Thread Content
The furnace design concept of Germany’s Krupp Koppers, 30 years ahead of its time – worth emulating. Basic concepts: Coke oven gas (rich gas): refers to a gaseous heating medium with a high calorific value, a low ignition temperature, and a high content of hydrocarbons; it is a gaseous product of the coking industry. Among them, hydrocarbons are prone to cracking into free carbon and hydrogen at high temperatures, and the free carbon tends to deposit in the gas flow channels, causing blockages in those channels ； Poor-quality gas: refers to a gaseous heating medium with a low calorific value, a high ignition temperature, and a high content of inert components. Since the inert components are high, the lean gas needs to be preheated above the ignition temperature in order to burn properly. Typical lean gas includes: blast furnace gas, generator gas, water gas, etc ； Flame temperature: The structure of a gas combustion flame is highly complex, and its temperature cannot be accurately determined. In engineering practice, the term \"flame temperature\" usually refers to the average temperature of the exhaust gases produced after the combustion of combustible gases. A coke oven is a highly complex thermal furnace; its structure, from bottom to top, consists of a regenerator chamber, an incline channel, a combustion/charring chamber, and a roof. Among them, the regenerator is used to recover the sensible heat of high-temperature exhaust gas and preheat the lean gas mixture with air. Based on the type of heating gas, they can be divided into single-reheat coke ovens and double-reheat coke ovens. Single-heating coke oven: A coke oven that uses only one type of gas for heating; it can be further divided into coke ovens that use rich coal gas for heating, and those that use lean coal gas for heating ； A regenerative coke oven is a type of coke oven that can be heated using either rich gas or lean gas. Internationally, coke ovens are basically heated directly using blast furnace gas (lean gas), coke oven gas (rich gas), or a mixed gas (rich gas or lean gas). Given the increasingly strict environmental regulations, various technologies for reducing NOx emissions have been developed. Front-end control methods primarily rely on well-established techniques in this field, such as exhaust gas recycling inside the coke oven, staged heating, and external exhaust gas recirculation. When using coke oven gas (rich gas) for heating: Since the hydrocarbons in coke oven gas (rich gas) tend to crack into free carbon and hydrogen at high temperatures, and the free carbon can accumulate in the gas channels causing blockages, it is not possible to preheat coke oven gas (rich gas) to very high temperatures. However, this gas contains trace amounts of organic compounds such as naphthalene and other impurities that can crystallize into solid particles at low temperatures; therefore, before entering the coke oven, the gas must be heated to above 50°C using an external heat exchanger (gas preheater) – this is done to prevent naphthalene from crystallizing and blocking the gas channels. After that, the gas enters the coke oven through the coke oven gas distribution system and the basement ； When using lean gas (blast furnace gas) for heating: Since blast furnace gas (lean gas) contains a large amount of inert components and has a high ignition temperature, it and air must first enter the regenerator (inside the coke oven) to be preheated to above the ignition temperature (usually above 1200°C), before entering the combustion chamber’s vertical flues where they combine to burn. \"Method for reducing the nitrogen oxide content in exhaust gases when heating gas-rich coke ovens or regenerative coke ovens, and coke oven system for implementing this method\" Application number: 89109066.5 This application relates to a method for reducing the NOx content in exhaust gases when heating gas-rich coke ovens or regenerative coke ovens. The original patent document (CN1043333A, published on June 27, 1990) discloses a method for reducing the NOx content in exhaust gases when heating gas-rich coke ovens or regenerative coke ovens (which is equivalent to disclosing \"an energy-saving and environmentally friendly coking process method with low NOx emission levels through pre-treatment\"); it specifically outlines the following (see paragraph 6 on page 2 of the specification to paragraph 1 on page 6): By combining external and internal exhaust gas circulation according to this invention, it is possible to reduce the flame temperature to levels that cannot be achieved using either of the two known methods alone. Due to the reduced flame temperature, especially when burning with rich gas at the bottom, it is possible to **reduce the generation of NOx in the coke oven without compromising its production capacity. This combination makes it possible to ideally increase the mixing ratio, and compared to external exhaust gas recirculation used alone, it significantly reduces investment costs and production expenses. A design has been developed to convert rich gas into lean gas by mixing in externally returned exhaust gas, followed by regenerative preheating. Using this method, a certain amount of returned exhaust gas can be used to mix with the rich gas (which is equivalent to disclosing \"mixing rich gas with exhaust gas from coke oven flues to form a mixed gas\"), so that the calorific value of the mixture equals that of the lean gas (2500 to 5200 kilojoules per standard cubic meter). Then, this \"diluted rich gas\" or \"artificially lean gas\" is regeneratively preheated just like a \"true\" lean gas (which is equivalent to the process of feeding the mixed gas