METHOD IN BURNING SOLID FUELS

Abstract: Method in burning solid fuels of different types in a furnace (10). The fuels is supplied to the upper end of an inclined grate (11) and is burnt thereon while primary air, possibly mixed with flue gas, is blown from below through the grate and the fuel bed (36) thereon to provide a zone of suspended fine fuel particles above the fuel bed. Unburnt material entrained into the escaping flue gas is separated from the flue gas and returned to the furnace for final combustion. Air and/or flue gas is blown into the upper portion of the grate transversely of the grate at a high pressure to provide an air or flue gas curtain at the location (12) of supplying the fuel while blowing-up existing fine fractions of the fuel from the fuel bed. At the same time the returned material together with air and/or flue gas is blown into the fuel bed in the lower portion of the grate for blowing-up remaining fine fractions of the fuel bed while blasting and stirring the fuel bed and simultaneously lowering the temperature of the hottest combustion zone located on the lower portion of the grate.

Description: METHOD IN BURNING SOLID FUELS The present invention relates to a method in burning solid fuels of different types in a furnace, wherein the fuel is supplied to the upper end of an inclined grate and is burnt on the grate wh le primary air, possibly mixed with flue gas, is blown from below through the grate and the fuel bed thereon to provide a zone of sus¬ pended fine fuel particles above the fuel bed. The fuels tjiat are possible in this case are uncut firewood, wood chips, bark, shavings, tilled peat, lump peat, cut straw, coal dust, lump coal, brown coal (briquettes), house waste (pellets, briquettes] to mention the most common solid fuels only, Practically all the fuels l sted above excepting uncut firewood, lump coal and briquettes, contain some propotion of fine fractions of a particle size between 0 and 3 mm. Thus, up to 60 % e.g.. of coal dust may com¬ prise such fine fractions. Tilled peat and' straw contain some proportion of fine fractions in the form of dust. Moreover, during the drying and gasification process of the combustion in the furnace large pieces of the fuel,, deteriorate to form smaller pieces and also a proportion of the fuel is comminuted in the final combustion phase. Hard pellets and briquettes are an exception, but for the rest it can be expected that fine fractions arise also during the combustion in the furnace. I.n the conventional burning method , the primary air is allowed to flow at a low speed through the fuel bed, the fine fractions of the fuel bed obstructing locally the air flow through the bed. This condition provides a slow drying and gasification progress in these local regions where the air passage is prevented or retarded, and a high temperature in the combustion zone due to insuf- ficient cool ng. OMPI In order to overcome the drawbacks mentioned above it has been proposed that the fine fractions are separated or are burnt more or less suspended in the combustion air and the flue gases in the furnace while the rest of the fuel is burnt in common manner on the grate. E.g. the fine fractions may be separated from the rest of the fuel before the fuel is supplied to the furnace, and may be blown into the furnace to be burnt by means of a separate burner. This system is expensive and complicated. Therefore, it has also been proposed to separate from escaping flue gas unburnt material entrained in the flue gas and consisting of the fine fractions, and to return this material to the furnace for the final combustion, The method a.ccording to the invention is of this last-mentioned type, and the object of the invention is primarily to make possible that humid fuels, dry fuels, fuels with high ash content, and fuels containing different size fractions, are burnt in an economic way and with the possibility of extensive automation of the combustion progress, at lower combustion temperature than that normally applied and with generation of a minor amount of NO than that obtained in conventional firing methods, no preheating of the air being required for h.umid fuels. Said object is achieved by the method according to the invention having obtained the characteristics appear¬ ing from cl aim 1. The flue gas curtain at the upper end portion of the grate then prevents backfiring where the supply of the fuel takes place, the fine fractions in the sus¬ pension above the fuel bed at the same time being burnt by blowing-up of the fine fractions in the fuel bed by means of the air and/or flue gas supplied. Since the returned material is blown into the fuel bed at the lower portion of the grate where the main combustion region of the fuel bed is located, the combustion temperature will be reduced there particularly if flue gas is blown into the bed, and accordingly the combustion