Electrical performance evaluation of Johansson biomass gasifier system coupled to a 150 KVA generator

A Johansson biomass gasifier system at Melani village in Eastern Cape South Africa was installed to assess the viability of biomass gasification for energy production in South Africa.. Mas

Electrical performance evaluation of Johansson biomass gasi fier

system coupled to a 150 KVA generator

a University of Fort Hare, Institute of Technology, P/Bag X1314, Alice 5700, South Africa

b University of Fort Hare, Physics Department, P/Bag X1314, Alice 5700, South Africa

a r t i c l e i n f o

Article history:

Received 4 November 2013

Accepted 12 June 2014

Available online

Keywords:

Gasification

Downdraft gasifier

Electrical performance

Conversion efficiency

a b s t r a c t The economic development of any community or society at large is directly linked to energy availability and usage Concern for climate change due to intense use of fossil fuel for energy production has increased interest in alternative energy technologies such as biomass gasification A Johansson biomass gasifier system at Melani village in Eastern Cape South Africa was installed to assess the viability of biomass gasification for energy production in South Africa This system utilizes chunks of wood coming from a sawmill industry located nearby, which produces large quantities of biomass waste that pose a challenge in terms of disposal A study on the implementation of the latter gasification project has been carried out Therefore this present study aims at evaluating the performance of the system when operated on a full electrical load A custom-built gas and temperature profiling system was used to measure the gas profiles from which the gas heating value was calculated A measuring balance/scale was used to measure the quantity of wood fed into the gasifier A dummy load bank was constructed using

12 kW water heating elements connected such that they draw maximum power from each of the three phases A power meter was used to measure the current, voltage, power as well as energy from the generator during operation A cold gas efficiency of 88.11% was obtained and the overall efficiency from feedstock to electrical power was found to be 20.5% at a specific consumption rate of 1.075 kg/kWh

© 2014 Elsevier Ltd All rights reserved

1 Introduction

Biomass is an organic material that stores solar energy via

photosynthesis and in turn creates a source of energy in form of

carbon, hydrogen and oxygen compound It contains less carbon

but more oxygen and a lower heating value than the conventional

fossil fuel[1] Conversion of biomass into other forms of energy

such as electrical energy and heat energy is a promising alternative to

fossil fuel due to its renewable nature and availability Two major

conversion processes (conversion to power, heat, transportation fuel,

chemicals and methane gas) exist but the choice of any is dependent

on the end use application, environmental impact, economic factor,

the type and properties of the biomass[2,3] These processes are

thermochemical conversion and Biochemical conversion

Biochemical conversion involves the fermentation of plant

ma-terial with the use of yeast or genetically modified microorganisms to

produce ethanol and anaerobic digestion of plant material to pro-duce methane gas Thermochemical conversion involves the ther-mal conversion of biomass material at different temperatures and oxygen environment It includes pyrolysis, gasification and com-bustion This research is focused on biomass gasification, which is the thermal conversion of carboneous material in a controlled ox-ygen, air or steam environment to yield a mixture of gases known

as producer gas and usually referred to as syngas This thermo-chemical process takes place in a gasifier/reactor which comes in different designs namely; downdraft, updraft and cross draft (fixed bed), bubbling, circulating and twin bed (fluidized bed), coaxial and opposed jet (entrainedflow)[4] The advantages and disadvantages

of these types of gasifiers are well known and documented[5,6] The downdraft gasifier has an advantage of producing gas with low tar concentration, which makes it suitable for operating gas engines and turbines used for electricity generation The concen-tration of tar in the producer gas generated from a downdraft gasifier is relatively low when compared to that from updraft gasifier It ranges between 10 and 100 g/Nm3for downdraft gasifier and 50 and 500 g/Nm3for updraft gasifier[7] Downdraft gasifiers are mostly used due to the earlier mentioned advantage Dogru

et al [8]investigated the gasification characteristics of hazelnut

* Corresponding author University of Fort Hare, Institute of Technology, P/Bag

X1314, Alice 5700, South Africa Tel.: þ27 833433195.

E-mail addresses: nnwokolo@ufh.ac.za , nwokolonwanne@yahoo.com

(N Nwokolo).

Contents lists available atScienceDirect Renewable Energy

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Renewable Energy 71 (2014) 695e700

shell using a pilot scale downdraft gasifier with 5 kW electrical

outputs An experimental study has been carried out by Sharma[4]

on a 75 kW th downdraft gasifier system The temperature profile

was obtained along side with gas composition, calorific value and

pressure drop across the porous gasifier bed Jayah et al.[9]

cali-brated a computer program from an experimental result in order

to investigate the impact of some operating and design

parame-ters on conversion efficiency of a downdraft gasifier The

oper-ating parameters were moisture content, particle size and inlet air

temperature and the design parameters were throat angle and

heat loss

The aim of this study is to analyze the performance of a 150 kVA

Johansson biomass gasifier system when operated on a full load

bank The load bank comprises of 12 kW heating elements

con-nected in parallel so as to draw maximum power from the

gener-ator on each of the three phases

2 Description of a Johansson biomass gasifier system

Fig 1represnts the different components of Johansson biomass

gasifier system This system comprises of a reactor/gasifier,

cyclone, scrubber, sawdustfilter, safety filter, condensate tank and

electrical generator

The reactor consists of four zones corresponding to the four

gasification processes, namely: drying, pyrolysis, oxidation and

reduction The feedstock (pine wood) is fed into the gasifier hopper

through the lid using an electrically controlled winch Air

con-taining oxygen and some non-reactive gases such as nitrogen is

blown into the oxidation zone through the air nozzles to start the

combustion process A 2 kW centrifugal blower is used to simulate

the engine suction when igniting the gasifier The gasifier is then

ignited by inserting two or three party sparklersfitted in a sparkler

holder through the ignition sleeve Once combustion has started

the oxygen content of the air reacts with solid carbonized fuel and

hydrogen in the fuel as represented in Eqs(1) and (2)to produce

carbon dioxide and steam respectively

These exothermic reactions then provide the necessary heat that drives the other processes For instance, in the reduction zone, the extent to which the endothermic reactions represented by Eqs

(3)e(4)occur depends on the quantity of heat it receives from the oxidation zone Furthermore this heat is used to drive off the moisture present in the wood at the drying zone[10,11]

Cþ H2O 4 CO þ H2 þ 131 kJ=mol (4)

The gas emanating from this zone (reduction) then goes through the gas purification system consisting of the cyclone, gas scrubber/ cooler, particle interference/sawdustfilters as well as a Donaldson

5mm paperfilter

2.1 Purification units 2.1.1 Cyclone The raw gas exiting through the bottom of the reactorfirst goes through the cyclone in a tangential manner Here about 80% of the coarse carbon particles and soot embedded in the raw gas are removed through centrifugal and inertia forces and exit through a pipe sealed by a rotary valve The centrifugal force causes the par-ticles to collide with the outer wall while moving downwards with the gasflow through inertia At the bottom of the cyclone the gas flow reverses its direction and begins to move up It then exits through a vortexfinder at the top of the cyclone while the particles exit through the bottom The particle collection efficiency of the cyclone depends on the size of the particles and the design of the cyclone as they come in different designs[11]

2.1.2 Gas scrubber/cooler The hot gas from the cyclone enters the scrubber through the bottom at a temperature of about 500C and exits through the top

at room temperature (25C) This loss of heat is undesirable in most applications, especially where the heat from the gas can be recycled and reused[20] In addition the scrubber removes the remaining

Gasifier

Cyclone

Gas Scrubber

Sawdust filter

Safety Filter

Electrical generator

Cooling pond

Pump

Fig 1 Schematic diagram of Johansson biomass Gasifier.

N Nwokolo et al / Renewable Energy 71 (2014) 695e700

fine carbon particles and soot in the gases that pass through the

cyclone This process washes off about 0.8 g/m3of gas, which is

translated to about 20% of thefine carbon particles These particles

(those less than 0.1mm) are collected by diffusion when water is

sprayed from the top of the scrubber while particles greater than

1mm settle by gravity and are collected gravitationally, by

impac-tion or by centrifugal means[11]

2.1.3 Sawdust and safetyfilter/paper filter

The sawdustfilter acts as a barrier and captures the very fine

carbon particles that exeunt with the gas through the scrubber The

sawdustfilters are filled with very fine sawdust that collects particles

through adsorption Lastly the clean gas goes through the safetyfilter,

a double cartridge Donaldson air tightfilter with a special gas tight

seal between the dust bowl and the body of thefilter

2.1.4 Electrical generator

The electric power generator is a self excited three phase

syn-chronous generator equipped with an automatic voltage regulator

This is an internal combustion gas engine, which was formerly

operated on diesel but modified to operate on a 100% producer gas

emanating from the gasifier The three phase alternator coupled to

the producer gas engine has a capacity of 150 kVA which works out

to be 120 kWe power It operates with a compression ratio of 14.5:1

Table 1presents the details of the engine configuration

3 Method and experiments

3.1 Gas analysis, ultimate and proximate analysis

The mass of the feedstock was determined using a measuring

balance/scale before feeding into the gasifier hopper The ultimate

and proximate analyses were done to determine the physical and

chemical properties of the pine wood used for this study The

calorific value of the material was determined using a CAL2K

oxy-gen Bomb calorimeter

Gas analysis was undertaken using a custom-built Gas and

Temperature Profiling System (GTPS), which employs non-dispersive

infrared gas sensors for measurement of methane, carbon

monox-ide, carbon dioxide and a palladium/nickel (Pd/Ni) gas sensor for

measurement of hydrogen The differential voltage outputs are

log-ged into a CR1000 data logger which comprises of a central

pro-cessing unit (CPU), analog and digital inputs and outputs It has eight

differential or 16 single-ended analog inputs for measuring

volt-ages up to 5 V The logged data are then downloaded into the

computer and transformed to percentage composition[12] The

calorific value of the gas (CVgas) was determined from the

per-centage composition of combustible gases as shown in Equation6

CVgas¼ðCOvol*COHVÞ þ ðH2vol*H2HVÞ þ ðCH4vol*CH4HVÞ

WhereCOvol, H2voland CH4volrepresents the volume concentration

of carbon monoxide, hydrogen and methane present in the pro-ducer gas respectively COHV, H2HVand CH4HVrepresent the heating value of these gases as stated in the standard gas table

The electrical performance of the generator was measured using

a portable energy meter which is capable of providing the load profiles of the generator phases when powering the load bank The energy meter recorded all the energy parameter from the three phase generator at a preset interval of 1 min The recorded data usually presented either in a graphical or statistical formats was downloaded into the computer via the powerTrack software for analysis This power track software serves as an interface between the computer and the energy meter, it allows communication to exist between the two devices

3.2 Mass balance/energy balance and efficiency determination The electrical output of the generator was measured when powering a load bank connected to it The load bank was con-structed using 12 kW water heating elements connected such that they draw maximum power from each of the three phases.Fig 1

shows an electrical circuit of the load bank designed using

a personal computer simulation program with integrated emphasis (pspice) Pspice simulates the behavior of electrical circuit, hence allowing the evaluation of circuits without physically building the circuit This in turn saves money and time for the designer The electrical circuit inFig 2represents the actual con figura-tion of the load bank connected to draw power from the three phase generator The power generated from the constant 400 V line to line voltages arranged in star connection was at a desired frequency range of 50 Hze55 Hz It can be deduced from the circuit that each line contained four 12 kW heating elements connected in series (total power demanded by loads on each line

is 48 kW) The line current under the full load condition was ideally 120 A and the total power dissipated by the elements from the three lines was 144 kW

The total weight of material (Min) that entered the downdraft gasifier was estimated as follows:

Where Wwis the weight of wood in kg that was consumed in the gasifier and Aw is the mass (kg) of air used The air flow rate was measured in Nm3using an anemometer and was latter con-verted to kg The total weight of output product Pout was also estimated as follows:

Where Gqand Fpare the total quantity of gas in kg andfine particles generated in kg respectively The total quantity of gas was deter-mined from the gas production rate of the gasifier, which is

300 Nm3/hr for Johansson biomass gasifier Fine particles were measured using a measuring balance

The energy balance of the downdraft gasifier was determined from total quantity of energy that went into the gasifier and the total quantity that came out as follows:

Where Einand Eoutis the total input energy and output energy in

MJ CVfuel is the calorific value of fuel in MJ/kg and CVgasis the calorific value of producer gas in MJ/Nm3

Table 1

Electrical generator configuration.

Type of coolant 50% ethylene glycol and 50% water

N Nwokolo et al / Renewable Energy 71 (2014) 695e700

The wood to producer gas conversion efficiency of the biomass

downdraft gasifier was estimated according to Eq(11)

CGE¼CVCVgas Ww

The gas to power efficiency of the system was determined

through Eq(12)as follows:

GPE¼ElEenergy

Where GPE represents gas to power efficiency and Elenergyis the

total electrical energy produced from the generator Wood to

electrical power production efficiency known as the overall

effi-ciency of the system is given as

WPE¼ElenergyE

4 Results and discussions

4.1 Wood analysis

Table 2 presents the proximate and ultimate analyses of a

random sample of wood generated from the sawmill industry The

proximate and ultimate analysis were determined to establish the

suitability of the feedstock for gasification purposes The obtained

moisture content is within the range required for downdraft

gasifier Usually high moisture content is unfavorable during

gasi-fication since it lowers combustion zone temperature and thus

leads to production of low quality gas High volatile matter content

shows the ease with which the wood can be ignited The fixed

carbon represents the carbon content of the wood which does not

decompose easily at low temperature Low ash content minimizes

the likelihood of slag formation at high temperature during the

gasification process No sulfur was detected in the wood sample

and oxygen was determined by difference The calorific value of the

wood was found to be 16.34 MJ/kg which is within the known

calorific value of wood This was used to determine the conversion

efficiency of the system

4.2 Gas analysis

Fig 3 shows the gas profiles as obtained from the gas and temperature profiling system On average the percentage compo-sitions of the gases are 29.6% of H2, 18.4% of CO, 18.57% of CO2and 2.6% of CH4 Nitrogen makes up for the remaining composition of the gas, which is relatively high The high percentage of nitrogen is because the Johansson biomass gasifier is an air blown gasifier The calorific value of the producer gas was calculated using Eq(6) This resulted in an average value of 6.3 MJ/Nm3, which is within the range (4e7 MJ/Nm3) reported for air gasification[13,7] This is attributed to the higher quantity of the combustible gases ob-tained The use of air introduces a high quantity of nitrogen to the gas which explains the reason for low calorific value of the pro-ducer gas

4.3 Electrical performance

The electrical output of the generator operated on a 100% pro-ducer gas was monitored using an energy meter During the operation of the generator the frequency varied between 50 Hz and

55 Hz The current in the three phase when the engine was oper-ated at full load (when the electrical demand from the engine is equal to the electrical output deliverable by the engine) varied between 104 A and 114 A The variation in current occurred in the red and yellow phase while the blue phase remained constant at

107 A The voltage was fairly constant all through the operation of the engine for 194 mins An average power of 121.93 kW was ob-tained which is 1.93 kW above the power rating of the engine This shows that the gas fed into the engine was able to drive the engine

to its power rating

4.4 Mass balance of the system

Fig 4shows the mass balance analysis carried out to account for the materials that entered the gasifier and the products that came out In total 718.64 kg of air was fed into the gasifier along side with

424 kg of wood This worked out to be 63% of air and 37% of wood

by mass fraction Translating further showed that every 1 kg of wood was gasified by 1.69 kg of air This therefore corresponds to an equivalence ratio of 0.29 bearing in mind that on average 5.74 kg of air is required for complete combustion of 1 kg of wood This lower equivalence ratio of 0.29 resulted in a gas heating value of 6.3 MJ/

Nm3mentioned earlier which is higher when compared to 4.6 MJ/

Nm3 obtained at an equivalence ratio of 0.4 by Ref.[7] This in-dicates that higher gas heating values are usually obtained at lower equivalence ratio Such is also the case with[14]findings where an

Fig 2 Schematic of the load bank circuit drawn using Pspice.

Table 2

Proximate and ultimate analysis of wood.

Pine wood Proximate analysis % by weight

Ultimate analysis% by weight

N Nwokolo et al / Renewable Energy 71 (2014) 695e700

increase in gasification air ratio from 0.16 to 0.26 resulted in

an increase in gas heating value from 3.6 to 4.2 MJ/Nm3

Equiva-lence ratio is an important gasification parameter that should be

carefully monitored

The output product comprises of producer gas, ash and fine

particles Out of 1142.64 kg of input material 89.77% was converted

to gas and the remaining percentage to ash and fine particles

Simplifying further showed that 1 kg of wood produced 2.29 Nm3

of gas while consuming 1.69 kg of air The obtained gas production

rate (GPR) of 2.29 Nm3is slightly lower than the average value of

2.5 Nm3reported[15].Table 3presents the summary of the input

material and output product for the mass balance analysis

4.5 Energy balance and efficiency determination of the system

The total energy input to the gasifier was estimated from the

total kilogram of wood (424 kg) consumed and the calorific value

of the wood (16.34 MJ/kg) This resulted in a total energy of

6928.16 MJ or 1924.49 kWh while the total energy output from the

gasifier was estimated from total quantity of gas (969 Nm3) and

heating value of the gas (6.3 MJ/Nm3) This gave a total energy of

6104.7 MJ or 1695.75 kWh This therefore indicates that 88.11% of

the energy contained in the wood was converted to gas energy

Hence energy lost while converting wood to gas was then

deter-mined by difference and amounted to 11.9% This loss is accounted

for by the heat lost during the process of cleaning the gas and

through the gasifier itself

The conversion efficiency of the system was evaluated in three stages: First stage from wood to producer gas which was deter-mined according to Eq(11) This resulted in an efficiency of 88.11% generally referred as cold gas efficiency or gasification efficiency Cold gas efficiency depends on the calorific value of gas and the quantity of gas generated as seen in the Eq(11) This value is much higher when compared to about 60e70% reported for wood

gas-ifiers[15]

In the second stage, producer gas to electric power generation

efficiency was evaluated based on the total electrical energy generated during the running of the system The 150 kVA generator coupled to the producer gas engine generated a total of 394.2 kWh

of electrical energy from 969 Nm3of gas supplied to it Working it out further then shows that 2.458 Nm3 of gas was required to generate 1 kWh of electrical energy This therefore gives a producer gas to electric power efficiency of 23.2% Lastly the overall efficiency

of the system was calculated from total fuel consumed in the gasifier and the electrical energy generated from the engine A total

of 424 kg of wood consumed resulted in a total electrical energy of 394.2 kWh Hence, a specific fuel consumption rate of 1.075 kg/ kWh and overall efficiency of 20.5% was obtained This is approxi-mately equal to the overall efficiency obtained for a dual fired downdraft gasifier system[16]but lower than that obtained in a two stage gasification system by 6% [14] The specific fuel con-sumption rate of 1.075 kg/kWh obtained compares very closely to reported values of 1.1 kg/kWh [16] and 1.21 kg/kWh [17] The comparison to other references showed that lesser kilogram of wood is required by the present downdraft gasifier to produce

1 kWh of electrical energy This is an evidence of stable gasifier operating conditions, low ash turn over and low charcoal yield of the Johansson biomass gasifier system.Table 4presents a summary

of some performance parameters obtained and their comparison with literature data

0 5 10 15 20 25 30 35

Time (Minutes)

CO H2 CH4 CO2

Fig 3 Percentage compositions of producer gas.

Downdraft Gasifier

Wood (424 kg)

+ Air (718.64 kg)

Producer gas (1025.76 kg)

Ash + Fine particles (116.88 kg)

Table 3 Summary of Mass balance analysis of the system.

Component Unit

(kg) Mass fraction (%)

Component Unit

(kg) Mass fraction (%) Pine wood 424.00 37.00 Producer gas 1025.76 89.77 Air 718.64 63.00 Ash þ fine particles 116.88 10.23 Total 1142.64 100.00 Total 1142.64 100

N Nwokolo et al / Renewable Energy 71 (2014) 695e700

5 Conclusion

The performance of a Johansson biomass gasifier evaluated in

this study showed that the system was producing power above its

power rating when operated at full load The calorific value of the

gas was estimated to be 6.3 MJ/Nm3at an equivalence ratio of 0.29

This agreed well with literature as summarized inTable 4 A cold

gas efficiency of 88.11% obtained indicates stability in the gasifier

operating condition This was evident also in the calorific value of

the gas obtained The study also revealed that every 1 kWh of

electrical energy generated consumed about 2.458 Nm3of gas

Acknowledgment

The authors would like to acknowledge South African Clean

Energy Solutions limited R8000 for funding the construction of the

load bank and the experimental part of the research We would also

like to acknowledge the National Research Foundation R40000,

Eskom R4.5million and Govan Mbeki Research and development

centre R8000 at the University of Fort Hare for Funding

References

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[3] Shrivastava V, Jha AK, Wamankar AK, Murugan S Performance and emission studies of a CI engine coupled with gasifier running in dual fuel mode Pro-cedia Engineering 2013;51:600e8

[4] Sharma KA Experimental study on 75 kWth downdraft (biomass) gasifier system Renew Energy 2009;34:1726e33

[5] Damartzis T, Zabaniotou A Thermo chemical conversion of biomass to second generation biofuels through integrated process designd a review Renew Sustain Energy Rev 2011;15:366e78

[6] Warnecke R Gasification of biomass: comparison of fixed bed and fluidized bed gasifier Biomass Bioenergy 2000;18:489e97

[7] Martınez DJ, Lora Electo Eduardo Silva, Andrade VR, Jaen LR Experimental study on biomass gasification in a double air stage downdraft reactor Biomass Bioenergy 2011;35:3465e80

[8] Dogru M, Howrath CR, Akay G, Keskinler B, Malik AA Gasification of hazelnut shells in a downdraft gasifier Energy 2002;27:415e27

[9] Jayah HT, Aye L, Fuller JR, Stewart FD Computer simulation of a downdraft wood gasifier for tea drying Biomass Bioenergy 2003;25:459e69

[10] Barrio M, Fossum M, Hustad JE A small-scale stratified downdraft gasifier coupled to a gas engine for combined heat and power production Norwegian University of Science and Technology, Department of Thermal Energy and Hydro Power; 2007 7491 Trondheim, Norway Sintef Energy Research, Department of Thermal Energy, 7465 Trondheim, Norwary

[11] Mamphweli NS, Meyer Edson Leroy Components and operation of the fixed bed downdraft system Johansson biomass gasifier Nova Science Publisher;

2012, ISBN 978-1-61209-681-0 [12] Mamphweli NS, Meyer EL Performance monitoring system for a biomass gasifier Journal of Engineering, Design and Technology 2013;11:7e18 [13] Holmgren MK, Berntsson T, Andersson E, Rydberg T System aspects of biomass gasification with methanol synthesis-Process concepts and energy analysis Energy 2012;45:817e28

[14] Wang Y, Yoshikawa K, Namioka T, Hashimoto Y Performance optimization of two-staged gasification system for woody biomass Fuel Process Technol 2007;88:243e50

[15] Rajvanshi AK Biomass gasification., published as chapter 4 in book In: Goswami DY, editor Alternative Energy in Agriculture, Vol II CRC Press;

1986 pp 83e102 [16] Raman P, Ram KN, Gupta R A dual fired downdraft gasifier system to produce cleaner gas for power generation: design, development and performance analysis Energy 2013;54:302e14

[17] Sridhar G, Paul JP, Mukunda SH Biomass derived producer gas as a recipro-cating engine fueldan experimental analysis Biomass Bioenergy 2001;21: 61e72

[18] Sheth NP, Babu VB Experimental studies on producer gas generation from wood wastein a downdraft biomass gasifier Bioresour Technol 2009;100: 3127e33

[19] Zainal ZA, Ali R, Quadir G, Seetharamu KN Experimental investigations of a downdraft biomass gasifier Biomass Bioenergy 2002;23:283e9

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Table 4

Comparing experimental data with literature.

Biomass

material

Optimum

ER

CV gas (MJ/

Nm 3 )

GPR (Nm 3 / kg)

CGE (%) Reference

Furniture waste 0.205 6.34 1.62 56.87 [18]

Hazel nutshell 0.276 5.15 2.73 80.91 [8]

Wood þ charcoal 0.388 5.62 1.08 33.72 [19]

study

ER ¼ Equivalence ratio.

N Nwokolo et al / Renewable Energy 71 (2014) 695e700