| Petroleum coke(PC) is the residue in crude oil refining.The output of high-sulfur PC increases due to the deterioration of crude,together with the expanding scale of delayed coking.Sulfur is an important chemical material;however,China is a country with poor natural sulfur resource.Scientific recovery of potential sulfur resources in PC is an important approach to alleviate the lack of sulfur resources,and also a method to promote the efficient and clean utilization of PC,which has high industrial value.Chemical looping gasification(CLG) realizes the partial oxidation of fuel by circulating lattice oxygen from oxygen carrier between gasification reactor and air reactor.CLG is a potential,efficient and clean technology to achieve fuel conversion with the advantages of auto-thermal balance,low cost and inhibiting the release of SO2.In this paper,based on the advantages of CLG,aiming at the clean utilization and sulfur recovery of high-sulfur PC,the characteristics of high-sulfur PC,the migration and conversion of sulfur during CLG process were studied.Besides,the evolution mechanism of sulfur model compounds of PC in CLG was explored,and the CLG system model of high-sulfur PC was established to provide guidance for the novel system coupled with sulfur recovery.The CLG experiments of PC using hematite as oxygen carrier were conducted in a small-scale batch fluidized bed,to evaluate the characteristics during different gasification processes,and the conversion of carbon and sulfur with the variables of temperature,flow rate of steam,and particle size.Compared with steam gasification,the carbon conversion efficiency(?c) and the sum volume of sulfur increase in CLG.Meanwhile,all of CO,CO2,H2S and SO2show the bimodal trend,which are derived from devolatilization and coke gasification,respectively.When PC of0.1-0.3 mm and hematite of 0.3-0.4 mm are employed at 900 °,the effective syngas fraction is more than 80%,and the mole ratio of H2S to SO2 is about 2,which meets the requirements of syngas production and sulfur recovery in Claus process.It verifies the feasibility of high-sulfur PC utilization via CLG based on hematite.Introducing K is a method to improve the gasification rate of PC,so catalysts were loaded on PC and oxygen carrier respectively to study the catalytic mechanism in gasification by batch fluidized bed and fixed bed.When potassium catalyst,such as K2CO3 or KNO3,is added into PC or oxygen carrier,the graphitization is inhibited with active phase formation of KHCO3 and KOCn on PC;K2O·x Fe2O3 eutectic in hematite generates to improve the reactions;and they lead to the increases in H2 yield and carbon conversion rate.Compared with CLG without K,the inorganic sulfur content on the surface of PC increases significantly after catalytic CLG of 10%K-PC;The increasing amount of K promotes the concentration of H2S and sulfur conversion efficiency.When adding K to hematite,K-S and K-Fe-S compounds are detected,realizing in-situ sulfur capture during CLG.The hematite with K has excellent stability with?c of more than 98% and H2 concentration of 66% after 21 cycles.It is revealed that the migration between PC and hematite and internal diffusion of K,the pores formation and autocatalysis are the fundamental factors to improve the gasification reaction activity,while sintering and potassium loss lead to the decrease of gasification rate.Fe-Cu oxygen carrier of high performance can also improve the reaction rate of PC during CLG.The characteristics of Fe-Cu oxygen carrier in CLG were theoretically analyzed by chemical thermodynamics.The increase of CuO in the oxygen carrier improves the conversion efficiency of PC,but excessive CuO would lead to the decrease of effective synthetic gas and sulfidation of oxygen carrier.The increase of steam concentration effectively inhibits the sulfidation and sintering of oxygen carrier with an increase in the production of H2 and H2S.However,the differences of oxygen carriers by two preparation methods can only be studied by experiments.In the CLG based on Fe-Cu oxygen carrier by wet impregnation,CO2 is conducive to the conversion of sulfur to SO2,and H2O to H2S.The selectivities of CO,CO2,H2S and SO2 are obviously different when using oxygen carriers prepared by different methods.Oxygen carrier by wet impregnation releases more oxygen and has stronger adsorption of CO with the enhancement of water-shift reaction,so the yields of CO2 and H2 increase,and S mainly releases as SO2 in the initial period;while the CO yield of oxygen carrier by sol-gel preparation is higher,and H2S is mainly produced,so the oxygen carrier by sol-gel preparation is more suitable for CLG process.The mechanism of sulfur conversion from sulfur-containing model compounds in CLG was investigated through chemical equilibrium analysis,TG-MS and fixed bed.Sulfur tends to convert to H2S instead of SO2 in the atmosphere with sufficient hydrogen sources such as H2O.According to the weight loss curve of sulfur-containing model compounds during CLG,the order of bond breaking is dibenzothiophene>dibenzothiophene sulfone>dibenzyl sulfide,dibenzyl disulfide>dibenzyl sulfoxide.The sulfur in mercaptan and benzyl sulfur(disulfide,sulfoxide,thioether)is easier to thermally migrate than that in thiophene and phenyl sulfur(sulfone,sulfoxide).During steam gasification,the proportion of H2S released was more than 87% except C12H8O2S.During steam CLG,Fe2O3 as oxygen carrier not only promotes the conversion of sulfur to SO2 in the middle temperature region of 500°-600 °,but also enhances the conversion of sulfur to H2S in the high temperature region of 750 °-900 °,leading to a decrease in the proportion of H2S.Therefore,CLG can achieve the selective conversion of sulfur by adjusting the parameters of steam and oxygen carrier to meet the various subsequent functional requirements.Finally,a CLG system model of high-sulfur PC with hematite was established to simulate the products and heat balance by Aspen Plus platform,which goal is the molar fraction of H2S/SO2 in the gasification product corresponds to the stoichiometric number of Claus reaction.The increasing molar ratio of O/C reduces the effective syngas yield(ysyn),but it is easier for the system to realize auto-thermal balance.The increase of gasification temperature is conducive to produce effective syngas,but it leads to the decrease of H2S concentration.When the ratio of O/C is 0.868,the mole ratio of H2S/SO2 is approximately 2,and the system almost maintains heat balance at 900 °.Considering the ratio of effective syngas and the heat balance,the operating ranges of the system are achieved when the molar ratio of O/C is 0.868-0.931 at the temperature range of 850-900 °.The ideal sulfur recovery rate of the whole system is about 95%.It is instructive to realize the high value-added industrial application of chemical looping gasification based on petroleum coke to generate syngas coupled with sulfur recovery in the future. |
PDF Full Text Request