A review of research on spontaneous combustion of coal

Coal oxidation at low temperatures is the heat source liable for the self-heating and spontaneous combustion of coal. This phenomenon has imposed severe problems in coal related industries. Attempts to understand this phenomenon by previous researchers have provided significant progress. It is wellknown that coal oxidation at low temperatures involves oxygen consumption and formation of gaseous and solid oxidation products. This process is majorly influenced by temperature, oxidation history of coal,coal properties, particle size distribution of the coal, etc. The current understanding of the phenomenon of self-heating and spontaneous combustion of coal is discussed along with the different experimental and numerical models established to predict self-heating characteristics of coal. This paper focuses on the global position of the study carried out by academics, research institutes and industries on spontaneous combustion of coal and coal mine fires. Within this framework, the generally used spontaneous combustion techniques to predict the spontaneous combustion liability of coal were evaluated. These techniques are well-known in their usage, but no specific method has become a standard to predict the spontaneous combustion liability. Further study is still needed to indicate a number of impending issues and to obtain a more complete understanding on the phenomenon.

Spontaneous combustion of coals and coal-shales

Spontaneous combustion of coal is a well-known phenomena around the globe. Apart from the coal itself,burning coal-shales is becoming a problem in the South African coal mines. Serious incidents of spontaneous combustion have been reported as a result of self-heating of reactive coal-shales. The intrinsic properties and spontaneous combustion tests of 28 selected coal and coal-shale samples were conducted and a relationship between the two has been established. Intrinsic properties were obtained by using the proximate and ultimate analysis, and spontaneous combustion liability tests results were obtained by using the Wits-Ehac and Wits-CT indices. The experimental results show that intrinsic properties of these materials complement to the spontaneous combustion liability tests results. Comparative analyses of intrinsic properties and spontaneous combustion characteristics indicate similarities between the mechanism of coal oxidation and that of the oxidative processes undergone by coal-shales. For the tested samples, coal samples have a higher intrinsic spontaneous combustion reactivity rating than the coal-shales.Furthermore, an increase in carbon, moisture, hydrogen, volatile matter, nitrogen and a decrease in ash content indicate an increased proneness to self-heating. The concentration of pyrite found in the coal-shales accelerates self-heating. The event of spontaneous combustion can occur if coal-shales absorb sufficient oxygen when subjected to atmospheric conditions.

Spontaneous combustion liability of coal and coal-shale: a review of prediction methods

This study presents a review of the various methods to predict the spontaneous combustion liability of coal and coal-shale. The relative propensity of coal to undergo self-heating can be established by different methods. These methods are well established in their usage, but the fact that no particular test method has become a standard to predict the spontaneous combustion liability indicates that doubt still exists as to the validity of all of them. The underlying principle of all the tests is that the more readily the coal undergoes exothermic oxidation, the more liable it is to self-heat. Comprehensive studies that centres on the international position on research being conducted by academics, different research institutes and industries on spontaneous combustion of coal and coal mine fires were evaluated. Relationships between the geochemical analysis (proximate and ultimate analysis, forms of sulphur, petrographic properties, X-ray diffraction and X-ray fluorescence) and spontaneous combustion testing methods (numerical and experimental approaches) used to predict the spontaneous combustion liability of coal were reviewed. The combination of these tests provides a better understanding of the mechanism that controls the spontaneous combustion phenomena. However, irrespective of the extensive studies that have been conducted over time, spontaneous combustion is still a major problem in the coal value chain.

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.

Influence of organic and inorganic properties of coal-shale on spontaneous combustion liability

Coal and coal-shale undergo low-temperature oxidation when exposed to air,potentially leading to spontaneous combustion.Coal-shale found in association with coal seams vary considerably in their intrinsic properties and spontaneous combustion liability index.Fourteen coal-shale samples collected from four different coal mines in Witbank Coalfield,South Africa,were experimentally investigated.The influence of coal-shale intrinsic properties and spontaneous combustion liability indices(determined by the WitsEhac Index and the Wits-CT Index)were established.The liability indices indicate relationships with the intrinsic factors and thus,identifying the major intrinsic factors affecting liability toward spontaneous combustion in these coal-shale samples.The XRF analysis indicated that the coal-shale samples are rich in Si O2,Al2O3 and Fe2O3,while the XRD showed that same coal-shale samples are generally dominated with kaolinite and quartz.The coal-shale occurred in association with medium Rank C bituminous coal and contained varying proportion of macerals.The Wits-Ehac Index was unable to reliably determine liability indices of some coal-shale samples,and hence the Wits-CT Index was developed.The results obtained from the characterisation tests may be used as a tool to predict the spontaneous combustion liability in carbonaceous material and may serve as a reference when comparing coal-shale from different coal mines.

Development of multiple soft computing models for estimating organic and inorganic constituents in coal

The distribution of the various organic and inorganic constituents and their influences on the combustion of coal has been comprehensively studied.However,the combustion characteristics of pulverized coal depend not only on rank but also on the composition,distribution,and combination of the macerals.Unlike the proximate and ultimate analyses,determining the macerals in coal involves the use of sophisticated microscopic instrumentation and expertise.In this study,an attempt was made to predict the amount of macerals(vitrinite,inertinite,and liptinite)and total mineral matter from the Witbank Coalfields samples using the multiple input single output white-box artificial neural network(MISOWB-ANN),gene expression programming(GEP),multiple linear regression(MLR),and multiple nonlinear regression(MNLR).The predictive models obtained from the multiple soft computing models adopted are contrasted with one another using difference,efficiency,and composite statistical indicators to examine the appropriateness of the models.The MISOWB-ANN provides a more reliable predictive model than the other three models with the lowest difference and highest efficiency and composite statistical indicators.

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.