Engineering

Flue and waste gas cleaning

Flue and waste gas cleaning

We are engaged in studies and consulting focused on technologies for reducing and eliminating the concentration of SOx, NOx, PCDD, PCDF, VOC and other pollutants in various industrial plants such as (power plants, heating plants, waste incinerators, foundries, cement plants, etc.). We process studies for small local sources up to large industrial units in the range of flue gas flows from 10,000 – 3,000,000 m3 / h. We can also prepare studies for modernization and intensification of existing resources.

Flue gas desulphurisation

In natural gaseous fuels, sulfur is most often found in the form of SO2 or SO3. For solid fuels, especially coal, which is used in most stationary heat sources, sulfur is already bound in the fuel as part of the combustible. In these fuels, sulfur can also occur in 3 different forms, mainly as sulfate (chemically bound to ashes), organic and pyrite. Pyrite sulfur is in coal in the form of sulfur which, in the case of non-dispersed occurrence in the fuel, is relatively well removed by conventional methods, such as treatment in hydro cyclones. However, the effectiveness of these methods is often limited. There are also other mechanical ways of removing sulfur from fuel, but it is a question of economic efficiency, because these methods are often not economical in terms of the selling price of heat in our market, precisely because of a large development of smaller environmental resources and use of cogeneration units. Today, practically the most used methods of removing sulfur compounds are from the flue gases entering the chimney. Sulfur removal can be performed in two ways (fundamentally different), either by catalytic oxidation to SO3 and subsequent removal in the form of H 2 SO 4 or by binding to a suitable solid additive.
Dry method of SOx reduction

Dry method of SOx reduction


The dry additive method is mostly used in combination with textile filters. The principle of this method is the dosing of the additive based on Ca2 + (most often slaked lime Ca (OH) 2), but also based on Na + (NaHCO3) into the flue gas stream into the flue or reactor, where the additive is intensively mixed with the flue gas and where to the primary reaction. The secondary reaction occurs on the filter fabric, which is intense, especially in the case of filters using fan blow regeneration.

This method is used for the desulphurisation of smaller combustion sources and for the reduction of HCl, HF, dioxins and other gaseous pollutants.

It achieves efficiencies of up to 75% for desulphurisation and over 90% for the reduction of HCl and HF.
This method has very low investment costs, but its disadvantage is lower efficiency and higher additive consumption.
Sometimes it is appropriate to supplement this method with intensification, whereby spraying water in the reactor we can achieve higher efficiency and lower consumption of the additive.

Semi-dry method of SOx reduction

Semi-dry method of SOx reduction


Another method used is the so-called semi-dry desulphurisation method. This method is preferred mainly for power units with an installed capacity of up to 300 MW. It is characterized in particular by the fact that the desulphurisation product is suitable for permanent storage in a normal landfill, but is not very suitable for further use as secondary raw material. In principle, this is a simple process that is easy to manage in practice. By injecting water into the flue gas stream, their temperature is reduced to a temperature 10-20 ° C lower than the flue gas saturation temperature (due to flue gas condensation and low-temperature corrosion in chimneys) and Ca (OH) is fed into the flue gas in powder or aqueous suspension. ) 2, which further reacts according to the relations shown on the right.

The advantage of this method is the reactivity of the reagents to other gaseous pollutants, such as hydrogen chloride or hydrogen fluoride, and thus to their partial removal from the flue gas.

Wet method of SOx reduction

Wet method of SOx reduction


Today, the most used method is the so-called wet limestone leaching. It is the most widespread method in coal energy and more or less the only method used today in modern power plants. The basic difference compared to previous methods is that it is a wet scrubbing of the flue gas stream with a reagent in the reactor at the same time to form the so-called end-product (energy gypsum), which can continue to be used as secondary raw material in construction, as a basis for road travel, or plasterboard.

