Feed duct for straw firing

Abstract: The feed duct (1) for a straw firing furnace comprises a slide valve (10) with a valve member (13) for closing the duct, a sprinkler (21) in a wall of the duct upstream of the slide valve, a water supply pipe (22) for the sprinkler, and means (14) for control and activation of the opening and closing of the valve member. The sprinker (2) is positioned close to the valve member and adapted to sprinkle said valve member by water, and the bottom (23) of the housing (12) of the slide valve (10) is positioned lower than the bottom (9) of the feed duct and is provided with drain openings (24) for draining off water supplied through the sprinkler.

Description: The present invention relates to a feed duct for a straw firing furnace comprising a slide valve with a valve member for closing of the duct, a sprinkler in a wall of the duct upstream of the slide valve, a water supply valve and a water supply pipe for the sprinkler, and means for control and activation of the opening and closing of the valve member and the water supply valve. From DK -A- 143,471 a feed duct of this type is known, in which a fire in the straw in the duct is extinguished by means of water supplied through the sprinkler. In US -A- 4.181.082 a plant for combustion of solid fuel, for instance wood, is disclosed, in which a thermostatically activated water supply valve is mounted in the duct, said valve opening for the addition of water to the fuel in the duct between a supply piston for fuel and the furnace space, when the temperature in the duct exceeds a certain level. DE-A- 2 933 786 reveals a feed duct with a sprinkler positioned close to the slide valve. These known plants suffer from the drawback that the water supplied can flow freely into the furnace space, in which it may cause damage, for instance by penetrating firebricks, by sudden cooling of heated parts, violent generation of steam, etc. From the text of the above accepted DK patent specification it appears that a fire in the feed duct may arise as a bale of straw ignites explosively, whereby flames flash through the inlet to the furnace space not yet closed by means of the slide valve. Due to this constant risk of fire the running of straw fired plants has required the constant presence of an operator for the monitoring of the plant. During normal operation of a plant a fire may, however, also arise in the feed duct for other reasons, for instance when the straw fired furnace is to be stopped, both in case of a planned stop or in case of an emergency stop, for instance caused by a too low water level in an associated boiler. In connection with a planned stop of the straw fired furnace, the supply of straw to the furnace is stopped, and the straw is removed the greatest possible extent from the feed duct, after which the slide valve is closed. As there will usually be some straw left in the duct, the valve cannot be completely closed. In case of an emergency stop, the valve member is closed directly down against the straw in the filled duct and stops in a position, which depends upon the degree of compression of the straw in the duct. Gradually, the fire will spread through surface burning from the furnace space to the feed duct upstream of the water-cooled part of the duct until also the straw under the valve member catches fire and has to be extinguished by the operators of the plant in order to prevent further spreading of the fire to other parts of the plant. Due to the often violent and prolonged generation of heat in the burning straw the involved parts of the slide valve and the feed duct may be damaged, for instance through deforming or cracking, resulting in an unintended operational break-down and considerable repair costs. The object of the invention is to provide a feed duct of the kind stated by way of introduction, by means of which water is prevented from entering the furnace space, when a fire in the duct is extinguished, and by means of which the slide valve is secured against damage on account of the heat generation. The invention provides a feed duct, which ensures that a fire is so effectively extinquished that a straw fired plant, of which the duct is a part, may be operated without the constant presence of an operator. The feed duct according to the invention differs from the known types in that that the sprinker is adapted to sprinkle said valve member by water, and that the bottom of the housing of the slide valve is positioned lower than the bottom of the feed duct and is provided with drain openings for draining off water supplied through the sprinkler. Thereby it is achieved that the water from the sprinkler, in addition to thoroughly drenching the straw in the duct upstream of the slide valve, cools off the slide valve so that it will not deform on account of the heat generated. The water supplied is drained off