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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 ) 7 9 9 4e8 0 0 1

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Laminar burning velocity and interchangeabilityanalysis of biogas/C3H8/H2 with normal andoxygen-enriched air

Cesar A. Cardona*, Andres A. Amell

Science and Technology of Gases and Rational Use of Energy Group, Faculty of Engineering, University of Antioquia,

Calle 67 N� 53-108, Bloque 20-447, Medellın, Colombia

a r t i c l e i n f o

Article history:

Received 4 February 2013

Received in revised form

3 April 2013

Accepted 13 April 2013

Available online 13 May 2013

Keywords:

Laminar burning velocity

Interchangeability

Oxygen-enriched combustion

Biogas

Propane

Hydrogen

* Corresponding author. Tel.: þ57 4 219 85 48E-mail addresses: ccardona@udea.edu.co

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.04.0

a b s t r a c t

Numerical and experimental measurements of the laminar burning velocities of biogas

(66% CH4 e 34% CO2) and a biogas/propane/hydrogen mixture (50% biogas e 40% C3H8 e

10% H2) were made with normal and oxygen-enriched air while varying the air/fuel ratio.

GRI-Mech 3.0 and C1eC3 reaction mechanisms were used to perform numerical simula-

tions. Schlieren images of laminar premixed flames were used to determine laminar

burning velocities at 25 �C and 849 mbar. The mixture’s laminar burning velocity was found

to be higher to that of pure biogas due to the addition of propane and hydrogen. An in-

crease in the laminar burning velocities of both fuels is reported by enriching air with

oxygen, a phenomenon that is explained by the increased reactivity of the mixture.

Additionally, an analysis of interchangeability based on both the Wobbe Index and the

laminar burning velocity between methane and a biogas/propane/hydrogen mixture is

presented in order to consider this mixture as a substitute for natural gas. It was found that

the variations of these properties between the fuels did not exceed 10%, enabling

interchangeability.

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

reserved.

1. Introduction established by Chen et al. [2]: (1) replacing fossil fuels with

The high production of gaseous pollutants such as NOx, CO,

CO2 and HC is due to the use of fossil fuels, which still

constitute the primary sources of global energy [1]. Both the

growing concern about the amount of these gases emitted into

the environment and the predictions of the exhaustion of

petroleum-derived fuels have motivated the inclusion of

alternative fuels, such as syngas (produced from biomass

gasification), biodiesel (produced from plants) and biogas

(generated fromorganicwaste), to the energy basket. In recent

decades, the use of these fuels has taken a leading role due to

a change in global consciousness, as evidenced by scientific

advances that have been focused on three aspects, as

; fax: þ57 4 211 90 28., cscardona@yahoo.com (2013, Hydrogen Energy P94

renewable fuels, (2) the development of more efficient tech-

nologies and (3) the combination of fossil and renewable fuels.

Unprepared for total replacement, the latter option is most

attractive because it requires no major changes in transport

infrastructure, distribution and storage, does not demand

significant changes in combustion equipment and provides a

fast reduction in greenhouse gas emissions. However, there is

a high level of complexity required for this alternative to be

efficient because these types of renewable fuels have low

energy density and heating values, very low burning veloc-

ities, high ignition energy and narrow flammability limits;

these features affect its combustibility when compared to

conventional fossil fuels.

C.A. Cardona).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 ) 7 9 9 4e8 0 0 1 7995

Among alternative fuels, biogas has certain advantages

with respect to its production because it is generated from the

anaerobic digestion of biomass and organic waste, such as

manure, garden waste and sewage, among others. It can be

used for heating, lighting, transportation, power generation

and as a complementary fuel for large turbines [2,3]. The

composition of biogas, though it varies depending on the

production process, is approximately two-thirds methane,

and the rest is carbon dioxide with some traces of other gases,

as shown in Table 1.

As a fuel, biogas has a low heating value, low burning ve-

locity and narrower flame-stability limits, mainly due to the

high amount of inerts present in the fuel. The auto-ignition

temperature is high, so it is resistant to self-ignition [3]. Due

to these characteristics, biogas presents certain problems

related to instability when used in conventional combustion

systems, which hinders their integration with traditional

methods of converting chemical energy to thermal energy [4].

