Influence of CO2 inleakage on the slight-alkalization of generator internal cooling water

Tianping Wang,Xuxiang Jia,Yu Wang,Chunsong Ye*

Department of Water Quality Engineering,School of Power and Mechanical Engineering,Wuhan University,Wuhan 430072,China

Keywords:Generator internal cooling water (GICW)Hollow copper conductor CO2 inleakage Slight-alkalization Temperature calibration

ABSTRACT The slight-alkalization of generator internal cooling water(GICW)is widely used to inhibit the corrosion of hollow copper conductor and thereby ensure the safe operation of the generator.CO2 inleakage is increasingly identified as a potential security risk for GICW system.In this paper,the influence of CO2 inleakage on the slight-alkalization of GICW was theoretically discussed.Based on the equilibriums of the CO2-NaOH-H2O system,CO2 inleakage saturation was derived to quantify the amount of the dissolved CO2 in GICW.This parameter can be directly calculated with the measured conductivity and the[Na+]of GICW.The influence of CO2 inleakage on the slight-alkalization conditioning of GICW and the measurement of its water quality parameters were then analyzed.The more severe the inleakage,the narrower the water quality operation ranges of GICW,resulting in the more difficult the slight-alkalization conditioning of GICW.The temperature calibrations of the conductivity and the pH value of GICW show nonlinear correlations with the amount of CO2 inleakage and the NaOH dosage.This study provides insights into the influence of CO2 inleakage on the slight-alkalization of GICW,which can serve as the theoretical basis for the actual slight-alkalization when CO2 inleakage occurs.

1.Introduction

Generator internal cooling water(GICW)system is widely used to removal the waste heat of copper conductor that produced during electricity generation both in thermal and nuclear power plant.A primary concern of this system is the corrosion of hollow copper conductor by the cooling water.The corrosion product can result in the flow restriction or even the plugging of the hollow copper conductor,and further frequent and expensive maintenance[1].Thus,the copper corrosion behavior and its inhibition methods in the cooling water have received considerable critical attention [2-8].Since the generator runs at a strong current and magnetic field,the corrosion inhibition of copper can only be achieved by conditioning the water quality of GICW.Among which,GICW slightalkalization is one of the most effective way.In this method,the pH value of GICW is conditioned to be between 8.0 and 9.0 (25°C) by adding trace amounts of the alkalization agent [6].In the alkaline aqueous media,a copper oxide passive film can be formed on the copper surface and inhibit the further corrosion,as confirmed both in thermodynamics and kinetics [9-12].

CO2inleakage is increasingly identified as the most common cause for the potential failure of GICW system.The inleakage may occur in many ways,such as the air ingress through the leaks,the supply of make-up water and so on [5,13,14].If the inleakage has not been well handled,GICW system may suffer the following potential troubles.Firstly,the dissolved CO2would damage the copper oxide passive film and further invalid its corrosion inhibition performance.Worse yet,the damaged film cannot be able to be completely repaired even if GICW is re-conditioned to be slightly alkaline [15].Secondly,CO2inleakage makes the slightalkalization of GICW to be more challengeable.To date,no guideline has been established for GICW treatment when CO2inleakage occurs.During the inleakage,the water quality of GICW can only be conditioned based on the historical experience in the field.However,the experience-based conditioning is somewhat unreliable,which would take a long time to condition the water quality of GICW to be in the operation ranges and account for the frequent fluctuation of that.The unstable water chemistry can result in dissolution,re-deposition and local accumulation of the copper oxides,which increases the risk of the flow drop of GICW system and the plugging of the hollow copper conductor [5,16,17].Moreover,CO2inleakage sometimes cannot be eliminated at once,for example,the repair of the leaks can only be conducted during the generator shut-down period.

Despite the possible damage of CO2inleakage on GICW system,the monitor of CO2inleakage in the field is once ignored due to the lack of a suitable characterization method.Recently,the failure cases of GICW system caused by CO2inleakage have aroused the urgent need to establish such a method[1,5,13,14].In fact,the conductivity and the pH value of GICW,which are daily monitored,can reflect CO2inleakage to a certain extent [6].Unfortunately,these two parameters are mainly used to indicate and guide the slightalkalization of GICW,which cannot quantify the specific amount of CO2inleakage.Though a few attempt had been made to characterize the CO2inleakage [14],a practical and widely accepted method has not been well established.

