Note: Descriptions are shown in the official language in which they were submitted.
3~
22819-504D
This is a division of our co-pending Canadian Patent
Application No. 423,544 filed on March 14th, 1983.
This invention relates to fast fluidized bed boilers.
More particularly, the invention relates to novel and improved de-
sign and control fea-tures of such boilers.
Fluidized bed boilers recent.ly have become of great
interest as a very attractive means of generating heat. Many
bubbling or turbulent fluidized bed boilers have proved to operate
very satisfactory.
However, when it comes to fast fluidized bed boilers,
serious problems have to be solved, particularly with regard to
heat transfer, particle separation and control of operation.
Thus, the common use of cyclone particle separators means
that the boiler has to be built from separated parts and cooling
systems. Also, cyclone particle separators have very huge dimen-
sions, at least at high boiler powers, giving serious lay-out
problems.
Also, existing power control systems which primarily con-
trol fuel feed as the load varies, tend to be inaccurate and to
give so-called over shoot.
It is previously known that ordinary fluidized bed
boilers can be provided with other particle separators than cyclone
particle separators; cf. for instance GB A 2 046 886 which dis-
closes a fluidized bed boiler having a baffle type separator locat-
ed in the effluent gas pass. The amount of separated material is
of course very low.
AEP 1170 CAN/IH DIV 1 ~ s
22819-504D
However, with regard to fast fluidized bed boilers,
those skilled in the art have been convinced that particle
separators of the cyclone type are necessary in view of the very
large amount of particles to be separated and the high velocity of
the gas-particle mixture.
SUMMARY OF INVENTION
It is an object of this invention to provide a novel and
improved fast fluidized bed system whereby the above-mentioned
problems are overcome and additional advantages are obtained, as
will be apparent from the following.
The invention provides in a process for operating a fast
fluidized bed boiler including a reactor, means for establishing
a fluidized bed within the reactor, a separator connected to the
reactor for receiving a gas-particle mixture from the reactor, the
improvement comprising mechanically separating particles from the
gas particle mixture in a non-centrifugal mechanical separator and
recirculating particles to the reactor through a path external of
the reactor.
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22819-504D
The boiler is designed as a completely cooled and inte-
grated unit, including the primary particle separator, being a
mechanical separator (preferably of a labyrinth type) not a cy-
clone separator. It has been found that a mechanical primary
particle separator of the labyrinth type can handle the very large
amount of circulating material, while simultaneously being inte-
grated both into a compact boiler structure and into one and the
same cooling system.
Recirculating the separated material via the non-
mechanical L-valve, enables accurate control of the recirculation
rate. In a preferred embodiment, separated material is collected
in a material or particle storage prior to being recirculated into
the reactor via said control valve. The storage can be integrated
with the boiler.
The boiler operation is controlled by maintaining the
reactor bed temperature essentially constant, or within a relative-
ly narrow temperature interval, by regulating the recirculation
rate dependent on the boiler load.
The above-mentioned aspects, as well as further
importan-t features of the invention, will be more fully under-
stood by the following description of presently preferred
but nonetheless illustrative embodiments of
,~
the invention, when taken in connection with the accomp-
anying drawings.
BRIEF DESCRIPTION OF THE DRAlVINGS
Fig. 1 is a schematic view of a preferred embodiment
of a fast fluidized bed boiler in accordance with the in-
vention.
Fig. 2 is a schematic perspective view of a fast
fluidized bed boiler, generally in accordance with Fig. 1,
further illustrating the novel design of the boiler.
Figs 3 and 4 are very schematic side and top views,
respectively, of the upper part of a fast fluidized bed
boiler according to the invention, illustrating the gene-
ral operation of an integrated mechanical labyrinth type
particle separator.
Fig. 5 is a schematic block diagram illustrating a
first embodiment of a novel method of controlling the
operation of a fast fluidized bed boiler in accordance
with the invention.
