As discussed in the section ondesign fuel, fuel properties vary. The pulverizer system shouldtherefore be designed to accommodate the fuel with the worstcombination of properties that still allow the steam generator toachieve the design steam flow. Three fuel properties affectpulverizer fuel processing capacity: moisture, heating value. AndHargrove grindability index. The fuel rank determines the pulverizedfuel fineness requirement, which in turn affects the fuel processingcapacity of a particular pulverizer. The following tabulation liststhe effects of fuel properties on pulverizer capacity:
Change in fuel propertyproduct fineness resulting change in pulverizer fuel processingcapacity
Moisture content increase decrease
Heating valueincrease increase
Hargrove Grindability index increaseincrease
pulverizer product fineness increase decrease
The worst-case fuelproperties, although they may not occur simultaneously, are used todefine the pulverizer design coal. The pulverizer design coal istherefore specified as follows:
Fuel moisture highest rangevalue
Fuel heating value lowest range value
Hargrovegrindability index lowest range value
pulverizer product finenesscorresponding to fuel rank
the range values used forpulverizer design fuel selection should be the range values of thesteam generator design fuel. if additional fuels are included in thespecification, the pulverizer system capacity should be checked withthese coals as well. However, to avoid over sizing the pulverizersystem, it is not recommended that the worst characteristics of theentire fuel list be used in defining the pulverizer design coal.
The application of twopulverizer selection criteria-one spare pulverizer with typical coal,and all pulverizers in service with the pulverixer design coal-mayresult in two different answers for the number of pulverizers. thismay occur if the extreme values of the properties that affectpulverizer capacity(moisture, Hargrove Grindability index, fuelheating value)cause a significant pulverizer derating when thepulverizer design coal is fired.
For example, the steamgenerator design coal may require four pulverizers in service at fullload. With the addition of one spare, the system then requires fivepulverizers. However, use of the worst or pulverizer design coalproperties may yield a coal that requires six pulverizers to achievefull load. in this case, the range values used with the steamgenerator design coal may be too wide, and thus indicate anunrealistic pulverizer design coal. However, if the range values aredetermined to be realistic, a system with two spare pulverizers(whenfiring the normal steam generator design coal)may be the best designsolution.
The design of the pulverizersystem determines the turndown capability of the steam generator. Theminimum stable load for an individual pulverizer firing coal is about50% of the pulverizer rated capacity. Normal utility minimum loadoperating practice is to operate at least two pulverizers. Thus theminimum steam generator load when firing coal without supporting fuelis equal to the full capacity of one pulverizer. A minimum of twopulverizers in operation is preferred, since a loss of one of twopulverizers will not require tripping the steam generator because ofloss of fuel and loss of flame.
The selection of thepulverizer size, therefore. Determines the steam generator turndown.A pulverizer system composed of a few large pulverizers results inpoorer turndown (higher minimum load) than a system composed of alarge number of smaller pulverizers. The capital cost, however,typically is less for a system with a few large pulverizers than fora system composed of a larger number of small pulverizers.
An additional consideration inthe selection of pulverizer number and size is the number ofpulverizers of the same model already in operation within the owner'ssystem. The use of the same pulverizer model number at other units orstations within the system offers the potential for sharing spareparts. Particularly during unplanned pulverizer outages.
The total number of aparticular pulverizer model sold and operating should also beconsidered. This aspect is especially important if the pulverizerselection is a new model with relatively few sold and operating. Theimproved performance and improved maintenance features of a newpulverizer model should be evaluated as part of the risk assessmentof purchasing a new pulverizer model.
Pulverizer Design Coal
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