Co-pelletization of Hemp Residues and Agricultural Biomass: Effect on Pellet Quality and Stability

Abstract

The rising interest in lowering the use of fossil fuels, which influence environmental pollution and global warming, is driving a substantial increase in renewable sources. Agricultural residues are the likely potential source for bioenergy generation. Some of them are already utilized for energy. Nonetheless, their potential is underutilized due to low biomass quality and high concentrations of sulfur and chloride, which induce the corrosion of adjoining equipment. However, their ash content and ash melting point make their utilization as renewable resources essential. Therefore, there is a need to find technologies to enhance biomass utilization for bioenergy processes. With the increase in hemp cultivation to extract phytocannabinoids, the amount of unused biomass has increased. The aim of this research was to investigate the use of hemp biomass for pellets and improve pellet quality by mixing them with lignin and oak sawdust. The results showed that the lowest amount of ash was found in pellets with 80% oak sawdust and 20% hemp residue compared with pellets made from mixtures of hemp residues, lignin, and oak sawdust. The highest calorific value was achieved by mixing hemp residues (20%) with lignin (80%).

1. Introduction

Climate change is one of the leading environmental challenges faced around the world. The only way to mitigate the negative impact on the planet is to reduce greenhouse gas (GHG) emissions drastically. This has necessitated the European Commission setting the goal to become CO₂ neutral by 2050 [1]. One of the crucial steps to fulfill these goals is the replacement of fossil fuels with renewable sources. Burning this fuel releases as much as 3.7 times more tons of CO₂ into the environment than burning biofuels [2]. In Europe, biomass heating systems appeared earlier than in other countries of the world, with mature utilization technologies and complete policy frameworks. Many EU countries, such as Lithuania, are located at high latitudes and high altitudes, which means they experience long, cold winters and high heating needs (Figure 1). Since they prioritized the development of biomass as a renewable heating source relatively early on, more and more households, industries, district heating networks, and buildings are turning to bioenergy [3].

In many countries, district heating systems have replaced fossil fuels with biofuels. Biofuels, mainly used for heat generation, are divided into three main groups: woody biomass, agricultural residues, and herbaceous energy crops. The main properties that describe the quality of biofuels are calorific value, ash content, fuel moisture, carbon and nitrogen content, and their ratio.

In most countries, wood is the most essential type of solid fuel. The part of tree stems that is unsuitable for industrial processing can be used as biofuel. This wood does not need any specific preparation, an advantage in the acceptability of wood waste as biofuel, with the main focus on logging residues [4]. The waste can be the above-ground part of the stump, tree tops, chopped sawn wood, or off-cuts from cutting tree stems [5,6]. As a rule, the distribution of wood waste is widely distributed on the map, so it is imperative to have economical and suitable transportation to the waste disposal plants for biofuel. For the pellets to be made from forest or wood industry production waste, it is important to ensure that the wood waste does not lose its technical and chemical properties during transportation and storage [7].

Even though firewood and wood residues are widely used, the attendant environmental issues and the need for biomass chemical composition and physical quality improvement have necessitated that attention should be paid to alternative sources of biofuels such as herbaceous plants, short-rotation trees, food waste, or even their mixtures. There is a wide range of biomass sources, but this research was focused on the analysis of the main three ones: hemp residues, oak sawdust, and lignin.

Hemp is a traditional industrial crop in many regions of the world. For centuries, hemp was primarily a source of strong stem fiber and seed oil [7]. Recently, this species has received a lot of attention for alternative uses, including hemp for the extraction of active compounds or even biofuel production. Hemp is an herbaceous plant that can grow to about 1–2 m in height, depending on variety, and environmental and agronomic conditions. Some authors have reported that hemp can reach yields higher than 22 tons per hectare [8], but usually, hemp yield is 7–15 tons per hectare [5,9]. In addition to a large amount of biomass, the plant has a well-developed tap root system, growing into the soil to a depth of about 2 m or more. This crop is rich in cellulose (73–77%) and low in hemicellulose and lignin (7–9% and 2–6%, respectively) compared to other herbaceous crops [6]. Hemp is recognized as a low-maintenance plant due to its short growing cycle, with the added benefit of reducing the need for pesticides [10]. With the increase of hemp areas for phytocannabinoid extraction, hemp residues have increased drastically. It is not very valuable to leave hemp residues as soil amendment due to its long degradation time in the soil. Therefore, the use of such biomass for bioenergy might be an option for more environmentally and economically friendly hemp management. Nevertheless, there is very limited information about the ability to use hemp for pelleting and heat generation.

Oak is known as one of the trees with the best heating properties. Nevertheless, it is very difficult to make pellets from oak sawdust because its durability is very low. Therefore, there is a need to look for alternative biomass sources to mix and have better quality pellets. Lignin is another potential organic material mined from the ground. The unique lignin mine in Lithuania has accumulated about 630 thousand cubic meters of lignin. Lignin has been stored for more than two decades, but due to the peculiarities of its chemical composition, it is perfectly preserved. This lignin differs from the lignin found in plants because it is mined directly from the ground. It has excellent bonding abilities and could be used as binding material for making pellets; however, the quality of such biomass needs to be studied.

All studied biomass sources have very low bulk density, which can be increased only by effective compression. The more the raw material is compressed, the better its transportation and combustion properties [11]. Pelleting is the most popular biofuel preparation technology. Pelleting converts potentially ground ingredients into dense, free-flowing, durable pellets. Generally, one of the most important indicators of granule quality is durability and mechanical durability [12,13,14]. Compaction of biomass using pressure and temperature, compression, and extrusion systems—pellet and briquette press—can increase the bulk density by 4–5 times [12]. World production of wood pellets has multiplied for over a decade and today stands at almost 30 million tons annually. Forecasts suggest that fuel pellet costs are rising rapidly worldwide [15].

For the pellets to be competitive in the market for their heating properties, the moisture content of the pellets must be around 11%. It is essential for durability and mechanical durability, with lengths of up to 40 mm and a diameter of 6 to 8 mm [15,16,17]. Grasses, plant residues, and nut peels can also be recycled into pellets, thus reducing waste [16]. The critical chemical composition of pellets produced from biomass includes nitrogen, carbon, cellulose, hemicellulose, starch, lignin, fat, and ash. These components affect the density and burning process. Compressed proteins and starch act as a binder that contributes to the durability of the granules [18,19,20]. Typically, pellets and briquettes have a bulk density of 600–750 kg·m³ and 350–450 kg·m³ [13]. In addition, due to the lower moisture content of dense biomass, it can be stored for a more extended period with minimal loss of quality [18]. High nitrogen content in the biomass might increase nitrous emissions, harming the environment. Therefore, mixing different biomass sources with wood sawdust is expected to improve the content of nitrogen or other chemical compounds.

It was hypothesized that hemp residues might be an appropriate source for pelleting and combining its biomass with oak sawdust or/and lignin will improve pellet properties. The aim of this research was to investigate the use of hemp biomass for pellets and improve pellet quality by mixing them with lignin and oak sawdust. The research objectives were: (1). To identify the potential biomass sources for pellet production; (2). To evaluate the influence of different biomass mixtures on pellet durability and quality; and (3). To assess the energy value of selected materials.

2. Materials and Methods

2.1. Research Design

Hemp (Felina 32) residues were collected from a field in Lithuania (Lithuanian Research Center for Agriculture and Forestry). Oak sawdust was collected from MB Biomica (Prienai distr., Lithuania), while lignin was collected from UAB Ligneniko (Kėdainai, Lithuania). This material was dried to a moisture content of 11%. The pellets were made using the ATLAS auto T40, and a pressure of 20 tons was applied. The composition of the different biomass samples used for pelleting is presented in

Table 1. The biomass mixtures used in the experiment.

Nomenclature	Percentage of Biomass in Mixtures
A80K20	Oak sawdust 80% hemp residues 20%
A50K50	Oak sawdust 50% hemp residues 50%
A20K80	Oak sawdust 20% hemp residues 80%
K80L10A10	Hemp residues 80% lignin 10% oak sawdust 10%
K50L25A25	Hemp residues 50% lignin 25% oak sawdust 25%
K20L40A40	Hemp residues 20% lignin 20% oak sawdust 60%

.

2.2. Calorific Value

The calorific value was measured using the equipment IKA 2000 (Königswinter, Germany) according to the standardEN 14918 [21].

