Six attempts to set fire to coal underground and tap the resulting gas have resulted in leaks, pollution, explosions and ill-health at plants around the world, according to the Scottish Environment Protection Agency (Sepa).

At El Tremedal in Spain an ignition system malfunctioned and a temperature gauge failed, causing “accumulation of methane and a subsequent explosion”, a draft Sepa report said. The 550-metre well was damaged and shut down in 1997.

The plant, which gasified 240 tonnes of coal, also contaminated groundwater. “This was a major technical and economic problem,” the report said.

An experimental 30-metre mine called Barbara in Poland suffered similar problems in 2013. “Cracks developed causing gases to leak and create explosive accumulations, igniting due to high temperatures,” Sepa reported. “Heavy metals, ammonia and cyanides found in effluents and groundwater near the site.”

The 150-metre deep Chinchilla mine run by Linc Energy in Queensland, Australia, from 2007 to 2013 caught fire because of a coal tar blockage. Toxic gases also leaked to the surface after well casings cracked under pressure.

According to Sepa, “uncontrolled leaks” of gas made workers ill. The Queensland government has launched a £3 million criminal court action on Chinchilla, alleging “extensive” pollution and “irreversible” harm, denied by the company.

A 150-metre mine at Bloodwood Creek that opened in Queensland in 2008 had a different issue. “An injection well blockage caused pressure to spike well above hydrostatic pressure, resulting in the emission of process water through the flare,” Sepa said.

At Hoe Creek in the US during the late 1970s a 50-metre well resulted in long term contamination. “Cavity collapse caused serious groundwater pollution and subsidence could be seen at the surface,” said the Sepa report.

A series of underground coal gasification trials in the former Soviet Union in the late 1950s and early 1960s contaminated groundwater. The pollution was “found to be widespread and persistent, even up to five years after production had ceased,” reported Sepa.

“Phenols were found within an aquifer which extended over an area of 10 square kilometres. There were significant gas losses due to leakage, and it was common for between five per cent and 25 per cent of the gas formed to be lost from the underground gasifier.”