Power demand

A primary concern in coal-fired power generation is the release of air pollutants. Limits on SO2 output, 0.52 g/MJ equivalent of coal input to a new plant, have been estabflshed. Eor a bituminous coal of 27.9 MJ/kg there is thus an upper limit of 0.72% sulfur content. Relatively few coals can meet this requirement. The U.S. Department of Energy indicated recoverable reserves of 420 x 10 t in 1987 (2) that were categorized by sulfur content 33.5% had 0.6% S or less, 15.4% had between 0.61% and 0.83% S, 16.1% had between 0.84 and 1.67% S, 12.4% had between 1.68 and 2.50% S, and 22.6% had more than 2.5% S. The lowest sulfur coal, 86%, is found west of the Mississippi River, mainly in Montana and Wyoming, quite distant from the electric power demand centers in the East. A trend to utilization of the western coals has developed. 

The development of improved supports and drive mechanisms has allowed gyratory crushers to take over most large hard-ore and mineral-crushing applications. The largest expense of these units is in relining them. Operation is intermittent so power demand is high, but the total power cost is not great. 

Thus, a cogeneration system is designed from one of two perspectives it may Be sized to meet the process heat and other steam needs of a plant or community of industrial and institutional users, so that the electric power is treated as a by-produc t which must be either used on site or sold or it may be sized to meet electric power demand, and the rejected heat used to supply needs at or near the site. The latter approach is the likely one if a utility owns the system the former if a chemical plant is the owner. 

Cogeneration systems will not match the varying power and heat demands at all times for most applications. Thus, an industrial cogeneration systems output frequently must be supplemented by the separate on-site generation of heat or the purchase of utility-supplied elec tric power. If the on-site electric power demand is relatively low, an alternative option is to match the cogeneration system to the heat load and contract for the sale of excess electricity to the local utihty grid. 

When an extraction-condensing turbine is decided upon, it may be specified in three different ways, depending upon process steam and power demand. Referring to Fig. 29-24, the usualpurchase is a unit in which rated capability can be carried either straight-... 

Normally such mills are installed in groups known as wind farms to provide a sizeable power source, except in remote areas, where power demand may be restricted to a very limited area and small mills may suffice. When mills are installed in groups, precautions are necessary to ensure that there is enough distance between any two mills so that there is no hindrance to routine maintenance, on the one hand, and obstruction of wind to other mills, on the other. For more details, refer to the literature available on the subject in the Further reading at the end of the chapter. 

To initiate operation of the next due machine when the power demand on the existing machine or bus exceeds its rating. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

Response Scale length (mm) Weight (kg) Dimensions (mm) Power demand (W) Current supply Remarks... 

Nakhamkin, M. Increasing Gas Turbine or Combined Cycle Power Production with Compressed Air to Meet Peak Power Demands, ASME Paper No. 2000-GT-0596. 

The energy delivered per seeond (joules/seeond or watts) must be suffleient to meet the eontinuous power demands of the load. This means that the energy stored during the ON time of the power switeh must have a high enough /pk to satisfy equation 3.8 ... 

Figure 4-17. Power demand for flooded screw compressors using slide valve and suction throttling [1]. (Reprinted by permission of the Councii of the institution of Mechanical Engineers from The Place of the Screw Compressor in Refrigeration )...

That means that the average heating power demand becomes... 

For a site with a fixed power demand throughout the year, the unfired plant illustrated in Fig. 9.2a is suitable for summer operation when the heat load is light. 

There are several types of steam turbine that can be used to meet widely varying steam and power demands. They can be employed individually or in combination with each other. 

The simple back-pressure turbine provides maximum economy with the simplest installation. An ideal backpressure turbogenerator set relies on the process steam requirements to match the power demand. However, this ideal is seldom realized in practice. In most installations the power and heat demands will fluctuate widely, with a fall in electrical demand when steam flow, for instance, rises. 

This is best illustrated by the following example for a rural development project. The electrical power demand is shown in Figure 15.21 and was developed from historical data generated from similar developments in other regions. The operating statistics indicated that... 

Since the electrical supply system carries the full apparent power (VA), a current higher than is theoretically necessary to supply the power demand needs to be supplied. Equipment must therefore be rated to carry the full apparent power plus the losses of the supply system, which are proportional to the square of the current. 

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