Downdraft Fixed-Bed Gasifier for Producing a Product Gas from Pourable Biomass Particles

Abstract

A downdraft fixed-bed gasifier for producing product gas from pourable biomass particles includes a gasifier container, a gasifier component, a feeder, an air supply inlet, a grate and a product gas vent. The gasifier container has a larger diameter than does the gasifier component. The lower open end of the gasifier component extends down into the gasifier container. The feeder is adapted to receive biomass particles into the upper closed end of the gasifier component. Combustion air is fed into the gasifier component through the air supply inlet located near the upper closed end. The grate supports the biomass particles and is disposed in a lower portion of the gasifier container below the lower open end of the gasifier component. Product gas generated from oxidizing the biomass particles exits the gasifier container through the product gas vent located in the side of the container below the level of the grate.

Claims

1-20. (canceled)

21. A downdraft fixed-bed gasifier for producing a product gas from pourable biomass particles, comprising: a gasifier container with a first diameter; a gasifier component with a second diameter, an upper closed end and a lower open end, wherein the lower open end of the gasifier component extends down into the gasifier container, wherein the upper closed end of the gasifier components projects up and out of the gasifier container, and wherein the first diameter is larger than the second diameter; a feeder adapted to receive biomass particles into the upper closed end of the gasifier component; an air supply inlet that enters the gasifier component near the upper closed end and through which combustion air is fed into the gasifier component; a grate adapted to support the biomass particles that is disposed in a lower portion of the gasifier container; and a product gas vent leading out of the gasifier container below the grate and through which the product gas generated from the biomass particles exits the gasifier container, and wherein the gasifier component is disposed in the gasifier container such that a first distance remains between the lower open end of the gasifier component and the grate.

22. The downdraft fixed-bed gasifier of claim 21, wherein each of the gasifier container and the gasifier component has a circular cross-section.

23. The downdraft fixed-bed gasifier of claim 22, wherein the gasifier component is arranged coaxially with respect to the gasifier container.

24. The downdraft fixed-bed gasifier of claim 22, wherein each of the gasifier container and the gasifier component has a constant diameter.

25. The downdraft fixed-bed gasifier of claim 21, wherein the grate is rotatable.

26. The downdraft fixed-bed gasifier of claim 21, wherein the first distance is 90% to 110% of the second diameter.

27. The downdraft fixed-bed gasifier of claim 21, wherein the second diameter is 50% to 80% of the first diameter.

28. The downdraft fixed-bed gasifier of claim 21, further comprising: a nozzle of an ignition device leading into the gasifier container above the grate and below the lower open end of the gasifier component, wherein the nozzle is adapted to inject hot air into the gasifier container.

29. The downdraft fixed-bed gasifier of claim 28, wherein the nozzle is ceramic.

30. The downdraft fixed-bed gasifier of claim 21, further comprising: a biomass particle storage container; and a biomass particle conveyor, wherein the biomass particle conveyor is connected to the feeder by a first airtight lock valve, wherein the biomass particle storage container has a second airtight lock valve, and wherein the biomass particle conveyor moves biomass particles from the biomass particle storage container through the feeder and into the gasifier component in an airtight manner sealed from the outside environment.

31. The downdraft fixed-bed gasifier of claim 21, further comprising: a gas processing device connected to the product gas vent, wherein the gas processing device is connected to a combustion engine that operates using the product gas.

32. The downdraft fixed-bed gasifier of claim 31, wherein the gas processing device includes a heat exchanger that cools the product gas received from the product gas vent.

33. The downdraft fixed-bed gasifier of claim 21, further comprising: a level sensor that detects a level of the biomass particles in the gasifier component.