of existing tar products will be accelerated. Moreover, the fuel bed is stirred and fine particles are turbulently blown up, said particles being burnt in suspension with the primary air and secondary air present in the region of the main combustion zone. It is particularly advantageous to apply the method according to the invention in accordance w th the development appearing from claim 2. Then, an effective preheating of the primary air to the first-mentioned grate is obtained, because the major proportion of this air has passed through the second grate section where the final combustion of the coal proportion and cooling of t.he slag takes place. The primary air to the first grate section is mixed with flue gas due to the combustion in the second grate section. The high temperature of this mixture of primary air and flue gas supplied to the first section facilitates or is a prerequisite for the com¬ bustion of the fine fuel fractions blown up from the fuel bed, in suspension above the fuel bed. Due to the fact that the flue gas from the final combustion on the second grate section together with the primary air is supplied to the first section also the following advantages are obtained: The final combustion is utilized for drying and gas fyi ng the fuel . The final combustion gas which is meagre as to oxygen but has a high drying and gasifying effect lowers the combustion temperature and reduces the the presence of local combustion progress in the fuel bed, i,e, the risk of slagging in the first grate section is reduced and lower values of NO can be expected. The necessity of air preheating is eliminated which is advantageous particularly in burning umi d fuels . The need of fresh air for drying and gasification is reduced, which means that the need of air excess will be reduced. The cool air is available for the coal combustion progress proper at the transition between the two..grate sections in order to secure • an oxidizing atmosphere where the greatest need thereof is prevailing and to reduce the combustion temperature in the rest of the fuel bed. The invention will be described in more detail with reference to the accompanying drawings in which FIG, 1 is a diagrammatic vertical sectional view ^ of a furnace for workinα the method according to the invention, FIG, 2 is an end view of a convection unit with cyclone separator, connected to the furnace, FIG. 3 s a sectional view along line A - A in FIG, 1 of the first grate section in the furnace,_ FIG. 4 is an enlarged cross-sectional view along line B - B in FIG. 1 of the second grate section in the furnace, and FIG. 5 is a plan view of the second grate section. In FIG, 1 , the furnace is designated generally with 10 and comprises a conventional water type system not shown in detail here. In the furnace, there is a water- -cooled grate consisting of an upper portion 11 which is arranged as an inclined grate and slopes downwards from a shaft 12 for the supply of solid fuel and connects at the lower end thereof to a lower grate portion (bottom grate) 13 at the upper end thereof, also the lower grate portion being arranged as an inclined grate with the lower end thereof connecting to an ash discharge 14. Thus, the grate is arranged as a two-section grate, the portion 11 forming a first grate section which is a typical drying and gasifying section and is formed as a self-feeding completely cooled stationary grate. As will be seen from FIG. 3, the grate portion 11 is formed as a trough (grate chute). Therefore, primary air can be supplied in a simple manner to the gasification zone above a fuel bed located on the grate. At the lower end of the grate portion 11 slag breakers 15 are arranged, because some fuels such as coal tend to cake to larger pieces together w th softened ash constituents, if any, in the main combustion zone which is located at the transition between the two section portions. The slag breakers 15 consist of circular sector plates which are arranged on end between the water tubes in the grate po tion 11 and are mounted to a shaft 16 which can be rotated back and forth by means not shown here in detail , e.g. a hydraul c cylinder, for oscillating the slag breakers 15 between the upwardly projecting position shown and a lowered position, The grate portion 13 is provided with a stoker for feeding the fuel bed progressively down towards the ash discharge 14. Referring also to FIGS. 5 and 4, this stoker comprises a number of mutually parallel rods 17 extending in the longitudinal direction of the grate,, which in groups of three rods are connected to a hydraulic cylinder 18 outside the furnace for recip¬ rocation of the rods. On the rods , transverse carriers 19 are arranged which have an edge chamfer on the upper side 20 thereof, which faces the upper end of the grate portion 13 such that the carriers when moving towards the upper end of the grate portion 13 