According to the statistics in 2013, about 66% of the total energy in China is provided by coal, 18.4% by petroleum and 5.8% by natural gas. The compositions of the flue gases differ according to the type of fuels. In cases such as the natural gas boilers, the latent heat occupies a large proportion of the total exhaust heat and will contribute a lot to energy saving if fully utilized. This paper introduces an open absorption system for waste heat recovery from flue gases using liquid desiccant as the cycling medium. The thermodynamic model was established to simulate the actual operation conditions of the system. The calculation results show that the heat recovery efficiency of the system is about 59% higher than that of the traditional condensing heat recovery method and is about 14% higher than the electrical or closed absorption heat pump system for natural gas boilers. The system also performs well when dealing with other type of fossil fuel boilers and the efficiency increases with the humidity ratio of the flue gas due to a higher latent heat proportion. The system also behaves well in water conservation and atmospheric pollutant purification.