Study On The Release Characteristics Of Fuel Chemical Energy Based On Energy Grade

Efficient and clean use of energy is the basis of China's economic stability,health and sustainable development.At present,China's energy structure is dominated by fossil fuels,such as coal and natural gas,and obtain heat in the form of direct combustion.However,because of the backward technology,the efficiency of this simple extensive use of energy is very low.At the same time,Combustion products: NOx,SOx,haze,and CO2,seriously affect people's normal production and life.Therefore,it is imperative to develop efficient and clean utilization technology of fossil fuels.Based on the previous studies,the energy reaction mechanism and the energy release path of coal direct combustion,gasification and reforming and combustion,supercritical water oxidation,chemical looping combustion and natural gas direct combustion,gasification and reforming and combustion and chemical looping combustion are discussed in this paper.At the same time,based on the theory of energy grade,by means of the Energy Utilization Diagram Methodology,the characteristics of the above combustion reactions of coal and natural gas are analyzed and compared,the energy conversion characteristics of various reactions are analyzed,and the characteristic of chemical energy release is revealed.The main research results are as follows.(1)Reasons for low energy utilization ratio of direct combustion:in the traditional thermal system,fuel is converted into physical energy mainly in the form of direct combustion,the chemical grade of fuel is high,while the other side of the system has a low grade heat source,the large difference between the two grade leads to the loss of energy in the direct combustion of fuel.(2)Compared with the direct combustion of coal,coal gasification reforming and combustion,supercritical water oxidation and chemical chain combustion reduce the grade difference between chemical energy and constant temperature heat source,and change the way of releasing chemical energy in coal: the gasification reforming reaction or reduction reaction can transform high grade coal chemical energy into a lower grade Ni or CO or H2 chemical energy.At the same time,the difference of grade is served as the force,to improve the reaction thermal grade of gasification reforming reaction or reduction reaction product(Ni or CO or H2).At last,chemical reaction products can be transformed into physical energy through oxidation,which reduces the whole process of energy loss.Compared to the direct combustion of coal,the exergy losses of gasification reforming and combustion,supercritical water oxidation and chemical looping combustion of coal were reduced by 14.32 kJ/mol,5.19 kJ/mol,8.27 kJ/mol,and the reductions were 17.92%,6.04% and 10.35%,respectively;thermal exergy increased by 9.99 kJ/mol,16.49 kJ/mol,4.95 kJ/mol,and the increases were 3.18%,5.25% and 6.52% respectively.Coal gasification,reforming and combustion can reduce exergy to the greatest extent,while chemical looping combustion of coal increased thermal exergy at most.Considering the practical commercial application,the coal gasification,reforming and reburning methods is most suitable.(3)By changing the way to chemical exergy release in natural gas,combustion modes of natural gas steam reforming and combustion and chemical looping combustion reduce the energy grade difference between energy receiving side and energy grade of chemical energy release side,therefore,the exergy efficiency is improved.Compared to the direct combustion of natural gas,the steam reforming and combustion and chemical looping combustion exergy losses are reduced by 32.43 kJ/mol,75.55 kJ/mol,and the reductions were 35.85% and 46.62%,respectively;thermal exergy increased 35.33 kJ/mol,20.96 kJ/mol,and the increases were 22.40% and 15.91%,respectively.The comparison results show that the best effect of reducing the exergy loss of combustion reaction is the steam reforming of methane,and chemical looping combustion of methane can release the most heat exergy,but considering chemical looping combustion is limited by oxygen carrier,steam reforming and burning of methane is more suitable to practical application.(4)Different reaction temperature of reforming reaction and reduction reaction in chemical chain combustion and steam reforming and combustion of coal and natural gas are compared and analyzed.The results show that the exergy losses of reforming reaction and reduction reaction increases and the thermal exergy decreases gradually with the increase of temperature.Through the study of the clean and efficient utilization of fuel(coal and natural gas)and its chemical energy release mechanism,this paper provide theoretical guidance to explore or study the new energy utilization mode and future practical industrial applications.