A BOILER WITH COMBUSTION RETORT

Abstract: There is described a boiler comprising a firing means, e.g. a feed screw conveyor, for feeding bio-fuel, e.g. wood pellets, from a fuel silo to a combustion retort (2) in the boiler, a furnace chamber (4), a water filled convection section (16) and a flue gas exhaust (10), where the furnace chamber (4) has smooth walls and is surrounded by the convection section (16), i.e. this and the furnace chamber (4) mainly have common walls, and where the flue gas exhaust (10) is disposed under the combustion retort at the bottom of the furnace chamber (4). By means of simple measures, hereby is achieved a boiler with lesser radiation and convection losses and which has an improved combustion technique and a more effective heat transmission to the boiler water, i.e. a greater efficiency.

Description: A BOILER WITH COMBUSTION RETORT The present invention concerns a boiler of the kind indicated in the introduction of claim 1. According to common principles for constructing biomass fired or oil fired boilers, the boilers are made with fire box for burning fuel and a convection section typically consisting of flue pipes for flue gas cooling with associated badly insulated cleanout doors, especially in the case of small and medium sized boilers. Flue boxes and flue gas exhaust are usually not insulated. Therefore, it is also well-known that particularly the uppermost part of the boiler yields very large radiation losses as flue boxes, flue gas exhaust and sundry cleanout doors for flue pipes breaks the insulation jackets, resulting in large radiation and con- vection losses. According to present rules, the flue connection for the boiler water with associated pressure expansion and safety valve, or possible open expansion vessel, has to be located at the top of the boiler, i.e. in the uppermost, hottest part of the boiler. If also the flue gas exhaust and sundry cleanout doors for flue pipes etc. are placed at the top of the boiler, this fact implies a rather complicated construction of the upper part of the boiler, resulting in great production and maintenance costs. The invention has the purpose of indicating a new and improved boiler which by means of simple measures makes possible to counteract the above mentioned drawbacks, and which has improved combustion technique and efficiency. The boiler according to the invention is characterised in that the furnace chamber has smooth walls and is surrounded by the convection section, i.e. this and the furnace chamber mainly have common walls, and that the flue gas exhaust is disposed under the combustion retort at the bottom of the furnace chamber. With simple measures there is obtained a boiler not encumbered by the drawbacks mentioned in the intro- duction, and which has an improved combustion technique and a more effective heat transmission to the boiler water. The heat radiation from the heat source, the retort, will cause an upward directed heat flow in the middle of the furnace chamber. When hitting the top of the furnace chamber, the heat flow will be divided and directed toward the water cooled heating surfaces and continue downward along the heating surfaces toward the flue gas exhaust. The strong heat flow from the retort will naturally rise upward. The descending heat flow, which has decreasing flue gas temperature, will appear at the spot in the furnace chamber with the lowest temperature, i.e. along the heating surfaces where furthermore there is the least ascending flow resistance. The upward directed pressure buildup in the furnace chamber will cause the relatively cool flue gases at the bottom of the furnace chamber to be forced out of the flue pipe. The boiler according to the invention will furthermore tend to keep itself clean, burning itself clean. If the fuel combustion, for example, occurs less than optimally, and the wall of the furnace chamber are sooted up for that reason, this will imply a reduction of the heat transmission to the heating surface, which in turn results in a rise in temperature with the consequence that the heating surface is burned clean. Suitably, the boiler according to the invention is designed so that the furnace chamber has shape as an upright, box-shaped furnace chamber with quadratic or rectangular cross-section, and that the surrounding water filled convection section has the same shape but with such size that the thickness of the water jacket surrounding the furnace chamber is about 50 mm. With such a design the boiler may easily be built together with a usual box-shaped fuel silo, for example with a volume of about 250 1. Alternatively, the boiler according to the invention is designed so that the furnace chamber is shaped as an upright, cylindric furnace chamber, and that the surrounding water filled convection section has the same shape but with such size that the thickness of the water jacket surrounding the furnace chamber is about 50 mm. With such a shape, the boiler may, for example, be placed adjacent to a wall, e.g. an outer wall, in a decorative way as the fuel silo in that case may be constituted by a larger outdoor silo. Experiments have shown that the boiler according to the invention may advanta- geously be designed so that the combustion retort is disposed at about the centre of the furnace chamber at a height corresponding to one third of the total height of the furnace chamber. With the purpose of regulating the temperature of the smoke in the flue, the boiler according to the invention may advantageously be designed so that flue gas exhaust comprises a horizontally disposed flue pipe arranged rotatably and having a number of radial apertures through which the flue has access to the flue pipe and a chimney. And for space reasons, it may be suitable that the boiler according to the invention furthermore is designed so that the