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SKMM 4453 Project Report Title: Combustion Characteristics of Biogas from Palm Oil Mill Effluent (POME) at Variable Equivalence Ratio Name: Ahmad Shukrie Md. Yudin Matric Number: 123456789 Date: 20 September 2016

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SKMM 4453

Project Report

Title: Combustion Characteristics of Biogas from Palm

Oil Mill Effluent (POME) at Variable Equivalence Ratio

Name: Ahmad Shukrie Md. Yudin

Matric Number: 123456789

Date: 20 September 2016

Combustion Characteristics of Biogas from Palm Oil Mill Effluent

(POME) at Variable Equivalence Ratio

A. S. Md Yudin 1, 2

, A. Saat1 and M.F. Mohd Yasin

1*

1High Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia,

81310, Skudai Johor Bahru, Malaysia. 2Energy and Sustainability Focus Group, Faculty of Mechanical Engineering, Universiti Malaysia Pahang,

26600, Pekan, Pahang, Malaysia.

*Corresponding author:

Mohd Fairus Mohd Yasin

[email protected]

Abstract

The use of different fuels in unmodified engines requires a thorough understanding of the

change of combustion characteristics that are introduced by the different fuel. In the present

study, the combustion characteristics of biogas at different equivalence ratio through

numerical analysis are studied. A non-premixed flame is simulated based on a lab scaled

burner with methane as a fuel for validation purpose. The turbulent non-premixed

combustion simulation was performed by using turbulence model coupled with Steady

Flamelet model integrated with GRI 2.11 detailed kinetic mechanism by using Probability

Density Function (PDF) approach. Good agreement was achieved between the numerical

prediction and experimental data where the temperature distribution in the axial and radial

direction of the burner was reproduced quite well. The combustion simulation of POME

biogas with 65% methane and 35% carbon dioxide composition at different equivalence ratio

ranging from 0.1 to 0.7 was simulated at fixed power output of 8.5kW. The fuel-bound CO2

consumption dominates the unique change of CO2 in the near-burner region of biogas flame

which has not been observed in non-premixed flame of hydrocarbon fuels. Due to the same

CO2 content of biogas, the specific heat of the mixture reduces and results in higher

maximum temperature and average temperature at the central axis and the outlet respectively

compared to that of methane. Surprisingly, the high average temperature at the outlet of

biogas flame produces low average NOx emission due to the reduction in the rate of NOx

production by the high concentration of CO2 in the reactant. At equivalence ratio of 0.1 to

0.6, the fuel switch from methane to biogas at fixed power results in more than 40%

reduction in NOx emission at the expanse of 25% increase in CO2 emission.

Keywords: CFD; biogas; methane; flame; combustion; NOx

1.0 Introduction

The concern on the pollution from burning of conventional fossil fuels that eventually

leads to global warming has triggered the major environmental issues worldwide [1]. The

primary greenhouse gas compositions in the atmosphere consists of water vapor (H2O),

carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3) [2,3].

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Pick an operating condition to be varied
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Introduce some novelty in the general study of biogas combustion
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Ignore this part

Concentrations of CO2, CH4 and N2O have shown large increase since few decades at an

average of 40%, 150%, and 20%, respectively, driven largely by economic and population

growth [4]. The Fifth Assessment Report (AR5) of Intergovernmental Panel on Climate

Change (IPCC) published in 2015 pointed out that methane was ranked second among the

most dominant greenhouse gas with a recent increase in emission level compared to that of

reported in the Fourth Assessment Report (AR4) of IPCC published in 2008 [2,4,5]. Besides

transport and energy sectors, the emission of CH4 from the agriculture and forestry are

among the dominant anthropogenic sources of CH4 [2,4].

In the palm oil industry, the methane gas generated from the anaerobic decomposition of

palm oil mill effluent (POME) is released into the atmosphere at an average rate of

1043.1 kg/day per anaerobic pond [6,7]. POME is the liquid waste with high biochemical

oxygen demand and chemical oxygen demand generated from the oil extraction process from

fresh fruit bunches in palm oil mills [1,7]. In a typical palm oil mill with annual production

capacity of 300,000 ton, it is estimated that 21,000,000 m3 biogas will be generated every

year [8]. Previous investigations illustrate that biogas production from anaerobic digestion of

POME is composed of 65% methane and 35% carbon dioxide [9]. In the midst of the

increasingly strict regulations on pollution, the palm-based biogas offers a renewable and

environment-friendly alternative to fossil fuels.

