CA1271945A

CA1271945A - Fines recirculating fluid bed combustor method and apparatus

Info

Publication number: CA1271945A
Application number: CA000490743A
Authority: CA
Inventors: Gary O. Goldbach; Jerry L. Cooper; John Louis Guillory
Original assignee: Combustion Power Co Inc
Current assignee: Combustion Power Co Inc
Priority date: 1984-09-14
Filing date: 1985-09-13
Publication date: 1990-07-24
Anticipated expiration: 2007-07-24
Also published as: EP0176293A2; DK429885D0; JPS61105007A; DK429885A; JPH0799250B2; EP0176293B1; EP0176293A3; DE3582385D1; NO853713L

Abstract

Fines Recirculating Fluid Bed Combustor Method and Apparatus Abstract A fluidized bed combustor for burning solid fuel or dirty liquid fuels comprised of a bubbling fluid bed with tubes in the bed, a hot recycle cyclone, a convective heat exchanger and a particle filter. The bubbling fluid bed is operated at low superficial velocities of 0.5 to 6 ft/sec and is composed of fine particulate 45 to 2000 microns in diameter with up to 40% less than 200 microns. Fines elutriated from the bed are isothermally recycled back to the bed resulting in high combustion efficiency and food sulfur oxide suppression from sorbents contained in the bed. The material recycled in one hour is equivalent to twice the weight of the bed. Ammonia injected upstream of the recycle cyclone suppresses nitrogen oxides with high efficiency because of the excellent mixing in the cyclone. The heat transfer coefficient on the tubes in the bed is increased at least 2 to 4 times because of the fine particulate in the bed. Fluidization occurs over a 10:1 range in superficial velocities.
The present invention relates in general to fluid bed combustors and more particularly to a bubbling fluid bed with high combustion efficiency and good sorbent utilization.

Description

, Descri~tion Fines Recirculating Fluid Bed Co~bustor Method and Apparatus Background of the Invention Fluid bed boilers burning high sulfur coal are well known in the art. These boilers use classical bubbling bed technology whereby the fluid bed operates with superficial velocities in the range of 4 to 12 ft/sec and the bed is composed of particles with an average diameter of approximately 1000 microns. Coal is burned in the bubbling bed and limestone or dolomite sorbent is added to suppress the sulfur oxide emissions. The sorbent is added in particle sizes of 1000 to 3000 microns and the bed is composed largely of coal ash, spent sorbent, partially spent sorbent and partially burned fuel particles. The bubbling bed contains tubes within it to transfer heat to the steam.
Tubes are also mounted above the bed in the freeboard to transfer heat from the hot combustion gases, thus cooling them. In operation, the bed elutriates fine particulates comprised of char, ash and partially spent sorbent. Many of these particles are captured by a recycle cyclone located downstream of the convective heat exchanger and these particles are returned to the bed in order to burn the fuel particles and allow unused sorbent to absorb more sulfur oxides. Very fine particles escape the recycle cyclone and are trapped in a filter system. The flow rate in the recycle loop is approximately equal to the total solids flow rate of the fuel and the sorbent fed into the combustor.
Conventional fluid bed boilers have several disadvantages. One disadvantage is that the combustion efficiency is low, approximately 97~, because small particles of unburned fuel escape to the combustion system. This problem would be vastly exacerbated if j~_ ~; , . ' .

~7~345 the boiler were to attempt to burn a low volatile content fuel such as petroleum coke which has 90~ fixed carbon compared to 42~ fixed carbon for coal. The second disadvantage is low sorbent utilization. A
calcium to sulfur molar ratio of at least 3:1 must be maintained to produce sulfur oxide suppression of 90%
to meet typical air pollution requirements. The reason for this is that the relatively large particles of sorbent only absorb sulfur oxides on their surface, leaving their interior material largely unused. A
third disadvantage is that these boilers emit nitrogen oxides as a pollutant; the nitrogen oxides are generated from fuel-bound nitrogen. In many parts of the country the nitrogen oxide emissions do not exceed local limits but in some areas, such as Southern California, they do.
To improve combustion efficiency of conventional fluid bed boilers, Stewart et. al. in U~S. Patent Number 4,177,741 teaches the agglomeration of the recycled fines before reintroducing them into the bubbling bed. The agglomerated fines are thus prevented Erom being blo~n out of the bed and are thus encouraged to burn in the bed. Jones, U.S. Patent Number 4,259,911 teaches agglomeration of coal fines plus recycled material before injection into the bed.
To improve the utilization of sorbent, Jones U.S.
Patent Number 4,329,234 teaches the removal of a portion of the fluid bed and grinding the sorbent particles to 50 microns in diameter to fracture them, exposing new surface for additional sorption of sulfur oxides. The fractured particles are reintroduced into bed by being agglomerated with the coal ~fuel). All of these approaches are simple modifications of the classic bubbling bed boiler described earlier.
Reh et. al. in German Patent Number DE 3,023,480 describes a different approach to obtain good sorbent utilization in suppressing sulfur oxides from :

