PLASMA IGNITION SYSTEM FOR LIQUID FUELS
Arkadiusz Dyjakon
Institute of Heat Engineering and Fluid Mechanics, Wroclaw University of Technology,
Wyb. Wyspiańskiego 27, 50-370 Wrocław, POLAND
E-mail: [email protected]
Introduction
Ignition of fuel-air mixtures is an important topic in combustion science from both
fundamental and practical points of view. Some of the applications of ignition phenomenon
include reliable ignition of fuel-air mixture in engines, burners and other devices [1-3].
The electrical discharge has been the ignition source of choice for most types of
propulsion and automotive combustion engines for over 100 years. It has many advantages
including simplicity, low cost, size and weight of the electronic elements, and it produces
sufficiently high temperatures to dissociate and partially ionize air-fuel mixture. Nevertheless,
there are also few disadvantages of electrical discharges, like: limited size of the discharge,
the necessity for supporting electrodes that may interfere with the flow or combustion
process, and quite high energy input efficiency (ratio of energy deposited in the gas to the
electrical energy consumed in producing the discharge). There are many types of electrical
discharges: dark, corona, spark, arc and others, however their application depends on process
parameters and expected results.
For instance, in combustion technology there is a tendency to the combustion of leaner
and leaner air-fuel mixtures, which lead to the financial benefits and lower natural
environment pollution. As a result the use of higher density of energy source is required to
perform the combustion process [4]. One of the propositions is application and development
of plasma ignition technology. This technology, which can initiate and stabilize the
combustion of solid, liquid, and gaseous fuels that previously could not be ignited by spark
energy discharges, will enable more sophisticated and cost effective combustion system
designs, and could lead to new ways to reduce combustion impurities.
Gliding arc discharge
For ignition of combustible mixture different kind of plasma can be applied (Fig. 1)
depending on the fuel used (gas, liquid, solid). In the case of liquid fuel ignition it is not
necessary to use high temperature plasma, which is able to ignite and create a pulverized fuel
flame in cold surrounding [5]. The suitable and sufficient source of ignition is low
temperature plasma generated for example during gliding arc discharge.
Temperature, K
10
5
non-thermal
plasma
thermal
plasma
T e = Tg = T j
Te
104
103
102
10-4
Tg
10-3
10-2
10-1
1
101
102
103
104
Pressure, kPa
Fig. 1. Type of plasma in dependence of pressure and temperature [6]:
Te, Tg, Tj – electrons, particles and ions temperature, respectively
Gliding discharge is an auto-oscillating periodic phenomenon developing between at
least two diverging electrodes located in a laminar or turbulent gas flow (Fig. 2). Self-initiated
in the upstream narrowest gap, the discharge forms the plasma column connecting the
electrodes of opposite polarity. This column is further dragged by the gas flow towards the
diverging downstream section. The discharge length grows with the increase of distance
between electrodes until it reaches a critical value, usually determined by the power supply
limits. After this point, the discharge extinguishes but momentarily reignites itself at the
minimum distance between the electrodes and a new cycle starts. The temperature of plasma
generated in the gliding arc discharge can be in the range 800-10000 K [7].
ANODE
WORKING
GAS
ignition
ARC
electrode
CATHODE
Fig. 2. Scheme of plasma igniter with gliding arc
Gliding discharges comprising both equilibrium and non-equilibrium plasma
conditions offer high energy efficiency and selectivity not only for ignition but also for other
chemical processes, like: gas conversions and decontamination processes (such as carbon
dioxide or steam reforming of methane to produce synthesis gas), oxidation H2S into SO2 for
pollution control or volatile organic compounds treatment for gas purification and
environmental protection.
Plasma igniter
Plasma igniter with gliding arc (Fig. 3) can be an alternative device used in the oil
burner for liquid fuel ignition, which is able to eliminate from main oil burner small gas
igniter. Gas igniter is not convenient because of strict safety regulations and possibility of
blow off of the gas flame during the attempt of hard oil ignition.
