FR2586086A1 - Device for combustion of solid fuels - Google Patents

Abstract

Device for combustion of solid fuels such as coal, lignite and peat, wood, bark, sawdust, straw, great reed, maize cobs or other agricultural by-products and domestic waste, comprising essentially a boiler with a vertical cylindrical furnace. This device is characterised in that the diameter of the refractory lining is, in its upper part, greatly reduced so as to form a partition comprising in its centre a perforation, and secondary air inlets open into the edges of this perforation. Combustion of solid fuels such as coal, lignite and peat, wood, bark, sawdust, straw, great reed, maize cobs or other agricultural by-products and domestic waste.

Description

 The present invention relates to a device for burning solid fuels such as coal, lignite and peat, wood, bark, sawdust, straw, cane de Provence, corn cobs or other sub-products. agricultural products and household waste, essentially comprising a cylindrical vertical furnace equipped at its base with a grate or hearth through which the primary combustion air is introduced, the lower part of the hearth being surrounded by a refractory lining.

 The fuels mentioned above are available at attractive prices but the combustion equipment making it possible to use them is generally expensive, cumbersome and therefore not compatible with many potential sites, or else have insufficient performance imperfect combustion, incompatibility with very high fuels humid, requires to accept a very high excess of combustion air.

 There are known vertical boilers with fire tube and smoke tubes; in these materials the combustion is carried out on a grate or hearth located at the bottom of the hearth tube and through which is injected the primary combustion air. The secondary air injection is made on the periphery of the hearth, below the fuel layer. In some versions, the bottom of the hearth tube is lined with a refractory lining adopting the shape of a trunk whose radiation allows the use of relatively wet wood (up to 45% compared to the gross weight).

 There are also known embodiments in which the refractory lining has in its upper part a slightly overhanging part relative to the focus.

 In all these devices, the combustion takes place almost entirely on the grid or the majority of the combustion air (150% of the stoichiometry) is brought in the form of primary air.

 The fumes then rise slowly into the hearth tube, due to the large section of it and the secondary air injection (representing a small part of the total air and being made from the periphery of the hearth) does not does not cause an energetic mixing of the gases allowing a complete combustion with limited excess of air.

This way of operating the combustion of biomasses is unsuited to their properties: in fact these fuels have a high level of volatile matter (80% of the dry matter) which spontaneously emerge during heating.

 As the air is introduced under the grate and the volatiles emerging above the grate, the mixture of air and volatile materials is imperfect and it is difficult to obtain complete combustion.

 Finally, because all the combustion is carried out in the vicinity of the grid, it does not develop in the upper part, a large flame, able to radiate energetically on the walls of the hearth tube and ensure a significant fraction of the interchange exchange. This deficiency must therefore be offset downstream by the presence of a large, bulky and expensive tubular exchanger.

 The object of the present invention is to provide a device and a combustion process of these solid fuels which overcomes these disadvantages, allows to use for combustion a quantity of air very close to the stoichiometry in an extremely compact installation.

 For this purpose, the invention relates to a device for burning solid fuels such as coal, lignite and peat, wood, bark, sawdust, straw, cane de Provence, corn cobs or other agricultural by-products and domestic refuse essentially comprising a cylindrical vertical furnace equipped at its base with a grate or hearth through which primary combustion air is introduced, the lower part of the hearth being surrounded by a refractory lining and its upper part constituting a secondary combustion zone to which smoke tubes are connected, characterized in that the diameter of the refractory lining is, at its upper part, greatly reduced so as to form a partition comprising at its center a perforation and, in the edges of this perforation, open outlets of secondary air.

 Compared to previous embodiments, this device represents a simplification of the fact that it reports all the secondary air in the narrowing which thus forms a real burner nozzle.

 According to another characteristic of the invention, the mouths of the secondary air inlets have a totally or partially tangential orientation with respect to the central perforation.

 According to yet another feature of the invention the central perforation is partially closed by a central core which leaves the gas an annular passage and the secondary air inlets open into the edges of this perforation and / or in the edges of the central disc.

 To use this device optimally and in accordance with the invention, for burning a fuel whose moisture content is less than 25%, the sole blows a primary air fraction equal to 55 to 80% of the stoichiometry and, by the secondary air inlets, a fraction of secondary air equal to 70 to 50% of the stoichiometry, the velocity of the gases from the primary combustion zone and passing through the central perforation being at least 8m and s being preferably between 10 and 15 m / s and the blowing speed by the mouths of the secondary air inlets being at least 25 m / s and preferably being between 30 and 40 m / s.

