Arrangement for converting a conventional oil boiler to a boiler with moist, granular and solid fuel

Abstract: The invention relates to an arrangement for converting a conventional oil boiler (1) to a boiler with moist, granular and solid fuel, in which arrangement a fuel combustion gas dryer (3, 4) is used, connected to the combustion gas line (6, 7) of the boiler, and a solid fuel combustion device (2) to replace the oil burner. The combustion gas dryer (3, 4) is a so-called circulating fluidized bed (CFB) dryer (4) together with a particle separator (3) and a combustion gas centre tap (6.2) has been fitted into the oil boiler, through which centre tap hot combustion gas has been set to be mixed with combustion gases of the normal outlet (6.1) in the desired proportion in order to regulate the temperature of the drying gas of the dryer (4). When most advantageous, the combustion devices (2) include a cyclone burner (2), known in itself.

Description: The invention relates to an arrangement for converting a conventional oil boiler to a boiler with moist, granular and solid fuel, which arrangement uses a fuel combustion gas dryer connected to the boiler's combustion gas line and a combustion device for solid fuel to replace the oil burner. Solid fuel means here mainly sawdust, peat and other biological fuel. Conventional heating, warm water and steam boilers which use oil are badly adaptable to solid fuel. Different pre-burner constructions are known, but they have several disadvantages. Full effect is normally not achieved, because the properties of fuel gases differ considerably from the measured values. The combustion gas temperature of a pre-burner that burns moist fuel stays much lower than that of oil heating which has been used as the standard measure. The share of water vapour is considerably greater than that measured. The situation will improve essentially if the moist fuel is dryed with combustion gas before it is burnt. Thereby the combustion temperature and together with it the combustion capacity will rise. However, no practical and economical way to convert a 0,5 - 10 MW oil boiler of low effect to a boiler with solid fuel has been found. It is difficult to make a fuel dryer work at the end of a conventional oil boiler, because the final temperature of combustion gas is too low. The aforementioned pre-burner solutions are quite awkward and expensive. The efficiency usually stays low and the greatest combustion capacity remains much lower than the nominal capacity of an oil boiler. Dust combustion does not normally come into question because it is so expensive. The object of the invention is to achieve a new sort of arrangement for converting a conventional oil boiler to a boiler with moist, granular and solid fuel, which gives a good efficiency and almost all the effect of the converted boiler. The characteristic features of the invention are presented in the accompanying patent claims. This invention makes use of solutions, known as such, in the way of a new combination, whereby the oil boiler can be made to function close to the optimal circumstances in a more simple way than before. Conventional oil boilers usually have several consecutive heat surfaces after the furnace. A centre tap can usually be installed somewhere in between these or even behind the furnace, for example in the maintenance hatch, whereby hot combustion gas is mixed in the desired proportion with cold combustion gases in order to obtain the desired temperature, 200 - 300°C. In this case the dryer can be a dryer, known in itself, which produces fuel with a moisture level of 10 - 15% for example to be used with a CMR burner. An especially advantageous application due to its simplicity can be obtained by using a CMR burner according to PCT publication WO 97/12177. CMR (Chemi mechanical reactor) is most suitable to replace an oil burner, because it gives a short flame length also with coarse fuel. The melt cyclone burners that are known are more complex and more expensive even if they could as such be used in this arrangement. In the following, the invention is presented with reference to the accompanying figures, which show some of the applications of the invention. Fig 1 shows a boiler arrangement which uses a CMR burner Fig 2 shows a boiler arrangement which uses a carburetor and a gas burner Fig 3 shows the solution in figure 1 in more detail Fig 4 shows the combustion gas and centre tap connection in a traditional flue - fire-tube boiler. The best application of the invention includes oil boiler 1, CMR burner 2, a fuel dryer attached to combustion gas line 6, which dryer is especially a dryer 4 with particle separator 3, in figure 1. The structure of the CMR burner has been explained in more detail in patent publication No 98854. In the publication, a selective delay is created so that the coarse particles stay in the swirl chamber for a longer time and until they are smaller than of boundary size. The length of the flame can be made short exactly by way of only