With the industrial growth, has become more and more urgent the need for a focused and detailed control of polluting components, including those released into the atmosphere. In this context, the industrial fumes treatment plant assumes a fundamental role, which has the aim to purify in the most effective way as possible, all impurities generated passing through the industrial production or the waste management process. The hazardous substances in suspension escaping from the processing cycles are different, so the purpose of the fume treatment plant designed by SMEA Engineering is the maximum respect for the law limits, pursuing a further reduction of emissions falling well below the government mandates, using the know-how accumulated for decades and the latest and most sophisticated available technologies.
Purify industrial fumes is therefore a duty towards nature and the whole community, to be realized in the most efficient and effective way to ensure a good and as healthy as possible life to all living beings.
FLUE GAS PURIFICATION
To illustrate the flue gas purification process we use, by focusing on this aspect, a concrete example of the design actually performed for an our customer, with the following goals:
- raise the efficiency of internal production cycles optimizing the use of the products obtained by distillation processes;
- Ensure the highest environmental protection, particularly with respect to greenhouse gases emissions;
- Implement further the principles of sustainable development;
- Reduce dependence on external energy supplies business;
- Align business performance to a more efficient exploitation of the potential possessed by the agro-energy.
The range of the polluting gaseous emissions produced from the combustion process can be divided into two categories:
1. Macro-pollutants, present in high concentrations. They depend on the fuel type and on the parameters that regulate the good combustion (temperature, residence time of the flue gas in the combustion chamber, volumetric capacities, etc.), and are traditionally detected by:
- Nitrogen oxides (NOx);
- Carbon monoxide (CO);
- Sulfur oxides (SOx);
- Any acid gases (HCl, HF);
- Particulate matter (PM10).
For the reduction and control of these elements, in addition to maintaining the ideal conditions for combustion (as required by legislation) we proceed to further reduction both in the combustion chamber (DeNOx with urea injection, SNCR), and downstream of the chamber (removal of any acid gases and particulates).
2. Micropollutants, which are treated if present in low or significant concentrations, but sometimes they are absent (as in the flue gas from biomass). They can be divided into:
- Inorganic micropollutants: mainly consisting of metals such as lead, cadmium, mercury and arsenic;
- Organic micropollutants: such as dioxins (PCDD, PCDF), polycyclic aromatic hydrocarbons and organochlorine compounds.
For the fuel type used in thermal power plants with biomass prevalence, dioxins are present in minimum conditions (are present in fact in so lower trace that are not revealed by the detection tools) or irrelevant. This is ensured by the requirements imposed by law on good combustion, the avoidance of the synthesis process conditions and the low concentrations of chlorine and metals in the fuel.
Treatment technologies must intervene effectively and, in particular, they are realized:
- Use of absorbent additives;
- Uptake of particulate (fabric filters and electrofilter);
- Development of condensation effects (favoring the passage of the pollutant from the vapor phase to the liquid phase or on the surface of the powders, by lowering the temperature, or by adding suitable additives).
NITROGEN OXIDE REDUCTION (NOx)
The formation of the nitrogen oxide ( nitric oxide (NO) and nitrogen dioxide (NO2), generally designated by the symbol NOx), when it occurs is due to the reaction, in the combustion chamber, of nitrogen and oxygen content in the combustion air and to a greater extent to the reaction of nitrogen contained in the organic waste with oxygen. The NOx reduction takes place through the SNCR system (non-catalytic reduction) of nitrogen oxides into molecular nitrogen (N2), to be made by injection of an aqueous urea solution in the gaseous flow of the combustion fumes.
The gases combustion temperature must be between 500 and 1000 ° C. Below you have a slowing of the reaction rate, resulting in decrease of NOx reduction, while above the urea would tend to react more with oxygen than with nitrogen oxides, forming more NOx. Another determining factor for the reduction is related to the distribution of urea in the camber.
