92%) in the installation was retained in the captured fly ash and bottom ash, with less as gypsum (0.69–7.94%); eventually, only 0.01–0.03% of Cr was emitted into the atmosphere with a concentration of 1.4–2.2 μg·Nm⁻³. The atmospheric emission factors of Cr for these utility boilers were as low as 14.86 ± 3.62 mg Cr·t⁻¹ coal, 7.72 ± 2.53 μg Cr (kW·h)⁻¹, and 0.70 ± 0.19 g Cr·TJ⁻¹, respectively. About 981 kg·y⁻¹ of Cr was discharged into the atmosphere from Guizhuo’s CFPPs in 2017, much lower than previous reported values. Most of the Cr in the CFPPs ended up in solid combustion products, identifying the need for the careful disposal of high-Cr-containing ashes (up to 500 mg·kg⁻¹) to prevent possible mobilization into the environment.

1. Introduction

Coal-fired power plants (CFPPs) are the largest coal consumers and the main energy source in China [1,2], which released huge masses of pollutants into the atmosphere each year, including hazardous trace metals and metalloids [3,4]. Hazardous trace metals that exist in coal, such as chromium (Cr), can migrate via and enrich coal combustion products (CCPs) during the coal combustion process, causing adverse effects on human health and the environment when released [5]. Inhalation cancer and non-cancer risks associated with chromium emissions from coal-burning are large compared with other elements [6], with it being identified as a hazardous pollutant in the USA 1990 Clean Air Act Amendments [7], in the Canadian Environmental Protection Act 1995 [6], and a hazardous atmospheric pollutant by the Chinese Ministry of Ecology and Environment [8]. Chromium (Cr), arsenic (As), and mercury (Hg) are the three elements recommended by the USA Electric Power Research Institute to be monitored in emissions, concerning carcinogenic and non-carcinogenic health risks [6]. However, compared to studies on Hg and As [9,10,11,12,13], studies regarding Cr emissions from CFPPs and the associated environmental impacts are limited, especially in China, which is the top coal consumer in the world [1].

Cr exists at trace levels in coal, with a typical range of 0.5–60 mg·kg⁻¹ [14]. Cr in coal is likely to be associated with clays (mainly illite) [15] or silicate matter [16], but is also present as Cr-oxy-hydroxides in organic matter [15] or as sulphides [15,17]; it is also associated with spinel group minerals (Fe(Cr,Al)₂O₄)) such as chromite (FeCr₂O₄) and chromian magnetite [15]. The volatility of Cr varies with the mode of occurrence of Cr in coals, with organic/sulphide-bound Cr partially evaporating as gaseous CrO₃ (g) during coal combustion, which is then stabilized with calcium oxide or other alkaline elemental oxides to form CaCrO₄ or (Mg, K, Na, Fe)CrO₄ (s) on the ash surface [17]. Huang et al. [18] found that Cr adsorption on fly ash is mainly chemisorption and the main Cr species is Cr₂O₃ at low temperatures (227 °C), which may react with CaO in fly ash to form CaO·Cr₂O₃. Chlorine facilitates this evaporation process, with the volatilization of Cr previously observed at >700 °C [19]. Although Cr in coal is trivalent (Cr(III)), which is relatively benign, the combustion process can convert some of Cr(III) to its more toxic form, the carcinogenic hexavalent (Cr(VI)) [20]. Furthermore, Cr exists in its organic form, with pulverized coal (PC) boiler combustion transforming more Cr(III) to Cr(VI) than the mineral-bound Cr and other boilers (circulating fluidized bed (CFB) boiler and stoker-fired boiler) [20,21]. In China, the contamination of surface water (e.g., upstream Zhujiang River) by leaching of Cr(VI) from industrial solid waste occurred in Yunnan province in 2011, which attracted wide attention.

In China, 47% of the total coal consumption (3.84 billion tons) in 2016 was by the direct combustion in power plants [2]. Although Cr concentration in coal is at trace levels, the large quantities of coal combustion result in enormous emission of Cr into the environment; 505 tons of Cr emissions from Chinese CFPPs were estimated in 2010 based on the emission factor method [22]. However, no onsite research was carried out regarding this topic in China until recently [23,24,25], with all of them conducted in North China. Guizhou Province is the fifth largest coal-bearing province and the largest in terms of production in South China [1]. Coal in Guizhou is mainly formed in Late Permian, with higher ash yields and sulfur contents than other parts of China [11,26]. In 2017, the output of coal was 163 million tons, with 66 million tons consumed by CFPPs in this province [27]. Although Cr contents in coal in this province were studied by several researchers [28,29], little is known about the behaviors of this element inside CFPPs in Guizhou and its atmospheric emissions.

