Biomass and Biogas - UNSOLVED PRACTICE SET

Q7. What is biomass? Give three examples of materials that count as biomass.

Q8. Describe the basic structure of a biogas plant (gobar gas plant): name the main parts and briefly state the function of each.

Q9. Why is biogas considered a cleaner fuel than directly burning cow dung cakes or wood? Give two reasons.

Q10. What happens to the leftover slurry after biogas is extracted from a biogas plant? Why is this byproduct valuable to farmers?

Q11. Why is biogas production called an 'anaerobic' process? What would happen if oxygen were allowed into the digester tank?

Q12. Explain why biomass and biogas are particularly well-suited as energy sources for rural Indian households compared to urban households.

Q13. Describe the construction and working of a biogas plant in detail. Your answer must cover:
(a) the materials fed into the plant (cattle dung, water, organic waste),
(b) the role of the digester/dome where anaerobic decomposition occurs,
(c) the bacteria involved and the conditions needed (absence of oxygen, suitable temperature),
(d) how the produced gas (biogas) is collected and piped for use, and
(e) what happens to the spent slurry and its use as a fertiliser.

Q14. A farming family in rural Punjab has a small dairy with several cows, producing significant amounts of dung daily. They are considering installing a biogas plant.
(a) Explain how the dung would be converted into usable cooking gas.
(b) What would the family use the gas for, and how would this change their daily routine compared to using firewood or LPG cylinders?
(c) Explain how the byproduct (slurry) could benefit their farming activities, potentially creating a 'closed loop' on the farm.
(d) What are the upfront costs and maintenance considerations the family should be aware of before installing a biogas plant?
(e) How might this investment pay off over several years?

Q15. Compare biomass/biogas with fossil fuels as energy sources.
(a) In terms of renewability — biomass regenerates relatively quickly (crops grow each season); fossil fuels take millions of years. Explain this difference.
(b) In terms of carbon emissions — burning biomass also releases CO₂, similar to fossil fuels. So why is biomass still often considered more 'climate-friendly' than fossil fuels?
(Hint: think about the carbon cycle — where does the carbon in biomass originally come from?)
(c) In terms of scale — can biomass/biogas realistically replace large-scale fossil fuel power plants for an entire country's electricity needs? Discuss limitations.
(d) What role do you think biomass/biogas should play in India's overall energy mix — large-scale or local/community-scale? Justify your view.

Q16. A family-sized biogas plant produces approximately 2 m³ of biogas per day from the dung of 4 cows. The calorific value of biogas is approximately 35,000 kJ/m³ (similar to natural gas, roughly).
(a) Calculate the total heat energy available from the biogas produced per day.
(b) If the family's daily cooking requires 50,000 kJ of heat energy, is 2 m³ of biogas sufficient for their needs (assuming 100% efficiency for simplicity)?
(c) If not sufficient, calculate the shortfall in kJ, and suggest one practical way the family might address this shortfall (e.g., supplementing with another fuel, or increasing the number of cattle).

Q17. A village biogas cooperative serves 20 households, with each household requiring 40,000 kJ of cooking energy per day. The biogas plant produces biogas with a calorific value of 35,000 kJ/m³.
(a) Calculate the total daily energy requirement for all 20 households.
(b) Calculate the volume of biogas (in m³) needed per day to meet this requirement (assume 100% efficiency for this estimate).
(c) If each cow contributes enough dung to produce 0.5 m³ of biogas per day, how many cows (combined across the village) are needed to meet this requirement?

Q18. A comparison is made between using 5 kg of dried cow dung directly as fuel (calorific value ≈ 6,000 kJ/kg) versus converting the same amount of dung into biogas through a biogas plant (assume the biogas produced from 5 kg of dung has a total energy content of 20,000 kJ, due to the more efficient extraction of energy).
(a) Calculate the total heat energy available from directly burning 5 kg of dried dung.
(b) Compare this with the energy content of the biogas (20,000 kJ) produced from the same 5 kg of dung.
(c) Based on this comparison, and also considering smoke production and the fertiliser byproduct, write 2-3 sentences explaining why converting dung to biogas is generally a better choice than direct burning.