Effect of the Water Vapour Addition in the Oxidant at the Flameless Oxidation Combustor
Bruno A. Bernardes
Mechanical Engineering Department, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal
Abstract
This thesis examines the effect of the water vapor concentration present in the oxidizing on the
performance of a combustor operating under flameless conditions. The combustion chamber
presents low NOx emissions and was developed for gas turbine applications. In the present
study, the major gaseous pollutant emissions were measured using gas analyzers for various
operating conditions of the combustor. The results showed that for low values of the excess air
coefficient the greater the water vapor concentration in the oxidant the greater the CO
emissions. For high values of the excess air coefficient the effect of the presence of water vapor
in the oxidant become negligible. For all conditions studied, NOx emissions were always below
10 ppm@15% O2 and combustion efficiencies greater than 99.8%. Overall, it was observed that
the greater the concentration of water vapor in the oxidant the lower the NOx emissions.
1. Introduction
Flameless oxidation is a combustion regime first observed by Wunning [1]. This mode of
combustion was obtained by increasing the recirculation ratio between the inlet air and
combustion products. The mixing provided by the recirculation create a distributed reaction
zone with a reduced temperature maximum, smooth temperature gradients and low oxygen
concentration. Since the reaction zone has a reduced temperature maximum the NOx formation
is sharply decreased. This technology has received various names, such as high temperature
air combustion (HiTac) [2], flameless oxidation, moderate or intensive low oxygen dilution
(MILD) combustion, and colorless distributed combustion (CDC).
Veríssimo et al. [3] performed a study of the flameless oxidation regime using OH*
chemiluminescence detector as reaction zone indicator. In their study was found that the
increase in the thermal load makes the reaction zone larger and closer to the combustor exit
and the NOx emission was found to be low and relatively independent of the fuel input in wide
range of studied conditions. Flamme [4] reported detailed measurements of temperature from a
semi-industrial furnace operating under flameless oxidation conditions. The results showed the
possibility to achieve low NOx emissions using the flameless oxidation technology even with
high preheated air levels. Arghode, et al. [5] studied the effect of achieving flameless oxidation
regime by starting with premixed and non-premixed flames. The results indicates that with
premixed configuration the burner got a lower overall emission when comparing with the nonpremixed case. Szegö et al. [6] reported measurements of temperature and flue-gas
composition from a MILD laboratory scale furnace. They found that air preheating is no
1 necessary to achieve MILD combustion, even with 40% of useful heat being extracted through a
cooling loop. Sánchez, et al. [7] studied the effect of the oxygen enriched air (35% O2)
compared to normal air (21% O2) in the flameless oxidation regime. They found that NOx were
below 5 ppm and the global efficiency increased almost 5 % for an oxygen enriched level of
30%. It was possible to reach flameless combustion due the fact that the oxygen concentration
in the reaction zone was always below 8%. In the present work was made a study of the effect
of the water vapour and oxygen concentrations in the oxidant at the emissions. Previous study
in the present chamber was made by Melo et al. [10-12].
2. Experimental Apparatus and Procedure
Figure 1 shows a perspective view of the combustor used in this study, whereas Figure 2
displays its cross section and major dimensions. The fuel injection system is formed by 15 1 mm
circular holes with a spacing of 2 mm. The fuel (natural gas) was injected through these holes to
the combustion chamber at an angle of 45º with respect to the r axis, as schematically depicted
in Figure 2. As shown in Figs. 1 and 2, the oxidant is admitted into the combustion chamber
through two staggered series of 14 4 mm circular holes (84 in a complete 360º combustor).
These oxidant inlets make angle of 120º and 210º with respect to the r axis in the right and left
section, respectively. The secondary air is admitted into the combustion chamber through a
series of 19 1 mm circular holes with a spacing of 2 mm, the secondary air inlet makes angle of
20º with respect to z axis. In this study, the air flow rates supplied through each of the staggered
series of 14 holes were identical.
In order to create the desired oxidant composition it was used a equipment called CEM
(Controlled Evaporation and Mixing) to introduce water vapor in the system. The water is
evaporated in a nitrogen flow creating a wet nitrogen and then air is added to it in order to
create the right proportion between the following species, O2, N2 and H2O. The CEM system is
constituted of two flowmeters and an evaporator equipment. It is possible to introduce at most
1200 g/h of water in the nitrogen flow. The CEM is controlled using an adequate software.
