ww.sciencedirect.com

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 0

Available online at w

ScienceDirect

journal homepage: www.elsevier .com/locate/he

The effect of the hydrogen presence oncombustion-induced rapid phase transitionof CO/O2/N2 mixtures

Anna Basco a, Francesco Cammarota a, Almerinda Di Benedetto b,*,Valeria Di Sarli a, Ernesto Salzano a, Gennaro Russo b

a Istituto di Ricerche sulla Combustione, CNR, Via Diocleziano 328, 80124 Napoli, ItalybDipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Universita degli Studi di Napoli

Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy

a r t i c l e i n f o

Article history:

Received 18 July 2013

Received in revised form

27 September 2013

Accepted 3 October 2013

Available online 5 November 2013

Keywords:

Explosion

cRPT

Rapid phase transition

* Corresponding author. Tel.: þ39 (0) 8176822E-mail addresses: almerinda.dibenedetto

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.10.0

a b s t r a c t

The explosion behavior of CO/O2/N2 mixtures was experimentally studied in a tubular

closed vessel at different oxygen contents, in the presence and in the absence of hydrogen.

We found that CO/O2/N2 mixture explosion does not exhibit cRPT whatever the oxygen

content thus demonstrating that this phenomenon is strictly connected to the water

produced by the combustion reaction. Conversely, the addition of even very small amounts

of hydrogen, triggers the cRPT phenomenon eventually leading to over-adiabatic peak

pressures. The obtained results confirm the key role of water in driving the cRPT

phenomenon.

In the presence of hydrogen (even small amounts), an intense explosion, the

combustion-induced Rapid Phase Transition (cRPT), was found. This highlights the key role

of water in driving the cRPT phenomenon.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Beside the clear environmental advantages of oxy-

The interest is growing in the development of an innovative

technology forpower generationnamed “oxy-fuel combustion”,

which uses highly oxygen-enriched air as the oxidizer. This

development is an important step towards zero-emission

combustion technology. In particular, the reduced content of

nitrogen in the oxidizer reduces the overall NOx emissions; the

higher reactivity of oxygen allows the use of fuel leanmixtures,

thus decreasing the flame temperature (i.e., lower equipment

cost and further decrease in NOx emissions); the stream of

exhaust gases containsmainly carbon dioxide andwater vapor,

thus favoring any subsequent CO2 sequestration process [1,2].

65.@unina.it, dibenede@irc.c2013, Hydrogen Energy P28

combustion, the use of highly O2-enriched air implies sub-

stantial cost penalty and potential corrosion concern [2].

These issues have forced process designers towards the

adoption of near stoichiometric conditions or even more

diluted mixtures in order to reduce the oxygen demand.

The use of oxygen-enriched air also poses severe safety

issues, as pure oxygen increases the laminar burning velocity

and enlarges the flammability range with respect to air [3].

These effects are even more dangerous in syngas oxy-

combustion because of the hydrogen presence, which makes

the mixture highly reactive, thus enhancing the explosion

risk.

nr.it (A. Di Benedetto).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Scheme of the experimental apparatus.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 016464

Explosion of methane/oxygen-enriched air mixtures in a

closed vessel shows anomalous behavior with oscillating

pressure histories both during and after the combustion re-

action and peak pressures much higher than the thermody-

namic values [4].

Similar results were found by Schildberg, H-P., Holtappels,

K., (2010) who studied the explosion behavior of CH4/O2/N2-

mixtures in a 20 l spherewith central ignition. They found that

at some conditions, pressure peaks higher than the thermo-

dynamic values were reached and also that before reaching

maximum combustion pressure, the pressure history was

oscillating [5].

