Learnings

Outcomes

Throughout the project, there have been a number of outcomes that have been reported. These are as follows:

·        Servicing, Calibration and Installation of Analytical Equipment – this report documented the process that will be undertaken in analysing the results that come through with the extraction tests. An overview of the extraction process has been highlighted and the role of each piece of equipment is detailed.

·        Extraction System Report – this report summaries each key stage of the extraction process and the role each piece of equipment represents.

·        Baseline analysis – this deliverable documented the expected ‘baseline’ levels of creosote present within a pair of poles prior to be being treated with the extraction vessel.

·        Extraction methodology – this document outlined the testing methodology that will be carried out throughout the whole testing period. 

All project outputs can be found on the National Grid Website: National Grid - Active Creosote Extraction (ACE) 

Lessons Learnt

Below is a summary of the key learning that have been generated in the reporting period.

It is estimated that 50% of a redundant wood pole can be recoverable through pre-treatment by selectively removing outer layer for treatment. In essence, the majority of the inside of a pole can already be classed as non-hazardous. 

Initial analytical results have shown that the highest levels of creosote (known as hot spots) concentration can be around 6500mg/kg. this is approximately 6.5 times greater than the classification levels for non hazardous wood. As per the waste acceptance criteria, the level for non-hazardous waste classification is below 1000mg/kg. this has shown us the areas to target for future tests.

Initial data has shown gravitational effects plays a part in the distribution of creosote hotspots. Towards the bottom end of the poles, the greatest concentration of creosote is observed. There are however, anomalies from time to time. This has opened up the scope of the anticipated scaled up system from a 2.5m vessel to a 5.0m vessel. It could prove more economical to have two separate vessels that focus on high and low creosote concentrated wood.

Fracturing in the wood significantly increases the creosote penetration into the wood. It has also been observed that there have been higher creosote levels deeped into the poles compared to the outside. This will be due to the inner section being 'shielded' from the environment. Similar to the above, this opens up scope as to what could we do with multiple vessels being used in tandem. i.e. one vessel used for higher creosote concentrated wood and one with less. 

The activity of procuring poles has proven to be quite a challenge a times. Local depots can be reluctant to retrieve poles from their pole skips which results in that they can only be collected before this is done. This leaves a narrow pickup window as local depots also dispose wood poles after a short time period too. This has shown us that, regular cooperation is needed with local depots in order to efficiently pick up poles that have been left for the project team.

Sample selection and preparation is fundamentally important and equally as important so to accurately quantify the creosote concentration in the samples. Because it is impracticable to analyse the whole of the pole, samples prepared for analysis must be a random, representative on the whole, and homogeneous. This is done through grinding the wood into a powder. This ensures that each sample was not only well ground, but also well mixed to ensure a true representation was produced, limiting any sample bias.

Initially the liquid CO2 storage tank must be set to the conditions of -17 ˚C and a pressure of 280 psi in order for the pumps to work efficiently. Heat exchangers are subsequently needed to get the supercritical conditions within the vessel of 1070 psi and 32˚C. Further details can be found in the extraction system overview report on the National Grid website.

Due to heat exchangers being needed, trace heating has been applied to the design as liquid CO2 cools under expansion from the transfer of the stored CO2 conditions of  -17 ˚C to the working conditions of 32 ˚C. Further details can be found in the extraction system overview report on the National Grid website (nationalgrid.co.uk/downloads-view-reciteme/637791).

In order to ensure safety venting lines and integrated safety relief valves have been attached to ensure safe release of gaseous CO2 in a case of a failed case of the refrigeration system. These safety relief valves are set to release the pressure before the pressure rating of the fittings, and the extraction vessel, are reached the spring loaded valve is opened and the pressure released.  This has assured a safety route, if testing elements were to go wrong. Further details can be found in the extraction system overview report on the National Grid website (nationalgrid.co.uk/downloads-view-reciteme/637791).

Procuring a CO2 storage tank compared to traditional bottles has increased complexity of the system with regards to the required redundancy measures to be put in place. This is due to the risk of creosote entering the CO2 tank once returned which carries severe consequences. Although the complexity has been increased with this change, it can be seen as an investment into what a scaled up system would potentially look like. For this to be BaU, it would be highly unlikely that the source CO2 delivery would be through bottles. In that regard, this addition will only contribute to the commercial potential and success of this project.