Direct contact pyrolysis of methane using nuclear reactor heat

摘要

An energy system based on fossil fuels is responsible for the generation ofmassive amounts of CO and CO2, (from oxidation of about 6 gigatons C per year), which is recognized as a threat to our environment. An alternate energy system based on hydrogen would be more environmentally benign but significant developmental work is required for commercial acceptability. These greenhouse gas emissions are recognized as a threat to our environment. The U.S. safely uses about 4 trillion cubic feet of H2, most of which is produced by the steam reforming of hydrocarbons, primarily methane (SMR). Two major disadvantages of the SMR technology are the concurrent generation of greenhouse gases and the energy costs. We are proposing to use direct contact pyrolysis (DCP) of methane/natural gas to minimize these disadvantages. In this process, methane is bubbled through a molten metal heated by the liquid metal coolant in Generation IV reactors to produce H2 and carbon, which is collected on the liquid metal surface. Coking of active sites does not occur. Proof-of-principle experiments were recently completed and critical experimental parameters were identified. Methane was converted to H2 and carbon when bubbled through lead at all temperatures studied, 600-900degC, without generating greenhouse gases. Conversion efficiency increased with temperature and residence time. The temperature dependence of the reaction was used to calculate the apparent energy of activation, 213 kJ/mol for temperatures between 750 and 900degC. This value is considerably lower than the bond dissociation energy of H-CH3 at 431 kJ/mol, indicating an autocatalytic reaction. The maximum conversion at 900degC was about 25%. Hydrogen is the only significant gaseous product in DCP, that is, the reaction, CH4-2H2+C goes to completion. No heavier gaseous hydrocarbons were observed. Results of tests with a commercial grade of natural gas were similar to those with methane, although the ethane in the natural gas was preferentially pyroloyzed. Currently we are identifying methods to increase conversion efficiency at lower temperatures, such as longer residence times.