FIRST RESULTS OF DEPOSIT AND CORROSION TESTS USING
A HIGH TEMPERATURE CORROSION PROBE IN A 30KW BFB
DURING COMBUSTION OF WOOD PELLETS DOPED WITH ZN
DOROTA BANKIEWICZa,*, ELISA ALONSO-HERRANZb,, PATRIK YRJASa, TOR LAURÉNa,
HARTMUT SPLIETHOFFb, MIKKO HUPAa
a
b
Process Chemistry Centre, Åbo Akademi, Biskopsgatan 8, 20500 Åbo, Finland
Lehrstuhl für Energiesysteme, Technische Universität München, Boltzmannstrasse 15,
85748 Garching, Germany
* Phone: +358 2215 4560, Fax: +358 2 215 4962, e-mail: dbankiew@abo.fi
ABSTRACT
The development and improvement of handling recovered wood waste (RWW)
towards higher economical and environmental standards such as use in order to produce
heat and electricity has increased in many countries. Waste wood originates mainly
from Construction and Demolition (C&D), MSW, and Commercial and Industrial
(C&I) sources. It is well known that recovered waste wood may contain substantial
concentrations of heavy metals, especially Zn and Pb. In this paper the importance of
zinc in the corrosion process was investigated, while lead will be studied later. In order
to better understand the fate of Zn and its role in high temperature corrosion of heat
exchanger tubes in waste wood fired fluidized bed boilers, high temperature corrosion
probe tests have been performed. Combustion tests were carried out in a 30 kW
bubbling fluidized bed reactor. A temperature controlled, air cooled probe was placed in
the freeboard section where the mean gas temperature was about 780ºC. The probe
surface was cooled to 450ºC, 500ºC and 550ºC, respectively. Three types of wood
pellets were burnt in the reactor. Two doped with varied amounts of ZnCl2, to obtain
different levels of Zn and one untreated. Two commercially used types of steel were
selected for tests: low alloy, ferritic (10CrMo9-10) and stainless, high alloy steel
(Sanicro 28). After 8h and 28h of exposure time, the steel rings were analysed using
scanning electron microscope and x-ray to determine the oxide layer thickness and the
elemental distribution in the oxide scale and in the deposit. The deposit samples from
both windward and leeward sides were collected for analysis. The flue gas composition
was monitored by means of conventional on-line gas analyzer for O2, CO, CO2, NO,
NO2, NOx and SO2. In addition, Electric Low Pressure Impactor (ELPI) measurements,
and online FTIR measurements were carried out in order to determine particle size
distribution and gas components (HCl and SO2), respectively. This paper describes
preliminary results obtained.
Keywords: Recovered Wood Waste, High Temperature Corrosion, Zinc
1. INTRODUCTION
Lately a lot of effort is being put to different aspects of using Recovered Wood as
raw material or energy carrier with the aim, to develop and improve the management of
Recovered Wood towards higher common technical, environmental and economic
standards in Europe. The main target is to develop strategies to avoid landfilling and
waste incineration without energy use of recovery. According to Rector (2006) other
factors supporting the use of waste wood as a fuel are: the increased cost of oil and
natural gas and the increased regulatory incentives to use renewable energy sources. An
estimation of the amount of Recovered Wood in 20 out of 22 EU members countries
(Austria, Belgium, Bulgaria, Croatia, Denmark, Finland, Germany, Greece, Hungary,
Ireland, Italy, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovenia,
Spain, Sweden and UK) shows that: about 30 Million tons of Recovered Wood are
produced annually which corresponds to about 13% of the annual round wood
consumption of 227 Million tons and about 444 PJ/a or 0.7% of the primary energy
consumption of 67,000 PJ/a. Currently, 34% of Recovered Wood is used for power
generation, 38% is being recycled and 28% is being composted or put into landfills.
Thus, energy recovery of RWW can reduce the amount of waste that goes to landfill and
produces heat, thereby establishing an economic outlet for the waste wood resource.
Moreover, RWW is considered CO2 neutral and its combustion emits less SO2 and NOx
than most fossil fuels. Whether the tree is rotting in nature or being burned for heating
purposes, the same amount of CO2 is always released (Fig1) (Krook et al., 2004 and
Wood pellets-basic training, 2007). Consequently, already 11 Million tons CO2emissions per year can be avoided through the substitution of fossil fuels and approx. 10
Million tons of fresh wood can be saved.
Figure 1. Demolition wood - the renewable source of energy. Wood pellets-basic training (2007).
RWW includes all kinds of wooden material that is available at the end of its use as a
wooden product. It is expected that the amount of RWW used in biofuels boilers might
increase in the coming years, because already in Sweden since 2002 it is illegal to
dispose combustible waste in landfills (Krook et al., 2004 and COST Action E31).
However, RWW is usually substantially contaminated with heavy metals, causing
pollution and downstream operational problems such as slagging, fouling, and corrosion
may occur. According to Krook et al. (2002, 2006) the wood-based materials, RWW
mainly consists of, can be divided into four main categories: untreated wood, surfacetreated wood, industrial preservative-treated wood and different types of building
boards such as plywood and particle boards. Of these materials only surface-treated
wood and preservative-treated wood contain heavy metals, thus being potential sources
of contamination with harmful components. Zinc and lead occur at the highest
concentration in surface-treated wood present in RWW, and originate mainly from
white pigments. Before RWW is used as a biofuel, the material is processed. Common
processes include: crushing to wood chips, treatment with magnetic separators,
eventually screening, and removing the finest fraction of the processed material.
However, even preprocessing treatment neither removes all metallic e.g. non-magnetic
parts nor contaminated surfaces.
The aim of this study is to investigate the effect and importance of problematic
RWW’s ash elements such as Zn and later on Pb regarding high temperature corrosion
processes. This study presents preliminary results of lab-scale corrosion tests carried out
at the Technical University in Munich, Germany.
2. EXPERIMENTAL
This study describes preliminary experiments carried out in a small-scale fluidized
bed reactor (30 kWth). Wood pellets doped with ZnCl2 were used as a fuel.
2.1. Lab-scale facility
The setup outline is shown in Fig. 2. The reactor had a cylindrical shape with an
inner diameter of 190 mm and a height of 550 mm. The freeboard section had diameter
of 310 mm and a height of 450 mm. At the bottom of the fluidization column a
Figure 2. Lab-scale setup with 30 kWth fluidized bed reactor (Klein WS)
perforated plate (91 holes; diameter = 1.8 mm) was used for the distribution of
fluidizing air. Fuel was fed by means of a screw feeder and pellets were falling on the
top of the bed with the secondary air flow. Electrical resistances (heater) placed around
the reactor heated up the bed up to 570 °C during the start up. The bed temperature was
regulated by the preheating of the fluidizing air and it was controlled by means of 4
thermocouples. Quartz sand (0.7-1.2 mm) was used as bed material in a total quantity of
10 l for each experiment. Bed material was changed before every test with a new fuel.
Combustion air was preheated up to 600°C by an electric heater and an additional
electrical resistance. Gas velocity in the reactor was about 0.5 m/s. Particulate matter
was separated from the flue gases in a cyclone. Reactor temperatures, pressure drop in
the bed, and, air flow were monitored and recorded continuously during the
experiments. Flue gas was sampled from the freeboard for online HCl and SO2 analysis
(FTIR). The flue gas composition was also monitored by means of a conventional online gas analyzer for O2, CO, CO2, NO, NO2, NOx and SO2. After the cyclone, flue gas
was filtered before being emitted into the atmosphere.
2.2. Experimental procedure
Fuel feeding rate and fluidization air were kept approximately constant and were
equal to 3 kg/h and 15 Nm3/h respectively. The aim was to obtain a constant lambda
value of about 1.2. When the bed temperature reached 570°C, fuel feeding was started.
Once the bed temperature reached a stationary value and freeboard temperatures
became stable, the corrosion probe was introduced into the freeboard. The freeboard
temperature varied from 740ºC to 770ºC during different tests. Combustion conditions
were kept constant for 7 hours. At the end of the test the corrosion probe was removed
from the reactor. After each test, samples of bed material, and ashes from cyclone and
flue gas duct after the freeboard were collected for SEM/EDX analysis.
2.3. Corrosion probe
A temperature controlled corrosion probe was used in the experiments. Temperature
controlled probe techniques and examples of results that can be achieved with this kind
of instrument were described by Laurén (2007). The working principle is shown in Fig.
3. The temperature of one of the test rings on the probe is controlled with a PIDcontroller. The temperature of the other test ring is monitored and logged during
Figure 3. Outline of corrosion probe
the test run. The cooling temperature of the probe was adjusted by the pressurized air
flow inside the tube. The probe was provided with two removable rings (Fig. 4). Tests
were performed with two commercially available superheater materials: one low alloy
steel 10CrMo9-10 and one austenitic steel Sanicro 28. Tab. 1 presents the detailed steel
compositions of the tested steels.
Figure 4. Corrosion probe tip with two removable rings after 7h exposure time (combustion of wood
pellet doped with 0.5 wt. % of ZnCl2).
Before the experiments all steel rings were cleaned in ethanol using the ultrasound bath.
Hereafter, the material rings (2 at a time) were fixed on the probe and introduced into
the freeboard for 7 or 28 hours when the freeboard temperature exceeded 700ºC.
Different cooling probe temperatures were set for each test (550ºC, 500ºC and 450ºC).
After the corrosion test, the test rings were cooled down to room temperature outside
the furnace. The rings were then placed in resin thinned with 99.8% ethanol. The aim of
thinning was in order to let resin to penetrate the very thin layer of collected deposit,
and preserve. The rings were then fully cast in resin and cut off in the middle. Similar
samples treatment prior to SEM analysis was described by Westén-Karlsson (2008).
The samples were then ready to be analyzed with SEM/EDX in order to indentify
elemental composition of the deposit and to estimate the oxide layer thickness.
Table 1. Detailed compositions of the steels used in the corrosion tests.
Composition, wt. %
Steel
element
10CrMo910
Sanicro28
Fe
Cr
Mo
Mn
Si
Ni
C
P
S
95-97
2-2.5
0.9-1.1
0-0.6
0-0.5
0-0.15
0-0.03
0-0.03
31-41
26-28
3-4
0-2.5
0-1
29.5-32.5
0-0.03
0-0.03
0-0.03
2.4. Fuel
Three types of wood pellets were burnt in the reactor. Two doped with varied
amounts of ZnCl2, to obtain different levels of Zn and one untreated as reference. Pellets
were doped with 0.1 and 0.5 wt. % of ZnCl2 what corresponds to 480 and 2400 mg of
Zn per kg of fuel respectively. Solid zinc chloride was dissolved in ion-exchanged water
and the solution was then evenly sprayed over a batch of wood pellets. After doping,
pellets were left for drying and then packed and stored in tight barrels. The aim of fuel
doping with ZnCl2 was to imitate demolition wood contaminated with Zn. The amount
of Zn in doped pellets corresponds to a mean and high content of Zn in demolition
wood found in the literature (Krook et. al 2006).
3. RESULTS AND DISCUSSION
The performed experiments are summarized in Table 2. Results of experiments with
untreated wood pellet are still under evaluation.
Table 2. Summary of the performed tests
Run
Fuel
Exposure time
[7 h]
1
wood pellet+0.1 ZnCl2 wt. %
28
2
wood pellet+0.5 ZnCl2 wt. %
7
3
wood pellet+0.5 ZnCl2 wt. %
7
4
wood pellet+0.5 ZnCl2 wt. %
7
5
wood pellet+0.5 ZnCl2 wt. %
7
6
wood pellet
7
7
wood pellet
7
8
wood pellet
7
Steel sort
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
10Cr-Mo9-10
S28
Probe temp.
[ºC]
450
550
500
450
550’
550
500
450
3.1. Oxide layer thickness, composition and structure
The thickest oxide layer growth was noticed on the 10CrMo9-10 low alloy steel
when combusting wood pellet doped with 0.5 wt. % of ZnCl2. The measurements taken
on a 45º angle of windward side showed growth of oxide layer over 70 µm thick after 7
hours exposure time (0 º - windward side, 180 º - leeward side). The rest of the
measurements taken in a one representative point of 0º, 45º and 180º angle on each
sample are presented in Table 3. (where wp means wood pellet doped with 0.1 and 0.5
wt. % of ZnCl2 respectively). The typical composition of the oxide scale was mainly
Fe2O3 with significant amounts of KCl on the top of the scale and Zn possibly as ZnCl 2
(as not much oxygen was found) in between of the mentioned compounds (Fig. 5, area
1). In all cases the thickest oxide layer growth was notice on 0º-45º section of the
windward side. S28 austenitic steel showed good resistance, with only negligible iron
and chromium oxides growth, difficult to measure and possible to be distinguished only
on the x-ray maps (Fig. 6). The visible deposit consisted mainly of KCl and significant
amounts of Zn most likely in a form of ZnCl2 rather than ZnO what is noticeable on the
x-ray maps. Also, in the impactor samples ZnCl2 was clearly present and more Cl was
found than K would be able to bind. This implies that not all ZnCl2 decomposed in the
combustion process.
ZnCl2, ZnO, Zn
KCl
1
Fe2O3
Steel ring
Figure 5. SEM image of the exposed 10CrMo9-10 steel rings. Wood pellets + 0.5wt. % of ZnCl2,
500⁰C-probe temperature and the analysis spectrum of the marked square in the image.
Table 3. Measurements of the oxide layer thickness (10CrMo9-10 steel)
Oxide layer thickness [µm]
Run
Fuel
Exp. time[h]
Probe temp. [ºC]
0º
45º
180º
1
wp+0.1
28
450
8
18
10
2
wp+0.5
7
550
40
73
47
4
wp+0.5
7
450
7
3
4
3
wp+0.5
7
500
25
22
9
5
wp+0.5
7
550
43
37
17
Note: wp = wood pellet
3.2. The deposit composition
The rate of deposit build-up during 7 h tests was negligible in all cases. Figure 7
presents the elements found in the wind- and leeward side of deposits after 28 h test.
Windward sides deposit analysis shows enrichment in ZnO as a main compound,
containing elevated amounts of KCl and most likely also K2SO4. Although, no sulphates
were detected on the oxide scale/deposit interface suggesting occurrence of solid K2SO4
further from the tube surface what was indicated in the studies done by Åmand et al.
(2006).
Cl
KCl
Cr
K
Fe
Zn
Ca
O
S
Figure 6. a) BSE-SEM image and b) x-ray maps of S28 steel ring when combusting wood pellet + 0.5 wt.
% ZnCl2, probe temperature 550oC, exposure time 7 h.
Sulfur released during combustion binds alkali metals or replaces chlorine in alkali
chlorides forming alkali sulfates, simultaneously preventing alkali chlorides formation.
Then, chlorine preferably tends to form HCl leaving the system with the flue gases.
Figure 7. SEM/EDX analyses of wind- and leeside deposits, probe temp. 450ºC, 28h test, wood pellets +
0.1 wt. % of ZnCl2
However, in this study addition of Cl (in a form of ZnCl2) enhanced formation of KCl
which appeared as an enrichment of KCl in the oxide scale/deposit interface (Fig. 4).
This is in agreement with Åmand et al. (2006).
4. CONCLUSIONS

Preliminary Test results showed that ZnCl2 has a profound impact on high
temperature corrosion

Higher ZnCl2 concentrations in the fuel resulted in higher corrosion rates of low
alloy steel

Zn was mainly found as ZnO on the windward side of the corrosion probe but
there are also indications for ZnCl2, which implies that ZnCl2 has not totally
decomposed during the combustion process.

Corrosion rate on the high grade steel seems to be negligible under the tested
conditions and exposure times.
ACKNOWLEDGMENTS
The tests presented in this paper were done at the Institute of Energy Systems at
the Technische Universität München as a part of project under INECSE Marie Curie
Early Stage Programme activities. This research was mainly financed by INECSE –
Marie Curie Early Stage Training Programme. The work was also partly financed by
Chemcom, a mainly Tekes financed project at the Process Chemistry Centre at Åbo
Akademi University. Other funders were Andritz Oy, Foster Wheeler Energia Oy,
International Paper Inc., Metso Power Oy, Oy Metsä-Botnia Ab, Clyde Bergemann
GmbH, amd UPM-Kymmene Oyj. SEM analyses and help given to this work was made
by Mr. Linus Silvander and is greatly appreciated. I would also like to express my
gratefulness to all TUM co-workers for their help and assistance during tests especially
to Mr. Matthias Abst. Special thanks are also given to Mr. Albert Daschner and Mr.
Martin Haindl for continuous help and fast problems solving. Mrs. Maria Zevenhoven is
also
acknowledged
for
her
help
and
valuable
comments.
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