At the height of the crisis caused by the continuous COVID-19 pandemic, an energy crisis has taken over the European Union. Not long after, due to geopolitical changes directly related to the EU energy market and gas supply, energy prices continued to reach unexpectedly high levels. In response to these problems, more attention was paid to the implementation of previously adopted policies, such as the European Green Deal, as well as new proposals, such as REPowerEU, that could prevent such scenarios in the future and reduce the EU’s dependence on existing energy supply models. However, most of those strategies involves a long-term process of construction and adaptation. EU residents, as well as industry stakeholders, are in need of momentary solutions that can mitigate the crisis at this time, by using available resources and technologies.
One of the constantly available and abundant resources in terms of renewable and CO2
-neutral energy, primarily heating, is biomass [1
]. In principle, all energy end-users that are converting fossil carbon can process renewable energy. Agriculture and forestry supply such raw materials, which are processed by traditionally bio-based sectors such as food and feed and the wood processing industry, which includes biogenic construction materials, cellulose and paper, and biogenic fibers [2
]. On the other side, following the European Commission’s proposal to revise the renewable energy directive in July 2021, the Parliament adopted its position in September, which aims to limit the amount of biomass that can be combusted. This is accomplished by excluding certain types of primary woody biomass from what is considered renewable energy, as well as limiting their use [3
]. Although there is a trend of dedicated production of biomass for energy from second-generation crops on soils of lower quality that meets REDII sustainability criteria, the cheapest sources of biomass, in addition to forest ones, are agricultural residues after harvest and/or processing, which are basically considered waste.
Agriculture, through the activities of production, processing, and storage of products, generates huge amounts of organic waste. This waste most often includes straw from different crops, low-quality and spoiled fruits and vegetables, animal excrement and different types of waste after harvesting and processing, as well as waste generated after caring for plantations (e.g., pruning). It is usually an unwanted resource and is often subject to improper and irresponsible disposal, which can cause unwanted effects on the environment and health. On the other hand, agricultural residues are of interest to various industries and sectors of the economy due to their energy content (i.e., for energy production), their potential use as raw material to produce biofuels and/or fine chemicals, and more [4
]. In terms of energy production, as this type of waste is biodegradable, it can be used to generate energy through several different processes. The most common ways of utilization are biomass, biogas, and gasification power plants. Agricultural waste can be used in biomass power plants (and cogeneration power plants) in order to generate heat (and electricity). This type of power plant uses a direct combustion, or sometimes pyrolysis, to convert biomass into fuel, which can then be used to generate electricity through combustion. Direct combustion represents a technically simple system (compared to others), and is widely used for energy production. A more technically advanced form of energy production system is biogas power plant, which uses anaerobic digestion process to produce methane gas, which can then be used to generate electricity, heat, or fuel. Compared to biomass power plants, biogas power plants are much more efficient in terms of energy production, produce cleaner energy with fewer emissions, and can produce fuel that can be stored. Waste can be valorized in gasification power plants to produce synthetic gas, which is then burned to generate heat and electricity. This type of power plant is often designed to process agricultural waste, such as crop residues. It produces a cleaner and more consistent fuel source, can use a wider variety of feedstock, which allows more diversified energy portfolio, and has a highest efficiency compared to previous described power plants [5
A significant segment of agricultural waste in Croatia that has not yet been covered by research; specifically, seed processing has the potential to be applied in a circular economy. The seed processing program in Croatia has a centuries-old tradition and is one of the oldest in Europe [6
]. Seed production implies the cultivation, processing, and sale of seeds, as well as the application of appropriate biological and technical interventions and compliance with appropriate legal regulations for the purpose of producing seeds of high genetic purity and quality [7
]. Seed processing is an important part of the production process of a particular crop. According to the current Law [8
], it is defined as the process of drying, cleaning, calibration, peeling, treatment with plant protection agents, packaging, sealing, and marking. It is inevitable that during harvesting, along with the targeted grain, a certain percentage of material that is not aim of the production, and harvesting machines are not able to separate them, physically contaminates the product. Therefore, after harvesting, the seeds are processed. The processing and quality of the processed seeds depends on the processing machines and equipment, as well as the control of the seeds during processing by the authorized laboratory for testing the quality of agricultural reproductive material. In general, seed processing consists of removing undesirable admixtures (straw, weeds, spikes, etc.), atypical grains, and broken grains, calibrating the seed, and carrying out seed treatment procedures depending on the crop [9
In Croatia, 60% of fuel/heat demand pertains to biomass (between 45 and 55 PJ), where most is used for residential heating. In general, around 10% of total energy supply is made from biomass, since around 34% of the land area refers to forests and 25% to agricultural land, split between arable land and permanent meadows/pastures [11
]. Energy use in industry is quite low in Croatia, due to the fact of poor industry development; hence, the use of biomass for industry heat is marginal compared to household consumption. On the other hand, reduction in heating oil consumption for heat generation is present, partly due to occasional co-financing programs for the replacement of heating oil boilers with those on biomass implemented by the Environmental Protection and Energy Efficiency Fund [12
]. In terms of utilizing some form of biomass as an energy source, in 2021, there were 120 biomass power plants, 71 biogas power plants, and 12 cogeneration plants in Croatia that possess a license for energy production, with a total production capacity of 1608.27 MW of heat and 1365.95 MW of electricity. There are installed biomass power plants with a heat capacity of 300.69 MWh, cogeneration power plants with a heat capacity of 180.78 MWh, and biogas power plants with a heat capacity of 1126.80 MWh [13
Biomass and cogeneration power plants are mostly designed to rely on the stable availability of mostly forest biomass (70% of total solid biomass feedstock in biomass and cogeneration power plants [14
]), wood waste, or agricultural residues. However, the majority of biogas plants rely on corn silage as the primary raw material (for co-digestion in combination with animal manure) [15
]. This form of production was sustainable until sudden disturbances in fertilizer and energy prices, which resulted in a sudden increase in silage prices. Given this predominant dependence on energy and input prices, producers are forced to adapt and rely on new alternative sources of biomass.
Strategically, the possibility of growing perennial energy crops is being considered as a preventive measure against future disturbances and uncertainty, while curatively using all available biomass feedstock. To a large extent, those are various wastes from the food industry. However, without increasing the capacity of food production, the available waste will not replace the bulk amounts of the silage, for which the installed facilities were designed and on which they rely. Therefore, it is necessary to investigate the possibilities of using other waste raw materials that can serve as a gradual and partial substitute for corn silage, and will not directly cause shortages in other markets [16
This paper aims to evaluate the potential of utilizing seed production wastes (SPWs) from Croatian seed production program for heat production in order to mitigate the energy crisis. The evaluation is made with a special focus on the existing RES infrastructure in Croatia, that is, directed towards those power plants that are using biomass as a feedstock (biogas and biomass power plants). It evaluates the thermal potential of the eight major crops being cultivated and determines which varieties offer the highest potential in current production system. Additionally, the paper attempts to determine which RES has the highest theoretical thermal output per unit area, to help the seed production and RES programs in Croatia maximize efficiency of the waste’s utilization.
3.1. Proximate and Ultimate Analyses of SPWs
Prior to energy potential calculations, initial characterization of biomass energy properties was carried out. Results of proximate analysis is presented in Table 1
, while results from ultimate analysis is presented in Table 2
. These analyses are common in the characterization of solid biofuels and provide a certain specification that characterize a particular feedstock as more or less suitable for use in a certain system for energy conversion. Additionally, values such as moisture or ash content can help in the design of combustion systems, while the elemental composition can serve as a point for the optimization of anaerobic digestion.
Statistically, a significant difference was calculated (at p < 0.05) for all researched proximate parameters. Moisture contents of researched SPWs ranged from 6.39% for soybean wastes to 14.39% for corn wastes. Ash content in corn, on the other hand, was lowest, i.e., 1.78%. The highest ash content, 6.58%, was recorded in bean waste samples. Corn waste had the highest shares of fixed carbon (17.20%), while soybean and rapeseed wastes contained least fixed carbon shares, i.e., 11.69 % and 11.27%, respectively. The highest content of volatile matter was recorded in soybean (77.51%) and rapeseed (78.12%), while the lowest content was recorded in corn waste (68.83%). Bean waste had the lowest HHV (16.3 MJ kg−1), while SPWs with high shares of oils had the highest HHV values, soybean 22.23 MJ kg−1, and rapeseed 22.51 MJ kg−1.
