Effect of operating parameters on performance of an intEffect of operating parameters on performance of an integrated biomass gasifier, solid oxide fuel cells and micro gas turbine system

Accepted Manuscript
Title: An experimental analysis on tar cracking using nano structured ni-Co/sip catalyst in a biomass gasifier based power generating system
Author: K Shanmuganandam, M Venkata Ramanan, R Saravanan, J Anichai
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DOI:
Reference:

S1359-4311(15)01211-9
10.1016/j.applthermaleng.2015.10.150
ATE 7267

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Applied Thermal Engineering

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13-5-2015
29-10-2015

Please cite this article as: K Shanmuganandam, M Venkata Ramanan, R Saravanan, J Anichai,
An experimental analysis on tar cracking using nano structured ni-Co/si-p catalyst in a biomass
gasifier based power generating system, Applied Thermal Engineering (2015),
10.1016/j.applthermaleng.2015.10.150.
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An experimental analysis on tar cracking using nano structured Ni-Co/Si-P catalyst in a
biomass gasifier based power generating system
Shanmuganandam K*,a, Venkata Ramanan Mb, Saravanan Rc, Anichai Jd
*,a

Corresponding Author, Research Scholar, Institute for Energy Studies, College of

Engineering

Guindy,

Anna

University,

Chennai

600025,

Tamilnadu,

INDIA.

E-mail: , Ph: +91-94865 17139.
b

Assistant Professor (Sr.), Institute for Energy Studies, College of Engineering Guindy, Anna

University, Chennai 600025, Tamilnadu, INDIA. E-mail: , Ph: +9144-2235 7912.

c

Professor, Institute for Energy Studies, College of Engineering Guindy, Anna University,

Chennai 600025, Tamilnadu, INDIA. E-mail: , Ph: +91-44-2235
7607.
d

Deputy Chief Engineer, Department of Mechanical Engineering, Saipem India Projects Ltd,

Chennai 600034, INDIA. E-mail: , Ph: +91-94452 84539.

Abstract
Adoption of biomass gasification based power generating systems for meeting the
power requirements of decentralised habitations on kW scale is not only a proven option but
is also regarded as an environmentally benign approach. One of the persisting issue till to be
resolved in biomass gasifiers is the formation of tar along with the producer gas. Tar is
regarded as highly carcinogenic and is observed to condense at room temperature thereby
blocking and fouling the downstream equipment’s. Among the tar mitigation methods,
catalytic tar mitigation method is highly effective and majority of the studies has been
conducted with bulk catalysts, which suffers due to inherent disadvantages. Hence it has been
proposed to experimentally analyse the impact of nano catalytic based tar reduction to
overcome the said drawbacks.
The objective of this study is to evaluate the effectiveness of a novel low cost
ecofriendly bimetallic nano structured Ni–Co/Si–P catalyst for tar removal in a downdraft 15
kWth biomass gasifier. The nano catalyst was synthesized by deposition–precipitation
method. Characterization of the catalyst has been accomplished using XRD, HR-SEM, HRTEM, BET and TGA analysis. Using XRD pattern the mean size of nano crystallite particles
has been observed in the range of 10 nm. HR-SEM and HR-TEM measurements concur with

Page 1 of 19

this value. BET analysis using N2 sorption studies revealed the surface area as128 m2 g-1.
TGA studies confirmed that the catalyst was thermally stable up to 900C. The gas generated
from the gasifier was made to pass through a catalytic tar cracking unit comprising Ni-Co/SiP nano catalyst. Experimentation with the nano catalyst resulted in a tar cracking of 99% as
compared to 91.5% from bulk mode. Hence it has been conjectured that nano Ni-Co/Si-P
catalyst is capable of mitigating the tar generated in biomass gasification systems
substantially.
Keywords: Biomass Gasifier; Catalytic Tar Cracking; Nano Catalyst; Ni-Co/Si-P.
1. Introduction
Considering the exponential increase in world's energy demand, gasification of
biomass is a promising, renewable energy alternative and is an eco-friendlier option
especially in view of bio energy production [1-8]. Biomass gasification is a thermo chemical reaction in which solid biomass reacts with sub - stoichometric air at high
temperature to generate producer gas. Furthermore, to improve energy utilization in biomass
gasification, polygeneration approach is adopted wherein three different forms of energies
such as heat, electricity production and hydrogen generation are accomplished from the same
biomass source [9-10]. To meet stringent gas purity requirements for polygeneration, the
produced gas has to be further cleaned as it is tar laden. Constituents of tar are identified as
all organic contaminants with molecular weight larger than benzene. Tar is highly
carcinogenic and condenses at room temperature creating frequent maintenance problems on
downstream equipment’s [11-16]. In addition application of producer gas in internal
combustion engines for power generation and hydrogen generation using membrane
separation technology requires minimum acceptable level of tar thereby, emphasizing the
urgent need to mitigate tar in biomass gasifiers [17 -18]. Numerous studies have already been
undertaken towards tar mitigation in producer gas [19-24]. Generally the different methods to
remove tar from producer gas are categorized as physical, thermal and catalytic processes. In
physical tar removal methods like wet scrubbing method, only 60% tar removal efficiency
was achieved. In addition only tar was trapped, while its energy content was wasted [25]. Tar
removal results were unsatisfactory when producer gas was passed through a physical tar
removal device like rotating particle separator [26]. In addition, thermal tar cracking method

requires very high temperature in the range of 900- 1250°C, to crack the tar which results in
energy penalty [27, 28]. On the contrary, catalytic tar cracking method operates at relatively
lower temperatures and results in high tar removal efficiency without creating waste water

Page 2 of 19


problems. Hence it is recognized as the most efficient method to diminish the tar formation in
the producer gas. Nickel based catalysts have proved to be very effective for tar mitigation
and can be availed at low cost [29-32]. However the above mentioned bulk material catalysts
suffer disadvantages of limited reusability, large formation of organic waste and associated
disposal problems. Hence, developments on nano nickel based catalysts with improved
performance are being carried out worldwide. Nano materials have attracted immense
interests due to their unique properties like enhanced surface area with higher catalytic
activity. Studies on catalytic tar mitigation using nano catalysts in biomass gasifiers are very
limited and needs intense investigation. The objective of this study is to evaluate the
effectiveness of a novel low cost ecofriendly bimetallic nano Ni-Co/Si-P catalyst for tar
removal in a downdraft biomass gasifier and for possible enhancement in the quality of the
producer gas to be suitable for usage in polygeneration. Nickel is an effective catalyst for dehydrogenation [33] and is deployed as nickel oxide (NiO) in this experiment which gets
reduced to nickel metal during the process. Cobalt carries unpaired “F” electrons by which it
chemisorbs the oxygen and can be used for oxidation [34]. In the present study,
silicophosphate was used as the support for the nano bimetallic catalyst. The effectiveness of
the catalyst in cracking the tar and thereby improving the producer gas composition and
calorific value are detailed.
2. Experimental & Instrumentation setup
The experimental set up comprises of the gasification system and the catalytic tar
cracking system. The gasification system consists of air blower, gasifier, and cyclone
separator, flaring duct, tar sampling set up and associated instruments. The catalytic tar
cracking system comprises of the catalytic tar cracking reactor, guard bed reactor and
associated instruments. The fixed bed catalytic tar cracking unit is located downstream of the

gasifier. A forced air, 15 KWth, downdraft, dry bottom fixed bed gasifier of 24 kg full load
capacity was chosen for generating the producer gas. The gasifier was fabricated as
cylindrical in shape, comprising of top and bottom shell. Casuarina wood was used as the
feed material and the manual feeding of wood material from top of gasifier using hopper was
done at the rate of 6 kg/h. A grate made of mild steel was used for holding the feed stock. A
highly efficient dry cyclone separator was employed to remove the particulates from the
producer gas. The dust laden producer gas enters the cyclone separator while the cleaned
producer gas leaves through the circular pipe at the top. An aerated burner was used for
flaring the producer gas. Provisions were provided at the producer gas exit line for inclusion

Page 3 of 19


of tar and sampling ports. Gasification of fuel was initiated by igniting a subtle charcoal bed
that was established prior to loading of Casuarina wood. Air supply was started by powering
the centrifugal blower. Once red hot condition was established inside the throat, feed material
was charged slowly into the gasifier and air supply was gradually reduced to attain an
appropriate equivalence ratio of 0.10 - 0.50. Flue gas emanates from the flare port within 5
minutes. Gasification of feed stock commences in 10 minutes and producer gas emanates
from flare port. The system attained stabilization in 45-60 minutes which was ensured by
observance of constant temperatures in raw gas and in various zones, after which
experimental analysis was initiated. Tar and gas sampling was conducted before and after the
catalytic reactor simultaneously to analyze the overall system performance. Schematic and
photograph of the experimental setup is depicted in Fig 1 and 2. In-situ approach (employing
catalysts inside the gasifier) has been reported ineffective [14] as the catalysts were easily
deactivated, so ex-situ approach was employed in the present study by which the catalysts
were employed downstream of the gasifier in a secondary catalytic reactor. The guard bed
reactor comprised dolomite stones (to improve the life of nanocatalyst as recommended by
Milne et al [17] ) and was arranged in series with a main catalytic reactor containing the
synthesized bimetallic nano Ni-Co/Si-P catalyst. Both the reactors were fabricated with

stainless steel. The dolomite used in guard bed captures the fine particulates and converts the
heavy tars, while the bimetallic nano Ni-Co/Si-P catalyst reforms the lighter tars into carbon
monoxide and hydrogen. Both the reactors were wound with electrical resistance coils so as
to maintain them in the desired temperatures.
A proximate analyzer comprising of muffle furnace and micro weigh balance
with associated auxiliaries was employed to establish Casuarina wood characteristics.
Parameters like moisture content (ASTM E 871-82), volatile matter (ASTM E 872-82) and
ash content (ASTM D 1102-84) were determined while rest was assumed to be fixed carbon.
Standardized (Benzoic acid based) bomb calorimeter was used to establish the calorific value
of feed material. The calorific value of producer gas was determined by a Junkers gas
calorimeter. Siemens make online gas analyzers viz Oxymat 61 (estimates O2 using
paramagnetic principle), Ultramat 23 (estimates CO, CO2 and CH4 using non dispersive
infrared multilayer technology) and Calomat 61 (estimates H2 using thermal conductivity
principle) was used to determine producer gas composition and was logged to the PC using
Siprom-GA software. Gas sampling system comprised of wash bottle, condensation pot,
coalesce filter, suction pump, fine filter, flame arrestor and diaphragm pump. Tar and

Page 4 of 19


particulate sampling and analysis was accomplished using a tar sampling and analysis setup
established as per the guidelines of International Protocol for measurement of organic
compounds in producer gas (Technical report CEN BT/TF 143, 2005). The temperatures at
different zones were measured using chromel-alumel (K- type) thermocouples which were
fixed permanently in all zones except in throat where thermocouples were inserted along
tuyueres at regular intervals and was logged to a PC using Agilent make (34907 A) data
acquisition system. Kane make Infrared thermometer (UEI-INF 200) was employed to
measure surface temperatures. Air flow to the gasifier was measured using orificemeter and
producer gas flow was measured by using a venturimeter. The required equivalence ratio
(ER) was established by controlling the air flow to the gasifier by a butterfly valve placed at

the discharge end of the centrifugal blower. Pressure measurement was accomplished by
deploying U tube manometers filled with water. The guard bed and catalytic reactor was
fixed with two thermocouples one at the center of fixed bed, which was moveable for
obtaining longitudinal temperature profiles and other at the perimeter of the bed.

2.1. Characterization of feed material
The casuarina wood was sized by an electric cutter to 40 mm diameter (approx.) and
53 mm length and the physical and chemical characteristics are presented in Table 1.
3. Synthesis and characterization of Ni-Co/Si-P nano catalyst
The tar cracking catalysts are divided into two major groups namely nickel based
catalysts and noble metal based catalysts [35]. Noble metals catalysts such as Pt/Ru/Rh are
more resistant to coking, but are very expensive and have limited availability, so they are not
preferable for use in industry. Ni-based catalysts are more suitable and widely used for tar
cracking, because of their enhanced catalytic activity, availability and low cost [36]. In
addition, it has been reported that the Nickel based catalysts have been extensively used for
tar cracking due to their strong ability for C-C bond rupture of tar compounds [37 & 38].
Hence, in the present study, nano Ni-Co/Si-P catalyst have been synthesized for tar cracking
reaction.
The Ni-Co/Si-P catalyst was synthesized by Deposition – Precipitation (DP) method.
Initially silicophosphate support was prepared by condensation of Tetraorthosilicates [TEOSMerck] and Triethyl Phosphite [TEPI] in ethanol solvent by adding 0.1 M of hydrochloric
acid. The volume ratios of (TEOS, TEPI), HCl and Ethanol was chosen as 1:0.25:6

Page 5 of 19


respectively. The mixture was stirred in a magnetic stirrer at 60°C for one hour. This
solution had transformed into silico phosphate gel. To incorporate metal ions of nickel and
cobalt on silicophosphate, hydrated nickel nitrate Ni(NO3)2.6H2O (Lobal) and hydrated cobalt
nitrate-Co(NO3)3.6H2O (Spectrochem) were chosen as precursor respectively. Nickel and
cobalt nitrates were dissolved in deionized water with a mole ratio of 0.15 and 0.05. Capping

agent cetyl trimethyl ammonium bromide (CTAB) in concentration of 2.1*10-4 mol/l were
also dissolved in above metal ion solution. This solution was transferred into a glass vessel
containing silicophosphate gel with constant stirring. After complete mixing of metal ions
and silicophosphate gel, sodium hydroxide (NaOH-0.1M) was added to precipitate the metal
ions as metal hydroxides into silicophosphate particles. Resulting precipitates were filtered by
using whatmann filter paper and washed with ethanol and deionized water for removing the
possible absorbed ions and chemicals. The resulting sample was dried in hot air oven at
120°C for two hours at heating rate of 10°C/min. Dried samples were calcined in muffle
furnace at 600°C for six hours at a heating rate of 20°C/min. This process resulted in
formation of Ni-Co/Si-P Nano catalyst. The above procedure yielded bulk Ni-Co/Si-P
catalyst in absence of capping agent. The obtained sample was pulverized, pelletized and
subjected to series of characterization and was used in the experiment.
3.1 Crystal Structure of Ni-Co/Si-P nano catalysts
The X- Ray Diffraction (XRD) pattern of the synthesised Ni-Co/Si-P Nano catalysts
is shown in Fig 3. All the diffraction peaks match with the standard data [JCPDS (Joint
committee of Powder Diffraction Studies) card no 47-1049 for Nickel Oxide, 22-0595 for
Cobalt Oxide, and 22-1380 for Silico Phosphate] and no characteristic peaks of any
impurities are detected in the pattern, which indicates that all the samples have high phase
purity. In addition, the peak width broadens due to the smaller particle size distribution. The
average crystallite size was calculated using Scherer formula [39] given in Equation (1).
L

0 . 89 

 cos 

(1)

where L is the crystallite size, λ, the X-ray wavelength, θ, the Bragg diffraction angle
and β, the peak width at full wave half maximum (FWHM). The average crystallite size ‘L’

calculated from the (111) diffraction peak was found to be 10 nm.

Page 6 of 19


3.2 Size and morphology of Ni-Co/Si-P nanostructures
High resolution scanning electron microscope (HR-SEM) observations confirm the
morphology of Ni-Co/Si-P nanostructures prepared using capping agent (CTAB), as
presented in Fig. 4 (a-b). It is obvious that the morphology of the nanocrystals changes in the
presence of capping agent. When the concentration of capping agent as 2.1×10−4 mol/l is
adopted, a high yield of Ni-Co/Si-P spherical nanoparticles (SNPs) are obtained with
diameters of 5 - 10 nm. The presence of small amounts of Co and Ni on Si-P inhibits the
growth rate and resulted in the formation of Ni-Co/Si-P with small size distributions.
To provide further evidence in the formation of Ni-Co/Si-P SNPs, High resolution
transmission electron microscope (HR-TEM) analysis was carried out. A HR-TEM image of
typical Ni-Co/Si-P SNPs is presented in Fig 5(a, b), indicating that the spherical nanoparticles
are self-assembled. The inset of Fig 5(b) shows the corresponding selected area electron
diffraction (SAED) pattern. The pattern implies that the Ni-Co/Si-P SNPs are good
crystalline material with single crystalline nature. For the purpose of particle size comparison,
the bulk Ni-Co/Si-P catalyst has been prepared without using capping agent. Thus when pure
precursors is adopted (without capping agent), a high yield of Ni-Co/Si-P micro-crystals are
obtained as shown in Fig 6 (a-b). A HR-SEM image of Ni-Co/Si-P with bulk morphology
clearly indicates that the catalysts grows along (001) plane and self-aggregated as microcrystals. The above result clearly indicates that the desired morphology could only be
achieved by suitably tuning the concentration of capping agent. It is presumed, the use of NiCo/Si-P SNPs may result in increased catalytic activity than the Ni-Co/Si-P micro-crystals.

3.3 Surface area analysis of Ni-Co/Si-P SNPs
The surface area, pore volume values and particle size are given in Table 2.
Generally, a high specific surface area has a beneficial effect on the activity for catalysts. In
this work, the surface area of Ni-Co/Si-P SNPs and Ni-Co/Si-P microcrystal are found to be
128 and 60 m2/g, as calculated adopting Brunauer - Emmett - Teller (BET) method. High

specific surface area of Ni-Co/Si-P SNPs would be beneficial to the catalytic activity via
enhancing the adsorption of reactant molecules, which is the determining step in the
heterogeneous catalytic reaction. The pore size distribution was determined using the BJH
(Barrett, Joyner and Halenda) method. The catalytic activity of Ni-Co/Si-P SNPs is relatively
higher than that of Ni-Co/Si-P micro-crystals due to the presence of mesopores.

Page 7 of 19


3.4 Thermal analysis of Ni-Co/Si-P SNPs
The result of Thermo Gravimetric analysis (TGA) of Ni-Co/Si-P SNPs is illustrated in
Fig 7. The initial weight loss below 100°C was due to desorption of water. The weight loss
between 100 and 400°C was due to decomposition of cobalt nitrates. Weight loss between
450-650°C was observed to be due to decomposition of nickel nitrate. The resulting cobalt
oxide and nickel oxide were verified to be stable up to 900°C as there was no weight loss
between 650°C and 900°C.

4. Results and Discussion
4.1. Base line fixation
Experiments were conducted without employing catalysts to fix the best optimum
range to yield the lowest concentration of tar and highest calorific value of producer gas. It is
well known that the gasifier performance is a function of bed temperature, moisture content
of feed material and equivalence ratio (ER). As the gasifier was operated on auto - thermal
mode, where the heat requirement for gasification was met from combusting part of the feed
material, the option of varying the bed temperature was ruled out. Non availability of the feed
material with variable moisture content has sealed the possibility of analysis of moisture
content on performance of the gasifier. A series of experiments were conducted by changing
the air flow rate, thereby varying the equivalence ratio in the range of 0.1 - 0.5, to investigate
the influence of ER on gasification behaviour, while other operating parameters such biomass
feed rate, bed temperature and biomass particle size were maintained constant. Equivalence

ratio is defined as the ratio of actual air flow rate to biomass flow rate to that of stoichometric
air flow rate to biomass flow rate. From Fig 8, it is evident that tar concentration decreases
with increasing ER. This may be attributed to the reaction between the volatiles and excess
oxygen in the pyrolysis zone, resulting in combustion of tar and increment in gasifier
temperature which also aided the thermal cracking of tar.

Tar cracking has resulted in increment of volumetric composition of gas. The CO
composition increases with increment in ER up to 0.3 and then decreases. The maximum
value of CO occurred as 14 vol. % at ER =0.3. The trend of CO2 was opposite to that of CO.
The composition of CO2 decreased up to ER = 0.3 and then increased. This might be due to
occurrence of reverse boudouard reaction (C + CO2 + heat =2CO) in the lower ER regions.

Page 8 of 19


Enhanced combustion reactions have predominated in the region of ER > 0.3, forming higher
CO2, owing to availability of excess O2. H2 composition increased up to an ER of 0.3 due to
cracking of tar and hydrocarbons, but decreased at higher ER values due to formation of H2O
as the excess O2 reacted with hydrogen to form water vapour. Due to higher temperature
formed in the gasifier at higher ER region, hydrocarbons like CH4 got thermally cracked, so
their values reduced consistently at higher ER. The zero O2 values expose perfect gasification
and non-occurrence of fuel bridging inside the gasifier. Higher ER value indicates increment
in N2 value which occurred due to the availability of quantum of N2 supplied along with the
air.
The calorific value of the producer gas is a function of combustibles present viz., H2,
CO & CH4. From Fig. 9 and 10, it is inferred that the minimum ratio of CO2/CO and
maximum value of combustibles occurred at ER = 0.3. From Fig. 11, it is inferred that the
calorific value of producer gas increases till an ER of 0.3 and recedes at higher ER due to
increment of CO2 and N2 composition. From Fig.12 it is inferred that the cold gas efficiency
increases up to ER = 0.3 and reaches 71.24%, while it recedes at higher ER value due to

decrement in combustibles and calorific value.

An ER of lesser than 0.3 yielded higher tar due to lesser temperature, while ER value
greater than 0.3 resulted in lesser tar possibly due to availability of more O2 (better
combustion resulting in high temperatures thereby facilitating thermal cracking of tar).
Higher ER values has also resulted in generation of more non combustibles such as CO2 and
N2, thereby reducing the calorific value of gas and cold gas efficiency. Hence the best
operating point with optimum tar concentration and highest caloric value and cold gas
efficiency was concluded as ER = 0.3. The typical gasification parameters at ER = 0.3 are
depicted in table 3. The presence of higher tar indicates requirement of frequent maintenance
and also possible premature failure of equipment’s associated with polygeneration systems.
Having fixed an optimal value of ER as 0.3, experimental analysis was carried out to
ascertain the capability of nano Ni-Co/Si-P SNPs catalyst in tar mitigation.
4.2 Analysis of catalytic tar mitigation
The tar cracking experiments were carried out using nano and bulk Ni-Co/Si-P
catalysts. From the generated producer gas, experiments were conducted with a slipstream
drawn at the rate of 0.016 l/sec, being passed across the catalytic tar conversion system
comprising of a guard bed containing dolomite stones (CaMgCO3) and main catalytic reactor

Page 9 of 19


containing Ni-Co/Si-P catalyst. Initially the catalytic reactor and guard bed reactor were
deployed with 10 grams of Ni-Co/Si-P SNPs catalyst and 100 grams of crushed dolomite
stones (CaMgCO3). The Ni-Co/Si-P SNPs catalyst was not subjected to prior reduction, as
the producer gas itself can reduce the catalyst. The catalytic reactor was operated at variable
temperatures of 725 - 825 °C in steps of 25 °C increment, while the guard bed reactor
comprising dolomite stones were maintained constantly at 650°C. The catalytic tar cracking
experiments were performed with the operating conditions , ER of 0.3, biomass feeding rate
of 6kgh-1 ,throat temperature of 720°C (avg), nickel loading of 15 wt % and gas residence

time of 0.3s. The results of tar cracking rate as a function of catalytic bed temperature is
depicted in Fig.13. As expected tar cracking rate steadily increased with increase in catalytic
bed temperature. At 800°C no visible tar was observed in the tar sampling lines after the
catalytic reactor. In addition isoproponal samples recovered from the impinger bottles
showed no hint of tar. The tar analysis conducted at 110°C on rotary flash evaporator
indicated 99.0% and 99.2% tar removal rates at catalytic bed temperature of 800°C and
825°C respectively as indicated in Fig 13. The tar cracking temperature of 800°C was fixed
as the optimum value, as further increment in temperature required additional energy input,
for a slight improvement in tar cracking rate. It is inferred that H2 and CO reached 24 and 16
vol %, indicating an increment of 77 % and 14 % respectively against non-catalytic mode,
whereas CO2 and CH4 decreased substantially as shown in Fig.14. It is inferred that the
calorific value, increased to 5.22 MJ/m3 as indicated in Fig 15. For identical experimental
conditions, the bulk catalyst exhibited tar removal rate of 91.5 %.

The significant decrement of tar is attributed to the secondary cracking of tar
constituents on the Ni-Co/Si-P SNPs catalyst in the catalytic reactor as denoted by
hydrocarbon reforming reactions (2-3) and water shift reactions (4-5).
CnHm + 2nH2O ↔ nCO + (n + m/2) H2

(2)

CnHm + nCO2↔ 2nCO +m/2 H2

(3)

CO + H2O ↔CO2 + H2

(4)

C + H2O ↔ CO + H2

(5)

Page 10 of 19


In addition, Hyun Ju Park et al. [40] reported very low benzene (model tar compound)
removal efficiency of 38.8% and 57.7% using Ni/ZrO2 and Ni-CeO2 catalysts. Abu-El-Rub et
al. [41] reported lower phenol (model tar compound) conversion efficiencies of 34.5% and
42.7% using silica sand and Olivine catalysts respectively.

These lower tar cracking

efficiencies might be due to the operating conditions and catalyst properties.
The higher tar cracking activity of the Ni-Co/Si-P SNPs catalyst has been attributed to
some important features of the catalyst such as particle size, surface area and porosity as
discussed below. The particle sizes of Ni-Co/Si-P catalysts play an important role in the
catalytic activity of tar cracking. Christensen et al. [42] reported that smaller particle size is
attributable to an increase in the catalytic activity of tar cracking. Moreover, as the particle
size decreases, the number of active surface sites increases. Thus, it is expected that NiCo/Si-P SNPs with very smaller particle size distribution would be a potentially efficient
catalyst than the bulk catalyst. In addition, Ni-Co/Si-P with spherical morphology had smaller
particle size distribution, as indicated by the XRD, HR-SEM and HR-TEM data, showed
considerably higher catalytic activity for tar cracking.
Usually, a high specific surface area has a beneficial effect on the activity for
catalysts. High-surface area catalyst [43] showed considerably higher catalytic activity for tar
cracking than the bulk catalyst. In this work, the surface areas of the Ni-Co/Si-P SNPs
catalysts exhibited an surface area increment of 113% more than the bulk catalyst. This high
incremental surface area of Ni-Co/Si-P SNPs was beneficial to tar cracking catalytic activity
via enhancing the adsorption of tar compounds on the surface of the catalyst.
Porosity of the catalyst is also one of the beneficial criteria for the catalytic cracking

of tar. The mesopores volume of Ni-Co/Si-P SNPs is 0.2469 cm3/g (as shown in Table 2)
provided more favourable mass transfer conditions than bulk catalyst, this is consistent with
the observations of Gao et al. [44]. The mesoporosity and large surface area of Ni-Co/Si-P
catalysts permit the active nickel sites to be dispersed in more areas that are accessible to
larger tar molecules, allowing these to be cracked into smaller molecules such as H2, CH4,
CO, and CO2 according to equations 1-4. As a result, Ni-Co/Si-P SNPs catalyst with
mesopores showed enhanced tar cracking catalytic activity than the bulk catalyst.
4. Conclusion
The tar quantity of 1.8 g/Nm3 was obtained when the gasifier was operated in absence
of catalyst at an optimum equivalence ratio of 0.3. The tar cracking performance of the NiCo/Si-P bulk and SNPs catalyst was evaluated. Tar mitigation is possible by both bulk and

Page 11 of 19


nano Ni-Co/Si-P catalyst. Tar removal efficiency reached 99% using Ni-Co/Si-P spherical
nano particles, at catalytic bed temperature of 800°C. The temperature of the catalyst bed
exerts due influence on tar cracking and it is concluded that the optimum tar cracking
temperature was 800°C. The gas composition patterns also increased remarkably. The H2 and
CO composition increased to 24 vol % and 16 vol % respectively while CO2 and CH4
composition decreased. The calorific value increased to 5.22 MJ/m3. The bulk Ni-Co/Si-P
catalyst exhibited lesser tar cracking rate of 91.5 %. This study revealed that Ni-Co/Si-P
spherical nano particles are more capable of tar mitigation and enhancing the producer gas
quality in biomass gasifiers, than bulk catalyst. In addition it was inferred that the low cost
Ni-Co/Si-P spherical nano particles are equally capable of tar mitigation like the high cost
Ruthium and Rhodium catalysts, enabling its application in industry based biomass gasifiers
employed for polygeneration.
Acknowledgment:
The authors gratefully acknowledge Department of Science and Technology (DST),
New Delhi, Govt. of India for providing financial support to carry out this research work
under PURSE scheme. One of the authors, Mr. K.Shanmuganandam is thankful to DST, New

Delhi for awarding DST PURSE fellowship to pursue this research.
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Page 16 of 19


Fig 1. Schematic of experimental setup
Fig 2. Gasifier in operation (Inset: Flame port as seen through sight glass)
Fig 3. XRD pattern of the as-synthesised Ni-Co/Si-P SNPs
Fig 4. HR-SEM images of the as-synthesised Ni-Co/Si-P SNPs
Fig.5 HR-TEM images & corresponding SAED pattern of as-synthesised Ni-Co/Si-P SNPs
Fig 6. HR-SEM images of the as-synthesized Ni-Co/Si-P micro-crystals
Fig 7. TGA pattern of Ni-Co/Si-P SNPs
Fig 8. Variation of tar and gas composition w.r.t. ER.
Fig 9. Variation of CO2/ CO w.r.t. ER.
Fig 10. Variation of combustibles and noncombustibles w.r.t. ER.
Fig 11. Variation of calorific value w.r.t. ER.

Fig 12. Variation of cold gas efficiency w.r.t. ER
Fig 13. Effect of catalytic bed temperature on tar cracking efficiency
Fig 14. Effect of catalytic bed temperature on gas composition
Fig 15. Effect of catalytic bed temperature on calorific value

Page 17 of 19


Table 1. Characterization of Casuarina wood
Proximate analysis (Wt %)

Moisture

10

Volatile
Matter
69

Ash

2.5

Fixed
Carbon
18.5

Other influential

Ultimate analysis (Wt %)

properties
Bulk

Carbon Hydrogen Nitrogen Oxygen Density
3

45.5

7

3.5

44

Calorific
Value

(kg/m )

(MJ/kg)

417

16.06

Table 2. BET analysis of Ni-Co/Si-P
S. No

Catalyst

Surface area

Pore Volume

Particle size

(m2/g)

(cm3/g)

(nm)

1

Nano Ni-Co/Si-P SNPs

128

0.246980

10

2

Bulk Ni-Co/Si-P

60

0.08760

>100

Page 18 of 19


Table 3: Typical gasification parameters at ER = 0.3
Gasification parameter

Unit

Value

Gas composition
CO

14

CO2

9

CH4

Vol %

3.5

H2

13.5

N2

60

O2

0

HHV of producer gas

MJ/m3

4.89

Concentration of tar

g/Nm3

1.8

Concentration of particulate matter Mg/Nm3

175

Specific gas generation (SGR)

m3/kg of fuel

Cold gas efficiency

%

Pressure drop across bed

mm Wc

Throat temperature

°C

720

Gas outlet temperature

°C

590

2.34
71.24
10

Page 19 of 19