Aspects of Cold Cap Melting - Hanford Site kinetics– TGA, EGA, and DSC Reaction rates from...
Transcript of Aspects of Cold Cap Melting - Hanford Site kinetics– TGA, EGA, and DSC Reaction rates from...
Aspects of Cold Cap Melting12th ESG 2014
Pavel Hrma
Pacific Northwest National Laboratory
Richland, Washington, USA
23 September 2014
Hanford Tank Waste Treatment
and Immobilization Plant
200,000 m3 of radioactive
waste from plutonium
production: 1943 − 1987
Photos provided by handfordvitplant.com from Feb. 2013 (top) and Aug. 2013 (bottom).
Pre-Treatment
Facility
HLW Vitrification
Facility
LAW Vitrification
Facility
Outline• Glass-melting furnace (melter) and cold cap (batch
blanket)
• Melter feed conversion to molten glass
• Mathematical modelling
• Data for modelling:– Gas generation kinetics: thermal and evolved gas analyses
– Effective heat capacity: differential scanning calorimetry
– Density and porosity
– Heat conductivity
– Viscosity
– Crystalline phases: XRD and SEM-EDS
• Model results: Cold cap temperature profile, glass production rate
• Cold cap produced in laboratory
• Bubbling
Glass-discharge
port
Electrode
s
Glass melting• Nuclear waste will be vitrified in all-electric (Joule-heated) melters.
• Glass-formers are mixed with the waste and charged into melters operating at 1150°C.
• Melter feed (the slurry batch) forms a cold cap floating on molten glass.
Picture courtesy of Jarrett Rice
Melter feed conversion to molten glass
T ≈ 100°C
T ≈ 800°C
T ≈ 960°C
T ≈ 1070°C
T ≈ 1100°C
Reacting
Feed
Primary Foam
Secondary Foam
Molten Glass Melt
Boiling Slurry
Picture adapted from R. Pokorny and P. Hrma, J. Nucl. Materials 429, 245-256 (2012).
Upper Heat Flux
Top Heat Flux
Bottom Heat Flux
Feed Mass Flux
Gas Mass Flux
Glass Mass Flux
Mathematical model: Mass and
energy balance
bbbb r
dx
vd
dt
d=+
)(ρρ 0=+ bg rr
( )2
2
dx
Td
dx
dTcjcj
dt
dTc
Eff
gg
Eff
bbbb λρ −−=
ρ is the spatial density
v is the velocity
r is the mass change rate (via chemical reactions)
c is the heat capacity
cbEff is the effective heat capacity (includes reaction heat)
x is the spatial coordinate (vertical position)
t is the time
j is the mass flux
subscriptsb and g denote the condensed phase and the gas phase
1. Cold cap gas phase and
condensed phases
(solids, molten salts,
glass-forming melt) move
vertically (1D model).
2. Condensed phases
(solids, molten salts,
glass-forming melt) move
with the same velocity.
3. Finite volume method
is simple, efficient and
adequate to problem
Coupling cold cap model with melter model
Reaction kinetics– TGA, EGA, and DSC
Reaction rates from thermogravimetric analysis (TGA), evolved gas analysis (EGA, and differential scanning calorimetry (DSC)
• The nth-order reaction kinetics satisfactorily describes most of the melting reactions
• EGA can be calibrated for quantitative analysis
7
( )
−−=∑
T
Bk
t
in
i
N
i
ii exp1
d
dα
α
effp p Tc c H α= + ∆ ∂
is the effective heat capacity
cp is the true heat capacity
∆H is the specific reaction enthalpy
Feed density and foaming
8
Triple foam layer occurs under the cold cap:
• primary foam (from trapped batch gases)
• gas cavities (moving sideways)
• secondary foam (from rising bubbles)
“Foaming curves” are obtained from
feed expansion experiments.
Feed pellets are photographed and
their profile area is measured. Pellet
volume, density, and void fraction
(porosity, p) is then computed.
m
bpρ
ρ=
ρb bulk density
ρm material density
Below: Effect of heat-treatment
temperature, T, on ρm: sample quenched
from T to room-temperature and sample
heated to T at 10 K min-1 (computed)
The rate of the feed-to-glass
conversion (the rate of
melting) is controlled by the
heat delivered to the cold
cap across the foam layer
and from the plenum space.
