A charging system for charging a reactor with air used energy produced by the reactor and includes a vessel having a hollow interior cavity partially filled with a liquid slug, a first air pocket within the cavity on a first side of the liquid slug, and a second air pocket within the cavity on a second side of the liquid slug. The liquid slug forms a water trap seal in the cavity between the two pockets and moves within the vessel in a cycle in which gas is loaded into the first air pocket in a first stroke and gas in the first air pocket is compressed in a second stroke. Movement of the liquid slug during the second stroke is caused by an increasing pressure in the second air pocket due to introduction of high-pressure gas from the reactor into the second air pocket.