Agricultural Plant Residues as Potential Co-Substrates for Biogas Production

<jats:p>Plant biomass can be used in many directions for bioenergy production. Biogas can be produced from a most diverse group of substrates compared to liquid or solid biofuels. The choice of substrates and technologies is crucial because it will allow getting the expected results. Not without significance is also the price and availability of substrates. Therefore, waste and residues are increasingly being used. Accordingly, the aim of the review was to analyze the potential of biogas production from agricultural plant residues and the effectiveness of using this feedstock as a co-substrate in anaerobic digestion. In this article, selected agricultural plant residues are collected, and their advantages and disadvantages as substrates for biogas production are described. Moreover, the effective technology of biogas production by anaerobic digestion on an industrial scale and calculations to obtain biogas and methane efficiency of the substrates are also included. In addition, the summarized biogas efficiency of selected plant agricultural waste under mesophilic conditions studied by many researchers is shown. On the basis of the analyzed results of this research, it can be concluded that agricultural plant residues have great potential as co-substrates for biogas production. It is important to experimentally determine both the biogas and the methane efficiency of the substrate, representing a potential raw material for the production of gaseous biofuels. The use of artificial neural networks in the prediction of biogas emission is future-proof and should facilitate the management of biogas plants. The use of waste from the cultivation and processing of plant raw materials will not only help to manage this waste rationally, but also contribute to the increase in production of renewable energy sources. Accordingly, the circular economy in terms of the management of agricultural plant residues to produce biogas will have a multi-faceted, positive impact on the environment. On the basis of this review, it can be concluded that numerous agricultural plant residues can be used as potential co-substrates for biogas production.</jats:p>

section_content: Agriculture is one of the first branches of the economy developed in any society. It has accompanied humanity for centuries; however, in recent decades, due to its intensification, agricultural production has been a large source of difficult-to-manage waste. Intensive production processes, as well as the use of artificial fertilizers, plant protection products, and specialized feeds in the biomass, heretofore used as fodder, have led to the generation of problematic waste. One possible solution is to use this waste to produce biofuels [1] [2] [3]. 

Plant biomass is widely regarded as an alternative feedstock to fossil fuels for energy production. Although it is necessary to provide significant amounts of energy for its production and processing, it is estimated that its energy management is less burdensome for the environment than the use of crude oil, lignite, or hard coal. This is due to the absorption of carbon dioxide during plant growth, which significantly reduces the overall balance of the impact of biomass production on the ecosystem [4] [5] [6] [7]. sawdust, etc., e. g., from short-rotated energy crops). Since then, humanity has been producing even more waste, due to the significant increase in population, and the problem of waste management is growing [22] [23] [24]. Therefore, selected agricultural plant residues are collected in Table 1, and their advantages and disadvantages as substrates for biogas production are described. The most common agricultural plant residue is grain straw, which is a waste derived from harvesting rye, wheat, rice, maize, sorghum, etc. for food purposes. It is common and easily accessible; however, for this reason, its price fluctuates throughout the year. Moreover, it is expensive because it is used for fodder purposes and as bedding. Fiber plant straw is cheaper because it is not suitable for animals. Nevertheless, due to the high content of lignocellulosic structures, it is difficult to fragmentize and decompose. Seed husks from corn, oat, or other seeds separated by winnowing or threshing are cheap but very light, which results in a low energy efficiency per unit volume. The dry pulpy residue left after the extraction of juice from sugar cane or residues from sugar beet processing may be classified as high-energy substrate for biogas production, but they are available locally and not suitable for transport and long-term storage due to the high water content and putrefactive processes taking place within them. The residues from agroforest-related branches of the industry can be used for carbon supplementation in bioreactors; however, because of their lightness and high lignocellulosic content, their decomposition is a longterm process. More uncommon substrates such as waste from sericulture or floriculture may be interesting local co-substrates to animal residues, mostly free or even attracting additional income for pickup. Just like any raw material, all the described agricultural plant residues have advantages and disadvantages. However, all of the above-characterized substrates can be used as a feedstock for biogas production.
section_name: Introduction

section_content: The most effective and widespread technology of biogas production on an industrial scale is anaerobic digestion carried out in adapted reactors under strictly defined conditions [25]. The gas produced in this process consists of about 55-60% CH 4 and about one-third CO 2. In addition, biogas may also contain other compounds, in different proportions. These are commonly small amounts of H 2 S, NH 3, H 2, and other gases [26] [27] [28]. 

The methane fermentation process, necessary to determine the biogas efficiency of the substrate, is carried out, for example, in accordance with DIN 38 414/S8 and VDI 4630. The volume of biogas produced is determined by summing up the daily production of biogas generated in the reactor during tests from the first day of the reaction to the time when the daily production of biogas accounts for less than 1% of the total biogas produced so far. During the experiments, the amount of biogas produced in the digesters is measured daily (Figure 1 ) [25, 26]. 

because of their lightness and high lignocellulosic content, their decomposition is a longterm process. More uncommon substrates such as waste from sericulture or floriculture may be interesting local co-substrates to animal residues, mostly free or even attracting additional income for pickup. Just like any raw material, all the described agricultural plant residues have advantages and disadvantages. However, all of the above-characterized substrates can be used as a feedstock for biogas production.
section_name: Biogas Production

section_content: The most effective and widespread technology of biogas production on an industrial scale is anaerobic digestion carried out in adapted reactors under strictly defined conditions [25]. The gas produced in this process consists of about 55-60% CH4 and about onethird CO2. In addition, biogas may also contain other compounds, in different proportions. These are commonly small amounts of H₂S, NH3, H2, and other gases [26] [27] [28]. 

The methane fermentation process, necessary to determine the biogas efficiency of the substrate, is carried out, for example, in accordance with DIN 38 414/S8 and VDI 4630. The volume of biogas produced is determined by summing up the daily production of biogas generated in the reactor during tests from the first day of the reaction to the time when the daily production of biogas accounts for less than 1% of the total biogas produced so far. During the experiments, the amount of biogas produced in the digesters is measured daily (Figure 1 ) [25, 26]. The process of biogas production consists of four main processes: hydrolysis, acidogenesis, acetateogenesis, and methanogenesis (Figure 2 ). In the last phase, the main biomethane production takes place. It is carried out by heterotrophic and autotrophic methane bacteria of the genera Methanobrevibacter, Methanobacter, Methanococcus, Methanosarcina, etc. [29] [30] [31]. The main chemical reactions occurring during the process of methanogenesis are shown and described below. The process of biogas production consists of four main processes: hydrolysis, acidogenesis, acetateogenesis, and methanogenesis (Figure 2 ). In the last phase, the main biomethane production takes place. It is carried out by heterotrophic and autotrophic methane bacteria of the genera Methanobrevibacter, Methanobacter, Methanococcus, Methanosarcina, etc. [29] [30] [31]. The main chemical reactions occurring during the process of methanogenesis are shown and described below. 

Two-thirds of CH 4 is formed from alcohols or acetates:

On the other hand, one-third is formed from the reduction of CO 2 with H 2 :

Figure 2. The process of biogas production [32]. 

Two-thirds of CH4 is formed from alcohols or acetates:

On the other hand, one-third is formed from the reduction of CO2 with H2:

The production of biogas, due to the reactions taking place during anaerobic digestion, can be carried out at different temperatures. However, it is very important to adjust and maintain a stable temperature in a range that is optimal for the development of a large group of microorganisms existing in a given inoculation [33] [34] [35] [36]. The extensive majority of methanogenic bacteria involved in the genesis of CH4 prefer mesophilic conditions. Optimum growth occurs at 25-45 °C [37] [38] [39]. 

The process of anaerobic digestion may be influenced by other factors such as reaction, availability of micro-and macroelements, carbon-to-nitrogen ratio, dry matter content, or the presence of anaerobic digestion inhibitors [28, 35, 39]. 

Many different substrates of organic origin are used for biogas production [40] [41] [42]. Biomethane is mostly produced from silage from corn and animal waste [43] [44] [45] [46]. However, biowaste and various plant biomass residues are increasingly being used for this purpose [47] [48] [49]. The production of biogas, due to the reactions taking place during anaerobic digestion, can be carried out at different temperatures. However, it is very important to adjust and maintain a stable temperature in a range that is optimal for the development of a large group of microorganisms existing in a given inoculation [33] [34] [35] [36]. The extensive majority of methanogenic bacteria involved in the genesis of CH 4 prefer mesophilic conditions. Optimum growth occurs at 25-45 • C [37] [38] [39]. 

The process of anaerobic digestion may be influenced by other factors such as reaction, availability of micro-and macroelements, carbon-to-nitrogen ratio, dry matter content, or the presence of anaerobic digestion inhibitors [28, 35, 39]. 

Many different substrates of organic origin are used for biogas production [40] [41] [42]. Biomethane is mostly produced from silage from corn and animal waste [43] [44] [45] [46]. However, biowaste and various plant biomass residues are increasingly being used for this purpose [47] [48] [49].
section_name: Biogas Production

section_content: The usefulness of selected plant agricultural residues may be assessed in terms of biogas efficiency, which is calculated as follows [50] :

where V b is the produced biogas volume (m 3 ), B e is the biogas efficiency of the substrate (m 3 /Mg), and M s is the mass of the substrate (Mg). Hence,

where V CH4 is the produced methane volume (m 3 ), V b is the produced biogas volume (m 3 ), and C CH4 is the methane concentration in produced biogas (%). 

The dynamics of the production of biogas and methane efficiency of the process may vary depending on the methodology used for the investigation. Nevertheless, a summary of the biogas efficiency under mesophilic conditions of fresh mass (FM) of selected plant agricultural waste is shown in Table 2. The investigation of substrates available in the Pozna ń University of Life Sciences experimental farm in Przybroda showed that the most effective substrate was silage produced from maize with cumulative biogas yield, reaching 218. 40 m 3 •Mg -1 FM, therein containing 130. 88 m 3 •Mg -1 CH4 per FM. Interpreting the obtained test results, among the analyzed substrates (pig slurry, caw slurry, and maize silage), the best biogas efficiency was found in maize silage (585 mm 3 •Mg -1 DM). Furthermore, the silage also showed the highest percentage of methane-55. 47%. In relation to substrate availability, every year on the Przybroda experimental farm, the total amount of biomethane produced from maize silage was 521,440 m 3, almost threefold higher compared to cattle manure. This produced 2212. 38 MWh•year -1 of electric energy and 2428. 22 MWh•year -1 of thermal energy. The calculated electric energy power was 0. 27 MW [51, 52]. 

The obtained research results on the efficiency of biogas production from maize straw silage proved that fermentation under thermophilic conditions is shorter (by 17%) compared to the mesophilic process. Furthermore, it permits increasing the production of biogas (by 8. 6%) and content of CH 4 (by 9. 3%). On the other hand, mesophilic fermentation has more stable pH fluctuations compared with the thermophilic process. Nevertheless, this is related to propionic acid content, which can inhibit the process and has great importance in continuing the fermentation. On the basis of energetic calculations, in this study, the substitution of both maize silage and straw is shown. The obtained results proved that it is possible to reduce the cost almost threefold, thus increasing the profitability of the biogas plant [53]. 

In another study [54], it was concluded that the potential methane production in maize and sorghum was similar. The obtained results supported the hypothesis that sorghum is a more energy-efficient crop compared to maize. Moreover, the irrigation could replace up to 36% of the evapotranspiration, and the fertilization nitrogen rate could be minimized to maximize the biomass production efficiency for anaerobic digestion in the area of Po Valley (Italy). 

The experiments by Wannasek's team comprised three vegetation periods of five sorghum varieties. Data on the development of crop yield such as maturity level and production of biogas were obtained. The biomass yield ranged between 15. 7 and 20. 67 Mg/ha of dry mass when growing sorghum as a main crop. The best methane yield (6500 m 3 /ha) was achieved by variety SOR 4 [55]. 

Ostovareh's team obtained the greatest biomethane yield of sweet sorghum stalks, which was achieved from the liquid fraction and bagasse (pretreated solids). The efficiency reached 278 mL CH 4 /g VS, corresponding to 92% theoretical yield. Furthermore, this result showed 270% improvement compared to the yield of methane obtained from untreated stalks [56]. 

The researchers from the Laboratory of Ecotechnologies, situated in the Poznan University of Life Sciences, focused on finding alternative biogas feedstock among residues from the food industry. The results of the methane efficiency gave 74. 33 m 3 CH 4 /Mg of fresh mass from empty fruit bunches of palm oil. Nevertheless, due to the high content of lignocellulose structures, a serious problem with preparation occurred [57]. 

According to theoretical calculations of biogas efficiency from obtained straw yield of hemp waste biomass, the production of 208. 8 m 3 /Mg of gaseous biofuel was assumed [58]. 

Another study demonstrated that silkworm breeding waste from sericulture generated a lower yield of biomethane compared to other agricultural residues, such as pig, cattle, and chicken manures. The fermentation of breeding waste from caterpillars of Bombyx mori L. under mesophilic conditions produced 65. 81 m 3 /Mg FM of CH 4 and 125. 59 m 3 /Mg FM of biogas (Figures 3 and 4 ; Table 3 ) [50]. Approximately 80% of accumulated CH4 production was recorded in the first 10 days of the experiment for analyzed materials. The hydraulic retention time for maize straw silage lasted 11 days longer [52] compared to sericulture breeding waste, analyzed by applying the same modified German standard [50]. 

In relation to the global situation caused by the COVID-19 pandemic, there emerged a need to find a solution to improve the economic situation of, for instance, flower producers. The rational usage of biowaste from floriculture and the effective use of their potential as a feedstock for bioenergy production were researched by Frankowski's team [59]. The investigation of floricultural waste included the leaves, stems, and flowers of different species: roses (Rosa L. ), hybrid tulips (Tulipa L. ), chrysanthemums (Dendranthema Des Moul. ), and sunflowers (Helianthus L. ). More than 80% of CH4 production was accumulated during the first 15 days of the study for each waste. Furthermore, the analyzed floricultural waste was characterized by high methane content, which was comparable with cattle (50-60%) and pig manure (60%) [36, 50]. However, in other studies, the methane content in gas produced reached 46-57% for chicken manure [60, 61] and 68-69% for fish waste [62]. This is caused by the fact that animal waste contains more fats and proteins, which are more effective sources of biogas production [59]. 

Due to the embargo on agricultural products from the European Union introduced by the Russian Federation, researchers from Poznan, Poland [63] presented the possibilities of utilizing vegetables in biogas plants, whose exports collapsed particularly strongly as a result of the embargo. The study of biogas and methane efficiency was carried out for the following vegetables: pumpkins, cauliflowers, eggplants, cabbage, peppers, cucumbers, and tomatoes. The fermentation process for all analyzed substrates ran without any process disruption. No factors which could inhibit methane fermentation were observed. The cumulative production of CH4 in terms of fresh mass is summarized in Table 2. Converting biogas and methane production per ton of fresh mass of selected vegetables, the obtained results were very diverse. The most efficient substrate of all the examined vegetables turned out to be pumpkin, whose cumulative biogas production was 42. 77 m 3 /Mg FM. On the other hand, cucumber was the least efficient substrate. Its biogas efficiency was on the level of 14. 94 m 3 /Mg FM due to the much lower content of dry mass. The most efficient substrate (organic mass on a dry basis in terms of cumulative production and methane) turned out to be tomatoes, while other vegetables showed a lower, albeit very similar, amount of produced biogas and methane. The higher yield of biofuel obtained from tomatoes compared to the yield from other vegetables was caused by the presence of a greater content of simple sugars and lower content of lignocellulosic substances. The highest percentage methane content was recorded for pumpkin at the level of 56. 55%, Approximately 80% of accumulated CH 4 production was recorded in the first 10 days of the experiment for analyzed materials. The hydraulic retention time for maize straw silage lasted 11 days longer [52] compared to sericulture breeding waste, analyzed by applying the same modified German standard [50]. 

In relation to the global situation caused by the COVID-19 pandemic, there emerged a need to find a solution to improve the economic situation of, for instance, flower producers. The rational usage of biowaste from floriculture and the effective use of their potential as a feedstock for bioenergy production were researched by Frankowski's team [59]. The investigation of floricultural waste included the leaves, stems, and flowers of different species: roses (Rosa L. ), hybrid tulips (Tulipa L. ), chrysanthemums (Dendranthema Des Moul. ), and sunflowers (Helianthus L. ). More than 80% of CH 4 production was accumulated during the first 15 days of the study for each waste. Furthermore, the analyzed floricultural waste was characterized by high methane content, which was comparable with cattle (50-60%) and pig manure (60%) [36, 50]. However, in other studies, the methane content in gas produced reached 46-57% for chicken manure [60, 61] and 68-69% for fish waste [62]. This is caused by the fact that animal waste contains more fats and proteins, which are more effective sources of biogas production [59]. 

Due to the embargo on agricultural products from the European Union introduced by the Russian Federation, researchers from Poznan, Poland [63] presented the possibilities of utilizing vegetables in biogas plants, whose exports collapsed particularly strongly as a result of the embargo. The study of biogas and methane efficiency was carried out for the following vegetables: pumpkins, cauliflowers, eggplants, cabbage, peppers, cucumbers, and tomatoes. The fermentation process for all analyzed substrates ran without any process disruption. No factors which could inhibit methane fermentation were observed. The cumulative production of CH 4 in terms of fresh mass is summarized in Table 2. Converting biogas and methane production per ton of fresh mass of selected vegetables, the obtained results were very diverse. The most efficient substrate of all the examined vegetables turned out to be pumpkin, whose cumulative biogas production was 42. 77 m 3 /Mg FM. On the other hand, cucumber was the least efficient substrate. Its biogas efficiency was on the level of 14. 94 m 3 /Mg FM due to the much lower content of dry mass. The most efficient substrate (organic mass on a dry basis in terms of cumulative production and methane) turned out to be tomatoes, while other vegetables showed a lower, albeit very similar, amount of produced biogas and methane. The higher yield of biofuel obtained from tomatoes compared to the yield from other vegetables was caused by the presence of a greater content of simple sugars and lower content of lignocellulosic substances. The highest percentage methane content was recorded for pumpkin at the level of 56. 55%, proving the high quality of biogas produced from the tested raw material. Important information for determining the biogas and methane efficiency of the biodegradable matter is also the hydraulic retention time. In the case of the tested vegetables, HRT ranged from 12 to 18 days. This is a relatively short period of using the substrate for biogas production [50, 63]. 

Potato processing waste can also be used for bioenergy purposes, e. g., for the production of liquid fuels or combined production of heat and electricity from burning biogas, which is a product of methane fermentation. Biogas efficiency studies for four substrates were carried out by Bartnikowska's team [64] in accordance with the applicable German standard (DIN 38 414/S8). The research material consisted of potatoes: (I) raw, (II) steamed, (III) raw peel, and (IV) steamed peel. For raw potatoes, the methane content was 47. 73%; from 1 ton of fresh matter, approximately 171 m 3 of biogas was obtained. For steamed potatoes, the methane content was lower and amounted to 42. 5%. Raw potato peel was characterized by the most satisfactory content of methane (48. 42%). For this form of potatoes, per 1 ton of fresh mass, approximately 139 m 3 of biogas could be obtained. On the other hand, biogas produced from steamed potato peel contained 46% of methane; from 1 ton of fresh mass, approximately 165 m 3 of biogas was obtained. According to Romuniak and Domasiewicz [65], the methane content in potatoes was 56. 5% on average. The biogas yield from 1 ton of fresh potato matter was 134 m 3. In the research led by Myczko [66], the highest amount of biogas was obtained from potato haulm (80-120 m 3 ), and the share of methane reached the value of 52-56%. Compared with other potato parts (i. e., peels (35-42 m 3 from 1 Mg; CH 4 : 25-44%) or potato juice (6. 5-15 m 3 from 1 Mg; CH 4 : 55-60%)), haulm was surprisingly a more favorable substrate for methane fermentation, mainly due to the low water content, which allowed obtaining better results per ton of dry matter of the raw material. For comparison, during the same investigation, the best results were achieved for maize silage, with biogas efficiency oscillating in the range of 170-200 m 3 per 1 ton of fresh mass. The methane content reached 52-56% [64, 66]. 

New perspectives in biogas systems were described by Marks's team [67]. The authors described new approaches of biogas installations which allow the effective fermentation of residues from animal and vegetable production, the agro-food industry, and municipal waste. For years, the basic substrate used in biogas plants in Poland or Germany has been maize silage [50, 68]. This was cut by a sudden rise in the price of maize silage because of growing demand from biogas plants and animal production feeding. Hay substrate is also used for biomethane production. However, it should be emphasized that the high content of lignocellulosic compounds in the plant biomass makes the anaerobic decomposition process more difficult, thus requiring, above all, a longer retention time [67]. 

Kowalczyk-Juśko's team described the application of a prediction model based on artificial neural networks to estimate the production of methane from various biomass in the form of silages [69]. This study determined the chemical and physical parameters of 13 silages made from various species and their biogas efficiency. Moreover, the most suitable model for the prediction of biogas production was indicated. This radial basis function model can be helpful in quickly estimating the energy value of different silages without long-term and expensive analysis, as proven for various substrates [59, 69]. 

Pilarski's team analyzed the possibility of using techniques of neural modeling to estimate the level of biomethane content in the gaseous fuel emitted over the fermentation process of silage [70]. As a result, using the tools available in the Statistica program, a set of 10 predictive topologies was generated with the use of the developed training set, estimating the methane level. Then, an optimal network was selected from them, i. e., a neural model of MLP type with a 5-12-1 structure. The analysis of the sensitivity of this model to input variables of the tested process revealed that the most important parameters characterizing the volume of biomethane emissions were (in order of capacity) volume of biogas produced, pH, conductivity, mass of dry matter, and mass of organic matter. The obtained research results confirmed the assumption that the predictive neural model, which describes the production of CH 4 during the silage fermentation process in a bioreactor, is an appropriate tool for assessing the forecast level of emissions [59, 70]. 

Agricultural biogas plants are a popular solution worldwide for energy production. Another particularly important aspect directly related to the functioning of a biogas plant is the production of digestate. Digestate is a residue from the process of anaerobic digestion. The fertilizer in question is characterized by a liquid form rich in nutrients, both macroelements and microelements. An additional benefit is the presence of organic matter. For this reason, using digestate, not only minerals, but also organic ingredients are supplied to the soil. Due to the growing prices of artificial fertilizers and problems with their availability, organic fertilizers, including digestate, are gaining importance. It should be mentioned that, in many biogas plants, the substrates for biogas production are manure and slurry. Therefore, the use of digestate is a rational solution in relation to the management of natural fertilizers. The use of digestate is safe for the environment, provided that the appropriate rules are observed. It should be remembered that digestate can be sold as fertilizer. This will be another source of income for biogas plant owners. It is good practice to transfer biomass and biodegradable waste to the biogas plant, collecting digestate in return. This action is in line with the principles of sustainable development and a circular economy.
section_name: Biogas Efficiency of Plant Agricultural Waste

section_content: On the basis of the analyzed results published in reputable scientific articles, it can be concluded that agricultural plant residues have great potential as co-substrates for biogas production. The results obtained by the researchers regarding both biogas and methane efficiency are satisfactory. According to data from the literature, biogas efficiency for the discussed substrates can be high. For example, for hemp straw, it is 208. 8 m 3 /Mg FM, and, for steamed potato peel, it is 165. 62 m 3 /Mg FM. The described substrates may be used as valuable feedstock in biogas plants. The use of waste from the cultivation and processing of plant raw materials will not only help to manage this waste rationally, but also contribute to the increase in the production of renewable energy sources. Accordingly, the circular economy in terms of the management of agricultural plant residues to produce biogas will have a multi-faceted, positive impact on the environment. 

Generally, the highest biogas efficiency per 1 Mg of fresh mass was obtained from agricultural plant residues characterized by low water content. Nevertheless, the methane content mostly resulted from the content of fats, proteins, and simple sugars, the decomposition of which enables obtaining the largest amount of energy in a short retention time. 

Most biogas calculators disregard the costs of labor and storage, as well as of the disposal of feedstock and residues. Moreover, they usually do not show the methods of counting, whereby the differences in the final results can exceed 60%. For this reason, biogas calculators should be treated as an advisory not decisive tool [71]. It is important to experimentally determine both biogas and methane efficiency of a substrate as a potential feedstock for the production of gaseous biofuels. Such tests should be carried out each time the supplier of the substrate changes or its chemical composition changes (e. g., the ratio of individual wastes in biomass). One of the future directions of biogas development is environmentally friendly motor gasoline as a trend to reduce emissions [72] [73] [74]. Moreover, the usage of artificial neural networks to predict biogas production is future-proof and should facilitate the management of biogas plants [59].
section_name: Conclusions

section_content: Data Availability Statement: All data generated or analyzed during this study are included in this published article.

section_name: Conflicts of Interest:

section_content: Author Contributions: Conceptualization, J. F. and W. C. ; methodology, J. F. ; software, J. F. ; validation, J. F. and W. C. ; formal analysis, J. F. ; investigation, J. F. ; resources, J. F. and W. C. ; data curation, J. F. and W. C. ; writing-original draft preparation, J. F. and W. C. ; writing-review and editing, J. F. and W. C. ; visualization, J. F. ; supervision, W. C. ; project administration and funding acquisition, J. F. and W. C. All authors have read and agreed to the published version of the manuscript. 

Funding: This research received no external funding.

section_content: The authors declare no conflict of interest.
section_name: Conflicts of Interest: