Control of Agglomeration and Defluidization during Fluidized-Bed

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Control of Agglomeration and Defluidization during Fluidized-Bed Combustion of South Australian Low-Rank Coals Philip J. van Eyk, Adam Kosminski, and Peter J. Ashman* South Australian Coal Research Laboratory, Centre for Energy Technology, School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: South Australian low-rank coals have high sulfur and high sodium contents, which cause operational problems when the coals are combusted. Fluidized-bed combustion (FBC) of these coals allows for efficient combustion and for convenient sulfur removal by the addition of in-bed sorbents, such as limestone or dolomite. However, the presence of sodium may result in operational problems for FBC because sodium compounds, such as sodium sulfate, which is present in the coal ash, may cause the bed particles to become “sticky” and lead to a loss of bed fluidization. Combustion experiments have been performed in a laboratoryscale fluidized-bed combustor for two South Australian low-rank coals: Kingston and Lochiel. This work was undertaken to compare the behavior of these two coals and to allow for comparisons to previous experience gained using Lochiel coal in both laboratory experiments and pilot-plant operation. Kaolinite-rich clay additives were used in these experiments in an attempt to alleviate the problems associated with sodium present in coals. The effect of refreshing and removing the bed material without interruption of the combustion process was also studied experimentally. Kingston coal showed better performance in the FBC process than Lochiel coal. The ash layer formed from FBC of Kingston coal was found to be less sticky than that formed by Lochiel coal, resulting in longer defluidization times for Kingston coal than for Lochiel coal when no clay additives were used. Kingston coal was able to be combusted at 850 °C with the addition of clay at 5% of the total feed rate and with the addition/removal of bed material at 5% of the total feed rate. Analysis showed that the sodium from the coal had reacted with the kaolinite in the clay to form nepheline, a highmelting-point solid compound, which thus restricted the formation of liquid sodium sulfates in the bed. The results of this study show good agreement with the results of previous studies that showed that the addition of kaolinite-rich clays led to problem-free combustion of Lochiel in both small- and pilot-scale operations at 800 °C.

1. INTRODUCTION The large reserves of South Australian lignite coals have the potential to be an economical and abundant source of fuel for power generation; however, they have unusually high contents of moisture (>50%), sulfur, sodium, and chlorine, and because of this, their use is difficult. These coals are currently not used for power generation in conventional pulverized-coal-fired furnaces because of the likelihood of unacceptable levels of heat-exchanger fouling and unreasonably high emissions of sulfur oxides. Fluidized-bed combustion (FBC) is a well-established process that offers a potential solution to these problems. FBC is wellknown to be capable of combusting a wide range of solid fuels at temperatures below the ash fusion point of these materials. Kingston coal from the south east of South Australia is now being considered as a fuel for a coal-fired boiler using FBC technology. The combustion and gasification of South Australian lignites, particularly Lochiel and Bowmans coals, have been investigated by the former State power generator Electricity Trust of South Australia (ETSA) and The University of Adelaide as part of the Cooperative Research Centre for New Technologies for Power Generation from Low Rank Coals and the Cooperative Research Centre for Clean Power from Lignite.1 4 Kingston coal and also Lochiel coal attracted serious attention as possible energy sources for power generation during the 1980s. Extensive research resulted in characterizing the behavior of Lochiel and Kingston coals during combustion in a pilot-scale pulverized-coal process; however, r 2011 American Chemical Society

problems because of fouling of heat-transfer surfaces and the substantial emissions of SO2 proved to be intractable.1,4 6 Further pilot-scale work focusing on Lochiel coal indicated that, under restricted conditions, the FBC process was able to successfully overcome these problems.4 A disadvantage of FBC is that agglomerates may form because of the formation of low-melting point inorganic compounds in the fuel ash, and this may lead to the entire bed defluidizing because of the presence of large agglomerates.7 Defluidization is a major inhibitor to the use of fluidized-bed technology for these coals. Controlling agglomeration and subsequent defluidization is thus critical for any commercial fluidized-bed process. Many investigations have been performed with the aim of understanding the high-temperature defluidization phenomena in fluidized beds in the combustion or gasification of coal,8 15 combustion of petroleum coke,16,17 combustion or gasification of biomass,18 27 and co-combustion of biomass and coal.28 In general, the mechanism of agglomeration and subsequent defluidization of an inert bed material during FBC is summarized as follows:29 33 (1) Inorganic transformations within the fuel particle result in the Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: November 11, 2011 Published: November 11, 2011 118

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formation of eutectic mixtures within the char as it is heated. (2) The eutectic mixture becomes molten once the temperature of the char exceeds the melting point of the mixture, thus forming a molten ash. (3) Collisions between the inert bed material and the char particles lead to a transfer of the molten ash onto the surface of the bed particles because of a greater affinity of the ash for the bed material.34 (4) Collisions between the coated bed particles result in the formation of bonds between particles, which form agglomerates, and can also lead to complete bed defluidization. During FBC, the sodium present in the Lochiel coal forms sodium sulfate or sodium calcium sulfates within the coal ash. These sulfates are liquid and, therefore, sticky at the process temperature and, hence, may cause agglomeration of the combustor bed material and the ash and, consequently, can also lead to defluidization: the loss of bed fluidization. Such operational problems must be avoided for smooth and economical operation of the FBC process. These mechanisms that lead to the formation of sodium compounds within the coal ash may be expected to vary from coal to coal because they are likely to depend upon the form of sodium initially present in the coal, as either water-bound sodium, salts dissolved in the coal moisture, or sodium bound within the organic structure of the coal. The propensity for agglomeration and subsequent defluidization during the FBC of Kingston coal has not been previously investigated, and the mechanisms of these phenomena are unknown. To avoid or decrease agglomeration within the fluidized bed, a number of studies have been conducted with additives introduced into fluidized beds (reviewed by Bartels et al.7). The underlying mechanism for these additives is either dilution of the fuel ash that leads to a less sticky ash layer on the bed particles or the reaction of the alkalis within the fuel with the additives, thereby forming higher melting point compounds rather than the formation of low melting point silicates or sulfates. The addition of these additives has been the focus of attention for combustion of Australian low-rank coals. Lindner et al.1 established that the addition of kaolin clay to coal helps to control fouling during the combustion of Lochiel coal. Vuthaluru et al.35 then investigated three control techniques in alleviating ash-related problems during combustion in a spouted-bed combustor for two lowrank coals. Among the additives tested, kaolinite was found to be the most effective. Vuthaluru36 also determined that the minimum additive requirement of kaolin should be 2 wt % of the coal to control fouling during the combustion of the Victorian Loy Yang coal. Kyi and Chadwick37 tested a suite of 12 mineral additives and determined that the best mineral additives on both performance and a cost basis were kaolin mineral types to bind various sodium compounds. Bhattacharya et al.4 showed that, to successfully combust Lochiel for long periods in a pilot-scale FBC, both agglomeration-controlling additives and bed renewal were required. From all of this work previously undertaken, it is clearly shown that the addition of additives to a fluidized-bed combustor can be used to successfully control agglomeration. However, the study by Bhattacharya et al.4 was the only work conducted specifically to show that stable operation was possible for extended periods for FBC of a high-sodium and high-sulfur coal (Lochiel). There has thus far been no knowledge gained on the effect of kaolinite additives on the operation of a fluidizedbed combustor with Kingston coal. There have been no fundamental studies conducted for extended periods to determine the effect of refreshing and removing bed material and ash from the bed during continuous operation on both Lochiel and Kingston coals.

Figure 1. Schematic diagram of the FBC system.

