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The design of an integrated burner-boiler system using flue-gas recirculation
T H E D E S I G N O F AN I N T E G R A T E D FLUE-GAS
BURNER-BOILER RECIRCULATION
SYSTEM
USING
J. G. MEIER* AND B. L. VOLLERIN Battelle Research Center, Geneva, Switzerland
The main goal of the project was the design of a series of burngr-boiler systems for domestic hot water heating, providing great compactness, high global thermal efficiency, low pollution and easy manufacture. This series has been designed on the basis of modularity of the elements, the use of a high thermal load pressurized combustion chamber (above 2.3 MW/m 3) with strong swirl and with external recirculation of flue gas. This system enables operation with light fuel oil or natural gas at very low excess air (below 10%) and achieves soot free combustion with significant reduction in emissions of CO, thermal NO~, unburned hydrocarbons, and combustion noise as compared to conventional burner-boiler systems. The boiler has been designed with computer models, and flow patterns were visualized and examined in a water model and also by means of Schlieren methods. The burner matched to the boiler provides a highly swirling flow (swirl number ranging from 0.6 to 1.3) of an adjustable mixture of flue gas and fresh air (mass flow rate ratio between 0 and 70%). Care has been taken to insure the stability of this system which uses a feed-back loop as a recireulation circuit. The complete series has been developed, built and tested. Examination of test results shows how this advanced concept may simultaneously fulfill low pollution and energy saving requirements.
Introduction
sphere, one arrives at a stalemate where conventional heating systems cannot achieve both high efficiency and low pollution levels.l Some attempts have been made to determine new designs of burners achieving efficient and clean combustion. 2-~ Systematic i n v e s t ~ a t i o n s of the influence of aerodynamics on pollutant formation in c o m b u s t i o n have been carriedout: effect of swirl, staged combustion, atomization of fuel or injection of gas in gas burners. It is not enough to consider the burner b y itself and try to improve its design in order to improve a c o m b u s t i o n system efficiency. The boiler design must be considered at the same time, in view of the fact that a slight improvement of the boiler performance plays an important role in the overall system's efficiency. In order to simultaneously fulfill low pollution and high thermal efficiency of the system, it is therefore necessary to develop an integrated concept of a burner-boiler system. In this way, it was here possible to conceive a system u s i n g external flue gas recirculation, a high thermal load pressurized
During the colder seasons domestic heating systems contribute significantly to the pollution of the atmosphere. A m o n g the pollutants generated are carbon monoxide (CO), soot, u n b u r n e d hydrocarbons (UHC), oxides of nitrogen (NO~) a n d s u l p h u r oxides (SO~). Control of the final pollution level of a system is ultimately a matter of setting up the right pattern of oxygen, hot or cold 'combustion products, temperature, and allowing the necessary time to complete the chemical reactions. Thus, it appears that the geometry and the aerodynamics of a c o m b u s t i o n system are of prior importance in order to obtain an overall clean operation. A m i n i m i z a t i o n of the pollutants emission is generally not compatible with high energy efficiency. As energy must be saved, and one must simultaneously bear in m i n d the protection of the purity of the atmo* present address: Solar, Division of International Harvester, San Diego, California 63
64
POWER SYSTEMS
combustion c h a m b e r and a strong s w i r l i n g flow in this c o m b u s t i o n chamber.
by excess air a n d C F G R . The velocity field also affects the formation of NO~. 7 G e n e r a l l y a critical level of excess air leading to high NO ~ concentration exists in combustion products. This is due to an " o p t i m u m " in 0 2 concentration and temperature in the flame. The typical N O ~vs excess air curve for a b u r n e r is a matter of fuel used and design. Peak flame temperature as well as m e a n flame temperature are reduced by using sufficiently cooled combustion products, m i x i n g these products with fresh air, and i n t r o d u c i n g the mixture into the combustion chamber. Other sources of NO~, like NO~ due to fuel b o u n d nitrogen, seem to be i n d e p e n d e n t of the C F G R rate. 11 Moreover, as the fuels used in our case are poor in fuel-bound nitrogen, no attention has here been p a i d to this correlation.
Basic Features of the Burner-Boiler System
Recirculation of the Flue-Gas The use of cold flue gas recirculation (CFGR) in c o m b u s t i o n systems provides one of the basic means for i m p r o v i n g c o m b u s t i o n efficiency and for reducing the p o l l u t i o n of the atmosphere as c o m p a r e d to conventional processes, s-12 A schematic view of a system using C F G R is represented in Fig. 1. The C F G R is characterized by the recirculation ratio R = rhy/rhas, where rhf is the flue-gas mass flow rate a n d rhas is the mass flow rate of stoichiometric air required. The principal effect of C F G R is the reduction of the oxygen content of the gas mixture fed into the burner, typically to a r o u n d 16% by volume (dry basis). The dilution of the oxygen leads to a general reduction of the flame temperature, a c c o r d i n g to the quantity of flue gas recycled. Let us summarize the major effects of C F G R on the emissions of pollutants other than SO~ from the burner-boiler system.
Carbon Monoxide and Unburned Hydrocarbons The process of formation and destruction of CO and U H C in burners is roughly similar. The well k n o w n parameters affecting these species are the degree of mixing, oxygen availability, residence time and temperature. One could expect high C O and U H C emission levels from the present system regarding the dilution effect of the C F G R which leads to a reduced c o m b u s t i o n intensity. As p r e v i o u s l y reported b y other authors, g low CO and U H C levels are achieved through a proper m a t c h i n g of excess air a n d C F G R rate. Although the use of C F G R seems to have a potential disadvantage regarding CO and U H C emissions, its influence can be completely counterbalanced b y the proper aerodynamic design of the system.
Nitrogen Oxides Since thermal N O ~is formed by the c h e m i c a l reaction of oxygen and nitrogen at a h i g h temperature, the temperature and oxygen concentration fields in the combustion c h a m b e r are of major importance. By reducing the flame temperature, a significant reduction of the NO~ formation via a thermal mechanism m a y be obtained. 5,9 T e m p e r a t u r e may be controlled
~5 3
1. Fan 2. Swirl generato~ ~
combustion head
3.
Combustion chamber
4.
Flue- gas collectors/heat
exchangers
S. Stack 6. Mixing box
nSf
7. Fan diffuser outlet r~o : instantaneous fresh-air mass flow rate rhf : Instantaneous recirculated f l u e - g a s mass flow rate
rh
: instantaneous mixture mass flow rate
1
FIG. 1. Schematic view of the system using the cold flue gas recirculation.
AN INTEGRATED BURNER-BOILER SYSTEM
Soot The influence of C F G R on soot emission has been known for m a n y years, s Global sootfree combustion can be achieved at very low excess air level w h e n the C F G R rate is set above a critical limit. F u r t h e r increase in the recirculation rate results in the elimination of carbon particles p r o d u c t i o n , leading to a b l u e flame combustion. T h e characteristic curves of Zero Bacharach Index vs excess air and recirculation ratio on the one hand, and the b l u e flame limit vs the same parameters on the other, depends on the burner design, e.g. the combustion staging and aerodynamics. T h e curves obtained in testing the present system will be shown b e l o w a n d compared to those obtained on previous systems.
Highlg Swirling Flame The basic advantages of the use of swirling flows in combustion chambers have been w i d e l y described b y m a n y authors, mainly for utilization of such flows in combustors or in b i g industrial b u r n e r s ? ~ It was decided to use such a type of flow (swirl number ranging from 0.4 to 1.3) in the present system's combustion chamber, in order to a) use the stabilization properties of swirl which anchors the flame in a purely a e r o d y n a m i c a l way, 14,15 b)
65
promote mixing b y setting u p an intense internal recirculation zone of hot combustion products, 16 c) obtain a more v o l u m i n o u s flame spreading u n i f o r m l y into the c o m b u s t i o n chamber, d) compensate the reduction in combustion intensity due to the C F G R and to m i n i m i z e CO and U H C emissions. In c o m b i n i n g strong swirl with C F G R , the conflict regarding emissions of CO (or UHC) and NOx a7 can be solved in that the temperature abatement due to C F G R yields to a reduction of NO~ emission a n d the internal recirculation due to the swirl promotes the completion of combustion. The high intensity flames obtainable with strong swirllS.19 generate a noise level which is sometimes unacceptable. It will be seen b e l o w that this noise level, mainly a combustion roar, can be drastically reduced below conventional b u r n e r noise levels, taking advantage of the simultaneous low excess air level and C F G R operation.
Pressurized Combustion Chamber In order to feed the convective channels of the boiler efficiently and to arrive at a system safely operable in various external conditions such as stack geometry, draft and weather, pressurization of the c o m b u s t i o n chamber of
2
333
1
1. 2. 3. /4. 5. 6.
High swirl burner Conical front element Intermediate element Back element Nozzles Heat transfer channels
7. 8. 9. 10. 11.
Reinjection slots Water-coded internal annulus Radial hollow segment Cold water inlet Hot water outlet
FIG. 2. Cross-sectional views of the boiler. On the left, longitudinal cross-sectional view showing the combustion chamber shape; on the right, transversal cross-sectional view showing the water passages in the intermediate elements.
