Energy content of biogas for rural energy planning: What is the typical heating value (approximate range) of biogas per cubic foot under normal composition?

Introduction / Context:
Biogas (primarily methane with carbon dioxide and trace gases) is widely used for cooking, heating, and small engines. Knowing its typical energy content is key to sizing digesters, estimating fuel equivalence, and comparing with pipeline natural gas.

Given Data / Assumptions:

• Typical biogas methane content is about 50–70% (balance mostly CO2).
• Heating value scales with methane fraction; pure methane is roughly 1,000 BTU/ft^3.
• We need the usual range, not upgraded biomethane.

Concept / Approach:
If methane is 50–70%, the biogas heating value is proportionally lower than natural gas. A practical rule-of-thumb range is about 400–700 BTU per cubic foot, depending on composition and moisture. Higher values like 1,000 BTU/ft^3 correspond to nearly pure methane (upgraded biogas), while 1,500 BTU/ft^3 and above are unrealistic for methane-based fuels at standard conditions.

Step-by-Step Solution:
Note methane fraction (0.5–0.7) multiplied by ~1,000 BTU/ft^3 for pure methane. Compute rough range: 0.5*1,000 to 0.7*1,000 ≈ 500–700 BTU/ft^3, allowing for dilution and conditions. Choose the closest common engineering range: 400–700 BTU/ft^3.

Verification / Alternative check:
Engineering handbooks and renewable energy guides list biogas heating value in the ~400–700 BTU/ft^3 range, varying with methane content and contaminants.

Why Other Options Are Wrong:

• 1,000 BTU/ft^3: Represents pipeline-quality methane/NG, not raw biogas.
• 1,500 BTU/ft^3 / >5,000 BTU/ft^3: Far above methane fuel values; not plausible.
• 100–200 BTU/ft^3: Too low for typical biogas; would imply extremely CO2-rich gas.

Common Pitfalls:
Confusing upgraded biomethane (nearly pure methane) with raw digester biogas that contains CO2 and moisture.

Final Answer:
The typical energy content of biogas is 400–700 BTU/ft^3.