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Thermodynamics of fossil, biomass and synthetic

fuels

Md. Mizanur Rahman School of Mechanical Engineering Universiti Teknologi Malaysia

Introduction

Fossil fuels

• Fossil fuels are hydrocarbons, primarily coal, fuel oil or natural gas, formed from the remains of dead plants and animals.

• Are buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.

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Composition of fossil fuels

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Fossil fuels are hydrocarbons comprised primarily of the following elements: carbon and hydrogen and some sulfur, nitrogen, oxygen, and mineral matter. Mineral matter turns into ash when burnt. The composition and the amounts of these elements change for different fossil fuels (coal, petroleum, and natural gas), but the elements are the same. For example, there is more hydrogen in liquid fuels than in coal per unit mass. For simplicity Natural gas is modeled as CH4 Coal model as C Gasoline model as C8H18

Combustion of fossil fuels

• The balanced chemical equation for the complete combustion of a general hydrocarbon fuel CnHm is given by

𝐶𝑛𝐻𝑚 + 𝑛 +𝑚

4(𝑂2+3.7𝑁2) → 𝑛𝐶𝑂2 + (

𝑚

2)𝐻2𝑂 + 3.7(𝑛 +

𝑚

4)𝑁2

• For pure carbon, m=0, for pure hydrogen n=0

• If 1.0 mole of CnHm is completely burned, see that 3.76[n + m/4] moles of (unreacting) nitrogen will be contained in the emissions

• 1 mole of oxygen entering a combustion chamber is accompanied by (0.79/0.21 5)= 3.76 mole of nitrogen

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Stoichiometric ratio

• Stoichiometric ratio= 𝑂𝑥𝑦𝑔𝑒𝑛 𝑚𝑎𝑠𝑠

𝑓𝑢𝑒𝑙 𝑚𝑎𝑠𝑠=

32𝑛+8𝑚

12𝑛+𝑚

• Stoichiometric ratio= 𝑎𝑖𝑟 𝑚𝑎𝑠𝑠

𝑓𝑢𝑒𝑙 𝑚𝑎𝑠𝑠= 4.319(

32𝑛+8𝑚

12𝑛+𝑚)

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Combustion of fossil fuels

• Combustion is a reaction with oxygen.

• In the case of the combustion of fossil fuels, the combustion reaction is what we think of as a burning process.

• In the combustion reaction, the species reacting with the oxgyen is oxidized. Fossil fuels are composed primarily of hydrocarbons (molecules containing primarily carbon hydrogen bonds).

• In these molecules carbon is in a very reduced state. During the combustion reaction, the hydrocarbon molecules are converted to carbon dioxide and water.

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• Bringing a fuel into intimate contact with oxygen is not sufficient to start a combustion process.

• The fuel must be brought above its ignition temperature.

• The minimum ignition temperatures of various substances in atmospheric air are approximately 260 °C for gasoline, 400 °C for carbon, 580 °C for hydrogen, 610 °C for carbon monoxide, and 630 °C for methane.

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Combustion inputs

Energy during combustion • During chemical reactions, energy is either released to the

environment (exothermic reaction) or absorbed from the environment (endothermic reaction).

• During chemical reactions, bonds are broken in the reactants and new ones are made in the products.

• Bond-breaking is an endothermic process and bond-making is an exothermic process. The average bond dissociation energies of some chemical bonds are shown in the following table

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Bond dissociation energies • For any chemical reaction, the overall energy change,

the enthalpy of reaction(DH), is the difference of all the energy absorbed in bond-breaking and all the energy released in bond-making.

DH = S BE(bonds broken)- SBE(bonds formed)

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1 mole of methane (16 g) releases 810 KJ of energy on burning

Average bond dissociation energies

Bond Bond Energy(kJ/mole)

Bond Bond Energy(kJ/mole)

H-H 432 C=O 799

O=O 494 C-C 347

O-H 460 C=C 611

C-H 410 C=C (aromatic)

519

C-O 360 N=O 623

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The negative value of the Enthalpy of Reaction

indicates that the reaction is a heat producing or

exothermic.

Fuel heating value

• The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. It may be expressed with the quantities: – energy/mole of fuel

– energy/mass of fuel

– energy/volume of the fuel

• For combustion processes, the enthalpy of reaction is usually referred to as the enthalpy of combustion hC, which represents the amount of heat released during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure which is 2393,520 kJ/kmol for carbon at the standard reference state.

• The enthalpy of combustion of a particular fuel is different at different temperatures and pressures.

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Thermodynamic properties of some fuels

Biomass fuel

• The two most common types of biofuel are bioethanol and biodiesel.

Bioethanol

• Bioethanol is an alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn, sugarcane, or sweet sorghum. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form (E100), but it is usually used as a gasoline additive to increase octane and improve vehicle emissions. Bioethanol is widely used in the United States and in Brazil.

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Biomass fuel

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Biomass fuel

Biodiesel • Biodiesel is produced from oils or fats using transesterification and

is the most common biofuel in Europe. It can be used as a fuel for vehicles in its pure form (B100), but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.

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Biomass fuel

• Biochar • What is biochar?

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• Biochar is a Powerfully Simple Tool to Combat Climate Change

• The carbon in biochar resists degradation and can hold carbon in soils for hundreds to thousands of years. Biochar is produced through pyrolysis or gasification — processes that heat biomass in the absence (or under reduction) of oxygen.

• In addition to creating a soil enhancer, sustainable biochar practices can produce oil and gas byproducts that can be used as fuel, providing clean, renewable energy. When the biochar is buried in the ground as a soil enhancer, the system can become carbon-negative.

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Synthetic fuels

• A Synthetic fuel is one that is manufactured from raw fuel to enhance its usefulness

• Synthetic fuels are usually fuel substitutes for gasoline and diesel fuel made from petroleum sources.

• The source of these synthetics can be any feedstock containing the combustible elements carbon or hydrogen. These include coal, oil shale, peat, biomass, tar sands, and natural gas.

• Examples: – Synthesis gas

– Methane

– Methanol

– Hydrogen

– Ethanol

–

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Fossil fuel synthesis

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