METHODS TO CONTROL FUEL QUALITY AT WOOD BURNING 

FACILITIES 

Monika Blassino 1, Helena Solo-Gabriele ~, and Timothy Townsend 2 

~University of Miami, Department of Civil, Architectural and Environmental 

Engineering, Coral Gables, FL 33146, United States 

2University of Florida, Department of Environmental Engineering Sciences, Gainesv

FL 32611, United States 

ABSTRACT 

Two sorting technologies including a chemical stain method and an x-ray fluorescen

(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 of those metals in the ash, when wood waste 

burned as source for boiler fuel at cogeneration plants. It could also prevent arsenic a

emissions known to occur when CCA treated wood is burned under certain condition

Stains (investigated in laboratory and field settings) could be used for screening fuel 

quality. XRF was tested in the laboratory using commercially available instrumentat

Laboratory scale experiments showed that XRF technology was able to detect CCA 

treated wood, which prompted further studies, including the initiation of pilot-scale 

experiments. 

Keywords: chromated copper arsenate, chromium, copper, arsenic, wood, sorting 

INTRODUCTION 

Chromated copper arsenate (CCA) is a wood treatment preservative containing copp

chromium, and arsenic. The purpose of the chemical is to prolong the useful life of 

by protecting it against biological deterioration. CCA treated wood comes in a rang

retention levels, from 4 kg/m ~ (0.25 pcf) to 40 kg/m 3 (2.5 pcf). 

The use of the CCA chemical in the United States grew by a factor of seven from th

1970' s until 1996. In 1970, CCA represented only 16% of the wood preservative m

whereas by 1996 this figure had increased to 79%. During this time frame the total 

volume of treated products also significantly increased. In 1970, the total volume of 

treated products was 7 million m 3 (250 million ft 3) of which 1 million m 3 (36 million 

were treated with CCA. By 1996, 17 million m 3 (610 million fi3) of treated wood 

products were produced, with CCA treated wood representing 13 million m 3 (470 m

ft3). Since CCA treated wood has a life span of about 25 years, it is projected that th
wood waste stream will mirror the production statistics of CCA treated wood and 

amounts of CCA treated wood being disposed will multiply (Solo-Gabriele and 

Townsend 1999). 

Since CCA contains metals, there are concerns with the ultimate fate of copper, 

chromium and arsenic once the CCA treated wood is disposed. In the State of Florid

USA, for example, much of the CCA treated wood waste is mixed with untreated wo

construction and demolition (C&D) debris recycling facilities. During the recycling 

C&D debris, CCA treated wood is inadvertently sorted out along with the rest of the 

wood. The two primary markets for recovered C&D wood are fuel in industrial furn

and landscape mulch. When CCA treated wood is burned with C&D wood at the 

cogeneration plants, the ash created contains high amounts of copper, chromium and 

arsenic, posing an environmental problem. Research indicates that a 5 % mixture of 

treated wood with untreated wood would cause a hazardous designation as per Unite

States regulations (Solo-Gabriele et al. 1999). Some studies also suggest that a 

considerable amount of arsenic can volatilize under certain conditions (McMahon et 

1986). 

One solution to avoid the problem associated with CCA treated wood in C&D wood 

develop technologies for separating the CCA treated wood from the rest of the wood 

waste. The rest of the wood waste can then be burned in cogeneration plants or recyc

as mulch without the added metals from CCA, whereas CCA treated wood can be 

recycled or disposed in a different manner (Felton and De Groot 1996). Visual 

identification followed by manual sorting is one option. The visual identification of 

treated wood based on its green hue is at times very difficult, especially when the wo

weathered, dirty, painted, or if the wood is characterized by low retention levels. Th

study addresses two alternative methods of sorting CCA treated wood from other wo

waste: use of chemical stains and x-ray fluorescence. The following sections will 

describe laboratory and field results of the chemical stains, and laboratory results for 

XRF. 

THE CHEMICAL STAINS 

The chemical stains developed through this study were modified from standards for 

determining depth of penetration of CCA in freshly cut borings of treated wood (AW

1996). In the disposal sector, the chemical stains would be used for two types of 

applications: identifying large pieces of CCA treated wood for sorting purposes and 

screening wood fuel quality. Even though the stains have several chemical names, h

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, 

indicator turns magenta, and rubeanic acid turns green. The color difference can be 

in Figure 1, where the top layer in not stained, followed by Chrome Azurol S stain, P

indicator, and rubeanic acid. 
Untreated 4 kg/m 3 9.6 kg/m 3 40 kg/m 3 Weathered 

No Stain 

Chrome 

Azurol S 

PAN 

Rubeanic 

Acid 

Figure 1. Untreated wood, new CCA treated wood, and weathered CCA treated wo

stained with chemical stains. 

For all chemical stains tested, the colors got darker with increasing retention levels. 

the laboratory setting, CCA treated wood pieces were readily separated from untreate

wood pieces based on the color created after the reaction with chemical stains. Those 

results prompted field experiments conducted at construction and demolition (C&D) 

debris recycling facilities. 

The objective of the field experiments was to determine whether the chemical stains 

could be implemented as tools for the sorting of CCA treated wood from other wood 

types. Three C&D debris facilities, located in the State of Florida, U.S.A., participate

the field study. All chemical stains were to be studied, but rubeanic acid was elimina

during the first study due to disadvantages associated with its use (use of two solutio

and dark green color that could be mistaken for other material). There were three typ

of experiments conducted at the C&D debris recycling facilities: a) separation of C

treated wood from other wood by product type, b) visual separation of wood 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 separati

wood waste loads into dimensional lumber, plywood, cutoffs, pallets, followed by so

treated from untreated wood. The purpose of this study was to evaluate the fraction o

products that were treated versus untreated. On average CCA treated wood represen

roughly 16 % of the wood waste on a weight basis. A considerable difference was n

between construction and demolition type wood waste. The construction piles have 

larger CCA fraction (20 % on average by weight) than the demolition piles (5 % by 

weight). 

In a second set of experiments, a wood waste pile was visually separated at each faci

visited without the use of stains and then checked with chemical stains. The purpose 

this experiment was to investigate the effectiveness and efficiency of separating CCA 

treated wood from other wood types based solely on visual discrimination. If any wo

sorting is presently done at the C&D debris facilities on routine basis, it is done by v
means without chemical stains. Results indicated that visual sorting without stains is 

beneficial. In all cases better than 50% sorting efficiencies were noted. 84% (by we

of the visually identified treated pile for facility A and 64 % (by weight) of the visua

identified treated pile for facility B was correctly identified. For the untreated piles, 

(by weight) of the wood was correctly sorted for facility A, 97 % (by weight) was 

correctly sorted for facility B, and almost 100 % was correctly sorted for facility C. 

In the third set of experiments, a total of three pre-sorted wood waste piles were 

analyzed, one at each facility visited. These piles were pre-sorted by employees at th

facilities and are considered untreated by those facilities. The piles consisted of woo

only, and their sizes are on an order of 10 or more tons. Given their large size, the w

for this experiment was randomly picked from the perimeter of the pile, then stained 

weighed. The intent was to show the amounts of CCA treated wood that would 

eventually be recycled with untreated wood. It was found that at facilities A and C t

amount of CCA treated wood in the pre-sorted piles was 10 % and 9 % respectively, 

whereas at facility B that amount was 30 % by weight. It is important to mention tha

facilities A and C, where pilot studies were conducted, implemented a considerable 

amount of employee training on sorting CCA treated wood from other wood types. 

Facility B, 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 trea

wood from untreated wood. Visual sorting (without the use of chemical stains) can 

potentially reduce the amount of CCA treated wood entering waste stream by 15 - 2

percent. 

/ \ 

J \ 

/ / ~ ~ ~I[EJ ected 

,/ / / Nucleus \, \ / \Electron 

Incide ned 

Emitted x-ray 

Figure 2. Emission of characteristic x-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 of an x-ray when an external source of x-rays is applied

XRF analyzers register this incoming energy and compare it to the known energies o

given element. 

The XRF detector used for experimentation in this study (ASOMA Instruments, Aus

TX) is one routinely used by the wood treatment industry to check the retention leve

treated wood products. The detector is a hollow metal cylinder fitted with a very thin 

along its radial axis (Williams 1987). It contains neon or argon gas at a pressure of 2 

atmospheres. Emitted photons are able to ionize the gas and create a pulse read by th

instrument. The detector senses every time a given atom (in this study copper, chrom

or arsenic) fluoresces and divides it by the total analysis time. This number is given 

unit of counts, and the analysis time is referred to as count time. The Model 400 requ

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 pr

head, which houses the detector and is attached to the main analyzer with a cable 

connecting the head. The remote probe (bead) weighs about 4.5 kilograms (10 pound

and its dimensions are 20.3 x 20.3 x 10.2 cm (8 x 8x 4 in). It includes a window for 

detector, which is approximately 2.5 cm (1 in) in diameter. 

The motivation for all the experiments conducted was to determine whether XRF 

technology could be used for on-line sorting of CCA treated wood from other wood 

types. The major objectives of experimentation were to find the best metal for the 

analysis (copper, chromium or arsenic), the shortest count time, the maximum spacin

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 poss

damage. 

Arsenic provided the largest number of counts for any given sample and was therefo

considered the best indicator for the presence of CCA in wood samples (Figure 3) 

followed by copper and chromium. For the shortest count time analysis, wood 

specimens at different retention levels were analyzed for chromium, copper, and arse

simultaneously at count times of 2 seconds, 3 seconds and 10 seconds. Figure 4, whi

typical of all samples tested, shows that the longer the count time, the higher the cou

readings for chromium, copper, arsenic. However, it is also clear that at a count time 

seconds it is possible to determine presence and concentration of metal just as well a

longer count times, for both shredded and whole wood. A further decrease of the 

analysis time for an on-line continuous operation is likely, given that for the instrum

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 continuous x-ray is used, opening and closing th

shutter would not be necessary thereby further reducing the analysis time and increa

the speed at which wood could be processed at a C&D debris recycling facility. For 

conservative count time of 2 seconds and length of wood specimen of 30 cm (1 It), t

speed of processing is about 590 m/hr (1900 tt/hr); this increases to 2300 m/hr (7700 

ft/hr) for a count time of 0.5 seconds. 
Figure 3. 

1500 

1000 

500 

Arsenic 

/ 

I/ ~ Copper 

Chromium 

0 0'.5 i f.5 2 23 

Retention Level (kg/m 3) 

Counts of arsenic, chromium and copper for CCA treated wood at count ti

of 2 seconds. 

Figure 4. 

2500 

20O0 

1500 

1000 

500 

0 

J I 

10 seconds / 

3 seconds 

2 seconds 

0 0.5 1 1.5 2 2.5 3 

Retention Level (kg/m 3) 

CCAtreatedwood at different retention levels and different count times 

counts ofarsenic. 

1200 

• ~ 1000 

800 

600 

400 

200 

0 0 

~ plastic shield 

4 kg/m 3, with a pla~ 

Untreated wood without a p as ~ " 

i i i i 1~ 

0 0.5 1 1.5 2 2.5 

Distance Away From Head (cm) 

Figure 5. Distance study on whole piece of 4 kg/m j (0.25 pcf) CCA treated woo

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 distanc

between the head and the wood specimen. The specimens utilized were untreated wo
and 4 kg/m 3 (0.25 pcf) CCA treated wood. A retention level of 4 kg/m 3 (0.25 pcf) w

chosen because it is the lowest concentration regularly manufactured in the United S

Figure 5 shows that the distance between the specimen and the sensor impacts the 

number of counts of arsenic read by the XRF analyzer; the larger the distance the low

the readings. It is still possible, however, to determine presence and concentration o

CCA in a given sample of wood, at a distance of about 2.5 cm (1 inch). Also when a 

plastic shield of thickness of- 2-3 mm (- 0.08 - 0.12 in) was placed on the sensor (t

protect it from damage), the count readings lowered but it was still possible to detect 

presence of CCA at a distance of about 2.5 cm (1 in). 

CONVEYOR DESIGN 

A conveyor system is currently being designed for semi-automated sorting of CCA 

treated wood from other wood types at facilities such as C&D debris recycling facili

It is anticipated that the conveyor system along with the detector and shelter will be 

constructed by December 2000. A successful design will be able to detect CCA trea

wood and automatically separate it from other wood types. Initially, however, part o

work would be done manually and the complexity of the system would be increased 

gradually. The experimentation site will be placed at a construction and demolition 

facility in Sarasota County, Florida. 

