Treatment of air emissions: how activated carbon compares with other technologies
Across Europe, industries are seeking effective solutions to solve the problems of contaminated air emissions.
Activated carbon filtration is an efficient and adaptable solution in many situations, either on its own or in combination with other technologies.
Typical challenges in air emissions
There are two main forms of air pollution which activated carbon can treat:
- Volatile Organic Compounds (VOCs). Many are subject to regulations as they can be harmful to human health or the environment.
- Odorous compounds. These can be VOCs as well, or inorganic compounds such as H₂S and NH3. Regulations aim to make conditions more pleasant for employees or nearby residents.
Plants are subject to emission limits depending on the industry, the type of compound, its toxicity level, the region, and proximity to residential areas. Benzene, for example, which is known to be potentially carcinogenic, is strictly controlled.
Companies may choose to go lower than the legal limits, for example to improve the smell of the workplace for their employees.
Sectors and sources
Activated carbon can be used to treat compounds from different sources in a range of settings.
Polluted air emissions are problematic for various industries. This is especially the case for chemical, petrochemical and pharmaceutical companies because of their wide use of solvents or base chemicals and the presence of intermediates. Producers of plastics, polymers and composites have to deal with very similar issues. However, for composite production, one issue is observed throughout the sector: odorous styrene emissions.
Generally speaking, the manufacturing industry has to deal with VOC emissions from, for example gluing, coating, drying and cleaning of products. VOC emissions and odour control are important topics for all companies involved in waste handling or recycling. These emissions are characterized by a wide variety of components, directly related to the type and composition of waste. Although the specific issues can differ from sector to sector, we’ve listed several key sources of air emissions throughout industry.
Production processes. VOCs and odours may be emitted at many stages, including from reactor tanks, coating lines and drying processes.
Tank storage. Products can be stored in gas or liquid form. Activated carbon filtration is most useful for liquids, such as petrol, benzene or more specialized organic chemicals. It can be used for emissions from huge storage facilities at ports, smaller tanks at industrial sites, and transportation containers. Emissions are typically discontinuous flows with high concentrations.
Production halls. If there are leaks in valves or sealants, tiny amounts of VOCs and odours can be released, creating an unpleasant working environment. This scenario typically produces low concentrations and high flow rates.
Aside from industry, other sources of VOCs and odour pollution include agriculture and transport. However, as these are not point sources, activated carbon filtration is not applicable.
In recent years, industry has made great strides in tackling air pollution. As limits have become more stringent, companies have sought to tackle even the smallest sources of emissions.
Air emission treatment technologies
These are the main air emission treatment technologies in use today:
Thermal oxidation, or Regenerative Thermal Oxidisers (RTOs) burn all organic compounds in the air at high temperatures (lower in a catalytic system). For continuous high VOC concentrations, this is generally an efficient and cost-effective method.
However, thermal oxidation is not ideal for treating fluctuating concentrations or discontinuous operations. In this case, natural gas needs to be added to the flow to keep the temperature steady. From an environmental and a cost perspective, this is problematic.
Biological treatments are fixed filters that contain a bed of microorganisms that consume the VOCs. They are well suited to treating airflows with low VOC concentrations from the food or feed industry, particularly if the aim is odour control. Nevertheless, it should be taken into account that a biofilter can emit some odour of its own.
However, the bacteria can only consume biodegradable VOCs. Toxic compounds will not be removed, and may even kill off the bacteria. The bacteria also require steady process conditions: too low concentrations, and they will starve; too high, and they will become overwhelmed. Furthermore, the microorganisms may have other requirements such as sufficient oxygen and humidity in the gas flow. This could result in additional equipment.
Scrubbers use water, acid or caustic solutions to absorb components. It is important to choose the scrubbing liquid based on the pollution. They are well suited for high concentrations, for example from a chemical factory. They can struggle to reach low emission limits or treat different types of VOCs, although several (different) scrubbers can be a solution.
However, in all cases they create a liquid waste stream that also requires treatment.
Condensation involves lowering the temperature so that the VOCs become liquid and can be removed. If liquid nitrogen is used for cooling, it is called cryogenic condensation. It is useful for treating flows with a single pollutant for recovery of this molecule. Such systems can be found at storage tanks, ships and silos, particularly for companies which already have liquid nitrogen on site. If not, it is unlikely to be used as liquid nitrogen is costly. Cryogenic condensation is not an easy technology to operate and while it easily removes the bulk of the concentration, it can be difficult to reach low emission limits.
Activated carbon filters do not burn, decompose or change molecules, but simply adsorb them onto the pores of the carbon filter bed. There are different carbon types depending on the compound, and the filters are dimensioned to suit the flowrate and inlet concentrations.
