**7. Biogas from Napier grass silage**

**Cultivar Dry matter yield (t/ha)**

**Table 2.** Annual dry matter (DM) yields of eight Napier grass cultivars.

Several studies had been examined via grass/grass silage as feedstocks to produce biogas as a renewable energy; however, if grass is to be used as raw materials for AD for energy production, it should be converted to silage due to the presence of lignocellulosic materials [26]. Lehtomaki et al. [27] showed that AD of grass silage in batch leach bed processes has the highest methane potential when compared with other potential crops. Smyth et al. [26] compared the net energy of the grass in biomethane systems with other energy crops, and they found that grass has higher gross energy than rapeseed biodiesel and wheat ethanol systems [28]. The yields of dry matter in vetiver grass provided the yield of ethanol at 1091.84 L/ha/year, whereas the leaves of dwarf Napier grass given the maximum yield of 2720.55 L/ha/year (0.98 g/L or

In numerous studies, grass silage has been recommended as an excellent substrate for biomethane production resulting from high-energy yields, low-energy input demand, long time storage, and usage of silage even for a whole year [29]. The higher potential of methane production from grass silage was confirmed both in batch and in semi-continuous experiments and batch leach bed processes [27]. In practice, grass silage is the most important substrate for agricultural biogas production following maize silage in Germany [30]. Though grass silage may be less energetically productive when compared to maize silage, it still offers a good energy balance and environmental advantages [31]. The key purpose of silage preparation is achieved by efficient preservation. It could keep high-energy content of a crop. And this is achieved by the combination of an anaerobic environment as well as the bacterial fermentation of sugar. The lactic acids formed in the latter progression lower the pH and avoid the prolif-

Generally, the fermentation under farm conditions was not involved in a controlled process. The silage fermentation characteristics were depending on the nutrients that allow the growth of microorganisms. The fermentation is usually characterized by a low pH, high lactic acid content, and low concentrations of butyric acid and ammonia-N. Additionally, the ensiled

**6. Potential of grass silage**

0.12 g/g substrate equivalent to 30.60%) [26].

eration of spoilage microorganisms.

Dwarf 27.1 Muaklek 35.1 Bana 49.1 Taiwan A148 51.5 Common 51.4 WrukWona 52.1 Tifton 58.4 Kamphaeng Saen 46.3

160 Advances in Silage Production and Utilization

Common cultivar of Napier grass was obtained from the agriculture farm which was cultivated at Mae Taeng district, Chiang Mai, Thailand. The grass was a first cut (cut at 45-day-old mature stage). Napier grass was crushed by machine into small particles. Stored grass was pulverized into small particles (1.0 mm) before use. Proximate, ultimate, chemical composition of Napier grass is shown in **Table 3**. The grass collecting and silage preparations are shown in **Figures 2** and **3**. The experiment was carried out in the Energy Research Center, School of Renewable Energy, Maejo University, Thailand. For all experiments, Napier grass (*Pennisetum purpur‐ eum*) was used as a monosubstrate.


b Dry basis; unit % by weight.

**Table 3.** Proximate, ultimate, chemical composition of Napier grass.

through improvements in the fermentation process using with Napier grass and water. Thirty kilograms of grass substrates was used in a leachate recirculation digester. The reactor working

Grass Silage for Biogas Production http://dx.doi.org/10.5772/64961 163

**Figure 5.** (A) Biogas yield (L/day VS) and cumulative biogas yield (L/kg VS) and (B) biogas compositions produced

Daily total biogas production of Napier grass as monosubstrate in the reactor is given in **Figure 3**. Energy crops and crop residues can be digested either alone or in co-digestion with other materials, employing either wet or dry processes. And after 85 days, the rate of biogas production was gradually declined. The biogas was accumulated throughout study period 20.62 L/kg fresh grass or 190.25 L/kg VS is the average total amount of gas 6.87 L/day (=6870

volume was 60 L.

from Napier grass.

**Figure 4.** Dry fermentation anaerobic digestion process.

**Figure 2.** Grass collection and silage preparation (A) cultivation, (B) transportation of grass, (C) grass crushing machine, and (D) small particle of grass.

