2 CombustionCombustion occurs when fossil fuels, such as natural gas, fuel oil, coal or gasoline, react with oxygen in the air to produce heat. The heat from burning fossil fuels is used for industrial processes, environmental heating or to expand gases in a cylinder and push a piston. Boilers, furnaces and engines are important users of fossil fuels.Fossil fuels are hydrocarbons, meaning they are composed primarily of carbon and hydrogen. When fossil fuels are burned, carbon dioxide (CO2) and water (H2O) are the principle chemical products, formed from the reactants carbon and hydrogen in the fuel and oxygen (O2) in the air.
3 Chemical BalanceThe combustion is a rapid oxidation process. The simplest example of hydrocarbon fuel combustion is the reaction of methane (CH4), the largest component of natural gas, with O2 in the air. When this reaction is balanced, or stoichiometric, each molecule of methane reacts with two molecules of O2 producing one molecule of CO2 and two molecules of H2O. When this occurs, energy is released as heat.
4 Three elements important to real combustion are missing in this simple equation. • Air is composed of about 79% nitrogen (N2), so there is a lot of N2 that enters the combustion with the O2 and is released with the flue gas. This has a huge effect on the basic size of a combustion system. One should keep in mind that the mass ratio of air to fuel is usually more than 10:1. Fuel is a very small part of the mass flow through the system.• Real systems don’t operate at a stoichiometric ratio = A little extra air is required for all combustion, so there is extra O2 (and N2) on both sides of the equation. How much extra air, how we control it and how we measure it are important subjects we will discuss later.• Most fuel has contaminants (ash, sulfur, nitrogen, etc.) that may or may not participate in the combustion, but which will appear in the flue gas as air pollutants.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
5 Excess AirThe amount of air chemically required to burn the fuel is frequently called the stoichiometric air flow. Alternatively, when the fuel and air flow are chemically or stoichiometrically balanced, we say that the equivalence ratio is equal to one.The equivalence ratio is defined as “the actual air-fuel ratio divided by theoretical or stoichiometric air-fuel ratio.”Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
6 Excess AirReal combustion systems operate with more air than the stoichiometric air requirement in order to avoid the emissions, such as CO, that result from incomplete combustion. Products of incomplete combustion result when some of the fuel does not mix with enough air.Excess air is normally expressed as a percent of the stoichiometric flow and can be any value from near zero to several hundred percent.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
8 Excess AirOperation at the minimum practical excess air is desirable for several reasons:Lower air flow means a lower stack flow rate – less hot gas going up the stack, which means improved thermal efficiency;NOx emissions are lower at lower excess air levels;Maximum load on some boilers is limited by the size of the air fans, so reducing the excess air requirement increases the maximum load.So two objectives of good combustion performance – and hence low emissions – are to:design and maintain a system which is capable of low excess air levels,continuously operate that system very close to the minimum practical air flow.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
9 Controlling excess air exactly is now an integral part of the air pollution control systems used in automobiles as well as many stationary sources.Theoretically, one could determine the excess air level in a system by measuring the fuel flow, calculating the air required for combustion and then subtracting this from the measured air flow.Fortunately, there is a far simpler and more precise method, and that is to measure the oxygen concentration (% O2) in the flue gas. Air enters the combustion zone with 20.9% O2 concentration. So whatever amount of O2 remains in the exhaust gas, by definition, represents the excess air. Using instruments developed since about 1970, O2 concentration in exhaust gas can be measured accurately, reliably and inexpensively. Note that when calculating air emissions it is standard practice to use O2 measured in a dry gas sample (% by vol., dry basis).Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
10 There is a simple relationship between excess air and flue gas oxygen concentration, given by Equation 4-8.This is more of a definition of excess air than a rigorously derived formula, but it is the form that pervades all the current air pollution data reduction methods. % O2 (by vol., dry basis) is the amount of oxygen remaining after combustion – %O2 is the initial minus the final amount of air, which, by conservation of mass, must be the amount of air consumed in the flame. So the excess air formula is simply the ratio of the amount of oxygen remaining divided by the amount consumed.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
