Patent Publication Number: US-2015083108-A1

Title: Cook stove assembly

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/756,386, filed Apr. 8, 2010, which claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/168,538, filed Apr. 10, 2009, which are hereby incorporated by reference in their entirety. The present application is related to U.S. Provisional Application No. 61/261,694, filed Nov. 16, 2009. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has rights in this invention pursuant to contract number DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to stoves and cooking apparatus for use in confined areas. 
     BACKGROUND 
     In many parts of the world, heating and cooking are performed using combustible biomaterial as a fuel source. Combustion with this type of fuel often is incomplete leading to production of poisonous gases, especially carbon monoxide. Within a living or enclosed space, use of biomaterials carbon monoxide may build-up causing sickness or death. 
     Carbon monoxide (CO) is a colorless, odorless, tasteless toxic gas produced by incomplete combustion in fuel-burning. CO poisoning may result in headaches, nausea, dizziness, or confusion. Left undetected, CO exposure can be fatal, and in the United States alone, accidental CO poisoning results in about 15,000 ER visits a year. 
     Because carbon monoxide is a byproduct of incomplete combustion, procedures that enhance combustion will reduce the production of carbon monoxide. Those of skill in the art will understand that enhancing combustion may generally be accomplished in three ways—by increasing the duration of combustion, raising the temperature at which combustion takes place, or optimizing the mixing of oxygen and fuel. Another contributor to incomplete combustion may be the presence of a heat sink that may quench combustion. In general, it is easier to control these factors when a gaseous fuel is burned as opposed to a solid fuel. Thus, in developed countries, solid fuel has been largely replaced by gaseous fuels for household use. But, as is evident from the carbon monoxide poisoning statistics presented above, even in the United States, improperly maintained natural gas or propane burners may produce significant amounts of carbon monoxide. 
     Carbon monoxide may be produced by combustion even under controlled conditions using modern appliances. For this reason, modern must be carefully engineered to properly mix air with gas and modern appliances are generally vented to allow exhaust to be directed out of the house. In contrast, in other countries, it is not uncommon for households to employ unvented, solid fuel biomass stoves for heating and cooking. Use of biomass creates a significant risk if the stove is used within the living quarters or an enclosed space. 
     Outside the United States, the predominant combustible material for household energy production come from solid fuels such as biomaterial (for example, without wishing to be limited, pelletized or compressed waste or wood, wood chips, coal, dung or other organic materials such as twigs, grasses, or rice husks). For example, it is estimated that over 70% of African households and 80% of Chinese households burn solid fuels for domestic energy needs. As described above, when solid fuel, especially wood, is burned in confined or poorly ventilated spaces, carbon monoxide levels may build to dangerous levels. It has been estimated that between 1.5 and 2 million people die each year as a result of exposure to indoor air pollution resulting from the use of solid fuels. 
     Poverty is one of the largest contributing factors to the use of solid biomaterial as a fuel source. For example, studies have shown that per capita gross national product (GNP) is closely correlated with dependence on biomass: countries with lower per capita GNP tend to rely on traditional fuel sources far more than countries with higher GNP. Thus, any solution to the problem of indoor air pollution from the combustion of solid fuels must be both cost effective and must not dramatically impact traditional behavior. 
     Various stove designs are available that may lessen the risk of using biomass for heating or cooking indoors. These stoves attempt to increase stove efficiency, and thus decrease pollution. Some stoves may be constructed of traditional materials such as brick, stone, or ceramics. Other stoves may be constructed of metal. Some stoves are designed to be constructed with either traditional or modern materials, such as, for example without limitation, “rocket” stoves. Rocket stoves employ an “L” design to control the combustion of fuel and mixing of air. In many rocket stoves, fuel, for example twigs, is slowly introduced to the combustion chamber at the bottom of the L. This slow addition of fuel helps to limit the rate of combustion by confining burning to the tips of the sticks. Rocket stove design may include insulation of the chimney to decrease quenching of combustion by cooler surfaces. Some stoves may be designed with a constant radius for both the upper and lower combustion chamber. While rocket stoves may be designed to control air flow passively, other stove designs use electric fans to force air through. 
     Gasification stove design may rely on passive air flow but more often employs forced air from electric fans to increase stove efficiency. Gasification stoves, (variously known as fan-stoves, semi-gasification stoves, etc.) offer an alternative to traditional stove designs. Gasification stoves replace direct combustion of biomass fuel with techniques that release volatile gases, which are then ignited separately. Gasification is a process that converts carbon containing materials, such as, for example without limitation, coal, petroleum, biomaterial, or biomass, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas mixture is itself a fuel and can be combusted. This process may reduce pollution by reducing incomplete combustion and the amount of material needed to fuel the stove. 
     Gasification techniques are potentially more efficient than direct combustion of the original fuel because it can be combusted at higher temperatures. In addition, the high-temperature combustion may refine out more corrosive elements such as chloride and potassium, allowing relatively cleaner combustion in some cases as well as higher efficiency. However, gasification stoves may be more difficult to construct than some other types of stoves, and therefore more expensive to produce. 
     To reduce costs of a solid fuel stove for household use and make it accessible to low income persons, requires that the materials used in its construction be inexpensive and that the manufacturing process be efficient and low cost. This is difficult because the combustion environment associated with the use of solid fuels is extreme, both in temperature and corrosiveness. Among other compounds, combustion of biomass produces highly corrosive nitrogen and sulfur compounds. 
     The combustion environment found in biomass stoves is unsuitable for most low-cost metals, therefore many stoves are constructed of ceramics, brick, or rock. The use of ceramic, brick, and rock, while reducing the cost of manufacture, may dramatically increase the cost of producing and distributing these stoves, decrease their durability, portability, limit combustion chamber geometry and may otherwise be undesirable. 
     Thus what is needed is a stove that is acceptable and accessible to persons with limited income, such as a stove that lessens the amount of toxic emissions, and may be produced from lightweight, inexpensive, corrosion-resistant materials, and that may be inexpensively and efficiently manufactured. 
     SUMMARY OF THE INVENTION 
     Many manufactured stoves, designed for use with solid fuels, are not specifically designed to lessen production of dangerous combustion products. Those manufactured stoves that do address indoor pollution are generally not ideal, either because they rely on drastic changes in traditional behavior (such as limiting use of solid fuels, moving the stoves out of doors, or depending on expensive or impractical venting), or they are financially out of reach for those with modest incomes. A cooking/heating alternative that is compatible with traditional behavior, inexpensive, and capable of lessening production of dangerous gases, may help prevent death and disease especially among persons of limited income. 
     A stove design is provided that reduces the amount of, at least, carbon monoxide gas emitted from burning a solid fuel energy source, especially biomass. The stove design may be used in either heating or cooking stoves. The inventive design comprises a combustion chamber with two parts, a first, lower combustion chamber and a second, upper combustion chamber. The lower combustion chamber may be configured to receive a solid biomass fuel. The upper combustion chamber may contain an annular constriction positioned within the second, generally cylindrical, upper combustion chamber. The constriction is designed to aid in completely combusting combustion gases as they travel through the upper combustion chamber by slowing the exit of incompletely combusted gases, re-directing uncombusted gases toward the center of the upper combustion chamber and into a flame, and by creating a hot surface that promotes combustion. In various embodiments, the constriction may comprise an orifice ring. 
     In many embodiments, the inventive design of the lower combustion chamber is a variety of shapes such as cylindrical, or pie shaped depending on the type of fuel used and the stove&#39;s intended purpose. A fuel grate or grill may be positioned within the lower combustion chamber to receive solid fuel. Solid fuel may be positioned, ignited, and partially or fully consumed within the lower combustion chamber. Flames and gases may be further consumed within the upper part and the resulting heat and exhaust gases directed out of the upper combustion chamber and toward a cooking vessel. 
     In various embodiments, constricting the flow of flames and gasses in the upper combustion chamber with an orifice ring, redirects partially combusted or uncombusted gases, such as for example, carbon monoxide, away from the wall of the upper part of the combustion chamber, back toward the center and into the flame where it may be consumed. The orifice ring may also create turbulence above the ring, so that gases near the wall of the upper combustion chamber remain in the upper combustion chamber longer, increasing the likelihood that they may be consumed before exiting the combustion chamber. The orifice ring may be positioned throughout the upper combustion chamber and more than one orifice ring may be positioned within the upper combustion chamber. In constricting the exhaust flow, redirecting it into the flame, and delaying its exit from the upper part of the combustion chamber, the orifice helps to reduce the amount of incompletely combusted gases produced. Thus, the inventive structure may help reduce the amount of at least carbon monoxide produced during heating or cooking by enhancing combustion of uncombusted, partially combusted, and dangerous gases within the upper part of the combustion chamber, reducing fuel use, and increasing energy efficiency 
     As described here, placement of the constriction or orifice ring within the upper combustion chamber helps to redirect and retard incompletely combusted gases, so that they might complete combustion before exiting the upper part of the combustion chamber. Solid fuel, especially biomaterial, is placed in the lower part combustion chamber and ignited. As the fuel burns, exhaust (comprising combustion gases, entrained air, particulate matter, as well as incompletely combusted gases) is formed and enters the upper part of the combustion chamber. The exhaust at the center of the second part continues to undergo combustion as it traverses the upper part of the combustion chamber, but combustion may be quenched near the walls of the upper combustion chamber leading to buildup of incompletely and uncombusted gases. However, these gases may be redirected into the hotter center of the upper part of the combustion chamber, or the flame, by the orifice ring and therefore may complete combustion. 
     Incompletely combusted gases that get by the orifice plate without being consumed have another chance to undergo combustion as their progress through the upper combustion chamber is retarded by the turbulence and/or recirculation produced above the constriction or orifice ring. To further reduce quenching by the upper part of the combustion chamber, the presently disclosed combustion chamber may be insulated by addition of various materials. For example without limitation, in some embodiments the insulating material may be stone, dirt, sand, clay, quarried materials, or a mixture thereof. In some embodiments, the quarried material may be, for example without limitation, perlite or vermiculite. 
     As the exhaust exits the upper combustion chamber it may be used to heat a cooking vessel placed atop a stove cooktop which may be in communication with the combustion chamber outlet. Because many households throughout the world use solid fuel in cooking and heating, even in confined spaces, the inventive device will help to lower, at least, levels of carbon monoxide and thus lessen the chances of death and disease resulting from carbon monoxide poisoning. 
     One of the many applications of the inventive structure is as part of an inexpensive, portable stove. When used in such a stove application the inventive structure may help reduce carbon monoxide production by as much as 60%. 
     In accordance with another embodiment a multi-burner cooktop is provided. The inventive stove may include a cooktop that sits atop the stove and directs exhaust from the combustion chamber to more than one opening such that multiple cooking vessels may be warmed at once. This inventive cooktop is designed to partially fit into the combustion chamber outlet and redirect heated exhaust through an exhaust chamber in communication with the two openings at the cooktop. The inventive cooktop may also have a third opening designed to allow exhaust gases to exit the exhaust chamber. In some embodiments, the third opening may be designed with a collar to receive an exhaust stovepipe or vent. 
