1 Introduction

Grasslands are one of the major vegetation types worldwide and cover about 30–40% of the land surface [1]. In the European Union, permanent grasslands occupy an area of 59 million ha, accounting for 35% of the utilized agricultural area [2] and are valuable ecosystems that provide a variety of important ecosystem services: (i) provisioning services, such as food supply, water supply, or habitat for wildlife species; (ii) regulating services, such as carbon sequestration, erosion control, or nutrient retention; (iii) cultural services, such as esthetic appreciation, recreation, or cultural heritage; and (iv) supporting services, such as water cycling, nutrient cycling, and primary production [3].

Semi-natural grasslands, a subgenre of permanent grasslands, have a particularly high biodiversity and are among the most species-rich habitats in the world [4]. Semi-natural grasslands are characterized by the fact that they are not intensively cultivated or fertilized. Yet, these areas have to be managed in order to maintain the typical vegetation, preserve the species composition, and prevent the natural successive growth of shrubs and trees [5]. Some types of semi-natural grasslands require extensive grazing, while others rely on mechanical harvesting or cutting [6].

Due to their high protein content, grass clippings are predominantly used for animal production as a feed for ruminants [7]. However, in many European countries, livestock and dairy farming are decreasing [8], leading to a lower demand for forage. Additionally, the EU Common Agricultural Policy (CAP) has introduced a cross-compliance system that establishes the preservation of permanent grassland as a prerequisite for direct payments to farmers. After a decreasing trend in the past, the area of permanent grassland has increased between 2005 and 2016 in Europe [2].

Although there is no exact information on the actual available biomass from grassland, it is supposed that there is increasing amount of surplus material which is not needed for feed chains. Meyer et al. [9] estimated that the amount of excess grass in the EU28 in 2030 would range between 20 and 110 Mt. Thus, grass offers great potential for alternative exploitation strategies. This applies in particular to grass from conservational and landscape management schemes, which is usually cut at a maximum twice a year in spring and autumn and largely unused.

Traditionally, grasses are used as animal feed, but they can also be processed for a wide range of other purposes. Depending on its chemical composition, biomass can be subject to a variety of thermo-chemical, biochemical, and physicochemical conversion processes [10]. Energy recovery from grass in biogas or incineration plants has been established as an alternative means of utilization. The composition of grasses also makes them a suitable feedstock for material recovery. Generally, the cell wall components cellulose and hemicellulose offer the possibility to produce fibers. An acid and/or enzymatic hydrolysis of both polymers results in monomeric sugars (e.g., glucose, fructose, galactose), which together with carbon-rich water-soluble extracts can be used as a full fermentation medium to produce single-cell protein, lactic acid, and other compounds of economic relevance [11]. Early-harvested grasses also contain proteins. As global protein demand is expected to increase drastically in the future [12], novel protein production strategies have to be developed. Grasses constitute a promising feedstock to helping fulfill the increasing demand since grassland can produce up to 50% more protein per hectare as peas, beans, or soya [11]; however, the protein content can be reduced in late-harvested grass.

To foster the use of grass, the aim of this review was to investigate and develop possible utilization strategies for grass from nature conservation areas. In this study, the term semi-natural grassland will be used for species-rich grasslands that are maintained for nature conservation purposes, in accordance with Bruinenberg et al. [13]. This includes grasslands in nature conservation areas as well as grasslands with other landscape management schemes. Grasslands that are managed in conservational schemes are mostly cut late due to the protection of endangered species, such as meadow breeders. The first cut may range from mid-June to February [14]. The focus was on a holistic use, taking the full potential as source of proteins, fibers, and further compounds in mind. This study explores and compares the potential uses of semi-natural grassland biomass. This study further links the value of grass for ecosystem services with the value as a material source and sheds light on the establishment of decentralized utilization processes operating in rural areas where grass is appearing.

2 Composition of grass

Grasses are composed of two main fractions: cell walls and cell contents. Cell walls mainly consist of the structural carbohydrates, cellulose and hemicellulose, and the organic polymer lignin. Cell walls can make up to 80% (w/w) of the dry material [15]. Cell contents are mainly comprised of sugars, proteins, lipids, and minerals. The average composition of grasses is shown in Table 1.

Table 1 Composition of grasses on a dry matter basis [11]
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The variation in the material composition of grasses is caused by multiple factors. Plant environmental factors such as temperature, water deficit, solar radiation, or nutrient availability in the soil exert an influence on the chemical characteristics [16]. The origin of the material (e.g., permanent grassland or roadside vegetation), the management intensity (fertilized vs. unfertilized), and the range of species also play a role [11]. Yet, the herbage maturity has by far the greatest impact on the composition [16].

The influence of the maturity on the characteristics of grass is illustrated in Table 2. Herrmann et al. [5] studied the composition of three typical types of grassland vegetation used in landscape management in north Germany at different harvest times throughout the season. The dry matter content generally rises with advancing maturity [15], which was confirmed by Herrmann et al. [5] although content fluctuated significantly and was influenced by precipitation. The organic dry matter content remained approximately constant at a high level of more than 90% (w/w) over the year.

Table 2 Composition of three types of grassland vegetation cut at different times (DM, dry matter; oDM, organic dry matter; n.d., not detected) (May 2006 and January 2007) [5]
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In the beginning of the growing season, cell contents may present up to 65% (w/w) of the biomass [7]. With advancing maturity, the cell wall concentration in the plant increases, and the proportion of cell contents decreases [16]. Consequently, the proportion of crude fibers rises over time, while the amount of crude protein, crude fat, and sugars declines. These claims are confirmed by the findings of Herrmann et al. [5]; the crude fiber content increased from < 30% (w/w) to up to > 40% (w/w). The protein content was reduced by about 60% (w/w) between May and January. The fat content was below 2% (w/w) in the beginning of the growing season and could not be detected in winter. The sugar content decreased from late autumn, and almost no sugar was detected in winter. Indeed, the sugar content is highly variable and depends mostly on the environmental conditions, such as light and temperature [7].

The ash content decreases over the course of the growing season. Kandel et al. [17] observed a rapid decline in the ash content from April to September in reed canary grass, and Burvall [18] reported a decrease from 6.4 to 5.6% (w/w) between summer and springtime. The decrease in ash is due to the loss of leaves in winter which have higher ash contents than the stems and the leaching of components that are readily dissolved such as Cl, K, and Na [19].

In contrast to biomass from intensive grassland, landscape management grass is a highly inhomogeneous material with extreme variable characteristics [5, 20]. The high species diversity results in mixtures of different growth stages, heading dates, and tissue compositions at a given harvesting date [13].

3 Use of grass as forage

Grasses are one of the principal forages and may be consumed on site by grazing or after conservation as silage or hay [21]. Most grasslands for forage production are nowadays intensively managed to provide high-quality forage. They are monocultures consisting of highly productive species such as perennial ryegrass (Lolium perenne) that are fertilized heavily and harvested early in the growing season [22]. Grasses of semi-natural, extensively managed grasslands can mostly no longer fulfill the demands of highly productive ruminants as they have a lower forage quality [23,24,25]. The major determinants of forage quality are the nutritive value, the digestibility, and the voluntary intake by ruminants.

The digestibility of forages from semi-natural sites is generally lower than from intensively managed grassland [13]. Tallowin and Jefferson [26] reported that the in vitro digestibilities from unfertilized semi-natural grasslands were at least 20% lower compared to the material from intensively managed grass. Bruinenberg et al. [22] compared the digestibility of grass derived from intensively managed grasslands with extensively managed species-poor and species-rich grassland and calculated digestibilities of 75.8%, 57.4%, and 54.5%, respectively.

Apart from that, the digestibility is strongly influenced by the plant maturity. Forage should be harvested immature to obtain the highest digestibility [16], as digestibility is dependent on the degree of lignification and the content of crude protein [15]. Thus, the delayed harvest of landscape management grass leads to a significant reduction in digestibility [13]. Bokdam and Wallis de Fries [27] reported that the digestibility was up to 45% lower for later harvests, while Waramit et al. [28] observed a decrease by up to 60% when harvesting was postponed from May to October.

The nutritive value of grass from landscape management was generally found to be low, especially with later dates of harvest. The content of crude protein declines with advancing maturity [28,29,30], so that forage necessitates partial supplementation with extra rumen degradable protein [24]. A decrease in content over the growing season in different natural grassland systems was also reported for the nutrients P [25, 29,30,31], N [25, 27], K [25, 27, 31], Mg, S, Cu, Fe, Mn, and Zn [31]. Phosphorus and other nutrients partially seem in short supply, and supplements might be necessary to prevent serious deficiency and to sustain high animal performance [26, 27, 30].

The nutritive value of forage is also characterized by the energy that is provided. The metabolizable energy of semi-natural grasslands is generally about 10–40% below that of intensively managed grass [26]. With advancing maturity, the energy content of the grass further decreases [29, 32].

As noted above, the quality of the forage is also influenced by the extent to which it is voluntarily consumed by ruminants. Armstrong et al. [33] have shown that the intake of forages from semi-natural grasslands is lower than intake from ryegrass and clover swards, which was attributed to lower digestibility. Intake further decreased with harvest date. Contrarily, Bruinenberg et al. [22] found that voluntary intake from species-rich and species-poor natural grasslands was not lower than from intensively managed grass. Nevertheless, even though intake of grass from landscape conservation might be comparable to intensively managed grass, the fact that it has poor forage quality and is not suitable as animal feed opens the path to material and energetic utilization strategies.

4 Use of grass for energy recovery

4.1 Combustion

Grass can be used as a material for combustion. The calorific value of grasses is only slightly lower than that of wood. However, the quality of grasses is significantly lower compared to wood (Table 3). Grasses contain high concentrations of the elements N, S, Cl, and K and higher ash contents, which are problematic for combustion. A high ash content can lead to deposit formation and increased fly ash emissions and causes more problems concerning ash storage and disposal. High concentrations of N, S, and Cl can lead to high emission levels of nitrogen oxides, sulfur oxides, and chlorine compounds. In addition to that, chlorine and sulfur can lead to deposit formation and corrosion of metal parts [34, 35]. K is the main mineral that causes sintering and boiler fouling [19].

Table 3 Biofuel characteristics of wood and grasses (HHV, higher heating value)
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The quality of grass as material for combustion is determined by various factors, the most important of which is the harvest time. The quality of grass generally increases with later date of harvest [35, 36]. The three main reasons for changes in characteristics over time are (i) decreasing proportions of leaf biomass containing higher nutrient concentrations than the stems; (ii) nutrient translocation and storage in plant roots; and (iii) leaching of elements by precipitation [35].

Significant reduction of the ash content with later harvest was reported for pure stands of different grasses, such as reed canary grass, miscanthus, switchgrass, perennial ryegrass, and cocksfoot [18, 37,38,39] as well as for semi-natural grassland sites [36, 40]. The decrease in ash content is attributed to a loss of nutrients. A significant decline in the contents of N, S, Cl, and K was observed in several studies [18, 36,37,38,39,40]. A significant increase in the ash fusion temperature from 1,070 to 1,400 °C was also shown [18].

The calorific value is not significantly affected by harvest time [18, 37, 39], making grass an energy-rich fuel even when harvest is delayed. However, delayed harvest of grasses leads to biomass losses resulting in less fuel available for energy production. It was observed that delayed harvest of reed canary grass and switchgrass leads to significant yield reductions by up to 24% and 43%, respectively [41, 42].

Fertilization has a negative impact on biofuel quality. Landström et al. [43] reported higher amounts of N and K in reed canary grass after fertilization. Accordingly, the use of fertilizers should be kept to a minimum [35]. Thus, the quality of grass from semi-natural grasslands for combustion is higher than from intensively managed grasslands due to late harvesting and lack of fertilization. Nevertheless, the concentration of problematic elements might still be beyond the requirements of conventional incineration plants. Combustion technologies require various adaptations for the processing of grass to reduce emissions, deposits, dust emission, and corrosion, control agglomeration, and achieve a more complete combustion [35].

4.2 Biogas production

Grass is a well-established and widely used feedstock in biogas plants. About 30–40% of the biogas plants in Germany are already operated with grass or grass silage as a substrate or co-substrate [44]. Substrate-specific methane yields of grasses usually range between 200 and 400 L per kg organic dry matter (oDM), with an average of about 300 L kg−1 oDM. The average methane yield of the most widely used feedstock maize is on average only about 20% higher at 370 L kg−1 oDM [44]. These values refer to intensively managed grassland, and the management intensity exerts great influence on the digestibility and the methane production potential of grasses. Even though biogas production is a rather well understood process, there is still room for improvement. In a recent study, possible ways of efficiency improvements of biogas production from grasses have been studied. Hansen et al. [45] found, for instance, that the hyperthermophilic anaerobic bacterium Caldicellulosiruptor bescii is effective in degrading and solubilizing lignocellulosic materials and that this pretreatment eventually increases biogas production. Conservational and landscape management grasses generally have significantly lower substrate-specific methane yields as indicated in Tables 4 and 5.

Table 4 Substrate-specific methane yield of different natural grassland vegetations at different times of harvest (oDM, organic dry matter)
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Table 5 Substrate- and area-specific methane yield of maize compared to common reed, reed canary grass, and reed sweet grass from protected wetlands in a Polish national park [124, 125] and three conservation areas with a periodic mowing scheme once a year [126] (oDM, organic dry matter)
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The most important factor influencing the yield is again the harvest time. The substrate-specific methane yield has shown to decrease significantly during the growing season [5, 17, 20, 46]. Table 4 shows the substrate-specific methane yield of natural permanent grassland vegetations at different times of harvest from early summer to the following winter. Methane yields decreased continuously by up to 60% over the course of the year. Lower methane yields are explained by increasing amounts of cell wall fractions of lower digestibility and lower contents of crude protein [5]. To maximize the substrate-specific methane yield, the biomass should be harvested at early stages of growth. Early harvest is, however, mostly not compatible with the requirements of landscape management areas.

