Effect of Exhaust Gas Recirculation and Air Staging on Gaseous and
Particle Emissions in a Domestic Boiler
Mafalda da Costa Bernardes Figueira Henriques
Supervisor: Mário Manuel Gonçalves da Costa
Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
Abstract
The present study aims to evaluate the effect of two distint methods in the reduction of the emission of gases and
particles from a domestic boiler fired with pellets. The methods studied are flue gas recirculation and air staging.
Both methods were examined for two boiler thermal loads. The application of RGE led to improvements in NO x
emissions, but its effect on the CO emissions was marginal. This method, however, is promising as a strategy to
reduce the emission of particles, in particular, ultra-fine particles. This requires, however, further investigation. The
air staging proved to be a very effective method to reduce PM 2,5 emissions. In this case, however, it is necessary to
give special attention to the CO emissions, which may increase when air staging is applied to the boiler.
Keywords: Biomass, domestic boiler, pollutants, flue gas recirculation, air staging
1.
Introduction
Fossil fuels are the prime energy source in the world. However, when fossil fuels are burned, they
release carbon dioxide and other gases into the atmosphere. Some of these gases contribute to climate change,
causing several damages, such as the greenhouse effect. Moreover, fossil fuels will deplete within the next
years.
In order to decrease our dependency on fossil fuels it is necessary to replace it by new energy
sources. Biomass has great potential to substitute fossil fuels, being one of the options to mitigate the
greenhouse gas emissions. This is because biomass combustion does not add carbon dioxide into the
atmosphere as it absorbs the same amount of carbon in growing as it releases when consumed as a fuel [1].
Biomass combustion in small-scale devices is a common source of heating and has become widely
used in countries where winter is more severe. However, despite the fact that biomass combustion contributes
to mitigate CO2 emissions, it is also an important source of air pollution. Particularly, pellet stoves are a
common type of small scale devices that are widely used in urban areas. These burning systems release high
amounts of gaseous and solid pollutants into the atmosphere.
Solid pollutants are known as Particulate Matter (PM) and consist of airborne particles composed of
acids (such as nitrates and sulphates), carbon, organic chemicals, and soil material. Particle emissions are
characterized according to their aerodynamic diameter, and can be distinguished into fine particles and coarse
particles. The fine particles have an aerodynamic diameter smaller than 2,5 µm, whereas the coarse particles
have a diameter higher than 10 µm. Fine particles aer mainly composed of organic matter, elemental carbon
(i.e., soot) and fine ash (i.e., inorganic compounds). The organic matter and the soot result from incomplete
combustion while the fine ash particles result from the inorganic material in the fuel ashes. The smallest
particles can travel high distances in the atmosphere. These particles are associated to serious health problems
as they can easily enter through the respiratory system.
Furthermore, small scale appliances originate high particle emissions when compared to the fluidized
bed systems. Despite the fact that particle emissions may lead to serious health problems, there is no method
capable of capturing efficiently the smallest particles. Facing the presented facts, the investigation of new
methods capable to reduce the finest particles formation reveals great importance. It is thought that the
modification of the combustion process may reduce particle emissions. Nevertheless, there are few studies
available dedicated to the particle emissions reduction by modifying the combustion process in small-scale
biomass-fired heating appliances.
Zandeckis et al. [2] performed a study related to the air staging influence in the combustion
efficiency, evaluating the gaseous emissions and the bottom ashes as well. The authors reported that the
application of air staging is very effective on domestic pellet boilers. The authors also concluded that CO and
particle emissions increase when the thermal input increases. Primary air reduction decreases the CO
emissions and also reduces the combustion temperature, diminishing the vaporization of the easiest volatile
elements present in the fuel ashes.
Becidan et al. [3] investigated the fuel ashes behaviour during biomass combustion with and without
air staging. The authors found out that air staging instigates a higher retention of the volatile elements present
in the fuel ashes, leading to minor particle emissions. The authors also concluded that the air staging is a
promising method concerning the smallest particle emissions reduction.
Wiinikka and Gebart. [4] studied the effect of air distribution rate on particle emissions during pellets
combustion in an experimental reactor. The small particle emissions diminished with the reduction on the
primary air. Due to the lack of oxygen, the temperature was lower and the vaporization of the fuel ashes was
reduced. The authors concluded that the vaporization of the fuel ashes was reduced if the primary air supply
close to the burner was reduced as well.
Lamberg et al. [5] investigated the effects of thermal input and air staging on particles and gaseous
emissions in a modern small scale pellet boiler. The authors concluded that CO and particle emissions
increased with the increase of the partial load, although the CO increase was more notable. Primary air
reduction resulted in a decrease on CO emissions, as well as in a decrease on the temperature in the primary
combustion zone. The outcome consisted on the reduction of the vaporization of the inorganic material from
the fuel. Secondary air reduction had a negative effect on the particle emissions and caused a higher
vaporization of the inorganic matter. Furthermore, incomplete combustion was another consequence of
secondary air reduction. A good mixture of the secondary air with the fuel inside the combustion chamber had
a strong impact on emissions’ reduction.
The present investigation focused on the influence that EGR and the Air Staging application have on
the gaseous and particle emissions. EGR is a common technique used to reduce NOx emissions by lowering
the temperature inside the combustion chamber and decreasing the O 2 concentration, thus making more
difficult the nitrogen oxidation [6] [7].
Air staging is a technique commonly used in large-scale energy production for NOx reduction. This
technique consists on creating a rich zone, known as primary combustion zone, where temperature and O2
concentration are low. Subsequently, the primary zone combustion products will be oxidized in an oxygen rich
zone. The O2 privation in the primary zone and the poor mixing of fuel and combustion air inhibits the NOx
formation. The lower temperature in the primary combustion zone also hinders the NOx formation.
2.
Experimental
The tests were performed in a domestic wood pellet-fired boiler with a maximum thermal capacity of
22 kW, with forced draught. In Figure 1, a schematic of the experimental set-up can be visualized. The pellets
were manually loaded into a hopper with a capacity of 45 kg and were fed to the burner through a screw
feeder which worked by impulses. The feeding rate of the pellets was regulated by the boiler load and the
pellets consumption was measured with the support of a loss-in-weigh technique.
The combustion of the pellets occurred within a hemispherical basket with a diameter of 120 mm.
Ignition was accomplished with the support of an electrical resistance placed close to the basket and the air
was supplied to the latter one by a dedicated fan through several small orifices located across the basket
bottom. Flue-gas concentrations of O2, CO, CO2, HC and NOx were measured with the support of a stainlesssteel probe and of analytical instrumentation.
The water flow rate circulating in the inner circuit of the boiler was measured with a rotameter and
the inlet and outlet temperatures were measured with thermocouples type K, T 1 and T2. Particles were
sampled isokinetically and the total mass concentration was achieved with the support of a low pressure threecascade impactor (LPI) with a flow rate of ≈ 8 L/min. Particles were sampled from the exhaust tube’s
centerline while the LPI was positioned horizontally. The LPI allows the collection of three particulate cut
sizes during the same sampling: Particles with diameters above 10 µm (PM10), particles with diameters
between 2.5 and 10 µm (PM2,5-10) and particles with diameters below 2,5 µm (PM2,5). In order to avoid
condensation along the line connecting the probe outlet to the impactor and also inside the impactor, a heating
jacket was used during sampling. Particles were collected with microfiber filters, which were dried before and
after each test. The filters were weighted in order to determine the quantity of particles captured.
Pellets
Flue gas
12 bit A/D
Termocouple K
T1
Probe
HC analyzer NOx analyzer O2 analyzer
Water
network
T3
T2
LPI3/LPI13
Air
Weighbridge
CO and CO 2
analyzers
BOILER
Condenser
Zero gas and
span gas
Silica gel
Water out
Water in
RS 232
Cotton
filter
220 V
Diaphragm
pump
Figure 1. Schematic of the experimental set-up.
In the present study, the combustion chamber of the boiler was modified in order to allow recirculating
the exhaust gases (Figure 2) and air staging (Figure 3).
Figure 2. Schematic of the domestic boiler with Exhaust Gas Recirculation.
During the EGR tests, a fraction of the flue gases re-entered into the combustion chamber along with
the combustion air. This was achieved by the complete opening of the valve which separates the flue gas pipe
from the air intake pipe, which allowed the mixture of the combustion air and the combustion products. In the
contrary, the air intake valve was partially closed, thus the percentage of flue gases desired along with the air
intake was controlled by the further closing or opening of that valve. The desired EGR percentage was
multiplied by the CO2 value which was measured with the support of a probe that was inserted in the flue gas
pipe. Afterwards, the probe was inserted in the air intake pipe, and the CO2 was again measured. The percentage
of CO2 present in the air intake pipe should be equal to the value obtained when EGR (%) was multiplied by the
percentage of CO2, where the latter one was firstly measured in the exhaust gases. In the case of a required
higher percentage of EGR, the air intake valve was further closed.
