Effect and Mechanism of Rare Earth Hydrotalcite Inhibiting Coal Spontaneous Combustion

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%.

Agglomeration of coal and polyethylene mixtures during fixed‑bed co‑gasification

The article presents the results of experimental studies on the gasification of mixtures of brown coal and polyethylene(up to 20 wt%fraction)in a laboratory reactor.The work aims to study the agglomeration process during the heating and oxidation of the mixtures.The measurement results(gas composition,pressure drop)provide indirect information on the dynamics of thermal decomposition and structural changes in the fuel bed.We have shown that the interaction between polyethylene and a coal surface leads to the formation of dense agglomerates,in which the molten polymer acts as a binder.Clinkers form as a result of interfacial interactions between components and filtration flow rearranging.The hydrogen/carbon ratio in the solid residue of coal-polyethylene co-gasification increases from 0.07–0.2 to 1.11,indicating the formation of stable hydrocarbon compounds on the carbon surface.The conducted research makes it possible to identify possible interactions between chemical reactions and transfer processes that lead to agglomeration in mixtures of coal with polyethylene.

Method for prevention and control of spontaneous combustion of coal seam and its application in mining field

Spontaneous combustion of coal seam has been and continues to be a big problem in coal mines. It could pose great threat to the safety of the whole mine and all miners, especially when it occurs in or nearby coal mines. Besides, environment of area surrounded mines during combustion can be threatened where large amount of toxic gases including CO_2, CO, SO_2 and H_2S can be leased by fire in mine. Hence, it is important and significant for scholars to study the controlling and preventing of the coal seam fire. In this paper, the complicated reasons for the occurrence and development of spontaneous combustion in coal seam are analysed and different models under various air leakage situations are built as well. Based on the model and approximately calculation, the difficulty of fire extinguishment in coal seam is pointed out as the difficulty and poor effect to remove the large amount of heat released. Detailed measurements about backfilling and case analyses are also provided on the basis of the recent ten years' practice of controlling spontaneous combustion in coal seams in China. A technical fire prevention and control method has been concluded as five steps including detection, prevention, sealing, injection and pressure adjustment. However, various backfill materials require different application and environmental factors, so in this paper, analyses and discussion about the effect and engineering application of prevention of spontaneous combustion are provided according to different backfilling technologies and methods. Once the aforementioned fire prevention can be widely applied and regulated in mines, green mining will be achievable concerning mine fire prevention and control.

Transformation of alkali and alkaline-earth metals during coal oxy-fuel combustion in the presence of SO_2 and H_2O

The occurrence modes of alkali and alkaline-earth metals（AAEMs） in coal relate to their release behavior and ash formation during combustion. To better understand the transformation of AAEMs,the release behavior of water-soluble,HCl-soluble,HCl-insoluble AAEMs during Shenmu coal（SM coal） oxy-fuel combustion in the presence of SO2 and H2O in a drop-tube reactor was investigated through serial dissolution using H2O and HCl solutions. The results show that the release rates of AAEMs increase with an increase in temperature under the three atmospheres studied. The high release rates of Mg and Ca from SM coal are dependent on the high content of soluble Mg and Ca in SM coal. SO2 inhibits the release rates of AAEMs,while H2O promotes them. The effects of SO2 and H2O on the Na and K species are more evident than those on Mg and Ca species. All three types of AAEMs in coal can volatilize in the gas phase during coal combustion. The W-type AAEMs release excessively,whereas the release rates of I-type AAEMs are relatively lower. Different types of AAEM may interconvert through different pathways under certain conditions. Both SO2 and H2O promote the transformation reactions. The effect of SO2 was related to sulfate formation and the promotion by H2O occurs because of a decrease in the melting point of the solid as well as the reaction of H2O.

Effect of weathering on physico-chemical properties and combustion behavior of an Indian thermal coal

An open air stockpile of conical shape was formed with 3.5 m base diameter and 5 m height using 500 tons of -200 mm size ROM thermal coal on a concrete floor. Coal sample from the stockpile were collected at 30 days interval for analysis of various chemical properties like;proximate analysis, ultimate analysis, gross calorific value (GCV) and various combustion related properties such as ignition temperature, peak temperature, burnout temperature, maximum combustion rate, ignition index, burnout index, combustion performance index and combustion rate intensity index. Experimental results show that, due to weathering of coal fixed carbon decreased from 35.6% to 19.9%, elemental carb on (C) decreased from 46.6% to 28.6%, hydrogen (H) decreased from 3.3% to 2.9% and GCV decreased by up to 55% of original value during 330 days of storage of coal in an open atmosphere. Ash con tent of coal increased form 29.2% to 46.6% due to loss of combustibles. Sulfur (S) of coal was found to get increased from 0.33% to 1.08% during storage. The activation energy of coal combustion increased from 22 kJ/mol to 54 kJ/mol. Variation in combustion parameters signifies that weathering has significant negative impact on coal combustion properties as coal become difficult to ignite.

