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

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. Predrying 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 a competitive heat source for the drying process. In a lignite-fired power system integrated with a steam dryer, the steam extraction is led to the dryer via a connection pipe and depressurization valve. To obtain a stable drying temperature at a variable load of power, 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 a compressor or ejector. Thermodynamic models were developed to calculate the net efficiency of a power system with different energy supply systems. The energy saving boundary—that is, the net efficiency of power system could be increased in some ranges of drying temperature by some energy supply system of dryer—was obtained. Furthermore, the exergy efficiencies were calculated.