Method and apparatus for extracting heat from a combustible material

A method and apparatus for extracting heat from a combustible material are disclosed which include the formation of a solid material strip of the combustible material, the formation of a series of spaced holes in the material strip, and the combustion of the material strip. The method and apparatus are particularly useful when a mixture of coal, clay, and lime is used as the combustible material, and the products of the combustion then include a structurally intact, solid residual strip composed substantially only of dehydrated clay and calcium sulfate, and a fluegas substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash. Because of the stable and uniform combustion characteristic, it is also suitable for the co-combustion and reduction of most toxic and solid wastes into safe, recyclable residues.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to methods and apparatuses for 
extracting heat from combustible materials, and, more particularly, to a 
method and apparatus for combusting a solid material strip at a controlled 
temperature. 
2. Description of the Prior Art 
Historically, solid fuel was burnt mainly by the flatbed combustion 
process. In the past two decades, solid fuel was replaced by liquid fuel, 
because liquid was less labor intensive and was cleaner in operation. The 
liquid fuel was burnt by spraying the liquid fuel through a high-speed 
nozzle to atomize the fuel. In recent years, as liquid fuel became more 
costly, many power plants were modified for both liquid and powdered solid 
fuel combustion using the spraying technique originally designed for the 
liquid fuel. For home heating, liquid fuel is still in wide use, mainly 
due to the handling and pollution problems of solid fuel. 
The fundamental problem with both liquid and powdered solid fuel combustion 
is that both types of combustion are unstable runaway forms of combustion 
which generally take place within 10 milliseconds, since it is nearly 
impossible to control the reaction rates inherent in these types of 
combustions. The resultant high temperatures of combustion promote 
excessive production of carbon monoxide, nitrogen oxides, and sulfur 
oxides, and the production of corrosive and coating micro flyash. These 
byproducts of combustion are the cause of many environmental and 
operational problems, i.e. the pollution of the atmosphere and the coating 
of heat exchangers resulting in reduced heat exchange efficiency, whose 
avoidance greatly increases the cost of combusting liquid and powdered 
solid fuels. 
Also, the processes used in the prior art combustion methods often result 
in combustion temperatures of over 2000.degree. C. At such a high 
temperature the combustion product is mainly CO instead of CO.sub.2. 
Consequently, a large power plant requires a mammoth combustion chamber to 
provide sufficient resident time for the CO gas to complete a secondary 
combustion into CO.sub.2 at a lower temperature of about 1000.degree. C., 
thus increasing the cost of operating the power plant. It is clear that a 
form of combustion is needed which avoids these problems. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to overcome the 
disadvantages of the prior art methods by providing a method and apparatus 
of extracting heat from a combustible material which includes combusting a 
solid material strip having regularly shaped holes of a predetermined size 
therein. 
Another object of the present invention is to provide a heat extraction 
method and apparatus which includes combusting a solid material strip at a 
desired temperature such that a fluegas produced by the combustion is 
substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, and 
micro fly-ash. 
Another object of the present invention is to provide a heat extraction 
method and apparatus in which one product of a combustion is a 
structurally intact, solid residual strip which may be easily removed from 
a combustion oven after combustion. 
Another object of the present invention to provide a heat extraction method 
and apparatus which is relatively inexpensive to operate and maintain. 
It is yet another object of the present invention to mix chemical compounds 
with the solid fuels for the purpose of complete and safe dissociation of 
toxic compounds either within the solid fuels or the additives into 
non-toxic solid residuals or flue gas. The stable and uniform combustion 
characteristics of the present invention is also ideal for the safe 
disposal of most garbage and toxic wastes. 
The method of the present invention of extracting heat from a combustible 
material includes the steps of forming a solid material strip of the 
combustible material, forming a plurality of spaced holes in the material 
strip, and combusting the material strip in a combustion oven. 
The method of the present invention may also include the steps of forming a 
solid material strip of the combustible material and providing a plurality 
of spaced holes punched into the material strip, and preheating the 
material strip in the absence of ambient air. The material strip is 
combusted in a combustion oven by two independent controls. The desired 
temperature of combustion is regulated by forcing air or oxygen into the 
spaced holes at a controlled rate, and a fluid is circulated at a 
controlled rate to carry heat from the combustion of the material strip to 
a heat exchanger at a desired rate. During the combustion, a structurally 
intact, solid residual strip is produced, and the residual strip is 
removed from the combustion oven. 
