TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The vent gas comprises methane, carbon monoxide, hydrogen, oxygen, hydrogen sulfide and sulfur dioxide.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The system further comprises: many compressors, many dryers and many distillation purification systems; and a heat exchanger where the vent gas is used to condense steam. In the system: the dryers lie downstream of at least one compressor and at least one distillation purification system lies downstream of the compressors; at least two compressors lie downstream of the dryers; and each compressor is in fluid communication with a heat exchanger that uses water as a cooling medium. The vent gas comprises two streams, a first stream that is recycled to the chemical looping system after being operated upon by the distillation purification system and a second stream that is recycled to the chemical looping system after exiting the dryer, where the second stream does not contact the distillation purification system. The distillation purification system is a cryogenic distillation purification system. In the system, the recycling of the vent gas to the chemical looping system facilitates further conversion of carbon monoxide to carbon dioxide and conversion of hydrogen sulfide and sulfur dioxide into calcium sulfide or calcium sulfate. Preferred Method: The method further comprises: cryogenically distilling a portion of the vent gas in the distillation purification system; drying the flue gas stream to remove water present; splitting the flue gas stream into two streams, of which one stream is recirculated to the chemical looping system via the distillation purification system while the other stream is directly recycled to the chemical looping system without contacting the distillation purification system, where the stream that contacts the distillation purification system is further split into two streams-one of which comprises the contaminants and the other comprising carbon dioxide, and the stream that comprises the carbon dioxide is further compressed in compressors; and contacting the stream emanating from the distillation purification system with calcium oxide to further remove carbon dioxide from the vent stream, where the vent stream is recycled to the reducer after removal of the carbon dioxide, or contacting the stream emanating from the distillation purification system with a semi-permeable to further remove carbon dioxide from the vent stream, and the vent stream is recycled to the reducer after removal of the carbon dioxide. In the method, calcium oxide is converted to calcium carbonate and is recycled to the chemical looping system.