(400n) One-Step Methane Separation and Purification of Landfill Gas Using Carbon Molecular Sieves

In recent years, with the increasing demand for nature-derived energy, biomethane, which is methane gas derived from biomass feedstock, has been attracting attention as an alternative energy source to conventional fossil fuels. Biomethane can be obtained by removing carbon dioxide, nitrogen, and other impurities from biogas generated by fermentation of biomass in an anaerobic environment. Carbon molecular sieves(CMS) are widely used in the process of purifying methane from biogas.The landfill gas biogases, which generated from organic waste in landfills, contains nitrogen and carbon dioxide as major impurities in addition to methane. But conventionally, when methane is separated and purified from the above-mentioned mixed gas, a process of removing carbon dioxide in the first step pressure swing adsorption(PSA) operation and removing nitrogen in the second step.We have developed a CMS that enables simultaneous removal of nitrogen and carbon dioxide in a single PSA step and report on the results.

Experimental Methods

CMS for evaluation was prepared by the following method. Cylindrical pellet-shaped coconut shell activated carbon was used as the precursor activated carbon, and CVD treatment was performed by injecting and vaporizing benzene into a quartz reaction tube at 1000 K with nitrogen gas distributed as the carrier gas. The degree of CMS pore narrowing was controlled by varying the benzene injection volume.The pore distribution of CMS was evaluated by molecular probe method. The adsorption kinetics of various gases on CMS were evaluated by the constant volume method. Separation and purification of test gas mixtures were evaluated using a small PSA system. Methane/nitrogen/carbon dioxide = 50/10/40 by volume ratio mixed gas was distributed to the PSA device as the raw gas, and the methane generation and methane concentration were evaluated.

Results

The adsorption rate curves of methane, nitrogen, and carbon dioxide were measured for each of the CMSs, and the results showed that the adsorption rate of carbon dioxide, nitrogen, and methane were faster in the order of carbon dioxide, nitrogen, and methane in each CMS. Comparison among the CMSs showed that the adsorption rate became slower as the amount of benzene in the CVD process increased.It was confirmed that nitrogen and carbon dioxide were simultaneously adsorbed on CMS and methane was selectively separated and recovered when PSA operation was performed using CMS with optimized benzene content and appropriately controlled adsorption rate of nitrogen, and methane concentration was over 98%.The evaluation results of the CMS and the demonstration results of the PSA operation will be reported in detail at the conference.