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LATVIAN JOURNAL OF PHYSICS AND TECHNICAL SCIENCES
2009, N 6
10.2478/v10047-009-0024-z
PHYSICAL AND TECHNICAL PROBLEMS OF ENERGETICS
EXPERIMENTAL STUDY OF THE COMBUSTION DYNAMICS OF
RENEWABLE & FOSSIL FUEL CO-FIRE IN SWIRLING FLAME
M. Zaķe1, I. Barmina1, V. Krishko2, M. Gedrovics2, A. Desņickis2
1Institute of Physics, University of Latvia,
32 Miera Str., Salaspils LV-2169, LATVIA
2Faculty of Power and Electrical Engineering, Riga Technical University,
1 Kronvalda Blvd., Riga LV-1010, LATVIA
The complex experimental research into the combustion dynamics of rene-
wable (wood biomass) and fossil (propane) fuel co-fire in a swirling flame flow has
been carried out with the aim to achieve clean and effective heat production with
reduced carbon emissions. The effect of propane co-fire on the formation of the
swirling flame velocity, temperature and composition fields as well as on the
combustion efficiency and heat output has been analysed. The results of experimental
study show that the propane supply into the wood biomass gasifier provides faster
wood fuel gasification with active release of volatiles at the primary stage of swirling
flame flow formation, while the swirl-induced recirculation with enhanced mixing of
the flame components results in a more complete burnout of wood volatiles
downstream of the combustor with reduced mass fraction of polluting impurities in
the emissions.
Key words: combustion dynamics, swirling flow, co-firing, renewables, fossil
fuel, greenhouse gases.
1. INTRODUCTION
As a result of human activities, greenhouse gases, such as carbon dioxide
(CO2) and methane (CH4), are still increasing in the Earth’s atmosphere causing its
temperature to rise, which would lead to even greater global warming during this
century. To minimize the potential impact of human activities on the global climate
changes, various types of greenhouse gas mitigation technologies are being eva-
luated with account of costs and impacts on the ecology systems [1–3]. Replacing
of fossil fuels with renewable energy sources is one of the most promising tasks.
Wood biomass is still an abundant self-renewing material throughout the world. As
a renewable energy resource, biomass has some indisputable advantages when
compared with other renewable sources; for example, wood biomass is widely
available, its conversion facilities require comparatively low capital costs and can
be integrated into existing fossil-fuelled power generation plants. The main
attractiveness of wood biomass as fuel is associated with carbon dioxide, because
the growth of trees and their conversion to energy as biomass fuels recycles
atmospheric carbon, thus not adding CO2 to the atmosphere. Besides, the low
sulphur contents of most biomass materials means that the emissions of SO2 (acid
gas resulting in the acid rains) are minimized [2]. However, conventional wood-
fuelled heating systems are not efficient because of dissimilar structure and
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different moisture content of wood fuels. As a result, up to half the gasified fuel
products and the heat of conventional wood fires can be lost up the chimneys. A
low-cost option for more effective and cleaner conversion of biomass − one of the
largest world’s energy sources − to the heat energy is the co-firing, i.e. the
simultaneous combustion of different types of fuels in the same boiler, with fossil
fuel partially replaced by renewable. Co-firing is an environmentally-friendly and
cheap method of mixing wood biomass with coal [4, 5] or the flow of volatiles with
gaseous fuel (natural gas, propane [6, 7]). Among the major factors affecting
combustion − its efficiency, formation of polluting emissions by co-firing the wood
fuel with gas, etc., is the flow dynamics that determines the formation of flame
structure. To improve combustion conditions with benefit to the ignition, flame
stability and emission reduction, the very promising technique is swirling com-
bustion [8–10]. Swirling flows with lean fuel & air premixing and the formation of
a central recirculation zone provide stable combustion with effective emission
reduction. The previous experimental study of co-firing wood biomass with
propane [11] has shown that the swirling flame structures and combustion
characteristics can be closely linked to the features of the swirling airflow field
determining the swirling flame flow formation with recirculation, which improves
mixing and combustion of the flame components [11]. Therefore the very im-
portant factor that determines the intensity of wood fuel gasification and the
development of combustion characteristics downstream of the combustor is the rate
of gas co-firing [12]. The previous investigations have also shown that the direct
propane co-fire of wood biomass with an additional heat supply into a wood layer
results in faster thermal decomposition of wood pellets, which facilitates ignition
and burnout of the volatiles. In fact, the measurements of the flame characteristics
at different rates of propane co-fire have shown that during the primary stage of
swirling flame formation the increase in the rate of propane co-fire can result in an
ignition delay with relatively high release of polluting CO and NO emissions. Such
being the case, the swirling combustion with recirculation can be used to improve
the combustion characteristics downstream of the swirling flame flow, completing
burnout of volatiles with greater heat output, higher temperatures inside the flame
reaction zone, and better release of CO2 emissions. For this reason, the motivation
of this research was to investigate the impact of the propane co-fire on the flame
velocity, temperature and composition fields and to reveal the main factors
affecting the balance between the enhanced wood fuel gasification by co-firing
with propane flame flow and the swirl-enhanced burnout of volatiles. As shown,
more effective burnout of volatiles and cleaner combustion of wood fuel can be
achieved by minimizing the impact of propane co-fire on the formation of polluting
emissions.
