Fabrication of the bench-scale biogas reactors proved more difficult than expected. We could have made it easier on ourselves by constructing larger reactors to give us more room to work inside the reactors, or by fabricating them using an opaque material. We chose to use glass so we could see the mixing and quiescent zones in the reactors. We overcame this challenge by slightly modifying the design and mixing fabrication materials. A positive result from this challenge was fabrication of two experimental reactors, rather than a single experimental reactor specified in the proposal.

I would like to tell you about a research project that we carried out here at Frontier Environmental Technology:

Title of grant: Biogas Generator Powered by Self-Sustaining Mixing Mechanism
Subtitle (more descriptive title): to develop a stand-alone high-rate biogas generator to provide fuel for daily cooking, reduce waste discharge, improve hygiene and eventually, improve the quality of life for the people who live in developing countries.

Name of lead organization: Frontier Environmental Technology
Primary contact at lead organization: Jianmin Wang, Ph.D., P.E.
Grantee location: Rolla, Missouri, USA
Developing country where the research is being or will be tested: Partners in Madagascar and Kenya are interested in more testing if resources are available.





Goal(s): The goal of this project is to develop a high-efficiency biogas generator capable of producing the quantity of biogas and quality of effluent typical of advanced biogas generators, but without their inherent power or maintenance requirements.

Objectives: (a) to develop a mixing device that uses the biogas generated from the lower portion of the biogas generator to automatically mix the generator content, without external energy input and mechanical moving parts; (b) to preliminary test the performance improvement of the biogas generator that integrates the self-mixing capability with the conventional biogas generators.

Start and end date: Started November 2011, final report submitted September 20, 2012.
Grant type: GCE Round 7
Funding for this research currently ongoing (yes/no): No
Research or implementation partners: N/A - still looking for partners for continued research

Links, further readings – results to date:
The major effort of this project was used to develop a self-mixing device, which has been tested in a small scale reactor. Initially air was used to mimic the biogas. Later on this self-mixing device was installed within a small-scale biogas generator to improve the reactor performance. Much modifications were made for the biogas generator during the 11 month testing period. The final results have indicated that the self-mixing biogas generator improved biogas production by 30% compared to the conventional, non-mixing biogas generators typically used in developing countries. Due to the small scale of the reactor used in the Phase I research, the advantages of the self-mixing device has not been fully demonstrated, mainly because of the low height/depth of the biogas generator used for testing. If further testing is conducted, a larger, more realistic reactor height will be used so that more biogas will be collected to drive the mixing activity.

How does it work?
Overall, I have a mixing device within the reactor. This device collects gas bubbles from the lower portion of the tank. At a certain gas volume, the entire amount of the gas is released all together, creating a suction from the tank bottom and mixing the reactor. It uses the lifting power of the biogas bubbles created within the reactor therefore it does not need external power to drive it. Also, it does not have any mechanical moving parts, and all function is accomplished hydraulically. Therefore it is expected to be maintenance-free.

Presentation at the FSM-2 Conference in Durban by Tim Canter:
www.susana.org/en/resources/library/details/1762

Video of presentation by Tim Canter:



Key components: (a) a sealed tank; (b) a surge-mixing device.

Please also find the poster I used in RTTF meeting in Seattle:


Entry in SuSanA library:
www.susana.org/lang-en/library/library?v...eitem&type=2&id=1762

It has more explanation on the self-mixing mechanism. Compared to all other gas mixing digesters, our system does not need external energy but provide more powerful mixing through the surge lifting device.

Possible applications:
We intend to use this unit for family use to replace old, large non-mixing digesters, because the old systems are so large that they have to be built on-site with causes lots of issues. Since our self-mixing units are small we can fabricate them in a central location to reduce cost and improve product quality.

Major frustrations:
Our project has a very good start but we were disappointed that we did not get follow up funding for continued development and testing. Since this technology is so different than conventional thinking it may take some time to understand.

The majority of the work during the research is to make the self-mixing biogas generator mechanically functional and to prove the self-mixing concept. Therefore, the actual biogas data is very limited. I really wished to have more funding to continue this research with a more realistic reactor, but it did not happen yet. We are welcoming supports from funding agencies and collaborators to continue this work.

Regards,
Jianmin
_______________________
Jianmin Wang, Ph.D., P.E.
Frontier Environmental Technology
900 Innovation Drive
Suite 203
Rolla, MO 65401-1110
USA

Dear Jianmin Wang,

I am Hamidatu Darimani, a PhD student at the Internationai Institutute of Water and Environmental Engineering,2iE, currently I am working on a JICA project in Burkina Faso. In my study, I am trying to determine the inactivation rate cofficients of pathogens in compost made from Human excreta using a rural model of urine diversion composting toilet. I am trying to make the compost sanitary for reuse in agriculture.

I am origially from Ghana. I am trying to write-up a business model for biogas generation. I was so happy when I saw your work. I will be happy to be your partner in Ghana if all goes on well with the project. I can help set-up a plant in Ghana to commercially sell out biogas using your digester. What do you think? We can have the business plan in place and use it for funding.

I wish you all the best in your search for funds. Hope to hear from you soon.

Hamidatu Darimani.

Hamidatu,

Thank you very much for your interest! Right now the project is stopped due to the funding issue. I will be more than happy to work with you and do field test in Ghana if there is funding to do so.

Jianmin Wang

Seems similar to this one:
www.supergas.dk/technicaldescription/supergasprincipe/

Thanks for sharing that information. It is a very interesting design, and the valve design is very smart. However, in terms of mixing intensity, it is similar to those traditional ones that have a large outlet tank.

Our self-mixing device continuously pump the sludge from the tank bottom to the top, and the scum layer could be minimized as a result of the surge action of the mixing device, which could help to improve performance. Also, the self-mixing biogas generator should be easier to set up due to the one-tank design. Unfortunately it has not been field tested to prove these points due to the limitation of the resources.

Jianmin Wang

This is very promising because it speeds up the fermentation. Currently the smaller size biogas installations with single fermentation chambers in combination with an overflow have large gas leakage into the atmosphere. The creation of a double fermentation chamber reduces the leakage, but also the stirring.
I suppose the fed-in liquid has to be really liquid and cannot contain long fibres such as may occur with adding kitchen waste.
I have tried to work out the coffee percolator principle but did not succeed.
Also I have thought about the waterwheel principle; the design is upside down and the water substituded by the raising gas. The other buckets on the wheel move down through the slurry ans cause the stirr. You need a rectangular tank to make it work. Also in such a case the slurry should be really thin. The disadvantage is that it will be difficult to maintain when getting stick.

sjoerdnienhuys,

Yes - you are right. The traditional biogas generators are so large that they have to be constructed in field using local materials and by local labor, which could easily cause leak. In one region of China (I forgot where it is), over 90% of biogas generator does not work due to leak. They are abandoned and could course safety issue.

The function is dependent on the digester size - That is why I wanted to test it in a field scale. When the unit is larger, all conduit is becoming larger, and less clogging could occur. I think, for a family size unit, I can make it work as long as the particle is less than 1 inch. I would also use a screen or a manual grinder as part of the intake so it can reduce the size of the feed particles. I think for most household waste, the particle size will be less than 1 inch.

We have tested very thick sludge up to 5% of solids. If the feed is very thick, the effluent can be returned to dilute it.

I get an idea what you want to achieve. However, both ideas will cause continuous action. Due to the low gas production, this continuous action may not be sufficient enough. In my design, I have all gas collected and release them at once, causing a strong surge action, which shakes the reactor and cause periodic mixing. Also, in my case, the surge mixer lift all sludge from the bottom of the reactor, to eliminate sludge deposition at the bottom.

I notice that you have done lots of thinking in biogas generator. I would be very interested in further discussing ideas with you.

Jianmin

On the issue of gas leakage I was merely referring to the single dome biogas reactors with good quality dome structure and only a pressure balance tank attached directly to the dome outlet. From field mesuring large amounts of gas escaped from the balance tank and when the main dome was not very regularly use for kitchen cooking.
A possible solution was to have a Z shaped wall structure inside and a second dome which also could capture gas. Using a simple fishtank airpump on a solar panel the gas from the second tank is pumped back to the first tank.
The Z profile in the two tanks increased the average retention period because there was no direct flow from the intake to the outlet as occurs of one volume domes. see sketches.
The internal PDM diaphragm turned out to be too costly and complicated.

You try to use the second dome to collect the biogas that could be otherwise wasted. There could be a number of things to consider:
1. What gas pump can you use to pump gas from one tank to the other one? Fish tank pump may not work because it takes gas from surrounding environment. The pump must have a intake port and discharge port, and has to be sealed all the time.
2. Second dome may not introduce good mixing, which is the key to differentiate high rate and low rate digesters.
3. A balance tank will be needed because, if the biogas is consumed the liquid level in the second tank will drop and air will be sucked to the second dome, then be pumped to the first dome - the tank will explode after air is mixed to the methane.

Jianmin

jmwang wrote: 3. A balance tank will be needed because, if the biogas is consumed the liquid level in the second tank will drop and air will be sucked to the second dome, then be pumped to the first dome - the tank will explode after air is mixed to the methane.

From a quick glance at sjoerdnienhuys sketch, that would be a concern in general, no? It specifically states that there is an air intake (to avoid sucking a vacuum in the second tank I guess), but that would clearly involve getting a dangerous gas/air mixture.

However overall the design seems similar to the "supergas" I linked above, so a similar mixing mechanism should be easy to include (sjoerdnienhuys design has slow mixing too, but the pressure release fast mixing is probably more efficient, even though higher gas pressures mean more gas leakage most likely too).

A bit OT: has anyone ever tried to make a single dome, but two step digestion system to work? From the biogas theory it would be beneficial to have the first acidification reaction separated from methanogenesis phase. I think this is done in some larger scale commercial biogas plants, and some rural biogas manuals suggest letting the ready feed material sit in the liquid solution a few hours before feeding it in the digestor, but is there also a design that includes this step automatically?
An overflowing inner ring might do the trick, while at the same time increasing the mean residual time of the material in the digestor... but I doubt that I am the first one to think of this...

air intake.
The sketch may not be clear enough, but the second dome has a membrane inside. Under the membrane is the biogas and above a dome to allow the membrane to be protected and expand. I got several quotations from the membrane manufacturers who use them in very large size thermophillic biogas reactors.
In this design three ways of mixing occurred; one by the z flow over the bottom sludge blanket; two by the flowing back and forth between the domes when gas is extracted, and three by the pedal operated mixer.
The design of the pedal operated mixer turned to be rather simple using the boat-drive shaft seal design. In urban environments there will be plenty of children willing to operate the bike a few times; maybe the bike can be made fancier. Other playing tools can be used for the stirring, like they use in South Africa for waterpumping.