The present invention relates to a solid fuel furnace according to the preamble to claim 1. Such a furnace is known from document SE-C-139 563.

Solid fuels, such as shavings, chips, peat, bark and the like, are usually very heterogeneous and the composition as well as the size of the material included may vary within wide limits between different deliveries but also in one and the same delivery. Moreover, as a rule the fuel contains a high and varying water content. For these reasons, among other things, it is therefore difficult to control the combustion of such fuels so as to obtain a complete combustion of the material and minimal emission of noxious substances, such as carbon monoxide, nitric oxide and dust, while obtaining a high degree of efficiency. In order to meet all these requirements, the combustion must be controllable as for temperature, speed of combustion, time of oxygen supply and the like, and the combustible gases which form must be thoroughly mixed with the supplied oxygen so that as low an amount as possible of incompletely burned gases are let out in the open.

In the combustion, the fuel is normally fed via a fuel feeder to a grate in the lower portion of the furnace. Air is then supplied through the grate for primary combustion of the fuel, while forming a bed of embers and releasing combustible gases. It is already known to inject an inert gas, in practice usually recirculated flue gases, from the front of the furnace parallel with the grate and in a direction opposite to the fuel feeding direction. As a result, a cooling effect is obtained, which is advantageous to avoid the forming of nitric oxides which form in large quantities at high temperatures by oxidation of the nitrogen in the air. At the same time, the hot gases are forced from the bed of embers towards the infeed end of the grate, which accelerates the vaporisation of the water content in the fuel.

In order to prolong the residence time of the combustible gases in the furnace and thus allow a late oxygenation and an efficient mixing of oxygen and combustible gases, combustion furnaces are known which are designed to provide, if possible, a substantially horizontal vortex or circulation of the gases in the furnace as they ascend to a so-called gas duct which is placed high up and which leads the hot gases to a boiler, heater or the like to transfer the heat to a suitable heat-carrying medium. By this circulation of the gases substantially parallel with the grate, a prolonged residence time is obtained for the gases in the furnace. The oxygenation of the combustible gases takes place successively during the displacement of the gases towards the gas duct, on the one hand through secondary air nozzles in the furnace walls and, on the other, through tertiary air nozzles in the gas duct. It has, however, been found difficult to obtain said horizontal circulation of the gases and as a rule they tend to ascend too quickly towards the gas duct, which results in an insufficient oxygenation and mixing. In addition, stationary pockets of gas tend to form in the furnace.

An object of the present invention is to obviate the problems and drawbacks of prior-art combustion furnaces for solid fuels. More specifically, the invention relates to a combustion furnace for combustion of solid fuels, in which the residence time of the combustible gases in the furnace is prolonged in relation to prior-art technique and in which a stable circulation of the gases is provided, while permitting a well-adjusted oxygenation and thorough mixing of the gases. This in turn serves the purpose of providing efficient combustion with minimal emission of noxious substances, such as carbon monoxide, nitric oxide and dust. At least these objects are achieved by means of a solid fuel furnace according to claim 1.

By the new and inventive features of the invention, a vertical circulation of the gases is provided in the furnace about a substantially horizontal axis. This has been found to render a number of positive advantages, such as a stable and controllable movement of the gases as well as a long residence time with the possibility of a late oxygenation. This in turn allows an almost complete combustion of the combustible gases with no emission or very little emission of noxious substances, such as carbon monoxide, nitric oxide and dust.

The stable, vertical circulation of the combustible gases is made possible by the gas duct extending substantially horizontally forward away from the combustion chamber and by the combustion chamber having an elevated portion which is located above an imaginary extension of an upper boundary wall of the gas duct. Furthermore, some kind of means are arranged to make the combustible gases circulate in such manner that the gases ascend in the rear portion of the combustion chamber whereas they descend in the front portion and sweep past the orifice of the gas duct, and a certain portion of the gas volume is then expelled out through the gas duct whereas the rest is circulated at least one more time. As a result, a stable, vertical circulation is provided in one single circuit or loop, which makes the combustion easy to control. The means which make the combustible gases circulate can be arranged in many different ways. In a preferred embodiment, recirculated flue gases are injected into the lower/front portion of the combustion chamber in the rearward direction and at a comparatively low speed, preferably lower than 50 m/s. Possibly, flue gases are also injected in a direction obliquely upward/forward from the region just above the rear portion of the grate.

The feature in claim 1 that the gas duct is directed forward does not have to imply that it should be strictly horizontal. In practice, this is preferred but certain deviations may occur without impairing the function of the furnace. The gas duct should, however, extend to a certain extent in the horizontal direction and have an inclination of less than 50°, preferably less than 30° and most preferably less than 10° in relation to a horizontal plane.

In a similar manner, it is preferred that the longitudinal axis of the gas duct should extend substantially in a vertical plane in which the circulation takes place, and in practice it is preferred that the extension of the longitudinal axis of the gas duct should be part of this plane. Certain deviations from this may, however, occur but the angle of the longitudinal axis of the gas duct in relation to said vertical plane should be less than 30°, preferably less than 20°, and most preferably less than 10°.

The long residence time of the combustible gases allows the air supply through mainly the secondary air nozzles to take place at a comparatively low speed from a relatively small number of air nozzles with relatively large cross-sectional dimensions. Consequently, there will be a minimal interference with the circulation in the furnace and the oxygen will be homogeneously distributed in the gas volume. In prior-art furnace types, the air supply has been provided at substantially higher speeds through air nozzles having smaller cross-sectional dimensions. Such air jets function almost as "pin-pricks" into the gas volume and have a clear tendency to form layers between combustible gases and air. In a preferred embodiment, the air speed in the secondary air nozzles is lower than 15 m/s and preferably lower than 10 m/s. The secondary air nozzles are suitably placed close to the centre of the formed vortex or circulation and air is injected transversely of the direction of motion of the gas volume.

For practical reasons, a combustion furnace for solid fuel has, as a rule, a rectangular or square section in a horizontal plane. By letting the gases circulate in the vertical direction according to the invention, it is ensured that the entire gas volume in the furnace participates in the circulation without any risk of stationary pockets being formed in the angles where incompletely burned gases can gather.

According to a preferred embodiment of the invention, the circulation of the gases is such that the lower portion of the circulation takes place in a direction opposite to the feeding direction of the fuel.

In a preferred embodiment, which is described below and shown in the drawings, the grate is stepped with movable grate elements and inclines somewhat downward from the infeed end. It should, however, be understood that the invention is also applicable to combustion furnaces with other types of grates, e.g. plane grates, fixed grates and also grates in which the fuel is fed from the underside through a hole in the centre of the grate.

The combustion furnace according to the invention is provided with a suitably dome-shaped elevation of the ceiling, the elevation being located above an imaginary extension of an upper boundary wall of the gas duct. The gas volume then has a vertical kinetic component as it passes, during the circulation, the inlet of the gas duct and this facilitates the recirculation of a great amount of gas down towards the grate and the bed of embers.

In a preferred embodiment of the invention, besides injecting a so-called front-edge jet of a substantially inert gas from the front of the furnace parallel with the grate in a direction opposite to the feeding direction of the fuel, a substantially inert gas is injected from the rear of the furnace obliquely upward/forward from the region just above the infeed end of the grate. The function of this so-called rear-edge jet is to facilitate the circulation of the gas volume in the furnace and counteract any tendency to back-flow and stationary pockets of gas gathering at the rear wall of the furnace. It should be understood that the inert gas in the front-edge jet as well as the rear-edge jet can, under certain operational conditions, contain a certain amount of oxygen and thus contribute to the combustion.

In the drawings

• FIG. 1 is a vertical cross-section in the longitudinal direction of a combustion furnace according to the invention,
• FIG. 2 is a horizontal cross-section of the furnace according to Fig. 1, and
• FIG. 3 is a vertical cross-section illustrating the inside of the front wall of the furnace.

