Industrial Combustion
Journal of the International Flame Research Foundation
Article Number 201503, October 2015
ISSN 2075-3071
CHARACTERISTICS OF NATURAL GAS COMBUSTION
IN A CIRCULATING FLUIDIZED BED
Dennis Y. Lu*, Edward J. Anthony,
Gerhard Löffler, Franz Winter
CETC-O/Natural Resources Canada
Vienna University of Technology
1 Haanel Dr., Ottawa
A-1060, Vienna
Canada K1A 1M1
Austria
* Corresponding author:
[email protected]
Abstract
Interest is growing in extending fluidized bed combustion (FBC) to fuels that are difficult to handle
and those that present difficulties because their combustion is associated with particularly
challenging air pollution problems. Such fuels include biomass (such as straw), plastic wastes, black
liquors and heavy liquid fuels. As all of these have very high volatiles contents, they tend to be
treated as easy to burn, and instead solid fuels and char combustion have received more attention in
the literature. Nonetheless, understanding the gas-phase chemistry of such fuels is helpful in
optimizing their combustion. This paper presents a study of natural gas combustion in a fluidized
bed, as a model system for investigating the gas-phase reactions involving C/H/N/O chemistry taking
place in the absence of char. The experimental work was conducted using a pilot-scale mini
circulating FBC (CFBC) unit of 0.1 m diameter and 5 m height. Combustion characteristics and
emissions were investigated by varying the operating conditions and in particular the combustion
temperature, fluidizing velocity and bed material. The results fit with the general current consensus
that FBC chemistry is associated with super-equilibrium free radical processes, similar to hightemperature flame systems. A CFBC model has been developed based on the general kinetic model
and a NO/N2O formation model. It uses the semi-theoretical approach with some measured
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parameters as inputs and appears to be capable of providing a reasonable description of the nitrogen
chemistry and the concentration profiles of NH3, HCN, NO, and N2O for the case of burning natural
gas.
Keywords
Natural gas, circulating fluidized bed, combustion, emissions
1. Introduction
The special features of fluidized bed combustion (FBC) have allowed very successful
commercial applications of the technology to environmentally challenging solid fuels such as highsulphur coal and petroleum coke. Recently, there has been interest in extending the application of
circulating FBC (CFBC) to a wide range of waste-derived fuels, including biomass fuels such as
straw, plastic wastes, black liquors and heavy liquid fuels, which either have difficult handling
properties or produce non-conventional air pollutant emissions problems [1-3]. Such fuels are all
very high in volatiles and are commonly considered readily combustible. In consequence, their
combustion in fluidized beds has received much less attention than that of coal and char. This paper
presents a study of natural gas combustion in a circulating fluidized bed as a model system for the
gas-phase reactions involving C/H/N/O chemistry, which will aid the study of the firing of biomass
fuels by CFBC.
In addition to the above, there are other reasons to examine the direct combustion of natural
gas in a FBC system. First, when burning gas fuels, the characteristics of a typical FBC, such as low
combustion temperatures and high solids recirculation, create a suitable environment for
simultaneous combustion and CO2 removal when burning gaseous fuels, as in the cases of chemical
looping [4] and CO2 looping combustion involving limestone [5]. Second, current flaring systems
have difficulties in dealing with gases that contain significant amounts of large droplets and/or high
sulphur contents. By contrast, FBC systems have no such problems and have for example been
successfully employed to burn atomized pitch [2-3, 6], as well as having been demonstrated (at least
in the bubbling bed form) as being capable of cleaning up gas streams contaminated by organic
vapours [7, 8]. Third, as further restrictions on gas phase emissions increase the need for better and
better performance, FBC technology can be used to achieve ultra-low emissions, e.g., CO and NOx
less than 10 ppm. Finally, as earlier workers have demonstrated the importance of superequilibrium
© International Flame Research Foundation, 2015
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radical concentrations in FBC, the natural gas system is an excellent choice to examine these
phenomena in the absence of char from solid fuels [9-11].
