DISPOSAL TECHNOLOGIES FOR CHROMATED COPPER ARSENATE (CCA)
TREATED WOOD WASTE
Lieve Helsen and Eric Van den Bulck
Katholieke Universiteit Leuven
Dept. of Mechanical Engineering,
Div. Applied Mechanics and Energy Conversion,
Celestijnenlaan 300A, B-3001 Heverlee, Belgium
Email address: firstname.lastname@example.org
Phone number: 00-32-16-32.25.05
Fax number: 00-32-16-32.29.85
OF THERMOCHEMICAL CONVERSION PROCESSES AS
Several alternative methods for the disposal of chromated copper arsenate (CCA) treated wood
waste have been studied in the literature, and these methods are reviewed and compared in this
paper. Extraction experiments have been carried out on CCA treated wood and evaluated as a
method to recover the metal compounds into either fresh wood preservatives or other useful
industrial materials. Recycling and recovery processes of the metals in the metallurgical industry
have also been studied, but not yet all metal products are transformed to usable forms. A study
about biorecycling of CCA treated wood through bioremediation and biodeterioration has been
initiated. Numerous studies and experiments have been carried out on burning contaminated
wood. Direct electrodialytic removal of the metals from CCA treated wood, as well as
electrochemical cleaning processes for ash resulting from combustion of CCA treated wood, are
under study. Pyrolysis processes (both slow and flash pyrolysis) have been investigated as a
major process for the disposal of cellulosic wastes, also CCA treated wood waste.
The authors performed a lot of experimental and theoretical work to get more insight in the metal
behaviour during the low-temperature pyrolysis of CCA treated wood waste. Experiments were
carried out with CCA treated wood samples, as well as with arsenic model compounds and
mixtures of arsenic oxides and reducing agents (glucose or activated carbon). The most important
conclusion is that zero arsenic release during pyrolysis of CCA treated wood seems to be
impossible since the reduction reaction (As2O5 → As2O3 + O2) can not be avoided in the reducing
environment, created by the presence of wood, char and pyrolysis vapours. Once the trivalent
arsenic oxide is formed, it is released. This release is driven by a vapour pressure controlled
volatilisation process: the higher the temperature, the faster the release.
The insights gained through these studies are used to evaluate other thermochemical conversion
processes (flash pyrolysis, gasification and combustion) with respect to their applicability to the
disposal of CCA treated wood waste. This evaluation is compared with observations and
calculations reported by other researchers in the literature. Finally, the most appropriate
thermochemical disposal technology is identified.
Keywords: chromated copper arsenate (CCA), wood waste, disposal, thermochemical conversion
It is estimated that world-wide the wood preservation industry presently treats approximately 30
million cubic metres of wood each year, consuming some 500 000 tonnes of preservative
chemicals. Approximately two-thirds of this volume is treated with chromated copper arsenate
(CCA) . CCA has been used to preserve wood from insects, fungi and water damage for many
years, and is still used today (almost exclusively as oxides), albeit restricted to a limited number
of industrial applications. Substantial amounts of CCA remain in the wood for many years and the
disposal of scrap wood is a growing problem in Europe, the United States, North America and
Japan. The quantities of discarded CCA treated wood will increase significantly in the future .
With respect to CCA treated wood at the end of its service life, the wood is classified as
hazardous in some member states of the EU and subject to stringent requirements and classified
as not hazardous in other member states and therefore subject to much less stringent requirements.
Current legislation in the classification of waste is thus imprecise thereby creating a lack of
consistency. For several wood products it has been concluded that the waste stage has a very
significant impact on the Life Cycle Assessment results .
Future waste minimisation focuses on the use of alternative wood treatment preservatives that
do not contain arsenic. However, these alternatives leach more copper than CCA treated wood.
From a regulatory perspective, they pose a lower risk than CCA treated wood within the disposal
sector and within terrestrial environments. Slightly higher risks are expected in aquatic
environments due to the toxicity of copper to aquatic organisms . A number of technical issues
still have to be resolved with several potential alternative treatments, including corrosion effects,
weathering properties and fixation characteristics. Viable alternatives are available for CCA
treated wood for the lower retention levels (4 - 6.4 kg/m3).