The timely slight-alkalization of GICW can effectively alleviate the impact of CO2leakage on the water quality of GICW.Thus,it is of great significance to reveal the influence of CO2inleakage on the slight-alkalization conditioning.Specifically,two key issues should be figured out if CO2inleakage exists: (i) the water quality correlations of GICW and (ii) the operation ranges of the water quality parameters.The water quality correlations of the highpurity water with the dissolved CO2at 25°C had been theoretically documented [4,18].Nevertheless,the correlations between the conductivity,the pH value,the amount of the dissolved CO2and the dosage of the alkalization agent of GICW have not been well profiled at the operation temperature of GICW.During the steadily operation,the water quality of GICW should be conditioned to be in the operation ranges that specified in current standards [6,19].However,few studies have attempted to analysis the influence of CO2inleakage on the operation ranges of the water quality parameters of GICW.

The conductivity and the pH value of GICW are the two most important parameters.They are highly temperature-dependent.To evaluate the operating status of GICW system,the measured conductivity and the pH value should be calibrated to the benchmark temperature of 25 °C [6,19].Adopting the appropriate temperature calibration methods for the conductivity and the pH value of GICW are vitally important,otherwise it may affect the accurate measurement of them and even misjudge the operating status of GICW system.For the conductivity of GICW,the constant temperature calibration coefficient,2%·°C-1,is widely accepted in the field[20].As for the pH value of GICW,the temperature calibration is achieved by adding a calibrated constant [6].However,the temperature calibration for both the conductivity and the pH value of the low-conductivity aqueous solution largely depends on its solutes [21,22].If CO2inleakage happens in GICW system,the solutes in GICW will be more complex and their concentration will also vary with the degree of CO2inleakage.Thus,CO2inleakage would affect the temperature calibrations for the conductivity and the pH value of GICW.Surprisingly,a systematic understanding on the effect is still lacking.

The purposes of this study were to(a)find out a simple parameter to characterize the amount of CO2inleakage;(b) analysis the influence of CO2inleakage on the slight-alkalization of GICW,including the water quality correlations and the operation ranges of the alkalization agent dosage,the conductivity and the pH value;(c)demonstrate the influence of CO2inleakage on the temperature calibrations of the conductivity and the pH value of GICW,and furtherly evaluate the applicability of the current temperature calibration methods for these two parameters.

2.CO2-NaOH-H2O Equilibrium System

GICW is essentially CO2-NaOH-H2O equilibrium system at the operation temperature in the presence of CO2inleakage.The slight-alkalization of GICW is achieved by dosing trace amounts of the alkalization agent such as NaOH into GICW,which is the most commonly used in the field [6].When CO2inleakage occurs,the gaseous CO2would dissolve into the slightly alkaline GICW.During the steadily operation of GICW system,the water temperature of GICW in the tank is typically 50°C[18].Thus,GICW can be regarded as the CO2-NaOH-H2O system that equilibrated at 50 °C.To reveal the water quality correlations of GICW,the equilibriums of this system are analyzed as the following.

In order to evaluate the degree of CO2inleakage,the limit condition where CO2reaches the dissolution equilibrium in pure water at 50 °C is selected as the benchmark and the total carbonate (CT)in the solution is used to quantify the specific amount of the dissolved CO2.Therefore,it is firstly necessary to analyze the dissolution equilibrium of CO2in pure water and calculate theCTunder this limit condition.

In the pure water dissolved with CO2,CTcan be defined as

whereKWof water is 13.27 at 50 °C [23].

Combined Eqs.(1)-(5),it can be iteratively calculated that the pH value of the solution is 5.76 and theCTof the solution is 7.63 μmol·L-1when CO2reaches the dissolution equilibrium in pure water at 50 °C.

When CO2inleakage occurs,the slightly alkaline GICW can be regarded as CO2-NaOH-H2O system.In this system,the ions in the solution include H+,Na+,OH-,According to the dissociation equilibriums of water and carbonic acid,and the electro-neutrality of the solution,there are actually only two independent variables among these five ions.

