INSIGHT: Ammonia and Fuel Oil Emulsification for Maritime Decarbonization

by Jose Femenia, PE
Friday January 20, 2023

It is estimated that there are over 50,000 commercial vessels operating in the world's  merchant fleets producing approximately three percent of the world's man made carbon dioxide mainly by burning heavy oils.  By volume ranking, the three main fuels used by commercial merchant vessels are low sulfur fuel oil, LSFO; high sulfur fuel oil, HSFO (with scrubbers); and low sulfur gas oil, (LSGO).  LSFO and HSFO require heating for appropriate atomization and combustion.

As a means of reducing the carbon dioxide emissions some ship owners have ordered new tonnage to operate on liquefied natural gas, LNG (mainly methane), or methanol.  Note both fuels are hydrocarbon fuels. 

A combustible inorganic compound that until recently has been overlooked as a ship fuel is ammonia, the world's second most produced chemical after sulfuric acid.  About 200 million metric tons of ammonia are produced worldwide annually, used mainly for fertilizers, munitions, and household products.  Due to its extensive use, many ports have the infrastructure and technical ability to handle ammonia.  Since ammonia is a discrete chemical compound, comingling of ammonia from different sources should not be a problem.

Since most of today's ammonia is produced from natural gas its price varies with the price of natural gas.  As the move to produce green ammonia accelerates, the price of the ammonia will decouple from the price of natural gas and be driven by the demand for world fertilizers. Green ammonia is produced from water and air using green electricity, electricity from nuclear, wind and solar power.

As a general statement, the more hydrogen a fuel molecule contains, the more heat it liberates upon combustion.  The approximate chemical expression for LNG is CH4 and it yields 21,500 BTU/lb when combusted.  The chemical expression for methanol is CH3OH and its heating value is approximately 9,800 BTU/lb.  Although producing less carbon dioxide than conventional marine fuels, both LNG and methanol produce carbon dioxide.  Note that heavy marine fuel oil's approximate chemical expression is CH1.5 and its approximate heating value is 18,500 BTU/lb.

Combusting ammonia, NH3, does not produce any carbon dioxide and produces 8,120 Btu/lb.  The fact that ammonia does not produce any carbon dioxide has been recognized by both engine manufactures and environmentally conscious ship owners.  Recently some engine manufacturers have announced they will have ammonia fuel engines developed by 2024 and available for ships by 2025.  The Italian shipowner, the Grimaldi Group, recently signed an order for the construction of up to ten ammonia ready pure car and truck carriers.

In addition to eliminating carbon dioxide, ammonia fuel also eliminates other hydrocarbon related emissions such as sulfur dioxides and particulate matter.

Shipowners starting to commit to building ammonia ready ships is a good sign, however, with new construction accounting for only two percent of the world's fleet replacement per year it will take decades for the marine industry to wean away from hydrocarbon fuels.  

Parallel Approach

A parallel approach should be considered, the development of a system of retrofitting existing vessels to reduce their hydrocarbon fuel consumption by modifying the vessels to replace a significant fraction of their oil consumption with ammonia.  As the oil consumption fraction decreases, the non-carbon dioxide pollutants, as well as the carbon dioxide emission, are proportionally reduced.

The use of emulsions for operating compression ignition engines, mainly using oil-water emulsions has been investigated for decades and shown to have some beneficial effects.  Oil-ammonia emulsions have also been investigated and shown to have positive effects with the added effect of  being a hydrocarbon fuel energy replacement.  Research has shown that oil derived energy replacement of over 90% can be achieved using oil-ammonia emulsions depending on engine load conditions.  It was also shown that carbon dioxide emissions were reduced monotonically for the same engine torque output as the amount of the ammonia in the fuel mixture increased.  Additionally, burning ammonia in engines does not necessarily increase NOx emissions despite the fuel-bound nitrogen. Lower levels of NOx emissions were observed if energy substitution by ammonia did not exceed 60%. This is thought to occur because of the lower combustion temperature of ammonia.

Over the years attempts have been made to emulsify liquid ammonia with very limited success due to the ammonia vaporizing prior to the emulsion reaching the high pressure fuel pumps.  Two of my colleagues, Jim Harbach and Vito Agosta experimented with low temperature diesel oil-ammonia emulsion on a small industrial size four-stroke compression ignition engine but were unsuccessful in achieving stable operation due to the ammonia volatility.

Appreciating the importance of reducing the marine industry's use of carbon dioxide producing alternative fuels has continued to be goal of myself, Jim Harbach and Dennis Brennan and we believe it can be achieved for existing commercial vessels in addition to new construction. 

Since most commercial vessels are not limed by auxiliary machinery space and weight, we are proposing a fuel oil-ammonia system designed to pressurize the fuel oil and liquid ammonia to pressures above the ammonia critical pressure 113.57 Bar  (132.41 oC) prior to emulsifying the two liquids thus allowing the fuel oil part of the emulsion to be heated to the temperature necessary to achieve the proper injection viscosity.

It should be noted that oil and ammonia emulsification element of the system must ensure the uniform dispersion of the ammonia within the oil, especially if the two liquids are immiscible.

The following table illustrates the carbon dioxide reduction that can be achieved by replacing part of the fuel oil combustion heat by combustion heat from ammonia for a 25,000 BHP compression ignition prime mover operating 300 days per year.

Whereas the above has focused on motor vessels, there is no reason why the concept can not be applied to steam vessels.

Prior to an owner retrofitting an existing vessel to use ammonia as a partial fuel substitution, plans must be discussed with the engine or boiler manufacture to ascertain the maximum amount of ammonia the engine or boiler can safely and efficiently utilize.

A logical question is "how do we store the ammonia on an existing vessel?"  My response is, utilize appropriately sized medium pressure ammonia tanks similar to the U.S. Department of Transportation approved 34,400 gallon ammonia railroad tanks.  The tanks can be located on deck if no other suitable space is available.  A 25,000 BHP engine operating with 10% ammonia fuel replacement would consume under seven tanks of ammonia every 30 days.

If one 25,000 HP ship substituting only 10 percent of its main engine heat input with green ammonia and operating 300 days at sea can reduce the global carbon dioxide emissions by nearly 7,000 metric tons per year,  think how much the marine industry's carbon dioxide emissions can be reduced if the practice becomes common place.  If 300 similar vessels using ammonia for 10% energy substitution were operated 300 days per year at sea, the marine industry's carbon dioxide reduction would be over 200,000 metric tons per year!  Furthermore, think of the oil that would be saved. 

From my perspective, oil is too valuable to burn for ship propulsion.  It is the feedstock for gasoline, light distillates, plastics, and synthetics and should be conserved to be used as feed stock.  By minimizing the consumption of hydrocarbons used to power ships, the marine industry can help reduce worldwide carbon dioxide emissions and conserve the feedstock necessary to sustain modern civilization.

In my opinion, the industry needs to get off top dead center and take strong but reasonable measures to help reduce the earth's carbon dioxide emissions such as by shifting to green nonhydrocarbon fuels such as ammonia. Shipowners, engine manufactures, classification societies, governments and fuel suppliers should all work together to make the decarbonization of the marine industry a reality.