“…It was found that methods that rely on adsorption with solid sorbents are promising in the enrichment of lean methane-air mixtures [21,32,[36][37][38]. The various adsorption beds were tested: activated carbon (AC) [39,40], MOF [38], honeycomb monolithic carbon fiber composites [37], zeolitic imidazolate frameworks (ZIFs) [41], carbon composites derived from macadamia nut shell (MNSs), carbon nanotubes (CNTs) [21], carbon molecular sieve (CMS) [42,43], Ba exchanged titanium silicate (Ba-ETS-4) [44], etc.…”