Waste Management xxx (2015) xxx–xxx

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Heavy metal enrichment characteristics in ash of municipal solid waste combustion in CO2/O2 atmosphere YuTing Tang a,⇑, XiaoQian Ma a,⇑, QuanHeng Yu a, Can Zhang a, Zhiyi Lai b, Xiaoshen Zhang a a b

Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China Guangdong International Engineering Consultant Corporation, Guangzhou 510095, China

a r t i c l e

i n f o

Article history: Received 16 March 2015 Revised 5 June 2015 Accepted 16 June 2015 Available online xxxx Keywords: Heavy metal MSW incineration CO2/O2 atmosphere Enrichment Bottom ash

a b s t r a c t This paper investigated the behavior of six heavy metals (Cd, Pb, Cu, Cr, Ni and Zn) in the bottom ashes of recycled polyvinyl chloride pellets (PVC), wood sawdust (WS) and paper mixture (PM), representing the common components of municipal solid waste (MSW), obtained during combustion in CO2/O2 atmosphere in a lab-scale electrically heated tube furnace. Replacement of N2 by CO2 did not obviously change the shape of relative enrichment factor (RE) curves and subsequent order of heavy metals, but increased enrichment of these heavy metals in bottom ashes of WS, PM and PVC. The increment of O2 concentration in CO2/O2 atmosphere further increased RE values. It was only when the temperature was higher than or equal to 700 °C that the increment of the combustion temperature reduced the RE values of heavy metals. The effect of temperature on heavy metals evaporation was the most pronounced for the medium volatile metal Pb, and the least for the low volatiles Cr and Ni. The effect of temperature was more pronounced for PVC ash than for WS and PM ashes. This paper contributes to the control of heavy metals during MSW incineration and management of MSW oxy-fuel residues. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Oxy-fuel combustion, which utilizes a CO2/O2 mixture as the oxidizer instead of air, is a promising technique to tackle the CO2 recovery (Ahn et al., 2011). Compared with the conventional air combustion, oxy-fuel combustion has the following advantages: (1) easier and cheaper CO2 recovery from exhaust gas (Irfan et al., 2012); (2) a higher boiler efficiency (Liu et al., 2000); (3) less NOx emissions (Tang et al., 2012). As a result, the application of the oxy-fuel technology on combustion systems is gaining in popularity (Hong et al., 2009; Pak et al., 2010). Oxy-fuel combustion can in principle be applied to any type of fuel utilized for thermal power production (Toftegaard et al., 2010). Most references related to the oxy-fuel combustion technology focused on the coal-fired combustion; while only a minority of researchers applied this technology in other combustion processes such as municipal solid waste (MSW) incineration, and therefore more knowledge is need in this field. One of the challenges and concern in MSW incineration is the emission of heavy metals (EU report, 2002). Heavy metals are ⇑ Corresponding authors at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. E-mail addresses: [email protected] (Y. Tang), [email protected] (X. Ma).

harmful to the environment and humans with the emission concentrations above what can be found in the natural environment. Compared to biomass and coal, the heavy metal concentration in MSW is relatively high (Sorum et al., 2003). Therefore, the formation and partitioning characteristics of heavy metals in MSW incineration system should be highly emphasized and extensively studied (Sorum et al., 2003; Zhang et al., 2011); however, the previous studies (Sorum et al., 2003; Zhang et al., 2011) were obtained only in traditional air combustion. Knowledge of the heavy metal behavior during MSW oxy-fuel combustion is important for the control of pollutant emissions, which is necessary for the integration of MSW treatment and the oxy-fuel combustion technology. Since the oxy-fuel combustion technology is still in development (Hu and Yan, 2012), the heavy metal behavior during MSW oxy-fuel combustion has been seldom investigated. Chen and Huang (2009) investigated the effects of the recycled flue gas rates on the partitioning characteristics and particle size distributions of five heavy metals in a MSW grate furnace, using equilibrium calculations. However, the experimental data on heavy metals behavior during MSW oxy-fuel combustion has never been explored. The subject of heavy metal emissions during oxy-fuel combustion is seen to have drawn less attention compared with other fundamental combustion issues. Only a minority of researchers have reported the heavy metals behavior under coal CO2/O2 combustion

http://dx.doi.org/10.1016/j.wasman.2015.06.024 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

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Y. Tang et al. / Waste Management xxx (2015) xxx–xxx

