Not only is agitation of the sludge an important aspect of anaerobic digesters, temperature is even more important. The sharp temperature fluctuations here in Colorado cause issues with the living organisms. The optimal temperature for these organisms is 85 F to 95 F. This is much higher than the average temperature here in Colorado for more than 3/4 of the year. Only parts of June, July, and August have these high enough temperatures. To combat this issue our team has determined that a temperature control system must be used. Using the high temperatures of the products from the gasifier we should be able to use heat integration. Heat integration is the process of using heat generated in process units to heat other streams in the process. This same idea can be used to cool certain streams that need to be cooled as well. This heat integration can be achieved with a immersed piping coil that allows for heat transfer to flow from the hot gases from the gasifier to the sludge being digested. Again, this adds more equipment to the once simple digester that has already had a mechanical agitator added.
One way that digesters existing on their own can stay within this optimal temperature range is by burning some of the gas that hey produce. Several ways of heating digesters are available internal and external. Internal processes leave the sludge in the tank, while external processes move the sludge outside of the tank. The sludge can be pumped through a heat exchanger, where heat will be transferred to the sludge via a heating fluid such as water. Types of heat exchangers that can be used include tube-in-tube and shell and tube exchangers. A boiler uses some of the produced biogas to heat water that is used to heat the sludge. In a tube-in-tube configuration the water flows in the annulus area counter current to the flow of sludge in the center tube. The diagram below shows this in more detail.
Shell and tube heat exchangers are similar where the sludge flows in a bank of tubes while heated water flows over the tubes. The diagram below shows this configuration. Again a boiler uses produced biogas to heat the water.
A second way of controlling the temperature of a digester without moving and pumping the sludge is by pumping the hot water into tubes within the digester. This system will not have as good of heat transfer but will keep the sludge from being pumped around. I feel that this system would be the best due to the fact that pumping the sludge around can put severe stresses on the organisms and may cause them to go dormant or even kill them. I am not sure of the pumps that would be used or if there are pumps that can be used that reduce this stress.
I really like this post--I especially like how you mix up personal, newsy, and technical posts on your blog.
ReplyDeleteI might say that you need to define some of your terms for a lay audience (most people don't know what a heat exchanger is, for example), or have follow-up posts that you can link to that explain some of these concepts through the use of storytelling or metaphor, if you don't want to weigh down this particular post.
Nice work.