1. Introduction
Environmentally friendly biomass, replacing fossil energy carriers as a fuel, is becoming increasingly important for the health and safety of society and for economic growth. What is becoming particularly important is the energy management of waste containing organic substances from agricultural and forestry production, the processing industry, and human society [
1]. Based on its water content, biomass is divided into wet and dry biomass [
2]. Wet biomass is mainly organic materials with such a high water content that they are unsuitable for direct combustion. However, they may be converted into gaseous fuel in biogas plants [
3]. Dry biomass wastes are most frequently pelleted or briquetted to stabilise their physical parameters. Biofuel briquettes and pellets differ in many respects from conventional solid fuels, such as coal or coke, due to higher moisture content, lower calorific values, and the different content of additional components such as chlorine, sulphur, phosphorus, nitrogen, and metals, that they contain. These factors can affect the content of the emission components of the gases produced during their combustion and the properties of the ash [
4]. Thus, these particular characteristics of biomass fuels cause many challenges; at the same time, in many cases, they also provide benefits.
Commercially available solid fuel for combustion in domestic boilers is mainly sold in the form of pellets, which are produced chiefly from wood or waste containing wooden mass. Cereal straw and typical energy crops such as miscanthus are also used in large quantities for pellet production [
5,
6]. Any material amenable to pressure agglomeration, such as tobacco post-harvest waste, can also be used for this purpose [
7]. To improve the mechanical parameters of pellets made from straw, natural binders are used, such as, for example, flour or waste biomass from food industry production [
8,
9,
10].
Biomass combustion in heating boilers can contribute to the emission of excessive amounts of substances that are harmful to humans and the environment, such as particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NO
x), and sulphur oxides (SO
x) [
11,
12]. Polycyclic aromatic hydrocarbons (PAH) and volatile organic compounds (VOC) are also emitted in the flue gas [
13]. The fuel composition mainly influences emissions but can also be affected by the combustion method, flue gas recirculation, and the boiler air supply [
14,
15,
16]. At the same time, the shape of the combustion chamber or the support of biomass combustion with gaseous fuel may reduce the number of emissions produced in the boiler [
17].
Pellets made from waste biomass may also present problems during combustion due to the quality and composition of the resulting ash, e.g., thermal sticking of the ash may occur, causing clogging of the grate [
7,
12,
15,
18]. One of the more common methods to reduce the environmental costs of biofuel combustion is the use of catalysts. These can be placed in the boiler flue gas lines [
19] or as catalytic admixtures to the solid biomass. Catalysts in this form are used to improve the combustion quality by accelerating the oxidation process and to improve the ash quality and flue gas [
20,
21,
22,
23,
24]. In the combustion of fuels produced from biomass, attempts have been made to use metal compounds as catalysts, for example, iron, calcium, or aluminum oxides, as well as vanadium, magnesium, copper, and titanium oxides as sodium and lithium chlorides [
20,
25].
Research into the use of iron oxides as catalytic additives has been pursued due to their low cost, environmental friendliness, high reactivity, and susceptibility to accessible surface and structural modifications. Three forms of iron oxide are known to occur: (III) Fe
2O
3, which occurs in nature as iron ore and most commonly as haematite containing approximately 70% pure iron, iron (II) FeO oxide, which infrequently occurs as a mineral-its mineral form is known as wüstite, and iron oxide (I) Fe
3O
4, which occurs in nature as magnetite, so-called because of its strong magnetic properties [
26]. Iron oxides are used to improve the combustion quality of many fuels and combustible substances, for example, in thermite. Iron oxide nanopowders are used as a catalyst in the burning of solid composite rocket fuels, as well as for the oxidation of soot formed during the combustion of liquid fuels and plastics [
27,
28,
29,
30]. Iron oxide in the form of Fe
2O
3 can accelerate the combustion process by being an oxygen carrier [
31]. Materials consisting of iron oxides in combination with another metal can also be used as catalysts to assist in the oxidation of solid fuels or gases. Au/Fe
2O
3 catalysts prepared using the co-precipitation method positively affect CO oxidation at low temperatures, and assist the combustion of volatile organic compounds [
32,
33]. Attempts have also been made to use iron oxides together with minerals, for example, Fe
2O
3-coated olivine ((Mg,Fe)
2[SiO
4]), as a catalyst in the gasification of biomass alone and co-gasification of biomass with coal to crack/reform tar and to increase H
2 yield [
34]. The action of iron oxide and copper as a catalyst in the thermal processing of biomass is also known [
35].
Carbon monoxide is used as a feedstock for iron smelting using coke. In the presence of burning coal, iron oxides are reduced with carbon, carbon monoxide, and carbon dioxide. The reduction starts as early as 300 °C and continues until all the carbon is consumed or escapes as gases [
36]. Further heating in the presence of oxygen can re-oxidise the reduced iron by diffusing oxygen from the surrounding atmosphere and causing it to return to its previous form [
37]. It has been found that iron oxide can also be reduced by biomass, and the reduction is temperature dependent and strengthened at temperatures above 1100 °C [
38]. In the case of biomass, iron oxide reduction is also enabled by carbon and hydrogen, which contain materials of organic origin. Research to date has been conducted on a micro-scale in many cases and has not included an analysis of solid biomass combustion in boilers as a fuel mixed with Fe
2O
3.
The aim of the study was to verify the hypothesis of whether the use of Fe2O3 iron oxide as a catalytic additive for biomass fuel has an impact on reducing the resulting fuel combustion emissions of NOx, CO, and H2. An additional aim was to establish whether adding Fe2O3 to biomass could improve the quality of fuel combustion in the boiler by causing better after-combustion of combustible substances contained in the fuel. An assumption was accepted that, during combustion, there would be a reduction of oxygen from Fe2O3, presuming that pure iron would be produced from a part of the catalyst, and the free oxygen would assist the biomass combustion process by reducing the amount of oxygen taken from the air.
3. Results and Discussion
3.1. Fuel Analysis
Prior to the tests related to the combustion and determination of flue gas components, the pellet density and elemental composition of the biofuel were analysed. These tests were carried out on the base fuels only, without any addition of the catalyst: pellets made from wood–denoted as WP (Wood Pellets) in the following descriptions, and pellets made from furniture boards–denoted as FBP (Furniture Board Pellets). The density of the dried wood pellets was determined as 1.31 g·cm
−3, and that of the furniture board pellets was 1.25 g·cm
−3. The averaged results of the carbon, hydrogen, nitrogen, sulphur, and oxygen contents and the combustion heat value, converted to the dry mass of the prepared products, are shown in
Table 2. The ash, volatile and solid fuel contents, and combustion heat values are shown in
Table 3. The sum of volatile and solid fuels represented the organic mass contained in the biofuel.
The moisture content of all the prepared fuels was also determined using the dryer method, for each type of biofuel and each Fe
2O
3 content, in accordance with PN-PN ISO 18134-1: 2015-11. The results of the pellet moisture content are presented in
Table 4. The higher moisture content of the pellets with the catalyst was due to the technological conditions of the pellet preparation with iron oxide.
3.2. Emission Analysis
The combustion of all the fuels in the boiler was carried out at the same rotational speed of the screw conveyor feeding the fuel to the retort burner. Using the data from the Wöhler A 550 flue gas analyser, graphs were prepared of the dependence of the contents of oxygen, carbon dioxide, hydrogen, carbon monoxide, and nitrogen compounds in the flue gas on the proportion of the catalyst in the fuel. The graphs are shown in
Figure 3 and
Figure 4.
To illustrate the results, the box plot of the values where the transverse line represents the mean value, the upper and lower border of the box are the ± value of the standard deviation, and the whiskers represent the minimum and maximum values obtained in the measurements. In order to facilitate a comparison of the results, the graphs of the same emissions from the combustion of wood pellets and furniture board pellets are shown side by side. The graphs show the trend lines determined from all the data collected from the measurement. For these lines, the linear equations of the dependence of the emission from the catalyst content of the fuel were determined.
The value of the p-coefficient was also calculated to determine the significance of the impact of the catalyst content on the emission content. The regression equations and the values of the p-coefficient are presented in
Table 5.
By analysing the graphs in
Figure 3 and
Figure 4, it can be found that the level of emissions in the flue gases, CO
2, H
2, CO, and NO
x, in the combustion of furniture board pellets compared to the emission level of these substances in the burning of wood pellets was always higher. The high nitrogen oxide emissions produced during the combustion of furniture board pellets were the result of the high nitrogen content of this raw material (
Table 2). The results in
Figure 3 indicate that there was always an excess of oxygen during the combustion of each granulate, resulting in low levels of CO emissions (
Figure 4).
Any disruption in the combustion process manifested itself with increased CO and H
2 emissions, as seen in
Figure 4, at 5% Fe
2O
3 content during the combustion of furniture board pellets (FBP). The carbon dioxide content during the combustion of wood pellets and furniture board pellets was coordinated with the oxygen content in the flue gases. A decrease in the carbon dioxide content of the flue gas was associated with an increase in the oxygen content. A higher catalyst content in the fuel stimulated a reduction in the carbon dioxide content and a growth in the oxygen content. The critical level was the 10% Fe
2O
3 content in the fuel because, beyond this value, the amount of carbon dioxide in the flue gas started to increase, and the oxygen content decreased. Statistical analysis demonstrated a decrease in the CO content in the boiler flue gases and the significance of the effect of an increase of the catalyst content in the fuel on reducing CO content. As can be seen from the graphs, the decrease in carbon monoxide content in the gases was not very pronounced; very frequently, its content values did not increase despite an increase in the catalyst content.
The regression equations for the dependence of oxygen and carbon dioxide contents from the catalyst content in
Table 5 indicate that only oxygen contained in the air was involved in the combustion process. The coefficients at “x” describing the effect of the Fe
2O
3 content in the fuel on the oxygen and carbon dioxide contents when combusting the same fuel are almost identical, which confirms that oxygen extracted from the chemical decomposition of the catalyst did not participate in the combustion process.
An analysis of the carbon dioxide content in the flue gas was carried out to verify whether the catalyst content in the pellets affects the quality of pellet combustion. A comparison was made between the CO
2 contents obtained in the tests (
Figure 3) and the values determined by stoichiometric equations from the elementary composition of raw fuel. The computations were based on the CO
2 content in the flue gas from pellets with no catalyst. In the comparative calculations, the identical amounts of biomass were accepted as those in pellets with the addition of Fe
2O
3 while assuming that the percentage catalyst content reduces the biomass content in the fuel by the same percentage. An analysis of the carbon dioxide content in the flue gases from the combustion of wood and furniture board pellets is presented in
Table 6.
After comparing the calculated and experimentally obtained results, it can be concluded that the calculated values are close to the experimental ones, which means that the catalyst content did not affect the combustion process.
Analysis of the effect of Fe
2O
3 content on gas emissions: CO, NO
x, and H
2 based on
Figure 4 showed a clear decrease in the hydrogen content of the flue gas during the combustion of wood pellets only at 15% catalyst content, and in the case of slab pellets at 10% catalyst content. This is applied to both wood pellets and furniture board pellets. There was also a noticeable effect of the catalyst content of the fuel on NO
x emission values.
To verify whether the catalyst used had a significant impact on reducing the emissions of such gases as CO, NO
x, and H
2 in the original version of the emission results, the results of the content of these gases recorded during the experiments were divided by the results of CO
2 contents, taking this quantity as a measure of the intensity of the combustion process. After a statistical analysis and an analysis of variance, the results obtained are presented in
Figure 5. The trend lines for the emission values are given in the graphs, and the calculated values of the coefficients are shown in
Table 7.
An analysis of the graphs in
Figure 5 and the regression equations in
Table 7 indicates that adding Fe
2O
3 iron oxide to the biomass fuel reduces emissions of the following compounds produced during fuel combustion: NO
x, CO, and H
2. However, the positive effect of the catalyst on reducing hydrogen emissions occurred only at a content of 15% Fe
2O
3 in the fuel. The impact of the catalyst on NO
x emissions is significantly more substantial with furniture boards used as fuel than wood. There is a negligible reduction in CO emissions in wood pellet and furniture board combustion.
3.3. Temperature Analysis
A statistical study of the results of the temperatures obtained over the burner and the temperature of the gases in the duct leading these out of the boiler is shown in
Figure 6.
Figure 6.
Influence of Fe
2O
3 content in the fuel on the combustion temperature and boiler flue gas temperature during the combustion of wood pellets (WP) and furniture board pellets (FBP): (
a) Wood pellets (WP); and (
b) Furniture board pellets (FBP). The graph shows one combustion temperature for each type of pellet, calculated as the average results obtained from the three thermocouples placed above the burner. The regression equations for the trend line and the calculated values for the value of the p coefficient determined as a result of an ANOVA analysis are shown in
Table 8.
Figure 6.
Influence of Fe
2O
3 content in the fuel on the combustion temperature and boiler flue gas temperature during the combustion of wood pellets (WP) and furniture board pellets (FBP): (
a) Wood pellets (WP); and (
b) Furniture board pellets (FBP). The graph shows one combustion temperature for each type of pellet, calculated as the average results obtained from the three thermocouples placed above the burner. The regression equations for the trend line and the calculated values for the value of the p coefficient determined as a result of an ANOVA analysis are shown in
Table 8.
Table 8.
Regression equations for the trend line of the dependence of pellet combustion temperature and exhaust gas temperature on the Fe2O3 content of the combusted pellet.
Table 8.
Regression equations for the trend line of the dependence of pellet combustion temperature and exhaust gas temperature on the Fe2O3 content of the combusted pellet.
Temperature | Wood Pellet | Furniture Board Pellet |
---|
Regression Equation | p-Value | Regression Equation | p-Value |
---|
Combustion temp., °C | y = 808.8 − 2.2x | 0.0000 | y = 721.5 + 1.5x | 0.0000 |
Flue gas temp., °C | y = 327.9 − 3.8x | 0.0000 | y = 328.9 − 4.1x | 0.0000 |
By analysing the trend lines, it was found that the combustion temperature of the pellets containing wood decreased with the participation of the catalyst in the fuel, and the combustion temperature of the granulate from furniture board waste increased. By analysing the mean values of the combustion temperatures of the fuel and their standard deviations, it was noted that the combustion temperature remained basically at the same level, regardless of the catalyst content.
The fuel combustion temperature was the resultant flame temperature. The flue gas temperature was the result of the heat generated from burning the fuel pellets and the heat that the flue gas gave up to the water jacket. As the amount of the fuel decreased with the increase in catalyst content, the amount of heat generated from burning the pellets was also reduced. In the case of the flue gas temperature, this was a downward trend line.
The average combustion temperature of each type of pellet in each case was below 900 °C, and its average value oscillated between 800 and 700 °C, so this was not the iron smelting temperature [
36]. Oxygen reduction in the catalyst at this temperature was not very intensive [
38], so its effect on the combustion process and reducing carbon monoxide and hydrogen were not very high, either. The low combustion temperature also decreased the group of nitrogen oxides of thermal origin.
After the study, the chemical and physical parameters of the combustion residues taken from the boiler ash pan were analyzed. The content of individual chemical compounds in the ashes from the combustion of wood pellets and furniture board pellets, depending on the proportion of the catalyst, is listed in
Table 9.
Fe2O3 was the dominant chemical compound in the combustion residues of fuels containing iron oxide. A small amount of Fe2O3 was also found in the ashes from pellet combustion without adding a catalyst. This proportion of iron oxide is due to the natural composition of the raw material used in the base fuel.
The analysis of ash content and unburned fuel residues in ashes from the combustion of both types of pellets is presented in
Table 10. In order to be able to assess the amount of unburned fuel in the residue material from the combustion of pellets without Fe
2O
3 and with this catalyst, the theoretical amount of organic matter that was burned to give an ash content identical to that in
Table 10 was determined. For this purpose, the ash content of both materials (WP and WBP, respectively) from
Table 3 was used. The Fe
2O
3 content of the pellet was also included in the calculation as the mineral component of the ash. The ratio of the unburned fuel (Total fuel of Ash from
Table 10) to the total calculated organic matter contained in the burned corresponding type of pellet was then determined. The results of the calculations are also shown in
Table 10 in the last column.
A comparative analysis of the ratio of unburned fuel in the ashes from pellet combustion with and without Fe2O3 content also indicates that the addition of a catalyst to the fuel pellet may have influenced changes in combustion quality but in a non-significant way. There was a slight increase in the unburned organic matter when burning wood pellets with Fe2O3 content compared to pellets without iron oxide (from 0.09% to 0.22%). There were also small changes in the ratio of dry organic matter in the residue from the combustion of pellets made from furniture board without iron oxide to the residue from the combustion of a similar pellet with an amount of this mineral (0.50 to 0.31%). In this case, there was a slight reduction in the amount of unburned fuel in the pellet combustion residues.
It is natural to have small amounts of unburned fuel in the residue from the combustion of solid fuels in boilers. During the combustion of fuels in heating boilers, there is also natural for this process to be disrupted, manifesting as temporary increases in the hydrogen and carbon monoxide content of the flue gases emitted (
Figure 5b).
It was impossible to determine from the Fe2O3 content of the remaining ash from combustion whether oxygen reduction occurred in the catalyst used and how it affected the emission results, as reoxidation of free iron may have occurred as the ash cooled. The oxygen reduction effect of the iron oxide may have happened during hydrogen and carbon monoxide reduction in the flue gas.