Another CO2 question 1

[boilerdoc]

Hallo everybody,
I got also a question similar to the thread of tangela from 02/13/03.
My problem is: I would like to separate CO2 from flue gases to be utilized as fertilizer in green houses. This method is common pratice with natural gas combustion. What about combustion of oil, coal or biomass? Beside CO2, N and small amount of CO and NOx there is nothing left when burning Natural gas.With other fuels there are probably a lot of other chemicals and aromates which might influence the taste of vegetables. Is there any feasable method of selective separation and release of CO2??
Please keep in mind that I am a boiler guy and no chemist, so understandable answers are highly apreciated.

Thanks

 

[TomSD]

I'm not an expert in the CO2 separation processes using amine based absorption solutions but I am curious about the fertilization methodology. I have heard of waste heat from large boiler/power installations being used for greenhouses located close by. I also recall reading of the use of CO2 enriched air being used in greenhouses; plants absorbing CO2 and releasing O2. If the latter method is referred to, I assume that the greenhouse air is only CO2 enriched to a certain degree while maintaining a safe atmosphere for workers. Then the gases from a boiler/power station might be used as the CO2 source without a complete separation of the CO2.

Although natural gas (essentially methane, CH4) is more easily combusted to CO2 and H2O, other hydrocarbons might be forced to burn completely but, as mentioned, other fuels may contain additional ingredients giving rise to harmful contaminants, especially sulfur compounds resulting in SO2 (forms acidic rain); coal has been particularly bad for sulfur. I have read in another thread that sulfur compounds can be removed from fuel oil. I believe NOx is more difficult to deal with.

I am wondering if it would be possible to "condition" the combustion gases from other fuels rather than do a CO2 separation.
Perhaps a first step would be to pass the combustion gas with added air or preferably oxygen through a catalytic converter to ensure maximum combustion of the organic constituents.
A second step might be contact with water which cools the gas and should absorb all SO2. I don't know how effective NOx absorption would be. Some organic residues will go with the water. There may some issues of disposal of the water; hopefully pH adjustment using caustic addition is sufficient.
A third step to further remove trace volatile organic carbon (VOC) species might be necessary. I believe that passing the gas through activated carbon beds would remove VOC's (and aromatics). The gas feed to the carbon bed should be heated slightly to be above the dewpoint for the moisture content, but too high a temperature with oxygen present could burn the activated carbon.
If harmful components can be sufficiently removed by such procedures, then could the "conditioned" gas (containing essentially CO2, nitrogen N2, some oxygen O2, and hopefully very little carbon monoxide CO) be used to enrich the greenhouse air for fertilization?

Possibly, the "conditioning" process doesn't have to be as complicated as described above, depending upon the "purity" of the fuel and/or allowable concentrations of contaminants in the diluted gas mixture to the greenhouse.
Perhaps the combusted gas can be passed directly through a carbon bed to remove residual organics and cooled sufficiently by simple dilution. I would be nervous about extra oxygen in the combusted gas causing a carbon bed fire at high temperatures but perhaps an indirect heat exchanger with cooling air upstream of the carbon bed is all that is needed. I don't know how effective activated carbon would be in removing the other possible contaminants but it may work if big enough and this could be practical for a small scale system. Unless someone else can verify this approach, a bit of lab work would be necessary.

How far am I off the mark?
Regards

 

[boilerdoc]

Hi TomSD,
thanks for your reply.There are helpful tips in it. You mentioned activated carbon to adsorb unwanted matters from the flue gas. If I remember correctly, Activated carbon is used to remove CO2 from fruit storage rooms with controlled atmosphere. Are there different coals available according to the molecular structure of the materials I want to separate? Also your idea of water spraying the flue gas might lead to absorption of CO2 in the water.

 

[JOM]

Hi.

A few persons have raised this idea of exploiting CO2 in combustion gases.

In Australia, there is research effort into disposing of CO2 by pumping it into exhausted oil/gas fields. Here is a URL that offers some discussion:-

http://www.apcrc.com.au/Programs/geodisc_res.html#top

So, is CO2 a useful resource or a nuisance waste product? Cheers,
John.

 

[TomSD]

Hi setra;
Activated carbon absorbs many, many things so the use of this material to remove CO2 is not too surprising. There experts who do much more work in this field than I have ever done and thus have better idea of what to expect, but I'm sure they would agree that only bench scale testing can verify the applicability for specific cases.

I would anticipate the activated carbon would absorb at least some CO2 but it should be saturated with respect to CO2 fairly quickly. The adsorbed CO2 will interfere with absorption of other contaminants to some extent but relative strength of absorption for the different species (or selectivity) is the important factor concerning the amount of absorption of the other species and the residual concentrations.

Operationally the initial CO2 absorption probably releases heat (any change of state has a heat of transition, the absorbed species go from a purely gas mixture state to the gas-solid state; such state changes are most commonly exothermic). A new carbon bed should be brought on line slowly (introduce process gas slowly) to avoid a high temperature; important for systems larger than bench scale. The absorption heat for CO2 (or the other major gas components, i.e. N2) may not large enough to be a problem but has to be checked (there may be existing data). I have heard of carbon bed fires where traces of nitrogen trichloride (a highly explosive contaminant) was being removed from chlorine gas due to rapid switching on-line and the heat of absorption of chlorine on activated carbon is "high".

If there is sufficient adsorption of contaminants to warrant this approach (also depending upon the scale of operation), other safety considerations have to taken into account. Especially vessel entry or handling during disposal (CO2 and other gases will be released for a long time and in enclosed spaces the atmosphere will not support animal/human life).

A carbon bed will become saturated with different species according to selectivities and inlet concentrations. The carbon bed may be regenerated by pulling a vacuum and/or passing a higher temperature inert gas through the bed. However, some species such as aromatics may not be easily removed and the carbon may simply have to be replaced. I'm still thinking of a relatively small scale application where the use of a simple carbon bed system without getting into regeneration, etc. would be convenient, and relatively inexpensive to install and operate.

Using a water spray to cool the gas will absorb some CO2. The amount of absorption will not be a significant portion of the gas stream. If you are going for commercial CO2 production, it would an recovery inefficiency. Again I'm still thinking a small localized system to enrich the atmosphere of a local greenhouse with the "conditioned" gas. If the exit water stream is a large amount, it should be diluted and downstream enclosed spaces avoided due to CO2 release.

On the other hand, perhaps the water stream might be used for mist in the greenhouse providing CO2 as well as moisture. I don't think the water should be used for irrigation since the water will be acidic due to presence of carbonic acid, the plant people would have to comment. A greater concentration of CO2 in the water would be effected by doing the gas-liquid contact in a pressurized vessel and more CO2 will be released during the misting. However, other contaminants absorbed into the water may be an issue (e.g. SO2, VOC's, etc.) although they may be removed by passing the water through absorption beds such as carbon or other absorbent materials that have greater specific selectivities. Low selectivity for CO2 in this case is more important.

Further thoughts; two or more stages in a water spray system might work where a first stage water spray removes the contaminants easily absorbed into water; the water stream then disposed. For a second stage water spray, the gas is now more easily compressed, if pressurization is desirable, and hopefully the concentrations of other contaminants is small enough to be tolerated so the final stage exit water can be used for mist in the greenhouse. A second stage spray under pressure may remove more contaminants. For small scale systems, a water ring compressor would act as a second stage contactor while compressing the gas for the next stage. More CO2 is absorbed at lower temperatures as well.

"Are there different coals available according to the molecular structure of the materials?" I take it, you are referring to activated carbon. There are certainly many forms of activated carbon depending upon the source material used in the manufacture of activated carbon; bone charcoal and coconut charcoal are examples of "natural" activated carbon but there is a huge variety of material sources plus differences in the products due to purification treatments, etc. and then there is doping with things for different effects including special catalytic results for reactions. So much I would have to learn to give a proper discourse. It is entirely possible to find an activated carbon product that is much better for removing the particular contaminants than other products. Norit,

http://www.Norit.com,

is a large supplier of activated and other forms of carbon for different purposes but there are many other suppliers.

Having an absorbent specific to molecular structure and/or size gets more into synthetic absorbents that can be "tailored" with such properties. These are commonly referred to as "molecular sieves", air drying absorbents are examples. There may be something worth considering in these products but they are usually expensive. It will be interesting to see if carbon nanotube chemistry will develop any such materials (of practical, economically viable use).

Regards

 

[boilerdoc]

Hi TomSD,
thanks for your detailed comments.That points the direction to go for.

 

[HRS]

Setra,

What you are looking to do is technically possible but economically is likely to be unviable. You may want to investigate the financial angle before spending too much time on your idea.

 

[boilerdoc]

HRS,
I am afraid you are right. I have just made some very optimistic calculations and found that with available technology it is far to expensive. Anyway it was worth thinking about.