1. Introduction
A developing society needs energy to sustain its growth. Growing consumerism, technological progress, volatile geopolitical situations, challenges posed by the supply of fossil fuels, continuous population growth, production of foodstuffs and other items of everyday life—all these drive up the global energy demand [
1]. Environmental protection has become the top priority, and sustainable waste management at various levels allows the reduction in biosphere pollution. Agricultural and forest waste, as well as waste from the agri-food industry, are becoming a more and more burning issue. Ligno-cellulosic materials, such as hay or straw, are not only intended for animal nutrition but are often considered farm waste [
2]. Energy transition and problems with the supply of fossil raw materials make their use for power generation purposes more than justified. Renewable energy has become the core element of sustainable development nowadays. The ecological benefits and the prevention of global warming are important, yet not the only advantages of RES (Renewable Energy Sources). Some others include access to nearby energy resources, new jobs, the country’s proper energy balance, or becoming independent of energy imports [
3]. By laying down new requirements limiting the emission of harmful gases from the burning of fossil fuels, the European Union has obliged the member states to introduce new, innovative technologies to increase the use of RES and control the consumption of cleaner energy in Europe. The laws made by the Polish legislator, in particular on extra financing options, have raised the share of RES in the national energy balance [
4]. However, the mere interest in RES does not suffice; therefore, the various types of available funding are aimed at transforming the curiosity and growing awareness of the general public into the practical use of renewable energy. The beneficiaries of green electricity programs can be companies, private individuals, and also farmers. The funding is sourced from, for example, the EU, domestic budgets or the Green Investment Scheme (GIS). The money supports many programs with specific beneficiaries, the nature of funding, or the conditions and indicators that must be met (e.g., to generate a certain amount of electrical energy per year). Some projects that have gained popularity in Poland and have created public interest are, for example, PROSUMENT, BOCIAN, ROP (Regional Operational Programs), Energia Plus [
5]. One of the ways of managing green energy is its use in agricultural biogas plants. Waste plant raw materials can be successfully used as a source of energy.
Biogas is a product of anaerobic digestion. Adequate hydration and temperature cause the development of bacteria that feed upon organic matter. The product of this process is combustible biogas, also known as mud/marsh gas [
6]. Agricultural biogas is produced from raw materials of animal and plant origin. Animal raw materials include manure and slaughter waste (liquid manure, slurry, and stable manure). Plant substrates are energy crops, food and garden waste, and grass clippings. The main substrate used in biogas plants is maize silage. It is diluted in slurry, which stabilizes the process of methane fermentation [
7,
8]. Energy crops used as silage, whole plants, tubers or seeds represent a wide group covering broad bean, grass, rapeseed, buckwheat, oats, mustard, beetroot, etc. [
9]. The most commonly used raw material in the production of agricultural biogas in Poland is maize silage and slurry. Comparing the number of agricultural biogas plants operating in Poland (117) and in Germany (about 10,000), a threat to the food industry is far from real; still, the number of such installations is growing, and the range of species providing an input for biomass is broad, similarly to the volume of waste generated by the agri-food industry [
10]. Post-harvest waste of rapeseed includes roots, siliques, dried leaves and stems, which, when left in the field, turn into natural fertilizer. Straw improves the soil structure by supplying micro- and macronutrients [
11]. Rapeseed straw consists of a hard, hemicellulose- and lignin-rich epidermis and a spongy interior, largely of cellulose [
12]. Buckwheat straw has a high content of volatile components and a large amount of oxygen. As a result, it has a very high concentration of released heat energy in the initial stage of the incineration process. For energy-generation purposes, grey straw is most often used, i.e., one left after harvest in the field for several days to enable rainwater to leach chlorine and potassium compounds from it. Thanks to this simple procedure, the corrosive effect of exhaust gases on the boiler surfaces is limited [
13]. As reported [
14], the most fertile soils in Poland (chernozem) account for less than 0.75% of arable land; therefore, in order to avoid fertilization of poorer-quality lands, pressure is put on energy crops that do not require high-quality soil.
According to the available literature [
15,
16], there are several methods of pretreatment of lignocellulose raw materials used to produce energy. However, it has not been determined yet which of them is the most efficient. Selection of the best available technology depends on various factors such as the type and size of biomass or the target product to be obtained as a result of processing [
17]. There are several basic methods of pretreatment, among them microbiological processing, which uses microorganisms occurring in natural conditions. Another method is chemical pretreatment, in which the action of chemicals results in preliminary hydrolysis. However, the latter requires special (non-metallic) tanks and that acid be recovered after the process has been completed. Among the mechanical treatment methods, there is micronization. It is commonly employed in the production of animal feed or biomass combustion. This method mainly proves effective with raw materials for the production of silage. One of the methods of pressure-thermal treatment is extrusion-cooking. Due to the extensive process configuration options, it can be adapted to a wide range of biomass inputs [
18]. The application of single- or twin-screw extruders may have an impact on obtained results. The influence of high pressure, temperature and the rotation speed of the extruder screws causes lignin bonds to break and releases cellulose and hemicelluloses, which increases the surface of active biomass and initiates hydrolysis. The extrusion-cooking process can be combined with other pretreatment methods, for example, the above-mentioned micronization. There are a number of factors that determine the ultimate effect of extrusion-cooking. These are, among others, the degree of raw material fragmentation and its moistening, the appropriate process temperature, and the choice of the proper extruder plasticizing system [
19]. The extrusion-cooking process is energy intensive, but under certain conditions, it can be very productive and cost-effective. The extruder cylinder can be heated using the waste heat from a biogas plant or excess generated electricity, which significantly reduces the costs of the entire process. The lignocellulosic raw material after extrusion can be stored and used at any time during the production of biogas. Owing to extrusion-cooking, the input material for a biogas plant can be supplied on a continuous basis, and the temperature ranging from 130 to 180 °C sufficiently protects the product against the development of microorganisms [
20].
The aim of the research was to determine the influence of modification of the plasticizing system on the course of the extrusion-cooking process and on the physicochemical properties of selected pretreated straws as well as the effect of pretreatment on biogas efficiency during methane fermentation.
2. Materials and Methods
As the main raw materials, rapeseed straw and buckwheat straw were used. The rape straw used in the tests was qualitatively assessed by 4.76% ash content, 19.41% lignin content, 41.33% cellulose content, 30.57% hemicellulose content, 42.47% carbon content and 0.51% nitrogen content. In the case of buckwheat straw, there was 5.73% ash content, 19.94% carbon content, 36.54% cellulose content, 28.26% hemicellulose content, 37.28% carbon content and 1.17% nitrogen content [
21]. The studies were carried out on extrudates made of rapeseed and buckwheat straws at three rotational speeds of the extruder screw (70, 90, and 110 rpm). The raw materials used were shredded to a size of 8–10 mm and moistened with tap water to the initial moisture content of 20, 25, 30, and 35% dry mass. A moisture analyzer (Radwag, MA 50.R.WH, Radom, Poland) was used to determine the initial moisture content of the straws.
A TS-45 single-screw extruder (Metalchem, Gliwice, Poland) with an L/D = 12 plasticizing system was used for the process. A standard and a prototype screw were used. It was equipped with an additional section of beaters to ensure better compaction of the processed raw material before its molding phase at the extruder head (
Figure 1).
During the treatment of straw, the processing efficiency and specific mechanical energy consumption of the extrusion-cooking process were determined, and the influence of process variables on selected physical and chemical properties of lignocellulosic raw materials was tested with the application of two different plasticizing systems (before and after modification). The followed tests also determined the effect of extruder screw modification (making additional cuts in the mixing zone) and process variables on the biogas efficiency of tested pretreated straws (
Figure 2).
2.1. Extrusion-Cooking Efficiency
The testing of the efficiency of the extrusion-cooking process focused on determining mass obtained over a specific time for all the raw materials used and at pre-set process parameters. The measurements were performed three times for each test series. The result was the mean value of all the measurements. The processing efficiency was determined with the formula:
where:
Q—process efficiency (kg h−1),
m—mass obtained during measurement (kg),
t—measurement time (h).
2.2. Specific Mechanical Energy Consumption of Extrusion-Cooking
Energy consumption of the extrusion-cooking process was determined based on the specific mechanical energy (SME) calculated according to the following formula [
22]:
where:
SME—specific mechanical energy (kWh kg−1),
n—extruder screw speed (rpm),
N—maximum extruder screw speed (rpm),
L—motor load (%),
P—power (kW),
Q—process efficiency (kg h−1).
2.3. Measuring WAI (Water Absorption Index)
The processed straw samples obtained through extrusion were ground with a laboratory grinder (ELDOM MK100S, Katowice, Poland) into particles with a diameter not greater than 0.8 mm. A suspension was prepared from a sample of 0.7 g and 7 mL of distilled water by continuous mixing for 20 min. The suspension was centrifuged at 15,000 rpm for 10 min in a laboratory centrifuge (Digicen 21, Madrit, Spain). Filtrate was collected from the obtained gel, and next, the gel was weighed [
23]. The water absorption index was calculated using the formula:
where:
2.4. Measuring WSI (Water Solubility Index)
The filtrate obtained during the measurement of the WAI was dried at 130 °C until total water evaporation [
24]. The water solubility index was calculated according to the formula:
where:
WSI—water solubility index (%),
ms—vessel mass after drying (g),
mps—vessel mass before drying (g),
mpp—sample mass (g).
2.5. Measuring Bulk Density of Processed Straws
Density was measured by pouring the tested material through a funnel into a 500 cm
3 vessel (any excess material was swept in as well) and then by weighing the vessel with the content. Based on the obtained mass, the bulk density was calculated as follows [
25]:
where:
2.6. Methane Fermentation of Processed Raw Materials
The study was conducted at the Laboratory of Ecotechnologies of the Poznan University of Life Sciences (PULS). Biogas efficiency was investigated in the methane fermentation process under mesophilic conditions (the most popular technology in Europe) in three replications with the use of proprietary biofermentors [
26] (
Figure 3).
The pretreated with the extrusion-cooking samples were analyzed for biogas efficiency in accordance with the generally recognized standards, namely DIN 38414/S8 and VDI 4630 [
27]. The tests were carried out in mesophilic conditions in sets of 3-vessel biofermentors [
28]. Fermentation reactors with a capacity of 2 dm
3 were filled with inoculum (with a dose of microorganisms from an operating biogas plant) and with rapeseed and buckwheat straws processed under various conditions of the extrusion-cooking process. The content of organic dry matter in the inoculum ranged from 1.5 to 2%. Dry mass and organic dry mass were measured prior to testing. After that, the substrates were placed in an airtight reactor for fermentation. The vessels with samples were immersed in water at a controlled temperature (about 39 °C), which simulated the actual operating conditions of biogas installations. The volume and qualitative composition of generated gases were measured every 24 h. The fermentation process was stopped when the daily biogas production lowered below 1% of the total amount of biogas produced. The samples were tested in three replications. The biogas yield was measured (m
3 Mg
−1) relative to fresh mass (rapeseed and buckwheat straws), dry mass, and dry organic mass, as described by Dach et al. [
29].
2.7. Analysis of Results
The data obtained during the tests were archived and analyzed statistically using Microsoft Office Professional Plus 2019 (Microsoft, Redmond, WA, USA, Excel) and Statistica 13.3 (TIBCO Software Inc., Palo Alto, CA, USA). The RSM method with surface approximation was used to investigate the dependence of specific results on variable process parameters.
The similarity of the screw systems (before and after modification) was analyzed using the principal component analysis (PCA) with Statistica 13.3. The PCA data matrix for the statistical analyses of both types of extrudates results had 11 columns and 24 rows. The input matrix was scaled automatically. The correct number of main components obtained in the analysis was determined based on Cattell’s criterion.
4. Conclusions
The following conclusions were drawn based on the conducted tests and the analysis of the results. The results in individual tests differed depending on the raw material used. Not only may this be attributed to the structure of the tested raw material, but it may also suggest the need to select appropriate process parameters for a specific material. In the case of buckwheat straw, the use of a prototype extruder screw increased the efficiency of the extrusion-cooking process and reduced its energy consumption. In the case of rapeseed straw, modification of the plasticizing system raised the energy requirement and reduced the efficiency of the process compared to when a standard screw was used. The opposite trend was reported only when 35% of the moisture content was applied.
When the prototype screw was used, in most cases, extruded buckwheat straw showed an increase in the WAI compared to the processing with the use of the standard screw. For rapeseed straw, the WAI index was noted to depend on the moisture level of the raw material. In the case of 20 and 25% of the moisture content, a higher WAI was observed in samples processed with the standard screw; and when moisture was raised, a higher WAI was noted in samples processed with the prototype screw.
In the case of extruded buckwheat straw, the use of the modified plasticizing system (prototype) increased the WSI in samples processed at 30 and 35% moisture (across the entire range of the screw speeds applied). In extruded rapeseed straw, this trend was observed for samples processed at a moisture level of 20%.
The use of the modified plasticizing system reduced BD in both tested raw materials. Only in buckwheat straw processed at the screw speed of 110 rpm and at 35% moisture was a decrease in the BD noted, unlike with the samples processed using the prototype screw.
In extruded buckwheat straw (obtained in the modified plasticizing system), there was an increase in the production of biogas and methane compared with the control sample (not subjected to the extrusion process) across the entire range of tests. For samples processed with the standard screw, the best yield of biogas across the entire range of tests was obtained for the sample processed at the screw speed of 70 rpm and 35% of the moisture content. Rapeseed straw processed with the prototype screw produced the highest biogas yield at the screw speed of 90 rpm applied for pretreatment. When testing rapeseed straw processed with the use of the standard screw, the top measured values were reported in samples processed at the maximum rotational speed of the extruder screw. In both cases, the best results were found in samples processed with a moisture level of 25%.
In future research attempts, more attention should be paid to the proper selection of process variables for specific raw materials. It would be reasonable to carry out a comparative analysis using various extruder plastification unit configurations (L/D) or screw lengths, as well as measuring the influence of modification of the plasticizing system on the extrusion-cooking process stability and on methane efficiency.