Aside from its use to supply medical oxygen, or as a substitute for bulk cryogenic or compressed-cylinder storage, which is the primary oxygen source for any hospital, PSA has numerous other uses. One of the primary applications of PSA is in the removal of carbon dioxide (CO2) as the final step in the large-scale commercial synthesis of hydrogen (H2) for use in oil refineries and in the production of ammonia (NH3). Refineries often use PSA technology in the removal of hydrogen sulfide (H2S) from hydrogen feed and recycle streams of hydrotreating and hydrocracking units. Another application of PSA is the separation of carbon dioxide from biogas to increase the methane (CH4) ratio. Through PSA the biogas can be upgraded to a quality similar to natural gas. This includes a process in landfill gas utilization to upgrade landfill gas to utility-grade high purity methane gas to be sold as natural gas.