1 A Short Account on Agricultural Natural Resources

GTZ-Sustainable Utilization of Natural Resources for Improved Food Security: Energy____________________________________________________________________________________________________________

1 A Short Account onAgricultural NaturalResources

IntroductionNatural resources can broadly be dividedinto Renewable and Non-renewable.Whereas the non-renewables includesuch resources as minerals, which cannotreplenish themselves once used, therenewables include plants, animals, soil,water and the atmosphere, which doreplenish themselves.

In addition to its economicalsignificance, forest-a renewable resourceplays a key role for ecological balanceand environmental sustainability ingeneral. Thus, the misuse of the resourcehas not only its economical implicationbut also influences the environmentalsecurity surrounding humanity. Thedestruction of forests negatively affectssoil erosion, climate change (local andglobal), agricultural productivity as wellas poverty and the environment ingeneral.

Ethiopia's Forest ResourceSituationIt is believed that more than 60% of thecountry's soil used to be covered withHigh Forest, Savannah Woodland andBushland forests before the last couplesof centuries. However, due to the unwiseutilization and destruction sustained forthe last several decades, the coveragedeclined to 40% in the turn of the 19thcentury. It further declined to 16% in thebeginning of the 1960's. Due to theincreased economical, social andpolitical activities following the changeof government in 1974, the situationworsened and in the late 70's theresource was found further dwindled to

3.5%. Currently, the coverage isestimated at a mere 2.7% since thedestruction is ever worsening. It istherefore evident that less emphasis isgiven to the protection and developmentof the resource in the country. For themajority of the society, the importanceof the resource in respect of theaforementioned benefits has yet to comeinto the picture. Still, we are in asituation where almost everybody hasfree access to the resource as a means ofincome. It is therefore implied that thegeneral awareness by the societytowards forest and its importance isminimal.

The causes of the destruction of forestsin Ethiopia are several and complex: theincrease for the need of arable andgrazing land, expansion of towns andnew settlements, and the increase in thefirewood demand as well as materials forconstruction are some of the main.These, mixed with the ever increase inthe population, exacerbate the situationfurther and especially the resulting soilerosion forced smallholding farminghouseholds move to areas alreadyaffected by soil erosion.

Except some sporadic endeavors,however, extensive and prudentmeasures towards solving the problemare yet to be taken. On the contrary, thepractice today is that more forested areasare being cleared in search of new

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cultivable lands to compensate for thedecline in agricultural productivity.

The majority of the population usesfirewood as a source of household fuel.Moreover, raw materials for suchapplications as building construction,household furniture manufacturing,animal feedstock, etc all come directlyor indirectly from the forest. Thesepractices of utilization, in unsustainablemanner, have obviously accelerated thedeforestation rate resulting in theenvironmental degradation, climatechange, and decline in the soil fertilityand agricultural productivity, which ledultimately to desertification, the impactof which have been eventually affectingthe livelihood of the people.

The following are among the basicfactors inherent to the problemssurrounding the forest resource of thecountry: Lack of policy addressing forest and

land use issues Increase in the population of both

inhabitants as well as animals Lack of awareness of the public for

the protection and development offorest

Lack of strong institutionalframework vested with theresponsibility of developing andadministrating the forest resources ofthe nation

In sum, we are in a situation locked withseveral problems surrounding our forestresource base.

The two universal solutions foralleviating the problems, agreed up onby many scholars and professionals inthe field, are:

1. Utilization of the existing resourcein a sustainable way and

2. Afforestation measures to increasethe level of the resource base

On the other hand, it has already beenmentioned that the majority of thepopulation is utilizing firewood andagricultural remains as their mainsources of fuel.

Since the last couples of decades, thegovernment of Ethiopia hasimplemented successful measures for thedissemination of improved stoves in abid to promote the efficient utilization offuel. With the same cause, theMoARD/GTZ-Sustainable Utilization ofthe Natural Resources for ImprovedFood Security-Energy (GTZ-SUN:Energy) project is currently implementedby the Federal Ministry of Agricultureand Rural Development and the GermanTechnical Cooperation (GTZ).

The GTZ-SUN: Energy projectThe project's general goal is tocontribute to the sustainabledevelopment and utilization of thenatural resources of the country. Withthese goals, the project intervenes tomeeting the following main objectives: Catalyze the developments of the

energy resources, especiallyRenewable Energy (RE) resourcesand technologies, to promote rationaluse of natural resources, povertyreduction and food security

Enhance the capacity of the differentpartner government agencies andother stakeholders to integrate andexecute energy developmentmeasures into their developmentprograms

Promote energy efficiency at all levels(especially the efficient use of

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biomass resources at the households)

The project embarks on the followingstrategies towards hitting its goal andmeeting its objectives: Undertake capacity building measures

for the different stakeholders indeveloping strategies for, andimplementation of energydevelopment measures

Promote strong participation of theprivate sector in the development andprovision of energy services

Promote the awareness about, and theadoption of different environmentfriendly energy technologies andmeasures

Actively promote the awareness ofdecision makers and the generalpublic on environment and energyissues

The project's strategy regarding thepromotion of Mirt improved Injerabaking stove include:

Focus on the commercialdissemination of the stove

Involving the private sector, throughthe formation of small holding stoveproduction and sales enterprises, forthe commercial dissemination of thestove

Promote and support the decentralizedproduction and dissemination of thestove

Close cooperation with governmentand non-government agencies

The project has commenced its activitiesin 1998 and has since established morethan 350 private stove productionfacilities in more than 200 towns inAmhara, Tigray, Oromia and SouthernNations Nationalities and Peoples'regions.

Moreover, the project has been closelycooperating with different agencies,institutions and organizations to promoteand disseminate the stove all over thenation.

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2 Fuel Saving StovesIntroductionEnergy from fire has long been used inactivities such as cooking, which requireenergy. In addition, especially in theancient civilizations, fire served as atypical means of protection againstpredator animals. Then, the sources ofenergy serving the purpose were mainlyfrom plants that are known, by theircollective name, as Biomass Fuels.

Energy is an indispensable part of everycivilization. However, according to thelevel of civilization, different sources ofenergy are utilized. Mineral fuels such asgasoline and gasoil have been used torun locomotives and different machinerywhereas to produce electricity,hydropower, solar energy, wind and coalhave been used.

Food, being a necessity for life, has to becooked before serving. For this purpose,different forms of energy such aselectricity, gasoline, natural gas andcoal, as well as biomass are being used.From all these, the share of biomassenergy, worldwide, is significant andabout 80% of the planet's populationdepend on this resource for their dailylivelihood. The picture is never differentin our country where about 95% of theenergy demand is satisfied by theutilization of these sources.

The direct burning of biomass fuels inthe traditional way emits partially burntgases, which are harmful to humanrespiratory and visual systems. This isnot only dangerous to women and theaccompanying children usually exposedto these gases but is also wastage interms of fuel utilization as the energythat could otherwise be obtained fromburning them, is lost. Moreover, fire

hazard associated with the three-stoneopen fire is among the commonhousehold problem especially in therural Ethiopia.

Currently, in some areas of Ethiopia,households are forced to expend 30 to40% of their income on cooking fuels. Inthe areas where fuel is acquired throughfree collection from the jungles, womenand children responsible for the dutyhave to travel 5 to 6 hours in search offuel.

As pointed out earlier, the causes of theproblem are the clearing of forests insearch of cultivable land, excessive useof the forest resource for constructionand fuel purposes, etc. These led tofirewood shortage and people have totravel farther away from their residencesto collect fuel.

In addition, the country has long beensuffering from recurrent droughts anderratic rainfall. Since the agriculturalremains and animal dropping outs areused as fuel, the soil fertility is decliningvery fast. The loss of soil fertility is not,however, remedied by chemical

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fertilizers and consequently theagricultural productivity is very muchlow.

In an effort to alleviate the problem,though only partially, fuel use efficiencyat household level is one of severalmeasures that need to be taken. It is wellknown that the three-stone open fire,which is common all over the nation, is avery inefficient way of cooking, fromenergy utilization point of view. Only 8-10% of the heat goes to cooking the foodwhere as more than 90% of the heat issimply wasted.

In this article, a brief account on fuelcharacteristics with due emphasis onfirewood, the combustion phenomenonand process, heat transfer mechanisms,heat dissipation mechanisms from stovesand the methods to minimize it arepresented. Finally, it is discussed aboutmirt stove, all its features, how it isproduced, in more detail.

2.1 How Do Fuels Burn?For it is the most relevant fuel in thiscontext, let's take the case of firewood toinvestigate the combustion phenomenon.

Typically, wood consists of cellulose,lignin-a compound of carbon andhydrogen, resin and water together withgum bases. To fire wood, oxygen isneeded in addition to the minimumamount of heat energy to start thecombustion process. When burningwood, the source of oxygen isatmospheric air. Atmospheric air is 79%nitrogen gas and almost 21% oxygen. Inany combustion process, the amount ofoxygen being supplied plays a crucialrole. Whenever there is enough oxygen,the wood burns well, with less smoke.

In the early stage of the combustion ofwood, water and carbon dioxide comeout of the wood and begin to cover itsexterior. This shielding prevents thecontact of the wood from oxygen andconsequently smoke is emitted.

However when the combustionprogresses, the amount of the heat ofcombustion increases resulting in theevaporation of the water and in theevolving of combustible gases and liquidtar like compound from the interior ofthe wood. More gases come out whenthe liquid tar is heated with the alreadycarbonized wood. Finally, the chemicalreaction of the emitted gases withoxygen produces chemical compoundsthat we see as flame.

For combustion, every combustible gasrequires its own minimum amount oftemperature and oxygen. In closedstoves if more than required amount ofoxygen enters the stove, the extraamount of air robs some of the heat ofcombustion on its way leaving thesystem. Also, if the speed of incomingair is more than necessary, then it will bedifficult to attain enough temperature inthe stove that would otherwise progressthe combustion. Nor is it possible tohave a good combustion in the stove ifthe amount of incoming air is less thanthe required.

