Download reduction of nitric oxide from domestic heaters

REDUCTION OF NITRIC OXIDE FROM DOMESTIC HEATERS
E. Baraldi, G. Berthold*
University of Modena, via G. Campi, 213/B 41100 Modena Italy
*Worgas Bruciatori, Via Coppi 17, 41043 Formigine, Italy
ABSTRACT
This paper reports combustion measurements on a laboratory prototype of an unvented
space heater. In particular, the influence of the air / fuel ratio, the exhaust gas temperature, the
gas type and gas sampling position on the formation of NO and NO2 is studied. The Butterfly
wing shaped flame technology having an extended bladed combustion front, characterised by
low average temperatures is applied to the unvented heater technology. Emission
measurements carried out on a prototype heater show that the NO2 / NOx ratios are up to 90%.
Measurements according the ANSI Z21.11.2 1996 standard (American National Gas Institute,
1996) resulted in NO2 =6.6 ppm for natural gas and in 9.8 ppm in the case of propane gas.
Keywords: unvented heaters, hyperstoichiometric flame, reduction NO thermal, NO2
reduction
INTRODUCTION
Indoor air pollution is receiving substantial attention as a public health concern
because of the high percentage of time that people spend indoors. In many cases, the
concentrations of pollutants in indoor air are higher than those found outdoors due to the
plethora of sources associated with human activities and building operations. The anthropical
sources of nitrogen oxides are represented by all combustion reactions for fossil fuels. Two of
these principle oxides are generated in a gas flame during high temperature combustion and
are usually referred to as NOx, with x = 1 or 2. They are mostly formed as nitrous oxide (x= 1,
NO  95 %) and the rest nitric oxide NO2. Nitric oxide is an important air pollutant due to its
potential effect on human health and its role in atmospheric chemical processes leading to
ozone formation and the production of atmospheric acids. Undesirable effects are never
described when normal NO concentrations are found in the environment, so from a health
point of view, the most important compound is NO2, because it is about four times more toxic
than NO. Various studies (Beard, 1982; Gilli et al., 1988; Saltzman, 1990; Admur, 1991
Uccelli e Mauro, 1992 and 1997) suggest that exposure to NO2 could lead to adverse health
effects in humans impairing respiratory defence mechanisms and increases susceptibility to
infection.
Therefore studies have been carried out to establish a correlation between toxic effects
and the levels of the pollutants which have induced them. On the basis of these considerations
the World Health Organisation (1987) has elaborated the air quality guidelines to provide an
information base to defend public health from harmful effects of pollution, or to reduce those
pollutants which are probably dangerous for health and the well being of human beings.
For a number of years several countries, like for instance the USA, Japan and
Australia have been concerned about the adverse health effects of low level NO2 exposure to
human being (CPSC, 1983; Porter and Rogers, 1988; Babich, 1990) caused by direct contact
with combustion gases which are a significant source of indoor air pollutants such as nitric
oxide, including emission from home heaters, unvented gas space heaters and kerosene
heaters; gas stoves and gas ranges used in catering appliances (Stolwijk et al., 1983; Leaderer
et al., 1986; Young et al., 1987; Koontz and Nadga, 1988; Spicer and Billick, 1995). To
prevent any negative health effects caused by NO2 it is recommended not to exceed levels
over 0.3 ppm per 8 hours, which correspond to an NO2 emission factor of 20 ppm air free in
the exhaust gas (ANSI Z21.11.2, 1996).
The NO formed in natural gas and propane flame comes from two sources (Reuther,
1995) «prompt» NO, which originates «promptly» in the combustion processes as a result of
the reaction of hydrocarbon radicals with atmospheric nitrogen, and «thermal» NO formed by
the high temperature reaction of nitrogen and oxygen. Also it is generally accepted that
prompt is weakly dependent upon temperature and dominates at primary aeration levels
<100% while thermal NO has a strong positive temperature dependence (the hotter the flame,
the more thermal NO) dominates at primary aeration levels > 100 %. However, the rate of the
NO formation depends on the flame temperature, oxygen concentration and the residence time
of the mixture in the combustion zone.
Whereas NO is formed early in a flame it is well known that under certain conditions a
high proportion of NO is converted to NO2 in exhaust gases from combustion systems such as
gas turbines (Johnson and Smith, 1978) and unvented space heaters (Bromly et al., 1984;
Traynor et al., 1985).
The amount of NO2 found in combustion products depends on the complex
interactions of the production and destruction mechanisms of all of the species involved (NO,
NO2, HO2, O2, O, etc.) as well as on the flue temperature and residence time. Extensive
studies of NO2 formation in combustion systems (Cernansky and Sawyer, 1975; Johnson et
al., 1979, Hargreaves et al., 1981; Sano, 1985; Hori et al., 1992; Amano and Hase, 1994;
Reuter and Billik, 1995) and in gas-sampling probes (Allen, 1975; Cernasky, 1977; Hori,
1980) regarding the temperature history, like cooling rate, residence time and high exhaust
gas temperature have suggested that i) NO2 is formed during the cooling of hot exhaust gases
and it increases with increasing cooling rate, ii) the conversion of NO to NO2 is promoted by
thermal quenching and by trace hydrocarbons (HCs), iii) NO is converted to NO2 mainly
through the oxidation reaction NO + HO2  NO2 + OH that occurs in the cooler regions
downstream of the flame, denominated post-flame zone, where the temperature is below
1000°C.
This paper emphasises how the NO2 formation in unvented heaters is influenced by
means of the air / fuel ratio, sampling position, temperature and gas type. The present
situation of emission is also shown for products that are currently available on the market.
The study is carried out by using a double slot Worgas burner whereas at the end of the article
an unvented heater is presented with very low NO and NO2 emissions.
EPERIMENTAL SET-UP
According to the American Standard (Z21.11.2, 1996) to measure the exhaust gas
products the heater is placed below a hood assembly as shown in Figure 1. The height of the
heater coincidences with the bottom line of the hood. As shown in the insert on the left of
Figure 1 the exhaust gases on their way downstream become highly diluted resulting in
typical oxygen concentration of roughly 20.2 % at the probe position P4. Such a high and
rapid dilution of the flue gases with cold ambient air is a typical characteristic for unvented
heaters and serves mainly to cool down hot flue gases to prevent hazards caused by they.
Assuming that in a typical heater the exhaust gas velocity is approximately 1 m/s an averaged
cooling rate of roughly 10 K/ms results.
2
probe
mixing chamber
hood
unvented heater
optional
heat exchanger
T (o C)
400
2
300
1
3
200
4
100
0
17
18
19
20
21
burner
O2 (%)
Figure 1: At the right schematically cross section of the experimental set-up is shown. The unvented heater is
equipped with a rectangular twin slot burner. To collect the exhaust gases a hood is installed above
the heater. Flue gas samples are taken at different measurement position indicated with P1 through
P4. The insert at the left shows the temperature at the different probe positions as a function of the
local oxygen content.
The burner used in the experiments is characterised by means of a rectangular twin
slot flame pattern. Such a slot pattern allows the creation of stable flames over a wide range of
the air/fuel ratio. The primary air p and gas flow rates are measured and controlled by means
of mass flow meters.
The combustion gas measurements have been carried out at different positions P1, P2,
P3 and P4, as indicated in Figure 1, using a chemiluminescent NO / NOx analyser (Rosemount
Analytical Inc. – Model 955), a non-dispersive infrared CO / CO2 analyser (Rosemount
Analytical Inc. – Model 880) and paramagnetic O2 analyser (SERVOMEX – Series 1100).
The temperatures were measured by Cr / Al type K thermocouples. Thus the temperatures and
the concentration of pollutants in the gas sample can be measured without substantially
disturbing the combustion. For the sake of comparing the results under different conditions all
the concentrations have been measured are conveniently been normalised to their air free
values. All the measurements of the combustion products are either carried out for natural gas
as well as for propane.
FLAMES SHAPES AND STABILITY AT DIFFERENT AIR / FUEL RATIOS
Figure 2 shows several flame shapes. Flame stability can be achieved depending on
the equivalence ratio and mixture flow rate, four different main flame configurations can be
3
generated: in fuel rich mixtures, depending on the flow rate, either yellow or blue flame
Bunsen type or M-shaped (double Bunsen) flames are present, as shown in Figure 2.
inverted flame
recirculation zone
Hyperstoichiometric flame
M-shaped flame
flame front
aerated combustion
inner cone
Classical bunsen
preheating zone
yellow
blue
Exstreme rich flame
Figure 2: Different flame shapes are shown as a function of the primary air p at constant input rate resulting
from a burner characterised by means of a rectangular twin slot pattern. Changing the primary air
concentration creates the different flames. At extreme rich conditions a yellow flame is formed,
while at primary air levels of around 80 % of the stoichiometric value two Bunsen type flames are
created. For lean fuel mixtures a V-shaped flame is formed.
