Comparison between aqueous- and vapor-phase reformation for thermochemical waste heat recovery of engine exhaust gas

Natural gas internal combustion engines release over half of the fuel's energy as waste heat and emit pollution that harms human health and accelerates climate change. Enriching natural gas with hydrogen has been shown to mitigate these impacts by reducing emissions and increasing engine efficiency. Thermal energy in the exhaust gas from natural gas engines can be used to drive chemical reactions to reform a biomass-derived feedstock into a hydrogen-rich gas. This gas can be blended with the primary fuel to enhance combustion and displace some of the natural gas demand. Two types of chemical reformation processes, aqueous-phase reformation (APR) and vapor-phase reformation (VPR), have been identified which can convert a biomass-derived sugar feedstock solution into a hydrogen-rich gas by recovering waste heat from engine exhaust gas. VPR operates at higher temperatures than APR, which limits the amount of heat that can be transferred from the exhaust gas to the reaction temperature. This study used a thermodynamic pinch analysis to compare the performance of these two processes based on their respective process heat demands and the thermal energy available from engine exhaust gas to determine how many moles of feedstock can be reformed. The calculations were performed using specifications for eight natural gas engines with reactor conditions from fourteen APR and ten VPR experiments, using glycerol as a model compound. The results predict that APR will perform better for engines with low exhaust gas temperatures, while VPR will perform better with higher exhaust gas temperatures. With exhaust gas at 873°C, VPR can convert 23% of exhaust gas waste heat into chemical energy while APR can convert 6.0%. With exhaust gas at 385°C, APR can convert 3.4% of exhaust gas waste heat into chemical energy while VPR cannot occur. At high exhaust gas temperatures, VPR is able to convert more waste heat into chemical energy than APR because the high quality heat consumed at the reaction temperature for VPR is used entirely for the heat of reaction. The heat consumed for APR at its reaction temperature is split between the heat of reaction and a heat consumed by water vaporization, thus a portion of the highest quality exhaust heat is consumed by vaporizing water. For APR, the rate at which waste heat can be utilized to reform glycerol is a strong function of reactor pressure. Higher pressures relative to the vapor pressure of water at the reaction temperature require less latent heat, and thus there is more high-quality thermal energy available for the heat of reaction. The rate of chemical energy production for VPR is a strong function of the reaction temperature, where lower reactor temperatures allow more heat to be converted into chemical energy. These results motivate future experimental work with vapor-phase reformation at low reaction temperatures and aqueous-phase reformation at high system pressures to maximize the rate of chemical energy production in this waste heat recovery system.