flue pipe is perpendicular to the firing means and is dis- posed under the firing means, preferably at one side of the furnace chamber. The invention also concerns a combustion retort and comprising an upper ignition vault supported on opposite side edges of a lower part of the combustion retort, which combustion retort is characterised in that the said ignition vault is made by moulding ceramic material, and that the ignition vault is reinforced with metal fibres that preferably consist of stainless steel. Hereby is achieved an ignition vault having great fracture strength and heat resistance as it does not crack even if subjected to rapid heating and/or cooling. With the purpose of achieving optimal burning out of the combustion gasses, the combustion retort according to the invention is designed so that a lower part of the ignition vault is designed with longitudinal, concave vaults at each side of a longitudinal, central edge for providing flow turbulence and ensuring mixture of combustion air and gasses. The invention is explained in more detail in connection with the drawing on which: Fig. 1 is a schematic perspective view showing heat flow in a furnace chamber in an embodiment of a boiler according to the invention, Fig. 2 is a schematic plan side view showing the heat flow in the furnace chamber in a boiler according to the invention, Fig. 3 is a schematic plan side view illustrating the heat transmission in a boiler according to the invention, Fig. 4 is a plan view showing the design of a preferred embodiment of a boiler according to the invention as seen from the front and with open door to the furnace chamber, Fig. 5 is a plan view showing the design of a preferred embodiment of a boiler according to the invention, as seen from the left side, Fig. 6 is a plan view showing the design of a preferred embodiment of a boiler according to the invention, seen from back, Fig. 7 is a plan front view of a preferred embodiment of a combustion retort for il- lustrating its function in a boiler according to the invention, Fig. 8 is a side sectional view of a lower part of the combustion retort showing how the retort is incorporated in the boiler, Fig. 9 is a top view of the lower part of the combustion retort shown in Fig. 8, Fig. 10 is a front view corresponding to Fig. 7 of a lower part of the combustion re- tort showing the air flow in cold condition, and Fig. 11 is a corresponding front view of a lower part of the combustion retort showing the air flow in hot condition. Fig. 1 shows how the heat radiation from the heat source 2 (the retort), as previously mentioned, will cause an ascending heat flow in the middle of the furnace chamber 4. When it hits the top 6 of the fumace chamber 4, the heat flow will be divided and directed toward the water cooled heating surfaces 8 and continue down along the heating surfaces 8 toward the flue gas exhaust 10. The strong heat flow from the retort 2 will naturally rise upward. The descending heat flow, having decreasing flue gas tempera- ture, will appear where the temperature in the furnace chamber 4 is lowest, i.e. along the heating surfaces 8 where also the least ascending flow resistance exists. The upward directed pressure build-up in the fumace chamber 4 will cause that the relatively cool flue gases at the bottom of the furnace chamber 4 will be forced out the flue pipe 10. Fig. 2 also shows the heat flow in the boiler furnace chamber 4. In the boiler flue gas exhaust 10 there is mounted a rotatable tube 12 provided with or designed with two short, radial pipe stubs 14 through which the smoke has access to the flue gas exhaust 10. The purpose of the rotatable tube 12 is to regulate the flue gas temperature which is highest when the pipe stubs 14 point upward, and which is lowest when the pipe stubs 14 point downward. The tube 12 may be removed when cleaning ashes from the boiler. Fig. 3 shows the heat transmission in the furnace chamber 4 (boiler). Above the retort 2, the heat transmission occurs by flame radiation. Under the retort 2, the heat transmission occurs by gas radiation and convection. A limited heat transmission also oc- curs by convection transition in the furnace chamber 4 (the firebox) when the cooled gasses are led downward along the heating surfaces 8 of the furnace chamber 4. As shown in Figs. 4-6, the furnace chamber 4 is surrounded by a water filled convection section 16 consisting of a water filled jacket 18 with the same shape as the fur- nace chamber 4 as the convection section 16 or the water jacket 18 and the fumace chamber 4 have walls in common constituting the top wall 6 and the heating surfaces 8 of the fumace chamber 4 which at the bottom and opposite to a door 22 is not surrounded by the water jacket 18. The water filled convection section 16 or the water jacket 18, which on the drawing is shown without strut connection, has e.g. a thickness of about 50 mm and is enclosed by an external insulation 20 (e.g. 25 mm mineral wool) which also extends under a bottom plate 24 of the fumace chamber 4. In other words, the boiler according to the invention appears outwardly as a closed, mainly smooth jacket insulated unit with minimal heat radiation and heat loss. The said door 22, which internally may be filled up with fireproof moulding into the furnace chamber 4, is shown in Fig. 4 in outward pivoted, open position so that the combustion retort 2, the flue pipe 10, 12 and an ash drawer 26 are visible. Furthermore, firing pipe 28, combustion retort 2, ash drawer 26, flue pipe 10,12 with nozzles 14 and a possible transverse partitioning wall 30 are shown in the side view, cf. Fig. 5, while Fig. 6 shows an end view of the firing pipe 28. The combustion retort 2 is shown from the front in Fig. 7 which serves as illustration of the function of the retort 2. This is upward provided with ignition vault 32 of