Combustion And NO_x Generation Characteristics Of Bituminous Coal Blended With Meager-lean Coal(Including Some Petroleum Coke)in Nanning

The uneven distribution of coal resources in China,coupled with the influence of coal market prices,has led many power plants to use mixed coal combustion for power generation.Because of the differences in the components of each coal type,if we simply rely on experience and mix randomly,it may cause difficulties in boiler ignition,inadequate combustion,coking in the furnace chamber and poor operating efficiency,which may even lead to furnace shutdown in serious cases.Therefore,it is necessary to carry out a targeted and in-depth study on the combustion characteristics of coal blending in power station boilers.In this paper,blending experiments are conducted for lean coal,bituminous coal and petroleum coke in Nanning,China,to study the influence of mixing ratio and reaction conditions on the combustion characteristics,coking characteristics and NO_X generation of blended coal.The combustion characteristics and kinetic parameters of the blends of the three kinds coals were studied under different conditions using a thermogravimetric analyzer.The experimental results show that the thermogravimetric curves of blended coals with different blending ratios are between those of bituminous coal and petroleum coke.In addition,the blended coal with 30%petroleum coke has the best combustion performance,but the combustion characteristics of the blended coal deteriorate sharply when the proportion of petroleum coke in the blended coal increases to more than 30%.Kinetic analysis showed that the activation energy of the stage before 500°C was higher than that of the stage after 500°C.The average activation energy of the samples increased with the increase of oxygen concentration.In addition,the average activation energy of the blended coals showed that the combustion characteristics of the blended coals could not be determined by the activation energy derived from the Coats-Redfern method alone.Bituminous coal,Nanning lean coal and petroleum coke were matched together in different proportions and different particle sizes and put into a tube furnace for pyrolysis to produce blended coal coke.Low-temperature nitrogen adsorption method and scanning electron microscopy were used to study the change patterns of surface morphology and fractal properties of the mixed coal coke.The analysis of the adsorption isotherms showed that the pore structure of the mixed coal coke mainly consisted of semi-open pores with semi-connectivity.The specific surface area of the blended coke increased with the increase of the final pyrolysis temperature,and then decreased and then increased again;the specific surface area of the blended coke increased with the increase of the proportion of bituminous coal blending,and the A11 blended coke had the maximum specific surface area.Increasing the content of petroleum coke in the blended coal samples would inhibit the generation of micropores.The micropores of the blended coal coke with different blending ratios were more concentrated than the mesopores.By calculating the fractal dimension of the blended coal coke,it was found that the trend of D1 was consistent with the trend of the ratio of microporous specific surface area to total specific surface area in the blended coal coke.The increase of bituminous coal in the blended coal coke makes its indicated structure complex and the increase of petroleum coke content is its surface becomes smooth,as depicted by SEM photographs.The effect of blending ratio and excess air coefficient on NO_x generation was investigated using the sinker,and the NO_x generation characteristics in the sinker were analyzed using CHEMKIN software.The results showed that the NO_x generation during the combustion of surface blended coals was related to the total nitrogen content of the blended coals,and it did not show a good pattern with the change of the blending ratio.Sensitivity analysis was used to study the components that play a major role in NO_x generation.radicals such as H,OH,and O are crucial in the reaction process.For the same radical reactions,the sensitivity coefficients showed positive and negative differences,which further illustrates the interaction of chemical reaction aspects in coal combustion due to coal blending;the main conversion products of HCN are NH and HNO,and NH₃ decomposition products NH and NH₂.As the excess air coefficient increases,the O₂ concentration increases,the generation of O and OH activation groups increases,and the whole reaction atmosphere is biased toward an oxidizing atmosphere,which is favorable to the rapid oxidation of HCN and NH₃ to NO.