What is Biogas?
Biogas is the gaseous emissions from anaerobic degradation of organic matter (from plants or animals) by a consortium of bacteria. Biogas is principally a mixture of methane (CH4) and carbon dioxide (CO2) along with other trace gases. Methane gas, the primary component of natural gas (98%), makes up 55-90% by volume of biogas, depending on the source of organic matter and conditions of degradation. Biogas is produced in all natural environments that have low levels of oxygen (O2) and have degradable organic matter present. These natural sources of biogas include: aquatic sediments, wet soils, buried organic matter, animal and insect digestive tracts, and in the core of some trees. Man’s activities create additional sources including landfills, waste lagoons, and waste storage structures. Atmospheric emissions of biogas from natural and man-made sources contribute to climate change due to methane’s potent greenhouse gas properties. Biogas technology permits the recovery of biogas from anaerobic digestion of organic matter using sealed vessels, and makes the biogas available for use as fuel for direct heating, electrical generation or mechanical power and other uses. Biogas is often made from wastes but can be made from biomass energy feedstocks as well.
Is biogas the same as biofuel?
Biogas is only one of many types of biofuels, which include solid, liquid or gaseous fuels from biomass. Any combustible fuel derived from recent (non-fossil) living matter (biomass) may be considered a biofuel, including ethanol derived from plant products, biodiesel from plant or animal oils, as well as, biogas from biomass. All biofuels are produced from sources which are renewable and are included as a subset of renewable energy sources that also include energy produced from solar, hydro, tidal, wind, and geothermal sources. Biogas, like natural gas, has a low volumetric energy density compared to the liquid biofuels, ethanol and biodiesel. However, biogas may be purified to a natural gas equivalent fuel for pipeline injection and further compressed for use as a transportation fuel. Methane, the principal component in biogas, has four times the volumetric energy density of hydrogen (H2) and is suitable for use in many types of fuel cell generators.
Energy Content of Biofuels
| Btu/lb |
Btu/cu ft |
Btu/gal |
|
|
Biogas (60% CH4) |
13,142 |
600 |
80 |
|
Purified Biogas (98% CH4) |
21,466 |
980 |
131 |
|
Pressurized Biogas (3000 psi, 98% CH4) |
21,466 |
196,000 |
26,205 |
|
Ethanol |
11,535 |
568,956 |
76,069 |
|
Biodiesel |
18,163 |
819,350 |
109,547 |
How do biofuels compare on energy crop yeilds?
There is no “silver bullet” in producing fuel from a sustainable energy crop. Fuel yields are limited by photosynthetic efficiency (less than 3% of solar energy is captured in even high yield crops), the efficiency of the conversion process, and the energy used in the production and conversion process (a significant cost for ethanol production). On a per acre basis, biogas production is far more efficient in capturing the energy found in energy crops. While the convenience and energy density of liquid fuels is an admirable target, if maximizing energy recovery from biomass and wastes is targeted, biogas production is the best choice. Further, even where ethanol and biodiesel production is used, biogas production from their waste products can improve the energy balance of the overall conversion process.
Energy Yields from Conversion of Energy Crops to Biofuels:
|
Biofuel |
Million btu/acre |
|
Biogas from sugarcane |
108 |
|
Ethanol from sugarcane |
54 |
|
Biodiesel from peanuts |
12 |