Post on 17-Jun-2018
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
mohdfairus@fkm.utm.my
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].
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
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
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)
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
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
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)
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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