Ash Treatment

Bottom Ash from Municipal Solid Waste Incineration

Conventional bottom ash (BA) management consists of a solidification process using inorganic binder reagents, such as cement. However, despite the heavy metal content, the use of BA as a natural aggregate has become increasingly more common. In particular, bottom ash is used as a raw material for clinker, cement mortar or frit production, as a drainage layer in landfills or as a sub-base material in road construction. In this study, the life cycle assessment approach was used to evaluate and compare ash solidification with ash recycling in Portland cement production as a clinker and gypsum substitute. The findings showed that the substitution of ash for clinker resulted in the lowest natural resources (NR) consumption and the lowest environmental burdens (EB). The decrease in the clinker substitution percentage generated a higher NR consumption and an increased EB. In ash recycling, the distance between the incinerator and the cement facility is an important parameter in the decision-making process. Specifically, ash solidification presented less favourable results than ash recycling (with a clinker substitution of 25 %), despite the increasing distance between the incinerator and the cement facility. However, when the clinker substitution decreased to 2.5 % or when ash was substituted for gypsum, the distance played an important role in the water impact.

Heavy Metal Characteristic of Bottom and Fly Ash

The characteristic of bottom and fly ash must complete the quality standards to protect landfill at final disposal. Based on the decree of the head of concerning the procedure for the requirements for hoarding of processing results, the requirements for ex-processing locations, and the location of the former landfill of hazardous and toxic materials must meet the waste disposal to landfills.

The results of characteristic bottom and ash incinerator showed that the Cd content had met the quality standard for waste disposal into type B landfills. The Zn content has the highest concentration followed by Pb, Cr, and Cu. Heavy metals such as Pb, Zn, Cd and Fe can be found in sharp objects, radioisotope shields, chemotherapy waste, laboratory chemicals and pigments and additives found in plastic packaging.

In any furnace, incineration generates cinders (incineration main/bottom ash) and dust (incineration fly ash) – about 10% of the ash – which combustion gas stirs up. This fly ash contains a variety of hazardous substances and needs to be handled carefully.

Fly ash also contains all sorts of hazardous heavy metals. Accordingly, it may be made insoluble through solidification with an agent called chelate or cement for transportation to the final disposal site, it may be transformed to reusable slag by melting furnace, or it may be reused as raw materials for cement. In other countries, (raw) fly ash refers to ash that does not contain lime, and a mixture of fly ash and lime or activated carbon is called Air Pollution Control residue (APC residue) which is not just cinders but something that has fulfilled a certain role.

In the field of construction, “fly ash” refers to incineration fly ash generated in a coal-fired power plant where electricity is produced as heat generated from the combustion of finely ground coal (powdered coal) turns a turbine. After incineration, the incombustible substances in coal remain as coal ash that corresponds to 10% of the total amount of coal depending on its quality. Coal ash falls into two types – fly ash and clinker ash – and occurs in a ratio of 9:1. Fly ash with a certain level of quality can be mixed with Portland cement and used as blended cement.

Here is a scanning electron micrograph of coal ash (left). The majority of the ash particles are characteristically round shape. As crushed particles are generally pointed like broken glass, this means that the particles have been melted at a high temperature. Cavities in the particles shown in the cross-sectional image suggest that a volatile component came out of the melted glass as bubbles. Glass reacts with cement, and accordingly provides strength, which results in strong and durable concrete.

When one looks at the image of combustible waste incineration fly ash (right), although there are some round particles which suggest melting, the ash contains a high proportion of irregular shaped particles and plant cinders-like particles that have rounded hollows of original biological structure. The ash also consists of a diverse range of components such as hygroscopic/deliquescent calcium chloride, household waste-derived sodium chloride (salt) and potassium chloride, and calcium hydroxide used for dechlorination. In spite of being known by the same general name of “fly ash,” coal ash and combustible waste incineration ash are very different.

This knowledge is not needed in everyday life. However, the components of fly ash are vital information for its disposal.