Low CO₂ Emission Combined Cycles With Natural Gas Reforming, Including NO_x Suppression

The present paper addresses the plant configuration, the thermodynamic performance and the economics of combined cycle power plants, having the following characteristics: (i) CO₂ emissions reduced by at least one order of magnitude, (ii) utilization of hydrogen produced by natural gas as the fuel for the gas turbine, (iii) acceptable NO_x emissions. Two plant configurations are discussed, based on high pressure reformers heated up by: (i) gas turbine exhausts with hydrogen firing, (ii) oxygen combustion of carbonated purge gases from a pressure-swing-absorber. In the first case CO₂ is separated by a chemical absorption plant, in the second from the reformer exhausts after water condensation. The fuel dilution by water/steam or nitrogen was properly kept into account, to achieve NO_x emission of about 30-45 ppmvd with a flame temperature of 2300 K. The two plant schemes show a net efficiency of about 48% (including CO₂ liquefaction) vs. 56% of the reference combined cycle having the same gas turbine and cycle technology, with a remarkable increase of the net power output (based on the same gas turbine unit). The ability of removing CO₂ from the exhausts yields to a 25-30% increase of the cost of electricity, i.e. 40-45 $/ton of CO₂ sequestrated.