EFFECTIVE SPECIFIC HEAT OF WOOD BRIQUETTES Daniela SOVA Transilvania University of Brasov, Faculty of Mechanical Engineering Str. Politehnicii nr. 1, 500024 Brasov, Romania E-mail: sova.@unitbv.ro Abstract: The effective specific heat of woo briquettes was etermine by using a mathematical moel evelope for bulk pellets/briquettes an by measurements mae in parallel an perpenicularly to the briquettes axis in the range of moisture contents below an above equilibrium moisture content. The mathematical moel was base on the assumption that bulk pellets/briquettes are a porous system, consisting of soli particles an gas, with effective thermal properties. The moel applie to the briquettes with the moisture content above the equilibrium moisture content is ifferent to that applie to the briquettes with the moisture content below equilibrium moisture content, since alongsie with woo fiber swelling, the magnitue an the number in the vois between chips increases. That is why, the briquettes ensity ecrease in this moisture content range. New equations are propose for this moisture content range consiering a larger woo cell lumen that inclues the chips interspaces. Experiments were performe at the temperature of briquettes ranging from 19 to 24 o C an the average moisture content from 0 to 22.7%, ry basis. When increasing the moisture content above equilibrium moisture content, the briquettes expane ue to fiber swelling, but also ue to the increase in the magnitue an number of chips interspaces. Both swelling an increase in briquettes vois influence the effective specific heat. Key wors: woo briquettes; effective specific heat; moisture content; transient line heat source metho. INTRODUCTION Romania is estimate to have a biomass energy potential of 7,594,000 toe/year corresponing to 19% of the total average primary consumption, which is ientifie in firewoo an woo waste from harvesting operations (1,175,000 toe/year), sawust an woo waste from woo processing operations (487,000 toe/year), agricultural waste (4,799,000 toe/year), biogas (588,000 toe/year) an househol waste (545,000 toe/year) (Borz et al. 2013). Woo-processing resiues (fine waste), such as sawust an woo shavings, are consiere to be of little economic value. They are either burne on site or transporte to isposal sites. Compressing the low ensity biomass into a soli fuel of a convenient size an shape allows burning them in the same way like woo or charcoal. In this situation, briquettes coul offer a means of waste management (Chaney 2010). Briquettes are manufacture by compression of resiues with a low moisture content (<25%, ry basis) at moerate to high pressure (>5MPa). Briquetting increases the bulk ensity of the biomass, increasing its energy ensity (the energy content per unit volume of material) (Chaney 2010). Biomass fuel properties for the combustion analysis are groupe into physical, chemical, thermal an mineral properties. The thermal properties are specific heat, thermal conuctivity an emissivity that vary with moisture content, temperature an egree of thermal egraation (Saiur et al. 2011, Raglan et al. 1991). Knowing the thermal properties of biomass briquettes an pellets is important for moeling the combustion process. Effective thermal conuctivity an specific heat of bulk woo pellets are also important properties for stuying self-heating uring their storage (Guo et al. 2012). A packe be of pellets is assume by Guo et al. (2012) an Guo (2013) to be a continuous homogeneous porous system with effective thermal properties. Sjöström an Blomqvist (2014) use the transient plane source technique to measure the specific heat an thermal conuctivity of bulk woo pellets within a temperature range of 22 an 120 o C. They also investigate the possibility of measuring those properties on iniviual pellets while stuying the moisture content epenence. The effect of moisture content on thermal properties of alfalfa pellets was stuie by Fasina an Sokhansanj (1995) using the line heat source metho for pellets moisture content ranging from 7.5 to 18%, wet basis. Specific heat an thermal conuctivity of softwoo, softwoo bark an softwoo char were comparatively measure at temperatures between 40 an 140 o C by Gupta et al. (2003). As compare to woo, which is an anisotropic an heterogeneous biological porous material, briquettes are consiere to be isotropic because of the ranom orientation of fibers uring the briquetting process. There is less information on specific heat of a single woo briquette an its epenence on moisture content an no information about its values if it is measure in parallel or perpenicularly to the briquette axis. 133
OBJECTIVE The objectives of the research reporte below were to investigate the specific heat of woo briquettes by measuring it using the transient line heat source metho an by moeling it using the mathematical moel evelope for the effective specific heat of bulk pellets/briquettes. Both, measurement an moeling were carrie out for moisture contents ranging from 0% to equilibrium moisture content an from equilibrium moisture content up to the maximum moisture content (24.5%, ry basis) permissible for briquettes to maintain their shape. Also, specific heat measurements were performe in parallel an perpenicularly to the briquettes axis. MATERIAL, METHOD, EQUIPMENT The briquettes were forme by compression of woo processing resiue, i.e. softwoo an harwoo chips in uncontrolle amounts, in a hyraulic briquetting press (MB4 Golmark). Compression is a continuous extrusion process which epens on the friction forces from the sie of the ie acting to prouce compression. The pressure use to form cylinrical briquettes with ensities between 750 an 800kgm -3 was 150bar. The resulting imensions of the briquettes were 40mm for the iameter an 30-75mm for the length. The maximum moisture content require by the proucer of the briquetting press is 17%. No biners were use in forming the briquettes. Twenty briquettes were selecte for thermal conuctivity an specific heat measurements from a lot of 200 extrue briquettes. They were store in the Laboratory of Heat Transfer at 20±1 o C an 45±2 %RH. Two lengths an two iameters of each briquette were measure using a igital pocket caliper (ULTRA, 0.01 mm accuracy). A stereometric metho (Rabier et al. 2006) was use to etermine briquette ensity. This metho was chosen an not a isplacement metho, in orer to preserve the structure of the briquettes which otherwise coul have alter the measurement of thermal properties. The stereometric metho consiste in briquettes weighing using a mass balance (KERN-EW 3000g, 0.01g accuracy), calculating the volume of the briquettes by using their main imensions an etermining the ensity. The briquettes were afterwars oven rie at 103±2 o C to constant mass in orer to etermine the moisture content (ry basis). In orer to prevent loss of material uring briquettes hanling, rying an weighing, previously rie glass pans were use. The moisture content was calculate base on wet an ry briquette masses (SR EN 13183-1-2003/AC-2004). The briquettes imensions were measure again after oven rying an briquettes ensity was recalculate. Thermal conuctivity an volumetric specific heat were measure with KD2 Pro analyzer (Decagon Devices Inc.) by using a SH-1 ual-neele sensor (30mm length, 1.30mm iameter, 2 neeles, 6mm spacing), base on the transient line heat source metho (Chaney 2010, Speyer 1996). This metho consists in the generation of a heat pulse by one probe, the response measure by the other, an a numerical analysis of the response behavior, which allows the thermal properties (volumetric heat capacity, thermal conuctivity an thermal iffusivity) to be foun. In orer to apply the metho, two Ø1.3mm x 30mm orifices were rille in each briquette. From the amount of twenty briquettes, nine were rille in parallel with the briquettes axis an the other nine perpenicularly to the briquette axis. The last two briquettes were rille both parallel an perpenicularly to the axis. Two or three measurements of the thermal properties were performe for each briquette, at 0% moisture content () an equilibrium moisture content (E). Thermal properties of briquettes epen on many factors, such as the material from which they are mae, the ensity to which they are compresse an the moisture content. In the case of biomass briquettes, ue to their varie nature in terms of constituent materials, the conitions uner which they are forme an their moisture content, stanar literature values are not available; there are likely to be significant ifferences between the thermal properties, not only for briquettes of ifferent materials, but also for briquettes of the same material forme at ifferent ensities an moisture contents (Chaney 2010). In orer to etermine in the present experiment the effect of moisture content on