Furnaces – Process – Burning pulverized fuel
Reexamination Certificate
2000-04-25
2002-07-16
Lazarus, Ira S. (Department: 3749)
Furnaces
Process
Burning pulverized fuel
C110S188000, C110S190000, C110S243000, C110S244000, C110S245000
Reexamination Certificate
active
06418866
ABSTRACT:
TECHNICAL FIELD
This invention concerns a method to operate a fluidized bed incinerator which incinerates waste containing solid carbon, such as sewage sludge, municipal garbage or industrial waste, and the incinerator employing this method. More specifically, it concerns a method to operate a fluidized bed incinerator which incinerates waste with a high moisture content, such as sewage sludge, and the incinerator employing this method.
TECHNICAL BACKGROUND
Fluidized bed incinerators can be divided into two types: those using fluidized beds of air bubbles, which are commonly employed to incinerate garbage and evaporated sewage sludge, and those using circulating fluidized beds, which are commonly employed in coal-burning boilers which generate electrical power and incinerators which burn a mixture of waste and fuel.
Fluidized bed incinerators employing air bubbles work as follows. When the velocity of the gas exceeds the speed at which the particles comprising the medium of flow become a fluid, air bubbles begin to form on the floor of the fluidized bed. These bubbles agitate the medium of flow, causing the interior of the bed to achieve an ebullient state, in which the fuel is combusted.
In circulating fluidized bed incinerators, the velocity of the aforesaid gas is forced to exceed the terminal velocity of the particles comprising the medium of flow. As the gas and the particles are vigorously mixed, the particles are entrained on the gas and dispersed and combusted above the fluidized bed. The dispersed particles are collected by a separating device such as a cyclone and recirculated in the incinerator.
These two types of fluidized bed incinerators account for most of the incinerators in use. Both are suitable for combusting low-quality fuel or waste. Most sewage sludge is processed in a fluidized bed incinerator, and municipal garbage and industrial waste tend to be burned in an incinerator connected in series with a stoker.
The configuration of the aforesaid air bubble-type fluidized bed incinerator is shown in FIG.
18
. The bottom of a vertical cylindrical tower is filled with a quantity of sand
50
a
, the fluidizing medium. This sand forms bed region
50
(the bubbling region or the dense region). A fluidizing gas is injected through air inlet
53
and thereafter forced uniformly through dispersion devices
52
, dispersion tubes feeding into the bottom of the bed. The velocity of the gas, which is the flow velocity at which the said gas is injected, is increased until it exceeds the speed at which the aforesaid fluidizing medium becomes a fluid. Air bubbles
50
b
form in the aforesaid fluidizing medium, agitating and fluidizing it, and causing its surface to assume an ebullient state.
The sludge to be incinerated is loaded into the furnace via sludge inlet
55
, which is above the aforesaid bed region
50
, now in an ebullient state. At the same time, an accelerant is loaded via inlet
54
and combusted. After the solid component of the sludge is combusted in bed region
50
, its volatile component is combusted in freeboard
56
, the space above bed region
50
. The exhaust gas from the said combustion is released through exhaust vent
57
on the top of the tower.
In an air bubble-type fluidized bed incinerator, waste such as raw garbage or sludge is combusted through the following process.
1) The air used to create a fluid is injected via gas dispersion devices
52
at the start of combustion. The sand is heated by a burner from the top layer down. As its temperature rises, the bed is fluidized by air bubbles.
2) Next, the garbage to be incinerated is loaded into the chamber. If the heat value of the garbage is too low, an accelerant is introduced to maintain the interior of the bed at the proper temperature.
3) After combustion has begun, the air heated by the exhaust gas is used as the aforesaid fluidizing gas. The garbage in the chamber is vigorously mixed and fluidized with the heated sand in the bed region. After a short time, part of it is gasified by dry distillation, and the remaining solids are combusted.
4) The uncombusted gases and the volatile or light portions of the garbage are conducted to freeboard
56
, the area above the fluidized bed, and there combusted.
When sewage sludge is incinerated in the aforesaid air bubble-type fluidized bed incinerator, the rate of combustion in the furnace is 60 to 80% in the fluidized bed, but it climbs to nearly 100% in the area of the freeboard.
Thus the combustion load of freeboard
56
is 20 to 40%, and the temperature of the freeboard is approximately 150° C. higher than that of the fluidized bed. Since the combustion energy required to incinerate raw garbage or sludge is likely to vary, parts of the freeboard may become too hot.
In an air bubble-type fluidized bed incinerator, the air heated by the exhaust gases to approximately 650° C. is reused in order to conserve energy and minimize pollution. To prevent harmful exhaust, the temperature at the vent of the incinerator must be regulated so that the average temperature of the uncombusted gases (mainly CO, dioxin and cyanogen) is around 850° C.
In order to maintain the sand bed fluidized by the medium at an appropriate average temperature, say between 700, and 750° C., the moisture load at the floor of the furnace must be less than 250 to 280 kg/m
2
h. Because of the limitations of the equipment, the aforesaid velocity of the gas must be at least 0.5 m/s (to maintain stable bubbling, it must be 0.5 to 1.5 m/s). Thus to incinerate waste with a high water content, such as sewage sludge, the floor of the furnace is made larger than is necessary for combustion, and more air is supplied than is actually needed for combustion. More exhaust gas is produced, and the extra air is wasted.
In many cases, the relative density of the substance to be incinerated is equal to or less than that of the fluidized bed. If the substance is less dense than the bed, when it is loaded into the chamber via the freeboard it will float on the surface of the fluidized sand on the very top of bubbling region, and the temperature within that region will not be conducive to effective combustion.
Sewage sludge has a relative density of approximately 0.8 t/m
3
. When it is loaded into the furnace, however, its moisture component immediately evaporates, leaving it with a density of 0.3 to 0.6 t/m
3
. Assuming silica with a relative density of 1.5 t/m
3
is used as the fluidizing medium, it will attain a relative density of 1.0 t/m
3
also assuming that the bed expands by a factor of 1.5.
In a case like this, where the substance to be incinerated is relatively light, it will float on the surface of the sand in the bubbling region even if it is loaded from the freeboard. The combustion of the substance will be limited to the top layer and will not extend to the interior of the bed. This imposes limitations on the maximum load which are not present when combustion can be extended effectively to the entire lower portion of the bed, including the bubbling region in the lower half of the air bubble bed and the dense layer below it.
Moreover, if combustion is achieved only in the upper portion of the aforesaid sand bed, the volatile component of the substance to be burned will be propelled through the splash region above the bed and combusted in the freeboard. There will be more combustion in the freeboard, which has a low thermal capacity, and less in the region which contains the dense layer of sand with its high thermal capacity. As a result, the temperature in the furnace will be unstable.
Another problem which can occur is that the waste product which falls onto the sand on top of the aforesaid bubbling region may not break up effectively. This results in some portions remaining uncombusted and leads to improper fluidization.
Also, waste matter like raw garbage and sewage sludge contains a high volume of volatile components. Since these sublimate, they are combusted in the freeboard. This causes the temperature of the exhaust gases to be too high.
In particular, if the temperature of the sand in
Goda Toshihisa
Honda Hiroki
Sasatani Shiro
Shimizu Yoshihito
Takuma Masao
Crowell & Moring LLP
Lazarus Ira S.
Mitsubishi Heavy Industries Ltd.
Rinehart K. B.
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