LOW EXCESS AIR TANGENTIAL FIRING SYSTEM

Abstract: A furnace (10) in which pulverized coal is burned in suspension. The coal is introduced along with primary air, tangent to an imaginary circle (42). The auxiliary air is introduced tangent to an imaginary circle (44) directly above the primary air, in a direction of rotation opposite that of the primary air. The auxiliary air is directed tangent to a circle of greater diameter than that of the primary air. There are a plurality of alternating levels within the furnace where primary air (38), and then auxiliary air (40), is introduced.

Description: Pulverized coal has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The technique involves introducing the coal and air into a furnace from the four corners thereof so that it is directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the fuel and air, stable flame conditions, and long residence time of the combustion gases in the furnace. In recent times, it has become important to minimize air pollution as much as possible. Thus, some proposed changes have been made to the standard tangential firing method. One such arrangement is set forth in document GB-A- 697 840. The subject matter of this document relates to pulverized fuel furnaces of the kind in which at each of a plurality of levels is a set of burners arranged to discharge into the furnace chamber tangentially to an imaginary cylinder. The whirling flame generated in such a furnace suffers from the defect that carrier air and dust have too low a relative velocity so that a long flame results and there is a tendency towards incomplete combustion. According to the teachings of document GB-A- 697 840 the burners at one level are arranged to discharge in a direction around the longitudinal axis of the furnace chamber opposite that in which the burners at an adjacent level discharge. With such an arrangement the unidirectional flow is destroyed shortly after ignition, being resolved into a large number of small eddies. As a result the fuel burns more quickly and the maximum rate of firing of the furnace chamber may be increased. Summary of the invention In accordance with the invention, a furnace is provided in which pulverized coal is burned in suspension with good mixing of the coal and air. In addition, all of the advantages previously associated with tangentially fired furnaces are obtained, by having a swirling, rotating, fire ball in the furnace. The walls are protected by a blanket of air, reducing slagging thereof. This is accomplished by introducing coal and primary air into the furnace tangentially at a first level, introducing auxiliary air in an amount at least twice that of the primary air into the furnace tangentially at a second level directly above the first level, but in a direction opposite to that of the primary air, with there being a plurality of such first and second levels, one above the other. As a result of the greater mass and velocity of the auxiliary air, the ultimate swirl within the furnace will be in the direction of the auxiliary air introduction. Because of this, the fuel, which is introduced in a direction counter to the swirl of the furnace, is forced after entering the unit, to change direction to that of the overall furnace gases. Tremendous turbulent mixing between the fuel and air is thus created in this process. This increased mixing reduces the need for high levels of excess air within the furnace. This increased mixing also results in enhanced carbon conversion which improves the units over all heat release rate while at the same time reducing upper furnace slagging and fouling. The auxiliary air is directed at a circle of larger diameter than that of the fuel, thus forming a layer of air adjacent the walls. In addition, overfire air, consisting essentially of all of the excess air supplied to the furnace, is introduced into the furnace at a level considerably above all of the primary and auxiliary air introduction levels, with the overfire air being directed tangentially to an imaginary circle, and in a direction opposite to that of the auxiliary air. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a sectioned perspective of a tangentially fired pulverized coal furnace incorporating the invention; Figure 2 is an enlarged sectional view of one corner of burners; Figure 3 is a view taken on line 3-3 of Figure 1; and Figure 4 is a view taken on line 4-4 of Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENT Looking now to Figure 1, a coal-fired furnace 10 is shown, having a plurality of levels of burners 12 therein with each level having a burner mounted in each of the four corners thereof. Air is supplied to the burners from fan 16 through ducts 18 and 20. Air is also supplied to pulverizer 22 through duct 24. Pulverized coal is transported to the burners in an air stream through ducts 26 and 28. There are separate air and fuel ducts leading to each individual burner, with separate valves and controls (not shown) also, so that each burner can be independently controlled. The combustion gases swirling upwardly in the furnace give up heat to the fluid passing through the tubes 30 lining all four of the furnace walls, before exiting the furnace through horizontal pass 32, leading to rear gas pass 34. Both the furnace and the rear pass contain other heat exchanger surface (not shown), for generating and superheating steam, as well known in the art. The specific manner of introducing the fuel and air into the furnace will now be described in more detail. Pulverized coal, generally ground to a flour-like consistency, is carried to each burner in a stream of air from the pulverizer mill 22. This air that carried the coal is generally referred to as the primary air. As best seen in Figure 2, more air, generally designated as secondary air, is through nozzles 36 introduced directly above and below the fuel-through nozzles 38. These nozzles 36 are tiltable along with the nozzles 38 through which the coal and primary air are introduced. This air is necessary for maintaining initial ignition and stable combustion conditions. The primary and secondary air constitutes about 20-30% of the total air required for complete or stoichiometric combustion of the coal. Still looking at Figure 2, positioned above and below each secondary air nozzle 36 are auxiliary, or tertiary air nozzles 40. The remainder of the air necessary for complete combustion, or stoichiometric conditions, is introduced through these nozzles 40. Generally, about 70-80% of the stoichiometric air is introduced through auxiliary nozzles 40. Looking now to Figures 3 and 4, the manner in which the coal and primary air, the secondary air, and the auxiliary air, is tangentially introduced into the furnace, is shown. As seen in Figure 3, the coal and primary air along with the secondary air, are introduced into the furnace tangential to an imaginary circle 42 in the central portion of the furnace. Looking now to Figure 4, it can be seen that the auxiliary air is introduced into the furnace tangential to an imaginary circle 44, at locations directly above and below the fire ball 42. The auxiliary air is introduced into the furnace rotating in a direction reverse, or opposite to the direction of rotation of the primary air and fuel. The result of this is a mixing and combustion efficiency much better than that realized with the usual tangentially fired furnace. This permits the use of less excess air in the furnace than previously required. The ultimate fire ball rising in the furnace rotates in a direction the same as that of the auxiliary air, since the mass introduced in this direction is several times that introduced in the opposite direction. The velocity of the auxiliary air is comparable to that of the primary and secondary air. The above feature, coupled with the fact that the auxiliary air is introduced tangential to a circle 44 larger than the circle 42, keeps a blanket of air adjacent to the furnace walls, thereby minimizing slagging on these walls. Looking again at Figure 1, all of the excess air is introduced into the furnace in the upper portion thereof. This excess, or overfire, air is introduced through nozzles (not shown), which are directed tangential to an imaginary circle (not shown), in a direction opposite to that of the rising fire ball; i.e., opposite to the direction of introduction of the auxiliary air 44. Since the amount of excess air is relatively small (5-20%), the flow leaving the furnace will still be swirling or rotating somewhat in the direction of rotation of the auxiliary air introduction. This causes some temperature unbalancing the gases leaving the furnace. Some statistical data of the proposed modified furnace will now be given. The primary air and fuel are introduced into the unit at a 6° angle to the radial line from the vertical centerline axis of the furnace. The auxiliary air is introduced at a 5-15° angle to the same vertical centerline of the furnace but opposite in direction. In this manner, the fuel and air are introducing swirl within the furnace in opposite directions. As stated previously, however, because of the greater mass and velocity of the auxiliary air, the ultimate overall swirl within the unit will be in the direction of the auxiliary air introduction. There can be as many as six elevations of burners; i.e., 24 in total, with six in each corner. These can be spread over a 9.14 meter (30 feet) height in the furnace beginning 15.24 meters (50 feet) above the opening in the coutant furnace bottom. The top wall of the furnace is approximately 30.48 meters (100 feet) above the top burner elevation, and the excess, or overfire, air is introduced about 18.29 meters (60 feet) above the top burner elevations.

