Tuesday, February 1, 2011

Alternate Fuels for Marine Propulsion Plants

By Louis Lemos

Commendable strides in the pursuit of alternative fuels and energy sources are being made, thanks mainly to private enterprise initiative and governmental endeavor, expressed by the US Department of Energy, to share information with interested members of the public. Meanwhile, public awareness of this progress is also assured by trade and consumer media reports on the topic of atmospheric pollution by exhaust emissions from ships. Proposed counter measures within the realm of feasibility and currently in use include legislation mandating that ships use low sulfur fuel within coastal waters such as Sulfur Emission Control Areas (SECA), or switching to shore power (cold ironing) while moored alongside the pier; increased use of Selective Catalytic Reduction (SCR) systems and electronically-controlled common rail fuel injection systems.

The overall effect of such measures has produced a significant drop in nitrous oxides, sulfur dioxides, hydrocarbons and particulate matter. The trend toward diesel-electric main propulsion plants involving medium speed engines of the trunk piston type, mainly for large passenger cruise liners, coastal ferries, research ships and some offshore support vessels, has also contributed significantly to the reduction of pollutant bearing exhaust emissions.

The difference in NOx emissions produced by diesel-electric plants can be as much as 20 percent lower than that of their straight-diesel-powered counter parts. This is attributable to the fact that the diesel engines in a diesel-electric propulsion plant, operate at a fairly constant speed, conducive to fairly stable combustion temperatures, maximum efficiency, optimum fuel economy and resultant uniformity of exhaust emissions with reduced pollutant content.

Cylinder Liner Lubrication
Currently, most large container ships, bulk carriers and super tankers are propelled by large slow speed, large bore diesel engines of the crosshead type. Many of them are now equipped with electronically controlled common rail fuel injection systems in addition to Selective Catalytic Reduction (SCR) systems of which there are several variations. Because of their immense size, these engines with cylinder bores ranging from 500 millimeters to 980 millimeters (19.68 inches to 38.58 inches) cannot rely on splash lubrication of the cylinder walls exclusively. For this purpose a separate cylinder liner lubrication method is employed using mechanical lubricators, independent of the main engine-driven lubrication system. The requirements for independently lubricating the cylinder liners of slow speed, large bore engines are to neutralize acids formed during combustion and thereby protect the cylinder liner from cold corrosion attack, to establish a reasonably stable oleous film between the cylinder liner and the piston rings and to preserve a degree of cleanliness of the cylinder liner surface and piston ring pack.

Low Sulfur Fuel Compatibility
It has been established that the requirement for continued reduction in allowable sulfur content of fuel, such as Low Sulfur Fuel (LSF), and Ultra-Low Sulfur Fuel (ULSF), adversely affects cylinder liner lubrication, particularly of the large-bore, slow-speed, main propulsion diesel engines. This is due to the use of a cylinder oil having a rather high Total Base Number (TBN), such as 70 TBN, that may result in excessive deposits on the pistons and scuffing of the cylinder liners. The severity of such adverse factors will vary in accordance with the degree of usage of LSF. For ships operating on trans-oceanic routes, wherein the majority of their running time is outside of the above-mentioned coastal areas, the use of conventional diesel fuels with relatively-higher sulfur content will be permissible. However, upon approaching such regulated areas it will become mandatory to switch to LSF for the duration of passage and/or presence therein. Conversely, ships engaged mainly in coastal trade on a full-time basis, and burning LSF, will require the use of a cylinder oil of a correspondingly lower Total Base Number such as 40 TBN. Given that the acidity of diesel fuel is proportional to the level of sulfur content, the Total Base number of the lubricant is relative to the oil’s ability to neutralize the acid. This is why the Total Base Number of an oil is also considered to be its Neutralization Value, and can be expressed as a measure of the acidity or alkalinity of the oil, whichever characteristic it possesses, and is also called the Acidity Number. A significant advantage of these mechanical lubricators is that, based on the known sulfur value of the fuel, the corresponding feed rate of the cylinder lubricant can be adjusted accordingly, for maximum effect. Should the need arise to change from the 70 TBN cylinder oil, commonly used for high sulfur fuels, to the lower 40 TBN cylinder oil, for use with low sulfur fuel, it is advisable to contact the lubricant supplier to determine the recommended feed rate of the lubricant, in order to ensure that the appropriate degree of cylinder liner lubrication is maintained.

