Talkin' Crap

To Heat or Not to Heat: The Anaerobic Digester Debate

• Dan Andersen • Season 1 • Episode 11

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Dan Andersen and Luke Soko discuss the potential of anaerobic digestion for manure management, focusing on whether to heat or not to heat digesters. Heating digesters can increase methane production but require insulation to mitigate heat loss. Find show notes here.

Unknown:

Music.

Dan Andersen:

Hello and welcome to Talkin' Crap, a podcast by Iowa State University Extension and Outreach. This institution is an equal opportunity provider for the full non discrimination statement or accommodation inquiries, go to www.extension.iastate.edu/diversity/ext. In this podcast, we discuss insights into the science technology and best practices surrounding manure management. Our objectives are to build awareness about the challenges farmers and the broader agricultural industry face around manure and to demonstrate solutions in areas of innovation. Hello. I'm Dan Anderson, associate professor and extension specialist in the Ag and Biosystems Engineering Department at Iowa State University. Your host of Talkin' Crap and joining me today, I have Luke Soko, PhD student in that same department working with me. Luke, could you introduce yourself for us?

Luke Soko:

Yeah, absolutely. My name is Luke Soko. I'm a PhD student at Iowa State University. I got my bachelor's at Iowa State in Biosystems Engineering. I grew up in the Quad Cities, Bettendorf, Iowa, and I grew interested in anaerobic digestion, in the idea that you can, you can take manure and convert it into renewable natural gas, and not only power the farm, power the grid, but also reduce emissions from that manure, while also powering whatever you need to power with renewable natural gas from the biogas.

Dan Andersen:

Perfect. And our topic for today is really going to be about anaerobic digestion, and some work Luke's been doing on deciding to heat or not to heat that anaerobic digester. And we'll talk a little bit more about what that means and why that and why that could be important. But before we dive into that, I wanted to talk about the forces driving anaerobic digestion. So we saw a boom of interest in the 1970s and the 2000s and now we're maybe seeing the third renaissance of anaerobic digestion. And previously we'd sort of seen them when fuel prices spiked or we got interested in renewable electricity in the 2000s what's making this time different? And why do you think there's more potential for biogas systems to stick around?

Luke Soko:

Large animal production companies such as Smithfield, Tyson, they've committed to a 30% reduction in their emissions by 2030. Companies, like JBS, have taken more aggressive pledges towards emissions reduction, so have Nestle. So you have Nestle, JBS, Smithfield, Tyson, all taking these pledges to reduce their emissions from their companies and an easy, not an easy way, but one way to do that is through anaerobic digestion. Typically, manure is stored in these large manure storages, and those manure storages digest anaerobically because of the function of oxygen diffusivity in the manure and biochemical oxygen demand, but the manure digests anaerobically and releases biogas. That biogas is roughly 60% methane, methane that goes into the atmosphere can be harmful to climate, but we can take that methane, we can take that biogas. We have the technology to scrub the biogas so that that 60% methane becomes roughly 99% methane, and that natural gas itself is 95%, 96% methane. And we can take that methane and put it into the pipeline, just like natural gas as renewable natural gas, and power the power our grid.

Dan Andersen:

Yeah, and I think that's a real big difference this time. We see that some of the larger agricultural suppliers, Smithfield, like you mentioned, JBS, Nestle, have all made these pledges, and it seems like they're taking this issue more seriously and thinking about what it means for their production practices, and that probably will lead to greater change in the past, where it hasn't been really self driven. It's been more fuel or energy production driven. The other thing I wanted to point out is there's two government programs, the RIN and the LCFS programs, that have really been put in place as policy by the governments, either federal or state, to support renewable fuels industry trying to wean ourselves off maybe gasoline to the extent we have been in the past, and looking for other alternative, renewable supplies and anaerobic digestion, has found a way to tap into that market

Luke Soko:

Absolutely, absolutely. LCFS, so there are two large markets, the RIN market, like you said, in the LCFS market. A RIN stands for Renewable Identification Number. It's the federal program to the Renewable Fuel Standard. Roughly a six year average of that price is about $2.20 per gallon of gasoline equivalent. That's roughly 77,000 Btu. That translates to about $1 per cubic meter of methane produced. The LCFS program is through California. LCFS stands for Low Carbon Fuel Standard. Rough year six year average is about $155 per ton of carbon dioxide equivalents abated. Currently, that price is about a third of that. You're looking at roughly$50 per ton of carbon dioxide abated. But what this means is, if you are making renewable natural gas, you're making methane. You're putting it into the pipeline rather than the atmosphere. You can make money through the RIN program, through getting dollars per cubic meter of methane, and you can also make money through you'll get what's called a carbon intensity score for your system, a CI score for your anaerobic digestion system, that will measure the amount of carbon dioxide abated per megajoule of methane that you produce. And then you can get low carbon fuel standard credits through that program based on the amount of carbon dioxide equivalence you reduce through anaerobic digestion.

Dan Andersen:

Yeah, and I think that's a huge change compared to when we've tried this in the past, these incentives are much bigger in magnitude than even we saw with renewable electricity generation in the 2000s and they seem to be more sustained in their support and that they've been around for a little while. I think USDA has sort of reiterated this with their terminology for climate smart and creating lots of opportunities for climate smart agriculture and what that means for our production practices, and it's trickled into the discussion of sustainable aviation fuel and the impact that that might have on ethanol, where it's really becoming widespread in our vernacular and impacting our production practices, and what that means for what we can turn things into. You mentioned a little bit about how we currently store manure. Most of the time we're using a deep pit slurry store or a lagoon of some type to hold that manure. It breaks down at various rates, depending on the conditions we put it in, making methane. Some of that methane is released to the atmosphere, and anaerobic digestion is a little bit different than that. So I wanted to walk through what anaerobic digestion is talking about, at least the basics of what's a lagoon digester? What are some other options that we have out there? So Could you walk us through a lagoon Digester?

Luke Soko:

Yeah, a lagoon digester, the simplest anaerobic digester. Well, you could review that. But a simple form of an anaerobic digester is you have a lagoon, and that lagoon is generating biogas. There's a lot of water in that lagoon. The manure is typically diluted in a lagoon. It's digesting anaerobically, without oxygen. It's making biogas by itself, just existing. So you can cover that lagoon, cover it with, typically, some sort of HDPE cover, or other cover, cover the lagoon, and that cover is going to capture the biogas so it's not being released into the atmosphere. You have it captured, and then under the cover you'd have, you'd siphon off that biogas to what's called a biogas upgrader. Typically, today, common ones the United States are pressure swing absorption biogas upgrade or there's also a membrane biogas upgrader, and there's other forms, like water washing and immune scrubbing. But you send it to what's called a biogas upgrader, and what that will do is within biogas, there's methane, carbon dioxide, hydrogen sulfide, and sometimes trace amounts of oxygen and nitrogen and water vapor. And you can't put all that into the pipeline. So what you're going to so the biogas upgrader scrubs out that carbon dioxide, hydrogen sulfide, oxygen, nitrogen, water vapor, so that you have a very concentrated stream of methane. And so that's what's leaving the biogas upgrader is a very concentrated stream of methane. And then if you're next to an injection point for natural gas, you can inject that methane directly into the pipeline, as long as it meets the specs of the utility company. If not, you'd have to put it into a truck, or you'd have to put it into a separate pipeline to an injection spot. So you transport it to the injection natural gas injection spot, where you put it into the pipeline. And that's how you go from the manure storage to putting renewable natural gas in the pipeline.

Dan Andersen:

So when you talk about a lagoon manure storage, you kind of implied it was simple. And the reason you're saying it's simple is it's really it looks like a storage you put a cover over the top of it, and that historically has been about what we do. We don't make any modifications to it where we're heating it or mixing it, although there is potential to make those modifications. Other digesters that people have sometimes used in agricultural industry are completely mixed digesters or plug flow digesters. What's the difference between that and a lagoon?

Luke Soko:

Absolutely yes. Good point. So the covered lagoon digester is very simple. It's not mixed. It's not heated. Typically, it's not mixed or heated. You can have variations of things. The plug flow digester is typically not mixed. Also, typically more common on dairy farms, solids, contents higher than that of a lagoon. It's a digester that's not mixed. The manure serpentines around a heated digester system. It's heated. That's the main difference. Is it's heated as as the manure approaches the temperature of a cow's stomach, roughly 37 degrees Celsius, that microbial activity is going to increase exponentially, that methane production is going to increase exponentially. So as it as we heat manure to around 37 degrees Celsius, we're going to generate significantly more methane, significantly more biogas in general. And if your goal is to make a lot of money by making a lot of methane, putting that methane into the pipeline, that could be something you'd want to do. So that's a plug flow, not mixed, and then you have the completely mixed digester. It could be all sorts of variations of things, but maybe a large, cylindrical digester, that's it is mixed. You have mixers in the bottom, and it is also heated. There's research that mixing digesters can increase gas production. There's a lot of research on how much to mix, when to mix, how to mix, but that is another form. So those are your three common forms of anaerobic digesters in the United States,

Dan Andersen:

Especially for agricultural systems. And I think when we look at some of those digesters, if we heat we might only store manure in that digester for 28-35, days, somewhere in that range, so a much shorter period of time. Lagoon digesters are really sort of long term storage, 180 day-360 days of storage. So even though we're not heating, we have that long term breakdown. So it's sort of a trade off between how long you're willing to hold it in this digester and and what temperature we need to get to to get it all done. The other big advantage of lagoon digesters, if you're on a farm that already has an outdoor manure storage. It can be relatively simple or cheap to cover it. One of the things you did for us, and we talked about before, and I want to briefly touch on again, was you looked through the EPA AgSTAR database and got gas production values from some different styles of digesters, heated digesters versus non heated. And since we're talking about, should we be heating today, I think that's what we want to focus on. Could you talk a little bit about what you found for typical lagoon effiencies compared to some of those heated digesters?

Luke Soko:

Absolutely. So, the California Air Resources Board, sometimes abbreviated as CARB, provides a model that estimates the amount of methane produced on a farm given the temperatures, given the outdoor temperatures, you can get baseline methane emissions for your farm. And if you have so a cow, a cow excretes roughly about 6.85 dry kilograms of volatile solids per day per cow. That's a milking cow, and that's an average between a milking cow and a dry cow, depending if you have a 60 day dry cycle on the farm, multiply that by .25 cubic meters of methane per kilogram of volatile solids, essentially your cow, daily cow. Biochemical methane potential is about 1.65 cubic meters of methane per cow per day. Now for a pig, that's roughly 0.15 cubic meters of methane per day per pig. Those are your like, essentially, your biochemical methane potential is your anaerobic digestion at roughly 100% digestion efficiency. But in practice, you're not going to get 100% digestion efficiency, especially if you're not heating. So what we'd see if you're emptying your manure storage once a year, and Dan made an excellent point that you when you're heating, you're trading retention time for heating. If you're not heating, you could have your manure in the in the storage for roughly a year and get very strong outputs of methane production, and that's because your attention times a year, as opposed to, if you have a completely mixed digester, your retention time might be 30 days, and you might say, Well, why don't I have my manure in a completely mixed digester for a year? And that's because you don't have the volume for that. If you empty a manure storage once a year, you're looking at, if you're in Florida, maybe 79% efficiency. And as you travel up the United States. Get to Minnesota, you're looking at roughly 65% efficiency. So for a cow, that's 65% times 1.65 cubic meters per methane, of methane per cow per day. So that's if you empty your manure storage once a year. If you empty your manure storage twice a year, you have significantly greater differences. You range from about 78% efficiency in your anaerobic digestion or anaerobic digester, up to about or down to about 42% anaerobic digestion efficiency if you're in Minnesota. So quite a range throughout the United States, and that's just based on temperature. That's just based on and that's an unheated anaerobic digester. That's that unheated lagoon sitting outside. You're ranging from, if you're emptying once, once per year, 79% down to 65% twice per year empty. You're ranging from about 78% efficiency down to about 42% efficiency.

Dan Andersen:

Yeah, yeah. So I think one of the things that you highlight there is there's a real geographic divide right as we go from south to north, or basically a temperature gradient. And we've seen anaerobic digesters really catch on in other countries that are warmer because they can use that simple technology, not heat. We've even seen as we look at implementation in the US, we've seen California dairies be a leader in putting covers on and certainly that's partly due to their legislative climate, but it's also due to their actual climate and the temperatures they have where it can be more proactive or productive. Similarly, we've seen in the swine industry, Missouri, North Carolina, be putting covered lagoons on and we haven't seen that system implemented it as easily in Iowa, we just don't have lagoons on swine farms to start with. But even where we do, they don't get as much gas production because of the colder temperatures, colder efficiency. So I think as you've thought about where this is going and what needs to happen, the question is, really, is it worth the price to heat that lagoon digester to do better or get better performance? And that's really what you're going to try and tell us about today, some work that you've been doing in that area. So what I wanted you to talk about is, first, you've done some work on just a typical anaerobic lagoon digester system, what it might cost and and for Iowa, what it might take for scales to be feasible. How many pigs are we talking about? How many cows do we need to start getting in the conversation for a digester?

Luke Soko:

Yeah, absolutely. So bare minimum, if you're looking to pay off a project in five years, you're looking at roughly 1200 cows. 1200 cows on a farm, and that's assuming that you're emptying the manure storage twice a year. Typically on a dairy farm, if you're having over 1000 cows, it's unlikely you have the manure storage capacity necessary to hold it for over six months, eight months. So you'd empty that roughly twice a year. So you need at least 1200 cows if you're emptying your manure storage twice a year. If you have swine, you need at least 8000 swine on the lagoon. If you have lagoon manure storage, you need roughly 8000 swine. And that's assuming that that lagoon is something you can just readily cover and then purchase the biogas upgrader and inject on site. If you have a deep pit farm, you're looking at over 35,000 swine to have an economically feasible system in five years. And the reason for that, one of the one reason for that is, if you have a deep pit, you know that manure is stored below the barn. It's not something you can just cover. You can't just cover the slats of the barn, so you need to build separate manure storage systems, and that would be expensive. Also, there's a difference in the carbon intensity associated with a deep pit versus a lagoon and the mitigations necessary, but we don't necessarily have to get into that. But one, one good point is that that data, that efficiency data, is based on the California Air Resources Board model, but we have actual data from the EPA. They have a livestock anaerobic digester database. And there are roughly 343, operating agricultural manure anaerobic digesters in the United States. And based on that data, if you have a covered lagoon, they estimate at roughly 50% anaerobic digestion efficiency, specifically for dairy. There's not a whole lot of data for swine in that database right now. So that's based on real data. So we estimated between 42% to 78% based on real data from the EPA. You're looking at about 50% efficiency. So that's 50% times your 1.65 cubic meters of methane per cow per day, and that's real data from the EPA reported.

Dan Andersen:

So I think definitely the scale has to get a little larger on some of these farms to make them feasible. But there are other means for innovation. Community digesters taking co-substrates, and while we're not going to talk about that today, certainly, I think this is a time where a lot of the projects that we're seeing implemented are picking farms where it's models that they know are going to work, that we're not working as hard to make it work. But there is room for innovation here. And the innovation that you've been thinking about is in some of these northern climates, what would happen if we wanted to heat our lagoon? Will that improve performance, or are we still going to need to be roughly the same animal numbers? So as you talk about that, what were some things that you had to think about in terms of heating lagoon that would change the cost structure, the cost flow at a digester project.

Luke Soko:

Absolutely. So, what I did is essentially a case study for a dairy farm with about 1200 1200 cows. If you want to translate the manure extruded from cows to pigs, the manure extruded from 1200 dairy cows is roughly 18,000 pigs. Biogas production from 1200 cows is roughly 14,000 pigs. If you have an existing manure storage that's holding six months worth of manure for 1200 cows, you're looking at something that's very large. You're looking at a manure storage that's 15 feet deep in one foot of freeboard, so you have six inches available for a 24 hour, 25 year storm event. You're looking at a very large manure storage, maybe roughly 90 meters long, 30 meters wide. You're only going to have about 3.5 meters of depth. So think about just the dimensions of that. You got about 3.5 meters of depth, like 90 meters long, 30 meters wide, essentially, I mean, it's like, it's like a disc, it's a square. It's like, it's like a floppy disk. It's a shape that's very difficult to retain heat in. So if you have an existing kind of lagoon manure storage with that, with the with that, just that geometry, that's a that's a shape that's difficult to retain heat. So what I looked at is, what if you could just, okay, so we have this manure storage, we know that if we heat this manure storage to roughly 37 degrees Celsius, so we're going to increase our anaerobic digestion efficiency, we're going to make more biogas. We're going to make more methane, why don't we just heat it? We could just get a heat exchanger system, a boiler system, heat this manure storage and we could make more gas, and we'll make more money. The problem with that is, when you have a shape that has terrible heat retention, and this is also an existing manure storage that's not insulated, what you get is a lot of heat loss. You're losing a lot of heat, and let's say to heat that system, you're using your own gas that you're producing, you're producing this methane, and then that methane, part of that methane stream, is going to go into a boiler system. It's going to heat hot water pipes. Those hot water pipes are going to go into the lagoon system. Or you could do a direct heat exchanger. But whichever you're, you're heating this system using your own gas. Okay, if you have, if you have a shape and with no insulation, that's losing a lot of heat. It's not essentially, it's not going to be worth your time to heat a manure storage that is not insulated, that is not designed for heat retention. The amount of gas that you use to heat your anaerobic digester divided by the amount of gas that you produce, in general, is called the parasitic load. It's the amount of gas that you need to use. That's that's taken away from what you're selling, the amount of gas you need to use. And that that parasitic load, if you're in Miami, and you're heating this 90 by 30 by three and a half system, if you're in Miami, you're looking at a parasitic load around maybe 15, 20% that's pretty typical of a digester, I guess, a real digester with insulation, you might get down to 10, 8% parasitic load. That's common now that's in Miami, though, if you, if you go up through the United States, you go up to Spokane, you could have a parasitic load around 60%; 60% of your gas is being used to heat your digester, and that is awfully silly. That's not a great system to produce gas and put it into the pipeline. So what we found is that, I mean, it seems intuitive that if you have a manure storage, all you would want to do is heat it up, and you'd make more gas, and you'd put that gas in the pipeline. Be happier. But if you have an existing manure storage, not an insulated or designed digester, it seems that you don't want to heat it, you're going to lose way too much heat. You have a you have a you're heating a disc that's not insulated. It's not going to retain the heat. Well, you're going to lose a lot of heat. And also keep in mind that when we have these lagoon digesters, if you have six months of storage, that's a very that's a long retention time. So it's, I mean, you can heat, but typically a heated digester, you don't have 180 days of retention. You'd normally have 30 to 60 days of retention, or less, even if it's mixed and heated,

Dan Andersen:

Yeah, and I think that's important to consider, right? So heating seems to be intuitive, like, I'm going to make more gas, but unless you have that waste heat hanging around for you to use, it's really difficult. And I think one of the things that I'd point out there is, with our current policy, we're really encouraging renewable natural gas production. If we go back to the model we were using in the 2000s where it was electricity production, well, if we're combusting it in a generator, a genset making electricity, we end up with a fair amount of waste heat that we can use to heat a digester. But in modern systems, that's not as practical with what's currently been encouraged. But big take homes from what you've done is certainly heating seems to make some sense. We could get more biogas production, more methane production, and in doing so, we get more consistent production that hopefully would help reduce some of the system some of the system costs, especially related to the biogas upgrader. But if we're going to do that, we're going to have to figure out ways to better insulate that digester, make it more practical for attaining heat so the parasitic load isn't as high. You've done this now. What do you think the next steps are? Where do we go from here? So I mentioned one of the things that we could do is thinking about making electricity instead of natural gas. Would that change the sort of outcome of where you ended up, or is that still not enough heat?

Luke Soko:

Yes, that's an excellent, excellent strategy is by using the combined heat and power models, where you have regenerating electricity. Right now, the electricity is not quite as incentivized as renewable natural gas production via the carbon credits, but that could change. People are looking into changing that.

Dan Andersen:

And especially if we saw that that was a effective way to operate a system. It provides some stability. The other thing I wanted to comment earlier on is we thought maybe biomass production and digestion might be feasible to hit some kind of economies of scale. But if we're going to put into a lagoon, that might not be our best option, and it would certainly have to be heated to do that if we even wanted to give it a try. Otherwise, we're going to generate a fair amount of sludge. But I think you gave us a lot to think about today, a lot of good thoughts on what the future of digestion might be, and certainly we've seen some examples of how it's implemented. But in order to reach more farms, we have to think about what it could look like, or what possibilities are out there.

Luke Soko:

Yeah. And in order to reach more farms, other possibilities so we could put in, we could increase the amount of biomass in the digester, and that could come from corn stover, perennial ground cover, other things. Or we could also look at systems, community systems, where we're leveraging pipelines and leveraging trucking to community injection points, community upgraders. That's another thing we could leverage. Also, so this, this case study I did. We didn't have any insulation. We could add insulation into these systems. This is just saying, hey, you need insulation, which, in hindsight, well, of course we need insulation. That's an excellent idea. So we could insulate. And one, one quick issue with unheated digesters is that you have massive peak flows in the summer, and your biogas upgrader has to be sized to handle these massive peak flows. If you have a heated digester, an advantage could be you have a steady flow rate of biogas going into that biogas upgrader. You could size that equipment more appropriately, maybe smaller equipment, maybe it's a cheaper upgrader. So that's an option. So what we see is, I mean, we need to insulate, we need to insulate, and we need to find ways to get multiple farms involved in the same project and achieve economies of scale so that we can get profitable projects.

Dan Andersen:

All right, that sounds great. I think there's a lot of hope on the horizon, but definitely room for innovation in figuring out what these systems can or need to look like to work for many farms within Iowa and the midwest. Thanks for your time today, Luke. And thanks to our audience for joining us. Thank you for joining this installment of Talkin' Crap. Be sure to take a look at the show notes on our website for links and materials mentioned in the episode. For more information or to get in touch, go to our website, www.extension.iastate.edu/immag/. If you found what you heard today useful or it made you think, we hope you subscribe to the show on your podcast app of choice. Signing off from a job that sometimes smells, but never stinks. Keep on talking crap. This material is based upon work supported by the US Department of Agriculture, Natural Resources Conservation Service under a federal award number NR233A750004G072. USDA is an equal opportunity provider, employer and lender. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect views of the US Department of Agriculture, Natural Resource Conservation Services. In addition, any reference to specific brands or types of products or services does not constitute or imply an endorsement by the US Department of Agriculture for those products or services.