66
POWER SYSTEMS
about 400 N / m z is used. A schematic view of the combustion chamber is given in Fig. 2. The chamber is of a cylindrical shape and connected to the burner by a conical front element. The swirling flows which expand towards the chamber's wall, promote internal recirculation of products located near the axis of the system. As concluded by Drake and Hubbard, 19 the mixing processes associated with swirling flames may give optimum combustion conditions if the combustion chamber is well matched to the burner, especially regarding the flow pattern. High thermal load has been achieved by developing the complete system as one unit. The system is rated to more than 2.3 M W / m a. The combustion chamber is connected to the heat transfer channels by small cross section nozzles. The role of these nozzles regarding the heat transfer is explained below. First of all, these nozzles create concentrated pressure losses between the combustion chamber and the channels, efficiently damping pressure waves occurring during the system start up, and therefore make it possible to increase the specific power rating of the boiler. Spiral Heat Transfer Channels The negligible level of soot formation in the combustion gases has permitted the design of narrow heat transfer channels, which are spirally wound around the combustion chamber, thus rendering the system very compact. Details of the design are explained
in the subsequent sections and Fig. 3 gives a schematic view of this particular geometry. In addition to the compactness gained by the use of this geometry, a great advantage is obtained regarding the curvature effect on the heat transfer coefficient. Various works 2~ initiated by W. R. Dean in 1927, deal with the flow in curved channels. Curvature causes secondary movements within the channel cross-section as indicated in Fig. 3. The centrifugal forces responsible for these secondary movements greatly increase the heat transfer between the fluid and the walls of the channel. This effect is substantial if the Dean number De of the flow is greater than a certain limit depending on the Prandtl number of the fluid. The Dean number is defined by the formula: De = Re ( D n / 2 . R c ) 1/z in which Re is the Reynolds number, Dn is the hydraulic diameter of the channel and R c is the radius of curvature of the duct. By way of an example, for combustion gases of Prandtl number of the order of 0.7, in the present system in which De is of order of 1000, the heat transfer coefficient can be four times higher than the coefficient expected in a straight channel. The nozzles injecting the combustion gases into the convective channels are located in such a way that they locally reinforce secondary movements thus increasing one more time the convective heat transfer rate to the walls of the channels. Correlations used to take
AA Sectional view
J
3
---6
1. Combustion chamber 2. Distribution channels 3. Nozzles
4. 5 6.
Connective channels Ftue- gas exit Secondary movements
FIG. 3. Spiral heat transfer channels arrangement.
AN INTEGRATED BURNER-BOILER SYSTEM these factors into account are reported in the following chapters.
Description of the Burner-Boiler System Modular Boiler As an example of the setup, the 60 kW boiler is shown in Fig. 2. This is a modular cast-iron boiler which comprises a hollow cover, a bottom, three intermediate annular elements, and an expansion vessel fastened to the boiler bottom. The bottom of the boiler, which closes off the combustion chamber provides access to six distribution channels having the shape of annular segments, which are concentric with the longitudinal axis of the combustion chamber. Each annular intermediate element is a hollow cast-iron body. The circumferential space of this hollow element communicates with two openings which are diametrically opposite each other and pass through each element parallel to the axis of the boiler. One opening is connected to the cold water feed circuit while the other is connected to the hot water distribution circuit. Six hollow radial studs connect the body of the element to an inner ring which surrounds the combustion chamber. A series of reinjection slots generate a film of gas along the wall of the chamber in a zone which is particularly exposed to the flame. As the reinjected gases are somewhat cooled, they may form a protective film locally, particularly if the boiler is powerful and has numerous intermediate elements. Aside from the curvature of the convection ducts, the existence of the nozzles has several advantages particularly in providing a uniform flowrate in all elements, in maintaining an intense turbulence in the convection ducts, thus increasing the heat transfer coefficient. The plurality of nozzles serves to increase the average temperature of the gases in the convective heat transfer channels, thus increasing the heat exchange between the gases and the water. One will also note the equiangular arrangement of the nozzles with respect to the longitudinal axis of the combustion chamber which distributes the hot points in the metal uniformly, thus significantly reducing the thermal stresses. The conical portion of the combustion chamber is cooled by the circulation of water within the hollow cover. The inner shape of the cover allows the swirling flame to spread out to the periphery of the combustion chamber. The suppression of recirculation
67
zones along the chamber's wall therefore makes it possible to fully utilize the total heat exchange surface of the boiler. Finally as can be seen from the cross sectional views of the boiler, its geometry is symmetrical both with respect to the water, feed, and discharge conduits and with respect to the convection or heat-transfer ducts and the exhaust collectors. The modular boiler covers the range from 22 to 132 kW. A large boiler covers the range 122 to 732 kW. In order to achieve a low-cost system, the same design is used over the entire power range from 22 to 732 kW.
The High Swirl Burner The burner is shown in Fig. 4. Flue gas is recirculated through the mixing box and ventilator back into the burner. To ensure the stability and quality of the combustion, homogenous mixing of flue-gas and fresh air is achieved in a mixing box operating by impingement of jets of these gases. Satisfactory operation is obtained when the perforated portions of the mixing zone are of about the same length and the recirculation ratio R can be adjusted continuously from 0 to 80% by means of sliding rings. The fan located downstream from the mixing box rotates at high velocity (around 9000 rpm for the small series of burner). As will be seen below, quite a high underpressure is required in the mixing box for the stability of the system. Moreover, to ensure complete stability, the slope of the fan at the operating point must be greater than a certain limit. Swirl is generated in a conventional way 24 and the swirl number, using the definition and calculation procedure described by Leuckel, 25 can be changed by replacement of the radial swirl generator. An axial jet Jf "air" helps in the protection of the fuel nozzle
5
I.
Flue gas collector
2.
Mixing box
3.
Fan
4.
Swirl generator
5.
Combustion chamber
6.
Fuel - oil gun
7.
Fuel nozzle aeration
8.
Can
FIG. 4. Schematic view of the high swirl burner.
68
POWER SYSTEMS
System's Design Methods
against the hot combustion gases reverse flows which might lead to coke deposit on this device. However, the jet axial momentum as well as the location of the outlet of the jet tube have to be well adjusted in order not to destroy the internal recirculation flow pattern. Final adjustments are found experimentally and the system is sufficiently flexible and easy to adjust around the optimal point, The can which maintains the flame root is of major importance in controlling the formation and expansion of the toroidal vortices generated by the swirl. Various geometries have been tested as reported in Fig. 5. The best results have been obtained with types 2 and 4 geometries.
Trumpet 1
Cylindro conical
Cylindro conical with retention 3
Design of the Combustion Chamber Once the major features of the chamber (cylinder--conical shape with high swirling flow) were settled, a model was built in order to observe the flow pattern within the chamber for various length to diameter ratios, swirl numbers and angles of the conical cover. This transparent model is supplied with water through an adjustable swirl generator and internal recirculation with reverse flows may be observed by seeding the flow with coloured polystyrene balls. A plane light beam illuminates the model either longitudinally or transversally, allowing observation of the axial/radial velocities or the tangential velocity of flow particles. Fig. 6 shows the experimental setup. The observations are mainly qualitative due to the complexity of fulfillment of the similitude criteria in such systems. 26 Namely, such a model only allows the simulation of an isothermal flow situation in the combustion chamber. Using models and numerical methods described in the literature, 27-29 a computer program predicting the thermo-physical parame-
Conical with retention 4
<-!. <-I.
Flc, 5. Various geometries of burner's can tested.
'~Lamp l Plane I--.~~---J
r---Water | 4--
light beam I %
- - - -
--I
I
Honeycomb
,= I-Water r =~P=*===~
!
--
I1
model
--Direction of observation
Swirlgenerator
~~
Water
lls
~
Direction of observation
Fie. 6. Experimental set-up hydrodynamic modelling of the combustion chamber.
AN INTEGRATED BURNER-BOILER SYSTEM ters field in the combustion chamber allowed us to optimize the shape of such a chamber. The main predicted variables are: a) axial, radial and tangential velocity components b) static pressure c) stagnation enthalpy d) absolute temperature 3) wall heat fluxes The flow is considered as axi-symmetrical and in steady-state. The input parameters are the mass flow rates of fuel and air-flue-gas mixture, the temperatures of these flows as well as the
~
69
temperature of the combustion chamber's walls, the swirl intensity, the static pressure level at the chamber's outlet and the dimensions of the system (burner throat and boiler). Fig. 7 gives a typical set of results of the computations. The predictions, particularly regarding the total heat transfer at the combustion chamber's wall, were quite well confirmed by measurements of the combustion gas outlet temperature and assured us of the possibility of covering the entire power range of the boiler series. The exit gas temperature from the combustion chamber serves as an input to the computer
+
+
+
+
Axial velocity isocontours (maximum value, 18m/sec; minimum value,-7 m/sec).
~
+
,
+
+
+
+
+
_
+~5
Radial velocityisocontours (maximum value, 6m/sec; minimum value,-0,25m/sec).
+
4-
+
Isotherms ( T inlet, 363 K;
T maxi, 1620 K ).
FIG. 7. Typical set of results of the combustion chamber's computation (60 kW boiler; combustion chamber dimensions: length, 0.638 m; diameter, 0.290 m; swirl number, 0.8).
70
POWER SYSTEMS such nozzles. The flow pattern in the mixing region of the injected gas and the i n c o m i n g flow in the channel has been examined b y a Schlieren method, as seen in Fig. 8. O p t i m i z a t i o n of the geometry of the channels has been carried out using a step-by-step procedure, f o l l o w i n g the flow path in the channels. T h e following hypotheses have been assumed: a) constant heat transfer coefficient obtained b y natural convection and constant temperature of the flow on the water side, and b) longitudinal exchange fluxes negligible with respect to the transversal fluxes (crosssectional averaged quantities considered). The influence of the nozzle's injection on secondary movements reinforcement was first considered b y taking into account the m o m e n tum difference between the main and the nozzle streams. But as the effect of the superposition of entrance, curvature and reinforcement factor are not yet well understood, it has been a s s u m e d that entrance effects are present after each nozzle and secondary movements have b e e n fully established. The m a i n correlations used are those of Rohsenow and Hartnett. 3~ Fig. 9 gives a typical set of results. C o m p u t a t i o n and experimentation have been found to be quite compatible. In particular, the cumulative heat exchanged by the hot gases to the water from the starting p o i n t of the channel is regularly distributed along the channel, thus s h o w i n g good heat flux distribution and optimal utilization of cast-iron.
FIG. 8. Examination of the flow pattern in a convective channel in the vicinity of an injection nozzle by a Schlieren method.
program for optimization of the convective channels.
Design of the Heat Transfer Channels In order to take advantage of the curvature effects, the reinforcement of secondary flows by combustion gas injection nozzles has been ensured by u s i n g high velocity jets t h r o u g h
D0r
800
w.-v
J
|
600
0
0
0.4
0.8
1.2
1.6
2.0
2.4
Channe! length, rn Fic. 9. Typical set of results of the heat transfer channels calculation (163 kW boiler; total heat transfer channel's length: 2.40 m; temperature of the injected gas: 1053 K). Curve 1, Temperature, K; curve 2, velocity • m/see; curve 3, heat transfered x0.1, W; curve 4, Reynolds number x0.05; curve 5, Nusselt number x20.
AN INTEGRATED BURNER-BOILER SYSTEM
Analysis of the Feed-Back Loop Influence on Stability Apart from c o m b u s t i o n - d r i v e n oscillations already investigated, ~1 the feed-back loop recirculating the flue-gas must satisfy specific conditions in order to prevent the occurrence of instabilities and oscillations. A s s u m i n g a moderate recirculation ratio (R < 90%) in order to avoid flame quenching, w h i c h reduces flame stability, and also a good mixing between flue-gas and fresh air, the characteristics of the system's components may be chosen in an optimal way in order to stabilize the system, suppressing the onset of feed-back-loop-driven oscillations. A linearized analysis of the response of the system to a slight disturbance has been carried out considering all parameters (pressures, flow rates, recirculation ratio and excess air) as time variable quantities, as assumed in Ref. 32. Moreover, as the m a i n dimensions of the system are small c o m p a r e d to the wavelength of the feed-back-loop-oscillation, at the frequencies encountered, no significant phase lag between the pressure signals at different points of the system has b e e n recorded. It is obvious that the instantaneous recirculation ratio, R* = rhf/rh a is a fluctuating quantity as well as the excess air )t = rh /rhas , where mas is the stoichiometric air mass flow rate. The analysis shows that R* varies with the pressure regime in the system, according to the values of the i m p e d a n c e s of the fresh air or flue gas circuits a n d the fan characteristic curve. To fulfill the well known Rayleigh criteria, 31 heat release and pressure in combustion chamber must vary inversely for stability.
..... .3.~ ....
C o n s i d e r i n g the fact that an increase of recirculation decreases the combustion intensity, t~ it appears that the system is stable w h e n R* and the c o m b u s t i o n chamber's pressure vary inversely. This implies a p r o p e r choice of the system's c o m p o n e n t s such as the characteristic curve of the fan and the impedances of the fresh air a n d flue-gas circuits. A simulation of the behavior of the fan has been carried out a c c o r d i n g to the correlations of Eck .3 and Eckert and Schnell. a4 The influence of instabilities on the orifices adjusting the fresh air and flue-gas mass flow rate has been found to be n e g l i g i b l e Y This stabilization process should be a p p l i e d to other purposes e n a b l i n g further investigation into the behavior of all the components to be carried out. Test and Main Results
Test Facilities F o u r test benches allow us to fire simultaneously four boilers of various p o w e r ratings. The water circuits are capable of transferring from 15 kW to 1 M W of heat. Figure 10 gives a schematic view of one test bench, showing the circuits of hot water, fuel and gases, as well as the gas s a m p l i n g location. The benches are e q u i p p e d for temperature measurements in water and gas passages in order to obtain a complete m a p p i n g of the system. Piezoelectric pressure transducers permit the measurement of pressure fluctuations in the hot combustion gas passages as well as in the burner. F u e l c o n s u m p t i o n is measured b y simply weighing the fuel injected in the system.
1 .. Boiler 2: Burner 3: External heat exchanger (finned tubes) 4: Stack with adjustable draft f.6
5: 3 - w a y
valve
6: Water pump 7: Gas analysis fines 8
,
1
??~q
71
8: Expansion vessel
2
FIG. 10. Experimental set-up for the burner-boiler system testing.
72
POWER SYSTEMS 1.50
1.t,O 0 1.30 Itl
Bachamch Index
u
t~
1.2o " ~ / /
Y ,~"~lor
I
Soot-fr.
/
/
transition /
flame
.7~rT~s~/
1.10
Sooty flame
~
//
~
I
I
I
10
20
30
BlueFlame
I " ~ " "~ I j
40
50
Recirculation ratio
I
I
I
60
70
80
90
%
FIG. 11. Various combustion regimes observed on a 60 kW boiler, with No. 2 fuel-oil (swirl number
1.1.).
Gas analyses are carried out c o n t i n u o u s l y after sampling, filtration and passage through a water trap for CO2, CO and 0 2 concentration measurements. Non-dispersive infra-red analyzers are used for CO 2 and CO and a c o n v e n tional amperometric apparatus is used for 0 2 measurement. T h e U H C s are measured by flame ionization, after sampling through a h e a t e d line. For NO x measurements, a heated sampling line is used between the s a m p l i n g probe and a set of filters a n d a water trap (liquid air vessel) a n d afterwards the line is connected to an N O , c h e m i l u m i n e s c e n c e analyzer.
16
14
"
"
~
,_ 13 *' u 12 ~, Q. ,11 U
-
E
[] CO
"1-
s0~ O ,<-
10
o Z
1.1
1.2
1.3
1.4 flame)
i
E r
FIG. 12b~ Emissions vs excess air, 30% recirculalion.
-I-
u
r
T5
Excess air, A (soot-free
9 CO 2 9 NOx 9 CO
~,~
1130
CO2 o NOx
1.o
100
15
16
Tests Carried Out tS
,ll
5O
o ~ 1o
x
o z
1.0
~
1.1
~
~
1.2
Excess
1.3
^
0
I.~
air,
Fzc. 12a. Emissions vs excess air, no recirculation (for h < 1.2. the Bacharach Index is > 1).
Steady state tests have been made at the n o m i n a l power of the system as well as plus or m i n u s 10% the n o m i n a l rating. T r a n s i t i o n from sooty to soot-free flame has been observed, as well as transition to complete blue flame operation in varying the excess air and recirculation ratio. C o m b u s t i o n noise has been measured with the burner operating on the boiler. The total mass flow rate through the burner has been kept constant i n order to compare easily the noise generated b y the b u r n e r operating with excess air only a n d the b u r n e r operating with excess air and recirculation.
AN INTEGRATED BURNER-BOILER SYSTEM
the main results gained on a 60 kW system. T h e CO 2 vs k curve is a function of the recirculation ratio a n d the curves s h o w n have b e e n computed. The emissions in the steady-state regime are reported in Fig. 12 (a to c). At zero recirculation, soot is emitted from the system If h < 1.25 whereas for recirculation ratios equal or greater than 30%, the Bacharach Index always remained at zero. C o m b u s t i o n noise abatement b y recirculation is shown on Fig. 13. The results obtained w h e n the system is operated at - 10% from its nominal rating are reported in Fig. 14. It can be noted that the overall thermal efficiency is always above 91% and is affected very little by p o w e r rating modifications. The start-up and s h u t - d o w n transient emissions are given on Fig. 15. However, due to the distance (2 m) b e t w e e n the boiler's exhaust gas collector and the s a m p l i n g tube, turbulent mixing occurs in the stack and the emissions recorded are p r o b a b l y somewhat too low. During the start-up the transition between the yellow flame and the b l u e flame at sufficient recirculation occurs in a very short time (approx. 0.5 sec).
100
is ~
*
c02
9 NOx ~ - ~ ~
+ CO *
E
~ UHC
~ ~-
o
D 11
50
~
x" o z +
+
-
1.o
u
1.2
Excess a i r , / ~
0
I~
73
I.~
( s o o t - f r e e flame )
FIG. 12c. Emissions vs excess air, 60% recirculation. Temperatures of c o m b u s t i o n gases in the convective channels have been measured with shielded thermocouples whereas hot water temperatures have b e e n recorded continuously in order to detect any hot p o i n t in the boiler. Some transient tests have been carried out in order to record the pressure variations in the combustion chamber, the pollutant emissions and water temperature variations.
