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Reviewing H Combustion: a Case Study for Non-Fuel Cell Power Systems and Safety in Passive Autocatalytic Recombiners Maria Hendrina du Toit, Alexander V. Avdeenkov, and Dmitri Bessarabov Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00724 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018
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Reviewing H2 combustion: a Case Study for Non-Fuel Cell Power Systems and Safety in Passive Autocatalytic Recombiners Maria H, du Toit*, Alexander V. Avdeenkov, Dmitri Bessarabov HySA Infrastructure CoC, North-West University, Faculty of Engineering, Private Bag X6001, Potchefstroom, 2531, South Africa, email:
[email protected]. Abstract This article presents a review of past and current research and development on H2 combustion in terms of power generation in gas turbines and safety in passive autocatalytic recombiners (PARs). The drive towards reducing greenhouse gas emissions has forced researchers to look at carbon-free alternatives for power generation. Fortunately, H2 is one such fuel source and has been proposed as a fuel in gas turbines for large-scale power production. The effects of H2 on the heating values, flame speed, burning velocity, flammability range, flashback, blow-off, ignition delay, and emissions have been reported, and the trends and gaps in R&D identified. Properties such as flame speed, burning velocities, and flammability limits at typical gas turbine conditions (high pressures, high temperatures, lean equivalence ratios, and turbulent conditions) still need to be determined experimentally for H2 fuel mixtures. Especially for mixtures with higher H2 concentrations and fuel mixtures with other fuels (such as biogas) as an evolutionary step towards adapting to a hydrogen economy. Much work has been done on premixed dry low emissions and diffusion combustion with dilution as means of accommodating high hydrogen content fuels. Staged combustion, vortex-stabilized combustion, multiple injection combustion, 1 ACS Paragon Plus Environment
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and catalytic combustion, have been proposed for high hydrogen content fuels, yet still further R&D is required. Finally, Oxy-fuel combustion provides a promising technology for high hydrogen content fuels and researchers should focus on testing and developing oxy-fuel combustion for high hydrogen content fuels. Although H2 is a good alternative to carbon-based fuel, its unique properties increase the probability of a fire hazard. Towards developing a hydrogen economy, we need to also ensure the safety of H2. Unfortunately, in unique scenarios, for instance in closed areas, venting and detection measures are no long adequate, nor possible, to ensure safety. Other intervention methods are required. H2 combustion in PARs is well known in the nuclear industry, but PARs can also be used for flammable gas control in other applications. The current status quo of PARs is discussed. Understanding and developing robust PARs for nuclear applications as well as alternative applications is vital for establishing a hydrogen economy. Gas turbine technologies that are H2 compatible have attracted much attention worldwide, both within academia and industry, especially in the USA, Europe, and some Asian countries. Presently, the key role-players in this field are the Department of Energy, Hitachi, Kawasaki, Siemens, and General Electric. Some of the initiatives towards developing H2 gas turbines are discussed, with the strongest being the potential use in clean integrated coal gasification combined cycle applications.
