Analysis of the use of waste heat obtained from coal-fired units in organic Rankine cycles and for brown coal drying

The ever increasing restrictions on greenhouse gas emissions necessitate the search for new energy technologies. One of the ways to meet the new requirements is to maximally improve the efficiency of power units. The paper presents two energy technologies which, if used, will increase the efficiency of electricity generation. One of the most effective ways to improve the efficiency of brown coal-fired units is drying of the coal fed into the boiler. Variants with a different share of dried coal in the loading of fuel fed into the boiler were analysed. In this technology, the waste heat obtained from exhaust gases is used. Calculations showed that in this case it was possible to achieve an increase in the overall efficiency of the unit by approx. 2 percentage points. The calculations were made with the use of an in-house code. The paper also presents an analysis of the feasibility and the potential for the use of waste heat obtained from exhaust gases to feed Organic Rankine Cycles (ORC's). Several low-temperature working fluids were considered, which due to their properties were best suited to use in this type of cycles. Moreover, they were selected so that steam could be produced effectively at the temperature of the exhaust gases directed to ORC exchangers. The impact of these working fluids on the efficiency and capacity of the ORC was examined. ORC's together with a regenerative exchanger, as well as the effect of regeneration on the increase in the efficiency and capacity of ORC power plants were analysed. The calculations for ORC's fed with the waste heat obtained from exhaust gases from a hard coal- and brown coal-fired boiler were compared. The calculations showed that for a large, brown coal-fired, steam unit an ORC power plant could gain a capacity equal to 1% of the overall capacity of the unit. The calculations were made with Aspen Plus software package.