The whole process consists of a series of sub-processes that implement the individual zones of the desulfurization reactor. This reactor is often called an absorber. The basic principle is the introduction of untreated flue gases into the absorber, where these flue gases are sprayed with a lime suspension in several levels. The design of the structure, the number of shower levels and the choice of the type of nozzles are usually based on CFD simulations to achieve the largest possible interfacial area of ​​the reagent and flue gas for the most perfect cleaning. The cleaned flue gas then leaves the upper part of the absorber to the existing chimney of the power plant. At the outlet of these flue gases from the absorber, there is a continuous measurement not only of flue gas emissions but especially of the flue gas temperature so as to ensure that this temperature is always at least 10 ° C higher than the dew point temperature of the flue gas at a given pressure. In practice, this flue gas temperature is in the range of 68-58 ° C. The absorber is usually a metal container with internal rubberizing in several layers. Shower levels are always at least 2, but in practice often 3. Above these shower levels there is still a device, the so-called droplet separator, which reduces the mass flow of water in the flue gas, and thus the loss of the working medium. These are mostly louvre grilles with rinsing nozzles, which are automatically performed by the ASŘTP system every few tens of minutes. The lower part of the absorber is formed by a collecting bottom, where a certain level of gypsum suspension remains. At these points, oxidizing air from the oxidizing air stirrers is introduced into the absorber. Furthermore, absorber stirrers are located here to mix the suspension and thus create a better environment for oxidation. This limestone-gypsum mixture is constantly recirculated by large recirculation pumps to the upper parts of the shower level nozzles. Due to the abrasive environment, these pipes are always made of fibreglass, referred to as FRP. The final product after flue gas showering is pumped by exhaust pumps into emergency sumps or into a thickener, where the resulting mixture is concentrated for dispatch outside the heating plant premises.

This method is very efficient and effective, but requires large premises for limestone management, to provide process water for flushing all pumps, construction of new buildings with gypsum and limestone slurry tanks and many other operating media needed for continuous flue gas cleaning. The method often achieves efficiencies of up to 98.5%. The usual pH value for proper desulfurization function is in practice around 5-5.5.

The resulting product is then obtained by sucking off the gypsum suspension from the collecting part of the absorber by suction pumps, which is further discharged to the mixing centre for dewatering. From the mixing centre, the suspension is further discharged into a thickener, which in practice can dewater the suspension up to 30% by water weight.

denitrification

Flue gas denitrification


Denitrification means the reduction of pollutants, especially NOx compounds from flue gases. These compounds are formed during the combustion of fuels at high temperatures (on the order of temperatures exceeding 1100 ° C), where thermal nitrogen compounds are most significantly formed. Fuel compounds bound in the combustible of a given fuel are also released into the flue gas by decomposition. Today, 3 different ways of reducing these pollutants are used (so-called primary methods of reducing NOx):

  1. Measures regulating the combustion system itself
  2. Design intervention in the combustion chamber
  3. A combination of the two previous methods

The basic elements for the primary reduction of nitrogen oxides are measures regulating the combustion system itself. These include, for example, flue gas recirculation, operation of combustion with a low coefficient of excess air, which is monitored by the ASŘTP system based on dynamic conditions of combustion itself, or various power operating values ​​of temperatures in individual floors of the combustion chamber.

The second way to reduce these oxides is to design the combustion chamber itself. It is mainly the replacement of existing burners with low-emission, stepped supply of combustion air, design of dead corners of the chamber, etc.

The third principle used is various methods combining the previous two categories, most often modifications of fuel grinding circuits together with regulation of the supply of primary, but especially secondary air to the combustion chamber.

Other methods of reducing NOx from flue gases are methods based on injecting an additive based on ammonia or urea into the flue gas.

Selective non-catalytic reduction of NOc

Selective non-catalytic reduction of NOc


Selective non-catalytic reduction consists in creating reduction conditions under which ammonia or urea injected into the boiler selectively (preferably) reduces nitrogen oxides to produce elemental nitrogen and water vapour. The NOx reduction efficiency is 40 to 60%. A characteristic feature of this method is that it takes place in the boiler in the temperature range of 900 to 1,050 ° C. The use of ammonia as a reducing agent has some disadvantages. Ammonia is a health-hazardous substance that requires more complex technological equipment for storage and handling when it leaks, the environment is disturbed by odours, the resulting compounds of ammonia and sulfur can form unwanted deposits on machinery. For these reasons, urea is used instead of ammonia in some processes.

Selective catalytic reduction

Selective catalytic reduction


The selective catalytic reduction is based on the same chemical reactions as the previous non-catalytic reduction, but thanks to the catalyst, the reactions take place at temperatures of 300 to 400 ° C. Ammonia is injected into the flue gas, which is then introduced into the catalyst reactor, in which the nitrogen oxides contained in the flue gas are again converted into nitrogen and water vapour. The NOx reduction efficiency is high at 80 to 90%. Catalysts are most often made of oxides of vanadium, molybdenum, tungsten and their combinations. Their price is relatively high and their lifespan is relatively low.