In lean fuel conditions either V-shaped or M-shaped flames are formed. The various
flame typologies present great differences not only from the point of view of flame front
morphology, but also concerning the main combustion features. As it has been observed in
previous works (Coghe et al. 1995), under lean fuel conditions, the mechanism of flame
stabilisation, transition between the two different flame shapes, and pollutant emission
reduction are mainly connected to the fluid dynamic features of the flame, particularly to the
value of air flow rate and to the formation of vortex structures outside the V-flame front. This
wing-shaped flame is attached over the burner and is extremely stable, owing to fluid dynamic
effects in a wide range of operating conditions and gives rise to low and uniform temperature
all over its area.
In the following a systematic analysis of the combustion products and in particular the
formation of NO2 is carried out for the case of an unvented heater. The input rate was fixed in
all experiments to 4.5 kW; natural gas as well as propane gas was used respectively at injector
pressures of 10 mbar and 20 mbar, respectively.
NO2 FORMATION IN TYPICAL UNVENTED HEATERS
The emissions of NOx, NO and NO2, respectively, for a typical unvented heater
without the heat exchanger are shown in Figure 3 as a function of the primary air p. The flue
gas samples are taken in position P4 downstream from the mixing chamber.
4
NO2 , NO, NO x (ppm)
120
Methane
NO x
NO
NO2
100
80
60
40
20
0
0.0
0.2
0.4
0.6
p
0.8
1.0
1.2
1.4
1.6
Figure 3: The NOx, NO and NO2 emissions are shown as a function of the primary air p. The data are taken
for natural gas with an heater input rate of 4.5 kW at the position P4.
For levels of 0<p<0.5 the NOx and the NO decrease with increasing primary air. In
this range a yellow flame is formed and changes to a blue flame with increasing primary air.
At p = 0.5 a local minimum in the NOx and NO concentration is observed. A «soft» blue
Bunsen type flame, where the inner and outer cones are not distinguishable and the NO
formation is subjected mainly to the prompt mechanism, accompanies this minimum. Further
increasing of the primary air in the burner up to p = 0.7 results in increasing NOx and NO,
while at even higher values a strong decrease in the NOx and NO formation is observed. This
behaviour reflects the well-known correlation with the flame temperature as a function of the
primary air levels and the formation of NO is explained by means of the Zeldovich
mechanism.
The NO2 concentration, also illustrated in Figure 3, shows a less pronounced
dependency over the wide range of primary air. In the regime of the Bunsen type flames
(0<p<1.2) the NO2 is almost constant, while above, in the regime of the inverted flames, a
strong decrease down to 2.5 ppm with increasing primary air is observed. This behaviour is
accompanied by a dramatic increase in the NO2 / NOx ratio up to values of about 90 %. Note
that as a comparison in a typical boiler application the NO2 / NOx ratio is normally lower than
5 %.
EXHAUST GAS TEMPERATURE
To study the effect of the exhaust gas temperature on the NO2 / NOx ratio a heat
exchanger was constructed to fit inside the unvented heater as shown in Figure 1. By
changing the amount of the water flux in the heat exchanger the temperature measured above
the heat exchanger in position P1 could be tuned from T1 = 150 °C up to T1 = 480 °C. The O2
concentration directly above the heat exchanger was measured at 12.9 % and did not
significantly change with temperature.
By increasing the outlet flue gas temperature T1 a strong decrease in the NO2 / NOx
ratio is observed as shown in Figure 4. The NO2 / NOx ratio decreases from 14 % to 3 % with
increasing T1 from 150 °C to 450 °C in the flue gas outlet.
5
16
Propane
NO2 / NO x (%)
14
12
10
8
6
4
2
0
100
200
300
400
500
T1 (oC)
Figure 4: It is shown the NO2 / NOx ratio as a function of the exhaust gas temperature at position P1. The data
are taken for propane gas at p = 0.8 with a heater input rate of 4.5 kW.
