Low emission flash-binary and two-phase binary geothermal power plants with water absorption and reinjection of non-condensable gases, Geothermics

The power production from geothermal resources with a high content of non-condensable gases (NCG) is concerning an increasing number of regions and may affect the sustainability relating to geothermal projects. The selection of an emission abatement system based on water absorption and reinjection of NCG back to the reservoir could solve the environmental impact, yet it implies significant power output penalty and water demand, as demonstrated in a recent paper by the authors (Manente et al., 2019) focused on the retrofit of existing flash steam power stations. A reconsideration of the plant design could result in a better integration with the absorption and reinjection section and finally reduce the associated requirements. To address this point, two “greenfield” plant layouts are investigated in this paper for utilization of a high enthalpy liquid dominated reservoir, where the single flash steam generation unit is complemented or even totally replaced by air cooled binary cycle units. The first layout is an “integrated flash-binary” plant based on a backpressure steam turbine and a bottoming ORC system, whereas the second layout consists in a “two-phase binary” plant. The performance of the two novel layouts is compared against that of a brownfield layout, first proposed in Manente et al. (2019), based on an existing single flash plant retrofitted with the absorption and reinjection system, using a consistent set of assumptions. The results show that the net power output remains almost unaltered because in the novel layouts the lower parasitic loads for gas compression counterbalance the lower production dictated by the air-cooled system and the irreversibility in the heat transfer between geothermal fluid and organic fluid. Thus, due to the better integration between power block and emission abatement section the net power output penalty of the novel layouts is contained within 2%. On the other hand, the hydrogen sulfide and carbon dioxide abatement efficiencies markedly increase as the removal of the cooling tower eliminates the secondary gaseous emissions and makes the entire steam condensate available for NCG absorption. In the integrated flash-binary plant design the overall H2S and CO2 abatement efficiencies reach 99% and 90%, respectively, without any external water demand, provided that measures are taken for a controlled precipitation of silica and other minerals from the geothermal brine prior to using it for NCG absorption.