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Optimization of drying of low grade coal with high moisture content using a disc dryer Seung Hyun Moon a In Soo Ryua Seung Jae Leea Tae In Ohmb a Korea Institute of Energy Research 71 2 Jang dong Yuseong gu Daejeon 305 343 South Korea b Hanbat National University 125 Dongseodaero Yuseong gu Daejeon 305 719 South Korea a b s t r a c ta r t i c l ei n f o Article history Received 12 September 2012 Received in revised form 3 March 2014 Accepted 7 March 2014 Available online 31 March 2014 Keywords Low grade coal Drying Disc dryer Coal Moisture content In this study low grade coal with highmoisture content wasdried ina novel disc dryer equipped with a heating plate and rotary blades Raw coal was fed into the center of the disc dryer and then was transferred from the center of the heating plate to the outside of the plate by the rotary blades during the drying process According toa numericalanalysisusing a modelofa singlesolid sphericalparticle withoutany porosity the temperatureof the coal particles reached that of the heating plate within 5 min The analysis also showed that at a heating plate temperatureof150 C themoisturecontentoftherawcoalwasloweredfrom34 tobelow3 within5min and all of the raw coal was dried after 10 min of drying time Experimental studies were used to investigate the infl uence of the following factors on the effectiveness of the novel disc dryer heating plate temperature coal feed rate rotational speed of the rotary blades drying environment and position of the coal on the heating plate When the heating plate temperature was high the moisture content of the dried coal was greatly decreased However on thebasis ofenergy effi ciency considerations itwasrecommendedthatthetemperature of the heating plate be maintained at 150 C Furthermore a decrease in the coal feed rate could lower the moisture content and a high rotational speed of the rotary blades could slightly reduce the moisture content as well In addition the reduction in moisture content could be remarkably enhanced by using a vacuum pumptoremove theevaporatedwatervapor fromthe insideof the dryer The position of thecoalon the heating platewasalsoimportant Thetemperatureoftherawcoalcouldbeincreasedwithoutevaporationuptoacertain distance from the center of the dryer However the moisture of the raw coal began to evaporate in a region near theoutsideoftheheatingplatebeforedischarging wheretheraw coaltemperaturereachedabout100 C Over all it was concluded that the size and temperature of the heating plate should be considered in the design and operation of a disc dryer for drying low grade coal In addition the dispersion of raw coal on the heating plate was important during the drying process The disc dryer could enhance the conductive heat transfer coeffi cient between raw coal and a heat source and mixing of the coal to reduce drying time 2014 Elsevier B V All rights reserved 1 Introduction Fossil fuel resources especially oil are rapidly depleting and thus oil prices are abruptly increased recently The price of high grade coal which is commonly used as a fuel is also increasing because high grade coal reserves are quite limited as well Because low grade coal is rela tively rich in reserves and low in price many researchers have been highly interested in upgrading it for use as a source of highly effi cient energy whileloweringtheconsumptionofoilandhigh gradecoal 1 2 The total reserves of coal in the world amount to 7 14 trillion tons of which 3 27 trillion tons is bituminous coal and anthracite and 3 87 trillion tons is subbituminous coal and lignite However only 0 98 trillion tons of coal is in recoverable reserves of which 47 3 or 0 46 trillion tons is low grade coal 3 In general low grade coal is classifi ed as such because it contains high levels of impurities such as ash and moisture that result in low energy values it is therefore less desirable for direct use as an energy source The use of low grade coal asan energy sourcerequires that itsmoisture contentfi rst be decreased in a drying process The water contained in coal is typically removed by mechanical and thermal methods including thermal drying and thermal dewatering The dewatering process of mechanical methods is primarily used to separate coal solids from slurry while that of thermal methods is used to produce dry coal by removing the moisture inherent in low grade coal In the thermal drying method combustion gas or superheated water vapor is