Subject: Methane from MSW Date: Thu, 25 Sep 2003 15:24:27 +0100 From: "Duncan.Martin" To: "Paul Harris (E-mail)" DIGESTION PROCESS DESIGN: FUNDAMENTALS Before you can design a process or calculate yields, you need to define what you are doing. Feedstock: sorted or unsorted MSW (municipal solid wastes)? Or manures? Or human sewage? Or organic industrial wastes? Or biomass crops? Or mixtures of all or some of these? Sorted MSW = organics only; unsorted includes packaging, ashes etc etc Hence sorted waste is usually referred to as OFMSW - OF = organic fraction. Will householders sort at home = separate collection? Or will you sort at the plant - this is much less effective but may reduce collection costs. Will your OFMSW include much garden (= yard) waste? Yes in rich countries? No in poor countries? Garden waste is best composted but it can be digested if any woody element is well shredded. (The woody parts will not digest but might easily cause blockages.) What do you plan to do with the digestate? Will it meet any local regulations for land spreading? It has fertilizer value but local regulations (mainly in developed countries) may limit its use because of: a) risk of spreading animal diseases from meat wastes (FMD, BSE etc) b) risk of adding to excessive P or N levels (mainly where too much artificlal fertilzer has been aplied in the past) and either groundwater or surface water quality is at risk). How secure are your plans for digestate disposal/feedstock supply? Think long-term here. Don't assume that farmers will queue at your gate to pay for digestate - they're all good businessmen/women! Once they realize you need them to take the stuff away as fast as you produce it, they'll want YOU to pay THEM. Transport costs make you dependent on a small local 'market' whose members can band together to optimize their position. You might do well if you get rid of it for free! Similarly with feedstock supply. Don't assume you'll be paid well for taking care of anyone's waste. If there are alternative outlets, you might not be paid at all! For these reasons, it can best to form a co-operative with other stakeholders, to ensure that all necessary parties are committed to the long-term success of the project. Now you can get more technical! What kind of digestion system are you considering? Batch or continuous? If continuous: plug flow or CSTR? What solids level: * low solids - up to about 6% - a 'liquid' system * high solids - can be much higher - maybe up to 20% - a porridge-like mixture * semi-dry - a bed of moist solids (this would be at most 40% solids because of the high moisture content of organic wastes) Semi-dry digesters can be unmixed - a kind of anaerobic compost heap. This is the kind of system I favour - but correct seeding is critical. If you plan some experimental tests (see below) I would be happy to advise on this option. All other kinds of digester need mixing - and mixing systems are a VERY common source of problems. Even the low-solids systems are usually "non-Newtonian" - and if you don't know what that means, don't even think of designing or selecting a mixer! Always expect solid layers, even in 'liquid' sytems - both floating and sinking solids occur. Even if your feed is a clear liquid, precipitates can form in the digester. Over time, these layers can build up if mixing is inadequate, until only a small fraction of the volume of your digester is effective. Do you plan to include a pasteurization step? At what stage? (Now common in Europe - more for fear of animal pathogens than human!) Finally, what will you use the biogas for? Is the demand 24/7/365? (Because storage for more a day is rarely economic) It's important to recognize that the economics of AD have little to do with the biogas - it is just "the icing on the cake"! Think of AD as a good waste treatment process - this is its main strength. EXPERIMENTAL CONSIDERATIONS - How large a scale can you work at - these systems can be temperamental at bench scale! I have had some success at 0.5L scale but I'd recommend 5L or more. - However, simple kit can give good results - beware of over-complicating! - The time scale of your project is critical: a single test can take many months, so it might be better to set up good number of SIMPLE tests in parallel than one high-tech one. - Where will you incubate your digesters? (Digesters can work at large scale without heating but even in hot countries incubation is probably essential for meaningful exptl results. Daily temperature variations affect small-scale equipment greatly while barely touching a large plant.) - Batch or continuous? Continuous feeding of snall-scale batch digesters is tricky - unless your feed is a clear liquid. Daily dosing is a reasonable compromise - but it is debatably meaningful unless done 7/7. Batch tests are much simpler and can give good results - especially for a preliminary study - but must be seeded correctly. - How you seed may be as important as what you seed with - see my papers listed below (especially the last three) - What will you measure? There may be little you CAN measure easily except biogas volume but this gives a lot of info IF done accurately and preferably daily. Remember that backflow is possible - esp at low output rates. - Gas composition is also useful but (even if you have access to such instruments as a GC) is laborious for a large number of samples. You can get quite useful info from simple chemical tests, as the main components are almost always CH4, CO2 & a little N2 (usually a sign of air ingress). Of course there might traces of O2, H2S, NH3, H2 and many others but a good idea of system performance can be gained by checking the volume loss on contact with NaOH (=CO2) and the volume change on combustion with a known volume of air (hence CH4 content). Dr Duncan J Martin, CEng, MIEI, MCIWM, MIChemE Senior Lecturer in Chemical Engineering Room L2009, CES Dept, University of Limerick, Ireland Tel/fax: +353 61 213123/202602 Email: duncan.martin@ul.ie; duncan.martin@icheme.org Useful publications: Martin, D. J., Potts, L. G. A., and Reeves, A .J. 1997. "Small-scale simulation of waste degradation in landfills", Biotechnology Letters 19:683-686, UK. Martin, D. J. 1999. "Mass-transfer limitations in solid-state digestion", Biotechnology Letters 21:809-814. Martin, D. J. 2000. "A novel mathematical model of solid-state digestion" Biotechnology Letters 22:91-94. Martin, D. J. 2001 "The site of reaction in solid-state digestion: a new hypothesis", Transactions of the Institution of Chemical Engineers 79 (B1): 29-37. Martin, D. J., 2001. "Accelerated biogas production without leachate recycle." Renewable Energy 24:535-538. Martin, D.J., Potts, L.G.A., and Heslop, V.A. 2003. "Reaction Mechanisms In Solid-State Anaerobic Digestion: I. The Reaction Front Hypothesis." Trans. I. Chem. E., Pt B, 81 (3): 171-180. Martin, D.J., Potts, L.G.A., and Heslop, V.A. 2003 "Reaction Mechanisms In Solid-State Anaerobic Digestion: II. The Significance Of Seeding." Trans. I. Chem. E., Pt B, 81 (3): 181-188. Martin, D.J., and Xue, E. 2003 "The Reaction Front Hypothesis in Solid-State Digestion: a Method of Predicting the Thickness of the Buffer Layer." Applied Biochemistry & Biotechnology, 109: 155-166.