Nitrogen rejection from landfill gas using Pressure Swing Adsorption

Landfill gas (LFG) produced from municipal solid waste substrates represents an important source of RNG and the market for its upgrade is facing significant challenges in terms of energy consumption and operating costs. To ensure higher CH4 yields and avoid its release in the atmosphere, the LFG is collected below the atmospheric pressure by the use of a vacuum pump that results in the contamination of the LFG by air and particularly N-2. Most of the proposed solutions, propose a two-step separation process in which the CO2 removal takes place in the first one while N-2 removal in the second. This study focuses on the removal of the N-2 from a decarbonated methane stream by a four-step PSA cycle. The impact of several parameters on process performance has been investigated using numerical simulations with the aim of simplifying the unit design and operational performances. In particular we investigate the effect of the pressure at the end of the desorption step showing that it is possible to operate the cycle with the desorption pressure slightly above atmospheric one. This allows avoiding the use of a dedicated vacuum pump with, however, a penalty in the energy required.