     In accordance with another embodiment, a metal alloy for use in the manufacture of a corrosion resistant combustion chamber for a stove is provided. The inventive combustion chamber lessens the cost of producing the stove and increases its durability in the extreme conditions found in biomass fuel consumption. The corrosion-resistant alloy is low cost compared to other corrosion resistant metals. Further, unlike corrosion resistant ceramic materials, the alloy reduces the weight of a stove manufactured with the alloy and therefore the cost of producing and shipping the stove. The alloy can be used in a wide range of heating and cooking stoves. For example, without limitation, the alloy may be used to produce rocket stoves, fan stoves, gasification stoves, coal stoves, and charcoal stoves. 
     in various embodiments, the metal alloy, includes iron (Fe), chromium (Cr), aluminum (Al). The alloy may be referred to as FeCrAl, and may also include other elements such as carbon and titanium. While FeCrAl is well known in the art as a metal alloy for use generally in non-structural applications such as wires or heating elements. FeCrAl has not been used in the construction of stoves, because it dramatically loses tensile strength at elevated temperatures. Rather, FeCrAl is often chosen for applications based on its superior electrical resistivity. Other characteristics of FeCrAl, such as for example, weldability, may be similar to other iron containing metals. 
     In the presently disclosed stove, FeCrAl is used to clad, line, or form the combustion chamber. However, FeCrAl may be used to clad, line, or form the combustion chamber of other types of stoves including, without limitation, rocket stoves, fan stoves, gasification stoves, semi-gasification stoves, coal stoves, and charcoal stoves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front perspective view of a stove. 
         FIG. 2  is a transparent perspective view of the stove liner including the combustion chamber and orifice ring. 
         FIG. 3  shows an alternative embodiment of the stove wherein the constriction in the upper combustion chamber is formed by an annular ridge in the wall. 
         FIG. 4  shows alternative positions of the orifice ring within the upper combustion chamber. 
         FIG. 5  depicts the current embodiment of the orifice plate. 
         FIG. 6  is an alternative embodiment of the orifice ring. 
         FIGS. 7A and 7B  show alternative embodiments of the orifice ring. 
         FIG. 8  is a sectional view of a stove in use with fuel being burned in the combustion chamber. 
         FIG. 9A  shows an alternative embodiment of the stove where a dual burner cooktop installed, also shown is an external grate to aid in supporting twigs, branches, and other long pieces of biomaterial. 
         FIG. 9B  section taken through line  9 B- 9 B of  FIG. 9A , and shows the flow path of the combustion materials through the dual burner cooktop. 
         FIG. 10  is a detailed exploded view of the dual burner cooktop. 
         FIG. 11  shows an alternative stove embodiment that may employ the inventive disclosures. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a stove  100  for use with the currently disclosed invention. The stove  100  may comprise a housing  200  and a cooktop  300 . The cooktop  300  may be seated atop the housing  200  and provide a structures  310  and a surface  320  for positioning and supporting a cooking vessel (not shown). The housing  200  may further comprise an external shell  210 , a mouth  220 , and an internal liner  400 . As seen in  FIG. 1 , the external shell  210  may include handles  230  to aid in grasping and/or transporting the stove. The external shell  210  may further define a lower base  240  portion and a bottom  250 . The bottom  250  may be designed to contact the ground or other support. In some embodiments the mouth  220  may be positioned near the lower base  240  of the shell  210 . In some embodiments, the mouth  220  opening may be covered by a door (not shown). 
     The internal liner  400  may further define a combustion chamber  500 , with openings at the mouth  220  and the surface  320  of the cooktop  300 . Positioned within the combustion chamber  500 , and visible through the mouth  220 , may be a grate  570 . The grate  570  may sit atop a slab  540 , which is positioned within the combustion chamber  500  and also visible through the mouth  220 . 
       FIG. 2  shows the stove  100  in sectional view. The combustion chamber  500  within the stove  100  is formed by the liner  400 . The liner may include an exterior surface  410  and an interior surface  420 . The interior surface  420  of the liner  400  defines the combustion chamber  500  with an inlet  510  and an outlet  520 . The inlet  510  is defined by the mouth  220 . The mouth  220  may be closed by a door (not shown), that may reduce access to the combustion chamber  500 . The combustion chamber outlet  510  defines an opening through the cooktop  300 . The exterior surface  410  of the liner  400  and the housing  200  may define a cavity  450 . The cavity  450  may be partially or fully filled with insulation material, such as without limitation, sand, stone, dirt, clay, quarried materials, or a mixture thereof. In some embodiments, the quarried material may be, for example without limitation, perlite, or vermiculite. 
     The combustion chamber  500  may further define a lower combustion chamber  550  and an upper combustion chamber  560 . The lower combustion chamber  550  may include a floor  535 , a ceiling  525 , and a sidewall  530 . The sidewall  530 , floor  535  and ceiling  525  may define the mouth  220  opening in the shell  210 . The sidewall  530  may be comprised of a single piece and have a generally pie-shape. In the embodiment shown in  FIGS. 1 and 2 , the lower combustion chamber  550  comprises a wider opening at the mouth  220  that tapers away from the mouth  220  to the boundary  555  with the upper combustion chamber  560 . In various embodiments, the pie-shaped lower combustion chamber  550  may help to provide room for the solid fuel and may also provide for adequate mixing of air. In other embodiments the lower combustion chamber  550  may take various shapes. In other embodiments, the sidewall  530  may define a plurality of wall structures. The floor  535  of the lower combustion chamber  550  may rest upon and be supported by the bottom  250  of the shell  210 . In  FIGS. 2 and 3 , a slab  540  is visible that is supported by the combustion chamber floor  535 . The slab  540  may be clay, stone, or some other suitable material, and it may be shaped to match the shape of the floor  535  of the combustion chamber  500 . 
