Apparatus for the combustion of poorly combustible fuels

An apparatus for the combustion of a poorly combustible fuel in a fluidized bed comprises a vertical front wall through which the poorly combustible fuel is delivered in particulate form and two lateral walls extending rearwardly from the front wall. A fixed grate extends from a front input end towards a rear discharge end between the lateral walls, and the two sides and the front input end of the grate define spaces with the lateral walls and the front wall, respectively. The fixed grate is upwardly inclined over at least a portion of its length from the input end to the discharge end whereby the discharge end is disposed above the input end, and the particulate fuel is delivered to the input end for forming a fluidized bed on the grate. A series of windboxes disposed under the grate along the length thereof blast combustion gas through the grate for fluidizing and burning the fuel while generating ashes moving rearwardly along the fluidized bed to the discharge end, and the discharge end constitutes a weir over which the ashes flow for easy removal. Plane surfaces are arranged in the spaces for supporting heaps of the particulate fuel.

The present invention relates to an apparatus for the combustion of a 
poorly combustible fuel, such as carboniferous or bituminous shales, in a 
fluidized bed. 
As is known, such fuels have a high content of ash and, in addition, when 
they are burned in a fluidized bed, the temperature of the bed is 
relatively low and insufficient to cause the formation of agglomerates of 
ash. 
In French patent application No. 2,528,543 filed June 15, 1982, it has been 
proposed to utilize a fluidized bed hearth on a mobile grate for burning 
such fuels, which is known under the trade name "Ignifluid", of Fives-Cail 
Babcock, of Paris, France. The essential concept of this apparatus resides 
in the easy removal of the slag or ashes produced during the combustion of 
the carbon with agglomeration of the ashes. When the carbons are poorly 
combustible, such a solution appears to be relatively complex. 
It is accordingly the primary object of this invention to propose a simpler 
and, therefore, more economical solution to this problem. 
The above and other objects are accomplished according to the invention 
with an apparatus for the combustion of a poorly combustible fuel in a 
fluidized bed, which comprises a substantially vertical front wall, means 
for delivering the poorly combustible fuel in particulate form through the 
front wall, two lateral walls extending rearwardly from the front wall, 
and a fixed grate having two sides and extending from a front input end 
towards a rear discharge end between the lateral walls. The two sides and 
the front input end of the grate define spaces with the lateral walls and 
the front wall, respectively, the fixed grate is upwardly inclined over at 
least a portion of its length from the input end to the discharge end 
whereby the discharge end is disposed above the input end, and the 
delivering means feeds the particulate fuel to the input end for forming a 
fluidized bed on the grate. A series of windboxes is disposed under the 
grate along the length thereof, blasts of combustion gas from the 
windboxes fluidizing and burning the fuel while generating ashes moving 
rearwardly along the fluidized bed to the discharge end, and the discharge 
end constituting a weir over which the ashes flow, and plane surfaces are 
arranged in these spaces for supporting heaps of the particulate fuel. 
The above and other objects, advantages and features of the present 
invention will become more apparent from the following detailed 
description of certain now preferred embodiments thereof, taken in 
conjunction with the accompanying, somewhat schematic drawing wherein 
FIG. 1 shows a fragmentary longitudinal section of one embodiment of the 
combustion apparatus along line I--I of FIG. 2; 
FIG. 2 is a transverse section along line II--II of FIG. 1 at the point of 
maximum depth of the fluidized bed; and 
FIG. 3 is a longitudinal section similar to that of FIG. 1, illustrating 
another embodiment of the combustion apparatus.

Referring now to the drawing and first to FIGS. 1 and 2, the illustrated 
apparatus for the combustion of a poorly combustible fuel in a fluidized 
bed comprises substantially vertical front wall 1, channel or chute 4 
constituting means for delivering poorly combustible fuel 17, such as coal 
or shale, in particulate form through front wall 1, and two lateral walls 
2 and 3 extending rearwardly from the front wall. Fixed grate 5 forms the 
bottom of a hearth chamber defined by the walls. The grate has two sides 
and extends from a front input end towards a rear discharge end between 
lateral walls 2 and 3. The two sides and the front input end of grate 5 
define spaces with the lateral walls and front wall 1, respectively, and 
the fixed grate is upwardly inclined over its length from the input end to 
the discharge end whereby the discharge end is disposed above the input 
end. Particulate fuel delivering means 4 feeds particulate fuel 17 to the 
input end for forming fluidized bed 11 on grate 5. In this embodiment, 
grate 5 ascends in a substantially rectilinear line towards the discharge 
end. 
A series of windboxes is disposed under grate 5 along the length thereof, 
blasts of a combustion gas, such as air, from the windboxes fluidizing and 
burning the fuel while generating ashes moving rearwardly along fluidized 
bed 11 to the discharge end. In the illustrated embodiment and as shown in 
FIG. 2, the windboxes are arrayed in a series of juxtaposed pairs of 
windboxes 9a, 9b disposed along the entire length of grate 5, each windbox 
9a, 9b extending over half the width of the grate between the two sides 
thereof. Oppositely extending ducts 10a, 10b are connected to the 
windboxes to supply the combustion gas thereto. It will be understood, 
however, that each pair of windboxes 9a, 9b may be replaced by a single 
windbox fed by a single duct or by two ducts from opposite sides. The 
discharge end of grate 5 constitutes weir 12 over which the ashes flow, as 
shown in FIG. 1. 
Plane surfaces 6', 7' and 8' are arranged in the spaces between the front 
wall and the input end as well as between the sides of the grates and the 
lateral walls for supporting heaps 6, 7 and 8 of the particulate fuel 
against the respective walls. These heaps of fuel form banks wherebetween 
fluidized bed 11 of the particulate fuel is held on grate 5, the fluidized 
bed being formed by the combustion gas blasts from the windboxes through 
the grate. 
At the same rate as the fuel is fed through channel 4 to fluidized bed 11, 
particulate carbon material flows over weir 12 at the upper discharge end 
of grate 5 into conduit 13 through which the material is removed from the 
combustion apparatus. In effect, this overflowing carbon material is 
constituted essentially by ashes generated while the fuel moves rearwardly 
along fluidized bed 11 to the discharge end. 
Preferably and as shown in FIGS. 1 and 3, the discharge end of the grate 
comprises short horizontal section 5' at the level of weir 12 to form an 
overflow ledge or ridge for the ashes. 
Windboxes 9c below short horizontal section 5' and close to the upper 
discharge end of grate 5 are arranged to provide a minimum blast to 
fluidize the ashes whereby they are maintained in motion to flow towards 
the discharge end. 
Normally, no agglomeration of ashes will occur in this apparatus during the 
combustion of poorly combustible fuels. However, if some agglomerates 
should accidentally be formed, they would settle at the deepest portion of 
fluidized bed 11 at the base of fuel heap at the input end of grate 5. To 
remove any such agglomerates, the apparatus is equipped with means for 
removing the agglomerates, the illustrated means comprising horizontal 
sliding register 14 cooperating with a port in plane surface 6' defined 
between front wall 1 and the input end of grate 5. The position of the 
sliding register is controlled by rod 15 (FIG. 3) operable from outside 
the apparatus. The sliding register normally closes the port in plane 
surface 6'. It may be slid open by rod 15 for very brief periods of time 
to cause a portion of fuel heap 6 to flow through the port and take along 
the agglomerates at the adjacent bottom of the fluidized bed to remove the 
same through duct 16 connected to the port in plane surface 6'. 
In FIG. 3, the same reference numerals as used in FIGS. 1 and 2 designate 
like parts operating in an equivalent manner to obviate redundancy in the 
description. This embodiment differs from the first-described embodiment 
in that the grate has a first horizontal portion 5"' adjacent the input 
end and succeeding portion 5" upwardly inclined towards the discharge end, 
short horizontal section 5' being disposed at the discharge end in the 
same manner as described in connection with FIG. 1. Furthermore, only a 
single pair of windboxes is disposed below the horizontal portion, the 
inclined portion and the short horizontal section of the grate, 
respectively. This reduces the number of windboxes (each pair of windboxes 
being replaceable by a single windbox, if desired) and provides a larger 
volume of fluidized bed per surface area. 
It is preferred to use a grate having a permeability descreasing from the 
input end towards the discharge end to adapt the grate to the 
concentration of carbon in fluidized bed 11, which varies in the same 
manner as the burning of the fuel proceeds. 
While the invention has been described in connection with two now preferred 
embodiments, it will be clearly understood that changes and variations may 
occur to those skilled in the art, particularly after benefitting from the 
present teaching, without departing from the spirit and scope of this 
invention, as defined by the appended claims. Any one of the described and 
illustrated structures may be replaced by equivalent means.