I n d . Eng. Chem. Res. 1988, 27, 2188-2190
2188
The Source of HCl Emission from Municipal Refuse Incinerators Shigeo Uchida* and Hiroshi Kamo Department of Chemical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432, Japan
Hiroshi Kubota Bio-cycle Laboratory, 1-26-3 Aobadai, Midori-ku, Yokohama 227, Japan
Inorganic chlorides such as sodium chloride are abundant in refuse and are an important source of HC1 produced in refuse incinerators. Study of the distribution of inorganic chlorides in classified municipal refuse samples shows that the main source of inorganic chlorides in municipal refuse is garbage and that a considerable portion of inorganic chlorides present in the refuse is due to the transfer from garbage with water. Organic chlorides stem from plastics, rubber, and leather. In the past, it was thought that most of the HC1 in flue gases from municipal refuse incinerators is due to combustion of organic chlorides, especially poly(viny1chloride) contained in refuse. However, the thermodynamical and experimental work of Uchida et al. (1983) has shown that inorganic chlorides such as sodium chloride are also an important source for HCl in refuse incinerators. In this study, the distribution of inorganic chlorides in classified municipal refuse samples is examined, using a new method for the quantitative separation of inorganic chlorides from organic chlorides. The data are then used to estimate organic and inorganic chlorides in municipal refuse and the concentration of hydrogen chloride in the flue gas when it is burned in an incinerator. Experimental Section Refuse samples were collected at the dumping site in Komoro-city, Nagano, Japan, where garbage, which was considered to be a main source of inorganic chlorides, and other combustible refuse were separately collected. One hundred and fifty-three kilograms of refuse was classified into six components: plastics, paper, cloth, wood, rubber and leather, and others. The compost made from the garbage and the leachate from the granulation process of the compost plant in the same city were used instead of using garbage directly. This is a convenient way to treat perishable garbage as a more stable form of the sample. However, since the separate collection of garbage is not complete, a considerable amount of garbage is found in the combustible refuse. The composition of combustible refuse collected at Komoro-city is shown in Table I. It is similar to that obtained at an incineration site in Tokyo (Bureau of Public Cleansing, 1981). The combustible refuse samples were pulverized to 7 mm or smaller. These and the compost sample were dried at 80 OC for 24 h, and the water content in each component sample was measured. Soluble salts in the dried samples were extracted by a Soxhlet extractor using water for 6 h. The difference between the concentration of calcium in the liquor extracted for 6 h and that for 12 h was negligible. Therefore, all samples were extracted for 6 h. Chlorine (the term “chlorine” is used hereafter for noncommital C1) in the liquor was analyzed by absorption spectroscopy using mercuric thiocyanate. Since the liquors had dark colors, their own absorptivities without the reagent were also measured and the values with the reagent were corrected. Cations which were considered to be combined with chlorine in the refuse were also analyzed. Na+, K+, and Mg+ were analyzed by atomic absorptiom*To whom all correspondence should be addressed.
