Heavy Metal Control by Natural and Modified Limestone during Wood

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Heavy metal control by natural and modified limestone during wood sawdust combustion in a CO2/O2 atmosphere Weihua Zheng, Xiaoqian Ma, Yuting Tang, Chuncheng Ke, and Zhendong Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03365 • Publication Date (Web): 24 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Heavy metal control by natural and modified limestone during wood sawdust combustion in a CO2/O2 atmosphere

Weihua Zheng, Xiaoqian Ma, Yuting Tang*, Chuncheng Ke, Zhendong Wu

Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, School of Electric Power, South China University of technology, Guangzhou 510640, China

*Corresponding author:

School of Electric Power, South China University of Technology

Guangzhou 510640, China

Tel.: +86 20 87110232;

Fax: +86 20 87110613.

E-mail address: [email protected]

ACS Paragon Plus Environment

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Abstract

This paper experimentally investigated the capture performance of natural and modified limestone for Cr, Cu, Pb and Zn during the wood sawdust combustion in an 80CO2/20O2 atmosphere. The addition dosage of limestone had significant effects on the capture efficiency for Cu and Pb, but no for Cr and Zn based on the two-factor analysis of variance method. The presence of NaCl always lowered the capture efficiency for Pb; whereas for Cu and Zn, the effects of NaCl turned from negative to positive as the temperature increased. Limestone modified with K2CO3 improved the capture performance for Cr and Cu, while limestone modified with Al2(SO4)3 improved the capture performance for all studied heavy metals. The best pretreatment for capturing Cu, Zn, Cr and Pb were modified limestone with Al2(SO4)3 (ion ratio γ=15), Al2(SO4)3 (γ=10), K2CO3 (γ=10) and Al2(SO4)3 (γ=10), respectively. The degree of improvement depended on the specific inorganic salt type and ion ratio. Therefore, choosing a reasonable inorganic modification method should depend on specific targeted heavy metals. The results conduce to understand the heavy metal control mechanism of limestone and explore a high-efficiency and low-cost method for heavy metal control during wood oxy-fuel combustion.

Keywords: heavy metal; CO2/O2 atmosphere; modified limestone; inorganic chloride; capture efficiency

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1.

Introduction

In the last decades, the biomass industry has developed rapidly. Biomass is the third largest natural source of energy in the world and total global potential is estimated at 100~440 EJ/year1. In China, the biomass power generation occupies 3% of the total installed capacity of power and more than 130 biomass incineration power plants have been put into use2.

Biomass contains numerous unwanted substances including heavy metals3, which mainly come from minerals and fertilizers in the soil, and are absorbed through the leaves from atmospheric dust and rain4. The emissions of metal compounds during combustion can cause environmental and health problems through contamination of air, soil and water in the areas surrounding biomass incineration power plants5, 6. Therefore, in the case of energetic utilization of the biomass, it remains a research challenge to control the emission of heavy metals7.

The methods of controlling heavy metals are divided into three categories: pre-control, mid-control and post-control8. It is difficult for most of post-control flue gas treatment devices to capture all the submicron particulates with high levels of heavy metals9, 10. The bottom ash could be gathered relatively easily5. Thus, as a mid-control method, inhibition of the volatilization of heavy metals by capturing them as non-volatile compounds in the bottom ash is an effective way to reduce the spread of metal pollution.


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Oxy-fuel combustion is potential to solve several gas emissions problems, and its importance is documented by numerous published papers 11-13. Combining the oxy-fuel combustion with biomass incineration can achieve an overall negative emissions balance and has the CO2 reduction potential of 800 million tons annually by 2050 14. Research on the heavy metal control during wood oxy-fuel combustion is helpful to reduce pollution, which is necessary for combining biomass utilization with the oxy-fuel combustion. Contreras et al.15 anticipated the partitioning behavior of mercury (Hg), arsenic (As), cadmium (Cd) and selenium (Se) during two biomasses combustion in oxy-fuel condition using thermodynamical equilibrium calculation; however, the capture behavior of sorbents during biomass oxy-fuel combustion still lacks of experimental data. Limestone was thought to have the potential to be a cost-efficient sorbent. Han et al.16 reported that porous Ca-based bead sorbents could remove heavy metal during sewage sludge incineration. Our previous paper 17 experimentally compared the capture behavior of natural limestone between N2/O2 and CO2/O2 atmospheres, and found that replacement of N2 by CO2 was favorable for capturing zinc (Zn), nickel (Ni) and copper (Cu) (except for Ni at 700 oC) during tire rubber combustion, but the capture performance of natural limestone still needed further improvement.

