Utilization of Ashes From Biomass Combustion and Gasification

Abstract

Utilization of ashes in useful applications will contribute to the sustainable use of biomass for power generation. Returning of the ashes to the locations where the biomass was harvested is presented as the most sustainable option to pursue, because it returns nutrients to the original soils. For ashes that cannot be recycled, alternative options are discussed: utilization as fertilizer or as raw material for the production of fertilizer, utilization as building material or as component in building products, and utilization as fuel. A separation between bottom ashes, fly ashes from combustion and carbon-rich fly ash from fluidized bed gasification was made. Primary conclusion was that the various ash streams from different fuels and installation types require different forms of utilization. Bulk utilization of ashes is most likely to be found in building materials or fuels (only high-carbon ashes). Niche applications and lucky matches between ash and special applications may be found, but are not contributing to the solutions for the bulk of the ashes. For certain ash streams after-treatment is needed. The key factors for success in all forms of utilization are consistency of ash quality and availability of large quantities.

Full text

ECN-RX--05-182
Utilization of ashes from biomass
combustion and gasification
Jan R. Pels
Danielle S. de Nie
Jacob H.A. Kiel
Published at 14th European Biomass Conference & Exhibition,
Paris, France, 17-21 October 2005

ECN-RX--05-182 2

UTILIZATION OF ASHES FROM BIOMASS COMBUSTION AND GASIFICATION
Jan. R. Pels, Danielle S. de Nie and Jacob H. A. Kiel
Energy Research Centre of The Netherlands, Biomass Department & Clean Fossil Fuel
Department
P.O. Box 1, 1755 ZG, Petten, The Netherlands
phone: +31-224-564884, e-mail: pels@ecn.nl, http://www.ecn.nl
* Author to whom correspondence should be directed
ABSTRACT: Utilization of ashes in useful applications will contribute to the sustainable
use of biomass for power generation. Returning of the ashes to the locations where the
biomass was harvested is presented as the most sustainable option to pursue, because it
returns nutrients to the original soils. For ashes that cannot be recycled, alternative
options are discussed: utilization as fertilizer or as raw material for the production of
fertilizer, utilization as building material or as component in building products, and
utilization as fuel. A separation between bottom ashes, fly ashes from combustion and
carbon-rich fly ash from fluidized bed gasification was made. Primary conclusion was
that the various ash streams from different fuels and installation types require different
forms of utilization. Bulk utilization of ashes is most likely to be found in building
materials or fuels (only high-carbon ashes). Niche applications and lucky matches
between ash and special applications may be found, but are not contributing to the
solutions for the bulk of the ashes. For certain ash streams after-treatment is needed. The
key factors for success in all forms of utilization are consistency of ash quality and
availability of large quantities.
Keywords: ash, bio-energy strategy, recovery of residues
1 INTRODUCTION
Utilization of ashes is part of
sustainable power generation from
biomass and contributes to the green
image, while landfill of biomass ashes
may be interpreted as wasting of
valuable nutrients. The search for
utilization options must deal with
largely different kinds of biomass ash.
Fuel composition and installation type
are the primary factors that influence ash
quality. Variations in the inorganic
fraction of fuels are directly reflected in
the ash compositions. The different
kinds of installations result in multiple
kinds of ash flows, each with differences
in morphology, composition and
leaching behavior. The main
classifications of ashes are between fly
ash and bottom ash, and between ashes
from fluidized bed combustion,
fluidized bed gasification, grate stokers
and entrained flow gasification.
In the waste management hierarchy,
see Figure 1, landfill is the least
attractive alternative. However for many
practical situations it happens to be the most
economic alternative. Landfill serves as the
bottom-line for comparisons of utilization
options. Any proposed kind of utilization must
have economic advantages compared to land
fill. Carbon-rich fly ashes from fluidized bed
gasification are particularly difficult kinds of
ashes. The unique position of this kind of fly
ashes is mainly due to the high caloric value as
a result of the relatively high amounts of
unburned carbon. Other characteristics are that
the gasification fly ash has a low density and
are difficult to handle. Contact with air should
be avoided to avoid spontaneous ignition and
possibly dust explosions. Disposal of high
caloric ashes in landfills is discouraged or
made nearly impossible (by means of very
high gate fees) in many European countries.
For high-carbon fly ashes, after-burning plus
landfill should serve as the bottom-line option.
The potential problems associated with
gasification fly ash should not be a deterrent
for implementation of gasification as a
technique for biomass conversion, but its fly
ashes require special attention.
At ECN, two projects were completed
recently: BIOAS aimed at utilization of ashes
ECN-RX--05-182 3

from combustion of clean biomass and
GASASH aimed at utilization of
gasification ashes. GASASH is a EU 5th
Framework project together with VTT,
Foster Wheeler, AICIA, PVO and
Essent Energy Production.
Figure 1 Proximity principle
2 RECYCLING OF NUTRIENTS
The most sustainable way to deal
with biomass ashes is to minimize ash
content in the fuel, i.e. to harvest
biomass in such a way that nutrients and
other ash-forming components remain
on site and are not removed together
with the biomass. Sustainable forestry
techniques that minimize the amount of
nutrients during harvesting of trees are
under development and should be
pursued from the point of view of ash
management.
Returning of biomass ashes to the
locations where the biomass was
harvested can be regarded as the next
best sustainable option. It brings back
the nutrients to the original soils and
hence closes mineral recycles. In the
long run, recycling avoids problems
with depletion and exhaustion of the
soils, unless the soils are fertilized in
other ways. For recycling, combustion
ashes seem more appropriate, but in
principle, gasification ashes can also be
used. The requirements of trees or crops
and sustainable conditioning of the soil
should be leading principles. E.g., ash
recycling should not result in an
uncontrolled pH shock in the soils.
Biomass ashes should be recycled
whenever possible, but the ash quality
must be high enough to prevent
pollution as a result of the spreading of
ashes. Most European countries with a
history in the use of biomass for energy
(Finland, Sweden, Denmark, Austria
and Germany among others) have
established legislation that enables and
controls the recycling of biomass ashes
to forests and agricultural areas. Ash recycling
is happening in a number of these countries,
but the total amounts that are recycled are very
small compared to the total production of
biomass ash. Often ashes contain too high
contents of heavy metals (Cd, but also Pb and
Zn) even when the ashes originate from clean,
untreated biomass. The problem exists mainly
for fly ashes and less for bottom ashes, but the
amounts of nutrients in bottom ashes are also
often lower.
It can be argued that under special
circumstances it might be acceptable to return
more ash than the legal limits allow. This
might be done, only if the biomass ashes are
recycled without any other material being
added to the soils, because in that case all
contaminants originate from that particular
area and there will be no net increase in
contaminants. Such a system is possible, but
probably difficult to monitor and enforce.
3 BULK UTILIZATION OPTIONS
Despite recycling of ashes being the most
sustainable form of ash utilization, there will
be large amounts of biomass ash produced that
cannot be recycled for a number of reasons.
Some landowners do not want to recycle ashes,
e.g. recreational areas or natural reserves. Also
there are large streams of biomass of which it
is not possible to trace back the original place
of harvest. Finally, there may be no legal
possibilities or ashes are simply not clean. For
all these streams of biomass ashes, an
alternative form of utilization must be found.
Three global utilization options have been
investigated and will be discussed in the next
sections:
• Utilization as (raw material for) fertilizer,
• Application as building material or as
component in the manufacture of building
material,
• Re-use as fuel
Niche applications will always exist, as
well as special cases where the characteristics
and quality of a certain kind of ash from a
perfect match with a certain form of
utilization. These “lucky matches” are not
discussed because they do not represent bulk
utilization options.
ECN-RX--05-182 4

