Boiler with cyclonic combustion

A boiler with a cyclonic combustor includes a first chamber which is substantially uncooled and refractory lined, a second chamber in fluid communication with the first chamber, ducts for supplying air and fuel directly into the first chamber and for forming a cyclonic flow pattern of hot gases for combustion within the first and second chambers, an exit throat at the end of the second chamber and a heat exchanger surrounding the second chamber for keeping low combustion temperature in both chambers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to boilers having cyclonic combustion. 
2. Description of the Related Art 
In the past, cyclone combustors have been used to produce a cyclone of 
turbulent gases within a combustion chamber for combusting various solid 
materials, including poor quality coal and vegetable refuse. Such 
combustors are disclosed in "Combustion in Swirling Flows: A Review", N. 
Syred and J.M. Beer, Combustion and Flame, Volume 23, pages 143-201 
(1974). A fluidized bed boiler having a cyclonic combustor is disclosed in 
U.S. Pat. No. 4,457,289 to Korenberg. These documents are incorporated by 
reference in this application. A fire tube boiler having a cyclonic 
combustor has been commercially marketed by Cyclotherm Division, Oswego 
Package Boiler Co., Inc. 
Although providing high specific heat release, known adiabatic cyclone 
combustors have the disadvantages that combustion temperature is high and 
NO.sub.x emissions are high. Combustion is unstable at low capacity 
burning and high hydrodynamic turndown ratios are not possible in 
non-adiabatic combustors. 
The hydrodynamic turndown ratio of the boiler is defined as the ratio of 
pressurized air flow at maximum load to pressurized air flow at minimum 
load and measures the ability of the boiler to operate over the extremes 
of its load ranges. A high turndown ratio would allow a wide range in the 
level of steam generation at a particular time. A wide range of steam 
generation is important to most efficiently allow the boiler to respond to 
varying steam demands. 
It is an object of the present invention to provide a boiler utilizing 
cyclonic combustion and having a very high specific heat release, low 
excess air and a relatively low combustion temperature at low CO and 
NO.sub.x emissions. 
It is also an object of the present invention to provide a boiler utilizing 
cyclonic combustion and which is stable at low capacity burning. 
It is another object of the present invention to provide a boiler utilizing 
cyclonic combustion and having a high turndown ratio. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims. 
SUMMARY OF THE INVENTION 
To achieve the foregoing objects, and in accordance with the purposes of 
the invention as embodied and broadly described herein, there is provided 
a boiler having a cyclonic combustor, comprising a first chamber having a 
front end, a rear end and a substantially cylindrical longitudinally 
extending outer wall which is substantially uncooled and refractory lined; 
a second chamber having a front end, a rear end, and a substantially 
cyclindrical longitudinally extending outer wall, the rear end of the 
first chamber in fluid combination with and substantially longitudinally 
aligned with the front end of the second chamber; means for supplying air 
and fuel directly into the first chamber and for forming a cyclonic flow 
pattern of hot gases for combustion within the first chamber and the 
second chamber; a substantially cyclindrical exit throat at the rear end 
of the second chamber aligned substantially concentrically therewith for 
exhausting hot gases from the second chamber, the exit throat having a 
diameter less than the inner diameter of the second chamber; and heat 
exchange means surrounding the second chamber for substantially cooling 
the wall of the second chamber without substantially cooling the wall of 
the first chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention as illustrated in the accompanying drawings. 
In accordance with the present invention, there is provided a boiler having 
a cyclonic combustor comprising a first chamber having a front end, a rear 
end and a substantially cyclindrical longitudinally extending outer wall 
which is substantially uncooled and refractory lined; a second chamber 
having a front end, a rear end and a substantially cyclindrical 
longitudinally extending outer wall, the rear end of the first chamber in 
fluid communication with and substantially longitudinally aligned with the 
front end of the second chamber; means for supplying air and fuel directly 
into the first chamber and performing a cyclonic flow pattern of hot gases 
for combustion within the first chamber and the second chamber; a 
substantially round exit throat at the rear end of the second chamber and 
aligned substantially concentrically therewith for exhausting hot gases 
from the second chamber, the exit throat having a diameter less than the 
inner diameter of the second chamber; and heat exchange means surrounding 
only the second chamber for substantially cooling the wall of the second 
chamber without substantially cooling the wall of the first chamber. 