The presence of chloride can significantly influence the capture performance of limestone for heavy metal during combustion 18, 19. Zhang et al. 20 reported that the presence of chlorine compounds significantly increased the volatilization of Cd during 4


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municipal solid waste incineration. Chen et al.21 studied on the effects of additives, including PVC, NaCl, and Na2SO4, on the capture performance of different sorbent, and they found that the presence of NaCl improved the capture efficiency of limestone for chromium (Cr), Cu, lead (Pb) and Cd. However, the experiments were carried out in an air atmosphere. The effects of inorganic chloride on the capture performance of limestone during oxy-fuel combustion are still unclear and deserves to be studied on.

Extensive research suggests that some inorganic additives can improve SO2 adsorption by Ca-based sorbent22-24. The pretreatment of limestone by additives may be a good choice to improve its capture performance for heavy metals, but the knowledge about the enhancement performance of additives on Ca-based sorbent for heavy metals control remains scarce. Li et al.8 found that the limestone modified by Al2(SO4)3 had an improved performance on capturing Zn, Cd and As. However, in their studies, only air atmosphere has been adopted and only three heavy metals from coal combustion have been focused on. Whether the modification methods are suitable for a wider range of heavy metals during the oxy-fuel combustion of wood sawdust must be investigated.

This study aimed to experimentally assess the capture performance of natural and modified limestone for heavy metals during wood sawdust combustion in a CO2/O2 combustion condition. The effects of furnace temperature, addition dosage of limestone, inorganic chloride, and modified method were taken into account. The 5


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capture mechanism of limestone, influence mechanism of NaCl and the enhancement mechanism of modification using inorganic additives on the heavy metals control of limestone were investigated. This work is helpful in exploring methods to reduce heavy metals emission from the oxy-fuel combustion system of biomass.

2.

Materials and methods

2.1. Materials

Materials tested in this paper is wood sawdust made from spruce. The limestone was made according to GB/T15897-1995 criterion in TianJin Fucheng Chemical Reagent Factory (TianJin city, China). The natural limestone had a high purity (99.0%) with the total amount of heavy metals less than 0.001%. Some limestone was immersed in solutions of NaCl, K2CO3 or Al2(SO4)3 based on the required ion ratio (γ= Ca2+/other metal ion) and stirred by a magnetic stirring apparatus (TianJin city, China) for 2 h, and then dried at 105oC for 12 h.

The experimental wood sawdust was pulverized using DFY-300 pulverizer (Wenling Linda Machinery Co., Ltd., China), and then passed through a sieve with a mesh size of 590 µm. The samples were dried at 105 oC for 3~4 h and stored in desiccators. Table 1 shows the ultimate and proximate analyses of wood sawdust.

2.2. Experimental methods

2.2.1. Combustion experiments 6


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The oxy-fuel combustion experiment used synthetic gas mixture (80% CO2 and 20% O2 by volume) as the feed gas with the flow rate of 0.06 m3/h, and wood sawdust with 0%, 5%, 10% or 15% sorbents combusted in a quartz tubular furnace. The experiment apparatus were also employed in our previous studies17, 25, 26. A thermocouple was installed at the center of the tube to control the furnace temperature. Once the furnace temperature reached to the desired value (600 oC, 700 oC, 800 oC or 900 oC), the sample holder which loaded 0.50± 0.001 g samples was pushed into the furnace. After fifteen minutes, 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

The samples and ash residue were digested in a WMX-III-B (Shaoguan Mingtian Instrument Co., Ltd China) microwave digestion system. 10 ml of acid mixtures was added to each teflon vessel in the oven. The contents of zinc (Zn), chromium (Cr), lead (Pb) and copper (Cu) in the raw wood sawdust and bottom ashes obtained by combustion were detected using Inductively Coupled Plasma Optical Emission Spectrometer (5100 ICP-OES) (Agilent Technologies Inc., USA). The guaranteed reagent (GR) and deionized water were used.

