• Waste management in Japan
  • Circular economy in Japan
  • Waste management in Asia
  • Disaster waste management
*The following is an English translation of an article from the December 2013 issue of the Center's online magazine (see the original text in Japanese).

Different Types of Fly Ash: Waste Incineration Ash and Coal Ash

December 2013 issue

Most people may think that once they bring combustible waste (municipal waste) to a collection site on a collection day it is done. However, it is important to see where municipal waste goes for safe disposal afterward. This article will introduce the final form (incineration fly ash in particular) of municipal waste by comparing it with coal ash discharged by coal-fired power plants.

Collected municipal waste is transported to an incineration plant where its volume is reduced by 70-80% through incineration. There are various types of incineration methods: including the most common method, a stoker furnace, where waste is combusted on the grate; a fluid bed furnace in which waste is quickly combusted using fluidized heated sand in the airflow; and a rotary kiln furnace that was also fully used for the incineration of disaster waste. 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.

The first issue is dioxin that is hazardous and can occur during incineration of chlorine-containing municipal waste depending on the incineration conditions. Therefore, in order to prevent the generation of dioxin, municipal waste is kept at a temperature of no lower than 900 ℃ for no less than 2 seconds to decompose organic matter, and cooled to a temperature of no higher than 200 ℃. While dioxin thereby does not occur, hydrogen chloride (HCI) remains during incineration, which is neutralized using slaked lime and collected in most cases using fine fabric bag filters. 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. Based on this information, the National Institute for Environmental Studies conducts basic research on safer waste disposal. The micro world - that nobody pays attention to in everyday life - may be important to ensure safety in waste disposal.

Left: cross-sectional image [top and right] and surface image [bottom left] of coal ash; Right: surface image of combustible waste incineration ash
Scanning Electron Micrograph of Incineration Fly Ash
(Left: cross-sectional image [top and right] and surface image [bottom left] of coal ash; Right: surface image of combustible waste incineration ash)