Assessment of Natural Radioactivity and Trace Element Composition of Coals and Ash and Slag Waste in Kazakhstan

Abstract

This article systematizes research data on the natural radioactivity of fossil coals and of ash and slag waste from coal power engineering in the context of radioecological safety. The relatively low energy efficiency of the operating thermal power plants in Kazakhstan has a significant impact on the environment. In addition to natural radioactive elements (U²³⁸ and its decay products, Th²³² and its decay products, and K⁴⁰), coal combustion waste also contains a significant amount of trace elements that have a negative impact on the atmosphere and the environment. In Kazakhstan, about 67% of electricity is generated by coal power engineering. However, in the process of burning coals, radioactive nuclides are concentrated in ash and slag waste. In the fuel power industry of Kazakhstan, high-ash coals with low concentrations of radionuclides are mainly used. The average contents of uranium and thorium are close to the clarke values. The natural radioactivity of coal and of ash and slag waste from Karaganda GRES-1, which consumes Ekibastuz coals with an ash content of 32–39%, was studied. The average values of the specific activities of U²³⁸, Th²³², and K⁴⁰ in 25 coal samples were 27.9 Bq/kg, 19.5 Bq/kg, and 81.0 Bq/kg, respectively. In ash and slag waste, the concentrations of these radionuclides were several times higher. The concentration coefficients of the studied radionuclides varied within the ranges of 4.7–5.5 for U, 3.8–5.7 for Th, and 4.2–8.6 for K⁴⁰. It was established that during coal combustion in thermal power plants, due to carbon combustion and the removal of volatile compounds, not only natural radionuclides but also many microelements, including toxic ones (Mn, Cd, Ni, Co, Zn, etc.), are concentrated in the ash.

1. Introduction

Kazakhstan is one of the ten largest coal producers in the world market. A significant portion of electricity is generated by coal power engineering, which mainly consumes coal from the Ekibastuz and Karaganda deposits. Coals from various deposits, regardless of their age, grade, degree of metamorphism, component composition, and quality, contain natural radioactive elements of the uranium, thorium, and actinouranium series and the long-lived radionuclide K⁴⁰. According to the UN Scientific Committee on the Effects of Atomic Radiation [1], the average concentrations of natural radionuclides in coals are (16–110) Bq/kg for U²³⁸; (17–60) Bq/kg for Ra²²⁶; (11–64) Bq/kg for Th²³²; and (40–850) Bq/kg for K⁴⁰. Concentrations of radioactive elements vary widely depending on the origin of coals, their quality, and the composition of their organic and mineral components. The problem of natural radioactivity of coals in Kazakhstan and the distribution of individual radionuclides has been poorly studied. In general, coals in Kazakhstan are weakly radioactive. Concentrations of natural radioactive nuclides (U²³⁸, Th²³², Ra²²⁶, K⁴⁰) are close to the clarke values. However, there are local areas near the Shubarkol and Maikube coal deposits with increased radioactivity [2,3]. The distribution of uranium, thorium, and potassium-40 in coals in Central Kazakhstan (Ekibastuz, Karaganda) is uneven and is determined by the combined influence of many factors, including different conditions of coal accumulation.

In coal power engineering, most developed countries of the world mainly use high-quality coals with a low ash content (less than 10%). In the fuel power engineering of Kazakhstan, high-ash Ekibastuz coals (above ~35%) and Karaganda coals (above ~26%) are predominantly burned. Therefore, coal power engineering is a source of environmental pollution due to fuel combustion waste, with radioactive elements being concentrated in ash and slag waste and emissions into the atmosphere along with many rare and toxic elements [4,5,6].

Studying the forms of uranium and thorium in coals came about due to the potential radioecological hazard associated with coal power engineering. There is a long-standing idea that there is a strong connection between uranium and organic matter [7,8,9], and for coals with a below-clarke uranium content, their mineral form is typical. In the process of coalification, the ratio of uranium forms changes, and the role of mineral forms increases. The main ash-forming aluminosilicate and carbonate minerals do not play a significant role in the concentration of uranium in coals. For thorium, a significant role of organic matter in its concentration during coal accumulation has been revealed. The radionuclide K⁴⁰ is mainly found in clay rocks. Its content is more correlated with mineral mass. In general, the distribution of uranium and thorium in coals is very uneven and is determined by the combined influence of many factors: coal accumulation conditions, the component composition of rocks, the degree of metamorphism, etc.

