Journal of Sustainable Bioenergy Systems, 2012, 2, 60-64 Published Online September 2012 (
The Direct Use of Post-Processing Wood Dust in
Gas Turbines
Alîne Doherty1, Eilín Walsh1*, Kevin P. McDonnell2
1School of Biosystems Engineering, University College Dublin, Dublin, Ireland
2Animal and Crop Sciences, School of Agriculture, Food Science and Veterinary Medicine,
University College Dublin, Dublin, Ireland
Email: *
Received April 5, 2012; revised May 7, 2012; accepted May 15, 2012
Woody biomass is a widely-used and favourable material for energy production due to its carbon neutral status. Energy
is generally derived either through direct combustion or gasification. The Irish forestry sector is forecasted to expand
significantly in coming years, and so the opportunity exists for the bioenergy sector to take advantage of the material for
which there will be no demand from current markets. A by-product of wood processing, wood dust is the cheapest form
of wood material available to the bioenergy sector. Currently wood dust is primarily processed into wood pellets for
energy generation. Research was conducted on post-processing birch wood dust; the calorific value and the Wobbe In-
dex were determined for a number of wood particle sizes and wood dust concentrations. The Wobbe Index determined
for the upper explosive concentration (4000 g/m3) falls within range of that of hydrogen gas, and wood dust-air mix-
tures of this concentration could therefore behave in a similar manner in a gas turbine. Due to its slightly lower HHV
and higher particle density, however, alterations to the gas turbine would be necessary to accommodate wood dust to
prevent abrasive damage to the turbine. As an unwanted by-product of wood processing the direct use of wood dust in a
gas turbine for energy generation could therefore have economic and environmental benefits.
Keywords: Wood Dust; Wood Processing; Gasification; Renewable Energy
1. Introduction
Woody biomass is a widely-used material for energy
production. Energy from wood is generally derived from
direct combustion or gasification of wood in pellet or
chip form. Due to its carbon neutral status, wood is a
very desirable alternative to the use of fossil fuels as a
source of primary energy. In addition to its environ-
mental advantages, utilising woody material as a primary
energy fuel can increase security of supply if the wood
material is obtained from a domestic source.
The potential for wood to be used as a primary energy
fuel in Ireland will be investigated, with particular em-
phasis placed on the feasibility of using wood dust to
generate heat and electrical energy. The current use of
wood as an energy source will be outlined, the volumes
of wood dust potentially available as a fuel source will be
explored, and how this material can be used as a fuel,
taking into consideration any amendments to existing
equipment that may be necessary, will be examined.
In 2004 Sustainable Energy Ireland conducted a sur-
vey of the bioenergy potential in Ireland and reported
that the forestry sector had undergone significant expan-
sion in previous years [1]. COFORD forecasted the Irish
forestry sector to further expand to 5 million cubic me-
tres by 2015 [2]: this expansion is evident in the almost
750,000 ha of forestry planted in 2010 [3]. It was con-
sidered unlikely that the increased availability of wood
material would be met by demand within current markets
which means an alternative outlet must be found for the
additional volume of woody material available. This ad-
ditional material could be used to great benefit by the
bioenergy industry to generate renewable energy from an
indigenous fuel source. To take advantage of the in-
creased availability and energetic potential of woody
biomass from the expansion of the Irish forestry sector, a
significant target for the integration of renewable electri-
cal energy has been set: 30% co-firing in three peat-
burning power stations by 2015 [4].
Wood-based bioenergy can take one of three forms:
direct biomass, for example chipping of trees removed
during thinning; indirect biomass, for example wood
by-products such as sawdust and bark recovered from
primary and secondary processing; or recovered waste
wood, for example construction and demolition waste
*Corresponding author.
opyright © 2012 SciRes. JSBS
wood, old pallets, etc. Recovered wood generally has a
lower moisture content than fresh biomass as a result of
the original processing [1], which increases the effi-
ciency of energy recovery from wood. Wood energy is
currently used in Ireland for heat and electricity genera-
tion: heat energy is primarily obtained from small-scale
combustion in domestic boilers while electricity is gen-
erated either by co-firing in power stations or by gasifi-
cation or pyrolysis to generate a synthetic gas or liquid to
be burned in a gas or steam turbine [1].
In Ireland energy is derived from wood material pri-
marily by direct combustion in either pellet or chip form.
Wood pellets are usually made from unprocessed, dry,
waste wood which can be either hardwood or softwood
in nature. Softwoods are more suitable than hardwoods
for pellet production due to the higher content of lignin
which acts as a binding agent; pellets made from hard-
woods such as willow require the addition of a binding
agent for durability [5]. Pellets have lower moisture con-
tent and higher energy density than chips (17.0 GJ/t vs.
