Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 13294−13302
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Changes in Physicochemical Properties and the Release of Inorganic Species during Hydrothermal Dewatering of Lignite Keji Wan,†,‡ Deepak Pudasainee,‡ Vinoj Kurian,‡ Zhenyong Miao,*,† and Rajender Gupta*,‡ †
National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, Jiangsu China ‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9, Canada
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S Supporting Information *
ABSTRACT: Hydrothermal dewatering (HTD) is a nonevaporative drying method mainly useful for lignite, which in general has high moisture content. The lignite upgraded with HTD is expected as a clean fuel for power station; however, the changes in physicochemical properties of the product still have not been systermetically studied. In this research, hydrothermal treatments of lignite were performed at 200, 250, 300, and 350 °C. The effects of HTD on chemical composition and physical structure of lignite were studied. After HTD treatment, about 30∼40% of carboxyl and 30∼80% of hydroxide compounds in lignite were removed. HTD significantly facilitated the development of mesopores in lignite, different from the change of macropores. HTD can effectively reduce the moisture holding ability in whole relative humidity range by removing oxygen-containing functional groups, and large amounts of monolayer water in coal were removed. The release of inorganic species during HTD was also studied. It showed that the HTD can greatly reduce Na, K, Mg, and Ca in lignite but the loss of organics may result in the enrichment of some elements mainly associated with inorganics in product. In case of trace elements, B, Ba, Sr, and As in lignite had the potential to be leached out during HTD.
1. INTRODUCTION Hydrothermal dewatering (HTD) is a nonevaporative drying method mainly useful for lignite with high moisture content. HTD is a simulating and accelerating the natural coalification process toward higher rank coal,1 which can effectively change chemical composition and physical structure of lignite. HTD upgrades the lignite as a fuel by reducing the moisture content and oxygen content, increasing both the upper and lower heating values of the product and reducing the levels of some of the troublesome inorganic elements.2,3 Temperature plays a vital role in HTD process, and it is generally carried out in the temperature range of 200−320 °C.4−6 An increase in temperature of HTD conditions causes greater changes in the chemical composition and physical structure of coal. With increase in process temperature, the lignocellulosic structure of lignite becomes less rigid and may facilitate the migration of loosely bound lignite components, such as wax, to the coal surface.7,8 When the temperature was more than 320 °C, large amounts of soluble organic substances were leached into the wastewater, producing the wastewater with high total organic carbon (TOC) concentration.9 The characteristics of organics in lignite after HTD have been studied,6,10,11 and these studies have shown that HTD can reduce the concentration of oxygen in coal and further decrease its hydrophilicity. As an important indicator to evaluate the quality of upgraded lignite, moisture holding capacity of lignite was always studied based on its water adsorption isotherm.5,12 However, though the monolayer © 2019 American Chemical Society
water has a strong interaction with coal surface and can greatly affect moisture holding capacity of lignite, its variation during HTD has not been further studied by using water adsorption isotherms. During coal combustion, the vaporized inorganic components are vital in the formation of a sticky layer to which the solid ash particles attach and initiate fouling deposition.13 Because the water in lignite is removed as a liquid during HTD process, large amounts of water-soluble inorganic substances can be leached out, decreasing the fouling potential during combustion. The removal of sodium (Na) from coal by HTD is a well-known advantage of the process, and more than 70% of Na in the coal can be leached out into the wastewater.14 The high Na removal is attributed to the decarboxylation reaction at the high processing temperatures.7 For other inorganic species, such as K, Mg, Ca, etc., Favas et al.14 proposed that the removal of K, Mg, and Ca during HTD are mainly determined by their forms in lignite. Trace elements such as arsenic and selenium present in coal are known to be of concern for public health. Because the humic acids can easily adsorb and accumulate the trace elements by a chelating reaction, the lignite, which is rich in humic acids, are thought to contain a Received: Revised: Accepted: Published: 13294