Early investigations into the FBC of gaseous fuels were undertaken in the 1970s and attention
was focused on hydrodynamic problems, such as whether the volatiles burn in the bubble phase or
in the particulate phase [12, 13]. Dennis et al. [14] examined the burning of propane/air mixtures in
a fluidized bed and explored general combustion phenomena, including the existence of a possible
critical temperature for complete combustion of gas mixtures in the bed. Bulewicz et al. [15] studied
100% natural gas combustion in a bubbling sand bed and confirmed the importance of combustion
temperature in terms of reducing emissions of hydrocarbons and CO, and also explored the effects
of bed particle size on the combustion process. They also explored the mechanisms of NO and NO2
formation. More recently, Baron et al. [16] studied CO and NOx emissions when burning methane
or liquid petroleum gas (LPG) in a quartz bubbling bed. Their results indicated that the temperature
for complete combustion of methane is higher than that for LPG and the highest of the fuels these
workers have examined to date [17].
Unfortunately, most of the work so far on the combustion of gases has been done with bubbling
beds where high solids contents or relatively low voidages are expected to inhibit combustion
dominated by superequilibrium radical processes. In such beds combustion occurs mainly in bubbles
inside the bed, or above it. This leaves open the question of what happens in a CFB, which can be
expected not to behave like a typical bubbling dense bed, given the absence of a well defined
bubbling bed in most of the riser and poor gas mixing in the combustor [18]. This paper attempts to
explore the distinguishing aspects of natural gas combustion in a CFB. The results from both
experimental trials and modelling are presented.
2. NO Formation
NO formation can be characterized by the origin of the nitrogen, which can come from either
the combustion air or the fuel. At high temperatures (> 1200°C) significant NO production occurs
through the Zeldovich mechanism by reactions of air nitrogen with either oxygen molecules or
radicals, such as O and OH [19, 20]. This can also be known as thermal NO.
NO formation can also occur through attack by hydrocarbon radicals, such as CH and CH2
(CHi), on air-derived N2, i.e. the so-called “prompt NO” [21-23],
CH + N2  HCN + N
(1)
CH2 + N2  HCN + NH
(2)
© International Flame Research Foundation, 2015
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followed by homogeneous oxidation of HCN, i.e. HCN is first converted to NCO and NH, which
are further oxidized to NO by O and OH radicals, or alternatively react with H atoms to produce N
atoms [24]. N atoms formed by this route and those produced by reaction (1) can produce NO via
the following reaction:
N + O2  NO + O
(3)
N + OH  NO + H
(4)
Prompt NO is usually ignored in coal combustion processes because of its relatively small
contribution to the total NO formation compared to thermal- and fuel-NO [20, 25]. However, it
should be noted that prompt NO might potentially result in significant NOx emissions from
combustion of natural gas due to the potential of high-level production of hydrocarbon radicals [20,
26]. It should also be remembered that this mechanism becomes significant under fuel-rich
conditions at temperatures above 1200°C [27]. However, at fuel-lean conditions NO can also be
attacked by CH radicals and reconverted to N2, i.e. the essence of NOx reburning strategies [27].
Wolfrum [28] and Malte and Pratt [29] also proposed extending the thermal-NO mechanism
for the N2O/NO reactions as follows:
N2 + O + M  N2O + M
(5)
N2O + O  NO + NO
(6)
N2O + O  N2 + O2
(7)
These reactions dominate at lower temperatures, whereas the Zeldovich-mechanism is more
significant at temperatures exceeding 1500°C [29-31]. Finally, the N2O/NO mechanism may be
significant under fuel-lean conditions [31]. Bozzelli and Dean [32] proposed a new route of NO
formation in hydrogen-air flames at low pressure and low temperature via NNH as an intermediate
for fuel-rich combustion:
N2 + H + M  NNH + M
(8)
NNH + O  NO + NH
(9)
which can produce NO both directly and indirectly, through subsequent reactions of NH. NNH is
the key intermediate in this route and can rapidly become balanced with H and N2 in flames. The
significance of these reactions has been confirmed by Harrington et al. [33] and Hayhurst and
Hutchinson [34] for fuel-rich flames cooler than 1800°C. Furthermore, these mechanisms were
recently proved by Baron et al. to be the only possible routes for NO formation when burning
hydrogen in a bubbling FBC (BFBC) [35].
