The furnace efficiency can be maintained or improved slightly by lowering the excess air ratio and raising the air preheat temperature in the heat furnace. The results obtained with a plant-size industrial furnace show that when the excess air oxygen concentration is reduced from 4% to 3%, the furnace efficiency is raised by 0.6%. However, raising the temperature of the pre-heated air will cause a higher furnace temperature and a faster flowing speed of the air in the furnace. Thus, the heat in the radiation zone of the furnace can be evenly transmitted to shorten the time for the fuel to reach the igniting point, and reduce the heat loss from the hot air stream. This will reduce the emission of CO2 and NOx. If the temperature of the pre-heated air is raised from 200 degrees C to 240 degrees C, about 2.3 x 10(6) m(3) of natural gas can be saved annually to reduce 3.1 x 10(3) t of CO2 and 14.3% of nitrogen oxide emissions. Such operational adjustments of the furnace using the recovered FG will save energy, reduce CO2 emission and alleviate the adverse impact to environment. (C) 2011 Elsevier B.V. All rights reserved.