Pulsed atmospheric fluidized bed combustor apparatus and process

A pulsed atmospheric fluidized bed reactor system is disclosed and claimed along with a process for utilization of same for the combustion of, e.g. high sulfur content coal. The system affords a economical, ecologically acceptable alternative to oil and gas fired combustors. The apparatus may also be employed for endothermic reaction, combustion of waste products, e.g. organic and medical waste, drying, calcining and the like.

FIELD OF THE INVENTION 
The present invention relates to apparatus and process for pulsed fluidized 
bed reactor for combustion of solid fuels and other operations in an 
efficient, environmentally acceptable, and economical manner. 
BACKGROUND OF THE INVENTION 
Many technologies have been developed and/or demonstrated for utilizing 
high-sulfur fuels in general and coals in particular. From a performance, 
emissions, and economics standpoint, fluidized bed combustion technology 
has emerged as a leading candidate for utilizing high sulfur fuels. Many 
fluidized bed combustion designs are available and are at various stages 
of commercialization. Such systems can be classified in terms of operating 
pressure (atmospheric or pressurized) and fluidization mode (bubbling or 
circulating). All the fluidized bed designs possess attributes such as 
in-situ sulfur capture, no slagging or fouling of heat transfer surfaces, 
high heat transfer rates to heat exchange surfaces, near uniform 
temperature in combustion zone, and fuel flexibility. These features have 
made it possible for fluidized bed combustion technology to compete 
successfully for the large industrial boiler market (6.3-37.8 kg/s or 
50,000-300,000 lb/hr steam). Large-scale (70 to 150 MW.sub.e) field 
demonstration projects are in progress to facilitate commercialization in 
the utility sector. The potential of fluidized bed combustion technology, 
and specifically, atmospheric fluidized bed combustion for small-scale 
(&lt;6.3 kg/s or 50,000 lb/hr steam equivalent) applications have, however, 
not been explored seriously until recently. 
Atmospheric fluidized bed combustion technology appears to have a great 
potential for oil and gas replacement in small-scale installations of less 
than 6.3 kg/s (50,000 lb/hr) steam equivalent. These smaller units can 
meet the needs of process heat, hot water, steam, and space heating in the 
residential, commercial, and industrial sectors. Currently, oil and 
natural gas-fired equipment are being used almost exclusively for these 
applications. Due to the large difference between the prices of these 
fuels and coal, coal-fueled atmospheric fluidized bed combustion 
technology engineered for small-scale applications has the potential of 
becoming very competitive under economic conditions in which the price 
differential overcomes the initial capital cost of the coal-based system. 
A successful coal-fueled system can not, however, only be more economical, 
but can also reduce the nations's dependence on foreign oil and open up 
new markets for domestic coal and the coal-fueled fluid-bed technologies. 
Market analysis indicates that a coal-based system that provides 
competitive levels of capital and operation and maintenance costs, 
performance, and reliability at the 0.126 to 1.26 kg/s (1,000 to 10,000 
lb/hr) steam generation rate can displace as much as 2.64 EJ (2.5 quad 
Btu) of gas and oil within the residential, commercial, and light 
industrial sectors. In the industrial sector, systems from 1.26 to 6.3 
kg/s (10,000 to 50,000 lb/hr) steam can displace another 1.16 EJ (1.1 quad 
Btu) of energy per year. 
As pointed out earlier, the atmospheric fluidized bed combustion systems 
can be classified into bubbling-bed and circulating bed systems. In a 
coal-fueled bubbling-bed system, it is critical to control the extent of 
fines (elutriable particles) in the coal and sorbent feed in order to 
limit particle carryover and its adverse effect on combustion and sulfur 
capture performance, emissions, and the size of solids collection 
equipment. Additionally, the higher Ca/S feed ratios typically required in 
bubbling fluidized combustion applications tend to increase sorbent and 
waste disposal costs, and turndown capability is rather limited. As 
regards a circulating fluidized bed combustion system, it exhibits higher 
combustion efficiency and sorbent utilization, lower NO.sub.x emissions 
due to multiple air staging, and greater fuel flexibility and turndown as 
compared to a bubbling type system. However, the circulating type system 
requires a tall combustor to accommodate sufficient heat exchange surface. 
