Ash Properties and Environmental Impact of Coal and Its Blend with Patent Fuel for Climate Sustainability

Abstract

Fossil fuels are still widely used today, and exploring more sustainable ways of using coal is crucial. One promising approach is to develop a patented fuel with reduced harmful gas emissions during combustion. This study aims to investigate the properties of the ash produced by burning steam coal mixed with various ratios of patent fuel. The combustion process was carried out using a standard solid fuel boiler with a power output of 70 kW. The ash samples were analyzed using various analytical methods, and ash leaching tests were conducted. The study found that adding patent fuel to coal affects combustion and ash composition. Determining the thermal stability of ash samples showed that adding patent fuel to coal results in decomposition starting earlier and in stronger combustion. The ash produced by patent fuel–coal mixtures contains lower concentrations of Ba, Cr, Ga, Li, Mn, V, and Zn than pure coal combustion. Leaching tests showed that coal ash leachates had higher concentrations of environmental pollutants, such as As, Cd, Co, Mn, Mo, Sb, and U, than patent fuel leachates. Adding patent fuel to coal affects combustion, ash properties, emissions, and disposal. Understanding these implications can help to develop sustainable coal usage and reduce environmental impact.

1. Introduction

Fossil fuels have been the primary source of energy for many years. Coal remains the most significant energy source for electricity generation, steelmaking, and cement production, playing a significant role in the global economy [1]. In 2022, the demand for coal reached its highest level ever, with over 8 billion tons of coal being mined in more than 40 countries annually. Currently, coal accounts for nearly 40% of the world’s electricity generation. It has become the preferred source for power generation due to its stability, reliability, low cost, easy logistics, and wide availability, making it a highly secure energy source [2].

However, coal-fired power plants are responsible for approximately 80% of the energy sector’s CO₂ emissions. Large city-scale coal-fired combined heat and power (CHP) plants are the biggest contributors to greenhouse gas emissions [3]. While eliminating coal from the energy consumption mix is a viable solution for achieving environmental sustainability, it may also lead to a decline in the economic growth and development of many countries globally [4]. Therefore, the goal is to find a feasible way to reduce the contributions from both environmental and economic perspectives.

The biggest challenges of the 21st century are fighting climate change and achieving a clean and just energy transition, and coal is one of the sectors that needs to be prioritized for energy transition. These are also the main priorities in Europe’s new green, sustainable growth strategy, known as the “European Green Deal” [5]. The EU has set the targets of cutting CO₂ emissions by at least 55% by 2030 and becoming climate-neutral by 2050, and substantial efforts will be required to achieve these international climate goals. While coal is still a primary fuel in the European energy mix, transitioning to cleaner forms of energy and innovative technologies, such as carbon capture and storage, is crucial in order to meet the EU’s commitments [6].

The world is currently facing an energy crisis that began in 2021 due to the rapid economic recovery after the pandemic, which was further intensified by Russia’s invasion of Ukraine in February 2022. Additionally, unexpected weather conditions led to lower hydro and nuclear power generation, which caused a surge in coal usage [7]. As a result, energy efficiency has become a crucial factor in energy management. Natural gas prices have surged to record levels, leading to an increase in electricity prices in some markets. Furthermore, oil prices are at their highest point since 2008 [8]. These high energy prices have also resulted in a sharp rise in inflation. Households are turning to coal as a source of heat, and high energy and gas prices have pushed many families into poverty. The situation has forced some factories to cut production or even shut down, and economic growth has slowed to the extent that some countries are on the verge of a severe recession.

Finding alternative raw materials and techniques that can reduce the use of natural aggregates and facilitate the move towards cleaner forms of energy is becoming increasingly important. Coal ash and desulfurization gypsum are waste products of coal mines and coal-fired power plants. One possible solution is to use bottom ash and fly ash as a partial or complete substitute for natural aggregates [9,10]. These materials are obtained at a minimal cost and are byproducts of coal processing, which is a more sustainable alternative to the resource-intensive processes involved in mining and processing natural aggregates. Studies have shown that these materials have significant potential in building and construction activities. Utilizing these waste products can lead to comprehensive resource utilization, reduce dependence on natural resources, and address the waste disposal problem.

One of the main reasons for the decreasing interest in coal usage in thermal power plants is its unfavorable environmental impact. The negative environmental effects of energy plants result from gaseous and particulate emissions, wastewater emissions, and liquid and solid waste production [11,12,13,14]. Nowadays, there are numerous technological possibilities for using coal in the industry in an environmentally acceptable way, including the choice of coal; temperature control in the combustion process to decrease volatile inorganic compounds emissions; the removal of pollutants before they enter and during the process; and the implementation of different working conditions during combustion to change the combustion products’ composition [15]. It is important to note that there is growing concern about safety conditions during coal mining and usage [16]. Important issues include paying attention to the characteristics of spontaneous coal combustion in order to prevent disasters that can harm production and usage safety and lead to significant losses [17].