The first aim of this work is to assess the tendency of Kingston coal to cause agglomeration and/or defluidization of the bed material during FBC. Lochiel coal is also assessed to compare the behavior of both coals and to allow the present work to be compared to previous experience gained using Lochiel coal in FBC during laboratory experiments and pilot-plant operations. As part of this objective, the effect of refreshing and removal of the ash and bed material from the fluidized bed during combustion will also be assessed. The second aim of this work is to assess the effect of the addition of kaolinite clays as additives to the bed material during combustion and their effectiveness in preventing the formation of sticky ash on the bed material and, hence, to control agglomeration and prevent subsequent defluidization for both Lochiel and Kingston coals.

2. EXPERIMENTAL SECTION 2.1. Apparatus. Combustion experiments were undertaken in a 77 mm diameter FBC system, shown schematically in Figure 1. The combustor consists of a 77 mm internal diameter stainless-steel cylindrical reactor, with a conical section at its base. A multi-hole distributor is connected to the bottom of the conical section and provides the spout of fluidizing hot air. Compressed air is heated using a Leister Electric Hot Air Tool type 5000. The distributor is designed to allow it to be dropped at the end of an experiment to remove the spent bed material from the combustor. The furnace is fitted with a coal feeder, primary and secondary cyclone separators for ash separation from 119

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Table 1. Summary of Experiments Performed for This Study overall result

temperature (°C)

800

850

a

sand added in feed

clay added in feed

(percentage of the total feed)

(percentage of the total feed)

test ID

Lochiel coal

test ID

Kingston coal

no

no

L1

3.4 h (defluidized)

K1

6 h (defluidized)

10%

no

L2

8.8 h (defluidized)

K2

30 h (NOPa)

a

10%

10% CW

L3

30 h (NOP )

no

no

L4

2.6 h (defluidized)

K4

7.5 h (defluidized)

10%

no

L5

13 h (defluidized)

K5

22 h (defluidized)

5%

no

L6

7.7 h (defluidized)

K6

8.1 h (defluidized)

5% 5%

10% CW 5% CW

K7 K8

30 h (NOPa) 30 h (NOPa)

5%

5% DV

K9

30 h (NOPa)

NOP means “no operational problems” and indicates a steady temperature and pressure drop during operation. (either 800 or 850 °C). The combustion air is then reduced to approximately 40 L min 1 STP for the superficial velocity in the combustor to be approximately 1 m s 1. It was verified experimentally during preliminary trials that a superficial velocity of 1 m s 1 was adequate to maintain stable fluidization. The combustion experiments were run for up to 30 h depending upon whether defluidization occurred in a particular test. For tests longer than 8 h, the test was continued over several 8 h days. On any given day, coal feeding occurred continuously for 8 h or until defluidization occurred. An abrupt change in the pressure drop across the bed, accompanied by a significant rise in the temperature above the bed, was taken to indicate that bed defluidization had occurred. At the end of each 8 h day, coal feed was ceased, the air supply was turned off, and the bed was allowed to slump and then cool overnight. The test was then recommenced on the next day by preheating the cooled bed material prior to restarting the air and coal flows. The maximum cumulative duration of any test was 30 h. Table 1 presents a summary of the experiments performed and shows, for each experiment, the period of stable operation that was observed prior to the occurrence of defluidization. During most experiments, because of the accumulation of ash and additives within the bed, refreshing and removal of the ash and bed material was carried out without interruption to the operation of the combustion process. Some bed material was routinely removed once every hour. Bed removal from the system was safely achieved using a pair of valves. While the bottom valve attached to the bed removal container was closed, the top valve attached to the reactor was opened to allow for bed material to flow into the bed removal section. The top valve was then closed, and the bottom valve was subsequently opened, to allow for the bed material to flow safely into the bed removal container. After the bottom valve was closed, the bed removal container could be removed for sampling. The amount removed was sufficient to allow for the pressure drop across the bed to be maintained within the target range of 550 650 Pa for any given test. Some typical sets of temperature and pressure data collected from the chart recorder are given by Figure 2. Figure 2a shows an example of an experiment that ended in defluidization. It can be observed that, at a time of approximately 2.6 h, there was an abrupt change in the pressure drop across the bed, accompanied by a significant rise in the temperature above the bed, which was indicative of bed defluidization. Figure 2b shows an example of a test where the temperature in the bed remained relatively constant at 800 °C and the temperature in the reactor freeboard, measured at a position 5 cm above the bed, was approximately 750 °C. Also shown in Figure 2b is the effect of renewal of the bed material. The pressure drop during each experiment that included bed renewal increased for a period of 1 h because of the combined effects of

