Inhibiting effect of CO2 on the oxidative combustion thermodynamics of coal.

A thermal analysis experiment was conducted in O2/N2/CO2 and O2/N2 atmospheres (O2 concentrations were 21, 14, 8, and CO2 concentrations were 0, 39, 46, 52) to investigate the thermal behavior of coal oxidation and combustion. Results demonstrated that an elevated CO2 concentration or decreased O2 concentration had a delaying effect on the thermogravimetric analysis and differential scanning calorimetry (DSC) curves; moreover, the characteristic temperatures were substantially augmented. When the O2 concentration was 21 vol%, the total heat released by coals A (highly volatile bituminous coal) and B (anthracite coal) decreased by 5.8% and 4.1%, respectively, after CO2 addition. The comprehensive combustion performance index was also lowered. The DSC curve can be divided into two exothermic peaks, and the ratio of the peak 1 to peak 2 areas decreased with the addition of CO2, which indicated that CO2 inhibited the oxidation of the active functional groups of coal structures. Apparent activation energy in O2/CO2/N2 was less than that in O2/N2. This journal is © The Royal Society of Chemistry.

Introduction

Coal is one of the most commonly used fossil energy resources in the world.[1] However, spontaneous combustion of coal is commonplace in many coal-producing countries, resulting in not only safety problems and resource waste[2-4] but also ecological damage and environmental pollution.[5,36] Therefore, understanding the characteristics of coal spontaneous combustion and developing preventative technology is crucial.[6] Conventional techniques adopted to prevent and control coal spontaneous combustion are irrigation with water and loess mud.[7] However, water and loess mud are unable to completely cover areas affected by the spontaneous combustion of coal. Consequently, a series of coal fire prevention and extinguishment materials, such as cellular grout, three-phase foam, gel foam, and suspension sand have been developed.[8,37] CO2 has not only a density greater than air but also an effective flame retardant effect. As a result, pouring CO2 into a goaf will completely bury the lower area, which is beneficial for the prevention and control of coal fires.[9] Therefore, CO2 firefighting technology has been developed, and used in the prevention and control of coal fires. Therefore, the inhibiting effect of CO2 on spontaneous combustion of coal under different conditions is the key to improving and using CO2 firefighting technology. But previous publications have focused on the characteristics of coal oxidation and combustion, and relevant experimental techniques such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been increasingly used.[10-12] Rotaru et al.[13] discovered that different types of coal demonstrated different apparent activation energies in the process of spontaneous combustion. Deng et al.[14] determined that a large difference in activation energy existed between the oxidation and combustion stages. The study of oxygen-enriched combustion characteristics involves examining the impact of CO2 on coal combustion.[15] Liu et al.[16] combusted two chars in O2/CO2 and O2/N2 atmospheres and found that substituting CO2 for inert nitrogen in the oxidizer had a subtle effect on coal combustion. Várhegyi et al.[17] demonstrated that during coal combustion in an O2/CO2 atmosphere, the net reaction rate of coal combustion was only proportional to the partial pressure of oxygen. Rathnam et al.[18] revealed that combustion reactivity prominently increased at high temperature stages in a 2% O2/CO2 atmosphere compared with a 2% O2/N2 atmosphere Zhou et al.[19] discovered that CO2 has a delayed effect on the maximum mass-loss rate and burnout temperature in the coal combustion process. Therefore, the influence of CO2 on characteristics of coal oxidation and combustion should be further study.

To further examine the inhibiting effect of CO2 on the characteristics of coal oxidation and combustion in different atmospheres, this study used the TGA and DSC methods to analyze the thermal behaviors and variation of apparent activation energies in O2/N2/CO2 and O2/N2 atmospheres. The main purposes of this research were to: (1) investigate the thermal behavior in different atmospheres; (2) analyze the effect of CO2 and oxygen concentration on apparent activation energies during oxidation and combustion of coal; (3) research the influence of CO2 and oxygen concentration on coal oxidation and combustion. The results are useful for understanding the oxidation and combustion characteristics of coal in O2/N2/CO2 and O2/N2 atmospheres and can aid development of CO2 fire prevention technology.

Experiments and methods

Experimental samples

Coal A was highly volatile bituminous coal and was collected from the Juye mining area in Shandong Province, China, whereas coal B was anthracite coal and came from the Furong mining area in Sichuan Province, China. The proximate and ultimate analyses of the two samples are listed in Table 1. The samples had a particle size of 0.067–0.079 mm and were stored in an airtight container.

