This invention relates to a fuel efficient biomass stove and a method of operating the stove to achieve a high efficiency of fuel use and relatively low undesirable emissions.
Wood and other forms of biomass have been used throughout history for cooking and space heating. In many parts of the world gas and electricity have replaced biomass as the preferred fuel for almost all cooking. In these regions, biomass stoves tend to be used only for recreational cooking, such as barbeques and when hiking or camping. However, it is estimated that there are more than 2 billion people who still rely on biomass as their primary fuel source for cooking; most are using traditional stoves that are slow, inefficient and hazardous to health. The smoke and other emissions of such stoves have been recognised as a significant cause of ill health, especially amongst women.
There have been many attempts to design more efficient biomass stoves. The challenge for those seeking to design a cooking stove to replace conventional biomass stoves is to provide a stove that not only bums the biomass efficiently, reducing fuel consumption and hazardous emissions, but is also easy and cheap to manufacture, use and maintain. There are many disclosures of biomass stoves of various designs. Some are designed as camping stoves, some as space heating devices and some are more directed to the specific needs of those communities that rely on a biomass stove as their primary means for cooking.
US Patent 5,842,463 discloses a portable wood burning camp stove comprising a combustion chamber with a lower grate for supporting the fuel and an outer wall around the combustion chamber such that an annular space is formed between the combustion chamber and the outer wall. In operation primary combustion air is drawn into the bottom of the combustion chamber and secondary air is drawn into the bottom of the annular space, passes up the outside of the combustion chamber and the heated secondary combustion air is introduced into the top of the combustion chamber. The primary and secondary air flows are dependent on natural convection and are not controlled.
US Patent 6,520,173 discloses a portable solid-fuel camp stove comprising a combustion chamber having a suspension screen for supporting the fuel. Primary combustion air flows into the combustion chamber from below the suspension screen. Extra oxygen can be supplied to aid in the ignition and acceleration of the rate of combustion of the fuel by

the user blowing air by mouth through a hose connected to an opening in the combustion chamber wall, above the suspension screen.
US Patent 6,336,449 discloses a burner system for burning granules, pellets or similarly sized solid biomass heating fuel meant largely for room heating applications. It uses an augur for feeding the fuel pellets and a fan for supplying combustion air. The combustion process is conducted by keeping the fuel temperature so that ash fusion does not occur. The air supply is distributed over the height and is arranged such that it is azimuthally poorly symmetric with potential for geometric distortions.
US Patent 5,137,010 addresses an improved grate and grate assembly for a stove fuelled by biomass pellets that overcomes the problem of accumulation of ash and clinkers encountered by conventional grates by introducing a movable elongated blade above the upper surface of a passive grate. The current invention does not utilize this type of grate.
US Patent 4,730,597 addresses a biomass stove which is essentially a free convection driven combustion system with secondary air being introduced through holes of varying sizes at multiple levels on the removable fuel basket. The distribution of holes is dependent on the nature of the fuel. One of the key features claimed is the introduction of reflector plates below the grate to reduce the heat loss from the stove bottom.
In the present invention, forced air is supplied using a fan both below the grate and at a fixed level close to the top of the stove. This results in gasification of the biomass and subsequent combustion of the evolved gases with additional air thus providing clean and complete combustion. The efficiency, emission levels and power level from the current invention are clearly documented hereinafter.
A number of papers have been published by T B Reed et al relating to a stove they called an "inverted downdraft gasifier", including:
La Fontaine, H and Reed, T. B., "An Inverted Downdraft Wood-Gas Stove and Charcoal Producer, in Energy from Biomass and Wastes", XV, D. Lass, Ed., Washington D. C, 1993;

Reed, T. B., and Larson, R., "A Wood-Gas Stove for Developuig Countrira", in "Developments in Thermochemical Biomass Conversion, Ed. A. V. Bridgewater, Blackie Academic Press, 1996;
Reed, T. B., and Walt, R., "The "Turbo" Wood Gas Stove" in Biomass: Proceedings of the 4th Biomass conference of the Americas in Oakland, Ca, Eds., Overend, R. P., and Choraet, E., Pergamon Press, 1999;
Reed, T. B., Anselmo, E. and Kircher, K., 'Testing & Modelling the Wood-Gas Turbo Stove", presented at the Progress in Thermochemical Biomass Conversion Conference, Sept. 17-22,2000, Tyrol, Austria.
The latter paper, in particular, discloses a forced draft inverted downdraft gasifier which they have called a "Turbo Stove". The stove comprises an inverted downdraft gasifier close coupled to a burner section. Ait passes up through the fuel and meets a flaming pyrolysis zone where the reaction generates charcoal and fuel gas. The Turbo Stove utilises forced convection to improve the mixing of air with fuel gas, resulting in more complete combustion; reducing soot and emissions. A 12 Volt, 3 Watt blower was used to provide 7.5 mm water pressure. The air supply to the gasification section and the air supply to the combustion section can be independendy adjusted.
US Patent Application 2003/0200905 discloses a device comprising a top burning gasifier with a close-coupled mixing-combustion chamber. In operation, primary air enters the fuel bed of the downdraft (or co-flow) gasifier from below and gasification starts at the top of the bed and proceeds down through the fixel. Fuel gas is generated at the top of the bed, rather than at the bottom as in conventional downdraft gasification. The hot gas flow is in the same direction as the flow due to natural convection. The gasifier can operate with natural convection alone, but forced convection increases the rate of gas production. The fuel gas produced in the gasification stage is mixed with secondary air in the combustion stage.
The efforts of many agencies and scientists all over the world have resulted in the development of a variety of stoves of different designs and materials of construction. However, there remains a need to provide improved stoves and methods of using the

stoves, which increase the overall quality of the combustion of the biomass by rightly mixing of fuel gas and air.
Based on detailed scientific research, it has now been found that the efficiency of biomass burners of the inverted downdraft gasifier type is improved wlien the fuel zone has an aspect ratio (diameter-to-height ratio) between 0.6 and 1.25. The biomass burners are particularly suitable for use as cooking stoves, but can also be used to heat water or for space heating. They can be combined with other burners, e.g. an LPG burner, to make a dual fuel stove.
Thus according to this inveation there is provided a fuel efficient biomass stove, which comprises of:
(a) an inner cylindrical combustion chamber (4) for the fuel(l);
(b) an outer container (5) with an annulus (12) between the combustion chamber (4) and the outer container (5); the height of the combustion chamber (4) fixed so as to accommodate fuel to a height which is nearly the same as the diameter of the combustion chamber, the fuel being any biomass with an ash content up to 12%;
(c) means for supplying a primary air supply to the bottom of the combustion chamber;
(d) means for supplying a secondary air supply to the top of the combustion chamber;
(e) a grate (3), with an option of positioning the grate at varying heights from the bottom offhe combustion chamber, and
(f) an ash tray (6) within the combustion chamber.
Stoves used according to the present invention can have significantly higher utilisation efEciencies than known stoves. Typically, known stove operation can give utilization efficiencies, based on a standard water boiling assessment protocol, of 30 to 40%. Under similar conditions, a stove operated according to the present invention can have an efficiency of 45 to 50%.
The fuel i.e. biomass is preferably selected firom a choice of high density biomass, briquettes or high-density pellets.

