Design And Experiments Of A Compact-fast Dual Fluidized Bed As Calcium Looping Reactors

Chemical looping technology is a near-and mid-term promising high-efficiency combustion and carbon dioxide capture technology.Calcium looping is one technical route of chemical looping,which can realize hydrogen-rich gasification,and have high efficiency,and produce flexibly regulated H₂/CO ratio syngas with in-situ carbon dioxide capture.Carbon dioxide sorbent material preparation and reactor design are two core research fileds for calcium looping development.Long-term stable operation and well-controlled characteristics of reactor are significant.Compact fluidized bed used as the gasifier or carbonator for calcium-looping is studied in this work,while fast fluidized bed riser is used as the regenerator.Design and experiments of the cold flow models and the lab-scale hot rig are conducted to help design and verify its feasibility,further to develop pilot-scale compact-fast dual fluidized bed system.Four generation cold flow models of compact-fast dual fluidized bed with different designs were built.The systems all have a compact fluidized bed and a riser,connected with standpipes,cyclones,loop seals,etc.The key parameters of cold flow model are designed according to Glicksman rule.According to experimental results from previous cold flow model experiments,the arrangement of a standpipe directly-inserted into the lower bubble fluidized bed is replaced with a straight pipe and a loop seal,and the variable-diameter standpipe connected to regenerator is changed to a straight standpipe.A long-term stable operation is achieved on the first generation cold flow model.Fluidization velocities in bubbling fluidized bed and regenerator are 4～10 u_mf and 3～7 u_t,respectively.Aeration in loop seals have significant effects on solid recirculation flux while fluidizations in bubbling fluidized bed and regenerator have no effects.The relationships between aerations and solid recirculation flux are built.The fourth-generation cold flow model is designed and built based on the previous cold flow model by scaling-up and the connection optimization between lower loop seal and regenerator with 3D arrangement design.A long-term stable operation with binary particles is achieved.Quartz sand and Chinese rice are used as model particles to simulated sorbent and char in sorption enhanced gasification process to keep similarity in particle size and density.Solids are sampled at upper standpipes,sieved and weighed to calculate mixing factor,and effects of fluidization number in reactor,solid recirculation flux and char concentration on this mixing factor are studied.The separation phenomenon on different particles in compact fliuidized bed is found.Fluidization number in the bubbling fluidized bed has primary influence on particle separation and mixing.A mixing factor model is built and several operating conditions were studied.A 10kW_th compact-fast dual fluidized bed hot rig is designed based on cold flow models and Glicksman scaling-up rule.Thermal expansions of working fluid and reactor pipes are considered in the hot rig system,and pre-heater,insulation and expansion joint are used.Dolomite is used as bed material and the system is tested by simulating cacium-looping for carbon dioxide capture process.The temperature in the bubbling fluidized bed and its upper riser,and in the regenerator are 700℃,600℃and 900℃,respectively.Carbon dioxide is effectively captured during the test,and a carbon dioxide capture efficiency model is developed for the compact fluidized bed.81%normonalized carbon capture efficiency has beed achieved on this hot rig.Compact fluidized bed carbonator has larger active space time comparing to independent bubbling fluidized bed or riser carbonator.The capture efficiency of the compact fluidized bed is relatively lower because of shorter solid residence time,larger active spacing velocity,and none optmal operation conditions.Test results indicate that upper riser has a positive effect on sorption enhanced process but most of carbon dioxide is captured in bubbling fluidized bed.Feasibility of long-term operation of the compact dual fluidized bed hot rig is confirmed.For biomass gasification,the feasibility of long-term stable operation of the compact-fast dual fluidized bed and its performance are studied.Dolomite is used as bed material.Temperature in the bubbling fluidized bed and its upper riser,and in the regenerator are 700℃,600℃and 890℃,respectively.Gasifier inlet:fuel 2 kg/h wood pellet,oxidation agent 1.6 kg/h steam mixed with 0.5Nm³/h nitrogen,and regenerator inlet:2 Nm³/h air.1.21Nm³/h,0.29Nm³/h and 0.1Nm³/h can be achieved during the biomass gasification.1kg of biomass can produce 0.6 Nm³ hydrogen,0.15 Nm³carbon monoxide,and 0.05 Nm³ methane.The volume concentrations of hydrogen,carbon monoxide,methane in product gas are 71.74%,17.39%,and 5.43%,respectively.Syngas heating value and cold gasification efficiency are 10.98MJ/Nm³ and 58%,respectively.Gasification efficiency is lower than that of previous simulation reesults and the advantage of the compact fluidized bed is not demonstrated exactly because of the shorter gas-solid residence time and relatively lower solid recirculation flux,and none optimal operation conditions.However,higher hydrogen concentration of syngas and long-term stable operation prove the feasibility of the compact-fast dual fluidized bed.Feasibility of compact dual fluidized bed as calcium looping reactors is proved according to above cold flow model and hot rig system tests.Based on the test results,minimum reactor volume design method is proposed for calcium looping reactor design.Balckbox model and time discrete analysis method are applied to calculate minimum reactor volume according to local fuel processing or gas production requirement as the boundary conditions.Minimum reactor volume can help to achieve low cost,high efficiency and high fuel conversion.Parameters of main reactors in pilot-scale compact-fast dual fluidized beds is proposed based on this method.