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Arsenic release and transformation in co-combustion of biomass and coal: Effect of mineral elements and volatile matter in biomass
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Arsenic release and transformation in co-combustion of biomass and coal: Effect of mineral elements and volatile matter in biomass ⁎
Tao Wanga, , Qin Yanga, Yinghao Wanga, Jiawei Wanga, Yongsheng Zhanga, Wei-Ping Pana,b a b
Key Laboratory of Power Station Energy Transfer Conversion and System, Ministry of Education, North China Electric Power University, Beijing 102206, PR China ICSET Solutions, Bowling Green, KY 42104, USA
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Co-combustion Arsenic release Mineral elements Volatile matter
After the co-combustion of tobacco stem/black bean straw/wheat straw/millet straw/corn stalk/rice straw and coal, it was found that all tested biomass in this study could inhibit arsenic release, but only rice straw promoted arsenic release. When the acid washed biomass was mixed with coal during combustion, the release of arsenic increased. When mineral metals (Na, K, Mg, Ca, Al and Fe) and Si elements were added to the coal, the mineral metals inhibited arsenic release. However, the release of arsenic was increased when the silicon content in biomass was high. The volatiles in the biomass also promoted the release of arsenic during co-combustion. The arsenic in the ash generated from co-combustion was mainly in the sulphide-bound state. Co-combustion of biomass and coal reduced the occurrence of an exchangeable state in the ash, and also significantly reduce the possibility of leaching.
1. Introduction Arsenic (As), a hazardous air pollutant, has received attention due to the risk of toxicity to both human health and the environment (AlAbed et al., 2008; Luo et al., 2019). The combustion of fossil fuel such as coal and petroleum become is the main source of air pollutants (Wang et al., 2018). In 2010, atmospheric emissions of As from primary anthropogenic sources was 2322 tons in China, and coal combustion accounted for 73.8% of this (Tian et al., 2015). The pyrolysis behaviour ⁎
of arsenic has a relationship with the type of coal and the arsenic distribution, and the volatility of arsenic is based on its thermal and chemical stability (Guo et al., 2004). Zhou et al. (2017) studied the effect of ash composition on arsenic release in a coal combustion system, and found that Fe, Ca, Mg, Al, Na and K in fly ash favour arsenic retention, while silicon reacts with Al, K and Ca to form KAlSi2O8, KAlSi2O6, KAlSiO4, CaSiO3, which exhibited a negative correlation with arsenic retention. Calcium compounds are considered to be effective inhibitors of arsenic release (Jadhav and Fan, 2001; Mahuli et al.,
Corresponding author. E-mail address: [email protected] (T. Wang).
https://doi.org/10.1016/j.biortech.2019.122388 Received 27 September 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tao Wang, et al., Bioresource Technology, https://doi.org/10.1016/j.biortech.2019.122388
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1998). Contreras et al. (2009) used HSC-Chemistry 5.0 software to evaluate the effect of minerals (Fe, Al, Na, K, Mg and Ca) on arsenic retention in thermodynamic studies. Biomass such as energy crops and agricultural waste are sustainable energy sources for future energy demands, and CO2 generated during combustion process of biomass is derived from plant photosynthesis (Wei et al., 2019). Biomass is regarded as an excellent resource for power generation. The co-utilisation of biomass with coal is a promising technology since it can reduce greenhouse gas emissions. It is estimated that CO2 emissions from coal-fired power plants will be reduced to 450 million ton/year when biomass content is added to 10% (Lee et al., 2019; Sahu et al., 2014). In the co-combustion system, Liu et al. (2016) found that the high volatility in the lignite also promoted the release of arsenic in a study on the arsenic release behaviours of co-combustion of lignite and bituminous coal. Lundholm calculated the distributions of arsenic by FactSage software (Karin et al., 2007), and found that Ca3(AsO4)2 was the major identified phase in the co-combustion of wood and peat. Zhou et al. (2016) found that the addition of biomass could inhibit the release of trace elements during coal combustion, and the inhibitory effect had a relationship with the biomass species and the addition ratio of biomass samples. Therefore, in the co-combustion of biomass and coal, few researchers have studied the effect of mineral elements in biomass on arsenic release. In addition, the volatile matter of biomass is usually higher than that of coal, and more experimental study is needed to determine the effect of volatiles on arsenic release with the addition of biomass to coal combustion. After co-combustion, certain forms of As in ash could leach out when the ash is exposed to water. Thus, studying the speciation of As in ash is essential to evaluate the risk of arsenic release into the natural environment. In this study, six kinds of biomass samples from different provinces of China were selected to study the effect of metal oxides and volatile matter of biomass on arsenic release. The forms of arsenic distribution in the ash were further investigated in the combustion with and without biomass. The arsenic content of biomass samples, coal samples and the co-combustion residue samples were digested and analysed by an atomic fluorescence spectrometer to determine the arsenic transformation behaviour and mechanism during the co-combustion process.
Table 2 The content of element. Sample
WS RS MS CS BS TS WS-H RS-H MS-H CS-H BS-H TS-H
The content of element (mg/g) Si
Na
Mg
Al
Ca
Fe
K
Mineral metals
52.97 82.04 54.07 20.09 5.12 0.44 44.83 78.73 58.20 20.81 5.17 0.48
2.38 0.26 1.55 0.68 0.42 0.47 1.54 0.10 0.96 0.53 0.14 0.36
3.11 1.61 5.64 3.59 6.42 5.78 0.71 0.22 0.53 0.41 0.27 0.19
7.00 0.77 5.22 2.18 1.06 0.11 3.98 0.51 4.49 2.00 0.69 0.10
11.64 4.28 12.87 8.96 16.60 28.23 0.69 0.99 0.46 0.41 0.29 0.76
5.19 0.81 3.91 1.69 0.94 0.20 1.85 0.22 2.88 1.08 0.46 0.19
18.51 23.69 21.46 17.08 16.43 39.78 0.36 0.27 0.33 0.71 0.37 0.25
47.83 31.42 50.65 34.18 41.87 74.57 9.13 2.31 9.65 5.14 2.22 1.85
Mineral metals = Na + Mg + Al + Ca + Fe + K.
is less than 1 ppm arsenic in the biomass in contrast to that of 69 ppm in the coal sample. The mineral metal content of raw biomass and acid washed biomass is listed in Table 2. As for acid treated biomass, 3 g of biomass was placed in a centrifuge bottle, then 100 mL of hydrochloric acid (1 mol/L) was added. After shaking for 8 h, the mixed solution was filtered and washed by deionised water until the pH remained unchanged (around 7). The treated biomass was dried at 50 °C in the oven for 12 h. The total amount of mineral metals in acid washed biomass dropped to 2–19% of that without acid washing. The Si oxides were almost unchanged after the acid wash.
