Simulation Study On Characteristics Of Coalbed Methane Reburning For Reducing NO_x Emission In Precalciner

Since the implementation of the reform and opening-up policy,the "extensive" economic development model has been adopted for nearly four decades,which caused increasingly serious energy and environmental problems threatening our country.The state also spares no effort to take control of the air pollutant emission.In order to meet the increasingly stringent NO_x emission standards for cement industry,developing efficient and low-cost denitrification technology for cement production is imperative.In this paper,the low-nitrogen combustion technology for precalciner using coalbed methane reburning is proposed and the numerical simulation is carried out.The aim is to provide guidance and reference for the development and application of denitrification technology in cement production process.The main contents and conclusions are as follows:(1)The simulation model was established on the basis of a TDF precalciner,software Fluent was used to analyze the velocity field,temperature field and concentration distribution in the precalciner under operation conditions.After data processing,the basis for reburning transformation scheme was acquired and the reliability of the model was verified by field test.(2)A scheme using coalbed methane reburning was proposed for precalciner transformation.The effects of the length of the main combustion zone,the length of the reburning zone,the excess air ratio of the reburning zone and the amount of coalbed methane on the fuel reburning technology were analyzed successively.The simulation result shows that,as the length of the main combustion zone increases,the denitrification efficiency of the coalbed methane reburning technology increases first and then decreases while the fuel burnout rate changes little.As for the length of the reburning zone,both the denitrification efficiency and the heat loss of incomplete combustion gradually increase with the increasing of the reburning zone.With the increase of the excess air coefficient in the reburning zone,the denitrification efficiency increases first and then decreases,and the decomposition rate of the raw material gradually increases.With the increase of coalbed methane,the denitrification effect gets stronger,but the combustion efficiency of the precalciner declines slightly.(3)The effects of four reburning factors on the raw material composition rate and denitrification efficiency were investigated by orthogonal analysis.The obtained results show that the increase of the reburning denitrification efficiency is based on the decrease of the decomposition rate of raw materials.Combined,the optimal transformtion scheme is acquired: the height of the coalbed methane reburning pipe is 7 m,the height of the OFA duct is 10.5 m,the excess air coefficient of the reburning zone is 0.75,and the heat of coalbed methane output accounts for 12.5%.(4)Comparing the reburning condition with the operation condition,the simulation results indicate that the coalbed methane reburning condition is more favorable to the gas-powder mixing and heat transfer.The main combustion zone temperature has slightly decreased,which is detrimental to the formation of fuel-NO.The temperature of the flue gas at the outlet of the precalciner decreased to some extent,which indicates the rise of the thermal efficiency in the precalciner;the fuel burnout rate was 91.57% and declined by 1.06%;the decomposition rate of raw material was 94.60% and increased by 2.94%.The NO concentration at the outlet of the precalciner is 378.15 mg/Nm3,and the denitrification efficiency is 49.27%,which reaches the current national standard.And the conclusion is drawn that the coalbed methane reburning technology can meet the demand for denitration of cement production.