Control of Agglomeration during Circulating Fluidized Bed Gasification

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Control of Agglomeration during Circulating Fluidized Bed Gasification of a South Australian Low-Rank Coal: Pilot Scale Testing Philip J. van Eyk,* Adam Kosminski, Peter J. Mullinger, and Peter J. Ashman Centre for Energy Technology, School of Chemical Engineering, The University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia ABSTRACT: In this paper, results are presented of experiments performed to assess control techniques to counteract the operational problems of agglomeration during gasification of a South Australian low-rank coal in a pilot scale circulating fluidized bed gasification (CFBG) plant. Kaolinite-rich clay additives were used in the experiments in order to reduce the problems associated with sodium present in coals. The effect of refreshing and removing the bed material during continuous gasification was also studied experimentally. Agglomerates were observed to form during tests that occurred without the addition of kaolinite; the joints of the agglomerates were found to be comprised principally of sodium and silicon, presumably as sodium silicates. Sodium silicates were also observed to contribute to deposition of bed material and ash in some parts of gasifier. Deposition occurred mainly just above the air delivery inlets to the riser and at the bottom of the recirculation leg. At both of these points, local higher temperatures than in the other parts of the gasifier would be expected, with temperature having a large influence on the viscosity and stickiness of silicate layers and their tendencies to stick to reactor surfaces. The addition of kaolinite clay reduces the formation of sodium silicates. Instead, sodium reacts with kaolinite to form sodium aluminosilicates, which do not melt at fluidized bed gasification conditions. Agglomeration during normal operating conditions was not observed during clay addition tests. These results further show that the formation of coating and agglomerates in the sand bed can be practically eliminated, and with fine-tuning of the Kingston coal gasification process, the problem of agglomeration and/or defluidization can potentially be overcome during operation of a commercial-scale gasifier. Under fluidized bed combustion conditions, it has been shown in several studies that the sodium present in the South Australian low rank coals forms sodium sulfate or sodium calcium sulfates within the coal ash.5−9,12 These sulfates are molten and, therefore, sticky at the process temperature above approximately 800 °C, and, hence, may cause agglomeration of the combustor bed material and the ash and, consequently, can also lead to defluidization. In comparison, relatively few studies have investigated agglomeration of similar coals under gasification conditions. One such group of studies, conducted by Kosminski et al.,30−33 performed thermodynamic calculations and subjected modified coal samples to different gasification atmospheres in a laboratory scale tubular furnace. It was concluded that sodium disilicate eutectic mixtures form in typical gasification environments. This low-melting-point compound is likely to be responsible for agglomeration and defluidization in a fluidized bed gasifier operated with these types of coals. Previous work in our laboratory13,14 was undertaken to investigate the mechanisms of agglomeration and defluidization during the spouted bed gasification of Lochiel coal in a laboratory-scale, spouted-bed reactor. Results of recent work, performed in a small scale fluidized-bed gasifier,15 showed that the formation of sodium silicates within the fluidized bed led to agglomeration and defluidization when

1. INTRODUCTION The low-rank coal deposits in South Australia have the potential to be a source of energy for many years, but due to their high content of moisture (>50%), sulfur, sodium, and chlorine, the utilization of these fuels is difficult. As an alternative to conventional pulverized coal combustion, fluidized bed processes offer a lower temperature operation that reduce the propensity of these coals to release volatile sodium compounds that cause severe fouling and corrosion on cooler heat-transfer surfaces. Additionally, there is significant interest in the conversion of these low-cost, low-rank coals into valuable chemical products such as liquid fuels, methanol, ammonia, and urea. Synthesis gas production via gasification is a crucial step in the manufacture of any of these products from low-rank coal. Kingston coal from the southeast of South Australia is now being considered as a feedstock for gasification using circulating fluidized bed gasification (CFBG) technology. A disadvantage of fluidized bed technology is that agglomerates may form because of the formation of lowmelting point inorganic compounds in the fuel ash, and this may lead to the entire bed defluidizing because of the presence of large agglomerates.1 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 agglomeration/defluidization phenomena in fluidized beds in the combustion or gasification of coal,2−15 combustion of petroleum coke,16,17 combustion or gasification of biomass,18−27 and co-combustion or co-gasification of biomass with coal.28,29 © XXXX American Chemical Society

Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 29, 2015 Revised: December 11, 2015

A

DOI: 10.1021/acs.energyfuels.5b02267 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Circulating fluidized bed gasifier (CFBG) piping and instrumentation diagram.

during CFBG of Kingston coal. Kaolin clay additives and removal of the ash and bed material while replenishing bed materials and additives during continuous operation were again assessed for the mitigation and control of these agglomeration related operational problems.

Kingston coal was gasified. However, there has thus far been no knowledge gained on the effect of larger scale continuous operation of a circulating fluidized bed gasifier with Kingston coal on the agglomeration/defluidization behavior. 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.1). 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.7−9,12 However, it is uncertain whether these additives will also be effective in a gasification environment and so the effect needs to be verified experimentally. Previous experimental work, performed in a small scale fluidized bed gasifier,15 determined that the addition of kaolinite-based clay to the coal feed appeared to control agglomeration within the bed, therefore allowing long-term continuous operation. These studies also determined that there was a positive effect of refreshing and removing bed material and ash from the bed on the agglomeration tendency of the fluidized bed while gasifying Kingston coal. However, due to the small scale of the previous tests, it is unclear whether the prevention of agglomeration and defluidization will be as efficient when the process is scaled up. Therefore, in the present work, tests have been performed in a pilot scale CFB reactor to more fully assess operational problems due to agglomeration, deposition, and defluidization

2. EXPERIMENTAL METHODS 2.1. Plant Description. A schematic diagram of the CFBG plant is depicted in Figure 1. The plant consists of an air-blown CFBG unit, a coal feeding system, and a downstream plant consisting of cyclones for particulate removal, a gas cooling system, and a syngas burner. 2.1.1. Coal Feeding. Dried brown coal of 20−31% moisture is first fed into the load hopper, which in turn feeds into the CFBG feed hopper. Once the feed hopper is full, it is sealed and pressurized with nitrogen to inert the coal within the hopper and also to stop the flow of gasification gases into the hopper. Coal is fed from the feed hopper into the gasifier reactor through a horizontal screw conveyor. The screw feeder is calibrated routinely for the coal used on an individual run to account for variation of moisture content in a given batch used. 2.1.2. Gasifier Reactor. The reactor has an internal diameter of 65 mm in the riser and a height of approximately 4 m. A bed of silica sand is fluidized using air which enters at the base of the riser. Coal that is introduced into the reactor is gasified within the silica sand bed using the fluidizing air as well as steam that is added to the air inlet stream as a secondary reactant. The syngas produced within the bed passes through a large cyclone that removes the silica sand and larger unburnt char particles from the gaseous stream. The silica sand and unburnt char are returned to the bed via a recirculation leg due to the combined effects of gravity and pressure from secondary air introduced at the bottom of the return leg. To maintain a satisfactory inventory of ash, char, and sand in the fluidized bed, bed material is removed during operation of the CFBG reactor. Below the bed, a valve and ash can B


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gasifier which occur in relatively small reactor compared to an industrial scale reactor. Feedback controllers are used to control the external reactor wall temperature using the installed cable heaters. Nitrogen is used throughout the plant for a number of purposes: to make inert the system prior to or after commencing an experiment, to pressurize the feed hopper system, and to ensure no ash or bed material interferes with the pressure measuring points attached to the unit. 2.1.3. Gas Cleaning, Cooling, and Disposal. In addition to the cyclone within the CFBG reactor section of the plant, a series of primary and secondary cyclones are utilized to remove the fine particulate matter from the syngas stream. The ash cans from each of the cyclones were routinely emptied to collect the fine particulate matter from the CFBG plant. Following the syngas cleaning section of the plant, the syngas is cooled in two parallel condensers. The condensers are large drums of water in which syngas is bubbled and cooled to close to ambient conditions. Tars and other condensable compounds contained within the gas stream are also captured in the condensers during the cooling process. Following cooling, the syngas stream is sent to a burner where it is combusted with air and then vented to atmosphere. Liquifed petroleum gas (LPG) is provided to the syngas burner as a pilot flame which provides stable operation of the burner even in cases where the fuel gas may be of low calorific value. 2.1.4. Operating Conditions. The plant is designed to operate at temperatures up to 1000 °C, although in practical terms the operation is within the range of 800−900 °C. The unit operates at atmospheric pressure and is designed to gasify solids either using air only or using a mixture of air and steam. Brown coal can be fed to the reactor at feed rates up to ∼15 kg/h. The air velocity within the riser ranges from 4 to 6 m/s, and typical ratios of air to dry, ash-free coal are approximately 1.8−2.4 on a mass basis. 2.1.5. Process Services. Air utilized in the CFBG plant is supplied by an in-house air compressor. For the primary air line, an electric honeycomb-design air preheater provides heat to raise the temperature of the inlet air to ∼600 °C. Steam is generated in an in-house boiler and is supplied to the primary air stream via a system of insulated pipework, condensate traps, and a steam mass flowmeter. Nitrogen is supplied to the plant by a series of manifolded nitrogen cylinders. Lastly, cooling water is supplied to the plant from suburban mains water.

Table 1. Proximate, Ultimate, Inorganic, and Ash Analysis of Coal Used in This Study Proximate Analysis % moisture volatile matter (% dry basis) fixed carbon (% dry basis) ash yield (% dry basis) Ultimate Analysis (% dry basis)

22−34 46.1 41.1 12.8

C H N S O (by difference) Inorganic Analysis (% dry basis) S Na Cl

58.60 3.90 0.52 3.30 20.88 2.9 0.93 0.16

Ash Analysis (% in ash) SiO2 Al2O3 Fe2O3 TiO2 K2O MgO Na2O CaO SO3

19.3 12.3 1.68 0.33 0.89 17.7 12.4 13.0 23.2

system is used to remove bed material during an experimental run. Another position for bed material removal was located at the bottom of the downcomer, where another valve and ash can were installed. The entire gasifier reactor, including the riser, cyclone, and return leg, are heated with cable heaters and insulated with ∼50 mm of high temperature fiber insulation in order to maintain temperature within the unit. The electric cable heaters are largely an experimental artifact utilized in order to counter high heat losses from the surface of the

Table 2. Kingston Coal Gasification Tests

test

coal feedvrate kg/h (wet basis)

moisture (%)

bed temperature (°C)

freeboard temperature (°C)

air flow rate (L/min at STP)

Steam Flow rate (kg/h)

1−12 13 14 15 16 17 18 19 20 21 22 23 24 25

8−15 11 12.8 12.7 11.5−13.9 10.1 11.9 11.9 11.9 11.6 11.6 15.5 11.4 11.4

20−24.6 24.6 24.6 24.6 24.6 25.7 25.7 25.7 25.7 25.7 25.7 48 31 31

810−880 850 820 830 850 850 800 830 800 800 800 < 800 810 810

500−850 850 820 830 850 850 800 830 800 800 800 < 800 800 810

200−350 200 250 250 250 200 200 200 200 240 250 250 210 250

0−5.7 0 0−5 0−5.7 0−5 0−6 0−4 2 2 2 0 0 0 0

26 27 28

11.2 11.2 11.2

20 20 20

810 810 810

810 810 810

250 210 230

0 0 3

Snobrite Snobrite Snobrite

29

11.2

20

815

815

210

3

Snobrite

C

clay additive

DV

Snobrite Snobrite Snobrite

result commissioning defluidization test for gas composition test for gas composition test for gas composition test for gas composition stable test for 1.5 h agglomerates agglomerates agglomerates agglomerates too wet to reach 800 °C sintering too wet to stabilize at 810 °C sintering sintering/deposition large agglomerates due to deposition continuous operation for 6h


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Energy & Fuels Table 3. Gasification Performance during Tests on Kingston Coal test no.

bed temperature (°C)

A/F

(S + W)/F

CO2

CO

H2

CH4

N2

15.1 15.2 15.3 15.4 16.1 16.2 16.3 16.4 16.5 17.1 17.2 17.3 17.4 17.5 18.1 18.2 18.3 19 20 21 0 h 21 0.5 h 21 1 h 21 1.5 h 21 2 h 25 1 h 29 0.5 h 29 1.5 h 29 4 h 29 5 h 29 6 h

830 830 830 830 850 850 850 850 850 850 850 850 850 850 800 800 800 830 800 800 800 800 800 800 810 815 815 815 815 815

2.14 2.14 2.14 2.14 2.36 2.36 2.36 1.96 2.14 2.3 2.3 2.3 2.3 2.3 1.85 1.85 1.85 1.85 1.85 2.28 2.28 2.28 2.28 2.28 2.89 2.12 2.12 2.12 2.12 2.12

0.38 0.62 0.86 1.06 0.38 0.78 1.04 0.38 0.38 0.4 0.72 1.05 1.37 0.56 0.4 0.66 0.92 0.66 0.66 0.67 0.67 0.67 0.67 0.67 0.52 0.71 0.71 0.71 0.71 0.71

16.5 18.2 18.3 15.9 16.1 16.5 16.4 15.5 15.1 17 17.9 19.1 19.6 17.8 16.7 17.7 18.9 16.9 19.4 18.6 18.7 19.7 19.8 20.2 17.5 16.7 16.7 16.8 16.8 16.8

12.4 11.4 11.2 13.8 12.9 12.7 13.7 14.8 14.6 12.2 12.4 11.7 11.8 12.2 9.7 9.6 8.9 10.4 9.9 9.5 10.1 10.3 9.9 10 9.1 9.2 9.5 11.2 11.3 11.4

12.1 17.4 20.2 19 11.6 15.2 21.4 12.6 11.2 13.9 16.8 23.8 27.4 16.8 9.6 13.5 18 12.3 18.6 14.3 15.7 17.2 17.5 16.8 12.1 17.2 19.5 24.1 24.8 24.8

1.9 2 1.9 1 1.9 1.8 1.2 2.2 1.9 2.4 2.4 2.2 2.1 2.3 2.1 2.1 2 2.2 2.4 1.7 1.7 1.8 1.9 1.9 2.2 1.7 2 2 2.2 2.1

56.4 50.3 47.8 50.2 56.8 53 46.9 54.4 56.6 53.7 49.9 42.8 38.7 50.4 61.2 56.5 51.7 57.6 49.1 55.3 53.3 50.6 50.4 50.4 58.4 54.7 51.9 45.5 44.7 44.7

2.2. Operational Procedure. 2.2.1. Plant Warm Up. At the start of each test, a series of activities are undertaken to warm the plant up. First, the inlet primary air to the installation was started at 400 L/min STP and the air preheater is turned onto full power to start the heating of the installation. The cable heating elements are then operated with a set point of 800 °C at the outside wall of the reactor to preheat the installation and insulation before a test. A load of 3 kg of washed silica sand of size range 0.2−0.5 mm is then added to the reactor using a secondary hopper, which then begins to fluidize with the 400L/min of air. Once cable heaters H2, H3, and H4 (freeboard, cyclone, and return leg) have heated the walls to 800 °C and cable heater H1 (bed) has heated the sand bed to ∼600 °C, the experiment is started. A batch of char (commercially sourced Auschar brand coal char) with very low mineral content is then loaded into the secondary hopper and then dropped into the bed in order to start the gasification of the char. The purpose of this char addition step is to raise the temperature of the sand bed to the experimental temperature without risking agglomeration and defluidization due to the very low mineral content. The char is added in small quantities (50 g at a time) to slowly raise the temperature of the bed to the required level (from 800 to 850 °C depending on the experimental parameters. The air is then reduced to approximately 200−250 L/min in order for the required gasification air flow to be established. 2.2.2. Start of Gasification. After stabilization of the bed temperature, coal is fed at the desired flow rate. Partial combustion of the coal provides additional heat, allowing the required temperature for the experiment (from 800 up to 850 °C) to be achieved within the bed. Steam was also utilized in some experiments to provide an additional gasification agent. Steam is introduced to the reactor through insulated pipework using a steam mass flow meter into the preheated air line. The gasification experiments were run for up to a

H2S

H2/CO

carbon conversion (%)

0.2 0.7 0.7 0.4 0.8

0.98 1.53 1.8 1.38 0.9 1.19 1.56 0.85 0.77 1.14 1.35 2.04 2.32 1.38 0.99 1.41 2.01 1.19 1.88 1.51 1.56 1.67 1.77 1.67 1.33 1.87 2.07 2.15 2.2 2.17

71 81 85 77 78 84 93 70 72 78 86 90 95 84 53 58 64 58 72 74 79 86 86 88 79 58 62 74 76 77

maximum of 6 h due to the extensive heat up and cool down times for the installation leading to a lengthy operation time. 2.2.3. Operation Activities. Trends in the key parameters were observed including: differential pressures, bed temperatures, freeboard temperatures, return leg temperatures, and differential pressures, visual observations of the burner, and condenser temperatures. For most tests, bed material is periodically removed to maintain bed level and gas samples are taken for subsequent analysis using the micro GC. An abrupt change in the pressure drop across the bed, accompanied by a significant rise in the temperature above the bed, indicated loss of fluidization, a phenomenon called bed defluidization. If this occurred during a test, the test was immediately stopped, and it was concluded that the operating conditions were not suitable for full-scale industrial application. 2.2.4. Shutdown. At the end of each test, the coal feed and external heating were stopped and the bed was allowed to cool down, first with nitrogen to purge the system of any remaining combustible gasification products for 10 min, and then naturally by cutting off all gas flows to the system. The bed contents were then removed (usually the following day), inspected, and analyzed. The presence of large clumps of sand particles in the bed material (termed agglomerates) implied that the operating conditions of the test will eventually lead to problems, regardless of whether problems were observed during the actual test. 2.3. Coal Preparation. The coal used in experiments was air-dried, crushed, and sieved to the range of 1.0−10 mm before each experiment. The coal used in this work was Kingston coal from South Australia. The coal was characterized using a range of analytical techniques. Moisture and ash yields were determined according to HRL method 1.6, an in-house method used by HRL Technology Pty Ltd. Fixed carbon and volatile matter were determined according to Australian Standard method AS 2434.2. Sulfur and chlorine content D


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concentrations of H2S implies that this pollutant will need to be captured in order for the gasification process to be successful in fullscale plants. Within these tests, it was observed that values of carbon conversion range from 53 to 95%. The conversion of carbon appeared to be governed by several factors. Carbon conversion was found to increase considerably with increasing bed temperature. Additionally, the carbon conversion increases with increasing air-to-fuel ratio (A/F) and with the addition of steam to the reactor. 2.7. Microscopic Examination of Bed Material. During the course of the experimental program, bed material samples were systematically examined using a scanning electron microscope (SEM). SEM proved to be a quick and efficient method of gaining information about the nature of the material depositing on the surface of the sand bed material, about formation of agglomerates in the reactor bed, and on the quality of deposits formed in the installation. 2.7.1. SEM Instrumentation. A scanning electron microscope, Phillips SEM XL-20, equipped with lanthanum hexaboride emitter and with energy dispersive X-ray detection system (EDAX) was the main instrument used for microscopic examination of samples of coated sand bed material, agglomerates, and deposits collected during the gasification tests. The XL-20 was operated in the secondary electron image to examine morphology of the samples, particularly formation of fused phases, and for elemental analysis. Accelerating voltage of 12−15 kV for the electron beam of 3−4 nm and working distance of 34 mm were used as main SEM operating conditions. Another scanning electron microscope, Phillips SEM XL-30, was used mainly for examination of the polished cross sections of samples. Both instruments allowed recording of images and both were equipped with appropriate software, for determining chemical analyses of the samples. 2.7.2. Sample Preparation. Samples for SEM secondary electron imaging were mounted directly with double-sided adhesive tape onto special aluminum stubs and then coated with carbon. Samples placed on stubs were examined for their morphology, particularly the formation of fused phases. Polished cross sections of samples of agglomerates or deposits were prepared by embedding samples in chlorine free epoxy resin blocks. The surface of the block was then ground, polished, and coated with carbon. Images of polished cross sections of samples were examined using the SEM XL-30 under backscattered electron image mode for clear identification of various phases, particularly identification of silicate-based coating layers on the silica grains and matrixes holding agglomerates together. Elemental analyses were conducted for each type of sample, and results were expressed on weight and/or atomic basis. As in the previous work,15 during the CFBG tests, the major emphases were on assessing the cause of agglomerate formation and the nature/chemistry of the agglomerates binding material. 2.7.3. Sample Imaging. Polished cross sections of the bed material samples were prepared for obtaining images clearly reflecting the composition of the examined materials and often the thickness of the sand coating. Backscattered electron images of these samples were obtained to clearly observe differences where heavier and lighter elements were present in examined samples. Despite the close proximity of sodium and silicon atomic numbers, reflecting the densities of their atoms in the images allow for observation of where sodium and other elements are concentrated around silica sand grains. Some of the backscattered images are accompanied by a map of X-ray fluorescence from an element present in the sample.

were measured according to AS 2434.6 and AS 1038.8, respectively. Ash elemental composition was determined by borate fusion of samples which are analyzed using inductively coupled plasma−atomic emission spectrometry (ICP−AES) according to AS 1038.14. The average proximate, ultimate, and ash analyses for this coal are shown in Table 1. The same methods for ash yield and ash elemental composition were also used for bed material analyses. The moisture content of the coal varied from sample to sample in a range as specified in the table. If additives or bed renewal was used in an experiment, the feed was prepared by mixing sieved coal with bed material and/or any additive that was used for the experimental test. This was undertaken using a rotating drum mixer to provide good mixing of the materials before gasification. 2.4. Refreshing/Removal of the Bed Material. During most experiments, due to accumulation of char, ash, and additives in the bed, refreshing and/or removal of bed material and of the ash was carried out without interruption to operation of the gasification process to maintain constant reactor bed volume. Bed refreshing was carried with sand addition to the feed at the rate of 5 wt % of the feed rate in some tests. Systematic periodic removal of the bed material, usually once or twice an hour, was also carried out to ensure constant pressure drop across the bed and to avoid excessive buildup of material in the bed. The amount removed at a given time was determined by observing the pressure drop across the fluidized bed. If the bed pressure drop had increased substantially over a period of time, a sufficient amount was removed to reduce the pressure drop back to a more appropriate level. Bed removal therefore kept the bed inventory within target, thus removing excess material as a potential cause of defluidization. This effectively simulates full-scale operation where incremental bed removal and replenishment is always undertaken online to maintain continuous operation. 2.5. Overview of Experiments Performed. A total of 29 tests were performed during the period of testing on the CFBG plant. Table 2 summarizes the tests on Kingston coal. The principal purpose of the tests was to establish the nature of ash related problems (e.g., agglomeration and deposition) that occur during continuous operation. Tests 1−12 were principally commissioning tests to determine the range of operational parameters that were required for continuous operation of the plant. Determining gasification performance in terms of the range of potential gas compositions and carbon conversions capable with this coal in the CFBG plant was the main purpose of tests 13−17. Tests 18−22 were conducted to determine the nature of the ash related problems during CFBG of Kingston coal, without the addition of additives. The aim of tests 23− 29 was to assess the suitability of the addition of a kaolinite-based clay to the coal feed for preventing the problems associated with agglomeration and deposition that were observed during tests 18−22. 2.6. Gasification Performance. Table 3 shows a summary of the gasification performance of tests where the gas composition was monitored. The outlet gas composition was analyzed by a Micro GC (Agilent; model 3000) with PoraPLOT Q and molecular sieve 5A columns. Chemical QA/QC software (Cerity NDS) was used to analyze the spectra and to calculate molar concentrations based on calibrated peak heights. The dry gas composition varied in the following range for the tests: 9−15% CO, 10−27% H2, 1−2% CH4, 15−20% CO2, 0.2−0.8% H2S, 43−61% N2. The major gases produced from gasification (CO, H2, and CO2) varied from test to test, and in some cases, within a single test. The most significant contributor to the variation from test to test was the addition of steam to the reactor. The ratio of H2 to CO (H2/CO) is an important parameter for the Fischer−Tropsch and other synthesis processes. Within these experiments, values of H2/CO ranging from 0.9 to 2.3 were observed, with the main contributor to variation in this ratio being the steam-tofuel ratio. Methane concentration was relatively consistent throughout the experiments at various bed temperatures, implying that CH4 evolution is predominantly from pyrolysis rather than gasification. Additionally, relatively high concentrations of H2S were measured (0.2−0.8%), as expected due to the high sulfur content in the fuel. Such high

3. RESULTS AND DISCUSSION 3.1. Agglomeration. Bed material was examined after tests to assess the chemical nature of any agglomerates that formed, and the matrix or fused material, which held agglomerates together. Figure 2 shows an example of a sample that was collected after test 21, containing agglomerates of bed material joined by fused joints between bed sand grains. Figure 2a shows a backscattered electron image of the cross section of an agglomerate from test 21, while parts b and c of Figure 2 E


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Figure 2. SEM images showing (a) the cross section of an agglomerate collected from test 21 and showing maps of (b) sodium and (c) silicon.

Figure 3. SEM backscattered electron images of the cross sections of agglomerates from (a) test 17 and (b) test 21. (c) Elemental composition of spots in (a) and (b). All EDS-results are on an O-free basis.

present images, based on elemental X-ray emissions, of the distribution of silicon and sodium in the agglomerate. The coating around silica bed material grains and the joints between them is clearly a material rich in sodium, as shown in Figure 2b. Examples of other agglomerates and the relevant joints are shown back scattered electron images of the cross section of agglomerates from test 17 and test 21 in Figure 3. The joints connecting the bed particles, and the coatings surrounding the particles can be clearly discerned in the figures from the silica grains in these cross sections by the difference in brightness (the silica grains are a darker gray than the coatings and joints). Bright inclusions in the joints indicate that there are other elements present with higher atomic numbers such as

magnesium or calcium. The numbered points and areas within Figure 3 were analyzed for elemental composition, as shown in Figure 3c. Analyses of points within the cross sections of various agglomerate joints show very similar concentrations of sodium of approximately 12−18 wt %, which is considerably higher than other elements except silicon. Silicon concentrations were in the range of ∼50−70 wt %. Although Figures 3 shows cross sectional SEM images of agglomerates, a clearer depiction of the means by which the particles stick together in an agglomerate is provided in the SEM images of the exterior of agglomerates shown in Figure 4. The numbered points and areas within Figure 4a,b were again analyzed for elemental composition, as shown in Figure 4c. It can be clearly seen that both the surface of the particles (shown F


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Figure 5. SEM backscattered electron image of cross sections of agglomerates from (a) test 21 and (b) test 22. Note the length scale showing agglomerates in the millimeter size range.

thickness. Despite the fact that there is no coating on some bed particles, or on part of their surface, the nonuniform coating that does occur is enough for formation of agglomerates and consequently an obstruction to the performance of the fluidized bed inside the gasifier. Therefore, the thickness and the viscosity and surface tension of the layers of sodium silicates are sufficient for agglomerates to form. The agglomerates that do form are able to grow to substantial sizes within short periods of operation. Figure 5 shows images of cross sections of agglomerates from test 21 and test 22. In both tests, the bed material collected after each test contained a significant number of such agglomerates. The cross section images presented here clearly show that the agglomerates which form in the fluidized bed can grow to a few millimeters in size. Kingston coal, unlike most of the other South Australian lignite coals, inherently contains kaolinite clay.12 The presence of kaolinite clay within the coal has proved to result in much less severe bed material deposition/coating/agglomeration in comparison with other South Australian lignite coal when combusted in fluidized beds.12 The presence of kaolinite reduces the formation of liquid sodium compounds under combustion conditions (principally sodium sulfate). Instead, sodium reacts with kaolinite to form sodium aluminosilicates, which do not melt at fluidized bed conditions. However, it was uncertain whether this same phenomenon would occur under gasification conditions. An image of the cross section of a char particle coming from test 16 is presented in Figure 6. The distribution of mineral matter within a coal particle is clearly shown. Large white areas inside that particle contain large

Figure 4. . SEM micrographs of agglomerates from (a) test 17 and (b) test 21. (c) Elemental composition of spots in (a) and (b). All EDSresults are on an O-free basis.

in Figure 4b) and the joints themselves (shown in Figure 4a) are enriched in both sodium and silicon. It is also clear from the SEM images that the joints and particle coating have a glossy appearance, which is an indication of the presence of a molten and viscous liquid fluid present during the high temperatures of gasification. From Figures 2−4, it is clear that sodium present in coal reacts to form silicates and is enriched in fused joints, with the ratios of sodium to magnesium and sodium to calcium being much higher than these ratios in the parent coal (as can be seen in Table 1). Such a high concentration of sodium suggests that when sodium contacts the silica sand bed material surface it reacts abundantly to form sodium silicates. From the figures shown here, it is also clear that the sodium silicate layers do not form a uniform coating on the sand grains and have a variable G


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shown in Figure 7c. The fused matrix of the deposit is made of silicates containing not only sodium but also large quantities of magnesium and calcium. Embedded in the matrix are silica grains, such as shown at point 1. Such an extensive formation of complex silicates can only happen if higher temperatures are involved in the process, which is consistent with the location of the deposit being near the air inlet to the gasifier. This is also shown in Figure 7b,d, where, although the deposit is not as extensive as that shown in Figure 7a, the coating of particles within the deposit contains not just silicon and sodium but also calcium and magnesium. The combination of these findings suggests that a similar chemistry is involved with deposit formation that was occurring with agglomeration. However, the higher temperatures of the points of the reactor where oxygen is entering leads to higher temperatures and more complex fused matrices containing calcium and magnesium in the sodium silicates. 3.3. Control of Agglomeration. As noted previously, kaolinite-based clay that is inherent in Kingston coal has a positive effect on ash related problems during gasification. The amount of clay which is naturally present in Kingston coal is, however, not sufficient to completely prevent the formation of sodium silicates. Therefore, kaolinite-based clay was used as an additive to Kingston coal during the gasification process to prevent bed material coating and subsequent agglomeration and deposition. A clay containing nearly equal quantities of kaolinite and quartz, DV clay, was added to Kingston coal during gasification test 17 at a rate of 5% of the total coal feedrate. The effect of DV clay was a reduction of the thickness of the coating on the sand particles and thinner layers of silicate fused matrix in the agglomerates but not a total elimination of them. Hence, a commercially available clay called Snobrite, which is richer in kaolinite than DV clay, was added to Kingston coal during the final series of gasification tests (tests 23−29) at a rate of 5% of the total coal feedrate. No more agglomeration was observed during those tests. Other problems occurred in some tests, causing these tests to be aborted, but it was not due to bed material agglomeration. The final test with clay addition (test 29) resulted in smooth continuous operation of the CFB gasifier, and no formation of agglomerates was observed during the 6 h of operation. This can be seen in the image of bed sand particles in Figure 8. The sand grains appear to be coated with fine ash, but that coating does not appear to be glossy as was the case for the nonclay addition tests (e.g., Figure 4), where this indicated the formation of sodium silicate glass. Backscattered images of cross sections of sand bed grains after test 29 show almost no silicate coating on the sand. This is shown in Figure 9. The coating present is very thin and finely coated with coal ash. The elemental analysis of the coating shown in Figure 9d shows that sodium silicate was likely still formed on sand grains and that the formation of silicates was not completely prevented during test 29. However, the sodium silicate coating was clearly not sufficient to cause agglomerates to form. Bed material samples collected after tests were analyzed for size distribution, ash yield, and ash composition. The samples that were analyzed were collected during tests 22, 24, and 29 to provide insight into the differences between the clay addition tests (tests 24 and 29) and a nonclay addition test (test 22). Because a significant amount of agglomerates were found in the test 22 bed material, the size analyses confirmed that larger portion of the bed material had sizes greater than that of the sand used to form the bed. Figure 10a shows that after test 22

Figure 6. (a) Backscattered electron imgae of the cross section of a char particle from test 16. (b) Elemental composition of frames in (a). All EDS-results are on an O-free basis.

quantities of sodium, aluminum, and silicon, indicating that they likely consist of sodium aluminosilicates. From our previous work on Kingston coal combustion in a CFB,12 it was determined that sodium that is present in Kingston coal is mostly organically bound to the coal structure and soluble in the inherent moisture. A very small proportion has been found to be associated with mineral matter such as clays. Hence, sodium is mostly not present in aluminosilicates within the original coal. Thus, the presence of signicant quantities of sodium aluminosilicates implies that sodium has reacted with the original kaolinite clay grains within the coal char. The reaction between sodium and kaolinite has been shown previously12,15,33 to form nepheline, a sodium aluminum silicate which melts above 1200 °C. 3.2. Deposition within the Gasifier. Sodium silicates formed during gasification of Kingston coal cause not only agglomeration but also deposition of bed material and ash in some parts of gasifier. In the current tests, deposition occurred mainly just above the air delivery inlets to the riser and at the bottom of the recirculation leg. At both these points, locally higher temperatures are expected than in the other parts of gasifier due to the combustion reactions that occur in these zones, and temperature has a large influence on the viscosity and stickiness of silicate layers and their tendencies to stick to reactor surfaces. Examples of cross sectional electron images of a deposit formed in the oxidizing region of the gasifier and collected after test 13 and of a deposit formed in inclined return leg and collected after test 21 are shown in Figure 7,b, respectively. The elemental analysis of frames 2, 3, and 4 shown in Figure 7a is H


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Figure 7. SEM backscattered electron images of the cross sections of (a) deposits collected from the bottom of the bed during test 13 and (b) deposits collected from the return leg during test 21. (c) and (d) Elemental composition of spots in (a) and (b), respectively. All EDS-results are on an O-free basis.

the particles of size 40%) of the Na2O is present in the larger particle sizes (>1 mm) for test 22, and a smaller proportion is present in the same bed size range for the clay addition tests (20−28%). The combination of these findings

agrees with the previous SEM analyses that suggested that sodium silicates were the dominant cause of bed agglomeration. For the clay addition tests, sodium is still retained in the bed within coal char (hence the relatively large Na2O content for the larger size range (>1 mm) for test 24 and 29), but sodium is not retained in silicate-based agglomerates (hence the very low proportion of SiO2 in the larger size range (>1 mm)). It is also interesting that for the clay addition tests Al2O3 has a greater proportion in the smaller size ranges (

Control of Agglomeration during Circulating Fluidized Bed Gasification

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