There are two alternatives for improving the combustion

properties of biogas: mixing it with other fuels with better

features [5] or increasing the amount of oxygen in themixture.

Addressing both alternatives, this study raises the possibility

of studying the combustion of biogas and a biogas/propane/

hydrogenmixture, with both normal air and air enriched with

pure oxygen. A mixture with natural gas is not considered

because, although it solves the problems of instability, the

calorific value remains low.

The chemical kinetics and laminar burning velocities of

propane, hydrogen and methane (the main constituent of

biogas) have been widely discussed separately [6e9], but the

study of their specific mixture has rarely been considered. For

this reason, studies should be performed to predict the

behavior of the combustion of this mixture in order to estab-

lish new properties to be used for equipment design that uses

this mixture as fuel. One of the most important properties is

the laminar burning velocity (SL), which characterizes all air/

fuel mixtures and is defined as the speed with which the un-

burnt gases move through the flame front [10,11]. This

parameter, which is characteristic of each fuel mixture, con-

tains essential information regarding reactivity and diffu-

sivity, and it is also used in the analysis of combustion

phenomena, such as stability, premixed flame structure, ki-

netic mechanism validation, turbulent burning velocity, fuel

interchangeability and burner design [12,13].

The idea of mixing these fuels is not new; hydrogen

[8,11,14e19] and propane [19,20] have been mixed with

methane to improve its combustion properties. However, the

combustion properties of the biogas/propane/hydrogen

mixture have been reported in few publications. Chen et al. [2]

Table 1 e Composition of biogas.

Methane 55e65%

Carbon dioxide 35e45%

Hydrogen sulfide 0e1 ppm

Nitrogen 0e3 ppm

Hydrogen sulfide 0e1 ppm

Oxygen 0e2 ppm

Ammonia 0e1 ppm

studied a hydrogen-enriched biogas diffusion flame subjected

to a slight oxygen enrichment, and Lee et al. [21] determined

the burning velocities of biogas and a biogas/propanemixture,

varying the amount of propane in themixture and the air-fuel

ratio. Other publications by the same authors focused on the

study of the flame stability of biogas and two landfill gas/

propane mixtures in laminar premixed and turbulent non-

premixed combustion [13]; the authors were also focused on

the interchangeability between some biogas/propane mix-

tures with natural gas for domestic appliances [22]. The

compositions of the mixtures were such that the calorific

value and the Wobbe index were the same as those of natural

gas. Tang et al. [6] and Park et al. [19] determined the burning

velocity and other combustion properties of the hydrogen/

propane mixture at different air/fuel ratios and hydrogen

fractions, and Milton and Keck [8], determined the laminar

burning velocity of the stoichiometric mixture of hydrogen

with some hydrocarbons, including propane, at different

pressures and temperatures.

In the field of reciprocating thermal machines, biogas has

enjoyed some popularity as a fuel for internal combustion

engines. Some authors [1,3,23,24] have considered the addi-

tion of hydrogen to biogas in different proportions in order to

study their performance in spark ignition engines, showing

benefits in terms of efficiency and reduction of greenhouse

gases.

To improve the combustion properties of methane, the

addition of oxygen to the fuel mix has also been considered

[25]. This addition increases the reactivity of the mixture and

therefore increases the laminar burning velocity.

The objective of this study is to numerically and experi-

mentally determine the laminar burning velocity of biogas

and a biogas/propane/hydrogen mixture with both normal

and oxygen-enriched air at different air/fuel ratios.

2. Experimental methodology

2.1. Gas composition

The chemical composition of the mixture has been defined

such that its High Wobbe Index (HWI) is equal to that of

methane (HWICH4 ¼ 8.14 kW h/m3st) as it is of interest to use

this mixture as a substitute for natural gas in burners for both

residential and industrial use and for internal combustion

engines. Table 2 presents the composition of the studied gases

and the oxygen enrichment level.

