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A Novel Fluidized Bed Drying and Density SegregationProcess for Upgrading Low-Rank CoalsNenad Sarunac a , Edward K. Levy a , Mark Ness b , Charles W. Bullinger b , Jonathan P.Mathews c & Philip M. Halleck ca Energy Research Center, Lehigh University, Bethlehem, Pennsylvania, USAb Great River Energy, Underwood, North Dakota, USAc Energy & Mineral Engineering & The EMS Energy Institute, The Pennsylvania StateUniversity, University Park, Pennsylvania, USAPublished online: 09 Jan 2012.

To cite this article: Nenad Sarunac , Edward K. Levy , Mark Ness , Charles W. Bullinger , Jonathan P. Mathews & Philip M.Halleck (2009) A Novel Fluidized Bed Drying and Density Segregation Process for Upgrading Low-Rank Coals, InternationalJournal of Coal Preparation and Utilization, 29:6, 317-332, DOI: 10.1080/19392691003666387

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International Journal of Coal Preparation and Utilization, 29: 317–332, 2009

Copyright © Taylor & Francis Group, LLC

ISSN: 1939-2699 print/1939-2702 online

DOI: 10.1080/19392691003666387

A NOVEL FLUIDIZED BED DRYING AND DENSITYSEGREGATION PROCESS FOR UPGRADINGLOW-RANK COALS

NENAD SARUNAC1, EDWARD K. LEVY1, MARK NESS2,CHARLES W. BULLINGER2, JONATHAN P. MATHEWS3,AND PHILIP M. HALLECK3

1Energy Research Center, Lehigh University,

Bethlehem, Pennsylvania, USA2Great River Energy, Underwood, North Dakota, USA3Energy & Mineral Engineering & The EMS

Energy Institute, The Pennsylvania State University,

University Park, Pennsylvania, USA

Lignite and sub-bituminous coals are attractive due to their lowcost, low emissions, and high reactivity. However, these coalscontain large amounts of moisture, which reduces calorific valueand lowers plant efficiency. A novel fluidized bed drying processwas developed that uses low-grade waste heat to reduce fuelmoisture content of low-rank high-moisture coals and concurrentlylowers sulfur and mercury content of dried coal through densitysegregation. This paper discusses quality improvement of low-rank coals by low-temperature thermal drying, describes changesin microstructure of coal particles and describes the reduction insulfur and mercury via density segregation during thermal drying oflignite in a specially designed fluidized bed.

Keywords: Coal microstructures; Computed tomography; Densitysegregation; Fluidized bed; High-moisture coals; Thermal drying

Received December 9, 2009; accepted January 29, 2010.Address correspondence to Nenad Sarunac, Energy Research Center, Lehigh

University, Bethlehem, PA 18015, USA. E-mail: ns01@lehigh.edu

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INTRODUCTION

Lignite and sub-bituminous coals from the western United States areattractive due to their low cost, low emissions, and high reactivity.However, they typically contain large amounts of moisture, whichreduces gross calorific value and results in a lower plant efficiencycompared to bituminous coals. According to the World Coal Institute,recoverable reserves of lignite and sub-bituminous coals are large, withUnited States having approximately 140 billion tons (52% of domesticcoal reserves), Russia 110 billion tons, China 50 billion tons, andGermany and Australia about 40 billion tons of recoverable reserves.Additionally, according to the U.S. Energy Information Agency useof western coals will continue to increase beyond the year 2030. Toreduce sulfur, mercury, and ash concentration, some bituminous coalsare washed and, after drying/dewatering, may still contain moisturelevels in excess of 20%. When high-moisture coals are burned in utilityboilers, a significant part of the fuel input (about 7% for northernU.S. lignites) is used to evaporate fuel moisture, resulting in lower plantefficiency.

Efforts are underway in countries having large reserves ofhigh-moisture, low-quality coals, such as Germany, Australia, and theUnited States, to develop efficient coal-dewatering and drying processes[1]. Most of these drying processes depend on high-grade or processheat to reduce coal moisture content, or employ complex equipmentlayout using expensive materials to recover latent heat of vaporization.This significantly increases the cost of thermal drying, which is the mainbarrier to large-scale industry acceptance of this technology.

Implementation of carbon capture and sequestration (CCS)technology at power plants using low-rank, high-moisture coals under-scores the need for efficient, inexpensive coal drying technology torecover a portion of the efficiency loss incurred by compression ofcarbon dioxide (CO2), air separation (in case of oxy-fuel combustion),or regeneration of a CO2 scrubbing reagent (in post-combustion CO2

capture). Therefore, new power plants, employing CCS and usinghigh-moisture fuel, would benefit from thermally dried coal.

TYPES OF COAL MOISTURE AND EFFECT ON DRYING RATE

Coal moisture can be classified into three broad categories: surface (free)moisture; inherent or physically bound moisture; and chemically bound

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moisture. Surface moisture is weakly bound to the surface of the coalparticle by adhesion and weak capillary forces. Bituminous and washedbituminous coals contain mostly surface moisture. In thermal drying ofbituminous coals, it is necessary to heat the surface of the coal particleto a high enough temperature to evaporate free moisture—there is noneed to heat the interior of the coal particle [2].

High levels of inherent moisture, characteristic of low-rank coals,such as lignites, are bound tightly in small capillaries and pores bycapillary forces. Chemically bound moisture is held in place by chemicalbonding between the coal surface and water. In thermal drying of low-rank coals it is necessary to heat the entire coal particle and its internalmoisture to a temperature that is sufficient to evaporate physically boundmoisture [3, 4]. Coal particles often disintegrate during drying in theseregimes.

The energy input that is necessary to dry high-moisture coal todesired residual water content is provided by heat transferred to the coalparticle. This heat includes thermal energy for heating the coal particle,for evaporating water, and for overcoming physical and chemical forcesthat bind water to coal. The proportion of various moisture bondingforces plays a dominant role in determining coal drying rate. Accordingto Vu et al. [5], coal is heated during the first drying phase (transientregion), where cohesion between fine- and wet-coal particles may causedifficulties in fluidization. For cohesive coals, this transient region ischaracterized by low drying rates, which improve after surface moistureis evaporated, and cohesion forces are reduced. During the second(constant drying rate) drying phase, surface (free) moisture is removedby overcoming adhesion and weak capillary forces. The drying ratein that region is a function of coal particle size and coal moisturecontent, while heat and mass transfer rates are directly proportionalto the driving forces of temperature and humidity gradient. In thethird (decreasing drying-rate region) the drying rate becomes diffusioncontrolled, because moisture held in small capillaries and pores has tobe diffused by overcoming capillary and dipole forces. As physical andchemical binding forces increase, the drying rate, under these conditions,decreases and eventually reaches zero. The three coal-drying regions arepresented in Figure 1. Binding force (energy) is highly dependent onthe coal structure (size and distribution of pores), oxygen functionality[5], and moisture content. For high-moisture German and Australianlignites, usually containing more than 55% moisture, binding energy

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Figure 1. Three coal drying regions.

becomes increasingly significant as coal moisture content is reducedbelow 20 to 25%.

LOW-TEMPERATURE COAL-DRYING PROCESS

A novel fluidized bed drying process that uses low-grade waste heatsources to reduce fuel moisture content of lignite, Powder River Basin(PRB), and other high-moisture coals was developed in the UnitedStates by Lehigh University's Energy Research Center (ERC) and GreatRiver Energy (GRE) under DOE Project DE-CF26-04NT41763 [6]. Thislow-temperature coal-drying process is based on a moving fluidizedbed. Crushed coal is fed to the first stage of the fluidized bed dryer(FBD) where nonfluidizable material such as rocks and stones and otherhigher density fractions are segregated to the bottom of the dryer. Thesegregated fraction is discharged as a stream of higher mineral mattercontent and hence also higher in sulfur and mercury in comparison tothe dried coal (product stream). Most of the coal fines are elutriatedin the first dryer stage. The fluidizable material enters a second stage

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of the fluidized bed dryer (FBD) where coal moisture is evaporated byheat supplied by the fluidizing air and an in-bed heat exchanger. The in-bed heat exchanger increases the temperature of the fluidizing (drying)air, increasing its moisture-carrying capacity. Partially dried coal, driedto the desired residual moisture content, is discharged from the FBD asthe product stream. Bed residence time and temperature are the mainparameters affecting residual moisture content. A schematic represen-tation of a two-stage moving fluidized bed dryer is presented in Figure 2.

The process was originally developed as a low-temperature dryingprocess to prevent spontaneous combustion (oxidation) of dried coal.Advanced drying designs were subsequently developed, operating athigher temperature to reduce the size (and cost) of the drying equipment.To prevent oxidation of coal, the high-temperature coal dryer can befluidized with an inert fluid (typically nitrogen from an Air SeparationUnit or a low-pressure CO2 stream) [7]. This is especially important forEuropean lignites, which are very reactive and prone to auto-ignition,and is probably one of the reasons why European designs of coal dryersemploy steam as the fluidization medium. The inert atmosphere prevents

Figure 2. Schematic representation of a moving bed fluidized bed dryer.