Claims: A method of operating a tangentially fired pulverized coal furnace (10), including, discharging (38) pulverized coal and primary air into the furnace (10) directed tangentially to an imaginary circle (42) in the center of the furnace (10) at a first level, discharging (40) auxiliary air in a mass more than two times and of greater velocity than that of the primary air into the furnace (10) at a second level directly above the first level, said auxiliary air being directed tangentially to an imaginary circle (44) of greater diameter than that at which the primary air and coal is and in an opposite direction to the coal and primary air, there being a plurality of first levels where primary air and coal are introduced, with each such first level being separated by a second level where auxiliary air is introduced, such as to produce fire balls rotating in opposite directions moving upwardly within the furnace (10), with the ultimate entire fire mass rotating in the direction of the auxiliary air introduction. The method set forth in Claim 1, wherein fluid is passed through tubes (30) lining all four walls of the furnace (10), so as to absorb heat from the burning coal in the furnace (10). The method set forth in Claim 1, wherein overfire air is introduced into the furnace (10) at a level considerably above all of the primary and auxiliary air introduction levels, with the overfire air being directed tangentially to an imaginary circle, and in a direction opposite to that of the auxiliary air. The method set forth in Claim 3, wherein the overfire air is essentially all of the excess air, and amounts to 5-20% excess air.

Inventors: ANDERSON, DAVID KENNETH, BIANCA, JOSEPH DAVID
Applicants: COMBUSTION ENGINEERING, INC.
Application year: 1987
Main classification: F23C5
IPC codes: F23D1, F23C99, F23C5