Dear Shalini

Can you provide little more details or any documents dealing with this.

Thank you

Please note that the traces of Hydrogen sulfide is very dangerous need to be purged out using proper separation method otherwise it may cause serious irreversible diseases if the out coming gas from the unit is passed through  a sponge iron filter media H2S will be converted in to FeS but a mass balance is required to design the filer media. Removing  Carbon dioxide is not easy unless the purged biogas is compressed , when the gas with low Critical pressure liquefies first I believe Methane can be separated first now feeding to the cooking stove or a portable generator use a pressure regulator to raise up to the required pressure . 

Arun: According to tests done, most biogas stoves achieve 50% efficiency. (See link.) That may seem "quite low", but it is not, at least as compared with stoves using other fuels, etc. The energy and effort of scrubbing biogas must be exceeded by the energy realized subsequently, and I do not think that is reasonably possible in small-scale applications where limited amounts of biogas are used primarily for uses such as cooking. One is far better served by attending to the nature of the pots used, the distance between pot and flame and similar issues, as contrasted with scrubbing. 

I am not convinced that scrubbing biogas makes any sense whatsoever for most common uses. The clear exception is where a very great deal of biogas is being produced (several tens of cubic meters per day), and is to be used for powering vehicles, or is being compressed for sale to those using it off-premises. As a demonstration that my opinion about this is widely shared, consider that of the millions of underground biogas plants being provided around the world, there are no reports of any with the capability of scrubbing the biogas before use.

Peter: Traces of H2S are not by any means necessarily "very dangerous indeed". It depends on the amount which is present, the per-day amount of biogas being considered, and the use to which the biogas is put. Proper selection of a substrate and manipulation of the pH of the digester are two easy and reliable means of preventing the over-production of H2S. See The Complete Biogas Handbook, p. 39: completebiogas.com.

https://www.google.com/search?sourceid=chrome-psyapi2&rlz=1C1CHFX_enUS574US574&ion=1&espv=2&ie=UTF-8&q=biogas%20stove%20efficiency&oq=biogas%20stove%20efficiency&aqs=chrome..69i57.4250j0j7

Dear Arun,

The biogas, in addition to methane contains carbon dioxide, in the form of water vapor and small amounts of other gases which are potentially harmful when burned in internal combustion engines. In biogas used for electricity and heat cogeneration, content in steam - hydrogen sulfide (H2S) and siloxanes - should be eliminated because of strong corrosive action as to reduce engine life endothermic (estimated at 80,000 h ), with an impact on maintenance costs and 'Motor stops.'

The choice and the design of devices for the treatment of biogas depends on the quantity to be treated, but above all of the fuel characteristics, substantially related to biomass departure.

Best Regards,

Andréa

Andréa,

Carbon dioxide will not harm engines, but it can reduce the efficiency of combustion, increasing heat released vs. mechanical power output. Depending on the engine's design and a number of other factors, running hotter may of course cause problems.

H2S will not harm engines, according to Walraven, when present at a tenth of one percent or less, and as mentioned, the amount of H2S in biogas can be adjusted in several ways. Where H2S is present in any detectable amount, however, oil changes must be more frequent, and that schedule can be determined by checking the pH of the engine oil, because H2S is converted to sulfuric acid following combustion.

Siloxanes are essentially NEVER present in biogas that comes from digesters, rather only in biogas that comes from (some) landfills. Further they are not corrosive, but rather they are changed into various forms of silica, which are usually abrasive.

d.

Dear David

Forgive my haste and my English.

And what good can you explain such differences.

Best Regards,

Andréa

Andréa,

Não se preocupe. We are all here to learn.

d.

There are at least two other alternatives that have not been discussed here: membrane separation and wood ash filtration. Membrane is simple and can be used in small scale, but one need energy to compress the gas. A compressor is also expensive.  If wood ash is available, this could be an alternative, but the method is young and probably need to be more developed. This link can be useful: http://www.hallbarconsulting.com/low-cost-biogas-to-biomethane-upgrading-system/.  

Best Regards, John

Thank you Dr. Morken

It was really useful

For household application, you can use adsorption/absorbtion technology by using liquid or solid adsorbent. The simple ways, you can use water by bubbling biogas into it before use on stove although it's not optimum purification for CO2 and H2S because it is physical adsorption only. The better option you can use alkaline solution, so it will react on acid-base reaction and reduce more CO2 and H2S. Another option you can use solid adsorbent such as wood ash, carbon active, metal organic framework, and zeolite (see one of my publication : https://www.researchgate.net/publication/269527050_Biogas_Filter_Based_on_Local_Natural_Zeolite_Materials). However, you should change them (for all of type of the adsorbents : liquid or solid) periodically because it will be saturated for some period.

I think the advanced technology (such as membrane technology, pressure swing adsorption, cryogenic technology, etc) do not suitable for household application, because it need more investment and not simple to operate.

Article Biogas Filter Based on Local Natural Zeolite Materials

Bubble the biogas through an aqueous solution of NaOH, KOH or lime. The acidic CO2 gets absorbed in the alkaline solution and is removed from the biogas. For removal of H2S, you may try steel wool. If you try these, do send us your feedback...

 Dear Arun Babu,

For some basic insights you may kindly have a look at this link:

http://link.springer.com/chapter/10.1007%2F978-94-007-2439-6_9#page-1

Regards

Krishna