Physico-chemical properties of Loy Yang lignite dewatered by mechanical thermal expression

Fuel 84 (2005) 1940–1948 www.fuelfirst.com

Physico-chemical properties of Loy Yang lignite dewatered by mechanical thermal expression Janine Hulston, George Favas, Alan L. Chaffee* CRC for Clean Power from Lignite, School of Chemistry, Department of Chemistry, Monash University, Wellington Road, Clayton, Vic. 3800, Australia Received 26 November 2004; received in revised form 23 March 2005; accepted 24 March 2005 Available online 22 April 2005

Abstract Mechanical thermal expression (MTE) is a pressure dewatering process, which is carried out at elevated temperatures. The process is being investigated as a means of lowering greenhouse gas emissions from existing lignite fired power stations by reducing the moisture content of lignites. The investigation was carried out to identify how variations in temperature and pressure during the MTE process affect the physicochemical properties of MTE treated Loy Yang lignites. Increases in MTE temperature (!250 8C) and pressure (!12.7 MPa) resulted in significant reductions in residual moisture content, moisture holding capacity and sodium levels, which have been largely attributed to the destruction of porosity in the lignite. q 2005 Elsevier Ltd. All rights reserved. Keywords: Mechanical thermal expression; Lignite; Physico-chemical properties

1. Introduction Electricity production in Victoria, Australia, is largely dependent upon the use of conventional boiler plants, which are fuelled with lignite sourced from the Latrobe Valley. While Latrobe Valley coals are considered clean (low N, S and ash) by world standards, they contain very high moisture levels, ranging between 1.5 and 2.3 g/g on a dry weight basis (db). During power generation, such high moisture levels result in reduced thermal efficiencies and increased CO2 emissions that contribute to the greenhouse effect. This is because a large amount of non-recoverable energy is utilised for the evaporation of water prior to combustion. As lignite is almost certain to remain a dominant energy source in Victoria for the foreseeable future, more efficient approaches to its use are required. One such approach is the development of non-evaporative drying technologies, including mechanical and thermal dewatering approaches, which conserve the latent heat of evaporation. Mechanical press dewatering at ambient temperatures was investigated by Banks and Burton [1], * Corresponding author. Tel.: C61 3 9905 4626; fax: C61 3 9905 4597. E-mail address: [email protected] (A.L. Chaffee).

0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.03.024

who found that up to 80% of water present in the coal structure could be removed. This approach, however, was impractical due to the high mechanical pressures and long pressing times required. Thermal processes, such as hydrothermal dewatering (HTD) [2] were equally effective in reducing moisture levels, but required temperatures in excess of 250 8C, which led to high organic carbon losses. Studies undertaken by Strauß and co-workers in the mid 1990s [3,4] found that the combined use of pressure and temperature could effectively lower moisture content levels, while requiring significantly lower pressures (!12 MPa) and temperatures (!200 8C). Investigations by Guo [5] and Favas et al. [2] have shown that, in addition to reducing the moisture content of lignites, MTE significantly alters the physico-chemical properties of these materials, including their porosity and ash yield. The purpose of this study was to further investigate the effects of MTE processing conditions, such as temperature and pressure, on the physico-chemical properties of MTE treated Loy Yang Low Ash (LYLA) lignite obtained from the Latrobe Valley. Of particular interest were the effects of temperature and pressure on material properties such as moisture content, surface area, porosity, moisture holding capacity (MHC), inorganic species removal, and loss of organic material to the wastewater stream. An understanding of such properties may play an important role in

J. Hulston et al. / Fuel 84 (2005) 1940–1948

determining the impacts of the MTE process on downstream combustion and gasification processes, as well as on storage and handling properties, including the propensity for spontaneous combustion.

