Combustor or gasifier for application in pressurized systems

Abstract: Fluidized bed combustor or gasifier including a combustion chamber(s) (12) with a polygonal horizontal cross section. The chamber (12) may have a trapezoidal or semicircular cross section, or may have five or more side walls (22,24,26,28) of at least two different lengths. The walls of the combustion chamber may be water tube panels. An external pressure vessel (18) surrounds the combustion chamber(s) and associated particle separator(s) (14;16), and may be spherical or cylindrical.

Description: BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to a circulating fluidized bed combustor or gasifier for application in pressurized combustion or gasification systems, according to the preamble of appending claim 1. The systems typically comprising at least one upright combustion chamber and one particle separator connected thereto enclosed in a common external upright pressure vessel. In conventional circulating fluidized bed processes high flow velocity and excellent mixing of particles and gases leads to efficient heat transfer and improved combustion efficiency. SO2 and NOx emissions are low due to desulphurizing sorbents used and due to staged combustion. Various fuels and refuse derived wastes may be burned or gasified and utilized in circulating fluidized bed combustion. The temperature is very stable and the heat transfer rate is high. In pressurized circulating fluidized bed processes principally all advantages from atmospheric circulating fluidized bed processes are maintained, whereas some additional advantages are achieved. The size of a pressurized steam generation plant, including combustion chamber and particle separators, can be made much smaller than a corresponding conventional atmospheric steam generation plant. Significant savings in material and investment costs are achieved. Further pressurized steam generation systems provide increased total efficiency compared to atmospheric steam boilers. Pressurizing of a circulating fluidized bed process provides a considerable increase in efficiency/volume ratio. In pressurized circulating fluidized bed systems fuel is combusted or gasified in a combustion chamber at high temperatures and high pressure. The external vessel provides pressure containment, which is cooled or insulated to enhance material strength and to thereby minimize costs of the pressure vessel. Combustion air pressurized in a compressor is directed into the pressure vessel into the space between the combustor and the peripheral wall of the pressure vessel. The pressurized air thereby provides for cooling of the walls of the pressure vessel. In the vessel the pressurized air is further directed through a grid into the combustion chamber for fluidizing and combusting of material therein. The pressure in the pressure vessel may be 8 - 30 bar, typically 10 - 14 bar. In a circulating fluidized bed system particles are separated in a particle separator, such as a cyclone or hot gas filter, from the hot gases produced in the combustion chamber and the separated particles are recycled into the combustion chamber. In a combined gas/steam power plant the hot gases discharged from the particle separator may be further cleaned and utilized in a gas turbine, thereby increasing the electrical efficiency of the power plant considerably compared with a conventional steam generation plant. The gas turbine may be connected to the compressor feeding pressurized air into the combustor. The peripheral walls of the combustion chamber are cooled by recovering heat in a water/steam circulation. Additional heating surfaces, such as superheaters, reheaters and economizers, connected to the water/steam circulation are usually arranged in the combustion chamber. In circulating fluidized bed combustors the additional heating surfaces are arranged in the upper part of the combustion chamber. A multitude of steam piping, including risers and downcomers, thereby have to be arranged within the pressure vessel. Steam generation systems for power plants are therefore large even if pressurized. The external pressure vessel can be a variety of shapes. Two common shapes are cylindrical and spherical. The price of a pressure vessel itself is high and the space inside the vessel must be utilized as advantageously as possible. The diameter of the pressure vessel should be kept as small as possible to minimize costs. The vessel wall thickness and hence material costs increase with the diameter of the vessel. When pressurizing a circulating fluidized bed combustor system all of the combustion chamber, particle separator, fuel feeding and ash discharge arrangements, as well as the piping for the water/steam circulation are preferably arranged in one single pressure vessel. A conventional combustion chamber, having a square, rectangular or circular cross section, leads to a very space consuming arrangement, which needs a large diameter pressure vessel, leaving a large volume of unused space in the vessel. The cost of the pressure vessel is a determining factor when calculating the total costs of the pressurized system. The bigger the system the more significant is the price of the pressure vessel. It has earlier been suggested in WO-A- 89/00660 to arrange a fluidized bed reactor and a filter in a back-to-back relation into a pressure vessel, the longest wall of the combustion chamber thereby being connected to the filter. A very compact combustion chamber and filter arrangement is achieved. Any elements, such as water/steam tubes, fuel inlets, gas outlets, that have to be connected to the combustion chamber, have to be connected to shorter side walls opposite to the longest wall, which is space consuming as there has to be space for the connection of these elements between the side walls and the peripheral wall of the pressure vessel. It is further known from US patent no. 4,479,458 to arrange a multitude of combustion chambers within one single pressure vessel and to arrange particle separator cyclones vertically above the combustion chambers. It is also known from international patent application WO-A-91/17388 to arrange a polygonal combustion chamber in the middle of a pressure vessel and to arrange particle separator cyclones vertically above the combustion chamber. It may, however, not always be possible to arrange cyclones at such high levels as suggested in these publications. It is therefore an object of the present invention to provide a circulating fluidized bed combustor or gasifier for application in pressurized systems in which the size of the pressure vessel is minimized. A circulating fluidized bed combustor or gasifier according to the present invention is thereby characterized by the features mentioned in the characterizing portion of claim 1. Typically a combustion chamber, having a non-symmetrical horizontal cross section and at least two adjacent walls forming an angle > 90°, is utilized in the pressurized combustor or gasifier according to the present invention. The arrangement of combustion chamber equipment within the pressure vessel together with related auxiliary equipment including cyclones, filters, steam piping, fuel feeding or other equipment can be enhanced by utilizing unconventional combustion chamber shapes. According to the present invention a trapezoidal, semi-cylindrical, hybrid trapezoidal/semi-cylindrical, or other semicylindrical-approaching multisided (e.g. five or more sides) polygonal cross section is provided to better conform the shape of the combustor to the external vessel. Advantages of the combustion chamber cross section of the invention include: Optimal utilization of plan area within the external pressure vessel, thereby minimizing the size, cost, and space requirements of the vessel. Minimization of the height of the combustor or gasifier, and of the external pressure vessel, by alternative configurations of the heat transfer surfaces. Such configurations include angling internal surfaces and maximizing wall area per unit height. Maximization of the perimeter area of the combustor or gasifier, enhancing circulation characteristics of the combustor or gasifier if it is cooled. Optimizing the cross sectional area of the combustor or gasifier, increasing the amount of usable space for location of heat transfer surfaces. Reducing the potential effects of erosion by increasing the angle and/or rounding edges and corners within the combustor or gasifier to reduce eddies. Increased wall area on the rear combustor wall for location of cyclone inlets, solids feeding or removal, and heat transfer surfaces. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematical vertical section of a pressurized combustor having an exemplary trapezoidal cross sectional combustion chamber in accordance with the invention arranged in a pressure vessel; FIGURE 2 is a cross sectional view taken along lines AA of the pressurized combustor of FIGURE 1; FIGURE 3 is a cross sectional view of another exemplary combustor system having a combustion chamber having straight walls forming a multi-sided polygon approximating a curved wall arranged in a pressure vessel; FIGURE 4 is a cross sectional view of still another exemplary pressurized combustor system having a trapezoidal combustion chamber arranged in the pressure vessel; DETAILED DESCRIPTION OF THE DRAWINGS The pressurized fluidized bed combustor shown in FIGURES 1 and 2 comprises a pressure vessel 10 having a combustion chamber 12 and two cyclone separators 14 and 16 arranged therein. The pressure vessel is formed of an upright cylindrical steel vessel 18 with external insulation 20 and a flanged cover plate 21 on top. The combustion chamber 12 has a trapezoidal cross section, and is mainly made of vertical planar tube panels forming a longest side wall 22, a short side wall 24 and two end walls 26 and 28. Of course in such a polygon at least two adjacent substantially straight walls form an angle greater than ninety degrees. The combustion chamber 12 is arranged in a first half of the pressure vessel, the long side wall or back wall 22 being arranged approximately in the middle part of the vessel 18 and the short side wall or front wall 24 and the end walls 26 and 28 being arranged close to the periphery of the pressure vessel 18. This provides a very space efficient arrangement of the combustion chamber 12, and cyclones 14, 16 and minimizes useless space in the first half of the pressure vessel 18. Further the total peripheral tube panel area is increased compared to systems where a rectangular or square combustion chamber with the same plan area is arranged in a similar pressure vessel. The lower end of the combustion chamber 12 is connected through a grid bottom 30 with a windbox 32 for introducing fluidizing and combustion air into the combustion chamber 12. An ash drain 34 is connected to the windbox 32 for discharging ash from the combustor 10. A fuel feeder 35 is connected to the combustion chamber 12 through the back wall 22. The upper part of the combustion chamber 12 is connected through two gas ducts 36 and 38 to cyclones 14 and 16 arranged mainly in the second half of the pressure vessel and adjacent the back wall. The cyclones 14, 16 have gas outlets 40 for discharging gas from the combustor 10, e.g. to a hot gas filter 41 or to a convection section (not shown). The cyclones 14, 16 are connected through return ducts 42 and 44 and loop seals 46 with the lower part of the combustion chamber 12. The tube walls 22, 24, 26, 28 of the combustion chamber 12 are connected through headers 48 with a steam drum 50. Additional heat transfer panels 56, e.g. superheaters, may easily be arranged in the combustion chamber 12, as the present invention provides enough space in the pressure vessel 18 for steam piping and other auxiliary equipment and ample space for additional heat transfer surfaces inside the combustion chamber. In FIGURE 3 components comparable to those in FIGURE 2 are shown by the same reference numeral. Fig. 3 shows a combustion chamber that almost completely fills the first half of the pressure vessel. The combustion chamber is constructed from flat panel walls. The cross section of the combustion chamber is a multi-sided polygon having six walls. In FIGURE 4 components comparable to those in FIGURE 2 are shown by the same reference numeral. The combustion chamber may, if desired, have a trapezoidal cross section, as shown in FIG. 4. A fuel feeder 35 is illustrated schematically in FIGURE 4, it being understood that the fuel feeder 35 will typically be located at the same level With respect to the chamber 12 as the fuel feeder 35 is with respect to the chamber 12 in FIGURE 1. Also, a filter 55 may be provided connected to a gas outlet of the particle separator, the filter being disposed adjacent the planar wall 22. Thus, the present invention provides a very flexible combustion chamber configuration, with a combustion chamber having four or more walls. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: A circulating fluidized bed combustor or gasifier for application in pressurized systems comprising an external upright pressure vessel (10) enclosing an upright combustion chamber (12), a particle separator (14, 16) connected to the combustion chamber and fuel feeding means (35), wherein the combustion chamber has a trapezoidal or multi-sided horizontal cross-section and is made of flat panel walls, including a longest side wall (22) and other side walls (24, 26, 28), at least two adjacent walls forming an angle > 90°, the pressure vessel is cylindrical and the horizontal cross section thereof has a first half and a second half, the combustion chamber being arranged in the first half such that the longest side wall (22) of the combustion chamber is located in the middle part of the pressure vessel, characterized in that the longest side wall (22) has a length substantially corresponding to the diameter of the pressure vessel, the other side walls (24, 26, 28) of the combustion chamber are arranged close to the periphery of the first half of the pressure vessel such that the combustion chamber almost completely fills said first half of the pressure vessel, the particle separators consist of two cyclone separators connected through gas ducts (36, 38) to the upper part of the combustion chamber and through return ducts (42, 44) to the lower part of the combustion chamber, and the cyclone separators (14, 16) and fuel feeding means (35) are arranged within the second half of the pressure vessel and connected to the longest side wall of the combustion chamber. A combustor or gasifier according to claim 1, wherein the walls of the combustion chamber are made of water tube panels. A combustor or gasifier according to claim 1 or 2, wherein steam piping (48) including risers and downcomers is disposed adjacent said longest wall (22) of said combustion chamber. A combustor or gasifier according to claim 1 or 2, wherein a filter (55) is connected to a gas outlet (40) of the respective cyclone separator and disposed adjacent to the longest wall, in the second half of the combustion chamber. A combustor or gasifier according to any one of the preceding claims 1 to 4, wherein the horizontal cross-section of the combustion chamber is a multi-sided polygon, having five or more side walls, the side walls being of at least two different lengths.

Inventors: PROVOL STEVEN, RUSSEL DAVID, PROVOL, STEVEN, RUSSEL, DAVID
Applicants: FOSTER WHEELER ENERGIA OY
Application year: 1993
Main classification: F23C11
IPC codes: F22B31, F23C10, B01J8