Theoretical Analysis of a Lignite-Fired Power Plant with Pre-Drying System in Terms of Energy Efficiency and Economy

Abstract

The coals with different initial moisture content to be used in the theoretical analysis were tested according to the related test standards and the results were used in the analysis. In the designed system, waste heat in the flue gas is used as the heat source. The fresh air is heated for drying in the shell-tube heat exchanger by means of flue gas. In fluidized bed dryers, the moisture of the coal was reduced by contacting the raw coal with the drying air. Due to the high initial moisture of the coals used in the theoretical analysis, the pre-drying degree could be at most 0.14 in order that the boiler feeding rate be the same as in the power plant feeding rate without pre-drying system. The system theoretically does not conform to the design parameters and the boiler feed rate is less than the flow rate of power plant without pre-drying system when the pre-drying degree is greater than 0.14. The pre-drying system had ten small capacity dryers working simultaneously to ensure the continuity of the boiler feed. In the study, the theoretical analysis was performed, and thermal performance of the power plant were formulized and graphically presented according to pre-drying degree. Thanks to the pre-drying system, owing to the decrease in the moisture content of the coal, a reduction in the flue gas flow rate and the amount of energy required to evaporate the moisture are realized. As a result, there was a decrease in the boiler losses and an increase in the efficiency of the boiler. The increase in boiler efficiency was 10.74% when coal 1 had been used and it was 7.92% when coal 2 had been used. Since the coals are dried in the same drying system under the same conditions, the moisture of coal 1 decreases more, thus the losses of the boiler decrease more. Due to the increase in the lower heating value and in the boiler efficiency, the efficiency of the power plant with pre-drying could increase by 3.04-4.34%. The coal pre-dried power plant had far more economic performance than a power plant without pre-drying system, since more electricity would be obtained thanks to the increase in efficiency of the plant. The redemption period of the system was determined as 2 years with coal 1 and it was determined as 3 years with coal 2. After the payback periods, the system makes a net profit and brings in an average of 55 million TL extra income per year. In addition, it was observed that when the degree of pre-drying was decreased, economic efficiency of the system also decreased. It is aimed that the study will provide principles of energy efficiency improvement in coal fired power plants with pre-drying system and will guide people who wants to do similar studies.