*1.3. The Course of the Combustion Process in Selected Grate Furnaces*

There are many constructions of grates and grate furnaces. The differences in the structures result mainly from the fact that grates and furnaces are adapted to the properties of the combusted fuel, to furnace efficiency and its functions (e.g., stove furnace or furnace in a water or steam boiler) [2,3,10,17,22,23].

The simplest structures of grate furnaces include furnaces with stationary grates and furnaces with flat movable grates. The first design is the traditional solution found in the furnaces of household stoves and low-power boilers. Such boilers are most often used by single-family houses, small farms, and small public utility buildings. The furnaces with flat movable grates constitute a difficult-to-estimate part of all movable furnaces, approximately hundreds of thousands [4–6,24]. The power of such furnaces ranges from a few to almost 100 MWt.

During the combustion of solid fuel in a combustion chamber with a stationary or movable grate, a number of processes take place, such as heating and drying of the fuel, degassing and partial gasification of the fuel, combustion of the obtained degassing and gasification products, and the combustion of the obtained carbonizate [17,23,25,26].

In the case of furnaces with a stationary grate, these processes take place for each portion of fuel fed to the furnace, and they dominate successively. There are also possible situations of a simultaneous occurrence of two or more processes in different parts of a given batch of fuel. During fuel combustion on movable grates, these processes take place along the length of the grate. In this case, these processes often overlap (in a given longitudinal section of the grate and the fuel, at different heights of the fuel layer and above it, different processes take place).

The combustion process changes the amount and properties of fuel over time (in the case of stationary grates) or along the length of the grate (in the case of furnaces with a mechanical grate). In each of the mentioned processes, there is also a different, often very different, air demand. The largest stream of air is required for the combustion of degassing and gasification products, and much less is needed for the combustion of carbonizate. On the other hand, for the heating and drying processes of fuel, an air supply is not necessary. The optimal amount of air necessary to be delivered to each of the subprocesses is additionally influenced by the thickness of the fuel layer and its properties [2,10,17,24–28].

In the grate furnaces in question, the majority of the oxidant is supplied to the combustion chamber as primary air. This air is supplied under the grate. The stream of this air also has the additional task of cooling the grate and protecting it from damage by excessively high temperatures.

Therefore, the regulation of the primary air stream is one of the most important control parameters for grate furnaces.

In the case of stationary furnaces, primary air regulation is possible by changing the efficiency of the primary air fan. In the absence of a fan, the control is possible only by adjusting the size of the opening through which primary air is sucked into the combustion chamber.

In order to be able to control the amount of primary air along the length of the movable flat grate, its supply is controlled in a specific zone. Depending on the structure, the space under the grate is divided into several zones [3,6,24,25,28]. The structure of the grate and that of the combustion chamber enable the feeding process of various primary air streams to individual zones. Most frequently, this is done with sector-based air boxes.

The basic problem related to the regulation of the amount of air supplied to specific zones results from the lack of knowledge of the curve describing the best air distribution along the length of the grate for a given fuel and furnace and for its efficiency. In the literature [3,29], we can find only qualitative information on the change in air demand along the length of the grate, without any description allowing for its more precise characteristics.

Another problem arises from the difficulty of assessing whether the amount of air supplied to a given zone is optimal or close to this value. Most often, in the case of industrial boilers and furnaces, the measurement of flue gas composition (or only the measurement of O2 and/or CO2 concentrations) is carried out at a very limited number of points, and often only at one point. This point, in turn, is often located at the exhaust outlet from the furnace or even from the entire installation. In such a case, the measurement results allow only for the assessment of the correctness of the course of the entire process, and they do not provide information on the possibility of its quick improvement. Such improvement is only possible by changing the setting of the control parameters by a trial-and-error method. Taking into account the efficiency change of the installation and the properties of the combusted fuel implies the necessity of frequent changes of the control parameters and long-term operation of the furnace in conditions significantly different from rational parameters. The exceptions include furnaces built in municipal waste incineration installations. In that case, the number of checkpoints measuring the most important (in terms of the regulation of the combustion process) components of the exhaust gas (i.e., CO, CO2, and/or O2) and the temperature is relatively large. However, even in the case of such installations, due to the turbulence of the process and often large dimensions of the combustion chamber, the adjustment is difficult [6,11,25,26].

The distribution of primary air in grate furnaces is generally viewed as one of the most important control parameters, e.g., [2,3,5,10,17,22,23,27]. However, most often the problem of primary air stream distribution is considered in the context of numerical simulations of combustion processes, e.g., [30–38].

There are few studies demonstrating the impact of primary air distribution on the obtained gaseous emissions along the length of the grate based on measurements. The [25,39] present the results of research studies carried out in industrial facilities. Due to the conditions of the carried-out research, the number of measurement points in the research presented in [39] was limited to five. In addition, only one primary air separation was taken into consideration in the publication.

In conclusion, we can state that one of the most important problems associated with the operation of grate furnaces is the supply of a suitable air stream under the grate. The mentioned stream is variable: in time (fixed grate furnaces) or along the length of the grate (mechanical furnaces).

The main objective of the study is to demonstrate the influence of air distribution on the formation dynamics of gaseous products of the combustion process. The dynamics is understood as the change in the stream of the above products over time. The impact in question was demonstrated based on the results of experimental tests for the theoretically determined air distribution function. It is also the basic innovative element of the study. The knowledge of the dynamics makes it possible to determine the total gaseous emissions of combustion products. The work also determined the effect of air distribution on the share of combustible parts in the slag.