© International Flame Research Foundation, 2015
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Combustion temperatures in FBC can be maintained at relatively low levels (typically 750900°C), and this has been found to be crucial for the capture of fuel-sulphur by direct addition of
calcium-based sorbents to the combustor. This combustion temperature window and fuel-lean
conditions are also beneficial in terms of control of NOx formations via the mechanisms described
above, with the possible exception of the N2O/NO route, which might serve as a significant source
for NO emissions in gas firing [16]. A caveat to this is that methane can be expected to require
higher combustion temperatures than other gaseous or vapour phase hydrocarbons, but this should
not significantly affect the argument provided above.
3. Modelling
In order to study the gas phase chemistry in the riser of a CFBC, a simple model for fluid
dynamics in the riser was combined with a detailed reaction mechanism for C/H/O/N chemistry.
These are described briefly below.
3.1 Reactor Model
In modelling the gas phase chemistry, plug flow of the gases was assumed. The assumption is
justified because in a CFBC the high gas velocities result in favourable turbulent radial gas dispersion
and minor axial dispersion. While there is some axial dispersion in the transport zone of a riser,
caused by the core-annulus structure of the flow, Sterneus et al. [36] consider it to be small and hence
in this work it will be ignored. In their work, Sterneus et al. also showed that horizontal gas mixing
appears to be limited, and suggested that this is likely to be part of the reason for elevated CO
emissions in CFBC. However, this should not be as significant for homogeneous NOx formation, as
these reactions are kinetically controlled. The riser was also assumed to be isothermal.
The axial distribution of the solids in the riser was obtained from the experimentally-observed
pressure drop with bed height according to:
p
 1      p  g
h
(Eq.10)
Equation 10 neglects friction and acceleration forces due to gravity. However, Hartge et al. [37]
compared -ray absorption measurements with the predictions of Equation 10 and found good
agreement.
© International Flame Research Foundation, 2015
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3.2 Reaction Mechanism
The homogeneous reaction mechanism was taken from Löffler [38], which has been shown to
describe NOx and N2O well under FBC conditions [39-41]. It is based on the mechanism of Bowman
et al. [42] with the H/N/O reactions replaced by the mechanism of Glarborg and coworkers [43, 44]
as suggested by Wargadalam et al. [41]. Moreover, the kinetics of the N2O destruction reactions
were adapted as recommended by Löffler et al. [39] and in addition, to account for the NOxsensitized CH4 oxidation, the changes proposed by Löffler et al. [40] were included.
The presence of particles in the fluidized bed leads to the surface recombination of free radicals
[45]. To account for this effect the approach presented by Löffler [38] and his coworkers [41] was
applied. Here, quenching of the radicals on the solid’s surface was calculated by the kinetic theory
of gases and the collision cross section of the particles.
rk   k  k f r ck S
k 
Vbed
8RT
M k
S
3 1
   1   
Vbed 2 d p
(Eq.11)
(Eq.12)
(Eq.13)
The recombination efficiency k gives the probability that a radical k, which comes in contact
with a surface, recombines. Kim and Boudart [46] give the recombination efficiency for H and O
atoms at 300-1250 K. Due to the lack of accurately measured values for the recombination
probability of OH and HO2 on quartz, an estimated value fr. k of 10-2 for OH and 10-3 for HO2 was
used [47]. The roughness factor fr for the quartz was estimated as 2.4 [47] and the particle density
was assumed to be constant.
4. Experimental
The CANMET pilot mini-CFB unit (Fig. 1) consists of a refractory-lined combustor, a hot
cyclone, an inclined L-valve return leg, an air feed system and a solid feed system. The combustor
consists of a 0.1 m internal diameter ceramic tube surrounded by insulating refractory material. The
internal length of the riser is 5 m. In addition to the refractory material, the two upper portions of the
combustor are externally insulated with Fiberfrax blanket insulation. Electric heaters (18 kW)
surround the bottom 0.95 m section (dense region) of the reactor, and provide supplementary heat
© International Flame Research Foundation, 2015
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during operation. The unit is designed to operate at temperatures of up to 1100°C and with superficial
gas velocities of up to 8 m/s.
Primary air enters the combustor through an air chamber at the bottom of the unit. The primary
air then passes through a stainless steel distributor plate, which supports the bed solids and evenly
distributes the fluidizing air by using capped nozzles with an open area of 6.7%. The solid feed inlet
is located approximately 1.05 m above the distributor plate, as is the secondary air inlet, the return
leg and the light-up burner port. Secondary air injected through one or more ports supplies additional
air to achieve complete combustion. The light-up burner is positioned at the same level as the
secondary air ports, and provides the heat necessary for heating up the bed.