2. Experimental 2.1. Coal sample description A homogenised bulk sample of Loy Yang Low Ash (LYLA) Victorian lignite was used for all experiments. The raw coal had a particle size of less than 6 mm, with a moisture content of 59.1 wt% on a wet basis (wb) and 1.44 g/g on a dry basis (db). The chemical analysis of LYLA on a weight percent dry basis (db) was C, 68.2; H, 4.8; N, 0.63; S, 0.27; ash, 0.8. 2.2. MTE experiments MTE experiments were performed using a custom built, temperature controlled pressure dewatering rig, capable of applying mechanical pressures up to 15 MPa at temperatures of up to 250 8C (Fig. 1). The rig consisted of a 5 cm diameter stainless steel sample chamber, which at its base, was fitted with a porous sintered frit and a water collection chamber. Samples contained inside the sample chamber were compressed by means of a compression device (INSTRON), which applied a known force onto a piston located inside the sample chamber. The applied pressure was monitored via a load cell located at the top of the compression device. The piston height, and therefore the sample height, was measured on a continuous basis. The temperature was controlled by means of an electrical heating mantle, which surrounded the sample chamber.

1941

The water released from the sample was expressed via a sintered frit located at the base of the sample chamber as well as through a frit located in the piston face. To prevent boiling and evaporation, the water pressure inside the system was at all times maintained above the saturation steam pressure at the temperature under investigation. MTE tests were carried out at a variety of temperatures and pressures, ranging between 125–250 8C and 2.5–12.7 MPa, respectively. Experiments were carried out by placing 100 g of raw lignite into the sample chamber, which was then filled with deionised water (100 ml) to expel any air present in the coal. The piston was gently inserted into the sample chamber and a small amount of pressure applied to expel any residual air trapped inside the system. The temperature was ramped to the desired value, and the sample compressed at a rate of 5 mm/min. Compression was continued until the desired pressure had been reached, after which the pressure was maintained at a constant level for a further 20 min. Upon completion of the experiment the sample was cooled to room temperature and the MTE product and wastewater collected for further testing. The wastewater was analysed for non-purgeable organic carbon levels (NPOC). Physicochemical properties of the MTE products were determined as described below. 2.3. Moisture content determination The moisture content was determined using a method based on the Australian Standard AS 2434.1 [6]. Measurements were carried out in duplicate, using 10 g sub-samples, which were crushed and placed into a 105 8C oven for 3 h under flowing nitrogen. The dried samples were allowed to cool in a desiccator prior to re-weighing. Oven-dried subsamples were subsequently used for surface area, helium pycnometry and mercury porosimetry experiments, as described below. 2.4. Surface area and micropore volume determination Surface area and micropore volume measurements were determined by CO2 adsorption using a Coulter Omnisorp 360 CX analyser. Experiments were carried out on ovendried 0.2 g sub-samples, which were further dried under vacuum at 105 8C for 24 h to ensure complete drying and removal of adsorbed gases. CO2 surface areas and micropore volumes (pores !2 nm diameter) were calculated using the Dubinin-Radushkevitch equation [7]. 2.5. Helium density determination

Fig. 1. Schematic diagram of custom built mechanical thermal dewatering rig.

The helium density, rHe, was determined by helium pycnometry using an AccuPyc 1330 (Micromeritics, Norcorss, GA, USA) Pycnometer. The unit was calibrated on a daily basis and measurements were carried out on oven-dried 1.5–2.5 g sub-samples. Prior to taking

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J. Hulston et al. / Fuel 84 (2005) 1940–1948

measurements, samples were purged with helium 99 times to ensure complete removal of adsorbed gases. The helium density was determined from an average of 10 measurements. 2.6. Pore size distribution determination Pore size distributions were obtained by mercury intrusion porosimetry (MIP), using an AUTOPORE II 9220 mercury porosimeter (Micromeritics). Experiments were carried out on oven-dried 0.5 g sub-samples, which consisted of approximately 2 mm diameter sized chunks. All measurements were carried out using calibrated 3 ml powder penetrometers. Measurements were performed at pressures ranging from 3.4 kPa to 414 MPa which, according to the Washburn equation [7], equates to a pore size range of 0.0036–437 mm at a surface tension of 485 m N mK1 and a contact angle of 1408 [8]. All results have been expressed on a volume per gram of dry coal basis. While the boundary between inter- and intra-particle porosity is a somewhat arbitrary distinction, in this study, it has been set at 60 mm based on experience with MTE products that show very little porosity in this pore diameter region (see Fig. 4).