the behavior of thermal properties of woo briquettes, they were humiifie in a climatic test chamber (KPK 200/FEUTRON) at 20 o C an 90% RH. The moisture content of each briquette was etermine accoring to the same metho (SR EN 13183-1-2003/AC-2004), the ensity was obtaine by using the stereometric metho an the thermal conuctivity an volumetric heat capacity were measure with KD2 Pro analyzer. The briquettes moisture content was increase from E up to the maximum moisture the briquettes coul absorb an measurements of thermal properties coul be performe. The present paper eals only with the etermination of briquettes specific heat, while the moels applie to the etermination of briquettes thermal conuctivity were explaine in etail in the paper reporte by Sova et al. Accoring to Guo et al. (2012), a packe be of pellets can be simplifie by assuming it as continuous homogeneous porous system with effective thermal properties. Similarly, for a porous material that consists of soli particles an gas, as briquettes can be regare, the effective volumetric heat capacity (rc p ) is approximate by the following equation: 134
p ( Pw ) rw cp w Pw rair cp air r c 1 + (1) where: c p is the effective specific heat of briquettes (Jkg -1 K -1 ), (Jkg -1 K -1 ) an c pair (Jkg -1 K -1 ) are specific heats of woo particles an gas (air), r (kgm -3 ), r w (kgm -3 ) an r air (kgm -3 ) are the ensities of briquettes, woo particles an air, P w is the volume fraction of gas in the wet porous material (wet porosity). Guo et al. (2012) also consiere that the effective specific heat of bulk pellets oes not iffer from the specific heat of a single pellet since the ensity of air is much smaller than the ensity of soli particles. The effective specific heat of briquettes can be therefore expresse from Eq (1) as: c p ( P ) The wet porosity is escribe by Eq (3), (Hunt et al. 2008): w ρw cp w + Pw ρaiρ cp aiρ 1 (2) ρ P ( 1 V % ) P bw w (3) 1 V % bw P where: V% bw is the volume fraction of the boun water in the cell wall an P is the ry porosity. The ry porosity is obtaine from: P r cw cw (4) r r r air where: r (kgm -3 ) is the oven ry ensity of briquettes, 3 ρ cw 1540kgm is the ensity of the cell-wall 3 substance (Siau 1995) an r 1.193kgm air is the ensity of air at 20 o C (Incropera et al. 2007). The volume fraction of the boun water, V% bw, is calculate as a function of the moisture content from the following equation (Hunt et al. 2008): V % (5) bw ρcw ρ + ρ bw cw 1115kgm 3 where: ρbw is the ensity of the boun water (Hunt et al. 2008). The ensity of woo particles, r w, is etermine using the rule of mixtures. Thus: w ( 1 V % bw ) ρcw + V bw ρbw ρ % (6) Accoring to Siau (1995) the specific heat of woo increases significantly with moisture content. When the moisture content is less than 5% the specific heat of woo may be calculate from the rule of mixtures (Siau 1995) as: 1260 + 4185 (7) where: <0.5, t30 o C, the specific heat of oven ry woo at 30 o C is 1260Jkg -1 K -1 (Siau 1995), the specific heat of free water at 30 o C is 4185Jkg -1 K -1 (Siau 1995). Specific heat also increases with temperature an accoring to Siau (1995) the specific heat of the ry cell wall may be calculate as: c o [ 0.004( t 30) ] 1260 C pw (8) 135
where: t o C is the temperature range from 0 o C to 100 o C. The temperature at which specific heat of briquettes was measure was 20 o C. Therefore, taking into account Eq (8) an the variation of the specific heat of water with temperature, Eq. (7) becomes: 1209.6 + 4181 If the moisture content of woo increases above that mentione in Eq (7), being in the range 5% an 24%, Siau (1995) suggests another relationship, base on a previous investigation, accoring to which an excess in specific heat must be taken in consieration. It correspons to approximately 418Jkg -1 K -1. Thus, Eq (7) changes into: 1260 + 4185 + 1674 After rearranging terms, Eq (10) becomes: ( 0.05) (9) (10) 1176 + 5859 (11) where: ranges from 0.05 to 0.3 an t30 o C. At t20 o C, Eq (11) may be written as: 1128 + 5808.5 The aforementione equations for the etermination of the specific heat are applie to the briquettes with 0% an E. In this moisture content range, the vois between chips remain almost unchange. When the moisture content increases above E, alongsie with woo fiber swelling the magnitue an the number of the vois between chips increase too. A consierable increase in the briquette overall imensions is notice an for that reason the wet porosity increases, even if the moisture content increases an thus a ecrease of porosity woul be expecte. Accoringly, in the range of moisture contents above E the wet porosity is not anymore calculate with Eq (3). In this range of moisture contents the woo cell has a larger lumen that inclues the chips interspaces. The new length of the woo cell is etermine from the proportionality between cell volume an briquette volume in oven ry conitions an the current moisture content conitions. It may be calculate from the following equation (Sova et al.): V V 136 b 1/ 2 (12) b L (13) where: L is the overall cell imension, V b is the briquette volume at current, V b is the briquette volume in oven ry conitions. The new lumen length, a, is calculate from the equality (Sova et al.): a a L L (14) where: a is the lumen length of the woo cell, L is the cell imension at a certain. The lumen length, a, is etermine from the ry porosity (Siau 1995): ( ) 1/ 2 The imension L is obtaine from the ry an wet porosities as follows: a (15) P P Pw 1/ 2 L (16) Detaile calculation of the overall cell imension, L, is shown in (Sova et al.). Accoringly, the wet porosity of the briquettes with >E is etermine from Eq (17):
a P w L In Eq (2), the specific heat of air at t20 o C is c pair 1006.86Jkg -1 K -1 (Incropera et al. 2007). 2 2 (17) RESULTS AND DISCUSSION From the twenty briquettes subjecte to thermal conuctivity an specific heat measurements, four briquettes were remove from the range of ata because they broke own uring humiification. The most affecte were those rille both parallel an perpenicularly to their axis. Therefore, the measurements were further performe only on 16 briquettes. The average moisture contents of the humiifie briquettes were 5.95% (E), 12.2%, 13.1%, 16.3%, 17.6%, 21% an 22.7%, ry basis. Commercial woo pellets an briquettes have a typical E of 4-8%, wet basis (Guo 2013). The results were ivie in two groups, one group comprising the briquettes rille in parallel with the axis an the other group comprising the briquettes rille perpenicularly to the axis. Fig. 1 shows the experimental an moele effective specific heat values corresponing to the longituinally rille briquettes with 0% an with E, as function of the briquettes ensity. It was ecie to plot the specific heat with respect to the briquettes ensity rather than with respect to their moisture content. The increase in moisture content from 0% to the E resulte in the linear increase of ensity an specific heat, which is in agreement with the statement of Siau (1995) regaring woo, that the specific heat increases significantly with moisture content. On the same figure, the specific heat of woo, escribe by Eqs (9) an (12), as function of ensity is represente too. The effective specific heat values are almost the same with the specific heat values of woo in this range of moisture contents. The low coefficients of etermination (R 2 ) can be explaine by the low number of ata in this range of moisture contents. The scatter in the experimental results inicates possible ifferences in the local ensity of briquettes. Fig. 1. Experimental an calculate effective specific heat of longituinally rille briquettes as a function of ensity for E. The experimental an moele effective specific heat results corresponing to the raially rille briquettes with 0% an with E, as function of the briquettes ensity are inicate in Fig. 2. The figure also shows the results of the specific heat of woo calculate with Eqs (9) an (12). The results of the effective specific heat of briquettes an specific heat of woo are very similar to those represente in Fig. 1. They increase with ensity increase in a linear regression. The experimental results are again scattere an they increase very slightly with ensity increase. Fig. 3 inicates the experimental an moele effective specific heat values of the longituinally rille briquettes with >E as linear function of briquettes ensity. The increase in moisture content 137
