The source of a mysterious stench that has pestered residents of Iran’s capital Tehran this year, is the usage of nonstandard mazut with high-density sulfur in the nearby town of Shahr-e Rey's industrial complexes, Tehran municipality has disclosed.

Last winter and again this fall a terrible foul smell gripped parts of Tehran and officials kept pointing fingers at different possible sources, but now they have admitted burning a low-quality fuel is the source of the stench.

Meanwhile, the head of the Environment Department in Shahr-e Rey maintains that using mazut in the city's cement plant has been banned.

Mazut is a heavy, low quality fuel oil usually used in heavy industrial plants and for ships. In the West mazut is broken down and turned into diesel.

"Since the volume of domestic consumption of natural gas goes up during the cold months of the year, the Natural Gas Company stops providing its industrial customers with the product," Zohreh Ebadati said on Sunday, December 22, adding, "Therefore, the owners of the industrial plants tend to use mazut instead of natural gas. And this leads to air pollution."

The level of sulfur density in mazut produced in Iran is nearly 3.5%, which is seven times more than the international standards for vessels on high seas, and its usage is strictly banned in the urban areas, specifically in the cities like Tehran that are struggling with air pollution.

It is for the first time that the Islamic Republic authorities confirm that during the cold seasons they disconnect the flow of natural gas to industrial plants.

The disclosure comes at a time when the same officials have repeatedly boasted of the country's "absolute self-sufficiency" in the consumption of natural gas.

As recently as December 8, President Hassan Rouhani proudly declared that the country had become self-sufficient in the production of natural gas, gasoline, and diesel, and has started exporting the three products.

"In the past, our gas exports had to be suspended in the winter months. Today exports continue despite the cold winters," he said before presenting the March (2020-21) fiscal budget to the Majlis (the Islamic Republic parliament).

It might be true that Iran exports some natural gas, but it is not because there is an abundance, rather it needs to make money and forces local industries to use mazut.

The Sad Story of Mazut

Mazut or Mazout is a Russian word, and it means a viscous liquid residue from the distillation of crude petroleum that is used chiefly as fuel oil.

The refined oil is exported as kerosene or petroleum; the heavier refuse (mazut) is used as fuel.

Since Iran's refineries are very old and deprived of new technologies, 24% of the crude oil has turned into mazut, which compared with gasoline and diesel has a very low value.

Moreover, the density of sulfur in mazut is exceptionally high.

In some of the Iranian refineries, the volume of mazut per crude oil refined is much higher. For example, 41% of the crude oil fed to the city of Kermanshah's refinery turns into mazut. Therefore, the value of its products is even less than the crude that it receives, i.e., it is an apparent loss-making complex.

Iranian refineries, on average, have produced more than 60 million liters (approximately 15.8 million U.S. liquid gallons) of mazut per day, which 35.5 million liters of (roughly 9.3 million gallons) of it have been exported, 4.3 million liters (1.1 million gallons) used for bunkering (vessels fuel), and the rest domestically consumed.

In 2016, Iran had a plan to invest fifteen billion dollars for upgrading eight old refineries across the country and reduce the volume of their mazut production from 24% to less than 10% of the crude oil fed to them.

Nevertheless, Tehran's contracts with the Chinese, South Korean, and other foreign companies for that purpose practically failed and shelved without any progress.

Based on the data provided by the Parliament Research Center, seven out of ten refineries in Iran (all built before the Islamic Revolution) are dilapidated, and beyond their service life.

The Problem of Getting Rid of Mazut

As said earlier, last year, Iran exported nearly 60% of mazut produced in the country and used almost 7% of it for bunkering. The main bulk of Iran's mazut is exported to the United Arab Emirates (UAE), and the UAE for refueling local and foreign vessels.

According to international data, contrary to the crude oil exports, the volume of Iran's exporting petroleum products has not significantly decreased under the U.S. sanctions imposed on the Islamic Republic.

However, the problem is that based on the International Maritime Organization's (IMO) regulations, the density of sulfur in the vessel fuel should be less than 0.5%, while it is 3.5% in the mazut produced in Iran.

The Research Center of the Islamic Legislative Assembly recently reported that Iran had no option other than selling mazut to the UAE and stop its bunkering operations. Therefore, Iran sells its mazut to the UAE, and there, the modern refineries reduce the volume of sulfur to the level required for bunkering.

Such development means that Iran's bunkering industry might totally collapse.

Moreover, if the U.S. decides to sanction Iran's mazut, the Islamic Republic would have no choice other than consuming its mazut in its local industrial complexes.

Besides cement plants, Iranian mazut is mainly used in the country's power stations. The Ministry of Energy's data shows that the volume of mazut used in Iran's power stations last spring increased up to 72% compared with the same period in the previous year, reaching 591 million liters (approximately 156 million U.S. liquid gallons) per three months.

However, in its latest weekly and monthly reports, the ministry has stopped referring to the volume of mazut used in the country's power stations.

Shahr-e Rey's thermal power plant that has been singled out as the main culprit and blamed for the stench that pesters residents of Tehran was constructed in 1977, two years before the Islamic Revolution that toppled Iran's pro-West king, Shah Mohammad Reza Pahlavi. At the time, the plant produced one-fourth of the electricity needed in Iran.

The plant has become so old and dilapidated that it has lost half of its capacity. Currently, the plant's capacity has dropped to nearly 600-700 megawatt. No reports have been published concerning the current fuel used in the station.

Presently, Iran has the world's fourth-largest proven deposits of crude oil and the world's second-largest deposits of natural gas. It also shares a massive offshore gas field in the Persian Gulf with Qatar.