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The Rate of Hydrogen Combustion in Supercritical Water Dimitrios T. Kallikragas, Igor M Svishchev, and Kashif I. Choudhry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09615 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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The Journal of Physical Chemistry

Title: The Rate of Hydrogen Combustion in Supercritical Water

Authors: Dimitrios T. Kallikragas a, Igor M. Svishchev*a, and Kashif I. Choudhry a

Affiliations:

a

Trent University 1600 West Bank Drive Peterborough ON Canada K9J 7B8 1-705-748-1011 Ext. 7163

* Corresponding Author [email protected] 1-705-748-1011 Ext. 7063

1 ACS Paragon Plus Environment


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Abstract Hydrogen is a clean burning energy source and an essential oxygen scavenger used in corrosion control chemistry for nuclear power plants. Here, we report on the combustion rate of hydrogen in supercritical water. The reaction of hydrogen oxidation by oxygen is studied in a flow-through reactor operating under continuous steady state conditions in terms of surface oxidation. Hydrogen released in-situ from the metal oxide reactor surface is accounted for in the overall reaction kinetics. Rate constants are presented for 500, 550 and 650 oC at 25 MPa. The activation energy for the homogeneous oxidation reaction is estimated to be 96.4 kJ mol-1. This value is comparable with the activation energies obtained for other small fuel molecules.


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Introduction

With the growth of worldwide environmental conscientiousness, there is an ever increasing demand for clean energy as well as for environmentally responsible methods for the destruction of hazardous wastes. In the past few decades, extensive research has been done in the areas of supercritical water oxidation (SCWO) 1-7 and thermochemical hydrogen production, 8-13

as well as using supercritical water (SCW) in power generation technologies, such as the

GEN-IV Supercritical Water Cooled Reactor (SCWR).

14-17

One of the potential uses of the

SCWR is for the large-scale co-generation of hydrogen gas, and the hydrogen thus produced can be stored and used as an alternative and clean burning energy source. Furthermore, hydrogen can act as an oxygen scavenger, and is added into the primary coolant to mitigate oxidative corrosion in nuclear power plants. Of fundamental importance to all these new technologies is the kinetics of the oxidation of hydrogen in SCW. These global reaction rates are necessary to predict the overall rate of hydrogen oxidation resulting from the sum of chain branching reactions. 18 Oxidation kinetics in SCW have been studied in the past for many organic compounds 19, hazardous materials such as benzene

1

and dichlorobenzene,

20

as well as for small molecules such as methane, carbon

monoxide, ammonia, and also for hydrogen. 18-19, 21-24 Holgate and Tester studied the oxidation of hydrogen in a flow-through reactor constructed from Inconel 625 tubing, and found the overall reaction rate to be first order with respect to hydrogen, and zeroth order with respect to oxygen. 25

One of the fundamental concerns in these studies is that the reactions of water with the surface

materials of the reactor may impact homogeneous oxidation kinetics. Thermochemical water splitting on the metal surface can lead to elevated levels of hydrogen produced in-situ, which can result in additional dosing of hydrogen in the reacting mixture. If not taken into consideration, 3 ACS Paragon Plus Environment


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these surface reactions can eventually result in the miscalculation of reaction rates.

To

experimentally quantify this effect we have recently studied hydrogen production in a flowthrough reactor, from deoxygenated SCW over stainless steel 316 and Alloy 800H. We have demonstrated that the hydrogen release rates are significant, particularly during the initial period of oxidation of the bare metal surface.

26

This work is the key to our current investigation of

hydrogen combustion, as it makes possible the estimation of dissolved hydrogen levels due to insitu surface reactions with the SCW. In this study, the reactor tube composed of Alloy 800H, was fed with oxygenated water until a steady state of complete surface oxidation was achieved. Concentrations of metals and oxygen at the exit of an Alloy 800H tube exposed to oxygenated supercritical water were monitored to study the initial stages of surface oxidation.

The steady state condition is

established when the oxygen concentration at the exit is approximately equal to that of the feed water, indicating that no further significant oxidation of the surface is occurring. Only then was hydrogen introduced into the system. By measuring hydrogen and oxygen levels in the reacting flow, and by accounting for the in-situ production, we evaluated the overall rate constants for the combustion reaction H2 (SCW) + ½ O2 (SCW) = H2O (SCW)

,

(Equation 1)

in which SCW denotes the supercritical water solvent. The rate of this pseudo first order reaction is defined as the rate of consumption of hydrogen. 25


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Experimental Section A flow through reactor of similar design as in our previous studies was used for the estimation of hydrogen combustion rates in SCW.

