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Thermal reactions
Chapter 9
Thermal reactions 9.1
INTRODUCTION
Arguably, the burning of substances produces the most toxic and largest quantity of air pollutants worldwide. No matter if the burning is intentional, for example, for energy production and cookstoves, or unintentional, for example, wildfires, air pollution at every scale is affected. The biogeochemical cycles considered in the previous chapter are greatly affected by combustion, especially the release of compounds of carbon, nitrogen, sulfur, and metals into these cycles. In particular, carbon compounds are included in metrics of pollution control efficiency and success, especially CO2, CO, and other products of incomplete combustion (PICs). As mentioned in Chapter 8, following the discussion of CO2’s role in global warming, it is somewhat ironic that the greater the amount of CO2 emitted from a source, the more efficiently the pollution control equipment is operating. Indeed, complete combustion results in the production of only CO2 and H2O. Likewise, the production of CH4 has been an indication of complete anaerobic digestion of organic compounds within the landfill or during fermentation. Both of these indicators of pollution control efficiency also happen to be greenhouse gases. The combustion of organic compounds is how most energy is generated. These carbon-based coupons undergo reactions that differ in rates and products, depending on temperature, mixtures with other substances, oxygen concentrations, and numerous other factors. As a result, the types and amounts of product released from combustion vary but include some of the most toxic and otherwise harmful pollutants. For this reason, the next chapter’s focus on combustion is also an extension of the carbon cycle.
9.2
AIR AND COMBUSTION
The theoretical, sufficient concentration of O2 to achieve complete combustion is that needed to react with the total C in the combustible material, that is, fuel. The air needed to achieve this is known as “theoretical air” or “stoichiometric air,” which depends on the chemical makeup of the fuel and the fuel feed rate. The feed rate is expressed as a volume or mass per time, for example, L hr1 and kg hr1, respectively. The ideal combustion process, that is, stoichiometric combustion, is that that burns the fuel completely. The deficit between stoichiometric combustion and incomplete combustion determines the percentage of combustion inefficiency (see Fig. 3.8 in Chapter 3). Therefore, at or above theoretical air, the process is 100% efficient. To avoid products of incomplete combustion (PICs), especially carbon monoxide (CO), excess air is usually added. The excess air or excess fuel for a combustion system is based on the stoichiometric air-fuel ratio, the precise, ideal fuel ratio in which chemical mixing proportion is reached. For safety reasons, prevention of explosive conditions from fouling and generation of high temperatures, and because fuel composition and conditions even in well-controlled reactors vary, combustors are designed to achieve “on-ratio” combustion, that is, requiring a known amount of excess air, often 10%–20% above the expect stoichiometric air value [1] (see Table 9.1). Fuel-lean mixtures have air content greater than the stoichiometric ratio; fuel-rich mixtures have air content less than the stoichiometric ratio. For fuels (CmHn) in the gas phase, the stoichiometric combustion reaction is [2] n n n (9.1) Cm Hn + m + ðO2 + 3:76N2 Þ ! mCO2 + H2 O + 3:76 m + N2 |fflffl{zfflffl} 4 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} 2 4 fuel
air
Air Pollution Calculations. https://doi.org/10.1016/B978-0-12-814934-8.00009-0 © 2019 Elsevier Inc. All rights reserved.
207
208
Air pollution calculations
TABLE 9.1 Approximate O2 and CO2 in flue gas at various excess air conditions Percent volume of CO2 in flue gas Percent excess air
Natural gas
Propane butane
Fuel oil
Bituminous coal
Anthracite coal
Percent O2 in flue gas for all fuels
0
12
14
15.5
18
20
0
20
10.5
12
13.5
15.5
16.5
3
40
9
10
12
13.5
14
5
60
8
9
10
12
12.5
7.5
80
7
8
9
11
11.5
9
100
6
6
8
9.5
10
10
Data from Engineering Toobox, Stoichiometric Combustion and Excess of Air, January 31, 2019, Available from: https://www.engineeringtoolbox.com/ stoichiometric-combustion-d_399.html.
For solid and liquid fuels (CaHbOcNdSe), the stoichiometric combustion reaction is [2] b d + 3:76x N2 + eSO2 Ca Hb Oc Nd Se + xðO2 + 3:76N2 Þ ! aCO2 + H2 O + |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} 2 2 fuel
(9.2)
air
where x ¼ a + b4 2c + e Note that the CaHbOcNdSe empirical formula of solid or liquid fuel is assumed to account of 100% of the mass and is calculated assuming the fuel is a dry, ash-free elemental composition. Combustion efficiency increases with excess air until heat loss in the amount of excess air reaches the heat loss to a point where the net efficiency begins to drop (see Fig. 9.1).
FIG. 9.1 Air-fuel ratio’s effect on combustion efficiency and generation of carbon monoxide. (Data from Engineering Toobox, Stoichiometric Combustion and Excess of Air. (2003). July 16, 2018. Available: https://www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html).
Thermal reactions Chapter
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209
What is the reaction of methane burned at 25% excess air? Since air is about 79% N2 and 21% O2, the N2-to-O2 mole ratio is 3.76. Thus, the stoichiometric combustion of CH4 is CH4 + 2ðO2 + 3:76 N2 Þ ! CO2 + 2 H2 O + 7:52 N2 Therefore, CH4 combusted with 25% excess air can be expressed as CH4 + 1:25 2ðO2 + 3:76 N2 Þ ! CO2 + 2 H2 O + 0:5 O2 + 9:4 N2 Note that the amounts of carbon dioxide and water remain the same after the theoretical air concentration is achieved.
Propane is being combusted at 80% theoretical air, that is, 20% air deficiency. What is the combustion efficiency of this reaction? C3 H8 + 5 ð0:80Þ ðO2 + 3:76N2 Þ ! 2CO + CO2 + 4H2 O + 15:04N2 Calculate the volume: Volume of product species is proportional to the coefficient of each species, coefficient of CO2 1 ¼ ¼ 0:33 or 33% by volume: CE of C3H8 gas ¼ coefficient of CO2 + coefficient of CO 1 + 2 Calculate the mass: From the stoichiometry, there is one mole of CO2 and 2 mol of CO in this reaction, thus, CE ¼
9.3
mass CO2 12 + 32 ¼ ¼ 0:44 or 44% by mass mass CO2 + mass CO ð12 + 32Þ + 2ð12 + 16Þ
VOLATILITY OF COMBUSTION PRODUCTS
These combustion principles must be applied to air pollution control technologies. For example, one of the cross media concerns that tie solid waste to air quality is the thermal treatment of waste. This is commonly done not only with excess O2, that is, incineration, but also when O2 is absent or at very low concentrations, that is, pyrolysis. Depending on the fuel, that is, waste stock, numerous volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs), notably polycyclic aromatic hydrocarbons (PAHs), dioxins and furan, are generated under high temperatures in waste combustors and in various industrial reactors. As evidence, Figs. 9.2 and 9.3 indicate that the highly varying optimal temperature ranges to form VOCs and SVOCS. The aliphatic (carbon-chain) compounds in this fire, notably 1-dodecene, 9-nonadecane, and 1hexacosene, are generated in higher concentrations at lower temperatures (about 800°C), whereas the aromatic (carbonring) compounds require higher temperatures. The total mass and the total concentrations of these thermally generated compounds continue to increase with temperature, but the chemical speciation changes [3, 4]. At some point, the temperature, pressure, sorbent, and oxygen conditions will supplant these compounds to form others that may be even more toxic, for example, the halogenated dioxins and furans. The engineer and plant operator seek the optimal combination of factors in the thermal process to completely convert as much of the organic mass to nontoxic products [5].
