Conversion of Wet Ethanol to Syngas and Hydrogen

Because of converging concerns about global climate change and depletion of conventional petroleum resources, many nations are looking for ways to create transportation fuels that are not derived from fossil fuels. Biofuels and hydrogen (H2 ) have the potential to meet this goal. Biofuels are attractive because they can be domestically produced and consume carbon dioxide (CO2 ) during the feedstock growth cycle. Hydrogen is appealing because its use emits no CO2 , and because hydrogen fuel cells can be very efficient. Today most hydrogen is derived from syngas, a mixture of hydrogen, carbon monoxide (CO) and carbon dioxide, which is produced through catalytic steam reforming of methane (CH4 ). Although effective, this process still produces CO2 . Another method used to generate hydrogen is water electrolysis, but this process is extremely energy intensive. Thus, finding an energy-efficient approach to producing hydrogen from biofeedstock is appealing. Though there are many biofuels, ethanol (C2 H5 OH) is a popular choice for replacing fossil fuels. However, many have questioned its value as a renewable fuel since it requires a significant amount of energy to produce, especially from corn. Producing pure ethanol requires substantial energy for distillation and dehydration to yield an appropriate “dry” fuel for traditional combustion engines. Wet ethanol, or ethanol that has not been fully distilled and dehydrated, requires significantly less energy to create than pure ethanol. In this paper, we present a non-catalytic pathway to produce hydrogenrich syngas from wet ethanol. The presence of water in the reactant fuel can increase the hydrogen mole fraction and decrease the carbon monoxide mole fraction of the product syngas, both of which are desired effects. Also, because there are no catalytic surfaces, the problems of coking and poisoning that typically plague biomass-to-hydrogen reforming systems are eliminated. The non-catalytic fuel reforming process presented herein is termed filtration combustion. In this process, a fuel-rich mixture of air and fuel is reacted in an inert porous matrix to produce syngas. Some of the ethanol and air mixtures under study lie outside the conventional rich flammability limits. These mixtures react because high local temperatures are created as the reaction front propagates into a region where the solid matrix has been heated by exhaust gases. These high temperatures effectively broaden the flammability limits, allowing the mixture to react and break down the fuel into syngas. The conversion of pure and wet ethanol is a novel application of this process. Exhaust composition measurements were taken for a range of water fractions and equivalence ratios (Φ) and were compared to equilibrium values. The water fraction is the volumetric fraction of the inlet fuel and water mixture that is water. Equivalence ratio is the ratio of the fuel to oxidizer ratio of the reactant mixture to the fuel to oxidizer ratio of a stoichiometric mixture. A stoichiometric mixture is defined as a mixture with proportions of fuel and oxidizer that would react to produce only water and carbon dioxide. The stoichiometric mixture (Φ = 1) of ethanol and oxygen (O2 ) is 1 mole of ethanol for every 3 moles of oxygen: C2H5OH+3O2 ↔ 2CO2+3H2O Hydrogen mole fraction of the exhaust gas increased with increasing equivalence ratio and remained nearly constant for increasing water-in-fuel concentration. Carbon monoxide mole fraction was also measured because it may be used as a fuel for certain fuel cells while it can poison others [1]. Species and energy conversion efficiencies were calculated, showing that significant energy savings could be made by reforming wet ethanol rather than pure ethanol into syngas. Also, it is shown that the hydrogen to carbon monoxide ratio increases with addition of water to the fuel, making this method attractive for the production of pure hydrogen.Copyright © 2008 by ASME