Chemical Recuperation of Low-Quality Waste Heats by Catalytic Dehydrogenation of Organic Chemical Hydrides and Its Exergy Analysis

Abstract

Any technological breakthrough on storage, transportation, and distribution of hydrogen is urgently required to popularize hydrogen energy systems. In the present paper, organic chemical hydrides with appropriate characteristics (e.g., high storage capacities, facile reversibility, and being safe and cost-competitive), consisting of reversible catalysis pairs such as methylcyclohexane dehydrogenation/toluene hydrogenation and decalin dehydrogenation/naphthalene hydrogenation, have been proposed as the suitable materials for carrying hydrogen. Efficient hydrogen generation from organic chemical hydrides at moderate heating temperatures lower than 300 °C, being low enough to avoid serious coke formation over catalyst surface, was accomplished only by using carbon-supported nanosize platinum catalysts in superheated liquid-film states. Desorption of products from catalytic active sites was enhanced with the temperature gradient formed in the superheated liquid-film state, so that both high catalytic conversions and reaction rates were attained simultaneously in spite of unfavorable temperatures for dehydrogenation. Adoption of the superheated liquid-film-type catalysis will make it possible to utilize low-quality waste heats below 300 °C as the heat source for endothermic dehydrogenation. Exhausted waste heat can be recuperated as chemical energy through the superheated liquid-film-type dehydrogenation of organic chemical hydrides, with hydrogen used for vehicles or stationary fuel cells. Exergy loss in the hydrogen storage and transportation systems ought to be suppressed through the chemical recuperation of waste heat by use of organic chemical hydrides. An exergy analysis in automotive application has revealed that organic chemical hydrides are superior to compressed hydrogen and liquefied hydrogen in terms of total exergy consumption.