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Gasification of tea (Camellia sinensis (L.) O. Kuntze)shrubs for black tea manufacturing process heatgeneration in Assam, India

Partha Pratim Dutta a,*, Debendra Chandra Baruah b

aDepartment of Mechanical Engineering, Tezpur (Central) University, Assam, 784028, IndiabDepartment of Energy, Tezpur (Central) University, Assam, 784028, India

a r t i c l e i n f o

Article history:

Received 1 May 2012

Received in revised form

29 March 2014

Accepted 31 March 2014

Available online 26 April 2014

Keywords:

Biomass

Gasification

Shrubs

Tea drying

Process heat

Payback period

* Corresponding author.E-mail address: ppdutta06@gmail.com (P

http://dx.doi.org/10.1016/j.biombioe.2014.03.0961-9534/ª 2014 Elsevier Ltd. All rights rese

a b s t r a c t

Gasification of uprooted tea shrub (Camellia sinensis (L.) O. Kuntze) is an attractive option for

partial substitution of thermal energy in tea manufacturing industries. Chopped and dried

uprooted tea branches with moisture content (X < 20%) have high energy contents suitable

to generate process heat. Good number of tea processing units in Assam use old design and

inefficient coal fired furnace and air heater with a low overall efficiency. Gasification of

uprooted tea shrubs may be beneficial partially to substitute these old design coal fired

furnaces. The calorific values of uprooted tea branches and generated producer gas were

found 18.50 MJ kg�1 and 4.2 MJ m�3, yielded products at 65% cold gasification efficiency.

Fermented tea samples with an average moisture content of 60% could be dried to 3%

moisture using biomass gasifier and tea dryer setup. Simple economic analysis shows

gasifier cum tea dryer technology may be economically favorable option with an annual

saving of 21,067 $ in a medium scale tea factory (990 t per year made tea) if 28% of total

thermal energy requirement is substituted by biomass gasification.

ª 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Tea cultivation and processing units are second most impor-

tant after oil and gas industries in Assam. Assam black tea

production in the year 2010e11 was estimated as 0.488 Mt

which alone accounted for about 50% of all India production

[1]. Tea drying is a highly energy intensive chemical engi-

neering unit operation amongst all the tea manufacturing

operations. The sources of thermal energy for tea drying

process in factories located in Assam has been fossil fuel

consisting natural gas, furnace oil (known as tea drying oil)

and coal. One or more than one sources are used based on

local availability and economy. The specific energy

.P. Dutta).062rved.

consumption (coal) in commercial tea drying has been

reported within the range of (0.8 to 1.13) kg kg�1 of made tea.

The reported variation could be due to varying level of per-

formances and overall efficiency of energy conversion de-

vices. Variations of specific energy requirements for tea

processingwere also observed amongst the fuel types. Specific

energy consumptions while using tea drying oil, coal and

natural gas had been reported as (23.88, 43.72 and 27.49) MJ

kg�1 of made tea respectively [2]. Overall, based on production

and average specific energy consumption rate, an estimated

9.42 PJ equivalent thermal energy was found to consume

annually for drying operation in tea factories of Assam. The

volatile prices and other environmental factors of fossil fuels

have caused uncertainties to continue relying on these fossil

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 828

fuels for tea processing in Assam. This has necessitated

searching for a reliable and sustainable thermal energy sys-

tem for tea drying. Uses of biomass as an alternative to con-

ventional fossil fuels have been attempted inmany cases with

success. However, success of such effort would require

comprehensive testing and analysis considering the fuel and

related technologies.

Combustion of cashew nut shell in furnace, semi open pit

and other open burning was poor with lower combustion

efficiency, high smoke emission, therefore efficient process

control was not convenient in such a situation [3]. Gasifica-

tion of biomass using a reactor (gasifier) to convert solid

biomass into gaseous fuel is almost a matured technology.

However, the application of biomass gasification technology

as a source of thermal energy for tea drying is only in

research and development stage till now. There are literature

available with reports of research and development on gasi-

fication technology covering (i) efficacy of thermal energy

generation, (ii) performance analysis and (iii) economic

feasibility analysis. Some of such related research works are

highlighted below.

Rubber wood feedstock was used in an 80 kW thermal

output downdraft gasifier for tea drying process heat genera-

tion. The moisture content of rubber wood was considered as

an important parameter which in turn affected gasifier per-

formance through reactor temperature and heat loss. The

optimum gasification zone length had to be selected for

maximum output in a given range of operating parameters.

The principal gasification reactions were pyrolysis, oxidation

and reduction and they produce combustible gases like CO,

H2, CH4, etc. The average calorific values of producer gas were

(4.18 to 4.62) MJ m�3 [4]. The use of open core gasifier

(1.25 GJ h�1) for process heat generation in pharmaceutical

industry had been reported in another study and it had stated

that (70e80) kg h�1 of wood could replace 20 L h�1 low density

diesel fuel [5]. In another study, babul wood (Prosopis juliflora),

groundnut shell briquettes, groundnut shell, mixture of wood

(P. juliflora) and cashew nut shell were satisfactorily gasified in

an open core throat less downdraft 50 kW gasifier that indi-

cated fuel dependency on performance [6]. Waste wood

(moisture content less than 20%) gasification and thermal

energy balance of a 150 kW downdraft gasifier had been per-

formed. An average cold gasification efficiency of 70% was

reported [7].

There are also reports of using by-products of coffee in-

dustry as fuel for gasification. Coffee grounds were success-

fully converted into gaseous and volatile matter by fast

pyrolysis at a temperature of 1073 K with about 88% conver-

sion efficiency. Further, it was reported that tar separation

was done by combustion to produce additional process heat in

allothermal gasification [8]. Similarly, coffee husk gasification

using high temperature air steam in a batch facility that was

maintained at three different gasification temperatures

900 �C, 800 �C, and 700 �C was studied. An increased gasifica-

tion temperature led to a linear increment of CO concentra-

tion in syngas for all gasification conditions. It was also

reported that kinetic parameters established the reaction

mechanism of zero order with apparent activation energy of

161 kJ mol�1 and frequency factor of 6.48 � 102 s�1 [9]. In

another investigation, a downdraft biomass gasifier with

furniture wood chip as feedstock was used to measure

equivalence ratio, gas composition, calorific value and gas

production rate, etc. A peak was seen at about 0.38 equiva-

lence ratios for optimum CO and CH4 yields; it showed first

increasing then decreasing trends of these constituents. At

equivalence ratio 0.38, they observed best performance of the

downdraft biomass gasifier. They also observed that gas pro-

duction per unit weight of fuel increased linearly with equiv-

alence ratio and a maximum cold gas efficiency of 80% was

achievable [10].

Techno-economic viability of cashew nut shells gasifica-

tion was reported for hot water generation in a local food

processing factory. They found that cashew nut shells were

excellent feedstock for gasification and it had high energy

content and similar composition to fuel wood. The payback

period for additional investment made in plant and machin-

ery was found less than one year. Studies revealed that 6.5 kg

of liquefied petroleum gas was fully replaced by gasification of

38 kg of sized wood on hourly basis. Fuel economic analysis of

gasifier showed that the saving was about 13,850 $ for 3000 h

of baking operation [11,12].

It is seen from the above discussion that there are several

efforts to use gasification technologies in diversified appli-

cations including selected food processing industries. Tea

estates in Assam generate biomass in the form of uprooted

tea branches, pruning litter and branches of shading trees.

The uprooting is done at certain intervals of plantation to

replant, so as to maintain optimum level of tea productivity.

Such uprooted tea branches are generally used as cooking

fuel through direct combustion in low efficiency traditional

cook stoves. The introduction of improved cook stoves with

higher conversion efficiency may lead to a substantial

saving of uprooted biomass and this saved biomass may be

used for tea manufacturing process heat generation. There

are two distinct options for generation of process heat using

surplus uprooted tea branches; viz. (1) Direct combustion

and (2) Gasification. Direct combustion of biomass in con-

ventional inefficient coal fired furnace of some tea factories

has socio-environment problem including greenhouse gas

emission and accumulation of tars and shoots in nearby

areas. Because the existing coal fired furnaces in the repre-

sentative tea factories had been observed running with a

very low overall efficiency. The reasons behind very low

overall efficiency in existing coal fired air heating furnaces

are inappropriate control of excess air for combustion,

improper insulation in flue gas path, inconsistent quality of

coal, inherently low flue gas to air heat exchanger heat

transfer rate, frequent fouling of cast iron tube heat

exchanger, and old conventional overall design of system.

Therefore, flue gases to air heat exchangers have very low

effectiveness compared to shell and tube heat exchanger

using steam as intermediate heat transferring medium to

heat tea drying air. Now, if these uprooted tea branches

were utilized through efficient gasification technology, then

lesser volume of greenhouse gases will be emitted compared

to inefficient fixed bed coal combustion furnace system. This

will contribute to a substantial saving of scared fossil fuels

such as coal or natural gas by application of renewable en-

ergy for tea drying and greenhouse gas emission balance,

etc.

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 8 29

Although there are adequateworks on biomass gasification

for different thermal and power applications as discussed

earlier, literature pertaining to gasification of biomass avail-

able in tea estates for process heat generation is very limited.

It is evident with the crisis of conventional fossil fuel both in

term of environment and economy, substitute renewable

fuels need to be searched for sustainability and viability of tea

industries. However, large scale applications of gasification

technology in tea processing using locally available biomass

would require systematic techno-economic investigation.

Keeping in view of these the present works have been un-

dertaken with the objectives of (i) investigation of the efficacy

of application of locally available tea estates biomass as fuel

for gasification; (ii) investigation of tea drying using gasifica-

tion technology and (iii) investigation of economic viability

gasification technology fueled by locally available biomass

compared with prevailing low energy efficient fixed bed coal

fired technology (Fig. 9) to heat air.

Fig. 1 e a) Experimental setup of producer gas based tea drying

thermal output downdraft gasifier using uprooted tea shrubs.

2. Materials and methods

An experimental study was carried out to gasify uprooted

tea shrubs (Camellia sinensis (L.) O. Kuntze), branches of

shading tree (Samanea saman (Jacq.) Merr.), etc. available

from an identified tea estate near Tezpur University (lati-

tude 26� 42003ʺ N and longitude 92� 49049ʺ E) in Assam (India).

The biomass sample was characterized for the required

parameters in the laboratories of Tezpur University. A

laboratory scale biomass gasification unit was operated

for process heat generation. This process heat from pro-

ducer gas combustion was used in laboratory scale tea dryer

(tray) to manufacture black tea. Further, the economic

feasibility of application of gasification technology for tea

processing was also assessed using a standard methodol-

ogy. The details of the materials and methodology are dis-

cussed below.

using uprooted tea shrubs. b) Experimental setup of 10 kW

Fig. 2 e Pressure drop across gasifier with gas flow rate.

Fig. 4 e Variation of calorific value with equivalence ratio.

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 830

2.1. Experimental setup for performance evaluation ofgasifier and operation of tea dryer

A detail of the experimental setup (Fig. 1a, b) and in-

strumentations used for (i) evaluation of the gasifier perfor-

mance and (ii) technical feasibility of tea drying by gasification

of uprooted tea branches have been presented in Table 1.

Description of different subassemblies and measuring in-

struments are briefly highlighted follows. With the studies of

above literature on gasification of various biomasses feed-

stock, a study had been made for gasification of locally

available uprooted tea shrubs etc., for process heat generation

in black tea manufacturing. An experimental setup for gasi-

fication of uprooted tea branches, tea pruning litter, etc. has

number of components as summarized below. The uprooted

tea shrubs were supplied by local tea estate for research

purpose.

Fig. 3 e Pressure drop across nozzle with gas flow rate.

2.1.1. GasifierAn Ankur make downdraft type woody biomass gasifier

(Model-WBG-10, 10 kg h�1) was used for the present study. The

gasifier was consisted of (i) hopper, (ii) a double walled air

insulated cylindrical reactor, (iii) air nozzles (iv) a rotatable

grate controlled by a comb-rotor, (v) ash pit and (vi) some

auxiliaries viz., scrubber and water pump assembly, vibrator,

blower, saw dust filter, etc. The controls of auxiliary opera-

tions were done with the help of a control panel. The total

electrical energy requirement to run the supplementary

equipment of the gasifier was 0.8 kW. Air was supplied by the

two air nozzles circumferentially at 180� apart. The nozzle

ends pointed into the combustion zone and they were

extended up to the minimum constriction of V throat inside

reactor. The gasifier setup was also provided with water filled

U- tube manometers to observe the pressure drop at air entry

nozzle and at reduction zone inside the reactive gasifier.

The gasifier was operated using uprooted tea shrubs for

performance evaluation. The air and fuel ratio was varied and

Fig. 5 e Variation of gas production and cold gas efficiency

with equivalence ratio.

Fig. 6 e Variation of gas composition and calorific value

with air fuel equivalence ratio.

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 8 31

thermal output in terms of calorific value; gas composition

and flow rate of producer gas were measured. The ash and

excess charcoal from the grate were rejected automatically

into the ash pit to maintain a constant reduction zone height

inside the gasifier. The gasification air discharge into the

gasifier at various loads was measured with rotameter. A

continuously running gasifier under study has been termed as

reactive gasifier.

2.1.2. Junker gas calorimeter for measurement of calorificvalue of producer gasJunker gas calorimeter (INSURF, Make: Instrumentation and

Refrigeration of India) was used for online measurement of

calorific value of producer gas. Producer gas contains five

principal components namely nitrogen, carbon dioxide, some

hydrogen, carbon mono oxide, and traces of methane out of

which last three gases are combustible. Producer gas calorific

value was measured by tapping a small amount of gas from

the main gas stream to feed Junker gas calorimeter and then

its calorific value was evaluated at an interval of 10 min.

Fig. 7 e GC Trace 600 for producer gas composition

analysis.

2.1.3. Measurement of gas flow using master turbine flowmeterA master turbine flow meter was used in addition to a rota-

meter to measure producer gas discharge for combustion in

the partially premixed gas burner (Fig. 8) as well as surplus

flared gas. Flow straighteners of 10D and 5D were provided at

upstream and downstream of flow meter installation for

minimizing flow fluctuation. A magnetically coupling meter

head directly gave producer gas discharge used for combus-

tion in tray dryer burner.

2.1.4. Measurement of air flow using rotameterTwo rotameters were used to measure both gasification air

coming through air nozzles and producer gas combustion air

required in the tray dryer. A blower was used at the outlet of

gasifier to adjust the gas flow rate. The flow of input air was

recorded using a Rotameter (Table 1) for all the test conditions.

The measurement of this air was very important aspect to

calculate the equivalence ratio for biomass gasification pro-

cess and system efficiency.

2.1.5. Measurement of pressure drops by U tube watermanometerPressure drop across the gasifier and nozzle were measured

with U tube water manometer at varying output gas flow rate.

With variation of air fuel equivalence ratio for gasification,

these two important pressure drops were indicative param-

eter for optimum performance of gasifier.

2.1.6. Measurement of biomass moisture and consumptionThe moisture of the woody biomass feedstock was

measured with established method [17]. The gasifier hopper

Fig. 8 e Flame of producer gas combustion at equivalence

ratio (ɸ [ 0.27) form gasification of uprooted tea shrubs.

Fig. 9 e A conventional inefficient fixed bed coal fire

furnace for tea drying.

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 832

had a capacity of 100 kg dry wood and average fuel con-

sumption ranged from (5 to 8 kg h�1). Once filled, it may

uninterruptedly run for 8 h. However, refilling of wood on

hourly basis was performed to calibrate hourly fuel

consumption data given by the manufacturer as standard.

Table 1 e Experimental setup and details instrumentations.

Sl. no. Items Technical specifi

1. V throat downdraft woody

gasifier and relevant accessories.

WBG-10 (thermal), make:

2. Junker gas calorimeter INSREF, fuel type: fuel gas

3. Temperature data logger CDL-28, Shivaki, India,

accuracy: �0.15% FS [15]

4. Gas turbine flow meter DN50-G65, (6e2500) m3 h�

India Ltd. [16]

5. Rotameter Acrylic body, range: (0.1e

accuracy: þ2% FSD

6. Wood moisture meter Moisture: (0 to 40) %, reso

accuracy: �(0.5% n þ 1), 1

7. U tube differential manometer Acrylic body, range: (250e

8. Tray dryer 10 kg h�1 size: (0.620 � 0.5

9. Digital weighing balance KERN: read out 0.01 g, ran

linearity �0.03

10. Gas chromatograph Thermo-fisher Trace GC:

11. Multifunction gas analyser Testo, Germany

Besides, to nullify the effect of excess air because of refilling

of wood, constant pressure drops across the nozzles

and gasifier were maintained by controlling the blower

outlet flap valve. During actual experimental gasification

studies of uprooted tea shrubs, gasifier was filled once after

every eight hours and normally tea drying experiments

complete before eight hours. Therefore, average hourly fuel

consumption was calculated by dividing total fuel consumed

by total hours of operation. Moreover, any minor variation of

excess air was controlled by suction blower flap valve

adjustment. Therefore in actual gasification of uprooted tea

branches, the hopper door was closed for entire period of

experiments.

2.1.7. Use of tea biomass generated producer gas for black teamanufacturingA laboratory scale tea dryer available in Tezpur University was

used to test the feasibility of manufacturing of black tea using

producer gas [18]. The dryer was fitted with a producer gas

partial premixed burner. The producer gas and combustion air

was supplied through a central tube and an annulus passage

respectively. Both of them were passed through a Venturi

constriction at a higher velocity to mix properly past a bluff

body for flame stabilization (Fig. 8). The flame height of the

producer gas burner could be optimized by controlling the

producer gas flow rate. The area of the each perforating plate

(10 numbers) of the tray dryer was 0.12 m2 and it had a ca-

pacity of 10 kg made tea per hour measured as per standard

procedure [19].

Fermented tea samples with initial moisture 60% (w.b.)

were collected in airtight container from a tea manufacturing

unit adjacent to the Tezpur University. Five kilograms of fer-

mented tea was thin layered in 10 trays. The producer gas

combustion products were directly fed into the drying

chamber of tray dryer by using a hot gas blower. Identified

samples from each tray were collected to measure moisture

loss until final moisture become constant [20,21].

cation Remarks

Ankur [13] Used for generating producer gas uprooted

tea shrubs feedstock

/petrol [14] Online measurement of calorific value of

producer gas

Online measurement of various temperatures

1 Rockwine To measure the producer gas flow rate

10) m3 h�1, To measure gas flow rate into burner and also

to measure gasification air

lution: 0.1

90 g

To measure moisture content of biomass feedstock

0e250) mm To measure pressure drop across the gasifier,

air nozzle and filter

33 � 0.38) m3 For fermented tea drying

ge: 1210 g, To measure moisture loss of fermented tea

600 To measure producer gas composition

To measure flue gas composition producer

gas furnace

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 8 33

2.1.8. Temperature data logger with computer interfaceThis is a multi-channel online temperature data logger,

(Model No: CDL-28, SHIVAKI, and India: accuracy �0.15%).

Sixteen number of K-type thermocouples (ChromeleAlumel)

were used to measure different temperatures such as gas

outlet, scrubber water outlet, dryer inlet gas temperature,

combustion product, ambient air temperature, and ten trays

average temperatures and dryer outlet temperatures over the

entire experimental periods. The temperature measurements

were performed as per ASTM [22] standard. The temperature

sensors were connected to computer using RS232 interface

through 16 channel data logger which recorded online tem-

peratures at the desired locations as shown in (Fig. 1).

2.1.9. Gas chromatographAn off line Gas Chromatograph (Trace GC: 600, Make: Thermo

Fisher, Italy) was used to measure producer gas quality at

different air fuel equivalence ratio (Fig. 7). Producer gas sam-

ples were collected in a bladder to inject it into the gas chro-

matograph and producer gas tapping point was fixed after fine

filter of gasifier setup to get desired quality of gas.

2.2. Feedstock characteristics and preparation

The uprooted tea (C. sinensis (L.) O. Kuntze) branches with

average age of 50 years were supplied by nearby tea estate

(latitude 26� 42003ʺ N and longitude 92� 49049ʺ E) was used as

gasification feedstock for present studies. They were chipped

by wood chipping machine to make sizes of (35 � 5) mm in

length, (20� 5)mm in diameter. It then followed thin layer sun

drying until moisture was less than 20% on wet basis to use it

as gasification feed or to store in gunny bags for future ap-

plications. The said biomass had the physical (Table 2) and

chemical properties as measured in laboratory conditions.

Proximate analysis (Table 3) of uprooted tea branches used for

gasification was performed to calculate the following param-

eters namely moisture content (ASTM D3172-73), volatile

matter (ASTM D3175-73) and ash content (ASTM D3174-73).

Similarly ultimate analysis of feed stock was performed as

per (ASTM E-777) for carbon and hydrogen, (ASTM E-778) for

nitrogen and oxygen by difference [23]. By using automatic

bomb calorimeter (Make: Changsha, China), the higher heat-

ing value of uprooted tea shrub was measured as

18.50 MJ kg�1.

2.3. Estimation of stoichiometric air fuel ratio

The ultimate analysis data of uprooted tea branches had been

used to compute stoichiometric air fuel ratio and equivalence

ratio for gasification. Themolecular formula (CH1.65O0.71N0.03)n

Table 2 e Average physical properties of uprooted teashrubs.

Feedstock

Chipsizes(mm3)

Apparentdensity(kg m�3)

Bulkdensity(kg m�3)

Porosity

Uprooted tea

shrubs

9425 1050 560 0.47

of feedstock was calculated by takingmole ratios of hydrogen,

oxygen and nitrogen to one mole of carbon. Stoichiometric

oxygen requirement is the amount of oxidizer to completely

form one mole of carbon dioxide and water.

The stoichiometric oxygen requirement [24] may be

computed from Equation (1).

Kilogram of oxygen per kilogram of fuelðMaÞ ¼ 2:68Cþ 8H�O

(1)

By using ultimate analysis data (Table 4), the stoichio-

metric air and biomass ratio is calculated from Equation (1) as

5.44 kg kg�1 of dry fuel.

2.4. Warm up and medium duration test

The gasifierwas operated according to the standard procedure

prescribed by the MNRE [25]. Since the producer gas generator

converts the solid biomass into gaseous fuel by a series of

thermo-chemical reactions, therefore to initiate proper gasi-

fication, some minimum warm up period is necessary. The

hopper of the gasifier was fully loaded with biomass through

top lid and then the lid was tightly closed. A firing torch

wrapped with cotton waste in support of kerosene oil was

used to fire the gasifier through air nozzles. The blower was

started immediately and combustion air was controlled by

means of the flap valve such that oxidation zone inside the

gasifier could be established. At the beginning, white opaque

smoke was released that was not combustible. After about

(15e20) min, the producer gas was less opaque with more

combustible and continued with sustained flame in producer

gas burners. The combustion products from the burner mix-

ing chamber was directly supplied into tea dryer for drying of

fermented tea. Measuring parameters were rate of fuel con-

sumption, pressure drop in gasifier and nozzles, calorific value

of producer gas, producer gas compositions, equivalence air

fuel ratio for gasification, different temperatures starting from

producer gas to dryer exhaust [26].

2.5. Maintenance of gas producer system

Various field studies and literature surveys revealed that

normally after certain hours of operations, the gasifier system

may give rise to certain operational problems. They are

excessive pressure drop across the gasifier, less porosity of

reduction bed, clogging of gas cleaning system, removal of

excessive charcoal from the ash pit, bridging of V throat inside

gasifier, congestion of scrubber pump nozzle, etc. It may be

noted that regular health checking and appropriate preventive

maintenance of gasifier as per operational manual is the

healthiest practices. Hopper feed door rubber seal may be

Table 3 e Average proximate analysis of uprooted teashrubs.

Feed stock Volatilematter %

db

Fixedcarbon %

db

Ashcontent %

db

Moisture %wb

Uprooted

tea shrubs

81.16 13.36 5.48 12

Table 4 e Average ultimate analysis of uprooted teashrubs.

Feed stock C % byweight db

H % byweight db

N % byweight db

O % byweight db

Uprooted tea

shrubs

44.43 6.16 1.65 41.9

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 834

checked after every 150 h. Similarly charcoal bed recon-

ditioning should be done every after 400 h operations and

checking for crack at cone may be maintained after elapse of

400 h as per manufacturers specifications [13].

2.6. Economic assessment

An effort had beenmade to investigate economic feasibility of

woody biomass based gasifier system for partial substitution

of conventional thermal energy in tea manufacturing. There

are different procedures available in literature for economic

feasibility analysis of a new energy system. However, in the

present investigation a specific procedure was followedwhere

(i) Net present value (ii) Benefit cost ratio and (iii) Payback

period pertaining to a new renewable energy technology were

assessed. The difference between the present value of all

future returns (Fn1) and present money required to make an

investment (Fn2) with rate of interest (i) for (n) years are

related with net present worth by Equation (2).

NPV ¼Xn¼n

n¼1

Fn1 � Fn1

ð1þ iÞn (2)

Benefit cost ratio defined as the present worth of benefit

stream to present worth of cost stream. An acceptable project

must have cost benefit ratio greater than one. Mathematically,

cost benefit ratio is expressed as in Equation (3).

Cost benefit ratio ¼Pn¼n

n¼1

Fn1ð1þiÞn

Pn¼n

n¼1

Fn2ð1þiÞn

(3)

The payback period is the total length of time from begin-

ning of the project until the net value of the incremental

production stream recovers total amount of capital invest-

ment. The following parameters were considered to carry out

economic analysis of gasifier (150 kW) cum tea dryer system

[7,27,and28].

(1) The life of biomass gasifier and heating system is 10

years.

Table 5 e Average temperatures at different points in experim

Gas flowrate (m3 h�1)

Ambient(�C)

Gas outlet(�C)

Scrubber outlet(�C)

Dr

16 35 380 42

18 35 390 44

20 35 405 45

22 35 425 48

24 35 450 50

(2) Salvage values a 10% of initial investment.

(3) Interest at 10% of initial investment.

(4) Depreciation at 20% of initial investment spread over 10

years.

(5) Repair and maintenance cost at 20% of initial invest-

ment spread over 10 years.

(6) Discount rate is assumed 10%.

(7) Electricity cost is 0.1 $ kW h�1 (Average, 2011).

(8) Annual operation is 250 days.

3. Results and discussions

Quality of output gas and rate of gas production are the

measures of gasification performance of a biomass gasifier.

The performance of a gasifier depends on the various factors

pertaining to feedstock characteristics (moisture content,

composition, sizes of wood chips, etc.), operating conditions

(air:fuel ratio), and design of gasifier (position of air inlet

nozzles, reduction zone volume, etc.). As stated earlier, pre-

sent investigation reports performance of a 10 kW thermal

output gasifier that used uprooted tea shrubs at varying air

fuel equivalence ratio. The pressure drops across gasifier bed

and air inlet nozzle weremeasured for varyingmeasured flow

rate of output gas. Simultaneously, the calorific value and

compositions of output gas was also recorded. From the

measured data, the performance of the gasifier was analyzed.

Further, the results of the use of producer gas for tea drying

are also presented below. Finally, the results of economic

analysis to examine the prospect of gasification technology

with locally available biomass feedstock are presented.

3.1. Pressure drop and gas flow rate

Pressure drop across a porous reactive gasifier bed, air nozzle

are important controlling variables of the system. The flow of

ambient air into the gasifier was assumed to be isothermal.

Fig. 2 shows the variation of gas flow rate with respect to

gasifier pressure drop for both a newly charged gasifier and a

reactive gasifier. It is clear that pressure drop is higher in a

reactive gasifier compared to a newly charged gasifier for

same output gas flow rate. This is because of less porosity and

higher resistance to gas flow inside a reactive gasifier. The

porosity in reduction bed of a gasifier gradually decreaseswith

continuous operations. If there is a continuous gasifier pres-

sure drop beyond certain value (DPG � 450 Pa) even with vi-

bration for a particular producer gas flow rate, then this is the

time for reconditioning of reduction bed either with new

charcoals or adjustment of reduction bed height by shaking it

ental setup.

yer inlet(�C)

Mixing chamber(�C)

Average tray(�C)

Dryer outlet(�C)

45 100 95 67

45 105 95 67

48 110 100 70

52 120 105 70

55 130 110 70

Table 6 e Computation of equivalence ratio forgasification.

Air flow rate(m3 h�1)

SAFR Biomass(kg h�1)

AAFR Equivalenceratio (f)

7.5 5.44 8 0.94 0.17

8.5 5.44 8 1.06 0.20

9.5 5.44 7.5 1.27 0.23

10.5 5.44 8 1.31 0.24

11 5.44 8.20 1.34 0.25

11.5 5.44 8.20 1.40 0.26

12.5 5.44 8.50 1.47 0.27

13 5.44 8.50 1.53 0.28

14 5.44 9.00 1.56 0.29

14.5 5.44 9.00 1.61 0.30

16 5.44 9.50 1.68 0.31

16 5.44 9.00 1.78 0.33

17 5.44 9.00 1.89 0.35

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 8 35

in certain cases. Therefore, the good health (optimum per-

formance) of the present gasifier was observed at

(DPG < 450 Pa).

Pressure drop across air nozzle is plotted against the gas

flow rate (Fig. 3) which is again another indicator of proper

gasification processes. If this pressure drop is increased

beyond certain values (DPN � 90 Pa) for a newly charged

gasifier then it might go into combustion mode. The flow of

output gas (25 m3 h�1) still increased but the quality of gas as

measured by Junker gas calorimeter was poor due to com-

bustion of a part of producer gas inside the gasifier. It also

increases corresponding pumping power of gasification pro-

cess leading to losses of net output energy and power. Hence

from this analysis it is opined that this gasifier must be

operated with optimum (DPN < 90 Pa).

3.2. Measurements of different temperatures

Sixteen different temperatures starting from ambient air and

finally to the dryer outlet temperatures weremeasured (Fig. 1).

Table 7 e Parameters used for estimation of payback period.

No Description of parame

1. Initial investment of 150 kW $6#down draft gas

2. Interest one initial investment (@ 10%)

3. Depreciation (@ 20%) for 10 years

4. Repair and maintenance cost (@ 20%)

5. Cost of electricity to run the accessories $6#of g

(8 kW h day�1 � 250 days $6#� 0.1 $ (kWh)�1

6. Fuel cost (wood, 100 kg h�1 � 15 h day�1 $6#� 2

7. Labor cost (2 � 2 $ day�1 � 250 days)

Total investment in first year

8. Cost of operation per day

Cost of operation by coal fired furnace for equivalent heating

1. Annual black tea production in an $6#average s

2. Total coal requirement (@ 0.8 kg kg�1) $6#made

3. Equivalent coal requirement (28%)

4. Annual cost of coal (222 t � 95 $ t�1)

5. Cost of electricity to run ID and FD fan etc.,

(15 kW h day�1 � 250 days � 0.1 $ (kWh)�1)

6. Labor cost 2 � 2 $ day�1 � 250 days)

7. Total cost

8. Cost of operation per day

Under varied output gas flow rate from (16 to 24) m3 h�1,

variation of different temperatures are presented in Table 5.

Minimumandmaximumdryermixing chamber recorded (100

and 130) �C, at corresponding gas flow rate and average tray

temperatures were varied from (95 to 110) �C. The average

dryer exhaust temperatures were varied from (67 to 70) �C.Higher dryer exhaust temperature (70 �C) was noted near the

completion of drying process.

3.3. Equivalence ratio and calorific value

The equivalence ratio is an important design controlling

parameter of a gasifier. It is defined as the ratio of actual air

fuel ratio (AAFR) to stoichiometric air fuel ratio (SAFR).

Different researchers found effective equivalence ratio for

wood gasification in the range of (0.25 to 0.35) in their

experimental downdraft gasifier [10,14and26]. If equivalence

ratio is below 0.20, it may give rise to incomplete gasification,

excessive char formation and lower heating value of pro-

ducer gas. For equivalence ratio above 0.4, it may result in

excessive formation of complete combustion products in the

expense of CO, CH4 and H2 of producer gas. The equivalence

ratio for the present study had been computed from ultimate

analysis data, biomass feed rate and gasification air flow rate.

The stoichiometric air requirement was calculated as

5.44 kg kg�1 of woody biomass. At biomass feeding rate of

8 kg h�1 and air flow rate of 7.5 m3 h�1, the least equivalence

ratio (0.17) was computed from experimental data. At

equivalence ratio 0.27 and wood consumption rate 8.5 kg h�1,

the best performance of the gasifier was observed. By the side

of variable gasification air flow rate and biomass feed rate,

the values of corresponding equivalence ratios are presented

in (Table 6).

The calorific value of producer gas obtained from gasifi-

cation of uprooted tea branches was similar to literature

[3,5,8,29,30] as reported by researchers. With increase in

equivalence ratio, the gas production rate continuously

ter Amount $6#(US $)

ifier for thermal mode [30] 10,000

1000

200

200

asifier 200

50 days � 11 $ t�1) [7] 4125

1000

16725 $

4.46 $ h�1

ize tea estate 990 t

tea 792 t

222 t

21067 $

375 $

1000

22,442 $

6 $ h�1

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 836

increased up to a particular value because it gave rise to a

higher air flow rate. After certain value of equivalence ratio

(0.27), calorific value of producer gas declined probably

because of higher concentration of CO2. The calorific value of

product gas wasmeasured 4.2 MJm�3 whichwas best calorific

value at 0.27 equivalence ratio (Fig. 4).

3.4. Gasifier efficiency and air fuel ratio

The cold gas efficiency of a biomass gasifier is defined as the

ratio of energy content of producer gas against unit weight of

biomass to higher heating value of unit weight of biomass as

given by Equation (4).

hcold_gas ¼Calorific value of producer gasð Þ � Producer gas production rate per unit weight of biomassð Þ

Higher heating value of the biomass(4)

It is evident from above expression that for a constant

calorific value of biomass, cold gas efficiency depends on

calorific value of producer gas and amount of gas produced

per unit weight of input biomass. Maximum cold gas effi-

ciency of 65% was observed woody biomass feed rate of

8.5 kg h�1 and 0.27 equivalence ratio. This gave rich yield of

producer gas for tea drying. The cold gas efficiency was then

falling gradually beyond this equivalence ratio (Fig.5) probably

complete combustion of a part of producer gas. Equivalence

ratio beyond 0.27, the bulk volume of producer gas mixture

sharply increased and gasification efficiency decreased.

3.5. Producer gas compositions

Fig. 6 shows variation of producer gas composition and

average calorific value with air fuel equivalence ratio for

gasification reaction. Both H2 and CO had increased with in-

crease in gasification air fuel equivalence ratio (F ¼ 0.27) and

maximum values of these two components were reported as

18% and 24% respectively. Beyond this equivalence ratio, CO

decreased and H2 contents almost remained constant. The

CH4 content nearly remained constant at about 1.5% and

Table 8 e Cash flow (US $) for heating system of a teadryer.

Year Cashoutflow

PW of cashoutflow

Cashinflow

PW ofcash inflow

NPV

0 10,000 10,000 0 0 �10,000

1 16,725 15204.55 22,442 20401.82 5197.27

2 16,725 13822.31 22,442 18547.11 4724.79

3 16,725 12565.74 22,442 16861.01 4295.27

4 16,725 11423.40 22,442 15328.19 3904.79

5 16,725 10384.91 22,442 13934.72 3549.81

6 16,725 9440.83 22,442 12667.92 3227.10

7 16,725 8582.57 22,442 11516.29 2933.72

8 16,725 7802.34 22,442 10469.36 2667.02

9 16,725 7093.03 22,442 9517.60 2424.57

10 16,725 6448.21 22,442 8652.36 2204.15

112767.88 137896.37 25128.49

maximum calorific value (4.5 MJ m�3) of producer gas was

reported at this air fuel equivalence ratio. The Gas Chro-

matograph setup used for analysis of producer gas has been

presented in Fig. 7.

3.6. Specific energy consumption of tea dryer

Thermal energy requirement for per kilogramofmade teawas

also accessed and it was found as 7.5 MJ kg�1 of water

removed from the fermented tea with initial moisture of 60%

on wet basis. It was also observed that 2.85 kg of water was

removed from 5 kg of fermented tea in 25 min by using 4 kg of

wood (moisture < 20%). The thermal energy required for

manufacturing one kilogram of back was found as

(9.95 MJ kg�1). The specific energy consumption was a bit to

higher side might be because of the full capacity of the dryer

was not utilized.

3.7. Exhaust flue gas emission of producer gas fireddryer

Producer gas fired tea dryer burner and conventional ID and

FD coal fired tea dryer furnace have been presented in Figs. 8

and 9. The average temperature of flue gas in the combus-

tion chamber of producer gas burner was 375 �C, at air fuel

equivalence ratio (F ¼ 1.25). The average value of carbon

monoxide was 8.5 ppm, carbon dioxide was 15.54% and

NOx emission was 150 ppm. The carbon monoxide emission

was within acceptable level as per ASHRAE standard [31]. The

NOx emission in the flue gas was higher probably because

presence of fuel bound nitrogen in the woody feedstock.

3.8. Economic analysis

Annual black tea manufactured by an average size tea estate

in Sonitpur district (Assam) was 990 t in the year 2010e11 as

computed by geographical information system mapping. Re-

ported tea plantation areas in a FCC image (band 2, 3 and 4)

were seen in dark-red to red tone depending on whether they

are directly planted or appearing below shaded trees in

different sizes with regular sharp edges indicating the pres-

ence of a fence around it. The average yield of black tea

Table 9 e Computation of payback period and benefit costratio.

Years Cash inflow Present worth Cumulativecash flow

1 22,442 20,402 20,402

2 22,442 18,547 4638

Payback period 1 year 3 months

Benefit cost ratio 1.22

b i om a s s a n d b i o e n e r g y 6 6 ( 2 0 1 4 ) 2 7e3 8 37

manufactured was assumed as 2 t ha�1 to compute annual

black tea production for a representative average size tea es-

tate. The total back tea production was calculated by multi-

plying yield of black tea by total mapped area of the tea estate

[32].

Annual black tea manufactured by an average size tea es-

tate in Sonitpur district (Assam) was 990 t (GIS mapping). The

corresponding coal requirement is 792 t (Coal 0.8 kg kg�1 of

black tea manufactured). It was also observed from this study

that thermal efficiency was 20% for a conventional coal fired

furnace and 80% for proposed producer gas fired furnace.

Therefore producer gas fired furnace is a better option over

coal fired system. Coal international prices was 95 $ t�1, Birol

et al. [33] and that of woody biomass was 11 $ t�1 in the year

2011. A 150 kW thermal woody biomass gasifier was consid-

ered that could substitute 28% of this thermal load [34]. The

payback period of the system is 15 months and benefit to cost

ratio 1.22 (Tables 7, 8 and 9). The annual carbon-dioxide

reduction 1299.5 t is achievable [35]. It is a very attractive op-

tion in addition to green fuel application in teamanufacturing,

emission reduction, waste utilization and clean development

mechanism.

4. Conclusions

There are potential advantages in application of gasification

technology for tea manufacturing industries in Assam, India.

The surplus uprooted tea branches, shrubs, shading trees are

in house generation of biomass in a tea manufacturing unit.

Therefore, a downdraft gasifier performance with uprooted

tea shrubs as feed stock and its economics in thermal mode

to substitute conventional inefficient coal fired furnace have

been summarized in the present studies. The uprooted tea

shrub has high energy densities (18 MJ kg�1, HHV) for gasifi-

cation. It was observed that gasifier under study performed

well in respect of gas quality given by gas chromatograph and

calorific value at 0.27 equivalence ratios. The maximum

volumes of CH4, H2 and CO were reported as 1.4%, 18% and

24% respectively at above equivalence ratio. The average

calorific value was observed as (4.2 MJ m�3) from Junker Gas

Calorimeter. This is a satisfactory result with uprooted tea

shrubs as gasifier feed stock. The maximum cold gasification

efficiency was found as 65% near air fuel equivalence ratio of

0.27 and then it declined by increasing the gas flow rate. It

may be concluded that if only 28% thermal energy for tea

drying comes from biomass gasification in a typical medium

scale tea estate, the annual net saving in fuel cost is

computed as 21,067 $ and equivalent reduction of CO2

emission of 1299.5 t. A 150 kW biomass gasification unit can

give rise to a payback period of 15 months and hence the

investment seems attractive and environmentally sustain-

able for tea estate of Assam, India. However, the proposed

28% thermal energy requirement may not be possible by in-

house generation of above biomass in a representative tea

estate. Supplementary plantation of some fast growing vari-

ety trees in addition to biomass generated from tea planta-

tion itself may be required. Wherever there is a scarcity of in

house biomass production, deficit biomass may be procured

from nearby market.

Acknowledgment

DST_SERB Research Project Government of India (2012e2015):

Development of an innovative model of Combined Heat and

Power from Purely Producer Gas Based Engine Alternator

System for Partial Conventional Energy Substitution of Tea

Processing Industries in North-East India.

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Nomenclature

AAFR: actual air fuel ratioASTM: American Society of Testing MaterialCV: calorific valueD: diameter of gas flowing pipeFC: fixed carbonFD: forced draftFn: future amount of money at end of n yearFSD: full scale deflectionGC: gas chromatographHHV: higher heating valueID: induced drafti: rate of interestkW: kilowattMNRE: Ministry of New and Renewable Energyn: numbers of yearsNPV: net present valueP: principal amountDPG: pressure drop across gasifierDPN: pressure drop across nozzleSAFR: stoichiometric air fuel ratioVM: volatile matterWBG: woody biomass gasifierX: moisture percentage (wet basis)

Greek symbols

F: air fuel equivalence ratiohcold_gas: cold gasification efficiency