Explosion safety during low temperature
pyrolysis of CCA treated wood
Figure C.1 presents the flammability diagram with the concentration of
nitrogen, oxygen and fuel (pyrolysis gasses and vapors) on each side
of the triangle. A mixture of oxygen, fuel (pyrolysis gasses and
vapors) and inert (nitrogen) is flammable, when the composition of
this mixture lies in the flammable region of the triangular diagram.
Two characteristics determine the flammability region of a lean fuel
mixture, like the pyrolysis gasses and vapors evolving from the
pyrolysis reactor. The Limiting flammability limit (LFL) or limiting
explosion limit (LEL), describes the leanest mixture (as a percentage
fuel in the mixture) that still sustains a flame. The limiting oxygen
concentration (LOC) describes the lowest oxygen concentration (as a
percentage oxygen in the mixture) that still sustains a flame. To
determine the LOC and LEL of the pyrolysis gasses and vapors
evolving from the pyrolysis of CCA treated wood, a theoretical
estimation method will be used, since the experimental determination
of the LOC and LEL was unfeasible (heavy metal contamination of the
pyrolysis gasses and vapors). Therefore, the LOC and LEL of the
individual constituents of the pyrolysis gasses and vapors have been
determined. Together with the composition of the pyrolysis gasses
from a number of literature sources, this allows to calculate the LOC
and LEL of the gas mixture.
Fig. C.1
flammability triangular diagram
Determination of the composition of pyrolysis
vapors
A complete composition of the gasses and vapors evolving from the
pyrolysis of CCA treated wood was not available in literature.
However, separate data on pyrolysis vapors and pyrolysis gasses could
be retrieved. The composition of the pyrolysis vapors showed to be
highly variable with a strong dependency on the type of wood and the
experimental conditions. Fu et al.[1] compared the composition of the
pyrolysis vapors evolving from untreated wood with those from CCA
treated wood at a process temperature of 350°C. The pyrolysis of CCA
treated wood produced a substantially higher amount of levoglucosan.
Zhurinsh et al.[2] compared the composition of the pyrolysis vapors
produced during the pyrolysis of 3 different types of wood: untreated
pine wood, creosote treated sapwood and Chromium-copper
impregnated sapwood. The latter is expected to exhibit a similar
pyrolysis behavior as CCA treated wood (wood pyrolysis is mostly
affected by copper and only to a limited extend by arsenic). The higher
amount of levoglucosan in CCA treated wood, reported by Fu [1],
could not be verified since levoglucosan was not part of the gas
species measured but there a significant influence of type of wood on
the pyrolysis vapor composition was observed. Since the differences in
pyrolysis vapor composition could have a profound effect on the
flammability of the pyrolysis gasses and vapors, The LOC and LEL of
the 5 different pyrolysis vapor compositions - CCA, CC and creosote
treated as well as untreated wood (as determined by [2] and [1]) - were
calculated.
The absolute amount of gaseous species was obtained from
multiplying the total amount of pyrolysis gas with the relative amount
of gaseous species. In turn, the total amount of pyrolysis gas was taken
as an average ratio of the amount of pyrolysis gas to the absolute
amount of tar. The relative amounts of gaseous species (to the total
amount of pyrolysis gas) were taken as determined by Williams et al.
[3].
Determination of the LOC
Since LOC values were not available for all liquid constituents present
in the pyrolysis vapors a group contribution method has been used to
estimate these data. The LOC of a mixture can be calculated based on
formula C.1
LOC mixture 
100  OS _ mixture
1  OS _ mixture  NT _ mixture
C.1
For which OS _ mixture , the stoichiometric amount of oxygen, equals the
amount of oxygen (mol) to oxidize 1 mol of fuel mixture and NT _ mixture
equals the amount of N2 (mol) to delute the stoichiometric amount of
oxygen to the maximal oxygen concentration. OS _ mixture and NT _ mixture
can be determined based on the values for the components which make
up the pyrolysis gasses and vapors:
OS _ mixture   OS _ i  Ci
i
NT _ mixture   NT _ i  Ci
i
With subscript i indicating the properties of a specific component of
the pyrolysis gasses and vapors and C the volumetric concentration of
the components.
OS _ i can be determined based on the bruto formula of the components,
while Subramaniam and Cangelosi [4] proposed a group contribution
method to calculate NT _ i . In this method, each functional group is
assigned a factor to which extend it contributes to NT _ i . However, the
parameter set described by Subramaniam and Cangelosi [4] result in
some cases in unconservative predictions of the LOC as indicated by
Fuβ et al. [5] Therefore the latter have proposed a new set of
parameters which result in more conservative figures but show a worst
fit with experimental data for most components. This is illustrated by
calculating the LOC for benzene and comparing with experimental
data. With the parameter set of Subramaniam and Cangelosi a value of
9.1% is abtained while with the method of Fuβ et al. [5] a value 8.7%
is obtained. Compared to the experimental value of 9.5% [6] the latter
shows a too conservative value. The group contribution method is not
suitabale for the calculation of LOC values for diatomic gasses like
hydrogen and carbonmonoxyde. For these gasses NT _ i values can be
calculated based on experimental literature data.
Determination of the LEL
Besides the LOC, the LEL (lower explosion limit) or LFL (lower
flammability limit) is a second flammability characterisitic of fuel lean
mixtures.
A first method to caluculate the LEL is based on LOC. Both
characterisics are related by following formula:
LEL = LOC*y/x
With x and y the coeficients in following reaction equation.
y fuel + x O2 -> a CO2 + b H2O
However, this formula can result in slightly unconservative values for
the LEL.
A second method uses the LEL values of the individual components of
the fuel mixture. The chaterlier’s law [7] describes the relation
between the LEL of the mixture and the LEL of each component i,
with volumetric concentration Ci
C
1
 i
LELmixture i LELi
C.3
The LEL values of the components can be found based on
experimental literature data [8-11] or can be calculated via the
simplified group contribution method proposed by Kondo et al. [12].
LEL values as well as LOC values are dependent of pressure and
temperature. Therefore, a 10% reduction in LEL or LOC values with
an increase of 100°C is generally accepted [13].
Results
LOC
Table C.1 shows the calculated LOC values. The difference between
LOC values calculated for gas compositions obtained from different
types of wood, is limited to less than 0.3 vol.%. Based on the LOC
values calculated for a process temperature of 390°C, an oxygen
concentration of 3 vol.% could be used during the experiments.
However, due to heat generation from oxidation reactions, the
temperature may well rise above 390°C. LOC values at 520°C show
that an oxygen concentration of 3 vol.% could result in unsafe process
conditions.
Table C.1 LOC values as a function of the type of wood. Values are
calculated for a process temperature of 390°C and 520°C, and are
calculated according to two different group contribution parameter sets
type of wood
untreated
pine
Cu/Cr
sapwood
Creosote
treated
sapwood
untreated
pine
CCA treated
wood
Reference
for
gas/vapor
composition
LOC (vol.%) for different
temperatures and group
contribution parameter sets
390°C, [4]
390°C,[5] 520°C,[4]
520°C,[5]
[1]
4.2
3.8
3.3
3.0
4.2
3.9
3.4
3.1
4.3
3.9
3.4
3.1
5.6
4.9
4.4
3.9
5.3
4.6
4.2
3.7
[1]
[1]
[2]
[2]
LEL
To run experiments at an oxygen concentration of more than 2 vol.%,
the heater gas flow can be adjusted so that the concentration of
combustible pyrolysis gasses and vapors is lower than the LEL value.
Table C.2 shows the LEL values for various types of wood. It can be
concluded that safe operation is requires a concentration of
combustible pyrolysis gasses and vapors lower than 1 vol.%. The
concentration of combustible pyrolysis gasses and vapors at a given
heater gas flow rate can be estimated based on the kinetic parameters
for tar and pyrolysis gas formation. This leads to a concentration of 0.5
vol.% at a heater gas flow of 1800l/h. Therefore experiments with a
concentration of 3 vol.% oxygen and a heater gas flow rate of 1800l/h
will not result in a flammable gas exiting the pyrolysis reactor.
Table C.2
LEL values at 520°C for various types of wood
type of wood
untreated pine
Cu/Cr sapwood
Creosote treated
sapwood
untreated pine
CCA treated wood
Reference
for
gas/vapor
composition
[1]
[1]
[1]
[2]
[2]
LEL
(vol.%)
@ 520°C
1.4
1.5
1.4
1.7
1.8
Implications for low temperature pyrolysis of CCA
treated wood on an industrial scale
This section aims at discussing the most important safety problems
that can arise during the operation of an industrial scale application of
the low temperature pyrolysis of CCA treated wood. Firstly, piping or
reactor rupture can result in the explosion of the pyrolysis gasses and
vapors evolving from the pyrolysis reactor. The pyrolysis gas at the
top of the reactor contains no oxygen (which is consumed at the
bottom of the reactor due to combustion reactions) during normal
operation. Therefore the pyrolysis gas on top of the reactor is not
flammable. However, in case of a piping failure or rupture of the
reactor, the pyrolysis gas may be mixed with air and a flammable
mixture may be produced. The flame propagation velocity in the
pyrolysis gasses and vapors can be approached by the flame
propagation velocity of methane/air mixtures which is around 0.3
m/s.[14] The heater gas velocity in an industrial application of the low
temperature pyrolysis is expected to be lower which allows flame
propagation in the reactor column. However, in case of small ruptures,
the gas exiting the crack will be higher and the flame will not be able
to enter the reactor column.
Secondly the proper functioning of the gas burner, supplying heat to
the process, is imperative. If the gas burner would blow out, and the air
supply to the burner is not cut off, air is supplied to the hot zone at the
bottom of the reactor. This will result in the combustion of char
particles at the bottom of the reactor and excessive heat production. If
the air supply is not cut of quick enough, the heat production will be so
intense that the reactor column will be difficult to cool down. It seems
imperative, that a nitrogen supply is provided on site the inertisise the
reactor column in case of an emergency.
Thirdly, dust poses a significant explosion risk. Char particles
extracted from the bottom of the reactor are compacted to produce a
char powder. This powder is claimed to have an minimum ignition
energy of more than 1 joule, ruling out dust explosions at room
temperature. However, the minimum ignition energy decreases
significantly at higher temperatures. In addition the Minimum Ignition
Temperature (MIT) of the char powder may lay below 370°C (the
expected temperature of the char particles). Wood dust, for example
has a MIT of less than 330°C. [15]
Therefore the char powder that is extracted from the bottom of the
reactor, is inertisised during the transported to the centrifuge hereby
excluding explosion of the char powder or smoldering combustion.
Wood dust poses a more significant explosion risk. Wood dusts can
have a minimum ignition energy as low as 2 mJ (depending on the
type of wood and the particle size of the wood dust). The static energy
build up during the discharge of the wood chips in the silo can be
enough to ignite the wood dust. Therefore, the conveyer belt and silo
containing the wood chips may best be inertisised, since static
electricity buildup can not be excluded. In addition pressure relief
panels should be incorporated in the design of the silo, to minimize
pressure build up in case of an explosion, hereby minimizing damage.
The heater gas velocity seems the most important factor to control
with respect to gas phase explosions. If the heater gas velocity is very
low, the pyrolysis gas will be lean in oxygen and will not support
flame spreading. If the velocity relatively high (>0.5 m/s) the fuel
mixture will be too lean to support flame spread and the flame velocity
will be lower than the bulk gas velocity (avoiding upstream flame
propagation).
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functional group distributions in pyrolysis oil of chromated copper
arsenate (CCA)-Treated wood, as elucidated by gas chromatography
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Chemistry Research, 46 (2007) 5258-5264.
[2] A. Zhurinsh, J. Zandersons, G. Dobele, Slow pyrolysis studies for
utilization of impregnated waste timber materials, Journal of
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Soc Chim (Paris), 19 (1898) 483-488
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pure compounds from their molecular structures, Journal of Hazardous
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Materials, 155 (2008) 440-448.
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on LEL and LOC values, (2007)
[14] A.D. Benedetto, V. Di Sarli, Laminar burning velocity of
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Hydrogen Energy, 32 (2007) 637-646.
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