2. EXPERIMENTAL
The pilot device for experimental study of combustion dynamics down-
stream of the swirling flame flow at co-firing discrete doses of renewable wood
fuel (up to 300–500 g) with fossil fuel (propane) is shown in Fig. 1 [12]. The main
elements of the device are: wood fuel gasifier (1), propane burner (2), and
combustor (3). The primary (4) and secondary (5) swirling air supplies below and
above the layer of wood biomass (wood pellets) are provided using two tangential
inlets 3mm in diameter determining the swirling flame flow formation and mixing
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of the flame components. The primary air
supply supports the wood fuel gasification,
while the secondary airs supply – combustion of
the volatiles downstream of the combustor. The
experimental study of the effect exerted by the
rate of propane co-fire on the combustion
dynamics was carried out at constant primary
and secondary air supply rates of 40 l/min and
70 l/min, with the mean air excess of up to 2.5–
3, by varying the rate of propane supply in the
range from 0 up to 0.83 l/min (corresponding to
the variations in the additional heat supply into
the upper part of the layer of wood pellets in the
range of 0−1.25 kJ/s and the ratio of additional
heat supply from propane combustion up to 30–
35%). The local measurements of the flame
velocity, temperature, composition and combus-
tion efficiency at different stages of the swirling
flow field formation were carried out to esti-
mate the effect of the propane co-fire rate on the
combustion dynamics.
3
6
5
2
1
4
The experimental study of the combus-
tion dynamics impact on the flame formation
and the local flame composition includes ex-
perimental research into the formation of the
flame velocity and temperature fields, heat production rate and composition of the
products at different stages of the wood fuel burnout. The diagnostic sections (6)
with peepholes are placed between the sections of water-cooled combustor (3) and
were used for the local input of different diagnostic tools (thermocouples, gas
sampling probes, Pitot’s tube) into the swirling flame flow to provide the local
measurements of the flame parameters (its velocity, temperature and composition)
at different stages of the swirling combustion. The local measurements of the flame
temperature were taken using Pt/Pt–Rh (10%) thermocouples and PC-20TR
software. In particular, the use of this software allowed for the estimation of the
average heat production rate at different stages of the swirling flame formation
from the calorimetric measurements of cooling water flow. The local variations in
the swirling flame velocity, temperature, composition of the products (NOx, CO2,
CO, O2) and combustion efficiency at different stages of the flame formation and
different rates of propane co-fire were on-line registered using the software of a gas
analyzer Testo 350XL with a 1 s time interval between measurements. Each of
the parameters: the average velocity and the composition, were estimated from 10–
15 measurements.
Fig. 1. Setup for experimental study
of the swirling flame flow dynamics
by co-firing of wood biomass with
the propane flame flow.
3. RESULTS AND DISCUSSION
3.1. The main factors of the formation of swirling flow velocity field.
The previous experiments have shown [11, 12] that the formation of swirling
flame flow velocity and composition profiles at the air excess supply into the
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combustor and the constant rate of wood fuel co-fire with propane are closely
connected with the swirl configuration and the primary & secondary air supply
rates determining the swirl intensity.
