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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) 267274 Corresponding author. Tel.: +82 42 860 3221; fax: +82 42 860 3134. E-mail address: shmoonkier.re.kr (S.-H. Moon). /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 VVKexp E=RT2 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 ? ? ? r0 03 k T r ? ? ? rR TTs 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/mK)0.26 Cv,v(J/kgK)1520 Cv,(J/kgK)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) 267274 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 13 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) 267274 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) 267274 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 ru