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Simulation of a flue gas driven open absorption system for waste heat recovery from fossil fuel boilers.

INTRODUCTION

According to the National Bureau of Statistics of China (NBC of China 2014), the total energy consumption in the year 2013 in China is up to 1,750 million tons (3,850 million lbs) of standard coal. About 66% of the total energy is provided by coal, 18.4% by petroleum and 5.8% by natural gas. Only about 9.8% is supplied by renewable energy sources such as the hydropower, wind power, etc. A great amount of industrial waste gas is emitted without being processed. Even though the Chinese government has taken necessary measures for energy conservation, there still remains significant heat recovery potential. The humidity ratios of the exhaust gases differ according to the type of fuels as fuels with higher hydrogen contents will produce more water vapor during combustion. The humidity ratio is up to 120 g per kg (0.120lbs/lbs) air in the case of natural gas and is even higher in the case of coke oven gas. The latent heat occupies a large proportion of the total exhaust heat and will contribute a lot to energy saving if fully utilized.

The condensing heat recovery system (Chen et al. 2012) recovers latent heat by cooling the flue gas below the dew point. The thermal efficiency of the condensing system is limited by the dew point of flue gas. When the temperature of the processed flue gas is above the dew point, the energy efficiency of the system remains at a low level while it increases rapidly as long as the flue gas can be cooled below the dew point. To further reduce the temperature of the cooling water, electrical and absorption heat pump systems are proposed (Qu et al. 2014). In these cases, an electrical heat pump or an absorption heat pump is used to produce cold water which is then applied to recover heat from flue gases. However, in these systems, additional high quality heat sources such as the electricity or direct-fired units are required which greatly increases the operating cost. Moreover, the heat recovery efficiency is still limited by the properties of indirect heat exchangers.

In 1992, Lazarrin et al. (Lazarrin et al. 1992) proposed an open absorption system for flue gas heat recovery using the liquid desiccant as the cycling medium. The system (Johansson and Westerlund, 2000) is mainly composed of an absorber, a generator and a condenser. The liquid desiccant absorbs the water vapor of flue gas in the absorber and gets diluted which is then regenerated by an external heat source in the vapor generator. Driven by the vapor pressure difference between the moist flue gas and the liquid desiccant, the system can recover the latent heat more effectively without cooling the flue gas down to the dew point. Lazzarin et al. (Lazarrin et al. 1992) did a theoretical simulation of the open absorption system and concluded that the energy efficiency of the system is much higher than the closed absorption system and the electrical heat pump system in the case of natural gas boilers. Wei et al. (Wei et al. 2015) set up a simulation model of the system and indicated that the open absorption system is the best choice considering the current price of electricity and natural gas in Beijing. Ye et al. (Ye et al. 2015) proposed a new process of the open absorption system which can switch between the single-stage mode and the double-stage mode according to the temperature of the heat source and concluded that the new open absrorption system outperforms both the ordinary single-stage open absorption system and the closed two-stage absorption heat pump system. Westerlund et al. (Westerlund et al. 2012) conducted a two-year-long actual operating test of the open absorption system for biomass boilers and found that the efficiency of the open absorption system varied with the water content of the raw fuel and the heat production increased by 40% when wet flues were used.

PRINCIPLE OF THE FLUE GAS DRIVEN OPEN ABSORPTION SYSTEM

Most of the existing open absorption systems use direct-fired units as the regeneration heat source. To avoid extra fuel consumption, this paper proposes a flue-gas-driven open absorption system shown in Figure 1.

The system includes three pipelines: the flue gas pipeline (marked in gray), the liquid desiccant pipeline (blue and pink solid lines), and the heating water pipeline (dash green line). High temperature flue gas from the boiler furnace first enters the steam generator and serves as the heat source for solution regeneration (FG1 [right arrow] FG2). Then, it flows into the gas-water heat exchanger (GWHX) and releases heat to the water system (FG2 [right arrow] FG3). The cooled flue gas then enters a counter-current packed tower, which got dehumidified after fully contact with the liquid desiccant (FG3 [right arrow] FG4). The liquid desiccant gets diluted after absorbing water vapor in the packed tower (S1 [right arrow] S2). The diluted solution is transported to the generator through the solution-solution heat exchanger (SSHX) (S3 [right arrow] S4), where it is boiled and regenerated (S5 [right arrow] S6). The concentrated solution then returns to the absorber through the SSHX (S7 [right arrow] S8) and the solution-water heat exchanger (SWHX) (S9 [right arrow] S10). The heating water supply system gains heat from the SWHX (W1 [right arrow] W2), gas-water heat exchanger (GWHX) (W3 [right arrow] W4), and the condenser (W5 [right arrow] W6) in sequence. The vapor (dot dash red line) from the generator condenses after releasing heat to the water system in the condenser and the condensate water is recycled.

This paper introduces the simulation results of the flue gas driven open absorption system, the system performance is analyzed and compared with existing condensing systems for different type of fossil fuel boilers. Important state parameters of the system are calculated with varying input parameters including the temperature and humidity ratio of the flue gas, the temperature and concentration of the liquid desiccant and also the temperatures at different positions of the water system. The results can be used for engineering design and actual operation control of the open absorption system.

THERMODYNAMIC MODEL

Combustion process

The moisture content of the flue gas is determined by the fuel type, the combustion completeness and the excess air coefficient. According to the compositions of different fuels shown in Table 1, the humidity ratios of different types of fuel gases can be calculated by applying the formula of combustion process and the results are listed in Table 2. According to the calculation result, the humidity ratio increases with the hydrogen content of the fuel. The dew point of flue gas generated by the coke oven gas is the highest, followed by natural gas, fuel oil, blast-furnace gas and coal.

Generator

The vapor generator regenerates the liquid desiccant with the high temperature flue gas. In an ideal situation, the water vapor reaches a heat and mass transfer balance with the liquid desiccant and the regeneration process should meet the law of energy and mass conservation. The correlation fomulas are shown as below.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

[[??].sub.S5][x.sub.S5] = [[??].sub.S6][x.sub.S6]. (3)

Condenser

The condenser is connected to the vapor outlet of the generator. During the condensing process, the water vapor condenses by releasing heat to the return water. The pressure loss from generator to condenser is neglected and the heat gain at the water side equals the latent heat of condensation.

Absorber

In the absorber, the heat and mass transfer happens at the same time between the liquid desiccant and flue gas. During the process, the temperature difference is the driving force of heat transfer, while the vapor pressure difference between the flue gas and the moist air layer at the solution surface is the driving force of mass transfer. According to the heat and mass exchange model of the countercurrent packed tower, the outlet parameters of solution and flue gas can be predicted by numerical simulation.

As shown in Figure 2, the absorber can be divided into small volumes along the solution flowing direction and the differential equations of heat and mass transfer are shown as below.

d[h.sub.a,i-1] = NT[U.sub.m]Le/H [([h.sub.a,i] - [h.sub.e,i]) + r (1/Le - 1) ([[omega].sub.a,i] - [[omega].sub.e,i])] dx. (4)

d[[omega].sub.a,i-1] = NT[U.sub.m]/H ([[omega].sub.a,i] - [[omega].sub.e,i]) dx. (5)

d[[??].sub.s,i-1] = [[??].sub.a,in]d[[omega].sub.a,i-1]. (6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

[[??].sub.s,i][x.sub.i] = [[??].sub.s,i-1][x.sub.i-1]. (8)

By assigning the Lewis number with value 1, the state parameters of solution and flue gas at the outlet of absorber can be determined according to the difference equations and inlet state parameters.

Heat exchanger

In the heat exchanger, the heat loss and gain in hot and cold sides meet the law of energy conservation. In the solution heat exchanger, the flow rate difference between the dilute solution and concentrated solution equals the vapor volume generated in the generator.

SIMULATION RESULT AND DISCUSSION

Simulation result

Five application cases are focused on during the simulation: the coke oven gas boiler, the natural gas boiler, the fuel oil boiler, the blast-furnace boiler and the coal fired boiler. The input parameters of the system are listed in Table 3.