34. A method of operating a downdraft fixed-bed gasifier, comprising: filling a gasifier component with biomass particles to a predetermined fill level, wherein a lower open end of the gasifier component extends down into a gasifier container, wherein the gasifier component is arranged coaxially with respect to the gasifier container, and wherein the biomass particles are supported by a grate disposed below the lower open end of the gasifier component in a lower portion of the gasifier container; supplying air through an air supply inlet into the gasifier component; discharging a product gas from a product gas vent leading out of the gasifier container below the grate, wherein the product gas is generated from the biomass particles; and adding additional biomass particles to the gasifier component so as to maintain the predetermined fill level of biomass particles in the gasifier component as biomass particles are consumed by generating the product gas.

35. The method of claim 34, further comprising: rotating the grate.

36. The method of claim 34, wherein the biomass particles are wood pellets.

37. The method of claim 34, wherein the biomass particles include a portion of kaolin, and wherein the portion is 1% to 5% by weight.

38. The method of claim 34, further comprising: igniting the biomass particles by injecting air with a temperature of more than 300° C. into the gasifier container below the lower open end of the gasifier component.

39. The method of claim 34, further comprising: stopping the adding of additional biomass particles; and stopping the supplying of air when the biomass particles have been consumed such that an upper extent of the biomass particles has dropped to a predetermined lower cutoff level in the gasifier component.

40. The method of claim 34, wherein ash is produced as product gas is generated from the biomass particles, further comprising: discharging the ash from the gasifier container in the product gas that is discharged from the product gas vent.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0016] The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

[0017] FIG. 1 is a schematic, cross-sectional diagram of an exemplary embodiment of the invention that includes the essential components.

[0018] FIG. 2 is a schematic representation of a combination of the downdraft fixed-bed gasifier of FIG. 1 together with a gas processing device and a combined heat and power unit (CHP).

[0019] FIG. 3 shows a bed of biomass particles in the gasifier of FIG. 1 and the different reaction zones.

DETAILED DESCRIPTION

[0020] Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

[0021] FIG. 1 is a schematic diagram of an exemplary embodiment of a downdraft fixed-bed gasifier 10. The downdraft fixed-bed gasifier 10 includes a cylindrical gasifier component 11 inside a cylindrical gasifier container 12. By supplying air 13 through a bed of biomass particles 14 in the tubular gasifier component 11, a uniform distribution of air is achieved throughout the biomass particles. By virtue of the uniform distribution of combustion air 13, hardly any temperature differences occur within the oxidation zone 15 of the gasifier container 12. As a result, even pyrolysis gases produced above the oxidation zone 15 flow through the oxidation zone in a uniform manner. The uniformity of the flow of air and gases allows a product gas 16 to be generated with low tar quantities.

[0022] By supplying air 13 from the top and by discharging product gas 16 from the lower part beneath a rotatable grate 17, air and gas flows through the fixed-bed gasifier 10 only from top to bottom. A drying zone 18 and a pyrolysis zone 19 are located in the gasifier component 11 which projects into the gasifier container 12. The oxidation zone 15 is situated beneath the open end 20 of the gasifier component 11, followed by the reduction zone 21 above the grate 17. The oxidation zone 15 is locally connected by gas flowing from top to bottom and by means of a cross-sectional jump from the gasifier component 11 outwards in the gasifier container 12 at the open end 20 of the gasifier component 11. Different flow speeds of the air and gas result from this cross-sectional jump. By expanding the cross-section, the flow speed of the air is slowed compared to conventional fixed-bed gasifiers. Conventional fixed-bed gasifiers have a necking beneath the oxidation zone 15, which increases the flow speed of the gas. In the fixed-bed gasifier 10, the different flow speeds within and outside the tubular gasifier component 11 virtually fix the oxidation zone 15 in front of the open end 20 of the tubular gasifier component 11.

