Research On Flow And Combustion Characteristics In 600MW MIMSC Technology Boiler With Vent Air Located On Front And Rear Walls

Undoubtedly,down-fired boiler has been applicable for burning low volatile coal,and there have been several schools of combustion technology at abroad on this kind of boiler,however,it turns out by practice test that those technologies have their own shortcomings,among which high NO_x emissions and carbon content in fly ash are shared.Therefore,a comprehensive down-fired combustion technology is worth studying,and Multiple Injections and Multiple Staging Combustion Technology is put forward with a series of principle advantages by a team from Harbin Institute of Technology.Aiming at further enriching and perfecting this technology for improving its flexibility to provide technical support for practical problems,in this thesis,a retrofited boiler,with vent air nozzles of a 600 MW supercritical down-fired boiler in China with the aforementioned technology laid on front and rear walls and 24 burners uniformly arranged,is taken as the research object.Then,under different ratios of primary air to vent air rate and different allocations between secondary air and tertiary air rate respectively,the reaearch on the flow and combustion characteristics in the furnace is carried out by means of cold singlephase modelling test and numerical simulation.A reduced cold single-phase moldelling test facility was built up with the ratio of 1:20 between it and the retrofited boiler,and the measurement of aerodynamical field in lower furnace was done under different ratios of primary air to vent air rate and different allocations between secondary air and tertiary air rate respectively through streamer tracing and hot-wire anemometer.As a result,with the decrease of primary air speed,the aerodynamic field in lower furnace is basically symmetrical,and the downward momentum of airflows is enhanced.When secondary air rate is 27%,33% and 39% respectivelty,the aerodynamic field in lower furnace is basically symmetrical,and the symmetry is especially better under the secondary air rate of 33% and 39%.While under the working condition of 45%,the W-type aerodynamic field is seriously skew,thus the condition was unallowed.Besides,with secondary air rate increased,the airflow rigidity above the tertiary air nozzles is better and better,but the airflow speed in the middle and lower area of dry bottom hopper becomes smaller.According to the physical dimension of the retrofited 600 MW down-fired furnace with on-wall vent air,a mesh model was established,and the research on thermal and NO_x generation characteristics in the furnace was done by numerical simulation software.The results show that the temperature field of lower furnace under different ratios of primary air to vent air rate is symmetrical.With the ratio of primary air to vent air rate reduced,the temperature level of lower furnace is lower and lower,the oxidizing atmosphere in the dry bottom hopper strengthened.Meanwhile,the concentration of NO_x emission at the furnace outlet decreases from 898.68mg/m3(PA:VA=4:6)to 847.68mg/m3(PA:VA= 3:7),then increases to 949.78mg/m3(PA:VA=2:8)at 6% O2,and the combustible content in fly ash is at a low level of about 4%.On the other hand,when the secondary air rate 27%,33% and 39% respectively,the temperature field of lower furnace diaplays a good symmetry,but the symmetry of the temperature field under the secondary air ratio of 45% is quite bad.With secondary air rate increased,the temperature level of lower furnace decreases under the first three conditions and finally increases,the oxidizing atmosphere in the dry bottom hopper consistently strengthenes and finally weakenes.In addition,under the four working conditions,the concentration of NO_x emission at the furnace outlet increases from 668.80mg/m3(SA:27%)to 898.68mg/m3(SA:39%),and then decreases to 809.90mg/m3(SA:45%)at 6% O2.The combustible content in fly ash under the first three conditions is at a low level of about 4%,while under the secondary air rate of 45%,it is relatively high of 6.83%.Comprehensively considering,it is recommended that the ratio of primary air to vent air rate is 3:7 and the secondary air rate 33%.