Start-up and Shutdown Protocol for Natural Gas-fired Power Stations with CO2 Capture

Publication Overview

In modern power grids, a power plant with CO2 capture will be required to operate as a low-carbon, flexible, dispatchable power generator. A recent IEAGHG study showed it is possible to achieve net-zero CO2 emissions from coal-fired and gas-fired power generation by employing higher capture rates and, in the case of coal-fired generation, by employing a mix of capture rates and biomass.

Publication Summary

• With increased penetration of intermittent renewables, however, greater load-following is likely and a rise in the frequency of start-up/shutdown (SUSD) cycles would be expected. If it transpired that frequent SUSD cycles resulted in appreciable CO2 emissions, it could undermine the value proposition of including modern, flexible, dispatchable fossil fuel power generation assets in the grid even though CCUS applied to them could enhance energy security and grid resilience.
• Limited work to-date has focused on the impact of start-up and shutdown of CO2 capture plants, particularly with respect to the time required and the fugitive CO2 emissions arising. This study broadens understanding of the process dynamics during start-up and shutdown, identifying key factors that impact CO2 capture performance and operability associated with a natural gas-fired combined heat and power3 (CHP) plant.
• To optimise CO2 capture performance during start-up and shutdown, factors shown to be important include the solvent inventory volume, the initial start-up temperature (cold vs hot) and the timing of steam availability.
• Using a larger solvent inventory can be beneficial in terms of maximising the cumulative CO2 capture rate from start-up, through steady state operation and shutdown. While the effect of solvent inventory volume is negligible for hot start-ups, there is significant improvement in cumulative CO2 capture in the case of cold start-ups.
• Overall, hot start-ups showed a significantly higher cumulative CO2 capture rate and lower specific reboiler duty compared to cold start-ups. With a low start-up solvent loading (0.05–0.11 mol CO2/mol amine), high online CO2 capture rates of 98–99% were achieved during start-up, occurring immediately after the introduction of flue gas to the system. The ability to sustain these high CO2 capture rates depends on the volume of the solvent inventory, the amine concentration, the starting solvent CO2 loading, and the timing of steam supply to the reboiler.
• Results indicate the importance of timely steam supply during start-up. Any delay in steam supply to the capture plant significantly reduces the cumulative CO2 capture rate, thereby greatly increasing the residual CO2 emissions. Start-up with preheating was shown to be a potentially valuable approach. Under specific operating conditions, preheating can provide higher cumulative CO2 capture rates during start-up of the capture plant, thereby minimising the residual CO2 emissions. To maximise the value of preheating, a higher flow rate of steam supply can heat the system much faster, but the duration of preheating must be optimised to minimise residual CO2 emissions associated with steam energy.
• Measures applied during hot start-up, such as high preheating and lower solvent loading, were shown to markedly improve capture performance. For example, a cumulative CO2 capture rate of 98.8% was achieved compared to the lower cumulative CO2 capture rates of between 90.0 to 90.3% using “conventional” hot start-up protocols. However, the specific reboiler duty was higher with the improved measures, at 5.58 MJ/kg CO2, compared to values of 3.76–4.03 MJ/kg CO2 (depending on solvent inventory volume) for the conventional start-up tests.
• In the context of a net-zero energy system, the need for operational flexibility will rise. Hence, the ability to maximise the CO2 capture rate during start-up and shutdown would be highly valuable as it will reduce residual CO2 emissions from power plants, thus easing the need for carbon offsets from CO2 removal technologies, e.g., bioenergy with CCS, or direct air capture.
• The evaluation has helped identify novel operational strategies that can minimise both the time taken for start-up and shutdown and the CO2 emissions associated with these operations.
• Finally, it should be noted that, while the detailed performance metrics, measurements and observations were plant, scale and solvent specific, the high-level insights drawn from this work should remain broadly applicable