DETERMINATION OF DRYING CHARACTERISTICS OF TURKISH PINE (PINUS BRUTIA) WOODCHIPS IN A SCREW DRYER IN VARIOUS DRYING CONDITIONS

Transcription

1 DETERMINATION OF DRYING CHARACTERISTICS OF TURKISH PINE (PINUS BRUTIA) WOODCHIPS IN A SCREW DRYER IN VARIOUS DRYING CONDITIONS ÖZGÜR KAPLAN 1, CENK ÇELİK 1, HAYATİ OLGUN 2, AWF AL KASSİR 3 1 Mechanical Engineering Department/ Kocaeli University Mechanical Engineering Department, Umuttepe Campus, Kocaeli, TURKEY ozgurkaplan1@gmail.com, cenkcelik@kocaeli.edu.tr 2 TUBITAK Marmara Research Center Tubitak Gebze Campus Marmara Research Center, 41470,Gebze-Kocaeli, TURKEY Hayati.Olgun@mam.gov.tr 3 Department of Energy and Chemical Engineering, University of Extremadura Avda. de Elvas, s/n, 06071, Badajoz, SPAIN aawf@unes.es Abstract: The aim of this study is to determine the optimum drying conditions and to investigate the drying characteristics of Turkish pine (pinus brutia) dried in a screw dryer. Hot air is used as drying medium. Experiments are carried out in various drying air temperatures, drying air flow rates and dryer types. Revolution of the screw is kept constant in the experiments. Change of moisture content of Turkish pine woodchip in these different drying conditions are determined. Woodchip is fed to dryer and dried 4 times in the same drying conditions in order to determine the change of moisture content of woodchip against drying time. Short analysis (ASTM E- 1755) is applied to the 4 times dried samples and the amounts of volatile matters are investigated. It is found that with increasing drying air temperature and flow rate, drying rate is increased and amounts of volatile matters in the samples stay same. Keywords: Woodchip drying, Screw conveyor dryer, Continuous drying

2 INTRODUCTION Wood has been used as a source of energy for cooking and heating for centuries. Today, wood is still being used in heating applications and power generation in both developed and developing countries [1]. Wood fuel has several environmental advantages over fossil fuel. The main advantage is that wood is a renewable and sustainable source of energy. Other advantages include the fact that the amount of carbon dioxide (CO 2 ) emitted during the burning process is typically 90% less than when burning fossil fuel. Wood fuel contains minimal amounts of sulfur and heavy metals. It is not a threat to acid rain pollution, and particulate emissions are controllable [2]. Lower heating value of dry wood is generally between MJ/kg. Increasing moisture content of wood decreases its lover heating value. For large scale combustors such as fluidized bed combustors, the maximum acceptable moisture content (MC) of wood is approximately mass%. Higher and significantly fluctuating MC in wood cause unstable combustion and higher CO and volatile organic compound (VOC) emissions. In gasification process, the higher MC value of wood results with the higher tar concentration and decreasing the lower heating value of synfuel [3]. In order to increase energy efficiency, improve energy production quality and reduce emissions during energy conversion, wood must be dried. Woodchips are most commonly dried in the industry via packed moving bed dryers, rotary dryers and pneumatic or flash dryers [4-6]. A detailed review about these dryers can be found in Pang et al, 10 [4]. And also further reading about industrial dryers can be found in Mujumdar, 06 [7]. In this study, a screw dryer is used for drying woodchips. Screw conveyors are enclosed devices that give good throughput control and provide environmentally clean solutions when dealing with fine, dusty and toxic materials. It s cost effective, easy to install and has a great volumetric efficiency compared with belt conveyors [8]. Thermal performance, hydrodynamic characteristics, design and residence time distribution of a pilot scale screw dryer is investigated by Waje et al [8,9,10,11]. The screw conveyor that has used in the experiments is designed to be integrated with a combustion facility such as a fluidized bed combustor. But in order to obtain more accurate results, ambient air is used as drying medium since inlet properties (temperature and humidity) of clean air can be measured more easily and accurately. Drying characteristics (change of moisture content and volatile organic compound) of woodchips in different drying medium flow rate, drying type and drying medium temperature are investigated in this present work. 2. MATERIAL and METHOD 2.1. Material Turkish Pine has been provided from The General Directorate of Forestry. It has been crushed and sifted to 0-2 mm woodchips in Fuel Processing Lab in TUBİTAK MAM. Elemental analysis has been applied to woodchip samples using LECO Truspec CHN-S Elemental Analysis equipment. Carbon (C), hydrogen (H) and nitrogen (N) contents of sample are investigated under ASTM 5373 standard and sulphur content investigated under ASTM D 4239 standards. Results of these analyses are given in table 1. Table 1 Elemental Analysis of Turkish Pine woodchip sample (dry basis, Biomass Type %C %H %N %S %O Turkish Pine 56,26 8,9 0, ,

3 In each experiment, the moisture content of the dried woodchip is measured. For moisture content measuring, Sartorious MA100 thermogravimetric moisture content measurement equipment is used. Also a short analysis (ASTM E-1755) is applied to the samples in Solid Fuels Research Lab. The volatile compound, ash and fixed carbon amounts are investigated in this analysis in order to determine if any drying based VOC emissions are released. Experimental setup consists of an air blower, electrical heating unit, screw conveyor and air distribution pipes. Schematic of experimental setup is given in fig Fig. 1 Schematic of experimental setup Ambient air is blown by the air blower to the electrical heating unit to heat the drying air and then hot drying air is delivered to screw conveyor where it meets with wet woodchips and drying occurs. Drying air can be delivered to the screw conveyor from both sides with pipe 1 and pipe 2. Dryer can be operated both co-current and counter-current setup. TT101 thermocouple measures drying medium inlet temperature in co-current and drying medium outlet temperature in countercurrent. Similarly, TT 105 thermocouple measures drying medium outlet temperature in co-current and drying medium inlet temperature in counter-current. TT106 thermocouple measures outside surface temperature of conveyor housing (on the aluminum sheet that covers insulation material). TT102, TT103 and TT104 thermocouples measure inside dryer temperature. Inside dryer temperature 1 is TT101 when it s co-current and TT105 when it s counter-current Fig. 2 Schematic of screw conveyor dryer The blower has an input power of 2.2kW and delivers approximately 0 m 3 /h of air at a maximum head pressure drop of 280 mbar. A frequency modulator controls the air flow rate. The drying air is heated as it passes through the electrical heater, which has a heat power of 4.5 kw and is controlled with contactors. A 0.37 kw electric motor and redactor set is used for rotating the screw and it s controlled by a frequency modulator. Air flow rate is measured with Bass 3

4 Instruments FOFT-0 orifice type flow meter. Humidity and temperature of ambient air is measured with Kimo TH-0 humidity sensor. Thermocouples along the dryer and at the pipes are NiCR-Ni type-k and have an accuracy of 0.1 C. All the measurement equipment is connected to Ahlborn Almemo MA data logger and data logger is connected to a PC via RS232 serial port. The data collected from measurement equipments are logged with AMR Win Control computer program. Cylindrical housing of the screw conveyor is made from 5 inch drawn steel pipe. Screw of the conveyor has an outside diameter of 1 mm and has mm pitch. Length of screw is 1.5 m and drying zone (between the inlet and the outlet pipes) of the dryer is 1 m long. Dimensions of the screw are given in fig 3. Air distribution pipes are made from 3 inch drawn steel pipe. Both the housing of the conveyor and the air distribution pipes are insulated with 50 mm ceramic wool Fig. 3 Dimensions of the screw 2.2. Method Experiments are conducted under m 3 /h and m 3 /h drying medium flow rate, 150 C, 175 C and 0 C drying medium temperature and co-current and counter-current dryer type conditions. Revolution of the screw is kept constant as 18 rpm in the experiments. 1.5 kg woodchip is fed in each experiment and drying time of 1.5 kg woodchip at 18 rpm screw speed is measured 3.15 min. In co-current TT101, in counter-current TT105 is the drying air inlet temperature and drying air temperature is adjusted according to these thermocouples. It s nearly impossible to operate the dryer over m 3 /h drying air flow rate. This is because higher flow rates cause woodchips to blow out from the chimney. 1.5 kg of woodchip is fed to the dryer and dried four times in the same conditions and at the end of each experiment a small amount of sample is collected from dried woodchips. Moisture content of this sample is measured on wet basis. Also, a short analysis (ASTM e-1755) is applied to the four time dried woodchip sample and its volatile organic compound (VOC) amount is investigated and compared with the original (non-dried) sample of woodchip. With the moisture content data collected from experiments, moisture content-drying step curves are drawn and a polynomial curve is fitted to these curves in order to determine the characteristic drying curve. 3. RESULTS 3.1. The effect of drying air temperature to drying process In fig 4, a bar graph of moisture content of woodchip against drying conditions is given. In this figure, it can be seen clearly that regardless from other drying conditions (drying air flow rate and drying type) the increase in drying air temperature increases moisture loss from the woodchip and decreases the moisture content of the woodchip. 4

5 Moisture Content Of Woodchip (wet basis, Moisture Content of Woodchip (wet basis, Fig. 4 The effect of drying air temperature to drying process The reason of this increase in the loss of moisture content can be explained with increasing heat transfer between the drying air and the woodchip The effect of drying air flow rate to drying process In fig 5, a bar graph of moisture content of woodchip against drying conditions is given. Regardless from other drying conditions (drying air temperature and drying type) the increase in the drying air flow rate increases moisture loss from the woodchip and decreases moisture content of the woodchip. When drying air flow rate is increased, the total energy that transferred with drying air to the dryer is increased. This could be the reason of the decrease in the moisture contents Fig. 5 The effect of drying air flow rate to drying process 5

6 Moisture Content of Woodchip (wet basis, The effect of drying type to drying process In fig 6, a bar graph of moisture content of woodchip against drying conditions is given. When co-current setup is used for drying, lower moisture contents can be reached compared to countercurrent drying. In counter current drying, drying air losses from material outlet section is greater than drying air losses from material feed inlet section in co-current. That s why with co-current setup, lower moisture contents can be reached Fig. 6 The effect of drying type to drying process 3.4. Drying Curves Four experiments are conducted in every drying condition. Including initial moisture content, five moisture content data obtained in every drying condition. With these data, moisture content against drying step curves is drawn. These curves are given in figures 7, 8, 9 and 10. These curves are drawn under constant drying type and drying air flow rate and variable drying air temperatures. In the constant-rate period of drying, moisture movement within the solid is rapid enough to maintain a saturated condition at the surface, and the rate of drying is controlled by the rate of heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated surface of the material across a stagnant air film into the environment. The rate of mass transfer balances the rate of heat transfer, and the temperature of the saturated surface remains constant [12]. Each curve is seemed to be in linear form. Thus, drying of the woodchip in these experiments occurred in the constant phase of drying. 6

7 Moisture Content Of Woodchip (wet basis, Moisture Content Of Woodchip (wet basis, m 3 /h, Co-Current Drying Step Fig. 7 Drying curves under m 3 /h drying air flow rate, co-current dryer type and various drying air temperatures m 3 /h, Counter-Current Drying Step Fig. 8 Drying curves under m 3 /h drying air flow rate, counter-current dryer type and various drying air temperatures 7

8 Moisture Content Of Woodchip (wet basis, Moisture Content Of Woodchip (wet basis, m 3 /h, Co-Current Drying Step Fig. 9 Drying curves under m 3 /h drying air flow rate, co-current dryer type and various drying air temperatures m 3 /h, Counter-Current Drying Step Fig. 10 Drying curves under m 3 /h drying air flow rate, counter-current dryer type and various drying air temperatures

9 Emissions In table 3, short analysis (ASTM E-1755) results are given. These ratios are given in dry basis and % mass. According to these data, all samples including original one has the same amount of volatile organic compound. That means there is no emission released during drying process even in 0 C. This could be explained with inside dryer temperature. When hot air meets with wet biomass, temperature inside the dryer drops immediately and because of that, there is no emission released. In 0 C experiments, inside dryer temperature 1 is around C. That could be the reason why there is no emission releasing. Table 3 Short analysis (ASTM E-1755) results of woodchip (dry basis, % mass) Drying Conditions (Drying Air Flow Rate, Drying Type, Drying Air Temperature) Ash Volatile Organic Compound Fixed Carbon m 3 /h,co-current,150 C m 3 /h,co-current,175 C m 3 /h,co-current,0 C m 3 /h,counter-current,150 C m 3 /h,counter-current,175 C m 3 /h,counter-current,0 C m 3 /h,co-current,150 C m 3 /h,co-current,175 C m 3 /h,co-current,0 C m 3 /h,counter-current,150 C m 3 /h,counter-current,175 C m 3 /h,counter-current,0 C Original Sample CONCLUSIONS The screw conveyor dryer is an alternative way to dry woodchips. Drying characteristics of Turkish Pine woodchips under various drying conditions is investigated in this study and it is found that the increase in both drying air temperature and flow rate increases the rate of evaporation during drying. Also co-current and counter-current drying types are investigated and when compared with each other, co-current dryer type provides increase in rate of evaporation. Drying air flow rate shouldn t exceed m 3 /h. Higher flow rates create operational problems such as woodchip blowing out from the flue and material feed inlet. If higher flow rates are needed, a cyclone and a bag filter should be added to the exit of flue. The best drying conditions in this study is m 3 /h drying air flow rate, 0 C drying air temperature and co-current dryer type. With this conditions, woodchip 9

10 dried from %60 MC to % MC (wet basis, mass %). There is no emissions released under all drying conditions Acknowledgements This project is funded by The Scientific and Technological Research Council of Turkey. Authors of this article wish to express their gratitude to Mrs. Berrin BAY, Mr. Erkutay TAŞDEMİRCİ, Mr. William Neuheisel and Ms. Nuriye KALEM for their help in writing of the article and overcoming technical problems during the experiments. References [1] Forestery in EU and the World: A Statistical Portrait; Eurostat Statistical Books, pp. 91 [2 ] pp. 1 [3] Svoboda K., Martinec J., Pohořely M., Baxter D., Integration of biomass drying with combustion/gasification techologies and minimization of emissions of organic compounds, Chemical Papers Vol:63, No:1, 09, pp.15 [4] Pang S., Mujumdar A.S., Drying of Woody Biomass for Bioenergy: Drying Technologies and Optimization for an Integrated Bioenergy Plant, Drying Technology, Vol:28, No:5, 10, pp [5] Pang S., Xu Q., Drying of Woody Biomass for Bioenergy Using Packed Moving Bed Dryer: Mathematical Modeling and Optimization, Drying Technology, Vol:28, No:5, 10, pp [6] Xu Q., Pang S., Mathematical Modeling of Rotary Drying of Woody Biomass, Drying Technology, Vol: 26, No:11, 08, pp [7] Mujumdar A.S., Handbook of Industrial Drying, CRC Press, 06 [8] Waje S.S., Thorat B. N., Mujumdar A. S., An Experimental Study of the Thermal Performance of a Screw Conveyor Dryer, Drying Technology, Vol:24, No:3, 07, pp [9] Waje S.S., Thorat B. N., Mujumdar A. S., Hydrodynamic Characteristics of a Pilot Scale Screw Conveyor Dryer, Drying Technology, Vol:, No:4, 07, pp [10] Waje S.S., Thorat B. N., Mujumdar A. S., Screw Conveyor Dryer: Process and Equipment Design, Drying Technology, Vol:, No:1, 07, pp [11] Waje S.S., Thorat B. N., Mujumdar A. S., Study of Residence Time Distribution in a Pilot Scale Screw Conveyor Dryer, Vol:, No:1, 07, pp [12] Perry H.R., Green W.D., Perry s Chemical Engineers Handbook, 7 th edition, pp 12-31,12-32, McGraw Hill,