(Zheng and Furimsky, 2003; Font et al., 2012; Zhuang and Pavlish, 2012). There have been some contradictory observations on the heavy metal emissions from coal CO2/O2 combustion. Some researchers experimentally showed a decrease compared with combustion in air (Krishnamoorthy and Veranth, 2003; Wen, 2007) whereas others on the basis of equilibrium calculations (Zheng and Furimsky, 2003) reported no apparent differences. Contreras et al. (2013) predicted that the Cd and Hg behavior during coal/biomass blends combustion using thermodynamic equilibrium calculations, and reported that total Cd and Hg vaporization were higher in oxy-fuel combustion than in air combustion. These studies revealed the differences of heavy metal emissions during coal or coal/biomass blends combustion under between air and CO2/O2 atmospheres. The above results are seen to be highly dependent on the fuel type, and whether the same conclusions are suitable for MSW combustion must be investigated. This study compared the behavior of lead (Pb), cadmium (Cd), zinc (Zn), chromium (Cr), nickel (Ni) and copper (Cu) during MSW combustion under both N2/O2 and CO2/O2 atmospheres experimentally. The above six metals were selected because: (1) they widely existed in many kinds of MSW components including the selected special wastes; (2) they represented metal families that behaved differently during incineration; and (3) they may hold a specific risk to the environment. The effects of waste fuel type, furnace temperature and O2 concentration in CO2/O2 atmosphere on the behavior of the heavy metals were explored. This paper aimed to judge whether or not MSW oxy-fuel combustion reduce the potential hazards of heavy metals by studying the behavior of the heavy metals during MSW combustion in CO2/O2 atmosphere. The results provided useful information for the future development of oxy-fuel combustion technology and the control of heavy metals during waste incineration. Since the bottom ash of MSW incineration can be widely reused as secondary construction materials such as landfill, road construction and cement production (Hjelmar, 1996; Yao et al., 2014), the study on the enrichment of the heavy elements in bottom ash is also imperative to select appropriate management strategies for MSW oxy-fuel residues.

2. Materials and methods

dried at 105 °C for 3–4 h and stored in desiccators. Vario EL-II chons elemental analyzer was used to determine carbon, hydrogen, nitrogen, sulfur content. The electronic balance was used for the proximate analysis. The ultimate analysis and proximate analysis are based on ASTM D5373-2008 criterion and GB212-91 criterion respectively. The chlorine content was tested based on IC (EN 14582-2007 addenda A) criterion in China National Analytical Center, Guangzhou. 2.2. Apparatus and methods 2.2.1. Combustion experiments Experimental apparatus schematic is shown in Fig. 1. In this paper, synthetic gas mixtures (CO2/O2) were used as the feed gas in oxy-fuel combustion, and combustion of fuel in different atmospheres (80N2/20O2, 80CO2/20O2, 70CO2/30O2) was performed. The flow rate of the mixed gas was 0.14 m3/h. The furnace was heated by silicon carbide from the room temperature to a desired temperature (600 °C, 700 °C, 800 °C, 900 °C or 1000 °C). The chamber temperature was monitored by a thermocouple mounted inside at the center of the tube. When the furnace was heated to the desired temperature, 0.20 ± 0.001 g samples were loaded into a sample holder and then the sample holder was inserted into the reactor. The sample was left in the furnace at the desired temperature for 1200 s (20 min), and the combustion and evolution of most components were generally complete in this holding time. At the end of the process, the ash residue was immediately moved out of the heating zone and then cooled to room temperature. 2.2.2. Heavy metal analysis in ash Lead (Pb), cadmium (Cd), zinc (Zn), chromium (Cr), nickel (Ni) and copper (Cu) concentrations in the raw PVC, WS, PM and their combustion ashes were investigated. The Cd, Cr, Cu, Ni, Pb and Zn contents were determined by flame atomic absorption spectroscopy using a TAS-990 atomic absorption spectrophotometer (AAS, Bejing Pgeneral Analytical Instrument Co., Ltd., China) after acid digestion using HNO3–HF–HClO4. The guaranteed reagent (GR) and deionized water were used. Each sample was analyzed in triplicate, with standard deviations less than 2%. In order to the clearly reflect the heavy metal enrichment in MSW ash, the relative enrichment factor (RE) was calculated using the Eq. (1) (Meij and Winkel, 2009):

