EP2318766A2

EP2318766A2 - Biomass stove apparatus and method for its use

Info

Publication number: EP2318766A2
Application number: EP09768244A
Authority: EP
Inventors: Hanasoge Suryanarayana Avadhany Mukunda; Srinivasaiah Dasappa; Palakat Joseph Paul; Nagamangala Krishnaiyengar Sriranga Rajan
Original assignee: First Energy Pvt Ltd
Current assignee: First Energy Pvt Ltd
Priority date: 2008-07-23
Filing date: 2009-07-20
Publication date: 2011-05-11
Also published as: WO2010029567A3; AP2011005595A0; AP3121A; WO2010029567A2

Abstract

A biomass stove comprising: an outer container; an inner combustion chamber for fuel being housed inside said outer container; and a fan for supplying air to the stove as a primary air supply to the bottom of the combustion chamber and a secondary air supply at or adjacent to the top of the combustion chamber; in which the combustion chamber comprises corrosion resistant and high temperature resistant, ceramic material; and the stove further comprises : a regulator for regulating the flow primary air supply and secondary air supply; and a grate at the bottom of the combustion chamber, for supporting fuel in the combustion chamber and being adjustable in height with respect to the bottom of the combustion chamber to accommodate fuel in the combustion chamber from the grate and up to the top or to near the top of the combustion chamber. The present invention also provides a method for operating such a biomass stove.

Description

BIOMASS STOVE APPARATUS AND METHOD FOR ITS USE.

FIELD OF THE INVENTION

The present invention relates to a biomass stove apparatus and a method for its use. The invention also relates to using biomass pellets as a fuel in a domestic single pan cooking stove of high efficiency and low undesirable emissions with user-friendliness and long life. The invention also relates to improvements in combustion chamber design, for example by means using a ceramic liner for the hot combustion chamber, which may increase the life of the stove whilst not compromising on high efficiency and low emissions and also for example, by an improved valve mechanism for controlling flow of primary and secondary air flow. The construction of the stove of the present invention provides flexible fuel capacity. Furthermore, an ash removal device may be incorporated in the stove which can facilitate easy removal of ash from the stove, for example at the end of the operation.

BACKGROUND

Two patents, US patent 5842463 and US patent 6520173 describe designs for a portable wood burning camp stove and a portable solid fuel camp-stove, respectively. These stove designs are aimed at reducing the weight of the stove for use as a backpack capable of burning as-is-available biomass in the field condition. The stove design of US patent 5842463 uses a double walled construction for air supply, with a primary combustion air passageway being provided proximate the floor of the combustion chamber and a secondary combustion air passageway in communication between an upper portion of the combustion-chamber and an upper portion of an air space defined by the combustion- chamber and an outer wall. US Patent 6520173 uses additional air blown into the combustion chamber by mouth through a hose and nozzle arrangement. There are other stove patents like US patent 6336449 meant for pellets largely intended for space heating applications.

Reed T. B., and Larson, R., in "A wood-gas stove for developing countries, in Developments in Thermochemical Biomass conversion", Ed. A. V. Bridgewater, Blackie Academic Press, 1996 describe inverted downdraft gasifier stoves. According to the authors, "riser sleeves" can usefully be used for constructing such stoves and whilst they are relatively soft, they can be "rigidized" by application of amorphous silica. Reed T. B., and Walt, R., in "The "turbo" wood gas stove" Biomass Proceedings of the 4^th Biomass conference of the Americas in Oakland, Eds., Overend, R. P., and Chornet, E., Pergamon Press, 1999 describe a forced convention wood-gas stove.

The descriptions indicate that the development process is still in progress. Principally, the stoves accept a range of fuels with thermal efficiencies generally at a maximum of only 37.5 %.

"Testing & Modelling the Wood-gas Turbo Stove" by T. B. Reed et al in Progress in Thermochemical Biomass Conversion Ed. A. V. Bridgwater Blackwell Science Ltd 2001 p693 ff relates to wood-gas stoves operating in several different gasification and combustion modes. In particular, a turbo stove with an inverted downdraft gasifier^and fuel magazine is described. Data for air fuel ratios for a research stove were presented in Table 1. The Primary to secondary air ratio is varying with power, not a desirable feature for efficiency and emissions. These are summarised in Table 1 below :

Table 1

This document does not disclose the use of insulated combustion chambers.

US patent 4,730,597 describes a biomass stove having an outer chamber formed by a continuous side wall which is connected to a bottom wall. An air inlet is formed in the bottom wall for allowing introduction of air into the interior of the outer chamber. A fuel basket, also formed as a continuous side wall attached to a bottom wall, is located within the outer chamber. In the fuel basket, near the bottom of the fuel basket, a grate is positioned. The fuel basket is positioned within an outer chamber, which has an air control mechanism to control air flow into the fuel basket.

US Patent 5,105,797 relates to a solid fuel burning stove having a hopper for storing pellet fuel, such as corn and a motor driven auger for moving the pellet fuel form the hopper to a burn pot. The burn pot has walls with apertures that allow forced air to flow into the combustion chamber to facilitate burning of the pellet fuel. A valved air inlet is used to regulate the amount of air flowing to the combustion chamber to vary the rate of combustion of the pellet fuel.

US Patent 4,471,751 discloses a small compact stove capable of burning a variety of carbonaceous fuels having a base and located on the base is a vertically oriented cylindrical wall formed of a heat conducting material and has open bottom and top ends. The interior surface of a portion of the wall forms a combustion chamber. A grate is located within the wall at the bottom end of the combustion chamber. The primary air chamber is located at the open bottom end of the wall and primary air is conducted through it and then up through the grate into the combustion chamber. A secondary air chamber surrounds at least a portion of the wall such that the wall forms one of the surfaces of the secondary air chamber. The wall includes a plurality of air passageways between the secondary air chamber and the combustion chamber such that air can flow within the secondary air chamber in contact with the exterior surface of the wall and be heated by heat conducted through the wall from the combustion chamber. The heated air from the secondary air chamber then flows through the passageways into the combustion chamber. The stove is said to be capable of burning solid, liquid and gaseous carbonaceous fuel by appropriately adapting the grate to support either the solid fuel or a container having a suitable surface for burning of liquid and gaseous fuels. In one embodiment the stove has an insulated outer wall, but this document does not disclose a stove with an outer chamber housing an inner combustion chamber comprising ceramic. Indian Patent application no.l365/CHE/2005 and corresponding International (PCT) publication WO 2007/036720 describe a biomass stove and method for its operation. The stove has a defined ratio of height of fuel to combustion chamber diameter.

There remains a need for a stove, which overcomes or at least mitigates problems associated with the life of the inner parts of the stove, removal of the ash from the stove and control of primary and secondary air supplies. With a stove comprising an inner combustion chamber and an outer annular space for supply of air to the combustion chamber, the inner wall defining the combustion chamber is subject to a corrosive environment, both oxidizing and reducing at different times and locations and is also subject to cyclic temperature changes due to repeated heating and cooling cycles. Also, the formation of large amounts of ash by the fuel pellets (which might be for example as high as up to 12 weight percent, for example up to 10 % by weight) might also adversely affect the inner wall defining the combustion chamber, which is in contact with the fuel pellets. It is also desirable to have a convenient method for removing ash from the stove, which avoids or reduces the need to invert the stove, which is considered a cumbersome and unsafe process. Also, it is desirable to have control of air for both the primary and secondary zones, which can assist in achieving, complete combustion and control of the power level as well as reducing emissions.

There is a need to provide for a stove which overcomes or at least mitigates these problems and in particular which can provide an increased the life of the stove, good fuel efficiency and low emissions and/or provide flexibility. It may not be out of place to mention that one of the standards that is applied to test fuel efficiency of a stove is the 'Water Boiling Test', using a IS standard (IS 13152).

SUMMARY OF THE INVENTION.

The present inventors have found solutions to these problems in the material of construction of the combustion chamber using for example, ceramic liners, and in control of the primary and the secondary air supply in the stove for example by means of a valve mechanism, which may (i) mitigate the corrosive environment and its effects, (ii) increase the fuel efficiency, (iii) provide low emissions and/or (iv) control the power level output of the stove.

The construction of the stove of the present invention provides flexible fuel capacity. In particular, a grate being adjustable in height with respect to the bottom of the combustion chamber can be adjusted to accommodate fuel for an intended purpose, for example so that fuel can be introduced into the combustion chamber from the grate and up to the top or near the top of the combustion chamber.

Furthermore, an ash removal device may be incorporated in the stove, which can facilitate easy removal of ash from the stove, for example at the end of the operation.

According to a first aspect of the present invention, there is provided biomass stove comprising: an outer container; an inner combustion chamber for fuel being housed inside said outer container; and a fan for supplying air to the stove as a primary air supply to the bottom of the combustion chamber and a secondary air supply to or adjacent to the top of the combustion chamber; in which the combustion chamber comprises corrosion resistant and high temperature resistant, ceramic material, and said stove further comprises :a regulator for regulating the flow primary air supply and secondary air supply; and a grate at the bottom of the combustion chamber, for supporting fuel in the combustion chamber and being adjustable in height with respect to the bottom of the combustion chamber to accommodate fuel in the combustion chamber from the grate and up to the top or to near the top of the combustion chamber.

