Biomass from Green Areas and Its Use for Energy Purposes

Abstract

In the current situation, fossil fuels are the primary source for electricity production. As a result of activities related to environmental protection, other sources are also used to produce energy. One of the renewable sources is biomass, which is becoming more and more popular for economic reasons. Biomass produced in green areas is a source of energy that has not been used in an appropriate way so far. This scientific article presents the possibility of using biomass from parks and gardens for the production of pellets and the assessment of their properties in terms of the possibility of using them for energy purposes. Coniferous sawdust was an additional component of the pellets. The produced pellets were tested for mechanical, thermogravimetric, and calorimetric properties. It was found that pellets made of biomass consisting of fir (493.12 N) and pine (450.84 N) cones with an addition of coniferous sawdust were the most resistant to mechanical damage. The amount of ash in the analyzed pellets was below 3%, and their calorific value ranged from 16.95 to 19.54 MJ·kg⁻¹. Additionally, during pellet combustion, the lowest emission of sulfur dioxide was recorded for pellets made of sawdust from coniferous trees and acorns (1.01 mg·m³), while the lowest emission of nitrogen oxides was recorded for pellets made of a mixture of coniferous sawdust and pinecones (65.33 mg·m³). The emission of the tested gases decreased as a result of the addition of coniferous sawdust to the tested types of biomass. On the basis of the conducted research, it was noted that waste biomass formed in green areas can be a raw material for energy production.

1. Introduction

The demand for electricity is constantly growing, and it is caused by civilization and technical progress. Currently, fossil fuels cover about 80% of the world’s energy demand [1]. Fossil fuels in the future will not be sufficient to cover the energy demand, therefore the future for the global economy may be renewable energy sources, which include, among others, energy from biomass [2]. Biomass is the oldest and most widely used renewable energy source. It is possible to convert biomass into various types of biofuels or energy using thermal, physical, and biological processes [3,4]. Biomass is a fuel that is easy to obtain and process. Biomass energy conversion technologies are relatively cheap, simple, and failure-free. Taking into account the environmental aspects in particular, there has been a continuous increase in obtaining energy from biomass using various technologies [5].

The promotion of the use of energy from renewable sources is included in the European Union Directive of 23 April 2009. This document defines the term biomass. According to this legal act, biomass is biodegradable waste or residues of biological origin from agriculture or forestry, as well as municipal waste of plant or animal origin [6]. The currently implemented energy strategy of the European Union is aimed at minimizing the harmful effects of energy technologies on the environment, keeping energy prices at their lowest possible level, and maintaining the security of the energy supply [7].

In accordance with the Green Book “Framework for climate and energy policy until 2030”, in order to avoid further degradation of the environment, it is necessary to gradually abandon conventional energy sources and increase the share of renewable energy sources [8]. Changes in the energy market resulting from the abandonment of conventional energy sources have now resulted in an increase in its prices. In addition, there have also been cases of negative impact of RES (renewable energy sources) on the operation of power systems. The recommended solution is to increase the share of biomass in energy conversion processes. The result of this action will be the preservation of the durability of the conventional system, taking into account the environmental protection resulting from the EU energy policy [9].

There are many types of biomass, such as wood and wood biomass, herbaceous biomass, aquatic biomass, and animal and household waste, which can be used to produce electricity and heat.

Table 1. Chemical composition of biomass used for energy purposes [10].

Type of Biomass	C (%)	H (%)	N (%)	S (%)	Volatile Compounds (%)	Ash (%)
Wood and woody biomass	49–57	5–10	<1–1	<1–1	30–80	1–8
Herbaceous biomass	42–58	3–9	<1–3	<1–1	41–77	1–19
Aquatic biomass	27–43	4–6	1–3	1–3	42–53	11–38
Animal and human waste biomass	57–61	7–8	6–12	1–2	43–62	23–34

presents the current state of knowledge on the parameters determining the quality of biomass used for energy purposes. On the other hand,

Table 2. Raw materials used for the production of solid fuels and their calorific value.

