Tugs built for northern European operations will require main diesel engines to be compliant with IMO’s toughest emissions requirements
All vessel newbuildings with diesel engines on board and their keels laid after 1 January 2021 will need to comply with IMO Tier III standards if they work in the North Sea and Baltic emission control areas (ECAs).
Their emissions restrictions will be similar to new tugs operating in the US, which must comply with US Environmental Protection Agency (EPA) Tier 4 requirements.
Both require nitrogen oxides (NOx) to be almost eliminated from diesel engine emissions. NOx control requirements apply to installed marine diesel engines of over 130 kW output power. Inside ECAs, tugs would either need to install a selective catalytic reduction (SCR) exhaust gas after-treatment system or another removal technology.
SCR systems process exhaust gases, removing particulates and NOx through a catalytic reaction using a reductant, usually a urea solution. This is injected in the exhaust and funnelled through a mixing chamber to a reactor containing a catalyst that enables a series of chemical reactions. Urea is converted to ammonia and carbon dioxide, and the ammonia subsequently reacts with nitrogen oxides to yield nitrogen and water.
SCR after-treatment technology can be retrofitted to tugs if there is sufficient space in the engineroom to accommodate storage tanks. They need urea tanks, dosing pumps, stainless-steel piping, mixing chambers and the SCR reactors.
Vessel operators need to handle and refill urea, control ammonia slip and regularly maintain these SCRs.
Several harbour tugs Damen Shipyards has built for European owners either have SCRs already installed or are ready for retrofitting in the future. Boluda Towage Europe took delivery of two new tugs for operations in Zeebrugge, Belgium with SCRs installed for IMO Tier III compliance. It is also retrofitting existing tugs in the fleet for similar requirements.
An alternative methodology to meet the IMO III or EPA Tier 4 standards is to minimise NOx formation in-cylinder by deploying exhaust gas recirculation (EGR) technology. In 2015, after six years of development, GE Transportation, a Wabtec company, was the first to launch a medium-speed marine diesel engine series, certified to IMO Tier III and EPA Tier 4 standards, through an advanced EGR system.
Depending on the engine load, a portion of the exhaust gas is cooled and mixed with fresh compressed and cooled combustion air, and subsequently routed into the cylinders.
This enables the reduction of the combustion temperature in the cylinder that minimises NOx formation. However, a lower combustion temperature could result in lower fuel efficiency if no other technologies are deployed.
GE’s engines and advanced EGR solution maintains fuel efficiency equal to or better than that of a comparable IMO Tier II emissions engine series through increased peak-cylinder-pressure, enabled by structural improvements and a two-stage turbocharging arrangement with intercooling and aftercooling.
GE’s engines have finer fuel atomisation, enabled by a 2,200 bar capable high-pressure common rail fuel system. They also have advanced combustion controls and an optimised Miller thermodynamic cycle.
Engines with EGR eliminate the need for space and weight provisions otherwise needed for an SCR system and urea storage tanks.
There are no additional operating expenses from urea use and catalyst replacements and no planning required for urea replenishment and handling urea on board.
There is also higher engine energy efficiency with more heat recovery available in the jacket water system from the EGR*.
For tugs built to operate outside of ECAs, diesel engines need to meet IMO Tier II emissions requirements. However, some tug owners have ordered harbour tugs to comply with IMO Tier III. Wilson Sons plans to build six tugs at its own shipyard in Brazil with SCRs for IMO Tier III compliance.
*More details in Riviera-hosted whitepaper EPA T4 / IMO III emissions compliance without urea after-treatment
Arguments for and against exhaust gas cleaning
During Riviera’s Diesel engine exhaust gases – exhaust gas cleaning, or is fuel selection good enough? webinar in December 2020, panellists discussed the positive and negative aspects of investing in emissions reduction technologies.
Exhaust Gas Cleaning Systems Association director Don Gregory highlighted how IMO had concentrated on SOx, NOx and particulate matter when producing its Marpol regulations, and not on other toxic compounds. “Non-government organisations, administrations and port authorities have not proposed any limits on the many toxic compounds contained in diesel engine exhaust,” said Mr Gregory.
Clean Shipping Alliance executive director Ian Adams said there were developments underway to remove other toxic elements and reduce greenhouse gas emissions from engine exhaust. There are developments underway testing using tourmaline beads in a second stage of an exhaust gas cleaning system (EGCS) for removing CO2 and engine manufacturers are developing gas recycling to reduce emissions and methane slip for dual-fuel engines.
EGCS producer Yara Marine Technologies director for sales and public affairs Kai Latun said the maritime sector should consider well-to-wake processes when considering emissions. He said marine gasoil has far higher emissions than very low sulphur fuel oil (VLSFO), or even heavy fuel oil (HFO) with onboard EGCS when considering well-to-wake. “CO2 emissions from a ship sailing with HFO and scrubbers is far lower than a ship with compliant fuels, and wash water from scrubbers does not harm the marine environment,” said Mr Latun.
Also during the webinar, Delta Corp Shipping senior vice president of shipmanagement Caroline Huot provided a shipowner’s perspective. She said the price differential between marine gasoil, HFO and VLSFO did not justify investment in exhaust gas cleaning. “Investment payback and capital allocation are necessities of everyday,” she said. “Legislation has been piling up and capital has to be invested including in ballast water treatment and scrubbers,” she said.
Ms Huot also highlighted the need for charterers to be prepared to pay premiums for vessels with emissions reduction technology on board. “The economics do not work,” she said.
Alternative fuel options
Tug owners have a selection of alternative fuels to test. LNG has emerged as the most available option now for cutting CO2 emissions by 25% and offering owners a path to carbon-neutral bioLNG and zero-carbon fuels green hydrogen and ammonia. Underpinning growth in its use in tugs is maturing LNG bunkering infrastructure in Asia, Europe and North America.
Liquefied petroleum gas (LPG) could be a breakthrough fuel as it resembles LNG in its availability and is easier to handle. Using LPG, vessels can reduce fuel consumption and CO2 emissions; lower SOx emissions by 99%, particulate matter emissions by 90% and NOx emissions by 10% when compared with conventional compliant fuel.
Methanol is also widely available, easy to handle and store. Adoption of IMO interim guidelines for using methanol as a marine fuel have increased its attractiveness to owners as a pathway to decarbonisation.
Ammonia is seen as frontrunner in the future fuels race as it contains no carbon, is easy to store and transport, and is an efficient hydrogen carrier. Drawbacks are its relative low energy density and toxicity.
Hydrogen fuel cells could also become an alternative for tugs. Its future adoption could follow trials with hydrogen-powered tug Hydrotug in Belgium in 2021. But there are operational challenges, including storage, handling, availability and cost.
Riviera will provide technical and operational webinars and virtual conferences in 2021. Sign up to attend on our events page