| At present,the global climate change caused by greenhouse gas emissions is one of the biggest challenge to human society,which seriously threatens the sustainable development of human beings.The power industry is one of the main sources of CO2 emissions,showing the characteristics of large carbon emissions and fast growth.In China's energy structure,coal has always played a dominant role,and the CO2 emissions from coal-fired power plants undoubtedly aggravate the harm brought by the greenhouse effect to people's production and life.Under the low carbon economy,coal power plant is bound to become an important object for carbon emission reduction.In order to reduce the CO2 emissions and improve the power generation efficiency of coal-fired power stations,it is urgent to develop new power generation technologies that are more energy-saving,environmental friendly,clean and efficient.The power generation system that directly burning demineralized coal can not only realize the CO2 capture from flue gas,but also reduce the emission of particulate matter and other pollutants.Therefore,the study on the pressurized pyrolysis process of demineralized coal and the reactivity of the char can provide new ideas for the optimization and upgrading of low rank coal,and also provide the theoretical basis for the clean utilization of demineralized coal.In reference to the domestic and foreign pressurized drop tube reactor(PDTR)system,a small size PDTR experimental platform(furnace is only 320 mm in diameter,and uniform temperature section is 500 mm in length;the design temperature is 1100 ?,and the pressure is 1.7 MPa)is designed and estiblished,which is more easily to operate than other PDTR systems.The pressurized feeder,sealing structure,temperature control system as well as the technical difficulties in the design of pressure balance between reaction tube inside and outside are fully considerated,and the commissioning and operating process of experiment system are introduced in detail.Finally,the PDTR experimental system can provide the reaction conditions of high temperature,high pressure and high heating rate,and can operate continuously and steadily at the feeding rate of 1?10g /h,which provides the experimental basis for the research of this subject.Leaching and washing coal with chemical reagents is a common way of demineralization,but different chemical reagents have different effects on the coal structure,resulting in different reactivities among coal samples.In order to explore the influence of four different acids and HCl-HF combined acid solutions on the chemical structure and pyrolysis reactivity of coal,the chemical structures of several coal samples leached by acid are semi-quantitatively characterized in this thesis,and the pyrolysis process and gaseous products of coal samples are studied in detail by means of TG-FTIR test.The results show that the combined acid leaching method has the best effect on the mineral matter removal and can reduce the ash to 0.19%.After acid leaching,the contents of carboxyl group and phenolic hydroxyl group increase,the contents of aliphatic and aromatic hydrogen decrease,and the pyrolysis reactivities of samples are improved.Among the acids,HF,H2SO4 and HCl have similar effects on the pyrolysis process of Zhundong coal,while HNO3 significantly reduces the length of the aliphatic chain in the coal sample,making the branching degree of the organic matter in the coal deep.The volatile content of sample leached by HNO3 increases,and the pyrolysis reactivity is significantly enhanced.According to the comprehensive effect of demineralization treatment on the coal structure and reactivity,the demineralization method is confirmed,and the demineralized coal samples are prepared for the further research.In this thesis,PDTR experimental system is used to prepare rapid pyrolysis char of raw coal and demineralized coal under different temperatures and pressures.The pore structure,organic functional group and carbon skeleton structure of coal chars are obtained by means of scanning electron microscope(SEM),nitrogen adsorption analyzer,automatic mercury porosimeter,Fourier transform infrared spectroscopy(FTIR)and Raman tests.The influence mechanism of demineralization treatment and influence of pyrolysis pressure on the structure of demineralized coal char are expounded at the molecular level,which enriches the theoretical research on the evolution of char structure during the rapid pyrolysis of demineralized coal at elevated pressure.Demineralization increases the pore structure of coal,and the pyrolysis pressure promotes the coalescence of pores in coal char.The pore structure of demineralized coal char decreases with pyrolysis temperature increasing,which is contrary to the change rule of raw coal.The hinder effect of pressure on the release of cyclic aromatic structure is greater than that of linear alkane structure.At the early stage of pressure elevated,the release of volatiles is inhibited,and at the later stage of pressure elevated,the condensation reaction is promoted.The changes of physical and chemical structure of coal char caused by pyrolysis pressure and demineralization have important effects on the subsequent reactivity.In this thesis,the reactivities of raw coal and demineralized coal char during rapid pyrolysis at elevated pressure are calculated and investigated by a TGA under constant temperature and programmed temperature test conditions.Under the condition of constant temperature test,with the pressure increasing,the intrinsic reactivity of raw coal and demineralized coal char prepared at 700?800? increases first and then decreases;while the intrinsic reactivity of coal char prepared at 900?1000? decreases monotonously.Under the condition of programmed temperature test,the combustion reactivity of raw coal and demineralized coal char increases first and then decreases with the pressure increasing.The combustion reactivity of coal char pyrolyzed at 700?800? reaches the maximum value at 0.5MPa,while that of raw coal and demineralized coal char pyrolyzed at 900?1000? reaches the maximum value at 0.5 and 1.0MPa respectively. |
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