The Cost of Compliance
While the enforcement of restricted exhaust emissions is of prime concern to ship-owners, so too is the difference in cost between conventional heavy fuel (380 centistokes) averaging 2.5% Sulfur Oxide (SOx), currently available for about $480.00 per ton, versus Marine diesel Oil (MDO) rated at less than 0.1% SOx, reportedly, around $695.00 per ton, depending upon the location of the bunkering port. Hence, compliance with the law becomes a rather expensive proposition. In addition to which, the contemporary solution to such outlawed exhaust emissions, involving switching from conventional heavy fuel (380 centistokes), or from No. 2 diesel fuel, to a distillate fuel with very low sulfur content, known as Ultra-Low Sulfur Fuel (ULSF), has caused problems of its own. Typically, related incidents involving ULSF have adversely affected cylinder liner lubrication of large-bore, slow-speed main propulsion diesel engines, and the diminished lubricity of ULSF has also been determined to be the cause of fuel injection pump binding of generator engines, resulting in loss of power. In one such incident, the generator failure resulted in a loss of steering power aboard a vessel heading into port.

Furthermore, it has been found that bio-fuel blends (also used as substitutes for conventional heavy fuel), are detrimental to elastomer sealing materials (a form of polymerized compounds), used in certain fuel transfer pump oil seals, apparently due to acidity of the bio-fuel due to oxidation. Given the temperature difference between MDO and ULSF, and the potential for fuel injection pump seizure, MAN diesel & Turbo has devised a diesel “switch” to handle changeover between high and low sulfur fuels, independent of engine load, and to automatically adjust fuel temperatures in the MGO cooler, for vessels entering or departing from SECA regulatory zones. The system is also designed to “log” the changeover process for official documentation for port authorities if and when required. This system is actually a later version of the “switch” originally developed by MAN diesel for the changeover from MDO to LNG, applicable to dual-fuel engines. Another alternative to the ultra-low sulfur fuel lies in the potential efficacy of “exhaust gas scrubbers”, such as those built by Hamworthy, Krystallon and the dry-scrubbing system recently certified by Germanischer Lloyd for MAN Diesel and Turbo, to reduce the levels of emission effluents by a factor not less than 95% sulfur oxide (S0x); 78% for nitrogen oxide, (NOx), and about 83% for particular matter (PM) within the next five years.

Dual Fuel
Yet another possible alternative lies in the feasibility of converting contemporary main propulsion diesel engines, whereby they become capable of burning “dual fuel”. This would involve retaining the conventional marine diesel oil (MDO) capability and modifying the engines to be capable of also burning liquefied natural gas (LNG). Several recently built European tankers, designed for the LNG trade with dual fuel (DF) capability plus a fleet of Norwegian ferries currently in operation are also burning LNG, from which the exhaust emissions are reportedly extremely low. While conventional wisdom appears to fault marine diesel engines for their exhaust emissions far more so than that of their highway or railroad counterparts, the truth is that for a riverine tugboat pushing a “fleet” of twelve of fifteen loaded barges upstream, the diesel engine exhaust emissions are estimated to be approximately 0.470 grams per ton/mile; which is about 27.70% lower than that of railroad trains and 35.60% lower than that of highway trucks, and are expected to become even lower if and when they eventually switch to burning LNG, given the current trend toward the adoption of LNG as a preferred marine engine fuel.

Since most ships are now powered by oil-burning diesel engines, either direct-drive, geared-drive or diesel-electric drive, the number of steam ships still in operation has diminished considerably. However, in the case of heavy oil tankers, such as VLCCs, (Very Large Crude Carriers), despite their large main propulsion diesel engines of 50,000-bhp or more, steam boilers play a prominent role in providing steam to sustain the cargo tank heating coils. This is due to the fact that in cold climates such as found in Northern European seaports, the relatively low sea water temperature has chilled the heavy black crude oil to a such a high viscosity that the cargo can not be pumped out without being pre-heated by the steam-heated tank coils. In such instances, the critical factors become (a) the oil seals of the fuel oil transfer pumps and boiler service pumps and (b) the steam boiler burner tips. Are they all currently designed for use with ULSF? Or do they also need to be replaced with ULSF-compatible components? The inevitable conclusion to all this is that solutions to certain problems that in turn, create problems of their own, are unacceptable and must be carefully evaluated before allowing one problem to be replaced by another potentially worse. Hence, responsible ship owners and ship managers cannot afford to ignore the above-mentioned factors, and have a moral obligation to alert their crews to such potential problems and take appropriate instructive and/or corrective measures to ensure that the required level of “readiness” is capable of sustaining the appropriate level of sea-worthiness.

Likewise, in view of the growing trend towards dual-fuel diesel engines, despite their several years of experience with various grades of diesel fuel, an appropriate level of readiness will be required of all those crew members involved with bunkering and monitoring of LNG fuel aboard ship. Meanwhile, ashore, it is reasonable to assume that as the demand for LNG as a marine fuel increases, and that suppliers develop the technology for economic mass production of same, that the price of such fuel will gradually decrease to a more competitive level. Meanwhile, considering their extensive experience with LNG as a marine fuel, with the design of ships and main propulsion engines specifically designed and built to be powered by LNG, Rolls-Royce is probably the most valuable source of information on this subject within the vast maritime community.