utilized to reduce the moisture content of the coal This can usually simplify the drying equipment but requires consumption of a huge amount of energy On the other hand in the thermal dewatering method the moisture of the coal is removed in a liquid state by using saturated water vapor or hot water in a pressurized reactor this is advantageous in terms of energy consumption but the Fuel Processing Technology 124 2014 267 274 Corresponding author Tel 82 42 860 3221 fax 82 42 860 3134 E mail address shmoon kier re kr S H Moon http dx doi org 10 1016 j fuproc 2014 03 009 0378 3820 2014 Elsevier B V All rights reserved Contents lists available at ScienceDirect Fuel Processing Technology journal homepage overall confi guration of the equipment is very complicated mostly owing to the pressurization Such thermal methods are often used in the SynCoal Process developed by the Western Energy Company in the United States Moreover other drying processes based on the thermal methods have been actively developed for instance the K Fuel process USA the binderless coal briquettes BCB technology Australia 4 the upgraded brown coal UBC process Japan 5 the Integrated Drying Gasifi cation Combined Cycle IDGCC HRL Ltd Australia 6 and so on These drying processes based on the thermal methods use microwaves 7 solvent 8 oil 5 or specifi c reactors for long residence time such as a fl uidized bed reactor 9 or a long duct 10 Most of the drying processes are operated with heating mediums and are complicated in the system constitution which causes high costs of construction Inthisstudy athermalmethodwasusedtodrylow gradecoalusing a disc dryer which was so designed that heat for drying coal can be transferred from a heat source to the coal without a heating medium during the drying process Thus no use of a heating medium in the dryer can result in simplifying constitution of the equipment with reducing drying time The bottom plate of the dryer was heated and the coal fed into the center of the dryer was transferred to the outlets by adjusting the rotational speed of the rotary blades The drying effi ciency of the process can be affected by the following factors coal particle size and feed rate drying temperature and residence time and drying environment within the dryer This study examined the infl uence of the coal residence time revolution speed of the rotary blades and drying environment in the dryer in order to determine theoptimalconditionsforacontinuouscoal dryingprocess Inaddition a numerical analysis was performed to predict the approximate drying timebycalculatingtheheattransfertimeofthecoalparticlesforvarious particle sizes and disc temperatures 2 Research methods 2 1 Numerical analysis Numerical calculations using a model of a single solid spherical particle without any porosity were used to resolve the particle size effect associated with coal drying 11 12 The Stefan model 13 and a two phase model 14 are often used for including the mass transfer effect of evaporated water vapor when drying a suffi ciently large coal particle However becausethis studyusedsignifi cantlysmallcoalparti cles it was necessary to use a simplifi ed numerical method mainly owing to the rapid mass and heat transfer phenomenon in a fi ne sized coal particle Because the transfer of heat to a coal particle on the disc can evaporate the moisture contained in the particle it is important to fi rst calculate the heat transfer from the disc to the coal particle For the numerical calculation it is assumed that the single coal particle is spher ical in shape and therefore the equations use spherical coordinates Assuming that there is no convective heat transfer inside the coal particle the numerical calculation considers only the conductive heat transfer and heat loss for coal drying The governing equation is as follows t cCpcT 1 r2 r kr2 T r qvGv Vp VpGvcpvT 1 where cpc and k are the density of the solid particle the specifi c heat at constant pressure and the thermal conductivity coeffi cient respec tively cpv is the specifi c heat of the evaporated component Gvis the drying rate qv is the volumetric fl owrate and Vpis the mass fraction of dried moisture The variables r t and T denote the radius of a coal particle time and temperature respectively The left hand side of the above equation is an unsteady state term and the right hand side includes terms for diffusion the latent heat loss due to evaporation and the sensible heat taken out by the evaporated component 15 The universal