**5.2 Removal of dissolved pollutants**

Quality control measures and pre-treatment processes to remove contaminants can improve the treatment of waste. This encompasses removal of dissolved pollutants from waste such as chlorine, alkali metals and sulfur which causes high temperature corrosion and ash deposition.

To sustainably and efficiently treat MSW in incinerators, minimum chlorineinduced adverse effects should be observed. Therefore, it is crucial to control the dissolved pollutants prior to the burning process such as the removing the alkali chlorides and separating plastic, than attempt to minimize their damage by the chemical dechlorination and catalytic separation. Since all the incineration hindrances are mainly due to the presence of chlorine, much research have focused on understanding the relationships between chlorine in the waste and the HCl formation. An investigation was conducted to determine the amount of HCl that is emitted from PVC and a mixture of waste without the PVC-fraction. It was observed that HCl-emission decreased by 40% to 1.7 mg Cl/g wet MSW [47]. Another authors Delay et al. [48] showed that the decrease in plastics, especially PVC in the MSW result in decreased boiler corrosion as well as decreased heavy metal emissions. At present, the techniques used to separate plastics, are grounded on density. In consideration of its accuracy, three advanced separation processes: the sink-float process, centrifuge and hydrocyclone separation are used, in order to find out how each process would distribute chlorine contained in plastic waste, to the overflow and underflow. It is concluded that with the current sorting technology, the 13% mixed waste plastic go to incinerators, 35% to recycling, and 52% to landfills. With the sink-float separation technology, 47.5% chlorine-poor plastic (<0.5%) goes to incineration plants as alternative fuel, and 16.9% (ca 4.7% Cl content) landfilling.

For the inorganic salt in the waste streams, a water-washing process can be a simple but practical method for inorganic salts removal. Numerous studies on waste and biomass washing have revealed that a large portion of chlorine could be released by elution tests. Jensen et al. [49] reported that approximately 90% of the salts can be removed from biomass char, within 20 min by water washing. Chen and Pagano [50], observed that application of a high temperature (93.7°C) leaching technique on a high chlorine coal (0.5 wt.%) result in chlorine decrease to about 0.2 wt.%. In MSW approximately 85% of easily water-soluble chlorine was leached out in the first washing procedure by distilled water [51]. The limitation of the water extraction method is on increasing the total costs of the operations as well as disposal of the effluent.

#### **5.3 Optimisation of combustion environment**

Optimization of combustion conditions in incineration plant had recently emerged as a method to reduce the corrosion impacts in waste to energy plants. Among the key methods, installations of the Segher Boiler Prisms in incineration facilities [47] has been utilized as a measure reduce corrosion in the boiler. The technology consist of a prism-shaped dynamic secondary air mixer, that is inserted at the transition of the combustion chamber and is cooled by flowing water, lined by refractory materials, and has the natural circulation system. The prism splits the flue gas into two channels where each is supplied by secondary air injection. Prism ensures homogenous injection of secondary air via multiple nozzles that are on the prism sides and boiler walls. This supply ensures that there is high turbulence and mixing of waste with excess air which prevents the deposition of ash and also facilitates sulphation process to occur [52], which in turn reduces the corrosion impact [47]. In addition, the prism results in a uniform distribution of flue gas speed, temperature and oxygen, thereby preventing the creation of hot spots within the boiler [32]. Moreover, the prism removes heat from the combustion zone, and reduces the temperature, which then reduces the volatilization of alkali chlorides, leading to generation of less chemically corrosive deposits [47, 52].
