A novel flue gas heat recovery system based on low-pressure regeneration liquid desiccant cycle

Flue gas from natural gas boilers contains much water vapor, so the latent heat occupies a large proportion of the total waste heat. This paper introduces a flue gas driven absorption system based on low-pressure regeneration liquid cycle to recover water and waste heat, especially the latent heat. The concentrated liquid desiccant is sprayed into the packed tower to absorb the vapor from the low-temperature flue gas and gets diluted itself. Then the diluted desiccant is heated and concentrated by the high-temperature flue gas in the vacuum regenerator. The evaporated water from the regenerator then releases condensation heat to the return water of the heating network. Based on the thermodynamic model of the new system, the simulation results show that the flue gas (200°C, 120 g/kg) is eventually released to the atmosphere at 53°C with a humidity ratio of 46 g/kg, which means considerable heat is recovered by the system. The heat and water recovery of the new system is not constrained by the dew point of the flue gas. It also lowers the requirement of the generation temperature due to vacuum regeneration. So in terms of heat recovery, the new system outperforms the traditional condensing system and the open-cycle absorption system by 28.3% and 23.1%, respectively. The new system also helps to reduce particulate emissions and recover water, with a recovery of 0.36 tons of water per hour according to the simulation result based on the boiler with a power of 2.8 MW.