Piedmont Biofuels Digester Design

[scumeye]

Hi Bob,
I think your site is great and wish I'd discovered it earlier. I'm an intern with Piedmont Biofuels in Pittsboro North Carolina. We're currently working on a project in which a digester would be used to break down any organic molecules (oils, glycerol, biodiesel (methyl-esters), and hopefully soaps (KOH reacted with FFAs) in our biodiesel wash water. Theres been an ongoing project to be able to treat our wash water on site at the co-op. The constructed wetlands (current treatment method) have never really worked. The theory is that oil is coating the roots of the plants. It gets the job done eventually but its usually not pretty and we're going to be doubling our production. With the current water wash method we would produce about 1000 gal a month of this wash water but we're also looking at doing some dry-washing. My goal is to get our wash water down to 300 gal a month and run it all through the digester. The start-up plan is pig and cow manure initially and then add some horse into the mix as well (I've heard that the bacteria in horse manure can handle more volatile/toxic substances.)
The design is two IBC totes (Liquid shippment containers) that are each about 1000 L. Theres about 1600 L of liquid contained in the full system so we calculated 40 L (about two 5 gallon buckets) a day going in. The plan is to wait till about 200 gal of slurry is in the system before we start inputing any of the wash water.
The cells are finished and we connected the system and filled it with water and it didn't leak and seemed to hold pressure. We disassembled the system to finish plastering the small structure that the digester will be housed in. Its a straw-bale building that should insulate well. The heating system is still somewhat on the drawing board but we should be ready to make it this coming weekend. It involves 2 solar hot water panels, a copper radiant floor system directly underneath the cells, and some form of additional heating for the mix tank(55 gal black metal drum outside the structure.)
Here's a link to a drawing of the system.
http://girlmark.com/forumphotos/piedmontbiodigester.gif
any advice of the best way to heat the influent. I was thinking that it would have valves to turn on the heat for the mixer only for a little while each day.
We also don't yet have a design/plan for the methane. We could easily get a gas hot water heater either for the co-op house or for supplemental heating of the digester or both. Theres also interest on this end for heating a greenhouse. I tried using the calculator but its a little difficult because we can get as much manure as needed for the project but i don't know how much gas to expect (seeing as how the gas in almost secondary for this system) or how fast our HRT will be.
Please let me know what you think.

[Bob]

Hi scumeye,

Wow. I'm honored, girlmark. Nice to see you here. I came across your biodiesel forum a couple of years ago, when I was involved with a community food production project for rural Alaska and was very impressed. Some of the best d-i-y biodiesel information available. (The project involved growing plants indoors (in the sub-Arctic) under lights powered by a diesel generator fueled with a combination of biodiesel and wood gas. Heat rejected from the diesel engine would be used to gasify the wood and maintain biodiesel process temperatures.) Anyway...

any advice of the best way to heat the influent.

First of all, I would calculate the heat loss from the system. So you have an idea of how much heat you need to make up. There are two primary sources of loss, transmission through the skin, and exchange of material.

To calculate transmission loss, use Q=u*A*(Ti - To) where Q is heat transfer rate (e.g. Btu/hr), u is conductivity (reciprocal of R-value), A is surface area, and To-Ti is difference in temperature between digester and ambient.

The other source of loss is associated with the difference in temperature between the incoming slurry and the desired digester temperature. This is

Q= mass * flow rate * specific heat * (Ti - To)

I was thinking that it would have valves to turn on the heat for the mixer only for a little while each day.

Manually? Or thermostatically controlled? The answers probably depend on how much daily maintenance you want to have to do, how much money you have to spend, and what kind of process temperature swing do you want to allow? (Which, in turn, will depend on the calculations above)

I tried using the calculator but its a little difficult because we can get as much manure as needed for the project but i don't know how much gas to expect (seeing as how the gas in almost secondary for this system) or how fast our HRT will be.

Maybe work backwards from the tank sizes that you already have to determine how much to put into them daily. A 25-30 day HRT should work fine. (1600L / 40 L/day = 40 days)

[girl Mark]

Thanks for the welcome, I'm hanging out at Piedmont Biofuels right now but I"m not Scumeye- that's Caleb, an interns, who's working on the biodigester project (I just hosted his diagram on my website since he had no easy way of posting it to the internet).

I'm very excited that he found this forum, and am following their progress with interest, and am quite excited to read the archives here too.

Mark

[Bob]

Glad to see you here, girl Mark.

I hope you find as much value here as I found on your forum.

[scumeye]

We're building wooden boxes to house 5/8" aluminum coils that will be packed in with clay (here in north carolina we have plenty of metal rich red clay) that will interface with the plastic tanks. We're concentrating more of the heat on the second cell to keep our methenogens happy. There will be one large solar hot water panel and a gas hot water heater in the system. The pump and blowers run off a pv panel and the whole system is off-grid. The big questions right now are in regards to safety and designing the gas storage/usage system.
I am also interested to know if many experiments have been conducted regarding the behavior of oils in biodigesters.
thanks for all your help

[Bob]

We're building wooden boxes to house 5/8" aluminum coils that will be packed in with clay (here in north carolina we have plenty of metal rich red clay) that will interface with the plastic tanks.

Are they sized based on the heat loss calculations above? Or flying by 'seat of the pants'? How will temperature be controlled? An aquastat in each tank that cycles pumps and/or heaters?

We're concentrating more of the heat on the second cell to keep our methenogens happy.

Is temperature difference between the first & second how you intend to culture methanogens in the 2nd? From your drawing, it appears that both tanks are the same size, implying that the retention time in each will be the same -- so you are not using difference in retention time to differentiate the cultures. What temperature do you plan to control each tank to? (If you don't heat the first one at all, it will grow mostly acidogens rather than methanogens -- as well as save you energy.)

The big questions right now are in regards to safety and designing the gas storage/usage system.

My own preference is to try to use the gas as soon as possible, as it is generated, to avoid the complications of having to store large quantities of flammable (and dangerous) gas. The simplest is to burn it in a hot water heater, storing the energy as heat. But of course you need to have a use for the hot water. Can you use it as a heat source for other processes -- i.e. producing biodiesel?

I am also interested to know if many experiments have been conducted regarding the behavior of oils in biodigesters.

I'm not familiar with that research. Though there may be some.

[scumeye]

As for heating system design we found your equation very helpful and put our on project engineer to work on it. The hot water from the solar hot water panels goes first into the hot water tank. The therm in the tank will kick on the heat if it gets below a certain temperature (said engineer is still working on equation.) The water will flow into the heat exchange for tank 2 first to ensure that there is enough heat. There will be a thermometer and another option for the system to apply additional heat if it falls below a set temp. We were thinking at least 110 degrees F to ensure that tank 1 would have enough leftover heat. Its interesting to hear that the acidogens don't require as much heat. At what temperatures do they work most effectively and what range of temperature can they survive? The tanks are the same size and the second tank actually holds less liquid volume. The guy that helped us design the system said that this was because of increased gas production in stage 2. I'm not familiar with the retention time strategy for specific colony growth. We were planning to store some of the gas and have it as on demand for extra heat when needed. I have a design for a gas delivery system that includes a gas bladder inside a container that will receive positive pressure when the pump kicks on from lack of heat. A certain amount of water will flow into the container and supply the required pressure for flow into the hot water heater. The water will drain back into the system after use.
This would be something I'd like to build and there's no guarantee that it would function properly or be safe enough. The biodiesel process is currently heated using solar hot water panels and a 500 gal hot water tank. We could use heat for the greenhouse right next to where the digester is set up. I'm also very interested in sterling engines and the potential to produce electricity without having to go to the trouble of turbines and scrubbers. Do you have any experience with systems that use sterling engines or know of anyone else that is using them.

[Bob]

Its interesting to hear that the acidogens don't require as much heat. At what temperatures do they work most effectively and what range of temperature can they survive?

This page provides an explanation. Basically they consist of a large and diverse group of hardier creatures. They can survive a wider pH and temperature range than can the methanogens. There are various so-called 'hybrid' designs in which there are multiple batch-fed 'first stages', which generate the "food" (volatile acids) for the methanogens in the biogas digester. And only the biogas digester needs to be carefully controlled for pH and temp -- a cost (and complexity) savings.

I have a design for a gas delivery system that includes a gas bladder inside a container that will receive positive pressure when the pump kicks on from lack of heat. A certain amount of water will flow into the container and supply the required pressure for flow into the hot water heater. The water will drain back into the system after use.

I'm not sure I get it, but it sounds complicated. The simplest way I know of to maintain constant gas pressure in the system is with a weighted, floating cover on a gas collector. (Which also provides a water trap between the gas and the digester.)

I'm also very interested in sterling engines and the potential to produce electricity without having to go to the trouble of turbines and scrubbers. Do you have any experience with systems that use sterling engines or know of anyone else that is using them.

I looked into them enough to discover that, at least for the scale of systems I was working with, the technology just isn't there yet. (Or wasn't anyway. I'd be interested in what new info you come up with.)

But, that said, talk to Ron Steele. Especially if you are a machinist or have access to a machine shop. For me, it just wasn't feasible.

{edited to add an afterthought. I see on the drawing that you are thinking of using a tube & shell heat exchanger. If you've come up with a way to keep it from fouling, I'd be interested in hearing more.}

[Bob]

There are various so-called 'hybrid' designs in which there are multiple batch-fed 'first stages', which generate the "food" (volatile acids) for the methanogens in the biogas digester. And only the biogas digester needs to be carefully controlled for pH and temp -- a cost (and complexity) savings.

Here is a video of one such design: "Hybrid" refers to a combo of batch and continuous feed, where you fill multiple primary stage tanks in batches spaced a few days apart, then feed the volatile acids to the methane digester. The same idea can be implemented on a smaller scale with, say, 55 gallon drums:

[girl Mark]

Scumeye,

Until Bob's post I'd forgotten all about a storage design I saw in Florida at UF Gainesville (There are two digester projects there, a huge dairy farm continuous one and a small hippie apro-tech one).

The appropriate technology folks with the digester at UF had a system for storing the methane that involved a 55 gallon drum partially filled with water that had a large hole cut in the lid, into which a smaller 15-gallon drum was inverted (ie the drum could bob up and down in the water, and the 15-gallon drum was upside down, open side into the water). They had a framework that kept the 15-gallon drum from tipping but could let it bob up and down, and some bricks or other weights on top of the drum to keep it in the water. The methane went into the 15-gallon drum. This meant that it could rise and fall depending on the amount of gas inside it (I think) and that the drum would slightly pressurize the gas.

[Bob]

That's it, girl Mark. I've used a 30 gal drum inverted inside a 55 gallon drum, with three vertical 1-1/2" PVC pipe guides bolted to the 55 gal drum to keep the inverted drum straight as it rises & falls.

The gas pressure under the weighted cover is a function of the weight of the cover and its horizontal cross-section area. For example, if the 30 gal drum has a diameter of 20" (and a cross-section area of (20/2)^2*pi = 314 Sq In), and weighs 100 lbs, the gas pressure would be 100/314 = .318 PSI. Or about .318 * 27.7 = 8.8 Inches of Water (W.G.)

A 57 lb total weight would give you about 5" W.G. -- a typical burner pressure (But N.B.: this is a gross simplification. Actual available pressure at the burner will depend on piping configuration, diameter and length. And required pressure will vary too -- depending on orifice diameter, etc.)