Deep Reduction Chemical Looping Hydrogen Generation With Pyrolysis Gas From Biomass Waste As Fuel:Technical And Mechanism Investigation

The annual production of biomass waste in China is about 5 billion tones,and one fifth is difficult to be biodegraded which is mainly disposed by thermal-chemical methods.Pyrolysis coupling with chemical looping hydrogen generation?CLHG?is an environmental-friendly process which can convert the biomass waste to hydrogen.The solid conversion of the reduction step is the key to its efficiency and cost.Here,when the main components in the pyrolysis gas of biomass waste is used as fuel gas,the behaviour of the reduction step in the packed bed CLHG is investigated to confirm the reason for the limitation of the solid conversion of the reduction step.According to this,the way to improve the solid conversion of is proposed and used to dispose the real pyrolysis gas from biomass waste.To achieve the production of high-purity hydrogen with CO₂ intrinsic separation,the fuel breakthrough in the reduction step should be forbidden.In this case,the solid conversion of the reduction step in the packed bed CLHG is limited to the range of 21.1%to 27.3%.The hydrogen production ability of the reduced oxygen carrier is only 11.2%to 18.2%of the theoretical maximum value.From the kinetic results of the reduction of the iron-based oxygen carrier,when the solid conversion is 27.3%,i.e.,the ceiling of the solid conversion of the reduction step,the rate of the reduction reaction is about 50%of the rate when the solid conversion is 11.1%?Fe₃O₄?.From thermodynamics,when the temperature is 850 ^oC and CO is used as fuel,the maximum solid conversion of the reduction step in the fluidized bed and the counter moving bed are respectively 11.1%and 31.2%.This shows that the solid conversion of the reduction step in the reactor is limited by the thermodynamics.However,the thermodynamic maximum solid conversion of the reduction step should be determined by both the thermodynamics and the mechanism of the reduction.There are three platforms in the breakthrough curve of the reduction step in the packed bed.This indicates that three separated reaction fronts,i.e.,Fe₂O₃-Fe₃O₄,Fe₃O₄-FeO,FeO-Fe,move at the same time during the reduction of the iron-based oxygen carrier.The movement of multiple reaction fronts is observed for the first time during the investigation of the temporal and spatial variation of the axial solid conversion in the packed bed.Basing on this,the reaction front movement model is raised and the model is also the operation curve in the packed bed.A new basic parameter of the reaction engineering,which is the merging temperature?T_m?,is found for the first time.When the reaction temperature is above T_m,Fe₂O₃-Fe₃O₄ front and Fe₃O₄-FeO front merge,leading to the disappearance of Fe₃O₄ zone.On the basis of the mechanism of the reduction step,the maximum solid conversion of the reduction step in the packed bed are respectively 40.9%and 55.4%when CO and H₂ is used as fuel.From the axial phase distribution in the packed bed,the deep reduction CLHG is proposed,in which multiple packed beds operating in series are considered for the reduction step.In this case,the deep reduction of iron-based oxygen carrier is realized with the full conversion of fuel,which overcome the limitation of solid conversion in the single packed bed.The experimental results of this technique indicate that the relationship between the hydrogen production and the solid conversion of the reduction step is positively linear,and the efficiency of hydrogen production is 92.8%to 97.5%.In the deep reduction CLHG with dual packed beds operating in series,the hydrogen production per cycle increases 49.3%.In a 30 kW_th pilot unit,the real pyrolysis gas from biomass waste is used as the fuel of the deep reduction CLHG.The maximum concentration of produced H₂ is 99.6%during the steam oxidization step.The production of dioxin during this process is only 0.7%of that in the air combustion.These results demonstrate that the deep reduction CLHG is feasible,environmental-friendly and high-efficiency.