Created in the gliding arc discharge plasma possess higher density of energy that
contribute to vigorous vaporizing and pyrolysis of the oil droplets. Hydrocarbons may be
removed or decomposed by oxygen (and hydrogen) plasma according the following reactions
[8]:
C m H n + O → CO + H 2 O ,
(1)
C m H n + H → CH 4 + H 2 .
a)
b)
Fig. 3. View of working plasma igniter tip:
construction (a), during the operation (b)
(2)
Research test bed
During the laboratory research the oil burner RG0R (Riello Burners company),
plasmatron with the gliding arc and electronic power supply (3 kW) of high frequency were
applied. The research procedure was, as follow (Fig. 4): in the combustion chamber sprayed
the air-oil mixture, which was ignited by plasma torch. Plasma igniter was installed in the
chamber at the distance of 0,01 m from front of the oil nozzle. The operation parameters of
the burner and plasma igniter (spraying pressure, spraying angle, equivalence ratio, oil rate
flow, plasmatron power) were constantly controlled during the experiment (Table 1). While
varying equivalence ratio in the mixture the measurements of ignition efficiency (IE) were
made:
N
IE = i ⋅ 100% ,
(3)
Ns
where:
Ni – number of sufficient ignition attempt of oil-air mixture,
Ns – total number of attempt.
4
1
7
2
3
9
10
6
GA – 40
8
5
Oil
11
Fig. 4. Schematic of the experimental set up:
1 – plasmatron, 2 – oil burner, 3 – combustion chamber, 4 – power supply of the plasmatron,
5 – compressor, 6 – electronic balance, 7 – manometer, 8 – oil filter, 9 – oil container,
10 – rotameter, 11 – exhaust-gas analyser
Table 1
The parameters and condition during the investigation
Unit
Oil stream
kg/h
Spraying pressure
bar
Equivalence ratio, φ
Power of the plasmatron, Nel
kW
Air stream for plasmatron
l/h
2
Viscosity (20 °C)
mm /s (°E)
Diesel fuel
L.C.V.
MJ/kg
o
Oil density (15 C)
kg/m3
Range
1.0 ÷ 1.8
8 ÷ 14
0.5 ÷ 0.35
1.0 ÷ 2.0
500
6 (1.5)
42.6
0.84
The experimental research were focused on the examination of the influence of nonthermal plasma on the ignition of liquid fuel in dependence from some operation parameters
of the plasma igniter and oil burner, such as: spraying ratio, equivalence ratio, electrical
power of plasma torch.
500
p = 1 atm
φ = 0,65
100
10
1
0,1
10
heavy fuel oil
diesel oil
iso-octane
100
SMD, µm
500
Minimum ignition energy Emin, mJ
Minimum ignition energy Emin, mJ
Influence of spraying quality on plasma ignition efficiency
One of the important factors influence on the ignition efficiency and flame
propagation rate of liquid fuels is spraying quality which decides both about the release of
volatile matters, rate of pyrolysis process and ignition energy (Fig. 5).
200
SMD, µm
100
100
diesel oil - air:
V = 15 m/s
p = 1 atm
φ = 0,65
60
10
6
4
3
30
0
0,25 0,5
0,75 1,0
Vapour concentration Ω
Fig. 5. Influence of pressure on spraying quality [9, 10]:
Decomposition of hydrocarbons improves ignition condition, especially in the case of
hard oil (mazout) ignition, for example butane pyrolysis can proceed as follow [11]:
C 4 H 10 → C 2 H 6 + C 2 H 4
(4)
C 4 H 10 → CH 4 + C 3 H 6
(5)
C 4 H 10 → H 2 + C 4 H 8
(6)
Experimental results confirms the consideration performed above, the deterioration of
spraying quality decreases the effectiveness of ignition and the range of ignitability shifts in
the direction of rich mixtures (Fig. 6).
