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Journal of Physics: Conference Series
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Technology of desorption of dissolved oxygen from water by boiler
exhaust gases
To cite this article: M M Zamaleev et al 2020 J. Phys.: Conf. Ser. 1683 042062
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The Third Conference "Problems of Thermal Physics and Power Engineering"
Journal of Physics: Conference Series 1683 (2020) 042062
IOP Publishing
doi:10.1088/1742-6596/1683/4/042062
1
Technology of desorption of dissolved oxygen from water by
boiler exhaust gases
M M Zamaleev1, R I Kamalova1 and O V Pazushkina1
1 Ulyanovsk State Technical University
Russia, 432027 U lyanovsk, Severny Venets, 32
Mansur_zamaleev@mail.ru
Abstract. A new technology of water degassing based on the use of boiler flue gases as a
desorbing agent is described. In the process of burning gaseous fuel, almost all oxygen is
consumed, which ensures its absence in the composition of exhaust gases. This makes it
possible to use the flue gases of a gas-tight boiler as a desorbent in the process of water
deaeration and to avoid using steam. The increasing concentration of carbon dioxide in
deaerated water is proposed to be reduced by metering an alkaline agent. The article presents
the results of evaluating the mass transfer efficiency of water degassing by the proposed
method, as well as the results of calculating the theoretically required specific flow rate of
exhaust gases for deaeration.
1. Water degassing technology with boiler exhaust gases
The process of thermal deaeration of the heating system make-up water and the additional boiler feed
water is a complex synthesis of the simultaneous flow of two components - heat transfer and mass
transfer. Heat transfer is the process of heating deaerated water to saturation temperature. Mass
transfer is provided by the release of corrosive gases from the deaerated water into the vapor medium.
At the same time, water vapor is considered a traditional type of desorbing agent and at the moment is
the only widely used medium that provides the process of degassing the additional feed water of
boilers and the make-up water of the heating system.
The need for additional steam costs for deaeration, additional heat losses with vapor, as well as
steam and condensate losses during the preparation of the make-up water of the heating network in
atmospheric deaerators are inevitable disadvantages of the thermal deaeration method due to steam. In
addition, in many medium and small cities of Russia, from 30 to 100% of heat energy for heat supply
is generated by boiler houses [1,2]. For boiler rooms with steam generators in the installed equipment,
the problem of thermal deaeration is solved quite simply. However, most hot water boiler houses are
forced to apply anti-corrosion treatment of water without thermal deaeration due to the lack of steam.
Thus, in order to ensure the required quality of the heating system make-up water, as well as to
simplify the deaeration technology itself in order to use it in hot water boilers, it is reasonable to
search for new energy efficient methods of water degassing [3].
In the research laboratory "Heat power systems and installations" of the Ulyanovsk State Technical
University (Research Laboratory "TESU" UlSTU), various technical and technological solutions are
being developed to improve the efficiency of the water deaeration process [4,5]. The use of
The Third Conference "Problems of Thermal Physics and Power Engineering"
Journal of Physics: Conference Series 1683 (2020) 042062
IOP Publishing
doi:10.1088/1742-6596/1683/4/042062
2
unconventional media as a desorbent in standard deaeration devices is one of the promising areas of
development in this area.
The authors proposed the use of a gas-tight boiler operating under pressurization and burning
natural gas as a desorbing agent of exhaust gases in a deaerator [6, 7].
The proposed technology makes it possible to increase the efficiency of the water degassing
process by eliminating the consumption of steam for deaeration, as well as the beneficial use of the
heat of the boiler exhaust gases during the deaeration process. The main advantage of the proposed
solution is the possibility of organizing the deaeration of the make-up water of the heating network at
hot water boilers without steam sources.
Note that the scope of the proposed technical solution is limited by the following conditions. The
highest efficiency can be achieved with non-stoichiometric combustion of natural gas [8], as well as in
the operation of a pressurized boiler, which avoids an increase in the oxygen content in flue gases due
to suction into the furnace and gas ducts.
2. Determination of the required flue gas flow rate to implement the proposed solution
The calculation is carried out on the basis of solving the balance equations of the processes of mass
transfer and heat transfer during thermal deaeration, provided that equilibrium between the phases is
achieved at the outlet of the deaerator [9].
For the calculation, the operating parameters of a typical boiler of the DKVR series and an
atmospheric deaerator of the DA-25 type are taken.
To assess the scope of application of the proposed solution, it is necessary to determine the values
of the theoretically required flow rate of the desorbing agent (boiler exhaust gases). It is assumed that
the maximum mass transfer and energy efficiency of the thermal deaerator is achieved at the lowest
possible consumption of the desorbing agent and the vapor removed from the deaerator.
The design scheme of the deaerator is shown in Figure 1.
Figure 1. Scheme of deaeration column of
countercurrent type: 1 - supply of source water; 2
- supply of exhaust gas; 3 - drainage of deaerated
water; 4 - deaerator outlet tap.
Let us determine the flow rate of flue gases to ensure the deaeration process with the flue gases of
the boiler, for this we compose the heat balance equation:
ventdwgasexsw QQQQ .