The state-of-the-art technology of high pressure boilers allows the operation with dual-

fuel capabilitywhere diesel or natural gas are used as supplementary energy source [10]. The

most remarkable feature offered by dual fuel diesel engine is the ability to switch over from

dual fuel operation to diesel mode almost promptly in case of shortage of the primary fuel

[11]. However, the change of combustion characteristics due to the fuel switch has to be

properly investigated to ensure that a proper level of combustion efficiency with acceptable

range of emission is achieved with the new fuel [12,13] . Numerous combustion research

works are carried out to investigate the compatibility of biogas from POME, non-edible oil

seeds, and many other biogas sources as substitute to natural gas in duel fuel diesel engine

[11,14–20] with very few studies have done a thorough comparison between the combustion

characteristics of methane and biogas at varying fuel-air mixture. The ultimate motivation

behind such investigation is to reduce the dependency on fossil fuel for power generation.

The excess presence of carbon dioxide in biogas acts as a diluent that reduces the

calorific value which in turn affects the combustion process [21,22]. Previous studies have

investigated the effects of CO2 dilution on emission of methane flame [23] and recent finding

showed that the high CO2 content in biogas deters the dominant reaction path [24]. Thus, the

present study aims to investigate the combustion characteristics of biogas driven from

POME. The combustion of POME biogas with 65% methane and 35% carbon dioxide will be

examined in depth with the help of computational fluid dynamics (CFD) at variable

equivalence ratio. The distribution of axial temperature, species and NOx emissions will be

analyzed to investigate the use of biogas as alternative fuel for energy generation. In the

present study, the configuration of a lab scale burner of Brookes and Moss [25] has been

adopted as the baseline boundary condition for methane flame validation. Then, a biogas

flame is simulated at varying equivalence ratio while maintaining the same power output as

the baseline methane flame.

2.0 Computational methodology

The burner geometry is shown in Fig. 1 where the main fuel burner had a 4.07 mm outlet

diameter and confined within 0.16 m width annular pilot flame surrounding the main burner.

The pilot flame is operated with 2% flow rate of the main fuel. The stabilized flame is

confined in a Pyrex tube of internal diameter 155 mm. The operating conditions for burner

are listed in Table 1. An axisymmetric computational domain with the boundary conditions

as shown in Fig. 2 were implemented in ANSYS FLUENT 14.0. The uniform mesh was

chosen by implementing mapped face meshing method with a systematic grid-refinement to

ensure that the simulation results are independent of the computational grid size. Mesh

independence was achieved with 64000 cells based on the axial temperature distribution.

Fig. 1. Piloted methane burner geometry in the present study [25].

The steady, turbulent, and incompressible flow is employed in the present study with

Reynolds Average Navier Stokes solver that computes the transport equations with the finite

volume method. In the present simulation, the model with standard wall functions is

employed with modification to turbulence coefficient . Second order upwind

discretization scheme is selected to obtain solution for the equations of mass, momentum,

energy, species, turbulent kinetic energy, and turbulent dissipation rate.

In modeling a non-premixed combustion, Steady Flamelet model with PDF approach was

used in the simulation. The GRI 2.11 detailed mechanism of methane combustion that

consists of 49 species and 277 reactions [26] is employed in the simulation. The thermal formation of Zeldovich describes the formation of as shown in Eqs. 1 to 3.

(1)

(2)

(3)

Pilot flow

Main flow

0.16 mm

4.07 mm

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Experimental data that is chosen has to be comprehensive enough for CFD analysis
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The use of detailed chemistry is needed for a proper validation
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Mesh independence study mentioned here. Detailed figures may be included in the appendix. Omitted here for brevity

The Coupled method is used for the pressure-velocity coupling scheme with least square

cell method for gradient and PRESTO! for pressure. CPU time is 0.58 hours on Intel Xeon

Quad-Core® processor (3.2 GHz). The residual reduction to more than six orders of

magnitude is set as the convergence criteria.

Table 1

Operating conditions for baseline methane flame

Fig. 2. Boundary conditions and 2-D structured mesh in the present study.