2.1. Materials Along with the improvement of life quality, various sorts of living goods have entered into people’s daily life. Correspondingly, the MSW components became complicated and disordered, which resulted in the great variety of heavy metal sources. On the one hand, some components like battery were reported to contain high heavy metal content, but they usually took very low proportions in MSW. On the other hand, some components had relatively low heavy metal content but their total amount took high percentage in MSW. Materials tested in this paper included recycled polyvinyl chloride pellets (PVC), wood sawdust (WS) and paper mixture (PM). PVC, WS and PM were all common MSW components in many regions. In this paper, the PVC represented the plastic as it was an important part of the plastic fraction contained in MSW (Liu et al., 2001), and 38–66% of the chlorine content of MSW originated from PVC. The paper mixture (PM) was prepared by mixing magazine paper, newspaper and copy paper in equal proportion (33.3 wt.%). The experimental materials were pulverized by DFY-300 pulverizer (Wenling Linda Machinery Co., Ltd., China), and then passed through a sieve with a mesh size of 178 lm. The samples were

RE ¼

C ash  Afuel C fuel

ð1Þ

where C ash , C fuel were the heavy metal contents in the bottom ash and fuel, respectively. Afuel was the ash content in the fuel. In order to clearly compare the extent of temperature effect on the heavy metal enrichment in MSW ash in temperature range of 700–1000 °C, the change rate of RE value (CRRE) was calculated using Eq. (2):

CRRE ¼

RE700  RE1000  100% RE1000

ð2Þ

where RE700 , RE1000 were the RE values of heavy metal in 700 and 1000 °C.

3. Results and discussions 3.1. Characterization of raw materials The materials’ ultimate and proximate analyses are shown in Table 1.

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Y. Tang et al. / Waste Management xxx (2015) xxx–xxx

temperature control system

heat preservation materials

thermocouple cylindrical quartz tube gas mixing apparatus

silicon carbide rod reactor

flowmeter

ice sink 5%HNO3+10%H2O2

O2

4%KMnO4+10%H2SO4

N2 CO2 Fig. 1. Schematic representation of experiments apparatus of heavy metals analysis.

Table 1 Ultimate and proximate analysis of PVC, WS and PM (on air dried basis). Materials

Ultimate analysis (wt.%)

Recycled polyvinyl chloride pellets (PVC) Wood sawdust (WS) Paper mixture (PM)

C

H

O

N

S

Cl

Moisture

Volatile matter

Fixed carbon

Ash

37.47 41.66 39.64

4.33 5.75 5.48

18.30 49.29 55.65

0.05 3.25 0.17

0.03 0.06 0.06

39.83 NDa ND

0.24 5.72 2.53

69.29 80.47 78.79

6.27 11.27 9.79

24.20 2.54 8.89

ND represented lower than the detection limit.

3.2. Effects of temperature and waste composition on heavy metals enrichment

the heavy metals contents was: Cu > Cr > Zn > Pb > Ni > Cd in WS and PM ashes, and this order in PVC ash was Pb > Cr > Cu > Zn > Ni > Cd. The contents of the heavy metals (Cr, Cu, Zn, Pb and Ni) in the bottom ashes generated from PVC combustion were the highest, followed by PM ash, and WS ash had the lowest content of the heavy metals. An exception was for Cd,

140

Cd Zn Cr Cu Ni Pb

120 100

in 80CO2/20O2 atmosphere

80 60 40 20 0

600

700

800

900

1000

300 250 in 80CO2 /20O2 atmosphere

200 150 100 50 0

600

700

Cd, Zn and Ni in PVC ash (mg/kg)

800

3000

200 Cd Zn Ni

150

Cr Cu Pb

in 80CO2/20O2atmosphere

100

2500 2000 1500 1000

50 500 600

900

1000

Temperature (ć)

Temperature (ć)

0

Cd Zn Cr Cu Ni Pb

700

800

900

1000

0

Cr, Cu and Pb in PVC ash (mg/kg)

Heavy metal content in WS ash (mg/kg)

The content curves of six heavy metals in WS, PM and PVC ashes obtained in 80CO2/20O2 atmosphere at different temperatures were presented in Fig. 2. In 80CO2/20O2 atmosphere, the order of

Heavy metal content in PM ash (mg/kg)

a

Proximate analysis (wt.%)

Temperature (ć)

Fig. 2. Variations of Cd, Cr, Cu, Zn, Pb and Ni contents with temperature for the combustion in 80CO2/20O2 atmosphere: (a) in WS ash; (b) in PM ash; and (c) in PVC ash.

