**5. Biogas production from permanent grassland**

The aim of supplying crop feedstock for biogas production is to achieve the highest possible biogas yields per area unit (m3 ha-1). The area biogas yield consists of the substrate biogas yield (l kg-1ODM; ODM - organic dry matter) and the biomass yield (kgODM ha-1). The substrate biogas yield depends on biomass quality on one hand and on biogas technology and process of engineering on the other hand (Prochnow et al., 2009b). It is possible to affect effectiveness of production of biogas by using suitable combination of biomass yields and its quality from the standpoint of biogas production. The most significant controllable factors, which determine the potential of yield of permanent grasslands, have been already described in the previous parts of the chapter. Quantity of biomass is, however, closely connected with its quality. The factors, which affect grasslands production, therefore have an impact on its quality as well.

## **5.1 Quality of biomass used for biogas production**

In general, all types of biomass (liquid manure, energy plants, bio waste, sewage sludge) can be used as substrates, as long as they contain carbohydrates, proteins, fats, cellulose, and hemicellulose as main components (Deublein & Steinhauser, 2008). According to Amon et al. (2007), the methane production from organic substrates mainly depends on the content of nutrients (crude protein, crude fat, crude fiber, N-free extracts), which can be degraded to CH4 and CO2. The biogas represents mainly the mixture of CH4 and CO2 with minority ratio of other gases (N2, O2, etc.). Ratio of CH4 in biogas usually varied from 60 to 65 % (Straka et al. 2006).

The organic matter is, the only source of utilizable energy, which was stored in it by the energy of sunlight while the plants were growing. This organic matter allows storing the energy up to the time when it is released again, in the process of its degradation while influencing of microorganisms in rumen or biogas plant.

Requirements on the biomass quality are different when crops are anaerobically digested in biogas plants compared to being fed to cattle. The digester at the biogas plant offers more time to degrade the organic substance than the rumen does. In addition, it is likely to assume that the microorganism population in the digester is different from that in the rumen (Amon et al., 2007). As with fodder for animals, chemical analysis of biomass can be considered as basic evaluation of quality, which assesses the content of individual nutrients. There is a difference in the specific methane yield of crude fat (850 l kg-1ODM), crude protein (490 l kg-1ODM), and carbohydrates (crude fiber and N-free extracts: 395 l kg-1ODM) (Karpenstein-Machan, 2005, as cited in Amon et al., 2007). The assessed content of nutrients gives no picture of how well they are degradable, which is dependent also on technological aspects of biogas transformation itself. For this reason, the chemical analyses of total content and mutual ratios of individual nutrients represent in the first place starting potential for following degradation. Real yield of biogas from plant biomass is then affected mainly by technological parameters of the process of degradation, from the size of input particles, parallel fermentation of various substrates, to compliance with suitable conditions for all levels of micro-bacterial degradation.

In the following sub-chapters 5.2 there are results of experiments with various types of management of permanent grasslands and the impact on yields of substrate and total production of biogas described.

#### **5.2 Substrate biogas yield**

188 Modeling and Optimization of Renewable Energy Systems

On the basis of the present results it can be concluded, that for swards with suitable botanical composition while the energy in form of fertilizers is added, it leads to adequate yield reaction and the value of energy gain reached is comparable with intensive crops on arable soil. In contrast to regular crops on arable soil, permanent grasslands reach noticeably higher levels of energy effectiveness as a result of very low energy inputs into system.

The aim of supplying crop feedstock for biogas production is to achieve the highest possible biogas yields per area unit (m3 ha-1). The area biogas yield consists of the substrate biogas yield (l kg-1ODM; ODM - organic dry matter) and the biomass yield (kgODM ha-1). The substrate biogas yield depends on biomass quality on one hand and on biogas technology and process of engineering on the other hand (Prochnow et al., 2009b). It is possible to affect effectiveness of production of biogas by using suitable combination of biomass yields and its quality from the standpoint of biogas production. The most significant controllable factors, which determine the potential of yield of permanent grasslands, have been already described in the previous parts of the chapter. Quantity of biomass is, however, closely connected with its quality. The factors, which affect grasslands production, therefore have