, combustion gases. Reh et. al. passes combustion gas through a fluidized bed of sorbent with particle size of 30 to 200 microns and a superficial velocity of 3 to 30 t/second, producing an entrained bed with a particle density 0.1 to 1Okg/cu m. The particulate entrained by the high gas velocity is removed by a recycle cyclone and returned to the bed, which is between 1300F and 2000F in temperature. The hourly recycle rate is approximately five times the bed weight. This approach achieves good sulfur oxide suppression by the use of fine particulate with large surface area and vigorous mixing. Reh however, does not teach combustion in the entrained bed of heat recovery with tubes from the entrained bed.
Reh in U.S. Patent Number 4,111,158 describes a fluid bed combustor based upon the principle of an entrained fluid bed which offers improvements in combustion efficiency, sulfur oxide suppression, nitrogen oxide control and turn-down. Whereas bubbling bed combustors operate with superficial velocities in the range of 4-12 ft/second and have a clearly defined upper surface, entrained bed combustors operate at superficial velocities of 15 to 45 ft/second and have no clearly defined upper surface but rather a gradation of particulate density from the bottom to the top of the combustor. The particulate is entrained with the gas flow in the reactor and separated from it by a recycle cyclone downstream of the reactor whereupon the particulate is reintroduced into the base of the reactor. Particle size ranges from 30 to 250 microns and the particle density is 10 to 40 kg/cu m in the upper portion of the reactor. Heat is not recovered from the particulate or gases in the reactor or recycle loop. Tubes in the reactor would be subject to high erosion and would not be effective in transferring heat because of the low particle density compared to that of a bubbling bed (500 kg/cu m). Heat is recovered by ,~

. , ~'~7~45 draining a portion of the bed from the base of the reactor and cooling it in a separate fluid bed heat exchanger optimized for that process. High combustion efficiency is obtained by completely burning small diameter fuel particles in the highly turbulent reactor and the hot recycle loop. Good sorbent usage is also obtained by using fine particulate and maintaining it at an effective temperature throughout the reactor and recycle loop. Limited nitrogen oxide control is obtained by progressively introducing co~bustion air along the length of the reactor. The disadvantage of the system is the need for the separate fluidized bed heat exchanger and large recycle cyclones.
Ammonia injection to suppress nitrogen oxides without a catalyst is taught by Lyon in ~.S. Patent ~umber 3,900,554. Lyon describes the basic gas phase reaction whereby ammonia selectively reduces nitrogen oxide in the presence of oxygen at 1742~F to 1832F and predicts a suppression of 20% at an ammonia/nitrogen oxide molar ratio of 2, Lyon does not teach the benefits of good mixing, as in the recycle cyclone, which produced nitrogen oxide ions of 95% at the same molar ratio of 2.

Disclosure of the Invention . . ~_ The object of the present invention is to achieve the benefits of high combustion efficiency and good ~ sorbent utilization without using a separate fluidi~ed - bed heat exchanger with a large recycle cyclone.
The present invention utilizes a bubbling fluid bed combustor with tubes in the bed for heat transfer but with bed particles whose average diameter is in the range of 100 to 800 microns wherein 20% to 40% of the particles, respectively, are less than 200 microns in diameter. The superficial velocity of the bed is 3 to 7 ft/second, well below the 15 to 45 ft/second of the entrained bed. The result of the relatively low ~7~

superficial velocity combined with a bed of small diameter particulate is to produce a bubbling bed but with a high rate of elutriation of the fines component of the bed. A particle density loading of 0.5 kg/cu m 5 is achieved at the top of the bed. This compares to ~
the 10-40 kg/cu meter typical of an entrained bed.
Hence, the present invention uses particulate sizes typical of entrained beds but a much lower superficial velocity and hence produces a bubbling bed with substantially reduced transport of bed material.
Compared to a conventional bubbling bed combustor, it uses a much smaller average particle size (500 microns versus 1000 microns) and has considerably higher bed transport. The recycle rate of a conventional bubbling fluid bed boiler is approximately equal to the combined solids feed rates whereas the recycle rate of the present invention is 20 times that value, equivalent to changing the bed every ~0 minutes.
Unlike the bub~ling bed combustors but similar to the entrained bed combustors, the present invention has no heat transfer surfaces between the bed and the recycle cyclone to cool the gas and particulate, hence contains an isothermal recycle loop operating at the ideal temperature for combustion or sulfur sorption. Unlike either the bubbling bed combustor or the entrained combustor the subject invention uses ammonia injection at the inlet of the recycle cyclone for control of nitrogen oxide emissions. Other benefits are a 100~ to 300% increase in heat transfer coefficient on the tubes in the bed because of the small particle size in the bed and a re~uction in tube erosion (compared to conventional bubbling beds) because of the low superficial velocities. (Tube erosion increases exponentially with superficial velocity). Another attractive feature is a 10:1 range of fluidization velocities which allows for a full fluidized start up at low system throughputs.
`'' ~7~45 The subject invention has demonstrated an 86 reduction in nitrogen oxides twithout the use of ammonia) by operating at a low combustion temperature of 1450F which reduces the evolution of nitrogen oxides from fuel bound nitrogen. In addition, it is also well known that char at 1450F will reduce nitrogen oxides in the presence of oxygen. At 1450F large quan,tities of char are elutriated from the bed and circulate in the recycle loop. A portion of the ion of the 86~
nitrogen oxides is contributed by its reaction with hot char but the degree of contribution is unknown.
Thus, we have discovered a unique fluid bed combustion method and apparatus producing superior performance and efficiency.

Brief Description of Drawings . .
Figure 1 is a schematic elevational sectional view of a complete system in which the present invention is embodied and utilized.

Best Mode for Carryinq Out the Invention The present invention is embodied and employed in a system comprised of a fluid bed combustor 10 having a combustion chamber 11 for containing a fluid particle bed B supported on a distribution plate 12. The combustor 10 includes cooling tubes 13 in the fluid bed B as well as a fuel feed 14, sorbent feed 15 and bed drain 16. Fluidizing air is introduced in the bottom of the combustor 10 at 17. The hot gas and elutriated particulate leaving the surface of the bed B pass through the freeboard 18 and are directed via a conduit 19 to a recycle cyclone 21 mounted above the bed to provide a straight dip leg 22 with adequate head in the dip-leg 22 to provide a free flowing return of fine particulate to the bed. The cut point of the recycle cyclone 21 is approximately 12 microns~

, ~, .

Ammonia is injected into the hot gas stream in the conduit 19 immediately upstream of the recycle cyclone by an ammonia injector 23 to suppress nitrogen oxides when burning fuel with fuel-bound nitrogen. Ammonia is supplied from a supply tank 24.
Hot gas leaving the recycle cyclone 21 via a conduit 25 passes through a convection heater 26 where the remaining heat is removed from the hot gas.
Downstream of the convection heater 26 the ~as passes through a filter system 27, such as a baghouse filter, to remove dust before being exhausted to the atmosphere through the stack 28~
According to this invention we provide fluidized bed combustion system method and apparatus which comprises a bubbling fluid bed combustor with a superficial velocity in the range of 0.5 ft/second to 7 ft/second but with bed material in the size range of 45 microns to 2000 microns in diameter whereby ~0~ to 20 of the bed material, respectively, is less than 200 microns in diameter. The large fraction is associated with dolomite feedstock size when dolomite is used.
The bubbling bed contains tubes to transfer heat from the hot fluid bed; the heat transfer coefficient on the outside of those tubes is in the range of 100 to 200 BTU/FT2-HR-F because of the fine particulate in the bed. Solid fuel or liquid fuel is fed directly into the bed with the solid fuel. If sulfur sorbent such as limestone or dolomite is used it is fed directly into the b~d as well.
The fluid bed combustor has a hot freeboard with no heat transfer surfaces. A large number of fines are elutriated from the fluid bed into the hot freeboard wherein excellent mixing conditions exist between the particles and the gas, and adequate residence time is available for chemical reactions7 Most of the particulate elutriated from the bed into the freeboard falls back into the bed but a significant amount is . ~ .

9~

transported by the gas flow into the recycle cyclone where it is separated from the gas and returned to the bed, all at substantially the same temperature as exists in the fluid bed. Particle loading of the gas entering the cyclone is approximately 0.5 kg/cu m. The extended residence time and the excellent mixing between the fine particulate char and the oxygen rich combustion gas in both the freeboard and recycle cyclone cause the char particles to burn to completion before they can exit the recycle cyclone. A benefit of the recycle char is believed to be the enhancement for highly efficient nitrogen oxide suppression at temperatures down to 1400F. Similarly, the extended residence time and the excellent mixing between the fine sorbent particles and the sulfur oxides in the combustion gas in the hot freeboard and the recycle cyclone promote good sulfur oxide capture by the sorbent. The fine particulate spend approximately 1 second in the bed and 3 seconds in the freeboard and recycle cyclone. The recycle cyclone is designed with a cut point of 5 microns and with a highly efficient dip leg to easily recycle most of the particles substantially larger than 5 microns, preventing their escape to the filter system. The flow rate of the captured particulate around the recycle loop is approximately twenty times the combined low of fuel and sorbent into the fluid bed. Its hourly flow rate is twlce the weight of the fluid bed itself.
If the fuel contains fuel-bound nitrogen and nitrogen oxide suppression is required, ammonia is sprayed into the hot combustion gas stream immediately upstream of the inlet to the recycle cyclone.
Although, ammonia selectively and most efficiently reduces nitrogen oxides without a catalyst in the range of 17~3 F to 1832F, efficient reduction is normally achieved in the present invention by operating the fluid bed at 1450F-1650F and achieving 0 to 150F of ;

~.~7~345 _g after burning above the bed. Injection of ammonia in ammonia/nitrogen oxide molar ratios of 1.5 to 2 provides nitrogen oxide suppression of 80~ to 95~
because of the excellent mixing occuring in the recycle cyclone. Under certain conditions nitrogen oxides are suppressed without the use of ammonia injection. When operating at low combustion temperatures of 1450F with fuel with a high percentage of fixed carbon, a substantial part of the recycled particulate is char.
This hot fine particulate char reduces nitrogen oxides such that a nitrogen suppression of 86% has been achieved by the present invention.
The hot combustion gas and dust escaping the recycle cyclone pass through a convective heat exchanger where the gases are cooled to their exit temperature~ Finally, a filter system removes the dust before discharging the combustion gas to the atmosphere.
The present invention thus provides the capability to burn cleanly a wide variety of solid and liquid fuels, some of which may be very difficult to burn (such as petroleum coke with 90~ fixed carbon, i.e., low volatiles) or fuels which may contain sulfur or nitrogen or the combination of sulfur and nitrogen, all of which cause air pollution. The present invention burns these fuels by using a conventional bubbling bed with a fine particulate composition and recycling a large portion of those fines through a hot recycle loop above the bubbling bed. Combustion efficiency of 99.4%
is obtained with petroleum coke with 90% fixed carbon, and 98~ suppression of sulfur oxides is obtained with a calcium sulfur molar ratio of 1.8. ~ 95~ suppression of nitrogen oxides is obtained with an ammoniafnitrogen oxide molar ratio of 2. All this occurs within the framework of the fluid bed recycle system and occurs simultaneous.

, - 1 o -A further benefit of the present invention is a large fluidization range of up to 15:1. Because the bubbling fluid bed is composed of fine particulate, its minimum fluidization velocity is as low as 0.5 ft/second.

