Abstract 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. 1 INTRODUCTION Traditional coal-fired boilers produce flue gas that contains a lot of sulfides, nitrogen oxides and particulates, which heavily pollutes the environment. Therefore, many coal-fired boilers have been replaced by natural gas boilers recently. However, natural gas combustion produces large amounts of water vapor and the humidity ratio of the flue gas can reach 120 g/kg. Besides, the temperature of the exhaust flue gas can be about 200°C. So it is of great significance to recover the waste heat of the flue gas, especially the latent heat. The recent methods to recover the waste heat include condensing method, closed absorption heat pump combined system, open-cycle absorption system and so on. The condensing method recovers heat by condensing the water vapor in the flue gas so that the vapor can release latent heat to the cold source. The cold source needs to be cold enough since the flue gas should be cooled to its dew point and a relatively big temperature difference between the flue gas and the cold source is needed due to the limited heat transfer coefficient of the indirect heat exchanger. It should be pointed out that the initial dew point of the flue gas of natural gas boilers is about 55°C and it will decrease with the condensation of water vapor. Researchers retrofitted a conventional natural gas fired boiler into a condensing boiler and the temperature of its cold source is as low as 10°C . If the heating network return water with a temperature of 45–55°C serves as the cold source, the condensing method can hardly recover latent heat from the flue gas. So some researchers proposed closed absorption heat pump combined system. The heat pump is driven by natural gas or electricity and can produce cold water at 20–25°C, so the flue gas can be reduced to 35°C considering the heat transfer temperature difference . Combined with the closed cycle heat pump, the heat recovery system can obtain low-temperature cold source and much latent heat is recovered along with the condensation of the water vapor in the flue gas. In order to enhance heat transfer effect, some scholars combined absorption heat pump with the direct-contact heat exchanger ; some proposed open-cycle absorption system resting upon the utilization of a packed tower where the flue gas contacts directly with liquid desiccants . Both systems achieved good effect, but high quality energy is consumed as heat source of the generator in the absorption system. Based on this, researchers retrofitted the open-cycle absorption system and replaced the high quality energy with flue gas [5, 6]. The liquid desiccant, serving as the working fluid, is regenerated and concentrated under atmospheric pressure. The boiling point is high and it requires high-temperature flue gas serving as the heat source. Besides, the temperature of the flue gas cannot be reduced below the desiccant’s boiling point under atmospheric pressure, which limits the heat release of the flue gas in the generator. In order to lower the temperature requirement of the heat source, this paper proposes a new kind of absorption system and the liquid desiccant is concentrated by relatively low-temperature flue gas in a vacuum regenerator. 2 SYSTEM DESCRIPTION As shown in Figure 1, a liquid desiccant like lithium bromide (LiBr) aqueous solution serves as the working fluid. Figure 1. View largeDownload slide Diagram of the absorption system with low-pressure regeneration. Figure 1. View largeDownload slide Diagram of the absorption system with low-pressure regeneration. Low temperature and high concentration solution has a higher moisture absorption capacity due to a lower vapor pressure on its surface. So the solution is cooled first in a water-solution heat exchanger (S-W HX) before the dehumidification process. The cooled solution is sprayed into the packed tower (absorber) to dehumidify the low-temperature flue gas and gets diluted itself. A proportion of the diluted solution flows into an indirect exchanger, that is, the regenerator, where the solution is heated and concentrated by the high-temperature flue gas from the furnace outlet. The concentrated solution is then cooled and sprayed into the absorber. The water evaporated from the solution releases condensation heat to the return water (~45°C) from the heating network. The return water flows through the water-solution heat exchanger, the