2. I. INTRODUCTION
The challenge facing the world is to safely dispose of the
generated solid waste. Combustion of energy-containing in-
dustrial wastes, such as deinking sludge residue from pulp
and paper processes, is considered as an immediate solution
since it is capable of effectively reducing more than 90% of
the overall mass/volume of the waste. Several studies were
performed to understand the structural evolution of wastes,
selected pollutant behavior during combustion, and their
emissions.1–6
However, one of the major drawbacks of the
combustion technique is that it emits hazardous solid resi-
dues (ash) and heavy metals,1
which need to be treated. This
has attracted the attention to improve the existing combus-
tion facilities and propose new approaches to reduce the
heavy metals emissions.
The new approach, proposed and tested by the authors,
is based on the concept of waste combustion within a multi-
zone-temperature fluidized bed reactor. Initially, the waste is
fed into a low-temperature zone ͑Ͻ1060 K͒, and then sub-
jected to a high-temperature treatment ͑ϳ1500 K͒, which is
followed by a sudden quenching in another low-temperature
zone ͑Ͻ1160 K͒. Earlier bench-scale studies focused on the
fundamental behavior of heavy metals within the fly ash
formed during burning of de-inking sludge.7–10
However, it
was not known if the observed metals encapsulation could be
obtained in pilot-scale reactors.
A new pilot-scale Multi-Mode Combustion Facility
(MCF) was designed and built to investigate the effect of the
LHL combustion on the heavy metals behavior during ther-
mal remediation of contaminated deinking sludge residue
generated by the pulp and paper industry. The MCF is a
“flexible” facility that allows operation in two different
modes: (i) as a fluidized bed combustor (FBC) and (ii) as a
single-burner furnace (SBF). The FBC nozzle plate and the
SBF burner are located on the opposite sides. When the MCF
is operating in the FBC mode, the burner is covered and vice
versa; when the facility is operating in the SBF mode, a
converging flange replaces the nozzle plate at the bottom. In
this article we report on the experimental work conducted in
the FBC mode. The article compares the LHL and the clas-
sical (no-LHL) approaches towards neutralization of heavy
metals during combustion of contaminated solid residues
generated by the pulp and paper industry.
II. EXPERIMENTAL FACILITY
The MCF experimental facility consists of five main
components: (1) a flexible combustion chamber, (2) air and
heating systems, (3) waste/fuel feeding mechanisms, (4) a
safety system, and (5) sampling and data acquisition devices.
A. Flexible combustion chamber
The entire facility assembled in the FBC mode is shown
in Fig. 1. The design of the primary air control system allows
the FBC facility to operate either in the bubbling or circulat-
ing modes. For the circulating mode, entrained particles are
returned to the bed of the reactor using a cyclone separator
and a loop seal arrangement. This is accomplished by con-
necting a flexible tube between the bottom of the cyclone
and a side opening of the combustor (see Fig. 1). Data re-
ported in this article come from experiments when the facil-
ity was operated in the bubbling mode, which dominates
industrial FBC applications.
Each section of the combustion chamber features a num-
ber of ports for sampling purposes. Each sampling port has
an inner diameter of 0.023 m and is located 0.10 m above
the bottom flange of each section. These ports provide access
for the insertion of probes to measure temperature, pressure,
and gas concentrations. In addition to the sampling ports, the
third and fifth sections have rectangular ports of 0.04 m
ϫ0.02 m, which are used for laser diagnostics and as view
ports. Moreover, in both sections, thermocouple probes and
pressure taps are inserted into small-sidelined ports, 0.05 m
apart, at variable depths in the reactor thus allowing for mea-
suring the temperature and pressure variations through the
walls of the reactor.
The combustion chamber’s height was 2.1 m with outer
and inner diameters of 0.5 and 0.15 m, respectively (the
maximum height is 9.50 m). It consists of six individual
flanged sections bolted together with 0.03-m-thick spacer
gaskets, Fiberfab ceramic papers that are capable of with-
standing high operating temperatures (up to 2000 K), and
can also prevent gas leakage.
All sections are made from type 304 L stainless steel.
They are lined with one layer of HPV castable refractory that
has a density of 1750 kg/m3
and 0.15 m in total thickness.
In addition to the refractory layer, each section of the cham-
ber features a concentric cooling zone of 0.027 m thickness
thus creating a water-cooling jacket. The use of cooling wa-
ter allows for maintaining temperature of the outer wall be-
low 330 K at the maximum internal temperature of the com-
bustion chamber of 1900 K. The flow of cooling water to
each zone is individually controlled so that each individual
section of the chamber may have a different internal tem-
perature than the rest of the sections.
The bottom section in the combustion chamber is 0.40 m
long. This section is used for heat supply. It has one opening
used for sampling purposes. The second section of the com-
bustion chamber is 0.4 m long (see Fig. 1). There are four
openings within this section, two of which are diametrically
opposed and 0.15 m above the base of the section. The first
FIG. 1. Overall setup of the multimode combustion facility (MCF) as-
sembled to work in the fluidized bed mode.
Rev. Sci. Instrum., Vol. 75, No. 12, December 2004 Combustion facility for waste and biomass 5309
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3. opening is 0.10 m in diameter and is connected to the waste/
biomass feed system while the other opening is used when
the facility is operating as a circulating fluidized bed com-
bustor (CFBC). It allows for unburned sludge particles and
fly ash to be recirculated back to the second section thus
increasing their residence time within the combustion cham-
ber. The same opening is also used as an exhaust opening
when the MCF is operating in the single burner furnace
(SBF) mode, since the natural gas burner is located at the top
of the facility. The diameter of the opening is 0.15 m. The
other two openings in the second section are used for solid
and gas sampling.
The three middle sections (3, 4, and 5) are used to create
the low–high–low temperature zones (LHL) within the MCF
(see Fig. 1). The third and fifth sections are similar in design
and are used to create the low temperature zone (Ͻ1060 and
Ͻ1160 K, respectively), while the fourth section is used to
create the high temperature zone ͑ϳ1500 K͒. Each of the
two low-temperature sections has two sampling ports, a rect-
angular view port and three side openings that are used for
temperature and pressure measurements. In addition, they
have a 0.95-m-diameter opening that acts as a cold air inlet
to create the low temperature zone in both sections. In the
third section, a controlled portion of cold air is supplied di-
rectly from the blower at a temperature of 298 K. However,
the opening of the fifth section is connected to a vortex de-
vice that supplies cold air at a temperature of 270 K. The
amount of cold air supplied to both sections is well con-
trolled and is used to create low temperature zones below
1160 K.
The fourth section is called the High Temperature Zone
(HTZ). This section has only one sampling opening at the
front and an angled side opening of 0.001 m in diameter. A
natural gas burner with a 70 kW capacity, a Midget Mixer
type MM, is inserted from the side opening to provide a
flame within the inner combustion chamber creating the high
temperature zone ͑ϳ1500 K͒. When the conventional com-
bustion setup is used (no-LHL), no natural gas burner is
added. Hence, no high temperature zone exists and no LHL
approach is applied during the conventional combustion ex-
periments.
The top section of the combustion chamber (section 6) is
the exhaust section (see Fig. 1). This section has two 90°-
angled sampling openings. Exhaust gases and solid particles
leave the combustion chamber through an opening located
0.15 m from the top flange (it is 0.15 m in diameter). The
opening is directly connected to a stainless steel exhaust pipe
fitted with an external water-cooling jacket. The exhaust pipe
is joined to a side opening of a cyclone by a flange so it can
be periodically removed and cleaned.
During the bubbling mode experiments, entrained solid
particles are collected in the cyclone while the exhaust gases
leave from an opening, 0.20 m in diameter, at the top of the
cyclone. The cyclone is made out of mild steel and it has a
maximum diameter of 0.5 and 1.2 m height. The cyclone is
designed to accept a flow of 40 m3
/h. In the circulating flu-
idized bed mode, the bottom of the cyclone is connected
directly to the side opening of the second section of the com-
bustion chamber (see Fig. 1).
B. Air supply and heating systems
The primary air inlet system is comprised of an air
blower, an air preheater, and a nozzle plate assembly (Fig. 1
illustrates the connection between the blower, preheater, and
the combustor). The design of section No. 1 permits easy
removal of the nozzle plate when needed. The nozzle plate,
suspended from the base flange that is used to support the
combustion chamber, has 37 uniformly positioned nozzles.
Each nozzle is made of a 0.038 m long stainless steel cylin-
drical rod with 12 mm diameter. It has a conical taper at its
top to avoid the settling of solids. The air exits from each
nozzle through four 0.0025 m diameter holes centered on
each face and located at 0.025 m above the nozzle base. The
nozzles also have two rectangular openings ͑0.004 m
ϫ0.0022 m͒ located below the four holes and are used to
provide extra airflow. To prevent the reverse flow of solids
into the nozzles, the holes are drilled facing downward at an
angle of 20°.
Primary air is supplied from an air blower with enough
capacity to cause solids’ circulation in both hot and cold flow
conditions. Under normal operating conditions, the blower is
capable of supplying a flow of 6 m3
/min, at a pressure of
10 kPa and 3450 rpm. The air passes through an air pre-
heater of 40 kW that is capable of increasing the air tempera-
ture up to 880 K. The preheater is a rectangular box
0.25 mϫ0.75 mϫ4.0 m. It is covered by a ceramic fiber
insulation material assuring the outside temperature lower
than 350 K. The hot air leaves the preheater and passes
through a laminar flow element. It is subsequently uniformly
distributed into the reactor through the nozzle plate.
A natural gas burner is installed in the fourth section to
create the high temperature region, which falls between the
two low temperature sections, thus creating the low–high–
low temperature zones. The gas burner is a suction-type pro-
portional air–gas mixer. Air passing through the jet produces
suction in the throat section to entrain a fuel gas, which is
regulated with an adjuster plug placed in the gas throttle
cock. Once set, the fuel-to-air ratio remains constant over a
wide range of airflows. The capacity of this burner is 70 kW.
C. Waste/fuel feed mechanisms
The FBC mode may be used to incinerate virtually any
type of waste material. However, each waste type needs a
different feeding system. In this article, a pulp and paper
deinking sludge was the standard waste material used. The
sludge consists of fibers ranging from 20 to 40 m in width
and 100–600 m in length forming larger ͑ϳ15 mm͒
spherical aggregates. It is a mixture of bark and pine-wood
residues typically used by a paper mill as a source of energy.
Sludge properties are listed in Table I. The sludge was doped
with aqueous solutions of metal salts Cd͑NO3͒2·4H2O and
Cr͑NO3͒3·9H2O (6000 ppm each) in order to quantify met-
als behavior. (These are the most typical forms of metals in
this type of biomass.10–12
) It is anticipated that energy costs
and legislative measures will stimulate an increase in the
usage of this biomass while increased recycling will enlarge
the heavy metal portion in its composition.
5310 Rev. Sci. Instrum., Vol. 75, No. 12, December 2004 Eldabbagh et al.
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4. The feeder consists of an electric motor, hopper, and an
Auger (see Fig. 1). The sludge particles are injected into the
combustion chamber 0.15 m above the nozzle plate by an
Auger feeder. The Auger is made from 304 stainless steel
rods. It is 0.10 m in diameter and 1.20 m long. The Auger is
connected to the bottom of the hopper. The hopper has a
height of 0.58 m and upper and lower diameters of 0.46 and
0.10 m, respectively. The capacity of the hopper is 50 kg of
the sludge. A 1 hp motor operating at 1750 rpm drives the
entire feeding system.
D. Safety system
During the design phase of the MCF, the safety of the
operating personnel as well as that of the facility was one of
the main objectives. The facility is continuously monitored
by a control system that will stop the electrical heaters in the
case of a sudden increase in the combustion chamber tem-
perature. The electrical heaters also have an on/off tempera-
ture controller. Once the temperature inside the reactor is
higher than the set temperature of 1300 K at the bed level,
the heaters will stop automatically until a lower temperature
is reached. The primary air preheater has a PID-type control-
ler in order to keep the preheater operational while control-
ling the temperature of the incoming primary airflow.
The gas burner is also equipped with a safety control
system that includes a gas pressure regulator. The system
will stop the gas flow in the case of low air pressure. In
addition, a flame detector is also included. A 15 min air-
purging period is used to push any natural gas leftovers from
the mixer’s tip before starting the burner. The facility is also
equipped with a rupture disk in case of an unusual increase
in pressure inside the combustion chamber. Moreover, an
inert gas, helium, is introduced into the combustion facility
in the startup and shutdown stages. This is to prevent any
potential accumulation of hydrocarbons within the combus-
tion chamber that might form explosive mixtures.
E. Gas and solid sampling techniques and
instruments
The gas and solid particle sampling probe was designed
based on the extractive sampling technique. The sampling
probe is used to extract both gas and solid particle samples
through a 0.025 m diameter and a 0.46 m long tube as
shown in Fig. 2. The probe consists of a water-cooling jacket
to protect the head and the stainless steel wall materials from
melting. The water flow rate is regulated with a valve to
ensure that the sample can be quenched to a temperature
below 410 K. Below this temperature, possible interactions
of the extracted samples will be prevented. The gas and solid
particle samples are extracted throughout the sampling
probe, until they reach the holding handle, which is located
outside of the reactor. The inside of this holding handle has a
stainless steel screen holder through which the extracted
samples pass. The solid particles are collected on the fiber-
glass filter attached to the holder, while the dry gases pass
through the filter and are sent to the gas analyzers. The on-
line analyzers measure the concentrations for CO2, NO, and
NOx. A list of gas analyzers used to obtain the gas concen-
tration profiles during the LHL and conventional combustion
experiments is presented in Table II.
III. RESULTS AND DISCUSSIONS
The aim of this section is to demonstrate proper opera-
tion of the MCF and its capabilities. In addition, interesting
features of the LHL combustion technique are briefly high-
lighted. Selected results, including combustion characteris-
tics obtained for deinking sludge and leachability of heavy
metals from fly ash formed during combustion, are discussed
in detail in other publications.13,14
Based on earlier fluidiza-
tion studies, an inert “garnet sand” (SiO2-40%, Fe2O3-35%,
and Al2O3-25%) with average particle size diameter of
1.1 mm was selected as bed material and fluidized to a
height of 0.4–0.5 m above the nozzle plate at a minimum
fluidization velocity of 0.41 m/s. Selected operating condi-
tions are listed in Table I. In order to achieve steady state
conditions, the facility was heated for 200 min prior to data
TABLE I. Properties of biomass and experimental conditions.
Ultimate analysis (wt %)
C: 26.9; H: 314; N: 0.26; S: 0.05; O: 18.47
Proximate analysis (wt %)
Volatiles: 46.89 Ash: 35.38
Fixed carbon: 1.92 Moisture: 15.81
Ash analysis (wt %)
SiO2: 38.12 Na2O: 0.30 K2O: 0.22
Al2O3: 28.33 CaO: 19.10
a
Cr, Pb, and Cd: 6000 ppm
Experimental conditions
Superficial velocity: 0.82 m/s Bed carbon: Ͻ2.5%
Res. time: 1.8 s O2: 8.4 vol % Air temp.: 823 K
a
Final contents after doping.
FIG. 2. Cross sectional view of the gas/solid sampling probe.
TABLE II. List of analytical instrumentation used during gas sampling.
Species Instruments Range
CO2 Horiba infrared analyzers 0%–25%
(Model VIA-510) ͑±0.5%͒
NO and NOx Teledyne analyzers 0–2000 ppm
(Model CEA-9001) ͑±0.5%͒
Rev. Sci. Instrum., Vol. 75, No. 12, December 2004 Combustion facility for waste and biomass 5311
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5. acquisition measurements. Data presented here were col-
lected during the stable period (typically 180 min).
A. Combustion characteristics
The main difference between the LHL and no-LHL com-
bustion regimes was the temperature in the high temperature
zone (HTZ). The temperature in the LHL case was ϳ470 K
higher than the maximum temperature obtained during the
classical fluidized bed combustion (Fig. 3). This difference
stimulated changes in the characteristics of the gas and the
solid phase, which, in turn, influenced the behavior of heavy
toxic metals.
Figure 4 illustrates the average radial temperature pro-
files in the freeboard region when LHL approach was ap-
plied. The measurements showed that, on average, about
300 K gradient exists between the high temperature and the
two low temperature sections. This is achieved with the use
of an efficient local cooling in both cold sections to stimulate
the quenching processes, which are essential in the LHL ap-
proach. The average temperatures in both low-temperature
sections were 1075 and 1156 K, respectively (standard de-
viations were 20.8 and 30.2). An average temperature in the
high temperature section and a standard deviation were 1476
and 80.45 K, respectively (this temperature exceeds the soft-
ening point of the generated ash particles). Average radial
profiles of velocity in the freeboard region in the LHL case
are illustrated in Fig. 5. The average velocity profiles are
similar in all sections although the velocity magnitudes are
different. The highest average velocity was found in the high
temperature section due to the presence of the natural gas
burner. The average velocity in the low temperature sections
is 1.4 times lower than that in the high temperature region,
which is due to the observed variations in the average tem-
peratures measured in the freeboard.
Lateral gas concentration profiles (average data), as well
as exhaust gas emissions of CO2, NO, and NOx during LHL
combustion are presented in Fig. 6. Despite the presence of
FIG. 3. Comparison of temperature profiles obtained during LHL and no-
LHL combustion.
FIG. 4. Average radial temperature profiles determined in the LHL experi-
ments.
FIG. 5. Average radial velocity profiles during LHL combustion experi-
ments.
FIG. 6. Average lateral gas concentration profiles and gas emissions ob-
tained during LHL combustion.
5312 Rev. Sci. Instrum., Vol. 75, No. 12, December 2004 Eldabbagh et al.
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6. the multizone temperatures, the final gas concentrations are
considered moderate at the cyclone’s exit at a temperature of
970 K. These data prove that the LHL helps improving and
completing the combustion process in the freeboard region
by maintaining the gaseous pollutant emissions below the
emission standards. Air staging15
and quenching in the ex-
haust section have reduced the presence of free radicals such
as OH−
, O͑−2͒
, HCN and H+
,16
which prevents the formation
of NO and NOx gases. Usually these free radicals are re-
leased during devolatilization, which is an early stage of the
combustion process, when the operating temperature is rela-
tively low ͑ϳ600 K͒. The levels of CO2 indicate that the
combustion reactions are almost completed due to the pres-
ence of the high temperature zone (the zonal average tem-
perature is 1476 K). The implementation of the multitem-
perature zones in the freeboard does contribute to an increase
in gas concentrations in the HTZ when compared to the gas
emissions during no-LHL combustion technique (Fig. 7).
However, the LHL balances this local increase by stimulat-
ing reduction in the final gas emissions due to the presence
of the high temperature zone which helps reducing NO
formed in the previous sections to N2.17,18
This approach also
helps in reducing both NO and CO levels by enhancing their
interaction, thus improving the combustion process accord-
ing to the following reaction:19
NO + CO → CO2 + N2. ͑1͒
B. Leachability of heavy metals from fly ash
The leachability of heavy metals (Cd, Cr, and Pb) from
the fly ash formed in the LHL process was determined using
the standard toxicity characteristic leaching procedure
(TCLP) specified by the U.S. EPA.20,21
The fly ash samples
collected from the exhaust section were analyzed. It was
found that the leached out amounts of Cd, Cr, and Pb were
0.14, 0.061, and 1.55 ppm, respectively. These amounts are
well below the acceptable limits for Cd, Cr, and Pb (0.5, 5.0,
and 5.0 ppm, respectively). Similar samples were collected
during the conventional combustion process where no LHL
was applied. It was determined that the leachability values in
this case were 30.7, 14.3, and 0.647 ppm for Cd, Cr, and Pb,
respectively. Thus, they were above the acceptable levels
(except Pb) causing no-LHL fly ash to be classified as
hazardous.22
Immobilization of heavy metals in the LHL was
likely due to their fixation within the inorganic matrix of the
fly ash particles. The leachability results indicate that the
LHL combustion significantly improves the containment of
the heavy metals within the fly ash as compared to the con-
ventional combustion technique.
The observed behavior of metals in ash is attributed to
the LHLs ability to create denser, more compacted and
smoother fly ash structures than those formed under no-LHL
scenario thus reducing significantly the heavy metals’
leachability.23,24
Figure 8 illustrates the morphological
changes of the fly ash occurring during the LHL combustion
[Figs. 8(a) and 8(c)] and the final particle formed during
no-LHL case [Fig. 8(d)]. The final fly ash formed in the
no-LHL tests was dominated by particles with diameters
5–25 m and highly porous surface [Fig. 8(d); ϳ42% po-
rosity]. The final particles formed in the LHL were different
[Fig. 8(c)]. They were spherical supermicron particles
͑10–30 m͒ with many submicron particles bound to their
surface. The particles’ interior was densely packed with the
inorganic phases.25
There are important practical implications of the LHL
method. This method allows for reducing ash particles’
leachability by two orders of magnitude. It means that the
LHL ash particles are classified as nonhazardous and can be
safely reused while the no-LHL ash would need to be treated
prior to safe disposal. Our findings may contribute to rede-
signing currently operating fluidized bed combustion units in
order to generate nonhazardous, nonleachable, reusable par-
FIG. 7. Average lateral gas concentration profiles and gas emissions ob-
tained during no-LHL combustion.
