Disclosed is a BOG reliquefaction system. The BOG reliquefaction system includes: a compressor compressing BOG; a heat exchanger cooling the BOG compressed by the compressor through heat exchange using BOG not compressed by the compressor as a refrigerant; a pressure reducer disposed downstream of the heat exchanger and reducing a pressure of fluid cooled by the heat exchanger; and a second oil filter disposed downstream of the pressure reducer, wherein the compressor includes at least one oil-lubrication type cylinder and the second oil filter is a cryogenic oil filter.

A method of processing biogas that includes obtaining a mobile tank containing biogas at a pressure greater than 1000 psig, connecting the mobile tank to a pressure let down system, and depressurizing the mobile tank to remove biogas therein. The depressurization includes removing gas from the mobile tank using the pressure let down system, and introducing a warming gas into the mobile tank.

A method of processing biogas that includes obtaining a mobile tank containing biogas at a pressure greater than 1000 psig, connecting the mobile tank to a pressure let down system, and depressurizing the mobile tank to remove biogas therein. The depressurization includes removing gas from the mobile tank using the pressure let down system, and introducing a warming gas into the mobile tank.