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Energy & Society Combustion 1 Toolkit 3: Combustion
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Toolkit 3: Combustion
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Table of Contents 1. Introduction 3 2. Energy Sources & Use 4
Energy use worldwide 4 2.1. Energy use in the United States 6 2.2. Sources of energy data 8 2.3.
3. Types of Fuels 9 Solid fuels 9 3.1.
Gas and liquid fuels 11 4. Combustion Stoichiometry 12
Combustion in an idealized atmosphere 12 4.1. Combustion in air 15 4.2. Real combustion 17 4.3.
Incomplete combustion 20 Air-fuel ratios 18 NOx formation 21 Fuel impurities 21
5. Air Pollution 22 6. References 23
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1. INTRODUCTION
What do we do with fossil fuels? Burn βem! Just how much fuel do we burn? Quite a lot, it turns out. Our first focus in this Toolkit will be to look at the types and amounts of fuels used globally and in the United States.
The energy content of many fuels, including wood, coal, natural gas, and petroleum, is made useful through combustion. In some cases, the heat and light resulting from combustion are the primary goals, as is the case with wood-fired cookstoves, natural-gas ovens, and kerosene lamps. In other cases, the heat resulting from combustion is utilized to generate steam, which turns a turbine, which produces electricity, which may be used for any number of uses. We will look much more closely at how power plants work in Toolkit 4, but for now we will set the stage by focusing on combustion.
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2. ENERGY SOURCES & USE
This section provides an overview of the energy sources that fuel our world and our country. In the context of this Toolkit, this overview should provide a clear sense of why understanding the basics of combustion is important for understanding energy.
The purpose of this section is to set the stage of what our energy landscape looks like.
β’ What are the main energy sources in use today globally, in the United States, in California?
β’ What are some of the key trends in energy use over time?
β’ What energy sources are used for what purposes?
Becoming familiar with some of these high-level energy statistics, and where to find them, will help you be a more critical consumer of energy data.
Although the emphasis of this section is simply to provide an overview, it is still worthwhile to pay attention to what is (and is not!) being represented by the table, graph, or chart in question. As you do so, you will begin to notice that there are many ways for data about energy to be collected, categorized, and displayed. For example, do the data represent primary energy or delivered energy? All energy consumption or just electricity? Being attuned to what the data do and do not represent allows you to be a more critical user of energy data.
Energy use worldwide 2.1.Letβs begin by taking a look at the global energy supply. Figures 1-3 each highlight a different aspect of global energy. Figure 1 shows how the global primary energy supply has changed over time in terms of fuel. What trends can you identify? At about what rate has the global energy supply been increasing in recent decades?
Comment [AK1]: Add spreadsheet from EIA and IEA
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Figure 1 Global primary energy supply by fuel (Mtoe), 1971-2009
Notes: * Other includes geothermal, solar, wind, heat, etc. Source: IEA, 2011, Key World Energy Statistics. International Energy Agency: Paris.
Figure 2 also depicts the global primary energy supply, but divides the data by region rather than fuel source. What trends do notice in this figure? Do population levels appear to explain regional variations in energy supply?
Figure 2 Global primary energy supply by region (Mtoe), 1971-2009
Notes: * Asia excludes China; ** Bunkers includes international aviation and marine bunkers. Source: IEA, 2011, Key World Energy Statistics. International Energy Agency: Paris.
Finally, Figure 3 focuses solely on trends in global electricity generation by energy source. The category of fossil thermal includes coal, natural gas, and oil. What trends emerge from these data? At about what rate has global electricity generation been increasing in recent decades?
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Figure 3 Global electricity generation by energy source (TWh), 1971-2009
Notes: ** Other includes geothermal, solar, wind, biofuels and waste, and heat. Source: IEA, 2011, Key World Energy Statistics. International Energy Agency: Paris.