The thermodynamic process of the system can be simulated according the model proposed above. The states of flue gases at the exit of the open absorption system as well as the heat and water recovery efficiencies of the system for different type of fuel boilers are summarized in Table 4.

From the calculation results, it can be seen that the natural gas boiler and the coke oven gas boiler outperform the other type of boilers. The heat and water recovery of the system increase with the moisture content of the flue gas. In natural gas boilers, the latent heat recovery accounts for about 37% of the total recycled heat while in coal fired boilers, it is about 14%.

To reach a mass balance of the system, the regeneration ability of the generator should match the dehumidification ability of the packed absorber, so in high humidity cases, more heat is required to regenerate liquid desiccant in the steam generator which means a more significant temperature drop of the flue gas in the generator. The flue gas temperature at the outlet of the generator in the five cases are summarized in Table 5.

In the system, the return water gains heat from the solution-water heat exchanger (SWHX), the flue gas-water heat exchanger (GWHX) and the condenser in sequence. The SWHX recovers the sensible and latent heat of low temperature flue gas, the GWHX recovers the sensible heat of medium temperature flue gas while the condenser recovers the sensible heat of high temperature flue gas with water vapor as the heat transfer medium. The distribution of the recycled heat in the simulated system are shown in Figure 3. It can be seen that the heat gains in the SWHX and the condenser increases while the heat gain in the GWHX decreases with the moisture content of the flue gas. This is because the higher moisture content of the flue gas will cause a higher solution temperature at the outlet of absorber, making it possible for more heat exchange in the SWHX. Moreover, in high humidity conditions, more water vapor will be generated in the generator and the latent heat of water vapor will cause a higher temperature increase of the return water in the condenser.

Comparison and discussion

Figure 4 gives the comparison of total heat recovery volumes for different type of fossil fuel boilers using three different type of methods. The calculation results show that the open absorption system outperforms the other two types of waste heat recovery systems and the advantage increases with the moisture content of the flue gases. In the case of natural gas boilers, the heat recovery efficiency of the open absorption system is about 59% higher than the traditional condensing system and about 14% higher than the electrical or closed absorption heat pump system.

The states of flue gases at the outlets of different kind of heat recovery systems are shown in Figure 5. It can be seen that in traditional condensing systems (circled by blue line), the flue gas cannot be processed under the dew point due to the limitation of return water temperature. By using the electrical or closed absorption heat pumps (circled by green line), the return water temperature can be reduced and flue gases with high humidity ratio (e.g. coke oven gas boiler and natural gas boiler) can be processed under the dew point while flue gases with low humidity ratio (e.g. coal fired boiler and fuel oil boiler) cannot. When the open absorption system (circled by red line) is applied, the humidity ratios of all kinds of flue gases can be reduced and the states of flue gases concentrate on the left upper side of the psychrometric chart, which means that the latent heat of flue gases can be recycled at a temperature higher that the dew point.

In the aspect of water recovery, the open absorption system also behaves well. According to the simulation result, the water recycled by the open absorption system is about 38.8% more than that recycled by the electrical or absorption heat pump system in the case of natual gas boilers. Moreover, atmospheric pollutants contained in the flue gases can also be effectively removed due to the purification effect of the liquid desiccant.

CONCLUSIONS

This paper introduces a flue gas driven open absorption system used for waste heat recovery from industrial flue gases. The thermodynamic model is established to simulate the operation performance of the system under different conditions. The efficiency of waste heat and water recovery is analyzed and compared for different type of fossil fuel boilers. The analysis results are summarized as below.

1. The system uses the high temperature flue gas at the outlet of the boiler furnace as the regeneration source of the liquid desiccant and does not need additional high quality energy input.

2. By using the open absorption system, the flue gas can be processed to a low humidity ratio at a temperature higher than the dew point. In high humidity cases such as the natural gas boilers, the heat recovery efficiency of the open absorption system is about 59% higher than the traditional condensing system and about 14% higher than the electrical and closed absorption heat pump systems.

3. The open absorption system can effectively recover the water vapor contained in the flue gas and the efficiency increases with the hydrogen content of the raw fuel. The system can also purify the atmospheric pollutants contained in the flue gas.

ACKNOWLEDGMENTS

The research was sponsored by the Ministry of Science and Technology of China (2016YFB0601601) and the National Natural Science Foundation of China (Grant No. 51321002, No. 51376097).
NOMENCLATURE

T         = temperature ([degrees]C)
h         = specific enthalphy (J/kg)
[??]      = mass flow (kg/s)
x         = solution concentration
[omega]   = humidity ratio (kg/kg)
NTU       = number of heat and mass transfer
Le        = Lewis number
H         = height ot the packed absorption column (m)

Subscripts

S         = solution
w         = water
FG        = flue gas
a         = dry air
e         = equivalent
vapor     = water vapor
in        = inlet of the packed absorber


REFERENCES

National Bureau of Statistics of China. 2014. China Statistical Yearbook. Beijing: China Statistics Press. (In Chinese)

Chen, Q., Finney, K., Li, H., Zhang, X., Zhou, J., and Sharifi, V. et al. 2012. Condensing boiler applications in the process industry. Applied Energy 89(1): 30-36.

Qu, M., Abdelaziz, O., and Yin, H. 2014. New configurations of a heat recovery absorption heat pump integrated with a natural gas boiler for boiler efficiency improvement. Energy Conversion & Management 87: 175-84.

Lazzarin, R. M., Longo, G. A., and Piccininni, F. 1992. An open cycle absorption heat pump. Heat Recovery Systems & Chp 12(5): 391-96.

Johansson, L., and Westerlund, L. 2000. An open absorption system installed at a sawmill. Energy 25(11): 1067-79.

Wei, M., Yuan, W., Song, Z., Fu, L., and Zhang, S. 2015. Simulation of a heat pump system for total heat recovery from flue gas. Applied Thermal Engineering 86: 326-32.

Ye, B., Liu, J., Xu, X., Chen, G., and Zheng, J. 2015. A new open absorption heat pump for latent heat recovery from moist gas. Energy Conversion & Management 94: 438-46.

Westerlund, L., Hermansson, R., and Fagerstrom, J. 2012. Flue gas purification and heat recovery: a biomass fired boiler supplied with an open absorption system. Applied Energy 96(3): 444-50.

Xu, S. R. 2009. Principle and equipment of boilers. Beijing: China WaterPower Press. (In Chinese)

Luo, Y., Yang, H., Lu, L., and Qi, R. 2014. A review of the mathematical models for predicting the heat and mass transfer process in the liquid desiccant dehumidifier. Renewable & Sustainable Energy Reviews 31(2): 587--99.

Zhenying Wang

Xiaoyue Zhang

Zhen Li, PhD

Member ASHRAE

Caption: Figure 1 Principle of the flue gas driven open absorption system (GWHX: gas-water heat exchanger; SSHX: solution-solution heat exchanger; SWHX: solution-water heat exchanger; Generator: heat recovery steam generator; Absorber: counter-flow structured packed tower)

Caption: Figure 2 Process of heat and mass transfer in a countercurrent packed tower (Luo et al. 2014)

Caption: Figure 4 Total heat recovery volumes using the three different type of methods

Caption: Figure 5 states of flue gases at the outlets of different kind of heat recovery systems (Black: Coal fired boiler; Blue: Blast-furnace boiler; Red: Fuel oil boiler; Green: Natural gas boiler; Light blue: Coke oven gas boiler)
Table 1. Composition of Different Type of Fossil
Fuels (Xu 2009, 15-28)

Composition         C     H     S    O    N

Coal                94%   5%    1%   --   --
Fuel Oil            88%   12%   --   --   --

Composition         C[H.sub.4]   [H.sub.2]   CO    [N.sub.2]

Natural Gas         100%         --          --    --
Blast-furnace Gas   --           15%         25%   50%
Coke Oven Gas       30%          60%         --    7%

Composition         C[O.sub.2]   [H.sub.2S]

