[Series] ① Principles of Biogas Production and Its Eco-friendly Benefits: Utilizing Microorganisms and Anaerobic Digestion to Generate Over 50% Methane > Notice

From the reaction equation above, the COD equivalent per mole of methane is 64 g. Consequently, the total COD equivalent for the 3 moles of methane on the right-hand side of <Reaction Equation 1> is 192 g, which corresponds to the 1 mole of glucose on the left-hand side. Under standard temperature and pressure (STP; 0°C, 1 atm), 1 mole of methane gas occupies a volume of 22.4 L. Therefore, the COD equivalent converted into methane during anaerobic digestion is 0.35 L/g COD.

Meanwhile, the lower heating value of methane is 8,580 kcal/Nm³ (10 kWh/Nm³, 35.9 MJ/Nm³), a value used in establishing the energy balance for anaerobic digestion processes.

The anaerobic digestion process begins with hydrolysis, in which organic macromolecules—such as carbohydrates, proteins, and fats—are broken down by extracellular enzymes secreted by bacteria into soluble substances (e.g., sugars, amino acids, and fatty acids) that the bacteria can absorb. Acid-forming bacteria then convert these sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Through the process of acetogenesis, these organic acids are further converted into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogenic archaea transform these products into methane and carbon dioxide. Figure 1 illustrates the stages of organic matter decomposition during anaerobic digestion.

Figure 2 illustrates the degradation stages of proteins, carbohydrates, and fats—the primary organic components—during the anaerobic digestion process. The numbers in the figure represent the microbial communities responsible for each reaction: 1, 2, 3, and 4 correspond to fermentative bacteria, hydrogen-producing acetogenic bacteria (hydrogen-producing acetogens), hydrogen-consuming acetogenic bacteria (homoacetogens), and methanogenic archaea (methanogens), respectively. Providing operational conditions that allow these microbial communities to grow in harmony is the most essential factor for the success of anaerobic digestion.

40% Reduction in Greenhouse Gases

Biogas is classified as a renewable energy source because it is produced using waste from industrial and daily activities as a raw material, requiring no additional energy input.

According to a report by the World Biogas Association (WBA), 105 billion tons of organic waste are generated globally each year. Processing this waste through anaerobic digestion could reduce greenhouse gas (GHG) emissions by 10% by 2030.

In other words, increasing biogas production is an effective way to combat global warming.

Biogas production can also reduce the pollution potential of wastewater by converting organic matter—which would otherwise consume oxygen and lead to low oxygen levels in surface waters—into energy. Nutrients such as nitrogen (N₂) and phosphorus (P) are retained in the byproducts of biogas production and can be used as a substitute for fertilizers in crop cultivation.

While the combustion of biogas—much like natural gas—releases carbon dioxide (a greenhouse gas), the carbon in biogas originates from plant matter that captured CO₂ from the atmosphere. Consequently, biogas production is carbon-neutral and does not increase net greenhouse gas emissions. Furthermore, replacing fossil fuels with biogas leads to a reduction in carbon dioxide emissions.

A 2007 assessment by the Swiss federal government on the environmental impact of biofuels found that biogas produced via anaerobic digestion offers significantly greater benefits in terms of greenhouse gas reduction and overall environmental impact compared to other biofuels, such as bioethanol or biodiesel. Figure 3, taken from a 2020 report by the European Biogas Association, compares greenhouse gas (GHG) emissions from various fuels used in trucks. Blending just 17% of bio-LNG—derived from biogas—into fuel reduces GHG emissions by 34% compared to diesel. Furthermore, using 100% bio-LNG results in a reduction of more than 130%.

Research by the European Parliament has confirmed that biogas is the form of bioenergy that yields the highest amount of energy per unit of cultivated land (Figure 4).

Using natural gas instead of gasoline or diesel can help reduce greenhouse gas emissions. For instance, switching the fuel used in heavy-duty trucks to CNG (compressed natural gas) or LNG (liquefied natural gas) results in a 15–16% reduction in greenhouse gas emissions. According to NGVA Europe, blending 20% ​​biogas into this natural gas can lower greenhouse gas emissions by 40% compared to conventional petroleum-based fuels.

Denmark is a leading nation that invests significant effort into the research and production of biogas. Biogas production in Denmark has grown rapidly since 2012, and it is projected that renewable gas will account for 30% of the country’s total gas supply by 2023.

The Danish government manages biogas production through a dedicated support system. This framework includes measures such as imposing excise taxes on fossil fuels, restricting the application of nitrogen and phosphorus on agricultural land, prohibiting the disposal of organic waste in landfills, and placing limitations on the use of energy crops for biogas production.

Source: Gas News(http://www.gasnews.com)