Proximate and ultimate analyses of the two coal samples

Coal sample	Proximate analysis (air-dried basis, mass%)				Ultimate analysis (%, daf.)
	Moisture	Ash	Volatile matter	Fixed carbon	C	H	O	N	S
Coal A	2.3	9.7	34.85	53.15	85.71	4.58	7.48	1.54	0.69
Coal B	1.4	26.02	8.85	63.73	89.69	3.61	1.71	2.85	2.14

Experimental details

A synchronous thermal analyzer, STA449F3 (Netzseh, Germany), was employed to conduct the experimental tests. The heating rates were set as 5, 10, and 15 °C min−1. The experimental samples were heated from 30 to 800 °C for each heating rate. The gases, mixtures of O2, N2, and CO2, were constantly supplied at a 100 mL min−1 flow rate. Table 2 presents the gas mixtures with different volume fractions of O2, N2, and CO2.

Gases supplied when undergoing thermal analysis experiments

Gas	Volume fraction (vol%)
	1	2	3	4	5	6
O2	21	14	8	21	14	8
CO2	0	0	0	39	46	52
N2	79	86	92	40	40	40

Analysis methods

According to the heating rate used in the experiments, single heating rate and multi-rate heating rate methods were concluded to calculate the activation energy. In addition, the single heating rate method must determine the mechanism functions before calculating the activation energy,[20] which may cause errors in the process of selecting mechanism functions. However, the multi-heating rate method can avoid the selection of mechanism functions to obtain more accurate values of activation energy. For instance, Kissinger–Akahira–Sunose (KAS) method and Flynn–Wall–Ozawa (FWO) widely used to the calculation of activation energy, as eqn (1) and (2), respectively.[21]

According to the analysis of Starink,[21] KAS method developed by Starink offers more accurate estimates of Eα, as described in eqn (3):

The comprehensive combustion performance index (K) was used to comprehensively evaluate the characteristics of coal spontaneous combustion, and is expressed in eqn (2):[22]

Results and discussion

Features on TG-DSC tests

According to the characteristic temperatures in Fig. 1, the mass variation of coal was divided into three stages: the low-temperature pyrolysis stage (stage 1), oxygen-adsorption stage (stage 2), and combustion stage (stage 3). The boundary temperatures for stage 2 were T1 and Tig, where T1 is the initial temperature when the coal's mass continuously increases and Tig is the ignition temperature.[20] Furthermore, T2 was defined as the temperature at which the maximum exothermic peak is completed on the DSC curve. Stage 1 was from the initial temperature to T1. Stage 3 was from Tig to T2.

Fig. 1

TG and DSC curves of a coal sample showing the temperature boundaries for different stages. This individual diagram is obtained from the thermal analysis of coal A conversion under 21 vol% O2/79 vol% N2 and heating rate of 5 °C min−1.

Mass variation

Fig. 2 illustrates the TG and differential thermogravimetry (DTG) curves, and the characteristic temperatures are given in Table 3 with a heating rate of 5 °C min−1.

Fig. 2

TG and DTG curves of the two coal samples with the heating rate of 5 °C min−1 under different atmospheres, (a) coal A and (b) coal B.

Characteristic temperatures during combustion of coal with heating rate of 5 °C min−1

Sample Atmosphere (O2 : N2 : CO2)	Coal A				Coal B
	T 1 (°C)	T ig (°C)	T max (°C)	T 2 (°C)	T 1 (°C)	T ig (°C)	T max (°C)	T 2 (°C)
21 : 79 : 0	81.3	308.4	482.9	579.5	83.35	367.6	523.2	617.9
14 : 86 : 0	79.1	313.6	500.1	591.1	81.45	370.1	539.7	627
8 : 92 : 0	83.1	323.1	516.2	616.4	70.25	382.3	567.1	649.2
21 : 40 : 39	71.9	309.2	485.4	583.4	84.55	371.7	524.9	618.8
14 : 40 : 46	80.6	317.9	499.7	598.7	81.45	378.6	546	634.7
8 : 40 : 52	75.7	325.5	523.9	627.5	80.55	387.4	576.5	660.2