Numerous papers and studies have revealed global problems connected to fly ash manipulation and disposal. It has been estimated that the world’s ash production is more than 550 Mt per year, and only a small portion of this is used for further commercial applications, while the majority ends up in waste disposal sites [18].

Many authors have described various applications of analytic instruments to determine ash characteristics from coal combustion, one of the most complex areas in the research of natural rocks and anthropogenic products. They highlighted the importance of understanding the behaviour of chemical species produced during coal combustion with regard to their impact on the environment, emissions, and transport. The complicated composition, small size, and variable morphology of particles cause problems in the identification, characterization, and data interpretation of ash [19,20,21,22,23,24,25,26,27,28].

J. Marrero et al. [29] highlighted that the determination of metals in ashes is a research area of permanent interest and is usually the first step in the subsequent evaluation of the associated environmental and biological risks. In their work, they have used different analytical approaches and instrumental techniques to provide morphological information, enrichment factors, and element concentrations regarding fly ashes.

N. Koukouzas et al. emphasised that the physical and chemical characteristics of fly ash, combined with the operational parameters of the power plant and the disposal environments in which the ashes are placed, control the susceptibility of these wastes to leaching and determine the potential for the contamination of groundwater aquifers [30].

An overview of the leaching behavior of elements from coal combustion fly ash provides insight into the factors that determine the leachability of elements and addresses the causes of their mobility. M. Izquierdo and X. Querol [31] found that the way in which a particular element occurs in the parent coal plays a crucial role in the leaching behavior of the fly ash. Additionally, the mobility of most elements in the ash is highly sensitive to pH.

According to N. Moreno et al. [32], leaching tests are very important for determining coal ash usage applications and their limitations because of the relatively high levels of leachable trace elements. Their study revealed that elements such as As, B, Ba, Cr, Mo, Se, and V, which have environmental significance, showed the highest mobility during the leaching tests.

The present study aims to investigate the influence of adding a patent fuel to steam coal on the properties of the resulting ash after combustion. Therefore, ash characterization and leaching tests were conducted utilizing various analytical techniques to determine the ashes’ chemical and phase compositions in order to identify potential applications.

What made this study unique was the use of a patent fuel in addition to coal and the comparison between ashes after the combustion of pure steam coal and mixtures and different ratios of coal and patent fuel.

2. Materials and Methods

2.1. Solid Fuels

Steam coal and patent fuel were used for the purpose of this study. The patent fuel was HYDROGEN INDUSTRIAL (HI fuel) (International Application Number: PCT/HR2010/000031; International Publication Number: WO 2012/032363 A1). According to the published literature, the gross calorific value of this HI fuel is about 95.69 MJ/kg [33,34].

The proximate and ultimate analyses of used coal, as well as its calorific value, are presented in

Table 1. Proximate and ultimate analyses of used coal.

Parameters	Test Methods	Results
Surface moisture	ASTM D 3302	2.96%
Inherent moisture	ASTM D 3173	3.25%
Total moisture	ASTM D 3302	6.21%
Ash content	ISO 1171	10.04%
Volatile matter	ISO 562	34.06%
C-fix	by calculation	49.68%
Total sulphur	ASTM D 4239	1.43%
C content	ASTM D 5373	70.00%
H content	ASTM D 5373	5.18%
N content	ASTM D 5373	1.42%
O content	by calculation	5.72%
Gross calorific value	ISO 1928	27.97 MJ/kg
Net calorific value	ISO 1928	26.69 MJ/kg
Emission factor	EC 2018/2066	96.17 tCO2/TJ

. All results are on an as-received basis.

The combustion experiments were carried out at the 70 kW boiler at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia. Details of combustion processes are described by Dobrovic et al. [35].

2.2. Ash Samples

Table 2. Ash samples from combustion in the experimental combustion chamber.