2.2. Experimental setup

The experiments were performed in a contoured slot burner,

and SL values were determined through the angle method

using Schlieren images of the flame front. The flames were

generated in three contoured slot burnerswith different outlet

geometries. The selection of the burner depends on the esti-

mated burning velocity of the mixture, considering that the

slot output speed is directly related to SL. The internal geom-

etry of these burners can maintain laminar flows for all air/

fuel ratios studied while reducing the effects of flame stretch

Table 2 e Composition of the mixtures.

Name Fuel HWI (kW h/m3st) Fuel mixture composition (%vol., dry)

Fuel Oxidizer

CH4 CO2 C3H8 H2 O2 N2

Bio21 Biogas 7.4 66 34 e e 21 79

Bio24 Biogas 66 34 e e 24 76

Bio32 Biogas 66 34 e e 32 68

M21 Biogas þ C3H8 þ H2 14.11 33 17 40 10 21 79

M24 Biogas þ C3H8 þ H2 24 76

M32 Biogas þ C3H8 þ H2 32 68

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 ) 7 9 9 4e8 0 0 17996

and curvature on the axis of the burner. The details of the

experimental setup can be found in the reference [26].

2.3. Determination of the laminar burning velocity

To determine the laminar burning velocity, the angle method

was used. Themeasurement is based on the principle that the

velocity at the nozzle exit of the unburnt gases is equal to the

velocity at which the flame front propagates from the burnt to

the unburnt zone at an angle q, that is SL ¼ Sug Sin q, where Sugis the mean velocity of the unburnt gases at the exit of the

burner and q is half of cone angle of flame.

Although in this method it is only possible to determine

average values of SL due to effects of the flame stretch and

curvature and heat loss to the walls of the burner, it has been

demonstrated that awell-designed contoured slot-type nozzle

burner can yield highly accurate laminar burning velocity

values, as demonstrated by Burbano et al. [26] and Pareja et al.

[26e28] in their experimental studies using the same type of

burner.

For each flame, it is possible to determine two q angles,

corresponding to the inner and outer edges of the flame front,

which are obtained through the processing of high resolution

images. The processing consists of subtracting a background

image previously captured from the flame image; the contrast

of the resulting image was increased in order to detect the

flame fronts. It was found that the difference between these

two angles is less than 1�, and therefore, the values reported

here correspond to the inner flame front angle.

0

5

10

15

20

25

30

0.5 0.7 0.9 1.1 1.3 1.5

Lam

in

ar b

urn

in

g v

elo

city (cm

/s), S

L

Equivalence ratio,

Bio21 (Exp.)GRI-Mech 3.0C1-C3

Fig. 1 e Laminar burning velocity of biogas with normal air

at 0.828 atm and 295 K.

2.4. Experimental conditions

The experiments were performed in the city of Medellın at

0.828 atm, 68% relative humidity and 295 � 2 K. The equiva-

lence ratios varied from lean conditions (B ¼ 0.6) to rich

conditions (B ¼ 1.6); within this range, it was possible to

obtain well-defined and stable flames. The response variables

were the mean velocity of the unburnt gases and the angle of

the flame. For each equivalence ratio, 60 images were

captured and processed in order to obtain reliable data.

Errors in the measurement of the laminar burning velocity

for each equivalence ratio were calculated as the propagation

errors from the measurements of the mean velocity of the

unburnt gases at the exit of the burner and the flame angle.

The error of the mean velocity was calculated from the mea-

surement error of the burner nozzle area and the flow of fuel

and air. The error of the flame angle was calculated by the

statistical treatment of the 60 images captured for each case,

where standard deviations were less than 1.5�.

3. Numerical methodology

The laminar burning velocity numerical calculations were

performed using the PREMIX code of the CHEMKIN-PRO

package. For comparative purposes, simulations were per-

formed with two detailed kinetic mechanisms, GRI-Mech 3.0

[29], whose effectiveness has been extensively tested for

methane oxidation [15,29e31], and C1eC3 of Qin [7], whichwas

developed for the combustion of heavy hydrocarbons,

including CH4, C2H4, C2H6, C3H4, C3H6 and C3H8. Due to the

presence of hydrogen in the mixture, transport properties

were evaluated using the multicomponent diffusion model

and the Soret effect.