Fig. 6 is a schematic block diagram illustrating a
second embodiment of a novel method of controlling the
operation of a fast fluidized bed boiler in accordance
with the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
General design and operation
Referring to Figs. 1-4, the boiler disclosed there-
in comprises the following main parts:
1. Startup burner
2. Air plenum
3. Bottom section
4. Reactor
5a,5b. Primary particle separator
6. Particle storage
7. U-pass, settling chamber, radiation heat
8. L-valve exchanger
9. Stand pipes
10. Convective heat exchanger
11. Secondary cyclone separator.
12. Fuel feed
13. Primary air supply
14. Secondary air supply.
15. Dome
81~
The boiler is built according to common boiler
manufacturers standards. The unit is completely cooled,
with standard membrane walls having water tubes. The
particle separator, particle recirculation line and
convective heat transfer parts are integrated with the
boiler body. The boiler is designed as a "slice" or
module, making it possible to increase power by connect-
ing several modules in parallel, with or without cooled
partitions.
The main combustion takes place in reactor 4 into
which gases and bed material enter from bottom section 3.
The bottom section is mainly a mixing and precombustion
chamber. Primary air is introduced through bottom nozzles
into the bottom section 3, while secondary air is supplied
~ 15 at the transition between the bottom section and the reac-
; tor. Fuel is introduced in the bottom section. Particles
are mainly separated in the particle separator. Bed ash
is 7ithdrawn from particle storage 6. New bed ma-terial is
fed into the particle storage as well. Additives come with
` 20 the fuel, typically coal.
The startup burner is oil fired, and burner air is
pri~ary air to the bed during operation.
The boiler is preferably connected to a heat ex-
changer for district heating.
The bed is designed to obtain a typical power dis-
tribution between the different cooling surfaces as
follows:
Bottom section ~ 10 %
Reactor ~ 45 ~0
Particle storage ~ 3 ~0
Convection parts ~ 30 O
Particles leaving reactor 4 are separated in three
stages and circulated, before final clean up in a bag
filter ~not shown). The first stage is the integrated
particle trap 5. The second stage is a settling chamber 7
combined with a change of gas direction below the convective
part. The third stage is an ordinary tangential cyclone 11
5 _
following the convection pass. Recirculation from separa-
tor 5 is provided via particle storage 6, into which the
separator feeds the particles directly. Settling chamber
7 and cyclone 11 feed the particles via stand pipes 9
directly into bottom section 3. The recirculation rate
from particle storage 6 is controlled by means of L-valves
8. The recirculation rates from settling chamber 7 and
cyclone 11 need not be controlled.
Bottom section 3 is enlarged to provide lower flui-
dizing velocities, and thus longer retention times. How-
ever, the velocity is high enough to provide a good mix-
ing of bed material and fuel. The bottom section consists
of membrane tubes as well. The bottom section is merely a
widening of the reactor, and ends with the secondary air
inlets.
Secondary air is introduced through holes in the
mcmbran~ walls of reactor 4.
Generally bottom section 3 should be as low and wide
~ possible to give low pressure drop and good mixing and
'~ng retention times. The height of reactor 4 is non-
-critical and should only be as high as wanted for any
c~nversion quotia for any specific fuel.
Reactor
A typical, reactor of a 2.5 MW boiler module is 6 m
high and 0.7 x 0.7 m in cross section. The walls are weld-
ed finned vertical tubes forming membrane walls.
Immediately at the entrance secondary air is intro-
duced. The air inlets are actually the spacing between
the tubes, with fins removed. These openings are provided
over the entire width of the front wall and back wall.
Two sets of openings are made to reduce pressure drop
when the bed is operated in the~cold mode. During warm
operation, one set of openings is used. The air velocity
is approximately 20 m/s in the slots.
The walls are absolutely smooth in the vertical
direction to avoid erosion on the tubes. Cooling is
provided by the membrane walls.
-- 6 --
38~3
The height of the reactor is chosen to provide a
reasonable burnout of the char particles. Since total
burnout is not achievable, there is no reason to make the
reactor too high. Typically, the width is chosen to give
2.5 MW output at ~ 8 m/s with 30 % excess air.
Mean gas speed is between 5 and 10 m/s, typically
about 8 m/s. Mean density of the gas-particle mixture
typically is from 5 to 100 kg/m3. Mean particle size
typically is 0.1 - O.S mm.
Pressure drop in the reactor is almost only depend-
ing on the mean density of the particle mixture and is
typically 3000 Pa.
-; The top of the reactor, which is the entrance to
particle separator 5 is quite simply a 90 degree bend,
rounded to give som smoothening of the change of direction.
The reactor walls continuously goes on into the particle
separator and bottom section walls.
Bottom section
As stated above, bottom section 3 is actually only
a widening of the reactor. The horizontal area is twice
the cross section of reactor 4. The height is determined
by the secondary air inlet and can be varied between 0.8 m
;~ and 1.2 m. The widening is directed by the desire to maxi-
mize the retention time for particles and still keep the
vertical height as low as possible. The bottom section
contributes to the pressure drop significantly. Pressure
drop for the bottom section typically varies from 15 kPa
to 7 kPa.
The distributor or bottom plate 21 is formed of
finned tubes and is only a prolonged part of the front
wall 23. These wall tubes continue further and form
back wall and floor 25 of air plenum 2. Thus air plenum 2
is cooled and can sustain a luminous flame from a startup
burner l.
Bottom section 3 serves several purposes such as
mixing chamber, sub-stoichiometric combustion/pyrolysis
area, dynamic dampener and low load combustion part.
-- 7 --
~2~8~3~
All recirculation lines 9 enter below the secondary
air inlet in the back wall. L-valves 8 and stand pipes 9
from settling chamber 7 and cyclone 11 extend through the
wall and end flush to the wall. Fuel is fed into bottom
section 3, which is advantageous for mixing, and residence
time causes. Also the NOX reduction and Sx reduction
abilities are favoured by this design. The back pressure
on the fuel line is though the highest anywhere in the
system. Fuel can be fed by a screw feeder through the wall.
Air distribution into the bottom section is provided
by air nozzles welded to the tube fins, with a 100 mm
spacing. The nozzles are formed of a short tube and a
head. The head is cylindrical with six holes drilled on
the circumference~ The holes are angeled downwards 15
degrees. preventing back leaking of bed material. The
pressure lrop is obtained in the holding tube, not in the
holes, t^ minimize jet momentum of the air.
Pri..ary particle separator
Particle separator 5 is located at the transition
between the top of reactor 4 and the first part of the
first vertical pass 7a into settling chamber 7. The sepa-
rator includes two sections~ the first one placed verti~
cally above particle storage 6 and the second one placed
in said first part of vertical pass 7a. Separator 5a
consists of vertically extending staggered U-beams and
separator 5b consists of similar U-beams extending in-
clined relative to the vertical direction. The U-s are
open towards the circulating gas-particle mixture.
Particle separator 5 separates the bed material and com-
bustion gas in a first step. The efficiency of the sepa-
rator depends on gas velocity and particle load. High
particle loading and low velocities increases efficiency.
Usually efficiency has proved to be better than 95 0.
The separated particles are fed directly into par-
ticle storage 6, whereas the gas continues to the vertical
radiation and conduction pass and the settling chamber 7.
-- 8 --
31~
The particle separator operates as a particle trap
or actually a labyrinth. The U-beams, positioned in a
staggered pattern provide the trapping. This makes the
gas to flow in serpent like stream lines while particles
tend to go straight and into the verticaliy positioned
or generally downwards directed U-beams. When the par-
ticles make impact on the bottom of the U they either
bounce back and come back or just fall down along the
beam which ends in a hole in the roof or back wall of
the particle storage.
The above-mentioned separation effect is illustrated
in Figs. 3 and 4, wherein arrows 31 indicate separated
material falling into particle storage 6.
This type of particle separator is truly two
dimensional, which is necessary for enabling/up or down
and still keeping the whole separator completely integrat-
ed with the totally cooled boiler body. Further this laby-
rinth type of separator has proved to/most suitable to
handle the extremely high particle loadings (such as 100
kg/m3) of a fast fluidized bed.
The particle separator beams are not cooled, and
they are only fixed len~hwise at their top end, suspended
in the cooled boiler roof and the cooled wall 7c between
the down- and upgoing passes 7a, 7b, respectively. The
other end of each beam is loosely fixed just by passing
through corresponding holes into particle storage 6.
TheU-beams above the particle storage roof are slight-
ly angled to the vertical direction, while the U-beams in
downward pass 7a typically have a 60 degree angle to the
horisontal direction.
Finally, with regard to the mechanical primary
particle separator it is to be emphasized that the word-
ing "labyrinth" is to be interpreted in a broad and
general way and, thus, is to include also so-called
shutter type separators, for instance, or in other
words gravity and momentum separation configurations,
that is, non-centrifugal configurations.
