Experimental Study On Combustion Characteristics And Nitrogen Transformation Of Coal/Biomass Circulating Fluidized Bed Oxy-fuel Combustion

The biomass-CO₂ capture and storage?Bio-CCS?technology with CO₂ negative emissions has become a new research focus in the field of low-carbon energy.Coal/biomass oxy-fuel combustion is an important part of Bio-CCS technology.High overall oxygen concentration combustion has obvious advantages in reducing the scale of combustion system,recirculating flue gas system scales,the building cost and the flue gas treatment cost.Therefore,the researchers have focused on the oxy-fuel combustion with high overall oxygen concentration.Moreover,the high overall oxygen concentration brings combustion temperature control,ash characteristics and nitrogen emission to oxy-fuel combustion.In this work,a systematic study on coal/biomass oxy-fuel combustion is carried out.Combustion characteristics,ash characteristics and nitrogen emissions of coal/biomass oxy-fuel combustion with high overall oxygen concentration are expected to obtain.Using TG-MS analysis method to study high oxygen concentration combustion characteristics and nitrogen transformation characteristics of coal/biomass oxy-fuel combustion,it is expected to obtain the mechanisms of nitrogen transformation and the effect on fuel nitrogen conversion.Compared with O₂/N₂,poor combustion characteristics of coal/biomass are obtained in O₂/CO₂ atmosphere with the same oxygen ratio.In O₂/CO₂ atmosphere,compared with coal combustion,coal/biomass releases more HCN,HNCO,NH₃ and NO.With the increase in the oxygen ratio in O₂/CO₂ atmosphere,the burnout temperature of coal/biomass decreases,and the complex characteristic index of combustion gradually increases.Increasing the oxygen ratio can promote the release of nitrogenous gases,shorten release time,and promote the conversion of the precursor to NO.With the increase in biomass share,the weight loss in the initial stage of combustion of mixed fuels increases.Increasing biomass share in mixed fuels can increase the relative yield of HCN,HNCO,NH₃ and NO.The 0.1 MW circulating fluidized bed experimental platform is used to conduct a feasibility study on coal/biomass?wheat straw,cotton stalk and corn straw?50%O₂/50%CO₂ and 50%O₂/50%recycled flue gas?RFG?oxy-fuel combustion.And a study on the effects of operating parameters is carried out,which is to investigate combustion characteristics and nitrogenous gases emission characteristics.Finally,methods for controlling nitrogen emission can be obtained.In the experiment of CFB oxy-fuel combustion,the temperature in the furnace decreases after adding wheat straw especially in the dense phase zone without changing other factor?air volume,secondary flow ratio and so on?.In the oxy-fuel combustion with high oxygen concentration?50%O₂?,the main compounds in fly ash are SiO₂,CaS04 and AlxSiyO1.5x+2y,while the form of K in fly ash is K3FeO₂.With the increase of combustion temperature,on the one hand,the fuel burns totally,fuel particles break to small,and make alkali metal easy to precipitate;on the other hand,the sintering phenomenon intensified,and inhibits the release of alkali metal.So the influence of temperature on the properties of fly ash temperature needs to consider the combined effect of the above two aspects.With the increase of biomass ratio in the mixed fuel,the alkali metal content in the fuel increases,but alkali metal in biomass is active,and easy to form volatile volatile,resulting in the decrease of alkali metal content in fly ash.Compared with air combustion,due to the inhibition of CO₂ to HCN formation,the time of fuel nitrogen release is prolonged in the oxy-fuel combustion process;then,the short residence time in the experiment resulted in the not enough time for HCN oxdiation,and HCN was detected in the flue gas of coal/biomass oxy-fuel combustion.In the experiment of CFB 50%O₂/50%CO₂ combustion,the reduction of excess oxygen ratio can effectively reduce the NO and N₂0 emissions of coal/biomass oxy-fuel combustion.The influence of flow staging on coal/biomass oxy-fuel combustion should be considered based on other factors such as the type of fuel,the combustion atmosphere,and so on.During high oxygen concentration?50%?oxy-fuel combustion,reducing the oxygen concentration?from 60%to 40%?in the primary flow can reduce the temperature of the dense phase zone,which is beneficial to furnace temperature control and stable operation,and reduces NO and N₂O emissions.In the process of combustion,increasing biomass share in the mixture will lead to the increase in nitrogenous gases emissions.The increase in combustion temperature can greatly reduce the emission of N₂O and HCN.Compared with 50%O₂/50%CO₂ atmosphere,the temperature distribution of the furnace is more uniform in 50%O₂/50%RFG atmosphere during coal/biomass combustion.The temperature difference between the dense phase zone and the dilute phase zone is lower than that of the O₂/CO₂,which is beneficial to the setting and adjustment of the operating parameters.In O₂/RFG atmosphere,the change of excess oxygen ratio can lead to the change of other operating parameters,and therefore it needs longer time to achieve stable operation than in O₂/CO₂ atmosphere.The addition of recycled flue gas can increase the residence time of nitrogenous gases in the furnace,ensure its full conversion,and promote the reduction of NO and N₂O.Unlike combustion in O₂/CO₂ atmosphere,in O₂/RFG atmosphere,NO and N₂O emissions increase with the increase in the secondary flow ratio.