1. Introduction
Growing human population and rising living standards lead to higher energy consumption. The use of renewable energy resources helps to solve energy-related challenges and contributes to lower greenhouse gas (GHG) emissions [1]. However, according to recent data, renewable energy sources account for only 14% of the total primary energy supply, of which bio-energy accounts for 67% [2].
As an alternative to fossil fuels, biomass is increasingly used as it is considered a neutral energy source in terms of carbon emissions [3]. Biomass is considered a zero-emission fuel because of the carbon cycle of plants, which binds CO2 during photosynthesis. Thermal processing, such as incineration, releases the same amount of carbon into the atmosphere [4]. Forest waste biomass, agricultural biomass, and biomass grown specifically for energy purposes will become more and more important for energy production [5]. According to bioenergy supply projections for 2050, reported by Errera et al. [6], the contribution of bioenergy to the global energy matrix in 2050 is expected to be 64–313 EJ. The composition of the bioenergy supply is also projected to change significantly. The use of firewood, which is the most widely used today, will be replaced by energy crops and biodegradable waste.
One of the important forms of biomass energy utilization is granulated biofuel [7]. Global pellet production in 2021 increased by 6.8% compared to 2020, and in the countries of the European Union, this increase was 9% [8].
Granulation helps to solve the problems caused by untreated biomass, such as high moisture content, irregular shape and size, and low bulk density. Densification produces more compact products with uniform shape and size [9]. Densifying biomass into pellets increases energy density, reduces particulate emissions, and improves biomass combustion and transport efficiency [10]. Pellets with a diameter of 6–8 mm and a length not exceeding 40 mm are the most popular [11]. The range of biomass feedstocks that can be used to produce pellet fuels is very broad, with both herbaceous plants and woody species suitable. The quality of the final product is influenced by a variety of factors, both in terms of the parameters of the granulation process and the properties of the biomass [12].
The use of fuel pellets is very wide, from household cook stoves to thermal electrical appliances. To ensure the quality of pellets, various national and international standards have been developed [13]. The properties of solid biofuel are essential to ensure its efficient use in power plants and low emissions. The physical and chemical properties of pellets (moisture content, ash content, calorific value, and others) strongly affect boiler operation, energy-efficient combustion, emissions, and costs. For example, high ash content and undesirable chemical composition of solid fuel can increase slag formation, furnace, and heat exchanger fouling and slagging [14,15].
To strengthen the economy and energy independence of rural areas, it is important to find alternative raw materials for wood biomass. It is also important that such alternative biomass can be granulated without pre-treatment and additives, thus ensuring a low raw material cost [16].
A potential source of biomass to produce solid biofuels is multi-crop plants. Multi-cropping is one of the most emphasized agronomic practices to improve soil health (structure, fertility, nutrient uptake, etc.) and increase yield and income [17]. Examples from around the world show that multi-cropping, like intercropping and rotations, provides numerous benefits: soil fertility is increased (when nitrogen is fixed using legumes), crops are less weedy, and more biomass is grown [18]. Growing two or more species of plants in one field is a practice that helps reduce the environmental impact of agriculture without reducing productivity [19]. Scientific studies confirm that it is a good practice to grow a certain variety of faba beans together with maize because, in this way, small farms can obtain a higher corn equivalent yield, increase food and nutrition security, and ensure the gross monetary value of the system [20]. A fibrous hemp (Cannabis sativa L.) is also suitable for use in multi-crops. Fibrous hemp can be a very sustainable and eco-friendly plant. It absorbs carbon very well and fights weeds well, so there is no need to use pesticides in crops [21]. This plant is grown not only for fiber but also for energy [22].
Even though biofuels are an alternative to fossil fuels to reduce the emission of anthropogenic GHG when making political decisions it should be justified that such biofuels are produced sustainably [23]. In some cases, results of the LCA show that, for the same heat output, the environmental impact of forest wood fuel is much lower than that of fuel oil but is still higher than natural gas [24]. This implies that new types of raw materials and management practices must be considered. The management of biomass raw material and the production of solid biofuel must be carried out following the principles of sustainable development, which are in line with global efforts to reduce GHG emissions and the impact of climate change [25]. The LCA helps to obtain information about the environmental impact of a product or service throughout its life cycle [26]. The LCA usually consists of four stages: (1) goal and scope definition, (2) inventory analysis, (3) impact assessment, and (4) interpretation of the results [27].
Although the LCA of wood pellets is commonly analyzed in scientific literature, the environmental impact of pellets produced from alternatives to wood raw materials is also investigated. For example, Wiloso et al. [28] performed the LCA of the use of sorghum pellets together with coal for electricity generation. Kylili et al. [29] performed an environmental assessment of the production process of biofuel pellets from waste olive husk. Li et al. [30] presented the LCA results of wheat straw pellets. The mentioned investigations showed promising results for these biomass sources in terms of sustainability. In scientific studies supporting the results of the environmental assessment of biomass preparation for solid biofuel, the goals are to compare the biomass energy yield of one plant with other energy plants [31]; to justify the social, economic, and environmental benefits of the biomass preparation process for solid biofuel [32]; to analyze the prospects of producing biomass pellets taking into account costs and energy requirements [33]; and to evaluate the possible reduction of GHG in the production of electricity from fossil fuels using it in combination with biomass pellets [28]. As pointed out by Martín-Gamboa et al. [34], when performing the LCA, the methodological choices of different authors differ significantly regarding the choice of functional unit (FU), system boundaries, impact categories, and other important components.
The conducted studies allow us to form a fragmentary picture of the use of alternative raw materials for wood biomass to produce solid biofuel. The results of the studies are difficult to compare due to the variety of research methods chosen by the authors and the different LCA conditions adopted. In addition, no studies could be found on the use of multi-crop biomass for solid biofuel production and the assessment of the environmental impact of the process. The novelty of our study consists not only of the unexplored raw material of multi-crop plants but also of the selected assessment of the life cycle of a complex system when the boundaries of the system from cradle-to-grave are supplemented with elements of circularity by returning the ash formed after burning the pellets to the soil.
The aim of this work is to substantiate the suitability of the biomass of the multi-crop plants to produce solid biofuel and to assess the impact of the granular fuel preparation process on the environment.
The results of the analysis are important for the development of the renewable energy sector, as they provide new knowledge about new biomass fuel sources that can be a good alternative to wood biofuel.
2. Materials and Methods
2.1. Design of the Research
Biomass pellets made from fibrous hemp, faba bean, and maize biomass grown in mono and polynomial crops were investigated in detail and an LCA of biomass preparation for solid biofuel was carried out.
A flow chart illustrating the performed studies is presented in Figure 1.
The 3 plants used for the research are suitable for multiple crops, are valuable and promising in terms of energy, and can produce a large amount of biomass in a short period. In the European Union, maize is the second most cultivated crop after wheat [35]. Its dry matter yields range between 22 and 26 t ha−1 [36]. Beans not only improve the soil but also reduce the need for chemical fertilizers for subsequent crops and contribute to reducing environmental pollution in agriculture [37]. Growing hemp for fiber can yield around 10.5 t ha−1 of raw material that can potentially be used for energy purposes [38].
The pellet biomass raw material used in our study was grown in 7 different fields in the multi-crop experiment at the Experimental Station of Vytautas Magnus University Agriculture Academy (Lithuania), in 2020–2022 by Romaneckas et al. [39] according to the methodology of the mentioned authors. Each of the mentioned plants was grown in three different fields as a monocrop; in another three fields, two plants were grown together in one field (3 fields with different combinations of plants); and in one field, all three plants were grown together (trinomial crop). According to the plot from which the biomass was taken, 7 types of pellets were produced: pellets from maize biomass (S-Mz), fibrous hemp biomass (S-FH), faba bean biomass (S-FB), maize and fibrous hemp (grown in the same field) biomass (MIX2-1), maize and faba bean (grown in the same field) biomass (MIX2-2), fibrous hemp and faba bean (grown in the same field) biomass (MIX2-3), and maize, hemp, and faba bean (grown in the same field) biomass (MIX3-1). Biomass samples were collected over a period of three consecutive years. Every year, biofuel pellets were produced from the collected biomass under laboratory conditions, and their quality parameters were tested. It was analyzed how the main parameters of the pellets met the requirements of the standard ISO 17225-6:2021 [40]. This standard defines the quality requirements for non-wood biofuels. After evaluating the quality of the pellets, in the next stage, the LCA of the use of biomass for solid biofuel was carried out.
2.2. Production of Biomass Pellets
The harvested biomass samples were dried to a moisture content of 12–16%. Pellets were produced under laboratory conditions using a low-power granulator (ZLSP200B, Poland, 7.5 kW, 6 mm matrix). Binders were not used in the granulation process. Before the biomass was granulated, it was crushed by a drum chopper of a forage harvester MARAL-125 (Landtechnik AG, Shönebeck, Germany) and ground using a Retsch SM 200 hummer mill (Retsch GmbH, Haan, Germany) with a sieve mesh of 2 mm in diameter.
2.3. Determination of the Main Characteristics of Biomass Pellets
2.3.1. Biometric Properties (Length, Diameter, Weight, Moisture, and Density)
In total, 10 pellets of each type were randomly selected. The length and diameter of the selected pellets were determined using a digital Vernier caliper (measurement accuracy 0.01 mm). Pellet weight was determined using a KERN ABJ scale (weighing accuracy 0.001 g) (ProfiLab24 GmbH, Berlin, Germany). After the determination of pellet measurements, the density of the pellets was calculated by dividing the sample weight by the sample volume.
The moisture content of the pellets was determined according to the standard LST EN ISO 18134-1:2016 [41]. All tests were conducted in triplicate to ensure the reliability and reproducibility of the results.
2.3.2. Elemental Analysis
The total amount of nitrogen (N), hydrogen (H), and carbon (C) were determined using a Flash2000 analyzer in accordance with the standard LST EN ISO 16948:2015 [42]. The column is prepared manually with quartz wool, electrolytic copper, and copper oxide every 200 measurements. Helium and oxygen (with 99.999% purity) as carrier gas was used by determining N, H, and C. The standard LST EN ISO 16994:2016 was used to determine the amount of sulfur (S) and chlorine (Cl) [43]. For the determination of the S and Cl Dionex ICS 5000 chromatography system with Dionex IonPac AS22, a 4 × 250 mm column was used. Diluted Dionex AS22 Eluent Concentrate in a ratio of 1 to 100 double-distilled water was used as the carrier fluid.
Analysis of the selected elements potassium (K) and phosphorus (P) was performed using an ICP-OES, Optima 8000 (Perkin Elmer, Waltham, MA, USA). Samples (0.4–0.5 g) were mineralized with 8 mL of concentrated nitric acid, 1 mL of hydrofluoric acid, and 3 mL of hydrogen peroxide at 800 W, 6 MPa, pRate: 50 kPa·s−1 (Multiwave 3000, Anton Paar GmbH, Graz, Austria). After the mineralization, the solution was poured into 50 mL flasks and diluted to 50 mL using deionized water.
2.3.3. Calorific Value and Ash Content
The calorific value was determined according to the LST EN ISO 18125:2017 standard [44]. An automatic bomb calorimeter IKA C6000 was used for the analysis.
The amount of ash was determined according to the standard LST EN ISO 18122:2016 [45]. A sample of ground pellets weighing about 1 g was burned to a constant weight. The ash content was determined by calculating the difference in sample weight before and after combustion.
2.3.4. Statistical Analysis
Pellet length, diameter, weight, and unit density were determined using 10 randomly selected pellets. The other tests described in Section 2.3.1, Section 2.3.2 and Section 2.3.3 were repeated 3 times each. Statistical results were processed using the MS Excel program. Differences between means were analyzed using EXCEL ANOVA tables for one-way analysis and Tukey’s HSD test. The accepted probability level was 95%.
2.4. Life Cycle Assessment
The LCA was conducted in accordance with ISO14040:2006 [46]. The impact assessment was performed with SimaPro 9.5 software. The CML-I baseline model [47] was used. In this study, 11 environmental impact categories were considered: global warming (GWP), eutrophication (EP), acidification (AP), ozone layer depletion (ODP), abiotic depletion (AD), photochemical oxidation (PO), terrestrial ecotoxicity (TE), freshwater aquatic ecotoxicity (FWAE), human toxicity (HT), abiotic depletion (fossil fuels) (ADF), and marine aquatic ecotoxicity (MAE). Data on biomass and crop cultivation, transport, fertilization, granulation, and other equipment were used from the Ecoinvent v3 database [48]. For a more detailed analysis, 4 impact categories were chosen as the most important ones, taking into account the nature of the processes and the related environmental problems, which were indicated and proposed by other authors [29,34,49]: ADF, GWP, AP, and EP.
2.4.1. Definition of the Aim and Scope
The objective of the LCA study is to assess the environmental impact of a pellet production process using biomass grown in single, binary, and ternary crops.
Environmental performance was analyzed for seven different scenarios. All scenarios include heat production in an industrial biofuel boiler by burning pellets made from multi-crop biomass. Each of the scenarios uses a different type of pellet, all 7 types of which are explained and discussed in Section 2.1.
2.4.2. Functional Unit
A functional unit (FU) describes the quantification of the product’s identified functions through inputs and outputs, and it stands as a reference point for comparing LCA results [33], according to internationally accepted recommendations [50,51,52,53]. In this study, the FU is 1 GJ of thermal energy produced by a pellet in a solid fuel boiler. A boiler efficiency of 91% was assumed, as such efficiencies can be achieved with optimal system design [54].
2.4.3. System Boundary
The LCA includes procedures to systematically assess the environmental impact of bioenergy production from both cradle-to-gate and cradle-to-grave (waste disposal). Usually, under the cradle-to-gate system boundaries, the LCA includes only a partial product life cycle from the raw material collection (“cradle”) to before the product is transferred to the consumer (“gate”) [55]. A full LCA of biomass preparation for pressed biofuel starts with biomass cultivation. It ends with waste (ash) disposal after briquette/pellet conversion to energy, including the environmental impact of the main equipment used during its operation [56]. Most studies apply the impacts of activities involved in the wood-based product granulation, transport, and use of materials and fuels. The comprehensive view provided by the LCA allows environmental impacts to be assessed on a whole system basis analyzing the agro-industrial value chains, such us in-tended crop cultivation, treatment, harvesting, collection, transportation, size reduction and screening, drying, granulation, storage, and utilization for heat [57].
The system boundaries considered for this study are shown in Figure 2. It includes the main processes: soil preparation and sowing, crop growing and harvesting of biomass, transportation of feedstock, pellet and ash, storage, pretreatment, granulation, combustion in a boiler, and granulation of ash. These processes in the LCA analysis are combined into the main phases: biomass production, biomass pellet transportation, biomass pellet and heat production, and ash utilization.
Material and energy inputs in specific operations and processes were based on mass/energy balances and efficiency data. The pellets resulting from the granulation stage are air-cooled down and packed in 15 kg bags or stored in a silo for bulk distribution (20% are packed and 80% are sold unpacked) [48].