Such makes it both impractical and expensive to scale-down circulating 
fluidized bed combustors to sizes significantly smaller than 12.6 kg/s 
(100,000 lb/hr) steam equivalent. 
Fluid bed systems in general tend to have large thermal inertia. Start-up 
for large fluid bed systems requires a considerable amount of time and 
also auxiliary subsystems to preheat the beds in a controlled manner. Both 
add to overall system cost and complexity. Concepts which provide a simple 
compact design for fast start-up with low-cost hardware and also have 
simple operational characteristics are a must for small-scale 
applications. Thermal inertia of fluid bed systems also affects load 
following to some extent and this has also been a serious shortcoming for 
scale-down to small end-use applications. System designs must provide fast 
response to load changes, particularly through auxiliary firing subsystems 
and methods of bed heating. Such designs should not require additional 
hardware and control systems if the system capital cost is to be 
maintained sufficiently low to compete favorably with existing oil and gas 
equipment. In addition, new designs capable of higher throughput for given 
combustor size will contribute to a reduction in capital cost per kJ/hr 
(Btu/hr) of fuel fired. This must be achieved, however, without 
compromising the pollution control performance of equipment intended to 
meet stringent requirements in some of these end-use applications. 
Simply scaling-down existing large atmospheric fluidized bed combustion 
systems to a size range suitable for small end-use sectors of interest 
will result in complex and expensive systems that will not be competitive 
with presently available oil and gas-fired equipment. New innovative 
approaches are needed to reduce cost and enhance performance. 
Such a new system should therefore possess a number of attributes, such as 
high combustion efficiency; high sulfur capture capacity; low NO.sub.x 
emissions; and should be capable of rapid start-up with load-following 
capability. Also such systems, as with most systems should be of a simple 
design with inexpensive, easily managed controls to afford a reliable, 
safe system. Last, but not least, the system should be at least 
technologically and economically equivalent to oil- and gas-fired packaged 
systems. 
The apparatus and process according to the present invention overcome the 
above noted problems of the prior art and possess the attributes set forth 
above. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to provide an improved 
combustor. 
Another object of the present invention is to provide an improved combustor 
that operates on high sulfur fuels such as coals while avoiding unwanted 
emmissions. 
Still another object according to the present invention is to provide an 
improved fluidized bed combustor. 
Yet another object according to the present invention is to provide a 
pulsed fluidized bed combustor capable of economical operation with high 
sulfur fuels. 
Another object of the present invention is to provide a pulsed atmospheric 
fluidized bed reactor. 
Still another object according to the present invention is to provide a 
pulsed fluidized bed combustor that may be down-sized to economically 
operate at 50,000 pounds per hour stress equivalent or less. 
Another object according to the present invention is to provide an improved 
process for the combustion of high sulfur fuels. 
It is still further another object according to the present invention to 
provide an improved process for combusting solid fuels in a fluidized bed 
environment. 
Generally speaking, apparatus according to the present invention includes a 
reactor vessel; means for feeding a fluidizable solid material into said 
vessel intermediate the height of same; means for supplying a fluidizing 
medium for said solid material into said vessel below said solid material 
entrance to said vessel to establish a fluidized bed of solid material 
therebetween; a pulse combustor unit extending into said vessel, said 
pulse combustor unit comprising a combustion chamber, valve means 
associated with said combustion chamber for admitting a fuel-air mixture 
thereto, a resonance chamber in communication with said combustion chamber 
and extending outwardly therefrom, an outer free end of said resonance 
chamber being located with respect to said fluidized bed to permit gaseous 
products from said resonance chamber to act thereon; heat transfer means 
located in said vessel with respect to said fluidized bed to withdraw heat 
therefrom; and flue gas exhaust means in communication with said vessel to 
exhaust products of combustion therefrom. 
