Quantitative Analysis of Heat Release during Coal Oxygen-Lean Combustion in a O2/CO2/N2 atmosphere by TG-DTG-DSC

doi:10.21203/rs.3.rs-1352759/v1

Coal combustion in an oxygen-lean and multi-gas environment is a common exothermic phenomenon for coalfield fires, leading to serious environmental destruction and loss of coal resources. Simultaneous thermal analysis experiments for Bulianta (BLT, high-volatile bituminous coal) and Yuwu coal (YW, anthracite) in 21vol.%O₂/79vol.%N₂ and 15vol.%O₂/5vol.%CO₂/80vol.%N₂were carried out to study the law of heat release. Based on the TG-DTG-DSC curves, the combustion characteristic parameters were analyzed. A delay of ignition and heat release existed during the coal oxygen-lean combustion in O₂/CO₂/N₂. Decreasing O₂ concentration caused a significant reduction of local reactivity and further the decreasing maximum heat release rate for low-rank coal, while increasing CO₂ concentration caused a significant thermal lag effect and further the increasing maximum heat release rate for high-rank coal. The relationship between the heat release rate and the reaction rate constant was quantitatively analyzed. At the increasing stage of the heat release rate, the heat release rate of the two coals increased conforming to ExpGro1 exponential model. At the decreasing stage of the heat release rate, the heat release rate of YW coal decreased exponentially with the reaction rate constant, while the heat release rate of BLT coal decreased linearly. Regardless of the atmospheres, the conversion rates corresponding to maximum heat release rate of BLT and YW coal were about 0.80 and 0.50, respectively, indicating that the coal rank played a dominant role. The results are helpful to understand the heat release process of coal oxygen-lean combustion in O₂/CO₂/N₂.

coal oxygen-lean combustion

multi-gas environment

heat release

apparent activation energy

quantitative analysis

Coal is an important energy source to meet the power demand as well as to promote the economy development because of its abundant reserves [1–3]. Coalfield fires triggered by spontaneous coal combustion also occur continuously when mining, and are considered a global crisis, which not only causes serious environmental destruction and loss of coal resources, but also poses a serious threat to human safety and health. [4–7]. Most coalfield fires occur in an oxygen-lean (oxygen concentration lower than air) and multi-gas environment due to insufficient oxygen supply and combustion product gases [8]. The development and expansion of coalfield fires closely relate to the heat accumulation of coal combustion. Obtaining the law of heat release during coal oxygen-lean combustion in a multi-gas atmosphere will be beneficial to understand and reveal the dynamic spread of a coalfield fire.

Coalfield fires start from coal low-temperature oxidation. Pan et al. [2] studied the heat release of the oxidation characteristics of pulverized coal using a C600 microcalorimeter. The results showed that the oxidative heat evolution of pulverized coal has obvious stage characteristics of first absorbing heat and then releasing heat. Other scholars [9–12] also came to a conclusion consistent with the above. Chang et al. [13] investigated the oxidation pyrolysis of coal below 150℃ in Yangquan through a thermal analyzer. The results suggested that the low-temperature oxidation process was an obvious oxidation process of heat release compared with the pyrolysis process. Zhai et al. [14] used the derivative of heat flow with time to record the endothermic and heat release changes of coal samples at different heating rates. The results presented that a low heating rate promoted the heat release effects of coal oxidation, resulting in a more thorough heat release process. In addition, some scholars studied the low-temperature oxidation of coal in sections based on heat release. Li et al. [15] determined the intrinsic reaction of Ximeng lignite at low temperatures, and divided the intrinsic reaction of coal and oxygen into three stages by using differential scanning calorimetry (DSC), including slow oxidation, accelerated oxidation, and rapid oxidation. The intrinsic reaction was to eliminate the heat release after water evaporation and thermal decomposition of internal oxygen-containing functional groups. The low-temperature oxidation of coal was also divided into water loss and weight loss stage, oxygen absorption stage and thermal decomposition stage by some scholars for research [16]. Totally, how to detect and prevent coalfield fires in the low-temperature oxidation stage of coal are the purpose of most scholars' research.

Exploring the overall heat release process of coal combustion is beneficial to understand the coalfield fires. Deng et al. [17] investigated the gas production and thermal behavior of weathered coal and fresh coal. The heat released by weathered coal continued to maintain oxidation under high temperature and oxygen-lean conditions. Su et al. [18] studied the main characteristic behaviors (temperature gradient, oxygen consumption, oxidation kinetics, gaseous products and heat release) of coal combustion, and divided the evolution process into five stages. Zhao et al. [19] divided the high-temperature oxidation process into four stages through using thermogravimetric differential scanning calorimetry (TG-DSC), including water evaporation and gas desorption, oxygen absorption and weight gain, thermal decomposition and combustion, and obtained detailed heat release characteristics. Most of research in the above was aimed at O₂/N₂ and O₂/CO₂ atmospheres. Because of the lower heat conduction coefficient and lower diffusivity of CO₂ than N₂, when N₂ in the atmosphere was completely replaced with CO₂ under the general supply in O₂/N₂, the reaction rate decreased, and the ignition and combustion delayed and the time to reach burnout extended [20–28]. Wang et al. [29] found that the TG curves of coal and coke combustion in O₂/CO₂ tended to be higher temperature range compared with conventional combustion. This was in accordance with the research results of Cahyadi et al. [30] and Ren et al. [12]. The above-mentioned studies only focused on the ignition, burnout, and weightlessness delay, the heat release of coal oxygen-lean combustion was not studied in details. As the O₂ concentration gradually decreased in the O₂/N₂ or O₂/CO₂ atmospheres, the heat release rate decreased and delayed, the coal combustion slowed down and the burnout time extended [10–12, 29]. The heat release process was not only affected by insufficient oxygen supply, it was also corrected with the presence of CO₂, H₂O, etc., when coalfield fires were in the oxygen-lean environment with multi-gas components such as O₂/CO₂/N₂, O₂/N₂/H₂O, O₂/H₂O/CO₂, etc. [31–34].

Heat release is the basis of coalfield fire spreading. However, there are currently few studies on the heat release of coal oxygen-lean combustion in a multi-gas environment. The purpose of this work is to analyze the law of heat release during coal oxygen-lean combustion in a O₂/CO₂/N₂ atmosphere. Simultaneous thermal analysis experiments were carried out for two coal samples in 21vol.%O₂/79vol.%N₂ and 15vol.%O₂/5vol.%CO₂/80vol.%N₂, respectively. Based on the TG-DTG-DSC curves, the combustion characteristic parameters were discussed, and the kinetic parameters were obtained. Furthermore, the relationship between the exothermic rate and the reaction rate constant was proposed. This work can provide theoretical support for revealing the spread of coalfield fires.

2.1 Preparation of coal samples

Two fresh coal samples were selected from the Bulianta colliery in Inner Mongolia and the Yuwu colliery in Shanxi, China, denoted as BLT and YW, respectively. The reason for choosing these two kinds of coal is that they belong to different rank coals and can show good experimental results. BLT coal belongs to high-volatile bituminous coal, which has a higher volatile matter (31.66%), lower fixed carbon (43.30%) and higher ash content (16.16%) than that of YW coal. YW coal belongs to anthracite. Coal samples were crushed in the laboratory, then sieved through 0.60mm, 0.45mm and 0.30mm gauze. The particle size between 0.30 ~ 0.45mm were selected as the experimental coal samples. The proximate analysis and ultimate analysis had been carried out in our previous research [35], as shown in Table 1.

Table 1

Proximate analysis and ultimate analysis of coal samples [35]
Coal sample	Proximate analysis (ad, %)				Ultimate analysis (ad, %)
Coal sample	Moisture	Ash	Volatile matter	Fixed Carbon	Nitrogen	Carbon	Hydrogen	Sulfur	Oxygen
BL coal	8.88	16.16	31.66	43.30	0.94	63.44	5.08	0.28	15.03
YW coal	0.71	9.40	9.90	79.99	1.30	83.48	4.05	0.24	3.72

2.2 TG-DTG‑DSC experiment

A synchronous thermal analyzer (NETZSCH STA 449 F3) was utilized. Two atmospheres, including 21vol.%O₂/79vol.%N₂ and 15vol.%O₂/5vol.%CO₂/80vol.%N₂ were set up. The coal sample was put in a container. Gases passed into the container from two inlets, one of which located the bottom with a gas flow rate of 50ml/min and another one located the middle with a gas flow rate of 20ml/min. Two kinds of coal samples, with a mass of about 13mg were heated from room temperature to 1100°C, at three heating rates of 10°C/min, 15°C/min, and 20°C/min, respectively, as seen in Table 2. Based on the synchronous thermal analyzer, the schematic diagram of the experimental system is shown in Fig. 1.