The experimental modelling of the effect of primary and secondary air
supply on the formation of the velocity field of cold confined swirling airflow
above the layer of wood pellets has shown that, as concerns the swirling airflow
velocity profiles formed, they are mainly sensitive to variations in the secondary
swirling air supply, and at constant primary & secondary air supply rates they
rapidly decay downstream of the combustor, indicating the formation of a
pronounced recirculation zone where the axial flow rate is balanced by
recirculation of the swirling airflow (Fig. 2a–f). The estimation of the cold airflow
conditions nearby a secondary air nozzle varying the rate of the primary air supply
in the range of 40–55 l/min and the secondary air supply rate in that of 60–
100 l/min shows the formation of a highly turbulent flow field above the wood
pellets with the air swirl number variations S = 0.6–1.4 (where S ≈ 2/3*vtg/vax)
and the Reynolds number being Re = 3600–9000. A typical formation of the cold
airflow velocity profiles close to the outlet of secondary air nozzle (L/D = 0.5) at
different primary & secondary air supply rates is illustrated in Fig. 2a,b. At the
cold conditions the axial and tangential flow velocity components exhibit similar
behaviour determining the formation of a pronounced central recirculation zone,
with the least flow velocity components near the flow centre (R = 0) and the peak
ones close to the channel walls (at r/R ≈ 0.8). At constant primary and secondary
swirling air supply rates the recirculation zone extends up to L/D ≈ 1 (Fig. 2c).
Further downstream, the air swirl motion close to the channel walls gradually
weakens with enhanced air swirl motion close to the flow centre. Moreover, the
local measurements of the air velocity components have shown that the secondary
swirling air supply into the combustor promotes the upstream and downstream
swirling airflow expansion, with the upstream swirl flow reversing from the layer
of wood pellets, which determines the observed increase of the air swirl velocity
near the flow centre (Fig. 2b,d). A similar swirl flow reversal downstream of the
axis was observed by the authors of [14] for the conditions when the number of
local swirls exceeds the critical value. For the conditions of an empty gasifier the
shape of tangential and axial velocity profiles is influenced by the primary swirling
air supply rates that determine formation of the primary recirculation zone close to
the primary swirling air nozzle with a reverse axial flow close to the bottom part of
the gasifier, and a high level of the axial flow velocity pulsations (Fig. 2e,f).
For the given configuration of our experimental device the wood fuel
gasification and burnout of volatiles is initiated by the radial injection of propane
flame into the upper part of the wood biomass layer, with direct impact on the
formation of the swirling flow velocity profiles downstream of the combustor
which depends on the combustion conditions nearby the gasifier outlet. First, the
experimental study of formation of the mentioned profiles was carried out for the
conditions of self-sustaining wood fuel burnout, when the propane flame injection
into combustor is interrupted immediately after ignition of the volatiles (prop = 0)
(t ≈ 200–250 s). Next, to assess the effect of propane co-fire on the flow dynamics,
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-2
0
2
4
6
0102030
Radial distance,mm
Velocity,m/s
-5
0
5
10
0102030
Radia l distance ,mm
Velocity,m/s
Fig. 2. Typical variations in the shape of tangential vtg and axial vax velocity profiles at different rates
of primary and secondary air supply above the layer of wood pellets (L/D = 0. 5) (a, b) and different
distances above (c, d) and below (L/D = –2) (e, f) the secondary air nozzle for the conditions of peak
mass load of the wood pellets (c, d) and empty gasifier (e, f).
the experimental research on the formation of the flame velocity profiles down-
stream of the flame channel flow was conducted at a constant rate of the additional
heat energy supply of 1.25 kJ/s into the flame of volatiles. It should be noted that
the propane co-fire with heat added to such flame accelerates the flame temperature
rise up to the peak value, increasing the temperature of the flame reaction zone by
150–200 oC and providing the wood fuel burnout at a nearly constant temperature
of the reaction zone and, respectively, a nearly constant rate of the heat energy
production in the time interval t ≈ 500–1550 s (Fig. 3a,b). At increasing the rate
of propane supply into the burner and the propane thermal capacity (Qprop) up to
1.25 kJ/s the ratio of propane co-fire (Qprop/Qtot, %) for given combustion
conditions gradually increases up to 30–35% of the total heat output produced
vax,40/70 l/min,L/D=0,5
vax, air-47/70l/min
vax,air-55/70l/min
vax,air-40/90 l/min
vax, air-40/100l/min
vax, air 40/70l/min;L/D=-0,3 upstr. flow
a
vtg,cold air-40/100l/min;L/D=0,5
vtg, air-40/90l/min
vtg, air 40/70 l/min;L/D=0,5
vtg,air-47/70l/min
vtg, air-55/70l/min
b
-2
0
2
4
6
0102030
Radial distance,mm
Velocity,m/s
-0.5
0.5
1.5
2.5
0102030
Radial distance,mm
Velocity,m/s
vax,40/70 l/min,L/D=0,5
vax,L/D=1
vax,L/D=4,5
c
vtg, air 40/70 l/min;L/D=0,5
vtg,L/D=1
vtg,L/D=4,5
d
0
0.5
1
1.5
2
0102030
Radial distance,mm
Velocity,m/s
-0.5
0
0.5
1
1.5
2
0102030
Radial distance,mm
Velocity,m/s
vax,40/70l/min,L/D=-2,empty gasifier
vax,0/70l/min
e
vtg,40/70l/min,L/D=-2,empty gasifier
vtg,0/70l/min
f
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downstream of combustor (Qtot) and can approximately be expressed as a square
function of the heat produced by propane co-fire (Fig. 3c).
