Analysis Of The Enhanced Lignite Gasification Process By Chemical Looping Gasification Technology With Transition Metal/Phosphogypsum Oxygen Carrier

Due to the fast industrial development in recent years,the consumption of fossil fuels such as coal is increasing.The resulting CO₂ emissions cause serious damage to the human society and ecological environment.Carbon Capture and Storage?CCS?is a technology for CO₂ capture.The technology mainly includes pre-combustion capture,post-combustion captureand oxyfuel combustion.Chemical Looping Gasification?CLG?is a kind of capture method before combustion.In order to reduce carbon dioxide emissions,lignite gasification can achieve“clean combustion”,but lignite direct gasification has the problem of hypoxia,resulting in low lignite conversion rate.Because of its high moisture and volatile content,lignite can be used in CLG.Oxygen carrier in CLG can solve the problem of hypoxia in lignite gasification process.Calcium sulfate in oxygen carriers has attracted much attention due to its strong oxygen carrying capacity.Among CaSO₄-based oxygen carriers,phosphogypsum?PG?is a secondary product of phosphoric acid preparation,and its main component is calcium sulfate dihydrate?CaSO₄·2H₂O?.Due to the demand of a large amount of phosphoric acid,a large amount of PG is produced.Thus,the resource utilization of PG is crucial to the phosphoric acid industry.According to the main components of PG,it is a recommended oxygen carrier.This method is the new way to recycle PG.Therefore,based on the sustainable economic development,it is worth to analyze the feasibility of using PG as oxygen carrier and lignite as fuel to prepare syngas by CLG process.According to the composition of phosphogypsum,its main active ingredient is CaSO₄.Therefore,phosphogypsum combined with CLG can promote lignite gasification,realize“clean combustion”of lignite in this process,and realize the recycling of phosphogypsum resources,which is more conducive to sustainable development.In this study,through theoretical analysis and experimental verification,the process of PG with CLG to promote lignite gasification,and discovered a novel method for the preparation of syngas from PG lignite CLG.The transition metal was screened by Ellingham figure to develop the new compound calcium based oxygen carrier to reduce the reaction temperature of PG directly used in lignite CLG.In addition,it is found that Cu/PG compound calcium based oxygen carrier releases molecular oxygen at high temperature and can provide more active sites to reduce the reaction temperature.Further study on the fluidized CLG process provides further basic data for industrialization.Based upon the results,following conclusions are drawn.?1?In the process of PG-lignite CLG,following conclusions have been obtained:The molar ratio of phosphogypsum/lignite should be about 0.2,the reaction temperature should be above 1123K,and the recommended size range of reactants should be 40-140 mesh.At temperatures above 1273K and water vapor content above20%,part of CaSO₄ converted into CaO,which is not conducive to PG recycling.The sintering phenomenon occurs when PG oxygen carrier is used for three times.Reaction temperature,ratio of PG/lignite,and water content have a large effect on the syngas generation.H₂ and CH₄ generation strongly depend on water vapor.The influence degree of these factors on CO yield is the PG/lignite's molar ratio>reaction temperature>the water vapor content.Among the three factors,the effect of reaction temperature on CO₂ generation was less than that of the others factors,according to ANOVA analysis.Under water vapor atmosphere,the mechanism of syngas generation is main via gas-solid reaction and solid-solid reaction.?2?In the process of PG compound oxygen carrier CLG,following conclusions have been obtained:In order to“save energy”,the compound oxygen carrier cerium,iron,nickel and copper with PG is recommended by Ellingham diagram to prepare compound oxygen carriers?Ce/PG,Fe/PG,Ni/PG and Cu/PG?.After CLG process,It was found that except cerium-based compound oxygen carriers,other compound oxygen carriers can promote lignite decomposition and reduce the temperature of CLG reaction process.Among the compound oxygen carriers,Cu/PG,which can also be used to capture S of the syngas and nontoxic of its products.Thus,it is an environmentally friendly oxygen carrier.Additionally,for higher selective conversion of carbon to carbon monoxide,it is suggested that the molar ratio of Cu to PG is about2.At 1023 K,the conversion rate of lignite can reach more than 90%and the selective conversion of carbon to CO is about 70%.?3?The mechanism of lignite CLG with Cu/PG have been analyzed and following conclusions have been obtained:The mechanism of syngas generation is main via gas-solid reaction?steam reacts with carbon?and solid-solid reaction?oxygen carriers react with carbon?.After deeper analyses,it concluded that the GB-SCM is the most suitable reaction model in this process,which belongs to the reaction rate control of diffusion of products away from reaction site.The regeneration process of the oxygen carrier belongs to gas-solid reaction,which belongs to CaS?Cu reacts with oxygen.?4?The experimental process of fluidized Cu/PG lignite CLG was analyzed and following conclusions have been obtained:It has been found that the single reaction time of 5min has been recommended for more a large amount of C selective conversion to CO.At the same time,the molar of H₂/CO with around 1 can be achieved.