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auto-ignition of coal in case the fluidizing flow is interrupted, and re-adsorption of moisture occurs. A number of coal-drying configurationshave been developed by the authors to accommodate different configu-rations of the power plant equipment and different available sources ofwaste heat.

EFFECTS OF COAL DRYING ON COAL CALORIFIC VALUE ANDPOWER PLANT PERFORMANCE

As coal moisture is reduced by thermal drying, the relative amounts ofcarbon, hydrogen, nitrogen, oxygen, and mineral matter increase. Resultsfor a northern U.S. lignite are presented in Figure 3 as functions of totalcoal moisture content expressed as lb moisture per lb wet coal. However,during drying in a moving bed FBD, some of the fine mineral grains areelutriated from the bed, while high-density mineral matter is segregatedout and discharged in the dryer's first stage. Experimental data show therelative increase in coal mineral-matter content is less than the generalcoal compositional schema presented in Figure 3.

Reduction of moisture content increases the higher heating value(HHV) of the coal. The effect of coal moisture content on HHV for

Figure 3. Lignite composition as a function of total coal moisture content.

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lignite, PRB, and washed Illinois coals is presented in Figure 4. As theresults show, drying raw lignite containing 38.5% moisture to about20% residual moisture level improves HHV by more than 30%, i.e., onaverage a 104BTU/lb improvement per 1%-point reduction in moisturefor this coal. The HHV of dried lignite is about the same as the HHV ofan as-received PRB coal. Similarly, drying PRB coal from 30% to 10%moisture content, improves HHV by approximately 30%, on averagea 120BTU/lb improvement per 1%-point moisture reduction. Thermaldrying of washed coals, for example Illinois coals, from 20% to 4%moisture content results in 20% improvement in HHV, i.e., on average a130BTU/lb improvement per 1%-point moisture reduction.

For a constant power plant output, higher HHV values require lowercoal flow rates and the burden on the coal-handling system, includingmills, belts, and feeders is reduced. Lower flow rate and improved grind-ability of dried coal also result in lower mill power requirements [8].

The flow rates of combustion air and flue gas decrease as moisturecontent of coal and coal flow rate are reduced. For a lignite, a 15%-pointreduction in coal moisture content results in reduction in a flue gasflow rate by 6% and a net unit efficiency improvement by up to 1.65%points. For washed Illinois coals, improvement in net unit efficiency

Figure 4. Coal Higher Heating Value as a function of coal moisture content.

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of up to 0.8% points and reduction in flue gas flow rate up to 3.5%are expected. In comparison, increasing steam temperature and pressurefrom subcritical to supercritical, and to ultrasupercritical conditions fora lignite-fired power plant yields improvement in net unit efficiency by2.8% and 4.5% points, respectively. The benefits of coal drying aredescribed in more detail in the literature [9–15].

Reduced flue gas flow rate also has a positive effect on performanceof the existing pollution control system by increasing flue gas residencetime, as well as on the size, and capital and operating costs of potentialpost-combustion CO2 capture or oxy-fuel systems. Also, improved HHVachieved by reduced coal moisture content has a significant impact oncapital cost of new (greenfield) power plants. For a lignite dried to18.5%, the plant capital cost is 8.2% lower compared to the wet (raw)lignite, while for dried Illinois coal the plant capital cost is 5.5% lowercompared to washed coal [16].

Removal of Sulfur and Mercury from Coal FeedstockThrough Density Segregation in a Fluidized Bed

If particles in a fluidized bed vary in size or density, the particlescan segregate; denser and larger particles (‘‘jetsam’’) tend to sink andaccumulate at the bottom of the bed, while less dense and smallerparticles (‘‘floatsam’’) tend to float. The density is the dominant drivingforce for segregation and competes with mixing in a fluidized bed [17].In first stage of the FBD designed by ERC and GRE, heavier densityfractions and nonfluidizable material (larger coal particles, stones, rocks,and tramp iron) segregate to the bottom of the dryer and are dischargedas a segregated stream. For northern U.S. lignites, a significant portionof the sulfur and mercury in coal is associated with iron sulfide (pyrite)forms. The segregated stream has significantly higher sulfur and mercurycontent (2.9 and 3.4 times, respectively) compared to the feed stream.Mass balances for sulfur and mercury, determined from a series ofprototype FBD scale field tests are presented in Figures 5 and 6. Coalsulfur, mercury, and moisture contents were determined by analyzingcoal samples collected from the feed, segregated, and product streamsentering and leaving a prototype FBD [6]. The average reduction insulfur achieved by density segregation in the first dryer stage (calcu-lated from the mass balance and taking into account evaporation ofcoal moisture) is approximately 22% (see Figure 5). The correspondingaverage reduction in mercury is approximately 34% (see Figure 6).

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Figure 5. Sulfur mass balance: Thermal drying of northern U.S. lignite in a FBD.

Figure 6. Mercury mass balance: Thermal drying of northern U.S. lignite in a FBD.

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As the data from Figures 5 and 6 show, only about 5% of the totalmoisture removed in a FBD is removed in first stage. This is becausethe coal is being heated and dense fractions segregated in first stage ofthe dryer. Most of the coal moisture is removed in second stage of thedryer. Also, on average, the segregated stream contains approximately15% more mineral matter (on a dry basis) compared to the feed stream,resulting in reduced ash concentration of the product stream.

CHANGES IN MICROSTRUCTURE OF COAL PARTICLES ANDMINERAL DISTRIBUTION CAUSED BY DRYING

Coal is complex and diverse, which imposes challenges in following itsbehavior. During the drying of lignite within a fluidized bed dryer, thereare changes and segregations that enhance the combustion properties,comminution, and reduced emissions. These changes are related to theloss of water, sizing, separation of coal and minerals, and separationof the high- and low-quality coal particles. While bulk parameters areessential to understanding chemical and physical transitions accompa-nying the partial drying, they do not reflect the diversity or providea 3-D distribution of density transitions, induced fractures in coal, orthe mineral dispersion that will aid in understanding drying transi-tions. X-ray computed tomography (CT) is a nondestructive method ofexamining the interior of objects. It is able to generate 3-D coal andmineral images [18, 19]. The data are quantitative and, based on the 3-Dimage analysis, the dispersion and quantity of the selected componentscan be determined and transitions can be determined.

Coal particles of about 3–5mm length were hand-selected fromthe feedstock, partially dried output, and segregated stream samples.Ten coal particles from each cut were scanned at high resolution.Particles were placed in sequential horizontal layers. Each sample layerwas supported and separated by X-ray transparent packing material.X-rays from a small focal-spot source pass through the sample to amultiline detector. Continuous sample rotation provides multiple viewsthat are resolved into a stack of 2-D images. Each image is generatedfrom an array of CT numbers that represent the X-ray attenuation ofthe individual volume elements, known as voxels. The X-ray attenu-ation depends on the density and elemental composition of the sample.Minerals differ from coal in that they have both a higher density andhigher atomic number. Thus, it is possible to identify those voxels that

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are rich in minerals. In this study, data collection was set to producevoxel dimensions of 35 × 35 × 39�6 microns. Minerals smaller than thiswill not be ‘‘seen’’ but will contribute to the higher X-ray attenuationof the voxel and, thus, may be classified as mineral or coal dependingon the contribution. Volumetric calculations were performed on thecoal particles and associated minerals. An additional five particles wereselected from the as-received coal and were air dried to below 3%moisture by mass and scanned and analyzed using X-ray computedtomography. The coal particles were further dried in an oven andrescanned.

Figure 7 shows a volumetric reconstruction of the three layers withall silica mineral voxels being false-colored grey. The packing materialsignal has been removed from the image. There is obvious segregationof minerals between the partially dried and segregated stream samples,with the segregated stream sample containing the greater quantity ofminerals; with the silica minerals delineating portions of the surface ofmany of the coal particles. From the X-ray attenuation it is possible todefine quantitatively the volume elements. Here low numbers are ‘‘coal,’’intermediate are ‘‘silicates,’’ and the high highest numbers are ‘‘iron.’’The threshold selected was somewhat arbitrary but was used consis-tently throughout the study. The volumes of each cut are provided inTable 1. The silicate volume is higher in the feed than either the segre-gated stream or the partially dried coal samples. There were less ironminerals, within or on the segregated stream coal, than in the feedstockor partially dried sample. The contribution is small however to all threesamples (around 0.01% by volume).

Figure 8 provides a reconstructed side view of the three layers ofthe three stacked sample cuts. The packing material has been removed

Figure 7. X-ray CT image of coal showing silicate minerals (light grey) for (a) feed coal,(b) partially dried coal, and (c) segregated stream sample, along with the coal.

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Table 1. Volume composition of the feedstock coal, partially dried coal, and segregatedstream coal samples determined from CT rendering

Sample stream Coal volume (%) Silicate volume (%) Iron volume (%)

Feedstock 95.22 4.83 0.008Partially dried coal 97.15 2.82 0.016Segregated stream 97.21 2.71 0.004

from the image. The top layer is the segregated stream, the middle is thepartially dried coal, and the bottom layer is the raw feed coal.

Several higher density ‘‘iron’’ particles were displaced during thecourse of the experiment and appear separated from the coal. On avolumetric basis there were very few iron minerals in the coals. Thecoals appear to be generally similar based on CT number (color). Thisis expected as the partial drying removing only some of the moisturewith partial shrinkage of the coal [20, 21]. Thus, the feed and thepartially dried samples appear to be similar. There are no noticeablecracks in the partially dried samples. Extensive drying of lignite coalswould further shrink the coal, potentially generating cracks that will aidin reducing energy for pulverization [22–24]. It is possible that otherparticle sizes will fracture and fragment but that would not have beenobserved with the sampling approach used here. Thus it should bepossible to ‘‘clean’’ the coal by removing minerals that can be liberated[25] by the shrinking/fracturing of the coal. The selected particles arenot expected to represent the bulk as mineral separation is expectedwith decreasing particle size [26]. Further it seems likely that as the coal

Figure 8. X-ray CT image of coal the three layers, top: feed coal, middle: partially driedcoal, and bottom: segregated stream sample, along with mineral matter.

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shrinks and cracks the particle size distribution will change generatingmore surface area and exposing more minerals. In the tumbling settingof a fluidized bed, opportunities exist for the loss of minerals from thecoal surface or from the expanding cracks that occur with shrinkage.

Four of the five particle air-dried samples had shrink-inducedfractures in contrast to the fluidized bed partially dried samples.A couple of these fractures are evident in this image on the uppermostparticle and the lower left-hand side particle (Figure 9). These were noteasily observed with the naked eye when samples were selected. All butone of the particles had multiple interior fractures that tended to bemostly linear, and directional with the long axis.

Some of the fractures passed through the length of the particlewhile others terminated in the interior. All the larger fractures reachedor emanated at the surface; none of these fractures were ‘‘contained’’within the interior volume of the particle. Following oven drying, thesamples were rescanned. No additional fractures were observed uponremoval of the <3% moisture by mass by oven drying. The surfacecracks are slightly more evident implying a slight aperture increase atthe surface. Recent X-ray CT work on lump-sized sub-bituminous dryingshowed extensive radial-proceeding fractures with drying at 50◦C [22].Thus particle size, induced mechanical strain, and rate of moisture lossare important parameters in the fracturing accompanying drying.

For the few particles selected the majority of the mineral grains areon the surface of the particles and the comminution is likely partiallycontrolled by the silicate distribution. The silicate volume is higher inthe feed than either the segregated or the partially dried samples. Therewere less iron minerals, within or on the segregated coal, than in thefeed or partially dried sample. The contribution is small however to all

Figure 9. X-ray CT image of the interior of the air-dried coal feed coal, bright objects arehigher CT-absorbing material (iron- and silicate-containing minerals), the dark elongatedfeatures are fractures.

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three samples (around 0.01% by volume). Heavy minerals may be absentfrom the larger particles by segregation into smaller sized fractions.No shrink-induced cracking was observed in the samples selected fromthe partially dried or segregated stream sample.

Four of the five particles air-dried (<3% moisture by mass) feedsamples had shrink-induced cracks in contrast to the partially driedsamples. Additional drying did not further alter the structures basedon visual comparison of the reconstructed images. All but one of theparticles had multiple interior fractures that tended to be mostly linear,and directional with the long axis of the sample. Some of the fracturespassed through the length of the particle while others ‘‘terminated’’ atthe particle interior.

CONCLUSIONS

Reduction of moisture content of high-moisture coals has a positiveeffect on power plant efficiency, reduces burden on coal-handlingequipment and, for newly constructed units, reduces equipment size andcapital cost. A novel moving fluidized bed drying process, developedby the ERC and GRE, uses low-grade waste heat sources to dry wetcoal (lignite, PRB, or washed bituminous coals) to the desired productmoisture level. In first dryer stage, nonfluidizable materials segregate tothe bottom and are discharged from the dryer as a segregated streamricher in mineral matter, sulfur, and mercury in comparison to the driedproduct stream. A number of coal-drying configurations were developedby the authors to accommodate different configurations of the powerplant equipment and different sources of waste heat.

X-ray computed tomography was employed to determine changesin microstructure of coal particles during the thermal drying process.Samples collected from the feedstock and segregated and dried productstreams from the prototype FBD were analyzed. Results show shrinkageoccurs as coal is partially dried. Drying to low-product-moisture levelsresults in shrink-induced fractures. For coal particles analyzed in thisstudy, it is evident that the majority of the minerals are on the surfaceof the particles observed and comminution is likely partially controlledby the silicate distribution. Additional work is needed to characterizechanges in coal microstructure for different coals and coal particles sizesduring thermal drying to different product moisture levels.

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