2.4.4. Inventory Analysis
Biomass Production
When evaluating the stage of biomass production, we used the data from Romaneckas et al. [39] on technological operations, seed and fertilizer rate per hectare in each of the 7 different crops, as well as the dry biomass yield of each crop (data on yield, fertilizer, seed rate, and yield are recalculated for 1 FU and presented in the inventory table). Data on biomass and crop cultivation and care, fertilization, and other equipment were used from the Ecoinvent v3 database [48].
Transportation
The transportation distance of biomass from the field to the pre-storage site was assumed to be 3 km and was performed by a tractor and trailer of 8 t loading capacity. The transportation from the pre-storage site to the granulation site was assumed to be 30 km and was performed by a 16–32 metric ton road lorry with EURO5 emission standard transport. The data for these processes were obtained from the Ecoinvent v3 database [48].
Pellet and Heat Production
The quantity B of each type of required biomass to obtain 1 GJ of energy was calculated using the equation:
B = Q/[LCV × ή] × 1000,(1)
where:B is the amount of fuel burned, kg;
Q is the amount of thermal energy produced, GJ;
LCV is the lower calorific value of the fuel, MJ kg−1;
ή—is the coefficient of efficiency of heat production in a boiler.
The adopted boiler efficiency is 91%.
The data of the pellet LCV in MJ kg−1 are taken from our research. The 3-year average value of the lower calorific value of each pellet type was used in the calculations.
Ash Utilization
After pellet incineration in the boiler, the obtained ash was granulated for further application in the fields as a fertilizer substitution. Dried cattle manure was used for ash granulation as a binding agent. The ash granulation was performed using 20% of ash + 80% of cattle manure by weight. The transportation of ash from the boiler house to the pellet production site was assumed to be 30 km and was performed by a 16–32 metric ton road lorry with EURO5 emission standard transport. Additionally, the ash pellet was transported 3 km to the fields by a tractor and trailer of 8 t loading capacity.
The research of Ruiz et al. [53] suggested that mineral fertilizer substitution with pellet ash is an important aspect not considered in most of the studies. The use of this mineral fraction as a fertilizer implies economic profits and environmental benefits.
According to the guidelines described in ISO 14044:2006 [46], a system expansion approach is used to account for the environmental benefits associated with the use of ash in agricultural activities. Thus, environmental credits are due to the avoided production of conventional fertilizers with the same amounts of nutrients calculated and subtracted from the system [54].
It was assumed that the ash formed after the burning of biomass pellets was used to produce granular fertilizers and spread them on the soil. After burning the pellets, the amount of ash obtained per FU was calculated according to the pellet ash data presented in this study. The 3-year average value of ash content of each pellet type was used in the calculations. Table 1 presents data on the concentration of K and P in the ash, and how much P and K was obtained from the amount of ash per FU.
The amount of potassium in the ash obtained after burning polynomial crop pellets varied from 156.1 to 245.1 g kg−1, and phosphorus from 21.3 to 45.5 g kg−1. In this way, the need for mineral (phosphorus and potassium) fertilizers required for biomass cultivation decreased.
Table 2 indicates the main inputs and outputs resulting from the multi-crop biofuel pellet production and final energy generation, in terms of FU. Material and energy inputs in specific operations and processes were based on mass/energy balances and efficiency data. The electricity mixes for consumed power considered in this evaluation were used from Ecoinvent v3 as the market for low voltage electricity in the European Network of Transmission Systems Operators for Electricity. All the energy and mass flows in the inventory are normalized to the FU.
Biomass loss due to harvesting, transportation, and pellet production is assumed to be negligible.
3. Results and Discussion
3.1. Biometric Properties of the Pellets
3.1.1. Length, Diameter, and Weight
When analyzing the quality characteristics of the pellets, the type of pellets and the year in which the biomass was grown are indicated in brackets. The average length of the pellets varied from 17.60 mm (MIX2-1 variant, 2021) to 26.78 mm (S-Mz variant, 2020). The average diameter of the pellets varied from 6.00 mm (S-FB and MIX2-2 variants, 2021) to 6.30 mm (S-Mz variant, 2021). The length and diameter of the pellets met the requirements of the standard ISO 17225-6:2021 [40] for class A pellets. According to the standard, the length of the A-grade pellets should be at least 3.15 mm and not more than 40 mm, and the diameter should be between 6 and 10 mm.
The lowest average weight of the produced pellet was 0.59 g (MIX2-2 variant, 2021), and the highest was 0.98 g (S-Mz variant, 2021).
3.1.2. Moisture Content and Density
The moisture content of the produced pellets varied from 3.86% (S-FH variant, 2020) to 16.63% (S-FH variant, 2021). It should be noted that the latter variant was the only one that exceeded the maximum value set by the standard ISO 17225-6:2021 for the moisture content of the pellets (≤12% for class A and ≤15% for class B pellets). All other variants met the requirements of the standard. The moisture content of all pellet variants is seen in Figure 3.
The density of all pellet samples was also determined. The lowest dry mass density (946.41 kg m−3 ± 79.68) was determined for S-Mz pellets (2021), and the highest (1195.75 kg m−3 ± 47.05) was for S-FH pellets (2020). The density of only three pellet samples was less than 1000 kg m−3, the density of all other variants exceeded this value. The bulk density of S-FB pellets produced from biomass harvested in 2022 was determined to be 643.43 ± 9.37 kg m−3. According to this parameter, these pellets can be classified as class A pellets. Other pellet variants meet the requirements for class B pellets (≥550 kg m−3), except for the S-Mz and MIX2-3 variants, whose bulk density was determined to be 507.80 ± 6.93 kg m−3 and 544.67 ± 14.84 kg m−3, respectively. The research results show that our chosen biomass raw material can be superior in terms of this parameter compared to wheat straw biomass. Azócar et al. [62] determined the bulk density of wheat straw pellets to be 469.00 ± 8.00 kg m−3, and Niedziółka et al. [63] found the bulk density of wheat straw pellets to be 386–420 kg m−3. The results obtained by other authors show that it is possible to produce pellets with a higher bulk density from fibrous hemp and corn biomass raw materials compared to our data. Rabbat et al. [64] determined the bulk density of fibrous hemp biomass pellets to be 740 kg m−3, and Theerarattananoon et al. [65] determined that corn stover bulk density was 597.9 kg m−3. Authors indicated that granule bulk density was also influenced by such granulation process variables as mill screen size and die thickness
3.2. Elemental Composition
The main chemical elements of biomass are oxygen, hydrogen, and carbon, with carbon being the primary determinant of the fuel’s heating value, contributing the most to the overall heating value [66].
The main parameters that indicate the suitability of biomass for energy production are humidity, ash content, and fixed carbon. Biomass elements such as N, S, and Cl lead to NOx, SO2, and HCl emissions during combustion, so the amount of these elements in biomass must be as low as possible to have an acceptable level of emission impact on the environment. Analysis of biomass minor and major elements (Si, Cl, P, K, Na, S, Mg, and Fe, Ca) is important in assessing slagging, dirt formation in the furnace, and its corrosion [67].
The O content in the produced pellets varied from 38.07% (S-FB variant, 2021) to 47.11% (S-FH variant, 2020). The H content ranged from 4.71% (S-Mz pellets, 2022) to 5.75% (MIX3-1 variant, 2022). The C content varied from 42.47% (S-FH variant, 2020) to 48.10% (MIX2-2 variant, 2022). The maximum permissible amount of N, S, and Cl in solid granulated biofuel was determined by the standards. Only in three pellet variants (S-FB variants in 2021 and 2022, and the MIX3-1 variant in 2022) did the N content exceed the maximum value (2.0%) set by the ISO 17225-6:2021 standard. The higher N concentration in the faba bean biomass in the second and third year of the experiment could be due to the ability of these plants grown in the same field during the experiment to accumulate nitrogen. In all other variants, the N content remained below this limit. It should be noted that the S levels in all pellets were very low and did not surpass the maximum limit of 0.20% for class A pellets, as established by the ISO 17225-6:2021 standard. In three pellet variants (S-Mz, S-FH, and MIX2-1 variants in 2022), the Cl content slightly exceeded the maximum limit of 0.40% set by the standard for class B pellets, while in the other variants, the Cl content adhered to the specified norms. As indicated by Podlesna [68], the concentration of Cl in plants and their tolerance to its excess in the soil varies. With its high availability in the environment, its excessive accumulation is possible because Cl is readily absorbed from the soil and directly from the air. The Cl concentration in plant biomass is also affected by nitrogen fertilization. To objectively assess the causes of Cl change in the biomass of the plants we studied, more detailed studies are needed. The Cl content of maize stover pellets was found to be 0.53%, by Zapata et al. [69], which was above the acceptable limit. This illustrates that higher than allowed Cl content in herbaceous biomass found in some cases is one of the problems for which solutions need to be proposed.
The content of C, H, N, and Cl in the produced pellets is observed in Figure 4.
Similar results from the study of the C, H, and O composition of plant biomass pellets were presented by Zapata et al. [69]. The C content in maize stalk pellets was 42.60% and in wheat straw pellets was 45.50%. The content of H was 5.20 and 5.80%, respectively, and O was 38.20% and 41.91%, respectively.
Certain measures can help to solve the problem of increased Cl in the pellets. Canabal et al. [70] indicated that the quality of pellets made from biomass mixtures characterized by the increased Cl content (the authors studied pellets made from pine wood with high proportions of eucalyptus wood) could be improved by torrefaction. Studies have shown that the torrefaction process can reduce the content of Cl in pellets by 63–77% [70].
As for the influence of the elemental composition of biomass on the operation of the boiler, the complexity of the reasons should be considered. Vasileiadou et al. [71] pointed out that the problems of slag formation, fouling, agglomeration, and related corrosion were multifaceted and depended not only on the fuel and ash composition but also on various factors such as combustion temperature and technology, fuel-air ratio, and boiler construction material.
3.3. Calorific Value and Ash Content
The calorific value is defined as the energy released during combustion. The LCV of dry wood fuel is usually 18.4–21.3 MJ kg−1 [9]. The LCV of our investigated pellets is seen in Figure 5.
The lowest calorific value (16.72 MJ kg−1) was found for S-FB pellets in 2021. The caloric value of this variant was the same as that of pellets made from sunflower stalks (16.72 MJ kg−1), which was determined by Zardzewiały et al. [72]. The highest calorific value (18.13 MJ kg−1) was established for S-Mz pellets in 2022. The calorific value of MIX3-1 pellets in 2022 was not far behind, which was 18.00 MJ kg−1. Thus, the calorific value of the latter two pellet variants was close to the calorific value of wood biomass pellets. It should be noted that all pellets met the requirements of the ISO 17225-6:2021 standard for calorific value (≥14.5 MJ kg−1).
The ash content can be considered the main parameter defining the quality of biomass used in power plants. Therefore, it is no coincidence that the quality classes defined in the technical specifications of solid biofuel always differ according to the ash content [73]. The results of the pellet ash test are seen in Figure 6.
The lowest ash content (4.49%) was determined in MIX2-1 pellets in 2020, and the highest (8.87%) in S-FH pellets in 2022. The ash content of non-wood biomass pellets was higher compared to pellets made from wood biomass. For example, according to Mack et al. [74], the ash content of spruce wood chips (without bark) pellets is 0.41%, pine (without bark) pellets is 0.39%, and oak (without bark) pellets is 0.27%. A higher ash content compared to wood biofuel may be one of the reasons why non-wood biomass biofuel is less attractive compared to wood biofuel. However, the higher ash content of non-wood biofuels is permitted by the standards and should not be a limiting factor for the use of non-wood biofuels. The ash content of all our produced pellets did not exceed the maximum permissible value of ash content (≤6% for class A pellets and ≤10% for class B pellets) defined in the standard ISO 17225-6:2021 [40].
3.4. Life Cycle Assessment—Environmental Analysis of Pellet Production
Results (Table 3) show the impacts associated with the generation of 1 GJ of thermal energy supplied by a pellet boiler. Results have been presented for the seven analyzed cases. Eleven impact categories have been included in these analyses.
As illustrated by the data in Table 3, case MIX2-1 had the lowest impact on the six impact categories: ADF, GWP, ODP, PO, AP, and EP. In the other five impact categories, the negative impact of this case was lower compared to the five other scenarios. MIX2-3 had the lowest impact on five impact categories: AD, HT, FWAE, MAE, and TE. However, for category EP, this option had a greater negative impact than the four other scenarios. The S-Mz scenario had the highest negative impact in as many as ten impact categories. The carbon footprint of this scenario per FU was 60% higher compared to the MIX2-1 scenario.
The impact of the different processes of pellet production and use for heat production has been investigated for four impact categories: ADF, GWP, AP, and EP.
Figure 7 illustrates the contribution of different phases to ADF.
In the case of six scenarios, the phase most influencing ADF was pellet and heat production (from 38.6 to 52.6%). Only in the case of S-Mz, biomass production had the greatest impact (51.45%).
When evaluating the environmental impact, the largest carbon footprint was recorded in the case S-Mz—29.11 kg CO2eq per FU. The lowest impact on the environment was determined in the case of MIX2-1 pellets—18.20 kg CO2eq. The determined carbon footprint of this variant was 37.48% lower compared to the case of S-Mz. Quinteiro et al. [75] determined that emissions through the combustion of maritime pine pellets in a pellet stove (7.79 kg CO2eq per GJ) were lower compared to the seven cases analyzed by us. However, it should be considered that different raw materials (wood biomass) were analyzed, and the limits of the system were from cradle-to-grave.
GWP is one of the most used environmental impact indicators in the LCA studies. It is a relative measure that shows how much heat GHG traps in the atmosphere in terms of carbon dioxide equivalent, which is one of the most pressing environmental problems [36]. The contribution of the different phases to the global warming potential is shown in Figure 8. Pellet and heat production had the largest negative impact on global warming (36.49–48.78%) in all cases except for S-Mz. In this case, biomass production was the biggest contributor to global warming (43.93%). This can be explained by the fact that the lowest dry yield (4461 kg ha−1) [39] was obtained in this scenario. In the case of S-FH and S-FB, ash utilization had only a slightly smaller effect than pellet and heat production. Sadaghiani et al. [49] showed that in the case of wood pellets, their production and burning processes had the greatest impact on the environment.
In addition, Sadaghiani et al. pointed out that acidification was one of the main impact categories of pellet burning [49]. Acidification refers to the release of acidifying substances that have negative effects on soil, groundwater, ecosystems, and materials [30]. Figure 9 illustrates the contribution of different phases to the acidification effect category.
For six scenarios, pellet and heat production contributed the most to acidification, and only in the case of the S-Mz scenario did biomass production have the largest contribution to this impact category.
Biomass cultivation, harvesting, and biomass preparation are stages that contribute to eutrophication. This is related to the use of fertilizers for biomass production, in addition, factors such as electricity production, compaction and production equipment, and fuel consumption are important for this impact category [76].
Eutrophication is one of the most frequently analyzed impact categories in the LCA of biofuel production [34]. The contribution of different processes to eutrophication is illustrated in Figure 10.
As can be seen in Figure 10, for scenarios S-Mz, S-FB, MIX2-2, and MIX2-3, the biomass production stage had the highest contribution to eutrophication. This can be explained by the high amount of maize or faba bean seeds needed.
Analysis of the four impact categories shows that pellet and heat production is usually the phase with the biggest contribution to the environment primarily due to the high consumption of electricity at this stage. Biomass production had the largest contribution to all four impact categories in the case of the S-Mz scenario, which obtained the lowest yield compared to the other six scenarios. This fact shows that to reduce the negative impact on the environment, biofuel production from agricultural plants should be focused on high-productivity crops.
The use of ash obtained from the burning of biomass pellets as fertilizer in the same bioenergy production system reduces the negative impact on the environment. Other studies [54] analyzed systems where ash products generated in the boilers were considered waste that was disposed of at a local landfill site. Such a point of view should not be considered in recent years of circular economy.
The production and use of solid biofuel inevitably had an impact in other categories as well, so for more detailed data it is appropriate to carry out a more detailed analysis covering each of the impact categories.
4. Conclusions
The results of the 3-year study of investigated multi-crop plant biomass pellets show that this biomass is a promising source of solid biofuel production. The use of this biomass allows the production of high-quality solid biofuels with physical-mechanical and chemical properties that meet the requirements of the non-wood biofuels standard ISO 17225-6:2021.
The LCA showed that scenarios MIX2-1 and MIX2-3 had the lowest environmental impact, using biomass from the binary crop of maize and hemp as well as hemp and faba bean for pellet production. The S-Mz scenario, using maize biomass grown in a single crop to produce pellets, had the highest carbon footprint per FU (29.1 CO2eq). This carbon footprint was as much as 60% higher compared to MIX2-1, which had the lowest carbon footprint of all scenarios (18.2 CO2eq). Overall, three of the four scenarios with biomass cultivation as a polyculture showed better environmental performance compared to the scenarios with biomass cultivation as a monoculture. The only exception was the MIX2-2 scenario, which used biomass from a binary crop of maize and faba bean and had one of the least favorable environmental impacts compared to the other scenarios. To ensure the sustainable production and use of solid biofuel, it is recommended to choose binary crop biomass as a raw material according to the MIX2-1 and MIX2-3 scenarios.
Conceptualization, R.P., A.J. and K.V.; methodology, R.P., A.J., K.V., E.L. and M.P.; validation, R.P. and A.J.; formal analysis, R.P. and A.J.; investigation, R.P., A.J., K.V., E.L. and M.P.; resources, K.V. and R.P.; data curation, R.P., M.P. and E.L.; writing—original draft preparation, R.P., A.J. and K.V.; writing—review and editing, A.J. and K.N.; visualization, R.P.; supervision, A.J.; project administration, R.P. and A.J. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Data are contained within the article.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 3. The moisture content of the produced pellets. In the figure, any two samples with a common letter are not significantly different, as assessed using the least significant difference.
Enlarge this image.Figure 4. The content of (a) C, (b) H, (c) N, and (d) Cl in the produced pellets. In the figure, any two samples with a common letter are not significantly different, as assessed using the least significant difference.
Enlarge this image.Figure 5. Lower calorific value of the produced pellets. In the figure, any two samples with a common letter are not significantly different, as assessed using the least significant difference.
Enlarge this image.Figure 6. The ash content of the pellet samples. In the figure, any two samples with a common letter are not significantly different, as assessed using the least significant difference.
Enlarge this image.Figure 7. Contribution of the life cycle phases to abiotic depletion (fossil fuels).
The content of P and K in ash and the use of ash for fertilization.
| Item | Unit | S-Mz | S-FH | S-FB | MIX2-1 | MIX2-2 | MIX2-3 | MIX3-1 |
|---|---|---|---|---|---|---|---|---|
| Content of K in ash | g kg−1 | 183.01 | 156.14 | 229.16 | 206.94 | 171.42 | 245.13 | 222.03 |
| kg FU−1 | 0.70 | 0.76 | 0.95 | 0.79 | 0.71 | 0.91 | 0.88 | |
| Content of P in ash | g kg−1 | 21.35 | 29.38 | 40.64 | 33.89 | 45.54 | 42.98 | 39.72 |
| kg FU−1 | 0.08 | 0.14 | 0.17 | 0.13 | 0.19 | 0.16 | 0.16 |
Inventory data associated with the multi-crop biofuel pellet production and final energy generation system per FU (1 GJ of thermal energy).
| Item | Scenarios | Data Source | ||||||
|---|---|---|---|---|---|---|---|---|
| S-Mz | S-FH | S-FB | MIX2-1 | MIX2-2 | MIX2-3 | MIX3-1 | ||
| A. Input | ||||||||
| Fertilizer: | ||||||||
| N, kg | 0.64 | 0.32 | 0.30 | 0.24 | 0.37 | 0.26 | 0.13 | [ |
| P, kg | 0.64 | 0.32 | 0.30 | 0.24 | 0.37 | 0.26 | 0.13 | |
| K, kg | 0.64 | 0.32 | 0.30 | 0.24 | 0.37 | 0.26 | 0.13 | |
| Maize seeds, kg | 0.90 | 0.36 | 0.74 | 0.10 | [ | |||
| Hemp seeds, kg | 0.16 | 0.08 | 0.12 | 0.04 | ||||
| Faba bean seeds, kg | 2.38 | 3.27 | 2.16 | 1.04 | ||||
| Biomass (dry basis), kg | 63.41 | 65.04 | 64.45 | 64.24 | 63.92 | 64.12 | 63.75 | [ |
| Maize biomass (dry basis), kg | 63.41 | 12.50 | 28.89 | 3.78 | ||||
| Hemp biomass (dry basis), kg | 65.04 | 51.74 | 50.89 | 20.13 | ||||
| Faba bean biomass (dry basis), kg | 64.45 | 35.03 | 13.23 | 39.85 | ||||
| Electricity for biomass granulation, kWh | 5.07 | 4.34 | 5.73 | 4.71 | 5.40 | 4.99 | 5.01 | [ |
| Electricity for ash-based fertilizers, kWh | 1.92 | 2.44 | 2.07 | 1.91 | 2.06 | 1.86 | 1.98 | [ |
| Cattle manure for ash granulation, kg 5 | 15.37 | 19.50 | 16.58 | 15.26 | 16.50 | 14.87 | 15.81 | - |
| B. Output | ||||||||
| Biomass pellets (dry basis), kg | 63.41 | 65.04 | 64.45 | 64.24 | 63.92 | 64.12 | 63.75 | Data source—our study |
| Ash, kg | 3.84 | 4.88 | 4.15 | 3.82 | 4.12 | 3.72 | 3.95 | |
| Ash fertilizer mixture, kg | 19.22 | 24.38 | 20.73 | 19.08 | 20.62 | 18.58 | 19.76 | |
1—The data source contains information on fertilizer application rate, kg ha−1; 2—The data source contains information on seed rate, kg ha−1; 3—The data source contains information on biomass yield, kg ha−1; 4—The data source contains information on electricity consumption for granulation, kWh kg−1; 5—It was assumed that manure and ash are granulated in a ratio of 80:20.
Characterized impacts associated with the pellet chain per FU.
| Impact Category | Unit | S-Mz | S-FH | S-FB | MIX2-1 | MIX2-2 | MIX2-3 | MIX3-1 |
|---|---|---|---|---|---|---|---|---|
| AD | kg Sbeq | 1.7 × 10−3 | 1.7 × 10−3 | 1.6 × 10−3 | 1.5 × 10−3 | 1.6 × 10−3 | 1.5 × 10−3 | 1.6 × 10−3 |
| ADF | MJ | 321.21 | 215.69 | 216.49 | 177.85 | 250.03 | 184.11 | 204.06 |
| GWP | kg CO2eq | 29.11 | 21.64 | 22.01 | 18.20 | 24.99 | 19.11 | 20.39 |
| ODP | kg CFC-11eq | 3.5 × 10−6 | 2.8 × 10−6 | 2.8 × 10−6 | 2.2 × 10−6 | 3.0 × 10−6 | 2.3 × 10−6 | 2.5 × 10−6 |
| HT | kg 1,4-DBeq | 82.19 | 80.62 | 76.85 | 73.71 | 78.55 | 73.26 | 74.88 |
| FWAE | kg 1,4-DBeq | 53.24 | 51.30 | 49.25 | 47.12 | 50.67 | 46.68 | 47.96 |
| MAE | kg 1,4-DBeq | 76,829.67 | 73,636.66 | 70,789.12 | 67,309.71 | 72,847.00 | 66,833.65 | 68,748.25 |
| TE | kg 1,4-DBeq | 1.1 × 10−1 | 9.4 × 10−2 | 8.7 × 10−2 | 8.4 × 10−2 | 9.4 × 10−2 | 8.2 × 10−2 | 8.8 × 10−2 |
| PO | kg C2H4eq | 1.1 × 10−2 | 1.0 × 10−2 | 9.8 × 10−3 | 8.7 × 10−3 | 1.0 × 10−2 | 8.7 × 10−3 | 8.9 × 10−3 |
| AP | kg SO2eq | 1.6 × 10−1 | 1.2 × 10−1 | 1.3 × 10−1 | 1.0 × 10−1 | 1.5 × 10−1 | 1.1 × 10−1 | 1.1 × 10−1 |
| EP | kg PO4—eq | 7.6 × 10−2 | 5.7 × 10−2 | 1.0 × 10−1 | 5.3 × 10−2 | 1.3 × 10−1 | 9.2 × 10−2 | 7.3 × 10−2 |
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; Venslauskas, Kęstutis 3
; Navickas, Kęstutis 3
; Praspaliauskas, Marius 1
; Lemanas, Egidijus 1
1 Laboratory of Heat-Equipment Research and Testing, Lithuanian Energy Institute, Breslaujos Str. 3, LT-44403 Kaunas, Lithuania;
2 Department of Agricultural Engineering and Safety, Faculty of Engineering, Agriculture Academy, Vytautas Magnus University, Studentu Str. 15A, Akademija, LT-53362 Kaunas, Lithuania
3 Department of Mechanical, Energy and Biotechnology Engineering, Faculty of Engineering, Agriculture Academy, Vytautas Magnus University, K. Donelaičio g. 58, LT-44248 Kaunas, Lithuania;