22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Quantifying the effects of inlet fine water droplet injection upon non-thermal
plasma treatment of exhausts
V. Gogulancea and V. Lavric
Chemical and Biochemical Engineering Department, University Politehnica of Bucharest, Bucharest, Romania
Abstract: The electron beam treatment of exhausts is a promising technology for the
simultaneous removal of sulfur dioxide, nitrogen oxides and several volatile organic
compounds. This paper presents the modelling strategy used to simulate sulfur dioxide and
nitrogen oxides’ removal process when fine water droplets are sprayed inside the irradiation
chamber.
Keywords:
absorption
electron beam, fine water droplets, mathematical modelling, gas-liquid
1. Introduction
Non thermal plasma depollution methods have been
developed successfully for the treatment of both gaseous
and liquid effluents. Treatment facilities based on electron
beam irradiation have been devised in Poland and China
for the industrial remediation of flue gases, while several
pilot plants are developing electrical discharge plasma
technologies for environmental applications throughout
Europe, Asia and the US [1].
The mechanism of non thermal plasma depollution is
based on the interactions between the pollutant molecules
and the reactive oxygen (O∙, ∙OH, HOO∙, O 3 ) and
nitrogen (N∙, ∙NH, ∙NH 2 ) species generated in the ionized
gas. The removal of sulfur dioxide and nitrogen oxides
occurs via oxidation and reduction reactions occurring at
atmospheric pressure and moderate temperatures with
efficiencies similar to those obtained using more
conventional treatment methods.
Despite proven efficiency and reliability, the plasma
depollution techniques are slow to make their entrance at
the industrial level, mainly due to the perceived high
energy consumption of the process. One of the several
methods projected for lowering both the investment and
operation costs of the process is the addition of fine water
droplets (FWD) into the irradiation chamber, proposed
originally by [2] and, more recently, by [3].
The fine water droplets are increasing the density of the
irradiation medium thus lowering the penetration depth of
the accelerated electrons. This strategy minimizes the
energy loss of the process and allows the use of high and
medium energy electron accelerators, without the risk of
irradiation contamination.
Moreover, the fine water droplet addition intensifies the
heterogeneous chemical phenomena occurring in non
thermal plasma, leading to a better removal of sulfur and
nitrogen oxides. The sulfuric acid, generated due to the
radiolysis phenomena, forms aerosol particles that grow
as a result of the absorption of other gas phase
components. The absorption of SO 2 and NO x as well as
other components from the gas phase leads to the
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pollutants being further oxidized or reduced to less
noxious compounds [4].
Nevertheless, experimental evidence, backed by
modelling data, shows that the gas to liquid ratio
occurring inside the irradiation chamber is as low as 10-610-5, having a relatively low impact on the pollutants’
removal efficiency inside the reactor. The experimental
works [2, 3] have shown that liquid ratios as low as 10-4
will significantly intensify these heterogeneous processes.
The
following
sections
will
present
the
phenomenological and numerical approaches used to
model the chemico-physical phenomena occurring during
the electron beam irradiation of a simulated flue gas
injected with fine water droplets.
2. Mathematical model
The model predictions will be presented along the
experimental findings of [3], using a flue gas with the
composition presented in [5] under the treatment
conditions of [3], summarized in Table 1.
Table 1. Treatment conditions and gas composition [3, 5]
Without FWD
With
FWD
Gas flow rate (L/h)
5200
5200
Absorbed EB power (W)
9
82.6
Gas to liquid ratio
~10-6
0.0096
Medium density (g/L)
1.06
10.64
SO 2 initial gas phase concentration
(ppmv)
812
NO initial gas phase concentration
(ppmv)
44
O 2 (volume %)
8.4
CO 2 (volume %)
9.3
H 2 O (volume %)
11.2
Even though complex kinetic systems of over 1200
reactions have been proposed for the interactions
occurring in the gas phase, we are currently employing a
reduced system. The system is only taking into account 90
1
chemical reactions and 40 chemical species, listed in [4],
and can mirror experimental results with sufficient
accuracy and reduced computational effort. For the
interactions between the accelerated electrons and the gas
components we have employed the radiochemical yields
presented by [6]. As a result, the mass balances for the
gas phase chemical species take the form of equation [1],
considering the irradiation chamber to be an ideal plug
flow reactor.
The modelling strategy for the sulfuric acid nucleation
is presented in [4], as well as the kinetic system for the
liquid phase reactions. The liquid kinetic system used in
this work was improved using the chemical reactions set
proposed by [7] to account for the NO and NO 2
transformations.
In our previous paper [4] we hypothesized that due to
the small dimensions of the droplets and the time scale of
the processes, thermodynamic equilibrium is attained
instantaneously between the gas and liquid phases,
neglecting the mass transfer resistances of the gas phase.
Another simplifying hypothesis was considering the
droplets identical to each other, which enabled the
treatment of the liquid phase as a continuous medium. As
a result, Henry’s law can be applied to evaluate the
absorption of the chemical species of interest into the
droplets. However, due to the high concentration of
sulfuric acid present in the droplets (~4 mol/L), the
partition coefficients proposed in the literature would not
hold and the use of solubility coefficients was preferred.
By spraying small water droplets (with a diameter
smaller than 10 µm) into the reactor, a second dispersed
liquid phase is formed, readily available for the
absorption of the soluble chemical species. The
hypotheses presented above were enforced in the case of
this second liquid phase, neglecting the interactions
between the sprayed and the condensed liquid droplets. In
the case of the fine water droplets, the objection posed by
the sulfuric acid concentration is voided, enabling the
direct use of Henry’s law.
The modelling results obtained neglecting the mass
transfer resistances showed that the amounts of ammonia
and sulfur dioxide absorbed in the sprayed solution were
unreasonably high. Re-examining the hypotheses led to
the dismissal of the instantaneous thermodynamic
equilibrium theory and the consideration of gas-phase
mass transfer resistances. Due to the small dimensions of
the droplets, the resistances on the liquid side were
neglected, assuming that the composition is constant at
any point inside the droplets.
The mass balance equations governing the
aforementioned phenomena can be deduced by writing
the flux equation for the mass transfer, equation [2] and
replacing in the general mass balance equation [3]. In this
way we obtain equation [5], by computing the partial
pressure at the interface using Henry’s law (equation [4])
for the liquid phase species. The resulting mass balances
must be written for both the spray and the condensed
droplets, switching from time to volume coordinates.
2
The gas phase mass transfer coefficients are calculated
using an approximation for Sherwood’s number (equation
[6]) valid for droplets with a diameter smaller than 100
μm. The diffusion coefficients for the species of interest
were taken from [8]. The resulting system of differential
equations is solved in Matlab, using an in-house written
routine.
Some of the absorbed species (SO 2 , NH 3 , HNO 3 ,
H 2 SO 4 ) undergo dissociation, transforming into their
corresponding ions (SO 3 2-, SO 4 2-, NH 4 +, etc.), which in
turn take part in the chemical reactions. The dissociation
is modelled by writing the mass and charge balances
using the dissociation constants proposed in literature. As
the dissociation is an equilibrium process, the resulting
system of algebraic equations is solved at every
integration step using Matlab’s fminimax solver for nonlinear equation systems.
3. Results and discussion
The results of the simulation in the case of no fine water
droplet injection show a 70.9% removal efficiency for the
NO x and a 93.6% SO 2 removal efficiency, compared to a
78% experimental efficiency for NO x and 85% for SO 2 ,
respectively.
The agreement between the experimental and modelling
values is satisfactorily high; the pollutants’ concentration
profiles are presented in Fig. 1, 2 and 3. Despite the low
concentration of NO, its removal efficiency is relatively
low as a result of the relatively low irradiation dosage
applied (~6 kGy). The NO concentration profile is quite
linear for the no fine water droplet case, when the NO 2
concentration is increasing steadily during the irradiation
treatment. However, its concentration is one order of
magnitude smaller than the initial NO concentration,
posing no serious environmental issues.
Fig. 1. Variation of NO gas phase concentration along the
reactor.
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Fig. 2. Variation of NO 2 gas phase concentration along
the reactor.
In contrast, the removal efficiency for SO 2 has a
reasonably high value despite the large initial
concentration, as both the ammonia and humidity content
are suited for SO 2 removal. In addition to this, the
residence time for the flue gases is higher than 64 s,
allowing the molecular reactions between SO 2 and NH 3
to have a larger impact on the overall efficiency. The
concentration profile of SO 2 exhibits an asymptotic
decrease, as a result of the overlapping contributions of
the radiation and thermal removal pathways.
For the second simulation, when fine water droplets are
sprayed into the reactor, the removal of NO greatly
increases, reaching a value of 99.1%. This is in good
agreement with the experimental observations which list
the removal efficiency at 98%. The profile for the NO
concentration shows a much faster removal than in the
dry case, with virtually all the pollutant being removed
after three quarters of the reactor. The profile shows also
a less linear dependency, as the absorption and chemical
reaction rates contribute to the removal.
In the case of NO 2 the profile is even more explicit:
after half of the reactor, the rate with which the NO 2 is
generated becomes smaller than the absorption rate and
the rates of the chemical reactions consuming it in the
liquid phase. As a result, the gas phase concentration
gradually drops after reaching its maximum, mirroring the
NO behaviour and showing an almost complete removal
of NO 2 .
In the case of SO 2 the model results show a better
efficiency than in the gas-only experiment, of 97.4%,
similarly slightly over-estimated compared to the
experimental value of 94%. The profile in this case is
significantly different from the dry experiment, showing
that the SO 2 is almost entirely absorbed in the liquid
droplets after less than a tenth of the reactor. Its gas phase
concentration still registers a slight decrease after the first
few meters of the irradiation chamber, mostly as a result
of the radiation induced pathway, as seen in Fig. 3.
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Fig. 3. Variation of SO 2 gas phase concentration along
the reactor.
The effect of the fine water droplet addition becomes
apparent in the case of ammonia as well (Fig. 4). In the
case of no water droplets, its concentration registers a
steeper decrease at first, decrease that becomes more
linear as the sulphur dioxide is removed from the gas
phase. However, with the addition of water droplets,
ammonia is entirely removed from the gas phase,
similarly to SO 2 , after less of a tenth of the irradiation
chamber. This behaviour is mirrored by its liquid phase
concentration, two orders of magnitude smaller than the
gas phase value, which increases abruptly and then
linearly decreases as ammonia undergoes dissociation.
The same performance is registered in the case of SO 2 ,
depicted in Fig.5, which dissociates to form the bisulfite
and sulphite anions, oxidized in the liquid phase to
hexavalent sulfur ionic compounds.
Fig. 4. Variation of NH 3 gas phase concentration along
the reactor.
..
.
3
5. Acknowledgements
This work is supported by the Sectoral Operational
Programme Human Resources Development (SOP HRD),
financed from the European Social Fund and the
Romanian
Government
under
the
contract
number POSDRU/159/1.5/S/137390.
The authors would like to thank Professor Ioan
Calinescu for the invaluable discussions and fruitful
exchange of ideas regarding the effects of fine water
droplet addition.
Fig. 5. Variation of NH 3 and SO 2 liquid phase
concentrations along the reactor.
Both nitrogen oxides are absorbed into the liquid phase,
where they are consumed in oxidizing reactions, as the
absorption rate is slower than that of the chemical
phenomena (Fig. 6).
6. References
[1] H. H. Kim, Plasma Processes and Polymers, 1, 2
(2004).
[2] B. V. Potapkin, M. A. Deminsky, A. A. Friedman, V.
D. Rosanov, Radiation Physics and Chemistry, 45, 6
(1995).
[3] I. Calinescu, D. Martin, A. G. Chmielewski, D.
Ighigeanu, Radiation Physics and Chemistry, 85, (2013).
[4] V. Gogulancea, V. Lavric, Plasma Chemistry and
Plasma Processing, 35, 1 (2015).
[5] I. Calinescu, D. Martin, D. Ighigeanu, A. M. Bulearca,
Revista de Chimie, 63, 6 (2012).
[6] H. Matzing, Advances in Chemical Physics, 80, 1
(1991)
[7] S. N. Pandis, J. H. Seinfeld, Journal of Geophysical
Research, 94, (1989).
[8]
http://www.nist.gov/data/PDFfiles/jpcrd1.pdf,
accessed on 25th January 2015.
7. Equations
Fig. 6.
Variation of NO and NO 2 liquid phase
concentrations along the reactor.
n
dNG i
= DG* ⋅ ρ ⋅ Gi ⋅ X i − ∑ ωijn j
(1 − f L )dV
j =1
[1]
4. Conclusions
The mathematical model proposed in this work aims to
Where N Gi is the molecular flux of the species i in the
provide a better insight into the chemico-physical
gas phase, f L is the liquid fraction, D G * is the dose rate, ρ
phenomena occurring as fine water droplets are injected
is the density of the gas phase, X i the molar fraction of
into the irradiation chamber during electron beam flue gas
component i and the sum represents the product of the
treatment.
stoichiometric coefficients and the reaction rates in which
The modelling results are in good agreement with the
component i takes part.
experimental findings for the case of no water injection
for both sulfur and nitrogen oxides. The relative deviation
N A kG aS ( p AG − p Ai )
[2]
is 9.1% for the NO removal and 9.9% for the SO=
2,
respectively.
In the case of water droplets injection, the model Where N A is the molar flux of component A, k G is the gas
predictions are strong for both the NO and SO 2 removal phase mass transfer coefficient, a S is the area of the
efficiency showing a 1.6% and 3.6 %, respectively droplets, p AG and p Ai are the partial pressures for A in the
relative deviation. Further modelling and experimental gas phase and at the gas-liquid interface.
efforts are required to ensure a better understanding of the
n
heterogeneous pollutant removal pathways during the
dc AL
NA
[3]
=
−
ωijn j
∑
electron beam flue gas treatment. In addition to this,
dt
Vdroplet j =1
careful consideration must be paid to the costs associated
with the water spraying process and with the subsequent
Where c AL is the molar concentration of A in the liquid
separation of the reaction products.
phase, V droplet is the droplet volume
4
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p Ai = c Ai H A
[4]
Where c Ai is the molar concentration of A at the interface,
equal to the molar concentration of A in the liquid phase
and H A is Henry’s constant for species A.
dcAL
dt
kG aS ( p AG − cAL H A ) n
− ∑ ωijn j
Vdroplet
j =1
[5]
Where k G is the mass transfer coefficient, a S is the area of
one droplet and V droplet is the droplet’s volume.
Sh
=
k AG d
= 2
D A , air
[6]
Where Sh is Sherwood’s number, d is the droplet diameter
and D A,air is the diffusion coefficient for species A in air.
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5