Co-Combustion Behavior of Paper Sludge Hydrochar and Pulverized Coal: Low Rank Coal and Its Product by Hydrothermal Carbonization

Abstract

In this paper, the combustion behavior of low rank coal and its product after hydrothermal carbonization with paper sludge hydrochar were studied. The Raman technique was used to compare the structural differences between raw coal and the product. Thermogravimetric analysis was employed to conduct experiments of single sample and their mixtures with different proportions at a heating rate of 20 °C/min, the activation energy of chemical reactions was calculated. The results showed that upgraded product had higher carbon ordering degree than raw coal and the ignition temperature and burnout temperature of the product were advanced. Compared with raw coal, the combustion characteristic parameters C and S of the product were higher, indicating that its combustibility was better. As for the mixture, when the paper sludge hydrochar ratio was not more than 10%, the mixed fuel combustion curve was still similar to coal curve. After the paper sludge hydrochar ratio exceeded 10%, the activation energy of the mixed combustion reaction of paper sludge hydrochar and upgraded coal was lower than that of raw coal and paper sludge hydrochar. These results indicated that the mixture of upgraded coal and paper sludge hydrochar as mixed fuel was a better option.

1. Introduction

Paper sludge hydrochar (PS) is a solid waste generated in the process of pulping and papermaking [1]. With the rapid growth of social economy, the demand for paper has risen sharply, which directly leads to the large accumulation of PS. The composition of PS is complex and it contains high level of harmful substances, including heavy metals such as mercury, cadmium, lead, and chromium [1,2], pathogenic microorganisms [3,4] and some compounds extremely difficult to decompose [4,5,6]. These characteristics result in great restrictions on the efficient use of PS. Traditional treatment methods mainly include natural drying [7], direct landfill with sludge backfill [2], and use as agricultural fertilizer [8,9]. These utilizations are not only complicated and costly in processing, but also cause more pollution to the environment. However, due to the relatively high volatile matter in the PS, it shows better combustion promoting performance. Therefore, using PS as a kind of fuel for industrial production, on one hand, can develop its utilization value to a greater extent, on the other hand, reduce the dependence on fossil energy consumption [2,10,11,12]. Therefore, making PS for combustion has become one key thermal treatment technology.

However, due to the low carbon content and low energy density in PS, it emits less heat during actual combustion. Mixing PS and coal for combustion can effectively solve this problem [13,14,15]. Co-combustion of PS and coal for industrial production will be simple and efficient. At the same time, because of the high volatile matter in PS, it is easy to ignite and react, this results in its improving reaction performance of coal when used in actual production such as thermal power generation. Liao et al. [16] studied the thermodynamic combustion behavior of PS and semi-bituminous coal, the result showed that PS had a high rate of volatilization compared with coal, and the oxygen-enriched environment could promote the process. Coimbra et al. [17] mixed PS at a ratio of 10% with bituminous coal and carried out thermogravimetric experiments, it was found that for mixture, the experimental value of activation energy during the combustion reaction was lower than the calculating value. Areeprasert et al. [18] studied the mixed combustion thermal behavior of PS and its upgraded product with low rank coal and high reactive coal n a fluidized bed, the conclusions indicated that the combustion behavior of PS and upgraded coal performed better. Results of these studies confirm that PS and coal co-combustion technology can not only make rational use of sludge resources, but also improve the combustion efficiency of pulverized coal.

As one of the important fossil energy sources, low rank coal resources have the advantage of abundant reserves and low development cost, which is widely used in direct combustion for power generation [19], dry distillation and pyrolysis [20], liquefaction [21] and gasification [22], etc. It is an ideal fuel for mixed combustion with PS. However, due to the low carbon content, high ash content and high content of harmful elements such as sulfur in the low rank coal [23], traditional utilizations will not only reduce the energy utilization efficiency, but also cause a lot of serious environmental problems such as the greenhouse effect. Hydrothermal carbonization technology (HTC), which has been considered as one clean and efficient technology, can improve the coal rank in reverse and increase the energy density and heating value. In addition, oxygen-containing functional groups and harmful elements in coal can also be removed after HTC, so the use value of low rank coal can comprehensively be improved. Meanwhile, compared with traditional treatment methods such as pyrolysis, hydrothermal carbonization and upgrading treatment technology has the characteristics of soft treatment process, low energy consumption and more environmental protection [23,24,25]. At present, studies on the combustion behavior of low rank coal and its product by HTC with PS are still relatively scarce. Therefore, further studies are needed.

