Pressurized Grade Conversion Of Coal In A Dual Fluidized Bed: Study On Semi-Coke Combustion Characteristics And Reactor Design

Coal resources are important in the development of China's economy.However,there have been problems of utilization of coal such as low efficiency,serious pollution and waste of resources all the time.As a clean and efficient using of coal,pressurized grade conversion can not only realize the coupling of coal pressurized pyrolysis and semi-coke pressurized combustion,but also bring more value-added products,higher efficiency and more economic benefits under limited resources conditions.In this paper,the research of the semi-coke pressurized combustion process in the pressurized coal grade conversion was carried out,aiming at understanding the characteristics of semi-coke pressurized combustion and providing theory and data support for the design,amplification and optimization of the combustion reactor in a pressurized dual fluidized bed.The low-temperature pyrolysis semi-coke prepared by a horizontal pressurized tube furnace was studied by proximate and ultimate analysis to obtain the composition characteristics of the semi-coke,and the pressurized combustion characteristics and dynamics of the semi-coke were investigated using a pressurized thermogravimetric analyzer.The influence of coal species on the combustion characteristics of semi-coke was analyzed.The patterns of weightlessness of semi-coke combustion under different working conditions were discussed.Moreover,the surface morphology and pore structure characteristics of the semi-coke were studied by scanning electron microscope and nitrogen adsorption instrument,respectively,and effects of different preparation conditions on the semi-coke pore structure were revealed.Using Shaanxi Heilonggou semi-coke as experimental material,an experimental system of semi-coke pressurized combustion was established to study the effects of combustion pressure,semi-coke varieties and semi-coke particle size on pullutant emission and mineral conversions.The results show that with the increase of combustion pressure,the emission of carbon oxides rises and NO and SO₂ drop.The NO emission concentration distributes in a multi peak distribution when pressure increases,and the SO₂ emission lags behind and the peak value decreases.With the increase of semi-coke particle size,CO₂ and NO produced during semi-coke combustion increase while CO and SO₂ decrease.Emissions of these four pollutants during combustion of semi-coke which is prepared by coal pyrolysis in gas atmosphere are more than those prepared in N₂ atmosphere.Meanwhile,as pressure increases or semi-coke particle size decreases,high temperature minerals in ash decline and low temperature minerals rise which results from that the combustion of semi-coke gradually transforms from heterogeneous combustion to homogenous combustion.No significant difference between two different kinds of semi-coke ash was observed.Based on the experimental mechanism research,the structure and process design of the semi-coke combustion reactor in pressurized dual fluidized bed were carried out.The structure size and thermodynamic calculations were performed first.The thermal efficiency was calculated to be 84.95%and the feeding rate of semi-coke in design was 210kg/h.Based on the designed process,ASPEN PLUS was used for preliminary simulations of semi-coke pressurized combustion.The effects of pressure and semi-coke species on pollutant emissions in the combustion process were investigated.The results provided a useful reference and guidance for the design of the reactor.Finally,based on the structural design and process simulation of the semi-coke pressurized combustion reactor,the combustion reactor of the pilot scale pressurized dual fluidized bed experimental platform was built.The system was firstly tested for the stable operation,and preliminary combustion experiments with semi-coke at different pressures,temperatures,and ratios of primary and secondary air were carried out in the pressurized dual fluidized bed to provide reference for further study of semi-coke combustion.