Nitrogen Release And Migration In Coal Chemical-looping Gasification

Coal is China's main energy source which is of great significance to China's industrial development and people's production and life.The energy status of China's "rich coal,lean oil and less gas"determines that the main position of energy consumption in coal will not change for a long time now.A large amount of CO2 and pollutants generated during the utilization of coal.NOx is one of the main polluting gases causing environmental problems such as acid rain and photochemical smog.To improve environmental and economic benefits,it is particularly important to develop a clean and efficient approach of coal resources.As a new type of coal utilization technology,coal chemical-looping gasification technology has the advantages of high energy utilization rate.With the development of chemical-looping gasification technology,the migration and release of nitrogen in the reaction process has received extensive attention.The low reaction temperature during the chemical-looping gasification reaction inhibits the formation of thermal NOx and rapid NOx,but the fuel-type NOx formed by nitrogen in coal is still a potential source of pollution.In this thesis,the migration of fuel nitrogen in the chemical-looping gasification process has been studied in detail.Firstly,the combined thermo gravimetric-infrared-mass spectrometry(TG-FTIR-MS)analysis was used to conduct chemical-looping gasification experiments of coal with an iron-based oxygen carrier under Ar atmosphere,and the mass change and gas composition during pyrolysis stage and steam gasification reaction stage of reduction process were monitored in real time.The surface element analysis of the solid phase was carried out by X-ray photoelectron spectroscopy,when the change of nitrogen functional groups in the solid phase at different stages of chemical-looping gasification reduction process was investigated.The results show that the oxygen carrier has an effect on the release of nitrogen-containing gas in different stages of the chemical-looping gasification reduction process.During the pyrolysis stage,the oxygen carrier promotes the generation of free radicals,which accelerates the release of nitrogen-containing gas in a pyrolysis stage.At high temperature,the oxygen carrier promotes the conversion of NH3 to HCN,and the addition of the oxygen carrier during the gasification stage reduces the degree of graphitization of the semi-coke and increases the release rate of the nitrogen-containing gas.For the nitrogen functional groups in the solid phase,the oxygen carrier promotes the decomposition and conversion of pyrrole nitrogen in the pyrolysis stage.At high temperature,as the graphitization and ordination degree of semi-coke decreased,the protonated pyridine embedded in coal macromolecules was exposed,the content of protonated pyridine decreased,and the content of pyridine nitrogen and pyrrole nitrogen increased greatly.Based on the chemical-looping gasification experiment,it is found that the valence state of the iron-based oxygen carrier in the reaction is complicated,which is not conducive to the study of the migration and release behavior of nitrogen.Therefore,the nickel-based oxygen carrier with simple valence change is selected.Summarizing the commonalities and differences by comparing the reaction characteristics of the two oxygen carriers.It has demonstrated that in the low temperature pyrolysis stage,the addition of nickel-based oxygen carrier and iron-based oxygen carrier promotes the reaction;in the high-temperature pyrolysis stage,both the nickel-based oxygen carrier and the iron-based oxygen carrier can react with the coke.The addition of two oxygen carriers in the gasification stage enhances the gasification activity of coal char.For the migration of nitrogen,at the same time as the reaction between the nickel-based oxygen carrier or the iron-based oxygen carrier and the coke in the high-temperature pyrolysis stage,the nitrogen element in the coke is released to produce a nitrogen-containing gas.Two kinds of oxygen carriers promotes the decomposition and conversion of protonated pyridine in the high temperature stage,and the iron-based oxygen carrier inhibits the decomposition of pyrrole-type nitrogen in the coal of low temperature pyrolysis stage.Finally,different C/O ratios were selected,and the effects of different oxygen carriers on the weight loss characteristics and on the release law of nitrogen-containing gas of the chemical-looping process were studied.Five cycles experiments were performed in thermogravimetric equipment to investigate changes in sample weight loss and nitrogen-containing gas releasing characteristics during the cycles.When the amount of oxygen carrier in the pyrolysis stage increases,the reactivity of the coal sample at 650? increases,the coke structure is more dense,and the initial time of the high temperature pyrolysis reaction is delayed.In the gasification stage,the coke gasification activity increases as the amount of oxygen carrier increases.For the release of nitrogen-containing gas,when the amount of oxygen carrier is increased,the release amount of NH3 and NO increases during the low-temperature pyrolysis stage,and the release of HCN does not change much.During the high temperature pyrolysis stage,the release of HCN and NO increased.The chemical-looping is more sensitive to the oxygen carrier cycle during the pyrolysis phase of the reduction process.With increasing number of cycles,the promoting effect of the oxygen carrier on the first-stage reaction performance of the low temperature pyrolysis decreases.At the beginning of the second cycle,the reactivity of the oxygen carrier with the coal decreases during the high temperature pyrolysis.For the release of nitrogen-containing gas at a low temperature pyrolysis,the circulation of the oxygen carrier is not conducive to the release of NH3 and NO,which is beneficial to the release of HCN.At the high temperature pyrolysis stage,the release of NO increases as the number of oxygen carrier cycles increases.