Effect and Mechanism of Rare Earth Hydrotalcite Inhibiting Coal Spontaneous Combustion

ZHANG Xiaojuan, LIU Bo, LUO Zhenmin, SUN Lu

(1.School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China)

Abstract: A hydrotalcite(layered double hydroxide, LDH) inhibitor which is suitable for the whole process of coal spontaneous combustion and a LDH inhibitor containing rare earth lanthanum elements were prepared.The inhibition effect and mechanism were analyzed by scanning electron microscopy (SEM),X-ray diffraction (XRD), thermal performance analysis, in-situ diffuse reflectance infrared spectroscopy and temperature-programmed experiment.The results have shown that the inhibitor containing lanthanum can play a good inhibitory role in every stage of coal oxidation.During the slow oxidation of coal samples, the inhibitor containing lanthanum ions can slow down the oxidation process of coal and increase the initial temperature of coal spontaneous combustion.At the same time, because the hydroxyl groups in LDHs are connected with-COO- groups on the coal surface through hydrogen bonds, the stability of coal is improved.With the increase of temperature, LDHs can remove interlayer water molecules and reduce the surface temperature of coal.CO release rate of coal samples decreases significantly after adding inhibitor containing lanthanum element, and the maximum inhibition rate of the inhibitor is 58.1%.

Key words: rare earth; hydrotalcite; coal spontaneous combustion; mechanism

1 Introduction

As an important global energy source, coal plays an important role in economic development.With the increasing mining intensity and mining depth, mine fire accidents occur frequently, which seriously threatens the safe and efficient mining of coal mines[1-5].Coal spontaneous combustion as a major cause of mine fire has become the main concern of the coal industry.With the development of science and technology, a series of coal spontaneous combustion prevention technologies and methods have been rapidly developed[6-8].Among them, the new inhibitor technology has attracted the attention of many scholars[9-15].In addition, with people paying more and more attention to environmental problems, it has become a research hotspot in the field of coal spontaneous combustion prevention and control technology to develop an environment-friendly and low-cost high-efficiency inhibitor.

Hydrotalcite (LDHs) is a kind of layered bimetallic hydroxide, which is halogen-free, non-toxic and environmentally friendly, and has attracted more and more attention in the field of flame retardants[16].LDHs not only have the excellent smoke suppression performance of aluminum hydroxide and magnesium hydroxide flame retardants but also have excellent physical shielding effect because of their unique twodimensional layered structure, which can effectively isolate oxygen, heat and combustible gases, so LDHs have more excellent flame retardant and smoke suppression performance[17].In recent years, many scholars at home and abroad have done a lot of research on the flame retardant properties of hydrotalcite.ZHU Ket al[18]studied the flame retardant mechanism of hydrotalcite-like flame retardant in asphalt materials and found that the addition of LDHs can significantly reduce the smoke emission and heat release rate of materials.To improve the flame retardant efficiency of hydrotalcite-like compounds, researchers often introduce cationic or anionic groups with high flame retardant efficiency between laminates[19-21].Edenharteret al[22]synthesized hydrotalcite intercalated with chloride ions by urea method, and on this basis synthesized hydrotalcite-like compounds intercalated with phenyl phosphate by anion substitution method,and found that phenyl phosphate hydrotalcite can reduce the smoke emission rate of polystyrene and increase the char formation rate.Because of the excellent flame retardant performance of LDHs, more and more scholars have applied it to the field of coal spontaneous combustion.WANG[23]put forward the mechanism of coordination inhibition, that is, capturing active groups in coal and forming complexes with metal ions in inhibitor, thus preventing the reaction between active groups and oxygen and inhibiting coal spontaneous combustion.Because the valence electron layer structure of rare-earth atoms has many empty orbitals, it is easy to accept lone pairs provided by various ligands to form coordination bonds.Therefore,the excellent chelating property of rare earth elements provides a theoretical basis for its coordination inhibition to prevent coal spontaneous combustion.

Based on this, in this paper, a hydrotalcite inhibitor with different rare earth lanthanum contents was prepared by the coprecipitation method.Based on the characterization of its structure and performance, the compound with raw coal was prepared by the coprecipitation method.The inhibition performance and effect of the inhibitor were studied by temperature-programmed oxidation experiment,thermogravimetric analysis andin-situdiffuse reflectance infrared spectroscopy, and its inhibition mechanism was discussed.The results show that hydrotalcite added with rare earth elements has better inhibition characteristics than ordinary hydrotalcite.The synthesis of the inhibitor further promoted the development of a new green flame retardant, which would provide a certain theoretical basis and reference for the application of hydrotalcite in the field of coal spontaneous combustion prevention.

2 Experimental

2.1 Materials

The main chemical reagents used in the experiment were zinc nitrate, magnesium nitrate,aluminum nitrate, lanthanum nitrate, anhydrous sodium carbonate and sodium hydroxide (all purchased from Sinopharm Chemical Reagents Co., Ltd.).

The coal sample was flame coal (SL coal) of Erdos Selian Mine Manager, and the crushing particle size was less than 10.0 mm.

2.1.1 Preparation of rare earth hydrotalcite-like compounds

Based on the coprecipitation method, hydrotalcite with different rare earth lanthanum contents was prepared by titration mixing, water bath crystallization,centrifugal washing and drying grinding, which were as follows:

where,n(La3+)/n(Zn2+)/n(Mg2+)/n(Al3+) is the proportion of metal ion substances in the salt solution.

2.1.2 Preparation of compound coal sample

To study the influence of La3+content in LDHs inhibitor and the addition concentration of inhibitor on the flame retardant performance, the inhibitors LDHs-1, LDHs-2, LDHs-3 and LDHs-4 with the same particle size were compounded with the coal sample of Selian by coprecipitation method, and the compounds SLLDHs-1 and SL-LDHs-4 with different compounding ratios were obtained.All samples were vacuum dried at 40 ℃ for 24 hours and stored in a silica gel drying oven for the next step.

2.2 Characterization and testing

X-ray diffraction analysis (XRD), scanning electron microscope analysis (SEM), temperatureprogrammed oxidation experiment, thermogravimetric analysis andin-situdiffuse reflectance infrared spectroscopy were used to explore the inhibition effect and mechanism of self-made rare earth hydrotalcite on coal spontaneous combustion.

2.2.1 X- ray powder diffraction analysis (XRD)

During the test, the synthesized product was placed on the aluminum frame to start XRD analysis,Cu target and Kα radiation, and the experimental samples were continuously scanned by continuous scanning.The tube voltage is 40 kV, the tube current is 30 mA, the intensity unit is CPS, the step size is 0.01,2θ= 10° - 80°, andw= 4/min.

2.2.2 Scanning electron microscopy

The accelerating voltage was 0.5 - 30 kV, each step was 0.1 kV, and the amplification factor was 20 -80 000.During the test, the surface of the conductive adhesive was coated with LDHs powder, then sprayed with gold, and then placed on the experimental device for observation.The equipment was equipped with an X-ray energy spectrum detector, and the detectable element area was Be4-U92, which was used to quantitatively detect the composition of elements in LDHs.

2.2.3 Thermal performance test of SL-LDHs

The TG209 thermogravimetric analyzer of Germany Naichi Company was used to conduct a thermogravimetric test on raw coal and its mixture.The initial temperature of the test was 35 ℃, the end temperature was 700 ℃, the temperature rising rate was 10 ℃/min, and the oxygen supply ratio (volume percentage) was 21%.

