Reprinted from FOREST INDUSTRIES, vol\BDe 98, number 4, pages 36.37 April 1971

Bark now burned as waste could help supply expanding energy require-ments; technology and economics indicate the feasibility of burningsouthern pine bark to generate steam in electric utility plants

By PETER KOCH and J.F. MULLEN

approximately the percentages (byweight) indicated: hydrogen 5.5, car-bon 56.5, nitrogen 0.4, oxygen 37.0,and ash 0.6. This same ovendry barkconsisted of 66% volatile matter, 0.6%ash, and 33.4% fixed carbon.

Southern pine bark contains no sul-fur. The absence of sulfur compoundsin the stack gases of a boiler would bean advantage in areas under stringentpollution regulations.

The bark typically contains con-siderable moisture. In living trees themoisture content averages close to70% of the dry weight of the bark.Storage of logs and bolts under watersprays, or wetting of piles by rain,probably raises this value. For presentpurposes, it can be assumed that barkwill have 100% moisture content. Thatis, a pound of wet bark may be halfwater and half dry matter.

Other tests at Pineville showed thatsouthern pine bark has a heat of com-bustion of about 8.900 British thermalunits (btu) per ovendry pound, orabout 3*% more than the heat con-tent of stemwood (about 8,600 btuper ovendry pound).

If the bark averages half water bywe~t, the heat content of a }>Qund isonly 4,450 bro. Since energy is lost inevaporating the water, changing thehydrogen content of bark to waterduring combustion, and discharginghot flue gases to the atmosphere, theusable heat from a pound of wet barkis at best about 70% of 4,450 btu, or3,115 bro.

BARK FROM SOUTHERN PINE TREEShas at present little economic value;huge quantities are burned in waste in.cinerators without thought of recover.ing energy. Annual consumption ofsouthern pine wood is expected to riseto 7 billion cubic feet or more by theyear 2000; accompanying this will beat least 20 million tons of bark if thebark is half water by weight (10 mil-lion tons ovendry). Because of nation-al concern over air pollution, it is like-ly that the practice of incineratingbark will be curtailed or discontinued.

Bark finds its way into a number ofindustrial products and these uses willincrease, but probably not very fast.By present lights, the most promising-way of disposing OJ large volumes ofbark is to make fuel of it. Some wood.using plants currently burn bark togenerate their own electricity, but theequipment investment is so heavy thatall but the biggest firms find it cheaperto purchase power. Large electricalgenerating plants, with their huge andreasonably constant demands for fuel,may be able to burn bark econom-ically.

Through a literature review andsome laboratory work, the SouthernForest Experiment Station at Pineville,La., has assembled limited data on thefeasibility of burning southern pinebark to generate steam in utilityplants.

Bark analysisAnalysis of a small sample from the

four major southern pine speciesshowed that ovendry bark is com-prised of the following constituents in

Technology of burning ~Technology is available to build

bark-fueled plants to generate electricpower, and several are operating suc-cessfully. Industrial boilers fueled withsouthern pine bark have been designedto produce predicted steam flows in

Dr. Koch is chief wood scientist. SouthernForest Experiment Station, USDA ForestService, Pinevllle, La. J.F. Mullen i. en en-gineer with Com~u.tion Engineering Inc.,Windsor, Conn.

the range of 50,000 to 1,000,000 lbs/hr. Gauge pressure and temperaturevary from 150 psi at 5200 F. to 1,335psi at 9580 F. or higher. For the lar.units, fuel consumption may be 200tons of wet bark per hour. If properlydesigned, these steam generators re-quire little manpower for operationand do not contaminate atmosphere,land, or water.

For efficient burning, the bark is re-duced to particles of the smallest sizepractical-preferably Y4 inch or small.er-and metered into the furnace onhigh-pressure air steams. The bark isinjected at least 8 feet-and prefer-ably 16 feet-above tl1e grate, so thatmost of it is burned before it reachesthe grate. The system is analogous tosuspension fIring of pulverized coal.

If moist bark (half water by weight)has a heat of combustion of8,900/2 =4,450 btu/lb, and if the boiler is de-signed to add 1,100 btu/lb to thewater and convert it to high.pressure,high-temperature steam. then the effi.ciency should be about 70%. Underthese conditions, about 2.83 lbs ofsteam can be generated from eachpound of moist bark, i.e., (4,450)(0.70) + 1,100. About 10,000 kw ofelectrical power can be generated from100,000 lbs/hr of such steam, and toproduce that amount of stearn wouldrequire nearly 20 tons of moist barkper hour.

