This article relates to the design of “end-of-pipe” air pollution control systems that are emitted from industrial processes of one kind or another.
The term ‘design’ specifically relates to the process aspect of design, and it is acknowledged that the full air pollution control system (often called abatement systems) requires other engineering design disciplines such as structural, mechanical and electrical.
As with any design, the cornerstone of the project is the design basis, and it is critically important to acquire not only the pertinent data for the design basis but also the knowledge of the variance of the data. This necessarily requires a fundamental understanding of how the process that is generating the pollution operates (e.g. any batch cycles, start-up, shut-down, discharging or other operational changes that can influence the process emissions).
The key data will typically be in the form of process monitoring that has been carried out. Most companies that provide monitoring services do so with a view to providing compliance reports. They are usually MCERT accredited and as such they have a methodology of reporting data to comply with the environmental legislation guidelines.
This usually means that available data is reported in terms of ‘reference conditions’ and the actual measured data is normalised to a set of conditions such as 273.15 K (0°C) and say, 11% moisture or a certain percent of oxygen.
“only “Actualised” data can be used to calculate the duty that the abatement has to handle in real terms”
Data in this format has to be converted to reflect the actual test conditions at the time of sampling. Only “Actualised” data can be used to calculate the duty that the abatement has to handle in real terms.
What can go wrong with referenced data?
The table below shows the relative quantity of moisture in a given fixed mass of air:
It also shows what the actual flow rate of air would be at the measured temperature.
There are two moisture scenarios in the table, 50% and 75% relative humidity. Each moisture scenario has three temperature scenarios, 0°C (Normalised), 20°C and 60°C.
You can see that the potential error between the design basis being 10,000 m3/hr and the actual flow that should be the basis, can grow significantly (up to almost 43% in this case).
It is essential that any engineer trying to design air pollution equipment has the right basis and they need to understand that data may be sent to them without this error removed (i.e. they may be told that the extract rate is 10,000 m3/hr without any indication as to whether or not the flow is normalised to the reference condition of 0°C (273.15 K).
This of course could lead to an exacerbated error if the temperature and relative humidity are then provided without the qualification of the flow rate. This is because an engineer may apply the correction in reverse to obtain a smaller air mass flow as well.
The importance of emission profiling
Another key mistake that we see in many applications and projects is the reliance on spot sampling for contaminant load data.
Many industrial processes have significant variation in contaminant load and even extraction rates and so it is vital that any spot sample has some context within. The graph above shows a typical FTIR trace for a process emission:
The dashed lines represent the maxima and minima of the FTIR response (red and green respectively), and the blue central line is the average TOC (Total Organic Carbon). The difference often exceeds a factor of 10 (i.e. an entire order of magnitude). This could give rise to an error which would easily direct the designer to the wrong abatement strategy.
The same sort of variation can occur with the flow and there are many processes with spurious emissions that lead to lower net average emissions.
It is therefore important to gain an understanding of the process cycles and define a test regime which will capture the relevant emission characteristics.
Limitations of test data accuracy
Testing is standardised within accredited, standard test methods, using specific types of instruments.
The inherent accuracy of the instruments is usually provided in the report generated by the emissions analyst. Some analytical companies also provide an error band in the results table they provide (example shown to the right). The error band, however, only relates to those parameters to which the equipment has been calibrated to analyse. Some instruments and test methods do not pick up certain compounds at all.
It is therefore important that when you request a given test regime (which may comprise various test methods) that you provide some indication of the sort of species of contaminant that you are likely to find and also the nature of the carrier gas (usually air or nitrogen) that analysis will handle.
Even air (or other gas) flow is difficult to measure accurately. Although instrument readings can be within +/- 1% accurate, the human capability of reading a varying display or the taking of ‘averages or averages’ across a velocity profile will typically lead to a variation of +/- 15% in the flow. Also the flow reading will vary with temperature, moisture and pressure or how the instrument was initialised. It is therefore important to manage the expectations of the level of precision that can be attained with testing and also ensure that the design considers expected variance and worst case scenarios. The example above shows an instrument whereby the temperature is programmed in to convert the accurate velocity pressure reading (top digits) to a calculated velocity reading (for the given test point).
