JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 42, Issue 8, Aug 2014 Online English edition of the Chinese language journal

Received: 20-Jan-2014; Revised: 05-May-2014. * Corresponding author. Tel: 027-87542417, E-mail: xian_li@hust.edu.cn. Foundation item: Supported by the Study on Independent Innovation Fund from Huazhong University of Science and Technology (2013TS077). Copyright 2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2014, 42(8), 897904

Degradative solvent extraction of demineralized and ion-exchanged low-rank coals LI Xian1, ZHU Xian-qing1, XIAO Li1, ASHIDA Ryuichi2, MIURA Kouichi2,*, LUO Guang-qian1, YAO Hong1 1State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China; 2School of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan; 3Institute of Advanced Energy, Kyoto University, Kyoto 615-8510, Japan

Abstract: Dehydration and upgrading are essential pretreatment methods for efficient utilization of low-rank coal. In previous works

the authors employed degradative solvent extraction method to dehydrate and upgrade low-rank coals and fractionate them into several

fractions. For further study of this method, two low-rank coals (MM and LY) were pretreated by acid washing for demineralization or

acid washing and Na/Co ion-exchange. The pretreated and raw coals were then extracted by 1-methylnaphthalene (1-MN) at 350°C and

fractionated into upgraded coal (UC), high molecular weight extract (deposit), low molecular weight extract (soluble), as well as a little

H2O and gas products. The results show that both acid washing and ion-exchange enhance the yields and carbon contents of the two

extracts. Ion-exchange obviously promotes the removal of oxygen-containing functional group during extraction. The yield of high

molecular weight extract of demineralized MM increases from 3.5% to 9.5%, and the carbon content and oxygen content of low

molecular weight extract of Na ion-exchanged LY are as high as 85.3% and less than 6.4%, respectively. Ion-exchange has a distinct

influence on physical and chemical properties of the extracts. The influence of Na ion-exchange is especially remarkable. Thus,

demineralization and ion-exchange have evident promotion for the degradative solvent extraction of low-rank coal.

Keywords: degradative solvent extraction; low rank coal; demineralization; ion-exchange; pretreatment

Coal is a valuable resource which can be used as both fuel and chemical feedstock. Due to the rapid increase in global coal consumption, the mineable high rank coals (such as bituminous coal) are being rapidly consumed. Hence, it’s extremely urgent to utilize low-rank coal such as lignite and sub bituminous coal. Currently the low-rank coal (especially lignite) is principally used for power generation with extremely low economic benefit. With the increasing energy demand and energy prices in China, the efficient utilization of low-rank coal has become one of the most important research fields in coal conversion and utilization. However, the high moisture content, high oxygen content and low carbon content of low-rank coal lead to low calorific value, inconvenience for storage and transportation, and also very difficult to be directly introduced to conventional conversion processes. Thus it must be dehydrated and deoxygenated before its efficient utilization, named as dehydration and upgrading of low-rank coal. It’s now widely recognized that poly-generation technique of coal grading conversion is the

most efficient means to utilize low-rank coal. Extensive researches have been carried out on its dehydration, upgrading and conversion. For example, Japan's JGCC corporation combined hydrothermal upgrading of low-rank coal and coal water slurry manufacturing process[1]. Zhang et al[2] proposed several poly-generation of low-rank coal gasification coupling coal low-temperature pyrolysis technologies. Wu et al[3] proposed a method of thermal catalytic conversion of lignite. The lignite was separated into 37% upgraded coal, 33% liquid product and 22% gas products. However, nowadays most upgrading methods do not correspond well with low-rank coal grading conversion poly-generation technique. Limited methods are mainly based on conventional high rank coal thermal conversion technologies, such as coal pyrolysis poly-generation, catalytic hydrogenation liquefaction and catalytic gasification. Owing to the great differences in chemical and physical properties between high rank and low-rank coal, problems arise when applying the conventional technologies to low-rank coal. For instance, high water yield

LI Xian et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 897904

and high CO2 emission, excess cross linking or polymerization reactions lead to low yield of low-molecular components for low-temperature pyrolysis or hydro-liquefaction with high hydrogen consumption. Thus, it’s of vital significance to develop an upgrading and multi-stage separation method to achieve grading conversion and poly-generation of low-rank coal.

Although abundant researches have been done on upgrading of low-rank coal, but most of them just aim at producing solid fuel with high calorific value[4,5]. Coal separation method under mild condition is very limited. Solvent extraction seems a feasible choice. But traditional solvent extraction of coal is generally used for the study of coal structure and is extremely difficult for commercial applications[6,7]. The authors have developed a degradative solvent extraction method of low-rank coal[8–11]. By this method, the low-rank coal was treated in nonpolar solvent below 350°C and was separated into 3 main solid fractions: 50%–60% upgraded coal (abbreviated as UC, insoluble at high temperature), 3%–10% high molecular weight extract (abbreviated as deposit, soluble

at high temperature but insoluble at room temperature) and 20%–30% low molecular weight extract (abbreviated as soluble, soluble at room temperature), as well as 10%–20% CO2 and H2O. More than 90% of carbon in raw coal was transferred into solid fractions (UC, deposit, soluble), while 30%–50% of oxygen was removed as CO2 or H2O. The feasibility of practical application of this method has been initially proven[12–15].

Previous studies showed that demineralization of low-rank coal could enhance the yield of coal solvent extraction[16–18]. Ion-exchange of low-rank coal could also influence liquefaction and pyrolysis process of low-rank coal. In this work, the effects of demineralization by acid washing and ion-exchange on the yield and properties of extraction products were investigated for understanding the mechanism of degradative solvent extraction of low-rank coal. 1 Experimental 1.1 Coal samples and solvent used

Table 1 Ultimate and proximate analyses of raw coal, pretreated coal and extracts

Sample Product Ultimate analysis wdaf/% Proximate analysis wd/%

C H N O+S V FC A

Raw-MM raw 66.4 3.9 1.9 27.8 50.2 24.0 25.8

UC 68.8 3.8 3.4 23.0 27.1 34.4 38.5

deposit 76.4 5.1 3.8 14.7 39.3 59.3 1.4

soluble 82.4 7.4 2.3 7.8 78.5 21.3 0.2

Dem.-MM raw 65.7 4.3 1.7 28.4 36.7 39.67 23.6

UC 73.0 4.1 5.5 18.4 26.5 37.7 35.8

deposit 77.7 5.0 3.7 13.7 43.0 56.2 0.8

soluble 83.7 7.1 2.7 6.5 78.9 21.0 0.1

Raw-LY raw 66.7 4.7 0.9 27.7 51.5 47.0 1.5

UC 77.4 4.0 1.0 17.6 31.8 66.0 2.3

deposit 77.5 5.0 1.0 16.5 39.4 59.9 0.7

soluble 81.8 7.5 0.5 10.2 83.4 16.3 0.3

Dem.-LY raw 69.1 4.8 0.5 25.6 51.9 48.0 0.1

UC 79.3 3.8 1.2 15.7 40.2 59.5 0.3

deposit 76.6 4.9 1.3 17.2 46.0 53.9 0.1

soluble 83.0 7.4 0.7 8.9 80.4 19.6 0.0

Na exc.-LY raw 66.9 4.7 0.7 27.7 48.7 45.5 5.8

UC 77.5 3.9 0.8 17.8 – – 4.9

deposit 80.6 5.7 1.0 12.7 40.3 59.6 0.1

soluble 85.3 7.8 0.5 6.4 85.3 14.3 0.4

Co exc.-LY raw 66.5 5.1 2.1 26.3 53.8 11.6 34.6

UC 77.1 4.5 2.4 16.0 36.5 11.7 51.8

deposit 77.7 5.0 2.2 15.1 43.9 55.6 0.5

soluble 84.4 7.4 0.9 7.3 81.4 18.0 0.6

: by difference; –: not accurately determined

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LI Xian et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 897904

Fig. 4 Carbon distribution in products from degradative solvent

extraction

The coal sample together with the solvent was stirred and

held for 60 min at 350°C. The final pressure was 3–5 MPa. Moreover, it was also found that the pressure hardly affected the degradative solvent extraction in our previous work. After 60 min the solvent along with extracts was transferred to the reservoir, thus the extracts were separated from insoluble substance at high temperature, termed as upgraded coal (UC). A portion of extracts which precipitated as solid at room temperature in the reservoir cooled by circulating cooling water, termed as high molecular weight extract (deposit). The rest of the extracts still solubilized in the solvent at room temperature was termed as low molecular weight extract (soluble). The deposit was recovered as solid by filtration. The filtrate was evaporated at around 150°C by vacuum rotary evaporator to remove the solvent and recover soluble as a solid. The obtained solid products (UC, deposit, soluble) were all dried under vacuum at around 150°C more than 3 h to remove residual solvent. The gaseous products were quantified by a gas chromatograph. The yield of liquid was calculated by difference. Figure 2 presents the schematic diagram of solvent extraction and product separation procedure.

1.2.3 Characterization of products

The proximate analysis and pyrolysis behavior of raw

sample and products were performed by a thermo gravimetric analyzer (Shimadzu, TGA50). The sample was heated to 900°C at 10 K/min in N2. The ultimate analysis was estimated on a CHN analyzer (Yanaco, CHN MT-6M). The thermoplastic behavior was examined using a thermo mechanical analyzer (Shimadzu, TMA) at 10 K/min till the sample was completely melted in N2. 2 Results and discussion

Fig. 5 Oxygen distribution in products from degradative solvent

extraction

2.1 Product yields of extraction

Figure 3 shows the product yields of degradative solvent

extractions. Compared with the extraction yields of raw coal, the yields of UCs from dem.-LY and dem.-MM decreased, while the yields of solubles and deposits of the two samples increased. Especially for dem.-MM, the yields of deposit increased from 3.5% to 9.5%, meanwhile the yield of liquid and CO2 reduced and almost unchanged. Large amount of oxygen-containing functional groups such as carboxylic and phenolic hydroxyl groups existing in low-rank coal which can be associated with divalent and trivalent cations (Mg2+, Ca2+, Fe2+ and Fe3+) and form rather stable metal cross links in coal molecules. These stable metal cross links thus inhibit the formation of small molecular weight fraction during solvent extraction of coal. It was reported that the cations can be substituted with the active H by acid washing, and the metal crosslink bond is then broken. So the yield of low molecular weight fraction from the degradative solvent extraction of coal is improved[18]. The yields of UCs from Na exc.-LY and Co exc.-LY was approximately equal with that of raw LY, but ion-exchange enhanced the yield of soluble and lowered that of deposit, particularly significant for the case of Na ion-exchange. This is probably because cation cross linkings were minimized in the Na-exchanged coal, which will suppress cross linking reaction and hence promote the transformation from deposit to soluble and CO2. The effect of Co is rather complicated. The divalent cation of Co2+ also form metal crosslink bond with oxygen-containing functional groups in coal molecules, thus suppressing solvent extraction of coal. Therefore the effect of Co ion-exchange on degradative solvent extraction of low-rank coal requires further study. Soluble has the highest added value among the three solid products as indicated in our previous works[12,14]. Hence ion-exchange improving the yield of soluble played positive role on degradative solvent extraction of low-rank coal.

LI Xian et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 897904

Fig. 6 Higher heating value of three solid products and raw coals

In addition, the increase in yield of CO2 indicated more

oxygen was removed from the raw coal which was upgraded further. 2.2 Elemental analysis and proximate analysis

The properties of pretreated coals (acid-washed and

ion-exchanged coals) and 3 solid products are shown in Table 1. The ash content of LY decreased significantly from 1.5% to 0.1% after ash washing. While the ash contents of Na exc.-LY and Co exc.-LY increased remarkably to 5.8% and 34.6%, respectively. The contents of Na and Co in Na exc.-LY and Co exc.-LY were calculated as 2.46 and 0.96 mmol/g coal, respectively. The slight loss of water-soluble substances during acid washing and ion-exchange contributed to small difference of the elemental and proximate analysis between pretreated and raw coals. The volatile contents of the solubles reached up to 80%, while the volatile contents of deposits and UCs were lower than that of their corresponding raw samples, except for the deposit from dem.-MM. The carbon contents of 3 solid products from demineralized coals were all higher than those from the raw coals, while the oxygen contents of the former were lower than the latter, except for the deposit from dem.-LY. The carbon contents of solubles from Na exc.-LY and Co exc.-LY further increased, and the oxygen contents of those were simultaneously further reduced. The carbon content and oxygen content of soluble from Na exc.-LY were as high as 85.3% and less than 6.4%, respectively. These results clearly demonstrated that the demineralization and ion-exchange not only improve the yields of extracts, but enhance the upgrading effect of low-rank coal.

2.3 Carbon and oxygen distribution to products

Figure 4 showed that demineralization increased the

transformation of carbon into soluble from dem.-MM and dem.-LY. This was mainly due to the improvements of the yields and carbon contents of the deposits from them.

Fig. 7 Higher heating value balance during the degradative solvent

extraction

The distribution of carbon in liquid is almost zero,

demonstrating that demineralization inhibits the transfer of carbon to small molecular extracts. Ion-exchange further improved the distribution of carbon in soluble, especially for Na ion-exchange. Demineralization and ion-exchange exhibited little impact on the distribution of carbon in UC. Figure 5 showed the oxygen distribution in products from degradative solvent extraction. 38.8% of oxygen in dem.-MM was transferred into the liquid fraction, much more than that (26.4%) in raw coal. While 38.7% of oxygen in dem.-MM was transferred into the UC, much less than that (51.5%) in raw MM. 26.3% of oxygen in dem.-LY was transferred into CO2, more than that (22.2%) in raw LY. Ion-exchange further promoted transfer of oxygen into CO2 and liquid fraction. For example, as much as 60.2% and 63.3% of oxygen were transferred into CO2 or H2O during extraction of Na exc.-LY and Co exc.-LY, respectively. Therefore, demineralization and ion-exchange not only increased the carbon content of the deposit and soluble fraction, but significantly facilitated the migration of oxygen in coal sample as CO2 or H2O. In other words, demineralization and ion-exchange further promote upgrading the low-rank coal during the degradative solvent extraction. The influence of metallic minerals on oxygen migration during degradative solvent extraction requires further study.

