Cow manure is often seen as a waste product, but it can actually be a valuable resource for renewable energy. With some planning and investment, cow manure can be converted into usable forms of energy like biogas and electricity. This provides environmental and economic benefits for cattle farms while reducing waste.
In this comprehensive guide, I’ll explain the full process of using cow manure for energy generation. First, we’ll look at why cow manure makes a good feedstock. Then, we’ll go through the main methods for energy conversion and equipment needed. Finally, I’ll provide real-world examples and cost considerations.
Why Cow Manure is Good for Energy Production
There are a few key reasons why cow manure can be effectively turned into energy:
High organic matter content – Cow manure contains high levels of organic matter like carbohydrates and volatile solids. This material can be broken down to produce combustible biogas.
Availability – Cattle farms have a constant, abundant supply of manure. Just one dairy cow can produce over 80 pounds of manure per day. This provides a reliable feedstock.
Renewability – Cows continue to produce manure that can be collected. So the process is renewable as long as there is a cattle herd.
Reduced waste – Many farms currently pay for manure disposal or store it in lagoons. Using it for energy reduces waste handling costs.
Methods of Converting Cow Manure to Energy
There are two main methods for converting cow manure into usable energy: anaerobic digestion to produce biogas and thermal conversion processes to generate electricity.
Anaerobic Digestion for Biogas Production
Anaerobic digestion uses bacteria to break down manure in an oxygen-free environment. This produces a gas called biogas which is mostly comprised of methane and carbon dioxide.
Here are the key steps involved in using anaerobic digestion for biogas generation:
Manure collection – Raw manure needs to be gathered using scrapers, vacuum trucks or flush systems. This feedstock then goes into the digester.
Pretreatment – The manure may need solids separation or grinding to prepare it for digestion. Adding water can also help optimize the process.
Anaerobic digestion – Inside the air-tight digester tank, bacteria break down the organic matter over several weeks. This produces the biogas.
Biogas storage – The biogas gets collected and stored in a gas holder, such as a balloon or tank.
Gas usage – The methane in the biogas can then be burned for heating, generating electricity, transportation fuel and other applications.
Typical anaerobic digester systems are able to convert around 25% of the organic matter in cow manure into usable methane biogas.
Thermal Conversion for Electricity Generation
Thermal conversion heats up dried cow manure to produce syngas and bio-oil. These byproducts can then be used to generate electricity.
Some examples of thermal conversion processes include:
- Gasification – Heats manure to high temperatures with a controlled oxygen supply. This produces syngas which can run generators.
- Pyrolysis – Heats manure in the complete absence of oxygen, creating a bio-oil.
- Combustion – Directly burns dried manure to heat boilers and produce steam for electricity.
These thermal conversion systems require cow manure with less than 20% moisture content. Drying preparation may be needed before the manure goes into the conversion reactor.
Gasification and pyrolysis are more complex but can convert over 75% of the cow manure’s energy into usable syngas or bio-oil. Combustion has lower efficiency but simpler equipment needs.
To convert cow manure into energy, cattle farms need to invest in specialized equipment matched to their chosen process.
For biogas from anaerobic digestion, the main equipment required includes:
Anaerobic digester – An airtight tank with temperature control and mixing. Needs to be sized based on the farm’s manure supply and energy needs.
Secondary treatment – A solid separator, grinder or other pretreatment equipment.
Biogas storage – A gas holder tank with adequate capacity.
Generator or boiler – Equipment to burn the methane for electricity and/or heat.
For thermal conversion processes, the key equipment needs are:
Drying system – A rotational drum, belt dryer or similar system to reduce manure moisture.
Conversion reactor – The gasifier, pyrolyzer or combustion chamber matched to the process.
Generator – A gas engine, steam turbine or other generator to produce electricity.
Emissions control – Scrubbers, filters or condensers to manage air pollutants.
Proper storage and handling systems are also required for both methods.
Many cattle farms around the world have successfully installed cow manure energy systems:
Rossdale Farms in the UK uses anaerobic digestion of dairy manure in two 5,500 m^3 tanks to produce 1,600 m^3 of biogas daily. This powers a CHP engine to meet over 60% of their electricity needs.
BioTown Ag in the U.S. partnered with Utah State University to demonstrate a 300-kW gasification system using manure from 4,500 cows. This generates 2,500 MWh of electricity annually.
Groot Zevert Vergisting in the Netherlands utilizes four digestion tanks to convert 100,000 tonnes of manure and 25,000 tonnes of organic waste per year into biogas. The gas fuels generators producing 8 million kWh of renewable electricity.
Costs and Considerations
For a farm to invest in cow manure energy conversion, some key economic factors should be considered:
System costs – Anaerobic digesters can range from $200,000 to $1 million+ depending on size. Thermal systems are $500,000 to $2 million. Maintenance is around 2-5% of capital costs.
Transport costs – Moving manure around the farm to pre-treatment and digesters requires front-end loaders and other equipment.
Revenue streams – Income can come from electricity sales, heating value, renewable energy credits and waste avoidance. Tipping fees can help offset costs.
Payback period – With incentives and credits, payback periods of 5-12 years are feasible for well-run systems. Ongoing profits are possible after payback.
Proper planning, system sizing and financial incentives are key to making cow manure energy generation economical. But the environmental benefits of waste reduction and renewable energy make these investments worthwhile for many farms.
Converting cow manure into energy is an innovative way for cattle farms to manage waste and generate income. Both anaerobic digestion and thermal processes like gasification can transform manure into usable forms of renewable energy.
While capital investments are significant, the potential benefits are huge: reduced waste and pollution, lower electricity costs, sustainable energy from an on-site resource, and new revenue. More research, demonstration projects and financial incentives could help make cow manure energy mainstream on farms across the world.