SE469144B - Method of maintaining low emissions of nitrogen oxides in combustion in a fluidized bed irrespective of the bed height, and power station in which the method is implemented - Google Patents

Abstract

Method and arrangement for combustion of fuel in a fluidized bed 10. A fluidized bed is supplied with fuel for combustion, energy released during the combustion is recovered by means of steam tubes 12a, 12b arranged in and downstream of the fluidized bed, the bed height varies on changes in the amount of power extracted so that the steam tubes are covered or not covered by the bed. The content of nitrogen oxides formed during combustion is reduced by thermal reduction using a reduction agent which is sprayed into the flue gases downstream of the fluidized bed. One or more uncooled zones 15, free of steam tubes, are arranged in the fluidized bed, alternately with the steam tubes. When an uncooled zone is not covered by the bed material, a reduction agent is sprayed into essentially uncooled flue gas in the region of said uncooled zone. <IMAGE>

Description

-Ps 2 \ 3 as well as the conditions for burning out residual carbon and carbon monoxide present in the flue gases.

THE INVENTION The invention aims to maintain good conditions for thermal reduction of nitrogen oxides and good burning of residual carbon and carbon monoxide, regardless of power output, so that variations in power output do not affect the plant's environmental performance. More specifically, the invention aims to maintain a low emission of nitrogen oxides and carbon monoxide from the power plant independent of the power output from the plant. A prerequisite for good nitric oxide reduction and combustion of residual carbon and carbon monoxide present in the flue gases is that a high flue gas temperature is maintained regardless of power output, bed height, in particular a high temperature must be maintained in connection with the supply means for injecting ammonium reducing agents, amine salts, amides or other reducing agent suitable for the thermal reduction of nitrogen oxides formed.

In the combustion of fuel in a fluidized bed, which is part of a power plant and includes; a fluidized bed of particulate material arranged to be supplied with fuel for combustion, - heat-transferring surfaces, preferably in the form of steam tubes, arranged in and downstream of the fluidized bed to recover energy released during combustion, and - where the bed height is arranged to vary with changes in power output, a reducing agent, such as ammonia, ammonium salts, amines, amine salts, amides or other reducing agent suitable for thermal reduction of formed nitrogen oxides, is arranged to be injected into the flue gases downstream of the fluidized bed, with a process according to the invention, a low emission of nitrogen and of residual carbon and carbon monoxide, independent of power take-off, -bed height-, by arranging alternately with the steam tubes in the fluidized bed and - that when one of the Uncooled zones in the event of a reduction of the power take-off are exposed from bed material, a reducing agent, suitable for reduction nitrogen oxides, are injected, in connection with said uncooled zone, into substantially uncooled flue gases to lower the content of nitrogen oxides in the flue gases.

Injection means for supplying reducing agent to uncooled flue gas, which preferably maintains a temperature of between 500 and 850 ° C at a combustion temperature in the bed of between 700 and 900 ° C, are according to the invention arranged in connection with said uncooled zones. When an uncooled zone with said injection means, at reduced bed height, is exposed from the fluidized bed, reducing agent can be injected into substantially uncooled flue gas. When the bed height is increased again and the injection means are covered by bed material, the injection of reducing agent through said injection means ceases, whereby the nitric oxide reduction is moved to the nearest higher level with injection means.

Because the flue gases remain substantially uncooled in said uncooled zones and that reducing agents will be injected into substantially uncooled flue gas in connection with these uncooled zones, residual carbon and carbon monoxide will be burned out and the content of nitrogen oxides in the flue gases will be reduced substantially independently of power output.

In a preferred embodiment of the invention, turbulence-forming grids are arranged in connection with said uncooled zones for mixing the flue gases. As a result, the conditions for, both a more complete combustion of residual carbon and carbon monoxide present in the flue gases and for thermal reduction of nitrogen oxides, are considerably improved.

Preferably, when the power outlet is reduced to partial load, the bed height is lowered to a level adjacent to the lower limit of an uncooled zone. After which the supply of 469 ”244 4 reducing agents for thermal reduction of nitrogen oxides can be started in the uncooled zone, the reducing agent being injected into, in the nearest uncooled flue gas, and, especially in the embodiment with turbulence-forming grids arranged in connection with the uncooled zones , very good conditions for both nitric oxide reduction and the burning of residual carbon and carbon monoxide in the flue gases are obtained. After burning out and thermal reduction in the uncooled zone, the flue gas then passes through at least one zone with steam tubes, the flue gas being cooled in the usual way, before the flue gas leaves the freeboard at the usual temperature.

Supply of reducing agent in connection with an uncooled zone is terminated before said zone is covered by the fluidized bed in connection with the bed height being increased by an increase in the power output.

FIGURES The invention is described in more detail below with reference to the accompanying figures. Figure 1 shows a fluidized bed for combustion of fuel supplied to the bed, where according to the invention uncooled zones are arranged alternately with steam tubes.