Biomass Waste	Calorific Value MJ·kg−1
Wood chips from vineyard pruning [11]	18.07
Corncob [12]	17.0
Olive husk [12]	20.9
Sunflower shell [12]	18.0
Walnut shell [12]	21.6
Hazelnut shell [12]	20.2
Tomato residue [12]	11.3
Wood bark [12]	20.5
Barley straw [13]	18.2
Sugarcane bagasse [13]	20.0
Pine bark [14]	20.4
Cotton stalks [14]	19.0
Bagasse [14]	21.2
Wood chips [14]	20.9
Rice husk [14]	15.1
Pine sawdust [15]	18.3
Saw dust [16]	17.7
Forest leaves [16]	12.2
Palm leaves [16]	15.4
Kanjaru weed [17]	9.8
Perry grass [17]	14.5
Bamboo leaves [17]	15.7

presents a list of selected raw materials used for the production of biofuels and their calorific values.

In addition to the commonly used types of biomass, such as wood and energy plants, there are other materials that need to be used to generate electricity. These include both household waste and residues from the care of urban greenery, falling leaves, grass, and tree branches [18,19]. The term “urban greenery” means vegetation complexes, especially parks and greenery in squares, streets, and allotment gardens. All managed waste resulting from plant care, such as leaves, mowed grass, tree branches, flowers, weeds, and seeds, is extremely important in economic and ecological terms. In most cases, this biomass, after being collected, is transported to landfills and composted there. It is worth paying attention to the energy potential of this raw material and its use as a source of energy because waste in green areas is generated regularly and throughout the year [2].

The management of waste from green areas Is a topic that is currently being discussed in many countries. In Italy, biomass from urban green areas is considered “hazardous waste”. This biomass is properly processed and disposed of [20]. In the United States of America, the great potential of biomass from urban greenery has been noticed. It was concluded that additional research is needed to determine its potential [21]. Green biomass is not used for energy purposes, and specialized companies deal with its disposal in Germany [22]. In Spain, attention has been paid to the potential of biomass from the maintenance of ornamental plants and urban trees [23,24,25]. The most popular form of waste management for green areas is composting [26], which results in the production of fertilizer [27]. Obtaining energy by incinerating waste from green areas is another method of managing this type of waste. In order to obtain energy, biomass that is fragmented or in the form of briquettes or pellets has been used [2]. Another way to use green waste is the production of biogas. Grass is best suited for this purpose because it does not contain too many impurities [28].

In order to valorize plant biomass, the process of so-called torrefaction is used. This involves gentle roasting of the biomass at a temperature of approx. 200–350 °C without access to oxygen, under atmospheric pressure. As a result of this process, a solid product is obtained, the so-called torrefied biomass, which, compared to raw biomass, has more favorable physicochemical properties when used as fuel in the power industry. In addition, torrefied is more durable and more homogeneous than solid biomass, which is important when using plant biomass [29]. Jarunglumlert et al. [30] studied the properties of sugar cane pellets subjected to dry and wet torrefaction. They found that the wet torrefaction process can significantly reduce the ash content of the finished product (ash content below 1% at a torrefaction temperature above 180 °C), resulting in a higher quality of the solid fuels tested. In turn, the calorific value of dry torrefied pellets was higher by 0.27 MJ compared to wet torrefied pellets. The production of pellets from heat-treated bagasse has been shown to be economically viable.

Another means of biomass valorization is the granulation and drying of compost from agricultural biowaste. As a result of research, a solution was designed that allows one to reduce the storage costs and environmental impact of biocompost. Granulation and drying is the technique to achieve these goals. In this work, the influence of the parameters of the drying and pelleting process on the density, crushing energy, and moisture diffusion in pellets produced from agricultural bio-waste was examined. In addition, the reaction of the soil and plants (i.e., basil) to the optimal dose of biocompost pellets was assessed. The biocompost pellet evenly released 80% of its nitrogen within 98 days. The applied solution can reduce the environmental impact of compost from agricultural bio-waste by more than 63%. The pelleting and drying process avoids methane emissions from unprocessed agricultural bio-waste compost. The proposed solution for the management of compost from agricultural bio-waste is a promising method of improving environmental and soil conditions on farms [31].

The aim of the work was to use waste biomass from parks and gardens for the production of pellets and to evaluate their properties in terms of the possibility of using them for energy purposes.

2. Materials and Methods

2.1. Materials

In this work, biomass from parks and gardens was used as a substrate for the analyzed pellets. Cones of silver fir (Abies alba Mill.) and Scots pine (Pinus sylvestris L.), chestnut nuts (Aesculus hippocastanum L.), and acorns of pedunculate oak (Quercus robur L.) were used in the study. The material for laboratory analyses were pellets produced in the technical laboratory of the University of Rzeszow

Scots pine and silver fir cones, chestnut nuts, and pedunculate oak acorns were collected in city parks and home gardens. In connection with the problem of managing the biomass generated during the growth and development of the above-mentioned trees, and in order to make practical use of the waste from the obtained biomass, solid fuel was produced and used for laboratory analyses. The biomass collected in parks and gardens was dried under the roof of an open shelter and ground in an Essa—CM 1000 biomass mill (Atest Sp. z o. o., Kielce, Poland). The moisture content of the biomass used for the production of pellets after drying ranged from 4.18% to 4.45%. An addition in the production process was coniferous sawdust, the humidity of which was 4.25%.

Then, eight types of pellets with different compositions were made in the Prime 200 granulator (Techno-MaszBud, ciemne, Poland):

• − 100% chestnuts—pellet P1;
• − 50% coniferous sawdust and 50% chestnuts—pellet P2;
• − 100% acorns—pellet P3;
• − 50% coniferous sawdust and 50% acorns—pellet P4;
• − 100% pine cones—pellet P5;
• − 50% coniferous sawdust and 50% pine cones—pellet P6;
• − 100% fir cones—pellet P7;
• − 50% coniferous sawdust and 50% fir cones—pellet P8.

In the measurements of mechanical properties, 50 pieces of each type of pellet, each with a length of 18 ± 1 mm, were used.

2.2. Static Tests of Pellets

In the strength laboratory, the mechanical properties of the eight types of pellets were tested in the horizontal axis in the process of uniaxial compression. Static tests were performed on a Zwick/Roell Z010 testing machine (Zwick Roell Polska Sp. Z o.o. Sp. K., Wroclaw, Poland) in accordance with the methodology developed by Gorzelany et al. [32]

2.3. Physicochemical Analyses

As part of the conducted research, the basic physicochemical parameters of the analyzed solid fuels were defined. The content of ash and volatile substances and the calorific value, as well as the total content of nitrogen, hydrogen, and carbon, were determined using the LECO TGA 701 thermogravimeter, the TrueSpec LECO CHN elemental composition analyzer (Leco, St. Joseph, MI, USA) and LECO AC 500 isoperibolic calorimeter (Leco, St. Joseph, MI, USA) in accordance with the methodology presented in [33].

The measurement of the content of carbon monoxide (CO), carbon dioxide (CO₂), nitrogen oxides (NO_x), and sulfur oxide (SO₂) was carried out using two ULTRAMAT 23 gas analyzers—A1 and A2. As a result of the use of two compatible analyzers, it was possible to examine all four compounds simultaneously, because one device is able to measure the concentration level of up to three active ingredients in infrared (e.g., CO, CO₂, NO_x, SO₂, CH₄, or freons) and determine the concentration of oxygen with an electrochemical cell. The test consisted of burning 100 g of the sample under controlled conditions at 800 °C using the LECO TGA 701 device in conjunction with the ULTRAMAT 23 system. This process lasted about 20 min until the selected fuel was completely combusted.

2.4. Statistical Analysis

Statistical analysis was carried out using the STATISTICA 12.5 PL software by StatSoft. For all analyses, a significance threshold of ≤0.05 was set. The results of laboratory tests were analyzed individually for each pellet. One-way analysis of variance (ANOVA) was used to check the significance of the interaction of diverse types of biomass obtained from green areas on the quality parameters of the pellets produced

2.5. Experimental Uncertainty

To analyze the repeatability and validity of the experimental data, uncertainty analysis was carried out [34]. This analysis was performed for the main parameters, which are summarized in

Table 3. Uncertainty values for the main parameters.

Parameter	Uncertainty
Destructive force (Fmax)	37.60 N
Energy needed to destroy pellets (W to Fmax)	1.33 MJ
Ash content	0.00068%
Volatile compounds	0.0021%
Calorific value	0.00022 MJ·kg−1
Nitrogen	0.0015%
Carbon	0.0018%
Hydrogen	0.004%
CO2	0.53 mg·m3
CO	0.00027%
SO2	0.00034 mg·m3
NOx	0.51 mg·m3

. The obtained uncertainty values were within acceptable ranges. It is worth noting that uncertainty values lower than 5% are considered acceptable for experimental study [35,36].

3. Results and Discussion

3.1. Measurement of Mechanical Properties

Mechanical properties determine important quality parameters of the tested solid fuels. Taking into account the process of automating the feeding of fuel to the boiler, the mechanical parameters provide an answer as to whether a given type of fuel will be appropriate for such a method of use. This article includes the outcomes of tests of the mechanical properties of the eight pellet types, in order to define the suitability of the analyzed biomass for their production. The analyses were performed 24 h after pellet production (see

Table 4. Mechanical parameters of the tested pellets in the horizontal axis compression process.

Pellet Type	Fmax (N)	W to Fmax (MJ)	Relative Deformation ε
P1	249.00 a ± 67.40	34.90 a ± 5.79	0.311 a ± 0.055
P2	282.00 a ± 66.30	77.60 b ± 19.70	0.562 b ± 0.055
P3	74.60 a ± 11.10	15.70 a ± 0.932	0.311 a ± 0.074
P4	63.60 a ± 18.80	11.30 a ± 0.582	0.262 a ± 0.074
P5	408.00 a ± 95.70	74.80 a ± 11.40	0.351 a ± 0.054
P6	451.00 a ± 141.00	108.00 b ± 35.40	0.441 a ± 0.141
P7	439.00 a ± 66.80	104.00 a ± 34.30	0.472 a ± 0.111
P8	493.00 a ± 135.00	119.00 a ± 39.40	0.511 a ± 0.131

Statistically significant differences marked by different letters (p ≤ 0.05). The data were analyzed separately for each type of material.

).

The values of mechanical parameters varied for the tested pellets. The lowest values of destructive force (F_max) were found for acorn pellets (P3 and P4). With regard to the admixture of coniferous sawdust, the lowest destructive force was recorded for these pellets (P4). In the case of solid fuels produced from chestnuts (P1, P2), it was found that pellets made of this type of material are definitely more resistant to damage compared to pellets from acorns (P3 and P4). On the other hand, pellets made of fir cones with the addition of coniferous sawdust (P8) were the most resistant to damage. The damage-causing force for this type of fuel was 493 N. For solid fuels made of pine cones (P5) and pine cones with the addition of coniferous sawdust (P6), similar destructive force values were noted as in the case of pellets of fir cone (P7) and fir cones with the addition of sawdust from coniferous trees (P8). The addition of coniferous sawdust to chestnut (P2), pine (P6), and fir (P8) resulted in a somewhat higher resistance to mechanical damage in comparison with solid fuels without the addition of coniferous sawdust (P1, P5, and P7).

Analyzing the results of pellet tests, it was found that the energy needed to destroy pellets (W to F_max) corresponds to the values of the damage-causing force. The lowest value of energy until destruction was characteristic for acorn pellets with an admixture of sawdust (P4), and the highest for pellets made of fir cones and sawdust from coniferous trees (P8). For chestnut (P2), pine (P6), and fir (P8) pellets, a 50% addition of sawdust from coniferous trees raised the value of the parameter under testing. In the case of acorn pellets, a 50% addition of sawdust from coniferous trees (P4) decreased the quantity of energy needed to destroy this pellet compared to pellets made of 100% acorns (P3). The relative deformation values up to the moment of destruction of the tested pellets were diversified. Pellets with the addition of coniferous sawdust were characterized by an increased value of the tested parameter (P2, P6, and P8) in relation to solid fuels made of 100% homogeneous materials (P1, P5, and P7). The exception here is acorn pellets: as a result of the addition of sawdust, the relative deformation parameter (P4) decreased compared to pellets that were 100% made of acorns (P3).

The mechanical properties of pellets depend in particular on the type of materials used for their production, the water content, the chemical composition of raw materials, and the addition of an adhesive [37,38]. Pellets made of raw materials containing large amounts of starch are characterized by greater resistance to mechanical damage. Starch is an excellent binder in the pelletizing process. The addition of coniferous (especially spruce) sawdust during the production of pellets has a positive effect on the durability of solid fuels, which increases the pellets’ resistance to mechanical damage [37]. Pellets made of waste biomass produced during the cultivation of sunflower and tobacco were distinguished by high resistance to mechanical damage. For the analyzed pellets, the destructive force oscillated in the range from 601.53 to 788.71 N. The addition of fir sawdust from coniferous trees during the production process of pellets from waste biomass of sunflower and tobacco improved the strength parameters of the tested pellets [33]. Analyzing the destructive force for pellets made of coniferous and deciduous sawdust and beech wood, it was found that 166 N to 654 N was needed to destroy different types of pellets [38]. The mechanical strength of pellets produced from straw increases as a result of the use of an additive in the form of coniferous sawdust during the production process [39]. It is advantageous to use different biomass mixtures for the production of pellets. Pellets made of more than one raw material are characterized by greater resistance to mechanical damage compared to pellets from a single type of biomass. Wood biomass pellets are more resistant to damage during transport and storage compared to agricultural biomass pellets [40].