500
700
900
1100
1300
1500
0 500 1000 1500
time,s
Temperature,K
0
300
600
900
1200
0 800 1600 2400
time,s
Heat power,J/s
Since the structure of the swirling flame flow is closely connected with the
formation of swirling airflow field, two factors are of importance: those
determining the formation of the tangential and axial velocity profiles of swirling
flame downstream of the combustor. First, the formation of such profiles is
influenced by the swirling airflow dynamics giving rise to the recirculation zone
near the flame axis with swirl-induced reverse axial motion of the hot products up
to the wood fuel layer where the flame components are actively mixed. The second
decisive factor is that the formation of the mentioned profiles is influenced by the
interrelated processes of swirl-induced reverse heat and mass transfer to the surface
of the wood layer. As a result, there are observed more intense wood fuel heating,
gasification, ignition and burnout of the volatiles developing at different rates of
the wood fuel burnout and of the heat production under the conditions of self-
sustaining wood fuel combustion and propane co-fire (Fig. 3b). It is estimated that,
while at the self-sustaining wood fuel burnout (prop.= 0) its average rate does
not exceed 0.135–0.14 g/s, at the co-fire with additional heat energy supply of
1.25 kJ/s it increases up to 0.2 g/s, which means enhancement of the burnout
process. Both for the conditions of self-sustaining wood fuel burnout and propane
co-fire a typical feature of the formation of flame velocity profiles is increased
thermal load of the flow field in comparison with non-reacting cold swirling
T1-5,K-L/D=0,5;r/R=0, prop.0
T1-4,K; prop.1,25kJ/s
a
Qsum-2,J/s, prop.0
Qsum-3,J/s, prop.1,25kJ/s
b
R
2
= 0,9393
0
10
20
30
40
0 0,5 1 1,5
Propane heat output, kJ/s
Propane co-fire
,
%
c
Fig. 3. The effect of propane co-fire (1.25 kJ/s)
on the flame temperature rise (a) at the initial
stage of the swirling flame flow formation
(R = 0, L/D = 0.5) and the rate of heat energy
production during the burnout of volatiles (b);
the variations of the ratio of propane co-fire by
varying additional heat supply into the
combustor (c).
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airflow. Such an increased thermal load promotes the radial and axial expansions
of the flame flow field that shift the peak values of the axial and tangential flame
velocity components closer to the channel walls up to r/R = 1 (Fig. 4a,d). In fact,
the differences of the wood fuel burnout rates under the conditions of self-
sustaining burnout and those of propane co-fire mean variations in the thermal load
of swirling flame, and, consequently, variations in the axial flow rates which
disturb the balance between the axial flow rates and recirculating flame com-
ponents. The self-sustaining wood fuel burnout indicates a faster establishment of
the balance between the axial flow rate and the recirculation with the least axial
0
1
2
3
4
0102030
Radial distance,mm
Velocity,m/s
0
1
2
3
0102030
Radial distance,mm
Velocity,m/s
Fig. 4. Formation of the swirling flame velocity profiles downstream of combustor at different stages
of the wood fuel burnout (2 − t = 500–700 s; 3 − t = 700–900 s; 4 − t = 900–1100 s;
5 − t = 1100–1300 s) and constant rate of propane co-fire (1.25 kJ/s).