[0023] Another advantage of the expanding cross-section is that the pyrolysis gases generated in the pyrolysis zone 19 above the oxidation zone 15 are not confined by a tube wall while flowing through the oxidation zone 15. The flow conditions over tube walls are not uniform, and thus the temperatures there would not be uniformly high. When pyrolysis gas flows on the edge of a tube wall through the oxidation zone 15, as is the case in the prior art, the long-chain hydrocarbons are not completely broken down. Additional long-chain hydrocarbon compounds are broken down by virtue of the absence of the tube wall in the oxidation zone 15 beneath the open end 20 of the gasifier component 11, thus leading to an improvement of efficiency when using the product gas 16 to power an engine 22.

[0024] The circular cross section and constant diameter 23 of the gasifier component 11 allows the combustion air 13 and product gas 16 to flow uniformly and to produce an oxidation zone 15 with a homogeneous temperature. A large portion of the long-chain hydrocarbon compounds are broken down in the oxidation zone 15, thereby generating high quality product gas 16.

[0025] By rotating the grate 17, clogging of the grate 17 is prevented and residual agglomeration is reduced. In addition, rotating the grate 17 allows ash to be removed from the gasifier container 12 as ash falls through the grate and is carried as particles by the product gas 16 out of the product gas vent 24.

[0026] The optimum distance 25 from the grate 17 to the open bottom end 20 of the gasifier component 11 was determined empirically. The distance 25 from the open end 20 of the gasifier component 11 to the grate 17 approximately equals the diameter 23 of the gasifier component 11. If the distance 25 is smaller than the optimum, the size of the reduction zone 21 is reduced, which negatively affects the quality of the product gas 16. If the distance 25 is larger than the optimum, the size of the reduction zone 21 increases, which likewise unfavorably affects to the quality of the product gas 16. Although the downdraft fixed-bed gasifier 10 even functions at a deviation of 40% (h=d+/−40%) from the optimum distance, the quality and yield of the product gas 16 is impaired.

[0027] The smaller the inner diameter 23 of the gasifier component 11 compared to the inner diameter 26 of the gasifier container 12, the larger is the difference of the gas flow speed within and outside the tubular gasifier component 11. If the difference in gas flow speed is too large, the material efficiency is reduced and more fuel is required to generate the same amount of product gas 16. If the difference in speed becomes too small, the gas flow speed exiting the gasifier component 11 is too high. In addition, the inner diameter 23 of the tubular gasifier component 11 must be large enough to form a bed of biomass particles 14 in the gasifier component 11. The optimum relative size of the inner diameter 26 of the gasifier container 12 compared to the inner diameter 23 of the gasifier component 11 was found empirically and results in a functional downdraft fixed-bed gasifier.

[0028] The biomass particle conveyor system 27 has airtight locks used for feeding biomass particles 14 into the tubular gasifier component 11. The biomass particle conveyor system 27 may also be used in other fixed-bed gasifiers independent of the present invention.

[0029] The wood gas or other product gas 16 generated in the downdraft fixed-bed gasifier 10 is preferably used in a combined heat and power unit (CHP) with a combustion engine 22 or a fuel cell for providing electrical and thermal power. The product gas 16 generated in the downdraft fixed-bed gasifier 10 is cooled and purified in a downstream gas processing device 28.

[0030] The cooled and purified product gas 16 is mixed with combustion air 13 in the gas mixing duct 29 of the combustion engine 22. Compared to the product gas 16 from the gas processing device 28, the combustion air 13 is cold, and the product gas 16 is further cooled. Such further cooling may lead to the undesired precipitation of solids or liquids and particularly of tar. By providing a condensate separator 30 after the combustion air 13 is added to the product gas 16 or wood gas but directly prior to combustion in the gas engine 22, the solid and/or liquid precipitations are deposited and separated from the product 16 and thus cannot harm the gas engine 22. This configuration of gas mixing duct 29, condensate separator 30 and gas engine 22 can also be used in other types of fixed-bed gasifiers independent of the present invention. The gas processing device 28 includes a heat exchanger for cooling the product gas 16 to a temperature that can be used as fuel in a combined heat and power (CHP) station.

