Every year over 2,000 tons of mercury are released though air emissions into the environment globally, 342/t of this being from Europe.
Bacteria in soils and sediments convert mercury released to the environment to methyl mercury. In this form, it is taken up by tiny aquatic plants and animals. Fish that eat these organisms build up methyl mercury in their bodies. As ever-bigger fish eat smaller ones, the methyl mercury becomes concentrated. This process is called “bioaccumulation”. Concentrations of methyl mercury in large fish can be over a million-fold larger than in the surrounding water. Methyl mercury (MeHg) is a deadly neurotoxin that even in low concentrations can cause serious learning difficulties in the young including an ability to use language, to process information, and in visual/motor integration. Recent research has also linked it with possible harmful effects on the cardiovascular, immune and reproductive systems.
Around 5% of the population in central and northern Europe, show exposure levels above the internationally accepted safe levels for methyl mercury with Mediterranean fishing communities and the Arctic population exceeding these limits significantly due to the high levels of fish consumption in these areas.
Figures for the damage caused by Hg are not available for Europe however based on a United States Congress report and adjusting differences between the EU-US population sizes we estimate that 90,000 cases pa of serious learning difficulties are caused this way and the economic benefits though IQ gains of reducing airborne emissions would be of the order of €485 million. The EC is continuing to press for solutions to this problem and imposing stringent regulations in line with the Community Strategy Concerning Mercury (2005), Incineration Directive (2000), and the Ambient Air Directive (2004) with plants such as power stations and crematoria previously regulated on a national level needing to be regulated on an EU level.
However achieving reductions is difficult. The core challenge is that elemental mercury (Hg0) is insoluble in water and cannot be captured effectively by wet scrubbers, so it must be captured with solid sorbent. Sulphur impregnated activated carbon(S-AC) is the best mercury sorbent currently available, however it is very expensive and so most organisations user cheaper and less effective non-impregnated activated carbon.
This project will make S-AC mercury removal affordable by demonstrating the use of the project innovative high quality sulphur impregnated activated carbon(S-AC) at low cost, by one step microwave activation of waste tyres.
The project sorbent has the following environmental advantages:
- The raw material has a ‘negative cost’ since we will be paid a disposal/gate fee of €1 for each tyre which provides a clear cost advantage over alternative sources of material for activated carbon production.
- The innovative project sorbent is obtained by a process which is a net generator of electricity since a diesel like oil and a light combustible gas stream are by-products of the one step activation process.
- High quality of S-AC product. The Sulphur impregnated activated carbon used in this project absorbs more mercury than that S-AC produced by multistep processes since it does not suffer the problem of pores being blocked during the impregnation process.
- High Hg loadings compared with non impregnated AC. Not only does this increase performance but it also means a smaller volume needs to be dealt with. After use activated carbon can be reactivated, land filled or when mixed with fly ash sold as for construction purposes. A lower volume of AC means lower reactivation, disposal costs and ensures fly ash is suitable as an aggregate removing the need for landfilling. The SOREME pyrolysis plant will produce innovative high quality sulphur impregnated activated carbon(S-AC) at low cost, by one step microwave activation of waste tyres. The use of a single stage process rather than several unit operations to perform the carbonisation, activation and impregnation steps avoids the need to reheat material. Combining impregnation with activation also improves the quality of the final activated carbon since the pores are continually formed during the impregnation process. More efficient heating and shorter residence times can be achieved by microwaves, which heat the processed materials from within obtaining faster more uniform heating than conventional methods.