The apparatus of the present invention for extracting heat from a 
combustible material includes a means for forming a solid material strip 
of the combustible material, a means for forming a plurality of spaced 
holes of predetermined diameters and spacings required for different rates 
of combustion in the material strip, and a means for combusting the 
material strip. The material strip combusting means includes a combustion 
oven. 
The apparatus of the present invention may include an extrusion screw for 
extruding a solid material strip of the combustible material and a 
conveyor extending away from the extrusion screw for carrying the material 
strip. Located along the conveyor are a hole punch for punching a 
plurality of spaced holes into the material strip, and a preheating oven 
for preheating the material strip. A combustion oven located along the 
conveyor for combusting the material strip includes an inlet for the 
material strip, an outlet for a structurally intact solid residual strip, 
means for intaking air into the combustion oven, and means for removing a 
hot fluegas from the combustion oven.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, the heat extracting apparatus of the present 
invention includes a hopper 10, an extrusion screw 14 located at the lower 
end of the hopper 10, and a conveyor 18 extending away from the extrusion 
screw 14 which runs the length of the apparatus. Located along the length 
of the conveyor are a hole punch 20, a preheating oven 24, a combustion 
oven 26, and an air-intake preheating chamber 27. The combustion oven 26 
is connected to the preheating oven 24 by a first conduit 25, and includes 
a material strip inlet 28, a material strip outlet 30, an inlet 32 for 
air/oxygen and a heat exchanging fluid/fluegas, and first and second 
outlets 34,35 for a fluegas produced in the combustion oven 26. A heat 
exchanger 36 is also located within the combustion oven 26. 
Piping 38 connects the first fluegas outlet 34 with a first blower 40 which 
in turn is connected by piping 42 to the air and heat exchanging fluid 
inlet 32. Piping 44 connects the second fluegas outlet 35 with the 
preheating oven 24. 
The preheating chamber 27 is connected to the combustion oven 26 by conduit 
46, and includes a preheating chamber residual strip inlet 48, a 
preheating chamber residual strip outlet 50, an air/oxygen inlet 52, and 
an air outlet 54. Air outlet 54 is connected to a second blower 56 by 
piping 58, and the second blower 56 is in turn connected to the air and 
heat exchanging fluid inlet 32 by piping 60. Both of the first and second 
blowers 40,56 are controlled by a computer 62, as is indicated generally 
at 64 and 66. 
in operation, basic data input to the computer 62 are the system heat 
output 90 at the heat exchanger 36 and the combustion temperature 92 of 
the gas at A. The computer 62 compares these parameters with the system 
heat output rate command 91 and combustion temperature command 93 to 
operate the respective blowers 40 and 56 for an automatic regulation of 
the combustion system. 
In operation, the hopper 10 holds a supply of a combustible material 12, 
generally in a granular or pulverized form. In the preferred embodiment, 
the combustible material 12 includes a moldable mixture of coal, clay, 
water and lime. As the extrusion screw 14 is turned, it extrudes a solid 
material strip 16 of the combustible material 12, and the preferred form 
of the material strip 16 is that of a continuous strip of uniform 
rectangular cross section. However, a material strip which is of a short 
finite length, or which has a non-rectangular cross section, may be used 
to achieve at least some of the advantages of the present invention. 
The extrusion screw 14 extrudes the material strip 16 onto the conveyor 18, 
and the conveyor 18 carries the material strip 16 along the length of the 
apparatus. As the material strip 16 is carried along the conveyor 18, the 
hole punch 20 operates to punch a plurality of spaced holes 22 into the 
material strip 16. As will be described below, the diameter and spacing of 
the holes 22 is critical for achieving the desired combustion of the 
material strip 16. the feed rate for the material strip 16 will be 
controlled by the computer 62 depending on the power demand 91 and other 
secondary factors relating to the composition of fuels. 
The material strip 16 then enters the preheating oven 24 wherein hot 
fluegas supplied by the piping 44 from the combustion oven 26 is 
circulated over the material strip 16. The preheating of the material 
strip 16 occurs in the absence of ambient air/oxygen, and so the 
temperature of the material strip 16 is raised to about 800.degree. C. in 
the preheating oven 24 without combustion of the material strip 16. While 
the material strip 16 is being preheated, the volatile components of the 
coal in the material strip 16 are extracted with the circulating fluegas 
and are removed from the preheating oven 24 as by piping 68 so that the 
volatile components may be processed to obtain side products. 
