Santino.diberardino@lneg.pt

MESTRADO INTEGRADO EM ENGENHARIA DA ENERGIA E DO AMBIENTE

Aula N 4-B

Calorific power

Santino Di Berardino

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PROPERTIES OF THE CARBON ( C )

Properties

Simbol C

Atomic Number 6

Valence Electrons

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Atomic number 12

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Covalent Bonds

Carbon can establish covalents bonds with four different atoms.

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.

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Bond Lengths (d)and Energies.*

Ligação O.L. d (Å = 10-10m) Edis (kJ/mol)

H-H 1 0,74 432

C-H 1 1,09 411

C-C 1 1,54 346

C=C 2 1,34 602

CºC 3 1,20 835

C-N 1 1,47 305

C=N 2 1,29# 615

C-O 1 1,43 358

C=O 2 1,20 799

N-O 1 1,40 1,40

N=O 2 1,21 1,21

N=N 2 1,25 418

NºN 3 1,1 942

Fonte: J.E. Huheey et al., Inorganic Chemistry, 4th. ed., 1993.* Com exceção das ligações H-H e NºN, todos os valores de d e Edisfornecidos são valores médios.#

Fonte: http://chemviz.ncsa.uiuc.edu/content/doc-resources-bond.html

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Hydrolysis

Reaction opposite of condensation. It divides a polymer into two parts by adding a water molecule (supported by an enzyme)

Breaking the connection releases energy

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Calorific power of a fuel

Calorific power of a fuel: is the amount of heat released per unit of mass (one kg) (or volume, m3)) to oxidize the fuel in complete form (complete combustion).

Carbon passes carbon dioxide and water forms if the fuel has Hydrogen.

CnHm+(n+m/4)O2 ------- nCO2 + m/2H2O

The calorific value of a fuel indicates its "energy content", being a characteristic of each substance.

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Units of Calorific Power

It is measured in units of energy per unit of substance, usually mass or volume.

For solid and liquid fuels, the calorific value is given in kcal / kg (metric units) or BTU / lb (British units).

For gaseous fuels, in kcal / m3 (metric units) or BTU / ft3 British units).

Also common is the use of molar units such as kcal / mol and kcal / kmol.

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Relationship between units

1 BTU / pound = 0,555 kcal / kg 1 libra = 453 g

1 BTU / ft3 = 8,9 kcal / m3

1 BTU = 252 cal

1 kcal / kmol = 1000 kcal/mol

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Combustion is a chemical oxidation reaction with oxygen (oxidizer), which converts chemical energy to thermal energy. The fuel is usually carbon or a carbon compound, such as hydrocarbons and other organic compounds. The complete combustion reactions of these are as follows:

Combustion

Methane: CH4 + 2 O2 → CO2 + 2H2OCarbon: C + O2 → CO2

Hydrocarbons: CxHy + (2x+y/2) O2 → x CO2 + y/2 H2Oo Oxygenated-compounds-Monoalcools:CxHyOz + (3x/2) O2 → x CO2 + y/2 H2O

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Calorific power-definitions

There are two ways of defining calorific power:

1 - Superior Calorific Power (P.C.S)2 - Lower Calorific Power (P.C.I).

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High Calorific Power

High Calorific Power (P.C.S): is given by the sum of the energy released in the form of heat and the energy spent in the vaporization of the water that is formed in a combustion reaction.

PCS = Ereleased + Evaporation

The higher calorific value presupposes that all combustion elements (fuel and air) and the resulting products (gases after combustion) are at 0 ° C, whereby the water vapor is fully condensed.

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High Calorific Power

Water vapor results from:

a) - specific humidity

b) water formed by the combustion of hydrogen from the fuel.

Cooling the combustion gases to 0 ° C causes condensation of the water vapor contained in the combustion fumes. Provides an amount of heat of:

597 kcal / kg condensed water vapor

Thus the PCS includes the heat of condensation of the water.

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Lower Calorific Power (P.C.I) of the fuel: it expresses the released energy (E) in the form of heat of oxidation of the fuel.