Energy use in the United States 2.2.Three fossil fuels figure prominently in both the United States energy supply: coal, natural gas, and petroleum. Each of these fossil fuels has a different history and pattern of use. In the United States, coal is primarily used to generate electricity; natural gas is used to generate electricity, as well as for residential, commercial, and industrial use; and petroleum is primarily used for transportation fuels, see Figure 4.
Figure 4 Primary U.S. energy consumption by source and sector in Quadrillion Btus, 2010
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Notes: 1. Does not include biofuels that have been blended with petroleum. 2. Excludes supplemental gaseous fuels. 3. Includes less than 0.1 quadrillion Btu of coal coke net exports. 4. Conventional hydroelectric power, geothermal, solar/PV, wind, and biomass, including biofuels. 5. Includes industrial combined-heat-and-power (CHP) and industrial electricity-only plants. 6. Includes commercial combined-heat-and-power (CHP) and commercial electricity-only plants. 7. Electricity-only and combined-heat-and-power (CHP) plants whose primary business is to sell electricity,
or electricity and heat, to the public. Includes 0.1 quadrillion Btu of electricity net imports not shown under βsource.β
Source: U.S. Energy Information Administration, 2011, Annual Energy Review 2010, Figure 2.0.
Importantly, Americaβs energy landscape has changed dramatically over time. These changes can be seen across a variety of metrics, including the types of energy sources in use, the magnitude of annual energy consumption, and per capita energy use. Changes in the energy landscape often follow social and technological shifts, such as those associated with technical innovations, e.g., the internal combustion engine, and new policies or regulations, e.g., clean energy standards. Consider what social and technological shifts might be associated with some of the trends seen in Figure 4. Throughout this course, we will discuss and explore some of the factors that have contributed to these shifts.
Figure 5 U.S. primary energy consumption estimates by source, 1775-2010
Notes: 1 Includes wind, solar, and geothermal. Source: U.S. EIA, Annual Energy Review. Tables 1.3, 10.1, and E1.U.S. Department of Energy, Energy Information Administration: Washington, DC.
0
5
10
15
20
25
30
35
40
45
Qua
drill
ion
Btu
Petroleum Coal Natural Gas
Hydroelectric Power Nuclear Electric Power Wood
Other Renewable EnergyΒΉ
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Sources of energy data 2.3.Energy statistics are updated regularly by various agencies and organizations. A few of the most useful sources of energy data include:
β’ The International Energy Agency, especially for international data: www.iea.org
β’ The U.S. Energy Information Administration, especially for U.S. data: www.eia.gov
β’ The California Energy Commission, especially for California data: www.energy.ca.gov
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3. TYPES OF FUELS
Fuels have typically been defined as substances that can be combusted to produce heat energy, which in turn can be used for any number of practical applications, ranging from cooking to electricity generation to transportation. Nuclear power has added a wrinkle to this standard definition, which has been expanded to include fissionable material, but for the purposes of this section, we will focus solely on fuels used in combustion.
Fuels can be naturally occurring substances, e.g., wood, or manufactured substances, e.g., gasoline. Although many things can be burned to produce heat, good fuels share certain characteristics:
β’ A high calorific value, β’ Low ignition temperature, β’ Low moisture content, β’ Low non-combustible matter content, β’ By-products of combustion that are not be toxic to people or the environment, β’ Combustion is controllable, β’ Readily available at low cost, β’ Safe to handle, β’ Easy and safe to transport.i
Our fuel choices involve balancing the attributes and availability of different fuel types, as well as the constraints of existing infrastructure and policies. Fuels are often classified based on whether they occur naturally, referred to as primary fuels, or they are manufactured, referred to as secondary fuels. In addition, fuels are often organized based on their state of matter: solid, liquid, or gas.
Table 1 Examples of primary and secondary fuels, by state of matter Primary fuels Secondary fuels Solid Coal
Peat Wood Dung
Charcoal Coke
Liquid Crude oil
Gasoline Diesel Kerosene
Gaseous Natural gas Biogas
Solid fuels 3.1.Solid fuels are used extensively around the world. In the United States, coal plays a central role in electricity generation. In many developing countries, biomass β wood, dung, and other materials β is collected daily and burned in a cookstove to meet household heating and cooking needs.
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Solid fuels consist of both combustible materials β the organic portions, including carbon, hydrogen, and sulfur β and incombustible materials β the moisture and mineral components of the fuel. The exact composition of a particular solid fuel can vary, sometimes substantially, from one sample to another. Thus, although we often talk about coal as though it is a uniform substance, its chemical composition can vary quite substantially from one site to another, as well as within a single site, as a function of various factors, including the composition of the original vegetation, the type of inorganic material present, and the specific conditions under which it has formed.
Coal is classified into four ranks, from lignite (immature) to anthracite (mature) based on the amount and type of carbon it contains and the amount of heat it can generate, see Table 2.
Table 2 Heating value by coal grade Coal grade Approximate heating values (kJ/kg coal) Anthracite 30,240-33,730 Bituminous 27,910-34,420 Subbituminous 19,310-23,620 Lignite 13,260-17,450
Table 3 Solid fuels and their characteristics Fuel type Key characteristics Peat Wood Moisture, volatiles, fixed carbon, ash Coal Moisture, volatiles, fixed carbon, ash, CH0.8 Charcoal Devolatilized wood Coke Devolatilized coal or petroleum
Table 4 Hydrocarbons in fossil fuels Name Molecular formula
Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12 Hexane C6H14 Heptane C7H16 Octane C8H18 Nonane C9H20 Decane C10H22 Undecane C11H24 Dodecane C12H26 Eicosane C20H42 Triacontane C30H62
Fuel type Composition notes Typical Use Simplified
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formula Natural Gas Mostly CH4; may include C2H6,
C3H8, and C4H10 prior to refining. Electricity; industrial use; commercial & residential heating and cooking.
CH4
Propane Small tanks, especially camping stoves, BBQs, etc.
C3H8
Butane Cigarette lighters C4H10 LPG Liquefied petroleum gas (LPG) is a
mix of C3H8 and C4H10. Rural heating and cooking, transportation fuel.
Gasoline Mostly C8H18, but with a mixture ranging from C7H16 to C12H26.
Wood/Cellulose C6H12O6
Gas and liquid fuels
Table 5 Gas and liquid fuels and their characteristics Fuel type Key characteristics Natural gas CH4, C2H6, N2, CO2 Propane C3 Butane C4 LPG A mixture of propane and
butane Synthetic gases From biomass and coal products Petroleum derived fuels β Gasoline β Diesel β Turbine fuels, kerosene β Heavy fuel oils
~CH2 C4 to C10, average C8 C12 C10
Shale oil derived liquids Alcohols, ethers Hydrogen
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4. COMBUSTION STOICHIOMETRY
What happens during combustion? In its simplest form, combustion involves the mixing of a fuel and an oxidizer and converts them into combustion products. Combustion is an exothermic process, which means it also releases energy, which can be used directly or converted into another form for some other use.
Equation 1 Fuel + Oxidizer β Combustion Products + Energy
In the following sections, we will draw upon some chemistry basics to explore the types and amounts of combustion products released when fossil fuels are combusted under idealized conditions. Then weβll look briefly at some of the complexities and messiness associated with combustion under real circumstances.
Handy tip: Learning the atomic weights of a few common elements (H, C, O, N, and S) will be useful for your homework assignments, see Table 6, although you can always look these up in the Appendix.
Table 6 Atomic weights of common elements in combustion Element Symbol Atomic Number Atomic Weight Hydrogen H 1 1.008 Carbon C 6 12.01 Nitrogen N 7 14.01 Oxygen O 8 16.00 Sulfur S 16 32.06
Combustion in an idealized atmosphere 4.1.First, letβs consider what combustion looks like in an idealized atmosphere consisting only of oxygen. To do this, weβll work through several examples in order to understand the basic form of combustion equations, the method for balancing combustion equations, and the set-up for calculating the amount of carbon dioxide associated with combustion of various fuels.
Example 1 Write the balanced equation for the combustion of cellulose (C6H12O6) in an oxygen-only (O2) environment.
Balancing this equation begins with identifying all of the chemical species on both sides. In this case, the left side of the equation has the fuel (C6H12O6) and the oxidizer (O2); the right side has just two chemical combustion products, carbon dioxide (CO2) and water (H2O).
πΆ6π»12π6 + π2 β πΆπ2 + π»2π
The next step requires balancing the equation, which we tackle one element at a time. To help highlight the steps taken to balance equations, the examples in this section will use colors to indicate the numbers (coefficients and subscripts) associated with the element being balanced. The underlined value indicates the coefficient added to balance the element of interest.
Comment [AK2]: Watch out for graphs that reverse rich and lean fuels.
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First, balance the carbon. A balanced equation should have the same number of each element on both sides of the equation. In this case, there are 6 carbons on the left, so we also need 6 carbons on the right. Remember, only the coefficients in front of the chemical species can be changed:
πΆ6π»12π6 + π2 β π₯πΆπ2 + π»2π
6 = π₯
πΆ6π»12π6 + π2 β 6πΆπ2 + π»2π
Next, balance the hydrogen working from the partially balanced equation:
πΆ6π»12π6 + π2 β 6πΆπ2 + π₯π»2π
12 = 2π₯
6 = π₯
πΆ6π»12π6 + π2 β 6πΆπ2 + 6π»2π
Finally, the oxygen, which requires a little bit of simple algebra:
πΆ6π»12π6 + π₯π2 β 6πΆπ2 + 6π»2π
6 + (π₯ β 2) = (6 β 2) + 6
6 + 2π₯ = 12 + 6
2π₯ = 12
π₯ = 6
πΆ6π»12π6 + 6π2 β 6πΆπ2 + 6π»2π
This gives us the balanced equation:
πΆ6π»12π6 + 6π2 β 6πΆπ2 + 6π»2π
Being able to balance the equations is only the first step. Once balanced, we can start asking more interesting questions.
Example 2 How much CO2 is produced when 1 tonne of methane (CH4) is combusted in an oxygen-only atmosphere?
Solving this problem takes several steps, which we will tackle one step at a time.
Step 1. Write out the equation.
πΆπ»4 + π2 β πΆπ2 + π»2π
Step 2. Balance the equation one element at a time: carbon, hydrogen, and oxygen.
πΆπ»4 + π2 β πΆπ2 + π»2π
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πΆπ»4 + π2 β πΆπ2 + 2π»2π
πΆπ»4 + 2π2 β πΆπ2 + 2π»2π
Step 3. Calculate CO2 emissions. To do this, we need a few additional pieces of information: the molar ratio of CO2 and CH4, which can be gleaned from the balanced equation, and the molar masses of CO2 and CH4:
Molar ratio =Moles of CO2
Moles of CH4=
ππΆπ2
ππΆπ»4
=1 πππ πΆπ2
1 πππ πΆπ»4
Molar mass of CO2 = ππΆπ2 = 12.01 πππππππ/πππ + οΏ½2 β 16.00 πππ₯π¦πππ/ππποΏ½ = 44.01 ππΆπ2/πππ
Molar mass of CH4 = ππΆπ»4 = 12.01 πππππππ/πππ + οΏ½4 β 1.008 πβπ¦ππππππ/ππποΏ½ = 16.042 ππΆπ»4/πππ
Now we can easily set up our calculation for determining CO2 emissions:
CO2 = οΏ½1πππ πΆπ2
1πππ πΆπ»4οΏ½ οΏ½
44.01π πΆπ2
1πππ πΆπ2οΏ½ οΏ½
1πππ πΆπ»4
16.042π πΆπ»4οΏ½ [106π πΆπ»4] οΏ½
1π‘ πΆπ2
106π πΆπ2οΏ½
= 2.74π‘ πΆπ2
So for every tonne of methane combusted, almost 3 tonnes of carbon dioxide are emitted. How do CO2 emissions from other fossil fuels compare? Letβs look at another example.
Example 3 How much CO2 is produced when 1 tonne of benzene (C6H6) is combusted in an oxygen-only atmosphere?
Step 1. Write out the equation.
πΆ6π»6 + π2 β πΆπ2 + π»2π
Step 2. Balance the equation one element at a time: carbon, hydrogen, and oxygen.
πΆ6π»6 + π2 β 6πΆπ2 + π»2π
πΆ6π»6 + π2 β 6πΆπ2 + 3π»2π
πΆ6π»6 + 7.5π2 β 6πΆπ2 + 3π»2π
Step 3. Calculate CO2 emissions.
Molar ratio =6 πππ πΆπ2
1 πππ πΆ6π»6
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Molar mass of C6H6 = (6 β 12.01) + (4 β 1.008) = 76.668 π/πππ
CO2 = οΏ½6πππ πΆπ2
1πππ πΆ6π»6οΏ½ οΏ½
44.01π πΆπ2
1πππ πΆπ2οΏ½ οΏ½
1πππ πΆ6π»6
76.668π πΆ6π»6οΏ½ [106π πΆπ»4] οΏ½
1π‘ πΆπ2
106π πΆπ2οΏ½
= 3.44π‘ πΆπ2
Now that we have worked through balancing the combustion equations for several specific fuels, letβs look at the general form used for balancing combustion equations using an unspecified hydrocarbon, CxHy, where x indicates the number carbon atoms and y indicates the number of hydrogen atoms.
Example 4 Write the balanced combustion equation for a general hydrocarbon (CxHy) burned in an oxygen-only atmosphere.
Step 1. Write out the equation.
πΆπ₯π»π + π2 β πΆπ2 + π»2π
Step 2. Balance the equation one element at a time: carbon, hydrogen, and oxygen.
πΆπ₯π»π¦ + π2 β π₯πΆπ2 + π»2π
πΆπ₯π»π¦ + π2 β π₯πΆπ2 + π¦2
π»2π
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ π2 β π₯πΆπ2 + π¦2
π»2π
Final balanced equation:
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ π2 β π₯πΆπ2 + π¦2
π»2π
Note that the ratio of carbon to hydrogen (x:y) determines the ratio of carbon dioxide to water produced during combustion.
Combustion in air 4.2.In Section 4.1, combustion was assumed to take place in an oxygen-only environment. However, most combustion takes place in air. The following calculations assume that air consists of 21% oxygen (O2) and 79% nitrogen (N2).
How does the assumption of combustion in air affect the stoichiometry? Letβs take a look.
Example 5 Write the balanced equation for cellulose (C6H12O6) combusted in air (O2 + 3.76N2).
We will follow the same basic steps used for combustion in an oxygen-only atmosphere, but we have to start with a somewhat more complicated base equation in step 1, and we will have to also balance the nitrogen in step 2.
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Step 1. Write out the equation.
πΆ6π»12π6 + (π2 + 3.76π2) β πΆπ2 + π»2π + π2
Step 2. Balance the equation one element at a time: carbon, hydrogen, oxygen, and nitrogen.
πΆ6π»12π6 + (π2 + 3.76π2) β 6πΆπ2 + π»2π + π2
πΆ6π»12π6 + (π2 + 3.76π2) β 6πΆπ2 + 6π»2π + π2
When you balance the oxygen and nitrogen, you have to maintain the ratio between oxygen and nitrogen in air, so the coefficient for air goes outside of the parentheses and applies to both gases:
πΆ6π»12π6 + 6(π2 + 3.76π2) β 6πΆπ2 + 6π»2π + π2
πΆ6π»12π6 + 6(π2 + 3.76π2) β 6πΆπ2 + 6π»2π + 22.57π2
This gives us our balanced equation:
πΆ6π»12π6 + 6(π2 + 3.76π2) β 6πΆπ2 + 6π»2π + 22.57π2
The final equation is very similar to the balanced equation from Example 1, except for the addition of diatomic nitrogen on both sides of the equation. The importance of nitrogenβs presence during combustion will become evident in Section 4.3 Real Combustion.
Letβs work through balancing the equations for methane (CH4) and benzene (C6H6) combusted in air as well, just like in Section 4.1.
Example 6 Write the balanced equation for methane (CH4) combusted in air (O2 + 3.76N2).
Step 1. Write out the equation.
πΆπ»4 + (π2 + 3.76π2) β πΆπ2 + π»2π + π2
Step 2. Balance the equation one element at a time: carbon, hydrogen, oxygen, and nitrogen.
πΆπ»4 + (π2 + 3.76π2) β πΆπ2 + π»2π + π2
πΆπ»4 + (π2 + 3.76π2) β πΆπ2 + 2π»2π + π2
πΆπ»4 + 2(π2 + 3.76π2) β πΆπ2 + 2π»2π + π2
πΆπ»4 + 2(π2 + 3.76π2) β πΆπ2 + 2π»2π + 7.52π2
This gives us our final balanced equation:
πΆπ»4 + 2(π2 + 3.76π2) β πΆπ2 + 2π»2π + 7.52π2
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Example 7 Write the balanced equation for benzene (C6H6) combusted in air (O2 + 3.76N2).
Step 1. Write out the equation.
πΆ6π»6 + (π2 + 3.76π2) β πΆπ2 + π»2π
Step 2. Balance the equation one element at a time: carbon, hydrogen, oxygen, and nitrogen.
πΆ6π»6 + (π2 + 3.76π2) β 6πΆπ2 + π»2π + π2
πΆ6π»6 + (π2 + 3.76π2) β 6πΆπ2 + 3π»2π + π2
πΆ6π»6 + 7.5(π2 + 3.76π2) β 6πΆπ2 + 3π»2π + π2
πΆ6π»6 + 7.5(π2 + 3.76π2) β 6πΆπ2 + 3π»2π + 28.21π2
This gives us our final balanced equation:
πΆ6π»6 + 7.5(π2 + 3.76π2) β 6πΆπ2 + 3π»2π + 28.21π2
Finally, we can work through the general equation for combustion in air.
Example 8 Write the balanced combustion equation for a general hydrocarbon (CxHy) burned in air (O2+3.76N2).
Step 1. Write out the equation.
πΆπ₯π»π¦ + (π2 + 3.76π2) β πΆπ2 + π»2π + π2
Step 2. Balance the equation one element at a time: carbon, hydrogen, oxygen, and nitrogen.
πΆπ₯π»π¦ + (π2 + 3.76π2) β π₯πΆπ2 + π»2π + π2
πΆπ₯π»π¦ + (π2 + 3.76π2) β π₯πΆπ2 + π¦2
π»2π + π2
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ (π2 + 3.76π2) β π₯πΆπ2 + π¦2
π»2π + π2
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ (π2 + 3.76π2) β π₯πΆπ2 + π¦2
π»2π + 3.76 οΏ½π₯ + π¦4
οΏ½ π2
Final balanced equation:
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ (π2 + 3.76π2) β π₯πΆπ2 + π¦2
π»2π + 3.76 οΏ½π₯ + π¦4
οΏ½ π2
Real combustion 4.3.Real combustion is much messier than either of our simplified scenarios suggest. For our purposes, these simplifications provide adequate approximations of combustion stoichiometry to solve problems. However, some of the additional complexities associated with real combustion still merit
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discussion and conceptual understanding. One factor that influences combustion is the ratio of air to fuel.
