Fs from the co-combustion of

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Environmental and Carbon Dioxide Issues

Emission characteristics of PCDD/Fs from the co-combustion of municipal solid waste in a lab-scale drop-tube furnace Xiaoqing Lin, Zhiliang Chen, Shengyong Lu, Shaorui Zhang, Mengmei Zhang, Xiaodong Li, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00408 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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Emission characteristics of PCDD/Fs from the co-combustion of municipal solid waste in a lab-scale drop-tube furnace Xiaoqing Lin, Zhiliang Chen, Shengyong Lu*, Shaorui Zhang, Mengmei Zhang, Xiaodong Li, Jianhua Yan State Key Laboratory for Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China *Corresponding author: Lu Shengyong ([email protected])

ABSTRACT Generally, coal-fired incinerators are characterized by a high combustion temperature and a well-equipped flue gas cleaning system. Co-combusting municipal solid waste (MSW) in coal-fired incinerators can reduce the emission of PCDD/Fs and help to enhance the treatment capacity of the MSW; more importantly, this approach provides a new orientation for upgrading and transitioning old coal-fired power plants. In this study, the simulated municipal solid waste (SMSW) is mixed with coal in proportions of 0 (without addition), 7.5, 15, 20, and 25 wt.% and then co-combusted in a drop-tube furnace. The PCDD/Fs emitted in both the flue gases and fly ashes from the combustion tests are well investigated. In the present study, the I-TEQ concentration of the PCDD/Fs in the flue gas can meet the emission limit (0.1 ng I-TEQ/Nm3) when MSW is co-combusted at less than 20 wt.%. The concentration of the PCDD/Fs in the fly ash is much lower than the permitting standard for disposal in sanitary landfill sites (3.0 ng I-TEQ/g), and even the percentage of SMSW increases to 25 wt.%. Particularly, a proper ratio of SMSW in a coal (7.5 or 15 wt.%) can increase the burnout efficiency of the organic carbon and reduce the concentration of PCDD/Fs in the flue gases and fly ashes. The dominant formation pathway of the PCDD/Fs in this study is de novo synthesis. The chlorine induced by SMSW is the main factor influencing the concentration of the PCDD/Fs both in flue gas and in fly ashes. These findings are helpful for the further development of co-combustion with renewable energy in coal-fired incinerators, yet more investigation on PCDD/Fs is still required to be conducted in the future. KEYWORDS: co-combustion, PCDD/Fs, MSW, coal combustion furnace

1. Introduction During the past decade in China, the amount of municipal solid waste (MSW) 1

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kept rising at an average rate of 2.6 % per year and reached 203 million tons by the end of 2016, which makes the issue of “garbage siege” worse. How to dispose MSW in a scientific and harmless way is currently a hot topic of concern for the Chinese society. Currently, landfilling and incineration are the two main methods of MSW disposal. MSW incineration (MSWI) has developed rapidly because of its advantages, such as mass and volume reduction as well as energy recovery[1]. However, MSWI is usually accompanied by the emission of contaminants, such as polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs)[2], heavy metals[3], HCl, CO, SOx, and NOx[4]. Among these contaminants, PCDD/Fs have received much more attention due to their extremely high toxicity and have been listed in the Stockholm Convention for elimination as persistent organic pollutants (POPs) [5]. During the combustion of MSW, PCDD/Fs can be formed through the homogeneous reaction in the temperature region of 500-800 °C [6]. It is generally agreed that keeping the gas residence time longer than 2 s at a combustion temperature over 850 °C, or longer than 1 s at a combustion temperature over 1000 °C, is an effective method to prohibit the formation of the PCDD/Fs during MSW incineration [7]. In coal-fired incinerators, the combustion temperature is usually higher than 1000 °C. Therefore, co-combusting MSW in a coal-fired incinerator may help to reduce the emission of PCDD/Fs. In addition, the selective catalytic reduction (SCR) system in coal-fired systems also has the ability to destroy the PCDD/Fs in the flue gas [8]. Moreover, the China Environmental Protection Administration puts great pressure on coal combustion power plants, commanding that they complete the transformation to ultra-low emissions and energy conservation by the end of 2020. Therefore, many small or even some big coal combustion power plants have been shut down in succession. Co-combusting part of the MSW in a pulverized coal combustor can not only increase the treatment capacity of the MSW but can also provide a new orientation for the transition and upgrading of the coal combustion power plants. Unlike coal, MSW contains a higher content of heavy metals, alkali metals and chlorine, so the interaction of these two fuels requires more investigation. So far, some experiments about the co-combustion of refuse-derived fuels (RDF) and coal 2


