A Combined-Type Fluid-Bed Dryer Suitable for Integration Within a Lignite-Fired Power Plant: System Design and Thermodynamic Analysis

Abstract

Lignite is becoming a competitive fuel for power plants, offering very high security of supply and cost-effectiveness. However, power plants firing lignite face some thorny issues, such as high carbon dioxide emissions, high investment in construction, etc. Lignite pre-drying is considered an attractive way to tackle these issues, but it consumes a lot of energy and has a high risk of ignition. Thus, a combined-type fluid-bed dryer, with which lignite could be dried safely using some waste from a power-generation process, is proposed in this paper. Both boiler exhaust flue gas and steam extraction of steam turbines are used as heat sources for this kind of dryer. To analyze its thermodynamics, a theoretical model was developed with which a reference case of a 1000 MW air-condensing power plant was performed. The results show that a dryer integrated within the power plant can evidently increase the plant efficiency by approximately 2.55%. The main factors, including the degree of pre-drying, dryer thermal efficiency, and the temperature of dryer exhaust, were analyzed. The results show that the degree of pre-drying has the most obvious influence. A 0.1 increase in pre-drying degree improves the plant thermal efficiency by about 0.62%, while a 10°C decrease of dryer exhaust temperature and a 10% increase in dryer thermal efficiency could improve the plant thermal efficiency by about 0.10% and 0.22%, respectively.