| With the rapid development and wide application of new dry process cement technology,the nitrogen oxides produced by cement industry have become the third largest source of atmospheric NO_x emissions.With the increasingly strict environmental control,the sustainable development of cement production faces great challenges.The precalciner is the key equipment for reducing NO_x in the cement industry,and the staged combustion technology is one of the most popular NO_x emission reduction technologies.Cement production has the characteristics of high temperature,long process,micro-negative pressure and alkaline environment.Using the high-temperature suspension combustion characteristics of cement precalciner to co-process solid waste(Refuse Derived Fuel,RDF),not only can save fossil fuels also have a certain nitrogen reduction effect under reasonable processes.However,the combustion characteristics of pulverized coal and RDF are very different.When the two are co-firing,it will inevitably have a certain impact on the temperature system and NO_x release of the decomposition furnace.In order to obtain good NO_x emission reduction effect under the premise of full combustion of fuel and full decomposition of raw materials,the key issue is to find the combustion characteristics and the mechanism of NO_x formation and reduction co-combustion of coal and RDF in the precalciner.In this paper,the combination of experimental research,CFD numerical simulation and cement plant thermal detection is used.On the one hand,based on the temperature environment of the precalciner and the oxygen concentration atmosphere,the high-temperature double-tube furnace is used to carry out combustion by means of high-temperature fuel injection.Combine with thermogravimetric analyzer-infrared spectrometer combined with(TG-FTIR)and pyrolyzer-gas chromatography-mass spectrometer(PY-GC/MS)to study fuel pyrolysis,cracking experiments.The combustion characteristics and NOx emission characteristics of fuel during co-firing with RDF were studied.The influence of coal quality,ambient temperature and oxygen concentration on NO_x release was studied and the optical RDF combustion scheme was obtained.On the other hand,the CFD numerical simulation software Ansys Fluent was used to simulate the multi-fuel combustion coupled with raw material decomposition and NOx emission process in the precalciner,and the simulation results were comprehensively analyzed by CFD-POST,FieldView,Tecplot,etc.The formation of three types of pollutant NOx,the conversion characteristics of fuel NOx,and the optimization scheme of NOx reduction during pulverized coal combustion were studied.What's more,the synergistic reduction of NOx by coal and RDF multi-stage combustion in precalciner were clarified.The optimization scheme of RDF and coal co-combustion were used in the actual production and got good application results.(1)The combustion characteristics of different bituminous coals and anthracites at different temperatures and oxygen concentrations,the formation of fuel NOx were studied by using a self-made double-tube furnace platform.The results show that the combustion effective of bituminous coal and anthracite at 900°C is high and the generated CO is less,but the corresponding fuel NOx is generated much more.The NOx production and conversion rate of bituminous coal is lower than that of the corresponding anthracite during combustion at different oxygen concentrations.For the bituminous coal,when the oxygen concentration is 14%,the NO_x yield and conversion rate are high and for anthracite,when the oxygen concentration is 16%,the NOx yield and conversion rate are high.By using TG-FTIR and Py-GC/MS,it is found that bituminous coal can release more types of hydrocarbons(alkanes,olefins,aromatic hydrocarbons)at lower temperatures,and the amount of NO_x produced and the conversion rate are low.(2)Using CFD numerical simulation technology to study the formation of NO_x in the combustion process of coal.The results show that the amount of fuel NOx generated in the precalciner environment is much higher than that of thermal NOx and prompt NOx,and self-generated fuel NOx can be efficiently converted by hydrocarbons in the precalciner.Through the optimization design of the coal inlet height,horizontal position,in-depth length and coal content,a suitable staged combustion scheme was found and the effect of denitration was 48%.(3)The characteristics of different RDF at different temperatures,oxygen concentrations,and their mixed combustion with bituminous coal and anthracite and the law of NOx formation were studied by using a double-tube furnace platform.During RDF combustion,at 700°C the maximum concentration of NOx and the total amount of NOx generated are high,and 700°C should be avoided when RDF is used as an alternative fuel.RDF has a low production of NOx and a low conversion rate at an oxygen concentration of 14%.During co-combution of RDF and coal,it's linear additional effect in the first stage,and synergistic effect in the later stage of the reaction.The combustion performance of anthracite and bituminous coal blended with RDF is improved,and the total amount of NOx and the conversion rate are significantly reduced.(4)According to the combustion characteristics of RDF in the precalciner environment,the co-combustion of RDF and coal coupling with raw material decomposition in the precalciner was carried out by using CFD numerical simulation technology.At the same time,based on the optimal scheme of staged combustion,the inlet position of RDF is optimized,and the RDF position is determined to be longer in the upper part of the swirl chamber and away from the position of the tertiary inlet,the path of upward and downward movement is longer,the particle dispersion effect is better,and the temperature system is more uniform,NOx reduction effect is better.By changing the mass substitution ratio of RDF,the influence of multi-stage combustion on NOx formation was explored.Studies have shown that when the RDF substitution amount is 50%,a good synergistic mechanism can be formed between RDF and coal co-combustion,which is beneficial to form an intensive combustion mode,and is also beneficial to form a CO reduction atmosphere and promote the reduction and conversion of NO_x. |
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