1 Ash related problems in biomass combustion plants
Inaugural lecture
Presented on May 20, 2005
at Technische Universiteit Eindhoven
prof.dr. Ingwald Obernberger
ash related
problems
in biomass combustion plants
2 prof.dr. Ingwald Obernberger 3 Ash related problems in biomass combustion plants
In recent years the thermal utilisation of solid biomass for heat and electricity
production has gained ever more importance. Within the European Union
in particular, biomass is seen as the most relevant renewable energy source
besides hydropower and is thus expected to substantially contribute to the
CO2 emission reduction targets defined in the Kyoto protocol. However,
according to data published by the International Energy Agency in 2004
[1], global primary energy consumption has increased from 6,034 Mtoe in
1973 to 10,230 Mtoe in 2002 and this trend is expected to continue. In 2002
combustible renewables and waste accounted for 10.9% of global primary
energy supply. There is a broad international consensus that the utilisation
of renewable energy sources should be further enforced in order to reduce
greenhouse gas emissions. In the EU, several political measures have been
initiated with the aim of supporting this requirement, including the White
Paper (1997), the Res-e Directive (2001), the Commitment on the Green
Paper on Security of Energy Supply (June 2002), the Directive on the Energy
Performance of Buildings (Dec 2002), the Directive on Liquid Biofuels (May
2003), the Commitment on Renewable Energies in Europe launched at the
Bonn conference (2004), and the Directive on the promotion of cogeneration
(2004).
Heat and power production from renewable energy sources –
the European situation
Renewable energy sources currently represent about 6.2% of total
primary energy supply within the EU 15, with combustible renewables
and waste as well as hydropower accounting for the major share.
However, EU policies are aimed at increasing the share of renewable
energy sources in gross energy supply from 6% in 1995 to 12% in
2010 [2]. To reach this aim, the utilisation of biomass should be tripled.
Furthermore, the share of green electricity should be increased from
14% (in 2001) to 22% in 2010 [3]. The use of biomass within electricity
production should thereby be increased tenfold (based on 1995 values)
to 828 PJ/a in 2010.
The leading countries in the renewable energy utilisation stakes in
Europe are Sweden (about 29% primary energy consumption from
Introduction
4 prof.dr. Ingwald Obernberger 5 Ash related problems in biomass combustion plants
Explanations: source [7]; data in MW
figure 1
Development of the
capacities of newly
installed biomass
combustion systems
per year in Austria
between 1989 and
2003
renewables), Finland (about 22%) and Austria (about 21%), while in
2003 the Netherlands produced only about 1.5% of their primary energy
from renewables [1]. The Dutch national targets aim to increase the share
of renewable energy in total energy use to 5% in the year 2010 and 10%
in 2020 [4]. Moreover, the Dutch government formulated the target to
increase the share of electricity from renewable energy sources in gross
electricity consumption from 3.5% (1997) to 9.0% in 2010, which is in
line with the targets of the respective EU directive [3]. An enormous
demand for appropriate measures and activities to meet these ambitious
goals will exist in the Netherlands as well as in the whole European
Union in the near future. In 2003, 70% of renewable energy
consumption in the Netherlands, which is about 1.1% of total primary
energy consumption, originated from bioenergy followed by wind (22%)
and heat pumps (3%) [5]. Austria for instance, covered 10.97% of total
primary energy consumption from renewable energy sources (excluding
hydropower) in 2001. 10.4% were covered by the thermal utilisation
of solid biomass. 60% of these solid biomass sources are used for
residential heating (small-scale combustion units) while 32% are utilised
for process heat and electricity generation in industry (heat only and
combined heat and power plants) and 8% are utilised in biomass district
heating plants [6].
Statistics [7] reveal an almost steady increase in the number and nominal
capacity of newly installed biomass combustion systems per year in
Austria since the early 1990s (see figure 1). In this period, more than
52,600 biomass combustion units with a total capacity of 3,600 MW
have been installed. This trend towards enforced utilisation of solid
biomass for energy production is supposed to continue in the next
few years.
A significant time point in these statistics is the year 1997, when
small-scale pellet furnaces started to penetrate the Austrian market.
The introduction of this new technology increased the number of
installed small-scale applications per year from 2,280 in 1997 to 7,751
units in 2003. This example nicely demonstrates the impact of a new
and innovative renewable energy technology on the market and also
shows that new concepts for renewable energy utilisation have a high
potential to gain broad public acceptance.
Technologies for thermal biomass utilisation
As already mentioned above, the thermal utilisation of solid biomass
is expected to play a major role in future concepts for the reduction
of greenhouse gas emissions from heat and electricity production. In
general, three different technologies for thermal biomass conversion can
be applied, namely pyrolysis, gasification and combustion. Combustion
is the most advanced and market-proven application, while pyrolysis
and gasification are still in the development or demonstration stages.
Combustion based systems
A broad spectrum of biomass combustion technologies for different
types of biomass fuels (woody biomass fuels, herbaceous biomass fuels,
biodegradable wastes and residues) covering a wide range of plant
capacities are currently available. The different applications can be
divided into three main fields.
Small-scale biomass combustion units (capacity range <100 kW) are
mainly applied for residential heating systems. Here, different types
of pellet and wood chip burners, log wood boilers, wood stoves and
fire-place inserts are commonly used. These systems mainly burn
woody biomass fuels such as log wood, wood chips and pellets.
The medium capacity range covers biomass heating and biomass
combined heat and power (CHP) plants in the capacity range between
100 kW and about 10 MWth. Underfeed stokers, grate-fired furnaces
6 prof.dr. Ingwald Obernberger 7 Ash related problems in biomass combustion plants
and dust burners are the main technologies applied in this capacity
range. Heat transfer is most commonly based on hot water boilers, but
also steam boilers and thermal oil boilers. These systems usually burn
woody biomass fuels such as wood chips, sawdust, bark, forest residues
and waste wood but also straw and other agricultural residues (e.g.
sunflower husks). In addition to heat production for process and district
heat supply, combined heat and power (CHP) production systems are of
increasing importance. A number of CHP technologies such as Stirling
engines, the ORC process, steam engines and steam turbines are
therefore available.
