Biomass Stove

Thermochemical conversion of biomass 729

Biomass stoves

Archaeological findings indicate that the use of fire was known to mankind400 000 years ago. The use of fire for cooking dates back to 100 000 yearsago, and cooking was presumably done over an open fire (mainly for cooking/roasting meat) with the fuel arranged in a pyramid shape. Despite lack ofcontrol on the fire and the smoky conditions, the open fire had the benefits ofpreserving food, protecting against animals, and providing warmth. A majordevelopment in open fire was the evolution of different-shaped vessels andlater of a shielded three-stone stove for holding the pot over the open fire.Subsequently, the shielded fire was changed to a U-shaped mud or mud/stonestove with a front opening for fuel feeding and combustion air entry. Sincethen, despite several developments in the wood stoves, a large population inthe developing world still employs the traditional three-stone or U-shapedshielded fire stove, and in many cases, they alternate between wood, cow dung

cakes, and agro-residue for fuel (FAO 1993).

Classification of biomass stoves

Stoves that burn biomass such as firewood and agro-residues are calledbiomass stoves. These are used at both the domestic and institutional levelsfor cooking, heating, and space-heating purposes.

Biomass stoves can be classified in several ways, based on their at-tributes, functions, material, fuel types, etc. (FAO 1993). Based on variouscharacteristics, stoves are classified as follows.

Figure 13.14 Circulating fluidized bed combustion systemSource last accessed on 20 April 2007

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730 Renewable energy engineering and technology

Fuel type Woody biomass, powdery biomass, briquettes, cow dung, etc. Function Mono function or multi-function stoves Construction material Metal, clay, ceramic, brick, etc. Portability Portable (metallic or ceramic) or fixed (mud, clay or brick) Number of pots Single, two pots, three pots, etc.

A stove designed for a particular fuel and a particular application can beused for different fuels and applications but may not perform with the sameeffectiveness.

Design criteria

A stove is a consumer-specific device. The effort on developing improvedstoves is mainly aimed at improving energy efficiency (saving fuel) or reducingemissions (improving working conditions and reducing an adverse impact onhealth). While designing a stove, both engineering and non-engineering or so-cial parameters are required to be considered.

Social factors

The interlinking of various criteria for stove design is shown in Figure 13.15.User need and availability of local biomass are the two important socialfactors that need to be taken into account while designing a stove. User needswould include factoring into the design various cooking operations (boiling,frying, baking, grilling, steaming, pressure-cooking, etc.) that have to beperformed on the stove. Apart from this, it is also important to know thecooking time and the process heat requirements, which would determine thepower range for the stove. Availability of local construction material, desiredportability, seasonal availability of local biomass, etc. also need to be con-sidered while designing a stove for the target group (Verhaart 1983).

Technical factors

A high-performance stove should be efficient from both the efficiency and theemission points of view. A fuel-efficient stove could reduce the drudgery ofcollecting fuel while reduction in emissions could save the users from theharmful impact due to exposure to smoke. However, as mentioned earlier, thegeneral strategy adopted in designing an improved stove is improving theenergy efficiency (by enhancing heat transfer) and providing a chimney for theremoval of smoke. Though this strategy improves the fuel efficiency and also

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Figure 13.15 Design considerations for a stove

the quality of the indoor air, the overall reduction in emissions is questionable.Therefore, in addition to improving the heat transfer efficiency, it is necessaryto ensure improvement in the combustion efficiency whereby unburnedharmful pollutants can be minimized. Complete combustion can be ensuredas follows. Maintaining higher temperatures in the combustion chamber Providing sufficient air and ensuring proper mixing for complete combustion Ensuring sufficient residence time for the completion of combustion reactions

Process factors

Composition of several biomass materials in terms of CHO content isquite similar on ash- and moisture-free basis (as mentioned in Chapter 12),

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but other parameters such as the moisture content and volatile fractionhave a profound effect on its combustion characteristics. In addition, thereare a number of other process factors, which need to be considered while de-signing the stove for maximizing efficiency and minimizing emissions. Theireffects are not easily quantifiable. Table 13.6 summarizes their qualitative ef-fect (Smith 1985).