2.2. Combustion testing In the combustion experiment, the air flow in the tube furnace (φ50 * L1000mm) was 1.0 L/min as this flow rate can eliminate the influence of external diffusion. One gram of sample was used for each biomass or coal single combustion test. The blending test included 1 g of coal and 1 g of biomass. The sample was placed in a corundum boat, and after the furnace temperature rose to the corresponding temperature at 900 °C for 10 min, the corundum boat was quickly pushed into the furnace to simulate boiler conditions. The heating rate is the most important factor to control the release of volatile organic compounds (VOCs) and other pollutants (Cheng et al., 2018). After 30 min, the corundum boat was taken out and cooled. The ash was weighed and tested. Experiments under the same working conditions were averaged three times to determine the experimental errors. The post-combustion ash sample was digested using aldrin digestion. The concentration of arsenic in samples were measured using an atomic fluorescence spectrometer (PSA10.055 Millennium Excalibur by PS Analytical). The mineral elements content in biomass was digested with in a microwave system (Speed Wave MWS-4, German). The content of mineral elements in the sample was measured by ICP-AES. TA Instruments Q600 SDT thermogravimetric analysis was also used to determine the best pyrolysis temperature to minimise the loss of mineral elements.
2. Material and methods 2.1. Sample description Six biomass samples and one coal sample were selected for the experiment. The biomass samples included wheat straw from Shanxi Province (WS), millet straw from Shanxi Province (MS), corn stalks from Shanxi Province (CS), black bean straw from Shanxi Province (BS), rice straw from Anhui Province and tobacco stems from Henan Province (TS). The coal sample was a high-sulphur coal from the Kaiyuan coal mine in Yunnan Province. Samples were air-dried, crushed and sieved to 120–150 μm. The ultimate/proximate analysis and arsenic content of samples is shown in Table 1. The volatile content for six biomasses is between 60 and 70% and for coal is around 40%. There Table 1 The ultimate/proximate of sample. Sample
WS MS CS BS RS TS Coal
Ultimate (AD, %)
Proximate (AD, %)
As(ug/g)
Moi
Vol
FC
Ash
C
H
O
N
S
7.34 7.64 8.43 8.51 7.85 9.36 4.59
69.45 66.74 67.64 70.09 63.70 59.80 39.59
16.22 16.06 16.88 15.13 14.89 16.01 41.64
6.99 9.56 7.05 6.27 13.56 14.83 14.18
40.70 39.62 40.90 40.43 38.50 34.09 44.74
5.84 5.86 6.47 6.23 5.50 5.60 5.33
45.05 45.05 42.61 44.65 40.74 42.88 32.94
1.25 1.71 2.61 2.29 1.49 2.47 1.23
0.17 0.21 0.36 0.13 0.21 0.13 1.58
2
0.59 0.52 0.36 0.29 1.07 0.15 68.75
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The speciation of arsenic in ash was also determined using the sequential chemical extraction method (Tessier et al., 1979). Arsenic in the ash was divided into exchangeable, sulphide-bound, organic-bound and residual fractions. The extraction procedures are listed in Supplementary material. The exchangeable state of arsenic is likely to be released during the leaching process, while the other three forms of arsenic are more stable.
Therefore, not all the biomass co-combusted with coal showed the inhibition of arsenic release. Thus, the effect of mineral in the biomass on the release of arsenic requires further study. The biomass was treated by an acid wash, and then the acid washed biomass was mixed with the coal sample.
2.3. Data analysis
As shown in Fig. 2(a), the release of arsenic in co-combustion of biomass with acid washed and coal was significantly higher than that of co-combustion of biomass and coal. After the acid wash, 81–98% of the mineral metals in the biomass was removed, as shown in Table 2. This shows that mineral metals can inhibit arsenic volatilisation during the co-combustion of mixtures. However, in order to study which element had the most significant effect on the inhibition of arsenic release, the additional elements Ca, Mg, Al, Fe and Si were added to the coal to study the effect of metals on the emission of arsenic. The elements Na and K were replaced by the corresponding nitrates. Table 3 lists the mass fraction of the added elements in the coal and the mass fraction of the corresponding reagents in the coal. The release of arsenic was assessed after the combustion of individual mineral elements added to coal (the mass of coal was 1 g). The arsenic release from coal at 900 °C was 17.36 μg/g, as shown in Fig. 2(b). When mineral metals such as Na, Mg, Al, Ca, Fe and K were added to the coal, the concentration of arsenic release was reduced to different degrees compared with the combustion of coal only. After the addition of calcium, the concentration of arsenic release during combustion was the lowest at 12.27 μg/g, and the retention rate reached 29.30%. Jadhav and Fan (2001) and Mahuli et al. (1998) indicated that calcium compounds are considered to be effective inhibitors of arsenic release. The amount of arsenic release after the addition Na, Mg, Al, Fe and K element was 14.71 μg/g, 14.43 μg/g, 13.46 μg/g, 13.77 μg/g and 14.76 μg/g, respectively. This shows that different kinds of mineral metals have different abilities to inhibit the release of arsenic. The inhibition of arsenic release during coal combustion from this study was in the order calcium, aluminium, iron, sodium, magnesium and potassium. When silicon was added, the retention rate was −12.4%, as shown in Fig. 2(c), indicating that Si promoted the release of arsenic. This result is in contrast to the other elements in this study. Calcium had an obvious inhibitory effect on arsenic in the combustion. It has been found that calcium-based materials have obvious effects on the removal of arsenic. In combustion, CaO reacts with arsenic oxides to form stable arsenate. Zhou et al. (2017) calculated by thermodynamic equilibrium that Ca3(AsO4)2 is the most likely form at 800 to 1200 °C. Most of the arsenic in coal exists in the form of pyrite. At 900 °C, Fe2O3 plays a catalytic role in the reaction process, which can promote the formation of FeAsO4. The most likely form of MgO reaction with arsenic oxides at this temperature is Mg3(AsO4)2. Na and K elements can form K3AsO4, KH2AsO4, KAs3O8, Na3AsO4, NaH2AsO4 and NaAs3O4 with arsenic oxides during combustion. However, K3AsO4 and Na3AsO4 are the most likely forms under constant temperature combustion conditions at 900 °C (Guo et al., 2004; Jadhav and Fan, 2001; Karin et al., 2007; Robinson et al., 1998; Zhou et al., 2017). On the other hand, silicon promotes the conversion of solid phase arsenic oxides (As2O3, As2O4 and As2O5) to gas phase arsenic (AsO, AsH and As2). Then, silicon reacts with Al, K and Ca to form KAlSi2O8, KAlSi2O6, KAlSiO4 and CaSiO3 (Zhou et al., 2017). Thus, the silicon reacts with Al, K and Ca much faster than the reaction between arsenic with Al, K and Ca. This is the main reason why a higher silicon content causes more arsenic release during coal combustion. As shown in Fig. 2(d), the contents of Ca and K in the TS was 28.23 mg/g and 39.78 mg/g, which were much higher than the other six biomasses. TS had the lowest silicon content of the seven biomass samples. Thus, a higher calcium and potassium content with a lower silicon content in the TS sample directly led to the lowest arsenic release in this study. The content of Ca in the BS was 16.60 mg/g, which is
3.2. Effect of mineral elements in the biomass on arsenic release
The arsenic content in the mixed samples and ash samples was used to determine the volatility of arsenic. Msample refers to the arsenic content in initial raw mixed samples, μg/g; Cash represents the arsenic content in ash obtained at temperature 900 °C, μg/g; and η is the corresponding ash yield per mass unit at cited temperature; Mash refers to the amount of arsenic in ash which is denoted on the basis of the initial mixed samples mass, μg/g. Thus, Mash could be represented as
Mash = Cash × η
(1)
Mreleasee refers to the amount of arsenic release during combustion which is denoted on the basis of the initial mixed samples mass, μg/g. M release could be represented as
Mrelease = Msample − Mash
(2)
The retention rate could be represented as
RE =
Vc − Va × 100% Vc
(3)
Vc refers to arsenic release during coal combustion, Va refers to arsenic release during coal combustion with addition of biomass or mineral element. 3. Results and discussion 3.1. Effect of biomass on arsenic release during coal combustion The release arsenic in co-combustion of biomass and coal was shown in Fig. 1. The concentration of arsenic release (M release) from the co-combustion of TS and coal was 5.67 μg/g. The total of arsenic release from combustion of TS and coal separately was 8.74 μg/g. This indicates that the mixture combustion of TS and coal inhibited the release of arsenic. The same results were observed in the case of the mixtures of BS, WS, MS and CS with coal. However, the arsenic release during the co-combustion of RS and coal was 9.64 μg/g, slightly higher than that of the single combustion of biomass and coal, indicating that the cocombustion of RS and coal did not inhibit the release of arsenic.
