PCFFs in municipal solid waste flue gas

Chemosphere, Voi.23, Nos.8-10, Printed in Great Britain

pp 1445-1452,

1991

0045-6535/91 $3.00 + 0.00 Pergamon Press plc

CATALYTIC D E S T R U C T I O N OF PCDDs/PCFFs IN MUNICIPAL SOLID WASTE FLUE GAS M. HIRAOKA, N. TAKEDA Department of Environmental and Sanitary Engineering Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan T. KASAKURA and Y. IMOTO NGK Insulators, Ltd. Suda-cho, Mizuho-ku, Nagoya 467, Japan H. TSUBOI, T. IWASAKI NKK Corporation Minamiwatarida-cho, Kawasaki-ku, Kawasaki 210, Japan

ABSTRACT Catalytic destruction technology was applied to reduce PCDDs/PCDFs concentration levels in the fluc gas from a municipal solid waste incinerator. Using common combustion techniques, high temperatures (>100012) are required to decompose PCDDs/PCDFs. However, from an energy saving perspective, it is more efficient to perform exhaust gas cleanup at low temperatures because the incinerator flue gas temperatures are comparatively low after leaving the waste heat boiler and air pollution control unit (250-300]2). The ability to remove PCDDs/PCDFs by catalytic destruction was previously confirmed by the authors, with the presented study conducting pilot plant experiments using exhaust gas (max. 1900 N n//hr) induced from the outlet of the electrostatic precipitator of a municipal solid waste incinerator. A PCDDs/PCDFs destruction ratio of at least 99% was obtained at temperatures less than 30(I ]2 , being dependent on space velocity and the catalyst geometric properties.

KEYWORDS PCDDs/PCDFs: gas

PCDDs/PCDFs

destruction;

catalytic destruction;

municipal solid waste

incineration:

lluc

EXPERIMENTS AND DISCUSSION Catalyst In the presented study, catalysts were used which primarily consisted of Pt supported on a Si/Ti carrier. Four different catalysts were tested, having either a honeycomb or corrugate form (See Fig. 1), with their geometric properties shown in Table 1.

1445

1446

ltoneyeomb Type

Cor rugat e Type B

I

A,

: Cell P i | c h l h ~ B 2 : \¥all T h i c k n e s s

AI~A2

B,

F-

Fig. 1

Form of catalyst 'crooss section)

Table 1. Catalyst geometric properties Nal 7.0 1. 0 Square 6.0 470

Cell Pitch (mm) Wall Thickness (mm) Cell Shape Hydraulic Diameter (ram) Geometric Surface Area (~/~)

Na 2 4.0 1. 0 Square 3.0 640

Na 3 3.2 0. 5 Square 2.7 910

No. 4 3.7~7.5 0.4~0.5 Corrugate I

860

Pilot Plant Exoeriments The main experimental components consisted of a catalyst reactor, a heat exchanger for maintaining a constant exhaust gas temperature, and a hot air generator, with Fig. 2 showing the pilot plant flow diagram. The gas temperature induced from the outlet of the electrostatic precipitator of the municipal solid waste incinerator was controlled by heat exchange between the generator's hot air and the induced gas, which had a maximum gas flow rate of 1900 N d/hr. The reactor had a square (560 mm) cross-section, was 3 m high, and contained four square (450 mm catalyst units, cach having a length of 400 ram.

e

Iff~

(?atalvst

/gT~ Fuel

[tot Air Generator

~-~ [---

Flue Gas (from electrostatic precipitator)

Fig. 2

.... , 2

I

'

L

.

.

.

.

.

®.

)

Fan

Pilot plant schematic flow diagram

1447

Destruction of PCDDs/PCDFs

Figure 3 shows the destruction ratio ( ~ ) of PCDDs with respect to temperature using catalyst 4 (cx)rrugate type) and ratios space velocities (SV). The SV is defined by the following equation. SV =

1,1a

Gas Flow (N rff/hr) Reactor Volume ( rr~ )

It is apparent that a lower SV resulted in a higher destruction. Figure 4 alternatively destruction of PCDDs with respect to SV using catalyst lqa 4 and various gas tempcratures.