The large-scale range (>10 MWth) mainly comprises CHP plants and
power plants with thermal capacities up to some 100 MW based on
grate-fired and fluidised bed combustion systems. These are usually fired
with woody biomass fuels (wood chips, sawdust, bark, forest residues
and waste wood) and straw but also with residues and wastes from the
agricultural industry such as fruit stones, kernels, husks and shells.
Finally it should be mentioned that co-firing of biomass fuels in largescale
coal fired power stations also offers an interesting option for
biomass utilisation. Due to the large plant sizes and the high amount
of biomass fired, co-firing offers great potential for CO2 reduction in
the short term if applied in existing power plants. A number of different
technologies for direct co-firing as well as co-firing in separate units
with junction of the steam have already proven their applicability in this
field. The biomass input in co-firing units is usually restricted to about
10-20% (by mass) of the whole fuel input if existing coal-fired power
stations are used.
State-of-the-art of biomass combustion
Combustion is the oldest biomass conversion technology and can
look back on a long history. It took mankind many thousands of years,
however, to develop really mature and ecologically friendly biomass
combustion systems. During the past decades, steadily increasing
requirements for the reduction of harmful emissions and the
improvement of plant efficiencies and availabilities have led to
extensive R&D in the following topics:
• Development of primary and secondary measures for emission
reduction (CO, NOx, SOx, HCl, PAH, PCDD/F).
• Improvement of the degree of automation of biomass combustion
systems.
• Improvement of the overall efficiency, reliability and availability
of biomass combustion plants.
As a result of these R&D efforts environmentally sound, energy efficient
and user friendly combustion systems are now available for a broad
range of capacities and a wide spectrum of different biomass fuels.
On-going research mainly focuses on
• Further optimisation of process control strategies for small-scale
combustion units.
• Utilisation of agricultural biomass fuels (grasses, crops) in small-scale
combustion units.
• Development of CHP technologies for small- and medium-scale
combustion systems.
• Further improvement of the efficiency and availability of large-scale
combustion plants.
• Solution of ash related problems.
The most relevant issue of the different research fields mentioned
above is the solution of problems related to the behaviour of ash forming
elements during biomass combustion, which include slagging, deposit
formation, corrosion, ash utilisation as well as particulate emissions.
As biomass fuels are ash rich in comparison to oil and natural gas,
ash related problems have a strong impact on the lifetime, availability
and operation of biomass combustion systems and consequently are
of great economic and ecological relevance. Due to their complexity
these issues are still not solved, but great strides have already been
made in understanding them and developing appropriate technological
mitigation measures.
Ash related problems in biomass combustion units
Solid fuels (biomass as well as coal) generally contain a considerable
amount of ash forming elements, which clearly distinguishes them from
liquid and gaseous fossil fuels. Bottom ashes and fly ashes emitted with
the flue gas are usually formed during combustion. The fly ash can be
divided into coarse fly ashes and aerosols (particles with a diameter
<1 μm). These ash fractions may cause both plant internal and emission
problems.
Appropriate dust separation devices must be applied to meet current
dust emission standards, which is not a problem for medium- and largescale
applications since highly efficient filter systems such as baghouse
filters and electrostatic precipitators are economically affordable for these
8 prof.dr. Ingwald Obernberger 9 Ash related problems in biomass combustion plants
table 1
Chemical composition
of different solid
biomass fuels
Explanations: ac … ash content; d.b. … dry basis (non ash free); GCV .. gross calorific value; mean
… mean value; s … standard deviation; no standard deviation is given for O, since this value is
calculated from the difference of the other main constituents
parameter wood chips
ac wt. % d.b.
C wt. % d.b.
H wt. % d.b.
O wt. % d.b.
N wt. % d.b.
S mg/kg d.b.
CI mg/kg d.b.
Si mg/kg d.b.
Ca mg/kg d.b.
Mg mg/kg d.b.
K mg/kg d.b.
Na mg/kg d.b.
Zn mg/kg d.b.
Pb mg/kg d.b.
GCV MJ/kg d.b.
bark
s
straw
0.9
50.40
5.91
42.65
0.12
242
1,317
56
3,195
395
907
61
35
1.1
20.07
s
0.7
0.32
0.28
0.02
225
854
28
2,317
256
376
40
29
0.7
0.27
3.5
50.31
5.79
40.12
0.24
499
3,936
202
11,287
1,351
2,368
176
115
2.1
20.10
0.6
0.51
0.26
0.20
156
2,417
52
2,525
391
603
117
26
1.0
0.56
mean s
5.6
45.82
5.38
42.65
0.58
981
14,791
3,597
3,105
867
6,603
547
23
0.8
17.96
1.7
2.17
0.43
0.83
419
6,321
3,945
1,131
590
4,471
786
25
0.7
0.97
mean mean
waste wood
(quality assorted)
mean s
3.2
48.28
5.54
41.53
1.40
835
4,068
696
4,846
994
875
1,002
405
149.3
18.75
1.5
0.43
0.14
0.53
479
2,343
430
1,676
464
227
721
151
108.8
0.32
installations. For small-scale biomass combustion systems, however,
no highly efficient and economically sound dust precipitators are
available on the market to date, which has led to significant R&D efforts
in this field. This is of special importance since many European regions
are currently facing severe air quality problems regarding fine particulate
matter (especially PM10), and small-scale biomass combustion systems
have already been identified as a major emission source together with
industry and traffic.
Plant problems caused by ashes mainly include deposit formation,
slagging and corrosion. Slagging, e.g. on the grate of a combustion
system, may disturb the combustion process and in extreme cases even
damage the grate. Deposit formation on boiler walls and tube surfaces
reduces the heat transfer and thus the efficiency of combustion units.
