| By means of the utilization of Oxy-fuel combustion technology,the volume fraction of CO2 in the exhaust gas after cooling treatment can reach approximately 90%.Oxy-steam combustion technology is normally regarded as an upgraded technology of oxy-fuel combustion.Steam is employed for replacing the circulating flue gas to participate in the reaction occurres in the main combustion zone,and the downstream flow gas can also achieve CO2 enrichment.The reaction temperature in the main combustion zone can be adjusted by additional steam,avoiding the formation of thermal NOx.Moreover,H2 O molecules can react with coal particles generating reductive gaseous products?H2 and CO?which can participate in the consumption of NOx at high temperatures.Thus,it is necessary to clarify the emission characteristics of coal-N during combustion process for the application of oxy-steam combustion technology.Based on the experimental tests and analytical testing methods,the emission mechanisms of coal-N during combustion process can be investigated systemically in this research.Additionally,the correlation among char particle surface behavior,microstructural characteristic and NO x formation characteristic during the oxy-steam combustion process will also be determined.Firstly,the char samples which were obtained from thorough devolatilization of two typical bituminous coals were employed for the investigation.Partially oxidized samples and partial-TPD samples were collected for the identification of Fourier transform infrared spectroscopy?FT-IR?,Temperatured programmed desorption/reduction?TPD/TPR?and X-ray photoelectron spectroscopy?XPS?.It was found that carboxyl,phenol,anhydride,ether,lactone and quinone were the main structures of oxygen containing functional group on the surface of char particles generated during the combustion process.The amount of C?O?on the surface of per unit mass sample reached the maximum,when the conversion ratio of char particle reached approximately 0.3.The primary structure and thermal stability of nitrogen containing complexes on particle surface were: N-5 < N-6 < N-Q.The reaction temperature generally expressed positive effects on the generation of C?O?,and expressed negative effects on the formation of C?N?.The interference of alkali metals and alkali earth metals on the identification of char combustion process was basically avoided after the demineralization treatment?HF-HCl-HF?.Based on the results obtained from the FT-IR analysis,the thermal stability and the centre of decomposition temperature of each type C?O?could be clarified: phenol?1000 K?< carboxyl?1150 K?< ether/anhydride?1400 K?< quinone?1600 K?< lactone?1650 K?.There were the most oxygen atoms attached to the surface of char particles at the conversion of 0.35,and the precursors of CO 2 were the dominant form.Additionally,the total amount of C?O?on particle surface could be regarded as an indicator reflecting NO emission characteristic in the process of char combustion.Due to the long term contact time,many NO molecules were reduced into N2 by the char particles.O2/H2O combustion tests of demineralized char sample were carried out in the horizontal fixed-bed experimental system in the temperature range of 1073 – 1473 K.The effects of reaction condition on the emission characteristics of nitrogen containing gaseous products during O2/H2O combustion process were determined.The TPD and Raman results illustrated that,with the enhancement of reaction temperature and O2 concentration in the reaction atmosphere,more active sites generated and larger aromatic ring structures decomposed into small ones.These variations in char particles expressed positive effects on accelerating NO consumption during the combustion process.Due to the attachment of ·H and ·OH to vacant active sites,the addition of steam had both positive and ne gative effects on C?O?formation,and the critical concentration of steam was 8.5 vol.%.Ether,phenol,quinone and lactone were the main form of C?O?generated during the O2/H2O combustion.In the early stage of the reaction,the reaction between oxidizing agents and char particles led to the enhancement of the relative amount of N-5 at the expense of N-Q and N-6.Due to the weak thermal stability of N-5,rapid decomposition occurred subsequently and the relative amount of N-5 decreased apparently at the middle and late stages of the reaction.There was also a good linear relationship between the combustion reactivity and the C?O?amount of each partially oxidized char sample.Finally,the O2/H2O combustion tests of SH demineralized coal were performed in this research.The results indicated that NO,N2 O,NH3 and HCN were the main types of nitrogen containing gaseous products released during the O2/H2O combustion process.Meanwhile,N2 O,NH3,HCN and a part of NO derived from the volatile-N,and most of char-N released in the form of NO during the O2/H2O combustion.Additionally,the enhancement of reaction temperature,oxygen concentration and steam concentration in the reaction atmosphere tend to promte the release of nitrogen content in the coal particles in the oxy-steam devolatilization stage.Meanwhile,there was generally a good linear relationship between the microstructural characteristic and NO conversion ratio.Consequently,it could be concluded that char combustion stage played a vital role in de termining the emission characteristics of nitrogen content in demineralized coal sample during the O2/H2O combustion process. |
PDF Full Text Request