Torrefaction is a thermal process that improves biomass performance as a fuel by property enhancements such as decreased moisture uptake and increased carbon density. Most studies to date have used very small amounts of finely ground biomass. This study reports the testing of a crucible furnace retort that was fabricated to produce intermediate quantities of torrefied material and to allow processing of wood chips. Yields ranging from 51 to 96% were impacted to a greater extent by differences in temperature than time of treatment. Although temperature-control issues (gradients, slow heating) were experienced with the crucible furnace retort, this equipment proved to be useful for its intended purpose

Eberhardt, Thomas L.

Reed, Karen G.

English

Wood and Fiber Science (E-Journal)

TECHNICAL NOTE: EVALUATION OF A CRUCIBLE FURNACERETORT FOR LABORATORY TORREFACTIONS OF WOOD CHIPS1Thomas L. Eberhardt*{Research Scientist and Team LeaderE-mail: teberhardt@fs.fed.usKaren G. ReedPhysical Sciences TechnicianUSDA Forest ServiceSouthern Research StationPineville, LA 71360-2009E-mail: kreed@fs.fed.us(Received May 2014)Abstract. Torrefaction is a thermal process that improves biomass performance as a fuel by propertyenhancements such as decreased moisture uptake and increased carbon density. Most studies to date haveused very small amounts of finely ground biomass. This study reports the testing of a crucible furnaceretort that was fabricated to produce intermediate quantities of torrefied material and to allow processingof wood chips. Yields ranging from 51 to 96% were impacted to a greater extent by differences intemperature than time of treatment. Although temperature-control issues (gradients, slow heating) wereexperienced with the crucible furnace retort, this equipment proved to be useful for its intended purpose.Keywords: Biofuels, biomass, moisture, particle size, pyrolysis, thermal degradation.INTRODUCTIONTorrefaction is a thermal process by which bio-mass can be made more amenable to fuel appli-cations. Conditions are typically anaerobic withatmospheric pressure and temperatures rangingfrom 200 to 300C. Yields of solid materialfrom torrefaction of woody biomass feedstocksare reported to range between 70 and 90%(Ciolkosz and Wallace 2011). Property enhance-ments include decreased moisture uptake, greaterresistance to microbial degradation, higher car-bon density, and increased grindability (Ariaset al 2008; Chen and Kuo 2011; Medic et al2012). Most torrefaction studies to date haveused small amounts of finely ground biomassin quartz tube furnaces. Little is known aboutthe processing of large particles (Ciolkosz andWallace 2011). Muffle furnaces equipped withbox-type chambers (retorts) have been used inonly a few studies to produce more substantialamounts of torrefied material (Pentananunt et al1990; Pimchuai et al 2010; Stelte et al 2011).Commercially practical particle sizes (woodchips) have been processed in box-type cham-bers (Phanphanich and Mani 2011; Singh et al2013) and in a pilot-scale torrefaction plant(Meng et al 2012). This study reports our effortsto develop equipment to produce intermediatequantities (50-500 g) of torrefied materialneeded for application assessments (adsorbents,soil amendments) and the ability to accommo-date wood chips.MATERIALS AND METHODSThe retort used in this study was fabricated byThermal Product Solutions (Williamsport, PA)for a programmable Lindberg Model 5661 cru-cible furnace with a Watlow EZ-Zone PM(Winona, MN) programmable integral deriva-tive (PID) controller. A basic schematic of theretort is shown in Fig 1. Fitted to the flanged* Corresponding author{ SWST member1 This article was written and prepared by US Governmentemployees on official time, and it is therefore in the publicdomain and not subject to copyright.Wood and Fiber Science, 46(4), 2014, pp. 600-604# 2014 by the Society of Wood Science and Technologyretort chamber was an insulated O-ring-sealedplug-type cover that, when installed, provided anapproximate internal volume of 1.6 L. The coverwas equipped with inlet and outlet tubes for purg-ing. The flow rate of the purge gas was monitoredusing a Key Instruments (Trevose, PA) P/N:60510 glass tube flowmeter. The cover was alsoequipped with a monitoring thermocouplesheathed in an Inconel 600W protection tube.Temperatures at different positions within theretort were determined by inserting a flexibleInconel 600W overbraided ceramic fiber-insulatedthermocouple through the purge gas inlet port.Thermocouple temperatures were measured withan Omega HH501AJK digital thermometer. Toassess the time needed for retort cooling, testswere conducted in which the temperature at thebottom of the heated retort (260C) was moni-tored with the retort remaining in the furnace orplaced in a cooling stand with or without aircirculation using a box fan.Pulp-grade pine wood chips (mostly Pinustaeda L.) were obtained from a local chip mill(Winnfield, LA). The moisture content (103 2C) of the air-dry wood chips was 9.6%. Woodchips (50 g) were transferred to a high-formporcelain crucible (250 mL) that was thenplaced in the bottom of the retort. The retortcover was installed with specific attention givento the placement of the outlet purge tube toensure that it was aligned past the side ofthe crucible (Fig 1). Under a constant purge oflaboratory-grade N2 (1 Lmin1), the retort washeated from room temperature to one of threetargeted operating temperatures (230, 260, and290C). Total run times ranged from 2 to 4 hincluding the time (less than 1 h) needed toreach the targeted operating temperature. Whenthe torrefaction run was complete, the retort wasremoved from the furnace and placed on a standto cool before stopping the flow of N2 andremoving the sample-containing crucible. Yieldswere based on the dry weights of both thetorrefied product and the starting wood chips. Atwo-way analysis of variance was conductedusing PROC GLM in SAS/STAT 9.3 (SAS Insti-tute, Inc., Cary, NC).RESULTS AND DISCUSSIONA concern with using a box-type chamber(retort) was very slow heat dissipation occurringafter turning off the electrical power to the muf-fle furnace (Stelte et al 2011); this would allowthe torrefaction process to continue, albeit at adecreasing rate as a function of the time neededto cool to a reasonable temperature before sam-ple removal. In other studies using muffle fur-naces, hot samples were immediately removedand allowed to openly cool (Pimchuai et al2010; Phanphanich and Mani 2011), the concernhere being the potential for sample oxidation.Thus, the retort used in this study was designedto be lifted from the furnace and placed in astand for faster cooling while maintaining a con-tinuous purge of N2. Testing and calibration ofthe retort are subsequently described; however,given the concerns related to retort cooling, tri-als with the retort remaining in the furnaceFigure 1. Schematic of crucible furnace retort with posi-tioning of crucible used for sample containment showing(a) fixed internal thermocouple, (b) purge gas inlet, (c)purge gas outlet, (d) O-ring seal, and (e) crucible.Eberhardt and Reed—RETORT FOR TORREFACTION OF WOOD CHIPS 601required 2.5 h to cool the torrefaction zone(lower 100 mm of retort) from 260C down to100C (Fig 2). When the retort was removed andplaced in the cooling stand, the time to reach100C was decreased to 30 min. With a fan forair circulation, this time was further decreasedto 10 min.Monitoring the measured temperature readingon the PID controller showed it to overshoot theprogrammed temperature setting by about 20Cbefore stabilizing at the programmed tempera-ture within 60 min of operation (Fig 3). Initialoperations also revealed a temperature gradientwithin the retort that was greater than antici-pated. After temperature stabilization (steady-state operation), the temperature at the fixedinternal thermocouple (Fig 1) was approxi-mately 75C lower than at the furnace thermo-couple. With a flexible thermocouple insertedthrough the retort air inlet port, the temperatureat the bottom of the retort was approximately35C lower (Fig 3). Thus, a torrefaction experi-ment with wood chips (265 g) placed directly inthe retort afforded a gradient of wood chipcolors, from light to dark brown, indicative ofdifferent degrees of torrefaction from top to bot-tom. Placement of an empty high-form porcelaincrucible (250 mL) in the retort gave a relativelyuniform temperature zone, ultimately limitingthe sample size to 50 g. Temperature calibrationwas performed and gave a linear (R2 ¼ 0.99)plot (not shown). The higher temperature settingprogrammed into the PID controller to achievethe target torrefaction temperature could then becalculated using the corresponding equation (t¼0.964c – 28.965, where t ¼ temperature experi-enced by sample and c ¼ temperature settingon the PID controller). For example, heatingthe feedstock to a target temperature of 260Crequired the PID controller to be programmed toa temperature of 300C.Torrefaction of the air-dry (9.6% MC) woodchips, contained in the aforementioned crucible,gave relatively homogeneous product colorations.Torrefied wood chip colors were increasinglydarker with increasing time and temperature. Themost dramatic difference in color change waswith increasing temperature for a given time.This observation was readily apparent in the datawith yields being impacted to a greater extent bytemperature as opposed to time (Table 1). Usingthe intermediate temperature of 260C, all yieldswere well within the range (70-90%) commonlycited in the literature (Ciolkosz and Wallace2011). At the lowest temperature (230C), allyields were greater than 90%, whereas at thehighest temperature (290C), all yields were lessthan 70%. At 290C, the yield at 5 h (50.14 0.81%) was the same as that at 4 h of treatment(51.24  1.27%). Torrefaction times longer than4 h were therefore not pursued. Given the highyield (low thermal degradation) when torrefyingFigure 2. Temperature monitoring of torrefaction zoneduring cooling of hot retort (260C target torrefaction tem-perature) under different conditions.Figure 3. Temperature monitoring of torrefaction zoneand at fixed internal thermocouple during initial 2 h ofheating cycle (260C target torrefaction temperature).602 WOOD AND FIBER SCIENCE, OCTOBER 2014, V. 46(4)for 2 h at 230C, shorter torrefaction times werenot pursued. It is acknowledged that commercialtorrefaction systems for the mass production ofbiochars would undoubtedly be designed forshorter residence times. Equipment temperature-control limitations demonstrated here precludedthe pursuit of kinetic studies. Also, condensationthat occurred within the plug-type cover pre-cluded the quantitative collection of condensablevolatiles for analysis.Monitoring the fixed internal thermocouple dur-ing the torrefactions showed that the tempera-ture lagged behind that obtained during testingwith an empty crucible (Fig 4). The temperatureof wood chips themselves could not be deter-mined given the current retort design; however,it was unlikely that any temperature deviationswere greater than those at the fixed internal ther-mocouple. Subsequent processing of oven-drywood chips gave plots that were essentiallyidentical to those with the empty crucible (plotsnot shown). The previously mentioned tempera-ture deviations at the fixed internal probe weretherefore attributed to moisture within the woodchips. Admittedly, it was not anticipated that aslittle as 5 g of water in 50 g of wood chips couldimpact the internal temperature of a cruciblefurnace retort with a volume of 1.6 L, with amass of 15 kg, and being operated under a con-tinuous purge of dry N2.CONCLUSIONTemperature-control limitations (gradients, slowheating) preclude the use of a crucible furnaceretort for kinetic studies; however, this equip-ment proved to be useful for producing interme-diate quantities of experimental torrefied woodsamples covering a wide range of yields.REFERENCESArias B, Pevida C, Fermoso J, Plaza MG, Rubiera F, Pis JJ(2008) Influence of torrefaction on the grindability andreactivity of woody biomass. Fuel Process Technol89:169-175.Chen W-H, Kuo P-C (2011) Torrefaction and co-torrefaction characterization of hemicellulose, celluloseand lignin as well as torrefaction of some basic constitu-ents in biomass. Energy 36:803-811.Ciolkosz D, Wallace R (2011) A review of torrefaction forbioenergy feedstock production. Biofuel Bioprod Bior5:317-329.Medic D, Darr M, Shah A, Potter B, Zimmerman J (2012)Effects of torrefaction process parameters on biomassfeedstock upgrading. Fuel 91:147-154.Meng J, Park J, Tilotta D, Park S (2012) The effect oftorrefaction on the chemistry of fast-pyrolysis bio-oil.Biores Technol 111:439-446.Pentananunt R, Rahman ANMM, Bhattacharya SC (1990)Upgrading of biomass by means of torrefaction. Energy15(12):1175-1179.Phanphanich M, Mani S (2011) Impact of torrefaction onthe grindability and fuel characteristics of forest bio-mass. Biores Technol 102:1246-1253.Table 1. Percentage yields of torrefied wood chips for different temperature and time conditions.Temperature (C)Time (h)a 230 260 2902 95.64 (0.25)b 85.30 (1.88) 67.36 (1.13)3 92.95 (0.76) 82.01 (1.47) 61.61 (0.45)4 91.53 (0.39) 78.41 (0.01) 51.24 (1.27)a Torrefaction time includes the time (less than 1 h) needed to reach the target torrefaction temperature.b Standard deviations shown in parentheses; two-way analysis of variance showed all values to be statistically different (P < 0.0001) across time and temperature.Figure 4. Temperature at fixed internal thermocouple dur-ing the first 2 h of heating cycle (to target torrefaction temper-ature) with empty crucible or crucible containing wood chips.Eberhardt and Reed—RETORT FOR TORREFACTION OF WOOD CHIPS 603Pimchuai A, Dutta A, Basu P (2010) Torrefaction of agricul-ture residue to enhance combustible properties. EnergyFuels 24:4638-4645.Singh T, Singh AP, Hussain I, Hall P (2013) Chemical char-acterisation and durability assessment of torrefied radiatapine (Pinus radiata) wood chips. Int Biodeter Biodegr85:347-353.Stelte W, Clemons C, Holm JK, Sanadi AR, Ahrenfeldt J,Shang L, Henriksen UB (2011) Pelletizing properties oftorrefied spruce. Biomass Bioenerg 35:4690-4698.604 WOOD AND FIBER SCIENCE, OCTOBER 2014, V. 46(4)

Technical Note: Evaluation of a Crucible Furnace Retort for Laboratory Torrefactions of Wood Chips

https://wfs.swst.org/index.php/wfs/article/download/995/995

Technical Note: Evaluation of a Crucible Furnace Retort for Laboratory Torrefactions of Wood Chips

Abstract

Similar works

Full text

Available Versions

Wood and Fiber Science (E-Journal)