A group of at least two gas-flow reactors (2.sub.1, 2.sub.2, 2'.sub.2, . . . , 2.sub.n, 2'n) preferably with an ascending gas flow (3), are supplied in parallel with gas and with a chemical neutralization and/or sorption agent of the pulverulent material type capable of adsorbing effluents present in the gas flow by bringing the chemical neutralization and/or sorption agent in contact with the gas flow, the at least two reactors having elements (10) for collecting the chemical neutralization and/or sorption agent after contact with the gas flow. At least one of the two reactors has elements (11) for discharging the chemical neutralization and/or or sorption agent, after contact with the gas flow, these discharge elements being connected to the collecting elements, and members (7, 16) for injecting the chemical neutralization and/or sorption agent into at least one other of the at least two reactors.