Schematic of Large Boiler

Water WallWater Wall

Drum InternalSuperheater

Types of FiringStoker/Grate Firing: Suitable for relatively small boilers (Steam Generation

Stoker Fired BoilerCombustion Efficiency Low

Coal lumps are fired.

Problem associated with oxygen diffusion.

Chance of incomplete combustion and unburnt fuel loss is high.

Pulverized Coal Fired BoilerPA carries the coal to the furnace through burner

PA is 30% - 50% of total air

Amount of PA depends on moisture content of coal

Pulverization SystemsCentral type and Individual typeCentral TypeIn central type:

cost is low

Reliability LowLogistics is complicated

Closed Type Pulverization System

Open type Pulverization System

Turbulent Burners and ArrangementsTwo Scroll Burners in which two scrolls are provided for whirling the dust-air mixture and secondary airSingle Scroll Burners in which the dust-air mixture is supplied in a straight flow and spread by a dissector and the secondary air is whirled in a scroll.1 kg to 3.8 kg of fuel per second 25 MW to 100MW of powerVorticity Parameter, n = 4Vt / Va Vt : maximum tangential component of the flow velocity at the burner exit Va : axial velocity component.n ranges from 1.5 to 5Primary flow velocity: 16 25 m/sSecondary flow velocity: 1.3 to 1.4 times primary velocity

Turbulent Burners and Arrangements

Straight Flow Burners and ArrangementsPrimary flow velocity: 20-28 m/sec Secondary flow velocity: 1.5 to 1.7 times primary flow velocity

Dry and Wet Bottom Furnaces

Cyclone FurnaceHigh heat release rate Water cooled surfaces are studded and covered with refractory chrome ore. Crushed coal used (95% will pass through a 4-mesh screen) Primary air is about 20% of the combustion air Secondary air velocity around 300 ft/s. The incoming coal particles are thrown to the walls by centrifugal force, held in the slag and scrubbed by the high velocity tangential secondary air. Very high heat loss through molten slag

Fluidized Bed CombustionCD Ac f (vf2/2) = Vs s g

CD is the drag coefficientAc is the cross sectional area of the particle f is the fluid density, vf is the fluid velocity Vs is the volume of the particles is the density of the particle

For spherical particlesvf = [(8/3CD)(s/f)(rg)]

r is the radius of the particle

Fluidized Bed Boiler

Fluidized Bed BoilerCaCO3 + SO2 + (1/2)O2 CaSO4 + CO2Desulphurization is accomplished by the addition of limestone directly to the bed together with the crushed coal. Limestone absorbs sulphur dioxide with the help of oxygen from the excess air. The calcium sulphate produced in this process is a dry waste product that is either regenerated or disposed off. Reductions in sulphur dioxide up to 90% have been achieved in fluidized bed pilot plants.

Fluidized Bed Boiler: AdvantagesLow combustion temperatures Allows inferior grades of coal to be used without slagging problems. The carbon and ash carryover in the flue gas does not reach temperatures at which they become soft and foul heat transfer surfaces. The low temperature combustion results in substantial reduction in the emission of oxides of nitrogen. Cheaper alloy materials may be used resulting in economy of construction.Absence of pulverization equipment, resulting in further economy through power savings. A fluidized bed combustor can be designed to incorporate the boiler within the bed, resulting in surface heat transfer rates that are 2 to 3 times higher than a conventional boiler. A fluidized bed steam generator is therefore much more compact than a conventional one of same capacity.Reduction in sulphur dioxide: lower stack temperature can be achieved because less acid is formed as a result of the condensation of water vapour.