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Show It is inferred that packaging coal-fired boilers in larger sizes would make coal more acceptable to industry. One question that arises is why can't a potential user simply buy several smaller conventional packaged coal units that are commercially available and still gain the same capital costs advantage? The answer is that the economy of scale for larger boilers cancels the advantage of packageability of conventional smaller units. In fact, the overall costs for several smaller units is more. For example, the cost for building one 150 MM Btu/h field-erected stoker-fired conventional system is about the same as 3-50 MM Btu/h stoker-fired package units (Table 1). On the other hand, three units take about 1-1/2 times as many people to operate0; thus, the overall cost is somewhat higher. It was felt that among other benefits, fluidized bed combustion offered an opportunity to package coal-fired boilers in larger capacities because the physical size of the boiler is not constrained by heat transfer but rather by the air flow rate. By pressurizing the fluidized bed, additional size reduction can be realized beyond that possible with atmospheric pressure designs. With this in mind, two industrial boiler concepts were developed utilizing FBC technology. One uses an atmospheric fluidized bed (AFB), the other is a pressurized fluidized bed (PFB) system. Each unit is designed to produce 146,338 lbm/h of superheated steam at 750°F, 650 psia (155,950,000 Btu/h). The basis for design was made as consistent as possible to effect a fair comparison. Some of the major design assumptions are presented in Table 2. THE PFB DESIGN ^(l&^S^jttG /_ U I The concept of fluidized bed combustion of coal involves burning the coal in a dense air-suspension of limestone particles. The mass flow of air is fixed by the combustion requirements and the superficial velocity of the air is selected according to fluidization mechanics. Pressurizing the air, with an exhaust gas-driven turbocharger, increases its density and allows a greater charge of air to be input per unit volume. Since the combustion heat release is a strong function of the air/coal feed rates, this method of supercharging the bed can be 21-5 |