2.2.3. Pore analysis in sorbents

The natural and modified limestone were calcined for 15 min at 800 oC under an

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80CO2/20O2 atmosphere, and then their pore characteristics were tested by Quantachrome SI-MP-10 surface areas and porosity analyzer (Quantachrome Instrument Co., Ltd, U.S.A). The sorbents were degassed at 300 oC for 6 h before gas adsorption measurements. The specific surface area was tested according to Brunauer–Emmett–Teller (BET). The pore characteristics of natural and modified limestone are showed in Table 2. The aforementioned experiments, including combustion experiment, heavy metal content detection and pore analysis, were all repeated twice or three times to ensure the reliability, with standard deviations less than 5%.

3.

Results and discussions

3.1. Effects of the temperature on capture performance of natural limestone

Fig.1 shows the variety of the capture efficiency of natural limestone with temperature during wood sawdust combustion in an 80CO2/20O2 atmosphere. The capture efficiency (CE) for heavy metals was described by the Eq. (1):

CE =

VRwithout − VRadded ×100% VRwithout

Eq. (1)

where VRadded , VRwithout were the volatilization rates of heavy metals during wood sawdust combustion with and without 10% sorbent, respectively. The volatilization rate (VR) was calculated based on the Eq. (2):

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VR =

C fuel − Cash C fuel

× 100%

Eq. (2)

where C fuel , Cash were the concentrations of heavy metals in the raw wood sawdust and its bottom ash obtained after 15 min combustion, respectively.

As shown in Fig.1, the order of average capture efficiency of natural limestone was: Cu>Zn>Pb >Cr. Blending with the limestone could reduce the volatilization rate of heavy metal (except for Cr at low temperatures), but the capture performance was not satisfying. The maximum capture efficiency of Cr and Pb was below 50 %. Therefore, choosing reasonable combustion conditions and modification methods to increase the capture efficiency of Ca-based sorbent was worth of deep study.

The effects of furnace temperature on the capture performance of natural limestone varied with heavy metal type. For Cr and Pb, the capture efficiency increased as the temperature rose up and reached the maximum at 900 oC. For Zn, the capture efficiency increased first and then decreased as the temperature rose and reached the maximum at 700 oC. When the temperature rose from 600 oC to 700 oC, the capture efficiency for Cu increased greatly. But when the temperature was higher than 700 oC, the capture efficiency for Cu was fluctuant around 50%.

The sorbent’s capture performance for heavy metals particularly associated with the volatility of the metals and their reactivity with the sorbent under a certain combustion condition 27. The capture mechanisms of sorbents during combustion

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contained chemical reaction and physical adsorption. The physical adsorption performed mainly through the condensation of heavy metals vapor and the particle capture by van der Waals forces, which were closely related to physical properties of heavy metals including particle size and relative molecular weight. The chemical reaction with the active center in the sorbent depended on the chemical properties of heavy metals. Whether chemical reaction or physical adsorption played a leading role was related to the specific combustion conditions and the heavy metal type. With the increment of temperature, the specific surface area and porosity volume first increased and then decreased. Higher temperature was unfavorable for physical adsorption of sorbent because of sintering28 and blocked pores. This indicated that moderate temperature favored physical adsorption; however, lower temperature was unfavorable for the chemical reaction. Therefore, Chen et al. 29 claimed that physical adsorption played the dominant role at around 700 oC, whereas chemical reaction played the dominant role at around 900 oC. Meanwhile, Chen et al. 30 proposed that the major capture mechanism varied with heavy metals type. Chemical and physical adsorption mechanisms were both important for capturing Cu, and Zn was captured mainly by physical adsorption; whereas the major capture mechanism for Pb and Cr was chemical reaction. Peng et al.31 claimed that the binding bonds among metallic atoms including O and OH were formed when Cr and Pb were in a high-temperature environment, and the chemical reaction between Cr/Pb and sorbent was the main adsorption mechanism. Chen et al.30 also agreed the chemical reaction was the major 10


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adsorption mechanism for Cr, and Li et al.8 claimed the physical adsorption was the major adsorption mechanism for Zn. So difference in the effects of temperature on capture efficiency between various heavy metals was attributed to their different major adsorption mechanism. In this paper, the capture efficiency for Zn reached maximum at 700 oC and capture efficiency for Cu kept a larger value at higher temperature, while capture efficiency for Cr and Pb increased as temperature rose. This results agreed with the previous researches30, 32 on the capture mechanism of heavy metals during combustion in air, indicating the replacement of N2 by CO2 hardly changed the adsorption mechanism.

3.2. Effects of addition dosage on capture performance of limestone

Fig.2 shows the capture efficiency of natural limestone at different addition dosage during wood sawdust combustion in an 80CO2/20O2 atmosphere. As it shown, the effect of limestone addition dosage on the capture efficiency depended on the heavy metal type. For Cr and Cu, the capture efficiency increased as the limestone addition dosage increased, except for the capture efficiency for Cr at 600 oC. The increment of capture efficiency for Cr was most obvious at 900 oC. For Pb, the variation of CE value was irregular when the limestone addition dosage changed from 5% to 10%. But when the limestone addition dosage increased from 10% to 15%, the capture efficiency for Pb increased greatly. For Zn, the capture efficiency was also fluctuant as limestone addition dosage increased when the temperature was lower than 11


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800 oC. As temperature rose to 800 oC and 900 oC, the CE value for Zn increased with the addition dosage.

In order to analyze the effects of addition dosage on the capture efficiency, a statistical analysis method, two-factor analysis of variance, was employed. The effects of temperature were also taken into account. The results are showed in Table 3. The significant level ( α ) was assumed to be 0.05 during calculation. When the F critical value is lower than the F value, it has a 95% probability to confirm that the factor has significant effects on the capture efficiency. The F critical value was 4.76 for factor temperature and 5.14 for factor addition dosage ( FT ,0.05 ( 3,6) = 4.76 ,

FA,0.05 ( 2,6) = 5.14 ). For the temperature factor, all F values were greater than FT ,0.05 ( 3,6) . Therefore, the furnace temperature has significant effects on the capture efficiency for all heavy metal elements. Different from the temperature factor, for addition dosage factor, only F values of Cu and Pb were greater than FA,0.05 ( 2,6 ) , indicating that the limestone addition dosage has significant effects on the capture efficiency for Cu and Pb, but has no significant effects on Cr and Zn.

Different effects of addition dosage on the capture efficiency between different heavy metal elements were resulted from the capture mechanism. As mentioned, Cu was captured by both physical and chemical adsorption and Zn was captured mainly by physical adsorption, while Cr and Pb were captured mainly by chemical reaction 30. Meanwhile, physical adsorption was affected by the specific surface area and total 12


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pore volume of sorbent, while chemical reaction was related with the reactivity of heavy metal. Furthermore, the dominant adsorption mechanism was physical adsorption at 700 oC, whereas it’s chemical adsorption at 900 oC. Therefore, the capture efficiency for Cr and Pb was less affected by the addition dosage of limestone at lower temperature. On account of the lower boiling temperature, Pb and its oxide evaporated first and had greater probability to contact with limestone. Accordingly, Pb was more likely to be captured by limestone and with addition dosage increased, the capture efficiency improved. The physical adsorption not only worked on Zn but also on other matters17. The incremental specific surface area and total pore volume may be consumed by matters like SOx, NOx or etc. Hence, the increment of addition dosage didn’t improve the capture efficiency for Zn. As for Cu, both of physical and chemical adsorption worked, so the capture efficiency was always improved at low and high temperatures.

