Chemical looping process for co-generation of hydrogen and electricity with simultaneous capture of carbon dioxide

Abstract

Hydrogen is considered as an environmentally friendly energy carrier and an attractive substitute for fossil fuel because it simply changes to water during consumption without emission of CO2. Carbon dioxide emission from fossil fuel has become an interest of worldwide concern due to global warning. The use of hydrogen as an energy carrier is a long-term option for energy system to reduce the risk of greenhouse effect. Despite simplicity and abundance of hydrogen, it does not occur naturally as a gas on the Earth - it is always combined with other elements such as water, many organic compounds such as gasoline, natural gas, methanol and propane. Today, most of hydrogen is produced from the hydrocarbon fuel and 80-85% of the world’s total hydrogen is provided by the steam methane reforming (SMR) process because of low cost, abundance of the natural gas, and proven technology. However, the SMR process has its limitations of contamination of H2 by CO due to water-gas-shift reaction equilibrium and undefined capital cost of heat integration at small scales of production. Even 50 ppm of CO can cause significant reductions in fuel cell performance. In contrast, cyclic operation of steam-iron process provides the prospect of generating very pure hydrogen viably from the small to large scale, allowing simultaneous capture of a pure steam of CO2. This process involves the use of iron oxide (Fe2O3) as an oxygen carrier and a three-step water-splitting cycle. Natural gas or syngas (CO: H2 = 2:1) is commonly used as a reducing agent for iron oxide. The gases are indirectly combusted in a fuel reactor (FR) to produce the carbon dioxide (CO2) and steam. The reduced form of iron oxide (FeO or FeO/Fe mixture) is subsequently oxidized by steam to generate hydrogen in a steam reactor (SR). Finally, partially oxidized iron oxide (Fe3O4) returns to its original form (Fe2O3) by reacting with air (O2) in a riser, or an air reactor (AR). The key issues in chemical looping process in...