are displaced under the fuel bed on the grate portion so as to dis- place the fuel bed downwards towards the ash discharge 14 when moving later towards the lower end of the grate portion. The frequency of the stroke of the cylinders 18 preferably should be adjustable. Under the lov/er grate portion 13 a conduit 21 is connected for the supply of primary air under the grate portion 13. The primary air after having passed through the grate portion 13 is supplied together with flue gases to the lower side of the grate portion 11 to pass through this portion to the combustion compartment above ' the double grate. When the primary air has passed through the grate portion 13 a mixture of flue gas and air is obtained, which means a high gas temperature in the region between the grate portions 11 and 13. This results in a many times increased volume of the gas flow through the fuel bed 36 and thus an improved blowing through the fuel bed. The flue gas outlet 22 of the furnace 10 is conno ed -through a cyclone separator 23 to a convection unit 24 the outlet of which can be connected through an economizer to a stack, which is not shown in more detail here. The cyclone separator 23 comprises a flame tube 26 to which flue gas is supplied from the furnace at the right hand end of the flame tube. Inside the flame tub.e there is provided between the ends of the tube a turbu- lator 27 while a cone 28 is arranged at the left hand end, said cone being formed with a number of slots 29 spaced around the curved surface thereof. The cone has a mouth portion 30 communicating with the convection unit while an annular space 31 between this mouth portion and the flame tube 26 communicates with a tangential outlet 32. When the flue gas passes through the flame tube 26 a turbulent movement is imparted to the gas by the turbu- lator 27, solid particles (dust) entrained into the flue gas being thrown towards the wall of the flame tube 26 according to known principles while cleaned gas passes centrally into the cone 28, The cleaned gas then passes through the mouth portion 30 to the convection unit while flue gas enriched as to impurities therein will be* drawn through the outlet 32 by means of a fan 33 connected to the outlet. A conduit 34 opening into the furnace 10 in the transition zone between the two portions 11 and 13 of the grate, is connected to the pressure side of the fan. A further conduit 35 opening into the furnace through the grate portion 11 at the upper region thereof. is also connected to the furnace 10, In .s.aid region, the conduit 35 can be connected to a distribution box or a nozzle ramp extending transversely of the grate portion 11 downwards of the location where the fuel is supplied from the fuel shaft 12. The conduit 35 is connected to a fan not shown here for the supply of air and/or flue gas under high pressure through the conduit 35. __. The solid fuel supplied to the furnace 10 through the fuel shaft 12 forms a fuel bed 36 on the grate portion 11 and a fuel bed 37 on the grate portion 13 as has been indicated by dash lines in FIC. .1. Air or flue gas is - supplied under pressure through the conduit 35 for_ providing a curtain screeni ng-off the fuel shaft from the furnace and also blowing-up fine fractions, if any, in the fuel from the fuel bed 36. In this fuel bed, substantially drying and gasification takes place while the main combustion shown is located at 38 at the transition between the two grate portions. The primary air is supplied through the conduit 21 under the grate portion 13 and passes through the fuel bed 37 on this grate portion where the final combustion takes place. The heated primary air mixed with flue gas from the fuel bed 37 continues to the grate portion 11 , passes through this portion and the fuel bed 36 located thereon to dry and gasify the fuel of said bed, and continues to the PI combustion compartment above the fuel bed 36. The temperature of the mixture of flue gas and air can be controlled by controlling the feeding speed of the fuel bed on the grate portion 13. A higher feeding speed provides a higher temperature of the final combustion and thus a higher temperature of the mixture as can be necessary when burning humid fuel. The flue gas escaping from the combustion compartment through the flue gas out¬ let 22 contains of course a great portion of solid particles (dust) ^ and the major part of these fine fractions is separated in the cyclone separator 23 to he transported by the fan 33 through the conduit 34 back to the furnace where flue gas and entrained dust areblown into the main combustion zone 38. Due to the supply through the conduits 34 and 35 there will be above the fuel bed 36 a zone indicated by a dot and dash line 39 i^FIG. 1, wherein fijie fractions of the fuel are sus¬ pended in a mixture of air (secondary air) and flue gas, and said fine fractions are burnt in said zone. Moreover, by