Nonisothermal catalytic liquefaction of corn stalk in subcritical and supercritical water

The pyrolysis of corn stalk was performed with thermogravimetry (TG), and its liquefaction was investigated in a semicontinuous apparatus with a nonisothermal fluid extraction technique, both with and without sodium carbonate (Na₂CO₃) as a catalyst. The results indicated that the main pyrolysis region of corn stalk is ∼500-650 K and the maximum rate occurs at ∼600 K on the differential thermogravimetry (DTG) curve. The presence of a catalyst has an obvious effect on the pyrolysis, especially in the temperature range of 550-650 K. When more than 1.0 wt % of catalyst was added, the DTG curve is altered greatly, from two peaks to one, for the catalyst, which has a greater effect on hemicellulose than on cellulose and lignin. Kinetic analysis shows that the activation energy in the main pyrolysis range (10-70 wt %) varies with different amounts of catalyst addition. The liquefaction conversion of corn stalk with 3 mL/min of water as a solvent at a pressure of 25 MPa, with or without the addition of 1.0 wt % of Na₂CO₃, is up to 95.7 and 95.4 wt %, respectively. The catalyst mainly improved the yield of bio-oil, from 33.4% without a catalyst, increasing to 47.2% with 1.0 wt % of Na₂CO₃. The catalyst has a positive effect on the liquefaction at relatively higher temperatures and can increase the yield of liquid product, as well as improve the quality of liquid product. More bio-oil and less gas can be obtained with a catalyst than that without a catalyst. A two-stage main reaction may occur for both the pyrolysis and liquefaction of corn stalk under the experimental conditions.