Both the increase of the CO2 concentration and decrease of the O2 concentration made the TG/DTG curve move to a high temperature range, as illustrated in Fig. 2. Tig and Tmax decreased with increasing O2 concentration. For example, when the O2 concentration was reduced from 21 to 8 vol%, the Tig of coal A increased from 308.4 to 323.1 °C, and the Tig of coal B increased from 309.2 to 325.5 °C (Table 3). The lower the oxygen concentration in the environment, the lower the number of activated oxygen molecules. Therefore, the opportunities of collision between activated oxygen molecules and active functional groups of coal is significantly reduced, and the reaction between coal and oxygen becomes difficult. This finding is in agreement with the reports by Deng et al.,[14] Wang et al.,[23] and Liu et al.[24] When the O2 concentration was constant, increasing the CO2 concentration had a delay effect on the TG and DTG curves, whereas the characteristic temperatures increased along with CO2 concentration. As Table 3 shows, when the O2 concentration was 21 vol% and the CO2 concentration was increased from 0 to 52 vol%, the ignition temperature of coal A increased from 323.1 to 325.5 °C and the ignition temperature of coal B increased from 382.3 to 387.4 °C. The diffusion rate of O2 in CO2 is lower than that in N2, which has an adverse effect on the transportation of O2 from the gas mixture to the coal particle surface.[23,25] In addition, CO2 is much easier for coal to physically adsorb than O2 and N2.[26,27] Therefore, adsorption of CO2 occupies some of the active sites on the coal surface, inhibiting the ability of O2 and coal to interact.

Exothermic properties

Differential scanning calorimetry

The DSC and first-order differential of DSC (DDSC) curves for the two coal samples from 30 to 800 °C were illustrated in Fig. 3. Total heat release is presented in Table 4.

Fig. 3

DSC and DDSC curves of the two coal samples with the heating rate of 5 °C min−1 under different atmospheres, (a) coal A and (b) coal B.

Factors used to fit sub-peaks for DSC curves and total heat release and the maximum of exothermic peak with heating rate of 5 °C min−1

Coal sample	Atmosphere (O2 : N2 : CO2)	Peak	v 0	S	t c	w	R 2	Q (kJ g−1)
Coal A	21 : 79 : 0	1	0.306	−172.495	66.473	35.291	0.9896	19.247
		2		−162.325	90.641	13.913
	14 : 86 : 0	1	0.156	−158.973	68.084	35.237	0.9939	17.891
		2		−159.263	93.457	15.907
	8 : 92 : 0	1	0.042	−139.887	69.772	34.700	0.9972	17.296
		2		−175.342	97.945	19.815
	21 : 40 : 39	1	−0.141	−162.444	66.061	34.582	0.9889	18.177
		2		−158.456	90.895	14.639
	14 : 40 : 46	1	−0.274	−118.875	67.107	33.344	0.9942	17.893
		2		−150.177	94.420	18.585
	8 : 40 : 52	1	−0.093	−131.845	70.106	34.198	0.9938	17.794
		2		−172.622	99.379	20.963
Coal B	21 : 79 : 0	1	0.038	−103.340	82.755	38.798	0.9972	15.308
		2		−138.154	97.773	12.381
	14 : 86 : 0	1	−0.165	−89.947	83.914	39.612	0.9984	13.812
		2		−141.981	100.100	13.747
	8 : 92 : 0	1	0.004	−68.126	84.428	39.436	0.9967	13.018
		2		−152.792	106.658	16.9603
	21 : 40 : 39	1	−0.200	−106.997	83.199	39.943	0.9976	14.779
		2		−144.890	98.404	12.736
	14 : 40 : 46	1	0.021	−76.145	83.285	37.229	0.9983	13.197
		2		−144.658	102.357	14.770
	8 : 40 : 52	1	−0.081	−68.306	87.135	42.810	0.9948	12.476
		2		−143.390	108.245	17.740

Fig. 3 shows that exothermic peaks occur when the temperature exceeds 200 °C. The exothermic process finished when the temperature was more than 600 °C. The DSC curve of coal exhibited typical bimodal exothermic behavior. However, the first exothermic peak of the DSC curves for coal B was not clear. This phenomenon is caused by differences in the types and quantities of active structures contained in coal. Some active structures, such as alkyl chains, methyl groups, carboxyl groups, and hydroxyl groups, are easily oxidized at lower temperatures.[28,29] However, the combustion or decomposition temperature of some stable structures, such as aromatic structures, is higher.[30] When the concentration of CO2 was zero, the maximum heat intensity and total heat increased significantly with an increase in the O2 concentration. When the O2 concentration was increased from 8 to 21 vol%, the total heat released by coal rose from 17.296 to 19.247 kJ g−1, as listed in Table 4. When the O2 was sufficient, the coal–oxygen reaction was more intense. With the increased O2 concentration, the coal and O2 were able to have more contact; therefore, the oxidation/combustion reactions of the coal produced a high proportion of stable oxidation products, increasing heat intensity. When the O2 concentration was kept constant, the DSC and DDSC curves moved to the high temperature region, whereas total heat release decreased substantially with increasing CO2 concentration. As Lee[31] noted, competitive adsorption occurs in coal when various gases exist. The adsorption of CO2 causes a reduction in the amount of O2 contacting coal. In addition, more unstable products are formed, leading to an apparent reduction in heat intensity.

