EP0110906B1

EP0110906B1 - Radiation shield for boilers for solid fuel

Info

Publication number: EP0110906B1
Application number: EP83901588A
Authority: EP
Inventors: Lars Leksander Slyngbom Jorgensen
Original assignee: JOERGENSEN LARS LEKSANDER S
Current assignee: JOERGENSEN LARS LEKSANDER S
Priority date: 1982-05-26
Filing date: 1983-05-17
Publication date: 1986-04-02
Also published as: FI73813C; FI840189A0; NO153346C; DK148668C; DK236582A; WO1983004296A1; JPS59500983A; DE3362762D1; NO153346B; FI73813B; FI840189A; EP0110906A1; NO840227L; DK148668B

Abstract

A boiler (1) with suspended arch at the firing opening and constructed for the combustion of solid fuel which, when heated and/or burned gives off combustible gasses, is provided with a radiation shield (3). The smoke and gas-tight radiation shield (3) branches from the front wall (6) of the firing chamber (11) and extends sloping downwards and with smoke and gas-tight connection to the front wall (6) and the side walls of the firing chamber in over that part of the hearth (10) from which the main part of the combustible gasses are given off. During operation, the radiation shield forms a firing chamber section which, for a water boiler, has a relatively high surface temperature, and where the ignition timeperature of the combustible gasses is exceeded and wherein these gasses with secondary air which is introduced through an inlet (9) above the hearth (10) and under the radiation shield (3) are burned, with the consequent possibility of increasing the thermal efficiency of the boiler (1) when compared with the efficiency which can be achieved with known water boilers.

