Combustion – Process of combustion or burner operation – Controlling or proportioning feed
Reexamination Certificate
2003-05-16
2004-10-19
Cocks, Josiah (Department: 3749)
Combustion
Process of combustion or burner operation
Controlling or proportioning feed
C431S005000, C431S006000
Reexamination Certificate
active
06805550
ABSTRACT:
The present invention is directed to a method of operating a furnace, particularly a multi-burner furnace, which utilises a hydrogen-rich gas as fuel.
BACKGROUND OF THE INVENTION
When hydrogen is mixed with air in a wide range of proportions, a violent explosion can result upon ignition thereof. Moreover hydrogen has the maximum laminar burning velocity of any gas. Thus, whereas the flame speed of an acetylene flame is approximately 3.5 times that of most hydrocarbon fuels, the flame speed of a hydrogen flame is approximately 6 times higher than that of most hydrocarbons.
In certain chemical operations, particularly those involving endothermic reactions, such as steam reforming of natural gas or another hydrocarbon feedstock, it is expedient to pass the reaction mixture, for example, a mixture of the hydrocarbon feedstock and steam, through the reaction tubes of a multi-tubular reactor which are positioned in a suitable furnace and which are heated by means of a multiplicity of burners. The burners in steam reformer furnaces and other furnaces used in chemical plant operations can be supplied with any appropriate fuel, such as gas oil, natural gas, or the like. If different fuels are to be burnt, then more than one type of burner can be installed in the furnace. Often it is convenient and economical to utilise an available source of combustible waste gas as fuel for the furnace.
The reaction tubes in a steam reformer furnace typically have a nominal diameter of 5 inches (12.70 cm). They are usually mounted with their axes arranged substantially vertically and widely spaced one from another in order to allow heating by radiation and convective heating to occur. The burners can be arranged near the bottom of the furnace so that the flame extends substantially vertically upwards, while the reactant mixture can be simultaneously fed down the catalyst-filled steam reformer tubes. In another more common arrangement the furnace is top fired. In this case the burners are mounted near the top of the furnace so that the flame projects downwards into the furnace along the length of the catalyst filled, vertically mounted reformer tubes.
Other types of chemical plant which have furnaces include steam crackers for ethylene and catalytic reformers. The furnaces in such forms of plant are generally top fired or side fired. Fired heaters for heating, in general, such as refinery crude heaters and vacuum unit heaters, also have multiple burners. They may burn any of a wide variety of liquid and gaseous fuels, often using more than one type of burner for different fuels.
In all such furnaces the burners are normally quite widely spaced one from another and it is conventional practice to light the burners one by one with individual pilot flames or with an igniter, which is often a retractable igniter, because the burners are normally spaced too far apart to allow for reliable flame propagation. To prevent burn out, the burners and pilot-light burners can be retracted into the refractory lining. Block valves are usually provided in order to allow ignition to be carried out in this way and to permit maintenance of the burners. This applies also to burners which consist of a burner array, in which multiple burning points are supplied by a single supply tube or pipe off a common header. In each case it is common practice to light the burners individually.
In some situations a hydrogen-rich gas is available as a waste gas stream. If, however, a hydrogen-rich gas stream is used as fuel for a furnace having a large number of burners, there is potential for a large volume of appropriately mixed hydrogen and air to form above the burners, which will give rise to a grave risk of an explosion upon ignition of the hydrogen-rich gas stream. This explosion is capable of damaging the ceramic lining to the furnace chamber or the reaction tubes or other components in the furnace and causing risk to the operators of the plant.
It is known to use a hydrogen-containing stream as a fuel for a furnace. For example, it is known to utilise a methanol plant purge gas as fuel for a conventional reformer furnace. However, the purge gas stream at start up of a methanol plant is hydrogen-lean and only when the plant is fully operational does a hydrogen-rich purge gas become available, by which time the burners in the furnace have already been lit. Accordingly any changeover from hydrogen-lean gas as fuel to hydrogen-rich purge gas as fuel occurs only after the burners have already been lit.
A so-called compact reformer is described in International Patent Publication No. 94/29013. This has a closely packed array of reaction tubes, which are typically considerably smaller in diameter than the reaction tubes in conventional steam reformers. Thus the reaction tubes in a compact reformer typically have, for example, a nominal diameter of 1½ inches (3.81 cm) in comparison with a nominal diameter of 5 inches (12.70 cm) which is typical for the reaction tubes of a conventional reformer. Moreover the reaction tubes are spaced much closer to one another in a compact reformer than in a conventional steam reformer with the burners correspondingly being positioned closer to one another within the reaction tube matrix.
Since the burners are so much closer to one another in a compact reformer than in a conventional reformer furnace, there is generally insufficient room to accommodate individual control valves for each burner fuel jet. Hence the burner fuel jets must in this case be supplied from a common manifold. Moreover, since space is limited, it is hardly practical to provide multiple igniters or pilot flames and there would be an increased risk of burn out of the pilot-light fuel jets compared with conventional reformer furnaces. Autoignition would be another possibility but then it is not clear how this can be safely achieved. A further possibility is to effect ignition at an outer burner of the array and then to rely on flame propagation to ignite the other burners. Although the burners in a compact reformer are close enough to permit flame propagation from one to another if conditions are favourable, it is important that the correct range of velocities, fuel compositions and air:fuel ratios are used if the risks of explosions and of non-reliable ignition of all burners are to be avoided, particularly when the fuel concerned is hydrogen or a hydrogen-rich gas.
SUMMARY OF THE INVENTION
The present invention seeks to provide a method of igniting the burners of a furnace containing an array of closely spaced burners, such as a compact reformer, in a safe and reliable manner. In addition, it seeks to provide a method of operating a furnace with a multiplicity of burners which are arranged in an array but which are not capable of individual control, in particular which are not provided with individual control valves. It further seeks to provide a method of initiating ignition in a furnace with a multiplicity of burners without using individual igniting devices for each burner. The invention also seeks to provide a method permitting safe operation of a furnace having multiple burners utilising a hydrogen-rich gas as fuel, particularly during start-up of the furnace. It further seeks to provide a method of operating a multi-burner furnace utilising a hydrogen-rich gas as fuel in which the risk of a potentially hazardous explosion is substantially obviated. An additional objective of the invention is to provide a method of utilising safely the calorific value of a hydrogen-rich waste gas stream.
According to the present invention there is provided a method of operating a furnace utilising a hydrogen-rich gas as furnace fuel, the furnace having a multiplicity of burners for burning fuel supplied thereto, which method comprises
(a) providing ignition means for lighting a flame at at least one predetermined burner selected from the multiplicity of burners,
(b) supplying to each of the multiplicity of burners an oxygen-containing gas and a combustible gas comprising a hydrocarbon gas in amounts capable of forming an ignitable mixture
Hilton Michael
Kippax John Wilson
Cocks Josiah
Davy Process Technology Limited
Senniger Powers
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