Activated carbon is suited to many applications, but if VOC concentrations are very high, then carbon consumption could be excessive. Thermal systems or a combination of treatments should be considered instead.
In many cases, activated carbon is used in combination with other treatments.
Thermal systems: Activated carbon can remove certain compounds from the airflow before thermal treatment, preventing the generation of flue gases that could exceed emissions limits.
This combination works well for emissions from storage tanks of petrochemical products, where there are high concentrations of hydrocarbons.
It can also be used not to capture VOCs, but as a sulphur guard. If sulphuric compounds and H₂S are burned in an RTO, it results in sulphur oxide. This greenhouse gas can cause acid rain and is subject to emission limits as well. Burning sulphur also decreases the life expectancy of the RTO.
DESOTEC filters act as sulphur guards for an RTO at petrochemical storage tanks at the Port of Rotterdam.
Biological treatments: Activated carbon can be used as a pre-treatment in two main ways.
Firstly, it can treat toxic compounds that would otherwise kill off the bacteria.
Secondly, it can act as a peak shaver. Biofilters require a steady inlet concentration, otherwise the bacteria may be overwhelmed. If concentrations fluctuate, activated carbon can filter out the peaks before the biofilter treats the remainder.
One DESOTEC client is a factory that makes PVC for windows, where the aim is to treat odour pollution. Our filters act as a peak shaver before biological treatments in this setting.
Thermal systems are dimensioned according to inlet concentrations and airflows. If these change, perhaps because production has increased, the system’s capacity might be exceeded. Activated carbon can polish the airflow to remove the last traces of VOCs.
Biological treatments do not always capture all VOCs. Certain compounds are not biodegradable, and fluctuations in production levels and seasonal temperatures can affect the system’s efficiency. Also, biofilters may themselves produce odour. In these cases, activated carbon can be used as a polishing step.
DESOTEC supplies filters to a factory that produces flavours and fragrances, whose biofilter was underperforming. It now uses activated carbon to polish and as a potential back-up in case the biofilter fails.
Scrubbers are sometimes insufficient to get VOC concentrations down to legal limits. They can be used to treat 90% of the VOCs, with activated carbon as a second step to polish.
Activated carbon can also be the final step in a train – for example, a caustic scrubber to remove H₂S, an acid scrubber to remove ammonia, then an activated carbon filter to remove organic components.
However, a dehumidifier might be required between the scrubber and the filter, making it often simpler to replace the scrubber with activated carbon.
Another DESOTEC client is a factory that produces fragrance blocks for toilets, whose water scrubber was proving ineffective. After a pilot scheme, the client decided to replace the scrubber with a DESOTEC filter.
Condensation may be insufficient to tackle the very last few milligrams of VOCs, so activated carbon can be used to polish. This might be the case with benzene, which is subject to stringent limits.
DESOTEC has a pharma client that uses our filters as an alternative to cryogenic condensation to remove methylene chloride.
Activated carbon filters can also be put in place as a back-up, and used during maintenance, due to a fault or a disaster situation, or in case concentrations of VOCs rise above the Lower Explosion Levels (LELs).
This is particularly useful in industries where production is 24/7 and any stoppage would be very costly.
DESOTEC activated carbon filtration solutions
DESOTEC provides a 24/7 service across Europe that takes care of clients’ air pollution concerns so they can focus on their core business.
Our filters are space-saving systems, far smaller than alternatives such as fixed biofilters.
They can be supplied swiftly, and installed on a temporary basis. This makes them well suited to urgent situations, such as sudden odour emissions. Once installed, they can be later tweaked to find the optimal solution.
DESOTEC filters are provided on a daily rental basis, so require very little upfront investment from companies.
As they are modular, they can be installed as an additional step in situations where concentration levels fluctuate, production might expand, or emissions limits might be lowered in the coming years.
They are easy to install and remove, and DESOTEC technicians handle filter exchanges so clients do not have to handle saturated carbon themselves. Instead, we transport spent carbon to our furnaces, where contaminants are destroyed and the carbon reactivated, making our filters an environmentally sound solution.
How can DESOTEC help you?
Across Europe, we are seeing clients switching to activated carbon for air emissions treatment, or including our filters in their thermal, biofiltration, scrubber or condensation systems.
To discuss how our solutions could work for your company, contact our engineers today.
At DESOTEC’s facilities, all used carbon is analysed so the right measures can be taken for handling and removing the saturated carbon out of the mobile filters. All molecules that were adsorbed on the activated carbon at the customers’ site, are desorbed inside DESOTEC’s reactivation furnaces. These contaminants are then fully destroyed, in accordance with National and European legislation, by an incineration and neutralisation setup. The entire installation and it's emissions are under continuous on-line monitoring, which guarantees that only harmless water vapour is seen exiting the chimney.