Lime in the industrial flue gas treatment
Thermal industrial processes give off polluting flue gases and these pollutants, which start out as part of the raw material being processed, become gaseous when the temperature rises during manufacturing. The drawback is if these gases are given off into the atmosphere without any treatment, and once the temperature of the gas drops, a moment is reached (the dew point) when the pollutants once again are in a liquid phase, and they have the potential to adversely affect the surroundings or enter the trophic chain. Hence, it is essential to capture them during the production process before they are emitted to the atmosphere.
Lime and its derivatives are one of the most suitable absorbents for the treatment of gases. Their natural origin, easy handling, high reactivity, richness, capacity to abate acid pollutants and the low solubility of the resulting products after the neutralization reactions, mean that they are widely used in many industrial processes. Another important function that lime and its derivatives carry out is that of facilitating the protection of the filter bags, used in almost all gas treatment systems.
When lime comes into contact with gas, it reacts by absorbing the pollutants on its surface. Lime, because of its great affinity, is specifically suited to the treatment of acidic gases, (Fluorine, Chlorine and Sulphur). Its neutralization capacity and its high reactivity make quicklime (Calcium oxide) and slaked lime (Calcium hydroxide), into perfect products in any of its formats.
The type of industrial thermal process and the characteristics of the gas that is emitted (temperature, flow rate, combination of pollutants…), determine the method of gas scrubbing to be implemented at any plant. Generally, the treatment procedures can be classified in the following way:
1. Dry scrubbing, where the reactant used is injected as a micronized dry dust, with the result that after the chemical reactions the waste product is completely dry.
2. Semi-dry scrubbing, the gas is washed and cooled with water, to retain the pollutants, before the dry micronized reactant is added (primarily, the efficiency of the desulphurization is enhanced as the additional humidity injected is deposited on the surface of the solid absorbent, “catalyzing” the desulphurization reaction), the final waste product is a dry dust.
3. Semi-wet scrubbing, in this case the absorbent injected is in the form of milk of lime The milk is pulverized and injected as a spray with air or by mechanical atomization. The lime milk is injected against the flow, in the form of very fine droplets (as a mist), the temperature of the gas evaporates the water and after filtering the final waste product is a dry inert dust.
4. Wet scrubbing, two reactants are normally used; milk of lime and micronized limestone/chalk milk. This procedure requires the installation of a gas scrubbing tower where the reactants are injected at different levels along with a significant volume of water. The suspension is applied in the form of counter-current dispersal where, as it mixes and reacts with the rising air, it is oxidized into Calcium sulphate. This is recovered mechanically and may, in some cases, be used as a construction material. The gas treated with this humid system must be, in general, reheated so that the dew point in the tower is not reached.
In the figure below, a generic diagram of a gas treatment plant can be seen, with examples showing the possible points for the injection of the NATURDEP® range of products.
Fig. Gas treatment plant and applicability of NATURDEP® products.
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