We attributed the nature of this phenomenon to cycles of

condensation and vaporization of the water produced dur-

ing combustion. More specifically, the water condenses at

the vessel walls and, if superheated [6,7] by the radiative

heat transfer from the flame, starts to evaporate explo-

sively. This rapid phase transition of water produces shock

waves which lead to over-adiabatic pressure peaks. This

phenomenon was named “combustion-induced Rapid Phase

Transition” (cRPT) [4]. We investigated the role of the main

parameters and conditions on the occurrence and severity

of cRPT [4,8e11] that have been found to be dependent on

the quantity and nature of diluents (Ar, He, N2 and CO2) and

on the surface to volume ratio of the vessel. The cRPT

severity increases with this ratio. Furthermore, sprinkling

talc (i.e., providing nucleation sites) onto the vessel walls

prevented the cRPT phenomenon. Finally, when inhibiting

the water condensation by increasing the vessel wall tem-

perature above the water boiling point at the water partial

pressure in the burned gas, the cRPT phenomenon

disappears.

Holtappels and Pasman [12] also found a similar phenom-

enon that they considered as a kind of transition state be-

tween the deflagration mode and the detonation mode.

Indeed, the mechanism through which a deflagration transits

into a detonation remains one of the most interesting unre-

solved problems in combustion theory.

In this paper, we tested the explosion behavior of stoi-

chiometric CO/O2/N2 mixtures, by varying the oxygen

enrichment factor E ¼ O2/(O2 þ N2) from 0.21 (air) to 1 (pure

oxygen), in the presence and in the absence of hydrogen. The

Table 1eMolar fractions of reactants and products, adiabatic temperature and adiabatic pressure for the testedmixtures ascalculated at equilibrium conditions by GASEQ code.

E Reactants Tad, K Pad, bar Products

YCO YO2 YN2 YN2 YCO2 YCO YO2 YNO

0.21 0.296 0.148 0.556 2699.6 7.92 0.627 0.276 0.061 0.024 0.010

0.60 0.545 0.273 0.182 3201.8 9.03 0.204 0.387 0.257 0.106 0.023

0.80 0.615 0.308 0.077 3292.1 9.23 0.083 0.413 0.319 0.134 0.018

1.00 0.667 0.333 0.000 3360.0 9.40 e 0.428 0.371 0.157 e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 0 16465

main aim was at demonstrating the key role of water pro-

duced by combustion (of H2) in driving the cRPT phenomenon.

2. Experimental

The experimental apparatus used in this work is schematized

in Fig. 1. The system is composed by two sections, a mixer for

the preparation of the reactive mixture and a reactor for the

explosion tests.

The mixer consists of a 5 dm3 cylindrical vessel and in-

cludes a magnetic-driven stirring system. The reactor is a

cylindrical AISI316SS stainless-steel tubular vessel (wall

thickness ¼ 2.5 cm). The diameter is 6 cm and the vertical

length is 120 cm. The reactor is equipped with rupture disk

(the maximum allowable operating pressure is 200 bar). For

pressure recording, which is the core measurement for the

experiments reported in this paper, a Kulite ETS-IA-375 (M)

series pressure transducer with a natural frequency of 150 kHz

was used. These transducers are specifically designed for

high-pressure, high- shock environments and blast analysis.

The electrical power for the transducer was supplied by

means of a chemical battery 12 VDC/7 AH in order tominimize

any disturbance on the output current, whichwas recorded by

means of a National Instrument USB- 6251 data acquisition

system (16 bit, 1.25 � 106 samples/s), with a frequency up to

1.0 MHz. No manipulations were performed on the analogical

signal output from the transducer or the digital data recorded.

