Study On The Integrated Process For Hydrogen Production From Biomass

Currently, catalytic gasification and fast pyrolysis of biomass following by catalytic reforming of bio-oil are considered as two kinds of promising technologies employed in hydrogen production from biomass. To decrease carbon deposition on the catalysts and energy consumption by the root, the integrated process technology is introduced for production of hydrogen, which incorporates fast pyrolysis of biomass, catalytic steam reforming and pressure swing adsorption. Five main parts are included in this research paper, which are tar formation mechanism from biomass pyrolysis/gasification under oxygen and steam, the integrated process for hydrogen-rich gas production from biomass, development of Ni-Co bimetallic catalysts and its catalytic mechanism, catalytic activities and stability of char catalyst, optimization technological parameters of PSA and the mathematical model for multi-component gas adsorption is established.The relationship between tar formation and SOR is very close. At high SOR, tar comes from biomass pyrolysis, and its component is very complex, main containing oxygen-contaning compounds; at relative lower SOR, oxygen-contaning compounds disappeared, and the structure of benzene with substituent groups reduced, and the structure of benzene with less chain increased; while SOR decrease to an extent, poly-aromatic compounds obviously increased. It shows that tar from biomass pyrolysis/gasification under high temperature is more stable, this theory gives support to the integrated process technology for hydrogen production.The hydrogen yield, H2, CH4 and CO composition could reach 88.84%,61.77%,0 and 1.2% under the optimal conditions of the integrated process technology which incorporates fast pyrolysis of biomass and catalytic steam reforming. Catalyst deactivation caused by coke deposit was not found, this result confirms that tar from biomass pyrolysis/gasification under low temperature have lower carbon deposition on the catalysts during catalytic reforming, it also confirms that irreversible condensation reaction during bio-oil condense cause bio-oil re-volatilized difficulty. Therefore, the integrated process technology is a good method to product hydrogen from biomass.Ni-Co/γ-Al2O3, Ni/γ-Al2O3, Co/γ-Al2O3, mechanical mixed Ni/γ-Al2O3 and Co/γ-Al2O3 catalyst (another name is Ni/γ-Al2O3|Co/γ-Al2O3) were prepared by wet impregnation method, and the catalytic performance for hydrogen production by catalytic reforming of volatile from biomass pyrolysis over the four catalysts were compared under the condition of different temperature and weight hourly space velocity. The experimental results show that hydrogen selectivity of 94.83% and hydrogen yield of 120.81g/kg biomass (dry basis) were obtained over Ni-Co/γ-Al2O3 under the condition of catalytic reforming temperature of 825℃and WHSV of 0.71b-1, higher than that over Co/γ-Al2O,Ni/γ-Al2O3|Co/y-Al2O3 and Ni/y-Al2O3 4.76% and 14.22%,6.90% and 24.13%,9.05% and 31.06%, respectively. Further, the effects of support type and weight percentage of Ni and Co metals in the catalysts were investigated, the results indicated that 3Ni9Co/Ce-Zr-O catalyst reached maximum hydrogen yield and lowest coke formation rate and had better stability than commercial Ni-based catalyst. Meanwhile mechanism of methanol and acetone (model compound derived from biomass pyrolysis) catalytic steam reforming over 6Ni-6Co/Ce-Zr-O catalyst were studied. The results indicated that methanol was firstly absorbed by Ce and transformed into methoxyl species. Methoxyl group further dehydrogenation produced formate. Parts of the formate separating from the Ce atom were combined with the absorbed H atoms to form formic acid, the others were transformed into absorbed CO2 after further liberating hydrogen. Finally, absorbed CO2 separated from the catalyst. Mechanism of acetone catalytic steam reforming as follows, first of all, the water molecule is broken up into OH and H under the synergic effect of the carrier and metal, the OH species then absorbed by Ce3+, and the H species are absorbed by active metal. Then the carbonyl group of acetone is absorbed by OH which is linked to Ce atom, while the methyl of acetone is absorbed by Ni atom meanwhile hydrogen is released. After that CO2 releases by breaking of Ce-O bond, then parts of the methyl absorbed from the neighboring Ni atoms polymerize to produce C2H4, the others after water assisted decomposition produce CO2 and H2.Biomass charcoal was used as primary steam reforming catalyst, the results indicated that when catalytic reforming temperature lower than 700℃, the bio-oil contents in outlet dry gas is very high, it isn't suitable for further catalytic steam reforming over metal catalyst. The catalytic activity of char became very prominent at high temperature(≥800℃). The first order kinetic rate constant of char for bio-oil conversion in the temperature range 800-900℃was found to have an apparent activation energy (Ea) of 56.98 kJ/mol and pre-exponential factor (ko) of 1.58x104 s-1. The char catalyst exhibited an excellent bio-oil removal rate (conversion>99.6%) and the bio-oil contents in outlet dry gas lower than 1.89 g/Nm3, thus it is suitable for further catalytic steam reforming over metal catalyst. The apparent carbon deposition rate equation for bio-oil steam reforming over char was obtained as:The pressure swing adsorption process research indicated that hydrogen yield of molecular sieve absorbent is higher than active carbon, but molecular sieve absorbent is more difficult to recovery. Therefore, we proposed that the pressure swing adsorption process was proceeding over molecular sieve absorbent after CO2 absorbed by alcohol-amine. The hydrogen yield increases when the adsorption pressure is less than 0.5Mp, whereas it decreases when the adsorption pressure exceeds 0.5Mp, increases dramatically as the bed height of adsorbent increases from 0.1m to 0.3m, whereas increases tardily as the bed height of adsorbent exceeds 0.4m, and the hydrogen yield decreases as the volume flow increases. The hydrogen yield of 97.86% was obtained under optimal operation conditions of 0.5MPa for adsorption pressure,0.4m for the height of adsorbent bed and 250ml/min for flow rate.