By Douglas J. Smith, IEng
THE GOAL OF THE Chariton Valley Biomass Project, an experimental energy program, is to process and supply an initial 4,000 tons of switchgrass to the Ottumwa generating station, Chillicothe, Iowa. Switchgrass is being evaluated to determine if it can be used to replace some of the coal currently burned in the Ottumwa generating station boilers.
Dust control system installed at Chariton Valley Biomass facility.
Switchgrass, a local agricultural resource, is delivered to the Chariton Valley Biomass Facility where it is crushed in a pulverizer and then conveyed to a storage/feeder hopper. From the hopper the biomass fuel is pneumatically conveyed to the boiler where it is co-fired with coal. Unfortunately dust is a major problem when switchgrass is processed. The dusting problem, however, has been resolved with the installation of a dust collection system. Dust is collected at two points after the switchgrass has been crushed.
During the design process, Sega Engineering evaluated a number of different dust collection systems including round and bag-house designs. Eventually, a round dust collection system, manufactured by Camfil Farr, Jonesboro, Ark., was chosen. Based on the original design, Sega Engineering, sized the dust collection unit for 20,000 cubic feet/min at an air to cloth ratio of 5.3:1. Because the Camfil Farr system uses a self contained cleaning system it did not require any compressed air. Sega Engineering also specified that a pulverizer be used instead of the tub grinder originally chosen.
During initial startup of the processing system, the pulverizer was unable to produce the required 12.5 tons/hr of switchgrass. However, Sega Engineering determined that this problem could be resolved by redesigning the ductwork from the pulverizer outlet and by eliminating the conveyor from the pulverizer to the feed/storage hopper.
With these changes the pulverized switchgrass now goes directly to the dust collector and is then discharged to the feed/storage hopper. According to Tom Montag, Sega project engineer, the modifications significantly increased the grain loading to the dust collector. Initially the grain loading of the dust collection system was expected to be 30 grains/cfm. However, after modifying the fuel processing system the dust collection system was seeing an average of 145 grains/cfm.
Despite the increased loading, however, the dust collector performed without any problems while maintaining less than 2 inches pressure drop during operation, Montag says. Not only has the dust collection system operated successfully in controlling the dust, it also can be used to store the processed fuel. As a result, a simpler, more effective and less costly process system design was possible.
According to Martin Braster, Biomass Coordinator for Chariton Valley Resource Conservation and Development, the dust control system provides excellent dust control. “The dust collector did more than it was it was originally called upon to do and we couldn’t be happier,” says Braster.
If the biomass project proves successful, the 650 MW coal-fired Ottumwa generating station, co-owned by Alliant and MidAmerican Energy, will replace 5 percent of the plant’s coal with switchgrass. This would amount to approximately 25 tons/hr and require 50,000 acres of land to grow a year’s supply of switchgrass.