12th North American Waste to Energy Conference

May 17-19, 2004, Savannah, Georgia USA

NAWTEC12-2212

12th North American Waste To Energy Conference (NA WfEC 12)

Waste-To-Energy Residues - The Search for Beneficial Uses

, Earth Engitluring Center, Coillmbia Uni�'er.'ity 500 If/ 12rY .rtreel, 918 Mudd

Neu! York, l\ry 10027 TeL: (212) 854-2142 e-maiL- km516@colll1nbil1.edu

2 Alarim Sdences lliset1rrh Center, Slate Unil'ersity 0lNe/v York At StOl!] Brook

.' Departmenl 'ifCilJil and En�irrmmenlal Engineering, Temple UllilJersif)'

Keywords: Waste-To-Energy, WfE , MSW Combustion, Beneficial Use, Bottom Ash, Fly Ash, Combined Ash

Abstract

In the U.S., about 28.5 million tons of municipal solid waste are combusted annually in waste-to-energy

facilities that generate 25-30% of ash by weight of the MSW feed. Since some residues were found to contain

high levels of lead and cadmium prior to the 1990s, they were commonly associated with environmental

pollution. However, for the last years nearly all ash samples have been tested non-hazardous. Research on the

beneficial use of combustion residue has been conducted for the past few decades yet the actual ash reuse rate

in the U.S. has remained close to 10%. Currently most of the ash is landfilled at considerable cost to the waste­

to-energy industry. A consortium of researchers at Columbia University, the State University of New York at

Stony Brook, Temple University, and other institutions seeks to develop and to advance the beneficial uses of

combustion residues, such as in construction materials or remediation of contaminated abandoned mines and

brownfields. This paper describes the search for beneficial use applications and provides an overview of the

first year of this consortium.

Introduction

In 2002, approximately 28.5 million tons or 7.7%

of the 370 million tons municipal solid waste (MSW)

in the U.S. were combusted in Waste-to-Energy

(WfE) facilities for the generation of electricity and

heat [1]. During the process about

25-30% of solid residues (by weight, or 10-12% by

volume of the MSW) is produced in form of bottom

and fly ash, which are mostly combined in the plant.

Since 1996, there have been no new WfE facilities in

the U.S. because of environmental and political

pressure. The major concern has been the perceived

75

release of hazardous toxic substances into the

environment In the past, primary focus of

environmental groups has been on air emissions,

especially of dioxins/ furans and heavy metals.

However, after the U.S. EPA enforced the

implementation of the Maximum Available Control

Technology (MACT) regulations in the 1990s, WTE

emissions have been reduced to a point that the U.S.

EPA named waste-to-energy "one of the cleanest

sources of energy" [2].

Prior to the 1990s, combined WTE ash was

found to contain high levels of lead and cadmium

Copyright © 2004 by ASME

and the U.S. EPA classified it as "potentially

hazardous waste", thus requiring analytical testing for

toxicity within the W1E facilities, prior to its disposal

or reuse. A compilation of analytical data has shown

that all the ash samples tested in the last decade have

been found to be non-hazardous [3] and have been

allowed to be disposed in MSW landfills or in

mono fills. However, the dioxins and heavy metals

that are removed from the process gas by means of

the MACT equipment are collected in the fly ash

component of the combined ash (about 15% of the

total ash for mass burn processes).

In addition to the importance of this issue for

existing WfE facilities, that spend a considerable part

of their income on land filling ash, successful ash

management will be a key factor for developing and

siting new W1E plants. It is expected that the focus

of opposition to new WfE facilities will shift from

air pollution to fate of contaminants in the ash

residues. Having decreased dramatically the au:

emissions by introducing better air pollution control

systems, the U.S. WfE industry has yet to develop a

strategy to avoid ash landfilling and to contradict the

perception that health risks have been transferred

from air emissions to potential emissions from the

land filling of ash.

Thus, the disposal or beneficial use of ash

remains the last obstacle in establishing W1E as a

clean and sustainable waste management option that

offers the additional benefit of recovering energy

from the waste stream. Several strategies for reusing

the WfE residues have been proposed and

developed with only partial of local successes.

However, in view of the classification of W1E ash as

potentially hazardous material public acceptance of

ash processing and of ash-containing products

continues to be a major concern.

In 2003, the Waste-To-Energy Research and

Technology Council (WTERT, [4]) founded the

University Ash Consortium to combine research

efforts at Columbia University, the State University

of New York at Stony Brook (SUNY), Temple

University, and other academic and industrial groups

76

concerned with sustainable W1E ash management.

The interdisciplinary research of the UAC comprises

three research areas: 1) WfE ash characterization and

comparison with residue from coal-fired power

plants; 2) the use of WfE ash in construction

applications such as granular fill, asphalt, or cement

blocks; and 3) the use of WfE ash for brownfield or

min remediation.

WTE combustion residues

WfE ash can be divided in the following groups

(after [3]):

1) Bottom ash as discharged from the

bottom of the furnace (mainly the grate)

and fallen through the furnace grates;

2) Heat recovery ash (HRA) as collected in

the heat recovery system including boiler,

economizer, and superheater. HRA is

frequently discharged into the bottom

ash stream and thus is often included in a

broader definition of bottom ash;

3) Fly ash as carried over from the furnace

and removed before sorbents are injected

to clean the flue gas;

4 ) Air pollution control (APC) residues as

collected in the APC equipment (e.g.,

scrubbers, electrostatic precipitators, and

baghouses) including fly ash, sorbents,

condensates, and reaction products. In

U.S., the term "fly ash" usually includes

APC residues;

5) Combined ash as a mixture of the above

categories. In the U.S., the majorities of

WfE facilities combine their ashes to

one residue stream.

WfE residues can be processed with either dry

or wet ash handling systems. Most facilities employ

wet systems because it is easier to control fugitive

dust emissions and air leakages. In May 1994, the U.S.

Supreme Court ruled that the WfE ash was not

exempt from testing to determine whether it is

hazardous. This ruling created a major obstacle in the

development of standard specifications for reuse of

Copyright © 2004 by ASME

WfE ash [3]. Also, as noted earlier, anti-incinerator

organizations have started to expand their objections

from dioxin emissions to combustion residues. For

example, Greenpeace reported that 97% of the total

dioxin emissions from an incinerator would be

present in the ash after a theoretical assessment of

releases from an incinerator in Sweden [5]. Yet,

dioxins and furans (pCDDs and PCDFs) are

generally considered to become strongly absorbed on

the surfaces of ash particles and therefore highly

insoluble in aqueous environments. Hence, it seems

unlikely that they will leach to a significant extent [6].

Concentrations of PCDDs and PCDFs in leachate

collected from ash residue mono fills have

characteristically been reported to range from non­

detectable to parts per quadrillion (ppq) levels, i.e. at

levels that are presendy considered to be below

regulatory concern [6]. Also, modem well-operated

WfE facilities are net-reducers of dioxins/furans [7].

WfE ash management and treatment

Most of the U.S. WfE ash is disposed of in

either mono fills, designated divisions of landfills

called monocells, or mixed landfills. An estimated

600,000 tons, or less than 10% of the total WfE ash,

is used beneficially, mostly in landfill applications

such as alternative daily cover or road base material

[8]. In contrast, WfE plants in Europe generally do

not combine the ashes because of relatively high

contaminant concentrations in the fly ash. The

bottom ash is considered non-hazardous and its rate

of reuse is well above 60% in Denmark, France,

Germany, and the Nethedands [9]. Most of the

bottom ash is used in road construction. On the

other hand, German WfE fly ash is mainly disposed

of as fill material in extinct underground salt mines

[10].

Prior to beneficial use application or disposal,

WfE ash can be treated to improve its physical or

environmental properties. Post-combustion materials

separation is commonly practiced. Especially the

number of WfE facilities that recover ferrous metals

from the ash (for mass-bum processes typically in the

range of 6-12% by weight) is very high. Other

materials such as non-ferrous metals are recovered

only at a few plants, due to economic restraints.

77

Inorganic salts that are highly soluble in aqueous

media have been found in residues. Consequendy, the

infiltration of water into WTE ash can produce

leachates that can be similar to dilute seawater [6].

Consideration of appropriate measures to mitigate or

control the emissions of inorganic salts to the

environment is required. One method to reduce the

inorganic salt content is washing the ash with surplus

water to extract about 50% of the salts [11]. Other

treatments for WfE residues commonly employ

stabilizing agents such as cement, chemical

compounds such as phosphoric acid to improve the

leaching properties of the ash, or thermal processes

that render the ash harmless [10, 12]. In Japan, most

WfE residues are vitrified in slag resistance electric

furnaces but this technology is associated with high

costs of over $100 per ton of processed ash and thus

has not been proven economically viable in the

European and U.S. markets [10, 13].

Beneficial uses of WfE ash

Generally, the beneficial use of WfE

combustion residues can be categorized in four main

applications:

1) in landfills as daily cover or road base

material,

2) as construction material,

3) for brownfield or mine remediation, and

4) in agricultural applications.