Knowledge of the inaccuracy and limitations of emissions analysis is important for the purpose of setting realistic design goals (whether as the designer or a tender specifier) and more importantly emission targets that can be realistically met.
The main available and well-established technologies are:
- Thermal Oxidation
- Catalytic Oxidation
- Dry Scrubbing (adsorption on either activated carbon or activated aluminium products)
- Wet (Chemical) Scrubbing
- Physical Dust/Particulate Filtration
There are of course other technologies that are less well established such as ozone injection, “ionised air” injection, U.V. light, membrane technology and plasma oxidation but there doesn’t seem to be enough successful installations with these technologies to merit further discussion, other than pilot trials for such an approach are strongly advised.
The following is a brief insight into the major abatement technologies.
“knowledge of the inaccuracy and limitations of emissions analysis is important for setting design goals”
Thermal oxidisers heat up the carrier gas and provide enough oxygen to combust the contaminants. This usually requires combustion chamber temperatures of around 800-900°C with an associated dwell time ranging from 0.6 to 2 seconds in this temperature range.
Oxidisers are grouped in terms of primary heat recovery type:
- Direct DFO – no primary heat recovery and hence the exhaust is at the combustion temperature.
- Recuperative – mechanical primary heat recovery (typically limited to 65% thermal efficiency).
- Regenerative (RTO) – ceramic, direct contact heat exchangers in a dynamic flow cycle
DFO and recuperative systems tend to be used on very high contaminant loads, where there is a high calorific value associated with the contaminant and hence it can greatly supplement the energy requirement. Such applications can often yield enough surplus energy for secondary heat recovery (i.e. generating steam, hot water or basic space heating).
Regenerative oxidisers have very high primary thermal efficiencies (up to 98%) and can operate auto-thermally (without the need for fuel) at lower concentrations of contaminant. They are limited to lower concentrations due to the destruction efficiency limitations imposed by the dynamic cycle operation.
We typically see thermal technologies used in applications with solvents and VOCs.
Catalytic oxidisers or “Catox” systems achieve combustion of the contaminants at lower temperatures (180°C to 300°C) by passing the heat gases across a bed of catalyst (usually precious metal monolithic media or earth-metal alumina media).
They have a lighter construction than the thermal oxidiser by virtue of this lower temperature but have a vulnerability to catalyst poisons such as sulphur, phosphorus, silicone, halogens and the catalyst will have as life of between 5 and 10 years depending on the duty.
Catox units are very good for single, clean VOC applications with moderate concentrations of contaminant.
Dry scrubbing currently offers the most reliable of all the available technologies and also one of the highest removal efficiencies of all technologies (readily removing down to ppb levels).
Activated carbon products are the most common dry scrubbing media with zeolites and alumina-based systems also available.
Such systems tend to be very simple to design and operate and tend to be the lowest capital cost of technology options that could handle the same duty.
However, dry scrubbing systems have limited capacity for holding the contaminant and hence the media has to be regularly changed to maintain compliance. It is the operational cost of the dry scrubber that tends to limit its application to low contaminant applications such as odour control or as a ‘polishing’ stage to another abatement technology.
Wet (chemical) scrubbing
Wet scrubbing, including wet scrubbing with the use of chemical reagents is a well-established and widely used approach for abatement.
The vast majority of wet scrubbers are aqueous (water as the scrubbing liquor base) but it is possible to utilise other liquids.
Scrubbing can be used for both gaseous contaminants (typically packed towers) or solids/particulate removal (typically venturi scrubbers).
Chemical reagents are used to fix gaseous contaminants as salts in the liquor and hence reduce or remove the vapour-liquid equilibrium resistance to mass transfer. Even very soluble species such as ammonia or ethanol can quickly reach an equilibrium that leaves an unacceptable amount of gas phase contaminant and so a reaction in the liquid phase is required to prevent this.
When chemicals are utilised a wet scrubbing system becomes a more sophisticated system requiring safe storage and control dosing of the reagents and this in turn tends to require a higher technical competence for the associated operators.
