An EU-funded project to recondition Kohtla-Järve's heaps of oil shale ash has caused a hotspot deep in the pile to flare up, making the project harder to complete.
The ash heaps at Kohtla-Järve are being covered with waterproof material, new topsoil and sod, as required by the EU's waste framework directive.
In the process, a layer of semi-coke - a residue from oil shale combustion - was removed from two ash heaps, providing more oxygen to a decades-old hotspot and intensifying it, reported ETV.
The largest amount of oil shale was mined during the 1970s and 1980s in Soviet Estonia, where subsidies were given out for the amount of oil shale burned, not for the amount of oil produced. The residue dumped in the heaps included partially spent oil shale that still had enough kerogen to catch fire.
Viru Keemia Group project leader Marti Viirmäe says the kerogen content of the Soviet-produced ash and coke heaps was at least twice the size as today's heaps and the deposition was done in a environmentally reckless manner. Current technologies prevent semi-coke from igniting because the waste is compressed and contains very little organic matter.
In the current flareup, hydrogen sulfide is being emitted, which is a poisonous gas. But the amounts are too small to pose a threat to human health, say officials.