Once the situation of good combustion isattained in the system, all thecombustible gases will come out of thewood and burn further developing theprocess. Eventually, carbon monoxideand nitrogen begin to burn and the

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carbonized wood gives off heat burningas charcoal.

When the combustion occurs very fast,the production of liquid tar andcombustible gases will be very fastwhich produces more heat in a relativelyshort period. Here the producedcharcoal is relatively smaller. On thecontrary, when the air supply is small thecombustion occurs slowly and results inthe production of more charcoal.

2.2 Heat TransferThe three modes of heat transfer thatoccur in nature are:a. Conductionb. Convection andc. Radiation

1. ConductionHeat transfer, by conduction, occurs byvirtue of temperature difference in thedirection from the body of highertemperature to that of lower temperature.This transfer will continue until a state,called thermal equilibrium, is reached atwhich the two bodies are at the sametemperature.

The amount and the rate of thetransferred heat depend on the heatconduction characteristics, expressed asthe thermal conductivity, of the twobodies. Some materials are goodconductors of heat but others are not.Materials such as steel and basalt aregood conductors of heat. Materials suchas clay, sand, earth (soil) as well aswood and sponge, are poor conductorsor good insulators of heat.

2. Convection

Hotbody

Coldbody

Q = kT AX

Q = Heat conductedk= thermal conductivity

T= Temperature differenceX= Distance of heat conduction

A= Surface area of heat conduction

Hotbody

Coldbody

Heat movesfrom hightemperaturezone down tolowtemperaturezone

Hot airmovesup

Flame

Charcoal

Combustiblevolatile gasesevolve here

Firewood

k

X

A

T1

T2

Q

X

7


To understand the transfer of heat byconvection, let's consider a certain liquidin a metal container whose bottom isexposed to a source of heat.

Under a state of high temperature,molecules of gases and liquids(collectively known as fluids) riseagainst the pull of gravity. Thismovement induces the movement ofcolder molecules in the downwarddirection. This continuous motion willkeep on so long as the heat is beingsupplied and every molecule of the fluidis at the same temperature. The resultingmotion is circulation in which hottermolecules move upwards and coldermolecules move downwards. Thisdynamics of material transport effectsthe distribution (i.e. the transfer) of heat,which we call convection, in the fluids.

Another instance of convective heattransfer is when a fluid is forced to flowover a hot/or cold solid surface.According to whether the fluid is hotteror colder than the solid, heat istransferred from the fluid or to the solid,respectively, by convection. Such kindof convection is called forced/or activeconvection. Examples of activeconvection include, cooling of water in

automotive radiator and the cooling ofhot pot by the surrounding air.

3. RadiationA good example of such energy transferis the sun's radiation coming to ourplanet.

No matter small it may be, everymaterial radiates heat energy as long asits temperature is above -273.15 oC.Materials that intercept the radiationreflect part of the energy while theyabsorb some of it. Some of the energycould also be transferred to other bodies,by either one or the combination of theother modes of heat transfer.

Radiation cannot be recognized withnaked eye when emitted and transferredat lower temperatures. But, it becomesmore visible when the temperatureincreases. When burning, wood gives offa flame radiating a lot of energy.

Q = Fe FA (T14 – T2

4)

= Stefan Boltzmann's constant(5.7x10-8)Fe= Emissivity coefficentFA= View factorT1=Temperature of body 1T2= Temperature of body 2

dET1

ET2

Q

T

X

T wA

Q = hATh = convective heat transfer coefficientA = heat transfer areaT = Temperature difference

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Radiative heat transfer

2.3 Heat Leakage from StovesAs already pointed out, the heat leakagefrom three-stone open fire is unmatched.Heat is lost through convection andradiation from the three stones, sincetheir temperature is very high. Since the'stove' is not closed, the hot gases burnand simply leave the system withoutheating the cooking pot (or mitad (plate)in the case of baking). The combustiblegases may even leave the stove withoutburning, making the fuel wastage evenworse. This phenomenon is observed asdark smoke coming out of the stove.

The common clay stoves in use alsohave problems. Their clay walls are thinand heat can be lost easily through that.Since the size of the stoves itself is notproperly designed, the amount of airflowing and fuel put into them is notproportional. This makes thecombustible gases escape the stove

without burning and hence the heat thatcould otherwise be obtained is lost.

In general, the following are the mainmechanisms of heat loss from stoves:

Mechanisms of Heat Dissipation fromStoves1. Incomplete combustionSome of the emitted gases from the fuelleave the system without burning. Theremaining fuel left unburned as charcoal,is considered also waste.

2. Heat loss due to loss of hot fluegases from stove

The inert parts in the incoming air suchas nitrogen rob some of the heatgenerated in the stove on their wayleaving the stove. The nitrogencontaining air comes into the stove witha lower temperature, say 23 oC. But,when leaving the stove its temperaturecould go as high as 400 to 500oC butnever less than 250oC. This means some

22

2 dEE T

T

ET1 = source radiation intensityET2 = radiation intensity at distance d

d= distance from the radiation source

2 Heatdissipatedfrom stove

4 Heatabsorbed bypot and stove

1 Unburnedfuel

3 Heatdissipatedfrom pot

Total availableheat

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of the heat is lost together with thesmoke. In addition to this, the moistureof the firewood is given off from thestove in the form of vapor, which cannotbe realized without stealing some of theheat of combustion.

3. Heat loss due to absorption by thestove and cooking utensil

During operation, beforeheating/cooking the food, the cookingutensils have themselves to be heated.Also the stove itself takes some of theheat of combustion. Since it is only thensome of the heat transfers to the food,the remaining heat contained in theutensils and the stoves, is consideredwasted.

4. Heat loss from the stove parts to thesurrounding environment

During operation, since the stove is athigher temperature than the surrounding

environment, it is obvious that heattransfers by the three modes of heattransfer, from the stove to thesurrounding environment.

5. Heat loss from the cooking utensilto the surrounding environment

By similar argument as in the case of thestove, heat transfers from the cookingutensils to the surrounding. In additionto that, heat of evaporation is one way ofheat loss in injera baking, wot (sauce)cooking and water boiling, especiallywhen there are not lids to cover theutensils*.

2.3 How can we minimize heatloss?

The strategies for minimizing heat lossfrom stoves are born from the causes ofthe problems of heat losses themselves.They are summarized as follows:

1. Shielding the fire against ambientIt has been already said that three-stoneopen fire is exposed from all its sides tothe surrounding environment. Thismakes it easy to give off the heat ofcombustion especially by convection andradiation. Therefore, the obvious remedyis to shield the fire against the ambient.This not only decreases the heat loss bythe aforementioned mechanisms but alsominimizes the creation of smoke, as theperipheries of the flame will not beexposed to the lower surroundingtemperatures that otherwise would coolthe flame and inhibit the burning of thecombustible gases evolving from thefirewood.

* The amount of heat energy needed to raise thetemperature of 1 liter water from 20oC to 100oCis about 335kJ, whereas 2260kJ (more than 6folds) of heat is required to evaporate the sameamount of water at the same temperature of100oC.

Heat absorbed by stoves and cookingutensilsQ= mCT

C = Specific heat capacity

m = Mass

T= Temperature difference

(Specific heat capacity (kJ/kg.K)

- Air 0.1- Concrete 1.0- Water 4.189- Full wood 1.8- Clay 0.92-1.0 (depending on its

moisture content)- Iron 0.45-0.5- Aluminum 0.9- Copper 0.39- Vermiculite 0.84-1.08 (Total condition)

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The selection of the proper material forthe shielding plays significant role, aswell. The common stoves with metalwalls loose heat to the surroundingeasily. However, such materials as earth,clay, concrete, cast iron are goodinsulators as they heat up slowly and donot give off heat fast either, thus can beconsidered as good candidates forshielding material.

2. Regulating the incoming airstreaming into the stoveIn biomass stoves, the place where thewood burns is called firebox. For wehave to obtain a good combustion, wehave to make sure that just enough air isreaching to the firebox. One strategy todo that is to design the stove air inlet sothat it allows just enough amount of airinto the stove.

Another important part of the stove inrelation to this issue is the flue gas(smoke) exit. Stoves 'breath' (i.e. suck infresh air and give out exhaust gases) ontheir own thanks to the pressuredifference between their two extremes(i.e. air inlets and exhaust gas outlets).When the pressure difference is beyondthe requirement, more air comes indisrupting the proper combustion thatwould otherwise occur, just asmentioned earlier. The over-pressuresituation may occur when, for example,the fuel outlet chimney is higher (inheight) or wider (in area) than necessary.

3. Sizing the fire boxWe have also to determine the properdistance between the fire bed and thecooking utensil to ensure maximumamount of heat being transferred to theutensil. To do this, we have to make surethat the utensil's part exposed to the

flame is not very far from the heatsource, i.e. from the burning fuel. This isso because the most relevant heattransfer mechanism is either conduction(through physical contact) or radiation orboth, which both need close distance.

On the other hand, the combustible gasesneed space and time to heat up andeventually burn. This calls for thedistance to be not very close.

General Characteristics ofImproved StovesThe general criteria an improved stovehas to fulfill are fuel saving, one thatprevents the user from heat and firehazards, one that is acceptable andaffordable by the users. It has also towork on the fuel, which the users cannormally afford.

Improved StoveLike any other product, one has toconsider many other elements, beyondheat characteristics, when designing an

Heat transfer in stoves(example)

When the height isincreased beyond thedesign value

Conduction: (fromflame to the baking pan)decreases

Convention: (from hotgases in the stove to thebaking pan) decreases

Radiation: (from thered-hot charcoal to thebaking pan) decreases

When the stove wall isthin or made ofconductive material

Conduction: (fromflame and red-hotcharcoal to the stovewall and to parts of thestove in contact with thewall) increases

Convention: (from hotgases in the stove to thestove wall and then tothe ambient air)increases

Radiation: (from thered-hot charcoal to thestove wall and then tothe ambient air)increases

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improved stove. The following pictorialrepresentation illustrates the problemone has to solve to provide an improvedstove.

