Basic Knowledge of Furnace Explosion
Furnace explosions are rare and unlikely. When compared with the total number of unit operating hours, the hours lost because of explosions are minimal. This desirable situation exists because (a) furnaces are supplied with an explosive accumulation only during a small percentage of their operating lives and (b) just a minute part of those explosive charges receive sufficient ignition energy to actually cause an explosion.
In suspension burning, the primary control of the combustion process is the admission rate of fuel and air to a furnace, independently of each other. The dynamic response of the combustion reaction, however, depends on the diffusion of the fuel and air to a flammable limit, and the elevation of this diffused mixture to its kindling temperature. The aerodynamic diffusion of fuel and air results from both the rate and method of admission. This admission flow pattern produces diffusion mechanically by inters rubbing of the fuel and air masses. Molecular diffusion is also present as a result of the elevated temperature level at which the combustion process takes place.
Furnace explosions result from a rapid rate of volume increase of the gaseous combustion products when too great a quantity of fuel and air reacts almost simultaneously in an enclosure with limited volume and strength. Avoiding furnace pressures in excess of furnace closure design pressure is, therefore, necessary to prevent furnace rupture.
The basis for any explosion-prevention system must be to limit the quantity of flammable fuel-and-air mixture that can exist in the furnace at any given instant. The rate of maximum pressure rise possible during the reaction is a function also of the available oxygen per unit volume of reactants. The effect of any oxygen density diluents (nitrogen, increased temperature, decreased pressure, excess fuel, inert gases) reduces the possible explosion pressure. Furnace-explosion prevention should be aimed at limiting the quantity of diffused flammable fuel-air mixture that can be accumulated in a furnace in proportion to the total volume and the mechanical strength of the furnace.
While fuel and air are being admitted to a furnace, there are only three possible methods of preventing excessive flammable diffused accumulations.
1. Igniting all flammable mixtures as they are formed, before their excessive accumulation.
2. Diffusing all flammable mixtures with sufficient additional air, prior to ignition, to a point beyond the diffused flammable-mixture ratio; and accomplishing this with a sufficient degree of diffusion before the flammable mixture occupies a critical percentage of the furnace volume.
3. Supplying an inert gas to diffuse simultaneously with the fuel and air, thereby diluting the oxygen content of the mixture below the flammable limit.
Implementation of these preventative methods requires operator action beyond the response, memory, and judgment capabilities of the normal operator controlling a plant in the manual mode. A fireside safeguard system must supervise the flow and processing of fuel, air, ignition energy, and the products of combustion. Satisfactory boiler operation requires that these four ingredients be properly prepared, ratioed, directed and sequenced so that the furnace cannot contain an explosive mixture. At supervised to check the results. Combustion must be kept efficient or the unconverted chemical energy may accumulate and subsequently become explosive.
The following factors influence the effective composition change of an explosive charge:
a) The facility for mixing
b) The inert material in the fuel
c) The fuel-air ratio
d) The kind of fuel
A furnace explosion requires both sufficient explosive accumulation within the furnace and sufficient energy for ignition.
The ignition requirements for an explosive charge are very small; making it impossible to protect against all possible sources of ignition, such as static electricity discharges, hot slag, and hot furnace surfaces.
Therefore, the practical means of avoiding a furnace explosion is the prevention of an explosive accumulation.
The potentially reactive furnace accumulation must be formed from an earlier buildup process which introduces reactive inputs not converted by oxidation to non-reactive or inert products. This buildup process must continue long enough to create a damaging accumulation. The accumulation composition, which must be within the limits of flammability for that particular fuel, is formed in one or more basic ways.
■ A flammable input into any furnace atmosphere (loss of ignition)
■ A fuel-rich input into an air-rich atmosphere (fuel interruption)
■ An air-rich input into a fuel-rich atmosphere (air interruption)
Furnace firing systems are designed to start up air-rich by introducing fuel into an air-filled furnace. Main fuel is introduced after the integral ignition system has satisfied permissive main-fuel interlocks that it can provide more ignition energy than the main fuel requires to be ignited or to remain ignited. Additional air is introduced around the primary-air/fuel mixture to take it beyond flammable limits, if it has not been ignited and reacted to inert combustion products; this is done to avoid a critical portion of the total furnace volume being occupied by a flammable mixture.
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