The reduction of harmful gases, dust particles or other types of components from industrial gas flows can be achieved by various methods. There are many types of air emission limiting techniques (see table as used by InfoMil and Vito) which are very specifically suitable for a very small range of the total spectrum of (harmful) components.
|WPrinciple||Components to be removed|
|Dry dust||Dry dust||Volatile
|Absorption scrubber (acidic, alkalic, oxidation)||++||++||++||++||++||++||++|
|Thermic oxidation (catalytic)||++||++||++|
|Photo oxidation and Ionisation||+||+||+||++|
|Chemical reduction (sncr, snr)||+||+||++||++|
The table shows roughly which components can be removed with the mentioned techniques. These techniques are classified by InfoMil and Vito as Best Available Techniques. Tin this table the components to be removed are considered as separate substances. However, it becomes more complex when the gas contains a composition of several components in which both various gaseous and dust-shaped components are carried along. When the gas also contains a lot of moisture, the options are greatly reduced.
The widest applicable technique in the sense of removal of various components are the groups of dust scrubbers and absorption scrubbers, also known by the collective name scrubbers. These techniques are widely applicable with regard to the gas flow rate and the material load of the gas and in the end are also simple in terms of construction.
Ravebo has been designing and constructing custom-made gas and dust scrubbers for various applications (https://www.ravebo.nl/industrie/gasbehandeling/engineering) for several years now. Depending on the gas composition, the available space and the requirements as set out in the Activiteitenbesluit, the gas scrubbers can be carried out both vertically and horizontally.
The difference between custom-made dust scrubbers and gas scrubbers is often in the details with regard to the construction. The gas and dust scrubbers will be constructed from a liquid-gas integration phase and a liquid-gas separation phase at all times. The operation is based on the fact that the components to be separated must first be captured by washing liquid droplets before they can be separated.
The amount of harmful particles - also called load of the gas - the particle size and the nature of the particles (dust or gas) determines the configuration and thus the construction of the gas scrubber. This applies to both horizontal and vertical gas scrubbers.
Liquid-gas integration means that with the help of a scrubbing liquid, as much contact surface as possible is created with which the gas is brought into contact. The incoming contaminated gas must be brought into contact with a washing liquid which – if necessary – is provided with one or more additives. These additives are only used when plain water as a washing liquid will not be sufficient. Acidic, alkaline or oxidizing chemicals can be used as a dosing agent. Depending on the composition of the gas, combinations are also used which are then dosed divided over several washing sections.
Another aspect of the gas is the particle size and the load. The larger the load and the larger the dimensions of the (dust) particles in the gas, the coarser the gas scrubber will have to be constructed. If the material carried with the gas has the property to be able to seriously pollute or even clog the gas scrubber, the gas scrubber must have an open structure.
Of course, there are also conceivable situations in which the gas carries both coarse particles and difficult to remove gas components. These latter components must be removed or neutralized from the gas scrubber internals using a very fine configuration – an extremely large contact surface. In the case of a horizontal gas scrubber, the gas scrubber must then be built from several sections in a row and in the case of a vertical gas scrubber, there will be mounted several washing stages on top of each other or several columns in a row. Of course, measures must be taken to prevent contamination between the washing liquids in the different sections or washing stages.
The liquid-gas integration process can be carried out in a variety of methods where it is important to create the largest possible exchange of contact surface under all circumstances. Today, this contact surface is often created by using random packings, structured packings or by an open-spray system with special nozzles that form very small droplets and are inserted with high velocity into the gas stream.
The composition and/or loading of the gas ultimately determine which of the systems are used, in case of a combination of coarse (dust) particles and difficult to remove (gas) components, a combination of the different systems can also be chosen. In the case of specific odor scrubbers, the same considerations for the configuration are basically made as for chemical scrubbers, on the understanding that the scrubber is not assessed on the absolute emission (cargo) but on odor load [ouE/m3]. However, the working principle is completely the same as a (chemical) gas scrubber in which the additives used are specifically chosen for the odor components to be removed.
In the food and biofuel industry, fat-containing emission flows can occur. This requires a completely different approach since in general a clean gas scrubber functions better than a (strongly) contaminated gas scrubber. This means that in many cases the fat component must be removed firstly before removing any other odor or gas components.
