PNNL-13501 - Pacific Northwest National Laboratory

Plasma
Particle
Dielectric Fiber
Electrode
HEPA Cartridge
Fiber-Bed
Plasma
(w/ Heating
Jacket)
Fluidized-Bed
Aerosol Generator
concentration at the outlet of the reactor with Fouriertransform
infrared spectroscopy. Water could be added to
the aerosol by introducing humid air, and the reactor
could be heated up to 425 K using a heating jacket if
desired.
Results and Accomplishments
FTIR
Spectrometer
API Particle Sizer
Air
H 2 O Bubbler
Figure 1. Test stand used for particulate collection/oxidation
measurements
Collection and oxidation of the graphite was measured as
a function of temperature, humidity, and voltage applied
to the plasma reactor. The collection results are shown in
Figure 2. Here, data obtained from an unfilled reactor are
compared to those from a fiber-filled reactor. In most
cases the collection efficiency is above 70%, and the
collection efficiency was above 90% at lower voltages.
We observed that increased temperature resulted in about
5 to 10% reduction in collection and that increased
humidity reduces collection at higher temperatures. We
also observed that the collection efficiency dropped,
particularly as the voltage was increased beyond 6 kV.
We believe this is a function of the power supply used to
drive the reactor.
Collection Efficiency
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
Plasma On
0 2 4 6 8 10 12 14 16
Applied Voltage, kV
Open Gap, 310K
Dry, 310K
Dry, 425K
0.6% Water, 310K
0.6% Water, 425K
Figure 2. Collection efficiency as a function of voltage for a
variety of conditions
The oxidation of carbonaceous particulate matter takes
place via the following reactions:
C(s) + O• → CO
C(s) + O3 → CO + O2
C(s) + •O2H → CO + •OH
C(s) + •OH → CO + H•
In all cases, one atom of carbon is oxidized per atom of
carbon monoxide evolved. Therefore, carbon monoxide
production should be roughly equivalent to carbon
oxidation. Figure 3 shows the measured levels of carbon
monoxide as a function of energy consumed for a variety
of stream conditions. At 310 K, the power consumption
per oxidized carbon atom did not decrease if the fiber
optic packing was added. This indicated that the silica
fiber optics offered no catalytic enhancement to the
oxidation. Other factors of interest included temperature
and humidity. Results showed that temperature
dramatically increased oxidation while the presence of
water lowered the oxidation rate at a given temperature.
At near room temperature (310 K), the plasma must
overcome the entire heat of reaction to convert carbon to
monoxide. At elevated temperature, the plasma must
overcome only part of the heat of reaction, so the energy
requirement is lower. When water is added to the gas
mix, some of the water molecules will reside on the
surface of the particulates and block oxidation sites, and
this effect lowers the oxidation rate.
Oxidation Rate, ppm CO
200
150
100
50
0
0 5 10 15 20 25 30
Energy Deposited, J/L
Summary and Conclusions
Open Gap, 310K
Dry, 310K
Dry, 390K
Dry, 425K
0.6% Water, 390K
1.2% Water, 390K
Figure 3. Oxidation rate as a function of energy deposited
into the gas
We demonstrated that organic particulates can be
collected and destroyed in alternating current-driven,
nonthermal plasmas. Humidity and temperature have a
slightly degratory effect on collection, but these effects
are overwhelmed by particle losses at high voltages. We
believe that this can be overcome by design of an
appropriate power supply. Oxidation of particles was
better at high temperature, but we observed that moisture
inhibited oxidation. Crushed fiber optics (SiO2) showed
no catalytic activity for carbon oxidation.
Human Health and Safety 277