In the solution,the electro-neutrality principle can be described as

TheCTof the solution can be separated from Eq.(7).

Eq.(8) confirms that theCTcan be directly calculated if the pH value and the[Na+]of the solution is known.However,GICW is the aqueous solution with low conductivity,low[H+]and poor buffering capacity,therefore its pH value is really difficult to be measured accurately [22].Thus,the conductivity of GICW,which can be easily and accurately measured,is chosen as the alternative variable.In order to ensure the insulation property toward the ground of the generator,the conductivity of GICW should be conditioned to be not more than 2.0 μS·cm-1(25°C)[6].For the aqueous solution with that conductivity,GICW can be regarded as the infinitely diluted solution,whose conductivity can be calculated by Kohlrausch’s law.Thus,the conductivity of GICW is the sum of the product of the ionic conductivity at infinite dilution of each ion and its corresponding molar concentration,which can be expressed as

where the ionic conductivity at infinite dilution of the ions at 50°C are listed in Table 1.

Before the slight-alkalization,the GICW is the pure water with dissolved CO2.For this acidic solution,[] can be ignored[24].Thus,combined Eq.(4),Eq.(9) and Table 1 [21,23],the conductivity of GICW before the slight-alkalization can be expressed as Eq.(10).

Combined Eq.(2),Eq.(3),Eq.(7),Eq.(9)and Table 1[21,23],the conductivity of GICW can be expressed as Eq.(11) taking the pH value and theCTas variables.

Combined Eq.(2),Eq.(3),Eq.(8),Eq.(9)and Table 1[21,23],the conductivity of GICW can be expressed as Eq.(12) taking the pH value and the [Na+] as variables.

Table 1 Ionic conductivity at infinite dilution of ions at 50 °C [21,23]

3.Characterization of CO2 Inleakage Amount

The amount of CO2inleakage can be calculated by the correlation analysis of GICW water quality.

Combined Eq.(8) and Eq.(12),there are only two independent variables among the conductivity,the pH value,the [Na+] concentration and theCT.Thus,theCTof GICW can be directly calculated with the known conductivity and the[Na+].To intuitively evaluate the degree of CO2inleakage,the percentage ofCT/CT,eq,which is denominated as CO2inleakage saturation,is used to quantify the amount of CO2inleakage.In this specific situation,theCT,eqat 50°C is 7.63 μmol·L-1as calculated in chapter 2.

Fig.1 shows the correlation surface of CO2inleakage saturation,the conductivity and the [Na+] of GICW.Fig.1 demonstrates that CO2inleakage saturation can be calculated if the conductivity and the [Na+] of GICW are known.Moreover,the contour map of the conductivity in Fig.1 also shows that the conductivity of GICW varies with the [Na+] and CO2inleakage saturation.These results prove that the dissolved CO2would affect the slight-alkalization conditioning of GICW.The detail deserves further discussion and will be presented in the following chapter.

The amount of CO2inleakage evaluated by GICW water quality correlation analysis is the most convenient way compared with other standard test methods.Firstly,a standard method was developed to measure theCTin boiler water and cooling water [25].In this standard method,the water sample is alkalized by NaOH solution to convert the total carbonate into the form of,and then SrCl2solution is added to precipitate the,and finally the precipitate is quantified by titration with HCl solution.However,this method requires the well-sealed devices for the cooling water sampling and theCTmeasurement,which are a little complex.Secondly,the dissolved CO2can be calculated by the conductivities of aqueous solution for the water sample of high purity [26]: acidize the water sample by strong-acid RH ion exchange resin to convert the total carbonate into the form of CO2(aq),and then degas the water to removal the CO2.The cation conductivity and the degassed cation conductivity of the water sample are on-line measured.A mathematic model was established between the difference value of these two conductivities and theCT[14].However,this method needs a set of elaborate on-line measurement device including a strong-acid RH resin column,two conductivity meters and an effective degas unit.The installation of this device would complex GICW water quality monitoring system.Thirdly,infrared adsorption method is also widely used to quantify the concentration of CO2.Typically,the water sample is acidized first,and then is degassed to blow out all the gaseous CO2.The gaseous CO2is dried and then quantified by infrared ray [27].Similar with the above two methods,the weaknesses of this method are that GICW water should be sampled off-line under well-sealed condition and a specified measurement device which is always unavailable in the field is required.In our method,theCTof GICW is calculated according to the inner correlations of GICW water quality.With the measured conductivity and the [Na+] of GICW,CO2inleakage saturation can be directly calculated.Among which,the conductivity of GICW is the parameter that should be daily monitored and is on-line measured[27].As for the[Na+]of GICW,it should be noted that this parameter is still not in the supervision list in current operation standards.However,the failure cases of GICW system proved that the additional measurement of the [Na+] can better interpret GICW water quality [4,28].Moreover,the measurement of the[Na+]is also of highly feasibility since this parameter is monitored as well in the main water-steam system of power plant and the measurement condition is available in the field[25,29].Thus,it is highly recommended to periodically measure the[Na+]of GICW.

During the operation of GICW system,CO2inleakage saturation can be used to(i)monitor the water quality of GICW more compre-hensively.CO2inleakage saturation intuitively characterizes the amount of leaked CO2in GICW.This parameter gives a better evaluation of the water quality of GICW;(ii) diagnose the abnormal state of GICW system.CO2inleakage saturation can be used to judge whether there is a inleakage in GICW system and furtherly find out the inleakage point.In a case study,researchers sampled GICW from different parts of the system and measured the amount of CO2in them.The interpretation of results pointed out the specific inleakage point and thus guided the maintenance of GICW system [14];(iii) make the slight-alkalization of GICW more predictable.The main target of the slight-alkalization of GICW is to condition and maintain the water quality to be in the operation ranges.Fig.1 shows that CO2inleakage would observably affect the slight-alkalization of GICW.If the amount of CO2inleakage can be quantified,the slight-alkalization of GICW can be more predictable and thus avoid the limitations of conventional experience-based conditioning.

Fig.1.Correlation surface of the CO2 inleakage saturation,the conductivity and the[Na+] of GICW.

4.Influence of CO2 Inleakage on the Slight-Alkalization Conditioning

Fig.2 displays the correlation curves of the water quality of GICW,including the conductivity,the pH value,theCTand the[Na+].

Fig.2.Correlation curves of GICW water quality.

By referring to Eq.(10),the black dotted curve shows the correlation of the conductivity and the pH value of the solution while CO2in the air dissolving into pure water at 50°C.The curve shows that the solution is gradually transformed from the neutral pure water into the slight-acidic and low-conductivity solution.The pH value is 5.76 and the conductivity is 0.940 μS·cm-1when the CO2reaches dissolution equilibrium in pure water at 50 °C.It has been calculated that the pH value is 5.66 and the conductivity is 0.863 μS·cm-1when the equilibrium reaches at 25 °C [18].The pH value and the conductivity seem do not change markedly at these two temperature.Though the dissociation constant of water and carbonic acid as well as the ionic conductivity of ions increase with the increasing of the water temperature,the solubility of CO2in pure water decreases.The Henry constant of CO2dissolving in pure water at 50°C is 3.49×10-3MPa-1,while the Henry constant is 6.06×10-3MPa-1at 25°C.It means that the solubility of CO2in pure water at 50 °C is only 57.6% of that at 25 °C.

By referring to Eq.(11),the red solid curves show the correlations of the conductivity and the pH value of GICW with the knownCT.With the increasing of the pH value,the conductivity increases in the absence of CO2inleakage,whereas the conductivity shows a trend of first decreasing and then increasing when CO2inleakage occurs.The results indicate that the dissolved CO2divides the actual slight-alkalization of GICW into two stages.In the first stage,the acid-alkali neutralization between the dissolved CO2and the added NaOH would react immediately.H+,the ionic conductivity of which is the biggest among all the ions,would be consumed gradually.That’s why the conductivity drops at the beginning of NaOH dosage.In the second stage,GICW slight-alkalizing is then achieved by the equilibrium between the subsequent added NaOH and the existed carbonate.Moreover,the greater CO2inleakage saturation,the longer the stage where the conductivity decreases.This phenomenon inspires to indicate the sign of CO2inleakage by monitoring the conductivity during the slight-alkalization of GICW.

By referring to Eq.(12),the blue solid curves present the correlations of the conductivity and the pH value of GICW with the known NaOH dosage.The intersections of the blue curves and the red curves are the water quality conditions that can be achieved by dosing the corresponding amount of NaOH under the corresponding CO2inleakage saturation.The distribution grids of the intersections show that the more the amount of the dissolved CO2,the more NaOH would be consumed in the stage of acid-alkali neutralization,and the lower pH value of GICW can be achieved with the same NaOH dosage.

Fig.2 can serve as a guidance for the actual slight-alkalization of GICW.Previously,the slight-alkalization depends more on the historical experience in the presence of CO2inleakage.Lacking the insights into the CO2inleakage influence on the slightalkalization,the experience-based slight-alkalization makes it difficult to accurately and timely condition the water quality of GICW to be in the operation ranges.Furthermore,it has been confirmed that the unstable water chemistry of GICW can result in dissolution,re-deposition and local accumulation of the copper oxides,and furtherly can account for the partial or even full plugging of the hollow cooper conductor [5,16,17].Thus,a reliable guidance for the slight-alkalization is urgently needed to maintain the water quality of GICW within the required ranges,especially under the condition of CO2inleakage.Fig.2 can provide a theoretical basis for the actual slight-alkalization of GICW.As represented in Fig.2,if the initial and target water quality conditions of GICW are known,the required NaOH dosage can be directly calculated.By referring to Fig.2,the slight-alkalization can be more predictable and the target water quality can be reached more timely.

The above discussion focuses more on the influence of CO2inleakage on GICW water quality during the slight-alkalization.In fact,the operation ranges of the water quality are specified in current operation standards.Thus,the influence of CO2inleakage on the operation ranges are also valuable for practice.Due to the corrosion inhibition of copper conductor and insulation property toward the ground of generator,the pH value (25 °C) of GICW should be conditioned to be in the range of 8.0-9.0,and the conductivity (25 °C) of that should be not more than 2.0 μS·cm-1[6,19].To illustrate the influence of CO2inleakage on the actual slight-alkalization,the operation ranges of the NaOH dosage,the conductivity and the pH value are calculated using the above specifications as the three boundary conditions.To meet all the conditions,we found that the lower limit of the operation range is restricted by the pH value (25 °C) of 8.0 and the upper limit is restricted by the conductivity (25 °C) of 2.0 μS·cm-1.

Fig.3 respectively presents the operation ranges of the NaOH dosage,the conductivity and the pH value under different CO2inleakage saturation.Firstly,Fig.3 clearly shows that both the lower limit and the upper limit of the NaOH dosage increase linearly with the increasing CO2inleakage saturation.The trend indicates that the dissolution of CO2makes it necessary to dose more NaOH to slightly alkalize GICW to the same pH value.For example,it needs only 0.99 μmol·L-1NaOH to condition the pH value of GICW to be 8.0 (25 °C) for the system without CO2.However,it would take 8.49 μmol·L-1NaOH to achieve the same pH value for the system saturated with CO2,which is 8.6 times of that without CO2.Secondly,as for the operation range of the conductivity,there are an obvious increase of the lower limit and a slight increase of the upper limit as CO2inleakage saturation increases.This is because the lower limit of the conductivity is reached by dosing a linear increasing NaOH,whereas the upper limit of that is restricted by the conductivity (25 °C) of 2.0 μS·cm-1.Thirdly,the operation limits of the pH value show a contrary trend.With the increasing of the dissolved CO2,the lower limit of the pH value slightly increases,while the upper limit decreases.Given the fact that the lower limit of the NaOH dosage increases more dramatically than that of the pH value,the result reveals that a significant portion of the dosed NaOH is consumed to neutralize the dissolved CO2and equilibrate the subsequent carbonate system.The decrease of the upper limit of the pH value is due to the restriction of the conductivity.Another interesting finding is that though the limits are calculated based on the same boundary conditions(25 °C) [6,19],the upper limit of the conductivities (50 °C) and the lower limit of the pH values (50 °C) are not exactly the same while CO2inleakage saturation changes.The result indicates that the temperature calibrations of the pH and the conductivity seem to be relevant to the water quality of GICW.The specific correlations will be discussed in the next chapter.

Fig.3.Operation ranges of GICW water quality under different CO2 inleakage saturation.

On the whole,what is also striking in Fig.3 is that the operation ranges of the NaOH dosage,the conductivity and the pH value all become narrow as CO2inleakage saturation increases.When CO2inleakage reaches the saturation,the conditioning ranges of NaOH dosage,the conductivity and the pH value become only 61.5%,58.3%,64.8%of that without CO2inleakage,respectively.Moreover,the impurity in water would also contribute to the conductivity of GICW.It can be inferred that the impurity would further narrow the operation ranges coupled with the leaked CO2.The narrower the operation ranges,the harder the slight-alkalization conditioning would be.Given the notable negative influence of CO2inleakage on the slight-alkalization of GICW,enough attention should be paid to CO2inleakage during the operation.Here are a few suggestions to reduce the negative impact of CO2inleakage.Firstly,the direct contact between GICW and the air should be avoided as much as possible.One of the effective strategy is to cut off the source of air,such as taking the fresh desalted water or the effluent of condensate polishing mixed bed as the make-up water,sealing GICW tank by N2-filling,installing an anti-fouling respirator to absorb CO2and so on.Secondly,it is recommended to periodically monitor the amount of CO2inleakage.CO2inleakage is the main factor that would result in the failure of GICW slight-alkalization.However,this important parameter is once ignored because of the lack of a suitable measurement method.Actually,the amount of CO2inleakage can be calculated with the known conductivity and[Na+],both of which can be measured accurately and availably in power plant.Thirdly,positive action should be taken in time if CO2inleakage occurs.To maintain the stability of the passive film on copper surface,the large-scale fluctuation of GICW water quality shall be avoided.If CO2inleakage exists,attention should be paid to the in-time water quality conditioning and trouble shooting.As can be seen in Fig.3,the range of the NaOH dosage will become very narrow under the severe CO2inleakage.In this specific situation,it is advised to replace the GICW by pure water and then slight-alkalify the GICW from the very beginning.

5.Influence of CO2 Inleakage on Temperature Calibrations of Conductivity and pH

The temperature calibration coefficients of the conductivity and the pH value between 50 °C and 25 °C can be theoretically calculated.In this theoretical calculation,the [Na+] and CO2inleakage saturation are selected as the two independent variables.These two variables can be regarded as the constant though the water temperature changes according to the mass balance principle.Other fundamental datum at 50 °C and 25 °C,including the dissociation constants of water and carbonic acid,and the ionic conductivities at infinite dilution of the ions,are referred to the relevant manuals [21,23,24,30].As confirmed in chapter 2,the conductivities and the pH values of GICW at these two temperatures can be directly calculated with the known [Na+] and CO2inleakage saturation.

Fig.4(a)shows the temperature calibration coefficient curves of the conductivity between 50°C and 25°C.The coefficient is in the range of 0.32-0.68,varying with CO2inleakage saturation and the NaOH dosage.With the increasing of the NaOH dosage,the coefficient gradually increases and then stabilizes at 0.68 in the absence of CO2,whereas it decreases firstly and then increases when CO2inleakage exists.The more severe CO2inleakage,the higher the coefficient and the [Na+] corresponding to the extreme point.In conclusion,the temperature calibration coefficient of the conductivity is in the non-linear correlation with the water quality of GICW.However,the linear calibration is widely used in the operation.For the cooling water,2%·°C-1is accepted for the temperature calibration of its conductivity,as advised in GB/T 6908-2018 [20].The corresponding coefficient is 0.67 by converting 2%·°C-1to the ratio of the conductivities between 25°C and 50°C,which is shown as the black dotted line in Fig.4(a).It is obvious that the linear temperature calibration of the conductivity is not applicable for all cases and the applicable scope of the linear calibration becomes narrower as CO2inleakage saturation increases.

Fig.4.Temperature calibration coefficients of (a) the conductivity and (b) the pH value of GICW between 50 °C and 25 °C.

