Boiler Plant Efficiency

Very large, modem WT boilers with sophisticated heat-recovery auxiliaries may attain efficiencies approaching 88 to 90%. However, the overall efficiency of a fossil fuel utility power generation plant system falls to only 32 to 38% when the efficiency of electricity generation and condenser cooling is included. Nevertheless, it only requires 10% more in fuel costs to operate a boiler at 1,250 psig than 

High heat-transfer rates and sufficient cooling via internal, two-phase flow and circulation are consequently necessary to ensure that modem boilers operate below their particular critical heat-flux (CHF) conditions. This minimizes poor thermal efficiency performance, risks of thermal metal fatigue, and probable tube failures from excessive temperatures. 

At constant heat flux, CHF occurs at lower steam qualities as pressure rises, thus reinforcing the need to provide the very highest quality of waterside conditions in HP industrial and power boilers. 

A drop in steam pressure necessitates an increase in the fuel supplied to the burner and vice versa. However, any change in the quantity of fuel supplied requires a corresponding change in the volume of air supplied to the boiler, and if the fuel-to-air ratio is not balanced within the 

The percentage of excess air required for complete combustion varies according to fuel types and burner design. 

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Close control of the amount of excess air is possible by the use of oxygen trim control equipment. Such equipment will control the flue gas oxygen content within the range of 2.0-3.0 per cent as compared to the normal 3.0-5.0 per cent. The improvement in boiler plant efficiency is of the order of 1.0-2.0 per cent. 

Many boilers do operate at low load factors and consequent poor annual efficiencies. This can be avoided by providing boiler plant with little or no margin over the actual required capacity and by installing multi-boilers or two or three smaller boilers. Boilers of this modular type are available for low-pressure hot water (LPHW) purposes, but obviously cost more than a single boiler. This will also increase the maintenance, number of examinations and the labor costs. Such an increase should be more than offset by the reduction in fuel costs due to the much higher annual boiler plant efficiency, which should be of the order of 20-30 per cent. 

Condensing boilers are now available for both gas- and oil-fired plant, the advantage of these being that the flue gases are further cooled down to below 100°C so that the latent heat available in the flue gas water vapor is recovered. The condensate has to be removed and the boiler capital cost is higher than for conventional plant. However, the boiler plant efficiency is increased to the order of 90 per cent, based upon the fuel gross calorific value. Where the flue gas exit temperatures are in excess of 200° C a further economy can be obtained by the provision of a spray recuperator in the case of gas and flue gas economizers for oil and coal. 

In many cases, boiler owners have run for years without appropriate pretreatment plants and therefore have unduly low expectations for boiler cleanliness, steam quality, and fuel consumption. But the fact is that recent changes in technology have brought down the costs of pretreatment equipment and increased quality and output so that even a relatively modest expenditure almost always results in significantly improved operational control and boiler plant efficiency. 

The use of FW that has been inadequately pretreated makes the mechanical operation of a boiler and the control of boiler water conditions innecessarily difficult. Additional quantities of internal treatment chemicals and higher rates of blowdown (BD) are usually required, which reduces boiler plant efficiency and raises the cost of generating steam. 

Of course, the overall boiler plant efficiency will be lower due to ... 

A second criterion of performance sometimes used is an artificial thermal efficiency (tja) in which the energy in the fuel supply to the CHP plant is supposed to be reduced by that which would be required to produce the heat load (Qu) in a separate heat only boiler of efficiency (tjb). i e- by (Qu/ b)- The artificial efficiency (tja) is then given by... 

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. 

Boiler plants are a major user of energy. The combustion efficiency of a boiler plant can easily be set at the optimum, and Table 30.2 suggests the parameters for this for various fossil fuels ... 

Annual fuel costs may be four to six times as high as the initial boiler plant capital cost. Thus, a small efficiency improvement of only, say, 1 % can produce significant savings in fuel costs. 

Because of their compact size, packaged vertical boilers can be custom-designed as a complete boiler plant system and simply shipped to the customer on a steel skid or platform. This type of system may comprise a dual boiler arrangement, with a pretreatment unit (water-softening and chemical-feed system), boiler blowdown and condensate return facilities, and also possibly a dual stack containing an economizer. This type of packaged system may reach 85% GCV efficiency. 

Many steam-generating plants have more than one type of direct or indirect FW heater to increase the FW temperature, which reduces the potential for boiler thermal shock and increases boiler operational efficiency. 

Clearly, therefore, the design and correct utilization of the FW tank and associated FW equipment is of paramount importance in ensuring the continuous, efficient, and smooth operation of the boiler plant system. As an aid to design, a schematic drawing of a FW tank indicating some of the important elements is shown in Figure 3.5. 

A fundamental problem that occurs even in fully industrialized countries and large international companies, is that whereas many smaller boiler plant managers purchase well-designed and efficient boilers, pumps, and auxiliaries from recognized quality manufacturers and distributors, the design and construction of FW tanks and even the overall boiler house system may be placed in the hands of persons with inadequate training and experience. 

For these medium to large operations, apart from taking measures to ensure the continuous efficiency of deaeration and oxygen scavenging, little more can be said because all that can be done practically to remove DO is being done. The problems of inadequate FW deaeration, the resultant corrosion risks entailed, and the consequent need for constant vigilance primarily affects those owners and operators of small to midsize steam-raising boiler plants where mechanical deaeration is not available. 

In any boiler plant system, the boiler itself typically is the single most expensive item of equipment and the source of generated steam. Thus, it tends to receive the highest percentage of whatever time, money, and human resources are available. This is understandable because any loss of heat transfer efficiency through deposition or other physicochemical waterside problems has the greatest impact here. 

The demand for alkali, phosphate, and/or polymer internal chemical treatments becomes significant, and unless extremely good control over BW chemistry and boiler plant operations is provided, the result most likely will be a scaled boiler, higher fuel costs, and reduced efficiency. A softener is required under these circumstances. 

Various internal treatment chemicals are employed as components of these programs. In all but the simplest or lowest efficiency boiler plants, they complement external conditioning processes (pre- and post-boiler) and BD control by providing a fine-tuning or polishing function. 

As with boiler plant waterside functions, a major operational fireside objective is to maximize efficiency and keep maintenance and related costs under close control. This means that all fuel system components, fireside, and heat transfer surfaces must be kept clean and in good working order. Also, the fuel delivery, combustion, and flue gas emission processes should run equally perfectly. 

It is, thus, to the second group that this book on boiler water treatment is primarily addressed. My key objective was to provide the reader with useful and practical boiler plant information that will help improve waterside cleanliness and add value to their facilities operational efficiencies. 

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 ... 

In the conversion of fossil and nuclear energy to electricity, the value of high temperature solution phase thermodynamics in improving plant reliability has been far less obvious than that of classical thermodynamics in predicting Carnot cycle efficiency. Experimental studies under conditions appropriate to modern boiler plant are difficult and with little pressure from designers for such studies this area of thermodynamic study has been seriously neglected until the last decade or two. 

To increase equipment reliability and plant efficiency, corrosion inhibitors are used in boiler and cooling water programs to control fouling and deposition on critical heat-transfer surfaces. In cooling systems, corrosion inhibition is commonly achieved through the use of passivators, which encourage the formation of a protective metal oxide film on the metal surface ( 1). ... 

Coal, as a low-price and abundant energy source, is an excellent choice for the production of electric power. The coal-fueled IGCC is one of the most promising technologies to convert coal to electricity with not only superior efficiency than PC boiler plants, but also the ability to meet the ever-demanding future environmental regulations including C02 emissions. 

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