According to another aspect of the present invention there is provided a fuel efficient biomass stove comprising: an outer container with an annular air space; an inner combustion chamber for fuel being housed inside said outer container; and a fan for supplying air to the stove as a primary air supply to the bottom of the combustion chamber and a secondary air supply to or adjacent to the top of the combustion chamber; in which the combustion chamber comprises corrosion resistant and high temperature resistant, ceramic material, and said stove further comprises :a regulator for regulating the flow primary air supply and secondary air supply; and a grate at the bottom of the combustion chamber, for supporting fuel in the combustion chamber and being adjustable in height with respect to the bottom of the combustion chamber to accommodate fuel in the combustion chamber from the grate and up to the top or to near the top of the combustion chamber.

The regulator may be of different types. According to one aspect, it may comprise a valve for regulating the flow of primary air supply and the flow of secondary air supply, the valve comprising an outer substantially cylindrical member and an inner substantially cylindrical member mounted co-axially within the outer member, the inner and outer portions having corresponding and variably alignable, radial orifices, wherein relative coaxial rotation of the outer and inner portions varies the extent of alignment of inner and outer orifices to regulate the ratio of primary air supply to secondary air supply, or to regulate the combined flow of primary air supply and secondary air supply in a constant, predetermined ratio, or to regulate the ratio of primary air supply to secondary air supply and the combined flow of primary air supply and secondary air supply. The valve may regulate the ratio of primary air supply to secondary air supply, the outer cylindrical member having at least two orifices comprising a first orifice operable connected to the bottom of the combustion chamber for supply of the primary air supply and a second orifice operable connected to the combustion chamber for supplying secondary air supply to or adjacent to the top of the combustion chamber. The valve may comprise a knob for rotating the inner cylindrical member.

According to another aspect of the present invention, the regulator may comprise a valve for regulating the flow of primary air supply and of secondary air supply, the valve comprising a gate slidably movable to partition an orifice to regulate the ratio of primary air supply to secondary air supply or to regulate the combined flow of primary air supply and secondary air supply in a constant, predetermined ratio. The valve may regulate the ratio of primary air supply to secondary air supply, the valve comprising a gate slidably moveable to partition an orifice between a portion for supply of the primary air supply and a portion for supply of the secondary air supply. The valve may comprise a rotatable knob having a pinion operable engaging a rack operably connected to the gate wherein rotation of the knob slidably moves the gate.

According to another aspect of the present invention, the regulator may comprise a first valve for regulating the flow primary air supply and a second valve for regulating the secondary air supply. The first and second valves may be operable by a single lever.

According to another aspect of the present invention, the regulator may comprise a single valve for regulating the flow of primary air supply, the remainder of the air supply being secondary air supply.

The fan may comprise at least one fan. The fan may comprise at least one fixed speed and/or at least one variable speed fan. The fan may comprise at least one variable speed electric fan and the regulator comprises a potentiometer for regulating the speed of the fan. The fan may comprise at least one blower.

The outer container and the inner combustion chamber may define therebetween a chamber, for example, an annular air space, through which secondary air supply may flow. The upper part of the combustion chamber may have a plurality of holes through which secondary air supply from the chamber may flow to or adjacent to the top of the combustion chamber. The diameters of the holes may be predetermined to provide predetermined flow rates of secondary air supply.

The regulator may regulate the ratio of primary air supply: secondary air supply in the range of from 1:4.6 to 1:3.6, for example about 1:4.

The ratio of the width of the combustion chamber to the maximum dimension of the combustion chamber may be in the range of from 0.6 to 1.25. Suitably, the combustion chamber has a square, rectangular or circular horizontal cross-section, preferably a square horizontal cross-section. Preferably, the combustion chamber has a square horizontal cross-section (that is, transverse to the longitudinal axis of the combustion chamber in the direction of air flow from the base to the top), with a ratio of the width of the combustion chamber to the height of the combustion chamber being in the range of from 0.6 to 1.25, for example 1 : 1. That is the combustion chamber may have a generally cubic shape, for example is a cube.

The combustion chamber comprises a corrosion resistant and high temperature resistant, ceramic material. Suitably, the combustion chamber is made of a corrosion resistant and high temperature resistant, ceramic material. The corrosion and high temperature resistant material may be in the form of ceramic blocks. The ceramic material (for example ceramic blocks) forming the combustion chamber may be contained in an outer metal container.

The regulator of the stove provides the ability to regulate the flow of primary air supply and secondary air supply. This may enable the combustion of the fuel to be controlled at different stages of the process, for example by regulating the ratio of primary air supply to secondary air supply. This may provide high efficiency and low undesirable emissions. Furthermore, the ability to regulate the flow of primary supply and the flow of secondary air supply may enable the operating power range of the stove to be varied over a significant range. For example, a stove with a nominal power output of 2 kW may be operated in the range of 1.5 to 2.5 kW by regulating the flow of primary air supply and the flow of secondary air supply.

The biomass stove may have a power level ranging from 1.5 to 2.5 kW.