2.2.4 Temperature programmed oxidation experiment

The development of coal spontaneous combustion mainly goes through three periods: latent period,self-heating period and combustion period[24].When monitoring the process of coal spontaneous combustion,the degree of coal oxidative spontaneous combustion is usually evaluated by the rate of CO production.Through the temperature-programmed experiment of coal spontaneous combustion, the characteristics of inhibiting coal spontaneous combustion of LDHs were compared and analyzed by comparing the changes of characteristic gas concentration and oxidation exothermic intensity of Selian coal samples before and after adding inhibitor.The resistivity (E) is calculated by Formula (1).

wherecr(ppm) is the amount of CO released from raw coal samples;ct(ppm) is the released amount of CO after inhibition treatment.

According to the test tube volume and coal sample density, each test sample weighed about 1 000 g.There were 13 groups of coal samples in total, of which 1# coal sample was the original coal sample without any inhibitor, and 2 # - 13 # coal samples with different lanthanum content and different inhibitor concentration.The heating rate of the system was controlled at 0.3 ℃/min to ensure uniform and slow heating.The sample placement of the temperature-programmed box is shown in Table 1.

Table 1 Temperature programmed box sample placement

2.2.5In-situdiffuse reflectance infrared spectroscopy

Using VERTEX 70vin-situdiffuse reflectance infrared spectrometer, the raw coal and 15% compound SL-LDHs-1 and SL-LDHs-4 were tested.The main functional groups and their distribution in the compound SL-LDHs-1 and SL-LDHs-4 were studied by blank control, and the changes of oxygen-containing functional groups in the low-temperature oxidation process of coal were tested to explore the inhibition mechanism of LDHs.

3 Results and discussion

3.1 X- ray powder diffraction analysis (XRD)

As can be seen from Fig.1(a), the diffraction peak characteristics of the XRD spectrum of LDHs-1 prepared are not much different from those of normal Zn-Mg-Al hydrotalcite except that there are several miscellaneous peaks near the (009) and (110) planes.The diffraction peak baseline of the spectrum is low and stable, the peak shape is sharp, and the symmetry is good, which indicates that water with single-crystal phase and high crystallinity has been successfully prepared.

Fig.1 XRD spectra of LDHs with different lanthanum contents: (a) LDHs-1; (b) LDHs 2-LDHs 4

The diffraction peaks in Fig.1(b) correspond to La3+content of 0.01 mol(LDHs-2), 0.03 mol(LDHs-3)and 0.07 mol(LDHs-4), respectively.Compared with the standard XRD of Zn-Mg-Al- LDHs, the characteristic peaks with layered double hydroxide(003), (006), (009) and (110) planes appeared near 2θ= 11.5°, 22.7°, 34.5° and 60.6°, which indicated that Al3+in standard Zn-Mg-Al-LDHs was replaced by La3+.From the XRD patterns of LDHs-2, LDHs-3 and LDHs-4, it can be seen that when Al3+is gradually replaced by La3+, the peak value of each characteristic peak decreases, and the characteristic peak of (110)plane near 2θ= 60.6° shows that the structural symmetry of the formed crystal decreases.When La3+content is 0.07, there are obvious peaks around 2θ=17.5° and 34.5°.Because the radius and charge density of the introduced rare-earth ions are quite different from those of Al ions, the structure of La-Zn-Mg-Al-LDHs formed is greatly affected with the increase of La3+concentration.

3.2 Scanning electron microscope analysis

The microstructure of rare earth hydrotalcite inhibitor was observed by scanning electron microscope at 50 K magnification, and the SEM image was obtained.From Fig.2(a), the layered stacked structure can be seen in LDHs without La3+addition, and the surface of LDHs presents the characteristics of a nanodisc shape.With the addition of La3+, the structure and micro-morphology of LDHs have been greatly affected.With the addition of La3+in LDHs, the nano-disc-like structure on the surface of LDHs showed a decreasing trend, and some of them turned into spherical or columnar clusters, and the layered structure gradually disappeared, but some of the surface layers remained smooth.Because the unique two-dimensional layered structure of LDHs is the main reason for the excellent physical shielding effect of hydrotalcite, the addition of La3+makes the layered structure of LDHs disappear,which can’t effectively isolate the transportation of oxygen, heat and combustible gas, and at the same time, it has a certain degree of influence on the inhibition effect of LDHs.

Fig.2 SEM images of LDHs with different La3+ ratios

The energy spectrum analysis results are shown in Table 2.From the energy spectrum analysis table, it can be seen that there are mainly four elements of O,Zn, Mg and Al in LDHs-1, and the La element added in the preparation process is more in LDHs-2.With the addition of La3+, the atomic percentage of oxygen elements in LDHs increases.As the O element mainly exists in the form of CO32-and bound water between plies, the increase of the O element means the increase of bound water between plies, and with the increase of bound water between plies of LDHs, more and more heat is absorbed in the endothermic decomposition process of LDHs, so hydrotalcite added with La3+can effectively inhibit the spontaneous combustion of coal.

Table 2 Energy spectrum analysis of LDHs

3.3 CO emission rate and inhibition rate

In the process of coal oxidation, slow oxidation occurs at about 70-80 ℃, which is the critical temperature of coal spontaneous combustion.When the coal temperature exceeds the critical temperature,coal spontaneous combustion enters the accelerated oxidation stage, and when the coal temperature reaches about 210-350 ℃, coal spontaneous combustion occurs[25].By monitoring the CO release rate in the process of coal oxidation, the inhibition effect of the inhibitor can be inferred.Fig.3 shows the influence of inhibitor concentration on CO release rate.From the figure, it can be seen that the CO release rate of coal samples after inhibitor treatment is lower than that of untreated coal samples, and the inhibitor effect of 15%inhibitor concentration is generally better than that of other additives.

Fig.3 Effect of inhibitor concentration on CO release rate

Fig.4 shows the influence of different inhibitors on the CO release rate when the concentration is 15%.Fig.4 shows the influence of La3+addition on the inhibition effect of hydrotalcite inhibitor more intuitively.15%LDHs-4 has the lowest CO release rate, and when the temperature is around 170 ℃, the CO release rate is only 8.5×10-10mol/cm3·s, which indicates that when La3+content is 0.07 mol, the inhibition performance of hydrotalcite is the strongest,and the inhibition effect is better than that of other added concentrations.According to the calculation, at 100 ℃, the inhibition rate of each inhibitor (LDHs-1 to LDHs-4) is 42.2%, 49.5%, 58.1% and 47.7%,respectively.When the concentration of the inhibitor is 15%, the inhibition effect is the best, so it was used in the next thermal performance test and infrared spectrum analysis.

Fig.4 Curve of CO release rate with temperature for different inhibitors

3.4 TG-DTG analysis

The TG-DTG curve of continuous raw coal with a heating rate of 10 ℃/min in the air atmosphere is shown in Fig.5.From Fig.5, seven characteristic temperatures can be determined in the whole process from normal temperature to complete combustion,which are respectively the critical temperatureT1,the first temperature at which the weight loss rate of coal reaches the maximum in the stage of water evaporation and desorption; Dry cracking temperatureT2, the initial temperature of mass increase caused by oxidation, which is the boundary temperature from the end of evaporation and desorption of water phase to the beginning of oxygen absorption and weight gain phase;temperatureT3, the temperature at which the weight of coal sample increases the fastest in the process of oxygen adsorption and weight gain; maximum mass temperatureT4, the boundary temperature from the end of oxygen adsorption weight gain to the beginning of thermal decomposition; ignition temperatureT5,which marks the beginning of coal combustion;maximum weight loss rate temperatureT6, the most intense temperature of coal combustion, and burnout temperatureT7: the temperature at which the mass of coal sample will not change after thorough combustion.

Fig.5 TG-DTG curve of raw coal and compound coal samples

The characteristic temperature points of raw coal and SL-LDHs-1 - SL-LDHs-4 are shown in Table 3.To study the influence of inhibitors on the thermal performance of raw coal, the characteristic temperature points of raw coal and mixture are compared more intuitively, as shown in Fig.6.

Fig.6 Comparison of characteristic temperature points between raw coal and compound

As shown in Fig.6, after adding four kinds of LDHs, the characteristic temperatureT1 of the raw coal rises, and the increase of temperatureT1 represents that the oxidation process of coal is delayed backward.Because there are a lot of hydroxyl groups in LDHs,the stability of coal is improved by connecting with-COO- groups on the coal surface through hydrogen bonds.Besides, the endothermic decomposition of LDHs during heating also leads to a decrease in the spontaneous combustion tendency of coal.At the same time, with the increase of La3+content in LDHs, the temperatureT1 of the maximum water loss rate increases slowly, and theT1 value of LDHs-4 reaches the maximum of 93.7 ℃.The maximum mass temperatureT4 of the mixture is different from that of the color-linked raw coal, which indicates that the weight loss caused by the absorption of heat by LDHs to remove hydroxyl groups on the laminate and the strong oxidative weight loss of condensed aromatic rings in the coal are at the same time, and the addition of five kinds of LDHs leads to the decrease of the color-linked coal sampleT4, and the effect follows the following order: LDHs-1 > LDHs-4 > LDHs-3 >LDHs-2;T5 andT6 respectively represent the starting temperature of combustion and the temperature at the peak of combustion, and both temperatures gradually decrease with the addition of inhibitor, which indicates that LDHs in the mixture absorbed the heat released by partial coal oxidation in the process of thermal decomposition, and the pyrolysis of LDHs is of great help to speed up the construction of the barrier layer for inhibiting coal spontaneous combustion.The above results show that LDHs have a good inhibition effect in both the low-temperature oxidation stage and the hightemperature stage of coal.Comprehensive comparison of experimental results shows that the inhibition effect of four inhibitors on coal spontaneous combustion follows the following order: LDHs-4>LDHs-3>LDHs-2>LDHs-1.

3.5 Infrared spectroscopic analysis

The oxidation of coal begins on the surface, and after the oxidation, the functional groups in coal also change.By observing the change of functional groups,we can guess the degree of coal oxidation and explore the mechanism of inhibitor inhibiting coal spontaneous combustion.Therefore, it is of great significance to test and characterize the functional groups of coal samples byin-situFTIR.To make the data more comparable,LDHs-1 and LDHs-4 inhibitors with a concentration of 15% were selected for infrared spectrum analysis and compared with the data obtained from the raw coal test.3.5.1 Distribution law of main functional groups

In the infrared absorption spectrum of coal, the larger the absorption peak area of easily oxidized functional groups and the more corresponding functional groups, the easier it is for coal to be oxidized,resulting in spontaneous combustion.Generally, the composition and percentage of functional groups in coal are determined by the quantitative determination method of the infrared spectrum, and the expression formula of the quantitative determination method of infrared spectrum is shown in Formula 2:

where,A(v) refers to the absorbance of the sample at wave numberv;T(v) is transmittance of the sample at wave numberv;K(v) is absorbance coefficient of the sample at wave numberv;bis sample thickness;cis sample concentration.

Use the peak area to judge the percentage of main functional groups in coal, and calculate the percentage of infrared spectrum absorption peak area of main functional groups in color coal and two SL-LDHs compounds (Table 4).

Table 4 The area percentage of the infrared absorption peak of main functional groups

Combined with Table 4, it can be seen that oxygen-containing functional groups such as -OH,C=O, C-O and -COO- account for the largest proportion at room temperature, which are 73.37%,60.12% and 69.44% respectively, indicating that the activity of oxygen-containing functional groups is relatively high.Oxygen-containing functional groups are prone to oxidation reaction after contacting with oxygen, thus releasing heat.The content of oxygencontaining functional groups decreases when inhibitor LDHs are added to the color coal, which indicates that adding LDHs can inhibit the oxidation and exothermic process of coal to a certain extent.

3.5.2 Simulation analysis of low-temperature oxidation of coal

To further discuss the change of active functional groups in coal during low-temperature oxidation, in the air atmosphere (flow rate of 50 mL/min),with the temperature rising rate of 5 K/min, thein-situinfrared spectrum tests of the color-linked coal sample and two compounds SL-LDHs under the experimental conditions of temperature from 20 to 430 ℃, were carried out with the results shown in Fig.7.Based on OPUS software, the trend curve of the peak intensity of oxygen-containing functional groups of SL-LDHs,and a blend of excellent blended raw coal and LDHs,the low-temperature oxidation process with increasing temperature was analyzed, and the changes of oxygencontaining functional groups were compared.The influence of LDHs on oxygen-containing functional groups in coal was also analyzed.

Fig.7 FTIR data of samples during low-temperature oxidation

The peak positions of aliphatic hydrocarbons(-CH3/-CH2-) are mainly between 2 915 - 2 975 cm-1and 2 850 - 2 880 cm-1.Using the data of stretching vibration at the peak position of 2 922 cm-1, the trend graph of the peak intensity of -CH3/-CH2- in colored coal before and after adding LDHs with temperature during low-temperature oxidation was obtained, as shown in Fig.7(a).With the increase of temperature,the number of active alkyl groups in coal samples gradually decreased, which indicates that active alkyl groups were rapidly consumed and the consumption rate gradually exceeded the generation rate of new alkyl groups.After adding LDHs, the intensity of -CH3/-CH2-peak in two SL-LDHs compounds decreased at 2 922 cm-1, indicating that LDHs slowed down the formation of active alkyl in coal samples, thus inhibiting the oxidation process of coal samples.Among them, the intensity of aliphatic hydrocarbon -CH3/-CH2- peak in the SL-LDHs-4 compound was the smallest, indicating that LDHs-4 was higher than LDHs-1.This may be because there is a difference in the ratio of metal ions in LDHs-1 and LDHs-4, and the endothermic decomposition temperature of LDHs-4 with a relatively better ratio of metal ions is relatively consistent with the temperature of heat released by the colorconnected coal sample participating in the oxidation reaction, and the heat released by the color-connected coal sample participating in the oxidation reaction is endothermically decomposed by LDHs-4, so the released heat is consumed so that it cannot accumulate in large quantities, avoiding the temperature increase of the color-connected coal sample and slowing down the decomposition of aliphatic hydrocarbons-CH3/-CH2- by heating.

The spectral peaks of -OH in oxygen-containing functional groups mainly lie between 3 625 - 3 700 cm-1and 3 200 - 3 550 cm-1[26].The peak position of 3 657 cm-1is selected, and the curves of the intensity of -OH peak with temperature in thein-situinfrared spectra of color-linked coal before and after adding two kinds of LDHs are shown in Fig.8.When the temperature of the raw coal sample is lower than 150℃, the number of hydroxyl groups decreases rapidly due to the evaporation and desorption of water and the reaction and consumption of some active hydroxyl groups with oxygen.After the inhibitor LDHs were added, the hydroxyl group decreased to about 200 ℃,which indicated that the inhibitor slowed down the oxidation reaction of coal in the later stage.With the increase in reaction temperature, aliphatic hydrocarbons combine with oxygen to form hydroxyl groups in the oxidation reaction, which increases slightly overall.However, due to the increase in temperature,LDHs absorb heat and remove some interlayer water molecules, which leads to the decrease of hydroxyl content, so the change curve of hydroxyl in coal after adding an inhibitor is wavy.Subsequently, due to desorption, the consumption rate of the hydroxyl group is higher than the generation rate, and the content of the hydroxyl group is further reduced.

