Experimental investigation on microstructure evolution and spontaneous combustion properties of secondary oxidation of lignite

Accepted Manuscript Title: Experimental investigation on microstructure evolution and spontaneous combustion properties of secondary oxidation of lignite Authors: Yibo Tang, Huae Wang PII: DOI: Reference:

S0957-5820(18)30379-3 https://doi.org/10.1016/j.psep.2019.01.031 PSEP 1650

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

26 June 2018 27 January 2019 30 January 2019

Please cite this article as: Tang Y, Wang H, Experimental investigation on microstructure evolution and spontaneous combustion properties of secondary oxidation of lignite, Process Safety and Environmental Protection (2019), https://doi.org/10.1016/j.psep.2019.01.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental

investigation

on

microstructure

evolution

and

spontaneous combustion properties of secondary oxidation of lignite

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Yibo Tang Huae Wang

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Taiyuan University of Technology, College of Mining Technology, Taiyuan, 030024, China

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Correspondence author e-mail: [email protected]

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Abstract: Effective prevention and control of spontaneous fire of low-rank coal at underground

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goafs are critical to ensure coal mine safety. The characteristics of secondary oxidation of two

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lignites types were investigated. The results show that the spontaneous combustion tendency of

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the preoxidized lignite is higher than that of the raw coal, especially in the early stages of low-temperature oxidation. After preoxidized by air under different conditions, coal samples were

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used in an adiabatic oxidation experiment (by H2O2) to simulate the self-heating process. The

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experimental results show that after being pre-oxidized 100–150 °C, the oxidation rate of coal sample re-oxidation increased significantly. and this effect is closely correlated to the treatment time and temperature. The TG/DSC tests show that preoxidation gradually increases the

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self-heating risk of coal; however, when the treated temperature exceeds 175°C, the calorific value of coal decreases sharply. The preoxidation step weakens some functional groups in coal to a certain degree, but the proportion of C–O structure fluctuates, decreasing the activation energy of the secondary oxidation of coal between 40 and 120 °C. The proportion of coal particles with

porosity 10 nm increases in coal samples studied after the treatment, which favors the oxidation diffusion of coal. In general, secondary oxidation causes microstructural changes in lignite and increases the risk of spontaneous combustion, but preoxidation at an excessively high

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Keywords: spontaneous combustion; lignite; secondary oxidation; coal mine safety

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temperature may over consume the organic constituents, decreasing the liability of self-heating.

1 Introduction

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Controlling spontaneous combustionof coal is a challenging task and attracts the attention of

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scientists worldwide[1–3]. Prevention and control of spontaneous combustion of residual coal in

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the goaf area and large-scale fire in coal seams incur huge cost[4–6]. In the past two decades,

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scholars have attempted to explain mechanisms underlying spontaneous combustion of coal

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from multiple perspectives and have investigated the internal and the external factors triggering spontaneous coal combustion process to develop efficient fire prevention strategies and

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extinguishing materials[7, 8]. Many studies have evaluated the efficiency of novel materials such as

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syntactic foams[9, 10], smart hydrogels[11], chemical inhibitors[12, 13], antioxidants[14–17], and aerosol materials[18] in controlling the spontaneous combustion of coal. However, further investigations

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are necessary to inhibit spontaneous fire in coal mines rapidly and efficiently. Self-heating of residual coal in a goaf is the most common accident in an underground mine,

which is mainly attributed to poor heat storage and ventilation conditions[19–22]. Because the goaf is a semi-enclosed space with no direct access, the ignition source remains concealed and imperceptible[23]. Although a series of mathematical models have been developed to predict the

evolution of spontaneous combustion and to evaluate the range of high-temperature zone by numerical simulations[24–26], it is difficult to directly quell spontaneous fires. Instead, an early warning given at the initial stage of the fire can be used to reduce the air leak into the goaf by applying methods such as sealing corners of the working face, adjusting the ventilation, and

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setting up closed walls[27]. By reducing the oxygen supply, combustion can be slowly controlled[28]. Once oxygen isolation fails and fresh air flows into the goaf, a more severe combustion process

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may ensue, increasing the fire risk[29]. These phenomena resemble the principle of a “backdraft”

in which the fire evolves into lean-oxygen combustion from normal-oxygen combustion when the

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enclosed oxygen in the room gradually runs out; however, once fresh air flows into the room,

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combustion is likely to be triggered again[29]. Wen et al. investigated early warning gas indicators

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for spontaneous coal combustion and concluded that the CO generation rate is higher in coal

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after secondary oxidation than that in raw coal, but that of CO2 is lower[30]. Deng et al. studied

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spontaneous combustion characteristics of four types of coal after secondary oxidation by programmed heating and found that the risk of spontaneous combustion is greater in coal after

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secondary oxidation[31]. Ma et al. compared the inhibition effects of inhibitors on the

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spontaneous combustion of both the preoxidized coal and raw coal samples and showed that inorganic chlorides have a better inhibition effect on raw coal[32]. All these investigations facilitate the understanding of secondary oxidation characteristics of coal, but the pore structure,

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thermodynamics and functional group changes related to spontaneous coal combustion characteristics before and after secondary oxidation have not been completely explained. In this study, two types of representative lignite samples were used to study the evolution of spontaneous combustion characteristics to reduce the risk of combustion of residual coal in the

coal mine goaf. 2 Experimental 2.1 Pretreatment Two types of lignite samples were used and their parameters are presented in Table 1. First, the

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lignite sample was placed in a vacuum drying oven and desiccated for 24 h at 40 C for further use. The coal was then placed in a metal container for preoxidation at 100, 125, 150 and 175 oC,

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respectively. Air was delivered at a flow rate of 20 mL/min. 2.2 Adiabatic oxidation

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After the pretreatment and to simulate the process of adiabatic coal oxidation, 5 g of preoxidized

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coal sample was placed in an insulated bottle (Fig. 1a) for treatment with hydrogen peroxide

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(H2O2). Hydrogen peroxide used in the experiments was obtained from Sinopharm Chemical

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Reagent Co.,Ltd. The initial temperature was 25 °C. After the bottle was instilled with 15 mL of

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30% H2O2, the magnetic mixing apparatus was turned on at a rotating speed of 120 r/min. When the coal sample mixes with the H2O2 solution, the oxidation reaction between these two mixtures

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begins almost immediately. The temperature of the mixture then rises sharply and temperature

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data are recorded continuously. 2.3 Microscopic testing TG/DSC tests were performed in the temperature range from room temperature to 800 °C under

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an air atmosphere at a heating rate of 10 K/min. The purge gas used in TG/DSC is dry air (flow rate 50 ml/min). Coal samples weighing 1 g were used to determine the infrared spectra, measured using an infrared spectrometer (Bruker VERTEX 70), and the changes in the organic functional groups of the coal before and after the treatment were analyzed in the 500–4000 cm-1

spectral range. Changes in the coal surface atoms before and after the treatment were analyzed by energy dispersive spectroscopy (EDS). 2.4 Gas emission Finally, using a self-developed experimental furnace (Fig. 1b), heating experiments of both

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preoxidized and raw samples were performed on 70-kg coal at temperatures between 30 and 200 °C at an air flow rate of 10 L/min and the change in the concentration of gases released from

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the coal before and after the preoxidation step during the heating process was measured. 3 Results and discussion

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3.1 Adiabatic heating

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Fig. 2 displays the adiabatic oxidation results obtained following treatment with hydrogen

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peroxide. The results indicate that the coal temperature gradually rises after treatment with

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hydrogen peroxide, the heating accelerates with time. The data vary considerably as a function of

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the preoxidized conditions.After preoxidation, the heating gradient of coal is greater than that of raw coal, and this tendency becomes more conspicuous with increasing treatment time. Clearly,

