Journal of the Ceramic Society of Japan

Technical report

114 [ 5 ] 408–410 (2006)fåÜáÄáíáçå çÑ aáçñáå cçêã~íáçå áå cäìÉ d~ë Äó oÉãçî~ä çÑeóÇêçÖÉå `ÜäçêáÇÉ rëáåÖ cç~ãáåÖ t~íÉê dä~ëëIsao TSUYUMOTO, Motoji KINOMURA and Kazuhiro KUZUHARA
Department of Environmental Chemistry, College of Environmental Engineering and Architecture, Kanazawa Institute of Technology,
7–1, Ohgigaoka, Nonoichi, Ishikawa 921–8501

᫘ᗡමᕮȴɱɁȡ֐᧸ǦǮद‫ك‬ᕮả⭒‫ڸ‬ǺȗȚᠻ⣔˛ǶǽɈȬȲȵȿɻⲲ᧯༔ཱ֝⯶ቊ΁Τᨇɿቆ⦕ቧәᗁɿ⊧‫ڤ‬ʶগ⦘ᖴకᏡগકᦹऴɿೈ᷂ક⤴ᦹऴ‫ك‬કᲇ⾻921–8501 ᮸఍᭔᮸఍⤭⦕ƼలᨊགǛˎ 7–1

We developed a foaming water glass FWG , a kind of alkaline sodium silicate hydrate lNa2O〔mSiO2〔nH2O
and applied it to a HCl remover and a dioxin inhibitor in flue of waste incinerators. HCl emissions from PVC
polyvinyl chloride proved to decrease substantially by incinerating the PVC with the FWG, indicating that theFWG is a useful HCl remover. HCl concentrations in flues of industrial waste incinerators were also reducedwith larger efficiency than the conventional calcium hydroxide particles by injecting a diluted FWG solution as acooling water. The reduction of HCl generation led to the decrease in dioxin concentrations in flue gas and fly=Received January 25, 2006; Accepted April 19, 2006?
ash.

Key-words : HCl, Dioxin, Water glass, Neutralization, Flue, PVC

rise to difficulties in the ash melting process.6 The clogging offine particles in bug filters is also a problem in using calcium
compounds in flues.
Recently, we have developedéfoaming water glass FWG êas a new wet sorbent for injection in a cooling tower to remove
HCl and simultaneously to inhibit the dioxin formation.7
FWG is a kind of alkaline sodium silicate hydrate lNa2O〔mSiO2〔nH2O and has a special property to form a glass foamor glass balloons over about 80c
C.7 Here we present theproperties of FWG as a HCl remover and investigate thepotential to replace Ca OH 2.

1. Introduction
The primary sources of polychlorinated dibenzo-p-dioxins
and polychlorinated dibenzofurans PCDDPCDF emissions at present are combustion and other high-temperatureprocesses such as municipal solid waste incinerators
MSWIs , industrial waste incinerators, crematories, electricarc furnaces in steel-making. Because most of the waste
incinerators in industrialized countries have been improved tokeep stable burning over 850cC, main focus on suppression ofdioxin emissions from the incinerators has been shifted to how
to inhibit the de novo synthesis in flues. A number of researchers have reported that nitrogen and sulfur containing chemicals were suitable inhibitors of chlorinated dibenzo-p-dioxin
formation on municipal incinerator fly ash.1 –3 They reactwith active sites on the catalytic fly ash surface to form stablecomplexes with metallic compounds and thereby inactivate
metallic catalysts such as CuO. Addition of the compoundswith lone pair electrons, e. g., those containing nitrogen orsulfur, is promising for the inhibition of the catalysts because
they can easily form stable complexes with copper, iron andother transition metals.4 Removal of raw materials for the denovo synthesis is also an effective measure to inhibit dioxin
formation. HCl is one of the acidic air pollutants emitted frommunicipal or industrial waste incinerators, and it acts as a raw
material of the de novo synthesis in flue gas. Prevention ofHCl generation in the flue is essential to reduce dioxins emissions as well as to avoid air pollution. Ca OH 2 is often used
as a dry sorbent injected in a spray reactor to remove HClfrom the flue gas.5 The addition of calcium compounds suchas limestone, calcium oxides, calcium hydroxides suppresses
dioxin formation by removing HCl. However, usage of calcium compounds has disadvantage in reaction efficiency, andits powder is usually injected in large excess over ten times
its chemical reaction stoichiometry. This brings about theincrease of fly ash and the rise in pH, which is undesirable toreduce solid wastes and to suppress the elution of amphoteric
metals such as Pb, Zn, Sn. Moreover, the melting point of the
fly ash increases with a rise in calcium content, and this gives