Furthermore, Table 4 indicates the heterogeneity of biomass from semi-natural grasslands, which has already been mentioned. Both studies were carried out with meadow foxtail vegetation under the same digestion conditions except for the retention time (28 days in [5] and 25 days in [20]). The methane yield in both studies is significantly different, especially in June, and cannot be explained by the retention time alone as the yield does not increase as much over time. Accordingly, the differences must be due to the composition of the biomass.

Other factors only have a minor impact on the substrate-specific methane yield. Fertilization had no significant effect on the substrate-specific methane yield of the different sward types [47]. Variations between substrate-specific methane yields of different grass species [46, 48, 49] and sward types [47] proved to be insignificant. Nevertheless, cultivated reed canary grass showed significantly higher biogas yields (406 L kg−1 oDM) than wild ones harvested in an unused meadow community in Poland (120 L kg−1 oDM) [50].

In order to maximize the biogas production, the highest possible area-specific methane yield (m3 CH4 ha−1) should be achieved. The area-specific methane yield consists of the biomass yield (kg oDM ha−1) and the substrate-specific methane yield [51]. The area-specific methane yield can be significantly affected by fertilization due to the resulting increase in biomass yield [47]. The area-specific methane yield is also affected by the time of harvest. The peak recorded for a pure stand of reed canary grass was observed to be in the end of July [17]. In a natural meadow foxtail vegetation, the maximum yield of 1,604 m3 ha−1 a−1 was recorded in September and strongly decreased to 155 m3 ha−1 a−1 in February. As a means of maximizing the overall methane yield, the grass should thus be harvested in late summer. This might conflict with conservation requirements in semi-natural grasslands as, e.g., purple moor grass and acute sedge vegetation require a later cutting date [5].

The substrate- and area-specific methane yield of different landscape management and conservation areas harvested at their designated date of management compared to maize are shown in Table 5. The conservational biomass is characterized by both low substrate-specific and low area-specific methane yield. It should be considered that the substrate-specific yield for maize referenced here is significantly higher than the average value (see above, 370 L kg−1 oDM). However, even assuming the average value, the area-specific yield (4,159 m3 ha−1) would still be significantly higher than in the conservation areas. Even though overall methane yields in landscape management and conservation areas are low, economic feasibility of biogas production might be given under certain circumstances [14].

4.3 Integrated generation of solid fuel and biogas

The integrated generation of solid fuel and biogas (IFBB) is a process aiming at producing a suitable feedstock for both anaerobic digestion and combustion from semi-natural grassland biomass. A schematic flow chart of the process is given in Fig. 1. The first step of IFBB consists of a hydrothermal conditioning of grass silage in which water is added and the mixture is kept at a defined temperature for a short time. This macerates the cell walls and produces a mash [52]. Subsequently, the mash is mechanically dehydrated by a screw press, resulting in a press fluid and a press cake. Minerals and organic compounds are transferred to the press fluid, while the press cake is rich in fibers and has a higher dry matter content [52]. The press fluid will undergo anaerobic digestion, whereas the press cake is dried, and the dry pellets can be used as a solid fuel for combustion or gasification [53]. The biogas is burned in an integrated combined heat and power plant, and the generated waste heat is used to dry the press cake. The digestate can be used as a liquid fertilizer that contains high concentrations of available nutrients [53].

Fig. 1
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Flow chart of the integrated generation of solid fuel and biogas (IFBB)

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The combustion characteristics of the press cake are significantly improved compared to the parent material. The concentrations of the detrimental elements N, S K, and Cl and the ash content are largely reduced. K and Cl, in particular, are almost completely transferred into the press fluid, whereas N, S, and ash are removed from the solid material to a lesser extent (Table 6). Despite the removal of these elements, the concentrations in the press cake might still be higher than suggested by Obernberger et al. [54] for solid fuels (see Table 3) [55].

Table 6 Proportion of mass flow of plant components in the press fluid
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The majority of the crude protein (57–82% (w/w)) and almost all of the fibers (neutral detergent fiber (NDF) 96% (w/w), acid detergent fiber (ADF) 96% (w/w), acid detergent lignin (ADL) 97% (w/w)) remain in the press cake [56]. The higher heating value of the press cake is slightly higher compared to the parent material [57, 58]. The ash softening temperature of the press cake is also raised [57,58,59]. An increase of 50 °C (from 1,000 to 1,050 °C) was reported by Bühle et al. [59] after conditioning at 25 °C, while Richter et al. [57] measured an increase of 143 °C (from 1,111 to 1,254 °C) after conditioning at 80 °C.

The mean substrate-specific methane yield of the press fluid is significantly higher than that of the whole-crop silage. Richter et al. [60] reported an increase of 82% after conditioning at 5 °C and of 96% at 60 °C. The press fluid generated up to 63% of the methane yield per ha produced by the whole-crop silage even though only 29% (w/w) of the organic material was extracted from the silage into the press fluid. The degree of degradation of the organic matter was increased from 55% (w/w) in the whole-crop silage to 89% (w/w) in the press fluid [60].

The overall energy yield of IFBB outperforms whole-crop digestion. The gross energy yield of IFBB in a double cropping system of winter rye and maize was estimated to be 91.2–94.4 MWh ha−1, whereas the biogas yield of whole-crop digestion amounted to only 63.4 MWh ha−1 [61]. Hensgen et al. [56] reported an energy yield between 4.08 and 12.55 MWh ha−1 for whole-crop digestion and between 9.75 and 30.19 MWh ha−1 for semi-natural grasslands. Direct combustion of hay however yielded even higher energy outputs between 10.51 and 31.66 MWh ha−1. While the energy conversion efficiency of IFBB is thus lower compared to direct combustion, the fuel quality is improved resulting in a lower risk of ash slagging and emissions, and the nutrients can partially be returned to the field as digestate fertilizer.

Richter et al. [62] predicted a maximum conversion efficiency at a temperature of 50 °C. At lower temperatures, more energy is needed for drying the press cake, and at higher temperatures, more energy is needed to heat the biomass water mixture during conditioning. In general, the temperature does not seem to have a large influence on the products. King et al. [63] stated that increasing the water temperature from 20 to 60 °C had little effect on the composition of the press products. Richter et al. [52] observed a trend towards higher mass flow into the fluid at higher temperatures, but the variation between the temperatures was low. Contradictory results were reported concerning the influence of the plant maturity on the products. King et al. [63] found that the fractionation was more efficient with more immature herbage, as soluble components could be removed more easily from the silage. According to Richter et al. [62], increasing maturity had a positive effect on the methane yield of the press fluid and the overall energy conversion efficiency (from 0.32 to 0.46).

5 Use of grass for material recovery

5.1 Green biorefinery

A green biorefinery (GBR) aims at a holistic utilization of biomass and combines the production of materials and energy. The raw material of a GBR is green biomass, which includes grass from the cultivation of permanent grassland, fallow lands, nature preserves, and green crops, such as lucerne, clover, and immature cereals from extensive land cultivation [64]. After harvesting, the biomass decays quickly due to uncontrolled fermentation processes, and the fresh material should best be processed within 8 h [65]. Alternatively, the feedstock can be preserved and stored as silage, allowing constant year-round supply and predictable feedstock quality [7]. The primary processing step is wet fractionation of the biomass by mechanical pressing resulting in a green (from fresh material) or brown (from silage) juice and a press cake. Both fractions contain a variety of valuable ingredients, and the further processing may vary depending on the desired products. Anaerobic digestion is generally integrated in biorefineries as an end-of-pipe technology to make full use of residues remaining after the refining processes [66].

The press juice contains proteins, lipids, lectins, sugars, organic acids, free amino acids, dyes, hormones, enzymes, minerals, and other materials [64, 67]. The composition of green and brown press juice varies considerably [7], leading to different valorization pathways. Schematic overviews on possible processing steps and products from green and brown press juice are given in Fig. 2 and 3.

Fig. 2
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Schematic overview of possible products from green juice

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Fig. 3
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Schematic overview of possible products from brown juice

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The main focus of a GBR for fresh biomass is the recovery of proteins [7]. After the pressing, proteins are precipitated by different methods such as heat coagulation, acidification, or fermentation and concentrated by separation and drying. An extensive overview on protein processing in a GBR is given by Santamaría-Fernández and Lübeck [67]. Typically, about 40–60% (w/w) of the total leaf proteins can be extracted by mechanical pressing [67]. The residual juice can be used as a medium for anaerobic digestion [68] and fermentation [69] or be recycled back to the field as fertilizer. However, as most of the nutrients remain in the press cake, the residual juice contains low amounts of nutrients, limiting the feasibility of applying sufficient fertilizer per area [70]. Alternatively, the green juice can directly be processed by fermentation to produce different chemicals such as lactic acid [71, 72] or L-lysine [73]. The press juice has also shown to be suited for direct use as co-substrate for the feed of monogastric animals. Barber et al. [74] and Adler et al. [75] showed that the inclusion of grass juice in the feed diet did not negatively affect the weight gain of growing pigs.

A green biorefinery for ensiled biomass mainly focuses on the production of amino acids and lactic acid due to their high concentrations in the press juice [7]. Ecker et al. [76] observed up to 40 g L−1 of lactic acid and around 20 g L−1 of amino acids in silage juice from grassland. The separation of lactic acids and amino acids can be realized by membrane technologies such as electrodialysis and ion exchangers [76, 77]. By changing the ensiling conditions, other platform chemicals such as butyric acid could also be recovered from the juice [78]. Alternatively, the juice can be digested [60] or be fed to monogastric animals [79, 80].

The press cake contains fibers, valuable dyes and pigments, crude drugs, and other organics [64]. There are a variety of possible uses for the press cake (Fig. 4). After drying, the cake can be processed to ruminant feed [81, 82] or applied as a solid fuel for combustion [39]. Alternatively, the press cake can directly be processed through anaerobic digestion [68]. Due to its high fiber content, the press cake can also be upgraded to manifold fiber applications such as insulation material [83, 84], building material [85], bio-composites [84, 86], pulp and paper [87], or the formation of cellulose nanocrystals [88]. Furthermore, the press cake fraction is a suitable feedstock for a lignocellulosic biorefinery and can yield a wide range of different products [89] and even for the production silver nanoparticles [90, 91]. It should further be admitted that the quality of fibers in terms of length and diameter does not only vary among different biomasses [92] but may also vary due to the late harvesting. Thus, further information on the quality of fibers from late-harvested grass is required to draw specific utilization approaches.

Fig. 4
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Schematic overview of possible products from press cake

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The green biorefinery concept is currently in an advanced stage of development in several European countries. Table 7 gives an overview on some of the operating GBR plants in Europe. The existing biorefineries use both fresh and ensiled grass and produce a variety of different outputs.

Table 7 Overview on some green biorefinery plants in Europe (FOS, fructooligosaccharides)
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To date, no GBR was described that uses late-harvested grass from semi-natural grasslands. For nature conservation reasons, these areas are cut only once or twice a year. This results in a significantly reduced operation time of the biorefinery compared to intensively managed areas with several cuts. Also, conservation areas are often small and less accessible which sets limits to fast processing. A mobile screw press could be useful to press the material in a decentralized manner shortly after harvest. The pressed juice could be preserved and processed in a central biorefinery. Alternatively, the material could be ensiled to ensure longer availability. It is supposed that biomass from semi-natural grasslands has a low content of exploitable ingredients and is therefore less suitable for ensiling [5, 93]. In addition to that, the suitability of grass for ensiling decreases with advancing maturity, resulting in a silage of poor quality [20, 94]. On the other hand, ensiling of late-harvested, heterogeneous grass was not considered problematic in the IFBB process.

Beyond that, it is assumed that grass clippings from semi-natural grasslands are less suitable for a GBR as the content of exploitable components is lower than in intensively managed areas [93, 95]. A theoretical mass balance of a GBR producing a fiber- and a protein-enriched fraction from semi-natural grassland is shown in Fig. 5. For simplification, it was assumed that no transformation of the plant components occurs during the processing. About 30 to 60% (w/w) of the original protein can be recovered in the protein concentrate [96], whereas 95% (w/w) of the fibers remain in the press cake [97]. As the protein concentration is generally low, the grass seems to be less appropriate for protein extraction [96]. The protein content of intensively managed grass can be about twice as high [22], resulting in twice the protein yield. Considering the high amount of fiber, the grass might rather represent an opportunity for fiber applications [96]. Late cut biomass could, for example, be suitable for the production of pulp and paper [98]. Due to the high fibers and lower water contents, the grass could possibly be a candidate for the lignocellulose biorefinery.

Fig. 5
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Mass balance of a green biorefinery, based on 1 kg dry material. The fiber and protein contents are based on data of 18 different semi-natural grasslands harvested between July and September [56]. Conversion efficiencies are adopted from [96] and [97]

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5.2 Lignocellulose biorefinery

A lignocellulose biorefinery (LCB) operates on dry lignocellulosic feedstock. Lignocellulosic biomass for a biorefinery comprises five categories: existing landscape species (e.g., softwood, hardwood, reed), fast growing plantations (e.g., poplar, willow), landscape conservation biomass (e.g., residual wood, grasses, straw), process lignocelluloses (e.g., press cake from crop drying plants, by-products from cereal mills), and used materials waste (e.g., recovered paper, used wood, cellulosic municipal solid wastes) [99].

The process steps in an LCB are developed based on the respective feedstock and the desired products and can thus vary greatly. The biochemical route is the most used process [100] and consists of four major processes: pretreatment, hydrolysis, fermentation, and downstream processing. The first step is the pretreatment, which is indispensable in order to overcome the inherent recalcitrance of the biomass to degradation. A variety of methods can be employed for the pretreatment of lignocellulosic biomass. The methods can be classified into four categories: physical (mechanical comminution, microwave, pulsed electric fields), physicochemical (CO2, steam, or ammonia fiber explosion), chemical (acid, alkaline, organosolv), and biological (fungi, enzymes). Each pretreatment results in specific modifications of the material, and the advantages and drawbacks have been addressed in several reviews [101,102,103].

Subsequently, the carbohydrates cellulose and hemicellulose are converted to monomer sugars by hydrolysis. Hydrolysis is usually carried out enzymatically by cellulases and hemicellulases or by employing diluted or concentrated acids [104]. Cellulose is degraded to glucose, and hemicellulose is degraded to C6 hexoses (glucose, mannose, galactose) and C5 pentoses (xylose, arabinose). The resulting sugars are converted, for instance, into ethanol, other alcohols, organic acids, and other fermentation products [99]. Enzymatic hydrolysis and fermentation are typically performed consecutively but can also be carried out partly or fully simultaneously [100].

Concepts for lignocellulosic biorefineries currently mainly focus on the production of ethanol as a biofuel [105]. However, there is great potential for the production of a virtually inexhaustible range of products. Due to the large number of possible products from an LCB, a general mass balance cannot be established. Isikgor and Becer [106] identified 16 different building blocks from C5 and C6 sugars that can be converted into more than 200 valuable compounds. Figure 6 shows the top 10 chemical opportunities from biorefinery carbohydrates according to Bozell and Petersen [107]. The furans hydroxymethylfurfural (HMF) and furfural are particularly interesting products, as they are, for example, the starting materials of Nylon 66 and Nylon [99]. Via modified acid hydrolysis at elevated temperatures, HMF, furfural, and levulinic acid can be produced from lignocellulosic feedstock [108, 109]. A pilot plant-producing HMF, furfural and phenolic compounds from Miscanthus, is currently in operation in Germany [110].

Fig. 6
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Top 10 target structures from biorefinery carbohydrates [107]

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The remaining lignin fraction after fermentation is of low purity and mostly employed for low-value applications such as energy generation [111, 112]. Depending on the pretreatment method, lignin can be extracted before the carbohydrate conversion [111] and constitutes a feedstock for high value-added products. Lignin and lignin-based materials can, for example, be used in sustainable construction as concrete and asphalt additives, polymeric foams, and corrosion inhibitors and in coating or phenol–formaldehyde resins [113]. By depolymerization of lignin, several aromatic monomers such as phenols (guaiacol, catechol, cresol), acids (muconic acid, cinnamic acid, vanillic acid), or aldehydes (syringaldehyde, cinnamaldehyde) can be synthesized [106, 114].

Protein recovery is not commonly included in an LCB. However, it is feasible to extract proteins also from dry lignocellulosic feedstock in a biorefinery [115, 116]. Instead of the mechanical pressing step in the GBR, biomass samples can be placed in an aqueous alkaline solution (pH 10–12) for 30–60 min at 40–90 °C. Protein extraction can be carried out at three different positions in the process (Fig. 7). If protein extraction is carried out after the pretreatment, it should be ensured that pretreatment does not impair protein quality.

Fig. 7
figure 7

modified from Chiesa and Gnansounou [115]

Process schemes of protein extraction in a lignocellulose biorefinery. a Extraction prior to pretreatment, b extraction after pretreatment, and c extraction after hydrolysis. Own representation

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There is little experimental data on protein extraction from dry grasses; thus, general conversion rates cannot be provided. Poor extraction yields of 30% (w/w) were reported for coastal Bermuda grass when extraction was performed prior to the pretreatment (Fig. 7a) [117]. If extraction was carried out after ammonia fiber explosion (AFEX) pretreatment (Fig. 7b), about 75% (w/w) of the original protein was solubilized. An additional 5% (w/w) was solubilized during hydrolysis. Bals et al. [118] reported that about 40% (w/w) of the original proteins from switchgrass was extracted after AFEX pretreatment. Almost the complete remaining amount was solubilized during the subsequent hydrolysis. If hydrolysis was performed before the protein extraction (Fig. 7c), about 60% (w/w) of the protein was solubilized during hydrolysis, and only 8% (w/w) could be extracted afterwards. Accordingly, option b appears to be most promising for high protein yields. Depending on the process used, protein extraction yield can be significantly higher than in a green biorefinery. Admittedly, the extraction is more complex, and the solubilized protein must be recovered from two different phases after protein extraction and hydrolysis to ensure high yields and profitability [119].

Previous research on LCB of grasses has focused on rather homogeneous feedstocks such as pure stands of energy grasses (for an overview of potential grass species, see [120]). Adler et al. [42] investigated 34 grasslands sites under the Conservation Reserve Program in the USA and found that the theoretical ethanol yield decreased with increasing species richness. Mezule et al. [121] studied the biomass potential of 67 semi-natural grassland plots in Latvia, harvested between June and August. They found that the amount of sugars produced by hydrolysis tended to decrease during the growing season. These findings suggest that species-rich and late-harvested semi-natural grasslands might be less suited for an LCB than energy grasses. On the other hand, Jungers et al. [122] reported that theoretical ethanol yield from mixed species conservational grasslands is similar to dedicated energy grass. Additional studies to explore the potential of semi-natural grasslands for the lignocellulosic biorefinery are required.

6 Conclusions and future perspectives

Late-harvested grass from landscape management areas is currently a largely unused biomass source. Compared to freshly harvested grass, it contains high amounts of fibers and reduced protein contents. Therefore, it is not suited for forage production. This opens the way to the variety of energy and material recovery options without competing with feed production. It is generally an interesting approach to combine ecosystem services in nature conservation areas with biomass production for an emerging bioeconomy.

As grass biomass has a high calorific value, it appears to be suited for combustion. Grass contains high concentrations of N, S, Cl, K, and ash, which can cause high levels of emissions, corrosion, and deposit formation in the boiler. Delaying harvest improves combustion properties, making semi-natural grassland biomass more suitable than intensively managed grasslands. However, adapted combustion technologies are required. Grass is a well-established and widely used feedstock in biogas plants. As late-harvested grass has increased amounts of fibers, the digestibility is significantly reduced. Additionally, the biomass yields are lower due to later harvest and less intensive management practices. Therefore, both substrate- and area-specific methane yields are comparably low. The IFBB process represents a combination of anaerobic digestion and combustion and was specifically designed for semi-natural grassland biomass. The treatment results in two fractions, each with improved properties for combustion and biogas production. For energy recovery purposes, the IFBB process represents the preferred conversion process.

While the suitability for energy recovery has already been practically investigated, only assumptions can be made about material recovery. For material use, the green biorefinery and the lignocellulose biorefinery are possible options. A green biorefinery separates fresh green or ensiled biomass in a press juice containing proteins, lipids, acids, minerals, and sugars and a press cake mostly containing fibers. Both fractions can be subjected to different processes depending on the desired products. To date, the use of landscape management grass in a GBR has not been reported. It was assumed that late-harvested grass is less suitable as the content of exploitable components is lower than in intensively managed areas. The lignocellulose biorefinery which operates on dry lignocellulosic feedstock offers an alternative valorization process. The processing steps can vary greatly depending on the feedstock and the desired products but mostly include pretreatment, hydrolysis, and fermentation. A wide range of carbohydrates can be produced, and protein could also be extracted. Again, no specific application of late-harvested grass in an LCB is known. Thus, with respect to the material utilization of landscape management grass, further research is needed to determine its suitability for a GBR or LCB to make full use of fibers, carbohydrates, and if possible, proteins.

The exploitation of late-harvested landscape management grass entails several challenges. As the areas are only cut once or maximum twice a year, availability of fresh biomass is limited. The operating time of a GBR processing fresh biomass would consequently be severely reduced. Alternatively, the biomass could be ensiled to ensure permanent processing. However, there seem to be contradicting opinions concerning the suitability of the biomass for ensiling. For an LCB, the material would be dried and could be stored and processed over a longer period of time. Another challenge is the high inhomogeneity of the material. Consistent feedstock quality and composition are a prerequisite for many conversion processes. It must be clarified to what extent the processes addressed can handle the variable biomass composition. Compared to intensively managed grasslands, landscape management areas are often smaller and geographically dispersed. Thus, centralized processing implies higher transport distances and costs. An adapted mobile biorefinery might be appropriate to use the harvested biomass demand-oriented in a decentralized way. Such a mobile biorefinery can focus on the most abundant fractions such as carbohydrates and/or fibers as well as on a concentration of proteins. Furthermore, to achieve a holistic utilization, energetic use of the remaining fractions should be considered.