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of which the content was lower in the PVC ash than that in PM ash. The above orders for three waste components suggested that incinerating MSW with high PVC contents became more prone to the formation of heavy metal compounds during combustion, thus increased the release of toxic metal compounds into the environment. The plastic content in total MSW fuel must be monitored to predict the partitioning of heavy metals. Table 2 showed three soil environmental quality standards in China and the soil environmental background value of Guangdong Province. Though Cd content in WS, PM and PVC bottom ashes generated from 80CO2/20O2 combustion was the lowest among six heavy metals, Cd content was still worse than the requirements of the Grade II national standards for soil environmental quality (GB15618-2008), the agricultural control standards for municipal waste (GB8172-87) and the soil environmental quality index for non-polluted vegetables (GB/T18407.1-2001). For PVC ash, the Pb content at 600 and 700 °C and Cr content at 700–1000 °C did not meet the requirements of the above three standards. Moreover, Zn and Ni contents in PM and PVC ashes met the requirements of the above three standards, but they were much higher than the soil environmental background values of Guangdong Province (red soil, neutral) (Xu and Liu, 1996). Consequently, the random dumping or direct use as construction materials of these MSW combustion ashes without essential stabilization treatment might be harmful to the environment and vegetation. It was crucial to monitor the quality of the MSW combustion ashes in heavy metal respect. At the same time, MSW combustion ashes contained large amounts of heavy metals, and they had the potential to recovery of metals (Diao et al., 2012). Therefore, the active development of metal extraction technique from the MSW combustion ashes contributed to solving the treatment problems of MSW combustion ashes. An increase of RE value means more elements in the bottom ash, thus a lower amount in the fly ash and in the emissions to atmosphere (Oboirien et al., 2014). As shown in Fig. 3, the sequences of RE values of heavy metals were Cu > Cr > Ni > Zn > Pb > Cd in WS ash, Cu > Ni > Cr > Zn > Pb > Cd in PM ash and Cr > Ni > Cu > Zn > Pb > Cd in PVC, respectively. The boiling points of Ni, Cr, Cu, Pb, Zn and Cd are 2732, 2672, 2567, 2212, 1740 and 765 °C, respectively. The sequence of RE values was to some extent consistent with the volatilities of heavy metals, but the relation between RE value and boiling point was not one-to-one. Zhang et al. (2008) obtained from a two year monitoring program on the partitioning of heavy metals in the incineration residues from two incinerators in Shanghai, and found that the subsequent order of heavy metal retained in the bottom ash was Cr > Ni > Cu > Zn > Pb > Cd, in complete accord with the order of PVC ash in this paper. According to Alcock et al. (1984), the tested heavy metals were divided with respect to their volatility into three subgroups: low volatile (Ni, Cr, and Cu), semi-volatile (Pb and Zn) and highly volatile (Cd). The order of RE values of PVC, PM and WS

Table 2 Soil environmental quality standards in China (mg/kg).

a

Standard

Cd

Zn

Cr

Cu

Ni

Pb

GB 15618-2008 (nonirrigated farmland, neutral)a GB8172-87b GB/T 18407.1-2001c Soil environmental background values of Guangdong Province (red soil, neutral)

0.45

250

200

100

90

80

3 0.3 0.034

– – 48.75

300 200 43.25

– – 14.8

– – 13

100 150 43.25

GB 15618-2008 is Grade II national standards for soil environmental quality. GB8172-87 is agricultural control standards for municipal waste. c GB/T 18407.1-2001 is soil environmental quality index for non-polluted vegetables.

ashes obtained in 80CO2/20O2 atmosphere were basically in accordance with the above classification. The increment of furnace temperature increased vapor pressure of heavy metals (Saqib and Bäckström, 2014). A presumption was that the increasing combustion temperature reduced the partitioning of the heavy metals to the bottom ash and impelled partial heavy metal to shift to the fly ash or the flue gases (Chiang et al., 1997). The heavy metals showed that trend in the temperature range of 700–1000 °C. As shown in Figs. 2 and 3, when the temperature was increased from 600 °C to 700 °C, the content and RE value of some heavy metals in the bottom ash remained the same or even increased. It was only when the temperature was higher than or equal to 700 °C that the increment of the combustion temperature enhanced the volatilization of heavy metals. These results confirmed that other factors from volatility/boiling points were also responsible for the enrichment of the heavy metals in the bottom ash, specifically in lower temperature range. Oboirien et al. (2014) and Peng and Lin (2014) also suggested that there were different control mechanisms for heavy metals during MSW combustion. As shown in Fig. 4, CRRE values of six heavy metals differed widely. In temperature range of 700–1000 °C, the effect of temperature on heavy metals evaporation was the most pronounced for the medium volatile metal Pb, specifically for PVC ash. Peng and Lin (2014) and Oboirien et al. (2014) also suggested that the effect of temperature on the partitioning of heavy metals was highly influenced by the metals specific. Unlike the opinion argued by Jianxin et al. (2007), the effect for highly volatile Cd was less pronounced than the medium volatile Pb in this paper, due to that Cd was almost entirely evaporated into gases at the temperature range in this experiment and no longer distinctly affected by furnace temperature. Belevi and Langmeier (2000) also found that the Pb release during waste combustion was temperature dependent and therefore significantly decreasing Pb enrichment in bottom ash with the increasing temperature. Thermodynamic equilibrium calculations predicted that in a combustion environment PbO(s) was a stable Pb compound at low temperatures (Pedersen et al., 2010); while Pb(g) and PbCl2(g) were the major stable species along with small quantities of PbS(g), PbO(g) and PbCl(g) at high temperatures (Menard et al., 2006). Regardless of fuel species, the smallest effect of temperature was noted for the low volatiles Cr and Ni. From the comprehensive review of related references, the major part of Ni was expected to be present as the stable condensed elemental Ni (cr,l) (Menard et al., 2006), and Cr always tended to stay in the bottom ash due to its interaction with zinc and cobalt forming ZnCr2O4(s) and CoCr2O4(s) (Saqib and Bäckström, 2014) in the investigated temperature range. As shown in Fig. 4, regardless of heavy metal species, the effect of temperature was more pronounced for PVC ash than that for WS and PM ashes. This result confirmed that the presence of chlorine in PVC fuel increased the volatilization of heavy metals, due to the formation of highly volatile metal chlorides (Chiang et al., 1997; Tomoda et al., 2006). Influence of chlorine on the heavy metals partitioning was temperature dependent, and chlorine had higher impact on the volatilization rate of heavy metals at high temperatures than at lower temperatures. Therefore, the enrichment of the heavy metals in PVC bottom ash was obviously temperature dependent. The connection between the chlorine influence on heavy metals partitioning and combustion temperature was also reported by previously papers (Chiang et al., 1997; Zhang and Kasai, 2004). 3.3. Effect of atmosphere type on heavy metals enrichment

b

The comparison of RE curves of the heavy metals in the bottom ash remained between 80CO2/20O2 and 80N2/20O2 atmospheres

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0.10 Cd Zn Cr Cu Ni Pb

0.09 0.08 0.07 0.06

Relative enrichment factor in PM ash

Relative enrichment factor in WS ash

Y. Tang et al. / Waste Management xxx (2015) xxx–xxx

in 80CO2/20O2atmosphere

0.05 0.04 0.03 0.02 0.01 0.00

600

700

800

900

6 5

3 2 1 0

1000

Cd Zn Cu Pb

Cr Ni

5 4

in 80CO2/20O2 atmosphere

0.3

3

0.2

2

0.1

1

0.0

0 600

700

800

700

800

900

1000

900

1000

Relative enrichment factor of Cr and Ni in PVC ash

Relative enrichment factor of Cd, Zn, Cu and Pb in PVC ash

6

0.4

600

Temperature (ć)

Temperature (ć)

0.5

in 80CO2/20O2 atmosphere

4

Cd Zn Cr Cu Ni Pb

Temperature (ć)

Fig. 3. Variations of the relative enrichment factor of Cd, Cr, Cu, Zn, Pb and Ni with temperature for the combustion in 80CO2/20O2 atmosphere: (a) in WS ash; (b) in PM ash; and (c) in PVC ash.

14000

Cd Zn Cr Cu Ni Pb

Change rate of RE (%)

12000 10000 8000 6000 4000 2000 0

WS

PM

PVC

Fuel species Fig. 4. Change rate of the relative enrichment factor of Cd, Cr, Cu, Zn, Pb and Ni for WS, PM and PVC ashes in 80CO2/20O2 atmosphere.

was shown in Fig. 5. When increasing the combustion temperature under 80N2/20O2 combustion, the change of RE was analogous to 80CO2/20O2 combustion. In 80N2/20O2 atmosphere, the sequences of RE values of heavy metals were Cu > Cr > Ni > Zn > Pb > Cd in WS ash, Cu > Ni > Cr > Zn > Pb > Cd in PM ash and Cr > Ni > Cu > Zn > Pb > Cd in PVC ash, respectively. The replacement of N2 by CO2 did not obviously change the shape of RE curves and subsequent order of heavy metals. Table 3 showed the average relative enrichment factor of Cd, Cr, Cu, Zn, Pb and Ni for WS, PM and PVC ashes obtained in 80CO2/20O2 and 80N2/20O2 atmospheres. The RE values of these

heavy metals in the bottom ashes generated from WS, PM and PVC combustion under 80N2/20O2 atmosphere were lower than those under 80CO2/20O2 atmosphere, and an exception was for Zn in PM ash. This result for WS, PM and PVC was similar to that for coal presented by Krishnamoorthy and Veranth (2003) and Wen (2007). They also found a decrease in metal evaporation during coal oxy-fuel combustion, when compared with coal conventional air combustion. The coexistence, integration and competition of various mechanisms led to the bigger RE value of the heavy metals in the bottom ashes generated from 80CO2/20O2 combustion. First, at the same O2 concentration, the combustion temperature of char particles in 80CO2/20O2 atmosphere was lower than that in 80N2/20O2 atmosphere. This decrease of particle temperature had been verified by experiments (Bejarano and Levendis, 2008) and modeling calculations (Hecht et al., 2012). It may be caused by the following reasons: (1) the specific heat capacity of CO2 was much higher than that of N2, thus more heat was needed for CO2 to reach the same temperature; (2) the oxygen diffusion rate in 80CO2/20O2 atmosphere was about 0.8 times that in 80N2/20O2 atmosphere at the same furnace temperature (Wall et al., 2009), which slowed down the burning rate; and (3) the gasification and CO2 dissociation consumed heat. Second, on the basis of reduction mechanism proposed by Quann and Sarofim (1982), refractory oxides may be reduced to sub-oxides by the following Reaction (Reaction (1)):

MOn þ CO () MOn1 þ CO2

ðReactionð1ÞÞ

where M referred to refractory mineral elements, such as Si, Ca and Mg. Actually, Reaction (Reaction (1)) not only governed the main mineral elements but also could be applied for heavy metals (Wang et al., 2014). Under 80CO2/20O2 combustion, the much

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Y. Tang et al. / Waste Management xxx (2015) xxx–xxx

1.0 PM in 80 CO2 /20O2 PM in 70CO2 /30O2 PVC in 80 N2/20O2 PVC in 80 CO2/20O2 PVC in 70CO2 /30O2

WS in 80 N2 /20O2 WS in 80 CO2/20O2 WS in 70CO2 /30O2 PM in 80 N2 /20O2

0.040 0.035 0.030

Relative enrichment factor of Zn

Relative enrichment factor of Cd

0.045

0.025 0.020 0.015 0.010 0.005

0.000

WS in 80 N2/20O2 WS in 80 CO2/20O2 WS in 70CO2/30O2 PM in 80 N2/20O2

0.9 0.8 0.7 0.6 0.5

PM in 80 CO2/20O2 PM in 70CO2/30O2 PVC in 80 N2/20O2 PVC in 80 CO2/20O2 PVC in 70CO2/30O2

0.4 0.3 0.2 0.1 0.0

600

700

800

900

1000

600

700

Temperature (ć)

8

PM in 80 CO2/20O2 PM in 70CO2/30O2 PVC in 80 N2/20O2 PVC in 80 CO2/20O2 PVC in 70CO2/30O2

WS in 80 N2/20O2 WS in 80 CO2/20O2 WS in 70CO2/30O2 PM in 80 N2/20O2

7 6 5

Relative enrichment factor of Cu

Relative enrichment factor of Cr

8

4 3 2 1 0

800

WS in 80 N2 /20O2 WS in 80 CO2 /20O2 WS in 70CO2 /30O2 PM in 80 N2 /20O2

7 6 5

PM in 80 CO2/20O2 PM in 70CO2 /30O2 PVC in 80 N2 /20O2 PVC in 80 CO2/20O2 PVC in 70CO2 /30O2

3 2 1 0

600

700

800

900

600

1000

700

0.6

4.5 4.0 3.5 3.0

PM in 80 CO2/20O2 PM in 70CO2/30O2 PVC in 80 N2/20O2 PVC in 80 CO2/20O2 PVC in 70CO2/30O2

2.5 2.0 1.5 1.0 0.5

Relative enrichment factor of Pb

WS in 80 N2/20O2 WS in 80 CO2 /20O2 WS in 70CO2 /30O2 PM in 80 N2/20O2

700

800

900

WS in 80 N2/20O2 WS in 80 CO2/20O2 WS in 70CO2/30O2 PM in 80 N2/20O2

0.5 0.4

900

1000

PM in 80 CO2/20O2 PM in 70CO2 /30O2 PVC in 80 N2/20O2 PVC in 80 CO2/20O2 PVC in 70CO2/30O2

0.3 0.2 0.1 0.0

600

800

Temperature (ć)

5.0

Relative enrichment factor of Ni

1000

4

Temperature (ć)

0.0

900

Temperature (ć)

1000

600

700

800

900

1000

Temperature (ć)

Temperature (ć)

Fig. 5. Variations of the relative enrichment factor of heavy metal with temperature for WS, PM and PVC ashes in 80CO2/20O2, 70CO2/30O2 and 80N2/20O2 atmospheres: (a) Cd; (b) Zn; (c) Cr; (d) Cu; (e) Ni; and (f) Pb.

Table 3 Average relative enrichment factor of Cd, Cr, Cu, Zn, Pb and Ni for WS, PM and PVC ashes obtained in 80CO2/20O2, 70CO2/30O2 and 80N2/20O2 atmospheres. WS

Cd Zn Cr Cu Ni Pb

PM

PVC

80N2/20O2

80CO2/20O2

70CO2/30O2

80N2/20O2

80CO2/20O2

70CO2/30O2

80N2/20O2

80CO2/20O2

70CO2/30O2

0.001 0.011 0.020 0.040 0.012 0.002

0.004 0.011 0.024 0.062 0.012 0.011

0.003 0.011 0.026 0.073 0.018 0.013

0.003 0.588 0.788 1.537 0.816 0.022

0.020 0.530 0.869 2.291 0.988 0.195

0.018 0.606 0.937 2.232 1.222 0.156

0.013 0.034 1.759 0.126 0.885 0.024

0.019 0.040 2.404 0.211 0.953 0.026

0.024 0.042 3.569 0.384 2.317 0.029

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Y. Tang et al. / Waste Management xxx (2015) xxx–xxx

higher CO2 concentration may impel the Reaction (Reaction (1)) to reverse direction, and consequently reduced the formation of the sub-oxides which had a relatively lower boiling point. Compared with 80N2/20O2 combustion, more heavy metals existed in the form of higher valence and then the volatilization was suppressed under 80CO2/20O2 combustion (Suriyawong et al., 2006). Meanwhile, CO2 could dissociate into CO and O2 through the strongly endothermic Reaction (Reaction (2)) (Glarborg and Bentzen, 2008).

CO2 () CO þ 0:5O2

ðReactionð2ÞÞ

Another pathway to the increase in CO concentration in CO2/O2 atmosphere was the gasification reaction between CO2 and char (Zheng and Furimsky, 2003), see Reaction (Reaction (3)):

C þ CO2 () 2CO

ðReactionð3ÞÞ

Though among researchers there was a difference in opinion on whether it was the thermal dissociation (Mackrory and Tree, 2008), or the gasification reactions (Zheng and Furimsky, 2003), that play the dominant role in the increment of CO concentration in CO2/O2 combustion, a higher CO concentration promoted Reaction (Reaction (1)) to the forward direction and consequently the volatilization of mineral oxides was promoted. It was worthwhile to note that neither the gasification nor the thermal dissociation played a role at lower temperatures (below 900 °C). This was due to a much lower rate of gasification or dissociation compared to combustion with O2 at these conditions (Varhegyi et al., 1996). Thus, the lower particles temperature and the suppression of Reaction (Reaction (1)) played the main role, and they resulted in the higher RE values of heavy metals in ashes obtained in CO2/O2 atmosphere at the temperature range in this experiment. Apart from the reasons mentioned above, the higher CO2 concentration in atmosphere hindered the development of porosity (Chen et al., 2007, Li et al., 2008). The poorer the porosity was, the bigger the diffusive resistance was. The increment of diffusive resistance could also suppressed the volatilization both volatile metals and metal sub-oxides, and consequently increased the content of heavy metals in the bottom ash remained.

7

30%, the concentration of all the heavy metals (Cr, Cu, Zn, Ni, Pb, Cd) in the bottom ash increased at the same temperatures in the most experimental cases, in agreement with Yu et al. (2012). There were the following counteracting effects of O2 concentration on partitioning of heavy metals. On the one hand, increasing O2 concentration inhibited the vaporization of heavy metals by forming involatile compound (Yu et al., 2012). The decrease of the absolute concentration of CO in 70CO2/30O2 atmosphere made the Reaction (Reaction (1)) to forward direction weaker, and consequently increased the partitioning tendency to the bottom ash, when compared with 80CO2/20O2 combustion. On the other hand, increasing the O2 concentration in the CO2/O2 atmosphere increased the particle surface temperature during combustion, due to a faster reaction rate between fuel and O2 (Suriyawong et al., 2006). A higher particle surface temperature reduced the partitioning of the metals to the bottom ash, and the decreased fraction shifted to the fly ash or the flue gases. The reduction in CO2 concentration in CO2/O2 atmosphere made the Reaction (Reaction (1)) to reverse direction weaker, and consequently reduced the formation of the sub-oxides. In sum, whether the furtherance or hindrance of O2 concentration on heavy metal retained in the bottom ash played the main role maybe depended on the given operating conditions, such as temperature, fuels type, combustion furnace, measurement and sampling methods. These factors were perhaps the causes of variation among different researches. The effect of O2 concentration in CO2/O2 atmosphere on heavy metals enrichment was worthy of further study by expanding variation range of O2 concentration. As shown in Fig. 5 and Table 3, compared with the ash obtained in 80N2/20O2 atmosphere, the 70CO2/30O2 atmosphere also increased enrichment of the heavy metals in bottom ash, and the difference was expanded. This paper only discussed the heavy metals enrichment characteristics in the bottom ashes obtained during combustion in 80N2/20O2, 80CO2/20O2 and 70CO2/30O2. The partitioning of target heavy metals in accordance with the fly ash and flue gas in different atmospheres is worth of further study.

3.4. Effect of partial pressure of oxygen on heavy metals enrichment 4. Conclusions To obtain flame and gas phase temperature profiles almost similar to those obtained from conventional combustion in air, the necessary oxygen concentration should increase to be about 30% in CO2/O2 atmosphere. It was essential to investigate the heavy metals behavior during MSW combustion in 70CO2/30O2 atmosphere. The RE curves of the heavy metals in the bottom ash remained between combustion and oxygen-enriched combustion in CO2/O2 atmosphere were compared in Fig. 5 and Table 3. When increasing the combustion temperature, the change of heavy metals in 70CO2/30O2 atmosphere was analogous to 80CO2/20O2 combustion. The variety of RE curves with the fuel type in 70CO2/30O2 atmosphere was also nearly the same as in 80CO2/20O2 atmosphere. The increment of O2 partial pressure did not change the order of RE values of heavy metals in PVC and PM ashes, and only Pb and Zn swapped order in WS ash. There were contradictory observations on the effects of O2 concentration compared with certain previous researches. Sheng et al. (2007) observed that the increasing the O2 concentration diminished the difference between air and oxy-fuel combustion. Oboirien et al. (2014) found that when the O2 concentration was increased, the concentration of all the heavy metals (Cr, Cu, Zn, Ni, Pb, Cd) in the bottom ash increased at 900 °C; however, the concentration of Cd remained the same while the concentration of Cu, Ni, Pb and Cr increased and the concentration of Zn decreased at 1000 °C. In this paper, when the O2 concentration was increased from 20% to

The following conclusions were made: (1) The contents of heavy metals in ashes obtained in CO2/O2 atmosphere, specifically for PVC ash, did not meet the requirements of soil environmental quality standards in China. Consequently, it was still essential to choose a proper treatment method for heavy metal in MSW oxy-combustion ashes, such as metal extraction technique, though the heavy metals in bottom ash could posed relatively less hazard to the atmosphere. (2) It was only when the temperature was higher than or equal to 700 °C that the increment of the combustion temperature reduced the RE values of heavy metals. These results confirmed that other factors from volatility/boiling points were also responsible for the enrichment of these heavy metals in the bottom ash. Regardless of fuel species, the effect of temperature on heavy metals evaporation was the most pronounced for the medium volatile metal Pb, and the smallest effect of temperature was noted for the low volatiles Cr and Ni. (3) The effect of temperature was more pronounced for PVC ash than that for WS and PM ashes, due to the fact that the chlorine in PVC had a higher impact on the suppression for the heavy metals enrichment in bottom ash at high temperatures than at lower temperatures.

Please cite this article in press as: Tang, Y., et al. Heavy metal enrichment characteristics in ash of municipal solid waste combustion in CO2/O2 atmosphere. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.024

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(4) Replacement of N2 by CO2 did not obviously change the shape of RE curves and subsequent order of heavy metals, but increased enrichment of these heavy metals in bottom ash during WS, PM and PVC combustion. The increment of O2 concentration in CO2/O2 atmosphere further increased enrichment of the heavy metals in bottom ashes and more different from 80N2/20O2 atmosphere than 80CO2/20O2 atmosphere.

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Please cite this article in press as: Tang, Y., et al. Heavy metal enrichment characteristics in ash of municipal solid waste combustion in CO2/O2 atmosphere. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.024