No.	Sample Description	Masss (Sample)/g	Day of Experiment
1.	Bottom ash after hard coal combustion	500	1st day of combustion
2.	Bottom ash Mixture: packages with a ratio of 0.44 kg hard coal + 10 ampoules	500
3.	Bottom ash Mixture: packages with a ratio of 0.54 kg hard coal + 8 ampoules	500
4.	Bottom ash from combustion chamber, after cleaning the combustion remains from the previous day.	500	2nd day of combustion
5.	Bottom ash Mixture: packages with a ratio of 0.54 kg hard coal + 8 ampoules	500
6.	Bottom ash Mixture: packages with a ratio of 0.44 kg hard coal + 10 ampoules	500
7.	Samples from the filter: 3 quartz filters with ash and soot from the flue gases—after hard coal combustion.	0.06
8.	Samples from the filter: 2 quartz filters with ash and soot from the flue gases—after combustion of 0.54 kg hard coal + 8 ampoules.	0.02
9.	Samples from the filter: 1 quartz filter with ash and soot from the flue gases—after combustion of 0.44 kg hard coal + 10 ampoules.	0.06

presents the ash samples obtained during the experiment. However, the analysis of the operating parameters of the experimental boiler indicated that the best, most consistent, and representative results were obtained on the second day of the test. Therefore, more representative ash samples were collected on that day. Based on this, only three ash samples from the bottom of the combustion chamber were selected (as described in

Table 3. Ash samples for characterization and leaching tests.

Samples	Description
Sample 1	Ash sample after combustion of pure steam coal
Sample 2	Ash sample after combustion block of 0.54 kg steam coal + 8 ampoules of patent fuel
Sample 3	Ash sample after combustion block of 0.44 kg steam coal + 10 ampoules of patent fuel

), which best represent the combustion products of pure coal, along with two coal mixtures with different proportions of patent fuel for complete ash characterization and leaching procedures. These samples are marked in Table 2 as 1, 5, and 6.

Other coal and patent fuel ratios did not show significant improvement during combustion, and those ash samples would not contribute to any conclusions about the impact of the addition of patent fuel to coal combustion and their residues. The number of fly ash samples from dust filters was insufficient to carry out all the characterization and leaching tests.

The procedure of preparing the ash analytical samples included grinding using the Cross-Beater Mill, Type SK 100R, dividing by sample divider PT 100, and sieving to a particle size less than 212 µm, with the AS 200Basic, DIN ISO 212 Micron Sieve. The ash samples for the characterization and leaching tests were additionally sieved to a particle size less than 50 µm using the RetschAS 200Basic and DIN ISO 50 Micron Sieve, Retsch, Verder Scientific, Haan, Germany.

2.3. Characterization of Ash Samples

Elemental CHNS analysis of ash samples was carried out using a LECO TrueSpec CHNS analyzer. The calorific values of the ash samples were obtained using the IKA C2000 calorimeter in an oxygen atmosphere.

The compositions of the ash samples were determined using an energy-dispersive X-ray fluorescence spectrometer (EDXRF): Oxford ED2000. Samples were prepared by pelletizing at 15 tonnes of pressure for 30 s with a P/N 25011, Specac manual hydraulic press, with Hoechst wax C micro powder (Merck) as a binder. The method was developed according to the standard ASTM D 4326 [36]. The calibration, validation, and estimation of measurement uncertainty were described in our earlier work [37].

For the determination of trace elements, a microwave-assisted acid digestion (MW-AD) was applied, followed by inductively coupled plasma-optical emission spectrometry (ICP-OES) [38,39]. A mixture of 9 mL HCl + 3 mL HNO₃+ 3 mL HF was employed in the MW-AD, after which 25 mL 4% H₃BO₃ was added in order to remove HF from the reaction mixture [40,41,42,43,44]. The microwave digestion was perfomed using a CEM MARSX oven with XP1500 vessels. An ICP-OES measurement was performed using a Thermo Electron Corporation ”IRIS Intrepid II“ XSP, Duo, Thermo Elemental. The mineralogical composition of the ash samples was obtained by XRD [45] using a Shimadzu XRD 600 spectrometer. The thermal stability of samples was determined by thermogravimetric (TG) and differential thermal analysis (DTA) measurements that were carried out simultaneously using a TA Instruments simultaneous TG-DTA apparatus, model SDT 2960.

2.4. Leaching Tests in Deionized Water and Acid Rain

Leaching tests were carried out by mixing the ash samples with ultra-pure water and acid rain as leachants for periods of 7 days, one month, and three months at room temperature (25 °C) [46,47,48,49]. All leaching tests were performed in triplicate.