100

,~

~'/5/"~/Jf

98

/

96

N. 4 94

~.

92

cat aly~l

@

Y,V

---- " O - - ---A---

SV=2940 sV=3520

1760

hr

i

. . . . --12---- S V = 4 7 0 0 X

SV-5880

90 250

300

350 ('i

Temperature

Fig. 3

PCDDs destruction ratio versus temperature

100

".

O

"~-, " \

98

2 -~

', \ A \ 96 \

,

\ \ \\

94

N, 4

2

(:atalysl

\\

350 ~C ~--

92

---'--(~---

300

----A----

250

O i

90

i

i

1000

t

3000 SV

Fig. 4

i

;

Space

Velocity

5000 [ilr

i

PCDDs destruction ratio versus SV

shows

the

1448

It was found that the destruction ratios were not dependent on the catalyst type, although they were dependent on the geometric surface area at a constant SV, with this relationship being shown in Fig. 5 and 6. The destruction of PCDDs is shown in Fig. 5, whereas Fig. 6 is tbr PCDFs. The curve designated as Na 2/Na 3 shows the results with two Na 2 catalysts being used in the reactor's upper hall and two Na 3 catalysts in the lower hall. It should be noted that the destruction ratio increases as the geometric surface area (Table 1) of the catalysts increases, ie., Na 1=470< No. 2/ No. 3 = 7 7 5 < No. 4=860( id / id ), with this tendency being shown for both PCDDs and PCDFs.

8V-2500

@

SV=2900

--

/ ..tji

100 --

/..~S /

SV-5000

//A

90-

i

I-

e"

.~ 2 7.

/

8V-2500

80 A 70

~,.1

Catalyst

O No2/No.3

Catalyst

•

Catalyst

Nn.4

250

300

350

Temperature Fig. 5

~'(j~

PCDDs destruction ratio versus temperature SV=2500

100

-

90

-

SV=2900

@ SV=5000 /

/

¢.

/

SV=2500

/ / ./

80 -

/

2

•/~ :

.o

70-

No.1

0

:.~.2/Nn.3

Catalyst

•

:

Catalyst

No.4

1

J

I

250

300

350

Temperature Fig. 6

Catalyst

['i~]

PCDFs destruction ratio versus temperature

1449

Surface Area Effects

Both cell geometry and total geometric surface area are very important factors in catalytic destruction. Figure 7 shows the destruction of PCDDs with respect to artificial velocity (AV), where AV is Actual Gas Flow Rate ( n ~ / h r ) Total Surface Area of Catalyst ( ~ )

AV =

As indicated in Fig. 7, the destruction ratio ( ~ ) can be expressed by the lollowing equation 7~ = K1 where K1

= -4.190 , K2

linearly decreases as AV increases. This relationship

(AV) + K z = 125.57 at 250~C

K~

= -1.928 , Kz

= 115.21 at 300~C

K1

= -1.526 , K s

= 111.24 at 350°C

100 ,

.2

90

*

80

~

~-

\

\\

" \

\

..'~

' 350'C

--0-6oi _

O

©

70

. .

'"

--A

\

- 3oo'(/

....

\ \

25o(~

\

O

I

L

I

l

I

5

10

15

20

25

AV

Fig. 7

"

Artificial

Velocity

~nL/hr~

PCDDs destruction ratio versus AV

Destruction of PCDD Homologues

The destruction of each paticular PCDD homologue by catalyst Na 1, Ne 2 and No 3 is shown in Fig. 8. Notice that at high destruction ratios the homologue ratios are nearly equal, whereas at lower ratios the T 4 CDD ratio is low, therely slightly lowering the TCDD equivalent (TEQ) destructkm ratio (the internationl TCDD equivalent factor (ITEF) was used as the equivalent factor).

