Combustion Properties of Fuelwood in Hot-Air Heaters

Transcription

1 ;Ji;~liiHi Bll. For. & For. Prod. Res. Inst. No. 33, 1984 Combstion Properties of Felwood in Hot-Air Heaters By Toshimi HIRATA' 1 ' and Yaso FUKUI' 2 ' Smmary : Following previos stdies on hot-water boilers, the combstion properties of felwoods of sgi and konara and Ogalite of wood briqette were obtained for two hot air heaters manfactred for trial. This was to yield basic data for the development and the improvement of hosehold combstion appliances. The frnace of heater I has a maximm capacity of six Ogalite briqettes. Heater II has a smaller frnace, which can contain one Ogalite briqette bt has an atomatic spplier of Ogalite. Since these heaters prodce hot air heated by inner heat exchangers, it is not dangeros to toch the otside srface of the heaters. Both the heaters are highly practical becase of sstained prodction of hot air at a stable temperatre, thogh the hot-air temperatre of heater II flctated more than that of heater I. The thermal efficiency is higher in heater I than heater II, bt the initial heating rate of hot air is larger in heater II than heater I. The emissions of carbon monoxide and smoke are greater in heater II than heater I, bt these emissions are considerably less than those from previosly reported hot-water boilers. The initial rising rate of the hot-air temperatre and the emission of carbon monoxide with sgi are the largest. The initial rising rate of the hot-air temperatre with Ogalite is the smallest de to the smallest combstion rate, and Ogalite is appropriate for heating for long time. Konara showed, for the most part, intermediate properties between sgi and Ogalite, bt the emission of nitrogen oxide from konara is the largest. The thermal efficiency and the emission of nitrogen oxide generally increased with increases in exhast temperatre. However, excessively high exhast temperatre made the thermal efficiency lower. Therefore, a mechanism to control combstion, depending on exhast temperatre, shold be developed for the ftre. 1. Introdction Potential combstion heat per nit volme of wood is generally eqivalent to one forth to one sixth of that of fossil fels, sch as petrolem and coal. This can be shown by comparing the specific gravity and the calorific vale of wood and the fossilfels. Wood's smaller combstion heat creates some disadvantages for the transportation and design of frnaces sing its solid state. Frthermore the solid shape makes exact control of the combstion rate difficlt. We have long sed charcoal, which has high calorific vales, and controlled the combstion rate. Now gasification and liqefaction of wood have been stdied on the basis of the pyrolysis to solve those difficlties. On the other hand, processes sing direct combstion of wood for indstrial and hosehold appliances are spreading since the rapid increase in oil prices. In spite of wider se, few research papers abot the direct combstion of wood in hosehold appliances have been pblished. It is necessary to develop hosehold appliances more sitable to the combstion of wood for more effective se of felwood. Following previos papers on hot-water boilersm>, this paper describes combstion properties of the same sorts of felwood in the most poplar hot-air Received April 12, 1984 (1) (2) Wood Utilization Division ;;j;:ft;fi]jil-38 Wood Utilization-38

2 -22- heaters. The aim was to obtain basic data for the development and improvement of hosehold combstion appliances. This stdy has been condcted nder the "Srvey Project for the Promotion of Utilization of nergy from the Forest" by the Japan Hosing and Wood Technology Center (Tokyo). 2. xperimental 2. 1 Fels The same fels sed in the previos stdiesm>, namely, woods of Konara (Qercs serrata THNs.) and Sgi (Chryptomeria japonica D. DoN), and Ogalite of wood briqette prodced in Japan, were sed for comparison among the stdies. They had been kept at a constant tern peratre and hmidity of l9 C and 42 r. h. %, respectively, for one year. The length of konara and sgi woods in strage was abot 3 em, and that of Ogalite was abot 4 em. The shapes, and the average vales of the moistre content, the calorific vale, and the specific gravity of these fels at time of se are listed in Table 1. The lower calorific vale in the table was obtained according to the well known eqation, H 1 H,-6(9h+MC)... ( 1) 1+MC where H 1 is the lower calorific vale, H" the higher calorific vale in the oven-dry condition, h the hydrogen content which was assmed to be 6% 8 ' in the present stdy, and MC the moistre content on a dry weight basis Hot-air heaters Two sorts of heaters were sed in the stdy. Both were manfactred for trial as hosehold appliances by Rocket Boiler, Inc. of Tschira, Japan, and Isolite Jki, Inc. of Osaka, Japan. These heaters were designed for combstion of Ogalite, bt felwoods other than Ogalite can be sed in them. They are shown in Figs. 1 and 2. Heater I in Fig. 1 is abot 68 em high, 81 em wide, and 38 em deep. To spply fel wood to the frnace the front lid, which has the hot-air otlet, mst be removed and then pt back in its place after igniting the fel wood. The frnace has a maximm capacity of six Ogalite briqettes. Dring combstion it is not dangeros to directly toch the otside of the heater de to inslation applied to the inside face of the frnace. Air for combstion is atomatically spplied by an electric fan, with a controlled rate, from the top of the frnace. The rate at Table 1. Fels sed in the stdy. Fels Ogalite I Shape at the se Abot 2 em long and 5,5 em diametrical octagonal colmn with abot 2 em diametrical empty cylinder at the center I MC*l I H,.*» H!*8 I r*~ (%) (kcal/kg) (kcal/kg) , 839*5 4, ~1.34) Sgi I Rectanglar parallelepiped, abot 3 X 3 X 3 em Konara I Logs or split ones, the approximate mean size, 3 X 3 em ,96*6 4,217 I.3~ , 562*6 3, ~) 1 MC is the moistre content based on oven-dry weight, 2 H,. the higher calorific vale at the oven-dry condition, 3 H! the lower calorific vale, *4 r the specific gravity in oven dry", 5 the vale was determined by means of Shimadz Bomb Calorimeter CA-3, and 6 the vales were cited from SATONAKA".

