Method and apparatus for gasifying solid biomass fuel

Abstract

A downdraft gasifier has a gas producer. The gas producer includes an introducing portion provided in an upper portion for introducing solid biomass fuel 4, an air inlet 17 provided in a central portion, a fire grate 14 provided in a lower portion, and a product gas delivery portion provided below the fire grate. In this gasifier, there are provided fire-proofball laid layers in which a plurality of metal balls 15 are disposed in layers on the fire grate substantially uniform, and a preheating unit (such as a burner) for preheating the fire-proof balls. The metal balls 15 are preheated in advance to a predetermined temperature ranging from 500° C. to 800° C. before the solid biomass fuel is introduced into the producer. After that, the solid biomass fuel 4 is introduced into the producer, and gas produced in the producer is delivered from a portion below the fire grate.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and apparatus for gasifying solid biomass fuel, and particularly relates to a gas producer and a gas producing method for use in a distributed power generator using solid biomass as fuel.

[0002] Biomass in which solar energy has been fixed and accumulated in-vivo by plant photosynthesis is notable as an alternative power source due to carbon or hydrogen contained therein.

[0003] There are some related-art power generation systems using biomass as fuel, such as 1) a system in which biomass fuel is burned directly in a boiler so as to generate power, for example, through a steam turbine; 2) a system in which biomass fuel using microbes is fermented, and methane gas extracted therefrom is, for example, fed to a fuel cell to thereby generate power; and 3) a system in which biomass fuel is gasified in a gas producer so as to produce combustible gas, and the combustible gas is, for example, fed to a gas engine or a diesel engine so as to generate power.

[0004] When biomass is used as fuel for a boiler in the system 1), firewood, sawdust or chaff may be used directly as fuel. From the point of view of efficiency, however, a power boiler of a medium size (500 kW) or larger is suited, and particularly this system is rarely used in a distributed power generator not higher than 300 kW. In addition, in the character of the boiler, thermal inertia is so high that it takes long time for starting and stopping. Therefore, long-term continuous operation is desired in this system, and the system is not adequate to a distributed power generator started and stopped in a short time.

[0005] On the other hand, in the system 2) in which methane gas is produced, the cost for equipment is high because large-scale equipment such as a methane gas producer is required. Further, the cost for disposal of a residue after the production of methane gas is also required. Thus, there is a disadvantage that the total cost becomes high.

[0006] As the system 3) in which biomass fuel is gasified, there was performed, in 1983, a so-called sawdust power generation system in which sawdust was gasified, and the product gas was fed to an engine through a gas cleaner so as to generate power of several tens of kVA (see Charcoal-Gas Car by Kozo Shionoya, 1996, POWERSHA Inc.,pp.17-18). In this case, there is a problem that the combustion efficiency is low because sawdust is powdery.

[0007] In this case, it is considered that sawdust is formed into solid chips, such solid biomass fuel is gasified, and this gas is used for power generation. By use of the solidified biomass fuel, it becomes easy to handle the fuel.

[0008] However, the solid biomass fuel is different from carbonized fuel such as charcoal in that a large amount of tar is contained in the product gas. There is a problem that such tar adheres to an intake valve or pipe arrangement of an engine so that long-term continuous operation becomes difficult. To solve this problem, it is necessary to remove the tar by use of a special filter or a spray shower (see the above document Charcoal-Gas Car, pp. 126-127). Alternatively, there is a method in which the gas producer is formed as a downdraft so as to burn and remove tar.

[0009] As for such a downdraft gas producer, some patent applications have been proposed (for example, see International Patent Publication No. 2000-505123). A gas producer disclosed in the International Patent Publication No. 2000-505123 is apparatus for gasifying chiefly an organic solid having a tendency to form slag. The same publication, however, discloses a downdraft gasifier in which wood or compost is formed into pieces, and the pieces are used as fuel. The gasifier has a gas producer. The gas producer includes a fuel feed portion provided in an upper portion, an air inlet provided in a central portion, a fire grate provided in a lower portion, and a delivery portion provided under the fire grate for delivering product gas. This gas producer is indeed a downdraft gas producer aimed at discharging the slag, but not aimed at burning and removing tar by downdraft. To burn and remove tar, producer structure and operation based on a combustion mechanism are required.

[0010] Next, description will be made below about the details of the mechanism of combustion in a gas producer (see the above document Charcoal-Gas Car pp. 30-35) and a distributed power generation system using solid biomass as fuel.

[0011] Generally, the gasifying method is defined as a method in which gas is produced chiefly by the reaction between carbon (C) and oxygen (O2) or between carbon and steam at a high temperature. When only the air is sent into the producer, the following reactions are involved.

C+O2═CO2   (1)

C+(½)O2═CO  (2)

CO+(½)O2═CO2  (3)

CO2+C═2CO  (4)

[0012] Of these reactions, the reactions (1) and (3) are combustion or oxidation reactions, and exothermic reactions. On the other hand, the reaction (4) is a reduction reaction and an endothermic reaction.

[0013] This reduction reaction is also called a producer gas reaction. It is considered that carbon dioxide (CO2) produced by the reaction (1) or (3) comes into contact with glowing carbon in a reduction layer so that (CO) is produced by the reduction action. In a downdraft gas producer, the air rich in oxygen comes into contact with carbon particles so as to carry out an oxidation reaction. When the oxygen is consumed and only carbon dioxide (CO2) is left, the carbon dioxide comes into contact with glowing carbon in a lower stage of the gas producer so that (CO) is produced. Thus, in a fuel zone in the downdraft gas producer, a reduction zone, an oxidation zone (or combustion zone) and a dry distillation zone (or preheating zone) are generally formed sequentially in the order of increasing height from a fire grate provided in the lower portion.

[0014] Next, description will be made on the outline of a distributed power generation system using solid biomass as fuel, with reference to FIG. 5. FIG. 5 shows a block diagram of the schematic configuration of the system. In FIG. 5, a gasifier 50 includes a gas producer 51, a gas cleaner 52 for cleaning product gas, and a gas cooler 53. Solid biomass fuel and the air are introduced into the gas producer 51.

[0015] Gas discharged from the gasifier 50 is fed to an engine 62 through a gas mixer 61 for mixing the gas with the air, and used as fuel gas for the engine. Thus, electric power is generated by a power generator 63 coupled with the engine. Alternatively, the gas mixer 61 may be provided integrally with the engine 62. In addition, the gas cleaner 52 and the gas cooler 53 may be arranged integrally. Further, exhaust heat in the gasifier 50 or the engine 62 may be utilized effectively in accordance with necessity so as to form a co-generation (electricity and heat feed) system.

[0016] In the related-art downdraft gas producer, when gas containing tar passes through a high-temperature combustion zone again, there is indeed an advantage that the tar is burned and removed. However, problems arise as follows, on the other hand.

[0017] Among them, there is one problem that gas with a stable CO concentration ratio suitable as reduction gas cannot be obtained because a combustion zone and a reduction zone are assumed to be mixed in the zone where the tar is burned, and it is therefore difficult to keep a stable combustion state. Further detailed description on this problem will be given later. In addition, there is another problem that the rise time required for the CO concentration to reach a high value suitable as engine gas is long because the CO concentration increases gradually when the gas producer is started.

SUMMARY OF THE INVENTION

[0018] The invention is achieved to solve the foregoing problems. An object of the invention is to provide a method and apparatus for gasifying solid biomass fuel, in which product gas contains no tar, starting and stopping of a gas producer are easy, gas having a stable composition can be obtained, and the rise time required for the stable composition is short.

[0019] To solve the foregoing problems, as in (1), the invention provides a downdraft gasifier for gasifying solid biomass fuel, having a gas producer, the gas producer including a solid biomass fuel introducing portion provided in an upper portion, an air inlet provided in a central portion, a fire grate provided in a lower portion, and a delivery portion provided below the fire grate for delivering product gas, wherein the downdraft gasifier further has: fire-proof ball laid layers in which a plurality of fire-proof balls such as metal balls or ceramics balls are disposed in layers on the fire grate substantially uniform; and a preheating unit for preheating the fire-proof balls.

[0020] By the gasifier, it is possible to form and keep a stable high-temperature reduction zone on the fire grate. Thus, as will be described in detail later, the rise time required for the CO gas concentration to reach a high level can be shortened, and the same concentration can be stabilized in the high level.

[0021] In addition, the invention stated in (2) and (3) are preferable for carrying out the invention. That is, as in (2), the preheating unit is a detachable burner for preheating the fire-proof balls by combustion heat in the gasifier defined in (1). Thus, the preheating structure and the preheating operation become simple and easy.

[0022] Further, as in (3), a cleaner/cooler for cleaning and cooling the product gas is provided in the gasifier defined in (1) or (2). Thus, high quality gas can be obtained. A cyclone or an oil filter can be used as a cleaner. The oil filter may be integrated with a cooler.

[0023] On the other hand, the invention stated in (4) is preferable as the method for producing gas. That is, as in (4), there is provided a downdraft gasifying method for gasifying solid biomass fuel, having the steps of: introducing solid biomass fuel from an upper portion of a producer; introducing combustion air from a central portion of the producer; and delivering product gas from a portion below a fire grate, the fire grate being provided in a lower portion of the producer; wherein the downdraft gasifying method further has the steps of: preheating a plurality of fire-proof balls such as metal balls or ceramics balls to a predetermined temperature in advance before the solid biomass fuel is introduced, the fire-proof balls being disposed in layers on the fire grate substantially uniform; and subsequently delivering gas from a portion below the fire grate, the gas being produced in the producer after the solid biomass fuel is introduced into the producer. Thus, as described previously, an intended combustion and reduction operation can be obtained so that high quality product gas can be obtained stably.

[0024] Further, the invention stated in (5) and (6) are preferable for carrying out the invention. That is, as in (5), in the gasifying method defined in (4), the preheating temperature of the fire-proof balls preheated ranges from 500° C. to 800° C. If the preheating temperature is lower than 500° C., the operation and effect can be obtained adequately. On the contrary, if the preheating temperature is higher than 800° C., the operation and effect are indeed equivalent to those conducted at a temperature ranging from 500° C. to 800° C., but unnecessary preheating is unfavorable from the point of view of reduction in total thermal efficiency.

[0025] In addition, as in (6), in the gasifying method defined in (4) or (5), after solid biomass fuel is introduced through a fuel feed gate from a solid biomass fuel feed hopper provided in an upper portion of the producer, the fuel feed gate is closed to stop fuel feed, and when a fuel level in the producer reaches a predetermined level due to fuel consumption in the producer, the fuel feed gate is opened to resume fuel feed. Thus, it is possible to produce gas continuously and stably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGS. 1A and 1B are views showing a schematic configuration of a gasifier according to an embodiment of the invention.

[0027] FIG. 2 is a view showing a schematic state when the gasifier shown in FIG. 1 starts.

[0028] FIG. 3 is a graph showing the result of an experiment of a combustion state according to the embodiment of the invention.

[0029] FIG. 4 is a graph showing the result of an experiment of a combustion state in a related-art gasifier.

[0030] FIG. 5 is a block diagram showing a schematic configuration of a distributed power generation system using solid biomass as fuel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] An embodiment of the invention will be described below with reference to the drawings.

[0032] FIGS. 1A and 1B are views showing a schematic configuration of a gasifier for gasifying solid biomass fuel according to the invention. FIG. 1A shows a schematic state in a normal operation. FIG. 1B shows a view for explaining a rough configuration of a fuel feed gate 12 in FIG. 1A. In addition, FIG. 2 shows a schematic state when the same apparatus is started. In FIGS. 1A, 1B and 2, the same constituent parts are referenced correspondingly. The configuration of the gasifier will be described below with reference to FIG. 1A.

[0033] In FIG. 1A, the gasifier includes a gas producer body 1, a cyclone 2, a suction fan 3, and solid biomass fuel 4. Gas produced in the gas producer body 1 is sucked by the suction fan 3. The sucked gas passes through a gas delivery pipe 21, the cyclone 2 and a gas discharge pipe 22, and is fed to the engine side, for example, via another a not-shown oil-filter/cooler. In the cyclone 2, solid impurities are removed by centrifugal action of the cyclone 2.

[0034] The gas producer body 1 includes a fuel feed hopper 11, a fuel feed gate 12, a producer combustion portion 13, a fire grate 14, metal balls 15, a burner port 16, and an air inlet pipe 17. At the starting time, a rod-like member of a burner simply illustrated as a member 18 in FIG. 2 is inserted into the burner port 16.

[0035] In this configuration, the metal balls 15 are disposed in layers substantially uniform. For example, balls of stainless steel or cast iron can be used as the metal balls 15. In addition, the material of the balls is not limited to metal. These balls may be made of a fire-proof material such as ceramics. The diameter of the balls is preferably in a range of about 30 mm to about 60 mm from the point of view of clogging prevention or soaking effect.

[0036] Preferably, the fuel feed gate 12 is designed to be able to feed the solid biomass fuel 4 from the fuel feed hopper 11 as uniformly as possible with respect to the radial direction of the producer combustion portion 13. For example, the fuel feed gate 12 preferably has a rotary gate structure, as shown in FIG. 1B.

[0037] In FIG. 1B, the reference numeral 12a represents a rotary gate; and 11a, a hopper bottom disposed on the rotary gate. In the rotary gate 12a, for example, a part of a rotary disc is notched at the angle of 300 centering the center of the disc, and a notch portion 12b formed thus is used as a feed port for the solid biomass fuel 4. On the other hand, a shield portion 11b for covering the notch portion is provided in the hopper bottom 11a.

[0038] In such a configuration, when the rotary gate 12a is rotated, the notch portion 12b moves sequentially in the circumferential direction. Thus, the solid biomass fuel 4 can be fed substantially uniformly. Incidentally, the fuel feed gate 12 comes into a closed mode when the shield portion 11b at the hopper bottom is superimposed on-the notch portion 12b of the rotary gate.

[0039] Next, description will be made below on the operation of the gas producer body 1.

[0040] At the starting time before the solid biomass fuel 4 is charged, the metal balls 15 are preheated to 500-800° C. by the burner 18 as shown in FIG. 2. At this time, since the combustion exhaust gas of the burner is discharged from a portion below the fire grate 14 through gaps among the metal balls, the metal balls are heated substantially uniformly. When the preheating of the metal balls 15 is completed, the burner 18 is extracted from the burner port 16, and the burner port 16 is closed. Then, the solid biomass fuel 4 is charged. After the fuel charge is completed, the fuel feed gate 12 is closed.

[0041] The fuel in the producer combustion portion 13 begins combustion in a zone above the preheated metal balls 15. Since the air inlet is provided in the central portion of the producer combustion portion 13, the combustion zone (oxidation zone) moves upward gradually. As a result, the zone right above the metal balls 15 becomes a reduction zone while a zone in the central portion near the air inlet becomes a combustion (oxidation) zone. A zone above the combustion zone (oxidation zone) becomes a preheated zone (dry distillation zone) Incidentally, tar contained in gas is burned and reduced into combustible gas when it is passing through the reduction zone.

[0042] When the solid biomass fuel in the producer combustion portion 13 is consumed so that the fuel level is lowered, a not-shown level sensor detects that the fuel level has reached a predetermined level. On the basis of this detection signal, the fuel feed gate 12 is opened to feed fuel again. For example, a known optical sensor can be used as the level sensor. Alternatively, the fuel level may be detected on the basis of the correlation between the temperature change of the metal balls 15 and the fuel level.

[0043] Next, description will be made below on an example of the result of an experiment in which chaff chips were gasified into fuel gas by the solid biomass fuel gasifier configured thus, in comparison with the result of an experiment using related-art apparatus having no layers having fire-proof balls such as metal balls laid.

[0044] FIG. 3 shows the result of the experiment according to the invention when the preheating temperature of the metal balls (stainless balls) is set at about 600° C. FIG. 4 shows the result of the experiment using the related-art apparatus. In FIGS. 3 and 4, the abscissa designates the lapsed time after combustion is started, and the ordinate designates the temperatures of an oxidation zone and a reduction zone and the CO concentration (volume %) of product gas. Incidentally, in FIG. 3, the temperature of the metal balls (ball layer temperature) is also shown.

[0045] First, description will be made with reference to FIG. 4. In the related-art apparatus, at the starting time, heated charcoal is put on a fire grate so as to start combustion. By the combustion start, the temperature in the vicinity of an air inlet (oxidation zone temperature) increased to about 1,400° C. at a blast, and the temperature is kept substantially. The temperature of a reduction zone increased gradually from about 400° C. The CO concentration increased gradually, but showed a low concentration value for about 1 hour after the point of time of the combustion start. After 1 hour had passed, the concentration increased comparatively. However, the CO concentration is not stable, but went up and down repeatedly.

[0046] This result can be considered as follows. That is, in the case of the related-art apparatus, the thickness of a combustion zone (including a preheated zone, an oxidation zone and a reduction zone) is easy to change in a small-scale producer, and the oxidation zone (combustion zone) and the reduction zone were mixed in the diameter direction of the same section (where the producer is sectioned crosswise). Accordingly, it is difficult to keep a stable combustion state so that the CO concentration is not stable.

[0047] On the other hand, according to the result of the experiment of the invention in FIG. 3, the temperature of a reduction zone is higher at the start of combustion with ignition made by the metal balls. When about 12 minutes had passed, the temperature of an oxidation zone and the temperature of the reduction zone crossed each other and then became reverse. In addition, at this point of time, the CO concentration increased suddenly and is stabilized. The CO concentration reached the ceiling at 50%, because 50% is the highest scale in a measuring instrument. Actually, the CO concentration not lower than the value shown in FIG. 3 could be obtained. The CO concentration fluctuated in the vicinity of the lapsed time of 30 minutes because fuel is charged again. Except the short time after the fuel is charged again, the CO concentration is stabilized again. There is no big change in the ball layer temperature. Incidentally, excluding CO, nitrogen (N2) in the air is mainly contained in the product gas.

[0048] As is apparent from the comparison between FIG. 3 and FIG. 4, an ideal combustion state is achieved by providing and preheating metal-ball laid layers and then burning solid biomass fuel. Thus, a stable high-temperature reduction zone can be formed and kept on a fire grate so that the rise time required for the CO gas concentration to reach a high level can be shortened, and the same concentration can be stabilized in a high level.

[0049] As described above, according to the invention, a downdraft gasifier has a gas producer. The gasifier includes a solid biomass fuel introducing portion provided in an upper portion, an air inlet provided in a central portion, a fire grate provided in a lower portion, and a product gas delivery portion provided below the fire grate. In this gasifier, there are provided fire-proof ball laid layers in which a plurality of fire-proof balls such as metal balls or ceramics balls are disposed in layers on the fire grate substantially uniform, and a preheating unit for preheating the fire-proof balls.

[0050] The fire-proof balls are preheated in advance to a predetermined temperature ranging from 500° C. to 800° C. before the solid biomass fuel is introduced. After that, the solid biomass fuel is introduced into the producer, and gas produced in the producer is delivered from a portion below the fire grate.

[0051] Accordingly, when the solid biomass fuel is gasified, the product gas contains no tar, and starting and stopping of the gas producer are easy. In addition, gas high in CO concentration and stable in composition can be obtained. Further, the rise time required for reaching the stable composition can be shortened.

Claims

1. A downdraft gasifier for gasifying solid biomass fuel, comprising:

a gas producer including a solid biomass fuel introducing portion provided in an upper portion,

an air inlet provided in a central portion,

a fire grate provided in a lower portion,

a delivery portion provided below said fire grate for delivering product gas,

fire-proof ball laid layers in which a plurality of fire-proof balls such as metal balls or ceramics balls are disposed in layers on said fire grate substantially uniform, and

a preheating unit for preheating said fire-proof balls.

2. The gasifier for gasifying solid biomass fuel according to claim 1, wherein

said preheating unit is a detachable burner for preheating said fire-proof balls by combustion heat.

3. The gasifier for gasifying solid biomass fuel according to claim 1, further comprising:

a cleaner/cooler for cleaning and cooling said product gas.

4. A downdraft gasifying method for gasifying solid biomass fuel, comprising the steps of:

introducing solid biomass fuel from an upper portion of a producer;

introducing combustion air from a central portion of said producer;

delivering product gas from a portion below a fire grate, said fire grate being provided in a lower portion of said producer;

preheating a plurality of fire-proof balls such as metal balls or ceramics balls to a predetermined temperature in advance before said solid biomass fuel is introduced, said fire-proof balls being disposed in layers on said fire grate substantially uniform; and

subsequently delivering gas from a portion below said fire grate, said gas being produced in said producer after said solid biomass fuel is introduced into said producer.

5. The method for gasifying solid biomass fuel according to claim 4, wherein

said preheating temperature of said fire-proof balls ranges from 500° C. to 800° C.

6. The method for gasifying solid biomass fuel according to claim 4, wherein

after solid biomass fuel is introduced through a fuel feed gate from a solid biomass fuel feed hopper provided in an upper portion of said producer, said fuel feed gate is closed to stop fuel feed, and when a fuel level in said producer reaches a predetermined level due to fuel consumption in said producer, said fuel feed gate is opened to resume fuel feed.