Sunday, July 31, 2011

How to Anaerobically Digest Biomass and Produce Biogas in 7 Simple Actions

Just how do you eat an elephant? The traditional response to that question is, "Just one bite at a time!" Actually, it is the same answer for how to complete any large task. When you look at the whole thing all at once, it appears to be impossible. Break it down into parts, steps or sub-tasks, and each of these is not nearly so formidable. Your specific steps can each be relatively simple, something that's no big deal, that can be done. And when you have done every one of the small-task steps, you gaze back and find out that now you have the entire formidable-task thing done. That's just exactly how it is with how to anaerobically digest biomass and produce biogas. Here is a method to tackle the formidable task of anaerobically digest biomass and produce biogas, in 7 simple steps.


Step 1. The waste is delivered to the plant and is initially sorted mechanically to remove remaining non-biodegradable contaminants. With this you will need to involve screens, air classifiers or magnets. In the event you neglect this or don't do it, you should expect a poor quality of feedstock delivery into the digester, and potential for blockages of the digester, causing downtime.

Step 2. The organic waste is then shredded and mixed with water and pumped to an enclosed vessel (reactor) known as the biogas digester. This step is important because the right mixture must be created for pumping into the digester ).

Step 3. In the digester it is heated, stirred, and held for up to three weeks whilst the bacteria digest the waste and emit a gas consisting of about two thirds methane and one third carbon dioxide. This will mean that during that period bacteria and other organisms are fermenting the organic material. This could also mean that biogas methane is produced, and this is the main product of, and reason for operating the process.

Step 4. After this the solid digested material is pressed to recover the added water. This will probably involve a mechanical press system.

Step 5. The solid digestate is placed in piles to aerate for for anything between two weeks and twelve weeks. Once the digestate has been aerated it can be used as a soil improver or growing media constituent in the same way as compost. A key point you will want to remember here is going to be that there may be regulations which limit the use of digestate due to concerns about possible transmission of infection, and other possible contaminants. The reason why this will be significant is, for example, that this limits the uses and value of digestate. If the material is derived from mixed wastes sources additional sorting may also be required to remove contaminates.

Step 6. The liquid fraction can be recirculated in the process but some excess is generated and depending of the feedstock this can be used as a fertiliser or if the waste is contaminated it has to be disposed of to sewer.

Step 7. The gas that's generated after a basic cleaning stage to get rid of hydrogen sulphide and water, can be burnt in gas engines to create electricity or in boilers to provide steam.

Or, in some locations it can make good business sense to purify the gas further by removing the CO2 so that the gas may be employed to fuel autos like automobiles, buses or vans, or the purified gas can also possibly be piped in to the natural gas network.. You are now almost there! Remember, that environmental groups and many governments are very keen to promote the use of Anaerobic Digestion due to its carbon neutral and renewable energy nature.

When you take the steps explained above, the massive elephant-problem you had will likely be "eaten up" one step at a time, "devoured" and dealt with. You will succeed in completing your project and can enjoy the fruits of victory and accomplishment! Congratulations on your victory! You took on a big challenge, conquered it and won, one step at a time!

Discover the best way to build and operate a biogas digester by going to the Biogas Digestion eBook Resources site

Steve Symes regularly writes on renewable energy issues. He feels that the environmental debate is too important to leave to the boffins. If you think so too then visit a Blog on the subject at Renewable Energy News.

1 comment:

Randy Mott said...

You are describing a very expensive and unnecessarily complex approach. Only a small fraction of the incoming substrates should require any treatment and this should be covered by user charges to the waste producer. Large, stable homogeneous waste streams should provide the base load (slaughterhouses, etc.). Adding water is also unnecessary: the substrates should only be about 15-22% TS. Dewatering the digestate is expensive and also unnecessary - liquid digestate is widely used as a fertilizer without dewatering. The capex and O&M for the system you describe what not make it economically feasible anywhere I know.
Randy Mott, President
CEERES (Warsaw)(Danish AD technology)