φ = 0.50
80
φ = 0.48
60
φ = 0.45
Ignition
efficiency, %
100
φ = 0.43
40
N el = 2.0 kW
20
o
Nozzle 40 60 W EN
0
6
8
10
12
14
Spraying pressure, bar
16
φ = 0.50
80
φ = 0.48
60
φ = 0.45
40
φ = 0.43
20
N el = 1.5 kW
Ignition
efficiency, %
100
o
Nozzle 40 60 W EN
0
Ignition
efficiency, %
6
8
10
12
14
Spraying pressure, bar
16
100
φ = 0.50
80
φ = 0.48
60
φ = 0.45
φ = 0.43
40
N el = 1.0 kW
20
o
Nozzle 40 60 W EN
0
6
8
10
12
14
Spraying pressure, bar
16
Fig. 6. Influence of spraying pressure and equivalence ratio on plasma ignition efficiency
Influence of plasmatron power on plasma ignition efficiency
Another parameter that also plays an important role during the combustion of liquid
fuels is ignition energy (Fig. 7). The higher ignition energy is the more thermal energy is
transferred to the combustible mixture and the larger is the degree of thermal pyrolysis of
hydrocarbons. Moreover, the comparison of plasma ignition system and spark ignition system
was performed to show the superiority of plasma igniter.
o
Nozzle 40 60 W EN
p = 14 bar
Ignition
efficiency, %
30
60
N el = 2.0 kW
N el = 1.5 kW
N el = 1.0 kW
100
Spark ignition
0,0
0,2
0,4
0,6
Equivalence ratio φ
0,8
o
Nozzle 40 60 W EN
p = 12 bar
Ignition
efficiency, %
30
60
N el = 2.0 kW
N el = 1.5 kW
N el = 1.0 kW
100
Spark ignition
0,0
0,2
0,4
0,6
Equivalence ratio φ
0,8
o
Nozzle 40 60 W EN
p = 10 bar
Ignition
efficiency, %
30
60
N el = 2.0 kW
N el = 1.5 kW
N el = 1.0 kW
Spark ignition
100
Ignition
efficiency, %
0,0
0,2
0,4
0,6
Equivalence ratio φ
0,8
o
30
Nozzle 40 60 W EN
p = 8 bar
60
N el = 2.0 kW
N el = 1.5 kW
N el = 1.0 kW
Spark ignition
100
0,0
0,2
0,4
0,6
Equivalence ratio φ
0,8
Fig. 7. Comparison of ignition efficiency of plasma and spark ignition system
The measurements showed that the increase of electrical power of plasma igniter Nel
facilitates the ignition of oil-air mixture. The other advantage arising from the higher input of
energy is the possibility of mixture ignition in worse operation condition of oil burner
(equivalence ratio φ << 1, higher flow velocity).
Conclusion
The stable ignition of oil-air mixture (especially hard oil) in the burner located in the
combustion chamber of coal-fired boiler is very important for its exploitation and production
of electricity. Inefficient ignition of oil during the start up of the boiler leads to fuel losses,
waterfall or slag funnel pollution and explosibility hazard. As a results power plant face the
additional costs which decrease its efficiency.
To limit the ignition failures new types of ignition system are searched and developed,
to which can be ranked plasma ignition systems (plasma igniters). The advantages of plasma
igniter with gliding arc are, as follow:
• higher power of ignition energy,
Spark energy in traditional high-energetic igniters varied in the range 4 ÷ 16J (spark
frequency: 3 ÷ 30 per second) [3]. The power of plasma is much higher (Fig. 6) and is
practically unlimited, it can be matched adequately to the given fuel and operation
condition of the burner (even during the work of plasma igniter).
• higher ignition efficiency in comparison to the traditional spark ignition system,
Because of higher energy density in plasma the ignition of liquid fuel is much more
effective (Fig. 7) which decreases the ignition failures of the burner.
• large volume of the “plasma flame” and lack of necessity of very precise location of
the ignition source in the outlet of the burner,
Low spark energy and its small volume requires very precise location of discharge
electrodes. Moreover, this kind of ignition system is sensitive on the feeding condition
of the burner. Plasma with gliding arc takes up larger volume (Fig. 3b) and therefore is
much more resistant on disturbances and rapid changes in the burner surroundings.
• the possibility operation in more difficult conditions (lean mixtures, high flow
velocities),
As a consequence of higher plasma energy and its large volume is high ignition
efficiency of the oil-air mixture in the conditions in which the spark ignition of oil is
impossible (Fig. 7). This factor is very important from practical and environmental
point of view.
• automation possibility and full control of the plasma ignition process.
The practical application can yield benefits, however the prosecution of the research in
technical scale is needed to eliminate eventual problems for the cooperation with other
devices or working systems and to choice proper conditions of the operation.
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