To Stack
To On-line Analyzers
Train and FTIR
Bypass
Cyclone
Weighing
Feeder
Injector
Air
Solid Drain
Baghouse
Start-up
Burner
Pressure or
Temperature
Ports
Distributor
Wind Box
Fluidizing Air
Figure 1. Schematic of CANMET Pilot Mini-scale CFB Unit
Natural gas is introduced into the combustor with the primary air at the inlet of the windbox
where they mix before entering the dense fluidized bed. Limestone sorbent (when used) is fed into
the combustor via an automated “loss-in-weight” weigh feeder. The feed rate of the screw feeder is
limited to 12 kg/h. Bed material passes from the combustor, through a transition section, into the
© International Flame Research Foundation, 2015
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cyclone, and is then reintroduced into the main combustion zone by means of a return leg, situated
directly above the bottom section of the reactor.
Prior to the start of heating, the combustor is pre-loaded with silica sand or limestone of ~4.55.0 kg with a static bed height of ~0.35 m. The size of the particles ranged from 0.3 to 0.75 mm for
both sand and limestone as bed materials. During the warm-up period the fluidizing air is introduced
solely through the windbox. An internal natural gas burner and the external electric heaters heat the
dense bed region of the combustor. When the bed temperature reaches 700°C, the primary fuel
supply, in this case natural gas, is started and once stable combustion has been achieved, the lightup burner is shut off. Secondary air (when used) is introduced through both the fuel feed injection
and the return-leg ports. The preheating period typically lasts around 2 to 2.5 hours and the preloaded
limestone can be expected to completely calcined by the time test condition are appropriate to
introduce the natural gas. This is important as Pilawska and Kandefer [7] observed a significant
impact of the limestone and precalcined limestone on the NOx formation from burning gas fuels.
When the bed temperature reaches the desired level, the external electric heaters automatically shut
off. The heaters automatically restart if the bed temperature drops below the pre-set point.
The unit is equipped with a data acquisition system which records the system temperature,
pressure drop and gas composition. Temperatures in the dense bed region are measured at four
different points by K-type thermocouples: 120, 240, 360 and 480 mm from the distributor plate, and
their average value is used as the average bed temperature, Tbed. Thermocouples are also situated
along the riser, cyclone and return leg. The pressure drop in the riser is measured by a series of
pressure taps. Flue gases are continuously analyzed for NO, CO, CO2 and O2 concentrations. CO
and CO2 analyzers operate on the non-dispersive infrared principle. O2 is measured using a
paramagnetic analyzer, and the NO analyzer is based on the effect of chemiluminescence. A Fourier
transform infrared (FTIR) spectrometer, Bomem 104 with an MCT detector and gas cell of 2.0 L
volume with an optical path length of 10 m, is employed for the analysis of hydrocarbons and for
determining the concentrations of NO2 and N2O. The detection limit for the NO and CO analyzers,
as well as for the FTIR for NO2 and N2O, is in parts per million (ppm), while it is 0.01% for the CO2
and O2 analyzers.
© International Flame Research Foundation, 2015
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5. Results and Discussion
5.1 Experimental Results
The combustion characteristics of natural gas were investigated by changing the operating
parameters, including the bed temperature, the superficial fluidizing velocity, the distribution of
combustion air and the nature of the bed material (see Table 1). To estimate the extent of combustion
of the natural gas injected, flue gas temperatures along the height of the combustor, from the bottom
of the bed (127 mm above the air distributor) to the outlet of the cyclone, were also measured.
The riser temperature generally decreased with increasing fluidizing velocity, which reduces
the residence time within the bed region, allowing a lower proportion of the natural gas to combust
in this region. However, too low a fluidizing velocity also lowered the riser temperature. This was
due to insufficient combustion air, resulting in a change from oxidizing to reducing conditions in the
dense bed region. It was found that the bed temperature fell dramatically when the bed stoichiometry
switched over to reducing conditions. With the combustion rate decreasing, the bed temperature
could not be controlled without switching on the external electric heater surrounding the bed zone.
Unlike the situation with coal combustion, there were no visible particles hotter than other bed
particles in the dense bed.
Distribution of combustion air, i.e. the ratio between primary and secondary air, also
significantly affects bed temperature, but the primary air remaining was always maintained above 34 times umf, to ensure good fluidization. Without changing the total air, the bed temperature
decreased with increasing secondary air, indicating that more of the natural gas was burning in the
riser region while the bed region remained at lower temperatures. Also, lower local temperatures
around the secondary air injection ports were as expected when more secondary air was provided,
resulting in a slowdown of the combustion reactions in this region and even lower bed temperatures.
© International Flame Research Foundation, 2015
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Table 1. Operating Parameters and Flue Gas Analyses
Bed
material
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Sand
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Tbed
C
842
845
855
842
838
840
775
777
781
820
751
752
844
864
857
865
856
866
894
899
889
862
883
781
Vf
m/s
1.50
2.07
1.98
2.08
1.75
1.86
1.62
1.33
1.26
1.19
1.33
1.17
3.23
3.02
1.75
1.88
2.21
1.16
1.51
1.51
1.61
1.62
1.50
1.01
O2
%
8.09
7.26
7.44
7.95
8.26
7.51
7.74
8.04
8.41
8.94
9.30
9.21
9.85
7.26
10.94
8.80
7.68
10.66
8.02
8.03
7.82
8.48
8.91
9.99
CO
ppm
37.8
107.5
141.6
45.6
39.0
53.4
33.7
37.2
27.1
31.7
33.9
32.1
577.4
186.6
84.1
94.9
79.4
713.4
498.9
59.6
937.5
1165.1
618.4
816.9
NOx
ppm
37.1
27.7
15.7
44.8
21.9
38.8
64.5
49.2
17.2
17.2
78.2
46.4
8.2
13.8
7.7
10.9
16.0
0.0
6.0
20.1
6.2
6.2
0.0
6.1
NO2
ppm
1.9
2.2
7.9
4.1
2.7
4.0
5.2
6.7
0.3
0.0
2.4
1.2
0.0
0.8
2.4
0.0
0.0
0.0
0.0
3.2
0.0
0.0
0.0
0.0
N2O
ppm
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND – not detected
Normally, no visible flame was seen through the view port located at the same level as the
return leg with a 30 degree decline, but a visible flame did appear if the superficial fluidizing velocity
was too high (>3.0 m/s) or the amount of primary air was too low. Then it was observed that the
flame colour was orange at high fluidizing velocities, but light blue at very low primary airflows.
The former effect was probably due to the continuum radiation from hot bed solids, while the latter
phenomenon was likely related to the CH and C2 radicals [15, 16], which may significantly increase
under reducing conditions (low gas velocity). Such effects have been described before for bubbling
beds burning natural gas [48].
This work demonstrated that a CFB could be successfully used to burn natural gas, although
the processes seemed to be considerably more sensitive to changes in operating conditions than for
a solid fuel, presumably due to the relatively poor gas mixing, which is well known in CFBC [49],
and the short residence times for such fuels. For example, emissions of unburned hydrocarbons and
carbon monoxide increased much more rapidly with decreasing oxygen concentration compared to
burning coal at typical FBC conditions. Effectively, complete combustion can be achieved at O2
© International Flame Research Foundation, 2015
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levels higher than 7%, compared to 3% typically for coal combustion in a FBC. Most past studies
have focused on bubbling fluidized beds where combustion occurs inside bubbles because the solids
inhibit normal combustion. If the oxygen and temperature are sufficiently high, the reactions occur
in the bubbles close to the distributor [50]. The observations in the CFB indicated that the lower
region could be regarded as equivalent to a gas burning nozzle, and the stream of premixed natural
gas and combustion air could be effectively “burned out” if the oxygen content was sufficient in the
premixed stream, and the temperature was appropriate.
In contrast to the combustion of solid fuels, where most of the combustion inefficiency is due
to the loss of unburned char in product solids, combustion inefficiency when burning natural gas in
a fluidized bed is primarily due to stack emissions of hydrocarbons and carbon monoxide. Unburned
hydrocarbons and CO are products of incomplete gas-phase combustion due to insufficient
temperature or mixing, or the inappropriate concentration of reactants. In our previous study on cofiring natural gas with solid fuels in FBC, the gas combustion inefficiency was mainly attributed to
CO rather than hydrocarbon emissions, except when injecting natural gas at the top of the riser, for
which the very limited residence time was believed to be the cause of elevated hydrocarbon
emissions [18].
Complete combustion of natural gas in circulating fluidized beds may not always be achieved
even when both oxygen and temperature should be sufficient for burnout. For example, switching
too much primary air to secondary air (>40% of the total air) can cause incomplete combustion since
there is typically poor gas mixing in the riser region compared to the dense bed. Investigations with
the mini pilot-scale apparatus also demonstrated that the combustion temperature for burning natural
gas should be higher than 780°C. When the temperature is lower than 725°C, which is below the
ignition temperature of methane (760°), the oxidation rate of fuel gas is too low to maintain the
combustion at all.
5.1.1 CO Emissions
The most critical factors with respect to emissions of unburned components are the bed
temperature and the combustion air distribution, particularly the amount of secondary air. Emissions
of hydrocarbons (shown in FTIR spectra) and carbon monoxide can dramatically increase with a
minor change in one of these parameters. Lower bed combustion temperatures (<780°C) and higher
© International Flame Research Foundation, 2015
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secondary air ratios (>50%) will typically increase hydrocarbon emissions. S high fluidizing velocity
in the dense bed may cause bypassing of unburned natural gas to the stack.
600
Sand bed
CO emissions, ppm
500
400
300
200
100
0
0
0.5
1
1.5
2
2.5
3
3.5
Superficial fluidizing velocity, m/s
Figure 2. CO Emissions vs. Superficial Fluidizing Velocity: Tbed at 751-865°C, staged air at 045% and excess air 10-80%
Figure 2 shows the CO emissions vs. the superficial fluidizing velocity. CO emissions from
burning natural gas in a sand bed were usually between 30-80 ppm, significantly lower than seen for
combustion of solid fuels in the same unit. The operating parameters were varied to achieve even
lower CO emissions (a few ppm); however, this goal was difficult to achieve. Increased CO
emissions were observed when the combustor was run at high fluidizing velocities and high
secondary air ratios.
Clear increases in CO emissions also occurred when the bed material was switched from quartz
sand to limestone. This was interesting given that the opposite behaviour was noted in our previous
coal studies and has also been reported by others for solid fuel combustion, where adding limestone
both ensures SO2 capture and promotes CO oxidation [18, 51]. It should also be noted that large
increases in CO emissions have been reported for a bubbling bed firing natural gas during the
calcination process [27]. In this case the limestone bed was fully calcined prior to the introduction
of natural gas. However, during the lime particle recirculation the lower temperature profiles and
high concentration of CO2 in the upper part of the reactor and cyclone can result in some
recarbonation of precalcined limestone, which during recycle back to the bed must again re-calcine.
This would help to explain the current results, and by contrast such a phenomena would not be
© International Flame Research Foundation, 2015
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expected to occur in a small bubbling bed when using limestone as a bed material, i.e. once calcined,
one would expect the bulk of the limestone particles to remain in this state and therefore not be able
to contribute to enhanced CO production [15, 27].
5.1.2 NOx Production
Figures 3 and 4 show the NOx emissions data versus combustion temperature and CO
emissions, respectively. Interestingly, NOx emissions varied widely from almost zero to 80 ppm.
These higher values are of particular interest since, because if there were no “hot spots”, these levels
must be associated with either prompt-NO, which is normally relatively low (at the ppm level), or
with the N2O formation routes described earlier. Considering that N2O is below detection limits in
this study (<1 ppm), both explanations seem unlikely. Instead it seems more likely that hot spots
may have occurred in this system, for instance near the secondary air ports where rapid combustion
may have occurred, resulting in the production of thermal-NO. The low end of the emissions range
was usually associated with a higher production of CO, particularly when limestone was used as the
bed material, and one could speculate that NOx is mainly controlled by the concentration of CO,
which is a strong reagent for reducing NO in the presence of char. At this moment we do not have a
good explanation for how CO could be associated with reduction of NOx in the absence of char
carbon. Bulewicz et al. [9] observed a similar tendency between CO and NO from burning gas fuels
in a bubbling bed, but they noted an increase of NO2 in the presence of CO at lower combustion
temperatures rather than a direct reduction via the reaction of NO and CO.
Dennis et al. [14] observed NO emissions from a few ppm to below detection when limestone
was used as the bed material in a bubbling fluidized bed. Pilawska et al. and Bulewicz et al. [7-9]
also noted a decrease of NOx emissions when limestone was introduced into the bed. In contrast, for
solid fuel burning in a fluidized bed, CaO is generally observed to promote NO formation, where the
pathways of NOx formation are dominated by relatively complicated fuel-N chemistry.
Another difference when looking at NOx emissions from coal combustion in a FBC in
comparison to our experiments was that NO2 concentrations from burning natural gas made a much
higher contribution to the total NOx (see Table 1). Typically it comprised 10%, and it was as high as
50% of NOx in some cases, compared to a few percent when burning coal. It has been suggested by
Baron et al. [16] that, when natural gas is burned at low temperatures in a FBC, combustion processes
© International Flame Research Foundation, 2015
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are dominated by HO2 radicals. While HO2 is less reactive than OH, it is very efficient in the
oxidation of NO to NO2 via reaction of HO2 + NO  NO2 + OH [52].
50
Sand bed
NOx emissions, ppm
40
Limestone
30
20
10
0
700
750
800
850
900
950
Bed temperature, °C
Figure 3. NOx Emissions vs. Bed Temperature: Uf at 1.1-3.1 m/s, staged air at 0-45% and excess
air 10-80%
50
NOx emissions, ppm
Sand bed
40
Limestone
30
20
10
0
0
200
400
600
800
CO emissions, ppm
Figure 4. NOx Emissions vs. CO Emissions: Uf at 1.1-3.1 m/s, Tbed at 751-899°C, staged air
at 0-45% and excess air 10-80%
© International Flame Research Foundation, 2015
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5.1.3 N2O Formation and Reduction
Nitrous oxide (N2O) might be produced from direct oxidation of air-derived N2 in combustion
gas, as in reaction 14:
N2 + O + M  N2O + M
(14)
Also, any N2O produced ought to have a better chance of surviving in the flue gas since there
is a limited heterogeneous reduction of N2O over nominally inert bed material and the bed
temperature is insufficient to promote significant thermal decomposition of N2O [15]. However, in
this study, N2O, as analyzed in situ by a FTIR spectrometer, was found to be low and in most runs,
below the detection limit regardless of the variations in conditions. N2O was not detected even when
the combustion temperature was lower than 750°C, which suggests that the kinetics for N2O
formation are simply too slow to be important in this system, despite being thermodynamically
favoured.
5.2 Modelling Results
In Figures 5 and 6 the concentrations of some species (calculated from our model) are shown
versus height in the riser of the CFB burning CH4. The bed temperature is assumed to be 850°C. At
1.15 m above the distributor the secondary air is added. Below this point the air to fuel ratio was set
to 0.8. It can be clearly seen that CH4 is partly converted to H2, CO and CO2. Below the point of
secondary air addition the fluidizing gas contains 6.5 vol-% CO and 2.5 vol-% H2. These are oxidized
rapidly after secondary air addition. Thus, approximately 25.5 % of the energy content of the gas,
i.e. 1.9 kW, is released immediately within 5 mm above the distributor. In reality, combustion cannot
be as rapid as might be expected based on such calculations as it is limited by mixing of the primary
gas stream from the bed region and the secondary air. Nevertheless, the assumption of rapid
combustion is justified and the riser can be regarded as isothermal, especially as solid concentrations
become relatively small above the dense bed.
Under the conditions assumed above, i.e. burning CH4 at 850C in a CFB, almost no NOx
formation is predicted (Fig. 6). This is in accordance with the expectation that temperatures are too
low for thermal or prompt NOx. However, as stated above it is likely that the temperature increases
significantly above the bed temperature of 850°C at the point of secondary air addition. Thus, it is
likely that the measured NOx may be formed in this region by the Zeldovich mechanism [16, 17].
This may also explain the findings that NO formation increases with increasing extent of air staging.
© International Flame Research Foundation, 2015
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O2
CH4
CO
H2
CO2
20
18
Concentrations [ppm]
16
14
12
10
8
6
4
2
0
0
1
2
3
4
5
Height [m]
Figure 5. (Calculated) Concentrations of the Main Species vs. Height during CH4 Combustion.
Tb = 850°C, prim = 0.8, sec = 1.3, U = 1.5 m/s.
