Effects of chlorides on emissions of hydrogen chloride formation in waste incineration

~

Chemosphere,

Vol. 38, No. 7, pp. 1571-1582, 1999 © 1999 Elsevier Science Ltd. All rights reserved 0045-6535/99/$ - see front matter

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P I h S0045-653508)00377-4

Effects o f Chlorides on E m i s s i o n s o f H y d r o g e n C h l o r i d e F o r m a t i o n in W a s t e I n c i n e r a t i o n

Kuen-Sheng Wang ~ Kung-Yuh Chiang2' Shin-Ming Lin ~ Chi-Chang Tsai t Chang-Jung Sun ~

1. Graduate Institute of Environmental Engineering, National Central University, Chung-Li, Taiwan, R.O.C. 2. Department of Environmental Engineering and Science, Feng-Chia University, 100 Wen-Hwa Road, TaiChung, Taiwan, R.O.C. (Received in Germany 16 March 1998; accepted 9 Jnrle 1998)

ABSTRACT This study investigates the effects of chlorides on the potential for hydrogen chloride (HCI) formation in a single heavy metal incineration system, using simulated municipal solid waste (MSW) with spiked organic and inorganic chlorides.

The experiments were conducted at 900°C with the chlorine content varying

from 0% to 1.6% (w/w as C1). The spiked chlorides include ionic and molecular bound compounds, such as PVC, C2C14, NaCI, KCI, and FeC13. The results indicate that an increase in the chlorine content enhances the potential for HCI formation in the flue gas and this potential is mainly affected by the characteristics of the chlorides, the presence of alkaline metals (Na and K) and the hydrogen content in the wastes.

Furthermore, the HCI formation potential due to the presence of various organic and inorganic

chlorides is found to be influenced in decreasing order as: PVC, C2C14, FeC13, NaC1, and KC1.

The

consistence is confirmed between the above experimental results and model predictions based on thermodynamic equilibrium analysis.

Keywords: Chloride, hydrogen chloride, heavy metal, incineration

INTRODUCTION

The increasingly stringent municipal solid waste (MSW) regulations in Taiwan, the scarcity of landfill sites 1571

1572 and constantly growing disposal costs have hastened eftbrts to adopt MSW combustion technologies and energy recovery strategies.

However, the public is concerned primarily about the potential emissions of

toxic compounds during such MSW incineration processes.

Hydrogen chloride emitted from incinerators

results primarily from the decomposition of various chlorides and from reactions with hydrogen resources in MSW, which is especially critical in Taiwan as the local MSW is characterized by a high plastic content (average of 17%) and food residue content (average of 26%) [1], which respectively contribute most of the organic and inorganic chlorides for HCI tbrmation and heavy metal partitioning. In the thermal degradation of polyvinyl chloride (PVC), the significant changes are the liberation of HCI and the development of intense discoloration resulting from the formation of conjugated polyene structures [2,3].

The cross-linking reaction has been investigated in laboratory tests to determine the products of

thermal decomposition and combustion.

Furthermore, previous studies also showed that the products were

formed independently of degradation, combustion temperature, heating rate, and thermal atmosphere.

The

major product of PVC combustion is HC1, except for other organochlorines (i.e. dichlorobenzene and trichlorobenzene).

On the other hand, several experimental results have indicated that HCI is emitted not

only from PVC but also t?om inorganic chlorides during the MSW incineration process.

Inorganic

chlorides such as NaC1 and KCI, abundantly present in fbod residues, have been considered as the mechanism determining the formation and the amount of potential HCI emissions. This fact suggests that incinerating wastes with high levels of plastics and food residues might consequently cause HCI formation, thus increasing the release of toxic compounds in the flue gas in incineration process. Therefore, to improve our understanding the potential for HC1 formation during MSW incineration, and to achieve a proper management and control of the toxic compounds released, this research focuses on the effects of the various organic and inorganic chlorides on the potential for HC1 formation during waste incineration.

MATERIALS AND METHODS

Materials In this work, the wastes were prepared according to the mean physical compositions of the island's MSW,

1573 shown in Table 1. The moisture contem in the wastes was adjusted to 50% with distilled water, to simulate the high moisture content of the local MSW.

Since the background heavy metal concentrations contributed

by components in the waste sample were extremely low, simulated heavy metal concentrations were prepared by adding standard solutions and compounds of tested heavy metals.