Table 2

Experimental design
Experiment number	Atmosphere	Heating rate (°C/min)	Heating range (°C)
BLT 1	O₂/N₂	10	room temperature ~ 1100
BLT 2	O₂/N₂	15
BLT 3	O₂/N₂	20
BLT 4	O₂/CO₂/N₂	10
BLT 5	O₂/CO₂/N₂	15
BLT 6	O₂/CO₂/N₂	20
YW 1	O₂/N₂	10
YW 2	O₂/N₂	15
YW 3	O₂/N₂	20
YW 4	O₂/CO₂/N₂	10
YW 5	O₂/CO₂/N₂	15
YW 6	O₂/CO₂/N₂	20

2.3 combustion kinetic theory

The coal combustion kinetic equation can be expressed as follows [36]

$$\frac{d\alpha }{\text{d}t}=k\left(T\right)\cdot f\left(\alpha \right)$$

1

Where, k(T) is the reaction rate constant. α corresponds to the conversion of coal, its expression is as follows

$$\alpha =\frac{{W}_{\text{O}}-W\text{i}}{{W}_{\text{O}}-{W}_{{\infty }}}$$

2

Where, W_i means the coal mass corresponding to the time of i.

The reaction rate constant of coal combustion can be expressed as follows [37].

$$k\left(T\right)=A\text{exp}\left(-\frac{E}{\text{R}T}\right)$$

3

Where, A corresponds to the pre-exponential factor (min^− 1); E corresponds to the apparent activation energy (kJ/mol), R corresponds to the universal gas constant.

The kinetics equation of non-isothermal reaction can be expressed as follows [38]

$$\frac{\text{d}\alpha }{\text{d}T}=\frac{A}{\beta }\text{exp}(-\frac{E}{\text{R}T})f\left(\alpha \right)$$

4

Where, β corresponds to the heating rate for non-isothermal experiments.

Due to the high accuracy, the Kissinger-Akahira-Sunose (KAS) method was utilized to calculate the apparent activation energy. Its expression is as follows

$$\text{ln}\left(\frac{\beta }{{{T}_{\alpha }}^{2}}\right)=\text{ln}\left[\frac{AR}{Eg\left(\alpha \right)}\right]-\frac{E}{\text{R}T}$$

5

Based on the plot of ln(β/T_α²) versus 1000/T, activation energies were calculated from the slope of the linear regression lines, pre-exponential factors were estimated from the intercepts.

3.1 The influence of the O₂/CO₂/N₂ atmosphere on the combustion characteristic parameters

Figure 2 gives the calculation method of combustion characteristic parameters, including ignition temperature (T_i), maximum heat release rate (v_p) and the temperature corresponding to maximum heat release rate (T_p). T_i corresponds to the temperature which coal samples begin to burn, and its value reflects the propensity of coal to spontaneous combustion. (T_p, v_p) is the point that the heat release rate of coal reaches to the maximum. It shows as the valley point on the DSC curve. The corresponding results are shown in Table 3. When the heating rate was constant, the values of T_i and T_p in the O₂/CO₂/N₂ atmosphere visibly increased compared with that in the O₂/N₂ atmosphere. This indicated that a delay of ignition and heat release existed during coal oxygen-lean combustion in the O₂/CO₂/N₂ atmosphere. This result was consistent with the literature [34–35, 39–40].

