Effect of the Growth Period of Tree Leaves and Needles on Their Fuel Properties

Abstract

The main advantage of using biomass for energy generation is the reduction in carbon dioxide emissions. For a fast reduction effect, it is important to use biomass characterised by an annual growth cycle. These may be fallen leaves. The fuel properties of the leaves can change during the growth period. These changes can result from both the natural growth process and environmental factors—particulate matter adsorption. The main objective was to determine changes in the characteristics of leaves and needles during the growth period (from May to October). Furthermore, to determine the effect of adsorbed particulate matter, the washing process was carried out. Studies were carried out for three tree species: Norway maple, horse chestnut and European larch. Proximate and ultimate analysis was performed and mercury content was determined. During the growth period, beneficial changes were observed: an increase in carbon content and a decrease in hydrogen and sulphur content. The unfavourable change was a significant increase in ash content, which caused a decrease in calorific value. The increase in ash content was caused by adsorbed particulate matter. They were mostly absorbed by the tissues of the needle and leaves and could not be removed by washing the surface.

1. Introduction

The use of biomass for energy generation is becoming increasingly important, and the amount of energy generated from this source is increasing [1]. The various types of biomass are characterised by different properties [2]. The use of a particular type of biomass is determined both by its properties and by its availability and price [3]. The price of biomass is a key factor for the development of biogas and biomethane production [3]. This raises the need to explore new sources of biomass, including waste biomass, which has not been used on an industrial scale so far [4]. Examples of this type of biomass include tea [5] and coffee waste [6], and marine biomass [7].

The main advantage of using biomass is the reduction in carbon dioxide emissions resulting from the substitution of fossil fuels [8]. However, it should be noted that for a fast reduction effect, it is particularly important to use biomass characterised by an annual growth cycle [9]. A significant source of this biomass is agro-industrial waste, such as straw [10] or nut shells [11]. Another type of annual growth biomass is fallen leaves [12]. The use of this type of biomass is interesting because its price will be relatively low. In addition, in some cases, its use can provide additional income, since cities can pay for its collection and utilisation. However, fallen leaves are characterised by a relatively high ash content, which contributes to a lower calorific value compared to high-quality biomass (wood, wood pellets) [13].

Conventional methods of leaf utilisation include composting or leaving them to decompose naturally [14]. In these processes, chemical enthalpy is lost, and carbon dioxide is released. Therefore, alternative methods of their utilisation are being investigated. Leaves can be used as a solid fuel [15], as a feedstock to produce hydrogen [14], or can be converted to many valuable chemicals and materials [16].

There is an observable gap in knowledge regarding the fuel properties of leaves and needles. In research papers addressing this issue, leaf characteristics are determined, mainly the ultimate and proximate analysis [17,18], in particular with respect to calorific value [19]. The bulk density [20], combustion characteristics [18], auto-ignition temperature [20], ash composition [18] and ash melting temperatures [20] are being studied. The heavy metal content is also being examined [20,21]. The need to complete the data is also due to the very large number of tree species, around 73,000 [22].

The presented results are related mainly to fallen leaves, and knowledge of the change in properties during the leaf growth period is limited. These changes can result from both the natural growth process [23] and environmental factors—particulate matter adsorption [24]. Most of the work on this issue did not address fuel properties. Changes in leaves were studied from a biological point of view, among others: growth rate [25], nitrogen content [25], and metabolite changes [26]. The second group of papers deals with the accumulation of contaminants in leaves, such as flavonoids [27], antioxidant capacity [27], and metallic pollutants [28]. Understanding these changes is crucial to predicting the properties of fallen leaves and needles, which will determine the processes of their use as fuel. Especially important is the process of adsorption of particulate matter on the surface of the leaves, resulting in a higher ash content and a lower calorific value. It is also necessary to assess the possibility of improving the quality of leaves by washing the surface.

The main objective of the paper was to determine changes in the characteristics of leaves and needles during the growth period (from May to October). In addition, to determine the effect of adsorbed particulate matter, the washing process of the leaf surface was carried out. Studies were carried out for three tree species: Norway maple, horse chestnut and European larch.

2. Materials and Methods

2.1. Examined Samples

The leaves of two tree species were sampled: Norway maple (Acer platanoides), horse chestnut (Aesculus hippocastanum), and the needles of European larch (Larix decidua). The trees were located on the territory of the city of Krakow (Lesser Poland Voivodeship, Poland). The choice of tree species was based on their prevalence and the size of their leaves (greater potential for adsorption of particulate matter). Both maple and chestnut are characterised by relatively large leaves. Maple has the largest share of the tree population in Krakow (11,412 trees), and horse chestnut is very common in parks (1964 trees) [29]. The European larch, the only coniferous tree species that drops its needles every year, was also selected for the study. The selected tree species are very numerous in Europe. These trees have also spread to other continents.

The leaves and needles were sampled at monthly intervals from May to October. The samples from May to September were taken directly from the trees, and the sample collected in October consisted of fallen leaves and needles (Figure 1). The sampling procedure was performed very carefully to eliminate the occurrence of undesirable components in the form of leaves of other tree species, branches, soil, or stones. One composite sample was taken each month for each tree species. To provide a representative composite sample, primary samples were taken from different places on one tree. Repeatability was performed for the selected horse chestnut sample. An additional composite sample was taken. The differences between the results obtained were within the repeatability limit of the method.

Each composite sample was divided into two parts. One part was passed to a further stage of research (raw sample), and the second part was subjected to the washing process to remove particulate matter adsorbed on the surface (washed sample).

2.2. Washing Procedure

The surface of the collected leaves and needles was covered with various amounts of adsorbed particulate matter. To determine their impact on biomass quality, a washing process was conducted. The leaves and needle surfaces were washed with tap water at about 20 °C. To thoroughly remove dust, a soft sponge was used, which allowed the washing process to be performed without damaging the surface of the leaves. The water used was characterised by high purity. The content of components that could affect the properties of the tested samples (e.g., mineral compounds, sulphates) was at a relatively low level. This limited the influence of water on the interpretation of the results obtained.

The following assumptions were adopted to ensure stable washing process conditions: (i) each leaf was washed separately and on each side; (ii) in the case of European larch, the washing process took 20 min, as technical difficulties made it impossible to wash each larch needle separately; (iii) the washing process was carried out over the screen to eliminate the loss of material due to crushing, etc.; (iv) washing was conducted under running water to avoid re-adsorption of dust from the water; (v) a steady stream of water was used.

2.3. Sample Preparation

The raw and washed samples were air-dried to remove external moisture (M_ex). The samples were then ground using an LMN-100 knife mill by Testchem (Pszow, Poland) to a grain size of less than 1.0 mm. The ground and homogenised samples were sealed tightly in plastic sample bags.

2.4. Sample Characteristics

Proximate and ultimate analysis was performed for the samples analysed, including inherent moisture (M), ash (A), carbon (C), hydrogen (H), total sulphur (S_t) and mercury (Hg) as well as the lower heating value (LHV). Moisture content was determined using an MA 110.R Moisture Analyser by Radwag (Radom, Poland). The ash content was determined at 550 °C using a muffle furnace by Czylok (Jastrzębie-Zdrój, Poland). The lower calorific value was calculated according to the PN-ISO 1928:2002 standard based on the higher calorific value determined using an IKA C 6000 calorimeter by IKA-Werke GmbH & Co. KG (Staufen im Breisgau, Germany). Carbon, hydrogen and sulphur content were determined using a CHS-580 analyser by Eltra (Haan, Germany). The mercury content (Hg) was determined using a DMA-80 mercury analyser by Milestone, according to the Milestone procedure. The characteristics of the samples analysed are presented in

Table 1. Characteristics of the air-dried samples (raw samples).

Tree Species	Month of Sampling	Mad [%]	Aad [%]	LHVad [kJ/kg]	Cad [%]	Had [%]	St.ad [%]	Hgad [µg/kg]
Norway maple	V	6.7	6.5	16,820	44.4	6.41	0.20	12
	VI	6.8	6.6	17,020	44.8	6.47	0.16	16
	VII	6.6	8.5	16,606	44.1	6.15	0.16	26
	VIII	6.8	9.2	16,357	43.6	6.05	0.15	30
	IX	6.9	10.0	16,279	43.4	5.84	0.14	46
	X	8.0	11.9	16,158	43.2	5.69	0.08	47
Horse chestnut	V	6.6	6.4	16,760	45.0	6.37	0.18	12
	VI	7.2	7.3	16,277	44.0	5.93	0.16	24
	VII	7.1	7.5	16,103	43.7	5.83	0.13	29
	VIII	7.2	8.0	16,290	44.0	5.86	0.13	38
	IX	7.7	8.1	16,097	43.6	5.80	0.11	41
	X	9.2	9.1	16,537	44.3	5.24	0.08	58
European larch	V	5.3	4.1	17,662	47.2	7.63	0.16	15
	VI	6.0	3.9	18,019	47.8	6.78	0.15	19
	VII	5.7	4.5	17,804	47.1	6.85	0.14	31
	VIII	5.9	5.3	17,688	46.8	6.9	0.14	36
	IX	6.5	6.5	17,552	46.2	6.51	0.14	41
	X	8.3	8.3	17,162	44.7	5.76	0.12	57

ad—air-dried basis.

.

3. Results and Discussion

3.1. Changes in Moisture Content

The external moisture content was very high at more than 50% (Figure 2). It reached its highest value in the early growth period (May–June). The exception was samples collected in October, which should be explained by the natural process of wilting of the leaves. The external moisture content ranged from 12.5% for chestnut leaves to 42.7% for larch needles. The observed differences are directly related to the size of the leaves and needles. Leaves are characterised by a higher specific surface area than needles (larger drying surface). The specific area is 342 [30], 250 [31], and 118 cm²/g [32] for Norway maple leaves, horse chestnut leaves, and European larch needles, respectively.

The opposite trend was observed for the inherent moisture (Figure 3). The lowest content was reported in May (from 5.3 to 6.7%) and the highest in October (from 8.0 to 9.2%). Such a variation can be explained by both changes in the structure of the examined samples, as well as by the humidity of the air in the laboratory.

3.2. Changes in Calorific Value

The changes in the lower heating value of the samples analysed are presented in Figure 4. A general trend can be observed that, with the duration of the growth period, calorific value decreases. This is related to the following factors: (i) an increase in moisture content (Figure 4); (ii) an increase in mineral matter content; (iii) changes in the structure of organic matter. The last two issues will be discussed in detail in the following sections. Changes in caloric value are not always evident due to simultaneous changes in mineral and organic matter. In the case of horse chestnut, an increase in calorific value was observed for the sample collected in October. An increase in carbon content was also observed for this sample (Table 1). The influence of sampling should also not be excluded, as a new portion of leaves was collected each time. The decrease in calorific value should be considered highly unfavourable, since mainly fallen leaves and needles (October) will be used for energy production.

There is an observable difference between the results obtained for the samples examined and those reported in the literature. Comparisons were made for samples collected in October because there is no data for samples taken during the growth period of leaves and needles. For horse chestnut leaves, the calorific value (16.5 MJ/kg) was within the reported range of 15.7–18.4 MJ/kg [33]. A higher calorific value was obtained for Norway maple leaves [34] and a lower value for European larch needles [35]. These differences were caused by the different ash content related to the different locations of the tree. The samples analysed by Maj (2018) were collected in the forest (lower pollution) [35], while the samples analysed by Zubkova et al. (2021) were taken from urban areas (higher pollution) [34].

