Solar co-production of H and C by thermal decomposition of CHsfera.sollab.eu/...of_H2_and_C_by_thermal_decomposition_of_CH4_M… · 2 Introduction Solar thermal cracking of natural

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Solar co-production of H2 and C by thermal decomposition of CH4

2nd SFERA Winter School, ETH Zurich, 24-25 March 2011, ,

Gilles MaagETH Zurich, Department of Mechanical and Process Engineering, 8092 Zurich, Switzerland

24th March 2011 [email protected] | www.pre.ethz.ch 1

Outline

Introduction

Part I: Radiation heat transfer model

Part II: Complete reactor model

Part III: 10 MW scale-up

Conclusions

[email protected] | www.pre.ethz.ch24th March 2011

SFERA Winter School Solar Fuels & Materials Page 169

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Introduction

Solar thermal cracking of natural gas

CH H

HH


75 kW/mol at 1500 K

carbon black

Motivation

Production of solar fuelt t bl transportable

storable reduced CO2 emissions compared to feedstock

Carbon in sequestrable form no CO2 emission in the entire process chain

Commerciable carbon quality (carbon black) no consumption of feedstock for process heat


no consumption of feedstock for process heat CO2-free production production of carbon black (CB) responsible for 0.1 % of World‘s

CO2 emissions

EU-project „SOLHYCARB“ design and testing of reactors, scale-up performance prediction


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PART IRadiation heat transfer model

Problem statement

Radiative heat transfer analysis

Conservation equations

Numerical results


G. Maag, W. Lipiński, A. Steinfeld,Particle-gas reacting flow under concentrated solar irradiation,Int. J. Heat Mass Transfer 52 (2009) 4997-5004.

Problem Statement

Radiative exchange in 1D i i fi it l b t i isemi-infinite slab containing a

non-uniform solid-gas medium

Non-participating surroundings

Perpendicular, collimated, external irradiation (Te = 5780 K)

S h i l di d


Spherical, monodisperse, and isothermal particles

1D convective mass flow, isobaric system

Optical properties vary with time


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Equation of radiative transfer

Radiative source function


extinction

emission in-scattering by particles

I dI I

dss

Application of Mie theory to complex refractive index of

Radiative properties (solid phase)

complex refractive index of propane soot

Spectral absorption and extinction efficiencies of carbon particles.

Dalzell, Sarofim, J. Heat Transfer, Feb 1969, 100-104.


Values approach 1 and 2 for λ →0 as expected for geometric optics.

„Cut-off“ wavelength increases with particle size


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Radiative Properties (Gas Phase)

Data from the HITRAN spectroscopic database

Ar and H2 assumed transparent

Scattering is neglected

106 points

CHf p ,T


Box model (5000 intervals)

Linear interpolation between discrete temperature, pressure, and wavelength ranges

4g CH g, f p ,T

Equation of radiative transfer (2) ,p ,g ,p b p ,g b g sca, ,p

ˆ ˆd d d dI I s I T I T s I s

s s

3 1W m srI

Integration over solid angle and wavelength:

sca, ,pi ,p i i

4

ˆ ˆ ˆ, d d4

I s

s s s

r ,p b p ,g b g

0 4 0 0

d d d 4 d 4 dI I T G I T Gq

ˆ dG I


Divergence of radiative flux

Source term to use in energy equation.

4

ˆ dG I s


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Monte Carlo Method

Statistical method.

Advantageous for complex problems Advantageous for complex problems.

Radiative energy is divided in Nray discrete „bundles“ (here 107).

Each bundle is characterized by: Starting point

Direction

Wavelength

These characteristics are assigned according to their probability


distribution.

Each „bundle“ path is followed.

If repeated for a large number of „bundles“, the solution approaches the exact one.

Mass conservation

Modeling – conservation equations

Particles conservation

Energy conservation

gas phase


solid phase


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Baseline parametersParameter Value Unit

Slab thickness L 0.1 m

Number of sublayers L/Δz 50Number of sublayers L/Δz 50 -

Number of rays nrays 106 -

Inlet particle volume fraction fv,0 10-5 -

Inlet CH4 molar fraction 0.5 -

Inlet particle diameter d0 2.5 µm

System pressure p0 101‘325 Pa

Incoming radiative flux 1.5·106 W·m-2eq

04CH ,x


g

Radiation source temperature Te 5780 K

Gas initial / inlet temperature Tg,0 300 K

Particle initial / inlet temperature Tp,0 300 K

Temperature of surroundings Tsurr 0 K

Emissivity of surroundings εsurr 1 -

eq

Results discussion – transient temperature response Temperature response of

ti l t L / 2particles at z = L / 2.

Both the heating rate and the steady state temperature increase with lower particle size.

Higher extinction of solar radiation.


radiation.

Lower value of “cut-off” wavelength, causing lower emission by particles being heated up.


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Results discussion – steady state profiles

Steady state particle t t fil l thtemperature profile along the slab for selected d0.

Smaller particle sizes lead to both higher heating rates and steady state temperatures.

Shadowing is observed for small particles.


small particles.

Inefficient absorption for big particles.


Steady state particle t t fil l thtemperature profile along the slab for selected fv,0 ,for d0 = 2.5 μm.

Extinction coefficient of the particle phase is directly proportional to fv.



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Steady state particle t t fil l thtemperature profile along the slab for selected .

Heating-up is faster for low

Gas phase absorbs about 100 times less incoming radiation than particles.

f CH i 5 ti hi h

4CH ,0x

4CH ,0x


cP of CH4 is 5 times higher than that of Ar at 1500 K.

Similar outlet temperatures are reached for all cases.

Summary – radiative behavior of particle-gas medium

Higher steady state temperatures are achieved for small particles and high volume fraction due to increased extinction of incoming radiation.

Temperature difference between particle and gas phase is insignificant.

External thermal radiation is predominantly absorbed by the particles.

Validation is difficult due to missing boundaries in model.



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PART IIComplete reactor model

Reactor

Problem statement

Cavity modeling

Absorber tube modeling

Experimental validation


G. Maag, S. Rodat, G. Flamant, A. Steinfeld,Heat transfer model and scale-up of an entrained-flow solar reactor for the thermal decomposition of methane,Int. J. Hydrogen Energy 35:13232-13241, 2010.

CNRS 10 kW reactor prototype

Tested in 2008/09 at PROMES, Od illOdeillo.


S. Rodat et al., Sol. Energy 83:1599-1610, 2009.


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CNRS 10 kW reactor prototype

1 MW solar furnace in Od ill (F )Odeillo (France).


Images: www.promes.cnrs.fr

Problem statement - cavity

Cubic receiver cavity W = H = D = 0 2 m

Material flow

W = H = D = 0.2 m

xabs = 0.17 m

graphite walls

4 cylindrical absorbers xabs = 0.17 m

dabs = 0.024 m

graphite

A t


Aperture at center of front face

dap = 0.09 m

Window quartz


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Problem statement – absorber tube

Two coaxial pipes

Material: graphite


Material: graphite

Modeling

Receiver cavity Tubes

Input: Inlet solar power

Net heat flux to tubes

Output: Wall temperatures

Input: Inlet mass flow

Inlet gas composition

Inlet gas conditions (T, p)

Tube temperatures

Output


Tube temperaturesOutput Outlet mass flow

Outlet gas composition

Outlet gas conditions (T, p)

Heat absorbed by fluidFinal output


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Modeling – receiver cavity

Radiative exchange between faces (windows – walls –t b )tubes): Radiosity method for windowed enclosures:

: overall emittance, reflectance, transmittance. Optical properties for graphite (walls, tubes) and quartz glass (windows)

t l i i di ti


: external incoming radiation : net radiative flux : view factor between faces j and k.

Monte Carlo method

3 discrete wavelength intervals to account for window properties. A system of 3·N equations is obtained and solved for T.

Modeling – tubes

N lt

dabsorber

d2,o

d2,i

d1

Nusselt

correlations

Fourier‘s Lawconduction

convection

T1,bulk

T2,bulk


RHT model (part I)radiation

annulus

tube


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2D radiation model (annnulus)

Boundary conditions for RTE: L

z

Ro

r

Ri


Modeling – chemistry

Considered reactions: # Ea(kJ/mol)

k0(s-1K-0.5(mol/m3)(1-m))

m(-)

1.

2.

3.

4

(kJ/mol) (s K (mol/m ) ) ( )

1 397.1 3.890·1012 1.280

2 135.6 7.178·108 1.101

3 68.18 2.832·106 1.126

4 31.38 3.258·101 1.970


4.

Rate law:

M. Wullenkord et al., Proc. WHEC2010, May 16-21, 2010, Essen, Germany. M. Wullenkord, personal communication.


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Governing equations – mass conservation

Steady-state mass conservation equation

rial f

low

(ρ,

w)

ρ: density (kg·m-3), w: flow velocity (m·s-1)

Assuming plug flow and ideal gas


Mat

er

d1

z

d2,o

Governing equations – energy conservation

Steady-state energy conservation equation.

Annulus: Annulus:

Tube:

h: specific enthalpy (J·kg-1) rial f

low

(ρ,

w)

q q


h: specific enthalpy (J kg ) hR: reaction enthalpy (W·m-3)

Mat

er

di

z

2,conductionq 1,conductionq


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Validation – CNRS 10

Comparison of outer absorber tube temperature

Standard deviation of relative errors: 1.7 %

Main sources of error:

Thermal transients in cavity insulation

Natural unsteadyness of


Natural unsteadyness of solar irradiation

Validation – CNRS 10

Comparison of outlet H2 concentration

Standard deviation of relative errors: 29.1 %

Main source of error:

Variations in system pressure and tube diameter due to carbon


depositions


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Summary – complete reactor model

A heat transfer model was developed to simulate a solar i it t i i f t b l b breceiver cavity containing an array of tubular absorbers

A tubular absorber model, accounting for radiative, conductive, and convective heat transfer through the tube walls, was built.

2D, two-phase radiation model was used to determine net radiative heat transfer from hot tube wall to fluid.


The model was validated with data obtained testing a 10 kW reactor prototype.

PART III10 MW scale-up

Scale-up geometry

Performance study


G. Maag, S. Rodat, G. Flamant, A. Steinfeld,Heat transfer model and scale-up of an entrained-flow solar reactor for the thermal decomposition of methane,Int. J. Hydrogen Energy 35:13232-13241, 2010.


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Scale-up

Objective: 10 MWth scale-up reactor

1 kg(C)/s

Treaction > 1500 K (better if > 1800 K)

To obtain such high temperatures, secondary concentrators (CPC) are required.

Graphite tubes Minimum size:


di = 8 cm

do = 14 cm

L = 2 m (tubes can be connected in series)

Window Maximum diameter: dwindow = 0.6 m

Receiver design

200

North field

286 heliostats 50m2

West field 3 910 MW

50

100

150

B

East field273 heliostats

West field273 heliostats

3.910 MW

3.560 MW3.560 MW

CPC concentrating factor: CCPC = 4 acceptance angle: Φ= 30°


-200 -150 -100 -50 0 50 100 150 2000

50

A

60°60°

top viewPower to apertures at design point (I = 800 Wm-2,noon)

Image: Dr. Akiba Segal, WIS


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Receiver design – proposed dimensions

bol

t ty

dimension

sym

b

unit

cavi

tce

ntra

l

late

ral

Depth D m 2.0 1.2

Height H m 4.0 4.0

Width W m 4.0 3.0

Number of absorbers N 26 20


Number of absorbers Nabs 26 20

Absorber outer diameter dabs,out m 0.14 0.14

Absorber inner diameter dabs,in m 0.08 0.08

Distance between absorbercenterlines

δ m 0.15 0.15

Performance – outlet temperature



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Performance – thermal energy efficiency


Performance – chemical energy efficiency



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Performance – chemical energy efficiency

The chemical energy efficiency can beefficiency can be increased from 43 % to over 60 % if Tout is reduced to 1600 K

Lower carbon quality

Incomplete conversion


Summary – scale-up

Based on simulation results, a 10 MW scale-up reactor was d i d d di i ddesigned and dimensioned.

The behavior of the reactor was simulated and predicted for the expected input parameter range.

At design point, a thermal energy efficiency of 75 % and a chemical efficiency of 42 % are predicted for Tout = 1870 K.

In the relevant range, chemical energy efficiency is strongly


affected by the chosen Tout. Lower Tout gives higher efficiency (59 % for 1600 K).

Optimum economical and or ecological operation range to be defined.


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Final conclusions

A radiative heat transfer model for a 2 phase solid-gas ti fl d l dreacting flow was developed.

The model was used to study the behavior of a reacting flow of CH4 laden with carbon particles exposed to highly concentrated solar irradiation.

A numerical model for a solar cavity receiver containing an array of absorber tubes was built.


Radiative heat transfer within the absorber tubes was approximated using a simplified correlation based on the first model.

Final conclusions (2)

Validation with a 10 kW solar reactor prototype was li h daccomplished.

The model was applied to optimize the design and simulate the performance of a 10 MW commercial-scale reactor.

The question of optimum economical and or ecological operation range is open.



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Acknowledgement

Prof. A. SteinfeldS

Dr. G. Flamant

Prof. W. Lipiński

Dr. S. Rodat, Dr. S. Abanades (CNRS)

M. Wullenkord (DLR)

Dr. A. Segal (WIS)


Funding European Commission (contract no. 019779 STREP-FP6)


Documents

Solar co-production of H and C by thermal decomposition of CHsfera.sollab.eu/...of_H2_and_C_by_thermal_decomposition_of_CH4_M… · 2 Introduction Solar thermal cracking of natural