First, reference is made to Fig. 1, which is a vertical cross-section in the longitudinal direction of a combustion furnace according to the invention. In the lower portion of the furnace, a grate generally designated 1 is positioned with an inclination forward/downward towards the front of the furnace. The grate is of prior-art kind and has individually movable, stepped grate elements for feeding the fuel from an infeed end 2 with a feeding device towards an outlet end 3 with a device for discharging the ashes. In the region above the discharge end, a gas duct 4 extends through the front wall of the furnace. The gas duct has a gas passage which communicates with the inner combustion chamber of the furnace and which is intended to be connected to a boiler (not shown in Fig. 1).

Reference numerals 5 and 6 relate to a front-edge nozzle and a rear-edge nozzle, respectively. Through these nozzles, recirculated flue gases are injected. The front-edge nozzle extends through the front wall of the furnace and is directed so that the flue gases are injected substantially parallel with the upper surface of the grate in the direction opposite to the feeding direction of the supplied fuel. The rear-edge nozzle 6 extends through the rear wall of the furnace in the region above the infeed end of the grate and is directed obliquely upward/forward.

Secondary air nozzles 7 are arranged in a line in the respective side walls of the combustion furnace. In addition, tertiary air nozzles 8 are arranged in the boundary walls of the gas duct.

As appears from the Figure, the combustion chamber of the furnace is formed with an elevated portion 9 which is located above an imaginary extension of the upper boundary wall of the gas duct. Furthermore, a constricted portion 10 is arranged in the inlet of the gas duct.

As appears from Fig. 2, which is a horizontal cross-section of the combustion furnace, the combustion chamber has a substantially rectangular design in the plane of the sheet of paper. Fig. 2, also indicates a portion of a boiler designated 11 communicating via the gas duct 4 with the combustion chamber of the furnace.

Fig. 3 is a vertical cross-section of the front wall of the furnace seen from the inside of the combustion chamber. In the preferred embodiment, the furnace is provided with three front-edge nozzles 5 but this number may vary depending on the width of the furnace. The gas duct 4 further is circular in cross-section.

Reference is once more being made to Fig. 1 to describe the inventive combustion. As previously mentioned, solid fuel in the form of e.g. chips, shavings, rests of trees or the like is fed to the grate 1 via an opening at the infeed end 2 at the rear wall of the furnace. By successive feeding by means of the movable grate elements, the fuel is fed downward towards the discharge end 3 so that a continuous fuel bed forms over the entire grate. Primary air is injected via holes (not shown) in the grate. This primary air provides a primary combustion of the fuel and a continuous bed of embers forms over the entire grate from which combustible gases are released which ascend in the combustion chamber. The flue gases which are injected through the front-edge nozzles 5 entrain the combustible gases from the bed of embers rearward towards the rear wall of the furnace, as suggested by the flow lines in the Figure. At the rear wall, the gases deflect upward and ascend, among other things, by means of the flue gas jets from the rear-edge nozzles 6 which are directed forward/upward and which, just as the front-edge nozzles, may vary in number depending on the width of the furnace. At the ceiling of the combustion chamber, the gases deflect forward and follow the inside of the elevated portion 9. At the orifice of the gas duct 4, a certain portion of the gas volume will be discharged through the gas duct whereas a certain portion will deflect downward towards the discharge end of the grate, where the gas is mixed once again with recirculated flue gases and combustible gases from the bed of embers in order to participate once again in a circulation cycle.

A number of factors contribute to the circulation of the gas volume in the furnace. On the one hand, the injection of recirculated flue gases from the nozzles 5 and 6, respectively, urges the movement of the gas volume and, on the other, the elevated portion 9 of the furnace ceiling results in the gas volume, while passing the orifice of the gas duct, having a kinetic component directed downward towards the grate. The constricted portion 10 at the orifice of the gas duct also contributes to a certain extent to the circulation, but its primary function is to increase the speed of the gas volume and provide a turbulent flow in the gas duct for efficient admixture of the tertiary air from the tertiary air nozzles 8. Owing to the described circulation of the gas volume in the combustion chamber, the gas will have a long residence time in the combustion chamber, which is advantageous and allows a well-adjusted oxygenation of the combustible gases via the secondary air nozzles 7 in the combustion chamber. The invention makes it possible to design the secondary air nozzles with greater cross-sectional dimensions than before and air may be supplied at an advantageously lower speed.