_ g
~2~1Q88~
Particle storage
_
Particle storage 6 is place6d ~between reactor 4 and
downward pass 7a. The comMon walls/are cooled. The cross
section is approximately 0.5 x 0.7 m. The storage volume
is determined by a suitable drain, which is the ash
withdrawal component. The particles enter the storage
from the roof and from the upper one third part of the
back wall. Particles leave the storage through exit holes
26 to the stand pipes. These holes do not cover more than
half the floor. The rest is a distributor plate for a
fluidizing agent, in this case air.
The storage can thus be fluidized. The amount of
air necessary is only 0.2 ~ of the main air supply.
The bed in the storage does not have to be fluidized
for any other reasons than providing smooth feeding to the
ash lin~ (not shown) and to the stand pipes leading to
L-valve~:8.
The height of the storage bed is determined by the
~ressuro-drop in the bottom section and reactor. The
volume is determined by the need for variations of materi-
al content in the reactor. In the embodiment disclosed
this gives a typical total particle content in the boiler
of approximately 1 metric tonne and a stored mass of
approximately 500 kg.
For startup and renewing of bed material a simple
hole having a valve can be provided in the wall of the
storage.
Settling chamber and radiative heat exchanger_
The gases passing through particle separator 5
enter a downward "empty" pass 7a. This pass is ended by a
conical bottom 28, where the gases pass an U-shaped bend
into upward pass 7b. The volume at the conical bottom acts
as a settling chamber, where some additional particles
are separated.
The downward vertical pass 7a thus does not include
any inserted heat exchangers. The walls are cooled and
quite dust loaded gas passes which implies that there is
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,,
both radiative and convective heat transfer to the walls
in approximately equal proportions.
The conical bottom can be uncooled partially.
Particles are drained through a simple hole into a stand
pipe9a directly into bottom section 3.
Convective heat exchanger
Convective heat exchanger 10 is located in the up-
ward pass 7b of the U-formed exhaust boiler part. The
walls are incorporated in the cooling system. The heat
exchanger includes a "flag" tyye of tubing, with no widen-
ing of the spacing but with in line pattern instead of a
staggered pattern.
L-valves and stand pipes
The particles from particle storage 6, settling
chamber 7 and cyclone 11 are reintroduced into bottom
section 3 via stand pipes 9. Usually these pipes are not
separately fluidized, but still operate in a fluidized
mode due to the operating conditions of the boiler.
The flow fromparticle storage 6 is controlled by
non-mechanical valves, so-called L-valves. These valves
are control valves, which can be shut off completely.
Flow rate is controlled by an external air supply just
above the sharp bend of the L, as indicated at 35. L-valves
are developed for granular material flow control in in-
dustry. However, they have proved to be most suitable in
this application, with high temperature material.
The controllability range is approximately 1 to 10.
During startup of the boiler there is need for a recircu-
lated flow rate of 1 to 5 ~ of the flow rate during
operation. Therefore a separate startup valve suitably is
installed in addition to the two full range valves used.
The cross-section of the full load stand pipes and L-
valves is square, while the small startup valve is
circular.
The three valves each have a separate air supply 35
and thus can be independently controlled. The horisontal
parts of the valves are approximately 4 times the stand
pipe diameter to enable an almost complete shut off, main-
taining the fu]l load capacity. The pressure drop back-
wards through the L-valve is somewhat smaller than the
total length of the horizontal and vertical parts, multi-
plied by the bed material density.
With regard to the design of L-valves, reference is
made to, for instance, "Solids flow control using a non-
mechanical L-valve" by T.M. Knowton and I. Hirsan, Insti-
tute of Gas Technology, Chicago, U.S. This is a paper
presented at the Ninth Synthetic Pipeline Gas Symposium,
Chicago, Illinois, Oct 31 - Nov 2, 1977, the contents of
this papper hereby being incorporated by reference.
It should be clear that the wording "so-called L-
valve" includes modifications such as J-valves giving the
same controllabilit~ by using concurrent fluidization.
The stand pipes7from settling chamber 7 and cyclone
11 have an angle to the horizon exceeding about 60 degrees~
; To avoid back flow of gas it is important that the pipes
end below the secondary air inlet, where a turbulent bed
exists. This means that a plug of bed material always
builds up in the stand pipe to balance the pressure
difference between the bottom section and the inlet of the
stand pipe. Thus the stand pipe length projected vertical-
ly must exceed the height corresponding to the bottom
section pressure. Then the back flow of gas through the
stand pipes cannot exceed the flow necessary for minimum
fluidization, which is very small compared to the main
gas flow.