1650

100

99.5

99.9

99.6

99.5

99.6

9,(}.3

99.5

@ 90

[

.'2 =

~

..

8,

Av

•

7O

'.8

60

Catalyst %,.

6 77

3o0

N,,2 / N~.3

2500

10. 19

250

N,~1

5000

22.32

300

'x,, 1

,1%00

8O

Temp.((~)

~.5

.0

5O T o t a l T4CDD PsC~)D H6CDI) ItTCI)D I)sCDD T~L~

Fig. 8

Destruction ratio of P C D D

homologues

N O x Destruction

If both P C D D and N O x reduction can be achieved by the same catalyst, a much more simple exhaust gas equipment configuration will result. Figure 9 shows the reduction of NOx and C O using two different catalysts, i.e., two ordinary NOx catalysts in the upper half and in the lower hall' two No. 4 catalysts. T h e N O x was completely reduced by the NOx catalyst, with the C O being reduced by the lqa 4 catalyst instead of the NOx one. However using these results, the ability to obtain NOx reduction by P C D D catalyst can not be conclusively determined.

150

Temp. 300°C SV(NOx catalysl) 3810hr I SV(PCDI) eatalyst)--3810hr I NI13/NOx mole ralil~ 1.1

1500

e100

q

1000 \\

, Z

50

I

500

x

<

I ',~ x x

Inlet

~liddle

Out let

~-- NOx ca,alys, +P('.Dl)~atalyst N~4~ Sampling Position of Rea(:Lor Fig. 9

NOx and C O reduction

1451

Figure 10 shows the NOx reduction by the P C D D catalyst, with the reduction being at least 50-70% at temperatures less than 3 0 0 ~ . Figure 11 shows the NOx reduction with same catalysts, although in this case the NH 3 / N O x molecular ratio was varied. It can be seen that the NOx reduction ratio increases as NH 3 / N O x molecular ratio increases. However when the NH 3 /NOx molecular ratio was significantly large, the NH 3 leakage from the reactor was also large. NH ~ , , o Temp Cc) (hSrV-,)i O A

350 300

-~--

275

3810 8810 3810 2940

V

250

2940 I

0

200

1.6

z. 150

100

50

0

"" ""2""

~

_ _ Inlet

.

k

Sampling

Fig. 10

3 _ _ _ ( ) u t let,

Middle Position

of

React. o r

NOx reduction

150

lok~ mti(

v

100

:5

-~

50

Temp 275~C SV = 8 8 1 0 /

-~ ~..~

Middle

Inlet

Sampling

Fig. 11

Position

Out let of

Reactor

NOx reduction

1452

CONCLUSIONS Pilot plant experiments were conducted at temperatures from 250-350 'C to confirm the reduction of PCDDs/PCDFs using a catalytic method. From the experiments it was determined that a sufficiently high destruction ratio can be attained even at temperatures below 300°C. It was also shown that a destruction ratio of greater than 99% could be achieved by selecting the proper artificial velocity (AV). The destruction ratio ( r~ ) and the AV are concluded to show a close correlation as dcfincd below 77 = K1

(AV) + K2

The presented destruction method can be utilized by installing a catalyst reactor inside an incinerator's exhaust gas duct, being both simple and highly effective. The destruction ratio's truc conccntration was approximately the same as the TEQ destruction ratio (TCDD equivalent). Additional NOx reduction experiments showed that both NOx and PCDDs/PCDFs reduction could bc achieved using the same PCDD catalyst.

REFERENCES

I)

M.Hiraoka, N.Takeda, S.Okajima, T.Kasakura. & Y.Imoto in Flue Gas, Chemosphere, Lg_, Nos. 1-6, pp. 361-366.

(1989),

Catalytic Destruction

of PCDDs