Influences Of Secondary Air On The Aerodynamical Field In A Down-fired Boiler Equipped With Swirl Burners

Down-fired boilers are designed to burn low volatility coals such as anthracite and lean coals.When a down-fired boiler is in operation,there is a high temperature and a long pulverized coal burning distance in the furnace for its unique structure and burner arrangement,which leads to a beneficial atmosphere for the formation of NOx.And for this reason,NOx emissions of down-fired boilers are always too high.Recent years,haze happens frequently in our country and the limits for polluta nts of burning coal is stricter.And for this reason,it's of great significance to decrease NOx emissions of down-fired boilers.For this purpose,the Combustion Engineering Research Institute(CERI)of Harbin Institute of Technology(HIT)did low-NOx combustion retrofits for a 300 MW B&W down-fired boiler.Equip the boiler with declinate over fire air near the boiler's throat to realize air gradation combustion in the furnace.To keep enough injection distance of the burners' air flow after the boiler is equipped with over fire air,arrange doroid baffles in the inner and outer secondary air nozzles,and the baffles' areas are all 25% of their own nozzles' areas.When the burners are arranged on the arches of the boiler,the baffles are close to the combustion area of the furnace.This article aims to investigate the characteristics of aerodynamic fields of the burner and the furnace after the boiler is retrofitted,and the investigation is done by cold air experiments on scale-models of the retrofitted burner and boiler.Using an IFA300 constant-temperature anemometer system,this article measured air velocities of the burner's air flow to investigate flow characteristics of aerodynamic fields of the burner;Using a series of thermocouples,this article measured air temperatures of the burner's air flow to investigate mixing characteristics of aerodynamic fields of the burner;Using an IFA300 constant-temperature anemometer system,this article measured air velocities in the whole furnace of the retrofitted boiler to investigate flow characteristics in the whole furnace.Firstly,this article investigated the flow characteristics of the retrofitted burner's air flow under secondary air rate 37.6%,41.7%,46.5% and 51.3% on a scale-model of the burner.The result shows that with secondary air rate improving,the diffusion and swirl of the burner air flow gets stronger and the injection distance of the burner air flow gets longer.The doroid baffles of the inner and outer secondary air lead to air flow differences between the burner side with baffles and the burner side without baffles.And the air flow velocities,diffusion velocities and swirl intensions of burner side without baffles are all bigger than that of burner side with baffles.However,the baffles doesn't change the swirl nature of the burner.Secondly,this article investigated the mixing characteristics of the retrofitted burner's air flow under secondary air rate 37.6%,41.7%,46.5% and 51.3% on a scale-model of the burner.The results show that it's benefit for the mixing of the burner's primary air and secondary air of both sides with and without baffles to increase the secondary air rate.But because there is more secondary air at the burner side without baffles,the mixing velocities of this side is bigger than the side with baffles.Finally,this article investigated the flow characteristics of the retrofitted boiler's air flow in the whole furnace under secondary air rate 37.6%,41.7%,46.5% and 51.3% on a scale-model of the boiler.The results show that with secondary air rate increasing,the aerodynamic field in the whole furnace changes from unsymmetrical field to symmetrical field,and finally changes to unsymmetrical field.And when secondary air rate is 46.5%,the aerodynamic field in the w hole furnace is a W shaped symmetrical field and the injection distance of the burner air flow is long enough.However,the change of secondary air rate does not influence the aerodynamic field in the upper furnace near over fire air nozzles,and under all secondary air rates there is a little W shaped field in this zone.Taking all the above results into consideration,choose secondary air rate 46.5% as the basic operating condition when the boiler is in operation.And by adjusting the boiler's air distribution on the basis of this condition,the boiler's operation can realize optimization.