A substantial rise in fuel consumption as the oil price collapsed has coincided with a rise in the sale of adulterated fuel, says the Department of Energy Business (DOEB).
Witoon Kulcharoenwirat, director-general of the department, said fuel demand grew 6-9% this year from 2015, particularly for petrol, which has encouraged profiteers to contaminate the fuel with other substances, lowering its quality.
The adulterated fuel is found in the North, Northeast and the South, mostly sold at non-brand retail fuel stations, said Mr Witoon.
For instance some retailers cheat by selling E20 gasohol, a 20% ethanol blend with 80% petrol, as E10 gasohol because the former is 2-2.50 baht cheaper.
Others cheat by selling fuels with the Euro 2 or 3 emissions standards on the domestic market, but the standard in Thailand is Euro 4, which is higher in quality and price. That helps profiteers gain 50-70 satang per litre, said Mr Witoon.
The total profit from these scams could be several million baht, he said.
For diesel, most of the cheating petrol stations were in the South, where smugglers brought in low-quality diesel to be sold as biodiesel grades with higher prices, said Mr Witoon.
He said the DOEB had been working closely with the Customs Department and the police to install tighter surveillance to suppress cheating at fuel stations and prevent further fuel adulteration.
Energy Minister Anantaporn Karnchanarat said the ministry had set up another 10 units of mobile fuel inspectors.
The department plans to phase out sales of gasohol 91 by 2018, with gasohol 95 to follow suit in 2027, said Mr Witoon. The plan will leave the country with two high-percentage ethanol fuels -- gasohol E20 and gasohol E85 -- that should spur demand for ethanol.
Average ethanol demand stood at 3.6 million litres per day in May. Demand is expected to drop as motorists switch to pure benzene while oil remains cheap.
For biodiesel, DOEB is preparing to launch the sale of B10, a 10% methyl ester blend with 90% diesel, instead of B7, to drive demand for biofuel.