Wet chemical scrubbing can be very efficient if the type of contaminants to be removed are conducive to fast liquid phase reactions or have low vapour pressure above aqueous solutions. Wet scrubbing is typically associated with inorganic gaseous contaminants for this reason.
Biofilters use gas/liquid interface biological reactions to improve the mass transfer of gaseous contaminants from the air to the liquid.
For a biofilter the gas phase containing the contaminant is in contact with the matrix supporting the biomass. The vast majority of biofilters are aerobic and require ambient levels of oxygen to work.
Bio-scrubbers, also use biological reactions but these are liquid phase reactions only and are used to ‘clean’ the recirculating liquor which is then used in a scrubber vessel. This means that the scrubber is collecting the contaminant, limited by vapour liquid equilibrium and are therefore less effective, in our experience than biofilters.
Biofilters can handle a wide range of contaminants, even those that are not fully soluble in water (some solid phase adsorption can take place depending on the type of support media).
The biomass is a ‘living thing’ that requires food, water and oxygen. The biomass will grow to match the net average input of contaminant that is a viable food substrate. Obviously, the biomass cannot immediately develop to handle a highly variable input and so biofilters tend to perform better in reasonably stable conditions.
The bacteria that are used in modern biofilters are largely available off-the-shelf and so a bio-matrix can be inoculated with the appropriate bacteria to reduce commissioning time and quickly out-strip the development of ‘wild seed’, redundant biomass which doesn’t provide any removal of contaminant.
Biofilters require around 10 -20 times the contact time of other technologies and tend to have a relatively large footprint. Their key ongoing costs are water, wastewater, and nutrient usage. This varies for each application as final effluent from a functioning effluent treatment plant can often provide an almost free source of both water and nutrient.
Biofilters don’t usually have the ability to meet stringent limits and often require a dry scrubber for polishing. However they can provide a low operational cost system for a broad range of contaminants.
Physical dust/particulate filters
These devices are very common and probably the abatement technology with which most people will have some familiarity and understanding of.
For low dust loadings static filters are common and usually have multiple banks of disposable filter elements starting with a coarse filter at the inlet and moving to finer filtration towards the outlet (this mitigates early blinding of filter elements).
For anticipated higher loadings, reverse jet, reverse air or shaker filter are used. These filters self-clean and the filter elements, although more expensive, remain in service for longer. For certain high loadings of heavier particulate then a cyclonic separator could be used to reduce the load to either a static or self-cleaning filter.
The approach taken tends to account for whether the particulate is a valuable solid or a solid that can be re-used within the process.
With high loadings whereby a concentration of more than 20,000 mg/m3 of dust could be generated, then dust explosion measures need to be considered. Often self-cleaning filters have ATEX rated components for this reason.
Dust filters are mainly used to protect downstream abatement technologies from particulate as it is rare that an abatement project is solely concerned with solid contaminant.
Summary of abatement technologies
Where abatement is required then the selection process has to account for the design considerations previously mentioned as well as any legislative requirements. Prescribed processes have abatement solutions that are listed as being BAT and hence are acceptable to the authorities handling the end-user process.
There is also the matter of safety and adaptability and so knowledge of the future plans for the process are important (e.g. will the process use water-based instead of solvent-based coatings, will the process have an extra production line installed etc.).
The above is a rough guideline and it is key to establish all of the design criteria discussed in this article to develop and justify the best solution.
Why do systems fail?
In our experience there are a few reoccurring reasons that lead to system failure:
- Incomplete or incorrect design basis and information acquisition
- Incorrect determination of the worst-case scenario
- Misinterpretation or lack of awareness of legislative targets
- Novel, untested technology
- Unreliable utility supplies (electricity, fuel gas, water)
- Poor installation of the designed equipment
- Lack of flexibility to process change or growth
However, the main cause of system failure is a lack of maintenance. This is often down to a mix of operator training and resource management. But, it can also be due to overly onerous maintenance regimes arising from a lack of design consideration for the maintenance of the system.
This is why any system design has to generate the appropriate quality of associated documentation and training to allow the system to be maintained as required.