Air-fuel ratios One factor that influences combustion is the ratio of air to fuel in the feed. The air-fuel (AF) ratio can be expressed in terms of mass or moles:
Mass of air in feed mixtureMass of fuel in feed mixture
= π΄πΉπππ π =ππππ
πππ’ππ
Moles of air in feed mixtureMoles of fuel in feed mixture
= π΄πΉππππ =ππππ
πππ’ππ
In sections 4.1 and 4.2, we balanced the general equations for the combustion of hydrocarbon fuels in oxygen and in air. From these equations, we can derive the stoichiometric ratio (AFstoich), the air-fuel ratio required for complete combustion.
Recall the balanced equation for combustion in oxygen:
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ π2 β π₯πΆπ2 + π¦2
π»2π
From this equation we can determine the stoichiometric ratio:
Stoichiometric ratio (oxygen) = AFstoich(π2) =ππ2
ππΆπ₯π»π¦
=οΏ½π₯ + π¦
4οΏ½ πππ1 πππ
= π₯ + π¦4
This ratio can also be expressed as a mass ratio (AFmass) by multiplying by the molecular masses:
Mass ratio (oxygen) = π΄πΉπππ π (π2) = οΏ½π΄πΉπ π‘πππβ(π2)οΏ½ οΏ½ππ2
ππΆπ₯π»π¦
οΏ½
= οΏ½π₯ + π¦4
οΏ½ οΏ½32.00 π/πππ
(12.01π₯ + 1.008π¦)π/ππποΏ½
=32.00π₯ + 8.00π¦
12.01π₯ + 1.008π¦
The mass ratio is thus revealed to be a function of the number of carbon and hydrogen atoms in the fuel. From this, the range of possible mass ratios can be determined based on a pure carbon fuel (y = 0) and a pure hydrogen fuel (x = 0).
π΄πΉπππ π (π2) (ππ’ππ ππππππ ππ’ππ) =32.00π₯ π/πππ12.01π₯ π/πππ
= 2.664
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π΄πΉπππ π (π2) (ππ’ππ βπ¦ππππππ ππ’ππ) =8.00π¦ π/πππ
1.008π¦ π/πππ= 7.937
From this we see that for complete combustion in oxygen the air-fuel ratios by mass range from about 2.7 to 7.9. Of course, since most fuels are burned in air, it is also important to know the air-fuel ratios associated with combustion in air. Recall the general formula for combustion in air:
πΆπ₯π»π¦ + οΏ½π₯ + π¦4
οΏ½ (π2 + 3.76π2) β π₯πΆπ2 + π¦2
π»2π + 3.76 οΏ½π₯ + π¦4
οΏ½ π2
In this case, the stoichiometric ratio represents the ratio of moles of air to moles of fuel, which remains the same:
Stoichiometric ratio (air) = π΄πΉπ π‘πππβ(πππ) =ππππ
ππΆπ₯π»π¦
=οΏ½π₯ + π¦
4οΏ½ πππ1 πππ
= π₯ + π¦4
The mass ratio in air can then be calculated as above or, since the stoichiometric ratios are equivalent, by multiplying the mass ratio in oxygen by the ratio of the mass of air to the mass of oxygen.
Mass ratio (air) = π΄πΉπππ π (πππ) = οΏ½π΄πΉπ π‘πππβ(πππ)οΏ½ οΏ½ππππ
ππΆπ₯π»π¦
οΏ½
= οΏ½π΄πΉπππ π (π2)οΏ½ οΏ½ππππ
ππ2
οΏ½
= οΏ½32.00π₯ + 8.00π¦
12.01π₯ + 1.008π¦οΏ½ οΏ½
137.3232.00π
οΏ½
= 4.29 οΏ½32.00π₯ + 8.00π¦
12.01π₯ + 1.008π¦οΏ½
Note that the air-fuel ratio can also sometimes expressed as a fuel-air (FA) ratio.