3 -23- which air is spplied can be changed with the rotation nmber of the fan, bt all the rns in this stdy were condcted at a constant rate of spply. The hot air is expelled from the heater with a constant velocity. Air in the room is introdced into the heat exchanger throgh an opening in the bottom of the heater, heated there, and sent back to the room as hot air by an electric fan. Heater II in Fig. 2 has a mechanism for atomatically spplying felwood into the frnace. CD --@ <D Blowed air inlet into frnace Hot-air otlet Heat Observation window Heat inslating grate (non-combstible) Felwood Air inlet Blower Smokestack Fig. 1. Schematic diagram of heater I. <D Smokestack Hot-air otlet Blowed air inlet into Heat inslating grate (non-combstible) Blower Ffillwood Gate for felwood spply Heat exchanger Felwood reservoir Fig. 2. Schematic diagram of heater II.

4 -24- Ogalite, whkh is piled in a line in the fel reservoir within the heater, is atomatically fed one-by-one to the frnace depending on the hot-air temperatre. Air for combstion is drown into the frnace from the top with an electric fan in a way similar to that of heater I. Air in the room is introdced throgh an opening nder the frnace, heated in the heat exchanger, and retrned to the room as hot air. one electric fan, and by two fans at its higher temperatre. throgh an ash door on the side face. At its lower temperatre, the hot air is expelled by The frnace can be cleaned Heater II is abot 78 em high. 85 em wide, and 46 em deep, and somewhat larger than heater I. Also heater II is not dangeros for toching directly with hands and so forth. Smooth ignition is necessary for good reprodcibility of the combstion of the felwoods. Therefore, bits of felwood, which absorbed kerosene with immersion, were placed on a felwood pile within the frnace and then ignited with matches. The amont of absorbed kerosene ranged from 1 to 3 g, depending on the type of fel wood Measrements Temperatre and velocity of hot air and thermal efficiency A wooden dct was closely inserted the heaters' hot-air otlet to minimize trblence in the hot-air flow for determination of its velocity. The inner sizes of the dcts are 15 em long, 7 em wide, and 9 em high for heater I, and 15 em long, 47 em wide, and 12 em high for heater II. A velocity distribtion was obtained by measring velocities on the centers of small sqares or rectangles which were given by dividing an opening section of a dct with a 4 or 5 em distance in both sides of the width and of the height. The velocities were measred with a hot-wire anemometer, the KANOMAX Anemomaster 1 (Nippon Kagak Kogyo, Inc., Osaka). From the velocity distribtion, average velocities of heater I and II were respectively calclated to be 1.7m/s. and.77m/s with one fan and 1.12m/s with two fans. rates were obtained from the opening areas of the dcts and these mean vales. Average hot-air flow Temperatres of hot air in the otlets and spplied air in the inlets of the heaters were measred with copper-constantan thermocoples. By integrating the difference between these two temperatres for the combstion dration, total heat obtained in the hot air was determined with the average flow rate, and the assmed vales of 1.16 J/(g- c) of the specific heat and of 1.93 g/l of the density of the hot air at 5 c of the temperatre. The total heat divided by the amont of the consmed fel is called the obtained heat here. The ratio of the ob tained heat against the lower calorific vale was calclated as the thermal efficiency Temperatres within the frnace and of the exhast To obtain information abot the combstion behaviors, the temperatre of the exhast and temperatre within the frnaces were measred by sing chromel-almel thermocoples. The temperatre of the exhast for both the heaters was measred at abot 1 em above the smokestack base in the center of the smokestack. The frnace temperatres of heater I and II were measred at an' pper or side portion within the frnaces, respectively missions of gases and smoke With the flow rate of abot ll/min, a part of the exhast was first introdced from the smokestack into a photometer, as shown in Fig. 3, and the concentration of the emitted smoke was determined as the optical density D, expressed as D =.l.log Io... ( 2) L I where L is the light path length in the photometer, I and I the illmination intensities before