Table 2e Composition of the reactivemixture, adiabatic temperconversion for the tested mixtures as calculated at equilibrium

E l Reagent mixture

YCO YH2 YO2 YN2

0.21 0.010 29.3 0.3 14.8 55.6

0.21 0.101 26.6 3.0 14.8 55.6

0.30 0.011 37.1 0.4 18.8 43.8

0.30 0.040 36.0 1.5 18.8 43.8

0.35 0.010 40.8 0.4 20.6 38.2

0.40 0.009 44.0 0.4 22.2 33.3

0.40 0.040 42.7 1.8 22.2 33.3

0.45 0.011 46.9 0.5 23.7 28.9

0.45 0.040 45.5 1.9 23.7 28.9

0.50 0.010 49.5 0.5 25.0 25.0

0.55 0.010 51.9 0.5 26.2 21.4

0.60 0.009 54.0 0.5 27.3 18.2

0.60 0.101 49.1 5.5 27.3 18.2

0.80 0.010 60.9 0.6 30.8 7.7

0.80 0.020 60.3 1.2 30.8 7.7

1.00 0.010 66.0 0.7 33.3 0.0

Thedatawerefiltered throughanon-linearalgorithmbased

on SavitzkyeGolay (SG) method [13] which utilizes an array of

weighted coefficients as a smoothing function to convolute m

uniformly spaced neighboring points. In the following, we set

m ¼ 21 which is a typical value adopted in explosion science.

The mixtures tested in this work were obtained by the

partial pressure methodology. Each mixture was sent to the

tubular reactor and ignited by a single spark [8 kV direct cur-

rent (DC)] through two electrodes positioned at the bottom of

the equipment (spark gap ¼ 1 mm). To this aim, an RC circuit

(Capacitor ¼ 590 pF; Current Intensity ¼ 5 mA) was used with

PC-based reed relays as high-voltage switches. The estimated

energy was 20 mJ.

Tables 1 and 2 give the compositions of the mixtures

investigated for different oxygen-air enrichment factors, E, in

the absence of hydrogen (Table 1) and in the presence of

hydrogen (Table 2). E was defined as follows:

E ¼ O2

O2 þN2(1)

The enrichment factor l (Table 2) was defined according to:

l ¼ H2

H2 þ CO(2)

In the same tables, for all mixtures, the maximum theo-

retical values of pressure (Pad) and temperature (Tad), as

computed by the Gaseq Chemical Equilibrium Program [14] at

adiabatic conditions and constant volume, and the

ature, adiabatic pressure, partial pressure of water and fuelconditions by GASEQ code.

Tad, K Pad, bar PH2O, bar XCO XH2

2696.8 7.91 0.0199 70.9 99.7

2691.6 7.89 0.2397 73.6 97.0

2904 8.38 0.0232 63.2 99.6

2900.1 8.37 0.1089 64.3 98.5

2980.6 8.55 0.0211 59.5 99.6

3040.4 8.68 0.0193 56.3 99.6

3035.3 8.67 0.1243 57.6 98.2

3089.3 8.79 0.0240 53.5 99.5

3083.8 8.77 0.1279 54.9 98.1

3129.8 8.87 0.0224 50.9 99.5

3165 8.95 0.0211 48.5 99.5

3194.8 9.01 0.0199 46.4 99.5

3185 8.98 0.4094 51.3 94.6

3284.3 9.21 0.0216 39.5 99.4

3280.5 9.20 0.0576 40.1 98.9

3351.2 9.37 0.0234 34.5 99.3

Fig. 2 e Pressure time histories as obtained during explosions of CO/O2/N2 mixtures with oxygen-air enrichment factor

varying from E [ 0.21 (air) to E [ 1 (pure oxygen). The dotted line (- - -) represents the adiabatic pressure.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 016466

corresponding molar compositions of gas products are also

reported. Finally, Table 2 gives the partial pressure of the

water produced by the combustion reaction ðPH2OÞ as calcu-

lated at equilibrium conditions. It is worth noting that the

water vapor pressure ðP0H2O

Þ at the wall temperature

(Twall ¼ 283 K) is equal to 0.022 bar.

Each composition was tested at least 3 times (great repro-

ducibility was found). For all tests, the initial pressure was set

to 1 bar. The temperature of the vessel walls was equal to the

ambient temperature (293 K).