Besides the use of alternative wood treatment preservatives other waste abatement,
elimination or reduction methods could be :
substitution of CCA treated wood by other materials such as untreated cedar, teak, plastic
lumber, concrete, steel, aluminium, brick, … for which complete and quantitative full life
cycle assessments are needed,
wood modification treatments (such as high temperature nitrogen or steam exposure or
thermal oil submersion) for which research on durability and weathering performance is
optimisation of preservation treatment for specific end-use conditions (better quality control,
selection of wood species),
designing details that will minimise the potential for decay and thereby the overuse of
service life enhancing technologies such as the use of stains and other surface protection
coatings and water repellents,
design to minimise waste during construction (reduction in off-cuts and other wastes from
Regardless of waste minimisation efforts, improved disposal-end management practices will play
a key role in minimising the impacts of CCA treated wood upon disposal within the short term
(25-40 y). The authors had the idea to give a critical overview of the different methods suggested
in literature as solutions for the disposal of CCA treated wood waste. While reviewing the papers
already published, an extensive review paper with a list of selected references, published by
Cooper  in 2003, was found. Because a good review already exists, the aim of this paper is not
to repeat this work. Therefore, in this paper the authors give a more detailed analysis of the
thermal processes and try to identify the most appropriate thermochemical disposal technology for
CCA treated wood waste. First, a short overview of the different methods under study is given,
based on the material published by Cooper [4,5,6] and other researchers.
LITERATURE REVIEW: OPTIONS FOR MANAGING CCA TREATED WOOD
A first question that arises when looking for disposal-end management options for CCA treated
wood waste is whether or not the CCA treated wood should be separated from mixed wood
sources. In Florida, for example, construction and demolition (C&D) wood can contain up to 30
wt% CCA treated wood . Sorting technologies have been studied [2,7,8,9,10,11,12,13,14] and
will become a greater challenge as more alternative preservatives are introduced. Visual sorting
based on the green colour is known to be not very effective, although it can potentially reduce the
amount of CCA treated wood entering waste streams by 15-20%. Chemical stains (e.g. PAN
indicator (C15H11N3O) producing an orange colour if sprayed on untreated wood and a magenta
colour if sprayed on CCA wood) were found to be effective for sorting small quantities (< few
tonnes/y) of CCA treated wood. Both laser and X-ray systems were shown to be very promising
technologies for sorting large quantities (> 8000 tonnes/y) of wood in a more automated way. The
detection limit of XRF is found to be 3-5% CCA treated wood. Moskal and Hahn  designed,
implemented and made a field evaluation of an online detector system using laser-induced
breakdown spectroscopy (LIBS) for the analysis of CCA treated wood. Discrimination between
CCA wood and untreated wood was based on the atomic emission signal of chromium. The
accuracy of the LIBS-based analysis ranged from 92% to 100% for sorting the waste at a
construction and demolition (C&D) debris recycling centre. The LIBS system did not prove
reliable for the detection of severely rotted wood samples or samples that were completely soaked
with water. Morak et al.  reported a very high spatial resolution for laser-induced plasma
emission spectrometry (LIPS) and found that the influence of the humidity and the species of the
wood on the results of the analysis is negligible.
The application of a permanent identification marking system similar to but more persistent than
for grade stamping may become a requirement. Whether this be indelible stamp, bar code or
embedded chip, it must be able to survive the service life exposure conditions to be of any use .
Industrial treated products, such as poles and railway ties, are easily recovered but CCA treated
residential lumber presents a challenge to collection and transportation because of the increasing
quantities and its widespread distribution. Eventually, it will be necessary to have a collection,
transportation and processing infrastructure for this material. Since at European level the sale of
arsenic-treated wood to consumer is banned and its use is restricted to a limited number of
essential industrial applications, the collection and transportation of CCA treated residential
lumber will be only a problem of the near future.
When looking for disposal-end management options for CCA treated wood waste, a hierarchy of
options should be considered with some options being more acceptable than others. The
acceptability can differ from location to location, e.g. in Europe a lot of treated wood waste is
incinerated while in North America almost all treated wood waste is landfilled. However, a
general order of preference can be defined:
1. waste abatement or elimination
2. waste reduction
3. waste reuse
4. waste refining for recycling
5. waste treatment and destruction
6. waste disposal
The first two points (abatement or elimination and reduction) have already been mentioned in the
introduction, the existing and emerging technologies for managing CCA treated wood waste are
summarised in Table 1, together with their barriers and prognosis with respect to implementation
Table 1 Existing and Emerging Technologies for Managing CCA Treated Wood Waste
management option barriers
wood waste is bulky and inefficient
to transport; contaminated sawdust
may be generated
used as garden borders,
posts, land piling,
retaining walls, …
dismantle; low quality wood
remanufacture – fence
other fasteners; high
dismantle; low quality wood
salvage and reuse through
transportation and storage
refining for recycling
wood based composites
issue of using metal containing and
contaminated wood and loss of
ownership of treated wood (product
should be identified as one containing
treated wood); landfill disposal is
only deferred, not avoided; CCA
tends to interfere with the adhesives
unlikely to be used since pulping of
treated wood releases the CCA
components into the spent pulping
liquor, unless it is mechanically
pulped; slow process due to long
good for industrial products but
of limited potential for residential
high contamination with nails and
other fasteners; high cost to
high contamination with nails and
material would have to be
refinished to even out differences
in weathering discoloration
limited potential high cost of handling sorting,
the market is not in favour of
using CCA wood in conventional
wood composite manufacturing,
questions about safety of workers
and environmental problems
CCA wood fibre cement products are
development of new composite
products; benefit from inclusion
of decay resistant wood fibre;
stabilisation of metals within a
cement matrix; improvement in
potential for the
curing time of the composite;
potential for hexavalent chromium
bending strength and stiffness,
internal bond strength, water
absorption and thickness swelling
benefit from inclusion of decay
resistant wood fibre; low cost and
high strength to weight ratio
processing and impacts on physical
and mechanical properties should be
it makes little sense to use CCA
wood since the decay hazard is too
low to justify it, except in the
presence of termites; in that case the
identification of the amount and
distribution of CCA particles is
required; an addition of 50% CCA
wood does not significantly affect the
it makes little sense to use CCA
wood since the decay hazard is too
low to justify it; use of CCA wood
would complicate the cleanup of
OSB is made from high quality
flakes; lumber products can not be
flaked properly; the presence of CCA
lowers all property
substantially; however, physical and
mechanical properties were enhanced
by spraying the flakes with a primer
just before spraying and blending of
part of the contaminants left in the
wood and loss in fibre quality;
absence of end use for extracted
wood and chemicals; problems with
contamination of the system by other
not 100% effective and slow;
recycling of CCA components is not
proved; not cost-effective at this
time; high cost of size reduction
almost complete extraction, only if
combined with solvent extraction =
dual remediation; several constraints
that limit efficiency and cost-
huge amounts of chemicals are used;
multistage extraction is required to
ensure complete removal of CCA;
chemicals is not disclosed (re-
oxidation + elimination of extracting
compounds), but mixing of recovered
solution and fresh CCA solution is
unproven and unlikely to be a
significant factor in the near term
fibreboard and MDF
unproven and unlikely to be a
significant factor in the near term
strand board (OSB)
unproven and unlikely to be a
significant factor in the near term
biodegradation by fungi
not economically feasible
some potential for treatment of
minor amounts of treated wood
such as that produced as a by-
product of milling
technically feasible but slow and
expensive (high cost of the
nutrient culture medium)
more research and development is
needed to improve, optimise and
evaluate the process; effects of
characteristics of wood residue
are not reported; extraction has
negative effect on the properties
of particleboard prepared from
feasible for surface removed
treated wood or sawdust by-
products of a re-sawing operation
to recycle CCA chemicals
does not increase the extractability of not economically feasible
the chemical components if used as a
pre-treatment prior to extraction;
leave some residual material in the
extracted wood (only 90% removal of
no field tests performed (pilot scale is
now being tested); expected cost is
high; after treatment the metals are
solution, the membrane and as a
precipitate on the electrode; total
removal of metals not achieved,
Cu/Cr/As ratio in the electrolyte
differs from the ration in the fresh
more leaching due to increased
surface area (less than 0.1% CCA
wood causes a mulch to exceed risk-
based direct exposure standard for
arsenic); CCA chemical is dispersed
into the environment; products will
only initial lab-scale experiments;
only 85% of the CCA is removed
not yet economically feasible;
difficult to compete with solvent
use for mulch, compost
or animal bedding
clear policies and regulations that
prohibit inclusion of CCA wood
in mulch should be developed
treatment and destruction
much more research is needed to
improve, optimise and evaluate
potential if the metals collected in
the ash are dealt with and arsenic
is trapped from the flue gas; most
common method in Europe but
strong resistance in Canada; more
favourable climate for this option
is expected in the future
advantage of energy recovery and
volume, but ash is considered as
monitoring are needed to meet air
grinding is required increasing the
energy consumption and cost
cost of grinding dirty material;
presence of arsenic in the emissions;
collection of metals in the ash where
it must be collected and dealt with
electrochemical processes or cyclone
melting); general resistance in some
countries to consider these options
Portland cement standards have
limitations on metal levels, chromium
being the limiting element; cost of
collection, transport, removal of
metal contaminants, getting a permit
arsenic is distributed over the three
pyrolysis gas); no time-temperature
threshold found for zero arsenic
high cost of pure oxygen; removal of
pure metallic arsenic in the vapour
not yet proven on a large scale;
arsenic emissions during start-up and
some potential, but requires
further development; lessens the
dependence on fossil fuels; metal
concentrations can be diluted by
mixing with other waste streams
(such as household waste) or
fuels (such as coal)
potential is limited to a fraction of
wood generated; appropriate for
milling residues and low retention
besides elimination of dioxins
and furans formation and possibly
additional advantages over the
other thermal destruction methods
pilot plant tests still have to be
performed; more research is
needed to evaluate the process
energy and raw materials
recovery by metallurgical
plant has to be well designed to scrub
all volatile and particulate arsenic
from the stacks; relatively low CCA
concentrations in the lumber make
infeasible; not yet all metal products
are transformed to usable forms
infrastructure for collection and
transportation of CCA wood
waste is developed;
research is needed to examine the
maximum amount of CCA wood
that can be mixed with copper
concentrates without interfering
not a preferred option because it
does not recover any value from
the used product; may not be
acceptable at individual landfill
sites (by 2005 no organic wastes
will be accepted at landfills in the
CCA chemical can leach from CCA
wood (both unburned and as ash) in
quantities that exceed regulatory
thresholds; monofill results in the
highest metal concentrations in the
leachate compared to C&D debris
landfill and MSW landfill; cost of
landfilling (hazardous waste sites,
lined landfills); shortage of landfill
As shown in Table 1 there are many technological options to manage waste of CCA treated wood,
but all have their limitations and problems. Instead of importing (the major part from China and
Mexico) considerable quantities of arsenic to Europe, it would be more reasonable to utilise the
arsenic recovered in whatever way (recycling process at the wood preservation sites, in the
metallurgical industry, arsenic containing solutions resulting from remediation processes, …).
However, the metals must be converted to their proper valence state before reuse. Such additional
processing adds to the cost of recycling which renders the current technologies not economically
feasible at this time. The main restriction on commercial exploitation of reuse or recycling
technology is the highly diffuse nature in which redundant treated timber enters the waste chain.
In the following sections the authors focus on thermochemical conversion processes as possible
alternatives for the treatment of waste of CCA treated wood. Thermal utilisation of the wood
waste offers the advantage of providing energy and concentrating wastes for recycling or disposal.
THERMOCHEMICAL CONVERSION PROCESSES: OBSERVATIONS
While the CCA preservative chemicals are relatively simple, inorganic reactions during the wood
preservation process produce complicated inorganic compounds and complexes. The thermal
decomposition behaviour of these inorganic compounds and complexes is unknown and difficult
to determine. The reactions and thermal decomposition of a system containing a volatile
compound, such as arsenic oxide, in a gas flow cannot be predicted solely based on equilibrium
data. Therefore, in practical disposal of CCA treated wood by thermal decomposition, the reaction
kinetics will likely determine the ultimate fate of arsenic in the system . Thermogravimetric
(TG) experiments with model compounds have been used to predict the thermal behaviour of the
CCA treated wood system by Helsen et al  and Kercher and Nagle . The main conclusions
are listed below.
1. Volatile As2O3 loss occurs below practical wood pyrolysis and combustion temperatures
(Tonset = 200°C), due to the high vapour pressure of As2O3.
2. Pure As2O5 does not reduce nor volatilise at temperatures lower than 600°C in air or nitrogen
atmosphere. Oxygen content of the atmosphere shows no effect on volatile loss, which
suggests a weight loss mechanism based on vapour pressure, not on the decomposition As2O5
→ As2O3 + O2. A hydrogen containing atmosphere (5% H2) causes As2O5 to volatilise at
much lower temperatures (order of 425°C) which suggests that reducing gases from thermal
decomposition of wood (e.g. CO), which behave similar to hydrogen, likely would
decompose As2O5 at lower temperatures.
3. The thermal decomposition of copper (II) oxide strongly depends on the oxygen content in
the atmosphere (Tonset is 775°C versus 1050°C in respectively nitrogen and air), indicating that
solid-state oxygen diffusion may be the limiting step. The onset of weight loss in a
hydrogen/nitrogen mix is around 200°C, which is confirmed by the Ellingham diagram
showing a driving force for the reduction of copper oxides by hydrogen (or carbon
4. Chromium (III) oxide does not undergo any significant reactions during heating in inert or air
5. When a mixture of copper (II) oxide and arenic (V) oxide is heated, part of arsenic (V) oxide
simply volatilises at slightly lower temperatures than in the pure As2O5 experiments; the
remainder of arsenic (V) oxide reacts with copper (II) oxide to form mixed copper arsenates
(2CuO.As2O5 and Cu3(AsO4)2). The atmosphere exhibits a strong effect on the thermal
decomposition of the copper arsenates; in air no weight loss is observed up to 900°C. During
thermal decomposition of CCA treated wood the formation of copper arsenates may be a
mechanism to limit arsenic loss up to 900°C.
6. When a mixture of chromium (III) oxide and arsenic (V) oxide is heated, free arsenic (V)
oxide is volatilised; some As2O5 reacts with Cr2O3 to form chromium arsenate (CrAsO4),
which however does not exhibit any temperature range of zero weight loss.