The ratio of the diameter of the combustion chamber to the height of the fuel bed is preferably close to xmity. However, the ratio may suitably be up to 1,3, preferably not more than 1.25, and may be as low as 0.6.
It has further been found that better heat transfer can be achieved when a larger diameter cooking vessel is used relative to the diameter of the stove. Thus, the vessel placed on the stove preferably has a diameter of from 2 to 4 times the diameter of the combustion chamber.
The invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a schematic sectional view of a biomass cooking stove suitable for use in the
present invention;
Figure 2 is a side elevation of the stove shown in Figure 1 in the direction X;
Figure 3 is a top view of the stove shown in Figure 1;
Figures I A, 2A and 3A illustrate the sectional view, side elevation and top view of the
biomass cooking stove with an alternative provision for air supply;
Figures IB, 2B and 3B illustrate the sectional view, side elevation and top view of the
biomass cooking stove with a second alternative provision for air supply;
Figure 4 illustrates the stove shown in Figure 1 in use for heating bath water;
Figure 5 illustrates a stove as shown in Figure 4 in use to maintain a volume of hot water;
Figure 6 is a schematic section of another stove suitable for use in the present invention;
Figure 7 is a side elevation of the stove of Figure 6 in the direction Y;
Figure 8 is a top view of the stove shown in Figure 6; and
Figure 9 is a schematic top view of a dual fuel buma-, which comprises in combination a
biomass stove and an LPG stove.
Figure 1 is a schematic section of a biomass cooking stove 7. Figures 2 and 3 are a side elevation and atop view of the stove of Figure 1. The stove 7 comprises a double-walled chamber 2, which comprises a substantially cylindrical inner combustion chamber 4 and an outer container 5. An annular space 12 is provided between the inner wall of the outer container 5 and the outer wall of the combustion chamber 4. A variable height grate 3 is provided in the bottom of the combustion chamber 4. Fuel 1 is loaded in the combiistion

chamber 4 above the grate 3 to a height lower than the diameter of the combustion chamber 4. At the bottom of the combustion chamber 4, below the grate 3, is an ash removal plate 6 for removing ash without distuibing the stove 7. The ash removal plate 6 preferably sits within the stove. Optionally, the ash removal plate 6 can be attached to or forms part of the grate 3 such that they can be removed from the stove 7 together. A blower, which can be a commercially available fan, 8 is fitted to the side of the stove 7 to provide an m supply. The jur supply is split into two parts. One part (the primary air supply) is introduced into the bottom of the combustion chamber 4 and flows up through e fuel bed 1 and controls the power of the stove. The other part (the secondary air supply) is introduced via the annular space 12 of the double walled chamber 2 and enters the inner combustioiv chamber 4 via holes 13 towards the top region, contributing to the quality of combustion. Controlling the average excess air is central to obtaining high combustion efficiency. The fan 8 may be driven, for example, by (i) a rechargeable battery 9 to operate when there is no electricity supply at the time of using the stove or (ii) an AC-to-DC converter (commonly called a battery eliminator), typically 12 V with a current rating of 0.2 amps, when an electricity supply exists; for a stove of capacity up to 3 kW (thennal). At higher stove capacity a higher fan rating is used.
Two valves 10 and 11 are shown for regulating the primary and secondary air supplies. The thennal power of the stove 7 can be regulated by use of the valves 10 and II or by a single valve 10a in combination with a distributor plate 11a as illustrated in Fig. lA. Altematively, the thennal power of the stove 7 could be controlled by a single electronic control such as a potentiometer 10b along with distributor tube 1 lb that changes the rpm of the fan 8. (Refer to Fig. IB).
The single control valve 10a regulates both primary and secondary air that is necessary for the stove operation. The valve is designed in such a manner that by varying its position (this alters simultaneously the primary & secondary air supply) one can vary the air flow that is required for regulation of thermal power output of the stove. The Distributor plate 11a provides adequate area for both primary and secondary ait streams, which in turn maintains the volumetric flow rate ratio of primary-to-secondary air between 1:4.6 and 1:3.6 over the range of power level. As an example in the case of 100 mm diameter stove, the nominal and maximum power levels are 2.0 and 3.0 kW respectively. Therefore in a single valve system, the primary-to-secondary air flow rate

ratio is obtained as a consequence of the difference in the areas of the inlet air entry for the primary and secondary air and single valve is used to distribute the single air stream between the two.
The control of primary & secondary air supply is acliieved using a Potentiometer 10b that varies the speed of the fan. The Distributor tube 1 lb provides adequate area for both primary and secondary air streams which in turn maintains the volumetric flow rate ratio of primary-to-secondary air between 1:4.6 and 1:3.6 for a power level over the range of power level. As an example in the case of 100 mm diameter stove, the nominal and maximum power levels aie 2.0 and 3.0 kW respectively. Therefore in a valve-less system, the primary-to-secondary air flow rate ratio is obtained as a consequence of the difference in the areas of the inlet air entry for the primary and secondary air and potentiometer controlled fan to distribute the single air stream between the two.
The fan rpm can be controlled up to the full value of 2500 rpm through the electric supply control. A small part of this air, i.e. primary air, enters the stove 7 under the grate 3 for the release of fuel gas. The remaining mr, i.e. secondary aii, goes to the annular space 12 of the double walled chamber 2. The air flow through the annular space 12 of the double walled chamber helps in regeneration of heat and thereby maintaining the outer wall of the stove at a lower temperature, this in tum would enhance the life of the biomass stove that is described here. Air for increasing the fuel gas generation enters from the bottom and for combustion from more than a dozen radially located holes 13 at the top of the inner combustion chamber 4 as shown in the Figures 1 to 3.
The fuel may be any suitable biomass. High bulk density biomass provides better performance. Suitably, pieces of biomass or pellets, which has an intrinsic density approaching one thousand kg/m The higher bulk densities allow the radiant heat transfer to the bottom of the vessel being heated to be more effectively exploited. Efficiencies in excess of 50% can be achieved based on standard water boiling tests. The fuel 1 may be in the form of (i) briquettes or high density pellets, 10 to 15 mm diameter and 10 to 15 mm long, made from coffee husk, rice husk, coconut husk, sawdust, peanut husk, pine needle waste, urban solid waste or a mix of these in any proportion (ii) broken coconut shells, or chopped pieces of firewood. Importantly, quite against normal intuition, this stove performs better when the biomass has an ash content typically 7 to 12

% due to the thamal radiation energy enhancement from the slowly convertijig ash filled material. That is why use of briquettes or high density pellets ensures better heat utilization efficiency.
The grate 3 can be a variable height grate. Possible means for providing variability in height would include, for example, telescopic legs or leg extensions. The height of the grate 3 can also be varied by placing it on a stand in the bottom of the combustion chamber 4, by providing supports at different heights in the combustion chamber or other means that will be apparent to a person skilled in the art. The primary air supplied to the bottom moves through the packed bed and increases the conversion of the solid fdel to gas with increased air flow through the bed. The variable height grate 3 allows to control the quantity of fuel charge and hence duration of operation. This also allows for high efficiency other than at the designed power level. Depending upon the duration of stove operation required, the height of the grate 3 is selected in such a maimer so as to position the fuel bed towards the top of the combustion chamber 4.
Initial ignition is facilitated by using fine pieces of light biomass, perhaps fire wood on the surface of the packed bed of the fuel briquettes, high density pellets or other biomass pieces. Spraying some liquid fuel {say, kerosene or alcohol) on the surface and lighting these fine pieces by a matchstick ensiu-es fast ignition and stabilization of the flames in the stove.
Once the stove is lit, the hot fuel vapours from the surface of the packed bed combine with the secondary air from the side holes 13 to bum up and generate hot flue gas at temperatures typically in the range of 1000 to UOOC. The stove operates at a thermal power that is about constant for 70 % of the operational duration of the stove. This power can be varied by adjusting the primary air supply to the bottom section as well as the secondary air supply for combustion. For many cooking applications, the stove can be used in a "fire and forget" mode for a large part of the time. This reduces enormously the time requhed for tending the stove, normally drawing away useful time of a person involved in cooking.
The stove operation comes to a close when all the fuel is converted to ash. After the stove has burnt up all the fuel, the ash remaining on the grate is removed using the

facility. The stove is ready for use again for the next batch of operations. The stove is characterized by a nominal duration of operation that depends on the amount of biomass loaded into the combustion space. The amount of biomass that can be loaded depends on the bulk density of the fuel used. This implies that briquette pieces or high density pellets that can be loaded are the maximum since they have densities up to 1000 kg/m'. The amount of fire wood chips that can be loaded will be lower since their densities go up to 600 kg/m. Coconut shells also have a high density (up to 1200 kg/m) and therefore are comparable to briquettes or high density pellets in terms of loading. Thus a stove that bums with briquettes or high density pellets for 45 minutes would bum only for 25 minutes in the case of wood chips. The relevant dimensions of the stove are shown in Table 1:
Table 1

Figure 4 shows the arrangement of a stove for bath water heating purposes. The biomass stove 7 is essentially the same as the stove illustrated in Figures 1 to 3. Above the biomass stove 7, is located a commercially available heat exchanger device 14 that consists of a coiled tube 15 with fins 16 that provides for very high heat transfer between the hot flue gases and the water flowing inside. The inlet 17 of the heat exchanger is connected to cold water from a tap and the heated water flows from the outlet tap 18.
In an alternative embodiment shown in Figure 5, the heat exchanger device 14 is connected via the inlet 17 to the bottom 19 of a water storage tank 20 of suitable capacity (typically 50 or 100 litres capacity for small domestic applications and larger capacity for other applications). Thewater tank 20 is provided with suitable insulation 21. The outlet 18 of the heat exchanger device 14 is led into the top 22 of the storage tank 20. This arrangement allows the heated water to be delivered to the top and the cooler water to be drained fix)m the bottom during the water heating operation. A separate line 23 allows

the hot water to be drawn off from the water storage tank 20. Other possible arrangements for drawing the hot water while the heating process is going on can also be used.
Figures 6 to 8 show another design of high power stove that is capable of continuously operating for an extended period, for example, more than 8 hours at a time, with the heed for varying the power during the operation. This design uses a horizontal segment 24 for feeding the biomass to die fuel feed port 25 of the stove during operation. Typically, the fuel will be in the form of fire wood, but other agro-residues are also suitable. Secondary air is introduced in the form of fine high velocity jets (velocities of 50 to 75 m/s) using a vertical tube 27 that terminates at the bottom region of the vertical combustion chamber 26 over the grate 3. The other end of the vertical tube 27 is connected to a blower 28 (typically of a pressure of 6000 Pa) with an air control valve 29. The air flow through the vertical tube 27 in turn draws primary air through the horizontal bed of fuel 1 through an ejector action. This ejector action is caused because of the momentum exchange between the high velocity jets of air and the surrounding zone filled with gaseous fuel at low velocity. This momentum transfer causes increased velocity of the gaseous fuel and hence lower pressitte. This lower pressure induces a flow fiom the ambient atmosphere through the horizontal segment containing packed solid fuel. This leads to a propagation of a flame fi'ont through the solid fuel bed much like in a gasification system whai the fuel bed is lit. This process generates combustible volatiles that bum in the vertical combustion chamber after mixing with the high speed jets of secondary air. The hot charcoal that falls on the grate 3will also get converted into a combustible fuel in terms of carbon monoxide and hydrogen. Conditions are created in the chamber for a "mild or flameless" combustion mode to dominate ensuring the lowest of emissions in terms of oxides of nitrogen. The combustion chamber 26 is insulated 21 to reduce heat loss and improve the efficiency of the stove. The ash is collected xiltimately at the bottom of the vertical segment in an ash tray 6. The tray can be taken out of the stove at the end of the operation.
Figure 9 illustrates a dual fuel combination stove with two burners 30 & 31. Burner 30 is a conventional LPG burner, whereas burner 31 is a biomass stove. The combination stove also has the conventional seaters/stands 32 for cooking vessels and an outer body

33. The LPG Line supply is connected to the inlet 34, which supplies die gas to bumCT 30. Burner 31 gets the supply of air through air control valve 35.
Examples
To illustrate the invention, three different cooking vessels and three different stoves were used, to replicate the use of the biomass stoves in domestic applications.
The cooking vessels were aluminium vessels of 10 litre volume (diameter of 320 mm, height of 160 mm, and 0.96 kg weight), 6 litre volume (diameter of 260 mm, height of 130 mm and weight of 0.61 kg) and 2.5 litre volume (diameter of 205 mm, height of 105 mm weight of 0.34 kg).
The three stovM, which were substantially as shown in Figures 1 to 3, were 100 mm, 150 mm and 200 mm inner diameter. The fuel loading zone height, i.e. the fuel bed height, in the three cases was as indicated in Table 1 above, i.e. 80 mm, 125 mm and 175 mm respectively.
The 100 mm diameter stove had a combustion space of 0.6 litres and could carry 130 g of wood chips, or 225 g of marigold based briquettes + 30 g of wood chips or 250 g of rice husk briquettes + 50 g of wood chips or 360 g of coffee husk briquettes + 30 g of wood chips. The wood chips are added to aid the ignition.
In the case of 150 mm diameter stove, the volume of the combustion space was 2.2 litres and it could carry larger amounts of the fuel in proportion to the volume.
In the case of 200 mm diameter stove, the combustion space volume was 5.5 litres and it could similarly carry increased amounts of fuel.
Three cooking vessels were used for determination of the thermal utilization efficiency. The consideration behind this choice is that small families may use smaller vessels and larger families, larger vessels. It would be valuable to determine the efficiency with vessel size. It can be expected that larger diameter vessels extract more heat compared to

smaller vessels and hence designs that allow greater heat extraction from the same stove would be the appropriate choice.
The standard procedure used for conducting the experiments was that the stove was lit and a suitable vessel filled with water and was placed on it after weighing the vessel with water in it on an accurate balance that provided the accuracy of 0.0001 kg over a total weight of 10 kg. The gasification air (primary air) was at a minimum and the combustion air (secondary air) closed for about a minute to minute-and-a-half to ensure that the combustion process got stabilized. After the flame had stabilized combustion air was raised to a level to provide for the required power. In the experiments, the stove and the cooking vessel with water were placed on an accurate electronic balance to obtain the weight loss with time. This was used to infer the instantaneous power level. The vessel with water had a stirrer and a thamometer to obtain the temperature of the water over time. Beyond about 50°C, water evaporation occurs slowly. To measure the loss of water due to evaporation &at is usually very small, typically 0.6 to 1 g/min, the vessel was taken off the stove and weighed on the balance to determine the amoimt of water evaporated. This was used in the calculations to account for the heat utilized. The heat utilization efficiency was calculated by dividing the heat extracted by the heating value of the biomass. The heat extracted has three components - the heating of the water, loss of water by evaporation (even below the boiling point), and the heating of the vessel. These heats were calculated and added. The heating value of the biomass is dependent on the moisture in the biomass and the ash content. Moisture was measured by a separate means by taking a part of the biomass used in the experiment for moisture determination. This was done by measuring the initial weight and putting the biomass into a fiimace at lOCC for a minimum of six hours. The material was taken out and weighed and again put into the fiimace. It was removed after another three hours and cooled and weighed. The difference between the initial weight and the final weight divided by the final weight gave the moisture fraction on dry basis. In the experiments several other measurements were also made - to determine the gas temperature and the oxygen fraction in the bottom section of the vessel towards the exit zone. These gave corroborative evidence to the heat utilization efficiency. More than a hundred experiments under a variety of conditions were performed with several containing detailed measurements, but several restricted to overall efficiency assessment.

In a typical case of 100 mm stove, the stove with wood chips of 130 g was lit and an aluminium vessel of diameter of 320 mm weighing 1 kg containing 10 litres of water was placed on it. This water was heated from 25 °C to 48 "C in 16 minutes. The moisture content in the wood chips was measured as 14 % and the ash content as 1.1 %. Hence the fiiel calorific value was obtained as 14.9 kJ/g. The avwage power level at which the stove delivered the heat was 2.5 kW. Just at the end of the operation, the amount of moisture that had evaporated was measured as 10 g. A calculation of the heat utilization efficiency was obtained as 52.5 %.
EXAMPLE I: The 100 mm stove
The principal results of experiments conducted using the 100 mm diameter stove over a number of biomass and vessels are summarized in the following Tables A, B and C; in which the following designations were used for the biomass fuel: Wo = Woodchips; M = Marigold based briquettes; CS = Coconut shell; CHB = Coffee husk briquettes.
Table A. Performance of 100 mm stove with dUferent biomass and a cooldng vessel containing 2.5 litres of water.

