ates latent heat. The amount of latent heat released is a function of the flue gas temperature and coal type. The amount of released latent heat increases as TM content of the coal increases and temperature of the flue gas decreases. The latent heat can be recovered in condensing heat exchangers (CXEs), but due to the low temperature of a cooling fluid, there are practical temperature limits (approximately 100F to 110F) that impose limits on the amount of latent heat than can be economically recovered from the flue gas. Available heat sinks limit the amount of low-temperature heat that can be beneficially used. The total (sensible and latent) heat of the flue gas is presented in Figure 6. As the flue gas is cooled below its saturation temperature, the amount of total heat greatly increases. However, as discussed previously, there are practical limitations associated with cooling of the flue gas to low temperatures and beneficial use of the recovered low temperature heat.

4.3. Flue Gas Desulphurization

One of the most commonly used FGD technologies for scrubbing pollutants from power plant gas emissions is a limestone forced oxidation (LSFO) scrubber system. In this process, many pollutants end up in the circulating water in the scrubber. To maintain appropriate operating conditions, a constant purge stream is discharged from the scrubber system, and the purge stream contains contaminants from coal, limestone, and make-up water. The purge is acidic and supersaturated with gypsum, with high concentrations of TDS, TSS, heavy metals, chlorides and occasionally, dissolved organic compounds. State and Federal laws regulate the concentration of pollutants in FGD wastewater prior to discharge to waterways. In some cases (e.g., power plants discharging to large rivers), the wastewater may be suitable for discharge after minor treatment for suspended solids and pH adjustment. However, in many cases, the wastewater requires treatment for the reduction of key contaminants, including suspended solids, COD/BOD, total nitrogen, and selected heavy metals to very low concentrations. The scrubber purge stream is most often treated in a dedicated wastewater facility rather than an existing treatment system.

Lime and limestone play a significant role in the removal of pollutants from flue gas streams of coal-fired power plants, incinerators and industrial facilities. Flue gas desulfurization (FGD) primarily refers to the removal of sulfur dioxide (SO2). However, lime and limestone are also used in the removal of other pollutants such as hydrogen chloride (HCl), sulfur trioxide (SO3), fine particulates and mercury. In the US, air pollution control applications were the second largest use of lime in 2003, consuming over 3.4 million tons of lime. Lime and limestone products are used in both wet and dry FGD processes to absorb the sulphurcontent.

4.4. Condensing Efficiency

A condensing heat exchanger operates at lower temperature in order to condense moisture from the flue gas stream. The Condensing Efficiency is plotted as a Function of Flue Gas Temperature as shown in the Figure 7. The acid dew point temperature is plotted against condensation as given in the Figure 8.The heat sink temperature imposes a limit on the amount of water that can be recovered by condensation from a flue gas stream. Condensation efficiency, the percentage of flue gas moisture condensed out the flue gas stream determined. The results indicate excellent agreement between the theoretically and experimentally determined values.

Condensation efficiency increases as sink temperature is reduced. Besides the flue gas temperature, condensation efficiency is a strong function of the coal type, which affects the initial moisture content of the flue gas. The results clearly show that high-moisture coals are prime candidates for water recovery from the flue gas.

5. Summary and Conclusion

The key step to enhance the performance of boilers in thermal power plant is the detailed study of boiler in the plant and then the efficiency calculation. The efficiency calculation by indirect method is the best way to account all the boiler losses. The flue gas loss is always higher than any other losses. The flue gas loss can be minimized by heat extraction and proper utilization. Moreover when the primary fuel is coal, it should be accounted that it is of higher calorific value, low moisture and low ash content. The heat recovery method is done by observing the amount of flue gas loss, temperature of the flue gas and layout of the plant. When the flue gas loss is reduced and

Figure 6. Flue gas heat vs gas temperature.

Figure 7. Condensing efficiency as a function of flue gas temperature.

Figure 8. Acid dew point temperature vs condensation.

the moisture content in the fuel is low, efficiency of the boiler can be as high as 80 percent.


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