Electrical Power Plant Design: Generation System

WEEK 04: GENERATION: AUXILLIARIES

Sections: Circulating-Water | Flue & Waste | Diesel Plant

Three classes of emissions are of major concern: (1) Nitrogen Oxides, (2) Sulfur Oxides and (3) Particulate Control. For more detailed exposition, click here.

Flue Gas Cleaning The products of combustion of coal-fed fires contain particles of solid matter floating in suspension. This may be smoke or dust. If smoke, the indication is that combustion conditions were faulty, and the proper remedy is in the design and management of the furnace. If dust, the particles are mainly fine ash particles called fly-ash intermixed with some quantity of carbon-ash material called cinder. Typical classification of particles by name are: (a) smoke particulates, at less than 0.2 microns; (b) dust, from 1 to 100 microns, and (c) cinder, from 50 to greater than 1000 microns. A critical characteristic of dust is its settling velocity in still air. This is proportional to the product of the square of micron size and mass density. Typical values: 18.3 cm/min at 10 microns, 18.3 m/min at 100 microns With a settling velocity of 61 cm/min or less the dust will be carried from 1.6 to 4.8 km in a light breeze, from an average chimney height. The gas loading is the weight of dust particles per cubic foot of flue gas. It can range from 4.6 to 27.6 grams per m³ with typical operating conditions represented by the minimum value. The removal of dust and cinders from flue gas can usually be effected to a required degree by commercial dust collectors. These can be classified as mechanical and electrical. The mechanical collectors are subdivided into wet and dry types. Wet types, called scrubbers, operate with water sprays to wast dust from air. It also produces a waste water that may require chemical neutralization before it can be discharged into natural bodies of water.

Hence the common mechanical dust collector is the dry type, which is subdivided according to the operating principle: Gravitational Separators act by slowing down gas flow so that particles remain in the chamber long enough to settle to the bottom. Not very suitable because of large chamber volume needed. Inertial Separators act by rapid change of direction of gas, which cannot be followed completely by the heavier particles. Common forms are the baffle, the louvre and the cyclone separators. Baffle separators are frequently improvised within a boiler setting in order to drop the large cinders from the gasses. The louvre is a high-speed gas type in which a small portion of the gas carries the bulk of the dust into a secondary chamber where velocities are low enough for a combination of gravitational and inertia forces to make the separation. Meanwhile the main gas flow passes out the side of the direct-flow chamber. The cyclone is a separating chamber wherein high-speed gas rotation is generated for the purpose of centrifuging the particles from the carrying gasses. Usually there is an outer downward flowing vortex which turns into an inward upward flowing vortex. Involute inlet and sufficient velocity head pressure are used to produce the vortices. Skimming cyclones shave off the dust at the periphery of the vortex, along with a small portion of the gas flow. This concentrated flow is then led to a secondary chamber for final separation. The absolute efficiency of a dust collector is the percentage of entering solids that will be removed by the collector. Some manufacturers prefer to rate their equipment on an efficiency curve, e.g., 91% at 15 microns.

Electrical Precipitator This separation principle consists of imposing an electrical charge on the particles as they pass near a collecting electrode of opposite polarity. Gas is made to flow beteen grounded collecting electrodes between which are suspended the highly charged ionizing wires. The particles are attracted to the collecting electrodes and stick there until removed by being jarred loose with electrode rapping or violent shaking.

Ash Handling All coal has more or less ash. Combustion of the coal is attended by the necessity of providing some means of removing the ash which is deposited in the ash hopper. Usually, from 5 to 40% of the combustion ash leaves the furnace with the gases carried in suspension. Ash handling is a major problem because, first, the ash is dusty, hence irritating and annoying to handle; second, it forms clinkers by fusing together in large lumps which must be broken; third, ash is abrasive and will wear all conveyor parts on contact with it. Ash disposal systems are designed for intermittent or continuous operation. An ash-handling system will consist of a means of removing ash from the furnace and loading it onto a conveyor system, the conveyor to deliver the ash to fill or storage, and a means of disposing of stored ash.

Auxiliary Systems: Diesel Plant