As shown by Amano et al. (1994) during the cooling process from flame temperature
down to the flue gas outlet temperature the amount of NO transformed into NO2 is mainly
determined through the balance of the following reaction:
HO2 + NO  NO2 + OH
NO2 + H  NO + OH
(1)
(2)
Whereas both reactions are in balance until the temperature reaches nearly 1000 °C,
below this temperature an effective formation of NO2 is present, because is favoured the
reaction (1); while at higher temperatures a decomposition of the NO2 occurs because the
reaction (2) is favoured. However, the reaction chemistry in the case of combustion in an
unvented heater is not under equilibrium conditions, therefore the time evolution of the
combustion gases is of importance. Numerical simulation carried out by Amano et al. (1994)
has shown that the cooling rate becomes important in explaining the NO2 formed by oxidation
of NO. In the case of our heater, cooling rates are estimated as follows: the flame temperature
as the upper limit is around (1200  1800) °C; while the lower temperature limit between 150
°C and 470 °C is given at position P1 above the heat exchanger, as shown in Figure 4.
Therefore, the extreme temperature drop is between 1650 K to 730 K and a typical residence
time of the flue gases before passing the heat exchanger is approximately 0.1 s. This results in
cooling rates in the range of 7 to 16 K/ms and according theoretical prediction to a conversion
ratio of NO into NO2 of 5 % to 10 %, which is in excellent agreement with the experimental
data shown in Figure 4.
POST HEATER ZONE
In the following the NO2 / NOx ratio is studied in the zone between the exhaust gas
outlet, position P1, and the probe position P4 in the hood. For the following experiments
propane gas is used and the heater without the heat exchanger is operated at 4.5 kW with
6
p=0.8.
Figure 5 shows the total NOx (air free) subdivided in its two components NO and NO2,
at the position P1 to P4. Within the accuracy of the measurement the total NOx can be
considered constant at all different positions while the ratio NO2 / NOx increases downstream.
The increase most pronounced is in the area where the exhaust gases get in contact with the
surrounding cold ambient air, as shown in Figure 5 for the measurements at position 1 with
NO2 = 14 ppm and at position 2 with NO2 = 25 ppm. Further downstream the NO2 is still
increasing to 32 ppm at position P3 which is at the top of the heater, while above no further
changes in the ratio is observed.
140
NOx (ppm)
120
NO
Propane
NO2
100
80
79
72
67
67
14
25
32
31
1
2
3
4
60
40
20
0
Position
Figure 5: The NO and NO2 components of the total NOx are shown at the different probe positions P1 to P4.
The data are taken for propane gas at p = 0.8 with a heater input rate of 4.5 kW.
Due to the dilution of the exhaust gases with the surrounding cold air the flue gas
temperature is decreasing downstream as it is shown in the insert of Figure 1. The largest
gradients occur in the vicinity of the exhaust gas outlet of the heater at the position P1 to P3,
while between position P3 and P4 no significant change in the oxygen content is noticed and
therefore also no temperature change observed. However, the average temperature gradients
from position P1 to P2 is estimated approximately to 1 K/ms, which at maximum results in a
transformation of less than 1 %. Further, the distance between the positions P2 and the burner
surface is roughly 400 mm downstream the flue gases, which is believed to have a low
concentration of the radicals HO2, OH and H. Thus the change of NO2 concentration from 14
to 25 ppm in this area seems to be improbable by means of the model discussed above.
However, at the outlet of the exhaust gas higher cooling rates up to (10  100) K/ms might
locally occur, which might explain the observed conversion also at a decreased presence of
the radical concentration.
Since it is of importance to determine the effective NO2 indoor room concentration it
is important that the measured NO2 at position P4 can be considered as stable for at least the
duration of the average air exchange time of a closed living area. The air exchange time is
typically in the order of some hours. As shown in the data of Figure 5 the NO2 does not
7
increase between position P3 and P4 and therefore can be considered as stable for at least the
air exchange time.
V-SHAPED FLAME FOR UNVENTED HEATER
According to health concern it was mentioned in the introduction part, that NO2 should
not be higher than 20ppm normalised to air free. As shown through the paper one effective
way to reduce the NO2 emissions can be the use of a high primary air ratio above 1.2.
