If the energy is free, you could just do electrolysis with huge carbon electrodes and immediately ignite the gases, bubbling the exhaust through your freshwater

If the energy is free, you could just do electrolysis with huge carbon electrodes and immediately ignite the gases, bubbling the exhaust through your freshwater reservoir. You can knock together a setup like that in your own backyard from junkyard parts. How you do it depends on whether you prioritize capital costs or operating costs.