Generally speaking, the process according to the present invention includes 
the steps of establishing and maintaining a fluidized bed of solid fuel 
within a vessel therefor and about a heat transfer means; pulse combusting 
a fuel-air mixture in a fashion to create a pulsation flow of combustion 
products and an acoustic wave therefrom; directing said pulsating flow of 
combustion products to act directly on said fluidized bed of solid fuel 
for combustion of said solid fuel; circulating a heat transfer medium 
through said heat transfer means to receive heat therefrom for 
predetermined treatment of said medium; and exhausting products of 
combustion from said vessel after separation of entrained solids therefrom 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred system according to the present invention integrates a pulse 
combustor with an atmospheric bubbling-bed type fluidized bed combustor as 
illustrated in FIG. 1. In this modular configuration, the pulse combustor 
burns the fuel fines which are less than 30 seive or 600 microns and the 
fluidized bed combusts the coarse solid fuel particles. 
As shown in FIG. 1, the pulsed atmospheric fluidized bed apparatus 
according to the present invention includes a refractory-lined vessel 
generally 10 in which the fluidized bed will be produced. A pulse 
combustor generally 30 is integrated with vessel 10 to afford the 
advantages set forth herein. 
Vessel 10 includes a lower section 12, an intermediate section 14, and an 
upper section 16. Located in lower section 12 of vessel 10 is a fluid 
distribution means 13 through which fluid may be introduced adequate in 
velocity to fluidize solids located in lower section 12. Generally 
speaking, it has been found that fluid velocities in a range of from about 
4 to about 13 feet per second are adequate for fluidization. Also located 
within lower section 12 where the dense fluidized bed will be formed are a 
plurality of tubes or conduits 60 through which a heat exchange medium may 
be passed to remove heat from the fluidized bed. Typically, air or water 
would be circulated through heat exchange tubes 60 to produce heated air, 
hot water or steam though other materials may be passed therethrough for 
an intended result. 
Intermediate vessel section 14 flares outwardly and connects lower section 
12 with upper section 16, with intermediate section 14 and upper section 
16 forming what is referred to as the freeboard area of a fluidized bed 
system, in which gas velocity decreases, gas residence time increases and 
elutriation decreases. Conversely, the dense fluidized bed in lower vessel 
section 12 operates in a bubbling, turbulent mode. 
Pulse combustor 30 includes valve means 32 which may be an aerodynamic 
valve or fluidic diode, a mechanical valve or the like, a combustion 
chamber 34 and a tailpipe 36. Additionally, pulse combustor 30 includes an 
air plenum 38 and a thrust augmentor 39. Tailpipe or resonator tube 36 may 
be a single tube as shown or a plurality of tubes and in a preferred 
embodiment has a diffuser section 40 located at a free end of same. 
Likewise in a preferred embodiment tailpipe 36 has a water jacket 41 
surrounding at least a portion of the length of same. 
Diffuser section 40 at the end of tailpipe 36 forms an expansion section 
which reduces the gas exit velocity from tailpipe 36 and prevents 
channeling in the fluidized bed. After the flue gas from the pulse 
combustor 30 exits the tailpipe 36 therefore, it enters the diffuser 
section 40 which provides fines recirculation and increased particle 
residence time in the bed. Vessel 10 also includes an overbed coarse fuel 
and sorbent feed system 70, preferably a screw conveyor, and a fuel 
classifier 71 for separating fuel feed into coarse fraction and fines. 
Fines are fed from classifier 71 via line 72 to pulse combustor 30 while 
the coarse fuel component is fed from classifier 71 to feed system 70. 
Sorbent such as crushed limestone is fed from a supply hopper 76 to feed 
system 70 for introduction to vessel 10 while the fuel sorbent mixture may 
vary, sorbent content is preferably maintained at a level of two to three 
times the sulfur ratio of the solid fuel, e.g. coal. 
Vessel 10 further includes a product gas exit conduit 80 having a gas 
solids separator 82, preferably an inertial separator at the entrance 
thereof to separate elutriated fines from the exit gas stream and return 
same to freeboard section 16. Waste rock, ash and the like are discarded 
from vessel 10 through port 17 located at a lower end of same. A burner 19 
is also provided for vessel 10, preferably fired by natural gas to be 
employed for operational safety and start-up of the system. 