Composition (vol. %) 100% CH4

Power output (kW) 8.5

Absolute pressure (atm) 1

Fuel mass flow (kg/s) 1.72 × 10-4

Air mass flow (kg/s) 1.18 × 10-2

Pilot fuel flow (kg/s) 3.43 × 10-6

Equivalence ratio, 0.25

Fuel temperature (K) 290

Air temperature (K) 290

Exit Reynolds number 5000

B

A F

E

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Detailed boundary condition specification
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Callout
axisymmetric mesh to save simulation time

3.0 Results and discussion

Validation of numerical modeling

In order to validate the present numerical study, the predicted temperature distribution in

the axial and radial directions of methane flame are compared with the measured data in

methane flame experiment with a co-flow burner [25]. Figs. 3(a) and 3(b) show the

temperature distribution from experimental measurement and numerical prediction. The

radial temperature distribution which is measured at a distance of 150 mm from the burner

shows good agreement with the data within a maximum error of 7%. The present prediction

which is based on a modified turbulence coefficient shows a better agreement

with the data compared to the modified turbulence coefficient suggested by Ziani and

Chaker [27] . The predicted radial temperature value shows good agreement with the

experimental values in a region close to the centerline of the flame. However, the predicted

temperature shows slight deviation from the experimental measurement at a far field region

from 17 to 30 mm from the centerline. The deviation might be attributed to the

underprediction of the heat transfer between the flame and the surrounding [28]. The

successful validation serves as a base to extend the model to simulate combustion of biogas

derived from POME with 65% methane and 35% carbon dioxide at variable equivalence

ratio.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 100 200 300 400 500

Tem

per

atu

re (

K)

Axial distance (mm)

Data

Prediction[27]

Prediction (present)

a)

Fig. 3. Temperature distribution along a) the centerline of the flame b) the radial distance at

150 mm height from the burner.

Effects of equivalence ratio on axial distribution of temperature and species

The global reaction of biogas combustion based on 65% CH4 and 35% CO2 by volume is

presented in Eq. 4 and the respective equivalence ratio is calculated based on Eq. 5 [29]

where and are the methane and air volumetric flow rates at 298K and 1 atm

respectively. Variable is the volumetric flow rate ratio of fuel to air at stoichiometry.

(4)

(5)

The combustion of biogas is simulated at an adjusted fuel and air flow rates as shown in

Tables 2 and 3(a) respectively to maintain the same power output and equivalence ratio as

the baseline methane flame. Referring to Tables 3(a) and 3(b), the study at varying

equivalence ratio for both flames is done by reducing the mass flow rate of air from

equivalence ratio 0.1 to 0.7 while the mass flow rate of fuel remains fixed. The comparison

of emission between the two flames are done at fixed power because the power output of

unmodified engines is normally maintained when the fuel switch is done while the

equivalence ratio is fixed to isolate the effects of the same parameter on flame temperature.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35 40

Tem

per

atu

re (

K)

Radial distance (mm)

b)

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Validation against the experimental data. The data is extracted from the published literature using plot digitizer software.

Table 2

Operating conditions for biogas flame

Table 3(a)

Inlet conditions for biogas flame at varying equivalence ratio

Table 3(b)

Inlet conditions for methane flame at varying equivalence ratio

mCH4

(kg/s)

mCH4, pilot

(kg/s)

mair

(kg/s)

0.1 1.70 x10-4 3.40 x10

-6 3.85 x10

-2

0.2 1.70 x10-4 3.40 x10

-6 1.54 x10

-2

0.3 1.70 x10-4 3.40 x10

-6 1.28 x10

-2

0.4 1.70 x10-4 3.40 x10

-6 9.62 x10

-3

0.5 1.70 x10-4 3.40 x10

-6 7.70 x10

-3

0.6 1.70 x10-4 3.40 x10

-6 6.42 x10

-3

0.7 1.70 x10-4

3.40 x10-6

5.50 x10-3

The effects of varying the equivalence ratio on the axial distribution of temperature and

species are shown in Figs. 4 to 7 with the close-up view shown in the inset. For both biogas

and methane flames, the temperature distribution along the axial direction shows the features

of a typical non-premixed flame where the temperature at the centerline gradually increases

Fuel composition (vol. %) 65% CH4, 35% CO2

Absolute pressure (atm) 1

Fuel temperature (K) 290

Air temperature (K) 290

mbiogas

(kg/s)

mbiogas, pilot

(kg/s)

mair

(kg/s)

0.1 4.45 x10-4 8.90 x10

-6 3.27 x10

-2

0.2 4.45 x10-4 8.90 x10

-6 1.42 x10

-2

0.3 4.45 x10-4 8.90 x10

-6 9.90 x10

-3

0.4 4.45 x10-4 8.90 x10

-6 7.59 x10

-3

0.5 4.45 x10-4 8.90 x10

-6 6.53 x10

-3

0.6 4.45 x10-4 8.90 x10

-6 5.44 x10

-3

0.7 4.45 x10-4

8.90 x10-6

4.67 x10-3

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Varying operating conditions

towards the reaction zone and then decreases towards the outlet. Figs. 4(a) and 4(b) show that

the deviation of temperature at different equivalence ratio can be seen quite significant in the

near outlet region for both flames. Maximum flame temperature of biogas and methane

shows steady increment as the equivalence ratio is increased due to the reduction of air

dilution and the increase in residence time as a result of the reduction in inlet mass flow rate.

