VGB POWERTECH 10 (2020) - International Journal for Generation and Storage of Electricity and Heat

Wood fly ash as cement replacement VGB PowerTech 10 l 2020
by Junckers Industries A/S. About 140.000
ton wood a year is incinerated at this plant.
The wood species incinerated are ash, merbau,
maple, oak, jatoba and beech, and the
incineration temperature in the wood-fired
grate boiler is 900 to 1,000 °C. The ash is
sprayed with water just after the incinerator
in order to avoid spontaneous ignition
and dust problems during handling, and
the WA sample for this investigation was
taken after this spraying. WA2 is fly ash
from HOFOR Amagerværket, which fires
wood pellets from mainly pine and spruce
(waste from wood industry so the fuel can
contain bark). The plant incinerates about
350.000 ton wood a year. The incineration
temperature is about 1,000 to 1,100 °C in
the grate boiler.
2.2 Ash characterization
The crystalline phases in of the WAs were
identified by X-Ray Diffraction (XRD). The
WA was loaded into the sample holder using
backloading. A PanAnalytical X-ray diffractometer,
sat at the PW3064 Spinner
stage for standard powder samples have
been used for the measurements. Cu-Kα
radiation with a wavelength of 1.54 Å was
used as the x-ray source. The samples were
measured between 4 °2θ and 100 5 °2θ
with a step size of 0.002 °2θ and a sampling
time per step of 24.8 s. The XRD plots were
qualitatively evaluated using X’Pert High-
Score Plus software, with data from the
International Centre for Diffraction Data
(ICDD). XRF and XRD were performed
with powder samples.
The WAs were dried at 105 °C for 24 hours
prior to the following characterization. The
content of major chemical element of the
WAs were measured by X-ray fluorescence
(XRF) spectrometry by the use of a SPEC-
TRO Gmbh X-LAB 2000 with a Pd-tube on
samples grounded to a particle size of ≤
200µm. The equivalent content of oxides
was calculated based on the element content.
The concentrations of Cd, Cr, Cu, Pb
and Zn were measured after pre-treatment
of the ash in accordance to DS259; 1.0 g
ash and 20.0 ml (1:1) HNO 3 was heated at
200 kPa (120 °C) for 30 minutes. Filtration
through 0.45 µm filter and the concentrations
measured with ICP-OES in the filtrate.
Ash pH was measured by suspending
10.0 g ash in 25 ml distilled water. After 1
hour agitation, pH was measured directly
in the suspension with a Radiometer pH
electrode. The suspension was filtered, and
the water soluble Cl and SO 4 concentrations
were measured in the filtrate by Ion
Chromatography while water soluble concentrations
of Ca, K, Mg and Na were measured
with ICP-OES. The acid soluble fraction
of Ca and Mg was found by suspending
5 g WA in 1.0 M HNO 3 , agitating the suspension
for 1 week and measuring the elemental
concentrations in the filtrate with
ICP-OES. Loss on ignition (LoI) was found
after 30 minutes at 550 °C. Water content
Tab. 1. Summary of WA pre-treatments.
As received
Dried
(105°C)
was measured as weight loss after 24 hours
at 105 °C (calculated as weight loss over the
weight of the wet sample). Three replicates
of each of these analyses were made. Solubility
in water was evaluated: 50.0 g ash
suspended in 500 ml distilled water and
agitated for 1 min. After settling, the water
was decanted. New 500 ml distilled water
added. This was repeated so the ash was
washed three times. Finally, the suspension
was filtered and the ash dried at 105 °C
and weighed.
2.3 Pre-treatments
The WAs were pretreated to see the effect
on the ash mineralogy and following the
influence on the compressive strength of
mortar. The major ash constituents after
the different pretreatments were identified
by XRD. The pretreatments were:
––
WA1 was dried at 105 °C for 24 hours to
see possible changes in chemical compounds
from this procedure.
––
WA2 was hydrated in the lab to see if the
same ash compounds would be obtained
in this ash as in WA2, which was hydrated
right after incineration. The WA1 was
mixed with water similar to WA1 as received
and left in a closed bucket for 2
weeks. The wet ash was mixed manually
every second day.
––
Both ashes contain organic compounds
and the ashes were fired in a muffle oven
550 °C to diminish the content of organic
material and to investigate if this second
heating changes the ash composition.
The loss of ignition at 550 °C is as a simple
method for estimating the residual
carbon in bio ashes [Zhao et al. 2013].
––
Both ashes were washed in water using
the same procedure as when evaluating
the solubility (paragraph 2.2).
––
Both ashes were washed in acid (1.0
M HCl). The same procedure as for evaluating
the solubility (see paragraph 2.2)
was used, except for the first washing
(out of the three subsequent washings)
being in this acid instead of distilled water.
The mass loss from the acid wash
was measured.
Ta b l e 1 is a summary of the ashes and
pretreatments.
2.4 Mortar sample preparation,
compressive strength and setting
Mortar was prepared with the different
ashes and pre-treated ashes as given in Ta -
b l e 1 . For mortar preparation a Portland
cement (CEM II/A-LL 52.5R) was used.
The sand used in the experiment was a
natural sea sand 0 to 4 mm with technical
Hydrated
Heated
(550°C)
Water
washed
(WW)
specification following EN 12620. The procedure
and recipe base for the mortar samples
was from EN 196-1, but we used sea
sand and not the standardized sand prescribed
in the standard. The mix used in
this investigation was: 225 ml water, 450 g
cement (ordinary Portland) and 1,350 g
sand. In the mortar with WA, 5 or 10 % cement
was replaced with ash (corresponding
to 22.5 g and 45 g respectively).
The mixing and moulding were sought carried
out as prescribed in EN 196-1, however
low workability experienced during the
casting of some of the mixes (mainly with
10 % cement replacement) was so low that
higher frequency and longer duration of
the vibration was necessary. The ease of
casting related to the workability in comparison
to the references were noted. The
mould consisted of three horizontal compartments
so three prismatic specimens
were prepared simultaneous for each recipe
(40 mm ×40 mm in cross section and
160 mm in length). Demoulding was carried
out after 20 to 24 hours and the samples
were cured horizontally in a water
bath. The ambient temperature during curing
was 21 to 25 °C, which was higher than
the 20 ±1 °C as prescribed in EN 196-1. After
7 days, the compressive strength was
measured. For compressive strength, the
mortar prisms were segmented into two in
a three-point loading setup, where after
the compressive strength was measured for
each segment, i.e. six measurements for
each mortar recipe. The testing machine
for compressive strength was a Mohr & Federhaff
AG-49.
3 Results and discussion
Acid washed
(AW)
WA1 X X X X X
WA2 X X X X X
3.1 Characterizations of investigated
ashes
Ta b l e 2 summarizes the overall results
from the characterization of the investigated
ashes.
3.1.1 Compliance of WAs with EN450-
1 for fly ash in concrete
The use of WA is not covered by the current
standard governing the use of fly ashes as
mineral additions in concrete (EN 450-1).
The regulations preclude the use of any
material not derived from coal combustion
and co-combustion ashes with coal. However,
as there is no standard covering WA in
concrete, this standard is often used to
evaluate the qualities for WAs (e.g. in
[Carević et al. 2019], and [Sigvardsen et al.
2019]). It is reasonable to think that a future
extension of the current regulations
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