In this study, different structural characteristics of PS, low rank coal and its HTC product were compared and analyzed through Raman technology. The combustion characteristics of individual samples and their mixtures were analyzed through thermogravimetric experiments. The different combustion features of rank coal and HTC products mixed with PS were investigated, and the Coats-Redfern kinetic calculation method was employed to calculate and analyze the combustion kinetic parameters of the mixed samples.

2. Material and Tests

2.1. Preparation of Sample

The PS used in the experiment was collected from Sludge treatment plant, and the low-rank coal sample named Shenmu (SHM) was produced in Shanxi Province, China. Both samples were ground with a mortar to the particle size of less than 74 μm for subsequent experiments. The hydrothermal carbonization (HTC) experiment was carried out using the HT-250FC-I-F001 flange magnetic reactor produced by Shanghai Huotong Experimental Instrument Co., Ltd, Shanghai, China. The structure diagram of equipment was shown in Figure 1. 40 g of raw drying coal and 160 mL of deionized water were put together into the high temperature and high-pressure reactor for the experiment. In order to eliminate the influence of air, high purity N₂ was passed through the reactor until the air was cleared. Before the experiment, the program temperature was set to 340 °C and magnetic stirring speed was set to 400 r/min. When the reaction reached the specified temperature, it was kept for 1 h and then naturally cooled to room temperature at 25 °C. The reaction product was collected and separated to obtain upgraded coal solid (SHM-HTC). Three samples of PS, SHM and SHM-HTC were all placed in the air-drying oven at 105 °C for 10 h to obtain dry basis samples [4,6,10,11,14,26]. Proximate, ultimate analysis and H/C atomic ratio of PS, SHM and SHM-HTC were as shown in

Table 1. Proximate, ultimate analysis and H/C atomic ratio of PS, SHM and SHM-HTC.

Sample	HHV (MJ/kg)	Proximate Analysis (wt.%)					Ultimate Analysis (wt.%)
		Ad	Vd	FCda	V/FC	Cd	Hd	Oda	Nd	Sd	O/C	H/C
SHM	30.30	1.65	43.05	55.30	0.78	74.95	5.09	17.05	0.86	0.40	0.17	0.81
SHM-HTC	31.23	1.25	34.43	64.32	0.54	78.29	4.69	14.58	0.85	0.35	0.14	0.72
PS	16.14	23.74	63.84	12.42	5.14	42.76	3.68	29.11	0.87	0.35	0.51	1.03

Note: A―Ash; V―Volatile matter; FC―Fixed carbon; ^a calculated by difference; d—drying base; C_d—carbon element; H_d—hydrogen element; O_d^a—oxygen element; N_d—nitrogen element; S_d—sulfur element; O/C—the ratio of moles of oxygen and carbon elements; H/C—the ratio of moles of hydrogen and carbon elements.

. Weigh the sample corresponding to the mass fraction and mix it evenly in the mortar. The mass mixing ratios of PS with SHM and SHM-HTC were 0 wt.%, 10 wt.%, 20 wt.%, 30 wt.% and 50 wt.%, respectively. The mixed samples were marked as 10PS90SHM, 20PS80SHM, 30PS70SHM, 50PS50SHM, 10PS90SHM-HTC, 20PS80SHM-HTC, 30PS70SHM-HTC, 50PS50SHM-HTC. All samples were tested for three times and averaged.

2.2. Raman Spectral Characterization of the Samples

The Raman spectroscopy detection was conducted by LabRAM HR Evolution (HORIBA) analyzer. The laser wavelength of the analyzer was 532 nm of visible light and 325 nm of ultraviolet light. To accurately measure the structural characteristics of different samples, the most commonly testing standards for carbonaceous materials are used including: the shift range was set between 800–3000 cm⁻¹, and the resolution was 0.67 cm⁻¹, the high temperature range of the experiment was 25–1500 °C.