[0031] A control device controls the biomass particle conveyor system 27, and a level sensor 31 detects a fill level 31 of biomass particles 14 in the tubular gasifier component 11. The control device and level sensor 31 ensure that the bed of biomass particles 14 in the gasifier component 11 is high enough to sufficiently distribute the air 13 flowing through the bed of biomass particles 14 in a uniform manner over the entire cross-section of the gasifier component 11 before the air reaches the first reaction zone 19. By continuously supplying combustion air 13 over the bed of biomass particles 14 and due to a continuous removal of product gas 16, the different reaction zones of the downdraft fixed-bed gasifier 10 remain stationary, and predefined reaction conditions are maintained.

[0032] By adding kaolin to the biomass pellets 14 in their manufacturing process, the melting point of the ash that results from the biomass gasification is increased and the likelihood that the grate 17 becomes clogged is reduced. Independent of the present invention, using such pellets 14 with kaolin may also be used in an advantageous manner in other types of wood gasifiers.

[0033] The downdraft fixed-bed gasifier 10 is started by first filling the gasifier component 11 with biomass particles 14 up to a predetermined target level 32 and then igniting the biomass particles 14 by blowing in hot air with a temperature of more than 300° C. into the bed of biomass particles 14 beneath the open end 20 of the tubular gasifier component 11. The temperature of the hot air is selected so that the biomass particles 14 are ignited safely.

[0034] The operation of the downdraft fixed-bed gasifier 10 is stopped by terminating the supply of biomass particles 14, continuing to supply combustion air 13 for a predetermined period of time or until the level of biomass particles 14 has dropped to a preset lower cutoff level, and then terminating the supply of combustion air 13. In the termination process, biomass gasification continues after the biomass particles 14 are no longer fed into the gasifier component 11, the level of the bed of biomass particles 14 in the gasifier component 11 drops, and as little as possible fresh biomass particles 14 remain. If after the supply of air 13 has ended, a large portion of fresh biomass particles 14 were to remain in the tubular gasifier component 11, the particles 14 would outgas humid gas that would cause the overlying biomass particles 14 to swell. Upon re-starting the gasifier 10, such swelling could lead to the gasifier component 11 becoming clogged.

[0035] FIG. 1 is a schematic diagram of an exemplary embodiment of a downdraft fixed-bed gasifier 10. The downdraft fixed-bed gasifier 10 includes a tubular gasifier container 12, the ends of which are closed by an upper cover 33 and a lower cover 34. The tubular gasifier component 11 has a lower open end 20 and a closed upper end 35 and projects with its open end 20 down into the gasifier container 12. The closed upper end 35 of the gasifier component 11 protrudes from the gasifier container 12 through the upper cover 33. The open end 20 of the gasifier component 11 lies approximately at mid height in the gasifier container 12. The rotatable grate 17 is positioned at a distance 25 below the open end 20 of the gasifier component 11. The rotatable grate 17 can be periodically moved by a motor drive 36 whose drive shaft penetrates the lower cover 34. A feeder 37, an air supply inlet 38 and the probe of the level sensor 31 all lead into the gasifier component 11 near its closed end 35. The biomass particles 14 are poured into the gasifier component 11 through the feeder 37. Combustion air 13 flows into the gasifier component 11 through the air supply inlet 38. And the level sensor 31 monitors the level of the biomass particles 14 in the tubular gasifier component 11.

[0036] An ignition device 39 and a closed inspection shaft 40 penetrate the outer wall of the gasifier container 12 and are disposed near the height of the open end 20 of the gasifier component 11. The ignition device 39 generates hot air with a temperature in a range of 300° C. to 600° C., which is used ignite the biomass particles 14 in the area beneath the open end 20 of the gasifier component 11 when starting the downdraft fixed-bed gasifier 10. The area beneath the open end 20 of the gasifier component 11 is within the oxidation zone 15. The ignition device 39 includes an air nozzle 41 for injecting hot air that aerates the gasifier container 12. By making the air nozzle 41 from ceramics, those parts of the ignition device 39 that are disposed within the gasifier container 12 are thermally decoupled from the parts outside the gasifier container 12. Maintenance and cleaning work can be performed inside the reactor vessel of the fixed-bed gasifier 10 during standstill of the reactor through the inspection shaft 40.