A preferred embodiment of the invention where a turbulence forming grid is arranged adjacent to the uncooled zones is shown in Figure 2 while Figure 3 shows the invention, in the preferred embodiment with turbulence forming grid, applied in a power plant with combustion in a pressurized fluidized bed a PFBC power plant energy from combustion is utilized by means of gas turbines arranged in the flue gas paths and steam turbines arranged in the power plant's feed water / steam system.

DESCRIPTION OF THE FIGURES A bed vessel with a fluidized bed arranged for the combustion of supplied fuel and heat-transferring surfaces arranged in the bed is shown in Figure 1 and Figure 2.

M di !!! 46 A fluidized bed 10 of particulate material, where, if the fuel is sulfur-containing, the particulate bed material at least in part contains sulfur absorbents such as lime, limestone or dolomite, is enclosed in a bed vessel 11. The fluidized bed 10 is supplied, by fluidizing means, not shown, gas for fluidizing the fluidized bed 10. During combustion, the fluidizing gas usually contains air or other oxygen-containing gas for combustion of fuel supplied to the bed 10.

The fuel, which may be solid, liquid or gaseous, is supplied to the fluidized bed 10, preferably by supply means arranged in connection with the lower region of the bed, not shown. At least a part of the heat released during combustion is taken out through heat-transmitting surfaces 12a, 12b arranged in and downstream of the fluidized bed 10, preferably arranged in the form of steam tubes connected to the power plant's feed water / steam system.

Figures 1 and 2 show only steam tubes 12a, 12b arranged in the fluidized bed 10, but steam tubes can also be arranged downstream of the fluidized bed 10, in a free table 13 arranged above the fluidized bed 10 or in the flue gas ducts.

At maximum power output, full load, all steam tubes 12a, 12b arranged in connection with the fluidized bed 10 are covered by the particulate bed material and the flue gases leave uncooled to the freeboard 13 where good conditions for burning residual carbon and carbon oxide present in the flue gases are obtained. The conditions for thermal reduction of nitrogen oxides are also good, which is why reducing agents, such as ammonia, ammonium salts, amines, amine salts, amides or other reducing agent suitable for reducing nitrogen oxides, are injected into the uncooled flue gases by means of injection means 14a arranged in connection with the freeboard 13.

In the event of a lowering of the power output, partial load, steam tubes l2a arranged in the upper parts of the fluidized bed 10 are exposed, said steam tubes 12a being cooled by flue gases emitting from the fluidized bed 10 so that the conditions for both thermal reduction of nitrogen oxides and carbon dioxide burning are reduced. By, according to the invention, alternating 469 4:41? 6 with the steam tubes 12a, 12b in the fluidized bed 10 arranged uncooled zones 15, i.e. zones free from steam tubes, are obtained even under very good conditions for thermal reduction of nitrogen oxides and for burning carbon monoxide and residual carbon in the flue gases.

When the power outlet is reduced to partial load, the bed height is lowered, preferably to a level adjacent to the lower limit of an uncooled zone 15, whereby injection means 14b arranged in connection with the uncooled zone are exposed.

When said injection means 14b are exposed, the supply of reducing agent through them starts.

Correspondingly, the supply of reducing agent is terminated by injection means 14b arranged in connection with an uncooled zone 15, before said injection means 14b and zone 15 are covered by the fluidized bed 10 in connection with the bed height being increased by an increase in the power output.

At a combustion temperature of between 700 and 900 ° C in the fluidized bed 10, regardless of whether injection takes place in the freeboard 13 at full load or further down in the bed vessel 11 at reduced bed height, partial load, the reducing agent for thermal reduction of nitrogen oxides should be injected at a flue gas temperature of between 500 and 850 ° C.

In order to further improve the conditions for both a good combustion of the flue gases and for the reduction of nitrogen oxides, in a preferred embodiment, as shown in figure 2, turbulence forming grid 16 is arranged in connection with said uncooled zones 15 to increase the mixing in the flue gases. In this way, the conditions for a good burn-out of carbon monoxide and residual carbon present in the flue gases are especially improved, but also the mixture of added reducing agents in the flue gases is improved and thus the conditions for a good nitric oxide reduction. 7 ~ 469 144 Figure 3 shows the invention applied in a power plant with combustion in a pressurized fluidized bed, a PFBC (gressurised Eluidised ßed Qombustion) plant.

In a PFBC plant, a particulate fluidized bed 10 is supplied with fuel, through fuel lines 21, and air, through airways 22, to fluidize the particulate bed material, which if the fuel is sulfur-containing at least in part contains sulfur absorbents such as lime, limestone or dolomite. and combustion of fuel supplied to the fluidized bed 10.

The fluidized bed 10 is contained in a bed vessel 11 which is arranged enclosed in a pressure vessel 23. To air supplied to the fluidized bed 10 is pressurized in a compressor 24 arranged in the airways, the pressure vessel 23 is supplied where it is supplied via fluidization and distribution means (not shown) in the lower part of the bed vessel. The vortex bed 10. Flue gases emitted from the combustion are emitted from the plant via flue gas paths 25. During the combustion, energy released is utilized partly by steam tubes 12 arranged in the vortex bed 10, which generate steam for steam turbines (not shown) and partly by one or more flue gases. gas turbines 26 arranged on the roads 25. The gas turbine 26 is advantageously arranged to drive the compressor 24 as shown in figure 3.

Before the flue gases are supplied to the gas turbine 26, they pass a flue gas purification equipment 27, preferably in the form of one or more cyclone separators, in which the dust content of the flue gases is separated. When changing the power output, the respective height is reduced, in the manner described above, whereby steam tubes 12 and zones cooled according to the invention are exposed and covered by the fluidized bed 10.

When the power outlet is reduced to partial load, the bed height is preferably reduced to a level adjacent to the lower limit of an uncooled zone 15, whereby injection means 14b arranged in connection with the uncooled zone are exposed.

When said injection means 14b are exposed, the supply of reducing agent through them starts. Correspondingly, the supply of reducing agent is terminated by injection means 14b arranged in connection with an uncooled zone 15, before the said injection means 14b and zone 15 are covered by the fluidized bed 10 in connection with the bed height being increased when the power outlet is increased.

Combustion of coal in a PFBC plant preferably takes place at a bed temperature of between 700 and 900 ° C. Regardless of whether injection of a reducing agent intended for thermal reduction of nitrogen oxides takes place in the freeboard 13 at full load or further down in the bed vessel 11 in a wide lowered bed height, partial load, exposed uncooled zone 15, the reducing agent is injected into flue gas with a temperature of between 500 and 850 ° C.

Turbulence forming grid 16 is advantageously arranged, in the manner described above, in connection with said uncooled zones 15 in order to increase the mixing in the flue gases.

A method and a device as described in the present patent application can be varied in many ways within the scope of the claims.

Claims (5)

PATENT REQUIREMENTS 1. Ways to maintain a low emission of nitrogen oxides and a good combustion of residual carbon and carbon monoxide, regardless of power output, when burning fuel in a fluidized bed (10), - where the fluidized bed is arranged to be part of a power plant, - where a fluidized bed of particulate matter is supplied with fuel for combustion, - where the energy released during combustion is recovered by means of heat-transferring surfaces (12a, 12b) arranged in and downstream of the fluidized bed, preferably in the form of steam tubes, - where the bed height is varied the bed and - where the content of nitrogen oxides formed during combustion is reduced by thermal reduction with ammonia, ammonium salts, amines, amine salts, amides or other reducing agent suitable for the reduction of nitrogen oxides, which is injected into the flue gases downstream of the fluidized bed, characterized by one or more zones (15) free from vapor tubes, uncooled zones are arranged alternately with the vapor tubes in the fluidized bed and that, when one of said uncooled zones is exposed to bed material during a reduction of the power take-off, a reducing agent suitable for reducing nitrogen oxides is injected into substantially uncooled flue gases, adjacent to said uncooled zone, to lower the content of nitrogen oxides in the flue gases. 2. A method according to claim 1, characterized in that the flue gases are mixed in said uncooled zone (15) by arranging a turbulence-forming grid (16) in connection with said zone. 3. A method according to claim 1 or claim 2, characterized in that reducing agent is injected into substantially uncooled flue gas at a flue gas temperature of between 500 and 850 ° C at a combustion temperature in the bed of between 700 and 900 ° C. 469 iii-LE »10 Combustion plant with combustion in a fluidized bed, - where a fluidized bed (10) of particulate material is arranged to be supplied with fuel for combustion, where heat-transferring surfaces (12a, 12b), preferably in the form of steam tubes, are arranged in and downstream of the fluidized bed for to recover energy released during combustion, - where the bed height is arranged to be varied in case of changes in the power output, - where a reducing agent such as ammonia, ammonium salts, amines, amine salts, amides or other reducing agent suitable for thermal reduction of formed nitrogen oxides is arranged to be injected into flue gases the fluidized bed, characterized in that uncooled zones (15), zones free from steam tubes, are arranged alternately with the steam tubes in the fluidized bed, that said uncooled zones are arranged to be exposed and covered when the bed height is varied as a result of changed power output, 14a, 14b) for the supply of reducing agents for the thermal reduction of nitrogen oxides, are arranged in connection to said uncooled zones (15) and that said injection means are arranged to supply reducing agents to substantially uncooled flue gas when the power outlet is exposed during a reduction of the power take-off. Power plant according to claim 4, characterized in that turbulence-forming grid plants (16) are arranged in connection with said uncooled zones (15).