9.4
FLUE GAS
Combustion is used primarily for heat by changing the potential chemical energy of the fuel to thermal energy. This occurs in a fossil fuel-fired power plant, a home furnace, or an automobile engine. Combustion is also used as a means of destruction for our unwanted materials. This decreases the volume of a solid waste by burning the combustibles in an incinerator. The combustion reactor, for example, kiln, accepts combustible solids, liquids, and gases, with undesirable properties such as odors, to a high temperature in an afterburner system to convert them to less objectionable gases. Note that incineration is often used to denote any thermal destruction of waste, but the process covers a large O2 range. As O2 concentrations approach zero, pyrolysis dominates, but combustion occurs at higher available O2 concentrations. As a general rule, a fuel is assumed to produce dry, stoichiometric gas emissions at a rate of 0.25 N m3 M1 J1, deducting for the water vapor that is formed during combustion [6]. The caloric value (Hinf) of the fuel is expressed in energy per mass (e.g., megajoules per kg, MJ kg1). Since air contains 21% O2, the dry flue gas at y% O2 is expressed as
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Air pollution calculations
FIG. 9.2 Selected hydrocarbon compounds generated in a low-density polyethylene pyrolysis; low-oxygen conditions, in four temperature regions. (Source: D.A. Vallero, Fundamentals of Air Pollution, 5th ed., Elsevier Academic Press, Waltham, MA, 2014, p. 999 pages cm; Data from R. Hawley-Fedder, M. Parsons, F. Karasek, Products obtained during combustion of polymers under simulated incinerator conditions: I. Polyethylene, J. Chromatogr. A 314 (1984) 263–273).
FIG. 9.3 Total aliphatic (chain) hydrocarbons versus polycyclic aromatic hydrocarbons (PAHs) generated in a low-density polyethylene pyrolysis in four temperature regions. Source: D.A. Vallero, Fundamentals of Air Pollution, 5th ed., Elsevier Academic Press, Waltham, MA, 2014, p. 999 pages cm; Data from R. HawleyFedder, M. Parsons, F. Karasek, Products obtained during combustion of polymers under simulated incinerator conditions: I. Polyethylene, J. Chromatogr. A 314 (1984) 263–273.
Thermal reactions Chapter
Dry flue gas in Nm3 kg1 ¼ Hinf 0:25
21 21 y
9
211
(9.3)
The wet flue gas at z% H2O is compared with saturation and is expressed as Wet flue gas in Nm3 kg1 ¼ Hinf 5:25
21 100 21 y 100 z
(9.4)
Actual flue gas volume (VAFG) at T (°C) is expressed as VAFG in m3 kg1 ¼ Hinf 525
21 y 273 + T 100 z 273
(9.5)
which is simplified to VAFG ¼ 2Hinf
273 + T ð21 yÞ ð100 zÞ
A waste with an average caloric value of 5 MJ kg21 is burned at 100°C in a reactor with 14% oxygen content and 15% water vapor content. What is the actual flue gas volume for this system? VAFG ¼ 2Hinf VAFG ¼ 2 5
273 + T ð21 y Þ ð100 z Þ
273 + 100 ð21 14Þ ð100 15Þ
VAFG ¼ 6:27 m3 kg1
Flue gas volume is important in the design and operation of air pollution control equipment. For example, it is often preferable to lower temperatures to prevent the formation of SVOCs during incineration; for example, chlorinated dioxins and furans can form catalytically in flue gas between about 250 and 350°C, so cooling to <230°C would substantially inhibit the formation of these toxic compounds [7]. There are a number of ways to cool flue gases, including evaporative cooling (humidification, for example, using water sprays), which increases the area of heat transfer. Addition of cooler air to the flue gas and heat recovery are also options. Addition of cooler air, however, would increase VAFG. Therefore, if a large volume of air is added, the combustion efficiency would be hampered.
9.5
THERMAL POLLUTANT DESTRUCTION
Plant managers, chemists, engineers, and regulators want to know the efficiency of combustion. This is important for determining the extent to which a process is functioning as designed. For example, is the fuel being burned as efficiently as needed to boil water to turn an electric turbine to produce a sufficient amount of energy? If the combustion efficiency is too low, the steam generation is insufficient to move the turbine, and fewer calories of heat are produced. Combustion efficiency is also crucial to air pollution, since incomplete combustion generates toxic substances. A common calculation of combustion efficiency (CE) is the ratio of the actual combustion relative to complete combustion of a fuel by comparing the CO concentration with combined CO and CO2 concentrations: CCO2 100 (9.6) CE ¼ CCO2 + CCO where CCO2 ¼ concentration of carbon dioxide and CCO ¼ concentration of carbon monoxide. Note that the concentrations can be in any volume or mass units, since the CE is a unitless value between 0 and 100%. After a certain temperature is reached, CE increases with increasing chamber temperature. Other factors that affect CE include mixing and available O2.
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Air pollution calculations
The efficiency of destroying toxic organic compounds by combustion is similar to CE, but rather than being a function of CO and CO2, waste incinerator performance is based on the destruction and removal efficiency (DRE), that is, the percentage of the number of molecules of an organic compound emitted to the atmosphere compared with the number of molecules entering the incinerator [8]. Commonly, for hazardous wastes, a minimum 99.9999% DRE is required. This means that one molecule of an organic compound is released to the air for every million molecules of the organic compound entering the incinerator. The DRE calculation is Win Wout 100 (9.7) DRE ¼ Win where Win is the mass feed rate of organic compound in the waste stream feeding the incinerator and Wout is the mass emission rate of the organic compound present in exhaust emissions prior to release to the atmosphere.
A regulatory agency requires that the combustion of 20 kg polychlorinated biphenyl (PCB) be incinerated in a hazardous waste incinerator at 99.9% efficiency and that the DRE be 99.9999%. The burn took 1 h. The exhaust rate was 1.5 L sec21. The emission rate was measured to be 950 μg m23 PCBs, 5 ppm CO, and 1% CO2. Does the incinerator meet the regulated standards? Since 1% ¼ 10,000 CO2 is 1%, combustion efficiency is as follows: 10000 CE ¼ 100 ¼ 99:95% 10000 + 5 The total volume of exhaust for the hour is 1.5 L sec1 3600 s hr.1 ¼ 5400 L ¼ 5.4 m3. Thus, at 950 μg m3, the total mass of PCBs exiting the stack is 950 μg m3 5.4 m3 ¼ 5130 μg PCB. Since 20 kg of PCB was fed into the incinerator, the DRE is 3 20 kg 109 μg kg1 5130 μg 4 5 100 ¼ 99:99997% 20 kg 109 μg kg1 2
Therefore, this incinerator appears to be meeting both CE and DRE standards.
Indeed, the number of nines in the DRE is an indicator of the toxicity of a compound. For example, a less toxic compound may only require four nines, that is, 99.99%, whereas a highly toxic compound like a PCB or dioxin would require more nines, for example, the six nines DRE (99.9999%). Combustion efficiency is defined in other ways, such as a measure of energy conservation from the fuel into useful energy, for example, steam generation [9]. This may be found by subtracting the heat content of the exhaust gases, expressed as a percentage of the fuel’s heating value, from the total fuel-heat potential, that is, 100%: stack heat losses 100 CE ¼ 100% fuel heating value Stack heat losses are calculated from measurements of gas concentration and temperature measurements, along with the fuel’s unique composition and heat content. The heat losses are primarily from the heated dry exhaust gases (CO2, N2, and O2) and from water vapor formed from the reaction of hydrogen in the fuel with O2 in the air. When water goes through a phase change from liquid to vapor, it absorbs a tremendous amount of heat energy in the process. This heat of vaporization, or latent heat, is usually lost and not recovered. This is evident from the white cloud that exits a stack on a cool day, that is, predominantly from condensing water vapor losing its latent heat to the atmosphere [9]. Table 9.2 gives combustion efficiency for fuel oil under varying conditions of temperature, CO2, and O2.
9.6
ACTIVATION ENERGY
Air pollutants can react with one another when they are forming (e.g., in a reactor), in the atmosphere or in any volume, for example, within a canister awaiting chemical analysis. Activation energy is needed to amount of energy needed for the reaction to take place, that is, the energy needed to bring all molecules in one mole of a substance to their reactive state at a given temperature. An exothermic reaction releases energy, whereas an endothermic reaction requires energy. If a catalyst is present, the reaction rate increases, that is, the catalyst decreases the demand for activation energy (see Fig. 9.4).
Thermal reactions Chapter
9
213
TABLE 9.2 Combustion efficiency of a fuel oil derived from net temperature (°C), that is, the difference of stack temperature (Tstack) and supply temperature (Tsupply), relative to carbon dioxide and oxygen percent and temperature Net temperature (Tstack 2 Tsupply) %CO2
%O2
149
160
171
182
193
204
216
227
238
249
260
288
316
15.6
0.0
31
31
31
30
30
30
30
30
29
29
29
28
28
14.1
2.0
31
31
30
30
30
30
29
29
29
29
28
28
27
13.4
3.0
31
30
30
30
30
29
29
29
29
28
28
28
27
12.6
4.0
31
30
30
30
29
29
29
29
28
28
28
27
27
11.9
5.0
30
30
30
29
29
29
29
28
28
28
28
27
26
11.1
6.0
30
30
29
29
29
29
28
28
28
27
27
26
26
10.4
7.0
30
29
29
29
29
28
28
28
27
27
27
26
25
9.6
8.0
29
29
29
28
28
28
27
27
27
26
26
25
24
Source of data: I. TSI, Combustion Analysis Basics: An Overview of Measurements, Methods and Calculations Used in Combustion Analysis. (2004). Available: http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Handbooks/CA-basic-2980175.pdf (Accessed 23 July 2018).