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have been carried out, to study the energy efficiency of plants[9], high temperature chlorine-induced corrosion[10], and the release features of regular emissions[11], heavy metals[12], and organic compounds[10, 13, 14], etc. Manninen et al. found that the RDF can be co-combusted with coal up to 20 % with very low heavy metals[15] and PCDD/Fs[16] emissions. Moreover, Jin et al. [17] studied the combustion efficiency and emission characteristics of SO2, HCl, NOx, PCDD/Fs, and heavy metals from a small coal-burning system with the addition of a different ratio of MSW. The results indicated that the combustion efficiency, the emission amount of HCl in the flue gas, and the concentration of the PCDD/Fs in the residues increased with the rising addition ratio of MSW. Conversely, the emission amounts of NO and SO2 decreased; particularly, the emission amount of N2O increased at first and then declined. Molcan et al. [18] found that the combustion efficiency of a pulverized coal combustor increased after adding 5-20 wt.% of biomass. The emission of NOx was related to the stability of flame, and the emission of SO2 was reduced in all the operating conditions. Bhuiyan et al. [19] indicated that the adding ratio of wood flour and straw can reach 30 wt.% in a full-scale coal-fired power plant. Therefore, the co-combustion of MSW and coal is feasible theoretically. However, the emission characteristics of PCDD/Fs during the co-combustion of coal and MSW has not been studied in detail, and the generation mechanism of the PCDD/Fs has not been revealed in other existing studies. Therefore, the main goal of this study is to investigate the emission characteristics and the synthesis mechanism of the PCDD/Fs both in flue gas and in fly ash thoroughly as well to identify the proper addition proportion of MSW in coal-fired plants from the perspective of PCDD/Fs control. Although the high combustion temperature of a coal combustion furnace can prohibit the formation of PCDD/Fs, PCDD/Fs could still be reformed though a de novo reaction and/or precursor routes in the post-combustion zone [6]. To control the emission of PCDD/Fs and find an optimal mix ratio of MSW and coal, it is important to investigate the emission characteristics of the PCDD/Fs and furthermore to discover the mechanism and key factors of PCDD/Fs generation. In the present study, a series of experiments are designed and conducted in a drop-tube furnace, and the 3


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emission characteristics of the PCDD/Fs both in flue gas and in fly ashes are studied in detail.

2. Materials and methods 2.1 Materials The coal utilized in the experiments was a mixture of Indonesia and Shenmu coal in a ratio of 1:1. The mixed coal was ground to less than 180 µm prior to the combustion. The raw MSW may not be suitable for being used in experiments since it is not homogeneous enough. The MSW used in the present study was simulated MSW (SMSW). Zhang et al. [20] studied the composition of MSW in Shanghai City, and the results are shown in Table 1. Based on their results, the SMSW was composed of kitchen garbage, plastics, paper, fabrics, woods, metals, and non-combustible materials in the percentages shown in Table 1. The SMSW was ground to less than 90 µm and then was stored in sealed plastic bags before use. The proximate and ultimate analyses for the mixed coal and SMSW are shown in Table 2; the Cl content in both the fuels are analyzed and determined through a high-temperature combustion hydrolyzing method (GB/T 3558-2014) and an ion chromatography method (Dionex Integrion HPIC, Thermo Scientific). It is obvious that the moisture content, ash content, fixed carbon and calorific value of the SMSW are lower compared to the mixed coal; however, the O, N, S and Cl contents of the SMSW are higher. Both the samples were milled and sieved individually to obtain a particle size less than 90 µm and were then mixed for different SMSW proportion experimental conditions. Table 1. Components of MSW in Shanghai City and simulated MSW MSW in Shanghai