[0037] Product gas 16 is removed from the gasifier container 12 through the product gas vent 24 located below the grate 17. The ashes falling through the grate 17 are discharged from the fixed-bed gasifier 10 by being swept up in the flow of product gas 16 out the product gas vent 24.

[0038] Both the tubular gasifier container 12 and the tubular gasifier component 11 have a circular and ring-shaped cross-section and are arranged concentrically to one another. The tubular gasifier component 11 has an inner diameter 23 that is smaller than the inner diameter 26 of the tubular gasifier container 12.

[0039] The feeder 37 for biomass particles 14 is connected to a biomass particle conveyor system 27 through a first lock valve 42 on top of the feeder 37 and a screw conveyor 43. The screw conveyor 43 is connected to a biomass particle storage container 44 in a airtight manner. The storage container 44 is loaded with biomass particles 14 through a second lock valve 45 that seals the storage container 44 airtight from the outside environment. Because the biomass particle storage container 44 is directly connected in an airtight manner to the screw conveyor 43, those two components maintain the air lock between the two lock valves 42 and 45 as the biomass particles 14 are fed into the fixed-bed gasifier 10.

[0040] FIG. 2 illustrates a combination of the fixed-bed gasifier 10 of FIG. 1 with a downstream gas processing device 28 and a combined heat and power unit (CHP) 46. The product gas 16 escaping from the product gas vent 24 is supplied to the gas processing device 28. The product gas 16 is cooled in a heat exchanger inside the gas processing device 28, and both solid and liquid impurities are separated to the extent possible.

[0041] The cooled and processed product gas 16 from the product gas processing device 28 is supplied via a product gas line 47 to a gas mixing duct 29 of a gas engine 22. The gas mixing duct 29 is connected to a combustion air supply manifold 48. By mixing comparatively cold external air and comparatively hot product gas 16 from the product gas processing device 28 in a mixed gas line 49, the resulting gas mixture is further cooled so that further liquid impurities precipitate out. The liquid impurities are deposited in a condensate separator 30 at the end of the mixed gas line 49 directly in front of the supply of the gas mixture to the gas engine 22 and can be conveniently removed. The quality of the mixed gas is thereby enhanced, and harmful impurities are prevented from being burned in and emitted from the gas engine 22.

[0042] FIG. 3 shows the downdraft fixed-bed gasifier 10 in normal operating conditions with the bed of biomass particles 14 in the gasifier component 11 as well as in the gasifier container 12 below the gasifier component 11. FIG. 3 also shows the positions of the different reaction zones. The oxidation zone 15 is disposed directly below the open end 20 of the gasifier component 11. Beneath the oxidation zone 15 is the reduction zone 21 that extends down to the grate 17. The pyrolysis zone 19 extends up from the oxidation zone 15 and is followed by the drying zone 18.

