H+H SCR Technology for lower NOx emissions
H+H SCR Technology for lower NOx emissions

Shipping and the Environment: From ZERO to IMO Tier III

The International Maritime Organization (IMO) Tier III NOx (Nitrogen Oxides) emission standards were established to regulate and reduce nitrogen oxide emissions (by e.g. SCR Systems) from marine engines in order to address air pollution and its impact on the environment and human health. Here is a brief history of shipping, the role of IMO and Marpol and finally IMO Tier III NOx limit values, to be realized with e.g. Marine SCR Systems, along with the main regulations. Nowadays NOx reduction plays an important role.


From the early voyages of ancient civilizations to the transformative era of the Industrial Revolution, ships have played an integral role in connecting the world. The steamship in the 19th century revolutionized shipping, increasing efficiency and allowing for faster and more reliable transportation of goods. The development of the marine diesel engine, which gradually supplanted steam engines took place the late 19th and early 20th centuries. Improved Efficiency, Simplicity, Reliability and Higher Power-to-Weight Ratio, just to name a few advantages of the diesel engines. Early marine diesel engines primarily ran on heavy fuel oil, which was readily available and cost-effective. 

Journey of Ships: Tracing Environmental Awareness at Sea 

Several factors, including significant events and accidents, contributed to the growing awareness of the environmental impact of shipping in the mid to late 20th century. Air pollution, oil spills, ballast water discharge, and habitat destruction were among the concerns. The establishment of the International Maritime Organization (IMO) in 1948 and its subsequent work on international regulations drew attention to the need for coordinated efforts to address shipping’s environmental impact and increase the safety at sea. IMO’s role in developing global standards for safety regulations and pollution prevention was instrumental in raising awareness. 

The Role of the International Maritime Organization (IMO) 

The International Convention for the Prevention of Pollution from Ships, commonly known as MARPOL, is one of the most significant international agreements regulating pollution from ships. In 1973, the IMO convened a conference in response to several high-profile oil spills, including the 1967 Torrey Canyon incident. The conference resulted in the adoption of the first MARPOL Convention on November 2, 1973 and has undergone several adaptions and revisions since its inception. 

It established regulations and standards to prevent pollution from oil, chemicals, sewage, garbage, exhaust gas and more. 

Since its adoption, MARPOL has undergone several amendments and protocols to strengthen and expand its provisions. Key amendments include Annex I (Oil Pollution), Annex II (Noxious Liquid Substances in Bulk), Annex IV (Sewage), and Annex V (Garbage). These amendments have introduced stricter regulations and improved environmental protection. 

Annex VI of MARPOL, which addresses air pollution from ships, was adopted in 1997 and came into force in May 2005. It sets limits on emissions of sulfur oxides (SOx) and nitrogen oxides (NOx) from ship exhausts. Within Annex VI, the NOx Technical Code was established to provide specific guidelines and requirements for controlling NOx emissions from marine diesel engines. With the entry into force of the IMO Tier III limits, SCR systems were also included, as NOx relevant part, in the certification of engines

The NOx Technical Code 

The NOx Technical Code was developed as part of MARPOL Annex VI to address the reduction of nitrogen oxide emissions from ship engines. It provides technical specifications for engines and systems that help shipowners and operators comply with the NOx emission limits set by MARPOL Annex VI. 

Entry into Force 

The NOx Technical Code entered into force on January 1, 2000. The rules set in regulation 13 apply to each diesel engine with a power output of more than 130 kW which is installed on a ship constructed on or after 1 January 2000. It is also applicable for engines with a power output of more than 130kW which undergoes a major conversion on or after 1 January 2000. 

In either case, this regulation does not apply to emergency diesel engines, engines installed in lifeboats and any device or equipment intended to be used solely in case of emergency. 


Like other parts of MARPOL, the NOx Technical Code has undergone revisions to align with technological advancements and more stringent emissions reduction targets. Shipowners and engine manufacturers must ensure that their engines comply with the latest code requirements. 

The main purpose of the NOx Technical Code, within the framework of MARPOL Annex VI, is to reduce the environmental impact of NOx emissions from ships by setting standards and promoting the use of emission-reduction technologies. These regulations are crucial for improving air quality and reducing the contribution of shipping to air pollution and its associated health and environmental risks.

Clear guidelines on how to certify or upgrade a marine diesel engine, including the Marine SCR system to IMO Tier III can be found in MEPC291(71) and it’s amendments. This will be detailed described in another post, so you get an overview what role the SCR system play.