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Municipal Solid Waste Incineration
Potential for Beneficial Reuse of Combustion Ash
Sarah Mahon, P.E.
Solid and Hazardous Waste Prevention and Control Engineering
Rensselaer Polytechnic Institute
MANE-6960, Spring 2014
Table of Contents
Abstract
1. Introduction
2. Types of Incinerator Ash
3. Incinerator Ash Characteristics
4. Case Study: Ash Reuse and Regulations
5. Public Opposition
6. Conclusions
Abstract
Incineration is a popular form of municipal solid waste (MSW) disposal since electricity can be
generated using the MSW as a fuel source. The incineration of MSW is generally considered to be a
renewable energy since MSW is regarded as a consistent, sustainable fuel source. Although a large
portion of the MSW incinerated in converted to energy, there is a significant amount of residue left
behind (ash) that needs to be handled. Currently in the United States, a majority of MSW incinerator
ash is landfilled. However, other countries have had success in reusing the MSW incinerator ash as
concrete aggregate and road asphalt base. Although some countries have had success with reusing
ash, there are challenges associated with MSW incinerator reuse, including the presence of heavy
leachable metals and public opposition. This paper explores options for ash reuse and challenges
associated with it.
1.0 Introduction
Incineration is a common method of municipal solid waste (MSW) disposal since the waste can be
converted to energy (also known as waste-to-energy). According to the US EPA, in 2011
approximately 250 million tons of MSW were generated of which approximately 29 million tons (or
12%) was combusted for energy recovery. Although a majority of the MSW processed in the
incineration plants is converted to energy, approximately 25% (by weight) of the input material is
converted to ash. The material is reduced by about 95% by volume; resulting in a very dense ash
with density between 1200-1800 lb/yd3 (Worrell). The ash requires further management after
incineration, and can either be reused or disposed in landfills. According to the Department of
Transportation (DOT), there are approximately 160 MSW incineration plants which generate 9
million tons of residual ash per year. (DOT)
Ash from municipal waste solid plants in the United States is currently landfilled. Landfilling of
incinerator ash is preferred to landfilling of the unprocessed MSW since the ash requires less space
(since incineration reduces volume by about 95%). However, some areas in Europe (where landfill
space is scarce and expensive) are beneficially reusing the ash for purposes such as concrete
aggregates. This report explores how ash is generated, the typical constituents and physical
properties of ash, options for beneficial reuse, and challenges associated with reuse.
2.0 Types of Incinerator Ash
The term “ash” is a generic term for several types of residues that are generated at incineration
plants. Bottom ash is the portion of material that remains after a combustion cycle is complete. It
can contain unburnt material, chunks of metal, glass, and ceramics. Up to 80% of the total ash
generated is bottom ash (DOT).
Boiler ash and fly ash are the portions of ash that get entrained in the exhaust stream and are
transported into the boiler and the air pollution control equipment. The boiler ash is the portion that
gets stuck to the boiler tubes and walls, and the fly ash is the portion that is collected in the air
pollution control equipment (such as scrubbers, baghouses, and electrostatic precipitators). (DOT).
Figure 1: Locations of Ash Generation in an Incineration Plant (DOT)
For the purposes of this report, both boiler ash and fly ash will be referred to as fly ash. The
distinction between bottom ash and fly ash are important due to the varying ash characteristics
(described in Section 3.0).
3.0 Incinerator Ash Characteristics
Incinerator ash can contain any of the pollutant parameters that were originally input into the
system. Since municipal solid waste can vary based on regional location and time frame, ash
characteristics can vary widely from plant to plant and even within the same plant. Ash
characteristics can also vary based on type of ash, age of incineration plants, and whether waste is
pre-sorted prior to incineration.
Bottom Ash Vs. Fly Ash
Bottom ash and fly ash from the same incineration plant have different properties. The bottom ash
is usually coarser with larger diameters than fly ash; bottom ash is too dense and heavy to enter the
exhaust stream. The fly ash is small and lightweight and gets caught in the exhaust stream. Bottom
ash can contain un-incinerated organic material, which is typically not observed in the fly ash. The
fly ash can contain volatile metals (such as lead, cadmium, and zinc) in greater quantities than
identified in the bottom ash. Fly ash is typically on the borderline of hazardous waste due to the
heavy metals content. To determine if ash is hazardous for metals, it is subjected to the toxic
characteristic leaching protocol (TCLP) procedure which exposes the ash to landfill-like conditions,
to determine what metals have the potential to leach from the waste if landfilled.
Fly ash can also contain dioxins and dibenzofurans (produced when plastics such as polyvinyl
chloride are incinerated) in much higher concentrations than bottom ash. This is of significant
concern due to the toxicity of these materials.
If the air pollution control equipment associated with the incineration plant contains lime based
materials (typically used as an acid gas treatment), the fly ash will contain lime. The lime causes the
fly ash to be extremely alkaline, to the point of being making the fly ash hazardous waste due to
alkalinity.
In ash reuse applications, the bottom ash is almost used exclusively; fly ash is rarely reused due to
alkalinity, high metals content, and high dioxin/dibenzofuran content. It is important to keep these
ash streams separate if reuse of ash is desired.
Currently in the United States, bottom ash and fly ash are combined into one waste stream. The
waste is combined so that when TCLP tested, the ash is determined to be non-hazardous and can
therefore be landfilled without any further treatment. Segregating the ashes would most likely cause
the fly ash to be hazardous and require treatment prior to disposal.
Age of Plants
The ash constituents can vary with the age of the incinerator plant. Newer facilities with more
advanced air pollution control technology are typically able to capture finer particulate matter; the
average diameter of fly ash will decrease and particle size distribution will contain more fine
particulates than older incineration plants. In addition, there is an inverse relationship between
reduced air emissions and fly ash toxicity; the better the air pollution equipment is at capturing
pollutants, the more pollutants the fly ash will contain. Improved furnace designs can also more
completely combust the waste, which can reduce the quantity of un-incinerated organic material that
is present in the bottom ash. (DOT)
Pre-sorting
Incinerator plants can either burn unsorted waste or presorted. When waste is presorted, it is
typically shredded, ferrous metals are removed, and non-ferrous metals are removed. The preprocessing activities can significantly alter the chemical and physical properties of the ash (as
opposed to unprocessed waste). For example, batteries are a large source of lead and cadmium, and
removing them prior to incineration can decrease the quantities of these hazardous constituents that
are in the final ash. Shredding the waste typically allows for more complete combustion, and metals
separation reduces the quantity of metals that are present in the ash.
Not only are potential contaminants (such as heavy metals) an important factor in determining
potential reuse, but physical properties and engineering characters are also important. Ash density,
crystallinity, particle size distribution, moisture content, and abrasion assistance can all impact
beneficial reuse options.
Due to the varying degree of ash (even without the same plant), it is important that ash be
characterized prior to reuse to ensure that it is non-hazardous in nature and that it will meet the
required engineering specifications for its intended reuse.
4.0 Case Studies – Ash Reuse and Regulations
The country of Taiwan did a comprehensive study of European countries and their management of
ash in order to develop their own management standards (Taiwan). The results are summarized
below.
Countries including the Netherlands, France, Germany, Denmark, and England currently reuse ash
in a variety of uses, such as coarse aggregate sub-bases, foundation and road fills, concrete aggregate,
building materials, asphalt, road construction projects, and embankment fills (Taiwan). According to
Veolia, a waste management company, England incentivizes the reuse of ash (most likely due to
limited and expensive landfill space).
According the Environmental Services Association, an England-based waste management company,
incinerator bottom ash can be used in multiple construction applications to replace primary
aggregates since the ash meets aggregate specifications. The ESA notes that plants need to sample
and analyze IBA to confirm it is non-hazardous and meets specifications. (ESA)
According to the Taiwan report, the bottom ash requires some pre-treatment prior to reuse,
including sieving to achieve appropriate sizes, pulverizing larger chunks, and separating out
undesirable chunks. The pre-processing can also include stabilization (Germany lets the material sit
for three months to reduce water content), heating, and rinsing. Most locations also test the
aggregate for leachable metals and dioxins to ensure the material is not hazardous. Each country has
different rules and regulations regarding the material usage.
The various restrictions regarding use of ash include restricting use in drinking water protected areas,
restricting distance from drinking water sources, restriction from stormwater catchment areas,
restriction for tap water source quality protection areas, and limitations regarding distance to
groundwater. (Taiwan)
The country of Taiwan has defined what bottom ash can be reused for, and also requires facilities
that reuse ash report on the quantity used, leaching and dioxin testing results, how ash is used, and
certifications if any treatment was used.(Taiwan)
5.0 Public Opposition
There are multiple groups that are opposed to incineration of MSW in general for multiple reasons;
including that it decreases recycling rates, it creates more greenhouse gases per unit of electricity
than coal-fired power plants, it is an overall inefficient means at producing electricity, and it burns
resources than can be recycled or composted. Some groups also argue that incinerations is not a
“renewable” source of energy since the waste is comprised of materials that are currently being
consumed at non-sustainable rates. Sweden currently needs to import waste since the demand to
operate incinerators outweighs the countries actual generation rate.
Not only is there opposition to incineration in general, but there is also opposition to reusing the ash
byproduct. According to the group Everything Connects, the incinerators make waste more toxic by
concentrating toxic portions of the waste. Although the volume of waste is reduced, the
concentration of toxic material increases.
Some groups point out that ash can vary considerably even within the same plant; even if waste is
sampled periodically to determine it remains non-hazardous, small proportions can be much more
toxic than average.
The incinerator bottom ash can also create health & safety issues for people who directly using the
bottom ash. According to the North Yorkshire Waste Action Group, incinerator bottom ash was
responsible for a hydrogen gas explosion that injured two workers. The bottom ash that was being
incorporated into concrete contained aluminum particles. As the bottom ash was mixed with water
and the concrete, hydrogen gas was formed, which ignited when a spark from construction
equipment ignited it.
Additional concerns include the long term leaching potential. Leaching is a highly complex process,
which can be affected by the product itself, the surrounding soil constituents, the soil type and void
ratio (e.g. sand vs. clay), etc. Current testing typically compares the material to landfill-like conditions
over a short time frame, it does test to specific soils or leaching potential over longer time frames.
For many areas, the long-term leaching potential given the localized conditions is not known.
There are a wide variety of public concern issues associated with the reuse of incinerator ash, which
may make reuse within the United States difficult.
6.0 Conclusions
Currently, it appears that countries that reuse MSW incinerator ash are located in areas where landfill
space is limited and expensive. A majority of opposition regarding ash reuse is due to the potential
for toxic materials to be applied in manners that negatively affect soil and groundwater quality. The
United States may be able to switch to reusing ash if it puts strict controls in place regarding reuse to
satisfy public opposition concerns; such as only using the bottom ash, frequent analytical testing to
confirm ash is non-hazardous, restricting areas the ash can be used to minimize contact with
stormwater, groundwater, and areas that may promote leaching.
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