Combustion systems, pollutants

Ozone, known for its beneficial role as a protective screen against ultraviolet radiation in the stratosphere, is a major pollutant at low altitudes (from 0 to 2000 m) affecting plants, animals and human beings. Ozone can be formed by a succession of photochemical reactions that preferentially involve hydrocarbons and nitrogen oxides emitted by the different combustion systems such as engines and furnaces. 

The high temperatures in the MHD combustion system mean that no complex organic compounds should be present in the combustion products. Gas chromatograph/mass spectrometer analysis of radiant furnace slag and ESP/baghouse composite, down to the part per biUion level, confirms this behef (53). With respect to inorganic priority pollutants, except for mercury, concentrations in MHD-derived fly-ash are expected to be lower than from conventional coal-fired plants. More complete discussion of this topic can be found in References 53 and 63. 

Nitrogen Oxides. From the combustion of fuels containing only C, H, and O, the usual ak pollutants or emissions of interest are carbon monoxide, unbumed hydrocarbons, and oxides of nitrogen (NO ). The interaction of the last two in the atmosphere produces photochemical smog. NO, the sum of NO and NO2, is formed almost entkely as NO in the products of flames typically 5 or 10% of it is subsequently converted to NO2 at low temperatures. Occasionally, conditions in a combustion system may lead to a much larger fraction of NO2 and the undeskable visibiUty thereof, ie, a very large exhaust plume. 

Unbumed Hydrocarbons Various unburned hydrocarbon species may be emitted from hydrocarbon flames. In general, there are two classes of unburned hydrocarbons (1) small molecules that are the intermediate products of combustion (for example, formaldehyde) and (2) larger molecules that are formed by pyro-synthesis in hot, fuel-rich zones within flames, e.g., benzene, toluene, xylene, and various polycyclic aromatic hydrocarbons (PAHs). Many of these species are listed as Hazardous Air Pollutants (HAPs) in Title III of the Clean Air Act Amendment of 1990 and are therefore of particular concern. In a well-adjusted combustion system, emission or HAPs is extremely low (typically, parts per trillion to parts per billion). However, emission of certain HAPs may be of concern in poorly designed or maladjusted systems. 

Boilers may be direct fired or indirect fired. Energy supply designs account for various combustion methods using fossil fuels, municipal waste, process residues, waste heat, and by-products. Special boiler combustion systems to reduce pollution or improve efficiency include fluidized-bed and combined cycle. 

A second important class of pollutant compounds resulting from combustion processes is the general class of the oxides of nitrogen (NO, NO2, NO3, N2O4, and so forth), typically denoted as NO . These NO molecules are key intermediates in the atmospheric conversion of VOCs into photochemical smog and ozone. There are three identified sources of NO molecules in combustion systems. 

Furthermore factors such as stoichiometric value, heat load and design of the burner as well as the combustion chamber have a significant impact on the emission of pollutant gases. Depending on the reaction of a combustion system to a changing equivalence ratio decisions can be made how to minimize the pollutant emissions by adapting the flow rate of air or gas. A combustion control system based on monitoring the CO fraction in the flue gas could thus be considered. 

Ash is the noncombustible residue remaining after complete coal combustion. Generally, the mass of ash is slightly less than that of mineral matter before burning. Sulfur is an undesirable constituent in coal, because the sulfur oxides formed when it bums contribute to air pollution and cause combustion system corrosion. 

The amount of prompt NO produced in combustion systems is relatively small compared with the total NO formation. However, prompt NO is still formed at low temperatures and is one of the features in producing ultra-low NO burners. The nitric oxide reacts with other species in the atmosphere to give various other nitrogen oxides, namely NO2 and nitrogen pollutants. 

In practice, it is the pyrolysis chemistry in the conversion system and the incomplete combustion of the pyrolysis products in the combustion system (Figure 14) which cause pollutant emission problems. In other words, the knowledge about the thermochemical conversion chemistry of single particles can be applied to the understanding of emission problems in PBC systems. 

This example shows that the equilibrium approach in general may work reasonably well for major species in combustion systems, provided that the overall process is diffusion controlled. Even under these conditions the equilibrium approach may fail, however, in predicting concentrations of minor components such as pollutants. 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

Recent tests provide an excellent example of the control of PCDD/F emissions from MSW combustion facilities.46 Tables 4 and 5 reproduce the results of a multiple regression analysis on operating variables relevant to the combustion system (i.e. before the combustion gases enter the pollution abatement equipment). 

The impetus for displacing fossil fuels with biomass in heat and power combustion systems is typically to reduce emissions of C02, and of NOx, SOx, and other air toxics, or to improve utilization of biomass residues and wastes. The threat of increased global warming has subjected the use of fossil fuels to increasing scrutiny in terms of greenhouse gas and pollutant emissions. Renewable and... 

Rupe, J. H., "System for Minimizing Internal Combustion Engine Pollution Emissions, U. S. Patent 3,906,913, September 23, 1975. 

Combustion systems are major sources of atmospheric pollutants. The oxidation of a hydrocarbon fuel proceeds rapidly (within a few milliseconds) and adiabatically to establish equilibrium among the H/C/O species (CO2, H2O, O2, CO, H, OH, O, etc.) at temperatures that often exceed 2000 K. At such high temperatures, the highly endothermic oxidation of N2 ... 

De Ruyck J, Koimov A. A., and El Asri R., Pollutants Reduction in Small Biomass Combustion Systems", EC project JOR3-CT97-0184, Twelve Months Progress Report, 1999. 

Technical Efficiency—major improvement potential with fuel cells Environmental advantage—no emissions of pollutants and C02 Overcome the thermodynamic limitations of combustion systems... 

---