Water in Brown Coal and Its Removal

Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.

Chapter 3 Water in Brown Coal and Its Removal David J. Allardice\ Alan L. Chaffee^'\ W. Roy Jackson^ "* and Marc Marshall^'"* ^Allardice Consulting, PO Box 88, Vermont, Victoria 3133, Australia ^School of Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia ^CRCfor Clean Power from Lignite, 677 Springvale Road, Mulgrave, Victoria 3170, Australia "^Centrefor Green Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia

3.1. INTRODUCTION The brown coals of Victoria represent an important state and national resource, as they did when the first critical review 'the Science of Victorian Brown Coal' was published in 1991 [1]. Chapter 3, The Water in Brown Coal [2], reminds readers that the high residual moisture content (in the 45 - 70 % range) critically impacts on virtually every facet of brown coal utilisation. This statement is even more pertinent today in view of the energy involved in drying the coal with its consequent increase in CO2 emissions. There are several policy drivers to facilitate reduced CO2 emissions from brown coal utilisation, which have contributed to a major shift in brown coal research emphasis both locally and internationally from the situation in 1991. These drivers have stimulated research into more efficient methods of drying brown coals and this in turn has revitalised fundamental studies of brown coal-water interactions. These fundamental studies have mainly been carried out in Victoria but there have also been important contributions from other Australian States, notably South Australia, and from overseas, particularly Japan. In contrast, many countries that bum low rank coals are active in developing new drying technologies. This chapter is divided into three parts. The first part deals with the physics, chemistry and analytical aspects of water in brown coal, the second with the drying and dewatering technologies and the third with the binderless briquetting technology for brown coal where the water in the coal plays a critical role. Emphasis will be placed on advances that have been made since the publication of Allardice's chapter in 1991 [2] but there will be some repetition where it is necessary for ease of comprehension.

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3.2. PHYSICS, CHEMISTRY AND ANALYTICAL ASPECTS OF WATER IN BROWN COAL 3.2.1. Isotherms and Hysteresis The simplest way of obtaining information on coal-water interactions is still by measuring the equilibrium moisture content of coals at different water vapour pressures at a fixed temperature [2]. Recent measurements have used both classical desiccator experiments [3-8] and an automated microbalance [4,7,8]. Victorian brown coals show sigmoid isotherms, typical for physical adsorption of condensable vapours on porous substances [9]. Figure 3.1 is an example based on results from a microbalance [10]. It shows the typical irreversible loss of moisture in the initial desorption isotherm from the bed moist state, which has been attributed to irreversible collapse of the pore structure of the coal during initial drying [3,10,11]. The subsequent adsorption cycle shows significant hysteresis, with the re-adsorption curve following a lower trajectory over the full relative pressure range. This has been attributed to swelling and shrinkage effects, which alter the structure of a colloidal gel such as low rank coal [11]. In contrast, the hysteresis observed in porous solids due to capillary filling is usually confined to relative vapour pressures above 0.5 p/po.

RAW COAL MOfSrUf^e 1 CONTfNJ 7'Q kg/kg T

a-e

^

0'? RtLATiVt

Oi VAPOR PfftSSURE

p/p^

Figure 3.1 Water sorption isotherms on Yalloum brown coal at 30°C [10].

Water in Brown Coal and Its Removal

87

0.16

^ 0.12 i

0.08

0.04

L

• Isotherm 1 :ads • Isotherm 2:des A Isotherm 3:acls

1 .

0

0.2

X Isotherm 4:des

,,,,,,,, 0.4

0.6

0.8

1

Relative vapour pressure P/PO

Figure 3.2 The effect of adsorption-desorption cycling on the isotherm shape for a Loy Yang coal sample previously dried in vacuo at 30°C to constant mass. Isotherms were obtained at 30°C in an automated microbalance [12].

Clemow and Chaffee [12] have constructed isotherms using an automated microbalance (the Hiden Intelligent Gravimetric Analyser). Commencing with vacuum dried coal, the irreversible portion of the isotherm is no longer present in Figure 3.2. The isotherm still exhibits the typical hysteresis loop, but the multiple adsorption/desorption cycles are nearly coincident, illustrating that, after the initial desorption, the process appears to be reversible. An extensive study of the relationship between several coal analytical parameters and sorption isotherms was carried out by Hall and co-workers [5] for four Victorian coals, Morwell, Loy Yang, Yalloum and Yallourn North Extension. This detailed report, commissioned by the Coal Corporation of Victoria, is available from the library of HRL Pty Ltd. The authors found that the Morwell, Loy Yang and Yalloum samples showed almost identical equilibrium water values at 9.2 and 25.3 % relative humidities and 21°C, irrespective of their total acidity, carboxylic group content and surface area. Yalloum North Extension showed a significantly lower sorption value. It must be noted that the variations in coal parameters were very small e.g. total carboxylic acid group content between 2.97 and 3.25 mmol/g db. Therefore it is not surprising that correlations of the type noted by Schafer [13] for a wide range of coals with a wide range of functional group parameters, e.g. total carboxylic acid group content from ca. 1.0 to 4.0 mmol/g db, were not observed.

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Only small effects of acid washing were observed, probably because of the low inorganic content of the coal samples. Use of ultrasonic mixing in leaching experiments generally led to increased sorption values. Addition of cations to the acid-washed coals increased the equilibrium moisture content over a range of relative humidity (9 - 75 %) [5,6], in agreement with previous observations made by Schafer [13] at a single relative humidity of 52%. The authors also examined four lithotypes (light, medium light, medium dark and dark) all from the same Yalloum seam [5]. There was clearly a trend for the darker lithotypes to hold more water in both desorption and adsorption experiments. The Yalloum lithotypes showed a greater variation in functional group content than the runof-mine (ROM) coals discussed previously (e.g. total carboxylic acid group content from 2.6 to 3.2 mmol g'^ db) and the adsorbed and desorbed water values showed some increase with acidity. Oxidised samples of the same lithotype showed increased sorption in all cases. The isosteric heat of sorption obtained from desorption plots at three temperatures for Yalloum and Morwell ROM coals were similar to values reported by Allardice and Evans [10] for a Yalloum coal. Both sets of values showed a similar decrease with increasing sorbed water content. The electrophoretic mobility of the ROM coals fell into two classes at lower pH with Yalloum and the Yalloum North Extension coals showing less negative values than the Morwell and Loy Yang coals. The coals with the less negative curves had lower equilibrium moisture contents [5]. The small number of coals used in the above work made it difficult to draw general conclusions. Allardice and co-workers [4] and Clemow and co-workers [14], as a prelude to a study of freezing and non-freezing water in Victorian brown coals, carried out desiccator experiments on 15 raw brown coals, 4 acid-washed coals and 7 hydrothermally dewatered (HTD) products from the coals. Statistically significant correlations were noted between the water contents at 30°C for a given coal or coal product at 15 % and 52 % relative humidity. Similar correlations were found between the 30''C water contents at 15 % and 92 % relative humidity. However, the water content of the 15 raw coals at 96 % relative humidity or 100 % relative humidity (bed moisture) did not correlate with the water content at lower humidities. This contrast suggests that the interactions that control water content at very high relative humidities (> 92 %) are different from those that operate at lower values. These interactions are further discussed below (see Section 3.2.4). These results also confirm the earlier conclusion that the bed moisture does not equal the moisture holding capacity, for raw coals with a moisture holding capacity greater than ca. 40 wt % (wet basis) i.e. 60 g water/ 100 g dry coal [2]. A compilation of the results from the two studies [2,4] is illustrated in Figure 3.3. Acid washing the coals to remove soluble inorganic salts and inorganic cations led to a consistent but variable reduction of equilibrium moisture content at 15, 52 and 92 % relative humidities but not at 96 % [4]. Schafer [13] had previously established this reduction in equilibrium moisture content on acid washing, but only at 52 % relative humidity.

89

Water in Brown Coal and Its Removal

Previous work has demonstrated that acidic functional groups in the coal are the major hydrophilic groups responsible for strong binding of water to the coal surface [2]. Clemow and co-workers [14] determined the non-acidic oxygen, phenolic (plus phenolate) and carboxylic (plus carboxylate) contents and correlated them with the equilibrium moisture contents of the coals at 15 % relative humidity, which approximate to the monolayer water as determined from the BET equation [9]. The only significant linear correlation was with the carboxylate plus carboxylic acid content, in general agreement with the results of Schafer [13] for measurements at 52 % relative humidity and those of Boger and co-workers [15] for heats of wetting. The authors made no attempt to differentiate between the effects of carboxylic acids and carboxylate groups but a regression coefficient of 1.2 ± 0.7 (90 % confidence limits) suggested that each of these functionalities was strongly associated with one water molecule. The correlation between Yalloum lithotypes and equilibrium moisture content noted by Hall and co-workers [5] was confirmed. However, the correlation was not found for lithotypes from other seams [4]. 3.2.2. Heats of Sorption from Isotherm Data Thermodynamic information on the water in the coal, such as heats of desorption and adsorption, can be obtained from moisture isotherms [10]. However, to get sensible

280

240 i i"220 o o

200

B 180 .io 160 I 140 CO

120 100

30

50

70

90

110

130

150

170

190

210

Moisture Holding Capacity (g/IOOg coal db) • This Study • Allardice

Figure 3.3 Relationship between bed moisture and moisture holding capacity for brown coals, including data (•) from Ref [2] and (•)fromRef [4].