During regular landfill operations, daily cover is

placed on all exposed solid waste at the end of each

working day to control vectors, odors, dust

emissions, and fires. Usually, natural soil is used but

there have been efforts to use alternative daily covers

(ADC) such as ash, auto shredder fluff, and foundry

sand [14]. Similarly, at the end of their service life,

landfills are required to be covered with a permanent

cover. WfE ash seems suitable for both applications

and has been utilized where permits allow such usage.

Generally, daily cover accounts for 10-25% of the

land filled MSW volume [15]. On the basis of data

presented in the most recent BioCycle "State of

Garbage in America" survey [1] and of the

assumption that the WfE ash represents 30% of the

MSW feedstock, the annual volume of ash that could

Copyright © 2004 by ASME

be used as ADC is about 7 million cubic yards. The

compacted volume of MSW landfilled in states with

WfE facilities is more than 300 million cubic yards

so that, theoretically, all WfE ash could be utilized as

ADC. However, with the long-run goal of the

WfERT to minimize landfilling of solid wastes, the

utilization of WfE combustion ash in landfill

applications IS not as attractive as other beneficial

uses.

Various studies have shown that it is possible to

use WfE ashes beneficially in construction;

summaries of these investigations can be found

among others in references [3, 16, 17]. Previous

projects included the use of WTE ash as structural

fill, aggregate in cement blocks, and as a constituent

of asphalt or concrete [18 - 29]. Typically, there is a

wide range for the ratio of WfE ash to aggregate in

beneficial use applications. In case of the WfE ash of

Polk County, MN combined ash replaced about 10%

of natural aggregate in asphalt pavements while the

Laconia project in New Hampshire utilized 50%

bottom ash [25, 28]. Similarly, cement blocks have

been produced with relatively small fractions of ash

to replacing the entire aggregate with ash. Generally,

the amount of fines, content of unburnt material,

water adsorption capacities, fugitive dust emissions,

and aesthetics limit the amount of ash to be used. On

the other hand, granular fill can be entirely composed

ofWTE ash.

A survey by Rogoff and Settar [30] reported

physical and chemical characteristics of WTE ash that

lend the ash capable for use in several applications,

including the remediation of acid mine drainage sites.

At Temple University, zeolite-like materials have

been successfully synthesized from combined WTE

ash. Their high alkalinity suggests using these ash­

derived products for the remediation of abandoned

mines, where acidic leachates could be neutralized.

WTE ash possesses characteristics that may lead to

the development of other beneficial uses, such as the

78

remediation of contaminated water and soils [31, 32].

WTE residues contain some nutrients, yet it will be

very difficult to gain public acceptance for projects

on using combustion ashes for agricultural purposes.

Overview of the University Ash Consortium

With the assistance of the WTERT and the

industrial members of the Integrated Waste Services

Association (IWSA, [7]), the UAC will evaluate the

technical, economic and environmental feasibility of

converting WTE ash into a valuable resource. The

organizational structure of the UAC is shown in

Figure 1. Currently, the UAC comprises mainly three

universities (Columbia, SUNY, and Temple) but its

structure is sought to be dynamic in order to allow

for other institutions to join. The close collaboration

of industry and multiple academic groups with

different foci enables the UAC to combine scientific

and applied as well as analytical and experimental

research that goes beyond developing a merely

technical (and often local) solution but includes other

aspects during the implementation of beneficial ash

applications, such as economic considerations, public

and political acceptance, and environmental impact.

In its first year, the UAC hosted a roundtable

discussion at the WfERT Fall Meeting 2003 and

applied for combined research funding. Because

beneficial use applications seem to be hampered by

lack of marketability rather than by technical and

environmental feasibility, the UAC will investigate

public acceptance and perception. In addition, risk

management and liability have to be addressed. The

economic incentives for ash reuse are provided

mainly through avoided landfill costs and savings in

virgin materials. The reuse of WTE ash will also save

natural resources and prolong the life of existing

landfills by further reducing the amount of material

to be deposed. In sum, questions and prejudices

against the use of WTE ash will be examined at the

UAC in depth and the information derived will assist

in the development of policies and markets for ash.

Copyright © 2004 by ASME

-_ ............ ..,.......--..J Columbi a

SUNY at Stony Brook Temple

Columbia I WTERT

Sustainable Ash Management Strategies (Beneficial Uses)

Figure 1: Cummt structure of the U nivcrsi!y Ash Consortium

The interdisciplinary research of the UAC

comprises three major research areas [33]:

1) IfI"fE ash dJaracterization and ronparison with roal

ashes (chemical and pl!ysical properties);

the work includes data collection, organization, and

statistical assessment from previous studies, and

verification of the analytical results. The experimental

program includes the chemical analysis of WfE

ashes. Different samples of bottom, fly, and

combined ash as well as residue from air pollution

control system have been obtained and then

examined by means of scanning electron microscopy

and EDX elemental analysis. These methods help to

gain a better understanding of the surface topology,

structure, and chemical composition of the ash. It is

planned to compare WfE ash properties with those

of residues from coal-fired power plants and if

necessary to develop, modify, or improve in-plant

treatment methods for ash, in particular fly ash, to

increase the opportunities for reuse. Columbia

University (Dr. Saugata Datta, Dr. Karsten Millrath)

will lead this project.

2) The lise of WIE ash in ronstmdion materials; the

work includes comprehensive data compilation of

79

prior projects and extensive experimental testing of

possible applications such as granular fill,

embankments, road base material, and aggregate in

either asphalt, concrete, or concrete masonry units

(cement blocks). The experimental part will mainly be

carried out under direction of Dr. Frank J. Roethel at

the Waste Reduction and Management Institute

(WRMI) of the State University of New York at

Stony Brook [34] as a continuation of previous

efforts [19-22]. It had been shown previously that

processed ash imposes marginal health risks when

used beneficially [22].

The paving and embankment demonstration

program at the WRMI recently entered its second

year. Field data obtained at a parking lot paved with

processed WfE ash in Farmingville, NY is being

assessed. Plans and bids are currently being finalized

for a second demonstration project beginning this

spring. Using construction quality cement blocks

fabricated with processed ash aggregate, hangars at

the Town of Brookhaven's airport will be

constructed. In addition, the program incorporates a

detailed investigation of asphalt using processed

WfE ash as an aggregate substitute in the apron and

taxiways leading to and from the new hangars.

Copyright © 2004 by ASME

Through the assistance of State Senator Caesar

Trunzo, the research program recendy received

additional funding from the New York State

Legislature to continue a third phase of this

investigation.

3) The use of WTE ash for remedial purposes; the

work of the Department of Civil and Environmental

Engineering at Temple University [35] on reclamation

of brownfields and abandoned mine drainage will be

continued [31, 32]. For his pioneering efforts into

using WfE ash to remediate and solve difficult and

cosdy environmental problems, the Environmental

Protection Agency (EPA) presented a bronze medal

to Dr. David Kargbo, leader of the research on WTE

ash at Temple University [36]. His research has

included the use of alkaline WTE ashes to synthesize

zeolites and other new materials that are economically

viable products for use in various industries and to

neutralize acidity from abandoned nune sites

(reclamation). Also, injection grouts for remedial

purposes have been formulated and geotechnical

testing and simulated weathering of the cured grouts

are currendy conducted to determine their long-term

stability.

After the technical, environmental, and

economical assessments of beneficial use applications

the UAC will assist in the implementation of

sustainable ash management plans and in specifying

recommendations for ash standards as a key to for

the WTE industry. The overall goal is to minimize

the amount of material to be landfilled. The aims of

the University Ash Consortium are described in the

web page of the Waste-to-Energy Research and

Technology Council [37].

Conclusions and Outlook

Environmentally acceptable yet economically

viable residue management practices will play a key

role in the advancement of WTE processes. Today,

most U.S. WTE plants combine their bottom and fly

ash and dispose of the mixture in co-disposal landfills

or mono fills. Although several studies on beneficial

uses on ash have been carried out in the past, the rate

of ash reuse in the U.S. has remained comparably

low. The University Ash Consortium intends to

80

develop sustainable ash management strategies based

on the research on beneficial use applications for

WTE ash. In a holistic approach, the UAC seeks to

expand the research from finding local technical

solutions to assessing the technical, economic,

environmental, and also sociopolitical feasibility of

ash reuse on a nationwide level.

The current projects underway at the UAC

member institutions focus on non-landfill

applications such as of WfE ash reuse ill

construction or remediation. Demonstrations

projects for the beneficial use of ash include the

production of asphalt paving and cement blocks to

build an aircraft hangar in Long Island, NY. Zeolites

have been synthesized that seem suitable to neutralize

acidic spills during the reclamation of abandoned

mines. The UAC will also investigate how to improve

processing techniques for WfE ash in order to gain

better public acceptance and marketability of ash

product A study on the influence of ash handling

systems (wet/dry) on effective and economic post­

combustion materials recovery will also be of interest

for the WfE industry.

The UAC has worked closely together with

representatives of the industry and other involved

parties thereby combining academic and applied

research. It is hoped to continue ongoing research

projects, to facilitate synergy effects for future

collaborations and proposals, to nourish knowledge

exchange, to educate the public and collaborate with

environmental and official organizations, and to lead

the U.S. WTE industry to a sustainable ash

management practice.

Copyright © 2004 by ASME

References

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81 Copyright © 2004 by ASME

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82 Copyright © 2004 by ASME