The Potential for Exhaust Heat Recovery by Thermochemical Recuperation for Hydrogen Enriched Internal Combustion

Abstract

In internal combustion engines a large portion of the chemical energy held in the original fuel is exhausted as waste heat. This exhaust heat represents a significant potential source of energy to be harnessed. The thermochemical recuperation process uses an endothermic reformation reaction to upgrade fuel into a hydrogen rich gas, thereby converting a portion of the exhaust heat into chemical energy. Enriching the primary fuel mixture with this hydrogen rich gas enables combustion with very lean or dilute mixtures resulting in low temperature combustion. The low temperature combustion regime can achieve higher efficiency and lower emissions than standard combustion regimes. Hydrogen enrichment via thermochemical recuperation does not require hydrogen refueling station infrastructure nor significant on-board hydrogen storage and could be used with existing engines. This technology shows promise in increasing the efficiency and reducing the emissions in internal combustion engines while also laying the groundwork for hydrogen production technologies and eventually for fuel cell systems. The promise of future application of this technology motivates further investigations. Thermochemical recuperation to produce the hydrogen required is carried out through a series of processes starting with fuel and water vaporization, superheating of the vapor and finally reformation of the mixture in a catalytic reactor. Based on the fuel being reformed and the catalyst being used, each of these processes takes place at a different temperature. The difference between the temperatures of these processes and the temperature of the exhaust stream drives heat transfer and determines the amount of thermal energy potentially recovered. This work will use computer models to explore various strategies for recovering thermal energy using a thermochemical recuperation process. The parameters used in this modeling effort come from reformation experiments and engine experiments underway in the Hydrogen Production and Utilization Laboratory at the University of California, Davis as well as engine and reformer models.