4. Results and discussion

4.1. Combustion with normal air

The experimental results of the laminar burning velocity of

Bio21 (66% CH4 e 34% CO2) according to the equivalence ratios

are shown in Fig. 1, along with the results of numerical sim-

ulations, which were carried out using the detailed kinetic

0

10

20

30

40

50

60

0.6 0.8 1.0 1.2 1.4 1.6

Lam

in

ar b

urn

in

g velo

city (cm

/s), S

L

Equivalence ratio,

Propane (Exp.)GRI-Mech 3.0C1-C3

Fig. 3 e Laminar burning velocity of propane with normal

air at 0.828 atm and 295 K.

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 ) 7 9 9 4e8 0 0 1 7997

mechanisms of GRI-Mech 3.0 and C1eC3. At 0.828 atm and

295 � 2 K, it was only possible to stabilize the laminar flame

between 0.9 < B < 1.2.

In both the stoichiometric and rich mixture regions, the

experimental results fit well with the predictions of GRI-Mech

3.0 because the mechanism has been adjusted to the oxida-

tion of methane, the main constituent of biogas.

The difficulty of working with biogas in conventional

burners was evidenced by the few equivalence ratios in which

it was possible to determine the burning velocity. This

behavior has already been reported by Qin et al. [32], who

varied the amount of CO2 in the biogas from 0% to 50%, rep-

resenting a 45% decrease in the burning velocity and by

reducing the adiabatic flame temperature. This phenomenon

is due to the addition of CO2 to the combustion of methane,

which absorbs energy from the reaction because of its high

specific heat and emits radiation to the surrounding due to its

high emissivity. This phenomenon significantly affects the

stability of the flame, leaving the biogas as a low flexibility fuel

for its use in commonly used burners.

Under the same experimental and numerical conditions,

Fig. 2 shows the burning velocity of the M21 mixture, whose

volumetric composition is 33% CH4, 17% CO2, 40% C3H8 and

10% H2, as shown in Table 2.

In the fuel lean region, the experimental results corre-

spond to those of Gri-Mech 3.0; but in the fuel rich region, the

results vary slightly from the prediction with the same

mechanism, although the differences never exceed 10%. Due

to the addition of propane and hydrogen to biogas, the mix-

ture’s laminar burning velocity at stoichiometric conditions

presents an increase of 65.6% compared to pure biogas.

Additionally, it was possible to obtain stable flames between

0.8 < B < 1.4.

In order to make comparisons and because propane rep-

resents 40% of the mixture, its burning velocity in a pure state

was also numerically and experimentally determined at the

same sub-atmospheric conditions, as shown in Fig. 3. The

experimental results are compared with those obtained in the

numerical simulations using the GRI-Mech 3.0 and C1eC3

mechanisms. It was possible to obtain stable flames within

the range 0.9 < B < 1.3.

0

10

20

30

40

50

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

La

min

ar b

urn

in

g velo

city (c

m/s

), S

L

Equivalence ratio,

M21 (Exp.)GRI-Mech 3.0C1-C3

Fig. 2 e Laminar burning velocity of M21 mixture at

0.828 atm and 295 K.

The predictions of the C1eC3 mechanism (designed for

heavy hydrocarbons) are closer to the experimental results of

propane, but similar trends are not evident between both re-

sults. However, the differences between the experimental and

numerical data for each equivalence ratio never exceed 10%;

in the stoichiometric region, the difference is only 2.8%, as

shown in Fig. 3.

Lee et al. [21] numerically and experimentally studied

biogas (CH4 54.5% e 37.5% CO2 e N2 7% e 1% O2), LPG (100%

C3H8) and their mixture in various proportions. The authors

used Gri-Mech 2.0 and C1eC3, among other mechanisms. In

the case of pure propane, the experimental results are greater

than those predicted by the C1eC3 mechanism, showing an

acceptable agreement with the results presented in Fig. 3. For

biogas, although the experimental results showed the same

trend as Gri-Mech 2.0, the absolute values are below the

numeric predictions. Note that, in this study, Gri-Mech 3.0was

used and the approximation was fairly good, making it clear

that this version is muchmore precise due to the adjustments

made by the authors.

