The principal products of combustion are CO2, water vapor, and ash, which are respectively oxidation-reaction products of carbon, and hydrogen, and non-combustible materials in the fuel. However, when the combustion reactions do not proceed to their fullest extent, other substances, some of which are potentially harmful, can be produced. The types and concentrations of contaminants in the waste stream (flue gas) flowing from any incineration process depend on the process type, the waste being burned, and combustion conditions. Such pollutants derive from three sources: they or their precursors are present in the waste feed, they are formed in the combustion process because of incomplete oxidation, or they are created by reformation reactions in the gas cooling or APCD.
As discussed in Chapter 5, the products of primary concern, owing to their potential effects on human health and the environment, are compounds that contain sulfur, nitrogen, halogens (such as chlorine), and toxic metals. Specific compounds of concern include CO, NOx, SOx, HCl, cadmium, lead, mercury, chromium, arsenic, beryllium, dioxins and furans, PCBs, and polycyclic aromatic hydrocarbons. In addition, the total quantities of particulate matter and acid particles (which may largely be liquids condensed after emission) that escape the APCD are also considered independently. The following discussion focuses on the source and control of the following pollutants: particulate matter, acid gases, mercury (Hg), lead (Pb), and products of incomplete combustion. They are used to represent the pollutants from incineration that are of concern for possible health effects.
Particulate matter consists primarily of entrained noncombustible matter in the flue gas, and the products of incomplete combustion that exist in solid or aerosol form (and discussed separately later). Particle concentrations in the flue gas in the absence of control devices have been found to range from 180 to more than 46,000 mg per dry standard cubic meter (0.08 to more than 20 grains per dry standard cubic foot).
Particulate matter from waste combustors includes inorganic ash present in the waste and carbonaceous soot formed in the combustion process. The inorganic-ash fraction of the particulate matter consists of mineral matter and metallic species. These materials are conserved in the combustion process and leave the combustion chamber as bottom ash or fly ash. Soot is a product of incomplete combustion that consists of unburned carbon in the form of fine particles or as deposits on inorganic particles. High-molecular-weight organic compounds condense on the surface of the particles, particularly on the carbon, downstream of the combustor.