Subtopic: Bio-fuels and their use in farm mechanization
Question Statement:
Which of the following comparative statements regarding the energy contents and storage properties of alternative gaseous farm fuels is/are CORRECT?
Options:
(A) Biogas containing $60\%\text{ CH}_4$ and $40\%\text{ CO}_2$ has an energy content of approximately $20\text{--}24\text{ MJ/m}^3$
(B) Producer gas has a lower energy density ($4.5\text{--}5.5\text{ MJ/m}^3$) due to high nitrogen dilution from atmospheric air gasification
(C) Pure compressed natural gas (CNG / biomethane) has a lower heating value of approximately $35\text{--}38\text{ MJ/m}^3$
(D) Direct compression of raw unpurified biogas to $200\text{ bar}$ is recommended without scrubbing $\text{CO}_2$ and $\text{H}_2\text{S}$
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
- **A is correct:** Raw biogas calorific value scales directly with methane content: $0.60 \times 37\text{ MJ/m}^3 \approx 22\text{ MJ/m}^3$.
- **B is correct:** Air-blown downdraft gasifiers produce gas with $\approx 50\%\text{ N}_2$, reducing LHV to $4.5\text{--}5.5\text{ MJ/m}^3$.
- **C is correct:** Methane has a standard volumetric LHV of $35.8\text{ MJ/m}^3$.
- **D is incorrect:** Compressing unscrubbed biogas creates severe corrosion in storage cylinders from acidic $\text{H}_2\text{S}$ and wastes compression energy on inert $\text{CO}_2$.