ash behaviour in a pulverized wood fired boiler
TRANSCRIPT
Ash behaviour in a pulverized wood fired boiler—a case study
Bengt-Johan Skrifvarsa,*, Tor Laurena, Mikko Hupaa, Rob Korbeeb, Per Ljungc
aCombustion and Materials Chemistry, Process Chemistry Centre, Abo Akademi University, Piispankatu 8, 20500 Turku, FinlandbECN, Petten, The Netherlands
cVattenfall Utveckling AB, Alukarleby, Sweden
Received 5 March 2003; revised 13 January 2004; accepted 13 January 2004; available online 5 February 2004
Abstract
The behaviour of the mineral matter in a fuel may crucially affect the availability of a boiler when the fuel is fired. The ash may cause
severe problems in the flue gas channel in forms of fireside deposits on heat exchangers. These deposits lower the efficiency of the boiler and
cause in the most severe cases an unscheduled shutdown.
In this paper we report results from a study where the ash behaviour was monitored in a pulverized wood fired boiler. Short-term deposit
sampling was combined with in situ fly ash and flue gas sampling as well as advanced fuel analyses.
By combining these three tools we could track down a chain of events the ash went through from the point where it was introduced into the
boiler with the fuel until the stage where it formed a deposit on a heat exchanger tube.
Sub-micron sized ash particles found in the flue gas with a Berner-type low-pressure impactor were enriched in alkali, sulphur and
chlorine. Similar particles were also found on the backside of the air-cooled deposit sampling probes, forming thin initial alkali, sulphur and
chlorine-rich deposit layer. These elements were further found by advanced fuel analysis to be associated with the moisture or the organic
phase of the fuel.
Larger ash particles of the size of 1–10 mm found in the flue gas with the low-pressure impactor were found to deposit on the front side of
the sampling probe. These particles consisted mainly of calcium, most likely oxide or carbonate. With the advanced fuel analyses we could
find these particles already as mineral particles in the wood fuel.
We also saw some indication that peat could act as a cleaning fuel. In general the results show that a detailed well-performed fuel analysis
is a key knowledge when ash behaviour predictions are to be made.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
The interest in firing biomass and waste alone or in
combination with other fuels for heat and/or power
production has increased continuously during the past 5
years. One driving force is the CO2 neutrality of these fuels.
Another is that they represent a new fuel potential for the
energy production. However, one great concern when firing
these opportunity fuels or mixes of them is the behaviour of
the mineral matter in the fuel. During combustion, the ash
may crucially affect the availability of a boiler by causing
severe fireside deposits on heat exchangers. These deposits
lower the efficiency of the boiler and cause in the most
severe cases unscheduled shutdown. Corrosion may
additionally occur if the deposits contain aggressive
constituents.
Traditional methods to do ash-related research such as
slagging, fouling and corrosion studies have included field
studies at full size boilers [1,2]. In such studies the air-
cooled probe technique has been a powerful tool [3–5]. By
short-term deposit sampling with the air-cooled probe
technique, researchers have gained information about the
quantity and quality of deposition. Based on these data,
more detailed and more phenomenological studies have
then opened ways for more accurate descriptions of the
mechanisms involved in slagging, fouling and corrosion.
In slagging, fouling and corrosion studies also fly-ash
particle measurements have played an important role. The
introduction of the on-line in situ particle amount and size
measurement device, the low-pressure cascade impactor
[6–8], opened even better possibilities to understand the
particle behaviour in the flue gas channel of a combustor.
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2004.01.008
Fuel 83 (2004) 1371–1379
www.fuelfirst.com
* Corresponding author. Tel.: þ358-2-215-31; fax: þ358-2-215-4962.
E-mail address: [email protected] (B.-J. Skrifvars).
With this device you could get quantitative size-segregated
measurements of the fly ash, as well as size-segregated
compositional data of the ash particles [8]. The impactor
complements well the short-term deposit probe
measurements.
A third area that has developed much during the last 15
years is the fuel analytics. Better analyses of the mineral
matter in the fuel have opened ways to describe and predict
more accurately the fuel mineral matter behaviour in a
combustor [9,10].
In this paper we report results from a case study where
the ash behaviour was monitored in a pulverized wood
fired boiler. Short-term deposit sampling was combined
with in situ fly ash and flue gas sampling as well as
advanced fuel analyses to describe the monitored ash
behaviour.
2. Experimental
2.1. The test runs
The boiler in which the tests were performed is an
80 MWth down-fired pulverized fuel boiler, for heat water
production situated in Drefviken-Jorbro, Sweden. The
measurement campaign consisted of eight different test
runs, under which the fuel and boiler load were varied.
The main fuel used was pellets or briquettes of wood,
which were grinded into powder. The pellets were
delivered by three different suppliers (referred to in the
further text as Suppliers 1–3). In four test runs peat was
added to the fuel. Furthermore, elemental sulphur was
injected into the boiler in three test runs. One of the main
ideas for using peat and elementary sulphur as additives to
the main fuel was to see if there would be any effect of
these additions on the ash and ash deposition behaviour in
the boiler.
The test runs are shown in Table 1 and the standard
analyses of the used fuels in Table 2.
For each test run a number of deposit probes were used.
The deposit probes were equipped with detachable rings. A
schematic picture of the boiler and the sampling locations is
presented in Fig. 1. Deposit probes were used at four
different positions in the boiler. In two of the positions (MP1
and MP2) the flue gas temperature was approximately
920 8C (average value from six deposition measurement
periods) and in two positions (MP3 and MP4) approxi-
mately 740 8C (average value from 14 deposition measure-
ment periods). Principles of the deposit probe are given in
the literature [3–5].
During the measurement campaign, fuel samples and
samples of precipitator ash were also taken for analysis, as
well as in situ fly ash sampling with a Berner-type low-
pressure cascade impactor. These samplings were per-
formed in the flue gas channel at a location where the flue
gas temperature was approximately 400 8C. Principles of
the Berner-type low-pressure cascade impactor as well as its
use are given in the literature [6–8].
3. Results
3.1. Fuel ash, ESP ash and deposit samples
Fig. 2 presents the deposition rates such as they were
measured with the short-term deposit probes. The 100%
wood chip firing case collected most deposits but also had
the greatest scattering of the results. A decrease in deposit
amounts seemed to take place when peat was added to
the fuel. Also addition of elemental sulphur to the furnace
seemed to decrease the amounts collected.
Fig. 3 shows the results of the fuel ash and precipitator
ash analyses. As can be seen the fuel ash does not differ very
much in chemical composition from the ash sample taken
from the precipitator catch (ESP) regardless of the fuel mix
that was fed. The deposit samples did, however, differ
significantly, compared to the fuel ash.
Fig. 4 presents the results of the chemical analyses of
the deposits collected from the hotter locations MP1 and
2. The vertical line on top of each bar indicates the
scattering of the value. In the front (wind) side deposits
calcium was the dominant element. Some potassium,
silicon, iron, and sulphur could also be found but no
chlorine. In the back (lee) side deposits potassium was
dominating and clearly higher shares of sulphur could be
detected than in the front side deposits. Also chlorine
was now found in significant amounts, up to 10 wt% of
the sample. An addition of peat to the furnace showed up
as a small increase in silicon and sulphur content of both
front- and backside deposits. The addition of sulphur
showed up as a significant increase in sulphur content,
correspondingly. It also caused a clear drop in the
chlorine content of the backside deposits.
Fig. 5 presents the corresponding results from the colder
locations MP3 and 4. Also here the front side deposits
Table 1
The different test runs performed during the measurement campaign at
Drefviken-Jorbro
Test no. Wood fuel Additions Load (MW) Remarks
1 Suppl. 1
þ Suppl. 2
68 Fuel shift at
approximately
13.00
2 Suppl. 1 Peat 3% 65
3 Suppl. 2 S 0.1% 75
4 Suppl. 3 S 0.1% 32 Instable burning
5 Suppl. 2 75
6 Suppl. 1 Peat 5% 64 No gas analysis
7 Suppl. 1 Peat 5%,
S 0.1%
67
8 Suppl. 1 Peat 5% 40
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–13791372
contained large shares of calcium. Potassium and sulphur
could now be found in higher shares compared to the MP1
and 2 front side deposits. Also chlorine showed up now on
the front side deposits in the MP3 and 4 samples. The
backside deposits contained again higher shares of potass-
ium and sulphur than the front side deposits and chlorine
was again present in significant shares. Addition of peat
showed up again as an increase in silicon and sulphur
content of front and backside deposits and the addition of
sulphur increased again the sulphur content of both deposits.
It also caused again a clear drop in the chlorine content of
the backside deposits.
3.2. Flue gas analyses
During the test runs also FTIR analyses of SO2 and HCl
were done. These results are shown in Fig. 6.
As can be seen from the figure, an addition of peat to the
wood chips did not change the amount of emitted SO2 and
HCl much but the addition of 0.1% sulphur did. The obvious
increase in SO2 emissions was accompanied by an increase
also of HCl emissions.
3.3. In situ fly ash sampling
During the test runs Nos. 3 and 5 in situ fly ash sampling
was also performed at a location in the flue gas channel
where the temperature was approximately 400 8C. These
results are shown in Figs. 7 and 8. As can be seen from the
figures, alkali sulphates and chlorides were enriched in the
smallest sized particles, ,1 mm, while silicon was found in
the largest particles, .10 mm. Calcium seemed to be
enriched in the size range 1–10 mm.
4. Discussion
It is interesting to note that the 100% wood chips seemed
to collect the highest amounts of deposits and that adding
either peat or elemental sulphur to the furnace seemed to
reduce the deposition rates (Fig. 2).
The decrease in deposition rates caused by peat might be
explained by an eroding (cleaning) effect of the peat ash.
This seems to be supported by the fact that no significant
change could be seen in the chemical composition of the
deposit when peat was added to the furnace, compared to
the case when 100% wood chips were fired alone. If the peat
ash had affected the deposit chemically one would have
expected to see a change in the deposit chemical
composition. Erosion caused by silicate type ash is a
known effect from coal firing [1].
The reason for the decreasing effect of sulphur addition
on the deposition rate is not as clear. One possible
Table 2
Standard fuel analyses of the Jorbro fuels
Peat Wood chips
Suppl. 1 Suppl. 1 Suppl. 2 Suppl. 3
Moisture (wt%) 35.4 7.3 5.9 8.0 7.5
Ash (wt% db) 12.8 0.4 0.7 0.4 0.4
Volat. (wt% db) – – – – –
C (wt% db) 52.1 47.1 48 – –
H (wt% db) 5.1 6.1 6 6.1 6.1
N (wt% db) 1.4 ,0.3 ,0.3 ,0.3 ,0.3
S (wt% db) 0.3 0.12 ,0.01 0.02
Cl (wt% db) 0.02 ,0.01 ,0.01 ,0.01 ,0.01
O (wt% db), diff 28.2 44.6 42.6 42.6 42.9
HHV (MJ/kg) 22.4 20.3 20.5 20.4 20.5
Ash analysis (wt% in ash)
SiO2 48.5 16.8 31.7 18.6 12.2
Al2O3 12.2 3.3 6.8 38.6 2.7
Fe2O3 12.1 2.8 9.3 3.4 3.1
CaO 8.6 29.3 22.5 30.5 33.2
MgO 1.2 6.1 4.0 6.2 6.1
Na2O 0.03 2.1 0.9 1.7 1.4
K2O 1.6 9.2 6.9 10.3 8.9
P2O5 1 2.6 2.6 2.6 2.2
Sum 85.2 72.3 84.6 111.9 69.8
Fig. 2. Deposition rates measured with the short-term deposit probe.Fig. 1. A schematic view of the boiler.
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–1379 1373
explanation to this effect could be a sulphation of the wood
ash, which after this would not be as prone to stick on to a
heat exchanger surface, as it would be in a non-sulphated
form. This kind of effect is supported by the chemical
analyses of the deposits. There is clearly the highest amount
of sulphur in the deposits collected from the firing cases
when elemental sulphur was added to the furnace.
Comparing the in situ fly ash sampling results with the
short-term deposit samples (Figs. 9 and 10) one can see
a number of other significant trends. For example, the
results show very clearly that the smallest ash size fractions
(,400 nm) showed up on the backside of the deposit probes
(Fig. 9) while the larger particles (1–10 mm) impacted on
the front side of the deposit probes (Fig. 10). This behaviour
is well known [1,11–13].
From the results it can also be seen that the chlorine did
not reach the backside of the hotter probe (Tsurf ¼ 520 8C)
but did reach the backside of the colder probe
Fig. 3. Fuel ash and ESP ash compositions of the wood chip firing case No. 1, wood chip þ peat firing case No. 8, and wood chip þ peat þ sulphur case No. 7
at Jorbro.
Fig. 4. MP1 and 2 deposit sample compositions during the wood chip firing cases 1 and 5, wood chip þ sulphur cases 3 and 4, wood chip þ peat firing cases 2,
6 and 8, and wood chip þ peat þ sulphur case No. 7 at Jorbro. Tsurf ¼ 520 8C. Front side (wind) and backside (lee) deposits separately. The vertical line on top
of each bar indicates the scattering of the analysis, based on 3–4 separately collected deposit samples.
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–13791374
(Tsurf ¼ 320 8C). This can be seen when comparing the
results presented in Figs. 3 and 4. It is further interesting to
note that the known effect of SO2(g) to inhibit chlorine to
form condensed alkali chloride compounds [14–16] can
also be seen from the results, both in the deposit analyses
(Figs. 3 and 4) as well as in the in situ fly ash analyses (Figs.
7 and 8). When sulphur was added to the fuel, chlorine
decreased in the smallest sized ash particles, disappeared
completely from the deposit samples and increased in the
flue gas as HCl (Fig. 6).
To get some further information of the fuels we
performed a chemical fractionation analysis on one of the
wood chip fuels. The results from these analyses are shown
in Fig. 11. In the fractionation analysis the fuel is leached
successively with increasingly aggressive solutions, starting
with water, continuing with ammonium acetate and
finishing with hydrochloric acid. Depending on what ash-
forming elements are solved into the solvents, indications
can be made on how the ash-forming elements have been
associated to the fuel. Details of this can be found in the
literature [10,17–20].
In Fig. 11 the analysed ash-forming elements are
shown on the x-axis and the amounts found in the three
solutions as well as in the insoluble rest are shown on the
y-axis. The y-axis values are expressed as mg per kg dry
fuel. The cross on top of each bar indicates the total
amount of an element analysed from the fuel while the
top of the bar indicates the same, however, reached
by analysing the element in the three solutions and the
insoluble rest fraction. If the top of the bar matches the
cross a good mass balance closure has been reached in
the fractionation analysis.
Fig. 5. MP3 and 4 deposit sample compositions during the wood chip firing cases 1 and 5, wood chip þ sulphur cases 3 and 4, wood chip þ peat firing cases 2,
6 and 8, and wood chip þ peat þ sulphur case No. 7 at Jorbro. Tsurf ¼ 520 8C. Front side (wind) and backside (lee) deposits separately. The vertical line on top
of each bar indicates the scattering of the analysis, based on 3–4 separately collected deposit samples.
Fig. 7. In situ fly ash sampling from the test run No. 5, 100% wood chips.
All analyses were performed with SEM/EDS. The size fractions 30 nm,
4.0 mm, 6.8 mm, 10.3 mm were not analysed.
Fig. 6. Flue gas analyses of SO2(g) and HCl(g) during the firing cases Nos.
1, 8, 7, 5, 3 and ‘extra’.
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–1379 1375
Fig. 8. In situ fly ash sampling from the test run No. 3, 100% wood chips þ 0.1% elemental sulphur. All analyses were performed with SEM/EDS. The size
fractions 30 nm, 4.0 mm, 6.8 mm, 10.3 mm were not analysed.
Fig. 9. A comparison of the composition of the 60, 100, 170, 260 and 400 nm sized particles with the backside (lee) deposits collected at MP3 and 4 in the boiler
during firing of wood chips alone.
Fig. 10. A comparison of the composition of the 1.0, 1.6, 2.5 and .10 mm sized particles with the front side (wind) deposits collected at MP3 and 4 in the boiler
during firing of wood chips alone.
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–13791376
As can be seen from the results, all alkali and chlorine
was solved out by water. Sulphur seemed to stay in the
rest. This can be explained by the fact that sulphur usually
forms covalent bounds, in this case most likely to the
organic structure of the fuel and could therefore not solve
out with any of the solvents used here. Calcium solved
out completely in water, ammonium acetate and HCl.
Almost no silica was solved out at all.
We also performed SEM analysis on the fuel. In Fig. 12
we show a SEM micrograph of the same wood chip fuel as
was analysed with the fractionation analysis. The surprising
result from this analysis was that we could find mineral
grains, consisting of almost pure calcium in the woodFig. 11. The chemical fractionation analysis of the wood chip fuel from
Supplier 3, fired in Jorbro alone or in combination with peat and/or
elemental sulphur.
Fig. 12. SEM/EDS analysis of the wood chip fuel from Supplier 3 (above), fired in Jorbro alone or in combination with peat and/or elemental sulphur. For
comparison a similar analysis of Scandinavian bark (below) [18].
B.-J. Skrifvars et al. / Fuel 83 (2004) 1371–1379 1377
structure, on top of other silicon-rich mineral grains, coming
from dust, soil and dirt. Later analyses have shown that
these kinds of calcium-rich mineral grains can also be found
in bark fuels to a much higher extent than in pure steam
wood [18,21]. There are further strong indications that these
calcium-rich mineral grains consist of calcium oxalate and
are released from the fuel as mineral particles of roughly
2–10 mm forming CaO particles at higher temperatures
(.800 8C), CaCO3 at lower temperatures (,800 8C), the
calcination–carbonation temperature being dependent on
the CO2 partial pressure [20].
The presence of these calcium-rich mineral grains in
the fuel may also explain the calcium-rich ash size
fraction of 1–10 mm that we measured in the boiler. It
can also explain the front side deposits rich in calcium
that we detected. It has earlier been shown that CaO
particles can deposit on cooled tubes, forming deposits by
rapidly carbonating (formation of CaCO3 from CaO) on
the surface [22].
5. Conclusions
By combining short-term deposit sampling with in situ
fly ash and flue gas sampling as well as advanced fuel
analyses, we could track down a chain of events the ash
went through from the point where it was introduced to the
boiler with the fuel until the stage where it formed a deposit
on a heat exchanger tube.
Sub-micron sized ash particles, found in the flue gas with
the Berner-type low-pressure impactor, were enriched in
alkali sulphur and chlorine. Similar particles were also
found on the backside of the air-cooled deposit sampling
probes, forming thin initial alkali sulphur and chlorine-rich
deposit layers. These elements were found by chemical
fractionation analysis to be associated with the moisture or
the organic phase of the fuel.
Larger ash particles of the size 1–10 mm, found in the
flue gas with the low-pressure impactor, were found to
deposit on the front side of the sampling probe. These
particles consisted mainly of calcium, most likely
carbonate. With the SEM/EDS analysis done directly
on the fuel we could show that these particles most
likely were present already as mineral particles in the
wood fuel.
We also got some indications that peat could act as a
cleaning fuel. It was speculated that if its ash was of right
type, i.e. contained a large enough number of silicate
particles, that could stay inert in the furnace, they could act
as eroding agents when passing through the heat exchanger
tube packages.
We could also again detect the effect of SO2(g) inhibiting
the formation of condensed alkali chlorides.
Acknowledgements
This work was partly done within the EU financed
project JOULE JOR3-CT98-0198 ‘Prediction of ash and
deposit formation for biomass co-combustion’ and the
Finnish TEKES national research program CODE (‘Com-
bustion Development by Modelling’). The financial support
is acknowledged.
The fractionation analysis work was coordinated and
performed by Dr Maria Zevenhoven. Her careful and
significant contribution to this work is gratefully
acknowledged.
Part of the results included in this paper were presented at
the United Engineering Foundation Conference, ‘Power
Production in the 21st Century, Impact of Fuel Quality and
Operations’, October 28–November 2, 2001, Snowbird,
Utah, USA.
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