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Separation of methane
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (MERO)ORCID iD: 0000-0001-5559-4983
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (MERO)ORCID iD: 0000-0002-7233-6916
2013 (English)Report (Other academic)
Abstract [en]

Gasification with gas cleaning and direct separation of methane, where the residual gas can be used in a conventional boiler, is an attractive scenario in regard to flexible production of methane, power and heat. The technological challenge is to separate methane from a product gas with a low methane content and also complex composition.

Membranes have several attractive properties for this application. They consume no energy input for their operation, as long as the feed is pressurized and no recirculation etc. is needed. They have attracted a great deal of interest in the last decades for separation of methane and carbon dioxide from anaerobic digestion biogas production and for treatment of natural gas. They have the potential to offer cost effect­tive solutions also for smaller plants, since they are delivered in modules that can be duplicated to fit the size of the plant. For these reasons, it was chosen to concentrate the study on membranes. Alternative separation methods that were considered are cryogenic separation and pressure swing adsorption (PSA).

Membranes separate gases based on differences in gas components’ diffusivity and solubility in the (polymer) membrane material. It was found that both these factors differ too little between CH4 and N2 to assume a possibility to separated them, if N2 is a large part of the gas composition, as is the case in air-blown gasification. Therefore, it was concluded that membrane separation is not an option for air-blown gasification. Instead, a gas from oxygen-blown CFB gasification or from dual bed indirect gasification was set as the input gas, thereby eliminating the problem with N2 in the search for suitable membranes. The main components in the gas are then H2, CO, H2O, CO2, CH4 and some higher hydrocarbon. An ideal separation would yield only CH4 and higher hydrocarbons in one stream and the other components in another.

There are two kinds of membranes, glassy, that separate mainly based on size, and rubbary, that separate mainly based on solubility which is closely connected to the boiling point of the gas components. Calculations were made on a typical glassy membrane. It was found that the problem is that the separation between CH4 and CO is insufficient. Therefore, a two-step membrane separation with a shift step between was tried. The shift reaction, CO + H2O -> H2 + CO2, introduces a gas composition to the second membrane that is more easily separated. Even with the possibly overoptimistic conditions set in this preliminary calculation, the result is unsatisfactory. The achieved CH4 percentage is only 80% and 30% of the methane is lost in the gas streams that will be combusted for heat and power production. Also, this separation option would require a rather complicated process. The conclusion is that it is unlikely that this concept would be attractive, even after modifications to achieve better percentage values. 

Rubbary membranes have also been considered. Based on the separation factors, it can be conclu­ded that they would require upstream separation of H2O and CO2 by a glassy membrane or other gas processing methods, before a rubbary membrane could perform the separation between CH4 and H2+CO. It is, however, unlikely that this separation would be sufficiently good, at least in a one-step system.

A possible way to increase performance of membranes is to use liquid membranes that has a liquid to absorb gas components that have permeated through the membrane. No publications have been found on specific liquid membranes that could be suitable for the separation in question. Liquid membranes intrinsically add the need for regeneration of the liquid phase. Further search for liquid membranes is assessed as one of the interesting routes for continued investigations on separation. 

Cryogenic separation can be expected to fulfil the requirements on gas quality and minimizing CH4 loss. The drawback is that a cryogenic plant would mean substantial investment cost and energy consumption. A conclusion from a rough cost analysis is that the cryogenic alternative seems to have the potential to be cost effective and should be included in further studies.

Liquid membranes, cryogenic and also PSA etc. are all worth considering for the separation process.

Place, publisher, year, edition, pages
2013. , 15 p.
Keyword [en]
gasification, biomass, gas separation, methane, membrane, glassy membrane, rubbary membrane, product gas, water gas shift
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:mdh:diva-23890OAI: oai:DiVA.org:mdh-23890DiVA: diva2:681690
Projects
Svensk Förgasningcentrum, SFC, Node SGV-CDGB
Funder
Swedish Energy Agency
Available from: 2013-12-20 Created: 2013-12-20 Last updated: 2015-11-13Bibliographically approved

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