The invention relates to a cyclone, i.e. a centrifugal separator, used to separate gas and solids from a gas/solids mixture.

Typical industrial applications of such a cyclone relate to its use in a power plant. Depending on the combustion process and the fuel used (for example: waste, coal, biomass) the flue gas (exhaust gas) of a power plant contains varying amounts of harmful substances like Nitrogen Oxides, for example NO, N0 ₂ or N ₂ 0 ₄ , hereinafter commonly referred to as NO _x .

Environmental aspects require to reduce the quantity of harmful NO _x by converting the Nitrogen oxides into harmless substances like Nitrogen and water and/or steam in a so- called DENOx-reaction, using ammonia (NH ₃ ) as a reduction agent.

Ammonia can be partly or completely replaced by urea, which is easier to handle and to store. The chemical reaction in the process is similar to that in the presence of ammonia:

NH ₂ CONH ₂ + H ₂ 0 -> 2NH ₃ + C0 ₂

The following reduction reaction (simplified) is:

4 NO + 4 NH ₃ + 0 ₂ -> 4 N ₂ + 6 H ₂ 0

As far as reference is made hereinafter to ammonia this always includes urea and/or equivalent reducing agents, if not explicitly excluded.

For the reduction process the following two different ways are known: - The so-called SNCR (selective non-catalytic reduction) process, which typically takes place at temperatures above 800°C degrees centigrade. in corresponding applications liquid ammonia as a reduction means is sprayed into the f!ue gas stream (comprising solid particles like minerals, ash etc.), in most cases (right) after the flue gas has been extracted from the combustion chamber. No catalytic converter is being used. This is why the reaction temperature must be in the range quoted above. A disadvantage of this technology is that effectiveness of the NO _x reduction is relatively poor and that the solids within the flue gas stream are contaminated by said ammonia.

The co-called SCR (selective catalytic reduction) process, wherein the denitrification is performed at temperatures between ca. 180°C and 550°C using ammonia in combination with a catalytic converter (catalyzer).

In this process a liquid ammonia and/or a gaseous ammonia may be used. Ammonia is fed into the gas stream shortly before the flue gas enters and passes the catalytic converter. To increase the contact between flue gas and ammonia it is known from EP 0 555746B1 to install corresponding distribution means within the gas duct, but these means increase the installation and service costs and worsen the gas flow as such.

The SCR process is used for automobile applications as well as for industrial applications like flue gas purification.

Although consuming less ammonia than the SNCR process, the efficiency in denitrification of this SCR method is up to 99%.

Therefor this technology has been applied for long; nevertheless a number of disadvantages remain, in particular when applied in connection with a flue gas treatment in a power plant: clogging of the catalyzer was observed, mainly caused by the solids within the yet untreated gas. Such clogging reduces the efficiency of the catalyzer. The solids further increase the wear and thus reduce the service time of the catalytic converter. Solids passing the catalyzer (reduction catalyst) may cause problems in subsequent installations like filters.

It is an object of the invention to further improve the denitrification of NO _x containing gases, in particular NO _x containing flue gases, in particular flue gases from industrial installations like a power plant, an incineration furnace or the like. The invention will now be explained by way of example with respect to a power plant application.

The basic idea of the invention is as follows: in a first aspect it seems favorable to extract any solid particles (solids) from the gas before it is brought into contact with a reduction agent. This reduces the required quantity of said agent. For this purpose the invention proposes to integrate a cyclone (centrifugal particle separator) into the way of the gas after leaving the combustion chamber. This avoids contamination of the solids by the reducing agent in the SCR and SNCR process. in the SCR method it further reduces the risk of clogging and/or excessive wear of the catalytic converter, as the solids are extracted from the gas (gas/solids mixture) before the gas undergoes the catalytic treatment, i.e. before the gas enters the catalyzer. By the same reason, any problems in subsequent gas treatments are avoided.

A second aspect is to combine the reduction-process with said particle separation; more precisely: charging a quantity of the reducing agent (ammonia, urea) into the purified gas stream, i.e. charging the reducing agent in the area of the outlet port of the cyclone, through which the purified gas leaves the separator.

In other words: The invention discloses a cyclone, which fulfils a double function: separation of any solids from the flue gas (exhaust gas) and contacting the pre-purified gas stream with a reducing agent.

In its most general embodiment the invention relates to a cyclone, comprising at least one inlet-port for a gas/solids-mixture, at least one gas outlet port, through which a pre-purified gas (separated from the gas/solids-mixture within the cyclone) leaves the cyclone, and at least one solids outlet-port for the solids (after being separated from the gas), characterized by means for introducing a reducing agent (like ammonia, urea) into the pre-purified gas (the gas freed from solids), leaving the cyclone. This will often be on its way through the gas outlet port.

The cyclone can be used in both methods (SCR, SNCR) and corresponding installations (apparatuses) as disclosed above.

As a component to perform an SNCR the cyclone can be arranged inside or after the combustion chamber or further downstream (in the flow direction of the gas/solids mixture) where temperatures above ca. 800°C prevail. The solids are then separated within the cyclone from the gas and extracted from the cyclone (via at least one corresponding solids outlet port). The remaining, more or less pure, but still O _x containing flue gas is then brought into contact with ammonia, preferably on its way !eaving the cyclone, to allow a selective non- cata!ytic reduction of the NO _x containing flue gas downstream of the cyclone.

An even more favorable use of the cyclone is as part of an installation (apparatus) by which an SCR process is performed. Again the flue gas is firstly separated from any solids and thereafter contacted with the reducing agent, wherein the means to provide said ammonia cart be an integral part of the cyclone and arranged at or close to its gas outlet port and in any case ahead (upstream) of a subsequent catalyzer.

The invention can be realized in a so-cailed uniflow separator as well as in a reverse flow separator, although the uniflow version is preferred not only because of its smaller size but also because of numerous advantages disclosed hereinafter.

The uniflow centrifugal separator features a (substantially) uniform main flow direction of the gas, which is still in combination with solid particles, i.e. the gas/solids-mixture , within the cyclone after the flue gas left the inlet port, and of the gas (without solids) which leaves the cyclone via the gas outlet port.

The main flow direction of the gas/solids mixture within the cyclone is defined as the flow direction perpendicular to the centrifugal forces and towards the outlet port for the solids, but not including the flow direction within the inlet port of the cyclone as this inlet port may be arranged in different ways as will be disclosed hereinafter, for example eccentrically to a central longitudinal axis of the cyclone.

The main flow direction of the gas, freed from the solids, is the flow direction of the gas when it leaves the cyclone via said gas outlet port.

It seems clear that both of said main flow directions (in a centrifugal separator) are not exactly linear, but easily to identify according to the preceding.

Best results may be achieved with a cyclone which is designed such that both main flow direction of the gas/solids stream and the gas stream leaving the cyclone are substantially parallel to the central longitudinal axis of the cyclone, wherein at least the gas flow through the gas exit port may be concentric to this central longitudinal axis.

The uniflow cyclone provides further advantages: While the solids are collected adjacent to the inner wall of the cyclone the gas flow is substantially centrically and substantially concentrically to the longitudinal axis of the cyclone. This implies a strong swirling effect onto and into the gas stream on its way to and through the gas outlet port.

This swirl (twist, angular momentum) is an extremely favorable aspect of the invention as it can replace any discrete means to optimize the distribution of the ammonia within the gas volume. The swirling effect is an in-situ effect which provides the required energy to homogenize gas and reducing agent. In other words: This uniflow separator takes as well the function of a mixer.

This embodiment only requires means for charging the reducing agent into the gas stream. These can be conventional spray nozzles or injection nozzles, which may be arranged at a distance to each other at one or rnore levels along the inside of the gas out!et port,, so-called atomization nozzles {German : Zweistoffdusen), feeding the reducing agent in an air stream, or similar means.

The cyclone can also be a counterflow centrifugal separator, featuring a main flow direction of the pre-purified gas leaving the cyclone via the gas outlet port in a counterflow to a main flow direction of the gas/solids-mixture within the cyclone, after its inlet port. "Counterflow" is only defined by the flow of the gaseous components within the cyclone, again not considering the flow behavior in the inlet port.

Typically the gas is urged to flow in a more or less cylindrical pattern adjacent to the inner wall of the cyclone (first main direction) thereby forming a central passageway through which the pre-purified gas flows in a reverse manner (second main direction) after separation of any solids and after having made a U-turn at that end of the cyclone, opposite to the inlet port and before leaving the cyclone. The solids may be extracted by wall integrated outlet ports and/or at that end of the cyclone, where the gas makes the U-turn as described above.

In both embodiments of a cyclone the means for introducing ammonia into the purified gas can be designed as spray nozzles arranged circumferentially at the gas outlet port, in particular along the inner wall of the gas outlet port. These charging means for the reducing agent can be oriented perpendicular to the gas flow or inclined to main flow direction to give the reducing agent the desired flow direction and speed.

Alternatively the charging means for the reducing agent can be arranged adjacent to the gas outlet port or around its free end. In a preferred arrangement the reducing agent is charged into the swirling gas stream, when it leaves the cyclone.

These charging means can be means like nozzles for introducing gaseous ammonia and/or gaseous urea as well as liquid reducing agents.

Especially in an embodiment without a reverse flow cyclone the gas outlet port can further be equipped with additional swirl means. These swirl/twist means can best be arranged inside the gas outlet port and can be realized by inclined blades, plates, helix structures etc..

As mentioned above the invention also provides for use of the cyclone in a power plant subsequent to its combustion chamber. This can be realized in such a way that the cyclone is arranged between the combustion chamber and a subsequent catalyst for selective catalytic reduction of an NO _x containing flue gas.

Alternatively the cyclone will be arranged inside or after the combustion chamber a for selective non-catalytic reduction of an NO _x containing gas after its treatment with ammonia and after having left the cyclone.

The combustion chamber as part of a power plant can be of any type, for example a combustion chamber (boiler/reactor) that burns at least one fuel of the group comprising waste, biomass and fossi!e fuels like coai.

Insofar the combustion chamber can be of a fluidized bed reactor type, a grate incineration furnace (for example for waste incineration), a pulverized coal (PC) boiler, etc.

The invention will now be described with reference to the attached drawing, which schematically displays in:

Fig. 1: a basic layout of a power plant, including a cyclone according to the invention

Fig. 2: a first embodiment of a cyclone (uniflow cyclone)

Fig. 3: a second embodiment of a cyclone (reverse flow cyclone)

In the figures identical parts and parts having the same or a similar function are displayed by the same numerals.

Fig. 1 displays a basic layout of a power plant in a very schematic way, wherein a combustion chamber of arbitrary type is identified by CC. In this embodiment coal is used as a fuel material and fed via corresponding burners (not displayed), which are arranged in the wall of the combustion chamber CC.

The flue gas, comprising a considerable amount of unburnt mineral solid particles, leaves the combustion chamber CC at its upper end, passed various heat exchangers HE made of so- called water tubes, before the gas duct making a first U-turn Ul, followed by a gas duct GD, which makes a second U-turn U2 before feeding the gas into a catalyzer (catalytic converter) CA. The gas path is symbolized by a dashed line and corresponding arrows.

In that part of the gas duct GD, which extends between the first and second U-turn (U1,U2), a cyclone 10 is installed.

A first embodiment of this cyclone, namely a so-called uniflow cyclone (also called uniflow centrifugal separator) is displayed in Fig. 2.

The gas, still containing the solids (i.a. minerals, ash), i.e. the gas/solids mixture is introduced into the separator via an inlet port 12, including swirling means 14 to give the gas/solids flow a corresponding angular momentum (twist, swirl). This is common knowledge and not further described here.

While the heavier solids tend to move towards the inner wall of the cyclone 10 before being extracted from the cyclone via corresponding solids outlet ports 16, the gas takes a more or less linear main flow direction from the inlet port 14 to a gas outlet port 18 opposite to the inlet port 14.

Thus the main flow direction of the gas/solids stream (after said inlet 12), displayed by arrow A _gS is substantially the same as the main flow direction of the gas leaving the cyclone, symbolized by arrow A _g .

Because of the swirling effect of the gas/solids stream within the cyclone 10 the gas, leaving the cyclone substantially coaxially to the central longitudinal axis A-A of the cyclone 10, also has a considerable twist.

This in-situ generated twist is used to distribute gaseous ammonia as a reducing agent for the subsequent DENOx-process (denitrification), wherein said ammonia is injected via corresponding nozzles 20, arranged in a ring like pattern and at a distance to each other along the inside of the gas exit port 18 close to its free end 18e.

In this embodiment the design and arrangement of the nozzles 20 is such that the reducing agent is sprayed in a substantially perpendicular direction compared with the main flow direction A _g of the gas.

The injection of ammonia into a swirling gas stream is favorable in view of costs, as no extra mixing means being necessary, and

efficiency, as it allows a very intensive and uniform mixture of ammonia and gas (ahead of the subsequent catalytic treatment)

As the solids have already been separated earlier (upstream within the cyclone) the ammonia can be used completely for the subsequent denitrification reaction in the subsequent catalyzer CA.

Fig. 2 also displays an alternative embodiment, wherein the gas/solids mixture enters the inlet port 14 (with its swirling means) radially and eccentrically, symbolized by dashed arrow R.

It becomes apparent that in this embodiment the main flow direction of the gas/solids stream changes when the gas/solids stream leaves the inlet port 14. Therefore the definition of the main flow direction only considers the flow pattern after (downstream) the inlet port 14, which then is equivalent to that in the uniflow version. Fig. 3 shows another type of a cyclone according to the invention, namely a counterflow (reverse flow) separator 10. The gas, still comprising solid particles, i.e. the gas/solids mixture is transported into the upper part of the cyclone 10 radially/eccentrically and thus similar to the alternative in Fig. 2.

While the gas and the solids then take a main flow direction substantially parallel to the centra! longitudinal axis AA of the cyclone (downwardly and adjacent to the inner wall of the cyclone 10), the heavier solids leave the cyclone via a bottom side solids outlet port 16, while the gas, now freed from the solids, makes a U-turn and moves upwardly in a countercurrent (reverse flow) and substantially coaxially to the centra! longitudinal axis A-A of the cyclone before entering the gas outlet port 18, which again is arranged coaxially to said axis A-A, passing the outlet port 18 and leaving it.

Before leaving it, the gas is brought into contact with a reducing agent, in this embodiment a urea based liquid, which is introduced into the gas stream by spray nozzles 20, arranged in the wall of said gas outlet port 18. These spray nozzles "atomize" the urea liquid into tiny droplets, which are injected into the gas under different angles, i.e. in the main flow direction, reverse to the main flow direction, perpendicular to the main flow direction and at any angles in-between.

This embodiment also allows a very intensive and uniform contact between gas and urea, which improves the DENOx-reaction when the gas/urea mixture enters the catalyst CA, which is arranged downstream of said cyclone (seen in the flow direction of the flue gas).

Fig. 3 displays the main flow directions A _gs (of the gas/solids mixture) and A _g (of the gas without solids) in their counterflow.

Fig. 3 also shows swirling means 22 at a distance below (upstream) said nozzles 20, which swirling means 22 are inclined plates, arranged at a distance to each other (similar to turbine blades) to give the gas stream a stronger twist.