13.4 GJ/t, [6]) and produce a predictable fuel with mini-
mum residual ash material [1], however processing costs
are higher when producing wood pellets. Wood chips are
usually produced from forestry logging residues and
purpose-grown energy crops [6]. Wood chip production
is more economical than pelleting as the required level of
processing is lower and can be carried out on a small-
scale, localised basis. Due to their lower energy density
and higher moisture content [6], wood chips must un-
dergo a degree of either active or passive drying prior to
use in a boiler [6]. The higher moisture content must also
be taken into consideration during storage to prevent
degradation of the feedstock [7].
The cheapest form of woody material for energy gen-
eration is wood dust produced as a by-product of proc-
essing as there is essentially no further pre-treatment
required. Wood dust is the most unfavoured by-product
in current wood industries as it is difficult to handle and
has the lowest energy density [2], so has the greatest po-
tential as an available bioenergy feedstock for Ireland.
Processing by-products including wood dust are cur-
rently consumed on-site by a number of sawmills in Ire-
land for the production of heat [1] to dry the incoming
wood. In addition, the two largest pellet producers in
Ireland convert approximately 200,000 tonnes of wood
dust material into wood pellets for direct combustion for
energy generation. Pellets can also be gasified or pyro-
lysed rather than combusted; gasification is a more effi-
cient process than combustion and therefore extracts
more utilisable energy [8]. Wood dust itself can be gas-
ified or pyrolysed to produce gas or liquid which can
then be combusted to generate power. One potential
problem with gasification of wood dust, however, is the
transportation of ungasified particles into the turbine
along with the produced gas [9]. Particulate matter mov-
ing through the turbine can cause disruption of the tur-
bine blades and can cause abrasion to the inner surfaces
of the turbine [10].
Wood dust is comprised of cellulose, hemicellulose,
lignin, and extractives. Lignin and extractives tend to be
more prominent in softwoods than hardwoods, translat-
ing into a higher heating value in softwoods [11]. Wood
dust has a relatively high volatiles content (60% - 70%)
and low heating value (17 - 18 MJ/kg) and thus does not
combust efficiently in conventional combustors [12].
From an ignition point of view, the ignition temperature
of wood dust is lower than for whole wood: between
204˚C and 260˚C [13] and between 350˚C and 600˚C
[14], respectively. Ignition point is influenced by multi-
ple factors including wood source, moisture content, par-
ticle size, molecular composition, dispersion, and con-
centration [13]. The smaller particle size associated with
dust is therefore advantageous for ignition, hence the
lower ignition point.
Moisture content is one of the most important factors
when considering wood dust as a fuel, both with regard
to the potential extractable energy and with regard to the
ignition point. The higher the moisture content of a feed-
stock for energy generation, the greater the required ini-
tial energy input to evaporate the moisture [15]. Fresh
wood dust has much higher moisture content than indus-
try-derived wood dust due to drying during preparation
for industrial use. The moisture content of fresh wood
material can be as high as 60% compared to approxi-
mately 10% for industry-dried wood dust [11]. Wood
dust moisture content influences five main characteristics
of wood dust explosions: maximum explosion pressure
(Pmax); maximum rate of pressure rise (KSt); minimum
ignition energy (MIE); minimum explosible dust concen-
tration (MEC); and minimum ignition temperature (MIT).
Increasing moisture content and particle size decrease the
maximum explosion pressure and maximum rate of pres-
sure rise and increase the minimum ignition energy [15].
In addition, increasing moisture content and particle size
increase minimum explosible dust concentration and
minimum ignition temperature.
The forecasted expansion of the forestry sector has
positive implications for biomass-based bioenergy in
Ireland. Due to the high volumes of wood dust produced
as a result of wood processing and therefore available
without additional treatment such as the necessary proc-
essing to produce wood pellets, it is considered that
wood dust is an under-exploited source of bioenergy ma-
terial in Ireland. The aim of this research, therefore, is to
investigate the potential use of wood dust directly in gas
turbines, i.e. the use of wood dust to generate energy
without first producing wood pellets.
Copyright © 2012 SciRes. JSBS
Copyright © 2012 SciRes. JSBS
HHV =HHVbulk
2. Methods and Materials
To determine the advantages of direct wood dust com-
bustion in a gas turbine system, three dust particle sizes
were investigated: 425 µm, 150 µm, and 63 µm. To iso-
late dust particles of these specific dimensions a mixed
sample of Russian and Irish (approximately 90% and
10%, respectively) birch plywood derived from the fur-
niture industry was sieved and the relevant fractions were
extracted. Moisture content of the dust samples (n = 3)
was measured by drying in a convection oven to a con-
stant weight.
The higher heating value (HHV) of the wood dust was
determined using a Parr 6400 (Parr Instrument Company,
Moline, Illinois, USA) bomb calorimeter. The HHV by
volume (HHVv) was used in conjunction with the spe-
cific gravity of each wood dust-air concentration to de-
termine the Wobbe Index of each wood dust-air concen-
tration using the following equations:
% Concentration by volume
Specific gravity of wood dust-air concentration
vbulkv air
where ρbulk = bulk density of wood sample, and ρair =
density of air (1.2041 kg/m3).
Wobbe Index was therefore calculated for each wood
dust-air concentration as:
Wobbe Index (4)
Simultaneous thermal analysis (STA) was conducted
to determine ignition points, weight loss due to ignition,
and loss of volatiles at each particle size and wood-dust
air concentration. This analysis was conducted using a
Rheometric Scientific STA 1000 (Rheometric Scientific
Inc, Piscataway, New Jersey, USA) apparatus on wood
dust samples of each particle size at a number of wood
dust-air concentrations: 50 g/m3, 500 g/m3, and 4000
g/m3. This gave a total of nine samples, each analysed in
triplicate (Table 1).
3. Results and Discussion
The moisture content of the wood dust was determined to
be 4%, a value which was expected due to the post-con-
sumer nature of the wood dust. As was described earlier,
low moisture content corresponds to a higher heating
value, a lower ignition temperature, and a greater loss of
volatiles [1]. A low moisture content such as that ob-
served here increases the likelihood of a wood dust ex-
plosion occurring and enhances the kinetics of the reac-
tion. The bulk density for the wood dust was also deter-
mined during this research and was calculated to be
380.23 kg/m3.
Results from the simultaneous thermal analysis are
shown in Table 2 and indicate that wood dust-air con-
centration and particle size have a considerable influence
on points of ignition and weight loss due to ignition. It
was observed that ignition temperature increased with
increasing particle size and that weight loss due to igni-
tion was greatest at 150 μm particle size. The analysis
indicates that at all three concentrations examined the
smallest particle size (63 μm) required the lowest ignition
energy and therefore recorded the lowest point of igni-
For all particle sizes the lowest point of ignition was
recorded for the stoichiometric concentration of 500 g/m3.
Minimum explosive concentration and upper explosive
concentration had the greatest percent weight loss at 425
μm particle size whereas percent weight loss was greatest
at 150 μm particle size at stoichiometric concentrations.
For all concentrations the greatest weight loss was re-
corded for 63 μm particle size, and for all particle sizes
the most pronounced weight loss was recorded at 50 g/m3
concentration. A more consistent pattern of weight loss
was observed for the stoichiometric and upper explosive
concentrations than that observed for the minimum ex-
plosive concentration, which indicates more stable com-
bustion at higher wood dust-air concentrations.
Fungtammasan et al. [12] reported the higher heating
value by mass (HHVm) of wood dust to be approximately
17 - 18 MJ/kg. Due to the low moisture content and the
species used in this research, a HHVm of 19.16 MJ/kg
Table 1. Experimental wood dust-air concentrations and partic le sizes teste d in STA.
Minimum explosive concentration
50 g/m3
Stoichiometric concentration
500 g/m3
Upper explosive concentration
4000 g/m3
Maximum particle size 425 µm 425 µm 425 µm
Mid particle size 150 µm 150 µm 150 µm
Minimum particle size 63 µm 63 µm 63 µm
Table 2. Results of calorific value and simultaneous thermal analysis conducted for each wood dust-air concentration and
particle size investigated.
Wood dust-air concentration Minimum explosive concentration
50 g/m3
Stoichiometric concentration
500 g/m3
Upper explosive concentration
4000 g/m3
HHVv (MJ/m3) 7293 7293 7293
Specific gravity 1.041 1.414 4.309
Wobbe Index (MJ/m3) 0.940 8.066 36.960
Ignition point (˚C) 249.85 252.14 240.04
Weight loss due to ignition (%) 7.761 0.625 0.175
Particle size (µm) 63 150 425
Ignition point (˚C) 235.722 246.847 259.469
Weight loss due to ignition (%) 2.343 3.211 3.006
was recorded. Present day turbines can typically operate
using gases with a HHVm between 9.4 MJ/kg (CO) and
54 MJ/kg (natural gas) [16], therefore a HHVm of 19.18
MJ/kg is well within operational range. The Wobbe In-
dex can be used to determine the interchangeability of
wood dust-air mixtures with other operational gases. The
results obtained indicate that both the stoichiometric and
upper explosive concentrations of wood are within the
limits of Wobbe Indices of current practical gaseous fu-
els (Table 2). The recorded Wobbe Index for the upper
explosive concentration falls within range of the HHV of
hydrogen gas, and wood dust-air mixtures of this con-
centration could therefore behave in a similar manner in
a gas turbine. Due to its slightly lower HHVm and higher
particle density, however, alterations to the gas turbine
would be necessary to accommodate wood dust as an
energy fuel.
