Simulation Research On Heat And Moisture Recovery And Nitrogen Reduction Performance Of Flue Gas Recirculation Gas Boiler By Membrane Heat Exchanger

The efficiency enhancement and cleaner emission technologies of gas boiler system are the development directions based on the social development needs and environmental needs.Since the existing low-nitrogen combustion technologies often have a negative impact on boiler efficiency,many current boiler systems cannot achieve efficient and clean production.Due to the low cost,flue gas recirculation technology is widely used in the cleaning renovation of existing boiler systems.However,it would bring obvious operational problems.A large amount of flue gas entering the furnace affects not only the formation of nitrogen oxides,but also the main reaction process in the furnace.The combustion characteristics of the furnace under the application of flue gas recirculation technology and the reaction process of the main pollutants were analyzed in this paper.Flue gas recirculation was comprehensively evaluated from five aspects:environmental impact,nitrogen reduction rate,radiation intensity in the furnace,burnout rate,and waste water/heat recovery based on the influence of the recirculation rate.The influence of membrane heat exchanger and flue gas recirculation sequence on system efficiency increase and nitrogen reduction was analyzed.The sustainability of the synergistic system under the dry and wet flue gas circulation was analyzed.The study found that the contribution rate of thermal NO decreased at high recirculation rates,while the proportion of N₂O NO increased significantly.The reaction rates of NCO,N,and HNO-NO pathways all decreased significantly.The main reactions of NO formation require the participation of OH and O,but the recirculation of flue gas can greatly reduce the content of the two groups in the furnace,thereby inhibiting the formation of NO.However,the effect of the recirculation rate on the concentration of the two groups gradually decreases with the increase of the recirculation rate,so that the change of NO emission is small under the high recirculation rate,and the nitrogen reduction rate is the highest under the condition of25%recirculation rate,which can reach 89.79%.The flue gas circulation has a great influence on the combustion characteristics in the furnace,and the static temperature in the furnace drops from 1927.24°C to 1601.85°C.The flame center moves up continuously with the increase of the furnace speed,which has a negative impact on the time for the gas to stay in the furnace to participate in the chemical reaction,and too much recirculated flue gas will lead to a decrease in the burnout rate.A small amount of flue gas recirculation can effectively reduce the CO content to achieve the treatment of pollutants.The optimal burnout rate occurs at 18%recirculation rate,which is99.25%.At this time,the CO content in the exhaust gas is 74.95ppm,and the NO content is20.79ppm.When the flue gas recirculation rate continues to increase,an increase in CO content is observed,and the burnout of the fuel deteriorates again.The increase in recirculation rate can enhance the water heat recovery process in the membrane heat exchanger.When the flue gas circulation is in front,the exhaust gas temperature can be as low as 32.94°C.The amount of water heat recovery increases with the increase of the recycling rate,the efficiency of waste heat recovery can reach 7.47%,and the efficiency of waste water recovery can reach 65.33%.The recirculation technology increases the water heat carried by the total flue gas.Putting the membrane heat exchanger at the outlet of the boiler flue gas can increase the waste heat recovery efficiency of the system by 2.56%,and at the same time recover an additional 1.05kg/h of condensed water.However,this will slightly reduce the moisture content of the recirculating flue gas,and the nitrogen reduction efficiency under the condition of 25%circulation rate is 4.02%lower than that of the system of flue gas recirculation former.The emergy gain of the system can reach up to 8.75E14se J/yr.the system of membrane heat exchanger former is more dependent on the energy value purchased from other places.As the recirculation rate increases,the system of membrane heat exchanger former will cause greater environmental pressure.Under a high recirculation rate,the ESI of the system of flue gas recirculation former is higher than that of the system of membrane heat exchanger former,which is 0.1246.The ESI of the two types of systems is much higher than the traditional tail denitrification technology,but lower than the ORC cycle system of waste heat power generation.