Heat conductivity
9
• Heat conductivity, λEff, estimated from crucible experiments
The glass-forming
melt became
connected at TP.
Foam evolved
between TP
and
TC
and collapsed
at T > TC.
Quartz dissolution and spinel formation
0.0
0.2
0.4
0.6
0.8
1.0
400 500 600 700 800 900 1000 1100 1200
Un
dis
solv
ed
qu
artz
frac
tio
n
Temperature, °C
A0-AN1 5
A0-AN1 45
A0-AN1 75
A0-AN1 150
A0-AN1 195
The legend indicates that the average
particle sizes
Quartz dissolution can be represented as
nth-order process:
0
0.01
0.02
0.03
0.04
0.05
0.06
0 200 400 600 800 1000 1200
Cry
sta
l ma
ss f
ract
ion
Temperature (°C)
Hematite, 5 K/min Hematite, 15 K/min
Spinel, 5 K/min Spinel, 15 K/min
Spinel under cold cap
11
ggss
M
F fff ϕϕη
η++= 0log
• ϕs – volume fraction
of dissolved quartz
• ϕg – volume fraction
of gas phase
• fg, fs, f0 – constants
Transient glass-forming phase viscosity
]/)(exp[ TBA SM ϕη =
Model resultsMelting rate versus cold cap
bottom temperature
No fitting coefficients were used.
Blue points represent melter data
reported by the Vitreous State
Laboratory (VSL).
0
200
400
600
800
1000
1200
0 0.01 0.02 0.03 0.04
Tem
pe
ratu
re (
°C)
Distance from cold cap bottom (m)
1100
1050
Cold cap temperature distribution
for two bottom temperatures
250
350
450
550
650
750
850
950
1050
1150
1000 1050 1100 1150
Me
ltin
g r
ate
(k
g m
-2d
ay
-1)
Bottom temperature (°C)
Updated CC
model
VSL at 1150°C
VSL at 1200°C
• Reacting feed with open pores
• Transition between reacting layer and
foaming layer
Laboratory scale melter (LSM)
Fracture through cold cap
LSM cold cap temperature profile
14
Cold cap temperature profile was obtained by
comparing optical and SEM images of
designated cold cap areas with samples heat
treated to various temperatures.
Additional check was performed by
comparing micro-XRD data of the cold cap
with XRD of heat-treated samples.
Temperature, crystallinity, and amorphous
phase distribution in LSM cold cap
15
0.4
0.5
0.6
0.7
0.8
0.9
1.0
300
500
700
900
1100
0 5 10 15 20
Am
orp
ho
us
ph
ase
fra
ctio
n
Tem
pe
ratu
re (
°C)
Distance from cold cap bottom (mm)
LSM cold cap temperature
Amorphous phase fraction
Left: micro-XRD of the LSM cold
cap compared with XRD of
heat-treated samples.
Below: Temperature and
amorphous phase distribution
in the LSM cold cap.
Effect of bubbling on melting rate
Bubbling
outlets
Production
rate
kg/m2/day
4 1060
5 1290
6 1400
1. Bubbling generates forced convection in
the molten glass • velocity gradients become steeper
• thermal boundary layer is suppressed
• cold cap bottom temperature rises
2. Bubbles from bubblers sweep the
secondary foam formed under the cold
cap into vent holes.
3. Feed is stirred into the melt at the edges
of vent holes, exposing a fraction of the
feed to high temperature.
Energy Solution
melter test data
No bubbling:
300-500 kg/m2/day
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Calculated specific bubbled area, m2 / m
2
Sp
ec
ific
pro
du
cti
on
ra
te,
kg
/m2-d
ay ♦♦♦♦ DM1200 y = 3703x + 245
���� DM100 y = 3256x + 400
LSM-model comparison of
temperature profiles
Red points: LSM data
shifted by secondary
foam and cavity
thickness; the slope
was adjusted assuming
that the unquenched
sample had 57%
porosity (primary
foam).
Important difference: The top surface of the LSM cold cap was dry
(400°C), whereas the model surface was wet (100°C).
y = -78.385x + 1056.2
y = -33.565x + 1396.7
200
400
600
800
1000
1200
0 5 10 15 20 25
Tem
pe
ratu
re (
°C)
Distance from cold cap bottom (mm)
Cold cap model
LSM cold cap
LSM-porosity correction