**2. Anaerobic digestion (AD) process**

Biogas is generated from a digestion process under anaerobic conditions whose application is rapidly emerging as a viable means for providing continuous gaseous fuel and power generation. Recently, there are many countries having move towards to utilize the renewable energy especially biogas production through AD. Basically in AD, the organic materials are biologically treated in the absence of oxygen. These processes were naturally occurring through bacteria to produce "biogas." Generally biogas component is a mixture of CH4 (40– 70%), CO2 (30–60%), and other trace gases, for example, hydrogen, hydrogen sulfide, and ammonia. The co‐product from the biogas fermenter is potentially useful fertilizer in the form of a liquid or solid "digestate" [8]. For biogas production, a variety of methods are applied which can be classified in wet and dry fermentation systems.

The AD cycle represents an integrated system of a physiological process of microbial and energy metabolism, as well as the processing of raw materials under specific conditions (**Figure 1**) [4]. However, the microbial community is sensitive to variations in the operating conditions applied. AD process can be possibly integrated with other conversion processes. It could be applicable to improve their sustainability and energy balance. On the other hand, biogas system is different from other biofuels like biohydrogen, bioethanol, and biodiesel which uses only carbohydrates and lipids. Biogas is produced from all the convertible biomass macromolecules under anaerobic conditions [8, 9].

**Figure 1.** Flow diagram of the anaerobic digestion process.

the best technologies for the production of biogas [4]. Because of the concerns regarding energy security and environmental impact of fossil fuels, utilization of renewable energy is signifi‐ cantly increasing which will leads to the upgradation of living standards of people [5].

Energy crops are the type of plants cultivated as raw materials for biogas production. Agri‐ cultural lands in Thailand are well suitable for growing annual crops. Usually, temperature is warm to hot weather year‐round in Thailand. The highest temperature recorded is generally during summer in the months of March till May. Most of the region receives an average rainfall of around 1100 mm. The annual crops can be used as an energy crop or raw material for biogas plant [1]. Among energy crops, grasses which belong to perennial crops are suitable due to their fastest growing rates even in infertile land, low cultivation costs, higher accessibility, consumption of whole plants, and lower environmental impacts when compared to other plants [6]. Some grass species are reported to have large amount of fibers and carbohydrates from which biogas can be produced. Many such types of grasses are popularly growing in Thailand [3, 7]. Grass substrates are converted to silage to be used as feedstock for anaerobic digestion. Energy production from silage has also attracted much interest in recent years. In the United States, perennial grasses have been stored as biomass to produce biofuels. This chapter illustrates the basic concepts of anaerobic digestion and addresses the overview of potential of grass as raw material for biogas production advance silage preparations and utilization for efficient biogas production with several digestion methods including dry and wet fermentation processes, monodigestions, and co‐digestions, along with environmental impact assessment. Consequently, the aim of this chapter was to provide an overview of how to efficiently utilize the grass silage for biogas production and helpful to reduce greenhouse

Biogas is generated from a digestion process under anaerobic conditions whose application is rapidly emerging as a viable means for providing continuous gaseous fuel and power generation. Recently, there are many countries having move towards to utilize the renewable energy especially biogas production through AD. Basically in AD, the organic materials are biologically treated in the absence of oxygen. These processes were naturally occurring through bacteria to produce "biogas." Generally biogas component is a mixture of CH4 (40– 70%), CO2 (30–60%), and other trace gases, for example, hydrogen, hydrogen sulfide, and ammonia. The co‐product from the biogas fermenter is potentially useful fertilizer in the form of a liquid or solid "digestate" [8]. For biogas production, a variety of methods are applied

The AD cycle represents an integrated system of a physiological process of microbial and energy metabolism, as well as the processing of raw materials under specific conditions (**Figure 1**) [4]. However, the microbial community is sensitive to variations in the operating conditions applied. AD process can be possibly integrated with other conversion processes. It could be applicable to improve their sustainability and energy balance. On the other hand, biogas system

gas effect with environmental benefits.

154 Advances in Silage Production and Utilization

**2. Anaerobic digestion (AD) process**

which can be classified in wet and dry fermentation systems.

AD is a collection of process achieved through bacteria that convert organic materials into biogas through four different stages (**Figure 1**) including hydrolysis, acidogenesis, acetogenesis, and methanogenesis [8, 9]. Organic matters are broken down step by step through these four stages towards methane production path. The complex macromolecules and components (carbohydrates, lipids, and proteins) available in organic matter are converted into simple sugars, long-chain fatty acids, and amino acids through first stage so-called hydrolysis. And second stage (acidogenesis) in turn converts these soluble micromolecules into volatile fatty acids, acetic acid, CO2, and H2. Third stage of acetogenesis converts the volatile fatty acids into more acetic acid, CO2, and H2S gas. The final stage of methanogenesis has the capability to generate methane by using the CO2 and H2S gas otherwise the acetic acid produced from either second or third stages [8, 9]. Thus, the AD process, if improperly managed, would become unstable and result in reduced biogas production. An overall review and assessment of AD techniques for biogas production and relevant research progress are necessary and imperative for further biogas development.

