Page 46 - Altogether Magazine Issue 4 English

46 and reintroduced into the anode paste recipe, while the desired temperature for the combustion of PAH. One the clean gases are released into the atmosphere. The or two seconds in the combustion chamber is enough recycling of the pitch loaded fines means there is neither time for the destruction of PAH. The clean gas is released waste stream nor product loss. The global capture though a second heat exchanger, while a third removes the efficiency of this method for heavy PAHs can be up to 99 remaining gas residues. Eolios is a combination of the dry per cent. scrubber and the RTO. At Qatalum paste plant, the RTO unit specifically treats the high concentrated fumes collected Of course, the drawback of the dry scrubbing system is that from the paste cooler – which contain mainly water vapour it is based on the ability of PAH to condensate. That works and light PAH fractions. The fumes are heated with hot well for the heavy fractions, which condensate easily: the air into a mixing pot situated on the paste cooler, to avoid global capture efficiency of this method for such fractions tar condensing in the duct. Following this process, diluted can be 99 per cent. But the lighter PAHs – emissions of fumes enter into the RTO unit at a temperature of 150°C. which, as we have seen, increase drastically when water Following treatment by the RTO, the clean gas is rejected to is added to cool the anode paste – do not condensate so the atmosphere. easily. If there is no condensation, no adsorption occurs and the pollutants pass through the bag filter without being To measure the performance of this method, a set of captured. Thus in order to improve the treatment efficiency flame ionisation detector measurements were taken to of light PAHs, we need to harness different technologies estimate the relationship between RTO temperature and using oxidation reactions. PAHs are comprised mainly of destruction efficiency. The gas sample was continuously C-H (carbon-hydrogen) bonds which are easily broken by sampled into the FID, which measures the concentration oxidation. This process converts them to carbon dioxide of total volatile organic compounds. As the table and water vapour. illustrates, the results were impressive: How does this happen? An RTO is the industrial process which does this. The equipment comprises three main RTO Combustion RTO Efficiency (%) components: temperature (°C) • The combustion chamber, where the oxidation 840 91.2 reaction occurs. The temperature in the combustion chamber is around 850-900°C. 870 96.2 • Ceramics beds, which are used as heat exchangers. The aim is to store heat and to recover it to preheat With environment emissions becoming ever more the inlet gas flow, thus saving energy. stringent, the importance of harnessing the full potential of innovative techniques and technologies • The valve box, which consists of two valves for inlet and cannot be overstated. Qatalum’s system sets a new outlet ducts and a valve for purge for each chamber. benchmark in the Gulf, combining the advantages of two established techniques, the dry scrubber and the Put briefly, the raw gas is introduced via the valve box and RTO, while minimising their weakness, all the while at a pre-heated as they pass through the ceramics to achieve substantially lower operating cost. Calcined Stack Dry Scubbing System Petroleum Coke + Liquid Pitch Paste Mixer RTO Coke Fines Injection Others suctions points (anoda forming area, Electrical Hot Air mixer...) Generator + Water Paste Cooler