'Making ultra-supercritical pulverized coal-fired power plants flexible': 'Assessment and optimization of sub-Benson turndown ratio':

by A.H.W.F. Van den Berg

Institution: Delft University of Technology
Year: 2015
Keywords: supercritical; power; part-load; off-design; Benson; pulverized; flexible; performance; dynamic load operating window
Record ID: 1252113
Full text PDF: http://resolver.tudelft.nl/uuid:21075b70-1e78-4d75-b472-329d4a2bcd33


Large-scale coal-fired power plants are typically designed as base-load units. This means that they are traditionally run at continuous high loading levels, most often full-load. The current energy market asks for more flexibility as renewable energy sources continue to penetrate the energy market. Coal-fired power plants are forced to operate at part-loads more frequently, in order to complement changes in the difference between energy demand and intermittent energy supply by renewable sources. The focus of this thesis is on the identification of limiting mechanisms determining the minimum load operating point, and on the thermodynamic performance in sub-Benson operation of a Benson-type coal-fired power plant. An increase of the dynamic load operating window, by lowering the minimum load, increases the adaptability and economic competitiveness to future energy dispatch. This is expected to be of high value in the future. This study assesses the feasibility to operate continuously on non-designed sub-Benson loads. First, the limitations that restrict the minimum load operating point of a once-through supercritical pulverized coal power plant are identified. Secondly, the lower limit of the dynamic load operating window is defined. Thirdly, the thermodynamic performance in sub-Benson operation, in terms of efficiencies and process behavior, is determined. This method is applied to case study Maasvlakte Power Plant 3. It can be concluded that the following limiting mechanisms are critical for low load operation; (1) The minimum flue gas inlet temperature requirement for the NOx-removal system. An indication of 21.7[%boil] minimum (percentage of the maximum live steam production) is required to ensure the minimum DeNOx flue gas inlet temperature. (2) The limited potential to extract steam from the cold-reheat for internal use in sub-Benson operation. Below 20[%boil], the auxiliary steam demand exceeds the maximum potential of internal steam extraction from the cold-reheat, meaning the system cannot operate autonomously anymore. (3) The minimum thermal input supplied by the individual burners which determines the overall thermal input into the boiler. The operating window of the individual burners limits the system to ~17.9[%boil] minimum. (4) The minimum live steam mass flow rate to maintain the minimum required pressure level of 103[bar]. This pressure level ensures homogeneous cooling of the evaporator pipes. The live steam mass flow rate to maintain the required pressure level is 15.9[%load]. The first constraint encountered is the flue gas temperature requirement at the NOx-removal installation. When considering the current minimum load of 25[%boil], it is concluded that the minimum load operating point can be decreased to 21.7[%boil]. This results in a 3.3[%boil] increase of the dynamic load operating window. A model of the case study power plant is developed to simulate the thermodynamic performance, focusing on the sub-Benson load regime. In addition, the simulation obtains an understanding of the…