- EXAMPLE - PETROLEUM COKE
Petroleum coke was burned with air in a fluidized bed combustor whose configuration is described in Figure 1. The fluid bed combustor was three feet in diameter and twelve feet tall with the recycle c~clone mounted above it. The combustor was refractory lined.
The bubbling bed was operated 3-1/2 to 4 feet deep and contained air tubes to transfer heat out of the bed.
The petroleum coke used in the test had the following composition and heating value:

Fixed Carbon 89.7~ by weight Nitrogen 1.9%
Sulfur 2.1~
Other Volatiles 4.4%
Ash 0.3~
Moisture 1.6%
HHV 14,270 BT~/LB
.

This Euel is difficult to burn because of the high fixed carbon with few volatiles. It also contains the elements of nitrogen and sulfur which produce nitrogen oxides and sulfur oxides as air pollutants. The fuel was introduced to the fluid bed through a fuel feed, the majority of the fuel being between 50 and 400 microns in diameter. Dolomite, a sulfur sorbent, was introduced into the bed through the sorbent feed. Its composition was:

. .

. . , -.; .

" - ,.

". ~ .

~7~4~

Calcium Carbonate 56.6% by weight Magnesium Carbonate 45.5~
Inerts 0.9%

Its size was between 4700 microns and 1200 -microns. This particular dolomite decrepitated in the bed into fine particles.
The fluid bed was initially composed of crushed dolomite with an average size of 800 microns.
After testing for approxi~ately 500 hours the bed was comprised of ash, spent sorbent and partially spent sorbent; average particle size had stabilized at approximately 300 microns. The fluld bed operated at an average superficial velocity of 4 ft/second. It was necessary to drain the bed periodically to maintain a constant level.
The recycle cyclone was designed to hold the majority of particles greater than 5 microns within the fluid bed combustor and was designed with a free ; flowing dip leg to provide little resistance in the ~0 particulate return path. As a result, high recycle flow rates of fines were achieved whereby the recirculation per hour was approximately twice the weight of the bed and twenty times the combined solids feed rate. The fuel particulate and sorbent particulate, unable to leave the fluid bed with the gas stream until they had reached a very small size, were contained in the bed and comminuted by the action of the bedO Fuel particles, restrained from leaving the fluid bed combustor, burned to completion providing high combustion efficiency even with a difficult fuel containing approximately 90% fixed carbon. Combustion efficiency was further enhanced by the isothermal nature of the recycle path. The fuel particle is heated to full combustion temperature in the bed and is not cooled either in the freeboard or the recycle cyclone. Operating at a bed temperature of 1600~F with ~7~ ~ ~ 5 20~ to 30% excess air, combustion efficiencies of 99.4 were achieved. Afterburning above the bed was in the range of 50 F to 100 F.
Comminution and retention of the sorbent particles provided a large surface area of the sorbent to absorb sulfur from gases in the fluid bed combustor. Ninety-eight percent sulfur oxide suppression was achieved at a calcium to sulfur molar ratio of 1.8. A further benefit of the fine particle size in the combustor was the increase in heat transfer coefficient on the surface of the tubes immersed in the bed. Heat transfer coefficients on the outside of the tubes ranging from 100 to 200 BTU/HR-FT2-F were observed compared to 40-60 BTU/HR-FT2-F for a conventional fluid bed boiler.
To suppress nitrogen oxides to meet local pollution control codes in Southern California, ammonia was in]ected upstream of the cyclone to mix with the combustion gas and selectively reduce nitrogen oxide to nitrogen and water according to the well-known reactions. At an NH3-to-NO molar ratio of 2, approximately 95% of the NO was suppressed.

EXAMPLE-UTAH COAL
Utah coal was burned in the same fluid bed combustor as previously described in the earlier example. The composition of the coal and its heating value were as follows:

Fixed carbon 43%
Nitrogen 1.3%
Sulfur 0.6%
Other Volatiles 37.1%
Ash 8.0%
Moisture 10.0%
HHV 11,500 BTU/LB

" The Utah coal had substantially less fixed carbon and substantially greater volatiles and hence was easier to burn than petroleum coke. The size of the coal was minus 1 5/8 inchesO The sulfur sorbent was the same dolomite as used in the prior example. Its composition was as follows:

Calcium carbonate 56.6% by weight Magnesium carbonate 45.5%
Inerts o.9%

Its size was between 1,200 microns and 4,700 microns but it decrepitated into fire particles in the bed.
Combustion efficiency with coal was 99~8% with 20%
excess air at a bed temperature of 1600F. For coal, the combustor could be operated as cool as 1400F with only 20% excess air and yet maintain good combustion characteristics. With petroleum coke, acceptable combustion characteristics could only be maintained at 1450F by increasing the excess air to 60~. For coal at 1600F, afterburning above the bed was reduced to 10-20F. Suppression of sulfur oxides and s~itrogen oxides was similar to that on petroleum coke.