The ultimate analysis provided an elemental composition outlook of SPW samples. These elements are directly related to combustion reactions and can significantly change the combustion process itself. Shares of carbon in researched SPWs ranged from 41.821% in rapeseed waste to 45.97% in bean, 46.11% in peas, and 46.32% in barley waste. Rapeseed had the highest content of hydrogen (8.35%), while wheat (6.22%), barley (6.08%), and corn (6.11%) had the lowest contents. Oxygen content ranged from 45.06% in wheat to 48.51% in corn waste. Wheat and oats had the highest shares of nitrogen, i.e., 2.19% and 2.20%, respectively. The lowest share of nitrogen was recorded in rapeseed samples, i.e., 0.36%. Barley contained the lowest amount of sulfur (0.06%), while rapeseed contained the highest (0.18%).
3.2. Total Theoretical Thermal Potential of SPWs
Using the input parameters from the above-mentioned analyses, in addition to the collected data related to the seed yield itself, the share of waste, and the total production capacity of the Croatian seed production program [18
], it is possible to provide an insight into the total theoretical thermal potential of SPWs. If the collected waste is completely directed as raw material for existing biomass and cogeneration power plants, the potential of thermal utilization per area unit (ha) and total production is shown in Table 3
. In the case that SPWs are used as raw material in existing biogas plants, the total heat potential for each waste per area unit (ha) and total production is shown in Table 4
shows the total theoretical thermal potentials of SPWs. The yields are calculated as the mean yields of all production varieties in relation to the production capacities of the varieties, i.e., crops. As a reference value of the percentage share of waste content during seed processing, the values were taken from the relevant facility described earlier. Using these parameters, in addition to the HHV values shown in Table 1
, the total theoretical thermal potential of SPW per hectare and in the total production registered by the national control agency was calculated. Total available SPW was calculated to be 1.317 t ha−1
. In total, 22.02% of the total available raw material per hectare consisted of oilseed rape, 21.56% of corn waste, and 18.83% of bean waste. Peas SPW has the lowest availability per hectare with 0.045 t ha−1
or 3.42%. Rapeseed SPW has the highest theoretical thermal potential per hectare, i.e., 6528 GJ ha−1
. The next SPWs of significant potentials are beans, with 4042 GJ ha−1
, and corn, with 4944 GJ ha−1
. However, considering the total production registered through the seed production program in Croatia, the highest theoretical thermal potential is represented by wheat SPW. Wheat seeds were produced on 7761.01 hectares in 2021 and represent the possibility of utilizing 14,415.46 GJ of theoretical thermal energy. Next is SPW of soybeans, with a production of 5970.95 hectares and a total theoretical thermal potential of 10,353.27 GJ. Barley and maize also represent significant SPWs, with total heat potentials of 6193.71 GJ and 5985.99 GJ, respectively. If the total production of investigated SPWs is considered, the theoretical thermal potential reaches level of 38,051.10 GJ.
Using laboratory scale bioreactors, the biogas and biomethane yield of an individual SPWs was determined. For the purposes of calculating the theoretical thermal potential of biomethane, a reference value of 39.8 MJ/m3 of methane was used. The highest biogas yield was recorded for wheat SPW, i.e., 901.75 mL gVS−1, while the lowest biogas yield was recorded for rapeseed SPW, i.e., 786.79 mL gVS−1. Despite this, rapeseed SPW had the highest proportion of biomethane in the composition of biogas, i.e., 56.79%. The share of biomethane in biogas produced from soybean SPW was 52.16%, while the lowest value of 47.84% was recorded for corn SPW. However, SPWs of rapeseed, bean and corn contain the highest theoretical thermal energy potential per hectare, i.e., 5.16 GJ ha−1, 4.33 GJ ha−1, and 4.55 GJ ha−1. If the total available energy potential from the seed production program in Croatia is calculated, SPW of wheat can provide a maximum of 13,708.61 GJ of theoretical thermal energy. SPWs of soy, barley, and corn have a significant production capacity and potential, i.e., 8147.20 GJ, 6284.91 GJ, and 5507.49 GJ. The total available potential of all SPWs is 34,727.91 GJ.
