VGB POWERTECH 7 (2020) - International Journal for Generation and Storage of Electricity and Heat

The Biofficiency Project | Part 1 VGB PowerTech 7 l 2020
Tab. 5. Overview on biomass ashes selected for Biofficiency.
Full-scale unit Capacity / MW el Biomass fuel Combustion
system
Rodenhuize
150 MW
Wood pellets 1
Wood pellets 2
Wood pellets 3
Wood pellets 4
of different techniques is important. For
example, with XRF analyses, the presence
of carbonate, hydrate, hydroxide and UBC
must be taken into account.
There may be some differences in the ashes
produced in laboratory-, bench-, pilot-, and
full-scale boilers. In small scale, the fuel
can be more homogeneous than in fullscale,
which consumes several truckloads
of biomass per hour. On the other hand, the
fly ash from the pilot-scale combustors may
contain significant amounts of stainless
steel corrosion products such as Cr and Ni.
At full-scale plants, the biomass fuels are
combusted at temperatures about between
850 °C and 1,500 °C depending on the combustion
technology, and include processes
of evaporation and condensation of compounds
of Na, K, Cl, S, P, etc. Evaporation
occurs at the high combustion temperatures
and condensation upon cooling of the
flue gases. At higher combustion temperatures,
there is more evaporation of K, Na,
Cl, S, P, which condensate on the ashes
upon cooling of the flue gases, and hence
more salts are present in the fly ash.
Fly ashes produced at higher combustion
temperatures (PF) differ markedly from
those produced at lower combustion temperatures
(CFB), not only regarding salt
content, but also, regarding reactivity at
temperatures above 850 °C. This was demonstrated
by TGA/DTA/DSC analyses,
which show exothermic reactions with
CFBC fly ashes at temperatures above 850 °C.
Problematic components regarding exact
and repeatable analysis are often B, Cl
and F, which require further attention.
Additives
Avedøre 2 400 MW Wood pellets PF Bituminous coal fly ash
Herning 78 MW Wood chips Grate none
Pulp&paper mill
power plant
PF
none
none
none
none
87 MW Bark CFB none
Pilot-scale unit Fuel input / kW th Biomass fuel
Valmet pilot plant
VTT pilot plant
TUM pilot plant
4000 kW
50 kW
200 kW
Bark
EFB
EFB and coal
Bark
Torrewashed bark
Torrewashed bark
Torrewashed straw
Straw
Bark
SE bark
SE bark
Combustion
system
CFB
CFB
PF
Additives
none
none
none
none
none
Sulphur
Kaolin
Kaolin
none
Kaolin
Coal fly ash
With increasing combustion temperature,
more Cr(VI) is anticipated. This is critical,
since Cr(VI) is known to be carcinogenic
and toxic. Consequently, more work on the
aspect of Cr(VI) in ash is required.
By water-leaching of the biomass fuel i.e. in
pre-treatment, fly ashes will contain less
Na, K, Cl and S, and hence less salts will be
present in the fly ashes.
2.4.2 Novel applications for biomass ash
In the course of the project existing valorisation
options for biomass ashes such as
use as fertiliser, filler materials or construction
materials were identified.
However, many of these applications can
be considered as “creative” land filling, at
least by authorities and NGOs. Some of
these applications are not specific for biomass
ash. New potential applications were
identified that exlude applications including
land filling and applications that are
not specific for biomass ash.
––
Leaching, with recovery of valuable elements
and compounds, especially P and K
––
Application in traditional ceramics and
traditional glass
––
Application in geo-polymers
––
Recycling elements back to the soil, for
limiting depletion
––
Application as or in fertiliser
––
Application in calcium silicate bricks
In the project these proposed valorisation
options were evaluated. Due to the new EU
fertiliser regulation 2019, the application
of biomass ashes in top-soils seems to be
restricted. Biomass ashes will most probably
exceed limits for allowed concentrations
of toxic heavy metals.
On the other hand, the use of biomass ash
in construction materials seems promising,
especially in calcium silicate blocks and
geo-polymers. To achieve a higher added
value in this field of application, biomass
ash should be used as binder rather than as
filler material. It could potentially replace
limestone, which is currently mined and
calcined with high fossil CO 2 emissions.
Fly ash from wood and bark were tested in
these novel applications during the Biofficiency
project. It was demonstrated that
ashes from biomass fuels can be used in
construction materials, like fired bricks,
calcium silicate blocks and geo-polymers
(see F i g u r e 6 ). Calcium silicate materials
and geo-polymers with high compressive
strengths of more than 50 MPa, even
more than 100 MPa, were obtained.
Additionally also the feasibility of the use
of biomass ash in top soils was investigated
Fig. 6. Fired bricks made from two different Dutch clays (Maas and Waal), and with 0, 5 and
25 wt.-% wood fly ash from a PF boiler.
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