Method and apparatus for burning poor gases.

0001The present invention relates to a method for burning poor gases by means of a gas which supports combustion, and to the combustion apparatus.

0002Various production processes and, in particular, iron-working processes are known which produce enormous quantities of poor combustible gas, that is, gas with a low calorific value (C.V.) which may be used for the production of steam and/or power. In particular the gas from blast furnaces contains about 25% by volume of carbon monoxide (CO) and 1% of hydrogen while the remainder is inert gas (CO₂,N₂) whereby its C.V. is of the order of 780kcal/Nmc, while the gas from converters for the production of steel has a C.V. of the order of 2000kcal/Nmc since it has a higher percentage content of CO.

0003In the prior art such poor gases are burnt in boilers, generally of very large size, with the use of ordinary burners for poor gases which use pre-heated air to support the combustion aided by about 10% of rich gas, for example, gas from a coke plant having a calorific value of the order of 4500kcal/Nmc. Boilers are also known which have a combustion chamber lined with a refractory material to facilitate the combustion of gases containing various percentages of CO given the hotter environment. Such boilers, if provided with means for achieving considerable turbulence, function without the use of any additional rich fuel after they have reached a certain temperature. This known method generally is of limited thermal efficiency because of the modest temperature of the combustion products.

0004On the other hand methods are known for recovering the residual heat from the exhaust gases from gas turbines which are still rich in oxygen and are at a high temperature. In the prior art these exhaust gases have been used directly in boilers to produce steam of medium temperature and pressure but do not yield high turbine efficiencies since the heat source is represented by exhaust gases with a low enthalpy (500 to 600°C).

0005In order to reduce this disadvantage, the temperature of the exhaust gases from the turbine is increased by suitable burners, so called "duct burners" inserted in the exhaust duct from the turbine upstream of the recovery boiler. These burners are supplied with a rich fuel and use the oxygen still contained in the exhaust gases as the combustion supporter. Thus it is possible to obtain the necessary heat exchange in the boiler to produce steam at a higher temperature.

0006A method is also known for burning the poor gas in a gas turbine. In this case the poor gas is previously mixed with gas from a coke plant, possibly mixed with gas from a converter for the production of steel. The mixture is then burnt together with about 24% of natural gas. The exhaust fumes are in turn then sent to a boiler for the recovery of the residual heat, producing steam at about 490°C, but resulting in a low efficiency in the steam turbine downstream.

0007This method has various disadvantages. First of all the gas from the steel-works must previously be freed from dust in enormous precipitating installations. Furthermore the coke plant gas must be cleaned by passage through washing stations to remove the tar and naphthalene which give problems in the combustion. These steps of filtering the gases require very burdensome installations. The gas turbine in turn requires very large, expensive compressors which, by absorbing a certain proportion of energy, reduce the efficiency of the gas turbine. Moreover, residual quantities of unburnt CO remain in the gases from the combustion chambers of the gas turbine because the stay times are too short at adiabatic temperatures which are too low. It is thus necessary for the mixture of combustible gases, including the natural gas, to have a calorific value of at least 1800kcal/Nmc.

0008With further improvements in the combustion installations in the gas turbines, some manufactures are moving towards achieving regular operation with mixtures of gases having calorific values of up to 1400 Kcal/Nmc by using even more costly technology.

0009The object of the invention is to provide a method and apparatus for achieving combustion of poor gases by means of a gas which supports combustion; this method and apparatus should be of the maximum simplicity and safety in operation so as to eliminate the disadvantages listed above of the prior art. A further object of the invention is to use the exhaust fumes from a gas turbine directly as the combustion supporter for the poor gases.

0010This object is achieved by the method of the invention, which is characterised in that it includes the following steps: supplying a flow of the gas which supports combustion to a combustion chamber in a predetermined direction and sense; at least in a preliminary phase in which a pilot flame is ignited in the chamber, directing the pilot flame in the said direction and sense; and supplying a flow of the poor gas to the chamber towards the pilot flame in a direction substantially transverse the said predetermined direction.

0011The apparatus for carrying out this method is characterised in that it includes a burner for a pilot flame located in a combustion chamber and arranged to direct the pilot flame in a predetermined direction and sense, and means for supplying a flow of the poor gas to the chamber towards the pilot flame in a direction substantially transverse the predetermined direction.