(1)
The heat balance equation for deaeration can be written as
The Third Conference "Problems of Thermal Physics and Power Engineering"
Journal of Physics: Conference Series 1683 (2020) 042062
IOP Publishing
doi:10.1088/1742-6596/1683/4/042062
3
ventventdwdwgasexgasexswsw hDhGhDhG ..
(2)
where
sw
G
and
dw
G
- the amount of initial and deaerated water, kg/h;
gasex
D.
- consumption of the
desorbing agent supplied to the deaerator, kg/h;
vent
D
- deaerator vapor consumption (mixture of
corrosive gases and flue gases released from water), kg / h;
sw
h
,
dw
h
are the enthalpy of water at the
entrance to the deaerator and at the exit from it;
gasex
h.
,
vent
h
are the enthalpy of flue gases at the
entrance to the deaerator and the enthalpy of vapor at the exit from the deaerator.
The flue gas flow rate is determined by the expression
gasex
swswventventdwdw
gasex h
hGhDhG
D.
.
(3)
The calculations show that to ensure the deaeration process of 25 t/h of water, the required flue gas
flow rate will be 71.5 m3/h.
3. Determination of the residual oxygen content in deaerated water
The material balance equation for deaeration for determining the oxygen concentration can be written
as
22
22 .. О
vent
vent
О
dw
dw
Оgasexgasex
О
swsw YDXGYDXG
(4)
where
sw
G
and
dw
G
- the amount of initial and deaerated water, kg / h;
gasex
D.
- consumption of
flue gases supplied to the deaerator, kg / h;
vent
D
- deaerator vapor consumption (mixture of corrosive
gases and flue gases released from water), kg / h;
2
О
sw
X
,
2
О
dw
X
- oxygen concentration in water at the
inlet to the deaerator and at the outlet from it;
2
.
Оgasex
Y
,
2
О
vent
Y
are the oxygen content in flue gases at the
inlet to the deaerator and in the vapor at the outlet from the deaerator.
Let us also express through the concentration of gas in water. According to Dalton's law, the total
pressure of a gas or vapor-gas mixture is equal to the sum of the partial pressures of gases and vapors
that make up the mixture. It follows from Henry's law that the concentration of a gas dissolved in
water is proportional to the partial pressure of this gas above the surface of the water.
The oxygen concentration in the flue gases at the inlet to the deaerator
2
.
Оgasex
Y
will be expressed:
pXКYО
dw
О
H
Оgasex /
22
2
.
(5)
where
2
О
H
К
is Henry's coefficient (constant of phase equilibrium for oxygen [10]), Pa; p - pressure
in the deaerator, Pa.
The oxygen concentration in the vapor leaving the deaerator depends on the flow pattern of water
and steam in the apparatus. With countercurrent flow, the molar fraction of O2 in the vapor-gas
mixture
2
О
vent
Y
is
pXКYО
sw
О
H
О
vent /
2
2
1
2
(6)
where
2
1
О
H
К
is Henry's coefficient (constant of phase equilibrium for oxygen), Pa; p - pressure in
the deaerator, Pa.
The material balance equation will take the form:
The Third Conference "Problems of Thermal Physics and Power Engineering"
Journal of Physics: Conference Series 1683 (2020) 042062
IOP Publishing
doi:10.1088/1742-6596/1683/4/042062
4
pXКDXGpXКDXG О
sw
О
H
vent
О
dw
dw
О
dw
О
H
gasex
О
swsw // 2
2
1
222
2.
(7)
Substituting the obtained values into the equation, we find the oxygen concentration in deaerated
water
3
.
mg/dm5.48
/
/
2
2
1
2
2
dw
О
H
gasex
sw
О
H
vent
О
sw
О
dw GpКD
GpКDX
X
(8)
In accordance with the PTE [11], the permissible oxygen content in the make-up water is 50
mg/dm3; therefore, the use of boiler flue gases as a desorbing agent in atmospheric deaerators makes it
possible to ensure the standard value of oxygen.
If the boiler flue gases are used as a desorbing agent, according to the above calculations, the
theoretically required gas consumption for deaeration will be 2,86 m3 per 1 ton of deaerated water.
The development of the proposed method for water degassing also includes solving the problem of
excess carbon dioxide content in flue gases and its negative impact on the quality of deaerated water.
To neutralize carbon dioxide in deaerated water, it is proposed to dose an alkaline reagent, for
example, sodium hydroxide, into the feed water pipeline.
Binding of carbon dioxide contained in feed water by alkali occurs in the form of a reaction:
СО2 + NaOH = NaHCO3 (9)
Thus, the introduction of alkali into deaerated water eliminates the presence of carbon dioxide in
the make-up water.
Conclusions
A new technology of deaeration of the heating system make-up water using the boiler
exhaust gases as a desorbing agent is proposed.
The theoretically required flow rate of the boiler flue gases for deaeration of the make-up
water was calculated, the residual oxygen concentration in the deaerated water was
estimated.
The performed calculations prove the applicability of the new technology for deaeration of
the make-up water of the heating network using the boiler exhaust gases as a desorbing
agent.
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The Third Conference "Problems of Thermal Physics and Power Engineering"
Journal of Physics: Conference Series 1683 (2020) 042062
IOP Publishing
doi:10.1088/1742-6596/1683/4/042062
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