condenser and the water-flue gas heat exchanger (G-W HX) in turn and gets heated. The waste heat of the flue gas is recovered in the regenerator, the flue gas-water heat exchanger (G-W HX) and the absorber in succession. Finally, the relatively clean and dry flue gas is released to the atmosphere. What is special for this system is that the regenerator is not connected with atmosphere in order to lower the boiling point of the solution. Take the lithium bromide water solution with a mass fraction of 40% as an example, its boiling temperature under 20 kPa and at the atmospheric pressure is 74°C and 117°C, respectively. In this system, the regenerator and condenser are connected together, forming an enclosed space. The space is filled with the water evaporated from the solution. The balanced vapor pressure in the enclosed space is determined by the temperature of the cold source in the condenser. The lower the temperature, the lower the pressure. A lower pressure corresponds to a lower boiling point. Figure 2 shows the one-to-one correspondence of the parameters. Figure 2. View largeDownload slide One-to-one correspondence of temperatures and pressure. Figure 2. View largeDownload slide One-to-one correspondence of temperatures and pressure. 3 THERMODYNAMIC MODEL 3.1 Heat balance The liquid desiccant (salt solution) can be regarded as the heat carrier and its enthalpy does not change after a cycle. The latent heat and the sensible heat are mainly transferred to the solution in the absorber and regenerator, respectively. Then the heat carried by the solution is transferred to the heating network return water via the condenser and solution-water heat exchanger. The enthalpy decrease of the flue gas, which is the quantity of heat recovery, is equal to the enthalpy increase of the return water, as shown in equation (1). The heat recovery rate is Qrecovery=mFGΔhFG=mwcpΔtw, (1)where Qrecovery is the heat recovery rate(kW), mFG is the mass flow rate of flue gas (kg/s), ΔhFG is the specific enthalpy change of flue gas (kJ/kg), mw is the mass flow rate of water (kg/s), cp is the specific heat capacity of water (4.2 kJ/kgK) and Δtw is the temperature change of water (K). 3.2 Absorber Most units in the new system are indirect heat exchangers except for the absorber, where the latent heat of the flue gas is transferred to the liquid desiccant. In the absorber, the flue gas is dehumidified by the liquid desiccant and the liquid desiccant is diluted. Figure 3 shows the finite difference model of the absorber . The absorber is an adiabatic counter flow packed tower. Figure 3. View largeDownload slide Finite difference model of the counter flow packed tower. Figure 3. View largeDownload slide Finite difference model of the counter flow packed tower. The main equations are listed below. Equations (2) (3) and (4) describe the energy conservation of the two contacting fluids, the solute mass conservation and the water mass conservation, respectively. Equations (5) and (6) are the mass transfer and total heat transfer equations. ms,ihs,i−ms,i−1hs,i−1=mFG,indhFG,i−1 (2) ms,ixi=ms,i−1xi−1 (3) dms,i−1=mFG,indωFG,i−1 (4) dωFG,i−1=NTUmH(ωFG,i−ωe,i)dx (5) dhFG,i−1=NTUmLeH[(hFG,i−he,i)+γ(1Le−1)(ωFG,i−ωe,i)]dx (6)where the subscript ‘s’ stands for solution and ‘e’ stands for the equivalent value of solution, x is the mass fraction of solution, ω is the humidity ratio (kg/kg), Le is Lewis number, NTUm is the number of mass transfer unit of the packed tower, H is the height of the packing. 3.3 Regenerator and condenser The regenerator is an indirect heat exchanger. The diluted solution flows into the regenerator and is heated by the high-temperature flue gas. The water absorbed by the solution is evaporated and fills the vacuum space of the regenerator and condenser. The water evaporation finally reaches equilibrium with the water condensation, and thus the pressure in the space is stabilized in a certain value. For example, if the cold source temperature is 55°C, the condensing temperature should be about 60°C considering a temperature difference of heat transfer. The condensing temperature is the saturation temperature under a certain pressure according to the Clapeyron equation. Then the boiling temperature of the salt solution can be obtained by formulations of the saturation vapor pressure of the solution . Figure 4 shows the relationship between the condensing temperature and the boiling point of the LiBr-H2O solution with a mass fraction of 40%. Inevitably, a little air will be carried into the regenerator and the pressure will increase gradually. So a vacuum pump should connect with the regenerate and operate at intervals to keep the pressure stable. The power consumption of the vacuum pump pales in comparison with the heat recovery rate because only a small quantity of air is carried into the regenerator. Figure 4. View largeDownload slide The relationship between the condensing temperature tc and the boiling point tb. Figure 4. View largeDownload slide The relationship between the condensing temperature tc and the boiling point tb. 4 RESULTS AND ANALYSIS The simulated system is based on a natural gas fired boiler with a rated power of 2.8 MW. In the original boiler system, 312 Nm3 natural gas is consumed per hour to heat the heating network return water with a flow rate of 80 m3/h from 45°C to 70°C. The flue gas is released to the atmosphere at a temperature of 200°C. Its humidity ratio is 120 g/kg. The application of the new system proposed in this paper can recover a lot of waste heat and the humidity ratio of the flue gas is reduced to 46 g/kg. The flue gas is finally released at 53°C. Besides, the water recovery rate is 0.1 kg/s. Figure 5 shows the simulation results. Figure 5. View largeDownload slide Simulation parameters and results of the absorption system with low-pressure regeneration. Figure 5. View largeDownload slide Simulation parameters and results of the absorption system with low-pressure regeneration. In order to compare the heat and water recovery effects of different kinds of systems, the calculation results are listed in Table 1. The equivalent thermal efficiency is the ratio of the heat obtained by the water to the consumption of the natural gas, whose low calorific value is 35.88 MJ/ Nm3. Table 1. Comparison of different kinds of heat recovery systems. System Flue gas final temperature (°C) Flue gas final humidity ratio (g/kg) Water recovery (t/h) Heat recovery (kW) Equivalent thermal efficiency (%) Original system 200 120 0 0 75.0 Condensing system 50 86 0.18 403 90.6 Open-cycle absorption system 62 75 0.24 420 92.4 Low-pressure regeneration system 53 46 0.36 517 97.3 System Flue gas final temperature (°C) Flue gas final humidity ratio (g/kg) Water recovery (t/h) Heat recovery (kW) Equivalent thermal efficiency (%) Original system 200 120 0 0 75.0 Condensing system 50 86 0.18 403 90.6 Open-cycle absorption system 62 75 0.24 420 92.4 Low-pressure regeneration system 53 46 0.36 517 97.3 As can be seen from the table, the complex open-cycle absorption system has little advantage over the simple condensing system when the temperature of the original flue gas is only 200°C. The new absorption system based on low-pressure regeneration liquid desiccant cycle performs best among all the existing systems in terms of water and heat recovery and it can reach a thermal efficiency of 97.3%. Although the condensing system can reduce the temperature of the flue gas to a lower level, its heat recovery rate is less than the absorption systems. This is because the latent heat occupies a large proportion of the waste heat and absorption systems perform better in recovery latent heat. 5 CONCLUSIONS Based on the thermodynamic model, a simulation was carried out on the new system. When the temperature and humidity ratio of the flue gas at the furnace outlet is 200°C and 120 g/kg, the flue gas 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 simulation results of the 2.8 MW boiler show that in terms of heat recovery, the new absorption 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. If the return water temperature is as high as 50°C, the condensing method can hardly recover latent heat and its thermal efficiency will decrease sharply. When the temperature of the flue gas from the furnace is 150°C or even lower, the open-cycle absorption system may be infeasible. So the new absorption system based on low-pressure regeneration liquid desiccant cycle has advantages in both heat recovery effect and applicability over the existing systems. FUNDING The research was sponsored by the National Basic Research Program of China ‘973’ (2013CB228301). REFERENCES 1 Che D, Liu Y, Gao C. 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International Journal of Low-Carbon Technologies – Oxford University Press
Published: Mar 1, 2018
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