This page lists questions and answers from the C-Change GrassToGas project’s PA Extension newsletter. To be added to the newsletter mailing list, contact GrassToGas@psu.edu.
1. Question: Is switchgrass a good feedstock for biogas digesters?
Answer: Not usually. Anaerobic digestion involves microscopic organisms that eat energy-rich components such as sugars. Lignocellulosic biomass, such as switchgrass, contains sugars, but is structured in a way that specifically prevents easy access to its energy-rich components. Because of the limited accessibility to the energy-rich components in switchgrass, it is not considered an ideal feedstock for biogas digesters.
2. Question: Why did my digester fill up with grey sandy material?
Answer: It’s not unusual for a PA farm digester to fill up slowly with a gritty solid material, necessitating a time consuming cleanout once every 10 years or so. This troublesome material is likely a combination of two things: Solids, and Struvite.
Dense Solids, such as sand or other grit, can be carried into the digester along with the regular feedstock, and owing to its higher density, settle down to the bottom.
Struvite is a mineral that actually is created inside the digester when components in the feedstock react to form Magnesium Ammonium Phosphate. According to Robb Meinen of Penn State Extension, Struvite can collect in the digester, and can also coat the inner surfaces of pipes, pumps, and other equipment. It can be a problem.
Ideally internal mixing in a digestion vessel would suspend solids in a manner that allows them to exit in routine effluent. The traditional way to get rid of unwanted accumulated material in a digester, such as dense solids or struvite, is by agitating the digester slurry and pumping it out. However, since most digesters operate on a continuous fill / continuous drain basis, this approach is difficult to make work. As a result, many farmers have been stuck with pulling the cover off their digester and cleaning it out with digging equipment. If you’ve come up with a better way to control solids buildup in your digester, please let us know so that we can pass on the word!
3. Question: What is buffering capacity and why does it matter?
Answer: Buffering capacity is the ability of a feedstock to handle the addition of acids or bases without its pH changing much. This is important in a digester because changes in pH can cause the digester’s microbes to stop growing well, leading to reduced biogas production and sometimes even worse problems such as “souring” (see the previous edition of the Grass to Gas Update for details). Cow manure tends to have a high buffering capacity, which makes more stable as a biogas feedstock. Food waste usually has a lower buffering capacity, which means that food waste digesters are more prone to having pH problems.
4. Question: Is the Hydrogen Sulfide in my biogas dangerous?
Answer: Yes it certainly can be dangerous if it is not properly contained. Hydrogen Sulfide (H2S) concentrations in dairy farm biogas typically range from about 1000 ppm to 3000 ppm. At 700 ppm, one breath of the gas is enough to make you pass out. At 1000 ppm or above, death is almost guaranteed from just one breath. Even if your digester has a scrubber to remove sulfide from the gas, that usually only reduces the Hydrogen Sulfide concentration – it doesn’t eliminate it completely. Also, keep in mind that adding a new feedstock (such as food waste) can increase or decrease your biogas Hydrogen Sulfide level, depending on the sulfur content of the new feedstock.
So be very careful if you have a leak in your system, and be sure to maintain all pipes and fittings so that they are well sealed. Keep any nearby sheds well ventilated so that there is no chance of unexpected leaks causing a buildup of gas. While trace levels up to ~100 ppm are detectible as a “rotten egg” smell, higher concentrations are actually odorless. Low levels of Hydrogen Sulfide can cause headaches and irritated eyes and throats. So, if you notice any of those symptoms cropping up, check for leaks. Be careful out there – we need more farmers, not fewer.
6. Question: What’s the difference between Biogas and Renewable Natural Gas?
Answer: Lately, a lot of people have been talking about Renewable Natural Gas, or RNG. What is it, and is it any different than Biogas? While there’s no “official” definition, we usually use the term “Biogas” to refer to the gas that comes out of a digester (~60% methane, 37% CO2, 3% water vapor and small amounts of Hydrogen Sulfide and other impurities). Renewable Natural Gas, however, is biogas that has been processed to make it “pipeline grade” for injection into the natural gas pipeline network. Upgrading Biogas to RNG usually involves removing the water vapor, removing Hydrogen Sulfide, and removing Carbon Dioxide. The resulting RNG is almost completely composed of methane.
7. Question: Is biochar good for my digester?
Answer: It seems that every time you turn around you hear something about the benefits of biochar. This is true of biogas production as well. Laboratory studies have noted that adding biochar to a digester can increase biogas production and improve methane content of the biochar. The scientists think that this is due to biochar improving the efficiency of the chemical reactions that occur inside the biogas-producing microbes. Is the magnitude of the improvement worth the cost and hassle of adding biochar to your digester? That’s a question that we’re still trying to figure out, and are actually in the process of doing some full-scale testing with partners to see if we can measure improvements to their digester when biochar is being added.
8. Question: What is Siloxane, and should I worry about it?
Answer: Siloxane is a silica compound (silicon+oxygen+alkane, for you chemistry buffs) that is used in a variety of common items, including beauty products and spray sealers used in construction. If present in biogas, they have a tendency to form oxides that deposit in the engine generator, leading to significant wear and expensive premature breakdown of your genset. Siloxanes can only end up in your biogas if there are siloxanes in your feedstock, and the good news is that manure and crop residues normally don’t have any siloxane. To our knowledge, siloxanes never occur spontaneously in nature – they have to be man-made. However, farmers that accept outside materials such as food waste or other organics for their digester need to be careful that there aren’t any siloxanes accidentally included with that material. Something as innocuous as “a bit of nail polish” can cause significant problems in your digester’s engine generator.
9. Question: What is “digester souring”, and should I worry about it?
Answer: “Souring” is a failure mode of a digester, during which the acid-producing microbes outpace the methane producing microbes. This causes the pH to drop, which damages the methane producing microbes even more, and causes the acid producing microbes to become even more dominant. The final result of this “downward spiral” is a sour, goopy mess in your digester that doesn’t produce any biogas. You definitely don’t want your digester to go sour.
While digesters that use 100% manure don’t often have this problem, quickly digestible materials like food waste can be more problematic in that respect. If you are concerned about your digester souring, it might be worthwhile to invest in a pH meter, take daily readings, and be ready to add a basic chemical such as caustic soda if the pH starts dropping.
10. Question: How can I remove Hydrogen Sulfide from my biogas?
Answer: Removing Hydrogen Sulfide (or H2S) from biogas is a good way to make your biogas less corrosive to your equipment and less hazardous to people. While there are many possible approaches to this, the three most common methods that farms use are:
- “Iron Sponge” – Pass the biogas through a container filled with rusty iron filings or similar rusty iron material (something with lots of surface area). The H2S reacts with the rust, forming an iron sulfide deposit that stays behind in the iron sponge.
- Air Addition – bleed a small amount of air into the digester headspace, to encourage naturally-occurring microbes to consume the H2S without harming the anaerobic bacteria that are in the liquid slurry of the digester.
- Bioreactor – pass the biogas through a large container that is filled with a porous medium on which microbes grow that feed on the H2S, removing it from the biogas.
Additives such as Iron Chloride can also be used, but are not a common approach for PA farm digesters. Biochar addition is another possible method that has been talked about lately, but it is not yet clear how well that works or how much biochar would be needed. Which method is best for your farm? That probably depends on the specifics of your digester and your desired level of H2S.
11. Question: What is an eRIN?
Answer: An eRIN is an “electronic Renewable Identification Number”, which is used to provide incentive payments to people who produce bio-based electricity that is used in electric vehicles. The US Department of Energy has recently proposed the creation of eRINs, which could be a new revenue source for digester owners who produce electricity. It remains to be seen how they will relate to the state-regulated “RECs” or “Renewable Energy Credits”, but it could be that a farmer will be able to receive payments for RECs and eRINs from their digester-based electricity.
12. Question: How does biochar help digesters?
Answer: While its benefits in farm-scale digesters are yet to be fully confirmed, many laboratory studies have confirmed several benefits from adding biochar to a digester. This includes:
- pH control – biochar increases the pH of the digester fluid and provides a buffering effect.
- Biogas quality – biochar adsorbs contaminants, and can reduce hydrogen sulfide levels in biogas.
- Biogas amount – biochar improves the environment for the microbes in the digester, resulting in more biogas production.
- Digestate quality – biochar is known for its value as a soil amendment, so when you spread your digestate on the fields, you can expect to see those benefits translated over to your crops.
Are those improvements to the digester’s performance worthwhile when used in a farm digester? Maybe, maybe not. We’re partnering with several of you to carry out a demonstration project this year, so we’ll be able to provide some first-hand perspective on it before long.
13. Question: What’s the best feedstock for making biogas?
Answer: Normally a biogas feedstock should have the following properties:
- high biogas yield: more biogas is better than less, after all
- low cost: ideally the feedstock should be free or even come with a “tipping fee”
- high availability: large amounts should be available for use in the digester
- easy handling: feedstock should flow well through a digester system without causing clogging or foaming problems
- stability: not prone to “upset” or related problems
No one feedstock excels in all of these areas.
14. Question: What should I be budgeting for maintenance of my digester?
Answer: The sad truth of the matter is that most of us don’t save up for maintenance costs, which means that as soon as something major breaks on the digester, we’re out of luck. The big ticket items tend to be genset overhauls and digester cleanout. Genset overhaul intervals will depend on your gas quality and engine characteristics – check with your equipment supplier and find out what they charge to provide service to the unit. As to cleaning out the digester, many digesters have a tendency to fill up with grit every 10 years or so. Pulling off the cover and scooping it clean is not an easy task, and can cost $100k or more, depending on the size of your system – in part because the flexible cover will probably have to be replaced as part of the job. A detailed maintenance plan will give you a more precise number, but as a rule of thumb, it is a good idea to save up, every year, 1.5-2% of the digester’s installation cost. That way you’ll have much if not all of the maintenance cost saved up when it is time to carry out those expensive but necessary repairs.
15. Question: What’s Going on with E-RINS?
Answer: E-RINs, which are financial payments for electricity used in electric vehicles, were showing a lot of promise as a new income stream for digester owners when the EPA proposed allowing biogas-based electricity to be the first and only type of electricity eligible for E-RINs in the United States. All indications were that this would be rolled out in the middle of 2023. Well, what happened? While we’re not privy to all of the discussions that have gone on, it seems that the E-RIN for biogas has been scuttled, or at least dramatically delayed. Why? One can only hypothesize that someone thought giving biogas digester owners this exclusive new incentive might not be the right thing to do. At any rate, it looks like digester owners will have to stick with utilizing existing incentives, such as the net metering provisions and Alternative Energy Credit system in Pennsylvania.
17. Question: What is recirculation and how does it affect digesters?
Answer (thanks to Elmin Rahic, postdoc at Penn State): Recirculation, as the name suggests, is the recycling of digestate liquid back into a digester. It can help reduce your water use (which is important with dry feedstocks like switchgrass), and it also recycles key nutrients, organic matter, microorganisms, and heat. This can help to reduce heating costs (important if you’re producing RNG instead of electricity and heat) and increase biogas production…sometimes. Like everything, it can get pretty complicated.
Whether it increases biogas production will be largely dependent on how and what you’re feeding into your digester. As a rule of thumb, if your digester is mostly fed with nutrient-dense feedstocks like dairy manure or certain food wastes, then recirculation could result in toxicity from the over-accumulation of certain nutrients (ex. ammonia), thereby inhibiting methane production. However, in “Grass2Gas” systems, where grasses and/or agricultural residues contribute a large portion of your digester inputs, recirculation could significantly increase biogas production by recycling those nutrients which may be lacking in your feedstocks (ex. nitrogen or trace minerals).
18. Question: What causes foaming in my digester, and what can I do about it?
Answer: Foaming in digesters is a mysterious and annoying problem for many digester owners. It can have many root causes, which makes it difficult to diagnose and treat. Ultimately, digester foaming is associated with higher levels of surfactants (compounds that cause bubbles to last longer). Surfactants can come from many sources. They include soap, oil, grease, or proteins from your feedstock, but they can also include volatile fatty acids (a product of the digestion process).
Preventing or treating foam in a digester can involve a variety of approaches. This includes reducing the amount of foam-inducing feedstock, reducing variations in temperature or feed rate that can cause volatile fatty acid content to rise, adding “nutrients” that improve the health of the microbial community, or adding “anti foam” products that counteract the effect of surfactants. It’s often hard to tell which approach will work best for a farm’s digester, and sometimes the solution that works for one farm will not be effective for another.
19. Question: What does Methane Concentration tell me about my digester?
Answer: Most biogas has about 55-65% methane, with the remainder being mostly carbon dioxide, a bit of water vapor, and small amounts of hydrogen sulfide (plus a few other things). The carbon dioxide is the byproduct of metabolism in the digester’s microbes. Thus, there has to be some carbon dioxide given off because otherwise the microbes wouldn’t be alive. If the methane concentration of your biogas drops, it may be a sign that your microbes are unhappy, and are putting more of their energy into surviving than producing methane. This could be due to changes in your digester’s feedstock, changes in the way the equipment is operating, or other factors. Of course, it could also be due to a malfunctioning gas sensor. Regardless, if you see your methane concentration dropping, it’s a strong indicator that you should take a look at what’s happening in your digester. If you don’t have a biogas sensor built into your digester but you’re using your biogas to generate electricity, keep in mind that a drop in methane concentration will likely result in a drop in the generator’s kW output (although other things can cause that to happen too).
20. Question: What kind of microbes do I have in my digester?
Answer: All digesters have a big variety of microbes – and that population could vary from time to time as well as from farm to farm. One research study found over 1,000 different species of microbes in a series of digesters that they analyzed. The only way to know for sure what’s in your digester is to carry out a genetic analysis of your digestate. That being said, most of the microbes in a digester will fall into two main categories: bacteria and archaea. Most of us are familiar with bacteria – that’s what tends to spoil our food or infect our cut fingers if we don’t wash out the cut thoroughly. Archaea are a less well known type of primitive microbe that was first recognized for its uniqueness back in the 1970s.
In your digester, bacteria tend to be the microbes that break down the biomass into smaller and more digestible molecules, while the Archaea eat those molecules and produce methane. This is of course a simplification of a pretty complex process, but it’s a reasonable starting point for understanding what those living creatures are inside your digester. One of your objectives as a “microbe farmer” is to keep the bacteria and the archaea both happy, and in balance with each other.
21. Question: How does income from Biogas Electricity compare to producing Renewable Natural Gas for pipeline injection?
Answer: To get an answer to this question, we pulled together some representative production numbers from the EPA’s Agstar biogas estimator tool, and combined them with fairly typical selling prices for electricity and natural gas in Pennsylvania. Keep in mind that actual numbers will vary from place to place and from time to time. That being said, a 1,000 cow dairy using manure only, with a digester and generator might be expected to produce about $200k of electricity, $17k of heat, and $33k of Renewable Energy Credits for a total of about $250k of income per year.
If the same farm was producing Renewable Natural Gas instead, they might be expected to produce about $36k of natural gas and $235k of Renewable Fuel credits (D3 RINS), for a total of about $271k of income per year – not too different. Keep in mind that pipeline gas could be eligible for additional incentives that are not available for electricity production. Also, income is not the same as profit, and RNG production tends to have higher expenses than electricity production. So, with all of those caveats in place, it’s interesting to note that income from electricity production is mostly a function of the value of the electricity, whereas income from RNG production is mostly a function of the value of the credits.
22. Question: How can I remove CO2 from my biogas?
Answer: Most farms’ biogas is about 40% carbon dioxide (CO2) – created by the natural respiration of the microbes in the digester. The carbon dioxide doesn’t have any combustion energy content, which means that engine generators for biogas are usually producing about 40% less electricity than if they were running on pure methane gas.
There are several ways that the CO2 can be separated out from biogas, including membrane systems (squeezes the CO2 through the membrane, leaving behind the natural gas), pressure swing absorbers (CO2 sticks to a special material, leaving only the methane behind), and water washing systems (water likes to absorb CO2 much more than methane). They are currently in use at industrial scale gas treatment facilities, but haven’t really found themselves onto Pennsylvania farms yet – although a few of our larger farms are working on adding them so they can purify their biogas and inject it into the natural gas pipeline. Should you consider one for your farm? Be cautious – the costs are significant, and the benefits may be uncertain.