To fill this research gap, we investigated six CFPPs with pulverized coal-fired boilers (PC) or circulating fluidized bed (CFB) boilers in Guizhou, with feed fuel, bottom ash, fly ash, limestone, gypsum, and stack flue gas collected simultaneously. The main purposes of this study were to (1) reveal the distribution behavior of Cr in these CFPPs, (2) to obtain the atmospheric emission factors of Cr, (3) and to estimate the total amount of Cr emitted from CFPPs in Guizhou. This study aims to provide a database for the assessment of the impact of Cr pollution, which would be conducive to environmental management and policy formulation.

2. Methodology

2.1. Power Plants and Sample Collection

Guizhou is the largest coal-bearing province in South China (Figure 1). Twenty coal-fired power plants in total were operating in this province in 2016 with an installation of 30.1 GW, of which two were CFB CFPPs and 18 were PC CFPPs. Most CFPPs were located in the central to western province where the coal mines are located (Figure 1). Six CFPPs, with 28% of the provincial installed capacity, were selected for investigation in this research; their locations are shown in Figure 1. Among them, four (#1–#4) are located in Western Guizhou, one is in Northwestern Guizhou (#5), and one is in Central Guizhou (#6). All of the CFPPs use local unwashed coal, except CFPP#1, which uses gangue and coal slime from the local coal preparation plant. The boiler type is either CFB (#1) or PC (#2–6), with a capacity from 200 MW to 600 MW. All CFPPs are equipped with air pollution control devices for NOx, SO₂, and particulate matter (PM) (

Table 1. Information regarding the six CFPPs surveyed in the present study.

Power Plants	Boiler Type	Installed Capacity	APCDs
#1	CFB	2 × 300 MW	SNCR + C-ESP–FF + WFGD
#2	PC	4 × 600 MW	SCR + C-ESP + WFGD
#3	PC	3 × 200 MW	SCR + C-ESP–FF + WFGD
#4	PC	4 × 600 MW	SCR + C-ESP + WFGD
#5	PC	4 × 300 MW	SCR + C-ESP–FF + WFGD
#6	PC	2 × 600 MW	SCR + C-ESP + OAD

Note: CFB, circulating fluidized bed boiler; PC, pulverized coal-fired boiler; SNCR, selective non-catalytic reduction; SCR, selective catalytic reduction; C-ESP–FF, cold side electrostatic precipitator and fabric filter; WFGD, limestone–gypsum wet flue gas desulfurization; OAD: organic amino desulfurization.

). In particular, NOx is controlled by selective catalytic reduction (SCR) for #2–#6, and selective non-catalytic reduction (SNCR) for #1. Cold-side electrostatic precipitators (ESP) or ESP–fabric filter (FF) are used for dust removal, and limestone–gypsum wet flue gas desulfurization (WFGD) is used for SO₂ treatment for all CFPPs except #6, which uses organic amino desulfurization to convert SO₂ into H₂SO₄.

In each CFPP, only one unit was sampled since the feed coal, boiler type and air pollution control devices (APCDs) are the same for all units. Solid samples, including the feed coal, bottom ash, ESP/ESP–FF fly ash, limestone, and gypsum, were collected simultaneously alongside the stack flue gas; the sampling points are shown in Figure 2. All samples were collected at least three times over a period of 2–3 days, with each solid sample weighing about 1 kg. The wastewater of WFGD was not collected since its contribution to the total output of Cr is negligible [25,31]. In addition, organic amine and sulfuric acid in CFPP#6 were not obtained for inaccessibility reasons. The vapor phase of Cr is thought not to exist at low temperatures (40–50 °C) of stack gas, so the gaseous phase Cr was not collected; the particulate-bound Cr in the stack gas was collected instead. PM in stack gas was withdrawn isokinetically by using the USA EPA Method 5 [32] as shown in Figure S1 in the Supplementary Materials and collected using a Teflon filter (Whatman^®, 0.45 μm pore size) for Cr analysis. Each flue gas sample was collected for ~3 h. This method collected most of the PM in the flue gas since the PM size is generally greater than 1 μm [33]. During the flue gas sampling, the sampling tubing was maintained at 120 ± 10 °C to prevent water condensation [32].

In addition, the operating parameters of each boiler were gathered, comprising information about the daily consumption/production of different solid materials (t·d⁻¹), the daily discharge of flue gas (Nm³·d⁻¹, cubic meter at normal conditions (0 °C and 1.01 MPa) per day), the concentration of PM in the stack flue gas (mg·Nm⁻³), and the load of the boiler (megawatt, MW), which was monitored by the online monitoring system.