Various organic chlorides

(PVC and C2C14)and inorganic chlorides (NaC1, KC1, and FeCI3), representing different bonding types, were used to simulate different chloride sources in the waste.

Table 1. Material in Weight simulatedMSW % sample

Ultimate Analysis,% by weight C

Paper Textiles Wood Food Waste

Composition of the simulated MSW sample

H

O

N

S

CI 0.12

35

37.3

5.4

45.0

ND

004

7

41.9

6.3

51.2

ND

0.05

5

473

6.4

49.0

ND

0.05

33

51.4

7.9

33.4

2.3

012

Proximate Analysis, % by weil]ht Moisture Ash

HeavyMetalConcentrations (mg/kg) Cd

Pb

Cr

Cu

Zn

73

12.3

0.35

1.5

0.5

15

7.8

7.5

0.6

0.3

4.7

0.4

6.6

11.4

0.09

9.8

0.2

ND

5.4

0.8

1.2

12.8

0.53

3.0

4.6

0.2

1.0

04

03

19.1

0.09

(milkpowder) Plastics

20

PVC pellet

45.89

5.38

22.2

0.08

ND

26

0.04

0.44

0.7

ND

ND

0.2

ND

PE

pellet

85.01

14.3

ND

ND

ND

ND

0.04

ND

15

ND

1.03

1.26

101

PE

Bag

85.5

13.9

0.1

ND

0.02

0.05

0.06

0.01

0.5

2.5

01

09

3.5

Experimental apparatus The experimental apparatus, as shown in Figure 1, consists of a tube furnace and a flue gas sampling train. The combustion chamber of the fumace is made of a quartz tube, 5.5 cm in diameter and 85 cm long, which is electric-heated, steel-shell housed, and glass-fiber lined. tube furnace by a quartz rod. the center of the tube.

The feeder is a quartz boat moved within the

The chamber temperature is monitored by a thermocouple mounted inside at

Downstream from the chamber is a quartz-fiber filter.

The tube between the

combustion chamber and the filter is heated to higher than 120°C, to prevent flue gas condensation during transfer. The flue gas sampling train consists of eight impingers connected in series.

Adopting the R.O.C.EPA

method 412.70A and the modified U.S.EPA method 5 (MM5 method), the first impinger is empty, the second and the third are filled with 0.1 N NaOH solution, while the fourth and the seventh are filled with a

1574

>

Z~ ~

-

I ~ ~ ,,: ~; ~-.:~ :~;?_

J

i~

, ~ ~;: ~

If

~ 2:._:+~.. : ~ ~ I " ~;~':~ - I ........, ~:~ ~

~

:-:

L

g

],' ~ = ~ - -

~

L

.~

E

~

::::

~

Z~

~2

~

,

- - ~ ~

~i

g~

......

II

i

~

I."

!

~

I

i~

o

1575 combined solution of 5% HNO3 and 10% H202, and the last with silica gel (300 grams).

Experimental procedure The combustion temperature in this experiment was controlled at 900°C.

The chloride content in the waste

sample varied from 0%, 0.4%, 0.8% to 1.6% (w/w) for organic and inorganic chloride.

The waste sample

was loaded in the quartz boat and pushed forward into the central part of the combustion chamber when the given temperature was reached.

The combustion air was supplied by an air compressor at a flow rate of

about 2.5 L/min at room temperature under atmospheric pressure, which corresponded to 50% excess air. The combustion time was pre-determined for each batch experiment as the time when the loss of incinerator residues on ignition became equal to or less than 3% (w/w),

Analysis Methods The residues in the quartz boat and the particulates captured by the filter were collected as bottom ash and fly ash samples after each batch combustion, while, flue gas samples were taken from the impingers.

All

the collected ash and aqueous samples were analyzed by ion chromatography (IC) and flame atomic absorption spectroscopy (FAAS) for chlorine ions (CI-) and the target metals, to determine their HC1 concentrations and metal partitioning characteristics.

The analytical methods used in this study are

summarized as follows:

HCI for flue gas sampling R.O.C.EPA NIEA A 412.70A Heavy metal sampling

U.S.EPA MM 5 Method

Recovery test

U.S.EPA SW 846

Moisture content

R.O.C.EPA Method 4215

Proximate analysis

R.O.C.EPA Method 4217

Loss on ignition

R.O.C.EPA Method 4216

Heavy metal content

R.O.C.EPA Method 301.1,330.1

HC1 digestion method

U.S.EPA Method 3051

1576

RESULTS AND DISCUSSION

Mass balance closure for chlorine content

As shown in Table 2 and Table 3, the mass balance for the chlorine contents during combustion is defined as the ratio between the amount o f chlorine recovered from the combustion discharges and that from the waste input.

The mass balance closure showed that a satisfactory recovery was obtained.

In cases with spiked

organic and inorganic chlorides, the recovery rates for chlorine were relatively high, ranging from 75.3% to 115.8% and from 73.4% to 104.7%, respectively.

Several possible explanations for incomplete closure

would include: (1) the chlorides may condense and be deposited on the tube incinerator walls or stay within the laboratory incinerator facility; (2) the chlorine concentrations are very close to the instrumental detection limits and thus missed and, (3) the preparation of representative simulated M S W samples is difficult [4].

Table 2.

Mass balance closure of organic chloride addition Organic chloride PVC

Metal

0%

0.4%

0.8%

C2CI4 1.6%

0%

0.4%

0.8%

1.6%

Cd

78,5

99.8

111.9

I 12.0

78.5

89.3

83.4

75.9

Zn

118.5

92.5

125.9

109.9

118.5

72.4

64.5

71.9

Cr

104.8

112.1

111.2

72.8

104.8

86.5

82.6

86.2

Cu

70.6

111.8

114.2

114.1

70.6

64.4

70.8

72.3

Average 93.1

104.1

115.8

102.2

93.1

78.2

75.3

76.6

Table 3.

Mass balance closure o f inorganic chloride addition Inorganic chloride

NaCI Metal

KC1

0.4% 0.8% 1.6%

Cd

78.5

80.5 81.0 63.3

Zn

118.5 92.0 93.3 107.7

118.5 114.6 67.8 108.5

Cr

104.8 93.2 102.8 7 5 . 8

1 0 4 . 8 90.1 7 0 . 2 9 1 . 1

1 0 4 . 8 116.2 106.2 62.5

Cu

70.6

72.8 74.1 85.1

average 93.1 84.6 87.8

83.0

0%

0 . 4 % 0.8% 1.6%

78.5

114.7 114.5 88.2

FeCI3

0%

0%

0.4%

0.8% 1.6%

78.5

103.8 96.7 72.0

118.5

85.1

71

86.7

70.6

99.3 102.7 115.7

70.6

91.2

82.1 72.2

93.1

104.7 88.8 100.9

93.1

99.1

89.0 73.4

1577 Effects of the organic chloride on HCl formation potential

In the waste incineration process, HC1 formation is a complex process, involving the availability of chlorine, the sources of hydrogen, and the competitive affinity of the tested metals. In general, the reactive affinity between hydrogen and chlorine is stronger than between the heavy metals and chlorine [5].

Therefore, the

presence of a large amount of hydrogen enhances the potential for HC1 formation, that is giving high level concentrations in the flue gases.

In this study, the HC1 formation potential (HFP) is defined as follows [6]:

HFP ( HCI formation potential, mg HCl/mg CI) = mass of HCI absorbed in the impingers / input chlorine in the waste sample

As shown in Figure 2 and Table 4, the formation ratio of HCI in a single-metal incineration system showed an increase when the PVC content increased.

Meanwhile, in the case of spiked PVC, the potential for HC1

formation was approximately 90%, which was more significant than other spiked chiorides.

However, in

the case when there was spiked single chromium (Cr), when 1.6% (w/w) chlorides (PVC) were added, the HC1 formation ratio decreased.

One possibility which would explain this result is that highly volatile

chromium chloride (i.e. CRO2C12)might be formed during waste incineration, which largely inhibits HC1 formation.

On the other hand, the recovery rate was just about 72.8% (Table 2), and an analytical error

might have occurred this test batch. C2C14content increased.

Figure 3 also shows that the formation ratio for HCI increased as the

Since C2C14is easily decomposed (b.p. 121°C), it was then possible for hydrogen

and C2C14to combine into HC1 [7].

