Effect of Temperature and CaO Addition on the Removal of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Fly Ash from a Medical Waste Incinerator

The effects of thermal treatment temperature and CaO addition on polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) content in the fly ashes from a medical waste incinerator with high chlorine content (7.5%) were investigated. Two kinds of batch mode were examined: without CaO addition under the temperature range of 50–850°C, and with CaO addition at various Ca/Cl molar ratios (CCR = 0.8–44.3) at 250°C. The results show that the PCDD/F content in raw fly ash was 6.20 ng I-TEQ/g. When elevating the temperature from 50 to 400°Cwithout CaO addition, the PCDD/F content dramatically increased from 5.38 to 575 ng I-TEQ/g. Even at 850°C, the PCDD/Fs were not destructed apparently and were still with a concentration of 401 ng I-TEQ/g. However, with the addition of CaO at CCR = 17.7 at only 250°C, the removal ratio of PCDD/Fs reached to 87.2% and the PCDD/F content was significantly reduced to 0.889 ng I-TEQ/g, which meets the soil pollution standard of Taiwan EPA (1 ng I-TEQ/g) and could be considered as backfill. CaO addition was proven efficiently to enhance the reduction rate of PCDD/Fs in fly ashes during the thermal treatment at a low temperature, as well as inhibit the formation of PCDD/Fs.

The typical chlorine level for municipal solid waste incinerators is between 0.2 and 0.8%, and that for medical waste is between 1.1 and 2.1% (Randall and Shoraka-Blair, 1994;Rigo et al., 1995).Hence, high concentrations of PCDD/Fs were formed, deposited (Wang et al., 2010;Huang et al., 2011) and found in small fly ash particles with large amounts of chlorine (Cobo et al., 2009).
There are two temperature windows in which PCDD/Fs are formed: the heterogeneous route (between 200°C and 400°C) and the homogeneous route (between 500°C and 800°C) (Stanmore, 2004).Three major mechanisms for PCDD/F emission from incinerators are: (1) high-temperature gas phase formation (300°C-600°C) (Everaert and Baeyens, 2002); (2) precursors transformation: such as chlorophenols, polychlorinated diphenyl ethers and chlorobenzenes (Lustenhouwer et al., 1980;Hutzinger and Blumich, 1985); and, (3) production via de novo synthesis in the low-temperature post-combustion zone (250°C-450°C) through residue carbon or metal catalysts in the fly ash reaction (Dickson et al., 1989;Everaert and Baeyens, 2002).For de novo synthesis, the important parameters are temperature, carbon, chlorine, oxygen and metal catalysis, especially copper, which act as a catalyst (Gullett et al., 1992;Stieglitz et al., 1989;Stanmore, 2004).De novo reaction shows that the PCDF/PCDD ratio is higher than one and the higher chlorinated congeners were dominant isomers (Everaert and Baeyens, 2002;Cunliffe and Williams, 2009).Additionally, the de novo reaction of PCDD/Fs was found in fly ash with a large change in PCDD/F concentration, but with a small change in temperature (Cunliffe and Williams, 2009).Therefore, the thermal treatment temperature is an important parameter related to the reduction rate of PCDD/Fs in fly ash treatment.
In order to reduce the PCDD/F emissions from incinerators, some additives, such as powder activated carbon, (NH 4 ) 2 SO 4 , urea, sulfur and calcium oxide (CaO) can be used.Adding (NH 4 ) 2 SO 4 to fuel (3%) can reduce PCDD/F emissions by 90% (Pandelova et al., 2005).The addition of urea resulted in the reduction of PCDD/F concentration by as much as 74% (Ruokojärvi et al., 2001).Sulfur is a very strong inhibitor of PCDD formation; it converts into SO 2 and reduces Cl 2 to HCl (Gullett et al., 1992).
Powder activated carbon injection is always used to reduce PCDD/F emission from MSWIs and can reach 92-99% removal efficiency of PCDD/Fs (Dong et al., 2001).However, activated carbon only provides PCDD/Fs with an adsorption surface, not a decomposition reaction.In contrast, calcium-based sorbents are another choice, which are widely employed not only to remove SO 2 , but also to inhibit PCDD/F formation due to their removing HCl and Cl 2 from flue gas or fly ash and reduce the production opportunities of PCDD/Fs (Gullett et al., 1994;Jozewicz and Gullett, 1995;Shemwell et al., 2001).CaO addition can reduce the PCDD/Fs formation by catalysis of fly ash from pentachlorophenol (PCP) at 300°C (Naikwadi and Karasek, 1989).Furthermore, a clear PCDD/F inhibition effect was observed when reacting dioxin precursors, PCP, hexachlorobenzene (HCB), and 2,3,4,5-tetrachlorophenol (2,3,4,5-T 4 CP) with CaO at temperatures of between 280°C and 450°C (Liu et al., 2005).Thermal treatment also effectively removes PCDD/Fs from heavily contaminated soil, with removal efficiencies greater than 99.99% at primary furnace temperatures of 750°C and 850°C (Lee et al., 2008;Lin et al., 2011).
The literature is limited on investigating the effects of temperature gradient on the reformation or removal of PCDD/Fs in fly ashes from medical incinerators with/without the addition of CaO.In this study, experiments were designed to investigate the effect of the treatment temperature and inlet Ca/Cl molar ratio (CCR) by adding CaO on the variation of PCDD/F content.The results provide useful information for dealing with PCDD/F reduction in the treatment of high chlorine content of fly ashes.