4 UTILISATION AS FERTILIZER
Biomass ashes may be used directly
as a fertilizer or soil improver or may be
used as a raw material in the production
of mineral fertilizer. The ashes are
returned to the soil, but the location is
not necessarily the same as where the
biomass originated. Utilization as (raw
material for) fertilizer saves primary
resources and can be seen as an example
of sustainable use of biomass.
Utilization of biomass ashes in all-
purpose fertilizers is limited. The three
elements in complete fertilizers are N, P
and K. Biomass ashes can only be a
significant source of potassium, because
a) ashes from thermal processes are
nearly free of nitrogen, and b)
phosphorus is present in a form that has
a very poor solubility at soil conditions.
For use as fertilizer in forests, the slow
release of P may not be a problem, in
particular on more acidic soils.
There are alternatives in fertilizer
production (other than direct utilization
as general-purpose fertilizer). Biomass
ashes may be blended with
complementary materials, or biomass
can be used in the same way as mineral
resources: dissolution of K and P at very
low pH and then processed in regular
fertilizer production.
Biomass ashes may also be used for
other elements than N, P and K. Many
ashes contain significant amounts of Ca,
Mg, Na and S, which represent an
agricultural value, In particular when
dolomite or limestone is added during
gasification or combustion, the ashes
can be a valuable source of Ca and Mg
and used as soil improver (pH control).
When used in fertilizer or soil
improver, existing legislation must be
applied. Wood ash from combustion of
untreated wood is permitted as fertilizer
according to the EU regulations for
biological farming. However, national
legislation must also allow fertilizers to
be used. In the Netherlands (and many
other countries) legislation is aimed at
environmental protection. The Dutch
Fertilizer Act allows the use of a listed
number of materials, but biomass ashes
are not on the list. In addition, an allowance
can be obtained by following an acceptance
procedure. In this procedure, first the
minimum dosage of fertilizer needed for
agricultural usefulness is determined based on
the content of nutrients. Secondly, a
permissible maximum dosage is determined
based on the content of contaminants. A
material can only be admitted as fertilizer
when the minimum useful dosage is less than
the maximum allowed dosage. In the BIOAS
Project, the potential for acceptance as
fertilizer was calculated for two kinds of fly
ash produced in combustion installations in the
Netherlands that use only clean biomass as
fuel. The result was that for both ashes in all
possible agricultural applications, there were
too many contaminants compared to nutrients.
Cadmium turned out to be the biggest problem,
but also Zn and As can prohibit direct
utilization as fertilizer. Bottom ashes of both
installations were not taken into account,
because these already found a useful
application as construction material. The
conclusion is that - at least in the Netherlands -
use of clean biomass result in production of
clean fly ashes. The result can be different for
bottom ashes other biomass ashes, e.g. grown
outside industrialized areas or from specific
kinds of plants or specific parts of plant that do
not take up contaminants from the soil. So far,
no-one has ever applied for allowance and
started the above-mentioned procedure for
recognition. Apparently, there is no need for
this kind of fertilizer.
Utilization as component in fertilizer or as
raw material in the manufacture of fertilizer is
in principle possible. In the Netherlands, the
end product is tested for acceptance as
fertilizer. The origin of the elements
composing the fertilizer is not relevant.
However, in practice, biomass ashes are not
attractive as raw material because they have a
low ratio of nutrients compared to
contaminants. Mineral sources are preferred
because they are cleaner. Exceptions may
exist, e.g. ash from chicken litter may be a
desired raw material for K and P. Another
example may be Ca-rich fly ash from fluidized
bed combustion of clean wood (no bark, etc.)
where dolomite was added to the bed.
Gasification ashes appear to be less
attractive as fertilizer than combustion ashes.
The ashes contain an inert carbon matrix that
lowers nutrient value. Other effects of the
ECN-RX--05-182 5

Fly ashes from biomass combustion are among
the more difficult materials for utilization.
Direct utilization as a bulk building material
replacing sand or gravel is almost not possible,
because it is such a fine powder. Utilization as
component in cement or concrete is a more
likely option. Typical biomass fly ash is
different from coal fly ash and does not
comply with the criteria of EN-450. Alkali and
chlorine are often the problematic components.
In addition, there are components present in
certain biomass fly ashes (e.g. phosphates) not
listed in EN-450, that have a negative effect on
the quality of the concrete when biomass ash is
used as substitute for coal fly ash.
carbon are that the ashes are to a certain
extend hydrophobic (poor contact with
water) and that certain trace elements
are bound, which can be an advantage or
disadvantage. The mere fact that the
ashes contain carbon is not enough to
prohibit utilization, but the possibility
that PAHs are adsorbed on the ashes can
be a problem.
In all forms of utilization as (raw
material for) fertilizer, production
volumes and consistency are key factors.
The profit margins are small in the
fertilizer industry, so it is important that
ashes become available at low prices, in
large quantities and with a predictable
composition. Currently, most ashes do
not fulfill these criteria. Typically,
installation owners buy fuels for the
lowest price and pay less attention to the
effects of fuel quality on ash quality.
One way of utilizing biomass fly ash is as
filler in cement blends or in mortars for special
applications. There are other possibilities. An
often-mentioned option is the manufacture of
lightweight aggregates, such as Lytag.
Currently, Lytag aggregates are made from
coal fly ash that includes a certain fraction of
biomass ashes resulting from direct co-firing at
the power plant. In The Netherlands, initiatives
to produce LWA using high temperature
processes (Lytag) or low temperature process
have not been very successful until now. It is
impossible to discuss all options, because there
is a large variation in ash composition and
building materials. For each kind of biomass
fly ash the possibilities must be investigated
separately.
The conclusion is that utilization of
biomass ash as (raw material for)
fertilizer should be pursued as a
sustainable option of ash utilization,
because nutrients are returned to the
biosphere and non-renewable sources
are saved. In practice, it will be very
difficult.
5 UTILISATION AS BUILDING
MATERIAL The possibilities for utilization of carbon-
rich fly ash from fluidized bed gasification in
building products is almost zero. However, at
least one niche product has been found: as
filler in C-FIX, a concrete-like material made
with heavy petroleum residue as binder. Made
with gasification fly ashes as filler, acceptable
C-FIX blocks with a good flexural strength
were produced.
Utilization of biomass ashes as
building material or as raw material in
the manufacture of building products
can be regarded as a sustainable form of
utilization when the use of ashes saves
the use of non-renewable resources.
When applied, biomass ashes must be
subjected to the same technical criteria
and environmental regulations as any
other material.
Besides technical requirements, there are
also environmental specifications for building
materials. In the Netherlands all stone-like
building materials must comply with the
regulations of the Dutch Building Materials
Decree. Compliance is based on leaching of
the end product. The origins and characteristics
of the individual components is not important,
only its behavior as an end product. A
percolation test exists for bulk granular
material and a tank test is used for shaped
building materials. Similar testing procedures
are expected to become part of the EU
Construction Products Directive. The liquids
Bottom ashes are the easiest ashes
for utilization as building material.
Bottom ashes from fluidized bed
combustion or gasification consists for a
large part of sand and may replace other
kinds of sand in road construction or
landscaping. Bottom ashes from grate
stokers and entrained-flow gasification
can be made into granulate and find its
way to road constructions and concrete.
ECN-RX--05-182 6

produced in the leaching tests are
analyzed for about 20 contaminating
components. For each component,
leached amounts (expressed as in mg
component per kg original material or
per square meter exposed surface) are
compared to an upper and a lower limit.
When all components stay below the
lower limit, the building material is
classified as ‘category 1’ and can be
used in unlimited quantities. When one
or more components exceeds the lower
limit, but all components stay below the
upper limit, the material is classified as
‘category 2’, which means that it can be
used when shielded from direct contact
with water (rain water or ground water).
If one or more component exceeds the
upper limit, the material cannot be used
as building material.
At ECN a large number of different
biomass ashes have been subjected to
leaching tests in the past years.
Generally, it was found that bottom
ashes from different kinds of
installations are cat. 1 of cat. 2 building
material. Fly ashes from biomass
combustion or gasification are more
difficult. None of the untreated biomass
fly ash samples tested at ECN has been
classified as cat. 1. Only a few complied
with the limits for cat. 2 applications.
The problem elements for combustion
fly ashes are typically Mo, Se, Cr and
Cl. For gasification ashes from
demolition wood the problem elements
were Pb and Ba.
For all applications where biomass
fly ash is used as a component in the
manufacture of building materials, the
leaching behavior of the fly ash itself is
not important. As mentioned before,
compliance tests for the DBMD are
done only on the end product, not on
raw material. It is important to avoid
mixing with the objective to dilute the
ashes. Blending and mixing is allowed,
when it has a technical function, i.e.
when the components contribute to the
useful characteristics of the end product,
e.g. improved strength or larger pH
buffer capacity.
Utilization as building material or
as component in the production of
building products currently offers the
best options for ashes from combustion of
biomass. The biggest challenge is consistency.
Together with the availability of large
volumes, consistency is the key factor for
success. Biomass ashes are only attractive
when it is available in larger quantities at a
predictable quality even when this is a lower
quality.
6 UTILISATION AS FUEL
Utilization as fuel is a viable option for
ashes with a significant amount of unburned
carbon. In practice, this option is limited to
carbon-rich fly ashes from fluidized bed
gasification. Utilization as fuel is a logical and
preferred option, because it is use ashes with
the same objective as the original material:
generating heat and power. Utilization as fuel
is not the same as waste incineration with
recovery of energy. First estimates indicate
that utilization as a fuel is possible when the
carbon content is larger than 35 wt% or the
caloric value is higher than 15 MJ/kg.
The fact that gasification fly ashes are fine
powder can be an advantage when co-fired in a
PF burner: no milling required. On the other
hand transport and handling of combustible
powder has increased health and safety risk,
e.g., dust explosion. When produced volumes
are small, gasification ashes of several
installations can be collected, possibly blended
and then sold as replacement fuel for
installations like cement kilns, steel mills or
even coal-fired power plants.
Logistics are the crucial factor in
utilization of gasification fly ash as fuel. The
fly ash is a very light material with a density of
about 200 kg/m3. Transport and storage of
such a light material is relatively expensive, in
particular when the distance between
production and utilization is large. One way to
improve this situation is to compact the fly
ashes into pellets, granulates or even
briquettes. At ECN, explorative pelletisation
tests were performed. Some gasification ashes
can be pelletised by adding only water, e.g.
when they are rich in Ca-compounds or have
other components that act as binders. Other
ashes may need addition of a binder. The
traditional binders for charcoal briquettes, e.g.,
starch, work very well. Pelletisation increases
the material density by about a factor 6.
Depending on the shapes of the pellets or
ECN-RX--05-182 7

granulates bulk density can increase up
to a factor 4. This means a significant
reduction in the number of trucks
needed for transportation. There are
additional advantages. Pellets have
better flowing characteristics. There is
no more compaction during
transportation. Dust is drastically lower,
so that exposure to ambient conditions
becomes possible. Health and safety
risks are much lower. The fly ash can be
transported in containers or big bags
rather than tanks. Storage is easier and
takes less space.
One of the most obvious ways to
improve the situation with respect to
logistics of high-carbon fly ash is to use
the fuel in the vicinity of the gasifier.
Even better would it be to combine
gasification and combustion, e.g.
indirect gasification (Battelle concept,
ECN’s Milena concept or VTT’s
integrated oxidizer).
Utilization as fuel in combustion
processes does not completely solve the
problem of finding a sustainable way of
utilizing biomass ashes. Combustion
creates low-carbon combustion ashes,
which are virtually identical to ashes
from combustion of biomass. Answers
to the question what to do with low-
carbon ashes produced in biomass-fired
installations can be found elsewhere in
this paper.
Co-firing of high-carbon ashes will
result in biomass ashes being
incorporated in end products or by-
products. Proper utilization of the low-
carbon ashes becomes the responsibility
of the buyer of the ashes. Also, the
buyer of the ashes must take care of
appropriate flue gas cleaning. Probably,
the combustion facility needs to be
licensed according to the local
implementation of the EU Waste
Incinerator Directive.
A market for fuels from gasification
fly ash is non-existent, but preliminary
estimates indicate that it is the most
economical way of utilizing high-carbon
fly ashes. Similar to utilization as
fertilizer or building material,
consistency and availability in large
quantities are key factors for success.
Water content and the caloric value are
most important, but the behavior of the (large)
inorganic fraction in the combustion chamber
is also of importance when considering using
biomass ashes as fuel.
7 AFTERTREATMENT
Direct utilization of fly ashes from
gasification as fuel is an attractive possibility,
but not likely the solution for all ash streams.
Certain ashes will require some kind of after-
treatment. This is particularly true for high-
carbon gasification ashes that have a low
caloric value or too many contaminants. For all
forms of after-treatment, the benefits should
outweigh the additional costs. The economics
will determine whether certain after-treatment
techniques will become successful. Gate fees
for land fill play a dominant role.
In the framework of the GASASH project,
ECN and the other partners have explored
several forms of after-treatment. These include
controlled leaching, high-temperature and low-
temperature combustion, screen sieving,
immobilization (e.g. C-Fix) and pelletisation.
All of the explored techniques were found to
be technically possible - to a certain extent -
and can be used to improve ash quality. The
latter two techniques have been briefly
introduced in the preceding sections. The other
techniques will be the subject of future
publications that will be done in co-operation
with the GASASH partners.
8 CONCLUSIONS
Return of ash from thermal conversion of
biomass to the original soils, where the
biomass was harvested, is the most ecological
and sustainable way to utilize the ashes and
should be pursued, since it returns nutrients
and closes the mineral cycle. Utilization as
(raw material for) production of fertilizer may
be a viable option for clean ashes with a high
amount of nutrients compared to contaminants.
For ashes from combustion of biomass,
utilization as building material or as
component in the manufacture of building
products is the most likely option. For carbon-
rich fly ashes from fluidized-bed gasification,
utilization as fuel is an obvious route. Some
ashes may require after-treatment before
utilization, but there will likely remains certain
ECN-RX--05-182 8

