O2 atmospheres

Fuel 165 (2016) 272–278

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Effects of sorbents on the heavy metals control during tire rubber and polyethylene combustion in CO2/O2 and N2/O2 atmospheres YuTing Tang a,⇑, XiaoQian Ma a,⇑, Can Zhang a, QuanHeng Yu a, Yunxiang Fan b a b

Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China College of Engineering, Boston University, 02215, United States

h i g h l i g h t s
Study on capture behavior for heavy metals of MSW combustion in CO2/O2 is novel.
Effects of atmosphere, fuel type, temperature and sorbent type were studied.
Replacement of N2 by CO2 increased heavy metal capture for TR but decreased for HDPE.
The increasing temperature increased capture efficiency for Zn but decreased for Ni.
This work helps control heavy metals and manages MSW oxy-fuel incineration.

a r t i c l e

i n f o

Article history: Received 6 May 2015 Received in revised form 17 September 2015 Accepted 2 October 2015 Available online 20 October 2015 Keywords: Heavy metal control CO2/O2 atmosphere Limestone Tire rubber High density polyethylene

a b s t r a c t This paper investigated the capture behavior for three heavy metals (Cu, Ni and Zn) during the waste tire rubber (TR) and high density polyethylene (HDPE) combustion in N2/O2 and CO2/O2 atmospheres in a lab-scale tubular furnace. Both the replacement of N2 by CO2 and addition of limestone decreased the evaporation of Ni, Cu and Zn during TR and HDPE combustion. Replacement of N2 by CO2 increased the capture efficiency of limestone for Zn, Cu and Ni from TR combustion (except for Ni at 700 °C), but decreased for capturing Zn, Cu and Ni from HDPE combustion. The waste composition highly influenced the capture efficiency of limestone for Zn, Cu and Ni, but the amount of sorbent only significantly affected the capture efficiency of limestone for Ni. The increment of the furnace temperature increased the capture efficiency of limestone for Zn but decreased for Ni. The sorption efficiency of limestone was worse than CaO. This work contributes to the control of heavy metals during municipal solid waste (MSW) incineration and management of MSW oxy-fuel combustion. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In order to solve problems arising from municipal solid waste (MSW), many developing countries are taking considerable interest in constructing MSW incinerators [1,2]. One of the challenges and concern in MSW incineration technology is the emission of heavy metals [3]. The emissions of heavy metals present potential threaten to both surrounding environmental and human health [4,5], because they are not biodegradable and tend to accumulate in living organisms [6,7]. Traditional air pollution control devices such as scrubbers, electrostatic precipitators and bag-house filters have been used ⇑ Corresponding authors at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. Tel.: +86 20 87110232; fax: +86 20 87110613. E-mail addresses: [email protected] (Y. Tang), [email protected] (X. Ma). http://dx.doi.org/10.1016/j.fuel.2015.10.038 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

to reduce fine particulates emitted from combustion processes. However, these control devices cannot effectively capture all the submicron particulates which are enriched with heavy metals [8–10]. An alternative control approach for heavy metal emission is to minimize the formation of metal vapors during combustion by suppressing metal volatilization and capturing volatilized metals using effective solid sorbents [11,12]. Possible sorbents should show high temperature stability, fast sorption kinetics, high loading capacity [13] and low cost. Activated carbon can absorb metal vapors at lower temperatures, say, around 380 K. Among the toxic heavy metals, only Mercury (Hg) is vapor at these low temperatures. Therefore, although activated carbon is effective for Hg, it fails to work for other heavy metals [14]. Moreover, the use of activated carbon is not suitable in developing countries due to its high costs associated with production and regeneration of spent carbon [15]. Consequently, the research on alternative low-cost materials as potential