The experimental arrangement used for measurement of the (mean) gas species concentration
at the combustor exhaust. The species considered in the measurements of the flue-gas data,
namely O2,CO,CO2, hydrocarbons (HC) and NOx were collected using a water-cooled stainless
steel probe installed in the exhaust plenum at approximately 1 m downstream from the
combustion chamber exit, in a location where the gas composition was nearly uniform. The wet
sample was drawn through the probe and part of the sampling system by an oil-free diaphragm
pump. A condenser removed the main particulate burden and condensate. A filter and a drier
removed any residual moisture and particles, so that a constant supply of clean dry combustion
gases was delivered to each measurement instrument through a manifold, to obtain species
concentrations on a dry basis. The analytical instrumentation included a magnetic pressure
2 analyser for O2 measurements, non-dispersive infrared gas analysers for CO2 and CO
measurements, a flame ionization detector for HC measurements, and a chemiluminescent
analyser for the quantification of NOx. The analogue outputs of the analysers were transmitted
via analogue/digital (A/D) boards to a computer where the signals were processed and the
mean values calculated. Zero and span calibrations with standard mixtures were performed
before and after each measurement session.
The maximum drift in the calibration was within +- 2% of the full scale. At the combustor exit,
where the gas composition was nearly uniform, probe effects were negligible and errors arose
mainly from quenching of chemical reactions and sample handling. Samples were quenched
near the probe tip to about 150ºC.
3. Working Principle of the Present Combustor
The working principle of the present combustor is based on the establishment of a large
recirculation zone within the combustion chamber, where part of the inlet air mixes with the
combustion products. The Figure 3.a illustrates conceptually the development of the
recirculation zone inside the combustor. The Figure 3.b represents schematically the
recirculation zone. The oxidant enters at station 1 and is split into two streams with identical flow
rates. One portion is entrained and its oxygen concentration is diluted by the recirculated
combustion products and directed toward point 2. At that point fuel is injected and mixed. The
combustible mixture ignites at station 3 after a certain ignition delay time. Combustion occurs
between points 3 and 4, and thereafter the combustion products are split, partially recirculating
with fresh air and partially exiting the combustor while diluting with fresh air (point 5). The two
streams mix and exit the combustor at point 6, at the required combustor’s exit temperature,
typically determined by the performance of the turbine located immediately downstream [8].
4. Results and Discussion
The goal of the experiment was the understanding of the effects of water addition and oxygen
dilution in the flameless oxidation regime. The tests were made varying the following
parameters: excess air from 1.2 to 1.8, oxygen percentage from 21% to 14% and water
percentage from 0 to 8%. In all tests, the fuel power of 4 kW and the secondary air flow of 40
l/min were kept constant.
Figure 4 shows the evolution of CO as a function of excess air coefficient to different conditions
of water and oxygen concentration in the oxidant. It is possible to see that CO evolution as a
function of λ, apart the conditions of oxygen and water concentrations, appear to have in any
case the same shape, higher values of CO emissions when λ is close stoichiometry and for λ
higher than 1.4 there is no relation between λ and CO emissions, in those cases the baseline is
3 near 20 dry volume ppm@15% O2. The experiment was made in a flameless oxidation
combustor which was design for higher recirculation ratios, therefore, when it is operating with
lowers values of λ, for example 1.2, it is possible to obtain in the recirculation zone very oxygen
diluted regions which inhibit the CO oxidation. Similar results was obtained by other authors
[5,13-16]. For a given water concentration in the inlet composition it was possible to see that the
more dilute the concentration of oxygen in the oxidant the higher is the CO emissions for lower
values of λ. This behave was expected for the same reason explained above. For λ values
higher than 1.4 there is no difference between different values of oxygen concentrations which
means that in any case the environment inside the combustor has enough oxygen to oxidize
almost all CO.
Figure 4 it could be seen that the water vapour addition increases the CO emissions for lower λ
values and the lower the oxygen concentration the higher the CO emission. This effect could
not be explained by the variation of the temperature, calculations of the adiabatic temperatures
show that, for a given O2 concentration the addition of water from 0 to 8 % have, at most, a
reduction of 30 ºC. The adiabatic temperature is more sensitive to excess air and it possible to
see that the CO evolution for higher λ values is almost independent in the given range of λ,
leading to an idea that temperature variation in those conditions does not play an important role
in emissions. It could be noticed that water vapour addition is more prominent to conditions of
O2 absence and the same does not happen with the nitrogen which works as inert. From this it
could be concluded that the water vapour must be chemically active to explain the observed
behaviour [9].
