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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 PII: DOI: Reference: S1359-4311(15)01211-9 http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.150 ATE 7267 To appear in: Applied Thermal Engineering Received date: Accepted date: 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), http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.150 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain 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: shanmugam_anandam@yahoo.com, Ph: +91-94865 17139 b Assistant Professor (Sr.), Institute for Energy Studies, College of Engineering Guindy, Anna University, Chennai 600025, Tamilnadu, INDIA E-mail: venkat@annauniv.edu, Ph: +9144-2235 7912 c Professor, Institute for Energy Studies, College of Engineering Guindy, Anna University, Chennai 600025, Tamilnadu, INDIA E-mail: rsaravanan@annauniv.edu, Ph: +91-44-2235 7607 d Deputy Chief Engineer, Department of Mechanical Engineering, Saipem India Projects Ltd, Chennai 600034, INDIA E-mail: anichai_j@yahoo.com, 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 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 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 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 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 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 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 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 and 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 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 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 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 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 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 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 (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 - 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 (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 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 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 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 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 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 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 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 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 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 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pyrolysis International Journal of Hydrogen energy 37(2012) 9590 - 9601 44 F Gao R, Dai W-L, Yang X, Li H, Fan K Highly efficient tungsten trioxide containing mesocellular silica foam catalyst in the O-heterocyclization of cycloocta-1,5-diene with aqueous H2O2 Appl Catal A Gen 45(2007) 332- 340 Page 16 of 19 Fig Schematic of experimental setup Fig Gasifier in operation (Inset: Flame port as seen through sight glass) Fig XRD pattern of the as-synthesised Ni-Co/Si-P SNPs Fig 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 HR-SEM images of the as-synthesized Ni-Co/Si-P micro-crystals Fig TGA pattern of Ni-Co/Si-P SNPs Fig Variation of tar and gas composition w.r.t ER Fig 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 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 45.5 3.5 44 Calorific Value (kg/m ) (MJ/kg) 417 16.06 Table BET analysis of Ni-Co/Si-P S No Catalyst Surface area Pore Volume Particle size (m2/g) (cm3/g) (nm) Nano Ni-Co/Si-P SNPs 128 0.246980 10 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 CH4 Vol % 3.5 H2 13.5 N2 60 O2 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