In the O₂/CO₂/N₂ atmosphere, the values of v_p for BLT coal samples decreased by 2.55 mW/mg, 4.18 mW/mg, and 16.51 mW/mg at 10°C/min, 15°C/min, and 20°C/min, respectively, compared with that in the O₂/N₂ atmosphere, because the decreasing O₂ concentration leaded to a reduction of local reactivity [28]. The values of v_p for YW coal samples increased by -3.52 mW/mg, 1.43 mW/mg, and 4.25 mW/mg at 10°C/min, 15°C/min, and 20°C/min, respectively, because the lower heat conduction coefficient of CO₂ than N₂ caused a thermal lag effect [41]. It can be seen that the influence of the O₂/CO₂/N₂ atmosphere on the maximum heat release rate was restricted by the coal rank. The low-rank coal burned faster due to its low carbon content, and O₂ had a significant impact on the maximum heat release rate. The high-rank coal contained more carbon and burned slowly, and CO₂ had a significant impact on the maximum heat release rate.

Table 3

Characteristic temperatures of BLT and YW coal
Experiment number	T_i (°C)	T_p (°C)	V_p (mW/mg)
BLT 1	401.16	491.94	32.75
BLT 2	394.45	522.18	41.01
BLT 3	370.52	539.18	45.36
BLT 4	431.16	521.05	30.20
BLT 5	425.75	549.20	36.83
BLT 6	385.53	573.96	28.85
YW 1	513.40	568.39	24.84
YW 2	519.18	608.91	26.68
YW 3	529.82	641.79	29.74
YW 4	530.42	598.98	21.32
YW 5	546.19	636.54	28.11
YW 6	544.22	654.27	35.99

3.2 The influence of the O₂/CO₂/N₂ atmosphere on the kinetic parameters by KAS method

Figure 3 shows the changes in the values of apparent activation energy and correlation coefficients (R²) by KAS method, in the two atmospheres. For BLT coal, as the conversion rate increased, the values of apparent activation energy all first decreased, then increased, and finally decreased. In the range of 0.5 ~ 0.15 conversion rate, it was in the low-temperature oxidation process. Since the R² was lower than 0.80, the values of apparent activation energy were not accurate and cannot be compared. When the conversion rate was higher than 0.15, the coal sample was ignited, and the values of apparent activation energy values in the two atmospheres appeared a sudden increase. During the combustion process, the values of apparent activation energy in the O₂/CO₂/N₂ atmosphere were approximately 33%~58% lower than that in the O₂/N₂ atmosphere. This was because the heat released by coal combustion accumulated more easily in the O₂/CO₂/N₂ atmosphere than in the O₂/N₂ atmosphere, as a result of the reduction of 6 vol.% O₂ and the addition of 5 vol.% CO₂ (low heat conduction coefficient).

For YW coal, as the conversion rate increased, the values of apparent activation energy in the two atmospheres kept decreasing. When the conversion rate was 0.05, it was in the low-temperature oxidation process, and the values of apparent activation energy in the O₂/CO₂/N₂ atmosphere were approximately 40% higher than that the O₂/N₂ atmosphere. This has been confirmed in the research of others [25, 35]. When the conversion rate was higher than 0.05, the coal was ignited, and the values of apparent activation energy in the two atmospheres were close. The influence of the atmosphere was no longer obvious.

In addition, the R² in the two atmospheres was greater than 0.99. However, for BLT coal, a decrease behavior was showed in the conversion rates ranges of 0.10 ~ 0.25 in O₂/N₂ and 0.15 ~ 0.45 in O₂/CO₂/N₂, respectively. The reason was that the precipitation of the remaining volatiles was promoted by the heat release of separated volatiles combustion [28, 42–43], and the precipitation and combustion of volatile was significantly deferred in the O₂/CO₂/N₂ atmosphere compared with that in the O2/N2 atmosphere, as a result of the slightly lower diffusivity of volatiles in CO₂ than in N₂ [28, 44–45] and the lower mass flux of oxygen to the volatiles flame [28, 46].

3.3 The influence of the O₂/CO₂/N₂ atmosphere on the heat release

The reaction rate between oxygen and coal is the key factor influencing the heat release rate [47]. Studying the relationship between the heat release rate and reaction rate is beneficial to understand in the heat release process during coal oxygen-lean combustion in the O₂/CO₂/N₂ atmosphere, which can provide a theoretical foundation for revealing the law of coalfield fire spreading. Since the value of the reaction rate constant can directly reflect the reaction rate, the reaction rate constant was used instead of the reaction rate in this study. According to our previous research [35], the kinetic mechanism functions of BLT and YW coal were Jander (Diffusional (3-D)) and three-level chemical reaction, respectively. The values of pre-exponential factor were calculated though Eq. (5), and then the values of reaction rate constant were obtained though Eq. (3).