through the valve housing and consequently does not penetrate into the furnace space or spread further upstream in the duct, whereby damages in these parts are prevented. The last-mentioned measure further has the additional advantage that water may be added in bigger amounts and over longer periods than is the case with the known plant. In a preferred embodiment of the feed duct according to the invention the valve member of the first slide valve may be hollow and adapted for being filled with water. In another embodiment there may be means for circulating cooling water through the space in the valve member. In these embodiments a further safeguard against damage of the valve member in the slide valve due to the high temperatures is achieved. In a third embodiment of the feed duct according to the invention there may, when the valve member, as described above, is independently cooled, be provided means for intermittent supply of water to the spray nozzle. As the fire only spreads comparatively slowly in the feed duct, it is in many cases not necessary to maintain a constant sprinkling of the straw once the straw has been thoroughly drenched, whereas it is necessary to keep the straw and the area at the slide valve constantly wet to prevent any remaining embers from starting the fire again. By this measure the fire is extinguished so effectively that the need for a constant presence of an operator is eliminated. A fourth embodiment of the feed duct according to the invention is characteristic in that upstream of the sprinkler a second slide valve for closing of the duct is mounted, that its valve housing is provided with drain openings for draining off water supplied through the sprinkler and means for controlling and activating the opening and closing of the valve members, and in that the means for control and activation of the valve members are adapted to simultaneously close both slide valves. Thereby an additional safeguard against fire penetrating further upstream in the feed duct is achieved, a further possibility of draining off water from the feed duct being simultaneously created. In a fifth embodiment of the feed duct according to the invention, the duct portion downstream of the first slide valve may be water-cooled, which has the effect that also this portion of the duct is secured against damage due to violent heat generation. The invention will be explained in detail in the following by means of the drawing, which shows an example of an embodiment of the gate valve arrangement according to the invention. The drawing shows a feed duct 1, which at one end is connected with a inlet duct 2, through which straw from a silo (not shown) is conveyed down in front of a piston 3, which moves backwards and forwards as indicated by the arrow A and thereby presses the straw into the duct. The other end of the feed duct 1 leads through the front wall 4 of a straw fired furnace and opens into a furnace space 5. A portion 6 of the duct 1 adjoining the front wall 4 is provided with a cooling jacket 7, and the space 8 between the cooling jacket 7 and the outer wall of the duct 1 is flowed through by cooling water. Upstream of the water-cooled duct portion 6 two slide valves 10 and 11 are mounted in the feed duct 1, mutually separated by a short duct section or intermediate piece 20. The downstream slide valve 10 is mounted directly on the water-cooled duct portion 6. The slide valve 10 comprises a valve housing 12, a valve member 13, and a pneumatic activating cylinder 14, which through its piston rod 15 displaces the valve member 13 between the open position shown in the drawing, in which the straw can freely pass through the valve housing 12 to the furnace space 5, and a closed position, in which the valve member 13 completely blocks the passage of the straw. Slide valve 11 is in the same way as slide valve 10 provided with a valve housing 16, a valve member 17, and a pneumatic activating cylinder 18 with a piston rod 19. The valve member 13 of the slide valve 10 consists of two plates with a common interspace, which during operation is kept filled with water. At the upper edge of the intermediate piece 20 at its end facing the valve housing 10 a sprinkler 21 with a feed pipe 22 for water is built in. The sprinkler 21 is directed such that water supplied through the pipe 22 is conveyed to the area, which is occupied by valve member 13, when the slide valve 10 is closed, as well as to the area of the intermediate piece 20 situated immediately upstream. Valve housings 12 and 16 are made in such a way that their bottoms 23 are positioned somewhat below the bottom 9 of the feed duct 1. The bottoms 23 are provided with drain openings 24 and 25, so that water injected through the nozzle 21 may be drained off through these openings. The gate valve arrangement works in the following way: During normal operation the