The paper describes a method for complying with emission quotas for a power producing or CHP producing system in an economically rational and optimal way. The method is based on extended use of a simulation model (SIVAEL) and a pre-processor which optimises the environmental aspects of each system unit. SIVAEL is a unit commitment/load dispatching model, which solves the problem of optimal scheduling of CHP production. On a weekly basis, the scheduling of power producing units, heat producing units and units with-CHP production is laid down. The pre-processor, PRESIVAEL, calculates the environmental parameters of each system unit. When handling the SO2 quota, for instance the pre-processor optimises the operation of each coal-fired (or oil-fired) unit, including possible deSO(x) plants. The application of the method is illustrated by examples. (C) 2001 Elsevier Science S.A. All rights reserved.