Hydrogen for fuel cells from ethanol by steam-reforming, partial-oxidation and combined auto-thermal reforming: A thermodynamic analysis

Thermodynamics of hydrogen production from ethanol by steam-reforming, partial-oxidation and combined auto-thermal reforming was investigated as a function of steam-to-ethanol ratio (0.00-10.00), oxygen-to-ethanol ratio (0.00-2.50) and temperatures (200-1000 degrees C) at atmospheric pressure. Thermodynamically ethanol is fully converted already at low temperatures. Main product at low ternperatures is methane, which changes to hydrogen with increased temperature. At elevated temperature also carbon monoxide content increases, which is in accordance with the water-gas-shift reaction. Coke-formation is a serious issue, especially at low steam-to-ethanol (S/E) ratios. Coke-formation free steam-reforming is possible above S/E>3. Steam -reforming achieves the highest hydrogen-yield, which is almost up to the theoretical value at high steam-to-ethanol ratios. Pure partial-oxidation shows similar trends of hydrogen and carbon monoxide content with temperature and oxygen-to-ethanol (O/E) ratio; therefore high hydrogen content is always accompanied by high carbon monoxide content. Partial-oxidation shows a low hydrogen yield and the avoidance of coke formation demands high temperatures or high O/E ratios, whereas nitrogen dilution increases strongly with O/E ratios. Increasing O/E-ratio from 0.00 to 0.75 in auto-thermal reforming shows no strong effect on the hydrogen and carbon monoxide formation at temperatures below 600 degrees C and over the whole S/E-ratio range. Auto-thermal operation reduces the coke-formation and reduces energy demand for the reforming process. (C) 2008 Elsevier B.V. All rights reserved.