2. Materials and methods
2.1 Foaming water glass FWG
FWG was prepared according to our developed procedure.8
In short, 150 g of NaOH and 20 g of NaF was dissolved in1000 cm3 of water, and 100 g of Si lump was added into theC. They reacted vigorously with the evolusolution at over 40ction of H2 gas, and pale yellow viscous FWG was obtained.
The active ingredient concentration and the density of thesolution were in the range of 35–45 mass㎪ and 1.3–1.5 gcm3,respectively. The solution showed high basicity at pH 12–13.
Heating the FWG over 80c
C yields a white glass foam just likestyrene foam whose density is 0.1–0.2 gcm3. This glass foam
is soluble in hot water, and the solution exhibits the same
properties as FWG again. The FWG contains no vesicant, andthe foaming gas is water vapour. The surface tensions andthe viscosities of the FWG solution 40 mass㎪ are summarized in Table 1. The special property to form glass foam isassumed to be due to its peculiar surface tension and viscosity
when the FWG solution is condensed around its boiling point.
Raman spectroscopy indicated that the silicate ion in the FWG
has a layer-structure,9 ,10 and the peculiarity of the FWG is
attributable to the layer-structure of the silicate ion.8
When the FWG solution is sprayed into a hot wind,
microballoon or microfilm of sodium silicate is formed asshown in Fig. 1. Thus the FWG contacts with the flue gas withwide area, and higher reaction efficiency is expected.
408

Isao TSUYUMOTO et al.

Journal of the Ceramic Society of Japan

114 [ 5 ] 2006

409

Table 1. Viscosity and Surface Tension of the FWG

Fig. 1. Optical microscope image of microballoon formed from theFWG in a hot wind.

Performance evaluation as HCl remover in incineration of PVC
In order to check performance as HCl remover, the HClemission was investigated when PVC polyvinyl chloride wasincinerated with the FWG in air. 0.5 g of PVC board wasdipped in the FWG and incinerated in air in a quartz tube. Theemitted gas was captured in 0.1 M NaOH aq , and the concentration of Cl⾼ was determined by the Mohr method. The
element distribution in the incineration residue was observed
by SEM scanning electron microscope and EPMA electronprobe microanalyzer .
The PVC-rubbers containing the FWG powder of 3 mass㎪,
5 mass㎪, and 10 mass㎪ were also prepared, and the amountof HCl emission was analyzed. The possibility to suppress theHCl emission from PVC incineration by FWG was investigated.
2.3 Performance evaluation as HCl remover and dioxin
inhibitor in industrial waste incinerators
Full scale tests were performed using three real industrial
waste incinerators with different incineration capacity of 12kgh, 40 kgh and 3300 kgh. The FWG was diluted in water,and it was sprayed as cooling water into the flue gas just after
a combustion chamber. The flue gas was sampled just before
the bug filter or electric precipitator. The concentrations ofHCl and dioxins in the flue gas were determined according to
the standard procedures JIS K 0107 .

Fig. 2. SEM photograph and X-ray fluorescence microprobe analysis of the incineration residue.

2.2

3. Results and discussion
Performance evaluation as HCl remover in incineration of PVC
The amount of HCl emitted from 0.5 g of PVC dipped with
0.5 g of FWG was determined at 0.116 g whereas that from
PVC only was 0.164 g. The HCl emission was reduced byabout 30㎪. Based on the assumption that 1 mol of Na in theFWG captures 1 mol of HCl by neutralization reaction, 1 g ofFWG solution is calculated to capture 3.0 mmol 108 mg ofHCl. The reaction efficiency in this case was calculated at85㎪. This high efficiency is owing to the foaming property
that the other alkaline solutions do not have.
The secondary electron image and the X-ray fluorescence
3.1Fig. 3. Relationship between HCl emission and the content of FWG
in PVC-rubber. The solid line is the theoretical line based on theassumption that 1 mol of Na in the FWG captures 1 mol of HCl gas,
and the dotted line is the fitted line of the experimental values.

microprobe analysis of the incineration residue are shown inFig. 2. NaCl generated by neutralization reaction between
HCl and the FWG was observed on SiO2 film. This demonstrates that the FWG captures HCl on SiO2 foam by the neutralization reaction.
Figure 3 shows the result of PVC-rubber containing theFWG powder. The reduction of HCl emission was decreased
linearly with increasing the content of the FWG powders.
Although the linearity indicates the reaction occurred basedon the chemical stoichiometry, the reaction efficiency wascalculated at considerably higher value, 194㎪ from the slope
in Fig. 3. A simple interpretation of this experimental result isthat 1 mol of Na in the solid FWG reduced 1.94 mol of HClemission. This result indicates that the neutralization reaction
is not the only reduction mechanism. Because it has beenfound that the FWG has fire retarding effect on polymers, theFWG hinders the decomposition of PVC to HCl gas, andsome chloride compounds may remain in the solid residue.
We can say that the FWG is appropriate as HCl removerbecause of its high efficiency. Although it is technically410

Inhibition of Dioxin Formation in Flue Gas by Removal of Hydrogen Chloride Using Foaming Water Glass