However, the reduction of maximum temperature is seen at equivalence ratio 0.6 and 0.7 of

biogas due to the change in the local velocity field due to the significant reduction in co-flow

velocity at the respective equivalence ratio. The same phenomenon is not seen in the methane

flame whose minimum flow rate of air is almost 20% higher compared to that of the biogas

flame.

Figs. 5(a) and 5(b) illustrate CH4 distribution of biogas and methane flames where the

mass fraction of CH4 for biogas at the burner is lower compared to that of methane due to the

CO2 dilution of the former. The fuel is consumed in the downstream direction and reaches

complete combustion after the reaction zone at 0.65 m downstream of the burner. Negligible

variation in CH4 mass fraction between the equivalence ratios was recorded for both fuels

since all the present cases involve complete combustion.

As expected in Fig. 6, 35% fuel-bound CO2 of biogas is seen near the burner where the

same species does not exist at the same location for methane. Therefore, the mass fraction of

CO2 decreases in the downstream direction for biogas while the opposite trend is seen for

methane. In both flames, the downstream increase in CO2 for biogas and methane flame is

due to the co-flow entrainment into the central region. The reduction of CO2 mass fraction for

biogas within the near-burner region is dictated by the more dominant consumption of the

fuel-bound CO2 compared to the production of CO2 from the fuel oxidation. The former CO2

process becomes less dominant after halfway of the burner height where CO2 is seen to be

unchanged before increases again downstream. The present analysis agrees well with

previous study which has confirmed the existence of a dominant CO2 consumption reaction

in the prediction of ignition delay of biogas-air mixture in shock tubes [24]. On the contrary,

the increase in CO2 for methane flame is only dictated by the latter process. As seen at the

exhaust, the mass fraction of CO2 increases by 33% in biogas compared to that of methane

which happens due to the fuel-bound CO2 of the former. The close-up of the inset in Figs.

6(a) and 6(b) shows the variation of the mass fraction of CO2 at the downstream region for

both biogas and methane at varying equivalence ratio that mainly governed by the difference

in the local velocity field.

Fig. 7 shows the mass fraction of OH as an indicator for local heat release rate along the

central axis [30]. The location of maximum OH concentration indicates the location of

maximum heat release rate which results in the high temperature region as shown in Fig. 4.

The maximum mass fraction of OH for biogas and methane flames (Fig. 7(a)) that shows

steady increment with the peak shifted downstream as the equivalence ratio is increased

agrees well with the temperature profiles in Fig. 4. A rare trend in the near-exhaust OH

profiles for biogas flame at equivalence ratio 0.6 and 0.7 are due to the same reason that has

been discussed for the temperature profiles of the same case previously. The difference in the

location of reaction zone between the two flames has been attributed to the CO2

concentration on the fuel side of the biogas flame that reduces the burning rate [31].

Fig. 8 shows the maximum flame temperature along the central axis at varying

equivalence ratio for both flames where the higher maximum temperature of biogas flame

compared to that of methane flame at almost all range of equivalence ratio is seen due to the

low mixture specific heat of the former. The high concentration of CO2 in biogas reduces the

heat capacity compared to methane thus allows a rapid heating during the combustion

process.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Tem

per

atu

re (

K)

Axial distance (m)

φ=0.1

φ=0.2

φ=0.3

φ=0.4

φ=0.5

φ=0.6

φ=0.7

1700

1900

2100

0.4 0.5 0.6 0.7 0.8

a) Biogas

Fig. 4. Influence of equivalence ratio on axial temperature profiles along the flame

centerline using a) biogas and b) methane.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Tem

per

atu

re (

K)

Axial distance (m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Ma

ss f

ract

ion

of

CH

4

Axial distance (m)

φ=0.1

φ=0.2

φ=0.3

φ=0.4

φ=0.5

φ=0.6

φ=0.7

1700

1900

2100

0.4 0.5 0.6 0.7 0.8

0

0.1

0.2

0.2 0.3 0.4

b) Methane

a) Biogas

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Analysis

Fig. 5. Influence of equivalence ratio on mass fraction of CH4 along the flame centerline

using a) biogas and, b) methane.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

ss f

ract

ion

CH

4

Axial distance (m)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

ss f

ract

ion

of

CO

2

Axial distance (m)

