Furnace especially well suited for burning straw, wood waste materials and the like

A furnace adapted for burning waste materials such as straw, brushwood or the like has a cylindrical housing enclosing a combustion chamber and a hopper above the chamber for supplying fuel thereto. A slidable partition member separates the hopper from the combustion chamber. A suction tube having a constriction in its wall to reduce the cross-section of the tube is provided to withdraw combustion gases from the combustion chamber. A conduit connected to a source of compressed air terminates in a nozzle disposed in the suction tube to direct compressed air in the direction of the flow of gases from the combustion chamber. Pillers are provided in the chamber to support a bale of straw or other fuel material.

This invention relates to a furnace for burning waste fuel such as straw or 
waste wood products. 
Straw has a caloric value of about 3,400 kcal/kg, that is to say one kg of 
straw when burned produces an amount of energy substantially equal to that 
of 0.36 liters of fuel oil. Thus one large round bale of straw of about 
500 kg will produce as much energy when burned as about 180 liters of fuel 
oil. Waste wood, for example brushwood, when burned will also produce 
about the same amount of energy per unit of weight as straw. In times of a 
constantly increasing energy shortage the utilization of such waste 
materials to produce energy is of increasing importance. 
Such wastes are amply available from various agricultural and forestry 
operations. No appreciable transport costs are involved. On the other 
hand, considerable heat is required by many agricultural operations. For 
example, large amounts of heat are necessary for drying grain such as corn 
and green fodder and also for the heating of large areas such as barns and 
other livestock housing. Thus the need for furnaces suitable for burning 
agricultural and forest waste is particularly important. 
A furnace adapted for burning such waste materials and particularly for the 
combustion of large straw bales is already known. This furnace is provided 
with a fan which is connected to an afterburning compartment through a 
socket for drawing off completely burned gases. The temperature of the 
burned gases flowing out of the afterburning compartment through the fan 
is about 800.degree. C. The fan can only withstand such high temperatures 
for a short time. Accordingly, it has been found, in practice, that after 
only a comparatively short operating time, the fan of the known furnace is 
destroyed so the furnace is not of any practical value. 
The feed hopper of the known furnace has a cylindrical shape which is 
closed at the bottom with a bottom member and at the top only with a 
cover. Therefore, it is impossible to replenish the fuel bed with material 
such as large straw bales while the furnace is in operation. Before 
refilling the furnace, it is necessary to burn the charge completely and 
to allow the furnace to cool down. The furnace therefore has to be closed 
down comparatively often and for a long time, which may likewise be 
undesirable. 
In the known furnace, the supply of primary combustion air is effected 
through lateral apertures in the wall of the feed hopper, which are in 
communication with the atmosphere. The apertures can be closed, for 
example by stoppers, in order to regulate the amount of primary combustion 
air to be supplied. In order to draw off the combustion gases and 
incompletely burned gases in the afterburning compartment, an aperture 
disposed axially in relation to the feed hopper is provided below the 
combustion and low-temperature carbonization region above the afterburning 
compartment. This aperture is covered with a hood to prevent ash from 
entering the afterburning compartment. 
The primary combustion air flows preferably from the lateral apertures in 
the wall of the feed hopper towards the central disposed above the 
combustion chamber and covered with a hood. In order that the primary 
combustion air will flow through the lower portion of the charge, that is 
to say, will flow through the combustion and low-temperature carbonization 
region over as long a path as possible on its way from the lateral 
apertures to the central aperture, the lateral apertures are provided 
above the combustion and low-temperature carbonization region in the wall 
of the feed hopper. Consequently, at the beginning of operation of the 
furnace, the lateral, lower portion of the charge burns away comparatively 
quickly so that a cone of unburned fuel or straw forms, the downwardly 
directed tip of which lies over the hood disposed over the offtake 
aperture, as a result of which further burning of the charge at this point 
is rendered difficult. This leads, as a further advantage of the known 
furnace, to the fact that great fluctuations occur in the heating capacity 
during the starting operation. 
It is an object of this invention to provide an improved furnace for 
burning waste materials. 
Another object of the invention is to provide a furnace particularly well 
suited for burning such materials as straw, brushwood and the like. 
The foregoing objects and others are accomplished in accordance with the 
invention generally speaking, by providing a furnace with an afterburning 
compartment, apparatus for supplying air to the afterburning compartment, 
a suction tube for withdrawing combustion products from the afterburning 
compartment which has an area reduced cross-section and a source of 
compressed air or other fluid such as a nozzle which discharges a 
compressed fluid into the area of reduced cross-section in the direction 
the combustion products are flowing. 
The following advantages are achieved by the invention: 
the nozzle provided in the suction tube and which is acted upon by 
fast-flowing gas, for example by compressed air produced by a compressor, 
is insensitive to heat with respect to the temperatures of the gas drawn 
off and flowing to the consumer device. Operational disturbances as a 
result of the offtake device are therefore eliminated in the furnace 
according to the invention. 
Replenishment of fuel is insured even during the burning, as the fuel 
slides in the feed hopper. Moreover, uniform burning of the lower portion 
of the charge is insured by the apertures distributed over the whole 
periphery of the feed hopper, disposed at the height of the combustion and 
low-temperature carbonization region and charged with compressed air as 
well as by the apertures disposed in the tip of each supporting piller and 
likewise charged with compressed air.

According to FIG. 1, the furnace provided by the invention has a feed 
hopper 1 which may be a sheet-metal cylindrical member and a lower widened 
portion which represents a combustion chamber 2. The feed hopper 1 is 
closed at the bottom by a bottom member 3 made of refractory brick, for 
example fireclay, and at the top by a cover 4. 
The cover 4 can be actuated by a cable 7 taken over a guide pulley 6 
secured to a jib 5. It is pivotally mounted by a spindle 8 on the upper 
end of the feed hopper 1. 
Between the combustion chamber 2 and the cover 4, at about half the height 
of the feed hopper 1, a slide member 9 is provided which is illustrated in 
broken lines in FIG. 1 and the diameter of which corresponds to the 
internal diameter of the feed hopper 1 so that when the cover 4 is open, 
the combustion chamber 2 can be closed at the top by the slide member 9. 
The slide member 9 travels on a guide rail 11 by means of rollers 10 
secured thereto and is actuated by a chain 14 acting on the slide member 9 
at 13 and guided by a guide roller 12 secured to the feed hopper 1, for 
example by means of an electric motor not illustrated. 
A plurality of pillars 15 are disposed at the bottom 3 of the feed hopper 1 
to receive pressed waste fuels, such as large round bales of straw or the 
like. The pillars 15 are made of refractory brick such as fireclay. 
Instead of or in addition to the pillars 15, a coil of pipe 16 (FIG. 2) 
can extend over the whole cross-section of the feed hopper 1, through 
which a coolant, particularly water, flows, as indicated diagrammatically 
by the two arrows in FIG. 2. The coil of pipe 16 serves to receive loose 
waste fuels, that is to say those which are not pressed, such as 
brushwood, bark or split firewood. 
The pillars 15 extend almost to the height of apertures 17 in the side wall 
18, likewise made of refractory brick, for example fireclay, of the 
combustion chamber 2 of the feed hopper 1. If the coil of pipe 16 is 
provided, it is at the same height as the upper ends of the pillars 15. 
The primary combustion air is supplied to the combustion and 
low-temperature carbonization region in the combustion chamber 2 through 
the apertures 17 in the side wall 18 of the combustion chamber 2. For this 
purpose, the apertures 17 are connected to a ring conduit 19 which is 
taken externally around the feed hopper 1 or the combustion chamber 2 and 
is in communication with a source of compressed air 22, for example a 
compressor, through a conduit 20 and a branch pipe 21. 
The pillars 15 are each provided with a point at their upper end. The 
charge, for example, the straw bales introduced into the feed hopper 1, 
rests on these points. Thus regions where the supply of primary combustion 
air is rendered difficult develop at the points of contact between the 
straw bales and the points of the pillars 15. It has therefore proved 
advantageous to provide apertures 23 at the points of the pillars 15, 
through which apertures the primary combustion air flows to the combustion 
and low-temperature carbonization region in the combustion chamber 2. The 
apertures 23 in the pillars 15 may likewise be connected to the source of 
compressed air 22 through conduits not illustrated. 
An afterburning compartment formed by a passage 24 is disposed immediately 
above the bottom of the feed hopper 1 or of the combustion chamber 2. The 
wall of the passage 24 likewise consists of refractory brick, such as 
fireclay. It extends substantially over the whole width of the bottom 3 
and is provided with lateral slits 25. 
A conduit 27 leads into the middle of the bottom 3 of the combustion 
chamber 2 through an aperture 26. The secondary combustion air is supplied 
to the afterburning compartment or the passage 24 through the conduit 27, 
via the branch 21 from the source of compressed air 22. In order to be 
able to regulate the throughput of compressed air for the secondary 
combustion air, a slide member or valve 28 is provided in the conduit 27. 
Such a slide member or such a valve may appropriate also precede the 
conduit 20 to the ring conduit 19 but is not illustrated in the drawing. 
A suction tube 29, through which the gases completely burned in the 
afterburning compartment are supplied to the consumer device projects 
through the side wall 18 of the combustion chamber 2 into the afterburning 
compartment or the passage 24. For this purpose, the suction tube 29 
comprises a constricted section 30. A fast flowing gas, for example 
compressed air, is supplied to the constricted section 30 through a nozzle 
31 disposed coaxially in the suction tube 29, in the direction of the 
outflowing gases, which is represented by an arrow in FIG. 1, the gas 
being supplied through a conduit 32 which may be connected to a 
compressor, not illustrated. The gas, such as compressed air, flowing 
through the nozzle 31 entrains the burned gases present in the suction 
tube 29 and in this manner produces a reduced pressure in the combustion 
chamber or passage 24, which leads to the drawing off of the combustion 
gas and incompletely burned gases formed in the combustion chamber 2 into 
the combustion chamber or passage 24. 
In operation, the furnace work as follows: 
After the cover 4 has been opened and the slide member 9 has been drawn out 
of the filling space 1, the filling space 1 is filled with two large round 
bales of straw, or the like. After setting fire to them, a primary 
combustion air is supplied through the apertures 17 and possibly 23 to the 
combustion chamber 2 and secondary combustion air is supplied through the 
aperture 26 to the afterburning compartment or passage 24. A fast flowing 
gas is also admitted to the nozzle 31. 
A combustion and low-temperature carbonization region develops above the 
pillars 15 or the coil of pipe 16, and pyrolignite, phenols or other 
combustible gases are driven out of the fuel by the heat. As a result of 
the suction produced by the nozzle 31, these incompletely burned gases or 
vapors are drawn through the apertures 25 into the afterburning 
compartment or into the passage 24. 
The afterburning then takes place in the passage 24 and can be controlled 
by adjusting the supply of compressed air by means of the valve or slide 
member 28. The completely burned gases then flow through the suction tube 
29 to a consumer device, for example, a grain drying installation. 
At the same time, the waste fuel drops down in the feed hopper. As soon as 
the charge, for example the upper of the two large round bales of straw, 
has sunk below the height of the slide member 9, the combustion chamber 2 
is shut off from the upper section of the feed hopper 1 by the slide 
member 9, the cover 4 is opened, the upper section of the feed hopper 1 
above the slide member 9 is filled with a fresh charge, for example a 
fresh large round bale of straw, then the cover 4 is closed and after the 
slide member 9 has been pulled out of the feed hopper 1, the fresh charge 
enters the combustion chamber 2. In this manner, the furnace according to 
the invention can be filled continuously that is to say it does not have 
to be stopped. 
In order to able to determine when the charge has sunk below the height of 
the slide member 9, an indicating device is provided below the slide 
member 9 on the feed hopper 1. This consists essentially of a bell-crank 
lever which is articulated on the feed hopper 1 and of which the first 
lever arm 33, represented in broken lines in FIG. 1, is disposed in the 
feed hopper 1 and of which the first lever arm 33, represented in broken 
lines in FIG. 1, is disposed in the feed hopper 1 and of which the second 
lever arm 34, extending from the feed hopper 1, carries a restoring weight 
35. 
If the lever arm 33 is loaded, that is to say there is fuel present at the 
height of the lever arm 33, then the lever arm 33 extends downwards with 
its free end between fuel and the wall of the combustion chamber 2, while 
the second lever arm 34 with the restoring weight 35 extends substantially 
horizontally away from the feed hopper 1. On the other hand, if there is 
no fuel in the combustion chamber 2 at the height of the lever arm 33, 
then the lever arm 33 projects horizontally into the feed hopper 1 and the 
lever arm 34 with the restoring weight 35 projects downwards. Thus the 
position of the lever arm 34 disposed externally on the feed hopper 1 or 
of the weight 35 indicates whether or not there is fuel at the height of 
the lever arm 33 in the interior of the feed hopper 1. 
Although the invention has been described in detail for the purpose of 
illustration, it is to be understood that such detail is solely for that 
purpose and that variations can be made therein by those skilled in the 
art without departing from the spirit and scope of the invention except as 
it may be limited by the claims.