Effect of Heat Recovery from Flue Gas on the Local Humidity and NOx Dispersion in a Thermal Power Station

The flue gas dispersion from the stack of a power station was investigated prior to addition of a high efficiency heat recovery facility. Decrease of the flue gas temperature from 115°C to 40°C by heat recovery could influence the local humidity and thermal NOx level. It might also result in the formation of a white large plume due to the early saturation of vapor around the stack, and deteriorate dispersion of thermal NOx. Numerical simulation revealed that the area influenced by flue gas has been enlarged, particularly in winter. The volume of visible plume indicating RH 1.0 increased to 120 m high and 80 m wide for the flue gas at 40°C, while a smaller plume was formed that was 85 m high and 50 m wide for that at 115°C. The humid air of the flue gas extended nearly 160 m further along the ground. The distance for the maximum NO2 concentration on the ground increased by 80 m and 50 m for 1 m/s wind and 3 m/s, respectively. The area influenced at the ground level expanded more than 250 m at 1 m/s wind after heat recovery. In particular, lowering the temperature of flue gas may affect the local environment more significantly in unusual cases including temperature inversions.


INTRODUCTION
With regard to global energy issues, the effective utilization and stringent management of energy sources are of great interest in Korea.Urban power stations are making various attempts to recover thermal energy even from the flue gas exhausting through stacks.However, heat recovery from flue gas may cause deterioration of the local air quality due to the formation of large plumes with sometimes fog or drizzle and the limited dispersion of pollutants such as NO x (Mokhtarzadeh-Dehghan et al., 2006).
The plumes discharged from large-scale industrial processes have occasionally been investigated in terms of climate condition and regional topography (Zhou et al., 2009).Cross winds, humidity, and atmospheric stability are critical meteorological parameters for the retention or propagation of the discharged pollutants.Main properties of flue gas influencing the dilution or dispersion of flue gas are moisture content, gas temperature, and emission.A computer-aided simulation for the flue gas condensation in the atmosphere has been carried out, and verified the effects by meteorological condition on the ground level humidity (Brown and Fletcher, 2005).While many numerical studies have observed on air pollutants dispersion from the emission sources (Konig and Mokhtarzadeh-Dehghan, 2002;Riddle et al., 2004;Yousif et al., 2006), the study focusing on flue gas temperature effects on pollutants dispersion or plume formation has not been found in open literature.
The test plant is a combined power station for electricity and steam generation in Ilsan, Korea.It burns liquefied natural gas (LNG).The combustion gas contains 60 ppm of NO and exhausts at 115°C with 20.5 m/s.Since extreme thermal recovery by a high efficiency heat exchanger will reduce the flue gas temperature to 40°C, it is essential to examine the anticipated effects including local humidity and NO x concentration in the vicinity of the study area prior to the field application of new facilities.The mathematical simulation using computational fluid dynamics (CFD) approached with combined parameters from three dimensional basis.

Computational Model
The local combined power station to be tested is adjacent to residential apartments and public buildings, as can be seen in Fig. 1  The quality of the meshes was ensured using different grid structures: hexahedral meshes for the hill and stacks, and tetrahedral meshes for the overall domain with approximately 1.2 million cells.
A standard κ-ε turbulent model was used based on the eddy viscosity with dispersion models for continuity and the Navier-Stokes equation (Konig and Mokhtarzadeh-Dehghan, 2002).The effects of energy and fluid transport are described in terms of temperature, kinetic energy and static pressure etc. as described in Eqs. ( 1)-(3) (Versteeg and Malalasekera, 1995): where u is velocity vector; ρ is fluid density (kg/m 3 ); p is static pressure (kg/ms 2 ); µ is dynamic viscosity (N•s/m 2 ); k is kinetic energy; Ф is dissipation function; ϕ is a general variable; Г is diffusion coefficient; S is source term.
The SIMPLE solver proposed by Patankar and Spalding (1972) was used to discretize the equations for momentum and continuity.The basic components for the numerical simulation, which was carried out using a commercial CFD package, are summarized in Table 1.
It was assumed that excess vapor condensed into water droplets, which formed white plumes at the stack exit when the relative humidity reached 100%.Relative humidity is the ratio of the partial pressure of water vapor actually present in an air-water mixture to the saturation pressure of water vapor at the mixture temperature.The saturation pressure is a relative function of temperature.The vapor pressure can be calculated using the ideal gas law.A vapor transport equation governs the vapor mass fraction, f, given by Eq. (4) (FLUENT 6.1 user's guide, 2003): where ρ is the mixture density, υ is the velocity vector of the vapor, γ is the effective exchange coefficient, and R e and R c are the water evaporation and vapor condensation rate terms.This rate is a function of local static pressure.Oxidation of NO can be described by the following two   Morrion et al. (1996): Many kinetic studies of NO conversion result in inverse proportionality of the rate constant, k, to the absolute temperature due to negative activation energy (Baulch et al., 1973;Tsukahara et al., 1999).Unlike other atmospheric chemical reactions, a low temperature makes the reaction rate in Eq. ( 5) faster, which can result in the production of more NO 2 .The most commonly accepted definition of k as a function of T is described by the Arrhenius equation, as studied by Atkinson and Baulch (1992).The used form of it in this study is as follows:

Boundary Condition and Meteorology
The detailed boundary conditions are summarized in Table 2.The wind rose of the test field is presented in Fig. 1(b).The prevailing wind direction in this region was eastward, and the most frequent wind speed on average from the last three years was 3 m/s.In practice, these results followed the principle that the wind profile increases with an increase in altitude, which can be defined by Deacon's equation, indicating exponential variation (Peterson and Hennesse, 1977): where the wind speed, U ref , corresponds to Z ref , which was 3 m/s at a height of 10 m, and α is the constant of atmospheric stability, which in this study was 0.2, depending on the weak instability of the atmospheric situation in the urban area (Irwin, 1979).A vertical temperature gradient was applied in order to estimate the plume rise in ambient atmospheric conditions, as defined by the following equation (EPA, 2005): where T 0 is the ground temperature (°C) and H is the altitude (km).Ilsan, in which the test power station is located, has four distinctive seasons with the variation of air temperature, -17.4°C to 35.7°C as summarized in Table 3.

RESULTS AND DISCUSSION
The present study simulated flue gas emission from a stack 80 m in height with an inner diameter of 5 m.The initial parameters for the atmosphere and flue gas for the calculations are listed in Table 4.The basic parametric variations at the stack inlet and outlet along the centerline of the stack are presented in Table 5.The pressure and temperature were gradually reduced with increases in height, and the thermal energy was transferred to the gravitational potential energy.The change in the flue gas velocity followed the law of mass conservation.The increase in the relative humidity was a result of a decrease in saturation vapor pressure due to the temperature drop.

Plume Formation
Warm and moist flue gas emitted from a power station stack forms visible plumes when it comes in contact with the relatively cold ambient air.High temperature of the flue gas leads to high saturation vapor pressure, which enhances   the storage capacity of water vapor.If the partial pressure of the vapor is greater than the saturation pressure at a certain temperature, the vapor can be more condensed, finally resulting in saturation and in increase of droplet size to approximately 10 µm (VanRenken and Nenes, 2009).
In order to predict the formation of plumes with the decrease of flue gas temperature, its shape and size were closely investigated by computational fluid simulation, and the effects on the humidity profiles in the surrounding region were studied.Fig. 2 shows a typical appearance of the potential relative humidity contours depending on the seasonal conditions of air temperature and relative humidity for the test region.It was assumed that the moisture content of the flue gas from the plant was constant at 5.8%.In the absence of any wind disturbance, a large plume could be formed vertically and dispersed gradually.Humidity of the stack plume could significantly influence the area for a distance of at least 100 m from the stack.As can be seen in pictures of Fig. 2, very apparent and large plumes appear in cold seasons, which were assumed to be winter (a1, b1), early spring, and late autumn (a2, b2), while the plume would be barely visible in the spring (a3, b3) and summer (a4, b4).In summer, the saturation vapor pressure of water usually increases to 41.17 mmHg under ambient conditions, which creates a high partial pressure in the flue gas rather than in the intrinsic moisture content.Therefore, a boundary layer of high relative humidity would form that would be smaller than on cold days according to the atmospheric humidity.
At 40°C, the flue gas would be much more humid even with the same moisture content, due to the low storage capacity for vapor.In addition, the gas would linger in the vicinity of the stack for a very short time due to lower driving momentum.The flue gas would penetrate into the atmosphere relatively slowly and would be less diluted.Therefore, the visible plume would be larger at 40°C than at 115°C, and thus the effects of humidity on the neighboring area is broaden.
The condensation rate of flue vapor is much lower in hot fluid.A visible plume could rarely be formed during the warm seasons due to the less difference between the temperature of the flue gas and the air outside.However, a very cold day with small difference between temperature of the flue gas and the outside would form a visible plume, even if the moisture content was identical.According to the psychrometric chart, low temperature can decrease the saturation pressure of the flue gas and thereby result in rapid reaching to the dew point.Consequently, visible plume would be found under 30°C of the atmospheric temperature at 40°C flue gas with 5.8% moisture content while 5°C is the critical temperature for 115°C in the present study.It implies very frequent visible plume could be seen all the year round near the plant.Thus, high efficiency demisters must be added and operated particularly in winter.

Effect of Flue Gas Temperature on Local Humidity
Fig. 3 presents the effect of the flue gas temperature on the relative humidity of the surrounding area with the wind, 3 m/s.As could be seen in the visual simulation, the moist flue gas was dispersed to the east according to the wind direction.The relative humidity of the stack bottom for moisture content of 5.8% was 5% at 115°C, but increased to 80% at 40°C due to decrease of saturation vapor pressure (see Table 5).Upon ejected to the air outside, the flue gas faces the dew point and the humidity increases steeply.However, as diluted in atmosphere, the relative humidity becomes lower.
Flue gas temperature affects the mass diffusivity of the gas fluid, according to modification of Fick's law (Poling et al., 2007) as follows: where T is the absolute temperature, M is the molecular weight of the species i and j, P abs is the absolute pressure, σ ij is the characteristic length, and Ω D is the diffusion collision integral, which is a measure of the interaction of the molecules in the system.Low temperature retards molecular motion due to a lowering of diffusivity, which causes the flue gas to aggregate in the exit for a moment.The influencing distance of the downstream humidity extended for 220 m due to the decrease in the flue gas temperature from 115°C to 40°C.During dispersion, the direct impact of the formed plume on the neighboring area was more apparent at high altitudes than at ground level (see Fig. 3).As was predicted earlier, cold flue gas influenced a larger area, particularly at ground level, as seen in Fig. 3(d).
Nevertheless, instantaneous dilution allowed an increase only of a few percent in the special humidity from the moisture out of the stack relative to the background value of 40%.The variation in humidity due to the downwash of the exhaust gas did not appear to be significant at any temperature.

Effect of Moisture Content on Local Humidity
Fig. 4 shows the relative humidity profiles with moisture content of the flue gas after heat recovery.It was found that an increase in moisture forms larger contours, while the dispersion pattern remains similar.The moisture content of the flue gas under ordinary operation of the power station is approximately 5.8%, but sometimes increases to more than 12%.This large amount of moisture may result in a denser plume at the stack exit, particularly at low temperatures.Therefore, an increase in moisture might facilitate super saturation of the flue gas and rapid condensation.The plume would be diluted and assimilated readily, as it was dispersed into the atmospheric air with 40% RH.
The effect of the moisture was also more significant at 12% than 5.8%.The contour line indicating the RH 42% of which just exceeds the ambient humidity did not fall the ground with 5.8% of moisture content till 400 m from the stack (Fig. 4(b)), but touched the ground in 12% (Fig. 4(d)).The high moisture stack gas initiates to be condensed even from the inside of the stack, resulting in sedimentation near the stack without dispersing far distant as well as larger visible plume formation.According to the definition of relative humidity, vapor pressure is a function of moisture content, and thereby more moisture may form denser vapor plume.Thus, the heavy vapor body created from lower temperature flue gas may settle down earlier than hot flue gas.In the long run it could be estimated that low temperature flue gas with more moisture would contribute directly to the local humidity increase.

Effects of Emission Velocity and Wind Speed on Local Humidity
Fig. 5 shows the dispersion of humid gas for 40°C with different emission velocities.Gas emission velocity seems not to directly influence the ground humidity, but to a certain extent on the space over the stack level.In other words, different size of discharged gas plume could be seen depending on emission velocity due to dispersion rate.It is quite obvious on the contour of 44% RH.The top view shows the RH profiles at the section of the top stack.High driving force could form wide gas plume resulting in greater influence even if limited in the atmospheric space.
Wind is one of the main factors to cause air turbulence which is significant for gas dispersion in the atmosphere.As can be seen in Fig. 6, the downwash possibly would be reduced as increasing the speed.The flue gas also could be diluted quickly and be flattened by huge dynamic energy of atmospheric air.It thus can be predicted that the flue gas plume at low speed would stay for a long time.

Dispersion of Nitric Oxides
Fig. 7 shows a concentration contour of NO 2 in which the initial concentration of NO in the intrinsic flue gas was 60 ppm.The thermal NO generated from the combustion processes of the power station could be partly converted to NO 2 , even inside the stack.As can be seen in the concentration distribution, flue gas containing nitrogen oxides would be instantaneously diluted at least 60 times as soon as released from the stack.The direct influencing area was found to be a distance up to 700 m from the emission stack at 115°C, but stretched over 900 m when the flue gas was cooled to 40°C.From a top view of a cross section above the stack exit at a height of 85 m, the elliptical body of the flue gas is larger and longer at lower gas temperatures.Hot fluid might cause greater dilution due to rapid dispersion into the atmosphere by cross winds.Since the conversion rate of NO to NO 2 would be higher at 40°C as based on the Arrhenius equation, cold flue gas might generate a higher amount of NO 2 .Fig. 8 illustrates the NO x concentration curves along the stack center in a windless conditions.The estimated conversion rate of NO to NO 2 increased up to 22.5% at 40°C at the exit of the stack, but was only 15.8% at 115°C.The concentration of NO x dropped steeply as soon as it exited the stack due to rapid dilution with air, in which assumed no NO x .The concentration would gradually decrease as NO x diffused into the air, even if a partial fraction of NO is consistently converted into NO 2 in ambient atmospheric conditions.From the height of 150 m, the NO 2 gas was completely diluted, and the concentration approached zero at 225 m, which is a distance of 145 m from the top of the stack.
Settling of NO 2 from the stack to the ground, which was distant 650 m to 900 m, was also more obvious at 40°C than at 115°C.A light wind (less than 1 m/s) might induce landing closer to the emission source.The ground level concentration of any pollutants would be directly associated with human exposure.The detailed comparison is demonstrated in Fig. 9.The highest NO 2 concentration was found at 500 m to 700 m from the stack.More rapid settling and slower dispersion at  40°C resulted in a higher concentration level of NO 2 on the ground.Therefore, lowering the flue gas temperature might cause an increase in the NO x concentration in the region.
According to the temperature distribution as seen in Table 3, daily difference is sometimes more than 10°C, and seasonal variation seems approximately 44°C maximum.The simulation results show, as seen in Fig. 10, the peak NO 2 concentration at 600 to 700 m away of the stack.For the lower temperature, the more NO 2 could be found at closer to the stack due to rapid down wash.The NO 2 level exceeding the Korean environmental standard, 0.1 ppm, appears 500 m to 1.1 km in summer at a typical temperature 35°C.Whilst in winter, the direct contribution of the flue gas would be reduced up to 900 m.Thus the seasonal variation of the NO 2 concentration is significant depending on the position.Thus the NO x control must be stringently prepared particularly in cold winter as expanded quite further than for the 115°C discharge.
While it is very unusual in the test area, the cases of temperature inversion occur sometimes in spring with a great daily temperature range.In this work, one of the worst cases has been reviewed with a variation: T = T 0 + 10 × H and plotted in Fig. 11.An obvious tendency comparing to Fig. 9 was found at 40°C with 3 m/s wind.As lowering the temperature of the flue gas, the NO 2 concentration may exceed the guideline to a certain extent with unexpected occurrence of temperature inversion.Fast wind facilitates horizontal migration of the discharged gas to some distance as being oppressed by high density air body.

CONCLUSIONS
Potential effects of heat recovery from the flue gas on the surrounding air quality were investigated by simulating the gas dispersion using a CFD technique in terms of humidity and NO x conversion.Decreased temperature may reduce  the fluid buoyancy which acts in part as a driving force, and thereby decrease the emission rate at the end of the stack.It finally may deteriorate the discharging gas dispersion.The increase of local humidity due to decreased temperature of flue gas was more than 10% depending on the weather condition.In particular, high humidity might form a large visible plume causing regional nuisances particularly in winter.The NO 2 converted from the stack NO was dispersed up to 900 m from the emission source with the basis of 0.1 ppm, which stretched 200 m more in case of 40°C than 115°C.It was convinced that the lower flue gas temperature could have direct impacts on the residents at ground level.In addition, it was found that gas dispersion with subsequent dilution becomes worse when temperature inversion occurs.Therefore, any additional methods making up deterioration of air quality due to extreme recovery of thermal energy must be prepared prior to the incorporation of a high-efficiency heat exchanger.
(a).A hill approximately 41 m high is situated on the upstream side of the stack, and a subway station is 450 m away to the southeast.A model domain 2000 m by 1000 m in plane and 300 m in height was made.

Fig. 1 .
Fig. 1.Geography and wind condition of the test power station site: (a) local map; (b) annual wind rose of Ilsan city in the past three years.

Fig. 8 .
Fig. 8. NO x concentration along the center line of the stack without wind.

Fig. 9 .
Fig. 9. NO 2 concentration of the ground level with flue gas temperature and wind speed (air temperature: 20°C).

Fig. 11 .
Fig. 11.NO 2 concentration curves on the ground level in case of temperature inversion.

Table 1 .
The basic components of the CFD model.

Table 3 .
Average temperature of test area in 2012.

Table 4 .
Simulation parameters and values.

Table 5 .
Parametric variation at the inlet and outlet of the stack.
Parameter T in /°C T out /°C V in /m•s -1 V out /m•s -1 P in /kPa P out /kPa RH in /% RH out /%