5 ~.iji111: mm Unit -s. -I.(') N Fig. 3. Photometer for measring smoke density. CD Smoke otlet, Light receptor, Glass Heating wire, Heat inslating glass fiber, Lens, Incandescent lamp, Smoke inlet. and dring the combstion. This method was cited from the Japanese Indstrial Standard (JIS) A Frthermore, the exhast flow passing throgh the photometer was sent into a Shimadz Infrared Gas Analyzer URA-2 S to measre the concentrations of carbon monoxide and carbon dioxide. and then into a Shimadz Magnetic Oxygen Analyzer MAGNOS-2 to measre the concentration difference of oxygen between the exhast and the atmosphere, and was finally released into the atmosphere. On the other hand, with the flow rate of abot 3//min, a part of the exhast was first introdced from the smokestack into Fji lectric Infrared Gas Analyzer UL TRAMA T -S ZAL to measre the total amont of two concentrations of nitrogen monoxide and nitrogen dioxide, and the concentration of slfr dioxide, respectively. The exhast introdced into the Shimadz and Fji lectric systems was dehydrated by magnesim perchlorate before the measrements. It is difficlt to obtain absolte amonts of these gases from the measred concentration. The gas concentrations were measred nder the above constant flow rates while the flow rate of the actal exhast did not remain constant de to change in the combstion rate with time. In addition, nitrogen dioxide and slfr dioxide wold be somewhat condensed in the paths to the analyzers. However, vales of the concentrations integrated for the combstion times per nit weight of fel consmption were calclated after the manner of the previos stdies 1 >&>, and assmed to be proportional to the actal amonts of these gases. 3. Reslts and Discssion 3. 1 Temperatre of hot air The changes in the temperatre of the hot air generated from heater I and II are respectively shown by representative examples in Figs. 4 and 5. In heater I, in Fig. 4, the felwood was spplied only one time at the beginning of the rn, while in heater II, in Fig. 5, the felwood was spplied two or three times inclding one before the ignition. It is fond from comparison between Figs. 4 and 5 that heater I generated hot air more stable in temperatre than heater II. The fel capacity of the frnace of heater II is abot one fifth or sixth of that of heater I. It was necessary several times to spply fels to the frnace of heater II for sstained generation of hot air. Therefore, the temperatre of hot air from heater II changed greatly for every spply of the fels, as compared with heater I. It is seen also in these figres that the initial elevation rates of the hot-air temperatre

6 -26-6.::_ 5 <11 L :::J ~ 4 < <11 +- L ;n I +- I -- Ogalite - - Konara Sgi 3 Time (hr) Fig. 4. Temperatre of hot air from heater I. Consmptions of Ogalite: 6,285g, of Sgi: 5,239g, and of Konara: 6,6g. 7 ~ 6 <11 5 L :::J +- ~ 4 j!i ~ 3 L... ', '. ' \ \ ' '. ' ' Ogalite - - Konara Sgi +- I Time (hr) Fig. 5. Temperatre of hot air from heater II. Consmptions of Ogalite: 2,346g, of Sgi: 2,878g, and of Konara: 2,41g. of both the heaters is greatest for the combstion of sgi and smallest for Ogalite. Stable combstion of the fels was sally established after the hot-air temperatre rose over 37 c. Therefore, the initial combstion time ntil the temperatre of the hot air rose to 37 c was one of indicators for evalating the practicalities of the heaters. The mean vales of these initial combstion times to 37 c are listed in Table 2. From the table the initial heating properties of sgi and of Ogalite are fond to be the best and worst, respectively. Moreover, the initial heating property of heater II is seen to be better than that of heater I. The initial heating property may be interpreted as depending on the pyrolysis rates of the fels. The specific heat of oven-dry wood was fond to be practically independent of species and is expressed as J/g at o cn. The differences in specific heat among the present fels is easily estimated to be less than 1% by sing the specific heat of this vale and of water.

7 -27- Therefore, the heat capacity per nit volme of Ogalite is conjectred to be larger than those of other fels, and that of konara is probably larger than that of sgi from the specific gravity in Table 1. The larger this heat capacity of the fels, the later the rise in the temperatre, especially in the inner part of them. For the reason of large temperatre dependency, namely, many reported high vales of Arrhenis activation energy of the overall pyrolysis of wood, probably sgi was most easily pyrolyzed and Ogalite most slowly pyrolyzed of all the fels. This interpretation of the initial heating property as dependent on the pyrolysis rate, which was controlled not by the chemical property bt by the physical property of the fels, agrees with reslts by AxiTAs>. He has shown ignition time of woods to increase with the specific gravity Thermal efficiency, obtained heat, and temperatres within the frnace and of the ex hast The mean vales of the thermal efficiencies and of the obtained heats calclated from individal rns are listed in Table 2. It is fond from the table that the thermal efficiency of heater I differes slightly among the fels, while that of heater II shows the large difference of abot 12% between the largest vale for sgi and the smallest one for konara. On the other hand, for the obtained heat per nit weight of the consmed fel of both the heaters, sgi gave the largest vale and konara the smallest. These vales for thermal efficiency are gener ally larger than previosly reported for a qick heating hot-water boiler 1 > and smaller than reported for a hot-water storage boiler 1 >. The vales of obtained heat for the most part are larger than corresponding vales of the two boilers, probably becase of the lower moistre contents of the present fels, except Ogalite, compared with ones in the previos stdies. The mean vales of the maximm temperatre within the frnace of heater I are higher than 7 C for all the fels, while in heater II only one given by konara of the corresponding vales rose over 7 C, as shown in Table 2. In heater I sgi and Ogalite gave respectively the highest and the lowest mean vale for the maximm frnace temperatre, agreeing with previos reslts with hot-water boilersm>. The inslating material, Kaowool (prodced by Isolite Jki, Inc., Osaka), which is applied to the inside faces of the frnaces of the two heaters. can be sed continosly at temperatre p to l,l C and for a moment at 1,26 C. Hence, these vales for the maximm temperatre sggest good maintenance of the applied insla ting material. The mean vales of the maximm exhast temperatre of heater II are considerably higher than those of heater I, as shown in Table 2. The largest mean vale of the exhast Table 2. Mean vales of initial combstion time to 37"C of hot air, of thermal efficiency, of obtained heat, and of maximm temperatres within frnace and of exhast. ~ I II Mea men Ogalite I Konara I Sgi Ogalite I Konara I Sgi Time to 37 C (min) II Thermal efficiency (%) 43, ,6 39, ,7 Obtained heat (cal/ g) 2,32 2, 169 2,378 2,132 I, 578 2,483 Frnace temperatre ("C) xhast temperatre ("C)

8 -28- temperatre was given by konara in both heaters. been shown to be the strongest in previos stdies on hot-water boilers 1 >s>. The flaming combstion of konara has Also in the present stdy the highest mean vale of this temperatre for konara is considered to indicate the point of most active flaming combstion of all the fels ffects of exhast temperatre and fel amont on thermal efficiency and obtained heat It is interesting that the thermal efficiency of heater I slightly increased with the increase in the maximm exhast temperatre, while the efficiency of heater II decreased with the increase in the maximm exhast temperatre, as shown in Table 2. Frthermore, the maximm exhast temperatres of heater I range from 221 to 248 C, and those of heater II from 383 to 436 C. The exhast temperatre is interpreted as rising with the growth of the flaming zone. From the above observations the thermal efficiency of hater I seems mainly to depend pon the growth of the flaming zone, bt that of heater II seems mostly determined by heat loss throgh high temperatre exhast. xcessively active flaming combstion wold increase heat lost into the atmosphere, bt too small a flaming zone wold make the heat exchanger work nsatisfactorily. In order to check on this estimation, the relationships between the thermal efficiency and the maximm exhast temperatre as an indicator of degree of the flaming combstion are shown in Figs. 6-8 for heater I and Fig. 9 for heater II. For Ogalite, in Fig. 6, the thermal efficiency has an obvios tendency to increase with increases in the maximm exhast temperatre in the whole range stdied. For konara and sgi the efficiencies increased with the maximm exhast temperatre in its lower range, and in its higher range they decreased with the increase in this temperatre. The maxima of the efficiency are shown in the temperatre range 24 to 27 C, bt the maximm for konara seems to be presented in a little higher range than for sgi, from a comparison between Figs. 7 and 8. Concerning heater II in Fig. 9, althogh it may involve some ncertainty to derive the following observations from the small nmber of the plots, the maxima of the efficiencies are fond in the ranges 39 to 4 C for Ogalite and 37 to 38 C for sgi. However, sch a maximm for konara cannot be fond in the figre. The thermal efficiency is thoght to have been affected also by the flow rate of the exhast, in addition to the temperatre. As stated above, the pyrolysis rate and then the combstion rate may generally decrease with the increase in the specific gravity of the fels. The flow rate of the exhast properly increases with the pyrolysis rate. Therefore, the smaller the specific gravity, the larger the rate of heat loss with the atmosphere de to exhast reslts at the constant temperatre. This sggests that in fel of smaller specific gravity the minimm effect of the heat loss on the thermal efficiency shold be fond at the lower maximm exhast temperatre, as shown as maxima in Figs It is thoght from this sggestion that the probable maximm thermal efficiency for Ogalite in Fig. 6 is at a higher temperatre than for konara, which may be ot of the range stdied. Frthermore, the corresponding maximm for konara in Fig. 9 is estimated to lie between those for sgi and Ogalite. ths observed and estimeted optimm maximm exhast temperatres for the highest thermal efficiency are listed in Table 3. For heater I, which is not eqipped with an atomatic fel feeder, the rns were carried ot at three levels of the fel spply, namely, abot 2.4, 4.4, and 6.4 kg (5.4 kg for sgi), to investigate the effects of the amont of fel. The These levels correspond to abot one forth, one half, and two thirds of frnace capacity, respectively. The rns were sally completed with the hot-air temperatre was lowered nder 3 C. At all levels the thermal efficiency is