fire- proof material reinforced with metal fibres, preferably of stainless steel, which imply great breaking strength and heat resistance, among others, so that crack formation may be avoided even by large and rapid temperature fluctuations. At the bottom and at each side of a central, longitudinal, projecting edge 31, the igni- tion vault 32 has double curved, concave ceilings 33 implying the shown double sided rotation and efficient mixing of combustion air and gasses. Via the firing pipe 28, the fuel is supplied into the combustion retort 2 of which the lower part 34 is made of cast iron or steel sheets with a central hollow 35 into which the fuel is supplied and pressed up like a molehill, i.e. the fuel protrudes up from the lower part 34 which is made of steel or cast iron. The advantage of the shown construction of the combustion retort 2 is a large degree of self-regulation of the ratio between primary and secondary combustion air. If there is too much primary combustion air at 36 compared with the amount of secondary combustion air at 38, the height of the combusting fire in the retort 2 will change to a lower level, causing a relatively larger supply of secondary combustion air. If the opposite is the case, the fire height in the retort 2 will climb. In other words, a balance between primary and secondary combustion air is achieved more or less automatically. I.e. if the air supply to the retort 2 is changed within a certain range, a stable situation between primary and secondary combustion air will arise within a short time, something which is possible because of oxygen control. Fig. 8 shows a side sectional view through the combustion retort 2 which via firing pipe 28 and feed screw conveyor is supplied with fuel being pressed up into the central hollow 35 of the retort 2 so that a molehill-like fuel heap is formed with a height exceeding the height of the hollow 35, i.e. the heap projects up in the hollow under the ignition vault 32. The firing pipe 28 is surrounded by a pipe 29 for supplying combustion air which is supplied via a lateral connection 39 to a ventilator, i.e. the cold combustion air surrounding the firing pipe 28 is cooling it, something which is also cooling the combustion retort 2 itself. Preheating of the combustion air occurs simultaneously, the air being supplied to the retort 2 through a sickle-shaped inlet 37. The cen- tral hollow 35 of the combustion retort 2 is also cooled by the external contact with the combustion air which is preheated simultaneously. The primary combustion air is conducted to the retort 2 through a very narrow, annular slit 40 along both longitudinal sides and at the front of the retort 2. The secondary combustion air is supplied via rows of nozzles or perforations 41 along both longitudinal sides and the front side of the top plate 42 of the retort, whereupon the ignition vault 32 is supported, something which is clearly seen in Fig. 9. Fig. 10 illustrates the flow of the combustion air by cold start of the combustion retort 2, whereas the flow of the combustion air in a hot combustion retort 2 is shown in Figs. 7 and 11.

Claims: CLAIMS 1. A boiler comprising firing means, e.g. a feed screw conveyor, for feeding solid fuel, e.g. wood pellets or other kinds of bio-fuel, from a fuel silo to a combustion retort in the boiler or other burner system, a furnace chamber, a water filled convection section and a flue gas exhaust, characterised in that the fumace chamber has smooth walls and is surrounded by the convection section, i.e. this and the fumace chamber mainly have common walls, and that the flue gas exhaust is disposed under the combustion retort at the bottom of the fumace chamber. 2. Boiler according to claim 1, characterised in that the furnace chamber has shape as an upright, box-shaped fumace chamber with quadratic or rectangular cross- section, and that the surrounding water filled convection section has the same shape but with such size that the thickness of the water jacket surrounding the furnace cham- ber is about 50 mm. 3. Boiler according to claim 1, characterised in that the fumace chamber is shaped as an upright, cylindric fumace chamber, and that the surrounding water filled convection section has the same shape but with such size that the thickness of the wa- ter jacket surrounding the fumace chamber is about 50 mm. 4. Boiler according to claim 1, characterised in that the combustion retort is disposed at about the centre of the furnace chamber at a height corresponding to one third of the total height of the fumace chamber. 5. Boiler according to claim 1, characterised in that the flue gas exhaust comprises a horizontally disposed flue pipe arranged rotatably and having a number of radial apertures through which the flue has access to the flue pipe and a chimney. 6. Boiler according to claim 1 and 5, characterised in that the flue pipe is perpendicular to the firing means and is disposed under the firing means, preferably at one side of the furnace chamber. 7. A combustion retort for a boiler according to claim 1 and comprising an upper ignition vault supported on opposite side edges of a lower part of the combustion retort, characterised in that the said ignition vault is made by moulding ceramic material, and that the ignition vault is reinforced with metal fibres that preferably consist of stainless steel. 8. Combustion retort according to claim 7, characterised in that a lower part of the ignition vault is designed with longitudinal, concave vaults at each side of a longitudinal, central edge for providing flow turbulence and ensuring mixture of combustion air and gasses.

Inventors: LARSEN ALF TAGE, LARSEN, ALF, TAGE
Applicants: LARSEN ALF TAGE, MASKINFABRIKKEN REKA AS, LARSEN, ALF, TAGE, MASKINFABRIKKEN REKA A/S
Application year: 2000
Main classification: F24H1
IPC codes: F23M5, F24H1