Environmental Engineering Reference
In-Depth Information
heating values of these fuels vary according to the fuel composition, none of them having a pure
molecular composition and some of them including inert components. The unit selling price of
these fuels may be based upon the volume (liquids, gases, and wood) or the mass (coal), but the
heating value may be a factor in the price. Their heating values are listed in Table 3.1.
In virtually all combustion systems, the water molecules in the products of combustion leaving
the device are in the form of vapor, not liquid, because the effluent temperature is high enough and
the concentration of water molecules is low enough to prevent the formation of liquid droplets. As
a practical matter, the heat of condensation of the water vapor is not available for partial conversion
to work, and the effective fuel heating value should be based upon the water product as a vapor, as
assumed in Table 3.1. Nevertheless, sometimes a
higher heating value
(
HHV
) is used in the sale of
fuel, based upon the assumption that the water product is in the liquid form. To determine this
HHV
for the fuel, we should add to the lower heating value (
FHV
of Table 3.1) the heat of vaporization
of water at the reference temperature, expressed as enthalpy per unit mass of hydrogen in water,
13
times the mass fraction of hydrogen in the fuel.
14
The distinction between higher and lower heating value is primarily a matter of convention.
Sellers of fuel like to quote their price in terms of dollars per million Btu of higher heating value,
a lower price than that per million Btu of lower heating value. On the other hand, users of fuel
who generate electricity prefer to rate their plant efficiency in terms of electrical energy produced
per unit of fuel lower heating value consumed, leading to higher efficiencies than when using the
higher fuel heating value. As long as the basis of the price or plant efficiency is stated, no confusion
should result.
3.10
IDEAL HEAT ENGINE CYCLES
Generating mechanical power from the combustion of fossil fuel is not a straightforward matter.
One must utilize the combustion process to change the temperature and /or pressure of a fluid and
then find a way to use the fluid to make mechanical work by moving a piston or turning a turbine.
The first and second laws of thermodynamics limit the amount of work that can be generated for
each unit mass of fuel used, and those limits depend upon the details of how the fuel is used to
create power.
To understand the implications of the thermodynamic laws for the conversion of fuel energy
to mechanical power, it is convenient to analyze ideal devices in which a fluid is heated and cooled,
and produces or absorbs work, as the fluid moves through a cycle. Such a device can be called a
heat engine
in that it exchanges heat with its environment while producing work in a cyclic process.
The combustion of fuel is represented in this idealized cycle by the addition of heat from a high-
temperature source. Some practical engines, like the gas turbine and the automobile engine, are not
heated from an external source. These are termed
internal combustion engines
(ICE). Nevertheless,
most of their features can be modeled as ideal heat engine cycles to help us understand their chief
attributes.
13
At 25
◦
C, this value is 21.823 MJ/kg H.
14
The difference in heating values is a maximum for hydrogen (21.823 MJ/kg fuel), but approaches 3.136
MJ/kg fuel for the heaviest hydrocarbons of Table 3.1.