**Figure 3.** Napier grass silage.

Leachate Recirculation Digester (LBR): A prototype of 100-L dry anaerobic batch digester was employed so-called LBR system, sometimes called percolating anaerobic or dry anaerobic digester [37], and experimental setup is shown in **Figure 4**. Specification of experimental parameters and biogas measurements are listed in **Table 1**. In this design, LBR was sequentially loaded with grass biomass and mixed with residual digested solids and leachate. For all experiments, prepared grass was used as a monosubstrate. Biogas production was received through improvements in the fermentation process using with Napier grass and water. Thirty kilograms of grass substrates was used in a leachate recirculation digester. The reactor working volume was 60 L.

**Figure 4.** Dry fermentation anaerobic digestion process.

**Figure 2.** Grass collection and silage preparation (A) cultivation, (B) transportation of grass, (C) grass crushing ma-

Leachate Recirculation Digester (LBR): A prototype of 100-L dry anaerobic batch digester was employed so-called LBR system, sometimes called percolating anaerobic or dry anaerobic digester [37], and experimental setup is shown in **Figure 4**. Specification of experimental parameters and biogas measurements are listed in **Table 1**. In this design, LBR was sequentially loaded with grass biomass and mixed with residual digested solids and leachate. For all experiments, prepared grass was used as a monosubstrate. Biogas production was received

chine, and (D) small particle of grass.

162 Advances in Silage Production and Utilization

**Figure 3.** Napier grass silage.

**Figure 5.** (A) Biogas yield (L/day VS) and cumulative biogas yield (L/kg VS) and (B) biogas compositions produced from Napier grass.

Daily total biogas production of Napier grass as monosubstrate in the reactor is given in **Figure 3**. Energy crops and crop residues can be digested either alone or in co-digestion with other materials, employing either wet or dry processes. And after 85 days, the rate of biogas production was gradually declined. The biogas was accumulated throughout study period 20.62 L/kg fresh grass or 190.25 L/kg VS is the average total amount of gas 6.87 L/day (=6870

ml/day), as shown in **Figure 5**. Bussabong et al. [38] stated the performance of the biogas production of ruzi grass (*Brachiaria ruziziensis*) as the monosubstrate had value of 244 ml/day with CSTR. This study results were demonstrated that biogas yield was 28 times higher than ruzi grass which was performed in CSTR. Batch reactors are often leach bed processes where solids are hydrolyzed by circulating leachate over a bed of organic matter. Recirculation of leachate stimulates the overall degradation owing to more efficient dispersion of inoculums, nutrients, and degradation products [27]. Accordingly, that is, main reason this study result confirmed was much higher than CSTR.

line gas (i.e., gas grid <1 ppmv) [39]. This study which verified H2S was extremely lower (i.e., 5 ppm). Therefore, the study approach is certainly applicable for CBG (compressed biomethane gas) engine. Consequently, this study investigated the potential of Napier grass biomass as a feedstock for biogas production. This suggested that it is possible to achieve stable operation using Napier grass, as a substrate for biogas production in pilot or large-scale biogas plant in the future. It was concluded that Napier grass as energy crop can be an alternative energy

Grass Silage for Biogas Production http://dx.doi.org/10.5772/64961 165

Recently, most of the agricultural biogas plants digest manure with the addition co-substrates to increase the content of organic material for achieving a higher gas yield [40]. For these reasons, co-digestion is commonly practiced and most recommended co-substrate was

Co-digestion has been defined as the anaerobic treatment of a mixture of at least two different substrates with the aim of improving the efficiency of the anaerobic digestion process. At present, there are an increasing number of full-scale co-digestion plants treating manure and industrial organic wastes. Co-digestion of mixed substrates offers many advantages, including ecological, technological, and economic benefits, compared to digesting a single substrate. However, combining two or more different types of feed stocks requires careful selection to improve the efficiency of anaerobic digestion [40]. The main resource is represented by animal manure and slurries from cattle and pig production units as well as from poultry, fish, etc. And

resource.

manure.

