Synergistic catalytic pyrolysis of biomass using molten salts and nickel for hydrogen-rich syngas

Abstract: The effects of Na2CO3-NaOH molten salts and nickel on the pyrolysis of 3 components of biomass, i.e. cellulose, hemicellulose and lignin, were studied in the thermogravimetric (TG) analyzer and fixed bed reactor. In the TG experiment, the addition of molten salt makes the peak of cellulose TG curve appear ahead of the TG curve of cellulose without catalyst. The corresponding differential thermogravimetric (DTG) curve has 2 separation peaks, while there is only one peak appearing in the DTG curve of cellulose without catalyst. In DTG curve of hemicellulose, compared with the hemicellulose pyrolysis alone, the temperature corresponding to the maximum pyrolysis rate of the peak is decreased by 82 ℃ after adding molten salt. As for lignin, the DTG and TG curve with catalyst basically coincide with that without catalyst, indicating that the low dose addition of molten salt and nickel had little effect on the pyrolysis of lignin. The results of fixed-bed experiment show that the molten salt can effectively absorb CO2, which promote the increase of H2 and the reduction of CO simultaneously in water-gas shift reaction. In the process of catalytic cellulose pyrolysis, the OH- contained in molten salt motivates the rearrangement of the ring-opening products of cellulose monomer at relatively low temperature, and the rearrangement products are easily decomposed, thus leading to an acceleration of cellulose decomposition. The Na+ contained in molten salt can react with phenolic hydroxyl groups in lignin structural units and decrease activation energy of ring-opening reaction of hemicellulose monomer, resulting in an easy decomposition of both lignin and hemicellulose. Under the optimum conditions, the amount of hydrogen produced by cellulose, hemicellulose and lignin can reach 913, 738 and 1 148 mL/g, respectively, and the content of H2 can reach 85.6%, 78.2% and 90.7%, respectively. The CH4 content can be reduced by the synergistic effect of molten salt and nickel, under which the CH4 yields of cellulose, hemicellulose and lignin are decreased by 35.0%, 24.5% and 12.0%, respectively compared with the single addition of molten salt. The synergistic effect of molten salt and nickel can effectively reduce the content of CO, CO2 and CH4 in syngas, which is beneficial to the pyrolysis of biomass to produce hydrogen-rich gas. Under the optimum conditions, the contents of CO, CO2 and CH4 in the lignin are only 0.5%, 0.4% and 5.9%, respectively. Meanwhile, in the presence of molten salt and nickel, the maximum hydrogen yields of cellulose, hemicellulose and lignin reach 910, 714 and 1 106 mL/g, respectively, and the contents of H2 are 77.6%, 77.8% and 91.6%, respectively. From 700 to 900 ℃, with the increase of temperature, the reaction of residual carbon and H2O, CO is promoted, so high temperature is conducive to the conversion of residual carbon. At the same time, CH4 production tends to decrease with the increase of temperature, because CH4 is easily decomposed at high temperatures. It can be seen that high temperature is conducive to the production of H2 and CO, and promotes the cracking of CH4, which is in favor of biomass pyrolysis to produce hydrogen-rich gases.