Article pubs.acs.org/EF
The Characteristics of Zinc and Arsenic from Co-firing of Municipal Sewage Sludge with Biomass in a Fluidized Bed Yazhou Zhao,†,‡ Hongjuan Jia,†,‡ and Qiangqiang Ren*,†,‡ †
University of Chinese Academy of Sciences, Beijing 100049, China Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China
‡
ABSTRACT: Fly ash from the incineration of municipal sewage sludge (MSS) contains high phosphorus content and can be a source of phosphorus resource. However, the utilization is limited due to the enrichment of trace metals in the fly ash. Co-firing of MSS and biomass was proposed to reduce the leaching toxicity of trace metals in the fly ash. The present study studies two typical trace metals, arsenic (As) and zinc (Zn), present in the co-firing of MSS and biomass in a bubbling fluidized bed. The influence of biomass share (the relative amount of biomass in the mixture of biomass and MSS), biomass species, and combustion temperatures on the concentrations, speciation, and the leaching toxicity of Zn and As in fly ash was investigated. The results show that Zn is more stable after co-firing of MSS and biomass. The stability of Zn species in the fly ash was the best at the incineration temperature of 1123 K. The leaching concentration of Zn in the fly ash decreased, from a maximum of 22.53 mg/L to the minimum of 11.37 mg/L, as the amount of biomass increased. For As, the presence of minerals such as Ca and Fe in biomass can provide chemical adsorption of As and promote the capture of As by forming arsenates. The stability of As species in the fly ash was better at the lower incineration temperature. Wheat straw and cotton stalk have the same influence on the leaching concentration of As in the fly ash. Co-firing MSS and biomass can effectively reduce the leaching toxicity of Zn in the fly ash, but the leaching toxicity of As decreased only at 50% biomass.
1. INTRODUCTION The disposal methods of municipal sewage sludge (MSS) include agricultural reuse, disposal in landfill or the sea, and incineration.1,2 Limited landfill sites and increasing disposal costs have promoted the development of incineration technology for MSS in China. Incineration is considered an environmentally friendly disposal option for MSS because it can reduce the volume and the mass of MSS by 90% and 70%, respectively, as well as destroy pathogenic agents and generate thermal energy.3 Fly ash, an MSS incineration residue, has good fertilizer properties due to the high content of phosphorus and is considered an important source of secondary phosphates.4,5 However, MSS contains a large amount of toxic trace metals (Zn, As, etc.) and fly ash from incineration is contaminated with trace metals above the legal limits for agricultural use.6−9 Additionally, trace metals can leach from the fly ash to contaminate the environment.10 Thus, it is essential to reduce the toxicity of trace metals in fly ash to increase the bioavailability of phosphorus and protect farmland. One common method to address this problem is the use of mineral sorbents to capture metallic vapors through either physical adsorption or chemical reaction.11,12 Yao et al.13−15 reported that aluminosilicate-based sorbents were more effective than calcium-based sorbents for capture of Pb and Cd, and the capture ability depended on chemical compositions and specific surface area of the sorbents. Scotto et al.16 studied the ability of sorbents to capture Pb at high temperature in a down-flow combustor, suggesting that the presence of chlorine reduced the reaction rate between kaolin and trace metals. Uberoi17 reported that cadmium could react with bauxite to form aluminosilicate compounds. This method can control the emissions of trace metals from combustion and incineration © 2016 American Chemical Society
