Hydrogen-Rich Syngas from Jatropha ... - ACS Publications

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Hydrogen rich syn-gas from Jatropha curcas shell biomass char in Fresnel lens solar concentrator assembly Sumit Sahitya, Hasan Baig, Ruchita Jani, Nirav Gadhiya, Prasanta Das, Tapas K. Mallick, and Subarna Maiti Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01406 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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Hydrogen rich syn-gas from Jatropha curcas shell biomass char in Fresnel lens solar concentrator assembly Sumit Sahitya a, Hasan Baig b, Ruchita Jani a, Nirav Gadhiya a, Prasanta Das a, Tapas K Mallick b, Subarna Maiti a* a

Process Design & Engineering Cell, CSIR-Central Salt & Marine Chemicals Research

Institute, G.B. Marg, Bhavnagar 364002, Gujarat, India. b

Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn

TR10 9FE, UK Corresponding author Tel. +91 278 2567760; Fax: +91 278 2567562; E-mail address: [email protected] (Subarna Maiti)

KEYWORDS- Hydrogen; Jatropha curcas shells; Fresnel lens solar concentrator assembly

Simulation; Pyrolysis; Steam gasification

ABSTRACT - Hydrogen-rich syngas was generated from Jatropha curcas shell biomass char in a Fresnel lens solar concentrator assembly using solar thermal energy. The assembly had two lenses arranged in such a manner that focal point of both coincided at a glass reactor, having a specially designed water inlet mechanism. Dual axis auto solar tracking system working in a closed loop was utilized for continuously maintaining the focal point on the reactor so that desired high temperature could be obtained for the reaction. The maximum temperature of 1087 °C with a geometrical concentration of 215 x was reached at the focal plane. The theoretical efficiency of the system was found to be 67.44 %. The simulated results of predicted temperature were first verified experimentally without any reaction mixture and they differed with an average error of 7.85 %. Jatropha curcas shell char was then steam gasified at the focus. A maximum char bed temperature of 736 °C could be achieved during the reaction and 42.77 % of hydrogen gas was obtained in the combustible 1 ACS Paragon Plus Environment

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gas mixture. In this work, a cost effective and modular assembly for high temperature solar steam gasification is designed and fabricated to make the process attractive and environmentally benign. GRAPHICAL ABSTRACT

■ Introduction

Several technologies are utilized today to harness energy from renewable energy resources including but not limited to solar, hydrogen, wind, biomass etc. Most of these sources occur naturally other than hydrogen, which despite its simplicity and abundance requires an energy intensive process for its extraction. The key factors which make hydrogen energy important

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include its use as a fuel for electricity generation 1, ease of transportation, high calorific value, and no pollution. Today, hydrogen is produced for industrial and commercial uses mainly from fossil fuels. The conventional methods adopted include reforming natural gas or electrolysis using electrical energy. Large-scale steam reforming of natural gas accounts for more than 50% of hydrogen production in the world. Although, sustainable methods like electrolysis (water splitting using electricity from renewable sources2, 3), biomass conversion (thermal/biochemical conversion4,

5

) and solar conversion6 (thermal gasification by

photolysis) exists, there is an increasing need to develop low-cost, highly efficient hydrogen generating technologies7 from diverse sources. Kumari et.al

8

demonstrated the potential of

extracting hydrogen using the water vapour above the seawater given the huge requirements of the fresh water for producing hydrogen. Lignocellulosic biomass is a renewable source of energy which can be utilised for producing hydrogen by thermo chemical conversion process like gasification. Steam, air or water are generally used as gasifying agents which produce hydrogen rich gas.9,10 Steam gasification is reported to be better than air gasification.11 However, during such process, about 25-30% of biomass is burnt to obtain the necessary energy to sustain the endothermic chemical reactions.12 The thermal energy required by these units can very well be achieved by concentrated solar energy13,14 where an optical element like a lens/ reflector is utilized to focus sunlight on a small area15. Several systems integrated with solar energy technologies have been presented in the literature16,

17, 18

and it is found that use of solar energy for

producing hydrogen has higher exergetic sustainability potential19. Table 1 represents a detailed review of some recent case studies where solar energy is employed for generating hydrogen.