3.2. Results of Calorimetric and Thermogravimetric Tests

Table 5. Ash content, volatile compounds, and calorific value of tested biomass pellets.

Pellet Type	Ash Content (%)	Volatile Compounds (%)	Calorific Value (MJ·kg−1)
P1	1.88 a ± 0.0911	78.9 a ± 0.611	17.5 b ± 0.111
P2	1.62 a ± 0.0422	80.2 b ± 0.101	16.9 a ± 0.0422
P3	1.81 a ± 0.102	79.3 a ± 0.122	19.5 a ± 0.0512
P4	1.76 a ± 0.0623	79.9 a ± 0.121	19.1 a ± 0.112
P5	1.40 a ± 0.211	76.8 a ± 0.0911	19.3 b ± 0.162
P6	2.02 b ± 0.131	77.9 b ± 0.0192	18.9 a ± 0.0635
P7	1.27 a ± 0.0821	74.7 a ± 0.0712	18.6 a ± 0.0554
P8	2.05 b ± 0.101	75.9 b ± 0.142	18.1 a ± 0.0221

Statistically significant differences marked by different letters (p ≤ 0.05). The data were analyzed separately for each type of materials.

presents the results for the percentage content of ash and volatile substances, as well as the calorific value of solid fuels containing waste biomass from green areas. In the case of the analyzed pellets, the ash values were at a very low level. Pellets with 100% fir cones (P7) had the lowest ash content. The highest amount of ash was recorded in solid fuels composed of 50% fir cones and 50% sawdust from coniferous trees (P8). In the case of chestnut and acorn pellets, the addition of coniferous sawdust reduces the ash content in pellets P1, P2, P3, and P4. The opposite tendency was observed for pellets from pine and fir cones. The addition of coniferous sawdust increased the ash content (P6 and P8) compared to pellets without this component (P5 and P7).

For volatile compounds, slight differences were noted between the tested types of pellets. The highest sum total of volatile compounds (80.20%) was found for pellets were made of 50% coniferous sawdust and 50% chestnuts (P2). On the other hand, the smallest amount of volatile compounds was recorded throughout the combustion of pellets made of fir cones (P7—74.64%). In the case of the analyzed pellets, the admixture of sawdust from coniferous trees increased the sum total of volatile compounds (P2, P4, P6, and P8) in comparison with pellets made without this addition (P1, P3, P5, and P7).

In the case of calorific value, its high value was found at the level of 16.95 MJ·kg⁻¹ to 19.54 MJ·kg⁻¹ (Table 5). Pellets made of 100% acorns (P3—19.54 MJ·kg⁻¹) had the highest calorific value. Pellets made of 100% pinecones (P5—19.30 MJ·kg⁻¹) also had a high calorific value. In the case of the analyzed biomass, the addition of sawdust from coniferous trees slightly reduced the calorific value of the tested pellets. In turn, the lowest calorific values were found in chestnuts pellet (17.5 MJ·kg⁻¹) and chestnuts pellet with 50% coniferous sawdust (16.9 MJ·kg⁻¹).

According to literature reports, the total ash content in wood pellets varies. In general, woody biomass and herbaceous biomass have ash contents of 1–8% and 1–19%, respectively [10]. The ash content in wood pellets is 0.55%, while the bark content is a maximum of 10% in the composition of this raw material [41]. Analyzing the pellets made of coniferous and deciduous sawdust, the ash content in the analyzed fuels was found to range from 0.31 to 0.56% [32]. Impurities present in the biomass for the production of solid fuels affect the high ash content in the produced pellet [41,42,43]. Pellets from agricultural waste were characterized by an ash content above 4%, and the addition of sawdust from coniferous trees in the amount of 50% significantly reduced the ash content in the tested pellets [33]. Pellets made from forest biomass (common spruce) are characterized by a significantly lower amount of ash compared to pellets whose basic component is leafy biomass (raspberry and red currant) [44].

The raw material used for the production of pellets is the main factor that determines the calorific value of the solid fuel produced. According to the available literature, coniferous wood has a higher calorific value compared to hardwood [45]. Solid fuel pellets produced from waste of agricultural origin are characterized by a calorific value in the range of 16.72–17.70 MJ·kg⁻¹ [33]. Scientists using household waste and raw material of wood origin made fuel mixtures whose calorific value oscillated around 16 MJ·kg⁻¹. With an increase in the share of raw material of wood origin in the pellet, the calorific value increased [46]. The analysis of 20 types of solid fuels available in Europe showed that wood pellets made of coniferous raw material had a calorific value above 18 MJ·kg⁻¹ [47]. The available studies show that the calorific values of the selected types of biomass are varied and amount to the following: for castor stalk, 14.6 MJ; for perry grass, 14.5 MJ; for rice husk, 15.1 MJ; for palm leaves, 15.4 MJ; and for saw dust, 17.7 MJ [14,16].

Solid biofuels have a high volatile content, on average 67% [48]. Waste that is generated in the production of sunflower seeds and is used for energy purposes produces large amounts of volatile substances at a level from 70.4% to 77.1% [49]. Studies on pellets made from agricultural waste have shown that they have volatile matter from 64.54% [50] to as high as 87.75% [51]. Wood pellet made of Norway spruce wood was characterized by an amount of volatile compounds at a level of slightly higher than 82%. Much lower levels of volatile compounds were recorded for pellets produced from raspberry and blackcurrant biomass: 78.09% and 76.25%, respectively [44]. Other studies on wood biomass pellets indicate that 67.60% of volatile compounds were recorded during the combustion process [52]. The content of volatile compounds in green biomass varies, ranging from 41 to 77%, while wood biomass has a higher content of volatile compounds—up to 80% [10].

Table 6. The content of nitrogen, carbon, and hydrogen for the analyzed pellets produced from biomass.

Pellet Type	Nitrogen (%)	Carbon (%)	Hydrogen (%)
P1	0.991 a ± 0.0311	46.3 a ± 0.211	6.43 a ± 0.311
P2	0.882 a ± 0.0111	45.2 a ± 0.412	6.83 a ± 0.921
P3	1.060 a ± 0.0421	50.6 a ± 0.282	6.71 a ± 0.111
P4	0.961 a ± 0.0512	50.6 a ± 0.272	6.72 a ± 0.821
P5	1.150 a ± 0.111	52.2 a ± 0.614	6.24 a ± 0.182
P6	1.170 a ± 0.131	51.7 a ± 0.352	6.31 a ± 0.132
P7	1.200 b ± 0.0521	49.5 a ± 0.233	6.13 a ± 0.212
P8	0.991 a ± 0.0621	48.2 a ± 0.172	6.20 a ± 0.181

Statistically significant differences marked by different letters (p ≤ 0.05). The data were analyzed separately for each type of materials.

presents the results of the percentage content of total nitrogen, total carbon, and hydrogen for individual solid fuels.

The level of total nitrogen in the analyzed samples was quite even and ranged from 0.88% to 1.20%. The addition of sawdust to the biomass obtained from green areas reduced the nitrogen content (P2, P4, and P8) in comparison with solid fuels composed of 100% biomass of chestnuts, acorns, and fir cones (P1, P3, and P7). The exception was pine biomass pellets. The admixture of sawdust (P6) slightly increased the content of this element compared to this type of pellet without the addition of sawdust (P5).

The percentage of total carbon content varied and depended on the composition of the pellets and the type of materials. The admixture of sawdust from coniferous trees did not importantly affect the carbon content in the analyzed solid fuels. Moreover, the percentage of hydrogen content in the analyzed solid fuels did not differ importantly. On the other hand, the addition of coniferous sawdust caused an increase in the hydrogen content for the analyzed pellets (Table 6).

Regarding the chemical composition of the biomass used for the production of pellets, it should be stated that it is diverse. In biomass from agriculture, which is intended for the production of solid fuels, the chemical composition is dominated by carbon [46]. The chemical composition of sunflower husk pellets is dominated by carbon and constitutes about 70%, nitrogen 9.1%, and hydrogen 8.8%. Sugar cane bagasse used for pellet production contains about 45% carbon, 12% hydrogen, and very low nitrogen content, below 0.5% [53,54,55].

The addition of coniferous sawdust to the material from which pellets are made reduces the nitrogen content in the final solid fuel by over 50%. In addition, there was no relationship between the addition of sawdust from coniferous trees and the content of carbon and hydrogen [33]. In turn, pellets made of Norway spruce contain 50.67% carbon, 7.16% hydrogen, and negligible amounts of nitrogen, at a level of 0.08%. In addition, pellets from deciduous biomass produced during the pruning of blackcurrants and raspberries contain, respectively: 55.13% and 52.30% carbon; 7.42% and 7.32% hydrogen, and 0.07% and 0.18% nitrogen [46].

Table 7. Average values of gaseous pollutants generated during pellet combustion.

Pellet Type	CO (mg·m3)	CO2 (%)	SO2 (mg·m3)	NOx (mg·m3)
P1	92.2 b ± 4.47	1.150 a ±0.06	1.44 b ±0.0721	97.3 a ± 4.87
P2	82.7 a ± 3.96	1.010 a ±0.05	1.29 a ±0.0711	91.2 a ± 4.56
P3	89.4 b ± 4.61	1.020 a ±0.05	1.11 a ±0.0623	96.9 b ± 4.84
P4	79.2 a ± 4.13	0.981 a ±0.05	1.01 a ±0.0632	88.9 a ± 4.45
P5	110.0 a ± 5.51	1.330 a ±0.07	2.68 a ±0.131	81.1 b ± 4.06
P6	102.0 a ± 5.07	1.250 a ±0.06	2.11 a ±0.111	65.3 a ± 3.27
P7	127.0 b ± 6.34	1.170 a ±0.06	4.12 b ±0.0912	119.0 b ± 5.61
P8	114.0 a ± 5.71	1.010 a ±0.05	3.24 a ±0.121	106.0 a ± 5.82

Statistically significant differences marked by different letters (p ≤ 0.05). The data were analyzed separately for each type of material.

contains data on gas emissions during the combustion of solid fuels from biomass.

Analyzing the research results, several significant relationships were observed. As a result of adding coniferous sawdust to the raw materials, a reduction in the amount of emitted gases was noted. For nitrogen oxides, the lowest emission was tested for solid fuels made of 50% pinecones and 50% coniferous sawdust (P6). On the other hand, the highest pollution of nitrogen oxides was recorded for solid fuels made out of fir cones— 119.14 mg·m³ (P7). Pellets made of chestnuts and acorns have sulfur dioxide emissions below 2.0 mg·m³. Pellets from fir cones were characterized by the highest emission of sulfur dioxide—4.12 mg·m³ (P7). In the case of carbon dioxide, pellets made of acorns and coniferous sawdust (P4) were characterized by the lowest emission of this gas. When burning the remaining pellets, the carbon dioxide emission was above 1%. Carbon monoxide emission for pellets P1–P4 was below 100 mg·m³. In turn, as a result of the combustion of pellets P5–P8, the emission of this gas exceeded 100 mg·m³. Taking into account the emission of sulfur dioxide and nitrogen oxides, pellets made of fir cones (P7) had the highest emission among the tested solid fuels (Table 7).

The emission of gases depends primarily on the composition of the wood pellets subjected to the combustion process. When burning waste biomass, we recorded various amounts of emitted gases. During the combustion of corn cobs, the emissions to the atmosphere were about 300 mg·m³ of carbon monoxide and 50 mg·m³ of nitrogen oxides [56]. As a result of burning coniferous wood, we note lower nitrogen oxide emissions compared to hardwood. This fact is confirmed by the results of emissions into the atmosphere of 30% less nitrogen oxides during the combustion of coniferous pellets compared to hardwood pellets [57].

Numerous scientific reports indicate that the formation of nitrogen oxides is determined by the type of solid, liquid, or gaseous fuel combusted in various types of boilers [58,59]. NO_x emissions depend largely on the amount of nitrogen in the fuel [60], and this is important when combusting nitrogen-rich solid, liquid, or gas fuels [61]. Pollutants such as NO_x and particulate matter are the result of nitrogen, potassium, chlorine, calcium, sodium, magnesium, phosphorus, and sulfur in the fuel [62]. The emission of nitrogen oxides during the combustion of pellets depends on the raw material from which the fuel was made. During the combustion of hardwood pellets, scientists recorded NO_x emissions to the atmosphere at a level of 265.86 mg·m⁻³. However, when burning pine pellets, the emission was significantly lower and amounted to 184.83 mg·m⁻³ [57]. Polonini et al. [63] studied the emission of pollutants in the form of CO and NO_x during the combustion of ENplus-certified pellets. As a result of the applied exhaust gas recirculation system, they recorded CO and NO_x emissions below 100 mg·m³ and 10 mg·m³, respectively.