The primary adjustment necessary would be a size al-
teration: fuels with lower heating values require a greater
volume of fuel to meet temperatures achieved by fuels
with higher heating values, and thus require a longer
combustion zone within the turbine [16]. In addition, to
prevent damage resulting from the use of a more abrasive
fuel, vertically-mounted cyclone combustors could be
used to burn fuels with a range of heating values which
ensure adequate particle entrapment. Conical-shaped com-
bustors collect particles at the base and avoid particle
infiltration to the turbine blades [9]. It is further recom-
mended that the combustor and blades be lined to protect
the blades against abrasion.
4. Conclusions
The results of this study show the use of wood dust as a
primary fuel in gas turbines for power generation to be
both feasible and advantageous. At the upper explosive
concentration investigated, the Wobbe Index was found
to be similar to that of hydrogen gas and, with small
enough particle size (63 µm), wood dust could behave
similarly in a gas turbine. Gas turbine design alterations
would be required to ensure proper injection, dispersion,
and mixing of the wood dust in the turbine as well as to
protect the turbine from abrasion caused by the wood
dust particles.
The moisture content of the wood dust used in this in-
vestigation was found to be 4%, which was unsurprising
given the post-processing nature of the wood dust. In
Ireland there are vast quantities of wood dust produced
annually during processing of wood into consumer prod-
ucts which would have a similar moisture content. This
abundance of waste woody material coupled with ad-
vancing bioenergy technology could contribute to reliev-
ing the import dependency faced by Ireland for primary
energy by generating energy from a domestic source
which is also carbon neutral.
[1] SEI, “Bioenergy in Ireland,” Sustainable Energy Ireland,
Dublin, 2004.
[2] Programme of Competitive Forestry Research for Devel-
opment, “Strategic Study: Maximising the Potential of Wood
Use for Energy Generation in Ireland,” 2004.
[3] Teagasc, “Forestry Statistics 2010,” Teagasc, Carlow,
[4] DCMNR, “Bioenergy Action Plan for Ireland,” Depart-
ment of Communications, Energy and Natural Resources,
Dublin, 2007.
[5] M. Peksa-Blanchard, P. Dolzan, A. Grassi, J. Heinimö, M.
Junginger, T. Ranta and A. Walter, “IEA Bioenergy Task
40: Global Wood Pellets Markets and Industry: Policy
Drivers, Market Status and Raw Material Potential,” In-
ternational Energy Agency, Paris, 2007.
[6] Sustainable Energy Authority of Ireland, “Wood Fuel and
Copyright © 2012 SciRes. JSBS
Supply Chain,” 2011. /Sources/ Wood_
[7] M. R. Wu, D. L. Schott and G. Lodewijks, “Physical
Properties of Solid Biomass,” Biomass and Bioenergy,
Vol. 35, No. 5, 2011, pp. 2093-2105.
[8] GTC, “Gasification: The Waste-to-Energy Solution,” Gasifi-
cation Technologies Council, Arlington, 2012.
[9] C. Syred, A. Griffiths and N. Syred, “Gas Turbine Com-
bustor with Integrated Ash Removal for Fine Particu-
lates,” Proceedings ASME Turbo Expo, Vienna, 14-17
June 2004, pp. 1-9.
[10] D. J. Flynn, J. J. Dillon, P. B. Desch and T. S. Lai, “The
NALCO Guide to Boiler Failure Analysis,” 2nd Edition,
McGraw Hill, Inc., New York, 2011.
[11] K. W. Ragland, D. J. Aerts and A. J. Baker, “Properties of
Wood for Combustion Analysis,” Bioresource Technol-
ogy, Vol. 37, 1991, pp. 161-168.
[12] B. Fungtammasan, P. Jittreepit, J. Torero and P. Joulain,
“An Experimental Study of the Combustion Characteris-
tics of Sawdust in a Cyclone Combustor,” Proceedings
EuropeanASEAN Conference on Combustion of Solids
and Treatment of Products, Hua Hin, 16-17 February
1995, pp. 1-18.
[13] Weyerhaeuser Company, “Wood and Wood Dust (With-
out Chemical Treatments or Resins/Adhesives). Material
Safety Data Sheet,” Weyerhaeuser Company, Washington,
[14] V. Babrauskas, “Ignition of Wood: A Review of the State
of the Art,” Proceedings Interflam, 9th International Fire
Science and Engineering Conference, Edinburgh, 17-19
September 2001, pp. 71-88.
[15] R. K. Eckhoff, “Dust Explosions in the Process Industry,”
Gulf Professional Publishing, Massachusetts, 2003.
[16] M. P. Boyce, “Gas Turbine Engineering Handbook,” Vol.
4, Butterworth-Heinemann, Woburn, 2011.
Copyright © 2012 SciRes. JSBS