Biogas Toolkit | US EPA
Biogas Toolkit | US EPA
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5 Things You Should Know Before Designing a Biogas Plant
We sat down and wrote this article on “5 things you should know before designing a Biogas Plant” to help our readers to create the best biogas plant design way back in . 10 years later, we look back, and we think this information is still relevant and useful. (And, we also added some additional hints and tips which we think you might find interesting!)
The list below contains our top 5 things that we think new anaerobic digestion plant developers should know about before designing a biogas Plant:
1. Feed Material Choice Implications When Designing a Biogas Plant
When starting a biogas plant project, you likely have some organic matter available as feedstock. However, most projects require additional feed materials (also called substrates) to operate efficiently.
While anaerobic digestion plants can process many types of organic materials, each plant has limitations on what it can handle. Your choice of feed materials will fundamentally shape the plant's design and process flow.
Seek a Dependable Long Duration Feed Supply Contract
An important consideration is the growing competition among biogas plant owners for premium waste materials. What starts as a waste stream—where plant operators can charge disposal fees—may become a valuable commodity that operators have to pay for.
To protect against this market shift, we recommend negotiating long-term contracts (10+ years) with biowaste providers. This timeframe typically allows for recovery of the initial investment while securing a stable feedstock supply.
Some wastes like food wastes are highly calorific (making them high gas-yielding and highly desirable for digestion) and may come without any need to comply with the Animal By-products Regulations (UK).
But, before assuming that the wastes of this type will always be freely available and bring in a gate fee, the promoter should note that this value may well quite soon be appreciated by the producer. When that happens the producer may start to charge a fee and not the other way around!
Once there is adequate anaerobic digestion capacity in any region it is common for a seller’s market to develop, and for the producer to start charging the AD Company for the honour of digesting their waste product!
For this reason, when designing a biogas plant always probe deeper and find maybe less high gas-yielding feedstock wastes which are less than ideal as a biogas plant feed material, but at the same time, such feedstocks can be much more secure as long-term economic digester feed sources.
2. Design Life of Plant
All biogas plant promoters should think very carefully about the design-life of their biogas plant. Many poor-quality biogas plants are being built which will suffer long-term problems and will close a long while before better quality AD plants, built to a longer “design life”. This can make “cutting corners” very bad value.
When is a Construction Cost Saving a Long Term Cost?
The majority of biogas plants are built to a budget as a necessity of funding, nevertheless, as the industry matures those buying biogas plants will have to stop buying the lowest priced tender and develop an in-depth understanding of value for money, and “lifetime maintenance” costs. It is ONLY by doing this and specifying the design life of biogas plants from the start, that better value can be obtained.
An example is the use of cheap mild-steel plate-based digester tanks that are inadequately corrosion protected later leading to corrosion penetration, leakage and plant commissioning for repair work.
Another common error is to spend too little on feedstock pre-processing for the removal of all non-organic contaminants especially when accepting food waste. A digester tank that fills with plastic, and inert materials can require hugely expensive emptying and specialist cleaning.
Fortunately, the latest low-destructive depackaging and separation equipment is far better than older models.
See also:5 ways pallet wrap can help your business
Want more information on Waste To Biogas? Feel free to contact us.
A reasonable design life to specify for AD plants is 15 to 20 years, maybe longer. However, few if any tank suppliers will provide a warranty for the continued corrosion-free performance of glass coated steel tanks beyond 10 years.
Avoid the Temptations of Short Lifetime Design
10 years is too short a design life for anaerobic digestion plants.
Hint: If you decide to specify a concrete-walled CSTR digester tank go for an in-situ cast reinforced tank which is cast all in one pour to avoid construction joints between the base/floor joint and the top of the wall. Precast concrete tanks which are usually multi-jointed and post-stressed look good on paper but the construction method is truly difficult to achieve. The wall units are imported to the site ready-made but they must be connected together with multiple water-tight vertical construction joints. Many such tanks have failed structurally well before the end of their design life due to invisible corrosion of the circumferential steel tendons.
3. Need for Mixing when Designing a Biogas Plant
Biogas plant substrates need mixing.
As offered by the cheapest AD Plant contractors, on-farm plants are frequently not supplied with any mixing equipment.
This is more often than not a mistake soon regretted and will shorten the life of the plant in between costly maintenance work.
Hint: We think that the best mixing systems for large CSTRs are those that are externally mounted and include a sparge cycle. In other words, in addition to providing a water jet to stir the contents of the digester, they also draw down some of the stored biogas and inject biogas at the jet nozzle. This is particularly good for getting any surface crust moving. Injecting gas into tanks for mixing is traditionally known as “sparging” in the biogas industry.4. Avoidance of Pump and Pipe Blockages
Novice designers of biogas plants can offer very low-cost AD plants, which work on paper, but not successfully when constructed.
Designing AD plant pipework is truly the domain of experienced pipe flow engineers only. To avoid problems later with pump and pipe blockages needs a designer who understands every aspect of designing-out blockages.
Blockage avoidance measures range from pump model selection to choice of pipe diameters, bends and specials.
5. Build Up of Grit
Often overlooked is the propensity for any biogas plant design which accepts waste material to become blocked-up due to the presence of grit that enters (wet AD) biogas digesters.
Once grit settles it won’t come out until the whole tank is dug out with a Tomcat type excavator, or similar!
Always ensure that any AD plant designer has made adequate provision for removing any grit build up.
Hint: Better still use one of the new grit separator units to remove it before it enters the digester. The grit takes up space that should be actively in use by the microorganisms, reducing efficiency. There are some innovative new products now available to perform this function. Some can also depackage food waste and remove the plastic.
Conclusion to Designing a Biogas Plant and the 5 Things You Should Know First
This list of tips doesn’t cover all the problems that can occur. Nevertheless, these are at least some of those that keep occurring and that we thought that our readers would benefit from knowing about.
Contact us to discuss your requirements of Organic Waste To Biogas. Our experienced sales team can help you identify the options that best suit your needs.