(231e) Optimal Design and Operation of an Ngcc Plant Integrated with Carbon Capture, Hydrogen Production and Long-Term Storage

With an increase in the penetration of renewable energy into the electric grid, variability of load of fossil fuel-based power plants is expected to keep rising. Additionally, it will be desired to decrease the amount of CO₂ emitted by fossil fuel-based power plants through post combustion capture (PCC) systems. For accommodating high penetration of renewables into the grid, fossil fuel-based power plants will have to rapidly ramp up and down their loads with frequent shutdown. Such extreme load following conditions can cause higher wear and tear of critical components in the power plant, lower efficiency, and higher CO₂ emission due to decreased efficiency. Furthermore, renewables are intermittent and, in particular, solar energy may be limited for several months in some regions. For reducing extreme load-following operation of power plants, improve their efficiency and for facilitating long term energy storage, one potential option is hydrogen generation and storage. Hydrogen can be produced by using electrolyzers when the electricity is in abundance while utilizing it when there is high energy demand. For retrofitting with the existing natural gas combined cycle (NGCC) plants, H₂ can be co-fired with natural gas thus generating lesser amount of CO₂ and leading to lower penalty for CO₂ capture and compression, which facilitates generation and more power when the electricity price is high thus improving the economics.

In this work, a dynamic model of the proposed configuration is developed by including a model of the NGCC plant that can simulate varying extent of H₂ co-firing with natural gas, a model of the solvent-based PCC that facilitates flexible capture, and a model of the H₂ generation process by using a proton exchange membrane (PEM) electrolyzer stack, and a model of the H₂ compression and storage system. For H₂ storage, above ground pressure vessels and caverns are considered as alternative options. An economic model of the integrated process is developed by including capital costs for the PCC, PEM electrolyzer and the hydrogen storage and fixed and variable operating costs of these units. Carbon dioxide emissions are penalized using a carbon tax. We undertake net present value (NPV) optimization in Python/PYOMO for 14 market regions that differ greatly in terms of locational marginal price (LMP) of electricity and carbon tax rates. Hourly data of LMP for a duration of one year is considered for NPV optimization. Decision variables include design variables such as the capture plant capacity, dimensions of storage systems, electrolyzer capacity, etc. as well as hourly operating profile of the power plant, capture plant, and the hydrogen system considering charging, discharging and idling as possible options. The underlying NPV optimization problem is a large-scale, highly nonlinear dynamic optimization problem. For computational tractability, several strategies are developed including model order reduction, surrogate model development, model reformulation, etc. The optimization algorithm also allows the plant to shutdown if needed when certain constraints are satisfied.

We compare the economic performance of multiple flowsheet configurations including NGCC only, NGCC+PCC, NGCC+PCC+H₂ generation and storage. We identify candidate regions based on forecasted LMP trends, where the integrated energy configurations will be effective. In regions where cost reductions are necessary, we identify the minimum reductions in operating and capital costs for the integrated system to be economically viable.

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