Stage characteristics of heat release

The DSC curves for the two samples at a heating rate of 5 °C min−1 and in O2 : N2 : CO2 = 21 : 79 : 0 and O2 : N2 : CO2 = 21 : 40 : 39 atmospheres are illustrated in Fig. 4. The sub-peaks of the DSC curves were fitted using eqn (5) (Gaussian equation). The fitting parameters of the sub-peaks are given in Table 4. Fig. 5 illustrates the ratio of the peak 1 to peak 2 areas.

Fig. 4

DSC curves from the pyrolysis domain stage (stage 1) through the combustion domain stage (stage 3) for the two coal samples, (a) coal A in atmosphere O2 : N2 = 21 : 79, (b) coal A in atmosphere O2 : N2 : CO2 = 21 : 40 : 39, (c) coal B in atmosphere O2 : N2 = 21 : 79, and (d) coal B in atmosphere O2 : N2 : CO2 = 21 : 40 : 39.

Fig. 5

Ratio of the peak 1/peak 2 areas.

The temperature corresponding to the maximum of peak 1 in coal A was in the range of 320–360 °C, whereas it was in the range of 410–440 °C for coal B, as listed in Table 4. Furthermore, the temperature corresponding to the maximum of peak 1 was close to the ignition temperature of coal. Some of the active structures in coal, such as alkyl chains, methyl groups, carboxyl groups, and hydroxyl groups, are oxidized at low temperatures, resulting in a clear oxidative-exothermic peak before coal combustion.[30] Peak 2 was caused by the breaking of numerous stable structures (such as aromatic rings) into small molecules that participated in the combustion reaction.[32] As illustrated in Fig. 5, the ratio for coal A was clearly larger than that for coal B, which indicates that attribution of peak 1 to the total heat release was greater than coal B. The primary reason for this phenomenon is that the metamorphic grade of coal A was lower than that of coal B, and the molecules of coal A contained numerous alkyl chains, methyl groups, carboxyl groups, hydroxyl groups, and other active structures.[33] The ratio decreased with a decrease of the O2 concentration or increase of the CO2 concentration, which indicates that a low O2 concentration or high CO2 concentration inhibits oxidation of some reactive groups in coal.

Comprehensive combustion performance index

The dependence of K on the O2 concentration for the two samples is illustrated in Fig. 6. The comprehensive combustion performance index of the two samples decreased with a decrease in the O2 concentration; furthermore, the higher the O2 concentration was, the more O2 participated in coal combustion, leading to the combustibility being strengthened. For the same O2 concentration, the addition of CO2 resulted in a significant decrease in the comprehensive combustion performance index. The physical adsorption of CO2 onto the coal surface occupied active sites, which hindering the physical adsorption and chemical adsorption of O2 by coal, and further inhibiting coal oxidation and combustion.

Fig. 6

Relationship between the comprehensive combustion performance index and O2 concentration.

Apparent activation energy

The calculation of the apparent activation energy uses the Starink method, which is a multiple heating rates calculation method. Therefore, a set of data points of multiple heating rates (5, 10 and 15 °C) calculated at intervals of 0.5 from 0.5 to 0.95 was obtained according to eqn (3), and ln(β/(T + 273.15)1.92) and 1/(T + 273.15) were calculated at different conversion rates as shown in Fig. 7. The apparent activation energies during stages 2 and 3 for the two coal samples are given in Fig. 8 and 9.

Fig. 7

Linear relationship between ln(β/(T + 273.15)1.92) and 1/(T + 273.15) of coal A in atmosphere O2 : N2 : CO2 = 21 : 40 : 39, (a) in oxygen-adsorption stage and (b) combustion domain stage.

Fig. 8

Relationship between apparent activation energy and conversion during the oxygen-adsorption stage (stage 2), (a) coal A and (b) coal B.

Fig. 9

Relationship between apparent activation energy and conversion during the combustion domain stage (stage 3), (a) coal A and (b) coal B.