ash fractions that have to be land filled.
Consistency and availability of large
quantities are the key factors for all
forms of utilization.
9 ACKNOWLEDGEMENT
The authors wish to thank
SenterNovem and the EU 5th
Framework for financial support, and
NMI, C-FIX, AMFERT, Van Werven
Recycling, the partners of the GASASH
project and many others for useful
discussions.
ECN-RX--05-182 9

ECN-RX--05-182 10

Utilization of ashes from biomass
combustion and gasification
Jan R. Pels, Danielle S. De Nie and Jacob. H. A. Kiel
210-11-2005
Biomass ash is “the third problem”
First problem: Where do I get a large enough supply of cheap fuel?
Second problem: How do I get the installation running and stable?
Third problem: What do I do with all those ashes?
PANIC!!!
Quick scan for solutions:
“Can’t we use the appropriate disposal routes?”
“In concrete, of course, just like coal as hes!”
“The recycling company is taking care of that.”
“Well, I think we need to landfill it, but that is so expensive”
310-11-2005
Objective
To investigate which options exist for utilization of ashes from
biomass combustion and gasification
Approach
•Results from project BIOAS: fly ash and bottom ash from
combustion of clean biomass
– “When viab le forms of utilization exist, they will certainly be found
for clean biomass ashes”
•Selected results from project GASASH: carbon-rich fly ash from
gasification (with VTT, PVO, Foster Wheeler, AICIA, EMC and
Essent Energy Production)
– Standard ash: gasification fly ash from AMER-CFB gasifier (85
MWth) fuelled with demolition wood
ECN-RX--05-182 11

410-11-2005
Gasifier at AMER-9 power plant
510-11-2005
Different kinds of ash, fuel and installations
Grate stokers:
•95% bottom ash: slags, sand and unburned wood
•fly ash: white powder, 80% soluble salts, accumulated heavy metals
Fluidized-bed combustion: equal amounts of
•Bottom ash: mainly sand, bed material and inerts/ash from the fuel
•Fly ash: grey powder, bulk of fuel-bound ash + fragmented sand
Fluidized-bed gasification:
•Bottom ash: mainly sand, bed material and inerts/ash from the fuel
•Fly ash: black powder, bulk of fuel-bound ash + char + fragmented sand
poeder, carbon content 10 - 70 wt%
And many more
•Entrained-flow gasification: bottom slag and flyslag
•Fly ash = filter ash, cyclone ash and cooler ash
610-11-2005
CHP Lelystad CHP Lelystad NARGUS AMER-CFB
Bottom ash Fly ash Fly ash Fly ash
Ash (wt%) 99.7 98.3 95 35
K (wt%) 7.5 37.5 2.8 0.7
P (wt%) 0.6 1.4 2.7 0.14
Ca (wt% ) 1 6.6 2.5 13 .7 4.9
Mg (wt% ) 1.7 0.4 1.2 0.6
S (wt%) 0.7 11.6 0.9 0.6
Si ( wt%) 22.1 0.23 1 9.3 7.6
Pb (mg/kg) 440 1300 120 4500
Zn (mg/kg) 450 10800 470 4000
Cd (mg/kg) 6 40 4 8
Cl (mg/kg) 7000 125000 0.3 10000
Composition of standard ashes
All numbers are on dry basis
Numbers are indications, real ash compositions show large variations
NARGUS = ECN’s BFB combustion facility
ECN-RX--05-182 12

