Mathmatic Modeling And Optimization On The Process Of MPS Coal Mill

Coal fine grinding system is one of the most important components of thermal powergeneration, cement production, coal chemical engineering and water-coal slurry preparation, etc,and this stage requires enormous energy consumption. Middle speed coal mill is the keyequipment of coal fine grinding system. Based on on-site sampling result and characterizationof coal breakage, the internationally most widely applided MPS middle speed coal mill wasmodeled.With tranditional middle speed coal mill testing method, only the samples outside the millbody, including new feed, fine coal product and mill reject can be sampled and it is impossibleto sample the circuit streams inside mill body. This problem hindered the development ofmiddle speed coal mill modelling. According to the structure and operational mechanism ofmiddle speed coal mill, the modification scheme of coal mill for mill inside circuit samplingand the tools for this purpose was designed. Plant aduit was conducted on the MPS middlespeed coal mill in Xutang Power Plant on2kinds of coal and under6different operationalconditions. Both the samples outside mill body (new feed, fine coal product and mill reject) andthe samples inside mill body (feed and reject of separator and feed of elutriator) were sampled.Size distribution analysis, density analysis, proximate analysis and Hardgrove Grindiability Testwere conducted on the samples.With JKMultiBal V1.0software the plant audit data was mass balanced to improve itsreliability. The classification efficiency curves of the two internal classification equipments(separator and elutriator) were modeled according to mass balance result and Whitenclassification efficiency curve function and a kind of Rosin-Rammle classification efficiencycurve function. The models indicated that the classification efficiency curves of the separatorand elutriator are influenced by primary air flow rate and coal density directly, and by millroller pressure indirectly.A Hardgrove mill was modificed by the addition of torque meter to measure the grindingenergy input. Grinding test of2kinds of coal from Xutang Power Plant was conducted on3factors4levels (4density fractions4size fractions4grinding time). A modifiedRosin-Rammler function was employed to fit the cumulative size distribution curve of groundproduct. The model describing the relation between product size distribution curve and specificcomminution energy was developed.The model of middle speed mill grinding section was developed, which is composed bymill power model, internal classicification model of grinding section and specific comminutionenergy distribution model. Through the fitting on plant audit data, the values of model parameters in the6sampling tests were obtained and modeled. The methodology to predictproduct size distribution of middle speed mill grinding section was given.The dynamic operation of middle speed mill can be simulated with the classificationefficiency curve model and grinding section model developed in this thesis. The simulationresult indicated that the circuit streams size distribution and flow rate of MPS middle speed mill,as well as the mill power, can be predicted well.The simulation reveals that the ratio of primary air to coal feed is the main factor affectingthe performance of middle speed mill. As the ratio of primary air to coal feed increases, thepower of coal mill increases while the power of primary air fan decreases. However, the totalpower of coal mill and primary air fan will decrease and the pulverizerd fuel will becomecoarser. The optimum ratio of primary air to coal feed should be determined by balancing thetotal power of coal mill and primary air fan and the combustion efficiency of boiler.