A portion of the 0.5 g ash samples were placed in a clean plastic container, which was filled with 50 mL of leachate. After the planned leaching time, the liquid phase (leachate) was separated from the solid (ash sample) by filtration through 0.45 μm filters. After a leaching period of 3 months, the pH value of each filtrate and the conductivity of the filtrates were measured. The concentration of Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni Cu, Zn, As, Se, Rb, Se, Mo, Ag, Cd, Sb, Ba, Ce, Tl, Pb, Bi, and U ions in leachates was determined by an inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent Technologies 7500cx ICP-MS system (Agilent, Waldbronn, Germany). The instrument was equipped with an integrated auto-sampler, a Scott Quartz spray chamber, and a MicroMist nebulizer (Glass Expansion, Australia). Verification of the accuracy and precision of the proposed method was performed using the following Standard Reference Materials (SRMs): SRM 1643e (Trace Elements in Water) from the National Institute of Standards and Technology (NIST) and SLRS-5 (River Water Reference Material for Trace Metals) from the National Research Council Canada (NRCC).

The obtained data were reported as averages ± SD. Validation data are shown in

Table 4. Certified and found values in µg/kg obtained for Standard Reference Materials: River Water Reference Material for Trace Metals (SLRS-5) and Trace Elements in Water (SRM 1643a) together with detection limits (DLs) of elements (in µg/kg).

Element	DLs	SLRS-5			SRM 1643e
		Certified (Mean ± SD)	Found (Mean ± SD)	% D	Certified (Mean ± SD)	Found (Mean ± SD)	% D
Li	0.0116	-	0.28 ± 0.02	-	17.00 ± 1.70	17.22 ± 0.23	101.3
Be	0.0009	0.005 2	0.003 ± 0.001	61.9	13.64 ± 0.16	8.91 ± 0.11	65.3
Na 1	1.8102	5.38 ± 0.10	4.75 ± 0.02	88.4	20.23 ± 0.25	20.01 ± 0.82	98.9
Mg 1	0.6240	2.54 ± 0.16	2.28 ± 0.14	89.7	7.84 ± 0.09	7.42 ± 0.15	94.7
Al	0.1774	49.50 ± 5.00	50.12 ± 0.52	101.3	138.33 ± 8.40	151.69 ± 0.60	109.7
K 1	6.5270	0.84 ± 0.04	0.77 ± 0.01	91.6	1.98 ± 0.03	2.01 ± 0.03	101.1
Ca 1	4.6519	10.50 ± 0.40	9.66 ± 0.04	92.0	31.50 ± 1.10	32.77 ± 0.28	104.0
V	0.0007	0.32 ± 0.03	0.30 ± 0.01	95.0	36.93 ± 0.57	35.73 ± 0.27	96.8
Cr	0.0052	0.21 ± 0.02	0.22 ± 0.02	104.0	19.90 ± 0.23	21.21 ± 0.16	106.6
Mn	0.0053	4.33 ± 0.18	4.17 ± 0.05	96.3	38.02 ± 0.44	39.27 ± 0.29	103.3
Fe	0.2430	91.20 ± 5.80	93.71 ± 0.74	102.7	95.70 ± 1.40	98.44 ± 5.57	102.9
Co	0.0006	0.05 2	0.05 ± 0.07	102.1	26.40 ± 0.32	27.04 ± 0.10	102.4
Ni	0.0138	0.48 ± 0.06	0.50 ± 0.02	104.9	60.89 ± 0.67	62.93 ± 0.74	103.4
Cu	0.0974	17.40 ± 1.30	16.74 ± 0.06	96.2	22.20 ± 0.31	16.20 ± 0.14	73.0
Zn	0.9186	0.85 ± 0.09	0.86 ± 0.54	101.6	76.50 ± 2.10	93.22 ± 10.87	121.9
As	0.0016	0.41 ± 0.04	0.40 ± 0.01	97.3	58.98 ± 0.70	61.48 ± 1.36	104.2
Se	0.0029	-	0.04 ± 0.00	-	11.68 ± 0.13	11.32 ± 0.46	96.9
Rb	0.0010	-	1.19 ± 0.01	-	13.80 ± 0.17	13.80 ± 0.13	100.0
Sr	0.0071	53.60 ± 1.30	52.92 ± 0.51	98.7	315.20 ± 3.50	331.66 ± 2.50	105.2
Mo	0.0278	0.50 2	0.38 ± 0.04	75.6	118.50 ± 1.30	119.75 ± 1.04	101.1
Ag	0.0007		0.01 ± 0.00	-	1.04 ± 0.07	0.78 ± 0.15	75.2
Cd	0.0002	0.006 ± 0.001	0.007 ± 0.000	123.3	6.41 ± 0.07	6.39 ± 0.06	99.7
Sb	0.0011	0.30 2	0.30 ± 0.01	100.1	56.88 ± 0.60	58.27 ± 0.63	102.4
Ba	0.0400	14.00 ± 0.50	13.87 ± 0.12	99.0	531.00 ± 5.60	554.59 ± 5.67	104.4
Ce	0.0005	-	0.31 ± 0.02	-	-	0.76 ± 0.02	-
Tl	0.0062	-	0.06 ± 0.00	-	7.26 ± 0.09	7.12 ± 0.30	98.1
Pb	0.0030	0.08 ± 0.01	0.09 ± 0.01	116.6	19.15 ± 0.20	18.09 ± 0.69	94.5
Bi	0.0033	-	0.03 ± 0.00	-	13.75 ± 0.15	12.10 ± 0.51	88.0
U	0.0008	0.10 2	0.12 ± 0.03	116.2	-	0.001 ± 0.000	-

