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

As clean energy, natural gas is used in many boilers. However, the flue gas from natural gas boilers contains much water vapor. The humidity ratio of the flue gas can reach 120 g/kg, so the latent heat occupies a large proportion of the total waste heat. Therefore, it will contribute a lot to energy saving to recover waste heat from the flue gas, especially the latent heat. This paper introduces a flue gas driven absorption system based on low-pressure regeneration liquid cycle to recover water and waste 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 is eventually released to the atmosphere at 53 ºC with a humidity ratio of 46 g/kg, which means considerable heat is recovered by the system. The new system recovers water and latent heat by absorbing vapor from the flue gas instead of condensing method, so it is not constrained by the dew point of the flue gas. It also lowers the requirement of the generation temperature due to vacuum regeneration, which is different from the open system. 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.