2.3. Thermogravimetric Analysis

The thermogravimetric (TG) experiment was carried out on the HCT-3 TG analyzer from Beijing Henven. The sample weight was 10 mg, the heating rate was set to 20 °C/min [14,27]. The sample was put in an alumina crucible and placed on the balance from room temperature to 900 °C. During the experiment, the air with the flow rate of 100 mL/min was kept ensuring the combustion reaction can end steadily. The device structure diagram was as shown in Figure 2.

2.4. Calculation of Combustion Characteristic Parameters

The thermogravimetric analysis method has been accepted by many researchers to evaluate the characteristic parameters of the whole combustion process [15,16]. In this study, flammability index (C, wt.%/(min·K²)) and comprehensive combustion characteristic index (S, wt.%²/(min²·K³)) were mainly used to characterize the mixed combustion of sludge and coal. C mainly indicated the reaction capacity of mixed samples in the initial stage of combustion reaction. S can fully reveal the characteristics of the beginning and end of the mixed sample. The better the combustion characteristic of the mixed sample, the larger the S value will be. The formulas of C and S were shown in Equations (1) and (2):

$$C=\frac{{(\mathrm{d}w/\mathrm{d}t)}_{\max }}{T_i^2},$$

(1)

$$S=\frac{{(\mathrm{d}w/\mathrm{d}t)}_{\max }\cdot {(\mathrm{d}w/\mathrm{d}t)}_{\min }}{T_i^2{T}_f},$$

(2)

Here, w means the mass of the sample during the reaction; t means the reaction time; ${(\mathrm{d}w/\mathrm{d}t)}_{\max }$ represents the maximum rate in the combustion reaction, which corresponding to the peak value of conversion rate curve (%/min); ${(\mathrm{d}w/\mathrm{d}t)}_{\min }$ represents the average combustion rate of samples (%/min); $T_i$ is the ignition temperature of samples (K); $T_f$ represents the burnout temperature of samples (K).

2.5. Description of Kinetic Model

Calculating the kinetics of chemical reaction is helpful to deeply understand the chemical reaction mechanism and accurately judge the reaction ability. In this study, Coats-Redfern kinetic model [28] was used to calculate the kinetic parameters of PS, SHM and SHM-HTC. The integral expression of the kinetic model is shown in Equations (3) and (4).

$$\mathrm{d}x/\mathrm{d}t=k\left(T\right)f\left(x\right),$$

(3)

$$k=A\cdot \exp \left(-E/RT\right),$$

(4)

Here, $k\left(T\right)$ represents the reaction rate constant; $x$ represents the degree of combustion reaction (%); $A$ refers to the pre-factor (min⁻¹); $E$ is the activation energy of the chemical reaction, which determines the difficulty degree of the reaction (kJ/mol); $T$ is the absolute value of the reaction temperature (K); $R$ is the universal gas constant, and its value is for 8.314 J/(mol·K). The expression $f\left(x\right)$ in Equation (3) can be transformed by Equation (5), where $x$ is the reaction order, and its expression is shown in Equation (6).

$$f\left(x\right)={\left(1-x\right)}^n,$$

(5)

$$x=\frac{m_0-m_t}{m_0-m_{\infty }},$$

(6)

Here, $t$ is reaction time, min; $m_0$ is the weight of the sample in the initial phase of the reaction, g; $m_t$ is the instantaneous weight of the reactants when the reaction proceeds to time $t$, g.

