Bioconversion of Waste Biomass to Biomethanol

Introduction

Today, the major environmental concerns are focused on the effective production and application of biofuels, biochemicals, biomaterials and bioproducts to replace fossil fuels and its derivatives at a global scale (Nanda et al. 2016d; Parakh et al. 2020; Okolie et al. 2020b). Because of massive amounts of greenhouse gas emissions, environmental pollution, global warming and climate change, rising crude oil prices and high carbon taxes associated with the exploiting usage of fossil fuels, there is a global impetus to seek eco-friendly and alternative renewable resources (Nanda et al. 2015; Rana et al. 2018; Rana et al. 2019; Rana et al. 2020; Okolie et al. 2020a). Harnessing the hydrocarbons present in the waste lignocellulosic biomass as carbohydrates is of great potential to sustain their conversion to biofuels and biochemicals (Nanda et al. 2013; Sarangi et al. 2017; Azargohar et al. 2018; Sarangi et al. 2018; Azargohar et al. 2019; Kang et al. 2019; Sarangi and Nanda 2019a; Sarangi and Nanda 2019b; Kang et al. 2020).

Lignocellulosic biomass containing cellulose, hemicellulose and lignin can be transformed to biofuels using several thermochemical technologies (e.g. pyrolysis, liquefaction, gasification, transesterification, torrefaction, reforming, etc.) and biological technologies (e.g. anaerobic digestion, enzymatic saccharification, photo-fermentation, dark fermentation, acetone-butanol-ethanol fermentation, ethanol fermentation, syngas fermentation, etc.) (Nanda et al. 2014b; Nanda et al. 2017a; Nanda et al. 2017b; Nanda et al. 2017c; Nanda et al. 2017e; Nanda et al. 2018; Sarangi and Nanda 2018; Sarangi and Nanda 2020). Based on the conversion methods, various fuel products obtained from lignocellulosic biomasses are bio-oil, biodiesel, biomethanol, bioethanol, biobutanol, biopropanol, biohydrogen, biomethane, syngas, etc. (Reddy et al. 2014; Nanda et al. 2014a; Reddy et al. 2016; Nanda et al. 2016c; Reddy et al. 2017; Reddy et al. 2018; Nayak et al. 2019; Okolie et al. 2019; Reddy et al. 2019; Yadav et al. 2019; Nanda et al. 2020; Nayak et al. 2020; Sarangi et al. 2020). Bioethanol, biobutanol, biomethanol and biopropanol are some of the alcohol-based bioproducts obtained predominantly from the microbial bioprocessing of waste biomass with the potential to be used as biofuels or commodity biochemicals.

Biomethanol, being one of the most dynamic and vibrant fuel substitutes, can be generated from waste biomass by utilizing specific microbial communities (Sarangi et al. 2020). With diverse applications in several industrial sectors worldwide, the environmental-friendly production of biomethanol is gaining global interest in research, development and innovation. This chapter describes some notable value-added applications of biomethanol as well as its fermentative production from waste biomass using selective microorganisms.

Applications of Biomethanol

Biomethanol (CH,OH) has many recognized applications in chemicals, fuels and other specialty sectors. Biomethanol is widely applied in the following sectors such as: (i) transportation fuels, (ii) blending with gasoline and diesel, (iii) conversion into dimethyl ether for diesel alternatives, (iv) electricity from fuel cells and (v) biodiesel production through transesterification process (Yanju et al. 2008; Matzen and Demirel 2016; Reddy et al. 2016; Reddy et al. 2018; Bhatia et al. 2020; Sarangi et al. 2020). In all the above-mentioned applications, a key attribute of biomethanol is associated with its sustainability, carbon-neutrality (if produced from bioresources) and cost-effectiveness compared to other alcohol-based hydrocarbon fuels.

Fuel properties such as research octane number, motor octane number and antiknock index of methanol are nearly 109, 89 and 99, respectively (Eyidogan et al. 2010). On the other hand, the research octane number and motor octane number of gasoline are in the range of 91-99 and 81-89, respectively (Nanda et al. 2017b). Having a greater octane number than gasoline, methanol is considered as a promising low-cost alternative fuel with satisfactory fuel performance and less environmental impacts (Fatih et al. 2011).

The blending of biomethanol is one of its significant applications found in the automobile sectors. Besides, blending 15% methanol with gasoline and 20% methanol with diesel requires slight modifications to the vehicular engines for use as a transportation fuel (Kowalewicz 1993). Moreover, methanol-ethanol-gasoline blended fuels can enhance engine performance and greater efficiencies along with lower CO and NO_v emissions than that of gasoline alone (Elfasakhany 2015).