[0043] In order to start the downdraft fixed-bed gasifier 10, the gasifier container 12 is first filled with biomass particles 14 up to a target level 32 via the feeder 37 so that the bed of biomass particles 14 is formed. By blowing in hot air from the ignition device 39, the biomass particles 14 are ignited directly below the open end 20 of the gasifier component 11 and form the oxidation zone 15. As soon as combustion of biomass particles 14 within the oxidation zone 15 is self-sustaining, the ignition device 39 is shut off. Beginning before the ignition of the biomass particles 14, combustion air 13 is supplied into the gasifier component 11 through the air supply inlet 38. The combustion heat released in the oxidation zone 15 gradually forms the remaining reaction zones. The product gas 16 flowing out the product gas vent 24 is supplied to the gas processing device 28. The quality of the product gas 16 is monitored in the gas processing device 28. During startup of the downdraft fixed-bed gasifier 10, the flow of product gas 16 from the product gas vent 24 is burned in an exhaust gas torch (not shown) on account of its poor quality. As soon as a sufficient quality of the product gas is reached, the product gas is cooled in the gas processing device 28 and is separated, to the extent possible, from solid and liquid impurities. During normal operating conditions of the fixed-bed gasifier 10, the target level 32 of the biomass particles 14 in the gasifier component 11 is monitored by level sensor 31 and, if necessary, biomass particles 14 from the biomass particle conveyor system 27 are refilled through the first lock valve 42 and the feeder 37 in order to fill the gasifier component 11 back to the target level 32.

[0044] Upon shutting down the fixed-bed gasifier 10, the resupply of biomass particles 14 is first stopped so that the level of biomass particles 14 in the gasifier component 11 drops below the target level 32. If a large amount of fresh biomass particles 14 were to remain in the tubular gasifier component 11 after the supply of combustion air 13 is shut off, the biomass particles 14 would outgas and the humid gas would cause the overlying biomass particles 14 to swell. Upon re-starting the gasifier, such swelling could then lead to the tubular gasifier component 11 becoming clogged. When the biomass particles are consumed such that the upper extent of the bed of particles drops down to a lower cutoff level 50 in the gasifier component 11, the air supply is shut off, and the gas production terminates. The lower cutoff level 50 corresponds approximately to the upper extent of the pyrolysis zone 19 during normal operating conditions. In this way, a small amount as possible of fresh biomass particles 14 remains in the gasifier container 12 and in the gasifier component 11 when the fixed-bed gasifier 10 is shut down.

[0045] The downdraft fixed-bed gasifier 10 is particularly suited for gasifying wood pellets and biomass pellets. When the biomass pellets are prepared, kaolin (hydrated aluminum silicate) is added to the pellets so that the finished pellets contain a mass proportion of kaolin of 1% to 5% and preferably 1.5% to 3%. Through the addition of kaolin, the melting point of the ash resulting from biomass gasification is increased so as to avoid the clogging of the grate 17 and any undesired deposit of ash at other components of the fixed-bed gasifier 10.

REFERENCE NUMERALS

[0046] 10 down-draft fixed-bed gasifier

[0047] 11 gasifier component

[0048] 12 gasifier container

[0049] 13 air

[0050] 14 biomass particles

[0051] 15 oxidation zone

[0052] 16 product gas

[0053] 17 grate

[0054] 18 drying zone

[0055] 19 pyrolysis zone

[0056] 20 open end of gasifier component 11

[0057] 21 reduction zone

[0058] 22 gas engine

[0059] 23 inner diameter of gasifier component 11

[0060] 24 product gas vent

[0061] 25 distance from grate 17 to open end 20

[0062] 26 inner diameter of gasifier container 12

[0063] 27 biomass particle conveyor system

[0064] 28 gas processing device

[0065] 29 gas mixing duct

[0066] 30 condensate separator

[0067] 31 level sensor for biomass particles

[0068] 32 target fill level of biomass particles

[0069] 33 upper cover

[0070] 34 lower cover

[0071] 35 closed end of gasifier component 11

[0072] 36 motor drive for grate 17

[0073] 37 feeder for biomass particles

[0074] 38 air supply inlet

[0075] 39 ignition device

[0076] 40 inspection shaft

[0077] 41 ceramic air nozzle

[0078] 42 first lock valve

[0079] 43 screw conveyor

[0080] 44 biomass particle storage container

[0081] 45 second lock valve

[0082] 46 combined heat and power unit (CHP)

[0083] 47 product gas line

[0084] 48 combustion air supply manifold

[0085] 49 mixed gas line

[0086] 50 lower cutoff level of biomass particles upon shut-down

[0087] Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.