Gasification chamber with mass flow wedge members

A gasifier includes a gasification chamber including an annular chamber wall with a top opening for introducing fuel into the gasification chamber. A restricted bottom outlet section of the gasification chamber has inwardly angled wedge walls encouraging a mass flow rather than a funnel-flow of fuel through the gasifier.

FIELD OF THE INVENTION

The subject invention relates to a down draft gasifier.

BACKGROUND OF THE INVENTION

In a typical downdraft gasifier, a vertically oriented gasification chamber receives feed stock (a fuel) which is combusted by pyrolysis in the gasification chamber. U.S. Pat. No. 6,647,903, incorporated herein by this reference, discloses a hollow cylinder with a lower cone-shaped section terminating in a restricted opening in order to retain the fuel in the gasifier and yet also allow spent fuel (char and ash) to exit the gasifier. In this way, fuel continually enters the gasifier and spent fuel continually exits the gasifier.

Steady fuel flow through a gasifier results in improved gasification, better gas quality, lower tar content, and higher yield. If the fuel flows too quickly through the gasifier, pyrolysis of the fuel is diminished and/or tar production increases. If the fuel flows too slowly through the gasifier, the fuel and/or char can build up in the gasifier and on the walls of the gasifier reducing gas quality. Gasifiers frequently suffer from bridging or channeling of the fuel. SeeHandbook of Biomass Downdraft Gasifier Engine Systems, Reed and Das, 1998 (Biomass Energy Foundation Press).

A gasifier with fairly steep long sloping walls defines a mass flow of fuel through the gasifier which provides an adequate flow of fuel through the gasifier but results in a gasifier with a restricted volume. The conical shaped gasifier thus must be rather tall and is not well adapted for use in portable, transportable, or on-site applications such as disclosed in co-pending U.S. patent application Ser. No. 12/070,032 incorporated herein by this reference.

A gasifier with short, less steep sloping walls at the outlet of the gasifier results in a funnel flow of the fuel. Fuel proximate the walls of the gasifier moves downward too slowly and interior fuel moves downward too quickly. Gasifier designs exhibiting a funnel-flow pattern results in material flowing preferentially through a funnel-shape channel located directly above the gasifier outlet while material outside this flow channel is stagnant. In some prior art designs, vibrators are provided for agitating the contents of the gasification chamber. See U.S. Pat. No. 7,736,402 incorporated herein by this reference.

There is a trade off, then, between the size of the gasifier and gasification parameters such as gas quality. For hoppers, mass flow and funnel flow are discussed in the paper“Solve Solids Flow Problems in Bins, Hoppers, and Feeders”by J. Marinelli and Dr. John W. Carson (June 2001), incorporated herein by this reference.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a new gasifier is provided which is configured to encourage a mass flow of fuel through the gasifier and yet, at the same time, results in less of a volume reduction than a gasifier configured with a curved chamber wall which slops inwardly.

The invention features a gasifier comprising a gasification chamber including an annular chamber wall with a top opening for introducing fuel into the gasification chamber, a plurality of air inlets opening into the gasification chamber, and a restricted bottom outlet section of the gasification chamber. Inwardly angled wedge walls encouraging a mass flow rather than a funnel-flow of fuel through the gasifier. Portions of the annular chamber wall between the inwardly angled wedge walls are not angled inwardly to increase the volume of the gasification chamber. In another aspect, a restricted bottom outlet section of the gasification chamber includes inwardly angled wedge walls with flat faces defining a discharge outlet having straight opposing edges co-joining curved sections of the annular chamber wall.

The air inlets may be flush with the annular chamber wall and typically there are air inlets through the inwardly angled wedge walls.

The inwardly angled wedge walls preferably slope inwardly at an angle of between 6° and 18° (e.g., 12°), the width of the wedge walls is between 90 and 95% of the diameter of the chamber wall, and the height of the wedge walls is between 50 and 60% of the height of the chamber. Typically, the top of each wedge wall is curved and smoothly co-joins the chamber wall. In one design, the wedge walls oppose each other in the chamber.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically depicts gasifier10with a hollow cylinder portion12and restricted bottom outlet14. See U.S. Pat. No. 7,736,402 incorporated herein by this reference. In such a funnel-flow design, as noted in the Background section above, fuel flows preferentially through a funnel-shaped channel located directly above outlet14while material outside this flow channel is stagnant. The result can be fuel proximate the walls of the gasification chamber moving downward too slowly through the gasifier and fuel interior to the walls of the gasification chamber moving downward too quickly.

FIG. 2schematically depicts gasifier10′ with fairly long and steep sloping curved wall16resulting in a mass flow of fuel through the gasifier which is preferred. But, now the volume of the gasifier is greatly reduced as noted in the Background section above. Thus, for gasifier10′,FIG. 2to have the same capacity as gasifier10,FIG. 1, gasifier10′,FIG. 2must be significantly taller which is disadvantageous in some applications such as portable, transportable, and/or on-site waste to energy systems.

In embodiments of the subject invention, an engineering compromise is made between the gasifier designs ofFIGS. 1 and 2. In one preferred embodiment, gasifier30,FIG. 3-4includes gasification chamber32with top opening34for introducing fuel such as dried waste in pellet form into gasification chamber32. A plurality of air inlets36open through annular inner chamber wall38and supply air into the gasification chamber via piping40.

Gasifier30includes restricted bottom outlet section44for maintaining fuel in the gasification chamber to maintain pyrolysis of the fuel. Restricted bottom outlet section44also allows spent fuel, char, and the like to exit the gasification chamber. A grate subsystem may be disposed below restricted outlet44as disclosed in co-pending U.S. patent application Ser. No. 12/586,830 incorporated herein by this reference.

Restricted bottom outlet section44is defined by inwardly angled wedge walls48aand48bdesigned to encourage a mass flow rather than a funnel-flow of fuel through gasification chamber32. To avoid a sharp decrease in the volume of chamber32, portions of the curve chamber wall38between inwardly angled wedge walls48aand48bare not angled inwardly as shown at50and instead continues straight down from the top of the gasifier to the bottom thereof.

In this preferred design, opposing inwardly angled wedge walls48aand48bhave flat faces as shown at60inFIG. 5for wall48a. This design defines a discharge outlet as depicted inFIG. 6with straight opposing edges66aand66badjoining curve sections68aand68bof annular chamber wall38,FIGS. 3-4. This design approximates a transition hopper design used in material handling. In a traditional transition hopper design, however, all the walls at the discharge portion slope inwardly.

Air inlets36may be flush with chamber wall38or short nozzles may be used. Inwardly angled wedge walls48aand48bmay also include air inlets as shown.

Inwardly angled wedge walls48aand48bmay slope inwardly at an angle of between 6° and 18°. In one prototype unit, angle θ,FIG. 5, was 12°. The width W,FIG. 5, of each wedge wall may be between 90 and 98% of the diameter of the gasification chamber inner wall. In the prototype unit, the gasification inner chamber wall was 19¼ inches in diameter and W was 18.12 inches. The length or height of each wedge wall may be between 50 and 60% of the length of the chamber. In the prototype unit, the chamber was 52 inches tall and h,FIG. 5, was approximately 30 inches. As shown inFIGS. 3-5, the top of each wedge wall may be curved at the top and smoothly co-joining with the chamber wall in a fashion such that there are no edges interfering with fuel flow through the chamber.

For the design shown inFIG. 2, with a gasifier 52 inches tall with a sloping wall, the volume of the gasifier would be 5.18 ft3. In the design shown inFIG. 3, gasifier30had a volume of 7.46 ft3. To achieve this same volume, the design shown inFIG. 2would need to be 65.62 inches tall.

The gasifier of the invention can be machined or formed to include integral sloping wedge walls or a hollow straight cylinder can be fabricated separately from wedge walls48aand48band the wedge walls inserted into the hollow straight cylindrically shaped chamber and secured to the non-inwardly sloping curved chamber wall.

In one preferred design, all the interior surfaces of gasification chamber32are lined with refractory material and polished. Stainless steel is the typical material used in the construction of the gasifier components.

The result is a gasifier with a steady flow of fuel therethrough providing better gasification, a higher gas quality, lower tar content and a higher yield. Gasifier30,FIGS. 3-4is configured to encourage a mass flow of fuel through gasification chamber32and yet at the same time there is less of a volume reduction when compared to the gasifier design shown inFIG. 2where the curved chamber wall slopes inwardly at a fairly shallow angle resulting in long inwardly sloping curved gasification chamber walls.