1

PlasTEP Final Conference

5. / 6. 12. 2012

Plasma for Gas Cleaning

Approaches by a Process

Engineer

Ulrich Riebel

Chair of Particle Technology

BTU Cottbus

Current Problems in Electrostatic Precipitation (10th Nordic Filtration Symposium, Trondheim 2006)

Transport of

Ionized Molecules

Ozone

Gas-to-Particle

Conversion

Electric Wind

„BEGA“

Corona Onset

Effects cm1011el

Particle

Reentrainment

„ABREA“

Corona Quenching

by space charges

cm10 4el

Back Corona

A Combination of Biofilter & ESP for Gas Cleaning Diss. Robert Mnich, BTU Cottbus, 2009

BEGA =

Bio – Elektrischer Geruchs–

Abscheider

Simultaneous removal

of aerosols and

organic vapours by

combination

of ESP and

biofilter

BEGA-subtopic: Direct Ionic Transport Does the corona discharge contribute significantly to VOC

abatement?

Experimental Evidence

• Atmosphere saturated with

various (organic) substances

• Controlled humidity and temp.

• Influence of discharge polarity

• Measurements of discharge

current and mass of deposit on

precipitation electrode (p. e.)

Substance

gnd

p.e. c.e.

+

-

Spannungs- Stromdiagramm

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

0,0 5,0 10,0 15,0 20,0 25,0

U in kV

I in

µA

Campher 33% rel. Luftfeuchte Raumtemperatur pos. Korona

Campher 33% rel. Luftfeuchte Raumtemperatur neg. Korona

rel. Luftfeuchte 33% Raumtemperatur neg. Korona

rel. Luftfeuchte 33% Raumtemperatur pos. Korona

Neg. Corona

Pos. corona –

current uptake

is reduced

significantly

Neg. corona –

no effect from

camphor.

Effect of VOCs on current-voltage characteristic of corona

This effect is

explained by the specific

attachment of VOCs to

the gas ions, leading to a

change of ion mobility

Direct Ionic Transport Exp. Evidence with Salol

A deposit of fine

particles is formed on

the precipitation

electrode 200x Vergrößerung 200 x Vergrößerung

OH

C - O

O

Influence of VOCs on Corona Discharge

• VOCs bind specifically to gas ions, forming ions of

lower mobility

• This is indicated by a lowered current uptake

• Hence ionized VOCs are transported to the

precipitation electrode at high velocity, v=50m/s

• Unfortunately, at typical ESP conditions, the corona

current (< 1 mA/m²) can only remove ppb concentra-

tions

….nevertheless…

Abscheidung vom Isopren im Elektroabscheider

0

2

4

6

8

10

12

14

16

18

20

15:47:15 15:48:41 15:50:08 15:51:34 15:53:01 15:54:27 15:55:53 15:57:20

Zeit

Ko

nzen

trati

on

vo

m Iso

pre

n [

pp

m C

3]

Konzentration vom Isopren im EA

ohne Spannung: "0kV"

Konzentration vom Isopren im EA

mit Spannung: "-30kV"

ESP – Removal of Isoprene by Corona Diss. Robert Mnich, BTU Cottbus 2009

Nevertheless…

Some VOC´s can be removed quite efficiently by

corona discharge in an ESP

Electrostatic Precipitator – Removal of Isoprene by Corona Discharge Diss. Robert Mnich, BTU Cottbus 2009

Trocken-EA, uLR=0,041m/s (tVWZ=7,2s), U=22 kV, negative/positive Polarität

24

,8

21

,9 37

,9 54

,1

12

1,0

3,2

3,3 7,0 2

0,1

49

,6

31,7

42,4

36,6

28,7 4

9,6

152,2

166,8

241,4 271,5

332,6

48

68

5250

29

96 97 95

89

80

0

40

80

120

160

200

240

280

320

360

400

48 68 78 107 171 71 94 147 184 249

Isopren-Rohgaskonzentration [mg/m³]

Iso

pren

-Re

ing

asko

nz. [m

g/m

³]

0

10

20

30

40

50

60

70

80

90

100

Ab

sch

eid

un

g V

OC

s [

%]

FID, Reingas [mg/m³] SMPS, Reingas [mg/m³] Abscheidung [%]

"positive Korona" "negative Korona"

0,0E+00

5,0E+03

1,0E+04

1,5E+04

2,0E+04

2,5E+04

3,0E+04

3,5E+04

4,0E+04

4,5E+04

5,0E+04

10 100 1000

x [nm]

C# [

#/c

m3]

20 ppm

23,5 ppm

31,1 ppm

48,4 ppm

60,6 ppm

81,9 ppm

300 ppm

Trend bei steigender Isoprenkonzentration

im Rohgas

Electrostatic Precipitators and VOC

reduction – Conclusions

• ESP performance is signifantly influenced by VOCs

• Some VOCs are removed significantly by oligomerization + particle formation behind the ESP

• Significant emission of intermediate reaction products and aerosols

• Hence typically, ESPs are not suited for VOC removal

In one of our next projects….