During the reaction, the partial pressure of air remains basically unchanged, and $f({\mathrm{P}}_{\mathrm{air}})$ is a constant value. The expression of the reaction heating rate $\beta$ is shown in Equation (7):

$$\beta =\mathrm{d}T/\mathrm{d}t,$$

(7)

By combining Equations (3)–(5), Equation (8) can be defined as:

$$\frac{\mathrm{d}x}{\mathrm{d}T}={(1-x)}^n\frac{A}{\beta }\exp [-E/(RT)],$$

(8)

The combustion reaction of pulverized coal can be regarded as a first-order reaction, thus Equation (9) can be obtained:

$$\ln \left|\frac{-\ln (1-x)}{T^2}\right|=\ln [(\frac{AR}{\beta E})(1-\frac{2RT}{E})]-\frac{E}{RT},$$

(9)

For most calculations of the activation energy of chemical reactions, $E/RT\ge 1$, $(1-2RT/E\approx 1)$. From $ln\left|-\ln (1-\alpha )/T^2\right|$ versus $1/T$ plot, the apparent activation energy of chemical reaction and pre-factor can be obtained by calculating the slope and intercept in the straight plot.

3. Results and Discussion

3.1. Structure Characterization Analysis

Raman technology was used here to study the structure order of the carbon material of the hydrothermal product and raw coal [29,30]. Five peaks, D₄, D₁, D₃, G and D₂ were selected to perform peak-split fitting operations on the Raman spectrum with band positions located at 1200 cm⁻¹, 1350 cm⁻¹, 1500 cm⁻¹, 1580 cm⁻¹, 1620 cm⁻¹. In these peaks, D₄, D₁, G, D₂ correspond to four Lorenz fitted peaks and D₃ was fitted with Gauss peak. The peak fitting results were shown in Figure 3. It could be seen from the Figure 3 that the Raman spectra for raw coal and HTC treated coal both had two obvious peaks at D₁ and G bands [31,32], the strength of the G peak was related to the graphitization degree of the carbon structure, the telescopic vibration of aromatic benzene ring in coal led to the emergence of the G band, the stronger the G peak indicated that its structure was closer to graphite, and the D₁ band corresponded to the disorder degree of carbon structure [33,34]. The G peak strength of the HTC treated coal was significantly higher than that of raw coal, but it was difficult to evaluate microstructure parameters accurately by comparing the peak intensity alone, so five peak strength parameters, which can be expressed as I_D4, I_D1, I_D3, I_G and I_D2, were extracted and I_D3+D4/I_G was calculated. The structural parameters were listed in

Table 2. Calculation results of Raman structure parameters of different samples.

Samples	ID4	ID1	ID3	IG	ID2	ID3+D4/IG
SHM	7497	10,178	4148	10,154	651	1.147
SHM-HTC	12,369	60,897	15,071	34,322	973	0.799

. Sheng et al. [35] used Raman technology to study the association between the microstructure of coke and its combustion reactions. The results showed that I_D3+D4/I_G was a suitable solution to indicate coke order, the smaller the value of I_D3+D4/I_G meant the higher carbon order of the material. It could be seen that compared with SHM, I_D3+D4/I_G value of SHM-HTC was reduced from 1.147 to 0.799, this indicated that HTC treatment could improve the coal order. In high temperature and high-pressure environment, the unstable side chain structure of the molecular group in low rank coal was susceptible to thermal break-off, and the fracture of aromatic base side chain and part of active groups will release a large number of free radicals, which accelerated the decomposition of unstable structures such as R-C=O-R’ and R-O-R’ [36]. In addition, the condensed aromatic nuclei of the basic structural units condensed to form solid products.

3.2. Separate Combustion of PS, SHM and SHM-HTC

From the results in Table 1, it can be found that the fixed carbon content of SHM-HTC increased significantly from 55.30 wt.% of SHM to 64.32 wt.%, and the volatile content decreased from 43.05 wt.% to 34.43 wt.%, which showed that during the HTC treatment, the volatile substances in the coal were partially removed and escaped in the form of gas. The study of Wu et al. [37] showed that the gas produced during HTC process mainly included carbide gases such as CO₂, CO, and C_nH_m type hydrocarbon compounds such as CH₄, C₂H₆, etc. This proved that the carbonization reaction of coal was promoted, thereby improving coal rank. From the ultimate analysis results, after HTC treatment, the carbon content of coal increased, while the hydrogen and oxygen content decreased, and the O/C and H/C atomic ratio were significantly reduced compared with raw coal. It reflected that the oxygen-containing active groups were decomposed and removed as the HTC had an effect, the proportion of carbon element was increased, and the energy density of coal was strengthened. In addition, it can be found that the content of sulfur element after HTC had decreased. Some studies [23,38] indicated that the sulfur in coal entered the liquid phase in the form of sulfate or was released into the gas phase as H₂S and SO₂. Compared with coal, the volatile content of PS was significantly higher than that of coal, and the carbon content was lower, which determined the ignition performance of PS was better than coal [39]. The H/C ratio of PS was 1.03, which was significantly higher compared with coal and its upgraded product, this also proved that PS was a highly hydrogen-rich energy.

