Flexibility of power generation and possibilities for feasible upgrades of coal power plants: a case of the Electric Power Industry of Serbia

Abstract

This paper deals with current status and possibilities for feasible improvement of the flexibility of power generation fleet consisting of aged coal-fired power plants and hydro power plants. The analyses are done by use of well-established deterministic metrics - the normalized flexibility indicator for each power unit is evaluated accounting for ramp up and ramp down power rates as well as the unit operating range. The method is applied to the case of the Electric Power Industry of Serbia (EPS). As expected, hydro power units are highly flexible with normalized flexibility indicator reaching the values as high as 0.78 (on the scale from 0 to 1). The units in power plants fired with lignite are of low flexibility - especially the units with the greatest power where the flexibility indicator is well below 0.2. This issue can represent a serious constraint for operation of these units in conditions of variable demand and future implementation of renewable energy sources with intermittent power generation. In units of lignite fired power plants the operating range (formulated as difference between maximal and minimal net electric power) is a stronger limitation than the value of ramp up / down rate. Therefore, the efforts for flexibility enhancement focus on lowering of minimal or increase of unit maximal power. In literature various technical measures are considered for this purpose. Despite their undoubtably positive impact, some of these measures (for example, implementation of additional boiler or replacement of thick-wall by thin-wall parts) would represent such a drastic surgery for the aged lignite fired power fleet that they cannot be considered as feasible. For that reason, we analyse sophisticated technical measures which contribute to flexibility increase, but do not require too high retrofitting costs. The solutions are based on (i) advanced operational procedures for feedwater redistribution (ii) installation of additional economizers for recovery of flue gas heat and (iii) implementation of steam accumulators.