Numerical Investigation On Low-load No_x Formation And Emission Characteristics And Flue-gas Temperature Deviation Of A 1000MWe Tower Boiler

The tower boiler is widely used in recent years in China because of larger unit capacity,smaller flue gas temperature deviation,less abrasion of heat exchangers.However,the issues of high NOx emissions under medium-low load and some degree of flue gas temperature deviation are still exist in actual operation of partial tower boiler.Numerical investigations were conducted on NOx formation and emission characteristics under low load and the mechanization of flue gas deviation of a 1000MWe ultra-supercritical tower boiler.Further,improvement of combustion and flue pass structure were studied to decrease NOx emissions at low load and flue gas temperature deviation.Firstly,a complete geometric modeling and detailed grid system were built for this boiler,a good consistency is showed between calculated results and measured results.Numerical simulations were conducted under different loads and actual operating parameters were considered to analysis the reasons of high NOx emissions at low load of the boiler.The results show the higher operation oxygen content and higher primary/coal ratio are the mean reasons.Secondly,the damper opening of surrounding-air was appropriately reduced to decrease the surrounding-air flow rate,then to decrease the stoichiometric ratio at the initial combustion stage.Correspondingly,the temperature,O2 concentration and NOx formation rate all decrease,and the NOx reduction rate increases.Both the calculation results and measured results show that the NOx emissions decrease about 20%under 700MWe and 500MWe without increase of fly ash carbon content.On this basis,the surrounding-air damper opening is set as a function of the boiler load to always maintain at a reasonable location.Operation results show no negative effects on the combustion and steam temperature and no overhearing of the primary air nozzle under the new control strategy.Finally,the influence of heat-exchanger structure on flue gas flow?temperature and heat exchange was analyzed by numerical simulation of the boiler.The results show that the flue gas would flow towards rear wall under the traction of induced drag fan.And the intensive heat exchangers would increase the deviation degree,hinder the lateral flow of flue gas,cause the flue gas flow to be uneven,finally,lead to flue gas deviation between left and right at the heat-exchange region.A flow-guided structure of flue gas which is similar to“furnace nose"is proposed to make the flue gas must turn before it enters back pass,thus to prevent the flue gas flow towards rear wall.The suitable depth of flow-guided structure make the flue gas flow and temperature to be more uniform and symmetrical between left and right,and has no negative effects on combustion and outlet parameters.