An effective method to solve the problem of biogas’s low

burning velocity is to mix it with other fuels with higher

burning velocities [11]. To this end, the biogas/hydrogen

mixture would offer better performance than the M21

mixture, but it should be noted that hydrogen is an expensive

fuel that is difficult to manage, and the combustion properties

would move away from those required to consider the new

mixture as a substituent of natural gas. At this point, it is

important to make an observation regarding the effect of the

addition of propane to the biogas/hydrogen mixture, which

would make the M21 mixture a good interchangeable alter-

native to natural gas.

When hydrogen and propane are added to biogas, the

laminar burning velocity is expected to increase because the

reactivity of the mixture increases along with the flame tem-

perature; the relation between themixtures’ reactivity and the

flame temperature can be explained by the Arrhenius form

one-step reaction, where the burning velocity increases with

the increase of the adiabatic flame temperature [33]. The

adiabatic flame temperature of M21 is 2229 K and is 95 K

105

SL

M21 (Exp.)M21

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 ) 7 9 9 4e8 0 0 17998

higher than that of Bio21, therefore the burning velocity of

M21 is higher, as presented in Figs. 1 and 2.

0

15

30

45

60

75

90

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Lam

in

ar b

urn

in

g v

elo

city (cm

/s),

Equivalence ratio,

M24 (Exp.)M24M32 (Exp.)M32

Fig. 5 e Laminar burning velocity of the biogas/propane/

hydrogen mixture with oxygen-enriched air at 0.828 atm y

295 K.

4.2. Combustion with oxygen-enriched air

This study also considered the enrichment of air with pure

oxygen for the combustion of biogas and the biogas/propane/

hydrogen mixture. Figs. 4 and 5 show respectively, the nu-

merical and experimental results of biogas and biogas/pro-

pane/hydrogen laminar burning velocities with 24% and 32%

oxygen-enriched air. The numerical results correspond to

those obtained with GRI-Mech 3.0.

In Fig. 4, it can be seen that the Bio24 and Bio32 experi-

mental results do not resemble the predictions made by GRI-

Mech 3.0. Although this mechanism has been widely used

formethane combustion, the addition of oxygen to the air and

its effect on the kinetics of combustion seems to have not

been contemplated in the development of the mechanism.

While trends of experimental and numerical results are

similar, there are significant variations in the absolute values

of burning velocity. Paradoxically, the results presented in

Fig. 5, which correspond to the biogas/propane/hydrogen

mixture with enriched air up to 32% oxygen, correspond quite

well to the predictions made by the mechanism, regardless of

the addition of oxygen to the air. In M24 and M32, there is a

significant increase of hydrocarbons in the mixture and a

considerable reduction of CO2 and N2 due to the addition of

propane, hydrogen and oxygen. In particular, CO2 is relegated

to 17% of the fuel mixture; this represents a reduction of heat

loss from the flame front, which depends on the amount of

CO2 present in themixture and on the associated emissivity of

its concentration, among others factors.

GRI-Mech 3.0 may be more sensitive to the addition of

hydrocarbons to themixture than to the increase of oxygen in

the oxidizer, and therefore, the numerical results of Fig. 5

more closely approximate the data compared in Fig. 4. In

both of the cases presented in Figs. 4 and 5, it is concluded that

the addition of oxygen to the air results in more stable flames

and a significant increase in the burning velocity, regardless

the nature of the fuel. This phenomenon is very important for

lean fuel industrial applications.

0

10

20

30

40

50

60

70

0.4 0.6 0.8 1.0 1.2 1.4 1.6

Lam

in

ar b

urn

in

g v

elo

city (cm

/s), S

L

Equivalence ratio,

Bio21 (Exp.)Bio21Bio24 (Exp.)Bio24Bio32 (Exp.)Bio32

Fig. 4 e Laminar burning velocity of biogas with oxygen-

enriched air at 0.828 atm y 295 K.

According to Fig. 4, moderate addition of oxygen has

greater impact on the burning velocity of pure biogas, i.e., for

biogas combustion, 24% oxygen in the air increases the

burning velocity up to 33% at the stoichiometric condition; the

same oxygen level for biogas/propane/hydrogen combustion

increases the burning velocity up to 17%. When the enrich-

ment level is slightly higher (32% oxygen in the air), the effect

of oxygen addition on both fuels is basically the same,

achieving an increase of 52% compared to the combustion

with normal air.

The increase in the amount of oxygen in the mixture re-

sults in increased adiabatic flame temperatures as follows:

15.6% of Bio21 with respect to Bio32 and 15.4% of M32 with

respect to M21 at the stoichiometric region, as shown in Figs. 6

and 7. Higher adiabatic flame temperatures make the burning

velocity to increase due to the increase of mixtures’ reactivity.

These increases are due in part to the fact that the addi-

tional oxygen results in less air being required tomaintain the

same stoichiometry [25]. The use of oxygen instead of air

represents a reduction in the inerts in the fuel and its

Ad

ia

ba

tic

fla

me

te

mp

era

tu

re

(K

)

Equivalence ratio, φ

Bio21

Bio24

Bio32

Fig. 6 e Adiabatic flame temperatures of biogas with

oxygen-enriched air at 0.828 atm y 295 K.

Ad

ia

ba

tic

fla

me

te

mp

era

tu

re

(K

)

Equivalence ratio, φ

M21

M24

M32

Fig. 7 e Adiabatic flame temperatures of the biogas/

propane/hydrogen mixture with oxygen-enriched air at

0.828 atm y 295 K.

La

min

ar b

urn

in

gvelo

city

(c

m/s

), S

(B

la

ck

ba

rs)

HW

I (

kW

h/m

st) (

Gra

y b

ars

)

Fig. 8 e High Wobbe Index, HWI (gray bars) and laminar

burning velocity, SL (black bars) of pure methane with

normal air and biogas and the biogas/propane/hydrogen

mixture with normal and oxygen-enriched air at B [ 1.0,

0.828 atm and 295 K.

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 ) 7 9 9 4e8 0 0 1 7999

consequent ability to absorb heat during the reaction, result-

ing in a higher flame temperature. In practical terms, the use

of additional oxygen in combustion results in increased pro-

ductivity in the industrial heating processes and improved

flame stability.

Due to the increased burning velocity and the widening of

the interval of stable flames, in particular for fuel lean condi-

tions, it is evident that the biogas combustion properties

significantly improved when air is enriched with pure oxygen

and when it is mixed with propane and hydrogen; however, is

not sufficient to achieve higher burning velocities if the aim is

to introduce this type of gas as fuel for thermal machines and

domestic and industrial appliances. There are other parame-

ters that must be taken into account, such as flame shape,

stability and power, efficiency, combustion products and

storage and transport characteristics [34]. For interchange-

ability purposes, theWobbe Index (WI) is regardedasoneof the

most important parameters because it can account for gas

quality without considering specific aspects. This index,

defined as the relationship between the heating value and the

square root of the relative density of the fuel, is proportional to

the thermalpowerof the system if theoperatingandgeometric

parameters remain constant [34]; i.e., for two gases with

identical WI, it is not necessary to make changes in the com-

bustion system geometry, injectors or gas supply conditions.

4.3. Interchangeability analysis

This analysis was performed considering the behavior of both

the Wobbe Index and the laminar burning velocity of the

biogas/propane/hydrogen mixture with respect to methane.

As a first approximation to their interchangeability, the ther-

mal input and the heat release rate must be approximately

equal, ensuring identical conditions of pressure, temperature

and equivalence ratio. The first condition is satisfied by the

equality between the Wobbe Indexes of the fuels and the

second, by the similarity of the laminar burning velocities.

