In recent years, increasing attention has been focused on the liquefaction of biomass in subcritical water by researchers. The authors try to obtain bio-oil of high quality and high yields using straw as starting material. This process was affected by many factors such as different conditions of pressure, heating rate, reaction time and addition of catalyst. In this paper, the effects of the dosage and kinds of catalysts, respectively, on the yield and Higher Heating Value (HHV) of the bio-oil were investigated and the influence of the mixture of different catalysts was also studied. The effect of catalyst on liquefaction of straw was studied using a high pressure batch autoclave under the subcritical water (T = 573 K, P = 18 MPa, t = 5 min) and the bio-oil obtained was observed and analysed. It was found that addition of appropriate amount of ZnCl2 could increase the yield of the bio-oil to as high as 32.90%, and addition of appropriate amount of Na2S could obviously improve the HHV of the bio-oil to the maximum of 34,056 kJ/kg. When the two catalysts of ZnCl2 and Na2S were added in a certain proportion at the same time, both the yield and the HHV of the bio oil could reach their maximum of 33.85% and 34,428 kJ/kg, and the residue of the reaction reached its minimum. GC-MS technology was also adopted to make qualitative and quantitative analyses on bio-oil as to further confirm the bio-oil's components. The main components of the bio-oil were ketones, phenol substances and their derivatives.
Keywords: catalyst, biomass liquefaction, subcritical water, bio-oil, straw, ketones, phenol