Flue gas is gas that exits to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. It often refers to the combustion exhaust gas produced from power plants. Its composition depends on what is being burned, but will usually consist of mostly nitrogen, carbon dioxide, water vapor and excess oxygen. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides.