What’s The Chemical Composition Of Biogas?

“Hey, I’ve been running a biogas digester at home for a few months, and it’s been working fine so far, but my gas production has started to drop off lately and the flame is burning yellow instead of the usual blue. I think the composition of the biogas might be off. Can you explain what the typical chemical makeup of biogas is and what can impact it, as well as what I can do to troubleshoot this issue?” Thanks, Frank, Hamilton, Canada.

Hi Frank! Sounds like you’re already familiar with biogas, which is great, but I totally get that it’s frustrating when things don’t run as smoothly as usual. Let’s get into the nitty-gritty of the chemical composition of biogas and explore what could be causing that yellow flame of yours. I’ll also touch on some troubleshooting tips so you can get that nice, clean blue flame back in no time.

What Is Biogas and Why Is Its Composition Important?

Biogas is essentially a mix of gases produced through the anaerobic digestion (when organic matter breaks down in the absence of oxygen) of organic materials like food scraps, animal manure, and plant remains. It’s an eco-friendly and renewable alternative to fossil fuels. However, the specific chemical composition of the biogas is what determines its usability and efficiency.

Analyzing and understanding the ratio of different gases in your biogas is crucial because it impacts the energy content of the gas and how cleanly it burns. For instance, a gas mixture with a higher concentration of methane (CH₄) will burn cleaner and hotter, leading to a blue flame, while a higher amount of impurities like hydrogen sulfide (H₂S) could cause safety concerns and even equipment damage over time.

The Main Gases in Biogas

Let’s break down the main chemical components that are typically found in biogas:

• Methane (CH₄) – 50% to 75%: The star of the show! Methane is the primary component of biogas and the one responsible for generating energy. The higher percentage of methane, the better the fuel quality. A typical household biogas system aims for a methane content of 60% to 70% for optimum output.
• Carbon Dioxide (CO₂) – 25% to 45%: CO₂ doesn’t contribute to the energy generation directly, but it’s always present. A biogas mixture with a very high CO₂ ratio produces a weaker flame, which could be what you’re seeing, Frank.
• Water Vapor (H₂O) – 0.3% to 5%: Water vapor usually makes its way into the gas as the organic material breaks down. Over time, excessive water can accumulate and lead to condensation issues within your pipes or digester system.
• Hydrogen Sulfide (H₂S) – 0% to 3%: This is the notorious rotten-egg-smelling gas. Although it’s typically a small percentage of the mix, hydrogen sulfide is important to monitor as too much of it can turn your biogas toxic and can corrode your equipment over time. Your yellow flame could be an indicator of higher levels of this impurity.
• Oxygen (O₂) – 0% to 1%: Ideally, you’ll want as little oxygen as possible in your biogas. Oxygen gets inside the system when there are leaks or flaws in the process. Not only does it reduce the efficiency of methane, but it creates the risk of explosion.
• Other Gases (N₂, NH₃, CO, H₂): In small quantities, gases like nitrogen (N₂), ammonia (NH₃), carbon monoxide (CO), and hydrogen (H₂) may also be present. While they wouldn’t normally pose a problem, high levels of any of these indicate something’s gone wrong with the anaerobic process.

What’s Causing Your Yellow Flame?

A yellow flame in your biogas system is usually a tell-tale sign of incomplete combustion, which, most often, means one of two things: either your methane concentration is too low, or another gas is interfering with combustion. So, Frank, here are a few key factors you’ll want to check:

1. Low Methane Content

If the methane concentration has dropped, your overall energy production takes a hit. This could be happening due to:

• Too much carbon dioxide: A high ratio of CO₂ steals space that should be occupied by methane. Look at the material you’re feeding your digester – food scraps, plant matter with high moisture or carbohydrates can sometimes produce more CO₂ than methane. Try to adjust your mix by adding some manure or drier materials.
• Insufficient organic material: If you haven’t been feeding enough organic matter into the digester, your system might not be producing methane at adequate levels. Make sure your digester is working with a healthy input volume every day.

2. Too Much Hydrogen Sulfide

A yellow flame with a detectable sulfur smell? That’s usually too much hydrogen sulfide at work. If H₂S levels get out of balance, you end up with a tainted biogas.

• Identifying H₂S: You’ll know this one by smell. Rotten eggs in and around your system indicate H₂S production is higher than it should be.
• Possible sources: Protein-rich materials like meat scraps or certain manures can spike your H₂S. If you’re adding things like that to your reactor, try reducing the quantity.

To remove H₂S, many home biogas users install a simple hydrogen sulfide scrubber (usually made with iron fillings or activated carbon); those absorb most of the H₂S before it reaches your burning equipment.

How to Test and Monitor the Chemical Composition of Your Biogas

Let’s talk about how you can measure what’s happening inside your digester. Professional gas analysis equipment might not be in the cards for smaller home setups, but don’t worry, Frank, there are easier ways to keep tabs on your biogas quality:

• Basic flame test: Continuing to notice flame color changes in your burner gives you a prime indicator of methane levels versus CO₂ or H₂S. Blue means it’s burning well, yellow means something’s out of balance.
• pH testing: Measure the pH levels in your digester regularly. Ideally, you want it to be in the range of 6.8 to 7.2 for optimal gas production. A drastic pH drop could mean that the acidic environment is leading to slower methane production and a higher CO₂ concentration. Using chalk or sodium bicarbonate can help balance the pH on the fly.
• DIY gas bag tests: Some people use plastic balloons or bags to trap biogas and then test its combustion properties separately — this can lead to a more controlled mixture and understanding of gas quality within certain time frames.

Maintaining Your Biogas System’s Balance

Getting good-quality biogas is all about balance. You’ll need to continue keeping close tabs on what you’re feeding into the reactor. Here’s a quick recap of material composition to focus on:

• Greens: Give these just enough. Food scraps with high nitrogen content (like veggie peels or coffee grounds) help keep the bacteria happy and producing methane. However, too much nitrogen can actually lower methane efficiency by increasing the non-combustible gases too much.
• Browns: Adding drier, carbon-rich materials, like straw or dry leaves, helps stabilize methane production and keeps CO₂ from getting too high. These sources provide a steady source of ‘bulk’ to your reactor.

Addressing Your Yellow Flame: Step-by-Step Troubleshooting

So, Frank, here’s a quick troubleshooting guide for getting that yellow flame back to the blue you want:

1. Check your feedstock: Review what you’ve been feeding into the digester in the past couple of weeks. Have you added large quantities of acidic or moisture-rich materials recently?
2. Test the pH: Test the pH to make sure you’re not dealing with too much acidity affecting methane production. Neutralizing acids might help.
3. Install a hydrogen sulfide filter: If you’re noticing strong sulfur smells, invest in a simple scrubber made with iron oxide or activated charcoal. This will attack the H₂S problem directly.
4. Increase your methane-producing feedstock: Introduce more manure or other high-methane producing materials to balance out excess CO₂. Make sure you are not over-filling the digester, as this can reduce the methane concentration.
5. Check moisture levels: Water vapor can accumulate in colder areas of the system and block gas flow. Dry things out before the gas hits the burner.

Final Thoughts…

Frank, I want to thank you for raising this awesome question. Understanding the chemical composition of biogas and how it affects your system is a huge step toward running an efficient and effective home energy setup. You’ve already got a running system, and that’s a huge win. Just keep tweaking the input, checking the gas quality, and watching that flame to make sure your methane levels stay in the sweet spot. Good luck!