The biomass stove may additionally comprise a removable ash tray in sealable engagement with the bottom of the combustion chamber. This avoids or reduces the need to invert the stove for removal of ash, for example at the end of the operation. The ash tray may be removable from the side of the stove or from the base of the stove. The ash tray should sealably engage the combustion chamber to reduce the opportunity for air supply or combustion gases to escape the combustion chamber through the bottom of the chamber.

According to a further aspect of the present invention there is provided a method for operating a biomass stove as hereindescribed which comprises: introducing biomass fuel into the combustion chamber in which the position of the grate is adjusted to accommodate the fuel for a predetermined purpose, and the fuel has an ash content of up to 12 weight percent and is introduced into the combustion chamber from the grate and up to the top or to near the top of the combustion chamber; igniting the fuel at the top of the combustion chamber; introducing primary air supply to the bottom of the combustion chamber and introducing secondary air supply to the combustion chamber at or adjacent to the top of combustion chamber.

The biomass fuel may be selected from the group consisting of wood chips, coconut shells, high density pellets derived from agro residues, briquettes derived from agro residues and combinations thereof.

The primary air flux may be in the range of 0.01 to 0.015 kg. m^"2 s^"1.

The construction of the stove of the present invention provides flexible fuel capacity. In particular, the grate being adjustable in height with respect to the bottom of the combustion chamber can be adjusted to accommodate fuel for an intended purpose, for example so that fuel can be introduced into the combustion chamber from the grate and up to the top or near the top of the combustion chamber. With the fuel in the combustion chamber extending up to the top or up to near the top of the combustion chamber combustion of the gases produced in the combustion chamber in combination with the secondary air is efficient. This can provide a stove with high efficiency and low undesirable emissions.

The height of fuel in the combustion chamber may be selected such that the burn time is in the range of from 15 minutes to 80 minutes using a variable grate, for example of from 50 minutes to 80 minutes. Typically, 600 gm of fuel may have a burn time of 70 to 90 minutes +/- 10%.

The water boiling efficiency of the stove may be at least 45% Other features, additional objects, and many of the advantages of this invention will readily be appreciated as the same becomes better understood from the following detailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 illustrates a schematic sectional view of a biomass stove according to the present invention.

Figure 2 illustrates a side elevation of the stove shown in Figure 1 in the direction X. Figure 3 illustrates a top view of the stove shown in Figure 1.

Figure 4 shows a schematic sectional view of an alternative stove according to the present invention.

Figure 5(a) illustrates a plan view of the ash tray of the stove shown in Figures 1 to 3 and

Figure 5(b) illustrates a side elevation of the ash tray shown in Figures 1 to 3. Figures 6(a), 6(b) and 6(c) illustrate in schematic form the air flow through the regulator of

Figures 1 to 3 in closed and open positions.

Figure 7 shows the regulator in Figures 6 (a) to 6(c) in perspective view and plan view respectively.

Figure 8 shows in end elevation the regulator of the stove in Figure 4 Figures 9(a) and 9(b) illustrate in schematic elevation, side and top views of an outer cylindrical member and an inner cylindrical member of a valve for regulating the flow of primary and secondary air according to the present invention.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Features in the drawings are identified with numerals as in the Table below.

Referring to Figures 1 to 8, the stoves illustrated have a combustion chamber with a square horizontal cross-section for a primary air supply mass flux in the range of from 0.10 to 0.15 kg.m^"2.s^"'. This implies the stoves have a power level in the range of 1.5 to 2.5 kW for a nominal 2 kW model and the burn time variation between 50 minutes to 80 minutes. A square horizontal cross-section compared with a circular one facilitates the stove design for ash removal and provides for simpler joints between the air supply duct and the stove body.

Figures 1, 2 and 3 are views of a biomass cooking stove (7). Figures 2 and 3 are a side elevation and a top view of the stove of Figure 1. Stove (7) comprises of a square horizontal cross-section combustion chamber (4) made of ceramic blocks (21). The ceramic blocks (21) forming the combustion chamber are contained in an outer metal container (5). The ceramic blocks have thermal properties to handle a high temperature corrosive environment in the combustion chamber and to ensure that the outer wall temperature of the outer container (5) is limited to a desired value, for example to less than

50 ⁰C.

In an another variation, the combustion chamber (4) is housed with the outer container (5) and has an annular air space (12) in between as shown in Figure 4. An air flow chamber (42) is provided at the bottom of the combustion chamber (2).

A grate (3) of square cross section is provided at the bottom of the combustion chamber (4). An ash removal tray (6) for removing ash is provided below the grate (3) in sealable engagement with the bottom (2) of the combustion chamber (4) in Figure 1 and with the air flow chamber (42) at the bottom of the combustion chamber in Figure 4. The ash tray (6) has a handle (51) for removing it from the stove for removal of ash.

