Method for reducing the tar content in pyrolysis gas

Abstract

Disclosed is a method for reducing the tar content in pyrolysis gas generated in a pyrolysis reactor (1). The method comprises the steps of: guiding the pyrolysis gas through a filter (2) to remove at least 90% of all the particles in the pyrolysis gas having a particle size down to 7μ and preferably down to 4μ from the pyrolysis gas, partially oxidizing the pyrolysis gas in a partial oxidation reactor (3) to remove tar from the pyrolysis gas, and guiding the pyrolysis gas through a coke bed (4) to further remove tar from the pyrolysis gas. Furthermore, a two-stage gasifier (6) is disclosed.

Claims

1. A method for reducing the tar content in pyrolysis gas generated in a pyrolysis reactor, said method comprising the steps of: guiding said pyrolysis gas through one or more filters to remove at least 90% of all the particles in said pyrolysis gas having a particle size down to 7 μm from said pyrolysis gas, partially oxidizing said pyrolysis gas in a partial oxidation reactor to remove tar from said pyrolysis gas, and guiding said pyrolysis gas through a coke bed to further remove tar from said pyrolysis gas, wherein said coke bed is arranged external to said pyrolysis reactor and external to a gasifier arranged to gasify pyrolysis coke generated in said pyrolysis reactor.

2. A method according to claim 1, wherein said method further comprises the step of guiding said pyrolysis gas through a cyclone to remove particles in said pyrolysis gas having a particle size down to 20 μm from said pyrolysis gas, before said pyrolysis gas is guided through said filter.

3. A method according to claim 2, wherein said particles that are removed by said cyclone are guided into said coke bed to form part of said coke bed and/or into a gasifier.

4. A method according to claim 1, wherein said particles that are removed by said one or more filters are guided into said coke bed to form part of said coke bed and/or into a gasifier.

5. A method according to claim 1, wherein said filter removes at least 90% of all the particles in said pyrolysis gas having a particle size down to 2 μm from said pyrolysis gas.

6. A method according to claim 1, wherein said partial oxidation raises the temperature of said pyrolysis gas to between 500 and 2,500° C.

7. A method according to claim 1, wherein said coke bed has a temperature of between 400 and 2,000° C.

8. A method according to claim 1, wherein said coke bed is a fluid bed coke bed.

9. A method according to claim 1, wherein said pyrolysis reactor is a fluid bed pyrolysis reactor.

10. A method according to claim 1, wherein said partial oxidation involves adding enough oxygen to combust all said pyrolysis gas between 5% and 70%.

11. A method according to claim 1, wherein said pyrolysis gas has a temperature of between 100 and 1,000° C. when entering said partial oxidation reactor.

12. A method according to claim 1, wherein said method comprises the step of generating said pyrolysis gas by heating biomass.

13. A method according to claim 1, wherein material in said pyrolysis reactor is heated by forming a flow of oxygen and/or superheated steam into said pyrolysis reactor.

14. A method according to claim 1, wherein said one or more filters remove at least 95% of all the particles in said pyrolysis gas having a particle size down to 4 μm from said pyrolysis gas.

15. A method according to claim 1, wherein said one or more filters comprises a candle filter.

16. A two-stage gasifier arranged to remove tar from pyrolysis gas by means of a method according to claim 1.

17. A method according to claim 1, said step of guiding said pyrolysis gas through one or more filters removes at least 90% of all the particles in said pyrolysis gas having a particle size down to 4 μm.

18. A method according to claim 1, wherein said method further comprises the step of guiding said pyrolysis gas through a cyclone to remove particles in said pyrolysis gas having a particle size down to 12 μm from said pyrolysis gas, before said pyrolysis gas is guided through said filter.

Description

FIGURES

(1) The invention will be described in the following with reference to the figures in which

(2) FIG. 1. illustrates a pyrolysis reactor followed by a gasifier where the coke bed is arranged internally in the gasifier, as seen from the front, and

(3) FIG. 2 illustrates a pyrolysis reactor followed by a gasifier where the coke bed is arranged externally to the gasifier, as seen from the front.

DETAILED DESCRIPTION

(4) FIG. 1 illustrates a pyrolysis reactor 1 followed by a gasifier 6 where the coke bed 4 is arranged internally in the gasifier 6, as seen from the front.

(5) In this embodiment fuel 8 is guided into the pyrolysis reactor 2 and superheated steam is guided into the pyrolysis reactor 2 at the bottom at a temperature at around 500° C. The superheated steam will then pyrolyze the fuel 8 as it moves up through the fuel 8 and the airborne pyrolyzed coke 7 will follow the pyrolysis gas out of the pyrolysis reactor 2 at the top of the pyrolysis reactor 2 where it in this embodiment enters a cyclone 5 that will separate the pyrolysis coke 7 from the pyrolysis gas and transfer the pyrolysis coke 7 to a gasifier 6.

(6) In this embodiment, the fuel 8 is straw material but in another embodiment the fuel could be wood chips, (raw or pre-dried) animal slurry, (raw or pre-dried) sewage, surplus material from biochemical production or food production, another natural plant material or any other form of organic material or plastic, fossil fuel or other.

(7) At the top of the pyrolysis reactor 2 the operation temperature will typically have dropped so that the pyrolysis gas and coke leaving the pyrolysis reactor 2 will have a temperature around 250-300° C.

(8) From the cyclone 5, the pyrolysis gas will then continue to a high temperature gas filter 2 which will filter substantially all dust and particles out of the pyrolysis gas before it enters a partial oxidation reactor 3 where the pyrolysis gas is partially oxidized in that air, oxygen enriched air or pure oxygen is added to the pyrolysis gas so that the pyrolysis gas is partially combusted, which in turn will raise the temperature of pyrolysis gas to—in this embodiment—around 1,000-1,100° C. before the gas leaves the partial oxidation reactor 3. The partial oxidation will result in an efficient tar decomposition.

(9) From the partial oxidation reactor 3 the hot pyrolysis gas is then reintroduced to the pyrolysis coke 7 in a coke bed 4 formed by a gasifier 6 arranged to gasify the pyrolysis coke from the cyclone 5.

(10) In this embodiment, the pyrolysis gas is not cooled before it enters the coke bed 4 which usually could be a problem in that temperatures above 950° C. could damage the gasifier 6. But by introducing the partially oxidized pyrolysis gas into the middle of the coke bed 4, or at least away from the walls, bottom and/or top of the coke bed 4, the hot pyrolysis gas can be introduced to the coke bed 4 directly from the partial oxidation reactor 3. However, in another embodiment the temperature of the partially oxidized pyrolysis gas would be lowered e.g. to around 900-1,000° C. (preferably around 950° C.) before it entered the coke bed 4 e.g. by means of quenching, by means of a heat exchanger (e.g. to supply heat to the superheated steam entering the pyrolysis reactor 1 and/or coke bed 4) or other.

(11) In this embodiment, the filter high temperature gas filter 2 is a hot gas candle filter but in another embodiment the filter 2 could also or instead comprise another type of hot gas filter such as sinter metal filters, sieves, strainers or other.

(12) In this embodiment, the coke bed 4 is also heated by means of superheated steam entering the coke bed 4 at the bottom at a temperature of around 900° C. to ensure gasification of the pyrolysis coke 7 and efficient decomposition of the remaining tar in the pyrolysis gas. The resulting substantially tar free product gas then exits the coke bed 4 at the top at a temperature around 700-750° C. in this embodiment.

(13) It should be underlined that the temperature examples mentioned above and below are specific examples relating to a specific type of wood chip being used as fuel 8 in this embodiment. However, if different fuel 8 was used some of the temperatures might be higher or lower.

(14) In the embodiments disclosed in FIGS. 1 and 2 both the pyrolysis reactor 1 and the coke bed 4 (which in this case is also the gasifier 6) are formed as fluid bed reactors. But in another embodiment the pyrolysis reactor 1, the coke bed 4 and/or the gasifier 6 could also or instead be in a counterflow configuration, a co-current fixed bed (“down draft”), entrained flow configuration, plasma configuration and/or other or the pyrolysis reactor 1, the coke bed 4 and/or the gasifier 6 could also or instead be based on the fixed coke bed principle.

(15) A “counterflow” configuration is to be understood any kind of pyrolysis reactor or coke bed where hot gas, air, steam or another gaseous substance is being fed in the bottom pyrolysis reactor or coke bed to either directly or indirectly drive the respective pyrolysis or gasification and the resulting gas is drawn from the top of the pyrolysis reactor or coke bed, while the fuel is fed at the top of the pyrolysis reactor or coke bed so that the closer the fuel moves to the bottom of the pyrolysis reactor or coke bed the more processed it is. I.e. fuel and gas moves in opposite directions—hence “counterflow”. “Counterflow” is also often referred to as “updraft”, “upward draft”, “counter-current” and other.

(16) FIG. 2 illustrates a pyrolysis reactor 1 followed by a gasifier 6 where the coke bed 4 is arranged externally to the gasifier 6, as seen from the front.

(17) In this embodiment, the system differs from the system disclosed in FIG. 1 in that the partially oxidized pyrolysis gas leaving the partial oxidation reactor 3 is guided into a separate coke bed 4 formed by means of coke from a separate process—i.e. not supplied from the preceding pyrolysis reactor 1. In this embodiment, the coke bed 4 could be formed from coke from an external combustion process or other.

(18) In the coke bed 4 the coke will be gasified in a similar manner as discussed in relation with the gasifier 6 to enable decomposition of at least some of the remaining tar in the partially oxidized pyrolysis gas.

(19) In this embodiment, the product gas leaving the external coke bed 4 will be mixed with the product gas leaving the gasifier 6 but in another embodiment the product gas leaving the external coke bed 4 could be drawn separately or it could be fed to the gasifier 6.

(20) In a further embodiment, the coke bed 4 could be formed by a combination of external coke and pyrolysis coke—i.e. a combination of the embodiments disclosed in FIGS. 1 and 2.

(21) The invention has been exemplified above with reference to specific examples of pyrolysis reactors 1, filters 2, gasifiers 6 and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

LIST

(22) 1. Pyrolysis reactor 2. Filter 3. Partial oxidation reactor 4. Coke bed 5. Cyclone 6. Gasifier 7. Pyrolysis coke 8. Fuel