PERFORMANCE ANALYSIS OF COAL MILLS

1. PERFORMANCE ANALYSIS OF COAL MILLS P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Correct Size, shape and quantity of Diet… For Complete Digestion..

2. Combustion Limits on Furnace Design • The lower limit of the furnace volume is dominated by the space required for burning the fuel completely, or • to an extent less than the allowable unburned fuel loss. • To complete the fuel combustion within the furnace space, the fuel injected into the furnace has to reside there for a time longer than critical time t*r. • The fuel residence time can also be estimated by the residence time of the combustion gas produced in the furnace. • An average residence time tr can be proposed.

4. Fuel combustion time is mainly dominated by the combustion reaction velocity and the rate at which oxygen is supplied into the reaction zone. • The combustion reaction velocity depends on chemical characteristics of the fuel. • Main technical factors that affect the combustion time are: • Combustion characteristics of the fuel. • Mixing characteristics. • Fluid flow characteristics of the furnace. • The combustion velocity of an oil fuel droplet is generally less than 0.1 msec. • In the case of coal combustion time is much longer.

5. 30%VM & 5 % Ash 30%VM & 15 % Ash 30%VM & 30 % Ash 30%VM & 40 % Ash Typical Combustion Reaction Velocity ( Flame Speed) of Pulverized Coal : Effect of Ash Fraction Flame speed m/s A/F ratio

6. 30%VM & 5 % Ash 20%VM & 5 % Ash 15%VM & 5 % Ash Typical Combustion Reaction Velocity ( Flame Speed) of Pulverized Coal: Effect of VM Fraction Flame speed m/s A/F ratio

7. Roller Bowl Coal Mill : A Controller of Combustion Time Coal 10 to 25 mm Size Hot Air ~ 2500C

8. Coal pulverizers • Coal pulverizers are essentially volumetric devices, because the density of coal is fairly constant, are rated in mass units of tons/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 furnace volume and mill capacity in a specific power station may dictate the need to purchase coals which are reactive and easily grind. • The common measure of mass in tons 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. • There are a number of possible remedial actions. • Operators can correct some deficiencies in the combustion system : • Biasing the performance of the coal pulverizing for variable coal qualities. • Use the spare mill into service for peak periods to ensure full output.

9. 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 around 5% of the overall energy consumption. • Pulverizing coal is no exception to this. • 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 particle size range necessary for fairly complete combustion in a modern pulverized coal fired boiler. • 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 % 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.

10. Ball & Tube Mill • The oldest pulverizer design still in frequent use. • 25% to 30% of cylinder volume is filled with wear resistant balls of 30 – 60mm. • The cylinder is rotated at a speed of about 20 rpm. • Specific power consumption 22 kWh per Ton. • Suitable for hard coals. • Highly reliable in requires low maintenance. • Bulky and heavy in construction.

11. mg+mw2R

12. Critical Angular Velocity wc mg-mw2R mwc2R = mg wc2 = g/R mg <mw2R : Ball will never fall down

13. mg+mw2R

14. a mw2R mg m(mw2R+mg Cos a) > mg sin a

15. mw2R mg

16. Pulverization due to ATTRITION a mw2R mg m(mw2R+mg Cos a) < mg sin a

17. Pulverization due to Impact mw2R mg a (mw2R-mg Cos a) = 0

18. Bowl Mill • The most widely used mill for grinding coal. • The raw coal is fed into the center of the mill. • This is an intermediate speed pulverizer. • The vertical shaft rotates at a speed 30 – 50 rpm. • Specific power consumption 12 kWh/ton.

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

20. The primary airflow measurement by round cross-sectional area venturis (or flow nozzles) should be provided to measure and control primary airflow to improve accuracy

21. Aerodynamic Lifting of Coal Particles

23. Carrying of Particles by Fluid Drag In view of the age of the technique it would be presumed that the subject of concurrent fluid-solid flow would be quite well defined and understood. Investigation of the published literature indicates, however, that such conveying is still an extremely empirical art.

24. 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 usually in the range of y = 0.5-0.6 kg/kg. • Assuming a coal density rc = 1.5 x 103 kg/m 3, and the density of the carrying gas as rg = 0.9 kg/m 3, the volume fraction of the coal can be shown to be very small, 0.036 % . • Dilute Phase Transport • The inter particle 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 = 16 -- 20 m/sec has to be chosen.

25. Pulverizer Capacity • Mill manufacturers provides a set of data or curves, which enable the capacity of a mill to be determined with a coal with specific properties. • The properties, which are of concern, are specific energy, HGI, moisture, particle size and reactivity. • Specific energy is necessary to determine the required nominal maximum mill capacity in tons/hour to ensure sufficient coal is delivered to the boiler. • A curve linking HGI and mill capacity provides information on mill performance with that coal. • A curve linking moisture content of the coal with mill capacity shows what reduction in capacity will arise if the moisture is excessive. • This is particularly important with ball mills. • The particle size distribution and top size may be of importance. • For ball mills there is a curve linking mill capacity with the top size of coal fed to the mill. • The reactivity of the coal, measured in the first instance by volatile matter is needed to determine if the mill can be set to provide standard 70% less than 75 micron or • a finer or coarser setting is necessary with corresponding alteration to mill capacity.