Fig.8 Changes of main functional groups of coal samples with temperature

3.6 Flame retardant mechanism

From the above experimental results, it can be seen that rare earth hydrotalcite can effectively inhibit the low-temperature oxidation stage and the hightemperature decomposition stage of coal, combined with the following facts:

a) There are many empty orbitals in the valence electron layer structure of rare-earth atoms, and they are easy to accept lone pairs provided by various ligands to form coordination bonds, which makes La3+have excellent chelating properties.

b) The proportion of element O in LDHs is the largest, reaching 63.07%.CO32-and interlayer bound water mainly exists in the interlayer.With the increase of La3+, the content of element O in LDHs increases,and the bound water between layers of rare-earth layered double hydroxides increases.

The following assumptions can be made:

a) Active groups in coal form complexes with metal ions in inhibitor LDHs.

b) Because of the selective complexation of-COOH with Zn2+, Mg2+, Al3+in coal macromolecules and the adsorption balance of coal to metal ions, La3+takes precedence over Zn2+, Mg2+, Al3+and other active groups in coal for adsorption and complexation.

c) The addition of La3+has an obvious influence on the structure and morphology of layered double hydroxide.Different rare earth layered double hydroxides LDHs-1, LDHs-2, LDHs-3 and LDHs-4 may form different complexes with acidic functional groups on the coal surface due to their different La3+content, thus showing different inhibition performance.

Based on the above assumptions and experimental facts, the coupling regulation mechanism of adsorption and complexation equilibrium between coal macromolecules and LDHs is proposed, as shown in Fig.9.

Fig.9 Mechanism diagram of hydrotalcite inhibiting coal spontaneous combustion

4 Conclusions

In this paper, hydrotalcite inhibitors containing different proportions of rare earth lanthanum elements were prepared.Taking the Selian coal as the research object, the inhibition effect of hydrotalcite inhibitors containing rare earth elements on spontaneous combustion of coal is studied and the inhibition mechanism is put forward, and the following results are obtained:

a) The self-made rare earth hydrotalcite is a nano-layered ordered crystal structure containing metal ions and a large amount of bound water, with obvious characteristic diffraction peaks of Zn-Mg-Al-LDHs, but the addition of La3+has a certain influence on the structure and morphology of layered double hydroxides.After adding lanthanum, the proportion of O element in LDHs is 63.07% at the maximum, and CO32-and interlayer bound water mainly exist in the interlayer.With the increase of interlayer bound water,the multistage decomposition process of LDHs will absorb more heat.

b) Lanthanum inhibitors can play a good role in inhibiting coal oxidation and decomposition at all stages.In the low-temperature oxidation stage, because LDHs contain a large number of hydroxyl groups, they are connected with -COO- groups on the surface of coal by hydrogen bonds, which improves the stability of coal.The temperature-programmed experiment shows that the inhibitor added with rare earth lanthanum can significantly reduce the CO release rate, and the decreasing trend of CO release rate is more obvious with the increase of lanthanum content.Because LDHs pyrolysis will absorb the heat released in the process of coal oxidation, after adding inhibitor-containing La3+,the temperature of characteristic temperature pointsT5 andT6 of coal is lower than that of raw coal.Compared with the raw coal sample, the number of aliphatic hydrocarbons and hydroxyl groups in the prepared LDHs added to the coal sample is reduced.LDHs can not only slow down the slow oxidation stage of the coal sample in the early stage but also absorb heat in the multi-stage pyrolysis process in the later stage, thus reducing the surface temperature of the coal sample.

c) With the increase of La3+content, the inhibition effect shows an increasingly significant-good trend.The flame retardant effect of the inhibitor is proportional to the amount of inhibitor.When the amount of inhibitor is 15%, the inhibition effect of coal spontaneous combustion is the most significant.When the addition of LDHs-3 with La3+content of 0.03 mol is 15%, the inhibition rate of the coal sample at 100 ℃ is the best,which is 58.1%.

Conflict of interest

All authors declare that there are no competing interests.