In general, all types of biomass (liquid manure, energy plants, bio waste, sewage sludge) can be used as substrates, as long as they contain carbohydrates, proteins, fats, cellulose, and hemicellulose as main components (Deublein & Steinhauser, 2008). According to Amon et al. (2007), the methane production from organic substrates mainly depends on the content of nutrients (crude protein, crude fat, crude fiber, N-free extracts), which can be degraded to CH4 and CO2. The biogas represents mainly the mixture of CH4 and CO2 with minority ratio of other gases (N2, O2, etc.). Ratio of CH4 in biogas usually varied from 60 to 65 % (Straka et

The organic matter is, the only source of utilizable energy, which was stored in it by the energy of sunlight while the plants were growing. This organic matter allows storing the energy up to the time when it is released again, in the process of its degradation while

Requirements on the biomass quality are different when crops are anaerobically digested in biogas plants compared to being fed to cattle. The digester at the biogas plant offers more time to degrade the organic substance than the rumen does. In addition, it is likely to assume that the microorganism population in the digester is different from that in the rumen (Amon et al., 2007). As with fodder for animals, chemical analysis of biomass can be considered as basic evaluation of quality, which assesses the content of individual nutrients. There is a difference in the specific methane yield of crude fat (850 l kg-1ODM), crude protein (490 l kg-1ODM), and carbohydrates (crude fiber and N-free extracts: 395 l kg-1ODM) (Karpenstein-Machan, 2005, as cited in Amon et al., 2007). The assessed content of nutrients gives no picture of how well they are degradable, which is dependent also on technological aspects of biogas transformation itself. For this reason, the chemical analyses of total content and mutual ratios of individual nutrients represent in the

**5. Biogas production from permanent grassland** 

**5.1 Quality of biomass used for biogas production** 

influencing of microorganisms in rumen or biogas plant.

an impact on its quality as well.

al. 2006).

Assessment of yield of biogas from biomass represents basic qualitative characteristic in this process of energy production. As mentioned before, the basic thing is content of individual nutrients. This fact leads to logical effort to theoretically calculate production of biogas from its content. Amon et al. (2007) described the methane energy value model, which estimates methane yield from the nutrient composition of energy crops in mono fermentation via regression models. This model investigates and considers the impact of the content of crude protein, crude fat, crude fiber and N-free extracts on the methane formation. It is necessary to put a reminder here, however, that the calculations based on content of nutrients or laboratory tests of amount of yield described below, show potential degradation of biomass with ideal conditions present. As above-mentioned, real values reached depend on technological aspects of fermentation in a specific biogas plant.


Table 4 presents results of substrate biogas yield in litter per kg of dry matter (1 kg-1DM) from two experimental locations (Nicov and Černíkovice) in 2009. Characteristics of locality conditions and design of these experiments are mentioned in chapters 4.1 and 4.2.

Table 4. Substrate biogas yield (l kg-1DM) from permanent grassland, 2009, Nicov and Černíkovice localities, Czech Republic

Utilization of Permanent Grassland for Biogas Production 191

species, to grasslands that are quite rich in species. This can lead to different management

Barring the content of nutrients in biomass, yield of biogas can also be influenced by modification of substrate. According to Hendriks & Zeeman (2009), pretreatment (mechanical, thermal, chemical) can be done to improve the hydrolysis yield and total methane yield. Mshandete et al. (2006) studied the effect of particle size on biogas yield from sisal fiber waste. Methane yield increased by 23 %, when the fibers were cut to 2 mm size,

Fig. 5. Methane yield (l kg-1ODM; m3 ha-1) from permanent grassland at two sites (hill and

The yield of biogas from one unit of area represents basic indicator for calculating economical effectiveness of the grown plants. Acceptable supply costs can be achieved at high grass yields, moderate distances of transport and favourable field conditions for machinery operation (Blokhina et al., 2011). As was noted by Deublein & Steinhauser (2008), it is only profitable from an economic point of view, when the materials are sourced from a

Results shown in Fig. 6 and Fig. 7 clearly demonstrate that although the effect of fertilization on substrate biogas yield was inconsistent, suitable doses of nutrients significantly increase production of biogas per hectare. Fig 6 shows furthermore that although higher frequency of mowing increased quality of biomass (Table 4), total production of biogas per hectare was comparable to two-cut and four-cut systems. Four-cut system has higher need for energy inputs (see chapter 4.3), which means the two-cut system can be evaluated as the more

valley) and under different management intensity (Amon et al., 2007)

focused on different goals, which takes into consideration environmental functions.

compared to untreated fibers.

**5.3 Area biogas yield** 

suitable system in this case.

location within a distance of 15 – 20 km.

The substrate biogas yield was assessed while using laboratory batch test. The plant material was processed in fresh state, immediately after harvest of monitored grasslands. Basic homogenization and grinding of matter followed. Tested biomass was put together with inoculum in doses into fermentors that were gas-sealed. Biomass from experimental locality of Nicov was tested in 2 litres bottles in three replications for each variant. Dosage of mixed substrate was 1000 g (100 – 200 g of biomass and 800 – 900 g of inoculum). Cultivation took place in thermo box at 37 °C. Time of delay for mixed substrate in fermentors was 35 days. Biomass from experimental location of Černíkovice was tested in 120 ml bottles in five replications for each variant. Two grams of tested biomass and 80 ml of inoculum were dosed into fermentors. Cultivation took place in thermo box at 40 °C for a period of 49 - 50 days. Production of biogas in laboratory tests of biomass was evaluated once a day from both locations, using gas-metric burette. Besides tests of production of biogas with substrates, cultivation of inoculum itself in the same conditions was done and it was subsequently discounted from production from test bottles with substrates. In this way, net substrate production of biogas was obtained. Active mesophile anaerobic sediment from biogas plant was used as the inoculum.

Values of substrate biogas yield were in range of 317 to 588 l of biogas for kgDM (Table 4). It is clear from the presented results, that the main influence on yield of biomass had term and sequence of mowing, and the highest values were reached when earlier term and higher frequency of mowing were applied. That is in concordance with results summarized by Prochnow et al. (2009b), who found that the yield of methane in general declines as the vegetation phase proceeds. Amon et al. (2007) came to similar conclusions that substrate methane yield declines from value around 300 l kg-1ODM during stem elongation and before inflorescence, to 171 l kg-1ODM during flower stage.

Results in Table 4 also show that in the framework of individual experiments, higher values in yield were reached during the third and the fourth mowing. The reason for that can be the generally applicable negative relationship between quality and quantity, where increasing yield decreases degradability of substrate, because of changes in chemical composition and higher share of lower quality tissues. The yield is usually lower during the third and the fourth mowing, with higher content of leaves in harvested material. Leaves are in general considered to have higher quality and are more easily degradable than stems (Pearson & Ison, 1997). Fertilization had no consistent effect on yield but it is possible to conclude that with strong increase in yield while fertilizing, there was also a slight decrease in yield of substrate. This difference was more apparent during earlier terms of mowing. It is therefore possible to affect quality of harvested biomass mainly by numbers and terms of mowing, and it is necessary to consider earliness and height of plants in the grassland as well.

Another influencing factors, however, has to be considered as well. The specific methane yields of grassland showed significant differences (Fig. 5) between the mountainous and the valley regions (Amon et al., 2007). A low specific methane yield (128 – 221 l kg-1ODM) only was measured from the biomass coming from the hill site, independent of the number of cuts. The grass grown at the valley site produced 190 – 392 l kg-1ODM. The highest specific methane yield was reached in the biomass from the second cut of the four-cut variant. The yield of biogas gained thus depends significantly on specifically varying compositions of species in permanent grasslands different locations. Grasslands always represent mixed associations of different botanical composition, from grasslands that are intense and poor in

