Air Monitoring

An Overview of a Relatively Simple Method to Determine the Biogenic versus Fossil Fuel Component of a Source Emission: BS ISO 13833: 2013

Jan 31 2022

Author: Roger Brown on behalf of Oakwood Environmental Services

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In this world, the origin of waste is becoming increasingly important to operators who dispose of waste. Often, taxation on the disposal of waste is based on its biogenic credentials. Rebates are sometimes given where energy is produced from sustainable renewable biomass or biogenic material. For waste sites, biogenic materials may be taxed very differently from non-biogenic materials.

As producers of goods get more adept at producing products either wholly from biogenic sources or from a mixture of biogenic and renewable sources, the difficulty in distinguishing between material that is fossil fuel based or biogenic based is becoming increasingly difficult; and yet there are fiscal reasons to distinguish the biogenic make up of waste materials.  
The standard methods are:
1. All the material is CO2 Neutral; this is ideal where the waste stream all arises from a single known sources such as biomass e.g. straw, willow etc.
2. Selective Dissolution Method (SDM); a small fraction of the waste is treated with an oxidising agent, typically a mixture of hydrogen peroxide and sulphuric acid. The biomass fraction is oxidised faster than non-biomass fractions and hence the amount of biomass can be calculated after correcting for moisture, inert materials (such as ash) and the amount of carbonates present.
3. The Reductionist Method; this can be applied where the biomass content is in the range 20-80%. It doesn’t work well where there are high calorific contents in the waste (such as fats). The calorific value (CV) of the fuel is determined along with the ash content, and moisture content. Then, knowing the CV of individual constituents of the waste, the percentage biomass can be calculated. Unsurprisingly this works well where the contents of the waste stream are well defined/characterised.
4. The Manual Sorting Method; the waste is sorted into sub-fractions and categorised. It is then sorted into material that is bigger than 1 cm, dried and weighed. The precision of this method can be assessed using the SDM method discussed earlier. This should be carried out in triplicate. Any significant differences in the SDM results suggest that either the sorting method is poor or that the waste is highly heterogenous.
If we consider the incineration of refuse, the tax associated with the disposal of waste is becoming crucial. Furthermore, there are tax benefits from the energy regulators where it can be proven that the waste was biogenically derived. Traditionally how was this achieved?
It hasn’t been a pleasant process, as anyone involved will tell you. Ideally, first take 50 tonnes of waste, then split it into a smaller amount and attempt to classify the material visually (remember this is the contents of your dustbin/wheelie bin etc.). Then aim to obtain a 10-20g sample which is representative of that waste! Once you’ve got the “representative sample” what next? Analyse it using the methods discussed above! Clearly, although there are Standards to help do this, the process is hugely manual and subjective. Now add to this the fact that manufacturers of plastics are aiming to make a recyclable or bioplastic which is indistinguishable from a fossil fuel-based product, and the process becomes even worse.
However, there are better ways to do this that are far more elegant. The Carbon-14 (14C) method is one such alternative. Is it difficult, dangerous, or not very good? Well, the answer to all these is no, it works really well. It relies on the fact that the naturally occurring isotope of 14C has a half-life of 5730 years. It is formed from cosmic neutrons interacting with nitrogen in the atmosphere. Plants then absorb the 14C. It is routinely used to carbon date archaeological artefacts. In addition, anything over 60,000 years old has no detectable 14C present. So, fossil fuels are devoid essentially of 14C, whilst more recent biomass has a lot of 14C relatively speaking.
Is there anything that makes the determination of 14C more difficult. Well of course there is, this is real life! In the 1950s & 1960s the nuclear weapon tests changed the natural background levels. The levels increased dramatically and have been decreasing since. The graph below shows how 14Cbio has changed since the 1960s. Other notable events which have affected the levels have included further nuclear weapon trials in the 1980s and the Chernobyl disaster.
It has subsequently been decided that the current level is 104-105% pmC (percentage of modern Carbon) of the 14C levels of 1950. This is accepted to be our datum.

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