Gas scrubbers using a washing liquid that is based on water only removes water soluble components. If this is not the case, additives can offer a solution in certain circumstances. Equally, there are conceivable situations in which the components carried along in the gas must be oxidized or reagented away. This is done in a buffer from which the washing liquid is also pumped around over the washing section. As mentioned above, the capturizing process must take place in the washing section itself as a result of the large contact surface and then the chemical reaction will have to take place in the buffer. This often requires more time than is strictly speaking available in the washing section; the residence time is too short for this. The content of the buffer is therefore of great importance in order to achieve sufficient residence time for the necessary reaction at a chemical gas scrubber. The chemical reaction can only take place when additives are dosed into the washing liquid. This requires a so-called dosing installation which adds the exact amount of chemicals in an accurate and safe manner. This chemical housekeeping must then be monitored with the help of various sensors.
The washing liquid will then pollute due to the process by the absorption of the carried (dust) particles and the gas components with the result that the liquid will become saturated. On the other hand, the residual products of the chemical reaction will also accumulate in the washing liquid. However, the washing liquid is continuously pumped around the washing section and will therefore, when there is saturation or contamination, then give components to the gas. This is of course undesirable and can easily be avoided by regularly changing the washing liquid. The washing liquid is kept at the right quality by regularly draining and then adding clean fresh water. The discharge is based on the measured values recorded by the sensors.
The refresh process then affects the chemical content of the washing liquid and so dosing will have to be done again to keep the liquid at the right quality. The quality of the scrubbing liquid is directly proportional to the efficiency of the scrubber; the cleaner the liquid, the cleaner the emitted gas will be.
The control of all sensors used, the discharge and supplementation valves, the pumps for the washing liquid and the dosing pumps is controlled by a PLC which is mounted in the control cabinet. Ravebo also takes care of this completely in-house and the control cabinet will be designed and built for each process specifically. The same applies to the PLC which is programmed by our own engineers, and if desired, can be monitored remotely during operation to support the operator of the installation.
The particles captured by the scrubbing liquid must be removed from the gas stream eventually in order to reduce emissions finally. The liquid droplets are removed from the gas stream by droplet separators based on mass inertion. The techniques used for this vary; from vane type separators to wire mesh demisters, each with their own specific characteristics and fields of application.
The yields for the removal of most gas components are often very high due to the use of additives in order to clean the gas in a simple and cost-effective way. In case of dust particles, enormous separation yields can be achieved so a gas scrubber can often replace two or more other techniques.
A custom-made gas scrubber has an enormous wide range of applications and excels in versatility. Especially in situations where the gas composition and load is very diverse, a gas scrubber can be used as the only applicable technique. The installations are hardly susceptible to pollution and due to their construction have a long service life, which makes them also very durable.
Another aspect is that the footprint is not an obstacle, so that a gas scrubber can be placed almost anywhere, both inside and outside. In addition, based on location, composition of the gas, applied chemistry and temperature, a choice of the construction material can also be made, such as various types of stainless steel up to several types of chemically resistant plastic. Stainless steel also makes it possible to clean very hot gases. In addition, this also has to do with the fact that a wet gas scrubber is an adiabatic process that causes the gas temperature to drop very quickly (this is also called quenchen) to the saturation temperature of the entered dry gas.
Gas scrubbers can also be used over a huge range when it comes to the size of the gas flow rate. From a few tens to even several hundred thousand cubic meters of contaminated gas can be cleaned per hour with a gas scrubber. Another aspect is the fact that fluctuating flow loads but also varying gas loads have hardly any influence on the operation of the gas scrubber due to the buffering effect of the liquid buffers. Gas scrubbers have a large turn-down ratio which means that the gas scrubbers can function in situations where the gas flow rate is 25% of the design conditions.
The system is therefore extremely suitable for badge processes and can even be used without adjustments in (production) processes that are shut down during the weekend. For winter operation, it may sometimes be necessary in these situations to apply insulation and /or tracing or to drain the installation.
The installations are ultimately easy for almost every operator to operate and maintain, while the inspection and maintenance intervals can also be particularly long. Thanks to the PLC control and the self-evident HMI, the gas scrubbers are easy to monitor. Provided that the sensors register the set values, this means that the gas scrubber functions excellently and with the exception of, subject to the control of some pumps, instruments and fittings, does not require maintenance.
Finally, Ravebo configures the gas scrubbers fully modular, adaptive and segmented in such a way that, in the event of the (production) process, activities decree or legislation being adjusted, they can be made suitable for the new (process) conditions without major investments.