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ELAINE T. HOWARD

ABSTRACT. As measured with an oxygen-bomb calorimeter, heats of combustionaveraged about 8,600 Btu/ovendry lb. for stemwood, rootwood, earlywood, and late-wood. In stemwood, 54 percent of the variation was related to extractive content. Valuesfor bark were greatest in samples from the base of the tree and decreased with in-creasing height in tree. Values for resinous wood varied directly with extractive con-tent. Needles and mixed bark had higher heats of combustion than stemwood. Sprucepine bark produced less heat than mixed bark from the other southern pine species.Samples from tops had lower values than other woody portions; of aU tree parts, oldcones ranked lowest. Dried kraft black liquor averaged only 5,965 Btu/ovendry lb.

heat obtainable from ovendry material, with nodeduction or losses. The heat obtainedby buming in open systems s considerablyess, as somescapesn the form of steamand other gases.Literature Review

WHEN A SOUTHERN PINE TREE is harvested,only about 60 percent by weight is put toimmediately useful human purposes. Stump,roots, and top are eft in the woods o decay,andbark creates major disposalproblem at mills re-ceiving the logs and bolts. Spent black liquorfrom the manufacture f kraft pulp is rich in un-utilized lignin. Thesematerialshave somevalueas uel, and arge-scale se or this purposewouldconserve ther energysources.This paper reports a study of the heats ofcombustionof various southern pine materials.The chief interest was in bark, rootwood, stem-wood, bark and wood from tops, and dried kraftblack liquor. In addition, ~ples of cones,needles, nots, resinous tumpwood,and charcoalwere tested. The information provides a basisfor comparing heat values of various southernpine materials and should be helpful in deter-mining their worth for fuel as comparedo otherpossibleuses.Heats of combustion were determined byoxygen-bombcalorimetry. In this procedure,aweighed sample s burned in an oxygen-filledmetal bomb submergedn a measured uantity ofwater, all held within an insulateddtamber. Byobserving he rise in water temperaturerom theexplosivecombustion f the sample, he numberof heat units liberated may be calculued. The

Heat of combustionvaries considerablywitchemical ontent. Resin, annins, ignin, terpen~and waxes have high heat values; carbohydrateare relatively ow in energy. Brown elal. (1952:state hat hardwoodscontain 8,300 to 8,700 Btu,ovendry b., whereassoftwoods ange rom 9,001to 9,700. The higher value for conifers is altributed to their greater esin and lignin content.The caloric value of wood is negativelyCOIrelated with moisture content. Kollmann anCote (1968, p. 150) give values of 8,100 Btulb. for ovendry wood, 6,840 or air-dry wood, an5,940 for wood at 30 percent moisture contenParr and Davidson (1922) reported 8,836 Btulb. for ovendry pine, and 8,050 for sampleswit8.88 percent moisture.Severalauthors have tested various portio[of southern pine trees. Otang and Mitche

The author is Associate Research Chemist, USDForest Service, Southern Forest Experiment St:tion, Pineville, La. This paper was received f(

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(1955) found that slashpine bark (at 6.4 percentmoisture) produced 9,002 Btu/lb. Madgwick(1970), working with Virginia pine, reported9,035 Btu/ovendry lb. for mature leaves,8,875for wood from live brandiesof 17-year-oldrees,8,695 for live branchwood rom trees 18 to 59years old, 8,530 for bole wood, 8,425 for bole~ and 9,018 for male flowers e.t pollen shed.Hough (1969) measured ,365 Btu for ovendrysandpine twigs, and found that needles veraged8,900 Btu for loblolly pine, 9,615 for sandpine,and 9,370 for slashpine.

MethodsSamples

Roots. A composite rom one tree of ead1 pe-cies, variety, and race. Sampleswere fromlarge lateral roots near the baseof the tree.Top. - Composite San1ple f wood and barkfrom the l-inm top of one tree of eachspecies, ariety, and rare.Needles. Compositesample rom one tree ofeach of the four major species. Sampleswere from the lower, middle, and uppercrown.Cones. Compositesampleof old, open conesfrom ead1of the four major species.SINmpwDod. Samples rom old-growth, resin-ous, ongleaf pines, separatednto three spe-cific gravity classes.Knols. - Resinous,decay-resistantnots pickedfrom the forest floor in a longieaf stand.Resin. - Liquid San1plesollected from mech-anically njured trees.Charcoal. Commercial riquettes roducedromsouthernpine bark and wood residues.Krafl black lif/Nor. - Dried residue of samplescollectedon six consecutive aysafter tall oilhad been removed.All samplesexcept stumpwood,knots, andresin were ground to passa 6O-mesh creen, ndovendried. The extremely resinous stumpwoodand knots could not be screened fter grinding.To minimize loss of volatiles, resin, stumpwood,and knotwood were dried at room temperatureover multiple mangesof desiccant.

CalorimetryHeats of combustionwere determinedwitha Parr 1241 adiabatic oxygen- bomb calorimeterwith automatic jacket controls. The manufac-turer's recommendationsor operation were fol-lowed. Approximately 0.5 g of sample wasplaced in an open combustioncapsule,spreyedlightly with a fine water mist, and placed n thebomb. The bomb was dJarged with 20 atmo-spheres of oxygen. Initial temperature afterstabilization and final tem~ after ignitionwere recorded. IntenOf' surfaceswere rinsed, hewashingswere titrated with sodium carbonate 0-lution for nitric acid correction,and the remain-ing fuse wire was measured.Grossheat of combustion,H., in oo./g wascomputedby the equation:

(t, - ta) W - el - e.H. =

Loblolly pine stemwood and spmce pine barkwere sampled in some detail. Most other ~pleswere compositesof equal weights of material fromeither the four major southern pines (loblolly,slash, longieaf, and shortleaf) or from all 12species, arieties, and races (Sternitzke and Nelson1970).Loblolly pine. - Stemwood samples rom 50 ma-ture trees were factorially divided as follows:Specific gravity: 0.49Growth rate: 6 rings/in.Rings from the pith: 0-10, 11-20, and

21-30Exact values for each of these variables, anddata on percentage of latewood and chemicalcomposition, had been determined previouslyfor each sample.Earlywooa' nd atewooa. A compositeplefrom one tree of each species, variety, andrace for each of the two tissue types. Early-wood and latewood were dissected fromwedges taken at three heights along thestem: 1 foot above ground, one-third treeheight, and two-thirds tree height.Sprllce pine bark. - Inner and outer bark from72 trees throughout the major range of thespecies. Variables were:Geographic location: 4 areasAge of trees: 15, 30, or 45 yearsGrowth rate: 6 rings/in. 1 foot

above groundHeight: 1 foot above ground, at a 4-inch top, and half-~y betweenthese levels.lfixea' bar~. - A composite sample for each ofthree hei~ts from 11 southern pines ( allspeciesxcept pruce ine)

mt. = temperaturet timeof firing, correctedfor thermometer scale error, of.

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in the proportions of other d1emicalconstituentshad only minor effects. A negative correlationwith number of rings from the pith reflects thedistribution of extractives n the stem:Alcohol-benzeneextractivesRings frompith Heat ofcombustion

(Btu/oveodry lb.)8,7958,5658,445

(%)9.64.83.8

0-1011-2021-30

t, = final maximum temperature, correctedfor thennometer scale error, of.W = energy equivalent of the calorimeter.calories per of.el = correction. in calories, for heat of for-mation of nitric acid. = m1 of 0.0725N alkali.e. = correction, in calories, for heat of com-bustion of fuse wire.m =OVendry eightof sample, .

Multiplication of wories per gram by 1.8 givesBtu/lb.Despite repeated efforts to eliminate theproblem.a very small carbon esidue emainedaf-tel combustion f most samples. Because f thisdifficulty, values may be slightly higher thanreported,but the differencewould not be greaterthen 50 Btu in the worst samples ccepted.Results

Averageheatof comOOstionor loblolly pinestemwoodwas 8,600 Btutovendry lb. Caloricvalues were regressed gainstdata on percentoflatewood,extractivecontent,growth rate, specificgravity (unextraded and extracted), and contentof holocellulose, alpha-cellulose,hemicellulose,and lignin. Growth rate, specific gravity, andpercentof latewoodproved nonsignifiant at the0.05 level. Extractivecontentwas positively cor-relatedwith heatof combustion nd accountedor54 percentof the variation (Fig. 1). Variation

y . .'00 + 50.9 Xr2: 0.54St= I~O B.T.U.~.S?;~~~\;)~~

Heatsof combustion or earlywood,atewood,and rootwood were not significantly different -all avera&ed bout 8,600 Btu/lb. Samples romthe I-jnd1 top (wood and bark) avera&ed ig-nifICantly ess 8,395 Btu (Table 1).Heat value of sprucepine bark was greatestin samples rom the baseof the tree. At greaterheights up the stem the outer bark becomeshin-ner and the inner bark - whim probablyhas essheat value thus makesup proportionatelymoreof the sample. Geographic egion, growth rate,age of tree, and bark specificgravity had no sig-nificant effect.Heat of comOOstion f the mixed bark sam-ples was greater than for sprucepine bark andalso was negatively correlatedwith height aboveground. The values are somewhat ower thanO1ang and Mitdtelrs (1955) results for slasbpine bark, but are higher than those reported byMadgwick (1970) for Virginia pine bark.Liquid resin had the highesthe8t of combus.tion, 14,625 to 16,250 Btu/lb. Variation invalueswas arge, as the sampleswere difficult tcmix thoroughly and thereforenot homogenous.Resinoos stumpwood had the followinEvalues/OVeDdryb.:

Alcohol-benzeneextrac-tives

(%)26.931.335...2 I I I I I I I I0 2 4 6 8 10 12 14EXTRACTIVES PERCENT)FI.-" 1. - ~ M:'- ~ .. C8IIIiIUItIonof oYeftdry loblolly pine sl8tllwood ~ alcohol-benzeneextradlw contetlt.

Sp. gr.(ovendry Heat of Coeff.wt. and vol.) combustion of var.

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