- Slides: 21
DIESEL FUEL Middle Distillate Fuel
Use of Diesel Fuel In Mobile Applications • • • Trucks Locomotives And now passenger cars Ships Stationary Applications Power generation Pump sets in agriculture
Refinery Capacity Increased consumption of diesel fuel will increase crude oil consumption because diesel oil density is more. Refinery energy consumption decreases slightly as diesel production increases (according to one estimate). But to cater to increased diesel production, existing facilities must be expanded and will increase costs (so lower diesel cost benefit will be lost). But there could be a reduced crude oil usage with increased diesel production.
Indian context In India we have two types of diesel fuels: 1. High speed diesel (HSD) used in automotive applications and 2. Light diesel oil (LDO) used in stationary applications. Diesel fuel properties are influenced by the source of the crude oil and the method of refining.
Properties of diesel fuel It is found that Specific gravity, flash point, viscosity, sulfur content, and carbon residue increase with increase in service severity. The cetane number (which measures the fuel’s ignition quality) decreases with increase in service severity. There is also a decrease in volatility with increase in service severity. Properties are, however, inter-related and it is difficult to isolate the effect of any single variable
Properties of diesel fuel (Continued) It is found that the self-ignition temperature of the normal paraffins decreases as the length of the chain increases. Since the cetane rating of the fuel is a measure of the ignition characteristics of the fuel, it can be concluded that the heavier members of the paraffin family have higher cetane ratings. In fact, cetane, C 16 H 34 (hexadecane) is the primary reference fuel in the cetane scale with an arbitrary cetane rating of 100 while other normal paraffins have cetane ratings that vary almost linearly with the length of the chain.
Octane versus Cetane Rating • It is generally observed that there is an inverse relationship of octane and cetane ratings. Wilkes, in 1940, gave the following relationship between the cetane number [CN] and the motor octane number [MON]: CN = 60 - 0. 5*MON
Characteristics of Hydrocarbons as Diesel Fuels S. No. General Formula Hydrocarbon Characteristics 1. n-paraffins Cn. H 2 n+2 (Straight chain) Low specific gravity & high cetane number 2 Iso-paraffins 3 Cycloparaffins Cn. H 2 n+2 (Branch chain) Cn. H 2 n Same sp. gr. as 1 but lower cetane no. & B. P. Higher sp. gr. than 1 but lower cetane number Higher sp. gr & B. P. but lower cetane no. than 1 -3 Highest sp. gr & B. P. & lowest cetane no. 4 Benzenes Cn. H 2 n-6 5 Naphthalenes Cn. H 2 n-12
Important Characteristics of diesel fuel 1. 2. 3. 4. 5. 6. Knock characteristics-requires high cetane number Starting characteristics-requires high volatility but will give lower power Smoking and Odor-high volatility will give better mixing and lower smoke but also lower power Corrosion and Wear-due to presence of sulfur and ash Ease of Handling-should have low pour point and viscosity for ease of handling but high flash and fire point for safety and fire hazard. Density, heat of combustion and cleanliness.
Important Properties 1. Relative density or specific gravity 2. API gravity given by: 3. 4. 5. 6. 7. Flash point (usually above 130 deg. C) Fire point (about 10 -15 deg higher) Sulfur content (must be very low) Carbon residue (must be low-deposit problem) Ash (must be low-abrasion problem)
Important Properties (Distillation) Low 50% point to prevent smoke and low 90% point to reduce carbon residue). End point temperatures less than 370 deg. C desirable The 50% temperature is usually regarded as a better index of the overall mixing problem than the 90% temperature.
Aniline Point This is an approximate measure of the aromatic content of a hydrocarbon fuel. It is defined as the lowest temperature at which a fuel oil is completely miscible with an equal volume of aniline. Aniline is an aromatic compound aromatics are more miscible in aniline than are paraffins. Hence, the lower the aniline point, the higher the aromatics content in the fuel oil. The higher the aromatics content, the lower the cetane number of the fuel. The aniline point can thus be used to indicate the probable ignition behavior of a diesel fuel.
Diesel Index The Diesel Index indicates the ignition quality of the fuel. It is found to correlate, approximately, to the cetane number of commercial fuels. It is obtained by the following equation Diesel Index and cetane number are usually about 50. Lower values will result in smoky exhaust
Other Properties 1. Pour point – mainly for low temperature applications 2. Calorific value – mainly for those fuels which come under the category of fuel oils and residual oils which have very high molecular weight and viscosity
Diesel Fuel Additives 1. 2. 3. 4. 5. Cetane Improvers, Detergents, Diesel Smoke Suppressants, Flow Improver Additives, and Other Additives like Anti-rust and Anticorrosion.
Cetane Improvers These are compounds that readily decompose to give free radicals and thus enhance the rate of chain initiation in diesel combustion. They promote fast oxidation of fuels and thus improve their ignition characteristics. Chemical compounds such as alkyl nitrates, ether nitrates, dinitrates of polyethylene glycols and certain peroxides are well known cetane improvers. In general, however, in view of their low cost and ease of handling, most commercial significance has been attached to different primary alkyl nitrates.