Table 2. Change in the Concentrations of HCl and Dioxins in the Flue Gas by Spraying the FWG. As for Incinerator C, the HCl and the Dioxins were Measured on a Different Day. Removal Efficiency is the Molar Ratio of the Reduced HCl to the Added Na of the FWG

difficult to capture all the HCl gas from PVC by the FWG,
the reaction between the FWG and HCl showed sufficientefficiency. We can expect that the addition in large excess is
unnecessary as for the FWG unlike the case with Ca OH 2.
3.2 Performance evaluation as HCl remover and dioxin
inhibitor in industrial waste incinerators
The capability of the FWG as HCl remover and the resulting reduction of dioxin formation are summarized in Table 2.
By spraying the diluted FWG aqueous solution into the flueas a cooling water, HCl concentrations were drasticallydecreased as for the incinerators A and B. The removal
efficiency of the incinerator B, the molar ratio of the removed
HCl to the sprayed Na in FWG, was calculated at 18.8㎪ muchhigher than that of the calcium hydroxide below 10㎪.6 Thisdifference is considered to be due to the shape of the sorbent
like microballoons. Spraying as a cooling water is also advantageous over spraying particles because HCl gas is soluble inwater and liquid-phase reactions are more efficient than gassolid reactions.
The removal efficiencies of the incinerator C for the injection rates 0.6 gNm3 and 0.07 gNm3 were 19.0㎪ and 38.4㎪,
respectively, whereas that for 1.2 gNm3 was much lower.
This drop for 1.2 gNm3 may be due to the low concentrationof HCl gas compared to the other two. The spraying was carried out from one nozzle located at the center of the section,and thus the incinerator C with large sectional area of the flue
was disadvantageous. This can be improved by the modification of the spraying technique that allows spread of the FWG
all over the sectional area. The dioxins concentration in the fly
ash collected in bug filters was also decreased from 4.4 ng–
TEQg to 0.5 ng–TEQg similarly to those in the flue gases inTable 2. The clogging in bug filters was not observed for theFWG unlike the fine particles of Ca OH 2.
Our result indicates that HCl and dioxins emission can becontrolled by spraying the FWG instead of using Ca OH 2 oractivated carbon. The FWG is advantageous over the other
HCl removers because the unreacted alkaline powders remaining in the fly ash is much less. Furthermore, no extra equipment is necessary other than a cooling tower because the FWG
is injected with cooling water. Thus the performance of existing incinerators is improved easily without any modificationof the equipment. It can be said that the FWG is a promisingneutralizer for acidic gases and a dioxin inhibitor in flues ofthe incinerators.

4. Conclusions
The foaming water glass FWG , a kind of alkaline sodium
silicate hydrate lNa2O〔mSiO2〔nH2O , was applied as a HClremover and simultaneously as a dioxin inhibitor. HCl emissions from PVC polyvinyl chloride incinerations weresubstantially decreased by dipping the PVC board in the FWG
or by preparing PVC-rubber containing the FWG powder of
3 mass㎪–10 mass㎪. The reaction efficiency was in the rangeof 85㎪–194㎪, indicating that the FWG will be a powerful
HCl remover. Full scale tests injecting a diluted FWG solutionin flue at industrial waste incinerators with different capacity
showed that the reaction efficiency of HCl depends on thesectional area of flues. This will be improved by modifying
spraying technique in accordance with the sectional area. The
removal efficiency was estimated at not less than ca. 20㎪.
Furthermore, the clogging in bug filters does not occur, andthe melting point of the fly ash in the ash–melting process
decreases owing to low calcium content and high silicon content based on the phase diagram.11 It can be said that theFWG is a promising neutralizer for acidic gases and a dioxin
inhibitor in flues instead of calcium compounds.
Acknowledgements The authors thank Mr. S. Tsuchida of N.
J. Chemicals Co., Ltd. for preparation of the FWG and Mr. K.
Ozaki of Trustlif Co., Ltd. for supplying the FWG and the PVCrubber.

1234567891011References
Dickson, L. C., Lenoir, D., Hutzinger, O., Naikwadi, K. P. andKarasek, F. W., Chemosphere, Vol. 19, pp. 1435–1445 1989 .
Addink, R., Paulus, R. and Olie, K., Env. Sci. Tech., Vol. 30,
pp. 2350–2354 1996 .
Ruokojarvi, P., Aatamila, M., Tuppurainen, K. and Ruuskanen, J., Chemosphere, Vol. 43, pp. 757–762 2001 .
Tuppurainen, K., Halonen, I., Ruokojarvi, P., Tarhanen, J.
and Ruuskanen, J., Chemosphere, Vol. 36, pp. 1493–1511
1998 .
Yan, R., Chin, T., Liang, D. T., Laursen, K., Ong, W. Y., Yao,
K. and Tay, J. H., Environ. Sci. Tech., Vol. 37, pp. 2556–2562
2003 .
Haikibutsu–Gakkai,éHaikibutsu Handbook,êOhmsha 1997
pp. 761–761.
Tsuyumoto, I. and Uchikawa, H., J. Ceram. Soc. Japan, Vol.
111, pp. 77–80 2003 .
Tsuyumoto, I., J. Am. Ceram. Soc., Vol. 88, pp. 1628–1630
2005 .
Nakamoto, K.,éInfrared and Raman Spectra of Inorganic andCoordination Compounds Part A, 5th ed.,êJohn Wiley & Sons,
New York 1997 pp. 257–261.
McMillan, P., Am. Mineral., Vol. 69, pp. 622–644 1984 .
Haikibutsu-Gakkai,éHaikibutsu Handbook,êOhmsha 1997
pp. 345–352.