Flue gases originated from different sources contains hazardous gaseous pollutants such as SO2, SO3, H2S, HCl, HF. Those gases have a serious impact on the human health and the environment. Human health may be imposed in many ways with both short-term and long-term effects. Examples of short-term effects include irritation to the eyes, nose and throat, and upper respiratory infections such as bronchitis and pneumonia. Long-term health effects can include chronic respiratory disease, lung cancer, heart disease, and even damage to the brain, nerves, liver, or kidneys. Continual exposure to air pollution affects the lungs of growing children and may aggravate or complicate medical conditions in the elderly. For the environment case, air pollutant gases can contaminate drinking water and vegetation, soil, damage aquatic life, and erode buildings. Also they lead to the climatic damages, global warming.
In order to obstruct harmful effects of flue gases, their concentrations have to be reduced to definite levels, mentioned in the regulations. Two main regulations for the whole world are EPA and EU 2000/76/EC standards.
SANTES offers cost-effective and reliable gas cleaning solutions to comply with the most stringent emission standards for plants thermally treating waste, offering various end product options. Also SANTES Flue Gas Treatment Systems are designed in such a way that incomplete combustion gases like carbon monoxide will not be produced.
There mainly two treatment methods for flue gases;
- Dry Scrubber
- Wet Scrubber
The selection of treatment method depends on fuel, incineration plant design and emission limits applicable to a specific case.
In dry scrubber the absorption agent (usually lime or sodium bicarbonate) is fed into the reactor as a dry powder. In the reactor, gaseous pollutants are absorbed by the agent. The dose rate of reagent may depend on the temperature as well as on reagent type.
In the design of SANTES Flue Gas Treatments, generally sodium bicarbonate is used as the absorption agent. Absorption agent is applied through the gas originating from combustion process for the removal of gaseous pollutants such as SO2, SO3, HCl and HF.
The reaction products generated are solid and need to be deposited from the flue-gas as dust in a subsequent stage, normally a bag filter. Also bag filter provides required residence time to the absorption agent and gaseous pollutants.
Wet scrubbers are used to absorb soluble gases into water or other liquids such as solutions and to remove particulates from gas streams by contacting with water or other liquid. The flue-gas flow is fed into water, hydrogen peroxide, or/and a washing solution containing part of the reagent (e.g. sodium hydroxide solution). In order to eliminate gaseous pollutants such as SO2, HCl, HF, flue gas is quenched with the special solutions in liquid droplets sprayed to the combustion gas alkaline solution is injected to the gas coming from the dry scrubber.
SANTES Flue Gas Treatment Systems are designed with the alkaline solutions. Moreover, they are designed in such a way that they are economical, simple to operate, and can easily be fabricated of corrosion-resistant materials.
Removal efficiency of pollutants is improved by increasing contact time in the scrubber or by the increase of surface area of the scrubber solution by the use of a spray nozzle or trays placed into the scrubbing towers. In SANTES Flue Gas Treatment Systems, there are 2 types of wet scrubber units:
- Spray Tower
- Tray Tower
They consist of empty cylindrical vessels made of stainless steel and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite direction is called countercurrent flow.
Countercurrent flow exposes the outlet gas with the lowest pollutant concentration to the freshest scrubbing liquid. Nozzles are placed across the tower at different heights to spray all of the gas as it moves up through the tower.
In the case of heavy particulate concentration together with gaseous pollutants, venturi scrubber is added to the system as it is shown in the above figure. First, flue gas enters to the venturi scrubber to eliminate particle matter problem. Then, it is directed to the spray tower for an effective gaseous pollutant removal from the flue gas stream.
Water is circulated within the system by means of circulation pump and pH of the system is controlled continuously and regulated by adding NaOH to the system.
Tray tower treatment method is used for the removal hazardous gases as well as deodorizing. They have the ability to adsorb flue gases such as HCl, HF, Cl2, SO2, H2S and working principle is the same with the spray tower; liquid with alkaline solution is given from the top of the tower and flue gas is given from the bottom of the tower namely countercurrent flow of gas and liquid.
In the tray tower there are some trays with circular holes which raises surface area and contact time, therefore better removal efficiency is obtained. Tray number in the tower depends on concentration of the flue gas that will be treated as well as the required removal standards.
Water circulation is achieved by the circulation pumps and pH of the system is kept constant by dosing NaOH automatically.