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

Renewable Energy 71 (2014) 695e700

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Electrical performance evaluation of Johansson biomass gasifier
system coupled to a 150 KVA generator
Nwabunwanne Nwokolo a, b, *, Sampson Mamphweli a, Edson Meyer a, Stephen Tangwe a
a
b

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

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 4 November 2013
Accepted 12 June 2014
Available online

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.

Keywords:
Gasification
Downdraft gasifier
Electrical performance
Conversion efficiency

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 material with the use of yeast or genetically modified microorganisms to

* Corresponding author. University of Fort Hare, Institute of Technology, P/Bag
X1314, Alice 5700, South Africa. Tel.: þ27 833433195.
E-mail
addresses:
,

(N. Nwokolo).
/>0960-1481/© 2014 Elsevier Ltd. All rights reserved.

produce ethanol and anaerobic digestion of plant material to produce methane gas. Thermochemical conversion involves the thermal conversion of biomass material at different temperatures and
oxygen environment. It includes pyrolysis, gasification and combustion. This research is focused on biomass gasification, which is
the thermal conversion of carboneous material in a controlled oxygen, air or steam environment to yield a mixture of gases known
as producer gas and usually referred to as syngas. This thermochemical 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 (entrained flow) [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 concentration 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/Nm3 for downdraft gasifier
and 50 and 500 g/Nm3 for updraft gasifier [7]. Downdraft gasifiers
are mostly used due to the earlier mentioned advantage. Dogru
et al. [8] investigated the gasification characteristics of hazelnut


696

N. Nwokolo et al. / 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] calibrated a computer program from an experimental result in order
to investigate the impact of some operating and design parameters on conversion efficiency of a downdraft gasifier. The operating 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 connected in parallel so as to draw maximum power from the generator on each of the three phases.

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].

C þ CO2 4 2CO

þ 172 kJ=mol

C þ H2 O 4 CO þ H2
C þ 2H2 4 CH4

(3)

þ 131 kJ=mol

þ 74 kJ=mol

(4)
(5)

The gas emanating from this zone (reduction) then goes through
the gas purification system consisting of the cyclone, gas scrubber/
cooler, particle interference/sawdust filters as well as a Donaldson
5 mm paper filter.

2. Description of a Johansson biomass gasifier system
2.1. Purification units
Fig. 1 represnts the different components of Johansson biomass
gasifier system. This system comprises of a reactor/gasifier,
cyclone, scrubber, sawdust filter, 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 containing 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 sparklers fitted 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.

C þ O2 / CO2
12O

=

H2 þ

2

À 394 kJ=mol

/ H2 O

(1)

À 284 kJ=mol

(2)

2.1.1. Cyclone
The raw gas exiting through the bottom of the reactor first 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 particles to collide with the outer wall while moving downwards with
the gas flow through inertia. At the bottom of the cyclone the gas
flow reverses its direction and begins to move up. It then exits
through a vortex finder 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 500  C and exits through the top
at room temperature (25  C). 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
Electrical
generator

Cyclone

Gas
Scrubber

Sawdust
filter

Pump

Cooling pond
Fig. 1. Schematic diagram of Johansson biomass Gasifier.

Safety
Filter

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/m3 of gas, which is
translated to about 20% of the fine carbon particles. These particles
(those less than 0.1 mm) are collected by diffusion when water is
sprayed from the top of the scrubber while particles greater than
1 mm settle by gravity and are collected gravitationally, by impaction or by centrifugal means [11].
2.1.3. Sawdust and safety filter/paper filter
The sawdust filter acts as a barrier and captures the very fine
carbon particles that exeunt with the gas through the scrubber. The
sawdust filters are filled with very fine sawdust that collects particles
through adsorption. Lastly the clean gas goes through the safety filter,
a double cartridge Donaldson air tight filter with a special gas tight
seal between the dust bowl and the body of the filter.

697

WhereCOvol, H2vol and CH4vol represents the volume concentration
of carbon monoxide, hydrogen and methane present in the producer gas respectively. COHV, H2HV and CH4HV represent 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

2.1.4. Electrical generator
The electric power generator is a self excited three phase synchronous 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 1 presents 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 oxygen 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 monoxide, carbon dioxide and a palladium/nickel (Pd/Ni) gas sensor for
measurement of hydrogen. The differential voltage outputs are logged into a CR1000 data logger which comprises of a central processing unit (CPU), analog and digital inputs and outputs. It has eight
differential or 16 single-ended analog inputs for measuring voltages 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 percentage composition of combustible gases as shown in Equation 6

CVgas ¼

ðCOvol *COHV Þ þ ðH2vol *H2HV Þ þ ðCH4vol *CH4HV Þ

100

(6)

Table 1
Electrical generator configuration.
Component

Detail/Units

Power output rating
Compression ratio
Nominal bore
Stroke
Cubic capacity
Number of cylinder
Dry weight
Type of coolant
Rated speed

120 kW
14.5:1
135 mm
152 mm
2611 L
12
2120 kg
50% ethylene glycol and 50% water
1500 RPM

The electrical output of the generator was measured when
powering a load bank connected to it. The load bank was constructed 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 in Fig. 2 represents the actual configuration 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:

Min ¼ Ww þ Aw

(7)

Where Ww is 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 Nm3 using an anemometer and was latter converted to kg. The total weight of output product Pout was also
estimated as follows:

Pout ¼ Gq þ Ash þ Fp

(8)

Where Gq and Fp are the total quantity of gas in kg and fine particles
generated in kg respectively. The total quantity of gas was determined 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:

Ein ¼ CVfuel  Ww

(9)

Eout ¼ CVgas  Gq

(10)

Where Ein and Eout is the total input energy and output energy in
MJ. CVfuel is the calorific value of fuel in MJ/kg and CVgas is the
calorific value of producer gas in MJ/Nm3.


698

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

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

The wood to producer gas conversion efficiency of the biomass
downdraft gasifier was estimated according to Eq (11).

CGE ¼

CVgas  Ww
 100
CVfuel  Gq

(11)

The gas to power efficiency of the system was determined
through Eq (12) as follows:

GPE ¼

Elenergy
 100
Eout

(12)

Where GPE represents gas to power efficiency and Elenergy is the
total electrical energy produced from the generator. Wood to
electrical power production efficiency known as the overall efficiency of the system is given as

WPE ¼

Elenergy
 100

Ein

(13)

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 compositions of the gases are 29.6% of H2, 18.4% of CO, 18.57% of CO2 and
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 obtained. The use of air introduces a high quantity of nitrogen to the
gas which explains the reason for low calorific value of the producer gas.
4.3. Electrical performance

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 gasification 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

Table 2
Proximate and ultimate analysis of wood.
Pine wood

Proximate analysis % by weight
MC
VM
14
67.72
Ultimate analysis% by weight
C
H
47.51
6.524

FC
17.88

AC
0.4

N
0.095

O
45.87

The electrical output of the generator operated on a 100% producer 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 operated 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 obtained 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. 4 shows 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/
Nm3 mentioned earlier which is higher when compared to 4.6 MJ/
Nm3 obtained at an equivalence ratio of 0.4 by Ref. [7]. This indicates that higher gas heating values are usually obtained at lower
equivalence ratio. Such is also the case with [14] findings where an


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

699

35

CO

H2

CH4

CO2

27

32

30

Gas Volume (%)

25
20
15
10
5
0
2

7

12

17

22
Time (Minutes)

37

42

Fig. 3. Percentage compositions of producer gas.

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. Equivalence 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 Nm3 is slightly lower than the average value of
2.5 Nm3 reported [15]. Table 3 presents 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 determined 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.

Downdraft Gasifier
Producer gas
(1025.76 kg)

Wood (424 kg)
+ Air (718.64 kg)

The conversion efficiency of the system was evaluated in three
stages: First stage from wood to producer gas which was determined 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 gasifiers [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 Nm3 of 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 approximately 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 consumption 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 4 presents a summary
of some performance parameters obtained and their comparison
with literature data.

Table 3
Summary of Mass balance analysis of the system.
Input

Ash + Fine particles
(116.88 kg)
Fig. 4. Schematic diagram for Mass balance of the system.

Output

Component Unit
Mass
Component
(kg)
fraction (%)
Pine wood

424.00 37.00
Producer gas
Air
718.64 63.00
Ash þ fine particles
Total
1142.64 100.00
Total

Unit
Mass
(kg)
fraction (%)
1025.76 89.77
116.88 10.23
1142.64 100


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

Table 4
Comparing experimental data with literature.
Biomass
material

Optimum
ER

CVgas (MJ/
Nm3)

GPR (Nm3/
kg)

CGE
(%)

Reference

Fuel wood
Furniture waste
Wood chips
Hazel nutshell
Wood þ charcoal
Pine wood

0.29
0.205
0.21
0.276
0.388
0.29

5.30
6.34
3.90
5.15
5.62

6.3

2.78
1.62
2.93
2.73
1.08
2.29

89.70
56.87
66.00
80.91
33.72
88.11

[16]
[18]
[14]
[8]
[19]
Present
study

ER ¼ Equivalence ratio.

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/Nm3 at an equivalence ratio of 0.29.
This agreed well with literature as summarized in Table 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 Nm3 of 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.
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