As shown in Fig.4(b),the temperature calibration coefficient of the pH value is also in the non-linear correlation with the water quality of GICW.With the increasing of the NaOH dosage,the coefficient gradually increases from 1.06 to 1.09 without the dissolved CO2,while the coefficient is in the range of 1.01-1.09 and shows aScurve once CO2inleakage occurs.The [Na+] corresponding to the inflection point increases as the amount of the dissolved CO2increases.To calibrate the measured pH value at 50 °C to that at 25 °C,the measured pH value is added to 0.36 in actual measurement,as advised in DL/T 1039-2016 [6].However,the calibration is somewhat rough without the consideration of the water quality of GICW.It is clearly showed in Fig.4(b) that the temperature calibration of the pH value by this simple way cannot cover all the situations.

The non-linear temperature calibration of both the conductivity and the pH value attributes to the non-linear correlations between GICW water quality and the temperature effect on the basic properties of the solutes.As presented in Fig.1 and Fig.2,the correlations between GICW water quality are non-linear.Restricted by the equilibriums of the NaOH-CO2-H2O system,the concentrations of the solutes are not proportional to the increase of the dissolved CO2or the NaOH dosage.What’s more,the water temperature affects the basic properties of the solutes,including the dissociation of the weak electrolytes and the conductivity of the ions.The dissociation equilibriums of water and carbonic acid have a lot to do with the water temperature.Fig.5(a) displays the dissociation constants of H2O (KW) and H2CO3(K1andK2) at different temperatures [23,24].As the temperature rises,the dissociation of the weak electrolytes are enhanced.Thus,the values ofKW,pK1and pK2all decrease with the increasing water temperature.It should be noted that the decreasing slopes of these constants are not the same.The water temperature also affects the conductivity of the ions.Fig.5(b)shows that the ionic conductivity at infinite dilution of all the ions increases linearly with temperature[21,30].The reason is that in the temperature range of GICW,the higher the water temperature,the weaker the ion hydration,the smaller solution viscosity,thus the lower migration resistance of ions.Whereas,the ionic conductivity of the ions increases to different degrees with the increasing water temperature.Among these ions,H+changes the most,followed by OH-,the others are relatively close.For the above reasons,the temperature calibration of both the conductivity and the pH value of GICW is non-linear with its water quality.

Fig.5.Equilibrium constants of (a) the weak electrolytes and (b) ionic conductivities at infinite dilution of the ions at 0-50 °C [21,23,24,30].

The linear temperature calibrations of the conductivity and the pH value of GICW are now still widely used in the field.However,as shown in Fig.4,the linear calibration may not be that appropriate especially under the serious CO2inleakage.To accurately indicate the operation status of the GICW system,the precise temperature calibration shall be achieved.Following are the two suggestions that can be implemented in the field.Firstly,profile the theoretical non-linear calibration curves and then upload the curves into the conductivity and the pH meter.By referring to the non-linear calibration curves,the measured conductivity and the pH value can be precisely calibrated to those at the benchmark temperature.Secondly,in-situcool GICW to 25 °C and then measure the conductivity and the pH value of the cooled water.In this way,the temperature influence on the measurement can be well avoided.The disadvantage of this method is the need to install anin-situwater cooling device for the monitor of GICW water quality.

6.Conclusions

CO2inleakage is one of the main obstacles to the safe operation of GICW system.This study set out with the aim of revealing the influence of CO2inleakage on the slight-alkalization of GICW.In this paper,a new parameter denominated as CO2inleakage saturation was derived based on the equilibriums of the CO2-NaOH-H2O system.This parameter can be used to quantify the amount of the dissolved CO2in GICW and to better interpret the operation status of GICW system.The influence of CO2inleakage on the water quality operation ranges of GICW and the temperature calibrations of the conductivity and the pH value of GICW were systematically investigated.The conditioning ranges of NaOH dosage,the conductivity and the pH value under the condition of saturated CO2in leakage become only 61.5%,58.3%,64.8% of that without the inleakage.The temperature calibration coefficient of the conductivity between 50 °C and 25 °C ranges in 0.32-0.68 and that of the pH value ranges in 1.01-1.09.Thus,the linear temperature calibration of these two parameters that used in the field cannot cover all the situations.Given the noteworthy negative influence of CO2inleakage on the slight-alkalization,it is strongly recommended to monitor the amount of CO2inleakage and sufficient attention should be paid to GICW slight-alkalization if the inleakage occurs.Further researches need to be done to develop an improved expert diagnosis system for GICW system and specify a guideline for the actual slight-alkalization of GICW during CO2inleakage.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.