Stove designing strategies

Forcing hot flue gases to flow past the surface area of a pot or griddle in anarrow channel is a stove design strategy popularized by Dr Samuel Baldwinand Dr Larry Winiarski. The two stove designers approach the problem ofsizing the channel gap differently. Winiarski in Rocket Stove Design Principle1997 advices designing a stove by maintaining a constant cross-sectional areathroughout the stove. Thus, the cross-sectional area at the opening into thefire is set first and then appropriate gaps are created around the pots based onmaintaining the same cross-sectional area (Bailis, Damon, and Still 2004).The experimental data currently available suggests that a power densityof 20 W

th/cm2 is feasible with chimney-less wood stoves and can be as

high as 50 Wth

/cm2 (Verhaart 1981). The Baldwin design uses the highfirepower of the stove as the starting point and the size of the channel gap

Table 13.6 Qualitative effect of various factors on stove design

Action to be taken to

Factor Maximize efficiency Minimize emissions

Stove factorsCombustion confinement Maximize MinimizeTemperature of combustion chamber Minimize MaximizeExcess air Optimize OptimizeThermal mass - for short cooking time Minimize Minimize - for long cooking time Maximize Maximize

Operational factorsFuel burning rate Minimize MaximizeFuel charge size Minimize MinimizeRatio of charge size to burn rate Minimize MinimizeVolume to surface ratio Maximize Maximize

Fuel actorsMoisture content Optimize @10% Optimize @25%Volatile matter content Minimize MinimizeAsh content Minimize Minimize

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is then determined accordingly. Table 13.7 shows the values of various stovegaps from Baldwins findings, which is an approximation meant to serve as aguide to the relationship between fire power, burning rate, length and width ofgap size, and stove efficiency.

Performance testing of biomass stoves

The overall thermal efficiency o of the stove is defined as the ratio of the

amount of useful heat absorbed for cooking food to the amount of energycontent in the fuel used (Khummomgkol 1986; Mande and Lata 2001). It isa combination of partial efficiencies such as the combustion efficiency

c

(fraction of energy content of fuel converted into heat through its combus-tion), heat transfer efficiency

h (fraction of heat generated that is transferred

to the pot), and pot efficiency p

(fraction of useful energy actually used forcooking food). Thus,

o c h p

Useful heat abosorbed by food

Heat conetent of biomass fuel used = =

Tabel 13.7 Baldwins suggested channel gap sizes for stove design

Burning rate Skirt gap# Length of gap# Stove thermal Firepower+

(kg/h) (mm) (cm) efficiency (%) (kW)

0.50 8 20 40 2.80.75 10 20 35 4.11.00 11 20 30 5.51.25 12 20 28 6.91.50 13 20 26 8.31.75 14 20 25 9.6

Source Bailis, Damon, and Still (2004)# See the accompanying sketch; + Burning rate x calorific value

Sketch of stove for parameters in Table 13.7

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The following three types of standard testing methods were evolvedafter critical review in the workshop organized by VITA (Volunteers inTechnical Assistance). WBT (water boiling test) under laboratory conditions CCT (controlled cooking test) involving actual cooking carried out

in the laboratory KPT (kitchen performance test) involving actual cooking in an actual kitchen

Water boiling test

WBT recommended by VITA is the most commonly used methodology tomeasure the power output of the stove and its efficiency (VITA 1985). Ittries to simulate the boiling and simmering operations commonly used duringcooking. A known quantity of water W is filled up to two-third level in a vesselof known weight V and is heated from ambient temperature T

a to boiling

point Tb.

The lid is kept closed, but is partially open during simmering. It isboiled for 15 minutes and then kept simmering (maintaining temperaturewithin 2 C of boiling point) for 60 minutes. The stove efficiency is com-puted using the following equation:

W =

where dW is the amount of water evaporated, Lw is the latent heat of water, F

is the amount of fuel burnt, CV is the calorific value of fuel, Cpw

and Cpv

arespecific heat values of water and the vessel, respectively.

Recently, a modified WBT version 1.5 is being developed under ShellHEH (Household Energy and Health) Programme (Bailis, Damon, and Still2004). This consists of separate measurements under three different phases,simulating various commonly used cooking processes instead of giving a singleefficiency number for a stove for indicating its performance. These areexplained as follows.

Phase 1: High power (cold start)Testing begins with the stove at room temperature and uses a pre-weighedbundle of wood to boil a measured quantity of water in a standard pot.

Phase 2: High power (hot start)The boiled water is then replaced with a fresh pot of cold water to perform thesecond phase of the test. Thus, water is now boiled on a hot stove in order to

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identify the differences in the performance of the stove in its hot and coldstages.

Phase 3: Low power (simmering)A measured amount of water is boiled and then, using a pre-weighed bundle ofwood, the water simmers at just below the boiling point for a measured periodof time (45 minutes). This phase tries to simulate the common cooking proc-ess that entails cooking of legumes or pulses. For a multi-pot stove, onlythe primary pot will be assessed for its performance during simmering ofthe water.

The combination of tests is intended to measure the stoves performanceat both high and low power outputs P

H and P

S, respectively, which are impor-

tant indicators of the stoves ability to conserve fuel. Thus, rather thanreporting a single number indicating the thermal efficiency of the stove, whichalone cannot predict stove performance, this test is designed to yield severalnumerical indicators including the following.

Time to boil Fuel burning rate (kilogram per hour) Specific fuel consumption (kilogram per task) Turndown ratio (ratio of high to low power output)

A well-designed stove should ideally perform with the same efficiencyover the entire power level range. However, most stoves do not have such idealperformance, making it necessary to evaluate stove performance under maxi-mum and minimum power levels.

Controlled cooking test

This test essentially gives the fuel consumption of a given stove for carryingout a typical cooking operation. It is done under laboratory conditions on astove with a typical vessel size and shape, normally used for cooking a typicalfood of the region, using a commonly used cooking operation. The amount offuel used up for cooking a known quantity of food and the time required forcooking are measured. The test is repeated at least three to five times toget average values. It can also be used to compare two different stoves forthe same cooking operation or compare stove performance for the variouscooking operations.

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Kitchen performance test

This performance test is carried out in the field under real-life conditions.The various cooking operations performed by the family during cooking aremonitored and recorded to arrive at the total fuel consumption per meal perperson. The test is performed for several days to get realistic average values.This test overcomes the drawback of stove performance for making a typicalmeal and takes into account the stove performance for a variety of cookingoperations such as boiling, frying, roasting, etc. encountered during mealpreparation. Though, for theoretical analysis, this is too gross a test, but itgives a realistic measure of stove performance comparison, namely, whetherthe improved stove actually saves fuel under field conditions (Prasad, Sangen,and Visser 1985).

All these tests can only be used for a relative comparison of stoves for agiven task under given operating conditions. These tests cannot be accepteduniversally for defining stove efficiencies as a small variation in the cookingpractice or operating conditions would significantly affect the performance.The procedure for determining the efficiency of the gas stoves includes oper-ating the stove at different constant power levels, using the water boiling testfor a turndown ratio (ratio of maximum to minimum power output level).Though it is easy to operate gas stoves at different constant power outputlevels, it is difficult and impractical to achieve such conditions for wood burn-ing stoves (Bussmann 1988).

Emissions from biomass stoves

Among the numerous pollutants emitted from biomass stoves, the most im-portant ones are carbon monoxide, TSP (total suspended particulates), PAH(polycyclic aromatic hydrocarbons), and formaldehyde. Several design andoperating parameters resulting in incomplete combustion contribute to theseemissions. These include: insufficient supply of combustion air, lowertemperatures in the combustion zone, improper mixing of air and fuel, etc.Many times, some design modification to improve the thermal performanceor efficiency, such as reducing the gap between the pan and the stove mouth aswell as between the pan and the grate, may actually result in increased emis-sions. Therefore, there is need to take into consideration the effect of anydesign modification on the stove as a whole. Generally, emissions are seen toincrease with increasing power levels. Similarly, higher emissions are observedwith smaller stoves. Often, this is due to the lower residence time of fuel in thecombustion chamber of the stove resulting in incomplete combustion andhence higher emissions.

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Emission testing of biomass stoves

Emissions from the stoves with chimney are measured by placing sensors inthe flue gas path. For the majority of stoves without chimney, the sensors arekept attached to the operator as emissions are measured to assess theirimpact on the health of the operator. However, these are non-standardtechniques and depend upon the local conditions of ventilation, positionof doors and windows, direction of the wind, and so on. The following twomethods available for monitoring emissions from a stove without chimney arecommonly used Hood method Chamber method

Hood methodThis is also called a direct method in which the stove is kept in the enclosedhood. The monitoring of air supply is done through pre-defined vents and theflue is extracted from the hood exit under isokinetic sampling conditions(Figure 13.16). The measurement of flow rates through vents is quite expensiveand not very accurate with natural draft flows. If air flow is induced using anexhaust fan then it can affect the normal operation of the stove. Since it is notalways possible to construct hood in situ under field conditions, this methodis normally used for laboratory experiments.

Chamber methodThe chamber method, first proposed by Ahuja, Joshi, Smith, et al. (1987) is alsocalled a simulated kitchen method or indirect method (Figure 13.17). This

Figure 13.16 Hood methodSource Smith et al. (2000)

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method can also be used in the field. However, for accurate measurements ofemission performance using this method, it is necessary to have relativelyconstant ventilation conditions throughout the experimentation period. Pol-lutants near the stove and in the background room conditions are measuredfor the particulate levels (using filter paper) and carbon monoxide (using asampler) at regular intervals throughout the experiment. The air exchange ratein the room is monitored using the standard exponential carbon monoxide de-cay method (where the carbon monoxide level is raised up to a certain leveland its decay after its source is monitored with time). Normally, emissionsfrom various stoves are compared for a given standard task rather than theemissions per unit fuel or heat output.

