A. T. WileyMesquite In tree tOrln.

F. G. Manwiller

Abstract

The heat of combustion of mesquite heartwood, sapwood,bark, and a mixture of stems and leaves was tested. Thevalues averaged 8,657, 8,021, 7,836, and 8,123 Btu per ODpound, respectively. If an industrial plant requiring 5O,{XX)pounds of steam per hour were located in an area averaging25 green tons of mesquite fiber per acre, a harvest radius ofabout 3 miles would be required to sustain the plant over aperiod of 10 years. Based on the costs of owning andoperating a 20,000 lb./hr. steam generator, the dollar value ofpreprocessed mesquite to a potential user was estimated to be$16.35 per ton (~ MC), or about $390 per acre of heavilyinfested land.

for rotating fuel crops. However, there occurs inportions of southwestern United States, a tree-mes-quite (Prosopis sp.)-which might overcome this disad-vantage.

Mesquite grows in two basic forms, a single-stemmed tree and a many-stemmed tree or brush. Ithas been estimated that in Texas alone, there areabout 34 million acres containing 10 to 40 tons peracre of green mesquite fiber (Stayton 1973). These treesare choking out forage crops and drastically reducingcattle production; thus, it is possible that the fibercould be obtained "free," with the cost of harvestingshared by landowner and a contract landclearer. Costincreases (over that of normal landclearingoperations) would be incurred in gathering, hogging,and transporting the fuel. The major economicquestion is whether the sale of mesquite fiber couldoffset these costs and yield a profit.

The purpose of this paper is to provide rough dataas to the market value of mesquite wood as a fuel. Todo this, the heat of combustion had first to bedetermined. It was then necessary to develop a methodby which a price could be determined at which woodcould sell competitively with other fuels. It has becomecommon practice to use the price per ton at which theprice per recovered Btu is the same as with thecheapest alternative fuel (Koch 1972, Anonymous1975a, Anonymous 1975b). It is widely recognized thatthis method does not account for the relatively highcost of owning and operating a wood-burning steamgenerator; however, the literature does not include a

WITH EACH INCREASE in the cost of fossil fuelsthere is a corresJKInding increase in the value of woodas a fuel. In 1972, Koch showed that at 32 cents perMft. 3 for natural gas, $2.45 per ton could be paid forgreen southern pine bark (10<JM> MC)I to obtain thesame number of Btu's in the form of steam. Today, at80 cents per Mft.3, the equivalent price of bark wouldbe $5.90 per ton, and in certain areas of Texas, theprice of natural gas may be as high as $2.00 per Mft.'.Since fossil fuel shortages wi11likely cause the price ofthese fuels to increase faster than inflation, andallocations may make them impossible to obtain, it isnot unreasonable to presume that wood fuel couldsomeday become a marketable commodity.

Previous authors have been pessimistic about theJKlssibility of harvesting wood for fuel purposes(Grantham and Ellis 1974). The primary objection isthe enormous acreage that would have to be set aside The authors are, res.--pectively, Wood Technolog!st, Texas

Forest Service, Texas Forest Prod. Lab.I.Lufkin, Tex.}, andResearch Scientist, So. Forest Expt. Sta., uSDA l"orest oerv.,Pineville, La. This paper was received for publication inOctober 1975.I All moisture contents are on an ovendry basis.

SEPTEMBER 197648

concise method of doing so. This paper presents such amethod, applied to mesquite; however, the method maybe used with other species and applied to industrialplants which produce fuelwood as a byproduct ofnormal plant operations.

Analysis of the expense of preparation of the fuelmust be left to others. An excellent discussion ofpresent methods of mechanical landclearing has beengiven by Fisher, Wiedeman, Meadors, and Brock(1973) and Meadors, Fisher, Haas, and Hoffman(1973).

The bRIStly tom! of ~

T8bIe 1. - HEAT OF COMBUSTION OF MESQUITE.

~..T-. A R8Ig8

~rtwood I- I onS~ 1,81 7,107-8,23588ft 7... 7,043-8,178Mixed atenla 8Id ~ 1,121 7,141-8.222

calculations are carried out a88uming a heat ofcombustion of 8,120 Btu/OD lb. To determine theeffective heating value of wood (Btu's recovered in theform of steam), deductions must be made for heat1088e8. Acrording to Reineke (1961), if the flue gaaesare 888umed to be at 4(X)°F, the losaee are 1,210 Btuper pound of water in the fuel, and 1,350 Btu/OD lb. ofwood for heat losses associated with ~ing thetemperature of flue gases and with water formed incombU8tion of hydrogen. In addition, about 1 percentof the total heat input may be assumed to be lost 88radiation in a 5O,<XX> lb./hr. boiler, and 1.5 percent ofthe total heat input may be assumed to go tounaccounted-for loues (de Lorenzi 1957). The expectedefficiency of meequite 88 a fuel at various MCs, e- isplotted in Figure 1. EfficienciM of southern pine bark(8,900 Btu/OD lb.) and stem wood (8,600 Btu/OD lb.)are also given for comparison. Efficiency multiplied byheat of combustion givM the effective heating valueper OD pound. At 60 percent and 15 percent MC, theeffective heating values of meequite are 5,840 and6,380 Btu/OD lb., respectively. This represents anincrease of 8 percent, for which it might be worthwhileto allow fallen mesquite to dry in the field beforehogging; however, it is doubtful that the averagelandowner would allow his land to be tied up for thelength of time required to reach this MC.

ExperimentalThe first step in determining the dollar value of

mesquite fuelwood was to determine its heat ofcombustion. The species selected is that most commonto west Texas-honey mesquite (P. juliflora). Since thewood has long been known by outdoorsmen to producean exceptionally hot flame, it was hoped that the heatof combustion would be very high. However, theheartwood proved normal for a hardwood, while theother tissues tested were somewhat low in heatingvalue.

Methods and EquipmentHeat of combustion was determined separately for

bark, sapwood, heartwood, and a mixture of stems andleaves. Samples of bark, sapwood, and heartwood weretaken from boards cut from several logs collected inBrown County near Brownwood, Texas. The sampleswere ground in a Wiley mill and screened to 40 meshand below. The particles from each sample werethoroughly mixed to form a representative sample.

To obtain a representative sample of mixed stemsand leaves, five stems were cut from 8pecimens ofmanY-8temmed mesquite trees growing in BexarCounty near San Antonio, Texas. The basal diametersof the branches ranged from about 1 to 4 inches. Thebranches with leaves were 8hredded, and the shredsthoroughly mixed. From the mix, 8everal randomsamples were taken and further reduced in a Wileymill and screened to 40 mesh and below.

Measurements of heat of combustion were takenwith a Parr adiabatic calorimeter with automaticjacket controls. The samples were sprayed with a imemist of water to 8low down combustion and preventportion8 of the specimen from being blown out of thespecimen dish. Nine measurements were taken of eachtissue type.

ResultsThe heats of combustion of the various tissues are

given in Table 1. The heat of combu8tion of mesquiteheartwood is nearly the same as that of loblolly pinestemwood-.,.8,600 Btu per ovendry (OD) pound (Koch1972). However, due to the relatively high density ofmesquite-« lb./it. ]-the heating value, per unit ofvol~e. of mesquite heartwood is about 1.4 times thatof pine.

Economic A8pectsSince branch wood is the form of mesquite which is

most likely to find use as fuel, the following

FOREST PRODUCTS JOURNAL 49Vol ~. No. .

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H. and H. are the heats of combustion of thealternative fuel and wood, respectively, and e. and e-are the efficiencies of the alternative fuel, and wood atzero percent MC, respectively. Typical values for e. aregiven in Table 2. The factor k in Equation 3, if verysmall, can cause p. to be very large; therefore, anupper limit must be sought. A reasonable limit wouldbe the price per ton at which the cost per recovered Btuis the same 88 that for the alternative boiler. This maybe calculated 88:

The price of wood fuel is calculated as follows970

u.'".778(I,(xx)=1.247 it.) lIb. Iteam

f.= 1.247(20,000)(6,000)(.00 19 )=$284,(XX)1 yr,

f.--9 ,6OOt284 ,000-34 ,900=$258,7001 yr.

~u- - 8,~ (~)=.147 lb. OD wood/lb. steam

p.= .~~(~~~'6iX»(I) = $29.36

p..=1.247(.~~!)(2.000) = $32.23/0D ton.147 .

Since p. does not exceed PwI. we use p. as tJ1e priceof wood fuel. At 60 percent MC, tJ1e priceis $29.36(.72)/(.808(1 +.60) )=$16.35/ton and the valueper acre of mesquite is $16.35 (15 OD ton/acre)( 1.60)=$3901 acre.

