There are different types of marine fuels available today on the market and marine industry will go into a major change sooner or later, as a response to the continuous change of the environmental rules and regulations. Therefore, the marine industry, among other industries, must respond and adapt to the changing and more strict regulations and become more efficient with regard to fuel consumption and emissions.
Many companies are researching and developing new types of fuels in liaison with engine manufacturers and shipyards. Some of the engine manufacturers strive to adapt and retrofit their existing engines to the new fuels, when possible, as it proves to be more economically practical. However, new built vessel will have to be equipped with new designed and versatile engines which can be used on dual fuels or new different type of fuels.
Fuel for marine operations of marine fuels is comprising of heavy fuel oils, distillates, LNG, ammonia, hydrogen, methanol, fuel cells and others. In the case of distillates, a distinction is made between marine gas oil (MGO) and marine diesel oil (MDO).
Residual Fuels
Residual fuel oil accounts for the vast majority of the fuel oil used by the world’s merchant fleet. This also applies to the vast majority of large diesel engines used on land. By definition, residual fuel oils are the byproducts of refinery processes that remain after the distillate or lighter fractions have been removed. These residues are complex mixtures that vary depending on the source of the crude oils processed and the refinery’s complexity.
After 2020 sulphur cap compliance regulation all conventional vessels are forced to use HFO + scrubber or LSFO and these fuels perform well on most parameters. There were some concerns about the availability of these fuels after 2020, but they are still significantly available than alternative fuels. The primary disadvantage of these fuels is their low environmental performance.
Scrubbers incur high investment and operating costs, but these are negligible in comparison to the costs of alternative fuels.
Fuel oil was initially derived from the residue of the atmospheric or vacuum distillation process in the early days of refining. By and large, the product entering the market for fuel oil was of consistent quality, with few issues. As demand for distillate products increased, refiners implemented secondary refining processes, altering the market characteristics of fuel oil. The secondary refining processes have some side effects like lesser amount of residual fuel, higher micro-carbon value, stability and sediment problems, higher density and catalyst fines contamination problems.
Individual country-specific product demand is extremely diverse, and cannot be met solely through crude oil selection due to the sheer volume required. Additionally, within a single geographical area, a variety of crude oil sources and refinery process configurations are used. As a result, fuel for industrial and marine markets exhibits considerable variation in its properties on a global scale. While this has been the case for the most part throughout history, the variations can be more pronounced than in the past.
An oil refinery can be thought of as a factory that transforms crude oil into a variety of useful products. It is designed to meet market demands in the most cost-effective and efficient manner possible. The first step in the manufacture of petroleum products is the atmospheric distillation of crude oil into its major fractions. When crude oil is heated, the lightest, most volatile hydrocarbons evaporate first, followed by the heaviest, least volatile hydrocarbons. After cooling, the vapors are condensed back into liquids. A fractionating column is used to carry out this distillation process. The column is divided into chambers by perforated trays that condense the vapors and allow the liquids to flow into storage tanks at each stage. The crude oil is preheated to a maximum of 350°C to avoid thermal cracking.
The residue from atmospheric distillation is sometimes referred to as long residue, and additional distillation at a reduced pressure and high temperature is used to recover more distillate product. This vacuum distillation process is critical for optimizing crude oil upgrading. The vacuum distillation residue, sometimes referred to as short residue, is used as a feedstock for further processing or as a component of a fuel. Unlike the fractionating column used in atmospheric distillation, the low-pressure vapors are condensed using a system of packed beds rather than trays.
Refineries that rely solely on atmospheric and vacuum distillation are referred to as “straight run” refineries, and the fuel oil produced is essentially either long or short run residue. The residue percentage varies according to the composition of the crude processed. The residue is 28 % for a typical “light” North African crude and up to 85 % for a “heavy” Venezuelan crude. The proportion of products produced does not always correspond to the demand for them and is largely determined by crude oil.
