Hydrogen Production by Internal Reforming Fuel Cells
Internal reforming fuel cells have been developed based on the Molten Carbonate Fuel Cell as well as on the Solid Oxide Fuel Cell. In these concepts effective use is made of the high temperature of the fuel cell and the surplus of heat produced by the fuel cell. The high temperature allows the reforming of hydrocarbons to hydrogen by a reaction with steam. This is an endothermic reaction consuming part of the (waste) heat produced in the fuel cell, which thereby increases the fuel cell's efficiency. The capability of internal reforming fuel cells to reform hydrocarbons can be exploited further by producing more hydrogen than necessary for their own consumption.This can be done by decreasing electric power output or by increasing the input of hydrocarbons. By producing hydrogen the Nernst loss of the fuel cell decreases since also the area near the fuel exit still contains a high partial pressure of hydrogen contrary to standard operation. Total efficiency in terms of hydrogen and power output can reach 95% as was shown by flowsheet calculations. Furthermore since waste heat can be converted into hydrogen the fuel cell can be operated in higher power density compared to standard operation and still obtain relatively high efficiencies in terms of hydrogen and power production. Overall a very flexible system is obtained in which operation can be optimized depending on (local) demand and market prices for power, hydrogen and heat.
A very interesting but more exotic form of hydrogen production using fuel cells is a concept in which carbon is electrochemically converted into carbon monoxide and subsequently reacted with steam to form carbon dioxide and hydrogen in the well known shift reaction. The electrochemical reaction takes place in a Direct Carbon Fuel Cell operated at high temperature to promote the formation of CO above CO2 following the temperature dependance of the Boudouard equilibrium.
The reaction is characterized by a negative enthalpy change and a positive entropy change. Therefor the reversible electrochemical reaction is endothermic and next to chemical energy also heat is converted into electric power. The conversion process can be called ‘electrochemical gasification’ and is a true Multi Source Multi Product conversion system in which chemical energy and heat are converted into electric power and CO (to hydrogen).