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Abstract—Experimental investigations of the DC electric field
effect on thermal decomposition of biomass, formation of the axial
flow of volatiles (CO, H2, CxHy), mixing of volatiles with swirling
airflow at low swirl intensity (S ≈ 0.2-0.35), their ignition and on
formation of combustion dynamics are carried out with the aim to
understand the mechanism of electric field influence on biomass
gasification, combustion of volatiles and heat energy production. The
DC electric field effect on combustion dynamics was studied by
varying the positive bias voltage of the central electrode from 0.6 kV
to 3 kV, whereas the ion current was limited to 2 mA. The results of
experimental investigations confirm the field-enhanced biomass
gasification with enhanced release of volatiles and the development
of endothermic processes at the primary stage of thermochemical
conversion of biomass determining the field-enhanced heat energy
consumption with the correlating decrease of the flame temperature
and heat energy production at this stage of flame formation. Further,
the field-enhanced radial expansion of the flame reaction zone
correlates with a more complete combustion of volatiles increasing
the combustion efficiency by 3% and decreasing the mass fraction of
CO, H2 and CxHy in the products, whereas by 10% increases the
average volume fraction of CO2 and the heat energy production
downstream the combustor increases by 5-10%
Keywords—Biomass, combustion, electrodynamic control,
gasification.
I. INTRODUCTION
IGHLY intensive use of the fossil fuels (natural gas, coal,
petroleum) for heat and energy production leads to a
gradual increase of the carbon-dioxide concentration in the
Earth's atmosphere and to global warming as well as to the
depletion of fossil fuels reserves in the proximate future [1],
[2]. Energy production from renewable fuels (wood, waste
biomass, agricultural residues, etc.) has minimum impact on
the environment and global warming, because the biomass
burning produces carbon-neutral emissions and can substitute
part of fossil fuel as an energy source [3]. Therefore, a clean
and efficient energy production with a partial substitution of
fuels by renewable energy resources is the driving force for
further development of the energy production technologies.
However, renewable fuels have lower heating values
compared to fossil fuel, dissimilar structure and wide-range
variations of the moisture content and chemical composition
that can cause variations of the combustion characteristics
with significant problems in boiler operation, which needs to
M. Zake is with the Institute of Physics, University of Latvia, 32 Miera
street , LV-2169, Salaspils, Latvia (corresponding author to provide phone:
+371 29891137; fax: +371 67901214; e-mail: mzfi@sal.lv ).
I. Barmina, A. Kolmickovs, and R. Valdmanis are with the Institute of
Physics, University of Latvia, 32 Miera street , LV-2169, Salaspils, Latvia (e-
mail: barmina@sal.lv, antons.kolmickovs@gmail.com, rww@inbox.lv).
be improved. One of the possibilities to improve the
combustion characteristics at thermochemical conversion of
biomass is co-firing of wood biomass with a small amount of
gas [4], [5]. Another way is thermal preprocessing of biomass
by a high-frequency electromagnetic field of 9 GHz [6], [7].
This method can provide uniform spatial heating of biomass
with a faster thermal destruction of hemicelluloses and
cellulose determining a faster thermochemical conversion of
the pre-treated biomass. An alternative biomass combustion
control technique is based on the electric field-induced “ion-
wind effect” determining the formation of interrelated
processes of heat/mass transfer in the electric field direction
with local variations of the combustion characteristics and
produced heat energy [8]-[12]. Previous experimental studies
of the DC electric field effect on the swirling flame formation
and combustion of volatiles at high swirl intensity have shown
that for such type of flames the field-enhanced mass transfer
of the flame species in the field direction allows to control the
process of products recirculation, with direct impact on the
flame shape and combustion characteristics [10], [11].
These investigations are focused on improvement and
control of the combustion characteristics at thermochemical
conversion of biomass (wood pellets) using DC electric field
effects on the biomass gasification and combustion of volatiles
at low swirl intensity with the aim to increase the total amount
of produced heat energy and minimize the environmental
impact of combustion systems.
II. EXPERIMENTAL
The DC electric field effect on the thermochemical
conversion of wood biomass, formation of combustion
dynamics and on the composition of the products is studied
experimentally using a batch-size experimental device along
with integrated processes of biomass gasification and
combustion of volatiles at an average heat power up to 2 kWh.
The main components of the experimental device are a
biomass gasifier (1), water-cooled sections of the combustor
of inner diameter D = 60 mm (2), a swirling propane/air
burner (3), inlet ports of primary (4) and secondary swirling
air supply (5), and diagnostic sections (6) with peepholes for
local placing of diagnostic tools (thermocouples, gas sampling
probes) into the flame (Fig. 1). Discrete doses of wood pellets
(m = 240 g) are added into the gasifier up to the inlet port of
the propane flame flow, which is used to initiate thermal
decomposition of biomass (wood) pellets and ignition of
volatiles. The duration of the heat input by the propane flame
flow into the biomass was limited to 350 s and it was switched
out when the self-sustaining processes of biomass gasification
Electric Field Impact on the Biomass Gasification
and Combustion Dynamics
M. Zake, I. Barmina, A. Kolmickovs, R. Valdmanis
H
International Science Index
International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:9 No: 7, 2015
822International Scholarly and Scientific Research & Innovation 9(7), 2015 scholar.waset.org/1999.2/10001811
International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811
and combustion of volatiles were developing. The primary and
secondary swirling airflow was supplied with the average rates
22 l/min and 40 l/min. The primary air supply was used to
initiate biomass gasification, which develops under fuel-rich
conditions (α ≈ 0.3-0.4). The secondary air supply was used to
provide mixing of the combustible volatiles with the air and
their complete burnout downstream the combustor at an
average air excess in the flame reaction zone of α ≈ 1.2-2.
Fig. 1 Digital image and principal schematic of the small-scale pilot
device
Electrical control of wood biomass gasification and
combustion of volatiles was provided using a single electrode
configuration when an electrode was axially inserted through
the biomass layer towards the combustor bottom. An electric
field was applied between the positively biased electrode (7)
and the grounded channel walls. The voltage of the axially
inserted electrode relative to the channel walls could be varied
from 0.6 kV up to 3 kV, whereas the ion current in these
experiments was limited to 2 mA.
The experimental study involves complex measurements of
the biomass weight loss rate (dL/dt) by using a moving rod
supplied with a pointer, spectral measurements of the
gasification products (C2H2, C2H4, CH4, CO) composition by a
FTIR spectrometer, and on-line with a time interval of 1 sec
measurements of the swirling flame velocity, temperature and
composition (CO, H2, CO2, NOx) profiles at the combustion of
volatiles using gas analyzers Testo 435 and Testo 350 XL.
Calorimetric measurements of the cooling water flow in the
water-cooled combustor were made to estimate the effect of
biomass characteristics variations on the heat production rates
during the combustion of the volatiles. The measurements of
the temperature and heat production rates were recorded using
a data plate PC-20 TR.
III. RESULTS AND DISCUSSION
A. Development of Combustion Dynamics at
Thermochemical Conversion of Biomass Pellets
The process of biomass thermochemical conversion
develops with the subsequent stages of endothermic processes
of biomass heating, drying, thermal decomposition and of
exothermic processes of volatiles and char combustion.
The primary processes of biomass heating and drying take
place in the temperature range T = 300-420 K and last up to
400 s. The biomass heating and drying result in a release of
moisture and light volatiles along with the variations of the
peak values of biomass weight loss from dm/dt = 0.1 g/s to 0.3
g/s (Figs. 2 (a) and (b)).
Fig. 2 Effect of different air supply on the variations of wood pellet
mass loss (a) and weight loss (b) rates; time-dependent variations of
the absorption intensity of the main products (c)
The next stage of biomass weight loss at t > 400 s is the
thermal decomposition of the main biomass constituents –
hemicellulose, cellulose and lignin at temperatures of 490-600
K, 580-650 K, and 520-800 K, respectively [12]. The biomass
thermal decomposition at this stage results in an increase of
the peak values of biomass weight loss up to 0.6 g/s at t ≈ 800-
1000 s (Fig. 2 (b)) with the correlating increase of infrared
absorbance of the main combustible gas-phase species (C2H2,
International Science Index
International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:9 No: 7, 2015
823International Scholarly and Scientific Research & Innovation 9(7), 2015 scholar.waset.org/1999.2/10001811
International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811
C2H4 ,CH4, CO, etc.) (Fig. 2 (c)). The biomass weight loss rate
at the biomass thermal decomposition with a high accuracy
(R2 = 0.97-0.99) can be presented as a linear dependence on
the primary air supply rate, which is responsible for the
development of the biomass gasification processes and for the
formation of the axial flow of volatiles above the biomass
layer at the thermal decomposition of hemicellulose, cellulose
and lignin.