26-28

The reactor tube itself was made from

Alloy 800H with an outer diameter of 3.175 mm and inner diameter 2.159 mm. The feed water and output lines consisted of stainless steel 316 (SS 316) tubing with outer and inner diameters of 1.59 and 0.75 mm respectively.

All connections were joined with zero dead volume

connectors (Valco®). The reactor was fed with ultrapure milli-Q water, saturated with either oxygen or hydrogen gas. Gas saturation of the feed water was achieved by sparging the two 10 L bottles with hydrogen and oxygen gas, (Praxair high purity gasses, UHP 5.0) resulting in dissolved gas concentrations of approximately 20 ppm (mg L-1) for oxygen, and 1.6 ppm for hydrogen. The bottles were continuously sparged during the duration of the experiment to ensure gas saturation of the water. Low pressure check valves (anti-backflow, Valco®) were installed on the sparging lines just outside the bottles to prevent water backflow from the bottles into the lines. The volumetric flow rate of the water was continuously monitored by weighing a collected volume of water over a set time period. Two separate HPLC pumps (Waters® 590) were used to control the volumetric flow rate of the oxygen and hydrogen saturated water. Different residence times were achieved by varying the total flow rate measured at the outlet section. In all cases, the flow rates were set to maintain a stoichiometric ratio of [O2] / [H2] = 0.56. Residence time in the reactor was calculated by dividing the reactor volume by the flow rate and normalizing by the SCW density. 26 The flow channels were combined prior to entering the reactor using a zero dead volume cross (Valco®). A switch was used to divert the flow either directly to the digital gas analyzer for calibration or to the programmable muffle furnace



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(Isotemp® 650 Series, Fisher Scientific) which contained the reactor tubing inside a custommade sand bath. Furnace temperature was maintained by the built-in proportional integral derivative (PID) controller equipped with a type K-thermocouple. The pressure in the system was controlled by an adjustable back pressure regulator (P-880, Upchurch®). Both the temperature and pressure of the system were monitored and recorded using a data logger (PrTC210, Omega®).

The effluent exiting the reactor was cooled to ambient temperature using a custom built heat exchanger, (length = 2.5 m, made of SS 316 tubing), coupled to a cold plate (TE Technology, Inc.) and depressurized prior to analysis. The dissolved hydrogen was measured using a gas chromatograph (SRI Instruments®, Model 8610C), equipped with a helium ionization detector (HID). Oxygen levels were monitored using a NeoFox system equipped with a FOSPOR optical probe (Ocean Optics®). For further information regarding the analysis of dissolved gasses, including the calibration methods, the reader is referred to our previous publications.

26-27

Samples of the effluent were collected and analyzed for dissolved metals

using inductively coupled mass spectrometry (ICP-MS, Agilent 8800).

Results and Discussion Initially, the flow-through reactor was fed with oxygen saturated water, with a dissolved oxygen content of ~ 20 ppm (mg L-1), in order to oxidize the surface. During this early stage of surface oxidation, both water and oxygen act as oxidants, generating H2, and consuming all of the added oxygen. Also indicative of this early stage of surface oxidation is the release of Fe, Al, Ni and Mn.28-29 Eventually as the oxide film thickness becomes sufficient to limit the access of oxygen to the metal surface, the film becomes protective, and oxygen and Cr appear at the test 6 ACS Paragon Plus Environment

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section outlet. The oxygen levels detected at the outlet and concentrations of Cr, Ni and Fe, in the effluent are shown in Figure 1.

Figure 1: O2 and dissolved metal concentrations measured at the exit of the reactor at a temperature of 650 oC and pressure of 25 MPa, with oxygenated feed water of [O2] = 20 ppm and volumetric at pump flow rate of 0.1 mL min-1.

This figure illustrates three distinct stages of surface oxide formation named as Initial, Transition and Steady State. The first stage corresponds to an initial period during which most of the oxygen from the feed water was consumed by the evolved hydrogen from the reactor surface, or diffused into the metal surface. The measured O2 concentration during the initial stage was less than 300 ppb. During the second stage named as the Transition stage, the O2 concentration at the exit of reactor increases. Finally, the third stage corresponds to a Steady State, a period during which the O2 concentration at the exit reaches that of the feed water and remains constant.



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For a more detailed and comprehensive discussion on the stages of oxidation and dissolved metals released, the reader is referred to our previous publication.29 Hydrogen was introduced only after the oxygen at the output equalled that of the input, which took roughly 500 hours at 650 oC, indicating a minimum rate of hydrolysis on the surface. The amount of hydrogen produced by the surface was estimated using Equation 2, 26 along with the effective rate constants obtained from the activation energy for Alloy 800H 27,

= 10

.

(Equation 2)

In Equation 2, [H2] is the molar concentration of hydrogen produced in mol L-1, τ is the residence time in s (seconds), (S/V) is the reactor surface-to-volume ratio in cm-1, and keff is the effective rate constant in mol cm-2 s-1, for hydrogen released from the surface oxidation by water.

27

The

residence time used in Equation 2, is a function of the reactor volume, divided by flow rate, and was normalized by the SCW density as shown in Equation 3,

,

, , = ∗ ∗ ,∗

.

(Equation 3)

In Equation 3, V is the reactor volume in cm3, f is the flow rate in mL s-1 (or cm3 s-1) and ρ is the SCW density, while ρ* is the water density at ambient conditions, both in g cm-3, while T and P denote temperature and pressure, with the asterisks corresponding to their ambient values. The effective rate constants for the hydrolysis reaction over the metal oxide surface were measured for 650 oC, and for 500 and 550 oC, were determined from Equation 4 below:

= !"

.

(Equation 4)


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Here the activation energy Ea of 41.7 kJ mol-1 and the pre-exponential factor A, were taken from the Arrhenius plot shown in our previous work.27 Table 1 presents the concentrations of hydrogen, both in the feed water and from hydrolysis over the metal oxide surface, as well as the total hydrogen available for the oxidation reaction given in Equation 1. Also shown in Table 1 are the concentrations of hydrogen gas measured in the effluent at the reactor outlet. The oxidation reaction was investigated at 500, 550 and 650 oC with corresponding densities of 0.090, 0.079 and 0.065 g cm-3, respectively.

The

concentrations of evolved hydrogen were calculated via Equation 2. In this work, the additional hydrogen doping due to surface reactions is estimated to be around 6 % of the concentration in the feed water at 500 oC with a flow rate of 1.25 ml min-1. It reaches 28 % of that in the feed water for the highest temperature of 650 oC with a flow rate of 0.75 ml min-1. The flow rates of oxygen and hydrogen saturated water were adjusted separately to maintain a stoichiometric ratio of [O2]/[H2] = 0.56, while obtaining total flow rates of 0.75, 1.00 and 1.25 mL min-1. This stoichiometric ratio was chosen to provide adequate oxygen for the reaction, with only a slight excess. As shown in our previous studies, excess oxygen can result in cracks in the oxide layer,

28

which would expose bare metal to the SCW resulting in a

deviation from steady state conditions in terms of surface oxidation, and increased hydrogen evolution within the reactor. Concentrations of hydrogen and oxygen in the feed water were those of saturated values determined by Henry’s Law and were normalized by the SCW water density to obtain the actual dissolved O2 and H2 levels within the reactor, due to the feed water. The concentration of hydrogen in the feed water ranged from 3.12 x 10-5 mol L-1 at 650 oC, to 4.31 x 10-5 mol L-1 at 500 oC. The evolution of hydrogen from the reactor surface decreased with



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increasing flow rate, as lower flow rates correspond to higher residence times, resulting in longer contact times for water with the metal oxide reactor surface.

Table 1: Experimental parameters of temperature, density, flow rate and residence time used in this study. Also shown are the [H2] in the feed water, produced from the surface, the corresponding total available in the reactor, as well as that measured in the reactor effluent.

Density

Flow

Residence

[H2]

[H2]

[H2]

[H2]

Rate

Time

Feed Water

From Surface

Total Available

Out

(g cm-3)

(ml min-1)

(s)

(10-5 mol L-1)

(10-6 mol L-1)

(10-5 mol L-1)

(10-5 mol L-1)

0.065

0.75

18.5

3.12

8.62

3.98

0.07

0.079

0.090

1

13.9

6.47

3.77

0.14

1.25

11.1

5.18

3.64

0.33

0.75

22.4

5.12

4.27

0.38

3.76

1

16.8

3.84

4.14

0.50

1.25

13.4

3.07

4.07

0.56

0.75

25.6

3.95

4.71

1.20

1

19.2

2.96

4.61

1.34

1.25

15.3

2.37

4.55

1.42

4.31

The rate constants for the first order reaction of hydrogen with oxygen

18

were obtained

using Equation 5 30: = −$% &

&' ()"

'