710-11-2005
General strategy for managing waste
Proximity principle used by EU for determining the best way to deal with waste
Similar to Dutch “Ladder of Lansink”
•Prevention = avoiding ash being formed Æfuel with low ash content
•Product re-use = using unburned part of fuel again as fuel
•Material recycling = recycling nutrients or other forms of utilization
810-11-2005
Landfill - The bottom-line
Combustion ashes can be landfilled without much difficulties
•Costs vary between countries
•Classification varies from inert to hazardous waste
•Bottom ash or sintered ash: from almost nothing to € 100 per ton
•Fly ashes: from € 50 - € 300 per ton
High-carbon fly ashes are high-calorific waste
•Discouraged by EU and made expensive or
impossible by national legislation
Extremely simplified calculation
•Ash content of average biomass: 4% (dry basis)
•Gate fee for average biomass ash: € 100/ton
•Cost of ash disposal: € 4 per ton dry fuel
•Compare: prices for biomass fuel € 0 - 40 per ton
910-11-2005
Best approach - To minimize the amounts of ash
Keeping nutrients and other mineral matter on site
•Sustainable harvesting in forestry
– Leaving branches, leaves and n eedles at harvest location
– Experience: no slowing down of growth = nutrient in balance
– Risk of nutrient depletion with maximized fuel extraction
– Owners must be driving force behind sustainable harv esting
•Agriculture presents more difficult situation
– Some parts of plants can stay on fields, e.g. straw
– W aste streams produ ced in food proc essing indust ry, e.g. sugar beet pulp
– Competition with utilization as cattle feed
– Fertilizer use
ECN-RX--05-182 13

10 10-11-2005
Next best - Recycling of ash
Ash from local combustion of wood waste or agricultural waste
•Legally possible for clean ash (local or national legislation)
– In Scandinavia, Austria and Germany in forestry
•Separate recycling system for ashes with (too) high contaminant levels
– no net increase of contaminants
– ashes are not contaminated (combusted separately)
– ashes returned to original area where biomass was harvested
– no oth er materials ar e added to the s oil
– strict monitoring
Recycling of ashes from imported biomass?
•Pellets contain very small amounts of ash
•Transportation can be costly
•Dependent on needs of the original soils
•Legal barriers – export of waste
11 10-11-2005
Bulk utilization options for ashes that cannot be recycled
Not all ashes can be recycled
•From exported biomass
•From unknown origins
•Contaminated biomass – contaminated ashes
•Landowners are not willing to accept ash
– farmers – incompatible with their wa y of using the land
– recreational areas – contact with public
– natural reserves – preservation of biodiversity
Three kinds of bulk applications
•Fertilizer
•Building material
•Fuel
Niche applications exist, but do not diminish
the need for bulk applications. White from combustion fly ash
Black from gasification fly ash
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Utilization of biomass as fertilizer - as old as agriculture
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Direct utilization as fertilizer
Recycling to soils, but at different location
Advantages:
•Nutrients from a biological source – ecological objectives
•Saving mineral sources (non-sustainable sources)
Disadvantages:
•Ashes are incomplete fertilizers – no nitrogen; non-soluble phosphorus
•Low nutrient content compared to heavy metals (especially Cd, also As, Zn)
•Large content of inerts
•Consistency in quality and quantity needed
Potential use as soil improver, especially when having high Ca and Mg, e.g. when
dolomite is used as bed material in combustion/gasification
Potential use as special fertilizer for energy plantations
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Utilization as raw material in fertilizer production
Utilization as raw material for fertilizer possible – only end product must comply
with legislation, origin of minerals not relevant.
The same issues apply:
•Ashes are incomplete fertilizers – no nitrogen; non-soluble phosphorus
•Low nutrient content compared to contaminants
•Larger amounts of inerts
•Mineral sources are cleaner and more reliable
•Consistency in quality and quantity
Gasification ashes (high-carbon ashes)
•theoretically possible – practical obstacles
– even more inert material
– poor contact with water
– possibility of PAHs adsorbed on char
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Current situation (utilization as fertilizer)
•Wood ash from combustion of uncontaminated wood allowed in EU for
biological farming.
•In the Netherlands, Fertilizer Act does not permit biomass ashes to be
used as fertilizer directly. No-one has petitioned for permission.
•Fertilizer industry not really interested, profit will not easily become the
reason for using ashes as fertilizer or as raw material for fertilizer.
•In Germany: clean wood ash mixed with dolomite is commercial fertilizer.
•Lucky matches possible, e.g. ashes from chicken manure or meat and
bone meal with high P content.
•Clean biomass in Netherlands does not produce clean ashes:
– fly ash too much Cd (and As, Zn, Pb),
– bottom ashes have low nutrient content, t oo much sand.
Finally: in practice, utilization of biomass ashes as fertilizer will be very difficult
and large ash streams will n ot be suitable.
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Direct utilization as building material
Utilization as building material is a sustainable form of utilization:
•Saving mineral sources (non-sustainable sources)
Bottom ashes are being utilized as building material
•From fluidized bed combustion/gasification: mainly sand
•From grate stokers in granulate (0-40 mm)
•Generally: replacing other sand, gravel and granulates in road construction
and landscaping, possibly used in concrete
Fly ashes are unlikely
to find direct application
as building material
Bottom ash used in road construction
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Utilization of fly ash in manufacture of building materials
The most likely form of utilization of combustion fly ashes
Examples:
•Special cement mortars
– Specifications outside EN-450
•Lightweight aggregates (LWA)
– Perh aps not in Nether lands, but viab le elsewhere
•High-temperature treatment into synthetic basalt
– Expensive and energy intensive process, but category 1 building material
Test block made at ECN Potential application in coastal defense
unrestricted use
18 10-11-2005
Gasification fly ash in C-FIX and other asphalt-like products
C-FIX is
•Concrete-like material
•In shaped blocks
•With petroleum residue as binder
Gasification ash as filler in C-FIX blocks
•7 wt% carbon rich fly ash
•Good technical quality
•Relatively expensive, but not due to
the fly ash
•Market introduction is bottleneck C-fix blocks as underlayer in road construction
Today a niche application, tomorrow…?
Future perspectives
•10 kT/y from AMER-CFB covers 2% of filler needed for new asphalt in NL
•Competition from other fillers, e.g. residues from waste incineration
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Re-use as fuel (for gasification fly ash)
•Direct use as fine black powder
•Pelletization/granulation
– Volume reduction by factor 4 - 6
– Storage in ambient air possible
– Strongly reduced health and safet y risks
– Some fly ashes need binders added
•Minimum carbon content of 35% or caloric value of over 15 MJ/kg
•Shifting ash utilization problem towards the buyer of the fuel
•Combustion facility needs license according to EU Waste Incinerator
Directive, or…
•Plant for production of re-usable fuel needs license for processing waste
.
•Preliminary calculations show that - when possible - re-use as fuel is most
economic option for carbon-rich ashes
Fuel pellets fromhigh-C fly ash
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Conclusions
In all applications, consistency is one key factor
•Ash delivered with a predictable, constant quality
•Constant low quality is better than fluctuations in high quality
Quantity is other key factor
•Only a few kiloton ash per year Æalmost nobody will contemplate using it
•More important for fertilizer, but also for building materials and fuels
Other conclusions
•Clean biomass does not guarantee clean ashes
•There is no single solution for “biomass ash”
– Solution to be found for each k ind of ash
•Utilization options are local options - transport limitations + legislation
•Landfill is often the most cost-effective alternative
THE END
Contact: pels@ecn.nl