1.1. Global Energy Scenario
The global energy scenario is one of the most challenging issues when it comes to the global development of a country. The current oscillations in energy production, due to many factors, such as geopolitics or international relations, have pointed out, in the cases of many countries (especially smaller or less powerful ones, which are usually more dependent and vulnerable compared to world powers, such as China or the USA [
1]), the need for modifications in energy patterns towards a lower energy dependency, which would imply a boost for their economies and industrial activities [
2], possibly reducing the influence of international oil companies (IOCs) in energy transition, as in the case of Europe [
3]. Notably, the possibility for many countries to become “prosumer actors” through a green transition could reduce any form of geopolitical concern [
4]. For instance, in the case of Europe (according to recent studies), it is vital to replace energy imports with domestic energy production, in order to reduce energy dependency. If import substitution takes place using domestic renewable energy sources, additional positive effects could be found in terms of energy dependency and security, as well as sustainable development [
5].
Additionally, there is an increasing concern (at both the national and international levels) about environmental issues such as climate change, with greenhouse gas emissions being among the most worrying aspects related to industrial activities and energy consumption. As a result, international organizations, such as the United Nations, have been fostering the Sustainable Development Goals (SDGs), for which the implementation of specific steps to promote sustainable economic and energy growth (especially in the cases of developing countries) have an essential role, promoting concepts such as green chemistry, circular economy, or atom efficiency, among others, in order to maintain the environmental quality of soil, water, and air [
6].
Specifically, the use of biomass could play an important role in the implementation of these SDGs or similar biomass-, bioprocessing-, or other bio-based product policies, implying, in many cases, the valorization of waste with difficult environmental management, through its energy use at local and industrial levels, and possibly contributing to bioeconomic strategies (of which many are still not completely coherent with common bioeconomic objectives), thus becoming an interesting resource through which to improve the use of marginal lands that can alleviate resource competition and soil deterioration [
7].
Additionally, the data included in
Figure 1 support the abovementioned reasoning. Thus, if worldwide pellet production is considered (
Figure 1a), an exponential increase can be observed over the last two decades, which points out the commercialization of biomass at the global level. Also, in the case of Europe, there was a steady increase in solid biomass primary energy production, from around 50 million tons of oil equivalent in 2000 to about 100 million tons in 2021 (that is, production doubled). This fact could imply an increase in the implementation of biomass power plants. For instance, regarding Germany (
Figure 1c), there was a 16% increase in the use of biomass power plants in the last decade, pointing out the commitment of this country to renewable energies, including biomass. Last but not least, the role of biomass in some developing regions or countries could be important, as shown in
Figure 1d, in the case of electricity generation from biomass and waste in Africa. As observed, electricity generation doubled in two decades, demonstrating this energy source as an interesting way to contribute, among other renewable energies, to the sustainable development of Africa. Indeed, interesting work has been carried out regarding the use of local biomass in boilers, as in the case of Cameroon [
8].
These data support the contribution of this energy source to sustainable development in general, increasing the energy independence of countries and regions, and managing some agricultural waste products, thus improving their valorization. In general, biomass energy use is devoted to local consumption, that is, many countries, such as Finland or Sweden, consume almost all of the biomass energy produced by themselves [
13,
14]. Nevertheless, there are other countries where biomass energy production exceeds its consumption, as in the case of Portugal [
15], whereas the opposite also takes place, with a higher consumption compared to biomass energy production (for instance, in Italy) [
16]. In any case, these differences are not significant, showing relatively similar biomass energy production and consumption.
Concerning Spain (whose biomass energy production/consumption ratio is also balanced, according to recent data [
17]), and specifically in the Extremadura region, a considerable amount of biomass waste is continuously generated due to diverse agro-industrial activities. Nevertheless, these waste products present high potential for energy generation (including barks, husks, corncobs, etc.), which represents a great opportunity for the implementation of circular economy or green chemistry policies [
18,
19]. Conventionally, these waste products are used for direct combustion and subsequent energy generation through steam cycles. In that sense, combustion optimization (for instance, that taking place in a stove) is essential to making this process energy and economically feasible at both the user and industrial levels.