The conversion curve and conversion rate curve of PS, SHM, SHM-HTC were shown in Figure 4. From the conversion rate curve, it can be found that the combustion process of PS was very different from that of coal. The combustion of PS can be considered as two stages. The temperature scope of the first stage was between 125.3 °C and 389.1 °C. During this stage, the weight loss proportion was 74.29 wt.%. and the burning of carbohydrates mainly occurred, which included the decomposition and burning of low-ignition hydrocarbon compounds and aliphatic side chains in the coal structure. The temperature scope of the second stage was from 389.1–559.2 °C. The decomposition reaction of some aromatic compounds with higher bond energy, such as aromatic alcohols, aromatic hydrocarbons and aromatic acids, occurred in this stage. Two peaks of the conversion rate curve of PS appeared at 302.3 °C and 420.1 °C, respectively, and the corresponding peak rates were 43.9 × 10⁻⁴ s⁻¹ and 7.01 × 10⁻⁴ s⁻¹. At the same time, it can be concluded that the combustion curve of PS and waste derivatives were relatively close [40,41]. For coal combustion, SHM combustion reaction mainly took place at 319.8–623.5 °C. This process started with the ignition of volatile matter, and the coal continuously absorbed heat, which was very rapid. Then was the intense combustion of carbon, during which process it released a large amount of actual heat and determined the combustion rate and burnout degree of coal. Compared with SHM, the temperature range of SHM-HTC combustion process was between 296.8–612.6 °C, the ignition temperature T_i of SHM-HTC was 366.9 °C lower than 375.7 °C of SHM, and the peak rate of SHM-HTC’s conversion rate curve was close to that of SHM of 22.2 × 10⁻⁴ s⁻¹, while the corresponding temperature of SHM-HTC’s peak rate, 478.1 °C, was significantly lower than that of SHM’s peak rate, 501.9 °C. Combustion characteristic parameters of PS, SHM, SHM-HTC were shown in

Table 3. Results of combustion reaction parameters for PS, SHM, SHM-HTC and their blend.

Samples	T1 (°C)	DTGmax−1× 10−4 (s−1)	T2 (°C)	DTGmax−2× 10−4 (s−1)	Ti (°C)	Tf (°C)	DTGmax× 10−4 (s−1)	DTGmean 10−4 (s−1)	C × 10−8	S × 10−14
PS	301.8	43.90	420.1	7.01	235.2	724.5	43.90	6.46	7.94	7.08
10PS90SHM	350.5	5.20	486.6	22.30	342.1	587.5	22.30	5.61	1.91	1.82
20PS80SHM	337.5	7.18	498.6	21.30	317.6	589.3	21.30	5.64	2.11	2.02
30PS70SHM	324.5	9.29	493.3	23.30	308.2	578.9	23.30	5.74	2.45	2.43
50PS50SHM	323.1	15.50	512.1	19.30	291.4	607.1	19.30	5.80	2.27	2.17
SHM	-	-	501.9	23.10	375.7	581.8	23.10	5.51	1.64	1.55
10PS90SHM-HTC	345.4	4.77	482.7	22.20	346.7	576.7	22.20	5.53	1.85	1.77
20PS80SHM-HTC	321.6	6.82	460.1	21.80	309.7	566.8	21.80	5.63	2.27	2.26
30PS70SHM-HTC	320.2	10.30	471.1	22.20	300.3	562.9	22.20	5.76	2.46	2.52
50PS50SHM-HTC	308.7	21.00	485.6	19.30	277.2	592.4	21.00	5.87	2.73	2.71
SHM-HTC	-	-	478.1	22.20	366.9	573.9	22.20	5.69	1.65	1.64