Characteristics of the stoveThe stove depicted in the above figurecan handle two pots at a time. Inaddition to the parts to serve theaforementioned purposes common to allimproved stoves, it features some otherunits making the stove even better:1. The combustible gases coming out of

the burning wood make the flamewhile the solid part burns formingcharcoal. Thus, the primary air,which burns the solid part (i.e.charcoal), comes into the stove from

beneath and the secondary air, whichburns the gases, comes into the stovefrom above the fuel. These are madepossible through the two differentdoors; one is at the bottom and theother is at the top. Note that the topdoor also serves to put in fuel. Wealso see that there are control bafflesused to regulate the incoming airflow.

2. The stove also features internalconduits usually made out ofconcrete or clay. Their purpose is totransport the exhaust gases whiletransferring their heat to the cookingpots on their way out. This effect isenhanced by making the conduitsspiral around the pots, which createsmore contact-area and -time betweenthe conduits and the pots.

3. There are exhaust regulators aroundthe exit of the stove just as there areon the inlet.

4. The caps fitted to the chimney canalso be multiple. This is to alter theheight of the chimney, in effect, toregulate the pressure differencebetween the inlet and the exit of thestove thereby regulating theincoming airflow according to thevarying power needs.

Fuelsavingstove

Durability Convenienceto produce

Strength

Cost

Heat transfercharacteristics

Aesthetics

Convenience to use

Chimney cap

Chimney

Smoke regulating baffle

Pot andplate rest

Grate

Fuel and primaryair inlet

Opening for primary airinlet and ash removal

Internal smoke duct

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Pot rest

Secondary air heater

Fuel and secondary inlet

GrateOpening for ash removaland primary air inlet

Chimney

Smokeregulating baffle

Pot rest Ceramic combustion chamber

HandleMetal Cladding

Grate

Opening for ash removaland primary air inlet

Door

Improved Charcoal Stove

Hypothetical improved firewood stove design

2.4 Stove sizing basicsThe examples and descriptions so farillustrate the general characteristics thataffect the fuel saving criterion of stovesand design considerations that should bekept in mind. Let's now see how we candetermine the basic parts influencing thefuel saving characteristics of stoves.

The following dimensions have beenobtained from literature. They have beendetermined through extensive researchand trials and therefore they can be used,at least, as the starting point to designdifferent fuel stoves.

The fire bed /grate/ in different woodstoves can be provided with perforations,which are used to filter down ash fromthe burning wood and let in primary airto the firebox. As can be seen from thefigures, the fact that the fire bed issmaller in diameter than around the topgives more room and more time for thecombustible gases to heat up andcombust before they leave the stove.Further, the fuel is made to keep itscentral position in the stove, which is

D= diameter of the pot (or anycooking utensil) on the stove

d= diameter of the fire bed

h= the height of the pot bottomfrom the fire bed

Efficiency %

Height (millimeter)

Radiation to pot

Convection to pot

Convection to potand to stove walls

Cumulative

Relationship between height and heattransfer rate in stoves

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needed for uniform heat distribution inthe stove. Another advantage is thelower cost of production materialbecause of smaller physical size of thegrate.

Fuel and Air InletIt has already been mentioned thatdetermining the proper size of the fueland air inlet is an important part of thedesign of stoves.

Before going directly into it, however,we have to see some facts regarding thereacting substances-the fuel and the air: Typically, wood is composed of 50%

carbon, 43% oxygen, 6% hydrogenand 1% ash. The fact that oxygen isavailable already in the wood in someamount makes the amount of externalair needed small.

To burn 1kg of firewood, thetheoretical amount of air necessary isonly 5m3. However, since thetheoretical conditions are notavailable in practice (such as the fuelis split into smaller pieces andvirtually every atom of carbon andhydrogen is contacted by that of theoxygen) more air, up to 40% extra isneeded; hence a total of 7m3 isneeded.

The rate of airflow for a naturallyaspirated stove (i.e. air flows into thestove without being blown) is about1m/s.

Moreover, we have to know about thecapacity of the stove we want to design: A stove capable of burning 1kg of

wood within an hour has about 5kWof power.

Knowing this we can determine the sizeof the air inlet, a, as:

timeneededspeedofairramountofaia

*

223

20002.03600*/1

7 cmmssm

ma

The air inlet area for a 5kW power stovethus should not be too much differentfrom 20cm2.

However, we have to determine the totalsize of air and fuel inlet, since both areusually put into the stove together. Wehave to employ a recommended ratio offuel to air inlet areas to determine thearea to be occupied by the fuel. Here wewill assume that 70% of the total areawill be occupied by fuel.

Thus, since we have already calculatedthe area of air inlet, to determine thetotal area of air and fuel inlet, A, we set,

Aa *3.0 or2

2

673.0

203.0

cmcmaA

This means that the area of the air andfuel inlet is about 8.5cm by 8.5cm.

