a substance which releases heat energy on combustion
chemical fuel
principal combustible elements of each fuel
carbon and hydrogen
this element is combustible but its presence on a fuel is considered undesirable
sulfur
factors in selecting the right type of fuel
availability, storage, handling, pollution and landed cost of fuel
Fuels can be classified according to whether:
- They occur in nature called primary fuels or are prepared called secondary fuels.
- They are in solid, liquid or gaseous state.
important requirement of a good fuel
- A good fuel should have a low ignition point.
- It should have a high calorific value.
- It should have high burning efficiency.
- It should not produce harmful gases.
- It should produce least quantity of smoke (ash) and harmful gases.
- It should be economical (cheaper).
- It should be easy to store and convenient for transportation.
used to measure the heating value of a fuel
calorimeter
measurement of calorimeter ranges from
288 - 298 K
what will happen if the product gases contain several percent H2O, considerably more than the saturation
vapor pressure of water at that temperature
water will condense in the calorimeter
The measurement requires complete combustion; that is, all carbon and hydrogen must be oxidized to form
CO2 and H2O, respectively
The measurement requires complete combustion; that is, all carbon and hydrogen must be oxidized to form CO2 and H2O, respectively
calorific value
the condensation of water is observed to _______________ due to the _________________
increase the apparent heat release due to the latent heat of vaporization
The measured calorific value thus depends on the
phase of the product water
The effect of this phase transition of water may be seen by examining the
heat transfer required to generate vapor-phase water in the products and the heat that is needed for the condensation of that vapor
this term is used to denote heat release due to combustion
calorific value
two measures of the enthalpy of combustion are generally specified in terms of the calorific value
NCV and GCV
corresponds to the heat of reaction when the latent heat of condensation of water is recovered
gross calorific value (GCV)
corresponds to the case when the water is present as vapor
net calorific value (NCV)
Three (3) sources of CV losses:
- Due to incomplete combustion
- Due to sensible heat
- Due to uncondensed water vapor
The presence of unburnt combustibles constitute a calorific value loss; had these substances been completely burned, they would have given off additional heat
CV Losses due to Incomplete Combustion
Part of the heat generated by fuel combustion is absorbed by the flue gases which leave the chimney or stack at high temperature.
CV loss due to sensible heat
By reviewing, we know that when water is in vapor form, the latent heat of vaporization of water is not recovered.
CV losses due to uncondensed water vapor
measurement of how well the fuel being burned is being utilized in the combustion process
thermal efficiency of combustion
how can we calculate thermal efficiency of combustion
from the gross calorific value of the fuel and the heating value losses, neglecting heat input from the fuel and air
