REP Bioenergy and Waste-to-Energy 3 — Questions and Answers
Question 1: In anaerobic digestion, the acetogenesis stage converts volatile fatty acids into which compounds?
- Methane and carbon dioxide directly
- Acetic acid, hydrogen, and carbon dioxide (Correct answer)
- Long-chain hydrocarbons
- Ammonia and sulfur dioxide
Correct answer: Acetic acid, hydrogen, and carbon dioxide
Acetogenesis converts propionate and butyrate into acetic acid, H2, and CO2, which methanogenic archaea then use to produce methane.
Question 2: What is the typical biogas methane content range produced from well-operated anaerobic digesters?
- 10–25%
- 30–45%
- 55–75% (Correct answer)
- 85–95%
Correct answer: 55–75%
Anaerobic digesters typically yield biogas with 55–75% methane, the remainder being mostly CO2 with trace H2S and water vapor.
Question 3: Which co-digestion feedstock is most commonly added to municipal sewage sludge to significantly boost biogas yields?
- Sand and grit
- Food waste and fats, oils, and greases (FOG) (Correct answer)
- Shredded paper only
- Inorganic industrial sludge
Correct answer: Food waste and fats, oils, and greases (FOG)
Food waste and FOG are carbon-rich substrates that enhance anaerobic digestion when co-digested with nitrogen-rich sewage sludge, increasing methane yields.
Question 4: The Boudouard reaction in gasification describes the equilibrium between carbon, CO2, and which product gas?
- Hydrogen
- Methane
- Carbon monoxide (Correct answer)
- Water vapor
Correct answer: Carbon monoxide
The Boudouard reaction (C + CO2 ⇌ 2CO) is endothermic and produces carbon monoxide from char and CO2 at high temperatures.
Question 5: Which type of biomass combustion system is best suited for fuels with widely varying particle sizes and moisture content?
- Pulverized fuel burner
- Stoker/grate-fired boiler (Correct answer)
- Entrained flow gasifier
- Pellet-only automated system
Correct answer: Stoker/grate-fired boiler
Stoker/grate-fired boilers can accommodate heterogeneous fuels with variable sizes and up to 60% moisture without extensive pre-processing.
Question 6: Which greenhouse gas accounting framework categorizes biogenic CO2 from biomass combustion as carbon-neutral?
- IPCC Scope 3 lifecycle analysis always
- The assumption that carbon absorbed during plant growth offsets combustion emissions (Correct answer)
- EPA Clean Air Act Section 112 definitions
- ISO 14001 environmental management standard
Correct answer: The assumption that carbon absorbed during plant growth offsets combustion emissions
Biogenic CO2 is considered carbon-neutral when the carbon released during combustion was recently sequestered from the atmosphere during biomass growth.
Question 7: What is 'digestate' in the context of anaerobic digestion systems?
- The raw biogas before upgrading
- The nutrient-rich solid and liquid residue remaining after digestion (Correct answer)
- The microbial inoculum added at startup
- Pre-treated feedstock before entering the digester
Correct answer: The nutrient-rich solid and liquid residue remaining after digestion
Digestate is the slurry remaining after anaerobic digestion, rich in nitrogen, phosphorus, and potassium, and used as an agricultural biofertilizer.
REP Bioenergy and Waste-to-Energy 3
In anaerobic digestion, the acetogenesis stage converts volatile fatty acids into which compounds?