A pulse combustor typically includes a flow diode, a combustion chamber and 
a resonance tube. Fuel and air enter the combustion chamber. An ignition 
source detonates the explosive mixture in the combustion chamber during 
start-up. The sudden increase in volume, triggered by the rapid increase 
in temperature and evolution of combustion products, pressurizes the 
chamber. As the hot gas expands, the valve, preferably a fluidic diode, 
permits preferential flow in the direction of the resonance tube. Gases 
exiting the combustion chamber and the resonance tube possess significant 
momentum. A vacuum is created in the combustion chamber due to the inertia 
of the gases within the resonance tube. The inertia of the gases in the 
resonance tube permits only a small fraction of exhaust gases to return to 
the combustion chamber, with the balance of the gas exiting the resonance 
tube. Since the chamber pressure is below atmospheric pressure, air and 
fuel are drawn into the chamber where autoignition takes place. Again, the 
valve constrains reverse flow, and the cycle begins anew. Once the first 
cycle is initiated, engine operation is thereafter self-sustaining. 
The flow diode utilized in many other pulse combustion concepts is a 
mechanical "flapper valve." The flapper valve is actually a check valve 
permitting flow from inlet to chamber, and constraining reverse flow by a 
mechanical seating arrangement. This served quite well for the purpose 
intended. While such a mechanical valve may be used in conjunction with 
the present system, an aerodynamic valve without moving parts is 
preferred. With an aerodynamic valve, during the exhaust stroke, a 
boundary layer builds in the valve, and turbulent eddies choke off much of 
the reverse flow. Moreover, the exhaust gases are of a much higher 
temperature than the inlet gases. Therefore, the viscosity of the gas is 
is much higher and the reverse resistance of the inlet diameter, in turn, 
is much higher than that for forward flow through the same opening. These 
phenomena, along with the high inertia of the exhuasting gases in the 
resonance tube, combine to yield preferential and mean flow from inlet to 
exhaust. Thus, the preferred pulse combustor is a self-aspirating engine, 
drawing its own air and fuel into the combustion chamber and auto-ejecting 
combustion products. 
Rapid pressure oscillations in the combustion chamber generate an intense 
oscillating flow field. In the case of coal combustion, the fluctuating 
flow field causes the products of combustion to be swept away from the 
reacting solid thus providing access to oxygen with little or no diffusion 
limitation. Second, pulse combustors experience very high mass transfer 
and heat transfer rates within the combustion zone. While these combustors 
tend to have very high heat release rate (typically 10 times those of 
conventional burners), the vigorous mass transfer and high heat transfer 
within the combustion region result in a more uniform temperature Thus, 
peak temperatures attained are much lower than in the case of conventional 
systems. This results in a significant reduction in nitrogen oxide 
(NO.sub.x) formation. The high heat release rates also result in a smaller 
combustor size for a given firing rate and a reduction in the residence 
time required. 
Performance of atmospheric fluidized bed combustors is affected by the rate 
of combustion of coal, which in turn is affected by coal properties 
(devolatilization, swelling, fragmentation, and char combustion), feed 
particle size range, feed system and combustion-enhanced mechanic 
attrition, heat and mass transfer rates, and unit operating conditions. 
Furthermore, for such systems the carbon carryover into the primary 
particle separator is generally high due to limited residence time of fuel 
fines in the combustor. To achieve high carbon utilization efficiency, 
recycling of fines to the bed has been often practiced. These recycle 
processes add to system complexity and cost and at times are prone to 
plugging. According to the present invention, higher combustion efficiency 
can be attained because the fuel fines are burned in the pulse combustor 
and only the coarse coal which has been classified is burned in the fluid 
bed. 
The three "Ts" of combustion, namely, 1) temperature, 2) turbulence, and 3) 
residence time for the pulse combustor and the bubbling fluid-bed 
freeboard are quite different, as shown below. 