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It should be born in mind that thecalculations presented are only to showthe methods of determination of thedifferent sizes of the stove and in reallife, the values may differ significantlyfrom one stove to another depending onthe specific cooking needs and fuels tobe used.

Exhaust outlet areaThe following should be in mind todetermine the exhaust area: The air entering the stove should exit

the stove although changing its form(i.e. it leaves the stove as a differentchemical compound)

Additionally, there are combustiblegases coming out of the firewood

These two facts call for the exhaust areato be greater than that of the air inlet. Asrecommended in literature we can makeit 30% larger than the air inlet, in thiscase. Therefore, it becomes, B:

22 2620*3.1*3.1 cmcmaB

ChimneyIf chimney is required for the stove, thenits area should obviously match that ofthe exhaust area. And since chimneysare usually cylindrical in shape, thediameter of their circular cross sectioncan easily be calculated employing theformula of area of a circle. Thus, callingthe diameter C, from

4

2CBarea ,

BC 2 , 141.3

Doing the math gives,

C=5.75cm2.

During operation, the chimney getsclogged with soot after some time andthis would impede the exhaust flow.Therefore, the diameter can be increasedup to 10 cm.

Regarding the height of the chimney, itshould not be higher than half a meterespecially for small kitchens. But, toeliminate smoke completely from thekitchen it can be made to rise up toanother half a meter beyond the roofingof the kitchen. This means that the totallength could go as high as 2 to 3 meters.If chimney is not to be used, a pot shouldbe rested on the pot rests provided on theexhaust outlet. This is to regulate thesmoke as a baffle as well as to utilize theheat of the flue gases before they leavethe stove.

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3 Mirt StoveMirt Injera baking stove was firstdeveloped by the Ethiopian RuralEnergy Development and PromotionCentre in early 90's, when the stove wasintroduced for the first time to the AddisAbaba Market. Compared to thetraditional open fire, the stove is provedto save fuel by 50%. All the fuels thatwe use on three-stone open fire(firewood, animal dung, branches,leaves, wood chip ...etc.) could also beused on mirt stove.

3.1 Main component parts ofMirt stove

Mirt stove is made of 6 different parts.Like any other improved stoves, it isimportant to maintain the dimensions ofthe stove, in addition to maintaining theraw materials and productionprocedures, to obtain the expectedefficiency and hence fuel saving.

1. Fuel inlet base width: 24 cm2. Fuel inlet central height: 11 cm3. Wall height (outer): 24cm4. Wall diameter: 62 cm5. Smoke outlet width: 19 cm

6. Smoke outlet height: 7cm7. Pot rest height: 14cm8. Pot rest hole diameter: 18cm9. Wall thickness: 6 cm

3.2 Production moldA set of mold consisting 8 different partsis used to produce mirt stove. The moldis to be manufactured from steel sheetmetal of 1.5 mm thickness. Dependingon its manufacturing quality and usagethe mold can serve for not less than 3years. Current market price of the mouldis estimated ETB 600-900.

1. Mold for pot rest2. Mold for wall3. Mold for fuel inlet4. Mold for smoke outlet5. Mold for pot rest support structure

3.3 Raw materials for mirt stoveproduction

Mirt is produced from cement and riversand. If red ash (scoria) is available, it ispreferred to river sand. For best qualitystove, the sand or the scoria should befree from dust or any other foreignmaterial. Otherwise, we may risk thestrength and the efficiency of the stove.The biggest particle size of either of thesand or the scoria should not be greaterthan 5mm.

Regarding river sand, it is seldomavailable free of dust. There could alsobe lustrous particulate materials called

3

2

51

411

Chimney & potrest

Smoke outlet

Mitad rest

Air & fuelinlet

16


silica which affect the strength and heattransfer characteristics. Thus, materialselection is an important element ofproduction to guarantee a good qualitystove.

To select a good quality river sand1. Observe a sample. If there is too

much silica in it then avoid it.2. To see the proportion of dust in the

sand, take a portion and put it in atransparent glass bottle. Pour somewater and shake well to mix ituniformly and let it settle. After 5 to10 minutes time, observe. If theamount of pure loam is not morethan 2% then the material could beused with some washing. If not thenavoid the sand.

To investigate the quality of scoria, themost important factor is the particle sizeof the material. Material consisting ofparticulate sizes either smaller than 3mmor bigger than 5mm should be avoidedsince both affect the physical strengthand the heat characteristics of the stove.

Non-fresh cement or that which isexposed to air and/or moisture is a goodrecipe for a bad quality stove. Care hasthus to be taken to avoid such material toproduce the stove as it greatly affects thestrength of the stove, especially at itshigher operating temperatures.

3.4 Preparation of productionplace

a. ShadeShade is needed as a workshop, toproduce and store the stove. The sizemay depend on the production capacityneeded but should not be less than 24m2

(6mX4m) in any case. For goodventilation, the minimum height shouldbe 2.5m.

The floor should be made plane, whethermade out of earth or concrete, to putfresh cast (which is wet and very fragile)without breakage or failure.

b. Space for screening sandRiver sand is to be sieved with 3mmmesh-size sieve. Scoria shall be siftedfirst with 3mm mesh-size sieve and thenthe remaining, which is retained on the3mm mesh-size sieve, shall be screenedon 5mm mesh-size sieve.

The sifted sand must be kept out of soilor any other dirt. For this purpose, amaterial, such as plastic canvas or anypiece of cloth could be used. If not aspecial place should be prepared wherethe floor is lined with a thin layer ofconcrete. As much as possible similarcare needs to be taken for the unsiftedsand.

c. StorageIf accumulation of produce is deemedunavoidable, a special place should beprepared. Care has to be taken in orderthat the produces are not eroded bywater especially during the rainyseasons.

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3.5 Tools and raw materials forstove production

a. Tools

Sieve for sand screening (3mm and5mm mesh sizes) Jug for water Shovel (for cement/sand mixing) Mason's bucket (for sand and cement

measurement) Mason's trowel Bowl (for river sand cleaning) Brush Stomping stick (wood or metal) Wooden board ( or Chip wood board ) Gloves Set of mold

b. Raw materials

cement (fresh) river sand or red ash(scoria) clean water (but not necessarily

potable) used car engine oil (for lubrication of

mold and for easy ejection of cast frommold)

3.6 ProductionRaw material preparationa. River sand Like already said, raw sand is first

screened using 3mm mesh-size sieve

The screened sand then shall be mixeddry with cement, with a sand to cementratio of 3:1. The mixing is such that itshould be homogenous.

The homogenously mixed material isagain mixed with water. Water ispoured in to the mixture little by littleand in steps while doing the mixing. Itis difficult to measure the amount ofneeded water in advance but it caneasily be known from the texture of themix by looking at a handful of sample.In any case, the water should not betoo much that the mix is very wet. Thisotherwise could lead to non-uniformcement distribution, more materialconsumption during production andheavy stove which all eventually leadto bad heat characteristics of the stove.

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Another disadvantage of excess wateris that it makes production difficult,especially to eject the cast from itsmold.

b. Red ash (Scoria) Raw scoria is first to be screened with

3mm mesh-size sieve. The retainedmaterial on the sieve then is put on5mm mesh-size sieve for anotherscreening. Note the step that thescreening should start from the smallermesh-size sieve. Then a homogeneousmixture is to be formed, by mixing,from both particle sizes of material in3mm to 5mm ratio of 3:1. The mixture is again mixed with

cement in 4:1 ratio before it is mixedwith water. The water is mixed just ina similar fashion as in the case of riversand; little by little while mixing andchecking the amount.

Production The producer shall first check that

he/she wears his/her gloves and that allthe necessary tools for production areavailable nearby. It shall be made sure also that the mold

is clean and free from any debris thathinder production and extraction ofcast stove part. If possible, one canmake use of used car engine oil beforestarting production by applying it inthin film on the interior of the mold.Be wary of the oil against contact withskin, however. Putting the mold on the appropriate

wooden board, mixed material is thento be poured into the mold. Whiledoing so it should not be forgotten topound the material with a piece ofblunt-ended wood or iron bar with amoderate force. It should not also to be forgotten that,

when producing the wall part of the

stove, the appropriate mold parts thatcreate the openings for fuel inlet orsmoke outlet are put.

After filling the material into the mold,then the cast is ejected from the mold.Note that the cast should not be ejectedbefore putting it on its place where it isto be dried. For easy ejection, hittinggently the sides of the mold with apiece of stick might help. The cast is then let to dry for about

24hrs before it is put off from thewooden board it has been standing on. After dismounting, the produce is then

ready for watering. Once produced astove part must be watered twice aday, at least for 7 days before putting itfor sale or into service. After each day of production, cleaning

the molds and all the working tools isimperative. Applying a thin film ofengine oil, especially, to the interior ofthe mold helps protect it againstdamage by the cement.

Notes:The following pieces of advice are goodfor quality product, safety againstpersonal injury and for prevention ofdamages to the working materials: Producers are advised always to lower

bending their legs, rather than bendingfrom their waste, to pick up or lowerheavy things, such as stove parts. Tools, such as shovels and mason's

trowel should be put in such a way that

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they are not to recoil when for exampleaccidentally stepped on them. Whenever watering the produces we

should not do that while the producesare on the wooden boards in order toavoid damage to the boards. Produces of different days are advised

to be sorted during watering andstoring. Orderly storage of parts makes easy

sorting and hence First-In-First-Out ofthem and makes the management ofthe store, efficient.

3.7 Transportation of mirt stovea. Man power 2-3 people can transport one full

set of stove for 1.5 to 2kms.

b. Wheel barrows It is possible to transport one full

set of stove for up to 6kms.

c. Pack animals A donkey, a mule or a horse can

transport a stove for up to 10kms. It is possible to transport one

more stove on a camel coveringthe same distance.

d. Animal powered wagons For up to 25kms it is possible to

transport 2 to 5 full sets ofstoves, according to theaccommodation capacity of thewagon.