Chapter 3

90

thermodynamic data, the isotherms must be constructed with a zero moisture datum point which is also isothermally determined i.e. by drying or evacuation at the isotherm temperature and not by subsequently determining the moisture content of the samples in an oven at a higher temperature such as 105°C. From a series of isotherms determined in this way, the Clausius-Clapeyron equation, modified for the equilibrium between a vapour and its adsorbed state on a solid, can be used to determine the heat of condensation or evaporation at different degrees of adsorption, i.e. the isosteric heat of sorption [2]. ainp_ q,t dT RT^

(3-1)

where qst is the isosteric heat of sorption (or the latent heat of vaporisation at constant moisture content V), p is the water vapour pressure, T is the absolute temperature and R the universal gas constant, qst can be determined from the slope of a plot of log p against 1/T for the selected moisture contents. Figure 3.4 shows that the isosteric heat of desorption of water from brown coal, from the saturated or bed moist state and through the capillary region of the isotherms, is

J-S

g

B-C

%

¥

I I

IB lATUn

2H>

RAW COAL MOISTURE

HEAT Of

-L 0^2

0-4 MOiSTUdE

CONTENT

I C'6

1

o-e

kgJHg DRY COAL

Figure 3.4 Isosteric heat of desorption of water from Yalloum brown coal as a function of moisture content [10].

Water in Brown Cool and Its Removal

91

essentially identical to the latent heat of vaporisation of liquid water. At lower moisture contents, there is a gradual increase in the heat of desorption through the multilayer and monolayer regions. 3.2.3. ^H NMR Experiments Early work by Lynch and his colleagues using ^H NMR techniques showed that most of the water associated with brown coal is mobile at temperatures above 0°C and not bonded to the surface but that water below 0.25 kg/kg dry coal is bonded through a continuous distribution of states [16]. More recent work by Lynch and co-workers [17] not only used ^H NMR to determine water binding capacities of coals but also to provide data on the extent to which the coal structure is changed by its exposure to water. The results showed that the fraction of mobile hydrogen increased with a decrease in coal rank as an increasing portion of the structure was 'plasticised' by interaction with water. Norinaga and coworkers [18] later reported similar results. Further ' H NMR work by Norinaga and coworkers [19] confirmed that the amount of mobile hydrogen decreased with the nonfreezable water content of both Victorian brown coals and two American lignites. Quantification of the hydroxyl group content of the coals by infrared measurements on D2O exchanged samples suggested that the amount of mobile coal hydrogen in asreceived Yalloum and Morwell coals exceeded the hydroxylic hydrogen content. Thus a significant proportion of hydrogen atoms that are not exchanged by D2O treatment appear to be mobilised by interaction with water. The authors do not speculate as to the chemical structure of these hydrogen atoms. ^H NMR studies of Yalloum, Beulah Zap and Illinois coals that had been partially dried and re-wetted were used by Norinaga and co-workers [20] to estimate the amounts of non-freezable pore water, freezable pore water and bulk water contents. The amounts of non-freezable and freezable pore water for the low rank coals were found to be in excellent agreement with values determined by DSC measurements (see below). It should be noted that the boundary between bulk water and freezable pore water was set at 260 K, somewhat lower than the 270 K used by other workers [14]. The freezing point distributions of water in the pores were used to calculate the pore size distribution following the Gibbs-Thompson equation (also see Chapter 2). The results suggested that removal of non-freezable water (but not freezable or bulk water) led to an irreversible decrease in the average pore diameter that was attributed to pore collapse. This observation can be linked with the irreversible change noted in the adsorption isotherm (see Section 3.2.1). Further work by Hayashi and his co-workers [21] using a different pulse sequence showed that the initial amplitude of the signal corresponded quantitatively to the amount of'mobile' proton. The signal was interpreted in terms of three components with different T2 values that were attributed to free water, pore water (freezable bound water and non-freezable water) and mobile coal hydroxyls. Analysis using a theory which relates pore dimension to pore liquid relaxation time concluded that the water-filled pores were slit-like rather than cylindrical, initially ca. 3

92

Chapter 3

nm in dimension but decreasing in dimension with loss of pore water in a roughly linear fashion. The rate of loss of water from two Victorian brown coals has been measured using ^H NMR relaxation time measurements [22]. A waxy, high atomic H/C ratio (1.13) Yalloum coal was shown to lose water at ca. 3 times the rate of lower H/C ratio (0.81) Loy Yang coal. 3.2.4. Differential Scanning Calorimetry The first attempt to characterise freezable water in Victorian low rank coals using differential scanning calorimetry (DSC) was published by Norinaga and co-workers [18]. The DSC traces showed two exothermic peaks centred at 226 and 258 K during cooling for Yalloum, Loy Yang and Morwell coals, whereas only a single broad endothermic peak centred at 273 K was observed during heating. The enthalpies of freezing associated with the 226 and 258 K peaks were determined by measuring the changes in heat generated due to congelation as a function of water content. Values of 333 J g"' water for the 258 K peak and 188 J g ' for the 226 K peak were obtained implying that the 258 K peak was due to water whose properties were very similar to those of bulk water. The amounts of bound (226 K) and bulk (258 K) water could be calculated and the amount of non-freezable water obtained by difference. Two other recent studies have reported on the distribution of freezing (freezable) and non-fi-eezing water in Victorian brown coals [4,14,23]. The non-freezing water contents of three brown coals, Loy Yang Low Ash (LYLA), two Yalloum Township coals, a

Table 3.1 Non-freezing water contents as measured by proton nuclear magnetic resonance spectroscopy ('H NMR) and differential scanning calorimetry (DSC) [4]. /->

1

1

Coal 01 product

LYLA HTD250 HTD290 HTDHI HTDLO YTP YTD

a

Non-freezing water (g/1 OOg coal db) DSC

'HNMR 56.2 25.5 17.9 10.3 9.5 50.0 77.8

56.2 ND^ ND 13.7 ND 60.9 89.2

a, LYLA, Loy Yang Low Ash coal, medium dark lithotype; HTD250, LYLA hydrothermally treated at 250°C to give a high-porosity product; HTD290, LYLA hydrothermally treated at 290°C to give a high-porosity product; HTDHI, LYLA hydrothermally treated at 320X to give a high-porosity product; HTDLO, LYLA hydrothermally treated at 320°C to give a low porosity product; YTP, Yalloum Township pale lithotype coal; YTD, Yalloum Township dark lithotype coal. References to coal analyses are given in [4]. b, ND = not determined.

Water in Brown Coal and Its Removal

93

pale lithotype (YTP) and a dark lithotype (YTD), and a high porosity hydrothermally dewatered LYLA coal product were determined by the DSC method [4,14]. These results together with ^H NMR determinations of non-freezing water are summarised in Table 3.1. Results for other hydrothermally dewatered products from LYLA coal are also included in the Table. The DSC results for non-freezing water were on average 12 % greater than those from ^H NMR measurements. Barton and Lynch [24] reported a similar difference for some bituminous coals, which was attributed to differences in sample preparation for the two techniques. Identical sample preparations were used in this later study and a probable explanation is that there is an underestimation of freezing water by the DSC method because of the assumption that the latent heat of fusion of all freezing water present in the coal is equal to that of bulk water. Other DSC studies [25] have shown that the heat of fusion of water adsorbed as thin films (>10 nm) on solid surfaces is lower than that of bulk water leading to an underestimate of freezing water. Norinaga and co-workers [18] also noted a component of adsorbed water with a low heat of fusion. Comparison of the non-freezing water (^H NMR values) with the equilibrium moisture content as determined by desiccators indicated that for the coals the nonfreezing water corresponded to the equilibrium moisture content at ca. 93 % relative humidity in all cases [4,14]. This is lower than the relative humidity of 96 - 100 % found for two bituminous coals by Barton and Lynch [24]. Correlation of the non-freezing water (^H NMR values) with the functional group contents indicated a dependence almost entirely on the carboxylic acid plus carboxylate content. The plot showed a linear regression coefficient of 10.9 suggesting that about 10 water molecules were associated with each acidic site sufficiently strongly to be inhibited from freezing. Combining this result with the correlation between monolayer water and carboxylic acid plus carboxylate content [14] (see Section 3.2.1) suggests a picture at equilibrium wherein each strong acid site has one very strongly bound water molecule attached with another 10 or so molecules still recognisably bound to the water molecule - strong acid complex. These differences between freezing and non-freezing water explain the change in the interactions that control the equilibrium moisture below and above 92 % relative humidity (see Section 3.2.1). The water that is lost at relative humidities greater than ca. 93 % is similar to bulk water and is not meaningfully associated with functional groups in the coal. Experiments carried out by these workers [4,14] involved heating the sample in the DSC and thus no peak at 230 K was observed as predicted by Norinaga and co-workers [18]. A further DSC study [23] of water in six brown coals in which the DSC samples were cooled as by Norinaga and co-workers [18] has reported one or two peaks at around 230 K. A similar peak was observed at the same temperature for solutions of sodium chloride, as previously reported by Hvidt and Borch [26]. However, the intensity of the 230 K peak for the coals did not correlate with the sodium ion concentration of the coal and indeed persisted with no change in coal samples which had been acid washed to remove soluble salts. Solutions of MgCl2, Mg(0Ac)2, CaCli, AICI3, FeCl2, succinic acid and benzene-1,2,4-tricarboxylic acid showed no peak other

