Catalytic Biomass Gasification And Syngas Methanation With Solid Heat Carrier

In present work,raw gas upgrading was decoupled from biomass/char steam gasification and a decoupled dual loop gasifier?DDLG?has been developed to facilitate biomass gasification and in-bed raw upgrading for qualified biogenous syngas production.Methanation of the as produced biogenous syngas was then performed to produce synthetic natural gas?Bio-SNG?.In DDLG,biomass gasification and hot gas upgrading are separated into two parallel loops and so that could be optimized individually.The DDLG reaction system consists of a bubbling fluidized bed gasifier,a moving bed upgrading reactor,a riser,a separator,etc.The study on fluidization behaviors of different units is crucial for optimizing operation and scaling up.The segregation of char in the gasifier was affected by superficial gas velocity,bed material circulating rate and char content.The pressure drop in the riser,as an indicator of the circulating performance,was influenced by superficial gas velocity and bed material circulating rate.The separator was designed basing on the different masses and terminal velocities of the circulating bed material from the two loops and its performance depended on superficial gas velocity and the mass ratio of coarse particle to fine particle.Desulfurization and tar reforming/cracking are two main aspects of hot gas upgrading.As in-bed desulfurizer of DDLG,an iron-bearing olivine supported ZnO?ZnO/olivine?was prepared and tested firstly in a fixed bed reactor.H₂S sorption over ZnO,adversely affected by H₂O,was accompanied by evident reduction of ZnO and vaporization of Zn at 550 ?,in comparison,no obvious ZnO reduction was observed at the same condition over ZnO/olivine.In addition,ZnO/olivine showed pronounced desulfurization ability at 550 ? during 3 circles of sulfidation-regeneration.In DDLG test with pine saw dust as feedstock and ZnO/olivine+Ni/olivine as upgrading bed materials,a synergy was found between desulfurization and tar destruction.The H₂0 evolved reactions such as steam gasification of char and steam reforming of tar/hydrocarbons were intensified at elivated gasification temperature and in presence of Ni/olivine-850.As a result,the decrease of H₂O favored H₂S sorption by ZnO/olivine,which in turn alleviated sulfur-poisoning of Ni/olivine-850.Biomass gasification and in-bed raw gas upgrading have been performed in DDLG,with aggregation-resistant nickel supported on calcined olivine?Ni/olivine-850?as the upgrading catalyst for simultaneous desulfurization and tar elimination of biogenous syngas.It was found that 850 ? calcined Ni/olivine was efficient for both inorganic-sulfur?H₂S?and organic-sulfur?thiophene?removal at 600-680 ? and the excellent desulfurization performance was maintained with wide range H₂O content?27.0-40.7%?.Meanwhile,tar was mostly eliminated and H₂ content increased much in the same temperature range.Calcined olivine supported nickel catalysts?Ni/olivine-350?were prepared for CO₂ methanation and its structure-activity relationship was evaluated.The FeOx shells formed on the surface of the calcined olivine were extracted from the olivine during its calcination,and acted as the interface between the calcined olivine and supported NiO.The Ni₃Fe as the most active component was formed on the FeO,shells from the reduced NiO and the partially reduced FeOx during the reduction of the Ni/olivine.The unreduced FeOx between the active phase Ni₃Fe and the olivine body,as the very support of the Ni₃Fe,plays an important role in CO2 methanation.The Ni/olivine with strong resistance to coke deposition and abrasion could be a promising methanation catalyst,especially for fluidized and moving bed operation.Methanation of hot raw syngas containing tar and H₂O over countercurrent moving bed was developed.Over the identical catalytic bed material of the moving bed,reforming of the trace amount of tar and syngas methanation occur.The bed material is heated by the exothermic methanation and delivered downward.As a result,an axial temperature gradient along the moving bed is formed,i.e.the upstage at relatively low temperature and the downstage at moderate temperature.The introduced tar is mainly reformed over the hot bed material in the inlet zone of the moving bed,meanwhile,the low-tar or even tar-free syngas and the reformer gas?H₂,CO,etc.?flow upward for further methanation.During raw biogenous syngas upgrading over Ni/olivine-350 in the countercurrent moving bed upgrading reactor,both CH4-rich gas production and tar elimination with good resistance towards carbon deposition were achieved.With the introduction of CO2 sorbent into the bed materials of the upgrading reactor further,CO/CO2 was deeply removed and CH₄/H₂ mixture was achieved.