The flame reaction zone is formed at swirl-enhanced mixing
of the axial flow of volatiles with the swirling airflow, which
leads to the ignition and combustion of the volatiles. The
modeling experiments have shown [13] that a typical feature
of the swirling flame formation for the given configuration of
the experimental device is the formation of downstream and
upstream swirling airflows above and below the swirling air
nozzles. With a low swirl intensity (S < 0.6), the upstream
swirl flow formation is responsible for primary swirl-enhanced
mixing of the axial flow of volatiles with an air swirl just
above the biomass layer and for the development of flame
velocity profiles. At the primary stage of flame formation
(L/D < 1) the upstream air swirl slows down the axial flow of
volatiles close to the channel walls (r/R > 0.8), where the
tangential velocity of the downstream and upstream swirling
air flows increases to the peak values (Fig. 3).
Fig. 3 The formation of flame velocity profiles downstream the
combustor with a low swirl number of the inlet flow (S = 0.25)
Fig. 4 Time-dependent variations of the heat power (a) CO2 volume
fraction (b) and excess air ratio in the flame reaction zone (c) at
thermochemical conversion of wood pellets
In contrast, the axial flame velocity increases towards the
centerline of the combustor (r/R < 0.5), where the tangential
flow velocity decreases to a minimum value with evident
spatial separation of the downstream axial and tangential
flows (Fig. 3). Hence, at the primary stage of the flame
reaction zone formation (L/D > 0.75) the mixing of the axial
flow of volatiles with the downstream swirling air flow is
incomplete that is confirmed by FTIR measurements of the
flame composition, indicating a high level of unburned
hydrocarbons (Fig. 2) in the flame reaction zone. The further
development of the downstream flow (L/D > 1) reveals the
radial expansion of the flame reaction zone with the
correlating expansion of the axial velocity profiles, which
occurs due to swirl-enhanced mixing of the axial flow of
volatiles with the downstream swirling airflow (Fig. 3).
The development of the flame reaction zone at biomass
thermochemical conversion results in time-dependent
variations of the produced heat power and CO2 volume
fraction in the products (Figs. 4 (a) and (b)) with correlating
variations of the air excess ratio (α) in the flame reaction zone
(Fig. 4 (c)).
As seen from Fig. 4, the highest values of the air excess
ratio in the flame reaction zone are observed at the primary
stage of the thermochemical conversion of wood pellets (t <
400 s) and rapidly decreases after the ignition of volatiles with
the correlating increase of the produced heat power and CO2
volume fraction. By analogy with the velocity profiles
formation the peak values of the flame temperature and CO2
volume fraction were found close to the flame axis (R = 0).
0
0.6
1.2
1.8
00.5 1
r/R
Velocity (m/s)
u; L/D=0.75; air-22/42 l/min
w; L/D=0.75
u; L/D =2. 7
w; L/D=2.7
u; L/D =4. 6
w; L/D=4.6
0
700
1400
2100
0 700 1400 2100
time (s )
Power (J/s
)
Qsum;air-2 2/42 l/min Qsum;air-3 0/34 l/m in
Qsum;air-3 3/31 l/min
a
2
8
14
20
0 700 1400 2100
time (s )
Volume fraction CO
2
(%)
air-2 2/42 l/m in air-3 0/34 l/m in
air-3 3/31 l/m in
b
1
2
3
4
0 700 1400 2100
time (s )
air-22/42 l/min air-30/3 4 l/min
air-33/31 l/min
c
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B. Electric Field Effect on Thermo Chemical Conversion of
Biomass
When a DC electric field is applied to the flame, the
increase of the average biomass weight loss from 0.14 g/s to
0.18 g/s evidences that the electric field induced current in the
space between the electrodes enhances the thermal
decomposition of biomass pellets (Fig. 5).
