Combustion systems waste from

Utility systems as sources of waste. The principal sources of utility waste are associated with hot utilities (including cogeneration systems) and cold utilities. Furnaces, steam boilers, gas turbines, and diesel engines all produce waste from products of combustion. The principal problem here is the emission of carbon dioxide, oxides of sulfur and nitrogen, and particulates (metal oxides, unbumt... 

Formation of Airborne Emissions. Airborne emissions are formed from combustion of waste fuels as a function of certain physical and chemical reactions and mechanisms. In grate-fired systems, particulate emissions result from particles being swept through the furnace and boiler in the gaseous combustion products, and from incomplete oxidation of the soHd particles, with consequent char carryover. If pile burning is used, eg, the mass bum units employed for unprocessed MSW, typically only 20—25% of the unbumed soHds and inerts exit the combustion system as flyash. If spreader-stoker technologies are employed, between 75 and 90% of the unbumed soHds and inerts may exit the combustion system in the form of flyash. 

When viewing effluent treatment methods, it is clear that the basic problem of disposing safely of waste material is, in many cases, not so much solved but moved from one place to another. If a method of treatment can be used that allows material to be recycled, then the waste problem is truly solved. However, if the treatment simply concentrates the waste as concentrated liquid, slurry or solid in a form, which cannot be recycled, then it will still need to be disposed of. Landfill disposal of such waste is increasingly unacceptable and thermal oxidation causes pollution through products of combustion and liquors from scrubbing systems. The best method for dealing with effluent problems is to solve the problem at source by waste minimization, as will be discussed in Chapter 28. 

The outer layers of the onion (the utility system) produce utility waste. The utility waste is products of fuel combustion, waste from the production of boiler feedwater for steam generation, and so on. However, the design of the utility system is closely tied together with the design of the heat exchanger network. Hence, in practice, the three outer layers should be considered as being the source of utility waste. 

Combustion of waste wood presents special problems from a regulatory standpoint, and research should continue on reducing pollutant emissions. There is some potential for blending with clean wood without substantially altering emission control systems in existing units. Better fuel characterization is needed for waste materials. 

The only available small-scale system is a packaged two chamber incinerator with waste heat recovery. This technique is practical at the 25 to 100 tons per day (TPD) scale. In these units, partial oxidation occurs in the first section of the unit and causes a portion of the waste material to degrade and give off combustible gases. These gases, as well as products of combustion and particulate from the first chamber, flow to a second chamber where they are combusted with excess air and a natural gas or oil pilot flame. The combustion products then flow through appropriate heat transfer equipment to produce steam, hot water, or hot air. Today, four small cities and more than sixty industrial plants use the technique with heat recovery equipment. 

Costs were estimated for a source separation system as shown in Figure 3. It was assumed that 50 percent of the theoretically available combustible solid waste could be recovered. The total costs ranged from 28.94/metric ton for a city of 10,000 population (gasoline at 0.26/liter) to 23.83/metric ton for a city of 20,000 population (gasoline at 0.53/liter). The computations and all assumptions made are summarized in Table III. The computations do not include a credit for savings on tipping fees and the extension of landfill life as these costs are site specific. Gasification of 50 percent of the available combustible solid waste in a community could reduce landfill requirements by 20 percent. 

OEC has been used in several different types of incineration applications to overcome thermal limitations in the process.2 One example is a fixed rotary incinerator where kiln instabilities were produced by variations in the incoming waste, which was sometimes cold and wet. The existing combustion system in the primary combustion chamber (PCC) was unable to handle these transient variations. 02 was injected into the kiln through a lance. The 02 flow was automatically controlled, based on feedback from the temperature at the exit. This improved the kiln stability. It also improved the refractory life in the afterburner due to the more uniform temperature profile in the overall system. In general, OEC can be used to increase incinerator throughput of low-heating-value waste materials. 

NOx andS,02 emissions. Oxides of nitrogen (NO and N02) and sulfur are emitted from most combustion systems including MSW and hazardous waste incinerators. The two principle mechanisms are shown in Figure 13. 

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