The ash fraction was obtained by measuring the burnt mass after the combustion process. ATw (°C) is the increment in temperature of the water in the heating up process. Multiple values of ATw (°C) refer to vessels being changed after the water reached the boiling point.


The important point to notice in the figures is that the water boiling efficiency varies between 41 and 58 %. Keeping the fuel quality and quantity about the game, larger efficiencies have resulted with the choice of a larger diameter vessel (vessel-to-bumer diameter ratio of 320/100 = 3.2) compared to the lowest of the efficiencies obtained with a vessel-to-bumer diameter ratio of 205/100 = 2.05).
Emissions of solid particulate matter (SPM), carbon monoxide (CO) and nitric oxide, NO {that is rqiresentative of oxides of nitrogen) measured in a standard hood meant for this purpose show values at a maximum of 2, 17 and 1 g/kg fuel burnt respectively- These are more universally reflected in terms of g/MJ of energy of the fuel to enable comparisons with different class of fuels (like kerosene or LPG). Since the calorific value of the fuels used in these experiments ranged between 14 and 15 MJ/kg, an upper estimate of the emissions can be obtained by using a calorific value of the biomass used as 14 MJ/kg. These correspond to 143 mg/MJ of SPM, 1214 mg/MJ of CO and 72 mg/MJ of NO. These values are in the lower range of values obtained by systems of the prior art.

EXAMPLE II: The 150 mm diameter stove
Experiments were done with 10 litre vessel since the power level was so large that it would make practical sense to apply it to this size vessel.
Table C. Performance of ISO mm stove with different biomass and a cooking vessel containing 10.0 litres of water.

EXAMPLE III: The 100 mm diameter stove in an LPG stove frameworlc
A classical LPG stove was chosen and one of the burners was replaced by a 100 mm biomass stove. Figure 5 shows the arrangement of the combination. The idea of this arrangement is that one can use the biomass stove for economy and energy availability at times when LPG is unavailable and one can xise LPG stove whenever the demand for use is immediate.
EXAMPLE IV: Bathwater heating stove in through-flow mode
Performance of 125 mm stove with following biomass and single coil heat exchanger, substantially as shown in Figure 4.


The important point to notice is that the water boiling efficiency varies between 72 to 75 %, which is much higher compared with the cooking vessel experiments. This is due to the higher heat transfer with the coil design heat exchanger.
EXAMPLE V: Bathwater heating stove in storage mode
Performance of 125 mm stove with following biomass and single coil heat exchanger substantially as shown in Figure 5.

The water boiling efficiency was around 40%. This lower efficiency is probably due to the steam generated during the heating operation - in some part, due to higher heating rate. Therefore this configuration calls for lower heating rate such that thermo siphon action could be sustained without formation of steam. The power level adequate for this mode of operation is about 1 kW. A 75 mm stove with a bum time of 50 minutes should suffice this requirement. The rise in temperature of water in this mode would be about 27 °C for 50 L capacity water.
EXAMPLE VI: Flameless combustion devices as cooking stoves of long bum duration

An efficiency of 33% is reported using a 500 mm vessel. It is possible to increase efficiaicy fiirther by using a vessel of about 700 mm whereby ttiere is higher heat transfer. Temperature measured in the flame zone was in excess of 1050- 1100 "C.

