Afterburning process

When pyrolytic operation is performed at air ratio for combustibles of about 0.6, it is possible not only to prevent formation of NOx, but also to reduce NOx by reducing reaction by NH or HCN in the furnace. In afterburning process of exhaust gas containing NHj and HCN, it is possible to prevent formation of NOx by two stage combustion under the condition of overall air ratio of about 1.1 and temperature at outlet of the chamber of below 950°C. 

Afterburning processes enable the removal of pollutants such as hydrocarbons and volatile organic compounds (VOCs) by treatment under thermal or catalytical conditions. Combinations of both techniques are also known. VOCs are emissions from various sources (e.g. solvents, reaction products etc. from the paint industry, enaml-ing operations, plywood manufacture, printing industry). They are mostly oxidized catalytically in the presence of Pt, Pd, Fe, Mn, Cu or Cr catalysts. The temperatures in catalytic afterburning processes are much lower than for thermal processes, so avoiding higher NOx levels. The catalysts involved are ceramic or metal honeycombs with washcoats based on cordierite, mullite or perovskites such as LaCoOs or Sr-doped LaCoOs. Conventional catalysts contain Ba-stabilized alumina plus Pt or Pd. 

Figure 10-4 illustrates schematically a typical course of an afterburning process. The curve shows the dependence of conversion on process temperature. There can be defined typical conversion values and areas ... 

Finally, the catalyst design procedure for an afterburning process can be described... 

You can select a suitable catalyst for a catalytic afterburning process from monoliths or pellets. Which process parameters are mainly influenced by your choice ... 

Liquid Injection. Liquid injection units are the most common type of incinerator today for the destmction of Hquid hazardous wastes such as solvents. Atomizers break the Hquid into fine droplets (100—150 microns) which allows the residence time to be extremely short (0.5—2.5 s). The viscosity of the waste is very important the waste must be both pumpable and capable of being atomized into fine droplets. Both gases and Hquids can be incinerated in Hquid injection units. Gases include organic streams from process vents and those from other thermal processes in the latter case, the Hquid injection incinerator operates as an afterburner. Aqueous wastes containing less than 75% water can be incinerated in Hquid injection units. 

The radicals and other reaction components are related by various equiUbria, and hence their decay by recombination reactions occurs in essence as one process on which the complete conversion of CO to CO2 depends. Therefore, the hot products of combustion of any lean hydrocarbon flame typically have a higher CO content than the equiUbrium value, slowly decreasing toward the equiUbrium concentration (CO afterburning) along with the radicals, so that the oxidation of CO is actually a radical recombination process. 

Three rapid oxidation methods are typically used to destroy combustible contaminants (1) flares (direct-fiame-combnstion), (2) thermal combustors, and (3) catalytic combustors. The thermal and flare methods are characterized by the presence of a flame during combustion. The combustion process is also commonly referred to as afterburning or incineration. ... 

Direct-flame afterburners are nearly 100% efficient when properly operated. They can be installed for approximately 350-700 per cubic meter of gas flow. Operating and maintenance costs are essentially those of the auxiliary gas fuel. On larger installations, the overall cost of the afterburner operation may be considerably reduced by using heat recovery equipment as shown in Fig. 29-16. In many industrial situations, boilers or kilns are used as entirely satisfactory afterburners for gases generated in other areas or processes. 

Afterburner A unit installed after the main combustion zone in a process to further provide combustion to reduce the emission of certain pollutants. It may be... 

The process is basically a rotary kiln design. Waste is first pretreated and then inserted in the rotary kiln, where it is incinerated with air. The chlorinated hydrocarbons are converted into H2O, CO2 and HCl. After that, in a wet scrubber the HCl is recovered as aqueous HCl. If needs be, natural gas or liquid energy carriers can be added in order to reach the necessary high temperatures in the afterburner. 

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

The enhanced volatilization process is operated by putting contaminated soil in contact with clean air in order to transfer the contaminants from the soil into an air stream. The air stream is further treated through the use of carbon canisters, water scrubbers or afterburners to reduce air emission impacts. Four methods are available that can achieve this effect19 ... 

Infrared thermal destruction technology is a thermal processing system that uses electrically powered silicon carbide rods to heat organic wastes to combustible temperatures. Any remaining combustibles are incinerated in an afterburner. One configuration made by ECOVA Corporation consists of four components65 ... 

ECC catalyst is subject to hydrothermal deactivation. This occurs when the A1 atom in the zeolitic cage is removed in the presence of water vapor and temperature. The result is a loss of activity and unit conversion. The effect of temperature on this process is nonlinear. The deactivation rate increases exponentially with temperature. Units that experience high afterburn have attributed high rates of catalyst deactivation on the higher dilute phase temperatures. This phenomenon is more apparent on units with high combustion air superficial velocities. The high velocity not only increases afterburn, but also increases catalyst entrainment to the cyclones and dilute area. COP is used to decrease afterburn and minimize catalyst deactivation. 

In recent years, a shift has been made away from Pt-containing CO combustion promoters to promoters containing elements other than Pt. This was driven by the requirements of various EPA Consent Decrees for several units to conduct trials of these new promoters. The desire is to use elements that are still effective in oxidizing CO to CO2 and preventing afterburn, but that do not generate NOx in the process. Many of the non-Pt combustion promoters that are currently... 

Several problems arose during full-scale demonstrations of the technology, including difficulties in monitoring particulate emissions and in monitoring the process. Wide fluctuations were observed in key process parameters, including carbon monoxide, carbon dioxide, and oxygen levels, as well as afterburner temperatures. 

The electric infrared incineration technology is a mobile thermal processing system that is suitable for soils or sediments contaminated with organic compounds, polychlorinated biphenyls (PCBs), and metals. Liquid organic wastes can be treated after mixing with sand or soil. Electrically powered silicon carbide rods heat organic wastes to combustion temperature while any remaining combustibles are incinerated in an afterburner. 

The heater process outlet temperature declines as airflow is reduced past the point of absolute combustion. In this situation we have products of incomplete or partial combustion such as aldehydes, ketones, and carbon monoxide going up the stack. This sets the heater up for afterburn in the stack, and the heating value of the fuel is also effectively reduced. 

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