94

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than the bulk water peak [27]. A mixture of the benzene tricarboxylic acid and NaCl also showed only the bulk water peak but a mixture of succinic acid and NaCl showed a significant peak at 230 K. The specific phenomenon, which causes the appearance of the 230 K peak in the coals, is thus uncertain. Norinaga and co-workers [18] attributed the 230 K peak to water whose properties were modified by enclosure in pores (also see Chapter 2). The DSC curves for the Victorian brown coal sample appear similar in the two publications [17,23] and the proportion of this pore water is calculated to be ca. 26 % of the total water by Norinaga and co-workers [18]. Miura and co-workers [28], using FTIR and DSC, attempted to estimate the strength of the brown coal-water interaction and reported that the enthalpy decreased as desorption progressed. 3.2.5. Molecular Modelling of Brown - Coal Water Interactions In a bid to develop a better molecular level understanding of brown coal-water interactions, two groups have applied molecular simulation approaches to the evaluation of model coal structures. Kumagai and co-workers [29] modelled the structure of Yalloum brown coal using two oligomers, namely a tetramer (MW 1540) and a pentamer (MW 1924) based on a monomer of composition C21H20O7, as illustrated in Figure 3.5. The unit structure was constructed on the basis of combined data from elemental analysis (C: 65.6, H: 5.2, O: 29.2 wt %) and ^^C-NMR spectroscopy. Initially, the two oligomers were combined with 360 water molecules, corresponding to 65.3 % moisture content (wet basis). After the minimum energy configuration was identified, the potential energy and volume occupation were calculated. The process was repeated in a step-wise fashion for analogous systems with progressively fewer water molecules, down to 0 (simulating dry coal). A monotonic decrease in the volume occupied by the brown coal-water model was observed, with the completely dried product occupying approximately one half of the volume of the original wet coal model (Figure 3.6). The volume occupied by the brown coal model itself (i.e. excluding the water from consideration) also decreased as water was removed, as a result of changes in molecular configuration of the oligomers. This change in molecular configuration was found to be irreversible; in other words, the reintroduction of water did not cause the configuration of the brown coal oligomers to revert to their original arrangement and the original volume of the brown coal-water model was not recovered. These model results were observed to correlate closely with the results of concurrent experimental measurements in which both the % moisture and volume of Yalloum brown coal were determined after equilibration at relative humidities over the range 0-85 %. Rather than attempting to model 'whole' coal, Vu and co-workers [30,31] used fossil wood as the basis of their modelling strategy to study brown coal-water interactions. In many parts of the Victorian brown coal deposits, morphologically distinct macroscopic fossil wood can be readily handpicked from the surrounding coal matrix. Chemical and spectroscopic data from such a sample were used to construct a model (C]ooH8o028) of

Water in Brown Cool and Its Removal

95

what is essentially degraded lignin (Figure 3.7). A 3-dimensional packing arrangement of three of these model units together with 470 water molecules (62 % by weight) into a periodic unit cell with dimensions 27.3 x 27.3 x 27.3 A is depicted in Figure 3.8. Vu and co-workers [31] used a dynamic modelling approach to investigate the interactions between water and structural subunits of degraded lignin and, also, three idealised, non-degraded lignin systems. Their work indicates that the diffusion of water molecules in the vicinity of lignin is substantially reduced compared to pure water, largely as a result of hydrogen bonding interactions (Table 3.2). The significantly reduced mobility of water molecules in proximity to carbonyl groups is attributed to

H3CO Figure 3.5 Monomer structure used as the basis for the Yalloum brown coal molecular model by Kumagai and co-workers [29], corresponding to C:65.6; H:5.2; 0:29.2 wt % (MW=384.4).

'""" >"' r k A

«' A

'^

• •

i '"""f a ik

t'" '»"" 1 '""* 4 ^ ^

1 -1

T

m

1 m\

ml

• j#OCX>MiWAraEl 1

1 1. »

m

t

m

i

1

m

J * s J

1

1

m

limn

1

tm

MiiLflure rentov^ wi% Figure 3.6 Monotonic reduction in volume occupied by the wet Yalloum coal model structure as it is dried from 65.3 % down to 0 % moisture (•). Upon rehydration, the volume returns to only ca. 80 % of its original value (o). The volume occupied by the coal itself (A) does not change until more than 80 % of the original water present has been removed. Reprinted from Ref 29 with permission from the authors.

96

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hydrogen bond formation. Conversely, the relatively enhanced diffusion of water molecules in proximity to methoxy groups is attributed to a hydrophobic effect. 3.2.6. Other Brown Coal - Water Surface Chemistry Studies Crawford and co-workers [32] measured the advancing and receding contact angles for light gas oil in water against the surfaces of a range of coals including a Loy Yang sample. Such measurements give information regarding the surface hydrophobicity, which is important in several coal beneficiation methods, e.g. aggregation, flotation.

• • • • • • a .

^ • ^ . . . • ^

Figure 3.7 Model of degraded lignin (C100H80O28) used to simulate brown coal-water interactions. The (degraded) lignin monomer units are circled.

Water in Brown Cool and Its

97

Removal

Figure 3.8 3-dimensional unit cell constructed from 3 degraded lignin structures (refer Figure 3.7) and water and used as the basis for molecular dynamics simulation.

Table 3.2 Time-averaged diffusion constants for water molecules in proximity to specific functional groups corresponding to the model depicted in Figure 3.8. Diffusion constants were determined fi-om molecular dynamics runs of 100 ps duration at a simulation temperature of 298 K. Local Water Group Bulk water*

Diffusion constant, cm^ s"^

Rate relative to pure water

1.94x10-'

0.49

< 2.85 A from the H of OH groups

1.84x10-'

0.47

< 4.65A from the O of all 0CH3 groups

2.88x10-'

0.73

<2.85AfromtheOofcarbonyl (C=0) groups

1.45x10-'

0.37

2.14x10-'

0.54

< 2.85A from the H of carboxylic acid groups * > 6 A from lignin surface.

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The Loy Yang coal showed a smaller contact angle hysteresis (i.e. the difference between advancing and receding contact angles) and a larger contact angle in the mineral free limit than higher rank coals. The larger concentration of oxygen-containing functional groups in the Loy Yang coal makes the organic component of the surface more hydrophilic than in higher rank coals, which will increase the contact angle, and closer in hydrophilicity to the mineral component of the surface, which reduces the hysteresis. Information concerning the structure of pores in brown coal that are open to fluid exchange has been obtained from small angle neutron scattering [33]. The influence of micro- and macro-pores on liquid exchange after four weeks of incubation with H2O/D2O mixtures was observed. It was suggested that this method could be used to measure liquid penetration into the coal matrix. 3.2.7. Factors Causing Variations in Bed Moisture Content Five factors that influence the bed moisture content of brown coal have been extensively discussed by Allardice [2] (depth of burial, compression by folding, petrographic, weathering and thermal effects). A phenomenon not noted previously which appears to influence compression and hence the coal moisture is sea level changes over the period of deposition. Evidence from marine microfossils and sand layers indicates periods when sea levels were high and flooding of the peat swamps occurred [34]. This flooding and sand deposition led to compression of the organic matter directly below and this has resulted in a decrease in moisture content and increase in rank for coal just below the marine boundary [35]. 3.2.8. Determination of Moisture Content 3.2.8.1. Definition of Moisture Content and Standard Methods of Determination Allardice [2] stated that the widely accepted definition of moisture content of brown coal is the amount of water that is released from the coal at 105 - 11 O^'C excluding water that is derived from the decomposition of functional groups. Allardice also pointed out that this definition is not an operational one, since the standard methods of moisture determination do not distinguish water originally present in the coal from that produced by decomposition up to 105-110°C. It is therefore, strictly speaking, necessary to quote the method used with any value given. The Australian Standard Methods for Moisture Determination in Lower Rank Coals have been changed. The method AS2434.5 [36] involving azeotropic distillation with toluene has been retained but only for determinations of moisture in chars from lower rank coals. A new standard AS2434.1 [37] was developed and incorporates three methods. The first is a two-stage procedure for bulk samples {ca. 500 g) in which equilibration on a tray at 38°C for 5 h and then at ambient temperature until constant weight is attained (stage 1) is followed by heating at 105 - 110°C for 3 h under N2 flowing at a fixed rate (stage 2) and final weighing. The second procedure for samples

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99

ca. 10 g involves drying at 105-110°C under a flow of N2 at a fixed rate for 3 h and final weighing. The third method is the well-established azeotropic distillation with toluene. It should be noted that sampling procedure has also been changed as of 1996. The current edition of AS2434.1 has removed the options of gas heating and the use of chromic acid to clean the glassware in the azeotropic distillation method. There is also a separate standard A2434.7 [38] for an analysis sample of coal (1 g, <212|Lim particle size, equilibrated in air), which involves heating the coal at 105110°C in a stream of nitrogen as above. The moisture is determined by weighing the sample or by absorption in magnesium perchlorate. 3.2.8,2. Rapid Methods for Moisture Determination There continues to be interest in developing rapid, on-line methods for moisture analysis for power generators, where a variation in moisture content from 64 to 68 % requires a 19 % increase in the coal feed rate to provide the same net heat input to the furnace [39]. This sensitivity has led to a target accuracy within 0.5 wt % for the moisture content of as-received coal. On-line methods of moisture determination will be even more critical for operators of advanced power generation plants. A large number of techniques have been evaluated and aspects of recent work emphasising applications to Victorian brown coals are summarised below. It should be noted that these methods all require calibration using samples analysed by the standard methods described above and are not a substitute for these methods. Normal-incidence-geometry X-ray diffraction has been used to measure moisture content of coals from several seams in the Latrobe Valley [40] but an accuracy of only 5 % was claimed. FTIR [41] in conjunction with a CIRCOM factor analysis package gave a standard error of 9 wt %, was slow and could only use very small samples. A different infrared technique, using the combination vibrations of water in the 1800 - 2100 cm"' range, has been patented [42], but its success in practice has not been reported. Fast neutron and gamma ray transmission (FANGAT) was evaluated using Yalloum and Loy Yang samples [43]. Accuracies of 0.8 to 1.8 wt % were obtained but were still considered unacceptable for raw brown coal moisture analysis. Prompt gamma-ray neutron activation analysis (PGNAA) can be used to simultaneously determine moisture, ash and the major components of Latrobe Valley brown coal (e.g. C and H values) [44,45]. It was hoped to develop an on-line estimation of a fouling index for brown coals based on this technique. Moisture determination was achieved with a root mean square (RMS) error of 0.6 to 0.8 wt % depending on the type of gauge used. Microwave transmission was considered by Allardice [2] to be a promising technique. A gauge developed for black coal determinations operating in the frequency range 2 - 4 GHz was shown to be too inaccurate for brown coal moisture determinations [41] with RMS errors of 0.7 to 1.7 wt % depending on the coal. More recent work, operating at a lower frequency of 1 GHz, gave determinations of brown coal moisture with a standard error of 0.67 wt % for bed depths of up to 350 mm [46].

