Afterburn

Mu/tihearth Furnace. Multihearth furnaces are most often used for incineration of municipal and industrial sludges, and for generation and reactivation of char. The main components of the multihearth are a refractory-lined shell, a central rotating shaft, a series of soHd flat hearths, a series of rabble arms having teeth for each hearth, an afterburner (possibly above the top hearth), an exhaust blower, fuel burners, an ash removal system, and a feed system. 

Burners and combustion air ports are located in the walls of the furnace to introduce either heat or air where needed. The air path is countercurrent to the sohds, flowing up from the bottom and across each hearth. The top hearth operates at 310—540°C and dries the feed material. The middle hearths, at 760—980°C, provide the combustion of the waste, whereas the bottom hearth cools the ash and preheats the air. If the gas leaving the top hearth is odorous or detrimental to the environment, afterburning is required. The moving parts in such a system are exposed to high temperatures. The hoUow central shaft is cooled by passing combustion air through it. 

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

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. 

To alleviate the air pollution problem associated with charcoal kilns and furnaces, the gases from the kiln and furnaces are burned (see Airpollution CONTROLMETHODS). They can be burned with additional fossil fuel to recover heat and steam (110,111), or in afterburners to nearly eliminate visible air pollution and odors (112). 

The devolatilized coal particles are transported to a direct-fired multihearth furnace where they are activated by holding the temperature of the furnace at about 1000°C. Product quaUty is maintained by controlling coal feed rate and bed temperature. As before, dust particles in the furnace off-gas are combusted in an afterburner before discharge of the gas to the atmosphere. Finally, the granular product is screened to provide the desired particle size. A typical yield of activated carbon is about 30—35% by weight based on the raw coal. 

Fig. 5. Effect of gas residence time on afterburning. The temperature change AT is the difference in temperature between flue and dense phase (30).

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. 

The increasing use of sihconized coatings for weather durabiUty caused severe masking problems for the all-metal, filter mesh-like catalyst elements available in the 1970s. Interest in catalytic afterburners increased when dispersed-phase precious metal—alumin a-on-ceramic honeycomb catalysts offered economically attractive results. 

Melting Smoke and particulates, fume afterburner, gas-cooling device and... 

Municipal and Single-chamber incinerators Particulates, smoke, volatiles, CO, SO, Afterburner, combustion controls... 

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. ... 

TABLE 25-28 Thermal Afterburners Conditions Required for Satisfactory Performance in Various Abatement Applications... 

Abatement category Afterburner residence time, s Temperature,... 

An inerease in ambient air temperature will deerease the available energy for the generator. This assumes that the fresh feed and eoke burn remains eonstant. The expander horsepower does not ehange, but the air blower horsepower inereases with inereased air temperature, eausing the exeess energy to deerease. Steam and water may need to be added to the flue gas flow at various points in the system to eontrol afterburning. In Figure 4-64, the solid eurves are for a normal flow of steam. The dotted eurves are for inereases in the steam rate by 3.05 times, 4.85 times, and 6.05 times the normal flowrate. 

What happens if the expander were operating at peak power so that the exeess power supplied to the generator is represented by line B1 Assume afterburning oeeurs to eause a temperature rise. In the events that follow, there will be variations deteetable in the blower horsepower requirements, but they will be small eompared to the others that oeeur. If the unit was in lined-out operation before the afterburning oeeurred, the operators will be reluetant to disturb the regenerator-reaetor balanee. 

Control acceleration of the string during an afterburn condition. The available energy of the string is increased as a result of higher inlet temperatures. 

Afterburn Control. Afterburn is the term for carbon monoxide burning downstream of the regenerator this causes an increase in temperature upstream of the expander. Temperature sensors in the gas stream cause the brake to energize. This provides sufficient resisting torque to prevent acceleration until the afterburn is brought under control by water or steam injection. 

The expander may be exposed to excessive inlet temperature conditions (afterburn) four times per year for 15 min each... 

Put in the necessary hooding, ducting, and equipment for an afterburner system which will bum the organic solvent vapors to a less polluting emission, but with no solvent recovery. 

Direct-flame afterburners are the most commonly used air pollution control device in which combustible aerosols, vapors, gases, and odors are to be controlled. The components of the afterburner are shown in Fig. 29-15. 

Fig. 29-15. Direct-fired afterburner. Source Los Angeles Air Pollution Control District.

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. 

Catalytic afterburners are currently used primarily in industry for the control of solvents and organic vapor emissions from industrial ovens. They are used as emission control devices for gasoline-powered automobiles (see Chapter 31). 

The main advantage of the catalytic afterburner is that the destruction of the pollutant gases can be accomplished at a temperature range of about 315°-485°C, which results in considerable savings in fuel costs. However, the installed costs of the catalytic systems are higher than those of the direct-flame afterburners because of the expense of the catalyst and associated systems, so the overall annual costs tend to balance out. 

Wet scrubber for formaldehyde and methanol only afterburner for organic vent gases Venturi scrubber followed by cyclone separator and packed countercurrent scrubber... 

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