Keywords: hydrogen, gas turbines, emissions, combustion, passive autocatalytic recombiners 1. Introduction Over the past few decades, there has been an international drive towards reducing greenhouse gas emissions and the effect it has on the climate. The United States Department of Energy (DOE) aims to reduce NOx emissions to 80% H2 in C3H8.40 The adiabatic flame temperature determines the maximum temperature in the combustor. This temperature determines the efficiency (Carnot efficiency) as well as the materials choice for the combustion chamber and turbine blades.16 This means that an increased adiabatic flame temperature could potentially increase the combustion efficiency,39 but also damage burner parts due to overheating.41,42 Table 1. Combustion properties of H2 compared to CH4 and C3H814,16,29,43,44 H2
CH4
C3H8
2.016
16.04
44.097
kg/m
0.0838
0.6512
1.87
K
845–858
813–905
760–766
Minimum ignition energy
mJ
0.02
0.29–0.33
0.26–0.305
Flammability range in air
vol %
4–75
5–15
2.1–10
0.1–7.1
0.4-1.6
0.56–2.7d
Stoichiometric composition in air
Φ vol %
29.53
9.48
4.02
Adiabatic flame temperature
K
2318-2400
2158–2226
2198–2267
Molecular weight
g/mol 3
Density at NTP Self-ignition temperature
a
Flammability range
b
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Burning velocitya
cm/s
237
42
46
Laminar flame speed (max)
cm/s
Lower heating value
MJ/kg
325 @ Φ=1.8 118.8–120.3
45 @ Φ=1.08 50
46.35
Higher heating value
MJ/kg
141.75
55.5
-
c
3
10.78
35.8
91.21
3
12.75
39.72
99.03
3
40.7
47.94
73.3
Lower heating value
Higher heating value
c
Lower Wobble index a
Stoichiometric conditions
b
Φ=Equivalence ratio
c
@ 273.15 K and 101.3 kPa
d
@ 298.15 K and 101.3 kPa
2.3
MJ/m MJ/m
MJ/m
Flammability range
Research into the determination of flammability limits is important because of fire and explosion dangers under different conditions,45 and the possible advantage of using lean combustion to improve emissions and efficiency.29 The upper flammability limit and lower flammability limit is the points at which the fuel-oxidant mixture can no longer propagate or self-support a flame.46 It is clear from Table 1 that the flammability range of H2 is very wide. This may be attributed to chemical kinetics and the enhanced diffusion coefficient which increases the strain resistance, resulting in a higher fuel concentration in the flame front.14 Therefore, adding H2 to slower burning fuels rises the upper flammability limit and lowers the lower flammability limit, which widens the flammability range of the mixture,31 resulting in a more efficient combustion.30 Extension of the flammability limits in H2 fuel mixtures makes leaner combustion possible7 due to more OH molecules,47 which can improve emissions and thermal efficiency.29 The flammability range of pure H2 at standard temperature and pressure is well known. However, very little information is available on the dependency of the flammability on the initial
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pressure and temperature. Liu and Zhang48 quantified the effect of temperature and pressure on the flammability limits of pure H2 and air. They showed that the upper flammability limit increases with pressure and temperature. However, Verhelst and Wallner12 found that the lower flammability limit rises with pressure, but that the upper flammability limit had an intricate behavior with pressure. Schroeder and Holtappels49 also experienced this anomaly for pure H2 fuel. Tang et al.29 demonstrated that the determination of the lower flammability limit of H2-CH4 mixtures by means of various experimental techniques was consistent. However, for the upper flammability limits, large inconsistencies exist, especially the predictions based on Le Chatelier’s principle. Wierzba and Wang45 showed that for H2-CO-CH4 mixtures at moderately low initial temperatures and low H2 concentrations the experimental data corresponded well with Le Chatelier’s rule. Conversely, at higher H2 concentrations and temperatures above 473 K, large differences were observed. Indications are therefore that there is clearly a gap in obtaining a full understanding of flammability limits, especially the upper flammability limits. Correct numerical models that can predict dependable values for the fuels and their mixtures with H2 at various temperatures and pressures, and for various compositions, are required.45 2.4
Flashback
