Oxygen generators for biogas hydrogen sulfide removal
Recent trends including the green transformation and rising energy costs have increased demand for biomethane from biogas. To produce biomethane also known as renewable natural gas (RNG), carbon dioxide (CO2), hydrogen sulfide (H2S), and water need to be removed from biogas. As part of a multi-step approach, oxygen (O2) generators can be used to remove H2S. These machines inject oxygen into a digester that processes feedstock waste. They are also used in conjunction with other technologies for best results.
In this article, we'll focus more specifically on oxygen generators for biogas in relation to H2S. This is because the gas is a byproduct of the waste used to generate biogas. If not dealt with, highly corrosive H2S can lead to issues in energy production. In fact, most natural gas grids have allowable limits of the gas. After a brief introduction to biogas, read on to learn about why oxygen is the optimal method for H2S removal.
What is biogas and biomethane?
When biogas is made, different types of waste are used. This includes agricultural, organic, and industrial waste. Biogas is essentially formed when this garbage decomposes, known as "anaerobic digestion." Since waste contains high levels of sulfur compounds, including H2S, they need to be separated out. In addition to causing corrosion, H2S is bad for the environment.
As mentioned, removing H2S is an essential part of turning biogas into biomethane. The multi-step process generally involves a digester, biogas blower, low pressure refrigerant dryer, biogas compression, and PSA or membrane CO2 removal and/or other technologies. CO2 recovery is also possible and is a way for biogas plants to add revenue.
Like described earlier, oxygen is introduced in the digester where it is microdosed for H2S removal. It is in the digester where waste decomposes and is turned into gas. Therefore, biomethane is biogas that's been cleaned and is considered the same as natural gas.
Why use oxygen for H2S removal?
There are several approaches for removing H2S from biogas. Common ones include oxygen dosing, and ferric dosing. When considering the best approach, it’s important to consider that oxygen dosing tends to require lower maintenance and is generally more cost effective.
With ferric dosing, ferric salt / chloride needs to be regularly replenished. Similarly, activated carbon filters require replacement, meaning additional operational expenses. You'll find these costs apply to other methods, described below. In addition, we explain why oxygen dosing is the optimal method.
Additional methods
Other approaches include iron hydroxide scrubbing, sodium hydroxide (NaOH) scrubbing, and iron oxide pellets. The first two involve a chemical process, requiring regeneration and careful waste management. With iron oxide pellets, it's important to correctly store pellets for use, and regeneration can be complicated.
Water scrubbing uses readily available water, but it is not good for biogas with high H2S levels. Also, the H2S saturated water needs to be properly disposed of.
Oxygen generation: an efficient, low-cost alternative
As an alternative to all these, it's possible to introduce oxygen as a means of lowering H2S levels. This occurs through a biochemical reaction between oxygen and H2S. This aerobic reaction converts H2S into sulfur and water. With this, it’s important to use the right oxygen levels. This is controlled within the range of 0.3-3%. Typically, 1% of oxygen is needed in the biogas to produce the best results.
As mentioned in the introduction, an on-site oxygen generator is optimal in this scenario. Without the need for chemicals, it's a more sustainable approach vs. ferric dosing, for example. In addition, the maintenance required is relatively low to ensure a low total cost of ownership (TCO). We further explain oxygen generators below.
Considerations for oxygen purity
When using oxygen to turn biogas into biomethane, it's important to pay attention to O2 purity. If the levels are too low, too much nitrogen (N2) can be introduced. This reduces the energy of the biogas. On the other hand, too much oxygen (purity and flow) can be unnecessary and might introduce safety risks. Therefore, it's recommended to work with knowledgeable gas specialists well-versed in correct oxygen purity levels for biomethane production.
On-site oxygen generators for biogas
As this article explains, the optimal way to control your oxygen supply is with an on-site generator. This equipment lets you set purity levels, which means mitigating the risk pointed out above. You'll also save money by not generating unnecessary oxygen purity.
While it is possible to work with oxygen deliveries, this can create bottlenecks in your production. You'll avoid logistics with on-site generation. In addition, delivery is more damaging to the environment through CO2 generated by transport vehicles. These, among other benefits, are covered in our related article on oxygen generation.
We're here to help
We hope this article provides a good overview as to why it's beneficial to use on-site oxygen generation to produce biomethane. If you'd like more information about the topics covered, our team is happy to help. We're available to discuss our full range of on-site oxygen generators. Feel free to contact us today.
