Energy and exergy analyses of a lignite-fired power plant integrated with a steam dryer at rated and partial loads

Lignite is a reliable raw energy material with abundant supply worldwide. However, the efficient use of lignite to generate power remains challenging, because lignite exhibits undesirable characteristics, including low heating value and high moisture content. Despite these disadvantages, the efficiency of lignite-fired power plants can be increased by predrying lignite. In this study, a predried lignite-fired power plant, which uses steam extracted from steam turbines as a heat source to dry lignite, was simulated using Aspen Plus. This study then analyzed the performance of the predried lignite-fired power plant at rated and partial loads on the basis of the thermodynamics principles. This study also examined the influence of lignite predrying on the exergy losses and destructions of the power plant components. Results indicate that the exergy destructed the combustion process decreases significantly because of lignite predrying, by 5.78 percentage points. The plant thermal efficiency based on the lower heating value of raw lignite relatively increases by 4.35%. The efficiency improvement of the power plant reduces as the load ratio decreases, because exergy is destructed in the admission opening, which is used to control the pressure of the steam leading to the dryer. The integration of the dryer saves energy only when the load ratio is >50%.