Coal mill pulverizer in thermal power plantsPresentation Transcript
COAL MILL/PULVERIZER IN THERMAL POWER PLANTS SHIVAJI CHOUDHURY
1.Introduction Coal continues to play a predominant role in the production of electricity in the world, A very large percentage of the total coal is burned in pulverized form. Pulverized coal achieved its first commercial success in the cement industry. At Wisconsin Electric Power Company, the use of pulverized coal was also shown to be a viable fuel for electrical power production after World War I Pulverized coal burns like gas, can be easily lighted and controlled.
2.Coal pulverizer/mill system One of the critical power plant components that is relied upon to convert the energy stored in coal into electricity is the coal pulverizer or mill. The coal flow is controlled by the feeder, allowing coal to flow into the pulverizer mill. The pulverized coal and air mixture is then transport to the boiler burner panels.
3.Boiler Boiler drum Reheater Final Super Platen Super heater heater LTSH Economizer Coal bunker Wind Box Secondary PA duct air duct Furnace Flue gas APH duct Coal feeder F D Fan P A Fan Coal Pulverizer
4.Pulverizer The pulverizer receives the raw coal and reduces it to a very fine, specified size consist, similar to face powder. There are four primary principals involved in pulverization: • Drying • Grinding • Circulation • Classification
4.1.Drying The coal that is passing through a pulverizer is entrained by the use of hot air usually from the air heater. During the pulverization process the surface area of the coal particles increases dramatically exposing fresh coal to the entrainment air. The inherent and surface moisture of the coal is reduced by the exposure to hot air.
4.2.Grinding There are three basic types of grinding: 1. Impaction – where the material to be ground is hit or impacted by an outside force 2. Crushing – where material is forced between two fixed objects 3. Attrition – where material is ground by rubbing or friction
4.3.Circulation The primary air, is the method of circulating the coal through the pulverizer. Circulating air is also important in allowing for the removal of heavy material such as pyrites, extraneous metal, etc. by centripetal force that otherwise might damage the grinding mechanisms.
4.4.Classification The circulating air is also used to classify the pulverized coal product prior to carrying it to the burners. The classifier, located on the top of a mill returns the over-size material back to the pulverizer but allows the proper-sized material to pass out of the mill to the burners. classifiers are critical in providing the desired quality of pulverized coal with the desired fineness
5.Types of PulverizersSpeed Low Medium High 10 to 20 40 to 70 900 to rpm rpm 1000 rpmType Ball tube Bowl Hammer mill Ball & race mill millDominating Attrition Crushing Impactprinciple
6.C-E Raymond Bowl Mill BOWL MILL
7.Ball and race mill
8.Foster Wheeler MBF Mill
10.Bowl Mill Bowl mills are employed to pulverize the pre-crushed raw coal to the required fineness before it is admitted into the boiler furnace for combustion. The mill output can be easily varied, as per the turndown ratio from its minimum to maximum load. Crushed raw coal at a controlled rate is fed into the revolving bowl of the Bowl Mill. Centrifugal force feeds the coal uniformly over the replaceable grinding ring where independently spun rolls exert the required grinding pressure. The rolls do not touch the grinding ring even when the mill is empty.
13.1.Fineness Fineness is an indicator of the quality of the pulverizer action. Specifically, fineness is a measurement of the percentage of a coal sample that passes through a set of test sieves usually designated at 50, 100, and 200 mesh A 70% coal sample passing through a 200 mesh screen indicates optimum mill performance. The mill wear and the power consumption are increased if the 70% value is exceeded. Values lower than 70% mean higher carbon loss and increased fuel consumption. In addition, coal retained on the 50 mesh screen should be in the 1–2% range. Higher values indicate worn internals or improper settings. Also, the higher percentages can cause boiler slagging and high unburned carbon.
13.2.Sieve distribution chart & Fineness testingscreen - Plot of coal fineness on Fineness testing screen Rosin and Rammler
13.3.Mill capacity vs Grindability Figure shows the effect of coal grindability (HGI) and desired fineness (percent passing a 200 mesh screen) on the mill capacity.
13.4.Moisture and grindabilityeffects on mill capacity Figure demonstrates how moisture and grindability affect pulverizer capacity. The dashed line estimates the increase in mill capacity in going from a high-volatile B bituminous coal with a 55 HGI, 12% moisture and a desired mill output of 70% through a 200 mesh screen to a highvolatile B bituminous coal with an HGI value of 60, a moisture content of 14% and the same fineness.
13.5.Mill Rejects The amount of pulverizer rejects is one indication of mill performance. Pyrites are the common mineral iron disulfide (FeS2) that has a pale brass-yellow color and metallic luster. However, it is not economical to attempt to grind and burn pyrites & Pyrite Scraper and Guard Assembly rock.
13.7.Logic tree – Mill Fineness Decrease in mill fineness SIEVE TEST RESULTS 70% THRU 200 MESH 99.2% THRU 50 MESHClassifier Loss of Ring or Classifier Exceeding Vane Roller Roller Vane Millposition tension wear wear capacity
14.Abrasion in pulverizer In coal pulverization, 5–20% of the material being crushed is abrasive mineral. Coal is not abrasive by itself. The minerals in coal that are the most abrasive are quartz and pyrite, which cause a abrasion or severe wear. carbides in the metal part, High chromium cast iron materials are used for improved abrasion resistance. Worn Journal Roll
14.1.CHEMICAL COMPOSITION OF THEINSERTS GRINDING ROLLS This specification of high chrome white cast iron inserts for making Insert Grinding Rolls. CHEMICAL COMPOSITION OF THE INSERTS: C = 2.6 – 3.0 % Cr = 15.0 – 22.0 % Mo = 1.0 – 2.0 % W = 1.0 – 2.0% Mn = 1.0 – 2.0 % Si = 0.5 – 1.0 % S = 0.1 % max P = 0.1 % max
15.Erosion in Pulverizer Erosion by mineral particles picked up in the air stream carrying pulverized coal through the mill, classifier, exhauster, and transport pipe is a recognized problem. Erosion can produce holes in steel liners and deep depressions in large section cast parts. There has been success in the industry using ceramic materials. Classifier cone with ceramic
16.Startup Procedures The startup procedures, in addition to the controls and interlocks, should follow the requirements of NFPA 85.
17.Condition Monitoring The main technologies used in condition monitoring are: Vibration analysis , Lubricant Oil analysis Particle count Viscosity Total acid measure Condition of oil additives Sediment in Lubricating Oils Any other test recommended by OEM.
18.Inspection Following inspection parameters are critical for bowl mill performance: • Classifier internal condition • Deflector ring length • Inverted cone clearance • Journal assembly condition • Grinding roll-to-bowl clearance • Spring pressure for rolls • Pyrite scraper clearance • Pyrite rejects chute and/or damper condition • Feeder settings • Air in-leakage sources
19.Test code of Pulverizer::ASME PTC 4.2 The purpose of this code is to establish procedures for conducting performance tests to determine: Capacity, Fineness of product, Raw coal feed, Grindability, Moisture, Sizing, Power consumption and Effect of changes in raw coal Characteristics on product fineness, pulverizer capacity, and power consumption. Effect of changes in pulverizer component settings on product fineness, pulverizer capacity, and power consumption.
Logic tree – Mill Fineness Decrease in mill fineness SIEVE TEST RESULTSClassifier Loss of Ring or Classifier Exceeding Vane Roller Roller Vane Millposition tension wear wear capacity