etry, and Ca+ was measured by chelatometry using EDTA reagent. From the results of the analysis of chlorine ion and cations extracted from samples by water, the amount of each ion in each 1-kg dry sample is calculated and shown in Table I. The ion ratio in the table is the ratio of the total moles of cations to the moles of C1- defined by Na+ + K+ 2Mg2++ 2Ca2+ (1) ion ratio =
+
c1-
where Na+, K+, etc., are concentrations of ions. Since the values of ion ratios are considerably larger than unity, the presence of anions other than C1- ions can be conjectured. In this study, however, a further analysis was not performed. The composition of the garbage given in Table I is estimated from the fact that 0.65 kg of compost is produced from 1 kg of dry garbage, yielding 1.67 kg of leachate. It is obvious that, except in the garbage, the concentration of C1- in classified components of high water content is higher than those of lower water content. The chlorine content in components other than garbage is considered to be small. Furthermore, the original water content in all components except wood is also considered to be low. Therefore, most of water was originally present in the garbage and transferred later to other components with the dissolved ions. The original water contents in the components are estimated from the data on the equilibrium water content, Le., 30% for wood; 2% for plastics, rubber, and leather; 8% for paper; 6% for cloth; and 5% for all others. The percentage of each ion in the garbage with respect to the total amount of the ions in the refuse is also shown in Table I. About 50% or more is contributed by the garbage for each ion except for K+, whose original content in wood is relatively high. For C1-, the contribution of the garbage is especially remarkable. Conversion of Inorganic Chlorides to Vaporized Chlorine. As mentioned previously, the vaporized chlorine (or combustible chlorine) in refuse was determined by burning and analyzing the chlorine in the resulting flue gas. In this study, burning experiments were performed with each of the six classified refuse components and the compost collected at Komoro-city. For each specimen, both the full sample and the extracted sample were examined. Experimental Apparatus and Procedure. The experimental apparatus used in this study is the same as that used in our previous study (Uchida et al., 1983) except for the sizes of the reactor tube and the sample holding boat. The reactor tube is made of a quartz pipe of 4%“ inner diameter and 1007-mm length. A sample of about 3 g which is pulverized into 0.15 mm or smaller, dried at 80
O888-5885/88/ 2627-2 188$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2189 Table I. ComDosition of Combustible Refuse (Wet Base) and Water and Ion Contents in 1 kg of Dry Component Sample compost leachate" garbageb (%), wood rubber, leather others compost plastics DaDer cloth component 9.8 3.3 7.8 33.7 12.3 29.8 3.3 composition (wet base), % 2.37 20.9 28.1 80.0 2.23 7.99 6.01 32.0 water content, % 14.2 0.704 3.29 6.03 19.3 (67.4) 1.63 1.21 1.53 0.954 Na+, g/kg 0.231 7.60 10.0 4.58 14.1 (33.6) 1.08 0.763 17.5 0.756 K+,g/kg 0.512 1.28 (43.2) 0.141 0.725 0.659 0.13 0.152 0.506 0.024 Mg2+,g/kg 3.08 0.928 3.10 5.82 11.7 (53.3) 1.05 0.879 0.977 3.29 Ca2+,g/kg 1.49 8.40 23.7 34.3 2.82 2.26 1.13 61.9 (75.0) 4.54 Cl-, g/kg 2.25 2.33 1.12 1.07 4.52 2.54 4.01 3.17 ion ratio Composition is calculated by data for compost and its leachate. Percentage of ion in garbage with a Values are given per 1 kg of liquor. respect to totar amount of the same ion in refuse.
"C for 24 h, and weighed accurately is put in a quartz boat of 31-mm height and 120-mm length. To prevent the scattering of the sample in the boat by sudden ignition, zeolite particles of about 5-mm diameter are used to cover the sample, and the boat is slowly inserted to the center of the reactor. At the same time, the air is introduced at a flow rate of 15 cm3/s. As seen later, however, the rate of chlorine production from samples containing inorganic chlorides is considerably different with and without zeolite. To see the effect of the presence of the zeolite on the chlorine production, some runs are planned without zeolite. The temperature is kept at 900 "C, and the chlorine in the flue gas is absorbed into water.