Description

This invention relates to a boiler for combustion of solid fuel which can give off inflammable gases, and which is built with a suspended arch over the firing place.
Boilers of this kind have previously had very widespread use. As a consequence of the recent years' violent increases in the price of fuel oil and natural gas, this type of boiler has again become economically attractive, particularly for larger heating installations. Not least, boilers for the burning of coal, which gives off inflammable gases when heated, are of great interest, the reason being that from the point of view of price, coal is a desirable fuel and is expected to remain so for a great number of years. It is known that coal-burning boilers of known construction normally have efficiencies of a maximum of 78% of the theoretical heat content of the coal, depending somewhat on the kind of coal and on the construction of the boiler. The reason that the efficiency is not greater is that the inflammable gases given off by the coal to a wide extent leave unburned by way of the chimney and are lost. Only in high-pressure boilers where the temperature of the firing chamber is very high, and where the temperature in the boilers burning-out opening therefore is considerably above the ignition temperature of the gases, can a generally complete combustion of the given-off gases be achieved with secondary air and an efficiency of around. 90%. In boilers for heating water, no effective and cheap construction for achieving such an efficiency is known.
The object of the present invention is to provide a construction which, when used in generally known boilers for heating water by burning solid fuel which gives off inflammable gases especially coal, results in an effective combustion of the inflammable gases given off, thus enabling the high efficiencies desired to be achieved. In addition, the construction according to the invention can be used in connection with various firing systems, for example travelling grate and stoker firing etc. Boilers are already known which are constructed with a view to increase the combustion efficiency. One such construction is shown in e.g. DE-C-460 763 which concerns an arrangement in suspended covers for firing chambers. The chambers are directed upwards in the direction of the exhaust gases. Although the construction reduces heat losses it has no arrangement similar to the ignition arch shown and disclosed in the following according to the subject invention.
US-A-983 510 describes a solid fuel boiler which is provided with an ignition arch and a roof which could be regarded as a kind of radiation shield. Air is blown in between the radiation shield and the ignition arch so as to be directed between the stones in the ignition arch and down towards the fire grate. Although this arrangement doubtless improves the combustion it also creates a considerable smoke gas velocity preventing the gases from being completely burned.
According to the invention, the above object is achieved with a boiler as presented and characterized in the main claim. The advantage of disposing a downwardly sloping, smoke-tight radiation shield in the manner described is that the combustible gases, which are given off mainly in the area covered by the radiation shield, rise up under this, are mixed with secondary air which is fed to this area, and are ignited, the reason being that the space under the radiation shield has a high temperature (both a high air temperature and a high radiation temperature) before the mixture of secondary air and combustible gases reaches forward to the relatively narrow burning-out opening rearmost in the boiler. Since the combustible gases are thus ignited and completely burned in the firing chamber in the boiler, it is possible to achieve the same high efficiencies with boilers of generally known construction as with those known from oil or gas-fired boilers and from coal-fired high-pressure boilers.
This is an advantage, providing that the radiation shield is built out so that it extends more than half-way backwards towards the rear wall of the . boiier, and particularly advantageous. if it reaches out at least 3/5 of the distance to the rear wall of the boiler, leaving a burning-out opening of 2/5 or less of the boiler length. The advantage is that the long extension increases the average residence time of the gases in the area in which they can be ignited, and thus also increases the possibility of a high efficiency in the boiler. Precisely how long the radiation shield must be depends on the construction of the boiler and the fuel being used, but the correct length can be found in a simple manner by those familiar with the technique, for example by analyzing smoke samples.
By suspending the radiation shield as presented and characterized in claim 3, a particular advantage is achieved in that the supporting construction for the radiation shield also is a heating surface in the boiler construction, whereby the materials thus at the same time fulfil several functional purposes, which contributes towards a cheap total construction.
Finally, the invention can be characterized in the manner presented in claim 4. The advantage of the loosely disposed but secured elements is that not only is it easy to undertake replacement, when this is required after a certain period of operation, but also it becomes possible to change in a simple and quick manner the size of the radiation shield by the removal or insertion of some of the ceramic elements. The latter can be advantageous when changing between types of fuels having different gas contents and varying ignition temperatures for the gases given off.
The invention will now be described in closer detail and with reference to the accompanying drawing which shows a vertical longitudinal section through an advantageous embodiment of a boiler according to the invention.
The boiler is designated generally by numeral 1. The suspended arch 2 is placed at the feed opening for the solid fuel which, in this boiler, is supplied by a travelling grate which is the bed of the hearth 10 in the firing chamber 11. In accordance with the invention, the radiation shield 3 extends from the front wall 6 of the firing chamber from a line above the arch 2, smoke-tight and with smoke-tight connection with said front wall 6 of the firing chamber 11 and two side walls sloping downwards in over the hearth 10.
The radiation shield 3 is built up of suitably-shaped ceramic elements 4 which comprise a non-selfsupporting shield which is supported by a number of parallel tubes 5 sloping upwards from the back wall 7 of the firing chamber 11, where the inside of each tube 5 in the supporting structure is in open connection with the water chamber in the back wall of the boiler. Foremost in the boiler 1, the tubes 5 which support the radiation shield in a corresponding manner are each connected to the water chamber in the front wall of the boiler. With this construction, the boiler water-helped by the siphon effect- can flow freely in an upwardly inclined manner through the tubes, thus holding these tubes sufficiently cooled during the operation of the boiler, so that they do not lose their supporting ability. In the shown boiler 1, the supporting tubes 5 used have a circular cross-section, but other cross-sections can be used, for example oval or rectangular. Each of the suitably-formed ceramic elements 4 have a breadth dimension at right angles to the longitudinal section shown in the drawing which is equal to the distance between the tubes 5. The cross-section of the elements 4 in the direction at right angles to the tubes 5 is similar to that of an hourglass, in that in each side of each element 4 there is formed a lengthwise recess or groove of a size and shape which corresponds in the main to the half cross-section of the supporting tubes 5. The thus suitably shaped elements 4 are hung on the tubes 5 and, when the boiler is in operation and the elements are heated, close to tightly together that the smoke discharge is forced essentially backwards in the boiler. An inlet 9 for secondary air is provided through the front wall 6 of the firing chamber 11 under the arch 2.
In the main, the boiler 1 functions like known boilers for solid fuel, and here shall only be described the difference in mode of operation due to the radiation shield 3 according to the invention.
The radiation shield 3 stops the combustible gases and the smoke gases rising from the foremost end of the hearth 10 and forces them backwards in the firing chamber 11, where they are combined with the surplus air and the gases rising from the rearmost end of the hearth 10 and flow through the burning-out opening 8, and from here forward through the boiler and into the smoke pipes. In this manner is established an expedient, uniform discharge of gas from the hearth. At the same time, with the construction shown radiation of heat to the water running through the tubes 5 is achieved. A surprising and advantageous effect is that with the smoke and gas-tight, somewhat heat-insulating shield of ceramic material or the like, a surface is obtained having a surface temperature which, for a water boiler, is relatively high.
The total effect is that these combustible gases are ignited and therefore can contribute to a pronounced increase in the thermal efficiency of the boiler. In a boiler with a radiation shield such as that shown, in operation there have thus been achieved thermal efficiencies of around 90%. It should be added that the ignition of the combustible gases can take place only when there . is sufficient oxygen present in the firing chamber. It is expedient to introduce this oxygen through the inlet 9, whereby the air introduced is heated in a suitable manner when flowing out under the suspended arch 2. Other forms of the inlet can be envisaged, but the secondary air must be introduced in such a way that it is mixed with the combustible gases under the radiation shield 3.
With generally known boilers having upwardly inclined radiation shields, the possibilities of varying boiler loading are quite small, for example because decreased loading gives increased 0₂% in the gases, thus resulting in less efficiency.
Therefore, with hot-water boilers with upwardly rising radiation shields, one recommends a maximum load reduction of 50%.
With downwardly inclined radiatipn shields according to the present invention, it has been shown to be possible to reduce the load right down to 25% of full load while still maintaining the very high efficiency, the reason being that the efficiency, in fact, increases slightly when the load is reduced. This is because the upwardly rising gases cannot avoid being burned when the radiation shield is constructed as described above.
For good order's sake it should be mentioned that by solid fuel is to be understood all kinds of solid fuel, for example wood, straw, pellets, peat, olive shells or briquettes.
Finally, the invention is not limited to the watercooled tubes having a certain cross-section. Tubes with all possible cross-sections can be envisaged, for example round, triangular, oval or square.

