Bottom ash-slag

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). 

Particulate matter may be removed from gas products by a number of means such as (1) water scrubbing or (2) a cyclone separator. Part of the ash from a conversion operation occurs as fly ash and is removed with particulate matter. Some gasifiers produce a melted bottom ash (slag) which is quenched in water while other processes produce a self-agglomerating ash consisting of softened particles. 

Studies of Baccini and colleagues have shown, that under long-term considerations even products like incineration bottom ash slag are not really inert. After the release of chloride and sulphate, two phases can be distinguished (Fig. 10.7) part of the calcium content, which was formerly present as hydroxide, is now carbona-tized. Iron and aluminium are transformed to oxides. On the long run, the carbonate buffer in the slag will be emptied. This will be correlated by relative low pH-values. Compared to the former conditions, elevated trace metals concentrations in the leachate can be expected. 

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 ... 

Torrey, W. 1978. Coal ash utilization fly ash, bottom ash and slag. Pollution Technology Review, 48, 136-145. 

Much along the same lines as the process described above, as proposed by Katsuyama et al. [57], Geerlings et al. recently described a process for producing CaC03 from various solid residues [58]. In this patent, two examples of the described process were described, one utilizing paper bottom ash and one steel slag, although neither example provided a reaction rate for the precipitation step. 

The extraction of calcium took place inside a water-filled, stirred reactor for 15 min, which resulted in a calcium hydroxide concentration of 1.1 gT1 for paper bottom ash and 0.46gT1 for steel slag. The formed hydroxide slurry was separated from the solids and carbonated by injecting it with C02 at a rate of 25 ml min-1. However, the feasibility of the process should be investigated with a cost and environmental assessment before any firm conclusions can be drawn, as was also the case for a process described by Gorset et al. [59]. 

WASTOXHAS has been applied to bottom ash from municipal solid waste incineration (MSWI) and matured slags from the second smelting of lead (Ferrari and Ferard, 1999 Ferrari, 2000), ... 

The WASTOXHAS procedure was applied in a case study involving two kinds of solid waste, a municipal solid waste incinerator Bottom Ash (BA) and a slag from a second Smelting of Lead (2SL). This case study describes the ecotoxicological portion of a multidisciplinary French national research program on the Waste... 

Table 5. Prerequisite study - Ecotoxicity data of leachates of a municipal solid waste incinerator bottom ash (BA) and a slag from a second smelting of lead (2SL) obtained after following the draft standardEN12457 (2002) using liquid-to-solid ratios (L/S) of 2 and 10.

Incineration is often regarded as a very efficient technique for municipal solid waste (MSW) management. However, the environmental impacts of MSW incineration need to be carefully taken into account. The most relevant problem with MSW incineration is flue gas treatment. However, another often overlooked issue is the disposal of solid byproducts of the incineration process. MSW incinerators essentially produce two types of solid by-products, that is, slag, or bottom ash, and fly ash, often mixed with various other chemicals used for flue gas treatment. Bottom ash and—even more—fly ash are regarded as dangerous wastes mainly due to their potentially toxic elements (PTE) content and their tendency to leach such PTE to the environment. 

Mineral impurities in coal are known to be primary contributors to the slagging and fouling of utility boilers, fly ash and bottom ash production as well as atmospheric pollution. They also produce undesirable effects in some parts of hydrogenation processes such as liquifaction and gasification (1,2,3). Despite a long history of investigation prompted by these observations, many questions remain unanswered. 

The fly ash formed in coal combustion also represents a disposal problem (see also Chap. 14). Although there are some uses such as in concrete and bricks, soil stabilization, soil conditioner, and landfill cover, more need to be found.24 Additional uses in wallboard, concrete blocks, and other construction materials should be possible. Other ashes include bottom ash and boiler slag. Experiments have been run on the recovery of iron, aluminum, and other metals from the ashes, but the processes may not be economical at this time. This could reduce the need to mine for these other materials. Coal-fired power plants produce over 100 million tons of ash annually in the United States. Coal fly ash is routinely mixed with water and put into settling basins. This process extracts some arsenic, cadmium, mercury, selenium, and strontium into water, which can then cause abnormalities in amphibians.25... 

---