3.3. Effects of inorganic chloride on capture performance of limestone

Fig.3 shows the capture efficiency of natural limestone during wood sawdust combustion in the presence of NaCl. A negative capture efficiency means that the promotion of NaCl on volatilization of heavy metal was greater than the capture effect of limestone on heavy metal and vice versa. As shown in Fig.3, at 600 oC, the capture efficiencies were all negative. As the temperature rose up, the capture efficiency for Cu and Zn turned positive at 700 oC, while at 900 oC, all capture efficiencies were 13


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positive. The results indicated that the promotion of NaCl on volatilization was weakened or the capture performance of limestone was improved as temperature rose. To illuminate the effects of NaCl, the capture efficiency obtained in the presence and absence of NaCl under the same conditions was compared and a parameter, DCE, was defined by Eq. (3):

DCE = CEwithout − CEwith

Eq. (3)

where CEwith , CEwithout were the capture efficiency of natural limestone in the presence and absence of NaCl, respectively. If the DCE value is positive, the presence of NaCl is considered to improve the capture efficiency of natural limestone for heavy metals. The DCE values are displayed in Fig.4. For Pb, the DCE values were negative, which indicated the presence of NaCl always weakened the sorption of natural limestone under the experiment conditions. The same phenomenon was observed by Hong et al.33 during sludge combustion in an air atmosphere. For Cu and Zn, the DCE values were negative first and then turned positive. The turning point was at 700 oC for Cu and 900 oC for Zn. The results demonstrated that the effects of NaCl on the capture efficiency of natural limestone turned from negative to positive as the furnace temperature rises. As for Cr, the DCE values varied from -8.98% to 0.56%. The capture efficiency of natural limestone for Cr was relatively less affected by NaCl.

The presence of NaCl affected the capture performance in two ways. On the one hand, the presence of Na had positive influences on the capture of heavy metals 14


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(except for Pb) because of the formation of low-melting-point eutectics on the particle surfaces and a significantly increment solid-state diffusivity. As Table 2 showed, in the presence of NaCl, the specific surface area of limestone increased by 181.0% and total pore volume increased by 117.2%. On the other hand, the presence of Cl had negative influences on the capture of heavy metals. The chlorine (Cl) derived from NaCl may form HCl, which would react with heavy metal to produce lower melting and boiling point metal chlorides, which had higher vapor pressures than their corresponding oxides34. Thus, the presence of Cl enhanced the volatilization of heavy metals. Moreover, HCl also reacted with limestone, eventually baffling the metal capture. Influence of Cl on heavy metal partitioning had been reported to be the volatility dependent, and Cl had a bigger promoting influence on volatilization of high volatile metals than on low volatile metals 17. According to Alcock et al. 35, Cr and Cu belonged to low volatile subgroup, whereas Pb and Zn were classed as semi-volatile subgroup. In other words, the capture efficiency for Cr and Cu was less affected by the presence of Cl than it for Pb and Zn. At 600 oC, because of the formation of low-melting point eutectics, the evaporation of heavy metals was promoted. Thus, the DCE values were negative. At 700 oC and 800 oC, the physical adsorption was enhanced. Cu can be captured by physical adsorption and was less affected by Cl, so the DCE value turned positive. For Cr, the primary adsorption mechanism was chemical reaction, and thus the improvement of physical adsorption didn’t work and the DCE value for Cr was still negative. At 900 oC, calcium carbonate decomposed 15


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into calcium oxide, which may react with metal chloride and produce calcium oxide metal chloride complex. This improved the chemical adsorption. Therefore, the DCE values of all heavy metals increased at 900 oC. Competition between alkali metal (K/Na) and Pb strongly favored alkali metal capture in the same temperature range, because of a higher reactivity of alkali metal. Alkali metal reacted with the sorbent before the deactivating melt, which was initiated by the alkali metal and Pb products, and significantly deactivated the sorbent 36, 37. Both Cl and Na had negative effects on Pb capture, and these negative effects led to a larger furtherance of NaCl than the hindrance of limestone on the evaporation of Pb.

3.4. Effects of modification methods on capture performance of limestone

Fig.5 shows the capture efficiency of natural and modified limestone during wood sawdust combustion at 800 oC in an 80CO2/20O2 atmosphere. Pretreatment of limestone by K2CO3 improved the capture performance for Cr and Cu, whereas limestone modified with Al2(SO4)3 shown an enhanced capture performance for all heavy metals. But the improvement depended on the specific modified method and ion ratio. Modification pretreatment had the greatest improvement effect on the capture efficiency for Cr. The optimal sorbent for capturing Cr was the limestone modified with K2CO3 (γ=10), closely following by the limestone modified with Al2(SO4)3 (γ=15). The capture efficiency for Cr increased from 7.68% in natural limestone to 83.20% in limestone modified with K2CO3 (γ=10) and to 82.28% in 16


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limestone modified with Al2(SO4)3 (γ=15). The limestone modified with Al2(SO4)3 (γ=15) had the best performace for capturing Cu, with the maximal capture efficiency of 82.14%. The limestone modified with Al2(SO4)3 (γ=10) had the best performace for capturing Zn and Pb, and its capture efficiency was 55.9% for Zn and 32.1% for Pb, respectively.

As shown in Table 2, the specific surface area, pore size and volume of modified limestone with Al2(SO4)3 were increased significantly, especially when γ was 10. Defects introduced during the modification process by Al2(SO4)3 caused ion breakdown and reduced the capability for bond recombination. The limestone modified with K2CO3 and raw limestone varies slightly in porosity characteristics.

The specific surface area of modified limestone followed the sequence of Al2(SO4)3 (γ=10) > Al2(SO4)3 (γ=15) > K2CO3 (γ=15) > K2CO3 (γ=10). The sequence of capture efficiency for Zn (shown in Fig.5) was to some extent consistent with the specific surface area of modified limestone, and this evidence supported the conclusion that the capture of limestone for Zn was due to physical adsorption. Compared with the limestone modified with Al2(SO4)3, limestone modified with K2CO3 had a worse pore structure but a higher capture efficiency for Cr, also indicating the major capture mechanism for Cr was chemical reaction rather than physical adsorption.

Compared with natural limestone, K2CO3 pretreatment favored the capture for Cr 17


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and Cu, but decreased the capture efficiency for Pb. Addition of alkali metal favored the capture for heavy metals (except for Pb) because of low-melting-point eutectics on the particle surfaces and physical co-condensation38, 39. According to Liu et al.40, heavy metals could combine with alkali metal to produce eutectics, be covered by the liquid phase eutectics, and adhere to liquid eutectics. The content of molten phases as the result of low-melting-point eutectics on the surfaces fixed the heavy metals more strongly in the matrix38, 41. Moreover, because of the replacement of partial Ca by K and their difference in the size of ion radius, limestone using K2CO3 pretreatment had more extrinsic point defects in the crystal lattices and more irregular crystal arrangement than that of natural limestone. Consequently, the ion diffusivity was higher and the diffusion resistance decreased42. Meanwhile, like the alkali metal in the NaCl, the addition of K in the Ca-based sorbent was also detrimental to Pb capture. Kuo et al.38 also found the modification using alkali metal for Ca-based sorbents was favorable to capture Cr and Cd but unfavorable to capture Pb in air atmosphere.

The limestone modified with Al2(SO4)3 had a higher capture efficiency for the heavy metals. Al2(SO4)3 pretreatment could possibly clean impurities in inner pores of limestone. This increased the pore size and specific surface area, and further improved the metal adsorption ability. Previous researches indicated that the presence of sulfur decreased the volatility of heavy metals because of stable metal sulfides 43 and improved the capture efficiency of some metals due to the formation of solid metal-sulfur-sorbent compounds44. Moreover, calcium aluminate had a higher activity 18


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and might further react with heavy metals 45.

It's difficult for either inorganic salts above to optimize the capture performance for a wide range of heavy metals with different characteristics at the same time. Therefore, choosing a reasonable modified method should depend on the specific targeted heavy metals, which should be judged by comparison of the hazards intensity and the content in the fuel among various heavy metals. For example, when Cu or Zn was the targeted heavy metal for a combustor, Al2(SO4)3 was a reasonable salt to modify limestone. The performance of organic acids and multiple modulated method using two or three inorganic salts to improve the capture efficiency for heavy metals were worthy of further study.

4.

