METHODS TO CONTROL FUEL QUALITY AT WOOD BURNING 

### FACILITIES
Monika Blassino 1, Helena Solo-Gabriele~, and Timothy Townsend2 ~University ofMiami, Department ofCivil, Architectural and Environmental

Engineering, Coral Gables, FL 33146, United States 2University ofFlorida, Department ofEnvironmental Engineering Sciences, Gainesville, FL 32611, United States
ABSTRACT
Two sorting technologies including a chemical stain method and an x-ray fluorescence (XRF) technique were investigated for separating chromated copper arsenate (CCA) treated wood from other wood types in the wood waste stream. The separation could potentially prevent high concentrations ofthose metals in the ash, when wood waste is burned as source for boiler fuel at cogeneration plants. It could also prevent arsenic air emissions known to occur when CCA treated wood is burned under certain conditions. Stains (investigated in laboratory and field settings) could be used for screening fuel quality. XRF was tested in the laboratory using commercially available instrumentation. Laboratory scale experiments showed that XRF technology was able to detect CCA treated wood, which prompted further studies, including the initiation ofpilot-scale experiments. Keywords: chromated copper arsenate, chromium, copper, arsenic, wood, sorting
INTRODUCTION Chromated copper arsenate (CCA) is a wood treatment preservative containing copper, chromium, and arsenic. The purpose ofthe chemical is to prolong the useful life ofwood by protecting it against biological deterioration. CCA treated wood comes in a range of retention levels, from 4 kg/m~(0.25 pcf) to 40 kg/m3(2.5 pcf).
The use ofthe CCA chemical in the United States grew by a factor of seven from the 1970' s until 1996. In 1970, CCA represented only 16% ofthe wood preservative market, whereas by 1996 this figure had increased to 79%. During this time frame the total volume oftreated products also significantly increased. In 1970, the total volume of treated products was 7 million m3 (250 million ft3) ofwhich 1 million m3 (36 million if3)
were treated with CCA. By 1996, 17 million m3(610 million fi3) oftreated wood products were produced, with CCA treated wood representing 13 million m3 (470 million ft3). Since CCA treated wood has a life span of about 25 years, it is projected that thewood waste stream will mirror the production statistics ofCCA treated wood and

amounts ofCCA treated wood being disposed will multiply (Solo-Gabriele and Townsend 1999).
Since CCA contains metals, there are concerns with the ultimate fate ofcopper, chromium and arsenic once the CCA treated wood is disposed. In the State ofFlorida, USA, for example, much ofthe CCA treated wood waste is mixed with untreated wood at construction and demolition (C&D) debris recycling facilities. During the recycling of C&D debris, CCA treated wood is inadvertently sorted out along with the rest ofthe wood. The two primary markets for recovered C&D wood are fuel in industrial furnaces and landscape mulch. When CCA treated wood is burned with C&D wood atthe cogenerationplants, the ash created contains high amounts ofcopper, chromium and arsenic, posing an environmentalproblem. Research indicatesthat a 5 % mixture ofCCA treated wood with untreated wood would cause a hazardous designation as per United States regulations (Solo-Gabriele et al. 1999). Some studies also suggest that a considerable amount ofarsenic can volatilizeunder certain conditions (McMahon et al. 1986).
One solutionto avoidthe problem associated with CCA treated wood in C&D wood is to develop technologies for separating the CCA treated wood from the rest ofthe wood waste. The rest ofthe wood waste can then be burned in cogenerationplants or recycled
as mulch without the added metals from CCA, whereas CCA treated wood canbe recycled or disposed in a different manner (Felton and De Groot 1996). Visual identificationfollowedby manual sorting is one option. The visual identificationofCCA treated wood based on its green hue is at times very difficult, especiallywhen the wood is weathered, dirty, painted, or ifthe wood is characterized by low retention levels. This study addresses two alternative methods of sorting CCA treated wood from other wood waste: use ofchemical stains and x-ray fluorescence. The following sections will describe laboratory and field results ofthe chemical stains, and laboratoryresults for XRF.

THE CHEMICAL STAINS The chemical stains developed through this study were modified from standards for determining depth ofpenetrationofCCA in freshly cut borings oftreated wood (AWPA 1996). Inthe disposal sector, the chemical stains would be used for two types of applications: identifyinglarge pieces ofCCA treated wood for sorting purposes and screening wood fuel quality. Even though the stains have several chemical names, here they will be referred to as Chrome Azurol S, PAN indicator, and rubeanic acid respectively. After a reaction with CCA-treated wood, Chrome Azurol S turns blue, PAN indicatorturns magenta, and rubeanic acid turns green. The color difference can be noted in Figure 1, where the top layer in not stained, followedby Chrome Azurol S stain, PAN indicator, and rubeanic acid.

### Untreated 4 kg/m3 9.6 kg/m3 40 kg/m3 Weathered

No Stain

Chrome Azurol S
PAN
Rubeanic Acid

Figure 1. Untreated wood, new CCA treated wood, and weathered CCA treated wood stained with chemical stains.
For all chemical stains tested, the colors got darker with increasing retention levels. In the laboratory setting, CCA treated wood pieces were readily separated from untreated wood pieces based on the color created afterthe reaction with chemical stains. Those results prompted field experiments conducted at construction and demolition (C&D) debris recycling facilities.
The objective ofthe field experiments was to determine whether the chemical stains could be implemented as tools forthe sorting ofCCA treated wood from other wood types. Three C&D debris facilities, located in the State ofFlorida, U.S.A., participated in the field study. All chemical stains were to be studied, but rubeanic acid was eliminated during the first study due to disadvantages associated with its use (use oftwo solutions and dark green color that could be mistaken for other material). There were three types ofexperiments conducted at the C&D debris recycling facilities: a) separation of CCA treated wood from other wood by product type, b) visual separation ofwood waste without chemical stains, and c) analysis of pre-sorted wood waste piles. Separation of CCA treated wood from other wood by product type involved separation of wood waste loads into dimensional lumber, plywood, cutoffs, pallets, followed by sorting treated from untreated wood. The purpose ofthis study was to evaluate the fraction of products that were treated versus untreated. On average CCA treated wood represented roughly 16 % ofthe wood waste on a weight basis. A considerable difference was noted between construction and demolition type wood waste. The construction piles have a larger CCA fraction (20 % on average by weight) than the demolition piles (5 % by weight).
In a second set ofexperiments, a wood waste pile was visually separated at each facility visited without the use of stains and then checked with chemical stains. The purpose of this experiment was to investigate the effectiveness and efficiency of separating CCA treated wood from other wood types based solely on visual discrimination. Ifany wood sorting is presently done at the C&D debris facilities on routine basis, it is done by visualmeans without chemical stains. Results indicatedthat visual sorting without stains is

beneficial. In all cases betterthan 50% sorting efficiencieswere noted. 84% (by weight) ofthe visuallyidentifiedtreated pile for facilityA and 64 % (by weight) ofthe visually identifiedtreated pile for facilityB was correctlyidentified. Forthe untreated piles, 60 % (by weight) ofthe wood was correctly sorted for facilityA, 97 % (by weight) was correctly sorted for facilityB, and almost 100 % was correctly sorted for facilityC.

In the third set ofexperiments, a total ofthree pre-sorted wood waste pileswere analyzed, one at each facilityvisited. These piles were pre-sorted by employees at the facilities and are considered untreated by those facilities. The piles consisted ofwood only, and their sizes are on an order of 10 or more tons. Giventheir large size, the wood for this experiment was randomlypicked from the perimeter ofthe pile, then stained and weighed. The intent was to show the amounts ofCCA treated wood that would eventuallybe recycled with untreated wood. It was found that at facilitiesA and C the amount ofCCA treated wood inthe pre-sorted piles was 10 % and 9 % respectively, whereas at facilityB that amount was 30 % by weight. It is importantto mentionthat facilitiesA and C, where pilot studies were conducted, implemented a considerable amount ofemployeetraining on sorting CCA treated wood from other wood types. FacilityB, while encouraging workers to separate CCA treated wood, did not have extensive training. The difference in training alone can be clearly noted in the percentages. This result suggests that training is an integral part of sorting CCA treated wood from untreated wood. Visual sorting (without the use ofchemical stains) can potentiallyreduce the amount ofCCA treated wood enteringwaste stream by 15 - 20 percent.