The process according to the invention is entirely suitable for the combustion of very humid products and, to burn a fuel whose moisture content is between approximately 25 and 40% of moisture, a primary air fraction is blown. equal to 50 to 90% of the stoichiometry and a fraction of secondary air equal to 80 to 50% of the stoichiometry and to burn a fuel whose moisture content is greater than 40% blows an equal primary air fraction 80 to 140% of the stoichiometry and a fraction of secondary air equal to 80 to 50% of the stoichiometry.On thus manages to burn the most varied fuels ranging from coal to green wood and even to pick up bark soaked with water on the slaughter yards
According to a particularly advantageous characteristic of the invention, the secondary air is injected into the perforation or shrinkage in a partially tangential manner to rotate the gases originating from the furnace, the flow conditions being defined by an additional number S, referred to as " Swirl "which measures the ratio between the kinetic moment of rotation M r and the pulse I according to the formula
M
r
S =
IRG in which Mr = R x DA2 x W2 and,
I = DG x W g
RG is the radius of the central perforation
R is the radius of the small circle tangent to the axis of the secondary air intakes.

DA is the mass flow rate of secondary air in kg / s.

DG is the mass flow of gases from the furnace in kg / s.

WA2 is the injection rate of the secondary air in m / s.

is the velocity of the gases from the furnace at the narrowing point in m's, and
0.5 <S <
This swirling motion improves the mixing of the gases but the centrifugal force at which it gives rise also has the effect of widening the base of the flame and of shortening it, which makes it possible to adapt the shape of the flame to the geometry of the flame. boiler, it may for example be much lower than a boiler of the same nominal power but designed according to the prior art. This spreading of the flame caused by the Swirlw is also accompanied by a certain recirculation of the gases downwards in the secondary combustion zone, which further improves the quality of combustion and heat exchange in this zone. .

 So we see that the way in which the combustion takes place is profoundly modified. While in the prior art, there is a combustion in a single step in the home, the device according to the invention achieves a staged combustion. Since primary air is insufficient to ensure complete combustion, fuel gasification occurs which gives rise to combustible gases with a high heating value, in the range of 3,350 to 6,300 Kjoules / m3. The addition of secondary air allows the combustion of these gases as in a conventional burner.

 The combustible gases arrive at the shrinkage at an air speed of less than 8 m / s and at this point the secondary air is injected at a high speed of between 25 and 40 m / s. Since the secondary air has little distance to penetrate into the upward flow of gas, a mixing and an excellent mixing of the gases is immediately obtained, a very complete combustion with a reduced excess of air and, consequently, a concentration The flame created radiates intensely and a large fraction (equivalent to 35 to 50% of the power of the boiler) of the heat exchange is carried out in the secondary combustion zone. It is also interesting to note that all the volatiles released in the hearth are burned at this level and contribute all the less to the subsequent formation of soot and fouling of the smoke tubes.

 The tangential arrangement of the gas injections also makes it possible to adapt the shape of the flame to the geometry of the boiler. The "swirl" makes it possible to widen the base of the flame and shorten it. In addition, the "swirl" causes the recirculation of gases within the flame and further improves the quality of combustion.

 The use of a central part in the narrowing allows, in large fireplaces, to ensure a sufficient passage section without increasing the penetration distance of the jets too much.

 Specialists will appreciate the monobloc and compact nature of this boiler which makes it particularly economical.

The invention will be better understood with the aid of the nonlimiting description hereinafter accompanied by drawings in which
FIG. 1 is a sectional view of the combustion zone of a boiler according to the prior art,
FIG. 2 is a sectional view of the essential part of the boiler according to the invention,
FIG. 3 is a top view in section along the line aa of FIG.
FIG. 4 is a sectional front view of another embodiment of the invention and
FIG. 5 is a sectional front view of another embodiment of the invention.

 In FIG. 1 partially representing a boiler according to the prior art, the fuel is introduced from the bottom of the boiler in the direction of the arrow 1 on the gate or sole 2 crossed by primary air from the box 3 in the direction of the arrows 4. The fuel burns in the firebox 5 constituting the lower zone of the combustion chamber surrounded by a refractory lining, either cylindrical 6 or having a slight thickness 7 at its upper part. In this refractory lining 6 or 7 are made, at two different levels, secondary air inlets, 8, 9, 10, 11. In such a boiler the majority of the combustion air is supplied in the form of air up to 150% of stoichiometry. The fumes then rise without substantial acceleration and the two-stage injection of the secondary air causes only a slight mixing of the gases and often is not sufficient not to quickly ensure complete combustion near the injectors. There is therefore, up to the combustion chamber 12 and the flue tubes 13, an excessive extension of the flame accompanied by a cooling and therefore a premature extinction before the completion of the combustion. The formation of soot and the loss of yield are the more or less visible consequences of these defects.

 Fig. 2 shows the device according to the invention in which the wood is introduced from the bottom of the boiler as in that described above, the identical parts having the same references. The refractory lining 14 of the present boiler is distinguished by its large widening 15 which delimits a narrowing or perforation 16 input on the vertical axis of the boiler. This perforation allows the exit of gases from the fireplace. It is provided with several secondary injection ports 17.

The secondary air injection orifices are therefore all brought into a horizontal plane at the level of the perforation 16. Their arrangement may be radial but it is preferably partially tangential, see
Fig. 3. There are schematically represented four injections 17 whose axis is tangential to a circle (immaterial) of radius R less than the radius RG of the perforation 16. The partially tangent character is given by the length of the radius R: the injection is radial if R - O and totally tangential if R = ri,
Figure 4 shows in section the variant of the invention in which the narrowing 18 of the gas passage from the fireplace to the chamber is annular. It is delimited on the one hand by the refractory lining 19 and on the other hand by a central core 20 held by arms 21. The air injections 22 are preferably partially tangential.

 FIG. 5 shows an embodiment of the invention, in which the refractory lining consists of a plurality of prefabricated moldings 23 forming the bottom portion and a concrete portion 24 cast on an opiate. Celleici comes to drown the metal structure of air intake consisting of a manifold 25, injection tubes 26 and a sheath 27 on which fits the fan 28.

 It should also be emphasized that this important narrowing alone profoundly modifies the nature of the boiler as until now and transforms it into a real wood burner. As in a burner, the respective flow rates of primary and secondary air, the height and the diameter of the flame are easily adjusted in response to the requirements of the fuels, which makes it possible to use the radiation of the flame at best and, to obtain the same power, reduce the dimensions of the boiler h build.

Claims (7)

1 - Combustion device for solid fuels such  coal, lignite and peat, wood,  bark, sawdust, straw, cane de Provence,  farmhouse cobs or other agricultural by-products  and household waste essentially comprising  a cylindrical vertical furnace boiler equipped with its  base of a grid or sole by which one introduces  primary combustion air, the lower part of the  fireplace being surrounded by a refractory lining and its  upper part constituting a combustion zone  secondary to which are connected tubes of  characterized in that the diameter of the  refractory lining is, at its upper part,  greatly reduced to form a partition  having in its center a perforation and, in the  edges of this perforation open arrivals  secondary air. 2 - Combustion device according to claim 1,  characterized in that the mouths of the arrivals  of secondary air have a totally or  partially tangential to the perfo  central ration. 3 - Device according to any one of claims 1  and 2, characterized in that the central perforation  is partially occluded by a central core that  leave the gas an annular passage and the arrivals  of secondary air open into the edges of this  perforation and / or in the edges of the central disk. 4 - Process implemented in the device according to one  any of claims 1 to 3, characterized in that  only to burn a fuel whose proportion  humidity is less than 25%, we blow by the  a fraction of primary air equal to 55 to 80% of  stoichiometry and, by secondary air inlets  a fraction of secondary air equal to 70 to 50%  stoichiometry, the velocity of gases from  the primary combustion zone and passing through the  central perforation being at least 8m / s and being  preferably between 10 and 15 m / s and the  blowing speed through the mouths of arrivals  of secondary air at least 25 m / s and being  preferably between 30 and 40 m / s. 5 - Process according to claim 4, characterized in that  that, to burn a fuel whose proportion  humidity is between about 25 and 40%  of humidity, we blow a fraction of primary air  equal to 60 to 90% of the stoichiometry and a fraction  secondary air equal to 80 to 50% of the stoichi  metry. 6 - Process according to claim 4, characterized in that  that, to burn a fuel whose proportion  humidity is greater than 40% one blows a  primary air fraction equal to 80 to 140% of the  stoichiometry and an equal secondary air fraction  at 80 to 50% of the stoichiometry. 7 - Process according to any one of claims 4 to  Characterized in that the secondary air is injected  in the perforation or narrowing way  partially tangential to rotate  gases from the furnace, the flow conditions  being defined by an addimensional number S says of  "Swirl" that measures the relationship between the moment  rotational kinetics Mr and the impulse I according to the  formula M r  S =  IRU  where M = R x DA x W and,  r 2 2  I G x WG  R G is the radius of the central perforation  R is the radius of the small circle tangent to the axis of  secondary air arrivals.  DA2 is the mass flow rate of secondary air in kg / s.  kg / s.  DG is the mass flow of the gases from the  W2 is the injection rate of the secondary air in m / s.  0.5 <S <1.5  W G is the velocity of the gases from the furnace at the narrowing point in m / s, and