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the secondary oxidation of coal is more sensitive to oxidant and heat than its initial oxidation,

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because in the first oxidation process, organic matter comprised of macromolecules is decomposed into smaller molecules that can be oxidized more easily. With increasing oxidation treatment temperature, the time needed for coal sample to reach a temperature of 90 °C

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decreases. For example, before treatment, temperature of raw coal increases from ambient temperature to 90 °C within 20.8 min. Minimally, with a preoxidation time of 360 min at 150 °C, only 9.8 min was required for the coal temperature to rise to 90 °C. Handling from 100 to 150 °C, the spontaneous combustion tendency of treated coal sample increased conspicuously.

Nevertheless, as the pretreatment temperature further rises and reaches 175 °C, the resulting heating rate starts to decrease. It is worth noting that at 175 °C, as the pretreatment time increases, the sensitivity of coal to secondary oxidation decreases and becomes similar to that of raw coal. This is because the oxidation temperature is close to the ignition point of coal, which

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results in the overconsumption of organic small molecules. 3.2 Thermal analysis

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As shown in Fig. 3, different temperature profiles can be distinguished due to the fact that the

mass loss rate differs in different temperature stages. In the early stage of the spontaneous

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combustion of coal, there is a critical temperature around 60-80 oC. When the temperature rises

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to the critical temperature, the heating rate of coal increases rapidly. When the temperature of

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SX lignite rises from room temperature to 72 °C (critical temperature)[33-34], the mass slowly

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decreases, and the coal experiences weak oxidation; between 72 °C and 267 °C (ignition

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temperature), the oxidation rate of coal accelerates, and organic contents decompose and oxidize into active small molecules. This stage is termed the preignition stage of coal. As the temperature

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increases from 267 °C to 606 °C (stable temperature), the proportion of mass loss is the largest; at

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this stage, coal reaches the ignition point and experiences vigorous reaction, during which not only the active functional groups such as side chains and bridge bonds participate in oxidation[35-36], but also stable aromatic structures decompose and oxidise. When the

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temperature increases from 595 °C to 800 °C, the coal mass loss decreases slightly, which indicates that combustion has almost finished. Correspondingly, the three characteristic temperature points of YN lignite are 79 °C, 259 °C, and 630 °C, respectively. In order to ensure the reliability of the data, the samples in Fig.3 are all processed for 360 minutes. From 100 oC to 150

o

C, the higher the pre-oxidized temperature, the greater the difference between treated and raw

coal. However, this trend changes when the pre-oxidized temperature reaches 175oC. The spontaneous combustion tendency of treated coal samples will be weakened and the difference between raw and treated coal will be smaller. Obviously, at treatment temperatures 175 °C, coal

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exhibits totally different oxidation behavior. Combined with DSC data, it is shown that the exothermic peak of the sample treated by 175°C reduces slightly compared to raw coal,

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demonstrating a decrease of the heat release potential during the combustion period. Although

the thermal analysis shows that pre-oxidation between 100-150°C activates coal, what can not be

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ignored is that high temperature treatment at 175 °C weakens the oxidation potential of coal.

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This can be attributed to the reduction in the heat value because of consumption of organic

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constituents during coal pre-oxidation. Consumption of organic matter in coal varies, especially

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during the high temperature stages, the consumption rate increases rapidly. Since the rate of

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organic consumption in coal is never linear at different temperatures, organic compounds are oxidised when the temperature exceeds a certain threshold. Thus, it can be believed that even

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secondary oxidation of lignite enhances spontaneous fire risks and it must satisfy one condition

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that the pre-oxidized temperature <175 °C. 3.3 Infrared spectra

Spontaneous coal combustion characteristics depend on the types and contents of functional

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groups in coal. As shown in Fig. 4, the main organic functional groups of both SX and YN lignite mainly include -OH, -CH2-, C=C, C–O and C–X. According to the Lambert–Beer law, the group concentration was calculated based on absorbance data. A comparison of infrared functional groups of coals treated under different conditions (Fig. 5) shows the -OH content in the SX raw

coal decreases as temperature rises. For pre-oxidized coal, the trend of functional groups with temperature is similar to that of the raw coal. As temperature rises, the -CH2- content first rapidly decreases and then becomes stable. As for the coal after pre-oxidation treatment at 125 °C, there is only a slight decrease in -CH2- content; however, as the treatment temperature reaches

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175 °C, the -CH2- content in coal quickly decreases, which indicates that the structures of aliphatic hydrocarbons in coal are sensitive to temperature in the low-temperature oxidation stage. After

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pre-oxidation, the content of aliphatic hydrocarbons in coal remains at a low level and will not

play a promoting role in the process of secondary oxidation. The change in the C–O structure is

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more complicated. As the temperature rises, the C–O decreases first, but increases with further

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rise of temperature, which is related to the strengthened oxidative activity of coal in the

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high-temperature regime. This phenomenon ensures continuous production of new

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oxygen-containing functional groups. In theory, spontaneous combustion of coal requires that

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functional groups in coal contact with O2 and emit heat. In the initial stage of spontaneous coal combustion the temperature is <200oC, and some stable chemical functional groups never react

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with oxygen. Therefore, functional groups such as aliphatic hydrocarbons and O-containing

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functional groups that flourish <200oC are particularly important in reacting with oxygen. Although the content of aliphatic hydrocarbons decreases significantly, the number of O-containing functional groups increases. The increase of O-containing functional groups

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counters the reduction in the number of aliphatic hydrocarbons, which are responsible for the fact that pre-oxidized coal has a higher spontaneous combustion tendency. Compared with other functional groups, as a relatively stable structure, C=C varied uniformly with the rise in temperature. Focus should be on the active structures that act as initiation sources at very initial

phases of spontaneous coal combustion. Heat released from their oxidation leads to a temperature rise and with that activates other functional groups. For pre-oxidized coal, O-containing functional groups are the focus that we should pay attention to. 3.4 Oxidation products

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Data on CO and CO2 emission from 40 oC to 120 oC are presented in Fig. 6. Specifically, at the initial stage, the concentration of CO produced by the pre-oxidized coal is higher than that of the

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raw coal. At 40°C, the maximum CO emission for pretreated SX coal is 2.6 times higher than that for raw coal. However, with the increase of reaction temperature, the concentration of CO

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produced by raw coal rises faster than that of the pre-oxidized coal. The ratio of the maxima of

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pretreated sample to raw coal drops to 1.2 when the temperature climbs to 120°C. Results from

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emission of the oxidation products suggest that pre-oxidized coal releases more CO when the

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temperature < 120 oC. After treatment, the treated coal samples showed a higher spontaneous

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combustion propensity than raw coal. But as the temperature increases, the disparity of CO emission from treated and raw sample decreases. The reduction of organic matter in treated coal

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leads to the potential of CO emission which continuously reduces as the temperature increases.

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For the gases, mainly CO and CO2, produced in the programmed heating oxidation process of coal, the reaction process can be written as follows:

coal  O2  mCO  gCO2  other products

(1)

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According to the Arrhenius equation, the reaction rate of coal oxidation at any temperature is given by Eq. (2):

v(Ti )  v(O2 )  v(CO) / m  v(CO2 ) / g  AcOn 2 exp( Ea / RTi )

(2)

where v is the reaction rate, mol/(m3∙s); T is the temperature of coal, K;

A is the

pre-exponential factor; cO 2 is the initial oxygen content in the reaction gas, mol/m3; n is the order of the reaction; E a is the activation energy, J/mol (in this composite reaction, the calculated activation energy is referred to as the apparent activation energy); and

R is the

molar gas constant and is equal to 8.314 J/(mol·K).