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Table 1 Recent case studies on hydrogen production using solar energy Authors

20

Solar

Geometric

Technology

Concentration

• Heliostat

• 1000 ×

Peak Temperature

Study

Key findings

Challenges

• 773.2 K

• Effects

operating

• Increase in solar light intensity and

• Variation of the solar intensity during

field

parameters, such as solar light intensity,

ambient temperature has a positive

system

ambient temperature and mass flow rate

impact on hydrogen production

of ZnS on the hydrogen production rate

rate. Little `impact on the H2

in a Cu–Cl cycle.

production due to the flow rate of

of

variation

of

the day leads to variable production.

ZnS.

21

• Parabolic

• 70 ×

• 573 K

Trough

• Effect of varying solar flux intensity on

• Increase in temperature of the along

with

reactor temperature and conversion rate

reactor

was studied along with the thermal

intensity has a useful impact on the

• Tracking errors.

performance of the solar receiver.

conversion rate of methanol to

• Transmittance losses glass tube.

hydrogen

• Shading losses.

and

the

solar

• Low optical efficiency due to optical

thermochemical

efficiency reaches maximum at a

imperfections.

• Absorption efficiency of the reciever.

certain value of flux intensity and then drops further. 22

• Parabolic

• 70 ×

Trough with

• 423-573 K

• The influence of varying solar flux intensity

on

methanol

conversion,

• Increase in solar flux intensity led to better hydrogen yield.

a

hydrogen yield and thermo chemical

• Some ranges of mole ratio of

single axis

efficiency was studied on a 5KW

water/methanol showed increase in

• Maintaining

desirable

ratio

of

water/methanol along with the varying solar intensity.

4

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tracking

reactor.

hydrogen yield.

• Influence

of

mole

ratio

of

water/methanol on hydrogen yield. • Results

compared

with,

kinematic

model.

• Steam reforming was found to produce more hydrogen yield than methanol decomposition. • 66-74% hydrogen found in the gas mixture.

23

• Refractive systems

• Variable Concentration

• The photo enhancement achieved using

(2-3.4 × due to the

• The

concentration

achieved

• Light intensity is not uniform on the

Dual-Chamber Bag Reactor (DCB) and

through the bag construction of the

receiver which affects the hydrogen production.

_ with further

reactor)

and

further

a Spherical Tank Reactor (STR) on the

DCBR and STR assembly was not

enhanceme

higher

concentration

hydrogen generation by photo electrical

sufficient enough to produce the

nt

using a Fresnel lens

water splitting was studied.

required quantity of hydrogen i.e.

using

Fresnel

• Further the effect of adding a Fresnel

Lens

0.5 kg/day.

lens concentrator assembly was also noted.

24

• Positive

• 300 ×

lens (laser _

light source)

• Adding nano catalyst instead of

–Methanol mixture with nano catalyst is

bulk catalyst helped improve the

attempted using a continuous Argon

hydrogen production rate.

Laser and solar light as heat source to generate hydrogen.

• Achromatic lens

• Methanol steam reforming from Water

(light

a

capillary

vial

higher hydrogen production rate.

also

enhanced hydrogen production.

• Capillary vial was designed to enhance hydrogen production.

from solar

• Adding

• Achieving smaller catalyst particles for

• Maximum hydrogen production efficiency of 5 % was achieved.

simulator) 16

• Compound

• 3.5×

• 873K

• Portable

CPC-based for

optical

• Efficiency in the range of 65–71%

• Reflectivity of the CPC surface was

Parabolic

concentrator

thermo-chemical

was found to drive methanol

found to reduce the optical efficiency of

Concentrat

applications like methanol reforming is

reforming with temperature from

the system.

or (CPC)

proposed.

250 °C to 300 °C.

5


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25

• Parabolic

• 80 ×

• 423-573 K

Trough

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• Synergetic Methanol steam reforming

• The solar energy to fuel conversion

(MSR) at 400 W/m2

and

electricity

• To ease operation it’s better to run the

irradiance for generating solar fuel.

efficiency was improved to 21 %

synergetic process in winter and let the

by incorporating the dual thermo

individual process run in all other

chemical processes.

months.

• The

solar

to

fuel

efficiency

is

determined for all seasons and a system

the

solar

to

capable enough to operate at varying

• By utilizing the irradiance as low

irradiance in all possible seasons is

as 100 -1000 W/m2 the solar fuel

developed.

conversion

efficiency

• The synergetic process was shown to benefit most during winter.

was

increased by 26 % making the system a reliable option in all seasons throughout the year. 26

• PV array

• 1×

_

• An off grid PV array powered PEM electrolyser to generate hydrogen is studied for eight different locations.

• All sites had very similar system efficiencies of around 80%.

the

• Two axis PV tracking increased the

• Field trial for one location was carried

annual H2 yield by 35 %. Annual

out to generate hydrogen and heat

hydrogen yields were dependent on

losses

local solar resource.

were

implementing

minimized automated

by thermal

insulation mechanisms.

capacity were studied.

240Wh

battery

for

effective

functioning during day and night.

• Using twice the PV capacity helped

• Effect of PV panel configuration and its

• Greater storage capacity is required than

double

the

hydrogen

production only if the excess power was stored in a battery and utilized at night hours.

27

• Heliostat

• 2500-5000×

• 1700 K

• Thermodynamic analysis of hydrogen from

H2S

• Solar

process

is

the

most

• Increased complexity related with the

field

production

the

advantageous with 45 % annual

hybrid reactor plus the associated

system

endothermic dissociation process using

saving as it minimizes the disposal

specific CO2 emissions of 0.42 kg/kW h

by

6


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solar energy is studied. • Hybrid solar and natural gas and Claus process are evaluated and compared. • An

economic

comparison

is

also

cost of sulphur as compared to

negate the economic advantage of the

Claus process.

hybrid scheme over the solar-only

• The hybrid Natural gas and Solar process

has

reduced

cost

design.

of

hydrogen production but the CO2

presented.

emissions as a result of this process are endangering. 28

• Parabolic

_

• 593 K

• Solar based multi generation system

Dish

comprising of the Rankine cycle,

Collector

organic

Rankine

cycle,

absorption

cooling and heating and hydrogen generation system was studied.

• Efficiencies do not change with

• The exergy efficiency of the multi-

changing receiver temperatures.

generation system was found to be

• Efficiencies increase with increase

higher than its energy efficiency. The key reason for this was the various

in reference temperatures. • Direct

solar

flux

affects

the

• Exergy destruction rates and exergy

efficiency of the system except for

efficiencies of each component of the

the Rankine and organic Rankine

system were evaluated.

cycle.

additional complementary processes.

• Parametric studies for studying the effect of reference temperature, direct solar radiation and receiver temperature are carried out.

7


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The composition of the producer gas obtained has been analysed by thermodynamic equilibrium method by 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

researchers17,29. However, to reach the high temperatures required for steam gasification, huge initial investment for power plants consisting of solar towers are involved and the interest is to design systems with low capital investment. Fresnel lens systems are a cheaper alternative for reaching the desired high temperature using solar concentration30. Point focusing Fresnel lens refracts the solar energy to a spot size small area and has several advantages which includes light weight, modularity and transmission. It has a high concentration ratio, and high temperature is obtained within very short interval of time31. The mass production of Fresnel lens is much cheaper than other point focusing concentrators. The lens is constructed by removing the extra material from bulge surface of convex lens while retaining its refractive ability32. Hence it is developed with a planar surface and other surface having sag type of profile called grooves or facets. The important parameters that define a Fresnel lens are optical efficiency, focal length, effective or clear aperture, number of grooves, the pitch (projection of grooves on horizontal plane), and angle of inclination and draft angle of each groove33,34. Fresnel lens has been used for various different solar applications35. In this work, a novel solar assembly to obtain high temperature for chemical reactions, by utilizing two point focusing Fresnel lenses is addressed. The objective is to generate hydrogen by biomass gasification by completely utilizing renewable energy resource so that it leads to sustainable energy production. An optical and thermal analysis at focal plane is performed in order to obtain the temperature profile of reactor placed at common focal point of two lenses. The simulation results are validated experimentally through hydrogen generation by biomass gasification. ■ Construction of the Device

