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A Novel Fluidized Bed Drying and Density Segregation Process for Upgrading Low-Rank Coals

A Novel Fluidized Bed Drying and Density Segregation Process for Upgrading Low-Rank Coals
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  1 A novel fluidized bed drying and density segregation process for upgradinglow-rank coals Nenad Sarunac and Edward K. Levy Energy Research Center, Lehigh University, Bethlehem, PA 18015 Mark Ness and Charles W. Bullinger  Great River Energy, Underwood, ND 58576 Jonathan P. Mathews and Philip Halleck Energy & Mineral Engineering & The Energy Institute,The Pennsylvania State University, University Park, PA 16802 Abstract Lignite and sub-bituminous coals are attractive due to their low cost and emissions and highreactivity. These coals, however, contain high amounts of moisture, which reduces calorificvalue of the fuel and lowers plant efficiency. A novel fluidized bed drying process that uses alow-grade waste heat to reduce fuel moisture content of low-rank high-moisture coals andremoves portion of sulfur and mercury from the coal through density segregation in a fluidizedbed was developed. This paper discusses quality improvement of low-rank coals by thermaldrying, describes changes in microstructure of coal particles and mineral distribution caused bydrying, and the reduction in sulfur and mercury content by density segregation during thermaldrying of lignite in a fluidized bed. Introduction Lignite and sub-bituminous coals from western U.S. are attractive due to their low cost andemissions and high reactivity, but typically contain high amounts of moisture, which reducesgross calorific value of the fuel and results in lower plant efficiency compared to the bituminouscoals. According to the World Coal Institute, recoverable reserves of lignite and sub-bituminouscoals are huge, with U.S. having approximately 140 billion tones (52% of domestic coalreserves), Russia 110 billion tons, China 50 billion tons, and Germany and Australia about 40billion tons each of recoverable reserves. Also, according to the U.S. Energy Information Energy(EIA) use of western coals will continue to increase beyond year 2030. Some bituminous coalsare washed to reduce sulfur, mercury and ash yields and, after drying, may contain moisturelevels in excess of 20%. When high-moisture coals are burned in utility boilers, a significant partof the fuel input (about 7% for northern U.S. lignites) is used to evaporate fuel moisture,resulting in lower plant efficiency compared to the low-moisture coals.Efforts are underway in countries having large reserves of high-moisture low-quality coals, suchas Germany, Australia, and U.S., to develop efficient coal dewatering and drying processes.Most of these drying processes depend on high-grade or process heat to reduce coal moisturecontent, or employ complex equipment layout and exotic (expensive) materials to recover latent  2 heat of moisture vaporization. This significantly increases cost of thermal drying, which is mainbarrier to industry acceptance of this technology.Implementation of carbon capture and sequestration (CCS) technology for power plants usinglow-rank, high-moisture coals, underscores the need for efficient, inexpensive, and reliable coaldrying technology to recover portion of the efficiency loss incurred by compression of carbondioxide (CO 2 ), air separation (in case of oxy-fuel combustion), or regeneration of CO 2 scrubbingreagent (in case of post-combustion CO 2 capture). Therefore, new power plants, employingCCS and using high-moisture fuel are being designed for thermally dried coal. Types of Coal Moisture and Effect on Drying Rate Coal moisture is classified into three categories: surface (free) moisture, inherent or physically-bound moisture, and chemically-bound moisture. Surface moisture is weakly bound to thesurface of the coal particle by adhesion and weak capillary forces. Bituminous and washedbituminous coals contain mostly surface moisture. In thermal drying of bituminous coals it isnecessary to heat surface of the coal particle to a high enough temperature to evaporate freemoisture - there is no need to heat interior of the coal particle [1].Inherent moisture, characteristic of low-rank porous coals, such as lignites 1 , is bound tightly insmall capillaries and pores by capillary forces. Chemically-bound moisture is held in place bychemical bounding between coal surface and water. In thermal drying of low-rank coals it isnecessary to heat the entire coal particle and its internal moisture to a temperature that issufficient to evaporate physically-bound moisture [2] and [3]. Coal particle often disintegratesduring drying.The energy input that is necessary to dry high-moisture coal to desired residual water content isprovided by a heat transferred to the coal particle. This heat includes thermal energy for heatingthe coal particle, for evaporating water, and for overcoming physical and chemical forces thatbind water to coal. The proportion of various moisture bounding forces plays dominant role indetermining coal drying rate. This is clearly shown inFigure 1where drying rates for U.S. lignite,Powder River Basin (PRB) and German lignite are presented as functions of time [4] and [5].Coal is heated during first drying phase (transient region), where cohesion between fine and wetcoal particles can cause difficulties in fluidization. For cohesive coals, transient region ischaracterized by low drying rate, which improves after part of surface moisture is evaporatedand cohesion forces are reduced. During the second, constant drying rate, drying phase,surface (free) moisture is removed by overcoming adhesion and weak capillary forces. Dryingrate in that region is function of only coal particle size and coal moisture content, while heat andmass transfer rates are directly proportional to the driving forces of temperature and humiditygradient. In third, decreasing drying rate region, drying rate becomes diffusion controlled,because moisture held in small capillaries has to be diffused by overcoming capillary and dipoleforces. As physical and chemical binding forces increase, drying rate decreases and eventuallyreaches zero value. Binding force (energy) is highly dependent on the coal structure (size and 1   High-moisture coals can also be classified as the capillary-porous colloids.    3 01020304050600 10 20 30 40 50 60 70 80 Time [min]    C  o  a   l   M  o   i  s   t  u  r  e   C  o  n   t  e  n   t ,   T   M   [   k  g   H    2    O   /   k  g  w  e   t  c  o  a   l  x   1   0   0   %   ] ND Lignite: Test L39PRB: Test 63Washed Illinois: Viper German Lignite: Test GL15 T air,in = 66 o C = 151 o F Lignite, PRB, and Illinois Coals:Top Size 6.35 mmGerman Lignite:Mean Particle Size = 0.45 mm T air,in = 60 o C = 140 o F distribution of pores) and moisture content. For high-moisture German and Australian lignites,usually containing more than 55% moisture, binding energy becomes significant for coalmoisture contents lower than 20 to 25%. Figure 1: Drying kinetics of various coals Low-Temperature Coal Drying Process  A novel fluidized bed drying process that uses a low-grade waste heat sources to reduce fuelmoisture content of lignite, PRB, and other high-moisture coals was developed in the U.S. by the Lehigh University’s Energy Research Center  (ERC) and Great River Energy (GRE) under DOE Project DE-CF26-04NT41763, [6]. This low-temperature coal-drying process is based on amoving fluidized bed, where crushed coal is feed to the first stage of the dryer where, owning toa novel design, non-fluidizable material, such as rocks and stones, and heaver-density fractions(pyrites, etc) are segregated to the bottom of the dryer and discharged as a segregated streamrich in sulfur and mercury. Also, most of coal fines are elutriated in the first stage. The fluidiziblematerial enters second stage of the fluidized bed dryer (FBD) where coal moisture is evaporatedby heat supplied by the fluidizing air and an in-bed heat exchanger. The in-bed heat exchanger increases temperature of the fluidizing (drying) air, increasing its moisture-carrying capacity.Dried coal, containing desired residual moisture content, is discharged from the FBD as aproduct stream. Bed residence time and temperature are main parameters affecting residualmoisture content. Schematic representation of a two-stage moving fluidized bed dryer ispresented inFigure 2.Dried lignite has a tendency to undergo spontaneous combustion. The key parameters thatinfluence spontaneous combustion are oxygen content, airflow velocity, particle size, moisturecontent, and humidity of the fluidizing air. In a FBD where air flows over the lignite at highvelocity, it is unlikely for the coal to spontaneously combust because the flowing