Therefore, the V-shaped flame realised on a twin slot burner has been chosen, whereas in our
case the primary air is supported only through natural draft caused by the gas jet. Such
burners are often called atmospheric burners and are typically used for unvented heaters.
In Figure 6 and Figure 3 it is shown, respectively, that the NO2 emissions are clearly
below the 20 ppm level for p above 1.2. Further, the low NO2 concentration is accompanied
by a NOx concentration below 20 ppm, resulting in high NO2 / NOx above 50 % up to 90 %.
Such high ratios at very low total NOx have been theoretically predicted by Amano et al.
(1994). However, even if the remaining percentage of NOx converts to NO2, still the NO2
level remains below the recommended concentration causing negative health effects.
NO 2 (ppm)
Methane
80
80
60
60
40
40
20
20
0
0.7
0.9
1.1
p
1.3
1.5
NO2 / NOx (%)
100
100
0
Figure 6: The NO2 concentration and the NO2 / NOx ratio at high primary air levels. A strong decrease of the
NO2 is observed for the V-shaped flame regime.
Measurements taken on the prototype-unvented heater at the American Gas
Association Research Laboratories (AGAR) have resulted in NO2 levels of 9.8 ppm for
propane and 6.6 ppm for methane.
CONCLUSIONS
A double slot burner applied to an unvented heater has been examined regarding the
NO and NO2 formation. It has been shown that the NO2 concentration can be clearly below a
20 ppm level for methane as well as for propane gas. However, the absolute values for the NO
and NO2 formations are higher in the case for propane, while the principal behaviour shows
the same dependency on the primary and secondary air levels.
8
For low primary air values the NO formation is strongly influenced by means of
prompt NO formation, while for higher primary air values the formation is well understood by
the Zeldovich mechanism. Within the accuracy of the measurements carried out the NO2 level
shows no dependency on the primary as well on the secondary air up to air / fuel ratios of 1.2,
while above a strong decrease down to 2.5 ppm is observed. This strong decrease in NO2 is
accompanied by a dramatic increase in the NO2 / NOx ratio.
The experimental data suggest, that the NO2 in the combustion chamber of an
unvented heater is formed via the oxidation of NO, which is mainly given by the HO2 radical,
while in the post heater zone the formation is not fully understood.
The NO2 and NO levels strongly depend on the sampling point of the flue gas, while
the total NOx remains constant for all different positions.
A V-shaped flame with hyperstoichiometric primary air concentration is applied to an
unvented heater to obtain low NO2 concentration. Measurements have shown that for propane
the NO2 results to be 9.9 ppm while for methane 6.6 ppm.
REFERENCES:
Admur, M.O. (1991) Air pollutants. Casarett & Doull's Toxicology, 4th Ed., Pergamon Press,
New York, pp. 854-871
Allen, J.D. (1975) Probe sampling of oxides of nitrogen. Combust. Flame, 24, pp. 133-136
Amato, T. and Hase, K. (1994) Cooling conditions of hot exhaust gas for low conversion of
NO to NO2. Journal of the Institute of Energy, 67, pp. 174-180
American National Standard Institute (1996) ANSI Z21.11.2 1996 Gas-Fired room heaters.
Unvented Room Heaters, Vol. 2, December
Babich, M.A. (1990) Exposure assessment of NO2 emissions from residential use of portable
kerosene heaters. U.S. Consumer Safety Commission, Washington
Beard, R.R. (1982) Inorganic compounds of oxygen, nitrogen, and carbon. Patty's Industrial
Hygiene and Toxicology, Vol. 2C, Clayton G.D. & Clayton F.E. (eds.), John Wiley &
Sons, New York, pp. 4053-4139
Bromly, J.H.; Barnes, F.J.; Johson, R.C.R. and Little, L.H. (1984) Nitrogen oxide emissions
from unflued space-heaters. J. Inst. Energy, 57, pp. 411-415
Cernansky, N.P. (1977) Sampling and measuring for NO and NO2 in combustion systems.