Combustion characteristics of unburned pulverized coal and its reaction kinetics with CO2

The combustion characteristics of two kinds of unburned pulverized coal (UPC) made from bituminous coal and anthracite were investigated by thermogravimetric analysis under air. The reaction kinetics mechanisms between UPC and CO2 in an isothermal experiment in the temperature range 1000–1100°C were investigated. The combustion performance of unburned pulverized coal made from bituminous coal (BUPC) was better than that of unburned pulverized coal made from anthracite (AUPC). The combustion characteristic indexes (S) of BUPC and AUPC are 0.47 × 10^-6 and 0.34 × 10^-6 %2·min^-2·°C^-3, respectively, and the combustion reaction apparent activation energies are 91.94 and 102.63 kJ·mol^-1, respectively. The reaction mechanism of BUPC with CO2 is random nucleation and growth, and the apparent activation energy is 96.24 kJ·mol^-1. By contrast, the reaction mechanism of AUPC with CO2 follows the shrinkage spherical function model and the apparent activation energy is 133.55 kJ·mol^-1.

The Production and Release of CFCs from Coal Combustion

The destruction of the ozone layer in the atmosphere caused by industriallysynthesized CFCs has aroused greatest concerns from the international society, but the CFCs formedfrom burning of coal containing fluorine have not been recognized by the world yet. In the presentstudy, we condensed the gas through cold traps and used the GC-MS to measure the gas composition,and found that the content of CFC-12 in the smog from coal combustion was significantly higher thanthe background value of the local atmosphere. This proves that CFC- 12 is formed in the process ofcoal combustion. This paper discusses a new source of non-synthesized CFCs.

A new apparatus to establish the spontaneous combustion propensity of coals and coal-shales

Coal and coal-shale both tend to undergo spontaneous combustion under favourable atmospheric conditions. The Wits-Ehac index has been developed in South Africa since the late 1980's to test the spontaneous combustion liability of coal. However, in some cases, the Wits-Ehac index fails to produce tangible results when testing coal-shales. To overcome this problem, a new apparatus has been developed to test carbonaceous materials such as coal and coal-shale under chemical reactions with oxygen and an index has been obtained. This index is called the Wits-CT index. The equipment emulates the influence of oxygen adsorption on carbonaceous material for a period of 24 h without a heating system.The Wits-CT index uses the total carbon content of the sample and the temperature variations obtained from the samples during reaction with oxygen to predict the spontaneous combustion liability. Eighteen samples have been analyzed using both indices and the results are in-line. It was found that coals and coal-shales with higher values of the Wits-CT index are more liable to spontaneous combustion.Further research on different coal-shales is underway in order to establish an extensive database for coal and coal-shales, together with known incidences of self-heating.

Emissions of volatile organic compounds and carbonyl compounds from residential coal combustion in China

Emissions of volatile organic compounds （VOCs） and carbonyls from residential coal combustion of five coals with different maturities were studied in a simulated room.The coals were burned in form of honeycomb briquettes in a domestic coal stove,one of the most common fuel/stove combinations in China.Through a dilution system,VOCs and carbonyls samples were collected by canisters and silica-gel cartridges and analyzed by gas chromatography and mass spectrum （GC/MS） and high performance liquid chromatography/ultraviolet （HPLC/UV）,respectively.The results show that the bituminous coals with medium volatile matter content produce the highest emissions while the anthracite yields the lowest.Among the identified carbonyls from the coal smoke,the aromatic compounds （benzaldehyde,2,5-dimethylbenzaldehyde and p-tolualdehyde,m/o-tolualdehyde,benzene,m,p-xylene and trimethyl-benzene） were relatively abundant,which might be due to the molecular structure of the coal.For formaldehyde,aromatic carbonyls and aliphatic alkanes,their concentrations increase up to the maximum values and then decrease with increasing coal maturity.The total carbonyls and VOCs have the same tendency,which was observed for the emission factors of organic carbon （OC）,elemental carbon （EC）,particulate matter （PM） and polycyclic aromatic hydrocarbons （PAHs） detected in the series study.

