The Direct Use of Post-Processing Wood Dust in Gas Turbines

doi:10.4236/jsbs.2012.23009

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Journal of Sustainable Bioenergy Systems, 2012, 2, 60-64

http://dx.doi.org/10.4236/jsbs.2012.23009 Published Online September 2012 (http://www.SciRP.org/journal/jsbs)

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: *[email protected]

Received April 5, 2012; revised May 7, 2012; accepted May 15, 2012

ABSTRACT

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.

A. DOHERTY ET AL. 61

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.

A. DOHERTY ET AL.

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:





(1)



Concentration

bulk









% Concentration by volume

C



(2)







Specific gravity of wood dust-air concentration

vbulkv air

air









(3)

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:

HHV V



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-

tion.

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

A. DOHERTY ET AL. 63

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.

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A. DOHERTY ET AL.

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