Fuels

FUELS: A substance which readily combines with oxygen to burn and gives off heat energy, can be used to heat a room, for cooking food, making steam in a boiler etc can be called a fuel.

Combustion: It is a phenomenon where elements (like carbon, hydrogen) or compounds or mixture of compounds/elements (like kerosene, petrol, diesel, LPG, coal gas etc.) combines with oxygen and reacts vigorously producing heat and light. Combustion is often accompanied by a flame.

Flame: Flame is the region in which chemical interactions among gases occur accompanied by evolution of heat and light. The flame occupies only a small portion of the combustible mixture at any time. Flame speed and thickness is largely affected by temperature, pressure and the type of fuel used. A flame has two zones, namely

  1. Pre-heat zone, where little heat is released.
  2. Reaction zone, where the bulk of chemical energy is released.

The calorific value of fuel: It is the heat released by a fuel during its combustion. For solid fuel, it is expressed in terms of Joule per Kilogram (J/Kg) or Kilocalorie per kilogram (Kcal/Kg). For a liquid fuel, it is expressed in Joule/liter or Kcal/liter, while for a gaseous fuel; it is in Joule/cubic meter or Kcal/cubic meter. There are two measurements of calorific value. One called net calorific value (NCV), is the useful heat we finally get by burning the fuel. The other is called gross calorific value (GCV), which is the NCV plus the heat that we could get back by condensing the steam as it comes out with the gaseous product of combustion. So, GCV is always higher than NCV. NCV is the parameter for comparing the heating values of different fuels.

Flashpoint: Flashpoint is the lowest temperature at which a liquid will give off sufficient vapor in air so that it ignites momentarily (without sustained burning) in presence of an external igniter like a flame or a spark.

Pour point: It is the lowest temperature of a liquid at which it will pour or flow when cooled under prescribed conditions. It is an indication of the lowest temperature at which the fuel oil is still readily capable of being pumped without getting excessively thick when cooled.

Specific heat: It is the heat required to raise the temperature of 1 Kg of substance through 1oC (in SI system). For fuels, it is usually expressed as Kcal/KgoC. This quantity is more important for liquid fuel as it is relatively high for them and the heat needed to heat it from room temperature to a desired high temperature for it to effectively burn. Light oils have low specific heat, while heavier oils have higher specific heat. These values usually vary from 0.22 to 0.28 for fuel oils.

Types of Fuels: Broadly there are three types of fuels, solid, liquid, and gaseous.

Solid fuel– Commonly used solid fuels are firewood, coal, and coke. Agro-residues like rice husk, bagasse, etc are also used as solid fuels. In missiles, rockets, etc. solid magnesium is used as a fuel.

In our country, solid fuels used in industries are mostly coal and coke. Coal can be classified as Bituminous, Anthracite and Lignite.

Anthracite is the geologically oldest grade of coal. It is a hard type consisting mainly of carbon with little volatile content and no moisture.

Lignite is the youngest from a geological perspective. It is a soft type consisting mainly of volatile matter and moisture with low fixed carbon.

Bituminous and semi-bituminous are common coal used in Indian industries. Bituminous and lignite produce a high quantity of smoke. Anthracite has less volatile matter and is smokeless. It is expensive but got a higher calorific value.

Another solid fuel is coke used in industries as fuel. Coke is produced by heating coal in a closed oven until its volatile content is driven off. Coke has a higher heating value and discharges little smoke. The calorific value is about 16,000 BTU/lb or 10,000 Kcal/Kg. 

Liquid Fuel– In industries liquid fuels used are Kerosene, Light Diesel Oil (LDO), High Speed Diesel (HSD), Petrol, Furnace Oil.

Gaseous Fuel– Commonly used gaseous fuels in India are LPG (Liquefied Petroleum Gas), Natural Gas, Producer Gas or Coal Gas. The calorific values of gaseous fuel is expressed Kcal per normal cubic meter (Kcal/Nm3), i.e., the volume corresponds to normal temp (200C) and normal atmospheric pressure (760 mm of Mercury)

LPG– It is a mixture of propane (C3H8) and butane (C4H10) plus small percentage of other hydrocarbons. It is a bye product of a petroleum refinery. It is a gas under normal temperature and pressure, but can be liquefied by moderate pressure and is stored, transported as liquids for ease of handling – takes 250 times less space than that taken by same mass in gaseous state. When the stored liquid comes out through the cylinders, comes under the reduced pressure in normal atmosphere, again evaporates to become gas to burn in ovens. To trace out leakage detection LPG is mixed with organic sulphide (Beta-mercaptan- C2H5SH). There should be adequate ground level ventilation where LPG is stored.

Natural Gas– Earth is the main source of natural gas. During exploration for petrol reserves, this is discovered. It is a very good fuel. Methane (CH4) is the main constituent (95% by volume). Other components are ethane (C2H6), butane (C4H10), propane (C3H8), pentane (C5H12), nitrogen (N2) and carbon di-oxide (CO2). Calorific value of natural gas ranges from 12,000 Kcal/m3 to 14,000 Kcal/m3.

Coal Gas or Town Gas- This is obtained as a bye product during the production of coke from coal in coke-oven plants. Coke is used for making iron from iron ore. Coal gas is a mixture of hydrogen, carbon monoxide, methane together with nitrogen, oxygen and carbon di-oxide and some other hydrocarbons, hydrogen sulphide and cyanide. This gas is toxic due to carbon monoxide. Due to hydrogen sulphide (smells like rotten egg) the leakage can be identified. Calorific value is about 4900 Kcal/m3.

