FIELD OF INVENTION
This invention relates to a class of biomass stoves and a method for its use that allow for continuous feed operation with lowest fan power with a quality of combustion associated with gasification mode of operation with very high efficiency of heat utilization. The invention also relates to the use of ceramic and insulation material in the high temperature zone for increased life without compromising of efficiency and ease of operation. The invention is primarily a portable device for domestic application.
BACKGROUND AND PRIOR ART ON CONTINUOUS BIOMASS FUELED STOVES
Continuous biomass stoves (i.e. stoves in which the fire-wood or other fuels are fed into or withdrawn fijom the stove as and when needed and the duration of operation as desired by the user) in various forms have been in domestic use for the last millennium all over the world. Yet the performance of most of these stoves has remained at low efficiencies of utilization (typically, less than 20 %) and substantial emissions of undesirable gases like carbon monoxide, soot and smoke. A large number of these continuous stoves designs are not portable.
Researchers, H S Mukunda, U Shrinivasa and S Disappear published a paper "Portable Single-pan Wood Stoves of High Efficiency for domestic use (Sadhana, v. 13, Part 4, December 1988, pp. 237 - 270) that contains the design of stoves named "Swosthee" (acronym drawn from the title as shown) that used the following principles:
• restricting the fuel port to limit the peak fuel load so that shooting tendencies at high power will be limited;
• a grate and metal enclosure insulated in one design and with an outer radiation exchange surface in another (the outer cover acted this way);

•

a height to ensure the natural draft (buoyancy effects) provided enough air flow through the port; and

• enough residence time for the gases for nearly clean combustion.
In one of the designs, for an input power of 1.5 kW, the water boiling efficiency was in excess of 45%, which had limitation of the fuel wood size and did not gain popularity. Further modification on this design led to a stove with an input power of about 4 kW and a water boiling efficiency up to 35 % at vessel diameter of 270 mm. The key limitation of this design is that depending on even minor most wind currents experienced in any kitchen, the air-to-fuel ratios would fluctuate leading to sooty combustion at times. This limitation is fundamental to domestic cook stoves all over the world because the momentum changes due to buoyancy are small and comparable to the momentum of the small wind currents in the kitchen.
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).
A stove design named "Rocket stove" credited to Larry Winooski (www.howtopedia.org/en/How to Build a Winooski Rocket Stove. www.bioenergvlists.org/en/stove design) seems to have happened in 1982 even though there is no documentary evidence for this. This stove design benefits from the use of insulation inside the vertical combustion port. It appears to have been built in numbers after 1996 and has efficiencies between 13 to 30 % depending on the materials and the peripheral geometry of the stove. This stove also depends on buoyancy to draw in the air. The air for combustion is drawn both under the grate as well over it. The performance limitations in terms of sooting tendencies would be about the same as in "Swosthee" noted above.
There are other US patents that discuss peripheral aspects of the stove or a domestic combustion device without providing attention to combustion aspects, US patent 4355587 discusses self-feeding process for wood logs below a certain size, but the combustion process improvements are not discussed.

us patent 4461275 discusses an apparatus and method for burning wood without, again addressing the combustion improvement. Both these patents address space heating applications.
US patent 2005/0183714 Al discusses wood burner with improved emissions. The description of the burner is very elaborate but neither the state of emissions from earlier burners nor those of improvements are presented in the document.
All the earlier patents and work, fail to implement the need for fuel and air to be mixed at micro-level to obtain as high a spatially uniform flame temperature as possible and it is the objective of this invention that such an important scientific principle is realized in a continuously operating portable biomass stove by converting solid fuels to gaseous fuel (i.e., the gasification process) and then burning it in a near-adiabatic environment
In recent times, it has been demonstrated that the use of small power fan and a design that leads to gasification of the solid fuel can substantially enhance the efficiency and reduce the emissions. Specifically, in a previous patent application no 1365/CEnB/2005 dated 27*'' September 2005 one of the inventions that were claimed was a continuous feed stove.
SUMMARY OF THE INVENTION
The present invention addresses the design of a continuous operation of the stove with wood or agro residue briquettes as the fuel, with high water boiling efficiency. The design is scalable to higher capacities and meeting various end user needs. The stove uses an ejector to draw the gaseous fuel from the gasification process and later the gaseous fuel is burnt. The ejector that uses the principle of pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid and then recompresses the mixed fluids by converting velocity energy back into pressure energy. The ejector is innovatively designed using commercially available fan of small power to blow the air and draw the gas, allow for mixing and later combustion.