Composition Analysis
Fly ash samples were collected from a bag filter in a medical waste incinerator.An inductively coupled plasma atomic emission spectrometer (Jobin Yvon JY-38 Plus ICP-AES) was used to analyze the metal species.The processes are as follows.The fly ash was pulverized to a particle size that passed through a mesh 100 sieve to ensure a consistent digestion efficiency.0.5 g samples were digested with an acid mixture (1 mL HBF 4 + 5 mL HNO 3 + 5 mL HClO 4 ) in a hermetically sealed Teflon tube by a microwave digester.The heating program was: 20-180°C at 10 °C/min and held isothermally for 30 min.The digestion solution was diluted to 25 mL with deionized water, filtrated by a cellulose ester filter, and then analyzed.The relative composition of Si and Ca was determined using scanning electron microscopyenergy dispersive spectroscopy (SEM-EDS) (Jeol JXA-840).The chlorine contents of fly ashes were determined by an energy-dispersive X-ray fluorescence spectrometer (EDXRF) (SPECTRO, XEPOS).Thermal gravimetric analysis (TGA) (TA, SDT Q600) was performed at a heating rate of 10 °C/min in a nitrogen atmosphere from room temperature to 1200°C.

Thermal Treatment Experiments
The experiments were divided into two parts: (1) 10 g of raw fly ash was treated at various temperatures (50°C-850°C); and (2) 10 g of raw fly ash was added various amounts of CaO (1, 5.25, 21, and 52.5 g of CaO, equal to Ca/Cl molar ratio (CCR) = 0.8, 4.4, 17.7 and 44.3, respectively) and treated at 250°C.All samples were put in a graphite crucible and placed in the furnace at atmospheric environment.The experiments were performed by increasing the furnace temperature by 6 °C/min from room temperature and maintaining it at 50, 100, 150, 200, 250, 300, 400, 500, 600, 750, and 850°C, respectively, for 1 h.Then, the samples were cooled by natural cooling process in the furnace.

Analysis of PCDD/Fs
Two PCDD/F samples were analyzed and averaged for each fly ash sample in this study.Fly ash samples were analyzed using the US EPA Method 1613B (US EPA, 1994).Samples were extracted with toluene for 24 hr followed by a series of sample cleanup procedures.The extract was transferred to a vial, and further concentrated using a N 2 stream.
High-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) was used for PCDD/F analysis.The HRGC (Hewlett-Packard 6970 Series gas, CA) system was equipped with a DB-5 fused silica capillary column (L = 60 m, ID = 0.25 mm, film thickness = 0.25 µm) (J&W Scientific, CA) with splitless injection.The HRMS (Micromass Autospec Ultima, Manchester, UK) system used a positive electron impact (EI+) source.The analyzer mode of the selected ion monitoring was used with a resolving power of 10,000.The electron energy and source temperature were specified at 35 eV and 250°C, respectively.The oven temperature program was set as follows: the temperature was initially 150°C (held for 1 min), then increased by 30 °C/min to 220°C (held for 12 min), and finally increased by 1.5 °C/min to 310°C (held for 20 min).Helium was used as the carrier gas.
EPA Method 5050 followed by EPA Method 9056 was used for the analysis of chlorine content in the fly ash.The fly ash is oxidized by combustion in a bomb containing oxygen under pressure.The liberated chlorinated compounds are adsorbed in a sodium carbonate/sodium bicarbonate solution.A small volume of combustate collection solution, 1.5 mL/min, was injected into an ion chromatograph to flush and fill a constant volume sample loop.The sample is then injected into a stream of carbonate-bicarbonate eluent of the same strength as the collection solution.Chlorine content was identified based on their retention times compared to standards.Quantitation is accomplished by measuring the peak area and comparing it to the calibration curve.

