Article pubs.acs.org/EF
Sulfur Transformation during Hydrothermal Dewatering of Low Rank Coal Junhong Wu, Jianzhong Liu,* Shao Yuan, Xu Zhang, Yan Liu, Zhihua Wang, and Junhu Zhou State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China ABSTRACT: The presence of sulfur in coals has raised serious environmental issues, which are obstacles to large-scale utilization of coals. Hydrothermal dewatering (HTD) is a promising upgrading method for low-rank coals (LRCs) to significantly remove oxygen-containing groups and irreversibly decrease the inherent moisture content. To uncouple the complex behavior of sulfur evolution during HTD processing of lignite and to elucidate the main mechanism, this research experimentally studied the characteristics of sulfur transformation in a Chinese lignite from Xiaolongtan coal mine during HTD upgrading. Results reveal that the HTD upgrading of raw coal within the temperature range from 200 to 300 °C can obtain a desirably upgraded coal with higher calorific value and lower inherent moisture. Compared with raw coal, organic sulfur content decreased significantly, whereas sulfate sulfur content gradually increased after HTD. X-ray photoelectron spectroscopy results showed that HTD promoted aliphatic sulfur decomposition and the release of sulfur-containing gases. The released gases, such as H2S, reacted with the organic matrix of coals to form thiophenic sulfur. As a result, thermally stable thiophenic sulfur increased with increasing HTD temperature. The increase of sulfate sulfur content after HTD was attributed to the release of SO2. The calculation of the mass balance on the sulfur revealed that the vast majority of sulfur remained in upgraded coals, and only a minimal amount was released into gaseous and liquid products. The sulfur-containing gases remarkably increased with increasing HTD temperature, whereas the sulfur in the wastewater decreased.
1. INTRODUCTION Low-rank coals (LRCs), including lignite and sub-bituminous coals, account for nearly half of the world’s coal reserves.1,2 The utilization of LRCs as cheap feedstock for pyrolysis, combustion, and gasification has recently been actively pursued because of increasing energy demands. High inherent moisture, high ash content, and low calorific value significantly restrict the large-scale application of LRCs. The direct use of LRCs leads to lower combustion efficiency, higher greenhouse gas emissions, and higher transportation costs.3 Upgrading these coals with respect to moisture, ash, and sulfur content is essential. Hydrothermal dewatering (HTD) is a promising upgrade method for LRCs and has recently attracted intense research attention. The outperformance of HTD upgrading compared with the other upgrading technologies for LRCs is twofold: First, inherent moisture is removed as liquid, and the latent heat is saved. Second, oxygen-containing groups and inherent moisture are irreversibly removed because of the chemical and structural changes of coal. In the past decades continuous efforts to investigate HTD are generally directed toward (1) the effect of HTD conditions on the treated coal properties;4−6 (2) the properties of gas-, solid-, and liquid-phase products;7−9 (3) the upgrading mechanism of HTD for low rank coals;10(4) the effect of HTD treatment for LRC on its slurrying, pyrolysis, combustion, and gasification behaviors;11−13 and (5) the disposal of wastewater produced from HTD.14,15 However, the research of sulfur transformation during HTD is completely lacking. Sulfur is one of the most notorious environmental contaminants. Sulfur dioxide emitted during coal combustion is the main source of acid rain. Sulfur in coal occurs in pyritic, organic, and sulfate forms. Pyritic and organic sulfur forms are the main components of the total sulfur in coal. A minor © 2015 American Chemical Society
amount of sulfate, which mainly consists of calcium and iron, occurs in weathered coals. The organic form, which is bound directly to the hydrocarbon matrix, generally occurs in the forms of sulfides (R−S−R), disulfides (R−S−S−R), thiols (R− SH), thiophenes (heterocyclic), sulfoxides (R−S−O−R), and sulphones (oxidized forms). A significant amount of research has been conducted on sulfur transformation behavior during the combustion, gasification, and pyrolysis of coal.16−23 Nevertheless, relatively few papers have reported the evolution of sulfur when LRCs are upgraded via HTD process. Temperature and atmosphere are the two crucial factors that affect the sulfur evolution. The aliphatic sulfur decomposed even at low temperature, and the aromatic sulfur decomposed at 400−700 °C, irrespective of coal type.16 Chen et al.18 reported that sulfur was removed from coal more effectively in hydropyrolysis than in pyrolysis under an inert atmosphere. Results indicate that hydrogen gas (H2) improves the formation of hydrogen sulfide (H2S).24 Li et al.25 performed thermal upgrading of Shengli lignite under simulated flue gas in a fluidized reactor from 200 to 500 °C. They found that the excellent selective desulfurization by the oxidizing atmosphere was attributed to facilitating the breaking of the C−S bond and the inhibition of the reaction of H2S with the coal matrix. For HTD, although increasing temperature can promote the removal of sulfur, it also leads to the additional loss of hydrocarbon compounds.5 On the other hand, the existence of oxidizing atmosphere will give rise to oxidation of lignite during HTD, but the oxygen functional groups in coal are unfavorable Received: June 7, 2015 Revised: September 15, 2015 Published: September 15, 2015 6586
DOI: 10.1021/acs.energyfuels.5b01258 Energy Fuels 2015, 29, 6586−6592
Article
Energy & Fuels Table 1. Proximate and Ultimate Analyses of Coal Samplesa proximate analysis (%)
ultimate analysis (%)
samples
Meq
Aad
Vad
FCad
Qb,ad (MJ/kg)
Cd
Hd
Nd
Od
St,d
Ss,d
Sp,d
So,d
AO/C
AS/C
raw coal HTD-200 HTD-250 HTD-300
18.34 10.85 9.14 5.96
12.11 13.70 14.61 15.65
39.09 38.24 36.48 34.83
32.35 36.80 39.62 43.31
18.15 20.59 21.98 23.28
56.00 59.05 60.83 63.96
3.64 3.76 3.84 4.14
1.49 1.71 1.78 1.86
22.05 17.76 15.19 11.04
2.318 2.278 2.255 2.212
0.379 0.394 0.484 0.581
0.306 0.321 0.333 0.347
1.633 1.563 1.438 1.284
29.5 22.6 18.7 13.1
1.55 1.45 1.39 1.30
Meq refers to equilibrium moisture content. A, V, and FC refer to ash, volatile, and fixed carbon contents, respectively. Qb refers to the bomb calorific value. “ad” refers to air-dried basis, and “d” refers to dry basis. St, Ss, Sp, and So refer to total sulfur, sulfate sulfur, pyritic sulfur, and organic sulfur, respectively. AO/C (or AS/C) refers to the oxygen (or sulfur) to carbon atom ratio, which is expressed as oxygen (or sulfur) atoms per 100 carbons. a
for the dewatering performance.26 Consequently, HTD experiments should be carried out at lower temperature (below 350 °C). Moreover, the air in the autoclave should be replaced by an inert atmosphere to prevent the coal oxidation. This study for the first time aims to gain insight into the evolution mechanism with regard to the different sulfur forms of LRCs during HTD upgrading. A thorough understanding of the transformation behavior of sulfur in the HTD process of LRCs may provide theoretical guidance for reducing sulfur dioxide emission in the follow-up combustions of upgraded coals.
Axis Ultra DLD spectrometer (Kratos, U.K.) using nonmonochromatic Al Kα radiation and a system of 1 × 10−8 Pa. Energy correction was made for sample charging in the case of coal based on the reference to the C−C species at binding energy (BE) of 284.8 eV. After the background was subtracted, curve fitting of Gaussian (70%)/ Lorentzian (30%) component peaks was performed using a leastsquares algorithm. 2.2.4. Gaseous and Liquid Product Analysis. The sulfur-containing gases were determined using a 7890A gas chromatograph (Agilent, U.S.A.) equipped with a flame photometric detector (FPD). The temperature of FPD was set to 250 °C, and it was supplied with 50 mL/min of hydrogen and 30 mL/min of air. Separation was performed using a GS-GasPro capillary column (30 m × 0.32 mm). Helium was used as the carrier gas at a constant flow rate of 2.5 mL/min. The column oven temperature was controlled as follows: initial hold at 60 °C for 5 min, increase at 25 °C/min to 250 °C and hold for 7 min. The measurement of sulfate ion in wastewater (liquid product) was conducted using an ICS-2100 ion chromatograph (Thermo Scientific, U.S.A.) coupled with a dual-piston pump, two valves (a 6-port valve and a 10-port valve) and a 25 μL sample loop. A DS6 heated conductivity detector was employed. The mobile phase was obtained by a Reagent-Free Controller and a KOH eluent generator. The eluent flow rate was 1.0 mL/min. An IonPac AG19 guard column (50 mm × 4 mm) and an IonPac AS19 separation column (250 mm × 4 mm) were used as analytical columns. 2.2.5. ICP Analysis of Mineral Elements. Analysis of major mineral elements (Ca, K, Na, Mg, Fe, Al) in coals and wastewater from HTD was performed using an ion-coupled plasma−mass spectroscopy (ICAP6000, Thermo Fisher). Prior to ICP-MS analysis, the coal samples and the liquid products were digested using a microwave digestion reactor. 2.2.6. XRD Analysis of Coal Samples. Mineralogies of raw coal and upgraded coals were recorded using a D/max 2550PC diffractometer. The measurements were conducted within the 10°−70° 2θ range with a step size of 0.02° and step time of 0.02 s, and a standard sample holder was used.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The lignite samples used in this study were obtained from the Xiaolongtan coal mine in Southwest China. The coal is a type of medium-sulfur coal with 1% to
