Energy Analysis for Hydrogen Generation with the Waste Heat of Internal Combustion Engine

Abstract:

The production of hydrogen with the exhaust energy of an internal combustion engine was investigated in this paper. Steam reforming of methanol is an efficient way to generate hydrogen at relatively low temperature. The reactants of this process are methanol and water, and the hydrogen yield may reach as high as 75% theoretically. However, this is an endothermic reaction, and additional energy has to be provided to this process. If copper oxides and zinc oxides are used as catalyst, the reaction may proceed at the temperature of 270°C. A heat exchanger was designed in this study to use the hot exhaust of a diesel engine to convert methanol to hydrogen. This system is composed of a reformer, a heating chamber, a by-pass valve, and a control valve. Methanol was mixed with pure water at the ratio of 1:1 to form methanol solution. The flow rate of the methanol solution was adjusted according to the engine speed and load such that the thermal energy of engine exhaust may be fully utilized. The reformer is made of copper tubes and compact alumina fins. Pills of catalyst were filled inside copper tubes. Hot exhaust gas flowed through fins and transferred heat to methanol solution. Methanol solution at room temperature was fed into the reformer at a specified rate. It was heated and vaporized inside the copper tube, and then converted to the final products. It was found that in our system the molar fraction of H2 in the reformed gas was 72.6%, while that of CO2 was 23.5%. The exhaust temperature of a diesel engine varies in the range of 250°C~450°C, depending on the load of engine. It is quite sufficient to generate hydrogen with engine exhaust in a methanol reformer. In our system, the hydrogen rate of 17.3 L/min can be obtained in the exhaust pipe of a diesel engine with the displacement volume of 6000c.c. It was found that 49.5% of thermal energy can be recovered, and 92.6% of the recovered energy can be converted. In total, 36.7% of the waste energy can be recovered and stored in the reformed fuel.