Energy & Society Combustion Toolkit 3: Combustion · PDF fileEnergy & Society Combustion 2 Table of Contents 1. Introduction 3 2. Energy Sources & Use 4 2.1. Energy use.....

Energy & Society 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 ‘em! 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.

• What are the main energy sources in use today globally, in the United States, in California?

• What are some of the key trends in energy use over time?

• 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.Let’s 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 “source.”

Source: U.S. Energy Information Administration, 2011, Annual Energy Review 2010, Figure 2.0.

Importantly, America’s 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 Energy¹


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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:

• The International Energy Agency, especially for international data: www.iea.org

• The U.S. Energy Information Administration, especially for U.S. data: www.eia.gov

• The California Energy Commission, especially for California data: www.energy.ca.gov

http://www.iea.org/

http://www.eia.gov/

http://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:

• A high calorific value, • Low ignition temperature, • Low moisture content, • Low non-combustible matter content, • By-products of combustion that are not be toxic to people or the environment, • Combustion is controllable, • Readily available at low cost, • Safe to handle, • 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 – wood, dung, and other materials – 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 – the organic portions, including carbon, hydrogen, and sulfur – and incombustible materials – 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 – Gasoline – Diesel – Turbine fuels, kerosene – 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 → 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 we’ll 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, let’s consider what combustion looks like in an idealized atmosphere consisting only of oxygen. To do this, we’ll 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).

𝐶6𝐻12𝑂6 + 𝑂2 → 𝐶𝑂2 + 𝐻2𝑂

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:

𝐶6𝐻12𝑂6 + 𝑂2 → 𝑥𝐶𝑂2 + 𝐻2𝑂

6 = 𝑥

𝐶6𝐻12𝑂6 + 𝑂2 → 6𝐶𝑂2 + 𝐻2𝑂

Next, balance the hydrogen working from the partially balanced equation:

𝐶6𝐻12𝑂6 + 𝑂2 → 6𝐶𝑂2 + 𝑥𝐻2𝑂

12 = 2𝑥

6 = 𝑥

𝐶6𝐻12𝑂6 + 𝑂2 → 6𝐶𝑂2 + 6𝐻2𝑂

Finally, the oxygen, which requires a little bit of simple algebra:

𝐶6𝐻12𝑂6 + 𝑥𝑂2 → 6𝐶𝑂2 + 6𝐻2𝑂

6 + (𝑥 ∙ 2) = (6 ∙ 2) + 6

6 + 2𝑥 = 12 + 6

2𝑥 = 12

𝑥 = 6

𝐶6𝐻12𝑂6 + 6𝑂2 → 6𝐶𝑂2 + 6𝐻2𝑂

This gives us the balanced equation:

𝐶6𝐻12𝑂6 + 6𝑂2 → 6𝐶𝑂2 + 6𝐻2𝑂

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.

𝐶𝐻4 + 𝑂2 → 𝐶𝑂2 + 𝐻2𝑂

Step 2. Balance the equation one element at a time: carbon, hydrogen, and oxygen.

𝐶𝐻4 + 𝑂2 → 𝐶𝑂2 + 𝐻2𝑂


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𝐶𝐻4 + 𝑂2 → 𝐶𝑂2 + 2𝐻2𝑂

𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2𝑂

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=

𝑛𝐶𝑂2

𝑛𝐶𝐻4

=1 𝑚𝑜𝑙 𝐶𝑂2

1 𝑚𝑜𝑙 𝐶𝐻4

Molar mass of CO2 = 𝑀𝐶𝑂2 = 12.01 𝑔𝑐𝑎𝑟𝑏𝑜𝑛/𝑚𝑜𝑙 + �2 ∙ 16.00 𝑔𝑜𝑥𝑦𝑔𝑒𝑛/𝑚𝑜𝑙� = 44.01 𝑔𝐶𝑂2/𝑚𝑜𝑙