______________________________________ 
Atmospheric 
Fluidized Bed 
Pulse Combustor 
Freeboard Zone 
______________________________________ 
Temperature &gt;1092.degree. C. or 
843.degree. C. or 1550.degree. F. 
2000.degree. F. (High) 
(Low) 
Turbulence Very High Moderate (Plug f 
(Oscillatory) with back mixing) 
Gas Residence 
10 to 100 2 to 3 seconds 
Time milliseconds 
______________________________________ 
Since the present invention employs both a pulse combustor and an 
atmospheric fluidized bed combustor, it can handle the full-size range of 
coarse and fines. The oscillating flow field in the pulse combustor 
provides for high interphase and intraparticle mass transfer rates. 
Therefore, the fuel fines essentially burn under kinetic control. Due to 
the reasonabily high temperature (&gt;1093.degree. C. but less than the 
temperature for ash fusion to prevent slagging), combustion of fuel fines 
is substantially complete at the exit of the pulse combustor. The 
additional residence time of 1 to 2 seconds in the freeboard zone of the 
fluidized bed unit ensures high carbon conversion and, in turn, high 
combustion efficiency. 
Devolatilization and combustion of fuel fines in the pulse combustor also 
enable the release of a significant portion of sulfur by the time the fuel 
fines leave the tailpipe or resonance zone. This sulfur has a high 
probability of capture in the dense fluid bed due to the pulse combustor 
effluxing into the fluid bed. The acoustic field radiated into the fluid 
bed enhances the mass transfer rate and in turn increases the reaction 
rate between the sorbent and SO.sub.2. Acoustic enhancement in the fluid 
bed mass transfer process, and the fines recirculation as a consequence of 
the draft tube design help achieve high sulfur capture efficiency at low 
Ca/S molar feed ratio, which leads to lower limestone and waste disposal 
costs. 
Pulse combustors are inherently low NO.sub.x devices. The rate of heat 
transfer in the pulsating flow is higher than that in conventional steady 
flow and helps create lower overall temperature in the combustion chamber. 
Also, the high rates of mixing between the hot combustion products and the 
colder residual products from the previous cycle and the incoming cold 
reactants create a short residence time at high temperature quenching the 
NO.sub.x production. These complementary mechanisms create an environment 
which approximates a well-stirred tank at relative low temperature and 
result in low NO.sub.x production. The dense fluid bed in the lower 
section 12 of vessel 10, due to operation at low temperature and with 
coarse fuel particles, enjoys a lower NO.sub.x production as well. 
Consequently, the NO.sub.x emissions from systems of the present invention 
are believed to be lower than that of conventional fluid bed combustors. 
The overall heat transfer coefficient in the water-jacketed pulse combustor 
tailpipe is of the same order as that for tubes immersed in the dense 
fluidized bed. The replacement of the inefficient heat exchanger in the 
freeboard zone of a conventional bubbling fluidized bed combustor by the 
water-jacketed pulse combustor tailpipe significantly decreases the heat 
transfer surface area requirement and cost. 
In order to establish the technical merit of the technology according to 
the present invention, a laboratory-scale system (1.58GJ/hr--1.5 
MMBtu/hr--coal firing rate) was designed, built and tested. A schematic of 
the unit is shown in FIG. 2. The primary objective of this work was to 
investigate the integration of a pulse combustor with the fluidized bed 
portion of a furnace. A convective section was not included since the 
additional expense was considered unjustified. Therefore, the steam output 
and the thermal efficiency of the unit tested are somewhat lower than 
those expected in normal practice. 
In FIG. 2, the apparatus as described with respect to FIG. 1 is illustrated 
with like members assigned, in conjunction with related process equipment. 
After classification of solids, e.g. coal, into fines and coarse particles 
(not shown), the coarse particles are maintained in coal bin 73 from which 
the particles are fed by a conveyor 75 into a sorbent feed bin 76 where 
sorbent is fed into the coal supply as noted hereinbefore. The 
coal-sorbent mixture is then fed to vessel 10 by feed conveyor 70 and 
falls onto the dense bed located in lower section 12 of vessel 10 which is 
being maintained in a bubbling fluidized state by fluid entering 
therebeneath through fluid distributor means 13. 