air prevents the  4 In-Bed HXE In-Bed HXE In-Bed HXE1 st Sta e2 nd Stage FeedStream ProductStreamSegregatedStreamFluidization Air andEvaporated Coal Moisture2 nd Stage Fluidizing Air In-Bed HXEIn-Bed HXE 1 st Stage Fluidizing Air Bed Surface development of “hot spots”. (When dry air flows ove r moist coal, moisture is removed from thecoal through evaporation, resulting in a decrease in temperature.) However, it is desirable tokeep moisture content of the product stream above the equilibrium moisture content, otherwisemeasures are needed to prevent re-absorption of the air moisture that could lead to auto-ignitionwhile dried product is stored in a silo. Figure 2 : Schematic representation of a moving bed FBDThe process was srcinally developed as a low-temperature drying process to prevent oxidationof dried coal. Advanced designs of the dryer are developed, operating at higher temperature toreduce size (and cost) of drying equipment. To prevent oxidation of coal, the high-temperaturecoal dryer is fluidized by an inert fluid (nitrogen from the Air Separation Unit (ASU), or low-pressure CO 2 stream), [7]. This is especially important for European lignites which are veryreactive are prone to auto-ignition, and is probably one of the reasons why European designs of coal dryers employ steam as a fluidization medium. The inert atmosphere prevents auto-ignitionof coal in case fluidizing flow is interrupted, and re-adsorption of moisture. A number of coaldrying configurations were developed to accommodate different configurations of the power plant equipment, and different sources of waste heat. Effects of Coal Drying on Coal Calorific Value and Power Plant Performance  As coal moisture content is reduced by thermal drying, carbon, hydrogen, nitrogen, oxygen, andash (mineral) content of the coal increase. Results for the northern U.S. lignite are presented inFigure 3as functions of total coal moisture content, TM, where quantity TM is expressed as lbmoisture per lb wet coal. However, during drying in a moving bed FBD, some of the fine ash iselutriated from the bed, while high-density mineral matter is segregated out and discharged inthe dryer first stage. Therefore, increase in coal mineral content is less than presented inFigure3.  5 37.5 35.0 32.5 30.0 27.5 25.0 22.5 20.0 17.5 15.0 12.5 10.0 Total Coal Moisture [% by weight]    C  o  a   l   C  o  m  p  o  s   i   t   i  o  n   [   l   b   /   l   b   ]  AshMoistureNOSHCarbon Reduction of coal moisture content increases higher heating value, HHV, of the coal. The effectof coal moisture content on HHV for lignite, PRB, and washed Illinois coals is presented inFigure 4. As the results show, drying raw lignite containing 38.5% moisture to about 20%residual moisture level improves HHV by more than 30%, i.e., on average a 104 BTUimprovement per 1%-point reduction in moisture. HHV of dried lignite is about the same as HHVof raw PRB coal. Similarly, drying raw PRB coal from 30 to 10% moisture content, improvesHHV by approximately 30%, on average a 120 BTU improvement per 1%-point moisturereduction. Thermal drying of washed coals, for example Illinois coals, from 20 to 4% moisturecontent results in 20% improvement in HHV, i.e., on average a 130 BTU improvement per 1%-point moisture reduction. Figure 3 : Lignite composition as a function of total coal moisture contentFor a constant power output of a power plant, higher HHV results in lower coal flow rate andreduced burden on the plant coal handling system, including mills and feeders. Lower flow rateand improved grindability of dried coal also result in lower mill power requirements.The flow rates of combustion air and flue gas decrease as moisture content of coal and coalflow rate are reduced. For lignite, a 15%-point reduction in coal moisture content results inreduction in flue gas flow rate by 6% and net unit efficiency improvement by up to 1.65%-points.For washed Illinois coals, improvement in net unit efficiency of up to 0.8%-point and reduction influe gas flow rate up to 3.5% are expected. In comparison, increasing steam temperature andpressure from subcritical to supercritical, and to ultrasupercritical conditions for a lignite-firedpower plant yields improvement in net unit efficiency by 2.8 and 4.5%-points, respectively. Thebenefits of coal drying are described in more detail in [8] to [12].
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