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e. Vehicles

On gravel roads, according to thespecific situation of the roads, wecan transport stoves for 50-100kms. The number of stovesbeing transported may varyaccording to the carryingcapacity and space of the specificvehicle.

However, for asphalted roads it ispossible to transport more than100kms of distance.

If stoves are to be loaded on theroof-racks of vehicles, care mustbe taken to avoid damage byrubbing against each other. Thus,used carton paper, straw or anyother thing serving the purpose,could be put between the partsand every thing must be tied tightwith rope. It is also possible andhelpful at the same time, to putlight loads (e.g. sacks of grains,etc.) on top of the stove parts.

3.8 Installation of Mirt stoveIt is recommended to install mirt, andany other stove for that matter, on anelevated platform. Therefore,preparation of such unit in the kitchenprecedes the installation of the stove.The following are among the advantagesof the platform: Tiredness and inconvenience during

Injera baking are greatly minimized It is easy to control the fire in the

stove It is possible to put around and to use

required materials for the bakingeasily and conveniently

Children and animals, which dwellusually in and around the kitchen,may be less risked of any fire danger

Wetting from spill over of water andother liquids inside and around thestove is less likely to happen.

Different Types of Platformsa. Wett (Solid) PlatformWett Platfrom from Earth and Stone Bigger stones are first put along

the apparent rim of the platform. The interior then is filled with

gravel and earth to the desiredlevel.

The top of the platform shall becompacted and (water) leveledwell, wetting it with water. All theexposed faces could also be linedwith mud to keep the aesthetics.

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Wett Platform from Stone/HollowBlock/Brick Like in the above case, the would-be

rim of the platform is set first byputting either stones, or blocks orbricks, all around, to the desiredlevel of the platform.

Again, the interior is to be filledwith gravel and earth.

The top most part is then to be filledwith a layer of concrete, by firstputting small pieces of stones andgravel beneath. The concrete is tobe made out of cement, sand andgravel, in the respective ratio of1:2:3. The alternative to theconcrete is brick or hollow block.Do not forget to water level themall, however.

b. Seqela (Hollow) Platform

This type of platform leaves a spaceunderneath; the space can be used forstorage.

Firewood, for example, may be storedin the space. This is important as dryfuel itself saves fuel.

Seqela Platform Made from Earth andStone Wooden poles, preferably of branches

of 'Y'-shape, are first erected indistances of 50 to 80 cm, dependingon the desired area of the platform.

A mesh made of wood is thenconstructed, put, and joined by anymeans, with the top of the erectedwooden poles.

A layer of gravel is applied on themesh before applying a layer of soilfinally and is finished.

Seqela Platform Made fromStone/Hollow Block or Bricks

The structure supporting the platformcan be constructed from stone, orbricks without any reinforcement, butwith reinforcement in the case ofhollow blocks or concrete.

The platform is usually made ofconcrete with steel or woodenreinforcement. The recommendedreinforcing steel mesh size is 10 to15cm2 using 10 to 12mm thick bars.

Once made, the concrete has to bewatered for 7 to 10 days.

Note:The fire bed in mirt stove is stronglyadvised to be made of clay material suchas used/broken Injera baking pan, Mitad.

Installation of Mirt Stove1. First mud mixed with straw is to be

prepared as a bonding material forthe parts

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2. Start the installation from the back ofthe stove, i.e., from the 'U' andfollow with the rear walls

3. Then assemble the rest of the wallsas shown in the figure

4. Continue with the baking pan, themitad

5. Then the last part to assemble will bethe pot rest. The opening that may becreated between the pot rest and themitad should be stuffed with mudtogether with small pieces of stone orbroken pieces of clay material.

6. Finally all the joints shall be filled,lined and finished with mud.

3.9 How to Use Mirt Stove Start firing the wood just outside of

the stove at its door in order todevelop it and push in as the firedevelops.

Do not stuff too much fuel into thestove. It suffices to put just a littleamount turn by turn.

Use dry wood. Always put something on the pot rest,

if not a pot of something to cook or

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boil. This not only allows the efficientutilization of the fuel but alsoregulates the speed of the smokeexiting the stove thereby optimizingthe heat distribution in the stove.

It is advised not to puff into the stoveto restart the fire. Use a flap like thing(e.g. a piece of hard paper) to blow inair. Otherwise, the flame may bounceback and harm the face.

Water or any similar liquid is bad forthe stove during or after operation.Contact against such things musttherefore be avoided.

After each session of operation, ashhas to be removed from the stove.This is important for the propercombustion and hence fuel saving ofthe stove.

It is desirable to repair the stove jointswhenever necessary. The onlyopenings on the stove should be thefuel/air inlet and the smoke outlet.

It is possible and necessary tosubstitute any failed part of the stove.

Whenever necessary the user mayseek assistance from a stove produceror the concerned technical staff fromthe nearby energy or agricultureoffices.

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Documents

1 A Short Account on Agricultural Natural Resources