Results and Discussion The experimental results on vaporized chlorine from the compost are shown in Figure 1. When water was added in the gas, some increase in the chlorine production was seen. The most considerable increase in the produced amount of chlorine was observed with the presence of water and zeolite. The amount of chlorine for the compost sample from which inorganic chlorides were almost completely removed by water extraction is very low. The production of chlorine in this case is considered due to the presence of a trace of plastics in the compost. The majority of chlorine produced from the compost sample without water extraction is, therefore, considered to be produced by the conversion of inorganic chlorides in the compost to hydrogen chloride. However, it should be acknowledged that the chlorides in compost are not necessarily the same as in garbage, as they change chemically and biologically in the composting process. Therefore, estimating chlorine vaporized when garbage is burned from the data for compost is a crude estimate. The mechanism of the conversion of inorganic chlorides into hydrogen chloride under the condition of the refuse incineration process has been proposed as follows (Uchida et al., 1983): 2NaC1+ nSiO, + A1203+ HzO 2HC1 NazO(Si02),A1203 (2) 2NaC1+ mSiOz + H 2 0
-
-
+
2HC1+ NazO(SiOz),
(3)
where n = 4 and m = 1 or 2. NaCl contained in the compost in large quantities reacts with SiOzand Alz03in the compost or SiOz in the reactor tube material according to reactions 2 and 3 and is converted to hydrogen chloride. Water vapor such as produced from the combustion of organic compounds is also necessary for the above reactions to proceed. As shown in Figure 1, the introduction of additional water vapor enhances the reactions. When zeolite particles are placed to cover the refuse components in the boat, inorganic chlorides in the sample
A
0
sample without Zeolite and H,O
60 120 180 240 300 360 420 time, m i n .
Figure 1. Effect of zeolite and water on chlorine release from compost sample burned a t 900 "C.
react thoroughly with the components of the zeolite, Si02 and A1203, and are converted to hydrogen chloride. Amounts of vaporized chlorine produced in the combustion of refuse samples in the presence of zeolite and water were then measured. The difference of the amount of vaporized chlorine produced from the sample from which inorganic chlorides are extracted by water (extracted sample) and that produced from the sample not extracted by water (non-water-extracted sample) should be the vaporized chlorine converted from inorganic chlorides. The difference was found to be nearly equal to the amount of the inorganic chlorides extracted by water from the sample. The analysis of the extract of the ash residue revealed that about 10% extractable chlorine originally present in the cloth or the compost sample and about 2% of that in other refuse samples remained in their ash. This supports the fact that most of inorganic chlorides in refuse samples convert to hydrogen chloride under the present combustion conditions. Only negligible amounts of chlorine were found in the ash of samples from which inorganic chlorides had been removed by water extraction.
Estimation of Amount of Chlorine in Refuse Component From the above, the amounts of organic and inorganic chlorides contained in classified refuse components were obtained and tabulated in Table 11. For comparison, the data reported by Hiraoka (1978) and Kondo (1978)are also presented in Table 11. The total chlorine in each compo-
2190 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table 11. Inorganic, Organic, and Total Chlorine in ComDonent (Gram/Kilogram of Dry Sample) chlorine inorganic organic total organica 45.6 33.4 36.2 plastics 2.76 4.07 1.9 1.81 paper 2.26 11.9 0.19 1.13 10.8 cloth 0.08 0.746 3.89 wood 3.14 68.9 70.3 90.8 rubber, leather 1.40 5.33 8.72 others 3.39 0.16 0.604 62.5 garbage 61.9
Each
totalb 41.43 1.45 2.89 9.08 6.35 7.97
From Hiraoka (1978). bFrom Kondo (1978).