3. Results

3.1. Explosion of CO/O2/N2 mixtures: effect of oxygen-air enrichment factor

In Fig. 2, the pressure time histories are shown as obtained

during explosions of CO/O2/N2 mixtures with oxygen-air

enrichment factor varying from E ¼ 0.21 (air) to E ¼ 1 (pure

oxygen). The plots are similar to typical pressure histories

registered during closed vessel explosions, with deviation

between maximum pressure and adiabatic pressure (i.e.,

maximum theoretical pressure), and pressure decay due to

heat losses towards the external environment. The increase of

E does not lead to a significant increase inmaximumpressure.

On the other hand, the rate of pressure rise (i.e., the mixture

reactivity) increases with increasing E.

3.2. Effect of H2 addition

Fig. 3 shows the pressure time histories as obtained during

explosions of CO/H2/O2/N2 mixtures with different values of E

and l. In the presence of hydrogen, the pressure trends are

qualitatively different from those shown in Fig. 2. Indeed, the

pressure histories exhibit a peak which over-takes the adia-

batic value. This peak characterizes the above cited phe-

nomenon of cRPT.

3.3. Discussion

Table 3, reports the values of the maximum rate of pressure

rise, (dP/dt)max, in the absence and presence of H2 in the

reactive mixture. The rate of pressure rise increases with the

oxygen (i.e., E factor) and hydrogen (i.e., l factor) contents

(Fig. 4).

The results of our tests show that the presence of H2 also

induces intense pressure peaks due to the occurrence of cRPT.

In Table 3, the occurrence and intensity (peak pressure) of the

cRPT phenomenon is reported, together with the partial

pressure of water in the given explosion conditions and the

equilibrium data (adiabatic pressure and adiabatic tempera-

ture) for all mixtures investigated.

A first observation is that, as l increases, the partial pres-

sure of water in the gaseous products increases and the

explosive phenomenon becomes more severe. Furthermore,

even in the presence of small amounts of hydrogen, the cRPT

Fig. 3 e Pressure time histories as obtained during explosions of CO/H2/O2/N2 mixtures with different values of E and l. The

dotted line (- - -) represents the adiabatic pressure.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 0 16467

phenomenon is observed. That agrees with our explanation of

cRPT, which considers the partial pressure of water as the

main parameter for the occurrence of the over-adiabatic

spike. Indeed, following our analysis, for l ¼ 0.01, the value

of the water partial pressure in the gaseous product ðPH2OÞ isgreater or about equal to the value of water vapor pressure

ðP0H2OÞ at the wall temperature (Twall ¼ 283e293 K;

P0H2O¼ 0.012e0.022 bar) and, thus, cRPT may occur.

As shown in our previous paper [4], the occurrence and

severityof thecRPTphenomenoncanalsobeexplained through

an analysis of three characteristic times: sreac, scond and srad.

sreac is the characteristic time required by the flame to

travel along the radial direction of the vessel.

sreac ¼ d2SF (3)

where d is the reactor diameter and SF the flame speed

calculated as a function of the laminar burning velocity, Sl,

and the expansion factor (i.e., the adiabatic pressure, Pad, to

initial pressure, Po, ratio):

SF ¼ Sl$

�Pad

Po

�(4)

In Eq. (4), the expansion factor was evaluated assuming all

gas as burned and at the maximum theoretical pressure (Pad).

scond is the characteristic time for water cooling and

condensation at the vessel walls and is defined as:

scond ¼ rcpV

hcA(5)

where r and cp are the density and the specific heat of the gas

mixture; V is the vessel volume andA is the surface of the side

walls of the vessel computed as A ¼ p D L (with D the tube

diameter and L the tube length); hc is the coefficient of heat

transfer due to condensation at the walls evaluated according

to the formula reported by Incropera and DeWitt [15].

srad is the characteristic time for heat exchange between

flame and walls by radiation and was computed through the

following formula:

srad ¼ rcpVðTF � TwÞs 3AR

�T4F � T4

w

� (6)

where TF is the flame temperature (adiabatic temperature), s

the StefaneBoltzmann constant, 3the emissivity and AR the

surface area enclosing the radiating gas volume (assumed as

equal to A).

In Table 4, the values of the characteristic times, the ratio

between the condensation time and the reaction time

(q1 ¼ scond/sreac) and the ratio between the condensation time

and the radiation time (q2 ¼ scond/srad) are also given.

According to our previous results [4,10], when the over-

adiabatic behavior occurs, q1 is higher than 1. On the con-

trary, at under-adiabatic conditions, q1 is lower than 1.

Therefore, q1 ¼ 1 can be viewed as a bifurcation point. This

result confirms the role of the synchronization between the

reaction phase and the water condensation phase in driving

the cRPT phenomenon.

Fig. 5 shows themap of the cRPT phenomenon in the plane

oxygen enrichment factor (E)-hydrogen content (l). The white

Table 3 e Adiabatic Temperature, Tad, Adiabatic Pressure, Pad, Rate of Pressure Rise in the early stage of reaction, dP/dt,Maximum Rate of Pressure Rise for cRPT, dP/dtcRPT, Partial Pressure of Water, PH2O, Peak Pressure of cRPT, PcRPT, for allcompositions analyzed.

E l Tad (K) Pad (bar) dP/dt (bar s�1) dP/dtcRPT (bar s�1) PH2O (bar) PcRPT (bar)

0.21 0 2699.6 7.92 24.5 e e

0.60 0 3201.8 9.03 88.1 e e

0.80 0 3292.1 9.23 81.2 e e

1.00 0 3360.0 9.40 92.1 e e

0.21 0.010 2696.8 7.91 53.3 0.0199 e

0.21 0.101 2691.6 7.89 206.6 2998.7 0.2397 7.1

0.30 0.011 2904 8.38 105.4 2798.4 0.0232 7.5

0.30 0.040 2900.1 8.37 175.1 1610.8 0.1089 6.6

0.35 0.010 2980.6 8.55 151.9 3323.0 0.0211 8.4

0.40 0.009 3040.4 8.68 192.5 3398.4 0.0193 8.0

0.40 0.040 3035.3 8.67 297.0 3382.8 0.1243 9.3

0.45 0.011 3089.3 8.79 224.1 1801.0 0.0240 7.0

0.45 0.040 3083.8 8.77 399.1 21,668.7 0.1279 38.8

0.50 0.010 3129.8 8.87 330.4 2169.9 0.0224 7.3

0.55 0.010 3165 8.95 322.6 1596.5 0.0211 7.7

0.60 0.009 3194.8 9.01 413.2 1893.4 0.0199 8.3

0.60 0.101 3185 8.98 2305.8 44,509.3 0.4094 337.8

0.80 0.010 3284.3 9.21 433.2 5335.2 0.0216 15.4

0.80 0.020 3280.5 9.20 3961.0 41,817.2 0.0576 184.5

1.00 0.010 3351.2 9.37 868.0 39,749.8 0.0234 20.2

Table 4 e Characteristic time of reaction, sreac,characteristic time of condensation, scond, characteristictime of flame radiation, srad, and dimensionlessparameters q1 and q2 for all compositions analyzed.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 016468

zone of the map, characterized by l < 0.01, was not investi-

gated because of experimental limits in obtaining the mix-

tures. The circle size is proportional to the maximum rate of

pressure rise for cRPT.

Three zonesmay be distinguished: zone 1ewhere no cRPT

is found; zone 2 e where cRPT is found with peak pressure

lower than the adiabatic value; zone 3 e where cRPT is found

with over-adiabatic peak pressures. In Fig. 5, the dark line

represents the limit between zones 2 and 3 and corresponds to

the line q1 ¼ 1. It is worth noting that, for E higher than 0.4, the

passage from zone 1 to zones 2 and 3 occurs with increasing

hydrogen content. It is also worth noting that a limit value of

hydrogen content (l ¼ 0.01) is required for driving the cRPT

phenomenon, whatever the oxygen-air enrichment factor.

In zones 2 and 3, as E and l increase, the severity of the

cRPT phenomenon (and, thus, the circle size) increases and

larger q1 values are found.

Fig. 4 e Rateofpressure rise versusE for different valuesof l.

Fig. 6 shows the same map in the plane adiabatic tempera-

ture (Tad)-water partial pressure ðPH2OÞ. If PH2O is lower than the

water vapor pressure at thewall temperature, no cRPT is found

(zone 1). Furthermore, if PH2O is higher than the P0H2Ovalue at

Twall, thecRPTphenomenonoccursand themaximumpressure

can be under-adiabatic (zone 2) or over-adiabatic (zone 3).

From this map, the role of the adiabatic temperature in trig-

gering cRPT is also found. At a fixed value of the water partial

pressure, the passage from zone 1 to zones 2 and 3 occurs with

increasing adiabatic temperature. As discussed in our previous

papers [4,10], as the adiabatic temperature increases, the

E l sreac (s) scond (s) srad (s) q1 q2 Notes

0.21 0 e e e e e NO cRPT

0.60 0 e e e e e NO cRPT

0.80 0 e e e e e NO cRPT

1.00 0 e e e e e NO cRPT

0.21 0.010 0.0180 0.0044 0.0032 0.25 1.38 NO cRPT

0.21 0.101 0.0076 0.0060 0.0191 0.78 0.31 under-adiabatic

0.30 0.011 0.0110 0.0047 0.0028 0.43 1.70 under-adiabatic

0.30 0.040 0.0066 0.0058 0.0100 0.88 0.58 under-adiabatic

0.35 0.010 0.0096 0.0045 0.0020 0.47 2.29 under-adiabatic

0.40 0.009 0.0086 0.0044 0.0015 0.51 3.00 under-adiabatic

0.40 0.040 0.0048 0.0059 0.0085 1.24 0.69 over-adiabatic

0.45 0.011 0.0072 0.0047 0.0019 0.66 2.52 under-adiabatic

0.45 0.040 0.0043 0.0058 0.0071 1.37 0.82 over-adiabatic

0.50 0.010 0.0067 0.0046 0.0015 0.69 3.05 under-adiabatic

0.55 0.010 0.0063 0.0045 0.0012 0.71 3.66 under-adiabatic

0.60 0.009 0.0061 0.0044 0.0010 0.73 4.32 under-adiabatic

0.60 0.101 0.0023 0.0060 0.0083 2.67 0.73 over-adiabatic

0.80 0.010 0.0049 0.0046 0.0009 0.94 4.95 over-adiabatic

0.80 0.020 0.0037 0.0055 0.0027 1.50 2.05 over-adiabatic

1.00 0.010 0.0041 0.0047 0.0009 1.13 5.38 over-adiabatic

Fig. 5 e Occurrence and severity of cRPT in Eel plane.Fig. 7 e Characteristic time ratio q2 versus Tad for under and

over-adiabatic behavior of cRPT.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 0 16469

heating of the condensed water (by radiation) becomes faster,

eventually synchronizing with the water condensation.

In Fig. 7, q2 is plotted as a function of the adiabatic tem-

perature, Tad. Two main trends are identified: a low temper-

ature zone and a high temperature zone. The low temperature

zone corresponds to zone 2 of Fig. 6, while the high temper-

ature zone corresponds to zone 3. Three states are identified.

The first state (State 1) lies at low temperature. In this case,

only the under-adiabatic cRPT behavior is possible. In the

second state (State 2), both the under and over-adiabatic cRPT

phenomena are possible. In this case, at the same flame

temperature, the cRPT is over-adiabatic when q2 is lower or

close to 1, suggesting that the peak pressure overcomes the

adiabatic values only if a strong synchronization between

condensation and evaporation is established. At very high

temperature (T > 3200 K, State 3), only over-adiabatic cRPT is

possible. In this case, the heating rate of the condensed water

is very fast, thus driving the explosive boiling.