vax-2,prop.0;L/D=1
vax-3,prop.0
vax-4,prop.0
vax-5,prop.0
vax-2,prop.1,25kJ/s;L/D=1
vax-3,prop.1,25kJ/s
vax-4;prop.1,25kJ/s
vax-5,
p
ro
p
.1,25kJ/s
a
vtg-2,prop.0;L/D=1
vtg-3,prop.0
vtg-4,prop.0
vtg-5,prop.0
vtg-2,prop.1,25kJ/s; L/D=1
vtg-3,prop.1,25kJ/s
vtg-4, prop.1,25kJ/s
vtg-5,prop.1,25kJ/s
b
-0.5
0
0.5
1
1.5
2
0102030
Radial distance,mm
Velocity,m/s
-0.5
0
0.5
1
1.5
2
0102030
Radia l distance,mm
Velocity,m/s
vax- 2, prop.0;L/D=2
vax- 3;prop.0
vax- 4;prop.0
vax- 5;prop.0
vax- 2,prop.1,25kJ/s;L/D=2
vax- 3, prop.1,25kJ/s
vax- 4;prop.1,25kJ/s
vax- 5;prop.1,25kJ/s
c
vtg-2,prop.0;L/D=2
vtg-3; prop.0;
vtg-4; prop.0
vtg-5;prop.0
vtg-2;prop.1,25kJ/s;L/D=2
vtg-3;prop.1,25kJ/s
vtg-4,prop.1,25kJ/s
vtg-5,prop.1,25kJ/s
d
-0.5
0
0.5
1
1.5
2
0 102030
Radial distance,mm
Velocity,m/s
-0.5
0
0.5
1
1.5
0102030
Radial distance,mm
Velocity,m/s
vax-2,prop.0;L/D=4,5
vax-3,prop.0
vax-4,prop.0
vax-5,prop.0
vax-2,prop.1,25kJ/s,L/D=4,5
vax-3,prop.1,25kJ/s
vax-4,prop.1,25kJ/s
vax-5,prop.1,25kJ/s
e
vtg-2,prop.0,L/D=4,5
vtg-3,prop.0
vtg-4,prop.0
vtg-5,prop.0
vtg-2,prop.1,25kJ/s,L/D=4,5
vtg-3,prop.1,25kJ/s
vtg-4,prop.1,25kJ/s
tg-5,prop.1,25kJ/s
f
v
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flame velocity as early as at L/D ≈ 1 and more active swirl flow reversing from the
wood layer near the flame centreline (r/R <0, 5), where the dominant mixing of the
flame components and combustion of the volatiles is initiated. Higher axial flow
velocity with reduced swirling flow reversal near the flame centre was detected for
the conditions of propane co-fire (Fig. 4a,b), when the enhanced wood fuel
gasification and combustion of volatiles restricts the swirl-induced recirculation
and shifts the balance between the axial flow of volatiles and the recirculation
further downstream – up to L/D ≈ 2. Time-dependent variations in the formation of
swirling flame flow structure immediately after ignition of volatiles (t = 500–700 s)
when the recirculation of hot products dominates, indicate the formation of the
minimum value of the axial flow velocity close to the flame centreline (r/R < 0, 3).
During the next stage of volatiles burnout (t > 700 s) the axial flame velocity close
to the flame centreline is gradually increasing with radial expansion of the flame
velocity profiles thus disturbing the local balance between the axial flow of
volatiles and the recirculation (Fig. 4c,d).
As could be seen, similar variations in the flame velocity profiles occur
downstream of the combustor (up to L/D ≈ 4.5): the peak values of the axial and
tangential flame velocity components along the outer boundary layer of recircu-
lation zone gradually decrease and the flow pattern approaches the flame centreline
with radial expansion of the flame reaction zone during the wood fuel burnout
(Fig. 4a–f). When comparing the swirl flame velocity profiles at this stage of the
flame formation for the conditions of self-sustaining wood fuel burnout (prop. =0)
and the propane co-fire it is seen that the most substantial difference in the
formation of these profiles is along the outer boundary layer. As follows from
Fig. 4, in the conditions of propane co-fire the tangential flame velocity profiles
indicate the formation of a more complex structure with two peaks of swirl velo-
city, separated by its least value close to the outer boundary of recirculation zone
(at r/R ≈ 0.7). As is seen in Fig. 4e,f, the least flame swirl velocity refers to the
peak value of the axial flame velocity that during the wood fuel burnout gradually
decreases and shifts to the flame centreline at the end stage of the wood fuel
burnout (t > 1200 s), when the enhanced burnout restricts recirculation (Fig. 4e,f).
3.2. The main factors of formation of swirling flow composition field
at co-firing the wood fuel with propane.
