*1.1. The Role of Combustion Processes in Grate Furnaces*

A significant part of the combustion processes of coal [1–6] and biomass [7–13] is carried out in grate furnaces. In the case of the above-mentioned fuels, such furnaces are most often used in heat plants and combined heat and power plants with small and medium power [1–5,8].

The combustion of solids, which will probably be implemented for the longest time in most countries of the world, will involve the combustion of waste and waste-based fuels. In recent years, a significant increase in the installations for energy management of municipal waste have taken place in the EU and China among other places [14–16]. In the EU, in the years 2008–2018, the number of such installations increased by 60, and currently only in Poland there are about 100 installations at various stages of planning and implementation (with only 8 operating so far) [14]. In China, after 2000, there was a rapid increase in the number of municipal waste incineration installations, resulting in the highest combined efficiency in the world currently [15]. At the same time, it should be emphasized that, depending on the country, grate furnaces are installed in about 70% to 100% of municipal waste incineration installations (e.g., 100% in Brazil and Australia, over 85% in the EU, and almost 70% in China) [15–17].

**Citation:** Kozioł, M.; Kozioł, J. Impact of Primary Air Separation in a Grate Furnace on the Resulting Combustion Products. *Energies* **2023**, *16*, 1647. https://doi.org/10.3390/ en16041647

Academic Editors: Monika Kosowska-Golachowska, Tomasz Czakiert and Andres Siirde

Received: 15 November 2022 Revised: 31 January 2023 Accepted: 3 February 2023 Published: 7 February 2023

**Copyright:** © 2023 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/).

## *1.2. Advantages and Disadvantages of Combustion Processes in Grate Furnaces*

The main advantages of grate furnaces include flexibility in terms of the properties of burned fuels, high reliability, ease of use, relatively low investment and operating costs in the case of structures with lower and medium power fired with coal [1,8,18,19]. In the case of waste incineration, only operating costs are usually lower [15,17].

The disadvantages of grate furnaces include increased emissions of CO, benzoalfapyrene, CxHy compared with dust and fluidized furnaces, as well as a greater share of combustible parts in the slag [4–6,13,19–21]. The above-mentioned disadvantages principally do not apply to structurally advanced furnaces in thermal waste treatment installations.

Another disadvantage of grate furnaces involves temperature limitation of the process aimed at preventing the melting of mineral fractions of the fuel, which would lead to conglomeration and sticking of slag to the grate [8,18]. To a large extent, the above-mentioned disadvantages result in another disadvantage, i.e., relatively low energy efficiency of installations equipped with the discussed furnaces [3–6,8,18].
