Acid Wash Scrubbing as a Countermeasure for Ammonia Emissions from a Postcombustion CO2 Capture Plant

Amine-based absorption desorption processes are considered to be the state-of-the-art technology for CO2 capture. However, the typical amines used are susceptible to oxidative and thermal degradation. Ammonia is formed as a result of oxidative solvent degradation. Because of the volatility of ammonia, significant emissions of ammonia can occur. Emission countermeasures such as water washing and use of demisters are ineffective against highly volatile components such as ammonia. The aim of this study is to demonstrate acid washing as a countermeasure for ammonia emissions. The tests were carried out at TNO's CO2 capture plant at Maasylakte, The Netherlands. Aqueous monoethanolamine (MEA, 30 wt %) was used as the absorption liquid. Typical ammonia emissions at the water wash outlet were in the range of 15-20 mg/Nm(3), as measured by Fourier transform infrared (FTIR) spectroscopy. An acid wash scrubber using sulfuric acid was installed downstream of the water wash. Parametric tests based on changing the pH and flow rate of the acid liquid and the temperature of the treated flue gas were performed to assess the efficiency of the acid wash scrubber. Additional ammonia (up to 150 mg/Nm(3)) was spiked in the treated flue gas to test the acid wash scrubber, even under extreme conditions. The results from the experiments were compared with a model made in Aspen Plus. The experimental results indicated that it was possible to reduce ammonia emissions below 5 mg/Nm(3) at a pH of 6 for both spiking and nonspiking experiments. The modeling results were in good agreement with the experimentally measured ammonia emissions. The breakthrough of ammonia capture, defined as less than 20% capture efficiency, was found to be at lower pH, 6.8 for nonspiking experiments and 8.1 for spiking experiments, than for the model, which predicted breakthrough at pH 8.7 for nonspiking experiments and 9.7 for spiking experiments. Changes in the acid liquid and treated flue gas temperature did not have an impact on the capture of ammonia. MEA emissions were also below 3 mg/Nm(3) at the acid wash scrubber outlet. It is estimated that the purchased equipment cost of an acid wash is about (sic)3.75 million for an 800-MWe-scale postcombustion CO2 capture plant.