Method and apparatus for operating a multiple hearth furnace

A multiple hearth furnace having a drying zone, a combustion zone and a cooling zone includes a recirculation loop that recycles exhaust gas from the drying zone to the cooling zone. In some embodiments, a first control loop including a temperature measurement device that measures temperature in the combustion zone controls fan speed of a recirculation fan that drives the recirculation loop. A second control loop monitors recirculation fan temperature and overrides the first control loop if the recirculation fan temperature exceeds a predetermined maximum. A third control loop controls air flow into the furnace.

TECHNICAL FIELD 
This invention relates to incineration, and more specifically, to a method 
and apparatus of controlling the incineration of sludge, slurry, and 
similar materials in multiple hearth furnaces such as those used in waste 
water treatment plants. 
BACKGROUND OF THE INVENTION 
The disposal of waste water sludge has become an increasingly difficult 
problem in recent years. With land fills becoming over filled, pressure 
from environmental groups mounting, and legislation directed at stopping 
ocean dumping, waste water from municipal sewage systems is often 
incinerated, thereby yielding inert ash material. By far, the overwhelming 
majority of such disposal is accomplished through the use of multiple 
hearth furnaces. 
FIG. 1 shows a very high level conceptual block diagram of a conventional 
multiple hearth furnace 101 comprising eleven hearths 1 through 11. 
Hearths 1 through 11 are constructed to support the many pounds of sludge 
or other material to be incinerated. The sludge is fed in through an input 
port 119 and is thereby placed on the top of hearth 1. In some systems, 
the sludge may be fed through an opening to enter the second hearth 
instead of the top hearth, thereby allowing the top hearth to be used as 
an afterburner for emissions control. The remainder of the operation of 
multiple hearth furnace 101 serves to move the sludge to be incinerated 
through the hearths one through eleven until an inert ash to be disposed 
of exits the system through output port 114. The technique of causing the 
movement will be discussed later herein. 
The eleven hearths shown in FIG. 1 are typically divided into three 
different major zones. These zones, from top to bottom, are termed the 
drying zone 120, the combustion zone 121 and the cooling zone 122. In the 
present example, the drying zone 120 comprises hearths 1 through 4 and is 
utilized to dry the sludge from a water content of approximately 70-85%, 
when the sludge is received through input port 119 in a typical waste 
water treatment plant, to a water content of approximately 45 to 65 
percent by weight. 
Once the sludge is dried enough to reach 45 to 65 percent liquid by weight, 
it is forced downwardly into the combustion zone 121 and combated. Most of 
the volatile material is combated in the upper hearths 5 and 6 of 
combustion zone 121, thereby producing temperatures in the range of 
approximately 1200 to 1900 degrees Fahrenheit. This removes most of the 
volatile portion of the combustible material and produces a material 
containing inert ashes and solid carbon residue. The lower hearths 7 and 8 
are used to burn any remaining carbon. Thus, the combustion zone is 
sometimes considered two zones, an upper combustion zone for burning most 
of the volatile material in the sludge, and a lower combustion zone for 
incinerating the remaining carbon. In the present example, hearths 5 and 6 
comprise the upper combustion zone, and hearths 7 and 8 comprise the lower 
combustion zone, thereby forming an entire combustion zone of four 
hearths. 
After combustion, the sludge, now essentially all inert ash, reaches the 
lowest hearths 9 through 11 which make up the cooling zone 122, and exits 
from opening 114. The cooling zone includes air, sometimes forced in from 
outside of the system with a fan. The final product exiting from output 
port 114 is inert ash at a temperature of approximately 100.degree. F. 
FIG. 2 shows a typical arrangement of four arms 201 through 204 on central 
shaft 115. Each arm contains a plurality of rabble teeth 210. 
During operation, the central shaft 115 rotates and the arms 201-204 move 
around the hearth, with rabble teeth 210 forcing the sludge toward the 
center of the hearth where it may be forced through opening 206 to the 
next hearth below. As can be appreciated from FIG. 1, some of the hearths 
include an opening 206 of FIG. 2 in the center of the hearth, while others 
include the openings 116 at the outer edge of the hearth, as shown in FIG. 
1. The rabble teeth 210 for each hearth are tilted inwardly or outwardly 
in such a manner that causes the sludge to be forced towards the outside 
of the hearth for those hearths where the opening is at the outer edge of 
the hearth, and towards the inside of hearth for those hearths where the 
opening is towards the inside of the hearth as in FIG. 2. 
In conventional multiple hearth furnaces such as that depicted in FIGS. 1 
and 2 hereof, the temperature required for each of the zones is, for the 
most part, manually controlled. Specifically, air is injected into the 
combustion zone, usually through the cooling zone, in a quantity which is 
sufficient to supply the required oxygen for proper combustion. 
Additionally, auxiliary burners may be provided on the furnace in order to 
make up any heat deficient in the drying or combustion of the materials. 
In recent furnaces however, due to higher capacity and dryer feed 
materials, additional excess air is often pumped into the combustion zone. 
The excess air is required to offset the hotter burning, increased 
capacity furnaces, and specifically, in order to appropriately limit the 
peak temperature thereof. The introduction of additional air into the 
combustion zone brings with it several disadvantages. 
One such disadvantage is that the additional air results in the consumption 
of additional energy to power the larger fans required to power the 
exhaust gas cleaning equipment. In addition, the higher oxygen 
concentration that results from air being pumped into the combustion zone 
causes an increase in the presence of nitrogen oxides in the exhaust gas, 
as well as the formation of melted residual ash near the end of the 
combustion zone. Moreover, the increased flow of air often results in 
extinguished combustion in the carbon burning zone which results in 
incomplete combustion. As a result, metal sulfides may be present in the 
ash exiting the multiple hearth furnace. Finally, the additional air being 
forced through the combustion chamber also leads to a quenching effect 
which causes lumps of partially dried but unburned material called sludge 
balls to pass through the incinerator and present themselves at the ash 
disposal system. 
It is an object of the invention to provide a technique for increasing the 
efficiency of multiple hearth furnaces. 
It is another object of the invention to provide for automatic control and 
adjustment of air flows in multiple hearth furnaces using flue gas 
recirculation. 
It is an object of the invention to increase the efficiency of multiple 
hearth furnaces without introducing so much oxygen into the combustion 
zone such that nitrogen oxide emissions are increased significantly. 
It is another object of the invention to reduce the melted ash (i.e.; slag) 
formed as the sludge makes its way through the numerous hearths. 
It is another object of the invention to increase the capacity of a 
multiple hearth furnace. 
It is still a further object of the invention to provide a technique for 
reducing or eliminating the formation of sludge balls present in the 
material as it presents itself at the lower most hearths. 
SUMMARY OF THE INVENTION 
The above and other problems of the prior art are overcome and a technical 
advance is achieved in accordance with the teachings of the present 
invention which relates to a multiple hearth furnace using a novel 
technique of flue gas recirculation in order to provide for increased 
incineration efficiency as well as a variety of other benefits. In 
accordance with the teachings of the present invention, a fan is installed 
in such a manner as to recirculate flue gases from the drying zone, 
preferably at the top hearth thereof, to the cooling zone, preferably to 
the bottom hearth of the cooling zone. Additionally, a fan may be utilized 
to pump air into the combustion zone. The recirculation of gas from the 
drying zone to the cooling zone results in a slightly heated cooling zone. 
This results in increased combustion without introducing additional oxygen 
into the combustion zone and thus increasing the production of Nitrogen 
Oxides. 
In an enhanced embodiment, a passive infrared detector (PAIR) is utilized 
to control the fan speed of the recirculation fan. Specifically, the fan 
speed utilized in removing gases from the drying zone and recirculating 
them to the cooling zone is adjusted based upon a feedback loop connecting 
such maximum speed adjustment to the output of the PAIR detector. As the 
temperature of the burning carbon increases, the fan speed, as controlled 
by the output of the PAIR detector, is increased. If the fan temperature 
increases too much, the fan may overheat. This problem is avoided by 
including an override such that increased fan temperature above a 
predetermined value results in decreased rotation speed, notwithstanding 
the aforementioned PAIR output. 
Finally, external air is introduced into the feedback path in a sufficient 
quantity to properly regulate oxygen content. A detector measures oxygen 
in an upper hearth and opens or closes an air valve in response thereto. 
Overheating of the recirculation fan results in an override, thereby 
greatly opening the air valve and cooling the fan, irrespective of the 
aforementioned oxygen detector. 