Energy
Activation energy (without catalyst)
Activation energy (with catalyst)
Reactants
Heat released to environment
Products
Direction of exothermic reaction
Energy Activation energy (without catalyst)
Activation energy (with catalyst) Absorbed heat
Products
Reactants
Direction of endothermic reaction FIG. 9.4 Exothermic reaction (top) and endothermic reaction (bottom), showing the effect of catalysis on activation energy.
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Air pollution calculations
This energy requirement is a reminder that reaction rates are temperature-dependent. Indeed, the rate constant k reflects this in the Arrhenius equation: Ea
k ¼ A eRT
(9.8)
where A is the preexponential factor (constant), Ea is the Arrhenius activation energy, R is the universal gas constant, and T is the absolute temperature.
Nitrogen dioxide and carbon monoxide are reacting in a chamber at 70. The measured rate constant for this reaction is at 701°K is 2.57 M21 s21 and at 895°K is 567 M21 s21. What is the activation energy? Convert the Arrhenius equation to natural log form: ln k ¼
Ea + ln A RT
(9.9)
ln k1 ¼
Ea + ln A RT1
(9.10)
ln k2 ¼
Ea + ln A RT2
(9.11)
Set up a “two-point” form of the Arrhenius equation:
The difference of Eq. (8.18) from Eq. (8.17):
1 1 T2 T1 1 1 567 M s Ea 1 1 ¼ ln 1 1 R 701 K 895 K 2:57 M s ln
k1 Ea ¼ k2 R
5:40 ¼
Ea R
Ea ¼ 5:40 Ea ¼ 5:4 ¼
3:09 101 K
(9.12)
K R 3:09 101
K J ¼ 1:45 105 J mol1 ¼ 145 kJ mol 1 8:314 1 mol K 3:09 10
We could have also solved for Ea at the outset: Ea ¼
9.7
RT 1 T2 k1 ln ðT1 T2 Þ k2
(9.13)
NITROGEN AND SULFUR
Nitrogen species important to air quality include both oxidized and reduced forms. Atmospheric N reactions can be more complicated than may be inferred from Reactions (8.11) and (8.12). For example, the NOx reaction from NO to NO2 likely involves water and an intermediate, that is, nitrous acid (HONO): 4NO + O2 + 2H2 O ! 4HONO
(9.14)
Combustion processes, both stationary (e.g., power plants) and mobile (e.g., automobiles), entail reactions that oxidize both atmospheric N2 and nitrogen in the fuel. As discussed in Chapter 8, the term “oxides of nitrogen” (NOx) does not include all oxidized species, just NO and NO2. However, the larger suite of N air pollution compounds, that is, total reactive nitrogen (NOy), includes not only NO and NO2 but also their oxidation products [10]. Combustion generates other vaporphase and aerosol species in NOy including HONO, nitric acid (HNO3), peroxyacetyl nitrate (PAN), organic nitrates, and particulate nitrates: NOy ¼ NO2 + NO + HNO3 + PAN + 2N2 O5 + HONO + NO3 + NO3 compounds + NO3 aerosols
(9.15)
Thermal reactions Chapter
9
215
The NOy compounds are products of reactions with NOx emissions or from transformations in the troposphere [11]. Most of the compounds originate from combustion processes. The N source is either molecular nitrogen in the troposphere, that is, in the air intake, or the N-content of the fuel. The tropospheric N2 makes up the largest share of gaseous content in the combustion air (79% by volume). When pressure and temperatures are sufficiently high, for example, in an internal combustion engine or industrial boiler, the plentiful N2 will react with O2. NO makes up about 90%–95% of this thermal NOx. Other nitrogen oxides can also form at high heat and pressure, especially NO2. Most motor vehicles around the world still employ high-temperature/highpressure internal combustion engines. Thus, such mobile sources contribute largely to the tropospheric concentrations of NOx, making it a major mobile source air pollutant in terms of human health directly (e.g., respiratory toxicity of NO2) and indirectly (i.e., NOx as the key components in tropospheric ozone production). These conditions of high temperature and pressure can also exist in boilers such as those in power plants, so NOx is also commonly found in high concentrations leaving fossil fuel power generating stations. In addition to the atmospheric molecular nitrogen as a precursor of nitrogen air pollutants of combustion, fossil fuels themselves contain varying concentrations of N. Nitrogen oxides that form from the fuel or feedstock are called “fuel NOx.” Unlike the sulfur compounds, which mainly exit stationary source stacks as vapor-phase compounds (e.g., SO2 and other oxides of sulfur), a significant fraction of the fuel nitrogen burned in power plants and other stationary sources remains in the bottom ash or in unburned aerosols in the gases leaving the combustion chamber, that is, the fly ash. Nitrogen oxides can also be released from nitric acid plants and other types of industrial processes involving the generation and/or use of nitric acid (HNO3). At temperatures far below combustion, such as those often present in the ambient atmosphere, NO2 can form the molecule NO2-O2N or simply N2O4 that consists of two identical simpler NO2 molecules. This molecular configuration is known as a dimer. The dimer N2O4 is distinctly reddish brown and contributes to the brown haze that is often associated with photochemical smog incidents. In addition to the health effects associated with NO2 exposure, much of the concern for regulating emissions of nitrogen compounds is to suppress the reactions in the atmosphere that generate the highly reactive molecule ozone (O3). Nitrogen oxides play key roles in O3 formation. Ozone forms photochemically (i.e., the reaction is caused or accelerated by light energy) in the lowest level of the atmosphere, known as the troposphere, where people live. Nitrogen dioxide is the principal gas responsible for absorbing sunlight needed for these photochemical reactions. So, in the presence of sunlight, the NO2 that forms from the NO incrementally stimulates the photochemical smog-forming reactions because nitrogen dioxide is very efficient at absorbing sunlight in the ultraviolet portion of its spectrum. This is why ozone episodes are more common during summer and in areas with ample sunlight. Other chemical ingredients, that is, ozone precursors, in O3 formation include volatile organic compounds (VOCs) and carbon monoxide (CO). Governments regulate the emissions of precursor compounds to diminish the rate at which O3 forms. Sulfur, like nitrogen, is oxidized during combustion but is also an important component of particulate matter (PM; see Table 9.3). For example, diesel particulate matter is formed by a number of simultaneous physical processes during cooling and dilution of exhaust, that is, nucleation, coagulation, condensation, and adsorption. The core of the particles is formed by nucleation and coagulation from primary spherical particles consisting of solid carbonaceous matter (known as elemental carbon, EC) and ash (metals and other elements). By coagulation, adsorption, and condensation, various organic and S compounds (e.g., sulfates) are added and combined with other condensed material [12]. The small diameter of diesel PM (<0.5 μm) makes for very large surface areas. <0.5 mm, these particles have a very large surface area per gram of mass, which allows them to adsorb large quantities of ash, organic compounds, and sulfate (see Fig. 9.5). The specific surface area of the EC core is approximately 30–50 m2 g1. The organic constituents originate from unburned fuel, engine lubrication oil, and small quantities of partial combustion and pyrolysis products. During combustion, S compounds in the fuel are oxidized to sulfur dioxide (SO2). From 1% to 4% of fuel, S is oxidized to form H2SO4. Upon cooling, this sulfuric acid and water condense into an aerosol that is nonvolatile under ambient conditions [12]. The oxidized chemical species of sulfur and nitrogen (e.g., sulfur dioxide (SO2) and nitrogen dioxide (NO2)) form acids when they react with water. This can occur in any media, for example, the atmosphere (i.e., acid deposition) and in mining waste and ash runoff to surface waters and groundwater. The lowered pH is responsible for numerous environmental problems. Many compounds contain both N and S along with the typical organic elements (C, H, and O). The reaction for the combustion of such compounds, in general form, is b d (9.16) H2 O + N2 + eS Ca Hb Oc Nd Se + 4a + b 2cÞ ! aCO2 + 2 2
216
Air pollution calculations
TABLE 9.3 Typical chemical composition (percent) of fine particulate matter (PM2.5) Eastern United States
Western United States
Diesel PM2.5
Elemental carbon
4
15
75
Organic carbon
21
39
19
Sulfate, nitrate, and ammonium
48
35
1
Minerals
4
15
2
Unknown
23
–
3
Source: US Environmental Protection Agency. Air quality criteria for particulate matter. External review draft, October 1999.
