(9a) Biomass Gasification In Supercritical Water

We gasified cellulose and lignin and model compounds (guaiacol and phenol) in supercritical water at 400-700 °C. The reactions were conducted in quartz tubes, at times with Ni added wires. These reactors allowed the homogeneous and heterogeneous SCWG rates to be quantified separately for the first time. Homogeneous SCWG of cellulose (no catalyst or metal present) produces CO₂ as the major compound, except at the lowest cellulose loadings (5 wt.%) where CH₄ became the major product. Manipulating the cellulose loading (wt %) and water density provides a means to control the product selectivity. In general, higher H₂ yields were obtained from cellulose at the longer reaction times, higher water densities, higher cellulose loadings, and higher temperatures. Results from SCWG of cellulose in quartz reactors differ from those obtained from nominally ?uncatalyzed? SCWG in stainless steel reactors. In quartz, the total gas yields are lower and the H₂ mole fraction in the gas is also lower. These comparisons indicate that the reaction surface influences both the rate of gas formation and the composition of the gas. Therefore, one needs both the homogeneous and heterogeneous reaction kinetics for SCWG to properly design, scale-up, or optimize a SCWG reactor. Guaiacol is mainly gasified into hydrogen, carbon dioxide, carbon monoxide, and methane. Nickel significantly changes the gas product compositions. Phenol is mainly gasified into hydrogen, carbon dioxide and methane. The gas compositions measured experimentally were consistent with those anticipated at chemical equilibrium. The results show that homogeneous gasification in supercritical water is slow, but rates are greatly increased by added Ni. The pseudo-first order rate constant at 600 °C for homogeneous gasification of phenol is 3.0 (± 0.4) x 10^-4 s^-1, and the rate constant for Ni-catalyzed gasification is 2.7 (± 0.7) x 10^-4 cm/s.