Interest in renewable natural gas (RNG) is growing throughout the United States and the world.
The production of RNG has a lot of upsides. RNG represents a renewable energy source that countries can use to reduce greenhouse gas emissions. RNG provides clean energy and effective waste management.
He walks through the steps of how to produce RNG using organic waste like crop residuals and explains how it can reduce methane emissions.
Intro to Renewable Natural Gas (RNG)
I want to talk a little bit today about renewable natural gas.
There’s a lot of technologies out there today that are being used to develop renewable natural gas. And I’m just gonna walk through a few of those for you so you can understand where it is that renewable natural gas comes from and how we can produce a product that can be sold into the natural gas market.
Hi, I’m Shane Torwalt. I’m the Engineering Director here at Vista Projects.
I want to talk a little bit today about renewable natural gas.
So renewable natural gas in general is basically just a product of biological decomposition.
Essentially you start with a feedstock and it can be any organic material practically and then you can get biogas out of that.
Biogas is not a value in its natural form. So it has to be refined and improved upon in order to make it into a saleable product.
But most of the biogas starts as a feedstock coming from either crop waste, animal waste, food waste it can come from, it can come from municipal waste and even wastewater sludge can be a bio stock for this type of biogas processing.
Any of these can be effective in starting as a starting point for your renewable natural gas project.
So the first stage in trying to generate this fuel gas is trying to get it into a gaseous state.
So you have all that material, it arrives at what’s known as an anaerobic digestion facility and within that facility are large tanks, everything gets dropped off and biologically the material starts to get broken down.
The thought is that over time you can start to generate methane up to 65% of the gas is methane as well as CO2.
These are the two primary gases that come off of a biologically processed feedstock material.
The thought is then that you still have impurities like H2S and siloxane and some volatile organic carbons. Those need to go, but you have to eventually produce a fairly refined methane stream.
There is still solids at the bottom of your anaerobic digester, and those are useful too. You can use those as fertilizer or any kind of soil improvement that is applicable for the nutrients that are left in those solids.
So the thought is that when once the RNG comes out of the anaerobic digester, it’s always saturated with water.
So the best way to handle your wet renewable natural gas is to cool it using some sort of technology and the technologies that are most common are the gas/liquid heat exchanger or you can use a refrigeration system or you can even use a desiccant style water removal system.
All three are very common and very effective at pulling that water out of that gas in order to eventually use it further downstream.
But even dry biogas has those impurities that need to be dealt with.
And those are dangerous gases like H2S which are poisonous to humans and people and in animals.
And the siloxane as a falling material that needs to be dealt with because if you use this gas with siloxane in it as a fuel, the fuel then is contaminated and it can damage machinery.
So the best purification technologies for getting rid of H2S and siloxane are Silica Gel Filters.
They can take care of siloxane and H2S via adsorption.
You can use activated carbon filters, those are very effective.
And then there’s some commercially available items as well like SulfaTreat or iron sponge both of those react chemically with the iron oxide to be able to get rid of your H2S and your siloxane.
These are pretty effective means of dealing with this primary impurity that needs to be removed.
The other one, of course, is what we talked about at the beginning of the presentation, which has to do with CO2 (carbon dioxide).
We mentioned that there’s 65% of the gas is methane and the other 35% of the gas is CO2.
So that’s a lot of CO2 that you go to deal with.
The most effective ways to get rid of CO2, of course, are either dry extraction or wet extraction.
The dry extraction comes in terms of membrane separation technology or pressure swing adsorption. In these cases, the size of the molecule, the CH4 molecule versus the CO2 molecule, the size difference as well as any kind of pressure difference in the system allows you to do the physical separation of those species.
If you’re looking at wet CO2 extraction you want to use amines or even a water wash, either one will do a good job in removing the CO2 ’cause the solubility of CO2 in those streams is much higher than it is for CH4.
So the methane will flow through easily.
This initial slide here talks a little bit about membrane separation technology.
Just to be more specific, you’re looking at multiple cylinders of membranes that allow the separation to occur physically.
It’s usually multiple stages. That’s a very effective and efficient way.
You can get better than 99% efficiency as far as CO2 removal goes.
Very simple and low maintenance system. You can do pressure swing adsorption.
Again, these are multiple vessels in different stages of service.
One vessel is gonna be in service at all times in the meanwhile, other vessels are gonna be in various stages of regeneration.
You have to be able to regenerate the adsorbing material.
It’s often, you know, a carbon-based molecule sieve or maybe a zeolite that’s in those vessels.
Either way, you have to be able to put a vessel in service regenerated and then get that vessel back into service in a continuous version.
A very effective way of getting rid of your CO2 from your renewable natural gas.
The liquid strategy as I mentioned, basically you’re circulating a liquid that can grab that CO2 because of the solubility of CO2 in that liquid stream.
Amine is a very effective and traditional approach. It also can grab residual H2S in case your upstream H2S capture was not entirely effective.
So in this case, the methane then just comes out of the first tower and can now be refined or purified slightly further in order to become your fuel gas.
The amine can be regenerated using heat and that way you’re not wasting amine all the time.
You can just recirculate it through the system very effectively.
The last one here is just a water wash. It turns out that water is also very inexpensive for low flows type of system that you might want to use.
Water gets circulated through the system.
Again, the CO2 dissolves into the water stream, and just like a pop bottle you can now extract the CO2 out of the water in order to reuse it as long as you drop the pressure in that tower two.
The idea is that then CO2 can be then captured and may be repurposed as another industrial gas.
So now that you have your renewable natural gas fairly purified at this stage, the goal then is to use it for some industrial purpose.
You can make power out of it using an internal combustion engine.
The idea here is that you can make up to five megawatts of power from just a single renewable natural gas facility.
That power then can be sold to the grid or it can be repurposed around the plant as well.
Either way, it’s a very cost-effective way of making power for your facility.
And then the other option with renewable natural gas is that it can be compressed and turned into a mixture that can be delivered into a pipeline system.
As long as you’re at the right pressure, it can be blended in with fossil fuel-based natural gas networks and become part of any aspect of a natural gas distribution network that people see at their homes to heat their homes and various other requirements.
So thank you for listening. Keep in mind that if you have an RNG project upcoming or any kind of biofuels project upcoming, Vista is eager to help.
We can provide you with all kinds of solutions around engineering and design, and we’re happy to be part of the next steps of your biofuels project.
RNG Facility Engineering
Renewable natural gas production is a promising sector. Ramping up saleable RNG volumes could have heavy-duty implications on the residential power and transportation sectors.
Vista Projects is an integrated industrial engineering and system integration firm with experience in gas processing, biofuels, and more.
Contact us today if you need help with your RNG project. We would be happy to discuss your processing facility.