0012In order to provide a better understanding of the invention, a preferred embodiment of the combustion apparatus will now be described by way of example with the aid of the appended drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a partial perspective view of a portion, or module, of combustion apparatus for burning poor gases according to the invention;</li><li>Figure 2 is a perspective view of the entire combustion apparatus;</li><li>Figure 3 is a cross section of a variant of the combustion apparatus of the invention;</li><li>Figure 4 is a sectioned taken on the line IV-IV of Figure 3;</li><li>Figure 5 is a partial perspective view of the combustion apparatus of figure 3.</li></ul>

0013The combustion apparatus enables a poor gas, for example the product of an iron-working plant, such as a blast furnace, to be burnt with the use of the exhaust gases or fumes from a gas turbine as the combustion-supporting gas. The combustion apparatus also enables other types of poor gas such as, for example, those produced by the atmospheric gasification of carbon or biomasses, to be burnt.

0014It is known that the exhaust gases from turbines have, on average, a content of about 13% of oxygen and a temperature above 500°C, in general about 540°C. With the use of these hot gases to support combustion, the heat in these gases is then automatically recovered, producing a higher temperature in the combustion.

0015In order to trigger and support this combustion, a pilot or support flame is used and is produced by burning a relatively rich gas, that is, a gas having an average-high calorific value such as coke plant gas or methane. The pilot flame is obtained with the same combustion-support gas as in the gas turbine exhaust.

0016With reference to figure 2, a combustion chamber for poor gases is generally indicated 6 and has a substantially square or rectangular section defined by a pair of longitudinal walls 7 and a pair of transverse walls 8. The chamber 6 is connected in the usual duct which conveys the combustion-support gas, normally constituted by the exhaust gases or fumes from a gas turbine. The combustion-support gas is supplied in parallel with the walls 7 and 8 in a pre-determined direction and sense indicated by the arrows C.

0017By way of example, the walls 7 and 8 may be vertical and the poor gas may be supplied upwardly as shown in the drawings. Alternatively the walls 7 and/or 8 may be inclined or horizontal, whereby the path of the combustion gas will be correspondingly inclined or horizontal.

0018A linear burner generally indicated 9 is located on the centre line of the chamber 6, parallel to the walls 7, and is arranged to produce a pilot flame 19 (Figure 3) which burns coke plant gas. In particular, the burner 9 may be of modular type, one module of which is indicated 9' in the drawing, so that if a series of modules 9' is placed end to end, a burner 9 of any length may be obtained.

0019The burner 9 (Figure 2) of known type and constituted by four modules, is constituted essentially by a tubular duct 11 which is provided along a generatrix with a series of holes or nozzles 12 (see also Figure 3) mutually spaced at a predetermined distance. For example a module 9' may contain four nozzles 12. The duct 11 is located within a channel 13 of upside-down U shape, whose transverse wall 14 has a series of holes 15 aligned with the nozzles 12.

0020The burner 9 also includes a pair of flame-distribution screens 16 symmetrical with each other. Each screen 16 includes a portion 17 parallel to the portion 17 of the other screen 16 and a portion 18 inclined so as to space the portion 17 from the duct 11. The two portions 17 are mutually spaced at a predetermined distance S, for example about 180mm. The burner 9 is located with the duct 11 parallel to the walls 7 on the centre line of the section of the chamber 6. The burner 9 is also oriented so that the holes 15 generate pilot flames 19 in the direction and in the sense of the arrows C.

0021The fuel constituted by the poor gas, for example that produced by a blast furnace, is supplied to the combustion chamber 6 through a pair of distributor casings 21 fixed to the two walls 7 of the chamber 6. Each casing 21 is supplied through four inlet tubes 22 perpendicular to the duct 11 and communicates with the chamber 6 through a series of through-apertures 23. These are preferably rectangular or square in shape, equidistant from each other and disposed at a height corresponding to the pilot flame 19 so that the flow of poor gas is supplied in a direction P transverse the direction of supply C of the combustion support gas.

0022A respective fixed fin 26 is fixed to the lower edge 24 of each aperture 23 and extends into the chamber 6. Moreover there is a common bar 28 in correspondence with the upper edges 27 of the apertures 23. Hence the fins 26 are disposed on those edges 24 of the apertures 23 which are upstream with respect to the direction C of flow of the combustion-support gas while the bar 28 is disposed on the downstream edges 27.