9 -29- so ro L J: f- ao ol ~--~ L ~- Maximm e,>xhast te,>mpe,>ratre,> ( c) Fig. 6. Dependency of thermal efficiency on exhast temperatre for combstion of Ogalite in heater L ~5 > c a, ro L a,.r: f-- ol ~ ~------~L ~~ zoo 25 3 Maximm e,>xhast te,>mpe,>ratre,> ( c) Fig. 7. Dependency of thermal efficiency on exhast temperatre for combstion of Konara in heater L

10 -21-6 > c <II $ "' ~ 4.r:. f- Fig. 8. Dependency of thermal efficiency on exhast temperatre for combstion of Sgi in heater I. Maximm exhast temperatre ( c), ~' I '\ I \, :, ' :' \,. : {;. ~.:.. Oga/ite Konara "Sgi (;-4 c <II ::j:: <II "' L <II.r:. 3 r- ol~----~ ~------~ Maximm exhast temperatre ( c) Fig. 9. Dependency of thermal efficiency on exhast temperatre for heater II.

11 -211- Table 3. Optimm conditions of combstion for thermal efficiency. ~ Heater I II Conditions ~ Ogalite I Konara I Sgi Ogalite I Konara I Maximm exhast temperatre ( C) Higher than to to to (38 to 39)? Amont of fel spply (kg) I Abov: I Abo! I Unde~ I -I -I Sgi 37 to 38 Table 4. Linear regression fnctions between thermal efficiency and fel consmption in heater I. Fel spply level (kg) I Y(%)*1 = a X(g)*9 + b(%) r*s ** , ,695* d Y is the thermal efficiency, *2 X the fel consmption, and *3 r the co-efficients of correlation, where the vales marked with ** and * are significant with the 1 and 5% levels of significance in the test of hypothesis, respectively. fond to increase with the consmption of the fels, as shown in Fig. 1, where this tendency is expressed with three lines by the linear regression. The regression fnctions are shown in Table 4. The statistical reslts in the table show that the dependence of the thermal efficiency on the fel consmption is large at the lower and higher levels of the fel spply, bt that at the middle level this dependence is small and cannot be jdged to be significant at the 1% level. It is noted for the design of a frnace that the dependence of the thermal efficiency on fel consmption changes with the amont of fel spplied. d 5 ~ >- c cv ::j:: cv <tl L cv f-- I o'l 2 Ogalite o Konara b. Sgi Consmption of f e I ( 9) 6 Fig. 1. Changes in fel-consmption dependency of thermal efficiency with three fel spply levels.

