devolatilization and ash comminution phenomena of sewage ... · besides, sewage sludge, due to its...
TRANSCRIPT
R. Solimene1, M. Urciuolo
1, A. Cammarota
1, R. Chirone
1, P. Salatino
1,2
1. Istituto di Ricerche sulla Combustione - C.N.R., Napoli - ITALY
2. Dipartimento di Ingegneria Chimica - Università egli Studi di Napoli Federico II - ITALY
1. Abstract
Wet sewage sludge has been characterized during fluidized bed combustion with reference to
devolatilization behavior and ash comminution phenomena with the aid of different and
complementary experimental protocols. Devolatilization/pyrolysis and char bun-out processes
have been analyzed by visual observation and by the time series of gaseous species measured
at the exhaust. Primary fragmentation pointed out the formation of a significant amount of
relatively large fragments as a function of the initial size of fuel particle.
2. Introduction
Sewage sludge management is one of the most important environmental problems.
Construction of new biological municipal wastewater treatment plants has resulted in a
continuous increase in the volume of sewage sludge generated, nevertheless a feasible
solution to the disposal of the expected enormous quantities of sludge must be found. The
restrictive environmental legislation that limits more and more the landfilling of this
biodegradable waste and the progressive decrease of the use of sludge in agriculture, that can
only be considered under very well controlled conditions due to the presence of heavy metals
and pathogens, indicates an ever more increasing interest in sludge thermal processes, in
particular, in sludge incineration. Combustion and co-combustion are the mostly viable
strategies to dispose sewage sludge [1]. Besides, sewage sludge, due to its biogenic nature, is
a promising substitute of fossil fuels in the light of the increasing concern to CO2 emissions.
Fluidized bed combustion/gasification has been indicated as one of the best option, due to
operation flexibility and to high efficiency and low pollutant emissions achieved with
different biogenic fuels, used either alone or in combination with fossil fuels [2-6]. The
success of fluidized bed combustion technology for sewage sludge can be attributed, apart
from minor specific advantages, by considering the great volume reduction of the produced
ash and the destruction of organic micro-pollutants and pathogens [1,7]. However even if
several bed units are in operation for sewage sludge combustion, fundamental work on the
comprehension of basic mechanisms (e.g. sludge drying, release and combustion of volatiles,
combustion of the high-ash content char) taking place during fuel conversion has received
considerably less attention and requires additional investigations [8-15].
The present paper aims at contributing to a better understanding of devolatilization and ash
comminution of wet sewage sludge under fluidized bed combustion conditions with the aid of
different and complementary experimental protocols. Devolatilization/pyrolysis and char
burn-out processes have been analyzed by visual observation and by the time series of
gaseous species measured at the exhaust. Primary particle fragmentation has been
Devolatilization and ash comminution phenomena of sewage
sludge during fluidized bed combustion
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Processes and Technologies for a Sustainable Energy
characterized in terms of number and dimension of the mm-sized generated fragments as a
function of the initial size of the fuel particle.
3. Experimental
A stainless steel atmospheric bubbling fluidized bed combustor 41 mm ID and 1 m high was
used for devolatilization, fragmentation and combustion experiments (Fig. 1A). A 2 mm thick
perforated plate with 55 holes (ID 0.5 mm) disposed in a triangular pitch was used as gas
distributor. A 0.6 m high stainless steel column for gas preheating and mixing was placed
under the distributor. Two semicylindrical 2.2 kW electric furnaces were used for heating the
fluidization column and the preheating section. Bed temperature, measured by means of a
chromel-alumel thermocouple placed 4 mm above the distributor, was regulated by a PID
controller. The freeboard was kept unlagged in order to minimize fines post-combustion in
this section. Gases were fed to the column via high-precision digital mass flowmeters. The
fluidization column top section was left open to the atmosphere. A stainless steel circular
basket could be inserted from the top in order to retrieve fragmented and un-fragmented
particles from the bed. The tolerance between the column walls and the basket was limited to
reduce as much as possible the amount of particles left in the bed when pulling out the basket.
A basket mesh of 0.8mm was used, so that the bed material could easily pass through the net
openings. A stainless steel probe was inserted from the top of the column in order to convey a
fraction of the exit gases directly to gas analyzers. A high efficiency cellulose filter was
inserted in the line to avoid particle entrainment into the analyzers. The probe, 2 mm ID, was
positioned 0.6 m above the distributor, approximately along the column axis. Data from the
analyzers were logged and further processed on a PC equipped with a data acquisition unit. A
quartz atmospheric bubbling fluidized bed combustor was also used. The geometrical features
of the fluidization column were the same of stainless steel column as well as the heating
system. An observation rectangular window (about 5x10cm) was proposely designed along
the reactor lateral insulation in order to visualize a part both of fluidized bed and of freeboard.
The visual observation was accomplished by a high-resolution video camera.
Quartz sieved in the size range 150-200m as bed material (bed inventory:180g) and a wet
sewage sludge, whose properties are reported in table 1, were used. The sludge was stabilized
1) gas preheating section;
2) electrical furnaces;
3) ceramic insulator;
4) gas distributor;
5) thermocouple;
6) fluidization column;
7) steel basket;
8) manometer;
9) digital mass flowmeters;
10) air dehumidifier (silica gel).
TC
1
2
3
4
air/nitrogen
4
55
6 7
1) gas preheating section;
2) gas distributor;
3) quartz fluidization column;
4) electrical furnaces;
5) ceramic insulator;
6) observation window;
7) Digital video camera;
A B
Fig. 1 Experimental apparatus. A) stainless steel fluidization column apparatus; B) quartz
fluidization column apparatus
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Ischia, June, 27-30 - 2010
and conditioned by inorganic and organic materials. Almost spherical particles (1-3cm) and
air or technical-grade nitrogen as fluidizing gas were used.
Experiments were carried out at fluidization velocity of 0.5m/s by injecting a single fuel
particle into the bed kept at 850 °C from the top of the column. After pyrolysis or combustion
process was complete, the resulting char or ash was retrieved by means of the basket in order
to investigate the number and the size of the produced fragments. Fragments smaller than
about 0.8 mm were lost through the basket net openings. However, these small fragments
were more likely to be generated by attrition rather than by fragmentation. The experiment
was repeated to collect a statistically significant number of fragments. Video recordings of the
combustion of fuel particles were also performed in the quartz bubbling fluidized bed
combustor. The experiments were characterized in terms of the initial size of the fuel particle.
4. Results and discussion
4.1. Phenomenology
Figure 2 reports some snapshots captured during the combustion of a 2cm sewage sludge
particle in the quartz atmospheric bubbling fluidized bed combustor. The upper part of the
bed and the first part of the freeboard are dark red and brighter red, respectively, in the
images. The origin of time scale corresponds to the time at which the fuel particle approaches
the bed surface. It can be observed that: i) the particle appears black (particle temperature
smaller than bed temperature) and often remains nearby bed surface during all the
devolatilization process which lasts about 210s; ii) volatile matter flames are absent; iii) bed
material partially covers fuel particle also when it is on the top of the bed (t=43.68s,
t=111.48s); iv) fuel particle become brighter when char burn-out takes place; v) at the end of
char burn-out, the particle appears only slightly darker of bed material (t=287.84). Taking into
account that devolatilization and drying mainly occur in parallel, the absence of volatiles
flames can be mainly due to the large amount of moisture present in the fuel particle which
reduces both the particle temperature during devolatilization and the heating value of the
released volatile matter. In these conditions, the volatile matter combustion takes place
homogenously mainly in the freeboard without hot spots (flameless combustion). It is also
worth to noting that the diagnostic techniques - the flame period and flame extinction time
methods - based on the detection of volatile flames around the devolatilizing fuel particles by
visual observation or UV detector, can not be applied to this kind of solid fuel. Instead, the
Tab. 1 Properties of tested fuel LHV (as received), kJ/kg 1245 Ash analysis (as received), %w
Proximate analysis (as received), %w Na 0.019
Moisture 77.8 Mg 0.081
Volatiles 14.5 Al 1.002
Fixed carbon 1.0 P 0.496
Ash 6.7 K 0.085
Ultimate analysis (dry basis), %w Ca 0.547
Carbon 34.7 Ti 0.009
Hydrogen 5.4 Cr 0.019
Nitrogen 5.1 Mn 0.006
Sulfur 0.5 Fe 0.328
Chlorine 1.2 Ni 0.062
Ash 30 Cu 0.025
Oxigen (by diff.) 24 Zn 0.023
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Processes and Technologies for a Sustainable Energy
techniques consisting of the analysis of the time-resolved gas concentration profiles measured
during devolatilization/pyrolysis and char burn-out are still adequate.
4.2. Devolatilization/pyrolysis and char burn-out experiments
Assessment of fuel devolatilization/pyrolysis and char burn-out of a sewage sludge single
particle at 850°C was based on the analysis of the time series of gas concentration measured
at the exhaust. Figure 3 reports the time series of gas concentration (CO, CO2, SO2, NO, H2,
CH4) corresponding to a typical pyrolysis test of an unfragmented 22mm fuel particle. The
time-history of release of measured species is different: shorter for NO and CH4, longer for H2
and CO, whereas it has an intermediate behavior for CO2 and SO2. As a consequence, the
time-resolved CO2 concentration profile has been adopted to estimate the pyrolysis time
corresponding to the 95% of the total released amount which, in this case, results in 305s.
Figures 4 and 5 report the time series of gas concentration (CO, CO2, NO) corresponding to a
typical combustion test of an unfragmented and a fragmented 22mm fuel particle,
respectively. Adopting the time-resolved CO2 concentration profile, it has been evaluated
both devolatilization and complete burn-out time which are 250 and 358s for unfragmented
particle and 160 and 230s for fragmented particle, respectively. The comparison between
combustion and pyrolysis results (Fig. 3, 4 and 5) highlights: i) CO2 profile presents a
shoulder at the end of the process due to char-burn-out; ii) the devolatilization time is smaller
than pyrolysis time indicating that also without the presence of flames volatile matter
t=0s t=5.8s t=43.68s
t=287.84st=210.36s t=211.36s t=214s
t=111.48st=0s t=5.8s t=43.68s
t=287.84st=210.36s t=211.36s t=214s
t=111.48s
Fig. 2 Snapshots captured during devolatilization and char burn-out of a particle of wet
sewage sludge. Fluidizing gas: air; fluidization velocity: 0.25m/s; fuel particle size:
2cm; frame rate 25fps.
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Ischia, June, 27-30 - 2010
combustion enables faster the
devolatilization process; iii) as
expected, particle fragmentation
reduces both devolatilization and
char burn-out time.
4.3. Primary fragmentation
experiments
The number of particle fragments
collected (multiplication factor) at
the end of pyrolysis as well as
combustion tests is reported in
figure 6A as a function of the
particle initial size. Results show
that in both oxidative and non-
oxidative atmosphere the number
of fragments generated by
comminution phenomena was
almost the same. In the
experimental conditions adopted,
there was a critical particle size
below which the sludge particle
do not fragment, 18-20mm.
Above this critical size, the
multiplication factor increases
exponentially. Figure 6B reports
the ratio between the largest
fragment size generated (with and
without particle fragmentation)
and the initial value of particle
size as a function of the particle
initial size. This ratio is quite
constant about 0.75 up to the
critical initial particle size (18-
20mm) then collapses because of
the fragmentation phenomena.
5. Conclusions
Devolatilization/pyrolysis and
combustion processes of a wet
sewage sludge in fluidized bed
have been characterized by visual
observation and by the analysis of
the time-resolved gas concentration profiles measured at the exhaust. Visual observation of
wet sewage sludge combustion pattern highlights the absence of flames during the volatile
matter combustion. The time-resolved CO2 concentration profile has been adopted to estimate
time, s
0 100 200 300 400 500
CO
CH
4 H
2, p
pm
0
2000
4000
6000
8000
10000
12000S
O2 N
O, p
pm
0
50
100
150
200
250
CO
2,
%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
CO, ppm
CH4, ppm
SO2, ppm
H2, ppm
CO2, %
NO, ppm
Fig. 3 Time-resolved CO, CH4, H2, SO2, NO and CO2
concentration during a pyrolysis experiment of
a unfragmented 22mm sludge particle.
time, s
0 100 200 300 400 500
CO
, p
pm
0
100
200
300
400
500
CO
2,
%
0
1
2
3
4
5
6
NO
, p
pm
0
50
100
150
200
250
300
CO, ppm
CO2, %
NO, ppm
Fig. 4 Time-resolved CO, NO and CO2 concentration
during a combustion experiment of a 22mm
unfragmented sewage sludge particle.
time, s
0 100 200 300 400 500
CO
, p
pm
0
100
200
300
400
500
CO
2,
%
0
1
2
3
4
5
6
NO
, p
pm
0
50
100
150
200
250
300
CO, ppm
CO2, %
NO, ppm
Fig. 5 Time-resolved CO, NO and CO2 concentra-
tions curves during a combustion experiment
of a 22mm fragmented sludge particle.
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Processes and Technologies for a Sustainable Energy
the pyrolysis, devolatilization and fuel burn-out time highlighting that: i) devolatilization is
faster than pyrolysis; ii) fragmentation enables faster conversion processes. Particle
fragmentation occurs during devolatilization whenever the particle initial size overcomes 18-
20mm under the investigated operating conditions.
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International Conference on
Fluidized Bed Combustion, Reno, NV (U.S.A.), p.1201 (2001).
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(2008).
11. Scott, S.A., Davidson, J.F., Dennis, J.S., Hayhurst, A.N.: Chem. Eng. Sci., 62:584 (2007).
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15. Solimene, R., Urciuolo, M., Cammarota, A., Chirone, R., Salatino, P., Damonte, G.,
Donati, C., Puglisi, G.: Exp. Therm. Flu. Sci., 34:387 (2010).
d, mm
10 12 14 16 18 20 22 24 26 28
No
ut
/ N
in
1
10
100
Nitrogen
Air
d, mm
10 12 14 16 18 20 22 24 26 28
do
ut
/ d
in
0.4
0.6
0.8
1.0
Nitrogen
Air
A B
Fig. 6 Fragmentation data as a function of the particle initial size. A) Multiplication factor;
B) Ratio between the largest fragment size generated and the particle initial size.
6