As shown above, the formation of flame velocity profiles for the given
configuration of the experimental set-up is highly influenced by the swirling flame
dynamics promoting the recirculation of hot products and determining the
formation of the reverse axial heat/mass transfer up to the wood layer with
enhanced wood fuel heating, gasification and burnout of volatiles developing at
different rates during the processes of the self-sustaining wood fuel burnout
(prop. = 0) and the propane co-fire. The measurements of the flame composition
profiles at different rates of propane co-fire confirm that the swirl-induced
formation of a central recirculation zone at r/R < 0.3 with enhanced reverse axial
heat/mass transfer of the hot products up to the wood layer results in an enhanced
wood fuel gasification and combustion of volatiles, determining the formation of
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the peak volume fraction of CO2 and the peak mass fraction of CO close to the
flame axis (at r/R < 0.3, see Fig. 5a,b). The enhanced wood fuel gasification and
burnout of the volatiles at propane co-fire results in a correlating increase in the
heat energy output and in the flame temperature (Fig. 3a,b) up to the top of the
flame recirculation zone (L/D ≈ 4.5). This promotes the radial expansion of the
flame velocity, composition and temperature profiles with correlating increase in
the combustion efficiency, while decreases the air excess downstream of the
combustor (Fig. 4e, 5e, 6a,b, 7a–f). As seen in Fig. 7, the dominant increase in the
combustion efficiency is close to the outer boundary layer of the recirculation zone
(r/R > 0.5), where the swirl-induced recirculation promotes enhanced mixing of the
flame components and burnout of the volatiles.
0
5
10
15
20
0102030
Radial distance,mm
Volume fraction,%
CO2-3,prop.0,L/D=1
CO2-3,prop.1kJ/s
CO2-3,prop.1,1kJ/s
CO2-3,prop.1,25kJ/s
a
0
3000
6000
9000
01020
Radial distance,mm
Mass fraction,ppm
30
CO-3,prop.0,L/D=1
CO-3,prop.1kJ/s
CO-3,prop.1,1kJ/s
CO-3,prop.1,25kJ/s
b
0
5
10
15
20
0102030
Radial distance,mm
Volume fraction,%
CO2-3,prop.0,L/D=2
CO2-3,prop.1kJ/s
CO2-3,prop.1,1kJ/s
CO2-3,prop.1,25kJ/s
c
0
1000
2000
3000
01020
Radial distance,mm
Mass fraction,ppm
Fig. 5. Effect of propane co-fire on the formation of CO2 and CO profiles downstream of the
combustor during the wood fuel burnout (3 for t = 700–900 s).
30
CO-3,prop.0,L/D=2
CO-3,prop.1kJ/s
CO-3,prop.1,1kJ/s
CO-3,prop.1,25kJ/s
d
0
6
12
18
0 102030
Radia l distance,mm
Volume fraction,%
CO2-3,prop.0,L/D=4,5
CO2-3,prop.1kJ/s
CO2-3,prop.1,1kJ/s
CO2-3,prop.1,25kJ/s
e
0
30
60
90
0102030
Radial distance,mm
Mass fraction,ppm
CO-3,prop.0,L/D=4,5
CO- 3, pro p.1kJ/ s
CO-3,prop.1,1kJ/s
CO-3,prop.1,25kJ/s
f
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As is clearly seen in Fig. 5b,d,f, the peak value of the CO mass fraction
rapidly decreases downstream of the flame axis (R = 0), which confirms that the
swirl-induced recirculation results in an enhanced mixing of the flame components
completing the burnout of volatiles downstream of the flame recirculation zone
(L/D = 4.5) (Fig. 5f). At this stage of the swirling flame formation a slight increase
in the CO mass fraction with high level of turbulent fluctuations in the flame at
increasing rate of propane co-fire is observed close to the outer boundary of the
recirculation zone (r/R ≈ 0.6–0.7), where the peak value of CO mass fraction
correlates with those of the axial flow velocity and combustion efficiency, while
the flame swirl velocity approaches the minimum value (Fig. 4e,f, Fig. 7e). The
formation of the peak CO mass fraction close to the outer boundary layer (at
L/D = 4.5 and r/R = 0.6–0.7) dominates during the primary stage of the enhanced
wood fuel gasification and burnout of volatiles (t < 1200 s) and rapidly decreases
during the wood fuel burnout when the wood layer thickness gradually decreases,
while the swirl flow reversing from the wood layer results in increasing peak
values of the axial and tangential flame velocity components close to the flame
centreline (Fig. 4e,f). The measurements of the flame temperature and composition
(CO2, NOx) profiles at different stages of the flame formation indicated the
correlations between the local variations in the volume fraction of CO2, mass
fraction of NOx, and flame temperatures in the flame reaction zone. The highest
flame temperature with, correspondingly, the highest combustion efficiency and
CO2 & NOx concentrations at the least air excess (Fig. 7b,d,f) and the mean O2
concentration are found near the flame axis (R = 0), i.e. within the central
recirculation zone. Moreover, the radial expansion of the flame temperature
profiles (Fig. 6a) at more intense propane co-fire correlates with the radial
expansion of the flame composition and combustion efficiency profiles (Fig. 5e,
Fig. 6b, Fig. 7e), thus indicating the influence of propane co-fire on the flame
characteristics.