FIG. 9.5 Diesel particulate matter. Source: US Environmental Protection Agency, Health Assessment for Diesel Engine Exhaust. Report No. EPA/600/8-90/057F. National Center for Environmental Assessment. Washington, DC, 2002.
Reaction (9.16) demonstrates the incremental complexity as additional elements enter the reaction. In the real world, pure reactions are rare. The environment is filled with mixtures and heterogeneous reactions. Reactions can occur in sequence, parallel, or both. For example, a feedstock to a municipal incinerator contains myriad types of wastes, from garbage to household chemicals to commercial wastes, and even small and sometimes large industrial wastes that may be illegally dumped. The N content of typical cow manure is about 5 kg per metric ton (about 0.5%). If the fuel used to burn the waste also contains S along with the organic matter, then the five elements will react according to the stoichiometry of Reaction (8.15). Redox occurs in certain regions of a combustion chamber, stack, or other locations inside the combustor or in the ambient air. Recall from Chapter 8 that the formation of sulfur dioxide (SO2) and nitric oxide (NO) by acidifying molecular sulfur, for example, in coal, includes the redox reaction: SðsÞ + NO3 ðaqÞ ! SO2 ðgÞ + NOðgÞ
(9.17)
The oxidation half-reactions for this reaction are S ! SO2
(9.18)
S + 2H2 O ! SO2 + 4H + + 4e
(9.19)
The reduction half-reactions for this reaction are NO3 ! NO
NO3 + 4H + 3e ! NO + 2H2 O +
Therefore, the balanced oxidation-reduction reactions are
(9.20) (9.21)
Thermal reactions Chapter
9
4NO3 + 3S + 16H + + 6H2 O ! 3SO2 + 16H + + 4NO + 8H2 O
4NO3 + 3S + 4H ! 3SO2 + 4NO + 2H2 +
217
(9.22) (9.23)
This indicates the importance of the sulfur content of a fuel in estimating the amount of SO2 that would be expected to be emitted during combustion.
A company receives weekly deliveries of coal and takes a sample of each week, with the analyses below. Calculate the weighted 28-day average that can be used to predict sulfur dioxide emissions.
Week
Percent sulfur
kJ kg21
kg SO2 per gigajoule (kg GJ21)
Coal burned (tonnes)
1
2.9
26,229
2.3
23
2
2.7
26,693
2.2
34
3
3.2
25,068
2.7
47
4
2.4
25,997
2.0
27
The emission rates will be weighted according to the amount of fuel burned each week. Calculate the heat input during the period represented by each sample for weeks 1 through 4: Week 1: Week 2: Week 3: Week 4:
23 t 26,229 kJ kg1 1000 kg tonne1 ¼ 6.03 108 kJ 34 t 26,693 kJ kg1 1000 kg tonne1 ¼ 9.08 108 kJ 47 t 25,068 kJ kg1 1000 kg tonne1 ¼ 1.18 109 kJ 27 t 25,997 kJ kg1 1000 kg tonne1 ¼ 7.01 108 kJ
Add the product of the heat input times the emission rate for each sample and divide by the total heat input for the 30-day period. Weighted 30-day average SO2 emission rate: h
6:03 108 kJ hr1 2:3 kg GJ1 + 9:08 108 kJ hr1 2:2 kg GJ1 + 1:18 109 kJ hr1 2:7 kg GJ1
i + 7:01 108 kJ hr1 2:0 kg GJ1
6:03 108 + 9:08 108 + 1:18 109 + 7:01 108 kJ 2:4 kg SO 2 per gigajoule Thus, this plant would be expected to have emitted, on average, a 2.4 kg of sulfur dioxide over this 4 week period, based on their fuel supply. These types of calculations are needed to ensure compliance with air quality standards. In the United States, most states conduct these rolling average calculations in units of BTU per pound, pounds of SO2 per million BTUs (mmBtu), and tons.
The air quality regulatory agency requests a 30-day rolling weighted average in addition to the previously reported 28-day weighted average. The company has purchased lower-quality coal the most recent 2 weeks. Calculate the rolling weighted average SO2 emission rates. Week
Percent sulfur
kJ kg21
kg SO2 per gigajoule (kg GJ21)
Coal burned (tonnes)
1
2.9
26,229
2.3
23
2
2.7
26,693
2.2
34
3
3.2
25,068
2.7
47
4
2.4
25,997
2.0
27
5
3.5
24,590
2.9
48
6
3.3
25,875
2.8
52
218
Air pollution calculations
There are four samples for the latest 30 days, so these will be used in the rolling average. The emission rates will be weighted according to the amount of fuel burned each week. Week 5: 49 t 24,590 kJ kg1 1000 kg tonne1 ¼ 1.20 109 kJ Week 6: 52 t 25,875 kJ kg1 1000 kg tonne1 ¼ 1.35 109 kJ The rolling, weighted 30-day average SO2 emission rate for weeks 2–5 is h
9:08 108 kJ hr1 2:2 kg GJ1 + 1:18 109 kJ hr1 2:7 kg GJ1 + 7:01 108 kJ hr1 2:0 kg GJ1
i + 1:20 109 kJ hr1 2:9 kg GJ1
9:08 108 + 1:18 109 + 7:01 108 + 1:20 109 kJ 2:5 kg SO 2 per gigajoule The rolling, weighted 30-day average SO2 emission rate for weeks 3–6 is h
1:18 109 kJ hr1 2:7 kg GJ1 + 7:01 108 kJ hr1 2:0 kg GJ1 + 1:20 109 kJ hr1 2:9 kg GJ1
i + 1:35 109 kJ hr1 2:8 kg GJ1
9:08 108 + 1:18 109 + 7:01 108 + 1:35 109 kJ 2:6 kg SO 2 per gigajoule This is problematic in that the SO2 emission rates are increasing, which means that the rolling, weighted average emission rates increase by 0.1 kg GJ1 each week on record.
The emitted nitrogen and sulfur compound can cause direct and indirect damage to health and ecosystems, for example, acid deposition, and to materials and structures (see Chapter 8).
9.8
METALS IN THERMAL REACTIONS
Metals like mercury (Hg) and lead (Pb) are emitted into the atmosphere from combustion and other thermal reactions. The metals are usually trace constituents of the fuel but may also be intentionally added as catalysts and for other purposes during the thermal processes, for example, organometallic compounds in gasoline or diesel fuel. Metals may also scavenge from the walls or components of the chamber or reactor. The reactions can be a single step, for example, the direct formation of a metal oxide from a reaction with the O2 in the air, or lead to various metal compounds through a series or branching of reactions depending on the conditions of the flue gas. The flue gas can have widely varying regions of flow rate (e.g., change in contact time), temperature, O2 concentration, particulate matter (PM) concentration, availability of sorbents, redox, and reactants (e.g., oxide gases and acids). These and other conditions can lead to different reactions, even if the fuel does not change.