2.2. Analysis Methods

In the laboratory, all solid materials were air-dried and ground to <150 μm. For the feed fuel samples, proximate analysis was accomplished after the implementation of the Chinese National Standard Method (GB/T 212-2008) [34] and ultimate analysis of carbon (C), hydrogen (H), and nitrogen (N) were conducted using an elemental analyzer (Vario MACRO Cube, Elementar, Germany), while the total sulfur (S) was measured following the Eschka method of GB/T 214-2007 [35]. The calorific value (Q) was quantified by GB/T 213-2008 [36]. The Cr concentration of different solid samples was determined by inductively coupled plasma mass spectrometry (ICP–MS, Analytik Jena, Germany) after digestion with a mixture of hydrofluoric acid (HF) and nitric acid (HNO₃) at 190 °C in an oven for 24 h [37].

2.3. Quality Assurance and Quality Control

The glassware, Teflon tubing and vessels used for sampling and sample digestion were soaked in 20% nitric acid overnight and rinsed with deionized water. The reagents used were trace metal grade, and HF and HNO₃ were double-distillated to remove impurities. During the digestion and analysis process, system blanks, duplicate samples, and certified reference materials (CRMs) were used to ensure quality assurance and quality control. CRMs for coal gangue (GSB 06-2182-2008-1), anthracite (GSB 06-2105-2007) and bituminous coal (GSB 06-2114-2007) were used during the proximate and ultimate analysis, and a recovery of 95–105% for different parameters was obtained. The CRMs of Coal (NIST SRM 1632d), fly ash (NIST SRM 1633c), and limestone (JDO-1) were digested and analyzed along with the solid samples, and the recovery of Cr in different CRMs was determined to be in the range of 91.7–113.8% (Table S1). The difference between the duplicate samples was less than 10%, and the analytical process blank was negligible (<0.1 mg/kg).

2.4. Relative Enrichment Index and Atmospheric Emission Factor of Chromium

2.4.1. Relative Enrichment Index

The relative enrichment index (REI) was calculated according to Wang et al. [5] to reveal the enrichment Cr between the bottom ash and fly ash during the combustion process, where a higher REI indicated the stronger enrichment capacity of Cr in the ash. The REI was calculated according to Equation (1):

$$REI=\frac{C_{ash}^{Cr}\times A_{ad}}{C_{coal}^{Cr}\times 100}$$

(1)

where $C_{ash}^{Cr}$ is the Cr concentration in the bottom ash or fly ash (mg·kg⁻¹), $A_{ad}^{}$ is the ash yield (%) of the feed coal, and $C_{Coal}^{Cr}$ is the Cr concentration in the feed coal (mg·kg⁻¹). If the REI is close to 1, the element has almost no volatilization in the process of coal combustion and basically remains in the coal combustion products [5]. The closer the REI is to zero, the more volatilization of the element, and the less is retained in the solid coal combustion products. According to REI, elements were classified into three groups [38], i.e., Group I, non-volatile; Group II, partially volatile; and Group III, volatile (Table S2). Since the PM in the flue gas after WFGD (e.g., stack gas) can mix with other impurities, such as gypsum and limestone [38], which constitute up to 55% of the total mass [39], the REI of PM in the stack gas was not calculated in this study.

2.4.2. Atmospheric Emission Factors

Emission factors (EMFs) of Cr were calculated based on three benchmarks [40,41], namely, the amount of coal consumption (EMF₁), the generated power (EMF₂), and the heat value of feed fuel (EMF₃), using Equations (2)–(4):

$$EM{F}_1=\frac{M_{\mathrm{Cr}}}{M_{\mathrm{coal}}}$$

(2)

$$EM{F}_2=\frac{M_{\mathrm{Cr}}}{P\times t}$$

(3)

$$EM{F}_3=\frac{M_{Cr}}{M_{coal}\times Q_{net,ad}}$$

(4)

where M_Cr is the quantity of Cr emitted into the atmosphere per day (g·d⁻¹), M_coal is the consumption of the feed coal (t·d⁻¹), P is the load of tested boiler (MW), t is the running time of a utility boiler (24 h·d⁻¹), and Q_net,ad is the heat value of the feed coal (MJ·kg⁻¹).

3. Results and Discussion

3.1. Proximate and Ultimate Analysis of the Feed Coal

The proximate and ultimate analysis results of the feed fuels of the six CFPPs are summarized in

Table 2. Proximate and ultimate analyses of feed coal (air-dried basis).