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3.2.8,3. Instrumental Methods of Moisture Determination Methods utilising nuclear magnetic resonance (see Section 3.2.3) or electron spin resonance can be faster than the standard methods described in Section 3.2.8.1. They may be able to distinguish between water present as such in the coal and that formed by chemical decomposition, but procedures that have been applied to brown coal to date require calibration standards like the on-line methods in Section 3.2.8.2. Barker and Smith [47], using a small commercial NMR spectrometer, found a good correlation between the water content of twelve Victorian low rank coal samples (55 70 wt % moisture) and the amplitude of the slowly-relaxing component of the NMR signal. A similar method was patented by an East German group [48]. Electron paramagnetic resonance determination of moisture content of low rank coal was patented by another East German group [49]. Thermogravimetric determination of moisture content automated e.g. in a LECO system, which has been used for many years as a rapid substitute for the standard determinations [2], could be developed as a standard technique to rank alongside them, but this has not yet been done.

3.3. DRYING OF LOW RANK COALS 3.3.1. General The high moisture content of brown coals makes drying an essential component of any upgrading or utilisation process. Water comprises a substantial proportion of the asmined coal (60 - 70 % in the case of Latrobe Valley brown coals) and the cost of evaporating or otherwise removing this moisture from low rank coals is frequently overlooked or under-estimated by developers. When more detailed evaluations are performed, the cost of drying and associated problems is often the major barrier to the development of competitive new brown coal technologies. The high moisture content of low rank coals has led to a plethora of drying processes being developed, but a major breakthrough on drying cost is still awaited. Water removal processes for low rank coals can generally be divided into evaporative drying or non-evaporative dewatering, where the water is removed from the coal in liquid form. Fohl and co-workers [50,51 ] reviewed both types of water removal from brown coal. The status of several processes in each category will be reviewed or updated below. In considering the drying requirements of low rank coals, it is informative to express the moisture content on a dry coal basis to indicate the quantity of water to be removed per unit mass of dried coal and hence the energy which has to be provided. Table 3.3 indicates the moisture contents of some low rank coal fields in Victoria and elsewhere. This shows that the moisture contents (per kg of dry coal) of Latrobe Valley brown coals can be more than 3 times that in a typical USA lignite.

101

Water in Brown Coal and Its Removal Table 3.3 Moisture contents of low rank coals, as received and dry basis. Field Yalloum, Victoria Loy Yang, Victoria Morwell, Victoria Anglesea, Victoria Leigh Creek, South Australia Lower Rhine, Germany Fort Union, Nth Dakota, USA

Typical moisture contents %, as - received

kg H20/kg dry coal

66.7 62.6 60.9 46.6 31 56 37.2

2.03 1.67 1.56 0.88 0.45 1.27 0.59

3.3.2. Evaporative Drying In evaporative drying, the water removal is achieved by applying heat to the coal, either directly or indirectly, comparable to the latent heat of vaporisation of the water, and removing the water in vapour form. One of the disadvantages of evaporative drying processes is the high energy requirement to evaporate the water, in some cases up to 25 % of the energy in the coal is required to dry off the moisture before any useful energy is obtained. This leads to correspondingly higher emissions of CO2 per unit of useful energy. However, in processes where a relatively pure steam effluent is achieved, vapour recompression and condensation can recover much of the evaporative energy. In evaporative drying processes, the moisture is removed predominantly as water vapour (and small amounts of CO2 and CO) from the coal. The ash forming elements in the coal are not removed, as they are not volatile at drying temperatures. Ash fouling in boilers may in fact be worse because of the higher temperatures encountered in dried brown coal flames. The dried product is normally used on-line because of difficulties in storage and handling due to safety risks from spontaneous combustion and dust explosions. If bulk storage and/or long distance transport is required, the dried coal needs to be agglomerated in some way, for example by briquetting, to facilitate safe transport, storage and handling. 3.3.2. /. Rate of Evaporative Drying Brown coal behaves as a porous solid with respect to drying kinetics. If the coal is dried slowly in a controlled humidity environment used to obtain moisture isotherms, there will be uniform moisture content and zero gradient through the coal. During air-drying under ambient conditions, the bulk moisture can diffuse to the surface of the coal particle via the pore structure and evaporate with minimal moisture gradient through the particle. The slow drying, which occurs in this phase, follows the

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classic constant rate drying period where evaporation occurs from the particle surface [52]. Following the removal of the bulk water in the constant rate period, the rate of drying will generally fall monotonically as the water is increasingly strongly bound to the coal. For higher rates of drying in thermal evaporative systems, particularly at elevated gas temperatures, the rate of evaporation exceeds the rate of diffusion to the particle surface. Under these conditions, the surface dries and evaporation occurs within the coal particle. This drying mechanism can be modelled as a dry shell/receding wet core with the drying rate controlled by conductive heat transfer through the dry shell, as established by Mcintosh [53,54]. The drying of brown coal is accompanied by substantial shrinkage (up to 50 % by volume for Latrobe Valley coals), particularly as removal of capillary water occurs at moisture contents below about 40 %. This shrinkage generates significant stress within larger particles during thermal drying under the (shrinking) dry shell/receding wet core regime, and explains the loss of lump strength, friability and breakage which frequently occurs as in association with the thermal drying of raw brown coal lumps or agglomerates. 3,3.2,2. Direct Evaporative Drying Processes 3.3.2.2. L Mill Drying Mill drying, also known as Flash Mill Drying, is the process currently used in conventional brown coal power stations where the coal is milled and dried simultaneously while entrained in a flow of hot gas recycled via the furnace gas off-take (Figure 3.9). Modelling studies by Mcintosh [53-56] demonstrated that, in the integrated mill drying systems in Latrobe Valley power stations, the bulk of the drying occurs after the feed coal has been milled to fine particle sizes, facilitating heat transfer from the hot recycle gas. This means that the 'drying shaft', where the coal is mixed with the hot gas before the mill, is in fact a misnomer, as most of the drying occurs in the mill and the subsequent burner feed ducts. A stand-alone version of the mill drying process is feasible using hot gas generated by burning some of the product and separating the dried coal product from the transport gas with cyclones and dust filtration devices. This was the basis of the Drikol development piloted by APM [57] at the Maddingley Brown Coal Mine in Victoria. 3.3.2.2.2. Hot Gas Rotary Drum or Fluidised Bed In both hot gas rotary drum and fluidised bed drying systems for brown coal, problems have been experienced with fires and explosions, with air contacting the hot coal particularly during start up and shut down [58,59]. It is interesting to note that the former SECV's first pilot plant venture in 1925 was a flue gas drum dryer at Newport, under the direction of Dr H Herman [60]. This project was abandoned after about 12

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months over concerns on operating safety. A hot gas fluid bed dryer demonstration project by AM AX at its Belle Ayr mine in Wyoming was eventually closed over concerns with fires and explosions in the drying plant and spontaneous combustion in transport and storage of the product [59]. 3.3.2.2.3. Steam Fluidised Bed Drying Steam Fluidised Bed Drying (SFBD) was invented by Prof Potter at Monash University in the 1970's [61,62]. The initial concept was for a cascading multistage fluidised bed dryer with heat recovery between successive beds. The concept was further developed as a single stage process with an 8 t h'^ (evaporation) pilot plant at Boma in the former East Germany [63]. German brown coal technologists recognised the safety advantages of the process where the drying occurs in an inert steam atmosphere, eliminating the principal risk of fires and explosions and enabling energy recovery to be achieved by vapour recompression.

GAS OFF TAKE

-20-25V^RF.(fines}and70^0 gas to 'inects burner

DE SWfRi VANES 75-BO'A ofPFand30Vnofgas to main burner

Classifier Volute - Coarse RF. return to mill Swirl Vanes in mill outlet

Figure 3.9 Typical mill drying and separation firing system developed for power stations burning Victorian brown coal. Reproduced with permission from D Clark, AusIMM Monograph Series 11, Victoria's Brown Coal, pp. 127-154. Copyright 1984 AusIMM.

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The single stage process involved drying the coal in a superheated steam fluidised bed with heat recovery through recompression of the product vapour. Some of the compressed vapour provides the fluidising steam while the bulk of the steam is passed through a heat exchanger submerged in the bed and condenses. As the evaporated moisture is recovered in liquid form, the process offers major efficiency advantages over conventional evaporative drying systems. Rheinbraun and Lurgi designed a commercial demonstration steam fluidised bed drying plant with an output of 20 t h"^ of dried brown coal. Two of these plants, with nominal annual capacities of 150,000 t yr"^ (dry coal), were built by Lurgi [64,65]. The first of these plants to come on-line was at Loy Yang in 1992 and the second at

Table 3.4 Typical properties of a steam fluidised bed dryer (based on Ref 64). Bed conditions Fluidising steam Heating steam Coal feed size Product size Product moisture Coal residence time

1 -lOkPag, 106-120°C 15-25 kPag, slightly superheated 400 - 500 kPag, saturated 0 - 6 mm 0 - 4 mm 10-20% about 60 minutes

(b)

(a) Raw brown coal

Steam

Condensate

Dry brown coal

Raw brown coal

Condensed vapours

Dry brown coal

Figure 3.10 Alternative configurations of 20 t hr'^ steam fluidised bed dryers (modified from Ref 65): (a), with external steam supply; (b), with integral energy recovery.

Water in Brown Coal and Its Removal

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Rheinbraun's Wachtberg briquette factory at Frechen near Cologne. The principal difference was that the Loy Yang plant was supplied with live steam from the Loy Yang A power station while the Wachtberg plant included energy recovery with vapour recompression. In the latter case, the only external energy supply is from the electric motors for the steam compression. Table 3.4 shows typical properties of a steam-fluidised bed dryer for brown coal, while Figure 3.10 shows the flow diagrams for the two variants. The product from the 150,000 t yr'^ (dry coal) steam fluidised bed drying plant at Loy Yang was milled to 75 % < 90 |im and pneumatically transported through a pipe to Edison Mission's Loy Yang B power plant for use as a start up and auxiliary fuel. The drying plant operated from 1992 to 2003 but Edison Mission has recently converted from dried coal to natural gas for this purpose. The Loy Yang drying plant is therefore closed at the time of writing (April 2004) and, unless another major customer can be located, seems unlikely to reopen. The performance of this plant was reviewed by Schmalfeld and Twigger [65]. The Wachtberg plant operated from 1993 to 1999 supplying dried coal into the briquette factory system, which supplied the briquette presses, and granulated coal for fluidised bed combustion and high temperature Winkler gasification. It is significant to note that basic laboratory measurements of the equilibrium moisture content of brown coal at elevated temperatures and pressures demonstrated significant differences between the Rhenish and Loy Yang brown coals, requiring modifications to the German design for use on the Loy Yang coal. While a typical product moisture content of 12 % is achieved with Loy Yang coal at a steam temperature of 107°C, the Rhenish coal, with lower initial moisture content, would require 112°C to achieve the same degree of drying [65]. The stable performance, reliability and flexibility of these two 20 th'^ plants exceeded expectations and demonstrated the commercial potential of the technology. As a result, the wider use of the technology in power generation was proposed in Germany and a 66 th'^ (dry coal) demonstration plant was built at RWE's Niederaussem brown coal power station in the Rhine area in 2000 but closed in 2002 [66]. As recently reported by Ewing and co-workers [66], further development of SFBD at the Wachtberg facility has demonstrated that even better performance and substantial cost reductions could be achieved by moving to a fine grain version of SFBD. Reducing the feed coal size from -6 mm to -2 mm results in an 80 % improvement in heat transfer due to the smaller particle size. As a result, the volume of the fluid bed can be reduced by 70 %, allowing investment costs for a commercial plant to be cut by an estimated 60 %. This improved version of the process is being piloted by Rheinbraun (now RE Engineering) at 17 t h'' dry coal scale at Wachtberg. The development is being fast tracked for inclusion as the pre-drying step for the brown coal feed to the next generation of German supercritical brown coal boilers, designated BoA Plus [66]. A demonstration plant integrated with a power station is scheduled for 2007 with full implementation in new plants from 2011.

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3.3.2.2.4. Pressurised Hot Gas Entrained Flow Drying This technology has arisen from the IDGCC project [67] where entrained flow drying occurs by injecting the moist coal via lock-hoppers into the high pressure hot gas stream exiting the gasifier. This technology provides a low cost pre-drying technique for incorporation into other high pressure processes for low rank coals such as gasification and high pressure fluidised bed combustion. There is a substantial cost reduction due to the simplicity of the drying plant (essentially a high pressure tube), relative to the cost of stand alone dryers. This drying technology has also been proposed as an option to retrofit to existing brown coal boilers. 3.3.2.2.5. NBCL/UBC Process An integral part of the Brown Coal Liquefaction technology (see Chapter 8), this process dries the moisture from crushed coal prior to hydrogenation by mixing the raw coal into an oil slurry and pumping under pressure through a heat exchanger to supply the evaporative energy [68,69]. As in the SFBD process, most of this energy can be recovered by recompression and condensation of the high pressure water vapour evolved. This concept has been further developed by Deguchi and co-workers [70] as a standalone upgrading process to produce ^upgraded brown coal' (UBC). If waste or low grade oil is used as the slurry heat transfer medium, the process can also upgrade the quality of the oil recovered from the process. The heavy oil components can be preferentially absorbed into the porous coal structure, leaving the lighter fractions available for recovery. The dry coal product is reportedly less reactive to spontaneous combustion because the residual heavy oil components block access to the pore structure. A 5 t d"' demonstration plant is reportedly under construction in Indonesia [71]. 3.3.2.2.6. Carbon Dry Process The Carbon Dry process was developed and piloted in North Dakota. Coal is heated in hot oil (Trench fried') to evaporate the water, with residual heavy oil fractions reportedly adsorbed into the coal, blocking the pores and thus reducing the spontaneous combustion risk. The economics of the process are strongly influenced by the degree of oil recovery, which is in turn affected by the porosity of the coal. Less than 5 % oil retained in the coal can more than double the cost of the dried product. As yet, we are not aware of a commercial plant to demonstrate this technology. 3.3.2.2.7. Microwave Drying Attempts to develop a viable microwave drying process for low rank coals [72] have not been successful, although it can be a useful technique for rapidly drying of laboratory samples. While microwaves can undoubtedly provide the energy to dry the

Water in Brown Coal and Its Removal

107

UtiE

'^y\^-^^,.?^^' €041

ri

Figure 3.11 Solar dried brown coal plant concept. Reprinted from Ref. 2 with permission from SECV.

coal, there is a substantial fire risk in the event of over drying, which led to the closure of a commercial microwave drying plant for peat in southern NSW. The intense energy transfer to the coal by microwaves can result in problems due to entrainment of fine particles out of the bed by the rapid evolution of water vapour. 3.3.2 J.8. Solar Drying Where the coal is left exposed to direct sunlight and unsaturated air, it will dry down to an equilibrium moisture content, about 15 % for Victorian brown coals. In the 1980s, the SECV operated a 2,200 t yr"^ Solar Dried Brown Coal pilot plant at Hazelwood in the Latrobe Valley. The process requires wet milling of coal to a pumpable slurry, which is dried in open-air ponds to provide a dense lump product [73,74]. Figure 3.11 presents a schematic illustration of the Solar Dried brown coal plant concept. The process requires large land areas and the production is seasonally variable depending on climatic conditions. It does not benefit from economies of scale to the same extent as other drying technologies because the major cost in civil works is effectively proportional to the production. However, the technology could be well suited to low rank coal deposits in arid areas with low labour costs.

108

Chapter 3 40 n

-;, 3oH

•

§ 20

a 10 E o o PH

Figure 3.12 Compressive strength of densified brown coal (10mm pellets) as a function of the pH of densified coal slurries [76]. •, Loy Yang coal; o, Morwell No 1 seam, H1317 bore coal; • , Maddingley coal.

3.3.2.2.9. Densified Brown Coal and Pellet Drying Another process was developed at Melbourne University by Johns and co-workers [75,76,77] to convert Victorian brown coal-water slurries into a material known as Densified Brown Coal (DBC). This process requires no addition of water, since the slurry is formed using only the inherent moisture. Attritioning in, for example, a kneader, sigma (or Z-arm) blender reduces the particle size to 5-10 [im and is said to 'release' the water from its matrix through physical disruption and collapse of the coal pore structure. A plastic mass is formed which can be readily extruded into pellets or blocks of convenient dimensions. The 'green' product is then air dried at ambient conditions, with or without the assistance of a draught, to form a product, which possesses a net wet specific energy similar to that of bituminous coal. During the drying step, the pellets/blocks shrink by an amount that is approximately equal to the volume of water that evaporates. The material behaves as a carbonaceous gel [78] in which, as the water evaporates, strong inter- and intra-particle attraction forces develop to produce a hard, dense product. The crush strength of the dried products was shown to be dependent on the pH of the initial coal slurry (Figure 3.12) and is attributed in part to the presence of ionic association between coal particles to give a stronger three-dimensional network. Wilson and Duane [79] reported further development of this concept as the Coldry Process, which proposed accelerating the air-drying of the raw pellets using hot gas and waste heat. However, they reported that the accelerated drying reduced the strength of the pellets although attempts are still being made to overcome this problem.

Water in Brown Coal and Its Removal

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Raw coal pellets can also be made in a drum pelletiser [80] but attempts to accelerate the drying of the wet spherical pellets using hot gas fluid bed drying by Swinburne University of Technology and the Coal Corporation of Victoria encountered similar problems of loss of strength, breakage and fires. The problem with accelerating the drying of raw coal pellets arises from the shrinking core drying mechanism that applies to drying of such pellets. The outer shell of the pellet dries first and shrinks, while the wet core does not change volume, thus weakening the structure of the pellet and leading to breakage and degradation. 3.3.2J. Indirect Drying Processes Indirect or contact drying occurs when the moist solid contacts a hot surface heated on the other side by a heat transfer medium such as steam, oil or hot gas. This contrasts with direct drying processes where the solid is contacted directly by the heat transfer fluid. The most significant indirect drying process is the steam tube rotary dryers used m brown coal briquette factories in Victoria and Germany, illustrated in Figure 3.13. The performance of these dryers has been reviewed in detail by Herman [81] and Kurtz [82]. These plants dry the bed moist brown coal from up to 66 % (in the case of Yalloum coal) to 15 % prior to briquetting.

#

-

*

^-^-CL^^-^-^J^^^-^-CL-:-^-^-^^^^

Figure 3.13 Steam tube dryer used in brown coal briquetting. Reprinted from Ref 81 with permission from SECV.

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Steam is passed through the shell of the rotary drum to heat the tubes through which crushed coal flows in a relatively inert atmosphere created by the evaporated water vapour. Individual briquette factory dryers can have heat transfer surfaces of up to 4,000 m^ in the tubes. The tubes are fitted with retarder 'rods' to slow the passage of larger particles through the dryer and provide a more uniformly dried product. This arrangement is the inverse of the usual shell and tube heat exchanger, where the steam would pass through the tubes and the material to be heated is in the larger volume of the drum. This latter arrangement has been used for brown coal drying but is perceived to have a higher risk of fires and explosions with the partially dried coal being tumbled around in the larger gas volume of the shell. The briquette factory dryers use back pressure steam from the steam turbines of an integrated power station. The briquette factory is thus a cogeneration plant, providing steam for power generation and coal drying, with a higher combined efficiency than a stand-alone power station. The driers effectively behave as the condensers for the power station steam cycle, while the power station provides the energy for drying the coal, at least in part from the 'waste' heat in the steam cycle which would otherwise finish in a cooling water circuit. There are also a number of other contact dryers, such as shell and tube dryers and heat screw conveyors which function by indirect heating. They have had limited application to low rank coal drying and in each case the safety aspects require careful consideration before committing to an installation. 3.3.3. Non-evaporative Dewatering Non-evaporative dewatering processes involve the removal of water from the coal in liquid form, thus saving the latent heat of vaporisation and hence reducing greenhouse gas emissions. Another benefit is that some of the 'dissolved' inorganics in the coal (particularly sodium) are removed in the liquid water, thus reducing the ash fouling propensity of the coal. However, this can generate effluent water treatment problems, particularly in thermal dewatering processes, which can contain organic as well as inorganic contaminants in the expressed water. The cost of treating the effluent water to acceptable standards has been a major impediment to the commercialisation of thermal dewatering processes. i. i. 3. L Thermal Dewatering Processes Thermal dewatering is a generic term to describe processes where the application of heat enables the removal of moisture from the coal in liquid form rather than as water vapour by evaporation. These processes in effect accelerate the coalification process and produce a harder product with lower moisture, oxygen and porosity, and higher carbon and heating values. The effect is attributed to thermal decomposition of the hydrophilic oxygen containing functional groups in low rank coals, accompanied by the release of CO2, shrinkage of the pore structure and expansion of the water in the pores. The processes typically involve heating the coal to temperatures in the 200 - 350°C range at

Water in Brown Coal and Its Removal

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pressures above the saturated steam pressure, i.e. high enough to prevent evaporative drying of the coal. Under these conditions, water can be exuded from the coal and separated physically in liquid form. Allardice [2] provided a detailed review of the research into the kinetics and mechanisms of thermal dewatering processes on brown coal. This section will therefore focus on the subsequent research and process development. 3.3.3.1.1. Fleissner Drying The earliest thermal dewatering process was developed in the 1920's in Austria and is known as Fleissner drying, after the inventor [83]. This involves batch autoclave treatment of coarse lumps of low rank coal in steam at 180 - 240°C to produce an upgraded hard lump fuel. The water expressed in the steam treatment is drained from the autoclave and the sensible heat remaining in the coal is used to evaporate more water on depressurising. The technology was licensed by Voest Alpine and several plants have operated on harder brown coals in central European countries [50,51], with one commissioned in the late 1980's. A continuous version of the process was developed in the 1980's but to our knowledge not commercialised. The process was piloted in Victoria [84] and South Australia and was close to commercial application to supply the South Australian Railways before the introduction of diesel locomotives. The difficulty in applying the process to the soft Victorian brown coals arose from the need for the feed to be in lump form and maintain its integrity through the process. Assessments indicated that less than 50 % of a Loy Yang feedstock would have sufficient lump size to facilitate drainage of the expressed water from the coal. A Japanese version of the Fleissner process (the DK process) with improved heat recovery was piloted in the 1980's by Kamei and co-workers [85]. A significant difference between the earlier Fleissner process and the subsequent developments was in the process temperatures. The Fleissner process focussed on temperatures around 200°C where water exclusion due to shrinkage is maximised but organic contamination of the water, which increases with temperature, is confined to acceptable levels. 3.3.3.1.2. Evans-Siemon Process To improve the heat transfer to the coal, a process was developed by Evans and Siemon at University of Melbourne in 1970. Their idea [86] was to heat the coal in water or oil rather than steam in a continuous flow reaction system under sufficient pressure to prevent evaporation. This concept could be applied to lump, granular or slurried coal systems. A pilot plant for lump coal operating at up to 280°C was designed [87] but the project lapsed because of the lack of incentive at that time to improve the efficiency of brown coal use unless it also reduced costs.

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3.3.3.1.3. Hydrothermal Dewatering or Hot Water Drying The slurry version of Evans-Siemon was developed further at University of North Dakota EERC [88] and SECV [89,90]. This version is generally referred to as Hydrothermal Dewatering (HTD) or less frequently as Hot Water Drying. In the process (Figure 3.14), a slurry of brown coal is heat-treated to around SOO^'C under sufficient pressure to prevent evaporation. After cooling and depressurising, excess water can be separated from the product slurry. The decomposition of the coal structure, which occurs under these conditions, could be regarded as analogous to accelerated coalification. Applied to high moisture brown coals, the product is an upgraded coal slurry with an energy content greater than the solid as-mined coal. Allardice and co-workers [90] reported the results of the hydrothermal dewatering (HTD) studies by the SECV/HRL in a 1 m^ hr' HTD slurry pilot plant. These studies confirmed that: • HTD treatment at 275 - 325°C resulted in non-evaporative moisture reduction and produced a pumpable coal water slurry which, for Latrobe Valley brown coals, had a lower moisture content and higher net wet specific energy than the starting coal. • The process also removed some inorganic constituents from the coal, which reduced ash deposit formation problems in combustion and liquefaction systems. Acid washing or the addition of multivalent exchangeable cations to the feed slurry further reduced the concentration of troublesome ion exchangeable elements such as sodium. • The overall energy recovery was about 97 %. • The average dry solids recovery was over 90 % including the losses due to soluble organic material and to water and gas released from the breakdown of coal functional groups during the HTD process. For Morwell and Loy Yang coals, the soluble organics emitted within the water was about 1 % (on dry coal basis) and the CO2 yield varied from 5.0 to 6.5 %. • Several treatment processes were investigated at laboratory and/or pilot scale using effluent water from the HTD pilot plant although with limited success. The processes tried included: - flocculation/filtration and membrane filtration; - reverse osmosis and electro dialysis reversal for inorganic removal, both of which suffered from membrane fouling by the organic wastes; - aerobic and anaerobic biological treatment for removal of the organic wastes; and - absorption onto feed coal or char. • Tests on a suite of international low rank coals demonstrated the wide applicability of the process for upgrading low rank coals, although the greatest beneficiation was achieved for coals with greater than 50 % raw coal moisture and 15 % oxygen. The HTD product was found to be technically suitable for use in coal fired gas turbine power generation, industrial boilers, pressurised fluidised bed combustion.

113

Water in Brown Coal and Its Removal WASTE GAS£S

^/'^^4^^^^^^''''^''''^^^^-'QM

SEPmAim

yATER

miEQ

COAL,/.;

CENfRIFUOE

yA"E«

SlUHRt crSl nOlSTURPi

PPODUCT SMiRRr CSO% MOISTURE)

Figure 3.14 Hydrotheraial dewatering process concept. Courtesy of SECV.

gasification and liquefaction applications. Unfortunately, the most prospective short term application of the technology, power generation in a coal slurry fired gas turbine [89], was abandoned by the SECV, in favour of other options such as IDGCC, for future power generation. The CRC for Clean Power from Lignite has also conducted further studies on the process [Chaffee and co-workers [91]), but its emphasis has shifted to MTE for predrying brown coal for power generation (Mcintosh, [92]). A Japanese HTD variant piloted by Hashimoto and Tokuda [93] reportedly produced higher solids density slurry products (> 60 % dry coal) from Loy Yang coal than achieved in Victoria. The use of surfactants contributed to this improvement, but differences in viscosity measurement techniques and the definition of 'pumpable' slurries may also have contributed to the difference. The Japanese have also studied the process's potential with respect to upgrading Indonesian brown coals but were discouraged by the cost and difficulty in cleaning up the effluent water. Companies such as Shell, Bechtel and Koppelman (K-Fuel) have patented other variants of the thermal dewatering process for low rank coals and the process has also been investigated for lignite upgrading in India and China. Recently Nakagawa and co-workers [94] reported that the organic contaminants in the recovered water from HTD of brown coal increased with increasing process temperature, reaching 1.5 % of the coal carbon at 300**C. They demonstrated that the organic carbon contaminants could be completely removed by pressurised hydrothermal gasification at temperatures as low as 350*'C using a novel Ni/carbon catalyst.

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Most of the studies on the development of pumpable brown coal slurries have involved adding water and, usually, surfactants. The addition of water is detrimental since it further reduces the already low heating value of the fuel or increases the quantity of water to be removed in the slurry upgrading process. Recent work by the Institute of Applied Energy, Japan, has demonstrated that a pumpable brown coal slurry can be produced from Latrobe Valley brown coals without the addition of more water as is normal in HTD studies. Katayama and Onozaki [95] achieved this by kneading the coal at elevated temperature (150 - 200°C) under saturated steam pressure to produce a stable slurry. This could make an improved feedstock for HTD treatment at higher temperatures. When the product is pumped into a hot reactor, the water in the slurry rapidly evaporates and the coal particles are 'atomised', so that the slurry is suitable as a direct feed to a coal gasifier. The authors claim that the slurry can be stored and transported and/or used as feed for briquette production. We suggest that the HTD process still has unrealised potential to convert brown coals into a clean, safe, exportable form. Trends in international coal markets are likely to make the technology more attractive as a means of accessing under-utilised low rank coal resources in the longer term, as black coals become scarcer and more expensive. The major concerns with generic thermal dewatering processes are the high capital cost of the plant, particularly the heat exchangers, the problems of treating the contaminated effluent water, and the low slurry densities achievable relative to black coal water slurries (> 70 %). 3.3,3.2, Mechanical Thermal Expression Initial Victorian studies of "press dewatering" at ambient temperatures by Banks and Burton [96,97] and later by Guo and co-workers [98] found that such processes could remove up to 80 % of the water in the coal. However, further development was abandoned as impractical because of the high pressures, which had to be applied to the coal for residence times of 20 minutes or more. Mechanical thermal expression (MTE) is a promising improvement initiated by Strauss and co-workers at the University of Dortmund, Germany [99-102]. The MTE process (Figure 3.15) combines the mechanical press dewatering concept with the use of elevated temperatures in the range 150 - 220°C. Such temperatures are high enough to 'soften' brown coal and enable dewatering at substantially lower mechanical pressures (e.g. 2 - 1 2 MPa) and residence times. The temperature is also low enough to avoid any appreciable chemical change, for example in elemental composition, during dewatering. The CRC for Clean Power from Lignite has recognised the potential of applying MTE dewatering to Victorian brown coal. Extensive studies on a laboratory scale have been carried out in Australia [98, 103-106] and in Germany [107,108]. Hulston and coworkers [106] have shown that the percentage water removal increases approximately linearly as a function of temperature within the range examined (Figure 3.16). The effectiveness of water removal is remarkably improved by doubling the applied mechanical pressure from 2.5 to 5.1 MPa. However, a further (approx) doubling of the applied mechanical pressure from 5.1 to 12.7 MPa does not have so marked an effect.

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Thus, the higher temperature used in MTE decreases the pressure and residence time required to manageable levels and enables significant moisture reduction in a realistic time frame and for a minimal expenditure of energy. Some energy can be recovered from the expressed water. MTE processing of brown coal produces strong, dense compacts with variable amounts of residual water, dependent upon processing conditions. Hulston and coworkers [106] found that, as the samples equihbrate with the atmosphere, they lose both

Steam

MTE

waste water 'cold'

Figure 3.15 Mechanical thermal expression of water from brown coal. Reprrinted with permission from Ref 102. Copyright 1999 VGB. MTE Product: Loy Yang Coal 100 90 80 70 60 50 40 30 20 10 100 120

140 160 180 200 220 240 260 Temperature (C)

Figure 3.16 Water removed from Loy Yang Coal by MTE as a ftinction of process conditions (after Hulston and co-worker [106]). Initial Moisture = 59.7 % (1.48 g/g db).

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"o

>

2.5 MPa

5.1 MPa

12.7 MPa

D Raw lignite to wet MTE product volume reduction D Wet to dry MTE volume reduction • Dry MTE product volume Figure 3.17 Volume reduction during MTE processing and drying [106].

moisture and volume. Figure 3.17 illustrates the volume losses that occur during and after MTE processing. The CRC for Clean Power from Lignite has also piloted a version of the process at 1 t h"' scale and plans for 15 t h ' and 100 t h'^ demonstration units have been developed with support of the local brown coal power generators and the State Government, as reported by Brockway and Jackson [110]. Few details of the Australian process configuration are publicly available, but it is targeted at a more continuous operation. The CRC is promoting the development of MTE as a practical concept to retrofit to existing boilers or to pre-dry the feed coal for an IGCC plant. As reported by Mcintosh [92], the CRC has concluded that MTE is less expensive and provides greater efficiency improvements in these applications than HTD or steam fluidised bed drying. Bergins and co-workers [109] calculated that potential power generation efficiency improvements of between 13 and 21 % could be achieved for a brown coal dewatered from an initial moisture content of 57.5 % and 65.5 % respectively to a final moisture content of 25 % as fired.

Water in Brown Coal and Its Removal

111

3.3.3.3. Solvent Dewatering Miura and co-workers [111] recently introduced a new concept of non-evaporative dewatering coal via solvent extraction. Using a fixed bed arrangement for contacting the coal, they investigated a range of solvents at temperatures up to 200°C. The extent of dewatering was governed by solvent polarity, temperature and solid-liquid contact time, but water contents as low as 2 % could be achieved with Morwell coal under appropriate conditions. Since the solvent is recycled, a practical system must employ a solvent that can solubilise substantial proportions of water at high temperature (150 - 200°C), but which is also substantially immiscible with water at low temperature. This facilitates separation of the organic and aqueous phases by decantation at ambient conditions. Polar solvents (e.g. methanol and ethyl acetate) provided good water removal, but the separation of water from the liquid mixture was difficult. With non-polar solvents (e.g. tetralin), good liquid separation efficiency was achieved, but large quantities of solvent were required to extract all the water. Using tetralin as the solvent, Miura and coworkers [111] claimed that this process had an energy requirement of < 1 MJ per kg of removed water. A recent report by Kanda and co-workers [112] indicates that this concept can be extended to the use of dimethyl ether (DME) as a solvent. Water is highly miscible in DME, which can be used to 'extract' the water from coal at ambient temperature (30°C) and modest pressures {ca. 0.8 MPa). The use of pressure is required to maintain DME in the liquid phase; since, at ambient pressure, it boils at -25°C. In its current stage of development, the process involves passing liquid phase DME through a fixed bed of brown coal. The liquid phase is then de-pressurised to evaporate DME, leaving the separated water as the liquid phase product. DME is recovered by compression and a series of heat exchangers are incorporated to facilitate recovery of most of the heat that would otherwise be lost during DME expansion. Kanda and co-workers [112] reported that, using this method, the moisture content of a Loy Yang brown coal sample could be reduced from 54 % to only 4 % in laboratory studies. The authors claim that the process consumes 948 kJ kg'^ of removed water, similar to Miura's estimate [111] and lower than the values reported by most other drying processes. The DME process has several other potential positive features. Because the process operates at ambient temperatures, the extracted water is unlikely to suffer from the organic contamination and clean up problems associated with thermal dewatering processes. DME is non-toxic and will become more widely available, because it is increasingly seen as a possible substitute fuel for diesel and LPG. The cost is also expected to fall as production grows. 3.3.3.4. Effluent Waterfi'omNon-Evaporative Dewatering Processes Unlike conventional evaporative processes, non-evaporative dewatering processes, such as Fleissner, HTD and MTE, produce a liquid by-product stream that must be

118

Chcq)ter3

managed and disposed of or re-used in an environmentally responsible way. The liquid water stream contains both organic and inorganic components. An understanding of the composition and concentration of species contained in this effluent water is clearly essential to a full evaluation of water quality issues associated with the various dewatering technologies. 3.3.3.4. J. Inorganic Components Inorganic components within the natural brown coal may be present in at least three forms (also see Chapter 2) as shown schematically in Figure 3.18. Some of the inorganic components are present as discrete mineral inclusions (e.g. pyrite and clays) and are largely unaffected by the dewatering processes. A second type of inorganic matter is present in the form of water-soluble salts. These are contained within the interstitial pore water and are removed with the dewatering product water. Favas and co-workers [104] found that this soluble inorganic matter is almost entirely removed in the product water stream in the HTD process. In the MTE process, Kealy and coworkers [103] reported that it was only removed in proportion to the reduction in total moisture. The situation is also depicted in Figure 3.18. For Latrobe Valley brown coals, the water-soluble material is mostly sodium chloride; but soluble sulphate salts may also be present [113]. A third type of inorganic matter consists of inorganic cations that are ion exchanged as cations at the site of carboxylate functional groups on the brown coal surface. Some of these cations can also be removed by MTE when acid is added [104], although it seems unlikely that this would be practicable on an industrial scale. Reduction in exchangeable cations also occurs with acid addition in HTD processing [90] and was found in earlier Evans-Siemon studies. In non-evaporative dewatering processes that operate at higher temperatures, more of these ion exchanged cations are released to the product water stream as a result of the decomposition of carboxylates [2]. Hence, the concentration of residual inorganics in HTD dewatered products (produced at higher temperatures) is generally lower than for MTE products [105]. 3.3.3.4.2. Organic Components Organic matter is also released into the product water during the dewatering processes. A comparison of the organic carbon levels in product water from a variety of non-evaporative dewatering studies has been recently assembled (Table 3.5). Where possible, the organic carbon data are compiled both in terms of product water concentration (g L"^) and in terms of organic carbon released from the raw coal (g kg" dry basis). The latter measure generally provides a better basis for comparison, as they are less influenced by experimental procedures (for example, different ratios of water to coal charged to experimental reaction vessels by different researchers). Table 3.5 demonstrates that higher yields and higher concentrations of organic carbon are produced at higher treatment temperatures. This same point can also be concluded from the specific reports cited in this table (see also Qi and Chaffee [114]

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119

and Favas and co-workers [104]). In fact, it is reasonable to conclude that the final treatment temperature is more critical to the degree of organic contamination in the effluent water than the particular thermal dewatering process used. Detailed analytical work has shown that the most abundant organic compounds that can be identified by gas chromatography-mass spectrometry (GC-MS) are low molecular weight organic acids and phenols. The detailed identification of individual

1. Mineral Inclusions

Surface Hydroxyl Group

^ ;) Multilayer Water

3. Dissolved Salts

"^ 2.Ton Exchanged

MTE

^ Surface Hydroxyl Group

Dissolved salts are expressed in proportion to expressed water Figure 3.18 Physico-chemical model of natural brown coal illustrating three different types of inorganic matter. Dissolved salts are removed in proportion to the volume of expressed.

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compounds has been reported for water from pressurised steam drying [117], HTD [115] and MTE [113]. However, it appears that only a fraction of the organic components present are usually accounted fr)r. For effluent water from the MTE process, the development of improved analytical techniques has shown that, of the identifiable components, aliphatic and aromatic acids are 1 - 2 orders of magnitude more abundant than phenolic components (Table 3.6). From a comparison of the GC-MS results with those from field flow fractionation (FFF) analysis, it has been estimated that the GC-MS identifiable components account for only ca. 25 % of the total organic carbon for a variety of Latrobe Valley coals [113].