. For the entire reaction process, SHM-HTC’s DTG_mean 5.69 × 10⁻⁴ s⁻¹ was higher than SHM’s DTG_mean 5.51 × 10⁻⁴ s⁻¹. In terms of C and S indexes, SHM-HTC was higher than SHM, which showed that the combustibility of coal can be improved after HTC treatment. These results illustrated that the HTC treatment can positively influence the combustion reaction process of the coal without losing the original good combustibility of low-rank coals. Both SHM and SHM-HTC had higher ignition and burnout temperatures than PS. Therefore, a certain amount of PS can be added to improve coal combustion.

3.3. Combustion of Mixed Samples

The conversion curve and conversion rate curve of co-combustion were shown in Figure 5. For the conversion curve, whether it was SHM or SHM-HTC mixture, because of the addition of PS, the combustion of the volatile matter in the mixture took place in advance. When the PS ratio was low, the combustion curve of the blends was similar to the coal sample, especially when the ratio was 10 wt.%, this was consistent with the results of Ge et al. [42]. As PS ratio increased, the overall weight loss curve moved toward the low temperature area, which showed that the addition of PS promoted the combustion reaction of the mixture, and the larger the PS ratio, the more significant this effect was. For conversion rate curve, due to the addition of PS, the sample curve was divided into two stages. The conversion rate curve of the blends showed two peaks. As the PS ratio increased, the conversion rate peak of the first stage of the mixture became more obvious, and the combustion and decomposition of the mixture gradually increased, which was attributed to the promoting effect of PS in co-combustion.

Combustion characteristic parameters were as shown in Table 3. According to the result, whether SHM or SHM-HTC, the addition of PS made the T_i of the mixture lower than the separate coal. With the increase of PS proportion, the decrease amplitude of T_i increased, this was the reason that the cleavage reaction of the active groups in the PS sample at low temperatures can quickly produce a large number of active free radicals, and the aliphatic compounds in the coal structure can also undergo decomposition reactions after the temperature exceeded 300 °C. These free radicals collided with each other at high temperature and accelerated the entire decomposition reaction [43]. The DTG_max of the mixture decreased along with the increase of PS ratio, but it was not particularly significant. While DTG_mean gradually increased with the increase of PS ratio, indicating that the addition of PS played a significant role in promoting the increase of the combustion conversion rate of the mixture. For SHM mixture, when the PS ratio was 10 wt.%, the value of C and S was slightly higher than SHM-HTC. However, when its ratio was more than 10%, C and S values of SHM-HTC mixture were higher than SHM mixture, as the PS ratio reached 50 wt.%, C and S of SHM-HTC reached 2.73 and 2.71, respectively, which demonstrated that after PS reached a certain proportion the combustion performance of mixed fuel of HTC product with PS was better than the raw coal mixture.

For SHM, when the PS ratio of the mixed sample reached 30 wt.%, its C and S reached the maximum values of 2.45 and 2.43, respectively. In addition, its T_f also increased sharply from 581.8–607.1 °C, which showed there was a certain ratio between PS and SHM. When this ratio was exceeded, the combustion process may be prolonged, which was not conducive to the full reaction of the sample. Similar phenomena can also be observed for SHM-HTC. when the PS ratio increased, although C and S was increased, the increase magnitude dropped significantly. These results all indicate that because of the complex composition of PS, although its being mixed with coal can promote the combustion reaction, excessive addition can inhibit the combustion promotion effect [16].

3.4. Kinetic Analysis

The calculation results of kinetic parameters for different samples were shown in

Table 4. Calculation results of kinetic parameters for PS, SHM, SHM-HTC and their blend.