**7.1. Co-digestion**

**Figure 6.** Wet fermentation (continuums type).


**Table 4.** Specification of experimental parameters and biogas measurements.

Biogas composition results are presented in **Figure 5**. Biogas composition from experimental measurements starting from 39 days of the experiment showed that the initial composition of the gas as possible. This term microbial methane was generated. (Methanogenic bacteria are not in the right conditions for growth.) The pH less than 6.5 was inhibit the growth of methanogenic bacteria are composed of methane, 7.9 after 54 days, the methane production increased due to the microbial production of methane. Theoretical and measured composition of methane and biogas production is presented in **Table 4**. The biogas composition of carbon dioxide (30.10%), methane (63.50%), and 5 ppm of hydrogen sulfide was estimated from the biogas.

H2S is commonly found in natural gas, biogas, and LPG. It is corrosive, toxic, and odorous; it can significantly damage mechanical and electrical equipment used for process control, energy generation, and heat recovery. Moreover, the combustion of H2S results in the release of sulfur dioxide, which is a problematic environmental gas emission [39]. The usages of biogas with H2S standard are as follows: steam and fired boilers (<1000 ppmv), steam and fired boilers (<1000 ppmv), fuel engines (<500 ppmv), motor fuels (i.e., CNG and CBG <23 ppmv), and pipe line gas (i.e., gas grid <1 ppmv) [39]. This study which verified H2S was extremely lower (i.e., 5 ppm). Therefore, the study approach is certainly applicable for CBG (compressed biomethane gas) engine. Consequently, this study investigated the potential of Napier grass biomass as a feedstock for biogas production. This suggested that it is possible to achieve stable operation using Napier grass, as a substrate for biogas production in pilot or large-scale biogas plant in the future. It was concluded that Napier grass as energy crop can be an alternative energy resource.

### **7.1. Co-digestion**

ml/day), as shown in **Figure 5**. Bussabong et al. [38] stated the performance of the biogas production of ruzi grass (*Brachiaria ruziziensis*) as the monosubstrate had value of 244 ml/day with CSTR. This study results were demonstrated that biogas yield was 28 times higher than ruzi grass which was performed in CSTR. Batch reactors are often leach bed processes where solids are hydrolyzed by circulating leachate over a bed of organic matter. Recirculation of leachate stimulates the overall degradation owing to more efficient dispersion of inoculums, nutrients, and degradation products [27]. Accordingly, that is, main reason this study result

Biogas composition results are presented in **Figure 5**. Biogas composition from experimental measurements starting from 39 days of the experiment showed that the initial composition of the gas as possible. This term microbial methane was generated. (Methanogenic bacteria are not in the right conditions for growth.) The pH less than 6.5 was inhibit the growth of methanogenic bacteria are composed of methane, 7.9 after 54 days, the methane production increased due to the microbial production of methane. Theoretical and measured composition of methane and biogas production is presented in **Table 4**. The biogas composition of carbon dioxide (30.10%), methane (63.50%), and 5 ppm of hydrogen sulfide was estimated from the

H2S is commonly found in natural gas, biogas, and LPG. It is corrosive, toxic, and odorous; it can significantly damage mechanical and electrical equipment used for process control, energy generation, and heat recovery. Moreover, the combustion of H2S results in the release of sulfur dioxide, which is a problematic environmental gas emission [39]. The usages of biogas with H2S standard are as follows: steam and fired boilers (<1000 ppmv), steam and fired boilers (<1000 ppmv), fuel engines (<500 ppmv), motor fuels (i.e., CNG and CBG <23 ppmv), and pipe

confirmed was much higher than CSTR.

164 Advances in Silage Production and Utilization

**Parameter Equipment or method**

Digesting system Dry anaerobic digester

**Table 4.** Specification of experimental parameters and biogas measurements.

Reactor type Leachate recirculation digester

Napier grass particle size 1.00 mm Grass substrate 30 kg

Volume of reactor 100 L Used volume of reactor 60 L

biogas.

Methane ASTM D 1945 Carbon dioxide ASTM D 1945-03 Hydrogen ASTM D 1945-03 Hydrogen sulfide ASTM D 5504-01 Oxygen ASTM D1945 Sulfur ASTM D 6667-04 Recently, most of the agricultural biogas plants digest manure with the addition co-substrates to increase the content of organic material for achieving a higher gas yield [40]. For these reasons, co-digestion is commonly practiced and most recommended co-substrate was manure.

**Figure 6.** Wet fermentation (continuums type).

Co-digestion has been defined as the anaerobic treatment of a mixture of at least two different substrates with the aim of improving the efficiency of the anaerobic digestion process. At present, there are an increasing number of full-scale co-digestion plants treating manure and industrial organic wastes. Co-digestion of mixed substrates offers many advantages, including ecological, technological, and economic benefits, compared to digesting a single substrate. However, combining two or more different types of feed stocks requires careful selection to improve the efficiency of anaerobic digestion [40]. The main resource is represented by animal manure and slurries from cattle and pig production units as well as from poultry, fish, etc. And agricultural substrate suitable for anaerobic digestion is represented by energy crops, of which most common are grain crops, grass crops, and maize. Grass crops are among the most promising energy crops for biogas production [41].

mgCH3COOH/L of VFA along with 6.66 of pH value. Wet fermentation (continuums type) is

Grass Silage for Biogas Production http://dx.doi.org/10.5772/64961 167

Gas samples were collected and analyzed, and gas components is presented in **Table 5** and **Figure 7**. The results obtained in this study suggest that co-digestion of microalgae and grass silage is a promising approach for improving biogas production. On 37 days, methane (CH4) content was reached over 70% and CO2 (10.05%), O2 (21%), and H2S 1205 ppm), which were met the standard of the Department of Energy. Efficiency criteria explained good performance

This study investigated the potential of Napier grass biomass as a feedstock for biogas production. Napier grass is fast-growing, high-yielding crops, and highly nutritious especially, so it is suitable for use as energy crops for biogas production. These results indicated that, Napier grass contains rich organic substances and these substances are suitable to use in the anaerobic fermentation process to be used to sustain microbial life and transform nutrients into biogas. Dry anaerobic digestion is a biological method used to convert organic substances into a stable product for land application without adverse environmental effects. The high content of methane (i.e., 63.50%) amount was found in total biogas from dry anaerobic fermentation in 90 days hydraulic detention time. But using with co-digestion of microalgae and Napier grass silage shows good results. In 37 days, methane content was 70%. This suggested that it is possible to achieve stable operation using Napier grass, as a substrate for

shown in **Figure 6**.

throughout the study.

**8. Conclusions**

**Figure 7.** Biogas compositions produced from Napier grass and microalgae.


**Table 5.** Biogas composition and fermenter characteristic of co-digestion of Napier grass and microalgae.

In this study, we used 40-L inoculums, 1000 L of microalgae and 200 Kg of Napier silage. Microalgae was cultivated in the open pond culture, and the mesophilic anaerobic inoculum was obtained from a working mesophilic anaerobic digester at Energy Research Center, Maejo University. The inocula had a TS concentration around 296.1 ± 0.4 mg/L, with 158.5 ± 1.02 mg/ L of VS. Total COD was 1241.6 mg/L, and 291.2 mg/L as CaCO3 of alkalinity, 136.4 mgCH3COOH/L of VFA along with 6.66 of pH value. Wet fermentation (continuums type) is shown in **Figure 6**.

Gas samples were collected and analyzed, and gas components is presented in **Table 5** and **Figure 7**. The results obtained in this study suggest that co-digestion of microalgae and grass silage is a promising approach for improving biogas production. On 37 days, methane (CH4) content was reached over 70% and CO2 (10.05%), O2 (21%), and H2S 1205 ppm), which were met the standard of the Department of Energy. Efficiency criteria explained good performance throughout the study.

