Regardless of the utilized organic matter the biomass can only be degraded by special groups of bacteria in the absence of oxygen and at a temperature of about 38°C. During this so called fermentation process organic compounds are decomposed into methane (CH4) and carbon dioxide (CO2). Several conversion steps are running during anaerobic digestion (AD):

• Hydrolysis of organic polymeric compounds (carbohydrates, proteins, fats) into monomers (sugars, amino acids)
• Acidogenesis further decomposes intermediates into hydrogen, CO2, alcohols and organic acids
• Acetat formation further converts the intermediate products into acetic acid, hydrogen and CO2
• In the final step methane is formed by splitting acetic acid (CH3COOH) into methane and CO2. Further hydrogen is oxidized with carbon dioxide to methane

All steps usually run in parallel in a biogas plant. The overall pace of the process is determined by the hydrolysis speed. If there are many easily degradable compounds in the reactor, methane formation can become the limiting step. In this case the addition of too much new material can lead to an excess of acid which causes the pH level to drop to unfavorable ranges. 

Important parameters influencing the process are dry matter content of the biomass, suspended and other content (e.g. mineral substances), inhibitor content (e.g. sulfur, heavy metals, aromatic compounds), process temperature, pH value, redox potential, lower fatty acids content and the ammonium content. Besides, some undesired elements, like hydrogen sulfide, water and CO2 occur during the process and have to be removed to receive the desired product gas. Depending on the further use of the raw biogas different cleaning and upgrading processes are needed.

To convert methane into a hydrogen and carbon monoxide rich product gas a reforming step is needed. There are different process options available that can produce product gasses with different H2/CO ratios:

Steam methane reforming

• Catalytic (nickel-based catalysts), endotherm process
• Bio-methan is mixed with steam under pressure of 20-25 bar
• Thermal energy (820-880°C) has to be provided from an external source
• Results in very high H2/CO ratios (4-7:1)

Partial oxidation

• Bio-methan is partially oxidized at high temperatures (1300-1400°C) and pressures of 25-40 bar
• Results in H2/CO ratios from 1.6-1.9 (e.g. not sufficient for further processing via FT synthesis)

Autothermal reforming

• Bio-methane is fed into the reactor together with steam and oxygen, where a part is burned (patially oxidized) to provide thermal energy
• The gas is passed over a catalyst and reacts to product gas
• Requires temperatures from 1020-1065°C and pressures of 25-29 bar
• Results in H2/CO ratios of 2.2-2.3

After reforming, cleaning and upgrading steps are needed to receive the desired syngas for further use, e.g. in FT synthesis (see chapter 2.3.1). Thereby mainly CO2 has to be removed and the H2 content has to be adjusted. Processes described in chapter 2.3.1.2 can be used for it. Generally, product gas from reforming is much cleaner than the product gases from thermo-chemical processes (like described in chapter 2.3.1). It does not contain tars or dust particles that need to be removed.