     The grate  570  may be placed within the lower combustion chamber  550  and be supported by the slab  540 . The grate  570  may be used to support solid fuel and may be constructed of, for example without limitation, mild steel. Solid fuel may include for example without limitation, coal, wood, charcoal, dung, leaves, grasses, pellets, wood chips, compressed or uncompressed biowaste, or other biomass material. A second, exterior grate  575 , shown in  FIG. 2 , may be positioned outside the stove  100  in front of the mouth  220 . The exterior grate  575  may be supported by the ground, and define a surface generally planar with the surface defined by the grate  570  positioned within the lower combustion chamber  550 . The exterior grate  575  may be designed to support sticks, twigs, or other long fuel that extends from within the lower combustion chamber  550  outside the mouth  220 . Other embodiments may have a plurality of mouths, formed as apertures that may or may not be large enough to accommodate fuel and may be predominantly designed to allow air to enter into the combustion chamber  500 . This is described in detail below with respect to gasification stove embodiments (See  FIG. 11 ). 
     The second, upper combustion chamber  560  may be positioned generally above the lower combustion chamber  550 . The lower combustion chamber  550  and the upper combustion chamber  560  are in communication at a boundary  555 . The upper combustion chamber  560  may be generally cylindrical. The upper combustion chamber  560  may be in communication with the cooktop  300  at the combustion chamber outlet  520 . In some embodiments, both the upper and lower combustion chamber may be generally cylindrical. In other embodiments as depicted in  FIG. 1 , the upper combustion chamber  560  may differ in shape from the lower combustion chamber  550 . 
     As described in more detail below, alternative embodiments of the stove may lack a mouth, and the upper and lower combustion chambers may both be generally cylindrical. In these alternative embodiments, the boundary between the upper and lower combustion chambers may lack obvious delineation. 
     The presently disclosed combustion chamber  500  may be constructed of several sections, parts, or pieces. As depicted in  FIG. 2  (and in greater detail in  FIGS. 3 ,  4  and  8 , below) the section defining the upper combustion chamber  560  sits atop a section(s) that defines the lower combustion chamber  550 . Here, the upper combustion chamber  560  piece extends, at least in part, behind and below the top edge of the sidewall  530  of the lower combustion chamber  550 . Here also, the ceiling of the lower combustion chamber  550  may be a separate section from the section that defines the sidewall(s)  530  of the lower combustion chamber  550 . The ceiling of the lower combustion chamber  550 , like the upper combustion chamber  560  piece, extends behind and below the top edge of the wall  530  of the lower combustion chamber  550 . The upper combustion chamber  560  piece is supported by at least part of the ceiling of the lower combustion chamber  550 . The sections or parts may be held together by corresponding tabs and slots, or may be spot welded. In various embodiments different methods are employed to connect the sections, parts, or pieces. In further embodiments as previously discussed, the combustion chamber  500  may be comprised of a single contiguous piece. 
     Also depicted in  FIG. 2 , the upper combustion chamber  560  may define a plurality of generally annular constrictions  600  that may reduce the cross-sectional area of the upper combustion chamber  560 . The constriction  600  may define an annular ridge  610  within the interior of the upper combustion chamber  560  that defines a diameter, d c , or the constriction diameter, which reduces the interior diameter, D i , of the combustion chamber  500 . 
     In the present embodiment, as shown in  FIG. 2 , the constriction  600  may support an orifice ring  650 . The orifice ring  650  may also be referred to as a plate. The orifice ring  650  may define an inner diameter, d o . In some embodiments, as shown in  FIG. 3 , the annular ridge  610  defining the constriction  600  of the upper combustion chamber  560  is much greater, and may alone define the constriction diameter, d c , similar to that defined by the orifice ring, d o , obviating the need for an orifice ring  650 . In various embodiments, the constriction  600  of the upper combustion chamber  560  defines an annular ridge  610  with a generally flat or planar upper surface and a generally flat or planar lower surface. 
       FIGS. 2 and 3  show a constriction  600  or orifice ring  650  positioned at or near the middle region of the upper combustion chamber  560 . In other embodiments, constrictions  600  or orifice rings  650  may be positioned other than the middle. Referring now to  FIG. 3 , the middle of the upper combustion chamber is designated “X,” the lower portion, “Y”, and the upper portion, “Z.” In other embodiments, as depicted in  FIG. 4 , multiple constrictions  600  may be positioned throughout the upper combustion chamber  560 . In other embodiments, as shown in  FIG. 4 , an orifice ring  650  may be positioned proximal the boundary  555  and lower combustion chamber  550  (marked as region “Y”), and/or at or near the top of the upper combustion chamber  560  (marked as region “Z), proximal the combustion chamber outlet  520 . 
     Various embodiments may include multiple constrictions  600 , orifice rings  650 , or combinations thereof positioned within the upper combustion chamber  560 . In some embodiments the multiple constrictions  600  and/or orifice rings  650  may define the same d o , in other embodiments the d o s may be different. In still further embodiments the orifice rings may be arranged to define converging or diverging nozzles. In still further embodiments it may be possible to alter the d o  or the size of constrictions  600  or orifice rings  650  to affect damping or control vortex formation. 
     In various embodiments with multiple constrictions  600 , the multiple constrictions  600  may define multiple constriction diameters, d c . In further embodiments with multiple constrictions  600 , constrictions  600  and orifice rings  650  maybe combined. In some embodiments the upper combustion chamber may be other than cylindrical, for example, without limitation, the upper combustion chamber may be square. 
     In various embodiments of the orifice ring  650 , the ring or plate may be removable or replaceable within the upper combustion chamber  560 . Replacement of orifice rings  650  may be aided by use of, for example without limitation, snap-fittings, press-fittings, and friction fittings.  FIGS. 2 and 4  depict orifice rings  650  held in place, at least in part by a plurality of protrusions  655  extending radially inward from the wall of the upper combustion chamber  560 . The protrusions  655  depicted in  FIGS. 2 and 4  are positioned above the orifice ring  650  or plate to aid in holding it in place against the annular ridge  610  positioned below the orifice ring  650 . In other embodiments the protrusions  655  may be positioned below the orifice ring  650  or plate and the annular ridge  610  may be positioned above the orifice plate  650 . In further embodiments, the orifice ring  650  may be supported below and above by protrusions  655 , or by annular ridges  610 . In various other embodiments, the orifice ring  650  may be welded in place and otherwise not easily removable. 
     Experiments have shown that an orifice ring positioned within the upper combustion chamber will reduce CO by a significant amount, such as by 25% in some instances, and by 70% in other instances. Depending on the design of the combustion chamber and the fuel used, the results may vary. 
       FIG. 5  shows the orifice ring  650  alone. The orifice ring  650  may define a generally flat annular ring that includes an inner (central) edge  660  and an outer (peripheral) edge  670 . The diameter of the outer edge  670  of the orifice ring  650 , depicted by the letter “D o ,” approximates the internal diameter of the upper combustion chamber  560  such that the orifice plate  650  may contact and/or fit close against the interior of the combustion chamber  500 . The inner edge  660  defines the orifice diameter denoted by “d o ,” and may further define an edge that is concentric and co-planar to the outer edge  670 . The inner edge  660  and the outer edge  670  thus define a flat, un-broken annular surface that is of generally constant width. One present embodiment of the orifice ring  650  has a ratio of d o /D o  0.75, while in other embodiments different ratios may also be used. In one present embodiment the thickness, T o , of the orifice ring  650  is 0.5 mm. In other embodiments the orifice ring  650  may be thicker or thinner. 
     The orifice ring  650  may be designed to slow heat transfer from the inner edge of the orifice ring  650  to the outer edge  670  of the orifice ring  650 . In various embodiments of the orifice ring  650 , as shown in  FIG. 6 , the ring may include a plurality of discontinuous circumferential slots  680  in the plate surface between the inner and outer edges  670 . The slots  680  may be designed to retard heat conduction/transfer from the inner edge  660  of the orifice ring  650  to the outer edge  670 . The circumferential slots  680  may, or may not, be formed all the way through the orifice ring  650 , for example the slots  680  may be defined by indentations of the orifice ring  650  where the thickness is generally less than the thickness of the plate in other areas. The slots  680  as shown here, are nearer the outer edge  670  than the inner edge  660 , however placement of the slots  680  may vary, and slots  680  on the same orifice ring  650  may also be placed in different locations on the ring surface. 
       FIGS. 7A and 7B  show further embodiments of the orifice plate  650 . In some embodiments, as depicted in  FIG. 7A , the orifice plates  650  have an inner edge  660  that is not concentric with the outer edge  670 , and where the width of the plate surface is not generally constant. Embodiments such as that shown in  FIG. 7A  may aid in redirecting the flow of exhaust, for example without limitation, from the edge of the upper combustion chamber  560  into the center. As shown in  FIG. 7B , other embodiments of the orifice plate  650  may be frustoconically shaped, and may include an inner edge  660  that defines a plurality of separated tabs  690  that extend radially inward. In still further embodiments of the orifice plate  650  the inner edge  660  may be serrated or discontinuous. Embodiments of the orifice plate  650  may also include edges that are not co-planar, for example, the inner ring may be positioned generally above the plane defined by the outer edge  670 , or the inner ring may define a plane that is below the plane of the outer edge  670 , or the plane defined by one edge may intersect the plane defined by the other edge. Additional embodiments may include an orifice plate  650  where either or both edges, do not define a single plane, or a plane at all, but may define a wave-like structure. In other embodiments the orifice ring  650  may include edges that are both discontinuous, for example without limitation, where the orifice plate  650  may define a spiral or corkscrew shape. Further orifice ring embodiments may have a combination of characteristics, for example without limitation, the outer edge may be continuous while the inner edge is discontinuous and defines a non-planar corkscrew shape. 
       FIG. 8  shows the stove  100  with fuel  101  burning in the combustion chamber  500 . In use, a fire built in the lower combustion chamber  550  may draw air (small unfilled arrows) into the combustion chamber  500  through the combustion chamber inlet  510  at the mouth  220 . In some embodiments a door (not shown) may be positioned to reduce the area of the mouth  220  in order to regulate the amount of air entering the combustion chamber  500 . Within the lower combustion chamber  550  the air may mix with gasses from the fuel  101  to promote combustion. In other embodiments the fuel may be heated to release gasses, as in gasification stoves. Exhaust from gasification or direct combustion (shown with solid arrows labeled “A,” and comprising entrained air, uncombusted gases, incompletely combusted gases, combusted gases, and particulate matter) rises up through the lower combustion chamber  550  into the upper combustion chamber  560 . Within the upper combustion chamber  560 , combustion may be quenched by the lower temperatures proximal the wall of the upper part. The gases from the quenched combustion, containing uncombusted and incompletely combusted gases such as carbon monoxide, continue to rise up through the upper combustion chamber  560  (depicted by arrows labeled “B”). When these incompletely or uncombusted gases reach the orifice ring  650  they are redirected toward the center of the flames and may then undergo combustion in the higher temperature or by passing through or into the flame (depicted by arrows labeled “C”). Additionally, incompletely combusted gases that are not consumed by the fire may linger in the upper combustion chamber  560  by the action of the turbulence front set up by and above the orifice ring  650  (depicted by arrows labeled “D”). Recirculation of the incompletely combusted gases in the region above the orifice ring  650 , depicted by arrow “D,” creates more opportunity for uncombusted gases to be combusted by the high temperatures or by passing into the flames. Finally, exhaust (depicted as single large unfilled arrow) exits the combustion chamber  500  at the combustion chamber outlet  520  defined by the opening in the cooktop  300 . After exiting the combustion chamber outlet  520 , it may be directed at a cooking vessel positioned above the outlet  520 . 
     In some applications, as explained above, the upper combustion chamber  560  may act as a heat sink and act to at least partially quench combustion of gases near the interior wall of the combustion chamber  500 . The orifice plate  650  may aid in helping redirect these gases back into the flame (arrows marked “C”), increasing the chance that they will undergo combustion. The inner edge  660  of the orifice ring  650  may be designed to become very hot to aid in promoting combustion of uncombusted gases flowing thereby. In addition, the orifice ring  650  creates a disruption in the flow of gases above the ring, such as by creating a turbulence zone (see arrows marked “D” in  FIG. 8 ) and thus impeding or delaying their travel through the combustion chamber  500 , thus also increasing the likelihood that they will also be consumed by the flame before leaving the combustion chamber  500 . In various embodiments, quenching may also be reduced by introduction of insulation material into the cavity  450 . 