systems and realize the solidification/stabilization of trace metals by converting trace metals into less soluble or less toxic forms.18 However, the use of sorbents is not appropriate for all trace metals because capture efficiency is related to the chemical speciation of metals and sorbents. Another approach to solve this problem is thermal treatment. The behavior of trace metals during the thermo-chemical process of sewage sludge ash has been widely studied. Vogel et al.19,20 found that trace metals were effectively removed at temperatures in the range of 1073−1223 K via the gas phase by utilization of PVC and gaseous hydrochloric acid as Cl donors. Nowak et al.5 reported that up to 97% Cu, 95% Pb, and 95% Zn could be removed at 1323 K using CaCl2 or MgCl2 as Cl donors in a rotary reactor. The thermo-chemical process reduced trace metal concentrations for a fertilizer containing mineral phosphates with high bioavailability. However, this method has disadvantages because it requires Cl donors, also polluting materials, and is more expensive. Therefore, a better solution is required to reduce the toxicity of trace metals contained in fly ash. Biomass is rich in mineral elements (including Si, Ca, K, and Al) and Cl and can provide both mineral sorbents and Cl donors. However, there have been only few reports of the use of co-combustion of MSS and biomass to reduce the toxicity of trace metals contained in MSS fly ash. Additionally, the alkali metal problems do not exist during co-firing of MSS and biomass because the phosphorus (P) in MSS can react with alkali metals to form high-meltingpoint compounds like Ca10K(PO4)7.21,22 The toxicity due to Received: September 22, 2016 Revised: November 8, 2016 Published: December 2, 2016 755
DOI: 10.1021/acs.energyfuels.6b02444 Energy Fuels 2017, 31, 755−762
Article
Energy & Fuels Table 1. Proximate and Ultimate Analyses of MSS and Biomass (wt %, as Dry Basis) proximate analysis
ultimate analysis
fuel
ash
fixed carbon
volatile
C
H
O
N
S
LHV (MJ/kg)
MSS wheat straw cotton stalk
33.39 6.97 11.79
9.69 19.73 20.02
56.92 73.30 68.19
34.85 45.10 43.22
4.92 5.60 5.13
19.92 41.56 38.79
5.99 0.57 0.92
0.93 0.20 0.15
13.67 16.46 16.02
Table 2. Main Ash-Forming Elements of MSS and Biomass (wt %, as Dry Basis) Fuel
Al
Ca
Fe
K
Mg
P
Si
Na
Cl
MSS wheat straw cotton stalk
1.76 0.03 0.43
1.77 0.34 0.96
1.58 0.07 0.34
1.06 1.98 1.01
0.95 0.18 0.34
1.06 0.11 0.14
5.27 2.37 2.48
0.33 0.20 0.27
0.03 1.14 0.22
trace metals in the fly ash cannot be assessed solely by measuring the trace metal contents in the fly ash because a greater residual content does not necessarily correlate to a greater leaching concentration. Leachability is dependent on the solubility of the trace metal-related compounds formed after incineration. Development of an effective approach to reduce the toxicity of trace metals requires investigating variation in the concentrations, chemical forms, and leachability of trace metals in fly ash during the co-firing of MSS and biomass. Zn and As are more highly concentrated in fly ash due to vaporization−condensation mechanisms.23 In China, Zn concentration is typically the highest of all trace metals in raw MSS incineration residue. Oxidized Zn is a soluble and migrating microelement under acidic conditions, making it one of the most phytotoxic trace metals, and Zn leaching toxicity is included in the criteria for hazardous waste identification in many countries.24 As is greatly toxic and harmful to both human health and the environment. The present study aims to investigate the characteristics of trace metals (Zn and As) during the co-firing of MSS and biomass in a bubbling fluidized-bed combustor. The focus is the influence of temperatures and the mass ratios of biomass to MSS on the transformation of Zn and As present in the fly ash. The results of this study will be useful to operators and researchers attempting to minimize trace metal pollution during MSS incineration.
Figure 1. XRD patterns of MSS and biomass.
