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Low sulfur on the high seas


Maritime shipping remains one of the most cost-efficient global transport methods, however its rapid growth since the 1970s has also increased its environmental impact. New regulations limiting the sulfur content of engine fuel mean that ships must either use cleaner, more expensive fuel; or install equipment to take sulfur out of exhaust gases, a process in which molybdenum plays a key role.

Ships are usually powered by diesel engines, typically run on heavy fuel oil which contains up to 3,500 times the amount of sulfur found in ultra-low sulfur diesel. Sulfur dioxide is released when the fuel is burned, leading to acid rain when dissolved in the atmosphere and to respiratory problems when inhaled.  

New regulations which came into force in 2015 in much of the northern hemisphere reduced the permitted level of sulfur in fuel to 0.1%, meaning that ships must now either use more expensive low-sulfur fuel, or opt to fit onboard flue gas desulfurization (FGD) equipment.

FGD units typically employ a process called wet scrubbing to remove up to 99% of sulfur dioxide from engine emissions. The SO2 from the exhaust turns into sulfuric acid which makes the environment within parts of the FGD units extremely aggressive, and it is here that molybdenum plays a crucial role by greatly increasing the corrosion resistance of stainless steel.

Several grades containing varying amounts of molybdenum up to 6% are typically used, depending on the potential for corrosion. Many recently-built ships and those in construction are designed with FGD units built-in while older vessels can be relatively easily retrofitted. The FGD route is cheaper than converting engines to run on gas oil or natural gas, or switching to low-sulfur fuel.  

Optimizing the global movement of goods is a key requirement in ensuring that the transport sector can make an overall positive contribution to sustainable development. Shipping has a large part to play in this, and the global reduction of sulfur emissions is a very significant step, one in which molybdenum is playing a facilitating role.

This post is adapted from an original article in MolyReview, in which we showcase some of the most interesting and amazing uses of molybdenum. Jump straight to the full version of this article here.

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