Using solar energy to enrich methane and generate electricity

Solar thermal decomposition technology may eventually help rid the world of its dependence on fossil fuels. In the meantime, some of these technologies may be integrated with existing infrastructure to help lower CO2 emissions. Scientists at the University of Rome proposed and modeled the integration of solar steam reforming technology with a steam turbine and existing natural gas (NG) infrastructure to create a tube and shell reactor (De Falco et al). Following decompression, natural gas with a volumetric hydrogen concentration of 17% (HCNG-17) can be fed straight into a low or medium pressure NG grid. Hydrogen enriched NG produces less CO2 when burned because a portion of the energy comes from hydrogen: hydrogen produces water, not CO2, when burned. The model found that concentrating a solar plant with an area of 16,000 m2 coupled with a tube and shell reactor with 4 reformers is capable of supplying the enriched methane and electricity demands of about 2930 domestic users. —Tim Fine
  De Falco, D., Giaconia, A., Marrelli, L., Tarquini, P., Grena, R., Caputo, G., 2009 Enriched methane production using solar energy: an assessment of plant performance. International Journal of Hydrogen Energy 34 98–109.

 Marcello De Falco and colleagues at the University of Rome and the ENEA Research Center modeled the integration of solar thermal methane reforming to enrich natural gas coupled with a steam turbine to generate electricity. In the model, a field of solar collectors concentrates sunlight onto solar receivers filled with molten salt. The molten salt is used as a heat transfer fluid and is transferred into a storage tank. From the tank the salt is pumped to either a steam reformer, where it heats the feedstock and drives the reformer, or to a steam turbine, to generate electricity. The model assumed an exit temperature of 550 ºC for the molten salt: the flow rate would be adjusted to meet this depending on the intermittency of solar radiation. The salt storage tank allows for the flow of salt to the reformers to be kept constant at 4 kg/s. Allowing longer residency times in the reformers and steam turbine increased their thermal efficiency but reduced the rate at which enriched methane was produced. The residence time would need to be set based on the enriched methane and electricity requirements, as well as the intermittency of solar radiation as measured by the availability of molten salt at the required temperature. Pressure was found to adversely effect the production of enriched methane while slightly increasing the electrical output from the steam turbine. While the reformers themselves don’t consume large amounts of heat, vaporizing their feedstock does, which puts constraints on the number of reformers that can be used on the same salt circuit. Examining the space requirements for the number solar collectors, the study found that the proposed plant would work for small municipalities: the space requirement could be a drawback for towns larger than 20,000 inhabitants.

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