letting out particles from the burner that are smaller than of boundary size. Certain dryers have been presented in the Finnish patent applications 852594 and 903097, and in reference No 04252654 of the Compendex database, Ruottu, Seppo; Sarkomaa, Pertti: "Present state of regenerative CFB heat exchanger development", Proceedings of the International Conference on Fluidized bed Combustion, ASME, New York, NY, USA, vol 1, pp. 419 - 422, 1995. The main principle is to circulate the fuel to be dryed with drying air until the characteristic weight of the fuel falls below such a level that it will, due to its lightness, leave the separation cyclone together with the outflow, after which the dry fuel is separated from the combustion gases in its own separation cyclone. A further drying apparatus is known from US 2 939 411. Referring to figure 1, even a simple dryer can be made to function quite effectively when a combustion gas centre tap is fitted to the oil boiler, the duct of which has been marked with reference number 6.2 in the figure, when the usual exit duct has been marked with reference number 6.1. These ducts include adjustment valves by which the hot (200 - 700°C) and cold (100 - 150°C) combustion gas is mixed together in order to adjust the temperature of the drying gas. When the humidity of the fuel, for example sawdust or peat, is 45 - 55%, the temperature of the combustion gas which is fed onto the dryer 4 is 200 - 300°C and the amount of heat is enough to dry the amount of fuel of corresponding combustion capacity to a level of humidity of 10 - 15% with a moderate air surplus. After separator 3, the temperature of the combustion gases in departing duct 8 is only 65 - 70°C which gives the whole arrangement a very good total efficiency (even 95%) despite the centre tap of the boiler. All of the outer components of boiler 1 of the arrangement, the circulating fluidized bed (CFB) dryer 4, the particle separator 3 and the CMR burner 2 are quite simple as to their structure. Especially the upper part of dryer 4 and the particle separator 3 must be made out of stainless steel or out of another material that can resist corrosion, because the combustion gases are close to dewpoint. Figure 2 presents an adaptation of the application in figure 1. A combination of carburetor 2' and gas burner 2 is used instead of the CMR burner in figure 1. The structure is more complex than that presented above, but still otherwise more advantageous than before. Also here, a great combustion capacity is obtained with dry fuel, when the water that is included in the oil does not circulate through the boiler. Figure 3 presents the principle of the connection in figure 1 in more detail. The inner structures of dryer 4 and of particle separator 3 are presented schematically. Their most important help devices, as those of CMR burner 2 are further presented. A conventional oil boiler 1, of the flue - fire-tube boiler type, is equipped with a CMR burner 2 instead of an oil boiler. Its fuel supply is explained later. The boiler can also be a plate or pipe boiler. In addition to the actual combustion gas exit 6.1, the boiler has been fitted with a centre tap 6.2 in order to catch the hot combustion gas. The capacity of the boiler is 80 - 90% of its nominal capacity with oil. The temperature in the combustion gas duct 6 that leads to dryer 4 is the abovementioned 200 - 300°C. The moist fuel is transferred from storage silo 10 by transporter 11 via sluice feeder 12 to feed connection 48 of the dryer. The main parts of dryer 4 are: dryer pipe 40, air division chamber 41, an air division plate upon it, separator cyclone 46, which has on its upper side the tangential inlets 45, wastepipe 43 and outlet connection 49. The fuel is fed through the aforementioned feed connection 48 onto the air division plate 42. In case big lumps need to be removed, this is done through outlet connection 49. When the fuel dries, it follows the strong flow upwards, arriving via the tangential feeding inlets 45 to cyclone 46, in which the heavier moist mass falls through wastepipe 43 back to air division plate 42, and the dry fuel that has circulated several times, and which is light, is let out through outlet 47. Outlet 47 is connected to transport duct 7 which transports the fuel-air mixture to particle separator 3 which here is formed out of cyclone 31, built on top of feeding silo 34. Intake connection 30 is tangential, bringing about the strong vortex that is required by the disparity. The bottom of cyclone 31 is slightly smaller in diameter than cone 32, the point of which shows upwards. The fuel particles flow along the cone surface down to the sides and further down to feeding silo 34. Screw transporter 35 that is included in the silo transports together with its sluice feeder the dry fuel to combustion air suction duct 21 of CMR burner 2. Blower 22 sucks the fuel up with the carrying airflow, and feeds it to the burner. The secondary airflow is produced with the help of blower 23. In the CMR burner, the