SD—standard deviations of measurements; DL—detection limits of method; %D- percentage difference between the observed concentration and the expected value; ¹ certified and found values are expressed in mg kg⁻¹. ² The values shown are not certified, but included for informational purposes only.

.

3. Results and Discussion

3.1. Chemical and Mineralogical Characterization

The elemental analysis results and calorific values provide information about the boiler’s combustion efficiency. These results are given in

Table 5. Elemental analysis and calorific values of the ash samples.

Sample	wt.(S)/%	wt.(C)/%	wt.(H)/%	wt.(N)/%	Gross Calorific Value/MJ/kg
Sample 1	0.85	59.80	0.48	1.04	20.60
Sample 2	1.37	63.07	0.48	1.17	21.66
Sample 3	1.44	64.80	0.31	1.06	21.47

.

Elemental analysis (Table 5) showed that ash samples obtained from the combustion of a mixture of coal and patent fuel (samples 2 and 3) have higher sulphur content, indicating the patent fuel’s ability to attach sulphur from combustion flue gases to ash. The high gross calorific values of all samples indicate that the combustion in the boiler was uncompleted.

The content of the main and minor elements in the ash samples determined by EDXRF is presented in

Table 6. Main and minor element content of the ash samples (wt.,%).

Sample	TiO2	Fe2O3	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	K2O	CaO
Sample 1	1.28	6.45	0.7	1.8	18.21	54.62	1.42	7.4	1.88	5.94
Sample 2	1.16	6.09	1.32	1.89	15.1	48.42	1.28	8.15	1.84	14.49
Sample 3	1.50	4.6	1.2	1.92	15.91	44.09	1.24	8.01	1.64	19.62

.

The greatest difference in the composition of these ash samples is in the CaO content, which is considerably higher in ash samples from the combustion of mixtures than in ash samples from pure steam coal. The concentration of CaO is associated with calcium, which exists in the composition of patent fuel.

The contents of trace elements were determined with inductively coupled plasma-optical emission spectrometry (ICP-OES).

Table 7. Concentration of trace elements in ash samples (mg/kg).

Element	Sample 1	Sample 2	Sample 3
	X ± SD	X ± SD	X ± SD
As	<QL	<QL	<QL
Ba	1053.34 ± 30.47	321.78 ± 11.34	312.76 ± 6.73
Cd	7.87 ± 0.19	7.63 ± 0.59	7.57 ± 0.10
Co	21.50 ± 0.41	17.22 ± 0.87	20.19 ± 0.65
Cr	242.88 ± 2.39	127.12 ± 6.84	113.62 ± 2.92
Cu	76.28 ± 1.06	55.48 ± 3.60	47.55 ± 1.29
Ga	343.81 ± 0.96	126.23 ± 6.48	162.39 ± 1.66
Li	110.78 ± 7.11	70.59 ± 3.73	75.10 ± 0.15
Mn	320.67 ± 2.47	114.81 ± 6.16	97.04 ± 1.52
Ni	154.69 ± 1.72	134.34 ± 5.70	117.36 ± 3.25
Pb	48.58 ± 3.31	35.46 ± 2.23	41.80 ± 7.70
Se	3.29 ± 1.16	3.05 ± 1.16	3.30 ± 0.47
Sr	402.52 ± 9.19	341.96 ± 13.85	408.10 ± 3.68
Tl	<QL	<QL	<QL
V	420.19 ± 2.90	207.14 ± 9.39	277.96 ± 3.61
Zn	93.51 ± 1.08	27.98 ± 2.03	27.23 ± 1.13

X—mean value of measurements; SD—standard deviation of measurements; QL—limit of quantification.

represents the results obtained for trace element concentrations in ash samples.

The highest measured concentrations in all ash samples were for Ba, V, and Sr. The lowest concentration was the Se content. The concentrations of As and Tl were below the quantification limit of the method used. After comparing the trace element content in the pure coal ash (sample 1) with ash samples obtained from mixtures of coal and patent fuel (samples 2 and 3), it was found that there were significant differences in the levels of Ba, Cr, Cu, Ga, Li, Mn, Ni, Pb, V, and Zn. In general, the levels of all trace elements were lower in the ash from mixtures than in the pure coal ash, where the highest differences were observed for Ba, Cr, Ga, Li, Mn, V, and Zn.

