where required by law, the sulfur and nitrogen oxide pollutants are removed by stack gas scrubbers which use a pulverized limestone or other alkaline wet slurry to remove those pollutants from the exit stack gas. other devices use catalysts to remove nitrous oxide compounds from the flue-gas stream. the gas travelling up the flue-gas stack may by this time have dropped to about . a typical flue-gas stack may be tall to disperse the remaining flue gas components in the atmosphere. the tallest flue-gas stack in the world is tall at the ekibastuz gres-2 power station in kazakhstan.