Boilers efficiency

Steam costs vary with the price of fuel. If steam is only generated at low pressure and not used for power generation in steam turbines, then the cost can be estimated from local fuel costs assuming a boiler efficiency of around 75 percent (but can be significantly higher) and distribution losses of perhaps another 10 percent, giving an overall efficiency of around 65 percent. 

The thermal efficiency of steam-tube units will range from 70 to 90 percent, if a well-insiilated cylinder is assumed. This does not allow For boiler efficiency, however, and is therefore not direc tly comparable with direct-heat units such as the direct-heat rotaiy diyer or indirect-heat calciner. 

Economizers Economizers improve boiler efficiency by extracting heat from the discharged flue gases and transferring it to feedwater, which enters the steam generator at a temperature appreciably lower than the saturation-steam temperature. 

Boilers. Boiler efficiency will determine how much fuel is used, an important operating cost parameter. Pin the supplier down on just what efficiency is quoted and whether the basis is higher or lower fuel heating value. 

The heat supply to the cyclic gas turbine power plant of Fig. 1.2 comes from the control surface Z. Within this second control surface, a steady-flow heating device is supplied with reactants (fuel and air) and it discharges the products of combustion. We may define a second efficiency for the heating device (or boiler) efficiency. 

For (a), calculations showed that the presence of feed heating made little difference to the overall efficiency. Essentially, this is because although feed heating raises the thermal efficiency %, it leads to a higher value of and hence a lower value of the boiler efficiency, tjb- The overall lower cycle efficiency (t o)l = Vb Hl expected to... 

A gas turbine plant with an overall efficiency t]cq = 0.25 matching a heat load Acc, = 2.25 is again considered as the basic CHP plant also implied is a non-useful heat rejection ratio (Cnu)cg( cg = [1 ( cg)( g + 1)1 =. 3/16. For FESR calculations, we again take the conventional plant efficiency as 0.4 and the conventional boiler efficiency as 0.9. At the fully matched condition the.se assumptions previously led to EUF = 0.8125 and FESR = 0.2. 

Economizer. The economizer is a tubular heat exchanger used to recover heat from the exhaust gases from boilers or some processes. It is used in boilers to recover much of the sensible heat for use in preheating the boiler feedwater. An increase in boiler efficiency of 4-6 per cent is typical. The design and materials of construction depend on the application. 

Each 10% of excess air typically results in a 1% loss of boiler efficiency. 

With the incorporation of an economizer into a boiler system, typically a 1% gain in boiler efficiency can be achieved for every 40 to 50 °F reduction in stack temperature. 

Boilers are designed to operate within a range of firing levels the greater this range (turndown ratio), the more flexible they are in operation. However, boiler efficiency changes with variation in steam output, and most boilers produce their maximum efficiency at about 80 to 85% of MCR. 

In addition, the use of water-wall design lowered gas exit temperatures, which increased boiler efficiency and reduced the potential for rapid degradation of refractory surfaces caused by the buildup of slag (molten ash). All larger WT boilers today employ water-wall membrane designs in their construction. 

Feedwater regulators are fitted to boilers as a means of controlling the rate of addition of FW. This is necessary to maintain a consistent water level that, in turn, reduces the risks of thermal shock to the boiler, reduces the potential for BW carryover with the steam, and improves boiler efficiency. 

Economizers are one of several types of FW heaters, all of which are designed to provide thermodynamic gains in the steam cycle. They typically are located in the exit gas system, where their use improves overall boiler efficiency, which tends to increase by 1 % for every 40 to 50 °F (22-28 °C) reduction in flue gas temperature... 

A boiler plant raises 5.2 kg/s of steam at 1825 kN/m2 pressure, using coal of calorific value 27.2 MJ/kg. If the boiler efficiency is 75%, how much coal is consumed per day If the steam is used to generate electricity, what is the power generation in kilowatts, assuming a 20% conversion efficiency of the turbines and generators ... 

Make a rough estimate of the cost of steam per ton, produced from a packaged boiler. 10,000 kg per hour of steam are required at 15 bar. Natural gas will be used as the fuel, calorific value 39 MJ/m3. Take the boiler efficiency as 80 per cent. No condensate will be returned to the boiler. 

As noted previously in Chapter 2, care should be exercised when considering boiler efficiency as to whether quoted... 

However, this is not likely to be the case. In this example, the two boilers might have different fuels, with different fuel costs and different efficiencies, and the gas turbine (perhaps, with supplementary firing) will have completely different characteristics from the steam boilers. Thus, there are degrees of freedom created by multiple steam generation devices with different costs of fuels, different boiler efficiencies and different power generation potential. Individual steam boilers and HRSGs will have minimum and maximum flows. 

Boiler Thermal Efficiency Traditionally, boiler thermal efficiency is calculated pour/pm, where in is the LHV (lower heating value) of the fuel. A rule of thumb for economizers is that boiler efficiency increases by 1 percent for every 22°C (40°F) drop in temperature of the dry flue gas. These two statements do not reveal the considerable quantity of additional heat, available to be recovered through condensation of the water vapor in the flue gas, which is lost to atmosphere with hot flue gas. Based on fuel HHV (higher heating value), the total latent heat loss can be substantial an additional 9.6 percent (natural gas), 8.0 percent (propane), 6.5 percent (heating ou). 

The three-step model was developed as a consequence of the extreme complexity of a PBC system. This author had a wish to describe the PBC-process as simple as possible and to define the main objectives of a PBC system. The main objectives of a PBC system are indicated by the efficiencies of each unit operation, that is, the conversion efficiency, the combustion efficiency, and the boiler efficiency. The advantage of the three-step model, as with any steady-state system theory, is that it presents a clear overview of the major objectives and relationships between main process flows of a PBC system. The disadvantage of a system theory is the low resolution, that is, the physical quantity of interest cannot be differentiated with respect to time and space. A partial differential theory of each subsystem is required to obtain higher resolution. However, a steady-state approach is often good enough. 

I have noted that the stack, flue-gas temperature of the boiler is 680°F. Let s assume that we have around 3 percent oxygen in the flue gas (or about 15 percent excess air). We can then apply the following rule of thumb For every 300°F that the flue-gas temperature exceeds the ambient air temperature, the boiler efficiency drops by 10 percent. 

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