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Generally, the emitting source of CO2 is not unique in underground collieries, and CO is more reliable as an indicator gas on evaluating the combustion behaviour of coal[37-38]. Conventionally,

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for coal-related fire problems, the rate of the coal–oxygen reaction is calculated based on the CO data[39-40]. As the result, the activation energy of coal oxidation be obtained according to the

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emission variation of CO. Assuming that in the whole process the coal mass is constant and the

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initial concentration of oxygen is constant before and after the reaction, the air flow is only axial

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along the coal canister, and the coal temperature in the canister is homogenously distributed, the

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production rate of CO at the axial coordinate dx in the coal canister can be written as[41] follows:

S  d x  v(CO)  kvg dc

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(3)

v(CO) is the production rate of CO,

where S is the sectional area of the coal canister, m2;

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mol/(m3·s); k is the unit conversion factor[41-42] and equals 22.4 × 109 mol/m4; v g is the air

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flow rate, m3/s; and c is the CO concentration, %; Substituting Eq. (2) into Eq. (1) leads to the following equation:

ASmcOn 2 exp(  Ea / RTi )dx  kvg dc

(4)

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Integrating both sides of Eq. (4) leads to Eq. (5):



L

0

ASmcOn 2 exp(  Ea / RTi )dx  

cout

0

kvg dc

(5)

where l denotes the height of the coal canister, m, and is equal to 0.1 m; and cout denotes the concentration of CO at the outlet of the coal canister.

By taking natural logarithm on both sides of Eq. (5), we get n

ln cout

ASLmcO2 E 1  a  ln( ) R Ti kvg

(6)

Eq. (6) shows that the relationship between ln cout and 1 Ti is linear; therefore, the

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activation energy E a of coal can be obtained by measuring the CO concentration at the outlet of the coal canister at different Ti in the experiments, plotting ln cout – 1 Ti and calculating

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the slope of the straight line.

As presented in Table 2, at temperatures between 40 and 120 °C, before and after treatment, the oxidation activation energies of SX and YN lignites declined by 14.3% and 7.8%, respectively.

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Compared with the initial oxidation of raw coal, coal subjected to secondary oxidation can more

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easily react with oxygen, and thus suffers from a higher risk of spontaneous combustion.

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3.5 Porosity and surface changes

The porosity of both lignites after treatment is given in Fig. 7. In YN raw coal, pores with a

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diameter 10 nm account for 42.34% of the total pore volume, which constitutes the highest

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fraction. After pre-oxidized treatment, this drops to 30.51%. Additionally, pores of 100-1000 and >1000nm reach 10.05% and 10.31%, respectively after treatment. It is apparent from Figure

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7 that the porosity in coal clearly increases after pre-oxidation notwithstanding that the proportionality of micropore, mesopore and macropore evolves in different directions. These

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pore evolutions are more moderate in SX coal, but the general trend is the same. According to some classical theories[43-45], during the early self-heating period, oxygen diffusion in coal is restrictedly controlled by porosity, which also means that spontaneous combustion performance for coal is closely related to its pore distribution. Besides, pores <10 nm are hardly become channels for oxygen diffused in coal[46-47]. Based on these results, it can be concluded that

pre-oxidation of coal increases the proportion of pores with diameter 10 nm and is conductive to the diffusion of oxygen in coal, thereby facilitating spontaneous coal combustion. Fig. 8 shows the changes in surface atoms of both lignites before and after treatment. Proportion of C and O in raw SX coal is the largest, and other atoms, like Si, Al, S, K, Ca and Fe,

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occupy less than 10% of the total. Clearly, the most remarkable competition occurs between C and O, while changes in heteroatoms can be ignored. The proportion of carbon atoms on the

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surface of raw SX coal reaches nearly 70%. After treatment at 100-150 °C, this number slowly dropped to nearly 60%. Once treated temperature climbs to 175 °C, proportion of C atoms is

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sharply declined to 48%, whereas data of O atoms rise to 46%. Data related with YN sample are

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the same in essentials while differing in minor points. Migration of atoms on coal surface before

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and after preoxidation illustrates that undergo treatment over 175 °C will lead to boosting

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enrichment of oxygen atoms on coal surface. Further, on the basis of mentioned performance for

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two kinds of pre-oxidized coal treated at 175 °C, excessively low C atom density (<50%) on surface

4 Conclusions

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is unfavorable to development on spontaneous combustion of coal.

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In order to investigate whether the risk of spontaneous combustion in the gob of a coal mine is increased after exposing the fire area, experiments were carried out for comparing the low-temperature oxidation behavior of raw coal and pre-oxidized coal. Often in the current

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literature, people prefer to agree with re-oxidation coal has a high spontaneous combustion propensity, with no consideration given to evolution at microscopic level, critical parameters and essential constraints during the self-heating process. Usual standpoints doubt that why pre-oxidized coal has more sensitive surfaces, especially under the fact that active groups are

consumed during initial treatment. According to the IR analysis, aliphatic hydrocarbon groups are compensated for by oxygen-containing functional groups formed during oxidation, which increases the spontaneous combustion risk of pre-oxdized coal. Based on experimental findings on porosity, with increasing of the treating temperature the ratio of pore <10nm tends to be

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continuously promoted, that is explicitly favors self-ignition of coal. Spontaneous ignitability of pre-oxidized coal does not monotonically increase with an increase in the pretreatment

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temperature. According to the Investigation from 100-175°C, once the temperature reaches 175 °C, the tendency of spontaneous combustion of coal will decline rapidly. Thus, this point

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serves as a threshold temperature for explicating the behavioral differentiation related to

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adiabatic oxidation for coal pre-oxidized under different temperatures.

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Acknowledgments

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This study is funded by the Project of National Natural Science Foundation of China (No.

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China(2015-037).

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51604185) and the Research Project Supported by Shanxi Scholarship Council of

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and Environmental Protection. 2018:1-27.

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(a) Set up for oxidation of coal

(b) Devices for programmed heating of coal samples Figure 1 The experimental setup

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(a) Samples pretreated at 100 °C

(b) Samples pretreated at 125 °C

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(c)Samples pretreated at 150 °C

(d) Samples pretreated at 175 °C Figure 2 Temperature rise of coal samples treated with H2O2 under different conditions

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(a) SX sample

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(b) YN sample Figure 3 TG and DSC curves of coal samples

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Figure 4 FTIR spectra of raw coal samples

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(a) Shanxi sample

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(b) Yunnan sample Figure 5 Changes in the functional groups in coal before and after treatment during low-temperature oxidation

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(a) CO

(b) CO2 Figure 6 Emission of CO and CO2 from coal samples before and after treatment.

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(a) SX

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(b) YN Figure 7 Pore structure of coal samples before and after treatment.

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(a) SX

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(b) YN Figure 8 Surface elements of coal samples before and after treatment.

Table 1 Parameters of coal samples Coal sample

Mad%

Aad%

Vad%

Fcad%

Q(MJ/Kg)

Shanxi lignite

17.56

20.38

32.67

29.39

19.11

Yunan lignite

26.74

11.19

34.52

26.55

17.35

Table 2 Activation energy of coal samples Activation energy (kJ/mol)

Coal samples

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* Mad = moisture content; Aad = ash content ; Vad = volatile matter content ; Fcad = Fixed carbon content; Q = calorific value.

YN

Raw sample

31.86 ± 0.52

33.58 ± 0.49

Preoxidation at 100 °C

29.78 ± 0.28

Preoxidation at 125 °C

27.82 ± 0.35

Preoxidation at 150 °C

27.29 ± 0.41

Preoxidation at 175 °C

30.72 ± 0.47

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SX

31.81 ± 0.37

31.17 ± 0.49

30.96 ± 0.56

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31.55 ± 0.43