Resumen de The potential of gas switching chemical looping technology to eliminate the energy penalty of CO2 capture
The growing concern in global warming caused by greenhouse gas emissions (GHG) has triggered substantial research and technology development efforts to curtail CO2 emissions from fossil fuel power plants, responsible of approximately 25% of the total CO2 emissions of the global economy. Coal has the highest carbon intensity of the different energy sources for electricity and heat generation and will remain a significant primary energy source in developing countries in the following decades. Compelling energy solutions with carbon capture and storage (CCS) will be a must to meet the global warming targets. Amongst the different solid fuel power generation technologies, Integrated Gasification Combined Cycles (IGCC) have the potential to reach higher efficiencies with a lower environmental impact than Pulverized Coal Boilers (PCB).
Removing the CO2 from exhaust combustion gases in a cost effective manner, with additionally low energy penalty, is a challenge. An alternative pathway to post-combustion CO2 capture is pre-combustion CO2 capture, where the gaseous fuel undergoes a shift reaction to produce a H2 rich carbon free fuel, while the CO2 is removed with absorbents more efficiently and cost effectively as the CO2 is not diluted in the air stream, typically at a higher partial pressure. This capture technology is well suited for IGCC plants where a gasification unit produces a pressurized syngas fuel. However, the energy penalty relative to an Unabated IGCC plant is still significant. This is a critical issue as a lower thermal efficiency leads to larger coal feed to the plant for a given electricity output increasing simultaneously the specific capital costs and the costs associated with fuel production and transportation.
Chemical Looping Combustion (CLC) appears as a promising technology to minimize the efficiency loss of carbon sequestration. This capture technology mode is also referred to as inherent carbon capture. In CLC, a metal oxygen carrier is exposed alternatively to a gaseous fuel and an air stream in two interconnected fluidized bed reactors. In the fuel reactor, the oxygen carrier is reduced and the products of combustion (CO2 and H2O) are obtained, which after water condensation, a relatively pure CO2 stream ready for compression and storage is delivered. In the air reactor, the exothermic oxidation reaction takes place, thereby heating the air stream to reactor temperature, suitable for power production in a gas turbine. However, the scale up of interconnected fluidized beds at pressurized conditions (required in power cycles) has been slow. Moreover, the complex hydrodynamics of solids transfer from one reactor to another makes the system have very low flexibility in part load operation of the power cycle. To circumvent these challenges, the gas switching (GS) technology was introduced. In this mode of reactor dynamic operation, the oxygen carrier is kept within the reactor volume, and a set of inlet and outlet valves exposes it to oxidant (air) and reduction (fuel) streams. In order to obtain a time constant averaged flow and temperature to the gas turbine for stable operation, a cluster of reactors is needed. The reactors are operated in bubbling fluidization regime as this has several advantages with respect to a fixed packed bed configuration. Alternative to Gas Switching Combustion (GSC), the Gas Switching Oxygen Production (GSOP) technology utilizes an oxygen carrier capable of releasing free oxygen in the reduction stream, which can be effectively utilized as an oxidant stream of a gasification process.
The goal of this Thesis is to determine the potential of gas switching chemical looping technology to eliminate the energy penalty of CO2 capture in IGCC plants, employing GSC and GSOP clusters. The modelling work of this research consists of developing a set of technology blocks that appear in power plant systems based on plausible performances reported in literature and synthesizing different power plant concepts by integrating the technology blocks. The novel power plant concepts are benchmarked against and Unabated IGCC plant and a Pre-combustion CO2 capture IGCC plant, the latter representative of an available and deployable CCS technology. In parallel, flexible power plants for H2 and power co-production integrating GSC and membrane reactors using advanced H-class gas turbine technology are developed, together with suitable plant benchmarks with and without CCS, with the purpose of presenting power plant concepts which can balance variable renewable energy (VRE), a critical aspect for the competitiveness of thermal plants in a future with high renewable penetration. With regards to the modelling of GS technology, a dynamic model is developed and connected to the stationary power plant simulation to obtain inlet stream data from it and deliver time-averaged output operating conditions. The 4E analysis methodology is employed to analyse and benchmark the novel plant configurations. 4E stands for Energy, Environmental, Exergy and Economic. Both Energy and Environmental analysis are performed to all of the synthesized plants. Exergy and Economic analysis have been performed to those plants that revealed a higher potential, depending on the project requirements.
It is recognized that substantial technology development is required to reach GS technology deployment. Many material related challenges must be overcome, i.e. the oxygen carrier must have a high mechanical stability and durability through many gas switching reduction and oxidation cycles, achieving a high conversion and preventing any fuel slip through the reduction stage. High temperature valves and filters must be available, to ensure a safe operation of the gas turbine. Furthermore, the reactor temperatures must be maximized in order to attain attractive thermal efficiencies. It is therefore highlighted that this Thesis constitutes and ex-ante assessment of GS technology for CCS, as a forecasting effort of its potential assuming that all technology showstoppers are overcome.
This Thesis is encompassed within the European ACT-GasTech Project (Grant Agreement No 691712) and has received funding from MINECO, Spain (reference PCIN-2017-013).