. . ~

Claims

The embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows:

1. A method of burning solid particulate or liquid fuel and recovering useful energy therefrom, comprising the steps of:
(a) maintaining a generally horizontal bed of inert particles, ash and some partially burned fuel on a distribution plate within an internal combustion chamber of a fluid bed combustor which also defines a freeboard immediately above the bed of particles;
(b) directing a continuous stream of fluidizing gas upward within said combustion chamber from below said distribu-tion plate so that the stream passes through the plate and said bed of particles for fluidizing the latter, the velocity of said stream of gas being such that the stream carries fine particles of a certain size and smaller upward with it into the freeboard from the fluidizing bed;
(c) capturing substantially all of said fine particles carried upward by said stream of fluidizing gas within a recycle cyclone which forms part of the combustor and which is positioned above said freeboard, and returning most of the captured particles to the fluidizing bed while exhausting the gas from said cyclone, whereby the captured and returned fine particles define a recycling path of movement from the bed, through the freeboard and cyclone and back into the bed;
(d) recovering heat energy from combustion taking place in said fluidizing bed but not from said recycling path of fine particles; and (e) operating the fluid bed combustor including its fluidizing bed and cyclone such that substantially all combus-tion of said fuel particles takes place only within said bed and not in either the freeboard or the cyclone and such that said recycling path of fine particles is maintained at a substan-tially constant temperature.

2. A method according to claim 1 wherein said fluid bed combustor is refractory lined, whereby said recycling path of fine particles can be maintained at said substantially constant temperature even though substantially no combustion takes place in either said freeboard or cyclone.

3. A method according to Claim 1 wherein said fluid bed combustor is operated such that the temperature within the fluidizing bed and the temperature along said recycling path are substantially equal.

4. A method according to Claim 3 wherein the temperature of said fluidizing bed and said recycling path is about 1400°F.

5. A method according to Claim 3 wherein the temperature of said fluidizing bed and said recycling path is about 1600°F.

6. A method according to Claim 1 wherein said fluid bed combustor is operated such that afterburning of fuel particles above said fluidizing bed amounts to no more than about 100°F.

7. A method according to Claim 1 wherein said fluid bed combustor is operated such that afterburning of fuel particles above said fluidizing bed amounts to no more than about 20°F.

8. A method according to Claim 1 wherein said fluid bed combustor is operated such that substantially no cooling of particles within either said freeboard or said recycle cyclone takes place.

9. A method according to Claim 8 wherein no heat transfer surfaces are provided within said combustion chamber between said fluidizing bed and said recycle cyclone.

10. A method according to Claim 1 wherein said fluid bed combustor is operated such that the temperature within said fluidizing bed is maintained between about 1450°F and 1650°F and afterburning of fuel particles above said bed amounts to no more than about 150°F, whereby to promote the efficient suppression of nitrogen oxide from fuel containing fuel bound nitrogen in the presence of ammonia, without the need for a catalyst.

11. A method according to Claim 10 including the step of spraying ammonia into said recycle path of particle-laden gas immediately upstream of the inlet to the recycle cyclone, without a catalyst.

12. A method according to Claim 10 wherein said fluid bed combustor is operated such that the temperature within said fluidizing bed is about 1450°F, whereby to promote the efficient suppression of nitrogen oxide from fuel having a high percentage of fixed carbon.

13. A method according to Claim 1 wherein said generally horizontal bed of inert particles, ash and partially burned solid fuel maintained on said distribution plate has an average particle size in the range of 100 microns to 800 microns with 20% to 40% less than 200 microns and wherein said continuous stream of fluidizing gas is directed upward within said combustion chamber and below said distribution plate at a superficial velocity in the range of 0.5 to 7 feet per second.

