already available and normally stored in the EMS database. In this case, it is sufficient to extract and merge them with process values read from plant historian. On the contrary, if PEMS are designed to be the only monitoring system, it is necessary to arrange a measuring campaign for the collection of emission data; this requires installing a temporary analyzer at plant site for a few days (typically 10-15 days) to obtain the baseline emission data, which will be used to build the mathematical model. Emission data have to be merged with process parameters values to complete the set of information needed for model building.