The radiation hazard of coal power engineering is associated with pollution of the atmosphere and environment by combustion waste: fly ash emitted with flue gases and ash and slag waste. The radioactivity of the waste depends on the concentration of radionuclides in the burned coal, the forms of their presence, the method and conditions of combustion, the quality of the coal, the efficiency of catching fly ash, the volatility of elements and their compounds, etc. Ash and slag waste from thermal power plants leads to an anthropogenic radiation background and, as a consequence, to additional irradiation of personnel and the population. According to the UN Scientific Committee on the Effects of Atomic Radiation, the production of 1 GW/a year of electricity costs humanity two people/Sv of the expected effective equivalent dose of radiation [1,10].

Despite the active use of renewable energy sources, the share of coal power generation in Kazakhstan and the world will still be significant in the next three decades. This exacerbates the environmental problems created by coal-fired thermal power plants. For Kazakhstan, this problem is further emphasized by the global strategy of achieving carbon neutrality and the decarbonization of the energy sector. According to the International Atomic Energy Agency (IAEA), the global total emission of uranium and thorium from coal combustion is about 37.3 tons per year [11]. According to [12], over 750 million tons of ash and slag waste have accumulated in the ash dumps of Kazakhstan over several decades of operation of coal-fired thermal power plants. It is easy to estimate that the existing ash and slag waste in fact turns into quasi-anthropogenic deposits of radioactive elements and many rare metals, including toxic ones.

The mechanism of coal ash formation during the combustion of solid fuel in thermal power plants and the concentration of rare elements, including radioactive ones, in ash and slag waste are considered in works [13,14,15,16]. When coal is burned, due to the combustion of carbon and the removal of volatile compounds, a concentration of radioactive and other rare elements occurs in ash and slag waste, which is carried out to ash dumps. Concentration of elements is also observed in fly ash. The ratio of concentrations of radionuclides and microelements in ash and slag waste and fly ash depends in a complex way on the physical and chemical properties of the coal being burned, its ash content, the degree of carbonization, the volatility of elements and compounds present, the combustion conditions, and the technical and technological features of the thermal power plant.

The degree of microelements’ enrichment in combustion products depends on the form of their presence in coal, the volatility of their oxides, and the other compounds formed during combustion. Weakly volatile compounds tend to accumulate in ash and slag waste, and more volatile ones are concentrated in emissions, that is, in fly ash [17,18]. The latter include Po, Ni, Zn, As, Cu, etc. The content of natural radioactive nuclides and toxic elements largely depends on the ash content of coal [19]. There are no clearly identified patterns. However, for specific coal deposits (Ekibastuz, Karaganda), a tendency for the specific activity of radionuclides to increase with the increasing ash content of coal has been revealed. In addition to radionuclides and toxic elements, increased concentrations of rare earth elements (Ce, La, Nd, etc.) also accumulate in coal ash. Studies have demonstrated the possibility of using coal and ash and slag waste as raw materials for extracting rare earth metals [20]. Coal power waste that contains naturally occurring radioactive elements and various micro- and toxic elements has a strong anthropogenic impact on the environment. The situation is aggravated by the fact that coal is burned without radiation and hygienic control. Measures are needed to utilize ash dumps and conduct systematic environmental monitoring.

The aim of this work is to study the natural radioactivity of coals and ash and slag waste from coal power engineering and to assess their trace elements in the context of their impact on the natural environment.

2. Materials and Methods

The coal-fired power plant Karaganda GRES-1 is located on the outskirts of the city of Temirtau, Karaganda Region. Its population is about 170,000 people and it is home to the largest mining and metallurgical enterprise, Karmet JSC. Representative sampling was carried out from the coal storage point and ash dumps of this power plant. A total of 25 coal samples and 17 samples of ash and slag waste were collected and analyzed. Samples were also collected from Topar GRES-2, which is located 3 km from the village of Topar, Karaganda Region, and is part of Kazakhmys Energy LLP, and the Karaganda mines, which are part of Karmet JSC. All collected coal and ash and slag waste samples were analyzed using various instrumental nuclear radiometric methods: gamma spectrometry for natural radioactivity; neutron-activation analysis; X-ray fluorescence analysis. Gamma-spectrometric analysis was carried out in the research laboratories of EcoExpert LLP in Karaganda and at the Institute of Nuclear Physics in Almaty.

On an “Aspect” gamma spectrometer (Dubna, Russia, manufacturer OJSC “LSRM”) with a semiconductor detector (Dubna, Russia) and SpectrlineGP software, at EcoExpert LLP, samples of about 1 kg with size of −3 mm were analyzed (Figure 1); the specific activities of Ra²²⁶, Th²³², and K⁴⁰ were determined, and the duration of the analysis was 5 h.

Gamma-spectrometric analysis of samples of coal and of ash and slag waste weighing ~100 g was used to determine the concentrations of U²³⁸, U²³⁵, Ra²²⁶, Th²³², and K⁴⁰. A CANBERRA spectrometer with Genie-2000 (Dubna, Russia) software was used. The duration of the analyses (12–15 h) was selected in terms of achieving the minimum statistical measurement error. Gamma lines were used to calculate the specific activity of radionuclides: the activity of U²³⁸ was determined by the intensity of the γ-radiation of its decay products ²³⁴Th (63 keV), ²¹⁴Pb (242, 295, 352 keV), ²¹⁴Bi (609 keV), ²²⁶Ra (186.2 keV); the activity of ²³⁵U (185.7 keV) and ²²⁷Th (236 keV); the ²³²Th activity determined by ²⁰⁸Tl (583 keV), ²¹²Pb (238 keV), ²²⁸Ac (911.2 keV); and the ⁴⁰K activity (1460.8 keV).

Instrumental neutron-activation analysis of coal and ash and slag waste was performed at the IRT-T research nuclear reactor at the Institute of Nuclear Physics and the nuclear reactor of Tomsk Polytechnic University. Concentrations of natural radionuclides and various trace elements were determined in finely ground samples of −0.1 mm. The duration of the highly sensitive neutron-activation analysis (the activation time, the cooling time, the measurement time) was more than a day.

The concentration of rare and toxic metals was estimated using an X-ray fluorescence spectrometer based on an X-ray tube with a tungsten anode of various intermediate targets and a semiconductor detector from the manufacturer AspapGeo LLC of Kazakhstan (Almaty Kazakhstan). The spectra were processed automatically using AspapGeo LLC’s software. The detection limits of trace elements were found. The errors of reproducibility of gamma-spectrometric and neutron-activation analyses performed in two different laboratories were estimated (21–27%). The relative average discrepancy of gamma-spectrometric and neutron-activation analysis data for natural radionuclides was 34%. Partial spectrometric analysis of coal and ash samples was performed using an MKS-01A MULTIRAD scintillation gamma spectrometer with PROGRESS software, implementing an improved method for selecting analytical gamma lines and optimizing energy characteristics, which allowed for increased sensitivity in the analysis [21].

3. Results

In recent decades, the legislative framework has changed, and the requirements for environmental safety of fuel energy using coal raw materials have increased significantly. This circumstance necessitates a comprehensive assessment of the fuel used and of combustion waste. When studying the radiation situation in the areas of thermal power plants, it is important to assess the specific activity of natural radioactive elements in the burned coal, solid waste (ash and slag), and fly ash. Radioactivity in areas adjacent to thermal power plants sometimes exceeds not only background but even maximum permissible values [22,23,24]. This depends on many factors: the component composition and ash content of coal, the concentration of radioactive nuclides in coal, the combustion conditions and technology, the efficiency of filtration systems, etc.

In addition to radioactive elements, coal contains many trace elements. Significant amounts of them accumulate in ash and slag waste and are released into the atmosphere [6,25,26]. The use of ash and slag for economic purposes is currently limited due to the toxicity and sometimes increased radioactivity of this waste. The disposal of ash and slag waste is currently one of the most pressing problems [10]. Uncontrolled use for construction and other purposes can lead to unforeseen environmental consequences. The presence of natural radioactive elements in coal and their concentration in combustion waste suggest the need for radioecological monitoring of the radioactivity level in burned coal and ash and slag waste in areas where coal power plants are located.

Studying radioactive elements in coals and their combustion products, as well as performing risk assessment for the population living in the immediate vicinity of a power plant, is a pressing task today. Coal contains natural radioactive elements of the uranium series (Th²³⁴, Ra²²⁶, Bi²¹⁴, Pb²¹⁰, etc.) and the thorium series (Th²³², Pb²¹², Ac²²⁸, Bi²¹², etc.), as well as the long-lived isotope K⁴⁰.

In the instrumental spectra of natural gamma radiation of coal and ash, taken with a gamma spectrometer with a semiconductor detector, the main gamma lines of U²³⁸ are clearly visible: Th²³⁴ (63 keV); Ra²²⁶ (186 keV); Pb²¹⁴ (242, 295 keV); Bi²¹⁴ (609 keV); Th²³²; Pb²¹² (238 keV); Tl²⁰⁸ (583 keV); and K⁴⁰ monoline (1460 keV) (Figure 2 and Figure 3).

The intensities of the specified gamma lines served as the basis for the algorithmic processing of instrumental signals and the output of data of the concentration of natural radionuclides in coal and ash.

During these studies, the data of the radioactivity of coals and ash and slag waste were obtained (

Table 1. Results of natural radioactivity gamma spectrometry.

Series	Radionuclide	Borly	Borly	Ekibastuz	Ekibastuz
		Bq/kg
		Coal	ASW	Coal	ASW
U238	U238	26.2	68.2	17.5	33.5
	Th234	26.2	68.2	17.5	33.5
	Pb214	26.3	65.2	24.1	34.5
	Pb210	43.3	75.1	18.3	24.3
	Ra226	22.0	64.2	18.4	32.9
	Bi214	23.4	59.7	24.6	33.0
Th232	Th232	21.8	45.4	15.2	30.4
	Ac228	21.8	45.4	15.2	30.4
	Pb212	20.9	44.3	16.3	32.2
	Ra224	20.1	42.8	17.0	30.2
	Bi212	16.2	46.1	18.5	34.3
	Tl208	7.2	15.2	5.3	11.1
	K40	<23	113.0	97.0	144.0

and

Table 2. Results of instrumental neutron-activation analysis.

Deposit	Number of Samples	Sample	INAA
			U, g/t	Th, g/t	K40, Bq/kg	Th/U, g/t
Ekibastuz	11	Coal	0.8–1.5	2.6–3.6	61.4–87.6	3.3–2.4
			1.2	3.1	79.8
	9	Ash and slag waste	4.2–8.2	11.8–17.9	112.1–243	2.8–2.9
			7.1	14.9	198.6

Note: the numerator indicates the range of values; in the denominator, there is the average value.

).

The results of gamma-spectrometric and neutron-activation analyses of coals show low natural radioactivity. Concentrations of radionuclides in coals of the studied deposits are close to the world average [27]. However, regardless of the coal, the concentration of natural radionuclides U, Th, and K⁴⁰ in ash and slag waste is significantly higher than in the original coal.

Table 3. Specific activity of radionuclides.

Deposit	Name	* Ad, %	Ra226	Th232	K40
			BQ/kg
Ekibastuz	Ash		53.0–70.0	50.0–74.0	170–267
	Coal	36.0	11.2–14.9	11.7–13.0	63
Karaganda, Kostenko mine	Ash		54.0–118.0	39.0–53.0	190.0–261.0
	Coal	35.0	20.0–24.0	12.0–14.0	39.2–47.0
Borly	Ash		185.0	131.0	294
	Coal	39.9	33.5	26	34

* Note: Ad—coal ash content, %.

presents the results of coal and coal ash studies performed by gamma spectrometry at EcoExpert LLC. An analysis was made of Ekibastuz coal, selected at the Karaganda State District Power Plant, as well as coal from the Karaganda Basin (Kostenko mine) and the Borly deposit, Molodezhny open pit. The concentration of radioactive elements in ash is approximately 2–7 times higher than in coal.

The natural radionuclide distribution in ash and coal from different deposits indicates significant variability of the specific activities of Th, Ra, and K⁴⁰ and their significant concentrations in ash (Figure 4).

The nuclide concentration factors (as the ratio of the specific activity of the nuclide in ash and the original coal) depend on many factors: the completeness of coal ashing, the ash content of coal and its component composition, the form of radionuclide in coal, the degree of its carbonization, and the level of radioactivity (Figure 5). The calculated concentration factors vary significantly: the minimum is 3.8 for Th²³²; the maximum is 8.6 for K⁴⁰. The specific activities of natural radionuclides in ash and slag waste exceed their average values in soil for U and K⁴⁰ by four to six times and for Th by two to three times. This indicates that waste from coal power generation poses a potential radioecological hazard to the environment.

The most common criterion for the radioactivity of waste coal for assessing the possibility of their utilization is determined by the value of the effective specific activity of radionuclides:

A_ef = A_Ra + 1.3A_Th + 0.09A_K

(1)

where A_Ra, A_Th, and A_K are the activity of Ra, Th, and K⁴⁰ in Bq/kg.

Ash and slag waste from Topar GRES with an effective specific activity of 383 Bq/kg falls into class II of radiation safety, which allows its use in road construction within populated areas. The acceptable safety limit of 370 Bq/kg creates an annual dose of about 1.5 mSv/year for the population [28].

The International Commission on Radioecological Protection [28,29] recommends a maximum annual dose of 1 mSv.

The assessment of the potential radiation risk for the population living in the area of the coal-fired thermal power plant and ash dumps was performed based on the calculation of the gamma radiation dose rate based on the measured natural activity of the specified radionuclides [27,29].

D = 0.462 A_Ra + 0.604 A_Th + 0.042 A_K

(2)

The calculated absorbed dose rate D was 176 nGy/h, which corresponds to an equivalent dose rate of 0.18 mkSv/h. This creates an annual radiation dose of 1.6 mSv.

The data obtained are slightly higher than the world average and are comparable with the results of radioecological studies of coal in Poland [30], India [31], Serbia [32], and Malaysia [33].

When solid fuel is burned at thermal power plants, microelements are concentrated in ash and slag due to carbon burnout and the removal of volatile compounds. The degree of concentration depends on the ash content of coal, the forms of microelements in the coal, their volatility, and temperature conditions.

The impact of coal power engineering is associated not only with the concentration of radioactive elements but also with the presence of toxic metals in coal and their combustion products. In general, studying the microelement composition of coal in Kazakhstan has been conducted relatively recently.

Table 4. Results of X-ray fluorescence analysis.

No	Element	%; μg/g	Coal			ASW
			Karaganda	Borly	Ekibastuz	Ekibastuz	Borly
1	Al	%	3.82–4.61	8.11	6.41–7.78	10.45	13.15
2	Si	%	2.56–3.47	15.79	14.33–17.92	26.79	25.78
3	Fe	%	0.5–0.9	0.7	1.2–2.3	9.1	2.3
4	Cu	μg/g	48.0–50.0	38.0	37.6–52.0	73.0	68.0
5	Zn	μg/g	33.4–94.8	195.4	23.2–25.0	31.0	129.0
6	Pb	μg/g	152.0	<10	<10	18.5	18.0
7	Cd	μg/g	5.7–6.2	5.0	4.4–4.7	3.3	4.3
8	K	%	0.16–0.19	0.35	0.53–0.68	0.6	0.56
9	Ca	%	1.19–1.28	<0.1	0.19–1.51	1.89	0.15
10	Ti	%	0.099–0.152	0.469	0.323–0.45	0.695	0.991
11	V	μg/g	120–180	<100	100–150	110	130
12	Mn	μg/g	100–117	<100	201–365	1680	<100
13	Ni	μg/g	38–64	16	35–65	34	22
14	Ga	μg/g	<10	21.6	14.0–19.9	19.9	32.9
15	Rb	μg/g	11.5–13.3	<10	13.4–18.1	16.3	18.3
16	Sr	μg/g	2520–3941	85.0	187.1–276.2	375.0	151.6
17	Y	μg/g	23.8–25.1	37.8	22.6–24.7	46	61
18	Zr	μg/g	30.8	236.2	125.0–167.3	265.6	403.8
19	Mo	μg/g	2.0–3.0	4.8	4.2–5.7	3.5	5.5
20	Co	μg/g	6.9–8.0	15.9	4.7–7.2	13.5	59.0

presents the results of X-ray fluorescence elemental analysis of coals and ash and slag waste. During combustion, microelements are concentrated in ash and slag waste. Their concentration in ash and slag waste is usually higher than that in the original coal.

Metals with higher boiling points (Pb, Cd, Zn, Cr, Ni) often remain in the solid phase or precipitate in ash and slag. This is caused by their low volatility and tendency to form stable oxides or compounds that condense on ash residue particles. Although lead is partially volatile, in our studies most of it precipitated in ash. Cadmium is a volatile element and can evaporate during combustion, and it often condenses and precipitates in ash, which is shown by our results. The obtained Cd concentrations (4.4–4.7 μg/g) exceed the toxicity threshold by three to five times [34]. Nickel in coal is mainly present in the form of mineral compounds, and during combustion, these compounds remain in coal as oxides or sulfates. Lead and nickel are metals with a high boiling point, which is confirmed by their presence in ash.

Our study confirmed that Zn is a volatile metal (in Ekibastuz, it is in the range of 23.2–48.0 μg/g in coals and in an amount of 31.0 μg/g in ash). As for Mn, its concentration in ash and slag waste (ASW) is five times higher than its concentration in coal. These data are comparable with studies of Chinese coals [25].

In addition to toxic metals, rare metals were studied. The concentration of titanium varies greatly in coals from different deposits in Kazakhstan. Karaganda coal had the maximum concentration of vanadium in coal of 120.0–180.0 μg/g. In the samples from the Ekibastuz basin, the concentration of vanadium was 100.0–150.0 μg/g in coal and 110.0 μg/g in ash and slag waste, which exceeds the toxicity threshold [34].

Thus, the distribution of trace elements in coals and combustion waste is determined by their physical and chemical properties and the combustion conditions, which lead to different degrees of their concentration in ash and slag waste. These features must be taken into account when developing emission cleaning systems and managing coal combustion waste to minimize their impact on the environment. To assess the impact of elements on the environment, enrichment factors were calculated that show how elements behave during coal combustion [31]. The enrichment factor in relation to Fe was calculated using the following formula:

$$\mathrm{REI}={\displaystyle \frac{\mathrm{Concetration}\mathrm{of}\mathrm{element}\mathrm{in}\mathrm{ash}/\mathrm{Concetration}\mathrm{of}\mathrm{Fe}\mathrm{in}\mathrm{ash}}{\mathrm{Concetration}\mathrm{of}\mathrm{element}\mathrm{in}\mathrm{coal}/\mathrm{Concetration}\mathrm{of}\mathrm{Fe}\mathrm{in}\mathrm{coal}}}$$

(3)

The relative enrichment factor relates trace metal concentration in combustion product to its concentration in coal.

Cobalt and copper have a high enrichment factor relative to Fe; the lowest factor is typical for Cd, Ni, and Zn; and chromium and manganese are in the average enrichment range (Figure 6).

Thus, coal power plants are powerful sources of environmental pollution in the form of radioactive elements and microelements, including toxic ones. To reduce their impact on the environment, it is necessary to provide measures for the disposal of ash dumps.

4. Conclusions

Studying the coals from Kazakhstan (Ekibastuz, Karaganda, Borly) has shown their low radioactivity. Concentrations of natural radioactive elements in the studied coals are close to clarke concentrations. In ash and slag waste from coal power engineering, concentrations of radioactive nuclides increase by three to eight times compared to the original coal. Not only radioactive elements but also many microelements accumulate in solid fuel combustion products. In Kazakhstan, coal products are not standardized for radiation and environmental safety. The existing radiation safety standards only limit the use of ash and slag waste for construction purposes. Ash and slag dumps from Kazakhstan thermal power plants that occupy vast territories are transformed into quasi-anthropogenic deposits of natural radioactive elements and various microtoxic elements. Ash and slag waste with an effective specific activity of radionuclides of 383 Bq/kg fall into radiation safety class II. The absorbed dose rate calculated based on the specific activity of radionuclides as a potential radioecological risk for the population in the ash dump area was 176 nGy/h, which creates an annual radiation dose of 1.6 mSv. In the areas where coal power plants are located, the systematic monitoring of the radioactivity level in coal and ash and slag waste is needed. Comprehensive measures are needed to reduce emissions of trace elements into the environment and to reclaim (to utilize) coal ash dumps.