WO2002033030A1

WO2002033030A1 - Method and device for gasifying biomass

Info

Publication number: WO2002033030A1
Application number: PCT/NL2001/000761
Authority: WO
Inventors: Abraham Van Der Drift; Christiaan Martinus Van Der Meijden
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2000-10-16
Filing date: 2001-10-16
Publication date: 2002-04-25
Also published as: NL1016411C2; AU2002214394A1

Abstract

Method and device for gasifying biomass in two stages. During a first stage, the biomass, mixed with an auxiliary medium, is exposed to an elevated temperature. In the process, fuel gases are released, as well as a mixture or auxiliary medium and charcoal-like material. The latter is then burned. The heat released is fed to the first stage. An additional stage, in which the tar which is present is separated from the gas production stream, is incorporated between these two stages, making use of the intermediate product, namely charcoal-like material.

Description

Method and device for gasifying biomass

The present invention relates to a method of gasifying biomass, comprising, in a first stage, mixing said biomass with an auxiliary medium and gasifying the biomass, discharging said gasification products, comprising tar, and the auxiliary medium from the first stage, separating said discharge stream into a stream of solid particles and a gaseous stream comprising product gas, discharging said gaseous stream, feeding said solid particles to a second stage, burning the solid gasification products (char) in the second stage, and feeding the said auxiliary medium to said first stage.

A method of this type is known from EP 0239589 Al, which describes the gasification of biomass using sand as auxiliary medium. In the gasifier, biomass is mixed with sand and steam is passed through. Decomposition into product gas and other components takes place at elevated temperature. These other components comprise charcoal-like material (char) which is burned in a second step. In principle, all that remains is auxiliary medium, such as sand, which can then be fed back to the first stage. Naturally, flue gases are also released in the second stage.

It has been found that in all situations where biomass is used as a fuel, tar is formed in the first product gas.

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In the prior art, it is proposed to remove this tar in a separate step after the process. In this context, the term tar is understood as meaning substances with long hydrocarbon chains which condense at a relatively high temperature and therefore cause problems.

The method used in the prior art, in which tar is removed in a separate stage, means that it is necessary to supply additional energy, with the result that the efficiency of the process decreases. Moreover, the environmentally friendly nature of the gasification of biomass appears to contradict the environmentally unfriendly nature of carcinogenic substances such as tar. Transporting tar and making it harmless involves a health risk. The object of the present invention is to improve the method which is known from the prior art, so that, firstly, the tar can be removed efficiently and without health risks and, secondly, the thermal efficiency of the method increases.

In a method as described above, this object is achieved in that between the first and second stages the discharge stream of the first stage is subjected to an intermediate stage for at least 1 second in a separate space at a temperature of between 700-900°C, in order to separate tar which is present in the discharge stream from the first stage from said gaseous stream.

The invention prevents the product gas from containing tar which subsequently has to be removed using a separate stage. In principle, according to the present invention tar can be removed in accordance with two mechanisms, optionally in combination. Firstly, it is possible for tar to be (partially) broken down in the intermediate stage during the relatively long residence time of more than 1 s. In the process, gaseous products which can still be taken up in the product gas are formed. This process may be catalytically enhanced, for example by the solid carbon-containing particles originating from the first stage. It is also possible for (some of the) tar to be adsorbed on the solid particles, and in particular the carbon-containing particles, and to be broken down during the combustion in the second stage. This carbon-like material is always derived directly from the gasification in the first stage and, unlike the auxiliary material, is not a reused material, with the result that the possible catalytic action or absorbing action is optimum.

In both cases, the energy content of tar is not lost, and lower levels of harmful components are formed. The auxiliary material which is fed back to the first stage comprises substantially no tar.

According to an advantageous embodiment, the product gas is separated out directly or, more particularly, during this intermediate stage. The remainder of the product stream from the first stage is subjected to conventional combustion in order to generate electricity and/or to produce heat or as a synthesis gas for, inter alia, the production of methane, methanol, diesel. According to an advantageous embodiment of the invention, the stages take place in a single reactor casing. This as far as possible prevents heat losses and enables the thermal efficiency of the device to increase. The intermediate stage is preferably carried out in a chamber which is located above the first reactor chamber in which the first reaction is carried out. This chamber preferably also lies above the second reactor chamber, in which the second stage, i.e. the combustion, is carried out. In this way, it is possible to achieve optimum circulation of, in particular, the auxiliary material, without it being necessary to use special transport means. In the gasifier, the transport takes place as a result of the supply of steam and biomass. In the second reactor, transport takes place by means of air fed to it. According to the invention, processing of biomass can be integrated in a single reactor which in principle has two outlets, one for product gas and one for flue gas. A device of this type is particularly easy to construct.

It will be understood that any other auxiliary medium can be used for the biomass instead of sand. The same applies to the abovementioned steam and air. Steam is used primarily to provide transport in the first reactor chamber. Air provides the oxygen required for the combustion of the solid gasification products (char).

According to an advantageous embodiment, a single reactor of this type can be insulated.

The time for which the product stream originating from the first stage is to be subjected to tar reduction is dependent on the quantity of tar present, which in turn is, of course, dependent on the gasified biomass. This time is preferably between one and four seconds, and the residence time in the tar-reduction space in question can be regulated by the speed of movement of the product stream and the volume of the space in question. These are partially defined during design.

The temperature at which this tar reduction takes place is preferably between 700 and 900°C, and more particularly is 850°C. The invention will be explained in more detail below with reference to the exemplary embodiment illustrated in the drawing, in which:

Fig. 1 diagrammatically depicts the various stages of the method according to the invention;

Fig. 2 diagrammatically depicts a device which can be used to carry out the method according to the invention; and

Fig. 3 shows the cross section on III-III in Fig. 2.

Fig. 1 illustrates the various stages according to the present invention on the basis of an example. In block 3, biomass, steam and energy are supplied in order in this way to gasify the biomass. The energy is stored in the auxiliary medium originating from block 4. Steam is supplied in order to produce a fluidized bed and in order to promote the gasification.

In block 4, the combustible solid fraction of product discharge stream originating from block 5 is burned. The auxiliary medium, such as sand, is also present in this product stream and is heated in the process. The combustion takes place with air being supplied, and flue gas is released.

Between blocks 3 and 4, according to the invention there is a block 5, where tar reduction takes place. Subjecting the product stream originating from block 3 to a temperature of approximately 850°C for at least one second causes tar which is present to break down into less harmful substances. These less harmful substances are discharged for beneficial use together with the product gas which is formed during the gasification. The residence time required is determined by the component in the product stream and the intensity with which carbon-containing material and tar- containing gas are in contact.

Fig. 2 shows an exemplary embodiment of a device which can be used to implement the method according to the invention. This device is denoted overall by 1 and comprises a single reaction chamber 3 which is provided with insulation (not shown in more detail). Within this housing 2, a number of chambers are delimited. The first reaction chamber is denoted by 3. As can be seen from the cross section III-III from Fig. 2, this is a single tube. On the underside, the housing 2 is closed off by a screen plate 15 through which gas but no material can pass. Steam is fed to the first reaction chamber 3 via inlet 8. Moreover, biomass is fed to the reaction chamber 3 via inlet 9, and auxiliary medium which is at a high temperature, such as sand, is fed to the reaction chamber 3 via inlet 11. As a result, the auxiliary medium is moved upwards via inlet 11 and, at inlet 9, comes into contact with the biomass, which is gasified as a result of the high temperature of the auxiliary medium (approximately 900°C). This leads to the formation of product gas, which leaves the tubular reactor 3 via the outlet 10. The product stream then passes into reaction space 5. The residence time in chamber 5 is approximately 1-4 seconds. The temperature is approximately 850°C. Various mechanisms can be used to make tar which is formed in reaction chamber 3 harmless during the relatively long residence time, i.e. to prevent tar from being entrained with the product gas. Firstly, it is possible to (catalytically) decompose tar and to discharge the gaseous decomposition products with the product stream, so that they can be used beneficially. It is also possible for tar to be adsorbed on the medium which is to be burned in the second stage and to be decomposed and burned in this second stage. Auxiliary medium and solid products which form after the gasification (charcoal-like products) are fed to reaction chamber 4 from chamber 5 via one or more inlets 12 (only one of which is illustrated). This reaction chamber 4 is closed off from below by a screen plate which is separate from screen plate 15. There, a fluidized bed is maintained by the supply of air denoted by 14. The flue gases released are discharged at 13. During the combustion, heat is formed, which is stored in the auxiliary medium and is released back to the reaction chamber 3 in the manner described above by transport of the auxiliary medium.

Moreover, the method described above has the advantage that the nitrogen present in the air introduced at inlet 14 is discharged via the outlet 13 for flue gas. Consequently, the calorific value of the product gas obtained at outlet 6 does not decrease, and the possible applications are widened. Compared to product gas obtained with a conventional gasifier, i.e. a gasifier which is not indirect, the calorific value is approximately twice as high, i.e. approximately 15 MJ/m_n ³. It has been found that the present invention makes it particularly easy to achieve a stable equilibrium.

Surprisingly, it has been found that a particularly substantial reduction of tar can be achieved in chamber 5. It is assumed that the solid material originating from the first reactor chamber, which is referred to as char has a catalyzing and adsorbing action. Since fresh solid material is being constantly supplied, a catalytic/adsorbing action of this type is continuously maintained.

The invention enables tar emissions to be greatly restricted and the efficiency of the gasification of biomass to be increased considerably, and the health risks can be limited since there is no need to handle various types of tar which are released.

It will be understood that various stages described above may be carried out in a slightly different way and that further stages may be connected upstream, as intermediate stages or downstream. All stages of this type are obvious on reading the above description and lie within the scope of the appended claims.

Claims

1. Method of gasifying biomass, comprising, in a first stage, mixing said biomass with an auxiliary medium and gasifying it, discharging said gasification products, comprising tar, and the auxiliary medium from the first stage, separating said discharge stream into a stream of solid particles and a gaseous stream comprising product gas, discharging said gaseous stream, feeding said solid particles to a second stage, burning the solid gasification products in the second stage, and feeding said auxiliary medium to said first stage, characterized in that between the first and second stages the discharge stream of the first stage is subjected to an intermediate stage for at least 1 second in a separate space at a temperature of between 700-900°C, in order to separate tar which is present in the discharge stream from the first stage from said gaseous stream.

2. Method according to Claim 1, in which said separation comprises the at least partial conversion of said tar.

3. Method according to Claim 2, in which said conversion is catalytically assisted by the char.

4. Method according to one of the preceding claims, in which said separation comprises adsorption on the said solid particles.

5. Method according to Claim 4, in which said separation comprises decomposition during burning of the said solid particles.

6. Method according to one of the preceding claims, in which the product gas is separated off after the intermediate stage.

7. Method according to one of the preceding claims, in which said stages take place in a single reactor housing.

8. Device (1) for gasifying biomass, comprising a first reactor chamber (3) provided with a feed (9) for biomass, a feed (11) for auxiliary medium and a discharge (10) for a first product stream comprising tar and auxiliary medium, a second reactor chamber (4) provided with an inlet (12) for the solid particles from the first product stream and auxiliary medium, a discharge (13) for combustion gases and a discharge for auxiliary medium connected to the feed for auxiliary medium leading into the said first reactor chamber (3), a separation device for gases, comprising a discharge (6) for product gas, being arranged between said first and second reactor chambers, characterized in that said separation chamber is designed in such a manner that the mean residence time of the said first product stream therein is at least 1 s.

9. Device according to Claim 8, in which the said separation chamber comprises a separation device for tar and product gas.

10. Device according to one of Claims 8 or 9, in which said device comprises a housing (2) in which said chambers are arranged.

11. Device according to one of Claims 8-10, in which said separation chamber is arranged above said first reaction chamber.

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12. Device according to one of Claims 8-11, in which said separation chamber is arranged above a second reaction chamber.

13. Device according to one of Claims 8-12, in which said first and second reaction chambers are arranged adjacent to one another.