**1. Introduction**

Due to the increasing number of renewable energy sources with a share in energy systems, research is being conducted on a large scale to increase conventional power plant flexibility [1]. This aspect was addressed in [2], where the authors analyzed increased flexibility in systems with a high share of renewable sources. In [3], the flexibility and economic aspects of power plant operation in new low-carbon systems were analyzed. Increasing the flexibility of coal-fired power plant operation by activities related to the regulation of the steam cycle are presented in [4]. In [5,6], the cooperation of conventional power plants was analyzed in terms of the power grid flexibility. The works mentioned here are broadly related to increasing the power range flexibility to lower the coal unit minimum load [7]. At present, most coal-fired unit operating power fits the range of 50–100% of nominal capacity. In case of a significant power increase in the system due to the power produced by units with priority, e.g., renewables, the currently running conventional power plants reduce their operation [8]. If the power supplied to the electricity network still exceeds the demand, some conventional units have to be shut down and prepared for restart [9]. Shutting down and restarting coal-fired units is economically inefficient, shortens the unit's lifetime, and causes increased emissions of harmful substances [7]. Therefore, research to reduce the minimum power block output to values below 50% of the nominal load increases their stable operation under current conditions.

At the same time, the emission standards, concerning mainly dust, sulfur oxides, and nitrogen oxides emissions, have recently been stringent [3]. Selective catalytic reduction

**Citation:** Kurkus-Gruszecka, M.; Krawczyk, P.; Lewandowski, J. Numerical Analysis on the Flue Gas Temperature Maintenance System of a Solid Fuel-Fired Boiler Operating at Minimum Loads. *Energies* **2021**, *14*, 4420. https://doi.org/10.3390/ en14154420

Academic Editor: Marcin Kami ´nski

Received: 26 June 2021 Accepted: 19 July 2021 Published: 22 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(SCR) technology is the most frequently used for reducing nitrogen oxides in coal-fired power boilers, as indicated in [10,11]. Such installations require operation in a specific flue gas temperature range [11], between 585 K and 670 K, depending on the catalyst type. The required range is also indicated in [12] (the authors analyzed the installation and its impact on the quality of flue gases), and [13] concerns the installation's operation optimization. The issue that arises relatively often during attempts to increase the boiler operation flexibility is the insufficiently high flue gas temperature before the SCR installation, which results in its incorrect operation. One of the solutions for a too low temperature before the SCR issue is to connect the higher temperature flue gas from another part of the boiler to the main flue gas stream before the SCR installation. In the solution mentioned above, the key is to effectively mix the flue gas streams at different temperatures to obtain a uniform flue gas temperature field before the installation. The routing of an additional duct for transporting hot exhaust gases requires the consideration of design possibilities. In many modernized coal boilers, the SCR systems were installed additionally, so it is necessary to introduce hot flue gases from the top of the duct. There is a high risk of not mixing the hot flue gas stream with the main, cooler stream in such a configuration. The solution presented in this paper in the form of an adequately profiled turbulizing flap enables effective mixing of exhaust gas streams with the introduction of hot exhaust gas from the top of the duct and obtaining a temperature field with appropriate uniformity before the SCR installation.

If the elements regulating the flue gas flow installation, e.g., control vanes or flaps, are planned in an existing boiler, a key parameter that should be considered is the flue gas pressure drop caused by the installed element, especially in boiler operation with high loads. Any additional pressure drop in the flue gas ducts increases the flue gas fans' power consumption, and in some cases, it can cause fan inefficiency. Increased power consumption also negatively affects the overall power unit efficiency. The technology developed by the authors makes it possible to regulate the pressure drop caused by the additional turbulence flap, with the possibility to fold the flap during the boiler operation with the nominal load. Thus, the impact of the device on the flue gas pressure drop is reduced to a minimum.

With the increasing availability of computational power, computational fluid dynamics (CFD) is increasingly being applied to the calculation of power boilers [14–18], characterized by relatively large calculation volumes and multiple physical and chemical phenomena. CFD methods are used in power boiler calculations for many purposes. In [15], the CFD method was used to optimize nitrogen oxide removal from exhaust gases. The temperature distribution in the boiler, validated by acoustic measurements, was modeled in [16]. In several studies, the main objective was to determine the flue gas flow character in the boiler. In [17], the influence of the NOx control installation on the flue gas flow in a boiler was examined. The authors of [18] investigated heat transfer by conduction and radiation from the flue gases to the evaporator and boiler superheaters. The flue gas and air mix flow through the power boiler was analyzed in [19]. In [20], the exhaust gas recirculation performance was determined. Many studies have used CFD methods to calculate the distribution, formation and reduction of nitrogen oxides [21]. Many works also model sub-systems of power boilers, such as SCR reactors [10,22] or dedusting systems. In [23], the mixing of the flue gas stream with primary air was modeled to increase the flue gas temperature before the SCR installation. However, the authors did not present the geometry of the mixing system. Despite many works on numerical modeling of power boilers, there is a lack of models in the literature concerning the mixing of flue gas streams with different temperatures on a large scale. As the flow is non-reactive, without heat exchange with the environment, and is single phase. In terms of the computational model complexity, this type of numerical analysis has been carried out and verified many times over recent years. Nevertheless, there are always limitations and risks inherent in the use of such a calculation method. Today, models of this degree of complexity often allow the elimination of experimental confirmation in industrial applications. In addition, the model has been validated for the current flue geometry.

This article presents the selected results of the calculations that led to developing the final turbulence flap concept. The novelty of the work is the development of a device allowing for effective mixing of the flue gas streams while maintaining the following criteria:


The developed shape of the flap allows for the flue gas stream to be mixed and obtain a uniform temperature field. Based on the analysis of the available literature, it is the first solution developed to mix the flue gas streams in the channel of a coal-fired boiler while maintaining the above criteria.