The stoves in Figures 1 to 4 are provided with supports (26) for cooking vessels to be heated by the stove. The stove in Figures 1 to 3 is also provided with a stand (30).

Features of the Drawings

Ash tray

The details of the ash tray (6) are as indicated in Figures 5(a) and 5(b). The square cross- section ash tray (6) in Figures 5(a) and 5(b) has an ash collection area (52) designed to cover the entire combustion chamber area, with a volume equivalent to hold up to 10 % from the fuel (1). At the end of the operation and after consuming all the fuel, the ash tray is pulled out from the stove using the handle (51) for removing the ash and emptying it from the tray.

The design of the ash tray (6) and its sealable engagement with the bottom of the combustion chamber (2) or the air flow chamber (42) at the bottom (2) of the combustion chamber (4) is such that no leakages can take place after positioning the ash tray in the stove. The interface between the ash tray and the stove may be sealed using a liner (53). This provides that there is no loss of power from the stove in use, for example, at any setting of the regulator. It is important that air leakage through the ash tray does not take place. The design is arranged in such a way as to prevent air leakage, for example by locking the tray into the stove. An insulated handle (51) is provided to the ash tray (6) to allow for easy and safe removal and unloading the ash.

Air flows and regulator

As shown in Figures 1 to 4, 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 (2) of the combustion chamber (4) directly as in Figures 1 to 3 or through the air flow chamber (42) and flows up through the bed of fuel (1) and controls the power of the stove (7). The other part of the air (secondary air supply) is pumped into the chamber or annular air space (12) for secondary air and flows from there into the combustion chamber at or adjacent to the top of the combustion chamber (4) through the holes (13). Referring to Figures 1, 4, 6(a) to (c), 7(a) and (b), 8 which show different versions of the regulator (14), primary air flow rate and the secondary air flow rate is regulated by valves (11) and (10) respectively. Independent regulation of the air flow rate is possible using these valves to provide varying power levels for the stove operation.

Figures 6(a) to (c), 7(a) and 7(b) show the details of the regulator (14) (air distribution system) for the stove shown in Figures 1 to 3. As mentioned above, the air supply (60) is split into two parts. Air requirement for primary air supply (62) which is about 25% of the total air flow, flows through grate (3) below the fuel bed (1). This air flow can be regulated by valve (11) depending upon the power level required. The primary air flow rate can be varied from 0.9 g/s to about 1.5 g/s at varying operating conditions. The primary air flow helps in the gasification process of the fuel. The primary air flow (62) and valve (11) are small compared with the secondary air supply and valve (10). The secondary air flow (61) is controlled by valve (10) at about 1.5 to 3 g/s of air which is pumped into the annular air space chamber (12) of the stove (7) via duct (15). The secondary air (62) is then introduced into the top combustion chamber beyond the fuel bed through a large number of holes (13) amounting to about 5.8 xlO^"4 m² (in one configuration, 60 holes of 3.5 mm diameter) located at or near the top (18) of the combustion chamber, arranged in 3 rows for proper distribution and mixing. Typical velocity of air in these holes (13) ranges from 2.5 m/s to 5 m/s. The secondary air flow distribution is designed to ensure proper mixing with gaseous fuel created by gasification of fuel (1) for complete combustion. This arrangement allows for control of burn time for the fuel loaded; for example up to a maximum burning time of from 50 minutes to 80 minutes, e.g. 70 to 80 minutes for 600 gm of fuel. This will give for example, a fuel consumption rate of 8 g/min to 12 g/min, corresponding to a power level varying from 1.5 kW to 3 kW, an essential requirement for preparing food with low flame intensity, for example pancakes and the like.

Apart from the independent operations of the valves as described above, another embodiment of the present invention describes a single lever (not shown) to operate both the valves. This provides a proportionate control of the primary and secondary air flow, maintaining the ratio in the range of from 1:4.6 to 1:3.6, for example about 1:4, between the primary and secondary air supplies over the range of operating power. Figure 8 shows the variation of a regulator (14) with single knob control (80) as shown in Figure 4. The regulator consists of a knob (80) which is connected by worm and gear (84) to a plate (81) slidable between different positions to obstruct in varying amounts a primary air orifice (82) for primary air flow and a secondary air orifice (83) for secondary air flow which flows are provided by fan (8) as shown in Figure 8. In an alternative embodiment a rack and pinion could be used. The knob (80) provides positions for nominal operations of the stove. The options are towards start-up, nominal operation and the char combustion period.

Figures 9(a) and 9(b) show an outer substantially cylindrical member (91) and an inner substantially cylindrical member (92) which may be mounted co-axially within the outer member in a valve of a regulator for regulating primary and secondary air flows. The outer cylinder (91) has two separate orifices (93, 94); a first orifice (93) which may be operable connected to the bottom of (2) of the combustion chamber (4) or the air flow chamber/manifold (42) and a second orifice (94), which may be operable connected to the top (18) of the combustion chamber (4) similar to the above described arrangement. The valve comprises of single valve knob (95) connected to the inner cylindrical member (92) so that rotation of the knob (95) regulates the flow of the primary and secondary air supplies to the stove (7). The valve provides air to both primary and secondary zones as described earlier.

A single control valve such as one comprising the outer and inner cylindrical members shown in Figures 9 (a) and (b) may regulate 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 and secondary air supplies) one can vary the air flow that as required for regulation of thermal power output of the stove. The volumetric flow rate ratio of primary flow to secondary air flow may be in the range of from 1:4.6 to 1:3.6 over the range of power level. As an example, in the case of nominal 100 mm square cross section stove, the nominal and maximum power levels are 2.0 and 3.0 kW respectively. Therefore, in a single valve system, the ratio of primary air flow to secondary is obtained as a consequence of the difference in the areas of the inlet air entry for the primary and secondary air and a single valve is used to distribute the single air stream between the two.

Alternatively, or additionally, referring to Figures 1 and 3, the thermal power of the stove (7) may be controlled by a single control such as a potentiometer (20) that varies the electric power from a source of electrical power, for example a rechargeable battery (9) and changes the speed of the fan (8).

The operational requirement for the stove demands the stove to be operational at low power level to high power level depending upon the usage. This can be achieved by regulating the air flow to the stove using the different valve designs indicated earlier. Some of the valves provide independent control of both primary and secondary air, depending upon the user requirement, while others have been operational on design which will provide the necessary operational flexibility.

Below in Table 2, is given the logic of stove operation which provides the operational status of the primary and the secondary air supplies.

Table 2

Referring to Figure 1, 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) (not shown), typically in the range of 6 to 12 V with a current rating of 0.1 to 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 may be used. The control of primary and secondary air supply is achieved using a potentiometer (20) that varies the speed of the fan (8). The distributor plate 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 square cross, the nominal and maximum power levels are 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 ratio in the areas of the inlet air entry for the primary air supply and of the inlet air entry for the secondary air supply whilst the and potentiometer controlled fan provides an air stream which is distributed between the two.

In a valve-less system, the primary-to-secondary air flow rate ratio may be 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 regulator may have two valves or a single valve or may be valve-less design. The two valve design will provide for a flame control at any fixed power level. This implies that at any fixed power level of the stove, if the air supply is brought down from the value that completes the combustion in the shortest region above the outlet of the combustion chamber, the flame size will become enlarged because additional oxygen from the atmosphere is needed to complete the combustion. This makes the flame volume larger and the heat release rate lower. This facility is specially needed while cooking some specific items (like, pancake, sugar candy, for example) wherein the reduction in local heat release helps in getting a food product of quality.

In the stove the use of ceramic wall prevents heat being taken away from the combustion chamber except during the initial heat up time. In any case, the magnitude of heat transfer constitutes less than a few percent of the heat generation rate and hence the effective utilization efficiency is acceptable. The fan speed can be controlled up to for example, a maximum speed of 2500 rpm by regulation of the electric supply.

Grate

Referring to Figure 1, the position of the grate (3) is adjustable in height with respect to the bottom (2) of the combustion chamber (4) to accommodate fuel (1) in the combustion chamber from the grate and up to the top (18) or near to the top (18) of the combustion chamber (4). Possible means for adjusting the height would include, for example, telescopic legs or leg extensions and the like. 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 flow supplied to the bottom of the combustion chamber moves through the packed bed of fuel (1) and increases the conversion of the solid fuel to gas (gasification) with increased air flow through the bed. The variable height grate (3) allows control of the quantity of fuel charged 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 manner so as to position the bed of fuel (1) extending up to the top or up to near the top of the combustion chamber (4). With the fuel in the combustion chamber extending up to the top or up to near the top of the combustion chamber combustion of the gases produced in the combustion chamber in combination with the secondary air is efficient. This can provide a stove with high efficiency and low undesirable emissions.

The fuel may be any suitable biomass. Biomass fuel may have an energy content of from 12 MJ/kg to 30 MJ/kg. High bulk density biomass provides better performance. Suitably, pieces of biomass or pellets, which has an intrinsic density approaching one thousand kg/m³. 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 bed (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.

Initial ignition is facilitated by using fine pieces of light biomass, for example fire wood on the surface of the packed bed of the fuel briquettes, high density pellets or other biomass pieces. Spraying some liquid fuel (for example, 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 fuel in the stove is lit, the hot fuel vapours from the surface of the packed bed combine with the secondary air through the holes (13) at the top of the combustion chamber (4) to burn up and generate hot flue gas at temperatures typically in the range of 1000 to 1200⁰C. 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 of the combustion chamber as well as the secondary air supply for combustion as described hereinabove. 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 an individual 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 ash tray. The stove is ready for use again for the next batch of operations.

The nominal duration of operation of the stove 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 pellet densities up to 1000 kg/m³. The amount of fire wood chips that can be loaded will be lower since their densities go up to 600 kg/m³. Coconut shells also have a high density (up to 1200 kg/m³) 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 60 minutes would burn only for 35 minutes in the case of wood chips.

Typical relevant dimensions of the different capacity stoves are shown in Table 3.

Table 3

Examples

To illustrate the invention, different cooking vessels and different stoves were used, to replicate the use of the biomass stoves in domestic applications.

A stove with a cross section area of 10000 mm² (100 mm square cross section) had a fuel loading space in the combustion chamber of 1.5 litres and could carry 400 g of wood pieces, or 600 g of pellets as fuel. The stove can operate at a nominal output of 2.0 kW.

A second stove with a cross section area of 40000 mm² area stove had a combustion chamber volume of 14 litres and could carry a total fuel capacity of 4.3 kg of pellets. The can operate at a nominal output of about 8 kW. Different cooking vessels were used to determine the thermal utilization efficiency. It can be expected that larger diameter vessels extract more heat compared to smaller vessels and hence testing with vessels that allow greater heat extraction from the same stove would be the appropriate choice.

Data for aluminium vessels of 10 litre volume (diameter of 320 mm, height of 160 mm, and 0.96 kg weight) is presented here. Different valve configurations were tested for efficiency and ease of operation. The test results are as indicated in the Tables 4 and 5 below.

Table 4.

Table 5

Efficiency test

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 supply) was set at a minimum and the combustion air (secondary air supply) 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 (secondary air supply) 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 50⁰C, 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 100⁰C 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.

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. In a typical case of 2 kW stove, the stove with about 500 g fuel 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 ⁰C to boiling temperature 97 ⁰C in 80 minutes. The moisture content in the fuel chips was measured as 10 % and the ash content as 8 %. Hence the fuel calorific value was obtained as 14.9 kJ/g. The average power level at which the stove delivered the heat was 1.5 kW. A calculation of the heat utilization efficiency was obtained as 50 %.

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 showed 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 conventional stoves.

The stove of the present invention has the following advantages:

• The regulator for primary and secondary air supplies may enable precise control of air and hence better control of the power level of the stove. This may allow flexibility in height to width ratio of the stove.

• The use of a ceramic walled combustion chamber is expected to enhance the life to beyond three years, compared to about two years or more for a cast iron combustion chamber.

• The grate and optional ash tray of the stove of the present invention helps in collecting the ash; the ash tray which can be separately removed from the stove body, the ash disposed of and the ash tray refitted with a seal preventing air leaking through the tray. This is more convenient that over-turning the stove itself.

• The optional square cross-section configuration may reduce waste and help optimize the cost of production of the stove compared to a circular stove. The invention has been described in a preferred form only and many variations may be made in the invention which will still be comprised within its spirit. The invention is not limited to details cited above. The components herein described may be replaced by its technical equivalent and yet the invention can be performed. The structure thus conceived is susceptible to numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.

Claims

WE CLAIMS ^

1. A biomass stove comprising: an outer container; an inner combustion chamber for fuel being housed inside said outer container; and a fan for supplying air to the stove as a primary air supply to the bottom of the combustion chamber and a secondary air supply to or adjacent to the top of the combustion chamber; in which the combustion chamber comprises corrosion resistant and high temperature resistant, ceramic material, and said stove further comprises : a regulator for regulating the flow primary air supply and secondary air supply; and a grate at the bottom of the combustion chamber, for supporting fuel in the combustion chamber and being adjustable in height with respect to the bottom of the combustion chamber to accommodate fuel in the combustion chamber from the grate and up to the top or to near the top of the combustion chamber.

2. A biomass stove comprising: an outer container with an annular air space; an inner combustion chamber for fuel being housed inside said outer container; and a fan for supplying air to the stove as a primary air supply to the bottom of the combustion chamber and a secondary air supply to or adjacent to the top of the combustion chamber; in which the combustion chamber comprises corrosion resistant and high temperature resistant, ceramic material, and said stove further comprises : a regulator for regulating the flow primary air supply and secondary air supply; and a grate at the bottom of the combustion chamber, for supporting fuel in the combustion chamber and being adjustable in height with respect to the bottom of the combustion chamber to accommodate fuel in the combustion chamber from the grate and up to the top or to near the top of the combustion chamber.