Samples	T (°C)	E (kJ/mol)	A × 105 (min−1)	R2
PS	350~600	5.91	1.16	0.9560
10PS90SHM	350~600	28.92	482.00	0.9758
20PS80SHM	350~600	24.79	166.00	0.9605
30PS70SHM	350~600	24.07	158.00	0.9532
50PS50SHM	350~600	18.81	40.70	0.9502
SHM	350~600	35.39	2950.00	0.9963
10PS90SHM-HTC	350~600	29.39	938.00	0.9729
20PS80SHM-HTC	350~600	25.66	423.00	0.9721
30PS70SHM-HTC	350~600	22.71	203.00	0.9606
50PS50SHM-HTC	350~600	15.76	29.40	0.9549
SHM-HTC	350~600	34.10	3300.00	0.9915

. The fitting curve of activation energy was shown in Figure 6. By calculating the activation energy and pre-factor of the chemical reaction process, the whole process can be understood more intuitively, and the difference of SHM, SHM-HTC and PS mixed combustion can be directly compared. R² indicated the reliability of the fitting results. It can be indicated from the calculation results that the fitting results of all samples were greater than 0.95, which verified the reliability of the results. According to the fitting data, PS had the lowest reaction activation energy of 5.91 kJ/mol, so it had the strongest reactivity and was most prone to chemical reaction, which was also the reason why PS had the highest C and S combustion characteristic index. E value of SHM was 35.39 kJ/mol, higher than that of SHM-HTC of 34.10 kJ/mol. The combustion of coal was considered to be the gas-solid reaction, the main heat generation was carbon. HTC treatment reduced the H/C atomic ratio of coal and at the same time reduced the V/FC ratio. A large amount of volatile matter was removed into the gas phase in the form of saturated or unsaturated hydrocarbon compounds such as alkanes, alkenes or alkynes [38], and the volatile matter released by the pyrolysis reaction in the coal particles at the early stage of combustion was reduced, which was more conducive to the contact of oxygen and coal, making the reaction easier to occur.

For mixed samples, when PS and SHM were mixed at a ratio of 10 wt.%, the E value was close to that of the SHM-HTC mixture. This was because when the PS content was low, it was pulverized coal that played a leading affect in the combustion process of the mixed fuel [44]. However, in contrast with the combustion reaction of a single sample, in the initial stage of the combustion of the mixed fuel, the surface of the SHM particles melted and collapsed to form a large number of pore structures, resulting in the release of a large amount of volatile matter. At the same time, both the volatile matter released by PS and coal freed a large amount of heat when contact with oxygen, which ensured the temperature conditions for the cracking reaction of organic matter inside the coal and PS particles. The HTC treatment will cause the side chains, functional groups, and bridge bonds around the basic structural units of low-rank coals to be cracked out. Compared with the upgraded SHM-HTC, the organic matter inside the SHM particles had smaller bond energy, poor thermal stability and easy to crack, forming low molecular compounds to escape in the form of gas, which resulted in the combustion reaction easier to proceed, and the corresponding activation energy was low. When the PS ratio exceeded 20 wt.%, the PS in the mixed fuel played a role in promoting combustion, and PS showed better reactivity at low temperature. Meanwhile, it can be known from the combustion reaction parameters C and S that the better combustibility of SHM-HTC also positively influenced the combustion of the mixture, so that the activation energy of the SHM-HTC in the combustion process was lower than the mixture of SHM and PS.

4. Conclusions

(1)

According to the results of Raman analysis, the I_D3+D4/I_G value of SHM-HTC decreased, which showed that HTC treatment had improved the carbon ordering degree of coal.

(2)

The conversion rate curve of PS combustion had two obvious peaks, so its combustion process was carried out in two stages, while the combustion of coal and its upgraded product showed only one peak, and the T_i and T_f of PS were both at low temperature. A better combustion performance of PS can be observed.

(3)

Compared with raw coal, upgraded coal had reduced volatile content and increased fixed carbon content, T_i and T_f were lower than raw coal, DTG_mean became higher, and C and S indexes increased, which showed that the combustion performance of upgraded coal was improved.

(4)

The mixture of PS and SHM-HTC had lower activation energy than the mixture of PS and SHM, which indicated that SHM-HTC and PS were better options as mixed fuel.