FIG. 8. Mechanism of the morphological changes occurring in fly ash par-
ticles during LHL combustion.
Rev. Sci. Instrum., Vol. 75, No. 12, December 2004 Combustion facility for waste and biomass 5313
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![Multi-mode combustion facility for thermal treatment studies of wastes
and biomass
Fadi Eldabbagh and Janusz A. Kozinskia)
Energy and Environmental Research Laboratory, Department of Mining, Metals and Materials Engineering,
Faculty of Engineering, McGill University, 3610 University Street, Wong 2160, Montreal, QC, H3A 2B2,
Canada
Michael Bourassa
Atlas Stainless Steel Inc., Sorel, Quebec, Canada
Jean-Pierre Farant
Department of Occupational Health, Faculty of Medicine, McGill University, Montreal, QC, H3A 2B2,
Canada
Peter Gangli
National Utility Investors (International), Pointe Claire, Quebec, Canada
Michael Groves
Bowater Canadian Forest Products Inc., Gatineau, Quebec, Canada
Eric Rosen
KMW System Inc., London, Ontario, Canada
Vic Uloth
Pulp and Paper Research Institute of Canada, Prince George, British Columbia, Canada
Jalal Hawari
Biotechnology Research Institute, National Research Council of Canada, Montréal, QC, Canada
Wes Hutny
Natural Resources Canada, Energy Technology Center-Ottawa, Ontario, Canada
(Received 8 October 2003; accepted 20 September 2004; published 30 November 2004)
This article describes newly built Multi-Mode Combustion Facility (MCF) used for investigating
thermal destruction of industrial wastes and combustion of biomass. A flexible, refractory-lined
combustion chamber consists of individual sections of various heights and diameter of 0.5 m. The
MCF can be used either as a fluidized bed combustor (FBC) to study the combustion of solid
residues or as a single-burner furnace (SBF) to study cofiring of biomass and natural gas. The
facility is designed such that the outer wall temperature should not exceed 327 K with the use of
water-cooling system and refractory materials. The inner temperature of each section is independent
of the rest of the sections and controlled individually. This arrangement allows for the combustion
process to be carried out in a multizone manner called low–high–low (LHL) temperature approach.
The LHL approach means that the waste/biomass is initially fed into a low temperature zone
͑Ͻ1060 K͒ and then subjected to the high temperature treatment ͑ϳ1500 K͒ that is followed by
another low temperature zone ͑Ͻ1160 K͒. The LHL setup allows for heavy metals encapsulation
and immobilization within the fly ash particles. The facility has 25 openings for sampling of solids
and gases at different stages of the combustion process, as well as in situ observation. Experiments
reported in this article were performed in the bubbling FBC mode with the purpose of testing the
leachability of heavy metals (Cd, Cr, and Pb) from fly ash generated during two different
combustion approaches: (1) multi-zone LHL treatment, and (2) no-LHL. Baseline fluidization
properties of different bed materials were tested. Axial profiles of temperature and gas concentration
(CO2, NO, and NOx) were compared. The results show that the leachability of the heavy metals (Cd,
Cr, and Pb) contained in the LHL-generated ash particles was negligible (0.14, 0.061, and 1.55 ppm,
respectively), while the leachability data from the no-LHL technique were 30.7, 14.3, and
0.647 ppm, respectively. It was concluded that the MCF facility is easy to operate, flexible, and
useful for studies of various waste-to-energy options. Our results also show an improvement in
heavy metals leachability when using the LHL combustion technique. © 2004 American Institute of
Physics. [DOI: 10.1063/1.1821645]
a)
Electronic mail: janusz.kozinski@mcgill.ca
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 75, NUMBER 12 DECEMBER 2004
0034-6748/2004/75(12)/5308/7/$22.00 5308 © 2004 American Institute of Physics
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