The preheated material strip 16 is next carried into the combustion oven 26 
by the conveyor 18 through the first conduit 25 and the material strip 
inlet 28. As will be discussed in more detail below with reference to FIG. 
2, air supplied by the air and heat exchanging fluid inlet 32 below the 
conveyor 18 flows into the spaced holes 22 in the material strip 16 and 
the material strip 16 begins to combust. The rate at which air is supplied 
from the preheating chamber 27 to the air and heat exchanging fluid inlet 
32 by the second blower 56 is controlled by the computer 62 such that the 
combustion of the material strip 16 occurs at a desired temperature. In 
the preferred embodiment, this desired temperature is substantially 
between 800.degree. C. and 1200.degree. C. 
As the material strip 16 is carried through the combustion oven 26 it 
continues to combust at the desired temperature until the material strip 
16 has completely combusted. At this point, the material strip is in the 
form of a structurally intact, solid residual strip 70 which, in the 
preferred embodiment, is composed substantially only of dehydrated clay 
and calcium sulfate. During combustion, a fluegas is produced as shown by 
arrows A, and, due to factors which are discussed herein below, the 
fluegas is substantially free of carbon monoxide, nitrogen oxides, sulfur 
oxides, and micro fly-ash. The fluegas circulates over the heat exchanger 
36 and is then removed from the combustion oven 26 by the first and second 
fluegas outlets 34,35. Fluegas which is removed from the combustion oven 
26 through the first fluegas outlet 34 is recirculated into the combustion 
oven 26 by the first blower 40 through piping 38, piping 42, and the air 
and heat exchanging fluid inlet 32. The first blower 40 is controlled by 
the computer 62 such that the rate of recirculation of the fluegas 
produces a desired rate of heat exchange between the fluegas and the heat 
exchanger 36. 
The residual strip 70 which remains on the conveyor 18 after complete 
combustion of the material strip 16 is then carried out of the combustion 
oven 26 through the residual strip outlet 30. The residual strip 70 is 
carried through the second conduit 46 and into the preheating chamber 27 
through the preheating chamber residual strip inlet 48. While the residual 
strip 70 is carried through the preheating chamber 27, air is cycled over 
and through the residual strip 70 from the air inlet 52 to the air outlet 
54, thereby preheating the air before it is supplied to the combustion 
oven 26. The residual strip 70 is then carried out of the preheating 
chamber 27 through the preheating chamber residual strip outlet 50, after 
which it can be removed for disposal or for further processing. In 
particular, the residual strip 70 is especially suitable to be reclaimed 
for the manufacture of cement, building block, and road pavement, since as 
discussed below it is substantially free of residual carbon. 
FIG. 2 shows a cutaway view of the material strip 16 as it is combusted in 
the combustion oven 26. As seen from FIG. 2, the combustion oven 26 
includes inside walls 72 made of, for example, heat insulating brick. The 
material strip 16 is carried on the conveyor 18 (not shown) through 
combustion oven 26 such that the walls 72 of the combustion oven 26 
closely contact the edges of the material strip 16. This contact prevents 
air from passing between the walls 72 and the material strip 16. As a 
result, as air and recirculated fluegas is supplied to the combustion oven 
26 as shown by arrows B. The air and fluegas mixture is thereby forced 
into and through the combusting holes 22. 
As mentioned above, the flow of air is regulated by the computer 62 through 
the second blower 56 such that the combustion temperature of the material 
strip 16 is between 800.degree. C. and 1200.degree. C. In the preferred 
embodiment, the combustion temperature is maintained substantially at 
1000.degree. C., which is measured by the radiating color of the material 
strip 16. This combustion temperature is in contrast to atomized fuel 
combustion processes, which often have combustion temperatures of over 
2000.degree. C. Such a high combustion temperature results in the 
production of large amounts of carbon monoxide, which requires a very 
large combustion chamber to allow the carbon monoxide to undergo a 
secondary combustion to carbon dioxide. Because the combustion process of 
the present invention occurs at a much lower temperature and at a much 
slower and steady rate, carbon monoxide produced during the combustion 
undergoes a secondary combustion to carbon dioxide substantially only 
within the spaced holes 22 and immediately thereabove, thus resulting in 
blue tongues of fire 80 extending from each of the holes 22. Also, a 
minimum stoichiometry of 1.0 is all that is needed to maintain ideal 
combustion in the present invention, as opposed to a minimum stoichiometry 
of 1.3 to 1.5 required for the conventional processes. As a result, flue 
heat loss is greatly reduced. 