#### **3. Grass: energy crop**

Compared with other feedstocks, grass has suitable and promising characteristics as energy crop for biogas production. Because of its assurance on availability of throughout year and conservation, ensilage or haylage are indisputable. Typically, compacting to extrude sheltered air and a plastic coverage is enough for conservation of fresh grass [10]. In general, the usage of grassland as a renewable source of energy during biogas production will provide considerable quantity of environment protection, owing to the capability of grass to sequester carbon into the soil matrix. Furthermore, various socioeconomic profits are possible to achieve without harming the food industry [11].

**Common name Scientific name Cultivation province Dry matter**

*Pennisetum purpureum* Schum. (common) Chiang Mai, Lampang,

*Pennisetum purpureum* Schum. cv. King Chiang Mai, Lampang,

*Pennisetum purpureum* Schum cv. Mott

*Pennisetum purpureum* Schum cv. Mott

*Pennisetum purpureum* Schum. cv.

*Pennisetum purpureum* Schum. cv.

*Pennisetum purpureum* Schum. cv.

*Pennisetum purpureum* Schum. cv. Taiwan

Pangola grass *Digitaria decumbens* Chiang Mai, Lampang,

Purple guinea grass *Panicum maximum* cv. TD 58 Chiang Mai, Lampang,

Ruzi grass *Brachiaria ruziziensis* Chiang Mai, Lampang,

Vetiver grass *Vetiveria zizanioides* cv. Kamphaeng Phet 1 Chiang Mai, Lampang,

*Vetiveria zizanioides* cv. Prachuap Khiri

Tifton Bermuda grass *Cynodon nlemfuensis* cv. Tifton Nakhon Ratchasima 58.4

(Dwarf)

(Dwarf)

Muaklek

WrukWona

A148

Khan

Information is not available in the literature.

**Table 1.** Types of grasses grown in Thailand.

Kamphaeng Saen

*Pennisetum purpureum* Schum. (common) Nakhon Ratchasima 51.4

Ratchaburi, or Phetchaburi

Nakhon Ratchasima 27.1

Nakhon Ratchasima 46.3

Nakhon Ratchasima 35.1

Nakhon Ratchasima 51.5

Chiang Mai, Lampang, Ratchaburi, or Phetchaburi

Ratchaburi

Ratchaburi

Ratchaburi

Ratchaburi

Ratchaburi

*Panicum maximum* cv. TD53 Nakhon Ratchasima N/Ab

*Vetiveria zizanioides* cv. Kamphaeng Phet 2 6.0 *Vetiveria zizanioides* cv. Loei 4.9 *Vetiveria zizanioides* cv. Nakhon Sawan 4.2

*Vetiveria zizanioides* cv. Ratchaburi 7.6 *Vetiveria zizanioides* cv. Roi Et 3.5 *Vetiveria zizanioides* cv. Songkhla 5.8 *Vetiveria zizanioides* cv. Sri Lanka 6.4 *Vetiveria zizanioides* cv. Surat Thani 5.5

*Brachiaria ruziziensis*: Ruzi grass *(B. ruziziensis)* used mainly for domestic animals grazing. Initially, ruzi grass was native to southern African continent. It came to Thailand in 1968 from Australia. Subsequently, the grass has become popular as cattle silage because of the

Napier grass (elephant

grass)

a

b

Banka et al. [14].

**yield (ton/ha/ year)a**

157

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

7.7

17.5

7.7

52.1

37.5

18.8

14.1

6.5

8.5

Perennial grasses, especially C4 grasses, are excellent candidate feedstocks for renewable energy production in support of several rationales such as high potential of dry matter yields, fast growth, and additional potential use of inputs compared to annual crops [12]. Furthermore, perennial grasses offer highest biomass yield which can be available for many harvests per year and give vital role in ecosystem services, for example, carbon sequestration in roots and soil, and to contribute the reduction of soil erosion due to massive perennial root systems that stabilize the soil. Lignin content which is negatively correlated with sugar release is lower in perennial grasses (161–192 mg g−1) when compared to woody plants (157–279 mg g−1) [13].

In Thailand, most of dairy cattle are grown by small-scale farmers and the grasses are used for cattle feeding. In common practice, para (*Brachiaria mutica*), ruzi (*Brachiaria ruziziensis*), guinea (*Panicum maximum*), and Napier grass (*Pennisetum purpureum*) are used in cattle feeding. Much of the prior research on candidate perennial grass biomass crops in Thailand has focused on *Brachiaria ruziziensis, Cynodon* sp., *Digitaria decumbens, Miscanthus sinensis, Panicum maximum, Paspalum atratum, Pennisetum polystachyon, Pennisetum purpureum, Pennisetum purpureum* × *Pennisetum americanum*, and *Vetiveria zizanioides*.