27. Pulverizer Capacity Curves Throughput, tons/hr Grindability Moisture content, %

29. Roller Bowl Coal Mill : A Controller of Combustion Time Coal 10 to 25 mm Size Hot Air ~ 2500C

30. Sizing of Pulverizers • Feeder capacity is selected to be1.25 times the pulverizer capacity. • Required fineness, is selected to be • 60% through a 200 mesh screen for lignite(75 mm), • 65% for sub-bituminous coal, • 70-75% for bituminous coal, and • 80-85% for anthracite. • Heat input per burner is assumed to be to 75 MW for a low slagging coal and • 40 MW for a severely slagging coal, • With intermediate values for intermediate slagging potentials. • General Capacity of A Coal Mill : 15 – 25 tons/hour. • Power Consumption: 200 – 350 kW.

31. Performance Calculations • Several performance parameters are calculated for the pulverizer train. • These include the following: • Effectiveness of Coal drying requirements. • Pulverizer heat balance. • Primary air flow requirements. • Number of pulverizers required as a function of load. • Auxiliary power requirements.

32. Prediction of Coal Drying • For predicting the amount of coal drying which is needed from the pulverizers the following methods were accepted. • For very high rank coals (fixed carbon greater than 93 percent), an outlet temperature of 75 to 80° C appeared most valid. • For low- and medium-volatile bituminous coals, an outlet temperature of 65 - 70° C appeared most valid. • Bituminous coals appear to have good outlet moisture an outlet temperature of 55 to 60° C is valid. • For low-rank coals, subbituminous through lignite (less than 69 percent fixed carbon, all of the surface moisture and one-third of the equilibrium moisture is driven off in the mills.

34. Energy Balance across pulverizer is very critical for satisfactory operation of Steam Generator.

35. Hot air Heat loss Puliverizer frictional dissipation Dry pulverized coal + Air + Moisture Coal Motor Power Input

36. Suggested Primary air fuel ratio

37. Hot air Heat loss Puliverizer frictional dissipation Dry pulverized coal + Air + Moisture Coal Motor Power Input Mill Energy Balance Tempering Air, Tatm

38. Pulverizer Heat Balance • To perform the necessary pulverizer heat and mass balance calculations, the following parameters are required: • Primary air temperature. • Primary air/fuel ratio. • Fuel burn rate. • Coal inlet temperature. • Coal moisture entering the mills. • Coal moisture content at the mill exit. • Mill outlet temperature. • Minimum acceptable mill outlet temperature. • Tempering air source temperature. • Tempering air flow.

39. Heat Losses and Gains in A Mill • Convection and Radiation Losses from the surface of the mill. • Heat losses are generally found to be at 5 percent of total thermal energy available. • Mills consume an electric energy of 60 kJ/kg. • The mill grinding heat dissipation, varies from 20 to 40 kJ/kg of coal.

40. Mill Heat Balance: Energy for Drying of Coal • Determine the latent heat per kg of water evaporated. • Calculate the total energy absorbed by evaporating the required amount of water from the coal.

41. Mill Heat Balance: Energy for heating of dry Coal • Determine the sensible heat increase of the coal.

42. Mill Heat Balance: Energy for heating of remaing Moisture • Determine the increase in sensible heat of remaining moisture.

43. Mill Heat Balance: Energy available • Determine the sensible heat available in the mill inlet air. • Calculate the the mill grinding heat generation

44. Mill Heat Balance: Energy gained by Tempering Air • Determine the sensible heat increase in tempering air air.

45. Mill Heat Balance: Energy losses • Calculate the heat lost from the surface of the purlverizer:

46. Mill Energy Balance: Verification • Total Energy Available: • Total Energy Consumed: • Calculate the difference: • Divide the difference by the total available to obtain the fraction

47. Validation of Design • For best desingn: X = 0. • Acceptable designs: X = +/- 0.05. • If X is not in the limits above, the design and performance calculations should be repeated. • At any time during Operation above conditions should be maintained for most efficient and reliable operation of mill.

48. Derate Analysis and Operating Concerns • Pulverizer capacity limitation : A derate is due to the fuel burn rate exceeding predicted pulverizer capacity with all pulverizers in service. • Feeder capacity limitation : A derate is due to the fuel burn rate is greater than the total actual feeder capacity with all pulverizers in service. • An exhauster mill limitation: A derate is due to the calculated airflow required with all pulverizers in service is greater than the actual exhauster fan flow. • Improper pulverizer outlet temperature: A derate is due to the heat available in the primary air for drying coal in the pulverizers is less than that required.

49. Auxiliary Power Requirements • The pulverizer system annual auxiliary power requirements are calculated in a multistep process. • The first step is to calculate the fractional load per pulverizer in service (Milllod) at load point i. where • FBRi = fuel burn rate, t/h, at load i, • Nmill,i= calculated number of pulverizers in service at load i, and • C mill = calculated capacity, t/h, per pulverizer. • The second step is to calculate the power required per pulverizer.

50. where • RPmill = Rated Power Consumption of mill, and • dP/dld = slope of pulverizer power Vs fractional pulverizer load curve based on manufacturer data. • The third step is to calculate the power required (MWh/yr) for the pulverizer system at load point i. where hmotor = motor efficiency. • Finally, the auxiliary power requirements for each load point are summed to obtain the total auxiliary power requirements for the pulverizer system.