Integración de un proceso de biometanización del CO2, generado en una planta de oxicombustión, con un ciclo combinado

Abstract

To reach the European Green Deal targets on climate neutrality by 2050 requires a wide variety of measures ranging from improving green finance rules to strengthening the EU Emissions Trading Scheme, to stimulating environmentally friendly innovation, and all the while ensuring cost-effectiveness. Climate neutrality means emitting only the greenhouse gases that nature can absorb; this means not only reducing these gas emissions, but also trying to capture those that are already present in the atmosphere. Achieving all these objectives implicitly entails transforming the current fossil fuelbased economy into an emission-neutral one. Therefore, some of the principal measures to be adopted, as a matter of urgency, are moving away from fossil fuels in favor of increasing the renewable energies and biofuels, and using systems for capturing the CO2 emitted and, also, for absorbing the one already present in the atmosphere. Renewable energy generation technologies have the inherent characteristic of following a variable production profile. Therefore, one of the ways to accelerate their use is to increase their management capacity (manageability), which implies among other measures, the need to promote the deployment of energy storage technologies. This thesis proposes for the first time a novel scheme for storing surplus renewable electrical energy by hybridizing a microbial electrosynthesis system with an industrial process operating in oxy-combustion mode. The hybrid process combines, in the same scheme, the valorization of CO2 emissions from an industry with the storage of intermitent renewable energy from the power grid, all under greener design parameters and a circular economy perspective. The present thesis aims to contribute to and continue the work initiated in previous research works, in which, on the one hand, novel concepts of Power-to-Gas energy storage systems were identified and, on the other hand, practical aspects for the design and operation of microbial electrosynthesis systems (MES) were addressed. The core part presents and studies a new hybridized system, OxyMES, which takes the oxy-combustion gases from an industrial boiler, which are composed mainly of CO2 and water vapor, and converts them into biomethane in a MES cell, thanks to the activity of microorganisms that are deposited in the biocathode. The result is a biogas suitable for direct use or for injection into the natural gas grid, once it has been cleaned of impurities. In addition, pure oxygen is also produced as a by-product of the anode chamber, as the bioelectrosynthesis is carried out in an aqueous medium. The system is intended to offer an "all-in-one" solution, since it converts CO2 into biogas and produces oxygen for oxyfuel combustion from electricity and water, all within a single system operating at ambient conditions. The second part of the thesis analyzes the coupling of a combined cycle, OxyCC, downstream of the OxyMES system, operating in oxy-combustion mode and composed of a gas turbine (OxyTG) and a heat recovery boiler (HRSG). Thus, the new OxyMESOxyCC hybrid scheme becomes a Power-to-Power system in which the energy charging and discharging stages are decoupled. In this part of the thesis, the aim was to evaluate the eventual application to the forthcoming new energy business models, for which the energy reconverted back into electricity and the effiency of the hybridized set were calculated (PtGtP). The methodology being followed was, first of all, to carry out the mass and energy balances of the OxyMES hybridized scheme, for a set of fixed starting assumptions regarding the fuel supply to the oxy-fuel boiler (coal) and the CO2-to-methane conversion and faradaic efficiencies in the MES cell. Then, a sensitivity analysis was performed considering several assumptions for the cell voltage (Vcell = 3.5 V/2.8 V/1.63 V/1.23 V). This parameter is used in the thesis as an indicator of the electrical power consumed by the MES cell requerided to produce the biogas. The greater the electric potential difference to be applied between the electrodes of the cell, the greater the electrical consumption. Once the inlet and outlet process streams and their conditions were defined, the mass and energy balances were performed and the Powerto- Gas OxyMES system efficiency was obtained, for the four Vcell case studies (PtG_MES). In the case of Vcell = 1.63 V, the performance obtained was found to be on the order of those reported by other research groups for Power-to-Gas systems conducting the methanation reaction in two steps: with an electrolyzer and with a methanation reactor. This Vcell scenario represents the trend that seems likely to be reached in the short to medium-term for these bioelectrochemical systems, if research efforts in this field continue progressing. The theoretical Power-to-Gas efficiency (PtG_MES) achieved in the MES cell is 51%, for a cell potential difference of 1.63 V, and that of the overall system integrating the oxyboiler, OxyMES (PtG&H_OxyMES), is 60%. Regarding the Power-to-Gas-to-Power system approach, or Power-to-Power for short, a simulation of