REFERENCES [1] Engineering Toobox, Stoichiometric Combustion and Excess of Air, July 16, 2018. Available: https://www.engineeringtoolbox.com/stoichiometriccombustion-d_399.html, 2003. [2] I.V. Ion, Energy Saving Technologies, Universitatea “Duna˘rea de Jos” din Galat¸i, Romania, 2013, Available from: https://scholar.google.com/ scholar?cluster¼4618665913383406658&hl¼en&as_sdt¼1,34 (Accessed on 31 January 2019). [3] D.A. Vallero, Fundamentals of Air Pollution, fifth ed., Elsevier Academic Press, Waltham, MA, 2014. p. 999 pages cm. [4] R. Hawley-Fedder, M. Parsons, F. Karasek, Products obtained during combustion of polymers under simulated incinerator conditions: I. Polyethylene, J. Chromatogr. A 314 (1984) 263–273. [5] EPA/540/R-92/074 B, Guide for Conducting Treatability Studies under CERCLA: Thermal Desorption, 1992. [6] W. Bank, Municipal Solid Waste Incineration: World Bank Technical Guidance Report, 1999. [7] EPA-45013-89-27e Municipal Waste Combustors: Background Information for Proposed Guidelines for Existing Facilities, 1989. [8] M.D. Erickson, P.G. Gorman, D.T. Heggem, Relationship of destruction parameters to the destruction/removal efficiency of PCBs, J. Air Pollut. Control Assoc. 35 (6) (1985) 663–665. [9] I. TSI, Combustion Analysis Basics: An Overview of Measurements, Methods and Calculations Used in Combustion Analysis, Available: http:// www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Handbooks/CA-basic-2980175.pdf, 2004. Accessed 23 July 2018. [10] B. Finalyson-Pitts, J. Pitts, Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Application, Academic Press, San Diego, CA, 1999. [11] EPA-452/R-11-005a, Policy Assessment for the Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur, 2011. [12] US Environmental Protection Agency, Health Assessment Document for Diesel Engine Exhaust, National Centre for Environmental Assessment, Washington, DC, 2002.
Thermal reactions 9.1
INTRODUCTION
Arguably, the burning of substances produces the most toxic and largest quantity of air pollutants worldwide. No matter if the burning is intentional, for example, for energy production and cookstoves, or unintentional, for example, wildfires, air pollution at every scale is affected. The biogeochemical cycles considered in the previous chapter are greatly affected by combustion, especially the release of compounds of carbon, nitrogen, sulfur, and metals into these cycles. In particular, carbon compounds are included in metrics of pollution control efficiency and success, especially CO2, CO, and other products of incomplete combustion (PICs). As mentioned in Chapter 8, following the discussion of CO2’s role in global warming, it is somewhat ironic that the greater the amount of CO2 emitted from a source, the more efficiently the pollution control equipment is operating. Indeed, complete combustion results in the production of only CO2 and H2O. Likewise, the production of CH4 has been an indication of complete anaerobic digestion of organic compounds within the landfill or during fermentation. Both of these indicators of pollution control efficiency also happen to be greenhouse gases. The combustion of organic compounds is how most energy is generated. These carbon-based coupons undergo reactions that differ in rates and products, depending on temperature, mixtures with other substances, oxygen concentrations, and numerous other factors. As a result, the types and amounts of product released from combustion vary but include some of the most toxic and otherwise harmful pollutants. For this reason, the next chapter’s focus on combustion is also an extension of the carbon cycle.
9.2
AIR AND COMBUSTION
The theoretical, sufficient concentration of O2 to achieve complete combustion is that needed to react with the total C in the combustible material, that is, fuel. The air needed to achieve this is known as “theoretical air” or “stoichiometric air,” which depends on the chemical makeup of the fuel and the fuel feed rate. The feed rate is expressed as a volume or mass per time, for example, L hr1 and kg hr1, respectively. The ideal combustion process, that is, stoichiometric combustion, is that that burns the fuel completely. The deficit between stoichiometric combustion and incomplete combustion determines the percentage of combustion inefficiency (see Fig. 3.8 in Chapter 3). Therefore, at or above theoretical air, the process is 100% efficient. To avoid products of incomplete combustion (PICs), especially carbon monoxide (CO), excess air is usually added. The excess air or excess fuel for a combustion system is based on the stoichiometric air-fuel ratio, the precise, ideal fuel ratio in which chemical mixing proportion is reached. For safety reasons, prevention of explosive conditions from fouling and generation of high temperatures, and because fuel composition and conditions even in well-controlled reactors vary, combustors are designed to achieve “on-ratio” combustion, that is, requiring a known amount of excess air, often 10%–20% above the expect stoichiometric air value [1] (see Table 9.1). Fuel-lean mixtures have air content greater than the stoichiometric ratio; fuel-rich mixtures have air content less than the stoichiometric ratio. For fuels (CmHn) in the gas phase, the stoichiometric combustion reaction is [2] n n n (9.1) Cm Hn + m + ðO2 + 3:76N2 Þ ! mCO2 + H2 O + 3:76 m + N2 |fflffl{zfflffl} 4 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} 2 4 fuel
air
Air Pollution Calculations. https://doi.org/10.1016/B978-0-12-814934-8.00009-0 © 2019 Elsevier Inc. All rights reserved.
207
208
Air pollution calculations
TABLE 9.1 Approximate O2 and CO2 in flue gas at various excess air conditions Percent volume of CO2 in flue gas Percent excess air
Natural gas
Propane butane
Fuel oil
Bituminous coal
Anthracite coal
Percent O2 in flue gas for all fuels
0
12
14
15.5
18
20
0
20
10.5
12
13.5
15.5
16.5
3
40
9
10
12
13.5
14
5
60
8
9
10
12
12.5
7.5
80
7
8
9
11
11.5
9
100
6
6
8
9.5
10
10
Data from Engineering Toobox, Stoichiometric Combustion and Excess of Air, January 31, 2019, Available from: https://www.engineeringtoolbox.com/ stoichiometric-combustion-d_399.html.
For solid and liquid fuels (CaHbOcNdSe), the stoichiometric combustion reaction is [2] b d + 3:76x N2 + eSO2 Ca Hb Oc Nd Se + xðO2 + 3:76N2 Þ ! aCO2 + H2 O + |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} 2 2 fuel
(9.2)
air
where x ¼ a + b4 2c + e Note that the CaHbOcNdSe empirical formula of solid or liquid fuel is assumed to account of 100% of the mass and is calculated assuming the fuel is a dry, ash-free elemental composition. Combustion efficiency increases with excess air until heat loss in the amount of excess air reaches the heat loss to a point where the net efficiency begins to drop (see Fig. 9.1).
FIG. 9.1 Air-fuel ratio’s effect on combustion efficiency and generation of carbon monoxide. (Data from Engineering Toobox, Stoichiometric Combustion and Excess of Air. (2003). July 16, 2018. Available: https://www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html).
Thermal reactions Chapter
9
209
What is the reaction of methane burned at 25% excess air? Since air is about 79% N2 and 21% O2, the N2-to-O2 mole ratio is 3.76. Thus, the stoichiometric combustion of CH4 is CH4 + 2ðO2 + 3:76 N2 Þ ! CO2 + 2 H2 O + 7:52 N2 Therefore, CH4 combusted with 25% excess air can be expressed as CH4 + 1:25 2ðO2 + 3:76 N2 Þ ! CO2 + 2 H2 O + 0:5 O2 + 9:4 N2 Note that the amounts of carbon dioxide and water remain the same after the theoretical air concentration is achieved.
Propane is being combusted at 80% theoretical air, that is, 20% air deficiency. What is the combustion efficiency of this reaction? C3 H8 + 5 ð0:80Þ ðO2 + 3:76N2 Þ ! 2CO + CO2 + 4H2 O + 15:04N2 Calculate the volume: Volume of product species is proportional to the coefficient of each species, coefficient of CO2 1 ¼ ¼ 0:33 or 33% by volume: CE of C3H8 gas ¼ coefficient of CO2 + coefficient of CO 1 + 2 Calculate the mass: From the stoichiometry, there is one mole of CO2 and 2 mol of CO in this reaction, thus, CE ¼
9.3
mass CO2 12 + 32 ¼ ¼ 0:44 or 44% by mass mass CO2 + mass CO ð12 + 32Þ + 2ð12 + 16Þ
VOLATILITY OF COMBUSTION PRODUCTS
These combustion principles must be applied to air pollution control technologies. For example, one of the cross media concerns that tie solid waste to air quality is the thermal treatment of waste. This is commonly done not only with excess O2, that is, incineration, but also when O2 is absent or at very low concentrations, that is, pyrolysis. Depending on the fuel, that is, waste stock, numerous volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs), notably polycyclic aromatic hydrocarbons (PAHs), dioxins and furan, are generated under high temperatures in waste combustors and in various industrial reactors. As evidence, Figs. 9.2 and 9.3 indicate that the highly varying optimal temperature ranges to form VOCs and SVOCS. The aliphatic (carbon-chain) compounds in this fire, notably 1-dodecene, 9-nonadecane, and 1hexacosene, are generated in higher concentrations at lower temperatures (about 800°C), whereas the aromatic (carbonring) compounds require higher temperatures. The total mass and the total concentrations of these thermally generated compounds continue to increase with temperature, but the chemical speciation changes [3, 4]. At some point, the temperature, pressure, sorbent, and oxygen conditions will supplant these compounds to form others that may be even more toxic, for example, the halogenated dioxins and furans. The engineer and plant operator seek the optimal combination of factors in the thermal process to completely convert as much of the organic mass to nontoxic products [5].