The catalytic effect of the Na and Ca-rich industrial wastes on the thermal ignition of coal combustion

The catalytic effects of four industrial wastes,namely,the soap residue(SR),brine sludge(BS),calcium carbide residue(CCR),and white lime mud(WLM),on coal thermal ignition were investigated.The acidity of palmitate anion associated with Na+in SR was lower than that of chloride anion combined with Na+in BS,which resulted in an improved the combustion of SR.The acidity of OH-anion combined with Ca2+in CCR was lower than that of CO32-anion combined with Ca2+in WLM,resulting in CCR exhibiting a better catalytic effect on coal ignition.The alkaline metal Na had lower initial ionisation energy than the alkaline earth metal Ca.Therefore,the Na-rich SR exhibited higher catalytic activity on coal ignition than Ca-rich CCR.The ignition temperature of coal with 0.5%SR decreased from 544 to 503°C.

Effects of fractal surface on rheological behavior and combustion kinetics of modified brown coal water slurries

The paper reports the effects of surface fractal structures on the rheological behavior and combustion kinetics of raw brown coal and three modified coal water slurries （CWSs）. The results show that the fractal structures and physicochemical properties of samples are dependent on various modification processes. The apparent viscosities of the coal water slurries increase with increasing surface fractal dimensions （D）, especially with decreasing shear rates. Fur- thermore, it has been proved that the ignition temperatures and apparent activation energies of modified CWSs are lower than that of raw coal water slurry. Compared with the traditional qualitative analysis of the effect of pore structures on CWSs properties, D can more efficiently indicate the quantificational effect of pore structures on the rheological behavior and combustion kinetics of CWSs.

Study on partition of spontaneous combustion danger zone and prediction of self-ignition in coalmine based on numeric simulation

By solving steady model of air flow diffusion and chemical reaction in loose coal, distribution of oxygen concentration and flow velocity magnitude were obtained. Compared the simulating results with critic value as well as duration of spontaneous combustion from large-scale spontaneous combustion experiment, 'three zones' of spontaneous combustion were partitioned and mining conditions to avoid spontaneous combustion were obtained. The above method was employed to partition 'three zones' in gob of fully mechanized top-coal caving long wall face and got fairly good result. Calculation of the above method is much smaller than simulating the whole process of coal spontaneous combustion, but the prediction precision can satisfy the demand of predicting and extinguishing spontaneous combustion in mining.

Geochemistry of Permian Coal and Its Combustion Residues in Huainan Coalfield, China

INAA (instrumental neutron activation analysis) was used to determine the contents of trace elements in coal samples from 11 workable Permian coal seams in Huainan coalfield. With XRFS (X-ray fluorescence spectrometry), the geochemical composition of its combustion residues from Tianjiaan Power Plant and Luohe Power Plant was analyzed. In addition, the electron microprobe was employed to study the chemical composition of some fly ashes and bottom ashes. Compared with those of the coals around the world, most trace elements of the Huainan coal are characterized by a normal abundance. Like most coals worldwide, the elements Se, S, As, Sb, Br, U and Cl are enriched in Huainan coal according to dark's value, while the other elements tend to be diluted. The large fluctuation in the contents of trace elements in this kind of coal demonstrates great differences in coal seams. On the other hand, the contents of these elements are very similar in different bottom and fly ash samples. The elements with low organic affinity tend to concentrate in fly and bottom ash than in coal, especially those elements associated with Fe. However, the elements with strong organic affinity tend to be volatile during the combustion. Moreover, the abundance of the elements in fly ash and bottom ash changes with different densities and with different particle sizes. The bottom and fly ashes consist of glass, lime, mullite, coal residues and ferrite minerals. Some residual chars contain higher contents of volatile elements, such as S, P, Cl and As, indicating that the ash containing more residual chars has a strong adsorption of some hazardous elements, suggesting a possible utilization of power plant ash in the field of water cleaning.