the supply through the conduits 34 and 35 the advantages extensively discussed above, which are specific for the firing method according to the invention are achieved. It should be added here that the supply of air and/or flue gas through the conduit 35 must take place at a high pressure and should not be started until the burning has started up. This supply as well as the supply through the conduit 34 contributes to the trans¬ portation of the dust entrained in the escaping flue gas, through the flame tube 26 in the cyclone separator, but the flue gas system can of course be provided in a con¬ ventional manner with a flue gas fan. Particularly, it should be noted that the dust returned through the con¬ duit 34 is supplied to the hot main combustion zone 38 where the temperature is considerably above the ignition temperature of the unburnt fuel particles included in the ^5ΕE ( OMPI dust and that there is obtained above the fuel bed 26 due to the supply of air and flue gas through the con¬ duits 34 and 35 a turbulent movement of the mixture of secondary air and flue gas above the fuel bed 36 in the suspension zone 39, which all contributes to an inten¬ sified combustion of the fine fractions. As will be easily understood, the method of the invention can be applied to furnaces of other types than that described here. However, the fundamental construc- tion of the furnace described is particularly well suited for the application of the method of the invention. As mentioned above the furnace has a conventional tube system and also in other respects it is constructed in the manner which is conventional as to furnaces of the type described here haying a reversing compartment between the combustion compartment and the flue gas out¬ let -^ith necessary doors., etc. although such details have not been menti oned -here . The dust separator can of course be of another type e.g. of the filter type, but the cyclone separator is preferred, because it is reliable in operation and requires negligible attendance, said separator at the same time being v e ry efficient. ..

Claims: CLAIMS 1. Method in burning solid fuels of different types in a furnace (10) , wherein the fuel is supplied to the upper end of an inclined grate (11) and is burnt on the grate while primary air, possibly mixed with flue gas, is blown from below through the grate and the fuel bed (36) thereon to provide a zone (39) of suspended fine fuel particles above the fuel bed, unburnt material entrained into the escaping flue gas being separated from the flue gas and returned to the furnace for final combustion, c h a r a c t e r i z e d in that air and/ or flue gas is blown into the upper portion of the grate (11) transversely of the grate at a high pressure to provide an air or flue gas curtain at the location (12) of supplying the fuel while blowing-up existing fine fractions of the fuel from the fuel bed (36), and that the Returned material together with air and/or flue gas is blown into the fuel bed in the lower portion of the grate for blowing-up remaining fine fractions of the fuel from the fuel bed while blasting and stirring the fuel bed and at the same time lowering the temperature of the hottest combustion zone (38) located on the lov/er porti-on of the grate. 2. Method according to claim 1, c h a r a c - t e r i z e d in that the fuel is burnt on said grate (11) in a first grate section then to be transferred at the lower end of the grate to the upper end of a second grate (13) inclined in the opposite direction and extend¬ ing towards the lower end thereof under the first- -mentioned grate, for continued combustion in a second grate section, and that primary air is supplied from below through said second grate (13) then to be supplied mixed with flue gas, from below through the first- -mentioned grate (11). 3. Method according to claim 2, c h a r a c - n t e r i z e d in that the feeding of the fuel on said second grate (13) is controlled so as to control the temperature of the mixture of primary air and flue gas supplied to the first-mentioned grate (11), 4. Method according to claim 2 or 3, c h a r a c t e r i z e d in that the moving direction of the fuel when moving from the fuel bed (36) on the first-mentioned grate (!!)_, is changed at the transition to the fuel bed (37) on said second grate (13) and that the returned material is supplied in the transition region (38) from one grate to the other. 5. Method according to any of claims 1 to 4, c h a r a c t e r i z e d in that the flue gas under the influence of the air and/or flue gas supplied is forced through a cyclone separator (23) while imparting a rotational movement to the flue gas, for separating unbu-tξp..t material entrained into the flue gas.

Inventors: FRILUND EYVIND, FRILUND, EYVIND
Applicants: LANDSKRONA FINANS AB
Application year: 1982
Main classification: F23B5
IPC codes: F23J15, F23L1