In any case, with the SNCR system in the combustion chamber it is possible to obtain NOx concentration in output lower than 200 mg / Nm3 required by legislation.
The industrial fume treatment plant process includes storage system, distribution and injection of the solution. The storage tank is typically a vertical tank provided with all accessories such as gauges, thermocouples, manhole, access stairs etc., and all connections and valves interception. The solution is transferred, using movement system, from the storage tank to the dosing module, in order to feed the solution and each injector. The injection lances are mainly constituted by an atomization zone where the air and the diluted solution come into contact, and a distribution area. The diluted solution is atomized with a passage through an orifice with the continuous addition of atomization air.
QUENCHING COOLING TOWER
The gaseous effluent coming out of the boiler convection section must be treated to reduce pollutants (macro and micro) that it carries. The first treatment the fumes undergo consists in passing through a cooling tower, in order to make their temperature suitable to the treatments that will be performed downstream of the tower itself.
The fumes go into the tower at a temperature which is between 180 and 200 ° C and leaves at a temperature of about 160 ° C. The cooling takes place by water injection, which is nebulized in an atomization zone through compressed air.
Despite the main purpose is to cool the gaseous flow, with the process described is obtained, at the same time, also the removal of a small amount of particulates (whose main abatement will still be the task of the following stages consisting of the sleeve filter and wet electrofilter) as the solid particles impinging the tower’s walls lose energy and fall on the bottom of the tower, together with a small proportion of micropollutants present in vapor state which condense (following a drop in temperature) and can thus be also collected on the bottom of the tower.
You also makes provision for the carbon treatment plant.
The temperature is around 160 ° C, a level that allows an effective abatement maintaining relatively low consumption of reagents and avoid having potentially damaging condensation.
In this section, you get an effective reduction of any macro-acids present in the fumes (SOx, HCl, HF).
Lime is stored in a silo, while the activated carbon in another one. Once obtained the adsorbing additive mixture, downstream of the storage silo, it is injected in a suitable reactor, which is able to optimize the contact with the gas to be purified.
The gaseous micropollutants in vapor state (as well as the macro-pollutants) that have been adsorbed, or condensed, thanks to the processes described previously, are now being culled due to their passage through a excludable cells bag filter (to ensure continuous operation during the sleeves cleaning). Each cell consists of a set of cylindrical modules, each with side wall covered with a fabric filter, through which is passing a current of gaseous substances that you want to treat.
In the passage through the fabric filter, the gaseous stream is freed of the suspended solid material, which is retained on the surface due to the action of a contemporary retention mechanism (as the solid particles are retained in the weave’s weft) and inertial and electrostatic mechanisms (for which the particles, although smaller than the fabric mesh, impact on the fabric structure due to their inertness and electrostatic attraction, remaining retained). While the filter is occluding, increases its filtering efficiency (since it decreases the size of the pores), but this also entails an increase of the load losses, for which, at regular intervals, it is necessary to clean it. The efficiency of the filter will be continuously controlled in the various compartments, by means of differential pressure instruments. The cleaning of the cells takes place by injection of compressed air, and the particulate retained is collected, by gravity, on the conical bottom and from here sent to disposal.
The efficiency of bag filters is obviously linked to the fabric choice, from which depends the pore size. In any case, the removal efficiencies of particulates are very high, normally more than 99%.
FINAL TREATMENT WITH WET ELECTROFILTER
It should be noted that design and installation of the wet electrofilter as the last stage of fumes purification is redundant in the plant: this choice is usually made in order to operate with criteria of absolute security in respect of the quality of the emissions into the atmosphere .
The stream of smoke coming out of the bag filter (and therefore already purified from the previous stages) is in fact further treated by the wet electrofilter that ensures additional acids, fine dust and other micro micropollutants reduction.
The smoke resulting from the purification phase of the industrial fume treatment plant will then be driven by means of two draft fans to the external chimney, on top of which will be applied a continuous analysis system for emissions dimensioned to measure the concentrations required remaining within the regulations limits.