The present inventors have found solutions to the use of wood or agro residue briquettes in a stove for continuous operation by designing the stove combustion chamber using for example, ceramic liners, and low pressure fan for the ejector in the combustion chamber which (i) allows for continuous operation of the stove to meet any duty cycle (ii) mitigates the corrosive environment and its effects, (iii) increases the fuel efficiency, (iv) provides low emissions and v) controls the power level output of the stove.
The present invention design of the stove provides fuel flexibility and operation time. In particular, the stove is designed to use wood sticks, briquettes by loading the fiiel in the fiiel port and subsequently feeding depending upon the usage, a grate at the bottom of the combustion chamber can be adjusted to accommodate fiiel for an intended purpose, an ejector to draw the gas from the fiiel storage, the combustion zone to bum the gas with the secondary air introduced and the top vessel support for the
Furthermore, an ash removal device may be incorporated in the stove, which can facilitate easy removal of ash from the stove, depending upon the usage.
According to a first aspect of the present invention, there is provided biomass stove comprising:
an outer container;
an inner combustion chamber for fiiel being housed inside said outer container; and
a fan for supplying air to the stove as a ejection air supply for drawing the fiiel gas from the fiiel zone and further combustion in the combustion chamber;
in which throat plate is chosen to ensure correct amounts of gasification and char oxidation air-flows for a given power level when combustion air just sufficient to oxidize the fuel vapors formed is supplied through the ejector a regulator for regulating the flow primary air supply and secondary air supply;
in which the ceramic combustion chamber to combust the gases is corrosion resistant& high temperature resistant, said stove further comprises:

a grate at the bottom of the combustion chamber, for supporting char discharged fi-om the fiiel chamber into the combustion chamber.
In the accompanying drawings:
Fig. 1 shows the stove described and claimed in Indian Patent application NO. 1365/CHE/2005;
Fig. 2 is a schematic diagram of the stove of the present invention;
Figs. 3A and 3B illustrates the fi-ont and top view of the stove; and
Figs 4 and 5 illustrate the ejection device for 1 or 2 kg/hour and 10 kg/hour respectively.
DETAILED DESCRIPTION:
Figure 1 shows the design of the stove that was described in the patent application no 1365/CHE/2005 dated 27th September 2005. This design uses a horizontal leg for feeding the biomass in the form of fire wood, or it could be other briquettes of sawdust or other agro-residues as well. This enables accessibility of the fiiel feed port for introducing fiiel when needed. Air was introduced in the form of fine high velocity jets (velocities of 50 to 75 m/s) at the bottom region of the vertical combustion chamber over the grate. This draws air through the horizontal bed of fiiel through an ejector action. A flame fi-ont will propagate through the solid fiiel bed much like in a gasification system when the fiiel bed is lit. This process g»ierates combustible volatiles that bum in the vertical combustion chamber after mixing with the high speed jets of air. The hot charcoal that falls on the grate will also get converted into a combustible fiiel in terms of carbon monoxide. Conditions are created in the chamber for a "mild or flameless" combustion mode to dominate thereby ensuring the lowest of emissions in terms of oxides of nitrogen.
The Present invention:
The earlier invention (1365/CHE/2005 dated 27* September 2005) of the stove described above was designed for large power level and it demanded high velocity jets

(50 to 75 m/s) to cause conditions for air ejection that is required for gasification mode of operation. To achieve this, it demands air supply pressure in excess of 6000 Pa. The present invention uses commercially available fans that are used in the industry. These industrial fans promise a pressure head of about 25 to 440 Pa under zero-flow conditions with a decreasing pressure head for increase in flow. This magnitude of pressure head will give jet velocities between 5 to 25 m/s depending upon the power level; when carefully designed. Using these devices to cause ejector action requires intricate understanding of the flow behavior in various sections in the stove (a) fi-om the fan to the air exit region and (b) flow behavior in the combustion zone as well as above it, particularly around the air injection region. These designs are illustrated by examples of 1.0, 2.0 and 10.0 kg/hr stove configurations. Designs for any power level can be built using these principles.
The stove configuration:
The invention is described with reference to the schematic diagram shown in Fig. 2. The configuration is chosen as square cross section to enable ease of manufacture. The process and performance are not limited to this geometry and can also be obtained with rectangular, circular or any other geometrical configuration. Fuel port 25 holds biomass in the form of split fuel wood sticks (or other agro-residues like cotton stalk or corncobs or other biomass in loose, pelletized or briquetted form) inserted longitudinally. The char zone 18 contains fuel wood initially but gets converted to char as the stove operation occurs. Ejector 13, which directs air vertically upwards, receives air fi-om^n 8. Pressure developed by the fan is augmented by nose cone 16 and bell mouthed entry 15 and inlet spheroid 17. Combustion air, just above the stoichiometric ratio required for complete combustion of products of gasification, supplied through ejector 13, creates a low pressure region above char zone 18. Ejector 13 blocks part of flow cross section area of combustion chamber and it creates conditions for gasification while minimizing the resistance to the flow of products of combustion. Also, air jets are distributed uniformly across the combustion chamber to effect proper mixing. The ejector induced low pressure region aids flow of gasification air in fuel port 1 and char-oxidation air through char zone 18. Buoyancy induced upward draft due to hot vertical inner walls 19 also aids these

flows. The high velocity air 5 to 25 m/s, issuing from ejector 13 also causes thorough mixing of air and fuel vapors aiding complete combustion within a short length of air jet. The size of throat plate 20 is chosen to ensure correct amounts of gasification and char oxidation air-flows for a given power level when combustion air just sufficient to oxidize the fiiel vapors formed is supplied through the ejector 13. The extent of opening 7 is allowed to be chosen to allow for sufficient air for the burning the char generated. Depending upon the extent of opening the amount of air flow varies and the extent to which the burning of char or oxidizing of the char will vary from, fiiom no oxidation to entire oxidation. By reducing the air flow that would be allowed to pass through the char bed, it is possible to reduce the oxidation of the char and retain it in the stove. Such an arrangement helps to produce char makine stove, if needed for the purpose of deriving economic benefits that char may offer or to bum away all the char to release the energy in the stove itself The velocity of combustion air through the ejector 13 affects the suction of fuel vapors and therefore ejector exit area is such that velocity is optimal to ensure near-stoichiometric air-to-fuel ratio. This creates highest possible temperature of products of combustion leading to high efficiency. The throat plate 20 receives energy from products of combustion and transmits it to the biomass stacked in the fuel port 25 to maintain pyrolysis at an appropriate rate in the feuel lport 25. The products of pyrolysis are forced through the char bed causing them to break up, in part, into simpler compounds like CO, H2 and methane by the process of gasification. The products of combustion leave the stove through exit 22. The size of exit 22 is optimized to maintain sufficient velocity of products of combustion with acceptable pressure drop. The vessel supports 23 are of optimal height to maximize heat transfer to vessel bottom and minimize emission of products of incomplete combustion. Shield 27 helps improve the utilization efBciency by a few percentage points by improving the velocity in the region below the vessel and thereby heat transfer coefficient. There is also some aid to the heat transfer by the creation of a hot wall which radiates back energy to vessel bottom. To achieve this, shield 27 is sloped suitably, typically between 6 to 10°. Insulation 21 is of very low density (70 to 150 kg/m3) ceramic wool leading to quick start up by faster creation of hot inner walls 19. The inner and outer structure of stove is made using weld mesh reducing its thermal inertia considerably. Stove is wrapped with a thin stainless steel cover 14. Ash port 28

below grate 3 facilitates ash extraction. By maintaining the use of low density insulation materials, the total inert mass of the stove is optimized, typically at about 4 kg for 1 kg/h (4 kWth) biomass burning stove. This is especially important for small power level stoves since heat loss affects the optimal performance of the stove.
Stove requires tending in the form of pushing of fuel wood to replenish oxidized char zone. In this manner the stove can be run indefinitely by loading the biomass through op&ifiiel port 1. During the ending phase of stove operation which consists of only char oxidation, fiiel port is kept closed by sliding doAvnwards the door 29, to prevent product gases firom getting diluted by induced air flow through fixel port.
While various materials were experimented for the interior insulation like light weight bricks and even lighter alumino-silicate insulation, small stoves benefit considerably fi'om light weiglht alumino-silicate insulation, the thickness of which is chosen to limit the outer wall temperatures to less than 100 °C. Further optimization is limited by the consideration that achieving low start up time and higher efficiency would be helped by smaller thermal mass and smaller thickness. The fuels to be used in these stoves are sticks of wood or other agro-residues like cotton stock or corncobs in loose or briquetted form. It is not limited to these only. Each stove can use any of the mentioned fiiels with an approximate size of 10 to 20 mm (1 kg/hr rating stove), 10 to 30 nmi (2 kg/hr stove) and 20 to 60 mm (10 kg/hr stove). In the case of 2 and 10 kg/hr stoves, the sticks used are of about the same size range as the biomass normally used in rural kitchens. The smaller power stove (1 kg/hr capacity) needs firewood to be split to appropriate sizes. A mix of sizes is considered desirable. Smaller size sticks help faster ignition and larger size sticks help maintain the power level. Starting the stove is straightforward. Small chippings of biomass are heaped on the biomass over the grate (See Fig. 2). A few wood chips doused in kerosene or fiiel oil spread on the top and ignited will start of the fire that takes 5 to 6 minutes to stabilize.
Turn-down Ratio:
Control of the thermal power of the stove, a very desirable feature expected from stoves, is achieved simply by regulating fiiel feed and also by reducing the fan power supply

voltage. In actual operations, the freedom will imply that precise operations to reduce the power and limit the soot emission may not occur always.
To ensure that the during the end of the stove operation, the wood sticks in the fuel port would continue to pyrolyse and reach the entrance zone which might lead to some smoke oozing out from the fuel port. This can be controlled by partially closing the fuel port using 29, or pushing the small amount of fuel towards the grate or withdrawing the smoking sticks from the fuel port do not pollute the kitchen environment, they can be plunged into a sand bed to ensure that the smoldering front end is extinguished. Such sticks can of course be used again in the stove. A recommended practice during such operations is to reduce the opening of the fuel entry zone to prevent back flow of the volatiles constituting the smoke. Any additional power demanded at this time can be met with by the addition of fuel. By bringing the fuel pieces into contact with the throat plate 20 one can hasten the change in power level required. The minimum power level that can be achieved through this method is about a third to fourth of the nominal power depending on the stove management sfrategy.
Examples:
The configuration described above can be optimized for various power levels demanded by domestic requirements, community kitchens or industries utilizing thermal energy for production purposes in addition to cooking tasks of various quantities. Also, the configuration can also work when the flow in the combustion chamber is along the horizontal axis provided higher jet velocity is provided to substitute for buoyancy induced suction due to hot vertical inner walls of the stove. Designs for three power level operations namely, 1 kg/hour, 2 kg/hour and 10 kg/hour are exemplified in the subsequent discussion. The principle dimensions of the stove body are shown in Fig. 3 in the form of symbols A, B, C etc.
The values of symbols for the stove body three different nominal power level operations are provided in Table. 1
Table 1: Dimensions of the stove body

