This specification discloses a method of producing petroleum from a petroleum-bearing formation. A heated fluid is generated in a heated fluid generator that is fired with a sulfur-containing fuel and the heated fluid is injected into the petroleum-bearing formation and petroleum is produced therefrom. The firing of the heated fluid generator with the sulfur-containing fuel produces a flue gas which contains sulfur oxides. The flue gas is passed through an emission scrubber vessel in counterflow with an alkaline water containing calcium ions to scrub the sulfur oxides from the flue gas, to reduce the alkalinity of the alkaline water and to form calcium sulfite. An oxidation inhibitor is added to inhibit the oxidation of the calcium sulfite. The scrubbed flue gas and the water of the reduced alkalinity are discharged from the emission scrubber vessel and the calcium sulfite is separated by mechanical means from the water of reduced alkalinity.