Experimental Study On Combustion Characteristics Of High Ash Content Oil Shale Semi-coke In The Circulating Fluidized Bed

Oil shale is the most potentially unconventional oil and gas resources,and the main usage of oil shale is retorting for shale oil product.In the retorting process,great amount of solid waste—oil shale semi-coke is formed.Because of high ash content,low calorific value,oil shale semi-coke is difficult to use.Dumping at open air not only affects the environment but also results in the waste of resources.The circulating fluidized bed combustion technology can use high ash,low calorific value of inferior fuel,so the combustion characteristics of oil shale semi-coke in circulating fluidized bed boiler has a clear engineering significance.The results can provide a reliable basis for the combustion and utilization of oil shale semi-coke.In this thesis,oil shale samples from three origins were retorted at 450℃-600℃ using a drop tube furnace.The TGA was used to study the influence of pyrolysis temperature,pyrolysis time and oil shale origin on the composition,calorific value and combustion characteristics of oil shale semi-coke samples.The combustion characteristics indexes and kinetic parameters of each sample were calculated.In addition,the co-combustion test of oil shale and semi-coke was carried out in a circulating fluidized bed middle demonstration plant(3.3MWth,22.5m in height).Some experimental results were obtained,which were the co-combustion characteristics of oil shale semi-coke and the pollutant emission characteristics.The results show that the pyrolysis reduces the calorific value by half for the oil shale from Huadian in Jilin province and Longkou in Shandong province.And there is not any effect on the oil shale from Yilan in Heilongjiang province,which is mainly related to its high fixed carbon content.Judged from the combustion characteristics indexes: ignition temperature Ti,burnout temperature Th,flammability characteristic index SW,the semi-coke from Yilan is of the best combustion characteristics.The results also show that,due to the different carbon content,there are three obvious stages in the combustion of oil shale and semi-coke samples from Huadian and Longkou including the combustion of volatile between 350℃ and 500℃,the combustion of fixed carbon between 500℃ and 700℃ and the decomposition of ash after 700℃,while the Yilan oil shale shows a continuous weight loss phenomenon.By comparing the Friedman-Carroll method,the Coats-Redlfern method and the direct Arrhenius method,it is more reasonable to use the direct Arrhenius method to calculate the kinetic parameters of the oil shale semi-coke combustion.With the increase of pyrolysis time and the pyrolysis temperature,the combustion characteristics of the semi-coke is deteriorated,and the activation energy of the volatile combustion is increased.And the effect of pyrolysis temperature is greater than the pyrolysis time.The results from the experiments in the circulating fluidized bed small demonstration plant indicate that the boiler can still operate stably under the condition of oil shale and semi-coke ratio is 2:1.The desulfurization efficiency reaches to 69.9%,the original emission value of NOX can be controlled under 150mg/m3.Due to the poor attrition characteristics of the semi-coke,the concentration of particles has to be reduced and the uniformity of the temperature distribution is deteriorated in the furnace The high ash content of the semi-coke is not conducive to burn out,so the carbon content in the bottom slag and fly ash is high.