14. A method according to Claim 1 including the step of feeding a sulphur oxide sorbent into said fluidizing bed along with the other particles therein and maintaining the fluidizing bed or inert particles, ash, partially burned fuel particles, partially spent sulphur sorbent and spent sulphur sorbent with an average particle size in the range of 100 to 800 microns.

15. A method according to Claim 1 wherein said fluid bed combustor is operated such that the temperature within said fluidizing bed and the temperature along said recycle path is between about 1400°F and °F whereby to encourage minimum nitrogen oxide formation during the combustion of said fuel and to encourage char production with the attendant suppression of nitrogen oxide by hot char in said fluidizing bed and along said recycle path.

16. In a method of burning solid particulate or liquid fuel and recovering useful energy therefrom, including the steps of maintaining a generally horizontal bed of inert particles, ash and partially burned fuel on a distribution plate within an internal combustion chamber of a fluid bed combustor which also defines a freeboard immediately above the bed of particles, directing a continuous stream of fluidizing gas upward within said combustion chamber from below said distribution plate so that the stream passes through the plate and said bed of particles for fluidizing the latter, the velocity of said stream of gas being such that the stream carries fine particles of a certain size and smaller upward with it into the freeboard from the fluidizing bed, and capturing substantially all of said fine particles carried upward by said stream of fluidizing gas within a recycle cyclone which forms part of the combustor and which is positioned above said freeboard, and returning most of the captured particles ot the fluidizing bed while exhausting the gas from said cyclone, whereby the captured and returned fine particles define a recycling path of movement from the bed, through the freeboard and cyclone, and back into the bed, the improvement comprising the steps of:
recovering heat energy in said fluidizing bed from combustion taking place in said fluidizing bed but not from said recycling path of fine particles while operating the fluid bed combustor including its fluidizing bed and cyclone such that substantially all combustion of said fuel particles takes place only within said bed and not within either the freeboard or the cyclone and such that said recycling path of fine particles is maintained at a substantially constant temperature.

17. In a method of burning solid particulate or liquid fuel and recovering useful energy therefrom, including the steps of maintaining a generally horizontal bed of inert particles, ash and partially burned fuel on a distribution plate within an internal combustion chamber of a fluid bed combustor which also defines a freeboard immediately above the bed of particles, directing a continuous stream of fluidizing gas upward within said combustion chamber from below said distribution plate so that the stream passes through the plate and said bed of particles for fluidizing the latter, the velocity of said stream of gas being such that the stream carries fine particles of a certain size and smaller upward with it into the freeboard from the fluidizing bed, and capturing substantially all of said fine particles carried upward by said stream of fluidizing gas within a recycle cyclone which forms part of the combustor and which is positioned above said freeboard, and returning most of the particles to the fluidizing bed while exhausting the gas from said cyclone, whereby the captured and returned fine particles define a recycling path of movement from the bed, through the freeboard and cyclone, and back into the bed, the improvement comprising the steps of:
operating the fluid bed combustor including said fluidizing bed and cyclone such that the temperature along said recycling path is substantially constant and at least ap-proximately equal to the temperature within said fluidizing bed while not providing any heat transfer surfaces within the freeboard above said fluidizing bed or otherwise cooling fuel particles either in said freeboard or in said recycle cyclone.

18. The improvement according to Claim 17 including the steps of recovering heat energy from combustion taking place in said fluidizing bed but not from said recycling path of fine particles and providing said fluid bed combustor so that it is refractory lined, and wherein said fluid bed combustor is operated such that substantially all combustion of said fuel particles take place only within said fluidizing bed and not in either the freeboard or the cyclone, whereby the temperature along the recycling path can be maintained substantially constant and equal to the temperature within the fluidizing bed without combustion taking place in the freeboard or cyclone and without the fuel particles in the freeboard or recycle cyclone having to be cooled.