7. In CCA treated wood, the thermal decomposition of the inorganic components can be
influenced by interactions with wood and its decomposition products. Therefore the influence
of the presence of glucose and activated carbon has been studied. The thermal decomposition
of As2O5 is highly influenced by the presence of glucose, both in a nitrogen atmosphere and
in a mixed nitrogen – oxygen atmosphere. The presence of glucose gives rise to a faster
decomposition, the effect being more pronounced the higher the oxygen concentration in the
purge gas is. The interaction of glucose and As2O5 is probably a combination of three effects:
mutual acceleration of the decomposition reaction, oxidation-reduction reactions and the
formation and decomposition of arsenate esters. Oxygen concentrations up to 10% are
sufficient to accelerate the decomposition of both As2O5 and glucose, but insufficient to
reverse the reaction As2O5 → As2O3 + O2. Also activated carbon influences the thermal
behaviour of As2O5, by promoting arsenic volatilisation at temperatures higher than 300°C.
Extrapolation of the behaviour of these model compounds to the real thermal decomposition
of CCA treated wood indicates that the reduction of pentavalent arsenic to trivalent arsenic is
favoured by the reducing environment, created by the presence of wood, char and pyrolysis
vapours. Therefore, the most important conclusion is that zero arsenic release during thermal
decomposition of CCA treated wood seems to be impossible since the reduction reaction
(As2O5 → As2O3 + O2) can not be avoided in the reducing environment. Once the trivalent
arsenic oxide is formed, it is released, obeying a temperature controlled solid-vapour
8. For a mixture of arsenic (V) oxide and yellow pine sawdust it was found that the products
from inert pyrolysis of wood promote the volatilisation of As2O5. By heating at 5°C/min
interaction between both compounds can be observed from 370°C, indicating that arsenic
volatilisation occurs above 370°C. However, if the mixture is held for longer time periods at
temperatures between 250°C - 370°C, it is observed that arsenic volatilisation occurs, the rate
of arsenic volatile loss increasing with dwell temperature.
9. For a mixture of copper (II) oxide and yellow pine sawdust inert pyrolysis causes the
reduction of copper (II) oxides at low temperatures (around 305°C).
These studies with model compounds may not take all effects into account, for example the
formation of complexes and hydrates of arsenic (V) oxide during preservative fixation that may
help to prevent arsenic loss below 400°C. Therefore thermal decomposition studies with real CCA
impregnated wood samples are necessary. A lot of researchers have studied the pyrolysis,
gasification or combustion / incineration of CCA treated wood and evaluated the fraction of
arsenic, copper and chromium released to the atmosphere and retained in the solid residue. This
work has varied in scale from laboratory to industrial installations and has included 100 % CCA
treated wood and mixtures with other waste timber sources or other industrial wastes. Both
experimental and modelling work have contributed to new insights.
Percentages of arsenic volatilised have been reported to range between 8 and 95 %
[16,30,31,32,33,34,35,36,37,38]. These percentages depend on temperature, residence time,
extended period of ash heating, presence of chlorine and/or sulphur, oxygen partial pressure, air
flow rate and the impregnation process. Amounts of copper and chromium volatilised are not well
documented, but are found to be much lower than for arsenic. In all studies arsenic is identified as
the problematic compound with respect to volatilisation. If working conditions can be determined
for which arsenic losses are predicted to approach zero, extensive flue gas cleaning equipment
(scrubbers and filters) is not required, resulting in a less expensive system. Therefore, a threshold
temperature, below which the arsenic volatilisation is zero, has been looked for. Hata et al. 
state that at 300°C already 20% of the total arsenic is volatilised, which is ascribed to part of the
arsenic being unreacted (as As2O5 compound) after impregnation of the wood. The remainder of
the arsenic has reacted during the impregnation process resulting in chromium arsenate
(Cr2As4O12) that decomposes only at temperatures higher than 700°C. Helsen et al.  conclude
that metal (Cr, Cu and As) release seems to be “zero”, but is inconclusive (because of the high
experimental uncertainty) at a temperature of 300°C which is held for 20 minutes. Residence
times of 40 minutes already result in non negligible arsenic releases. Furthermore, they show that
the major part of arsenic in the solid pyrolysis residue (350°C, 20 minutes) is present in trivalent
state . Pasek and McIntyre  reported that arsenic volatilisation is predicted (through linear
extrapolation) to approach zero under conditions of limited air flow and high combustion
temperature in excess of 1100°C. No volatilisation of copper or chromium was observed. The
residual ash is indigestible even under the strongest acidic conditions, which is thought to be due
to the formation of transition metal arsenides at the higher combustion or calcination
temperatures. The results from this work are contrary to other studies. Moreover, arsenic balances
were far below 100%, which is suspected to be due to incomplete sampling and/or analysis of the
metals released, a problem also appearing in several other studies [37,38,40,41,42,43]. These
studies show that a threshold level (temperature-time) below which zero arsenic release is
guaranteed will be very difficult or even impossible to reach in large industrial installations
without flue gas cleaning.
The mechanism responsible for arsenic release during the thermal decomposition of CCA treated
wood is not yet fully understood, although a lot of researchers have tried to identify the arsenic
compounds released and to postulate some hypotheses. McMahon et al.  reported that
negligible amounts of arsine (AsH3) are formed during CCA wood combustion. Essentially all of
the volatilised arsenic recovered was found in the condensed (particulate) form and consisted of
both arsenites and arsenates. The volatile arsenic trioxide, however, could not be trapped
efficiently. They stated that arsenic release is not so much a function of how the fuel is burned,
but rather how long the residual ash is exposed to high temperature. Hirata et al.  stated that
arsenic compounds are first reduced to As2O3 with heating, after which it is gasified according to
the equilibrium 2As2O3 ↔ As4O6 and generally accepted to be As4O6 for temperatures up to
1073°C. For minimising gaseous toxicants from arsenic, CCA treated wood must be burned at
low temperatures with reduced air supply. Cornfield et al.  did not detect arsine or other metal
compounds in volatile nonparticulate form. They suggested that the metals released are all present
in particulate form. Helsen and Van den Bulck  concluded that the release of arsenic during
pyrolysis of CCA treated wood is controlled by the reduction of pentavalent to trivalent arsenic,
which is accelerated by the presence of reducing compounds originating from the pyrolysing
wood. Once arsenic trioxide is formed, it will be released at temperatures as low as 200°C. In
freshly treated wood arsenic is fixed in pentavalent state, but in weathered wood the arsenic may
be partly reduced to the trivalent state. The only way to avoid or limit arsenic release (at low
temperatures) is to control the reduction reaction. Once arsenic trioxide is formed, it is not easy to
re-oxidise it. For example, during combustion with a high air/fuel ratio oxygen is present in the
flue gas, but arsenic trioxide does not get oxidised into arsenic pentoxide as the reaction is known
to happen only under pressure .
Besides experimental studies modelling contributes to a deeper understanding of the metal
behaviour during thermal decomposition of CCA treated wood. Sandelin and Backman 
studied the high temperature equilibrium chemistry involved when CCA treated wood is burned
by utilising an equilibrium model based upon minimising the Gibbs free energy of a hypothetical
combustion system. They revealed that partial pressures of arsenic-containing compounds
dominate in the temperature range from 500 to 1600°C. At temperatures between 500 and
1150°C, As4O6(g) is the dominating species, but at higher temperatures AsO(g) takes over. The
following explanation was given: arsenic pentoxide is stable at low temperatures but ''forms''
gaseous As4O6 at about 580°C. They concluded that chromium and copper in impregnated wood
are unlikely to volatilise at common combustion temperatures. At 1200°C only 0.05 % of the total
chromium and 0.51 % of the copper was found in the gas phase. Arsenic was more volatile,
existing 86.89 % in the gas phase at the same temperature. Supplementary calculations showed
that magnesium, copper and chromium compounds may prevent arsenic from volatilising. In
addition, reducing conditions within the char particle may affect the tendency of the metals to
vaporise. Conclusions with respect to low-temperature chemistry were not given. Kitamura and
Katayama  combined experimental studies and thermodynamic analyses and concluded that
the higher retention of arsenic in charcoal (after pyrolysis in nitrogen atmosphere) compared to
ash (after combustion in air) is due to absorption of arsenic in the charcoal. Thermodynamic
calculations resulted in the identification of vaporised arsenic species in nitrogen and air
atmosphere: As4, As2 and As3 dominate up to 1100 K in nitrogen atmosphere, while AsO2, AsO,
As, As4O7 and As4O6 appear at temperatures above 1100 K in air. These results do not agree with
the results published by Sandelin and Backman .
Since thermal processes inherently lead to volatilisation of arsenic, appropriate arsenic capturing
devices have to be installed. These devices are said to be commercially available, but very few
tests have been carried out on industrial scale for the specific case of thermal conversion of CCA
treated wood that is characterised by the production of submicron aerosol fumes which are
difficult to effectively collect. Even on lab-scale it is very difficult to obtain arsenic mass balances
of 100%. The most important conclusions drawn from an extensive literature review are given
elsewhere . Syrjanen and Kangas  emphasised the need to change existing flue gas
cleaning equipment when impregnated timber is burned. A venturi scrubber was found to be
insufficient in combination with a grate boiler; the average arsenic concentration in the exhaust
gas was 2.8 mg/Nm3 . Additional investments are needed for better cleaning systems, tuned in
to the type of burner, gasifier or pyrolyser, and for measurements to control emissions. Industrial
experience with other feedstocks can be helpful in the design of an appropriate arsenic capturing
device. When incinerating arsenic containing waste an efficient filter (electrostatic filter) does not
succeed in capturing all the arsenic. Around 5.4% of the arsenic originally present in the waste
passes the electrostatic filter and is captured in the downstream wet scrubber (using lime and
NaOH) by absorption and/or chemisorption . Sorbent injection is a very attractive method to
reduce arsenic emission during coal combustion [46,51,52,53]. Arsenic reacts, while still in the
vapour state, at high combustion temperatures, with various sorbents to form larger particles
which can be collected effectively by particulate collection devices. The sequestering action of the
sorbents reduces the vapour form and/or fine particle form of the metal . These sorbents can
be fly ash, activated carbon or mineral material. Hydrated lime (Ca(OH)2) and limestone (CaCO3)
are found to be very effective. While Ca is responsible for the reaction of As with these solids, it
is the availability of active Ca sites at the surface of these solids that determines the rate of
reaction . At temperatures below 600°C tricalciumorthoarsenate (Ca3As2O8) is formed, while
temperatures between 700 and 900°C give rise to the formation of dicalciumpyroarsenate
(Ca2As2O7), which is unstable and therefore responsible for a decrease in As capture at higher
temperature . Sterling and Helble , however, reported a maximum capture of As with
calciumoxide at 1000°C.
