Scholarly article on topic 'Estimate research on co-carbonization of blend coal with waste plastics'

Estimate research on co-carbonization of blend coal with waste plastics Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Earth and Planetary Science
OECD Field of science
Keywords
{"waste plastics" / anthracite / co-carbonization / "coke strength" / "coke reactivity" / porosity}

Abstract of research paper on Materials engineering, author of scientific article — Zhou Min, Liu Cai-hong, Meng Lei, Wei Xiao-mei, Zheng Zhi-lei

Abstract The 50 g co-carbonization test of blend coals with two high-volatile bituminous coals and two low-volatile bituminous coals and added waste plastics(PE, PP, PS and PET) under 1.33 kPa was investigated at a plastic addition ratio of 2%~10%. Co-carbonization was performed from 200 °C, at a heating ratio of 3 K/min, to 900 °C. In the study of coke strength, coke reactivity index and porosity, we appended the anthracite of different addition ratios of 0%, 5%, and 10%. The result shows that 1) In the case of anthracite addition ratio of 5%, the effect of PE and PP on blend coals is similar and the coke strength reaches the maximum at the plastic addition ratio of 6% and then decreases; 2) PS and PET deteriorate the coke strength, observably at the addition ratio of 5%; and 3) both the coke reactivity and porosity increase with the increase of addition ratio of waste plastics. This study shows that mirco strength of coke with 5% anthracite addition can effectively distinguish the effect of different addition ratio and types of waste plastics on coke property.

Academic research paper on topic "Estimate research on co-carbonization of blend coal with waste plastics"

Available online at www.sciencedirect.com

.«ttmiasmamm.

ELSEVIER

Procedía Earth and Planetary Science 1 (2009) 807-813

ScienceDirect

Procedia Earth and Planetary Science

www.elsevier.com/locate/procedia

The 6th International Conference on Mining Science & Technology

Estimate research on co-carbonization of blend coal with waste

plastics

Zhou Mina*, Liu Cai-hongb, Meng Leia, Wei Xiao-meia, Zheng Zhi-leia

"Key Laboratory of Coal Processing and Efficient Utilization, School of Chemical Engineering & Technology, China University of Mining and

Technology, Xuzhou 221008, China bCNNC China Nuclear Power Engineering Co. Ltd., Hebei Branch, Shijiazhuang 050021, China

The 50 g co-carbonization test of blend coals with two high-volatile bituminous coals and two low-volatile bituminous coals and added waste plastics(PE, PP, PS and PET) under 1.33 kPa was investigated at a plastic addition ratio of 2%~10%. Co-carbonization was performed from 200 °C, at a heating ratio of 3 K/min, to 900 °C. In the study of coke strength, coke reactivity index and porosity, we appended the anthracite of different addition ratios of 0%, 5%, and 10%. The result shows that 1) In the case of anthracite addition ratio of 5%, the effect of PE and PP on blend coals is similar and the coke strength reaches the maximum at the plastic addition ratio of 6% and then decreases; 2) PS and PET deteriorate the coke strength, observably at the addition ratio of 5%; and 3) both the coke reactivity and porosity increase with the increase of addition ratio of waste plastics. This study shows that mirco strength of coke with 5% anthracite addition can effectively distinguish the effect of different addition ratio and types of waste plastics on coke property.

Keywords: waste plastics; anthracite; co-carbonization; coke strength; coke reactivity; porosity

1. Introduction

As a resource compatible and environmentally less hazardous method to treat waste plastics, co-coking technology of coal and waste plastics has been developed. The authors [1] have developed a recycling process for waste plastics using coke ovens. Here, the waste plastics are thermally decomposed with coal at a high-temperature reducing atmosphere in the coke oven chamber then converted to coke, tar, light oil and gas. Nippon Steel Corporation has successfully operated a waste plastics recycling process using coke ovens at Nagoya and Kimitsu Works in 2000 and at Yawata and Muroran Works in 2002 [2]. M.A. Diez et al. have shown that the composition of plastics wastes and the coking conditions (i.e. bulk density of the charge) have an effect on coke quality [3]. However, no method to forecast the effect of waste plastics added on coke quality for co-carbonization of coal/plastic mixtures has been reported.

* Corresponding author. Tel.: +86-516-83591055; fax: +86-516-83591059. E-mail address: zm@cumt.edu.cn.

Abstract

1878-5220/09/$- See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proeps.2009.09.127

In this paper, we will discuss the coke property (coke strength, coke reactivity and porosity) of coke from co-carbonization of coal/plastics mixtures with different addition ratios of anthracite of 0%, 5%, and 10% to clearly differentiate the effect of addition ratio and types on coke. We expect that these can provide basic data for industrial application.

2. Experimental

2.1. Sample used

Two high-volatile bituminous coals (gas coal and fat coal) and two low-volatile bituminous coals (coking coal and lean coal) on a dry basis were used as the basic components of blend coal. The blend coal (following refer to BC) samples were prepared by mixing the four bituminous coals according to the proportion of gas coal 45%; fat coal 20%; coking coal 20%, and lean coal 15% for industrial production. The properties of coals are listed in Table 1.

The waste plastics selected are onefold polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) which were crashed mechanically to <0.2 mm. Characterization data of plastics are given in Table 2 which is referred to coal.

Table 1. Proximate and ultimate analyses of coals

Proximate analysis (%) Ultimate analysis (%, daf)

Mad Vdaf Ad FCdaf C H N S O

Gas coal 1.57 37.47 6.41 62.53 84.14 5.60 1.40 0.47 8.40

Fat coal 1.60 35.09 8.04 64.91 84.92 5.47 1.02 1.87 6.72

Coking coal 0.70 21.03 9.03 78.97 89.56 4.87 1.46 0.93 3.18

Lean coal 0.68 15.49 9.31 84.51 91.39 4.45 1.33 0.34 2.48

Table 2. Proximate analysis of plastics

Proximate analysis (%) Plastic -

Mad Vdaf Ad

PE 0.94 100 1.15

PP 0.21 100 0.01

PS 0.70 100 0.08

PET 0.60 92.95 0.04

2.2. Co-carbonization tests of the blend coal with waste plastics

The coal/plastic mixtures were carbonized in an electrically heated muffle furnace. The coals and plastics were crushed to 100% <2 mm and <3 mm respectively except that PS was treated cryogenically and ground to <2 mm because of its very low density and large volume. In this study, the anthracite addition mass of 0%, 5%, and 10% were chosen in order to enhance/enlarge the effect of plastic type and ratio on coke quality more clearly. The anthracite was mechanically broken to <0.25 mm. The 50g mixed samples were charged in a steel cup (height 70 mm, and 057*4 mm) under 1.33 kPa and carbonized under the heating condition of 3 °C/min from 200 °C up to 900 °C. Several holes of 5 mm in diameter were punched at the bottom of the cup so that the gas released could evolve freely during the carbonizing process.

2.3. Coke compressive and micro strength measurements

The cokes obtained were put on the compression-testing machine to note the pressure as coke compressive strength index. After that, the coke sample were broken and 5 g coke blocks of >3 mm were chosen to put into a steel tank (diameter 64 mm, and height 69 mm) together with five steel balls (diameter 28 mm). We took the weight percentage of >0.6 mm granularity to the total mass after drum test of 800 r for 3 min as micro strength index. Experimental data obtained were the average of two parallel tests.

2.4. Coke reactivity measurement

The coke blocks used were obtained from the 50 g co-carbonization of coal/plastic tests. About 20 g samples of 3~6 mm in particle size were placed in a reaction tube and heated from room temperature to 750 °C at a rate of 20~25 °C/min and then keep 5 min. After that, 500 mL/min carbon dioxide gas was introduced to the reaction tube for 2.5 min. We collected the gas with a gas analyzer in one minute interval then stopped CO2 flowing. Repeat the process of rising temperature, keeping temperature and aerating CO2, then collecting gas above by per 50 °C until 1100 °C. Experimental data obtained were the average value of two parallel tests.

2.5. Porosity measurement

Mercury intrusion method was adopted to measure and evaluate the pore structure of the cokes from the 50 g co-carbonization tests. Mercury was pressed into coke pores by external pressure. The coke pore diameter (y) and porosity (9) were calculated as follows:

2acosç 730

r =--=--(1)

where a is the surface tension (N/m2); ç the angle of tension effect; and P the external pressure (MPa). In the tests, a = 0.48 N/m2 and y = 140°.

0 = 1 - p (2) Pz

where pp is the particle density (kg/L) and pz is the true density (kg/L). 3. Results and discussion

3.1. The compressive strength and micro strength analysis of coke

Fig. 1 shows the effect of plastics added on compressive strength and micro strength of coke from 50 g co-carbonization of coal/plastic mixtures.

Among the four plastics, the added PE and PP cause compressive strength of coke increase observably compared with the coal sample only, where it is similar with the <5% addition mass of PS and PET in Fig. 1(a). This could be due to the increasing of internal plastic component of plastics to the blend coal, and will reduce the caking property if without it [4]. But the cokes are friable rapidly as addition mass of PS and PET increasing, even equal or lower than that of BC when 8% addition.

As shown in Fig. 1(b), a fall trend occurs as the micro strength of cokes, especially for that from PS and PET mixed. For PP addition, the micro strength indexes are all higher than that of BC.

This suggests that coke strength method can distinguish the effect of added PS and PET in respect that they destroyed coke quality obviously, which is consistent with the study of E.S. Uzumkesici [5] and Valentina V [6]. However, the effect of added ratio of PE and PP on coke can not be differentiated clearly by this method. So we performed the co-carbonization tests again by appending 5% and 10% anthracite respectively to magnify the effect of PE and PP. This can be due to the stability of anthracite which doesn't melt itself but absorbs liquefaction products of part active component from the pyrolysis process on the surface as a high-rank coal. This led the liquid matter decreased and fluidity and swelling property of the liquid matter changed.

I I 0% addition V77A 3% addition f\ 1 5% addition FFFFFI 8% addition 10% addition

CD 60-

I I 0% addition YZZA 3% addition [\~~1 5% addition Effl 8% addition KK^ 10% addition

<D 2 -

(a) (b)

Fig. 1. (a) The effect of waste plastics addition on compressive strength with no anthracite; (b) The effect of waste plastics addition on micro strength with no anthracite

Compressive and micro strength index of cokes obtained from 50 g co-carbonization test of coal/plastic mixtures with 5% anthracite are shown in Fig. 2.

Relative to the sample without plastics (namely BC), it is clear that the value of compressive strength of cokes from co-carbonization of coal/PE and coal/PP mixtures with 5% anthracite is improved markedly as the plastic mass increasing before the maximum value of 6% plastics addition, and then becomes inferior rapidly. In addition, PE decreases compressive strength to a greater extent than PP with the same addition mass. It is similar for PS and PET that reduce coke compressive strength more significantly than others. In Fig. 2(a), we can easily find that added PS and PET engender the negative effect when the addition ratio higher than 5%.

(a) (b)

Fig. 2. (a) The effect of waste plastics addition on compressive strength with 5% anthracite; (b) The effect of waste plastics addition on micro strength with 5% anthracite

The micro strength of coke from coal/plastic mixtures with 5% anthracite is shown in Fig. 2(b). For added PP and PE, the highest values are both at 3% and then decrease significantly after 6% addition. But in the case of PS and PET, micro strength decrease drastically when the PS addition up to 4% and PET 3%.

