Research On The Synergistic Control Of Sulfur And Nitrogen Pollutants In Circulating Fluidized Bed Combustion

With increasingly strict environmental requirements,it is important to synergistically reduce sulfur dioxide（SO₂）and nitrogen oxide（NO_x,N₂O）emissions from circulating fluidized bed（CFB）boilers for clean and efficient coal utilization.However,there are contradictions in suppressing sulfur and nitrogen pollutants during CFB combustion.In particular,different key issues need to be resolved to decompose contradictions when boilers operate under different loads.To address this,two technical ideas of"staged limestone injection"and"in-furnace desulfurization with carbide slag"were studied to evaluate their effectiveness in synergistic control of sulfur and nitrogen pollutants."Staged limestone injection"involves separately injecting desulfurization limestone into the dense and dilute phase zones of the furnace,mainly to address the issue of increased NO_xemissions from in-furnace desulfurization under high boiler loads.Regarding this technical idea,combustion experiments of coal/coke with limestone addition were first conducted in a tube furnace,to study the influence mechanisms of limestone on sulfur and nitrogen transformations during different coal combustion stages.By testing sulfur and nitrogen functional groups in coal and coke,the pathways for sulfur and nitrogen conversions of coal combustion under the influence of limestone were revealed.Next,considering factors including sulfur and nitrogen partitioning in volatile matter and char during coal pyrolysis,nitrogen species in volatiles,two-step conversion mechanisms for char nitrogen,effects of desulfurization product layers,and catalysis of Ca O on nitrogen reactions,a model was developed for predicting sulfur and nitrogen pollutant emissions from CFB combustion with in-furnace desulfurization.Combustion tests were then carried out in a 30 k W CFB test rig and numerical simulations were used to study the effects of staged limestone injection on SO₂,NO and N₂O emissions.Finally,a staged limestone injection scheme was proposed for a 350MW CFB boiler,with the original limestone inlet as the primary inlet and the secondary air port as the secondary inlet.Numerical simulations were performed to predict the impacts on SO₂,NO and N₂O emissions.Results show that staged limestone injection can effectively reduce SO₂,NO and N₂O emissions compared to single-stage injection."In-furnace desulfurization with carbide slag"utilizes carbide slag instead of limestone as the in-furnace desulfurizer,mainly to address the issues of low desulfurization efficiency and high N₂O emissions under low boiler loads and bed temperatures.Regarding this technical idea,combustion experiments of coal with carbide slag addition were first conducted in a tube furnace,to study the desulfurization characteristics of carbide slag during coal combustion and its influences on fuel nitrogen conversions to NO and N₂O.Results show that besides SO₂removal,carbide slag also promotes fuel nitrogen conversion to NO and inhibits fuel nitrogen conversion to N₂O.In-furnace desulfurization tests with carbide slag were then conducted in a 30 k W CFB test rig.The test results show that carbide slag has the highest desulfurization efficiency at a bed temperature of 800°C,and can reduce N₂O emissions while increasing NO emissions.Finally,scanning electron microscopy was utilized to characterize the surface morphological changes of carbide slag before and after desulfurization,revealing the microscopic desulfurization mechanisms.