Power Plants	Fuel Type	Proximate Analysis (%)				Ultimate Analysis (%)						Calorific Value (MJ·kg-1)
		Moisture	Volatile Matter	Ash Yield	Fixed Carbon	Carbon	Hydrogen	Nitrogen	Total Sulphur	Chlorine a	Chromium a
#1 (No. = 4)	Gangue	2.89 ± 0.42	18.77 ± 0.27	45.15 ± 2.83	33.19 ± 2.87	47.24 ± 3.43	3.21 ± 0.13	0.85 ± 0.05	0.45 ± 0.07	183 ± 41	85.6 ± 12.2	19.29 ± 1.28
#1 (No. = 4)	Coal slime	9.06 ± 3.18	17.53 ± 0.63	43.94 ± 4.18	29.47 ± 1.69	41.86 ± 1.96	2.96 ± 0.15	0.82 ± 0.03	0.29 ± 0.04	254 ± 47	101.5 ± 9.6	17.03 ± 0.79
#2 (No. = 4)	Bituminous	0.66 ± 0.14	15.05 ± 0.72	39.56 ± 2.22	44.72 ± 1.76	51.21 ± 1.72	3.04 ± 0.07	0.85 ± 0.04	2.50 ± 0.27	261 ± 25	68.6 ± 5.4	20.74 ± 0.53
#3 (No. = 4)	Bituminous	1.22 ± 0.11	15.96 ± 1.14	45.73 ± 1.94	37.09 ± 1.18	47.04 ± 3.34	3.04 ± 0.20	0.79 ± 0.06	1.37 ± 0.17	227 ± 61	95.6 ± 2.5	19.19 ± 1.32
#4 (No. = 4)	Bituminous	1.08 ± 0.13	18.07 ± 1.70	30.68 ± 2.85	50.17 ± 4.50	62.38 ± 2.86	3.19 ± 0.32	1.07 ± 0.08	0.64 ± 0.06	198 ± 27	40.3 ± 7.0	24.47 ± 1.19
#5 (No. = 3)	Anthracite	1.43 ± 0.20	8.77 ± 1.21	38.08 ± 8.00	51.73 ± 9.01	52.56 ± 12.81	3.64 ± 0.70	1.18 ± 0.28	3.41 ± 0.86	169 ± 56	39.5 ± 3.7	21.89 ± 4.96
#6 (No. = 6)	Bituminous	1.26 ± 0.13	22.44 ± 0.90	32.27 ± 1.96	44.03 ± 1.51	57.73 ± 1.65	4.18 ± 1.07	1.19 ± 0.44	3.82 ± 0.52	499 ± 10	44.9 ± 2.5	24.25 ± 1.29
Min–Max		0.66–9.06	8.77–22.44	30.68–45.73	29.47–51.73	41.86–62.38	2.96–4.18	0.79–1.19	0.29–3.82	169–499	39.5–101.5	17.03–24.47
Mean ± SD		2.51 ± 2.75	16.66 ± 3.89	39.34 ± 5.64	41.49 ± 7.84	51.43 ± 6.43	3.32 ± 0.41	0.96 ± 0.16	1.78 ± 1.35	256 ± 104	68.0 ± 24.8	20.98 ± 2.55

Note: ^a unit for chlorine and chromium is in mg·kg^-1.

. Fuel used in the CFB boiler (CFPP#1) was characterized by a higher moisture content (2.89–9.06%), higher ash yield (43.94–45.15%), lower fixed carbon (29.47–33.19%), and lower calorific value (17.03–19.29 MJ·kg⁻¹) compared to CFPPs #2–#6 with PC. In general, the coal quality used in PC boilers (moisture content: 0.66–1.43%; ash yield: 30.68–45.73%; fixed carbon: 37.09–51.73%; calorific values: 19.19–24.47 MJ·kg⁻¹) was better than that of the CFB boiler. Moreover, the total sulfur in the feed coal of CFPPs #2, #3, #5, and #6 were as high as 1.37–3.82% (average: 2.78%), which are grouped into medium-to-high sulfur content (2.01–3.00%) according to GB/T 15224.2-2010 [42]; these values were much higher than those of CFPPs #1 and #4 (0.29–0.64%). The calorific value of the feed coals of the six CFPPs ranged from 17.03 to 24.47 MJ·kg⁻¹ with an average of 20.98 MJ·kg⁻¹, which were classified into low-to-middle calorific coal (16.7–21.3 MJ·kg⁻¹) according to GB/T 15224.3-2010 [43]. In addition, the coal used in the six CFPPs belonged to medium-to-high ash coal (average: 39.34%) according to GB/T 15224.1-2018 (30.01–40.00%) [44]. The range of chlorine in the feed fuels was 169–499 mg·kg⁻¹ with an average of 256 ± 104 mg·kg⁻¹, which was close to the national average (255 mg·kg⁻¹, [45]) and belonged to ultra-low (<500 mg·kg⁻¹) chlorine coal according to GB/T 20475.2-2006 [46]. Overall, the feed fuels featured high ash yield, low–medium calorific value, and with high sulfur contents for some, with generally poor coal quality.