The substrate biogas yield was assessed while using laboratory batch test. The plant material was processed in fresh state, immediately after harvest of monitored grasslands. Basic homogenization and grinding of matter followed. Tested biomass was put together with inoculum in doses into fermentors that were gas-sealed. Biomass from experimental locality of Nicov was tested in 2 litres bottles in three replications for each variant. Dosage of mixed substrate was 1000 g (100 – 200 g of biomass and 800 – 900 g of inoculum). Cultivation took place in thermo box at 37 °C. Time of delay for mixed substrate in fermentors was 35 days. Biomass from experimental location of Černíkovice was tested in 120 ml bottles in five replications for each variant. Two grams of tested biomass and 80 ml of inoculum were dosed into fermentors. Cultivation took place in thermo box at 40 °C for a period of 49 - 50 days. Production of biogas in laboratory tests of biomass was evaluated once a day from both locations, using gas-metric burette. Besides tests of production of biogas with substrates, cultivation of inoculum itself in the same conditions was done and it was subsequently discounted from production from test bottles with substrates. In this way, net substrate production of biogas was obtained. Active mesophile anaerobic sediment from

Values of substrate biogas yield were in range of 317 to 588 l of biogas for kgDM (Table 4). It is clear from the presented results, that the main influence on yield of biomass had term and sequence of mowing, and the highest values were reached when earlier term and higher frequency of mowing were applied. That is in concordance with results summarized by Prochnow et al. (2009b), who found that the yield of methane in general declines as the vegetation phase proceeds. Amon et al. (2007) came to similar conclusions that substrate methane yield declines from value around 300 l kg-1ODM during stem elongation and before

Results in Table 4 also show that in the framework of individual experiments, higher values in yield were reached during the third and the fourth mowing. The reason for that can be the generally applicable negative relationship between quality and quantity, where increasing yield decreases degradability of substrate, because of changes in chemical composition and higher share of lower quality tissues. The yield is usually lower during the third and the fourth mowing, with higher content of leaves in harvested material. Leaves are in general considered to have higher quality and are more easily degradable than stems (Pearson & Ison, 1997). Fertilization had no consistent effect on yield but it is possible to conclude that with strong increase in yield while fertilizing, there was also a slight decrease in yield of substrate. This difference was more apparent during earlier terms of mowing. It is therefore possible to affect quality of harvested biomass mainly by numbers and terms of mowing, and it is necessary to

Another influencing factors, however, has to be considered as well. The specific methane yields of grassland showed significant differences (Fig. 5) between the mountainous and the valley regions (Amon et al., 2007). A low specific methane yield (128 – 221 l kg-1ODM) only was measured from the biomass coming from the hill site, independent of the number of cuts. The grass grown at the valley site produced 190 – 392 l kg-1ODM. The highest specific methane yield was reached in the biomass from the second cut of the four-cut variant. The yield of biogas gained thus depends significantly on specifically varying compositions of species in permanent grasslands different locations. Grasslands always represent mixed associations of different botanical composition, from grasslands that are intense and poor in

biogas plant was used as the inoculum.

inflorescence, to 171 l kg-1ODM during flower stage.

consider earliness and height of plants in the grassland as well.

species, to grasslands that are quite rich in species. This can lead to different management focused on different goals, which takes into consideration environmental functions.

Barring the content of nutrients in biomass, yield of biogas can also be influenced by modification of substrate. According to Hendriks & Zeeman (2009), pretreatment (mechanical, thermal, chemical) can be done to improve the hydrolysis yield and total methane yield. Mshandete et al. (2006) studied the effect of particle size on biogas yield from sisal fiber waste. Methane yield increased by 23 %, when the fibers were cut to 2 mm size, compared to untreated fibers.

Fig. 5. Methane yield (l kg-1ODM; m3 ha-1) from permanent grassland at two sites (hill and valley) and under different management intensity (Amon et al., 2007)

#### **5.3 Area biogas yield**

The yield of biogas from one unit of area represents basic indicator for calculating economical effectiveness of the grown plants. Acceptable supply costs can be achieved at high grass yields, moderate distances of transport and favourable field conditions for machinery operation (Blokhina et al., 2011). As was noted by Deublein & Steinhauser (2008), it is only profitable from an economic point of view, when the materials are sourced from a location within a distance of 15 – 20 km.

Results shown in Fig. 6 and Fig. 7 clearly demonstrate that although the effect of fertilization on substrate biogas yield was inconsistent, suitable doses of nutrients significantly increase production of biogas per hectare. Fig 6 shows furthermore that although higher frequency of mowing increased quality of biomass (Table 4), total production of biogas per hectare was comparable to two-cut and four-cut systems. Four-cut system has higher need for energy inputs (see chapter 4.3), which means the two-cut system can be evaluated as the more suitable system in this case.