Fig. 1. The arsenic release in coal combustion and co-combustion of biomass. 3
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Fig. 2. Effect of acid washed biomass on arsenic release (a), effect of mineral elements on arsenic release (b) and retention rate (c), the content of mineral elements of biomass (d).
lower than that of TS. The content of Mg was 6.42 mg/g, which was the highest of the seven biomass samples, and the content of K was 16.43 mg/g, which was the average of the seven biomass samples. The content of Si in BS was 5.12 mg/g, which was much lower than that of WS, CS, MS and RS. This can well explain the excellent inhibitory effect on arsenic release during the co-combustion of coal and BS. The content of Ca and Mg in WS, MS and CS was much lower than that of TS and BS, but their contents of Na, Al and Fe were higher than those of TS and BS. The contents of Na, Al and Fe in WS, MS and CS were mostly within 5.00 mg/g. At the same time, the content of Si in these three biomasses was much higher than in TS and BS. This may be the reason why the inhibition of arsenic release by WS, CS and MS was less than that of TS and BS. The total content of the mineral metals in RS was 31.42 mg/g,
Table 3 The mass fraction of the added elements in the coal. Element
Quality Score
Reagent
Quality Score of Reagent
K Al Mg Fe Ca Si Na
5% 5% 5% 5% 5% 5% 5%
KNO3 Al2O3 MgO Fe2O3 CaO SiO2 NaNO3
12.9% 9.4% 8.3% 7.1% 7.0% 10.7% 18.5%
Fig. 3. The relationship between the content of mineral elements of biomass and arsenic release during co-combustion. 4
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compounds. All samples exhibited similar mass losses except TS, which had less weight loss compared with the others before 400 °C, indicating that its volatile matter content was the least; these results are consistent with the ultimate analysis. Thus, the pyrolysis temperature was set at 400 °C to remove the volatile matter in the samples. The biomass samples after heat treatment were called biochar, and 1 g of coal mixed with biochar derived from 1 g of biomass in this study. Fig. 4(c) shows that the arsenic release concentration decreased by 7.6%, 12.4%, 13.4%, 15.6%, 9.0% and 19.1% in the co-combustion of TS/BS/WS/MS/CS/RS biochar and coal, indicating that the volatiles in the biomass promote the release of arsenic during co-combustion. As shown in Fig. 4(d), the calorific value was enhanced by 37% during the co-combustion of coal and biomass and the decomposition process of arsenic-containing minerals was promoted due to the large amount of heat from the combustion of volatiles to accelerate combustion process (Li, 2013; Liu et al., 2006). Furthermore, the process of arsenic release was advanced.
while that of Si was as high as 82.04 mg/g. This may be the reason for the worst arsenic inhibitory effect in the case of RS. The relationship between the content of mineral elements and arsenic release is presented in Fig. 3. It is clear that arsenic release is controlled by the combination of mineral metals and silicon. The content of arsenic release decreased with an increasing mineral metals content, while it increased with a high Si content. The mineral metals have the capability to react with arsenic, but silicon will consume the mineral metals, which serve as active sites for arsenic gas formation. Thus, the competition between the two reactions will determine the release of arsenic. In addition to the mineral metals and silicon content in the biomass and coal, it is believed that the volatile contents also play an important role. Thus, the effect of volatile matter in the biomass on the release of arsenic in mixed samples is presented the following section.
3.3. Effect of volatile matter in the biomass on arsenic release The volatile matter content in the six kinds of biomass was as around 60–70%. In order to explore whether the volatiles in the biomass promote arsenic release during co-combustion, the effect of the volatiles content in biomass on the release of arsenic was also studied. However, when the volatiles were removed from the biomass, the mineral elements may also escape during the pyrolysis process. In order to minimise the effects of the escape of mineral elements, it was necessary to reduce the pyrolysis temperature of the biomass as much as possible. The mass loss as function of temperature (TG and DTG (derivative of TG)) is shown in the Fig. 4(a) and (b). The first mass loss around 100 °C is attributed to the loss of moisture, the second one is attributed to the release of volatile matter before 400 °C and the last peak at a high temperature around 800 °C may be the decomposition of salt
3.4. Effect of biomass on As speciation in the ash The speciation of arsenic in the samples may also play an important role for the release of arsenic during combustion. In order to study the changes in the forms of arsenic in co-combustion ash and coal ash. Coal ash was generated from 2 g of coal after combustion. The co-combustion ash was obtained from 4 g of biomass and coal co-combustion. It can be seen in Fig. 5 that the arsenic in the seven kinds of ash was mainly in the sulphide-bound state (F2) since arsenic has a high affinity for sulphides and arsenic is combined with pyrite (Fang et al., 2014; Yudovich and Ketris, 2005). Coal ash exhibited the highest content of exchangeable state (F1) at 3.4%. The exchangeable state was reduced to
Fig. 4. TG (a) and DTG (b) results of seven kinds of biomass in N2 atmosphere, effect of volatile matter on arsenic release (c) and calorific value (d). 5
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combustion ash. The exchangeable state of arsenic in ash decreased after co-combustion with six biomasses, indicating that the co-combustion of coal and biomass reduces the risk of arsenic release from ash into the natural environment. CRediT authorship contribution statement Tao Wang: Conceptualization, Methodology, Writing - original draft, Funding acquisition. Qin Yang: Resources, Supervision. Yinghao Wang: Validation, Formal analysis. Jiawei Wang: Investigation. Yongsheng Zhang: Project administration, Funding acquisition. WeiPing Pan: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 5. Evolution of As speciation in ash (TS, BS, WS, MS, CS and RS represent the co-combustion of TS, BS, WS, MS, CS and RS with coal).
Acknowledgements
less than 1% during the co-combustion of biomass. The sulphide-bound state decreased after the addition of biomass, except for TS and BS cocombustion. As shown in Table 1, the volatile matter in biomass was at least 50% more than that in coal, For example the volatile matter for BS and coal is 70.09% and 39.59%, respectively. Volatile matter combustion produces large amounts of heats, which promoted the decomposition of arsenic-containing minerals in the sulphide-bound state. However, the sulphide-bound state increased from 90.4% in coal ash to 93.9% in TS co-combustion and 91.6% in BS co-combustion. There was no significant residual state (F4) in the co-combustion of TS/BS and coal. This may be attributed to the arsenic oxide in the gas phase that combined with the Ca in TS/BS to form sulphide-bound arsenic during the co-combustion process. On the other hand, the residue state was also converted into the sulphide-bound state (Zeng et al., 2001). The content of the residual state sharply dropped, which may be related to the high calcium content in TS and BS. Calcium is often used as a desulphurising agent, and it has a strong affinity for sulphur. Therefore, calcium, with the highest content in TS and BS, reacts with the sulphur on the surface of the residual state and promotes the conversion of the residual state to the sulphide-bound state (F2) during co-combustion. Arsenic incorporated into silicates is considered to be a residual state (Keon et al., 2001), as shown in Table 2. The content of silicon in TS and BS was only 0.44 and 5.12 mg/g, respectively. WS, MS, CS and RS had a high silicon content ranging from 20 to 82 mg/g; RS had the highest silicon content of 82.04 mg/g. Thus, co-combustion of coal with RS led to the highest residual arsenic content of 6.2% in the ash. From these results, the mineral metal content and the silicone content play the most important roles in the release of arsenic during co-combustion. The retention of arsenic in ash is in the form of the sulphide-bound state. The volatile content also plays a second important role. The heat generated from the combustion of volatiles is another factor that enhances the release of arsenic in the gas phase. The decrease in the exchangeable state during co-combustion may also significantly reduce the risk of arsenic leaching during the landfilling of ash.
This work was supported by National Natural Science Foundation of China (51706069), China Shenhua Research Project (SHGF-17-87). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122388. References Al-Abed, S.R., Jegadeesan, G., Scheckel, K.G., Tolaymat, T., 2008. Speciation, characterization, and mobility of As, Se, and Hg in flue gas desulphurization residues. Environ. Sci. Technol. 42 (5), 1693–1698. Cheng, J., Zhang, Y., Wang, T., Xu, H., Norris, P., Pan, W.P., 2018. Emission of volatile organic compounds (VOCs) during coal combustion at different heating rates. Fuel 225, 554–562. Contreras, M.L., Arostegui, J.M., Armesto, L., 2009. Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations. Fuel 88 (3), 539–546. Fang, T., Liu, G., Zhou, C., Sun, R., Chen, J., Wu, D., 2014. Lead in Chinese coals: distribution, modes of occurrence, and environmental effects. Environ. Geochem. Health 36 (3), 563–581. Guo, R., Yang, J., Liu, Z., 2004. Thermal and chemical stabilities of arsenic in three Chinese coals. Fuel Process. Technol. 85 (8), 903–912. Jadhav, R.A., Fan, L.S., 2001. Capture of gas-phase arsenic oxide by lime: kinetic and mechanistic studies. Environ. Sci. Technol. 35 (4), 794–799. Karin, L., Dan, B.M., Anders, N., Andrei, S., 2007. Fate of Cu, Cr, and As during combustion of impregnated wood with and without peat additive. Environ. Sci. Technol. 41 (18), 6534–6540. Keon, N.E., Swartz, C.H., Brabander, D.J., Harvey, C., Hemond, H.F., 2001. Validation of an arsenic sequential extraction method for evaluating mobility in sediments. Environ. Sci. Technol. 35 (13), 2778–2784. Lee, S., Lee, T., Jeong, S., Lee, J., 2019. Economic analysis of a 600 mwe ultra supercritical circulating fluidized bed power plant based on coal tax and biomass cocombustion plans. Renew Energy 138, 121–127. Li, C.Z., 2013. Importance of volatile–char interactions during the pyrolysis and gasification of low-rank fuels – a review. Fuel 112 (3), 609–623. Liu, H., Wang, C., Yue, Z., Huang, X., Guo, Y., Wang, J., 2016. Experimental and modeling study on the volatilization of arsenic during co-combustion of high arsenic lignite blends. Appl. Therm. Eng. 108, 1336–1343. Liu, S., Wang, Y., Yu, L., Oakey, J., 2006. Volatilization of mercury, arsenic and selenium during underground coal gasification. Fuel 85 (10–11), 1550–1558. Luo, M., Lin, H., He, Y., Li, B., Dong, Y., Wang, L., 2019. Efficient simultaneous removal of cadmium and arsenic in aqueous solution by titanium-modified ultrasonic biochar. Bioresour. Technol. 284, 333–339. Mahuli, S., Agnihotri, R., Chauk, S., Ghosh-Dastidar, A., Fan, L.S., 1998. Mechanism of arsenic sorption by hydrated lime. Environ. Sci. Technol. 31 (11), 3226–3231. Robinson, A.L., Junker, H., Buckley, S.G., Sclippa, G., Baxter, L.L., 1998. Interactions between coal and biomass when cofiring. Symposium Combust 27 (1), 1351–1359. Sahu, S.G., Chakraborty, N., Sarkar, P., 2014. Coal–biomass co-combustion: an overview. Renew. Sustain. Energy Rev. 39 (6), 575–586. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51 (7), 844–851. Tian, H.Z., Zhu, C.Y., Gao, J.J., Cheng, K., Hao, J.M., Wang, K., Hua, S.B., Wang, Y., Zhou, J.R., 2015. Quantitative assessment of atmospheric emissions of toxic heavy metals
4. Conclusion Tobacco stem, black bean straw, wheat straw, millet straw and corn stalks inhibited arsenic release in the co-combustion of coal, but rice straw promoted arsenic release. Mineral metals (Na, Mg, Al, Ca, Fe and K) in the biomass inhibited the release of arsenic during combustion, especially calcium, while Si promoted the conversion of solid phase arsenic oxides into gas phase arsenic. This is also supported by finding that the majority of the sulphide-bound state occurred in the co6
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7
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Arsenic release and transformation in co-combustion of biomass and coal: Effect of mineral elements and volatile matter in biomass ⁎
Tao Wanga, , Qin Yanga, Yinghao Wanga, Jiawei Wanga, Yongsheng Zhanga, Wei-Ping Pana,b a b
Key Laboratory of Power Station Energy Transfer Conversion and System, Ministry of Education, North China Electric Power University, Beijing 102206, PR China ICSET Solutions, Bowling Green, KY 42104, USA
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Co-combustion Arsenic release Mineral elements Volatile matter
After the co-combustion of tobacco stem/black bean straw/wheat straw/millet straw/corn stalk/rice straw and coal, it was found that all tested biomass in this study could inhibit arsenic release, but only rice straw promoted arsenic release. When the acid washed biomass was mixed with coal during combustion, the release of arsenic increased. When mineral metals (Na, K, Mg, Ca, Al and Fe) and Si elements were added to the coal, the mineral metals inhibited arsenic release. However, the release of arsenic was increased when the silicon content in biomass was high. The volatiles in the biomass also promoted the release of arsenic during co-combustion. The arsenic in the ash generated from co-combustion was mainly in the sulphide-bound state. Co-combustion of biomass and coal reduced the occurrence of an exchangeable state in the ash, and also significantly reduce the possibility of leaching.
1. Introduction Arsenic (As), a hazardous air pollutant, has received attention due to the risk of toxicity to both human health and the environment (AlAbed et al., 2008; Luo et al., 2019). The combustion of fossil fuel such as coal and petroleum become is the main source of air pollutants (Wang et al., 2018). In 2010, atmospheric emissions of As from primary anthropogenic sources was 2322 tons in China, and coal combustion accounted for 73.8% of this (Tian et al., 2015). The pyrolysis behaviour ⁎
of arsenic has a relationship with the type of coal and the arsenic distribution, and the volatility of arsenic is based on its thermal and chemical stability (Guo et al., 2004). Zhou et al. (2017) studied the effect of ash composition on arsenic release in a coal combustion system, and found that Fe, Ca, Mg, Al, Na and K in fly ash favour arsenic retention, while silicon reacts with Al, K and Ca to form KAlSi2O8, KAlSi2O6, KAlSiO4, CaSiO3, which exhibited a negative correlation with arsenic retention. Calcium compounds are considered to be effective inhibitors of arsenic release (Jadhav and Fan, 2001; Mahuli et al.,
Corresponding author. E-mail address: [email protected] (T. Wang).
https://doi.org/10.1016/j.biortech.2019.122388 Received 27 September 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tao Wang, et al., Bioresource Technology, https://doi.org/10.1016/j.biortech.2019.122388
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1998). Contreras et al. (2009) used HSC-Chemistry 5.0 software to evaluate the effect of minerals (Fe, Al, Na, K, Mg and Ca) on arsenic retention in thermodynamic studies. Biomass such as energy crops and agricultural waste are sustainable energy sources for future energy demands, and CO2 generated during combustion process of biomass is derived from plant photosynthesis (Wei et al., 2019). Biomass is regarded as an excellent resource for power generation. The co-utilisation of biomass with coal is a promising technology since it can reduce greenhouse gas emissions. It is estimated that CO2 emissions from coal-fired power plants will be reduced to 450 million ton/year when biomass content is added to 10% (Lee et al., 2019; Sahu et al., 2014). In the co-combustion system, Liu et al. (2016) found that the high volatility in the lignite also promoted the release of arsenic in a study on the arsenic release behaviours of co-combustion of lignite and bituminous coal. Lundholm calculated the distributions of arsenic by FactSage software (Karin et al., 2007), and found that Ca3(AsO4)2 was the major identified phase in the co-combustion of wood and peat. Zhou et al. (2016) found that the addition of biomass could inhibit the release of trace elements during coal combustion, and the inhibitory effect had a relationship with the biomass species and the addition ratio of biomass samples. Therefore, in the co-combustion of biomass and coal, few researchers have studied the effect of mineral elements in biomass on arsenic release. In addition, the volatile matter of biomass is usually higher than that of coal, and more experimental study is needed to determine the effect of volatiles on arsenic release with the addition of biomass to coal combustion. After co-combustion, certain forms of As in ash could leach out when the ash is exposed to water. Thus, studying the speciation of As in ash is essential to evaluate the risk of arsenic release into the natural environment. In this study, six kinds of biomass samples from different provinces of China were selected to study the effect of metal oxides and volatile matter of biomass on arsenic release. The forms of arsenic distribution in the ash were further investigated in the combustion with and without biomass. The arsenic content of biomass samples, coal samples and the co-combustion residue samples were digested and analysed by an atomic fluorescence spectrometer to determine the arsenic transformation behaviour and mechanism during the co-combustion process.
Table 2 The content of element. Sample
WS RS MS CS BS TS WS-H RS-H MS-H CS-H BS-H TS-H
The content of element (mg/g) Si
Na
Mg
Al
Ca
Fe
K
Mineral metals
52.97 82.04 54.07 20.09 5.12 0.44 44.83 78.73 58.20 20.81 5.17 0.48
2.38 0.26 1.55 0.68 0.42 0.47 1.54 0.10 0.96 0.53 0.14 0.36
3.11 1.61 5.64 3.59 6.42 5.78 0.71 0.22 0.53 0.41 0.27 0.19
7.00 0.77 5.22 2.18 1.06 0.11 3.98 0.51 4.49 2.00 0.69 0.10
11.64 4.28 12.87 8.96 16.60 28.23 0.69 0.99 0.46 0.41 0.29 0.76
5.19 0.81 3.91 1.69 0.94 0.20 1.85 0.22 2.88 1.08 0.46 0.19
18.51 23.69 21.46 17.08 16.43 39.78 0.36 0.27 0.33 0.71 0.37 0.25
47.83 31.42 50.65 34.18 41.87 74.57 9.13 2.31 9.65 5.14 2.22 1.85
Mineral metals = Na + Mg + Al + Ca + Fe + K.
is less than 1 ppm arsenic in the biomass in contrast to that of 69 ppm in the coal sample. The mineral metal content of raw biomass and acid washed biomass is listed in Table 2. As for acid treated biomass, 3 g of biomass was placed in a centrifuge bottle, then 100 mL of hydrochloric acid (1 mol/L) was added. After shaking for 8 h, the mixed solution was filtered and washed by deionised water until the pH remained unchanged (around 7). The treated biomass was dried at 50 °C in the oven for 12 h. The total amount of mineral metals in acid washed biomass dropped to 2–19% of that without acid washing. The Si oxides were almost unchanged after the acid wash.
2.2. Combustion testing In the combustion experiment, the air flow in the tube furnace (φ50 * L1000mm) was 1.0 L/min as this flow rate can eliminate the influence of external diffusion. One gram of sample was used for each biomass or coal single combustion test. The blending test included 1 g of coal and 1 g of biomass. The sample was placed in a corundum boat, and after the furnace temperature rose to the corresponding temperature at 900 °C for 10 min, the corundum boat was quickly pushed into the furnace to simulate boiler conditions. The heating rate is the most important factor to control the release of volatile organic compounds (VOCs) and other pollutants (Cheng et al., 2018). After 30 min, the corundum boat was taken out and cooled. The ash was weighed and tested. Experiments under the same working conditions were averaged three times to determine the experimental errors. The post-combustion ash sample was digested using aldrin digestion. The concentration of arsenic in samples were measured using an atomic fluorescence spectrometer (PSA10.055 Millennium Excalibur by PS Analytical). The mineral elements content in biomass was digested with in a microwave system (Speed Wave MWS-4, German). The content of mineral elements in the sample was measured by ICP-AES. TA Instruments Q600 SDT thermogravimetric analysis was also used to determine the best pyrolysis temperature to minimise the loss of mineral elements.
2. Material and methods 2.1. Sample description Six biomass samples and one coal sample were selected for the experiment. The biomass samples included wheat straw from Shanxi Province (WS), millet straw from Shanxi Province (MS), corn stalks from Shanxi Province (CS), black bean straw from Shanxi Province (BS), rice straw from Anhui Province and tobacco stems from Henan Province (TS). The coal sample was a high-sulphur coal from the Kaiyuan coal mine in Yunnan Province. Samples were air-dried, crushed and sieved to 120–150 μm. The ultimate/proximate analysis and arsenic content of samples is shown in Table 1. The volatile content for six biomasses is between 60 and 70% and for coal is around 40%. There Table 1 The ultimate/proximate of sample. Sample
WS MS CS BS RS TS Coal
Ultimate (AD, %)
Proximate (AD, %)
As(ug/g)
Moi
Vol
FC
Ash
C
H
O
N
S
7.34 7.64 8.43 8.51 7.85 9.36 4.59
69.45 66.74 67.64 70.09 63.70 59.80 39.59
16.22 16.06 16.88 15.13 14.89 16.01 41.64
6.99 9.56 7.05 6.27 13.56 14.83 14.18
40.70 39.62 40.90 40.43 38.50 34.09 44.74
5.84 5.86 6.47 6.23 5.50 5.60 5.33
45.05 45.05 42.61 44.65 40.74 42.88 32.94
1.25 1.71 2.61 2.29 1.49 2.47 1.23
0.17 0.21 0.36 0.13 0.21 0.13 1.58
2
0.59 0.52 0.36 0.29 1.07 0.15 68.75
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The speciation of arsenic in ash was also determined using the sequential chemical extraction method (Tessier et al., 1979). Arsenic in the ash was divided into exchangeable, sulphide-bound, organic-bound and residual fractions. The extraction procedures are listed in Supplementary material. The exchangeable state of arsenic is likely to be released during the leaching process, while the other three forms of arsenic are more stable.
Therefore, not all the biomass co-combusted with coal showed the inhibition of arsenic release. Thus, the effect of mineral in the biomass on the release of arsenic requires further study. The biomass was treated by an acid wash, and then the acid washed biomass was mixed with the coal sample.
2.3. Data analysis
As shown in Fig. 2(a), the release of arsenic in co-combustion of biomass with acid washed and coal was significantly higher than that of co-combustion of biomass and coal. After the acid wash, 81–98% of the mineral metals in the biomass was removed, as shown in Table 2. This shows that mineral metals can inhibit arsenic volatilisation during the co-combustion of mixtures. However, in order to study which element had the most significant effect on the inhibition of arsenic release, the additional elements Ca, Mg, Al, Fe and Si were added to the coal to study the effect of metals on the emission of arsenic. The elements Na and K were replaced by the corresponding nitrates. Table 3 lists the mass fraction of the added elements in the coal and the mass fraction of the corresponding reagents in the coal. The release of arsenic was assessed after the combustion of individual mineral elements added to coal (the mass of coal was 1 g). The arsenic release from coal at 900 °C was 17.36 μg/g, as shown in Fig. 2(b). When mineral metals such as Na, Mg, Al, Ca, Fe and K were added to the coal, the concentration of arsenic release was reduced to different degrees compared with the combustion of coal only. After the addition of calcium, the concentration of arsenic release during combustion was the lowest at 12.27 μg/g, and the retention rate reached 29.30%. Jadhav and Fan (2001) and Mahuli et al. (1998) indicated that calcium compounds are considered to be effective inhibitors of arsenic release. The amount of arsenic release after the addition Na, Mg, Al, Fe and K element was 14.71 μg/g, 14.43 μg/g, 13.46 μg/g, 13.77 μg/g and 14.76 μg/g, respectively. This shows that different kinds of mineral metals have different abilities to inhibit the release of arsenic. The inhibition of arsenic release during coal combustion from this study was in the order calcium, aluminium, iron, sodium, magnesium and potassium. When silicon was added, the retention rate was −12.4%, as shown in Fig. 2(c), indicating that Si promoted the release of arsenic. This result is in contrast to the other elements in this study. Calcium had an obvious inhibitory effect on arsenic in the combustion. It has been found that calcium-based materials have obvious effects on the removal of arsenic. In combustion, CaO reacts with arsenic oxides to form stable arsenate. Zhou et al. (2017) calculated by thermodynamic equilibrium that Ca3(AsO4)2 is the most likely form at 800 to 1200 °C. Most of the arsenic in coal exists in the form of pyrite. At 900 °C, Fe2O3 plays a catalytic role in the reaction process, which can promote the formation of FeAsO4. The most likely form of MgO reaction with arsenic oxides at this temperature is Mg3(AsO4)2. Na and K elements can form K3AsO4, KH2AsO4, KAs3O8, Na3AsO4, NaH2AsO4 and NaAs3O4 with arsenic oxides during combustion. However, K3AsO4 and Na3AsO4 are the most likely forms under constant temperature combustion conditions at 900 °C (Guo et al., 2004; Jadhav and Fan, 2001; Karin et al., 2007; Robinson et al., 1998; Zhou et al., 2017). On the other hand, silicon promotes the conversion of solid phase arsenic oxides (As2O3, As2O4 and As2O5) to gas phase arsenic (AsO, AsH and As2). Then, silicon reacts with Al, K and Ca to form KAlSi2O8, KAlSi2O6, KAlSiO4 and CaSiO3 (Zhou et al., 2017). Thus, the silicon reacts with Al, K and Ca much faster than the reaction between arsenic with Al, K and Ca. This is the main reason why a higher silicon content causes more arsenic release during coal combustion. As shown in Fig. 2(d), the contents of Ca and K in the TS was 28.23 mg/g and 39.78 mg/g, which were much higher than the other six biomasses. TS had the lowest silicon content of the seven biomass samples. Thus, a higher calcium and potassium content with a lower silicon content in the TS sample directly led to the lowest arsenic release in this study. The content of Ca in the BS was 16.60 mg/g, which is
3.2. Effect of mineral elements in the biomass on arsenic release
The arsenic content in the mixed samples and ash samples was used to determine the volatility of arsenic. Msample refers to the arsenic content in initial raw mixed samples, μg/g; Cash represents the arsenic content in ash obtained at temperature 900 °C, μg/g; and η is the corresponding ash yield per mass unit at cited temperature; Mash refers to the amount of arsenic in ash which is denoted on the basis of the initial mixed samples mass, μg/g. Thus, Mash could be represented as
Mash = Cash × η
(1)
Mreleasee refers to the amount of arsenic release during combustion which is denoted on the basis of the initial mixed samples mass, μg/g. M release could be represented as
Mrelease = Msample − Mash
(2)
The retention rate could be represented as
RE =
Vc − Va × 100% Vc
(3)
Vc refers to arsenic release during coal combustion, Va refers to arsenic release during coal combustion with addition of biomass or mineral element. 3. Results and discussion 3.1. Effect of biomass on arsenic release during coal combustion The release arsenic in co-combustion of biomass and coal was shown in Fig. 1. The concentration of arsenic release (M release) from the co-combustion of TS and coal was 5.67 μg/g. The total of arsenic release from combustion of TS and coal separately was 8.74 μg/g. This indicates that the mixture combustion of TS and coal inhibited the release of arsenic. The same results were observed in the case of the mixtures of BS, WS, MS and CS with coal. However, the arsenic release during the co-combustion of RS and coal was 9.64 μg/g, slightly higher than that of the single combustion of biomass and coal, indicating that the cocombustion of RS and coal did not inhibit the release of arsenic.
Fig. 1. The arsenic release in coal combustion and co-combustion of biomass. 3
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Fig. 2. Effect of acid washed biomass on arsenic release (a), effect of mineral elements on arsenic release (b) and retention rate (c), the content of mineral elements of biomass (d).
lower than that of TS. The content of Mg was 6.42 mg/g, which was the highest of the seven biomass samples, and the content of K was 16.43 mg/g, which was the average of the seven biomass samples. The content of Si in BS was 5.12 mg/g, which was much lower than that of WS, CS, MS and RS. This can well explain the excellent inhibitory effect on arsenic release during the co-combustion of coal and BS. The content of Ca and Mg in WS, MS and CS was much lower than that of TS and BS, but their contents of Na, Al and Fe were higher than those of TS and BS. The contents of Na, Al and Fe in WS, MS and CS were mostly within 5.00 mg/g. At the same time, the content of Si in these three biomasses was much higher than in TS and BS. This may be the reason why the inhibition of arsenic release by WS, CS and MS was less than that of TS and BS. The total content of the mineral metals in RS was 31.42 mg/g,
Table 3 The mass fraction of the added elements in the coal. Element
Quality Score
Reagent
Quality Score of Reagent
K Al Mg Fe Ca Si Na
5% 5% 5% 5% 5% 5% 5%
KNO3 Al2O3 MgO Fe2O3 CaO SiO2 NaNO3
12.9% 9.4% 8.3% 7.1% 7.0% 10.7% 18.5%
Fig. 3. The relationship between the content of mineral elements of biomass and arsenic release during co-combustion. 4
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compounds. All samples exhibited similar mass losses except TS, which had less weight loss compared with the others before 400 °C, indicating that its volatile matter content was the least; these results are consistent with the ultimate analysis. Thus, the pyrolysis temperature was set at 400 °C to remove the volatile matter in the samples. The biomass samples after heat treatment were called biochar, and 1 g of coal mixed with biochar derived from 1 g of biomass in this study. Fig. 4(c) shows that the arsenic release concentration decreased by 7.6%, 12.4%, 13.4%, 15.6%, 9.0% and 19.1% in the co-combustion of TS/BS/WS/MS/CS/RS biochar and coal, indicating that the volatiles in the biomass promote the release of arsenic during co-combustion. As shown in Fig. 4(d), the calorific value was enhanced by 37% during the co-combustion of coal and biomass and the decomposition process of arsenic-containing minerals was promoted due to the large amount of heat from the combustion of volatiles to accelerate combustion process (Li, 2013; Liu et al., 2006). Furthermore, the process of arsenic release was advanced.
while that of Si was as high as 82.04 mg/g. This may be the reason for the worst arsenic inhibitory effect in the case of RS. The relationship between the content of mineral elements and arsenic release is presented in Fig. 3. It is clear that arsenic release is controlled by the combination of mineral metals and silicon. The content of arsenic release decreased with an increasing mineral metals content, while it increased with a high Si content. The mineral metals have the capability to react with arsenic, but silicon will consume the mineral metals, which serve as active sites for arsenic gas formation. Thus, the competition between the two reactions will determine the release of arsenic. In addition to the mineral metals and silicon content in the biomass and coal, it is believed that the volatile contents also play an important role. Thus, the effect of volatile matter in the biomass on the release of arsenic in mixed samples is presented the following section.
3.3. Effect of volatile matter in the biomass on arsenic release The volatile matter content in the six kinds of biomass was as around 60–70%. In order to explore whether the volatiles in the biomass promote arsenic release during co-combustion, the effect of the volatiles content in biomass on the release of arsenic was also studied. However, when the volatiles were removed from the biomass, the mineral elements may also escape during the pyrolysis process. In order to minimise the effects of the escape of mineral elements, it was necessary to reduce the pyrolysis temperature of the biomass as much as possible. The mass loss as function of temperature (TG and DTG (derivative of TG)) is shown in the Fig. 4(a) and (b). The first mass loss around 100 °C is attributed to the loss of moisture, the second one is attributed to the release of volatile matter before 400 °C and the last peak at a high temperature around 800 °C may be the decomposition of salt
3.4. Effect of biomass on As speciation in the ash The speciation of arsenic in the samples may also play an important role for the release of arsenic during combustion. In order to study the changes in the forms of arsenic in co-combustion ash and coal ash. Coal ash was generated from 2 g of coal after combustion. The co-combustion ash was obtained from 4 g of biomass and coal co-combustion. It can be seen in Fig. 5 that the arsenic in the seven kinds of ash was mainly in the sulphide-bound state (F2) since arsenic has a high affinity for sulphides and arsenic is combined with pyrite (Fang et al., 2014; Yudovich and Ketris, 2005). Coal ash exhibited the highest content of exchangeable state (F1) at 3.4%. The exchangeable state was reduced to
Fig. 4. TG (a) and DTG (b) results of seven kinds of biomass in N2 atmosphere, effect of volatile matter on arsenic release (c) and calorific value (d). 5
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combustion ash. The exchangeable state of arsenic in ash decreased after co-combustion with six biomasses, indicating that the co-combustion of coal and biomass reduces the risk of arsenic release from ash into the natural environment. CRediT authorship contribution statement Tao Wang: Conceptualization, Methodology, Writing - original draft, Funding acquisition. Qin Yang: Resources, Supervision. Yinghao Wang: Validation, Formal analysis. Jiawei Wang: Investigation. Yongsheng Zhang: Project administration, Funding acquisition. WeiPing Pan: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 5. Evolution of As speciation in ash (TS, BS, WS, MS, CS and RS represent the co-combustion of TS, BS, WS, MS, CS and RS with coal).