Cr concentrations in the feed fuel of the six CFPPs ranged from 39.5 to 101.5 mg·kg⁻¹, with higher values for CFPP #1 and #3 (85.6–101.5 mg·kg⁻¹) than other CFPPs (39.5–68.6 mg·kg⁻¹). The average Cr concentration in the feed fuel of the six CFPPs was 68.0 ± 24.8 mg·kg⁻¹, about four times higher than the global average of 16 mg·kg⁻¹ [47] and Chinese coal (15.4 mg·kg⁻¹, [45]) and twice that of Western Guizhou coal (32 mg·kg⁻¹, [29]) (Figure 3), but similar to the arithmetic average of Guizhou’s coal (59.7 ± 61.0 mg·kg⁻¹, No. = 107, internal unpublished data) (Figure 1). To disclose the enrichment of Cr in the feed coals of these CFPPs, a ratio of Cr concentration in the feed coal to the national average (15.4 mg·kg⁻¹, EF1) or the provincial average (59.7 mg·kg⁻¹, EF2) was calculated [5]. If the EF was less than 0.5, the concentration level was considered to be low, whereas if 0.5 < EF < 2, it was considered to be at a normal level, and EF > 2 indicated a high concentration level [5]. Cr in the feed coal of all investigated CFPPs demonstrated high EF1 values of 2.6–6.6, indicating the obvious enrichment of Cr compared to the national average. The EF2 values were normal (FE2 = 0.7–1.7) regarding Guizhou’s average, meaning the feed coals of these CFPPs were highly representative of Guizhou’s coal. Cr in coal is closely associated with ash-forming minerals, such as clay [48], as demonstrated by the significantly positive correlations between Cr concentration and the ash yield of feed fuel (Figure 4a). Hence, due to a greater ash yield in the feed fuel in the present study (39.34 ± 5.64%) compared to the national average (16.85%) [26], much higher Cr contents were found in the feed fuels versus the national average.

In addition, Cr in feed fuels was found to be closely related (p < 0.01) to vanadium and nickel (Figure 4b,c), indicating these elements have similar geochemical constraints during the coal-forming process. In contrast, sulfur showed an opposite trend with Cr in feed coal (Figure 4d), suggesting that sulfide compounds are not primary constituents of Cr in feed fuel. Other research also indicated similar close relationships between Cr and ash yield, iron, and vanadium in coals obtained from nine provinces in China [16].

3.2. Cr in Bottom Ash, ESP/ESP–FF Fly Ash, Limestone, and Gypsum

Cr concentrations in different solid samples (bottom/fly ash, limestone, and FGD gypsum) collected from the post-combustion process are illustrated in

Table 3. Cr concentration in bottom ash, ESP/ESP–FF fly ash, limestone, and gypsum.

CFPPs	#1 (No. = 4)	#2 (No. = 3)	#3 (No. = 3)	#4 (No. = 3)	#5 (No. = 3)	#6 (No. = 3)
Bottom ash (mg·kg−1)	187.1 ± 11.2	192.4 ± 8.0	198.6 ± 4.6	550.1 ± 99.3	141.7 ± 7.2	157.3 ± 6.2
ESP/ESP–FF fly ash (mg·kg−1)	195.1 ± 5.2	155.1 ± 3.0	213.5 ± 2.1	106.0 ± 5.2	130.1 ± 7.9	136.4 ± 5.1
Limestone (mg·kg−1)	15.5 ± 0.5	2.3 ± 0.2	20.2 ± 1.4	26.9 ± 4.1	10.9 ± 1.1	n.d.
Gypsum (mg·kg−1)	38.9 ± 13.2	36.6 ± 0.7	31.2 ± 3.0	42.0 ± 1.0	24.0 ± 4.4	n.d.

n.d, no data.

