The solar thermal decarbonization of natural gas

The endothermic decomposition of natural gas into a carbon-rich condensed p hase and a hydrogen-rich gas phase, using concentrated solar energy as the source of high-temperature process heat, is considered as a model reaction for conducting a 2nd-law analysis of a solar decarbonization process in whi ch carbon is removed from fossil fuels prior to their use for power generat ion. The theoretical maximum closed-cycle exergy efficiency, defined as the ratio of the Gibbs free energy change of the reaction to the solar power i nput, can be as high as 35% for a black-body solar cavity-receiver/reactor operating at 1500 K and under a mean solar flux concentration ratio of 1000 , and decreases to 21% if the products exiting the solar reactor are quench ed without recovering their sensible heat. Four technically viable routes a re examined for extracting power from the chemical products of the solar de composition of CH4: (1) carbon is sequestered and only H-2 is used in a fue l cell; (2) carbon is used to fuel a conventional Rankine cycle and H-2 is used in a fuel cell; (3) carbon is steam-gasified to syngas in a solar gasi fication process and the syngas further processed to H-2, which, together w ith H-2 from the CH4-decomposition reaction, is used in a fuel cell; and (4 ) carbon serves as a reducing agent of ZnO in a solar carbothermic process for producing Zn and CO that are further converted via water-splitting and water-shifting reactions to H-2 for use in a fuel cell, The open-cycle ener gy efficiency, defined as the ratio of electric power output to the thermal energy input (solar + heating value of reactants), exceeds 65% for the 3rd and 4th power generation routes. Both of these routes offer a net gain of 40% in the electrical output and, consequently, an equal percent reduction in the corresponding Specific CO2 emissions, vis-A-vis the direct use of CH 4 for fueling a 55%-efficient combined Brayton-Rankine cycle. For route nr. 1, the energy penalty for avoiding CO2 emissions amounts to 30% of the ele ctrical output. (C) 2001 International Association for Hydrogen Energy. Pub lished by Elsevier Science Ltd. All rights reserved.