Abstract
A furnace for performing an endothermic process comprises tubes containing a catalyst for converting a gaseous feed, said tubes positioned inside the furnace, inner burners mounted to a furnace roof between the tubes, and outer burners mounted to the furnace roof between the tubes and a furnace wall. The outer burners are positioned close to the furnace wall, and configured to operate with 45-60% of the power of the inner burners and with an inlet velocity between 90 to 110% of the inlet velocity of the inner burners.
Claims
1. A furnace for performing an endothermic process comprising tubes containing a catalyst for converting a gaseous feed, wherein said tubes are positioned in inside the furnace, inner burners mounted to a furnace roof between the tubes, and outer burners mounted to the furnace roof between the tubes and a furnace wall, wherein the outer burners are positioned such that the distance of the central axis of each outer burner to the furnace wall is less than 25% of the distance between the outermost tubes and the furnace wall, and that the outer burners are configured to operate with 45-60% of the power of the inner burners and with an inlet velocity between 90 to 110% of the inlet velocity of the inner burners.
2. The furnace according to claim 1, wherein the tubes are positioned in rows and that the ratio of the distance between the furnace wall and the first tube row to the gap between two adjacent tube rows corresponds to the ratio between the power of the outer and the power of the inner burners.
3. The furnace according to claim 1, wherein at least a part of the furnace roof is provided with a temperature resistant, high emissivity solid surface.
4. The furnace according to claim 3, wherein the high emissivity solid surface contains silicon carbide or ceramic porous foams.
5. The furnace according to claim 1, wherein at least some of the burners are jet flame burners.
6. The furnace according to claim 1, wherein at least some of the burners are burners with high swirl ball flame technology.
7. The furnace according to claim 1, wherein at least some of the burners are arranged such that the flame is formed on a porous radiating shield.
8. The furnace according to claim 7, wherein at least some of the burners are arranged in a square or a hexahedral configuration with regard to the catalyst tubes.
9. The furnace according to claim 7, wherein the length of radiating shield is between 10 and 40% of the catalyst containing tube length.
10. The furnace according to claim 1, wherein at least a part of the furnace roof is designed to have a convex or concave shape.
11. A process for operating a furnace for performing an endothermic process with catalyst containing tubes positioned in inside the furnace for converting a gaseous feed and which are heated by inner burners mounted to a furnace roof between the tubes and by outer burners mounted to the furnace roof between the tubes and a furnace wall, wherein the outer burners are positioned such that the distance of the central axis of each outer burner to the furnace wall is less than 25% of the distance between the outermost tubes and the furnace wall, and that the outer burners are operated with 45-60% of the power of the inner burners and that an inlet velocity of the outer burners is adjusted to be between 90 and 110% of the inlet velocity of the inner burners.
12. The process according to claim 11, wherein at least some of the burners' flames are directed from the top to the bottom of the furnace.
13. The process according to claim 11, wherein the feed flows through the vertically arranged catalyst tubes from the top to the bottom of the furnace.
14. The process according to claim 11, wherein the inlet velocity is adjusted by air injection.
15. The process according to claim 11, wherein the endothermic process is a steam reforming process.
Description
BRIEF DESCRIPTION OF THE FIGURES
(1) In the drawings:
(2) FIG. 1 shows the typical design of a furnace before an endothermic reaction in catalyst containing tubes;
(3) FIG. 2 shows the typical vertical tube heat flux and temperature profile;
(4) FIG. 3 shows an illustration of the flame bending;
(5) FIG. 4 shows the average tube duty row per row for a reformer furnace (8 tube rows) and a virtual 24 tube rows firebox;
(6) FIG. 5 schematically shows the section of the furnace including the proposed design;
(7) FIG. 6 shows the normalized tube duty row per row average for reference and optimized reformer design;
(8) FIG. 7 shows the normalized duty for standard tube bay (17 tubes) in an 8 tube row reformer ((a) actual design (b) optimized design);
(9) FIG. 8 shows the implementation of a burner in combination with a high emissivity refractory layer;
(10) FIG. 9 shows the high swirl flame concept;
(11) FIG. 10 shows the radiant burner concept for diffusion flame and premix;
(12) FIG. 11 shows the radiant burner to tube arrangement;
(13) FIG. 12 shows the implementation of the furnace roof as radiant wall;
(14) FIG. 13 shows options for configurations of a concave roof;
(15) FIG. 14 shows options for configurations with a convex roof;
(16) FIG. 15 shows a linear radiant roof burner.
DETAILED DESCRIPTION
(17) FIG. 1 shows a typical arrangement of a top-fired furnace 1 used to obtain a synthesis gas from a feed (educt) comprising, e.g., methane and steam. Catalyst tubes 2 are provided in several rows within the furnace 1. The feed is supplied through tubes 2 from the top to the bottom from where the resulting product, e.g., a synthesis gas comprising hydrogen, carbon monoxide and residuals, is withdrawn. Between the tube rows, burners 3 fire vertically downwards from the top. The resulting flue gases are withdrawn through exhaust tunnels 4.
(18) The typical vertical profiles for heat flux and temperature are plotted in FIG. 2. It is evident that the heat flux and the temperature profile are coupled to each other. The stiffer the heat flux and the temperature profile is in the upper part of the tube, the higher is the heat duty to the tube at the same temperature and the higher is the process gas flow rate capacity per tube at the same conversion rate.
(19) FIG. 3 is an illustration of the flame bending for 4 and 8 tube rows (only half of the firebox has been simulated for symmetry reasons). The fluid mechanism and jets theory will define the typical flow arrangement within a top-fired firebox, meaning the side burners hot burnt gases jet aspiration towards the middle center of the firebox. If the lower power or flow rates along the side walls yields to a lower velocity, this will reinforce the bending effect of the side flames to the center, due to the weaker momentum of the side jets.
(20) FIG. 4 shows the simulated average tube duty per row for a reformer (furnace) having a firebox with 24 tube rows (only half of the firebox has been simulated for symmetry reasons). To counter the phenomenon of the bending effect, the momentum from the side burner rows has been increased step by step, up to 78% of the inner burner power. The flame bending is not suppressed, and the power increase creates an overheated region at the side of the furnace, whose peak value is located on the second tube row from the wall due to the hot flue gas flowing through the first tube row and heating the next row.
(21) FIG. 5 shows the design modification as proposed with the present invention, wherein two channels 5, 6 are defined by the furnace wall 1a and the catalyst tubes 2. The distance d between two catalyst tube rows 2.sub.1 and 2.sub.2 defines the middle channel 5. Inner burners 3a are centrally positioned between tubes 2.sub.1 and 2.sub.2 on the roof 1b of the furnace 1. In channel 6 defined between the outer catalyst tube 2.sub.1 and the furnace wall 1a outer or side burners 3b are arranged on the roof 1b of the furnace. The dimension d.sub.1 of outer channel 6 is set so that its ratio to the distance d is the same as the ratio of the power of the outer and the inner burners, namely 45 to 60%, such as 50 to 58%, and such as approximately 55% of the diameter d.
(22) The configuration described above has been simulated using the SMR3D simulation tool (Air Liquide proprietary tool based on a Computational Fluid Dynamic (CFD) tool coupled to a reforming tube model). The result of the simulation is presented in FIGS. 6 and 7 and compared to a reference design. The optimized design of the present invention results in a much better duty homogeneity at the reformer scale. The tube duty standard deviation has been decreased and optimized torow per row1% vs. 4% in reference case and, as shown in FIG. 7 tube per tube, 3.5% vs. 6.5% in reference case.
(23) FIG. 8a shows the simplest implementation of a burner 3 in combination with a high emissivity refractory layer 7 provided at the inside of the furnace roof 1b. The high emissivity refractory layer 7 may be formed from bricks, thin sheets or a coated layer, wherein the high emissivity results from intrinsic material properties, for instance using SiC sheets, or from surface treatment or texturing, for instance using ceramic foam sheets. A row of reduced power separated jet burners 3 form a continuous flat flame. Compared to the prior art, the proposed innovative implementation uses jet flame burners arranged in a continuous flame with massive number of staged air and fuel injection, e.g. between 10 to 30 fuel injectors per meter, compared to one burner every 2 to 6 meters as proposed in prior art designs.
(24) For the most efficient heat transfer implementation, wall burners 8 can be based on a radial burner technology as shown in FIG. 8b, or in a ramp wall burner technology as shown in FIG. 8c, likely disposed in a staggered way with tubes.
(25) As presented in FIG. 9, also the implementation of swirl ball flame combustion technology associated with a high emissivity layer 7 on the furnace roof 1b is possible. Therein the flame itself is captured in a recirculation zone, when fuel as well as the oxygen source and/or steam is recirculated from a lower point of the flame back into the direction of the furnace roof.
(26) FIG. 10 shows two implementations of the invention using unit cylindrical radiant burners of two possible kinds: FIG. 10a shows a diffusion flame enclosed in a radiating shield 9. Passages 10 to let flue gases enter the flame root by a Venturi effect is recommended to achieve low NO.sub.x amounts. The dilution of the combustion reactants with flue gas will reduce the maximum flame temperature. Both dilution and temperature drops the NOX formation kinetics.
(27) Premixed combustion with flame attachment in a porous form itself is presented in FIG. 10b. The combustion reaction occurs inside the porous media, which is heated up and emits radiation towards the tubes in front of it. The main advantage of this technology is that the radiation could be located at the optimum place regarding heat transfer to tubes.
(28) The radiant shell is made of high temperature resistant material such as porous ceramic foam with high emissivity (SiC, Al.sub.2O.sub.3, ZrO.sub.2).
(29) The use of radiant burners allows designing the furnace with optimized burner tube to tube pavement that minimizes the circumferential flux inhomogeneities as presented in FIG. 11. Two tube pavements are proposed: FIG. 11a shows a square tube pavement while FIG. 11b shows a hexahedral tube pavement with a standard burner.
(30) FIG. 12 shows the implementation of burners heating the furnace roof 1b itself and uses this roof as a radiant wall. As shown in FIG. 12a, it is possible to design the high insulation, low heat conductivity refractory lining in a convex or as shown in FIG. 12b in a concave form.
(31) FIG. 13 shows different burner configurations which can be implemented in a concave formed burner roof. The cross-section view for two radiating oblique walls 1c is illustrated in FIG. 13a, wherein the optimum angle Ca is below or about 50, and the horizontal cave roof width Cw is designed based on the tube corridor width W and the Ca angle so that enough space is kept to arrange the burners accordingly.
(32) In FIG. 13b, typical jet burners 3 such as shown in FIG. 8 are disposed in lines to be fired downwards along the oblique radiating walls 1c on each side of the caved roof.
(33) FIG. 13c presents the same kind of design with a continuous linear wall burner.
(34) In FIG. 13d, typical radial wall burners are disposed in the caved roof with spacing L so that the best compromise between heat fluxes homogeneity and burner number reduction is found. It can typically correspond to one burner every 2 to 8 tubes.
(35) Finally, FIG. 13e presents a configuration, where classical high swirl burners are installed in the caved roof.
(36) FIG. 14 shows a convex roof design (spike roof). In FIG. 14a, wall burners 3 are arranged around the convex form firing downwards; in FIG. 14b, the burners 3 are arranged inside the convex roof so that the flames are fired upwards from the spike end to increase the radiating area covered by the flames, and thus the transfer efficiency to the tubes 2.
(37) As shown in FIG. 15, it is also possible to provide a porous combustion chamber in which at least one large power burner is fired. Openings close to the flame root (furnace side walls 1a) can be added to let flue gases recirculate from the furnace atmosphere into the gas generator chamber by venturi effect section pressure. The configuration presented in FIG. 15a shows a single burner arrangement with staggered firing from one tube-to-tube-corridor to another. This configuration saves several unit burners capital costs compared to the prior art, where up to 15 or 20 per row are used in large top-fired reformer, and that can be replaced by one or two in the presented embodiment. A more reliable configuration is to have two burners per radiant channel as shown in FIG. 15b so that the furnace performance would be less critically affected in case of burner unaccepted downward.
LIST OF REFERENCE NUMBERS
(38) 1 furnace 1a furnace wall 1b furnace roof 1c oblique furnace wall 2 tube 3 burner 3a inner burner 3b outer burner 4 exhaust tunnel 5 middle channel 6 outer channel 7 high emissivity refractory layer 8 wall burner 9 radiating shield 10 passage