The particle mass flow from cyclone 11 and settling
chamber 7 is thus uncontrolled but selfadjusting to keep
the material level at the one directed by the pressure
drop in the rest of the system.
All stand pipes and L-valves are uncooled and in-
sulated.
The construction of the stand pipes may differ
widely, but the basic principle must be kept: hig}l angle
and smooth tubing with bottom end connected to a dense bed
region.
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388~
Cyclone
Cyclone 11 is a standard cyclone for high dust load-
ings. It operates at temperatures below 250C and is thus
a non-critical component. It should have a high efficiency
just to keep the dust loading low for subsequent bag
filter (not shown)~
The cyclone feed particles directly back into the
stand pipe and gas leaves directly to the conventional bag
house.
Ash withdrawal system
Ash and sorbents are continuously enriched in the
bed. The mineral matter have to be removed continuously.
The carbon concentration in the bed material is in the
order of 1 %. Depending on conversion quotia the concen-
tration varies, but all the time the concentration is low-
est in the recirculation lines. The bed material is thus
suitably drained from the particle storage.
Control of operation
The heat transfer in the reactor is variable and
controllable. The heat transfer in the rest of the system
~ varies according to common rules.
; The coal concentration in the bed material varies
and depends on:
Particle size
Oxygen concentration
Bed temperatures.
This implies that for every load and excess air
ratio there exists a given carbon concentration. If the
temperature is increased this leads to a lower "equili-
brium" concentration. If different fuels are fired the
balance of the cooling effect between the reactor and
convective parts is altered if the bed temperature is
remained at 850C for optimum combustion results, If the
Wlll
load changes the temperatu~-s of the cooling water/change.
This does not affect the cooling effect significantly.
Conventionally, a load change is allowed to change
the fuel feed, which in its turn affects the carbon
L~8~8
concentration in the bed initially. This will give a
change in generated heat by chemical reaction, and
of course a change in temperature of the bed. This
leads to an alteration of the cooling effect.
However, an increase of the fuel flow means that
the carbon concentration, liberated heat and temperature
also increase. With an elevated temperature, however, an
even lower carbon concentration than the initial is
called upon for equilibrium. This leads to a typical
"over shot" in control.
The classical way of control where boiler load
controls the fuel feed and air flow thus does no~ work
very well with fast fluidized beds. The link connecting
chemical reaction and energy production is missing,
giving the draw backs of over shoot and cocurrent initial
reaction.
Now, actually it is not the carbon concentration in
the bed which controls the amount of chemical reaction,
but the mass content in the combustion zone. If the cir-
culation flow rate is increased, this automatically leads
to increased density in the reactor. Increased density
` means also increased mass of carbon in the reactor.
An increase of circulation rate thus leads to both
a higher heat flux to the walls and a higher heat pro-
duction, until the carbon is consumed. This phenomenon
is rapid and much faster than the way of increasing
; carbon concentration by increasing fuel feed.
Consequently, in accordance with the invention it
is suggested that the recirculation rate should be in-
cluded in the control system, bearing in mind that the
fast fluidized bed does not react as any other boiler type
due to the accumulated carbon in the bed material and the
change in heat transfer with particle recirculation mass
flux.
Thus, the novel control method in accordance with
the invention means that the temperature of the bed mate-
rial is maintained essentially constant, or within a
-- - 14 -
88~3
relatively narrow temperature interval, by regulating
the recirculation rate dependent on the boiler load.
Typically, the bed temperature is maintained within a
50C interval, the interval optionally being set within
an operating temperature interval of from about 700C
to about 900C.
Two preferred embodiments of the method involving
recirculation rate regulation are envisaged.
The first embodiment means that the boiler load
regulatesthe recirculation rate or flow, effecting the
bed temperature~ while bed temperature regulates the
supply of fuel and air to the reactor.
The second embodiment means that the boiler load
regulates the supply of fuel and air to the reactor,
while the bed temperature regulates the recirculation
rate or flow.
This novel method gives almost instantaneous response
to a load variation, and the problems with the missing
link, the heat transfer and over shoot and negative res-
ponse are eliminated.
The regulation of the supply of fuel and air suitably
means that primarily the fuel flow is controlled, the air
flow being controlled in response to the fue~ flow. How-
ever, the opposite way is also possible.