PLASMA appeared to be the last hope

• Off-gas from smoking

• Wet scrubber does not reduce VOCs sufficiently

• Postcombustion not wanted

…so we collected a lot of literature on plasma

reactions and built a lab reactor for a feasibility

study.

We had to learn some lessons about…

• Energy input may be calculated from Lissajous measurement or from Manley formula…

• The results are quite different

• Calorimetric measure-ment confirm Lissajous results

Plasma – measurement of energy input

BA Roman Eibauer, BTU 2011

Lissajous

Manley

„Cold Plasma“ – Specific Energy Input and

Reactor Temperature I

• Many Cold Plasmas are not quite so cold…high energy inputs:

1 J/ltr ~ 1 °C temperature rise

Magureanu et al. Appl.Catalysis B, 2007

F. Holzer, U. Roland H.D. Kopinke, UFZ-Ber. 1998/99

Kim et al. Int. J. Plasma Env. Sci. & Technol. 2007

„Cold Plasma“ – Specific Energy Input and

Reactor Temperature II

• Temperature effects on plasma

reaction kinetics?

• Research reactors should have a

defined temperature…

heating/cooling

• Technical (big) reactors are

adiabatic…

the transition to thermal

combustion will occur beyond

300 ..500 J/Ltr.

adiabatic

Kim et al. Int. J. Plasma Env. Sci. & Technol. 2007

- Small lab reactors – significant

temperature losses

- Many reference experiments

were run at a non-defined

temperature

DBD plasmas…we still have the aerosol problem

450 ng/m³ @ 15 kV

140 ng/m³ @ 17,5 kV

Aerosols

remain after VOC oxidation

and require more energy

input

Plasma was not a full success…

• Complicated interaction of

temperature, humidity and

catalysts decisive for energy

input.

• Methane was not oxidized

efficiently and we could not

find a catalyst for this task.

• Compared to wet scrubber,

comparable methane emission

but higher energy consumption

Methane above

TOC limit

TiO2 catalyst

active above

85°C

BA Simon Marschall, BTU 2012

DBD plasmas…what we were missing

• Kinetic constants for diverse VOCs and reaction conditions

• Many reactor designs, but almost no comparative data on reactor efficiency

• Generally accepted „model substances“ for testing reactor efficiencies

• More knowledge about byproducts, CO, phosgen, heterocyclic VOCs, particle production

Kim et al, J. Phys. D:

Appl. Phys 2005, 1292

Magureanu et al. Appl.

Catalysis B, 2007

Plasma für Gas Cleaning - Outlook

- Energy efficiency is critical, plasma will be restricted to

very low concentrations and special applications

+ Compact reactors/short residence time

+ Function without additional media (water, gas…)

+ No residues (ideally)

+ A natural process – atmospheric oxidation – is imitated

++ A lot of research is needed!

The End

Plasma in Gas Cleaning

Specific transport of ionized molecules

Could it be relevant in gas cleaning?

ESP - typical current density: I ≤ 1 mA/m2

elementary charge e = 1,60210-19 As

typical electrode spacing a = 0,1 m

typical residence time = 1 s

cC

a

τ

e

I

gasprocessed volume of

es (ions)argchnumber of

315 /m1025,6Cc

mC25107,2

0VΑv

Ν

gasvolume of

moleculesnumber of ppbv4ˆ

9104

mC

cC

Strom- Spannungsdiagramm

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

0,0 5,0 10,0 15,0 20,0 25,0

U in kV

I in

µA

1- Butanol 33% rel. Luftfeuchte 20°C pos. Korona 1- Butanol 33% rel. Luftfeuchte 20°C neg. Korona

rel. Luftfeuchte 33% Raumtemperatur neg. Korona rel. Luftfeuchte 33% Raumtemperatur pos. Korona