Claims

1. Boiler with suspended arch at the firing opening and constructed for the combustion of solid fuel which, when being heated and/or burned, gives off combustible gases, characterized in that an essentially smoke- and gas-tight radiation shield (3) having an essentially smoke-and gas-tight connection with the front wall (6) and the side walls of the firing chamber (11) and placed in a line above the suspended arch (2), extends downwardly inclining in over the hearth (10) and covers a greater part of this than the suspended arch (2), and in that an inlet (9) for secondary combustion air is disposed above the hearth (10) and under the radiation shield (3), preferably opening out in the firing chamber (11) under the suspended arch (2).

2. Boiler according to claim 1, characterized in that the length of the radiation shield is at least half of the distance from the front wall (6) to the rear wall (7) of the firing chamber (11), and preferably at least 6/10 of this distance.

3. Boiler according to claims 1 or 2, characterized in that the radiation shield (3) is built up of suitably-shaped, temperature resistant elements (4) of ceramic material, and in that this non-selfsupporting radiation shield (3) is suspended in a supporting structure, for example of tubes (5) which, by having water passed through them when the boiler is in operation, are kept cooled to so low a temperature that the supporting ability of the tubes (5) is maintained.

4. Boiler,according to claim 3, characterized in that the suitably-shaped, ceramic elements (4) are formed in such a way that they can develop the desired degree of smoke- and gas-tightness in the . radiation shield (3) without mortar filling between the elements (4) or between these and the walls of the firing chamber, and that the radiation shield (3) is, moreover, formed in such a way that its effective extension in the direction towards the back wall (7) of the firing chamber (11) can be changed by the insertion or the removal of some of the suitably-shaped, ceramic elements (4).