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AS-FIRED FUELSAMPLING METHODSPart of the Stock Product Line

BackgroundAs-fired sample analysis provides one part of the in-formation required to determine boiler performance. Many plants also use as-fired sampling methods in the fuel stream as part of plant betterment programs developed to improve boiler efficiency. System wide heat rate dispatch of generating capacity, as per-formed by some utilities, mandates monthly perfor-mance tests requiring as-fired sample analysis. The Btu variations in the waste fuels, burned at many of the fluid-bed boiler installations, make it impossible to check boiler performance without analysis of an as-fired fuel sample.

Performance guarantees for new boilers often require efficiency tests as part of contract completion and boiler acceptance. A large number of new pulverized coal-fired and fluid-bed boilers are being supplied with feed systems having- as a minimum- provisions for manual collection of as-fired samples. In several cases, the as-fired sample analysis has been critical to successful completion of the boiler performance acceptance tests. Many plants are also interested in as-fired sampling for emission monitoring.

Sampling Requirements

Over the past few years, it has become increasingly impor-tant for installations to obtain an accurate analysis of fuel just prior to final feeding into the combustion process. In the case of pulverized coal-fired boilers, the last point is in the feeder just before the bed feed chute. The further upstream the sample is collected from the combustion process, the greater possible deviation the sample will have from its true as-fired fuel condition.

A covered conveyor to the day bin would be the upstream limit for what many plants consider an as-fired sample. The fuel characteristics are relatively stable for most in-stallations once the material is moved into the day bins; however, the time between sample collection and actual firing can vary greatly, making it very hard to correlate boiler performance with sampled material characteristics. Computer modeling programs are available for tracking material locations within the day bins based on in/out feed rates and geometry. However, these would only be practical for mass flow bins with first-in, first-out flow patterns. A more practical, time-oriented upstream limit for as-fired sample collection is after discharge from the day bin. The time period between this point and entering the boiler is relatively short for a set firing rate.

The quality of the sample is dependent upon the method of collection as well as the sampling location. It is also important to take the proper steps to maintain sample quality after collection. The ASTM Standard D2234 for the collection of a gross sample of coal describes two sample collection categories. In Type I, the sample selection is not subject to human discretion. In Type II, the sample is influenced by human discretion in the selection. The type of sample is further divided into four conditions: (A) a stopped belt sample, (B) a full material stream cut, (C) a partial stream cut, and (D) a stationary material sample. The best quality sample would be a Type I, Condition A sample obtained by stopping the feeder belt and remov-ing all the material contained on a six-inch length selected at random. During the boiler operation it is not possible to use this method. The sampling methods most frequently used would be classified Type II, Condition C. These would include samples collected manually from connections on the downspout and shovels or small scoops at feeder dis-charge. These methods do not all provide accurate sam-ples. Some degree of sample bias (error) is usually caused by sampling a partial stream in which size segregation may have occurred. To provide a minimum bias sample, it is necessary to remove material equally from the entire cross-section of the material stream.

Stock Equipment Company, Inc.schenck process group16490 Chillicothe RoadChagrin Falls, Ohio 44023-4398phone: 440.543.6000fax: 440.543.5944email: stock.sales@stockequipment.comwww.stockequipment.com

Stock Manual CollectionStock has developed a manual sample collection system for use on pressurized feed systems. The sampler consists of an as-sembly using 4” diameter PVC pipe for an outer housing and an internal scoop made of 11-guage, 304 stainless steel. Camlock fittings are provided for pressure tight, quick connection of the scoop to the feeder body. A spring operated cover on the inside of the feeder body is used to provide a seal during connection and removal of the scoop assembly.

The procedure for an op-erator to take a sample is: 1) Remove the sample connection port cap, 2) Connect the sampler hous-ing assembly, 3) With the scoop opening in the down position, insert the scoop through the internal cover into the material stream in the feeder, 4) Rotate the

scoop 180 degrees, 5) Fill the scoop with material and pull from the feeder by reversing steps 1 through 3.

The sample is classified as Type I, Condition C because it is tak-en from the full width of the material stream but only from a three-inch portion of the approximately 8” full, cross-sectional, length of the falling stream. The full width sample provides rel-atively little bias compared to partial stream cuts as one might get with a shovel at the feeder discharge or a pipe connection on the downspout.

COALLECTORTM SamplerIn response to requirements for automatic as-fired sam-pling, Stock has developed the COALLECTOR Sampler. Available as an option on new feeders or as a retrofit for cur-rently installed equipment, the sampler is designed to remove a six pound sample from the entire cross section of material on the belt.

The COALLECTOR Sampler is located at the discharge end of the feeder. A pneumatically operated scoop lowers through the coal at the belt discharge. In the full open po-sition, any material entering the scoop during opening is dropped out. The rate of closing of the scoop is set to cor-respond with the normal op-erating feed rate of the feed-er. The scoop closes, passing through the falling stream, and then lifts up through the coal on the end of the belt. In the fully raised position, the scoop forms the bottom of the four-inch diameter hous-ing containing an auger used for removing the sample from the feeder. Feeder internal pressure is isolated from the sample discharge connection with an isolation valve.

A complete sampling operation and auger purge is accom-plished in approximately one minute. The base sampler is provided with manual and push button controls to initiate

the sample collection cycle. Timing relays, pneumatic solenoids, and flow control valves regulate the scoop and auger operation.

For unattended sample collection, automatic controls are available to provide programma-ble sampling intervals

based on time or material feed. For example, a sample can be programmed for collection every two, four, six, or twenty-four fours in the timed mode, or programmed for collection at 50, 100, 200, or 1000 ton intervals in the total mode. The controls are housed inside a NEMA 4 control box.

The COALLECTOR Sampler meets ASTM requirements for a Type I, Condition C, partial stream sample and approach-es a Type I, Condition B, full stream sample. The COALLEC-TOR Sampler has been tested in com-parison to a Condition A stopped belt sample and found to provide good proximate analysis results for the op-erating conditions simulated.