. A range of Cr concentrations were found for bottom ash (142–550 mg·kg⁻¹) and ESP/ESP–FF fly ash (106–213 mg·kg⁻¹). Comparatively, the Cr concentration was identical for the paired ESP/ESP–FF fly and bottom ash for most CFPPs (#1–#3, #5–#6), and a slight fluctuation in the Cr concentration was observed between bottom ash and fly ash, possibly caused by the fact that fly ash is finer compared with bottom ash, providing more surface area for the condensation of Cr [5], therefore, Cr existing in organic/sulfide-bound form may partially volatilize and coagulate on the surface of fly ash, resulting in higher concentrations in fly ash than in bottom ash. In addition, a possibility is that bottom ash is enriched with iron oxide (e.g., chromite (FeCr₂O₄), magnetite (Fe₃O₄), and trevorite (NiFe₂O₄)) with varying amounts of Cr₂O₃ and a higher density (4.3–5.2 g·cm⁻³) than that of fly ash (1.9–2.9 g·cm⁻³) [5,15,49], thereby resulting in Cr being preferentially retained in the bottom ash. High Cr in coal ash might also stem from the grinding media or as a result of stainless steel erosion of power plant installation, which would result in high Fe-, Cr-, and Ni-containing particles [50]. Since the Fe content was not determined in this study, we checked the concentrations of Cr, V, Co, and Ni in the feed fuels, bottom ash, and captured fly ash in the investigated six and other eight CFPPs in Guizhou (internal unpublished data), as well as the ratios of Cr/Co, Cr/V, and Cr/Ni in these samples. Cr was found to only increase abnormally in the bottom ash (Table S3), with no such Cr increase in fly ash or enrichment of V, Ni, or Co in the bottom/fly ash, therefore allowing the exclusion of the boiler erosion or introduction from grinding media theories. The most probable reason for abnormal high Cr in bottom ash is the occurrence of Cr compounds (e.g., FeCr₂O₄) in the feed fuel, which are highly dense and are preferentially detained in the bottom ash during coal combustion. In summary, the partitioning of Cr during coal combustion is largely dependent on the occurrence of Cr in the feed coal [18]. The Cr concentration is either higher in the bottom ash than in the fly ash (e.g., CFPP #2, #4, #5, #6), or higher in fly ash than in bottom ash (CFPP#1, #3), and these two situations were both found in other CFPPs worldwide (Table S4). This phenomenon is different to other semi-volatile elements (such as Pb and Cd), which demonstrate consistently higher concentrations (up to seven times higher) in fly ash than bottom ash [51,52]. The astonishingly high Cr contents in bottom ash (550 mg·kg⁻¹) compared to fly ash (106 mg·kg⁻¹) in CFPP#4 may have been caused by the existence of Cr in spinel [49]; similar phenomena were observed in a Canadian CFPP and an American CFPP, where Cr was observed in the feed coal (52 mg·kg⁻¹), bottom ash (344 mg·kg⁻¹), and FF ash (192 mg·kg⁻¹) for the former case [53] and in the feed coal (195 mg·kg⁻¹), bottom ash (374 mg·kg⁻¹), and fly ash (131 mg·kg⁻¹) for the latter case [54]. Hence, coal properties, especially with regard to the occurrence of Cr, are important influencing factors for Cr redistribution between bottom ash and fly ash [6,18]. Although the boiler temperature of the present study (800–1500 °C) caused less impact on the enrichment of Cr in the bottom or fly ash, it significantly affected the enrichment/depletion of semi-volatile elements (e.g., Cd and Pb) in the fly/bottom ash [11,51,52]. In addition, such high Cr in bottom ash has not been reported in China before, suggesting the unique coal properties in Guizhou.

Both Cr concentrations in ESP/ESP–FF fly ash and bottom ash were positively correlated with the Cr in feed coal in most cases (Figure 5), suggesting that Cr in bottom ash and fly ash are dominantly inherited from the feed coal. The average Cr content in the bottom and fly ash in the present study was 197 ± 111 mg·kg⁻¹, much higher than the world average (100 mg·kg⁻¹) [47].

There was considerable overlap between the groups for Cr in a previous study [3], showing that it either belonged to Group II (partial volatile) or Group I (non-volatile). Based on Equation (1), the REI values of Cr in the bottom ash and fly ash were all close to 1 (

Table 4. Relative enrichment index (REI) values of Cr in ESP/ESP–FF fly ash and bottom ash of the six CFPPs.

Coal Combustion Products	#1	#2	#3	#4	#5	#6
Bottom ash	0.86	1.11	0.95	4.19	1.37	1.13
ESP/ESP–FF Fly ash	0.90	0.89	1.02	0.82	1.25	0.98

), indicating that it belonged to the non-volatile element (namely Group I) according to the classification by Meij [38] (Table S2). The anomalous high REI (4.19) of the bottom ash of CFPP #4 demonstrated the extra enrichment of this element. In addition, the low volatilization of Cr in this study was thought to be due to the presence of high mineral phases in coal, such as aluminosilicates, thereby depressing the volatility of Cr by chemical immobilization and competition with Cl [55], with the latter observed to be low in this study.

Compared with the Cr concentration in feed fuels, fly ash, and bottom ash, the Cr contents in limestone and FGD gypsum were relatively low, with ranges of 2.3–26.9 mg·kg⁻¹ and 24.0–42.0 mg·kg⁻¹, respectively. Generally, the Cr concentration in gypsum was higher than in paired limestone by 10–30 mg·kg⁻¹ (Table 3), indicating that some of the Cr found in the gypsum was introduced by flue gas downstream of the dust collectors.

3.3. Atmospheric Emissions and Mass Balance of Cr

Particulate-bound Cr in the stack gas of the six CFPPs was in the range of 1.4–2.2 μg·Nm⁻³, with a mean of 1.8 ± 0.3 μg·Nm⁻³ (

Table 5. Particulate-bound chromium in the stack gas of the six CFPPs and atmospheric emission factors (EMF).