Natural Gas         --           --
Blast-furnace Gas   10%          --
Coke Oven Gas       3%           --

Table 2. Humidity Ratio of Different Type of Flue Gases (excess air
coefficient=1.5)

Fuel type                              Coal      Fuel Oil

Humidity ratio (kg/kg, lbs/lbs)        0.031      0.067
Dew point ([degrees]C, [degrees]F)   32 (89.6)   45 (113)

Fuel type                            Natural Gas   Blast-furnace Gas

Humidity ratio (kg/kg, lbs/lbs)         0.120            0.062
Dew point ([degrees]C, [degrees]F)    55 (131)        43 (109.4)

Fuel type                            Coke Oven Gas

Humidity ratio (kg/kg, lbs/lbs)          0.163
Dew point ([degrees]C, [degrees]F)     60 (140)

Table 3. Input parameters of the simulated open absorption system

Flue gas temperature at the furnace outlet
400[degrees]C (752[degrees]F)

Return water temperature
40[degrees]C (104[degrees]F)

Solution concentration
55%

Pressure in generator
101.325kPa (14.692psi)

NTU of the packed absorption column
2

Flue gas volume
1kg/s (2.2lbs/s)

Gas-solution ratio
0.2

Table 4. Performance of the Open Absorption System for Different
Types of Boilers

Fuel type                               Coal         Fuel Oil

Outlet air humidity ratio              0.014          0.020
(kg/kg, lbs/lbs)
Outlet air temperature              49.1(120.4)    50.8(123.4)
([degrees]C, [degrees]F)
Water recovery (kg/s, lbs/s)        0.017(0.037)   0.047(0.103)
Heat recovery in absorber (kW, HP)   61.7(83.9)    137.6(187.1)
Total heat recovery (kW, HP)        427.9(581.8)   527.3(716.9)

Fuel type                           Natural Gas    Blast-furnace Gas

Outlet air humidity ratio              0.030             0.019
(kg/kg, lbs/lbs)
Outlet air temperature              53.5(128.3)       50.6(123.1)
([degrees]C, [degrees]F)
Water recovery (kg/s, lbs/s)        0.091(0.200)     0.043(0.095)
Heat recovery in absorber (kW, HP)   251(341.3)      126.6(172.1)
Total heat recovery (kW, HP)        676.4(919.6)     512.9(697.3)

Fuel type                           Coke Oven Gas

Outlet air humidity ratio               0.039
(kg/kg, lbs/lbs)
Outlet air temperature               55.7(132.3)
([degrees]C, [degrees]F)
Water recovery (kg/s, lbs/s)        0.124(0.273)
Heat recovery in absorber (kW, HP)  335.8(456.6)
Total heat recovery (kW, HP)        788.7(1072.3)

Table 5. Temperature of flue gases at the outlet of the generator

Fuel type                                 Coal     Fuel Oil

Flue gas temperature at the outlet of     372.5      308.0
the generator ([degrees]C, [degrees]F)   (702.5)    (586.4)

Fuel type                                Natural Gas

Flue gas temperature at the outlet of       231.2
the generator ([degrees]C, [degrees]F)     (448.2)

Fuel type                                Blast-furnace Gas

Flue gas temperature at the outlet of          324.7
the generator ([degrees]C, [degrees]F)        (616.5)

Fuel type                                Coke Oven Gas

Flue gas temperature at the outlet of        173.2
the generator ([degrees]C, [degrees]F)      (343.8)

Figure 3 Heat gains in different parts of the open absorption
system used for different type of flue gases

                    Solution-water HX   Flue gas-water HX   Condenser

Coal                65.02               337.8               27.4
Fuel oil            135                 282.4               97.11
Natural gas         229.2               208                 192.3
Blast furnace gas   116.4               297.3               78.36
Coke oven gas       317.7               144.5               276.2

Note: Table made from bar graph.
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Author:Wang, Zhenying; Zhang, Xiaoyue; Li, Zhen
Publication:ASHRAE Transactions
Date:Jan 1, 2017
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