12 -212- The mean vales of the obtained heat per nit fel consmption were calclated for each spply level, and the changes in the mean vales with the amont of fel spplied are shown in Fig. 11. xcept for the highest fel spply level, the largest obtained heat and the smallest one were respectively given by sgi and konara. Frthermore, the optimm fel spply level for the largest obtained heat and then the highest thermal efficiency is sggested for each fel from the figre. It is noted that this optimm fel spply level rose with the specific gravity of the fel "" Iii 24 i 23 L " ~ 22.;;......Q 21 LI Ogalite o Konara t:,. Sgi -~ --~ ~ ~ "'.. ~ o ", a ~ e... \. v..., ~ ~ ~~--~--~~--~--~--~--~~ Consmot!on of fel ( ~) Fig. 11. Changes in obtained heat with fel spply amont. -'!... Ql L. :::J 3 Ogalite o Konara 6 Sgi A -ro ~ 25 A Ql... o... <ll eo A :::l Ill..c X Ql. 2 A X Ill eaa ~ ( ol..., A A A Consmption of fel (g) Fig. 12. Plots of fel consmption vs. maximm exhast temperatre in heater I.

13 -213- The maximm exhast temperatre generally rose with the fel amont, as shown in Fig. 12. The rise in the exhast temperatre not only raises the efficiency of the heat exchanger bt also increases the heat lost by the exhast. For felwood of the higher specific gravity, sch as Ogalite, the rise in the exhast temperatre may have more contribtion to heat ex change than to the heat loss in the stdied range. Bt for sgi of the lower specific gravity, if this rise is high, it may increase the heat loss rather than the heat exchange. Also the optimm fel spply amonts for the highest thermal efficiency are listed in Table 3, where the optimm combstion conditions are fond to differ considerably among the fels. Therefore, a hot-air heater mst be designed to sit the different optimm conditions of varios fel woods missions of gases and smoke Mean vales of the maximm concentrations of gases and smoke, and of the maximm differences in concentrations of oxygen between the exhast and the atmosphere are listed in Table 5. Frthermore, mean vales of the integrated concentrations per nit fel consmp tion are listed as the emissions of the gases and the smoke. and the oxygen consmption in the same table. For both the heaters the maximm concentration of carbon monoxide is the highest in the combstion of konara and the lowest in Ogalite, while the emission of this gas is the largest in sgi and the smallest in Ogalite, as shown in the table. In addition, heater II emitted more carbon monoxide than heater I. The emissions from the heaters are considerably smaller than the 2, to 8, ppm min g-1 from the previosly reported hot-water boilersml. The emission of carbon dioxide and the consmption of oxygen depend on the efficiency of the combstion. These two qantities shold increase with the growth of the flaming com bstion. From Table 5 the emission of carbon dioxide and the consmption of oxygen are fond to be larger in heater II than in heater I. For both the heaters the differences of the two qantities among the fels are small. Since air for combstion was sent into the frnaces Table 5. Mean vales of emissions of gases and smoke, and oxygen consmption. =- Heater I II Gas~ Ogalite Konara Sgi Ogalite Konara co Maximm (ppm)*1 4,371 6,465 5, ,876 16,581 Total (ppm min g-1) *» COs Maximm (%) Total(% min g-1).59.61,61.26,23 :~ Maximm (%) Total(% min g-1),95.97 o SOs Maximm (ppm) Total (ppm min g-1) , 56 NOx Maximm (ppm) Total (ppm min g-1) , Smoke Maximm (m-1) density Total (m- 1 min g-1).82.57,55, Sgi 14, o. 31 I For all the gases and the smoke, the maximm means the maximm concentration (for 2 the maxi mm concentration difference between the exhast and the atmosphere), and *2 the total means the emission or consmption per nit consmption of the fel.

14 -214- at constant rates, the combstion wold proceed at a constant rate in comparison with frnaces withot fans. Therefore, it is not thoght that the overall growth of the flaming combstion was very different among the fels. For the maximm concentration of slfr dioxide, konara gave the highest vales in both heaters. For the emission of this gas, however, sgi and Ogalite gave the largest vale in heater I and Heater II, respectively, as shown in Table 5. Slfr dioxide is a polltant in its own right, and also can combine with rain to form dilte slfros acid. The average slfr content of wood is.13% 9 >, and can be calclated to be less than two hndredths and one hndredth of those of coal 9 > and of crd oillol, respectively Therefore, the emission of slfr dioxide from wood is not thoght to have as important an environment impact as other fels. For smoke emission, Ogalite gave the largest vales in both heaters. In heater I. however, Ogalite gave the lowest vale of all the maximm concentrations, as shown in Table 5. This may be cased by Ogalite's relatively lowest combstion rate. The smoke emission in heater II is larger than in heater I, bt even the emission in heater II is considerably smaller than the above-mentioned boilers mission of nitrogen oxide Both the maximm concentration and emission for konara are remarkably greater than those for the other fels, as shown in Table 5. mission of nitrogen oxide (NOx) from combstion generally refers to nitrogen in the fel and in the air. Nitrogen oxide from the former sorce is called fel NOx and that from the latter is called thermal NOx. The emission of the thermal NOx increases with the temperatre of the combstion. The remarkably high emission of this gas from konara may be attribtable to the fel NOx. Nitrogen content of a tree differs considerably among the species 11 >-w. Most of the reported valesl1l-l&> are less than 1%. Oak has been reported by NIKITINm to be one of the woods with the highest nitrogen content, and the bark has been fond to contain twice nitrogen as mch as pine barkw. Ths, the present fel, konara with its bark, contains more nitrogen than the other fels. The emission of the thermal NOx is thoght to increase with the growth of the flaming combstion. For each fel, plots were made of the maximm concentration of nitrogen oxide against the maximm exhast temperatre, as shown in Figs These figres show that the maximm concentration of emitted nitrogen oxide generally increases with the rise in the maximm exhast temperatre. Regression lines between these two qantities were determined, as shown by the solid lines in the figres. The corresponding linear regression fnctions are given in Table 6. The emission of nitrogen oxide from konara has a prominently higher dependency on the exhast temperatre than those of the other fels, as shown in the table. The coefficients of correlation for konara and sgi are both highly significant, while that for Table 6. Linear regression fnctions between maximm concentration of NOx and maximm exhast temperatre. Fel Y(ppm)* 1 a X("C)*S + b(ppm) r Ogalite o. 14-2,5.468 Konara, ,74** Sgi o. 11-1,,699* d Y is the maximm concentration NOx and *2 X the maximm exhast temperatre.

15 c)4 z '+- c 3... rl! L.... a; 2 c 8.o ~7r~~--~~--~~--~~--~~--~----~--~~---- O Maximm exhast temperatre (oc) Fig. 13 Relationship between maximm concentration of NOx and maximm exhast temperatre for Ogalite in heater I. 15 "' "'- 1 z... c ~... c Ill ~ 5 ::l ol~~~~~ 1~ ~ ~ ~ Maximm exhast temperatre (oc) Fig. 14. Relationship between maximm concentration of NOx and maximm exhast temperatre for konara in heater I.

16 <> <>- ~ 3 c5 z..,_ 25 :;:... ~ c (I) 2 ::J :;j 15 ::: Maximm O>xhast to>mpo>ratro> Fig. 15. Relationship between maximm concentration of NOx and maximm exhast temperatre for sgi in heater I. ( c) Ogalite lies within the acceptance region, also seen in the same table. The relation between the maximm concentration of nitrogen oxide and the maximm exhast temperatre for Ogalite may more correctly be expressed by a polynomial regression crve rather than the straight line, as shown by a broken line in Fig. 13. The corresponding fnction is Y = (ppm) (ppm/"C) X-.7 4(ppmf"C 9 ) X 9 ( 3 ) where Y is the maximm concentration of nitrogen oxide in ppm and X the maximm exhast temperatre in c. The correlation coefficient for Ogalite was calclated to be small, probably for this reason. 4. Conclsions 1) Two hosehold hot-air heaters, which were manfactred for trial, yielded hot air at a considerably stable temperatre for a long time, by brning wood of sgi and konara, and Ogalite of wood briqettes. Therefore, both heaters are jdged to be highly practicable. For heater I the mean thermal efficieny is almost eqal among the fels, while for heater II that of sgi is the largest and that of konara the smallest. The initial rising rate of hotair temperatre is the largest in the combstion of sgi and the smallest in Ogalite. 2) For both the heaters, generally, the thermal efficiency increased with increases in the maximm exhast temperatre, de to increases in efficiency of the heat exchange. Too high an exhast temperatre, however, made the thermal efficiency lower becase of increased heat loss into the atmosphere. Frthermore, the optimm fel amont for obtaining the highest thermal efficiency of heater I was fond to be different among the fels sed. Therefore, from consideration of thermal efficiency, combstion conditions shold be different among fel woods. 3) For both heaters the emission of carbon monoxide from sgi is the largest and that