4. Conclusions and Perspectives

Due to the growing demand for electricity, the consumption of fossil fuels is growing, which is associated with the emission of greenhouse gases that pollute the natural environment. Considering the huge amounts of biomass produced in parks and gardens, it is worth paying attention to its potential and use as a source of renewable energy. By producing pellets from this type of raw material, we obtain a solid fuel that is environmentally friendly.

The results of analyses of pellets produced from biomass obtained from green areas (i.e., chestnuts, acorns, and pine and fir cones) indicate that these types of biomass can be used for energy purposes. The results of mechanical properties tests indicate that pellets made of chestnut and pine and fir cones are of appropriate quality to be used in boilers with an automatic feeding system. The low value of the destructive force for acorn pellets proves that this type of pellet is very susceptible to crushing and breaking, which significantly hinders its storage, transport, and use in the power industry. In the case of the mixture of sawdust from coniferous trees, no increase in the mechanical strength of the analyzed pellet was noted. It would be advisable to use other admixtures with the raw material of acorns in order to check the resistance to mechanical damage of the produced pellet.

The calorimetric analysis showed that the tested solid fuels are of high quality. This is evidenced by the very low ash content for the analyzed types of pellets—below 3%—and their high calorific value. The emission of NO_x and SO₂ in the combustion of pellets corresponds to the current emission standards for fuels produced from biomass in the area of the European Union.

Biomass from green areas is mainly composted in landfills. Determining its energy quality allows one to determine the direction of its use and thus solve the problem of undeveloped biomass from green areas. The research results are promising for an attempt to produce solid fuels from biomass and use it in small household installations and in the power industry. The use of waste biomass, and thus the replacement of non-renewable raw materials perfectly, fits into the ideas of the circular economy. The depletion of natural resources and the increase in their prices are the main reasons for the creation of the concept of a circular economy. Biomass is considered carbon-neutral because it is assumed that its combustion emits the same amount of carbon dioxide that is fixed in the next vegetation period during plant growth. Therefore, it is claimed that biomass or biofuels have a closed carbon dioxide cycle. It is worth paying attention to the problematic issue of biomass production. It is important that it complies with the principles of sustainable development. Appropriate management of waste from green areas will reduce environmental pollution resulting from composting and thus mitigate climate change. Another good example of the idea of closed-loop waste management is the use of leachate from landfills as a medium for anaerobic digestion of coal substrate. As a result of this process, methane is used for energy production and fertilizer.

The presented results require further research in order to obtain information about the potential of biomass from green areas, which has not been fully understood and used so far. In addition, it is important to disseminate information about the high quality of fuels produced from waste biomass from green areas that have been tested so far. It is worth paying attention to other types of biomass produced in urban and rural green areas as well. There are many different wastes generated during maintenance work, the properties of which should be tested in order to determine their suitability for energy purposes.

This experiment was a pilot study in order to demonstrate the usefulness of the analyzed biomass materials for the possible industrial production of solid fuels. Based on the obtained results, the raw materials used in the form of chestnuts, pinecones, and fir cones showed their suitability for the production of solid fuels for energy purposes. However, the production of pellets from biomass obtained from green areas requires further research. After further analyses, we plan to patent and implement pellets from waste biomass from green areas with an admixture of coniferous sawdust in specific proportions.

In addition, we plan to perform analyses that will be used to assess the pellet production process in technical and economic terms. As a result of the technical and economic analysis, the technological parameters of bioenergy and bioproducts will be compared with economic indicators. Additionally, when considering biofuels in the form of pellets, it is worth paying attention to the assessment of the impact of their production process and energy use on the natural environment. An energy analysis will be carried out with a view to defining all energy flows involved in the production of pellets. It is worth paying attention to the energy analysis, the results of which will help to improve the biomass processing process, taking into account sustainable development. The performance of the above-mentioned analyses will allow us to define the risks associated with the production process and to develop an optimal biofuel production technology with improved thermodynamic, economic, and environmental properties.