3. A stove as claimed in claim 1 and 2 in which the regulator comprises a valve for regulating the flow of primary air supply and the flow of secondary air supply, the valve comprising an outer substantially cylindrical member and an inner substantially cylindrical member mounted co-axially within the outer member, the inner and outer portions having corresponding and variably alignable, radial orifices, wherein relative co-axial rotation of the outer and inner portions varies the extent of alignment of inner and outer orifices to regulate the ratio of primary air supply to secondary air supply, or to regulate the combined flow of primary air supply and secondary air supply in a constant, predetermined ratio, or to regulate the ratio of primary air supply to secondary air supply and the combined flow of primary air supply and secondary air supply.

4. A stove as claimed in claim 3 in which the valve regulates the ratio of primary air supply to secondary air supply and the outer cylindrical member has two orifices comprising a first orifice operable connected to the bottom of the combustion chamber for supply of the primary air supply and a second orifice operable connected to the combustion chamber for supply of the secondary air supply to or adjacent to the top of the combustion chamber.

5. A stove as claimed in claim 3 or claim 4 in which the valve comprises a knob for rotating the inner cylindrical member.

6. A stove as claimed in claim 1 or claim 2 in which the regulator comprises a valve for regulating the flow of primary air supply and of secondary air supply, the valve comprising a gate slidably movable to partition an orifice to regulate the ratio of primary air supply to secondary air supply or to regulate the combined flow of primary air supply and secondary air supply in a constant, predetermined ratio.

7. A stove as claimed in claim 6 in which the valve regulates the ratio of primary air supply to secondary air supply and comprises a gate slidably moveable to partition an orifice between a portion for supply of the primary air supply and a portion for supply of the secondary air supply.

8. A stove as claimed in claim 6 or claim 7 in which the valve comprises a rotatable knob having a pinion operable engaging a rack operably connected to the gate wherein rotation of the knob slidably moves the gate.

9. A stove as claimed in claim 1 or claim 2 in which the regulator comprises a first valve for regulating the flow primary air supply and a second valve for regulating the secondary air supply.

10. A stove as claimed in claim 9 in which the first and second valves are operable by a single lever.

11. A stove as claimed in claim 1 or claim 2 in which the regulator comprises a single valve for regulating the flow of primary air supply, the remainder of the air supply being secondary air supply.

12. A stove as claimed in claim 1 or claim 2 in which the regulator comprises a plate slidable between different positions to obstruct in varying amounts a primary air orifice for primary air flow and a secondary air orifice for secondary air flow.

13. A stove as claimed in any one of the preceding claims in which the fan comprises at least one fixed speed and/or at least one variable speed fan.

14. A stove as claimed in claim 1 or claim 2 in which the fan comprises at least one variable speed electric fan and the regulator comprises a potentiometer for regulating the speed of the fan.

15. A stove as claimed in any one of the preceding claims in which the regulator regulates the ratio of primary air supply to secondary air supply in the range of from 1:4.6 to 1:3.6.

16. A stove as claimed in any one of the preceding claims in which ratio of the width of the combustion chamber to the height of the combustion chamber is in the range of from 0.6 to 1.25.

17. A stove as claimed in any one of the preceding claims having a power level ranging from 1.5 to 2.5 kW.

18. A stove as claimed in any one of the preceding claims additionally comprising a removable ash tray in sealable engagement with the bottom of the combustion chamber or with an air flow chamber at the bottom of the combustion chamber.

19. A method for operating a biomass stove as claimed in any one of the preceding claims which comprises: introducing biomass fuel into the combustion chamber in which the position of the grate is adjusted to accommodate the fuel for a predetermined purpose, and the fuel has an ash content of up to 12 weight percent and is introduced into the combustion chamber from the grate and up to the top or to near the top of the combustion chamber; igniting the fuel at the top of the combustion chamber; introducing primary air supply to the bottom of the combustion chamber and introducing secondary air supply to the combustion chamber at or adjacent to the top of combustion chamber.

20. A method as claimed in claim 19 in which the biomass fuel is selected from the group consisting of wood chips, coconut shells, high density pellets derived from agro residues, briquettes derived from agro residues and combinations thereof.

21. A method as claimed in claim 19, in which the biomass fuel is selected from the group consisting of high density pellets derived from agro residues, briquettes derived from agro residues and combinations thereof.

22. A method as claimed in any one of claims 19 to 21 in which the height of fuel in the combustion chamber is selected such that the burn time is in the range of from 15 minutes to 80 minutes.

23. A method as claimed in claim 20 or claim 21 in which the height of fuel in the combustion chamber is selected such that burn time for 600 g of fuel is 70 to 80 minutes +/- 10%.

24. A method as claimed in any one of claims 19 to 23, in which the primary air supply is in the range of 0.01 to 0.015 kgm^"V ' .

25. A biomass stove as herein substantially described and illustrated with reference to the accompanying figures.

26. A method as herein substantially described and illustrated with reference to the accompanying figures.