into a heat exchanger for heat exchange, and then feeding the gas that has undergone this heat exchange into a coke oven for combustion), and according to the present invention, a circulating gas flow is further introduced into the combustion products within the dual-chamber furnace. The heating system of the present invention is used in regenerative coke ovens. Its advantage is that it eliminates the rich gas distribution system and the underground nozzle chamber. The coke oven or regenerative coke oven system used to implement the method of this invention has a combustion chamber composed of dual flues, with one or more stages of combustion within these dual flues. It also features an exchange heat exchanger for recovering waste gas heat, as well as an exchange heat exchanger designed to preheat the burning medium at the bottom. Such a coke oven system is equipped with various devices for circulating external waste gas, such as fans, intake pipes, distribution pipes, and shut-off pipes, as well as measuring and control devices. In addition, the coke oven system includes intermediate connection walls between the dual flues; these walls have rotation points, and at the bottom of the flues there is one or more holes through which waste gas flows from a downward direction to the upward-moving or burning flue (this corresponds to the concept of “reintroducing a portion of the waste gas from the coke oven flues back into the combustion chamber of the coke oven during operation, in order to control the oxygen level in the combustion chamber”). The patent document also reveals (see paragraph 6 on page 2 of the specification) that, through the combination of external and internal exhaust gas recirculation according to the present invention, it is possible to reduce the flame temperature to a level that cannot be achieved solely using one of the two known methods. Due to the reduced flame temperature, especially when burning with rich gas at the bottom, **the generation of NOx in the coke oven can be reduced. Analysis of patent documents: The statements in the patent documents that \"the rich gas is converted into lean gas by mixing it with externally returned exhaust gas\" and that \"this diluted rich gas or artificially created lean gas is preheated regeneratively as if it were a ‘true’ lean gas\" indicate that the technology described in the patents alters the properties of the heating gas. Additionally, the fact that the patent documents mention that \"it is possible to eliminate the rich gas distribution system and the underground nozzle chamber\" also shows a change in the properties of the gas, and it further indicates that regenerative preheating corresponds to the regenerator in coke ovens ； Furthermore, adjusting the calorific value of gas by mixing waste gas is common knowledge in related industrial fields (see: \"Code for Design of Urban Gas Systems\" GB50028-2006, 8.4 Gasification Stations and Mixing Stations) ； The patent document should state that \"the mixed gas (lean gas) is fed into a heat exchanger (regenerator) for heat exchange, and the mixed gas (lean gas) after this heat exchange is then fed into a coke oven for combustion.\" Additionally, it is common engineering practice for the mixed gas (lean gas) to first enter a regenerator before being fed into various industrial furnaces, in order to be preheated to a temperature above the ignition point (see: \"Modern Coke Production Technology Manual\", Metallurgical Industry Press, Chapter 3: Furnaces and Equipment, Section 6: Thermal Characteristics of Coke Ovens) ； The patent document states that \"it is equivalent to disclosing a method in which, during the operation of the coke oven, a portion of the waste gas from the coke oven flue is redirected back into the combustion chamber of the coke oven in order to control the oxygen level within that combustion chamber.\" This description refers to the recirculation of waste gas inside the coke oven; moreover, such recirculation is a common practice in international coke oven design (see: \"Modern Coke Production Technology Manual\", Metallurgical Industry Press, Chapter 3: Furnaces and Equipment, Section 6: Thermal Properties of Coke Ovens) ； The research and analysis conclusions in the patent document are as follows: The core technical aspect disclosed in the patent document is the conversion of rich gas into mixed gas (lean gas) by mixing it with externally returned exhaust gas. The adjusted mixed gas (lean gas) can be preheated and distributed using the existing lean gas heating system of the regenerative coke oven, while the equipment for the rich gas system can be eliminated (see: patent document “The rich gas distribution system and underground nozzle chamber can be removed”). What is disclosed in the patent documents is the mixing of exhaust gas returned from outside into the rich gas (coke oven gas), thereby reducing the richness of the rich gas (coke oven gas) and causing a fundamental change in its properties. The physical and chemical properties of the so-called \"mixed gas\" resulting from this change in the properties of the rich gas (coke oven gas) are not very different from those of true lean gas. As stated in the patent documents, the \"diluted rich gas\" or \"artificial lean gas\" is preheated in a regenerative manner, just like \"true\" lean gas (which corresponds to the description of \"feeding the mixed gas into a heat exchanger for heat exchange, and then feeding the heated mixed gas into a coke oven for combustion\"). Patent document technical route: Rich gas (coking oven gas) —> Blended flue gas —> Mixed gas (lean gas & reduced calorific value) —> Suitable for lean gas heating systems.