Bottom ash

Similar to oil-fired plants, either low NO burners, SCR, or SNCR can be appHed for NO control at PC-fired plants. Likewise, fabric filter baghouses or electrostatic precipitators can be used to capture flyash (see Airpollution controlmethods). The collection and removal of significant levels of bottom ash, unbumed matter that drops to the bottom of the furnace, is a unique challenge associated with coal-fired faciUties. Once removed, significant levels of both bottom ash and flyash may require transport for landfilling. Some beneficial reuses of this ash have been identified, such as in the manufacture of Pordand cement. 

In ECS s 1986 repowefing project Babcock and Wilcox (B W) constmcted a bubbling-bed section to ECS s existing 125 MWe pulverized-coal furnace to produce 31.3 t/h of lime, usiag cmshed coal as the source of heat to calciae limestone ia the fluidized bed. A portion of the lime is drawn from the bed as bottom ash and a portion is collected as fly ash. Both portions are transferred to a cement (qv) plant adjacent to the boiler. The hot flue gas from the EBC flows iato the existing main pulverized-coal furnace, ia which a B W LIMB system was also iastaHed to absorb sulfur dioxide dufing those times when the EBC is not operating. 

Alternatively, combustion catalysts may be formulated to act primarily as furnace area catalysts, in which case they reduce the carbon in fly ash and bottom ash. This, in turn, lowers the degree of fouling in the boiler convection area and the economizer. 

The management or disposal of metals and ash, other by-products of the combustion process, also causes concern. Ash is an inert solid material composed primarily of carbon, salts, and metals. During combustion, most ash collects at the bottom of the combustion chamber (bottom ash). When this ash is removed from the combustion chamber, it may be considered hazardous waste via the derived-from rule or because it exhibits a characteristic. Small particles of ash (particulate matter that may also have metals attached), however, may be carried up the stack with the gases (fly ash). These particles and associated metals are also regulated by the combustion regulations, as they may carry hazardous constituents out of the unit and into the atmosphere. Since combustion will not destroy inorganic compounds present in hazardous waste, such as metals, it is possible that such... 

Additives often form a problem in recycling processes. Material recycling is often not possible or only with a considerable loss of quality. Plastics recycling is notoriously difficult due to the mixed composition of the plastics waste stream. The recycled material can only be used in certain applications that do not demand a pure material. Recycling of the additives themselves is theoretically possible only for metals, but in practice this type of recycling is not feasible. The metals occur only in low concentrations. Recovery from fly ash and bottom ash is possible, but expensive in view of growing scarcity problems it may become a viable options for at least some metals. 

The primary solid byproduct of combustion processes is bottom ash, which primarily consists of mineral matter and minor amounts of unreacted carbon. Because the leaching property of the ash, the bottom ash from combustion of most material is considered hazardous. An exception is the bottom ash from combustion of biomass. 

PCDD/PCDFs are found not only in stack gases but also in solid residues from any combustion process such as bottom ash, slag and fly ash. With advanced technology and better burnout of the ashes and slag (characterized by a low content of organic carbon), PCDD/PCDFs concentrations have declined (Fiedler, 1999). 

Fig. 7a-e. Alkyl-substituted phenanthrene series (GC-MS key ions mlz 178,192,206,220, and 234) from a bottom ash sample from a coal fired power plant... 

There are 720 coal-fired power plants in the USA. When coal is burned in these power plants, two types of ash are produced coal fly ash and bottom ash. Coal fly ash is the very fine particulate matter carried in the flue gas bottom ash (or slag) is the larger, heavier particles that fall to the bottom of the hopper after combustion [261-264]. The physical and chemical characteristics of these ashes vary depending on the type of coal burned. These ashes are characterized by the following ... 

Bottom ash has a similar chemical makeup to fly ash but has a much coarser gradation. A recent study on its use as a sub-base material showed that it had sufficient engineering properties to perform adequately. 

Combined ash - when fly ash and bottom ash are placed in landfills, they are generally combined. The physical properties of combined ash (including gradation, specific gravity, and loss on ignition) can vary considerably depending on the type of plant and source of coal. 

In 1980, 2.8 million tons of municipal solid waste was burned in the USA, yielding approximately 33% municipal waste combustion (MWC) ash. By 1990, the amount burned had increased to 32 million tons, creating approximately 25% of MWC ash or residue [265-267]. Controlled combustion of municipal solid waste produces two types of ash fly and bottom ash. Most MWC ash (80-99%) is bottom ash however, it usually contains a high percentage of toxic materials, and the leachates may not meet environmental standards. 

Fig. 12. Isotherm sorption models for bottom ash solid waste, representing Langmuir (C/Cs vs C), Freundlich (logCs vs log C), and Linear (Csvs C) models...

Isotherm plots of TOC data for only Bottom Ash Solid waste and isotherm equations for the different solid phases are shown in Fig. 12, and the isotherm parameters determined from statistical regression analyses with their coefficients are given in Table 4. 

Vassiladou EE (1993) Utilization of fly and bottom ash as a partial fine aggregate replacement in asphalt concrete mixtures. Symposium proceedings Recovery and Effective Reuse of Discarded Materials and By-products for Construction of Highway Facilities. Federal Highway Administration, FHA 101... 

Steam electric power plants using oil or coal as a fuel produce ash as a waste product of combustion. The total ash product is a combination of bottom ash and fly ash. Because the ash... 

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