The present disclosure provides biorefining systems for co-producing activated carbon along with primary products. A host plant converts a feedstock comprising biomass into primary products and carbon-containing co-products; a modular reactor system pyrolyzes and activates the co-products, to generate activated carbon and pyrolysis off-gas; and an oxidation unit oxidizes the pyrolysis off-gas, generating CO2, H2O, and energy. The energy is recycled and utilized in the host plant, and the CO2 and H2O may be recycled to the reactor system as an activation agent. The host plant may be a saw mill, a pulp and paper plant, a corn wet or dry mill, a sugar production facility, or a food or beverage plant, for example. In some embodiments, the activated carbon is utilized at the host plant to purify one or more primary products, to purify water, to treat a liquid waste stream, and/or to treat a vapor waste stream.