1. Overview
Overview of CDQ (Coke Dry Quenching) system
The coke oven consists of plate-like carbonization chambers alternately arranged in a sandwich form to achieve higher thermal efficiency in carbonization.
Raw material placed in the carbonization chambers is heated to a temperature between approximately 1100 and 1350oC through the combustion of blast furnace gas in the combustion chambers, which are located on both sides of the carbonization chambers beyond the refractory brick. The heated raw material is not exposed to air for approximately 12 to 14 hours to allow carbonization to proceed. In this process, the fixed carbon contained in the raw material fuses and solidifies to become red- hot coke in the lower section of the carbonization chambers. The volatile component in the raw material vaporizes and decomposes, becoming gas. After escaping from the coke surface, the gas is collected through a pipe located in the upper section of the carbonization chambers. When carbonization is complete, the red- hot coke (approximately 1,050oC) is discharged from the coke oven and then carried to the top of the chambers. The coke is then fed to the chambers and while it descends through the chamber, is cooled with circulating gas blown from the bottom of the chamber.
After it has cooled to approximately 200oC, the coke is ejected from the bottom, while the circulating gas that has been heated to 800oC or higher generates high-temperature and high- pressure steam in the boiler. The gas is purified by a dust collector and then sent back to the chambers for recycling. The generated steam is used as process steam or for power generation.
2. Features of CDQ system
Conventional system
The red-hot coke extruded from the coke oven is cooled by spraying it with water. The water used for cooling is vaporized and released into the atmosphere. An issue with this conventional system is the energy loss when the thermal energy of the red-hot coke is converted into heat that is vaporized and released unused. Another drawback is that the conventional system also produces airborne coke dust.
CDQ system
In the CDQ system, the red-hot coke is cooled by gas circulating in an enclosed system, thereby preventing the release of airborne coke dust. The thermal energy of the red-hot coke, which is lost in the conventional system, is collected and reused as steam in the CDQ system. This technology uses less fossil fuel and results in lower CO2 emissions, thereby contributing to the prevention of global warming.
3. Dissemination of coke dry quenching (CDQ) systems
CDQ systems have been installed in many steel works and coke ovens in Japan as energy-efficient, environmentally-friendly technology.
Through NEDO model projects, the effectiveness of CDQ has also been recognized in China. The Chinese government specified CDQ technology as one of the targets in the Tenth 5-year plan in 2000. Steel works in Hanfang, Beijing, Chengde and Hangzhou have already introduced Japanese CDQ systems.
4. Future prospects
The Asian region is expected to continue increasing its production of crude steel. Efforts to introduce CDQ are being made in China and India. CDQ is an established technology that can help Japan to achieve its Kyoto Protocol target via the use of CDM projects.
Source: New Energy and Industrial Technology Development Organization (NEDO), Japan Coal Energy Center (JCOAL)
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