Combustion Chemistry

13.3 Combustion Chemistry

Numerous chemical reactions can occur during combustion as illustrated by the following discussion. Consider, for example, one of the simplest combustion processes, the burning of methane in the presence of air. The overall chemical reaction is represented by:

CH 4 + 2 O 2 Æ CO 2 + 2 HO 2 (13.2) In fact, many more chemical reactions are possible. If the source of the oxygen is air, nitrogen will be

carried along with the oxygen at a ratio of approximately 79 moles (or volumes) of nitrogen for each

21 moles of oxygen. The nitrogen is a diluent for the combustion process, but a small (but important) fraction also oxidizes to form different oxides, commonly referred to as NO x . In addition, if the com- bustion is less than complete, some of the carbon will form CO rather than CO 2 . Because of the presence of free radicals in a flame, molecular fragments can coalesce and form larger organic molecules. When the material being burned contains elements such as chlorine, numerous other chemical reactions are possible. For example, the combustion of carbon tetrachloride with methane can result in the following products:

CH 4 + CCl 4 + O 2 + N 2 Æ CO 2 + 2 H O HCl N + + 2 + CO Cl + 2 + CH Cl 3 + (13.3)

CH Cl 2 2 + CHCl 3 + 2 5 C H Cl C H Cl + 2 4 2 + 2 3 C H Cl + ? 3

where “?” refers to a variety of trace and possibly unknown compounds that could potentially form. The goal of a well-designed combustor is to minimize the release of the undesirable products and convert as much of the organics to CO 2 , water, and other materials that may safely be released after treatment by an APCD. The combustion products of a typical properly operating combustor will contain on the order of 5 to 12% CO 2 , 20 to 100 ppm CO, 10 to 25% H 2 O, ppb and parts per trillion (ppt) of different POHCs and PICs, ppm quantities of NO x , and ppm quantities of SO x . If the combustion is poor (poor mixing of the oxygen and fuel or improper atomization of the fuel), localized pockets of gas will form where there is insufficient oxygen to complete the combustion. CO will form in these localized pockets, and because the reaction of CO to CO 2 is slow outside the flame zone (on the order of seconds at the postflame zone conditions), it will not be completely destroyed. This mode of failure is commonly termed “kinetics limiting,” because the rate at which the chemical reactions occur was less than the time that the combustor kept the constituents at the proper conditions of oxygen and temperature to destroy the intermediate compound.

Similar explanations can be offered for the formation of other PICs. Many are normal equilibrium products of combustion (usually in minutely small amounts) at the conditions of some point in the combustion process. Because of the similarity, PIC formation is commonly associated with CO emisssions.

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Test data (EPA, 1991, 1992) have shown that PICs rarely if ever occur when the CO level is less than 100 ppm (dry and adjusted to 7% O 2 ). They sometimes occur at CO levels over 100 ppm. It must be noted that PICs occur during the combustion of all fuels, including wood, petroleum products, and coal. Their formation is not characteristic just of the combustion of hazardous wastes.

Hydrogen forms two major products of combustion, depending on whether or not chlorine or other halogens are present in the waste. If chlorinated organic wastes are burned, then the hydrogen will preferentially combine with the chlorine and form HC1. The thermodynamics of HC1 formation are such that all but a small fraction (order of 0.1%) of the chlorine will form HC1; the balance will form chlorine gas. The reaction between free chlorine and virtually any form of hydrogen found in the

combustion chamber is so rapid that the Cl 2 :HC1 ratio will be equilibrium limiting in virtually all cases. Organically bound oxygen will behave like a source of oxygen for the combustion process. Fluorine, which is a more electronegative compound than chlorine, will be converted to HF during the combustion process. Like chlorine, it will form an equilibrium between the element and the acid, but the thermodynamics dictates this equilibrium to result in a lower F 2 :HF ratio than is the Cl 2 :HC1. Bromine and iodine tend to form more of the gas than the acid. Combustion of a brominated or iodinated material will result in significant releases of bromine or iodine gas. This fact is important to incinerator

design because Br 2 and I 2 will not be removed by simple aqueous scrubbers. Furthermore, because the production of the elemental gases is equilibrium limiting, modifications to the combustion system will not reduce their concentration in the flue gas significantly. It is, sometimes, possible to increase acid form by the addition of salts, although this is a relatively experimental procedure.

Organic sulfur forms the di- and trioxides during combustion. The vast majority of the sulfur will form SO 2 , with trace amounts of SO 3 also forming. The ratio of the two is equilibrium limiting. SO 3 forms a strong acid (H 2 S0 4 , sulfuric acid) when dissolved in water. It is thus readily removed by a scrubber designed to remove HC1. SO 2 forms sulfurous acid (H 2 SO 3 ), a weak acid that is not controlled well by

a typical acid gas scrubber that has been designed for HC1 removal. Nitrogen enters the combustion process both as the element, with the combustion air, and as chemically

bound in the waste or fuel. During combustion, the nitrogen forms a variety of oxides. The ratio between the oxides is governed by a complex interaction between kinetic and equilibrium relationships that is highly temperature dependent. The reaction kinetics are such that the reactions to create, destroy, and convert the various oxides from one to the other occur at a reasonable rate only at the high temperatures of the flame zone. Nitrogen oxides are, therefore, controlled by modifying the shape or temperature distribution of the flame and by adding ammonia to lower the equilibrium NO x concentration, and to

decrease N 2 emissions. NO x formation and control as well as the concept of equilibrium are discussed below.