WO1985003281A1

WO1985003281A1 - Combustion heating apparatus for steam reforming

Info

Publication number: WO1985003281A1
Application number: PCT/GB1985/000032
Authority: WO
Inventors: Stuart Ross Martin
Original assignee: Stuart Ross Martin
Priority date: 1984-01-25
Filing date: 1985-01-23
Publication date: 1985-08-01
Also published as: GB8401989D0; EP0169227A1; AU3887085A

Abstract

Combustion heating apparatus for fluids of the kind in which the fluid is passed through a heated zone (15), which zone is heated by a combustion zone (20) to which air and fuel is fed. The invention is characterised in that said fluid is passed through the heated zone (14') in multipe substantially parallel streams, in that there is a heated zone for each stream, in that there is an associated combustion zone (20) for each heated zone, and in that the associated heated and combustion zones are preferably elongate and arranged one within another with a high degree of radial symmetry, preferably concentrically. Heat may be produced in a desired distribution in the combustion zones by introduction either of air or fuel, or both, through pores (5) or ducts in a combustion wall bounding said zones, and the combustion reaction may be stabilised close to this wall by turbulence-promoting projections. The apparatus may be for steam reforming, in which case an endothermic chemical reaction between steam and hydrocarbons is carried out in each heated zone to form hydrogen and oxides of carbon, which reaction may be promoted by catalytically active surfaces disposed within each heated zone.

Description

Combustion Heating Apparatus for Steam Reforming

This invention relates to combustion heating appar¬ atus for fluids, and has particularly advantageous applica¬ tion to the process of steam reforming of hydrocarbons.

The steam reforming process consists of heating a mixture of a hydrocarbon feedstock with steam in the presence of a solid catalyst. The catalyst generally used is nickel, in finely divided form and dispersed on the surface of porous support bodies made of alumina. Dependent upon the application, the process may be carried out at any pressure up to 40 bar or even more with known apparatus. The •temperature of the process stream reaches its maximum as it leaves the catalyst bed, this temperature being limited in practice by the corresponding metal temperature at which adequate mechanical properties of the containing duct .can be relied upon. Typically, these temperatures are about 850°C and 900°C respectively. The product stream consists of a mixture of hydrogen, carbon oxides, steam, and methane, together with any inert components such as nitrogen which may be present. The product composition may approach quite closely the equilibrium composition for the conditions at exit from the catalyst bed.

The steam reforming process is of great industrial importance. Among its applications are the production of hydrogen and reducing gases for metallurgical and fuel refining processes, and for fuel cells, and the manufacture of synthesis gases for conversion to ammonia, ethanol, and other chemicals and fuels.

Steam reforming is usually carried out in a fired tubular furnace. The catalyst is normally contained within a number of identical tubes, typically of 0.12 m outside diameter and more than 12 in length, suspended within a refractory lined furnace box which is heated by burners.

Various arrangements of the tubes and burners are successfully employed, so as to meet competitive standards of reliability, performance, and cost. A principal require¬ ment is that, at any given horizontal plane, the heat flux should be rather evenly distributed from tube to tube and around each tube. This requirement leads to wide spacing of the tubes and hence large dimensions of the furnace. There is thus a large incentive to develop more compact, lower cost, apparatus.

The main deficiency of the conventional reformer furnace, however, is that it is wasteful in its use of energy. Typically, less than half of the heat released by the burners goes to provide reaction heat. This part of the heat is directly useful, since it is potentially available as fuel value of the end products. The balance of the heat goes to heat up the exiting process and flue gas streams and the surroundings; and while a high proportion of this is usually recovered to process heat, and steam raising, large costs and large losses of work potential are involved in such recovery. In particular, only some 25% of heat transferred to a steam system is recovered as work. Work is also lost as a result of an undesirably high frictional pressure drop through the catalyst bed. This has to be accepted in conventional reformer practice, because the cost penalty of using more numerous or wider tubes to reduce the pressure drop is too high.

It is known from U.S. Patent specification No. 3,713,784 that the 'heat to be transferred to the reformer tubes may be reduced ^"by some 20% by the use of a recupera¬ tive counter-current heat exchange duct within each tube,

10 through which the product stream is passed. Such recupera¬ tive reformer tubes have found very few applications so far, probably because of considerations of cost and of access to replace the catalyst. It may be however, that in the context of a radically new .reformer design the recuperative '^••-5 reformer tube arrangement will have significant advantages.