Conclusions A new integrated burner-boiler system of a p o w e r rating ranging from 22 to 730 kW, firing No. 2 fuel-oil or natural gas has been designed, built and tested, taking advantage of a high swirling flame and the external recirculation of cold flue-gas.
Main Results Figure 11 gives the various combustion regimes observed and Fig. 12 (a to c) shows
_8
\
w 100 0 ~ L. rn "0 > "U C
90
80 70
60
_8 50 > 0
o
7
I? so
I-;
,
100
I 500
Frequency,
I000
, 5000
Ioooo
cps
FIG. 13. Octave Band Noise spectrum (60 kW boiler). Curve 1: Background noise; curve 2: Blue flame combustion (k = 1.10; R = 60%); curve 3: yellow flame combustion (~. = 1.7; R = 0%).
74
POWER SYSTEMS 100
.-e*
483
v
g 2
473
90
/
Itl en
E
-6 o t9
463 80 453 I
I
I
5O
60
70
Power rating,
kW
FIG. 14. Variations of the performances of the system vs power rating (nominal power rating 60 kW).
25 14
CO /~ // ,A r \Xx ~
13 12
11 9 g
__N~
70 NOx
\
UHC
,7
g6 o
I00
CO 2
5
\
5o "
o'~ z 40 "\
~U_.HC_ I
I
i
J
I
I
5
I
10
so E
I
I
"-ti
i
I
15
I
I
I
10
I"~
20
time, sec. Fro. 15. Emissions as CO2, CO, UHC and NO~ vs time during the start-up and the shut-down of the system. (Solid lines, start-up; dashed lines, shut-down). Extensive testing of the entire system confirmed the original hypotheses that the comb i n e d use of cold flue gas recirculation and swirl enables one to obtain a simultaneous reduction of NO~, CO and U H C at very low excess air levels, leading to a close control of the emission levels at optimum thermal efficiency of the system. The external recirculation, yielding an oxidant dilution and flame temperature reduction require to adopt a design where good mixing of fuel and oxidant and stable, pulsation-free combustion conditions can be ensured. It has b e e n demonstrated that the use of intense swirl allows at least twice as high firing
rates in the boiler than with a c o n v e n t i o n a l design, and that the soot-free c o m b u s t i o n allows the design with narrow convective heat exchange channels. The external recirculation can be simply used without adding any significant complexity to the entire system.
Acknowledgments This work has been carried out in the Combustion Research Laboratory of the Battelle Research Center, Geneva, Switzerland, on behalf of the Officine Termetecniehe Br6da, Italy. The authors wish to thank the Management of this Company for having granted their permission to publish this paper and
AN INTEGRATED BURNER-BOILER SYSTEM are grateful for the cooperation of many colleagues at the Battelle-Geneva Research Center during the course of the work reported here. 12. REFERENCES 1, BARRETT,R, E. AND MILLER, S. E.: Field Investigation of Emissions from Combustion Equipment for Space Heating. Environmental Protection Agency Report No. EPA-R2-73-084a, June 1973. 2. DICKERSON,R. A. AND OKUDA, A. S.: Design of an Optimum Distillate Oil Burner for Control of Pollutant Emissions. Environmental Protection Agency Report No. EPA-650/2-74-047, June 1974. 3. SCHIASSL A.: For Energy Preservation-Low Excess Air Combustion. Intern. Flame Research Foundation R e p o r t - - F 25/ha/3, January 1974. 4. BROWN,T. D.: The Performance of Vane Swirlers in Domestic Oil Burners. Canadian Combustion Research Laboratory Report--FCR 72/59, Department of Energy, Mines and Resources, Ottawa (Canada), July 1972. 5. MOORMAN,R. J. ANDLONG, C. H.: Design, Development and Testing of a Swirl Type Gas Burner with Flue-Gas Recirculation for NO x Control. Am. Soc. Mech. Engrs. Paper 73-Pwr-21, July 1973. 6. BOWMAN,C, T. AND COHEN, L. S.: Influence of Aerodynamic Phenomena on Pollutant Formation in Combustion; Volume I: Experimental Results, Environmental Protection Agency Report No. EPA-650/2-75-061-a, July 1975. 7. HEAP, M. P., LOWES, T. M. AND WALMSLEY,R.: The Effect of Burner Parameters on Nitric Oxide Formation in Natural Gas and Pulverized Fuel Flames. Paper presented at the American Flame Research Committee Flame Days, Chicago, II1., September 1972. 8. COOPER, P. W., KAMO, R., MAREK, C. J. AND SOLBRIG,C. W.: Recirculation and Fuel-Air Mixing as Related to Oil-Burner Design. American Petroleum Institute Publication 1723, May 1964. 9. ANDREWS,R. L., SIEGMUND, C. W, AND LEVINE, D. G.: Effect of Flue-Gas Recirculation on Emissions from Heating Oil Combustion. Paper presented at the 61st Annual Meeting of the Air Pollution Control Association, Saint-Paul, Minn. June 23-27, 1968. 10. HEDLEY,A. B. ANDJACKSON,E. W.: The Influence of Recirculation in Combustion Processes. Paper presented at the 1966 American Petroleum Institute Research Conference on Distillate Fuel Combustion, Chicago, Ill. June 13-15, 1966. 11. TURNER, D. W. AND SIEGMUND, C. W.: Staged Combustion and Flue Gas Recycle: Potential for Minimizing NO x from Fuel-Oil Combus-
13.
14. 15.
16.
17.
18. 19. 20.
21. 22. 23. 24.
25.
26.
27.
28.
29.
30. 31. 32.
75
tion. Paper presented at the American Flame Research Committee Flame Days, Chicago, II1., September 1972. MONAGHAN,M, T. ANn McGaAxH, I. A.: J. Inst. Petrol. 55, 303 (1969). BEER, J. M. AND CmGIER, N. A.: Combustion Aerodynamics, Applied Science Publishers Ltd., 1972. LEUCKEL, W.; Revue G~n6rale de Thermique. 84, 1367 (1968). BEER,J. M. ANDLEUCrEL, W.: Turbulent Flames in Rotating Flow Systems. Paper presented at the North American Fuel Technology Conference, Ottawa, Canada, May 31-June 3, 1970. SYREO, N., CHIGIER, N. A. AND BEER, J. M.: Thirteenth Symposium (International) on Combustion, p. 617, The Combustion Institute, 1971. LOWES, T. M., HEAP, M. P. AND SMITH, B. B.: Reduction of Pollution by Burner Design. Intern. Flame Research Foundation Report--K 2 0 / a / 7 4 , 1974. KERR, N. M. AND FRASER, D.: J. Inst. Fuel. 38, part I and If, 519, (1965). DRAKE,P. F. AND HUBBARD,E. H.: J. Inst. Fuel. 39, 98 (1966). MORI, Y. ANDNAKAYAMA,W.: Intern. J. Heat Mass Transfer, 8, 67 (1965). ITO, H.: Trans. Am. Soc. Mech. Engrs. Journal of Basic Engineering, 81, 123 (1959). MORI,Y. ANDNAKAYAMA,W.; Intern. J. Heat Mass Transfer, 10, 681, 1967. PATANr,AR, S. V., PRATAP,V. S. AND SPALDING, D. B.: J. Fluid. Mech., 62, 539 (1974). FRICKER,N. AND LEVCKEL,W.: Flow and Mixing Patterns in Gas Flames with Swirl in the Annular Air Stream. Intern. Flame Research Foundation Report G 0 2 / a / 1 8 , 1969. LEUCKEL,W.: Swirl Intensities, Swirl Types and Energy Losses of Different Swirl Generating Devices. Intern. Flame Research Foundation Report--G 0 2 / a / 1 6 , 1967. SPALDING,D. B.: Ninth Symposium (International) on Combustion, p. 833, Academic Press, 1963. GOSMAN, A. D., Put,', W. M., RUNCHAL, A. K., SPALDING,D. B. AND WOLFSHTEIN, M. W.: Heat and Mass Transfer in Recirculationg Flows, Academic Press, 1969. PATANr,Aa, S. V. AND SPALDING,D. B.: Heat and Mass Transfer in Boundary Layers, 2nd Ed., Intertext Books, 1970. LAUNDER,B. E. AND SPALDING,D. B.: Mathematical Models of Turbulence, Academic Press, 1973. ROHSENOW,W. M. ANDHARTNE'I-F,J. P.: Handbook of Heat Transfer, McGraw-Hill, 1973. PUTNA.VI,A. A.: Combustion-Driven Oscillations in Industry, Elsevier, 1971. MAUSS, F., PERTHUIS, E. AND SALE, B.: Tenth
76
POWER SYSTEMS
Symposium (International) on Combustion, p. 1241, The Combustion Institute, 1965. 33. Ecr, B.: Ventilatoren, Springer Verlag (Berlin), 1972.
34. ECKEt~T,B. AND SCHNELL, E.: Axial-und RadialKompressoren, Springer Verlag (Berlin), 1961. 35. EARLES,S. W, AND ZAREK,J. J.: Proc. Inst. Mech. Engrs. (London) 177, 997, 1963.
