Investigation On Pollutant Emission Characteristics Of Coal Combustion In Pressurized Oxy-fluidized Beds

The global warming caused by greenhouse gas has seriously threatened the survival and development of human beings.Coal-fired power plants are the largest concentrated emissions source of CO₂ from human production activities.And it is of great significance for China to develop coal-fired CO₂ reduction technology for the development of low-carbon economy.Since now,oxy-fuel combustion is considered to be one of the most promising technologies for CO₂ reduction of coal-fired power plants.The conventional oxy-fuel combustion technology still has some problems,such as the excessive power consumption of air separation unit,the lower purity of CO₂ caused by the air leakage in furnace and flue gas duct.All these problems will greatly reduce the net efficiency of power plants.The pressurized oxy-fuel fluidized bed combustion(POFBC)technology is proposed in this context,and it has the advantages of small work loss,high net efficiency and easy recovery of latent heat.So the POFBC technology,which combines the advantages of pressurized oxy-fuel combustion and fluidized bed combustion technology,is a novel CO₂ capture technology with a prospect for the industrial application.Investigation on POFBC is significant for the development and extension of this technology.The present research work focuses on four main gaseous pollutants of coal combustion,NO,N₂O,SO₂ and SO₃.And a combination method of experimental research and theoretical analysis is used to study the conversion pathway of N and S during the coal combustion with high pressure and high concentration of CO₂.The results obtained are summarized as follow:(1)The N/S conversion characteristics during coal pyrolysis under CO₂ atmosphere were studied in a 20 k W_thpressurized fluidized bed.It was found that the generation of HCN and NH₃ increase simultaneously with the increase of overall pressures.High pressure and the existence of high partial pressures of CO₂ enhanced the fuel-N conversion rate in pyrolysis process.The high-pressure CO₂atmosphere promoted the formation of CO,and the presence of high concentrations of CO further accelerated the decomposition of pyrite.Meanwhile,high-pressure pyrolysis also promoted the self-desulfurization of coal ash,and enhanced the conversion reaction of gaseous S and pyrite to thiophene.(2)The effect of temperature,pressure,atmosphere and other operating parameters on the direct sulfurization of limestone was studied by using a pressurized horizontal fixed bed.The results showed that when the proportion of each component in the reaction atmosphere is kept constant,the conversion rate of limestone will increase with the increase of total pressure.When the total pressure was kept constant,increasing the CO₂ partial pressure had a certain inhibitory effect on the direct sulfurization reaction of limestone.This is because the existence of high partial pressure of CO₂ inhibits the migration of anion vacancy such as CO₃^2-/SO₄^2-and solid ions in the product layer,and reduces the generation rate of direct desulfurization reaction.Also,the effect of operation parameters on the homogeneous reaction of SO₃ was studied in a pressurized vertical fixed bed.It was found that high reaction pressure greatly promoted the production of SO₃ and the conversion of SO₂.As the reaction pressure increased,the increase of SO₂ conversion ratio slowed down gradually.Increasing the concentration of SO₂ and O₂ at the reactor inlet would increase the final production of SO₃,while increasing the concentration of H₂O had little effect on the production of SO₃.(3)The effects of operating parameters such as temperature,pressure,concentration of reductant(CO and NH₃)on the homogeneous formation and reduction of NO_x were studied by the reaction kinetics calculation and the experiments on a pressurized fixed bed.The results showed that the two most important pathways of NO_x homogeneous formation are NH₃?NH₂?H₂NO?HNO?NO and HCN?CN?NCO?N₂O when the reaction pressure ranges from 0.1 MPa to 0.9 MPa.With the increase of reaction pressure,the formation pathway of N₂O remained unchanged,however the secondary formation path of NO changed to some extent.Part of the H₂NO intermediates and NO₂ would further generate HONO with the increase of H₂O partial pressure,and then HONO would decompose rapidly into NO under the catalysis of H₂O.For the homogeneous reduction and decomposition of NO_x,increasing the reaction pressure promoted the NO reduction reactions,and it also promoted the generation of NO₂ in the flue gas to a certain extent.While for N₂O,the increase in reaction pressure would not change its main reduction path,but it greatly promoted the thermal decomposition rate of N₂O,and this promotion was more obvious when the system was lower than 0.5 MPa.(4)More than 100 hours of stable combustion experiments were carried out in a 10 k W_thcontinuous-feeding pressurized fluidized bed,and the effects of operating parameters such as pressure,temperature,atmosphere and oxygen concentration on the coal combustion and the emission of gaseous pollutants were investigated.Experimental results proved the synchronous reduction of unburned carbon,CO,NO_x and SO₂ in pressurized oxy-combustion compared with the atmospheric oxy-combustion,and demonstrated that POFBC is a new combustion mode for high combustion efficiency and low emissions.And the reducing effect of pressure on NO and SO₂ emission was more pronounced in the lower pressure range(?0.3 MPa)than that in the higher pressure.The influence of temperature on the main gaseous pollutants under oxy-fuel combustion was the same as that under air combustion,that was the concentrations of NO and SO₂ correlated positively with temperature,while for N₂O,it had a negative correlation.Compared with air combustion,NO and SO₂ emission dropped sharply in 21%O₂/79%CO₂atmosphere.However,N₂O concentration during oxy-combustion was slightly higher than that in air combustion in the range of experimental pressure.Increasing the inlet O₂ concentration enhanced the overall oxidizing atmosphere in the furnace,which would increase the emission of NO and SO₂,but it would also make the fuel-N much more easily converted to NO rather than N₂O,and this would reduce the emission of N₂O.The above results will enrich the scientific understanding of pressurized oxy-coal fluidized bed,and lay a foundation for the industrial application of this technology.