Fuel-air ratio = πΉπ΄πππ π (πππ) =1
π΄πΉπππ π (πππ)=
πππ’ππ
ππππ
The stoichiometric ratio gives the ratio of air to fuel necessary for complete combustion. The stoichiometric ratio serves as a reference against which actual air-fuel ratios can be compared. When a feed mixture has more fuel than necessary, it is called a rich mixture. When a feed mixture has more air than necessary, it is called a lean mixture.
Rich mixture: π΄πΉπππ₯π‘π’ππ < π΄πΉπ π‘πππβ
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Lean mixture: π΄πΉπππ₯π‘π’ππ > π΄πΉπ π‘πππβ
The equivalence ratio (Ξ¦) provides a measure of the deviation of the actual air-fuel ratio and the stoichiometric ratio:
Equivalence ratio = π =π΄πΉπ π‘πππβ
π΄πΉπππ₯π‘π’ππ=
πΉπ΄πππ₯π‘π’ππ
πΉπ΄π π‘πππβ
Most combustion systems operate under lean conditions. Consider Figure 6, which illustrates how power, fuel consumption, and various chemical products vary as a function of the air-fuel ratio. Based on this graph, what advantages are there to using a lean mixture rather than a stoichiometric or rich mixture?
Figure 6 Combustion products, fuel consumption, and power as a function of air-fuel ratios
Source: Unknown β taken from lecture slides.
Incomplete combustion To this point, we have assumed complete oxidation of the fuel during combustion. Without complete oxidation, the products of combustion become messier, including not just H2O, CO2, and N2, but also a variety of other trace products, such as carbon monoxide (CO), nitrogen monoxide (NO), and hydrocarbons (HC). Incomplete combustion might more accurately be described by Equation 2.
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Equation 2 πΆπ₯π»π¦ + π2 + π2 β mostly(πΆπ2 + 2π»2π + π2) + traces(πΆπ + ππ + π»πΆ + β― )
Incomplete oxidation can occur under a variety of circumstances, including when:
β’ There is poor mixing fuel and air, β’ The temperature is too low, β’ Oxygen levels are insufficient, β’ Combustion occurs too rapidly.
NOx and CO formation At higher temperatures, atmospheric nitrogen begins to react with oxygen to form nitrogen oxides (NOx), also known as thermal NOx when formed at high temperatures. Oxidation of atmospheric nitrogen is governed by two equations:
π2 + π2 + βπππ‘ β 2ππ
ππ + 12π2 β ππ2
In addition to the oxidation of atmospheric nitrogen, NOx can also form during combustion when nitrogen compounds in the fuel are oxidized.
Carbon monoxide can also be formed during incomplete combustion at high temperature:
πΆπ2 β πΆπ + 12π2
Fuel impurities Up to this point, we have assumed that the fuels themselves are pure, consisting only of carbon and hydrogen. However, this is rarely the case. Fossil fuels can contain a variety of different impurities, including sulfur, mercury (Hg), and ash. These impurities can have significant impacts on the environment and public health, so we will return to them again throughout the course. For now, however, we will only consider them in terms of increasingly complex chemistry. Among the products that result from these impurities are volatile organic compounds (VOCs).
Equation 3 Fuel(πΆ, π», π, π, ππ β) + air(π2 + π2) β (πΆπ2, π»2π, πΆπ, πππ₯, πππ₯, πππΆπ , ππππ‘πππ’πππ‘ππ ) + ππ β
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5. AIR POLLUTION
Figure 7 Combustion products as a function of the air-fuel ratio
Source: Seinfeld, J. Atmospheric Chemistry and Physics of Air Pollution.
In 1800 London, the dominant fossil fuel in use was coal. About 1 ton of coal was burned per person. With a population of one million, how much β¦ was released annually?
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6. REFERENCES
i Chatwal, Gurdeep R., and Madhu Arora. 2008. Analytical chemistry. Mumbai [India]: Himalaya Pub. House. http://public.eblib.com/EBLPublic/PublicView.do?ptiID=588240. Kaur, H. 2008. Analytical chemistry. Meerut: Pragati Prakashan. http://site.ebrary.com/id/10417456.