1.2. Combustion: Foundations and Optimization
Combustion is a chemical reaction in which oxidizing air is combined with oxidable elements together in a combustible substance. Through this chemical reaction, energy is released as heat. The chemical species that are obtained are considered reaction products, exhaust gas, or combustion gas. For this purpose, pure oxygen is not usually used, whereas oxidizing gas is normally employed (for instance, air). Thus, there are different kinds of combustion, including complete combustion (in which the combustible is completely oxidized), neutral combustion (a complete combustion in which stoichiometric oxygen is used), and incomplete combustion (in which the combustible is not completely oxidized).
Moreover, the heating value of a fuel is the heat released in a complete combustion for one unit of fuel. Thus, the heating value can be determined as a high heating value (HHV) or a low heating value (LHV), depending on the consideration of steam condensation due to combustion or water content in the fuel. Thus, the only difference between HHV and LHV is the condensation heat of steam (produced during combustion or included in the fuel). As a consequence, this parameter is extremely important in this context, as the use of fuels with specific heating values will determine the efficiency of combustion processes in stoves.
Combustion facilities, such as those including stoves, are usually designed to obtain the highest yield possible, that is, an optimum yield, in order to make combustion as efficient as possible. For that purpose, energy loss should be minimal. Thus, loss due to sensible heat, or solid and gaseous non-burnt products, can be reduced if the excess air coefficient (n, included in Equation (1)) is adjusted, which is difficult to achieve in a specific facility.
where air
r is the real air in the facility and air
t is the theoretical air that should be provided. Thus, loss due to sensible heat is due to the fact that a percentage of the generated heat during combustion is not useful, as the exhaust gases evolve at higher temperatures compared to their behaviors at the ambient temperature. Considering the exhaust gases (P
s) as a mixture of ideal gases, this loss can be assessed according to Equation (2), as follows:
where:
hh—ha represents the enthalpy change in the exhaust gases.
mh is the exhaust flow.
Cph is the average specific heat of the exhaust gases.
Th—Ta are the inlet and outlet temperature, respectively, of the exhaust gases and the air.
Vh and ρh are the total volume generated and the exhaust gas density, respectively.
Thus, loss could be avoided if Vh = 0 or Th = Ta. The way to reduce these losses is to decrease the outlet temperature and the total volume of the exhaust gases. In the first case, heat recovery systems are used for the exhaust gases to decrease Th, or for air heaters to increase Ta. Also, superheaters or reheaters can be used for this purpose. The total exhaust gas volume reduction can thus be achieved by reducing excess air. In fact, this loss is minimal if the minimum air necessary for the combustion of a specific combustible is used.
Losses due to non-combusted gas occur on account of the presence of combustible gas, mainly CO. An LHV in a solid fuel can be calculated, according to its elemental analysis (dry basis), through Equation (3), as follows:
where P
C is the percentage, by weight, of carbon obtained in the ultimate analysis, P′
H2 is the hydrogen available to be burned, P
H2 is the percentage, by weight, of hydrogen obtained in the ultimate analysis, and P
S is the percentage, by weight, of sulfur obtained in the ultimate analysis; all of these values are expressed in kg of each element per kg of fuel. If the total combustion of carbon takes place, the following reaction occurs (see Equation (4)):
If combustion is incomplete (obtaining CO), the P
C value would be P
C∙(1 − x), and the following reaction takes place (Equation (5)):
where x is the burned gas ratio, (1 − x) is the unburned gas ratio (which is evolved), P′
H2 = P
H2 − P
O2/8 (for wet basis), and P′
H2 = P
H2 for dry basis. Thus, according to Equation (6):
the loss due to unburned gas (P
ub) would be obtained according to Equation (7), as follows:
where P
C is the amount of carbon in the utilized biomass. From the previous equation, it can be inferred that the lower the value of x, that is, the more incomplete the combustion, the higher the loss will be. Therefore, this loss will be avoided when carrying out a complete combustion. This can be achieved by pulverizing the fuel, provoking turbulence to increase the contact between the oxidizing agent and the fuel, providing a suitable excess air coefficient, increasing the heat in the stove, or increasing the combustion time.
Therefore, concerning these losses, an optimum excess air coefficient is needed to contribute to a maximum yield, as can be observed in
Figure 2. The indirect method through which to determine the yield of the process is based on the calculation of all losses that take place in the oven through Equation (8), as follows:
The graphical representation of loss through sensitive and latent enthalpy presents a disadvantage, that is, the impossibility of finding a general mathematical expression that correlates both losses according to excess air (n). For this reason, the only reliable way to represent these equations is through experimental sampling of each combustion facility, in order to test different excess air ratios.
1.3. Ostwald Diagram: Theoretical Foundations
In order to optimize combustion processes, a mathematical tool (developed by Wilhelm Ostwald and used in the literature for some time to determine the performance of biomass facilities [
20]) will be used with the fuel parameters obtained for each combustion stage. If the flue gas composition is known, in percentage, it is possible to represent the stage of the process. Thus, in
Figure 3, an example of an Ostwald diagram can be observed.
Thus, the two catheti, or legs, represent the values of VCO2 and VO2 in the exhaust gases, and the hypotenuse represents the complete combustion line. The triangle is divided, in turn, by the so-called air line, into two parts: one representing the incomplete combustion points due to the lack of air (triangle corresponding to the ordinate–abscissa–air lines), and another one representing the incomplete combustion due to excess air (triangle corresponding to the air–abscissa–complete combustion lines).
Regarding the complete combustion line, it is the geometric line corresponding to V
CO2 and V
O2, that is, corresponding to complete combustion (see Equation (9)).
Thus, this line has a value, at the abscissa axis, of 0.21, whereas its value at the ordinate axis is CO
2 max. This first point, CO
2 max, V
O2 = 0 (which corresponds to combustion in the absence of oxygen, that is, a neutral combustion with η = 1), depends on the kind of fuel (its composition), whereas the value at the abscissa axis is always 0.21, regardless of the fuel used and, therefore, it is common to every Ostwald diagram. Concerning the air line, it is the line whose excess air coefficient equals one. For its representation, previous equations are required. If n = 1 and CO
2 max and O
2 max are calculated, the intersection points with the abscissa and ordinate axis are obtained (Equations (10) and (11)) as follows:
Line n = 1 divides the triangle into two parts, the one on the left corresponding to incomplete combustion due to the lack of air, and the one on the right corresponding to incomplete combustion due to excess air. It should be noted that another property of line η = 1, as VCO = 2VO2, if the VO2 values are correlated to this line, the VCO scale is obtained by multiplying these values by two, which is a scale that can be projected over an additional axis, perpendicular to the hypotenuse of the triangle. Regarding the lines obtained when VCO is constant, even though these are theoretical curves, they can be considered as straight lines, as the difference is negligible. Thus, these lines are obtained by drawing parallel lines to the hypothenuse. When attributing other values to n and replacing it in the previous equations, the lines when n is constant are obtained.
1.4. Novelty and Aim of This Work
As previously explained, the role of a combustion process is vital to increasing the efficiency of domestic or industrial stoves and, subsequently, the sustainability of the process, reducing the amounts of pollutants released into the atmosphere, depending on factors such as biomass composition [
21,
22], previous conditions [
23], stove design (including its evolution to improve performance) [
24,
25,
26], or burning conditions [
27,
28,
29]. Thus, some studies have previously dealt with this subject, as can be observed in
Table 1. These efforts focused on evolved pollutants applied to the use of domestic or industrial stoves, or simulations [
30], in addition to focusing on variables such as the kind of biomass, biomass load, or pre-treatments, like wood washing or drying [
31].
In that sense, the novelty of this work lies in the optimization of a pellet stove, considering both the raw materials and parameters utilized, depending on the stove’s power type. Thus, an adapted optimization of a specific stove for each kind of biomass is offered, as well as an assessment of the influence of raw material on combustion optimization. Considering the above, in more detail, the aim of this work was to carry out the optimization of the combustion process in a commercial stove. For that purpose, different kinds of fuels were used at different air ratios and power levels, followed by assessing efficiency according to the flue gas flow and composition.