Grate bar and grate for combustion plants

A grate bar support for use in combustion furnaces, particularly rubbish incinerators, has a hollow-box shaped member having a trapezoidal cross-section along a longitudinal direction and trapezoidal cross-section along a direction transverse to the longitudinal direction, a front and a rear end and a nozzle shaped air outlet opening located at the front end of the hollow box shaped member. A grate is formed by a plurality of overlapping alternately stationary and movable rows of grate bars having a stroke which corresponds to the distance two adjacent rows of grate bars are offset with respect to each other. Preferably, the grate bar supports are arcuately shaped so as to provide a tight grate surface in both cold and hot operating conditions.

FIELD OF THE INVENTION 
The present invention relates to a grate bar for use in combustion 
furnaces, such as rubbish incinerators, and, particularly, to hollow 
box-shaped grate bars having a trapezoidal contour and angled air nozzles. 
BACKGROUND OF THE INVENTION 
A grate for use in refuse combustion furnaces is customarily formed by rows 
of grate bars lying one behind the other and extending transverse to the 
direction of transport of the material being burned. These grate bars are 
subjected to considerably higher thermal and mechanical loads than grate 
bars used in power plants fueled by fossil fuel, since the materials 
burned in these combustion furnaces, vary considerably, in size, weight, 
calorific power, burning behavior and the like. The life of a grate bar 
should be as long and constant as possible for reasons of repair and 
overall availability of the furnace. 
In a refuse combustion furnace, there are a myriad of different operational 
requirements, which are attempted to be satisfied, by varying the shape of 
the grate bars, and by utilizing grate bars of different materials. 
A hollow box-shaped bipartite grate bar having air fed conduits on its rear 
and outlet openings on its surface is described in European Patent B1-0 
205 658. Grate bars of this type produce a constant minimum air pressure 
in order to prevent the material being burned from penetrating into the 
grate bars. To date, however, no attention has been paid to the problem 
caused by melting non-ferrous metals and small high density steel scraps. 
A grate bar in accordance with European Patent A1 0 170 803 comprises a 
support part and a head part which is flanged to the front of the grate 
bar and which is made of a material having great thermal resistance. 
During oxidation of the materials being burned, the grate bars receive 
part of the air from the bottom of the grate and, expel the air from the 
head part of each grate bar through an air outlet opening. Relative to the 
bottom of the grate there is a vacuum within the combustion chamber of the 
furance. The grate bar, which is otherwise open at the bottom, is divided 
by a rib at the center for reasons of stability and conduction of air. 
However, the above-described grate bar has certain defects. Molten, 
non-ferrous metals and small parts of steel scrap can be trapped in the 
space between the support part and the head part of the grate bar. Because 
the grate bar supports are disposed on the bottom of the grate bars, and 
because of the special shape of the head part of the grate bars, the 
movable rows of grate bars cannot, under these circumstances, be fully 
moved over the top surfaces of the adjacent rows of grate bars. Moreover, 
the air cooling of the bars is not constant due to variable inflow cross 
sections, as a result of the movement of the grate bars. 
German Patent A1-33 13 615, discloses grate bars which have, at their head 
part, an outlet opening which permits the emergence of a stream of air at 
an angle which extends beyond the horizontal plane of the grate as well as 
beyond the head of the adjacent row of grate bars. The grate bar is 
otherwise open at the bottom and therefore does not permit a constant air 
cooling of the grate bars. 
The top surfaces of the grate bars are arranged in a step-wise manner and 
the stroke of each row of grate bars is limited to about one-half of the 
free top surface of the grate bars. 
German Patent A1 38 13 441, discloses a grate bar which is configured as a 
double layer construction for admitting air in two planes, and 
consequently, with two planes permitting the air to exit at the protruding 
head part of the grate bar. The front surface of the lower grate bar above 
the air outlet plane of the upper grate bar can never be cleared from any 
material including material to be burned, non-ferrous metals and steel 
scrap. Thus, such grate bars can only be used effectively on oblique 
grates and not on horizontal grates. No consideration has been given to 
the problem of melting non-ferrous metal. 
SUMMARY OF THE INVENTION 
An object of the present invention is to create a grate having grate bars 
which are thermally resistant, and prevent the underived emergence of air 
between the grate bars. Another object of the present invention is to have 
the tightest possible surface fit between the grate bars in both hot and 
cold conditions so that the grate bars are not susceptible to failure by 
the flow of non-ferrous metals or by small pieces of steel scrap. Yet, 
another object of the present invention is to permit the largest possible 
offset with respect to the adjacent row of grate bars so as to permit 
complete clearing of the rows of grate bars. The term offset refers to the 
distance by which a preferably movable row of grate bars is overlapping a 
preferably stationary row of grate bars. 
The grate bar in accordance with the present invention has a hollow 
box-like trapezoidal cross section in both the longitudinal and transverse 
axes and includes an air inlet opening at the rear of the grate for 
introducing an air and air outlet opening at the front thereof. 
Furthermore, the grate bar can be provided with one or more ribs extending 
along the longitudinal direction which divide the hollow box-shaped bar on 
the inside. Additionally, the grate bar can be configured to include a 
front part which is arranged below the surface upon which the material 
being burned lies and which can be replaced when it becomes worn. 
A significant advantage of the grate bar of the present invention lies in 
its trapezoidal shape, in particular along its longitudinal axis, which is 
the result of the bevelling of both the front and rear ends of the grate 
bar. The drive point is located on the rear, bottom side of the bar, 
opposite the rear end point of the top surface of the grate bar and the 
lower edge of the front part of the grate bar will still remain in contact 
with the upper or top surface of the preceding grate bar so that 
consecutive rows of grate bars still overlap each other at all times. This 
arrangement makes it possible to impart a stroke which extends the grate 
bar to more than two-thirds of its entire length. Nevertheless, the tip of 
the head part which lies on the preceding row of grate bars can completely 
clear the material to be burned from the grate bar lying in front of it. 
Upon the backward movement of the grate bar, its surface is cleared by the 
grate bar lying above it. Due to the sizable length of the stroke, the 
advancement of burning material is improved and the frictional wear on the 
grate bars is reduced. 
The oxidation and the turning around or tumbling movement of the material 
being burned is aided by the air outlet nozzle. The air outlet nozzle and 
the corresponding stream of air are directed downward with respect to the 
horizontal design plane of the grate. Even when the bottom air flow is cut 
off or reduced, the penetration of the material being burned such as 
liquid non-ferrous metal or the like into the nozzle is impossible as a 
result of positioning the nozzle approximately in the center of the front 
surface of the grate bar. The stream of air entering the rear of the grate 
bar is directed towards the head part by the hollow box-shape and one or 
more inner ribs of the grate bar. Consequently, the grate is optimally 
cooled, especially when the inflow and outflow cross sections of the grate 
bar remain constant as the bottom air passes therethrough. 
For manufacturing reasons or weight considerations in facilitating the 
installation of heavy grate bars, I-shaped or U-shaped cast sections can 
be employed to form a hollow box-like grate bar in accordance with the 
present invention. 
The grate bar of the present invention has a trapezoidal cross section 
along the transverse direction while in the cold condition, so that the 
top or upper surface on which the material to be burned lies, is narrower 
than that of the bottom surface. However, at operating temperatures, the 
grate bar expands more on the top surface which is heated by the burning 
material than on the bottom side which receives a stream of cooling bottom 
air. Under operating temperature, the side surfaces of the adjacent grate 
bars expand so as to cause the side surfaces to lie parallel to each 
other. Thus, tight joints between the adjacent bars are established. 
Moreover, because under actual operating temperatures the head part of the 
grate bar is hotter than the rear part of the grate bar, the head part of 
the grate bar is also made correspondingly narrower than the rear part of 
the grate bar. Consequently, a grate bar is obtained which is tight in 
both the hot and cold conditions. 
In order to prevent the grate bars from vertically rising during operation, 
as a result of having material to be burned wedged between two adjacent 
rows of grate bars, the grate bar support is configured in an arcuate 
shape so that the surface of the grate is trough-shaped, and remains 
trough-shaped during operation. The arcuate formation of the grate bar 
support is achieved by configuring the grate bars so as to have a 
trapezoidal cross section along the transverse direction while tapering 
the grate bars towards the head part thereof. 
If the grate surface is required to be flat during the operation of the 
furnace, the height of the arc of the grate bar support can be selected to 
be correspondingly smaller. 
It is important that the entire surface of the grate bar which can be 
cleaned or cleared by the reciprocating movement of and adjacent 
overlapping grate bar be as flat as possible so that no material to be 
burned will remain in a recess in the surface. A flat clearable surface 
permits the preceding row of grate bars to be cleared completely of 
material to be burned, as the grate bars are displaced with respect to 
each other. In order to preclude melting non-ferrous metal or small pieces 
of steel scrap from depositing in recesses in the surface of the grate, 
the replaceable head part of the grate bar is placed in a form-locked or 
form-fitted manner (mechanical interference fit) below the surface of the 
grate bar. The head part of the grate bar can be connected to the upper 
part of the hollow-box shaped grate bar by, for instance, pins. In this 
case, the insertable head part is made narrower than the grate bar so that 
an air outlet nozzle results between the adjacent bars. As an alternative 
embodiment, several head parts can be associated with one grate bar, so as 
to form between each other, as a recess, a common air nozzle.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The grate bar schematically shown in FIG. 1 has a surface 1 on which 
material to be burned is disposed and a bottom side 2 which at least in 
part rests on a row of preceding grate bars. A central rib 4 divides the 
grate bar in the longitudinal direction into two chambers, each of which 
can be fed with air through an open rear side 3 through which the air is 
supplied air to the grate as so-called bottom air. The air stream flows 
through the hollow-box and emerges at slot nozzle 6. This nozzle 6 is 
located on the front side of the grate bar above the front edge 7. Upon 
each stroke, the front edge 7 moves the burning material away from the 
preceding row of grate bars and thus assures the advancement of burning 
material and produces an additional raking effect on the material being 
burned. The horizontal plane H of the grate which is shown in FIG. 1 
illustrates that the stream of air is directed at an angle .alpha. away 
from the horizontal plane and into the material being burned. Experiments 
have shown that such a direction of a stream of air leads to an optimal 
burning of the material. Preferably, the cross sections of the air feed 
and the cross sections of the air outlet always remain the same size 
regardless of the stroke which is transmitted by a grate bar support (not 
shown) or by a driver to the grate-bar at drive point 5. 
FIG. 2 illustrates the head part of another grate bar having a replaceable 
front part 10 disposed between the upper surface 1 and the bottom side or 
lower surface 2, and which matingly engages in a form-locked or 
form-fitted manner around a projection 11 on upper surface 1. The front 
part 10 is loosely held in place by a threaded pin (not shown) inserted 
through hole 8. 
The insertable front part 10 and a parallel front part (not shown) inserted 
on the other side of the central rib 4 or a front part of an adjacent 
grate bar, form an air channel 12 on the front edge of the grate bar or 
bars, respectively, and an air outlet nozzle 15 which permits a stream of 
air to emerge at an angle .alpha., as previously described with respect to 
the horizontal plane H of the grate. 
FIG. 3 shows the above-described arrangement along section line III--III of 
FIG. 2 with respect to two adjacent grate bars 13 and 14. 
FIG. 4 diagrammatically illustrates the top view of a grate bar similar to 
that shown in FIG. 1. HU is the rear lower edge of the grate bar and HO is 
the rear upper edge of the surface of the grate bar. Similarly, VO is the 
front edge of the grate bar on its surface and VU is the front edge of the 
grate bar on its lower surface. In the cold state and in order to 
accommodate the different thermal expansions of the grate bar during the 
operation of the furnace, the front edge of the grate bar is made narrower 
than the rear edge while the upper surface is made narrower than the lower 
surface. The dashed lines of FIG. 4 illustrate the expansion of the grate 
bar. As illustrated, the side surfaces have expanded in both a vertical 
and parallel manner and no longer appear trapezoidal in the transverse 
direction. Under these circumstances, the grate surface would be flat. 
It is understood that the illustration as shown in FIG. 4 is somewhat 
exaggerated. The actual volume of material removed from the bar to form 
the trapezoidal shape is a few tenths of a millimeter which can be 
produced by non-parallel grinding of the surfaces. 
If the trough position of the grate surface (FIG. 5) is to be retained 
while under hot operating conditions, then each grate bar 16 must be 
shaped so that it retains its trapezoidal shape along the transverse 
direction and the sides of the grate bars remain non-parallel in the 
vertical direction. The grate-bar support 17 is then also formed in an 
arcuate shape. 
It should be understood that the preferred embodiments and examples 
described are for illustrative purposes only and are not to be construed 
as limiting the scope of the present invention which is properly 
delineated only in the appended claims.