CO2 Extraction from Landfill Gas 3

[bonzoboy]

A landfill gas produced at 1,000 cfm is roughly 50% CO2. What technologies would be applicable to extracting the CO2 from the gas? Any idea of how much $ such a system might cost (ignoring the cost of CO2 compression)

 

[Montemayor]

Bonzo:

You don’t furnish sufficient basic data to allow answering your questions. Therefore, we have to make a series of assumptions. This makes it tiring, non-specific, and probably inaccurate – but that’s a result of little or no data. Here’s what’s missing and what I have to assume:

1. What is the flat, accessible land area producing the gas?
2. What do you mean by cfm? Do you mean standard cubic feet per minute as measured at 60 ^oF and 14.696 psia? I think you do, and that would mean 1,000 Scfm.
3. You don’t state the basis of your percentage. Do you mean mass % or volumetric (molar) %? I believe you mean the latter and will assume so.
4. What are the remaining components in the landfill gas besides the CO₂? I have to assume something so I’ll have to take methane (CH₄ as being the remaining substance. If you have significant oxygen, sulfur, chlorine, or other halogen compounds in the gas you are better off forgetting about this idea. I suspect you have (or would have) to contend with oxygen creeping into the mix.

The simplest and most direct technology would be absorbing the CO₂ with a 10 – 15% wt. solution of Monoethanolamine or other Amine. Your subsequent CO₂ product would be “food grade” and be very competitive quality-wise. The cost to produce this product, however, would be atrocious. Depending on the land area involved, your capital investment would easily run in the $millions. And this assumes you would have total access to the land. What is involved is:

1. You have to totally cover the land mass with a hermetic covering in order to keep invading oxygen out. This means a structural covering – however low in height. It would have to withstand the rigors of nature – such as snow loads, winds, rain, heat, etc. – and remain hermetic.
2. You have to maintain a positive pressure (however slight) within the structure in order to permit the positive suction of the gas and also keep oxygen out. Of course, this means structural integrity and more cost.
3. You have to furnish various compressors to suck the gas from the contained structure and raise the pressure sufficiently to allow its subsequent purification, cooling, drying, and ultimate liquefaction.
4. To carry out the compression you need at least one main compressor to take the gas from several inches of water pressure to 250 psig. This involves approximately 150 to 200 hp.
5. You will require low pressure steam (approx. 50 psig) to regenerate the Amine solution.
6. You will require an Amine absorber, 3 Amine heat exchangers, 1 cooler-condenser, an Amine Stripper and its attached reboiler.
7. You will need an adsorption dryer unit to dry the compressed CO₂.
8. You will need approx. a 150 hp refrigeration unit complete with a CO₂ condenser.
9. You will require sufficient liquid CO₂ storage tanks (250 psig & -8 ^oF).
10. All liquid CO₂ piping and equipment has to be cold-insulated.
11. You will need a building to house the above process equipment.
12. You will need pumps, weigh scales, and other handling equipment to transfer and sell the liquid CO₂.

You will need more investment but I’m not going to bore you and other readers any longer with all the details. Suffice it to say that the idea is a bummer. The only thing that might help to justify such a large investment would be if the remaining 500 Scfm were pure, recoverable Methane and there were a convenient natural gas pipeline nearby that could accept custody transfer. But I doubt that very seriously. Realistically, I believe your methane recovery would be much lower than that and the cost of recovering it could easily surpass its inherent energy content.

I hope this answers your query.

 

[zdas04]

One economic concept that Mr Montemayor skipped over is electric generation. People are having very good success taking raw landfill gas (about 45% methane, 50% CO2) and using that trash gas to run an engine pushing an electric generator.

On the other hand you can separate the CO2 from the stream, run the 90% methane into a genset, use the waste heat to provide steam for amine regeneration, and sell the electricity. A properly designed facility can have good economics and astounding thermodynamic effeciency.

But you only asked about the CO2.

David

 

[bonzoboy]

Thanks for the feedback. The 1,000 cfm is at ambient pressure and temperature, but it is extracted from a fill, so there is no scavenging involve. I could burn it on site, and generate power, but the infratstructure for electricity is also a problem.

But this answers my basic question. It's not cheap!!!