Composition and PCDD/F Content in Medical Waste and Raw Fly Ash
The compositions of medical waste showed that the contents of moisture, ash, and volatile matter with fixed carbon were 9.3%, 17.3%, and 73.4%, respectively, indicating this type of ash contained high amount of unburned organicmatter.The element analysis of medical waste shows that the major metals were Ca, Na, Fe, Si, and Al, reaching 9619, 7497, 5872, 5658, and 1914 ppm, respectively (Table 1).
From the thermal gravimetric analysis (Fig. 1), the patterns indicate the weight loss of fly ashes are apparently found at the intervals of room temperature-100°C, and 400-550°C, indicating the water and volatile matter are the major compositions in the fly ashes.In addition, the metal analysis of fly ash shows that high amount of catalysts for forming PCDD/Fs via de nova synthesis process, including Fe and Cu was found, reaching 48282 and 554 ppm, respectively (Table 1).Moreover, a large amount of Na was also measured  (73152 ppm) and is relative with the high concentration of chlorine content 7.5% which is close the value (8.1%) (Zhao et al., 2010).The high quantity of Cl was presented perhaps because the usage of polyvinyl chloride disposable infusion devices and NaCl that was used frequently in medical treatment.However, high Cr content (9550 ppm) will conduct to the fly ash being belong to hazardous waste.
The PCDD/F content in raw fly ash is shown in Table 2.The mean PCDD/Fs and I-TEQ were 58.6 ng/g and 6.20 ng I-TEQ/g, respectively, which exceeds the regulatory standard of the Taiwan EPA (the limit of 1 ng I-TEQ/g) (Taiwan EPA, 2008).The PCDD/F content in the fly ash is close to that in Mininni's study (1.7-147 ng I-TEQ/g), which investigated PCDD/F formation in fly ash collected from a hospital incinerator (Mininni et al., 2007).The ratio of PCDD/PCDF was 0.251, indicating that the dominant congeners were PCDFs.

PCDD/F Content in Thermally Treated Fly Ash under Various Temperatures without CaO Addition
The PCDD/F content in thermally treated fly ash under various temperatures is shown in Table 2.The PCDD/F content in thermally treated fly ash was between 50.5-57.8ng/g under low temperatures (50-150°C), which was close to that of raw fly ash (58.6 ng/g).There was a dramatic increase in PCDD/F content in thermally treated fly ash under middle temperatures (200-400°C) when the temperature reached 300°C (Fig. 2).The total PCDD/F content and total I-TEQ content reached 6252 ng/g and 381 ng I-TEQ/g at 300°C, respectively, which are 107 and 62 times higher than those for the raw fly ash.Furthermore, the highest PCDD/F content in thermally treated fly ash was 8251 ng/g, with a corresponding I-TEQ of 621 ng I-TEQ/g at 500°C, which is 100 times larger than that for raw fly ash.The PCDD/F content in thermally treated fly ash ranged from 401 to 544 ng I-TEQ/g under high temperatures (600°C-850°C), which is over 60 times higher than that for the raw fly ash.
Fig. 3 showed that the pattern of the PCDFs/PCDDs ratio and PCDFs/PCDDs I-TEQ ratio was similar for  temperatures from 50 to 200°C.This illustrates that the variations between concentration and I-TEQ were consistent when the temperature was less than 250°C.However, the de novo reaction was the dominant mechanism when the temperature was greater than 250°C and led to more PCDF formation.This resulted in an obvious increase in the PCDFs/PCDDs ratio and PCDFs/PCDDs I-TEQ ratio, which implies that the de novo reaction was probably the dominant mechanism (Huang and Buekens, 1995).
The de novo synthesis reaction temperature ranged from 250 to 400°C.Stanmore (2004) indicated PCDD/F congeners are effectively destroyed at temperatures over 500°C.Dechlorination, desorption or destruction might be responsible for the reduction of PCDD/F content in the fly ash during heating (Cunliffe and Williams, 2007).However, the PCDD/F content reached its highest at 500°C and then decreased by only around 26% at 850°Cin this study.This was probably because the de novo synthesis led to PCDD/F being formed again during the natural cooling process in the furnace.Lee's study (Lee et al., 2008) indicated that the reduction rate of PCDD/Fs in contaminated soil reached 99.99% at the temperatures of 750°C and 850°C.This may be due to the different properties between soil and medical fly ash.Stieglitz's study reported that in municipal waste incineration particulate organic carbon is the primary source for PCDD/PCDF formation (Stieglitz et al., 1989).The medical fly ash contained much Fe and unburned carbon, which acted as a catalyst to promote the reactions of adsorbed organic compounds, resulting in the formation of high amounts of PCDD/Fs.