Fines separated from the coal are stored in a bin 74 therefor and are fed 
to an eductor where they are transported via line 72 to pulse combustor 
30. Assuming that pulse combustor is in an operational state, aerodynamic 
valve 32 pulls in an air-feed mixture on demand. As shown in FIG. 2, 
natural gas is also fed to pulse cobustor valve 32 where it also serves as 
fuel. Products of combustion from pulse combustor 30 then proceed with an 
oscillating acoustic pressure wave through resonance tube or tailpipe 36, 
through diffuser section 40 and into the fluidized bed. Generally 
speaking, heat release rate in pulse combustor 30 is in a range of from 
about 2 to about 6 MM Btu/hr/ft..sup.3 ; with product gas temperature in a 
range of from about 1400.degree. to about 3500.degree. F. Gas velocity in 
the resonance tube is in a range of from about 150 to about 1600 feet per 
second with velocity oscillation in a range of from about 20 to about 150 
Hertz. 
In the vessel, acoustic pressure wave level in a range of from about 100 to 
about 185 dB are achievable. Likewise temperatures are achievable in the 
vessel 10 up to about 2000.degree. F. based on volumetric heat releases 
from the pulse combustor in a range of from about 100,000 to about 200,000 
Btu/hr/cu.ft. Temperatures in the freeboard zone 14, 16 of vessel 10 may 
then exceed 2000.degree. F., and are capable of destruction of organic 
materials. In the fluidized bed, temperatures in a range of from about 
1500.degree. to about 1700.degree. F. are desired to minimize nitrogen 
oxides. 
The acoustic wave exiting diffuser 40 and impacting in the fluidized bed 
brings about enhanced mixing and heat transfer. The solid fuel in 
fluidized state is combusted while temperatures in the bed may be 
controlled by heat transfer medium passing through tubes 60 submerged in 
the fluidized bed. Obviously heat transfer from the bed to the medium may 
be used to both control the overall temperature of the fluidized bed 
and/or to create a desired resultant effect on the medium, i.e. to heat 
water or air, to produce steam or the like. 
Products of combustion then rise above the fluidized bed into the freeboard 
zone, where further heat transfer or reaction may take place, and from the 
freeboard zone through entrained solids separator 82 and out the flue gas 
exit 80 to cyclone 90. Since the fuel has been classified, minimal fines 
are elutriated into the freeboard zone, thus again lessening the release 
of sulfur. 
Also in the overall scheme of operation, the fluidizing medium, e.g. air or 
steam may be preheated in preheater 92. Fluidizing medium is supplied to 
preheater 92 by a primary air blower 94 and/or return of excess air or 
other fluid from pulse combustor 30. Likewise as illustrated, steam 
generated in tubes 60 passes therefrom to a steam drum 96 and from drum 96 
as desired. 
In a high sulfur coal burning process, limestone and coarse, classified 
coal are fed onto the fluidized bed within vessel 10 while the fines are 
fed, as noted above, to pulse combustor 30 as a fuel source. Sulfur in the 
fines is basically removed in the pulse combustor and is picked up by the 
limestone in the fluidized bed. Likewise sulfur in the coarse coal is 
captured by limestone in the bed in a more efficient manner than in prior 
art systems. To accomplish such, temperatures in the fluidized bed are 
preferably maintained in a range of from about 1400.degree. to about 
1750.degree. F. Likewise in this temperature range less nitrogen oxide 
byproducts are produced. 
A total of 28 tests were performed on apparatus as shown in FIG. 2, 
including shakedown and characterizations tests. The unit was tested both 
with and without the pulse combustor, and test parameters are given in 
Table 1. The system was on-line for more than 200 hours and combusted 
nearly 9 tons of coal. N.sub.2 O emissions measurements were made in 
collaboration with Drs. L. J. Muzio and G. Shiomoto of Fossil Energy 
Research Corporation, Laguna Hills, Calif. 