PC = TheIt assumes that the water vapor contained in the flue gases does not condense and therefore does not include the heat derived from its condensation.It only expresses the heat oxidation of the fuel.

Lower Calorific Power (P.C.I)

For fuels which do not contain Hydrogen in theircomposition, the value of P.C.S is the same as that of P.C.Ibecause there is no formation of water and no energy isspent in their vaporization.

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Relation between calorific powers

PCI = PCS - 597 x G PCI = Lower calorific value (kcal / kgcomb)

PCS = Higher calorific value (kcal / kgcomb)

597 = Heat of condensation of water at O ° C (kcal / kg water)

G = Percentage by weight of the water formed by the combustion of H2 plus the own humidity of the fuel (kgwater / kgcomb)

G = 9H + H209: kg of water that is formed by oxidizing one kg of hydrogen.

H: Percentage of hydrogen contained in the fuel.

H2O: Percentage of fuel moisture.

The previous equation is:

PCI = PCS - 597 x ( 9H + H2O)

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Values of the Calorific Power

The calorific value of a fuel depends on the thermodynamic conditions of the determination,

Constant Pressure Calorific Power (PCp) - The amount of heat released in the combustion that is processed at constant pressure (open environment).

Constant Volume Calorific Power (PCv) - The amount of heat released in the combustion that is processed at constant volume (closed environment).

The value of the calorific value at constant volume and greater than the value of the calorific value at constant pressure for the same fuel, since a part of the heat generated in the process at constant pressure, is consumed in the work of expansion of the gases under pressure.

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Determination of calorific value

There are several procedures for determining the calorific value of the fuel:

1 – Analitic Method

3 - Practic Method

2 - Empiric Method

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Calculation of calorific value

Can be made based on:

composition of fuel, and in the combustion heats values of fuel fractions (Table 1).

Empirical formulas.

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Combustion Heat

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Analytical method

The analytical method applies the Principle of Energy Conservation:

"The calorific value of a compound body is equal to the sum of the calorific values of the individual elements forming it, multiplied by its percentage value, discounting the total amount of hydrogen fuel which is combined with its oxygen"

The application of this method requires the elemental analysis of the fuel, whose pre-calorific power is intended to determine:C % - H % - 02 % - S % - humidity%

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Calorific power of carbon

If carbon (C) if combined with sufficient oxygen is totally burned to form carbon dioxide and releasing heat. The complete combustion reaction of carbon is as follows

C + O2 CO2 + 8.140 kcal/kgcarbon

If the oxygen available for combustion is not sufficient, the carbon partially oxidizes and forms the carbon monoxide, releasing a smaller amount of heat of carbon, according to the reaction:

C + ½ O2 CO + 2.440 kcal/kgcarbono

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Hydrogen calorific power

HIGH HEAT POWER

Hydrogen combines with oxygen in its entirety,

which results in water with heat release.

This value includes the heat released by

condensation of the water vapor formed in the

combustion, so that according to the above, it

corresponds to the calorific value of the hydrogen:

H2 + ½ O2 H2O + 34.400 kcal/kgHidrog.

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Hydrogen calorific power

LOWER CALORIFIC POWER

In the case where it is not possible to take advantage of the

heat of condensation, the heat released in the hydrogen

oxidation will deduct the heat lost by not condensing the

water vapor in order to reduce the lower calorific value of the

hydrogen.

being : H2O = 0, Fuel without moisture

H = 1 kg hydrogen

Results: PCI = 34 400 – 600 x 9

PCI = 34 400 – 5400

PCI = PCS – 600 x (9H + H2O) H2 + ½ 2.

