Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
MONITORING FLUIDIZATION QUALITY AND COMBUSTION
EFFICIENCY OF INVASIVE SPECIES PELLETS
Tânia Ferreira, [email protected]
Edmundo Marques, [email protected]
Oporto University – Portugal
Diana Almeida, [email protected]
Carlos Pereira, [email protected]
João Monney Paiva, [email protected]
Polytechnic of Viseu – Portugal
Carlos Pinho, [email protected]
Oporto University – Portugal
Abstract. A transparent fluidized bed with 10 cm ID was used to burn two different type of pellets, pine and acacia, in
the bubbling regime. The fluidized bed setup had to be adapted to allow for the different linear expansion
characteristics between the metallic distributor and the quartz tube.
The combustion was initiated using a sandwich made of a batch of pellets, a burning alcohol embedded tissue and,
again, a batch of pellets. Both the visual observation of the bed and the pressure drop measurement enabled ensuring
fluidization quality. The bed temperature, as well as CO, NOx and CO2 emissions were monitored during the
combustion process. The results obtained with both pellets were benchmarked with those obtained in previous work
using a domestic pellet boiler. The results show that the former high level of CO and NOx emissions were not due to
the species particular properties but to the ill adapted boiler design, concerning CO, and to the species inherent nitrogen
content, concerning NOx.
Keywords: invasive species, fluidization quality, combustion efficiency.
1. INTRODUCTION
During the last decades, worldwide energy needs have been supported mainly by fossil fuels (Mahmoundi et al.,
2010; Saidur et al., 2011). According to the annual report of the International Energy Agency (IEA, 2013), it is
estimated that the world's energy consumption will grow 56% between 2010 and 2040, being fossil fuels responsible for
almost 80% of this value. Regarding carbon dioxide emissions, and taking into account current policies and regulations,
a 46% increase in these emissions is expected (IEA, 2013). The rising prices and the higher emissions arising from the
use of fossil fuels have been causing economic and environmental concerns, which have motivated the increasing use of
renewable energies (Verma et al., 2012). Nonetheless, there is still large room for improvement, both in efficiency and
environmental impact (González et al., 2006). The use of biomass for energy purposes can minimize the dependence on
fossil fuels and reduce the emissions of pollutants. Its combustion is characterized reduced CO2 atmosphere addition,
due to photosynthesis, if complemented by a corresponding adequate forestry biomass plantation (Klason and Bai,
2007; Ozil et al., 2009). Ever since mankind has discovered fire, biomass has been used as the main source of energy,
for heating and cooking particularly, although its use is often characterized by low efficiencies and high emissions
(Demirbas, 2005). The densification process of biomass, specifically pelletization, improves some of the undesirable
features that biomass presents in its original state, which usually complicates its use as a fuel (Mediavilla et al., 2009;
Theerarattananoon et al., 2011). Over the last few years, the use of wood pellets has been continuously increasing; in
Europe, it is estimated that in 2020 an increase of over 10% will take place in pellets production, compared to 2009
(Lestander et al., 2012) and Pinus pinaster, the leading species used in the export pellets industry. Nevertheless its area
of occupancy has been undergoing a significant reduction due to its extensive use in paper and furniture industries
(Monteiro et al., 2012). The use of some invasive species exhibited some issues concerning CO and NO x emissions
when burnt in a domestic pellet boiler, which lead to non-compliance of some European standards, as showed in a
previous study (Ferreira et al., 2014).
Fluidized bed combustion is a flexible combustion system burning different fuels. Fluidized bed combustors allow a
more uniform temperature distribution as well as greater solid-gas contact area, thus ensuring low pollutant levels.
(Obernberger, 1998; Khan et al., 2009). This work uses a bubbling fluidized bed to evaluate the use of some invasive
species as a fuel. The emission results were compared with those obtained in the above mentioned work (Ferreira et al.,
2014).
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
2. EXPERIMENTAL SETUP AND PROCEDURES
2.1 Experimental setup
The experimental setup consisted of a cylindrical fluidized bed with 100 mm internal diameter, 1230 mm long,
made of 10 mm thick fused quartz (Fig. 1). The distributor was made of F10 steel with 38 3mm ID orifices. The
fluidization gas used was air from a laboratory distribution line.
The metallic distributor was of the tuyère type, completely covered up to the orifice level with 2 mm silica ballotinis
in order to minimize conduction heat transfer to the glass-metal interface during the running tests. The fluidized bed
was composed by 1000 g of silica sand particles in the 355-425 µm range, corresponding to a static bed height of 11
cm.
The bed temperature was measured by a K type thermocouple that was connected to Picolog Recorder through a
TC-08 USB datalogger. The mass flow rate of the fluidizing air was measured with a set of three orifice plate flow
meters, which allows obtaining superficial air velocities ranging from 0.6 to 2.3 m/s, using both a U-tube water pressure
manometer and a Furness Control FCO 34 differential pressure transducer, connected to a data acquisition system. A
pressure probe allows the measurement of the pressure drop of the bed, which was also recorded during the tests. A
stainless steel probe sampled the exhaust gases from the center of the reactor at 50 cm above the distributor and a Testo
350 Emission Analyzer measured the flue gases, using the software Easy Emission. The continuous upper feeding of
pellets was ensured using a vibratory feeder Retsch DR 1000.
Figure 1. Experimental setup.
2.2 Fuel characterization
For the present work, two different type of pellets were used: Pinus pinaster and Acacia dealbata. The first ones
were acquired from a Portuguese company and are currently being traded, while Acacia dealbata pellets were
manufactured specifically for this work at the lab facilities. Both types of pellets were previously tested as a fuel in a
domestic pellet boiler (Ferreira et al., 2014).
The main properties of the pellets used in this work are shown in Tab. 1. The proximate and ultimate analysis of
Pinus pinaster pellets was provided by the manufacturer of these pellets, the analysis of Acacia dealbata pellets was
previously made by LNEG – National Laboratory of Engineering and Geology in Lisbon, Portugal. The LHV was
determined under DIN EN 14918. The results reveal that pine pellets present a better heating value when compared with
acacia pellets; however, both types of pellets have an attractive heating value. In the proximate analysis, pine revealed a
higher volatile content as well as lower ash content, compared to acacia pellets. Relatively to the ultimate analysis,
higher nitrogen content was observed in acacia pellets, as well as higher sulphur and chlorine content when compared to
pine pellets.
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
Table 1. Fuel properties.
Proximate analysis (as received), % (w/w)
Moisture content
Volatile matter
Fixed carbon
Ash
Ultimate analysis (dry basis), % (w/w)
Carbon
Hydrogen
Nitrogen
Sulfur
Chlorine
Oxygen
LHV, kJ/kg
Pinus pinaster
Acacia dealbata
6.6
85.3
14.1
0.60
6.6
74.7
17.3
1.4
50.7
6.9
0.2
0.01
0.01
42.19
18.8
48.8
1.1
0.065
0.12
43.6
18.1
2.3 Startup system
As there was no external source of energy, it was necessary to establish an efficient system to promote the
combustion startup of the installation. Pellets ignition was initiated using a batch of pellets sandwich with a burning
alcohol embedded tissue in between. The first layer of pellets had about 4 cm high. During approximately 6 minutes the
bed was kept at incipient fluidization conditions to ensure that all the pellets combustion had sufficient oxygen with a
fixed bed condition. Then the air flow would momentarily, but significantly, increase, so that the bed would become
vigorously fluidized, thus swallowing the mass of burning pellets. Only then the continuous feeding was initiated.
Figure 2 represents the successive steps of that fluidized bed combustion process, from the ignition phase of pellets
inside the bed till the steady state combustion bubbling regime.
Figure 2. Stages of fluidized bed combustion.
3. RESULTS
Some tests were made using pine and acacia pellets in a bubbling fluidized bed. In the work mentioned previously
(Ferreira et al., 2014), both type of pellets were tested in three different thermal loads which were designated as
“reduced”, “average” and “high” loads. However, in this work, due to the operational capacities of the fluidized bed,
namely its internal diameter and height, only the “reduced” load was possible to test.
Figure 3 shows the carbon monoxide (a) and nitrogen oxides (b) emissions for pine and acacia pellets during
approximate stationary periods in two different combustion systems: pellet boiler and fluidized bed.
For both type of pellets, the combustion in fluidized bed revealed a better, more efficient combustion, with lower
carbon monoxide emissions. On average, those CO emissions were 55 and 42% lower for the combustion in fluidized
bed, respectively for pine and acacia, when compared with the same process taking place inside a conventional fixed
bed pellets boiler.
Regarding NOx emissions, Fig. 3(b) represents those pollutant levels during the combustion of pine and acacia
pellets in both combustion systems. When these two combustion situations are compared, the nitrogen oxides display a
slight decrease for the acacia pellets case. In contrast, in the case of pine pellets, for the operational conditions tested, an
increase in NOx emissions was noticed when the combustion of pellets is carried out in the fluidized bed. Like the
obtained results for NOx emissions in a domestic boiler, the fluidized bed produces emissions considerably higher when
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
acacia is burned compared to pine. This is probably the result of the high nitrogen content of the substance itself, as can
be observed in Tab. 1: Acacia dealbata has a significantly higher N2 content than Pinus pinaster (five to six times
more).
(a)
Pellets boiler
4000
NOx (dry volume ppm
@13% O2)
CO (dry volume ppm @ 13%
O2)
5000
Fluidized bed
3000
2000
1000
0
Pine
450
400
350
300
250
200
150
100
50
0
Pellets boiler
Fluidized bed
Pine
Acacia dealbata
(b)
Acacia dealbata
Figure 3. CO and NOx emissions for pine (a) and acacia (b) pellets for the two different combustion systems.
As can be observed in Fig. 4, for both pellets, the combustion in the fluidized bed allows a reduction in CO2
emissions. On average, the CO2 emissions were quite similar, regardless the pellets type burned.
CO2 (dry volume % @13%
O2)
10
Pellets boiler
Fluidized bed
8
6
4
2
0
Pine
Acacia dealbata
Figure 4. CO2 emissions for pine and acacia pellets for the two different combustion systems.
Table 2 shows three examples of some tests performed for both pellets in the fluidized bed, with the average of
pollutant emissions and the combustion parameters. In the case of the acacia results presented, the fluidizing air and the
fuel mass flow rate were approximately the same in the three tests, once the feeder was in the same flow control
position. Although the fuel supply was performed continuously and automatically, there were periods of time, relatively
small, in which the fluidized bed was not being fed and the combustion was poor, and others in which the fuel supply
was in excess and therefore combustion was rich. The volumetric feeding system used (an Archimedes screw) was
considerably influenced by the length of the pellets that were intermittently being dropped inside the bed.
Table 2. Pollutant emissions and combustion parameters of some tests performed in a fluidized bed.
Pinus p.
Acacia d.
O2 (dry vol.
%)
CO (dry vol.
ppm)
NOx (dry vol.
ppm)
Fluidizing air mass flow
rate (g/min)
11.88
1826
94
217
Bed
temperature
(°C)
-
13.09
1167
79
189
837
14.25
980
70
254
824
13.82
1474
296
147
734
14.41
963
290
147
671
16.38
575
182
147
765
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
These fluctuations in the fuel mass flow rate lead to extremely high levels of instantaneous emissions, as shown in
Fig. 5, where the pollutant emissions (CO and O2) as a function of time are represented, during a specific test, using
acacia pellets. The most significant variations in CO emissions, specifically the steep increases, were followed by a
significant reduction in oxygen concentration in the exhaust gases, which leads to the conclusion that, in these
moments, the fuel mass flow rate was excessive for the fluidized air flow available in the bed.
Regarding to the bed temperatures, these were relatively low, particularly in the tests of acacia pellets.
14000
25
CO
O2
12000
15
8000
6000
10
O2 (dry volume %)
CO (dry volume ppm)
20
10000
4000
5
2000
0
0
0
10
20
30
40
50
60
70
80
Time (min)
Figure 5. Fuel emissions in a test of Acacia dealbata.
4. CONCLUSIONS
The pollutant emissions resulting from the fluidized bed combustion of pine and acacia pellets were analyzed and
compared with the results obtained previously in a domestic pellets boiler using the same type of pellets.
With the combustion in fluidized bed the obtained carbon monoxide emissions were lower than those obtained
using the domestic pellets boiler, on average 49% lower, for both pellets substances. Regarding to NOx emissions, a
decrease of these emissions using the fluidized bed combustion system was also verified for acacia pellets.
The significant fluctuations of CO emissions during the running tests are due to intermittences of the fuel supply, as
the value is very small and, being a volumetric system, very dependent of the pellets average size, namely the pellets
length.
Taking into account the obtained results, both pellets presented a better combustion behavior, with lower emissions,
using the fluidized bed. The higher CO emissions obtained previously were due to the boiler design and not to the
inherent properties of these species. Concerning NOx emissions, the combustion emissions are strongly and unavoidably
dependent on the nitrogen content of the Acacia invasive species. However, this is a work in progress and it will be
necessary to carry out more tests in the fluidized bed using a more constant fuel supply and achieving higher bed
temperatures, looking forward to further reducing pollutant emissions, mainly CO.
5. ACKNOWLEDGEMENTS
This work was partially supported by the PTDC/AGR-CFL/114826/2009 grant from the Portuguese Foundation for
Science and Technology (FCT). The tests were carried out in the laboratory facilities of ESTV/IPV. The authors wish to
express their gratitude to the ESTV board. Tânia Ferreira and Carlos Pinho are thankful to the FEDER funds through
the Operational Programme for Competitiveness Factors – COMPETE, ON.2 - O Novo Norte - North Portugal
Regional Operational Programme and National Funds through FCT - Foundation for Science and Technology under the
projects: PEst-C/EQB/UI0511, NORTE-07-0124-FEDER-000026 - RL1_ Energy.
Proceedings of ENCIT 2014
Copyright © 2014 by ABCM
15th Brazilian Congress of Thermal Sciences and Engineering
November 10-13, 2014, Belém, PA, Brazil
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7. RESPONSIBILITY NOTICE
The authors are the only responsible for the printed material included in this paper.