Additional benefits of the invention will be seen from an examination of 
the following description of the preferred embodiment and drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 shows a conceptual block diagram of the arrangement of the present 
invention comprising a plurality of hearths 401 to 411, several external 
burners 412 through 416, and a central shaft 417. Additionally, a fan 420 
is shown as introducing additional air into the furnace often through 
multiple nozzles. 
In operation, as the sludge material to be treated makes its way through 
the system from upper hearth 401 of the drying zone to lower hearth 411 of 
the cooling zone 122, flue gases recirculate via fan 421 and piping 422 in 
order to be returned to the cooling zone at lower hearth 411. Ideally, 
cooling zone 122 comprises three or four hearths, the combustion zone 
comprises three or four hearths, and the drying zone comprises three or 
four hearths. 
Additionally, fan 421 should be arranged in order to provide sufficient 
power to force between 25 and 125 percent of the normal exhaust gas volume 
which would typically exit the drying zone 120 back into the cooling zone. 
Those of skill in this art will be familiar with how to select such a fan. 
While forcing air from anywhere within the drying zone to the cooling zone 
results in improved performance, ideally the system operates by forcing 
air from the top hearth of the drying zone sometimes termed the feed 
hearth, to the lowest hearth of the cooling zone. 
Additionally, it has been found that the recirculation fan 421 should 
provide enough force to recirculate approximately 25 percent to 125 
percent of the normal exhaust gas volume which would exit the drying zone 
if no recirculation fan had been present. The recirculation may also 
provide that gas being recirculated is forced into a plurality of hearths, 
only one of which is the lower most hearth of the cooling zone. For 
example, gas may be recirculated from one hearth in the drying zone to 
plural hearths in the cooling zone, one of which is preferably the lower 
most hearth. Additionally, gas may be recirculated from plural hearths 
within the drying zone to one or more hearths within the cooling zone. 
As an additional improvement, it may be desirable to adjust the amount of 
gas being recirculated based upon parameters such as the highest 
temperature within the combustion zone, which may include one or more 
hearths. Specifically, it has been found that a control loop with feedback 
may be utilized to allow adjustment of the volume of gas recirculated 
based upon the temperature of the combustion zone. An exemplary embodiment 
of such an arrangement will now be discussed. 
FIG. 4 shows an exemplary embodiment of the present invention utilizing an 
enhanced control system for providing control of a flue gas recirculation 
fan 501. The arrangement of FIG. 4 includes a feed hearth 516 which is 
part of the drying zone. As indicated, path 517 depicts the flue gas 
recirculation path from the drying zone back to the cooling zone 518. 
Temperature elements 511, and 513 are preferably passive infrared (PAIR) 
detectors, well-known heat sensing devices for monitoring the temperature 
of the solid material on the hearth. Temperature element 505 is typically 
a thermocouple. 
The arrangement also includes a temperature indicating controller 506, 
temperature transmitters 510 and 512, and variable frequency drive 515. An 
oxygen detector 507 is arranged to measure oxygen content at top hearth 
521, which, in the example of FIG. 4, is an afterburner hearth. As 
indicated by the discontinuities, any number of hearths is possible. 
In operation, FAR fan 501 begins operating with torque supplied by motor 
522 and causes gases from feed hearth 516 in the drying zone to be sucked 
out and recirculated to the cooling zone 518, preferably the bottom hearth 
thereof as shown. The concept behind the control electronics indicated in 
FIG. 4 is to control the speed of the fan based upon the bed temperature 
detected at hearths 508 and 509, which represent the lower combustion zone 
where carbon is combusted as previously described. 
Each of temperature elements 511 and 513 outputs a temperature signal and 
with the assistance of temperature transmitters 510 and 512, transmits a 
voltage or current indicative of such temperature to decision block 523. 
At decision block 523, the greater of the two temperatures is sent to a 
temperature indicator controller 525, which typically outputs a low 
voltage signal. The output 524 of temperature indicating controller 525 is 
therefore a voltage in the range of, for example, 0 to 5 volts. 