2.9. Non-purgeable organic carbon analysis The total non-purgeable organic carbon (NPOC) content of the MTE wastewater was determined using a Shimadzu TOC-5000 analyser. In this process, samples were acidified to liberate the inorganic carbon as carbon dioxide, which was subsequently purged. It is possible that some volatile organic substances are lost during this process. The organic carbon content was determined by oxidation and the evolved carbon dioxide determined using infrared detection. 2.10. Inorganic analysis The concentration of acid extractable ions such as Al, Fe, Ca, Mg and Na present in the raw lignite and MTE processed lignites was determined using a method based on the Australian Standard AS 2434.9 [10]. To determine the concentration of water-extractable sodium present in the raw LYLA lignite, 100 g (wb) sub-samples were water washed over a 24 h period with 500 ml of distilled water. Samples were subsequently filtered and the washing process repeated twice more.

3. Results and discussion 2.7. Total volume and pore volume determination for wet and oven-dried MTE products To determine how the total product volume of LYLA lignites was affected by varying MTE processing conditions and subsequent drying in a 105 8C oven, calipers were used to directly measure the volume change between freshly prepared and oven-dried MTE products. Comparisons were then made to the raw lignite, the volume of which was taken as the summation of the specific volume of the carbonaceous matter (determined by helium density measurements) and the water volume (determined from moisture content measurements). The pore volumes of MTE products (whether wet or dried) were determined from the difference between the total product volume (determined using calipers) and the specific volume (determined by helium density measurements on the dry product). 2.8. Moisture holding capacity determination Moisture holding capacity (MHC) experiments were performed on 5 g sub-samples, which were exposed to a constant relative humidity of 96% at a temperature of 30 8C, according to a method based on the Australian Standard AS2424.3 [9]. Experiments were carried out by weighing the sub-sample into a glass dish, which was placed inside a desiccator containing a 30–40 mm thick potassium sulphate pulp. The desiccator was evacuated and submerged in a 30 8C bath for 9 days. The sample was then removed, weighed and the moisture content determined.

3.1. Effect of MTE on residual water content The MTE process significantly reduced the moisture content of LYLA lignites. Under the most severe processing conditions (12.7 MPa/250 8C), the water content of the raw lignite was reduced from 1.44 to 0.22 (g/g) (db), corresponding to an 85% reduction of the amount of water originally present in the raw lignite. The moisture content decreased approximately linearly with increasing temperature, for each of the applied pressures investigated (Fig. 2(a)), so that between 125 and 200 8C, the moisture content was reduced by 0.035 and 0.047 g/g (db) for every 10 8C increase in temperature. The moisture content decreased significantly up to applied pressures of about 5 MPa, beyond which a further doubling in pressure had relative little impact (Fig. 2(b)). This is in agreement with the findings of Guo [5] and Bergins [11]. Thus beyond an applied pressure of about 5 MPa, further moisture reductions are more effectively achieved by increasing the processing temperature rather than pressure. Bergins [12] showed that the reduction in moisture content is due to both thermal and mechanical dewatering processes. During the application of mechanical pressure, water is expelled from the coal structure via particle consolidation and collapse of the coal structure, while during thermal dewatering water is expelled via several mechanisms [13], including † expansion of water due to a decrease in water density with increasing temperature;

J. Hulston et al. / Fuel 84 (2005) 1940–1948

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(b) 1.6

(a) 1.6 12.7 MPa 5.1 MPa 2.5 MPa

1.2 1.0 0.8 0.6 0.4

125˚C

1.4

Residual water content (g/g, db)

Residual water content (g/g, db)

1.4

200˚C

1.2 1.0 0.8 0.6 0.4

0.2

0.2

0.0 100 120 140 160 180 200 220 240 260 Temperature (˚C)

0.0

0

2

4 6 8 10 12 Applied Pressure (MPa)

14

Fig. 2. Effect of MTE temperature (a) and pressure (b) on the residual water content of LYLA lignite.

† decomposition of oxygen containing functional groups, which leads to the formation of CO2 which forces water out of the coal structure; and † possible collapse of the coal structure. 3.2. Effect of MTE on helium density, surface area and micropore volume Under the conditions investigated, changes in MTE temperature and pressure had relatively little effect on the helium density, as well as the surface area and limiting micropore volumes of MTE treated LYLA lignites (Table 1). However, as these analyses can only be carried out on dried samples, differences between raw lignite and wet MTE products must be interpreted with caution, as the shrinkage on drying of these two materials is not necessarily the same. 3.3. Effect of MTE and oven drying on lignite volume Shown in Fig. 3 are the effects of MTE temperature and pressure on the reduction of lignite volume, which was taken as the volume difference between the raw lignite volume and the volume of the wet MTE product (Fig. 3, white bars). The raw lignite volume was taken as the summation of the specific volume plus water volume (88.2 cm3). The wet MTE product volume was determined by measuring the external product dimensions using calipers. As expected from the residual moisture content trends, increases in both MTE temperature and pressure resulted in significant lignite volume reductions (white bars, Fig. 3). Even under quite mild MTE processing conditions of 2.5 MPa and 125 8C a 16% reduction in lignite volume was achieved. Under the most severe conditions tested (12.7 MPa/250 8C), this was increased to 53%. The loss of lignite volume was found to directly correlate to the volume of water lost during the MTE process. Upon subsequent oven drying of the wet MTE products at 105 8C, a further reduction in lignite volume was

observed as a function of both increasing temperature and pressure (grey bars, Fig. 3). This volume reduction was calculated from the volume difference between the wet and oven-dried MTE products measured using calipers. It should, however, be noted that the volume measurement of the oven-dried MTE product also includes the volume of cracks that may have formed during oven drying. From Fig. 3 it is clear that, while the volume of the oven-dried MTE products (black bars) decreases with increasing temperature and pressure, the effect is not as pronounced as for the wet MTE products (black plus grey bars). This behaviour has been investigated in more detail by Hulston et al. [14]. 3.4. Effect of MTE on pore size distribution One means of characterising the pore volume of MTE products is via mercury intrusion porosimetry (MIP). The advantage of this technique is that it allows the rapid Table 1 Effect of MTE temperature and pressure on helium density, CO2 surface area and micropore volume (determined by CO2 adsorption method) of LYLA lignite Process conditions

rHe (g cmK1) G0.003

Surface area (m2 gG1) G11

Microporosity (cm3 gK1) G0.003

Untreated LYLA 125 8C/2.5 MPa 180 8C/2.5 MPa 200 8C/2.5 MPa 125 8C/5.1 MPa 150 8C/5.1 MPa 180 8C/5.1 MPa 200 8C/5.1 MPa 125 8C/12.7 MPa 150 8C/12.7 MPa 200 8C/12.7 MPa 250 8C/12.7 MPa

1.393 1.394 1.395 1.394 1.401 1.400 1.400 1.401 1.398 1.397 1.398 1.396

218 205 205 218 202 216 214 216 198 200 216 184

0.058 0.054 0.054 0.058 0.054 0.057 0.057 0.057 0.053 0.053 0.058 0.048

Values for untreated LYLA have been shown for comparison.

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J. Hulston et al. / Fuel 84 (2005) 1940–1948 Table 2 Calculation method used to determine macro-, meso- and micropore volumes of MTE treated LYLA lignite

Dry MTE product volume Raw lignite to wet MTE product volume reduction Wet to dry MTE volume reduction 90

Volume

IUPAC diameter range

Diameter range used

Method

Vmicro Vmeso

d!2 nm 2 nm!d! 0.05 mm dO0.05 mm

0.5 nm!d!2 nm 3.6 nm!d!0.05 mm

CO2 adsorption MIP (Cumulative pore volume) MIP (Cumulative pore volume)

3

Lignite volume (cm )

80 70 60 50

Vmacro

40 30 20 10 0

125˚C 180˚C 200˚C

125˚C 150˚C 180˚C 200˚C

2.5 MPa

5.1 MPa

125˚C 200˚C 250˚C

12.7 MPa

Fig. 3. Effect of MTE process and oven drying on the volume reduction of LYLA lignites.

probing of pore volumes over a broad pore size range. Unfortunately, as samples need to be dried prior to testing, pore size distributions could only be obtained for oven-dried MTE products. Furthermore, care needs to be taken when analysing results for very small pore diameters (i.e. lower mesopore size region) as the high mercury intrusion pressures required to scan this size region may result in sample compression or degradation. Irrespective of these limitations, the technique is a valuable tool for comparing trends between the various MTE processing conditions. Shown in Fig. 4 is the pore size distribution of MTE products prepared under both mild (125 8C/2.5 MPa) and severe (250 8C/12.7 MPa) MTE processing conditions. 0.70 Raw LYLA

0.60

dV/dlogD (cm3.g-1) (db)

0.05 mm!d!60 mm

200˚C/12.7 MPa 125˚C/2.5 MPa

0.50 Inter

Intra

Macro

Meso

Micro

0.40 0.30 0.20 0.10 0.00 1000

100

10

1

0.1

0.01

0.001

Pore diameter (µm) Fig. 4. Pore size distribution of raw LYLA lignite and MTE products prepared under mild (125 8C/2.5 MPa) and severe (250 8C/12.7 MPa) processing conditions. Intra- and inter-particle, as well as macro-, mesoand macropore diameter boundaries have been shown for reference.

For comparative purposes the pore size distribution of a raw LYLA lignite has been included. To aid in the interpretation of the pore size distribution data, the pore size range has been divided into several regions. The first distinction has been made between inter- and intra-particle porosity, the boundary of which has been set at 60 mm. Whereas the inter-particle porosity represents the volume contribution from the interstices between particles, the intra-particle porosity represents the volume contribution from within particles. The second distinction subdivides the intra-particle porosity into macro-, meso- and micropore size regions as defined in Table 2. The lower bound to the mesopore size range has been set at 3.6 nm, slightly higher than the conventional IUPAC [15] boundary of 2 nm (Table 2), as the instrument cannot intrude mercury into pores smaller than 3.6 nm in diameter. The micropore volume in turn was estimated from CO2 adsorption isotherms using the Dubinin-Radushkevitch equation [7], where the pore diameter was taken to range between 0.5 and 2 nm [16]. Whereas the pore volume in the 2–3.6 nm region was not measured, it is believed to be relatively small. From Fig. 4 it can be seen that the porosity of raw LYLA lignite is mainly made up of macropores, with diameters ranging between 0.05 and 2 mm. Under mild MTE processing conditions (125 8C/12.7 MPa), the macropore volume is significantly reduced, and there is a shift to smaller pore diameters. Under severe MTE processing conditions (250 8C/12.7 MPa) little macropore volume remains, which is associated with a substantial shift to smaller diameter pores and an increase in mesopore volume. To allow easy comparisons to be made between the various MTE processing conditions, the findings have been expressed on a volume basis, calculated from the relationships provided in Table 2. The effect of MTE pressure and temperature on the macro-, meso- and micropore volumes are shown in Fig. 5. The macropore volume decreased as a function of increasing temperature (125–200 8C) at all three test pressures investigated. At 12.7 MPa, however, no further reduction in macropore volume was observed above 200 8C. The macropore volume decreased with pressure up to an applied pressure of about 5.1 MPa with little further change when the pressure was increased to 12.7 MPa. The mesopore volume increased with increasing temperature and pressure, although the effect of pressure

J. Hulston et al. / Fuel 84 (2005) 1940–1948

considering changes in the gel structure or the onset of chemical decomposition reactions [13]. The total pore volume was taken as the summation of the macro-, meso-, and micropore volumes defined above. Increased destruction of macropores with increasing process severity outweighs formation of mesopores, resulting in an overall decrease in total pore volume with increasing MTE temperature and pressure.

V(Macropores) 0.05 µm< d <60 µm V(Micropores) 0.5 nm < d< 2 nm V(Mesopores) 3.6 nm < d < 0.05 µm 0.8 0.7 0.6 0.5

3

Pore volume (cm /g of dry coal)

1945

0.4

3.5. Effect of MTE on moisture holding capacity 0.3 0.2 0.1 Raw LYLA

125˚C 180˚C 200˚C

125˚C 150˚C 180˚C 200˚C

2.5 MPa

5.1 MPa

125˚C 150˚C 200˚C 250˚C

12.7 MPa

Fig. 5. Effect of MTE temperature and pressure on the macro-, meso- and micropore volumes of oven-dried LYLA MTE products.

was less pronounced at 200 8C. In contrast, the MTE process had little effect on the micropore volume as determined from CO2 adsorption experiments (also see Table 1). Furthermore, the results indicate that while the macropore volume contribution under mild MTE conditions is significantly larger than that of meso- or macropores, under more severe MTE conditions, the volume contribution for each size range becomes more comparable. The observed decrease in macropore volume and increase in mesopore volume, suggests that some of the macropores are being converted into mesopores. A possible mechanism could involve the deformation of pore shape, whereby under the application of elevated pressures, cylindrical pores become deformed into more oval shaped pores, a process, which would further be facilitated by the softening of the coal structure with increasing temperature. This hypothesis is further supported by the fact that the surface area remains constant with MTE processing conditions. Other perspectives on these changes might be gleaned by (a) 1.0 0.9 0.8

(b) 1.0 12.7 MPa

MHC (g/g, db)

0.7

0.5 0.4

200˚C

0.8

2.5 MPa

0.6

125˚C

0.9

5.1 MPa

0.7

MHC (g/g, db)

0.0

The moisture holding capacity (MHC) is defined as the equilibrium moisture content of a wet sample at a relative humidity of 96% and a constant temperature of 30 8C [9]. It provides a valuable tool for monitoring changes occurring in the coal structure as a result of the MTE process. The moisture holding capacity of MTE treated LYLA lignites followed a similar trend to the residual moisture content, decreasing approximately linearly with increasing temperature (cf. Figs. 6(a) and 2(a)) and decreasing significantly with pressure up to an applied pressure of about 5 MPa, with little change occurring at higher pressures (cf. Figs. 6(b) and 2(b)). Shown in Fig. 7(a) is the relationship between the MHC and moisture content of wet MTE products. Up to a moisture content of about 0.8 g/g (db), the two parameters follow a linear relationship, with the MHC value being slightly lower than the moisture content. The slightly lower MHC value is to be expected as MHC experiments were performed at a relative humidity of 96% and as such not all pores will be fully filled with water. Above a moisture content of 0.8 g/g (db), however, the MHC became substantially lower than the moisture content value of the wet MTE products. These findings can be compared to the observations made by Ode and Gibson [17], who found a direct correlation between the moisture holding capacity and bed moisture content of USA sub-bituminous coals, whereas for brown coals and lignites the MHC was significantly lower than the bed moisture content. A reduction in MHC may be attributed to a reduction in surface area, a reduction in pore volume or a loss of oxygen

0.6 0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0 100 120 140 160 180 200 220 240 260 Temperature (˚C)

0.0

0

2

4 6 8 10 12 Applied Pressure (MPa)

14

Fig. 6. Effect of MTE temperature (a) and pressure (b) on the moisture holding capacity (MHC) of LYLA lignite.

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J. Hulston et al. / Fuel 84 (2005) 1940–1948

3

Moisture holding capacity (cm /g)

1.4 1.2 1.0

Raw LYLA

0.8 0.6 0.4 0.2

Moisture content - MHC (cm3/g) (db)

(b) 0.5

(a) 1.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.3

0.2

0.1

0.0 0.0

0.0 0.0

0.4

1.6

0.1

0.2

0.3

0.4

0.5

Macropore volume(cm 3/g) (db)

Residual moisture content (cm3/g) (db)

Fig. 7. Relationship between MHC and residual moisture content (a) and macropore volume (b) of MTE treated LYLA lignite.

containing functional groups (i.e. a decrease in hydrophilic character) [18]. The relatively low temperatures used during the MTE process, and the reduction in MHC with increasing pressure at constant temperature, suggest that changes occurring in the oxygen functionality of the MTE products are not the main driving force for the observed trends. No changes in surface area with processing conditions were observed for the dried MTE products, but it is not known whether this will carry over to wet MTE products. Since the MHC followed similar trends to the residual moisture content, it is most likely that the decrease in moisture holding capacity is controlled by changes occurring in pore volume. This is more clearly illustrated in Fig. 7(b), where the difference between the moisture content and MHC has been plotted against the macropore volume determined from MIP. The calculated moisture content difference corresponds to the volume of water not retained during the MHC experiment, and as shown in Fig. 7(b), decreases significantly with a decrease in the macropore volume. While the measured macropore volume was determined for dried MTE products, which are known to have undergone shrinkage, the trends are expected to be similar for wet MTE products. The results suggest that at higher moisture contents, the water in the macropores behaves like bulk water, which is in equilibrium with the surrounding atmosphere, thus leading to only a partial filling of the available pore space. As the moisture content decreases, the corresponding macropore volume decreases. This is associated with a corresponding increase in the proportion of water held more tightly to the coal matrix, thus explaining the direct correlation between the MHC and residual moisture content of the wet MTE products with low residual moisture content (Fig. 7(a)). 3.6. Effect of MTE on inorganic species removal Latrobe Valley lignites contain three main forms of inorganic species. The first form exists as discrete mineral inclusions in the coal matrix (e.g. silica, clays, pyrite).

The second form includes salts, such as sodium chloride dissolved in the interstitial pore water; while the third form includes species, which are ion exchanged at carboxylic acid and phenolic sites (e.g. Na, Ca and Mg). While the ash yield of most Victorian lignites is low, these inorganic species, in particular the alkali and alkaline earth metallic (AAEM) species, play an important role in pyrolysis and combustion processes, where they can act as excellent gasification catalysts, but can also give rise to slagging and fouling problems [19]. As shown in Table 3, under the conditions tested, the MTE process had little effect on inorganic species such as Mg, Ca and Al. However, Na concentrations decreased linearly with decreasing moisture content (Fig. 8). As the water is expelled from the lignite structure, one would expect the sodium concentration to decrease in proportion to the amount of water removed. However, as can be seen from Fig. 8, this was not the case, suggesting that only some of the Na is present as a soluble salt, with the remainder tightly bound to carboxylic and phenolic functional groups. This was confirmed by water Table 3 Effect of MTE temperature and pressure on the elemental composition of MTE treated LYLA lignite Process conditions Raw LYLA 125 8C/2.5 MPa 180 8C/2.5 MPa 200 8C/2.5 MPa 125 8C/5.1 MPa 150 8C/5.1 MPa 180 8C/5.1 MPa 200 8C/5.1 MPa 125 8C/12.7 MPa 150 8C/12.7 MPa 200 8C/12.7 MPa 250 8C/12.7 MPa Washed LYLA a

NP, non-pyritic.

Elemental analysis wt% (db)G0.01 wt% (db) Na

Mg

Ca

Al

FeNPa

0.09 0.07 0.07 0.06 0.06 0.06 0.06 0.05 0.06 0.06 0.05 0.06 0.04

0.08 0.08 0.08 0.08 0.07 0.07 0.08 0.08 0.08 0.08 0.07 0.06 0.07

0.04 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.02 0.01 0.01 0.03 0.00 0.01 0.02 0.00 0.00 0.02 0.02 0.00 0.02

0.07 0.07 0.07 0.06 0.07 0.07 0.06 0.07 0.06 0.07 0.07 0.07 0.07

J. Hulston et al. / Fuel 84 (2005) 1940–1948 0.10 Raw LYLA

0.09

0.07 0.06 0.05 Water washed

0.04 0.03 0.02 0.01 0.00 0.0

0.2

0.4 0.6 0.8 1.0 1.2 1.4 Residual watercontent (g/g, db)

1.6

Fig. 8. Relationship between sodium concentration and residual moisture content of MTE treated LYLA lignite.

washing experiments carried out on the raw coal, which showed a direct correlation between the water extractable sodium concentration and the moisture content of the raw coal, thus supporting the notion that the MTE process only removes the water soluble form of Na. 3.7. Effect of MTE on NPOC concentrations in wastewater Non-evaporative drying processes, such as the Fleissner process and the hydrothermal dewatering (HTD) technique, which rely on high processing temperatures, have been shown to result in significant losses of organic material to the wastewater stream. Studies undertaken by Racovalis on HTD treated Loy Yang lignites have shown non-purgeable organic carbon (NPOC) levels in the wastewater to range between 2 and 50 g/kg of dry feed coal [20]. Such high organic carbon losses not only reduce the amount of combustible product, but may also lead to high wastewater cleanup costs. To determine the impact of MTE on organic matter loss, NPOC levels in the wastewater stream were

(a) 1.4

monitored as a function of MTE temperature and pressure. As shown in Fig. 9(a), NPOC levels increased approximately linearly with increasing temperature, and increased significantly up to an applied pressure of about 5 MPa, beyond which the values started to plateau Fig. 9(b). It can be seen that the MTE process generally yielded lower NPOC levels than those reported for the HTD process. However, at the highest MTE temperature of 250 8C, the observed level was 1.3 g/kg db, just slightly lower than the value reported by Racovalis for HTD at the same temperature. Thus, it appears likely that the NPOC values in the wastewater stream are predominantly influenced by processing temperature. Gas chromatography–mass spectrometry (GC–MS) studies undertaken by Qi [21] showed the organic content of the wastewater stream to predominantly consist of low molecular weight organic acids. The effects of MTE on wastewater quality and management are currently being investigated by Butler [22].

4. Conclusions MTE significantly reduced the moisture content of raw lignite, with the moisture loss increasing with both temperature and pressure. Under the most severe conditions tested (250 8C/12.7 MPa), 85% of the water originally present in the raw lignite was removed. Under the conditions tested, the MTE process had little effect on material properties such as surface area, micropore volume and true density. With increasing MTE temperatures and pressures, the total pore volume decreased, due to the progressive collapse of macropores, which was associated with a slight increase in mesopore volume. While highly dewatered MTE products had MHC values equivalent to the residual moisture content, slightly dewatered MTE products had MHC values significantly below the residual moisture content. This latter effect has

(b) 0.9 0.8

TOC (g/kg feed coal) (db)

1.2 1.0 0.8 0.6 0.4 12.7 MPa

0.2

TOC (g/kg feed coal) (db)

Na content (wt%, db)

0.08

1947

0.7 0.6 0.5 0.4 0.3

0.2 125˚C

5.1 MPa 2.5 MPa

0.0 100 120 140 160 180 200 220 240 260 Temperature (˚C)

0.1

200˚C

0.0 0

2

4 6 8 10 Applied pressure (MPa)

12

14

Fig. 9. Effect of MTE temperature (a) and pressure (b) on the total organic carbon content of MTE wastewater.

1948

J. Hulston et al. / Fuel 84 (2005) 1940–1948

been attributed to the presence of macropores, which contain bulk water that can be lost more readily to the surrounding atmosphere. In addition, the MTE process was effective in removing the water-soluble Na fraction, which decreased linearly with the proportion of water removed. NPOC levels in the MTE product water increased with temperature, and were generally lower than those reported for HTD. This is believed to be largely due to the lower temperature regime used in the MTE process.

Acknowledgements The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Clean Power from Lignite, which is established and supported under the Australian Government’s Cooperative Research Centre Program. Many thanks also go to Dr Tim Kealy for preparing the MTE pellets and wastewater samples and to Dr Christian Bergins and Dr Marc Marshall for their valuable comments.

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