Acknowledgements
less than 1% during the co-combustion of biomass. The sulphide-bound state decreased after the addition of biomass, except for TS and BS cocombustion. As shown in Table 1, the volatile matter in biomass was at least 50% more than that in coal, For example the volatile matter for BS and coal is 70.09% and 39.59%, respectively. Volatile matter combustion produces large amounts of heats, which promoted the decomposition of arsenic-containing minerals in the sulphide-bound state. However, the sulphide-bound state increased from 90.4% in coal ash to 93.9% in TS co-combustion and 91.6% in BS co-combustion. There was no significant residual state (F4) in the co-combustion of TS/BS and coal. This may be attributed to the arsenic oxide in the gas phase that combined with the Ca in TS/BS to form sulphide-bound arsenic during the co-combustion process. On the other hand, the residue state was also converted into the sulphide-bound state (Zeng et al., 2001). The content of the residual state sharply dropped, which may be related to the high calcium content in TS and BS. Calcium is often used as a desulphurising agent, and it has a strong affinity for sulphur. Therefore, calcium, with the highest content in TS and BS, reacts with the sulphur on the surface of the residual state and promotes the conversion of the residual state to the sulphide-bound state (F2) during co-combustion. Arsenic incorporated into silicates is considered to be a residual state (Keon et al., 2001), as shown in Table 2. The content of silicon in TS and BS was only 0.44 and 5.12 mg/g, respectively. WS, MS, CS and RS had a high silicon content ranging from 20 to 82 mg/g; RS had the highest silicon content of 82.04 mg/g. Thus, co-combustion of coal with RS led to the highest residual arsenic content of 6.2% in the ash. From these results, the mineral metal content and the silicone content play the most important roles in the release of arsenic during co-combustion. The retention of arsenic in ash is in the form of the sulphide-bound state. The volatile content also plays a second important role. The heat generated from the combustion of volatiles is another factor that enhances the release of arsenic in the gas phase. The decrease in the exchangeable state during co-combustion may also significantly reduce the risk of arsenic leaching during the landfilling of ash.
This work was supported by National Natural Science Foundation of China (51706069), China Shenhua Research Project (SHGF-17-87). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122388. References Al-Abed, S.R., Jegadeesan, G., Scheckel, K.G., Tolaymat, T., 2008. Speciation, characterization, and mobility of As, Se, and Hg in flue gas desulphurization residues. Environ. Sci. Technol. 42 (5), 1693–1698. Cheng, J., Zhang, Y., Wang, T., Xu, H., Norris, P., Pan, W.P., 2018. Emission of volatile organic compounds (VOCs) during coal combustion at different heating rates. Fuel 225, 554–562. Contreras, M.L., Arostegui, J.M., Armesto, L., 2009. Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations. Fuel 88 (3), 539–546. Fang, T., Liu, G., Zhou, C., Sun, R., Chen, J., Wu, D., 2014. Lead in Chinese coals: distribution, modes of occurrence, and environmental effects. Environ. Geochem. Health 36 (3), 563–581. Guo, R., Yang, J., Liu, Z., 2004. Thermal and chemical stabilities of arsenic in three Chinese coals. Fuel Process. Technol. 85 (8), 903–912. Jadhav, R.A., Fan, L.S., 2001. Capture of gas-phase arsenic oxide by lime: kinetic and mechanistic studies. Environ. Sci. Technol. 35 (4), 794–799. Karin, L., Dan, B.M., Anders, N., Andrei, S., 2007. Fate of Cu, Cr, and As during combustion of impregnated wood with and without peat additive. Environ. Sci. Technol. 41 (18), 6534–6540. Keon, N.E., Swartz, C.H., Brabander, D.J., Harvey, C., Hemond, H.F., 2001. Validation of an arsenic sequential extraction method for evaluating mobility in sediments. Environ. Sci. Technol. 35 (13), 2778–2784. Lee, S., Lee, T., Jeong, S., Lee, J., 2019. Economic analysis of a 600 mwe ultra supercritical circulating fluidized bed power plant based on coal tax and biomass cocombustion plans. Renew Energy 138, 121–127. Li, C.Z., 2013. Importance of volatile–char interactions during the pyrolysis and gasification of low-rank fuels – a review. Fuel 112 (3), 609–623. Liu, H., Wang, C., Yue, Z., Huang, X., Guo, Y., Wang, J., 2016. Experimental and modeling study on the volatilization of arsenic during co-combustion of high arsenic lignite blends. Appl. Therm. Eng. 108, 1336–1343. Liu, S., Wang, Y., Yu, L., Oakey, J., 2006. Volatilization of mercury, arsenic and selenium during underground coal gasification. Fuel 85 (10–11), 1550–1558. Luo, M., Lin, H., He, Y., Li, B., Dong, Y., Wang, L., 2019. Efficient simultaneous removal of cadmium and arsenic in aqueous solution by titanium-modified ultrasonic biochar. Bioresour. Technol. 284, 333–339. Mahuli, S., Agnihotri, R., Chauk, S., Ghosh-Dastidar, A., Fan, L.S., 1998. Mechanism of arsenic sorption by hydrated lime. Environ. Sci. Technol. 31 (11), 3226–3231. Robinson, A.L., Junker, H., Buckley, S.G., Sclippa, G., Baxter, L.L., 1998. Interactions between coal and biomass when cofiring. Symposium Combust 27 (1), 1351–1359. Sahu, S.G., Chakraborty, N., Sarkar, P., 2014. Coal–biomass co-combustion: an overview. Renew. Sustain. Energy Rev. 39 (6), 575–586. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51 (7), 844–851. Tian, H.Z., Zhu, C.Y., Gao, J.J., Cheng, K., Hao, J.M., Wang, K., Hua, S.B., Wang, Y., Zhou, J.R., 2015. Quantitative assessment of atmospheric emissions of toxic heavy metals
4. Conclusion Tobacco stem, black bean straw, wheat straw, millet straw and corn stalks inhibited arsenic release in the co-combustion of coal, but rice straw promoted arsenic release. Mineral metals (Na, Mg, Al, Ca, Fe and K) in the biomass inhibited the release of arsenic during combustion, especially calcium, while Si promoted the conversion of solid phase arsenic oxides into gas phase arsenic. This is also supported by finding that the majority of the sulphide-bound state occurred in the co6
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