In the study of [9] it can be seen that the water addition reduces the CO emissions due the
importance of the reaction (R1) which improves the quantity of hydroxyl radicals in the reaction
zone. The reaction (R4) is the more relevant reaction for the CO oxidation.
→2
(R1)
→
(R2)
→
(R3)
→
(R4)
The increase of CO emissions with the water addition could be explained by the reduction of the
reaction rate of (R4). This happens for two main reasons. The first one, is that in the flameless
oxidation combustor operating under excess air of 1.2 it was possible to notice the lack of
oxygen in the reaction zone, this could lead to a reduction of the reaction rate of (R3). Since
4 (R1) is a global reaction of the elementary reactions (R2) and (R3) it is possible to see that the
hydroxyl formation which was of second order of reaction came to be of first order of reaction
only due (R2). And the second one, since the flameless oxidation combustor works at higher
recirculation ratio it is expectable higher concentrations of CO2 in the reaction zone against the
diffusion flame case [1], and since the reaction (R3) does not play an important role in this
condition it could be found more quantities of hydrogen in the reaction zone. Due to this it could
be expected that in the equilibrium condition the presence of CO2 and H improves the
importance of the backward direction of the reaction (R4), which leads to a lower reaction rate
of the forward direction which is the CO oxidation. Of course, this is only a speculative idea,
further studies must be made in order to validate it.
Figure 4 shows the evolution of NOx as a function of excess air coefficient to different conditions
of water and oxygen concentrations. For a given water and oxygen concentration it is possible
to notice that the NOx emissions increase with λ equal 1.2 to 1.4 and then it stabilizes. This
trend is in concord with those obtained by other authors [5, 13, 16]. For a given water
concentration it is possible to see that the higher the oxygen concentration the higher is the NOx
emission. This result was expected due the oxygen dependence in the NOx formation. For a
given oxygen concentration it could be seen that the increasing in water concentration reduces
the NOx emissions. As explained above the temperature does not change considerably to
different water concentrations, the NOx reduction is due not by the temperature decrease but to
the fact that while increasing the water concentration the nitrogen concentration decreases in
the same proportion.
5. Conclusions
This work deals with the characterization of a low-NOx combustor for gas turbine when
operating with a large range of oxidant composition. In the present work was studied the effect
of the water vapor addition and the oxygen dilution in the oxidant. The present study shows that
the combustor can operate at different levels of oxygen concentration and water vapor without
breaking the flameless oxidation regime. Despite the CO increase with the water vapor
concentration and oxygen dilution for lower values of λ, the combustor still get very low values
of CO emissions. The emissions of NOx was always below 10 ppm@15% O2 and the
combustion efficiency near 100%. This results shows the promising future of the gas turbine
operating with flameless oxidation regime.
References
[1] J. Wunning and J. Wunning, "Flameless Oxidation to Reduce Thermal NO‐formation," Prog. Energy Combust. Sci , vol. 23, pp. 81‐94, 1997. 5 [2] M. Katsuki and T. Hasegawa, "The Science and technology of combustion in highly preheated air," Proc. Combust. Inst. , vol. 23, pp. 3135‐3146, 1998. [3] A. Veríssimo and A. C. M. Rocha, "Experimental Study on the Influence of the Thermal Input on the Reaction Zone under Flameless Oxidation Conditions," Fuel Processing Technology, vol. 44, pp. 423‐428, 2013. [4] M. Flamme, "New combustion system for gas turbine (NGT)," Appl. Therm, vol. 137, pp. 1551‐1559, 2004. [5] V. Arghode, A. Gupta and K. Bryden, "High Intensity Colorless Distributed Combustion for Ultra Low Emissions and Enhanced Performance," Applied Energy, pp. 822‐830, 2012. [6] G. Szegö, B. Dally and G. Nathan, "Scaling of NOx emissions from a laboratory‐scale mild combustion furnace," Combustion and Flame, vol. 154, pp. 281‐295, 2008. [7] M. Sánchez, F. Cadavid and A. Amell, "Experimental evaluation of a 20 kW oxygen enhanced self‐regenerative burner operated in flameless combustion mode," Applied Energy, no. 111, pp. 240‐246, 2013. [8] Y. Levy, V. Sherbaum and P. Arfi, "Basic Thermodynamics of FLOXCOM, the Low‐NOx Gas Turbine Adiabatic Combustor," Applied Thermal Engineering, vol. 24, pp. 1593‐1605, 2004. [9] J. Richard, J. Garo, J. Souil, J. Vantelon and V. Knorre, "Chemical and physical effects of water vapor addition on diffusion flames," Fire Safety, vol. 38, pp. 569‐587, 2003. [10] M. Melo, J. Sousa and M. Costa, "Flow and Combustion Characteristics of a Low‐NOx Combustor Model for Gas Turbines," Journal of Propulsion and Power, vol. 27, no. 6, pp. 1212‐1217, 2010. [11] M. Melo, J. Sousa, M. Costa and Y. Levy, "Experimental Investigation of a Novel Combustor Model for Gas Turbine," Journal of Propulsion and Power, vol. 25, pp. 609‐617, 2009. [12] M. Melo, Desenvolvimento e Optimização de uma Câmara de Combustão de para Turbina a Gás, Lisboa: Instituto Superior Técnico, 2008. [13] G. Szegö, B. Dally and G. Nathan, "Operation characteristics of a parallel jet MILD combustion burner system," Combustion and Flame, pp. 429‐438, 2009. [14] V. K. Arghode and A. K. Gupta, "Effect of flow field for colorless distributed (CDC) for gas turbine combustion," Applied Energy, vol. 87, pp. 1631‐1640, 2010. [15] V. K. Arghode and A. K. Gupta, "Development of high intensity CDC combustor for gas 6 turbine engines," Applied Energy, pp. 963‐973, 2011. [16] A. Veríssimo, A. Rocha and M. Costa, "Operational, Combustion, and Emission Characteristics of a Small‐Scale Combustor," Energy&Fuels, pp. 2469‐2480, 2011. ant
Oxid H O)
N , 2
(O 2, 2
Fuel
Air 2
r
st
Exhau
z
Figure 1 – Perspective view of the combustor.
194
120
Oxidant
o
ø4
210
o
Right inlet
R180
Left inlet
ø4
R35
45
Air 2
20
o
o
Fuel (+ air)
20
Exhaust
z
r
θ
R40
130
164
Figure 2 – Cross-sectional schematic and key dimensions of the
combustor.
7 Figure 3 – Schematic of the internal aerodynamics of the combustor: a) section of the annular
combustor and b) main flowpaths, sketched as stream channels.
8 12
CO vs lambda (%H2O = 0)
NOx (ppm @ 15% O2)
CO (ppm @ 15% O2)
120
18%O2
100
16%O2
80
14%O2
21%O2
60
40
20
1
1.2
1.4
1.6
1.8
2
18%O2
14%O2
1
CO vs lambda (%H2O = 4)
100
NOx (ppm @ 15% O2)
CO (ppm @ 15% O2)
80
60
40
20
1.2
1.4
1.6
16%O2
21%O2
1.8
2
1.8
2
1.8
2
NOx vs lambda (%H2O = 4)
10
8
6
4
2
0
1
1.2
1.4
1.6
1.8
2
CO vs lambda (%H2O = 6)
100
12
NOx (ppm @ 15% O2)
CO (ppm @ 15% O2)
4
12
0
80
60
40
20
1
1.2
1.4
1.6
NOx vs lambda (%H2O = 6)
10
8
6
4
2
0
0
1
1.2
1.4
1.6
1.8
2
CO vs lambda (%H2O = 8)
100
18%O2
80
16%O2
14%O2
60
12
NOx (ppm @ 15% O2)
CO (ppm @ 15% O2)
6
2
120
120
8
0
0
120
NOx vs lambda (%H2O = 0)
10
40
20
1
1.2
1.4
1.6
NOx vs lambda (%H2O = 8)
10
18%O2
8
16%O2
6
14%O2
4
2
0
0
1
1.2
1.4
λ
1.6
1.8
2
1
1.2
1.4
1.6
1.8
2
λ
Figure 4 – Emissions of CO and NOx as a function of λ for different water and oxygen
composition in the oxidant.
9