Secondary air
Secondary air
Pellets
Primary air
Figure 3. Schematic of the domestic boiler with Air Staging.
The air staging was accomplished with the support of a supplementary device. The referred device’s
shape is presented in Figure 3. The stainless steel structure presents holes in all its structure. The secondary air
went through this device, and its flow rate was measured by a second rotameter. Secondary air was fed into the
flame through the holes, and the secondary air jets direction achieved 45 degrees with the vertical direction.
The primary air entered through the small orifices located across the basket bottom. The amount of air
supplied was kept constant. The increase of secondary air flow rate led to a decrease on primary air flow rate
which was controlled by the boiler itself.
3.
Results and Discussion
The main characteristics of the pine pellets composition used in the present work can be visualized in
Table 1. The pellets were produced in the north of Portugal.
Parameter
Carbon
Hydrogen
Nitrogen
Sulphur
Oxigen
Volatile
Fixed Carbon
Moisture
Ashes
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
P2O5
K2O
Na2O
Cl
Other oxides
LHV
Diameter
Length
Table 1. Characteristics of pine pellets.
Value
Ultimate Analyses (%wt, daf)
46
6.2
0.5
<0.01
47.3
Proximate Analyses (%wt, ar)
80.5
10.9
7.3
1.3
Ash composition (wt%, db)
20.9
6.2
21.6
26.2
0.3
4.3
4.2
11.5
2.5
0.04
2.3
17.1
Average Dimensions of the pellets (mm)
6
18
3.1. Operating Conditions
Tables 2 and 3 summarize the boiler operating conditions for the tests where EGR and air staging were
applied, respectively. The variations on the thermal input caused a variation in the excess air and consequently
on the flue gase O2 concentration. This was due to the fact that the fan velocity was kept constant, despite of the
pellets feeding rate. This situation implies that when the fuel consumption increases, the excess air decreases [8].
Table 2. Operating conditions for EGR tests.
Thermal
input (kW)
Pellts feed
rate (kg/h)
13
2,6
17
3,4
Oxidant
O2 (%)
(vol.%)
20,9
20,39
20,13
19,59
19,30
18,35
20,9
20,09
19,79
19,39
18,31
17,76
17,11
CO2 (vol.%)
0
0,47
0,98
1,31
1,81
2,49
0
0,47
0,91
1,27
2,08
2,67
3,17
Flue gas O2
(vol. %)
17,28
17,2
17,28
17,05
17,59
16,71
16,1
15,6
16,0
16,4
15,9
16,0
16,3
EGR
(%)
0
10
20
30
45
60
0
10
20
30
40
50
60
Observing Figure 2 it can be noticed that an increase on EGR produced a decrease on the flue gas O2
concentration. This situation is related to the fact that the increase of EGR incited a higher O2 dilution. It can
also be noticed that the increase on EGR is accompanied with a CO 2 increase on the flue gas.
Table 3. Operating conditions for Air Staging tests.
Thermal input
(kW)
Pellets feed rate
(kg/h)
13
2,6
17
3,4
Flue gas O2
(vol. %)
Air Staging
(%)
17,4
17,3
17,5
17,1
17,3
16,11
16,31
16,12
16,67
16,44
0
3
6
9
12
0
3
6
9
12
Table 3 shows the operating conditions for the tests in which air staging was applied. For both the
thermal inputs the O2 concentration in the flue gas was kept approximately constant. This is related to the fact
that air supply was kept approximately constant.
3.2.
Effects of EGR and Air Staging on gaseous and particle emissions
The effect of both EGR and air staging application on gaseous and particulate emissions can be
visualized from Figures 4 to 7. CO and NOx emissions concentration were measured and corrected at 13% O 2.
The gaseous and particle emissions were normalized relatively to the baseline which corresponds to the situation
where no EGR or air staging were applied. In the present investigation the HC emissions were not taken into
account as they were considerable reduced, and therefore could be neglected.
3.2.1
Effect of EGR on the gaseous and total particle emissions
PM /PM baseline
EGR , 13 kW
1.5
1.5
1.0
1.0
0.5
0.5
PM
CO
NOx
0.0
0.0
baseline
10
20
30
45
60
2.0
2.0
PM
CO
NOx
EGR, 17 kW
1.5
1.0
1.0
0.5
0.5
0.0
0.0
baseline
10
20
EGR (%)
30
40
50
60
EGR (%)
2.0
2.0
EGR, 13 kW
1.0
0.5
PM < 2.5 µm
EGR, 17 kW
PM < 2.5 µm
1.5
PM /PM baseline
PM /PM baseline
1.5
CO/CO baseline, NOx/NOx baseline
2.0
2.0
CO/CO baseline, NOx/NOx baseline
PM /PM baseline
In Figure 4 the effect that EGR has on the CO, NOx and total particle emissions is revealed, for 13 kW,
as well as its effect on the finest particle emissions (PM 2,5). Figure 5 shows the effect of EGR application on the
gaseous and particle emissions for a 17 kW thermal input.
1.5
1.0
0.5
0.0
0.0
baseline
10
20
30
EGR (%)
45
60
Figure 4. Effect of EGR on gaseous and
particle emissions for 13 kW.
baseline
10
20
30
EGR (%)
40
50
60
Figure 5. Effect of EGR on gaseous and
particle emissions for 17 kW.
It can be verified that the effect of EGR on NOx emissions was slightly superior when applying a
thermal load of 13 kW. A study performed by Buddeker [8] revealed that the excess air decreases when the
thermal input increases. The author also demonstrated that NO x emissions reduction was more significant for
higher excess air ratios, i.e., for smaller thermal inputs. The referred fact was related to the high air concentration
inside the combustor chamber [9]. The high air concentration led to a decrease in the temperature inside the
combustion chamber, thus freezing the chemical reactions which inhibit the formation of new species.
Increasing EGR produced a higher O2 dilution in the oxidant. When a thermal load of 13 kW was
applied the excess air was elevated, thus the decrease of O2 concentration caused by the increase of EGR might
have had a strong contribution on lowering NOx emissions. This is due to the fact that NOx depends strongly on
the local O2 concentration [10]. The referred reason, along with the combustion temperature reduction resultant
from the raise of the thermal capacity of the burned gases, might have had a strong influence on the NO x
reduction.
When a thermal load of 17 kW was applied, the initial O2 availability was lower. Therefore the EGR
application might have had a smaller influence on NOx emissions. This motive is possibly explained by the fact
that EGR application produced a stronger effect solely on the temperature reduction. So, nitrogen oxidation
might have had a diminished relevance when compared to the decrease on temperature.
For both the thermal inputs, CO emissions demonstrated no reduction with the EGR application.
However, CO emissions were superior when a thermal load of 13 kW was applied. Rabaçal et al. [11] verified
that for the higher thermal inputs CO emissions tended to decrease. This fact indicated that the incomplete
combustion was encouraged by the higher excess air ratios, i.e., by the lower thermal inputs.
Lamberg et al. [5] observed that a higher excess air ratio provoked a decrease on temperature, leading to
a reduction of the CO oxidation rate. This motive can be explained by the fact that the oxidation rate depends
strongly on temperature, which decreases with the excess air increase. Consequently, the oxidation reactions
were processed at slower combustion rates, and the complete fuel oxidation was not possible to acquire, leading
to the fuel incomplete combustion. The temperature reduction, caused by the EGR increase, might also have had
a negative effect on CO emissions due to a more incomplete combustion.
Finally, when observing the total particles evolution, it is easily verified that it was similar to the CO
emissions behavior. This has already been discussed by Fernandes and Costa. [12]. The similarity between both
the pollutants trend evolution suggests that CO emissions might be a good indicator of the boiler efficiency
regarding the particle emissions. Thus, the particles evolution behavior might be related to the soot formation,
i.e., to the fuel incomplete combustion. The incomplete combustion is probably due to the excess air and to the
EGR augment which provoked a temperature reduction.
The EGR increase presented a contrary effect of what was desired regarding PM2, 5 emissions, and there
was no clear evidence of the PM2,5 reduction with the EGR intensification. Possibly due to a more incomplete
combustion, occurred a poorer oxidation of the organic matter, and the soot formation was favored.
For the higher thermal inputs, EGR application produces better results regarding the PM2,5 emissions.
This fact might be related to the lower excess air, which resulted in a higher temperature inside the combustion
chamber, allowing a better soot oxidation.
Comparing total particles evolution with the PM2,5 evolution it can be seen that both presented a similar
behavior. Such fact suggests that total particles were dominated by particles whose sizes are in the order of the
submícrons.
1.5
2.0
Air Staging, 13 kW
1.5
1.0
0.5
1.0
0.5
PM
CO
NOx
0.0
0.0
baseline
0.03
0.06
0.09
0.12
2.0
2.0
Air Staging, 17 kW
PM /PM baseline
PM /PM baseline
2.0
1.5
1.5
1.0
1.0
0.5
PM
CO
NOx
0.5
0.0
0.0
baseline
0.03
Sec. air/Prim. air
0.09
0.12
Sec. air/Prim. air
2.0
2.0
Air Staging, 13 kW
Air Staging, 17 kW
PM < 2.5 µm
1.5
PM /PM baseline
PM /PM baseline
0.06
CO/CO baseline, NOx/NOx baseline
Effect of Air Staging on the gaseous and particle emissions
CO/CO baseline, NOx/NOx baseline
3.2.2.
1.0
0.5
0.0
1.5
PM < 2.5 µm
1.0
0.5
0.0
baseline
0.03
0.06
0.09
Sec. air/Prim. air
0.12
Figure 6. Effect of Air Staging on gaseous
and particle emissions for 13 kW.
baseline
0.03
0.06
0.09
Sec. air/Prim. air
0.12
Figure 7. Effect of Air Staging on gaseous
and particle emissions for 17 kW.
The effect of air staging on gaseous and particulate emissions for both 13 and 17 kW can be seen in
Figures 6 and 7, respectively. Similarly to what was said previously, the gaseous emissions were normalized at
13% O2 and were compared to the situation in which no air staging was applied.
In Figure 6, it can be observed that the NOx decreasing with the air staging application was not
successful when a thermal load of 17 kW was applied, similarly to what had happened when EGR was applied.
Once again the results are consistent with those obtained by Rabaçal [13]. The authors observed that, for the
same pellet fired-boiler, the NOx emissions increased with the raise of the thermal input, due to the decreasing of
the excess air ratio. For the higher thermal inputs, there is a minor availability of O 2 in the secondary combustion
zone. This results in a higher combustion temperature which leads to better nitrogen oxidation conditions,
originating higher NOx emissions.
Air staging did not present improvements concerning the CO emissions when a thermal load of 13 kW
was applied. Despite this fact, the combustion efficiency increased significantly for a thermal load of 17 kW,
thus presenting a reduction on the CO emissions. Such fact proves, once again, that combustion conditions are
more effective when lower excess air ratios are applied.
This indicates that the rise on the secondary air does not have a great influence on the fuel oxidation.
The same finding was reported by other authors [2], [14]. According to Chaney and Liu [14], the higher the
secondary air flow rate is, the bigger has to be the required area for the fuel complete oxidation. Due to boiler’s
small dimensions, the mixture between fuel and secondary air became more difficult to achieve. The requirement
for a larger combustion area augments with the increase of the secondary air flow rate, resulting in a less
efficient combustion process. On the other hand, Lamberg et al. [5] indicated that the increase of the secondary
air brings good benefits concerning the emissions due to incomplete combustion. In fact, when applying a
thermal load of 17 kW, CO emissions had a significant reduction when the secondary air flow rate was
intensified. Possibly, due to the lower excess air present in the combustion chamber, the secondary air flow rate
diminished. Moreover, due to the minor excess air ratio, the secondary air flow rate was inferior to that when a
thermal load of 13 kW was applied. Thereby, the mixture conditions inside the combustion chamber, when
applying a thermal load of 17 kW, might have been enhanced comparatively to the case of 13 kW.
However, contrary to what occurred for the tests where EGR was applied, CO and particle emissions
evolution present different trends. Such situation can be related to a minor vaporization of the inorganic elements
present on the fuel, caused by the temperature reduction in the primary combustion zone. The soot formation
reduction, related to a better oxidation, might also have contributed to the particle emissions abatement.
Moreover, it was verified that the particles reduction was more efficient when a thermal load of 17 kW was
applied. The better mixing of the secondary combustion air with the fuel and the higher temperatures in the
secondary combustion zone are possibly the main reasons for the emissions from incomplete combustion
reduction, including soot.
Contrasting to what occurred when EGR was applied, the variation of PM2,5 emissions shows a different
trend from the total particle emissions evolution, as it can be seen in Figures 6 and 7. In fact, when comparing
the particle emissions evolution presented in both pictures, it is clear that the decreasing on PM 2,5 is very
noticeable, whereas the total particles reduction is less obvious.
This difference may suggest that a displacement on the particle mass concentration from the small
towards the biggest particles occurred. Wiinikka and Gebart. [4] and Žandeckis et al. [2] verified that the coarse
fly particle sizes augmented when higher secondary air flow rates were applied. The authors explained that this
situation was due to the increase of the air velocity inside the combustion chamber. The velocity increase
provoked an extra force, in such a way that a large fraction of small particles was removed out from the fuel bed.
These particles were then carried out by the gas flow, contributing to the coarse fly particles increase.
Furthermore, an increase on the secondary air flow rate produces a reduction on the primary air flow
rate, leading to a temperature decrease in the primary combustion zone. Similarly, Lamberg et al. [5] verified
that the secondary air reduction produced negative effects on the fine particle emissions and on the vaporization
of the alkaline metals. The authors state that the primary air reduction causes a secondary air flow rate elevation,
thus reducing the temperature in the primary combustion zone.. The temperature reduction could inhibit the
vaporization of the alkaline metals present in the fuel ashes. The authors indicated as well that the secondary air
increase resulted in a better mixture in the secondary combustion zone. This way, according to the authors, a
better mixture in the secondary zone was promoted, thus the gaseous and particle emissions due to the
incomplete combustion of the fuel decreased. Due to the temperature reduction, the vaporization inhibition of the
inorganic elements from the fuel might have occurred. Hence, these elements might have merged in the glowing
particle remaining in the fuel bed. These elements could later be carried out by the fuel gas, contributing to the
raise of the coarse fly particles or could remain in the burner, originating the bottom ashes.
With the increment of the air staging it can be verified that that the PM2.5 emission decreases
significantly. This finding is consistent with the studies performed previously by Wiinikka and Gebart [4], who
found out that the fine particle emissions decrease when the secondary air increases with regards to the primary
air.. According to the authors, this could be related to the fact that enhancing the secondary air increases the
velocity inside the combustion chamber. Also, a better mixture between the fuel and the secondary air might
occur, resulting in a better soot oxidation. This fact had possibly a high contribution on the particles reduction.
Possibly, the mixture was more effective when the excess air ratio was smaller, which occurred for the boiler
thermal input of 17 kW.
3.
Conclusions
The present work consisted on the study of the effects caused by the application of EGR and air staging
in the gaseous and particle emissions resultant from the biomass combustion process in a pellet stove firedboiler, using pine pellets. For that purpose and in order to perform the current research, the burning system was
modified and the change of the oxidant admission system was executed.
Gaseous emissions were analyzed by means of proper instrumentation. The sampling and classification
of the particles was accomplished by a three stage low pressure impactor (LPI). The LPI classified the particles
accordingly to their aerodynamic diameter in three different size ranges.
It was observed that the strategies applied produced effects on gaseous and particle emissions, due to
the reduced temperatures which were achieved inside the combustion chamber. The temperature reduction
brought benefits related to the minor vaporization of the inorganic matter present in the biomass ashes. However,
the temperature reduction could also have a negative effect associated with the soot emissions that might have
increased as a consequence of incomplete combustion.
The EGR application showed an improvement on the NOx emissions, but had small effects on the CO
emissions. The EGR application as a strategy for particles reduction seemed to be an effective method, in special
for the case of particles in the order of the microns. Nevertheless, further investigation in this area is necessary.
Air staging was proved to be an effective method regarding the PM 2,5 emissions reduction. However,
special attention must be given to the CO emissions that have increased when compared to the situation in which
no staging was applied. However, this situation was only observed when higher excess air ratios were applied.
This reveals that air staging can affect negatively the combustion efficiency, if applied in an inappropriate way.
Furthermore, due to the fact that this method inhibits the vaporization of a part of the inorganic matter from the
fuel, it may cause ash agglomeration in the burner, obstructing the primary air passage and affecting the fuel
oxidation. In addition, it may promote higher flame instability, resulting in a less efficient combustion.
In conclusion, the modification of the combustion process demonstrates a good potential in lowering the
particle emissions, without severe consequences in the gaseous emissions. However, attention to external factors
must be given, as these may produce perturbations into the combustion process, decreasing its efficiency.
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