Environ. Sci. Technol. 2001, 35, 2001-2005
Release of Hydrogen Chloride from Combustibles in Municipal Solid Waste XIAO-FEN GUO,* XUE-LIAN YANG, HAIBIN LI, CHUANG-ZHI WU, AND YONG CHEN Guangzhou Institute of Energy Conversion, Chinese Academy of Science, No. 81 Xianlie Zhong Road, Guangzhou, 510070 China FAN LI AND KE-CHANG XIE Taiyuan University of Technology
Study of the inorganic chlorides in municipal solid waste (MSW) shows that the main source of inorganic chlorides in MSW is food. The main organic source of HCl emission from MSW is plastic. But wood, textiles, and food also produce a large amount of HCl when they are combusted. Each combustible shows a different HCl releasing temperature range. At 973 K, there are 30-70% of the total chlorine left in the char of each combustibles in MSW.
TABLE 1. Proximate Analysis of Samples (Dry Basis, wt %) sample
ash
VM
FC
calorific value (kJ/g)
paper textiles wood food plastic
2.28 1.21 4.2 15.38 1.0
77.35 80.8 80.5 70.8 81.23
20.37 17.99 15.3 13.72 17.77
14.874 18.660 14.791 10.707 40.058
TABLE 2. Ultimate Analysis of Samples (Dry Basis, wt %) sample
carbon
hydrogen
oxygen
sulfur
nitrogen
paper textiles wood food plastic
40.68 44.89 42.39 32.96 80.73
6.06 6.49 5.98 4.86 13.32
46.45 44.83 41.65 36.18
0.275 0.191 0.257 0.357 0.218
0.35 1.33 0.91 1.85
Introduction Municipal solid waste (MSW) is increasing significantly in the world. As the complexity of waste increases, so does the complexity of waste management. Currently due to increasing regulatory pressures on landfills and the growing problem of the disposal of diverse solid wastes, it has led to research into the potential of utilizing waste as a supply of energy or of converting it to a usable fuel substitute. Since the heating value of MSW is also increasing, it can be thermolyzed, and its energy is recovered through power generation. The problems associated with incineration are assessed and dealt with. Among these is concern for the removal of air pollutants such as hydrogen chloride (HCl). HCl is one factor for the acid rain formation. The efficiency of the power generation is extremely lower than 20% as compared with 40% of the coal-fired plants, since a considerable amount of the chlorine is contained in MSW, which may cause high-temperature corrosion of the boiler tube. But in the past, it was thought that most of HCl in flue gas from a MSW incinerator was due to the combustion of organic chloride that has been removed from refuse. HCl still existed in flue gas. There is alarge amount of literature about HCl released from coal (1-3). Details about HCl emission from other combustibles in MSW has rarely been reported (4, 5). Many studies have focused on the release performance of HCl from combustibles in MSW in Western countries; however, only rare attention was paid in China during the past years. Because of the different economics and culture, the components of MSW in China are different from Western countries. There are a lot of water and lower heating values for combustibles in MSW in China. Too much food is contained in MSW in China, and the components in food are more varied than in Western countries. With this background, the HCl emission charac* Corresponding author e-mail:
[email protected]; fax: +86-02087608586. 10.1021/es991208r CCC: $20.00 Published on Web 04/19/2001
2001 American Chemical Society
FIGURE 1. Schematic diagram of reaction installation: 1, heating controller; 2, thermocouple; 3, water; 4, furnace; 5, sample; 6, reactor; 7, gas cylinder. tersitics of combustibles in MSW have been studied. The HCl emission characteristics at different temperature have been concluded.
Experimental Section Samples. The samples were collected at the dumping site in Guangzhou, China. The combustibles were separately collected. They were classified into six components: paper, textiles, wood, food, rubber, and plastic, which are the main combustibles in MSW. Prior to the experiment, these samples were pulverized and dried in a drier at 393 K for 8 h. The proximate analysis and ultimate analyses of each component sample were measured, and the results were shown in Tables 1 and 2. HCl Emission Experiment. The experimental apparatus used in this study is given in Figure 1. The reactor tube is made of a quartz pipe of 40 mm i.d. and 650 mm length. A weighted amount of sample is put in the ceramic boat located in the reactor. The carrier gas (air) is introduced. The heating temperature is raised to the experimental temperature at the rate of 10 K/min and kept there for 30 min. The hydrogen chloride (HCl) in the flue gas is absorbed into water and analyzed by chloride electrode (96-17B Orion). Chlorine in MSW. The potentiometer method was used to determined total chlorine in MSW. The sample was VOL. 35, NO. 10, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. DTGA curves of each combustible. extracted by a extractor using water at 473 K for 6 h. Chlorine in the liquor was analyzed by potentiometry. The inorganic chlorine in sample was measured. The alkali was added to the sample, the compost was burned at 833K, and the ash was extracted by a extractor using water at 373 K for 6 h. Chlorine in the ash was analyzed by potentiometry. The total chlorine in sample was measured. Pyrolysis-FTIR. By using the combined pyrolysis-FTIR instrumental system, the volatile species produced on CDS2000 pyrolysis reactor during of each combustibles in MSW were analyzed by a Bio-Rad FTS165FTIR spectrometer. The system is able to continuously measure the mass change of each combustible sample with the temperature increase at the heating rate of 1473 K/min as well as to identify qualitatively and determine quantitatively the individual gaseous species such as HCl emitted during combustibles pyrolysis. It is noted that, since a significantly amount of water is released during pyrolysis, the HCl gas could be dissolved in the water and condensed on the tubing transfer 2002
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wall. Thus, the gas cell is heated by heating coils up to 473 K, and the temperature can be controlled continuously by a temperature controller. The FTIR spectrometer is also able to automatically scan the pyrolysis gas mixture, and the spectra can be taken automatically every 20s. TG Experiments. To investigate the combustion process, a TG system (WCT-2, Beijing) was used. The experiments were carried out under an atmosphere of air. All samples were subjected to temperature ramps in an inert gas over a temperature range of 298-1173 K when the TG curve reaches a plateau and no further weight loss occurred. Air at a flow rate of 30 mL/min was used as inert atmosphere, and the heating rate was 10 K/min.
