Catalytic Gasification Of Lignocellulosic Wastes With Ni-Co Bimetallic Catalysts In Subcritical Water

Ni-Co bimetallic catalysts were synthesized for the gasification of agricultural lignocellulosic wastes(e.g.rice straw,cotton straw and peanut shell)to gas fuel in subcritical water.In the hydrothermal gasification,Ni/Al2O3 was widely selected as the catalyst due to its low cost and high catalytic activity.To further improve the gasification efficiency of the catalyst,Co was added into Ni/Al2O3 to form Ni-Co alloy.Firstly,cellulose was selected as the model compound of agricultural lignocellulosic wastes for the preliminary gasification study.The order of Ni-based catalysts activity with different supports was Al2O3,SBC(spent bleaching clay)ash and SiO2,with corresponding H2 yields of which were 80.6%,69.0%and 57.0%,respectively.The Ni-based catalyst supported on Al2O3 showed the highest catalytic activity in the hydrothermal gasification of cellulose at 350℃,and the H2 yield and gasification efficiency were 80.6%and 81.9%,respectively.The catalytic performance of 10Ni/Al2O3 catalysts were greatly enhanced by the addition of Co.The H2 yield reached the maximum value of 88.4%when 6 wt.%Co was added,which is 1.44 times of that with 10Ni/Al2O3 catalyst.The influence factors(e.g.weight of raw materials and the residence time)were further investigated using l0Ni6Co/Al2O3 catalyst.The best gasification efficiency was achieved under the condition of 0.5 g cellulose and 20 min residence time,with H2 yield of 94.9%.After the 1st used,10Ni6Co/Al2O3 catalyst was recycled with H2 yield of 62.6%higher than that(61.2%)for the fresh 10Ni/Al2O3 without adding Co.Secondly,catalysts were characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),hydrogen temperature programmed reduction(H2-TPR),ammonia temperature programmed desorption(NH3-TPD),N2 adsorption desorption and CHNS elemental analyzer.It was found that the gasification performances of Ni-based catalysts were positively related to the acid value and specific surface area of the catalyst.20Ni/Al2O3 showed the highest acid value and specific surface area of 468.6μmol/g and 175.39 m2/g,respectively.Ni-Co alloy structure was formed by the modification of 10Ni/Al2O3 catalyst with Co promoting the gasification performance of the catalyst and reducing the deactivation by carbon deposition on the catalyst surface.Finally,cotton straw,rice straw and peanut shell were gasified using 10Ni-6Co/Al2O3 catalyst.The highest H2 yield of 82.6%was achieved with cotton straw.Compared with the gasification without catalyst,H2 yield for cotton straw,rice straw and peanut shell was increased by 67.8,51.4 and 76.0 times,respectively.Moreover,10Ni-6Co/SBC and 20Ni/SBC catalysts were synthesized using SBC ash as the support for the gasification of different model compounds.The gasification of glycerol,cellulose and pentamethylolfurfural with 10Ni-6Co/SBC catalyst reached H2 yields of 106.6%,49.9%and 48.2%,respectively.Using 20Ni/SBC,cellulose,fructose,arabinose and pentamethylolfurfural reached H2 yields of 77.5%,44.2%,57.0%and 47.0%,respectively.The results showed that using SBC ash as the support for catalysts is feasible for the hydrothermal gasification to produce hydrogen.It can be concluded that the synthesized Ni-Co bimetallic catalyst was effective for the hydrothermal gasification of agricultural wastes,and catalytic hydrothermal gasification technology is feasible for the energy utilization of lignocellulosic wastes.