Water extraction from high moisture lignite by means of efficient integration of waste heat and water recovery technologies with flue gas pre-drying system

The flue gas pre-dried lignite-fired power system (FPLPS) integrates the fan mill flue gas dryer with an open pulverizing system and yields an increase of the boiler efficiency. Particularly, the dryer exhaust gas contains a large amount of vapor removed from high moisture lignite, which exhibits great potential for waste heat and water recovery. Two available options are considered to realize the extraction of water from lignite: the low pressure economizer (LPE) for water-cooled units and the spray tower (SPT) integrated with heat pump for air-cooled units. This paper aims at evaluating the energy saving and water recovery potentials of the FPLPS integrated with both schemes. Results showed that the plant efficiency improvement of the FPLPS at base case varied from 1.14% to 1.47% depending on the moisture content of raw lignite. The water recovery ratio and plant efficiency improvement in the optimal LPE scheme were 39.4% and 0.20%, respectively. In contrast, 83.3% of water recover ratio and 110.6 MWth heat supply were achieved in the SPT system. Both schemes were economically feasible with discounted payback periods of around 3 years. Moreover, parametric analysis was conducted to examine the economic viability of both schemes with different lignite types and market factors.