**2. Operational aspects of methane fermentation**

Methane fermentation occurs naturally and is carried out in bioreactors. Medium optimization and mineral requirement are treated in Refs. [140–142].

The reactions can be described as follows for methane limitation 4.9 g is for bacteria and products together [5]:

*Value-Added Products from Natural Gas Using Fermentation Processes: Products… DOI: http://dx.doi.org/10.5772/intechopen.104643*

$$\begin{array}{rcl} \text{CH}\_4 + \text{1.8O}\_2 & \rightarrow & \text{Bacteria} \\ \text{16 g} & \text{56 g} & \text{16.1 g dry wt.} \end{array} + \text{0.34} \text{CO}\_2 \tag{1}$$

Under oxygen limitation, it was found [5]:

$$\underset{\text{16. g}}{\text{CH}\_4} + \underset{\text{9.6 g}}{\text{O}}\_2 \rightarrow \underset{\text{4. g}}{\text{Bateria}}\_{\text{4. g}} + \text{Products} + \underset{\text{2. 2 g}}{\text{O}} \tag{2}$$

By contrast, [96] found for methane limitation:

$$\text{4.25 g }\text{O}\_2 + \text{1.245 g }\text{CH}\_4 \to \text{1 g cells }\ + \text{2.0 g CO}\_2\tag{3}$$

For oxygen limitation, this formula could be determined [72]:

$$\text{2.5 g }\text{O}\_2 + \text{1.01 g }\text{CH}\_4 \rightarrow \text{1 g cells }\, + \text{1.216 g }\text{CO}\_2\tag{4}$$


#### **Table 23.**

*RNA and DNA in yeast cells (*C. lipolytica *YB-423) treated by heat shock.*

For fermentation technology in general, see Refs. [143–146], for its economics [147].

#### **2.1 SCP production process**

As SCP has already been produced at a large scale some 50 years ago, see above, there is ample experience with gas fermentation. SCP production is summarized in **Table 24**.

As **Table 24** shows, different approaches are being followed. For a review on SCP, see Refs. [85, 151].

A major difficulty with methane is its low solubility in water, resulting in low productivity. Paraffin oil as a "methane vector" for improved mass transfer and higher cell densities was suggested in the literature [64]. Koutinas et al. have proposed to use of γ-alumina pellets to improve methane fermentation [152]. Emulsion-based fermentation to enhance the mass transfer of methane is presented in Ref. [153].

For the bioprocess, **Table 25** contrasts two reactor configurations, the "classic" CSTR (continuously stirred tank reactor) and the loop reactor.

