Fluidized-bed combustion system

A fluidized-bed combustion system for a boiler, a heating furnace for heating steels or the like, in which a plurality of gas distribution pipes are disposed at the bottom of the combustion chamber. The lower end of each gas distribution pipe is communicated with an air supply line with an air flow rate control valve and a fuel supply line with a fuel flow rate control valve and the air and fuel admitted into the gas distribution pipe are mixed within the same and the air and fuel mixture is injected or sprayed into the fluidized bed through injection ports of the gas distribution pipe which are positioned adjacent to the bottom of the fluidized bed, whereby the complete combustion within the fluidized bed can be ensured and the combustion can be controlled over a wide range and optimized depending upon a load.

BACKGROUND OF THE INVENTION 
The present invention relates to a fluidized-bed combustion system for use 
with a heating furnace, a boiler or the like. 
In FIG. 1 is schematically shown a prior art fluidized-bed combustion 
system in which a fluidized bed c of solid particles is produced and 
maintained by the air introduced into the vessel through an air supply 
line a and distributed through a gas distributor b in the form of a 
perforated plate or an assembly of gas distribution pipes. Fuel supplied 
through fuel lines d is distributed through the gas distributor b into the 
fluidized bed c and burned. 
The prior art fluidized-bed combustion system of the type described however 
has the defect that it takes a relatively long period of time for making 
the combustion mixture of air and fuel so that it becomes difficult to 
attain the full or 100% combustion of fuel in the fluidized bed c. As a 
result, the combustion zone expands beyond the fluidized bed c into the 
freeboard e and therefore sufficient transmission of heat produced by the 
combustion to the solid particles in the fluidized bed c cannot be 
attained. As a consequence, the fluidized bed c cannot reach a 
sufficiently high temperature so that the heat of radiation emitted from 
the solid particles in the fluidized bed c and the contact or direct heat 
transmission capability are inevitably decreased and subsequently the 
solid particles in the fluidized bed c cannot be heated satisfactorily and 
the complete combustion of fuel cannot be attained. In addition, if the 
flow rate of fuel is reduced while the flow rate of air is maintained 
unchanged so as to vary the load, the mixing between air and fuel is 
degraded so that it becomes difficult to burn the fuel within the 
fliudized bed c of a predetermined height. Consequently it becomes 
difficult to vary the load over a wide range. 
The present invention was therefore made to overcome the above and other 
problems encountered in the prior art fluidized-bed combustion system and 
has for its object to provide a novel fluidized-bed combustion system in 
which a plurality of gas distribution pipes are disposed at the bottom of 
a furnace shell or combustion chamber, each gas distribution pipe being 
communicated with an air supply line with an air flow rate control valve 
and a fuel supply line with a fuel flow rate control valve so that the air 
and the fuel admitted into the gas distribution pipe are mixed therein and 
the resulting combustion mixture is injected or sprayed into the fluidized 
bed through injection ports of the gas distribution pipe adjacent to the 
bottom of the fluidized bed, whereby the combustion efficiency can be much 
increased and the combustion can be controlled over a wide range and 
consequently optimized depending upon a load. 
The present invention will become more apparent from the following 
description of the preferred embodiments thereof taken in conjunction with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 2, a mass of solid particles 1 which form a 
fluidized bed 2 are charged at the bottom of a reaction or combustion 
vessel 2. A plurality of gas distribution pipes 4 extend into the 
fluidized bed 2 through the bottom of the reaction vessel and are spaced 
apart from each other by a predetermined distance. Each gas distribution 
pipe 4 is provided at the upper end thereof with a plurality of 
equiangularly spaced gas injection ports 5 which are radially outwardly 
and horizontally directed and are located closer to the bottom of the 
fluidized bed 2 than to the top thereof. The lower end of the gas 
distribution pipe 4 is connected to a fuel supply line 7 with a fuel flow 
rate control valve 6 and to an air supply line 9 with an air flow rate 
control valve 8 so that the fuel and the air are introduced and mixed with 
each other in the gas distribution pipe 4. The combustion mixture of air 
and fuel is injected into the fluidized bed 2 through the injection ports 
5. A workpiece such as a steel 11 to be heated is disposed at the top 
portion of a freeboard 10 of the furnace vessel 3. 
Referring next to FIG. 3, the construction of the gas distribution pipe 4 
will be described in more detail. The gas distribution pipe 4 comprises in 
general an inner injection pipe section 12 which is disposed within the 
fluidized bed 2 in the furnace shell and an outer mixing pipe section 13 
which is disposed exterior of the furnace shell and is connected to the 
inner injection pipe section 12 through a connecting socket or unit 15 
which extends gas-tightly through the bottom 14 of the furnace shell. Both 
the inner and outer mixing pipe sections 12 and 13 may be threaded into 
the socket or unit 15. The inner pipe section 12 has its top closed and is 
formed with a plurality of equiangularly spaced and radially outwardly and 
horizontally directed combustion-mixture injection ports 5 adjacent its 
top. 
The outer mixing pipe section 13 is formed adjacent to its lower end with 
an air inlet port 16 which extends radially outwardly and is connected to 
the air supply line 9 so that the combustion air can be introduced into 
the outer mixing pipe section 13 in the radial direction. A fuel injection 
pipe 18 with a flange 18a is detachably attached to the lower end of the 
outer mixing pipe 13 and is communicated with the fuel supply line 7. More 
particularly, an internal thread 18b of the flange 18a of the fuel 
injection pipe 18 is engaged with an external thread 19 at the lower end 
portion of the outer mixing pipe section 13 and the fuel injection pipe 18 
extends into the outer mixing pipe section 13 substantially coaxially 
thereof and beyond the opening of the air inlet port 16 by a predetermined 
length or height. A fuel injection port 17 is opened at the top of the 
fuel inlet pipe 18. Thus the fuel flows through the fuel supply line 7 and 
the fuel injection pipe 18 and is sprayed through the fuel injection port 
17 and mixed with the combustion air introduced through the air supply 
line 9 and the air inlet port 16. The combustion mixture thus prepared 
flows upward into the inner injection pipe section 12 and is sprayed 
through the injection ports 5 into the fluidized bed 2. 
Referring next to FIG. 4, a variation of the gas distribution pipe used in 
the present invention will be described in detail below. The gas 
distribution pipe 4 comprises in general an outer pipe section 21 and an 
inner or fuel inlet pipe section 22. The air inlet port 16 extends 
radially outwardly of the outer pipe section 21 and is communicated with 
the air supply line 9 and a plurality of equiangularly spaced 
combustion-mixture injection nozzles 20 each for providing the injection 
port 5 are extended radially outwardly from the outer pipe section 21 
adjacent the top end thereof. The inner or fuel inlet pipe section 22 
extends through the bottom of the outer pipe section 21 into the same 
coaxially thereof and has its upper end connected to the closed top end of 
the outer pipe section 21. The lower end of the inner or fuel inlet pipe 
section 22 is communicated with the fuel supply line 7 and a plurality of 
fuel injection ports 17 are formed through the wall of the inner or fuel 
inlet pipe section 22 adjacent to the top end thereof and in opposed 
relationship with the nozzles 20 of the outer pipe section 21. Thus the 
fuel sprayed through the fuel injection ports 17 is mixed with the 
combustion air supplied through the air inlet port 16 into the outer pipe 
section 21 and the thus prepared admixture of air and fuel is sprayed 
through the nozzle ports 5 into the fluidized bed 2. 
As described above, according to the present invention, the air supplied 
through the air supply line 9 and the fuel supplied through the fuel feed 
line 7 are mixed well in the gas distribution pipe 4 and then sprayed 
through the nozzle ports 5 into the fluidized bed 2 so that the solid 
particles in the fluidized bed 2 are sufficiently fluidized and the 
combustion mixture is burned by the heat from the solid particles which 
are heated at high temperatures. The solid particles in the fluidized bed 
2 are in turn heated by the products of combustion while the latter rise 
through the bed 2. Since the air and the fuel have been premixted, the 
smooth and efficient combustion of fuel is ensured so that no combustion 
takes place in the freeboard 10 and consequently the heat liberated by the 
combustion of fuel can be efficiently transferred to the solid particles 
in the fluidized bed 2. Thus radiation of heat can be much enhanced. In 
addition, the height of the fluidized bed 2 can be reduced. Furthermore 
since the combustion is efficiently carried out within the fluidized bed 
2, partial or local combustion can be well controlled by controlling the 
ratio of the air and the fuel in the gas distribution pipe 4 so that the 
load can be varied over a wide range. 
It is to be understood that the present invention is not limited to the 
embodiment described above and that various modifications can be effected 
without departing the true spirit of the present invention. For instance, 
the design, construction and location of the gas distribution pipes can be 
varied within the scope of the present invention. So far the present 
invention has been described in conjunction with the heating of steel 
products 11 (See FIG. 2), but the fluidized-bed combustion system of the 
present invention may equally be applied to an incinerator for burning 
refuse, a boiler or the like. 
The novel effects and advantages of the present invention may be summarized 
as follows: 
(i) The combustion air and the fuel are mixed within the gas distribution 
pipes and then injected into the fluidized bed so that the fluidized bed 
may be maintained and the combustion mixture may be burned within the 
fluidized bed. Therefore the combustion velocity is fast and the complete 
combustion is ensured even in a fluidized bed of a low height so that the 
heat of combustion can be efficiently transferred to the solid particles 
in the fluidized bed. 
(ii) Since the combustion is carried out in a fluidized bed which is low in 
height, the power required for charging the fluidizing and combustion air 
into the furnace can be lowered. 
(iii) A plurality of gas distribution pipes are provided which can be 
individually controlled, so that an optimum combustion can be attained. In 
addition, some of the gas distribution pipes can be selectively 
deactivated so that the combustion can be carried out at a desired 
localized portion in the fluidized bed and consequently the load can be 
varied over a wide range. 
(iv) The solid particles in the fluidized bed heated to high temperatures 
start the combustion of the air and fuel mixture so that even the fuels 
with a low heat value can be completely burned in a stable manner.