Comparative fuel cost

Fuel cost can be calculated by using a formulae by arriving at the cost of a useful “therm”.

1 Therm = 100,000 BTU

Coke:- If coke cost  Rs. 12000 per ton or Rs. 4/- per lb then

1 therm cost = Cost per lb x 100000 / 12000  =  (4 x 100000 / 12000)  =  Rs. 33.33

A useful therm is the amount of heat output put to good use. Generally coke is considered as 60% efficient fuel. So, one useful therm costs =  Cost per therm x 100 / 60 = Rs. 33.33×100/60 =

Rs. 55.55

Gas:- To change cu.ft to therm, the heating power or calorific value of the gas to be known. This is generally shown on the gas cylinders.

Cost of Therm = Calorific value x Hundred of cu. ft / 1000 = A

Generally gas is calculated on 80% efficient.

So, one useful therm cost = Cost per therm x 100/80 = A x 100/80

Electricity:

1 unit of electricity produces 3412 BTU. Electricity is calculated as 100% efficient.

So, 1 therm costs = Cost per unit (B) x 100000 / 3412  = B x 100000/3412

Oil:-  1 gallon produces  165000 BTU. Oil is calculated as 75 % efficient.

So, 1 therm costs = Cost per gallon x 100000 / 165000 = C

So, one useful therm costs = Cost per therm x 100/75 = C x 100 / 75

Gas:  Stoves and burners are the appliances that are two of the arterial equipment in the kitchen of a hotel or a catering establishment. The correct choice of the type of ovens and burners are critical for energy efficiency, safety and functional performance of a kitchen. Burners are also used in boilers and many other equipments of hotel industry.

Oil stoves are not used for cooking in hotel industry. Oil burners are used in oil-fired furnaces of boiler and water heaters in a hotel. Purpose of oil burner is to make fine particles of fuel-oil, burnt at the mouth of the burner. This is known as the atomization which is different from vaporization. In atomization liquid remains liquid in tiny droplets form, but in vaporization liquid changed to gaseous state.

Out of above types of burners we will concentrate only on the twin-fluid atomizer type burners.

  1. Low air pressure burner- Air is provided by a blower, air acts as an atomizing agent. Oil is injected through a nozzle at the open end of burner where it gets atomized. Flow of air and oil regulated by valves. When oil flow rate is low, efficiency becomes lower. Air pressure in these types is 8 lb/ sq. in (about 330 mm of mercury- half the normal atmospheric pressure)
  2. Medium and High pressure burner:- Air is provided by a compressor. When load changes, the quantity of atomizing air does not change, only secondary air entering the system changes. So at lower loads also these burners are efficient.

Medium pressure burners have air pressure – 12 lb/sq in (80% of the atmospheric pressure).

High pressure burners have air pressure – 15 lb/sq.in ( 1 atmospheric pressure).    

Some chefs prefer gas cook tops since those can control heat more finely and more quickly. But some chefs prefer electric ovens as those can heat food more uniformly.

Low pressure burners use gas at low pressure (less than 0.15 Kgf/cm2 or 2 lb/sq. in or psi – about 1/7 th of 1 atm pressure). They are usually multijet type, gas is supplied from manifold / cylinder to a number of small single jets or circular rows of small jets around the inner circumference of circular opening in a block of heat resisting materials.

In high pressure burners, the gas jet draws air into mixing chamber and delivers proportional mixture to the burner. When the regulating valve opened, gas flows through a small nozzle into a venturi tube (a tube with a constriction at some section). At the narrow section, gas velocity becomes high when pressure drops – sucks air from outside through openings at the narrow section. This gas-air mixture flows through pipe to the burner.  

Different types of gas burners – There are five types gas burners like

  1. Giant: For rapid heating, for large utensils. Heat rate- not less than 12000 BTU/hr.
  2. Regular/Standard: For general cooking in average size pans. Heat rate- not less than 9000 BTU/hr.
  3. Simmering: For domestic in medium size pans. Heat rate- not less than 1200 BTU/hr.
  4. Pilot: For small families with small size pans. Heat rate- not more than 300 BTU/hr.
  5. Mini Pilot: For very small use with very small size pans. Heat rate- not more than 125 BTU/hr.

Now-a-days big size ranges available with heat output as 40,20,000 BTU/hr., some heavy duty fryers have 1,20,000 BTU/hr.

Safety Precautions (while using Gas Equipment)

  1. Equipments should be cleaned regularly, since gas contains sulphur, which corrodes metals.
  2. Gas cylinders should be placed in upright vertical position and never in horizontal position.
  3. Gas cylinders should not be tilted to an inclined position while being used, in order to completely utilize the gas.
  4. Cylinders must not be hammered.
  5. Cylinders and ovens to be placed in well-ventilated place.
  6. Inflammable materials should not be kept very near to gas bank.
  7. Empty gas cylinders to be immediately removed and kept at a place away from fire or other source of heat.
  8. Hose connecting the gas valve and oven should be checked at regular intervals.
  9. When work is over, regulating valve and burner switch should be in ‘OFF’ position.
  10. Everyday burner heads should be removed from ovens and pots to be cleaned with steel wire brushes.
  11. Open doors and windows before lighting burners.
  12. To put off oven, first regulating valve of the gas line is to be closed and then gas burner knob to be kept in ‘OFF’ position.

 Steps for efficient operation of LPG / other stove and burners

  1. Correct size of burner / nozzle should be ensured.
  2. If there is no pilot flame, ignition torch to be used.
  3. Attention should be given to fading or pulsation of flame.
  4. Flame size to be adjusted as per requirement.
  5. Proper shut down procedure to be followed

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