Utilization of Permanent Grassland for Biogas Production 193

Therefore, it is possible to conclude that the basic element for determination of suitable management methods is still permanent grassland with specific composition of species, on specific locality, with respect to its environmental functions. According to variability of permanent grasslands, there is no universal optimal management for production of biomass for animals or energy use. The first and the basic step for optimizing production of biogas is a well-chosen number of mows, which will suitably utilize present vegetation period and natural fertility of locality. Terms of mows consequently must be based on planned number of mows considering earliness of sward and actual biomass yield. It is necessary to understand that frequent mows in early vegetation periods do increase substrate biogas yield (l kg-1ODM), but because of lower yield from higher number of mows, reduction of area biogas yield (m3 ha-1) can occur. This aspect of reduction in yield can be partially eliminated on grasslands with suitable composition of species by using adequate fertilization. This significantly increases yield in various regimes of harvest. At the same time, it does not

Utilization of permanent grasslands for production of biogas represents a system with different final adjustment in comparison with utilization of forage for feeding purposes. The basic management of permanent grasslands abides preserved, however optimization of this process can differ from traditional use of biomass. The chapter shows that the term grassland is very wide and includes varied groups of stands. Therefore, it cannot be provided any all-purpose instructions for biogas produce from permanent grasslands. It is also necessary to point out that optimal management, which would cover both productive

According cited literature and our own results, it is evident that for determination the optimal management of permanent grasslands for production of biogas it is necessary to take into consideration an influence of locality, species composition and other reciprocal biological relations. The system of biogas production from the permanent grasslands can fulfil productive as well as non-productive functions of grasslands. Considering the type of stand it is possible to modify the management towards maximization of production or

Research was supported by projects MSM 6046070901 and NAZV 1G58055. Publishing was supported by project Klastr Bioplyn No. 5.1 SPK02/019 financed by CzechInvest,

Amon, T.; Amon, B.; Kryvoruchko, V.; Machmüller, A.; Hopfner-Sixt, K.; Bodiroza, V.;

Hrbek, R.; Friedel, J.; Pasch, E.; Wagentristl, H.; Schreiner, M. & Zollitsch, W. (2007). Methane production through anaerobic digestion of variol energy crops grown in sustainable crop rotations. *Bioresource Technology*, Vol.98, No.17, (December 2007),

significantly reduce yields of substrate.

strengthen their environmental value.

Investment and Business Development Agency.

pp. 3204–3212, ISSN 0960-8524

**7. Acknowledgment** 

**8. References** 

and non-productive functions of this vegetation, does not exist.

**6. Conclusion** 

Amon et al. (2007) found that area methane yield tends to increase with increasing number of cut and fertilization levels. The biomass yields seem to be more important factor to achieve high area methane yield. According to Gerin et al. (2008), extensity of management of permanent grasslands leads to decrease in yield of dry matter and substrate biogas yield. However, too high intensity does not necessarily have to produce satisfactory results either.

Fig. 6. Area biogas yield (m3 ha-1) from permanent grassland under different management intensity, 2009, Nicov locality, Czech Republic

Fig. 7. Area biogas yield (m3 ha-1) from permanent grassland under different management intensity, 2009, Černíkovice locality, Czech Republic

Therefore, it is possible to conclude that the basic element for determination of suitable management methods is still permanent grassland with specific composition of species, on specific locality, with respect to its environmental functions. According to variability of permanent grasslands, there is no universal optimal management for production of biomass for animals or energy use. The first and the basic step for optimizing production of biogas is a well-chosen number of mows, which will suitably utilize present vegetation period and natural fertility of locality. Terms of mows consequently must be based on planned number of mows considering earliness of sward and actual biomass yield. It is necessary to understand that frequent mows in early vegetation periods do increase substrate biogas yield (l kg-1ODM), but because of lower yield from higher number of mows, reduction of area biogas yield (m3 ha-1) can occur. This aspect of reduction in yield can be partially eliminated on grasslands with suitable composition of species by using adequate fertilization. This significantly increases yield in various regimes of harvest. At the same time, it does not significantly reduce yields of substrate.