2.3. Durability of Pellets

Biomass mixtures were granulated by Specac Atlas auto T40 equipment (Fort Washington, PA, USA). First, 3 g of biomass samples were put in the pressure vessel and compressed. After pelleting, all pellets were left to be stable for 24 h. Before conducting durability tests, pellets were weighed. The pellets were placed in the DELTA equipment for durability tests at 50 rpm 500 times. After this, the pellets were weighed again, and the stability was calculated. Each treatment consisted of three replicates. The durability of the pellets was calculated by Formula (1).

$$D_U=\frac{m_A}{m_E}·100$$

(1)

where m_E—pellet weight before the durability tester (g); m_A—pellet weight after durability tester (g).

2.4. Carbon, Nitrogen, and Sulfur Content

Carbon and nitrogen content was measured using the CNS Elemental combustion system equipment (Amsterdam, The Netherlands). First, 5 g of the samples was weighed in the alov capsule, then folded and placed in the equipment.

2.5. Chlorine Content

Chlorine content was determined using Thermo Fisher ICAP Q ICP-MS (Waltham, MA, USA). First, 3 g of the sample was diluted with 2 mL of HCl and 10 mL of HNO₃ and mineralized for 1 h after mineralization. The resulting solution was diluted with water to make up to 2% HNO₃ in a 50 g flask.

2.6. Statistical and Numerical Analyses

The observed data were statistically processed using SAS 9.4 software. The Tukey HSD test was applied to determine significant differences between means at an alpha level of 0.05. Lowercase letters that differ denote significant differences at p < 0.05.

3. Results and Discussion

3.1. Chemical Composition of the Pellets

Energy generation is one source of CO₂ and even more dangerous nitrous oxide emissions. It has been noted that biomass nitrogen content, which could be emitted to the environment during thermal conversion processes, depends on biomass origin and quality, incineration temperature, and oxygen concentrations. According to BALTPOOL (biomass trade), the nitrogen content of the standard biomass cannot exceed 1% of the dry weight. The best pellets have a nitrogen content of less than 0.3% of the dry weight (https://www.baltpool.eu/lt/birzoje-prekiaujami-produktai/, accessed on 5 August 2023).

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

The nitrogen and carbon content of the different mixed observed during this research is shown in Figure 2 and Figure 3. The yield of annual crops is mainly influenced by nitrogen availability in the soil, which is one of the key elements for biomass and plant proteins accumulation [22,23,24,25]. Therefore, hemp presented the highest nitrogen content in its biomass (0.59%), while the lowest amount of nitrogen was observed in oak sawdust at 0.23%. Mixing hemp with the selected biomass helped reduce nitrogen content. Similarly, in the group of mixtures consisting of only hemp residues and oak sawdust, the lowest nitrogen content (0.26%) was observed when oak sawdust comprised 80% and hemp residues 20% (A80 K20). However, there were no significant differences between the different amounts of hemp residues and oak sawdust as observed in the mixtures (Figure 2). In the mixtures consisting of hemp residues, oak sawdust, and lignin, the lowest nitrogen content was determined at 0.35% when hemp residues were 20%, lignin 20%, and oak sawdust 60%. When comparing this mixture with others, there were significant differences in N concentration in all the biomass mixtures except for the combination where oak sawdust was 50% and hemp residues 50%. In comparison, nitrogen content in wood pellets varies from 0.04% to 0.26% [26].

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

The quality of biomass used for bioenergy is mainly described by the carbon content. The carbon content in biomass is directly correlated to heating or calorific value. The higher the carbon content is, the more heat will be produced [27]. It was determined that the highest carbon content was in lignin (41.93%), and the lowest was found in hemp residues (33.53%). However, there were no significant differences between the evaluated mixtures (Figure 3). It can be stated that all evaluated biomass sources can potentially be used for bioenergy generation because of the carbon content.

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

In order to protect the boiler from corrosion, it is very important to control the chlorine and sulfur levels in the biofuel. When pellets are burnt with high levels of sulfur and chlorine, these chemicals form acid compounds that settle on the walls and eventually break them down by forming corrosion [28]. According to ISO requirements, the amount of chlorine in the granules cannot exceed 0.02% [29]. The highest chlorine content was determined at hemp and lignin, with over 0.1% content (Figure 4). The lowest chlorine content was found in oak sawdust, with statistical differences in the chlorine content of different raw materials. Additionally, when comparing pellets, the lowest chlorine content was found in the mixtures made from A20K80. There were no significant differences compared to combinations made from hemp residues and oak sawdust.

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

Another important parameter in pellets is sulfur composition. Sulfur not only causes corrosion to the boilers in which the pellets are burnt, but during combustion, sulfur oxidizes and emits harmful substances into the environment. The substances emitted in the form of sulfur hexafluoride, which oxidized to SO₂ and the rest to SO₃, are very aggressive components that contribute to greenhouse gases. They are acutely harmful to human health and directly contribute to the acidification of agricultural lands, forests, oceans, and the corrosion of heating machines. The sulfur content of the pellets should not exceed 0.04% according to ISO requirements. The highest sulfur content was found in lignin at 0.21%, with the lowest content found in oak and hemp at 0.02–0.04%, respectively (Figure 5). Comparing the sulfur content of lignin with the results obtained by other researchers, it was observed that the sulfur content is very similar at 0.20% [30]. The amount of sulfur in wood sawdust pellets differs from other researchers. Previous studies found 0.1% sulfur in the pellets, while in our studies, it was 0.02%. The variation in the content may vary with different woods [31]. No significant difference was observed in the sulfur content of the different pellets from different constituent components made from hemp residues, oak sawdust, and lignin.

3.2. Ash Content in the Pellets and Different Raw Materials

Ashes are the non-burning residue left after the fuel or combustible material has not completely burned. They consist of non-flammable and non-volatile (non-volatile during combustion) mineral substances. According to ISO 17225-2:2021 standards [32], biofuel ash content cannot exceed 2%. In all other cases, when using fuel with higher ash content, the ash must be removed continuously, which increases energy consumption and costs.

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

The highest ash content was determined in lignin (9.49%), while the lowest ash content was observed in oak sawdust at 0.70% (Figure 6). Comparing the results of lignin ash content with those obtained by other researchers, it was observed that the obtained ash content is similar. Scientists from the USA found 6.07% ash content in lignin [30]. This determined ash content is 3% lower than the lignin ash content obtained in our studies, but this may be due to the different lignin compositions in different countries. Furthermore, comparing our experimental results of wood pellets with other previous studies, it was observed that the ash contents obtained from wood amounts ranged between 0.31 and 1% [30,33]. The results are very similar to those obtained in these studies when the ash content of oak sawdust pellets was determined. The hemp ash content obtained in our study was 2.59%, while other researchers obtained 3.60% ash content in hemp residues of the same variety [34]. This difference may be due to differences in the preparation of hemp residues, and it is unclear whether the fiber was removed entirely in other studies.

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

The high ash content in lignin influenced the pellet quality of all mixtures. The lowest amount of ash was found in pellets with 80% oak sawdust and 20% hemp residue. However, there were no significant differences compared to other oak sawdust and hemp residue pellets. The highest amount of ash was determined in the mixture consisting of hemp residues 50% lignin, 25% oak sawdust 25% (K50L25A25) (Figure 7). Although research has been conducted in areas where wood sawdust is mixed with different grasses or straws, our study presented research in the ash content of pellets made from oak sawdust and hemp stalk residues These biomass mixtures from wood sawdust and different grasses presented ash content in the range of 3.29–7.49% [31,34]. However, it was noticed that the ash content of these mixture pellets was determined to be 1.24–1.76%. The ash content of wood sawdust and other herbaceous plant pellets is two times higher than the pellets produced in this experiment.

3.3. Calorific Value of Pellets and Different Raw Materials

Calorific value directly indicates energy and economic properties. The upper and lower calorific values can be calculated from the calorimeter results. When calculating the upper calorific value, we assume that water vapor has been released from the combustion products so that the wood is completely dry or formed as a product of hydrogen combustion, fully condensed. In contrast to calculating the upper calorific value, when calculating the lower calorific value, we do not consider the heat of condensation of water vapor of combustion products. The upper and lower thermal differences in wet material and large volumes tend to be mostly water [35].

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

The highest calorific value is determined in lignin at 21.35 MJ·kg⁻¹ with the lowest calorific value found in hemp residues at 16.80 MJ·kg⁻¹. After comparing the granule mixtures, the highest calorific value was determined in the biomass mixture, hemp residues 50%, lignin 25%, and oak sawdust 25%—19.94 MJ·kg⁻¹. When comparing these pellets with other pellets mixed with hemp residues, lignin, and oak sawdust, significant differences (p < 0.05) were found only in the mixture where the highest calorific value was determined. Meanwhile, the lowest calorific value was in pellets, where hemp residues make up 80% and oak sawdust 20%. However, compared to other pellets made only from oak sawdust and hemp residues, there were no significant differences between these pellets (Figure 8).

Additionally, after comparing the mixtures of different pellets, the pellets mixed with hemp residues 50%, oak sawdust 25%, and lignin 25% have the highest calorific value. These pellets, however, showed no significant differences between pellets mixed with hemp residues 50% and oak sawdust 50% and hemp residues 80% and oak sawdust 20%. It was also noticed that the calorific value of the same variety of hemp in our experiments was higher (17.37 MJ·kg⁻¹) compared to 16.80 MJ·kg⁻¹ obtained by other scientists [24]. In pellets from wood sawdust, the calorific value in other scientists’ experiments reached 14 MJ·kg⁻¹ compared to 18.67 MJ·kg⁻¹ observed in our study. This may be due to different wood and different moisture content. When comparing the calorific value results of pellets made from different biomass mixtures with the studies conducted by other scientists, it was observed that in all cases, the calorific value was relatively low, from 10–15 MJ·kg⁻¹ [36]. In this study, the calorific value varied from 18–20 MJ·kg⁻¹. These significant differences can be explained by the fact that the mixtures of this experiment contained lignin with a calorific value of 21.35 MJ·kg⁻¹.

3.4. Durability of Pellets

Pellet durability is an important property to determine the quality and usability of pellets. One of the biggest problems associated with durability occurs during transport and straining work. It is necessary to ensure that the pellets or briquettes are sufficiently durable. These problems arise due to moisture absorption and falling [37]. The biomass of plants rich in lignin might improve pellet durability properties. Considering the biofuel exchange requirements, pellets’ durability should be no more than 98% and no less than 97.5%, but usually, most biomass sources have higher durability of more than 95% [27].

Data are presented as means ± standard error; different letters correspond to significant differences (p < 0.05) between means according to Tukey’s test.

After carrying out the pellet durability test, it was observed that the most durable pellets at 95% were found in the biomass mixture comprising hemp residues 20%, lignin 20%, and oak sawdust 60%. In contrast, the lowest durability at 78% was found in pellets with hemp residues 10%, lignin 10%, and oak sawdust 80% (Figure 9). Very similar results were obtained by granulating pellets from biomass with a stability of less than 93% [38].

The heat map shows how the pellets in this experiment correspond to the main parameters of the pellets (Figure 10). Yellow squares present lower levels of different pellet parameters, while green colors indicate the highest levels, n.a means that the durability of lignin was not determinate. Pellets made from various organic materials follow the standard requirements for fuel pellets. Nevertheless, mixing hemp with oak sawdust and lignin might increase the pellet quality and energy value.

4. Conclusions

This paper proposed the potential improvement of hemp residue pellets by mixing the hemp biomass with oak sawdust and lignin to increase hemp pellets’ quality (chemical composition), durability, and heating value. The present research results confirmed the hypothesis that adding more oak sawdust and lignin biomass to hemp residues increases the hemp pellet quality. The mixture of hemp residues with oak sawdust and lignin lowered nitrogen content by 59% in pellets, positively affecting the environment. At the same time, lower NO_x emissions would be generated during the burning process. Nevertheless, all mixtures with lignin had higher sulfur content, which might increase equipment corrosion. Additionally, adding lignin and oak sawdust to hemp biomass improved pellet durability and caloric value. Nevertheless, we faced the problem of excessive ash content only in pellets where hemp residues are mixed only with oak sawdust following EU requirements.

Generally, we deduced that pellets made from various organic materials (hemp residues, lignin, and oak sawdust) follow the standard requirements for fuel pellets and can reduce the high amounts of residual biomass with a positive environmental effect. Mixing different biomass sources might be an issue for increasing pellet quality and energy potential.