*+*,-*.-

,

(Equation 5)


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where k is the rate constant in s-1, τ is the residence time in s, and [H2]OUT and [H2]AVAILABLE are, respectively, the concentrations of H2 measured at the exit, and the total H2 available in the reactor, both in mol L-1. A plot was made of -ln (H2OUT / H2AVAILABLE, in mol L-1,) vs. residence time in s, and the rate constants were determined from the slopes, as shown in Figure 2. The amount of hydrogen available was obtained from the sum of the hydrogen in the feed water as well as that evolved from the surface, as shown in Table 1. Total hydrogen available for the oxidation reaction ranged from 3.64 x 10-5 mol L-1 for 650 oC and 1.25 ml min-1 to 4.71x 10-5 ml min-1 at 500 oC and 0.75 ml min-1. Although the evolved hydrogen was greater at higher temperatures, the total hydrogen available was greatest at 500 oC as the contribution from dissolved hydrogen in the feed water is an order of magnitude greater than that of hydrogen produced over the surface. However, the ratio of hydrogen evolved to that in the feed water increases with temperature, affecting the measured kinetics to a greater degree at higher temperatures and thus lower densities.

Figure 2: Plot showing the consumption rate for hydrogen in SCW at 500, 550, and 650 oC and 25 MPa. H2AVAILABLE is defined as the sum of the H2 introduced in the feed water, as well as that evolved by water splitting over the oxide layer. The ratio of [O2] / [H2] in the feed water was maintained at 0.56. The linear regression done on each set of three data points for 650, 550, and 500 oC yielded R2 values of 0.999, 0.989, and 0.999 respectively. 11 ACS Paragon Plus Environment


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The first order rate constants obtained for the oxidation of hydrogen in SCW were, 0.0199, 0.0510, and 0.2287 s-1 for 500, 550, 650 oC respectively. Figure 3 is an Arrhenius plot, showing our activation energy compared to Holgate and Tester’s, as well as those for methane and ammonia. As can be seen from Figure 3, our activation energy for the oxidation of hydrogen in SCW is 96.4 kJ mol-1, and is comparable to that of other small molecules which have similar bond dissociation energies, such as ammonia,22, 31

and methane 23. Although our result for hydrogen is similar to Lee’s value for methane, one

should note that other references in the literature give higher values for methane ranging from 184 to over 400 kJ mol-1.32-34 Holgate and Tester obtained an activation energy for hydrogen of 372 kJ mol-1. 25 At this point we note that our rate constants for H2 oxidation are somewhat close to Holgate and Tester’s data at low temperatures. However at higher temperatures, namely in the regime of 650 oC, our rate constants are much smaller. And it is at higher temperatures, the additional doping of hydrogen due to surface reactions becomes significant. This is, basically, the primary assumption of this paper that the in-situ hydrogen production should be explicitly accounted for in SCW studies. Otherwise, it may result in underestimating the actual hydrogen level in the reacting flow, and thus overestimating the rate of its consumption, especially at high temperatures.


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Figure 3: Arrhenius plot showing the activation energy for the oxidation of hydrogen in supercritical water. Also shown is the value obtained by Holgate and Tester 25 as well as those for methane obtained by J.H., Lee et al. 23, and for ammonia by A. Lee et al. 22, 31.

At a flow rate of 0.1 ml min-1, an initial concentration of dissolved hydrogen from the surface reaction of Alloy 800H with degassed SCW water at 650 oC and 25 MPa, can be estimated at 8.94 x 10-4 mol L-1, dropping to 6.47 x 10-5 mol L-1 during steady state operation. 27 Holgate and Tester used a reactor constructed of Inconnel 625, which also has high nickel content, and they used two long preheater sections, each made of SS 316 and Hastelloy C276 tubing. The temperature of the first and the second sections, reached as high as 500 and 700 oC, respectively. Based on our previous measurements of hydrogen evolution in SS 316 tubing, the concentrations of hydrogen in the preheater sections in Holgate and Tester’s experiments can potentially reach 10-5 mol L-1 or higher. 26 This is a significant fraction of the hydrogen levels in the feed water used in their study. Also effecting evolved hydrogen rates is the fact that their reactor had a much larger surface to volume ratio of 234 cm-1, which impacts the hydrogen evolution according to Equation 2 above.

Moreover, an accurate assessment of evolved

hydrogen in their system cannot be made without rate constants, or activation energies for water 13 ACS Paragon Plus Environment


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splitting over Inconnel and Hastelloy alloys. In our experience, even at the higher concentrations of hydrogen and oxygen that can be safely dosed in to a tubular SCW reactor, the in-situ release of hydrogen at the reactor walls will have a substantial impact on the measured oxidation kinetics.


