What is Heat engine ? It's Classification & Difference between 2-stroke engine and 4-stroke engine :

Heat Engine: The machine which converts heat produced by burning of fuel into useful work is known as heat engine. In other words, heat engine is basically a equipment which generates thermal energy and transforms the same into mechanical energy form.
 
There are different types of classification of heat engines based on various criterias.
There are two types of classification based on working cycle. The same have been mentioned in the table below.

In 2-stroke engine, the whole sequence of events i.e. suction, compression, power and exhaust are completed in two strokes of piston i.e. one revolution of crankshaft.
In 4-stroke engine, all the four events (suction, compression, power and exhaust) take place inside the engine cylinder. The 4 events are completed in 4 strokes of the piston i.e. two revolutions of the crankshaft.

The figure-1 depicts 2 stroke engine (petrol type) and 4 stroke engine (petrol/diesel type). In 4 stroke engine all the four events viz. suction, compression, power and exhaust occur in four different strokes of piston. In 2 stroke engine, suction and compression occur in upward stroke of the piston while power and exhaust occur in downward stroke of the piston. 

Working Principle of Gas Turbine ::

The working principle of gas turbine is to convert the chemical energy of fuel into mechanical energy through combustion process, and then the mechanical energy is converted by a generator into electrical energy. Gas turbine works with Brayton cycle and its working fluid is gas. The simplest gas turbine system consists of 3 (three) main components: compressor, combustor and turbine, with the arrangement as shown in Figure 1.

 

 

The working principle of gas turbine system is the atmospheric air goes into the compressor which serves to suck air and raise the pressure of air, so that its pressure will rise. Then the high pressure air flow into combustion chamber. In the combustion chamber the fuel is sprayed into the air flow, resulting in the burning/combustion process. The combustion process takes place at constant pressure, so it can be said that the combustion chamber is used only to raise the temperature of air. High-temperature combustion gas is then entered into a gas turbine in which its energy is used to rotate turbine blades. As much as ± 60% of power generated by turbine is used to rotate compressor itself, the remainder is used to turn generator.

This ideal cycle consists of 2 (two) Isobars processes that occur in the combustion chamber and exhaust process of used gases, and 2 (two) isentropic processes that occur in the compressor and the expansion of gases in the turbine.

Source :  http://steamofboiler.blogspot.com/2011/09/working-principle-of-gas-turbine.html

What is heavy fuel oil (HFO) ?

HFO, also known as “residual fuel oil”, is based on the high viscosity, tar-like mass, which remains after the distillation and subsequent cracking of crude oil in order to produce lighter hydrocarbon products, such as petrol , distillate diesel fuels and heating oil or feedstocks for lubricants.
The main components are alkanes,  cycloalkanes and different carbon hydrides. The boiling range is between 300°C and ~700°C.
Due to its semi-fluid consistence, HFO has to be preheated to make it combustible in engines.
RMA, RMB, RMD, RME, RMG or RMK are the international trade names.


Main ref : http://powerplants.man.eu/fuels/hfo
Another Ref : https://www.anton-paar.com/corp-en/products/applications/viscometry-of-heavy-fuel-oil-hfo-and-residual-fuel-oil-rfo/

Cheap, but challenging

As a residual product, HFO is a relatively inexpensive fuel – it typically costs 30% less than distillate fuels (MDO/MGO) (Verlinkung). It thus became the standard fuel for large marine diesel engines during the oil crisis in the 1970s and 1980s, and it required extensive adaptation of the injection system and other components of low and medium speed engines – which are still the only reciprocating engines capable of running on HFO.
 Most of  MAN medium speed liquid fuel engines  can burn heavy fuel oil (HFO). Of course,  medium speed dual  fuel engines are capable of burning HFO in liquid fuel mode as well.


What is HFO or RFO?

Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Fuel oil is classified in six classes, numbered 1 through 6, according to its boiling point, composition and purpose.

•  No.1 is similar to kerosene and is the fraction that boils off right after gasoline.

•  No.2 is the diesel fuel that trucks and cars run on, leading to the name "road diesel".

•  No.3 is a distillate fuel oil and is rarely used.

•  No.4 fuel oil is usually a blend of distillate and residual    fuel oils, such as No.2 and 6; sometimes it is just a heavy distillate (~ diesel distillate or residual fuel oil).

•  No.5 is a mixture of 75-80% of No.6 and 25-20% of No.2.

•  No.6 is called residual fuel oil (RFO) or heavy fuel oil   (HFO). It is the remainder of the crude oil after gasoline   and distillate fuel oils were extracted through distillation. It fuels thermal power stations or robust engines.

Why measure the viscosity and density of RFO?

The viscosity of an oil is a measure of its resistance to flow. In fuel oil it is highly significant since it indicates both the relative ease with which the oil can be pumped, and the ease of atomization.

Density alone is of little significance as an indication of the burning characteristics of fuel oil. Used in conjunction with other properties, it is of value in mass-volume relationships and for the specific energy of an oil.

