Soot Filtration: How can you test for it?

In a recent blog on aerosols and dust I mentioned briefly soot testing. In this blog, I want to go into more detail about soot testing and some of the challenges and possible solutions that you can find out there. 

What is soot?
Soot is the result of the incomplete combustion of organic materials such as coal, wood, or liquid hydrocarbons. Soot particulates are associated with much of the urban pollution in both the developing and developed world and are often a major contributory factor to pollutants known commonly as "PM 2.5". As we will see, this is not technically correct for soot but the implications of these particulates on health cannot be underestimated, contributing significantly to premature deaths in extreme cases. 

The soot particle is not like a typical dust where the material is a defined material with a defined particle size range. Typically starting at about 50nm the hot soot particle from incomplete combustion will grow through particle coalescence over a short interval of time to around 100-120nm cooling from a hot, sticky particle to a cool, resinous material which is much harder. 

The result is that the behaviour of soot when interacting with filter materials can and does alter as a function of time so, a smaller hotter particle will stick to the surface of a filter whilst a cooler, harder particle will behave in a much more traditional way. 

The other issue to be aware of is that the chemical composition, it's softening point and particle size also depends upon the chemical nature of the combusting material as well as the level of oxygen at the point of combustion. This means that use of poorly defined materials such as kerosene can lead to significant differences in the nature of the soot. 

How does soot impact my filter? 
Soot in the air has a significant impact on the lifetime of filters, this is a well known and accepted fact. It leads to shorter operating lives of all air filters. Soot build up is also seen in lube/oil filtration where it enters into the oil and, again, can impact the lifetime of the element. 

Beyond this hot soot also passes through air filters and builds up on the airflow and other sensors in the air management system of a car or truck.   

How can I measure soot filtration? 
Simulating soot is one of the hardest challenges in filtration testing today. The inherent instability of soot particles makes standardising the testing protocols very challenging and this is, as of 2014, still in development. There are a number of different solutions in the market, the simplest being a small kerosene burner (Often known in the US as "Tiki Torches") feeding soot into an airflow. A typical example from Ahlstrom is shown below. 

Soot test stand set up (from Ahlstrom Particles Q1 2012 p3)

Other companies also have such a test set up including the test institute IBR. These systems can generate a very consistent particle size distribution between 90 and 150nm (if measured).

However the only problems that these set ups have is the probable inconsistency of the soot generated due to the reasons stated above due to the fuel, face velocity factors meaning that particle growth time and particle temperature at the surface mean that the nature of the particles can and will be different based on test scenario. Inconsistency and non traceability is a key issue that has to be addressed.

One final challenge with soot is the inherently long run times (30 minutes testing led to little or no pressure drop increase at 15cm/s on a typical flatsheet air filter sample). 

The issue of slow loading is a fundamental one in soot testing. If the concentration of soot is too high, it is not easy to measure the particle size because the number of particles per volume of air becomes too high and the actual particle size will be larger as the close proximity of the soot allows for more agglomeration of the particles. This is seen in all soot testing set ups. 

Exhausts
The obvious source of soot is the source that generates it. Some set ups use diesel engines as a source of soot (one ex colleague of mine once tried to use this as a justification for the company to buy a Bugatti but that didn't really work), funnelling the soot through to the test stand from there. Another, less subtle option used by a filter manufacturer with labs in central Stuttgart was to apparently syphon the air from the polluted street outside the building. However in both of these cases reproducibility elsewhere is the major challenge.    

Gas soot generators- Chilling off
In trying to standardise soot testing the ISO standards committee have tried to remove the inherent variabilities of such approaches and standardise the protocol. The current proposals are to generate a soot standard to ISO DIN 12103 which would be an annex to the current automotive air test standard (ISO 5011). The fuel here is ethylene or propylene burned in a controlled oxygen environment using nitrogen to control the degree of combustion. The soot is then quenched to fix the particle size and stop further coalescence and agglomeration of the particles. 

Such systems are already in commercial production by companies such as Matter  (known as REXS) and are used for both filtration testing and for testing of other systems (such as DPF catalysts by Bosch for instance).  This system can generate between 50mg/hour at 80nm and 3g/hour at the larger 200nm by increasing the concentration of the particles. 

The advantages of such a system is clear. The fuel is well known and chemically controllable, using a controlled quenching enables the particle size to be fixed easily. The biggest disadvantage however is the cost of the generator itself. 

Graphite erosion
One proposal from Palas is for a spark erosion of graphite. Such a system would use a high voltage system to erode a graphite rod in an inert atmosphere to generate particles of average particle size 98.9nm. The only disadvantage of this system is the relatively low rate of soot generation (0.06mg-7mg/hour) but this can be rectified by use of a small sample size. A concept developed by me shows the layout of such a system. 

concept soot test stand using spark erosion (Tony Lawson)

No soot at all? 
A more traditional approach to evaluating soot is to find an alternative aerosol with a similar particle size such as KCl. This has been used successfully to replicate soot. The biggest disadvantage is that it isn't soot. 

Soot Testing to EN779
A variant of the ISO 12103 proposed protocol was demonstrated to me recently by Unifil and really impressed me. Using propane to generate a quenched sample of average particle size 95nm which is able to test flat sheet samples very effectively to 450Pa. A typical sample on a F8 rated media showed the impact of soot with a DHC of around 2gsm to soot compared with a typical DHC of 237gsm using SAE ISO fine test dust on the same material using a flatsheet test stand. This is a significant demonstration of how  rapidly soot cant lead to a loss in performance. 

So in summary, soot is a nasty material that we have to remove from air. However it is a difficult material to simulate in a lab compared to real life and whilst innovative solutions exist to test soot these are yet to be formalised into documented, approved ISO standards. 

Thanks again for following my blog...until next time

Tony