Document 93j7YJQ9yMbMnvoG0xvM4kYj3

BURNER EMPLOYS A ROTARY. IMPELLER mounted on Igni tion tube ee coal spreader, plue adjustable air vanes \TURBULENT BURNER for coal and oil firing rotates Incom- Ing fuel and olr mixture as well as secondary air CRUSHED COAL AND AIR enter eyelene burner with a TANGENTIAL BURNER directs flame tangent to an Imag* whirling motion snd burn. Blag sticks to the burner wall Inary elrele in the furnace Interior, aide turbulence in recent year# has the unit system been applied successfully to anthracite. Suc cess hinges on removing excess air from the transporting air before it en ters the burner to become primary air. Burners. There ire broadly two types of burners for firing powdered coals: (1) nozzle (2) turbulent burner. The so-called "fan-taiiH or U flame issues from a nozzle burner. Mixing of air and entrained fuel occurs within the furnace chamber. Usually the mixing force is secondary air admitted through separate nozzles. Occasionally a tertiary > is directed through inlet vanes grouped around the primary air and coal inlet. The turbulent, flare-type burner im parts a rotary motion to (I) the coal-air mixture in a central nozzle (2) the sec ondary air issuing from o circular cham ber around that nozzle--all within the burner. This gives some premixing for coal end air, Turbulence occurs outside the burner. Burners are further grouped accord ing to location in the furnace, that is. vertical, horizontal, intertube, corner or opposed. Early vertical firing had the primary air and coal nozzle at top of COAL AND PRIMARY AIR discharge through the Coal spreader of this horizontal flare-type burner, make turbulent mixture with Incoming eecendary air furnace chamber pointing downward, more or less vertically. Secondary air nozzles in a wall of the furnace di rected air horizontally into the chamber. '; Horizontal, nozzle-type burners, lo cated in opposite walls, to give "op posed firing" or in corners to give "tangential firing" play tbe flames against one another in the first case and tangent to an imaginary circle in the second. Several advantages seem op- , parent from such firing (1) a pro- J nounced turbulence can be set up (2) any unburned combustible in the "tail" of one nozzle flame is caught up in the second. Cyclone Burner. A very recent en try in the field of pulverized fuel firing is the cyclone burner {Power, Jan 1947, pp 77-80). Thia burner receives crushed, not pulverized, coal in a stream of high-velocity air tangent to the circular burner housing. The coal thrown to the rim of the burner by'centrifugal force and held by a coating of molten ash receives a vigorous scrub bing by the fast moving air until the energy of the incoming air is speat. Secondary air to complete combustion enters also at a high velocity and paral lel to the path of the primary air-coal mix. With a temperature in the burner chamber high enough to promote'and ' sustain combustion the stream of pri mary sir combines with the distilled volstiles snd the parallel secondary-air flow completes combustion to leave only 8lh. If the fusion temperature of the. coal POWER January 1948 is below ihst reached during combus tion the ash becomes molten and under centrifugal force clings to the surface of the burner. By inclining the burner tbe ash proceeds to the low point of tbe burner where it discharges con tinuously. Cemblnotlen Burners. Tbe war years emphasized the need for fuel flexibility and since most pulverized-fuel furnaces lend themselves to oil or gas firing, combination burners have been devel oped. Most combination burnera for oil, gas or eoal admit all the air for com bustion through the burner. Only the primary air carrying coal to the burner Is premized. furnace Typos. The modern watercooled furnace, such as is often used in firing pulverized coal, has two functions. It must (!) permit economical burning el fuel and (2) take away or absorb enough of the heat released to bold Bue-gas temperatures below the slag ging point. Successful pulverized-fuel firing resuits from on intensive mixture of coal snd sir within the furnace proper to give completo combustion in the short est time snd within the least space, first requirement that the flame must meet is not to impingo on combustionchamber walls. Beyond that blanket requirement for all firing systems, the pub-erized-coal installation is character^cd by a necessary balance between available coal transport time end com bustion time. Here is why that balance U to im portant. Temperatures realized in tbe combustion of most coals are well in excess of the ash-fusion temperatures. Coal burned in suspension, then, has its ash content at one time or another in a molten state. It is highly desirable for the ash to be dry if it is to enter the gas passages of tho steam generator. Transfer of heat from the auspended coal portiele by radiation to the boder surface is the only way of attaining this dry stale. And it takes time. Can it be sped up? Yes, fully watercooled furnaces absorb heat faster than other furnaces. But the time for ab sorbing the heat usually exceeds the combustion time. This time in a system of suspended-fuel firing is roughly equivalent to distance. So the distance between the burner paint and the en trance to the gas surface is a function of the lime required for heat absorp tion. The magnitude of Ihis'time or of furnace distance is largely determined by ash content of a given-sized fuel. There is still one other faetor grow ing out of the ash in suspended coal firing. In the course of combustion, ash particles, while still molten, bump into other ash portieles to form slUl larger pieces. They are often too large to atay in suspension, so drop out. Tbe accumulation of these pieces presents a problem of removing them from the furnace. At this point a cholco may be made whether to keep the ash, when once molten, in (bat state or to return it to the dry state before removal. That in a nutshell is (he reason for the two different designs--the slag tap for molleo-ash removal, She dry bottom for dryash removal. What steps can be taken to reduce atmospheric pollution from pulverizedcoal firing? This pollution problem Is almost entirely one of flue dust, or flyash as it is often called. Smoke con and does occur in arrested combus tion--where flame Impinged on tubes, or fuel feed went badly out of balance from, say, moisture hanging up the raw-coal supply. But the major prob lem is one of Rue dust Flue dust Is a byproduct of sus pension firing of a solid fuel. Its quan tity, by weight, can be redueed by striv ing for complete combustion. Here are some operating aids. The pulverizing mill promotes good combustion by delivering uniformly fine coal to the coal transport system. This system, in turn, should distribute cost uniformly within the feeder aided by orifice or directional vanes. Good burning depends on suitable coal, clean burners, proper, mixing. De signs should permit a primary air qs close as possible to the optimum igni tion air ratio for oil loads. For ex tremely low loads use separato burners. Design improvements, represent tbe principal hope for reduced flue dust to atmosphere from pulverized-eoel burners. Removal of entrained flyash from flue gas by external collecting equipment will be discussed in a later article in the series. fOWER January 1748 |I7| 79