Flashback commences when the flow speed of the fuel mixture becomes slower than the laminar burning velocity. The flame will burn faster than the incoming fuel, which will cause the flame to propagate upstream41 and damage the fuel injectors.1 Due to the extremely high burning velocity of H2, flashback is more probable. It has been shown that adding H2 definitely increases the risk for flashback.31,50 One study showed that flashback is more pronounced for pure H2 than
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for H2 mixtures, and that even a small fraction of CH4 will reduce the flashback risk considerably.51 One would argue that using H2 as a fuel additive instead of a pure fuel is advantageous when the flashback risk is the major consideration. Flashback can be avoided by increasing the fuel flow, which increases the burner exit velocity, and which in turn results in a larger pressure drop over the burner.19 2.5
Blow-off
When the flow velocity of the mixture is faster than the laminar burning velocity, the flame will blow off the burner rim and propagate at a distance from the burner, which is called blow-off. H2 fuel mixtures have exhibited enhanced blowout limits and a lower blowout flame temperature.31 H2 addition makes gas mixtures more resistant to blow-off, therefore H2 addition has been suggested as a method to decrease the lean blowout limit in lean premixed burners.52 Studies have recommended use of the Damkohler number as a measure of the occurrence of blow-off. The Damkohler number is defined as “the ratio of a characteristic flow time scale and a chemical time scale”.16 One study suggests that blow-off generally occurs at a Damkohler number of 0.6,41 while another study found that blow-off occurs at 0.4, depending on how the Damkohler number is defined. The Damkohler number should not be taken as an absolute because the standard deviation can be as high as 55%.53 Increasing the H2 fraction results in a lower blow-off limit, which means that blow-off will occur at lower equivalence ratios. Increasing the pressure will have the same effect on the lean blow-off limit. This provides the gas turbine with a more flexible operating range.41 2.6
Flame speed
The laminar flame speed is the speed at which the flame is traveling from a fixed reference point, or the rate of expansion of the flame front. The laminar flame speed is another vital property of 10 ACS Paragon Plus Environment
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combustion due to its effect on the burning rate, location of the flame front, flashback risk, and flame stabilization.19,54 The maximum laminar flame speed of pure H2 is about seven times faster than that of CH414 due to kinetic, thermal, and diffusion effects (of which the kinetic effect is the greatest).55 The laminar flame speed can be determined experimentally or numerically. The majority of experimental studies have been carried out at ambient conditions for H2-CH4 mixtures31,56–58 and natural gas-hydrogen mixtures.30 However, Sun et al.,60 Krejci et al.,59 Kéromnès et al.17and Zhang et al.,54 determined the laminar flame speeds for hydrogen-syngas mixtures at higher pressures. Hu et al.61 also determined the laminar flame speed experimentally at elevated temperatures and pressures for H2-CH4 mixtures. Recent developments in chemical kinetics mechanisms to predict the laminar flame speed are described in Section 2.9. The majority of laminar flame speed models are functions of pressure, temperature, and equivalence ratio. Existing models will not be accurate for fuel mixtures because the temperature and pressure dependencies are projected to be nonlinear in the fuel blends.62 One study calculated the laminar flame speed for H2-CH4 mixtures at various temperatures, pressures, and equivalence ratios using a gravimetric mixture ratio approximation.63 Overall, irrespective of the mixture, H2 will increase the laminar flame speed of the mixture;31,56,58,64,65 however, the flame speed does not necessarily increase linearly with H2 addition.66 There still remains a gap for experimental work on laminar flame speed to be carried out at typical gas turbine conditions of high temperatures, pressures, and lean equivalence ratios,14,54 especially for hydrogen-natural gas and hydrogen-biofuel mixtures. Such studies will become vital in validating numerical models at high temperature and pressure conditions.