11 This formula is not exactly the form that results if one started with a balanced chemical equation and went through a rigorous derivation. An assumption implicit in Equation 4-8 is that the combustion air volume is the same as the flue gas volume – which is only approximately correct. The exact formula for excess air includes fuel composition as well as % O2. Equation 4-8 is simple, it does not depend on fuel composition and it is widely accepted. However, note that 20.9% is the dry volume concentration of O2 in the atmosphere, so O2 should be measured dry. In most sampling systems the gas sample is extracted and cooled before it enters the instrument, yielding a dry measurement. If O2 is measured wet (hot flue gas with no condensation or using a dilution type sampling system), the O2 value must be corrected upward by the ratio of wet to dry flue gas volumes before it is used in standard data reduction formulas.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
13 Combustion-generated pollutants Combustion sources emit a number of different pollutants that can be divided into three categories:• Products of Incomplete Combustion (PIC): black smoke/soot, CO, organic compounds, and in some cases particulate matter.• Pollutants resulting from inorganic contaminants in the fuel: SOx, particulates, HCl, etc.• NOx: nitrogen oxides from atmospheric N2 and O2 breaking down in the combustion zone and reactingSource: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
15 ACID GASES: SULFUR OXIDES AND HCl Most fuels contain at least a trace of sulfur and many waste fuels contain significant amounts of chloride. These inorganic contaminants convert mostly to SO2 and HCl gases in the combustion zone. The potential emissions can be calculated from a fuel analysis as described in the preceding section.
17 Hydrochloric AcidMuch of the chloride in fuel will convert to HCl (hydrochloric acid) in the combustion zone. One may generally assume that all of the organic chlorides will be converted to HCl. Compounds like NaCl (sodium chloride) may pass through the combustion zone unchanged. In theory, a fuel that is low in hydrogen (coke or anthracite coal) can suppress HCl formation and drive the chloride to form Cl2 (chlorine gas). However, in practice, there are not many sources that fire low hydrogen fuels with significant amounts of chloride. Cement kilns can fire low hydrogen fuels like pulverized coke, but when they burn hazardous wastes with significant chlorides, there is enough hydrogen in the wastes that the chloride is emitted as HCl rather than Cl2. Sources that emit significant amounts of chlorine gas appear to be a special case that is left to other references. Chlorine gas in small amounts can be a factor in dioxin formation.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
18 Particulate MattersCombustion-generated particulate matter falls into two groups:• Large particles – those larger than 1-2μm that are descended primarily from large fuel particles and,• Fine particles – those smaller than about 1μm that are composed primarily of material that was in vapor form in the combustion zone and condensed as the exhaust gases cooled.Large particles derive almost entirely from the breakdown of even larger particles of fuel or ash. As a general rule, large particles are responsible for most of the weight of measured particulate emissions (Method 5).Fine particles derive primarily from the agglomeration or condensation of vapors or large molecules – the opposite process from the formation of large particles. Fine particles are frequently responsible for most of the visible emissions (Method 9).Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
19 Formation of Coke and Ash Particulate Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
20 Themost important particle formation mechanisms in air pollution sources include the following: • Physical attrition/mechanical dispersion• Combustion particle burnout• Homogeneous condensation• Heterogeneous nucleation• Droplet evaporation
21 Fine particles are formed “from the bottom up”; that is, they start as atoms or molecules that condense or agglomerate into particles. These particles can grow to be large particles, but their growth rate slows by the time they reach about 0.5μm in size. This size is a kind of temporary ceiling in particle growth. The fundamental explanation for this limit is that 0.5μm is about the size where particles become too big to undergo Brownian motion. They stop bouncing around, so they stop running into other particles to combine with; therefore, further growth is much slower. Although some fine particulate can be formed by the breakup of larger particles, for the most part, fine particles are composed of chemicals that are in a gaseous or vapor phase in the combustion zone. Obviously this includes carbon, sulfur oxides, and any heavy hydrocarbons that survive combustion. It also includes any metals and minerals that are heated to liquid or vapor phase during combustion.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