3.3. Changes in Ash Content

The changes in ash content in the analysed samples are presented in Figure 5. Along with the duration of the growth period, the mineral matter content increased (increased ash content). This change should be explained by adsorbed particulate matter from the air rather than assimilated mineral components from the soil. Leaves and needles have the ability to absorb particulate matter in their tissues [36]. Particulate matter can also be adsorbed on the surface. Only a slight decrease in the ash content was observed in the washed samples. In some cases, the ash content was not reduced. The share of particulate matter adsorbed on the surface in relation to the increase in ash content from May to October ranged from 12 to 21% for Norway maple and European larch, respectively. Most of the particulate matter was absorbed inside the leaves and needles. The ash content in the leaves can be even higher, even greater than 20% [20].

For Norway maple leaves, the ash content (12.9%) was within the reported range from 11.2 [37] to 16.3% [34] (dry basis). The recorded ash content of horse chestnut leaves was 11.8% [37] (dry basis) and was higher than that of the sample examined in the paper (10.0% dry basis). It was noted that the tested sample of European larch needles had a much higher ash content (8.3%) than that reported in the literature (0.5% [35]). As already mentioned, the low ash sample was collected in a forest [35], and the sample studied in the paper was collected in one of the biggest cities in Poland (Krakow).

3.4. Changes in Organic Matter

Changes in carbon, hydrogen, and sulphur content in the analysed samples are presented in Figure 6. Significant changes in the organic matter of the leaves and needles analysed are observable. With growth time, there is an increase in carbon content and a decrease in hydrogen and sulphur content. Such changes should result in an increase in calorific value. However, not only is the content of combustible elements important (mainly carbon and hydrogen), but also their binding to oxygen [38]. The calorific value will be negatively affected by an increase in the ash content (Figure 5) and moisture content (Figure 3). The decrease in sulphur content, which was above 50% for horse chestnut and Norway maple leaves, should be considered a favourable change. As assumed, the washing process did not change the composition of the organic matter.

Despite the differences in ash and calorific content, the composition of organic matter for the samples collected in October was in good agreement with data from the literature. For Norway maple leaves, the C/H ratio was 7.6 and 7.4, for the investigated and comparison samples, respectively [34]. For European larch needles and maple leaves, the C/H ratio was 7.9 and 7.8, for the investigated and comparison samples, respectively [35]. For chestnut leaves, the recorded carbon content was within the reported range of 43.18 ± 1.5% [39]. For sulphur content, the reported values are also at very low levels [34,35].

3.5. Changes in Mercury Content

Together with the particulate matter, the leaves and needles captured mercury from the air. Changes in mercury content in the analysed samples are presented in Figure 7. The mercury content of the leaves taken in October was relatively high and even exceeded the mercury content in the hard coals [40]. Compared to the ash content (Figure 5), the proportion of mercury adsorbed on the surface was higher, i.e., 22 to 24%, for the horse chestnut and Norway maple, respectively.

The same origin of adsorbed particulate matter and mercury is suggested by a significant correlation for the relationship between the ash content and the mercury content (Figure 8). The leaves and needles of the different tree species analysed were characterised by different properties (relationships obtained are characterised by different slopes of the linear function). The order in which the curves are placed on the graph corresponds to the specific leaf/needle area. European larch needles are characterised by the smallest specific area, while Norway maple leaves are characterised by the largest specific area. Interestingly, despite having a much smaller surface area, the European larch needles absorbed an amount of mercury similar to that absorbed by the much larger horse chestnut leaves. Larch needles are covered with wax [41], which likely supports the capture of mercury from the air.

The very high mercury content in leaves and needles can be a limitation to their use as fuel. This issue is particularly important for combustion plants not equipped with a flue gas cleaning system. These include residential solid fuel boilers. In such cases, it may be necessary to remove mercury before combustion. This can be performed in the process of torrefaction, and the efficiency achieved is nearly 100% [21]. Between 29 and 50% of the cost of torrefaction is the cost of the raw material [42]. The cost of waste biomass in the form of fallen leaves and needles will be characterised by a low price. In some cases, obtaining such raw material can be profitable, as fees can be charged for the disposal of green waste. Therefore, improving the properties of leaves in the torrefaction process may be reasonable.