Fig. 6. Effect of propane co-fire on the formation of flame temperature and NOx mass fraction
profiles in the flame reaction zone at different stages of wood fuel burnout
(3 − t = 700–900 s; 4 − t = 900–1100 s; 5 − t = 1100–1300 s).
0,4
0,6
0,8
1
0 102030
Radial distance,mm
Normalized temperature
T-3,prop.0,L/D=4,5
T-4,prop.0
T-5,prop.0
T-3,prop.1,25kJ/s,L/D=4,5
T-4,prop.1,25kJ/s
T-5,prop.1,25kJ/s
a
0
0,2
0,4
0,6
0,8
1
010203
Radial distance,mm
Normalized mass fraction
0
NOx-3,prop.0,L/D=4,5
NOx-4,pr op.0
NOx-5,pr op.0
NOx-3,pr op.1,25kJ/s,L/D=4,5
NOx-4,pr op.1,25kJ/s
NOx-5,pr op.1,25kJ/s
b
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-20
0
20
40
60
80
0102030
Radial distance,mm
Efficiency,%
eff -3,prop.0,L/D=1
eff -3,prop.1kJ/s
eff -3,prop.1,1kJ/s
eff -3,prop.1,25kJ/s
a
0
0
0
0
0
0
0
0
0102030
Radial distance,mm
20
40
60
80
100
120
140
Air excess,%
a.e.-3,prop.0,L/D=1
Fig. 7. Effect of propane co-fire on the formation of combustion efficiency and air excess profiles
at different stages of the burnout of volatiles.
Moreover, the radial expansion of the flame temperature profiles with
correlating expansion of the NOx mass fraction profiles along the outside part of
the swirling flame flow (Fig. 6a,b) indicates that the formation of NOx under the
given combustion conditions mostly refers to the temperature-sensitive two-stage
mechanism. In the near-stoichiometric or lean systems this mechanism looks as
follows:
O2 ↔ O+O
O+N2 ↔ NO+N
N+O2 ↔ NO+O
a.e.-3,prop.1kJ/s
a.e.-3,prop.1,1kJ/s
a.e.-3;prop.1,25kJ/s
b
-10
0
10
20
30
40
50
60
70
0102030
Radial distance,mm
Efficiency,%
eff -3,prop.0,L/D=2
eff -3,prop.1kJ/s
eff -3,prop.1,1kJ/s
eff -3,prop.1,25kJ/s
c
0
100
200
300
400
500
600
700
0102030
Radial distance,mm
Air excess,%
a.e.-3,prop.0,L/D=2
a.e.-3,prop.1kJ/s
a.e.-3,prop.1,1kJ/s
a.e.-3,prop1,25kJ/s
d
30
40
50
60
70
0102030
Radial distance,mm
Efficiency,%
eff -3,prop.0,L/D=4,5
eff -3,prop.1kJ/s
eff -3,prop.1,1kJ/s
eff -3,prop.1,25kJ/s
e
0
500
1000
1500
2000
0102030
Radial distance,mm
Air excess,%
a.e.-3,prop.0,L/D=4,5
a.e.-3,prop.1kJ/s
a.e.-3,prop.1,1kJ/s
a.e.-3,prop.1,25kJ/s
f
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The most active NO formation by this mechanism occurs in the flame zone
with the highest temperature and the air excess of 30–100%.