etermine the ecrease of the ensity because of the increase in the vois between the chips an the increase of the specific heat. The specific heat is not so much influence by the increase in the vois as it is influence by the increase of the moisture content. Only at moisture contents exceeing approximately 20%, the experimental results of the specific heat start to ecrease because of the increase in the vois magnitue. The same ascenant tren has the specific heat of woo. Comparing the specific heat results with the thermal conuctivity results, the tren is ifferent; i.e. the briquettes thermal conuctivity ecrease with moisture content increase, because the ensity ecrease an the influence of the increasing vois became preominant (Sova et al.). It also can be physically explaine by the fact that the ecrease in the heat transfer by conuction etermines the increase in the energy store in the briquette an less heat is require to raise the temperature by the same amount. Fig. 2. Experimental an calculate effective specific heat of raially rille briquettes as a function of ensity for E. Fig. 3. Experimental an calculate effective specific heat of longituinally rille briquettes as a function of ensity for >E. 138
Fig. 4 inicates the experimental an moele effective specific heat results corresponing to the raially rille briquettes with >E, as function of the briquettes ensity. Fig. 4. Experimental an calculate effective specific heat of raially rille briquettes as a function of ensity for >E. The increase in briquettes moisture content etermine the ecrease of their ensity an the linear increase in the effective specific heat. The specific heat of woo has the same tren an also is in linear regression with ensity. The experimental specific heat results increase with moisture content increase an with ensity ecrease from 650kgm -3 to about 500kgm -3 ; instea they ecrease with ensity ecrease starting with about 500kgm -3. The variation of experimental specific heat with ensity is polynomial. Sova et al. inicate the same polynomial variation of the experimental thermal conuctivity results with respect to the ensity of briquettes. They explaine that the woo fiber swelling was more important on the briquettes length than in raial irection ue to the woo swelling characteristics an that the effect of moisture content on specific heat, an thermal conuctivity as well, was significant for the briquettes with the ensity ranging from 650kgm -3 to about 500kgm -3. As the briquettes ensity ecrease below 500kgm -3, the influence of the vois became preponerant on both specific heat an thermal conuctivity. The experimental specific heat values are higher in case of the raially rille briquettes than in case of the longituinally rille briquettes, thus epening on the measurement irection. The ratio of their average values is 1.7 (3.29kJkg -1 K -1 : 1.936kJkg -1 K -1 ). On the other han, the average values of the moele specific heat of raially rille briquettes an longituinally rille briquettes are in a ratio of 1 (1.976kJkg -1 K -1 : 1.973kJkg -1 K -1 ). This shows that the moele specific heat values are not sensitive to the measurement irection, perpenicular or parallel to the briquettes axis. It is to observe that the average experimental specific heat value of the longituinally rille briquettes is very close to the average moele specific heat values. The experimental results of specific heat are therefore epening on moisture content, ensity an measurement irection. The briquettes moisture content range from 0 to 24.5%, ry basis an the ensity range from 330kgm -3 (at maximum ) to 802kgm -3 (at E). The typical range of moisture contents of woo use for fuel is from 5% to 20% (Raglan an Aerts 1991). The specific heat of longituinally rille briquettes range from 1.217kJkg -1 K -1 to 3kJkg -1 K -1 an that of raially rille briquettes from 1.485kJkg -1 K -1 to 6.843kJkg -1 K -1. Guo et al. (2012) etermine effective specific heat values of woo pellets with the moisture content between 1.7 an 9% that range from 1.074 to 1.253kJkg -1 K -1, increasing with moisture content linearly. The bulk ensities range from 650 to 675kgm -3. The specific heat of ry woo pellets was foun by the same authors to be 1.01±0.05 (kjkg -1 K -1 ). In the present paper the measure specific heat of ry briquettes range from 1.3 to 1.99kJkg -1 K -1 (the ensity 139
