Graham
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I agree with all the comment thus far. High ammonia levels are prohibitive for biogas production. High total solids content which may need a lot of water. Some bedding materials are not great in terms of digestibility.
I think some additional policy support will be needed for continued growth. We should see enhancement of technologies for digestion, upgrading and LCO2 which will bring down cost and increase efficiency. We may see rising demand for RNG in the transport sector to replace significant portions of heavy duty vehicle diesel consumption. I am optimistic we will see strong growth over time but will need to overcome economic and infrastructural challenges on the way there!
Fully agree with all the comments. It requires a mutli-faceted approach. Identify your risk sources, implement engineering controls, wear PPE, carry gas analyzers and add a healthy dose of common sense.
I agree with Joerg… Here in the US we see landfills dominating together with recycling and composting. To my mind, probably due to the fact that there is a robust existing infrastructure in place for this already, which helps make it cost effective.
From my perspective the pre-treatment of waste prior to it entering the digester can indeed affect biogas production. If for example you have inadequate pre-treatment and therefore too much inert materials are finding their way into the digester, you are effectively taking up space with materials that cannot produce biogas. In the dairy environment we see this with sand which is used for bedding. Too much sand in the digester can result in sediment build up, which reduces the amount of digester capacity you have, and thereby reduces biogas production. Similar parallels exist in with packaged food waste. I have completed a number of projects in a past life on sewage sludge. Here, pre-treatment is often the explosion of the cells via steam which greatly increases methane potential. Thermal hydrolysis is another example where biogas production can be significantly increased. For each application the tradeoffs would need to be considered.
Its a critically important item of course and so we at Host Bio-energy put considerable thought into this. We utilize double membrane digester roofs which are cost effective and extremely durable. They provide protection from extreme weather and UV rays. They offer several hours of biogas buffering which from an operational perspective is helpful when downstream systems are offline. They are also extremely easy to install and maintain. We can in colder climates up the anti on heat insulation too, helping us maintain optimal temperatures within the digester. Our membrane roofs also has efficient gas collection with minimal losses which is exactly what you need on an RNG project!
Nice topic of discussion. For me, in instances where a small scale upgrader was the feasible alternative, we have often found that other project related costs hindered progress. If for example the interconnect costs are not proportional, then on smaller flows, we have a project that in its totally may not pencil out even though the upgrading technology is proven, robust and cost effective. When to can hub and spoke a few small sites together you may be able change the math!
At Host Bio-Energy we utilize a mechanical mixing system which includes rotating propellors and paddle type mixers in combination with one another. We feel this is a robust solution that enhances heat transfer through the digester contents and allows for circular and horizontal mixing. Our paddle type mixers actually have their gearbox and motor on the outside of the tank for ease of maintenance.