Symbol Description

Dimensions, mm

1 kg/hr 2 kg/hr 10 kg/hr

B

Fuel port height
Un insulated length of fuel port

75
75

100
100

200
300

Insulation thickness

30

60

75

75
70
70
170
15
85
D Insulated length of fuel port
E Height of combustion zone
F Inner size of cover plate
G Outer size of cover plate
Q Height of vessel support
H Distance of air inlet from stove top
I Opening for air duct

100
200
70
200
90
150
300
350
20
25
140
300
45x45 45x45 45x250

J Distance of air inlet from stove bottom
K Square grate size
L Height of restrictive plate at fuel port throat
M Ash port height
N Height of stove leg
O Fuel port width
P Ash tray width

115 190 260
100 150 200
35 50 120
50 60 75
10 25 40
100 150 200
95 140 180

Figure 4 and Table 2 provide dimensional details of ejection device used for stoves with nominal power levels of 1 kg/hour and 2 kg/hour operation. Figure 5 provides dimensional details of ejector device for 10 kg/hour system. All these designs have a power flux throughput of 0.5 to 1 MW/m2, but can be extended to 2.0 MW/m2 making the designs very compact (the reference area used here is the area of cross section of the combustion zone). The power flux is limited for domestic applications by the need to limit the particulate emissions without elaborate appendages.
Table 2: Dimensional details of ejection device for 1 & 2 kg/hour systems



The specifications of the fans can be taken to be indicative and fans with slightly different specifications can be used to design the air flow passages appropriately.
Efficiency of operation:
Water boiling experiments were conducted on Ejector Induced GAsifer- Stove-n kg/hour (EIGAS-n) for n=l and n=2.
Procedure: Tests are conducted using three aluminum cylindrical vessels as below.
320 mm diameter and (1.2 kg vessel weight including lid and stirrer)
265 mm diameter (0.82 kg vessel weight including lid and stirrer)
230 mm diameter (0.54 kg vessel weight including lid and stirrer)
600 mm diameter (15.8 kg vessel weight including lid and stirrer)
A 470 mm maximum diameter conical vessel with nearly flat bottom with 35 1 volume (1.88 kg weight including lid and stirrer is also used)
The biomass used is about 9 to 24 mm nominal size sticks about 300 mm long obtained from splitting casurina poles. 30 g of kindling material and 2 ml of kerosene is used to ignite the sticks and vessel is placed on the stove in the first minute after sticks are ignited. All the biomass used is sundry at a moisture fraction of 9 ± 1 %.

EIGAS-1 has 100 mm fuel port width, 40 mm fuel port height and 63 x 2 mm width ejector area.
EIGAS-1 (reduced fuel and air areas) has 70 mm fuel port width, 40 mm fuel port height and 55 X 2 mm width ejector area.
The test procedure consists of two phases.
Phase 1: Water is heated from ambient temperature to around 85 °C and un-charred biomass is extracted from the stove.
Phase 2: The char oxidation is completed with a second vessel (with ambient temperature water) placed on the stove. During this period fuel port is kept closed. Experiment is stopped when water temperature rise is not observed in two consecutive minutes.
Biomass used with mean moisture of 9 % moisture and 1% ash with lower energy content of l6 MJ/kg.
Efficiency includes accounting for water evaporated (AH Evaporation = 2235 kJ/kg) and energy absorbed by the Aluminum vessel (specific heat of Alimiinum is taken as 0.9 kJ/kg.K) The results are as follows: The efficiency values depend on the vessel size as already discussed in the earlier patent 1365/CHE/2005 dated 27 th September 2005. These values, shown in Tables 4 - 6 are the highest recorded for continuous stoves when tested under standard conditions.