Biomass pyrolysis

The thermal decomposition of organic matter in vacuum or in an inert atmos-phere is called pyrolysis. The products of biomass pyrolysis include charcoal,condensable liquid (generally called pyroligneous liquid), and gaseous prod-ucts. The proportions of these components vary depending on the operationalconditions and the type of biomass used.

As mentioned in the earlier chapter, the composition based on ash- andmoisture-free basis (CHO) is similar for most biomass materials. Biomassconsists of three major constituents, namely cellulose, hemicellulose, andlignin. In most biomass materials, cellulose is in the form of glucan polymer

Figure 13.17 Chamber methodSource BallardTremeer and Jawurek (1999)

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with an average molecular weight of 100 000. Hemicellulose has a molecularweight (


In practice, it is easy to control the highest temperature achieved andthe residence time in the reactor, but it is not easy to control the heating rateto maximize either the char or the tar yields. The heating rate depends ontwo main factors, namely, particle size and reactor type. For a given reac-tor, the larger the particle size, the lower is the heating rate (due to lowconductivity of wood) which results in more char yield (Figure 13.19). Forpyrolysis reactors, where the temperatures are normally below 1000 C ra-diation heat transfer does not play a major role and the dominant heat transfermode is conduction and convection. Some reactors like fluidized bed can han-dle only smaller/finer particles (Villermaux 1982). The following observationscan be made from Figures 13.18 and 13.19. Charcoal can be produced by using stacking kilns, multiple hearth kilns,

and rotary kilns with low heating rates, low temperature, and high resi-dence time.

Pyrolytic liquid can be obtained using transported bed reactors with highheating rates, low temperature, and shorter residence time.

Gas can be obtained by means of fluidized bed, circulating bed, or evencyclone reactors with high heating rates and higher temperatures.

Figure 13.18 Effect of various operating parameters on products of pyrolysis

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Charcoal production

Pyrolysis processes are generally called carbonization processes when theobjective is to produce charcoal from the pyrolysis of biomass. Nowadays,though charcoal is mainly used for industrial and chemical applications indeveloped countries, it is still an important fuel for cooking and heating indeveloping countries. It is estimated that through the control of pyrolysisreactions it is possible to obtain 3040 kg each of char and pyroligneousliquid, and 1520 kg gases per 100 kg. The major parameters for charcoal mak-ing are as follows. Yield The ratio of the weight of charcoal obtained to the dry weight of

feed material. Normally, it ranges from 1:3 to 1:5, depending on the qualityof feed material and the operating conditions maintained in the reactorused.

Volatile content The weight loss of charcoal when it is heated up to 900 Cper unit weight of charcoal.

Fixed carbon content The weight of charcoal minus volatiles anduncombustible ash fraction per unit weight of dry charcoal.

Charcoal is used for a variety of applications, depending on its varyingproperties. Some applications are as follows. Domestic for cooking and heating Agricultural for drying and processing

Figure 13.19 Various operating zones of different pyrolysis reactors

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Metallurgical copper, bronze, nickel, aluminium, etc. Chemical carbon monoxide, carbon dioxide, activated carbon, calcium

carbide, absorbent, soil conditioner, pharmaceutical, etc.More than 100 charcoal-making processes are known today (Emrich

1985). They can be classified as follows. Kilns Used for producing charcoal alone. Converter Used for pyrolyzing smaller particles and recovering

by-products. Retort Used for pyrolyzing billets or logs reduced in size to about

30 cm length and 18 cm diameter.In modern technology, the following three types of heating methods are

used in order to initiate the carbonization process and maintain the requiredtemperature. Internal heating Normally used when the raw material is cheap. Part of

the material used for charcoal making is burned within the kiln usingcontrolled air supply.

External heating Normally used in retorts to indirectly heat the material tobe carbonized. Fuel for this can be obtained from the gases or liquid re-leased during the pyrolysis process itself.

Heating with gas recirculation Normally expensive and used for large sys-tems. Part of the gas produced is burned and directed inside the reactor forits carbonization under controlled conditions.

Traditional charcoal-making kilns

Most of the charcoal-making processes adopted in developing countries aregoverned by low investment and low operating cost, with limited range ofbiomass material available locally. As a result, these processes are generallylabour intensive, have low yield, and take a longer operating time. These canbe classified basically as stacked bed kilns and are generally operated inbatches.

A huge quantity of locally available biomass is cut and stacked on theground or inside a shallow pit. It is then covered with layers of leafy materialand soil layers. It is slowly ignited through the holes provided, and once theignition spreads throughout the fuel bed the air supply is cut off by closingthese holes and gradually the entire fuel bed gets carbonized in the absence ofair for oxidation. The charcoal is then removed after the cooling down of thekiln (Figure 13.20).

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Modern charcoal Missouri kiln

The Missouri kiln is widely used in developed countries (Massengale 1985).Here pieces of wood of approximately 20 cm are neatly stacked and about15 cm of air space is ensured at regular intervals. The wood-holding capacity ofthese kilns is in the range 100180 tonnes and it takes about 820 days forcompleting a batch of charcoal-making with 20% yield. Unlike the earthmounted traditional charcoal kiln, it is a permanent structure made up ofbrick or concrete construction and the same kiln can be used for severalbatches with minor maintenance (Figure 13.21).

Lambiotte retort

It is a continuous carbonization process suitable for industrial-scaleapplication requiring huge investments, power, water, etc. The retort isnormally characterized by automation, energy, as well as labour-savingtechniques (Carre, Herbert, Lacrosse, et al. 1985) and is a multi-stage re-tort (Figure 13.22). Uniform-size wood of 10 cm diameter and 30 cm length isfed from the top into the cylindrical retort. It gets dried in the first stage fol-lowed by carbonization in the second stage and cooling in the third stage.Pyroligneous gases are taken out from the carbonization zone and burnedwith controlled air supply in the drying zone. For cooling, gases are removedfrom the carbonization zone and are re-injected through the bottom of theretort after the tars get cracked and gases get cooled by a water-cooling system.

Fluidized bed carbonizer converter

This is a rapid pyrolysis (short residence time) process for forming finecharcoal powder from particles. Biomass particles (less than 6 mm in size) aredirected to the glowing charcoal bed, which is fluidized with an oxygenatedairgas mixture. A cyclone removes dust and pyrolytic liquid from the gases

Figure 13.20 Traditional charcoal-making kiln

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Figure 13.22 Lambiotte: multi-stage pyrolyser retort

Figure 13.21 Missouri kiln

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for further recovery. Charcoal particles are withdrawn continuously from theoverflow pipe situated inside the reactor and are normally briquetted beforefurther use (Figure 13.23).

Flash carbonization

Professor Michael Antal Jr at Renewable Resources Research Laboratory, Uni-versity of Hawaii, has developed a new pyrolysis process of flash carbonizationto convert green biomass waste into liquid fuel and solid charcoal (Vrhegyi,Szab, and Antal 1994). This process involves the ignition of flash fire at anelevated pressure in the packed bed of green biomass. Because of the elevatedpressure, the fire spreads quickly throughout the biomass bed, triggering thetransformation of biomass into biocarbon within a short time span of 3045minutes. In order to minimize carbon monoxide and other pollutantemissions, an after burner is provided where temperature levels of 1200 C aremaintained (Figure 13.24).

Charring drum pyrolyzer

For use in developing countries, a simple charring drum had been used to car-bonize loose biomass, which can later be briquetted for densification to use itas fuel for charcoal substitution. A used oil barrel/drum of 200-litre capacity is

Figure 13.23 Fluid-bed carbonizer

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used for this purpose (Figure 13.24). Loose biomass is placed inside it and theair is distributed either by providing controlled air entries at various levels(TER I 1990) or by placing an inverted perforated cone inside the drum withthe chimney (Neinhuysm 2003). Loose dry biomass is placed around the in-verted perforated cone placed inside the charring drum and is ignited. Oncethe layer catches fire, more biomass is added layer by layer till the entire drumis filled. White smoke vents out through the chimney extension. Once thesmoke turns from white (containing moisture) to grey colour, the chimney ex-tension is removed and a lid is put on the drum to slowly extinguish the fireinside it. It takes about two hours each for igniting the biomass and for cool-ing down the drum for charcoal removal.

Biomass gasification

Thermochemical biomass gasification is a process of converting solid biomassfuel into combustible gas (called producer gas) by means of partial oxidationcarried out in a reactor called gasifier. The first gasifier units were built inFrance during 1850s. The first vehicle to be powered with producer gas wasmade by J W Parker in Scotland in 1901. Producer gas plants (using coal orpeat), used during World War I, are described in detail in literature (Rambrush

Figure 13.24 Schematic of drum pyrolyser

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Biomass Stove

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