exhaust gases, an ash recirculation system, and a bed material removal section. The reactor is also fitted with thermocouples and differential pressure probes along its height. These devices are interfaced to a data acquisition unit. There is a glass-covered viewing port at the top of the combustor. External heating is used during startup to aid in combustor preheating and also to minimize heat loss from the reactor during operation. An electrically heated furnace houses the middle section of the reactor. The outer walls of the furnace consist of thermal ceramics insulating firebricks. Two pairs of Kanthal Crusilite electrical heating elements provide heating; the elements are situated within each corner of the furnace and are suspended vertically from the top plate. Coal is fed above the bed, at a height of 550 mm above the entrance to the conical base, via a pair of lock hoppers and a screw feeder. Nitrogen is used to ensure an inert atmosphere within each of the hoppers, to prevent combustion of these very reactive coals in the hoppers and to provide backpressure to prevent flue gases from entering the hoppers. The coal feed line is water-cooled near the entrance to the reactor. Type-K thermocouples (3.0 mm in diameter) were used to measure the temperature at various locations along the centerline of the reactor. Pressure tappings are located 15 mm below the gas inlet and 190 mm above the gas inlet. The absolute pressure of the combustor is measured using a Wika pressure transmitter at the point 15 mm below the gas inlet. The differential bed pressure drop is measured between the upper and lower pressure tappings using an ABB Kent Deltapi K Series electronic transmitter. Both pressure transmitters provide a 4 20 mA signal to a Mann Industries PM350 industrial process monitor. Temperatures and pressure signals were monitored using a Pico Technology eight-channel data logger (model TC-08) and logged at 1 Hz using the supplied PicoLog software to a computer. Combustion of coal takes place in the lower part of the furnace in a fluidized bed of silica sand. Exhaust gases are passed through primary and secondary cyclone separators. Material collected in the primary cyclone, consisting of particles approximately larger than 0.1 mm, is returned to the furnace via a recirculation pipe using recirculation air. Finer ash material is separated in the secondary cyclone and collected in the ash can. 2.2. Procedures. At the start of each experiment, 200 g of sand is sieved to a size range of 0.85 1.0 mm, loaded into the combustor, and fluidized with 60 L min 1 standard temperature and pressure (STP) air. The sand bed is heated to about 400 °C using the air preheater and furnace external heating elements prior to the introduction of the coal. Once this temperature is reached, coal feeding is started at a feedrate of 500 g h 1, with the combustion of the coal supplying the extra heat needed to raise the bed to the required temperature for the experiment 120

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Table 2. Proximate, Inorganic, and Ash Analyses of the AirDried Coal Samples Used in This Study Lochiel

Kingston

Proximate Analysis moisture (%)

22.1

23.6

volatile matter (% dry basis) fixed carbon (% dry basis)

47.0 36.9

45.9 40.9

ash yield (% dry basis)