4. Conclusions

Theexplosionbehavior of CO/O2/N2mixtureswas investigated

in a tubular closed vessel by varying the oxygen content, in the

Fig. 6 e Occurrence and severity of cRPT in TadePH2O plane.

presenceandabsenceofhydrogen. In theabsenceofhydrogen,

regardless of the oxygen content, no cRPT was observed, thus

further demonstrating that this phenomenon is strictly con-

nected to the water produced by the combustion reaction. On

the contrary, when adding even very small amounts of

hydrogen, the cRPTphenomenon is excited and over-adiabatic

peak pressures are reached. The obtained results confirm the

key role of water in driving the cRPT phenomenon.

In the next future a model for the simulation of the

coupling between the flame propagation and the heat

condensation and rapid phase transition will be developed.

Acknowledgments

The authors gratefully acknowledge the financial funding

provided by MiSE (Ministero per lo Sviluppo Economico)

within the framework of the project “Carbone pulito”.

r e f e r e n c e s

[1] Dennis R. The gas turbine handbook. National EnergyTechnology Laboratory. US Department of Energy; 2006.

[2] Lieuwen TC, Yang V, Yetter R. Synthesis gas combustion:fundamentals and applications. CRC Press. Taylor & Francis;2010.

[3] Scott FE, Van Dolah RW, Zabetakis MG. In: Sixth symposium(International) on combustion. Pittsburgh: The Combust.Inst.; 1957. p. 540e5.

[4] Di Benedetto A, Cammarota F, Di Sarli V, Salzano E, Russo G.Anomalous behavior during explosions of CH4 in oxygen-enriched air. Combust Flame 2011;158:2214e9.

[5] BASF or to refer to the paper of Schildberg, H-P., Holtappels,K. Schildberg Peter. The course of explosions of CH4/O2/N2-mixtures in a 20 l sphere. In: Proceedings 13th internationalsymposium on loss prevention June 2010 [Bruges, Belgium]

[6] Reid RC. Rapid phase transitions from liquid to vapor. AdvChem Eng 1983;12:105e208.

[7] Reid RC. Superheated liquids. Am Sci 1976;64:146e56.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 4 6 3e1 6 4 7 016470

[8] Di Benedetto A, Cammarota F, Di Sarli V, Salzano E, Russo G.Reconsidering the flammability diagram for CH4/O2/N2 andCH4/O2/CO2 mixtures in light of combustion-induced rapidphase transition. Chem Eng Sci 2012;84:142e7.

[9] Di Benedetto A, Cammarota F, Di Sarli V, Salzano E,Russo G. Combustion-induced rapid-phase transition (cRPT)in CH4/CO2/O2-enriched mixtures. Energ Fuels2012;26:4799e803.

[10] Di Benedetto A, Cammarota F, Di Sarli V, Salzano E, Russo G.Effect of diluents on rapid phase transition of water inducedby combustion. AIChE J 2012;58:2810e9.

[11] Salzano E, Basco A, Cammarota F, Di Sarli V, Di Benedetto A.Explosions of Syngas/CO2 mixtures in oxygen-enriched air.Ind Eng Chem Res 2012;51:7671e8.

[12] Holtappels K, Pasman HJ. Interpretation of gas explosiontests; extremes in explosion severity, SAFEKINEX: safe andeffcient hydrocarbon oxidation processes by kinetics andexplosion expertise. FP7 Programme “Energy, Environmentand Sustainable Development”. Delft, The Netherlands: DelftUniversity of Technology; 2007. Contract EVG1-CT-2002e00072.

[13] Savitzky K, Golay MJE. Smoothing and differentiation of databy simplified least squares procedures. Anal Chem1964;36:1627e39.

[14] GASEQ. A chemical equilibrium program for windowsAvailable at: http://www.c.morley.dsl.pipex.com/.

[15] Incropera FP, DeWitt DP. Fundamentals of heat and masstransfer. 4th ed. New York: Wiley; 1996.