Combustor with riser

TECHNICAL FIELD

The invention relates to a power plant with combustion of a solid fuel in a fluidized bed enclosed in a combustor. It is intended for a plant with combustion at atmospheric pressure as well as at a higher pressure. In the latter case, the combustor is usually enclosed in a pressure vessel, the combustion gases being utilized for driving a gas turbine. Power plants of this kind are commonly called PFBC plants.

BACKGROUND ART

In a PFBC power plant the combustion takes place in a flui¬ dized bed of particulate material, usually substantially consisting of limestone or dolomite which serves as sulphur absorbent. The combustion takes place at a pressure which considerably exceeds the atmospheric pressure. The com¬ bustor is suitably enclosed in a pressure vessel and is connected to a gas turbine . The gas turbine drives a com¬ pressor, which compresses the combustion air, and a gene¬ rator. The bed section of the combustor acco odates tube coils, which absorb heat from the bed, cool this and gene¬ rate and superheat steam for a steam turbine which drives a generator. Above the bed section, there is a freeboard for collection of combustion gases, which are forwarded from the combustor to a cleaning plant, usually consisting of cyclo- nes connected to the uppermost portion of the freeboard in the combustor.

The combustion gases which leave the bed cause particulate material from the bed to be brought along with them. This material may contain unburnt fuel particles, which are not completely burnt out because of too short residence time in the bed. It is known to return material from dust separ¬ ators to burn such fuel and thus increase the combustion

efficiency. It is also known to arrange coarse separators at the combustor. Such a coarse separator is disclosed in European patent application 87102200.0.

A cleaning plant in a PFBC plant may consist of a large number of cyclones placed along the periphery of the com¬ bustor, with connections to the freeboard at the uppermost portion of the combustor. Because of uncontrollable flows within the freeboard, flow zones or flow paths may arise within the freeboard .which means that the distribution of combustion gases with contents of particulate material is not uniform to the different cyclone outputs. Certain cyclones or cyclone groups may be subjected to gas flows with a higher speed or gas with a larger proportion of erosive particles or a larger proportion of unburnt fuel particles or a larger proportion of unused combustion air. In this way, the cyclones or corresponding cleaning devices are non-uniformly worn.

As mentioned above, it occurs that unburnt particles accom¬ pany the combustion gases under certain load conditions more than what is normal. Under certain circumstances, these un¬ burnt particles may cause fire in cyclones or filters in a cleaning plant for flue gases. To avoid such drawbacks, it has been proposed, among other things, to use secondary com- bustors, connected between the freeboard and the cleaning plant. In the secondary combustor, the incompletely burnt- out fuel particles are then burnt. See, for example, the patent specification DE 3 503 603.

The nitrogen in the oxidizing gas, preferably air, supplied to the primary process reactor in the form of a combustor, together with any nitrogen present in the fuel, gives rise to undesirable nitrogen oxides upon gasification or com- bustion. This formation of nitrogen oxide may be suppressed by limiting the supply of gas to the primary process reac¬ tor, especially the supply of the oxygen included in the

gas . However, this increases the amount of unused, unburnt .fuel residues in the flue gases.

The content of nitrogen oxides in the flue gases may be re- duced by additives, for example ammonia or water-mixed ammonia. If this is done non-catalytically, the addition may be made in the freeboard of the process reactor or downstream thereof. However, demands are placed on careful temperature control of the flue gases, normally in a narrow interval around 970°C, for this reduction to be efficient. Thus, the demands on the flexibility of the primary flui- dized bed, with respect to changes in the load, conflict with the demands from subsequent process steps for treatment of the flue gases in, for example, inertia separators and for nitrogen reduction, for more stable conditions.

All the above-mentioned difficulties, such as unbalanced load of components in a cleaning plant, coarse separation and reduction of the content of nitrogen oxides in flue gases, by, for example, injection of ammonia, are examples of technical difficulties which have to be overcome by processing the flue gases in some way on their way from the freeboard to the cleaning plant, by arranging devices or taking measures somewhere along the path of the flue gases up to the cleaning plant. In the publications mentioned above, certain separate solutions to some of the mentioned problems are presented by means of devices which are arranged adjacent to the freeboard of the combustor. However, according to prior art this has been difficult to achieve, since one problem is that, for example, cyclones included in a cleaning plant have been connected directly to the freeboard of the combustor via connections in the upper¬ most portion of the combustor. This has made it difficult to provide space for the mentioned forms of treatment be- tween the freeboard and the cyclones and to distribute the flue gases to the cyclones after the flue gas treatment .

SUMMARY OF THE INVENTION

By arranging a substantially vertical duct or riser exten¬ ding from the uppermost portion of the combustor as an ex- tension of the freeboard, a favourable outflow and mixing of the combustion gases as well as combustion of accompanying unburnt fuel are achieved by longer residence times for fuel particles in the combustion gases before these are passed to the components of a cleaning plant. A smaller cross-section area in the riser than in the freeboard of the combustor results in an increased gas speed and increased turbulence in the riser. This, together with devices along the riser which set the gas in rotation before it reaches cyclones in a cleaning plant, creates a more uniform distribution of combustion gas with contents of dust particles to the cyclones and a more uniform loading of the individual cy¬ clones and initiates a rotation of the gas flow even before the gas flow reaches the cyclones. Further, the separa¬ tion of coarse particles may be achieved in the duct with simple separation devices. Reduction of nitrogen oxides in the combustion gases may be performed in the riser by the injection of additives, for example ammonia or water-mixed ammonia. Because of a turbulence created in the riser, a favourable mixing of flue gas and a nitrogen-reducing substance may be obtained. The riser may also be utilized as a secondary combustor to achieve an increase of the gas temperature and hence an increase of the gas turbine power.

With an extended freeboard in the form of a riser with a smaller area than that of the freeboard, combined with the higher gas speed and the greater turbulence in the riser, where flue gas outlets for cyclones or other variants of flue gas cleaning devices are arranged, a more homogeneous composition of the gas is obtained before the gas is distributed to the cleaning devices. This provides a uniform load of the component in a subsequent cleaning plant, such as cyclones, which under certain conditions is far from what may be attained in current designs .

The riser has a height of the order of magnitude of the height of the freeboard or greater than that, which also provides space for a long zone for injection of, for example, ammonia for the purpose of reducing No _x . The powerful turbulence in the riser also accomplishes a uniform temperature distribution along the riser, which provides a possibility of No _x reduction with, for example, the addition of ammonia.

The injection of secondary combustion air along the inner sides of the riser to create a secondary combustion zone for unburnt fuel residues in the flue gases is also possible.

By rotating the flue gases in the riser, good opportunities are created for the above-mentioned addition of ammonia or a nitrogen reducing agent to the flue gases, because the turbulence contributes to a good mixing and distribution of the additive.

The riser eliminates the conflict between the demands of the reactor process on flexibility and the demands of the sub¬ sequent processes on stable flow and temperature conditions, while at the same time avoiding the risk of uncontrolled- combustion as a result of unburnt fuel residues in the process paths. In present-day systems, incomplete com¬ bustion of fuel particles in the actual combustor is avoided by the supply of excess air to the process with an air quan¬ tity determined by the air requirement for the fuel quan¬ tity, supplied to the bed, of that fuel feed nozzle which, at the particular moment, feeds in the largest fuel quan¬ tity. This leads to excess air at other points in the bed. By the formation of flow zones there is a possibility that certain cyclone paths in a cleaning plant are loaded with an excess air included in the flue gases, whereas other cyclone paths in their flue gases may contain an excess of unburnt fuel. When bringing together the flue gases of the diffe¬ rent cyclone paths after a cleaning plant, a risk of unwel¬ come fires arises when partially unburnt particles may en-

counter air with contents of oxygen, which may be the case, for example, at the inlet of a gas turbine or inside such a gas turbine. These drawbacks mentioned are avoided with the described more efficient intermixing of the flue gases ahead of a cleaning plant. The need of the supply of excess air is eliminated by the mentioned more efficient mixing of the flue gases, which in turn entails a more favourable combus- tion by, for example, reducing the No _x contents in the flue gases. In addition, a possibility of increasing the degree of utilization of the fuel, while maintaining a low forma¬ tion of nitrogen oxide, is provided.

The riser is formed as an essentially cylindrical pipe. The ^■ flue gases in this pipe are brought to rotate by the arrangement of flow-directing and/or turbulence-creating means at the inlet of the riser and/or along the extent of the riser. Guide vanes in the inlet of the riser, or additional nozzles, which blow in an additional gas, are examples of such flow-directing means . At the outlet of the riser there may be arranged guide means, which guide the outflowing flue gases essentially rectilinearly out into connections to, for example, cyclones in a cleaning plant. This may be achieved with tangentially located outlets or guide vanes.

An extended freeboard contributes to improved possibilities of either forming the riser with means for coarse separation of particles in the flue gas, or providing space between the freeboard and a cleaning plant for one or more separate coarse separators in the flue gas paths.

A riser connected to the uppermost portion of the freeboard in the combustor also contributes to a very favourable design solution of the problem of conducting the flue gases from the freeboard of the combustor to cyclones in those cases where the cyclones, because of space problems, are placed on top of the combustor or with the cyclone inlets higher than the combustor.

BRIEF DESCRIPTION OF THE DRAWING

The figure schematically illustrates the location of a riser connected to a combustor enclosed in a pressure vessel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the figure, 1 designates a cylindrical pressure vessel which encloses a combustor 3, a container 5 for receiving and storing bed material, a cleaning plant 7 consisting of a number of groups of series-connected cyclones 7a - 7f, as well as other auxiliary equipment. The lower portion of the combustor 3 forms the bed section 3a of the combustor. This bed section contains heat-absorbing tubes which form a steam generator. The upper portion of the combustor 3 forms the freeboard 3b of the combustor, which freeboard receives combustion gases leaving the bed of the combustor 3. The freeboard 3b of the combustor 3 is connected to a vertical duct, which constitutes a riser 9 connecting the combustor 3 to the cleaning plant 7, which is located in the space 11 between the riser 9 and the surrounding pressure vessel 1. Cleaned gases are collected in the conduit 13 and are led to a gas turbine included in the power plant . Separated dust is discharged through a pressure-reducing cyclone ash cooler 15. The combustor may be suspended at its corners 17 from pendulums 19, which may be attached to brackets or beams 21 attached to the pressure vessel 1.

The roof of the combustor 3 is suitably made with sides which are obliquely sloping upwards towards the riser 9.

There is, of course, nothing preventing the arrangement of more than one riser. However, this would create a risk of the desired uniform load of the cyclones not being attained because of an unbalanced flow through the multiple risers. Nor is it possible to arrange the riser centrally, as in the embodiment shown. However, the symmetry which is obtained with a riser 9 of circular cross section, centrally con¬ nected to the combustor 3, simplifies the achievement of a

symmetrical flow of the flue gas flowing out f the combustor 3.

The cross-section area of the riser 9 is adapted in relation to the area of the freeboard cross section such that the desired gas speed through the riser 9 is obtained. In the freeboard 3b the gas speed amounts to about 1 m/s . A suitable value to be aimed at for the gas speed in the embodiment is 6 m/s. The height of the riser is suitably the same as the height of the associated cyclones 7. The height of the riser 9 is at least as great as the diameter of the riser.