Heating – Processes of heating or heater operation – Of heating a fluid
Reexamination Certificate
2000-04-14
2001-06-26
Lu, Jiping (Department: 3749)
Heating
Processes of heating or heater operation
Of heating a fluid
C431S011000, C431S215000
Reexamination Certificate
active
06250916
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to gaseous fuel burners. More specifically the invention relates to energy efficient burning of fuel using such burners.
2. Related Art
Oxy-fuel burners and technologies are being used more and more in high temperature processes such as, glass manufacturing, incineration of wastes, steel reheating, aluminum smelting, and iron smelting, for the benefits they provide:
high heat transfer rates;
fuel consumption reductions (energy savings);
reduced volume of flue gas;
reduction of pollutants emission, such as oxides of nitrogen (NOx), carbon monoxide (CO), and particulates.
Oxygen used in these high temperature processes can be technically pure oxygen (99.99%) or various grades of industrial oxygen, with purities down to 80%.
Despite the reduction of the flue gas volume that the substitution of combustion with air by combustion with pure oxygen yields, a significant amount of energy is lost in the flue gas, especially for high temperature processes. For example, in an oxy-fuel fired glass furnace where all the fuel is combusted with pure oxygen, and for which the temperature of the flue gas at the furnace exhaust is of the order of 1350° C., typically 30% to 40% of the energy released by the combustion of the fuel is lost in the flue gas. It would be advantageous to recover some of the energy available from the flue gas in order to improve the economics of operating an oxy-fuel fired furnace.
A number of techniques to recover energy from flue gases are available. Those techniques have been proven or described for air-fuel fired furnaces. Similar techniques have yet to be demonstrated for oxy-fuel furnaces, because of difficulties that will become apparent from the following discussion.
One technique consists in using the energy available in the flue gas to preheat and dry out the raw materials before loading them into the furnace. In the case of glass melting, the raw materials consist of recycled glass, commonly referred to as cullet, and other minerals and chemicals in a pulverized form referred to as batch materials that have a relatively high water content. The energy exchange between the flue gas and the raw materials is carried out in a batch/cullet preheater. Such devices are commonly available, for example from Zippe Inc. of Wertheim, Germany. Experience shows that this technology is difficult to operate when the batch represents more than 50% of the raw materials because of a tendency to plug. This limits the applicability of the technique to a limited number of glass melting operations that use a large fraction of cullet. Another drawback of this technique is that the inlet temperature of the flue gas in the materials preheater must be generally kept lower than 600° C. In the case of an oxy-fuel fired furnace where the flue gas is produced at a temperature higher than 1000° C., cooling of the flue gas prior to the materials preheater would be required.
Energy efficiency of air-fuel furnaces is greatly improved if the energy available from the flue gas is used to preheat the combustion air. Recuperators, where some of the heat from the flue gas is transferred to the combustion air in a heat exchanger, and regenerators, where some of the heat from the flue gas is accumulated in a ceramic or refractory material for later preheating of the combustion air, are the most common techniques encountered in the industry for this purpose. Such techniques are difficult to apply in the case of oxy-fuel fired furnaces because of the hazards of handling the extremely reactive hot oxygen.
Thermochemical energy recovery (also known as fuel reforming) is another technique that consists in increasing the heat content of a fuel by reacting it with steam or carbon dioxide or a mixture of the two in a reactor (reformer), and generating a combustible mixture that contains hydrogen (H
2
) and carbon monoxide (CO) and has a higher heat content than the initial fuel. The reforming reaction occurs at high temperature (typically 900° C.), is endothermic, and takes advantage of the high temperature of the flue gases to generate the high temperature gases required by the process, and to provide the energy for the reforming reaction. Practically, the fuel consumption in a glass plant is not high enough to provide an economical justification to the high capital cost of installing a fuel reforming system. The complexity of the reformer, and safety constraints linked to handling hot H
2
and CO, are additional drawbacks of this technology. In the case of oxy-fuel furnaces, the energy available from the flue gas is typically not sufficient for reforming all the fuel, and an additional energy source is generally required in addition to the flue gas, which adds to the complexity of the apparatus.
Co-generation of power and heat (i.e. the simultaneous generation of electricity and steam using the hot flue gases) is another technique that is available to recover the energy from flue gas, and use it for other purposes than recycling into the furnace. The disadvantage of this approach is that the capital costs tend to be very high. This option is, however, viable for very high heat output furnaces (those which produce greater than 30 megawatts of power).
With stricter environmental regulations, a number of industries are required to install pollution abatement systems. Those devices typically cannot handle the very high temperatures found at the exhaust of an oxy-fuel furnace used for a high temperature process. For instance, at the outlet of an oxy-fuel fired glass tank furnace, the temperature typically ranges from about 1300° C. to about 1450° C. Before the flue gases can be treated by the pollution abatement system (which can be an electrostatic precipitator or a baghouse in the case of cleaning the flue gas from particulate matter) it is highly preferable to cool down the gases. This is generally performed by diluting the gases with ambient air, or spraying of water that vaporizes upon contact with the hot gases, to yield a cooling of the gases, or by a combination of these techniques. Dilution with air increases the amount of gas to be treated by the pollution abatement system, which increases its cost. Water injection elevates the dew point of the gases and forces the pollution abatement device to operate at high temperature. This is especially true for oxy-fuel fired furnaces where the water content of the flue gases can be as high as 60% by volume.
What is needed then is a method and apparatus (or system) which efficiently and at relatively low capital cost recovers at least a portion of the available heat which otherwise is wasted to the atmosphere, particularly in high temperature processes where oxy-fuel burners are employed, and simultaneously cools down the flue gases.
SUMMARY OF THE INVENTION
In accordance with the invention, methods and apparatus are presented which combine one or more oxy-fuel burners operating with either hot oxidant, hot fuel, or both, with use of a primary heat exchanger disposed in a flue gas channel. As used herein the term “oxidant” is used to mean either pure oxygen (as defined in the industry) or oxygen enriched air. “Process gas” as used herein refers to gases and particles including all gases which are not combustion products. The primary heat exchanger employs an intermediate safe fluid (air or nitrogen for example) to transfer at least a portion of the heat from the hot flue gases to either the oxidant, the fuel, or both, used in the burners. The function of the primary heat exchanger is to transfer at least a portion of the heat from the hot flue gases to the intermediate safe fluid (hereinafter referred to simply as the intermediate fluid). Removing the energy of the flue gases in a heat exchanger is a convenient means of cooling of the flue gases without increasing the quantity of flue gases or increasing the water content of these gases. The dimension of pollution abatement device that may be installed before the gases are exhausted to the atmosphere can be smaller, an
Duchateau Eric L.
Illy Fabien S.
Philippe Louis C.
American Air Liquide Inc.
Lu Jiping
Wendt Jeffrey L.
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