PCI = 29 900 kcal/kgHidrog

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Calorific Power of Sulfur

Sulfur is a contaminant of the fuel and its

presence is undesirable, however, when this

element is present it is oxidized according to

the following chemical reaction and releases

heat:

S + O2 = SO2 + 2.220 kcal/kgsulfur

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Dulong Formula - Calorific power of a dry fuel

According to the principle of energy conservation, Physicist Dulonghas established that the upper calorific value of a dry solid or liquid fuel containing carbon, hydrogen and sulfur in the composition is expressed in the formula:

PCS = 8.140 x C+34.400 x(H - O/8)+2.220 x SWhere:

C: quantity centesimal in weight of carbon in kg of fuel •

H: amount centesimal of hydrogen per kilogram of total weight of fuel •

O: quantity centesimal of oxygen per kilogram of stable weight burning of fuel •

S: centesimal value of sulfur in fuel combustion per kilogram of weight

(O / 8: amount of hydrogen by weight, which is combined with the oxygen of the same

fuel resulting from the "combination water"

(H-O / 8): quantity of available hydrogen "by weight"

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Calculation by combustion heats

Once the mass composition (or amount of matter) of the fuel is known, the amount of heat evolved in the combustion of each fraction is determined. The sum of these amounts of heat will be the estimated value of the calorific value.

Important:

In calculating the lower calorific value of fuels containing both moisture and / or water, the amount of heat required to vaporize the combined water mass and / or the mass of moisture should be subtracted from the value found.

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Determination of calorific power-experimental method

The experimental measurement in the laboratory is made with the aid of calorimeters, which are adiabatic devices (which do not exchange heat with the environment).

A known mass of the fuel is combusted. The heat generated and transferred to a mass of water, whose temperature is elevated. From the experimental data collected, the calorific value is calculated.

For both liquid and liquid fuels, the Berthelot-Mahler calorimeter (or "calorimeter pump") is used.

For gas fuels the Junkers calorimeter is used.

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Calorimeter

A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types.

A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber. It is one of the measurement devices used in the study of thermodynamics, chemistry, and

biochemistry.

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Calorimeter

Basically a calorimeter is a device used to measure either

specific heat capacity or the amount of energy produced or

absorbed in a chemical reaction.

It functions by putting a said liquid of known amount into a

thermally insulated container, then by producing a known

amount of heat, the temperature rise can be measure. Specific

heat capacity can be calculated easily by dividing the energy

supplied by the temperature rise.

Likewise, it is also used to measure the heat absorbed or

produced of a chemical reaction (or the enthalpy change).

By letting the reaction occur in a calorimeter with a liquid of

known heat capacity, the energy produced or absorbed can be

easily worked out by multiplying temperature change and heat

capacity.

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Calorimeter

To find the enthalpy change per mole of a substance A in a reaction between two substances A and B, the substances are added to a calorimeter and the initial and final temperatures (before the reaction started and after it has finished) are noted. Multiplying the temperature change by the mass and specific heat capacities of the substances gives a value for the energy given off or absorbed during the reaction. Dividing the energy change by how many moles of A were present gives its enthalpy change of reaction

q = Cv (Tf-Ti) Where q is the amount of heat according to the change in temperature

measured in joules and Cv is the heat capacity of the calorimeter which is a value associated with each individual apparatus in units of energy per temperature (Joules/Kelvin).

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Junkers Gas calorimeter

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Heating power of different fuels

Fuel Poder CaloríficoSuperior (a 25ºC e 1 atm) (kJ/g)

Poder CaloríficoInferior (a 25ºC e 1 atm) (kJ/g)

Hydrogen 141,9 119,9

Methane 55,5 50,0

Propane 50,4 45,6

Gasolinae 47,5 44,5

Gasoil 44,8 42,5

Methanol 19,96 18,1

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Biofuels

Heat power is a very important physicochemical property for biofuels.

The calorific value of diesel is 46 MJ / kg, biodiesel is 33-40 MJ / kg and vegetable oil (rapeseed) is 36.9 MJ / kg.

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Solid Biomass

The most important quality feature, for any power source, is its calorific value. In the case of solid biomass this characteristic is directly influenced by the water content.

The lower calorific power PCMB can be calculated using the following mathematical formula :

Where PCseco is the calorific value of the wood without water

and% H2O is the water content in the wood in the state in which

it is found.

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Water content and calorific value of biomass

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Humidity

Being biomass is a natural product, it has a very varied water content varies, even if it does not suffer external influences. The fastest way to estimate this content is based on values collected over several years.

The typical water content for fresh woody biomass is between 40 and 60%.

Green plants may have a higher water content, up to 80%.