Comparison Of Energy Release Between Coal Indirect And Direct Combustion Based On Energy Utilization Diagram Methodology

As the largest energy source in China,coal resources are abundant in reserves and have a long history of exploitation,which provides a source of power for the social progress and economic development of our country.Coal direct combustion is the main utilization,however energy utilization is low,accompanied by atmospheric pollution problems such as CO₂,SO₂ emissions.In order to solve the problem of effic ient utilization of coal resources and integration of environment,clean coal utilization technology emerged as the times require.At present,great progress has been made in the research of clean coal technologies such as integrated coal gasification comb ined power generation and supercritical technology.However,the characteristics of energy release in the reaction are still unclear.On this basis,this paper researches several clean coal utilizations—supercritical water gasification and re-combustion and chemical-looping combustion.The Energy Utilization Diagram Methodology is used to explore the difference of coal energy release between indirect and direct combustion,and provides an in-depth analysis of the internal causes of exergy losses.Then,from the idea of reducing the loss of chemical energy,the research on the principle of physical energy and chemical energy cascade utilization was carried out.Through the general exergy function and the Gibbs free enthalpy function in the thermodynamic system,the three internal correlations of endothermic reaction characteristics,Gibbs free grade and heat exergy increase in indirect combustion processes.The main results are as follows:?1?Typical indirect combustion methods such as coal supercritical water gasification and re-combustion and chemical-looping combustion can not only reduce the emission of harmful gases,but also divide one step of traditional direct combustion into a new form of endothermic reaction and exothermic reaction.It can change the traditional release pathways of chemical energy in coal,reduce grade difference between the heat source and the coal chemical,then reduce the exergy losses during the reaction process,increase heat exergy,and more chemical energy is converted into physical energy,achieving a step-by-step release of energy.Compared with the coal direct combustion process,the exergy losses of supercritical water gasification and re-combustion and chemical-looping combustion can be reduced by 23.9%and 17.12%,respectively,and the increase of heat exergy in these processes are 5.2%and 1.8%,respectively.The energy efficiency was 82.3%and 79.5%,and the exergy efficiency was 70.7%and 65.2%respectively.?2?According to the basic theory of thermodynamics,the heat exergy gain function based on optimal endothermic reaction temperature in indirect combustion process is derived and constructed.The heat exergy gain,endothermic reaction characteristics and the Gibbs free energy grade are combined in the same function,in order to explore the influence of the temperature of the heat source in the process of endothermic reaction on heat exergy gain,endothermic reaction characteristics and Gibbs free energy grade,and to find the optimum temperature state point of each indirect combustion processes.Among them,when the endothermic reaction temperature in the process of coal supercritical water gasification and re-combustion is about 750?⁸25?,there is a higher combustion gain,and the maximum gain can reach 35%at 800?;The optimum gain of 28%is achieved at temperature of 900?⁹50?in the coal chemical-looping combustion process.Continuous increase of temperature will cause the cementation of CaSO₄ oxygen carrier,resulting in a decrease in reducibility.In this paper,the energy release characteristics of coal direct combustion and indirect combustion process are compared,and the influence of the endothermic reaction temperature on the heat exergy is constructed.It provides a theoretical support for exploring the new indirect combustion mode of fuel.