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/si-p catalyst in a biomass gasifier based ni-Co/si-power generating system Author: K Sha

Accepted Manuscript

Title: An experimental analysis on tar cracking using nano structured

ni-Co/si-p catalyst in a biomass gasifier based ni-Co/si-power generating system

Author: K Shanmuganandam, M Venkata Ramanan, R Saravanan, J Anichai

PII: S1359-4311(15)01211-9

DOI: http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.150

Reference: ATE 7267

To appear in: Applied Thermal Engineering

Received date: 13-5-2015

Accepted date: 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

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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 M b , Saravanan R c , Anichai J d

*,a

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

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:

+91-44-2235 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, SEM,

HR-TEM, 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

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/Si-P 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

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

de-hydrogenation [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

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

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

[TEOS-Merck] 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

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)





 cos

89 0



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

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

micro-crystals 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

Ni-Co/Si-P SNPs may result in increased catalytic activity than the Ni-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

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

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