PCDD/F Content in Fly Ash for Various Amounts of CaO Addition at 250°C
Due to a dramatic increase in PCDD/F content in thermally treated fly ash was found when the temperature was greater than 250°C (Fig. 2), a low temperature of 250°C was set by adding CaO for removing PCDD/Fs.After adding 0, 1, 5.25, 21, and 52.5 g of CaO (CCR = 0,0.8,4.4,17.7 and 44.3,respectively) into 10 g of fly ash and heating at 250°C for 1 hr, the total PCDD/F content values in the fly ash were 86.9, 31.0,24.3, 7.53, and 4.47 ng/g, with corresponding total PCDD/F I-TEQ content values of 5.41, 3.88, 3.08, 0.889, and 0.535 ng I-TEQ/g, respectively (Table 3 and Fig. 4).The results showed that the PCDD/F content in the fly ash could be decreased to lower than the soil pollution standard of Taiwan EPA (1 ng I-TEQ/g) when enough CaO was added.
The removal efficiencies (compared with raw fly ash) of total PCDD/Fs increased from 58.5% to 92.4% with corresponding I-TEQ values of 50.3% to 91.4% when the CCR elevated from 0.8 to 44.3.However, when the CCR was set at 17.7, the PCDD/F content in the fly ash decreased to 0.889 ng I-TEQ/g, which meets the soil pollution standard of Taiwan EPA (1 ng I-TEQ/g) and could be considered as backfill.Furthermore, the removal ratio (compared with non-CaO addition at 250°C) of total PCDD/Fs ranged from 64.4% to 94.9% with corresponding I-TEQ values of 28.3% to 90.1%.Increasing CCR increased the removal ratio and CaO addition was shown to reduce the formation of PCDD/Fs.
CaO is often used as a good absorber of HCl and SO 2 .Results show that CaO inhibits PCDD/F formation for capturing HCl or Cl 2 during the thermal treatment process.It has been reported that CaO inhibits PCDD/F formation from PCP, HCB, and 2,3,4,5-T 4 CP at temperatures in the range of 280°C to 450°C (Liu et al., 2005).Chlorophenols (CPs) and chlorobenzene (CBs) play decisive roles in PCDD/F formation (Altwicker, 1996).Liu indicated CaO absorbed PCP, then decomposed it and inhibited the PCDD/F formation (Liu et al., 2005).Accordingly, we inferred that the high chlorine content (7.5%) and volatile matter (17.3%) in the fly ash from a medical incinerator apparently increased the possibility to form precursors, such as CPs or CBs.However, the CaO addition may have increased the reaction or absorption rate between CaO and chlorine, decreased the formation of precursors, and then inhibited the PCDD/F formation.
Based on Stanmore's study, PCDD/F congeners should be effectively destroyed when the temperature is over 500°C (Stanmore, 2004); however, the PCDD/F content (> 500°C) in this study was higher than 6,000 ng/g and 400 ng I-TEQ/g.It is probable that the thermally treated fly ash passed the de novo synthesis window when the system cooled down naturally and a lot of PCDD/F congeners formed again.Adding CaO apparently inhibited the PCDD/F formation, especially when the CCR was greater than 17.7, then the resultant PCDD/F content in fly ash could be less than the standard of the Taiwan EPA (1.0 ng I-TEQ/g) at low treatment temperature as 250°C

CONCLUSION
The mean total PCDD/F I-TEQ in raw fly ash from a medical incinerator was 6.20 ng I-TEQ/g.After thermal treatment (50-250°C), the total PCDD/F content in treated fly ash was 5.38-7.99ng I-TEQ/g.However, after hightemperature treatment (300-850°C), the total PCDD/F content in thermally treated fly ash elevated and reached 401-621 ng I-TEQ/g.It is probable that the thermally treated fly ash passed the de novo synthesis window when the system  8, 4.4, 17.7, and 44.3.When the CCR was at 17.7, the PCDD/F content in the fly ashes decreased to 0.889 ng I-TEQ/g, which meets the Taiwan EPA standard (1.0 ng I-TEQ/g).Accordingly, CaO addition with thermal treatment at low temperature (250°C) could be taken into consideration as an alternative to reduce the PCDD/F content in fly ashes with high chlorine levels.

CCR
Ca/Cl molar ratio APCD air pollution control devices

Fig. 1 .
Fig. 1.The pattern of thermal gravimetric analysis of raw fly ashes.

Fig. 2 .
Fig. 2. PCDD/F content in medical fly ash by thermal treatment at 50-850°C without CaO addition.

Fig. 4 .
Fig. 4. PCDD/F content in medical fly ash treated with CaO addition at various Ca/Cl ratios and at 250°C.

Table 1 .
The element analysis of medical waste and fly ashes (mg/kg)

Table 2 .
PCDD/F content in raw fly ash and treated fly ash at various temperatures without CaO addition.

Table 3 .
PCDD/F content in fly ash treated at various Ca/Cl ratios and at 250°C.