Prog. Astron. And Aeron., 53, pp.83-102
Cernansky, N.P. and Sawyer, R.F. (1975) NO and NO2 formation in a turbulent
hydrocarbon/air diffusion flame. 15th symp. (int.) on Combustion, The Combustion
Institute, Pittsburgh, pp. 1039-1050
Coghe, A.; Zizak, G.; Solero, G.; Sebastiani, E. and Beghi, M. (1995) Low-NOx natural gas
burner for domestic and semi-industrial appliances: investigation on an advanced purely
aerodynamic NOx reduction technology. 3th International Conference on Combustion
Technologies for a Clean Environment, Lisbon, Portugal, pp. 14.2
CPSC Consumer Product Safety Commission (1983) Kerosene heater report. U.S. Consumer
Product Safety Commission, Washington
Gilli, G.; Corrao, G. and Scursatone E. (1988) Inquinamento atmosferico e salute. Franco
Angeli, Milano
Hargreaves, K.J.A.; Harvey, R.; Roper, F.G. and Smith, D.B. (1981) Formation of NO 2 by
laminar flames. 8th symp. (int.) on Combustion, The Combustion Institute
9
Hori, M. (1980) Effects of probing conditions on NO2/NOx ratios. Combust. Sci. Tech., 23,
pp. 131-135
Hori, M.; Matsunaga, N.; Malte, P.C. and Marinov, N.M. (1992) The effect of low
concentration fuels on the conversion of nitric oxide to nitrogen dioxide. 24th symp.
(int.) on Combustion, The Combustion Institute, Pittsburgh, pp. 909-916
Johnson, G.M. and Smith, M.Y. (1978) Emission of nitrogen dioxide from a large gas turbine
power station. Combust. Sci. Tech., 19, pp. 67-70
Johnson, G.M.; Smith, M.Y. and Mulcahy, F.R. (1979) The presence of NO2 in premixed
flames. 17th symp. (int.) on Combustion, The Combustion Institute, Pittsburgh, pp. 647660
Koontz, M.D. and Nagda N.L. (1988) A topical report on a field monitoring study of homes
with unvented gas space heaters. Methodology and results Vol. 3, Gas Research
Institute, Chicago, IL
Leaderer, B.P.; Zagranski, R.T.; Berwick, M. and Stolwijk, J.A.J. (1986) Assessment of
exposure to indoor air contaminants from combustion sources: methodology and
applications. American Journal of Epidemiology, 124, pp. 275-289
Porter, W.K. and Rogers, P.L. (1988) Exposure assessment for nitrogen dioxide from
kerosene heaters. U.S. Consumer Safety Commission, Washington
Reuther, J.J. and Billick, I.H. (1995) Measurement, modelling, and mitigation of oxides-ofnitrogen emissions from gas-appliance flames. International gas Research Conference,
pp. 280-289
Saltzmman, L.E. (1990) Health effects associated with nitrogen dioxide exposure-an overall
discussion, 1990. U.S. Consumer Safety Commission, Washington
Sano, T. (1985) NO2 formation in the mixing region of hot burned gas with cool air. Combust.
Sci. Tech., 43, pp. 259-269
Spicer, C.W. and Billick, I.H. (1995) Persistence and fate of natural gas appliance emissions.
International Gas Research Conference,
Stolwijk, J.A.J.; Leaderer, B.P.; Zagranski, R. and Berwick M. (1983) Kerosene heater field
studies. Final report to the Consumer Product Safety Commission, John B. Pierce
Foundation Laboratory, Yale University School of Medicine
Traynor, G.W.; Girman, J.R.; Apte, M.G.; Dillworth, J.F. and White, P.D. (1985) Indoor air
pollutant due to emissions from unvented gas-fired space heaters. J. Pollution Control
Assn, 35, pp. 231-237
Uccelli, R. (1997) Fonti, livelli ed effetti dell’inquinamento atmosferico. Energia Ambiente
Innovazione, 1 / 97
Uccelli, R. and Mauro, F. (1992) Ossidi di azoto. Appunti sugli agenti nocivi, Serie Noxiae 1,
ENEA
World Health Organisation (1987) Air quality guidelines for Europe. WHO Regional
Publication, European Series n.23, Copenaghen
Young, G.S.; Menzies, K.T.; Yamac, R.F.; McCarthy, S.M. and Yocom, J.E. (1987)
Unvented gas space heater study. For the U.S. Consumer Product Safety Commission,
Washington
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