nent reported by Kondo nearly agrees with the present value except for rubber and leather, garbage, and cloth samples. The value for chlorine in rubber and leather found by Kondo is much smaller than that obtained in the present study and by Hiraoka. His data were obtained in 1974 and are older than the other data. Therefore, less synthetic leather was probably present in Kondo's sample. Currently, most synthetic leather is made of poly(viny1 chloride). As for the cloth, the present data show the very large content of organic chlorides. It is reasonable to consider that the large content of chlorine in textiles was accidentally mixed in the sample. Total chlorine in the garbage reported by Kondo is considerably lower than the present value. He analyzed the chlorine of the garbage classified from the totally collected refuse sample. Therefore, a considerable part of inorganic chlorides in the garbage had been transferred to other components with water, and therefore, the chlorine content in the garbage would be lowered. The content of chlorine in each refuse component estimated with the assumption of no separation of garbage shows that the weight ratio of organic chlorides to inorganic chlorides in the refuse is 43/57, and the total amount of chlorine in 1000 kg of refuse (wet base) is about 13 kg. This corresponds well to the value of 14 kg, obtained by Hiraoka (1978) at an incineration site. The estimated concentration of hydrogen chloride in the flue gas of the refuse of the composition estimated with the assumption of no separation of garbage is given in Table 111. In the calculation, 6000 nm3 of air is assumed to be required to burn 1000 kg of combustible refuse (wet base). The percentage of the conversion of inorganic chlorides t o hydrogen chloride in the incinerator is assumed to be 30%, while the percentage of the conversion estimated from the data obtained by Hiraoka (1978) is 40%. The percentage of the conversion becomes higher as the temperature in the incinerator increases. It also varies with other operating conditions and the design of the incinerator. If plastics, rubber, and leather were removed from the refuse collected in Komoro-city, the concentration of hydrogen chloride in the flue gas would be that shown in the same table. In this calculation, according to the actual result achieved in Tokyo (Bureau of Public Cleansing, 1980), it is assumed that 76% plastics, rubber, and leather was separately collected from the overall refuse. Even then, the concentration of hydrogen chloride in the flue gas exceeds the 430 ppm limit set in the emission regulation in Japan.
Table 111. Estimated €IC1 Concentration in Flue Gas Obtained by Burning of 1000 kg of Combustible Refuse with 6000 nm3 of Air flue gas composition C1, conversn, contribusource kelton % HC1, ppm tion, % organic 5 914 100 622 73.5 total refuse' 2 189 100 202 44.6 refuse w/o P, R, Lb inorganic total refuse" 7 094 30 224 26.5 7 941 30 251 55.4 refuse w/o P, R, Lb total 100.0 total refuse" 13008 846 refuse w/o P, R, 10 130 453 100.0 Lb Refuse from a Refuse containing plastics, rubber, and leather. which 76% plastics, rubber, and leather was removed.
Conclusions The following conclusions are drawn from this study. 1. The main source of inorganic chlorides in municipal refuse is garbage. A considerable amount of inorganic chlorides present in the refuse other than garbage is due to the transfer of chlorides from garbage with water. 2. The amount of inorganic chlorides contained in refuse can be determined by the analysis of the water extract from sample refuse. 3. The amount of organic chlorides contained in refuse can be determined by the analysis of the flue gas produced when the residue of the water-extracted refuse sample is burned. 4. Organic chlorides stem from plastics, rubber, and leather. 5. The weight ratio of organic chlorides to inorganic chlorides in the municipal refuse including garbage was estimated to be 43/57, and 1000 kg of refuse (wet base) contains about 13 kg of chlorides. Acknowledgment The authors are grateful to T. Tsubone, A. Katayama, K. Tsuchiya, T. Suzuki, and S. Inukai for their experimental contribution and for the financial assistance from The Association for Promotion of Plastic Waste Treatment and Utilization in Japan. Registry No. HC1,7647-01-0; Na, 7440-23-5; K, 7440-09-7; Mg, 7439-95-4; Ca, 7440-70-2.
Literature Cited Bureau of Public Cleansing, "Outline of Activities". Pamphlet, Tokyo Metropolitan Government, 1980. Bureau of Public Cleansing, "About Cleansing '81". Pamphlet, Tokyo Metropolitan Government, 1981. Hiraoka, M. Paper presented at the 12th Fall Meeting of the Society of Chemical Engineers, Okayama, Japan, 1978; p 80. Kondo, K. Kotai Haikibutsu 1978,26, 30. Uchida, S.; Kamo, H.; Kubota, H.; Kanaya, K. Ind. Eng. Chem. Process Des. Dev. 1983,22, 144. Received for review August 12, 1987 Revised manuscript received April 8, 1988 Accepted August 31, 1988
