EP3041919B1

EP3041919B1 - Arrangement and method for burning fuel

Info

Publication number: EP3041919B1
Application number: EP14781612.8A
Authority: EP
Inventors: Timo Honkola
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2013-09-03
Filing date: 2014-09-03
Publication date: 2017-11-08
Anticipated expiration: 2034-09-03
Also published as: WO2015033022A1; SI3041919T1; FI20135887A; EP3041919A1; HUE035411T2; FI126917B; HRP20171737T1

Description

Background

The invention relates to an arrangement for fuel burning, the arrangement comprising gasification equipment for converting fuel into a product gas by gasification and a combustion plant for burning the product gas.
The invention also relates to a method for fuel burning, in which method the fuel is converted into product gas by gasification in gasification equipment and the product gas is burned in a combustion plant.
A growing tendency is to replace conventional fuels by renewable fuels, such as agro fuels, waste, or the like, as a replacing energy source.
However, burning of such replacement fuels causes problems. With agro fuels, for example, problems may arise due to sintering of the fluidized bed, soiling of the walls and other thermal surfaces of the burner, and superheater corrosion, the problems being caused by an attempt to achieve complete combustion and low CO emissions. In EP1136542 A1 , an arrangement for burning fuel is disclosed, having a gasifier, a separator and a combustion plant.

Brief description

The arrangement and method of the invention are characterized by what is disclosed in the characterizing parts of the independent claims. Other embodiments of the invention are characterized by what is disclosed in the other claims.
Inventive embodiments are also disclosed in the specification and drawings of this application. The inventive contents of the application may also be defined in ways other than those described in the following claims. The inventive contents may also consist of several separate inventions, particularly if the invention is examined in the light of expressed or implicit sub-tasks or in view of obtained benefits or benefit groups. In such a case, some of the definitions contained in the following claims may be unnecessary in view of the separate inventive ideas. Features of the different embodiments of the invention may be applied to other embodiments within the scope of the basic inventive idea.
The idea of the invention is that fuel is gasified at a lower temperature so that alkali metals contained in the fuel remain in a solid state and in that the gasified fuel is conveyed to a separator comprising means for purifying the product gas of solids and thus enabling to remove the alkali metals from the product gas to be conveyed into combustion equipment.

Brief description of the drawings

Some embodiments of the invention are explained in more detail in the accompanying drawings, in which

Figure 1 is a schematic side view of an arrangement and method according to the invention,
Figure 2 is a schematic side view of a second arrangement and method according to the invention,
Figure 3 is a schematic side view of a third arrangement and method according to the invention, and
Figure 4 is a schematic view of a combustion plant arrangement comprising an arrangement of the invention.

For the sake of clarity, the figures show some embodiments of the invention in a simplified manner. In the figures, like reference numerals identify like elements.

Detailed description

Figure 1 is a schematic sectional side view of an arrangement and method of the invention. The arrangement 1 may be intended for burning poor-grade fuels, such as wet biomasses, sludges, recycled fuels, and waste coals; naturally other fuels can also be used. According to an idea, a boiler is used for burning what are known as agro fuels. Agro fuel refers to straw, straw pellets, palm oil waste or any other waste produced in agricultural production, for example. Agro fuels typically originate from fast-growing plants and, thus, contain lots of alkali metals, chlorine and phosphor.
The arrangement comprises gasification equipment 3 for converting the fuel to a product gas by gasification and a separator 4 comprising means for purifying the product gas of solids.
The gasification equipment 3 may be any gasification equipment known per se, such as a circulating fluidized bed gasifier, i.e. a CFB gasifier. The gasification of the fuel F takes place in a reactor 7 of the gasification equipment.
Fuel gas P generated in the gasification equipment 3 is conveyed to the separator 4, where solids are separated from it. Product gas G thus produced is conveyed into the combustion plant. Substantially all, most preferably 100%, of the gasified fuel flows from the gasification equipment 3 through the separator 4 into the combustion plant.
The arrangement 1 comprises adjustment means 10 for maintaining the gasification temperature in the reactor 7 under 800 °C, preferably within a temperature range of 600 to 750 °C, i.e. below the prior art CFB gasification temperature. According to an idea, the adjustment means 10 adjust the ratio of air to fuel to be conveyed into the reactor 7.
In the conditions prevailing in the reactor 7, i.e. within said temperature range and in the sub-stochiometric conditions, the alkali metals contained in the fuel remain in a solid state. Alkali metals in solid state flow with the fuel gas P into the separator 4, where they are removed from the product gas G. The alkali metals content of the product gas to be conveyed into the combustion plant is very low, even to the extent that the product gas G is in practice preferably free of alkali. The low alkali metals content of the product gas G essentially reduces, or, in practice, even abolishes altogether the corrosion risk of the combustion plant. This allows e.g. corrosion of superheaters possibly provided in the combustion plant to be reduced.
A particularly high amount of alkali metals is found in agro fuels. Agro fuels show a good reactivity in gasification, which is partly due to the catalysing effect of alkali metals which provides a good carbon conversion in the low temperature range mentioned above.
If the temperature exceeds 800°C, alkali metals begin to gasify and become essentially more difficult to remove from the product gas. On the other hand, carbon conversion may decrease significantly if the temperature is below 600°C.
In addition to alkali metals, other harmful substances or compounds, such as chlorine and phosphor, can be removed.
A further advantage of the low temperature to be mentioned is that it is below the typical sintering temperature of alkali metals, so alkali metals are unlikely to sinter on the surfaces of the reactor 7. This allows the cleaning interval of the reactor 7 to be extended.
If the reactor 7 walls are made of masonry material, their temperature is rather high. Hence the reactor 7 does not have cold surfaces to which ash, or the like, would attach.
The product gas G may have a fairly high CO content, but this is not a problem because the fuel burns to extinction in a combustion plant following the separator 4.
According to an idea, a heat exchanger 14a, 14b may be arranged between the gasification equipment 3 and the separator 4 and/or between the separator 4 and the combustion plant. The heat exchanger 14a arranged between the gasification equipment 3 and the separator 4 reduces the temperature of the fuel gas P before the separation step. This allows the temperature prevailing in the separator 4 to be reduced, which makes it possible to manufacture the separator 4 using affordable materials. The heat exchanger 14b arranged between the separator 4 and combustion plant reduces the temperature of the product gas G to be conveyed into the combustion plant, which in turn decreases the combustion temperature and reduces the melting of the ash in the combustion plant. Naturally, the same advantage may be gained with the heat exchanger 14a arranged between the gasification equipment 3 and the separator 4. In addition, the heat exchangers 14a, 14b allow heat energy to be recovered for utilization in the arrangement 1 or outside it.
The separator 4 may be an apparatus based on a cyclone, multi-cyclone, separator, ceramic filter, electric filter, or a similar device in which solid particles contained in the fuel gas and the alkali metals they carry can be removed or at least the amount of them substantially reduced so that the product gas G to be supplied into the combustion plant contains substantially less alkali metals than the fuel gas P.
Figure 2 is a schematic side view of a second and Figure 3 of a third arrangement and method of the invention. In both figures the gasification equipment is a CFB-type gasifier.
Fuel F is conveyed through one or more fuel supply channels 8 into the reactor 7 of the CFB gasifier. The walls of the reactor 7 are most preferably of a masonry material, although they may also be composed of a membrane wall comprising cooling pipes that are typically arranged side by side and parallel with one another and interconnected by fins, or as a plate structure or as a combination of these wall structures.
The operation of the CFB gasifier is based on recycling of the bed material. This kind of situation is achieved by increasing the flow rate of the fluidization gas to be higher than the terminal rate of the particles. As a result, the bed becomes turbulent and the particles are carried in the fluidization gas to the top part of the gasifier 7 and from there into the cyclone 6 of the gasifier.
The CFB gasifier naturally comprises components and structures not shown in the figures for simplicity of presentation. These include nozzles for supplying primary air and/or circulation gas into the reactor 7, channels for supplying reagents and other additives possibly needed in the gasification, etc.
In the cyclone 6 the bed material is separated from the gas flow, whereby fuel gas P is produced. The operation of the cyclone 6 is based on a strong rotating motion in which what is known as centrifugal force separates the bed material outward onto the cyclone walls from where it accumulates onto the bottom of the cyclone 6. The bed material is returned through a return channel 9 back into the bottom part of the reactor 7. The fuel gas P is conveyed from the cyclone 6 into the separator 4.
The separator 4 may comprise a cyclone, or one of the other separators mentioned above, which separates solids S from the fuel gas. The product gas G produced as a result of the separation is conveyed into the combustion plant.
The solids separated in the separator 4 may contain inert ash, nutrients usable for plants, incombustible coal, and the like. The solids may be utilized e.g. as such or further processed into products for use or returned into the nature.
The embodiments shown in Figures 2 and 3 differ from one another in the placement of the channel conveying the fuel gas P into the separator 4. In Figure 2 the channel begins at the top part of the cyclone, whereas in Figure 3 the channel begins at the bottom of the cyclone. Both solutions, as any other structural solution of a CFB gasifier, may be applied to the equipment and method of the invention.
Because of the low gasification temperature in the reactor 7, the bottom ash on the bottom of the reactor 7 may contain coal. It may be removed by means of a bottom ash oxidizer 11, such as a wind sieve known per se. The oxidizer blows air and, with it, the light coal upward for gasification in the reactor 7.
The bottom ash that accumulates on the bottom of the reactor 7 may be removed by bottom ash removal means 12 known per se and not disclosed here for simplification of the disclosure.
Figure 4 is a schematic view of a combustion plant arrangement in which the arrangement and method of the invention is applied. The combustion plant arrangement comprises the separator 4 already disclosed above and the combustion plant 5 arranged after it. The product gas G is burned in the combustion plant 5 and the thermal energy generated by the combustion is recovered.
The combustion plant 5 may be a boiler, known per se, in which the product gas G may be burned and the generated thermal energy is recovered into water and/or steam by means of different thermal surfaces, such as the boiler walls, superheaters, boiling surfaces and economizers. According to another idea, the combustion plant 5 may be a furnace, such as cement kiln, in which the energy obtained by burning the product gas is used for carrying out a chemical or physical process.
The combustion plant arrangement may comprise fuel pre-processing means 2, which may include drying means for reducing the moisture content of the wet fuel. The drying means may comprise a belt drier, for example.
In some cases, features disclosed in this application may be used as such, regardless of other features. On the other hand, when necessary, features disclosed in this application may be combined in order to provide different combinations.
In summary, the arrangement of the invention is characterized in that: the gasification equipment comprises means for gasifying fuel to produce a product gas at a low temperature and in that the arrangement further comprises a separator arranged between the gasification equipment and the combustion plant so that substantially all the product gas flows through it from the gasification equipment into the combustion plant, the separator comprising means for purifying the product gas of solids.
In addition, the method of the invention is characterized by adjusting the temperature of the gasification equipment so that it is at most 800°C, conveying the fuel gas generated in the gasification from the gasification equipment into the separator, purifying the fuel of solids, whereby product gas is produced, and conveying the product gas into the combustion plant for combustion.
The drawings and the related description are only intended to illustrate the idea of the invention. A person skilled in the art will find it obvious that the invention is not restricted to the above embodiments, which disclose the invention by means of some embodiments, but various modifications and different applications of the invention are possible within the inventive idea defined in the accompanying claims.

