The Challenge
Decarbonising energy generation is critical to reducing greenhouse gas emissions. The Allam Cycle is novel method for power generation where carbon-based fuels are combusted in a way that allows for relatively easy capture of the two product streams: carbon dioxide and water. The cycle combines oxygen and natural gas in a combustor, with the exhaust expanded through a turbine to generate power. Water is separated from the exhaust stream, and the CO2-rich exhaust is then compressed to 100 bar. Part of the compressed stream CO2-rich is recycled, at up to 400 bar and around 700 °C for injection into the combustor to control the reactor temperature, while the remainder is sent for geological storage. The Allam cycle has the highest energetic efficiency of the oxy-fuel cycles, and has recently been demonstrated in a 25 MW test facility developed in La Porte, Texas [1, 2].
A challenge with this cycle, however, is managing the water produced during combustion. If water exists in a liquid phase anywhere in the cycle, it can react with the CO2 to form carbonic acid and corrode the process equipment. Therefore, it is important for longevity of the plant to understand how much water needs to be separated from the recycled stream.
Engineers look to equations of state to make phase behaviour predictions. For carbon dioxide rich mixtures, there are significant inconsistencies between the recommended equations of state as well as differences between the outputs of thermodynamic calculation software used to implement the same equation. For example, in a 95 mol% mixture of CO2 with water present at 0.1 mol%, the aqueous dewpoint predicted by the two EOS shown in Figure 1 varies by up to 20 K at 40 bar. So, which equation of state and software implementation should engineers rely on?
The Solution
The Future Energy Exports CRC developed an apparatus to measure phase behaviour of low humidity carbon dioxide rich mixtures. Microwave resonators are used to measure the dielectric permittivity of fluid mixtures. The advantage of a dielectric measurement is that the sensor response caused by liquid water can be 80 times higher than most other fluids. In this case, the resonator can detect liquid water on the order of nanolitres, which is a very small fraction of the sample total volume (2 – 15 mL). Such sensitivity is important for characterising the phase behaviour of mixtures with trace quantities of water.
Outcome, Impact and Future Horizons
This project produced some important results relating to measurement capability, thermodynamic modelling and how to design robust Allam cycles. A reliable technique for preparing and handling multi-component mixtures with trace amounts of water with validated composition was established and demonstrated. The experiments reveal that the equation of state which best matches the measured data depends on the concentration of carbon dioxide in the mixture. Finally, we used the measurements to advise our industry partner how to modify the inlet conditions to their compressors to avoid condensed water, and how to modify the pump inlet conditions to keep any fluid entering them in single phase.