With the introduction of practical electrical power generation and distribution during the 1880s, demands were created for larger and more efficient boilers. Most power generation in the United States relied on (and continues to rely on) the burning of fossil fuels in steam boilers to drive high-speed turbine generator sets. Low cost and ready availability have made coal the fuel of choice for most electrical power generators. During 1920s, commencing with Lakeside Power Plant of Wisconsin Electric Power, pulverized-coal firing (the burning in suspension of finely ground coal particles) evolved as a means of providing more complete fuel combustion, higher system efficiencies, and facilitated the use of larger boilers than had been practical with stoker firing. About this same time, Arthur J. Stock, a 1922 mechanical engineering graduate of the University of Michigan, was applying for patents on a non-segregating coal distributor to supply coal in even-sized distribution to stoker-fired boilers.
The demand for larger, more efficient boilers continued throughout the first half of the twentieth century. A typical pulverized-coal-fired unit fuel system consisted of storage bunkers holding two-inch or smaller sized coal, feeders to control the flow, pulverizers to dry and grind the coal to a fine powder, pneumatic transport lines (burner lines), burners, and the boiler furnace in which combustion took place. Air supply fans, control dampers, and combustion control systems supplied combustion air as required for optimum combustion. A variety of mechanical feeders, including drag chain conveyors, table feeders, and rotary pocket feeders, historically have been used to volumetrically control the flow of fuel to the pulverizes.
In any firing process, accurate fuel feed is required to correctly proportion combustion air and thereby ensure total oxidation of all combustible elements. There would have been no need for gravimetric feeding by weight if coal were a more uniform material. However, coal is a blended mixture of coarse and fine particles, exhibiting dramatic changes in flow density (material bulk density under flowing conditions) with small changes in surface moisture. At any point in the transport and storage of coal, the coal is likely to be rained upon. Under other weather conditions, drying of the coal may occur and change its bulk density. Changes in the bulk density of coal limit the accuracy to which combustion fuel-to-air ratios can be held, unless compensated for by the feeder ahead of the combustion process.
By monitoring weight indications at a scale dial and manually adjusting delivery rates, evidence was obtained that feeding on a weight rather than volume basis could significantly improve boiler control and combustion efficiency. During this same period, increased emphasis was being placed on the need to maintain accurate inventory records as to total fuel consumed. These inventory records provided valuable data for calculation of unit operating costs and system efficiencies. In response, batch-type weighing scales, such as those manufactured by Stock Equipment Company and others, were finding use in pulverize feed systems placed ahead of the volumetric feeders. Problems were encountered with the inherent complexity of such systems, including coal pluggage at transfer points. The increased building elevations required for such multiple component systems were also expensive.
