Everything You Need To Know About Flue Gas Cleaning

07 Mar Everything You Need To Know About Flue Gas Cleaning

In the incineration plant, there is a mixture of gases containing pollutants such as dust, carbon dioxide, sulphur dioxide, and soot, as well as nitrogen oxides, heavy metal bearing fumes and unburned hydrocarbons.

One of the most important parts of the incineration plant is its final unit as it cleans off the pollutants that have been produced. The contaminants are removed from the flue gases by using the flue gas cleaning systems. Thus ensures that harmful substances are discharged into the atmosphere.

Removal of the particles

Removing the solid particles is the first step in the flue gas treatment. There are three removers commonly used these days. They are the cyclone, the fabric filter and the electrostatic precipitator.

In the hurricane, there is an immersed tube around which the gas revolves. The particles are then carried towards the wall of the cylinder by inertia. From there the clean gas finds it through the top and the particles exit through the bottom.

In the electrostatic precipitator, the particles that the flue contains make contact with a high voltage and as a result the particles are electrically charged. The charging causes them to move toward the precipitation electrode. After this, the power supply is suspended, and the precipitator is hit by a hammer so as to push the dust down. The fabric filter has the same operation as the vacuum cleaner.

The gas is passed through a filter and air is allowed to flow through it, leaving the particles behind. The particles left in the filter remain there until the filter is cleaned by compressed air that blows in the opposite direction, and as such the dust fall off from its collection point.

Removal of gaseous contaminants

The recent technologies used for this process either employs absorption or adsorption. In the intake process, additives are mixed with the flue gas. The additives act on the contaminant gases transforming them to products that no longer that pollutants. The contaminants of the flue gas attach themselves to the surface of adsorbent in the adsorbing process.

The non-pollutants, however, pass through. The choice of which methods will depend on the advantages and disadvantages they have. Your choice of method to use will depend on the location and the specific conditions- wet or dry systems.

Dry System Process

The dry scrubbing process is one that neither needs wet scrubber nor the electric precipitators. In the dry flue gas cleaning system, sodium bicarbonate is used as the additive. Over a heat of 140 degrees Celsius, the sodium bicarbonate in the gas decomposes to give sodium carbonate, carbon dioxide and water.

The gaseous products of the decomposition of bicarbonate (water and carbon dioxide) leave holes and gaps in the particle grain of the reagent. The rate of the dose of the additive reagent depends on the reagent types. It also depends on the temperature. The stoichiometric dosage used is often a 0.1 to 0.4.

Another reagent that can be utilised is the calcium hydroxide. Sodium bicarbonate has a stoichiometric value of 1, while calcium hydroxide has a stoichiometric value of 2.

The decomposition of sodium bicarbonate under the action of heat brings the actual reagent Na2CO3 to life. The gaseous products of the decomposed products, CO2 and H2O, will be emitted in the flue gas stream so as to decrease the total mass of reagent.

The process requirements for effective treatment of gas state that the bicarbonate should have a large surface area for grinding and also as materials exchange area. Also, a minimum residence time of 2 seconds at a temperature over 140°C must be ensured.

With calcium hydroxide used in the deposition of contaminants, the efficiency of treatment could be improved by increasing the relative humidity of the flue gas.

Introduction and discussion of the different techniques

Both the dry and wet systems have their advantages and disadvantages and as such will require consideration in the selection process. The wet method has a high potential for disposal of exhaust gas.

There is the construction of a multi-stage wet system in the waste- and EBS-incineration plants because their requirements in flue gas cleaning are broader and stricter. Also in the thermal waste treatment plants, there must be zero waste water emission. As a result of this, cleaning water left must be reprocessed and vaporised.

The multi-stage systems are however costly and require lots of energy. So the systems that have been in use in the last years are the single stage. In the utilisation of the single-stage dry sorption technology, when the concentrations of the raw gas HCl and SO2 are up to 2.500 mg/m³N  and 2.000 mg/m³N respectively under the economic conditions, the emission limits are met.

Flue Gas Cleaning Systems Optimization

There are several measures being taken to increase the efficiency of the flue gas cleaning systems.

Increasing the potential of lime based systems. Increased relative humidity is the greatest potential in a lime-based system. Measures to be taken are –  Recuperative gas cooling (for instance, with a heat exchanger, injection of water into a cooling tower (this is associated with a steam cooling system) and steam injection.

Regardless of the relative flue gas humidity, an optimised removal requires a good filter flow and also a suitable recirculation of the reaction salts, especially CI-salts. Which optimisation is the most feasible and appropriate should be determined from the practical application and evaluation?

Increasing the potential of sodium bicarbonate systems – Ensure there is high temperature, warranty of a homogeneous distribution, realisation of sufficient residence time, and top surface or small particle sizes.

With the flue gas cleaning systems, pollutants are continuously removed from the environmental cycle – pollutants that were initially released to the environment from wastes.

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