Experimental Study On Generation Characteristics And Release Mechanism Of Hydrogen Gas From Coal Oxidation Process

Coal spontaneous combustion caused by coal oxidation is a major disaster prevalent in the coal industry,which seriously restricts the healthy development of coal industry.When coal is in contact with oxygen,oxidation reaction will take place and release a variety of gases including CO,CO2 and H2.However,molecular hydrogen,as one of the important gas products,has been studied in the initial stage.Based on this,five different kinds of metamorphic coal samples were selected as the research objects.According to the theory of oxidation kinetics,thermodynamics and intermediate complexes,the formation rule and release mechanism of hydrogen from different coal samples were systematically studied.Meanwhile,the feasibility of forecasting coal spontaneous combustion by multi-index gases based on hydrogen was explored.Then some main conclusions were obtained.(1)The release of hydrogen from the coal oxidation mainly comes from the oxidation reaction process of coal,rather than the thermal decomposition of hydrogen-containing functional groups in coal.The release of hydrogen in coal can be divided into two stages:the slow release phase at coal temperatures below 100℃ and the accelerated release phase at temperature of 100-200℃.(2)During the oxidation process,the release of hydrogen is affected by coal temperature,coal rank,particle size and quality.The coal temperature and coal rank fundamentally affect the release of hydrogen.The increase of coal temperature not only promotes the release of hydrogen by increasing oxygen consumption,but also improves the hydrogen release capacity under the unit oxygen consumption.With the increase of the degree of metamorphism of coal,the ability of coal to release hydrogen show the characteristics of alternative variation of increase and decrease in the form of "waves"(3)During the whole low-temperature oxidation process,the change range of H element is gradually increasing,while the increase of O element is decreasing.At the stage of physical adsorption and chemical adsorption,the change of O/C dominates.While in the chemical reaction stage(T>100℃),the change of H/C occupies the main position.Meanwhile,the activation energies of C,H and N in the coal are positive,and the activation energy of H element is the smallest,indicating that the H-functional group is prone to oxidation.O and S elements are negative.(4)It was found that only the aldehyde group(-CHO)released hydrogen gas,which indicates that the oxidation reaction of the aldehyde group in the air can produce hydrogen directly,and the aldehyde group-containing compound is a hydrogen-releasing precursor compound.When the aldehyde group solution is mixed with coal,the amount of hydrogen released increases obviously.In the case of hydrogen release,the coal sample can catalyze aldehyde group to release hydrogen gas.At the same time the carbonyl C=O bond in aldehydes,will take further oxidation reaction,which is more likely to generate CO2,rather than directly decomposed into CO under coal catalysis.(5)The release capacity of coal is not related to the amount of hydrogen in the coal,but it is directly related to the content of the aldehyde group in the coal.Namely,the higher the aldehyde contents of the coal sample,the stronger the hydrogen release capacity.The aliphatic C-H component oxidation reaction can be divided into two different stages and 100℃ is the critical temperature point.The effect of coal characteristics on the oxidative activity of aliphatic CH components is mainly reflected in the first stage.Therefore,when evaluating the spontaneous combustion characteristics of different coal,the dynamic parameters in the first stage should be used as the main theoretical basis.(6)Based on the evolution of active functional groups in coal,the mechanism of hydrogen release during coal oxidation was studied.First,when coal samples contact with air,oxygen molecules attack the aliphatic C-H components,producing intermediates such as peroxides or perhydrides.Then with the increase of coal temperature,these intermediates are easily oxidized and decomposed into a variety of oxygen-containing functional groups such as hydroxyl,carboxyl,and hydrogen-releasing precursors.Some of the hydroxyl groups in the coal are also easily oxidized and produced directly aldehyde group.Finally,the aldehyde group mainly releases hydrogen through two sources.In the first route,the C-H bond in the aldehyde group is subjected to oxygen attack under the catalysis of coal,and the hydrogen radicals are combined with each other to form hydrogen.The second way is that the aldehyde group reacts with the hydroxyl-containing compound to release hydrogen.(7)By analyzing the correlation between the conversion of aliphatic CH components and the release of hydrogen,it was found that the methylene functional groups exhibit the highest reaction rate in the aliphatic CH component,but the release of hydrogen will involve more hydrogen atoms on the methine functional groups.At the same time,the relationship between the aliphatic C-H component and hydrogen release was deduced:ECH+ECH3-2ECH2/RT/a’T+b’-(lnAH2/A+lnAH2/A-2lnAH2/A).(8)Based on relationship between the generation rate of CO,HZ and C2H4 gas and the oxygen consumption rate,the M value method is proposed to predict the coal temperature:M=O2%0.265N2%×RCO+RH2+RC2H4/(1+η)RO2.The whole coal oxidation stage can be divided into three different stages,namely,the slow oxidation stage,accelerated oxidation stage according to the change trend of M value and the temperature point producing CO,H2 and C2H4.