CFPPs	#1	#2	#3	#4	#5	#6	Mean + SD
Cr in stack gas (μg·Nm−3)	2.0 ± 0.3	2.2 ± 0.1	2.1 ± 0.9	1.4 ± 0.0	1.9 ± 0.0	1.4 ± 0.0	1.8 ± 0.3
EMF1 (mg·t−1 coal)	12.64	17.04	14.77	13.07	20.85	10.77	14.86 ± 3.62
EMF2 (μg·(kW·h)−1)	9.81	8.02	8.16	5.25	10.80	4.28	7.72 ± 2.53
EMF3 (g·TJ−1)	0.71	0.82	0.77	0.53	0.95	0.44	0.70 ± 0.19

). There were no significant differences in the emitted Cr concentrations between CFB and PC CFPPs. Emission standards for Cr from CFPPs do not exist in China, however, the Integrated Emission Standard of Air Pollutants (GB 16297-1996) [56] enacted by the National Environmental Protection Agency of China in 1996 specified the limit of Cr as 0.08 mg·m⁻³. Compared to this regulation, Cr emission concentrations from these six CFPPs were far less than the limit. The concentration of Cr in the stack gas observed in this study was in the range determined by previous research (0.44–5.5μg·m⁻³, [24,25,31,41,57,58,59,60,61] as seen in Table S4), but was much lower than the 55–156 μg·m⁻³ observed at two CFPPs in the USA which used ESP or venture wet scrubbers (Table S4) [62]. Additionally, particulate matter discharged from the stack flue gas of CFPPs #1-6 was in the range of 10.0–14.8 mg·Nm⁻³ (average: 12.0 mg·Nm⁻³), which was slightly higher than a CFPP equipped with an ultra-low emission device of wet ESP (0.48–4.02 mg·Nm⁻³) [63] and comparable to a CFPP installed with a low–low temperature electrostatic precipitator (<15 mg·Nm⁻³, [64]). Overall, the PM emitted from these CFPPs was lower than the national emission standard for CFPPs (30 mg·m⁻³, GB 13223-2011) [65].

Based on the input and output material flow information (Table S5) and Cr concentrations in the various samples discussed above (Table 2, Table 3 and Table 5), the Cr flow in six CFPPs was calculated (Table S6). Basically, the inputs and outputs of Cr were balanced, with an output/input ratio of 95.6–129.7% (Figure 6 and Table S6), well within the acceptable range of 70–130% [59]. The output Cr prevailing over input might be caused by the inhomogeneity of the chemical composition of the combusted fuel and the small sample size in a relative short time period (2–3 days). Feed fuel was shown to be the majority Cr input (>93.62%) (Figure 6 and Table S7) compared to limestone (0.17–6.38%) due to a relatively high Cr concentration and a large feed fuel consumption volume (Table 3 and Table S5). For the Cr output, the average proportion of Cr in the ESP/ESP–FF fly ash (78.4%) of PC boilers (CFPPs #2-6) was higher than the CFB boiler (67.0%, CFPP#1) due to the greater amount of fly ash produced by PC boilers; an opposite trend was observed for bottom ash, with 17.5% of PC boilers and 32.3% of the CFB boiler (Table S7). For CFPP #4, the share of Cr in the bottom ash (45.10%) was obviously higher than other PC boilers (7.11–15.45%) due to the high Cr concentration in the bottom ash, as discussed above (Figure 6 and Table 3). A similar partitioning share of 39% for bottom ash was observed in a PC utility boiler in the USA [54]. Cr in WFGD gypsum contributed 0.69–7.94% of the total Cr output in these CFPPs (Figure 6 and Table S7). Only 0.01–0.03% of Cr was finally emitted into the ambient atmosphere through the stack, with an atmospheric emission rate of 30–86 g Cr·d⁻¹ for the six utility boilers (Table S6). The atmospheric emission ratios of the present study were lower than CFPPs in the Ningxia Hui Autonomous Region (6.41%, [23]), North China (0.14–1.5%, [25,66]), Japan (0.421%, [31]), and Canada (0.17–1.00%, [6,58]), while comparable to CFPPs observed by Zhao et al. (<0.1%, [41]; <0.05%, [24]).

The emissions of trace elements from stationary combustion sources are affected by the occurrence of elements in fuels, transformation into vapor and particles in furnaces, and the ability of these vapors and particles to penetrate APCDs [67]. The low emission of Cr in this study can be ascribed to the non-volatility of Cr during combustion and the high removal efficiency (over 99.9%) of ESP/ESP–FF combined with the additional removal (30–56%) by downstream WFGD for PM [68,69,70]. Therefore, most Cr (mean: 99.7%; range: 99.95–99.99%) in the flue gas was captured by these devices in this study. The removal of PM was much higher in the present study than that observed using the ESP (97%) or venture wet scrubber (99.2%) several decades ago reported by another study [62]. Therefore, the atmospheric emission factors of Cr for these utility boilers were as low as 10.77–20.85 (mean: 14.86 ± 3.62) mg Cr·t⁻¹ coal, 4.28–10.80 (mean: 7.72 ± 2.53) μg Cr·(kW·h)⁻¹, and 0.44–0.95 (mean: 0.70 ± 0.19) g Cr·TJ⁻¹ (Table 5). Compared with other onsite investigations (Table S4), the emission factors of Cr (14.86 ± 3.62 mg·t⁻¹ coal; 7.72 ± 2.53 μg·(kW·h)⁻¹; 0.70 ± 0.19 g·TJ⁻¹) of this study were comparable or slightly higher than a CFPP in the Netherlands with EMFs of 3.4 μg·(kW·h)⁻¹ and 0.38 g·TJ⁻¹ [40] and a CFPP in Japan (1.68 μg/(kW·h), [31]), and other Chinese CFPPs (3.81–10.71 mg ·t⁻¹ coal; 0.25–0.52 g·TJ⁻¹ [24,41,59,61], Table S4), which were all observed in field studies.

Coupled with the atmospheric emission factor and the activity level in Guizhou, such as coal consumption by CFPPs [27], the atmospheric emission of Cr from this source in Guizhou was estimated to be 981 kg·y⁻¹ (P10-P90: 773–1250 kg·y⁻¹) in 2017. However, this figure is around one-tenth of that estimated by other researchers for Guizhou’s CFPPs in 2010 (9.51 tons/yr) [22], and this may be due to the lower dust removal for ESP (98.54%) and FF (95.13%) adopted by them [22] and a further reduction of 81% in PM emissions from Chinese CFPPs in recent years [70].

Apart from the atmospheric emissions, about 3950 tons of Cr was estimated to be present in the different solid by-products of CFPPs in Guizhou in 2017, including 747 tons of Cr in bottom ash, 3045 tons of Cr in ESP/FF fly ash, and 159 tons of Cr in gypsum. During coal combustion, up to 43% of Cr(III) is transformed into Cr (VI) [20], therefore, Cr in solid coal combustion products should be paid more attention since Cr in the leachate of coal ash is “nearly 100 percent hexavalent Cr(VI)”, as observed in the USA [71]. In North China, 20–30% of total Cr in fly ash and FGD gypsum is exchangeable [5,72], and Cr in some coal ash leachate (up to 77 ng·mL⁻¹) exceeds Chinese underground water limits (50 ng·mL⁻¹) [23]. The situation may be even worse in Guizhou, both due to the higher Cr concentration in the bottom/fly ash and the lower pH in the precipitation (range: 4.6–6.9; mean: 5.4) [73] in Guizhou than that of North China (e.g., mean pH 6.73 in rural Beijing) [74].

4. Conclusions

Based on the onsite investigation, the behaviors and atmospheric emissions of Cr from six utility boilers in Guizhou Province was investigated. The results showed that, due to the high ash yield (31–46%) and the affinity of Cr with ash-forming minerals, the Cr in feed fuels of this study (mean: 68 mg·kg⁻¹) were approximately four times the national average (15.4 mg·kg⁻¹). Cr concentrations in bottom ash and ESP/ESP–FF fly ash were roughly the same, with the exception of much higher (550 mg·kg⁻¹) Cr in bottom ash than fly ash (106 mg·kg⁻¹) found for one CFPP (#4); probably due to the occurrence of Cr in chromite (FeCr₂O₄) in feed fuels. Cr in limestone was relatively low (2.3–26.9 mg·kg⁻¹) but was slightly higher in the flue gas desulfurization gypsum (24.0–42.0 mg·kg⁻¹). Cr in the stack gas was in the range of 1.4–2.2 μg·Nm⁻³. Feed fuels contribute the majority (>93.62%) of Cr input, while, ESP/ESP–FF fly ash (49.00–89.50%) represents the main discharge pathway, followed by bottom ash (7.11–45.10%), gypsum (0.69–7.94%), and stack emissions (0.01–0.03%). The atmospheric emission factors of Cr were 10.77–20.85 mg Cr·t⁻¹ coal, 4.28–10.80 μg Cr·(kW·h)⁻¹, and 0.44–0.95 g Cr·TJ⁻¹. The atmospheric emission of Cr from CFPPs in Guizhou was estimated to be 981 kg·y⁻¹ in 2017. Around 4000 tons of Cr enter different coal combustion products each year. Due to the high Cr concentration in some ashes (e.g., >500 mg·kg⁻¹) and the possible conversion of trivalent Cr(III) into hexavalent Cr(VI) during coal combustion, extreme caution should be exerted regarding the treatment of such materials to prevent the possible leakage of Cr into surrounding waters and soils. In the future, the speciation of Cr in the combustion ashes/gypsum and the leachability of Cr in these coal combustion products should be studied to assess the possible environmental impacts of these materials.