Severe deposit formation can also lead to the blocking of heat exchanger
sections, necessitating unexpected plant shutdowns for deposit removal
and boiler cleaning. Furthermore, deposits can be corrosive and may
thus reduce the lifetime of the installations. Considerable R&D efforts
are being made to find technological solutions for these problems,
especially for large-scale combustion units using waste wood or
herbaceous biomass fuels.
This paper presents the latest research results on ash and aerosol (fine
particulate) formation and behaviour. The knowledge of these processes
forms the basis for R&D regarding all relevant ash related problems
mentioned above, and is therefore essential for the development of
measures concerning the reduction of fine particulate emissions as
well as concerning the reduction of deposit formation and corrosion
problems in biomass furnaces and boilers.
As mentioned above, biomass fuels contain considerable quantities
of ash forming elements in addition to their main organic constituents
(C, H, O, N). The most important of these elements are Si, Ca, Mg,
K, Na, S, Cl as well as heavy metals such as Zn and Pb. Table 1 shows
typical compositions of the most commonly used solid biomass fuels.
These data reveal extreme variations in ash content and ash composition
between as well as within the different types of biomass fuels. Wood
has a much lower ash content, for example, than bark, waste wood
and herbaceous fuels.
Ash formation in biomass combustion plants
10 prof.dr. Ingwald Obernberger 11 Ash related problems in biomass combustion plants
There are generally two sources for inorganic ash forming matter in
biomass fuels. On the one hand, ash forming elements originate from
the plant itself, as they are part of the structure of the fibres (e.g.: Si,
Ca) or are macro or micro plant nutrients (e.g.: K, P, S, Zn). On the
other hand, inorganic matter in biomass fuels can also come from
contamination with soil, sand or stones. Coatings, paints, glass pieces
and metal parts are major sources of contamination in waste wood.
During the combustion of solid biomass fuels, the behaviour of ash
forming elements follows a general scheme, which is depicted in figure 2.
The following gives a brief description of the basic principles of ash
formation during biomass combustion. A discussion of the detailed
mechanisms relevant to aerosol formation will follow in the next section.
Upon entering the combustion unit, the fuel is first dried, followed
by devolatilisation of the volatile organic matter. Subsequently, the
remaining fixed carbon is oxidised during heterogeneous gas-solid
reactions, which is called char combustion. During these steps the ash
forming elements behave in two different ways according to their volatility.
Non-volatile compounds such as Si, Ca and Mg are engaged in ash fusion
as well as coagulation processes. Once the organic matter has been
oxidised, these elements remain as coarse ash structures. Easily volatile
species such as K, Na, S, Cl, Zn and Pb generally behave differently.
A considerable proportion of these elements is released to the gas phase
due to the high temperatures occurring during combustion. There they
undergo homogeneous gas phase reactions and later on, due to
supersaturation in the gas phase, these ash forming vapours start to
nucleate (formation of submicron aerosol particles) or condense on
surfaces of existing particles. The submicron particles are so-called
aerosols and form one important fraction of the fly ashes.
The second fly ash fraction is formed by small coarse ash particles from
the fuel bed entrained with the flue gas. Depending on their particle size
they are either precipitated from the flue gas in the furnace or boiler
or are entrained with the flue gas forming coarse fly ash emissions.
It is also important to mention that a share of the volatile ash forming
elements is not released to the gas phase but undergoes secondary
reactions with the non-volatile species (e.g.: Ca, Si), thus being embedded
in the coarse ash. The mechanisms of these secondary reactions and the
parameters influencing the release of volatile species from the fuel are of
great relevance for the modelling of aerosol formation and therefore are
a major issue in ongoing European research projects [8].
Another relevant issue is that almost complete combustion must be
achieved regarding charcoal as well as flue gas burnout. Otherwise
tars and hydrocarbons can also form aerosols by nucleation and
condensation processes when the flue gas temperature decreases,
causing a significant increase in aerosol emissions. In modern biomass
combustion systems these organic emissions can be kept to a minimum
using appropriate combustion and process control technologies and will
therefore not be discussed in more detail in this paper. Nevertheless,
organic fly ash emissions can be a concern in old biomass combustion
figure 2 devices [8, 9, 10].
Ash formation during
biomass combustion
12 prof.dr. Ingwald Obernberger 13 Ash related problems in biomass combustion plants
1000
800
600
400
200
0
0.01 0.10 1.00 10.00 100.00 1000.00
wood chips
bark
waste wood
dm/dlog(dp) [mg/Nm3]
dp [μm ae.d.]
figure 3
Particle size
distribution of
coarse fly ashes
and aerosols
Coarse fly ashes and aerosols – particle size distributions and
concentrations in the flue gas
As explained above, the fly ash consists of aerosols and coarse fly ashes.
The formation pathways and the particle sizes of these two fractions are
completely different.
The particle size distribution of coarse fly ashes at boiler outlet is
typically in the range of some μm to about 200 μm with a distribution
peak at about 30 to 70 μm (see also figure 3) [11]. The exact shape of
the particle size distribution curve as well as its peak depend on the fuel,
the geometry of the furnace and the particle precipitation in the different
furnace and boiler sections. The mass of coarse fly ashes at boiler outlet
can range between some mg/Nm3 up to more than 1,000 mg/Nm3
depending on different parameters like for instance the ash content
of the fuel, the load and the operating conditions of the combustion
unit (see next section).
Explanations: dp … particle diameter; ae.d. … aerodynamic diameter; data related to dry flue gas
and 13 vol.% O2, results from measurements at boiler outlet of a grate-fired combustion unit; aerosol
measurements performed with a low-pressure cascade impactor; coarse particle sampling carried out
with a cyclone and subsequent analysis of the particle size distribution using a sedimentation method
Coarse fly ashes and aerosols
The typical particle size range of aerosols is <1 μm. The mass mean
diameter of the particle size distribution is usually in a range between
0.1 and 0.5 μm and increases with increasing aerosol mass as a result
of coagulation effects. The concentration of aerosols in the flue gas
depends on the amount of aerosol forming elements released from
the fuel during combustion. Typical values are 20 – 50 mg/Nm3 for
softwood, 50 – 100 mg/Nm3 for hardwood and bark as well as >100
mg/Nm3 for straw and waste wood (all data related to dry flue gas and
13 vol.% O2) [12].
