Oxidation and self-reaction of active groups in coal

Abstract：The direct burn-off of coal and sorption reaction sequences between coal and oxygen had been confirmed to occur in this phenomenon.However,whether the two squences can comprehensively reflect spontaneous combustion of coal is still being suspected.The dissertation focused on the oxidation and self-reaction of active groups in coal.Firstly,the active groups in coal were analyzed.According to the formed time,the active groups in coal were separated to initial ones and secondary ones.The groups in initial coal samples were tested by FTIR.The real-time changes of active groups were obtained using an in-situ FTIR.An in-situ testing system of infared spectrogram was designed based on traditional FTIR.In the in-situ testing system,a chamber was used to load coal sample;a gas providing system and an electronical heater was used to simulate different conditions.Using the in-situ testing system,the real-time changes of active groups under different conditions were tested.Under oxygen atmosphere,some active groups decreases at initial stage and then increases with the rise of temperature later.The break points of different active groups are also different.Under oxygen-free atmosphere,the change trends of active groups are similar to oxygen atmosphere.In contrast with oxygen atmosphere,the quantity of active groups under oxygen-free atmosphere is smaller and the temperatures at break points are lower.Under oxygen atmosphere after oxygen-free atmosphere,the decrease at initial stage almost disappears.The analysis on real-time change of active groups indicates some types of active groups can react at low temperatures under oxygen-free atmoshphere.At the initial stage of coal reactions under oxygen and oxygen-free atmospheres,initial active groups in coal are consumed but the same types of secondary active groups cannot form very much until a high enough temperature.So the quantity of active groups decreases at the initial stage.During the reaction under oxygen atmosphere of coal samples which had reacted under oxygen-free atmosphere,most of the initial active groups had been consumed during the previous reaction under oxygen-free atmosphere.So the self-reaction of active groups is weak.Consequently,the quantity of active groups would increase with the production of secondary active groups.The decrease phenomenon at the initial stage of coal reaction disappears.In order to further approve the self-reaction of active groups,the products were tested by a gas chromatography.Gaseous production of coal reaction under different conditions of oxygen atmosphere,oxygen-free atmosphere and oxygen atmosphere after oxygen-free atmosphere were tested.The reactions under the three different conditons can produce different types of gaseous products.It indicates self-reaction of active groups exists and can produce products.Based on comprehensive analysis,a reaction model including three reaction sequences was proposed for spontaneous combustion of coal.The model suggests the self-reaction of active groups exists in spontaneous combustion of coal besides direct burn-off and sorption sequences between coal and oxygen.And the chemical reactions which generate CO,CO2 and H2O were proposed.Spontaneous combustion of coal is the result of oxidation and self-reaction of active groups.However,self-reaction of active groups in coal can not exist individually.The heat released from oxidation of active groups provides initial enery for the self-reaction of active groups.Based on the theory of oxidation and self-reaction of active groups,some macroscopic characteristics were analyzed.At early stage of coal self-heating,only initial active groups and few secondary active groups take part in reaction.In contrast,plenty of secondary active groups take part in chain reactions at later stage.Consequently,self-heating of coal is a nolinear process.For high ranks of coal,initial active groups are few and secondary active groups are difficult to form.It results in the decrease of the quantity of active groups.As a result,their oxygen consumption rate decreases during 30~50 ℃.Moisture inhibits the transfer of active groups,which decreases the probability of active groups to contact and react.Consequently,the coals with high moisture content present a delay phenomenon in temperature rise.The unreacted sulphur components form a film that covers the coal surface and would slightly inhibit the reaction between active groups.The theory of oxidation and self-reaction of active groups can explain the macroscopic characteristics very well.