Thermodynamic analysis of hydrogen production from aqueous-phase glucose reformation as applied to waste heat recovery from natural gas internal combustion engines

A thermodynamic model was designed to explore the viability of utilizing hydrogen-rich gas produced by aqueous-phase reformation (APR) of glucose to provide hydrogen enrichment which can improve efficiency and reduce emissions in natural gas internal combustion engines (ICE). Furthermore, this model assessed whether the endothermic APR process could be driven using waste heat recovered from engine exhaust effectively converting this heat energy into usable chemical potential energy. The reactor conditions modeled are based upon published peer reviewed experimental results which were designed to be just below the vaporization point of water for two reactor environments at 498K and 2.9MPa, and 538K and 5.6MPa. The model was designed to match experimental alkane production and calculate the resulting hydrogen, carbon dioxide, and water vapor production. Results from this model for several different APR performance scenarios are presented, as well as for two different ICE sizes. ICE application results are applied with a constraint on the intake fuel mixture of 20% hydrogen by volume. The model more closely emulated the percent molar composition of the gaseous effluent for the high temperature environement yet consistently overestimated the hydrogen and alkane selectivity, the difference likely due to unknown reaction pathways. The lower temperature environment is predicted to favor total chemical coefficient of performance (1.04 to 1.08 compared with 1.02 to 1.03 for the high temperature environment) along with a lower rate of destroyed exergy averaging 58% of the higher temperature environment exergy destruction rate. Total destroyed exergy across an APR catalyst bed control volume averages 12% to 23% of the total exergy transfered into the reactor as heat from ICE exhaust. A net gain in flow exergy is predicted at reactor conditions for both environments, which is lost when throttling and cooling the APR products down to standard temperature and pressure before entering an ICE, some of which could be recovered by requiring the products to do work during the throttling and cooling process. Across all scenarios and both reactor environments, the predicted formation of water vapor will demand 90% or more of the energy required to drive the APR reaction. If heat is recovered from the APR liquid and gas effluent, ICE exhaust stack temperatures are expected to be high enough to provide the required amount of energy to drive the APR process at steady state. To obtain an additional gain in engine efficiency, on top of that obtained from the combustion benefits of hydrogen, from the utilization of exhaust heat a feedstock conversion of over 85% is required. The predicted volume of catalyst required to serve an ICE, although a crude estimate, is large using first generation catalysts and may be uneconomical. Different feedstocks as well as second and third generation catalysts have the potential to significantly improve catalyst activity, reducing required catalyst volumes and costs, and should be a primary goal for future work in assessing the potential of APR for ICEs. Furthermore, the quantity of water vapor generated, and therefore the thermal energy demand, can be reduced by increasing the reactor pressure, yet at the possible expense of reduced hydrogen selectivity. Exploration of an increased reactor pressure should also be a high priority.