It should also be noted that leaves and needles can also adsorb other heavy metals [43]. On the one hand, this is unfavourable, because it can lead to the re-emission of captured pollutants. On the other hand, the ability to bioaccumulate heavy metals allows tree leaves and needles to be used as a biomonitor of environmental pollution [44]. This effect can also be used to clean up the environment. However, this requires further research and the development of an effective and cost-efficient method.

3.6. Implications from Observed Changes in Leaves and Needles During the Growth Period

The results presented demonstrate the general trend of changes in the properties of the leaves of Norway maple and horse chestnut and the needles of European larch during the growth period. The same changes should be expected for other tree species. However, the magnitude of the changes should not be generalised to other tree species. Differences in ash, carbon, hydrogen, and mercury content, as well as in calorific value, were observed between the tree species analysed (the sulphur content was at a very low level for all analysed samples). The differences are caused by both the characteristics of the leaves and needles (specific surface area) and the location. The lower particulate matter content in the air will result in a lower amount of adsorbed dust, and thus a lower ash and mercury content and a higher calorific value.

Collecting this type of biomass from forested areas is difficult. Additionally, in the European Union, the RED III Directive [45] creates the need to collect waste biomass from other sources. Solid fuel in the form of fallen leaves and needles obtained from urban areas will be of lower quality. In light of the results, washing the particulate matter off the surface of leaves and needles will not provide a significant benefit. Hence, the effective use of this type of biomass requires the development of an appropriate treatment. Ryšavý et al. (2021) [15] have conducted research on the combustion of beech leaf briquettes in a residential boiler. The authors point out the disadvantage of such briquettes, namely the high ash content (12.9%). This is much higher than in wood. The authors also pointed out the high content of nitrogen oxides in the flue gas. For these reasons, it is appropriate to use this type of fuel as a secondary fuel during the co-combustion process [15].

However, the stability of the organic matter composition should be considered a significant advantage. Regardless of the sampling location (different ash content), thermochemical conversion processes should proceed in a similar course for leaves or needles from the same tree species. The higher mineral matter content (ash) can be considered from different points of view. On the one hand, mineral matter can be beneficial, for example, in catalysing the gasification process [46]. On the other hand, it can cause difficulties in operation due to the risk of slagging [47].

3.7. Comparison of Leaves and Needles with Other Types of Waste Biomass

To evaluate the fuel characteristics of fallen leaves and needles in comparison with other types of waste biomass, a comprehensive analysis was carried out that focused on key parameters such as moisture content (M), ash content (A), lower heating value (LHV), and elemental composition (C, H, S). These values are summarised in

Table 2. Comparison of fallen leaves and needles with other types of waste biomass.

Type of Biomass	Mad [%]	Aad [%]	LHVad [kJ/kg]	Cad [%]	Had [%]	St.ad [%]	Ref.
Leaves (avg.)	8.6	10.5	16,695	43.7	5.50	0.08	from this work
Needles	8.3	8.3	17,162	44.7	5.76	0.12	from this work
Beech leaves (briquetted)	8.9	12.9	15,640	42.6	4.62	0.09	[15]
Pine needles	4.0	3.4	17,550	53.7	5.52	0.20	[48]
Neem leaves	12.1	18.0	15,580	29.3	4.38	0.22	[49]
Banana leaves (briquetted)	7.2	10.7	16,150	44.3	6.23	<0.30	[17]
Willow leaves	3.5	0.7	18,140	45.8	6.24	n.d.	[50]
Bark	7.9	4.7	15,975	50.0	6.28	0.04	[21]
Rice husks	6.7	13.0	n.d.	39.3	5.70	n.d.	[51]
Wheat straw	6.3	16.5	n.d.	36.7	5.10	n.d.	[51]
Used cotton	3.4	0.6	15,357	43.2	7.64	0.10	[52]

ad—air-dried basis; n.d.—not defined.

.