Fig. 5 The electric field-induced current effect on the time-dependent
variations of the biomass weight loss
FTIR measurements and analysis of the gas-phase
composition at the bottom of the combustor (L/D = 0.75)
confirm the field-enhanced thermal destruction of the main
biomass constituents - hemicellulose, cellulose and lignin
resulting in an enhanced release of the main combustible
species (CO, CH4, C2H2, C2H4, etc.) and CO2 (Fig. 6). As
follows from Fig. 6, during the first stage of biomass (wood
pellets) thermal decomposition (t < 500 s), when the processes
of hemicellulose depolymerization and decarboxylation are
initiated, the field-enhanced release of CO dominates. The
absorbance of CO acquires its peak value at t = 500 s and
starts to decrease at t > 700 s. The next stage of the field-
enhanced thermal decomposition of wood pellets (t > 600 s)
can be related to cellulose depolymerization and dehydration
of hemicellulose monomers, oxidation, decarboxylation and
cycle opening, when other volatiles like CH4, C2H2 and C2H4
are found in the gaseous products.
Fig. 6 Time-dependent variations of the combustible volatiles
absorbance at field-enhanced thermal decomposition of biomass
pellets
In addition to the field-induced variations of the biomass
thermal decomposition, the field-induced variations of the
flame dynamics were observed (Fig. 7).
As seen from Fig. 7, the dominant electric field-induced
variations of the flame dynamics downstream the combustor
are observed along the outside part of the flame reaction zone
close to the channel walls, with the correlating field-induced
decrease of the axial and tangential velocity components (Fig.
7). This correlation allows to suggest the field-enhanced
formation of the upstream swirling air flow by limiting the
formation of the axial flow of volatiles, with a faster decrease
of the downstream swirling air flow close to the channel walls
and towards the flame centerline. The field-enhanced
formation of the upstream air swirl motion is confirmed by
0
85
170
255
0 500 1000 1500
time (s )
Mass (g)
I=0 I=0.3 mA I=1.2 mA I=1.7 mA
CO
0.00
0.09
0.18
0.27
0 500 1000
time (s )
Absorbance
(rel .un.)
I=0 mA I=0.4 mA
I=0.85 mA I=1.8 mA
a
CH
4
0.00
0.12
0.24
0.36
400 700 100
0
time (s )
Ab
sor
b
ance
(rel .un.)
I=0 mA I=0.4 mA I=0.85 mA
I=1.35 mA I=1.8 mA
b
C
2
H
2
0
0.15
0.3
0.45
400 600 800 1000
time (s )
Absorbance
(rel .un.)
I=0 mA I=0.4 mA I=0.85 mA
I=1.35 mA I=1.8 mA
c
C
2
H
4
0
0.04
0.08
0.12
400 700 1000
time (s)
Absorbance (rel.un.)
I=0 mA I=0.4 mA I=0.85 m
A
I=1.35 mA I=1.8 mA
d
International Science Index
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International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811
measuring the average values of the axial and tangential
velocity components below the swirling air nozzles so proving
the decrease of the axial flow velocity average value, whereas
the average value of the upstream swirling air velocity
increases, with the peak value of the field-induced current
increase by 1-1.2 mA and positive bias voltage of the axially
inserted electrode by 1.5-1.8 kV.
Fig. 7 Field-induced variations of the axial (a) and tangential (b)
velocity profiles above (a, b) and below (c) the swirling air nozzles
The field-enhanced thermal decomposition of biomass
pellets and the field-enhanced upstream swirl motion
determining the swirl-enhanced mixing of the axial flow of
volatiles with the air result in field-induced variations of the
combustion dynamics providing a variation of the produced
heat energy and heat release rate at the thermochemical
conversion of wood pellets (Fig. 8).