For comparison purposes, Fig. 8 shows the burning veloc-

ities at stoichiometric conditions and the High Wobbe Index

(HWI) of biogas and the biogas/propane/hydrogen mixture

with normal and oxygen-enriched air compared with pure

methane with normal air.

The High Wobbe Index of the biogas/propane/hydrogen

mixture, whichwas established in advance at the beginning of

this investigation, corresponds to a gas composition defined

following these parameters: biogas and propane (the main

component of LPG) must have greater participation in the

mixture, and the HWI of the mixture should be as close as

possible to that of pure methane. The bars that quantify the

properties analyzed for methane and M21 are quite similar:

the HWI of methane and M21 are 14.08 kW h/m3st and

14.11 kW h/m3st, respectively; the burning velocities of

methane and M21 are 41.65 cm/s and 46.33 cm/s, respectively.

These differences do not exceed 10% and are a good, though

not unique, indicator of the interchangeability of these fuels.

Other properties shall be explored to ensure that the effi-

ciency, safety and quality of the combustion of the mixture of

biogas/propane/hydrogen used here do not show significant

changes with respect to the combustion of methane.

5. Conclusions

Measurements of laminar burning velocity of biogas (66% CH4

e 34% CO2) and a biogas/propane/hydrogen mixture (33% CH4

e 17% CO2 e 40% C3H8 e 10% H2) were made with normal and

oxygen-enriched air at 0.828 atm and 298 K while varying the

air/fuel ratio. Numerical calculations of the laminar burning

velocity were also performed using detailed kinetic mecha-

nisms and were compared with the experimental results.

From the analysis of these results, the following conclusions

can be stated:

The predictions of the GRI-Mech 3.0 mechanism adjusted

quite well to the experimental results, except those of oxygen-

enriched biogas combustion, where certain differences were

possibly due to the high amount of CO2 in the fuel mixture.

The maximum burning velocity value experimentally ob-

tained for the biogas/propane/hydrogen mixture was 46 cm/s

atB¼ 1.0 with normal air. Themaximumvalue for biogaswas

28 cm/s atB¼ 1.1 with normal air. The increase of the laminar

burning velocity of themixture compared to that of biogas can

be explained by the addition of propane and hydrogen. When

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 ) 7 9 9 4e8 0 0 18000

hydrogen and propane are added to biogas, the burning ve-

locity is expected to increase due to increased reactivity of the

mixture which may be explained by the Arrhenius form one-

step reaction, where the rate of reaction of the mixture and

the burning velocity are directly proportional.

The burning velocity is significantly affected by the

amount of oxygen in the oxidizing mixture. Biogas and the

biogas/propane/hydrogen mixture were subjected to com-

bustion with oxygen-enriched air, where a remarkable in-

crease in the burning velocity was evidenced. For biogas, an

increase of 3% oxygen in the air reflects a 33% increase in the

burning velocity at stoichiometric conditions. For the same

increase of oxygen for the combustion of the biogas/propane/

hydrogen mixture, the burning velocity increased 17%. The

addition of oxygen to the fuel mixture results in the increase

of its reactivity, causing the burning velocity to increase.

The M21 mixture (33% CH4 e 17% CO2 e 40% C3H8 e 10% H2

with normal air) presents good signs of interchangeability

with methane (the main component of natural gas) because

the HighWobbe Index and the laminar burning velocity do not

show considerable differences (less than 10%). However, these

two properties are not the only properties that must be

considered when studying the possibility of exchanging two

fuels for the same application, and it is suggested that other

analyses be performed in order to establish the interchange-

ability of these fuel gases.

Acknowledgments

The authorswould like to acknowledge the valuable economic

contributions of COLCIENCIAS through the program “JOVENES

INVESTIGADORES E INNOVADORES 2010e2011 VIRGINIA

GUTIERREZ DE PINEDA” and the University of Antioquia

through the program “SOSTENIBILIDAD 2013e2014 DE LA

VICERRECTORIA DE INVESTIGACION”. The financial support

of GASURE Group of the University of Antioquia is also

acknowledged. The authors also thank the “Jovenes Inves-

tigadores U. de A.” for their support in the experimental

measurements.

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