Reference numbers

1: arrangement
2: fuel pre-processing means
3: gasification equipment
4: separator
5: combustion plant
6: cyclone
7: reactor
8: fuel supply channel
9: return channel
10: adjustment means
11: bottom ash oxidizer
12: bottom ash discharge means
13: solids discharge channel
14a, b: heat exchanger

F: fuel
G: product gas
P: fuel gas
S: solids

Claims

An arrangement for burning fuel, the arrangement (1) comprising gasification equipment (3) for converting fuel (F) into product gas (G) by gasification and a combustion plant (5) for burning the product gas (G), wherein the gasification equipment (3) comprises means for gasifying fuel at a low temperature, the arrangement (1) further comprising
a separator (4) arranged between the gasification equipment (3) and the combustion plant (5) so that substantially all product gas (G) conveyed into the combustion plant (5) flows from the gasification equipment (3) through the separator into the combustion plant (5);
the separator (4) comprising means for purifying the product gas (G) of solids,
between the gasification equipment (3) and the separator (4) and/or between the separator (4) and the combustion plant (5) there is arranged a heat exchanger (14a, 14b), characterized in that
the combustion plant (5) for burning the product gas (G) is arranged to receive the product gas (G) produced in the gasification equipment (3) and cooled in the heat exchanger (14a, 14b).
An arrangement as claimed in claim 1, characterized in that the gasification equipment (3) is a CFB gasifier.
An arrangement as claimed in claim 1 or 2, characterized in that said low temperature is below 800°C, preferably within a temperature range of 600 to 750°C.
An arrangement as claimed in any one of the preceding claims, characterized in that the bottom part of the gasification equipment (3) is provided with means (11) for oxidizing bottom ash.
An arrangement as claimed in any one of the preceding claims, characterized in that the gasification equipment (3) comprises masonry walls.
An arrangement as claimed in any one of the preceding claims, characterized in that the combustion plant (5) is a boiler.
An arrangement as claimed in any one of claims 1 to 6, characterized in that the combustion plant (5) is a furnace.
An arrangement as claimed in any one of the preceding claims, characterized in that the separator (4) comprises a cyclone or a multi-cyclone.
An arrangement as claimed in any one of claims 1 to 7, characterized in that the separator (4) comprises a separating tank.
An arrangement as claimed in any one of claims 1 to 7, characterized in that the separator (4) comprises a ceramic filter or an electric filter.
A method for burning fuel, in which method the fuel (F) is converted into product gas (G) by gasification in gasification equipment (3) and the product gas (G) is burned in a combustion plant (5), by
adjusting the temperature of the gasification equipment (3) so that it is at most 800°C;
conveying the fuel gas (P) produced in the gasification from the gasification equipment (3) into the separator (4);
purifying the fuel gas (P) of solids, whereby product gas (G) is produced; and
conveying the product gas (G) into the combustion plant (5) for combustion,
reducing the temperature of the fuel gas (P) and/or the product gas (G) by a heat exchanger (14a, 14b) arranged between the gasification equipment (3) and the separator (4) and/or between the separator (4) and the combustion plant (5),
characterized by
burning the product gas (G) produced in the gasification equipment (3) and cooled in the heat exchanger (14a, 14b) in the combustion plant (5).
A method as claimed in claim 11, characterized in that fuel (F) comprising agro fuel is supplied into the gasification equipment (3).
A method as claimed in claim 11 or 12, characterized by gasifying the fuel in a CFB gasifier.
A method as claimed in any one of claims 11 to 13 characterized by adjusting the temperature of the gasification equipment (3) to be within a range of 600 to 750°C.
A method as claimed in any of claims 11 to 14, characterized by purifying the fuel gas (P) of solids in a cyclone (6), multi-cyclone, separating tank, ceramic filter or electric filter.