Solid fuel combustion is a major source of Polychlorinated dibenzo-p-dioxines (PCDDs) and polychlorinated dibenzofurans (PCDFs) that are highly toxic compounds even in trace amounts. In addition, the complex conditions of the flue gases could favor, in same cases, PCDD/F formation. Thus the presence of chlorine and metal catalysts (i.e. copper) in the flue gases, could lead, through heterogeneous reactions, to PCDD/F formation between 250-400°C. These major theories have been established to elucidate the formation of PCDD/Fs in combustion systems:
(i) PCDD/Fs are already present in the incoming feed and are incompletely destroyed or transformed during combustion
(ii) PCDD/Fs can be formed during combustion and
(iii) PCDD/Fs can be formed by de novo mechanism that is in the low-temperature post-combustion zone of incinerators through some heterogeneous catalytic reactions that occur in the flue gas-fly ash environment.
Post-combustion and Pre-combustion techniques have been elaborated to minimize the PCDD/F emissions. Post combustion techniques utilize gas-cleaning devices to capture or destroy them after formation, while certain compounds could be added in the raw, prior the combustion zone, to inhibit PCDD/Fs formation (pre-combustion measures). In this work the PCDD/F emissions during the combustion of lignite, olive Kernel and blends were measured and the efficiency urea to act as potential inhibitor in PCDD/F formation was investigated also.