Fig. 8 Electric field-induced variations of the produced heat
energy at biomass gasification (a) and combustion of volatiles (b);
and heat power time-dependent variations (c)
The development of the endothermic processes of field-
enhanced thermal degradation of wood pellets (Fig. 5) results
in an intensive heat energy consumption at the biomass
gasification stage, thus decreasing by about 13 % the produced
heat energy at this stage of thermochemical conversion of
biomass pellets (Fig. 8 (a)). In contrast, the field-enhanced
mixing of the axial flow of volatiles with the upstream air
swirl assures the field-enhanced ignition and combustion of
volatiles downstream the combustor, so increasing the
produced heat energy by about 11 %. In fact, the peak value of
the produced heat energy (Fig. 8 (b)) correlates with the peak
value of the upstream field-enhanced air swirl motion (Fig. 7
(c)), which leads to swirl-enhanced mixing of the flame
components and to faster burnout of volatiles, as confirmed by
0.6
1
1.4
00.51
r/R
Velocity (m/s)
u;I=0;L/D=4.6 I=0.96 mA
a
0
0.5
1
00.5 1
r/R
Velocity (m/s)
w;I=0;L/D=4.6 I=0.96 mA
b
0
0.8
1.6
00.81.6
I (mA)
Average velocity (m/s)
uav ;L/D=-0 ,9 wa v
c
2.2
2.3
2.4
2.5
2.6
012
I (mA)
Heat energy (MJ/kg)
Qg
asifier
a
7.5
7.75
8
8.25
8.5
012
I (mA)
Heat energy (MJ/kg)
Qcombustor
b
300
800
1300
1800
0 1000 2000
time (s)
Power (J/s)
I=0 I=1.25 mA
c
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International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811
the measurements of the biomass weight loss (Fig. 5),
determining the faster heat power release at thermochemical
conversion of biomass pellets (Fig. 8 (c)). The local
measurements of the flame temperature have shown that the
produced heat energy field-enhanced increase correlates with
the decrease of the temperature average value near the flame
axis by about 11 %. This correlation confirms that the
mechanism, which is responsible for the field-enhanced
increase of the produced heat energy, can be related to the
field-induced ion wind motion [8] determining the formation
of the interrelated processes of field-enhanced heat/mass
transfer in the field direction. Regarding [14], this leads to
field-enhanced flame homogenization, which decreases the
peak flame temperature with the correlating increase of the
produced heat energy.
With the air excess supply into the flame reaction zone (α ≈
1.2-1.4), the biomass field-enhanced thermochemical
conversion with the enhanced release and combustion of
volatiles correlates with the field-enhanced variation of the
product composition and combustion efficiency (Fig. 9). The
local measurements of the flame composition confirm the
field-enhanced combustion of volatiles, which leads to
increasing the average CO2 volume fraction in the products by
10% and combustion efficiency by 3 %, whereas the excess air
ratio in the products decreases by about 10 %, which is the
evidence of a more complete combustion of volatiles (Fig. 9).
Moreover, the field-enhanced decrease of the peak
temperature of the flame reaction zone limits the temperature-
sensitive NOx formation, which, in accordance with the two-
step Zeldovich mechanism, exponentially increases with an
increase of the flame temperature [15]. Hence, the peak flame
temperature decrease with the homogenization of the flame
reaction zone results in a decrease of the average value of the
NOx emission mass fraction in the products by about 12 %.
Finally, the presented results allow to conclude that the field-
induced variations of the combustion dynamics at the
thermochemical conversions of biomass pellets determine a
cleaner and more effective heat energy production.
Fig. 9 The DC electric field effect on the volume fraction of CO2 (a),
on the combustion efficiency (b) and air excess ratio (c) in the
products at different stages of swirling flame formation
IV. CONCLUSIONS
The performed experimental study of the electric field
effect on the thermochemical conversion of biomass (wood
pellets) leads to the following conclusions:
The primary electric field effect on the thermo-chemical
conversion of biomass can be related to the biomass field-
enhanced thermal decomposition promoting the enhanced
release of combustible volatiles.
The field-enhanced upstream airflow formation ensures the
enhanced mixing of the upstream air swirling flow with the
axial flow of volatiles so determining a faster ignition and a
more complete combustion of volatiles downstream the
combustor and increasing the produced heat energy at the
thermo-chemical conversion of biomass pellets.
The field-enhanced variation of the products composition
and combustion efficiency confirm that the electric field effect
on the flame can be used to obtain a cleaner and more
effective heat energy production.
ACKNOWLEDGMENT
The authors would like to acknowledge the financial
8.8
9.8
10.8
00.61.21.8
I (mA)
Volume fraction CO
2 (%)
L/D=6.9 L/D=7.8
a
65
68
71
0 0.6 1.2 1.8
I (mA)
Efficiency (%)
L/D=6.9 L/D=7.8
b
1.8
2.1
2.4
00.61.21.8
I (mA)
L/D=6.9 L/D=7.8
c
International Science Index
International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:9 No: 7, 2015
827International Scholarly and Scientific Research & Innovation 9(7), 2015 scholar.waset.org/1999.2/10001811
International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811
support from the Latvian research grant No. 623/1.
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International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:9 No: 7, 2015
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International Science Index, Chemical and Molecular Engineering Vol:9, No:7, 2015 internationalscienceindex.org/publication/10001811