Results and Discussion TG Curve. Figure 2a-e is the TG curves of each component. The TG curve in Figure 2a-c shows that there are two weightlosing stages in the combustion process. For plastic, its combustion curve is different from the other three compo-
TABLE 3. Compostion of (Dry) MSW; Chlorine Analyses fraction paper textiles wood food rubber plastic
inorg Cl (g/kg)
wt % 7.71 5.99 7.15 57.43 1.50 20.21
2.7 3.2 3.5 6.3 0.45 1.3
Cla
inorg (g/kg)
2.26 1.13 3.14 61.9 2.76
total a
inorg Cl (g/kg of MSW) 0.21 0.192 0.25 3.62 0.007 0.26 4.539
TABLE 7. Characteristic of HCl Emission from Combustibles in MSW components
total chlorine (g/kg)
estd HCl concn (mg/g)
HCl conversion (%)
paper textiles wood food rubber plastic
2.7 3.2 4.0 6.7 3.55 5.4
0.12 2.10 2.16 1.97 1.08 3.77
4.44 65.6 54 29.4 30.4 69.8
From Uchida (5).
TABLE 4. Compostion of (Dry) MSW; Chlorine Analyses fraction
wt %
paper textiles wood food rubber plastic
7.71 5.99 7.15 57.43 1.50 20.21
org Cl (g/kg)
0.5 0.4 3.5 4.1
org Cla (g/kg) 1.81 10.8 0.746 0.604 33.4
total a
org Cl (g/kg of MSW)
0.036 0.23 0.05 0.83 1.146
From Uchida (5).
FIGURE 3. Relation of HCl emission and temperature for each sample.
TABLE 5. Chlorine in Each Combustible in MSW (g/kg of Dry Sample) chlorine sample
inorganic
paper textiles wood food rubber plastic
2.7 3.2 3.5 6.3 0.15 1.3
organic
total
0.5 0.4 3.5 4.1
2.7 3.2 4.0 6.7 3.55 5.4
TABLE 6. Characteristics of HCl Emission from Combustibles in MSW components
wt %
estd HCl concn (mg/g)
estd HCl concn (mg/g of MSW)
paper textiles wood food rubber plastic
7.71 5.99 7.15 57.43 1.50 20.21
0.12 2.10 2.16 1.97 1.08 3.77
0.0092 0.1258 0.1544 1.1288 0.0162 0.7619
nents. There is significant weight loss in the TG curve. The TG curve of food shows that the weight loss is decreasing with the temperature increasing. Inorganic Chlorine in Each Sample. The experimental results on inorganic chlorine in each component sample are shown in Table 3. From the table, it can be seen that a large amount of inorganic chlorine exists in food. The reason is that there is a lot of NaCl in food. As for the wood, the present data show the large content of inorganic chlorine. It is reasonable to consider that it is transferred from the inorganic chlorine in the food by water. So the main reason to consider is that it is transferred from the inorganic chlorine in the food by water. The main source of inorganic chlorine in MSW is food as compared with the data reported by Uchida. The inorganic chlorine in paper and wood nearly agree with the present value. But for food, the value of the present study is
FIGURE 4. HCl emission with time during wood pyrolysis at the heating rate of 1200 K/min. much smaller than that obtained by Uchida. It was thought that the food for the present study came from Guangzhou, China. There is less salt used in their food. So the inorganic chlorine contained in food is smaller. Organic Chlorine in Each Sample. The experimental results on organic chlorine in each component sample are shown in Table 4. Large amounts of organic chlorine exist in plastic. For paper and textiles, the organic chlorine was too small to measure. The amount of organic chlorine that existed in wood and food is also less. Compared with the data reported by Uchida, the organic chlorine in paper, wood, and food nearly agree with the present value. But for textiles and plastic, the value of the present study is much smaller than that obtained by Uchida. It is reasonable to consider that, as a main organic chlorine source, there is not much less poly(vinyl chloride) used in China than used in Japan. Less plastic is made of poly(vinyl chloride) in China. But in VOL. 35, NO. 10, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. HCl emission with time during food pyrolysis at the heating rate of 1200 K/min.
