Prevention of explosive mixtures in the regeneration of sulfur oxide acceptors

In a process wherein (A) a flue gas containing sulfur dioxide and oxygen is passed through a first inlet duct and open valve communicating therewith to a first reactor containing a copper-containing acceptor on a refractory carrier at a temperature of between 200.degree. and 600.degree. C, to remove sulfur dioxide from the flue gas, and then the purified flue gas is passed from the first reactor through an open valve communicating with a first outlet duct, and (B) wherein the valves communicating with the first inlet duct and the first outlet duct of said first reactor are closed, and the flue gas containing the sulfur dioxide and oxygen is then passed through another first inlet duct and open valve communicating therewith to a second reactor containing a copper-containing acceptor on a refractory carrier at a temperature between 200.degree. and 600.degree. C, to remove sulfur dioxide from the flue gas, and then the purified flue gas is passed from the second reactor through an open valve communicating with another first outlet duct, while, (C) concomitantly passing a hydrogen-containing gas through a second inlet duct and open valve communicating therewith to said first reactor at a temperature between 200.degree. and 600.degree. C to regenerate the acceptor and then passing the hydrogen-containing gas from said first reactor through an open valve-communicating with a second outlet duct, the improvement comprising, preventing hydrogen build-up from valve leakage into the first inlet duct of the first reactor during step (C) by passing a quantity of flue gas sufficient to prevent hydrogen build-up through the first inlet duct of the first reactor and then to the first inlet duct of the second reactor.