A note should be made regarding tJ1e flexibility ofthe method. Different situations are accounted for bymanipulating the figures in tJ1e cost columns. Forexample, if a plant already owns a gas-burning systemand is considering converting to a wood-burningsystem, the annual payment and tax credit figures arezero for the gas system. Based on cost data obtainedfrom the same sources as those above, tJ1e price whichcould be paid for mesquite to fire a 00,000 lb./hr.system would be $28.77 IOD ton.

~e...(l+m)

where e.a is the efficiency at the MC under questionand m is the MC, expressed 88 a fraction.

If market conditions have fixed the price of woodfuel, then the annual OD tonnage t, of fuel which mustbe produced 88 a byproduct to keep costs in line withthose incurred in owning and operating an alternativeboiler is given by:

t = UwBh - 1.- [7]p.

Here, p. is the market price of OD fuelwood.Example: Assume that a 20,000 lb./hr. wood-burningsystem has been installed which is run at full capacityfor 6,000 hours per year. The least expensive alter-native was a gas-burning system. The cost of naturalgas is $l.90/Mft. 3 All fuelwood is to be purchased.

What price should be paid for fuelwood?Solution: The estimated coat figures are as follows:l

~ ~

ConclusionThe market value of mesquite, calculated as $390

per acre, is somewhat tenuous as it is highlydependent upon the volume per acre of mesquite andthe price of natural gas, which appears quite unstable.However, it is substantial enough to indicate thatmesquite is an energy resource which should not beoverlooked. Any eventual use will be dependent upondevelopments of economic harvesting and fuelpreparation methods. It must also be kept in mind thatfuel allocations may soon drive the value of fuel that isreadily available far above that which can be derivedfrom simple cost comparisons.

Total cost ofsystem installed $260,000 S60.(MX)

4,6002O,(XX)

26,<XX>500

lO,<XX>

6,(XX)100

3,800

2,600 800

-9,600 -2,200

-12,(XX) -2,900

--

Interest (1~ on 100yr. loan)Depreciation (average over

100yr. period)InsuranceEl~tricityMaintenance (average over

10-year. period)Tax creditsInterest (48% of averageannual interest payment)

Depreciation (48% of averageannual depreciation)

Investment (10% of investmentspread over 10-yr. period) -2.600

Subtotal 34.~Fuel 258.700

-600

9,600284,000

$293.600 $293.600Total

Uterature CitedANoNYMous. 1975&. Fuel value caJculator. USDA FOr8t Service, BE.

Area, State and Private Forestry, PineviDe, La.ANONYMOUB. 1975b. How to compare fuel val_. North Central

Forat Expt. Sta., U.S. Dept. A.n. St. Paul, Minn.FISHER. C. E., H. T. WIEDEMANN, E. H. MKADO_, aDd oJ. H. B8ocL

1973. Mechanical amllol of ~lJite. 1" y.q1Jit.-Cr(;w'.h aoddevelopment, management, ~nOlDiC8, _1101, -. TUa8 AIri.Expt. Sta., Re8ean:h Mon08T8ph No.1. TUa8 A. .. M. Univ. pp. 46-52.

GRANTHAM, JOHN B., and THOMAS H. EI.l.I8. 1974. Potential. of woodfor producing energy. J. of Foreatry. September p. 562.

KOCH, PETER. 1972. Utiliation of the M»uth.n ~_. U.s. Dept. Acri.,AIri. Handbook 43). p. 1385.

DE l.oUNZi, Orro. 1~7. ComOO8tion Rft,i-"D8. ComblJ8tion~neering, Inc., New York, N.Y. Chap. ~,p. 23.

M&ADO_, C. H., C. E. FI8HKB, R H. HAAS, and G. O. HoFFMAN. 1973.Combinations of methods of maintenance control of m~lJite. 1"~uite-crowth and development, maD88'emelJt, KODomica,0001101, -- T- Acri. Expt. eta., ~ MoDQ8raph No.1.T- A. .. M. Univ. pp. 53-59.

R&IN&X&, L. H. 1~1. WoCMi fuel combaation practjce. USDA FOr8tService, Forest Prod. Lab., Madi8On, Wia. Report No. 1666-18.

STAYTON, C. L. 1973. Saltcedar and m~lJite for wood pulp, boardproduct., animal nutrition and loil amendment-a propoeal forimproving range and watershed environment.. Texaa A.n. Ext.Service, Texas A. .. M. Univ.

~e a.t fjpre8 u8ed are a compilation of data obtained from onemanllfacturer and two eqineerinc firms.

FOREST PRODUCTS JOURNAL 51Vol... No.

Both p. and p.. may be adjusted to various MCs bymultiplying by