Among the samples examined, leaves (8.6%) and needles (8.3%) exhibit moderate moisture content, suggesting acceptable drying characteristics for combustion. In contrast, used cotton (3.4%), willow leaves (3.5%) and pine needles (4.0%) exhibit significantly lower moisture, indicating better natural desiccation or pretreatment efficiency. On the other hand, neem leaves (12.1%) show the highest moisture content, potentially requiring additional drying before combustion. However, compared to typical reference fuels recommended for small-scale combustion appliances—such as wood logs or briquettes—all listed biomass types still fall well within acceptable limits. According to user manuals, the reference moisture content for such fuels is often specified as below 20% [53].

Ash content, which is directly related to fouling and heavy metal deposition, varies markedly across samples. Needles (8.3%) perform better than leaves (10.5%), but both are surpassed by low-ash biomasses such as used cotton (0.6%), willow leaves (0.7%) and pine needles (3.4%). Neem leaves (18.0%) and wheat straw (16.5%) exhibit particularly high ash content, making them less favourable from an ash management perspective, particularly in systems without advanced ash handling technologies. Biomass with such high ash content typically requires combustion units specifically designed for this purpose, often equipped with automatic ash removal systems [54].

The lower heating value of needles (17,162 kJ/kg) is slightly higher than that of leaves (16,695 kJ/kg), reflecting a marginally better energy yield. Notably, willow leaves have the highest calorific value (18,140 kJ/kg) among all samples compared. This can be attributed, in part, to their exceptionally low ash content (0. 7%), which means that a larger proportion of combustible organic matter remains in the fuel. Conversely, neem leaves (15,580 kJ/kg), beech leaves (15,640 kJ/kg) have among the lowest heating values, which correlates with their elevated ash content (18.0% and 12.9%, respectively). These variations illustrate how a higher mineral (ash) content dilutes the combustible fraction, thus reducing the energy available per unit mass—an important consideration for fuel selection in energy-efficient applications. Interestingly, the used cotton had a low calorific value (15,357 kJ/kg), but it was characterised by a low ash content (0.6%). This is due to oxygen binding to combustible elements.

Needles (44.7%) and leaves (43.7%) exhibit a carbon content typical of lignocellulosic materials. A significantly higher carbon content is seen in pine needles (53.7%), aligning with their superior calorific performance. In contrast, neem leaves (29.3%) have a substantially lower carbon content, suggesting reduced energy density and altered combustion behaviour. Hydrogen content across the samples ranges from 4.38% in neem leaves to over 7.64% in used cotton. Needles (5.76%) and leaves (5.50%) again sit in the mid-range, indicating a balanced H/C ratio suitable for clean combustion [55].

4. Conclusions

During the leaves’ and needles’ growth period, beneficial changes in organic matter were observed: an increase in carbon content (1.5–2.8 pp. on dry, ash–free basis) and a decrease in hydrogen (0.3–1.5 pp. on dry, ash–free basis) and sulphur content (0.04–0.13 pp. on dry, ash–free basis). The unfavourable change was a significant increase in ash content (almost a twofold increase; 3.2–5.9 pp. on dry basis), which resulted in a decrease in lower heating value (223–662 kJ/kg on air-dried basis). The decrease in calorific value should be considered highly unfavourable, since mainly fallen leaves and needles (in October) will be used for energy production. The increase in ash content was caused by adsorbed particulate matter. They were mostly absorbed by the tissues of the needle leaves and could not be removed by washing the surface.

The very high mercury content in fallen leaves and needles (in October) can be a limitation to their use as fuel. This issue is particularly important for combustion plants not equipped with a flue gas cleaning system. These include residential solid fuel boilers. In such cases, it may be necessary to remove mercury before combustion. This can be performed in the process of torrefaction, and the efficiency achieved is nearly 100%.

It should also be noted that leaves and needles can also adsorb other heavy metals. On the one hand, this is unfavourable, because it can lead to the re-emission of captured pollutants. On the other hand, this effect can be used to clean up the environment. However, this requires further research and the development of an effective and cost-efficient method.