The phase composition of the investigated samples was determined by qualitative X-ray diffraction (XRD). Many mineral phases were identified: quartz, cristobalite, hematite, rutile, and portlandite. The XRD patterns of ash samples from the combustion of a mixture of coal and patent fuel show higher diffraction maximum intensities for calcite, anhydrite, and lime, which is in accordance with earlier determinations of a higher calcium content in ash samples of mixtures with patent fuel (Table 5). The identified mineral phases are presented in Figure 1.

Figure 1 clearly shows that the contents of quartz (SiO₂) and anhydrite (CaSO₄) were very similar in all samples. Different relative intensities of diffraction maximums were observed for the other mineral phases, especially calcite (CaCO₃), which showed the most significant difference between all XRD patterns.

3.2. Thermal Analysis

Thermogravimetric measurements were obtained in the temperature interval from room temperature (25 °C) up to 1200 °C, which allowed for the identification of the main changes in the mass of the samples according to temperature increases.

The results obtained from the TGA/DTA characterization of the pure coal ash sample—sample 1 (Figure 2a)—show three main mass losses:

• A loss of mass from the sample due to the evaporation of absorbed water during the temperature interval from room temperature up to 80 °C;
• A two-step loss of mass in the temperature interval between 80 and 320 °C due to the evaporation of chemically bonded water (water which occurs in the sample in hydrate form);
• A loss of mass due to coal oxidation and combustion in the temperature interval between 320 and 850 °C.

Several of the losses from the mass samples, which are about 10% in the temperature range up to 320 °C, concur with three endothermic peaks in the DTA curve, indicating the evaporation process of physically and chemically bound water and other volatile compounds from the sample. The exothermic peak beginning at 450 °C with a maximum of 582 °C corresponds to an 80% mass loss from the ash sample (Figure 2a). This broad exothermic peak is the result of the oxidation/combustion of pure coal ash. The exothermic peak in the DTA curve is not symmetric; the local maximum is visible at around 700 °C, indicating two steps of decomposition. Despite this, it can be seen that the DTA curve continues to increase in the range above 900 °C. However, loss from the mass sample was not detected in this area of the TGA curve. This change in energy may be due to some structural changes in the sample. Similar thermal behaviour was noticed for the ash samples that inclided mixtures with patent fuel: samples 2 and 3 (shown in Figure 2b,c). However, during the temperature interval between 320 and 1100 °C, more significant changes in the two-step decomposition of samples (Figure 3) were identified as a consequence of the presence of patent fuel in the samples. These changes were particularly emphasized in the ash sample with a higher content of patent fuel (sample 3).

A comparison of the DTA thermograms of all three samples is presented in Figure 3.

The TGA/DTA curves of the ash samples taken from the fuel mixtures exhibit minor overlapping but offer a better differentiation between mass and energy changes with temperature compared to the pure coal ash. Nonetheless, an additional energetic component in the ash with patent fuel was observed in the temperature range of approximately 800 °C, resulting in increased energy production.

3.3. Leaching Tests

For these tests, batch leaching was conducted using both ultra-pure water and synthetic acid rain [50]. The leaching was carried out for set durations of 7 days, one month, and three months, after which the obtained leachates were separated from the residues, and their pH values were measured. The pH values continued to decrease throughout the leaching period, reaching their lowest point at the end of the three-month period.

Figure 4 and Figure 5 illustrate the changes in pH value with the leaching time of the ash samples in ultra-pure water and acid rain. In both cases, the pH values increased after a leaching period of 7 days due to the dissolving of alkaline oxides, such as CaO, MgO, Li₂O, Na₂O, K₂O, and BaO. After 30 days of leaching, pH values began to decrease due to various reactions in the water–ash system, such as the dissolving, hydrolysis, precipitation, and sorption of some elements from water to ash particles [51,52]. The pH value of the leachate and the concentration of metals in leachate are inversely related, and the pH value can significantly vary with the source of fly ash [53,54].

Table 8. Leachates’ conductivities after leaching period of three months (µS/cm).