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Emitted x-ray Figure 2. Emission ofcharacteristicx-ray.X-RAY FLUORESCENCE

X-ray fluorescence (XRF) (Figure 2) is based on the ability of an atom to emit its characteristic energy in form ofan x-ray when an external source ofx-rays is applied. XRF analyzers register this incoming energy and compare itto the known energies ofa given element.
The XRF detector used for experimentationinthis study (ASOMA Instruments, Austin, TX) is one routinelyused by the wood treatment industryto check the retentionlevel of treated wood products. The detector is a hollow metal cylinderfitted with a verythinwire along its radial axis (Williams 1987). It contains neon or argon gas at a pressure of2 atmospheres. Emitted photons are able to ionize the gas and create a pulse read by the instrument. The detector senses every time a given atom (in this study copper, chromium
or arsenic) fluoresces and divides it by the total analysistime. This number is given a unit ofcounts, and the analysistime is referred to as count time. The Model 400 requires considerably less pre-processing and therefore would be more suitable for use by the disposal sector. The Model 400, which utilized the detector, includes a hand-held probe head, which houses the detector and is attached to the main analyzerwith a cable connectingthe head. The remote probe (bead) weighs about 4.5 kilograms (10 pounds) and its dimensions are 20.3 x 20.3 x 10.2 cm (8 x 8x 4 in). It includes a window for the detector, which is approximately 2.5 cm (1in) in diameter.
The motivationfor all the experiments conducted was to determine whether XRF technologycould be used for on-line sorting ofCCA treated wood from other wood types. The major objectives ofexperimentationwere to find the best metal forthe analysis (copper, chromium or arsenic), the shortest count time, the maximum spacing between window (located in the head on the instrument) and wood samples, and evaluating whether a plastic shield could be used to protect the window against possible damage.
Arsenic provided the largest number ofcounts for anygiven sample and was therefore considered the best indicatorforthe presence ofCCA in wood samples (Figure 3) followed by copper and chromium. For the shortest count time analysis, wood specimens at different retention levelswere analyzed for chromium, copper, and arsenic simultaneouslyat counttimes of2 seconds, 3 seconds and 10 seconds. Figure 4, which is typical ofall samples tested, shows that the longer the counttime, the higher the count readings for chromium, copper, arsenic. However, it is also clear that at a counttime of2 seconds it is possible to determine presence and concentrationofmetaljust as well as at longer count times, for both shredded and whole wood. A further decrease ofthe analysistime for an on-line continuous operation is likely, given that forthe instrument tested, time is needed for an internal "shutter"to open and release the x-ray and then close. In an on-line system, where a continuousx-ray is used, opening and closingthe shutter would not be necessarythereby further reducing the analysistime and increasing the speed at which wood could be processed at a C&D debris recycling facility. For a conservative count time of2 seconds and length ofwood specimen of 30 cm (1 It), the speed ofprocessing is about 590 m/hr (1900 tt/hr); this increases to 2300 m/hr (7700 ft/hr) for a counttime of0.5 seconds.Figure 3. 1500

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### RetentionLevel(kg/m

3) Counts of arsenic, chromium and copper for CCA treated wood at count time of 2 seconds.

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RetentionLevel(kg/m3) CCAtreatedwood at different retention levels and different count times v. counts ofarsenic.

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### DistanceAwayFromHead(cm) Figure 5. Distance study on whole piece of 4 kg/m j (0.25 pcf) CCA treated wood utilizing a count time of 5 seconds and with and without a plastic shield present.
When it comes to an on-line conveyor design, the most important design information came from the experiment that determined the maximum possible separation distance between the head and the wood specimen. The specimens utilized were untreated woodand 4 kg/m3 (0.25 pcf) CCA treated wood. A retention level of4 kg/m3 (0.25 pcf) was

chosenbecause it is the lowest concentrationregularly manufactured in the United States. Figure 5 shows that the distance between the specimen and the sensor impacts the number ofcounts ofarsenic read by the XRF analyzer; the larger the distance the lower the readings. It is still possible, however, to determine presence and concentrationof CCA in a given sample ofwood, at a distance ofabout 2.5 cm (1 inch). Also when a plastic shield ofthickness of- 2-3 mm (- 0.08 - 0.12 in) was placed on the sensor (to protect it from damage), the countreadings lowered but it was still possible to detect presence ofCCA at a distance ofabout 2.5 cm (1 in).
CONVEYOR DESIGN A conveyor system is currentlybeing designed for semi-automated sorting ofCCA treated wood from other wood types at facilities such as C&D debris recycling facilities. It is anticipated thatthe conveyor system alongwith the detector and shelterwill be constructed by December 2000. A successful design will be able to detect CCA treated wood and automatically separate it from other wood types. Initially,however, part ofthe work would be done manually and the complexityofthe system would be increased gradually. The experimentation site will be placed at a construction and demolition facilityin Sarasota County, Florida. Figure 6 shows a cross-sectional view ofthe conveyor. The conveyorwill be about 0.9 m (3 feet) offthe ground for easy manual operation. The first component ofthe conveyor would be a table, where wood would be pushed to one side (the side where the detector would be located) and onto a roller conveyor. The roller conveyorwould remove significant amount ofdust and dirt that is inherit to C&D waste. Removal ofthe dirt is necessary for more accurate readings as well as the protection ofXRF instrument. The XRF detector would be placed between the rollers towards the end ofthe roller conveyor. If CCA is detected, a buzzer would sound and a treated piece ofwood could be removed manually and placed in a designated CCA treated wood pile. An inclinebelt conveyor would allow for small as well as large and longuntreated wood pieces to be dropped less than 3.6 m (12 feet) to large container, also referred to as a roll-off

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# I I Inclined I R°ll-°ff Belt Conveyor Figure 6. Cross-sectional view ofthe conveyor system. Figure 7 shows the set up ofthe operation. The incomingwood would be loaded manually onto the conveyor, and manually sorted (ifnecessary) on the flatportion ofthebelt conveyor. The controlroom located tothe left.ofthe conveyor systemwould monitor

the XRF instrument, buzzer system, and the conveyor.



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Figure 7. Layoutofthe XRF sorting operations.

To allow for a gradual increase ofcomplexityofthe conveyor, i.e.the transitionfrom manual operationsto automated operations, the conveyorwould have a speed control. Also there would be enough overhead space to add other conveyorcomponents. The conveyordesign would be such to accommodatepossible changes to the technologyused. For example, one alternativetechnologythat isbeing considered is laser induced breakdown spectroscopy (LIBS). A major differencein the conveyorlayoutwould be the locationofthe detectionequipment; it could be located either onthe bottom ofthe roller conveyoror above eitherthe roller conveyoror the fiat portionofthe belt conveyor.
CONCLUSIONS The disposalpredictions suggest that CCA treated wood will be found in the waste stream in increasing amounts (Solo-Gabrieleand Townsend 1999).CCA treatedwood mixed with other wood types limits its disposal through cogeneration. The use of technologiesfor distinguishingCCA treated wood from otherwood types should be

### encouraged in recycling operations where the quality ofthe recycled wood impacts its

potential for re-use. Chemical stains are suitable for sorting small quantities ofwood waste and as a method for checking fuel quality. XRF technology has the potential for sorting large quantities ofwood waste using an on-line automated system. Development of such a system is in progress. Other technologies, which would automate wood sorting
are also being explored.

. REFERENCES American Wood Preservers' Association. 1996.

### American WoodPreservers' Association Book ofStandards. American Wood Preserver's Association, Grandbury, TX.

2. Felton, C., De Groot, R.C. 1996. 

### The Recycling Potential ofPreservative Treated Wood. 

### ForestProductsJournal 46(7/8): 37-46.
3. McMahon, CK., Bush, P.B., Woolson, E.A. 1986. How Much Arsenic is Released When CCA-Treated Wood Is Burned. 

### ForestProductsJournal 36(1912): 45-50.

. 

### Solo-Gabriele, H.M., Townsend, T. 1999. Disposal Practices and Management Alternatives for CCA-Treated Wood Waste. 

### WasteManagementResearch, 17: 378-389.

. Solo-Gabriele, H.M., Townsend, T., Kormienko, M., Calitu V., Messick, B. 1999. 

### Disposal ofCCA-Treated Wood: AnEvaluation of Existing andAlternative Management Options. Final Technical Report #99-06. Submitted to the Florida Center for Solid and Hazardous Waste, Gainesville, FL.
6. Williams, K.L. 1987.

### An Introduction toX-ray Spectrometry:X-ray Fluorescenceand ElectronMicroprobeAnalysis. Winchester, MA, U.S.A.: Allen & Unwin, Inc.