**Figure 7.** Biogas compositions produced from Napier grass and microalgae.

## **8. Conclusions**

agricultural substrate suitable for anaerobic digestion is represented by energy crops, of which most common are grain crops, grass crops, and maize. Grass crops are among the most

**Day Cumulative biogas (cb-m) Biogas component Temp (°C) pH**

14 0.2618 4.9 29.6 0.9 823 31.8 5.43 15 0.6952 6.0 33.0 0.5 3877 30.5 5.65 16 1.0864 5.8 32.6 0.6 3562 29.5 5.61 17 1.4983 6.3 33.2 0.0 2325 29.2 5.45 18 2.0725 6.3 32.0 0.5 5 29.6 5.56 19 2.6462 6.9 32.0 0.1 38 30.4 5.64 20 3.2223 7.8 32.0 0.0 310 31.1 5.39 21 3.8514 8.6 32.1 0.0 423 31.7 5.48 22 4.4955 8.6 32.0 0.0 1073 31.5 5.54 23 5.1493 10.5 32.0 0.0 1458 31.9 5.42 24 5.8107 11.3 32.3 0.0 1693 30.7 5.66 25 6.8659 13.1 31.9 0.0 3715 28.5 5.68 26 7.8239 14.1 33.8 0.0 4143 29.6 5.71 27 8.4877 16.6 33.6 0.0 3972 29.9 5.74 28 9.1979 20.2 33.5 0.0 4067 28.9 5.64 29 9.7640 22.8 34.3 0.2 5345 29.7 6.08 30 10.2390 29.4 33.2 0.0 4117 28.4 6.25 31 10.8979 35.6 34.4 0.0 3623 30.1 6.08 32 11.3843 42.4 33.3 0.0 3713 30.6 6.76 33 11.8339 53.4 29.4 0.0 2522 30.6 6.78 34 12.1919 58.8 27.1 0.0 1996 27.5 6.51 35 12.7557 64.9 23.8 0.0 1592 25.1 6.85 36 13.2300 68.9 22.3 0.0 1700 24.6 6.89 37 13.5053 **70.2** 21.9 0.0 1205 25.4 6.51 38 14.1023 66.9 23.0 0.0 775 26.5 6.92 39 14.7192 62.9 26.9 0.0 1200 27.8 6.84 40 15.2051 56.9 30.4 0.0 1223 29.0 6.72

**Table 5.** Biogas composition and fermenter characteristic of co-digestion of Napier grass and microalgae.

In this study, we used 40-L inoculums, 1000 L of microalgae and 200 Kg of Napier silage. Microalgae was cultivated in the open pond culture, and the mesophilic anaerobic inoculum was obtained from a working mesophilic anaerobic digester at Energy Research Center, Maejo University. The inocula had a TS concentration around 296.1 ± 0.4 mg/L, with 158.5 ± 1.02 mg/ L of VS. Total COD was 1241.6 mg/L, and 291.2 mg/L as CaCO3 of alkalinity, 136.4

**CH4 CO2 O2 H2S (ppm)**

promising energy crops for biogas production [41].

166 Advances in Silage Production and Utilization

This study investigated the potential of Napier grass biomass as a feedstock for biogas production. Napier grass is fast-growing, high-yielding crops, and highly nutritious especially, so it is suitable for use as energy crops for biogas production. These results indicated that, Napier grass contains rich organic substances and these substances are suitable to use in the anaerobic fermentation process to be used to sustain microbial life and transform nutrients into biogas. Dry anaerobic digestion is a biological method used to convert organic substances into a stable product for land application without adverse environmental effects. The high content of methane (i.e., 63.50%) amount was found in total biogas from dry anaerobic fermentation in 90 days hydraulic detention time. But using with co-digestion of microalgae and Napier grass silage shows good results. In 37 days, methane content was 70%. This suggested that it is possible to achieve stable operation using Napier grass, as a substrate for

biogas production with co-digestion method in pilot or large-scale biogas plant in the future. The biogas digested material is excellent source for fertilizer and it is beneficial for environmental safety and management aspects as well. It was concluded that Napier grass as energy crop can be an alternative energy resource.

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