As should be realized, the carbon mass content, tem-
perature and oxygen concentration in the reactor determine
the amount of fuel combusted. Therefore, if the air flow
is controlled only by the fuel flow, a fluctuating excess
air ratio will result. Thus, the oxygen concentration also
should control the air flow.
The above-indicated two embodiments of the novel
method according to the invention are illustrated on
Figs. 5 and 6, respectively.
The two control systems of Figs. 5 and 6 both com-
prise the following main components; Dome pressure sensor
51; dome pressure regulator 53; L-valve air flow regulator
55; L-valve air flow control valve 57; L-valve air flow
- - 15 -
i8~
sensor 59; bed temperature sensor 61; bed temperature
regulator 63; fuel flow regulator 65; total air flow regu-
lator 67; total air flow control means 69; total air flow
sensor 71; secondarY air flow. regu~ator 73; secondary air
~low control means ~/5 secondary alr
flow sensor 77; secon~ary air ratio adjuster 79; O2-sensor
81; O2-regulator 83; and fuel-total air flow-ratio adjuster
85. All regulators are of PI~ type.
Referring now to Fig. 5, the power need is measured
by dome pressure sensor 57. An increasing dome pressure
indicates that the combustion power exceeds the power con-
; sumed by the load and vice versa.
The difference between the dome pressure actual
value and the dome pressure reference value (set manually)
is the input to dome pressure regulator 53, the output of
which controls the recirculation rate by being one input
(reference value) to L-valve air flow regulator 55, the
other input being the L-valve air flow actual value re-
ceived from sensor 59. Regulator 55 controls valve 57 and
thus the L-valve air flow.
Thus the dome pressure controls the recirculation
such that instantaneous changes of the power need are met
by changes of the particle density of the reactor. Increa-
sed particle density means improved heat transfer which
in its turn means that the bed temperature decreases,
requesting increased fuel and air flow. Decreasing partic-
le density gives the opposite effect.
The bed temperature is measured by means of sensor
61, thus giving an actual bed temperature value as one
input to bed temperature regulator 63, -the other input
being a reference value set manually. The output of regu-
lator 63 is the fuel reference value and is fed to fuel
regulator 65. The fuel system is a separate system provid-
ing a constant fuel flow in accordance with the -fuel
reference value.
The output of regulator 65, that is, the actual
fuel flow value, controls the air flow and thus is one
input to total air flow regulator 67. The air flow is
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B
controlled such that a suitable ratio between air and fuel
is obtained. The other input to regulator 67 is obtained
from total air flow sensor 71, via 2 adjuster 85. I`hus,
said ratio is adjusted dependent on the 2 content of the
flue gases. The 2 content is measured by means of sensor
81, giving one input to 2 regulator 82, the other input
to regulator 83 being a manually set 2 reference value.
The output of 2 regulator 83 controls adjuster 85. Regu-
lator 67 controls the total air flow by means of a suit-
able flow control means 69.
The total air flow includes the primary air flow
and the secondaTy air flow. The secondary air flow is
controlled by secondary air flow regulator 73 operating
on a suitable flow control means 75. The reference input
to regulator 73 is the actual total air flow value ob~in-
ed from sensor 71, the other input being obtained from
actual secondary air flow sensor 77 via manual adjuster
79. ~djuster 79 makes it possible to set a desired ratio
between secondary air and total air.
Referring to Fig. 6,,the system shown differs from
that of Fig. 5 in two respects. Firstly, the output of
dome pressure regulator 53 is the fuel flow reference
value instead of the L-valve air flow reference value.
Secondly, the output of bed temperature regulator 63 is
the L-valve air flow reference value instead of fuel flow
reference value. Also, bed temperature regulator 63 is
adaptive and has externally controlled parameters, for
instance dependent on boiler power. The total and secon-
dary air flow control is the same.
Finally, it is to be emphasized that a control method
in accordance with the invention means that various fuels
can be fired without any changes being necessary with
regard to the boiler or fuel supply means. In fact, ope-
rational effects due to fuel substitution quite easily can
be handled by regulating the recirculation rate.
While certain preferred embodiments of the invention
have been described, it should be apparent that various
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~2~
modifications, changes and substitutions are possible
within the spirit and the scope of invention as given
by a broad interpretation of the appended claims.
. ..
.
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