TABLE 1 
______________________________________ 
Test Parameters 
______________________________________ 
Coal Type: Pittsburgh No. 8, 
W. Kentucky Nos. 9 
and 11 
Coal Size Distribution: 
9.5 mm (3/8") by 0 
with 15-40% fines 
by wt. 
Limestone: Shasta 
Limestone Size Distribution: 
3.2 mm (1/8") by 0 
Superficial Gas velocity: 
1.52-2.13 m/s (5- 
7 ft/s) 
Bed Temperature: 815-871.degree. C. (1500- 
1600.degree. F.) 
Ca/S Ratio: 2.5-2.7 
Bed Area: 0.61 m .times. 0.61 m 
(2' .times. 2') 
Furnace Height: 3.05 m (10') 
Pulse Combustor Fuel: 
Coal, Gas 
______________________________________ 
A summary comparison of the performance and emissions data from the 0.61 
m.times.0.61 m (2'.times.2') facility according to the present invention 
with those from conventional bubbling fluidized bed combustors (taller 
freeboard and recycle operation) and circulating fluidized bed combustor 
units is given in Table 2. The comparison is for typical high-volatile 
bituminous coals and sorbents of average reactivity. The values indicated 
for the prior art fluidized bed combustors are based on published 
information. It is seen that the system according to the present invention 
exhibits superior performance in relation to the prior art. The higher 
combustion efficiency translates into reduced coal consumption and lower 
system operating cost; the improvement in sulfur capture implies less 
sorbent requirement and waste generation and in turn lower operation cost; 
lower NO.sub.x and CO emissions mean ease of siting; and greater 
steam-generation rate translates into less heat exchange surface area and 
reduced capital cost. Also, it seems that N.sub.2 O emissions from 
fluidized bed technology are not insignificant but are comparable to 
published data on NO.sub.x emissions. The mode of operation does not have 
much influence on N.sub.2 O emissions. In summary, the present system 
performance generally (i) surpasses those of the conventional systems, 
(ii) is comparable to circulating fluidized bed combustion in combustion 
and N.sub.2 O emissions, and (iii) is better than circulating fluidized 
bed combustors in sulfur capture and CO and NO.sub.x emissions. 
TABLE 2 
______________________________________ 
Performance Characteristics of Fluidized 
Bed Combustors 
______________________________________ 
Pulsed Circulat- 
ATM ATM Bubbling* 
ing* 
______________________________________ 
Combustion 
89-93 92-97 90-97 93-99 
Efficiency % 
SO.sub.2 Capture 
70-85 90-98 70-85 75-95 
Effic. (%) 
NO.sub.x 155-620 110-265 400-500 100-300 
Emissions 
(ppmv)+ 
N.sub.2 O 
70-100 70-100 10-220 10-220 
Emissions 
(ppmv)+ 
CO Emissions 
400-1600 180-800 400-1200 
500-1500 
(ppmv)+ 
Steam Rate 
227-317 363-372 
kg/s (lb/hr) 
(500-700) (800-820) 
______________________________________ 
Test Parameters 
______________________________________ 
Bed Temperature 815-871.degree. C. (1500-1600.degree. F.) 
Ca/S Ratio 2.5-2.7 
Coal Bituminous (high volatile) 
______________________________________ 
*Based on literature data 
These factors indicate the present invention to be an attractive option at 
any scale. The fact that it is impractical and expensive to scale-down a 
circulating fluidized bed combustor to the 0.126 to 6.3 kg/s (1,000 to 
50,000 lb/hr) steam equavalent range as noted above. 
Apparatus as described heretofore has been directed primarily to a system 
for combusting high sulfur content coal. Such apparatus, particularly as 
described with respect to FIG. 1, can also be employed for improved 
combustion of other products such as by way of example, biomass, waste 
products exemplified by medical waste, industrial waste, organics and the 
like, for endothermic reactions, drying, calcining and the like. 
It will be understood, of course, that while the form of the invention 
herein shown and described constitutes a preferred embodiment of the 
invention, it is not intended to illustrate all possible form of the 
invention. It will also be understood that the words used are words of 
description rather than of limitation and that various changes may be made 
without departing from the spirit and scope of the invention herein 
described.