range from 720 to 763kgm -3 ). The specific heat of ry woo at 20 o C, as earlier mentione in the paper, is 1.21kJkg -1 K -1. Pauner an Bygbjerg (2007) assume in their paper a specific heat of 2.2kJkg -1 K -1 for stuying selfheating of biofuel pellets. Sjöström an Blomqvist (2014) measure the thermal properties of woo pellets at elevate temperatures using the transient plane source technique. For bulk ensities ranging from 502 to 693kgm -3 an 6.6%, ry basis, the specific heat range from 1.35 to 1.63kJkg -1 K -1. They also measure the specific heat of a single pellet with the ensity 1290kgm -3 at 2.9% an 11.7% moisture content an obtaine the values 1.44kJkg -1 K -1 an 1.77kJkg -1 K -1, respectively. Fasina an Sokhansanj (1995) reporte specific heat values of alfalfa pellets ranging from 1.636 to 2.021kJkg -1 K -1 within the moisture content range of 7.5 to 18%, wet basis an at 30 o C temperature. They use the line heat source metho to obtain bulk thermal conuctivity an thermal iffusivity. The specific heat of pellets was calculate from values of thermal conuctivity, thermal iffusivity an bulk ensity. Chaney (2010) measure the specific heat of newspaper briquettes by using the ual probe heatpulse metho over a range of ensities (175-375kgm -3 ) an obtaine the mean value of 1.612kJkg -1 K -1 in a range of values between 1.2kJkg -1 K -1 an 2.3kJkg -1 K -1. He assume that the specific heat of briquettes was constant across the ensity range teste, because the contribution of the porosity ha an insignificant effect on specific heat. The experimental results reporte by the above mentione authors are comparable to those obtaine for the longituinally rille briquettes, as escribe in this paper. As regars the results of the raially rille briquettes, they are much higher than those reporte in the above mentione papers. The literature offers no mention that any other author woul have measure the thermal properties of pellets or briquettes perpenicularly to their axis. Therefore, the research must be continue with more measurements at ifferent temperatures an moisture contents in orer to completely justify the results obtaine in the experiment escribe within this paper. CONCLUSIONS Accoring to experiments an moels, the effective specific heat of briquettes increase with ensity increase, when the moisture content increase from 0% to the equilibrium moisture content. The same conclusion can be rawn for both measurements, in parallel an perpenicularly to the briquettes axis. When the moisture content increase from the equilibrium moisture content to the maximum moisture content the ensity ecrease an the specific heat increase. Accoring to the experiments mae in parallel with the briquettes axis, the specific heat is not so much influence by the increase in the vois as it is influence by the increase of the moisture content. Only at moisture contents exceeing approximately 20%, the experimental results of the specific heat start to ecrease because of the increase in the vois magnitue. For the experiments mae perpenicularly to the briquette axis, the results showe the polynomial increase of the specific heat with moisture content increase an with ensity ecrease from 650kgm -3 to about 500kgm -3. Then, the specific heat ecrease in a polynomial regression with moisture content increase an with ensity ecrease from about 500kgm -3. In this case the woo fiber swelling was more important on the briquettes length than in raial irection ue to the woo swelling characteristics an the effect of moisture content on specific heat was significant for the briquettes with the ensity ranging from 650kgm -3 to about 500kgm -3. As the briquettes ensity ecrease below 500kgm -3, the influence of the vois became preponerant on the specific heat. The experimental specific heat values are higher in case of the raially rille briquettes than in case of the longituinally rille briquettes, thus epening on the measurement irection. The ratio of their average values is 1.7. The average experimental specific heat value of the briquettes measure in parallel with the axis is very close to the average moele specific heat value. REFERENCES Borz SA, Derczeni R, Popa B, Nita MD (2013) Regional profile of the biomass sector in Romania. www.foropa.eu. Chaney J (2010) Combustion characteristics of biomass briquettes. PhD thesis. University of Nottingham, Great Britain. Fasina OO, Sokhansanj S (1995) Bulk thermal properties of alfalfa pellets. Canaian Agricultural Engineering 37(2):91-95. Guo W (2013) Self-heating an spontaneous combustion of woo pellets uring storage. PhD thesis. University of British Columbia, Vancouver, Canaa. Guo W, Lim CJ, Bi X, Sokhansanj S, Melin S (2012) Determination of effective thermal conuctivity an specific heat capacity of woo pellets. Fuel 103:347 355. 140
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