22 NITROGEN OXIDESNOx is an acronym for the sum of two compounds: nitric oxide (NO) and nitrogen dioxide (NO2).NOx = NO2 + NOBoth compounds are formed in a combustion zone, but NO usually accounts for 95% or more of the total. However, once emitted into the atmosphere, NO promptly oxidizes to NO2. Thus from a regulatory perspective it does not matter which compound is emitted because it will all show up as NO2 in the atmosphere. This has led to the regulation of the sum of the two or NOx. When determining the mass of emissions we need to assign the molecular weight – in this case the number is 46, which is the molecular weight of NO2.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
23 NOx is the one pollutant emitted by nearly all combustion sources independent of what is in the fuel or the configuration of the combustor. The reason is that even when there is no nitrogen in the fuel, NOx is created from the breakdown of atmospheric N2 and O2 in the combustion zone and the subsequent reaction to form NO.N2 + O2 ↔ 2NOFormation and control of NOx emissions will be discussed in detail latter.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
24 Dioxin-Furan Formation Polychlorinated-dibenzo-dioxins (PCDD) and polychlorinated-dibenzo-furans (PCDF) are very toxic compounds that are chemically stable and environmentally persistent. Their toxicity combined with their ability to accumulate in the food chain has led to regulation at levels measured in nanograms per cubic meter of exhaust gas.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
25 PCDD and PCDF are produced almost exclusively by combustion systems when the fuel contains chlorine. In addition, there must be some level of incomplete combustion because organic species cannot be created once all the carbon has oxidized to CO or CO2.Solid waste incinerators are the sources most likely to meet these conditions and, indeed they have been a significant source of PCDD and PCDF. Other sources include forest fires and automobiles – where the amount of chlorine in the fuel is very small, but the amount of fuel burned is very large. Utility boilers probably emit some PCDD, but the amount of organic material in their exhausts is undetectable by normal test methods and PCDD would have to be generated from carbon.
26 Dioxin-furan formation can be eliminated by completely burning the fuel. The most efficient method of generating dioxins is to incompletely burn chlorinated aromatic compounds such as chlorinated phenols once used as coolants in electric transformers. Any time there is a fire in an old transformer, downwind or adjacent areas are heavily contaminated with dioxins. Perhaps the next best way to emit dioxins is to incompletely burn any chlorine containing waste – such as PVC plastic. Municipal and hospital waste incinerators built before 1980 frequently fit in this category. Most of these sources generated substantial amounts of dioxins during combustion, although most of them have been regulated out of existence.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
27 Research since about 1990 has shown that dioxins and furans can be created in the exhaust gas after the combustion zone – primarily at temperatures in the range of °C (480° - 570°F), which can include the normal operating temperature of a dust collector.The mechanism is a chemical reaction between organic PIC and molecular chlorine (Cl2) in the presence of fly ash that apparently acts as a catalyst. This synthesis apparently requires a time span of several seconds.The way to avoid dioxin formation is (1) to completely burn the fuel, and (2) to cool the gas rapidly through the critical temperature range of 250°-300°C.This basically means avoiding the installation of a dust collector or a long duct where the exhaust gas is in the critical temperature range. A baghouse operating in the critical temperature range can be particularly troublesome because flue gas passes through the filter cake, which appears to be a catalyst for dioxin formation.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.
28 There is some evidence that dioxins might be formed from carbon; although this appears to be much less efficient than forming dioxins from organic material. This possibility reinforces the need for the dust collector to operate well outside of the 250°-300°C temperature range.Source: USEPA, APTI, 2012, Combustion Source Evaluation Student Manual.