Y. Tang et al. / Fuel 165 (2016) 272–278

sorbents which can effectively capture a wide range of heavy metals has been emphasized recently. The sources of heavy metals in MSW fuel [16,17] and behavior of heavy metals during MSW combustion [2,3,18] have been studied to a certain extent. Some researchers have reported the sorbents’ behavior to control the emission of heavy metals. Ho et al. [19] investigated the capture efficiency of different sorbents in the sand bed of a fluidized bed incinerator, and showed that the adsorption efficiency of limestone for Lead (Pb) and Cadmium (Cd) was better than silica sand and aluminum oxide. Gullett and Ragnunathan [20] showed the reduction of Sb, As, Hg and Se in sub-micron particles when hydrated lime and limestone were injected into a pilot scale high-temperature furnace. Chen et al. [21] showed that the best sorbent for the Pb, Cd, Chromium (Cr) and Copper (Cu) was limestone, especially as the feed waste contained organic chloride. Cheng et al. [22] adopted the sorbents to control the emissions of Pb, Cd and Cr, and showed that 4% (by mass) of CaO was the most effective on the control of Pb, and 6% (by mass) CaO was the most effective on the control of Cd. Zhou et al. [23] investigated the capture efficiency of oxides of Al, Si and Ca to control the Cu emission at 900 °C, and found that the sorption efficiencies of the oxides follow a sequence of CaO > Al2O3 > SiO2. In the previous studies relating to the capture efficiency of the sorbents during MSW combustion, only air atmosphere has been adopted [11]. The references related to effects of other reaction atmosphere such as the oxy-fuel combustion condition on the capture characteristics mainly focused on the coal-fired or biomass combustion. Font et al. [24] evaluated the abatement capacity of Hg and other trace elements in a 90 kWth bubbling fluidized bed (BFB) oxy-combustion pilot plant fed with coal and limestone (bed material), and they suggested that the relatively low temperatures and high Ca content from limestone promoted condensation and sorption of trace element compounds. Chen et al. [25] discussed the mechanism of Cr(VI) formation upon the interaction with metal oxides and they found that Fe2O3 and CaO exhibited a larger capability than MgO on the capture of Cr vapors during coal oxy-fuel combustion. Contreras et al. [26] proved that the increase in SiO2 and Al2O3 concentration promoted the formation of CdSiO3 and CdOAl2O3 to collect Cd, during pure fuels and coal/biomass blends combustion under oxy-fuel conditions by equilibrium calculations. The capture characteristics results from above references may be highly dependent on the fuel type and heavy metal type, and whether the same conclusions are suitable for other heavy metals during MSW combustion must be investigated. Oxy-fuel combustion, which utilizes a CO2/O2 mixture as the oxidizer instead of air, is a promising technique to tackle the CO2 recovery [27]. This combustion technology can in principle be applied to any type of fuel including MSW utilized for thermal power production [28], however, the capture mechanisms of sorbents in MSW oxy-fuel combustion condition have remained scarce. This study compared the capture behavior of limestone during combustion of two common MSW components under both N2/O2 and CO2/O2 atmospheres experimentally. The effects of waste fuel type, furnace temperature, the amount of sorbent added and sorbent type in CO2/O2 atmosphere on the capture behavior of sorbents were explored. The results provided useful information for the future development of oxy-fuel combustion technology and the control of heavy metals during waste incineration.

2. Materials and methods 2.1. Materials The MSW components was complicated and disordered, which resulted in the great variety of heavy metal sources. Materials

