Elsevier

Fuel

Volume 202, 15 August 2017, Pages 296-306
Fuel

Full Length Article
Treatment of lignite and thermal coal with low cost amino acid based ionic liquid-water mixtures

https://doi.org/10.1016/j.fuel.2017.04.051Get rights and content

Highlights

Abstract

Coal is a major resource in Australia, both for power generation and as an export good, however its environmental impact is of significant concern. The structure of coal is inherently complex and contains many heteroatoms, which negatively affect the combustion efficiency and cause pollution. This study reveals that pretreatment of two types of Australian coal, a lignite coal and a thermal bituminous coal, with low cost choline amino acid based ionic liquids, markedly reduces the particle size and heteroatom content. This method is rapid (3 h) and proceeds at a low temperature of 70 °C. FTIR, NMR and TGA analysis revealed that pretreated coal has reduced aliphatic carbon, carbonyl groups and sulphur compared to untreated coal. Choline arginate was the best performing pretreatment agent for both types of coal. This is attributed to multiple hydrogen bond acceptor sites on the arginate anion, which facilitates strong interaction with the coal surface. These results open new pathways towards more energy efficient and cleaner coal, using low cost ionic liquids.

Introduction

Coal is considered a major resource for the Australian economy, providing both domestic energy (32%, second to oil which was 38%) and revenue from export [1]. The structure of coal is inherently complex and poses significant processing challenges [2]. In general, it is composed of primarily carbon, together with hydrogen, oxygen, nitrogen, sulphur and many other trace elements [3]. It is the presence of these extra elements that reduces the energy efficiency of coal, with oxygen reducing the amount of energy released per kg [4], and sulphur and other elements contributes to the release of toxic gases to the environment [5]. Thus, coal is often pre-treated by some physical or chemical processes prior to combustion to address this concern. Two key objectives of the pre-treatment of coal are softening, swelling and dissolution of coal particles and removal of the extra elements.

Ionic liquids (ILs) are a class of solvents that consist of cations and anions and have melting points under 100 °C [6]. ILs possess some unique characteristics, such as negligible vapour pressure, non-flammability and structural stability. Most importantly, certain ILs can be functionalised for different applications, simply by modifying their cation and anion. Recently, a select class of ILs have received significant attention in the field of lignocellulosic biomass energy conversion [7], [8], [9]. It has been shown that certain types of ILs partially dissolve lignocellulosic biomass [7], thereby facilitating their conversion to biofuels. Lignocellulosic biomass is derived from living plant materials, and are primarily comprised of cellulose, hemicellulose and lignin. It was suggested that these ILs work by forming hydrogen bonds with the various components of the lignocellulose, consequently disrupting the cross-linked network present in plant cell walls and thereby facilitating dissolution [9]. Furthermore, the anions of the ionic liquids appeared to play a more important role than the cations in dissolving the biomass, with suitable anions including chloride [10], carboxylates [11], phosphates, phosphonates [12] and amino acid anions [13], [14]. As some types of coal are fossilized lignin, coal and lignin share many common structures, including the high composition of carbon, the presence of aromatic rings and the types of linkage between their monomeric units. Hence it can be hypothesized that the ILs that solubilize the lignin present in lignocellulosic biomass will have some effect on coal as well.

Study on the use of ILs in coal improvement has been rather limited [15], [16], [17], [18], [19]. In our previous investigations [17], [18], [19], four types of ionic liquids, namely 1-butylpyridinium chloride ([Bpyd][Cl]), 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCM]), 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), and 1-butyl-3-methylimidazolium tricyanomethanide ([Bmim][TCM]), were able to alter the thermo-physical state of two types of coals. Treatment with ILs results in a reduction in the size of the coal particles, as well as an alteration in the elemental composition of the coal. TGA analysis showed that the treated coal was composed of a larger portion of lower molecular weight fragments.

So far, one of the major barriers in the utilization of ionic liquids in large-scale operations is their relative high cost, contributed by the cost of the starting materials, their synthesis, purification and relatively low recovery and reuse [20]. Therefore, the objective of this study is to explore the use of cheaper ionic liquids; made from simple acid-base reactions between two naturally occurring chemicals, choline and amino acids; for the pre-treatment of coal. This class of ILs have produced positive results in the field of lignocellulosic biomass processing [13], and have shown success in a single step reaction from biomass to ethanol. In addition, in the present study, instead of using pure ILs, mixture of IL-water has been utilised with the aim to further reduce the overall cost of the process. Comparison of the current work has been established with our previous results with [Bmim][Cl] in order to gauge the efficacy of this relatively simple and inexpensive class of ILs [18].

Section snippets

Synthesis of ionic liquids

Choline hydroxide (46 wt% in water), all amino acids, namely l-alanine, l-arginine, l-aspartic acid, glycine, l-leucine, l-lysine, l-phenylalanine, l-valine were purchased from Sigma Aldrich and used in the current work without any further purification. In the synthesis process firstly equal amount choline hydroxide and amino acids (by mole percentage) were mixed together. The resultant mixtures were then heated up to 70 °C and stirred for continuous 3 h. Specific amount of water was also added to

3.1. Dissolution of coal

The eight amino acids chosen cover the three categories of amino acids, namely non-polar side chain (Ala, Gly, Leu, Val), polar side chain (Phe) and electronically charged side chain (acidic: Asp; basic: Arg, Lys). While there have been a few studies on the treatment of coal with ionic liquids [15], [17], [18], [19], [21], there has been a very little attempt to illuminate the interaction between the ILs and the coal bulk structure, largely due to the very complex nature of the coal. By

Conclusion

We have demonstrated the use of relatively low cost ionic liquid-water mixtures in the treatment of coal. Choline arginate and choline glycinate were the two best ionic liquids in this series. Treatment of coal by ionic liquids reduces the decomposition temperature of coals, the size of the coal particles, and removes Sulphur and oxygenated functional groups, all of which are beneficial to downstream combustion of coal. Comparison of results obtained for the different ionic liquids reveals that

Acknowledgements

The authors acknowledge the financial support provided by the Priority Research Centre for Advanced Fluids and Interfaces, the University of Newcastle Australia, for the work presented in this paper. The authors also acknowledge Dr. Christopher McRae from the Department of Chemistry and Biomolecular Sciences at Macquarie University for carrying out the microanalysis of the raw and treated coal samples. Dr. Blake Simmons acknowledges the financial support of the DOE Joint BioEnergy Institute (//www.jbei.org

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