valve members 13 and 17 are displaced to the open position show in the drawing, and the straw is by means of the piston 2 intermittently pressed through the feed duct 1 to the furnace space 5, in which it is burned. When it is desired to stop the firing, the supply of straw is interrupted at 2, and by means of the piston 3 the straw in the feed duct 1 is pressed so far into the duct as possible. Then the valve members 13 and 17 are by means of the pneumatic activating cylinders 14 and 18, respectively, brought to their closed positions. Remaining straw, if any, in the valve housings and in the intermediate piece 20 is compressed thereby and may, if the amount of straw is too big, prevent a complete closing of the valve members. Then water is supplied intermittently to nozzle 21, which causes a complete drenching of the straw, if any, in the intermediate piece 20. Surplus water is drained off through the drain openings 24 and 25. As straw is not supplied any more to the furnace space 5, the fire in there will gradually spread backwards through the mouth of the feed duct 1, and the straw in the water-cooled duct portion 6 will catch fire. As, however, the straw below the valve members is strongly compressed and drenched, the fire will not spread by surface burning in the area between the slide valves, and on account of the heavy drenching burning of the straw in the duct piece 20 is prevented. The closed, water-filled valve member 13 is also sprinkled by water from the spray nozzle 21 and is thereby kept at a temperature so low that the valve member will not be damaged by the burning straw. The valve housing 12 is also cooled, partly by being connected with the water-cooled duct portion 6, partly by water draining off through the drain opening 24. As the fire spreads comparatively slowly in the feed duct, it is in the case, in which the valve member 13 is independently cooled, as shown in the drawing and described above, sufficient to supply the water intermittently to the spray nozzle 21, as there has already been established sufficient safety against burning of the straw in the duct piece 20, when this straw once has been thoroughly drenched. To prevent embers in the feed duct from starting the fire again, it is, however, necessary at regular intervals to supply water to the spray nozzle. Tests with a gate valve arrangement according to the invention has shown that the fire is stopped so effectively from spreading backwards through the feed duct that it is possible to leave the plant unmanned during operation. The necesary control and monitor functions, which among others causes a closing of the slide valves, when required, are of a conventional nature and need no further explanation.

Claims: A feed duct (1) for a straw firing furnace comprising a slide valve (10) with a valve member (13) for closing of the duct, a sprinkler (21) in a wall of the duct upstream of the slide valve, a water supply valve and a water supply pipe (22) for the sprinkler, and means (14) for control and activation of the opening and closing of the valve member and the water supply valve, the sprinker (21) being positioned close to the valve member (13) characterized in that the sprinkler (21) is adapted to sprinkle said valve member with water, and that the bottom (23) of the housing (12) of the slide valve (10) is positioned lower than the bottom (9) of the feed duct (1) and is provided with drain openings (24) for draining off water supplied through the sprinkler (21). A feed duct according to claim 1, characterized in that the valve member (13) of the slide valve (10) is hollow and adapted for being filled with water. A feed duct according to claim 2, characterized in comprising means for circulating cooling water through the space in the valve member (13). A feed duct according to any of the preceding claims, characterized in that the means for controlling and activating the water supply valve are adapted to intermittent supply of water to the sprinkler (21). A feed duct according to any of the preceding claims, characterized in that upstream of the sprinkler a second slide valve (11) for closing of the duct (1) is mounted, that its valve housing (16) is provided with drain openings (25) for draining off water supplied through the sprinkler (21) and means for controlling and activating the opening and closing of the valve members (17), and in that the means for control and activation of the valve members (13,17) are adapted to simultaneously close both slide valves (10,11). A feed duct according to any of the preceding claims, characterized in that the duct portion (6) downstream of the first slide valve (10) is water-cooled.

Inventors: SVENDSEN JENS JORGEN BAGGE, SVENDSEN, JENS JORGEN BAGGE
Applicants: BURMEISTER & WAINS ENERGI, BURMEISTER & WAIN ENERGI A/S
Application year: 1991
Main classification: F23K3
IPC codes: F23K3