fuel is set to be fed together with a sub-stoichiometric primary air amount into the swirl chamber and the secondary airflow is set to be fed in a concentric whirl into the swirl chamber, around the outflow. The rate of air is regulated with nominal effect with the help of secondary airflow to the area of 1,2 - 1,35. Figure 4 presents an arrangement in connection with a conventional oil boiler in detail. The oil boiler is marked with reference number 1 and it is of the flue - fire-tube boiler type. It has three passes, I, II and III, which have also been marked with reference numbers 13, 14 and 15. The first pass is formed by flue 13 itself. The second and third passes are formed by the fire-tubes. CMR burner 2 blows the hot combustion gases to flue 13. Return chamber 16 is placed at the other end, and it leads the combustion gases to the second pass 14. At the end of this, between the second pass 14 and the third pass 15, there is another return chamber 17. At the end of the third chamber there is an outlet chamber 18, onto which the normal combustion gas outlet connection has been fitted. Chambers 16, 17 and 18 are usually either provided with a maintenance hatch or they can be opened so that the fire surfaces can be cleaned. The abovementioned normal combustion gas duct 6.1 is connected to outlet chamber 18 connection. It is essential in relation to this invention that a centre tap 6.2 is made to the conventional boiler, which centre tap is connected to some centre chamber, here chamber 17. It would alternatively be possible to connect the centre tap also to chamber 16 by duct 6.3 which has been drawn by broken lines in the figure, for example through maintenance hatch 19, if a higher centre tap temperature were needed. Practical experiments have shown that the temperature after the second pass has been sufficient and that the required 200 - 300°C temperature is reached in all load circumstances before the dryer. Some other oil boiler might require an earlier centre tap. The arrangement can be fitted with a scrubber in order to further enhance the combustion gases or with a heat recovery unit in order to advance the efficiency.

Claims: Arrangement for converting a conventional oil boiler (1) to a boiler with moist, granular and solid fuel, in which arrangement a fuel combustion gas dryer (3, 4) is used, connected to the combustion gas line (6, 7) of the boiler, and a combustion device (2) to replace the oil burner, wherein the combustion gas dryer (3, 4) is a so-called circulating fluidized bed (CFB) dryer (4) together with a particle separator (3) and wherein a combustion gas centre tap (6.2) has been fitted into the oil boiler, through which centre tap hot combustion gas has been set to be mixed with combustion gases of the normal outlet (6.1) in the desired proportion in order to regulate the temperature of the drying gas of the dryer (4), and wherein the combustion devices (2) alternatively include either a cyclone burner (2) or a carburetor/gas burner combination (2, 2') with its fuel feeder devices. Arrangement according to patent claim 1, characterised in that the temperature of the combustion gas before the dryer (4) has been fitted to the area 200 - 300°C and after the separator to 60 - 80°C when the air surplus is 1,2 - 1,35 at nominal effect. Arrangement according to patent claim 1 or 2, characterised in that the particle separator (3) is a cyclone (31) that has been formed on top of the fuel silo (34) in an integrated fashion, in which there is, on the bottom of the cylinder space, a centrally smaller cone (32) that points upwards, whereby the separated mass flows down to the silo (34) from in between the lower edge of the cone (32) and the cylinder. Arrangement according to one of patent claims 1 - 3, characterised in that the dryer (4) includes a vertical dryer pipe (40), the lower part of which is an air division chamber (41) and an air division plate (42) and the upper part of which has been centrally fitted with a separating cyclone (46), the central wastepipe (43) of which reaches down close to the air division plate (42). Arrangement according to one of patent claims 1 - 4, characterised in that the cyclone burner is a CMR burner, known in itself, in which the fuel has been set to be fed together with a sub-stoichiometric primary air amount into the swirl chamber and the secondary airflow has been fitted to be fed in a concentric whirl around the outflow of the swirl chamber. Arrangement according to one of patent claims 1 - 5 in connection with an oil boiler (1) of flue - fire-tube boiler type, in which there are one or more centre chambers (16, 17) on the way of the combustion gases, characterised in that the centre tap (6.2) is connected to one of the centre chambers (16, 17).

Inventors: MIETTINEN MARKKU, OKSANEN MAUNO, PAJUNEN KARI, PYYKKOENEN MARKKU, MIETTINEN, MARKKU, OKSANEN, MAUNO, PAJUNEN, KARI, PYYKKOENEN, MARKKU, Pyykkönen, Markku
Applicants: VAPO OY
Application year: 1998
Main classification: F23K1
IPC codes: F23K1