Pressure variable adsorptive method for separating hydrogen nitrogen and pure carbon dioxide from exchange air

The present invention relates to a pressure swing adsorption method for separating hydrogen, nitrogen and pure CO2 from conversion gas. Said conversion gas is one produced by using coal to make gas. It uses the series-connection operation of first and second groups of adsorption towers, in which its first group is used for preparing CO2, its outlet is hydrogen and nitrogen mixed gas containing CO2, the mixed gas is fed into second group of adsorption tower to prepare hydrogen and nitrogen. Said two groups of adsorption towers in one cyclic period includes the adsorption step A in turn: pressure-equalizing and reduction ED, vacuum-pumping VC, pressure-equalizing and boosting ED, and final boosting FR, and first group of adsorption tower possesses the CO2 conversion step P after pressure-equalizing ED and before vacuum-pumping VC.