Sandeep Kumar, Adam Byrd, and Ram B. Gupta. Chemical Engineering, Auburn University, 212 Ross Hall, Auburn, AL 36849-5127
Sub- and super-critical water (critical point: 374 °C, 221 bar) provide a novel reaction medium for the efficient conversion of lignocellulosic biomass to usable liquid and gas fuels. Recently, this medium has attracted much attention as a non-toxic, environmentally benign, inexpensive and tunable reaction medium for conducting ionic/free radical reactions. Dielectric constant of water near the critical point decreases considerably, which enhances the solubility of organic compounds. Almost complete conversion of crystalline cellulose ( >90%) to water-soluble products above 330 °C in a short residence time (3-5 s) was possible, and high yield (65-67%) of hydrolysis products (glucose and oligomers) was achieved in subcritical water (335 - 354 °C) (Kumar and Gupta, Ind. Eng. Chem. Res., 2008).Subcritical water was used for the pretreatment of switchgrass in a flow through reactor in temperature range 150 to 180 °C and pressure 35 to 136 bar. The process mainly removed hemicelluloses causing structural changes, which improved the accessibility to enzymes to cellulose. This pretreatment method can be effectively used for ethanol production.At a higher temperature, subcritical water converts biomass to biocrude, a mixture of oxygenated hydrocarbons. Liquefaction of switchgrass for biocrude production in subcritical water (230-260 °C) was studied. More than 80% of switchgrass was solubilized in only 20 minutes.At even higher temperature, supercritical water can effectively convert carbohydrates into hydrogen fuel (Byrd, Pant, and, Chem. Res., 2007). Biocrude produced from switchgrass liquefaction was reformed in supercritical water. The gaseous products contained mainly hydrogen and CO2.
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