φ=0.1

φ=0.2

φ=0.3

φ=0.4

φ=0.5

φ=0.6

φ=0.7

0.1

0.12

0.14

0.16

0.18

0.5 0.6 0.7 0.8

0

0.1

0.2

0.2 0.3 0.4

b) Methane

a) Biogas

Fig. 6. Influence of equivalence ratio on mass fraction of CO2 along the flame centerline

using a) biogas and, b) methane.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

ss f

ract

ion

of

CO

2

Axial distance (m)

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

ss f

ract

ion

of

OH

Axial distance (m)

φ=0.1 φ=0.2 φ=0.3 φ=0.4 φ=0.5 φ=0.6 φ=0.7

0.08

0.1

0.12

0.5 0.6 0.7 0.8

b) Methane

a) Biogas

Fig. 7. Influence of equivalence ratio on mass fraction of OH along the flame centerline

using a) biogas and, b) methane.

Fig. 8. The maximum flame temperature along the centerline for both biogas and

methane flames.

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

ss f

ract

ion

of

OH

Axial distance (m)

1980

1990

2000

2010

2020

2030

2040

2050

2060

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Ma

xim

um

tem

per

atu

re (

K)

Equivalence ratio

Tmax (BIOGAS)

Tmax (METHANE)

b) Methane

Formation of NOx

Figure 9 presents the average concentration of thermal NOx and average temperature at

the exhaust of both flames at varying equivalence ratio where the thermal NOx and

temperature increases with equivalence ratio. A sudden reduction of biogas NOx was

observed at equivalence ratio of 0.7 due to the reduction of temperature near the outlet.

Interestingly, NOx emission for methane is higher compared to that of biogas at an average of

44% within equivalence ratio of 0.1to 0.6 though the average flame temperature of the

former is lower compared to the latter. This phenomenon may be explained by the high

concentration of carbon dioxide in biogas flame that triggered the reduction in NO

production. Experimental studies with different CO2 dilutions in methane stream by Erete at

al [23] shows that there is a reduction in the reactant mass fraction in regions of high

temperature within the flame, thereby leads to low radical species and reaction rates which in

turn reduces NOx level. Since Tables 3(a) and 3(b) show that the air mass flow rate of biogas

flame is almost 20% lower compared to that of methane flame, the residence time is not

likely to be the cause of the lower NOx emission of the former flame compared to that of the

latter.

Based on the analysis above, it can be observed that combustion of biogas at the same

power capacity as methane would ensure low emissions of NOx at different equivalence ratio.

However, the reduction in NOx emission of biogas flame is achieved at the expanse of the

increase in CO2 emission. It is worth noting that the difference in NOx emissions of the two

fuels grows larger as the equivalence ratio increases.

Fig. 9. Average thermal NOx emission and average temperature at the outlet.

0

200

400

600

800

1000

1200

1400

1600

1800

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Av

era

ge

tem

per

atu

re (

K)

Av

era

ge

NO

x (

pp

m)

Equivalence ratio

NO (BIOGAS)

NO (METHANE)

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Emission analysis

4.0 Conclusion

The numerical simulation of the non-premixed turbulent combustion of pure methane

was performed and validated with the experimental work of Brookes and Moss [25],

adopting the same burner geometry and operating conditions. The predicted temperature

distribution agrees well with the data. The combustion of biogas derived from POME with

65% methane and 35% carbon dioxide composition at different equivalence ratio ranging

from 0.1 to 0.7 was simulated with the fixed power output of 8.5 kW. Comparison was made

with methane flame at fixed power output and the results were assessed in terms of flame

axial temperature distribution, reactant and product species, and the NOx emission. The

conclusions that can be drawn from this study are as followed:

1. The low specific heat of the gaseous mixture in biogas flame due to the fuel-bound

CO2 results in higher maximum temperature along the central axis and average

temperature at the outlet compared to that of methane flame.

2. In the near-burner region of biogas flame, the consumption of the fuel-bound CO2

dictates the unique reduction of CO2 which does not exist in the hydrocarbon

flame.

3. The fuel-bound CO2 in biogas produces 25 % higher CO2 emission at the outlet

compared to that of methane flame.

4. Despite the high average temperature at the outlet, biogas flame produces lower

NOx emission compared to that of methane flame due to the high concentration of

CO2 that reduces the NOx production reaction.

5. Thermal NOx in biogas flame was reduced by about 40% compared to that of

methane flame within equivalence ratio 0.1 to 0.6 at the expanse of the increase in

CO2 emission.

Acknowledgement

The authors would like to thank Ministry of Higher Education of Malaysia and Universiti

Teknologi Malaysia for supporting this research activity under the research grant

scheme R.J130000.7824.4F749.

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