Kitchen garbage

Plastics

Paper

Fabrics

Woods

Metals

Non-combustible

wt. %, wet basis

42.4

23.5

21.5

7.5

3.5

0.1

1.5

wt. %, dry basis

15.7

37.2

27.0

9.8

6.2

0.3

3.8

Simulated MSW

Flour

Veggie

PVC

PE

Books

Newspaper

Nylon

Redwood

Iron

Glass

Quartz

wt. %, dry basis

7.7

8

18.6

18.6

13.5

13.5

9.8

6.2

0.3

1.9

1.9

Table 2. Proximate and Ultimate Analysis for mixed coal and simulated MSW Proximate analysis (mass %) Sample

Mad

Aad

Vad

FCad

Qnet, ad, (MJ/kg)

Ultimate analysis (mass %) C

H

O

N

S

Cl 4


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Energy & Fuels

coal Simulated MSW

5.49

12.28

30.41

56.07

27.19

63.88

4.16

16.68

0.85

0.31

0.57

1.96

8.36

54.91

34.77

19.32

41.28

5.69

40.35

1.58

0.78

2.39

Note: ad-air dried; Qnet-lower calorific value; M%-moisture content; A%-ash content; V-volatile content; FC-fixed carbon content.

2.2 Experimental procedures The experiments were carried out in a drop-tube furnace (Figure 1) with an inside diameter of 100 mm and a feeding capacity of 0.5-2 kg/h. The main part of this furnace consists of an ignition (1500 mm in length) and a burnout section (2000 mm in length). The ignition section provides suitable ignition conditions for various coals due to its temperature control up to 1000 °C and has an electrical heating power of 20 kw. The maximum temperature of the burnout section is 1500 °C, which is close to the maximum temperature of a full-scale incinerator, making it suitable for the slagging and burning out tests. The supporting instruments of this furnace include a pressure fan, an induced draft fan, an air-preheater, an air-flow controller and a screw feeder. The air-preheater can heat compressed air up to 450±5 °C and adjust the temperature as required. The feeding flow of the fuels is controlled by the screw feeder at a range of 0.5-20 kg/h. The in-blowing volume of the primary and secondary air is controlled by the air-flow controller. The temperature in the furnace is measured by nickel-chromium/nickel-silicon thermocouples at the ignition section and by platinum-10% rhodium/platinum thermocouples at the burnout section.

5


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Figure 1. Schematic diagram of drop-tube furnace

The SMSW was homogeneously mixed with the coal in percentages of 0, 7.5, 15, 20, and 25 wt.% (based on the total mass) prior to the combustion, and they were labeled as test 1, 2, 3, 4, and 5, respectively. The mixed fuels were blown into the furnace by the primary air with a feeding rate of 0.6 kg/h and then combusted at the same condition as the secondary air. The preheated air was set to 350 °C, and the total air was adjusted to ensure the flue gas oxygen content ca. 6% (primary air is approximately 20 vol.% of the total air). The temperatures of the ignition and burnout 6


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section were set at 800 and 1300 °C, respectively. The fly ash was collected from a cyclone separator. For each test, three fly ash samples were subjected to PCDD/Fs analysis. The PCDD/Fs in the flue gas were sampled by adsorption on XAD-II polymeric resin after the filtration by a filter membrane, and two bottles of toluene were placed downstream to ensure the complete collection of the PCDD/Fs. In addition, the sampling pipe was washed with toluene and then, the eluent was collected for future analysis. Finally, the eluent, the resin, and the two bottles of toluene were combined together to analyze the concentration of the PCDD/Fs in flue gas. For each test, the collection of the PCDD/Fs in the flue gas was conducted three times sequentially, to obtain three parallel samples for the subsequent analysis.

2.3 Analytical methods PCDD/Fs analysis. The cleanup procedure of the PCDD/Fs samples in the gas phase and fly ash was conducted according to USA EPA 23A [21] and USA EPA 1613 methods [22], respectively. Prior to the purification procedure, the fly ash was soaked for 4 h in 2 mol/L hydrochloric acid with a liquid to solid ratio of 40 mL/1 g in order to remove the metal salts in the fly ash. The PCDD/Fs were identified and quantified by HRGC/HRMS by using a 6890 Series gas chromatograph (Agilent, USA) coupled to a JMS-800D mass spectrometer (JEOL, Japan). A DB-5ms (60 m×0.25 mm I.D., 0.25µm film thickness) capillary column was used for separating the PCDD/Fs congeners. The detailed cleanup procedure and PCDD/Fs analysis can be found in our previous papers [23-25]. The concentration of HCl in the flue gas was analyzed with a Fourier Transform Infrared Spectroscopy (FTIR) flue gas analyzer (Gasmet, Dx4000, Finland). The concentration chlorine and other elements in the fly ashes was analyzed by X-ray fluorescence equipment (XRF, Thermoscientific ARL ADVANT’X IntelliPowerTM 4200, Wilmington, DE, USA). The concentration of C in the fly ash was measured by EDS (energy dispersive spectroscopy). 2.4 Statistics analysis The I-TEQ concentration of PCDD/Fs in the flue gas (Cg) is calculated using the 7