Oxygen-adsorption stage

The physical and chemical adsorption of O2 and CO2 is easier and the energy required for the chemical reaction between O2 and coal is lower at the beginning of the oxygen-adsorption stage. The reaction between coal and oxygen is a step-by-step reactive process in which functional groups are gradually activated and oxidized. At low temperatures, only some active structures are activated and react with O2. Moreover, these oxidation reactions require less reaction energy, resulting in lower apparent activation energy of coal. As the temperature increases, more stable structures are gradually activated and react with O2. These oxidation reactions require more energy, leading to an increase in apparent activation energy. Therefore, we observed that with an increase of the conversion rate, apparent activation energy increased. Furthermore, the tendency and value of apparent activation energy changed relative to the conversion rate at all percentages of O2 in O2/N2 or O2/CO2/N2, while tendency of apparent activation energy was almost the same, as illustrated in Fig. 8. When the CO2 concentration was 0, the apparent activation energy decreased with an increase in the O2 concentration. In light of the criteria of competitive reactions, the primary reason for this behavior is that the active structures consume a considerable amount of O2. The decrease of the O2 concentration inhibits the reaction of the stable structures in coal with O2, which requires more energy. In addition, coal B was anthracite, which provides more surface area for strongly competitive chemisorption,[34] resulting in complex changes of the apparent activation energies. When the O2 concentration was the same, the apparent activation energy of coal oxidation in O2/N2 was greater than that in O2/CO2/N2. Liu et al.[24] considered chemisorption in O2/CO2 conditions to be mainly controlled by O2 chemisorption at the oxygen-adsorption stage. Moreover, CO2 competes with O2 when they are adsorbed by active sites on the coal surface, which hinders the chemical reaction of coal and O2. In summary, O2 competed with CO2 for active sites; thus, more energy was required.

Combustion stage

The apparent activation energy of coal A first increased and then decreased with an increase in the conversion rate. Coal B exhibited a similar tendency when the O2 concentration was more than 8 vol%. When the O2 concentration was 8 vol%, the apparent activation energy of coal B decreased with an increase in the conversion rate, as illustrated in Fig. 9. Temperatures when the apparent activation energy reached the maximum were close to the devolatilization temperature of coal. When the temperature exceeded the devolatilization temperature, a large volume of volatiles evaporated out, leading to an increase in the diffusion resistance of O2 to the coal pores.[35] Then, the apparent activation energy of coal combustion tended to decrease due to the large number of active structures resulting from pyrolysis. Values of the apparent activation energy in N2/O2 atmospheres were larger than those in O2/N2/CO2 atmospheres. The cause of this phenomenon is that competition chemisorption of O2 and CO2 on the coal surface during combustion in the O2/N2/CO2 atmosphere, which causes the chemical adsorption process of oxygen in the O2/N2/CO2 atmosphere significantly longer than in the O2/N2 atmosphere.[24] Consequently, the probability of oxygen contacting with the active structure is reduced. Therefore, the combustion rate of coal in the O2/N2/CO2 atmosphere is lower than that in the O2/N2 atmosphere (Fig. 2).

Conclusions

• With an increase in the CO2 concentration or decrease in the O2 concentration, the thermal analysis curves of coal were delayed and the characteristic temperatures increased.

• The DSC curve can be divided into two exothermic peaks, and the ratio of the peak 1 to peak 2 areas decreased with the addition of CO2, which indicated that CO2 inhibited the oxidation of active functional groups of coal structures. Furthermore, the total heat release decreased. Higher concentrations of CO2 or lower concentrations of O2 resulted in a lower comprehensive combustion performance index.

• At the oxygen-adsorption stage, the apparent activation energy increased with an increase in the conversion rate, whereas it first increased and then decreased at the combustion stage. The apparent activation energy in the O2/CO2/N2 atmosphere was less than that in the O2/N2 atmosphere.

Apparent activation energy (J mol−1)

Constant

Comprehensive combustion performance index (%2 min−2 °C−3))

Total heat release (kJ mg−1)

Gas constant (J mol−1 K−1))

Peak area (mW min mg−1)

Temperature (°C)

Initial temperature when the coal's mass continuously increases (°C)

Temperature is exothermic completed on DSC curve (°C)

Burnout temperature (°C)

Center of the peak time (min)

Ignition temperature (°C)

Baseline of peak (mW mg−1)

Full width at half maximum (min)

Change rate of exothermic (min)

Average rate of mass loss (% min−1)

Maximum rate of mass loss (% min−1)

Conversion rate (%)

Heating rate (°C min−1)

Conflicts of interest

There are no conflicts to declare.