Char making operation of the stove
The operation as indicated above is carried out by closing the bottom ash extraction chamber preventing air ingress from through it. At the end of the operation the amount of char left behind amounts to 20 to 24 % of the initial biomass used in the fuel port. This can deliver the heat to the vessel as well as leave behind the char. Such char can be collected as soon the stove operation is completed and can be doused in a vessel containing sand. This operation will enable extraction of dry char that becomes cold and is not further oxidized by the ambient air.
Emissions from the stove.
The emissions of Carbon monoxide (CO) and Particulate matter (PM) were measured in a standard hood apparatus for a typical operational period of one hour and averaged over this period. The results showed 1.1 g/MJ of CO and 0.06g/MJ of PM for the domestic stove. These values are lower than most wood stoves whose emissions vary between 1 to 10 g/MJ of CO and 0.2 to 10. g/MJ of PM.

WE CLAIM:
1) A biomass stove comprising:
an outer container;
an inner combustion chamber for fuel being housed inside said outer container;
a fan for supplying air through an ejector that draws the fuel gas from the fuel zone for further combustion in the combustion chamber;
and
a throat plate to ensure correct amounts of gasification and char oxidation air-flows for a given power level when combustion air just sufficient to oxidize the fuel vapors formed is supplied through the ejector a regulator for regulating the flow primary air supply and secondary air supply;
the combustion chamber is corrosion resistant & high temperature resistant to combust the gases, said stove further comprises:
a grate at the bottom of the combustion chamber, for supporting char discharged from the fuel chamber into the combustion chamber.
2) A stove as claimed in claim 1, wherein the combustion chamber is being made of either a ceramic material or has a lining, which is made of ceramic material or other suitable insulating material.
3) A stove as claimed in claim 1, wherein a very low pressure fan is used to create a high velocity jet which sustains the stove operation in gasification mode.
4) A stove as claimed in any one of claims 1 to 3 wherein the fan pressure being augmented using an aerodynamically designed nose cone along with a bell mouth entry and inlet spheroid.

5) A stove as claimed in 1 to 3 with the turn-down ratio (the ratio of peak to low power) that can be made as large as a factor of 3.5 by choosing to reduce the fuel load in the fuel port after the system has been started and is running for a stabilization period, often minutes.
6) A stove as claimed in claim 1 to 5 wherein the range of fuels used include firewood such as coconut shell, corncobs or any agro-residue either in loose form or pelletized or briquetted form.
7) A stove as claimed in 1 to 6 is wherein the rating may vary between 1 to 100 kg/hr and above for various thermal applications including cooking.
8) A set of designs of the stoves biomass stove wherein the power flux can be varied between 0.5 to 2.0 MW/m2.
9) A method for operating a biomass stove continuously as claimed in any one of the preceding claims which comprises:
introducing biomass fuel into the fuel chamber in which can accommodate the wood sticks or briquetted fuel;
igniting the fuel near the grate in the combustion chamber;
introducing ejection air supply in the combustion chamber to support drawing the fuel vapour firom the fuel zone and allowing combustion to take place towards the top of combustion chamber; and
the ejection air apart from drawing the gas for combustion, controls the gasification process and char combustion process for complete conversion.

10) Method as claimed in claim 9, in which the biomass fuel is selected from the
group consisting of wood sticks, or agro residue briquettes and combinations
thereof.
11) A method as claimed in any one of claims 9 and 10, in which the stove being
operated for long duration hours continuously and has a control on the power
level depending upon the requirement.
12) A method as claimed in any one of claims 9 to 11, in which the total flux in the
combustion chamber is in the range of 0.5 to 2.0 MW/m^ depending upon the end
use.
13) A biomass stove as herein substantially described and illustrated with reference
to the accompanying figures.
14) A method as herein substantially described and illustrated with reference to the
accompanying figures.