Fuel oil specification

Heavy fuel oil

ISO 8217, ISO-F-RM

K700

Fuel-system related characteristics values

Viscosity (at 50 °C)

mm2/s (cSt)

max.

700

Viscosity (at 100 °C)

mm2/s (cSt)

max.

55

Density (at 15 °C)

kg/m3

max.

1,010

Flash point

°C

min.

60

Pour point

°C

max.

30

Hydrogen sulfide

mg/kg

max.

2

Acid number

mg KOH/g

max.

2.5

Total sediment aged

mass %

max.

0.10

Engine-related characteristic values

Carbon residues (Conradson)

m%

max.

20

Sulphur

m%

max.

5

Ash

m%

max.

0.15

Vanadium

mg/kg (=ppm)

max.

450

Water

Vol.%

max.

0.5

Additional parameters

Aluminium and silicon

mg/kg

max.

60

Asphalts

m%

max.

2/3 of carbon residues (Conradson)

Sodium

mg/kg

Sodium < 1/3 vanadium, sodium < 100

Used lubricating oil (ULO)

mg/kg

Ca >30 and Zn >15 or CA >30 and P >15

What are Liquid Fuels ?

All petroleum products, natural gas liquids, biofuels, and liquids derived from other hydrocarbon sources (coal to liquids and gas to liquids). Not included are compressed natural gas (CNG), liquefied natural gas (LNG), and hydrogen.

What is Diesel Fuel ?

Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. The important properties which are used to characterize diesel fuel include cetane number (or cetane index), fuel volatility, density, viscosity, cold behavior, and sulfur content. Diesel fuel specifications differ for various fuel grades and in different countries.

Natural gas ?

Natural gas - like oil and coal - is a fossil fuel. It is a “nonrenewable” fuel because it takes millions of years to form deep in the earth before it can be used again. When microorganisms, sea plants and animals died over 300 million years ago (even before the dinosaurs!), they settled on the bottom of the oceans, which covered most of the earth. Layers of these dead sea plants, animals, sand, mud, and other debris built up over time and the pressure and heat from the earth turned them into petroleum and natural gas. The natural gas would get trapped in pockets of underground rocks. Sometimes the gas would seep up to the earth's surface and dissipate into the air or ignite from lightning strikes.
Natural gas in its pure form is colorless, odorless and shapeless. Only after processing is the "rotten egg" odor added for safety. Natural gas is actually a mixture of many gases, including propane, butane, carbon dioxide, ethane, oxygen, nitrogen and hydrogen sulfide, but it is primarily made up of methane gas. When you burn natural gas, it gives off heat energy that is stored in the gas molecules. There are some renewable sources for methane, like garbage in landfills, that produce some gas.

Difference between steam and gas?

Steam is a mixture of Gas and Liquid(water) while gas Could be an element in its molecular form or in a mixture/compound form.

O2 is oxygen gas and it can be found as oxygen. While Air is a mixture of many gases.

steam on the other hand is a mixture if air n water. When water is heated above its boiling point, it vaporizes and mixes with air. upon cooling (condensation) water regains its liquid form. Thats why when you go into a steam room you get wet.

Smoke on the other hand is a mixture of Solids and Gases. When you burn something solid like wood or coal you get black smoke. That smoke hurts you because it has solids particles in it which precioitate on the walls of your nose/eyes etc.

Steam does not hurt rather it soothes because it has water which moistens the walls of eye/nose/throat etc.

Steam is water in gas phase.

What are gases?

Gas is everywhere, and it surrounds us. The air around us is a kind of gas. The atmosphere surrounding the earth is a gas too. Helium, Oxygen, Carbon dioxide and water vapour are all gases.

The particles in gases are very different from that of solids and liquids.
In gases, the particles are far apart from each other and arranged in a random way. The particles also move quickly in all directions. Gases can fill up any container of any shape and size. Gases can be compressed or squashed because the molecules are far from each other. When gas is compressed, the gas molecules move from an area of high pressure to low pressure. 

Vapour is also a gas. Gases that are liquid at room temperature, like water, can be classified as vapour. This means they are usually liquid, but can vaporize (turn into gas) under certain conditions.

The table below is a great guide to the relationship between solids, liquids and gases:

 

COMBINED CYCLE POWER PLANT: HOW IT WORKS!

A combined-cycle power plant uses both a gas and a steam turbine together to produce up to 50 percent more electricity from the same fuel than a traditional simple-cycle plant. The waste heat from the gas turbine is routed to the nearby steam turbine, which generates extra power.

HOW A COMBINED-CYCLE POWER PLANT PRODUCES  ELECTRICITY ?

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1.     Gas turbine burns fuel. 
·         The gas turbine compresses air and mixes it with fuel that is heated to a very high temperature. The hot air-fuel mixture moves through the gas turbine blades, making them spin. 
·         The fast-spinning turbine drives a generator that converts a portion of the spinning energy into electricity.

2.     Heat recovery system captures exhaust.

·         A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine that would otherwise escape through the exhaust stack.

·         The HRSG creates steam from the gas turbine exhaust heat and delivers it to the steam turbine.

3.     Steam turbine delivers additional electricity.

·         The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity