WO1988009364A1

WO1988009364A1 - A method for generating producer gas from straw and like materials, and apparatus for carrying out the method

Info

Publication number: WO1988009364A1
Application number: PCT/SE1988/000256
Authority: WO
Inventors: John MUNCK AF ROSENSCHÖLD
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-05-27
Filing date: 1988-05-19
Publication date: 1988-12-01
Anticipated expiration: 1989-11-27
Also published as: SE455423B; SE8702233D0

Abstract

According to this invention producer gas is manufactured from a straw-chip fraction obtained from crude straw. The straw-chip fraction is introduced into a pyrolysis space or chamber (13) together with primary air heated to at least about 400°C, therewith to engender so-called flash pyrolysis. The resultant gas, together with contaminants, is passed down through a dolomite bed in a tubular reaction vessel (8) in which reduction of carbonized flakes takes place and the tar present is converted to combustible gas form.

Description

A method for generating producer gas from straw and like materials, and apparatus for carrying out the method.

The present invention relates to a method for generating producer gas from straw or straw-like material by pyroly¬ sis of the straw, and to apparatus for carrying out the method.

It is well known that the progressively increasing surplus of straw in cereal producing countries has become problem¬ atic and that the attempts to use this straw as a source of fuel is beset with serious diffuculties.

Among other things, when pyrolysing crude straw there is obtained a particularly dense bed of charcoal or carbon¬ ized matter through which the crude pyrolysis gas is un¬ able to pass, due to the high resistance presented by the bed. Furthermore, crude straw contains significant quantities of nitrogen which if remaining in the producer gas generated will result in higher quantities of nitrogen oxides (NO ) in the exhaust gases of the engine driven by the producer gas. The larger quantities of ash produced also presents a problem, which ash, furthermore, has a low melting point.

Consequently, it is an object of the present invention to provide a method and apparatus by means of which producer gas can be generated from straw and like materials while avoiding the aforementioned drawbacks.

This object is achieved with the inventive method and ap¬ paratus set forth in the following method and apparatus claims.

By treating the straw or comparable straw-like material in a mill, e.g. a mill of the kind manufactured by the Danish Company United Milling, it is possible to divide straw having a moisture content of, e.g., 10 % into a fraction consisting of straw cuttings, referred to here- inafter as the straw-chip fraction and a powder or flour fraction.

The inventive method utilizes the straw-chip fraction, which can be suspended in an upwardly rising carrier air- flow and introduced thus suspended into a flow of primary air having a temperature of at least about 400 C. The chip-air suspension is then subjected to flash pyrolysis in pyrolysis chambers, i.e. pyrolysis of essentially an explosive nature. The pyrolysis chambers, or channels, preferably increase progressively in cross-sectional area in an upward direction. Secondary air is introduced into the gas fe into the dolomite bed and combusted so as to produce the heat required for the reactions involving carbon dioxide and water. The upper part of the dolomite bed will have a high temperature and in order to decompose the tar or pitch present effectively the lower half of the bed should be maintained at a temperature of at least about 800 C. The ash produced during the combustion pro¬ cess is taken up optionally by the dolomite, which is replaced with fresh dolomite either continuously or at specific intervals.

The invention will now be described with reference to the accompanying drawing, the single figure of which illus- trates very schematically a plant which incorporates a gas generator according to the invention.

The drawing illustrates a producer gas generator which comprises a container vessel 1 with an internal thermal insulation 2 comprised, e.g., of mineral wool. The con- tainer 1 is cylindrical and has an upper opening 3 in which there is arranged a feeder gate 4, shown very schematically in the drawing, which can be opened and closed with the aid of operating means (not shown) so that when open dolomite can be fed from a source not shown, through the gate 4 and into an infeed chute 5 provided with an outlet 6. The outlet 6 of the infeed chute 5 is surrounded by a hollow tubular reactor vessel 8, which in the illustrated embodiment comprises two truncated cones 8', 8" with the cone bases facing one another and joined together. The vessel 8 may be made of steel, a ceramic material or some other wear-resistant and heat-resistant material. The conical wall of the lower cone 8¹ has passing therethrough a multiple of apertures 9 through which the producer gas generated in the conical vessel 8 can depart, while the lower, open end of the cone 8¹ joins with an opening 10 which can be opened and closed, essen¬ tially in a gas-tight fashion, by means of a further gate or valve device 11 of the same kind as the gate 4. The lower part of the container 1 incorporates infeed channels 12 through which hot primary air is fed to an annular space 13 between the upper cane 8" and the inner wall sur¬ face of the container 1, of which channels only one is shown. The channels are distributed around the whole of the container 1. The capacitor of the generator can be varied, by varying the number of channels. The essenti¬ ally annular space 13 is divided by substantially vertical walls (not shown) into a number of mutually separated pyrolysis chambers, into each of which straw-chips and air are introduced. Opening into each pyrolysis chamber is at least one channel 14 for conducting pre-heated secondary air. The horizontal cross-sectional areas of the annular space 13 divided up by the aforesaid walls will therefore progressively increase. This increase in area is neces- sary, since the volume of gas present will increase pro- gressively during the course of pyrolysis. Obviously, this increase in area can be achieved in various ways, e.g. by giving the container a conical configuration and the centrally located tubular vessel the form of a straight cylinder. The channels 12 of the illustrated embodiment are isolated from, a Lower gas outfeed space 18 by means of an annular partition 19 located between the inner wall surface of the container 1 and the vessel 8. Located above the channels 12 is at least one infeed channel 15 for feeding straw-chip fraction into each section, this straw-chip fraction being passed to the channel 15 with the aid of a screw conveyor or some other suitable conveyor means. In the case of the illustrated embodiment a gas burner 16 is arranged in each section in the space 13, slightly above the associated channel 12. The gas burner may optionally be constructed to effect a very rapid pyrolysis reaction. Located in the lower part of the container 1 is an outfeed channel 17 through which generated producer gas is conducted to a cyclone 27 for extracting solids from said gas. The exiting gas has a temperature in the order of 800 C and is cooled in a heat exchanger 20, where heat is transferred, e.g., to the return water of a hot water system. In the case of the illustrated embodiment the gas is passed through a further gas cleansing filter 21. Naturally, the gas shall not be cooled to a level at which water vapour in the gas will begin to condensate, unless a condensation receiver is provided. The gas is then passed to an engine 22 which drives, e.g., an electric generator 23. The engine ex- haust gases, which maintain a temperature of e.g., 450-

500 C, are passed to a heat exchanger 24 which heats pro¬ cess air to a temperature of e.g., 400°C which air is introduced as primary air into respective infeed channels 12, via a pipe 25, and flows upwards in associated pyroly- sis chambers in the annular space 13. The gas engendered by the pyrolysis process is drawn in through the upper opening 26 of the vessel 8 by the suction generated by the engine 22 in operation. The cooled engine exhaust gas will still contain thermal energy subsequent to passing through the heat exchanger 24 and can be passed through a further heat exchanger 28 for transferring heat to, e.g., a water- carried heating system or may also be used to pre-heat the air-chip suspension fed to the container 1 and to the space 13. Pre-heating of the straw chips is effected primarily with the aid of the hot exhaust gases from the heat exchanger 28, these gases being passed through the chip store to this end.

It is assumed that the vessel 8, in which the reactions take place, is filled to the level of the opening 6 with dolomite stones ranging from, e.g., 10 to 100 mm in size, these stones forming a bed which extends down to the valve gate 11. The two sealing valve gates 4 and 11 are closed. As beforementioned, the straw-chip faction, optionally pre-heated, is fed through the inlet openings 15, and in order to engender flash pyrolysis, should the primary air not hold the temperature desired, the respective gas burners 15 are ignited, which will then enhance the suc¬ tion of primary air in through the channels, as a result of the ensuing ejector effect, and ignite the straw chips. The chips are caught up in the air flow. The temperature of the upwardly moving turbulent flow will rise rapidly and the resultant pyrolysis process will be exothermic and strong oxidation will take place. The continually in- creasing pyrolysis reaction will result in an increase in gas speed, so that the straw chips are entrained forcibly by the gas flow while becoming progressively more carbon¬ ized. The straw chips are converted to charcoal flakes in the upper part of the pyrolysis zone in the sectioned space 13, which flakes are sucked down into the centrally located vessel 8 and into the dolomite bed. As a result of the pyrolysis and due to the introduction of pre-heated secondary air through openings 14 is each section, the dolomite bed is maintained at a temperature of at least about 900°C (flame temperature about 1400°C). The combustion of secondary air provides the heat required for reducing the charcoal flakes within the dolomite bed and the desired thermal cracking of the tar or pitch produced during the pyrolysis process in addition to carbon mon¬ oxide (CO), carbond dioxide (CO-) and water (H-O) is converted totally, or at least substantially, to burnable gases.