FIGURE 7. HCl emission with time during plastic pyrolysis at the heating rate of 1200 K/min.
FIGURE 6. HCl emission with time during paper pyrolysis at the heating rate of 1200 K/min. the future, there will be more poly(vinyl chloride) used in China. The organic chlorine contained in plastic will be increased. Comparing the data from Tables 3 and 4, it was found that the inorganic chlorine contained in MSW is more than organic chlorine. From Table 5, paper, textiles, wood, and food mainly contain inorganic chlorine. For rubber and plastic, organic chlorine is the main chlorine component. HCl Emission of Each Sample. The experimental results on vaporized chlorine from the combustibles in MSW are shown in Table 5. The estimated HCl concentration (mg/g of MSW) of combustibles in MSW (wt %) is equal to the product of the ratio and estimated HCl concentration (mg/ g) since the HCl emission from combustibles in MSW is measured after the combustibles were burned at 1173 K. From Table 6, it can be seen that, when the sample was heated, some increase in the hydrogen chloride production was seen. The most considerable increase in the produced amount of hydrogen chloride was observed when the plastic was combusted. But a large amount of HCl was also produced when textiles, wood, and food were combusted. In past, the HCl emission source was thought only to be food and plastic, but in our study there were also a lot of HCl produced when 2004
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FIGURE 8. HCl emission with time during food pyrolysis at the heating rate of 1200 K/min. wood and textiles were combusted. So as a HCl emission source, wood and textiles should not be neglected. The chlorine release reactions did not seem to be dependent on the heating rate of the fuel since no significant changes in the loss of chlorine by increasing the heating rate could be detected. From Table 7, it can be concluded that at 973 K 30-70% of the total chlorine was still left in the char of each combustible in MSW. Effect of Temperature on HCl Emission. The experimental results on HCl emission from the sample at different temperaturea are shown in Figure 3. When the temperature was increased, some increase in the HCl emission was seen. It has been found that the temperature range is different for the main procedure of HCl produced from each sample. For plastic and paper, the range between 673 and and 773 K is the main HCl emission stage. But for wood, textiles, and rubber, the main stage is from 773 to 873 K. A lot of HCl was produced in the range of 873-973 K when food was combusted. It is well-known that the temperature for organic chlorine converting to hydrogen chloride is lower than that
with both CH4 and paraffin. Several peaks in the HCl absorption region can be used to determine the HCl. Since the maximum peak is at 2819 cm-1 in HCl absorption region, it is not possible to determine the HCl release profile. By contrast, although the 2798 cm-1 peak overlaps slightly with both CH4 and/or paraffin absorption region, its absorbance intensity is significantly larger than those of CH4 region I and/or paraffin region. Therefore, the HCl 2798 cm-1 peak is most useful in determining the HCl release profile (Figures 4-9). It can be seen that the peak at 2798 cm-1 increases as the temperature increases. For each sample, the HCl release rate that reaches the maximum is different for each sample. For each sample, the shape of curve that the HCl releases with reaction time is different. It can be guessed that it is caused by the different mechanism of HCl emission. From the figures, it can be seen that the concentration profiles of HCl never go back to baseline. The reason is that the HCl emission from combustibles in MSW has been absorbed in the windows of FTIR. The concentration of HCl in windows of FTIR increased with time. So the concentration profiles of HCl cannot go back to the baseline. FIGURE 9. HCl emission with time during textile pyrolysis at the heating rate of 1200 K/min. for inorganic chloride. So as the main inorganic chlorine source, the major HCl produced temperature from food is lower than the main organic source of plastic. FTIR Results. One of the major thrusts of the experiment is to determine the HCl emission from the combustibles pyrolysis. From Dakang Shao (1), it was known that the HCl absorption region I (R-branch) (3116-2893 cm-1) overlaps completely with the CH4 absorption band (3218-2836 cm-1) and the paraffin absorption region. The absorbance intensity of HCl absorption band is much lower than those of both CH4 and paraffin. Thus, no peaks in the HCl from absorption region I could be used to identify the HCl from the combination of CH4 and paraffin. However, HCl absorption region II (P-branch) (2873-2565 cm-1) only overlaps partially
Acknowledgments The authors thank the Chinese Academy Sciences Fund (KY957-03-03) and the GuangDong government foundation for financial support during this investigation.
Literature Cited (1) Shao, D. K.; Hutchinson, E. J.; Cao, H. B.; Pan, W. P. Energy Fuels 1994, 8, 402. (2) Edgcombe, L. J. Fuel 1956, 38, 48. (3) Bjorkman, E.; Stromberg, B. Energy Fuels 1997, 1026, 11032. (4) Kanters, M. J.; Nispen, R. V.; Louw, R. Environ. Sci. Technol. 1996, 2121, 2126. (5) Uchida, H.; Kamo, H. Ind. Eng. Chem. Res. 1988, 27, 2190.
Received for review October 25, 1999. Revised manuscript received November 6, 2000. Accepted December 11, 2000. ES991208R
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