Leachate	Sample	Conductivity, µS/cm
Ultra-pure water	1	370.1
	2	394.30
	3	441.33
Acid rain	1	325.50
	2	398.67
	3	411.15

shows the results of the measurement of the conductivity of leachates after a three-month leaching period. It is assumed that a precipitation reaction occurred in the leachates due to hydrolysis reactions caused by dissolved elements and hydroxide ions in the acid rain (Figure 6). Additionally, the particles in the ash samples absorbed certain elements from the solution, leading to a reduction in the conductivity of the acid rain solutions. These combined reactions resulted in a decrease in the conductivity of the acid rain solutions.

3.3.1. Leaching in Ultra-Pure Water

The concentrations of elements obtained in the ultra-pure water show increased concentrations of major and trace elements during the whole leaching period (shown in Figure 6 and Figure 7).

Figure 6 presents a comparison between the concentrations of Na, Mg, K, and Ca in leachates after leaching periods of 7 days, one month, and three months.

Figure 7 presents the concentration of potential toxicological elements in ultra-pure water leachate (As, Cr, Cu, Fe, Ni, Pb, and Se), which may negatively impact human health and the environment [13].

The highest concentrations for all three samples after a leaching period of three months were measured for Ca (5339.53, 6790.67, and 8022.96 mg/kg for sample 1, 2, and 3, respectively); Al (13.53, 1015.90, and 2992.41 mg/kg for sample 1, 2, and 3, respectively); and K (3174.94, 1404.07, and 764.51 mg /kg for sample 1, 2, and 3, respectively).

Interestingly, the addition of the patent fuel significantly reduced the As (arsenic) level in the ash leachates. Among the three samples, the highest concentration of As was found in the leachate of sample 1 (2.32 mg/kg) after a leaching period of three months. On the other hand, the concentration of As in the leachates of samples 2 and 3 was significantly lower, at 0.06 mg/kg and 0.02 mg/kg, respectively. This observation may have implications for the environmental impact of the combustion process.

In the leachates of sample 2 and sample 3, the highest trace element concentration was measured for Cu. After three months of leaching, the Cu levels in the leachates of samples 2 and 3 were 1.25 and 0.97 mg/kg, respectively. Also, in these leachates, considerable increases in the Cr (0.75 mg/kg for sample 2, and 0.86 mg/kg for sample 3) and Fe contents (0.61 mg/kg for sample 2, and 0.89 mg/kg for sample 3) were determined.

In the leachate of sample 3, an increase in Se content was detected, and its final concentration after three months of leaching was 0.41 mg/kg. After the same period, the Se concentrations in the leachates were 0.11 mg/kg for sample 1 and 0.16 mg/kg for sample 2.

3.3.2. Leaching in Synthetic Acid Rain

Figure 8 and Figure 9 present a comparison between the concentrations of the main (Na, Mg, K, Ca) and trace elements (As, Cr, Cu, Fe, Ni, Pb, Se) in the acid rain leachates after leaching periods of 7 days, one month, and three months.

Although it was anticipated that the concentration of elements in leachates would increase throughout the leaching period, it has been observed that the concentration of certain elements has either stagnated or decreased after one month of leaching. The most significant decreases were observed for K, Mg, and Cu. This decrease can be explained by the precipitation process of these cations with anions present in the solution.

The highest values in the acid rain leachates after three months of leaching were measured for Ca (5445.77 mg/kg in sample 1; 6795.46 mg/kg in sample 2; and 10,437.34 mg/kg in sample 3), K (3077.41 mg/kg in sample 1; 1489.20 mg/kg in sample 2; and 714.76 mg/kg in sample 3), and Al (15.56 mg/kg in sample 1; 648.70 mg/kg in sample 2; and 1739.02 mg/kg in sample 3). These elements also had the highest concentration levels in the ultra-pure water leachates.

The highest concentration of trace elements was found to be Cu. Interestingly, the same Cu levels were observed in the ultra-pure water leachates. After a three-month leaching period, the Cu concentrations were 0.29, 0.73, and 3.79 mg/kg for samples 1, 2, and 3, respectively. After one month of leaching, the Cu content in the leachates of samples 1 and 2 started to decrease.

Increases in the concentrations of other elements were also shown. The highest increasing rates of concentrations were measured for Cr and As; their concentrations increased 30 times. The final concentrations of Cr were 0.39, 0.82, and 0.91 mg/kg for samples 1, 2, and 3, respectively, whereas the concentrations for As were 2.06, 0.17, and 0.06 mg/kg. A tendency towards increasing levels during the whole leaching period was also observed for Fe. After 7 days of leaching, the increase in the Fe concentration was 40–50%. The final concentrations of Fe were 0.70, 0.74, and 0.84 mg/kg in the leachates of samples 1, 2, and 3, respectively. Other significant increases in concentrations were noticed for Cd, Se, Ce, Tl, and Co.

It is important to mention that the leaching of As and U in both water and acid rain was observed only in the leachate of the pure coal ash and not in the leachate of the ash from the mixtures. Both leachates (ultra-pure water and acid rain) showed that the leachate of the pure coal ash contained higher concentrations of Li, K, V, Mn, Zn, As, Rb, Mo, Sb, Ba, and U. On the other hand, in the leachates from the mixtures (sample 2 and sample 3), the concentrations of Al, Ca, Cr, Cu, Se, Sr, and Zn were higher.

The affinity of different elements for leaching in ultra-pure water or acid rain was determined by calculating the total amount of leached-out metal ions. The following metal ions were considered: Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Mo, Ag, Cd, Sb, Ba, Ce, Tl, Pb, Bi, and U. The results obtained from both leachates are presented in Figure 10 and Figure 11.

Most elements have a higher tendency to leach in acid rain than in water, particularly Co, Ag, Cd, Ce, Tl, Pb, and Bi. Co showed the biggest difference, leaching 13 times better in acid rain than in water, followed by Ag and Cd, which leached 7 times better in acid rain. However, for V and Rb, better leaching was observed in water compared to acid rain.

In Figure 10, it can be seen that the concentration of ions in the ultra-pure water leachates increased over time. This increase is more noticeable in the leachates of samples 2 and sample 3 (ash samples resulting from the combustion of coal and patent fuel) and less in the leachate of the pure steam coal.

In Figure 11, it can be seen that the concentration of ions in acid rain changes similarly to leaching in water. However, after 30 days of leaching, the overall ion concentration begins to decrease. This is due to certain elements reacting with precipitation and being absorbed by ash particles. The decrease is more evident for the pure coal sample (sample 1) and sample 2, while for sample 3, it is smoother.

The release of trace elements from ash into the solution, as well as the rate of release, is influenced by three factors [55]:

• The concentration of elements in the solid phases;
• The distribution of elements among ash particles;
• The possibility of the incorporation of elements into secondary solids.

The ratios of the content of trace elements in leachates after the overall leaching period to the total content of trace elements in ash samples (wleach/wash × 100) were also calculated for ultra-pure water and acid rain. The results show that all ratios are below 1% except for Cu (1.32–7.97%), Se (3.28–12.42%), Sr (12.44–24.60%), and V (1.14–17.60%), which indicates that these elements have the highest leaching activity. From an environmental perspective, special consideration should be applied to these elements during ash disposal.

4. Conclusions

This study aimed to evaluate the effect that adding patent fuel to steam coal has on the composition and properties of the ash resulting from combustion. The conclusions of this research are summarised below:

(1)

Patent fuel can absorb sulphur from flue gases during combustion, with 0.85 mas.% of sulphur in the ash sample after the combustion of pure steam coal (Sample 1) compared to 1.44 mas.% in the ash sample (Sample 3) after the combustion of 0.44 kg steam coal with ten ampoules of patent fuel.

(2)

The addition of patent fuel led to lower concentrations of trace elements in the ash, specifically for the elements Ba, Cr, Ga, Li, Mn, V, and Zn. For Ba, Ga, and Zn, the reduction in these elements in the ash produced by patent fuel–coal combinations compared to the ash produced by pure coal combustion was up to 70%.

(3)

Qualitative XRD diffraction identified that the main mineral phase in ash samples is quartz (SiO₂). Other minor mineral components were mulite (3Al₂O₃ × 2SiO₂), calcite (CaCO₃), lime (CaO), anhydrite (CaSO₄), and hematite (Fe₂O₃). Ash samples after the combustion of mixtures of coal and patent fuel also had cristobalite (SiO₂), portlandite (Ca(OH)₂), and rutile (TiO₂).

(4)

Adding patent fuel to coal results in an earlier start of decomposition, stronger combustion, and faster oxidation, resulting in the release of more energy in the same temperature range.

(5)

The leaching tests revealed that most of the elements, particularly Co, Ag, Cd, Ce, Tl, Pb, and Bi, were found to leach more easily in acid rain compared to water. Co showed the biggest difference, with thirteen times better leaching in acid rain than in water, followed by Ag and Cd with seven times better leaching in acid rain.

(6)

The leaching characteristics of the measured elements were found to be dependent on the pH value.

(7)

The coal ash leachates contained higher levels of environmental pollutants, including As, Cd, Co, Mn, Mo, Sb, and U, than the patent fuel leachates.

After conducting a comprehensive series of tests, the key findings of this study indicate that the addition of a patent fuel to steam coal significantly enhances its properties during combustion. This not only improves the efficiency of the process but also reduces the negative environmental impact associated with ash disposal.