0023The two series of fins 26 enable the poor combustible gas to mix effectively with the combustion-support gas before it reaches the flame 19. Moreover, together with the two bars 28, they promote the penetration of the combustion-support gas to the pilot flame 19 which is obtained with the same combustion-support gas as that used to burn the poor gas, that is with the exhaust gas from the turbine.

0024The size of the chamber 6 and the dimensions of the apertures 23, the fins 26 and the bars 28 are selected according to the characteristics of the gases in the two flows. For poor combustible gas from a blast furnace and for a combustion-support gas provided by the exhaust gases of a turbine, a distance D between each wall 7 and the corresponding vertical portion 17 of the screen 16 of between 3 and 5 times the height A of the apertures 23 may, to advantage, be chosen.

0025The length L of the fixed fins 26 may in turn, to advantage, be between 0.3 and 0.6 times the distance D, while the width T of the bar 28 may be between 0.15 and 0.5 times the length L of the fins 26. Consequently, the width T may be between 1/15 and 1/5 of the distance D.

0026According to the composition of the two gases in the mixture, once the surfaces of all the zones over which the flame and the burnt gases flow have reached a certain temperature, the system no longer needs the pilot flame 19 to support the combustion of the poor gas. The supply of coke plant gas may then be stopped automatically in any known manner, either in dependence on the temperature reached or when a predetermined period of time has lapsed after the ignition.

0027In the variant of figures 3 to 5, each aperture 23 has an associated series of auxiliary fins 29 which are normally in an inactive position such as that shown by the broken lines in Figure 3. These are moved into their active positions shown in Figures 3 to 5 when, for any reason, the flow of exhaust gases from the turbine stops and it is wished to replace the combustion-support gas with ambient air containing 21% oxygen. This air is generally at ambient temperature but if the mixture of combustible gas had too low a calorific value, it could be pre-heated to increase the efficiency of the combustion process.

0028For this purpose, the auxiliary fins 29 are moved by a corresponding bar 31 which is rotatable on at least two supports 32 carried by two pillars 33 fixed to each wall 7. The bar 31 is also rotatable on two other supports carried by the walls 8. Thus the bar 31 is rotatable about an axis parallel to the duct 11 and is located adjacent the free ends of the fixed fins 26. The auxiliary fins 29 have the same spacing and substantially the same width as the corresponding series of fixed fins 26 while their length may, to advantage, be between 0.8 and 1.2 times the length L of the fixed fins 26.

0029One end of each bar 31 projects from the front wall 8 and is fixed to a handgrip 34 which is manually operable to rotate the bar 31 so as to dispose the auxiliary fins 29 of the corresponding series substantially coplanar with the fixed fins 26. Thus the auxiliary fins 29 of each series, when they are made active, reduce the passage cross-section for the flow of combustion air into the chamber 6.

0030From what has been explained above, it will be clear that the combustion apparatus achieves a method of combustion including at least the following operations: the supply of a flow of combustion-support gas into the combustion chamber 6 in a predetermined direction and sense C; the lighting of a pilot flame 19 in the chamber 6 at least in a preliminary ignition phase, the pilot flame 19 being directed in the direction and sense C; and the supply of a flow of poor gas into the chamber 6 towards the pilot flame 19 in a direction substantially transverse the direction C.

0031Furthermore, the advantages of the method of the invention and of the combustion apparatus which achieves this method over the prior art will be obvious. First of all, poor gases obtained from particular production processes and the exhaust gases from gas turbines are used simultaneously so that the low energy contents of both of these are recovered. Furthermore, the same combustion-support gas is used both to burn the poor gas and to burn the rich gas which gives the pilot flame. Finally, in the variant of Figures 3 to 5, any temporary interruption in the flow of exhaust gases from the turbine may be overcome with another combustion-support gas without the need to waste the poor gas flowing in the meantime.

0032It is understood that various modifications and improvements may be made to the method and apparatus described without thereby departing from the scope of the claims. For example, the pilot flame 19 may be directed horizontally and the poor gas may be supplied upwardly with respect to the pilot flame 19. Moreover the burner 9 may be replaced by another type of burner. Finally, the fins 26 and/or 29 may be replaced by other means for creating turbulence and hence mixing the two gas flows, and the dimensions given are entirely exemplary and must not be understood in any limiting sense.