Modeling of hydrogen-rich gas production by plasma reforming of hydrocarbon fuels.pdf

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International Journal of Hydrogen Energy 31 (2006) 769–774

www.elsevier.com/locate/ijhydene

Modeling of hydrogen-rich gas production by plasma reforming of hydrocarbon fuels

M.S. Benilova,∗, G.V. Naidisb

a Departamento de Física, Universidade da Madeira, Largo do Municipio, 9000 Funchal, Portugalb Institute for High Temperatures, Russian Academy of Sciences, Moscow 125412, Russia

Available online 9 August 2005

Abstract

The work is devoted to simulation of plasma reforming of hydrocarbon gases by means of a kinetic approach. It is shown

that the use of standard kinetic schemes, without inclusion of specific plasma processes, is sufficient for simulation of hydrogen

production in discharges with hot plasmas, in particular, in low-current arc discharges. Results of simulation of methane

and octane reforming are compared with data of experiments with low-current arcs available in the literature. An amount of

hydrogen produced in octane reforming is calculated for a wide range of mixture compositions, temperatures at the reactor

entrance, and discharge powers.

᭧ 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Plasma-chemical methods of hydrogen production from

natural gas and liquid hydrocarbon fuels through partial oxi-

dation and steam reforming processes are considered among

the most promising. Recently, a number of works have ap-

peared on the experimental study of plasma reforming of

mixtures of hydrocarbons with air and steam by means of

gas discharges of various types [1–9]. Typically, the effi-

ciency of hydrogen production with the use of cold plas-

mas, at gas temperatures in the discharge being lower than

400–500K (dielectric barrier discharges, coronas), is rela-

tively low: the values of QH2 , the discharge energy requiredfor production of one gram of H2, exceed 30 kJ/g (or 0.6 eV

per one H2 molecule). Higher efficiency has been reached

with the use of arc discharges at low currents (of the order

of 10−2–10−1 A) and near atmospheric pressures [6,7]. In

particular, values of QH2 in the range 3–13kJ/g were ob-

tained in partial oxidation of diesel fuel [6]. Note that though

the plasma state in low-current arcs (at discharge currents

∗ Corresponding author. Fax: +391 291 705269.

E-mail address: [email protected] (M.S. Benilov).

0360-3199/$30.00 ᭧ 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2005.06.018

about several tens of milliamperes) in molecular gases atatmospheric pressure may deviate from equilibrium, the gas

temperature T c in the arc core is relatively high. For example,

values of T c in atmospheric-pressure arcs in air at currents

in the range 10−2–10−1 A are about 2000–3000 K [10].

Similar T c values have been obtained for this current range

also in our calculations of arc discharges in mixtures of air

with hydrocarbons [11].

The effect of plasma may consist both in gas heating

and in generation of chemically active species (excited

molecules, radicals, ions, electrons) in collisions of gas

molecules with electrons. The latter process is, evidently,

dominating in cold plasmas, where the gas heating is small.Both experiments and calculations show that the energy

cost of active species in discharges with cold plasmas is

usually rather high, about several electron Volts [12]. The

energy cost of active species in discharges with hot plas-

mas (arcs) is even higher than that in discharges with cold

plasmas, because in arc discharges a major part of the

energy input goes to gas heating. Therefore, the plasma

processing of hydrocarbon gases due to production of ac-

tive species could be energetically effective only in the case

if these active species are capable of participating in chain

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770 M.S. Benilov, G.V. Naidis / International Journal of Hydrogen Energy 31 (2006) 769– 774

reactions, that is, to serve as catalysts. (Note that possi-

ble mechanisms of plasma catalysis of hydrocarbons due

to ion–molecule reactions are discussed in [13,14].) Ac-

cording to the experimental data cited above, discharges

with cold plasmas (dielectric barrier discharges, coronas),

where the discharge effect is mainly in generation of active

species, produce hydrogen less efficiently than those with

hot plasmas (arc discharges). Therefore, one may assume

that for hot plasmas the effect of gas heating plays the major

role.

In [15,16] an analysis of the reforming process, aimed

at the search of optimal conditions for hydrogen produc-

tion, has been performed on the basis of thermodynamic ap-

proach. However, in general such approach is insufficient:

the effective time of the overall chemical process should

not exceed the residence time of reacting mixture inside the

chemical reactor, the latter being typically in the range of

seconds. Thus, more realistic estimates of parameters of the

reforming process are required. Such estimates can be ob-tained using a kinetic approach.

In this work, the known kinetic schemes for oxidation of

hydrocarbon gases are used for simulation of methane and

octane reforming in discharges with hot plasmas. On the ba-

sis of above considerations, the effect on hydrogen produc-

tion of active species generated by discharges is assumed

to be much smaller than that of gas heating. Therefore, no

discharge-related non-equilibrium reactions are taken into

account. The conditions are considered close to those real-

ising in experiments with low-current arcs.

2. The model

To simulate the plasma reforming process, a simple ap-

proach is used in which the discharge is accounted for as

an additional, with respect to chemical processes, energy

source inside the reactor. Calculations of chemical transfor-

mations, based on assumption of a perfect mixing of species

inside the reactor chamber, are performed using the code

PSR from the CHEMKIN-II package [17]. Input parame-

ters for the code are the gas composition and temperature

T ent at the reactor entrance, pressure p, the reactor volume

V , the total mass flow rate G and the power loss from the

reactor W loss. To account for the energy input from the dis-charge, the value of W loss in our calculations is taken nega-

tive, W loss =−W , where W is the discharge power. Output

parameters are the gas composition at the reactor exit and

the temperature inside the reactor.

Partial oxidation of two hydrocarbon fuels has been sim-

ulated, methane and octane (the latter is conventionally con-

sidered as a substitute of gasoline, e.g., [1]), in empty reac-

tors in the absence of catalysts inside the reactor chamber.

Kinetic schemes of reforming are based on mechanisms of

fuel oxidation available in the literature. For simulation of

methane reforming, two kinetic schemes have been used:

GRI-Mech 3.0 [18] and the Leeds methane oxidation mech-

anism version 1.5 [19]. Simulation of octane reforming has

been performed using the kinetic scheme [20].

Composition of the mixture of hydrocarbon CnHm with

air and steam at the reactor entrance is characterized by

two parameters, O/C ratio, defined as 2[O2]/n[CnHm],

and H2O/C ratio, defined as [H2O]/n[CnHm]. Here [O2],

[H2O] and [CnHm] are the volume flow rates of oxygen,

water and hydrocarbon.

3. Results of simulation

3.1. Methane reforming

Plasma reforming of methane–air mixtures has been sim-

ulated for conditions close to those of experiment [6]. The

results of simulation are given in Figs. 1 and 2. O/C ratio

was varied from 0.4 to 0.8, H2O/C ratio was taken equal to

zero. Fig. 1 shows the ratio H2/CO of the molar fractionsof H2 and CO in the reforming products versus O/C ratio;

two lines correspond to the results obtained using two above

mentioned mechanisms of methane combustion [18,19]. In

the same figure experimental data are shown, for two O/C

ratios ([6], Table 10, variants 3 and 4). It is seen that both

kinetic schemes give the values of H2/CO ratio rather close

to those measured. However, the results obtained with the

Leeds scheme [19] are in a better agreement with the ex-

periment concerning the trend of variation of H2/CO ratio

with the change of O/C.

Fig. 1. The ratio of the molar fractions of H2 and CO at methane

reforming versus O/C ratio at p=1bar, V =10−3 m3, W =1kW,

T ent = 300 K, the ratio of the discharge power to the total

volume flow rate 0.59kWh/m3(n). Lines—simulation, points—

experiment [6].



M.S. Benilov, G.V. Naidis/ International Journal of Hydrogen Energy 31 (2006) 769 – 774 771

Fig. 2. The ratio of the discharge power to the sum of volume

flow rates of produced H2 and CO at methane reforming versus

O/C ratio at conditions the same as in Fig. 1. Lines—simulation,

points—experiment [6].

In Fig. 2 the parameter SER, defined in [6] as the ra-

tio of the discharge power to the sum of volume flow rates

of H2 and CO produced, is shown versus O/C ratio. Both

schemes give the values of SER somewhat lower than those

measured. That is, the calculated efficiency of syngas pro-duction is higher than that obtained experimentally. Possi-

bly, this discrepancy occurs because the calculation does not

account for the thermal losses that are inevitably present in

experiment.

Note that the calculated gas temperature inside the re-

actor is rather high under the conditions considered above,

in the range 1400–1600 K. Such strong gas heating at rela-

tively small O/C ratios is mainly due to the energy supplied

by the discharge. At low energy inputs from the discharge,

when the gas heating is mainly due to the energy released at

methane oxidation process, considerably larger O/C ratios

should be employed. In [15], on the basis of the thermo-

dynamic analysis, it has been shown that optimal hydrogenyields correspond to the ratios of molar fractions of air and

methane in the range 3–3.5, that is, to O/C ratios in the range

1.3–1.5. The adiabatic gas temperature, calculated using the

relation of equality of the total enthalpies of reactants and

products, in these conditions is lower than 1200 K. (Note

that the adiabatic gas temperature is an increasing function

of O/C ratio.) The kinetic analysis shows that at such low

temperatures the time of establishment of equilibrium in an

empty reactor (without catalysts) is very large. As an exam-

ple, in Fig. 3 the results are given for simulation of chemical

transformations for O/C = 1.6, p = 1 bar at constant time

temperature 1270 K (the equilibrium adiabatic temperature).

Fig. 3. The molar fractions of H2, CO, CO2 and CH4 versus the

time of methane reforming at O/C = 1.6, p = 1bar, T = 1270 K.

Calculations have been performed using the code SENKIN

from the CHEMKIN-II package and the Leeds scheme of

methane oxidation. The molar fractions of various compo-

nents are shown as functions of time elapsed since the be-

ginning of chemical processes. It is seen that formation of

final products, H2 and CO, is a result of a complex chain of

chemical transformations, with a wide spectrum of reaction

times. At the first, fast stage of the reforming, at t < 0.1 s,

there is a steep decrease of XCH4 and a steep increase of

XH2 . However, at this stage only about one-third of hydro-

gen molecules are formed. Most of hydrogen is produced at

the second stage, which takes much more time. Therefore,

in order to provide fast enough methane reforming in the

absence of catalysts and at a relatively small energy input

from the discharge, O/C ratios should be taken substantially

larger than those corresponding to optimal hydrogen yields

estimated using thermodynamic approach.

3.2. Octane reforming

In Fig. 4 the composition of products of octane reforming

is given versus O/C ratio, simulated for conditions close

to those of experiment [7]; H2O/C ratio is taken equal to

zero. In the same figure, the molar fractions of products

are presented, measured in [7] at O/C = 1.32. It is seen

that the calculated composition of products is consistent

with that obtained in the experiment. Calculated temperature

inside the reactor increases with O/C ratio from 1300 K

(at O/C = 0.8) to 1600K (at O/C = 1.32) to 1900 K (at

O/C = 1.8).

Carbon monoxide obtained at partial oxidation of a hy-

drocarbon fuel may produce hydrogen in water shift reaction




Fig. 4. The molar fractions of products (in %) of octane re-

forming versus O/C ratio at p = 1bar, V = 2 × 10−3 m3,

W = 0.27kW, T ent = 300K, Gfuel = 0.26 g/s. Lines—simulation;

points—experiment [7]: — H2, — CO, ♦ — CO2, — CH4.

CO+H2O → CO2 +H2. (Note that the equilibrium in this

process is shifted to the right-hand side only at rather low

temperatures, T < 800 K. As partial oxidation proceeds fast

enough in the absence of catalysts at much higher tempera-

tures, the water shift can proceed only after cooling of par-tial oxidation products. At the stage of water shift, the use

of a catalyst is required because the time of overall process

in the gas phase at T < 800 K is unreasonably large.) One

of the parameters characterizing the efficiency of reforming

process is the ratio GH2/Gfuel of the mass flow rate of pro-

duced hydrogen, potentially accessible after water shift, to

the mass flow rate of fuel. In Fig. 5 the ratio GH2/Gfuel

is given versus O/C ratio, calculated for octane–air mixture

(without water) for various values of T ent and W . Optimal

O/C ratios, resulting in the highest hydrogen yield, are in

the range 1.3–1.5, corresponding to the ratio of air and fuel

mass flow rates at the reactor entrance Gair/Gfuel = 6–7.

It is seen that preheating of air–fuel mixture leads to a no-ticeable increase of hydrogen yield. The effect of discharge

power is relatively weak due to smallness of the energy in-

put provided by the discharge in comparison with the chem-

ical energy released in partial oxidation process. The role of

the discharge in these conditions is, evidently, mainly in the

formation of high-temperature regions where an ignition of

the reacting mixture occurs.

At small values of H2O/C ratio, the molar fraction of

H2O in the products of partial oxidation is smaller than that

of CO. In this case an additional amount of water should be

injected before the water shift stage. The effect of addition of

water to fuel–air mixture at the reactor entrance is illustrated

Fig. 5. The ratio GH2/Gfuel at octane reforming versus O/C ratio

at p = 2bar, V = 3× 10−3 m3, Gfuel = 1 g/s.

Fig. 6. The molar fractions of products (in %) of octane reforming

versus H2O/C ratio at O/C = 1.4, V = 3× 10−3 m3, p = 2bar,

T ent = 800K, W = 0.3kW, Gfuel = 1 g/s.

by Fig. 6, where the calculated composition of products is

given versus H2O/C ratio. Increase of H2O/C ratio results

in decrease of the molar fraction of CO and increase of that

of H2O. Fig. 6 shows that the use of mixtures with H2O/C

ratio about 0.5 (corresponding to the ratio of water and fuel

mass flow rates at the reactor entrance GH2O/Gfuel about

0.6) allows one to get, after partial oxidation, the mixture

with nearly equal molar fractions of H2O and CO. Note



M.S. Benilov, G.V. Naidis/ International Journal of Hydrogen Energy 31 (2006) 769 – 774 773

Fig. 7. The ratio GH2/Gfuel at octane reforming versus the dis-

charge power at O/C = 1.4, p = 2bar, V = 3× 10−3 m3.

that the effect of H2O/C ratio on hydrogen yield is not

substantial. It follows that the amount of water required for

water shift process may be added to the fuel–air mixture not

necessarily after fuel reforming in partial oxidation process,

but already at the first stage, before reforming.

The effect of discharge power on hydrogen yield is shown

in Fig. 7 where the ratio GH2

/Gfuel, potentially accessible

after water shift, is given versus W , for various values of T ent

and H2O/C ratio. It is seen that increase of W by about an

order of magnitude results in an increase of GH2 not exceed-

ing 10%. Such weak effect of W is due to smallness of the

energy input provided by the discharge in comparison with

the chemical energy released in partial oxidation process.

4. Conclusions

A kinetic approach to analysis of plasma reforming of

hydrocarbon gases based on the use of standard kinetic

schemes available in the literature without any additionaladjustments gives results which are in a reasonable agree-

ment with experimental data on low-current arc discharges.

This approach allows one to specify optimal conditions for

reforming process. In particular, for partial oxidation of oc-

tane in empty reactor (without a catalyst) the initial mix-

ture composition with O/C ratio about 1.3–1.5 is prefer-

able. Two stages of reforming, partial oxidation (stage 1)

and water shift (stage 2) should proceed separately. Before

the second stage, the mixture is to be cooled to temperatures

lower than 800 K. For water shift the use of a catalyst (e.g.,

based on iron oxide) is required. Cooling of the products of

partial oxidation before water shift stage may be provided

by means of heat exchange with the species before they en-

ter the reactor. Such heat exchange would also preheat the

mixture, thus enhancing hydrogen yield at the first stage of

reforming.

A use of regimes with high O/C ratios, where the re-

quired energy is provided by exothermic chemical reactions

of oxidation of fuel, allows one to work at relatively low

discharge powers. In these regimes the role of discharge

is mainly in mixture ignition, like in spark-ignited gaso-

line engines. Some threshold value of the discharge power

must exist that is required to ensure ignition. An extrapola-

tion of experimental data [7] gives, for the fuel mass flow

rate 1 g/s, the discharge power, sufficient for ignition, about

1 kW. Corresponding value of the energy consumption for

hydrogen production is about 5 kJ/g (about 0.1 eV per one

H2 molecule).

Acknowledgements

The work was supported by the EC through the project

No. ENK5-CT-2000-00346.

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Modeling of hydrogen-rich gas production by plasma reforming of hydrocarbon fuels.pdf