22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Warm plasma aided ignition of coal powders at atmospheric pressure
H.-P. Li1, Z.-X. Sun2, H. Guo1 and Z.-X. Li2
1
2
Department of Engineering Physics, Tsinghua University, 100084 Beijing, P. R. China
College of Mechanical Engineering, Hebei United University, 063009 Tangshan, Hebei Province, P. R. China
Abstract: The unique features of the atmospheric air warm plasmas include the medium
gas temperature level, the high chemical reactivity, the low cooling load for electrodes of
the plasma generators, etc. In this paper, a plasma-aided ignition method with the
employment of the atmospheric warm plasmas is proposed. The discharge characteristics
of the air warm plasmas under different air flow rates, and the coal combustion flame
features with the feeding of the air-coal powder mixture into the plasma generator are
studied experimentally. The preliminary results show that it is possible to develop a
warm plasma-aided ignition technology for igniting the pulverized coal during the
start-stop operation period in thermal power plants in future.
Keywords:
warm plasma, plasma-aided ignition, atmospheric pressure discharge
1. Introduction
Generally, light oils are used for igniting coal powders
during the start-stop operation period or stabilizing
combustion under a low load operation mode in thermal
power plants. It was reported that for a 600 MW
pulverized coal boiler unit, 300 tons of oil is necessary
in a cold starting process [1]. Due to the very limited
oil resources, it is an urgent demand to reduce the oil
consumption, and to develop little-oil or even oil-free
ignition technologies in the thermal power plants.
Among different types of oil-saving pulverized coal
ignition technologies, plasma-assisted ignition (PAI)
technology has attracted much attention of researchers in
the academic community and engineers in industries due
to its unique features of high energy density, high
chemical reactivity, fast start-stop process, etc. It is
known that the ignition point of coal powders is around
600-800 K, while the temperature of the employed arc
discharge thermal plasmas for the pulverized coal
ignition usually ranges from 6000 to 10000 K [2, 3].
This not only leads to a low ignition efficiency, which is
defined as the power assumption of plasmas for igniting
1 kg of pulverized coal in one hour, but also results in a
heavy burden of the cooling system for the electrodes of
plasma generators, a low thermal efficiency and a short
lifetime of the plasma torches.
All of these
disadvantages retard the actual applications of the
thermal plasma coal powder ignition technologies in the
thermal power plants.
In our opinion, it is indispensable to develop an
appropriate plasma source with a suitable combination
of the plasma energy density level and the concentration
level of the chemically reactive species for promoting
the large scale applications of the PAI technologies in
the thermal power plants. Recently, a non-equilibrium
warm plasma source, which possesses a medium gas
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temperature level about 2000-3000 K and high chemical
reactivity, is developed using different types of plasma
generators, such as microwave discharges, low current
arcs, sparks, gliding arcs, etc. [4-6]. So, the purpose of
this paper is to study the characteristics of a pulverized
coal ignition system using an atmospheric warm plasma
source [3].
2. Experiment setup
A schematic of the experiment setup is shown in
Fig. 1. A high-voltage alternative-current (HVAC)
power supply is employed for the generation of the
atmospheric air warm plasmas.
Since the gas
temperature is around 2000 K, an air-cooling water
circulation system is used to cool the electrodes of the
plasma torch. The pulverized coal is mixed with air in
a powder feeder and is fed into the plasma torch under a
fixed air flow rate. The feeding rate of the coal
powders is controlled by adjusting the applied voltage of
the powder feeder. The waveforms of the discharge
current and voltage are measured using a current probe
(Tektronix TCP202) and a high voltage probe
(Tektronix P6015 A), respectively, and are recorded on
a digital oscilloscope (Tektronix DPO4034). The
images of the plasma jet or flames at the outlet of the
plasma torch are taken by a digital camera (Fujifilm
FinePixS9600).
3. Experimental Results
In this section, the characteristics of the plasma jet
without the feeding of the pulverized coal is studied
first, since the features of the air plasmas would
influence the coal powder ignition process under
different operating conditions. The images of the
plasma jet under different gas flow rates at a fixed
driving frequency of the power supply, f = 26.0 kHz, are
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22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Fig. 1.
Schematic of the experiment setup.
shown in Fig. 2 which are taken with an exposure time
of tExp = 1/7 s. The corresponding variation of the
power input with the air flow rate is shown in Fig. 3.
It is seen from Figs. 2 and 3 that: (i) under a lower
flow rate (e.g., Q<3.5 splm in this study), the length of
the plasma jet increases with increasing the air flow rate,
accompanied by an increase of the power input from 182
to 275 W; (ii) with a further increase of the air flow rate
(e.g., Q>3.5 splm in this study), there exists a slight
decrease of the plasma jet length, and the variation of
the power input is also small (on the level of 280 W).
Fig. 3.
rates.
Fig. 2. Jet images at the outlet of the plasma torch
under different air flow rates.
At the constant values of Q=8.0 slpm and f=26.0 kHz,
the waveforms of the discharge voltage and current with
air and air-coal powder mixture are shown in Fig. 4. It
can be seen that there exists a significant influence on
the discharge characteristics, especially the waveforms
of the discharge voltages, with the addition of the coal
powders in the air. Compared with the air discharges,
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Variation of the power input with the air flow
the addition of coal powders reduces the discharge
voltage and current from Vd,p-p = 2.6 to 1.6 kV and
Id,p-p = 1040 to 940 mA, respectively, and
correspondingly, the power input also decreases from
281 to 232 W.
The flame images under different feeding rates of the
pulverized coal are presented in Fig. 5 with
Q = 8.0 slpm, f = 26.0 kHz and tExp = 1/2 s. Fig, 5
shows that a long combustion flame can be formed due
to the sufficient mixing of the coal powders with the
discharge plasmas inside the plasma generator. And
the length and diameter of the flames become larger
with increasing in the coal powder feeding rate.
2
6. References
[1] J.-H. Tu.
Energy of China, 11, 38-40 (1996)
[2] H. Li, W. Guo, P. Xu, X. Yuan and T. Zhao.
Nuclear Techniques, 27, 626-629 (2007)
[3] Q. Chen, J. Zha, Z. Zhou, B. Bai and W. Xia.
J. Univ. Sci. Technol. China, 40, 1301-1304 (2010)
[4] A. Gutsol, A. Rabinovich and A. Fridman.
J. Phys. D: Appl. Phys., 44, 274001 (2011)
[5] Z.-B. Wang, G.-X. Chen, Z. Wang, N. Ge, H.-P. Li
and C.-Y. Bao. J. Appl. Phys., 110, 033308
(2011)
[6] L. Fulcheri, J.-D. Rollier and J. Gonzalez-Aguilar.
Plasma Sources Sci. Technol., 16 183-192 (2007)
Fig. 4. Voltage and current waveforms with air (a) and
air-coal powder mixture (b).
Fig. 5. Images of the coal combustion flames under
different feeding rates of the coal powders. (a) 3.4
g/min; (b) 4.7 g/min; (c) 5.6 g/min
4. Conclusions
In this paper, an atmospheric warm plasma-aided
ignition (wPAI) method is proposed for igniting the
pulverized coal. The preliminary experiments show
that a stable warm plasma can be sustained with the
feeding of the air-coal powder mixture into the plasma
torch, and a long coal combustion flame is obtained at
the downstream of the plasma generator. These results
demonstrate the possibility to develop a new wPAI
technology used in the thermal power plants with a
further understanding to, as well as control of, the
complicated physical-chemical processes in this wPAI
system.
5. Acknowledgment
This work has been supported by the National Natural
Science Foundation of China (11035005).
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