Active Creosote Extraction (ACE) is an environmental based innovation project aligning with decarbonisation and net zero. Disposal of creosote impregnated wood poles is a significant hazardous waste stream that is only going to escalate in the coming years, due to more stringent legislation likely to be put in place and the disposal method through landowners inevitably becoming unviable. Leaving high temperature incineration the only option, this method is a deterrent for DNOs needing to reduce their carbon emissions. By creating a method, where creosote can be extracted from redundant wood poles to such levels they are deemed non-hazardous waste, high temperature incineration can be avoided resulting in significant carbon emissions savings and taking a significant step to net zero.
Benefits
Project ACE will aim to create a new method of disposing creosote impregnated wood poles through an extraction process. This new method will significantly reduce the carbon emissions compared to the current incineration process. This method will create a new pathway for wood pole disposal and will also avoid any impact on DNOs if changes in legislation are made in the future. If the wood poles are deemed as non-hazardous waste, they will be able to be re purposed, donated to charities and utilised to make fences or furniture. This project will signicantly contribute towards WPDs target of a net zero business by 2028. Project ACE will aim to create a commercially viable product that can then be used as a service by other DNOs and businesses that use wood poles.
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 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 over the project.
Pre treatment
It is estimated that 50% of a redundant wood pole can be recovered through pre-treatment by selectively removing the outer layer for treatment. In essence, the majority of the inside of a pole can already be classified as non-hazardous. The full report can be viewed here.
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 play 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 creosote penetration into the wood. It has also been observed that there have been higher creosote levels deeper 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 we could do with multiple vessels being used in tandem. i.e. one vessel used for higher creosote concentrated wood and one with less.
Sample selection and preparation is fundamentally important and equally important is accurately quantifying 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 of the whole, and homogeneous. This is done by 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.
Logistics
The activity of procuring poles has proven to be quite a challenge at 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.
System Learning
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. (nationalgrid.co.uk/downloads-view-reciteme/637791).
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. Rather than dispensing numerous CO2 bottles, a single tank is able to be used. 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.
Testing
Due to the design being completely bespoke, establishing a methodology into the quickest and most effective way to achieving operating pressurisation took more time than anticipated.
Incremental pressure testing is required to achieve the desired operational pressure to solve leaking. Leaking occurs due to fittings not being tight enough and higher pressures causes further leaks, hence the need to incremental testing to address these. Due to the temperature deltas being used (range of 50 degrees C difference), compression / expansion is observed.
A chiller unit has had to be procured due to CO2 warming and state change during the introduction to the pump head. This resulted in cavitation and some CO2 leakage. By introducing the chiller unit, the CO2 can continuously be pumped in a liquid state.
5.0m pole sized needed to be re scoped due to the labour-intensive process of loading a 5.0m pole into a 5.0m pressure vessel. This process was deemed inefficient due to the size issue and also the variability of creosote concentration with a 5.0m poles. i.e. parts of the pole are non-hazardous during the test compared to other areas. This highly reduces space efficiency.
It was decided to design and build a skid to initially load the poles into the vessel in an easier way but to also introduce a barrier between the wood and the extracted creosote that was still left in the vessel. This would reduce surface contact and yield more creosote out of the wood.
Pulsed SFE – this method of extraction seemed to be more consistent than the standard Supercritical Fluid Extraction (SFE) and indicates extraction during the additional depressurisation periods.
Methanol modified SFE. – Methanol as an additive to the Supercritical Fluid Carbon Dioxide (SFCO2), seems to have an effect, with the possibility for the requirement for additional to be added, but once again the physical condition of the poles seems potentially to be a contributing effect on the extraction efficiency.
Acetone modified SFE – the addition of acetone seemed to have no additional effect on the extraction efficiency, and so can be discounted as an influencing factor.
Looking at the poles as a whole it can be seen that the core, which contributes between 40 and 70% of the total mass, are devoid of creosote and so do not require processing, and generally can be reused. Removal of this core would seem an obvious next step, along with the “mulching” of the remaining wood which would make it more uniform and also result in a much greater surface area for extraction, both of which would improve extraction.
The highest extraction efficiency was seen at 89% for a pulsed SFE test reducing the concentration from 17,206 mg / kg to 1816 mg / kg.
The best way for creosote removal is through the use of chipped wood due to the increased surface area available for the carbon dioxide to target.
The inclusion of the carbon recovery unit significantly improved the extraction effectiveness with the ability to operate at higher pressures and temperatures. One test resulted in an extraction from 9000 mg / kg to 300 mg / kg.