Methanol production from CO, is a sustainable option for recycling C0₂ and reducing its concentration as a potent greenhouse gas from the atmosphere (Nguyen et al. 2020). Methanol can be produced using C0₂ from industrial flue gas. Nearly about 0.19 t of methanol was produced per ton of fossil fuel, thereby resulting in a reduction of 0.42 MT of C0₂ emissions per year (Ptasinski et al. 2002). The application of biomethanol is also found in the power generation sector for gas turbines (Galindo and Badr 2007; Suntana et al. 2009). In another attribute, methanol can be used for biodiesel production via transesterification (Reddy et al. 2018). Besides, residual methanol and glycerol obtained as waste effluents from biodiesel industries can serve as a precursor for hydrogen production through supercritical water gasification (Reddy et al. 2016). Methanol is also used as an anti-frost agent, organic solvent and precursor for producing several fine chemicals (Sarangi et al. 2020).

Production of Biomethanol

The conversion of biomass to methanol is achieved through catalytic thermochemical processes and biological processes mediated by methanotrophic bacteria. In the thermochemical processes, waste biomass undergoes gasification to produce synthesis gas or syngas, which comprises CO. C0₂, H₂, CH₄ and traces of C₂₊ gases (Nanda et al. 2016a; Nanda et al. 2016b; Nanda et al. 2017d; Okolie et al. 2020c). In the next step, the conditioning of syngas is performed to remove various impurities such as tar and other undesired gases to optimize the ratio of H₂:CO (Okolie et al. 2019). Through Fischer- Tropsch catalysis, the conditioned syngas is converted to hydrocarbon fuels, chemicals and alcohols including methanol (Venvik and Yang 2017; Singh et al. 2018). The conversion of pectin produces uronic acid that aids in methanol production by combining with ether (Bhattacharyya et al. 2008). Another novel approach for the production of biomethanol from bio-oil has been reported through СО-rich bio-syngas, which in turn was produced from C0₂-rich bio-syngas (Xu et al. 2011).

On the other hand, the biological production of methanol deals with the utilization of waste biomass and anaerobic bioprocesses mediated by methanotrophic bacteria. Lignocellulosic biomass including agricultural crop residues and forestry refuses to act as promising resources and storehouses of fermentable hexose and pentose sugars for conversion to biofuels and biochemicals. Methanotrophic bacteria have been explored for the conversion of methane to methanol (Hanson and Hanson 1996). A few examples of methanotrophic bacteria are Methylocalduin, Methylococcus, Methylogaea, Methylohalobius, Methylomarinovum, Methylopara- coccus, Methylothermus, etc. (Bjorck et al. 2018). Bacillus methanicus is one of the notable methanotrophic bacteria.

Depending on the availability of methane in the environment, two population types of methanotrophs have been considered for methane conversion to methanol (Bender and Conrad 1992). The first category of methanotrophs is found in soils having high methane concentration. Such microorganisms convert methane at a level of more than 40 ppm concentration, thereby being regarded as the low-affinity methanotrophs. On the other hand, the second category of methanotrophs grows at a low methane concentration of about 2 ppm, thereby being known as the high-affinity methanotrophs. By the catalytic action of methane monooxygenase (MMO), the methanotrophs convert methane into methanol through the oxidation reaction (Figure 5.1).

Soluble cytoplasmic form (sMMO) as well as particulate membrane-bound form (pMMO) are the two forms of methane monooxygenase. There are other possible routes for the conversion of methanol into C0₂ via formaldehyde and formic acid

FIGURE 5.1 Conversion of methane to methanol via methanotrophic bacteria

and three different types of enzymes, i.e. methanol dehydrogenase (MDH), formaldehyde dehydrogenase (FADH) and formate dehydrogenase (FDH) (Hanson and Hanson 1996; Xin et al. 2009). The selection of the methanotrophic bacteria and the standardization of its growth conditions can affect the final recovery of biomethanol. More exploration of potential microbial communities and understanding their bio- catalytic activities could aid in the large-scale production of biomethanol from waste biomass sources.

Conclusions

The implementation of microbial communities to utilize waste biomass sources for the production of biofuels and biochemicals is a sustainable alternative to extraction, processing and utilization of fossil fuels. Waste biomass can act as an alternative feedstock for the production of biomethanol by methanotrophic bacteria via methane conversion. Various applications of methanol have been recognized in fuels, chemicals and other specialty applications. Noteworthy developments in the biological conversion processes could result in large-scale commercial production of biomethanol for industrial applications. Moreover, methane is considered as the second most potent greenhouse gas after C0₂. Hence, its conversion to biomethanol has great advantages in reducing its atmospheric concentrations and mitigating global warming.