March 27, 2019 June 24, 2019 July 1, 2019 July 1, 2019 DOI: 10.1021/acs.iecr.9b01691 Ind. Eng. Chem. Res. 2019, 58, 13294−13302
Article
Industrial & Engineering Chemistry Research higher proportion of organically bound trace elements compared to bituminous coals.15 Xiang et al.16 proposed that HTD process condition is similar to subcritical water condition, and trace elements may also be removed through HTD processing. Although Xiang et al.,16 Butler et al.,17 and Chen et al.18 have reported the fate of trace elements in lignite during HTD, the changes of inorganic substances in raw coal, HTD product, and wastewater were not simultaneously measured and analyzed. Besides, the previous studies about HTD have dealt with a restricted range of coals, and thus it is useful to test whether the conclusions previously reached hold for other coals. Hence, on the basis of the characteristics of hydrothermal treated lignite, the objectives of this paper are to (1) study the changes of inorganic substances in raw coal, HTD product and wastewater at the same time; (2) investigate the effect of HTD on monolayer water in lignite and then analyze the interaction between coal and water; and (3) obtain a general conclusion about the changes in physicochemical properties of lignite during HTD based on a series of lignite samples from different coalfields;
Figure 1. Changes of pressure in autoclave with increase in temperature (50 g of dry lignite sample and 150 g of deionized water in the autoclave).
2. EXPERIMENTAL SECTION 2.1. Samples and Experimental Conditions. Two lignite samples, Shengli (SL) and Xiao Longtan (XLT) from China and one lignite sample from Canada (CAN) were used in this study. All lignite samples were crushed and sieved to particle size below 150 μm and stored in airtight containers. The hydrothermal treatment was conducted in a 500 mL stainless steel autoclave (Parr 4843, Parr Instrument, Moline, IL) equipped with a pressure gauge and a thermocouple. The hydrothermal upgrading experiment started with the equivalent of 50 g of dry lignite with its as-received moisture content. And then deionized water was added into the autoclave to meet that the ratio of dry lignite to water was 1:3. The sealed autoclave was flushed three times with nitrogen gas at room temperature to get an inert atmosphere. After that, the gas pressure in autoclave was increased and maintained at 10 bar, and then the autoclave was leak-tested and heated to the desired temperature. Hydrothermal treatments were performed at 200, 250, 300, and 350 °C. The change of pressure in autoclave with increase in temperature is shown in Figure 1. After temperature had reached the desired value and been maintained for 30 min, the autoclave was then actively cooled by passing water through cooling coils. The pressure in the autoclave needed to be first released, and then the solid sample was removed from the autoclave and filtered on 0.45 μm filters. The hydrothermally treated lignites at 200, 250, 300, and 350 °C were assigned sample names as “HTD200”, “HTD250”, “HTD300”, and “HTD350”, respectively. 2.2. Proximate and Ultimate analysis. Proximate analysis of raw and upgraded lignites were conducted in accordance with the standard test methods for proximate analysis of coal, ASTM D7582, by thermogravimetric analysis (TGA) (LECO 701, LECO Corporation, St. Joseph, MI). About 1.0 g of sample was loaded into crucible. The purity of both of nitrogen and oxygen used for proximate analysis was 99.998%. About 1.5 mg of sample was loaded into the Thermo Flash 2000 coupled with a Mettler XP2U microgram balance for ultimate analysis. 2.3. Pore Structure Analysis. A Brunauer, Emmett and Teller (BET) instrument (Autosorb IQ, Quantachrome Instruments, Boynton Beach, FL) was used for porosity
determination. Prior to adsorption analysis, about 2 g of samples was degassed under vacuum at 150 °C for 720 min. The changes in pore structure of lignite were characterized by a nitrogen adsorption and desorption at −196 °C. The total pore volume was measured from the amount adsorbed at a relative pressure of 0.99. 2.4. Measurements of Oxygen Functional Groups. A FTIR spectrometer (NEXUS 670, Thermo Nicolet Corp., U.S.) equipped with the Diffuse Reflectance Infrared Fourier Transform (DRIFT) module was used to identify the variation of organic components in lignite after HTD treatment. Pure KBr was used to collect the background before test. About 500 mg of KBr powder and 1.5 mg of dry sample were placed in an agate mortar, thoroughly ground and mixed and then kept in the diffuse reflectance test apparatus. The FTIR spectra of the samples were recorded over range of 4000−650 cm−1 at a resolution of 4 cm−1. 2.5. Moisture Holding Characteristics of Lignites. The measurements of moisture holding characteristics were conducted using a dynamic vapor sorption (DVS) analyzer (Aquadyne DVS supplied by Quantachrome Instruments). It was a very useful means of determining accurate water sorption isotherms and using a range of preset relative humidity values (P/P0). Approximately 70 mg of lignite sample was used for each measurement at 30 °C. The relative humidity started from 0% and then increased to a series of fixed values (0%, 2%, 4%, 6%, 8%, 11%, 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 96%). The water uptake versus time was recorded under isothermal conditions until equilibrium. 2.6. Concentration of Inorganic Species in Lignite and Wastewater. The concentrations of inorganic elements in lignites and wastewater were measured using high-resolution laser ablation inductively coupled plasma mass spectrometry (PerkinElmer’s Elan 6000, ICP−MS). Prior to the elements determination of coal by ICP−MS, the samples were digested with HF, HNO3 and HCl and all acids are trace metal grade. First, 0.2 g sample was put into beaker and 8 mL HF and 2 mL HNO3 was added. The beaker was sealed and heated on 130 °C hot plate for 48 h. The beaker was then opened and heated on 140 °C hot plate until the solution was completely dried. 13295
DOI: 10.1021/acs.iecr.9b01691 Ind. Eng. Chem. Res. 2019, 58, 13294−13302
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Industrial & Engineering Chemistry Research
it was increased for the Canadian lignite. Thus, the geographical origin of samples had an effect on the concentration of sulfur in HTD lignite. As reported in previous researches,20,21 oxygen-containing functional groups, especially carboxyl and hydroxyl groups on the surface of coal, are the major attachment sites for water in lignite, because they can form strong hydrogen bonds with water molecules and present strong hydrophilicity. The organic oxygen content in coal can be calculated approximately by difference, that is, the sum of the organic C, H, N, and S and ash yield subtracted from 100%, but the error will be approximately the sum of all the errors in the directly measured quantities, so that the value obtained would not be accurate. Besides, ultimate analysis cannot give the information on oxygen-containing functional groups. To avoid these problems, in this study, FTIR was used to determine the concentrations of oxygen-containing functional groups in lignite. Figure 3 shows an example of the FTIR spectra of CAN raw and hydrothermally upgraded lignite at 300 °C, with major peaks labeled. The absorption peaks of hydroxyl and carboxyl groups correspond to 3000−3700 cm−1 and 1700− 1760 cm−1; respectively, and the stretching band of aromatic carbon is near 1577 cm−1.22,23 Because aromatic carbon has high chemical stability and is not greatly affected by upgrading procedures,22,24 the area of aromatic carbon peak (1577 cm−1) was used for reference to quantify the changes of other components. Table 1 shows the changes in the amounts of carboxyl and hydroxyl groups based on the area of the peak in FTIR spectra. For all of the three lignite samples, the amounts of both carboxyl and hydroxyl groups decreased sharply after HTD upgrading. About 30∼40% of carboxyl and 30∼80% of hydroxyl in lignite were removed in HTD. The high process temperature facilitated the breakup of hydroxyl and carboxyl groups and then produced water