Conclusions

Replacing N2 with CO2 didn’t change the primary adsorption mechanism of limestone for heavy metal during combustion. The capture performance of natural limestone was unsatisfying with its maximum capture efficiency for Cr and Pb of below 50% during wood sawdust combustion in an 80CO2/20O2 atmosphere. The addition dosage of natural limestone had significant effects on the capture efficiency for Cu and Pb, but had no significant effects for Cr and Zn. When oxy-fuel combustion technology was applied, the presence of NaCl always lowered the capture efficiency of limestone for Pb, while it improved the capture performance of limestone for Cu and Zn at a higher temperature. However, capture efficiency for Cr 19


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was less affected by NaCl. The capture performance was partly improved by modifying with inorganic salts. The best pretreatment for capturing Cu, Zn, Pb and Cr were modified limestone with Al2(SO4)3 (γ=15), Al2(SO4)3 (γ=10), Al2(SO4)3 (γ=10) and K2CO3 (γ=10). The maximum capture efficiency was 83.20% for Cr, 82.1% for Cu, 32.1% for Pb and 55.9% for Zn. None of the modified limestone solution reconciled the adsorption demands of all the tested heavy metals, therefore, choosing a reasonable modification should depend on specific targeted heavy metals.

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Acknowledgement

This work was supported by the Natural Science Foundation of China [51606071], Natural Science Foundation of Guangdong Province, China [2016A030310424], Fundamental Research Funds for the Central Universities, China Postdoctoral Science Foundation funded project [2015M582382], China Postdoctoral Science Foundation specific funded project [2016T90781], and Guangdong Key Laboratory of Efficient and Clean Energy Utilization [2013A061401005].

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22. Laursen, K.; And, J. R. G.; Lim, C. J., Enhancement of the Sulfur Capture Capacity of Limestones by the Addition of Na2CO3 and NaCl. Environmental Science & Technology 2001, 35, (21), 4384-4389. 23. Wang, C.; Shen, X.; Xu, Y., Investigation on sulfation of modified Ca-based sorbent. Proceedings of the Csee 2002, 79, (2), 121-133. 24. Chen, C.; Ye, Z.; Wang, C., Enhancement of direct sulfation of limestone by Na2CO3 addition. Fuel Processing Technology 2009, 90, (7–8), 889-894. 25. Tang, Y.; Ma, X.; Yu, Q.; Zhang, C.; Lai, Z.; Zhang, X., Heavy metal enrichment characteristics in ash of municipal solid waste combustion in CO2/O2 atmosphere. Waste Management 2015, 43, 247-254. 26. Ke, C.; Ma, X.; Tang, Y.; Zheng, W.; Wu, Z., The volatilization of heavy metals during co-combustion of food waste and polyvinyl chloride in air and carbon dioxide/oxygen atmosphere. Bioresource Technology 2017, 244, (Pt 1), 1024-1030. 27. Ming-Yen, W.; Jiann-Harng, H.; Chen, J. C., The behavior of heavy metal cr, pb and cd during waste incineration in fluidized bed under various chlorine additives. Journal of Chemical Engineering of Japan 1996, 29, (3), 494-500. 28. Chen, C.; Zhao, C.; Liang, C.; Pang, K., Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere. Fuel Processing Technology 2007, 88, (2), 171-178. 29. Chen, J. C.; Wey, M. Y.; Lin, Y. C., The adsorption of heavy metals by different sorbents under various incineration conditions. Chemosphere 1998, 37, (13), 2617-2625. 30. Chen, J. C.; Wey, M. Y.; Liu, Z. S., Adsorption mechanism of heavy metals on sorbents during incineration. Journal of Environment Engneering 2001, 127, (1), 63-69. 31. Peng, T. H.; Lin, C. L.; Wey, M. Y., Development of a low-temperature two-stage fluidized bed incinerator for controlling heavy-metal emission in flue gases. Applied Thermal Engineering 2014, 62, (2), 706-713. 32. Peng, W.; Liu, Z.; Motahari-Nezhad, M.; Banisaeed, M.; Shahraki, S.; Beheshti, M., A detailed study of oxy-fuel combustion of biomass in a circulating fluidized bed (CFB) combustor: Evaluation of catalytic performance of metal nanoparticles (Al, Ni) for combustion efficiency improvement. Energy 2016, 109, 1139-1147. 24


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44. Ho, T. C.; Chuang, T. C.; Chelluri, S.; Lee, Y.; Hopper, J. R., Simultaneous capture of metal, sulfur and chlorine by sorbents during fluidized bed incineration. Waste Management 2001, 21, (5), 435-441. 45. Samaksaman, U.; Peng, T. H.; Kuo, J. H.; Lu, C. H.; Wey, M. Y., Thermal treatment of soil co-contaminated with lube oil and heavy metals in a low-temperature two-stage fluidized bed incinerator. Applied Thermal Engineering 2015, 93, 131-138. Table Captions

Table 1 Ultimate and proximate analysis of wood sawdust.

Table 2 Pore characteristics of natural and modified limestone after 15min combustion in an 80CO2/20O2 atmosphere.

Table 3 Results of two-factor analysis of variance.

Figure Captions

Fig.1 Capture efficiency of natural limestone at different temperatures during wood sawdust combustion in an 80CO2/20O2 atmosphere.

Fig.2 Capture efficiency of natural limestone at different addition dosage during wood sawdust combustion in an 80CO2/20O2 atmosphere.

Fig.3 Capture efficiency of natural limestone during wood sawdust combustion in the presence of NaCl in an 80CO2/20O2 atmosphere.

Fig.4 Effects of NaCl on the capture efficiency of natural limestone during wood sawdust combustion in an 80CO2/20O2 atmosphere. 26


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Fig.5 Capture efficiency of natural and modified limestone during wood sawdust combustion at 800₂ in an 80CO2/20O2 atmosphere.

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Figure

Fig.1 Capture efficiency of natural limestone at different temperatures during wood combustion in an 80CO2/20O2 atmosphere.

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Energy & Fuels

(a) Capture efficiency for Cr.

(b) Capture efficiency for Cu.

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(c) Capture efficiency for Pb.

(d) Capture efficiency for Zn. Fig.2 Capture efficiency of natural limestone at different addition dosage during wood sawdust combustion in an 80CO2/20O2 atmosphere. 30


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Energy & Fuels

Fig.3 Capture efficiency of natural limestone during wood sawdust combustion in the presence of NaCl in an 80CO2/20O2 atmosphere.

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Fig.4 Effects of NaCl on the capture efficiency of natural limestone during wood sawdust combustion in an 80CO2/20O2 atmosphere.

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Energy & Fuels

Fig.5 Capture efficiency of natural and modified limestone during wood sawdust combustion at 800₂ in an 80CO2/20O2 atmosphere.

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Table Table 1 Ultimate and proximate analysis of wood sawdust. Materials Ultimate analysis (wt%) C

H

Oa

N

S

wood 46.85 6.57 46.22 0.02 0.34 sawdust a By difference.

Proximate analysis (wt%) Volatile Fixed Ash matter carbon 82.31

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12.79

4.99

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Energy & Fuels

Table 2 o Pore characteristics of limestone combustion at 800 C under an 80CO2/20O2 atmosphere Specific surface Total pore volume Ion Mean pore Material -6 ratio (*10 L/g) diameter (nm) area (m²/g) Natural limestone Limestone modified with NaCl Limestone modified with K2CO3 Limestone modified with Al2(SO4)3

0

0.627

0.174

11.112

10

1.762

0.378

8.584

10

0.646

0.235

14.529

15

0.71

0.217

12.246

10

11.317

4.81

17.002

15

6.245

2.572

16.471

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Table 3 Results of two-factor analysis of variance. Heavy Different source SSa dfb metal Temperature 585.43 3 Addition dosage 34.55 2 Cr Error 58.26 6 Total 678.24 11 Temperature 4194.84 3 Addition dosage 1938.76 2 Cu Error 393.67 6 Total 6527.27 11 Temperature 2273.42 3 Addition dosage 385 2 Pb Error 101.04 6 Total 2759.46 11 Temperature 4131.39 3 Addition dosage 33.41 2 Zn Error 75.02 6 Total 4239.82 11 a SS represented sum of squares of deviations; b Df represented degree of freedom; c MS represented mean square deviation.

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MSc

F

P-value

195.14 17.28 9.71

20.1 1.78

0 0.25

1398.28 969.38 65.61

21.31 14.77

0 0

757.81 192.5 16.84

45 11.43

0 0.01

1377.13 16.71 12.5

110.15 1.34

0 0.33

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Heavy Metal Control by Natural and Modified Limestone during Wood

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