     As depicted in  FIGS. 1 ,  2 , and  8 , a cooktop  300  may be positioned above and in communication with the combustion chamber outlet  520  in order to receive a cooking vessel (not shown) and position that vessel to be heated by the heated exhaust. The cooktop  300  may further include a plurality of structures  310  designed to support the cooking vessel above the cooktop surface  20  and over the combustion chamber outlet  520 . The position of the combustion chamber outlet  520 , cooktop  300 , and support structures  310  directs the exhaust to the underside of the cooking vessel to facilitate efficient heating of the cooking vessel. 
       FIG. 9A  shows an alternative embodiment of the stove  100  wherein the dual burner cooktop  700  is designed to support two cooking vessels simultaneously.  FIG. 9B  is a sectional view of the stove  100  in  FIG. 9A . The dual burner cooktop  700  defines a generally elongated structure, having an elliptical-like shape. The dual burner cooktop  700  has two ends; a first rounded end  710  of the dual burner cooktop  700  is positioned above the stove  100 , and a second tapered end  720  of the dual burner cooktop  700  extends away from the stove  100 . The tapered end  720  positioned away from the stove  100  may be supported by legs  730  attached at or near the tapered end  710  of the dual burner cooktop  700 , which extend down to contact the ground. The legs  730  are sufficiently long to support the dual burner cooktop  700  in a horizontal, and generally planar position. The dual burner cooktop  700  defines a generally flat surface  750 . At the edge of the cooktop surface  750 , an apron  740  extends downward. The lower edge  745  of the apron  740  is designed to rest upon the cooktop  300  of the stove  100  and provide protection from contact with a dual burner liner  765  described below. Other means for supporting the dual burner cooktop  700  are contemplated. In further embodiments, for example without limitation, supports such as a leg or legs may be positioned at or nearer the second end. 
     The cooktop surface  750  may define three openings, which may be in communication with an exhaust chamber  760  defined by the underside of the cooktop surface  750  and the liner  765 . A first opening  770  may be positioned near the rounded end  710 . A second opening  780  may be positioned near the middle of the dual burner cooktop  700 , and a third opening  790  may be positioned near to the elongated end  720 . The first  770  and second  780  openings may be surrounded by annular ridges  775  designed to support a cooking vessel. The third opening  790  may define a collar  795 . The third opening  790  is smaller than the first  770  and second opening  780 , and acts as an exhaust outlet. The collar  795  of third opening  790  radiates upward from the cooktop surface  750  and may be designed to receive a stovepipe or vent (not shown). The annular ridges  775  of the first  770  and second opening  790  extend upward from the cooktop surface  750  and are generally concentric to the openings. 
       FIG. 9B  shows a cutaway of the dual burner cooktop  700  showing exhaust gases as they travel from the lower combustion chamber  550  through the stove  100  into and through the dual burner cooktop stove  700 . The exhaust chamber  760  may be defined by the liner  765  and under surface of the cooktop. The liner  760  defines an opening  756  that is designed to fit with the combustion chamber outlet  520 . The liner opening  756  further defines a sleeve  754  that extends downwardly into the upper combustion chamber  560  of the stove  100 . When engaged, the sleeve  754  extends into the upper combustion chamber  560  so that the upper combustion chamber  560  may be in communication with the exhaust chamber  550 . The diameter of the liner opening  756  may be smaller than the interior diameter, D i , of the upper combustion chamber  560 . The exhaust chamber  760  may help to direct the exhaust from the combustion chamber  760  to the first  770  and second  780  openings in the cooktop surface  750  where it may be used to heat a cooking vessel positioned above those openings. The exhaust not passing through the first  770  and second  780  openings may then exit the exhaust chamber  760  by way of the third opening  790  in the cooktop surface  750 . Alternative embodiments may have more or fewer openings used for cooking formed in the extended cook top surface. 
     Arrows in  FIG. 9B  show the path of heated exhaust product as it: “a”—travels to the upper combustion chamber  560 ; “b”—passes through the exhaust chamber opening  756  defined by the sleeve  754  and into the exhaust chamber  760  of the dual burner cooktop  700 ; “c”—leaves the exhaust chamber  760  through the first opening  770 ; “d”—travels through the exhaust chamber  760 ; “e”—leaves the exhaust chamber  760  through the second opening  780 ; “f”—continues through the exhaust chamber  760 ; and “g”—leaves the exhaust chamber  760  through the third opening  790 . 
       FIG. 10  shows an exploded view of the dual burner cooktop  700 , liner  760 , sleeve  754 , and stove cooktop  300 . Here can be seen the interior shape of the exhaust chamber  760  and a narrow channel  752  in the exhaust chamber  760  defined by the liner  765  between the first  770  and second openings  780 . The narrow channel  752  helps direct the exhaust gases toward the openings. 
     FECRAL 
     In various embodiments of the stove, the combustion chamber may be clad in FeCrAl. FeCrAl is a metal alloy containing iron, chromium, aluminum, and other elements in varying ratios depending on the intended purpose. FeCrAl is known in the art to be resistant to corrosion in both reductive and oxidative environments. FeCrAl may form two oxide layers, one iron and another of aluminum that help guard against corrosion. Normally used for its electrical resistivity characteristics, FeCrAl is typically not used for applications with high temperatures where structural load is applied because of its poor structural performance at high temperatures. For example, FeCrAl alloys may have a very high melting point (&gt;1000° C.) and easily forms stable aluminum oxides which resist corrosion. When used as part of the combustion chamber of the present invention, FeCrAl performs well. 
     The present embodiment uses FeCrAl to form, line, or clad the combustion chamber as well as to form the orifice ring (if present). In at least one embodiment, the wall thickness of the combustion chamber may be 0.7 mm. Further embodiments may possess combustion chamber wall thicknesses greater than 0.7 mm or less than 0.7 mm, such as without limitation, 0.5 mm. In some embodiments the wall thickness of the upper portion of combustion chamber may be from 0.5 to 0.3 mm. In further embodiments the wall thickness of the upper combustion chamber may be less than 0.3 mm. In various embodiments the thickness of walls in the upper chamber may differ from the wall thickness in the lower chamber. In some embodiments, the thickness of the combustion chamber walls may vary. Use of this inexpensive, corrosion-resistant metal alloy allows production of an inexpensive, long-lasting, corrosion-resistant alternative to ceramics or specialized metals. Use of FeCrAl alloy may allow the construction of an inexpensive metal combustion chamber for biomass stoves as opposed to a combustion chamber of other metals or ceramics which are heavier and more problematic when manufacturing and shipping stoves. The reduced mass may also allow for faster heating of the chamber, reducing emissions and improving efficiency. 
     The ratio of compounds within the alloy may be changed depending on the desired application. For example, one FeCrAl alloy embodiment may contain a mixture of Al (˜5-15%), Cr (˜3-8%), and Fe (balance). Another embodiment may have a weight percent ratio of 13% Chromium: 4% Aluminum, with the balance being mostly Iron. Other ranges include Al (2%-8%): Cr (10%-20%): Other (&lt;1%): and Fe (Balance). In other embodiments the ratios of chromium, aluminum, iron, and other elements may vary. 
     In various embodiments, the FeCrAl may contain carbon, titanium, or zinc. In some embodiments, the FeCrAl may contain less than 0.1% carbon. In embodiments where FeCrAl contains less than 0.1% carbon, the FeCrAl may further comprise titanium. In embodiments with FeCrAl containing carbon and titanium, the titanium may be less than 1%. In some embodiments, the FeCrAl may contain about 0.08% or less of carbon and about 0.5% titanium. In various embodiments, titanium may help increase the oxidation resistance of FeCrAl containing carbon. FeCrAl may have the trade name FECRALLOY, OHMALOY (manufactured by Allegheny Ludlum), or KANTHAL. In the present embodiment, the orifice ring may also be constructed of FeCrAl. In other embodiments, the orifice ring may be constructed of other suitable materials. 
       FIG. 11  shows an alternative embodiment of a portable biomass stove  1000 . This stove  1000  embodiment is comprised of an exterior shell  1100 , a cooktop  1200 , and a liner  1300 . The shell  1100  may further include handles  1110 , a base  1120 , and a generally flat bottom  1130  for supporting the stove on the ground or other suitable surface. The cooktop  1200  may include a cooktop surface  1210  and a plurality of support structures  1220  for supporting a cooking vessel above the cooktop  1210 . The liner  1300  is further comprised of an interior surface  1305  and an exterior surface  1310 . Positioned between the shell  1100  and the exterior surface of the liner  1310  is a cavity  1400 . The liner  1300  may define a combustion chamber  1320  that is generally cylindrical and opens through the cooktop  1200  at a combustion chamber outlet  1325 . The combustion chamber may be comprised of a lower combustion chamber  1350  and an upper combustion chamber  1360 . The lower combustion chamber  1350  may include a floor  1335  that may be designed to hold solid biomass fuel  1001 . 
     Positioned at the base  1120  of the stove  1000  may be a plurality of apertures  1140  in the shell  1100 . The apertures  1140  may open into an intake chamber  1410  defined by the bottom of the shell  1100  and a divider  1420 , which divides the cavity  1400 . The divider  1420  may define an opening  1430  into which may be placed a fan  1440 . The fan  1440  may be connected to a wire(s)  1445 , which are in turn connected to a battery  1450  or other device to provide electricity to the fan  1440 . The fan  1440  may aid in drawing air through the apertures  1140  into the intake chamber  1410 . The fan  1440  may further force air from the intake chamber  1410  into the cavity  1400  above the divider  1420 . The battery  1450  may also be connected by wire(s)  1445  to a heating element  1330 . The heating element  1330  may aid in heating solid biomass fuel  1001 . The heated solid biomass fuel  1001  may give off volatile gases that mix with air that may be forced or drawn in from the cavity  1400  that may enter the combustion chamber  1320  through a plurality of inlets  1340  positioned near the floor  1335  of the lower combustion chamber  1350 . Air from the cavity  1400  may also enter the combustion chamber  1320  through a second plurality of inlets  1345  positioned near the top of the upper combustion chamber  1360 . In some embodiments a plurality of doors (not shown) may be movably and selectively positioned over the apertures and/or inlets to reduce the area of these openings and aid in regulation of the amount of air entering the combustion chamber. 
     When the stove in  FIG. 11  is in use, air is drawn into the cavity  1400  at the intake chamber  1420  (arrows marked “α”) through a plurality of apertures  1140 , at least partially by the action of the electric fan  1440 . In some embodiments the fan may have variable speed to help regulate the flow of air. The air may pass through the fan  1440  (arrows marked “β”) and be forced toward the liner  1300 . Some of the air may travel up the cavity, through the inlets  1340 , and into the lower combustion chamber  1350  (arrows marked “γ”), where the air γ may mix with volatile gases from the heated solid biomass fuel  1001  to form a combustible gas (large empty arrows marked “ε”). Some air may travel further up the cavity (arrows marked “δ”). Some air may enter the combustion chamber  1320  through the inlets  1345  positioned near the top of the upper combustion chamber  1360  (arrows marked “ξ”). The air ξ entering at the top of the combustion chamber  1360  may mix with the combustible gas ε rising up from the lower combustion chamber  1350 , and when ignited may form a flame. This flame may be used to heat a cooking vessel positioned above the outlet. 
     In the embodiment shown in  FIG. 11 , fuel may be added in batches through the combustion chamber outlet. Further, in this embodiment the combustion chamber may be lined, clad, or formed of FeCrAl while the shell may be manufactured from some other material. The use of FeCrAl in the combustion chamber beneficially allows the combustion chamber to better withstand the very high temperatures and corrosive effects of the combustion process, such as in a gasification stove. Use of FeCrAl may allow the presently disclosed cook stove to last longer than with typical materials such as stainless steel. FeCrAl will also allow production of a less expensive stove by reducing costs associated with stoves that use combustion chambers made of other materials such as ceramics. 
     EXAMPLE 1 
     Thermal efficiency and particulate matter production was analyzed in cookstoves with and without an orifice ring. In this experiment the amount of time needed to boil water was measured along with the amount of wood used and particulate matter produced for each stove. Results from the tests were used to calculate thermal efficiency for biomass stoves with and without an orifice ring. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Wood 
                 Time to 
                   
