A direct contact cooler design for simultaneously recovering latent heat and capturing SO_x and NO_x from pressurized flue gas

Integrated SO_x and NO_x removal technology using a direct contact cooler is a promising alternative for cleaning coal combustion flue gas, especially for pressurized combustion systems. Past investigations of integrated SO_x and NO_x removal have been limited to low-temperature processes, where the flue gas has been cooled upstream of the removal unit, and the moisture in the flue gas has been condensed out. This work presents a novel design that recovers the flue gas heat while simultaneously removing SO_x and NO_x in a single reactive-absorption column – direct contact cooler. A validated model was used to evaluate different direct contact cooler designs. Modeling results suggested that a counter-current column with a single water inlet can scrub 83% of the NO_x and 96.5% of the SO_x in 115 s, while an optimized design with multiple water inlets enhances the scrubbing of NO_x by 9 percentage points and SO_x by 3.5 percentage points, due to a 34% increase in residence time. It is also observed that the liquid-phase reaction between absorbed SO_x and NO_x plays a significant role in the removal of SO₂ from the flue gas for high-temperature scrubbing compared with low-temperature scrubbing. The design reinforces the potential of high-temperature SO_x-NO_x removal and heat recovery and suggests a means of reducing capital costs for such columns.