Superfine Pulverized Coal Oxy-fuel Combustion And NOx Heterogeneous Reduction Mechanism

With the development of the society, more and more attention has been paid to ecological environment. Nowadays, mankind is facing three serious environmental problems: greenhouse effect, acid rain and ozone depletion. The latter two have ever caused serious environmental crisis, such as the London smog rain event in 1952,Los Angeles smog event and Antarctic ozone hole which have aroused the attention of the international community during the 60s and 70s of 20th century. From then on, a series of effective technologies and measures for control and reduction of pollutant have been developed. However, the greenhouse effect, caused by emission of carbon dioxide from fossil fuel combustion especially coal combustion, has gained attention and credibility in recent years. Many technologies for controlling CO₂ emission are developing, among which O₂/CO₂ combustion technology is an effective, high cost-effective, highly competitive comprehensive clean combustion technology. Superfine pulverized coal O₂/CO₂ combustion technology is proposed in the paper which applies superfine pulverized coal to the traditional O₂/CO₂ combustion technology. We attempt to combine the advantages of both in order to achieve good combustion properties and lower pollutant emission. The research results could provide technical support for a new round of research and design of high efficiency and low combustion equipments and might become a new widely promoting technology for simultaneous removal of contaminants.A series of exploratory studies have been conducted on many key issues concerning superfine pulverized coal O₂/CO₂ combustion technology. Surface properties of superfine coal/char, the pyrolysis and combustion process of superfine coal in O₂/CO₂, pollutants emission characteristics and NO heterogeneous reduction mechanism have been studied using hot stage microscope, thermogravimetric analysis（TGA）, fixed-bed reactor bench and entrained flow comprehensive combustion bench combined with X-ray photoelectron spectroscopy（XPS） and quantum chemistry calculation software. Mechanochemical effect is not only a research focus, but also has been applied in inorganic preparation. However, there are few reports in coal research field. The mechanochemical effect during the grinding process was studied in the first chapter. The element types,the functionalities of carbon, nitrogen, oxygen and sulfur on coal surface and their variations with coal particle size have been tested and analyzed using X-ray photoelectron spectroscopy （XPS）. The results showed that mechanical force have different effects on different elements. The research could enrich the comminution theory and surface chemical characteristics of superfine pulverized coal.Pyrolysis is the initial stage of coal conversion process. Therefore a thorough study on pyrolysis helps to understand coal utilization processes such as combustion, gasification, liquefaction. Pyrolysis experiment of superfine pulverized coal in CO₂ using the fixed bed reactor bench were carried out. Moreover, the effects of coal type, particle size and temperature on the evolved gas composition and the gas release mechanism were analyzed. The results showed that nitrogen in coal evolves mainly in the forms of NH3, HCN and N2O while the occurrence of N2O and NO are closely related to the existence of CO₂ atmosphere. The atmosphere, temperature and coal particle size have great influence on the release of CO.A deep insight into the combustion characteristics of fossil fuel provides the prerequisite and basis for their usage. The ignition and combustion characteristics of superfine pulverized coal were investigated using a thermogravimetry and a hot stage microscopy. Ignition flame characteristics of superfine pulverized coal in the O₂/CO₂ atmosphere have been observed using a hot stage microscope attaching a camera. The effects of atmosphere, particle size, oxygen flow rate and oxygen concentration on the ignition characteristics of micro-pulverized coal were analyzed. It is found that it is easier for coal to ignite in N2/O₂ mixture than in CO₂/O₂ mixture and for the coals particle size above 20μm, the ignition process become more violent for the volatile matter evolves intensively. Particle size of coal affects the ignition mode. Homogeneous ignition occurred in the case of 33.68μm while heterogeneous ignition took place in the cases of lower than 20μm. On the thermogravimetry the effects of oxygen concentration, particle size and heating rate on the coal combustion characteristics under O₂/CO₂ atmosphere were analyzed. The results indicated that oxygen concentration played the most important role. As the particle size decreases, the ignition and burnout temperatures decrease while the comprehensive combustion index S increases. Higher oxygen concentration helps to promote the effect of heating rate.An entrained flow comprehensive combustion test bench has been designed and built to further study the combustion and emission characteristics of superfine pulverized coal in the O₂/CO₂ atmosphere. The effects of particle size, furnace temperature, stoichiometric ratio, recycled NOx on the CO₂, CO and NOx emissions have been analyzed. Grey correlation method has been applied to calculate and sort the relation degrees of the factors over the gases CO₂, CO, N2O, NO₂ and NO and nitrogen conversion rate. The results indicated that the concentration of recycled NO only have apparent impact on the formation of NO₂ .Other three factors have great influence on the gases and nitrogen conversion rate for Tiefa bituminous coal while it is slightly different for Neimenggu coal. Furnace temperature and stoichiometric ratio have obvious influence on CO₂ and NO concentrations and nitrogen conversion rate while particle size has apparent influence on CO₂ and CO concentrations and nitrogen conversion rate.The variations of functional groups on the surface of superfine pulverized coal/char during pyrolysis in CO₂ atmosphere promote the understanding of the conversion process of the functional groups during pyrolysis and combustion in the O₂/CO₂ atmosphere and it is also an important reference to the simulation of quantum chemistry. We have acquired coal char in the high concentration of CO₂ at different temperatures on the fixed bed reactor. Afterwards, XPS tests have been conducted to reveal the variations of the functional groups of coal/char at different stages of pyrolysis.The heterogeneous mechanism of NO reduction during superfine pulverized coal O₂/CO₂ combustion has been investigated from a microscopic view based on the experimental results and combined with the density functional theory and organic reaction theory. Simplified models of coal char during gasification and combustion have been proposed and the thermodynamic, structural parameters and adsorption selectivity of adsorption process of small molecules the surfaces of coal char and coal char during gasification. It is found that small gas molecules tend to wholly or partly adsorb onto the char surfaces between two active sites in a side parallel way. The ungasified coal char surface is more favorable of NO adsorption than the coal char during gasification. On the basis of reaction heat and population analysis, heterogeneous reduction reaction steps and pathways are further analyzed in detail and finally the NO heterogeneous reduction mechanism is concluded which could provide a reference for the conversion of NOx in the oxy-fuel combustion process.