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formula (eq. 1). n

Cg =

Cni

∑V i =1

sd i

(1)

n

where,

Cn = the I-TEQ concentration of PCDD/F in the sample (ng); Vsd = the sampling volume of flue gas(m3); n = the number of parallel samples. The concentration of PCDD/Fs in the fly ash (Cf) is calculated as follows: n

Cf =

Cni

∑M i =1

fi

(2)

n

where,

Cn = the I-TEQ concentration of PCDD/F in the sample (ng); Mf = the mass of the fly ash subjected to PCDD/Fs analysis (g); n = the number of parallel samples. The pearson correlation coefficient (R) between variable X (x1, x2, … , xn) and Y (y1, y2, … , yn) is calculated as follows: n

n

n

n(∑ xi yi ) − (∑ xi )(∑ yi ) R=

i =1 n

i =1 n

i =1 n

(3)

n

[n∑ xi − (∑ xi ) ][n ∑ yi − (∑ yi ) ] 2

i =1

2

i =1

2

i =1

2

i =1

3. Results and discussion 3.1 Concentration of HCl in the flue gas and the elementary composition of fly ashes For the generation of the PCDD/Fs, the main pathway could be simply speculated by the ratio of PCDFs to PCDDs: if the ratio is bigger than 1, the main pathway is more likely to be a de novo reaction; otherwise, the generation from precursors plays a leading role[26]. In the post-combustion zone of the furnace, the 8


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PCDD/Fs can be formed through a de novo reaction, and it has been considered to be a major formation pathway of the PCDD/Fs[6, 27]. For the de novo reaction, some key factors have been found, including the element sources (Cl, C, O)[28-30], the metal catalysts (Cu, Fe) [31, 32], and the reaction temperature [33]. Chlorine is almost the most important element for the formation of the PCDD/Fs, and its concentration directly affects the generation and emission of the PCDD/Fs. The concentration of HCl in the flue gas and the main elemental composition of the fly ashes are shown in Table 3a and 3b, respectively. The concentration of the HCl varies within the range of 0.85-1.68 mg/m3 with an increasing addition amount of the SMSW below the percentage of 20 wt.%, while it dramatically rises to 5.72 mg/m3 when the percentage of the SMSW reaches 25 wt.%. The chlorine content in the fly ashes roughly increases with the augment of the SMSW content in the mixed fuels. However, the two important catalytic metals Cu and Fe change little, except for the Cu in the fly ash from test 3 (15 wt.% SMSW).

Table 3a. The concentration of HCl in the flue gas (converting to 6% oxygen content) Tests

1

2

3

4

5

HCl (mg/m3)

1.50±0.08

1.68±0.15

1.42±0.11

0.85±0.09

5.72±0.39

Table 3b. The main elemental composition in the fly ashes. Elements a

O Caa Sia Ca Fea Ala Cla Sa Mga Ka Naa Tia

Test 1

Test 2

Test 3

Test 4

Test 5

34.0±0.31 13.8±0.15 22.2±0.14 5.8±0.08 6.4±0.14 10.1±0.15 1.6±0.06 0.8±0.03 2.2±0.04 0.87±0.04 1.0±0.03 0.4±0.03

49.3±0.38 12.8±0.14 10.4±0.09 10.1±0.15 7.8±0.16 3.8±0.05 1.9±0.06 0.6±0.02 0.7±0.01 0.73±0.04 0.6±0.02 0.5±0.03

49.5±0.38 12.5±0.13 9.1±0.08 10.7±0.15 7.8±0.16 2.7±0.04 3.4±0.09 0.8±0.03 0.6±0.01 0.74±0.04 0.4±0.02 0.4±0.03