Additional refining processes were introduced to meet product demand. Apart from atmospheric and vacuum distillation, a modern refinery may also include secondary refining processes such as cracking, which may be thermal or catalytic. Below is an illustration of a typical modern refinery installation.
Copyright: researchgate.org
Thermal cracking is the oldest and, in theory, the simplest method of refinery conversion. It is carried out at temperatures ranging from 450 to 750°C and pressures ranging from atmospheric to 70 bar. Temperature and pressure are determined by the type of feedstock and the desired product. At these elevated temperatures, the large hydrocarbon molecules become unstable and self-destruct. Another critical factor in the process is the length of stay.
The feedstock can be either atmospheric or vacuum distillation residue, or a combination of the two. The thermal cracking process has three primary applications in modern refineries: visbreaking, thermal gas oil units, and coking. Visbreaking is the most critical process in the production of residual fuel oil. It is a relatively mild type of thermal cracking that is frequently used to reduce the viscosity of straight-run residual fuels. Typically, such fuels are extremely viscous and must be blended with a relatively high-value distillate to meet the finished product specification. Visbreaking significantly reduces the amount of distillate required as diluent or “cutter stock,” which can then be diverted to a more profitable product stream.
A thermal gas oil unit’s primary objective is to produce and recover the maximum amount of gas oil possible. In extreme cases, the residues viscosity may exceed that of the feedstock. Coking is a form of thermal cracking that is quite severe. It is intended for the conversion of straight-run residues to more valuable products such as naphtha and diesel oil. Additionally, gas and coke are produced, and as a result, this process is not used in the production of residual fuel oils.
In the petroleum refining industry, catalytic cracking has become the primary process for converting heavy hydrocarbon fractions, primarily into high-quality gasoline and fuel oil components. These are more valuable than the feedstock because they are lighter and less viscous. There are numerous catalytic cracker designs, however, the final product output can be separated into gases, gasoline blending components, catalytically cracked cycle oils, and cycle oil slurry in all cases. Cycle oils are critical in relation to residual fuel oil because they act as cutter stocks, reducing the viscosity of residues. Prior to being used as a cutter stock, the cycle oil slurry must be treated to remove any entrained catalyst particles.
There are numerous residues and diluents available in a modern refinery for the production of fuel oil. Typically, the fuel is made up of visbroken residue that has been diluted with cycle oils and trace amounts of other distillates. Below figure depicts the major feedstock, diluent, and residue streams in a modern refinery.
Copyright: researchgate.org
Obviously, if a refinery lacks a thermal cracking facility (visbreaker or thermal gas oil unit), the fuel oil will be derived from long or short residue. Apart from the primary residual fuel streams in a modern refinery, it is worth noting that additional developments have been made to maximize the amount of gasoline, kerosene, and diesel produced from a barrel of oil. One of these is through residue hydro conversion, which converts residual fractions into feedstock that can then be processed further in conventional crackers to yield lighter products. Production optimization for lighter products is accomplished at the expense of residual fuel oil.
The British Standard Institute was the first to issue a specification for ship fuels in 1982 (BS MA 100). It was developed collaboratively by suppliers and engine manufacturers, with quality and price trade-offs.
The International Maritime Organization (IMO) initiated ISO 8217 “Petroleum products – Fuel (class F)” in 1987, which has since become the general standard. The standard makes a distinction between residue fuels (RMA, RMB, RMC…, RMK) and distillates (DMX, DMA, DMB and DMC). It categorizes residue fuels based on viscosity and establishes class-specific limits (maxima for density, flash point, pour point, coke residue, water, ash content, and sulphur content). Viscosity classes are defined as viscosity values of 30, 80, 180, 380, and 700 mm2/s at 500 C.
Critical values for “cat fines” (80mg/kg for aluminum plus silicon) and a test for quantifying possible total sedimentation content were adopted in March 1996 and we will talk about “cat fines” on a later post.