Temperature indicating controller 525 varies such voltage according to the 
difference between the predetermined set point and the hottest solids 
temperature of combustion hearths 508 or 509. This voltage is fed into 
decision block 514 and utilized to control the VFD 515 in order to 
increase the speed of the fan as the solids temperature in the hotter of 
hearths 508 and 509 rises. An exemplary set of parameters might be to 
increase the fan speed linearly between 500 RPM and 1350 RPM, as the 
hottest combustion hearth increases from 1400.degree. F. to 1850.degree. 
F. It is preferable to monitor at least two hearths, to be sure the 
maximum temperature is detected. 
As the temperature of the solids in combustion zone 121 increases, so does 
the speed of revolution of fan 501. However, the hot fan presents a danger 
of mechanical failure. Thus, if the fan 501 itself begins to become 
overheated, then the speed of the fan should not be increased. In 
accordance with this goal, temperature element 505, which is typically a 
thermocouple, senses the temperature at the gas input of FGR fan 501 and 
with the assistance of a temperature indicator controller 504 and inverter 
527, sends an inverted voltage signal to comparator 514. If the 
temperature of the fan becomes too hot, then comparator 514 will send 
input 526 as the control signal to VFD 515, thereby decreasing the speed 
of the fan. 
Thus, the rotation speed of the fan is controlled in accordance with the 
maximum solids temperature being generated in combustion hearths 508 and 
509 unless and until that heat becomes so hot that the increased 
revolution of the fan causes the fan to be at risk of mechanical damage or 
failure. In such a case, the fan temperature will take over as the 
controlling signal for fan revolution, thereby slowing down the speed of 
the fan. 
An additional feedback loop is utilized to control an air valve 531 for 
supplying air from external to the system into the FGR path 517. 
Specifically, an oxygen detector 507 and inverter 532 are input into the 
comparator 503. The detector 507 is set to output a voltage in the range 
of 0 to 5 volts DC based upon the oxygen content present in the gas at the 
top of the highest hearth in multiple hearth furnace 502. Specifically, as 
the oxygen content measured by detector 507 increases above a 
predetermined set point, typically in the range of 3 to 8 volume percent, 
the inverter 532 will send a decreased signal to the comparator 503, which 
will normally send the decreased input 533 to a valve 531, thereby closing 
the valve slightly. Accordingly, as the oxygen content measured by 
detector 507 increases, the amount of air, and thus oxygen, allowed in 
from external to the system will decrease because valve 531 will close 
slightly. Conversely, as oxygen content measured by detector 507 
decreases, the valve will open slightly, thereby increasing the input of 
oxygenated air into the system. 
As an override, temperature indicating controller 506 is set to a 
predetermined maximum value of temperature permitted by the fan. For 
example, many stainless steel fans are limited to 1400 degrees Fahrenheit 
when their rpm reaches 1350. If the fan continues to overheat, then 
comparator 503 will receive a greater signal from input 534 than from 533. 
Accordingly, the air valve 531 will be forced open almost entirely when 
the temperature of the fan 501 becomes too hot. This forcing open of the 
air valve, and the flooding of the recirculation path with cool air from 
external to the system, occurs notwithstanding the oxygen content measured 
by detector 507. 
Thus, while the oxygen content in the drying zone is normally used as the 
feedback parameter for adjusting valve opening, the valve opening is 
adjusted by high temperature sensor 506 if and when fan 501 overheats. In 
accordance with the foregoing techniques, a first parameter is therefore 
used to control the valve opening, until that parameter is no longer 
useful, after which a second parameter is used to control the valve 
opening. 
While the above describes the preferred embodiment of the invention, 
various other modifications or additions which are apparent to those 
skilled in the art may be made. For example, while the temperature at the 
combustion zone has been utilized to control the feedback path between the 
drying zone and the cooling zone, the temperature at any zone may be 
utilized to control a feedback path between any other two zones. 
Additionally, while the specific parameters for control being utilized are 
fan temperature and oxygen content, any hierarchy of parameters may be 
utilized. Indeed, the feedback may be controlled by a plurality of 
different parameters in order to form a hierarchy. Parameter 1 may be 
utilized as long as certain conditions are met, in which case parameter 2 
takes over as long as certain conditions are met. When those conditions 
are not met, a third parameter may take over as well. 
The above describes the preferred embodiments of the invention, however, 
various other modifications will be apparent to those of ordinary skill in 
the art. It is intended that such modifications be covered by the appended 
claims.