The HCI formation ratio for spiked C2C14,ranged from 50% to 70%,

which was lower than case for spiked PVC.

Table 4.

HC1 formation ratio (HFP) under organic chloride addition Organicchloride

Pvc Metal 0% 0.4% 0.8% 1.6% Cd 13.2 73.3 93.2 Zn 26.9 .75.6 96.2 Cr 7.9 92.2 84.7 Cu 16.9 91.4...90.1

95.5 94.1 56.2 98.8

c~c~ 0% 0.4% 0.8% 1.6% 13.2 56.9 67,5 26.9 52.9 54.5 7.9 65.7 66,8 1.6.9 53.5 58.2

68.4 51.8 74.8 56.5

1578 10(

7~

61] 50 41]

u

30 20

Cd

Zn

Metal

Cr

Cu

• 0% • 0.40% O 0.80% Im 1.60% Figure 2, PVC effect on the HCI formation

100(/~- . . . . . . .

i 90~80~

~

70

•~

60

~ -~

50

j

40

~

30 20 10 Cd

Zn

Cr

Cu

Metal • 0% •0.40% 1-10.$0% • 1,60% Figure 3. C2CI4 effect on the HCI formation

Effects of inorganic chloride on the HCI formation potential

Similarly, with the addition of NaCI or KC1 in a single-metal incineration system, the availability of the chlorine ions in NaC1 or KC1 and the amount of hydrogen during combustion determine the potential for HC1 formation.

Being ionic compounds, NaCI and KC1 have a lattice energy (786 kJ/mol and 715 kJ/mol,

respectively) higher than other tested chlorides with molecular bonding (e.g., PVC and

C2C14).

Consequently, the chlorine ions provided by the organic chlorides (PVC and C2C14) are more readily

1579 available than those from the inorganic chlorides (NaCi and KC1). The batch results, shown in Figures 4 and 5, indicate that the formation ratio of HCI decreased as the NaC1 or KC1 content increased, especially for the case with 1.6% spiked chlorides (w/w).

The reasons for this

were the high lattice energy of the ionic compounds and the slow release rate of the chlorine ions, which might increase the formation potential of heavy metal chlorides and largely decrease HC1 formation. According to the heavy metal partitioning characteristics found during the previous study, increasing the chlorine content by adding NaCI and KCI increases heavy metal partitioning in the gas phase [8].

That is,

in the case of spiked NaCI and KC1, the potential of the HCI formation may be reduced. In the case of spiked FeCI3, since the energy of the molecular bond is lower than for the ionic bond, the release capacity of chlorine ions and the HC1 formation ratio are increased.

Another possibility is that the

hygroscopic character of FeCI3 might form a hydrate providing the hydrogen content and increasing the potential for HCI formation [9].

As shown in Figure 6 and Table 5, the HC1 formation ratio was higher

than for the other spiked inorganic chlorides, ranging from 40% to 60%.

In summary, the potential for

HC1 formation and the characteristics of heavy metal partitioning are mainly affected by the characteristics of the chlorides, the presence of alkaline metals (Na and K) and the hydrogen content in the wastes.

In this

study, the presence of various organic and inorganic chlorides was determined, in decreasing order as: PVC, C2C 14, FeCI~, NaC1, and KC1, to increase the HC1 formation potential.

Table 5.

HCI formation ratio (HFP) under inorganic chloride addition Inorganicchloride NaCI

KCI

FeCIj

Metal 0% 0.4% 0.8% 1.6%

0%

0.4% 0.8% 1.6%

0%

Cd

13.2 31.9

13.3

18.7

13.2 25.5 33.5 11.6

13.2

56.3 44.1 61.0

Zn Cr

26.9 35.7 32.8 7.9 21.5 19.5

8.2 18.0

26.9 35.0 22.1 13.2 26.9 7.9 38.2 16.8 18.6 7.9

34.9 33.6 55.0 70.5 49.1 31.1

Cu

16.9 19.7 28.1

12.0

16.9 32.7 16.1 19.4

50.9 62.3 46.0

16.9

0 . 4 % 0.8% 1.6%

Comparison w i t h data from a t h e r m o d y n a m i c equilibrium model

A thermodynamic equilibrium analysis of the incineration system was used the Gibbs free energy.

The

method was based on each species having a chemical potential influenced by the temperature, pressure, and

1580

100~,~

.

