Process for incinerating solids on a water-cooled thrust combustion grate, and a grate plate and grate for accomplishing the process

A process wherein primary air supplied to the combustion bed through the thrust combustion grate is deflected after exiting from a surface of the thrust combustion grate by deflector elements mounted on the surface of the thrust combustion grate. The grate required for this purpose has grate plates made from a permeable hollow element with connection pieces for supplying and draining cooling water, with primary air supply ducts that run through the grate plate from a bottom to a top. Deflector elements against which the primary air exiting the outlet is intended to impact, are disposed over openings of the primary air supply ducts.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for incinerating solids on a 
water-cooled thrust combustion grate of the type installed, for example, 
in waste incinerators. This invention also relates to a specific grate 
plate and a grate having of a number of such grate plates for carrying out 
the process. The solids to be incinerated can be all kinds of different 
solids, for example lignite, shavings, chips of wood or rubber, residues 
of all kinds, industrial waste, sewage sludge, hospital waste or domestic 
refuse, and the like. 
2. Description of Prior Art 
In the case of conventional thrust combustion grates of the type installed 
in waste incinerators and which have layers of grates that rest on top of 
each other in the manner of a stairway, of which every second one can be 
moved in a thrust direction, primary air is blown from underneath and 
through the grate and into a combustion bed. In the case of cast grates, 
which are still the most widely used type of grate, where the individual 
layers of grates have of a row of cast grate bars positioned loosely next 
to each other or screwed to each other, the primary air reaches the upper 
surface of the grate through holes in the sides and/or the head portion of 
the cast grate bars. The primary air is blown through the grate by large 
ventilators in the zones underneath the grate which generate excess 
pressures equivalent to a column of water of the order of approx. 40 mm to 
250 mm. Approximately 2% of each grate surface is reserved as a passage 
for the primary air, and the volume of air blown through can be up to 
2,500 m.sup.3 of air per hour per square meter of grate surface. As the 
air flows through, it can reach peak speeds of over 30 m/s. This air that 
flows through the grate serves, on the one hand, as primary air for the 
fire, and, on the other hand, as cooling air for the cast grate. One of 
the disadvantages of this concept is that the penetration of the 
combustion bed by the air is very irregular. If, for example, a wire or 
any other small item lodges itself between two adjacent grate bars, the 
gap between them is widened at the cost of the gaps between the other 
grate bars. This means that the volume of air flowing through this gap 
will not be the same as the volume flowing through the gaps between the 
other grate bars. Another disadvantage is that, where the calorific value 
of the combustible material is high and the combustion bed is thin, as 
occurs repeatedly from spot to spot as the combustible material is 
transported along the flow of primary air breaks through the combustion 
bed at that point, creating a high darting flame which carries dust and 
ash with it far up into the boiler room without completely delivering all 
the oxygen to the fire. This causes a local excess of air, which has a 
negative impact on the flue gas. 
A substantial improvement in the incineration process is achieved with 
water-cooled grates comprising hollow grate plates preferably made from 
sheet metal which advantageously extend over an entire width of the grate. 
The grate plates have primary air supply ducts, for example, primary air 
supply pipes that pass through the grate plate, possibly tapering towards 
the top, or the primary air supply ducts are formed by holes for blowing 
primary air through, so that the primary air can be blown through the 
grate from underneath and directed out onto its upper surface. Because the 
grate plates extend over the entire width, slag can no longer fall through 
the individual grate elements to end up underneath the grate, as can 
happen when the layers of grates are made up of a number of grate bars 
positioned loosely next to each other. This virtually eliminates the 
problem of falling slag. The great advantage of a water-cooled grate, 
however, lies in the fact that the air blown through it need only fulfil 
the function of supplying air for combustion, for example, need not fulfil 
any cooling function whatsoever. As a result, the volume of air needing to 
be supplied can be drastically reduced, leading to a much quieter and more 
efficient fire. The distribution of primary air across the individual 
primary air supply ducts remains largely even. One remaining disadvantage, 
however, is that, especially in the event of high calorific values and/or 
a combustion bed which is thin from spot to spot, the primary air flow 
exiting from a primary air duct opening located at such a spot can break 
through the combustion bed. 
The overall requirements made of incineration processes are increasing 
constantly. Because the composition, and hence the calorific value, and 
also the volume of, for example, domestic waste fluctuates greatly from 
region to region and season to season, as do its physical characteristics 
such as specific weight, article size distribution, permeability to air, 
moisture, ash content, percentage of non-ferrous metals etc., it is not 
easy always to achieve good combustion of the combustible gases and slag 
while remaining within the values prescribed by regulations. One objective 
is to achieve an even distribution of temperature within the gas flow in 
the boiler room, for which purpose it is essential that the combustion 
process on the grate and in the furnace chamber above the grate is 
controlled and even. The finite number of primary air supply lines 
respective openings, the periodic blockage of individual openings, the 
irregular volume of loose material and the resultant differences in the 
heights of the layers of combustible material, plus variations in its 
calorific value, often lead, however, to uneven combustion. 
An insufficient supply of primary air to air-cooled grates can cause the 
grate to overheat. The combustion zone is prolonged, leading to 
unsatisfactory combustion of the slag. The lack of air in the furnace 
chamber has a negative impact on the combustion of gas and on the flow 
patterns in the boiler room. This in turn leads to excessive soiling of 
the boiler walls. If individual primary air supply openings become blocked 
up, this leads to an increase in the speed of the air exiting from the 
other unblocked openings and, wherever the flow of primary air breaks 
through the combustion bed (blow-by), to the formation of streaks in the 
furnace chamber, increased formation of CO and NO.sub.x and an increase in 
dust emissions. If the nature of the combustible material causes total or 
partial blockages in the openings on one side of the grate, the combustion 
bed is rendered uneven, and the combustion process is only satisfactory on 
one side. 
SUMMARY OF THE INVENTION 
It is one object of this invention to provide a process by which means 
primary air can largely be prevented from breaking through the combustion 
bed, as can blockages of the primary air supply openings, and which makes 
it possible to reduce the volume of air blown through, to improve the 
combustion process and hence also to improve the quality of the flue gas. 
It is another object of this invention to provide a grate plate and a 
grate that comprises such grate plates, on which this process can be 
carried out. 
The above and other objects are solved by a process for incinerating solids 
on a thrust combustion grate wherein primary air supplied to the 
combustion bed through the thrust combustion grate is deflected after the 
primary air flows through the grate by deflector elements disposed on the 
surface of the grate. In one preferred embodiment, there is a grate plate 
and a grate comprised of such grate plates for carrying out the process in 
line with the features described below and in the claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Thrust combustion grates have stationary and movable layers of grates 
comprising grate plates or a row of grate bars, with the layers of grates 
resting on top of each other like a stairway. Thrust combustion grates of 
this kind can be installed in such a way that the combustion bed lies 
essentially horizontally, or at an angle with angles of up to 20 degrees 
or more being common. European latent Reference EP 0 621 449 discloses a 
water-cooled thrust combustion grate with grate plates made from sheet 
steel which form panel-shaped hollow elements extending over an entire 
width of the grate path, through which water is directed as a cooling 
medium. Every second grate plate is movable, and can therefore execute a 
scraping or a transporting stroke. In the case of a forward feed grate, 
the leading edge of the movable grate plates can push combustible material 
forward onto the next grate plate down. In contrast, a reverse feed grate 
forms something like a sloping stairway built in the wrong way round. In a 
reverse feed grate, the leading edges of the movable grate plates 
transport the combustible material behind them backwards, which then rolls 
back down in the direction of the slope of the grate. The movable grate 
plates, grate plates disposed in-between two stationary grate plates, are 
usually moved collectively to and from in the downward direction of their 
inclination. This ensures that burning refuse lying on the grate for high 
dwelltimes of 45 to 120 minutes is constantly turned over and distributed 
evenly over the grate. 
One advantageous embodiment of this thrust combustion grate and its main 
elements is shown in FIG. 1, which shows a cross-section of part of a 
thrust combustion grate. The grate comprises layers of grates disposed in 
a stairway manner, each layer being formed by one hollow, water-cooled 
grate plate 1,2,3,4. Every second grate layer, for example grate plates 2 
and 4 in FIG. 1, is movable, while the grate plates in-between are 
stationary, suspended on crossbars 5. The movable grate plates 2,4 are 
each mounted at the side on a roller 6 and rear portions of the moveable 
grate plates 2, 4 rest on vertical rollers 7, which are disposed along the 
barriers that define the sides. Each movable grate plate 2,4 is driven by 
its own hydraulic piston-cylinder unit 8. Pipes 9 for supplying primary 
air from the zone underneath the grate run through the grate plate and 
open out at the leading edge of each grate plate. The primary air supply 
pipes 9 open out slightly above the surface of the grate plate and have a 
cross-section like an oblong hole, as will be illustrated below. This 
prevents excessive amounts of slag from falling into the pipes 9. The 
openings of these primary air pipes 9 or corresponding primary air supply 
ducts, as shown here, have deflector elements 10 in the form of caps made 
out of bow-shaped deflector plates, which are simply welded onto the 
surface of the grate plates. The top section of the deflector plates has a 
V-shaped cross-section. The flow of primary air impacting from below on 
the deflector plates is divided by the deflector plates and deflected to 
the side. At the same time, the bow-shaped deflector plates cover the 
opening in the direction of movement of the grate, so that the combustible 
material is guided around the deflector plates and does not pass directly 
over the primary air openings. 
FIG. 2 shows a perspective view of part of the front edge of a grate plate 
where the deflector elements are designed in the form of welded on 
bow-shaped deflector plates 10. The primary air supply pipes 9 shaped like 
oblong holes, which open out one or a few millimeters above the surface of 
the grate plate. The opening or nozzle caps 10 in the form of the 
bow-shaped deflector plates 10 are welded on over the openings. The 
deflector plates 10 are made from sheet steel and, when welded on and 
viewed from the side, they form a trapezoidal shape, with the piece of 
sheet metal that forms the top of the trapezium being contrived with a 
V-shaped cross-section, which can be achieved by a simple bevelling. With 
this shape the primary air flow impacting from below is divided in two as 
indicated by the arrows. deflected to the side and whirled up as well. The 
effect is that the air penetrates the combustion bed diffusely, so to 
speak, and at a substantially reduced speed. The air which flows through 
the primary air openings disposed in a row is able to penetrate the 
combustion bed diffusely across its entire width, so that the oxygen in 
the air is supplied to the combustion much more homogeneously than 
previously. Instead of the bow-shaped deflector plates 10 shown in FIG. 2, 
they can also be shaped in the form of a semicircular arch or an angle 
welded onto the grate plate over the opening like a gable. The deflector 
plates can be mounted in any direction, for example so that the plane of 
the angle can also run at a right angle to the direction of thrust. By 
mounting the deflector plates as shown in FIG. 2, one can also prevent 
blockages in the primary air supply openings. 
FIG. 3 shows a grate plate where the deflector elements are designed as 
flat, welded on deflector plates 12. This embodiment fulfils the given 
objective, to deflect the primary air and diffuse it, as indicated by the 
arrows in FIG. 3. The flat deflector plates 12 can also act as barbs, and 
with every forward thrust of the moveable plates can carry with them the 
combustible material lying in the area in front of the flat deflector 
plates 12, while they then clear this area again as they pull back, 
whereupon the primary air can again flow against and cool the flat 
deflector plates 12. The combustible material lying on the grate in the 
vertical direction above flat plates 12 is separated by this carrying 
along action, and a horizontal displacement of the layers of the 
combustion bed takes place. Blockage of the primary air supply openings 
can also be prevented because when the next grate layer down moves away 
relative to the supply openings, any material that has lodged itself under 
flat deflector plate 12 during the previous opposite relative movement 
works itself free and unblocks the opening again. 
FIG. 4 shows another embodiment of a deflector element, where a sawtooth 
shaped sheet 13, similar to the shape of a mower blade, is welded onto the 
front edge of the grate plate across the width of the grate. Each sawtooth 
projects over a primary air supply nozzle so that the exiting flow of 
primary air impacts against a sawtooth and is deflected forwards and 
around the two sides. A horizontal displacement of the combustion bed 
layers can be achieved with this embodiment too, and blockage of the 
primary air supply openings can be prevented as well. 
FIG. 5 shows an embodiment with screwed on opening or nozzle caps 14. In 
this embodiment, the primary air supply ducts or pipes are circular and 
the pipe openings, which project slightly beyond the grate plate surface, 
have an outer thread onto which the nozzle cap 14 is screwed. The nozzle 
caps 14 can be conventional fittings with a hexagonal outer shape which 
have radial holes 15 for this application. The fittings are only screwed 
on over a small part of their thread so that the primary air can freely 
flow out through radial holes 15. After exiting the pipe opening, the 
primary air is then deflected by the fittings and flows radially through 
and out of the holes, of which there are six in the embodiment shown in 
FIG. 5, whereby it is diffused on all sides into the surrounding 
combustible material as indicated by the arrows. Any blockage of the 
openings around the nozzle caps 14 is eliminated because of the movement 
of the opening and nozzle caps 14 relative to the transported combustible 
material. Nozzle caps of this type can have other shapes, and can be 
welded instead of screwed. 
FIG. 6A shows another embodiment of the deflector elements. The pipes 16 
have a cross-section 17 like an oblong hole. The pipes 16 are sealed at 
one end, where they form a rounded cap 18. The pipes 16 have an open end 
inserted downwards into corresponding oblong holes in the top and bottom 
grate plate sheet and welded imperviously into these oblong holes. The 
length of the pipes is greater than the thickness of the grate plate and 
as they are welded into the latter with their bottom end flush with the 
underside of the grate plate, the cap end projects beyond the surface of 
the grate plate. On both sides of the section of pipes 16 that projects 
beyond the grate plate, slots 19 are below the caps 18 in the straight 
portions, which in pipe 16 are directed from inside to outside and 
downwards. First, this ensures that the air is deflected inside the cap 
18, and then flows, depending on how the slots 19 in the caps 18 are 
positioned, upwards, horizontally or downwards at an angle through the 
slots 19 onto the bed of refuse. Secondly, this arrangement largely 
prevents the slots 19 from becoming blocked by combustible material 
because the slots 19 only move along the combustible material and are, as 
already mentioned, directed downwards. Because of the rounded caps 18, the 
sections of pipe that project beyond the surface of the grate plate can 
virtually travel through the combustible material along with the 
transported grate plates, and the combustible material can be pushed past 
these sections of pipe without sticking on sharp edges and causing damage 
to the pipe 16 or even dislocating the pipe 16 completely. 
As a general rule, it is only possible to realize deflector elements like 
the ones described in the Figures, such as positioned on the surface of 
the grate, on water-cooled grates which remain at a low temperature in 
operation so that a large part of the heat is conducted away from the 
deflector elements to the grate. On air-cooled grates, however, elements 
of this type would burn within a very short time. 
A thrust combustion grate comprising water-cooled grate plates can, 
therefore, be fitted with deflector elements of this type, thereby 
ensuring that the primary air supplied to the combustion bed through the 
thrust combustion grate is deflected immediately after exiting from the 
surface of the thrust grate. The resultant diffusion of the primary air 
and consequently more homogeneous penetration of the combustion bed is 
enormously advantageous for the quality of the combustion. The qualitative 
impact of the supply of oxygen is discussed below. 
FIG. 7 shows a diagram for assessing the quality of the combustion, showing 
the flue gases G, and the efficiency of the incinerator E, as a function 
of the O.sub.2 content in the flue gases G. The CO value is taken as the 
predominant measure of the quality of the combustion. The diagram shows 
that the CO limit value (CO.sub.max) is adhered to over a relatively large 
bandwidth of the O.sub.2 content in the flue gas. As the O.sub.2 content 
decreases, the NO.sub.x content decreases, too, and the efficiency F of 
the incinerator increases while the gas volume flow V decreases 
simultaneously. If, however, the O.sub.2 content is reduced beyond a 
certain degree, the CO value suddenly increases sharply. The aim, 
therefore, of the combustion control process is to keep the O.sub.2 value 
low enough to minimize the NO.sub.x content while simultaneously adhering 
to the CO limit value. Such an ideal working point is shown on the 
diagram. It guarantees both compliance with the flue gas values required 
by regulations and high operating efficiency. This process optimizes the 
supply of oxygen so that less air has to be blown through the combustible 
material. Hence one moves closer to the basic objective of achieving 
stoichiometric combustion. Dust emissions are also reduced, as is the 
speed of the dust particles. This reduces the erosion of the boiler walls 
because many fast-moving dust particles impact on the boiler walls like 
sandblasting. 
Tests in a waste incinerator have shown that by using this process, the 
excess pressure below the grate was able to be reduced to a third of the 
value otherwise required, while it was still possible to adhere to the 
flue gas quality prescribed by the law. This means that instead of large 
volumes of air flowing at high speed through the grate and the combustible 
material in an uncontrolled manner at certain points, a controlled volume 
of oxygen is diffused very gently, such as at low flow rates, through the 
combustible material. This prevents unnecessary volumes of flue gas from 
developing, substantially reduces the speed of the flue gas and hence the 
occurrence of fly ash as well. Furthermore, any small amount of fly ash is 
no longer whirled up high into the boiler. All this allows the boiler and 
all downstream plant components to be made smaller, thereby achieving 
greater cost-efficiency.