Performance Studies on a Downdraft Biomass Gasifier with Blends of Coconut Shell and Rubber Seed Shell as Feedstock

Accepted Manuscript Title: Performance studies on a downdraft biomass gasifier with blends of coconut shell and rubber seed shell as feedstock Author: V.. Joseph Sekhar, Performance st

Accepted Manuscript

Title: Performance studies on a downdraft biomass gasifier with blends of

coconut shell and rubber seed shell as feedstock

Author: V Christus Jeya Singh, S Joseph Sekhar

PII: S1359-4311(15)01026-1

DOI: http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.09.099

Reference: ATE 7096

To appear in: Applied Thermal Engineering

Received date: 24-4-2015

Accepted date: 25-9-2015

Please cite this article as: V Christus Jeya Singh, S Joseph Sekhar, Performance studies on a

downdraft biomass gasifier with blends of coconut shell and rubber seed shell as feedstock,

Applied Thermal Engineering (2015), http://dx.doi.org/doi:

10.1016/j.applthermaleng.2015.09.099

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Performance Studies on a Downdraft Biomass Gasifier with

Blends of Coconut Shell and Rubber Seed Shell as Feedstock

V Christus Jeya Singh 1* and S Joseph Sekhar 1

1Department of Mechanical Engineering, St Xavier’s Catholic College of Engineering,

Nagercoil, Kanyakumari, Tamilnadu, India

Corresponding author: *christjsingh@yahoo.co.in

Keyword: Coconut shell, Rubber seed shell, Downdraft gasifier, Equivalence ratio,

Conversion efficiency

Highlights:

 Analytical and experimental investigation on the performance of biomass gasifier

 Blends of coconut shell and rubber seed shell as feedstock in gasifier instead of wood

 Two-zone kinetic model to predict the performance of biomass blends in gasifier

 Impact of equivalent ratio on the performance of gasifier with blends of biomasses

Abstract

The use of biomass gasification system for the generation of combined heat and

power has gained importance, because it isconsidered to beone of the most promising

renewable energy technologies Widespread research has been already carried out on

downdraft gasifier with single biomass as feedstock However a limited work is available to

ascertain the feasibility of utilizing blends of biomasses In this paper, theoretical and

experimental studies have been carried out on a 50kWth downdraft gasifier with the blends of

coconut shell and rubber seed shell, which are available abundantly in the rural villages of

South India Two-Zone kinetic modelling is followed for the theoretical studies. Experimental

study has been carried out to prove the validity of the modelling approach.The results show

that the mixing of rubber seed shell and coconut shell with various compositions yields the

performance which is on par with the woody biomasses Besides, the maximum values of the

performance parameters are obtained when the equivalence ratio is maintained between 0.2

and 0.3

Introduction

The fossil fuels contribute 80% to world total energy demand, whereas biomass

resources cover only 10-15% [1] India has an immense opportunity for energy generation

from the available 500 metric tons of biomass per year The rural areas of India depend on

conventional biomasses such as firewood, animal dung, agricultural residue and forest

products for cooking, heating, lighting and other related applications It is also observed that

the biomass energy technology may contribute to one-third of total energy consumption in

India [2&3]

Biomass gasification process comprises of biochemical reactions inside the gasifier

with limited supply of air, and generates combustible gases such as CO, H2 and CH4 The

energy content of the combustible gases may be conveniently used for heat and power

applications These gases can be effectively utilised in advanced technologies like gas

turbines and fuel cells in order to get increased system efficiency [4] The concept of

combined heat and power production from biomass is especially useful to agriculture based

process industries and rural electrification in developing countries [5&6]

The performance of the gasification process depends on the types of feedstock and its

characteristics such as moisture content, composition (ultimate analysis), equivalence ratio

and so on Inappropriate selection of these parameters may lead to excessive presence of tar

and soot in the producer gas These unwanted materials in the producer gas may disturb the

continuous and smooth operation of gas engines [7&8] The fixed bed gasifiers are classified

depending on the supply of gasifying agent into the reactor They are updraft, downdraft,

cross draft and multistage Among the commercially produced gasifiers, downdraft, fluidized

bed, updraft and other gasifiers have the share of 75%, 20%, 2.5 % and 2.5% respectively [9]

Compared to other gasification technologies, the downdraft gasifier is the most sustainable

option for decentralized heat and power generation because the producer gas obtained from it

contains very low content of tar and particulates

Theoretical and experimental studies have been carried out to analyse the behaviour

of gasifiers The minimization of Gibbs free energy has been followed [10] to predict the

performance of wood waste (saw dust) This approach was found to be good to analyse the

gasification process above 1500 K [11] Another model developed based on thermodynamic

equilibrium approach by Jarungthammachote and Dutta [12] has considered the equilibrium

constant with correction coefficient for predicting the composition of producer gas in a

downdraft gasifier It is also suggested that the moisture content and equivalence ratio have to

be maintained at 10 to 20% and 0.3 to 0.45 respectively for optimal energy conversion

Chemical equilibrium approach has been used to analyse the influence of parameters such as

moisture content, equivalence ratio and heating value on the quality of producer gas [11&12]

The experimental works have been carried out to predict the influence of various

factors on the performance of downdraft biomass gasifier with wood [13, 18], waste wood,

wheat straw, coconut shell [14, 18], saw dust [15], cashew nut shell [16], agricultural and

forest residues [17], rubber seed kernel, coir-pith [18], etc as feedstock A numerical and

experimental study conducted to analyse the behaviour of reduction zone shows that the

conversion efficiency of the gasifier decreases as the throat angle increases The optimum

length of the reduction zone has been reported as 22 cm for efficient operation of the gasifier

[13]

The gasifier running with single biomass throughout the year suffers with issues such

as risk on transportation, non-availability of a particular biomass throughout the year and

incomplete utilization of various biomasses available in a region [19] Therefore blends of

biomasses in a gasifier can improve the continuous and steady operation of gasifiers in

remote areas A analytical model developed to analyse the performance of

saw-dust/cow-dung mixture in a downdraft gasifier shows that the mixing of cow saw-dust/cow-dung would reduce the

gas production rate and heating value, however, having 40 - 50 % cow dung in the mixture is

technologically and economically viable [20] To the further extent of this approach, through

this paper, the performance of a 50 kWth downdraft gasifier has been analysed with the

blends of coconut shell-rubber seed shell Equilibrium modelling concept has been used in

the theoretical studies Experiments were also conducted to investigate the effect of mixture

composition and equivalence ratio on the species concentration and heating value of the

producer gas

Methodology

The total gasifier model has been simplified into two different zones as shown in

Fig.1 The thermodynamic equilibrium approach is followed in the zone-1 while chemical

kinetics of reactions has been considered in zone-2 [20] In order to implement the

equilibrium analysis, the assumptions used in this analysis are steady state gas flow inside the

gasifier, adiabatic wall, infinite residence for the reactions to take place, uniform species

temperature at each level of gasification, negligible amount of tar or unburnt hydrocarbon in

the exhaust, ideal gas behaviours of gases And major species in the product gas are CO, CO2,

H2, CH4 and N2

The blends of two biomasses in various proportions have been used as feedstock in

the gasifier instead of single biomass, and the performance parameters like heating value,

species concentration, gas production rate and conversion efficiency were studied While

using two different biomass energy sources, the global reaction of gasification process for the

mixture can be written as [20],

(1)

Where l, p, q, t, u, and v are the number of atoms of the respective chemical

component in the feedstock The representative chemical formulae (CH l O p N q , CH t O u N v) of

different biomasses are derived from their ultimate analysis, using the generalized procedure

[21] The equations used for calculating the number of moles of second biomass (b), number

of moles of oxygen in air (X g ) and the number of moles of moisture (M w) have been taken

from published literature [20] The ultimate analysis results of feedstock materials are shown

in the Table 1

The higher heating value of the biomass is calculated using the Friedl’s equation

(2)

The numbers of moles of respective species, x1 to x7 in the gasification process have

been calculated by solving the global reaction [20].A uniform temperature has been assumed

to the species at the outlet of zone-1 and the same is determined from the energy balance

𝐶𝐻𝑙𝑂𝑝𝑁𝑞 + 𝑏𝐶𝐻𝑡𝑂𝑢𝑁𝑣 + 𝑀𝑤𝐻2𝑂 + 𝑋𝑔 𝑂2+ 3.76𝑁2

→ 𝑥1𝐶𝑂 + 𝑥2𝐻2+ 𝑥3𝐶𝑂2+ 𝑥4𝐻2𝑂 + 𝑥5𝐶𝐻4+ 𝑥6𝑁2+ 𝑥7𝐶

(1)

𝐻𝐻𝑉 𝑀𝐽 𝑘𝑔 = 3.55𝐶2− 232𝐶 − 2230𝐻 + 51.2𝐶𝐻 + 131𝑁 + 20600

across the zone considering that there is a negligible amount of kinetic and potential energy

changes

Based on the global reaction given in equation 1, the energy balance equation is

expanded as [20],

(3)

The energy carried out by individual species (CO, H2, CO2, H2O, CH4, and N2), char

and ash are considered separately in the above equation Standard equations are used to find

the heat of formation of fuels [22] Specific heat for ash and char is considered as 840 J/kgK

and 21.86 J/mol K respectively [20]

The species formed in the zone-1 enter the zone-2 and undergo the following

endothermic reactions to form the final composition of the producer gas [20]

(4) (5) (6) (7)

The above equations are assumed to be reversible and the rate of reaction is calculated

using Arrhenius type kinetic rate equations The mass flow rate of the species (i=1 to 7) at the

inlet of the reduction zone depends on the feed rate of biomass The rate of formation of

species and the temperature of the species formed in the zone-2 are calculated from the

relations used in the previous literature [20] The temperature of the species leaving zone-2 is

obtained by applying energy balance across zone-2 The linear and non-linear equations are

solved using appropriate tools in the open source software SCILAB

The equivalence ratio plays a vital role on the performance of the gasifier [23] If the

equivalence ratio is below 0.2, the conversion efficiency and gas production rate reduce

𝐻𝑓1 + 𝑏𝐻𝑓2+ 𝑋𝑔 𝐶𝑝𝑂2

𝑇𝑎

𝑇 0

𝑑𝑇 + 3.76𝑋𝑔 𝐶𝑝𝑁2

𝑇𝑎

𝑇 0

𝑑𝑇 + 𝑀𝑤ℎ𝑓𝐻2𝑂 + 𝑄𝑙𝑜𝑠𝑠

= 𝑥𝑖

6

𝑖=1

ℎ𝑓𝑖 + 𝐶𝑝𝑖

𝑇

𝑇 0

𝑑𝑇 + 𝑥7𝐶𝑝𝐶 𝑇 − 𝑇0 + 𝑚𝑎𝑠ℎ𝐶𝑝𝑎𝑠 ℎ 𝑇 − 𝑇0

𝐶 + 𝐶𝑂2 ⇔ 2𝐶𝑂

𝐶 + 𝐻2 𝑂 ⇔ 𝐶𝑂 + 𝐻2

𝐶 + 2𝐻2 ⇔ 𝐶𝐻4

𝐶𝐻4 + 𝐻2 𝑂 ⇔ 𝐶𝑂 + 3𝐻2

drastically [24] Therefore in this analysis, the equivalence ratio has been taken between 0.2

and 0.45, and the variations in performance parameters are studied The blends used in this

study and their compositions are given in Table 2

Experimental Setup

The schematic diagram of the experimental setup consists of an imbert downdraft

biomass gasifier (50 kWth), scrubber with cyclone separator and filters is shown in Fig 2

The feedstock to be used in this gasifier as per supplier specification is wood chip A gas

analyser, gas chromatograph and a gas flow meter are provided to measure the quality and

quantity of the producer gas Calibrated K-type (chromel-alumel) thermocouples are placed

in different locations along the axis of the gasifier to measure the temperature at various

zones To prevent the leakage of producer gas and to reduce its temperature, a water seal

arrangement is made under the reactor A special metering rod is also provided to calculate

the flow rate of feedstock To maintain a homogeneous mixture, the biomasses were mixed

before being filled in the gasifier Using the special arrangement, the flow rate of feedstock

was also recorded A data logger was used to record the temperature in an interval of 5

minutes Samples of the gas were taken every 10 minutes to analyse its composition In

addition to that, the quantity of the output gas was also recorded Each blend was studied for

a minimum of three times and the average values were taken The equivalence ratio was

regulated by controlling the air flow rate and it was measured with an accuracy of ± 1 %

HHV and moisture content of feedstock material were checked before each loading

Results and Discussion

The theoretical work is carried out to study the variation in performance parameters

for a wide range of equivalence ratios between 0.2 and 0.45 However, the experimental

studies were carried out by maintaining the equivalence ratio between 0.2 and 0.34 The

comparison of species concentration obtained from the present model with the previous

works [12] shows that the deviation of present result is within 12% Thus the validity of the

2-zone kinetic equilibrium approach to simulate the gasifier has been proved The results

obtained from modelling and experimental studies have been analysed, and the influence of

the equivalence ratio and composition of biomass blends on the performance parameters are

evaluated

a) Species concentration

The variations in species concentration (CO, H2, CO2, CH4 and N2) with equivalence

ratio for coconut shell and rubber seed shell are shown in Fig 3 and Fig 4 respectively The

results show that the species concentration of CO, H2 and CH4 from the modelling has a close

agreement with that of experimental observations It is also observed that the combustible gas

composition for both biomasses is high when the equivalence ratio is close to 0.2 However the

species concentration of the combustible gases produced using coconut shell is higher than the

other one This might be because of the higher carbon content in coconut shell

The reduction zone plays a vital role in the performance of the gasifier The endothermic

reactions [24] that take place in the reduction zone cause the reduction in the temperature and

release CO, H2 and CH4 Since the concentration of CH4 does not vary significantly, the

variations of CO and H2 are plotted in Fig 5 and Fig 6 respectively and considered for the

discussion A steady increase in CO concentration is observed along the reduction zone whereas

the concentration of H2 is almost constant This is due to the difference in the rate of reaction of

each reaction The concentration observed for CO and H2 is almost constant when the reduction

zone distance is beyond 0.24 m A similar trend has also been observed in previous works [25]

The quality of the producer gas leaving the gasifier depends on the concentration of

the combustible gases (CO, H2 and CH4) at the exit of the gasifier Even though the

theoretical analysis was carried out for all the blends given in Table 2, only the results of the

blend D1 is plotted in Fig 7 and considered for discussion It is observed that the species

concentration of combustible gases is high when ф is kept at 0.2 Therefore, the airflow rate

should be adjusted to maintain the equivalence ratio close to 0.2 for both of the feedstock

materials.The trend observed from the experimental study is similar to the prediction from the

simulation

b) Gas production rate

The Fig 8 demonstrates the variation of gas production rate with equivalence ratio for all

compositions used in the study It is observed that there is a good agreement between the

predicted and measured values Even though the combustible gas composition decreases due

to the increase in ф, an increase in gas production rate is observed This is caused by the high

concentration of non-combustible gases such as CO2 and N2 in producer gas When the

equivalence ratio increases, the quantity of O2 increases, and due to the domination of

combustion reactions, the production of CO2 increases This causes a reduction in the

concentration of CO and H2

c) Higher Heating Value and Conversion Efficiency

The heating value depends on the concentration of CO, H2 and CH4 in the producer

gas The variation of HHV of producer gas with equivalence ratio (ф), for various feedstock

compositions is plotted in Fig 9 It is observed that the HHV reduces with the increase in

equivalence ratio (Fig 9) Since the gas production rate is almost constant until the

equivalence ratio reaches 0.3(Fig 8), to maintain a high heating value of the producer gas,

equivalence ratio may be kept between 0.2 and 0.3 Next to the pure biomass (blend D0), the

blend D1 gives better performance at this range

The impact of equivalence ratio on conversion efficiency is depicted in Fig 10 The

conversion efficiency decreases with the increase in equivalence ratio This is due to the poor

quality of producer gas as discussed in the previous section (Fig 9) It is observed that the

conversion efficiency is maximum when the biomass blend is rich in coconut shell When

rubber seed shell is mixed with coconut shell, the conversion efficiency is reasonably good

for the equivalence ratio between 0.2 and 0.3 This prediction shows that the equivalence

ratio should be kept in the above said range for the feedstock combinations used in this study

Moreover the temperature obtained from the simulation and experimental studies at

the reduction zone is matching with the values reported in the literature (26) For instance the

temperature at the reduction zone entry for blend D1 is 1285K and 1267K for simulation and

experimental observations respectively This proves the validity of energy equations used in

this study

Conclusion

Theoretical and experimental studies were conducted with various binary blends of

coconut shell and rubber seed shell as feedstock in a 50kWth downdraft gasifier Based on the

present study the following conclusions are made

1 The concentration of combustible gases in the producer gas is maximum at equivalence

ratio close to 0.2 and a significant reduction in the concentration of CO and H2 is observed

when the equivalence ratio is beyond 0.3

2 Irrespective of the compositions in the blend, to obtain maximum conversion efficiency,

the equivalence ratio should be maintained between 0.2 and 0.3

3 Among the two energy sources, coconut shell shows maximum combustible species

concentration and conversion efficiency

4 The present study proves that the coconut shell and rubber seed shell can be used as a

single biomass or blends in a biomass gasifier, which is designed for wood

Nomenclature

ɸ - Equivalence Ratio

DAF - Dry Air Free

HHV - Higher Heating Value (MJ/kg)

D0, D1, D2, D3 - Blends of Coconut Shell and Rubber Seed Shell

MC - Moisture Content of Biomass

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