After looking into these processes several issues have come up with anaerobic digesters. Agitation is first. The time that it takes for the feed stocks to be digested into CH4 and CO2 varies greatly from digester to digester. One important aspect of this time is agitation. If the sludge is agitated and well mixed the digestion takes place at a faster rate. This complicates a simple anaerobic digester from a simple air tight vessel to a vessel with moving parts. Mechanical agitators can be used to achieve this, but cost more money and expertise on operating the digester. Because there are living organisms within this sludge, to much agitation can also be harmful to the organisms. Walking this fine line of how much agitation is needed to speed the process up is making things more complicated. We hope to find this information soon so that we can begin sizing and pricing the agitation that is needed. Any suggestions from other processes that people know about in any industry would be greatly appreciated.
Monday, March 21, 2011
Senior design and complications with anaerobic digesters: agitation
I chose my topic of waste to energy at the beginning of the semester having a interest in the industry and technology. Later I was able to pick from four senior design topics for my Chemical and Biochemical major, one including a gasification of waste streams to energy. One stipulation of the project was to integrate a biochemical process with the thermochemical process of gasification to obtain energy from the waste feed stocks. My group has been looking at the possibility of implementing an anaerobic digester and gasifier to obtain energy from multiple waste feed stocks. Feed stocks include farm wastes (cow manure), plastics, and woody biomass. The process will have two feed prep areas where the biodegradables will be fed to the anaerobic digester and the non-biodegradables will be fed to the gasifier. The left over sludge from the anaerobic digester will then be dried and added to the gasifier to obtain the full amount of energy possible.
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I like that this post is very thorough on how you are accomplishing this technology, although you might have lost some people. As for a suggestion, depending on the density and viscosity of the sludge this might not be applicable but you could try bubbling up a gas through the sludge from the bottom. This gas could obviously not be air but you might be able to use C02 or some other thing.
ReplyDeleteThat is a very good suggestion as the CO2 will have to be separated from the CH4 anyway. I will look into this and see if that has been used in any existing processes.
ReplyDeleteAnother good post.
ReplyDeleteIt does remind me of the importance of scalability for projects like this. A lot of times environmentalists argue that we have all the technologies we need to make things like waste-to-energy work. This may be true on a small scale, and when our time-scales are long (slow food, slow industry, etc.). But if we really want to scale up, things have to get faster, probably more expensive, and, as you point out, more technically complex. This is good to keep in mind.