Molar mass of CH4 = 𝑀𝐶𝐻4 = 12.01 𝑔𝑐𝑎𝑟𝑏𝑜𝑛/𝑚𝑜𝑙 + �4 ∙ 1.008 𝑔ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛/𝑚𝑜𝑙� = 16.042 𝑔𝐶𝐻4/𝑚𝑜𝑙

Now we can easily set up our calculation for determining CO2 emissions:

CO2 = �1𝑚𝑜𝑙 𝐶𝑂2

1𝑚𝑜𝑙 𝐶𝐻4� �

44.01𝑔 𝐶𝑂2

1𝑚𝑜𝑙 𝐶𝑂2� �

1𝑚𝑜𝑙 𝐶𝐻4

16.042𝑔 𝐶𝐻4� [106𝑔 𝐶𝐻4] �

1𝑡 𝐶𝑂2

106𝑔 𝐶𝑂2�

= 2.74𝑡 𝐶𝑂2

So for every tonne of methane combusted, almost 3 tonnes of carbon dioxide are emitted. How do CO2 emissions from other fossil fuels compare? Let’s look at another example.

Example 3 How much CO2 is produced when 1 tonne of benzene (C6H6) is combusted in an oxygen-only atmosphere?


𝐶6𝐻6 + 𝑂2 → 𝐶𝑂2 + 𝐻2𝑂


𝐶6𝐻6 + 𝑂2 → 6𝐶𝑂2 + 𝐻2𝑂

𝐶6𝐻6 + 𝑂2 → 6𝐶𝑂2 + 3𝐻2𝑂

𝐶6𝐻6 + 7.5𝑂2 → 6𝐶𝑂2 + 3𝐻2𝑂

Step 3. Calculate CO2 emissions.

Molar ratio =6 𝑚𝑜𝑙 𝐶𝑂2

1 𝑚𝑜𝑙 𝐶6𝐻6


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Molar mass of C6H6 = (6 ∙ 12.01) + (4 ∙ 1.008) = 76.668 𝑔/𝑚𝑜𝑙

CO2 = �6𝑚𝑜𝑙 𝐶𝑂2

1𝑚𝑜𝑙 𝐶6𝐻6� �

44.01𝑔 𝐶𝑂2

1𝑚𝑜𝑙 𝐶𝑂2� �

1𝑚𝑜𝑙 𝐶6𝐻6

76.668𝑔 𝐶6𝐻6� [106𝑔 𝐶𝐻4] �

1𝑡 𝐶𝑂2

106𝑔 𝐶𝑂2�

= 3.44𝑡 𝐶𝑂2

Now that we have worked through balancing the combustion equations for several specific fuels, let’s 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.


𝐶𝑥𝐻𝑌 + 𝑂2 → 𝐶𝑂2 + 𝐻2𝑂


𝐶𝑥𝐻𝑦 + 𝑂2 → 𝑥𝐶𝑂2 + 𝐻2𝑂

𝐶𝑥𝐻𝑦 + 𝑂2 → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂

𝐶𝑥𝐻𝑦 + �𝑥 + 𝑦4

� 𝑂2 → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂

Final balanced equation:


� 𝑂2 → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂

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? Let’s 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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𝐶6𝐻12𝑂6 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 𝐻2𝑂 + 𝑁2

Step 2. Balance the equation one element at a time: carbon, hydrogen, oxygen, and nitrogen.

𝐶6𝐻12𝑂6 + (𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 𝐻2𝑂 + 𝑁2

𝐶6𝐻12𝑂6 + (𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 6𝐻2𝑂 + 𝑁2

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:

𝐶6𝐻12𝑂6 + 6(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 6𝐻2𝑂 + 𝑁2

𝐶6𝐻12𝑂6 + 6(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 6𝐻2𝑂 + 22.57𝑁2

This gives us our balanced equation:

𝐶6𝐻12𝑂6 + 6(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 6𝐻2𝑂 + 22.57𝑁2

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 nitrogen’s presence during combustion will become evident in Section 4.3 Real Combustion.

Let’s 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).


𝐶𝐻4 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 𝐻2𝑂 + 𝑁2


𝐶𝐻4 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 𝐻2𝑂 + 𝑁2

𝐶𝐻4 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 2𝐻2𝑂 + 𝑁2

𝐶𝐻4 + 2(𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 2𝐻2𝑂 + 𝑁2

𝐶𝐻4 + 2(𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 2𝐻2𝑂 + 7.52𝑁2

This gives us our final balanced equation:

𝐶𝐻4 + 2(𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 2𝐻2𝑂 + 7.52𝑁2


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Example 7 Write the balanced equation for benzene (C6H6) combusted in air (O2 + 3.76N2).


𝐶6𝐻6 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 𝐻2𝑂


𝐶6𝐻6 + (𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 𝐻2𝑂 + 𝑁2

𝐶6𝐻6 + (𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 3𝐻2𝑂 + 𝑁2

𝐶6𝐻6 + 7.5(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 3𝐻2𝑂 + 𝑁2

𝐶6𝐻6 + 7.5(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 3𝐻2𝑂 + 28.21𝑁2

This gives us our final balanced equation:

𝐶6𝐻6 + 7.5(𝑂2 + 3.76𝑁2) → 6𝐶𝑂2 + 3𝐻2𝑂 + 28.21𝑁2

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).


𝐶𝑥𝐻𝑦 + (𝑂2 + 3.76𝑁2) → 𝐶𝑂2 + 𝐻2𝑂 + 𝑁2


𝐶𝑥𝐻𝑦 + (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝐻2𝑂 + 𝑁2

𝐶𝑥𝐻𝑦 + (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂 + 𝑁2


� (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂 + 𝑁2


� (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂 + 3.76 �𝑥 + 𝑦4

� 𝑁2

Final balanced equation:


� (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂 + 3.76 �𝑥 + 𝑦4

� 𝑁2

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

= 𝐴𝐹𝑚𝑎𝑠𝑠 =𝑚𝑎𝑖𝑟

𝑚𝑓𝑢𝑒𝑙

Moles of air in feed mixtureMoles of fuel in feed mixture

= 𝐴𝐹𝑚𝑜𝑙𝑒 =𝑛𝑎𝑖𝑟

𝑛𝑓𝑢𝑒𝑙

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:


� 𝑂2 → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂

From this equation we can determine the stoichiometric ratio:

Stoichiometric ratio (oxygen) = AFstoich(𝑂2) =𝑛𝑂2

𝑛𝐶𝑥𝐻𝑦

=�𝑥 + 𝑦

4� 𝑚𝑜𝑙1 𝑚𝑜𝑙

= 𝑥 + 𝑦4

This ratio can also be expressed as a mass ratio (AFmass) by multiplying by the molecular masses:

Mass ratio (oxygen) = 𝐴𝐹𝑚𝑎𝑠𝑠(𝑂2) = �𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ(𝑂2)� �𝑀𝑂2

𝑀𝐶𝑥𝐻𝑦

�

= �𝑥 + 𝑦4

� �32.00 𝑔/𝑚𝑜𝑙

(12.01𝑥 + 1.008𝑦)𝑔/𝑚𝑜𝑙�

=32.00𝑥 + 8.00𝑦

12.01𝑥 + 1.008𝑦

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).

𝐴𝐹𝑚𝑎𝑠𝑠(𝑂2) (𝑝𝑢𝑟𝑒 𝑐𝑎𝑟𝑏𝑜𝑛 𝑓𝑢𝑒𝑙) =32.00𝑥 𝑔/𝑚𝑜𝑙12.01𝑥 𝑔/𝑚𝑜𝑙

= 2.664


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𝐴𝐹𝑚𝑎𝑠𝑠(𝑂2) (𝑝𝑢𝑟𝑒 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑓𝑢𝑒𝑙) =8.00𝑦 𝑔/𝑚𝑜𝑙

1.008𝑦 𝑔/𝑚𝑜𝑙= 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:


� (𝑂2 + 3.76𝑁2) → 𝑥𝐶𝑂2 + 𝑦2

𝐻2𝑂 + 3.76 �𝑥 + 𝑦4

� 𝑁2

In this case, the stoichiometric ratio represents the ratio of moles of air to moles of fuel, which remains the same:

Stoichiometric ratio (air) = 𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ(𝑎𝑖𝑟) =𝑛𝑎𝑖𝑟

𝑛𝐶𝑥𝐻𝑦

=�𝑥 + 𝑦

4� 𝑚𝑜𝑙1 𝑚𝑜𝑙

= 𝑥 + 𝑦4

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) = 𝐴𝐹𝑚𝑎𝑠𝑠(𝑎𝑖𝑟) = �𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ(𝑎𝑖𝑟)� �𝑀𝑎𝑖𝑟

𝑀𝐶𝑥𝐻𝑦

�

= �𝐴𝐹𝑚𝑎𝑠𝑠(𝑂2)� �𝑀𝑎𝑖𝑟

𝑀𝑂2

�

= �32.00𝑥 + 8.00𝑦

12.01𝑥 + 1.008𝑦� �

137.3232.00𝑔

�

= 4.29 �32.00𝑥 + 8.00𝑦

12.01𝑥 + 1.008𝑦�

Note that the air-fuel ratio can also sometimes expressed as a fuel-air (FA) ratio.

Fuel-air ratio = 𝐹𝐴𝑚𝑎𝑠𝑠(𝑎𝑖𝑟) =1

𝐴𝐹𝑚𝑎𝑠𝑠(𝑎𝑖𝑟)=

𝑚𝑓𝑢𝑒𝑙

𝑚𝑎𝑖𝑟

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: 𝐴𝐹𝑚𝑖𝑥𝑡𝑢𝑟𝑒 < 𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ


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Lean mixture: 𝐴𝐹𝑚𝑖𝑥𝑡𝑢𝑟𝑒 > 𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ

The equivalence ratio (Φ) provides a measure of the deviation of the actual air-fuel ratio and the stoichiometric ratio:

Equivalence ratio = 𝜙 =𝐴𝐹𝑠𝑡𝑜𝑖𝑐ℎ

𝐴𝐹𝑚𝑖𝑥𝑡𝑢𝑟𝑒=

𝐹𝐴𝑚𝑖𝑥𝑡𝑢𝑟𝑒

𝐹𝐴𝑠𝑡𝑜𝑖𝑐ℎ

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 – 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 𝐶𝑥𝐻𝑦 + 𝑂2 + 𝑁2 → mostly(𝐶𝑂2 + 2𝐻2𝑂 + 𝑁2) + traces(𝐶𝑂 + 𝑁𝑂 + 𝐻𝐶 + ⋯ )

Incomplete oxidation can occur under a variety of circumstances, including when:

• There is poor mixing fuel and air, • The temperature is too low, • Oxygen levels are insufficient, • 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:

𝑂2 + 𝑁2 + ℎ𝑒𝑎𝑡 → 2𝑁𝑂

𝑁𝑂 + 12𝑂2 → 𝑁𝑂2

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:

𝐶𝑂2 → 𝐶𝑂 + 12𝑂2

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(𝐶, 𝐻, 𝑁, 𝑆, 𝑎𝑠ℎ) + air(𝑂2 + 𝑁2) → (𝐶𝑂2, 𝐻2𝑂, 𝐶𝑂, 𝑁𝑂𝑥, 𝑆𝑂𝑥, 𝑉𝑂𝐶𝑠, 𝑝𝑎𝑟𝑡𝑖𝑐𝑢𝑙𝑎𝑡𝑒𝑠) + 𝑎𝑠ℎ


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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 … 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.

http://public.eblib.com/EBLPublic/PublicView.do?ptiID=588240

Energy & Society Combustion Toolkit 3: Combustion · PDF fileEnergy & Society Combustion 2 Table of Contents 1. Introduction 3 2. Energy Sources & Use 4 2.1. Energy use.....

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