It is also known that the efficiency of a furnace can be raised by recuperative heat exchange between flue gas and combustion air. Combustion reaction (flame) temperature is generally increased as a direct result. The conventional

^2υ approach is to have a single recuperator for the furnace, and distribute heated air by ducts to the burners. The burners tend to be more bulky, heavy, and expensive as the preheat is increased. The hot air ductwork becomes problematical and expensive as desirably high temperatures

²⁵ are reached, and the higher flame temperatures put increased demands on the burner components and any refractories which are close to the flames.

Pressurisation of the combustion space of a steam reformer is also known, for instance from USP 3,958,951. Advantages include size reduction, and the possibility of recovering work by expansion of combustion product gas. However, up to the present, pressurised combustion reformers have not been used in large scale applications, probably because the proposed designs have been considered to be too complicated.

Turning now to the present invention. Applicant has found that selected features of these and other known arrangements and designs can be utilised, in combination with other features which are novel with respect to fired heaters, to produce substantial benefits.

Thus, an object o_^f the present invention is to provide an improved combustion heating apparatus for fluids e.g. for carrying out steam reforming, with a higher thermal efficiency and lower cost than is achieved by the known kinds of apparatus.

According to the present invention, there is provi¬ ded combustion heating apparatus of the kind in which the fluid is passed through a heated zone, which zone is heated by a combustion zone to which air and fuel is fed, characterised in that said fluid is passed through said heated zone in multiple substantially parallel streams, in that there is a heated zone for each stream, in that there is an associated combustion zone for each heated zone and in that the associated heated and combustion zones are arranged one within another with a high degree of radial symmetry.

Thus, in combustion heating apparatus in accordance with the invention, instead of a single large firebox, each individual heated zone through which a fluid stream passes is provided with its own combustion zone or chamber and, where required for steam reforming, catalyst zone or ■ chamber, the combination being preferably concentric.

Since such an arrangement of combustion heating apparatus requires a large number of combustion chambers of unconventional size and shape, further beneficial features of the invention may include: 0 a) ensuring a substantially equal heat release between parallel streams, and, b) ensuring a controlled distribution of heat release within each combustion chamber.

These features will be discussed in further detail 5 hereinafter.

The main benefits foreseen for combustion heating apparatus according to the present invention are as follows:-

1. Relatively close grouping of catalyst chambers with 0 associated combustion chambers, with resultant economies of space, materials, and energy.

2. Relatively moderate refractory temperatures are sufficient to transmit the heat.

3. Relatively low thermal inertia, giving good control 5 response as well as reduced times for starting up and cooling down.

4. It is readily achieved to heat the first, coolest, part of the catalyst zone by counter-current exchange with the exiting flue gas. Combustion efficiency is directly increased by this feature, at the expense of some addition to the surface area required for heating the catalyst zone.

5. Simple and effective recuperative air heaters can be readily incorporated.

6. Alternatively to 5, relatively simple designs of pressurised combustion heating apparatus are made possible, with resultant economies of materials and energy-.

With regard to feature a) above, that heat release be substantially uniform between parallel combustion units, this may be met by having each fuel, air, and flue gas stream follow a substantially identical path through a combination of plenum chambers and manifolds.

With regard to feature b) above, that ther is a controlled distribution of heat within a combustion chamber, this may be met in one of at least two distinct ways, which will now be discussed with reference to Figures 1 to 6 of the accompanying drawings, in which:-

Figure 1 is a fragmentary sectional view of one form of combustion chamber,

Figure 2.is a fragmentary sectional view of another form of combustion chamber.

Figures 3 and 4 are enlarged, contour views of turbulence-promoting shapes, and

Figures 5 and 6 are enlarged, contour views of catalyst bodies.

1. Referring to Figure 1, flue gas may be recirculated axially by the jet action of incoming air (a) and fuel (f) at the burner nozzle 1. Circulation along the length of the chamber is promoted by a baffle 2 which divides the chamber into separate channels 3 and 4 for forward and reverse flow. 2. Referring to Figure 2, either the air (a), or the fuel (f), may be introduced gradually through the radiant surface of a porous wall 5 of the combustion chamber.The resulting extensive flame zone can be stablised close to the wall by turbulence-promoting shapes. Catalytic bodies can also be incorporated, so as to maintain the flame reaction at conditions unfavourable to stable non-catalytic combustion. Figures 3 and 4 illustrate examples of turbulence-promoting shapes, while Figures 5 and 6 show examples of catalyst bodies one comprising spaced catalytic metal gauze discs G and the other having an enclosing catalytic metal spiral S. These bodies may also be shaped to function as turbulence promoters.

Within the scope of the present invention, for steam reforming, the basic geometry of the catalyst and combustion chambers may be any one of at least three possible configurations:

1. The catalyst may be contained in tubes, with a combustion chamber around each tube.

2. Combustion may take place within tubes, with the catialyst either in a surrounding annulus, or in a continuous space containing other combustion tubes. In the latter case, provided the flow through the catalyst bed is axial rather than transverse, the feature of substantial radial axial symmetry can be preserved. 3. The catalyst may be in an annular space between inner and outer combustion tubes.

Practicable designs can no doubt be developed using the above heat distribution and geometric possibilities in various combinations. Study of alternatives and possibily experimental work will be required to make sure that the best combination for a given application is selected.

With respect to means of control of heat distri¬ bution within the combustion chamber, the porous wall arrangement is generally preferred over the jet recircu- lation arrangement because: a) heat distribution is directly controlled b) less space is required c) reliability is potentially higher, since none of the components is required to operate at such high temperatures as occur in the throat of a conventional burner nozzle.

It is generally preferred to add air through the porous wall rather than fuel because: a) it is thus easier to ensure equality of fuel flow to each combustion chamber, b) the greater flow through the porous wall makes it easier to design this component, c) the fuel supply system is simpler and therefore easier to make safe. The solution which is being proposed involves heating the fuel to high temperatures. Sooting must there¬ fore be considered as a possible problem.

It may be difficult to avoid soot deposition in case where the fuel has a significant content of hydrocarbons with two or more carbon atoms. This limitation may in practice not be too serious because: a) a hydrogen-rich fraction separated from the reformer product gas is often available as fuel, b) for start-up, a mixture of natural gas and steam may be used. c) if necessary, some of the reformed gas may be used as fuel. In order that the invention may be readily under¬ stood, three embodiments of combustion heating apparatus in accordance therewith will now be described with reference to Figures 7 to 10 of the accompanying drawings in which:-

Figures 7a and 7b are elevation and plan views respectively of the first embodiment of apparatus

Figure 8 is an enlarged cross-sectional view of one tube/combustor unit of the apparatus shown in Figure 7, and

Figures 9 and 10 are sectional elevations of the second and third embodiments of apparatus. Referring to Figures 7 and 8, the heating apparatus is a steam reformer intended to operate with near atmospheric combustion pressure. The apparatus comprises a group of some 40 identical tube/combusion units 6 arranged around central inlet and outlet process ducts 7 and 8. Fuel is distributed via a pipe manifold 9. Air and flue gas duct connections are respectively at 10 and 11. Referring to

Figure 8, one tube/combustor unit 6 is shown with a part of surrounding structure common to all units. Hydrocarbon and steam feed mixture enters at 12 and is heated against reformed gas in a recuperative exchanger 13. It then flows downward through a catalyst bed 14. Reformed gas returns upward through a thin-walled tube 15, giving up heat to the catalyst bed before leaving at 16. To enhance heat transfer, the tube 15 is formed to have fluted surfaces, and is fitted with an internal restricting cylinder 17.

The catalyst tube has an outside diameter of 0.12m and it is packed with catalyst over a height of 10 m. In respect of throughput, outside surface, and weight, it is comparable with a conventional reformer tube, but average heat flux is lower because of the recuperated heat supplied from the central tube 15.

Fuel entering at 18 is preheated against flue gas in a helical coil 19, and enters the combustion chamber 20, in this embodiment via a porous wall 5 as described herein, before flowing through a bed of ceramic pellets 21 to ensure an even flow distribution.

Flue gas is partly cooled in a convection heating section 24 by exchange with the inlet portion 14 of a cata- lyst tube 14'. Catalyst bodies are located in the section

24 so as to ensure complete combustion, as well as to promote good heat transfer by mixing action.

Flue gas is then further cooled against incoming air in a recuperative exchanger- 25 before flowing at low velocity through a plenum chamber 26 to the edge of the cylindrical outer casing of the apparatus. Here it is further cooled in an outer recuperator 27 which is common to the group of tube/combustor units, before exiting at 28. These units are suspended from above and are free to expand without restraint.

Access for replacement of the catalyst is obtained by removing cover plates 29. It will be appreciated that, for a large plant a number of such reformers would be used.

Referring now to Figure 9 the second embodiment is in the form of a pressurised combustion heater for delivering product gas at a high temperature. The steam reforming reaction is carried out within a bed 30 of catalyst resting on a perforated grid 31 and filling a continuous space outside a number of identical heating tubes, such as 32, and within the pressure enclosure 33. In operation, process gas enters at a port 34, flows upwardly through a bed 30, where reaction takes place, and leaves at a port 35. Fuel gas supplied at 36 is segregated from product gas by a baffle 37 and is drawn into extensions 38 of the heating tubes 32. Gaps in the baffle 37 ensure that only a small pressure difference exists across the wall of the heating tubes 32. Of course, if it is desired to use product gas as a fuel, baffle 37 and tube extensions 38 are omitted. Air enters at 39 and is distributed via a plenum chamber 40 to air tubes 41 each of which is arranged concentrically within a heating tube 32 and each of which has a porous wall section at 42. Combustion takes place under the influence of a catalyst 43 in the annular space between each air and heating tube unit, which catalyst rests upon a perforated grid 44. Combustion gases exit at 45.

Referring now to Figure 10, the third embodiment is a variant of the second embodiment, and is suited to the case where it is desired to recuperate heat from the product gas. Like parts have, therefore, been given the same reference numerals.

The steam reforming reaction is carried out within a number of identical beds such as 46, each resting on a grid 47, and filling a space between a heating tube 32 and a catalyst container 48.

In operation, process gas flows upwardly through the beds 30, then downwardly through the gaps between the catalyst containers 48, before leaving at a port 49. The catalyst containers may be circular or hexagonal in cross- section, and they may be corrugated or ribbed to provide rigidity and to improve recuperative heat transfer. Their cross-section is reduced at 50, so as to form a plenum chamber for the outlet gases.

Claims

1. Combustion heating apparatus for fluids of the kind in which the fluid is passed through a heated zone, which zone is heated by a combustion zone to which air and fuel is fed, characterised in that said fluid is passed through said heated zone in multiple substantially parallel streams, in that there is a heated zone for each stream, in that there is an associated combustion zone for each heated zone, and in that the associated heated and combustion zones are arranged one within another with a high degree of radial symmetry.

2> Combustion heating apparatus, characterised in that said associated heated and combustion zones are elongate and.arranged concentrically.

3. Combustion heating apparatus according to Claim 1 or 2, characterised in that each combustion zone is of annular or circular cross-section and contained within the heated zone.

4. Combustion heating apparatus according to Claim 1 or 2, characterised in that each heated zone is of annular or circular cross-section and contained within the combustion zone.

5. Combustion heating apparatus according to Claim 3 or 4, characterised in that the outer, heated or combustion zones are provided by one continuous space.

6. Combustion heating apparatus according to any one of Claims 1 to 5, characterised in that mixing in each combustion zone is produced by jet action of incoming air and/or fuel.

7. Combustion heating apparatus according to any one of Claims 1 to 5, characterised in that heat is produced in a desired distribution in the combustion zones by introduction either of air or fuel, or both, through multiple perfora¬ tions or pores in a combustion wall bounding said zones, or ducts inserted in said zones.

8. Combustion heating apparatus according to Claim 6 or 7, characterised in that the combustion reaction is stabilised close to the perforated boundary wall or duct, by turbulence-promoting projections or bodies.

9. Combustion heating apparatus according to any one of the preceding claims, characterised in that combustion is promoted by catalytically active surfaces disposed within each combustion zone.

10. Combustion heating apparatus according to any one of the preceding claims, characterised in that an endothermic chemical reaction is carried out in each heated zone, which reaction is promoted by catalytically active surfaces disposed within each heated zone.

11. Combustion heating apparatus according to claim 10, characterised in that the endothermic reaction is a reaction between steam and hydrocarbons to form hydrogen and oxides of carbon.