               
               
                   
                   
                   
                 Use 
                 Boil 
                 Thermal 
               
               
                   
                 CO (g) 
                 PM (mg) 
                 (g) 
                 (min) 
                 Efficiency 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 ElBv10 (shortened 
                 15.4 
                 529 
                 449.1 
                 31.5 
                 40.1 
               
               
                 elbow 3″ orifice) 
               
               
                 ElBv11 (shortened 
                 20.5 
                 1518 
                 463.1 
                 34.5 
                 31.3 
               
               
                 elbow no orifice) 
               
               
                   
               
            
           
         
       
     
     Table 1 shows experimental results of stove performance with and without an orifice during a three phase modified water boil test. Wood was used as a bio-mass source. Carbon monoxide (CO) emissions are measured in grams, particulate matter (PM) is measured in milligrams, wood use in grams. The results presented in Table 1 show that the presence of an orifice ring led to decreased CO and PM production from the stove while increasing thermal efficiency. 
     EXAMPLE 2 
     The effect on carbon monoxide (CO) production of stoves with and without an orifice ring was tested using the Testo system. This experiment used a FeCrAl 100 mm standard rocket stove having an elbow. From a cold start, the tests showed that the orifice plate resulted in a 2.51 g of CO produced while the rocket elbow without the orifice plate resulted in production of 8.5 g of CO. CO production was measured by Fourier transform infrared (FITR) spectroscopy. 
     All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, inner, outer, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the example of the invention, and do not create limitations, particularly as to the position, orientation, or use of the invention unless specifically set forth in the claims. Joinder references (e.g., attached, coupled, connected, joined, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. 
     In some instances, components are described with reference to “ends” having a particular characteristic and/or being connected with another part. However, those skilled in the art will recognize that the present invention is not limited to components which terminate immediately beyond their points of connection with other parts. Thus, the term “end” should be interpreted broadly, in a manner that includes areas adjacent, rearward, forward of, or otherwise near the terminus of a particular element, link, component, part, member or the like. In methodologies directly or indirectly set forth herein, various steps and operations are described in one possible order of operation, but those skilled in the art will recognize that steps and operations may be rearranged, replaced, or eliminated without necessarily departing from the spirit and scope of the present invention. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims. 
     It will be apparent to those of ordinary skill in the art that variations and alternative embodiments may be made given the foregoing description. Such variations and alternative embodiments are accordingly considered within the scope of the present invention.