Thermodynamic Analysis on a Lignite-Fired Power System Integrated with a Steam Dryer: Investigation on Energy Supply System of the Dryer

Abstract

Lignite is becoming a competitive fuel for power generation with high security of supply and a low price. But a power plant firing lignite directly always has some weaknesses, including low thermal efficiency and high construction investment. That pre-drying lignite before feeding it to the boiler is a potential method to tackle these weaknesses, and low pressure steam extracted from steam turbines is considered as a competitive heat source for the drying process. In a lignite-fired power system integrated with steam dryer, the steam extraction is led to the dryer via connection pipe and depressurization valve directly. To obtain a stable drying temperature in a variable load of power plant, the steam extraction is often used by the dryer after depressurization, which indeed causes exergy loss. To reduce the exergy loss, the steam extraction was proposed to be supplied to the dryer via compressor or ejector. The thermodynamic models were developed to calculate the net efficiency of power system with different energy supply system of the dryer. The energy saving boundary, that the net efficiency of power system could be increased in some ranges of drying temperature by some energy supply system of dryer, Downloaded by [New York University] at 05:02 08 June 2015 2 was gained. Furthermore, the exergy efficiencies were calculated to get the essential of the results obtained. NOMENCLATURE Abbreviation CLPS Conventional Lignite-fired Power System PLPS Pre-dried Lignite-fired Power System PLPSC Pre-dried Lignite-fired Power System that steam energy supplied to the dryer via Compressor PLPSD Pre-dried Lignite-fired Power System that steam energy supplied to the dryer Directly PLPSE Pre-dried Lignite-fired Power System that steam energy supplied to the dryer via Ejector THA Turbine Heat Acceptance condition NHV Net Heating Value Symbols h enthalpy, kJ/kg p pressure, MPa t d the drying temperature, defined as the heat source temperature (saturated temperature of low pressure steam) of the dryer, °C Downloaded by [New York University] at 05:02 08 June 2015 3 E exergy, kJ/kg P electrical power, kW W mechanical power, kW δ relative variation ζ ap station service power consumption rate, %.η cs isentropic efficiency of compressor, % η ex exergy efficiency, % η sg gross efficiency of power system, % η sn net efficiency of power system, % λ pre-drying degree, i.e., the moisture mass pre-dried out of 1 kg raw lignite, kg/kg ω entrainment ratio of the ejector, °C

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