Water re-circulation to the scrubber and co-combustion of coal with petroleum coke are the most significant factors that affect the retention capacity for trace elements in a power plant equipped with a forced-oxidation flue gas desulphurisation (FGD) system. The water re-circulation favours the progressive saturation of most elements in the gypsum slurry with the subsequent increase in emission by entraining particles and droplets in the outgoing gas of FGD (OUT-FGD). The co-combustion increases the content of HCl and SO2 in the gaseous stream, which enhances the formation of gaseous chlorides species, especially of Hg, and the formation of condensed species of Cu, Cr, Ni, Na, Cd, and Zn OUT-FGD, respectively. High gaseous retention capacity (83–100%) for S, Cl, F, As, and B is attained by the whole plant with a subsequent reduction in their gaseous emissions below the limits established by the European directive 2001/80/EC for large combustion plants, and also below the Pollutant Release and Transfer Register (PRTR) threshold values. The retention of Se increases from 2007 (75%) to 2008 (97%). The emissions of gaseous Hg and particulate As, Zn, and Ni exceed the PRTR values. Remediation actions for the gaseous and PM emissions are relevant given the role of the filtered water re-circulation increasing the particles emissions OUT-FGD.