The pyrolysis process takes place explosively in the de- scribed and illustrated gas generator and the use of a tubular magazine filled with dolomite eliminates the presence of tar in the gas, or at least results in a gas that contains less tar, and enables the gas to be used in a motor.

Claims

1. A method of generating producer gas from straw or like material by subjecting said material to pyrolysis, charac- terized by grinding dry straw to form a chip fraction and a flour fraction, said chip fraction being suspended in an air flow; separating the chip fraction from the flour fraction; introducing the separated chip fraction into an upwardly moving flow of primary air having a tempera- ture of at least about 400°C in at least one pyrolysis chamber having an upper outlet opening, said air flow being brought to a state of turbulence and said pyrolysis chamber being in heat transfer contact, through the medium of a wall surface, with a reaction chamber which contains a dolomite bed; igniting and pyrolizing the ingoing chip fraction; introducing pre-heated secondary air into the gas engendered by pyrolysis and subsequently discharging the gas through the outlet in the pyrolysis chamber and through an upper opening in the reaction chamber and causing said gas to flow down into and through the dolo¬ mite bed; and maintaining the upper part of the dolomite bed at a temperature such as to effect between the carbon, water and carbon dioxide present a reaction which will generate combustible gases; and maintaining the lower part of the dolomite bed at a temperature of at least about 800 C for converting the tar present in the gas to com¬ bustible gas form.

2. A method according to claim 1, characterized by pre- heating the primary and secondary air by means of exhaust gases taken froman engine driven by the producer gas gener¬ ated.

3. A method according to claim 2, characterized by pre- heating and surrounding the straw-chip fraction with said exhaust gases, which are practically inert.

4. Apparatus for carrying out the method according to claim 1 and for generating producer gas by pyrolyzing a straw-chip fraction, characterized in that the apparatus includes a thermally insulated container (1) having ar¬ ranged centrally therein a hollow tubular reaction vessel (8) which defines with the opposing inner wall of the con¬ tainer (1) at least one pyrolysis space (13), in that the vessel (8) has an upper open end through which dolomite and gas generated in the pyrolysis space (13) are fed to the interior of the reaction vessel (8); in that the vessel (8) has a lower end (8¹) which is separated from the pyrolysis space (13) and provided with an outlet aperture (9) for generated producer gas; in that arranged in the lower part of the container (1) above said lower end of the vessel (8) is an inlet (15), through which the straw-chip fraction is introduced into the pyrolysis space, and a primary air inlet (12) located beneath said straw-chip inlet (15); in that arranged in the upper part of the container is an inlet (14) through which secondary air is introduced into the pyrolysis space; in that ar¬ ranged in the lower part of the container (1) is a pro¬ ducer gas outlet (17) which is isolated from the pyrolysis space (13); and in that heating devices (22, 24) are pro¬ vided for heating the primary air to a temperature of at lleeaasstt 440000°CC prior to introducing said air into the pyroly- sis chamber.

5. Apparatus according to claim 4, characterized in that the heating devices comprise at least one heat exchanger (24) which is intended to receive exhaust gas from an engine (22) driven by the generated producer gas.

6. Apparatus according to claim 4 or claim 5, character- ized in that the container (1) has an upper opening (3) and a lower opening (10) each of which is provided with a respective valve gate (4, 11) for respectively supplying dolomite to and removing dolomite from said reaction vessel (8).

7. Apparatus according to any of claims 4-6, character¬ ized in that the cross-sectional area of the pyrolysis space increases progressively in an upward direction.