Recommendation for the design of the boiler-turbine unit for faster load rejection specifically in the Captive Power Plant Abhisheik Malhotra AMIMechE, Deputy Manager - Mechanical at Lahmeyer International India Pvt. Ltd; Affiliate Member - Energy Institute, UK We are presently working on the design engineering of coal fired captive power plant comprising of two (2) units of 100 MWe capacity with sub-critical technology and provided with natural circulation, single reheat, vertical, two pass design, pulverized coal fired and balanced draft type boiler along with the emission control system viz. Urea based SCR (Selective Catalytic Reduction), ESP (Electrostatic Precipitator) and Limestone base WFGD (Wet Flue Gas Desulphurization) System. Further as the plant shall be constructed and installed in the smelting plant, Client has specified that the power plant shall be suitably designed to take care of the smelter load rejections, the frequency of which has been mentioned as follows: 0-5 MW = 20 times/hour; 5-15 MW = 5 times/hour; 15-20 MW = 1/hour and 20-25 MW = 0.1/hour. Client has explicitly stated that the load rejections up to 20 MW shall have to be carried out without operating the steam turbine bypass system and the resulting increase in the steam pressure in the boiler and piping network must no result in the lifting of the pressure safety valves.

In view of the above mentioned, we believe such an operation mode of the power plant, shall result in the over designing of the boiler pressure parts so as to absorb the excess pressure transients and thermal stresses during the load rejections up to 20 MW (with no steam turbine bypass operation and also considering comparatively, the slow response of the boiler fuel and combustion system during the load rejection operation). Request the group members expert opinion on the subject design and operation issue. Do share your experience if you have come across such an operation scenario and design requirements considered in tackling the issue.

I myself have worked in the captive power plant operation department, but every time there was a load rejection, we used to operate steam turbine bypass, and only in case of prolonged load rejection, boiler fuel and combustion system were being put into operation. Further, also provide your opinion / advice whether sliding operation shall be beneficial for this plant as it may result in reducing the excess pressure surges during the transient condition as compared to the constant pressure operation and also whether once through evaporator design will be beneficial for this plant considering faster load rejection requirements.