and light volatile components such as carbon dioxide, carbon monoxide and so on.11 3.1.2. Changes in Physical Structure of Lignite During HTD. The pore types in porous media are generally classified as micropore (50 nm).25 Based on the previous studies of scholars,25−27 a general conclusion that HTD can effectively decrease the macropores in lignite with increasing temperature can be given. In this study, the change in mesopores of lignite during HTD was mainly studied, and Figure 4 shows the variations of mesopore structure of coals after HTD at different temperatures. It can be seen that HTD significantly facilitated the development of mesopores in lignite, different from the variation of macropores. The collapse of macropores in lignite during HTD has been attributed to shrinkage forces,26 and the resuting reduction in macropore size will increase the mesopore volume, but, as described above in Section 3.1.1, devolatilization occurred during HTD, and the effects of thermal decomposition and liberation of the resulting volatiles on the mesopore volume should also be considered. In Figure 4, the pore volume increased for each pore diameter interval in the mesopore range for CAN and SL coals upgraded at 200 °C, but compared to upgraded CAN coal, the mesopores of upgraded SL coal included many small pores of diameter 2−10 nm and therefore the average pore diameter was smaller. From SI Table S1, the yield of CAN (94.95%) coal after HTD at 200 °C was greater than SL coal (90.50%), so that more material was released from SL coal, which might result in an increase in smaller pores in SL coal. When the upgrading temperature exceeded 200 °C, the pyrolysis of coal was greatly developed
After that, 5 mL HCl and 5 mL HNO3 was added into the beaker and heat at 130 °C for 24 h, and then completely dried at 140 °C. Finally, 10 mL HNO3 was added and heat at 130 °C for couple of hours. One mL this solution was taken, and 0.1 mL HNO3, 0.1 mL internal standards (In, Bi, and Sc) and 8.8 mL deionized water was then added. The sample was ready for analysis.
3. RESULTS AND DISCUSSION 3.1. Effects of HTD on the Physicochemical Properties of Lignites. 3.1.1. Changes in Chemical Composition of Lignite During HTD. The proximate analysis of raw and upgraded lignite is shown in Supporting Information (SI) Table S1. Because unstable organic compounds in coal would be removed at higher temperature and pressure, the volatile content decreased and fixed carbon content increased with temperature for all lignite samples. In SI Table S1, as the temperature increased, the yields of lignite slightly decreased until HTD temperature of 300 °C and then decreased sharply at 350 °C. The lower yield of lignite after HTD upgrading means more loss of substances from raw coal, and for SL, XLT, and CAN lignites, the losses from raw coals at hydrothermal treatment temperature of 350 °C even reached 32.50%, 33.06%, and 25.83%, respectively. Much of this loss was due to extraction of organic material into the wastewater, which causes difficulty during water treatment,19 and thus the subsequent study of hydrothermal treatment was limited below the temperature of 350 °C. The water of slurry often becomes slightly acidic during HTD,14 and the weak acid environment can promote the release of minerals (e.g., calcite) in coal. That may be the reason why the ash content of SL coal slightly decreased after HTD at 200 and 250 °C. Figure 2 shows the ultimate analysis of different coals before and after HTD (300 °C), where Y-axis is a percentage on a dry
Figure 2. Elemental composition of different coals before and after HTD treatment (300 °C).
ash free (daf) basis. As expected, the carbon content increased after HTD, which indicates that the energy density of coal was enhanced. From Figure 2, the nitrogen was slightly enriched for all HTD products, and it was not beneficial to control the emissions of pollutants during combustion. The changes in sulfur element did not show a uniform trend. For two Chinese lignites, the sulfur content in HTD product was decreased, but 13296
DOI: 10.1021/acs.iecr.9b01691 Ind. Eng. Chem. Res. 2019, 58, 13294−13302
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Industrial & Engineering Chemistry Research
Figure 3. FTIR spectra of CAN raw lignite and its HTD product at 300 °C.
(EMC) of HTD coals was also far lower than that of raw lignite, and their differences also increased with RH. Therefore, the HTD method provides a way to decrease the moisture holding ability of lignite in whole RH range. According to Table 1 and Figure 5, the reduction of water adsorption ability of upgraded coal was increased with the more removal of carboxyl and hydroxide in coal after HTD. Specifically, as shown in Table 1, the A(−COOH)/A(Ar−C) and A(−OH)/ A(Ar−C) of SL lignite was reduced by 0.505 and 4.277 after HTD treatment (in Table 1), respectively, which had the largest reduction of −COOH and −OH. At the same time, among the three samples, the moisture content of SL lignite during adsorption had the largest reduction after HTD (in Figure 5). The polar sites including carboxyl and hydroxide in coal surface can strongly bond with water molecules and form monolayer water content (MWC) which can be described by the classic BET equation28,
Table 1. Change in Oxygen-Functional Groups in Lignite After HTDa sample
A(−COOH)/A(Ar−C)
A(−OH)/A(Ar−C)
SL-RAW SL-HTD300 CAN-RAW CAN-HTD300 XLT-RAW XLT-HTD300
1.219 0.714 0.690 0.474 0.798 0.463
5.371 1.094 1.537 1.113 2.591 1.153
a
Notes: A(−COOH)-the peak area of carboxyl; A(−OH)-the peak area of hydroxyl; A(Ar−C)-the peak area of aromatic carbon.
and tar was then generated. However, the release of tar was hindered by the high pressure environment in the HTD autoclave, which could retain tar and block coal pores, resulting in the decrease of pores of all diameters in CAN and SL lignites as the upgrading temperature was changed to 250 °C and then 300 °C. Compared to CAN and SL lignites, with increasing temperature, the pore structure in XLT lignite was greatly developed. Because large amounts of organics was released (in SI Table S1), more small mesopores in the pore size range of 2−16 nm were generated after HTD with higher temperature. Overall, the HTD can facilitate the development of mesopores in lignite, but its trend is greatly affected by lignite properties. 3.2. Effect of Htd on Moisture Holding Characteristic of Lignite. the moisture holding characteristics and readsorption ability of HTD lignites were studied by the water adsorption isotherms (see Figure 5). According to the BET classification,28 water adsorption isotherms on lignite had the sigmoidal-shaped profile, including three regions, namely monolayer water, multilayer water and capillary water. The moisture content at maximum RH of 96% was defined as moisture holding capacity of lignite at 30 °C.29 From Figure 5, the moisture holding capacity fell dramatically from 0.225 g/g dry coal in raw lignite to 0.08 g/g dry coal in products. However, because of the substantial hysteresis effect, the moisture holding capacity of lignite measured by adsorption process is likely to be lower than the conventional moisture holding capacity determined by desorption. In the whole range of RH from 0 to 96%, the equilibrium moisture content
P 1 C−1 P = + V (P0 − P) CVm CVm P0
where, V is the moisture content of sample in dry basis, C is constant, Vm is MWC, P is the partial absolute pressure, P0 is saturation pressure of water vapor, and P/P0 is relative humidity. C and Vm can be calculated from the above equation by plotting the adsorption isotherm as P/(V(P0 − P)) vs. P/P0 in the relative humidity range of 0.05−0.40 and measuring the slope, which was shown in Figure 6. The results obtained by fitting BET equation are shown in Table 2. From Table 2, the MWC in raw coal was in the range of 0.036−0.057 g/g dry coal and the MWC in HTD coals was about 0.024 g/g dry coal. Large amounts of monolayer water lost the ability to attach to coal surface after HTD. The decrease of monolayer water corresponded to the reduction of A(−COOH)/A(Ar−C) and A(−OH)/A(Ar−C) of HTD coal in Table 1. 3.3. Effect of HTD Processing on the Release of Inorganic Elements. 3.3.1. Effect of HTD on the Removal of Major Metallic Elements. The seven cations listed in Table 3, were the most abundant metallic elements in lignites and named as major elements, which are also related to the fouling of boiler during combustion.17 Table 4 shows the concen13297
DOI: 10.1021/acs.iecr.9b01691 Ind. Eng. Chem. Res. 2019, 58, 13294−13302
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Industrial & Engineering Chemistry Research
Figure 4. Changes in porosity parameters of (a) CAN, (c) SL, and (e) XLT coal and pore size distribution of (b) CAN, (d) SL, and (f) XLT coal after HTD treatment.