It can clearly be seen that coarse fly ash and aerosol formation differ
significantly when using different biomass fuels. The reasons as well
as the parameters influencing the formation and behaviour of these
two fly ash fractions are explained in the following sections.
Both fractions must always be considered, however, in regard to ash
related problems in biomass combustion units, because they either
both contribute to these problems or influence each other during their
formation process.
Coarse fly ashes
Formation and influencing parameters
During combustion, ash particles and charcoal particles are entrained
from the fuel bed with the flue gas. These particles form the coarse fly
ash fraction. Particles which are too large to follow the flow patterns
of the flue gas in the furnace are precipitated by impaction on furnace
walls or by gravitational settling. Entrained charcoal particles undergo
combustion processes and should reach a complete burnout before the
flue gas enters the convective boiler section. In modern biomass
combustion systems, the amount of organic carbon in the fly ash is
usually well below 5 wt.% (d.b.), which is an important guiding value
for complete combustion.
The remaining coarse fly ash particles enter the heat exchanger section
where again precipitation and impaction of particles on boiler tube
surfaces take place.
The particles which pass the heat exchanger section are emitted with
the flue gas and form the coarse fly ash emissions (right hand peak in
figure 3).
Consequently, the concentrations of coarse fly ashes in the flue gas
increase with increasing disturbance of the fuel bed, which means e.g.
• increasing load of the combustion unit,
14 prof.dr. Ingwald Obernberger 15 Ash related problems in biomass combustion plants
Explanations: bark (left image): particle sampling with polycarbonate filter, picture width: 38.2 μm;
waste wood (right image): particle sampling with a cyclone; particles embedded in resin, the resin
block was cut and subsequently polished, picture width: 110 μm
figure 5
SEM image of coarse
fly ash particles from
bark and waste wood
combustion in a
grate furnace
figure 4
SEM image and X-ray
mapping images
of a coarse fly ash
particle and aerosols
from wood chip
combustion in a
grate furnace
K P Si Al
Explanations: wood chips: spruce; picture width: 8.82 μm,
electron energy: 7 keV; the intensity of the colour indicates
the concentration level of an element
SEM image
• uneven distribution of the primary combustion air flow over the fuel
bed (in fixed-bed combustion systems),
• uneven distribution of the fuel over the fuel bed (in fixed-bed
combustion systems).
Furthermore, coarse fly ash emissions usually increase with increasing
ash content and decreasing particle size of the fuel. Furnace designs
providing areas with low flue gas velocities or sharp turns of the flue
gas duct can, however, help to enhance coarse fly ash precipitation in
the furnace and thereby decrease emissions.
Finally the combustion technology itself also influences the coarse
fly ash emissions. Dust injection burners or fluidised bed incinerators,
where the combustion concept is based on the dispersion of the fuel in
a gas flow, show significantly higher fly ash emissions than fixed-bed
systems such as grate-fired furnaces and underfeed stokers.
Structure and chemical properties of coarse fly ashes
figure 4 shows an image of a typical coarse fly ash particle surrounded
by aerosols sampled during spruce combustion on a polycarbonate filter.
Coarse fly ash particles sampled during bark (left image) and waste wood
(right image) combustion are presented in figure 5. The images were taken
with a scanning electron microscope (SEM). Figure 4 also shows x-ray
mapping images obtained by energy dispersive x-ray spectrometry (EDX).
The data in figure 4 clearly indicate that the coarse fly ash particle mainly
consists of Ca, Mg, P, Si and Al, while the surrounding aerosols are mainly
formed by K, S and Cl. The main ash matrix elements (Si, Ca, Mg) are
usually bound as oxides, but also phosphates and sulphates are formed.
Coarse fly ashes provide surfaces for the condensation of ash forming
vapours during their passage through the boiler sections, and their
surfaces can therefore be enriched with alkaline and heavy metal salts.
Aerosols
Formation and influencing parameters – general aspects
The formation pathway of aerosols is much more sophisticated.
As mentioned above, aerosols are formed by gas-to-particle conversion
processes of ash forming vapours, which have been released from the fuel
during combustion. Consequently, the concentration of aerosol forming
elements in a fuel, or more precisely, the release of aerosol forming
Ca Mg S Cl
16 prof.dr. Ingwald Obernberger 17 Ash related problems in biomass combustion plants
1 2
atom% atom%
K 28.5 27.1
Na 2.7 7.6
S 9.0 9.2
Cl 1.6 5.4
Zn 7.3 2.8
O 50.4 47.8
1 2
atom% atom%
K 27.3 37.1
S 8.7 4.7
Cl 18.1 34.5
Zn 3.4 2.4
Ca 1.3 0.0
O 40.7 21.3
1 2
atom% atom%
K 8.7 13.1
Na 4.4 0.0
S 0.0 3.2
Cl 36.6 44.6
Zn 12.6 10.4
Pb 25.5 6.5
O 12.6 22.5
SEM images and
results from EDX
analyses of aerosols
figure 6
Waste wood
(picture width: 2 μm)
Bark
(picture width: 4 μm)
Wood chips (spruce)
(picture width: 4 μm)
elements from the fuel during combustion, is the most relevant
parameter for aerosol formation. Aerosol formation cannot be influenced
to a relevant extent by primary measures using state-of-the-art plant
designs and process control strategies.
Results from recent research have shown [11, 12], however, that different
aerosol formation pathways can be defined for different types of biomass
fuels. The following section gives a short characterisation of aerosol
particles from biomass combustion to provide a better understanding
of the subsequent detailed discussion of aerosol formation mechanisms.