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tested in this paper included waste tire rubber (TR) and high density polyethylene (HDPE), which were common MSW components and ones of the main sources of heavy metals in MSW fuel. HDPE mainly used in shopping litter bags and films is one of the principal types of plastic waste [29]. Due to the excellent flexibility, elasticity and other superior mechanical and chemical properties, both natural and synthetic rubbers in vulcanized form are used to produce various rubber products, such as tires, hoses, cushions, and gloves [30–33]. HDPE and TR were obtained from Dongguan Qisheng plastic materials Co. Ltd. (Guangdong Province, China) and Weicheng Rubber GongMao Co. Ltd. (Zhejiang Province, China) respectively. Limestone and quicklime/calcium oxide (CaO) prepared as sorbents were obtained from TianJin Fucheng Chemical Reagent Factory (TianJin city, China). The results of chemical analysis on limestone indicated that limestone had high amount of CaCO3 (99.0%) with less amount of impurities. The total amount of heavy metals in limestone and CaO was less than 0.001% and 0.005%, respectively. The limestone and CaO were made based on GB/T15897-1995 and GB1262-92 criterion, respectively. The experimental materials and sorbents 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 uniformity of the samples which was added by sorbents was ensured by a micro rotary mixer (Nanjing Quntong drying equipment factory, Jiangsu Province, China). The samples were dried at 105 °C for 3–4 h and stored in desiccators. The materials’ ultimate and proximate analyses are shown in Table 1. All runs were repeated to have 2 or 3 replicates to ensure the reliability, with standard deviations less than 3%. 2.2. Apparatus and methods 2.2.1. Combustion experiments Synthetic gas mixtures (CO2/O2) were used as the feed gas in oxy-fuel combustion, and combustion of fuel in two different atmospheres (80N2/20O2, 80CO2/20O2) was performed in a lab-scale tubular furnace. Four levels of sorbents (0%, 5%, 10% and 15%) were added to the TR and HDPE sample to study their effects on heavy metal volatility at a desired temperature (700 °C or 900 °C) in 1200 s (20 min), which is long enough for TR and HDPE to burn out. The flow rate of the mixed gas was 0.14 m3/h. 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, 0.50 ± 0.001 g samples were loaded into a sample holder and then the sample holder was inserted into the reactor. 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 Zinc (Zn), Nickel (Ni) and Copper (Cu) concentrations in the raw TR and HDPE and their combustion ashes were investigated. The Cu, Ni and Zn contents were determined using a TAS-990 atomic absorption spectrophotometer (AAS, Bejing Pgeneral Analytical Instrument Co., Ltd, China) after acid digestion using HNO3-HFHClO4. The guaranteed reagent (GR) and deionized water were used. 2.2.3. Pore analysis in sorbents The pure sorbents also combusted in an 80N2/20O2 or 80CO2/20O2 for 20 min in the above quartz tube furnace, and then the pore characteristics of sorbent char were tested using Quantachrome SI-MP-10 surface areas and porosity analyzer (Quantachrome Instrument Co., Ltd, U.S.A). Prior to gas adsorption measurements, the sorbents were degassed in the adsorption

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Table 1 Ultimate and proximate analysis of TR and HDPE (on air dried basis). Materials

Ultimate analysis (wt%)

Tire rubber (TR) High density polyethylene (HDPE) a

Proximate analysis (wt%)

C

H

O

N

S

Cl

Moisture

Volatile matter

Fixed carbon

Ash

81.72 82.83

7.51 14.42

5.32 2.74

0.50 0.02

1.71 0.06

3.25 NDa

1.21 0.04

84.98 0.82

3.94 98.89

9.82 0.25

ND represented lower than the detection limit.

system at 300 °C for 6 h. The specific surface area (SBET) of the samples was determined by Brunauer–Emmett–Teller (BET). The repeatability deviations in all cases were less than 2%. 3. Results and discussions 3.1. Heavy metal enrichment characteristics in ash without sorbents The volatilization rate (VR) for heavy metals was calculated using Eq. (1):

VR ¼ ðC fuel  C ash Þ=C fuel  100%

ð1Þ

where C fuel ; C ash were the contents of heavy metals in the original fuel sample and the bottom ash after 20 min combustion, respectively. The content and volatilization proportion of three heavy metals in original HDPE and TR ashes without sorbents obtained in 80N2/20O2 and 80CO2/20O2 atmospheres were presented in Table 2. In both atmospheres, the order of the heavy metals contents was: Zn > Ni > Cu in HDPE ash, and this order in TR ash was Zn > Cu > Ni. Although there was no obvious contrast of volatilization proportion between TR and HDPE, the contents of the heavy metals in the bottom ashes generated from TR combustion were bigger than those from HDPE combustion, especially of Zn. This due to that the original sample of TR contained much more heavy metals than HDPE sample. The increment of furnace temperature increased vapor pressure of heavy metals. Regardless of fuel species and atmosphere, when the temperature was increased from 700 °C to 900 °C, the content of the heavy metals in the bottom ash declined and their volatilization proportion increased, indicating that the increasing furnace temperature reduced the partitioning of the heavy metals to the bottom ash and the decreased fraction shifted to the fly ash or the flue gases. This was due to the fact that the increment of furnace temperature increased vapor pressure of heavy metals [34]. Saqib and Bäckström [34] and Chiang et al. [35] also found that the increment of combustion temperature would enhance the vaporization rate of heavy metals. At the same furnace temperature, the contents of the heavy metals in the bottom ashes generated in an 80N2/20O2 atmosphere were lower than those in an 80CO2/20O2 atmosphere. This result for TR and HDPE was similar to that for coal presented by

Krishnamoorthy and Veranth [36] and Wen [37]. They also found a decrease in metal evaporation during coal oxy-fuel combustion, when compared with coal conventional air combustion. The following results led to the smaller volatilization rate of the heavy metals in the bottom ashes generated from 80CO2/20O2 combustion [38]. First, at the same O2 concentration, the combustion temperature of char particles in 80CO2/20O2 atmosphere was lower than that in 80N2/20O2 atmosphere [38]. The lower combustion temperature may be caused by higher specific heat capacity of CO2, lower oxygen diffusion rate [39] and heat consumption by the gasification and CO2 dissociation. Second, the much higher CO2 concentration in 80CO2/20O2 combustion 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 [40]. Whether it was the lower particle temperature or less sub-oxides that play the dominate role in the smaller volatilization rate of the heavy metals in an 80CO2/20O2 atmosphere was worthy of further study. 3.2. Effects of the sorbent amount and waste composition on capture behavior of sorbents Fig. 1(a) and (b) displayed the capture efficiency of limestone for Zn, Cu and Ni during TR and HDPE combustion at 900 °C in an 80CO2/20O2 atmosphere, respectively. The capture efficiency of limestone for the three metals followed the sequence of Ni > Cu > Zn during HDPE combustion, and of Ni > Zn > Cu during TR combustion. The capture efficiency (CE) for heavy metals was calculated using Eq. (2):

CE ¼ ðVRwithout  VRadded Þ=VRwithout  100%

ð2Þ

where VRwithout ; VRadded were the volatilization rates of heavy metals without or with sorbent in a certain operating condition, respectively. The capture efficiency of limestone for Ni and Zn increased monotonously with the increment in the amount of sorbent added. As the amount of sorbent added increased, the capture efficiency of limestone for Cu initially increased and then decreased with the maximum value obtained in the 10% case. In order to quantitatively reflect the effects of the sorbent amount and waste composition on the capture behavior of limestone, the analysis of

Table 2 Content and volatilization rate of heavy metals for HDPE and TR. Material

Temperature (°C)

Atmosphere

Zn

Ni

Zn

Ni

Cu

HDPE

900

80CO2/20O2 80N2/20O2 80CO2/20O2 80N2/20O2

31.6 24.6 40.9 38

10.9 4.5 18.5 18.1

5.1 2.3 6.1 5.8

52.4 63.0 38.4 42.8

57.5 82.4 27.8 29.4

32.0 69.3 18.7 22.7

80CO2/20O2 80N2/20O2 80CO2/20O2 80N2/20O2

21,765 21,800 25,600 23,400

11.0 9.1 25.0 20.7

77.5 78.0 118.0 99.7

53.2 53.1 44.9 49.7

64.1 70.3 18.4 32.4

44.8 44.5 16.0 29.0

700 TR

900 700

Heavy metals content in bottom ash (mg/kg)

Volatilization rate (%)

Cu

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Fig. 1. Capture efficiency of limestone for Zn, Cu and Ni at 900 °C in an 80CO2/20O2 atmosphere: (a) during HDPE combustion; (b) during TR combustion.

