Combustion Efficiency and Air Quality

At the beginning were the prophets. Heaven told them the future by hotline.

Recently, Gilardi and Karle (1991) published a review on structural investigations of energetic materials in the crystalline state. The purpose of the study was to promote the understanding of the relationship between structure and function and to facilitate the synthesis of new materials. This study was based on the Cambridge Structural Database (Allen et al., 1983) and also on the Energetic Materials Structural Database of the (U.S.) Naval Research Laboratory (NRL) with over 300 relevant structures (Gilardi et al., 1992).

Inorganic azides can be classified into (i) ionic salts (e.g., NaN₃), (ii) heavymetal azides (e.g., AgN₃, PbN₃), and (iii) covalently bound nonmetal azides (XN₃: X = H, R₂B, R₃Si, NO, NO₂, R₂P, halogen; R = alkyl, aryl) (Jones, 1973). Whereas the ionic salts are reasonably stable materials and sodium azide is prepared on a commercial scale, the major use of the heavy-metal azides depends on their explosive nature (Greenwood and Earnshaw, 1984). Particularly, lead azide is used for detonators because of its reliability under a variety of adverse, especially damp conditions (Köhler and Meyer, 1991). Although the covalent azides have been known since the beginning of this century, it is only in recent years that these azides have found use in preparative chemistry and that their structures have been elucidated (Tornieporth-Oetting and Klapötke, 1993a). The characterization and usage of the covalent nonmetal azides have obviously been hampered by their thermodynamic and kinetic instability. HN₃ and all of the halogen azides are very hazardous. Especially dangerous are the pure halogen azides in the condensed phase (Dehnicke, 1983). Violent explosions can also occur in the gaseous state upon sudden variations of the pressure (Dehnicke, 1983).

Combustion of energetic solids is the basis of rocket propulsion for space exploration and military technologies. Accurate models of combustion that contain chemical and fluid-mechanical details are greatly needed because atmospheric contamination and cost considerations limit ground-based testing. International disarmament treaties mandate disposal of stockpiled energetic materials. However, conventional disposal methods, such as open-pit burning and detonation, are increasingly restricted by environmental regulations. Description of the gaseous emission products frequently must be given before combustion is authorized. Manipulation of the combustion process may be necessary. Hence, combustion processes must be understood and predicted with ever greater accuracy.

The partial oxidation of methane known as the Sachsee process, later modified by others several times, is a procedure for the production of acetylene from methane In the course of the process a mixture of methane and pure oxygen in a ratio of 2:1, preheated to 973 K, is fed into a flame reactor, where in an autotherm reaction the combustion takes place within 0.02-0.03 s, producing acetylene (Fig. 1). The product gas is quenched to protect acetylene from polymerization. Acetylene and oxidation products are separated from the cooled product gas by suitable methods (Németh et al, 1987).

The relations between energetics, the environment, and firing technology are extremely broad. The most important problems of environmental pollution caused by the production of energy are as follows:

Among the many coal conversion processes are those that produce liquid fuels from solid coal. It is not easy to do this, and one underlying problem is apparent even from a casual inspection of the mass or elemental formulas of coal and some other common fuels. A typical value for the carbon/hydrogen mass ratio of a bituminous coal is about 15; for gasoline, the value is about 6, and for natural gas, 3. In terms of an elemental formula, coal might be CH_0.8, gasoline CH₂, and natural gas CH₄. To make liquid fuel from coal requires either addition of hydrogen or loss of carbon.

Although environmental factors have had an impact on industrial production throughout history, their role has only recently become widely known owing to local, regional, and global environmental problems. The nature of the relation between production and environmental problems is highly empirical, as evidenced by the relatively undeveloped description and management system of these problems (Adonyi, 1993). An outstanding example of this is that industrial production is often blamed for generating toxic wastes, but the inducting role of the consumption structure is hardly taken into consideration. The result is the low efficiency of environmental activities and a lack of methodology for licensing environmental technologies (Gauweiler, 1992).

Human exposure to combustion particulates is a risk imposed on society justified by the need for the beneficial uses of energy. The respiratory system is the major route for this exposure in the form of airborne suspensions (aerosols) of these particles (U.S. EPA, 1982). Like Prometheus (Fig. 1), we find ourselves chained to the rock of the essential benefits of combustion while at the same time suffering the deterioration of the internal human systems necessary for life support. It is common knowledge that particle exposure, particularly from combustion, is a source of lung disease. In broad terms, outdoor particulates exhibit a bimodal size distribution ((U.S. EPA, 1986a) consisting of fine particles (less than 2.5 μm in diameter, with peak size concentration about 0.9 p.m) and coarse particles (> 2.5 μm in diameter with peak concentrations in the size range of 10-20 μ.m). Figure 2 illustrates this distribution, indicating some of the common constituents in each size range. The coarse particles include reentrained surface dust, salt spray, and particles formed by mechanical processes such as crushing and grinding. Particles from combustion general fall into the fine range and occur in two size categories: condensation nuclei and accumulation mode. The condensation nuclei are generally considered to range in size from 0.005 to 0.05 μm in diameter and result from cooling condensation of vapors or plasmas produced by high-temperature processes in combustion. Accumulation mode particles generally range from 0.5 to 2.0 μm in diameter and form principally by coagulation or are grown through vapor condensation of short-lived particles originally in the nuclei mode. Particles in the accumulation mode normally do not grow into the size range of the coarse mode ((U.S. EPA, 1986a). It is well known that fine particles evade the natural defenses of the human respiratory tract. Often this fraction of exposure is designated respirable suspended particulates (RSP) to distinguish this feature from exposure to the total quantity of suspended particulates (TSP). In 1987, the U.S. Environmental Protection Agency (EPA) introduced new annual and 24-hour standards for particulate matter, using a new indicator, PM₁₀, that includes only those particles with an aerodynamic diameter smaller than 10 μm ((U.S. EPA, 1991). An additional measure, PM_2.5, was also developed that includes only particles smaller than 2.5 μm in diameter.

Coal and oil combustion produces fly ash particulates whose dispersion into the environment represents one of the major sources of pollution.

Ambient airborne particulate matter has associated with it thousands of organic compounds (Graedel et al.,1986). These compounds are derived from primary emissions, secondary reactions from gas-phase components to yield particulate-bound species, and reactions on the particulates themselves. In attempting to assess the health impacts of particulate-bound organic matter, one might consider acute or chronic human illness as end points. Our studies have largely focused upon carcinogenic substances thought to be associated with the chronic disease cancer.

The quality of indoor air is influenced by chemical contamination from several sources. The main sources of indoor air pollution are infiltration of the outdoor air, volatile compounds from contaminated ground, and emissions from building materials. Naturally, the indoor use and generation of chemicals are relevant sources too. The research activities of the Institute for Ecological Chemistry have been focused on the behavior of semivolatile organic compounds (SVOCs) coming from wood preservation paints used indoors, case studies following problems dealing with indoor air quality, and the occurrence of indoor volatile organic compounds (VOCs) in the Bavarian region.

Both nationally and internationally, there is a growing interest in decision-support tools to assist decision makers in defining optimal combinations of abatement measures concerning a wide range of pollutants and their adverse effects. The importance given to cost-effectiveness and optimization has increased as the size of the investments needed to reduce environmental deterioration has been realized. Usually, each abatement measure influences the emissions of a number of pollutants, and some components are closely related in terms of abatement measures. This fact requires that different pollutants and abatement measures are considered as far as possible in an integrated way.