9.4
FLUE GAS
Combustion is used primarily for heat by changing the potential chemical energy of the fuel to thermal energy. This occurs in a fossil fuel-fired power plant, a home furnace, or an automobile engine. Combustion is also used as a means of destruction for our unwanted materials. This decreases the volume of a solid waste by burning the combustibles in an incinerator. The combustion reactor, for example, kiln, accepts combustible solids, liquids, and gases, with undesirable properties such as odors, to a high temperature in an afterburner system to convert them to less objectionable gases. Note that incineration is often used to denote any thermal destruction of waste, but the process covers a large O2 range. As O2 concentrations approach zero, pyrolysis dominates, but combustion occurs at higher available O2 concentrations. As a general rule, a fuel is assumed to produce dry, stoichiometric gas emissions at a rate of 0.25 N m3 M1 J1, deducting for the water vapor that is formed during combustion [6]. The caloric value (Hinf) of the fuel is expressed in energy per mass (e.g., megajoules per kg, MJ kg1). Since air contains 21% O2, the dry flue gas at y% O2 is expressed as
210
Air pollution calculations
FIG. 9.2 Selected hydrocarbon compounds generated in a low-density polyethylene pyrolysis; low-oxygen conditions, in four temperature regions. (Source: D.A. Vallero, Fundamentals of Air Pollution, 5th ed., Elsevier Academic Press, Waltham, MA, 2014, p. 999 pages cm; Data from R. Hawley-Fedder, M. Parsons, F. Karasek, Products obtained during combustion of polymers under simulated incinerator conditions: I. Polyethylene, J. Chromatogr. A 314 (1984) 263–273).
FIG. 9.3 Total aliphatic (chain) hydrocarbons versus polycyclic aromatic hydrocarbons (PAHs) generated in a low-density polyethylene pyrolysis in four temperature regions. Source: D.A. Vallero, Fundamentals of Air Pollution, 5th ed., Elsevier Academic Press, Waltham, MA, 2014, p. 999 pages cm; Data from R. HawleyFedder, M. Parsons, F. Karasek, Products obtained during combustion of polymers under simulated incinerator conditions: I. Polyethylene, J. Chromatogr. A 314 (1984) 263–273.
Thermal reactions Chapter
Dry flue gas in Nm3 kg1 ¼ Hinf 0:25
21 21 y
9
211
(9.3)
The wet flue gas at z% H2O is compared with saturation and is expressed as Wet flue gas in Nm3 kg1 ¼ Hinf 5:25
21 100 21 y 100 z
(9.4)
Actual flue gas volume (VAFG) at T (°C) is expressed as VAFG in m3 kg1 ¼ Hinf 525
21 y 273 + T 100 z 273
(9.5)
which is simplified to VAFG ¼ 2Hinf
273 + T ð21 yÞ ð100 zÞ
A waste with an average caloric value of 5 MJ kg21 is burned at 100°C in a reactor with 14% oxygen content and 15% water vapor content. What is the actual flue gas volume for this system? VAFG ¼ 2Hinf VAFG ¼ 2 5
273 + T ð21 y Þ ð100 z Þ
273 + 100 ð21 14Þ ð100 15Þ
VAFG ¼ 6:27 m3 kg1
Flue gas volume is important in the design and operation of air pollution control equipment. For example, it is often preferable to lower temperatures to prevent the formation of SVOCs during incineration; for example, chlorinated dioxins and furans can form catalytically in flue gas between about 250 and 350°C, so cooling to <230°C would substantially inhibit the formation of these toxic compounds [7]. There are a number of ways to cool flue gases, including evaporative cooling (humidification, for example, using water sprays), which increases the area of heat transfer. Addition of cooler air to the flue gas and heat recovery are also options. Addition of cooler air, however, would increase VAFG. Therefore, if a large volume of air is added, the combustion efficiency would be hampered.
9.5
THERMAL POLLUTANT DESTRUCTION
Plant managers, chemists, engineers, and regulators want to know the efficiency of combustion. This is important for determining the extent to which a process is functioning as designed. For example, is the fuel being burned as efficiently as needed to boil water to turn an electric turbine to produce a sufficient amount of energy? If the combustion efficiency is too low, the steam generation is insufficient to move the turbine, and fewer calories of heat are produced. Combustion efficiency is also crucial to air pollution, since incomplete combustion generates toxic substances. A common calculation of combustion efficiency (CE) is the ratio of the actual combustion relative to complete combustion of a fuel by comparing the CO concentration with combined CO and CO2 concentrations: CCO2 100 (9.6) CE ¼ CCO2 + CCO where CCO2 ¼ concentration of carbon dioxide and CCO ¼ concentration of carbon monoxide. Note that the concentrations can be in any volume or mass units, since the CE is a unitless value between 0 and 100%. After a certain temperature is reached, CE increases with increasing chamber temperature. Other factors that affect CE include mixing and available O2.
212
Air pollution calculations
The efficiency of destroying toxic organic compounds by combustion is similar to CE, but rather than being a function of CO and CO2, waste incinerator performance is based on the destruction and removal efficiency (DRE), that is, the percentage of the number of molecules of an organic compound emitted to the atmosphere compared with the number of molecules entering the incinerator [8]. Commonly, for hazardous wastes, a minimum 99.9999% DRE is required. This means that one molecule of an organic compound is released to the air for every million molecules of the organic compound entering the incinerator. The DRE calculation is Win Wout 100 (9.7) DRE ¼ Win where Win is the mass feed rate of organic compound in the waste stream feeding the incinerator and Wout is the mass emission rate of the organic compound present in exhaust emissions prior to release to the atmosphere.
A regulatory agency requires that the combustion of 20 kg polychlorinated biphenyl (PCB) be incinerated in a hazardous waste incinerator at 99.9% efficiency and that the DRE be 99.9999%. The burn took 1 h. The exhaust rate was 1.5 L sec21. The emission rate was measured to be 950 μg m23 PCBs, 5 ppm CO, and 1% CO2. Does the incinerator meet the regulated standards? Since 1% ¼ 10,000 CO2 is 1%, combustion efficiency is as follows: 10000 CE ¼ 100 ¼ 99:95% 10000 + 5 The total volume of exhaust for the hour is 1.5 L sec1 3600 s hr.1 ¼ 5400 L ¼ 5.4 m3. Thus, at 950 μg m3, the total mass of PCBs exiting the stack is 950 μg m3 5.4 m3 ¼ 5130 μg PCB. Since 20 kg of PCB was fed into the incinerator, the DRE is 3 20 kg 109 μg kg1 5130 μg 4 5 100 ¼ 99:99997% 20 kg 109 μg kg1 2
Therefore, this incinerator appears to be meeting both CE and DRE standards.
Indeed, the number of nines in the DRE is an indicator of the toxicity of a compound. For example, a less toxic compound may only require four nines, that is, 99.99%, whereas a highly toxic compound like a PCB or dioxin would require more nines, for example, the six nines DRE (99.9999%). Combustion efficiency is defined in other ways, such as a measure of energy conservation from the fuel into useful energy, for example, steam generation [9]. This may be found by subtracting the heat content of the exhaust gases, expressed as a percentage of the fuel’s heating value, from the total fuel-heat potential, that is, 100%: stack heat losses 100 CE ¼ 100% fuel heating value Stack heat losses are calculated from measurements of gas concentration and temperature measurements, along with the fuel’s unique composition and heat content. The heat losses are primarily from the heated dry exhaust gases (CO2, N2, and O2) and from water vapor formed from the reaction of hydrogen in the fuel with O2 in the air. When water goes through a phase change from liquid to vapor, it absorbs a tremendous amount of heat energy in the process. This heat of vaporization, or latent heat, is usually lost and not recovered. This is evident from the white cloud that exits a stack on a cool day, that is, predominantly from condensing water vapor losing its latent heat to the atmosphere [9]. Table 9.2 gives combustion efficiency for fuel oil under varying conditions of temperature, CO2, and O2.
9.6
ACTIVATION ENERGY
Air pollutants can react with one another when they are forming (e.g., in a reactor), in the atmosphere or in any volume, for example, within a canister awaiting chemical analysis. Activation energy is needed to amount of energy needed for the reaction to take place, that is, the energy needed to bring all molecules in one mole of a substance to their reactive state at a given temperature. An exothermic reaction releases energy, whereas an endothermic reaction requires energy. If a catalyst is present, the reaction rate increases, that is, the catalyst decreases the demand for activation energy (see Fig. 9.4).
Thermal reactions Chapter
9
213
TABLE 9.2 Combustion efficiency of a fuel oil derived from net temperature (°C), that is, the difference of stack temperature (Tstack) and supply temperature (Tsupply), relative to carbon dioxide and oxygen percent and temperature Net temperature (Tstack 2 Tsupply) %CO2
%O2
149
160
171
182
193
204
216
227
238
249
260
288
316
15.6
0.0
31
31
31
30
30
30
30
30
29
29
29
28
28
14.1
2.0
31
31
30
30
30
30
29
29
29
29
28
28
27
13.4
3.0
31
30
30
30
30
29
29
29
29
28
28
28
27
12.6
4.0
31
30
30
30
29
29
29
29
28
28
28
27
27
11.9
5.0
30
30
30
29
29
29
29
28
28
28
28
27
26
11.1
6.0
30
30
29
29
29
29
28
28
28
27
27
26
26
10.4
7.0
30
29
29
29
29
28
28
28
27
27
27
26
25
9.6
8.0
29
29
29
28
28
28
27
27
27
26
26
25
24
Source of data: I. TSI, Combustion Analysis Basics: An Overview of Measurements, Methods and Calculations Used in Combustion Analysis. (2004). Available: http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Handbooks/CA-basic-2980175.pdf (Accessed 23 July 2018).
Energy
Activation energy (without catalyst)
Activation energy (with catalyst)
Reactants
Heat released to environment
Products
Direction of exothermic reaction
Energy Activation energy (without catalyst)
Activation energy (with catalyst) Absorbed heat
Products
Reactants
Direction of endothermic reaction FIG. 9.4 Exothermic reaction (top) and endothermic reaction (bottom), showing the effect of catalysis on activation energy.
214
Air pollution calculations
This energy requirement is a reminder that reaction rates are temperature-dependent. Indeed, the rate constant k reflects this in the Arrhenius equation: Ea
k ¼ A eRT
(9.8)
where A is the preexponential factor (constant), Ea is the Arrhenius activation energy, R is the universal gas constant, and T is the absolute temperature.
Nitrogen dioxide and carbon monoxide are reacting in a chamber at 70. The measured rate constant for this reaction is at 701°K is 2.57 M21 s21 and at 895°K is 567 M21 s21. What is the activation energy? Convert the Arrhenius equation to natural log form: ln k ¼
Ea + ln A RT
(9.9)
ln k1 ¼
Ea + ln A RT1
(9.10)
ln k2 ¼
Ea + ln A RT2
(9.11)
Set up a “two-point” form of the Arrhenius equation:
The difference of Eq. (8.18) from Eq. (8.17):
1 1 T2 T1 1 1 567 M s Ea 1 1 ¼ ln 1 1 R 701 K 895 K 2:57 M s ln
k1 Ea ¼ k2 R
5:40 ¼
Ea R
Ea ¼ 5:40 Ea ¼ 5:4 ¼
3:09 101 K
(9.12)
K R 3:09 101
K J ¼ 1:45 105 J mol1 ¼ 145 kJ mol 1 8:314 1 mol K 3:09 10
We could have also solved for Ea at the outset: Ea ¼
9.7
RT 1 T2 k1 ln ðT1 T2 Þ k2
(9.13)
NITROGEN AND SULFUR
Nitrogen species important to air quality include both oxidized and reduced forms. Atmospheric N reactions can be more complicated than may be inferred from Reactions (8.11) and (8.12). For example, the NOx reaction from NO to NO2 likely involves water and an intermediate, that is, nitrous acid (HONO): 4NO + O2 + 2H2 O ! 4HONO
(9.14)
Combustion processes, both stationary (e.g., power plants) and mobile (e.g., automobiles), entail reactions that oxidize both atmospheric N2 and nitrogen in the fuel. As discussed in Chapter 8, the term “oxides of nitrogen” (NOx) does not include all oxidized species, just NO and NO2. However, the larger suite of N air pollution compounds, that is, total reactive nitrogen (NOy), includes not only NO and NO2 but also their oxidation products [10]. Combustion generates other vaporphase and aerosol species in NOy including HONO, nitric acid (HNO3), peroxyacetyl nitrate (PAN), organic nitrates, and particulate nitrates: NOy ¼ NO2 + NO + HNO3 + PAN + 2N2 O5 + HONO + NO3 + NO3 compounds + NO3 aerosols
(9.15)
Thermal reactions Chapter
9
215
The NOy compounds are products of reactions with NOx emissions or from transformations in the troposphere [11]. Most of the compounds originate from combustion processes. The N source is either molecular nitrogen in the troposphere, that is, in the air intake, or the N-content of the fuel. The tropospheric N2 makes up the largest share of gaseous content in the combustion air (79% by volume). When pressure and temperatures are sufficiently high, for example, in an internal combustion engine or industrial boiler, the plentiful N2 will react with O2. NO makes up about 90%–95% of this thermal NOx. Other nitrogen oxides can also form at high heat and pressure, especially NO2. Most motor vehicles around the world still employ high-temperature/highpressure internal combustion engines. Thus, such mobile sources contribute largely to the tropospheric concentrations of NOx, making it a major mobile source air pollutant in terms of human health directly (e.g., respiratory toxicity of NO2) and indirectly (i.e., NOx as the key components in tropospheric ozone production). These conditions of high temperature and pressure can also exist in boilers such as those in power plants, so NOx is also commonly found in high concentrations leaving fossil fuel power generating stations. In addition to the atmospheric molecular nitrogen as a precursor of nitrogen air pollutants of combustion, fossil fuels themselves contain varying concentrations of N. Nitrogen oxides that form from the fuel or feedstock are called “fuel NOx.” Unlike the sulfur compounds, which mainly exit stationary source stacks as vapor-phase compounds (e.g., SO2 and other oxides of sulfur), a significant fraction of the fuel nitrogen burned in power plants and other stationary sources remains in the bottom ash or in unburned aerosols in the gases leaving the combustion chamber, that is, the fly ash. Nitrogen oxides can also be released from nitric acid plants and other types of industrial processes involving the generation and/or use of nitric acid (HNO3). At temperatures far below combustion, such as those often present in the ambient atmosphere, NO2 can form the molecule NO2-O2N or simply N2O4 that consists of two identical simpler NO2 molecules. This molecular configuration is known as a dimer. The dimer N2O4 is distinctly reddish brown and contributes to the brown haze that is often associated with photochemical smog incidents. In addition to the health effects associated with NO2 exposure, much of the concern for regulating emissions of nitrogen compounds is to suppress the reactions in the atmosphere that generate the highly reactive molecule ozone (O3). Nitrogen oxides play key roles in O3 formation. Ozone forms photochemically (i.e., the reaction is caused or accelerated by light energy) in the lowest level of the atmosphere, known as the troposphere, where people live. Nitrogen dioxide is the principal gas responsible for absorbing sunlight needed for these photochemical reactions. So, in the presence of sunlight, the NO2 that forms from the NO incrementally stimulates the photochemical smog-forming reactions because nitrogen dioxide is very efficient at absorbing sunlight in the ultraviolet portion of its spectrum. This is why ozone episodes are more common during summer and in areas with ample sunlight. Other chemical ingredients, that is, ozone precursors, in O3 formation include volatile organic compounds (VOCs) and carbon monoxide (CO). Governments regulate the emissions of precursor compounds to diminish the rate at which O3 forms. Sulfur, like nitrogen, is oxidized during combustion but is also an important component of particulate matter (PM; see Table 9.3). For example, diesel particulate matter is formed by a number of simultaneous physical processes during cooling and dilution of exhaust, that is, nucleation, coagulation, condensation, and adsorption. The core of the particles is formed by nucleation and coagulation from primary spherical particles consisting of solid carbonaceous matter (known as elemental carbon, EC) and ash (metals and other elements). By coagulation, adsorption, and condensation, various organic and S compounds (e.g., sulfates) are added and combined with other condensed material [12]. The small diameter of diesel PM (<0.5 μm) makes for very large surface areas. <0.5 mm, these particles have a very large surface area per gram of mass, which allows them to adsorb large quantities of ash, organic compounds, and sulfate (see Fig. 9.5). The specific surface area of the EC core is approximately 30–50 m2 g1. The organic constituents originate from unburned fuel, engine lubrication oil, and small quantities of partial combustion and pyrolysis products. During combustion, S compounds in the fuel are oxidized to sulfur dioxide (SO2). From 1% to 4% of fuel, S is oxidized to form H2SO4. Upon cooling, this sulfuric acid and water condense into an aerosol that is nonvolatile under ambient conditions [12]. The oxidized chemical species of sulfur and nitrogen (e.g., sulfur dioxide (SO2) and nitrogen dioxide (NO2)) form acids when they react with water. This can occur in any media, for example, the atmosphere (i.e., acid deposition) and in mining waste and ash runoff to surface waters and groundwater. The lowered pH is responsible for numerous environmental problems. Many compounds contain both N and S along with the typical organic elements (C, H, and O). The reaction for the combustion of such compounds, in general form, is b d (9.16) H2 O + N2 + eS Ca Hb Oc Nd Se + 4a + b 2cÞ ! aCO2 + 2 2
216
Air pollution calculations
TABLE 9.3 Typical chemical composition (percent) of fine particulate matter (PM2.5) Eastern United States
Western United States
Diesel PM2.5
Elemental carbon
4
15
75
Organic carbon
21
39
19
Sulfate, nitrate, and ammonium
48
35
1
Minerals
4
15
2
Unknown
23
–
3
Source: US Environmental Protection Agency. Air quality criteria for particulate matter. External review draft, October 1999.
FIG. 9.5 Diesel particulate matter. Source: US Environmental Protection Agency, Health Assessment for Diesel Engine Exhaust. Report No. EPA/600/8-90/057F. National Center for Environmental Assessment. Washington, DC, 2002.
Reaction (9.16) demonstrates the incremental complexity as additional elements enter the reaction. In the real world, pure reactions are rare. The environment is filled with mixtures and heterogeneous reactions. Reactions can occur in sequence, parallel, or both. For example, a feedstock to a municipal incinerator contains myriad types of wastes, from garbage to household chemicals to commercial wastes, and even small and sometimes large industrial wastes that may be illegally dumped. The N content of typical cow manure is about 5 kg per metric ton (about 0.5%). If the fuel used to burn the waste also contains S along with the organic matter, then the five elements will react according to the stoichiometry of Reaction (8.15). Redox occurs in certain regions of a combustion chamber, stack, or other locations inside the combustor or in the ambient air. Recall from Chapter 8 that the formation of sulfur dioxide (SO2) and nitric oxide (NO) by acidifying molecular sulfur, for example, in coal, includes the redox reaction: SðsÞ + NO3 ðaqÞ ! SO2 ðgÞ + NOðgÞ
(9.17)
The oxidation half-reactions for this reaction are S ! SO2
(9.18)
S + 2H2 O ! SO2 + 4H + + 4e
(9.19)
The reduction half-reactions for this reaction are NO3 ! NO
NO3 + 4H + 3e ! NO + 2H2 O +
Therefore, the balanced oxidation-reduction reactions are
(9.20) (9.21)
Thermal reactions Chapter
9
4NO3 + 3S + 16H + + 6H2 O ! 3SO2 + 16H + + 4NO + 8H2 O
4NO3 + 3S + 4H ! 3SO2 + 4NO + 2H2 +
217
(9.22) (9.23)
This indicates the importance of the sulfur content of a fuel in estimating the amount of SO2 that would be expected to be emitted during combustion.
A company receives weekly deliveries of coal and takes a sample of each week, with the analyses below. Calculate the weighted 28-day average that can be used to predict sulfur dioxide emissions.
Week
Percent sulfur
kJ kg21
kg SO2 per gigajoule (kg GJ21)
Coal burned (tonnes)
1
2.9
26,229
2.3
23
2
2.7
26,693
2.2
34
3
3.2
25,068
2.7
47
4
2.4
25,997
2.0
27
The emission rates will be weighted according to the amount of fuel burned each week. Calculate the heat input during the period represented by each sample for weeks 1 through 4: Week 1: Week 2: Week 3: Week 4:
23 t 26,229 kJ kg1 1000 kg tonne1 ¼ 6.03 108 kJ 34 t 26,693 kJ kg1 1000 kg tonne1 ¼ 9.08 108 kJ 47 t 25,068 kJ kg1 1000 kg tonne1 ¼ 1.18 109 kJ 27 t 25,997 kJ kg1 1000 kg tonne1 ¼ 7.01 108 kJ
Add the product of the heat input times the emission rate for each sample and divide by the total heat input for the 30-day period. Weighted 30-day average SO2 emission rate: h
6:03 108 kJ hr1 2:3 kg GJ1 + 9:08 108 kJ hr1 2:2 kg GJ1 + 1:18 109 kJ hr1 2:7 kg GJ1
i + 7:01 108 kJ hr1 2:0 kg GJ1
6:03 108 + 9:08 108 + 1:18 109 + 7:01 108 kJ 2:4 kg SO 2 per gigajoule Thus, this plant would be expected to have emitted, on average, a 2.4 kg of sulfur dioxide over this 4 week period, based on their fuel supply. These types of calculations are needed to ensure compliance with air quality standards. In the United States, most states conduct these rolling average calculations in units of BTU per pound, pounds of SO2 per million BTUs (mmBtu), and tons.
The air quality regulatory agency requests a 30-day rolling weighted average in addition to the previously reported 28-day weighted average. The company has purchased lower-quality coal the most recent 2 weeks. Calculate the rolling weighted average SO2 emission rates. Week
Percent sulfur
kJ kg21
kg SO2 per gigajoule (kg GJ21)
Coal burned (tonnes)
1
2.9
26,229
2.3
23
2
2.7
26,693
2.2
34
3
3.2
25,068
2.7
47
4
2.4
25,997
2.0
27
5
3.5
24,590
2.9
48
6
3.3
25,875
2.8
52
218
Air pollution calculations
There are four samples for the latest 30 days, so these will be used in the rolling average. The emission rates will be weighted according to the amount of fuel burned each week. Week 5: 49 t 24,590 kJ kg1 1000 kg tonne1 ¼ 1.20 109 kJ Week 6: 52 t 25,875 kJ kg1 1000 kg tonne1 ¼ 1.35 109 kJ The rolling, weighted 30-day average SO2 emission rate for weeks 2–5 is h
9:08 108 kJ hr1 2:2 kg GJ1 + 1:18 109 kJ hr1 2:7 kg GJ1 + 7:01 108 kJ hr1 2:0 kg GJ1
i + 1:20 109 kJ hr1 2:9 kg GJ1
9:08 108 + 1:18 109 + 7:01 108 + 1:20 109 kJ 2:5 kg SO 2 per gigajoule The rolling, weighted 30-day average SO2 emission rate for weeks 3–6 is h
1:18 109 kJ hr1 2:7 kg GJ1 + 7:01 108 kJ hr1 2:0 kg GJ1 + 1:20 109 kJ hr1 2:9 kg GJ1
i + 1:35 109 kJ hr1 2:8 kg GJ1
9:08 108 + 1:18 109 + 7:01 108 + 1:35 109 kJ 2:6 kg SO 2 per gigajoule This is problematic in that the SO2 emission rates are increasing, which means that the rolling, weighted average emission rates increase by 0.1 kg GJ1 each week on record.
The emitted nitrogen and sulfur compound can cause direct and indirect damage to health and ecosystems, for example, acid deposition, and to materials and structures (see Chapter 8).
9.8
METALS IN THERMAL REACTIONS
Metals like mercury (Hg) and lead (Pb) are emitted into the atmosphere from combustion and other thermal reactions. The metals are usually trace constituents of the fuel but may also be intentionally added as catalysts and for other purposes during the thermal processes, for example, organometallic compounds in gasoline or diesel fuel. Metals may also scavenge from the walls or components of the chamber or reactor. The reactions can be a single step, for example, the direct formation of a metal oxide from a reaction with the O2 in the air, or lead to various metal compounds through a series or branching of reactions depending on the conditions of the flue gas. The flue gas can have widely varying regions of flow rate (e.g., change in contact time), temperature, O2 concentration, particulate matter (PM) concentration, availability of sorbents, redox, and reactants (e.g., oxide gases and acids). These and other conditions can lead to different reactions, even if the fuel does not change.
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