FIG. 1 shows a horizontally disposed fire tube boiler having a cyclonic 
combustor in accordance with one preferred embodiment of the invention. 
The combustor includes a central fire tube, also known as a Morison tube, 
with a combustion chamber 21 including areas defined by first chamber 22 
and second chamber 24. First chamber 22 includes a front end 26, a rear 
end 28 and a substantially cylindrical longitudinally extending outer wall 
30 which is substantially uncooled and refractory lined. 
Second chamber 24 has a front end 32, a rear end 34 and a substantially 
cylindrical longitudinally extending outer wall 36, which is preferably 
constructed of metal. The rear end 28 of the first chamber 22 and the 
front end 32 of second chamber 24 are in fluid communication and 
longitudinally aligned with each other. 
Means for supplying air and fuel directly into first chamber 22 such as 
plenum or manifold 38 and tangential nozzles 40 form a cyclonic flow 
pattern of hot gases within the reaction chamber defined by first chamber 
22 and second chamber 24. The fuel, which is preferably liquid or gaseous, 
is introduced tangentially by nozzles 43 and may additionally or 
alternatively be introduced into first chamber 22 radially by nozzle 41. A 
substantially cylindrical exit throat 42 is positioned at rear end 34 of 
second chamber 24 and aligned concentrically with second chamber 24 so 
that the exit throat has a diameter less than the inner diameter of second 
chamber 24. As embodied herein, a source of pressurized air such as a 
blower (not shown) feeds air from plenum or manifold 38 through nozzles 40 
into the first chamber 22. 
In accordance with the invention, it is critical that the cross sectional 
area of the tangential air nozzles 40 and the geometric characteristics of 
first chamber 22 and second chamber 24 be adapted to provide a Swirl 
number (S) of at least about 0.6 and Reynolds number (Re) of at least 
about 18,000 which are required to create a cyclone of turbulence in first 
chamber 22 and second chamber 24 when operating at maximum capacity. On 
the other hand, the Swirl number and Reynolds number at maximum capacity 
must not exceed those values which would result in an unacceptable 
pressure drop through the tangential air nozzles 40 and the combustion 
chamber constituted by first chamber 22 and second chamber 24. 
It is the cyclone of turbulence which enables the achievement of specific 
heat release values up to and higher than 3.5.times.10.sup.6 Kcal per 
cubic meter per hour and NO.sub.x concentration in flue gases of about 
60-120 ppm and about 120-180 ppm when firing natural gas and light fuel 
oil, respectively. Exit throat 42 and the interior of second chamber 24 
must exhibit certain geometric characteristics together with the cross 
sectional area of the tangential air nozzles of first chamber 22 in order 
to provide the requisite Swirl and Reynolds number. 
All of the above-noted features are explained in greater detail below and 
are discussed generally in the article by Syred and Beer mentioned above, 
and the references noted in that article which are hereby incorporated by 
reference. 
If the combustion chamber 21 comprising first chamber 22 and second chamber 
24 is designed and operated so as to achieve a Swirl number of at least 
about 0.6 and a Reynolds number of at least about 18,000 in such chamber 
and the ratio of the diameter of the exit throat 42 (De) to the diameter 
of the inner wall of the second chamber 24 (Do), i.e., De/Do defined 
herein as X, lies within the range of about 0.4 to about 0.7, the first 
chamber 22 and second chamber 24 will, during operation, exhibit large 
internal reverse flow zones with as many as three concentric toroidal 
recirculation zones being formed. Such recirculation zones are known 
generally in the field of conventional cyclone combustors. This coupled 
with the high level of turbulence results in significantly improved heat 
exchange and, therefore, a relatively uniform temperature throughout 
combustion chambers 22 and 24. 
The value of ratio X preferably lies within the range of about 0.4 to about 
0.7 because as X increases, the pressure drop decreases through the 
combustion chamber and the Swirl number increases. Higher values of X are 
preferred. However, for values of X in excess of 0.7, the internal reverse 
flow zones are not formed sufficiently. 
