Joining semi-closed gas turbine cycle and tri-reforming: SCGT-TRIREF as a proposal for low CO2 emissions powerplants

Methane conversion to a rich H-2 fuel by reforming reactions is a largely applied industrial process. Recently, it has been considered for applications combined to gas turbine powerplants, as a mean for (I) chemical recuperation (i.e. chemical looping CRGT) and (II) decarbonising the primary fuel and make the related power cycle a low CO2 releaser. The possibility of enhancing methane conversion by the addition of CO2 to the steam reactant flow (i.e. tri-reforming) has been assessed and showed interesting results. When dealing with gas turbines, the possibility of applying tri-reforming is related to the availability of some CO2 into the fluegas going to the reformer. This happens in semi-closed gas turbine cycles (SCGT), where the fluegas has a typical 14-15% CO2 mass content. The possibility of joining CRGT and SCGT technologies to improve methane reforming and propose an innovative, low CO2 emissions gas turbine cycle was assessed here. One of the key issues of this joining is also the possibility of greatly reduce the external water consumption due to the reforming, as the SCGT is a water producer cycle. The SCGT-TRIREF cycle is an SCGT cycle where fuel tri-reforming is applied. The steam due to the reformer is generated by the vaporization of the condensed water coming out from the fluegas condensing heat exchanger, upstream the main compressor, where the exhausts are cooled down and partially recirculated. The heat due to the steam generation is recuperated from the turbine exhausts cooling. The reforming process is partially sustained by the heat recovered from the turbine exhausts (which generates superheated steam) and partially by the auto thermal reactions of methane with fresh air, coming from the compressor (i.e. partial combustion). The effect Of CO2 on methane reforming (tri-reforming effect) increases with decreasing steam/methane ratio: at very low values, around 30% of methane is converted by reactions with CO2. At high values of steam/methane ratio, the steam reforming reactions are dominant and only a marginal fraction of methane is interested to tri-reforming. Under optimised conditions, which can be reached at relatively high pressure ratios (25-30), the power cycle showed a potential efficiency around 46% and specific work at 550 kJ/kg level. When the amine CO2 capture is applied, the specific CO2 emissions range between 45 and 55 g(CO2)/kW h. (C) 2009 Elsevier Ltd. All rights reserved.