During the 2019 regular outage of unit 3 of the Cogeneration power plant Ljubljana TE-TOL, a high pressure water-tube boiler was upgraded by a selective non-catalytic system for reduction of NOx emissions. This changed the conditions of fuel combustion, therefore it was necessary to redefine the boiler efficiency. The key issue at determining the efficiency are thermodynamic properties of flue gases at the boiler outlet, which define the energy flows through the flue tract. This work describes the procedure for determining representative values of flue gas parameters, which are needed to calculate the boiler efficiency according to a standardized procedure. A 2-point grid measurement method is used and its adequacy is demonstrated. A test section for the flue gases composition and their temperature in a large channel is presented. The results of the measurements are appropriately evaluated.
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