The idea that heat and work are equivalent was proposed by Julius Robert von Mayer in
1842 in the leading German physics journal and independently by James Prescott Joule in
1843 in the leading British physics journal.
"The Mechanical Equivalent of Heat", in which he specified a numerical value for the
amount of mechanical work required to produce a unit of heat.
In particular Joule had experimented on the amount of mechanical work needed to raise
the temperature of a pound of water by one degree Fahrenheit and found a consistent
value of 772.24 foot pound force (4.1550 J·cal-1).
Joule contended that motion and heat were mutually interchangeable and that, in every
case, a given amount of work would generate the same amount of heat. Meyer also
published a numerical value for mechanical equivalent of heat in 1845 but his
experimental method wasn't as convincing.
Though a standardised value of 4.1860 J·cal-1 was established in the early 20th century,
in the 1920s, it was ultimately realised that the constant is simply the specific heat of
water, a quantity that varies with temperature between the values of 4.17 and 4.22 J·g1
·°C-1. The change in unit was the result of the demise of the calorie as a unit in physics
and chemistry.
COAL
Coal is classified by geologists as a mineral. But most minerals, like salt or iron ore, were
formed by inorganic matter . Coal, on the other hand, came from organic matter-plants,
that lived about 300 million years ago.
A compressed layer of dead, decaying plants in the water.
TYPES OF COAL
Coal is a very complex and diverse energy resource that can vary greatly, even within the
same deposit. In general, there are four basic varieties of coal, which are the result of
geologic forces having altered plant material in different ways. These varieties descended
from the first stage in the formation of coal: the creation of peat or partially decomposed
plant material.
Lignite: Increased pressures and heat from overlying strata caused buried peat to dry
and harden into lignite. Lignite is a brownish-black coal with generally high moisture and
ash content and lower heating value. Geologically, is the youngest and the lowest ranked
coal, containing 25 to 35 percent carbon and the lowest heating value-4,000 to 8,300 Btus
per pound. However, it is an important form of energy for generating electricity,
particularly in the American Southwest, and to produce synthetic natural gas and liquids.
Lignite is found in the Gulf Coast and Northern Plains of the United States. Lignite
reserves account for 9 percent of the United States' coal reserves.
Subbituminous: Under still more pressure, some lignite was changed into the next rank
of coal: subbituminous. This coal is a combustible mineral formed from the remains of
trees, ferns and other plants that existed and died during the time of the dinosaurs. A dull
black coal with a higher heating value than lignite that is used primarily for generating
electricity and for space heating. It contains about 35 to 45 percent carbon and has a
heating value between 8,300 to 11,500 Btus per pound. Subbituminous coal is
predominately found in Montana, Wyoming, UTAH, Colorado, New Mexico,
Washington, and Alaska. Subbituminous coal accounts for about 37 percent of the coal
reserves in the United States.
Bituminous: Sometimes called "soft coal" it is the most common type of coal found in
the United States. It is 45 to 86 percent carbon, softer than anthracite, and has a heat
content between 10,500 and 14,000 Btus per pound. This is the type most commonly
used for electric power generation in the U.S. and for producing coke for the steel
industry. About 52 percent of the United States' coal reserves are bituminous coal.
Anthracite: Sometimes called 'hard coal," anthracite was formed from bituminous coal
when great pressures developed in folded rock strata during the creation of mountain
ranges. Anthracite has the highest energy content of all coals. It contains 86 to 97 percent
carbon, and has a heat content of nearly 15,000 Btus (British thermal units) per pound. It
is used for space heating and generating electricity. There are about 7.3 billion tons of
anthracite reserves, located in 11 counties of northeastern Pennsylvania. Anthracite
accounts for about 2 percent of the coal reserves in the United States.
All coal has carbon and sulfur to some degree. Coal that was formed in swamps covered
by seawater contains a higher sulfur content; low sulfur coal was generally formed under
freshwater conditions.
The Lower Calorific Value (or Net Calorific Value - NCV) suppose that the products of
combustion contains the water vapor and that the heat in the water vapor is not recovered.
Fuel
Higher Calorific Value
(Gross Calorific Value - GCV)
kJ/kg
Btu/lb
Acetone
29,000
Alcohol, 96%
30,000
Anthracite
32,500 - 34,000
14,000 - 14,500
Bituminous coal
17,000 - 23,250
7,300 - 10,000
Butane
49,510
20,900
Carbon
34,080
Charcoal
29,600
12,800
Coal
15,000 - 27,000
8,000 - 14,000
Coke
28,000 - 31,000
12,000 - 13,500
Diesel
44,800
19,300
Ethanol
29,700
12,800
Ether
43,000
Gasoline
47,300
Glycerin
19,000
Hydrogen
141,790
61,000
Lignite
16,300
7,000
Methane
55,530
Oils, vegetable
39,000 - 48,000
Peat
13,800 - 20,500
Petrol
48,000
Petroleum
43,000
Propane
50,350
Semi anthracite
26,700 - 32,500
Sulfur
9,200
Tar
36,000
Turpentine
44,000
Wood (dry)
14,400 - 17,400
20,400
5,500 - 8,800
11,500 - 14,000
6,200 - 7,500
kJ/m3
Acetylene
56,000
Butane C4H10
133,000
Hydrogen
13,000
Natural gas
43,000
Methane CH4
39,820
Propane C3H8
101,000
Town gas
18,000
Btu/ft3
kJ/l
Btu/gal
Gas oil
38,000
164,000
Heavy fuel oil
41,200
177,000
Kerosene
35,000
154,000


1 kJ/kg = 0.4299 Btu/ lbm = 0.23884 kcal/kg
1 Btu/lbm = 2.326 kJ/kg = 1.8 kcal/kg