Research On Chemical Looping Gasification Of Biomass With Calcium Ferrite As Oxygen Carriers

The chemical looping gasification of calcium ferrite with biomass is an efficient way of the use of renewable energy,while also producing syngas rich in CO,which has high energy utilization efficiency.At present,the reaction mechanism and characteristics between calcium ferrite and biomass are not clear.Studies on the reduction kinetics of calcium ferrite mainly focus on gas-solid reaction,which is limited to coal,graphite and other fuels with a high degree of graphitization.The reaction mechanism between calcium ferrite and biochar is still unknown.Therefore,the kinetic of the chemical looping gasification of biochars with calcium ferrite as oxygen carriers were studied,and the effects of different kinds of calcium ferrite and biochars were investigated by TGA.Meanwhile fixed-bed chemical looping gasification and oxygen carrier oxidation were studied when poplar sawdust and corn straw were used as fuel,CaFe2O4 and Ca2Fe2O5 as oxygen carriers.The reaction characteristics of biomass and calcium ferrite in the process of chemical looping gasification were investigated.The effects of the experiment were analyzed like the phase composition of oxygen carrier during the reaction and the physicochemical structure of biochar by XRD,SEM,BET and etc.The ?kvára-?esták method was used to determine the kinetic mechanism function.The characteristics of the chemical looping gasification reaction between calcium ferrite oxygen carrier and biomass were revealed.It provides theoretical reference for practical application of chemical looping gasification of biomass and calcium ferrite.The results show that:The reaction rate of oxygen carriers followed the sequence Ca2Fe2O5>CaFe2O4>Fe2O3,and the oxygen transport capacity decreased in the order of CaFe2O4>Ca2Fe2O5>Fe2O3.The oxygen carriers could be completely reduced to Fe and CaO by biochar.There were more reactive sites on the surface of corn straw and corn straw char during the reaction compared with poplar sawdust and poplar straw char,so the reaction rate of corn straw was faster.The activation energy of CaFe2O4 was within the range of 167.44-600.83 kJ/mol;The activation energy of Ca2Fe2O5 was within the range of 413.62-583.51 kJ/mol.The average activation energy of CaFe2O4 was more than that of Ca2Fe2O5,CaFe3O5 was generated during the reduction of CaFe2O4 and may have a negative influence on the lattice oxygen diffusion.The reduction of CaFe2O4 could be divided into two stages:When the conversion degree a was less than 0.15,the CaFe2O4 was reduced to Ca2Fe2O5.When reducted by PSc,it followed the random nucleation and nuclei growth model(n=3):the mechanism function was G16(α)=[-ln(1-α)]3.When reducted by CSc,it followed the random nucleation and nuclei growth model(n=2):the mechanism function was G15(α)=[-ln(1-α)]2.When a was more than 0.15,the Ca2Fe2O5 was further reduced to CaO and Fe,which followed the 3-D diffusion mechanism.When reducted by PSc,the mechanism function was G7(α)=1-2/3α-(1-α)2/3.When reducted by CSc,the mechanism function was G8(α)=[(1+α)1/3-1]2.The mechanism function of the reduction of Ca2Fe2O5 followed the same as the second stage of the reduction of CaFe2O4.With the increase of oxygen carrier proportion in biomass gasification,the syngas yield increased,and the cold gas efficiency of syngas increased.When the mass ratio of oxygen carrier to biomass was 1:1,the syngas yield was 34.70 mmol/g and reached the maximum,which increased to 1.5 times of that when without oxygen carrier.The high oxygen carrier ratio and biomass ratio may lead low utilization efficiency,the utilization efficiency was highest when the mass ratio of biomass to oxygen carrier is 1:1.With the increase of reaction temperature,the gas yield of the biomass gasification and the calcium ferrite oxidation increased.When the temperature rised from 800 ℃ to 900 ℃,the syngas gas yield increases to about 1.34 times.When it rised from 900℃ to 1000℃,it increased to about 1.07 times.The utilization efficiency is high when the reaction temperature is 900℃.The chemical looping gasification of biomass and oxygen carrier can be divided into two stages:0-10 min:the yields of H2,CH4 and C2+reached 100%,99.59%and 97.77%of its final yields respectively,it was almost the final yields.The main reaction of this stage was biomass pyrolysis.The second stage was 10 min-end:the gas yield of CO and CO2 at this stage was 19.79%and 21.60%of its final yield respectively.The main reaction of this stage was the gasification of biochar with oxygen carrier.Compared with the gasification of biomass,the oxidation of oxygen carrier was more smooth and slow.CaFe2O4 can release more lattice oxygen during the gasification,which the total syngas yield was higher,but the reaction rate of Ca2Fe2O5 was faster.There are more volatile in poplar sawdust and it could generate more syngas,while the reaction rate of corn straw was faster and could finish the reaction in a shorter time.When oxidized by CO2,the oxidation of CaFe2O4 was CaO+Fe→Ca2Fe2O5+CaFe3O5,which the lattice oxygen released in CaFe2O4 could not fully recovered.While Ca2Fe2O5 can be completely oxidized and regenerated by CO2.Comparing CaFe2O4 and Ca2Fe2O5 as oxygen carriers,CaFe2O4 can release more lattice oxygen,the reaction rate of Ca2Fe2O5 is faster.CO2 can completely oxidize and regenerate Ca2Fe2O5 oxygen carrier,its cycle stability is better than Ca2Fe2O5.