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Practically, the turbulent flame speed is more important than the laminar flame speed. The flame speed is considerably increased in a turbulent environment because mixing is improved and the flame surface wrinkles, which increases the surface area and reaction rate. The turbulent flame speed can be calculated by the unstretched laminar flame speed and the turbulence intensity.14 It is reported that the turbulent flame speed is also increased upon H2 addition;67 in fact, at H2 contents >35 vol % the turbulent flame speed increases drastically, and nonlinearly, compared to the increase in the laminar flame speed.14 The increased flame speed of H2 reduces the combustion duration of H2-CH4 mixtures65 and results in shorter flames.52 The higher flame speed of H2 provides an opportunity for the length of the combustion chamber to be reduced, which, in turn, will reduce the combustion residence time (less NO formation) and cooling requirements.3 The dependence of the turbulent flame speed on the laminar flame speed, turbulence intensity and stretch, are intricate and remain difficult to understand.22 Considerably less research into turbulent flame speed has been carried out compared to the existing work on laminar flame speed determination. This provides researchers with the opportunity to further study, and better understand, the determination of turbulent flame speed using models and calculations. 2.7
Burning velocity
The burning velocity is another key property used to characterize combustion. It is described as “the velocity at which unburned gases move through the combustion wave in the direction normal to the wave surface”.46 Due to the reactivity of H2, gas mixtures thereof will also have very high burning velocities (rates).30,31 This increased burning velocity can cause the combustion of H2 in gas turbines to become unstable.24 However, the increased burning velocity can also lead to stable combustion of lean mixtures.68 Salzano et al.69 determined burning
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velocities for H2-CH4 mixtures using a formula based on the Le Chatelier’s Rule. They achieved good estimates at any initial pressure, but not for fuel mixtures >50% H2. Their study showed that at the initial pressure, the burning velocity decreases and that the effect becomes less prominent for hydrogen-enriched fuel mixtures. The burning velocity can be determined by either experiments or combustion mechanisms. The majority of the existing experimental investigations were determined at room temperature and atmospheric pressure for H2-CH4 mixtures.30,40,57,58,70 Only a few other studies looked at the effect of higher pressures71 or temperatures61 for H2-CH4 and hydrogen-natural gas.72 One research suggestion is to determine the laminar burning velocity at different pressures, temperatures, and equivalence ratios by using correlations. Gu et al.73 and Salzano et al.69 suggested using power law correlations for CH4 and H2-CH4 mixtures, respectively. This is however more complex because the correlations need to be defined for each different H2-CH4 (or other fuel) blend. Most investigations to date have evaluated the flame properties of H2 addition up to 30%; only a few studies have evaluated H2-CH4 mixtures >50%. The latter should be the focus of future research. H2 addition increases the laminar7,40,57,58,72,74 and turbulent75 burning velocity. The turbulent burning velocity depends on the laminar burning velocity.67 The turbulent burning velocity is also dependent on laminar flame thickness, temperature, pressure, and turbulence intensity.16,76 As is the case with the determination of the flame speed, studies determining the turbulent burning velocity are scarce. Researchers should focus on obtaining a better understanding of the effect of H2 on the turbulent burning velocity.
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2.8
Ignition delay
Ignition delay time indicates how much time is required for premixing before self-ignition and combustion occurs. Currently, time of premixing of the fuel and air ranges between 1 and 5 ms.16 A high level of premixing is ideal for low emissions; however, if mixing takes too long, the mixture could self-ignite and damage the components. This becomes a problem with H2 which is very reactive and has a very low self-ignition energy,7 as shown in Table 1. When the H2 content of a fuel mixture is increased, the ignition delay time is decreased.2 The ignition delay time of H2 depends on the fuel choice and also the pressure.41 Generally, at temperatures >1350 K and 8 vol % H2 in air), deflagration in the PARs can be initiated due to the hot surfaces of the catalyst. Continuing research focusses on designing PARs to reduce the probability of ignition. Based on the technical characteristics of PAR, the threshold of the H2 fuel mixture ignition is 8–12% of the volume (depending on the manufacturer). This does not correspond to regulatory