The main constituents of assembly were the solar concentrator assembly, dual axis auto solar tracker, reactor unit, water inlet mechanism, and auto locking mechanism along with electronic and electrical system with power supply entities. The solar concentrator assembly consisted of a primary Fresnel lens and a secondary Fresnel lens as solar concentrator, a flat reflector reflecting sunrays to secondary Fresnel lens, glass reactor as the absorber for the concentrated solar energy as shown in Fig.1.The primary 8 ACS Paragon Plus Environment

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Fresnel lens was positioned normal to sunrays by dual axis tracking, while secondary Fresnel lens was 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

positioned at an angle from 123° to primary Fresnel lens. Both the lenses had same dimensions, 640 mm × 480 mm with focal length of 600 mm. Flat reflector made of anodized aluminium sheet pasted on hollow PVC (Poly Vinyl Chloride) block was attached at an angle of 60° - 65° from secondary Fresnel lens. The particular angular arrangement was done so that both the lenses in spatial arrangement had a common focal point and both the lenses concentrated beam or direct solar radiation at the common point of focus. Reactor unit consisted of silica glass reactor which resisted high thermal shocks, when water contacted the surface of glass at high temperatures during gasification. It had capacity of 3-5 g of biomass char depending on type of biomass. The silica glass reactor could be adjusted along linear way in two axis and in rotational way along third axial direction. Outlet of silica glass reactor was integrated to a gas analyser (Electronic Systems Tech, India) via activated charcoal column for moisture absorption. The line diagram of entire assembly is shown in Fig.2a and the photograph of the system in shown in Fig.2b. The frame of primary Fresnel lens had LDRs (light-dependent resistors), one pair of which was for azimuthal rotation and another pair for altitude. During isolation, the voltage difference was compared by comparator circuit and provided the positive or negative output, which in turn, operated the switching device made up of power transistors and group of relays. Switching device controlled the operation of DC (direct current) motors with gear box and rotated the supporting structure accordingly. Switching device also controlled the operation of auto locking mechanism. Two semicircular aluminum plates containing grooves were attached along with two solenoid coils for auto locking of the system. Two 12V- 26AH batteries along with solar charge controller of 24V- 10A and two 75W PV panels were utilized to supply power to the system. The total power utilized in tracking the system for an entire day of operation was 180 W.


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Fig.1. Positioning of the Fresnel lenses in the assembly

(a)

(b)


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Fig.2. (a) Line diagram of the entire Fresnel lens assembly comprising of 1. Primary Fresnel lens, 2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Secondary Fresnel lens 3. Flat Reflector, 4. Focal Reactor Unit, 5. Dual Axis Auto Solar Tracker, 6. Auto locking mechanism, 7. Solar Driven Electronics System (b) Photograph of the Fresnel lens solar concentrator assembly The dual axis auto solar tracker started its operation from home position. It tracked the sun throughout the day so that the desired temperature could be obtained continuously at reactor unit by concentrating the solar energy. It covered the azimuthal and altitude angle as per solar position. After the operational cycle was completed, it regained its home position and was shut down. Auto locking mechanism held the supporting structure during its operation to protect against wind load. The LCD (liquid crystal display) display was attached to PIC16F877 microcontroller (peripheral interface controller), ADC (analog to digital converter) module and K-type thermocouples that were used to measure the temperature of reactor and its surrounding area. Program code was developed for LCD interface and ADC interface by using microchip’s MPLABX program development tool and the program was loaded in PIC16F877 microcontroller through PICkit3 programmer cum debugger. The LDRs, voltage divider circuit, comparator circuit, relay units, limit switches, power transistors, DC motors with gear box - one for altitude angle and one for azimuth angle, solenoid coils used in auto locking mechanism, LCD, SSR (solid state relay) were part of electrical and electronic unit.


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Fig.3. Tracking and locking circuit of the Fresnel lens assembly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

■ System Modelling

In order to model the system, both optical and thermal analysis were required. The optical analysis could be used to predict the solar concentration process and its effectiveness. The thermal analysis when coupled with this could be used to predict the temperature available for gasification. ■ Optical Analysis

An optical analysis is an effective way to estimate amount of concentrated sunlight reaching the absorber surface. Typically, an optical analysis involves ray tracing using in house codes36 or commercial packages37, 38

. Several studies can be found in literature which makes use of commercial software like ASAP, Tracepro,

Opticad, Zeemax and Apex for carrying out ray tracing. In the present study, Apex 201339 was utilized to carry out ray tracing analysis of the assembly. The ray tracing method involved a source of light and emanating parallel rays of light along with an optical component through which these rays travelled undergoing refraction/reflection and then reached the target absorber surface. Optical losses experienced due to absorption, diffraction and ray scattering phenomenon were considered accordingly. In the present study, Two Fresnel lens and reflectors were used as the optical components. The focal area of each of the lenses was 78.5 mm2.


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Fig.4. Fresnel lens and parameters of design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For the optical analysis, the linear prism structure of the Fresnel lens was taken. The prisms were considered to be aligned in such a way that equally spaced centre points of the prism’s slope were on straight line orthogonal to optical axis i.e. zero draft angle (ideal case). All prisms were considered to have same width.

As shown in Fig.4, the collecting angle ( ) was calculated between the central ray from each prism and the optical axis on the basis of Eq. (1):

= 90° −

.

Where =

(1)

Where is number of prisms, ϕ is F-number and CA is the clear aperture diameter of the Fresnel lens.

Further wedge angle (α) for each prism was calculated by Eq. (2):

= arccos

"#$%&

'()"* " #$%&

)

(2)

Where n is refractive index of material (1.49 for PMMA). The above angles were computed with dimensions of Fresnel lens as 640 mm × 480 mm, pitch 0.5 mm, the number of prisms 800 and the wedge angle at the end as 40.35° which reached 0.39° at the centre. The clear aperture diameter for rectangular face was obtained by considering a circumscribing circle with centre at intersection of diagonals of rectangle

40,41

. Hence the clear aperture diameter was 800 mm. The

circular 3-dimensional Fresnel lens geometry was attained by rotating the above profile containing concentric bands which was further cut to obtain the required rectangular aperture as shown in Fig.4.Using the above Fresnel lens, the system assembly was generated mimicking the actual experiment design. Two Fresnel lenses were positioned; the primary lens was placed normal to the source emitting collimated rays as shown in Fig.5 (a). The reflector used in the study had an average reflectance of 80%. The reflectance measurements were carried out using a Perkin Elmer Lambda 900 UV-Vis spectrometer. This instrument is capable of scanning a material for transmittance, reflectance and absorption properties within a wavelength range from 200nm to 3000nm at 1 nm intervals. For reflectance measurements, samples were mounted in rear sample mount. A light beam from a light source was allowed to pass through a 13 ACS Paragon Plus Environment

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monochromator and filters before reaching the sample. An integrating sphere was used to collect all the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reflected light and compare against an established reference to evaluate the reflectance of the sample. The variation of its reflectance with respect to wavelength is shown in Fig.5b.

Fig.5. (a) Ray tracing of the Fresnel lens assembly, (b) Variation of its reflectance with respect to wavelength The rays incident on Fresnel lens converge along the absorber is shown in Fig.5a.The source used in the study was modelled using AM 1.5D spectrum. The simulation was carried using 1000000 rays in order to carry out ray tracing with a source having an intensity of 1000 W/ m2. Upon ray tracing, it was found that the concentration obtained on the top semi-circular plane was 145000 W/ m2 .Optical efficiency (ƞ) is the ratio of energy available at absorber to that of energy incident on the concentrator Fresnel lens which can be calculated using Eq.(3). ƞ=

+,- .-/0-//+,- "0-/ 2

× 100 %

(3)

where Cg is geometrical concentration and is given as 34 =

567 $ #$"#6"/57/$5

$,,6#/"4 7567 $ 56#6865

,

(4)


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The collecting area of receiver was the semi curved surface area of cylindrical glass reactor kept at the

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

focus, Ac = : r l where radius r = 14 mm of the cylinder absorber and l, the length of the absorber = 65 mm, substituting values in Eq. (4), Cg was found to be 215 whilst ratio of flux input to output was 145, making the optical efficiency 67.44 %. ■ Thermal Analysis

Based on the results of the optical analysis, the thermal analysis of the system was carried out in order to estimate the temperature of air inside the absorber tube. The geometry considered for CFD analysis consisted of an absorber silica glass cylindrical tube having a thickness of 1.5 mm. The absorber received point focussed solar thermal radiation from the two Fresnel lenses and the thermal energy was assumed to be utilized for the proposed reaction and part of the energy was lost to the surroundings by convection and radiation. The entire process was considered to take place in a closed system and the energy balance equation could be written as: = > ?@ A?B C > DE < J=K F B GB> ? ℎ> GI? G>

= > ?@ L> B?II = > ?@ > > DE N BR>O @? @ ?M ℎ> IN @ >I P+Q S G?M II D I@ ? ?@ ℎ> GI? G> ? ℎ> IN

?N O D

(5 )

The geometry of the glass receiver is shown in Fig.6a. It consisted of two regions - glass and air. From the optical analysis results, it was noted that the light concentrating from the Fresnel lens assembly was mainly received at the centre of the absorber and only in half of its length. Hence, the geometry was created in such a manner that focus could be applied to the central area within the receiver. Hence, the flux receiving part was divided into sixteen sections; half of them were treated as flux receiving zones and the other half were given convection boundary conditions. For the analysis, a Quad type mesh consisting of 101619 elements was used. The solution was considered to be converged, when the values of scaled residuals reached values less than the prescribed requirement of 10-6. Computations were performed in parallel on a dual processor workstation with Intel R Xeon (3.3 GHz, 24GB RAM) at the Environment & Sustainability institute, University of Exeter. The 2 D view of the grid is shown in Fig.6b. The thermal properties considered are mentioned in Table 2.


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Fig. 6 (a) Geometry of the absorber considered for thermal analysis (b) Grid of the geometry Table 2 Properties considered for thermal analysis Material

Density (kg/m3)

Thermal

Specific Heat(J/kg

Conductivity(W/mK)

K)

Viscosity (kg/m-s)

Thermal Expansion Coefficient(1/K)

Air

1.225

0.0371

1021

Glass

2203

1.38

703

2.485E-05

3.3E-03

Boundary conditions were then applied to determine the temperature of the air inside the absorber. Table 3 shows the boundary conditions given to the mesh for the thermal analysis. Table 3 Boundary conditions for thermal analysis Sr. No

Region

Boundary Condition

1

Regions receiving flux as per optical analysis

Heat flux

result 2

All other exterior surfaces

Convective heat transfer coefficient hconv , Free stream temperature Tamb

The steady state equation for conservation of mass and momentum were solved for the thermal model in Fluent. They were valid for incompressible flow i.e. flow within air inside the absorber. For considering heat transfer in the system, the energy equation was taken into consideration and for modelling the natural 16 ACS Paragon Plus Environment

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convection of air inside and outside the absorber and predicting the effect of falling heat on it, the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

boussinesq approximation model was considered. The governing equations as considered from the software are presented in Eq. 6 - 13. The equation for conservation of mass or continuity equation can be written as, TU

T/

+ ∇. X FY = AZ

(6)

The momentum equation is, T

T/

XFY + ∇ X FY FY = −∇0 + ∇. [̿ + XDY + ]Y

(7)

Where p is the static pressure, [ ^is the stress tensor, while XDY O ]Y are the gravitational body force and the external body force respectively. Here the stress tensor is given by, [̿ = _ 0 (Tar cracking reaction)

(21)

Before carrying out the solar steam gasification experiments, studies were conducted in a fixed-bed and tubular horizontal reactor attached to a peristaltic pump. Since temperature was a vital factor in the steam gasification process, total gas production, gas composition and heating value were checked at temperatures of 700 and 800 oC and the water flow rate through the pump was set at 1 ml min -1. Overall, the hydrogen yield improved significantly at 700 oC, and dropped off at 800 oC The maximum hydrogen yield (63.66 %) and CV (22.41 MJm-3) was recorded at 700 oC Higher H2 might have been due to tar cracking and endothermic reforming reactions at 700 oC. Boudouard reaction and carbon gasification reaction at that temperature might have also been responsible. At 800 oC, reactions Eqs. (17) & (19) were dominating as most of the reduction reactions occurred at this higher temperature resulting in higher CO and CO2 as compared to 700 °C. From Fig.12, it is clear that hydrogen yield lasted for only few minutes at 800 °C.

Fig. 12. Hydrogen yield at 700 and 800 oC and water flow rate of 1ml min-1


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The effect of water flow rates had significant changes on biochar conversion, total amount of gas produced and its composition at different flow rates of 1, 2, 3, 4 and 5 ml min -1 and temperature of 700°C as in Fig. 13.

70 60 50 40 H2(%V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 20 10 0 0

-10 1 ml/min

10

20 2 ml/min

30 3 ml/min Time (min)

40

50

4 ml/min

60 5 ml/min

Fig. 13. Hydrogen yield at 700 and water flow rates of 1, 2, 3, 4 and 5 ml min-1 As observed from Fig. 13, H2 yield decreased with increasing water flow rates. It is possible that at high flow rate, the temperature inside the reactor dropped, and also the high rate might have caused improper reaction with the char and residence time might be playing crucial factor. Steam gasification of Jatropha shell char in the solar Fresnel lens assembly As mentioned in the previous section, during steam gasification, the focus was on promoting high temperature hydrogen-forming reactions such as water gas, CO– shift, steam methane reforming, Boudouard and tar cracking. For solar-driven steam gasification in the Fresnel lens assembly, the elevated temperature required for such reactions was attained at the focal region due to the concentration of solar rays and highquality syngas was obtained without any external heat source. As reported by de Lasa

43

, the optimal

temperature for H2 generation with minimal tar and CH4 generation is between 627 °C and 827 °C .To replicate the experiments in the Fresnel lens solar assembly, the water flowrate was kept at 1ml min-1.


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Experiments were conducted from 10th to 20th February continuously and Fig. 14 depicts the 800

45 40

700 600

30 500

25 20

400

15

300

10 200

Reactant bed temperature, oC

35

5 100

0

11.00

12.00

13.00

14.00

15.00

16.00

-5

17.00 0

12.21 12.28 12.34 12.40 12.46 12.52 12.58 13.04 13.10 13.16 13.22 13.28 13.34 13.40 13.46 13.52 13.58 14.04 14.10 14.16 14.22 14.28 14.34 14.40 14.46 14.52 14.58 15.04 15.10 15.16 15.22 15.28 15.34 15.40 15.46 15.52 15.58

% v/v of H2, CO & CH4 in evolved gas mixture

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time of day Temperature

H2

CO

CH4

Fig. 14. Steam gasification of Jatropha curcas shell char at the focus of the Fresnel lens assembly (%v/v of combustible gases evolved vs. temperature at focus) on a typical sunny day % v/v combustible gases evolved and temperature of the char bed when the steam gasification was carried out from 12.21 h to 16.13 h on 19th Feb, as representative figure. The average solar intensity was 639 W/m2 and ambient temperature 32.09 °C during the experiment. As soon as water was inserted inside the reactor, temperature dropped down from 297 to 108 °C. As can be observed from Fig.14, hydrogen yield significantly increased with increasing temperature inside the reactor and maximum %v/v of hydrogen was 42.77%, CO 8.87% and CH4 0.13%. The rate of increase in temperature was not linear, as it depended upon the solar intensity and ambient conditions. The maximum temperature reached in the char bed during reaction was 736 °C.

To confirm the repeatability, reaction was carried out in the month of April

continuously for a week, using the same biomass char. The results for 3 consecutive days when the average solar intensity was ca. 650 W/m2 are given in Fig. 15. From the plots it is clear that temperature had direct correlation to H2 production and about 30 – 40 % v/v H2 was obtained in the combustible gas mixture, if the temperature of the reactor bed was 600 – 700 oC. However, control of temperature was not possible in theses cases, as solar intensity varied with time. Each of the experiments were carried out during afternoon (12.00 27 ACS Paragon Plus Environment

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noon to 16.00 h). It was also found, that drop in solar intensity, due to passage of cloud etc., dropped the

400

20

200

10

0 0 12.00 12.10 12.20 12.30 12.40 12.50 time of day

Temperature

H2

600

30

400

20

200

10

0 0 13.00 14.00 15.00 16.00 17.00 time of day Temperature

H2

50 40 30 20 10 0

800 600 400 200 0 12.00

14.00 time of day Temperature

16.00

%v/v H2 in gas mixture

30

40

Reactant bed temp. oC

600

800

%v/v H2 in gas mixture

40

Reactant bed temp. oC

800

% v/v H2 in gas mixture

reactor bed temperature and consequently H2 yield.