E K model suggested by Fu et al 16 to calculate the volatilization rate of a component in a coal particle can be applied in this study by assuming that the volatile component is water at a tem perature of less than 160 C 17 This model is expressed by an Arrhenius type equation see Eq 2 having an activation energy E term and a proportional constant K which are universal values inde pendent of the type of coal The volatilization rate in this equation is therefore a function only of the temperature of the coal particle 16 dV dt V V Kexp E RT 2 where V is the volume of the evaporated component and V is the fi nal volume produced by the evaporation The boundary conditions on the particle are as follows T r r 0 0 3 k T r r R T Ts F T4 T 4 s 4 T t r T0 t 0 5 Eq 3 presents a symmetry condition at the center of the coal parti cle and Eq 4 implies that the heat conductively transferred at the sur face of the coal particle is the same as the sum of the heat given off by radiation and convection at the outside of the coal particle In Eq 5 regarding the initial conditions the initial temperature inside the coal particle is given as T0 Thenumericalanalysiswasperformedwithcoalparticlesizesof1 3 and 5 mm and thetemperatures of the bottom heatingplate of the disc weresetto130 140 150 and160 C Thephysicalpropertiesofthecoal used in thisstudy aresummarized in Table 1 and were used to calculate the temperature and moisture content of the coal particle along with time 11 Table 1 Physical properties of coal used in this study 7 v kg m3 1500 k W m K 0 26 Cv v J kg K 1520 Cv J kg K 550 V m s 0 5 emissivity 0 9 TD disc temperature C 130 140 150 160 Table 2 Characteristics of Indonesian low grade coal Proximate analysis wt Elemental analysis wt MoistureVolatile matterAshFixed carbonCarbonHydrogenNitrogenOxygenSulfur 34 2733 642 0129 9970 505 140 9921 330 03 268S H Moon et al Fuel Processing Technology 124 2014 267 274 2 2 Experimental 2 2 1 Experimental setup ThecoalusedinthisstudywaslignitefromtheregionofKalimantan Indonesia this lignite is commonly classifi ed as a low grade coal The diameter of the coal particles was in the range of 1 3 mm Proximate and elemental analyses of the Indonesian lignite sample were carried out in accordance with Korean standard test methods for coal samples KS E 3705 3706 3707 and 3712 using a TruSpec elemental analyzer LECO Corporation USA an SC 432DR sulfur analyzer LECO Corpora tion USA and a TEA 701 thermogravimeter LECO Corporation USA The analysis results are presented in Table 2 Inthedisc dryer thebottom heatingplate was heatedbysteam pro duced from a steam generator The temperature of the bottom heating plate and the interior temperature of the dryer were measured with a k typethermocouple Ascrewfeederanda gearedmotorwereinstalled to quantitatively feed the coal into the dryer The four arms supporting therotarybladesofthedryerwere40cminlength andtheirrevolution Fig 1 Schematic drawing of the disc dryer Fig 2 Details of rotary blades installed in the disc dryer 269S H Moon et al Fuel Processing Technology 124 2014 267 274 speed was controlled by a timing belt connecting to a control speed motor A schematic drawing of the disc dryer is shown in Fig 1 Each supporting arm had four spindles installed that contained two curved blades at the bottom Each spindle was connected to a gear attached on the supporting arms in order to transfer the rotational power from the control speed motor to each spindle The installed blades were designed to spread coal evenly over the bottom heating plate and at the same time to push dried coal out the outlets Fig 2 shows the details of the structure of the supporting arms and blades The moisture content of coal samples dried in the disc dryer was mea sured by a moisture analyzer Metrohm 841 KF Titrando Karl Fischer 2 2 2 Drying methods First the moisture content and residence time of the coal samples were examined by varying the temperature of the bottom heating plate of the disc dryer The temperature of the plate was raised in 10 C increments from 130 C to 160 C while the coal feed rate was held constant at 5 kg h and the rotational speed of the blades was held constant at 0 84 rpm Second the coal feed rate was varied from 5 to10kg h ata blade rotationalspeed of 0 84rpmin thesametemper aturerange Third theeffectofthebladerotationalspeedwasexamined beginning at 0 37 rpm and increasing to 0 84 rpm for the conditions where the heating plate temperature was 150 C and the coal feed ratewas5 kg h Intheabovedryingexperiments thewatervaporevap oratedfromthecoalwasdischargedusingavacuumpump However in the fi nal experiment investigating the effect of the drying environment inside the disc dryer no vacuum pump was used and the water vapor evaporated from the coal was allowed to stay inside the dryer This experiment was conducted using a heating plate temperature of 150 C a feed rate of 5 kg h and a blade rotational speed of 0 84 rpm In each experiment a dried coal sample was collected from the outlet of the heating plate after 10 min of drying time and the moisture content of Fig 3 Sampling positions for collection of dried coal from the heating plate of the disc dryer Fig 4 Calculated heat transfer ratio from heated disc TD to coal particle TC over time for various particle sizes and disc temperatures TD a 130 C b 140 C c 150 C and d 160 C Fig 5 Calculated temperature variation over time for a 3 mm diameter coal particle at various disc temperatures 270S H Moon et al Fuel Processing Technology 124 2014 267 274 the sample was measured To determine the moisture content of coal samples relative to the position of the heating plate samples were col lected at three intermediate points along a line between the center of the heating plate and the outlet see Fig 3 3 Results and discussion 3 1 Numerical analysis results 3 1 1 Heat transfer calculation Itisnecessaryfi rsttoascertainthemosteffectivetimeforheattrans ferfromthebottomheatingplatetoacoalparticle sincethedryingpro cessforcoalparticlesstartsonlywhenthemoistureinthemreceivesthe heat transferred from the bottom heating plate Fig 4 shows the calcu lation results for temperature variations at the center of coal particles thatare1 3 and5mmindiameter Ittakes2 4 and6min respectively for the temperature at the center of these coal particles to reach that of the bottom heating plate However the time for the temperature at the particle center to reach the heating plate temperature is not affected by the set point of the heating plate temperature for a given coal particle size The temperature at the center of a 3 mm coal particle was obtained by numerical calculation for heating plate temperatures of 130 140 150 and 160 C see Fig 5 The tendency of the heat to transfer from the heating plate to the coal particle was similar in all examined condi tions of the heating plate temperature and thus the temperature at the center of the particle becomes the same as that of the heating plate within 4 min However the difference in heating time to reach the plate temperature was insignifi cant varying the temperature of the heating plate The interior temperature distribution of the coal particle in this study is steeper in slope than that suggested by Agarwal et al for coal drying in a fl uidized bed 18 which is attributed to the fact that the heat in this study is transferred mainly by conduction while the heat transfer in a fl uidized bed is by convection Moreover as the heat loss due to the evaporation heat of water is not negligible when the amount of coal to be dried becomes larger it is expected that the increase in coal particle temperature would level off at a particle tem perature of 100 C 7 3 1 2 Coal drying calculation Although the mathematical model of an Arrhenius type equation employed in this calculation was originally specifi ed for the volatiliza tion of coal the volatile component in the model could be assumed to be water vapor at a temperature as low as 150 C Prior to drying the moisture content of the coal samples was 34 27 as presented in Table 2 Fig 6 Calculated moisture content variation of coal particle over time for various particle diameters a 1 mm b 3 mm and c 5 mm Fig 7 Calculated moisture content variation over time of various coal particle sizes at a disc temperature of 150 C Fig 8 Effectofdisctemperatureonthemoisture contentofcoaldriedatafeedrateof5 kg h and a blade rotational speed of 0 84 rpm while running a vacuum pump Table 3 Residence time of coal in disc dryer at various heating plate temperatures with coal feed rate of 5 kg h and blade rotational speed of 0 84 rpm Temperature of the heating plate C Residence time min 13010 14010 1509 5 1609 4 271S H Moon et al Fuel Processing Technology 124 2014 267 274 Fig 6 shows that the moisture content decreases with increases in drying time for all coal particle sizes However the drying rate at a heating plate temperature of 130 C diminishes slowly while those at 150 C and 160 C drop sharply for complete drying within 10