Analysis of Pulverizers P M V Subbarao Professor Mechanical Engineering Department Multi Task Machines to meet the rate of rapid coal combustion ……

Coal pulverizers Coal pulverizers are essentially volumetric devices. As the density of coal is fairly constant, are rated in mass units of tonnes/hr. A pulverizer accepts a volume of material to be pulverized which is dependent on the physical dimensions of the mill and the ability of coal to pass through the coal pulverizing system. The common measure of mass in tonnes enables matching of energy requirements with available coal properties and mill capacity. Increased combustible loss can occur if the furnace volume or mill capacity is less than desirable for a particular coal. The furnace volume and mill capacity in a specific power station may dictate the need to purchase coals which are reactive and which can be ground easily. Size reduction is energy intensive and generally very inefficient with regard to energy consumption. In many processes the actual energy used in breakage of particles is less than 5% of the overall energy consumption.

Mills There are basically four different types of pulverizing mills which are designed to reduce coal with a top particle size of about 50 mm to the necessary particle size range. Ball&Tube Mill, Ball &Race Mill, Bowl Mill & Impact Mill Each type has a different grinding mechanism and different operating characteristics. There are four unit operations going concurrently within the mill body, coal drying, transport, classification and grinding. For coal pulverizers the capacity of a mill is normally specified as tonnes output when grinding coal with a HGI of 50, with a particle size of 70% less than 75 micron and 1 or 2 % greater than 300 micron and with a moisture in coal of less than 10%. A few manufacturers specify 55 instead of 50 with respect to HGI.. This standardization enables selection of an appropriate mill for a specific duty.

Coal Mills

Typical Layout of Pulverizer Circuit

Schematic of typical coal pulverized system A Inlet Duct; B Bowl Orifice; C Grinding Mill; D Transfer Duct to Exhauster; E Fan Exit Duct.

Aerodynamic Lifting of Coal Particles

Pressurized and exhauster-fan grinding mill circuits

Carrying of Particles by Fluid Drag

Pneumatic Carrying of Particles The major goal of pneumatic conveying of solids is to maximize the carrying capacity of the installation and carry flows with high-solids concentration ("dense-phase flow"). In pulverized coal combustion the ratio of coal to carrying gas is determined by systems and combustion considerations and is usually in the range of y = kg/kg ("dilute phase transport"). Assuming a coal density  c = 1.5 x 10 3 kg/m 3, and the density of the carrying gas as  g = 0.9 kg/m 3, the volume fraction of the coal can be shown to be very small, %. The interparticle effects can therefore be neglected for steady state operation. An important aerodynamic characteristic of the particles is their terminal velocity (the free-fall velocity in stagnant air) which for a spherical particle of d = 0.1 mm has an approximate value of 0.3m/sec. Experience shows that due to non-uniformities of flow behind bends, and to avoid settling of solids in horizontal sections of the transport line, a gas velocity of ~ V = 20 m/sec has to be chosen.

Mill Pressure Drop : A Measure of Gas Velocities The pressure loss coefficients for the pulverized-coal system elements are not well established. The load performance is very sensitive to small variations in pressure loss coefficient.

Particle Size Distribution--Pulverized-Coal Classifiers The pulverized-coal classifier has the task of making a clean cut in the pulverized-coal size distribution: returning the oversize particles to the mill for further grinding but allowing the "ready to burn" pulverized coal to be transported to the burner. The mill's performance, its safety and also the efficiency of combustion depend on a sufficiently selective operation of the mill classifier.

Classifiers The classification of solid particles according to their size in the spiral house of a cyclone is illustrated by Figure. The particles can migrate toward the outer wall or the exit tube; From the force balance on a particle and knowing the path of the gas in the cyclone, the radius r l can be determined, which will be the limiting radius for the radial penetration of a particle of diameter, .

Force Balance on Particles At values of r > r l, the centrifugal force, which is inversely proportional to the third power of the radius, grows faster than the radial component of the gas velocity, which is inversely proportional to the radius. The limiting radius can be given as : where  is the particle size,  p is particle density, V t,0 is tangential inlet velocity to the cyclone, r 0 is the cyclone radius, and  the dynamic viscosity of the gas. with

Assuming that the cyclone precipitates only particles of sizes larger than those whose limiting radius coincides with the cyclone diameter (r t = r o ), the limiting (smallest) particle size precipitated can be given as

The total pressure loss coefficient of a cyclone, i.e. the number of lost velocity heads calculated on the basis of the inlet gas velocity to the cyclone is between 10 and 18:

For a straight pipe of diameter D the pressure drop per unit length of pipe is given by Where, * is a coefficient, analogous to coefficient of friction. Where air is the coefficient of wall friction for air; ; pf, is the coefficient of friction for the particle in the pipe; and pg  is a coefficient corresponding to the weight of the particle. Transport between Mill and the Burner

The friction coefficient for air, lair, can be calculated after Prandtl from The wall friction coefficient of the particles, pf, is a function of the Froude number of the flow, calculated with the gas velocity, as

Settlement of Coal Particles in a Pipe Cook and Hurworth suggest that the source of deposits in long horizontal pulverized-coal pipelines is the phenomenon of "roping". Roping is the segregation of pulverized coal and air with the coal forming a band traveling along the bottom of the horizontal pipe. This band of fuel is then slowing down due to wall friction. It was found that as the air to coal ratio increased, the tendency for deposition decreased and at values of air/coal > 3.5 and at conveying velocities of 22-31m/sec, they could run their test rig free of deposition. Unfortunately air/fuel ratios required for grinding mill operation, and for purposes of the primary mixture injected through the burner, have much higher coal concentrations so that the use of these lean concentrations is not practicable.

Division of pulverized-coal mixture: (a) by alternate layers; (b) after turbulence-promoting screen. Splitting of Dust-Laden Gas Flows

Pulverizer Sizing The suggestednumber of pulverizers is as follows. Unit Size (MW) Initial Number of Pulverizers < In Operation at MCR with Typical Coal 456 Spares with Typical Coal 112 In Operation with Worst Case Coal 568