Study Of Surface Chemistry And Microstructure And The Influence On NO_x Emissions From Oxidant Staged Combustion Of Superfine Pulverized Coal In O₂/CO₂ Atmosphere

Energy and environment are two global challenging issues which we are facing. China is the largest country on the aspects of producing and consuming coals. At present, 67% of the total Chinese energy needs are fulfilled by coals and this situation will last for a long time in the future. The air pollutants released from coal combustion is one of the largest pollutive sources and has caused serious damages to the environments. This situation has been a serious restriction to the development of society and economy. Hence, how to use coals with higher efficiency and lower pollutive emissions becomes extremely important. Researchers over the world have been working on NO_x and SOx which can cause direct damages such as acid rain and photochemical smoke for years and have obtained lots of meaningful achievements. In recent years, the greenhouse effect caused by emissions of carbon dioxide has attracted more and more attention. So there will be a huge market potential to explore one kind of new combustion technique which can realize reducing pollutive emissions efficently, capturing CO₂ easily and having good combustion performances simultaneously in the same facility.A series of studies had been conducted on the surface morphology, microstructure and chemical characteristics of superfine pulverized coal by applying several advancing equipments such as SSRF, AFM, NMR, EPR, XPS and XRD. Combustion and pollutants emission properties were also investigated using fixed bed reactor and coal combustion system. Fractal theory, mechanochemical effect analysis and abundant testing methods were also adopted based on detailed experimental works. The results could provide a reference for refining the oxy-fuel combustion theory, revealing the NO_x reduction mechanism and studying the influence of surface chemistry and microstructure on the combustion process.The physical characteristics of pulverized coal is an important influencing factor which determine the mass, heat transferring rate in the particle and the reactive areas. Huge evolutions of physical characteristics will occur during processes of combustion, gasification and liquefaction. So it is meaningful to study the physical characteristics of superfine pulverized coal which is helpful for further understanding the ignition and combustion processes. Several advancing equipments such as AFM, SSRF were applied to study the physical characteristics combining fractal theory. The surface morphology, particle size distribution (PSD), comminution theory and pore structures were studied in detail. The results show that with increasing of the particle size, the surface fractal dimensions increase and the surface roughnesses also increase. On the contrary, with decreasing of the particle, the PSD fractal dimensions increase, which represents that the higher degree of particle fragmentation will be and the distribution is more concentrated. The piecewise fractal model is successfully adopted to study the PSD of the superfine pulverized coal and the results show that there are three different fractal dimensions in the whole range and each of them is corresponding to one kind of comminution theory and fragmentation mechanism. A new economic fineness of pulverized coal is defined which can better evaluate the relationship between the particle size and performance of the combustion apparatus. After studing the pore structures of superfine pulverized coal, it is found that the surface fractal dimension decreases with decreasing of the particle size which indicates that the smaller the mean coal particle size is, the smoother the interfacial boundary will be. On the other hand, with decreasing of the particle size, the pore structure fractal dimension increases which indicates that the more complex pore network will be. Therefore, the conclusion can be drawn that with the decrease of the coal particle size, the channels for reactant gas transportation become smoother and the mass transfer resistance lowers down which is easier for the gas transportation.Consequently, the reaction rate increases. On the other hand, the increase of the pore structure fractal dimensions can provide more reactive surfaces which are also advantageous for coal combustion.Mechanochemical effect occurs during the comminution process of superfine pulverized coal and the chemical properties of coal determine the combustion and pollutants emission characteristics. Some advancing equipments such as SSRF, XPS, NMR and EPR were applied to study the chemical characteristics from several aspects such as elements constitution, carbon skeleton structure, free radical and surface organic groups. The results from this study can provide the foundation and guidance for further study of coal/char strutrure model and pollutants emission characteristics. EPR results show that with decrease of particle size, the free radical increases. The free radical increases in coal during the acid washing process. The particle size influences the oxygen containing type most. NMR spectra indicate that with decrease of particle size, the aromatic carbons and hydrogens increase, the aliphatic carbons decrease, protonated aliphatic decrease obviously and aliphatic carbons bonded to oxygen decrease while aromatic carbons bonded to oxygen increase. XPS results suggest that with decrease of particle size, pyridine nitrogen decreases while pyrrole nitrogen increases. It is concluded from the SSRF results that there is surface interfacial layer in coal particles which is caused by the organic groups linked to the matrix of the coals. The interfacial thickness of superfine pulverized coal particles decreases with increasing coal quality and particle size.For the char particles, the interfacial thickness decreases with increasing pyrolysis temperature. Furthermore, the interfacial thickness of NMG chars formed in CO₂ atmosphere is thinner than that of the chars formed in N₂ atmosphere. Pyrolysis is the initial stage of coal conversion process and has important, sometimes crucial influence on the whole combustion process. Pyrolysis experiments of superfine pulverized coal were carried out using the fixed bed reactor. Moreover, the effects of coal type, particle size and temperature on the gas releasing mechanism and evolution of gas compositions were analyzed. The results show that there are three main stages during CH4 releasing process and each type of CH4 consists with NMR analysis very well. During pyrolysis of pulverized coal, the amount of CO increases with decrease of particle size but a best size exists. The amount of CO yielding in CO₂ atmosphere is much higher than that in N2 atmophere because of the gasification effects. It is found that the gasification effects have a close relationship with pore structures of coal. With decrease of the particle size, there are more pores in coal which can intensify the gasfication effects. The results show that there are four main stages during CO releasing process and each type of CO has a close relationship with carbon contained organic groups in coal. The formation of NH3 is related with the free radical in coal which has to be actived by hydrogen contained radical outside. The smaller the coal particle size is, the more easily the fuel nitrogen converts to volatile nitrogen.A thorough study on O₂/CO₂ staged combustion was conducted on the one dimensional coal combustion experimental system. The results show that in O₂/CO₂ atmosphere, when it is under fuel lean combustion condition, volatile nitrogen increases with decrease of particle size which results in the increase of total NO_x. On the other hand, when it is under fuel rich combustion condition, volatile nitrogen lowers down with decrease of particle size and the reducing capability on NO_x of chars increases which induces the decrease of total NO_x. The reduction of NO_x is obvious when oxidant staged technique is adopted in O₂/CO₂ combustion and the superfine pulverized coal can reduce NO_x emissions more efficiently than larger particles in this new technology. It is found that the NO_x emissions along the furnace appear as'M'type. The homogenious reduction effect of NO_x is more efficient at higher temperature while it is disadvantageous for the heterogeneous reduction effect.Oxidant staged combustion of superfine pulverized coal in O₂/CO₂ atmosphere is a brand new technology which realize simultaneously the removal of NO_x and capturing CO₂ easily. Also this technology can solve the inherent disadvantages in normal air staged combustion or O₂/CO₂ combustion such as low combustion efficiency, high combustion loss and slagging issues to a certain extent. Furthermore, this technology is feasible and easy to realize with few reconstructing on the exsiting devices. Therefore the oxidant staged O₂/CO₂ combustion technology will become a useful and promising method on controlling CO₂ and NO_x emissions in the future.