Boilers power plant, steam temperature

As of the mid-1990s, many older conventional steam plants have been converted to combined cycle. The old boiler is removed and replaced by a combustion turbine and heat recovery steam generator. Although the cycle efficiency is not as high as completely new plants, substantial capital cost is avoided by the modification and reuse of existing steam turbine and auxiHary equipment. In many combined cycle power plants, steam is injected into the combustors of the combustion turbine to lower peak flame temperatures and consequendy lower NO. ... 

Example 2.4 A typical 500 megawatt coal-fired power plant burns about 200 tons of bituminous coal per hour (50 kg s 1) with a heating value of 12,500 btu lb-1 (28,700 kJ kg-1). The high-temperature boiler operates at 538 °C while the heat exchanger cools the condensed steam to about 50 °C. Calculate the theoretical (maximum) efficiency of the process and compare with the efficiency that is actually achieved. 

In a Rankine power plant, the steam temperature and pressure at the turbine inlet are 1000°F and 2000 psia. The temperature of the condensing steam in the condenser is maintained at 60° F. The power generated by the turbine is 30,000 hp. Assuming all processes to be ideal, determine (1) the pump power required (hp), (2) the mass flow rate, (3) the heat transfer added in the boiler (Btu/hr), (4) the heat transfer removed from the condenser (Btu/hr), and (5) the cycle thermal efficiency (%). 

Steam Purity. The trend toward higher pressures and temperatures in steam power plant practice imposes a severe demand on steam-purification equipment for elimination of troublesome solids in the steam. Carryover may result from ineffective mechanical separation and from the vaporization of boiler-water salts. Total cany-over is the sum of the mechanical and vaporous carry-over of all impurities. 

A boiler that supplies a steam turbine used in an electric power plant must raise the temperature of water from the ambient temperature passing through a physical state change and then to live (very hot) steam. How much energy (kcal and kJ) would be required to raise 15,000 L of water from 25°C to steam at 175°C (steam requires 0.5 cal/g K) ... 

In this demonstration plant, the boiler feedwater can be preheated by geothermal heat if available. Depending on the availability and temperature of the geothermal heat in the area, the geothermal plant s design would vary. If the groundwater temperature exceeds 150°C (302°F), a "flash steam" power plant would be used, and if it is between 100 and 150°C (212 and 302°F), a "binary cycle" power plant would be used. 

Figure 3. The minimum of the unit cost of essergy T/S with respect to the output steam temperature T for the power plant boiler.

The thermal efficiency of a steam power cycle is increased when the press and hence the vaporization temperature in the boiler is raised. It is also increas by increased superheating in the boiler. Thus, high boiler pressures and te peratures favor high efficiencies. However, these same conditions increase capital investment in the plant, because they require heavier construction more expensive materials of construction. Moreover, these costs increase ev more rapidly as more severe conditions are imposed. Thus, in practice po plants seldom operate at pressures much above 10,000 kPa and tempera much above 600°C. The thermal efficiency of a power plant increases as pressure and hence the temperature in the condenser is reduced. However, condensation temperature must be higher than the temperature of the coo medium, usually water, and this is controlled by local conditions of climate geography. Power plants universally operate with the condenser pressure as 1 as practicable. 

Example 16.5 The operating conditions of a practical steam power plant are described in Example 8.1, parts (b) and (c). In addition, steam generation is accomplished in a fumace/boiler unit where methane is burned completely to C02 and H20 with 25 percent excess air. The flue gas leaving the furnace has a temperature of 460 K, and T0 = 298.15 K. Make a thermodynamic analysis of the power plant. 

Paths of various processes are conveniently traced on a thermodynamic diagram. For example, consider the operation of the boiler in a steam power plant. The initial state is liquid water at a temperature below its boiling point the final state is steam in the superheat region. As the water goes into the boiler and is heated, its temperature rises at constant pressure (line 1-2 in Figs. 6.2 and 6.3) until saturation is reached. From point 2 to point 3 the water vaporizes, the... 

Steam power plants produce electricity with rather low thermal efficiency. An increase in efficiency leads to savings in fuel costs and minimizes environmental effects. The two basic approaches in increasing the thermal efficiency of a cycle are (i) design a process that transfers heat to the working fluid at high temperature in the boiler and (ii) design a process that transfers heat to the working fluid at low temperature in the condenser. These may decrease the temperature differences, and hence the level of irreversibility. 

The suspended superheater below the furnace roof worked well in regulating the steam temperature. Three 75 t/h CFBC boilers of similar design have been operating in Anshan Second Thermal Power Plant during... 

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