variance was shown in Table 3. The significant level (a) was assumed to be 0.05. Compared with F 0:95 ð2; 5Þ value (=5.79), it’s 95% chance that the effect of amount of sorbent added on capture efficiency for Zn and Cu was not enough distinguished, but difference for Ni was marked among different amount of sorbent added for this given experimental combustion conditions. The more the limestone was added, the bigger the specific surface area could participate in adsorption. Limestone could capture HCl, SOx and various heavy metals; the added specific surface area might be used to capture the Ni, HCl and SOx rather than Zn and Cu when the limestone amount was above a certain value. Other factors

had much more effects than the amount of sorbent added on the capture behavior of limestone for Zn and Cu, and it was costeffective to choose a small amount of sorbent added for capturing Zn and Cu. The capture efficiency of only three levels of sorbents (5%, 10% and 15%) was compared in this paper, and the effect of the amount of sorbent added in CO2/O2 atmosphere on the capture of different heavy metals was worthy of further study by expanding variation range of the amount of sorbent added. According to the comparison between F-value and F 0:95 ð1; 5Þ value (=6.61), it’s 95% chance that the capture efficiency of limestone for Zn, Cu and Ni had significant difference between HDPE and TR, indicating that the capture efficiency of limestone was highly influenced by the waste composition. The capture efficiency of limestone for Zn and Ni during TR combustion was higher than that during HDPE combustion, while the capture efficiency of limestone for Cu during HDPE combustion was better. Chen et al. [21] also claimed that the adsorption efficiency of the sorbents varied with different feed waste compositions. As shown in Table 1, there was different content between TR and HDPE, such as the Cl and S contents. There were the following counteracting effects of Cl and S contents in fuel on heavy metal capture. On the one hand, the existence of S and Cl enhanced the capture of some metals by the formation of solid metal-sulfursorbent compounds [41]. The transient formation of a mobile halide ion-containing phases (i.e. CaCl2) modified the surface of the partially sulfated sorbent particles to form more voids [42], which provided diffusion paths for some heavy metal toward the interior of a limestone particle. Moreover, the sticky surface of low melting point compounds by mixing CaCl2 and CaSO4 at high temperatures favored partial metal particles capture. On the other hand, with the progression of sulfation, the CaSO4 layer on the surface of the limestone particle became thicker, and diffusion into the sorbent particle became more difficult, eventually baffling the reaction between sorbent and heavy metals. Whether the furtherance or hindrance of Cl and S contents in fuel on heavy metal capture played the main role maybe depended on the metal type or other given operating conditions. Therefore, comparing TR combustion with HDPE combustion, the result of capture efficiency for Cu was different from Zn and Ni. The promoting effect of chlorine on volatilization rate of high volatile heavy metal was bigger than that on low volatile heavy metal, and this phenomenon reflected that during TR combustion the decrement of HCl and other chlorides by limestone increased the capture efficiency for Zn and Ni but not for Cu which had the smallest volatilization rate. The effect of fuel composition in CO2/O2 atmosphere on the capture of different heavy metals was worthy of further study by expanding variation range of fuels type.

Table 3 Two-factor analysis of variance for the capture efficiency of limestone.

a b c

Heavy metal

Different source

SSa

Dfb

MSc

Cu

Waste composition Amount of sorbent added Error Total

1185.34 178.38 124.09 1487.81

1 2 2 5

1185.343 89.189 62.045

19.105 1.437

0.049 0.410

Ni

Waste composition Amount of sorbent added Error Total

157.34 2323.43 5.78 2486.55

1 2 2 5

157.338 1161.716 2.891

54.427 401.866

0.018 0.002

Zn

Waste composition Amount of sorbent added Error Total

191.09 146.38 82.58 420.06

1 2 2 5

191.092 73.191 41.291

4.628 1.773

0.164 0.361

SS represented sum of squares of deviations; Df represented degree of freedom; MS represented mean square deviation.

F-value

P-value

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3.3. Effects of atmosphere type and furnace temperature on capture behavior of sorbents Fig. 2 display the differences of the capture efficiency of limestone for Zn, Cu and Ni during TR and HDPE combustion between 80CO2/20O2 and 80N2/20O2 atmosphere, respectively. The influence of temperature on the capture efficiency of limestone varied with heavy metal type, even for the same waste composition in the same atmosphere. Regardless of atmosphere, for TR, Ni and Cu were captured more effectively by limestone at 700 °C than at 900 °C, and the capture efficiency of limestone for Zn performed better at 900 °C. For HDPE, when the furnace temperature was increased from 700 °C to 900 °C, the capture efficiency of limestone for Cu remained the same in an 80N2/20O2 atmosphere but increased obviously in an 80CO2/20O2 atmosphere. The influence of temperature on the capture efficiency of limestone for Zn and Ni during HDPE combustion was the same in 80CO2/20O2 and 80N2/20O2 atmospheres. The increment of the furnace temperature increased the capture efficiency of limestone for Zn but decreased the capture efficiency for Ni, the same as TR.

The sorbent mechanisms in capturing metals during incineration included chemical reaction and physical adsorption. In general, whether chemical reaction or physical adsorption played the main role depended on the given reaction conditions. When the temperature increased, the physical adsorption process of sorbent for the three metals was inhibited, because higher temperature enhanced sintering [43] and obstructed the development of pore structure. However, the decreased temperature also decreased the chemical adsorption reaction. Thus, Chen et al. proposed that physical adsorption was dominated at lower temperature of 700 °C, and chemical adsorption was dominated at higher temperature of 900 °C [44]. The physical adsorption mainly included the condensation of heavy metals vapor and the particles capture by van der Waals forces which were affected by physical properties of heavy metal particles such as particle size and relative molecular weight. The chemical properties of heavy metal distinctly affected the chemical interaction with the active center in the sorbent. Therefore, even for the same type of sorbent, the major adsorption mechanism varied with different heavy metals [45]. The capture for Ni was mainly due to physical sorption; whereas the capture for Zn was mainly due to chemical reaction. Different major adsorption mechanism was the possible reason for different effect of temperature on their capture efficiency. Whether at 700 or 900 °C, Zn, Cu and Ni from HDPE combustion were captured more effectively by limestone at the 80N2/20O2 atmosphere condition than at the 80CO2/20O2 atmosphere condition. For TR, only Ni at 700 °C was captured more effectively by limestone in an 80N2/20O2 atmosphere than in an 80CO2/20O2 atmosphere, and the capture efficiency of limestone for Zn and Cu performed better in an 80CO2/20O2 atmosphere at both furnace temperatures. In 80N2/20O2 atmosphere, limestone usually decomposed into CaO and CO2 at 700 and 900 °C, and the pore structure increased until when the sintering of CaO happened and aggravated. The sintering caused the surface area of limestone decrease [46]. The behavior of calcination and sintering under high CO2 concentration was different from that in conventional air combustion [46,47], and these differences physically caused the different capture efficiency between two atmospheres. The decomposition temperature of limestone became higher with the increase of CO2 concentration [48]. Therefore, in 80CO2/20O2 atmosphere, the calcination of limestone was inhibited at 700 °C and the calcination time for limestone’s complete decomposition to produce nascent lime was extended at 900 °C. First the pore surface area of limestone in 80CO2/20O2 atmosphere was poorer than that in 80N2/20O2 atmosphere, as time went by, and then it reversed due to a less severe sinteration in 80CO2/20O2 atmosphere [43]. As shown in Table 4, the SBET and pore volume of limestone after 20 min combustion in 80CO2/20O2 atmosphere were bigger than those in 80N2/20O2 atmosphere. As shown in Table 1, the sum of content of Cl and S in TR was much bigger than that of HDPE. Limestone could capture HCl, SOx and heavy metals. The capture for HCl and SOx might postpone the occurrence of heavy metals capture. The capture for heavy metals released from HDPE was earlier than TR combustion. Zn, Cu and Ni from HDPE and TR combustion were captured by limestone at different times, and this difference might be one of the reasons for different variety of their capture Table 4 Pore characteristics of limestone after 20 min combustion under both atmospheres.

Fig. 2. Comparison of capture efficiency of limestone during TR and HDPE combustion between 80CO2/20O2 and 80N2/20O2 atmosphere: (a) Zn; (b) Ni; (c) Cu.

Temperature (°C)

Atmosphere

SBET (m2/g)

Total pore volume (106 L/g)

Mean pore diameter (nm)

900 900 700 700

80CO2/20O2 80N2/20O2 80CO2/20O2 80N2/20O2

12.377 0.003 41.919 1.937

133.4 0.017 166.1 3.423

43.120 23.715 15.853 7.067

Y. Tang et al. / Fuel 165 (2016) 272–278

efficiency with atmosphere, because the replacement of N2 by CO2 was only favorable for the porosity development of limestone after a certain calcination time. Moreover, the chemical reaction and products of different heavy metal species, waste components and limestone in 80CO2/20O2 atmosphere were worthy of further study, because they also affected the variety of the capture efficiency of limestone with atmosphere. 3.4. Effects of sorbent type on capture behavior Fig. 3 display the differences of the capture efficiency for Zn, Cu and Ni during TR and HDPE combustion between limestone and CaO, respectively. CaO had higher sorption efficiency on Zn, Cu and Ni than limestone, with an exception for Zn from HDPE combustion at 900 °C in an 80CO2/20O2 atmosphere. Regardless of fuel species and metal type, the capture ability of CaO in an 80CO2/20O2 atmosphere always performed better at 700 °C than 900 °C. The physical adsorption reaction was the dominated mechanism of CaO, because the molecular attraction between the sorbent and metal compounds was favorable at lower temperature. This result agrees with the thermodynamic equilibrium calculation result presented by Zhou et al. [23]. They found that the addition of CaO did not change the liquid CuCl, indicating that CaO had little chemical influence on Cu speciation, so that the sorption of CaO was due to physical effect [23]. Regardless of fuel species and furnace

277

temperature, the capture efficiency of CaO for the three metals followed the sequence of Ni > Cu > Zn. The capture efficiency of CaO for Zn, Cu or Ni during HDPE combustion was always higher than that during TR combustion. For the different type of sorbents, both specific surface area and chemical compositions were responsible for their different capture ability [9], moreover, the content of molten phases which was raised by low-melting-point eutectics on the surfaces made the heavy metals better fixed in the matrix [4,49]. The variety of the capture efficiency of limestone with atmosphere, temperature or waste composition wasn’t as the exactly same as CaO which was mainly determined by physical effect, indicating that the capture mechanism of limestone was more complicated. Although the performance of limestone was not as good as CaO for capturing Cu and Ni, the price of limestone was lower than CaO. Therefore, limestone still offered the potential for low cost effective media to control heavy metals during MSW combustion. According to Lu Huan Liang et al. [50], the performance of limestone for heavy metals capture could be improved using some modification methods, such as thermal treatment and salt soaking-calcination. Choosing the correct modification method and operating conditions to improve capture performance of limestone in MSW oxy-fuel combustion was worth of further study. 4. Conclusions The following conclusions were made: 1. Both the replacement of N2 by CO2 and addition of limestone decreased the evaporation of Ni, Cu and Zn during TR and HDPE combustion. 2. In an 80CO2/20O2 atmosphere, the capture efficiency of limestone for Zn, Cu and Ni was highly influenced by the waste composition, but the amount of sorbent added only significantly affected the capture efficiency of limestone for Ni. 3. Replacement of N2 by CO2 was favorable for capturing Zn, Cu and Ni from TR combustion (except for Ni at 700 °C), but unfavorable for capturing Zn, Cu and Ni from HDPE combustion. Different fuel component and capture time caused the difference in the effect of atmosphere type on heavy metals capture between TR and HDPE. 4. Regardless of waste species and atmosphere, the increment of the furnace temperature increased the capture efficiency of limestone for Zn but decreased the capture efficiency for Ni; however, the variety of capture efficiency for Cu with temperature depended on waste species and atmosphere. 5. The sorption efficiency of limestone was worse than CaO and still offered the potential for low cost effective media to control heavy metals during MSW combustion with the correct modification method and operating conditions.

Acknowledgements The authors are grateful to the support given by the National Basic Research Program of China (973 Program) (2011CB201500), China Postdoctoral Science Foundation funded project and Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (KLB10004). References

Fig. 3. Comparison of capture efficiency of limestone during TR and HDPE combustion between limestone and CaO: (a) at 900 °C in 80CO2/20O2; (b) at 900 °C in 80N2/20O2; (c) at 700 °C in 80CO2/20O2.

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