Because the rate of combustion of the material strip 16 is controlled such 
that combustion occurs more slowly and steady than atomized combustion 
processes, the combustion process of the present invention is much more 
complete and produces far less toxic byproducts than the atomized 
combustion processes. Substantially all of the carbon in the material 
strip 16 is combusted in the combustion oven 26. Also, substantially all 
of the sulfur present in the coal of the preferred embodiment is captured 
in the material strip 16 because of the intimate physical contact between 
the lime and the coal particles in the material strip 16, and 
substantially none of the nitrogen present in the air forms nitrogen 
oxides during the combustion process. As a result, the products of the 
combustion include a fluegas which is substantially free of carbon 
monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash, and a 
structurally intact solid residual strip composed substantially only of 
dehydrated clay and calcium sulfate. Because the residual strip 70 is 
substantially free of residual carbon, it is particularly suitable for 
reclamation for the manufacture of cement, building block, and road 
pavement. On the other hand, the ash from present power plants is not 
suitable for these applications because of its high carbon content. 
FIGS. 3 and 4 give sectional views of one of the plurality of spaced holes 
22 as the material strip 16 undergoes combustion at a low rate, and at a 
high rate, respectively. The combustion process of the present invention 
is necessarily ablative in nature, and so as the combustion progresses the 
effective reacting surface available within the hole 22 will increase. 
This tendency for the combustion rate to increase is counterbalanced, 
however, because the diffusion path which oxygen from the air entering the 
hole must travel to reach the reacting surface and which carbon monoxide 
from the reacting surface must travel to reach the hole 22 increases as 
the combustion progresses. It provides a stable combustion environment for 
further regulation of its combustion rate and temperature by a computer. 
As will be seen from FIGS. 3 and 4, the diameter to length ratio of the 
hole 22 is critical to the achievement of proper combustion of the 
material strip 16. With reference to FIG. 3, when the material strip 16 
undergoes combustion at a low combustion rate the gas flow speed in the 
hole 22 is very slow. Consequently, the CO gas formed in the wall of the 
hole 22 will have sufficient TIME to diffuse across the center line D of 
the hole 22 to react with the O.sub.2 gas in the hole 22. The O.sub.2 gas 
is therefore nearly exhausted near the exhaust end 74 of the hole 22. 
Thus, to achieve proper combustion at a low combustion rate, the diameter 
to length ratio of the hole 22 should be sufficiently large to prevent the 
complete depletion of O.sub.2 in the hole 22 so that at least some oxygen 
will be present at the exhaust end 74 of the hole 22. 
With reference to FIG. 4, under a high combustion rate there will be a much 
faster gas flow speed in the hole 22, and the boundary layers of the 
O.sub.2 and the CO will be very thin. Consequently there will be a high 
concentration of O.sub.2 at the exhaust end 74 of the hole 22, and most of 
the secondary combustion of CO to CO.sub.2 will take place immediately 
above the hole 22, which gives rise to a long blue tongue of fire 80. 
Therefore, for a high combustion rate, more holes with a small diameter to 
length ratio must be punched in the material strip to insure complete 
secondary combustion of the CO to CO.sub.2. 
While this invention has been illustrated and described in connection with 
the preferred embodiments, it is recognized that variations and changes 
may be made and equivalents may be employed herein without departing from 
the scope of the invention as set forth in the claims. For example, 
instead of carrying away the volatile components of the coal in the 
material strip 16 for processing after they have been extracted in the 
preheating oven 24, the volatile components may be separately carried to 
the combustion oven 26 to be combusted. Also, the heat exchanger 36 need 
not be immediately within the combustion oven 26, as the fluegas may be 
carried from the fluegas outlet 34 to a separate heat exchanger before the 
fluegas is recirculated into the combustion oven 26. 
Additionally, the fluegas from the combustion oven 26 need not necessarily 
be used as the heat exchanging fluid which is supplied to the air and heat 
exchanging fluid inlet 32, as one skilled in the art would realize that 
other fluids may be substituted for the fluegas while retaining at least 
some of the advantages of the present invention. Also, in the preheating 
chamber 27 the air need not be directly physically passed over and through 
the residual strip 70 to preheat the air, although this is the most 
efficient method of heat transfer.