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compliance requirements that demand controlled flameless recombination at all possible limits of the H2 concentration in the premises of the containment. The results of studies of some possible scenarios for severe accidents, conducted by the JSC SSC IPPE (Institute of Physics and Power Engineering; Obninsk, Russia), show that the average H2 concentration in nuclear containments may exceed 14 vol %, while in some rooms it may reach 20 vol %. For the conditions of severe accidents, related to the melting of the nuclear core as well as the melting of the reactor vessel, along with an interaction of the melt with concrete, the concentration of H2 in the containment will be significantly higher. Thus, even for a limited list of considered severe accidents, accompanied by draining, but not melting of the core (steam-zirconium reaction), the existing PAR systems are not capable of providing a hydrogen-safe nuclear plant. Such a low threshold of the ignition of the H2-containing mixture of developed PARs is caused by the design features, which require a passive functioning of the system, thus pumping the hydrogen-containing mixture by natural circulation. Natural circulation is characterized by a small expenditure of the pumping hydrogen-containing mixture, which reduces the PAR efficiency and reduces the heat extraction that is produced by catalytic reactions. Thus, considerable heating of the PAR’s active element takes place, to temperatures that can cause an ignition of the H2-containing mixture. It is expected that, in the event of a severe accident, a huge amount of H2 will be released, causing core melting, followed by melted core interaction with concrete. According to the estimations made by JSC SSC IPPE (Russia) and Research Center “Kurchatov Institute”, up to 2400 kg of H2 can be released under containment within 24 h in the event of an accident at a VVER-1000 nuclear power plant. This results in about 20 vol % H2 concentration in
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containment. In order to recycle this amount of H2 with 90 PAR units, and decrease the H2 concentration to 4% during the first 24 h, the productivity of a single PAR should be at least 1.1 kg/h. Currently available VVER-1000 PARs (RVC-1000) (catalytic surface size 1.2 m2) are able to recombine about 0.157 kg/h of H2 at a concentration of 4 vol % and 0.3 kg/h at 8 vol % H2. Therefore, the available PAR performance cannot guarantee that the indoor containment concentration will not exceed the nuclear safety limits (4 vol % is the lower limit for deflagration and 18 vol % is the lower limit of detonation). The PAR produced by Siemens, FR 90/1-960,137 exhibits the required performance: 1.2 kg/h, for a catalytic surface area of 5,376 m2. Besides its large dimensions and large catalytic surface, the FR 90/1-960 has a low ignition threshold: 8 vol % of H2 in the mixture. In this case, the problem of an adequate response to a fast unexpected situation during the severe accidents arises. In rooms of large volumes, there is a great probability of uneven distribution of H2. Hence the volumetric H2 concentration in certain areas may differ significantly from the average volumetric concentration. Therefore, in some areas with locally high concentrations of H2, the FR 90/1-960 may initiate the explosion of the mixture. Because of the significant weight and size of FR 90/1960, there are some problems associated with placement, which has an important effect on the strength characteristics of the containment. Therefore, to ensure reliable hydrogen safety for all types of possible severe accidents and to reduce the mechanical impact on the nuclear containment, it is necessary to have PARs that, at low overall weight and dimensions characteristics (a small area of the catalytic surface), would have a productivity of 2–4 kg/h at 4 vol % of H2 and an ignition threshold at least at 10–12 vol % of H2.
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There are three main characteristics of a PAR governing its work capacity: productivity, start characteristics (the minimal hydrogen concentration from which PAR performs a stable recombination and the time to reach a full capacity at a given hydrogen concentration), and the ignition threshold. The experimental data138,139 obtained make it possible to determine the specific productivity of the catalytic elements (productivity per unit surface area of the catalyst). Figure 9 shows the dependence of the specific productivity (G) for two types of PAR, RVK, and FR from AREVA, as a function of the inlet volume concentration.
Figure 9. Specific productivity of the RVK and AREVA type PARs, depending on the vol % of H2.
The dependence of productivity was determined, using the results of experiments carried out with FR1-1500 and FR1-750 recombiners. The catalytic surfaces are 5.7 m2 and 11 m2, respectively. Approximation is given for the case when the concentration of H2 is less than the concentration of O2. Results of the comparison demonstrate that the specific productivities, taking into account possible errors in approximations, are fairly similar. This indicates that the 42 ACS Paragon Plus Environment
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absolute productivity is mainly determined by the area of the catalytic surface, but only at rather low H2 concentrations. None of the types of recombiners considered here appear to have significant advantages compared with each other. One of the start characteristic is the minimal volumetric concentration of H2, from which PAR performs a stable H2 recombination. For NIS and AECL PARs, the recombination start is set at a concentration of
