DESIGN AND TESTING OF CASHEW NUT SHELL GASIFIER STOVE

Miguel M. Uamusse, Jonas Matsinhe

Universidade Eduardo Mondlane, Mozambique

miguel.meque@uem.mz

ABSTRACT

Biomass has already emerged in the renewable energy area as one of the promising

candidates for the future energy source(Basu, 2010). Historically, biomass has been a

major source of fuel from the existence of making rapid urbanization, but widespread

use of fossil fuels in the industrialization has relegated it to a minor source of energy

The CNS Gasifier Stove developed has a thermal efficiency of 35.5 % and an energy

output of 2.19 kW. The corresponding values for LPG stove and Kerosene stove are

53.53 % (0.69 kW) and 39.3 % (0.6 kW), respectively. Though the use of CNS gasifier

as a fuel is less efficient than the aforementioned, with the rising oil prices and the

search for alternatives to contemporary cooking fuels, it is still a viable alternative in this

comparison.

LIST OF ABBREVIATIONS

ASTM American Society for Testing and Materials

CNS Cashew Nut Shell

FAO Food and Agriculture Organization of the United Nations

GHG Greenhouse Gases

HHV Higher Heating Value

IEA International Energy Agency

LHV Lower Heating Value

LPG Liquefied Petroleum Gas

MC Moisture Content

NBCB North Bank of Cahora Bassa

NOMA NORAD’s Program for Master Studies

Pi, Po Power input and out put

Qs,Ql Sensible and Latent Heat

TGA Thermal Gravimetric Analysis

UNEP United Nation Environmental Programe

VM Volatile Matter

WBT Water Boiling Test

WHO World Health Organization

1. INTRODUCTION

Energy is a basic necessity for human activity, economic and social development. But global

strategies to meet this basic need for the world's rapidly growing population are lacking. The

major sources of energy to supply such a scenario are oil, coal, natural gas, hydro energy, nuclear

energy, renewable combustible wastes and other sources. The global energy supply in 2010 was

estimated to 11,500 mtoe (million tons of oil equivalent) and Projected to be 13,700 mtoe in the

year 2020. Under the 2010 scenario, the global energy mix was: oil 35.1 %, Coal 23.5 %, Natural

gas 20.7 %, Renewable combustible wastes 11.1 %, Nuclear 6.8 %, Hydro 2.3 %, other sources

0.5 % (Economy Watch, 2010).

Energy is a basic necessity for human activity, economic and social development. But global

strategies to meet this basic need for the world's rapidly growing population are lacking. The

major sources of energy to supply such a scenario are oil, coal, natural gas, hydro energy, nuclear

energy, renewable combustible wastes and other sources (Wilson et a,.l 2010). The global energy

supply in 2010 was estimated to 11,500 mtoe (million tons of oil equivalent) and Projected to be

13,700 mtoe in the year 2020. Under the 2010 scenario, the global energy mix was: oil 35.1 %,

Coal 23.5 %, Natural gas 20.7 %, Renewable combustible wastes 11.1 %, Nuclear 6.8 %, Hydro

2.3 %, other sources 0.5 % (Economy Watch, 2010).

1.1.Biomass Energy Technologies

Biomass to energy routes are numerous and can mainly be divided in three groups, as shown in

Figure 2.1: thermochemical, biochemical conversion routes and physical or mechanical

extraction route (Tsamba, 2008). The first group includes combustion, gasification, and

pyrolysis. The second group incorporates digestion and fermentation. Only thermochemical

technologies are discussed in the following section.

Figure 1.1 Thermochemical Conversion Routes (Tsamba, 2008)

1.2. Gasification

Gasification is thermo-chemical process at high temperature that converts carbon containing

fuels, such as coal and biomass, into a combustible gas containing mainly carbon monoxide,

hydrogen, methane and inert gases, through incomplete combustion and reduction. At the

beginning of 1812’s, gasifiers were used for industrial power and heating applications until in the

1920’s, when oil-fueled systems gradually took over the systems fueled by producer gas (Quaak

et al., 1999). It is also noted that unreliable supplies of petroleum gas revitalized gasifiers during

World War II and it was intensively used in transportation and on farm systems (Rajvanshi,

1986). Gasification is found to be appealing for the reasons listed below (Quaak et al., 1999;

Rajvanshi, 1986):

i. The combustible gas can be used in internal combustion engines or gas turbines (enabling

high efficient power generation), burned directly or used in the production of methanol or

hydrogen.

ii. A gaseous fuel needs little excess air in combustion and has low levels of contaminants.

iii. It enables solid fuels to replace oil, considering the rising oil price and climate change

effect.

Eventually, gasification technology is currently identified as a sustainable renewable alternative

for energy generation with zero impact to the environment.

1.3. Type of Gasifiers and Process

Due to the interaction of air or oxygen and biomass in the gasifier, gasifiers are classified

according to the way air or oxygen is introduced in it. There are three types of gasifiers,

Downdraft, Updraft, and Crossdraft . As the classification implies updraft gasifier has air passing

through the biomass from bottom and the combustible gases come out from the top of the

gasifier. Similarly in the downdraft gasifier the air is passed from the tubers in the downdraft

direction.

Table 1.1 Advantages and Disadvantages of Gasifiers (Quaak, 1999; Rajvanshi, 1986

Gasifier type Advantages Disadvantage

Updraft (i) Small pressure drop.

(ii) Little tendency towards slag formation

and simplicity in construction.

(iii) High charcoal burnout with internal

heat exchange leading to low gas exit

temperature and high gasification

efficiencies.

(iv) Can run on wide range of moisture

content and some size variation in feed

stock.

(i) Moisture Content of fuel.

(ii) Relatively long time required for

start up of engine utilizing syngas

(iii) Poor reaction capability with

heavy gas load.

(iv) High amounts of tar and

pyrolysis products.

(v) Demanding excessive cleaning

for the case of engine application

Downdraft (i) Flexible adaptation of gas production to

load.

(ii) Low sensitivity to charcoal dust and tar

content of fuel.

(iii) Produces gas with low tar content

suitable for internal combustion engine

(i) Needs uniform and not very

small particle size of fuel 1 – 2

mm.

(ii) High amount of ash and dust

particles remain in gas

(iii) The relatively high temperature

of exit flue gas

(iv) Results in lower gasification

efficiency biomass moisture

content must be less than 25 %

on dry basis

Crossdraft (i) Fast response time to load.

(ii) Flexible gas production.

(iii) Can be operated in small scale

(i) High sensitivity to slag formation

(ii) Demanding clean charcoal

(iii) High pressure drop.

(iv) Charcoal gasification results in

extremely high temperature.

2. DESIGN OF THE GASIFIER STOVE

2.1. Development Process

The development of Cashew Nut Shells gasifier stove was executed in accordance to the

calculation done in section 3. The size of the reactor was scaled down to 0.15 m reactor diameter,

0.60 m height of reactor and 2.1 kW power output, just enough for a family with 3 to 4 members.

However for the ultimate analysis the chemical formula for the CNS was determine as; From

ultimate amalysis, C = 63.2 %, H = 6.74 and O = 21.9 %. For the purpose determining the

amount of air for combustion of the biomass, empirical formulae was deployed as stated in

Equation 4.1

OHbCOaAirNOCH 2201.023.02.1 +⇒+ ………………………………… (4.1)

BiomassofkgperAirofkgTotal

NONOAir

wOW

zyX

ZYMrmXc

N

OH

47.4:

47.379.021.0:

0085.0267.5041.0041.0.

1463.

23.0267.5369.12.1

267.574.61

267.5267.5

369.116

9.2174.6174.6267.5

122.63

2222 +⇒+

====

======

=======

The schematic configuration diagram of the said gasifier stove is depicted in Figure and its main

dimension given in Figure .

Fig.2.1. Gasifier Reactor

2.2. The Major Parts of the CNS Gas Stove.

i. The Gasifier Stove Reactor

The gasifier stove reactor is shown schematically Figure 4.1. The reactor is cylindrical in shape

having a diameter of 0.15 m and a height of 0.60 m. The cylinder is made from a galvanized

iron sheet gauge 1.8 mm, thickness on the outside and of a stainless steel sheet gauge 2.0 mm

thickness in the inside. This cylinder is provided with an annular space of 2 cm, where the

burned CNS or any other materials is placed to serve as insulation in order to prevent heat loss in

the gasifier..

Fan assembly

Ash chamber

Reactor

Port support

ii. The Ash Chamber

The ash chamber (Figure 4.1) serves as the storage for ash produced after each operation. It is

located beneath the reactor to easily catch the ash that is falling from the reactor. This chamber is

provided with a door that can be opened for easy disposal of ash and it must be kept always

closed when operating the gasifier. The ash chamber is tightly fitted in all sides to prevent the air

given off by the fan from escaping the chamber hence, minimizing excessive loss of draft in the

system in gasifying the fuel.

iii. The Fan Assembly

The fan assembly is the component of the stove that provides the air needed by the fuel during

gasification. It is fastened on door of the chamber directly push the air into the column of CNS in

the reactor. The fan used for the standard model is a 76.2 mm (3 inch) diameter axial-type fan

that is used in computers. It has a rated power input of 16 W using a 220 volt AC. A manually

operated rotary switch is used to control the speed of the fan which, in turn, controls the flow of

gas to the burner during operation

iv. The Burner

The burner converts the gas coming out from the reactor to a bluish flame, where combustible

gas is allowed to pass through. The secondary holes located at the periphery of the burner are

used to supply the air necessary for the combustion of gases. On top of the burner is a pot

support that holds the pot in place during cooking. The burner is removable for easy loading of

fuel into the reactor and is set in place during operation.

3. METHODOLOGY

the methodology used for this work was first study the raw material use( CNS)and calculation

the size of stove suitable to supply a power output of 2.1 kw for household cooking. The required

time to operate the stove was set at 40 minutes, before discharging char and reloading Cashew

Nut Shells.

1. Power Input (Pi):The amount of power to supply the energy needed by the stove,

considering a thermal efficiency for the stove of 30 %, will be

ηPoutPi =

where: Pout-power output, kW; Pi-power in put, kW; η-efficiency of stove

2. Fuel Consumption Rate:(FCR) Based on the power input, the amount of Cashew Nut

Shells to be burned per hour will be

HVFPiFCR×

=0012.0

where: FCR -fuel consumption rate, kg /hr Po -power output, kW HVF-heating value of

cashew nut shell, kJ/kg.

3. Cross-Sectional Area of the Reactor (Ar) assuming a specific gasification rate of 87

kg/hr-m2 , the cross-sectional area for the CNSG stove will be

SGRFCRA =

Where: FCR - fuel consumption rate, kg/hr, SGR-specific gasification rate of Cashew

Nut Shells,

4. Height of Reactor (Hr)

In order to attain the 40-minute operating time required, considering a 1.5 cm per min

fire zone rate for the stove, the height of the reactor will be

FZRTH or ×=

Where: Hr-Height of reactor, m; To-operating time, FZR-fire zone rate.

5. Diameter of Reactor (Dr)

Using the computed Ar, the diameter of the reactor inner cylinder will be

5.04

=πADR

Where: D - diameter of reactor, m FCR - fuel consumption rate, kg/hr, SGR-specific

gasification rate of Cashew Nut Shell

6. Airflow Requirement (Ar)

The amount of air needed to gasify the fuel, considering an equivalence ratio for the stove

of 0.35 m/s, will be

Rr ESAFCRA ××=

where: AFR - air flow rate, m3/hr, FCR - rate of consumption of Cashew Nut Shells, kg/hr, SA-

stoichiometric air of Cashew Nut Shells, 4.5 kg air per kg cashew nut shells

Table 4.1 gives the summary of empirically parameter of gasifier. The values were calculated

using the correlation as introduced in above.

4. RESULTS AND DISCUSSIONS

During the entire operation, the quality of gas varied from the start to the end. In the initial stage,

the flame color was yellowish to pink; in the middle stage, the flame turned pinkish-to bluish;

and towards the end of operation, the flame became bluish. The color of the flame is affected by

the moisture and hydrocarbon present in cashew nut shells being burned in the fire zone

Figure 4.1 Table 2.2 Advantages and Disadvantages of Gasifiers (Quaak, 1999; Rajvanshi,

1986 Color of Flame in CNSG

Table 4.1. result of design parameter.

Parameter Calculation result

Power input 6.1 kW

Fuel Consumption Rate 1.35 kg/hr

Cross-Sectional Area of the Reactor 0.0154 m2

Height of Reactor (Hr) 0.60 m

Diameter of Reactor (Dr) 0.130 m

Airflow Requirement (Ar) 0.20 m3/min

The power imput is around 6,1kW and Fuel consumption rate are good for this

gasification, and the CNS gas stove emits a flammable and poisonous gas. It is thus

recommended that the gas produced during operation is properly burned. It is

recommended that inhalation of the gas emitted from the stove is prohibited since it is

toxic and injurious to health.

5.CONCLUSIONS AND RECOMMENDATIONS

a) The fuel properties of the cashew nut shell waste revealed its suitability for gasification.

The sampled cashew nut shell had a moisture content (4.2 %), low ash content (7.5 %),

and a heating value of 24.2 MJ/kg against the recommended values 6.7%, 26.2%,

21.5MJ/kg respectively.

b) The proper utilization of the cashew nut shell waste through gasification route will

conserve the biomass fuel and make its use self-sustainable for thermal energy. They can

be used for domestic cooking, heating and for keeping the surrounding warm, instead of

relying on depleting fossil fuel, highly cost gas and electricity.

REFERENCES

Basu, P (2010), Biomass Gasification and Pyrolysis ,Piratical design and Theory,

Elsevier USA

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Combustion and Gasification Technologies, Energy Series: World BankTechnical Paper

No. 422.

Rajvanshi, A.K (1986), “Biomass Gasification”, in Goswami, D. Y Alternative Energyin

Agriculture Vol. II, Ed. CRC Press, pp. 83-102

Tsamba, A. J (2008). Fundamental study of two Selected Tropical Biomass for

Energy: Coconut and cashew nut shells. Stockholm: KTH Industrial Engineering and

management.

Wilson, L, Yang, W, Blasiak, W, W, Geoffrey R. J and Mhilu, C. F (2010), Thermal

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