BURNER-BOILER RECIRCULATION
SYSTEM
USING
J. G. MEIER* AND B. L. VOLLERIN Battelle Research Center, Geneva, Switzerland
The main goal of the project was the design of a series of burngr-boiler systems for domestic hot water heating, providing great compactness, high global thermal efficiency, low pollution and easy manufacture. This series has been designed on the basis of modularity of the elements, the use of a high thermal load pressurized combustion chamber (above 2.3 MW/m 3) with strong swirl and with external recirculation of flue gas. This system enables operation with light fuel oil or natural gas at very low excess air (below 10%) and achieves soot free combustion with significant reduction in emissions of CO, thermal NO~, unburned hydrocarbons, and combustion noise as compared to conventional burner-boiler systems. The boiler has been designed with computer models, and flow patterns were visualized and examined in a water model and also by means of Schlieren methods. The burner matched to the boiler provides a highly swirling flow (swirl number ranging from 0.6 to 1.3) of an adjustable mixture of flue gas and fresh air (mass flow rate ratio between 0 and 70%). Care has been taken to insure the stability of this system which uses a feed-back loop as a recireulation circuit. The complete series has been developed, built and tested. Examination of test results shows how this advanced concept may simultaneously fulfill low pollution and energy saving requirements.
Introduction
sphere, one arrives at a stalemate where conventional heating systems cannot achieve both high efficiency and low pollution levels.l Some attempts have been made to determine new designs of burners achieving efficient and clean combustion. 2-~ Systematic i n v e s t ~ a t i o n s of the influence of aerodynamics on pollutant formation in c o m b u s t i o n have been carriedout: effect of swirl, staged combustion, atomization of fuel or injection of gas in gas burners. It is not enough to consider the burner b y itself and try to improve its design in order to improve a c o m b u s t i o n system efficiency. The boiler design must be considered at the same time, in view of the fact that a slight improvement of the boiler performance plays an important role in the overall system's efficiency. In order to simultaneously fulfill low pollution and high thermal efficiency of the system, it is therefore necessary to develop an integrated concept of a burner-boiler system. In this way, it was here possible to conceive a system u s i n g external flue gas recirculation, a high thermal load pressurized
During the colder seasons domestic heating systems contribute significantly to the pollution of the atmosphere. A m o n g the pollutants generated are carbon monoxide (CO), soot, u n b u r n e d hydrocarbons (UHC), oxides of nitrogen (NO~) a n d s u l p h u r oxides (SO~). Control of the final pollution level of a system is ultimately a matter of setting up the right pattern of oxygen, hot or cold 'combustion products, temperature, and allowing the necessary time to complete the chemical reactions. Thus, it appears that the geometry and the aerodynamics of a c o m b u s t i o n system are of prior importance in order to obtain an overall clean operation. A m i n i m i z a t i o n of the pollutants emission is generally not compatible with high energy efficiency. As energy must be saved, and one must simultaneously bear in m i n d the protection of the purity of the atmo* present address: Solar, Division of International Harvester, San Diego, California 63
64
POWER SYSTEMS
combustion c h a m b e r and a strong s w i r l i n g flow in this c o m b u s t i o n chamber.
by excess air a n d C F G R . The velocity field also affects the formation of NO~. 7 G e n e r a l l y a critical level of excess air leading to high NO ~ concentration exists in combustion products. This is due to an " o p t i m u m " in 0 2 concentration and temperature in the flame. The typical N O ~vs excess air curve for a b u r n e r is a matter of fuel used and design. Peak flame temperature as well as m e a n flame temperature are reduced by using sufficiently cooled combustion products, m i x i n g these products with fresh air, and i n t r o d u c i n g the mixture into the combustion chamber. Other sources of NO~, like NO~ due to fuel b o u n d nitrogen, seem to be i n d e p e n d e n t of the C F G R rate. 11 Moreover, as the fuels used in our case are poor in fuel-bound nitrogen, no attention has here been p a i d to this correlation.
Basic Features of the Burner-Boiler System
Recirculation of the Flue-Gas The use of cold flue gas recirculation (CFGR) in c o m b u s t i o n systems provides one of the basic means for i m p r o v i n g c o m b u s t i o n efficiency and for reducing the p o l l u t i o n of the atmosphere as c o m p a r e d to conventional processes, s-12 A schematic view of a system using C F G R is represented in Fig. 1. The C F G R is characterized by the recirculation ratio R = rhy/rhas, where rhf is the flue-gas mass flow rate a n d rhas is the mass flow rate of stoichiometric air required. The principal effect of C F G R is the reduction of the oxygen content of the gas mixture fed into the burner, typically to a r o u n d 16% by volume (dry basis). The dilution of the oxygen leads to a general reduction of the flame temperature, a c c o r d i n g to the quantity of flue gas recycled. Let us summarize the major effects of C F G R on the emissions of pollutants other than SO~ from the burner-boiler system.
Carbon Monoxide and Unburned Hydrocarbons The process of formation and destruction of CO and U H C in burners is roughly similar. The well k n o w n parameters affecting these species are the degree of mixing, oxygen availability, residence time and temperature. One could expect high C O and U H C emission levels from the present system regarding the dilution effect of the C F G R which leads to a reduced c o m b u s t i o n intensity. As p r e v i o u s l y reported b y other authors, g low CO and U H C levels are achieved through a proper m a t c h i n g of excess air a n d C F G R rate. Although the use of C F G R seems to have a potential disadvantage regarding CO and U H C emissions, its influence can be completely counterbalanced b y the proper aerodynamic design of the system.
Nitrogen Oxides Since thermal N O ~is formed by the c h e m i c a l reaction of oxygen and nitrogen at a h i g h temperature, the temperature and oxygen concentration fields in the combustion c h a m b e r are of major importance. By reducing the flame temperature, a significant reduction of the NO~ formation via a thermal mechanism m a y be obtained. 5,9 T e m p e r a t u r e may be controlled
~5 3
1. Fan 2. Swirl generato~ ~
combustion head
3.
Combustion chamber
4.
Flue- gas collectors/heat
exchangers
S. Stack 6. Mixing box
nSf
7. Fan diffuser outlet r~o : instantaneous fresh-air mass flow rate rhf : Instantaneous recirculated f l u e - g a s mass flow rate
rh
: instantaneous mixture mass flow rate
1
FIG. 1. Schematic view of the system using the cold flue gas recirculation.
AN INTEGRATED BURNER-BOILER SYSTEM
Soot The influence of C F G R on soot emission has been known for m a n y years, s Global sootfree combustion can be achieved at very low excess air level w h e n the C F G R rate is set above a critical limit. F u r t h e r increase in the recirculation rate results in the elimination of carbon particles p r o d u c t i o n , leading to a b l u e flame combustion. T h e characteristic curves of Zero Bacharach Index vs excess air and recirculation ratio on the one hand, and the b l u e flame limit vs the same parameters on the other, depends on the burner design, e.g. the combustion staging and aerodynamics. T h e curves obtained in testing the present system will be shown b e l o w a n d compared to those obtained on previous systems.
Highlg Swirling Flame The basic advantages of the use of swirling flows in combustion chambers have been w i d e l y described b y m a n y authors, mainly for utilization of such flows in combustors or in b i g industrial b u r n e r s ? ~ It was decided to use such a type of flow (swirl number ranging from 0.4 to 1.3) in the present system's combustion chamber, in order to a) use the stabilization properties of swirl which anchors the flame in a purely a e r o d y n a m i c a l way, 14,15 b)
65
promote mixing b y setting u p an intense internal recirculation zone of hot combustion products, 16 c) obtain a more v o l u m i n o u s flame spreading u n i f o r m l y into the c o m b u s t i o n chamber, d) compensate the reduction in combustion intensity due to the C F G R and to m i n i m i z e CO and U H C emissions. In c o m b i n i n g strong swirl with C F G R , the conflict regarding emissions of CO (or UHC) and NOx a7 can be solved in that the temperature abatement due to C F G R yields to a reduction of NO~ emission a n d the internal recirculation due to the swirl promotes the completion of combustion. The high intensity flames obtainable with strong swirllS.19 generate a noise level which is sometimes unacceptable. It will be seen b e l o w that this noise level, mainly a combustion roar, can be drastically reduced below conventional b u r n e r noise levels, taking advantage of the simultaneous low excess air level and C F G R operation.
Pressurized Combustion Chamber In order to feed the convective channels of the boiler efficiently and to arrive at a system safely operable in various external conditions such as stack geometry, draft and weather, pressurization of the c o m b u s t i o n chamber of
2
333
1
1. 2. 3. /4. 5. 6.
High swirl burner Conical front element Intermediate element Back element Nozzles Heat transfer channels
7. 8. 9. 10. 11.
Reinjection slots Water-coded internal annulus Radial hollow segment Cold water inlet Hot water outlet
FIG. 2. Cross-sectional views of the boiler. On the left, longitudinal cross-sectional view showing the combustion chamber shape; on the right, transversal cross-sectional view showing the water passages in the intermediate elements.
66
POWER SYSTEMS
about 400 N / m z is used. A schematic view of the combustion chamber is given in Fig. 2. The chamber is of a cylindrical shape and connected to the burner by a conical front element. The swirling flows which expand towards the chamber's wall, promote internal recirculation of products located near the axis of the system. As concluded by Drake and Hubbard, 19 the mixing processes associated with swirling flames may give optimum combustion conditions if the combustion chamber is well matched to the burner, especially regarding the flow pattern. High thermal load has been achieved by developing the complete system as one unit. The system is rated to more than 2.3 M W / m a. The combustion chamber is connected to the heat transfer channels by small cross section nozzles. The role of these nozzles regarding the heat transfer is explained below. First of all, these nozzles create concentrated pressure losses between the combustion chamber and the channels, efficiently damping pressure waves occurring during the system start up, and therefore make it possible to increase the specific power rating of the boiler. Spiral Heat Transfer Channels The negligible level of soot formation in the combustion gases has permitted the design of narrow heat transfer channels, which are spirally wound around the combustion chamber, thus rendering the system very compact. Details of the design are explained
in the subsequent sections and Fig. 3 gives a schematic view of this particular geometry. In addition to the compactness gained by the use of this geometry, a great advantage is obtained regarding the curvature effect on the heat transfer coefficient. Various works 2~ initiated by W. R. Dean in 1927, deal with the flow in curved channels. Curvature causes secondary movements within the channel cross-section as indicated in Fig. 3. The centrifugal forces responsible for these secondary movements greatly increase the heat transfer between the fluid and the walls of the channel. This effect is substantial if the Dean number De of the flow is greater than a certain limit depending on the Prandtl number of the fluid. The Dean number is defined by the formula: De = Re ( D n / 2 . R c ) 1/z in which Re is the Reynolds number, Dn is the hydraulic diameter of the channel and R c is the radius of curvature of the duct. By way of an example, for combustion gases of Prandtl number of the order of 0.7, in the present system in which De is of order of 1000, the heat transfer coefficient can be four times higher than the coefficient expected in a straight channel. The nozzles injecting the combustion gases into the convective channels are located in such a way that they locally reinforce secondary movements thus increasing one more time the convective heat transfer rate to the walls of the channels. Correlations used to take
AA Sectional view
J
3
---6
1. Combustion chamber 2. Distribution channels 3. Nozzles
4. 5 6.
Connective channels Ftue- gas exit Secondary movements
FIG. 3. Spiral heat transfer channels arrangement.
AN INTEGRATED BURNER-BOILER SYSTEM these factors into account are reported in the following chapters.
Description of the Burner-Boiler System Modular Boiler As an example of the setup, the 60 kW boiler is shown in Fig. 2. This is a modular cast-iron boiler which comprises a hollow cover, a bottom, three intermediate annular elements, and an expansion vessel fastened to the boiler bottom. The bottom of the boiler, which closes off the combustion chamber provides access to six distribution channels having the shape of annular segments, which are concentric with the longitudinal axis of the combustion chamber. Each annular intermediate element is a hollow cast-iron body. The circumferential space of this hollow element communicates with two openings which are diametrically opposite each other and pass through each element parallel to the axis of the boiler. One opening is connected to the cold water feed circuit while the other is connected to the hot water distribution circuit. Six hollow radial studs connect the body of the element to an inner ring which surrounds the combustion chamber. A series of reinjection slots generate a film of gas along the wall of the chamber in a zone which is particularly exposed to the flame. As the reinjected gases are somewhat cooled, they may form a protective film locally, particularly if the boiler is powerful and has numerous intermediate elements. Aside from the curvature of the convection ducts, the existence of the nozzles has several advantages particularly in providing a uniform flowrate in all elements, in maintaining an intense turbulence in the convection ducts, thus increasing the heat transfer coefficient. The plurality of nozzles serves to increase the average temperature of the gases in the convective heat transfer channels, thus increasing the heat exchange between the gases and the water. One will also note the equiangular arrangement of the nozzles with respect to the longitudinal axis of the combustion chamber which distributes the hot points in the metal uniformly, thus significantly reducing the thermal stresses. The conical portion of the combustion chamber is cooled by the circulation of water within the hollow cover. The inner shape of the cover allows the swirling flame to spread out to the periphery of the combustion chamber. The suppression of recirculation
67
zones along the chamber's wall therefore makes it possible to fully utilize the total heat exchange surface of the boiler. Finally as can be seen from the cross sectional views of the boiler, its geometry is symmetrical both with respect to the water, feed, and discharge conduits and with respect to the convection or heat-transfer ducts and the exhaust collectors. The modular boiler covers the range from 22 to 132 kW. A large boiler covers the range 122 to 732 kW. In order to achieve a low-cost system, the same design is used over the entire power range from 22 to 732 kW.
The High Swirl Burner The burner is shown in Fig. 4. Flue gas is recirculated through the mixing box and ventilator back into the burner. To ensure the stability and quality of the combustion, homogenous mixing of flue-gas and fresh air is achieved in a mixing box operating by impingement of jets of these gases. Satisfactory operation is obtained when the perforated portions of the mixing zone are of about the same length and the recirculation ratio R can be adjusted continuously from 0 to 80% by means of sliding rings. The fan located downstream from the mixing box rotates at high velocity (around 9000 rpm for the small series of burner). As will be seen below, quite a high underpressure is required in the mixing box for the stability of the system. Moreover, to ensure complete stability, the slope of the fan at the operating point must be greater than a certain limit. Swirl is generated in a conventional way 24 and the swirl number, using the definition and calculation procedure described by Leuckel, 25 can be changed by replacement of the radial swirl generator. An axial jet Jf "air" helps in the protection of the fuel nozzle
5
I.
Flue gas collector
2.
Mixing box
3.
Fan
4.
Swirl generator
5.
Combustion chamber
6.
Fuel - oil gun
7.
Fuel nozzle aeration
8.
Can
FIG. 4. Schematic view of the high swirl burner.
68
POWER SYSTEMS
System's Design Methods
against the hot combustion gases reverse flows which might lead to coke deposit on this device. However, the jet axial momentum as well as the location of the outlet of the jet tube have to be well adjusted in order not to destroy the internal recirculation flow pattern. Final adjustments are found experimentally and the system is sufficiently flexible and easy to adjust around the optimal point, The can which maintains the flame root is of major importance in controlling the formation and expansion of the toroidal vortices generated by the swirl. Various geometries have been tested as reported in Fig. 5. The best results have been obtained with types 2 and 4 geometries.
Trumpet 1
Cylindro conical
Cylindro conical with retention 3
Design of the Combustion Chamber Once the major features of the chamber (cylinder--conical shape with high swirling flow) were settled, a model was built in order to observe the flow pattern within the chamber for various length to diameter ratios, swirl numbers and angles of the conical cover. This transparent model is supplied with water through an adjustable swirl generator and internal recirculation with reverse flows may be observed by seeding the flow with coloured polystyrene balls. A plane light beam illuminates the model either longitudinally or transversally, allowing observation of the axial/radial velocities or the tangential velocity of flow particles. Fig. 6 shows the experimental setup. The observations are mainly qualitative due to the complexity of fulfillment of the similitude criteria in such systems. 26 Namely, such a model only allows the simulation of an isothermal flow situation in the combustion chamber. Using models and numerical methods described in the literature, 27-29 a computer program predicting the thermo-physical parame-
Conical with retention 4
<-!. <-I.
Flc, 5. Various geometries of burner's can tested.
'~Lamp l Plane I--.~~---J
r---Water | 4--
light beam I %
- - - -
--I
I
Honeycomb
,= I-Water r =~P=*===~
!
--
I1
model
--Direction of observation
Swirlgenerator
~~
Water
lls
~
Direction of observation
Fie. 6. Experimental set-up hydrodynamic modelling of the combustion chamber.
AN INTEGRATED BURNER-BOILER SYSTEM ters field in the combustion chamber allowed us to optimize the shape of such a chamber. The main predicted variables are: a) axial, radial and tangential velocity components b) static pressure c) stagnation enthalpy d) absolute temperature 3) wall heat fluxes The flow is considered as axi-symmetrical and in steady-state. The input parameters are the mass flow rates of fuel and air-flue-gas mixture, the temperatures of these flows as well as the
~
69
temperature of the combustion chamber's walls, the swirl intensity, the static pressure level at the chamber's outlet and the dimensions of the system (burner throat and boiler). Fig. 7 gives a typical set of results of the computations. The predictions, particularly regarding the total heat transfer at the combustion chamber's wall, were quite well confirmed by measurements of the combustion gas outlet temperature and assured us of the possibility of covering the entire power range of the boiler series. The exit gas temperature from the combustion chamber serves as an input to the computer
+
+
+
+
Axial velocity isocontours (maximum value, 18m/sec; minimum value,-7 m/sec).
~
+
,
+
+
+
+
+
_
+~5
Radial velocityisocontours (maximum value, 6m/sec; minimum value,-0,25m/sec).
+
4-
+
Isotherms ( T inlet, 363 K;
T maxi, 1620 K ).
FIG. 7. Typical set of results of the combustion chamber's computation (60 kW boiler; combustion chamber dimensions: length, 0.638 m; diameter, 0.290 m; swirl number, 0.8).
70
POWER SYSTEMS such nozzles. The flow pattern in the mixing region of the injected gas and the i n c o m i n g flow in the channel has been examined b y a Schlieren method, as seen in Fig. 8. O p t i m i z a t i o n of the geometry of the channels has been carried out using a step-by-step procedure, f o l l o w i n g the flow path in the channels. T h e following hypotheses have been assumed: a) constant heat transfer coefficient obtained b y natural convection and constant temperature of the flow on the water side, and b) longitudinal exchange fluxes negligible with respect to the transversal fluxes (crosssectional averaged quantities considered). The influence of the nozzle's injection on secondary movements reinforcement was first considered b y taking into account the m o m e n tum difference between the main and the nozzle streams. But as the effect of the superposition of entrance, curvature and reinforcement factor are not yet well understood, it has been a s s u m e d that entrance effects are present after each nozzle and secondary movements have b e e n fully established. The m a i n correlations used are those of Rohsenow and Hartnett. 3~ Fig. 9 gives a typical set of results. C o m p u t a t i o n and experimentation have been found to be quite compatible. In particular, the cumulative heat exchanged by the hot gases to the water from the starting p o i n t of the channel is regularly distributed along the channel, thus s h o w i n g good heat flux distribution and optimal utilization of cast-iron.
FIG. 8. Examination of the flow pattern in a convective channel in the vicinity of an injection nozzle by a Schlieren method.
program for optimization of the convective channels.
Design of the Heat Transfer Channels In order to take advantage of the curvature effects, the reinforcement of secondary flows by combustion gas injection nozzles has been ensured by u s i n g high velocity jets t h r o u g h
D0r
800
w.-v
J
|
600
0
0
0.4
0.8
1.2
1.6
2.0
2.4
Channe! length, rn Fic. 9. Typical set of results of the heat transfer channels calculation (163 kW boiler; total heat transfer channel's length: 2.40 m; temperature of the injected gas: 1053 K). Curve 1, Temperature, K; curve 2, velocity • m/see; curve 3, heat transfered x0.1, W; curve 4, Reynolds number x0.05; curve 5, Nusselt number x20.
AN INTEGRATED BURNER-BOILER SYSTEM
Analysis of the Feed-Back Loop Influence on Stability Apart from c o m b u s t i o n - d r i v e n oscillations already investigated, ~1 the feed-back loop recirculating the flue-gas must satisfy specific conditions in order to prevent the occurrence of instabilities and oscillations. A s s u m i n g a moderate recirculation ratio (R < 90%) in order to avoid flame quenching, w h i c h reduces flame stability, and also a good mixing between flue-gas and fresh air, the characteristics of the system's components may be chosen in an optimal way in order to stabilize the system, suppressing the onset of feed-back-loop-driven oscillations. A linearized analysis of the response of the system to a slight disturbance has been carried out considering all parameters (pressures, flow rates, recirculation ratio and excess air) as time variable quantities, as assumed in Ref. 32. Moreover, as the m a i n dimensions of the system are small c o m p a r e d to the wavelength of the feed-back-loop-oscillation, at the frequencies encountered, no significant phase lag between the pressure signals at different points of the system has b e e n recorded. It is obvious that the instantaneous recirculation ratio, R* = rhf/rh a is a fluctuating quantity as well as the excess air )t = rh /rhas , where mas is the stoichiometric air mass flow rate. The analysis shows that R* varies with the pressure regime in the system, according to the values of the i m p e d a n c e s of the fresh air or flue gas circuits a n d the fan characteristic curve. To fulfill the well known Rayleigh criteria, 31 heat release and pressure in combustion chamber must vary inversely for stability.
..... .3.~ ....
C o n s i d e r i n g the fact that an increase of recirculation decreases the combustion intensity, t~ it appears that the system is stable w h e n R* and the c o m b u s t i o n chamber's pressure vary inversely. This implies a p r o p e r choice of the system's c o m p o n e n t s such as the characteristic curve of the fan and the impedances of the fresh air a n d flue-gas circuits. A simulation of the behavior of the fan has been carried out a c c o r d i n g to the correlations of Eck .3 and Eckert and Schnell. a4 The influence of instabilities on the orifices adjusting the fresh air and flue-gas mass flow rate has been found to be n e g l i g i b l e Y This stabilization process should be a p p l i e d to other purposes e n a b l i n g further investigation into the behavior of all the components to be carried out. Test and Main Results
Test Facilities F o u r test benches allow us to fire simultaneously four boilers of various p o w e r ratings. The water circuits are capable of transferring from 15 kW to 1 M W of heat. Figure 10 gives a schematic view of one test bench, showing the circuits of hot water, fuel and gases, as well as the gas s a m p l i n g location. The benches are e q u i p p e d for temperature measurements in water and gas passages in order to obtain a complete m a p p i n g of the system. Piezoelectric pressure transducers permit the measurement of pressure fluctuations in the hot combustion gas passages as well as in the burner. F u e l c o n s u m p t i o n is measured b y simply weighing the fuel injected in the system.
1 .. Boiler 2: Burner 3: External heat exchanger (finned tubes) 4: Stack with adjustable draft f.6
5: 3 - w a y
valve
6: Water pump 7: Gas analysis fines 8
,
1
??~q
71
8: Expansion vessel
2
FIG. 10. Experimental set-up for the burner-boiler system testing.
72
POWER SYSTEMS 1.50
1.t,O 0 1.30 Itl
Bachamch Index
u
t~
1.2o " ~ / /
Y ,~"~lor
I
Soot-fr.
/
/
transition /
flame
.7~rT~s~/
1.10
Sooty flame
~
//
~
I
I
I
10
20
30
BlueFlame
I " ~ " "~ I j
40
50
Recirculation ratio
I
I
I
60
70
80
90
%
FIG. 11. Various combustion regimes observed on a 60 kW boiler, with No. 2 fuel-oil (swirl number
1.1.).
Gas analyses are carried out c o n t i n u o u s l y after sampling, filtration and passage through a water trap for CO2, CO and 0 2 concentration measurements. Non-dispersive infra-red analyzers are used for CO 2 and CO and a c o n v e n tional amperometric apparatus is used for 0 2 measurement. T h e U H C s are measured by flame ionization, after sampling through a h e a t e d line. For NO x measurements, a heated sampling line is used between the s a m p l i n g probe and a set of filters a n d a water trap (liquid air vessel) a n d afterwards the line is connected to an N O , c h e m i l u m i n e s c e n c e analyzer.
16
14
"
"
~
,_ 13 *' u 12 ~, Q. ,11 U
-
E
[] CO
"1-
s0~ O ,<-
10
o Z
1.1
1.2
1.3
1.4 flame)
i
E r
FIG. 12b~ Emissions vs excess air, 30% recirculalion.
-I-
u
r
T5
Excess air, A (soot-free
9 CO 2 9 NOx 9 CO
~,~
1130
CO2 o NOx
1.o
100
15
16
Tests Carried Out tS
,ll
5O
o ~ 1o
x
o z
1.0
~
1.1
~
~
1.2
Excess
1.3
^
0
I.~
air,
Fzc. 12a. Emissions vs excess air, no recirculation (for h < 1.2. the Bacharach Index is > 1).
Steady state tests have been made at the n o m i n a l power of the system as well as plus or m i n u s 10% the n o m i n a l rating. T r a n s i t i o n from sooty to soot-free flame has been observed, as well as transition to complete blue flame operation in varying the excess air and recirculation ratio. C o m b u s t i o n noise has been measured with the burner operating on the boiler. The total mass flow rate through the burner has been kept constant i n order to compare easily the noise generated b y the b u r n e r operating with excess air only a n d the b u r n e r operating with excess air and recirculation.
AN INTEGRATED BURNER-BOILER SYSTEM
the main results gained on a 60 kW system. T h e CO 2 vs k curve is a function of the recirculation ratio a n d the curves s h o w n have b e e n computed. The emissions in the steady-state regime are reported in Fig. 12 (a to c). At zero recirculation, soot is emitted from the system If h < 1.25 whereas for recirculation ratios equal or greater than 30%, the Bacharach Index always remained at zero. C o m b u s t i o n noise abatement b y recirculation is shown on Fig. 13. The results obtained w h e n the system is operated at - 10% from its nominal rating are reported in Fig. 14. It can be noted that the overall thermal efficiency is always above 91% and is affected very little by p o w e r rating modifications. The start-up and s h u t - d o w n transient emissions are given on Fig. 15. However, due to the distance (2 m) b e t w e e n the boiler's exhaust gas collector and the s a m p l i n g tube, turbulent mixing occurs in the stack and the emissions recorded are p r o b a b l y somewhat too low. During the start-up the transition between the yellow flame and the b l u e flame at sufficient recirculation occurs in a very short time (approx. 0.5 sec).
100
is ~
*
c02
9 NOx ~ - ~ ~
+ CO *
E
~ UHC
~ ~-
o
D 11
50
~
x" o z +
+
-
1.o
u
1.2
Excess a i r , / ~
0
I~
73
I.~
( s o o t - f r e e flame )
FIG. 12c. Emissions vs excess air, 60% recirculation. Temperatures of c o m b u s t i o n gases in the convective channels have been measured with shielded thermocouples whereas hot water temperatures have b e e n recorded continuously in order to detect any hot p o i n t in the boiler. Some transient tests have been carried out in order to record the pressure variations in the combustion chamber, the pollutant emissions and water temperature variations.
Conclusions A new integrated burner-boiler system of a p o w e r rating ranging from 22 to 730 kW, firing No. 2 fuel-oil or natural gas has been designed, built and tested, taking advantage of a high swirling flame and the external recirculation of cold flue-gas.
Main Results Figure 11 gives the various combustion regimes observed and Fig. 12 (a to c) shows
_8
\
w 100 0 ~ L. rn "0 > "U C
90
80 70
60
_8 50 > 0
o
7
I? so
I-;
,
100
I 500
Frequency,
I000
, 5000
Ioooo
cps
FIG. 13. Octave Band Noise spectrum (60 kW boiler). Curve 1: Background noise; curve 2: Blue flame combustion (k = 1.10; R = 60%); curve 3: yellow flame combustion (~. = 1.7; R = 0%).
74
POWER SYSTEMS 100
.-e*
483
v
g 2
473
90
/
Itl en
E
-6 o t9
463 80 453 I
I
I
5O
60
70
Power rating,
kW
FIG. 14. Variations of the performances of the system vs power rating (nominal power rating 60 kW).
25 14
CO /~ // ,A r \Xx ~
13 12
11 9 g
__N~
70 NOx
\
UHC
,7
g6 o
I00
CO 2
5
\
5o "
o'~ z 40 "\
~U_.HC_ I
I
i
J
I
I
5
I
10
so E
I
I
"-ti
i
I
15
I
I
I
10
I"~
20
time, sec. Fro. 15. Emissions as CO2, CO, UHC and NO~ vs time during the start-up and the shut-down of the system. (Solid lines, start-up; dashed lines, shut-down). Extensive testing of the entire system confirmed the original hypotheses that the comb i n e d use of cold flue gas recirculation and swirl enables one to obtain a simultaneous reduction of NO~, CO and U H C at very low excess air levels, leading to a close control of the emission levels at optimum thermal efficiency of the system. The external recirculation, yielding an oxidant dilution and flame temperature reduction require to adopt a design where good mixing of fuel and oxidant and stable, pulsation-free combustion conditions can be ensured. It has b e e n demonstrated that the use of intense swirl allows at least twice as high firing
rates in the boiler than with a c o n v e n t i o n a l design, and that the soot-free c o m b u s t i o n allows the design with narrow convective heat exchange channels. The external recirculation can be simply used without adding any significant complexity to the entire system.
Acknowledgments This work has been carried out in the Combustion Research Laboratory of the Battelle Research Center, Geneva, Switzerland, on behalf of the Officine Termetecniehe Br6da, Italy. The authors wish to thank the Management of this Company for having granted their permission to publish this paper and
AN INTEGRATED BURNER-BOILER SYSTEM are grateful for the cooperation of many colleagues at the Battelle-Geneva Research Center during the course of the work reported here. 12. REFERENCES 1, BARRETT,R, E. AND MILLER, S. E.: Field Investigation of Emissions from Combustion Equipment for Space Heating. Environmental Protection Agency Report No. EPA-R2-73-084a, June 1973. 2. DICKERSON,R. A. AND OKUDA, A. S.: Design of an Optimum Distillate Oil Burner for Control of Pollutant Emissions. Environmental Protection Agency Report No. EPA-650/2-74-047, June 1974. 3. SCHIASSL A.: For Energy Preservation-Low Excess Air Combustion. Intern. Flame Research Foundation R e p o r t - - F 25/ha/3, January 1974. 4. BROWN,T. D.: The Performance of Vane Swirlers in Domestic Oil Burners. Canadian Combustion Research Laboratory Report--FCR 72/59, Department of Energy, Mines and Resources, Ottawa (Canada), July 1972. 5. MOORMAN,R. J. ANDLONG, C. H.: Design, Development and Testing of a Swirl Type Gas Burner with Flue-Gas Recirculation for NO x Control. Am. Soc. Mech. Engrs. Paper 73-Pwr-21, July 1973. 6. BOWMAN,C, T. AND COHEN, L. S.: Influence of Aerodynamic Phenomena on Pollutant Formation in Combustion; Volume I: Experimental Results, Environmental Protection Agency Report No. EPA-650/2-75-061-a, July 1975. 7. HEAP, M. P., LOWES, T. M. AND WALMSLEY,R.: The Effect of Burner Parameters on Nitric Oxide Formation in Natural Gas and Pulverized Fuel Flames. Paper presented at the American Flame Research Committee Flame Days, Chicago, II1., September 1972. 8. COOPER, P. W., KAMO, R., MAREK, C. J. AND SOLBRIG,C. W.: Recirculation and Fuel-Air Mixing as Related to Oil-Burner Design. American Petroleum Institute Publication 1723, May 1964. 9. ANDREWS,R. L., SIEGMUND, C. W, AND LEVINE, D. G.: Effect of Flue-Gas Recirculation on Emissions from Heating Oil Combustion. Paper presented at the 61st Annual Meeting of the Air Pollution Control Association, Saint-Paul, Minn. June 23-27, 1968. 10. HEDLEY,A. B. ANDJACKSON,E. W.: The Influence of Recirculation in Combustion Processes. Paper presented at the 1966 American Petroleum Institute Research Conference on Distillate Fuel Combustion, Chicago, Ill. June 13-15, 1966. 11. TURNER, D. W. AND SIEGMUND, C. W.: Staged Combustion and Flue Gas Recycle: Potential for Minimizing NO x from Fuel-Oil Combus-
13.
14. 15.
16.
17.
18. 19. 20.
21. 22. 23. 24.
25.
26.
27.
28.
29.
30. 31. 32.
75
tion. Paper presented at the American Flame Research Committee Flame Days, Chicago, II1., September 1972. MONAGHAN,M, T. ANn McGaAxH, I. A.: J. Inst. Petrol. 55, 303 (1969). BEER, J. M. AND CmGIER, N. A.: Combustion Aerodynamics, Applied Science Publishers Ltd., 1972. LEUCKEL, W.; Revue G~n6rale de Thermique. 84, 1367 (1968). BEER,J. M. ANDLEUCrEL, W.: Turbulent Flames in Rotating Flow Systems. Paper presented at the North American Fuel Technology Conference, Ottawa, Canada, May 31-June 3, 1970. SYREO, N., CHIGIER, N. A. AND BEER, J. M.: Thirteenth Symposium (International) on Combustion, p. 617, The Combustion Institute, 1971. LOWES, T. M., HEAP, M. P. AND SMITH, B. B.: Reduction of Pollution by Burner Design. Intern. Flame Research Foundation Report--K 2 0 / a / 7 4 , 1974. KERR, N. M. AND FRASER, D.: J. Inst. Fuel. 38, part I and If, 519, (1965). DRAKE,P. F. AND HUBBARD,E. H.: J. Inst. Fuel. 39, 98 (1966). MORI, Y. ANDNAKAYAMA,W.: Intern. J. Heat Mass Transfer, 8, 67 (1965). ITO, H.: Trans. Am. Soc. Mech. Engrs. Journal of Basic Engineering, 81, 123 (1959). MORI,Y. ANDNAKAYAMA,W.; Intern. J. Heat Mass Transfer, 10, 681, 1967. PATANr,AR, S. V., PRATAP,V. S. AND SPALDING, D. B.: J. Fluid. Mech., 62, 539 (1974). FRICKER,N. AND LEVCKEL,W.: Flow and Mixing Patterns in Gas Flames with Swirl in the Annular Air Stream. Intern. Flame Research Foundation Report G 0 2 / a / 1 8 , 1969. LEUCKEL,W.: Swirl Intensities, Swirl Types and Energy Losses of Different Swirl Generating Devices. Intern. Flame Research Foundation Report--G 0 2 / a / 1 6 , 1967. SPALDING,D. B.: Ninth Symposium (International) on Combustion, p. 833, Academic Press, 1963. GOSMAN, A. D., Put,', W. M., RUNCHAL, A. K., SPALDING,D. B. AND WOLFSHTEIN, M. W.: Heat and Mass Transfer in Recirculationg Flows, Academic Press, 1969. PATANr,Aa, S. V. AND SPALDING,D. B.: Heat and Mass Transfer in Boundary Layers, 2nd Ed., Intertext Books, 1970. LAUNDER,B. E. AND SPALDING,D. B.: Mathematical Models of Turbulence, Academic Press, 1973. ROHSENOW,W. M. ANDHARTNE'I-F,J. P.: Handbook of Heat Transfer, McGraw-Hill, 1973. PUTNA.VI,A. A.: Combustion-Driven Oscillations in Industry, Elsevier, 1971. MAUSS, F., PERTHUIS, E. AND SALE, B.: Tenth
76
POWER SYSTEMS
Symposium (International) on Combustion, p. 1241, The Combustion Institute, 1965. 33. Ecr, B.: Ventilatoren, Springer Verlag (Berlin), 1972.
34. ECKEt~T,B. AND SCHNELL, E.: Axial-und RadialKompressoren, Springer Verlag (Berlin), 1961. 35. EARLES,S. W, AND ZAREK,J. J.: Proc. Inst. Mech. Engrs. (London) 177, 997, 1963.