Structure and chemical properties of aerosols
The basic mechanisms of aerosol formation in combustion processes
are generally well known from previous research work [13, 14]. As already
mentioned, aerosols are formed by nucleation of ash forming vapours.
The particles subsequently grow by condensation of vapours on their
surfaces as well as by coagulation processes.
Figure 6 shows SEM images and results from EDX analyses of aerosols.
It can be derived that aerosols consist of almost spherically shaped
particles and agglomerates thereof, which is typical for particles formed
by nucleation and subsequent coagulation.
K, Na, S and Cl are the most relevant aerosol forming species in the
specific case of chemically untreated wood combustion. Consequently,
the formation of K and Na sulphates and K and Na chlorides followed by
nucleation of these compounds are the most relevant processes leading
to aerosol formation. Carbonates are formed if there is not enough
S and Cl available to bind all the K and Na released from the fuel. This
behaviour is confirmed by the results of the EDX analyses presented in
figure 6.
Additionally, easily volatile heavy metals such as Zn and Pb significantly
contribute to aerosol formation, especially with increasing concentration
of these elements in the fuel. Aerosols formed during the combustion of
waste wood (which is very rich in heavy metals), for instance, may contain
more heavy metal compounds than alkaline compounds [11, 12, 16].
Figure 6 also shows, however, that non-volatile compounds such as Ca
can also be found in the submicron aerosol fraction. This phenomenon,
which is generally in contradiction with the classic aerosol formation
theories based on gas-to-solid phase transition, is discussed in the
next section.
Explanations: particle sampling with polycarbonate filters at a 440 kWth grate-fired combustion plant
Detailed aerosol formation pathways
In order to investigate aerosol formation processes in fixed-bed combustion
units, the Institute for Resource Efficient and Sustainable Systems, Graz
University of Technology, performed a considerable number of test runs
in biomass combustion plants equipped with different furnace and boiler
technologies using a broad range of solid biomass fuels [12, 16, 17].
The particle size distributions of aerosols downstream of the boiler as well
as the chemical compositions and shapes of the particles have thus been
determined. The results and experiences gained from these test runs as
well as theoretical considerations provided the basis for postulating
different aerosol formation processes for different biomass fuels, which
are discussed in the following and are summarised in figure 7.
18 prof.dr. Ingwald Obernberger 19 Ash related problems in biomass combustion plants
coarse fly ashes aerosols
cooling
phase
gas phase
reactions
K, Na, S, CI, easily volatile heavy
metals released from the fuel to the
gas phase
condensation
of vapours on surfaces of
coarse fly ashes
surface reactions
of vapours with coarse
condensation
of vapours on surfaces of
existing aerosols
nucleation
of supersaturated
ash vapours
coarse fly ash
particles entrained
from the fuel bed
bottom ash
refractory species and
volatile compounds bound
in the bottom ash
bark combustion:
submicron ZnO and CaO particles
waste wood combustion:
submicron ZnO particles
figure 7
Fly ash and aerosol
formation pathways
in fixed-bed
combustion systems
The first step of aerosol formation in biomass combustion processes
is always the release of aerosol forming elements from the fuel.
K, Na, S, Cl as well as easily volatile heavy metals such as Zn and Pb
are released from the fuel bed to the gaseous phase. These elements
subsequently undergo homogeneous gas phase reactions.
The most important compounds formed are K and Na chlorides and
sulphates. As soon as the partial pressure of a single compound exceeds
the saturation pressure, which can be due to a high formation ratio of
the specific compound, due to different thermodynamic properties of
a new compound formed or due to the cooling of the flue gas, gas-toparticle
conversion takes place. This can happen either by homogeneous
nucleation (the formation of aerosol particles) or condensation of these
vapours on existing surfaces (see also figure 2). These surfaces can
either be other particles (aerosols or coarse fly ashes) or, e.g., boiler
tubes. Nucleation and condensation are always competing processes,
which means that if the existing particles provide enough surface area,
nucleation might be suppressed and the dominating process is then
particle growth by condensation. Once particles have been formed,
however, they start to coagulate due to Brownian motion, diffusion and
turbulent impaction. The typical particle size distribution of the aerosol
fraction measured at boiler outlet thus depends on the time-temperature
profile of the biomass combustion plant.
The mechanism explained usually prevails during the combustion of
chemically untreated wood chips and straw, where K, Na, S and Cl are
the dominating elements involved in aerosol formation. This formation
pathway has been confirmed by the shape and chemical composition
of the aerosols sampled at boiler outlet and results from theoretical
modelling of the aerosol formation process [12, 15].
Zn-rich fuels (such as bark and especially waste wood) make aerosol
formation more complex, which is mainly due to the exceptional
position of Zn within aerosol formation. Under reducing conditions,
as commonly prevalent in the fuel bed of a fixed-bed biomass furnace,
elemental Zn can be released to the gas phase. As soon as the vapour
pressure of oxygen rises, Zn is oxidised into ZnO. Since the saturation
pressure of ZnO is low, ZnO nucleation takes place immediately,
resulting in the formation of particles in a size range of some nm.
This process is assumed to happen directly after the ash vapours have
left the fuel bed. The higher the concentration of Zn in a fuel, the higher
the relevance of this ZnO particle formation step for all subsequent
processes, because the ZnO particles provide a large surface area for
the condensation of ash forming vapours. Especially during waste
wood combustion, the amount of ZnO particles can be so high that
subsequent formation of aerosols by nucleation of other vapours (e.g.:
KCl, K2SO4) is totally suppressed by condensation on the ZnO surfaces.
As a result, particle formation takes place immediately above the fuel bed
(in the primary combustion zone) when burning Zn-rich fuels, while the
combustion of biomass fuels with low Zn concentrations is supposed
to result in nucleation of ash forming vapours upon cooling of the flue
gas (in the boiler section).
This behaviour has been confirmed by measurements with a newly
developed high-temperature low-pressure impactor (HT-LPI), which
was specially designed for the purpose of taking particle samples directly
from the hot furnace at temperatures up to 1,100°C in order to get more
insight in particle formation processes [11]. Figure 8 shows a SEM image
as well as EDX analyses of aerosol particles sampled with this new
impactor upstream of the boiler inlet. The results of the EDX analyses
clearly indicate that at this position, ZnO particles have already been
formed but almost no nucleation or condensation of alkaline compounds
has occurred.
20 prof.dr. Ingwald Obernberger 21 Ash related problems in biomass combustion plants
Explanations: grate combustion; analytical results excluding C
figure 9
SEM image and
EDX analyses of a
charcoal particle from
bark combustion
SEM image and
results from EDX
analyses of particles
sampled with a
high-temperature
low-pressure impactor
figure 8
SEM picture width: 1.1 μm; particle sampling took place in the secondary combustion zone of
a grate-fired combustion unit at a temperature of about 1,100°C; fuel: waste wood; analytical
results normalised to 100% without oxygen
Zn 93.6 wt. %
Si 2.4 wt. %
K 1.4 wt. %
S 1.1 wt. %
Fe 0.5 wt.%
Ca 0.5 wt.%
P 0.3 wt.%
Al 0.2 wt.%
Another process relevant to aerosol formation during bark combustion
was investigated in connection with the high Ca concentration in bark
(see figure 6). SEM/EDX analyses of aerosols from bark combustion
have shown that they contain CaO particles in a size range > 0.3 μm.
Since Ca cannot be evaporated under the conditions prevailing in a
fixed biomass bed, another mechanism must be responsible for the
formation of these particles. SEM/EDX analyses of fuel bed chars from
bark combustion indicate that the char contains Ca structures consisting
of submicron and almost spherical CaO particles (see figure 9) [18, 19].
It can therefore be assumed that CaO particles from these structures are
entrained from the fuel bed due to thermal defragmentation within the
process of char combustion. These particles can then act as seeds for
surface condensation of other ash forming vapours.
A comprehensive characterisation of aerosols and coarse fly ashes
formed during biomass combustion is now available as a result of the
extensive research carried out during the past decade. These data have
provided the basis for identifying the formation mechanisms of aerosols
and coarse fly ashes.
With respect to the formation of coarse fly ashes, it has been shown
that the application of a state-of-the-art combustion system enabling an
even distribution of the fuel and the combustion air over the grate may
help to reduce particle emissions in fixed-bed furnaces. Furthermore,
appropriate fuel preparation and storage measures for the reduction of
mineral impurities will also help to decrease coarse fly ash formation.
Due to their comparatively large particle sizes (between some μm and
about 200 μm) coarse fly ashes can easily be precipitated from the flue
gas with cyclones and multi-cyclones, but the reduction of coarse fly ash
formation may be of relevance with respect to the reduction of deposit
formation in furnaces and boilers.
In case of a complete combustion aerosols are formed by gas-to-particle
conversion of ash forming vapours. However, if no complete combustion
is achieved, aerosols can also originate from condensation of gaseous
organic compounds (tars, hydrocarbons). Particle precipitation as well as
deposit formation are important issues in regard to aerosols. The small
particle sizes of significantly less than 1 μm require highly sophisticated
dust separation devices for emission control. Furthermore, aerosols
contribute to deposit formation and may contribute to corrosion.
The research results on aerosol formation clearly indicate that the fuel
used is the governing parameter for the mass of aerosols formed during
biomass combustion. The K, Na, S, Cl, Zn and Pb concentrations in the
fuel determine the mass of aerosol emissions as well as their chemical
compositions. Consequently, the amount of aerosols formed during
combustion can be ranked as following: softwood < hardwood < bark <
waste wood and straw.
The experiences and results of the test runs and the application of a
newly developed aerosol formation model revealed different formation
mechanisms depending on the chemical composition of the biomass
Summary and conclusions
atom%
Ca 33
K 24
Mg 2
Mn 1
Cl 1
O 39
22 prof.dr. Ingwald Obernberger 23 Ash related problems in biomass combustion plants
fuels. The nucleation of K sulphate and chlorine vapours is the
most important particle formation mechanism in the combustion of
chemically untreated wood and straw. CaO particles released from the
fuel as well as ZnO particles formed directly above the fuel bed influence
aerosol formation during bark combustion by providing additional
surface area for the condensation of K compounds. The high Zn
concentrations in waste wood lead to the formation of a large number
of ZnO particles in the flue gas right above the fuel bed, which usually
provide sufficient surface area to suppress nucleation of ash forming
vapours by surface condensation.
As the different formation mechanisms indicate, the first steps of
aerosol formation may occur in different furnace and boiler regions
depending on the fuel used. While aerosol formation during waste wood
combustion starts right above the fuel bed, particle formation during
the combustion of chemically untreated wood and straw is assumed to
occur during the cooling phase in the boiler. These results concerning
the different pathways of aerosol formation and the properties of
aerosols formed during biomass combustion are to be used in the
future to develop more sophisticated models for deposit formation in
furnaces and boilers, which combine aerosol formation modelling and
computational fluid dynamics (CFD).
However, there is still a lack of reliable data and models describing the
release behaviour of aerosol forming species from the fuel, which are of
relevance for the modelling of aerosol formation. At the moment, aerosol
formation can only be predicted if release data from measurements
are available, which is usually not the case, and therefore assumptions
concerning the initial conditions for aerosol formation must be made.
Future research should therefore focus on the development of models
which are able to describe the release of ash forming elements from the
fuel in order to increase the prediction accuracy of aerosol formation
modelling.
Finally it should be mentioned that it is well known, that the impact of
particulates on the human organism increases with decreasing particle
size. Consequently, aerosols are predestined to contribute to health
risks. However, actually no detailed information about the health risks
of aerosols from biomass combustion compared with fine particulate
emissions from other sources are available. In this field a high demand
for additional research is given.
The knowledge about the formation pathways and chemical
compositions of aerosols formed during the combustion process,
significantly contributes to a better understanding of the mechanisms
involved in deposit formation and corrosion processes in biomass
boilers. By combining advanced fuel characterisation tools with the
knowledge available concerning aerosol and fly ash formation, models
to predict the risks for deposit formation and corrosion can be worked
out with respect to the fuels used in a combustion unit, and appropriate
measures to reduce these ash related problems can be considered already
during the design phase of a plant.
24 prof.dr. Ingwald Obernberger 25 Ash related problems in biomass combustion plants
My main aims and objectives for my work as a part-time professor
for “Thermochemical Biomass Conversion” in the Department of
Mechanical Engineering at Technische Universiteit Eindhoven (TU/e)
are the following:
First of all I would like to strengthen and improve the knowledge and
competence in the field of “Thermochemical Biomass Conversion” at
TU/e by supplying expertise and experience in this field, which I have
gained through my work at Graz University of Technology and the
engineering company BIOS BIOENERGIESYSTEME GmbH in Graz
over the past 10 years.
A lecture entitled “Thermochemical Biomass Conversion” has been
established at TU/e in the course of my appointment. This lecture aims
at the education of students in this new and important field of energy
utilisation from solid biomass and at the initiation and supervision of
internships and MSc theses in this working area.
Another focus of my work will be the co-ordination and further
development of R&D activities and the support of ongoing activities in
the field of thermochemical conversion of solid biomass in the Division
of Thermo Fluids Engineering at TU/e. Within this aim, international
and national R&D projects covering MSc and PhD thesis work will be
carried out.
In addition, I am committed to improving the co-operation between
TU/e and interested companies active in the field of thermochemical
biomass conversion (on an international level). Drawing on my
background in basic and applied research, development and the
demonstration of new technologies (as a first step towards market
introduction), my aim is to establish strong links between the university
and industry in order to cover the entire development chain from
basic research through to the demonstration of new developments.
An important step towards this goal is the participation of TU/e in the
Austrian Bioenergy Centre, an Austrian Centre of Competence, where
scientific and industrial partners (on an international basis) carry out
joint R&D projects in the field of “Thermal Biomass Utilisation”.
Finally, I would like to strengthen the co-operation and the utilisation
of synergies between TU/e and Graz University of Technology in Austria
through my work at both institutions. Examples of how this objective is
being achieved are ongoing student exchanges, joint (international) R&D
projects and the shared use of laboratory and measurement equipment.
This international co-operation between universities will be of increasing
importance in the near future and is also supported by the EU.
Aims and objectives for my work at TU/e
26 prof.dr. Ingwald Obernberger 27 Ash related problems in biomass combustion plants
First and foremost I would like to thank the Executive Board of
Technische Universiteit Eindhoven (TU/e) for my appointment as
part-time professor for “Thermochemical Biomass Conversion” in the
Department of Mechanical Engineering. It was a great honour for me to
accept this appointment.
Moreover, I would like to thank the Dean of the Department of
Mechanical Engineering, prof.dr.ir. Dick van Campen, for his support
during the start of my work at TU/e.
My special thanks are due to the three full-time professors at the
Division of Thermo Fluids Engineering, prof.dr.ir. Bert Brouwers
(process technology), prof.dr.ir. Anton van Steenhoven (energy
technology) and prof.dr.ir. Philip de Goey (combustion technology),
for the good co-operation and the valuable information exchange.
I would like to extend my special thanks to prof.dr.ir. Bert Brouwers,
for the long and successful R&D co-operation between us, for his
valuable comments and new ideas concerning specific R&D related
topics, for the excellent working atmosphere in the process technology
section and for the friendship that has developed between us during
the past few years.
My thanks also go to Prof.Dipl.-Ing.Dr. Michael Narodoslawsky, who
has supported and still supports my scientific career at Graz University
of Technology and who has introduced me to the subject of sustainable
energy utilisation and renewable energy sources.
Furthermore, I would like to thank my colleague Dipl.-Ing. Thomas
Brunner, and dr.ir. Erik van Kemenade for their scientific assistance and
co-operation in R&D projects and for their much valued friendship.
I would also like to thank my parents for their support, which has made
it possible for me to study.
Finally, I would like to thank my wife, Evanthia, for her strong support
throughout my career and her love, which is the most important support
I receive.
1 INTERNATIONAL ENERGY AGENCY, 2004: Key World Energy
Statistics, 2004 Edition, www.iea.org/dbtw-wpd/Textbase/nppdf/
free/2004 keyworld2004.pdf
2 EUROPEAN UNION, 1995: Energy for the future - renewable sources
of energy, White Paper, COM(95) 682, 13.12.1995
3 EUROPEAN COMMISSION, 2001: Directive 2001/77/ec of the
European Parliament and of the Council of 27 September 2001 on
the promotion of electricity produced from renewable energy sources
in the internal electricity market
4 Minister of Economic Affairs, 1995: Derde Energienota, Ministry of
Economic Affairs, The Hague, The Netherlands
5 http://www.duurzame-energie.nl/downloads/RES2003.pdf
6 SEDMIDUBSKY A., LUTTER E., 2003: Daten zu erneuerbarer
Energie in Österreich 2003, Energieverwertungsagentur, Vienna,
Austria
7 JONAS A., HANEDER H., 2004: Zahlenmäßige Entwicklung der
modernen Holz- und Rindenfeuerungen in Österreich, Gesamtbilanz
1989 bis 2003, Forstabteilung der Niederösterreichischen Landes-
Landwirtschaftskammer (Ed.), St. Pölten, Austria
8 OBERNBERGER I. (ed.), BRUNNER T. (ed.), 2005: Proceedings
of the international workshop “Aerosols in Biomass Combustion”
18th March 2005, Graz University of Technology, Austria. Series
“Thermal Biomass Utilization”, Volume 6, ISBN 3-9501980-2-4,
Graz, Austria
9 JOHANSSON L. S., LECKNER B., GUSTAVSSON L., COOPER D.,
TULLIN C.,POTTER A., BERNTSEN M., 2005: Particle Emissions
Acknowledgement References
28 prof.dr. Ingwald Obernberger 29 Ash related problems in biomass combustion plants
from Residential Biofuel Boilers and Stoves – Old and Modern
Techniques. In: Proceedings of the international workshop “Aerosols
in Biomass Combustion” 18th March 2005, Graz University of
Technology, Austria. Series “Thermal Biomass Utilization”,
Volume 6, ISBN 39501980-2-4, Graz, Austria
10 OSER M., NUSSBAUMER TH., MÜLLER P., MOHR M., FIGI R,
2004.: Mechanisms of Particle Formation in Biomass Combustion.
In: Proceedings of the Second World Biomass Conference, 10-14 May
2004, ISBN 88-89407-04-2, pp.1246–1249, ETA Florence and WIP
Munich (ed.), Rome, Italy
11 OBERNBERGER I., BRUNNER T., JÖLLER M., 2001:
Characterisation and Formation of Aerosols and Fly-Ashes from
Fixed-Bed Biomass Combustion. In: Proceedings of the international
IEA Seminar “Aerosols in Biomass Combustion”, Zuerich,
Switzerland, ISBN 3-908705-00-2, pp.69-74, IEA Bioenergy
Agreement, Task 32 “Biomass Combustion and Co-Firing” c/o TNOMEP
(ed.), Apeldoorn, Netherlands
12 BRUNNER T., JÖLLER M., OBERNBERGER I., 2004: Aerosol
formation in fixed-bed biomass furnaces - results from
measurements and modelling. To be published in: Proc. of the
Internat. Conf. "Science in Thermal and Chemical Biomass
Conversion", Sept 2004, Victoria, Canada
13 FRIEDLANDER S.K., 1977: Smoke Dust and Haze, ISBN: 0-19-
512999-7, John Whiley and Sons, New York, USA
14 CHRISTENSEN K. A., 1995: The Formation of Submicron Particles
from the Combustion of Straw, Ph.D. Thesis, ISBN 87-90142-04-7,
Department of Chemical Engineering, Technical University of
Denmark, Lyngby, Denmark
15 JÖLLER Markus, BRUNNER Thomas, OBERNBERGER Ingwald,
2005: Modeling of Aerosol Formation in Biomass Combustion in
Grate Furnaces and Comparison with Measurements. In: Energy and
Fuels, Volume 19 (2005), pp. 311-323
16 OBERNBERGER I., BRUNNER T., FRANDSEN F., SKIFVARS B.-J.,
BACKMAN R., BROUWERS J.J.H., VAN KEMENADE E., MÜLLER
M., STEURER C., BECHER U., 2003: Aerosols in fixed-bed biomass
combustion – formation, growth, chemical composition, deposition,
precipitation and separation from flue gas, final report, EU project
No. NNE5-1999-00114, European Commission DG Research (ed),
Brussels, Belgium
17 FRANDSEN F., 2005: Utilising biomass and waste for power
production – a decade of contributing to the understanding,
interpretation and analysis of deposits and corrosion products.
In: Fuel, Volume 84 (2005), pp. 1277-1294
18 WERKELIN J., 2002: Distribution of Ash-Forming Elements in
Four Trees of Different Species, Master’s Thesis, ISBN 952-12-10079,
Åbo Akademi, Process Chemistry Centre, Combustion and Materials
Chemistry, Abo, Finland
19 ZEVENHOVEN-ONDERWATER M., 2001: Ash-forming matter in
biomass fuels, Ph.D. Thesis, ISBN 952-12-0813-9, Grafia OY, Abo,
Finland
30 prof.dr. Ingwald Obernberger 31 Ash related problems in biomass combustion plants
Prof.Dipl.-Ing.Dr.Ingwald Obernberger has been appointed parttime
professor at Technische Universiteit Eindhoven (TU/e) for
“Thermochemical Biomass Conversion” in the department of Mechanical
Engineering as of June 1st 2003.
Prof.Dipl.-Ing.Dr.Ingwald Obernberger (1962) graduated from Graz
University of Technology in Austria (GUT), where he completed his PhD
with distinction in 1994. Since then he has been Managing Director
of the engineering company BIOS BIOENERGIESYSTEME GmbH
and Head of the Thermal Biomass Utilisation Group at the Institute for
Resource Efficient and Sustainable Systems (GUT), where he since 1995
has lectured on Thermal Biomass Utilisation. His appointments include
scientific adviser to the European Commission, DG TREN, section
Biomass and Waste, since 1996; Assistant Professor for Environmental
and Energy Engineering at GUT, since 1997; Austrian representative
in the International Energy Agency (IEA), Bioenergy Agreement,
TASK 32, Biomass Combustion and Cofiring, since 1998; and member
of the editorial board of the international scientific journal Biomass
and Bioenergy. Since June 2003 he has been Professor (part-time) for
“Thermochemical Biomass Conversion” at TU/e and key researcher at
the Austrian Bioenergy Centre, Graz.
Moreover, Prof.Dipl.-Ing.Dr.Ingwald Obernberger is coordinator of
several national and international R&D projects in the field of thermal
biomass utilisation and author of a considerable number of scientific
publications. His main research areas are the characterisation of the
physical and chemical properties of biomass fuels and ashes, ash related
problems in biomass combustion plants, the development of biomass
furnace and flue gas cleaning technologies, the simulation of reacting
flows in the gas phase of fixed bed biomass furnaces and boilers as well
as the development of innovative small-scale biomass CHP technologies.
Curriculum Vitae
32 prof.dr. Ingwald Obernberger
Colophon
Production:
Communicatie Service Centrum TU/e
Photography cover:
Fotohaus Furgler, Graz (A)
Design:
Plaza ontwerpers, Eindhoven
Print:
Drukkerij Lecturis, Eindhoven
ISBN: 90-386-1443-8
Digital version:
www.tue.nl/bib/