Table 3.5 Comparison of organic carbon levels reported in the product water from nonevaporative dewatering Victorian brown coalfromQi [113]. K/iTr: n 1 -51 D ^ u UTT^ n i ^i ^TD pilot plant MTE[n3] BatchHTD[115] f"^^^^^ Temperature (°C) Brown coal TOC (g/L) OC (g/kg dry coal)

120-200 Loy Yang 0.08 - 0.4 0.4-2.2

Pressurised steam drying**[in] 182-222 Loy Yang NA 0.1*-2.3

300 Loy Yang L32 NA

250-350 Loy Yang 0.3*-7 2*-50*

* estimated from charts presented in the papers. ** this procedure is similar to a Fleissner steam autoclave procedure. NA - not available. Table 3.6 Quantities of major organic groups in MTE water produced under a range of conditions,fromQi [113]. 150°C 25MPa

Loy Yang A 200^C 200^C 25MPa 6MPa

200X 6 MPa

Morwell 200"C 6 MPa

200°C 25 MPa

Total mono- & dihydroxy phenols (mg/kg, db)

0.31

3.0

13

18

22

52

Total tri-hydroxy phenols (mg/kg, db)

0.038

0.26-0.85

0.32-1.1

0.89-3.0

1.0-3.4

5.6-19

Total aliphatic acids (g/kg, db)

0.091

0.43

0.65

0.83

0.74

1.3

Total aromatic acids (g/kg, db)

0.070

0.19

0.38

0.47

0.54

0.67

Total of compounds identified by GC-MS (g/kg, db)

0.16

0.62

1.0

1.3

1.3

2.0

Total organic carbon (gC/kg,db)

0.71

1.3

2.2

2.5

2.9

3.6

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121

3.3.3.4.3. Effluent Water Utilisation The quantity of water that would be released by any of the non-evaporative drying processes is large. For example, under modest MTE conditions (180°C and 6 MPa), it is expected that 60 - 70 % of the original water would be removed. For a typical Latrobe Valley brown coal (with 60 % moisture), if MTE is applied to the output from a new mine producing say 30 million tonnes per annum of brown coal (comparable in scale to the Loy Yang development), it can be estimated that the MTE plant would produce ca. 13 Gigalitres (GL) of effluent water per year. This is a substantial portion of the total water usage in the Latrobe River Basin (currently ca. 130 GL) and it is clear that a plan would be needed to manage this by-product water in an appropriate manner. Butler and co-workers [118] have considered the potential uses for MTE water and identified industrial cooling water, agricultural irrigation in the region and environmental recharge (re-injection to the underground water table) as possibilities. Since the concentrations of both organic and inorganic components exceed guideline levels for these applications, it is clear that some remediation will be required [118]. However, the extent of remediation for MTE water will not be as great as required for the effluent water derived from dewatering processes that function at higher temperatures such as HTD. The cost and difficulty in treating HTD effluent water has been a major obstacle to its commercial development, compared for example with the lower temperature Fleissner process effluent. MTE by-product water has now been reasonably well characterised by classical procedures. However, there is a further range of industrially relevant physical characterisation parameters (e.g. colour, turbidity, conductivity, biochemical oxygen demand, total dissolved solids) for which systematic data must still be gathered to enable comparison with guidelines/limits for the envisaged applications. These parameters form the basis for specification of public water quality classifications. It is understood that the water produced in the demonstration scale MTE trials at Frechen and Niederaussem was treated using a microbiological fixed bed process. This approach was adapted from a process to treat wastewater from coke manufacture. Only limited details are available from work by Reich-Welber and Felgener [119]. The pressurised hydrothermal gasification process recently reported by Nakagawa and co-workers [94] for treatment of HTD effluent offers a novel treatment alternative for MTE water. In this process, the organic components in the effluent are gasified under water at temperatures as low as 350''C and pressures above saturation to prevent evaporation of the water.

3.4. MOISTURE AND BINDERLESS BRIQUETTING OF BROWN COAL 3.4.1. Background to the Briquette Industry Brown coal briquettes have been produced in Europe for centuries, initially by kneading and moulding brown coal and water to form 'mud bricks' which were dried in

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the sun and stockpiled for later use. 'Modem' binderless briquetting of dried brown coal was developed in central Germany around 1850 and is now the second largest use for brown coal around the world (after power generation). Brown coal briquettes led the industrial development of Germany as the production increased from 750,000 tonnes in 1885 to 60 Mtpa during World War II [81 ]. In recent years, world briquette production has been substantially reduced by the decimation of the industry in Germany, where production fell from over 50 Mtpa in 1990 to less than 4 Mtpa in 1998. This resulted from the restructuring of the German economy following unification, the environmentally unacceptable quality of much of the coal available for briquetting in eastern Germany and the expansion of the natural gas grid through areas previously reliant on brown coal for industrial and domestic fuel. Large-scale briquetting of Victorian brown coal commenced at Yalloum in 1924 using the German technology. This followed abortive attempts at Yalloum North in the 1890s. The original Yalloum plant closed in 1970. The Morwell briquette and power plant commenced operation in 1959, with a nominal production capacity of 1.2 Mtpa of briquettes and 170 MW of electricity. It operated until December 2003 when a major fire in the dry coal handling sections closed the briquetting plant. The Morwell briquette and power complex was one of the largest cogeneration plants in the Southern Hemisphere, but 4 months after the fire, a decision is still awaited on whether to partially rebuild the briquetting capacity. This is a salutary lesson on the risks associated with the production, storage and handling of dried brown coal, even for an experienced operator. Briquette production in Victoria peaked at 1.9 Mtpa in 1966 prior to the introduction of natural gas. Before the fire, production had fallen to below 400 ktpa, primarily for

CRUSHING 3jfc AND SREENING

Figure 3.19 A schematic diagram of brown coal briquetting. Courtesy of SECV.

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123

industrial and commercial heat applications, char production and occasional niche market export opportunities because of the high quality of the product. Briquettes, as a cheap source of energy, underpinned the industrial development of Victoria and its manufacturing industries after the first and second world wars. 3.4.2. Briquetting Process In the briquetting process, shown schematically in Figure 3.19, the raw brown coal is crushed to - 8 mm, dried in rotary steam tube driers from 66 % to about 15 % moisture. The dry coal is then cooled to about 40°C and briquetted without a binder, using an Exter reciprocating extrusion press at 1200 kg cm'^ pressure to form hard compacts with an energy content (21 MJ kg'^ net wet basis) comparable to many higher rank coals. The briquette factory dryers use low grade 'waste' energy (backpressure steam from the integrated power station) to dry the coal. This cogeneration process therefore produces lower greenhouse gas emissions than stand alone brown coal briquetting processes, which use high grade heat to dry the coal. Herman in his classic 1952 book 'Brown Coal' [81] devotes 279 pages to a detailed description of the binderless briquetting process. More recent reviews of the technology have been published by Kurtz [120-123] and others [2,124]. Because of the poor strength and weathering characteristics of briquettes made from Morwell brown coal, production at the Morwell briquette factory from the outset used Yalloum coal transported to Morwell. However, because of changes in the quality of Yalloum coal with the development of Eastfield and the competition between the privatised coal mines in the 1990's, Loy Yang coal has recently provided 90 % of the briquetting coal, with Yalloum Eastfield providing the balance. With appropriate selection of coal from the Loy Yang mine, an improvement in briquette quality, with ash yields generally below 1.5 % dry basis, has been achieved. A number of alternative agglomeration technologies for brown coal have also been proposed. These could supplement or eventually replace the current technology. Processes such as double roll pressing, pellet milling and drum pelletising could offer improved economics for a new installation, although questions remain on product quality and self-heating characteristics of these products. To date none of these alternative briquetting processes have replicated at a pilot scale the strength and weather resistance of the commercially produced binderless extruded briquettes. Another approach has been to use binders to briquette the dried brown coal. Unfortunately, the high cost of suitable binders and the relatively high concentrations required because of the porosity of the dried coal make this an unattractive route to develop a low cost bulk commodity fuel. 3.4.3. Impact of Moisture on Briquettes Moisture is a critical factor in the binderless briquetting or pelletising of dried low rank coals. The bonding mechanism is generally accepted as hydrogen bonding between

124

Chapters

coal particles via the oxygen containing functional groups on the coal surfaces and the moisture remaining in the coal after drying down to the multilayer moisture region [125-128]. Control of the moisture content is critical to optimising the briquette strength and minimising the shrinkage (and breakage) on equilibration with the atmospheric humidity in storage and handling. For Victorian brown coals, the optimum moisture content for briquetting is around 15 %. The inferior weathering behaviour found with briquettes made from Morwell coal, relative to briquettes from Yalloum or Loy Yang coal, is also associated with the coal moisture. The inferior performance of Morwell in this regard is attributed to rapid swelling of the briquettes on wetting, due to the different exchangeable cation composition of this coal producing greater cation hydration, higher swelling and loss of strength. Morwell coal contains much higher levels of exchangeable cations, particularly calcium and magnesium, than Yalloum or Loy Yang. Moisture can also be a contributing factor in the spontaneous combustion of briquettes (and dried brown coal) in storage. Heat (of adsorption) is generated when moisture is re-adsorbed onto briquettes. This can occur if there is an increase in humidity or even light rain, if storage under low humidity conditions has partially dried the briquettes. The resulting increase in briquette temperature can accelerate oxidation by air to the point where spontaneous combustion occurs. As a rule of thumb, the rate of oxidation approximately doubles for each 10°C rise in temperature. The industry has developed safe operating practices to minimise and manage the risk of spontaneous combustion in storage and transport, which can be a serious concern for new operators with unproven products. The subject of spontaneous combustion in brown coal based materials was reviewed in detail by Mulcahy and co-workers [129] and case studies on the shipping of brown coal briquettes have been reported by Cunningham and co-workers [130]. Procedures for the safe shipping of brown coal briquettes have been incorporated into the International Maritime Organisation, Code of Safe Practice for Shipping Bulk Cargos [131].

3.5. CONCLUSION In a Royal Commission on Coal in 1890, the Victorian Government Analyst Cosmo Newberry commented that "the brown coal in the Latrobe Valley would be an excellent fuel if it could be rid of its water'' [129]. Over 100 years later, this is still the major impediment to the economic development of Victorian brown coal and other low rank coal resources. A huge market exists for a technology, which can convert low rank coals to high-energy transportable fuels at a comparable cost to exported black coals. Despite the development of many innovative drying technologies, none have yet managed to clear this hurdle. It is important to note that any dewatering or drying process that can remove the water from brown coal in liquid form, i.e. without the need to supply the evaporative

Water in Brown Coal and Its Removal

125

energy to dry the coal, has the potential to reduce the greenhouse gas emissions from use of these coals by up to 25 %. The current emphasis on reducing greenhouse gas emissions has added an extra factor for consideration in the selection of brown coal drying processes for further development. The two attracting the most interest at present for application to Victorian brown coal are mechanical thermal expression at the CRC for Clean Power from Lignite in Victoria [92] and the fine grain version of steam fluidised bed drying in Germany [66]. In addition, two emerging Japanese developments, the upgraded brown coal pilot plant in Indonesia [71] and the concept for solvent dewatering with DME [112], also warrant further evaluation.

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