Figure 4 and 5 show the DSC-k(T) curves of BLT and YW coal, respectively. The conversion rate corresponding to the maximum heat release rate was taken as a segment point, and the DSC-k(T) curves were divided into two stages: the increasing stage and the decreasing stage of the heat release rate. The conversion rate corresponding to the maximum heat release rate of BLT and YW coal was always about 0.80 and 0.50, respectively., indicating that the conversion rate corresponding to the maximum heat release rate was only related to the coal rank, and not corrected to the atmosphere.

At the increasing stage of the heat release rate, the heat release rate of two coals increased exponentially with the increasing reaction rate constant. At the decreasing stage of the heat release rate, the heat release rate of YW coal decreased exponentially with the increasing reaction rate constant, whereas the heat release rate of BLT coal decreased linearly with the increasing reaction rate constant. This was because the ash on the coal surface obviously hindered the continued diffusion of O₂ into the coal pores, resulting in a decrease in the subsequent heat release rate. BLT coal contained less carbon and less ash, so its heat release rate decreased more slowly than that of YW coal and decreased linearly with the increasing reaction rate constant. ExpGro1 exponential model (see Eq. (6)) was selected to fit the DSC-k(T) curves at the increasing stage of heat release rate for the two coal samples. The model showed a high degree of fit, with the R² for both BLT and YW coal sample above 0.94. Therefore, the relationship between the heat release rate and reaction rate constant for both BLT and YW coal sample can be effectively expressed by the model. The relationship between the heat release rate and the reaction rate constant is approximately as Eq. (7).

$$y={y}_{0}+{A}_{1}\text{e}\text{x}\text{p}\frac{x}{{t}_{1}}$$

6

$$\left|\text{D}\text{S}\text{C}\right|={y}_{0}+{A}_{1}\text{e}\text{x}\text{p}\frac{\text{d}\alpha /\text{d}t}{{t}_{1}f\left(\alpha \right)}$$

7

Where, y₀ is the offset. A₁ is the amplitude, t₁ is the width.

In order to quantitatively analyze the relationship between y₀, A₁ and t₁ and heating rate, Figs. 6 and 7 show the changes in y₀, A₁ and t₁ with the heating rate, respectively. There was a linear relationship between y₀, A₁, t₁ and heating rate for YW coal. For BLT coal, y₀, A₁ and t₁ were basically linear with the heating rate in the O₂/N₂ atmosphere, whereas there was a non-linear relationship between y₀, A₁, t₁ and heating rate in the O₂/CO₂/N₂ atmosphere. Furthermore, y₀-β, y₀-A₁ and y₀-t₁ curves were fitted respectively. The reaction rate constant was calculated using Eq. (1). On the whole, there was a following relationship between the heat release rate and reaction rate of coal, as follows

$$\left|\text{D}\text{S}\text{C}\right|={a}_{1}\beta +{b}_{1}+({a}_{2}\beta +{b}_{2})\times \text{e}\text{x}\text{p}\frac{\text{d}\alpha /\text{d}t}{\left({a}_{3}\beta +{b}_{3}\right)f\left(\alpha \right)}$$

8

Where, a₁ and b₁ are constants related to y₀, a₂ and b₂ are constants related to A₁, a₃ and b₃ are constants related to t₁, as seen in Table 4.

Table 4
The value of constants in equation. (8)
Coal samples	Atmosphere	Conversion rate range	a₁	b₁	a₂	b₂	a₃	b₃
BLT	O₂/N₂	0.05–0.80	1.59	20.78	-1.67	-13.35	-0.09	0.19
BLT	O₂/N₂	0.80–0.95	1.28	21.04	-0.04	0.83	/	/
YW	O₂/N₂	0.05–0.50	0.33	22.12	0.08	-25.54	-0.00	-0.01
	O₂/N₂	0.50–0.95	0.75	0.84	-0.35	21.32	-0.42	2.89
	O₂/CO₂/N₂	0.05–0.50	1.47	6.42	-1.23	-12.41	-0.00	0.00
	O₂/CO₂/N₂	0.50–0.95	1.31	-4.48	0.05	13.32	-0.42	3.07

In this work, simultaneous thermal analysis experiments for BLT coal (high-volatile bituminous coal) and YW coal (anthracite) in the 21%O₂/79%N₂ and 15%O₂/5%CO₂/80%N₂ atmospheres were carried out. Based on the TG-DTG-DSC curves, the combustion characteristic parameters were discussed, the values of apparent activation energy were obtained using Coats-Redfern method, and the relationship between the heat release rate and reaction rate constant was quantitatively analyzed. The following conclusions can be drawn:

(1) A delay of ignition and heat release existed during the coal oxygen-lean combustion in O₂/CO₂/N₂. Decreasing O₂ concentration caused a significant reduction of local reactivity and further the decreasing maximum heat release rate for low-rank coal, while increasing CO₂ concentration caused a significant thermal lag effect and further the increasing maximum heat release rate for high-rank coal.

(2) During the combustion process, the values of apparent activation energy in the O₂/CO₂/N₂ atmosphere were approximately 33%~58% lower than that in the O₂/N₂ atmosphere for BLT coal, while the values of apparent activation energy in the two atmospheres for YW coal were close. For BLT coal, the values of correlation coefficients were less than 0.80 in the conversion rates ranges of 0.10 ~ 0.25 in O₂/N₂ and 0.15 ~ 0.45 O₂/CO₂/N₂, respectively, which was because that the precipitation of the remaining volatiles was promoted by the heat release of separated volatiles combustion, and the precipitation and combustion of volatile was significantly deferred in the O₂/CO₂/N₂ atmosphere compared with that in the O₂/N₂ atmosphere due to the slightly lower diffusivity of volatiles in CO₂ than in N₂ and the lower mass flux of oxygen to the volatiles flame.

(3) Regardless of the atmospheres, the conversion rates corresponding to maximum heat release rate of BLT and YW coal were about 0.80 and 0.50, respectively, indicating that the coal rank played a dominant role. At the increasing stage of the heat release rate, the heat release rate of the two coals increased conforming to ExpGro1 exponential model. At the decreasing stage of the heat release rate, the heat release rate of YW coal decreased exponentially with the reaction rate constant, while the heat release rate of BLT coal decreased linearly.

Acknowledgments

This work is supported by the National Natural Science Foundation of China [52004257], the 111 Project [B17041], and the Fundamental Research Funds for the Central Universities [2652018098].

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No competing interests reported.

Quantitative Analysis of Heat Release during Coal Oxygen-Lean Combustion in a O₂/CO₂/N₂ atmosphere by TG-DTG-DSC

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experiments And Methods

2.1 Preparation of coal samples

2.2 TG-DTG‑DSC experiment

2.3 combustion kinetic theory

3. Results And Discussions

3.1 The influence of the O₂/CO₂/N₂ atmosphere on the combustion characteristic parameters

3.2 The influence of the O₂/CO₂/N₂ atmosphere on the kinetic parameters by KAS method

3.3 The influence of the O₂/CO₂/N₂ atmosphere on the heat release

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

Quantitative Analysis of Heat Release during Coal Oxygen-Lean Combustion in a O2/CO2/N2 atmosphere by TG-DTG-DSC

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experiments And Methods

2.1 Preparation of coal samples

2.2 TG-DTG‑DSC experiment

2.3 combustion kinetic theory

3. Results And Discussions

3.1 The influence of the O2/CO2/N2 atmosphere on the combustion characteristic parameters

3.2 The influence of the O2/CO2/N2 atmosphere on the kinetic parameters by KAS method

3.3 The influence of the O2/CO2/N2 atmosphere on the heat release

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

Quantitative Analysis of Heat Release during Coal Oxygen-Lean Combustion in a O₂/CO₂/N₂ atmosphere by TG-DTG-DSC

3.1 The influence of the O₂/CO₂/N₂ atmosphere on the combustion characteristic parameters

3.2 The influence of the O₂/CO₂/N₂ atmosphere on the kinetic parameters by KAS method

3.3 The influence of the O₂/CO₂/N₂ atmosphere on the heat release