Biogas is a renewable fuel made when microorganisms break down organic waste without oxygen. A biogas energy machine uses food scraps, manure, crop residue, or wastewater sludge as its input instead of coal or petroleum. This matters because it turns waste into useful energy while reducing methane leaks from landfills and manure piles.
The same system can also produce nutrient-rich digestate that can be used as fertilizer.
Key Facts
- Anaerobic digestion happens without oxygen and produces mainly methane, carbon dioxide, and traces of other gases.
- Typical raw biogas contains about 50 percent to 70 percent methane, CH4, and 30 percent to 50 percent carbon dioxide, CO2.
- Energy in biogas comes mostly from methane combustion: CH4 + 2O2 = CO2 + 2H2O + energy.
- Power output can be estimated by P = E/t, where P is power, E is useful energy, and t is time.
- Electrical efficiency of a generator is efficiency = electrical energy out/chemical energy in.
- Biogas cleanup often removes water vapor, hydrogen sulfide, H2S, and excess CO2 before the gas is burned or upgraded.
Vocabulary
- Anaerobic digester
- A sealed tank where microorganisms break down organic material without oxygen to produce biogas.
- Biogas
- A renewable gas mixture produced by decomposition of organic waste, mainly containing methane and carbon dioxide.
- Methane
- Methane is the energy-rich gas, CH4, in biogas that can be burned to produce heat or electricity.
- Digestate
- Digestate is the leftover liquid and solid material from a digester that contains nutrients and can often be used as fertilizer.
- Combined heat and power
- Combined heat and power is a system that uses a fuel to make electricity while capturing useful waste heat.
Common Mistakes to Avoid
- Calling biogas pure methane is wrong because raw biogas is a mixture that usually contains a large amount of carbon dioxide plus small impurities.
- Adding oxygen to the digester is wrong because the key microbes that make methane work in anaerobic conditions without oxygen.
- Ignoring gas cleanup is wrong because water vapor and hydrogen sulfide can corrode pipes, valves, engines, and generators.
- Treating all organic waste as equally useful is wrong because feedstocks differ in moisture, carbon content, digestion rate, and methane yield.
Practice Questions
- 1 A digester produces 120 m3 of biogas per day, and the gas is 60 percent methane by volume. How many cubic meters of methane are produced each day?
- 2 A biogas generator receives 900 MJ of chemical energy from methane in one day and converts 30 percent of it into electricity. How many MJ of electrical energy are produced?
- 3 A farm wants to use biogas for both electricity and building heat. Explain why a combined heat and power system can be more useful than a generator that only makes electricity.