Article pubs.acs.org/EF
Impact of Sodium and Sulfur Species on Agglomeration and Defluidization during Spouted Bed Gasification of South Australian Lignite Daniel P. McCullough, Philip J. van Eyk,* Peter J. Ashman, and Peter J. Mullinger South Australian Coal Research Laboratory, Centre for Energy Technology, School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia, Australia ABSTRACT: Transformations of inorganic elements during steam gasification of a high-sodium, high-sulfur lignite were investigated in a 77 mm i.d. spouted bed reactor. The role of sodium and sulfur in agglomeration and defluidization of the bed material is assessed by analysis of inorganics in the bed char, in cyclone dust, in any agglomerates formed, and in deposits formed at the gas inlet to the reactor. It was found that a sodium disilicate-quartz eutectic was the key species in causing stickiness of the surface of mineral particles within the coal. The present data indicate that sodium and silica from the coal are reacting to form sodium silicate species, which forms the “glue” causing solid ash particles to cohere, initiating particle growth and agglomeration. This finding agrees with a fundamental study conducted by other authors. Sodium−calcium−sulfur eutectics, which have been found by other authors to cause agglomeration for similar coals under combustion conditions in a fluidized bed, were only found to form near the steam−air inlet to the reactor and thus do not play a significant part in agglomerate chemistry under the reducing conditions which exist throughout the majority of the bed under gasification conditions.
1. INTRODUCTION 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. Due to unacceptable levels of heat exchanger fouling and high emissions of sulfur oxides, along with high energy-specific greenhouse gas emissions, these coals are currently not utilized for power generation in conventional pulverized coal-fired furnaces. As an alternative to pulverized coal combustion, fluidized bed gasification offers more efficient utilization of South Australian low-rank coal since gasification occurs at lower temperatures permitting more precise control over emissions, such as by addition of dolomite to the bed to control the emission of oxides of sulfur. Gasification also allows flexibility of process choice: electricity production is possible at relatively high efficiency, via integration within an IGCC (integrated gasification combined cycle) process, or the production of high-value products such as synthetic transport fuels (e.g., synthetic diesel) or commodity chemicals (e.g., methanol, dimethyl ether, etc.) using coal-derived synthesis gas as the feedstock.1 Spouted beds are an alternative to fluidized beds that were originally developed by Mathur and Gishler2 for handling coarse particles. The utilization of spouted beds for various applications has since been applied including biomass pyrolysis,3−6 biomass and coal gasification,7−10 waste plastics recycling,11−14 polymerization,15 and bio-oil reforming.16,17 However, agglomeration and defluidization are major inhibitors to the efficient industrial use of both fluidized and spouted bed technology.18 Agglomeration generally occurs when bed particles enter a softened or sticky state, thus reducing the relative movement between particles and often resulting in particle growth.19,20 Under worst-case conditions, the bed ceases to fluidize effectively, a phenomenon often termed “defluidization.” Prevention or © 2015 American Chemical Society
control of agglomeration and defluidization are thus critical for any commercial fluidized bed process, and hence many investigations have been attempted to understand this hightemperature agglomeration−defluidization phenomenon. Such studies have focused on the combustion21−29 and gasification of coal,30−35 combustion of petroleum coke,36,37 combustion and gasification of biomass,38−46 and cocombustion of biomass and coal.47 Several previous studies have investigated agglomeration and defluidization of similar coals to the present study (viz. highsodium, high-sulfur lignite) under fluidized bed combustion conditions.25−29 These studies have shown that low melting sulfate eutectics, containing sodium and calcium, are the predominant species causing ash sintering and hence agglomeration. 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 laboratoryscale 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 laboratory9 has investigated the physical mechanisms of agglomeration and defluidization during the spouted bed gasification of a high-sodium, high-sulfur Australian low-rank coal in a laboratory-scale, spouted-bed reactor. That work showed that agglomeration proceeds by the coating of mineral particles, which leads to particle growth and eventual Received: February 15, 2015 Revised: April 29, 2015 Published: April 30, 2015 3922
DOI: 10.1021/acs.energyfuels.5b00367 Energy Fuels 2015, 29, 3922−3932
Article
Energy & Fuels
the letter “B” were designed to gain an overview of the gasifier bed behavior over a wider range of operating settings. During an experiment, the reactor9 was heated by a combination of air that flowed through a preheater (700 °C) and a set of four external heating elements. Steam was introduced to the vessel upon reaching steady state temperatures inside the reactor. When the reactor reached an equilibrium temperature, air flow was replaced by nitrogen to create an inert atmosphere. As soon as the reactor was inert, the screw feeder for the coal was turned on to the desired coal feed rate and was fed until 40 g of char was present in the reactor. Once the pressure and temperature readings were stabilized for this initial char bed, the nitrogen input was replaced with air, thus starting gasification. Gasification experiments were conducted for a 4-h duration, regardless of whether defluidization occurred, except for run B05, which was stopped after approximately 3.7 h of operation due to a rapid bed temperature excursion that exceeded 1000 °C. Monitoring of the bed temperature (at fixed positions in the bed) and bed pressure drop occurred at a frequency of one reading per second. Bed char, cyclone dust, ash deposits, and agglomerate samples were collected at the completion of each run and subjected to a variety of analysis methods. Based on the samples collected of the bed char, and the measured inorganic content (presented previously9), it is estimated that a range of 10−20% of the coal ash was retained in the bed within the experiments depending on the operational parameters. The remainder of the ash was entrained from the spouted bed (80−90%). 2.2. Analyses. Coals 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 content 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 2. The mineralogy of each sample was determined using semiquantitative X-ray Diffraction (XRD) by the CSIRO Division of Land and Water. Samples for analysis were oven-dried to 60 °C then ground with a 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.
defluidization of the bed. It was found that for a given fluidizing gas composition, there appears to exist a “high temperature defluidization limit” which specifies a minimum superficial velocity to avoid defluidization for a given bed temperature. Consequently, at bed temperatures or gas velocity conditions beyond this limit, particle growth and agglomeration occurs during stable operation. This particle growth appears to arise mainly with the coating of mineral particles by a molten inorganic phase. However, the chemical processes responsible for the molten ash coating on the mineral particles have not yet been established. The aim of this current paper is to investigate the chemical mechanisms of agglomeration and defluidization of South Australian lignite under spouted bed gasification conditions. A spouted bed gasifier used previously by McCullough et al.,9 which is similar to that used in the studies by Manzoori et al.,25−28 is utilized in this work to provide a basis for comparison of agglomeration and defluidization phenomena under gasification conditions with that under combustion conditions. The present study examines the gasification of a high-sulfur, high-sodium Australian low-rank coal within a spouted bed environment and aims to determine the role of sodium and sulfur in agglomeration and defluidization, and other ash-related problems, under gasification conditions.
2. METHODS 2.1. Experimental Section. Experiments were conducted in a 77 mm internal diameter, spouted bed gasification reactor, the details of which are reported by McCullough et al.9 In summary, air-dried Lochiel coal from South Australia was sieved to obtain a particle size distribution between 1.00 and 3.35 mm in diameter and fed continuously into the spouted bed. The coal was gasified using steam-air mixtures of specific proportions, under controlled temperature conditions. Table 1 shows the basic operating settings used in each experiment, including inlet coal feed rate, ratio of superficial velocity to minimum spouting velocity (Us/Umin), air/fuel, and steam/fuel. Umin was determined empirically to be 0.3 m/s. Also included is the maximum bed temperature before defluidization, run times, and classification of the bed of each of the experiments into one of three different behaviors (discussed in more detail in section 3). The experiments labeled by the letter “A” were designed to target operating conditions that were shown to be conducive to agglomeration and defluidization, whereas the experiments labeled by
Table 1. Operational Settings for Experimental Program
a
run
coal rate (db) (kg/h)
Us/Umin
air/fuel (w/w)
steam/fuel (w/w)
Tbed_max (°C)
run time (h)
classification
A01 A02a A03a,b A04a,b A05a,b A06a,b B01 B02a,b B03 B05a,b B06 B07 B08a,b B09 B10 B11 B12
0.92 0.77 0.77 0.77 0.77 0.77 1.09 0.75 0.88 0.75 0.83 1.09 0.88 1.27 1.05 0.92 0.88
2.17 1.97 2.27 2.17 2.07 2.03 1.97 1.90 1.90 1.67 1.70 1.90 2.07 2.00 1.93 2.13 2.03
2.7 3.0 3.5 3.2 3.2 3.2 2.2 2.8 2.5 2.5 2.5 2.1 2.8 2.0 2.2 2.8 2.5
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.4 0.5 0.4 0.4 0.5 0.5
926 920 939 967 915 909 831 914 844 861 789 823 887 799 872 826 910
4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 3.7 4.0 4.0 4.0 4.0 4.0 4.0 4.0
GRWT DEFL DEFL DEFL DEFL DEFL STBL DEFL GRWT DEFL GRWT STBL DEFL STBL STBL STBL GRWT
Defluidization detected. bAgglomerates (>3.35 mm diameter) detected. 3923
DOI: 10.1021/acs.energyfuels.5b00367 Energy Fuels 2015, 29, 3922−3932
Article
Energy & Fuels
crucibles. The mixture was then heated slowly to 700 °C to oxidize the organic matter and then further heated to 1050 °C for 12 min. The resulting melt was poured into a 32 mm Pt/Au mold heated to a similar temperature. The melt was cooled quickly over a compressed air stream, and the resulting glass disks were analyzed on a Philips PW1480 wavelength dispersive XRF system using a dual anode Sc/Mo tube with spectra interpreted using algorithms developed at CSIRO Land and Water. 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. Further to this, an X-ray Mapping (XRM) function in the SEM software was used to gain overall elemental distribution data for each selected area.
Table 2. Lochiel Coal Properties
a
coal analysisa
composition
proximate analysis moisture (ROM, wb) moisture (air-dried, wb) ash yield (db) fixed carbon (db) volatile matter (db) ultimate analysis C (db) H (db) N (db) S (db) O (db) (by difference) ash composition SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 SO3
wt % 60.3 12.7 15.7 38.1 46.2 wt % 58.2 4.4 0.5 3.5 33.4 wt % in ash 31.40 8.30 4.05 8.58 9.97 8.68 0.30 0.48 27.50
3. RESULTS AND DISCUSSION Each test shown in Table 1 was classified into one of three different categories based on observations of the bed behavior during each run and a subsequent analysis of the particle size distribution within the bed. These classifications represent an increasing tendency of the bed toward defluidization. Thus, the tests were classified as either “stable runs” (STBL), where the bed char particle size remained essentially constant during the experiments, “growth runs” (GRWT), which are stable runs with observed particle growth in the bed, indicating a possible tendency toward defluidization, and “defluidization runs” (DEFL),
ROM, run-of-mine coal; db, dry basis; wb, wet basis.
X-ray Fluorescence (XRF) analyses were carried out to determine the elemental composition (in oxide form) of each sample as a percentage of the total weight of the original sample. Analysis was carried out by the CSIRO Division of Land and Water. The sample for analysis was ovendried at 105 °C, and approximately 1−2 g of each sample was accurately weighed with 4 g of 12−22 lithium borate flux directly into Pt/Au
Table 3. XRD Analysis Results of Char Beds from All Experiments run
dominant/co-dominant (>60%)a
subdominant (20−60%)a
minor (5−20%)a STBL oldhamite, anhydrite, amorphous oldhamite, magnetite, monticelliteb anhydrite, monticellite, oldhamite, amorphous augite, amorphous
B01 B07 B09
quartz quartz quartz
gehlenite gehlenite
B10
quartz
oldhamite, periclase
B11
quartz
oldhamite, periclase
A01 B03 B06 B12
quartz quartz quartz quartz, gehlenite
gehlenite, augite gehlenite
anhydrite, monticellite, amorphous GRWT oldhamite oldhamite, anhydrite, magnetite oldhamite, anhydrite, gehlenite periclase, augite
A02 A03 A04 A05 A06 B02
gehlenite, augite augite gehlenite, augite augite augite
DEFL nepheline nepheline, forsterite, magnetite nepheline, magnetite gehlenite, magnetite gehlenite, magnetite, nepheline, calcite augite, anhydrite, monticellite, amorphous
B05
quartz quartz, gehlenite quartz quartz quartz quartz, gehleniteakermanite quartz
B08
quartz
oldhamite
gehlenite
oldhamite, anhydrite, magnetite, amorphous, augite anhydrite, gehlenite-akermanite, periclase
trace (
