Acidic wastes with acid production from waste incineration
The common way to produce HCl is to convert chlorine with pure hydrogen in a HCl synthesis unit. The hydrogen required for this process is generated typically in a steam reformer using natural gas as feed stock. However in case wastes with higher chlorine concentrations are available, an economical alternative for HCl production by HCl synthesis is the HCl recovery from waste incineration.
At high temperature and certain oxygen excess in the combustion chamber, the organic part of the waste will be oxidised to CO2 and H2O while the chlorine will be converted to HCl. The hot fluegas first passes a quench system where already most of the HCl will be absorbed in water. In the subsequent absorption step, remaining HCl will be solved in circulation water. With such systems, a very pure HCl solution can be recovered from the waste combustion, a concentration of more than 30 w% can be achieved.
As the combustion process follows the Deacon equilibrium, the chlorine content of the waste will not be converted completely but a small amount remains as Cl2 in the fluegas. These impurities have to be removed in order to meet the emission limits. The fluegas downstream of the HCl recovery stage is sent to an alkaline scrubber where the chlorine reacts with sodium hydroxide and sodium bisulphate to sodium chloride. In order to keep the salt concentration below its solubility limit, a certain waste water will be bleed off while fresh water is added to the system.
In case no waste water is acceptable or no Cl2 impurities are allowed in the recovered HCl, a staged combustion process avoiding the formation of Cl2 is an alternative. In a first combustion stage, chlorinated waste is burned under excess air. This leads, as described above, to remaining Cl2 in the fluegas. In this modified process now the fluegas from the first stage is sent to a second combustion stage together with excess fuel. With this arrangement, the waste is safely destructed under controlled excess oxygen, and all Cl2 formed at this stage is transformed into HCl in the secondary reducing stage. The HCl will be recovered in the same way as per above description. Because of the substoichometic condition of the combustion process, the flue gas leaving the HCl-recovery system contains CO, a further simple incinerator may be required in order to meet emission limits. The heat of combustion can be recovered by steam generation.
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