Biomass Stove

This invention relates to a 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 burns 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. The system comprises a burner comprising upright co-axial inner and outer cylindrical walls providing a confined central gasification/combustion chamber surrounded by an annular combustion air manifold. The annular combustion air manifold is closed at the top and bottom, has an air inlet in the outer wall and a plurality of spaced apertures arranged in upward rows in the inner cylindrical wall providing air inlets from the combustion air manifold into the combustion zone of the chamber. A grate at the bottom of the chamber supports the fuel. An auger can supply fuel to the combustion/gasification chamber. Air is drawn into the region of the grate where the fuel pellets are pyrolyzed to produce combustion gases which are drawn upwardly into the combustion zone of the gasification/combustion chamber where they are mixed with heated air entering through the spaced apertures. A fan can be used to produce a negative pressure in the combustion zone and control the air flows.

A number of papers have been published by T B Reed et al relating to a stove they called an "inverted downdraft gasifϊer", 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 Developing Countries", 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 4thBiomass conference of the Americas in Oakland, Ca, Eds., , Overend, R. P., and Chornet, 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. Air 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 independently 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 fuel. 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 quality of the combustion of the biomass and the mixtures of fuel gas and air.

It has now been found that the efficiency of biomass burners of the inverted downdraft gasifier type is improved when the fuel zone has an aspect ratio (diameter-to- height ratio) in the range of one or more. 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 the present invention, a method for operating a biomass cooking stove which stove comprises (a) a substantially cylindrical combustion chamber for the fuel, (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,

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 introduced into the combustion chamber, to a height which is about the same as the diameter of the combustion chamber. Stoves used according to the present invention can have significantly higher efficiencies than known stoves. Typically, known stove operation can give efficiencies, based on a standard water boiling assessment, of 30 to 40%. Under similar conditions, a stove operated according to the present invention can have an efficiency of 45 to 50%.

The ratio of the diameter of the combustion chamber to the height of the fuel bed is preferably close to unity. 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;

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 burner 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 a top 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 grate 3 is provided in the bottom of the combustion chamber 4. Fuel 1 is loaded in the combustion 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 disturbing the stove 7. The ash removal plate 6 preferably sits within the stove. Optionally, the ash removal plate 6 can be attached to or form 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 air supply. The air 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 the porous 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 combustion 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 (thermal). 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 thermal power of the stove 7 can be regulated by use of the valves 10 and 11. Alternatively, the thermal power of the stove 7 could be controlled by a single electronic control (not shown) that changes the rpm of the fan 8. 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 air, i.e. secondary air, 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 turn 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 hoϊes 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, the pieces of biomass or pellets have an intrinsic density approaching one thousand kg/m ³ . 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 thermal radiation energy enhancement from the slowly converting 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 fuel 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. The height of the fuel 1 is selected to attain a certain burn time and the height of the grate 3 is selected 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 ensures 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 burn up and generate hot flue gas at temperatures typically in the range of 1000 to 1200 ⁰ C. 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 required 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 ³ . The amount of fire wood chips that can be loaded will be lower since their densities go up to 600 kg/m ³ . Coconut shells also have a high density (up to 1200 kg/m ³ ) and therefore are comparable to briquettes or high density pellets in terms of loading. Thus a stove that burns with briquettes or high density pellets for 45 minutes would burn 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). The water 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 from 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 need for varying the power during the operation. This design uses a horizontal segment 24 for feeding the biomass to the 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 pressure. This lower pressure induces a flow from the ambient atmosphere through the horizontal segment containing packed solid fuel. This leads to a propagation of a flame front through the solid fuel bed much like in a gasification system when the fuel bed is lit. This process generates combustible volatiles that burn 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 ultimately 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 stands 32 for cooking vessels and an outer body 33. The LPG Line supply is connected to the inlet 34, which supplies the gas to burner 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 stoves, 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. hi 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 thermometer 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 that 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 amount 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 furnace at 100 ⁰ C for a minimum of six hours. The material was taken out and weighed and again put into the furnace. 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. Li 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 fuel calorific value was obtained as 14.9 kJ/g. The average 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 different biomass and a cooking vessel containing 2.5 litres of water.

The -ash fraction was obtained by measuring the burnt mass after the combustion process. δTw (°C) is the increment in temperature of the water in the heating up process.

Multiple values of δTw ( ⁰ C) refer to vessels being changed after the water reached the boiling point.

Table B. Performance of 100 mm stove with different biomass and a cooking vessel containing 6.0 litres of water.

Table C. Performance of 100 mm stove with different biomass and a cooking vessel containing 10.0 litres of water.

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 same, larger efficiencies have resulted with the choice of a larger diameter vessel (vessel-to-burher diameter ratio of 320/100 = 3.2) compared to the lowest of the efficiencies obtained with a vessel-to-burner diameter ratio of 205/100 = 2.05).

Emissions of solid particulate matter (SPM), carbon monoxide (CO) and nitric oxide, NO (that is representative 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 150 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 framework

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 use 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 burn 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 burn duration

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