Study On Chemical Looping Combustion Mechanism Of Coking Coal And Weak Coking Coal

In China,coal resources are abundant and the reserves of medium-low-rank coal with poor caking property are especially huge.As environmental pollution becomes increasingly severe,its use is under tremendous environmental pressure.Developing clean coal technology is an inevitable choice to alleviate environmental pressure and achieve sustainable development.Chemical looping combustion is a new type of anaerobic combustion technology that achieves indirect combustion of fuel through the lattice oxygen released by oxygen carriers,which is clean,efficient and unpolluted.Coal chemical looping combustion is an important technical direction for coal clean utilization,and has received extensive attention from researchers in recent years.At present,the researches on coal chemical looping combustion at home and abroad focus on the macroscopic reaction characteristics,and there are few studies on microscopic reaction mechanisms of coal chemical looping combustion.Using coking coal and weak coking coal as raw materials,the reaction characteristics,reaction kinetics,precipitation process of gas phase products and the evolution of main functional groups in coal were discussed in this paper.It is of great significance for understanding and revealing the reaction mechanism of chemical looping combustion in coking coal and weak coking coal.The main research contents are as follows:?1?The chemical looping combustion experiments of coking coal and weak coking coal were carried out in the thermogravimetric analyzer.The weight loss characteristics and reaction kinetic parameters were obtained;the kinetic equations of chemical looping combustion reaction of caking coal and weak coking coal were established.The results showed that chemical looping combustion process of coking coal and weak coking coal could be divided into three stages:drying stage,starting pyrolysis stage and gasification combustion stage.In the three reaction stages,the activation energy E and the pre-exponential factor A remained the same change law,indicating that the chemical looping combustion reaction of coking coal,weak coking coal and Fe4Al6 oxygen carriers had a kinetic compensation effect.The activation energy and the pre-exponential factor were both large in the drying stage,indicating that the effective collision frequency of the coal that absorbed heat was high,so the reaction was easy to proceed.The activation energy at starting pyrolysis stage and gasification combustion stage reduced slightly,but the reduction of the pre-factor was more significant,which indicated that the effective collision frequency of the activated molecules was very low,so the reaction was not easy to carry out.?2?The TG-FTIR combined technique was used to analyze the precipitation characteristics of gaseous products in the chemical looping combustion process of coking coal and weak coking coal,and the precipitation law of typical gas products was studied.The results showed that the weight loss rate peak of coking coal and weak coking coal corresponded to the absorption peak of gas products.The time of weight loss peak and absorption peak of weak coking coal were earlier than that of coking coal,and its reaction mainly occurred in the early stage of combustion.At low temperature,the water came from free water and combined water in coal,and at high temperature,the water was formed by the reaction of pyrolysis gas with Fe4Al6oxygen carriers.The precipitation of CO₂ was the largest and the reaction temperature range was wide.The CO₂ of weak coking coal was mainly precipitated at 500⁸00°C,and the CO₂ of coking coal was mainly precipitated at 750⁹00°C.The precipitated amount of tar-like substances was small,and the precipitation temperature of weak coking coal was lower than that of coking coal.?3?The fixed-bed reactor was used to investigate the chemical looping combustion reaction characteristics of coking coal and weak coking coal.The evolution process of surface functional groups in coal was analyzed by Fourier infrared analyzer,and the quantity change of main carbon-containing functional groups was studied by peak-matching technique.The results showed that at 900°C,the cumulative carbon conversion ratios of coking coal and weak coking coal were close to 100%.The maximum carbon conversion rate and CO₂ content of weak coking coal were0.103%/min and 5.91%,respectively.The maximum carbon conversion rate and CO₂content of coking coal were 0.061%/min and 5.06%,respectively.So the reaction rate of weak coking coal was faster.In the first 3 minutes of the reaction,a large amount of associated hydroxyl groups,aliphatic methyl and methylene groups were cleaved,which was the main source of gas phase products in the early stage of chemical looping combustion.During the first 1 to 6 minutes of the reaction,the aromatic hydrocarbon structure was gradually graphitized and vaporized by steam,which was the main source of gas phase products in the middle a nd late stages of chemical looping combustion.Weak coking coal had many aliphatic hydrocarbons with a high degree of branching and some aromatic carbons with oxygen-substituted functional groups,so the reaction rate of carbon-containing functional groups was faster.The branching degree of aliphatic hydrocarbons in coking coal was low and its aromatic carbons arranged closely,so the reaction rate of carbon-containing functional groups in coking coal was slower.