Characteristic And Mechanism Study Of Enhanced Hydrogen Production By Ca-Based Additive From Biomass Pyrolysis And Gasification

Hydrogen is one kind of clean energy with high energy density, effective and low cost hydrogen production technology attracts more and more attention. Due to the advantages of high yield and renewable, biomass thermal conversion method for hydrogen production has become the research hot spot. Calcium based additives can effectively promote the conversion of biomass to hydrogen, but in this process, the regularity and mechanism of different calcium based precursor on biomass thermal conversion, the influence of calcium based additives on biomass pyrolysis kinetics, reactor structure design, research and development of new type additives, decomposition of tar and other issues still need be further studied. In this thesis the above problems were mainly discussed, it will beof great significance in promotion of biomass and hydrogenutilization.By combination of experimental study and theoretical analysis method, biomass pyrolysis and gasification characters of the pine were studied by using TGA and Fixedbed system. The influence of different calcium based precursor on biomass pyrolysis was examined, and the key dynamics parameters of biomass pyrolysis reaction were obtained. By using thermodynamic analysis, the effect of calcium based additives on hydrogen production with in-situ directional biomass gasification was studied and its influence mechanism; Two stages biomass gasification reactor with dual temperature zone was designed, and the influence of the setup type on biomass high temperature gasification for hydrogen productionwas examined; In order to strengthen calcium based additives for hydrogen production and tar elimination, a compound NiO/dolomite additivewas developed, its reactivity on biomass gasification reaction and the influence on tar conversion wasdiscussed. The enhance mechanism of calcium based additives on hydrogen production from biomass wasstudied. The following main conclusions wereobtained:First of all, based on researches of reaction temperature, drying time, CaO added amount, different calcium based precursor on directional pyrolysis of biomass, itis found that: adding CaO can effectively improve the H₂ concentration and yield. CaO fixed moisturefrom the low-temperature pyrolysis, and in middle temperature range it was released with volatile matters a nd contact with volatile fully which strengthened the water gas shift reaction;In addition, CaO captured CO₂ from pyrolysis process, broke the reaction equilibrium in the system, also leaded to the enhanced water gas shift reaction. However, cause of too much water content in biomass, H₂ production of per unit biomasswas low.Adding calcium based additives during biomass pyrolysis showed little effect on decomposition intensity and characteristic temperature of biomass.Dolomite showed the highest pyrolysis reactivity, in the high temperature range pyrolysis activation energy was 110 kJ/mol, less than other additives, with dolomite the pyrolysis conversion rate was as high as 90%. Under high temperature, the Boudouard reaction promoted consumption of the biomass coke, reduced the CO₂ partial pressure.Secondly, combined with the thermodynamic analysis and experimental research the biomass in-situ directional steam gasification for produce hydrogen was studied, results showed that: H₂ yield from biomass gasification increased by 79% dueto addition of CaO when the reaction temperature was 327 ?.Under P = 1atm and temperature of 757 ? the maximum energy efficiency was more than 90%, exergy efficiency was more than 80%, further increase the pressure and temperature efficiency was reduced. By using natural dolomite and limestone prepared a one-body additive, with mass ratio of 1:1 the additive showed a large specific surface area, moderate pore volume and diameter. Hydrogen concentration from biomass gasification increased by 8% compared withthat use dolomite and limestonealone.Next, two stages reactor with dual temperature zone was designed, and its influence on biomass gasification was examined, results shown that: the reactor was divided into two periodsas biomass gasification bed and absorption/reforming bed.Gasification bed temperature was 900 ?, absorption/reforming bed temperature was kept at 730?by using waste heat from gasification bed, but without additional heat source alone. When dolomite was added in both gasification and absorption/reforming beds, H₂ concentration and yield in the syngas reached 70.8 vol.% and 471 ml/g, CO₂ concentration lowered to 18.8 vol. %, indicates that using the dolomite this arrangement can effectively increase producingof hydrogen from the biomass steam gasification.Finally, novel NiO/dolomite additive was prepared, and studied its effect in directional steam gasification of biomass and its basic mechanism. It is found that: by using two stages reactor, with impregnate of 2wt.% NiO on dolomite in the absorption/reforming bed, the H₂ yieldwas improvedto 509.2 ml/g. With increasing of the NiO content, the amount of carbon deposition rate increased and average pore diameter reduced, resulted in decrease of hydrogen production. From XRD analysis, it is found that the NiO-MgO phase was formed during calcination, and it was reduced to Niduring gasification which improved its catalysis. The new type additive showed little positive effecton methane steam reforming, butexhibited strongpromotion on the water gas shift reaction, increased the activity of dolomite in low temperatures. With NiO, benzene steam reforming reaction waspromoted, at 900 ? carbon conversion rate reached 28%, the H₂ concentration was more than 50 vol.%.High reforming temperature leaded tolow carbon deposition, when temperature increased from 800 ? to 900?,carbon deposition lowered 63%, however, carbon deposition was increased with increase of space velocity and steam amount.Based on calcium based additives biomass thermal conversion for hydrogen production was studied, we obtained reaction kinetics of the key parameters, revealed the effect mechanism of different precursor effecton pyrolysis and gasification, designed a new type reactor, developed the NiO/dolomite additive to effectively improve the efficiency of hydrogen production from biomass, illuminated the strengthen the mechanism of calcium based additives on hydrogen production. The results can provide theoretical support to the development ofbiomass thermal conversion for hydrogen production technology.