Research On The Transformation Of Sulfur And Nitrogen During Co-pyrolysis And Co-gasification Between Coal And Biomass

Based on existing energy conditions in China,it is an important research topic that how to achieve the integration of traditional energy and renewable energy,promote greater openness,integrate higher level and deeper in the energy field to ensure energy security,reduce environmental pressure and adapt to the national energy security strategy with the development of“the Belt and Road”and international cooperation.Pyrolysis is a more efficient and cleaner way to be used than direct combustion.The co-pyrolysis of coal and biomass can not only reduce the emission of CO₂,SOx,and NOx,but also improve the shortcoming of biomass to individual pyrolysis with the higher moisture content,low calorific value and low density.However,how to transfer of sulfur and nitrogen of coal and biomass in co-pyrolysis process,which were not clear at current researches,and much attention was paid to study the individual fuel pyrolysis or the transfer of single element.In this study,cornstalk（RC）and cellulosic ethanol residues（FC）are chosen to pyrolysis as raw biomass materials.The transfer of sulfur and nitrogen and pyrolysis mechanism between high sulfur coal with low rank（BC）and biomass are studied during co-pyrolysis process,and co-gasification.The objective of this study is to gather useful data and valuable knowledge that can be used to industrialize application.The thermal gravimetric analyzer（TGA）was used to test for biomass and BC during co-pyrolysis process,it was found that the range of pyrolysis temperature for the blend was divided into four stages by derivative thermogravimetric（DTG）curves.Synergistic interaction occurred during co-pyrolysis process due to free radial and catalytic of biomass,and then which was obvious for the blend when the ratio of BC/RC and BC/FC were 4:6 and 6:4,respectively.The mean value of theoretical activation energy for the blend with 40%RC and the blend with 40%FC were 265.84 kJ/mol and 272.31 kJ/mol during co-pyrolysis,which were higher than that of experimental activation energy about 14.89%and14.79%,respectively.It meant that energy barrier for the blend was be reduced with biomass,promoting the process of pyrolysis.The kinetics equation of pyrolysis of the blends were obtained by KAS（Kissinger-Akahira-Sunose）,FWO（Flynn-Wall-Ozawa）and master-plots method.Afterwards,theoretical enthalpy was higher than experimental enthalpy,indicating that the total energy barrier became lower in a pyrolysis system.Meanwhile,the entropy of blend was increase during pyrolysis process.The quadratic polynomial equations were obtained for desulfurization rate to optimize pyrolysis conditions using response surface methodology（RSM）.And the conditions of co-pyrolysis were temperature of pyrolysis,heating rate and the ratio of coal in the blend（CR）.The optimum desulfurization rate of 85.97%for the blend（BC/RC）was achieved at temperature of 819℃,heating rate of 15℃/min,CR of 55%.And then the optimum desulfurization rate of 87.21%for the blend（BC/FC）was achieved at temperature of 813℃,heating rate of 15℃/min,CR of 40%.The sulfur content of char,H₂S and COS on the basis of total sulfur of the blend（BC/RC）were 14.03%,59.84%and 12.57%,respectively,under optimum pyrolysis conditions.The cumulative conversion of H₂S and COS were increased with the increase of the temperature.Meanwhile,the nitrogen content of char,NH₃ and HCN on the basis of total nitrogen of the blend（BC/RC）were 41.47%,7.85%and 2.38%,respectively.Under optimum pyrolysis conditions,the sulfur content of char,H₂S and COS on the basis of total sulfur of the blend（BC/FC）were 12.79%,68.50%and10.23%,respectively.And cumulative conversion of H₂S and COS were more gentle.The nitrogen content of char,NH₃ and HCN on the basis of total nitrogen of the blend（BC/FC）were 30.74%,8.04%and 2.43%,respectively.The yield of char was deceased with the increase of the ratio of biomass under rapid pyrolysis process,the experimental value of the blend was lower than that of theoretical calculation,indicating synergistic interactions occurring to promote decomposition of char.The sulfur content of char of blend（BC/FC）was decreased and then increased with the increase of the temperature.When CR was 80%,the cumulative conversion of H₂S and COS were increased and then decreased with the increase of the temperature.The maximum of H₂S was 69.21%at 800℃,which of COS was 10.84%at 700℃.Meanwhile,the cumulative conversion of NH₃ and HCN were 9.85%and3.95%.Comparing with desulfurization rate and denitrification rate under slow heating rate,H₂S,NH₃ and HCN were accelerated to release,and more active oxygen was transferred into tar to the disadvantage of COS under fast heating rate.Char of the blends from BC/RC and BC/FC were prepared by RSM under the optimum desulfurization rate,the yield of which gasified were 31.76%and 23.35%with steam as gasification agent.The sulfur content of gasified char,H₂S and COS on the basis of total sulfur of the blend（BC/RC）were 9.75%,5.63%and 1.26%,respectively.The nitrogen content of char,NH₃ and HCN on the basis of total nitrogen of the blend were 22.57%,8.35%and 3.73%,respectively.The sulfur content of gasified char,H₂S and COS on the basis of total sulfur of the blend（BC/FC）were 7.04%,6.61%and 0.83%,respectively.The nitrogen content of char,NH₃ and HCN on the basis of total nitrogen of the blend were 16.18%,11.37%and 2.54%,respectively.The content sulfur of char was reduced by optimizing pyrolysis conditions to desulfurization during co-pyrolysis between coal and biomass.It has more important practical significance to control effectively emission of sulfur and nitrogen by studying the transfer of sulfur and nitrogen during co-pyrolysis and the co-gasification process.Meanwhile,the new research direction was opened up to utilize biomass waste resources.