Abatement of streams resulting from complex pollutants
The abatement of streams resulting from complex pollutants implies knowledge of the pollutants themselves in order to design a solution capable of tackling them. These substances include organic halogenated, nitrogenous and organic silicate compounds (silanes, siloxanes).
Specific objectives of the order, specific issues addressed
The abatement of streams resulting from complex pollutants requires identifying two particular groups of objectives: the first related to environmental conformity by building a plant with emissions below the required value; the second is related to the duration of the plant itself which, due to the presence of strongly corrosive inorganic acids, is always put to the test.
Intervention and proposal
In agreement with the customer, a technology focused on the quality of the emissions was chosen.
The chosen process was therefore based on:
- a combustion section, with dedicated burner;
- a post-treatment section of combustion gases including a quencher and a dual stage neutralisation column to abate the inorganic compounds produced (HCl and free chlorine).
|COMPONENT||FLOW RATE (kg/h)|
Thanks to a tail ventilator, we work in slight negative pressure to avoid leakage of any contaminant.
The following factors were taken into account in this design:
- Guarantee complete oxidation to CO2 avoiding the formation of CO or other products resulting from incomplete combustion;
- Limit the formation of thermal NOx;
- Minimise the formation of Cl2, reducing it as much as possible to HCl;
- Minimise auxiliary fuel consumption.
Searching for a balance between these 4 factors was the most challenging aspect of the project. Individual needs were at odds one with another (low concentration of CO implicates high combustion temperature, which however would increase the formation of NOx; the oxygen concentration should be as high as possible in the first case, but low in the second and fourth case, etc.).
The best operating parameters, determined in the design stage and tested at start-up, were therefore the following:
|COMBUSTION TEMPERATURE||1.050-1.100 °C (max di progetto 1.400 °C)|
|OXYGEN CONCENTRATION||3 %|
|RESIDENCE TIME||> 1.25 sec|
|FUEL CONSUMPTION||15 kg/h|
Special attention was paid to the choice of the internal refractory lining, to avoid any risk of corrosion and to make the plant highly reliable.
The gas washing phase included an initial cooling section (to avoid the formation of dioxins) made with a double stage quencher. The first stage brings to mind a tubular heat exchanger, while the second is with packing bodies (Raschig rings). The construction material chosen was graphite due to its high corrosion resistance. To reduce water consumption to a minimum (which must be softened to lower dust emissions as much as possible and to avoid calcium carbonate scaling), a liquid recirculation circuit was made with relative cooling. The water exiting the quencher ends up directly in the bottom tank of the neutralisation column, from whence it is pumped to a cooling exchanger and then sent both to the quencher section and to the top of the first stage of the column itself.
The soda solution is dosed in this first section of the washing column (made of fibreglass) to abate the hydrochloric acid formed in the combustion chamber.
The pH values maintained here are not ideal for the abatement of free chlorine: this is why we introduced a second separate washing section.
To optimise this process, but also to respect the limits of liquid drains and to prevent CO2 from being absorbed in the washing solution, we used sodium bisulfite as a reducing agent.
The high reliability of the plant, guaranteed by a safety PLC and the necessary control loops, the very high abatement efficiency and the reduced fuel consumption make this plant a model for the treatment of high concentration chlorinated emissions.
|Values measured at test-run|
|Vinyl chloride CVM + 1.2-dichloroethane DCE||1||<0,5|
|Total dusts||40||Not detected|
|Volatile organic substances VOS||20||1|
|Sulphur oxides, SOx||300||Not detected|
|Nitrogen oxides NOx||100||66|
|Carbon monoxide CO||100||<1|
|Ethyl chloride EtCl||20||<0,2|
|Values measured at test-run|