Table 3. Concentrations of Trace Metals in MSS and Biomass (mg/kg, as Dry Basis) Fuel
Zn
As
Cd
Pb
Cu
Cr
MSS wheat straw cotton stalk
804.67 28.45 32.82
8.40 0.69 0.57
1.17 0.12 0.13
25.32 3.55 3.84
168.90 50.61 18.68
70.11 26.58 22.12
the highest concentration, at toxic levels. The concentration of As was lower, but arsenic is extremely poisonous. Zn and As are easily enriched in fly ash.23 The characteristics of Zn and As in the fly ash were investigated. From the data shown in Tables 2 and 3, although the contents of Si, Ca, and Al in biomass were lower than those in MSS, there was little Zn and As in biomass. It could be seen from Figure 1 that the crystalline phases in MSS and biomass were different. The main phase components of MSS were SiO2, Ca9Fe(PO4)7, KAlSi3O8, MgCa2(PO4)2(H2O)2, and NaAlSi3O8. The main components of wheat straw were SiO2, KCl, K2SO4, CaAl2Si2O8, and KAl3Si3O10(OH)2. Unlike the wheat straw, the cotton stalk contained some CaO but without KAl3Si3O10(OH)2. The interactions between the minerals with different forms and trace metals were different. The biomass was chosen as a mineral additive during MSS incineration. 2.2. Apparatus and Methods. Experiments were performed in a 5 kW electrically heated bubbling fluidized-bed combustor with a height of 1750 mm and an inner diameter of 100 mm, as shown in Figure 2. Details of the apparatus and experimental process were described in the previous report.22 The bed temperature was controlled in the temperature range of 1123−1223 K. Each test was maintained for 2 h. Fly ash was sampled from the ash bucket. The temperature for the fly ash sampling in the cyclone was in the range of 753−773 K. The concentrations of Zn and As in fly ash were analyzed by the same methods as used for MSS. The speciation of Zn and As in
2. EXPERIMENTAL SECTION 2.1. Materials. The dewatered MSS used in this study was obtained from the Qinghe Wastewater Treatment Plant in Beijing. The biomass fuels were wheat straw and cotton stalk from Hebei province in north China. The three kinds of raw materials were crushed into pellets ( co-firing of MSS and 50% wheat straw > co-firing of MSS and 50% cotton stalk. The co-combustion of MSS and cotton stalk can enhance the stability of Zn in the ash better than the co-combustion of MSS
Figure 6. Influence of different biomasses on Zn and As concentrations in the fly ash from co-firing of MSS and biomass. 759
DOI: 10.1021/acs.energyfuels.6b02444 Energy Fuels 2017, 31, 755−762
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Figure 7. Influence of different biomasses on the speciation distribution of Zn and As in the fly ash from co-firing of MSS and biomass.
Figure 8. Influence of different biomasses on the leaching ratio and leaching concentration of Zn and As in the fly ash from co-firing of MSS and biomass.
reduced, as shown in Figure 9. The nonstable fractions (acidsoluble and reducible fractions) of Zn increased greatly when the incineration temperature increased from 1123 K to 1223 K (displayed in Figure 10a). At the higher temperature, the oxidizable and residual fractions of trace metals gradually decomposed to simple compounds, increasing the acid-soluble and reducible fractions.36 Therefore, the stability of Zn species in the fly ash samples was weakened as the incineration temperature increased. The leaching ratio and the leaching concentration of Zn in the fly ash samples increased as the temperature increased (Figure 11a,b), as determined by the variation of Zn speciation, as shown in Figure 10a. The leaching toxicity of Zn in the fly ash was lowest at the incineration temperature of 1123 K. 3.3.2. As in the Fly Ash. The effects of incineration temperature on the content of As in the fly ash samples are shown in Figure 9. The concentration of As in the fly ash samples increased with the rise of incineration temperatures from 1123 to 1223 K. Arsenic was predominantly present as a vapor, As2O3(g), in the incinerator at temperatures higher than 900 K.30 Related research results37,38 suggested that Ca-based compounds could react with As2O3 to form arsenates and to capture As effectively. In addition to Ca-based compounds, other inorganic compounds, such as iron oxides, could react with As2O3 to form FeAsO4. Hu et al.30 claimed that the chemical oxidation of As2O3(g) with Ca-, Fe-, or Al-based compounds facilitated the formation of various arsenates in the fly ash samples, simultaneously, and the considerable amount of As in the cyclone ash was also attributed to the rapid
Figure 9. Influence of incineration temperature on the concentrations of Zn and As in the fly ash from co-firing of MSS and wheat straw.
trace metals. From 1123 to 1223 K, the elevated temperatures reached the boiling point of some Zn compounds, like ZnCl2, and a large amount of Zn compounds evaporated into flue gas, and were converted into the condensed phase and adsorbed on the surface of fly ash particles when the flue gas temperature decreased below the melting point and boiling point of Zn compounds, resulting in an increase in Zn concentration (Figure 9). However, at the incineration temperature of 1223 K, the flue gas temperature is much higher than the boiling point of some Zn compounds such as ZnCl2, which leads that the volatile Zn compounds cannot condense on the surfaces of fly ash particles. As a result, the Zn content in the fly ash is 760
DOI: 10.1021/acs.energyfuels.6b02444 Energy Fuels 2017, 31, 755−762
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Figure 10. Influence of incineration temperature on the speciation distribution of Zn and As in the fly ash from co-firing of MSS and wheat straw.
Figure 11. Influence of incineration temperature on leaching ratio and leaching concentration of Zn and As in the fly ash from co-firing of MSS and wheat straw.
4. CONCLUSION The feasibility of using biomass to drive the harmless transformation of Zn and As from sludge incineration was investigated, and the effects of biomass blending ratio, different biomass species, and incineration temperature on the characteristics of Zn and As in the fly ash produced from co-firing of MSS and biomass were studied. The leaching toxicity of Zn and As in the fly ash was effectively reduced by co-firing MSS and biomass especially at 50% biomass. The presence of minerals (Si, Ca, and others) in biomass can provide chemical adsorption of Zn and facilitate the formation of stable Zn species from co-firing of MSS and wheat straw. The leaching toxicity of Zn in the fly ash is reduced with the cocombustion of MSS and wheat straw/cotton stalk, and the effect of cotton stalk is better. The stability of Zn species in the fly ash is weakened as the incineration temperature increases. For As in the fly ash from co-firing of MSS and biomass, the capture of As2O3(g) by forming arsenates is accelerated by the presence of minerals such as Ca and Fe in the biomass. Arsenic in the fly ash exists mainly in the form of unstable acid-soluble and reducible states. The leaching toxicity of As was the lowest at the biomass blending ratio of 50%. The wheat straw and cotton stalk showed almost the same effect on the leaching toxicity of As. The leaching toxicity of As in the fly ash increased as the incineration temperatures increased.
condensation of As2O3(g) on the surface of ash particles and/or the stabilization of As to ash matrix, such as the glass phase. Therefore, the increase of As content in the fly ash samples could be due to the enhancement of oxidation and physical adsorption of As2O3(g) at the higher temperature. As shown in Figure 10b, the As in the fly ash samples was mainly in the unstable acid-soluble and reducible fractions; the proportion was 83.87, 82.39, and 89.55% at 1123, 1173, and 1223 K, respectively. The oxidizable and residual fractions of As decomposed gradually to simple compounds at 1223 K, leading to the redistribution of speciation to acid-soluble and reducible fractions, as stated previously. As a consequence, the stability of As in the fly ash sample is highest at 1173 K. The leaching ratio of As in the fly ash samples decreased first and then increased from 1123 to 1223 K, as shown in Figure 11a, which confirms the results of Figure 10b. Moreover, combining the variation of the concentration and leaching ratio of As (Figures 9 and 11a), the variation of As leaching concentration showed the same trend as the variation of As concentration, indicating that the leaching concentration of As is mainly controlled by the concentration of As in the fly ash samples at different incineration temperatures. Measures should be taken to suppress the increase of As concentration such as by effective control of operating temperature. 761
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-82543055. Fax: +86-10-82543119. E-mail:
[email protected]. ORCID
Yazhou Zhao: 0000-0002-4460-6683 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51476169), the Youth Innovation Promotion Association, the Chinese Academy of Sciences (No. 2015120), and the International Science and Technology Cooperation Program of China (Grant No. 2014DFG61680). The authors also thank the Advanced Standards Technical Services Company Limited in Beijing for the help on the experimental measurements.
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