Example VII
As an example for the 125 mm diameter stove, the air flow rate measured at various voltage input provided to the fan (this varying voltage is achieved using the Potentiometer)

We Claim:
1. A biomass stove, which comprises of:
(a) an inner cylindrical combustion chamber (4) for the fuel(l);
(b) an outer container (5) with an annulus (12) between the combustion chamber (4) and the outer container (5); the height of the combustion chamber (4) fixed so as to accommodate fuel to a height which is nearly the same as the diameter of the combustion chamber, the fuel being any biomass with an ash content up to 12%;
(c) means (10) for supplying a primary air supply to the bottom of the combustion chamber(4);
(d) means (11) for supplying a secondary air supply to the top of the combustion chamber(4);
(e) a grate (3), with an option of positioning the grate(3) at varying heights from the bottom of the combustion chamber(4); and
(f) an ash tray (6) within the combustion chamber(4).
2. The biomass stoves as claimed in Claim 1, wherein the biomass is selected from
briquettes or high density pellets for the fuel.
3. The biomass stove as claimed in claim 1, wherein the ratio of the diameter of the combustion chamber to the height of the fuel bed is between 0.6 and 1.25.
4. The biomass stove as claimed in any one of claims 1 to 3, wherein a vessel is placed on the stove, the diameter of the vessel being from 2 to 4 times the diameter of the combustion chamber.
5. The biomass stove as claimed in any one of claims 1 to 4, wherein an ash removal plate (6) is either attached to or form part of grate 3, such that they are removed together.
6. The biomass stove, as claimed in any one of claims 1 to 5 wherein, with its grate provided with telescopic legs or leg extensions to adjust the height of the grate (3).

7. The biomass stove as claimed in claim 6, wherein the height of fuel bed is fixed by selecting the grate height to position the fuel bed towards the top of the combustion chamber.
8. The biomass stove as claimed in any one of the preceding claims, wherein means for providing the primary air supply and the means for providing the secondary air supply are at least one fixed or variable speed fan that is driven by a rechargeable battery or an AC-to-DC converter.
9. The biomass stove as claimed in any one of the preceding claims, wherein means for providing the primary air supply and the secondary air supply is controlled using valves.
10. The biomass stove as claimed in claim 9, wherein the means for air supply is either a two valve system or a single valve system or a valveless system.
11. A combination stove comprising at least one conventional burner, air inlet and vessel seater in combination with one or more biomass stove, as claimed in any one of claims 1 to 10.
12. The combination stove constituting of single or multiple burners of LPG or biomass stove as claimed in any one of claims 1 to 11.
13. A water heating system comprising the biomass stove as claimed in claim 1, 2, 3 & 4, wherein the ratio of the diameter of the combustion chamber to the height of the fuel bed is to 0.6, the water heating system device is compact with high heat utilization efficiency.
14. A biomass stove having continuous mode of operation comprising: (a), a horizontal segment (24) for feeding the fuel (1);
(b) a combustion chamber (26) to accommodate fuel, the fuel being any biomass, briquette or high density pellet with an ash content up to 12%;

(c) means for supplying a primary air through the horizontal fuel bed (1) through ejector
action;
(d) means for supplying a secondary air supply;
(e) a grate(3), with an option of positioning the grate at varying heights, placed at the bottom of the combustion chamber;
(f) an ash tray (6) within the combustion chamber, and
(g) an ejector based arrangement to draw the oxidatively pyrolized gases from a horizontally
arranged biomass container and bum them up in flameless mode with the very low emissions
including oxides of nitrogen.
15. The biomass stove as claimed in claim 14, wherein the secondary air supply is through a
blower of a pressure of up to 6000Pa.
16. A method for operating a biomass cooking stove (7), which comprises (a) a cylindrical
combustion chamber (4) for the fuel (1), (b) means for providing a primary air supply to the
bottom of the combustion chamber and (c) means for providing a secondary air supply to the top
of the combustion chamber, the method comprising introducing biomass fuel into the
combustion chamber (4), igniting the fuel at the top of the bed and introducing the primary air
supply to the bottom of the combustion chamber and the secondary air supply to the top of
combustion chamber, the method being characterised in that the fuel is any biomass having an
ash content of up to 12% , said fuel being introduced into the combustion chamber, to a height
which is nearly the same as the diameter of the combustion chamber.
17. A method as claimed in claim 16, wherein the biomass is selected from briquettes or high
density pellets for the fuel.