Figure 6 shows a cross-sectional view of the conveyor. The conveyor will be about 

(3 feet) off the ground for easy manual operation. The first component of the convey

would be a table, where wood would be pushed to one side (the side where the detec

would be located) and onto a roller conveyor. The roller conveyor would remove 

significant amount of dust and dirt that is inherit to C&D waste. Removal of the dirt 

necessary for more accurate readings as well as the protection of XRF instrument. T

XRF detector would be placed between the rollers towards the end of the roller conv

If CCA is detected, a buzzer would sound and a treated piece of wood could be rem

manually and placed in a designated CCA treated wood pile. An incline belt convey

would allow for small as well as large and long untreated wood pieces to be dropped 

than 3.6 m (12 feet) to large container, also referred to as a roll-off 

i ,., =' , ' 10' i 15' 

3_-t .................... 

Table Roller Conveyor Belt Conveyor 

I 

I Inclined I R°ll-

Belt Conveyor 

Figure 6. Cross-sectional view of the conveyor system. 

Figure 7 shows the set up of the operation. The incoming wood would be loaded 

manually onto the conveyor, and manually sorted (if necessary) on the flat portion o
belt conveyor. The control room located to the left. of the conveyor system would mo

the XRF instrument, buzzer system, and the conveyor. 

30' 

E 

0 

0 

r~ 

o 

0 

0 5' 

10' 

> 

cO 

0 

(D 

cO 

cRoll-off 

4[3' 

5' 

1 

10' 

15' 

CCA-Treated 

Wood © 

Other Wood 

,7 Types 

Figure 7. Layout of the XRF sorting operations. 

To allow for a gradual increase of complexity of the conveyor, i.e. the transition from 

manual operations to automated operations, the conveyor would have a speed contro

Also there would be enough overhead space to add other conveyor components. The 

conveyor design would be such to accommodate possible changes to the technology 

For example, one alternative technology that is being considered is laser induced 

breakdown spectroscopy (LIBS). A major difference in the conveyor layout would b

location of the detection equipment; it could be located either on the bottom of the ro

conveyor or above either the roller conveyor or the fiat portion of the belt conveyor. 

CONCLUSIONS 

The disposal predictions suggest that CCA treated wood will be found in the waste 

stream in increasing amounts (Solo-Gabriele and Townsend 1999). CCA treated woo

mixed with other wood types limits its disposal through cogeneration. The use of 

technologies for distinguishing CCA treated wood from other wood types should be 
encouraged in recycling operations where the quality of the recycled wood impacts it

potential for re-use. Chemical stains are suitable for sorting small quantities of wood 

waste and as a method for checking fuel quality. XRF technology has the potential f

sorting large quantities of wood waste using an on-line automated system. Developm

of such a system is in progress. Other technologies, which would automate wood sor

are also being explored. 

. 

REFERENCES 

American Wood Preservers' Association. 1996. American Wood Preservers' 

Association Book of Standards. American Wood Preserver's Association, 

Grandbury, TX. 

2. Felton, C., De Groot, R.C. 1996. The Recycling Potential of Preservative Treated 

Wood. Forest Products Journal 46(7/8): 37-46. 

3. McMahon, CK., Bush, P.B., Woolson, E.A. 1986. How Much Arsenic is Release

When CCA-Treated Wood Is Burned. Forest Products Journal 36(1912): 45

. Solo-Gabriele, H.M., Townsend, T. 1999. Disposal Practices and Management 

Alternatives for CCA-Treated Wood Waste. Waste Management Research, 

378-389. 

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

Disposal of CCA-Treated Wood: An Evaluation of Existing and Alternative 

Management Options. Final Technical Report #99-06. Submitted to the Flor

Center for Solid and Hazardous Waste, Gainesville, FL. 

6. Williams, K.L. 1987. An Introduction to X-ray Spectrometry: X-ray Fluorescence 

Electron Microprobe Analysis. Winchester, MA, U.S.A.: Allen & Unwin, Inc