4.1. Proximate and Ultimate Analyses of SPWs
Despite significant differences in all investigated parameters of proximate and ultimate analysis, seed production wastes (SPW) of eight major agricultural crops (wheat, soybean, rapeseed, oat, bean, peas, barley, and corn) mostly corresponded to the reference values of solid biofuels general requirements for fuel specifications and classes for grains (BS EN ISO 17225-1:2014). Typical ash content (AC) should be expected to be between 1.20% and 4.00% (3.75% to 5.50% for rapeseed), and all of the investigated SPWs had AC in range, with the exception of soybean (4.34%), oats (4.24%), and bean (6.58%). AC, the solid residue after the combustion, is one of the most researched properties of biomass due to its catalytic influence on biomass combustion systems, as well as some other conversion processes. A reduced AC improves the energy properties of biomass and enables its more efficient utilization [19
]. In this research, corn had the lowest ash content of 1.78%, which is in range of conventional forest biomass often used for combustion and heat production.
The volatile matter (VM) and fixed carbon (FC) of a fuel provide data to aid in understanding the combustion behavior, which has a significant impact on the design of a conversion plant. VM represents the organic material in a fuel that can be vaporized and burned and is an important parameter that has a significant impact on the combustion process. High VM content in fuel can lead to faster combustion and more efficient burning, and according to the literature, biomass contains up to 2.5 times more volatile matter than coal, which has a significant impact on the conditions of ignition and combustion [20
]. FC is part of the biomass which remain after the volatile components are burned off and form coke during combustion, where high value indicates that the fuel combustion will last longer and act as the main heat generator. Combustion of the fixed carbon provides a high-energy source of heat, which makes it a good fuel source for combustion applications [20
]. Unlike for AC, there are no normative values for grains. Corn SPW had highest FC content (17.52%) and lowest VM content (68.83%), which presupposes its suitability for use as a feedstock in thermal energy production. However, compared to values of various biomass samples in literature, where VM usually ranges between 63.70% and 86.50%, while FC ranges between 12.50% and 18.00% [21
], all researched SPWs had comparative values, with the exception of FC in rapeseed, which was 11.27%.
The higher heating value (HHV) represents the amount of heat energy produced during combustion and is important energy property of fuels that defines the energy efficiency of feedstock use [23
]. According to BS EN ISO 17225-1:2014, HHV values of grains are usually in the range between 16.50 MJ kg−1
and 19.60 MJ kg−1
(rapeseed grains from 27.50 MJ kg−1
to 29.00 MJ kg−1
). Starch-rich SPWs had comparable HHV values, ranging from 17.35 MJ kg−1
for oat SPW, to 18.21 MJ kg−1
for wheat SPW. Oil-rich SPWs had higher HHVs, specifically 22.23 MJ kg−1
for soybean SPW and 22.51 MJ kg−1
for rapeseed SPW, which is lower than used in normative. The reason for this is probably the fact that impurities present in SPW are of lower oil content, which is high in calories, and of a more complex structure (straw, leaves, twigs), which can significantly reduce HHV, considering the proportion in the waste. However, all researched SPWs meet the basic biofuel quality assumptions and as such can be used efficiently in conversion systems.
The ultimate analysis, or the carbon, hydrogen, oxygen, nitrogen, and sulfur analysis, determines some properties of the biomass that can direct further conversion steps. In terms of combustion, carbon and hydrogen content increases the heating value, oxygen content reduces it, while nitrogen and sulfur contents are significant sources of NOx and SOx pollutants. In terms of biogas production, the ultimate analysis of the organic part of feedstock can be carried out in order to set up the empirical formula, which can be used, via the stoichiometric relationships, to calculate maximum biogas yield and composition and the oxidation oxygen theoretically required (VDI 4630:2016-11). Normative values for ultimate analysis of grains are stated as following ranges: C from 42.00% to 50.00%, H from 5.50% to 6.50%, O from 43.00% to 50.00%, N from 1.00% to 3.00%, and S from 0.05% to 0.10%. Carbon share in SPWs analyzed in this research ranged from 41.85% for rapeseed, which was the only SPW to lower the content of C compared to normative typical variation range, to 46.46% for wheat. Most of the SPWs were in accordance to the normative concerning hydrogen content of bean (6.52%) and oat (6.63%) SPWs slightly exceeded the upper variation, while rapeseed contained rather more hydrogen than it was expected, i.e., 7.12%. All SPWs had oxygen content in accordance to the normative values and their typical variations, ranging from 45.06% in wheat SPW to 49.19% in rapeseed SPW. Starch-rich SPWs had nitrogen content in range of typical variations, according to normative values, while oil-rich SPWs contained less: 0.36% in rapeseed and 0.92% in soybean SPWs. Sulfur content exceeded normative variations in oat (0.11%), corn (0.14%), and rapeseed (0.18%) SPWs.
4.2. Total Theoretical Thermal Potential of SPWs
The aforementioned data are crucial for the quality design of plants and pathways for biomass conversion. Additionally, using data on the availability of raw materials per production area and the total annual production capacity, it is possible to calculate the theoretical thermal potential of an individual SPW, as well as the total potential. Taking the average yields of each crop used in the seed production program and the average amount of waste generated using conventional seed processing equipment, it is possible to calculate how much SPW we can expect for further conversion steps if we produce it on one hectare. The SPWs of rapeseed (0.290 t ha−1), corn (0.284 t ha−1), and beans (0.248 t ha−1) have the highest availability per production unit (ha). On the other hand, the lowest availability is provided by the production of soybeans (0.078 t ha−1) and peas (0.045 t ha−1). Accordingly, since the HHV values are not drastically different but only statistically justified, the theoretical thermal potentials of individual SPW are consistent with the distribution of the availability of feedstock, so the highest theoretical thermal energy potential is contained in the rapeseed SPW (6528 GJ ha−1). After that, the SPW of corn has a potential of 4944 GJ ha−1 and of beans of 4042 GJ ha−1. The lowest theoretical thermal potential is contained in peas SPW, ie 0.753 GJ ha−1, which is also the only SPW that does not exceed the potential of 1.00 GJ ha−1.
However, these potentials help in planning the production and use of raw materials in the future, while the total production area per crop can be taken to show the possibility of using the current amounts of SPWs. Wheat had the largest production area in the seed production program in Croatia in 2021, with a total of 7761.01 ha. After that, soybean production was represented on the total production area of 5970.95 ha. The production programs of rapeseed, peas, and beans had the smallest representation with areas of 13.69 ha, 14.34 ha, and 15.21 ha, respectively. In total, theoretically available total SPWs can be collected from 18,274.61 ha. Accordingly, the total theoretical thermal potential of SPWs, if used in biomass power plants or cogenerations, provides a possible production of 38,051.10 GJ of thermal energy. Of this, SPW of wheat includes 37.88% of total energy, i.e., 14,415.46 GJ, while that of soybeans includes 27.21%, i.e., 10,353.27 GJ. Peas have the lowest thermal potential of the entire seed production program, with 10.80 GJ of available thermal energy.
Based on the available amounts of individual SPW and production capacities, and using the results of anaerobic digestion of samples, an insight into the theoretical potential of using these wastes in biogas plants, where the produced CH4
is converted into thermal energy, is given. Using a laboratory system for biogas and biomethane production, the range of biogas yields ranged between 841.22 mL gVS−1
for corn SPW and 901.75 mL gVS−1
for wheat SPW. These data are a rough indicator, but it cannot be used for more detailed calculations because biogas is composed of several gases, mainly methane and carbon dioxide. From the aspect of (thermal) energy production, it is methane that combusts and converts, so it is necessary to know the composition of the produced biogas, i.e., the proportion of methane in it. The highest methane content was in rapeseed SPW of 56.79% and soybean of 52.16%, probably due to higher proportions of high-calorie fats and oils in the biomass. The lowest proportion of methane was recorded in corn SPW. When calculating the total theoretical thermal potential of an individual SPW per hectare, peas, which have the lowest availability, also have the lowest potential of 0.78 GJ ha−1
. On the other hand, beans, corn, and rapeseed have the highest potential, i.e., 4.33 GJ ha−1
, 4.55 GJ ha−1
, and 5.16 GJ ha−1
, respectively. However, for the same reasons as explained in the section discussing the theoretical potential for use in direct combustion processes, the total theoretical potential in the entire seed production program was calculated. Thus, in Croatia, in 2021, the total theoretical thermal potential of using SPWs in the production of thermal energy from biogas amounted to 34,727.91 GJ. The highest single theoretical thermal potential was related to wheat SPW, which was 13,708.61 GJ, while the lowest was 11.12 GJ, for peas SPW. These biomethane yields per mass of feedstock correspond to the characteristic yields of energy crops such as grass silage, corn, or fodder beet that are conventionally used in biogas production, so it can be considered that SPWs can replace a certain amount of primary raw material [24