2.4 Heating value of products and thermal equilibrium

The higher heating value (HHV) of raw coal, pretreated

coal and solid products was calculated by Dulong equation[19] as follows:

HHV[MJ/kg, daf]=(338.1wC+1441.8wH–180.2 wO)/1000 (1) Where wO, wH, wC is mass fraction of oxygen, hydrogen and

carbon, respectively. Figure 6 showed the HHVs of 3 solid fractions were all obviously higher than those of their corresponding raw coals or pretreated coals. The HHVs of the deposits and solubles exceed 29.9 and 36.6 MJ/kg, respectively.

LI Xian et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 897904

Fig. 8 TG curves of MM raw coal, pretreated coal and three solid products

Fig. 9 TG curves of LY raw coal pretreated coal and three solid products

The HHVs of the deposit and soluble from Na exc.-LY

reached up to 33.2 and 37.9 MJ/kg, respectively. The heating value balance was estimated based on the yield of solid products and their corresponding HHVs in Figure 7. The total HHVs of the three solid fractions were similar to those of their corresponding raw coals or pretreated coals, indicating that the HHV was not lost during degradative solvent extraction.

2.5 Thermal decomposition properties of solid products

Figure 8 shows TG curves of MM raw coal, pretreated

coal and the three solid fractions. The TG curves of dem.-MM and raw MM, two deposits and two solubles were extremely similar to each other, respectively.

LI Xian et al. / Journal of Fuel Chemistry and Technology, 2014, 42(8): 897904

Fig. 10 TMA curves of LY deposit and LY soluble

While the TG curves of the two UCs were rather different,

mainly due to higher volatile content of MM than that of dem.-MM. In addition, decomposition of mineral substances over 700°C also has certain influence. Figure 9 shows TG curves of LY raw coal, pretreated coal and the three solid fractions. The weight decrease of Co exc.-LY notably exceeded that of raw LY. The weight decrease of UC from Co exc.-LY was obviously higher than those of UCs from dem.-LY and raw LY, because of catalytic effect of Co on coal pyrolysis. Hence the Co remaining in UC could further expedite its thermal conversion. This may contribute to the utilization of UC. In general, the TG curves of all deposits or solubles showed no distinct difference, except slightly less weight loss of the deposit from Na exc.-LY, and lower starting temperature and slightly higher weight loss of soluble from Na exc.-LY. It can be concluded that Na ion-exchange has certain influence on thermal properties of the extraction products. 2.6 Thermo mechanical analysis of solid products

Figure 10 shows the TMA profiles of deposit and soluble

from raw LY and pretreated LY. All of the deposits and solubles completely melted at 230 and 90°C, respectively. The deposits from dem.-LY and raw LY had lower melting points than the deposits from ion-exchanged LY. The deposit from Na exc.-LY had highest melting point. The TMA profiles of the four solubles are rather similar, among which the melting point of the soluble from Na exc.-LY was relatively lower. These results clearly showed that the thermo plasticities of the extracts were little affected by demineralization and divalent Co exchange, while significantly affected by univalent Na exchange. The thermo plasticities of the extracts are mainly determined by the content of small volatile molecules[20,21]. So the results coincide with the volatile contents of extracts shown in Table 1 and TGA curves. 3 Conclusions

Two low-rank coals (MM and LY) were pretreated by acid washing for demineralization or acid washing and Na/Co ion-exchange. The raw and pretreated coals were then extracted by 1-methylnaphthalene (1-MN) and fractionated into UC, deposit, soluble, a little amount of H2O, and gaseous products. Acid washing not only increased the yields and carbon contents of the two extracts but promoted the oxygen removal as CO2 and H2O during the extraction. Na and Co ion-exchange further enhanced the yields and carbon contents of the two extracts, and promoted the oxygen removal as CO2 and H2O. Furthermore, demineralization and ion-exchange had a distinct influence on elemental compositions, physical and chemical properties of the two extracts, especially for Na ion-exchange. Demineralization and ion-exchange revealed little impact on the yield and elemental composition of UC, but the cation was almost completely retained in UC after the extraction, which can dramatically catalyze the subsequent thermal conversion of UC, contributing to its further utilization. References

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