Fly ash, the fme particles of amcarried out of the stack by flue gases,may contain highly visible particulatecharcoal or char in addition to less visi-ble mineral ash. A typical sieve analy-sis of char carried by flue gas enteringtl1e dust collector of a boiler fired withsouthern pine bark is shown in thetable.

This charcoal results from incom-plete combustion. Because southern

FOREST INDUSTRIES/April 1971~

Burner Msembty for simultaneous orsolo inj~ion of bark and oil (or gas).Hot air is introduced in a jet aroundeach fuel jet. Fuel nozzles (withaccompanying air jets) can be tiltedvertically to alter trajectory for desireddistribution of fuel in the furnace.Normally: one burner assembly is lo-cated at each of four corners of rec-tangular furnace. (Drawing from Com-bustion Engineering Inc.)

pine bark has about 10 to 13% lessvolatile matter than wood, it alsotends to have a correspondingly great-er fIXed carbon content and hence toproduce more char.

If furnace temperatures are high,fuel particles are small, oxygen supplyis ample, and overflfe air is turbulent,the amount of char leaving the com.bustion chamber will be held to a min-imum. What does enter the flue gasescan be cleaned by mechanical or elec-trostatic devices. Either will do a goodjob; a combination of the two types ismost effective, though most costly.

Economics of bark-fired power plMtsFossil fuels, with which bark must

compete, can be burned more effi-ciently than bark and have higherheats of combustion, but cost moreper ton (see table). To determine thedelivered price at which bark is com-petitive with the fossil fuels, the fuelcost to generate 1,000 lbs of steam canbe calculated from the data in table byassuming that 1,100 btu (usable) arerequired to generate I lb of steam. Adelivered price of about $2 per greenton of bark (half water) appears to becompetitive. To be directly equivalentin price to coal, green bark would haveto sell for $1.96 per green ton. Theequivalent to oil would be $2.18, andto gas $2.45.

Since bark-burning furnaces arelarger than oil. or gas-fired units, theycost more to build; because of materi-al-handling equipment, coal-fired boil-ers cost more than oil. fired uni~. Theapproximate relative constructioncosts (installed) are subject to consid-erable variation according to steamgenerating capacity and manner ofassembly, i.e., whether shop-assembledor field-erected.

comparable to that now used to con-vey pulp chips from sawmills to pulpmills. Haul distances would necessarilybe short.

While revenue to the mill operatorfrom sale of bark would be small, theuse of bark as a fuel for utility plantswould solve the disposal problem with-out contributing greatly to air pollu-tion; moreover, such use would pro-mote conservation of fossil fuel. The20 million tons of green southern pinebark expected to accrue annually bythe year 2000 would be sufficient tofuel 12 generating plants, each with acapacity ()f 1 million lbs/hr of steam(1,000° P.). In total, these 12 plantscould supply the present electrical de-mands of the cities of Savannah, Mo-bUe, and Pensacola. 8

A bark price of $2 per green ton(delivered into the fuel pile) permitsonly minimal transportation charges.Bark from pine-using mills wouldprobably be conveyed into open-topfreight cars or motor vans and deliv-ered to power generating plants forbulk discharge into outdoor fuel piJes.The delivery system would be closely

Byproducts from b.-k combustionTheoretically, about 730 Ibs of air

are required to burn 225 Ibs of moistbark. This quantity of bark will yield 1million btu of heat. The resulting fluegas will be comprised of nitrogen, car-bon dioxide, and water vapor.

If the inplt (no excess air) com-prises 225 Ibs of fuel, 730 Ibs of dryair, and 9.5 Ibs of water vapor in thecombustion air for a total of about964 Ibs, the output should be heatplus 561 Ibs of nitrogen, 233 Ibs ofcarbon dioxide, 169 Ibs of watervapor, and 0.7 Ib of ash.

When analyzed according to Desig-nation 271-58 of the AmericanSociety for Testing and Materials, alimited sample of southern pine barkash was found to contain about 27%CaO, 21% A1103, 19% sand (Si01),9% K10, 6% S03, 5%MgO,4%P10S,3% Na10, 1% Fe103, and 5% othermaterial.

If bark ash is treated with water,the potassium compounds dissolve andcan be separated from the remainingsolids; on evaporation of the solution,potash is recovered.

Ash from coal.fued plants is in-creasingly utilized in mixes yieldinghigh-strength concrete; a lightweightfraction of the ash (floatable on water)has been shown suitable for the manu.facture of high-grade refractories. It islikely that ash from bark can be simi.larly marketed.

37FOREST INDUSTRIES/April 1971