.

.

.

.

90: S0~ 70~ 60~ 50. 40 ~

u

30 20 10 0 Cd

Zn

Cr

Cu

Metal B 0 % B 0 . 4 0 % O0.S0% B 1.60%

Figure 4. NaC1 effect on the HCI formation

100~f~

.

.

.

.

.

.

.

.

.

.

.

.

90~ ,

s0ii , 70~ ! 60/

~o~ ._i

Cd

Za

Cr

Cu

Metal BO% B0.40% r~O.gO%B 1.60% F i g u r e 5. K C I effect on the H C I formation

chemical composition characteristics of the incineration system. Accordingly, the speciation of heavy metals or other compounds during incineration can be calculated and predicted.

Based on the parameters

of the equilibrium model, including the combustion temperature, the amount of excess air, the waste compositions, the spiked chloride content, the single-metal content, and the pressure, the prediction results indicated that the dominant species were H:O(~), H2(s), HCl(g), KCI(~), and NaCl~).

Furthermore, the

1581 potential for the HCI formation also showed the same tendency in the experimental results with spiked organic and inorganic chlorides (as shown in Figure 7).

ioo~ 90~ 80~

~oI 60 50 40 30 ~0 10 Cd

Zn

Cr Metal • 0% • 0.40% [3 0.80% ! 1.60%

Cu

Figure 6. FeCl3 effect on the HCI formation

100

10

J

! -~

f

z

z

~

L 0.1 I 0.0%

0.5%

1.0%

1.5%

2.0%

Chlorine content(%) i - ~ - c2c14 +

FeCI3 ~

NaCI +

KCI 1

Figure 7. HCI formation prediction by equilibrium model

CONCLUSIONS

Laboratory studies and the thermodynamic equilibrium prediction analysis were employed to investigate the potential for HCI formation during a simulated MSW incineration process. The facts suggest that potential

1582 for HCI formation show a close relationship between the experimental results and the model prediction.

In

addition, the characteristics of the spiked chlorides, the presence of alkaline metals, and the hydrogen content in the wastes all have strong effects on the HCI formation.

In this study, the potential for HCI

formation in the presence of the spiked chlorides is found to be in decreasing order: PVC, C2C1~, FeC13, NaCI, and KC 1.

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K. Kishore and R. Nagarajan, Polymer Combustion: A Review, J. Polym. Eng. 7, 319-347 (1987).

3. J.B. Adeniyi and G. Scott, The Effects of Structural Defects on the Stability of Poly Vinyl Chloride: A Critical Review, Poly. Deg. Stab. 17, 117-129 (1987). 4.

T.C. Ho, H. T. Lee, T. H. Kuo, D. Chen and W. D. Bosticle, Analysis of Incinerator Performance and Metal Emissions from Trial and Test Bums, Hazard Waste Hazard 11, 53-70 (1994).

5. C.Y. Wu, P. Biswas and N. J. Fendinger, Model to Assess Heavy Metal Emission from Municipal Solid Waste Incineration, Hazard Waste Hazard 11, 71-92 (1994). 6.

M.Y. Wey and T. J. Fang, The Effect of Organic and Inorganic Chlorides on the Formation of HC1 with Various Hydrogen Containing Sources in a Fluidized Bed Incinerator, Environ. Internat. 21,423-431 (1995).

7.

D. A. Tirey, P.H. Taylor and B. Dellinger, Products of Incomplete Combustion from the High Temperature Pyrolysis of Chlorinated Methanes ( Edited by R.E. Clement and R.O. Kagel), Emissions from Combustion Processes: Origin, Measurement, Control, pp.109-126 (1990).

8.

K.S. Wang, K. Y. Chiang, S. M. Lin and C. C.Tasi, Chlorides Affecting the Speciation and Partitioning of a Single Heavy Metal in a Waste Incineration Process, Journal of the Chinese Institute of Environmental Engineering 8, (1998) (in press).

9.

J.D. Johnson, R. G. Fuerst, T. J. Logan, M. R. Midgett, M. R. Peterson, J. Albritton and R. K. M. Jayanty, Development of a Laboratory Method for Estimation of Hydrogen Chloride Emission Potential of Incinerator Feed Materials, Hazard Waste Hazard 12, 61-69 (1995).