19. In a method of burning solid or liquid particulate fuel and recovering useful energy therefrom, including the steps of maintaining a generally horizontal bed of inert particles, ash and partially burned fuel on a distribution plate within an internal combustion chamber of a fluid bed combustor which also defines a freeboard immediately above the bed of particles, directing a continuous stream of fluidizing gas upward within said combustion chamber from below said distribution plate so that the stream passes through the plate and said bed of particles for fluidizing the latter, the velocity of said stream of gas being such that the stream carries fine particles of a certain size and smaller upward with it into the freeboard from the fluidizing bed, and capturing substantially all of said fine particles carried upward by said stream of fluidizing gas within a recycle cyclone which forms part of the combustor and which is positioned above said freeboard, and returning most of the captured particles to the fluidizing bed while exhausting the gas from said cyclone, whereby the captured and returned fine particles define a recycling path of movement from the bed, through the freeboard and cyclone, and back into the bed, the improvement comprising the steps of:
maintaining said fluidizing bed of inert particles, ash and partially burned fuel with an average particle size in the range of 100 microns to 800 microns with 20% to 40% less than 200 microns, directing said continuous stream of fluidizing gas upward within said combustion chamber from below said distribution plate at a superficial velocity in the range of 0.5 to 7 feet per second, and providing heat collecting tubes within said fluidizing bed for recovering the heat resulting from combustion taking place therein, whereby the relatively small particle size within said fluidizing bed results in efficient heat transfer, from the particles to the tubes while the relatively low flow rate of said gas stream through said bed and across said tubes minimizes erosion to the latter.

20. The improvement according to Claim 19 including the step of feeding the sulfur oxide sorbent into said fluidizing bed concurrent with said fuel and maintaining said fluidizing bed of inert particles, ash, partially burned fuel particles, partially spent sulfur sorbent and spent sulfur sorbent with an average particle size in the range of 100 to 800 microns.

21. The improvement according to Claim 19 including the step of introducing ammonia into said recycle path immediately upstream of the recycle cyclone.

22. The improvement according to Claim 19 wherein said fluid bed combustor is operated such that the temperature within the fluidizing bed and the temperature along the recycling path are approximately 1400°F to 1500°F, whereby to encourage minimum nitrogen oxide formation during combustion of said fuel and to encourage char production with the attendant suppression of nitrogen oxide by hot char in said fluidizing bed and along the recycling path.

23. An improvement according to Claim 19 wherein said fluid bed combustor is operated such that the amount of particles within said stream which is returned to bed is equivalent to one to five times the weight of the fluid bed.

24. In a fluid bed combustion apparatus including housing means defining an internal combustion chamber, means including a distribution plate for maintaining a generally horizontal bed of inert particles, ash and partially burned fuel on said distribu-tion plate and within said internal combustion chamber directly below a freeboard within the chamber, means for directing a continuous stream of fluidizing gas upward within said combus-tion chamber from below said distribution plate 80 that the stream passes through the plate and said bed of particles for fluidizing the latter, the velocity of said stream of gas being such that the stream carries fine particles of a certain size and smaller upward with it into the freeboard from the fluidiz-ing bed, and means including a recycle cyclone located within said combustion chamber above said freeboard for capturing substantially all of said fine particles carried upward by said stream of fluidizing gas and for returning most of the captured particles to said fluidizing bed while exhausting the gas from said cyclone, whereby the captured and returned fine particles define a recycling path of movement from the bed, through the freeboard and cyclone and back into the bed, the improvement comprising:
the fluidizing bed, cyclone and freeboard therebetween which are configured such that substantially all combustion of said fuel particles takes place only within said bed and not within either the freeboard or the cyclone and such that said recycling path of fine particles is maintained at a substantial-ly constant temperature; and means for recovering heat energy only from within said fluidizing bed and not from said recycling path of fine particles.

25. In a fluid bed combustion apparatus in which a fluid bed of inert particles, ash and partially burned fuel is fluidized by means of a continuous stream of fluidizing gas passing through the bed, the improvement comprising:
(a) said fluidizing bed which is configured 60 that fuel particles therein substantially completely combust before being carried off by said gas stream;
(b) means including a recycle cyclone for capturing substantially all of the particles carried upward by said stream of gas and for returning most of the captured particles to said fluidizing bed such that the temperature at any point along the path of movement of the recycled particles is substantially constant; and (c) means for recovering heat energy only from the combustion taking place in said fluidizing bed.