Energy for Sustainable Development 13 (2009) 166–173
Contents lists available at ScienceDirect
Energy for Sustainable Development
Air gasification characteristics of coir pith in a circulating fluidized bed gasifier
K.N. Sheeba a,⁎, J. Sarat Chandra Babu a, S. Jaisankar b
a
b
Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli-620015 Tamil Nadu, India
Department of Mechanical Engineering, OXFORD Engineering College, Tiruchirappalli, Tamil Nadu, India
a r t i c l e
i n f o
Article history:
Received 18 June 2009
Revised 19 June 2009
Accepted 19 June 2009
Keywords:
Coir pith
Gasification
Equivalence Ratio
Gas yield
Conversion
Hydrogen
a b s t r a c t
The gasification characteristics of coir pith are studied in a circulating fluidized bed gasifier using air as the
gasifying medium. The effect of various parameters such as temperature, Equivalence Ratio (ER), and
Oxygen/Carbon ratio (O/C) on gas yield, gas composition, gas heating value, carbon conversion, cold gas and
overall thermal efficiency is studied. The ERs employed in the study are 0.18, 0.21, 0.24 and 0.31. It has been
found that temperature has an influence on the cracking reactions, as indicated by the increased yield of
hydrogen. The maximum yield of hydrogen is obtained at a temperature of 1028.6 °C, and as the Equivalence
Ratio (ER) increased, hydrogen yield decreased. The gas yield also increased with increase in temperature
and ER. Also it has been observed that the gas heating value increases with increase in temperature and
decrease in ER. Carbon conversion is found to increase with increase in temperature and increase in ER. It is
found that the increase in temperature favours both the cold gas thermal and overall thermal efficiencies.
Also the cold gas and overall thermal gasification efficiencies decrease with increase in Equivalence Ratio.
© 2009 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
Introduction
Gasification is termed as the conversion of a fuel through
thermochemical mechanism to a product, usually syngas or fuel gas.
The syngas is produced in a reducing atmosphere using various
gasifying agents. Some of the commonly employed gasification agents
are air, CO2, steam and O2.
The performance of a biomass gasifier is influenced by a number of
parameters such as feed property, physical dimensions of the gasifier,
operating parameters etc. Gasifiers suitable for various fuel types, fuel
characteristics, operating conditions, end product applications have
been developed, demonstrated and are commercially accepted.
Some of the notable works on syngas production through
gasification are discussed in this section. McKendry (2002) reviewed
gasification technologies for energy production from biomass and
concluded that biomass properties and pretreatment are the key
parameters in selecting a viable process. Similarly, Demirbas (2004)
reviewed combustion characteristics of various biomass fuels,
examined the properties of the biomass as relevant to combustion
and analyzed the concept of cofiring biomass with coal. Wander et al.
(2004) gasified saw dust in a fixed-bed, downdraft, stratified and
open top gasifier of 12 kg/h capacity. It had an internal gas
recirculation which can burn part of the gas produced to raise the
gasification reaction temperature. Saxena et al. (2008) reviewed
⁎ Corresponding author. Tel.: +91 431 2503113; fax: +91 431 2500133.
E-mail addresses: sheeba@nitt.edu (K.N. Sheeba), jaisankar13@yahoo.com
(S. Jaisankar).
gasification technologies for maximizing H2 yield and discussed the
effect of various parameters such as temperature, catalyst activity,
and biomass/steam ratio on the hydrogen gas yield. Lv et al. (2007)
experimented with hydrogen production from biomass by air and
oxygen/steam gasification in a downdraft gasifier and showed that
the use of steam/oxygen improved the hydrogen yield compared to
air gasification. The maximum heating value of the product gas
reached 11.11 MJ/Nm3 for biomass oxygen/steam gasification and the
maximum hydrogen yield is reported to be 45.16 gH2/kg biomass.
They found that the H2/CO ratio, for biomass oxygen/steam
gasification reaches 0.70 to 0.90, which is lower than that of
biomass-air gasification (1.06 to 1.27) because of higher hydrogen
yield. Mahishi and Goswami (2007) conducted experiments on
gasification using CO2 as a sorbent in the process, pine bark as the
biomass material in the presence of calcium oxide, at atmospheric
pressure with a temperature range of 500 °C to 700 °C. They observed
that the hydrogen yield, total gas yield and carbon conversion
efficiency increased by 48.6%, 62.2% and 83.5%, respectively, in the
presence of sorbent at a gasification temperature of 600 °C. They
concluded that this increase in the product is due to the reforming of
tars and hydrocarbons in the raw product gas in the presence of
calcium oxide. Typical research on the effect of three different gasifying
agents on the gasification of biomass in atmospheric and bubbling
fluidized bed experiments using air, pure steam, and steam-O2
mixtures were reported by Gil et al. (1999b), Herguido et al. (1992),
Gil et al. (1997), and Narvaez et al. (1996). It was concluded that the use
of steam resulted in an increase in the hydrogen yield, whereas O2 as a
medium improved the gas yield. Use of air as a gasification medium
resulted in a decreased gas yield compared to O2.
0973-0826/$ – see front matter © 2009 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.esd.2009.06.002
K.N. Sheeba et al. / Energy for Sustainable Development 13 (2009) 166–173
A circulating fluidized bed gasifier is used in the present study.
They are characterized by high superficial gas velocity, and higher
solids flux so that there is no clear interface in the riser between the
dense and dilute phases. In addition to this, circulating fluidized beds
are advantageous compared to fixed-bed gasifiers, because it can
offer the process for medium and large capacity plants, improve the
carbon conversion efficiency through better solid-gas mixing and
residence time. It also can adapt to change in biomass feedstock, with
less impact on performance. Extensive reviews on circulating fluidized
beds are available in literature (Kwauk, 1994; Grace et al., 1997;
Yoshida and Mineo, 1989; Berruti et al., 1995). Bridgwater (1995)
compared the notable characteristics of a circulating fluidized bed
gasifier with other gasifiers for air blown gasification. Garcia-Ibañez
et al. (2004) reported the gasification of olive oil waste, leached orujillo
in a pilot plant circulating fluidized bed reactor of 300 kWthermal
capacity using air as the oxidizing agent and reported the lowest
heating value of 3.8 MJ/Nm3 at a temperature of 780 °C. The carbon
conversion was found to be high, within 81.0%–86.9%, and that the
increase in the Equivalence Ratio increased gas yield but with less
impact on carbon conversion.
The literature shows that considerable amount of research on
gasification has been undertaken. Various designs of biomass gasifiers
have been operated using a wide variety of fuels like firewood,
agricultural residues such as bagasse, crop stalks, animal dung and
wastes generated from agro-based industries. Low density fuels like
saw dust, coir pith etc. pose resistance to gravity flow. Hence fluidized
bed system is preferred for them. The fuel employed in the present
study is coir pith, one of the agro wastes generated from the coir
industry. Around 7.5 million tonnes of coir pith is available annually in
India, with a heating value of 129 PJ of heat and 8.7 million tonnes of
CO2. A number of coir industries are located in the pockets of south
Tamilnadu. One such location is Samayapuram, in the Tiruchirappalli
district from where the samples were collected for experiment. Lumps
of coir pith are currently dumped outside these industries as waste.
Moreover they are bulky and they require more space and cost for
storage and transportation.
They are burnt to restore space for further dumping and this is
the current disposal method in Samayapuram. Coir pith is biodegradable, but the rate of degradation is very slow, because of the high
lignin content. On an average, it takes around 20 years to decompose.
If coir pith is not disposed in time, rain water percolates through it.
This will absorb tannin and other chemicals and pollute the ground
water.
Studies on coir pith as a source for thermochemical energy are
limited because it has been considered extensively as soil manure. The
compost of coir pith is used for crop husbandry as a source of plant
nutrition, especially rice. Also the activated carbon that can be
prepared from coir pith is used as an adsorbent for removal of toxic
metals from industrial effluents (Kadirvelu et al., 2001). The yet
another use of coir pith is the preparation of coir particle boards using
high compressive pressure (Jayadeep et al., 1991). One of the studies
on coir pith as a thermochemical source in a pyrolysis process proved
that the biomass with higher lignin content yields more char. Also
potassium oxide which is present in considerable quantity in coir pith
is a known catalyst for steam reforming reaction (Raveendran et al.,
1995). Hence considering the fact that coir pith has a long term value
because of the large quantity generated from coir industries, an
attempt is made to gasify coir pith to elucidate the gas production
characteristics. A study of the physical properties of coir pith is
necessary in evaluating the thermochemical characteristics of the
material. Manickam and Subramanian (2006) evaluated the physical
properties of coir pith such as bulk density, particle density and the
coefficient of friction. The particles selected for the above study had
moisture content ranging from 10.1% to 60.2% by weight. Correlations
were developed to explain the effect of particle size and moisture
content on the bulk density, porosity, coefficient of friction. Ramadhas
167
et al. (2006) experimentally studied the suitability of using coir pith as
a feed material along with the conventional wood chips in a
downdraft gasifier and analyzed the gas produced for its use in
Internal Combustion (IC) engines. The engine is tested in dual fuel
mode using combinations of diesel – coir pith and diesel – wood chips
separately. Based on the experimental results, the system is optimized
for maximum diesel savings. Also the performance of the IC engine is
compared for diesel alone and dual fuel mode conditions. It was found
that the specific energy consumption was high for dual fuel mode than
the diesel-alone operation.
Hence even though the availability of coir pith is huge in the
southern states of India, studies using coir pith as a single fuel for
syngas production is limited and the raw material is considered only
for biological composting. Since the biological degradation process of
coir pith is slow, thermal decomposition may be considered as an
alternative. Also gasification of coir pith is accomplished in R&D
organizations in fixed-bed gasifiers through the use of briquetted
blocks. Briquetting consumes huge amount of energy (Subramanian
et al., 2004). Hence gasification of powdery coir pith in fluidized bed
can be considered.
Hence in the present study an attempt is made to gasify powdery
coir pith in a circulating fluidizing bed gasifier and the various
parameters that affect the quality and quantity of product gas have
been studied.
Fuel and bed materials
A typical sample of coir pith is as shown in Fig. 1. Samples of coir
pith are taken and analyzed for its moisture content. The moisture
content in the raw coir pith is initially high in the order of 30%–40%.
The final moisture content after solar drying is reduced to 12%–17%.
Fibres if any present in the mixture are removed. These samples are
then stored in air tight gunny bags.
A representative sample is taken from the gunny bags and their
ultimate, proximate and ash analysis are carried out by M/s BHEL,
Tiruchirappalli. The empirical formula of the coir pith based on the
ratio of H, O and C is calculated to be CH1.022O0.7417.The auxiliary fuel
used to initially sustain the temperature is charcoal. The samples are
subjected to sieve analysis and the average particle size is determined
(Cheremisinoff and Cheremisinoff, 1985). The bulk density of coir
pith is found to vary from 78 to 110 kg/m3 with a moisture content of
12%–17% and a porosity of 0.63–0.78. The particle density varies from
400 to 500 kg/m3 depending on the moisture content.
A suitable bed material such as sand is employed in the gasification
process to facilitate the fluidizing quality of coir pith. Sand of suitable
size (211 μm, density 1467.33 kg/m3) is selected and is well mixed
with the biomass material. Approximately, 0.3–3 kg of silica sand is
used as bed inert material and is dependent upon the amount of coir
pith handled during the gasification tests.
The optimum particle size selected from the cold study is 360 μm
for coir pith because too small a size of the material resulted in more
Fig. 1. Sample of coir pith.
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unburnt particles as tested in the experiment and 211 μm for sand.
They are mixed in the ratio of coir pith with 12% sand.
Table 1
Physical dimensions of the gasifier.
Parts
Experimentation
Feed cylinder
The work is carried out at Centre for Energy and Environmental
Science & Technology (CEESAT), a UK-India-REC project located at
National Institute of Technology, Tiruchirappalli, India. A systematic
procedure is adapted to ensure the continuity of the process and the
reliability of the data. The pretreatment of the coir feed such as
removing the unwanted fibres and sieve analysis is carried out prior to
each run of the experiment.
A schematic diagram of the experimental set up of a circulating
fluidized bed gasifier is as shown in Fig. 2. The physical dimensions of
the gasifier are presented in Table 1.
The biomass material is charged to the feed cylinder through a
hopper. The feed cylinder is connected to a reactor and the feed to the
reactor is entrained in air from the feed cylinder. Coupled to the reactor
is a riser. The product gas with the unburnt particles passes through the
riser. The topmost part of the riser has a lean bed of particles. The riser
is connected to a high temperature cyclone wherein the unburnt
particles are separated from the gas and returned back to the feed
cylinder through the downcomer. The product gas from the top of the
cyclone separator is fed to a burner.
The entire unit is flushed with N2 gas to ensure that the combustion
products are eliminated from the system prior to each experiment. A
known weight of the fuel is fed through the hopper to the feed cylinder
initially. The auxiliary fuel (charcoal) is fed to the grate in the reactor
manually and is burnt to initially sustain the temperature in the
system. For this purpose, air from the bottom of the reactor is passed by
keeping the secondary air closed. After attaining a stable temperature
(through air preheating) which is monitored through the thermocouples, the experimental set up is ready for the gasification process.
The coir pith material is fed into the reactor along with sand entrained
in air.
The product gas along with unburnt coir and sand passes through
the riser and reaches the cyclone. Here the gas is collected from the top
and the unburnt coir and sand particles collected at the bottom are fed
back to the feed cylinder. Hence sufficient residence time is provided for
the particle cracking thus ensuring complete gasification. Monitoring of
temperature at various zones is carried out simultaneously. The
temperature readings are taken for an interval of 3 min and the time
Reactor
Dimensions
Diameter
Height
370 mm
170 mm
Diameter
Height
190 mm
840 mm
Diameter
Height
50 mm
1300 mm
Height
Inlet type
Inlet dimensions
410 mm
Square
70 mm × 70 mm
Riser
Cyclone separator
averaged values are used to represent the temperature. The gas
composition is analyzed by gas chromatography for every 45 min.
Data collection
The riser is provided with five numbers of temperature tappings in
addition to the one provided in the reactor port. Five temperature
probes (T1 to T5) measure the temperature variations within the riser.
The probe T6 monitors the temperature at the reactor. The thermocouple probes are connected to a digital temperature recorder. Special
provisions for the insertion of thermocouples are provided in the
gasifier. Thermocouples are inserted in the special tubes of diameter of
5 mm and protrude 100 mm from the riser column and integrated with
the riser. In order to reduce the heat losses through the integrated tube,
gypsum salt surrounds the inserted thermocouple. K-type thermocouple (Chromel-Alumel) of Make Omega and having an accuracy of
±0.5 °C, commonly used in many industrial applications is used to
measure the temperature. Further the pressure difference is measured
by suitable pressure tapings connected between the reactor and the
riser with U-tube manometer. The rotameters used for the gasification
study are of capacity 0.04–100 m3/h with an accuracy of ±5%. The
main gas constituents, CO, CO2, H2, CH4, and the contents of hydrocarbon components (C2+) are analysed by gas chromatography (GC).
This 5890 Series II Hewlett-Packard Chromatograph has two detectors
(TCD and FID) connected in series.
Results and discussions
The process is conducted using air as the gasification agent. Here
the study is focused on the variation of the dependent variables with
temperature and ER.
Fig. 2. Schematic diagram of a circulating fluidized bed gasifier.
Fig. 3. Effect of temperature on gas composition (ER 0.18).
K.N. Sheeba et al. / Energy for Sustainable Development 13 (2009) 166–173
169
Fig. 6. Variation of gas heating value with temperature (ER 0.18–0.31).
Fig. 4. Variation of H2 composition with temperature (ER 0.18–0.31).
Influence of temperature
The effect of varying temperature on gas composition, gas yield,
gas heating value, tar yield, carbon conversion, cold gas and overall
thermal efficiency are studied and the results are presented below.
Gas composition
A typical variation of gas composition with increase in temperature
for an ER of 0.18 is shown in Fig. 3. When gasifying with air as the
medium, the results showed that the yield of gaseous product is
higher. This is due to the enhanced cracking and catalytic action of
potassium oxide which is exceptionally higher (32.6%) in coir pith.
This is in agreement with the studies conducted by Raveendran et al.
(1995), for pyrolysis of similar kind of biomass materials. As observed
from the figure, beyond 750 °C, there is a marginal decrease in the
concentration of CO2 and marginal increase in the concentration of CO
indicating the contribution of char gasification reaction or Boudouard
reaction.
cracking reactions, other reaction such as the water gas shift reaction
also decides the final gas composition.
CO + H2 O Y CO2 + H2
C + 2 H2 O Y CO2 + 2H2
It is observed from the Fig. 3, that there is a steep rise in the H2 yield
from 650 °C to 915 °C, due to the cracking reactions and as the
temperature increases, the rate of H2 yield is found to be slow. The yield
of H2 stabilizes and no significant change is observed after 915 °C. It is
also to be noted that beyond 700 °C, apart from the reforming and
Two molecules of hydrogen released by the latter reaction may also
contribute to the increased yield of H2 at higher temperatures.
Methane composition is found to decrease at higher temperatures.
Methane decomposition is favoured by the possible cracking and
reforming reactions at higher temperatures. It is also to be noted that
the hydrocarbons exhibit minor variations over the entire temperature range and is insignificant to draw any conclusion. Similar trend is
observed for all the tested ER.
Fig. 4 shows the variation of H2 composition with temperature
and the influence of ER on the same. H2 yield is reported on a N2 free
basis. It is observed from the figure that lower the ER, higher the H2
yield. It is because the shift reactions are prominent at lower ER.
Simell et al. (1992) reported a maximum H2 yield of 14% at 850 °C for
an ER of 0.3, when tested using peat fired gas with air as the
gasification medium and catalysed by carbonate rocks like dolomite
and limestone. However while gasifying coir pith in a fluidized bed,
H2 is found to be 7.5% at 810 °C, for an ER of 0.31. The difference may
be attributed to the use of in bed dolomite with secondary air
injection (Simell et al., 1992).
Fig. 5. Effect of temperature on gas heating value (ER 0.21).
Fig. 7. Effect of temperature on gas yield (ER 0.18).
C + CO2 Y 2CO
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K.N. Sheeba et al. / Energy for Sustainable Development 13 (2009) 166–173
Fig. 10. Effect of temperature on carbon conversion (ER 0.21).
Fig. 8. Variation of gas yield with temperature (ER 0.18–0.31).
air may contribute a theoretical gas yield of 1.511 Nm3/kg as per
the reaction
Gas heating value
The influence of temperature on the gas heating value is shown in
Fig. 5 and the influence of ER on the same is given in Fig. 6.
Gas heating value improves slightly with increasing temperature
due to the increase in the yield of H2. It ranged from 5.00 to 5.31 MJ/
Nm3 for a temperature range of 651.55 °C–1020 °C with an ER of 0.18.
Narvaez et al. (1996) reported a value of 4 to 6.3 MJ/Nm3 over a
temperature range of 700 °C to 810 °C with an ER of 0.31. Similarly
gas heating value in the range of 5.8–6.3 MJ/Nm3 at this temperature
is reported by other researchers (Maniatis et al., 1988; Var der Aarsen
et al., 1982; Rensfelt and Ekstrom, 1989). Also it is observed that the
yield of hydrocarbons is less in the present studies, thus a lower
heating value is expected. The marginal increase in heating value of
the gas is due to the contribution of increased H2 yield at higher
temperatures. It is observed from Fig. 6 that lower the ER, higher the
gas heating value. For a given temperature and an ER, the gas heating
value stabilizes at 810 °C and no appreciable change is observed
above 810 °C. It may be concluded that a useful product gas can be
obtained by restricting the optimum temperature to 810 °C with an
ER of 0.18.
Gas yield
The influence of temperature on the gas yield is shown in Fig. 7.
It is observed from the figure that gas yield increases with increase
in temperature. As the temperature increases, more of the solid
fuel is converted into gaseous products thereby increasing the
product gas yield. It is also observed from the figure that till
810 °C, the rate of increase of gas yield is high. The rate is lower at
temperatures higher than 810 °C. The gasification of coir pith using
Fig. 9. Effect of temperature on tar yield (ER 0.21).
CH1:022 O0:7417 + 0:12915O2 Y CO + 0:511H2
Literature on biomass gasification reports a maximum gas yield of
1.52 Nm3/kg at a temperature of 950 °C and an ER of 0.18 for saw dust
(Turn et al., 1998) whereas the coir pith gasification is found to yield
1.12 Nm3 of gas/kg of biomass at a temperature of 915 °C and an ER of
0.18 and a temperature of 1028.6 °C could produce a gas yield of
1.62 Nm3/kg at an ER of 0.31. This exceeds the theoretical gas yield and
hence may be concluded that the excess yield may be contributed by
N2. It is observed from Fig. 8 that the highest ER improved the gas
yield, for the highest temperature.
Tar yield
The influence of temperature on tar yield is as shown in Fig. 9. With
increase in temperature, the tar yield decreases. At higher temperatures
the tar may be cracked to produce hydrocarbon compounds. Further
decrease in tar yield could be obtained with the use of commercial
catalysts such as Nickel, Dolomite and Olivine which enhances tar
cracking (Gil et al., 1999a; Courson et al., 2002; Devi et al., 2003, 2005;
Świerczyński et al., 2007 and Zhang et al., 2007). Similar study using
saw dust as fuel reported a value of 15–0.4 g/Nm3 with temperatures
ranging from 700 °C to 815 °C (Li et al., 2004). For the range of operating
conditions employed, the tar yield is found to be higher at the lowest
temperature tested, 12.58 g/Nm3 in the present study.
Carbon conversion efficiency
The influence of temperature on carbon conversion efficiency for an
ER of 0.21 is as shown in Fig. 10. It can be observed from the figure that
Fig. 11. Effect of temperature on cold gas thermal efficiency and overall thermal
efficiency (ER 0.21).
K.N. Sheeba et al. / Energy for Sustainable Development 13 (2009) 166–173
171
Fig. 14. Effect of O/C on gas heating value.
Fig. 12. Effect of Equivalence Ratio on gas composition.
the conversion of carbon from the feed to the product gas improves
with increase in temperature. The highest carbon conversion obtained
for coir pith is 83% with an ER of 0.31. It is also notable from the figure
that there is a slight change in the trend in carbon conversion efficiency
with increase in temperature above 700 °C, because of the decrease in
CO2 concentrations favoured by the reduction reactions. An exponential relationship fits the curve with a factor of 0.9387.
Cold gas thermal efficiency and overall thermal efficiency
The effect of temperature on the cold gas thermal efficiency and
overall thermal efficiency is plotted in Fig. 11. It is obvious that the
increase in temperature favours both the cold gas thermal and overall
thermal efficiencies. The highest cold gas thermal efficiency of 62.48%
is obtained for lowest ER of 0.18. Similar trend is observed for all the
ER. Results from other work indicate a value of 68% for the fixed-bed
downdraft gasifier using saw dust as the fuel (Wander et al., 2004).
Influence of Equivalence Ratio
The various ER studied are 0.18, 0.21, 0.24 and 0.31. The influence
of ER on gas composition, gas heating value, gas yield, tar yield,
carbon conversion efficiency, cold gas thermal and overall thermal
efficiencies are studied and the results are presented below.
Gas composition
The influence of ER on gas composition is presented in Fig. 12.
It is found from the figure that
• H2 and CO concentrations decreases
• CO2 increases
• Hydrocarbon species decreases
Fig. 13. Effect of Equivalence Ratio on gas heating value.
H2 composition is found to be 8.35% (inert free basis) at a temperature of 810 °C with an ER of 0.18 which is lower than reported by
Narvaez et al. (1996) but agrees well with that reported by Li et al.
(2004). The difference may be due to the difference in the design of
the gasifiers employed.
Gas heating value
The influence of ER and O/C on the gas heating value is shown in
Figs. 13 and 14 respectively. As observed from the Fig. 13, at higher ER
the gas heating value decreases because the total fractions of both
oxidizing and reducing species decreases, while the inert nitrogen
content increases. An exponential relationship is obtained between
the gas heating value and ER from the figure.
HV = 11.8884 e− 4.4821ER for which the correlation factor is
R2 = 0.995. The exponential part of the correlation shows the
sensitivity of the gas heating value to ER. Another way to correlate
gas quality with the degree of oxidation is by the use of O/C molar
ratio.
An alternate correlation is obtained in terms of O/C Ratio HHV =
86:569 e − 1:9315ðO=CÞ for which the correlation factor is R2 = 0:995: Gas
heating value is found to have exponential decrease with increase in
ER and O/C.
Gas yield
The influence of ER on gas yield is as shown in Fig. 15. As the ER
increases, the gas yield increases. The increase in ER increases the
temperature and hence the conversion of the solid fuel to gaseous
species is more with higher ER as compared to the unconverted fuel at
lesser ER. If ER is further increased and approaches 1, the useful gas
production ceases, because the product gas will then be a mixture of
Fig. 15. Effect of Equivalence Ratio on gas yield.
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Fig. 16. Effect of Equivalence Ratio on tar yield.
air and combustion gas. This is evident from Fig. 15. Typical value of
about 1.57 Nm3/kg is observed for an ER of 0.31 and temperature of
810 °C. This agrees with the reported results on biomass-air
gasification (Ergudenler and Ghaly, 1992).
Tar yield
The influence of ER on the tar yield is shown in Fig. 16. As ER
increases, the oxygen available for cracking the residual species
increases, tar cracking is effective and hence the tar content decreases,
thereby increasing the yield of gases. Even though increase in ER can
significantly reduce the amount of tar produced by the system,
sacrifice must be made in the useful heat output i.e., heating value of
the gas. Narvaez et al. (1996) report a tar yield of 10 g/Nm3 at a
temperature of 800 °C and an ER of 0.2. Observation from the present
study reveals a value of 9.76 g/Nm3 for the same conditions. This
decrease may be due to the efficient tar cracking accomplished by the
increase in temperature attained due to the increase in ER.
Carbon conversion efficiency
The influence of O/C on carbon conversion efficiency is shown in
Fig. 17. The carbon conversion is determined from the product gas
composition and the feed data. The air ratio or the O/C is a primary
parameter that influences the carbon conversion. As the air ratio
increases, the carbon conversion increases. As noted from the figure,
the carbon conversion is fitted with a correlation of
0:2941TðO=CÞ
CC = 47:171 e
2
Fig. 18. Effect of ER on cold gas thermal efficiency and overall thermal efficiency.
Cold gas thermal efficiency and overall thermal efficiency
The influence of ER on cold gas thermal efficiency and overall
thermal efficiency is shown in Fig. 18. It is observed from the figure
that the gasification efficiencies decrease with increase in ER. As ER
increases, the carbon conversion increases, but the gas heating value
decreases with increase in ER. Hence the gasification efficiencies
decrease and it will finally be reduced to zero when the process
becomes combustion, in which case the carbon conversion will approach
unity. Hence it may be noted that the gasification efficiencies determine
the performance of a gasifier rather than the carbon conversion.
Influence of O/C on CO/CO2, H2/CO, CH4/H2
The influence of O/C on the product gas ratios are as shown in
Fig. 19. Three molar ratios are required to characterize the gas
composition. They are CO/CO2, H2/CO, and CH4/H2. As more oxygen is
supplied, more carbon is oxidized to CO2 instead of forming CO,
causing a decrease in CO/CO2. H2 content in the raw gas is mainly
determined by the CO shift reaction, hence the variation observed for
H2/CO is less sensitive with change in O/C ratio. Hence H2/CO molar
ratio increases with O/C whereas CH4/H2 decreases with O/C. It is also
to be noted that the air blown gasification produces a value of H2/CO
less than 1.0 as proved in the earlier works with H2/CO ranging
between 0.53 and 0.7 (Van der Drift et al., 2001).
Conclusions
with a coefficient of R = 0:8414:
Present study agrees with the experimental results reported by
Van der Drift et al. (2001). The conversion observed for coir pith may
vary about 5% from this literature data. This may be due to the
difference in the operating conditions and the reactor configuration.
Fig. 17. Effect of O/C on carbon conversion.
Coir pith material is gasified in a circulating fluidized bed, using air.
Effect of temperature and ER on the gas composition, gas heating
value, gas yield, tar yield, carbon conversion, cold gas thermal
Fig. 19. Effect of O/C ratio on CO/CO2, H2/CO, CH4/H2.
K.N. Sheeba et al. / Energy for Sustainable Development 13 (2009) 166–173
efficiency and overall thermal efficiency are studied. For biomass-air
gasification, it is found that the increase in temperature favoured the
H2 gas composition, gas yield and the gas heating value. H2 yield is
detected at a minimum temperature of 721.6 °C with 3% (inert free)
composition at an ER of 0.31. The highest H2 composition is found to
be 11.2% (inert free) at a temperature of 1020 °C and at an ER of 0.18.
Increase in ER reduced the H2 content with minimum values reported
at ER of 0.31 for all the temperatures tested. Also the yield of H2 gas
stabilized at 915 °C and no appreciable change is observed above
915 °C. The increase in gas heating value is insignificant with increase
in temperature and the highest heating value observed is 5.31 MJ/
Nm3 for a temperature of 1020 °C and an Equivalence Ratio of 0.18. The
gas heating value also stabilized at 810 °C after which the change
(5.25–5.31 MJ/Nm3) is insignificant. But as the ER increases, gas
heating value is found to have an exponential decrease. Gas yield is
found to increase with temperature and ER due to the effective
cracking reactions. A maximum gas yield of 1.624 Nm3/kg is obtained
at a temperature of 1028.6 °C and at an ER of 0.31. Highest carbon
conversion is achieved at the highest temperature and ER and is
83.36% at a temperature of 1028.6 °C and at an ER of 0.31. The overall
thermal efficiency is observed to be increasing with increase in
temperature with a maximum value of 66.11% at a temperature of
1028.6 °C and at an ER of 0.18. Similar trend is observed for cold gas
thermal efficiency with a maximum value of 62.48% observed at a
temperature of 1028.6 °C and at an ER of 0.31. However with increase
in ER (0.18–0.31), both cold gas thermal and overall thermal efficiency
decreased. Also tar yield decreases with increase in temperature and
ER because of the effective tar cracking at the oxidizing atmosphere.
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