Production and characterization of ash-free coal from low-rank Canadian coal by solvent extraction Moshfiqur Rahman, Arunkumar Samanta, Rajender Gupta ⁎ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2V4, Canada abstract article info Article history: Received 26 June 2012 Received in revised form 30 March 2013 Accepted 3 April 2013 Available online xxxx Keywords: Solvent extraction Low-rank coal Ash-free coal Vitrinite content Mean maximum vitrinite reflectance In this work, ash-free coal (AFC) was extracted from low-rank Canadian coals with mean maximum vitrinite re- flectance (MMVR) in the range of 0.38–0.69 using non-polar organic solvent, organic solvent combination (polar–nonpolar solvent mixture), and with and without hydro-treated heavy aromatic hydrocarbon solvents from coal–tar industry to study the effect of type of coal and solvent type on the production yield of AFC. High temperature solvent extraction was carried out in 0.5 L autoclave in the temperature range of 473 to 723 K. It was observed that 1-methylnaphthalene (1-MN), a non-polar solvent did not give any significant difference in yields [~30% (daf)]. However, an extraction yield of 73% (daf) AFC was achieved using hydrotreated aromatic hy- drocarbons at 673 K. The performance of extraction yields was correlated by the vitrinite content and MMVR of the coal and it was observed that higher proportions of vitrinite and a lower MMVR value of coal produced higher extraction yield. Proximate and ultimate analysis, FTIR, ICP-MS, 13 C CP/MAS NMR, thermogravimetric analysis and particle size distribution were used to characterize AFC. The heating value of the AFC was estimated to be in the range of 36–37 MJ/kg and a substantial decrease of sulfur content (ca. 12.5–61.1%) is also observed in AFC. AFC showed a narrower particle size distribution with a d 50 of 7.0 μm. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The natural abundance of coal reserves and its low cost-effective availability are increasing the demand of coals and its utilization to pro- duce energy and liquid fuels. There is a recent need in using low-rank coals, such as lignite and sub-bituminous, because of the increased demand of power and other applications. However, these coals have sev- eral limitations, such as higher mineral matter and moisture contents and low calorific value. Moreover, the utilization of such coals aggravates various environmental problems, such as SO x emission and high GHG emissions. It is also necessary to remove the mineral matter from coal to be combusted directly in the new generation integrated gas combined cycle (IGCC) gas turbines to overcome issues like erosion and corrosion of turbine blade and fouling due to coal ash deposition. Thus, it is advan- tageous to upgrade coals in terms of mineral mater and moisture con- tent. A new cost-effective and efficient process is therefore essential to remove the mineral matter and upgrade the low-rank lignite and sub-bituminous coals. One such technology could be the production of ash-free coal (AFC). It could be a preferred feed for some applications, such as direct combustion in the gas turbines [1]. Utilization of AFC directly in a gas turbine as fuel can generate a power system of higher thermal efficiency without damaging the turbine blades [1,2]. There are two main types of chemical upgrading of coals to produce clean coal. The first one, aiming to produce the upgraded coal using strong acids or alkalis to dissolve all the minerals leaving the organic coal matrix under hydrothermal conditions [3–6], is termed as UCC. The second process uses organic solvents to dissolve organic matter and precipitating back the ash free coal known as hyper-coal [1,7,8]. It is termed here as ash free coal (AFC). However, the coal from the UCC process may contain around 0.5% ash [7,8] and cannot be directly fired in the gas turbines. Another possible concern could be associated with the corrosiveness and biodegradability of strong acids and alkali reagents used and consequently disposal of the waste solution. On the other hand, solvent extraction of coal using organic solvents can produce AFC that has significantly much lower ash content than that obtained from UCC process. Besides, this process helps to remove alkali and heavy metals and almost all inorganic sulfurs. However, there are some limitations of the ash free coal preparation process, such as low product yield and use of residual coal discharged. A subsequent process is still needed to reduce alkali metal (Na and K) contents to less than 0.5 ppm, which is the current acceptable level for introduction to gas turbine [9]. But, since the residual coal with high ash content contains no moisture and it has high heat value, they can be utilized effectively for power genera- tion or steam generation using fluidized bed combustors. The residue coals also have high reactivity and can be used as reducing agent in the synthetic rutile production from ilmenite resources [10]. Therefore, con- tinuous research efforts are being made to identify the coal and solvent, in particular, low-cost industrial solvent to maximize the extraction yield and reducing the amount of residual coal. Fuel Processing Technology 115 (2013) 88–98 ⁎ Corresponding author. Tel./fax: +1 780 492 6861. E-mail address: rajender.gupta@ualberta.ca (R. Gupta). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.04.008 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc