FGD and DeNOx
NEWSLETTER       

                                                                                                         March 2005
                                                                                                             No. 323

Riley Power Coal Pulverizers

Coal pulverizer design and operation is an important element integral to the long-term success of a low NO_x combustion system. Riley Power Inc. (RPI), a Babcock Power Inc. company, has been developing improved coal pulverizer technology during the past several years to meet these challenging demands. All three types of coal pulverizer systems supplied by RPI have undergone design upgrades and improvements. These machines include Ball Tube Mills (BTM), MPS mills and Atrita^® Pulverizers. The Atrita^® Pulverizer has been upgraded for better coal fineness and longer service life. BTM systems have been upgraded for more reliable operation and MPS mills have been upgraded for increased capacity.

MPS mills have experienced their “third generation” design by increasing mill grinding force by over 60 percent. As a result, mill capacity increases of 20 to 50 percent, depending on the coal application, have been experienced since the mill grinding capacity is directly proportional to available grinding force. This implies, for example, that only six or seven mills of the same physical size are required today for a boiler that previously needed eight mills to supply the same coal flow. This significantly reduces the initial capital investment for mills and coal piping systems. The adjustable grinding force capability with hydropneumatic loading system design enables MPS mills to vary the grinding load as mill load demand changes.

The Atrita^® Pulverizer is a horizontal type high speed coal mill which consists  primarily of three sections:  crusher, grinding and fan section. The coal feed into the mill is first reduced in size and dried in the crushing section. Screened by the grid segments under the hammers, the reduced-size coal is introduced to the grinding section for pulverization. The conveying air or primary air, developed by an integral fan in the mill’s fan section, transports the pulverized coal from the grinding section to the burners.

A new rejector arm assembly design has been developed, the DynaRing™ Classifier. A field installation test of the DynaRing™ Classifier was conducted on an Atrita^® 550D Pulverizer installed at a utility located in the Northeast. The data showed that with the DynaRing™ Classifier, the coal fineness improved from the previous 67.6 percent to 80 percent passing through 200 mesh at approximately the same fuel flow.

Experience indicates that coal fineness improvement will reduce LOI by 5 to 10 percent, which accordingly increases boiler efficiency up to 1 percent and improves flyash quality as well. Better wear-resistant materials have been applied to RPI pulverizers. Typical  operating life of critical components in medium speed machines has increased by 20 percent, while operating life in high speed machines has increased 30-40 percent.

The RPI Ball Tube Mill (BTM) is a cylindrical low-speed grinding mill. It consists of a steel barrel with cast abrasion-resistant liners and partially filled with hardened steel balls. Coal mixed with preheated primary air enters both ends of the mill from a crusher/dryer or feeder. As the mill rotates, the balls cascade and pulverize the coal by impact and attrition. The pulverized coal is then conveyed by primary air to external centrifugal classifiers. Fine coal particles exit the classifier into coal piping for transport to the furnace, while oversized coal particles return to the mill.

The RPI Model 80 Centrifugal Static Classifier, which contains no moving parts and is almost maintenance free, is integrated into the BTM system design to establish a complete pulverizer system. With sufficient mill grinding capacity, BTM systems equipped with Model 80 Centrifugal Classifiers are capable of producing a coal fineness of 99 percent or higher through 50 mesh and 80 percent through 200 mesh.

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