Renewable methane. Integrated configurations of power-to-gas and carbon capture by means of renewable energy surplus

Resumen

Carbon dioxide is the largest single contributor to global warming [1] and current atmospheric concentrations are increasing at the fastest ever observed rate (2.0 ppm/yr) [2]. To reverse this situation, the International Energy Agency (IEA) stated as top priority the decarbonization of electricity and heat generation sectors, because they produce more than two-fifths of global CO2 emissions [3]. In Europe, the decarbonization of the energy system is based on largely deploying renewable energy sources (RES) [4]. This deployment has already allow to reduce CO2 emissions a 12% with respect to 2009 levels [5]. However, moving forward in this direction implies high shares of intermittent sources in energy production that might cause temporary electricity surpluses, and make the system operation more complex [6][7]. Hence, the objective of the first part of the thesis is to quantify the potential energy surplus that might exist in the future Spanish energy mix due to the renewable penetration. This study was published in Energy, and presented in the 13th International Conference on Greenhouse Gas Control Technologies with an associated paper in Energy Procedia: i Energy storage in Spain: forecasting electricity excess and assessment of Power-to-Gas potential up to 2050, Energy 2018, 143, 900-910 ii Power to Gas technology under Spanish future energy scenario, Energy Procedia 2017, 114, 6880-6885 Facing this situation, the European Commission proposed the development of large scale energy storage as a solution [8]. Energy storage can balance renewable electricity surplus from low to high demand periods, as well as displace fossil fuels in applications that traditionally were hardly covered by renewable energies (e.g., transport). Nevertheless, current energy storage technologies are hindered at large scale either by practical (e.g., special locations required, potential hazards) or technical constrains (e.g., low rate power, short storage durations) [9][10][11]. The most promising technique to overcome these limitations is the hydrogen energy storage. It uses electricity to feed electrolyzers that dissociate water and produce hydrogen (energy carrier), which can be later used to re-generate electricity [12]. Still, two main barriers must be overcome to make hydrogen storage feasible: the high cost (investments above 1000 €/kW, and lack of distribution infrastructure) and the low round-trip efficiency (36.5% - 66.5%, LHV) [13]. To that end, a new concept known as Power to Gas (PtG) has stood out in the last years [14]. Power to Gas combines the H2 from electrolysis with CO2 to produce CH4 (main component of natural gas), thus transferring the electricity surplus from the electric network to the gas infrastructure [15]. This synthetic natural gas widens the final uses of the stored energy, potentially enabling greater reconversion efficiencies and economic profits. Besides, safety measures and transport costs associated to hydrogen are avoided. So, the second part of this thesis reviews worldwide existing PtG projects. The objective is to gather practical experiences regarding the construction and operation of Power to Gas plants, in order to evaluate the remaining challenges facing industrial development. The review was published in Renewable & Sustainable Energy Reviews: iii Power to Gas projects review: Lab, pilot and demo plants for storing renewable energy and CO2, Renew Sust Energ Rev 2017, 69, 292-312 To date, 46 experimental PtG projects have been developed worldwide, but the high cost of the equipment and the low efficiency still restrict the profitability of the concept, and so do the number of field experiences at industrial scale. The motivation of the thesis is to overcome these barriers. Otherwise, PtG deployment will be restricted until future favorable energy scenarios become a reality (those in which renewable sources predominate and economic penalizations on CO2 emissions strongly apply). The hybridizing potential of Power to Gas [15] (heat from methanation and oxygen from electrolysis) is the key that allows to broaden the suitable scenarios for industrial development. Thus, the core objective is to propose novel PtG hybrid concepts to rise efficiency, make a better use of the available resources, remove part of the initially required equipment, and widen the potential uses of this technology. The thesis analyses and characterizes the proposed systems to show that significant improvements in PtG systems can be achieved by means of proper hybridizations. Some remarkable hybridizations to enhance PtG are those that avoid carbon capture energy penalties, since these are significant when CO2 has to be captured from diluted sources. The efficiency drops between 9 and 12 points [16][17], what hides the positive aspect of CO2 recycling in PtG, and its environmental advantages with respect to the other energy storage technologies (CO2 recycling puts carbon dioxide emissions in a closed loop to continuously regenerate the fuel that is consumed). A suitable option to avoid the carbon capture penalty is the hybridization with an oxy-fuel combustion facility. In oxy-fuel combustion, pure oxygen is used as comburent instead of air [18]. The large N2 content present in conventional air-fired combustion is substituted by the combustion products (CO2 and H2O), so flue gas achieves high concentrations of carbon dioxide. Energy penalty associated to this capture process mainly comes from the air separation unit (ASU) that produces the required oxygen (typically 190 kWh/tO2) [19]. Hence, Power to Gas-oxyfuel combustion systems use the oxygen from electrolysis to suppress the electrical consumption demanded by the ASU. Besides, since CO2 is recycled to fuel, the energy consumption of compression stage required to store carbon dioxide is avoided as well. When this concept is implemented into an application that includes a single boiler, the exothermal heat from methanation can be directly integrated as a useful output of the system. In the case that the concept is used in a power plant, the heat can be integrated in the thermal power cycle to increase the overall electrical efficiency. The PtG-Oxycombustion hybridization is the first proposal studied in this thesis. The ratio between the sizes of electrolysis and oxy-fuel combustion processes is used as a key parameter to define the operation strategy of the hybridization. Besides, different simulations are run to determine the influence of the type of fuel fed to oxy-fuel combustion. Then, the study of the proposal is completed with an application case that comprises the energy integration and efficiency evaluation of a hybrid PtG-oxy-fuel combined cycle power plant. The results were published in three papers: iv Power to gas-oxyfuel boiler hybrid systems, Int. J. Hydrogen Energy 2015, 24, 168-175 v Power to Gas-biomass oxycombustion hybrid system: Energy integration and potential applications, Appl Energy 2016, 167, 221-229 vi Future applications of hydrogen production and CO2 utilization for energy storage: Hybrid Power to Gas-Oxycombustion power plants, Int. J. Hydrogen Energy 2017, Vol.42, 19, 13625-13632 Another option to avoid the drawback of attaining pure CO2 is the integration of Power to Gas with amine scrubbing post-combustion capture. In this capture technique, flue gas from an air-fired boiler enters to an absorption column where inert gases bubble out, and CO2 is absorbed by amine. The CO2 rich solvent is then regenerated in a stripping column with counter flowing steam at 100 – 200 °C, thus obtaining a highly concentrated CO2 flow [20]. The main efficiency penalty comes from the thermal energy required to regenerate the solvent, but it can be diminished by using the exothermal energy generated during methanation. Amine capture is the most established and commercially deployed capture technology, hence there already exists studies regarding the application of PtG to power plants with amine capture systems coupled [21][22]. Therefore, the last part of the thesis proposes the novel application of PtG-Amine systems to the chemical industry. The study of this proposal comprises the evaluation of the technical and economic viability of this application for the first time, in which hydrogen is a byproduct coming from electrolytic lines of production of a chemical plant. The study was published in Applied Energy: vii Power to Gas-Electrochemical industry hybrid systems: A case study, Applied Energy 2017, 202, 435-446