Heat exchange means surround second chamber 24 for substantially cooling 
the wall 36 of second chamber 24 without substantially cooling the wall 30 
of first chamber 22. The heat exchange means preferably includes an outer 
boiler shell 48, gas tubes 50 between outer shell 48 and Morison tube 54 
for conducting hot gases from second chamber 24, and space 52 within shell 
48 outside gas tubes 50 and second chamber 24 for conducting water which 
is heated by the heated gases in the second chamber 24 and the gas tubes 
50, all in a conventionally known manner. 
Stable combustion, even at low boiler capacity, is achieved by not cooling 
the walls of first chamber 22 where the air and fuel are injected, but 
cooling only the walls of the second chamber 24. This stable combustion 
enables high turndown ratios to be accomplished. For example, as a result 
of this construction, the turndown ratio can be increased from 4:1 up to 
and higher than 10:1. Excess air can be decreased from 25-30% to 5% and 
kept constant at 5% over the high turndown ratio of 10:1. The flame 
temperature can be decreased to 2000.degree. F. and lower, as opposed to 
about 3000.degree. F. for conventional fire tube boilers. Therefore 
NO.sub.x emission is lower than in the standard burner/boiler unit due to 
the lower flame temperature and lower excess air. 
The central fire tube preferably includes a cylindrical tube 54 extending 
from, aligned and continuous with wall 36 of second chamber 24. Hot gases 
from the second chamber 24 pass through exit throat 42 into cylindrical 
tube 54. 
The heat exchange means also preferably includes means such as rear 
compartment 56 for directing the flow of hot gases exiting cylindrical 
tube 54 from second chamber 24 through a first predetermined set of the 
gas tubes 50 such as those in the lower part of FIG. 1 and front 
compartment 58 for directing the hot gases from the first set of gas pipes 
50 to a second set of gas pipes 50 in the upper portion of FIG. 1 in the 
opposite direction. This is shown by the arrows indicating gas flow and is 
conventionally known for fire tube boilers. 
As shown in FIG. 3, the means for supplying air and fuel preferably 
includes separate conduits for supplying air and fuel separately and 
directly into first chamber 22 and for mixing and combusting them in the 
first and second chambers 22 and 24. 
The ratio between the length of the first and second chamber 22 and 24 
affects the combustion temperature within the combustion chamber 21. In 
general, as the length of first chamber 22 decreases relative to the 
length of second chamber 24 the combustion temperature decreases. This 
ratio is important because lowering the combustion temperature lowers the 
NO.sub.x formation. This ratio for natural gas and fuel oil is normally 
less than 1.5 for a low capacity combustor and can become less than 0.5 
for a very high capacity combustor. It is preferable that the first to 
second chamber length ratio is substantially in the range of about 0.2:1 
to 1.5:1. Because of this, the combustion temperature can be less than 
2000-2200.degree. F. even for high capacity combustion. 
In order to prevent damage to metal wall 36 of second chamber 24 due to 
overheating, the front end portion 32 of wall 36 is lined with a 
refractory material 60. 
Although it is preferable that the cyclonic combustor described above be 
positioned within a boiler system, it is contemplated that it can be used 
for purposes of combustion or reaction without substantial boiler 
apparatus. Such a cyclonic combustion comprises a first chamber having a 
front end, a rear end and a substantially cylindrical longitudinally 
extending outer wall which is substantially uncooled and refractory lined; 
a second chamber having a front end, a rear end and a substantially 
cylindrical longitudinally extending outer wall, the rear end of the first 
chamber in fluid communication with and longitudinally aligned with the 
front end of the second chamber; means for supplying air and fuel directly 
into the first chamber and for forming a cyclonic flow pattern within the 
first chamber and the second chamber; a substantially cylindrical exit 
throat at the rear end of the second chamber and aligned substantially 
concentrically therewith, the exit throat having a diameter less than the 
inner diameter of the second chamber; and means for substantially cooling 
the wall of the second chamber without substantially cooling the wall of 
the first chamber. 
Additional advantages and modifications will readily occur to those skilled 
in the art. The invention in its broader aspects is, therefore, not 
limited to the specific details, representative apparatus, and 
illustrative examples shown and described. Accordingly, deparatures may be 
made from such details without departing from the spirit or the scope of 
applicant's general inventive concept.