17 -217- from Ogalite the smallest. Bt the smoke emission from Ogalite is larger than those of sgi and konara, despite Ogalite having the lowest vale of the maximm concentration of all the fels. These two emissions, however, are considerably smaller than those for the previosly stdied hot water boilers. 4) Slfr dioxide was prodced from the combstion of the felwoods, bt the emission is not considered serios in comparison with that from coal and petrolem. The emission of nitrogen oxide from konara is remarkably greater than from the other fels in both heaters. This may be attribted to the high nitrogen content of konara. In general the maximm concentration of nitrogen oxide increased with the maximm exhast temperatre becase of the increase in the thermal NOx. 5) For the ftre a control system for combstion of felwood needs to be developed to obtain reasonable exhast temperatre, and conseqently high thermal efficiency and the sppression of the emission of nitrogen oxide. Frthermore, it is desirable to se sgi and konara in the initial heating for a rapid rise in hot-air temperatre, and then to se Ogalite for sstained heating with fewer fel-spply problems. Acknowledgement The athors wish to express their thanks to Rocket Boiler Inc. (Tschira, Ibaraki, Japan), Isolite Jki Inc. (Osaka), and Japan Heating Indstrial Association (Tokyo), for the spplied heaters. Cited References 1) HIRATA, T. and G. SG!URA : Bll. Forestry Forest Prods. Res. Inst., No. 322, 31, (1983) 2) HIRATA, T. and G. SGIURA : ibid., No. 322, 55, (1983) 3) M!GITA, N.: "Wood Chemistry", Vol. 1, d. by Migita. Yonezawa, and Kondo, Kyorits, Tokyo, p. 65, (1968) 4) "Handbook of Wood Indstry", d. by Forestry Forest Prods. Res. Inst., Marzen, p. 62 and 98, (1982) 5) SATONAKA, S. : Res. Bll. College xp. Forests, College of Agricltre, Hokkaido Univ., 22, 69, (1963) 6) HIRATA, T., Y. Fx1, S. Usm, and H. AB : Mokzai Gakkaishi, 27, 419, (1981) 7) STAMM, A. J.: "Wood and Celllose Science", Ronald Press, New York, p. 283, (1964) 8) AKITA, K. : Rept. Fire Res. Inst. Japan, 9, No. 1~2. (1959) 9) TILLMAN, D. A.: "Wood as an nergy Resorce", Academic Press, New York, p. 77, (1978) 1) "Handbook of Chemistry", Div. Application, d. by Chern. Soc. Japan, p. 516, (1973) 11) NIKITIN, N. I.: "The Chemistry of Celllose and Wood", Israel Program for Scientific Translation, Jersalem, p. 34, (1962) 12) FRUKAWA, T. : J. Japanese Forestry Soc., 45, 99, (1963) 13) KATo, Y., K. Do1, and T. FRUKAWA : ibid., 45, 191, (1963) 14) TILLMAN, D. A.: "Wood as an nergy Resorce", Academic Press, New York, p. 71, 73, and 84, (1978) 15) BTHL, J. S. et al.: "nergy from Wood" (Rept. to Office Technol. Assessment Congress U.S.), OTA-C , p. 246, (1979) 16) KARCHSKY, J. and P. KocH: U.S. Forest Serv. General Technical Rept., S-24, (1979)

18 林業試験場研究報告第 33 号 温風暖房機における木質燃料の燃焼特性 平田利美 ω 福井康夫惜 摘要 小型燃焼器具の改良, 開発 l 乙資するため, 本研究では既報の温水ボイラーの燃焼試験 iと続き, 温風暖房機における木質燃料の燃焼特性を求めた 使われた試作の 2 機種は, 木質燃料を用いるものとしては, 初めての本格的な温風暖房機である 河機種の主な相違は, 暖房機 I がオガライトを最高 6 本収容出来る大きな燃焼炉を有し, 燃料の中途補給が不可能なのに対し, 暖房機 はオガライトを 1 本収容可能な炉を有し, 温風温度に応じてオガライトを 1 本ずつ自動供給する機構を備えている点にある 燃料として, スギ, コナラ, およびオガライトを用いた 暖房機 の温風温度は暖房機 I より変動するが, 実用性を損なうほどではない しかし, 温風の初期昇温速度は暖房機 の方が大きい 暖房機 は熱効率では暖房機 I より小さく, 煙と一酸化炭素の発生量は暖房機 I より多いが, 既報の温水ボイラーよりは少ない 両機種に共通した各燃料の特性を示すと次のとおりである スギでは温風の初期昇温速度は大きいが, 一酸化炭素発生量も多い オガライトでは燃焼速度が小さいため, 温風の初期昇温速度は小さいが, 長時間暖房 lζ 適している コナラの燃焼特性は, スギとオガライトの特性の中聞に相当するが, 酸化窒素発生量が著しく多い 一般 1<:. 熱効率は排気温度が高くなるにつれて大きくなるが, 排気温度が高過ぎると, 大気への放出熱のため減少する 酸化窒素は排気温度に比例して多く発生する 以上の ζ とから, 排気温度によって燃焼を制御する機構の開発が望まれる 1984 年 4 月 12 日受理 (2) 木材利用部