42.5±0.35 10.6±0.11 10.6±0.09 11.9±0.16 5.8±0.12 4.7±0.06 9.8±0.16 0.9±0.03 1.2±0.02 0.65±0.03 0.5±0.02 0.3±0.02

44.3±0.36 10.0±0.11 10.3±0.09 11.7±0.16 6.5±0.14 4.6±0.06 8.7±0.015 0.8±0.03 1.1±0.02 0.64±0.03 0.5±0.02 0.3±0.02 9


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Pa Mnb Srb Mob Znb Crb Bab Zrb Cub Nib

0.2±0.01 1260±67 1460±75 600±30 696±33 402±20 320±82 219±10 193±9 165±8

0.3±0.01 1450±78 1990±98 821±41 933±47 323±16 711±112 310±15 239±11 193±10

0.3±0.01 1430±77 2100±110 1250±66 2470±122 371±18 845±121 348±17 462±21 260±13

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0.1±0.01 1000±56 724±36 285±15 1520±81 576±28 54±12 127±6 276±14 577±27

0.1±0.01 1010±56 686±33 297±15 1260±68 879±41 64±14 104±5 297±15 699±33

Notes: aUnit: %(w./w.); bUnit: mg/kg.

3.2 Emission characteristics of PCDD/Fs in gas phase The International Toxic Equivalent Quantity (I-TEQ) concentration of the PCDD/Fs in the flue gases under different experimental conditions is shown in Figure 2. Without the addition of the SMSW, the concentration of PCDD/Fs is merely 0.05 ng I-TEQ/m3, which is lower than the Chinese emission limit of 0.1 ng I-TEQ/m3 for a municipal solid waste incinerator. The concentration of PCDD/Fs reduces to 0.008 and 0.009 ng I-TEQ/m3, respectively, with the addition of 7.5 and 15 wt.% SMSW. As is well known, the PCDD/F formation mechanism is complex and, affected by lots of factors, including the element sources (Cl, C, and O), the metal catalysts, and the reaction temperature[28-33]. The PCDD/F formed on the fly ash surface will also be further desorbed into the flue gas under certain conditions, leading to the change of the gas phase dioxin concentration[34]. The chlorine and metal catalysts in the SMSW seem to show less facilitation in the formation of the PCDD/Fs at the current percentages, which might be due to the concentration being lower than the reaction limit. The previous study also found that organic compounds (PCDD/Fs) were not generated when the chlorine was lower than the threshold limit[35]. As the percentage of the SMSW increases to 20%, the concentration of the PCDD/Fs rises again and reaches 0.051 ng I-TEQ/m3, which is still lower than the emission limit, but higher than the concentration detected by Ruud et al.[36] and Nick et al.[37] in coal-fired power plants (0.0001-0.041 ng I-TEQ/m3). We can conclude that it will not affect the emission concentration of the PCDD/F under a co-combustion proportion less than 10


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20%. However, the emission concentration of the PCDD/Fs exceeds the regulation level and reaches 0.193 ng I-TEQ/m3 when the percentage of the SMSW rises to 25 wt.%. This phenomenon is in line with the obvious augmentation of the HCl concentration in the flue gas for test 5 (Table 3a), indicating that the chlorine might be the dominant factor affecting the PCDD/Fs formation. Furthermore, any SMSW that is not fully burned might also lead to the increasing of the PCDD/Fs concentration. In this test, 25 wt.% SMSW might be the limit of the co-combustion. With more waste, the combustion needs a larger furnace and more reaction time. Once produced, the unburned carton or PCDD/F-precursors (chlorobenzenes, chlorophenols), without full combustion, easily form PCDD/Fs within the post-combustion zone, either from precursors or by de novo synthesis on the surface of fly ash particles[26, 34]. Overall, the addition of the MSW below 20 wt. % will not significantly influence the emission of the PCDD/Fs. In 2016, according to the data of the National Bureau of Statistics of the People’s Republic of China, approximately 1.84 billion tons of standard coals were consumed in coal-fired plants, and approximately 7.38 million tons of MSW was treated in waste incineration plants. Therefore, if the addition proportion of the MSW reaches 20 wt. %, the theoretical co-combusting capacity of the MSW in coal-fired plants will be 36.8 million tons, which is far greater than the total treatment capacity of the MSW incineration. Particularly, an appropriate proportion of the MSW (

Fs from the co-combustion of

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