Besides the mechanism responsible for arsenic release and options available for arsenic capture,
the characteristics of ash resulting from combustion of CCA treated wood and combustion of a
mixture of untreated and CCA treated wood have been studied. It is concluded that the
environmental impact of the ashes investigated (bottom ash, boiler ash, fly ash) is remarkable,
none of them meeting the requirements for above-ground disposal [54,55]. Leachates
concentrations according to the DIN 38414 part 4 leaching standard exceed the limits for arsenic
and chromium. Moreover, chromium is present in the toxic hexavalent state . Bottom ash
from wood mixed with minimum 5% CCA treated wood is characterised as hazardous waste
under US regulations . To dispose the ash in an environmentally sound manner two options
1. the elements enriched in the ash after the combustion process are recycled;
2. the ash is landfilled after pretreatment, e.g. solidification with cement, concrete, …
Different theories exist about the formation of polychlorinated dibenzo-p-dioxins (PCDD) and
polychlorinated dibenzofurans (PCDF), but about the role of copper in the pathways all
researchers are unanimous: copper is identified as a catalyst for PCDD/F formation
[34,56,57,58,59,60,61]. Due to the presence of copper in CCA treated wood, the formation of
toxic PCDD/Fs has to be taken into account . Wunderli et al.  examined solid residues
(bottom ash and fly ash) from wood (native and waste) combustion and concluded that wood
burning is always accompanied by unwanted production of PCDD/F, the amount being dependent
on the type of wood burned and the construction of the combustion system. Low carbon burnout
and zones with low temperatures seem to support the formation of PCDD/F strongly [60,62].
Consequently, grate boiler fly ashes contain higher levels of PCDD/F than either bubbling or
circulating fluidised bed boiler fly ashes . One way to avoid the formation of PCDD/F in
incinerators is by blocking the catalytically active sites of copper species by poisoning, for
example through the addition of small amounts of sulfamide to the fuel . Since PCDD/F
formation is the combination of the elements C, H O and Cl under favourable conditions, another
way is to ensure working conditions that eliminate one or more of the essential elements (C, H, O,
Cl) or essential parameters (temperature 250-400°C), for example pyrolysis is performed in an
oxygen-free environment or flue gases are immediately quenched to very low temperatures. In
this aspect pyrolysis has an advantage over gasification and combustion.
BEST AVAILABLE THERMOCHEMICAL CONVERSION TECHNOLOGY
For an inert pyrolysis process to be a reasonable disposal method for CCA treated wood, volatile
arsenic loss has to be controlled and the solid pyrolysis product must be suitable for recuperating
the inorganic compounds. SEM-EDXA studies have shown [30,64], that during pyrolysis the
metal compounds form agglomerates, which suggests that the metals can be easily recuperated
from the charcoal in a dry way . However, arsenic losses are already observed for
temperatures as low as 275°C . Lower temperatures give rise to very slow wood
decomposition rates and thus extremely long reaction times. Therefore, in practice pyrolysis leads
to non zero arsenic volatilisation. However, the amount of arsenic volatilised is much less
compared to gasification or incineration and therefore the arsenic released may be easier captured
by for example chemisorption. The use of flue gas cleaning equipment that captures all arsenic
volatilised can thus not be eliminated. With respect to the formation of PCDD/Fs and maybe to
recovery of the metals, pyrolysis could be a better option than gasification or combustion.
Flash pyrolysis, that aims at producing as much pyrolysis oil as possible, is not an option for CCA
treated wood since a non negligible percentage of arsenic (between 5 and 18% ) is collected
in the oil. The advantage of pyrolysis oil is that it can be stored, but substantial concentrations of
arsenic make it useless.
Incineration of CCA treated wood can be coupled to a recycling process, provided that an
extensive gas cleaning system is used to control air emissions. The arsenic containing solution,
collected in the scrubber, is recycled to the CCA solution production unit and the ash containing
arsenic, copper and chromium is processed in a copper smelter [38,66] or recycled through
chemical or electrochemical processes . The arsenic trioxide dust collected in filters still poses
problems with respect to occupational health. As far as occupational health is concerned the use
of wet methods to capture arsenic is preferred. Incineration is thus an option for the disposal of
CCA treated wood waste or mixed wood waste if three requirements are satisfied:
1. the arsenic and PCDD/F emissions are avoided by using an appropriate gas cleaning system
and appropriate cooling trajectories for the flue gas,
2. the arsenic captured (scrubber solution and filter dust) can be recycled in a safe way,
3. an environmentally sound ash treatment technology is available.
A disadvantage of incineration is that it generates heat that has to be used immediately or
converted to electricity (efficiency is relatively low), instead of producing a secondary fuel.
Co-incineration is often presented as the best solution for the treatment of wood waste.
the attraction of co-incineration is the economy of scale; power stations are huge compared to
low investment cost since the incineration plant already exists, only the gas cleaning
equipment has to be extended or adjusted. In Norwegian waste incinerators, for example, the
combination of bag filters with activated carbon and wet scrubbers is used .
the installation can be designed and installed on a short term.
the availability of CCA treated wood waste is not an issue since co-incineration is highly
flexible with respect to the fuel used.
if different waste streams are mixed, e.g. CCA treated wood waste and municipal solid waste
(MSW), arsenic may be scavenged by the calcium present in the other waste stream.
it is easier to comply with emission legislation due to the dilution effect.
However, it is not advisable to mix CCA treated wood with other fuels, such as coal, since CCA
treated wood contains much more arsenic than coal. Consequently, the incineration process would
deliver more bottom ash that has a higher concentration of water-soluble arsenic and the volatile
arsenic has to be removed from a larger amount of flue gas . Moreover in some countries (like
Denmark) legislation prescribes that impregnated wood waste must be sorted out and treated
separately. For these countries co-incineration is not an option. In other countries, like the
Netherlands, a mixture of coal and up to 40% of wood waste (including CCA treated wood) can
be used as input fuel for power plants, receiving green certificates . In the European waste
classification system, however, CCA treated wood waste is defined as dangerous waste and
excluded form the biomass category for which green certificates can be handed out. Most
European countries, except the Netherlands, follow this EU directive.
Gasification is characterised by higher energetic efficiencies (electricity generation efficiency is
enhanced by burning a combustible gas in a gas turbine instead of fuelling a boiler) and lower
environmental impact compared to incineration. If CCA treated wood is used as feedstock,
appropriate gas cleaning equipment is still needed , but the amount of gas to be cleaned is
lower than for incineration. During high temperature gasification the arsenic may be totally
converted to metallic arsenic, which is much easier to capture than arsenic trioxide since metallic
arsenic does not go through a liquid phase upon cooling and has a higher sublimation temperature
than arsenic trioxide . It is essential that the total amount of arsenic is released from the CCA
treated wood and reduced to the metallic form. A cleaning system that captures all the arsenic is a
very critical point in this gasification unit. Due to the high temperature (1100-1500°C) all organic
compounds are cracked, eliminating the danger for PCDD/F formation. When a metallurgical
furnace is used the chromium and copper can be caught in a slag, which can be applied as
abrasive. The syngas (H2 + CO, diluted by CO2 + H2O + N2) can be used or sold as fuel and the
pure metallic arsenic can be recycled in the CCA impregnation process. A disadvantage of the
process is the high temperature needed, but the heat required can be recovered from the gas
produced. This process has still to be proven at pilot scale.
The authors conclude that the best available thermochemical conversion technology for the
treatment of CCA treated wood waste is:
on the short term: co-incineration as long as CCA treated wood waste has not to be treated
separately and dilution is allowed.
on the long term a sustainable solution has to be found: preference is given to recycle as much
material as possible but it has do be done in a cost-effective way. Dependent on the results of
further research work one of the following methods will be identified as best available
1. low-temperature (380°C) pyrolysis in a moving bed ;
2. high temperature gasification (1100-1500°C) in a metallurgical furnace .
Both technologies aim at recuperating the metals and the energy (as secondary fuels:
combustible gas and charcoal or syngas) contained in the CCA treated wood waste, but both
technologies still have to be proven.
The optimal scale of application is determined by a balance between the high investment cost of
the reactor and flue gas cleaning equipment on one hand and the high transport cost to collect the
waste timber on the other hand. The important issue is whether or not it is better to transport the
wood waste over long distances to gain economy of scale for the operation of large thermal
L. Helsen is a post-doctoral research fellow of the Fund for Scientific Research of Flanders
(Fonds voor Wetenschappelijk Onderzoek - Vlaanderen) (Belgium). The authors are grateful to
the company ARCH Timber Protection Limited (UK) for the financial support of the research
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