Combustion performance of pulverized coal and corresponding kinetics study after adding the additives of Fe_(2)O_(3) and CaO

Combustion performance of pulverized coal(PC)in blast furnace(BF)process is regarded as a criteria parameter to assess the prop-er injection dosage of PC.In this paper,effects of two kinds of additives,Fe_(2)O_(3) and CaO,on PC combustion were studied using the thermo-gravimetric method.The results demonstrate that both the Fe_(2)O_(3) and CaO can promote combustion performance index of PC including igni-tion index(C_(i)),burnout index(D_(b)),as well as comprehensive combustibility index(S_(n)).The S_(n) increases from 1.37×10^(−6) to 2.16×10^(−6)%2·min^(−2)·℃^(−3) as the Fe_(2)O_(3) proportion increases from 0 to 5.0wt%.Additionally,the combustion kinetics of PC was clarified using the Coats-Redfern method.The results show that the activation energy(E)of PC combustion decreases after adding the above additives.For instance,the E decreases from 56.54 to 35.75 kJ/mol when the Fe_(2)O_(3) proportion increases from 0 to 5.0wt%,which supports the improved combustion per-formance.Moreover,it is uneconomic to utilize pure Fe_(2)O_(3) and CaO in production.Based on economy analysis,we selected the iron-bearing dust(IBD)which contains much Fe_(2)O_(3) and CaO component to investigate,and got the same effects.Therefore,the IBD is a potential option for catalytic PC combustion in BF process.

Physicochemical,mineralogy,and thermo-kinetic characterisation of newly discovered Nigerian coals under pyrolysis and combustion conditions

In this study,the physicochemical,microstructural,mineralogical,thermal,and kinetic properties of three newly discovered coals from Akunza(AKZ),Ome(OME),and Shiga(SHG)in Nigeria were examined for potential energy recovery.Physicochemical analysis revealed high combustible but low levels of polluting elements.The higher heating values ranged from 18.65 MJ/kg(AKZ)to 26.59 MJ/kg(SHG).Microstructure and mineralogical analyses revealed particles with a rough texture,surface,and glassy lustre,which could be ascribed to metals,quartz,and kaolinite minerals.The major elements(C,O,Si,and Al),along with minor elements(Ca,Cu,Fe,K,Mg,S,and Ti)detected are associated with clays,salts,or the porphyrin constituents of coal.Thermal analysis showed mass loss(ML)ranges from 30.51%to 87.57%and residual mass(RM)from 12.44%to 69.49%under combustion(oxidative)and pyrolysis(non-oxidative)TGA conditions due to thermal degradation of organic matter and macerals(vitrinite,inertinite and liptinite).Kinetic analysis revealed the coals are highly reactive under the oxidative and non-oxidative conditions based on the Coats-Redfem Model.The activation energy(Ea)ranged from 23.81 to 89.56 kJ/mol,whereas the pre-exponential factor(kQ)was from 6.77×10^(-4)/min to 1.72×10^(3)/min under pyrolysis and combustion conditions.In conclusion,the coals are practical feedstocks for either energy recovery or industrial applications.

Experimental and mechanistic study on chemical looping combustion of caking coal

Under high-temperature batch fluidized bed conditions and by employing juye coal as the raw material,the present study determined the effects of the bed material,temperature,OC/C ratio,steam flow and oxygen carrier cycle on the chemical looping combustion of coal.In addition,the variations taking place in the surface functional groups of coal under different reaction times were investigated,and the variations achieved by the gas released under the pyrolysis and combustion of Juye coal were analyzed.As revealed from the results,the carbon conversion ratio and rate were elevated significantly,and the volume fraction of the outlet CO_(2)remained more than 92%under the oxygen carriers.The optimized reaction conditions to achieve the chemical looping combustion of Juye coal consisted of a temperature of 900℃,an OC/C ratio of 2,as well as a steam flow rate of 0.5 g·min^(-1).When the coal was undergoing the chemical looping combustion,volatiles primarily originated from the pyrolysis of aliphatic-CH_(3)and-CH_(2),and CO and H_(2)were largely generated from the gasification of aromatic carbon.In the CLC process,H_(2)O and CO_(2)began to separate out at 270℃,CH4 and tar began to precipitate at 370℃,and the amount of CO_(2)was continuously elevated with the rise of the temperature.