The measurements of the flame composition have shown that at the rates of
propane co-fire up to 25–30% the peak value of the NOx mass fraction in the
products is relatively low and does not exceed 100 ppm, which is quite acceptable
for the wood fuel burnout. The average value of the NOx mass fraction in the flame
reaction zone approaches 66 ppm at the average value of the CO mass fraction
during the burnout of volatiles 28–30 ppm and combustion efficiency 60%, indi-
cating that the propane effect of co-fire on the flame swirling flame formation can
be used to achieve clean and effective wood fuel burnout.
4. CONCLUSIONS
The developed laboratory-scale combustor for the wood fuel & propane co-
fire allowed the complex experimental research into the propane co-fire effect on
the formation of the flame velocity, temperature and composition profiles.
Measurements of the flow patterns demonstrate that the formation of the
flame velocity profiles is dictated by the air swirl, i.e. by induced formation of the
central recirculation zone with intensive mixing of the flame components and
combustion of the volatiles.
For the given combustion conditions the propane co-fire promotes the radial
expansion of the flame velocity profiles with correlating increase in the combustion
efficiency, flame temperature and local concentration of the main products (CO2
and NOx) along the outside part of the flame reaction zone and resulting increase in
the total heat output downstream of combustor (Qtot).
It is found that for the given rates of propane co-fire the dominant CO2
release (up to 80%) refers to the carbon-neutral emissions produced during the
burnout of renewable wood fuel at the least average CO mass fraction in the
products (28–30 ppm). Moreover, for the given combustion conditions the average
NOx fraction in the products does not exceed 60–70 ppm. Hence, the propane co-
fire of the wood fuel downstream of the swirl flame flow can be used to provide
cleaner and more effective burnout of wood fuel.
REFERENCES
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Energy in Reducing Greenhouse Gas Build-up, 1–4.
http://www.tva.gov/environment/air/ontheair/renewable.htm.
3. Climate Change Home (2009). U.S. Environmental Protection Agency,
http://www.epa.gov/climatechange/index.html.
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http://www.primenergy.com/reference_BioMassFiring.htm.
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Swirling-Flow Burner. PhD Theses, Brigham Young University, 1–230.
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particulate emissions 24%, saves $0.13/MMBtu. Journal Articles by Fluent Soft Users,
1–4.
7. Babu, S.P. (2001). Role of Natural Gas in Promoting Bioenergy as a Component of the
Sustainable Energy Scenario. Natural Gas/Renewable Energy Hybrids Workshop,
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15
NETL, Morgantown, WV, 1–12, http://www.netl.doe.gov/publications/proceedings/
01/hybrids/ ngbm8-01.pdf.
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combustion. Combust. Sci. and Tech., 175, 545–578.
9. Vanoverberghe, K.P. (2004). Flow, Turbulence and Combustion of Premixed Swirling
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Leuven, Belgium, 1–225.
10. Littlejohn, D., Majeski, A.J., Tonse, S., Castaldini, C., & Cheng, R.K. (2002).
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KOMBINĒTĀ ATJAUNOJAMĀ UN FOSILĀ KURINĀMĀ DEGŠANAS
PROCESA DINAMIKAS EKSPERIMENTĀLIE PĒTĪJUMI VIRPUĻPLŪSMĀ
M. Zaķe, I. Barmina, V. Kriško, M. Gedrovičs, A. Descņickis
Kopsavilkums
Veikti kombinētā atjaunojamā (koksnes biomasa) un fosilā kurināmā
(propāna) degšanas procesa dinamikas eksperimentālie pētījumi virpuļplūsmās ar
mērķi izveidot ekoloģiski tīrus un efektīvus koksnes biomasas degšanas un siltuma
ražošanas procesus, mainot papildus siltuma padevi gaistošo savienojumu degšanas
zonā. Eksperimentālo pētījumu komplekss apvieno ātruma sadalījuma, liesmas
temperatūras, sastāva un degšanas procesa efektivitātes radiālā un aksiālā sada-
lījuma veidošanās pētījumus dažādās kombinētā degšanas procesa attīstības
stadijās, mainot propāna padevi liesmā. Pētījumu rezultātā parādīts, ka propāna
padeve koksnes biomasā ierosina ātrāku koksnes gazifikāciju, nodrošinot pilnīgāku
gaistošo savienojumu sadedzināšanu virpuļplūsmas recirkulācijas zonā, kurā notiek
intensīva gaistošo savienojumu sajaukšanās ar liesmas komponentēm, palielinot
degšanas zonas temperatūru, kas atkarīga no papildus siltuma padeves gazifikātora
izejā.
09.11.2009.
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