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Conclusions The accurate rate constants and activation energy obtained in this study will provide necessary data in the consideration of emerging SCW technologies. By establishing steady state conditions in terms of surface oxidation, and by including evolved hydrogen in the determination of the rate constants, this new data is more representative of actual conditions in SCW power plants, oxidation reactors, and thermochemical water splitting systems. Further confirmation of our data can be obtained by determining water splitting rates over both fresh and oxidized Inconel 625 tubing to account for evolved hydrogen rates form nickel-based alloys. However, since the majority of the oxidation of hydrogen occurs in the bulk volume of the reactor, and not on the surfaces, our activation energy will likely be valid in a reactor of any material, but reaction rates themselves may differ according to the amounts of hydrogen evolved from the surface of the materials.

Acknowledgements: The authors are grateful for the financial support of the Generation IV Energy Technologies Program. Funding to the Generation IV Program was provided by Natural Resources Canada through the Office of Energy Research and Development, Atomic Energy of Canada Limited, and the Natural Sciences and Engineering Research Council of Canada.



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Lee, A.; Saulters, O. S.; Castillo, H. G.; Connon, C. S. In Destruction of Ammonia and Acetic Acid by Hydrothermal Oxidation, International Mechanical Engineering Congress and Exhibition, Atlanta, GA, Nov 17-22, 1996; Pepper, D. W., Douglass, R.W., Heinrich, J.C., Ed. American Society of Mechanical Engineers: New York, 1996. Lee, J. H.; Foster, N. R. Direct Partial Oxidation of Methane to Methanol in Supercritical Water J. Supercrit. Fluids 1996, 9, 99-105. Holgate, H. R.; Tester, J. W. Oxidation of Hydrogen and Carbon Monoxide in Sub- and Supercritical Water: Reaction Kinetics, Pathways, and Water-Density Effects. 2. Elementray Reaction Modelling. J. Phys. Chem. 1994, 98, 810-822. Holgate, H. R.; Tester, J. W. Fundamental Kinetics and Mechanisms of Hydrogen Oxidation in Supercritical Water. Combust. Sci. Technol. 1993, 88, 369-396. Choudhry, K. I.; Carvajal-Ortiz, R. A.; Kallikragas, D. T.; Svishchev, I. M. Hydrogen Evolution Rate During the Corrosion of Stainless Steel in Supercritical Water. Corros. Sci. 2014, 83, 226– 233. Choudhry, K. I.; Svishchev, I. M.; Orlovskaya, L. Alloy 800H Under Supercritical Water Conditions: a Flow Reactor Study of Corrosion and Hydrogen Evolution. Mater. Corros. 2015, 66, 1430-1434. Choudhry, K. I.; Mahboubi, S.; Botton, G. A.; Kish, J. R.; Svishchev, I. M. Corrosion of Engineering Materials in a Supercritical Water Cooled Reactor: Characterization of Oxide Scales on Alloy 800H and Stainless Steel 316. Corros. Sci. 2015, 100 222–230. Choudhry, K. I.; Guzonas, D. A.; Kallikragas, D. T.; Svishchev, I. M. On-line Monitoring of Oxide Formation and Dissolution on Alloy 800H in Supercritical Water. Corros. Sci. 2016, 111, 574–582. Levenspiel, O., Chemical Reaction Engineering, 3 ed.; John Wiley & Sons: New York, 1999. Tester, J. W.; Cline, J. A. Hydrolysis and Oxidation in Subcritical and Supercritical Water: Connecting Process Engineering Science to Molecular Interactions. Corrosion 1999, 55, 10881100. Savage, P. E.; Yu, J.; Stylski, N.; Brock, E. E. Kinetics and Mechanism of Methane Oxidation in Supercritical Water. J. Supercrit. Fluids 1998, 12, 141-153. Brock, E. E.; Savage, P. E. Detailed Chemical Kinetics Model for Supercritical Water Oxidation of C1 Compounds and H2. AlChE J. 1995, 41, 1874-1888. Webley, P. A.; Tester, J. W. Fundamental Kinetics of Methane Oxidation in Supercritical Water. Energy Fuels 1991, 5, 411-419.



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TOC Graphic:

H2 (SCW) + ½ O2 (SCW) = H2O (SCW)


Rate of Hydrogen Combustion in Supercritical Water - The Journal of

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