Reactant bed temp, oC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H2

Fig. 15. Steam gasification of Jatropha curcas shell char at the focus of the Fresnel lens assembly (%v/v of H2 vs. temperature at focus) during 3 days in the month of April ■ Conclusion

The main advantage of solar-driven steam gasification of biomass is deliberation of higher syngas output per unit of feedstock, as no portion of the feedstock is combusted for process heat. Generally, large solar concentrators are required to bring about the high temperature necessary for the reaction. In this work, we have described the design of an economic -mobile solar device with dual axis auto tracker and Fresnel lens solar concentrator, with the aim to carry out high temperature chemical reactions. Optical analysis was carried out using ray tracing analysis of the assembly. Geometrical concentration ratio was found to be 215 whilst ratio of flux input to output was 145, and the optical efficiency was computed to be 67.44 %. The maximum temperature of the absorber reached on a particular sunny day was predicted as 1087 °C. To validate the model, experiments was performed without reactant mixture at the focus and it was seen that the modelled temperatures matched fairly well with the experimental with an error of 7.85 %. Jatropha curcas shells were chosen as the biomass for experiments of high temperature steam gasification to produce H2 rich syn-gas. Fuel characterization was performed and the calorific value of the shells was estimated to be 15.58 MJ.kg-1. Pyrolysis was carried out in a fixed bed reactor and based on yield and H/C value, bio-char prepared at 400 °C was selected for solar steam gasification. On a typical day in February steam gasification was carried out from 12.21 h to 16.13 h, when the average solar intensity was 639 W/m2 and ambient temperature was 32.09 °C during the experiment. The composition of the product syn-gas was H2 max. 28 ACS Paragon Plus Environment

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Energy & Fuels

42.77%, CO max.8.87% and CH4 max.0.13%. The maximum temperature reached in the char bed during 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reaction was 736 °C. The experiments were repeated and it could be concluded that temperature had direct correlation to H2 production and about 30 – 40 % v/v H2 was obtained in the combustible gas mixture, if the temperature of the reactor bed could be maintained at 600 – 700 oC. Future work involves scale up of the system and techno-economic analysis. Nomenclature: ]Y –External body force (N) FY – Velocity (m/s)

AM – Air Mass Ratio CA – Clear Aperture Diameter of Fresnel lens (mm) Cp – Specific heat (J/kg K) E – Energy (J) f- Focal length (mm) g- Acceleration due to gravity (m/s2) l - Length of the glass reactor (mm) L –Characteristic length (m) n – Refractive Index of the Fresnel lens p – Pressure (Pa) r-

Radius of the glass reactor (mm)

Ra – Rayleigh number Sh – Radiation contribution Sm – Mass added to continuous phase from dispersed phase 29 ACS Paragon Plus Environment

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x – Number of prisms in Fresnel lens 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

t – Absorber or Receiver area (mm2)

3D – Geometrical Concentration ratio

Greek Letters µ - kinematic viscosity (kg/m-s) ƞ − Optical Efficiency

α – Thermal diffusivity (m2/s) α – Wedge Angle of Fresnel Lens facet (Degree) β – Coefficient of thermal expansion (1/K) κ – Thermal conductivity (W/m-K) κeff – Effective thermal conductivity (W/m-K) ρo – Operating density (kg/m3) Φ – F- Number - Collection Angle of Fresnel lens facet Degree) XDY –Gravitational body force (N)

■ ACKNOWLEDGEMENT

This work was supported by DST-UKIERI collaborative project (DST/INT/UK/P-83/2014) and CSIR India is acknowledged for infra-structure support. This is CSIR-CSMCRI 098/2016. ■ REFERENCES

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