Decoupling Gasification Of Biomass And Coal For Hydrogen-rich Gas Production With Catalyst

In order to efficiently remove tar in gasification process of biomass and coal,and also selectively adjust composition of product gas from the process,a novel decoupled dual loop gasification(DDLG)system has been proposed based on the concept of decoupling-optimizing-reorganizing.The system decouples gasification process into fuel pyrolysis/gasification,tar cracking/reforming and residual char combustion,which occur in three isolated reactors,i.e.a gasifier,a reformer and a combustor.Both the gasifier and the reformer are separately interconnected with the combustor,forming two bed material circulation loops in parallel.In this way,both the gasifier and the reformer could be optimized individually under appropriate reaction conditions.With pine sawdust and Shenmu bituminous coal as feedstocks,and calcined olivine both as circulating solid heat carrier and in-situ tar destruction catalyst,steam gasification of the biomass and the coal for hydrogen-rich gas production with low tar content has been investigated.For steam gasification of the pine sawdust,separation of the reformer from the gasifier increased the residence time of the volatiles and the contact between the volatiles and the catalyst.As a result,tar destruction was markedly improved by increasing the reformer temperature and in the presence of in-situ tar destruction catalyst.Specifically,at the gasifier temperature 800 ?,reformer temperature 850 ? and steam to carbon mass ratio(S/C)1.2,the tar content in product gas decreased to 14.1 g/Nm3,and the dry gas yield and H2 concentration reach 1.02 Nm3/kg daf and 40.8 vol.%,respectively.The steam gasification performance of the Shenmu bituminous coal could be further improved by raising the gasifier temperature,S/C and equivalence ratio(oxygen to fuel weight ratio used in the test/oxygen to fuel weight ratio for stoichiometric combustion,ER)to enhance char gasification on the basis of the optimization of the tar destruction.Specifically,at the gasifier temperature 800 0C,reformer temperature 850 ?,S/C 1.2 and ER 0.2,the dry gas yield and carbon conversion increased to 0.94 Nm3/kg daf and 38.1%,respectively.The gasification performance and product gas composition of co-gasification of the pine sawdust and the Shenmu bituminous coal changed linearly with the pine sawdust blending ratio.There was no synergy effect observed during the co-gasification process of the pine sawdust and the Shenmu bituminous coal in the DDLG.NiO/olivine and NiO-CaO/olivine catalysts were prepared to strengthen tar cracking/reforming and adjust product gas composition in steam gasification of the pine sawdust and the Shenmu bituminous coal.The structure-activity relationship and deactivation mechanism of the catalysts were also discussed.It has been indicated that the strong interaction between NiO and Fe2O3 and MgO on the surface of the calcined olivine benefited to disperse and anchoring Ni grains.Moreover,MgO and partially reduced Fe2O3(denoted as FeOx)on the surface of the calcined olivine facilitated chemisorption and dissociation of H2O and CO2.As a result,the 5NiO/olivine catalyst showed an excellent performance in terms of activity,stability,and coke-resistance ability,which was comparable to that of commercial catalyst Z418.However,the Ni grains on the surface of the calcined olivine also suffered from slight sintering after three reaction-regeneration cycles,which resulted in decrease of activity of the NiO/olivine.Additionally,5NiO/olivine catalyst acted also as an oxygen carrier to consume part of the product gas,which restrained the conversion of H2O and CO2 in steam gasification of the pine sawdust and the Shenmu bituminous coal.There was a synergy effect observed between Ni and CaO in the NiO-CaO/olivine for dry reforming of toluene.The match between Ni and CaO could be easily balanced through adjusting the ratio of Ni/(Ni+Ca).The performance of 3NiO-1CaO/olivine catalyst was superior to that of the 5NiO/olivine,which resulted in evident increase of H2O conversion and decrease of CO2 concentration in steam gasification of the pine sawdust.Specifically,at the gasifier temperature 800 ?,reformer temperature 850 ? and S/C 1.2,the tar content and concentrations of CH4 and CO2 reduced to 1.5 g/Nm3,4.5 vol.%and 21.2 vol.%,respectively,and the dry gas yield,water conversion and H2 concentration increased to 1.46 Nm3/kg daf,26.9%and 51.0 vol.%.Simulation model of steam gasification of the pine sawdust and the Shenmu bituminous coal in the DDLG was established using Aspen Plus software.The distribution of circulating solid heat carrier of both two circulation loops and energy balance of steam gasification of the pine sawdust and the Shenmu bituminous coal in the DDLG have been investigated.In order to maintain the gasifier at 800 ?,reformer 850 ? and combustor 920 ?,the energy balance of the DDLG can be achieved when co-gasification of the pine sawdust and the Shenmu bituminous coal at the pine sawdust blending ratio(weight of pine sawdust/total weight of pine sawdust and Shenmu bituminous coal,BR)of 70.8%(Shenmu bituminous coal blending ratio of 29.2%).