O
H
OH
NO
N2O
160
0,50
Concentrations [ppm]
0,40
120
0,35
100
0,30
80
0,25
0,20
60
0,15
40
0,10
20
NO, N2O Concentrations [ppm]
0,45
140
0,05
0
0,00
0
1
2
3
4
5
Height [m]
Figure 6. (Calculated) Concentrations of Radicals and Nitrogen-containing Species vs.
Height during CH4 Combustion. Tb = 850°C, prim = 0.8, sec = 1.3, U = 1.5 m/s.
© International Flame Research Foundation, 2015
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A lower air-to-fuel ratio in the primary zone causes a higher fraction of the incoming gas to be
oxidized with secondary air addition. Figure 7 shows the (calculated) composition of the gas below
the secondary air addition point for different primary air-to-fuel ratios.
Model calculations also indicate that the measured CO emissions are affected by mixing
limitations between the fluidizing gas and the secondary air, which is reported, for instance, by
Löffler et al. [36]. At lower primary air-to-fuel ratios the CO concentration below the secondary air
addition point increases with an increasing primary air-to-fuel ratio. This results in higher CO
emissions with increasing NOx formation, where normally CO emissions decrease with increasing
NOx emissions in the presence of char. The assumption of hot spots as the reason for NOx formation
might be one of the factors for the observed CO and NOx emissions, along with a possible limestone
effect [8].
10
9
Concentration [v-%]
8
CO
7
6
5
H2
4
3
CH4
2
1
0
0,5
0,6
0,7
0,8
0,9
1
Pirmary Air-to-fuel Ratio [-]
Figure 7. (Calculated) Concentrations of CO, H2, and CH4 vs. Primary Air-to-Fuel
Ratios. Tb = 850°C, prim = 0.8, sec = 1.3, U = 1.5 m/s.
6. Conclusions
Premixed natural gas can be effectively combusted in a circulating fluidized bed, and bed solids
promote mixing for complete combustion. This work provides a simple model for the gas-phase
reactions involving C/H/N/O chemistry, and should be helpful in understanding the combustion
characteristics of volatiles when burning other liquid/solid fuels in a CFB. It should be noted that
© International Flame Research Foundation, 2015
17
complete combustion of natural gas in a fluidized bed is not always achieved and its combustion is
considerably more sensitive to changes in operating conditions than is the burning of solid fuels.
NOx emissions from the combustion of natural gas in the CANMET CFB were occasionally
high, and certainly much higher than expected from prompt NO processes seen when burning solid
fuels. These higher levels may be associated with local hot spots in the bed. NO reduction with CO
and CH/CH2 in the post-combustion zone is a dominant factor in controlling NOx emissions. This
mechanism for NO reduction is more pronounced in the presence of limestone.
A model has been successfully employed to describe the chemistry of burning methane under
circulating FBC conditions. The model predicts that the methane is converted to CO2, CO and H2 in
the dense bed region and further rapid oxidation is expected when the fuel gas stream meets the
secondary air at the bed’s surface. However, relatively little NO and N2O can be formed via the
suggested NOx production routes.
The proportion of NO2 to NO from the burning of natural gas is much larger than seen with
coal combustion. Measured N2O emissions are below the detectable level, indicating that although
N2O is thermodynamically favoured, its production is kinetically limited in this system.
Acknowledgments
Funding by Natural Resources Canada through the Program on Energy R&D (PERD) is
gratefully acknowledged. The comments from and fruitful discussions with Prof. E. M. Bulewicz
from Cracow University of Technology, Poland, are also highly appreciated. The authors also wish
to acknowledge the contributions of G. Lett and V. Ko during the experimental work.
Nomenclature
ck
= concentration of the species k, mol/m3
dp
= diameter of particle, m
fr
= roughness factor,
g
= gravity, g/m2.s
h, h
= height, bed height, m
k
= radical species
Mk
= mass of species k, g
p, p
= pressure, pressure drop, kPa
rk
= recombination rate of the species k, mol/m3s
© International Flame Research Foundation, 2015
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R
= gas constant, 8.314 × 10-3, kJ/mol.K
S
= surface area of the reactor, m2
T
= temperature, K
vk
= velocity of the species k, m/s
Vbed
= volume of bed, m3

= voidage of bed, -
k
= recombination efficiency for species k, -
p
= density of particle, kg/m3
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