Figure 5. Water adsorption isotherms of raw lignite and HTD coals at 30 °C.
the removal rates of Mg reached 81.75% for XLT lignite. In Table 4, the concentration of Ca in wastewater reached about 841 mg/L, which was the most abundant metallic element in wastewater. The Al and Fe in lignite mainly existed in minerals which had stable chemical state, and the concentrations of Al and Fe in wastewater was very low.
trations of these major elements in wastewater produced from HTD process. From Table 3, the most common inorganic metallic elements in raw lignites were magnesium (Mg), aluminum (Al), calcium (Ca), and iron (Fe), though their concentrations varied from coal to coal. It was shown that the HTD can reduce the concentration of Mg and Ca in coals, and 13298
DOI: 10.1021/acs.iecr.9b01691 Ind. Eng. Chem. Res. 2019, 58, 13294−13302
Article
Industrial & Engineering Chemistry Research
Figure 6. Fitting of adsorption isotherms (0.05−0.40) of coals before and after HTD using BET equation.
Table 2. Results Obtained by Fitting BET Equationa sample
slope b
intercept, a
adj. Rsquare
MWC (g/g dry coal)
XLT-RAW XLT-HTD300 CAN-RAW CAN-HTD300 SL-RAW SL-HTD300
19.16 39.85 24.93 3.16 15.31 38.99
2.57 3.22 2.77 37.95 2.35 2.87
0.994 0.992 0.998 0.995 0.999 0.993
0.046 0.023 0.036 0.024 0.057 0.024
XLT lignite. The higher loss of organic carbon in lignite indirectly made a contribution to concentrating the elements in products, which resulted in the increasing percentage of K in XLT lignite. The small amount of titanium (Ti) in the original lignite was still retained in the product and, similar to Al and Fe, little of Ti was leached into wastewater. Most of Ti in lignite may be present in a nonsoluble mineral form. The above observations are not completely consistent with previous report of Butler et al.17 who found that HTD water contained greater amounts of Al, Ca, Fe, K, Mg, Na, and Ti. The Al, Fe, and Ti was low concentration in wastewater in this study, which is consistent with the results of Favas et al.,14 and that would be as the results of different chemical forms of elements in different lignites. Overall, the HTD can remove a significant proportion of the Na, K, Mg, and Ca in lignites in some circumstances, but the loss of organic carbon may result in the elements enrichment, which is undesirable for its application to subsequent combustion or gasification. From Table 3 and Table 4, the amount of the metallic element in CAN coal was higher than the sum of the amounts in the wastewater and HTD product. For SL coal, the mass balance of metals was reasonable. In XLT coal, the amount in the coal was greater than that in the water and product. The poor mass balance of CAN and XLT lignite may be as the results of coal properties. As described in Section 3.1.2, at the upgrading temperature of 300 °C, for CAN lignite, the release
Adj. R-Square: adjusted correlation coefficient square of fitting, MWC- monolayer water content.
a
Generally, the forms of sodium (Na) and potassium (K) in lignite can be classified into four fractions; namely the soluble, the carboxylic matrix-associated, the macromolecular organic group-associated, and the inorganic silicate mineral fraction.30 Hydrothermal treatment may ease to leach the soluble Na and K into wastewater and remove the organic Na and K by decarboxylation. In Table 3, the concentration of Na decreased after HTD. For SL lignite, the percentage of Na reduced sharply from 0.187% in raw lignite to 0.043% in products and about 77% of Na was removed. According to Table 3, the concentration of K in CAN and SL lignite also decreased in HTD. Although the concentration of K in XLT lignite increased after upgrading, still lots of K existed in wastewater, which indicates that the HTD can also effectively reduce K in
Table 3. Concentration of Major Metallic Elements in Colas before and after Upgrading (Dry Basis, g/100g)a analyte
Na
Mg
Al
K
Ca
Ti
Fe
CAN-RAW CAN-HTD300 SL-RAW SL-HTD300 XLT-RAW XLT-HTD300
0.064 0.057 0.187 0.043 0.002
