HOW DO YOU REMOVE H2S FROM BIOGAS?


Feb 11, 2021

The gaseous compound, hydrogen sulfide (written as H2S), is a hazardous byproduct of many biological and industrial processes including the creation of biogas. It’s usually critical to separate the H2S gas from desirable or less hazardous gasses before they can be further refined. This is not, however, a simple matter. Fortunately, modern techniques are making it easier and more economical to eliminate hydrogen sulfide from other gaseous products.

WHAT IS HYDROGEN SULFIDE?

As its chemical formula suggests, H2S is composed of two hydrogen atoms and one sulfur atom. At room temperature, it manifests as a colorless gas with a characteristic “rotten-egg” odor. More critically, hydrogen sulfide is highly flammable and is toxic if inhaled. Once released into the atmosphere, it combines with oxygen to form one of the major components of acid rain. 

Hydrogen sulfide is released from natural sources such as hot springs and volcanic hotspots. It is also a byproduct of the anaerobic bacteria that break down organic matter in the absence of oxygen. This means H2S shows up in significant quantities in the guts of herbivores like cows, in landfills and wastewater treatment facilities, and even in poorly aerated swamps or peat bogs. The latter is the reason that hydrogen sulfide occurs in natural gas and crude petroleum – those products were simply buried plant life from millions of years ago.

H2S is also a byproduct of some industrial processes. For many years this undesirable gas was either released or flared off along with methane, but that practice led to the dangerous rise in acidic rain that took a concerted international regulatory effort to control. 

Even if contained, H2S poses risks to onsite workers. Though it initially smells strongly of rotten eggs, the gas can quickly overwhelm the brain’s olfactory center. Even short-term exposure to high concentrations can render a person unable to detect it without special equipment. And since H2S is heavier than air, it tends to accumulate in poorly ventilated areas. Breathing highly concentrated hydrogen sulfide can lead to quick incapacitation and permanent lung and brain damage.

As if that weren’t enough, the byproducts created by H2S combustion are corrosive. Though it is often associated with methane fuels, hydrogen sulfide will damage power generators if it is not first removed from the fuel gasses.

Nowadays, most industrial H2S is required to be scrubbed out before waste products can be safely used or released. 

H2S REMOVAL

Unfortunately, separating H2S from other, more useful gasses like methane is not as simple as running it through a filter. The hydrogen sulfide must chemically adhere to some other material and precipitate out of the gas mixture.

For decades, the most common extraction media has been composed of iron oxide (FeO). The raw output gas is passed through beds of FeO pellets. This gives the H2S an opportunity to bind to the media before the gas is forwarded to more delicate equipment. 

Unfortunately, this traditional process is quite inefficient, so a significant amount of removal media is required in order to extract much H2S. Additionally, iron oxide gets saturated quickly, so it must be replaced often. 

Improved H2S removal options were clearly needed. Extensive research and experimentation ultimately yielded a product called FerroSorp which improved on nearly every one of the old FeO’s performance statistics.

FERROSORP H2S REMOVAL MEDIA

FerroSorp is based around iron oxide-hydroxide, or FeO(OH). In contrast to its simpler cousin, this iron hydroxide formula can absorb far more H2S per unit volume and does so at a higher rate. Even so, it does not get saturated nearly as quickly, so beds of FerroSorp can remain in place up to three times as long as traditional FeO. The FerroSorp material is also less dense, which confers a dual benefit; it is easier to transport than FeO media, and it offers more surface area onto which the hydrogen sulfide can adhere.

Following the initial capture of H2S, the loaded FerroSorp will undergo a secondary “regeneration” reaction as long as enough moisture and oxygen is present in the surrounding air. The H2O and O2 will ultimately replace the sulfur in binding with FerroSorp’s iron atoms, after which the sulfur itself is left in a powdery elemental form. The regenerated iron hydroxide can then once again bind with new hydrogen sulfide molecules, extending its useful lifetime.

As with any chemical removal media, FerroSorp does eventually become saturated to the point where it can no longer be regenerated. Fortunately, since FerroSorp converts the hydrogen sulfide into relatively harmless elemental sulfur, it is easy and inexpensive to dispose of. The FerroSorp and its byproducts are environmentally nontoxic.

FERROSORP MODE OF OPERATION

FerroSorp undergoes two distinct reactions when it encounters hydrogen sulfide. First is the initial absorption reaction:

Absorption:      2 Fe(OH)3 + 3 H2S → Fe2S3 + 6 H2                                – 62 kJ/mol

Note that the above equation assumes the FerroSorp’s active iron hydroxide has already encountered enough moisture to convert it into Fe(OH)3. Three moles of water vapor are required per mole of iron hydroxide in order to prep it. Without this moisture, the FerroSorp will still absorb H2S, but at a far less efficient rate.

Following the absorption phase, the loaded FerroSorp will typically undergo a secondary regeneration reaction:

Regeneration:  Fe2S3 + 1½ O2 + 3 H2O → 2 Fe(OH)3 + 3 S                         – 603 kJ/mol

Regeneration will naturally take place as long as there is enough water vapor and free oxygen available around the FerroSorp bed. As the iron hydroxide is restored to its original form, molecular sulfur precipitates out as a powdery coating on the FerroSorp pellets.

The two reactions combined can be expressed as:

3 H2S + 1½ O2 → 3/8 S8 + 3 H2                         ∆G° = – 665 kJ/mol

It must be noted that both reactions are exothermic, and the regeneration reaction gives off about ten times as much energy as the absorption one. Temperatures around the FerroSorp bed must be monitored for safety purposes; in the presence of oxygen, sulfur can ignite above temperatures of 190° Celsius (374° Fahrenheit).

MOISTURE

As mentioned above, the moisture level in the gas stream plays a significant role in the hydrogen sulfide removal process. A minimum water moisture level of 40% is necessary to keep the reaction as efficient as possible. The hydrogen sulfide will ionize as it interacts with H2O in the gas stream. Ionization primes the sulfur content to bond quickly with the removal media’s iron atoms. 

Not every gas stream in need of purification is the same, of course. Some have outputs with exceedingly high moisture content while others are much drier. Naturally, it’s rare for one product to deal well with all situations. To accommodate various site needs, there are two versions of FerroSorp, designed to be used at different relative humidity levels. FerroSorp SD works best in 40%-60% humidity gas streams while FerroSorp SK is designed for 60%-100% levels. 

OXYGEN LEVELS

Because free oxygen is an important part of the regeneration reaction as well, it also must be present in the right quantities to promote maximum H2S removal rates. If a concentration of 1000 ppm H2S must be removed, 0.2%-0.4% oxygen in the gas stream will help to yield the maximum removal rates. It can be injected into the gas stream as pure O2 or ambient air can be used. In the case of the latter, between 1% and 2% of air would be sufficient.

Removal of higher H2S concentrations, of course, requires higher concentrations of oxygen. Your O2 input should be tailored to your specific H2S requirement.

CONSTANT SYSTEM PRESSURE

Some H2S removal solutions tend to expand and clump up as they extract hydrogen sulfide from the gas. This can cause the pre-filtration pressure to rise as the media becomes increasingly saturated. 

The presence of the pellet bed does, of course, have some effect on the overall system pressure. Interra Global has tested a range of flow rates to help customers estimate the expected pressure drop. But over the life of any given batch of pellets, you can expect this pressure differential to remain effectively constant. 

DISPOSAL

Though the FerroSorp pellets last longer than other H2S removal media, they will eventually become saturated. A powdery grey coating of sulfur precipitate will develop on the pellets and eventually prevent new H2S from reaching the iron hydroxide surface. Once it gets to this point, the batch of pellets needs to be replaced.

FerroSorp media is less dense than other iron oxide media, so it is easier to handle and transport. As long as it has not encountered much in the way of liquids, the expended pellets can be easily vacuumed out of their bed. 

A dust mask, safety glasses, and gloves are recommended when handling the media. This is particularly important for the saturated pellets because the sulfurous dust that collects on their surfaces is easily brushed off. The sulfur is environmentally nontoxic but is not good for the lungs or eyes.

The spent FerroSorp media does not require hazardous-material handling during transport or disposal. Many landfills that accept household waste will also readily take the FerroSorp.

FERROSORP BENEFITS AT A GLANCE

FerroSorp is superior to other existing hydrogen sulfide removal methods in nearly every metric. In summary of its advantages: 

  • FerroSorp’s H2S removal loading rates far exceed any similar product. It can be expected to absorb >40% by weight when properly configured.
  • Iron hydroxide is the highest-purity iron compound that functions as an H2S removal agent.
  • Properly employed, FerroSorp brings the outlet H2S concentration down to safe levels, ensuring that it meets or exceeds strict environmental regulations.
  • The regeneration reaction helps FerroSorp last longer. Case studies indicate that the same volume remains effective 2.5-3 times as long as competing media.
  • Users experience less caking/clumping than with other media. This makes FerroSorp easier to change out, reducing maintenance time and costs.
  • Under most site specific local regulations, loaded pellets are environmentally nontoxic. No post-processing is needed, so disposal is simple and inexpensive.
  • Pre- and post-purification pressures do not vary significantly over the life of the media. Because FerroSorp pellets do not expand or cake up as they extract H2S, they don’t create a significant pressure differential.
  • Minimal maintenance is needed between changeouts. FerroSorp requires no daily water sprays like some media, so no water disposal or treatment is necessary.

If you’re in an industry that produces methane in any form, you probably know all too well the challenges of hydrogen sulfide removal. FerroSorp has proven itself to be the most efficient and cost-effective H2S removal media on the market. As North America’s FerroSorp provider, Interra Global can answer any further questions you may have on its use. Contact us today at:

Interra Global
800 Busse Hwy
Park Ridge, IL 60068
Phone: 1(847)892-6593
Email: sales@interraglobal.com

The Author | InterraGlobal

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