the combined cycle in oxy-combustion mode has been carried out in the thesis according to two gas turbine outlet gas recirculation schemes (RC: hot/wet and RF: cold/dry recirculation) and, in each of them, with different percentages of oxygen content at the gas turbine combustion chamber inlet (degree of oxy-combustion, Oxc). In this model, a maximum limit of 2% by volume has been imposed on the oxygen content in the oxy-combustion product gases to avoid inhibiting the activity of methanogenic microorganisms. After carrying out the balances of all the case studies, those corresponding to the oxy-combustion degree of 14%vol were chosen as a reference, since they achieved the highest yields of the set. This corresponds to a recirculation ratio of 80% in the hot recirculation scheme (Oxc_RC14) and 90% in the cold recirculation scheme (Oxc_RF14). Since one of the main objectives of the proposed OxyMES process is to avoid one of the units that most penalize energetically the oxy-combustion CO2 capture processes (air separation unit, ASU), several scenarios of gas turbine load are analyzed and, therefore, different oxygen consumptions. If the gas turbine operates at a load close to 80% of the nominal one, external oxygen supply to the plant can be avoided, since the MES cell is able to cover simultaneously the demand of the oxy-fired boiler and the combined cycle. The OxyMES-OxyCC system can be made self-sufficient and independent of external supplies by adequately sizing the tanks system of the main process gaseous fluids (oxygen, CO2 and biogas). The overall efficiency or "round trip" efficiency of the Power-to-Gas-to-Power system (PtGtP), when the Vcell is in the range of 1.63 V and 2.8 V, is between 22.8% and 14.6% (RC_Ox14 case) and between 23.7% and 15.2% (RF_Ox14 case), both cases with all three power cycles operating at full load. If the operation of the OxyTG is at partial load (self-sufficient mode), then this efficiency decreases by up to three percentage points (between 19.5% and 12.5% in RC_Ox14 and between 20.2% and 12.9% in RF_Ox14, at 79.6% OxyTG load). The transformation to electrical energy of the gas stored during the discharged step reaches its maximum Gas-to-Power efficiency value (GtPe) at full OxyTG turbine load, with 37.0 %. At turbine partial load (self-sufficient mode), this efficiency decreases to 36.5 % and if, the chemical energy of the stored biogas is accounted for, the Gas-to- Power&Gas efficiency (GtP&G) increases to 51.6 %. The proposed OxyMES system is a system that, simultaneously, neutralizes CO2 emissions and stores energy, capable of valorizing CO2 emissions without energetically penalizing the process, assuming as a reference a conventional oxy-combustion plant for CO2 capture. It has been concluded that with CO2-to-methane conversion factors greater than 85%, the specific emissions can be improved with respect to the reference plant. For a conversion factor of 95%, the value of CO2 converted per unit of chemical energy stored as methane is 208 gCO2/kWhth. In addition, if the fuel used in the oxy-fuel boiler were a biogenic waste, overall negative emissions from the process could be achieved. The main limiting factors of the proposed process that have been identified are the high overpotentials of the microbial cell, which penalize the overall efficiency, and the need to use storage tanks for the process gases. The latter factor is inherent to the power-togas technology. With respect to overpotentials, improvements in materials and new cell configurations suggest that it will be feasible to achieve considerable reductions in the coming years. As for tanks, with the right size it is possible to achieve 100% autonomy, free of external supplies of process fluids (O2, biogas and CO2). Therefore, the capabilities of OxyMES are expected to be maximized when integrated into larger systems forming hubs of different industrial processes. Finally, it has been analyzed whether the possible energy advantage of not needing ASU or CPU at the times when the MES cell operates, compensates for the high energy consumption of the MES cell. It has been found that this energy advantage exists as long as the energy that feeds the system comes from renewable surpluses. In these cases, the energy savings from not installing both units (ASU and CPU) can also offset the energy consumption of the biogas to biomethane upgrading system, which is necessary to inject it into the natural gas grid. Bio-Power-to-Gas energy storage system In general, systems such as the one proposed require improving the overall performance results of the whole system (roundtrip efficiency) for which, necessarily, improvements must be achieved in each subunit or subprocess. The self-sustainable configuration of the hybrid system provides flexibility in energy storage and connectivity between the electricity and gas sectors (sector coupling), since it maintains a percentage of stored biogas (20%) ready to be used in the combined cycle or to be injected into the natural gas grid. In short, this thesis contributes to strengthening the position of microbial electrosynthesis technologies hybridized with industrial systems as a promising solution for non-dispatchable renewable energy storage systems.