Analysis of spontaneous combustion liability indices and coal recording standards/basis

The characterization of the physical and chemical properties of coal on a standard provides an understanding of its characteristics towards spontaneous combustion.The trend of linear relationships between coal recording standards(%air-dried(ad),%dry(db)and%dry ash free(daf)basis)of 30 selected coal samples from the Witbank coalfields and spontaneous combustion liability indices were evaluated.The spontaneous combustion liability indices of these samples were evaluated by crossing point temperature(XPT),Stage II Slope,FCC(Feng,Chakravorty,Cochrane)and the Wits-Ehac tests,while the coal properties were determined from the proximate and ultimate analyses.The results obtained from these coal properties were related to different liability indices to develop trends of linear relationships using regression analysis.The ad basis indicated higher correlation coefficients than the db and daf basis for the XPT and FCC index,while the daf showed higher correlation coefficients than the ad and db basis for the Wits-Ehac index.It was found that the trend of linear relationships of these coal properties differs from one liability index to the next.The XPTs showed a better trend followed by the Stage II Slope on the coal properties among the spontaneous combustion liability indices evaluated.

Combustion mechanism and control approaches of underground coal fires:a review

With the large-scale mining of coal resources,the huge economic losses and environmental problems caused by underground coal fires have become increasingly prominent,and the research on the status quo and response strategies of underground coal fires is of great significance to accelerate the green prevention and control of coal fires,energy conservation and emission reduction.In this paper,we summarized and sorted out the research status of underground coal fires,focused on the theoretical and technical issues such as underground coal fire combustion mechanism,multiphysics coupling effect of coal fire combustion,fire prevention and extinguishing technology for underground coal fires,and beneficial utilization technology,and described the latest research progress of the prevention and control for underground coal fire hazards.Finally,the key research problems in the field of underground coal fire hazards prevention and control were proposed in the direction of the basic theory,technology research,comprehensive management and utilization,with a view to providing ideas and solutions for the management of underground coal fires.

Migration of sulfur in in-situ gasification chemical looping combustion of Beisu coal with iron-and copper-based oxygen carriers

Chemical looping combustion(CLC)is an energy conversion technology with high efficiency and inherent separation of CO_(2).The existence of sulfur in coal may affect the CO_(2) purity and the performance of oxygen carrier due to the interactions between sulfur contaminants and oxygen carrier.The migration of sulfur in Beisu coal during the in-situ gasification chemical looping combustion(i G-CLC)process using two oxygen carriers(iron ore and Cu O/Si O_(2))was investigated respectively.The thermodynamic analysis results showed the formation of metal sulfides was thermodynamically favored at low temperatures and low oxygen excess coefficients,while they were obviously inhibited and the production of SO_(2) was significantly promoted with an increase in temperature and oxygen excess coefficient.Moreover,part of sulfur was captured and fixed in the forms of alkali/alkaline earth metal sulfate due to the high amount of alkali/alkaline earth metal oxides in the coal ash or/and oxygen carrier.The experimental results showed that the sulfur in coal mainly released in the form of SO_(2),and the sulfur conversion efficiency(XS)in the reduction stage were 51.04%and 48.24%when using iron ore and Cu O/Si O_(2) respectively.The existence of metal sulfides was observed in the reduced oxygen carriers.The values of XSin the reoxidation process reached 3.80%and 7.64%when using iron ore and Cu O/Si O_(2) respectively.The residue and accumulation of sulfur were also found on the surfaces of two oxygen carriers.

Characterization of bio-coal briquettes blended from low quality coal and biomass waste treated by Garant■bio-activator and its application for fuel combustion

Experimental research was carried out on the manufacturing of bio-coal briquettes from a blend of two different types of low-quality coal and biomass waste in the absence of coal carbonization,where the third blend of the material was fermented by adding a bio-activator solution before pressurizing the components into briquettes.The coal samples from Caringin-Garut Regency(BB-Garut)had a low calorific value and a high sulfur content(6.57 wt%),whereas the coal samples from Bayah-Lebak Regency(BB-Bayah)had a higher calorific value and a lower sulfur content(0.51 wt%).The biomass added to the coal blend is in the form of fermented cow dung(Bio-Kohe),and it had a calorific value of 4192 kcal/kg and a total sulfur content of 1.56 wt%.The main objective of this study is to determine the total decrease in the sulfur content in a blend of coal and biomass in which a fennentation process was carried out using a bio-activator for 24 h.The used bio-activator was made from Garant■(1:40)+molasses 1 wt%/vol,and its used amount was 0.2 L/kg.Also,the total sulfur content in the blend was 1.00 wt%-1.14 wt%,which fulfills the necessary quality requirements for non-carbonized bio-coal briquettes.The pyritic and sulfate content in the raw coal was dominant,and the organic sulfur,when fermented with Garant■,was found to be less in the produced bio-coal briquettes by 38%-58%.