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DNV GL has provided a list of the pros and cons of using alternative fuels in order to comply with emissions regulations.

As reported last week, the classification society published a new brochure that is designed to offer a set of best practices for the design of ships subject to NOx Tier III requirements.

In the document, DNV GL explains that the use of alternative fuels - such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), methanol or ethylene in lean-burn gas engines - offer a means to reduce NOx emissions.

Natural gas engines are capable of reducing NOx emissions by 80 to 90 percent, thereby complying with Tier III. However, the company notes that NOx emissions depend highly on the combustion principle applied in the engine, irrespectively of fuel type.

When operating on natural gas or other alternative fuels, peak cylinder temperatures are reduced, relative to diesel operation, through the use of a lean-burn Otto-cycle combustion process. These lower peak cylinder temperatures restrict the formation of NOx.

Of all available alternative fuels, DNV GL suggests that LNG has "the highest potential" when lean burning Otto-principle engines are used.

DNV GL also mentions that low-flashpoint liquids such as methanol - which has a flashpoint of around 12 degrees Celsius - are gaining interest. As well as having low SOx and NOx emissions, DNV GL says a methanol fuel system is "easy to retrofit on a ship".

Pros and cons

Below is DNV GL's list of pros and cons of alternative fuels, which focuses on LNG in particular.

One issue not included in DNV GL's list of cons, however, is the leakage of methane from the engine - also known as 'methane slip' - which is said to be present in Otto-cycle engines, both dual-fuel and spark-ignited.

Earlier this month, Bunker Index referred to a study in Norway, which warns that methane slip from the engine is a potent greenhouse gas (GHG) that could pose additional challenges.

The report also stresses that a "one-sided focus on LNG" could result in the sector ending up with a high-carbon infrastructure. As a carbon-based fuel of fossil origin, it is worth noting that the combustion of LNG still results in CO2 emissions.

Alternative fuels - DNV GL's pros

- A potentially a low-OPEX solution, which benefits owners who pay for fuel directly

- Both SOx and NOx compliance achievable (ECA and CARB regimes), approx. 100% SOx removal and approx. 85% NOx removal from four-stroke engines (approx. 40% NOx removal in high-pressure two-strokes, need EGR for NOx compliance)

- Access to multiple fuel markets (DF engines) and may use liquid cargoes as fuel.

- Emerging regulations and standardization support LNG as a safe fuel.

- Boosts operator company's corporate social responsibility (CSR) profile.

- Constantly growing merchant fleet with LNG as fuel and gas ready.

Alternative fuels - DNV GL's cons

- LNG fuel tanks are currently rather investment intensive.

- LNG bunkering grid is still under development.

- LNG fuel tank requires additional space, which might reduce cargo space.