The micro and compressive strength of cokes from co-carbonization of coal/plastics with 5% anthracite are almost higher than that without any anthracite addition as shown in Fig. 1 and Fig. 2. The reason for this is considered as a better ratio of active/inert component in the case of anthracite addition which favors the fluidity [7]. This induces gas pores filled by colloid, and then the shrinkage of semicoke decrease and the porosity improve to enhance the strength of semicoke and coke [8]. However, coal caking property and coke strength will deteriorate if

too much anthracite added. Therefore, the added amount of anthracite should be moderate [9].

Co-carbonization of coal/plastics with 10% anthracite induces pulverization of cokes which resulted in the very low compressive strength. Hence, a slight difference in compressive strength as the addition mass of plastics can not be easily distinguished. It is implied that the anthracite addition of 10% is excessive.

The result indicates that for 5% anthracite addition, distinguishing ability of coke compressive and micro strength on plastic added is better than that with no addition. The cokes of 5% anthracite addition were selected for evaluating the reactivity and porosity.

The test indicates that the micro strength index is better as an estimated method for the effect of plastics added on coke quality than compressive strength index which only perform once for one coke sample, and could not be tested repeatedly. But the granularity size and the test conditions such as rotate speed and test duration should be confirmed to minimize the error range of the micro strength index.

3.2. The porosity analysis of coke

The pore characters of the coke samples from co-carbonization of coal/plastic mixtures with 5% anthracite were listed in Table 3 and Fig. 3.

Table 3. Pore characters of coke from co-carbonization with 5% anthracite

Addition mass(type)

Character

Average porosity (%)

Total intrusion volume (mL/g) Average pore diameter (^m)

100%BCA 97%BCA+3%PE 95%BCA +5%PE 98%BCA +2%PET 98%BCA +2%PS

39.64 41.56 45.39 41.74 42.33

0.3859

0.4106

0.4626

0.4301

0.2241 0.1387 0.1905 0.1909 0.2171

5 0.06E

0 0.05 -

g 0.04. 0

8. 0.03'

0 0.02E

o 0.( 0.00-0.01

2.71828 7.38906 20.08554 54.59815 148.41316 403.42879

ln(R), M m

Fig. 3. The effect of waste plastics addition on micro strength with 5% anthracite

All of the three plastics added result in coke porosity increase in this study as shown in Table 3, which is consistent with the study of Seiji Nomura [10]. The reason may be as follows: firstly, large amount of contraction cavities was formed when plastics melt during co-carbonization; secondly, during the process of softening, de-volatilization and swelling of coal/plastic mixtures, the volatile is entrapped into the hardening viscous coal mass; last, the pores and anti-fissure are formed [11]. As shown in Fig. 3, the macropore interval of coke (>100 ¡um ) broadens but volume minishes by 3% PE added. Both the mesopore (20~100 fm ) and macropore volume increase when addition mass up to 5%. In the case of PS and PET, a large number of mesopore, which is considered to

determine reactivity, are formed in the coke structure, while the macropore volume of coke decreases. This is accordant with the result of coke strength and coke reactivity measured foregoing.

The coke porosity test can effectively distinguish the effect of waste plastics added on the coke quality. However, as the complex operation, it is not suitable as a routine test measure.

3.3. The coke reactivity analysis

Fig. 4 shows the coke reactivity of former cokes from co-carbonization of coal/plastic mixtures with 5% anthracite.

+3%PE i?

-»—I—1-1-»—I-1—I—»-1—»—I—1-1—

750 800 850 900 950 1000 1050 110C

temperature, °C

801 7060 50 4030 20 100

temperature, °C

Fig. 4. The effect of PE / PP / PET / PS addition on coke reactivity

temperature, °C

temperature, °C

We can easily find that the coke is much inferior because of the added plastics. It is important to point out the existence of a correlation between this reactivity index and that most frequently employed by the industry, the Nippon Steel Corporation procedure [12]. As Fig. 4 shown, PP and PE cause coke reactivity rise irregularly as the addition mass increases. On the other hand, coke reactivity increase regularly along with the addition mass of PS and PET. But in the case of added PS, coke reactivity of 4% addition is lower than 2% addition inversely in high temperature of up to 1000 °C. The reason is that the density of plastics is much lower than that of coal samples. During pyrolysis of plastics around coal, the bulk of plastics shrank and large numbers of volatile formed. These both induce crack and cavity of coke, which is proved by porosity test. CO2 gas enters into the inside of coke more easily, and then the reactivity increases compared with the coal samples only.

Due to the technical limit, it is not successful to industrial application for the high addition mass of plastics. Therefore, coke reactivity is not a good method to estimate the effect of added plastics on coke quality.

4. Conclusions

+ 10% PE

+4% PET

-I111111111111111111 111111111111111111

700 750 800 850 900 950 1000 1050 1100 1150 700 750 800 850 900 950 1000 1050 1100 1150

(1) Estimate method on co-carbonization of coal/plastic mixtures with 5% anthracite is better than 0% and 10%

addition ratio of anthracite.

(2) The coke compressive and micro strength index could distinguish the effect of added plastics of different ratios and types more obviously than coke reactivity index and porosity with the addition mass of 5% anthracite to co-carbonize. The coke reactivity and porosity may be considered as a routine method for forecasting the coke property. However, because of the irregularity and complex operation respectively, the coke reactivity and porosity are not advised to be used as a routine evaluation target.

(3) The micro strength index is better as an estimated method for the effect of plastics added on coke quality than compressive strength index which only perform once for one coke sample, and could not be tested repeatedly. But the granularity size and the test conditions such as rotate speed and test duration should be tested to minimize the error range of the micro strength index.

References

[1] S. Nomura and K. Kato, The effect of plastic size on coke quality and coking pressure in the co-carbonization of coal/plastic in coke oven. Fuel. 85 (2006) 47-56.

[2] S. Nomura and K. Kato, Basic study on separate charge of coal and waste plastic in coke oven chamber. Fuel. 84 (2005) 429-434.

[3] M.A. Diez, R. Alvarez, S. Melendi and C. Barriocanal, Feedstock recycling of plastic wastes/oil mixtures in cokemaking. Fuel. 88 (2009) 1937-1944.

[4] U. S' wietlik, G. Gryglewicz, H. Machnikowska, J. Machnikowski, C. Barriocanal and R. Alvarez, Modification of coking behaviour of coal blends by plasticzing additives. Journal of Analytical and Applied Pyrolysis. 52 (1999) 15-31.

[5] E.S.Uzumkesici, M.D. Casal-Banciella, C. McRae, C.E. Snape and D. Taylor. Co-processing of single plastic waste stream in low temperature carbonization. Fuel. 78 (1999) 1697-1702.

[6] V.V. Zubkova, Influence of polyethylene terephthalate on the carbonization of bituminous coals and on the modification of their electric and dielectric properties. Fuel. 85 (2006) 1652-1665.

[7] J.G. Wang, J.H. Zhou, Y.M. Hu, C.H. Peng and Z.W. Zhang, Property Study on Anthracite in Coal Blend for Coke-making. Fuel & Chemical Processes. 1 (2007) 1-4.

[8] C.H. Peng, Coking Test for Coal Blend of Anthracite. Coal Chemical Industry. 6 (2005) 47-51.

[9] L.J. Fan, Study on the Influence of Mixed Anthracite Coal on Coke Function. Shanxi Coking Coal Science & Technology. 7 (2004) 27-29.

[10] S. Nomuraa, K. Kato, T. Nakagawa and I. Komaki, The effect of plastic addition on coal caking properties during carbonization. Fuel. 82 (2003)1775-1782.

[11] J.J. Cai, G.W. Yu, H.Q. Liao, K. Qian, P. Zhao and Y.B. He, Disposal of Waste Plastics With Traditional Coking Process. Journal of Iron And Steel Research, International. 1 (2006) 05-09.

[12] J.A. Mene'ndez, R. Alvarez and J.J. Pis, Extended Abstracts of Eurocarbon. 1 (1998) 221.