16.1

13.3

Inorganic Analysis (% Dry Basis) S

3.0

2.9

Na

0.84

0.93

Cl

0.37

0.16

Ash Analysis (% in Ash) SiO2

37.0

19.3

Al2O3 Fe2O3

6.11 4.48

12.3 1.68

TiO2

0.75

0.33

K2O

0.42

MgO

8.41

Na2O

9.11

0.89 17.7 12.4

CaO

11.2

13.0

SO3

19.7

23.2

oven-dried to 60 °C and then ground with mortar and pestle before being lightly pressed into aluminum sample holders. The XRD patterns were recorded with a Philips PW1800 microprocessor-controlled diffractometer using Co Kα radiation, variable divergence slit, and graphite monochromator. The diffraction patterns were recorded in steps of 0.05° 2θ with a 3.0 s counting time per step and logged to data files for analysis. CSIRO technicians interpreted the diffraction patterns. Individual particles of interest were analyzed using scanning electron microscopy with electron-dispersive spectroscopy (SEM EDS). Agglomerates were mounted in resin and placed in a vacuum oven to ensure that the resin reached all open pores. Finished samples were cut using a diamond saw to obtain cross-sections for mounting on stubs. Samples were carbon-coated to reduce charging of the sample. The samples were then inserted into a Philips XL30 field emission gun scanning electron microscope, operating in backscattered electron (BSE) mode using an accelerating voltage of 15 20 kV and a beam spot size of 4 5 Å. Point analysis could then be carried out using energy-dispersive spectrometry, so that specific mineral structures could be identified. 2.4. Materials. Coals were air-dried to reduce the moisture content from >50% in the raw coal samples to 22 24% in the samples combusted. The samples were then crushed and sieved to the range of 1.0 3.35 mm before each experiment. The coals used were Kingston and Lochiel coals, both of which are low-rank coals from South Australia. Analyses of the air-dried coal samples are shown in Table 2. Additional analyses were conducted for the coals to determine the possible chemical nature of some elements present in coal, as described in section 2.3. The solubility of inorganic components in water, ammonium acetate, and HCl for Kingston and Lochiel coals is presented in Figure 3. This was undertaken to indicate the proportion of inorganic components that are present as salts dissolved in the inherent moisture or attached to the coal structure. Solubility of a component in water indicates the proportion of that component that is dissolved in the coal moisture. Solubility of a component in ammonium acetate indicates the combined proportion of that component that is attached to the coal structure (organically bound) and dissolved in the coal moisture. Solubility of a component in HCl indicates the total quantity present in

Figure 2. (a) Typical data collected from a combustion run that ended in defluidization and (b) typical data from a combustion run, with bed renewal, which did not end in defluidization. ash accumulation and sand addition to the bed. It increased approximately linearly within each hour from ∼550 to ∼650 Pa. At the end of each hour, the pressure drop was reduced to approximately 550 Pa by the removal of some bed material from the reactor. In this way, the bed weight was maintained within a relatively narrow range, and thus, any influence of the defluidization time is expected to be minimal. 2.3. Analyses. Coals and bed material were characterized using a range of analytical techniques. Moisture and ash yields were determined according to HRL method 1.6, an in-house confidential method used by HRL Technology Pty Ltd. Fixed carbon and volatile matter are determined according to Australian Standard method AS 2434.2. Sulfur and chlorine contents are carried out according to AS 2434.6 and AS 1038.8, respectively. Ash composition was determined using borate fusion of samples and analyzed with inductively coupled plasma atomic emission spectrometry (ICP AES) according to AS 1038.14. Acidextractable (AS 2434.9), water, and ammonium-acetate-extractable (modified AS 2434.9 method, excluding the ash matrix) elemental analyses were carried out to indicate the quantity of various inorganic elements within the coal or bed material that were soluble in acid, water, or ammonium acetate. The mineralogy of samples was determined using semi-quantitative X-ray diffraction (XRD). Analysis was carried out commercially by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Division of Land and Water. Samples for analysis were 121

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Table 3. Chemical Analysis of Clays Used in This Study3 constituent

CW clay (wt % in clay)

DV clay (wt % in clay)

SiO2

55.35

64.42

Al2O3

40.88

29.36

Fe2O3

0.43

2.28

TiO2

1.34

1.26

K2O

0.74

0.45

MgO

0.09

0.22

Na2O

0.60

0.95

CaO SO3

0.10 0.15

0.27 0.17

this work, an inorganic analysis of the coals was performed, as seen in Table 2. It is observed that the coals have relatively similar contents of sodium and sulfur but very different levels of chlorine. Additionally, the ash analyses of the coals show that Kingston coal has significantly more aluminum and magnesium, whereas Lochiel coal has significantly more silicon. Figure 3 also shows that there are differences between the forms of sodium in the coals. Lochiel coals appear to contain mostly water-soluble sodium. This is consistent with the higher chlorine content in the coal, as shown in Table 2, which implies the presence of a significant quantity of NaCl. Conversely, for Kingston coal, there appears to be a significant quantity of sodium that is soluble in ammonium acetate but not soluble in water and, therefore, indicates the presence of a significant quantity of sodium bound to the organic coal structure. It is clear from these differences that the ash formed from the FBC of the two coals will be significantly different, and hence, the mechanism of defluidization of the two coals will likely be different. To assess the differences in defluidization tendencies between the coals, experiments were first conducted without the addition of fresh bed material or the use of additives in the coal feed for both coals, to compare the combustion behavior of Lochiel and Kingston coals directly (tests L1, L4, K1, and K4 in Table 1). These experiments were performed to provide a benchmark for the later experiments with bed renewal and with clay additives. FBC of Lochiel coal lasted for very short times because the loss of bed fluidization had taken place after only 2.5 3.4 h. FBC of Kingston coal lasted 2 3 times longer at any given experimental temperature. The addition of fresh bed material to the coal feed and subsequent removal of bed material from the reactor were used in some experiments to help prevent the buildup of ash in the bed that would occur because of ash coating of the bed particles. The continuous addition of bed sand material with coal also diluted the ash buildup in the bed and ensured a consistent ratio of sand to ash in the bed while using the bed removal system. The main variable in these experiments was the quantity of sand added to the feed to partly “renew” the bed material. As observed by Table 1, all of the experiments combusting either Lochiel or Kingston coal without clay additives resulted in defluidization, except test K2 for Kingston. It was generally observed that the major impact of renewing the bed material was to delay the defluidization of the bed. The summary of these results is shown in graphical form in Figure 4. Figure 4 shows that, in general, the greater the percentage of sand in the feed, the greater the time until defluidization. For tests K2 and K5 for Kingston coal, the resulting long periods without defluidization were considered to be due to the large quantity of bed renewal (10% of the total feed rate).

Figure 3. Solubility of various inorganic components present in (a) Lochiel and (b) Kingston coals in water, ammonium acetate, and 50% HCl. the coal, including the two previous forms, as well as the proportion that is incorporated in minerals, such as carbonates and clays that are present in the coal. Figure 3 also shows the total sodium as measured by borate fusion of samples and subsequent analysis using ICP AES. This shows that not all of sodium has been accounted for in the solubility analyses and implies that there is a small amount of uncertainty in the solubility measurements. However, the solubility analysis gives a good indication of the relative amounts of water-soluble and organically bound forms of sodium. Before each experiment, the feed was prepared by mixing sieved coal with bed material and, in some cases, with the additive used for that particular experiment. Mixing was undertaken by hand until a good extent of mixing of the materials was observed. Kaolin clay-based additives were used in the experiments to assist with the control of bed agglomeration. The additives tested in this work were CW clay, a clay rich in minerals kaolinite and sillimanite, and DV clay, a clay rich in minerals kaolinite and quartz. These clays are identical to those used for previous work in our laboratory.3,35 The chemical and mineralogical compositions of these clays are given in Tables 3 and 4. Both clays had particle sizes 60%; co-dominant, two or more components of equal quantity; sub-dominant, 20 60%; minor, 5 20%; and trace, 60%; co-dominant, two or more components of equal quantity; sub-dominant, 20 60%; minor, 5 20%; and trace, 60%; co-dominant, two or more components of equal quantity; sub-dominant, 20 60%; minor, 5 20%; and trace,