The iron and steel sector is the second-largest industrial user of energy, consuming 616 Mtoe in 2007 and is also the largest industrial source of CO2 emissions. The five most important producers – China, Japan, the United States, the European Union and Russia – account for over 70% of total world steel production.
The iron and steel sector is the second-largest industrial user of energy, consuming 616 Mtoe in 2007 and is also the largest industrial source of CO2 emissions. The five most important producers – China, Japan, the United States, the European Union and Russia – account for over 70% of total world steel production. In general steel works, the coke ovens account for 7-8% of energy consumption. Approximately 45% of this amount is the sensible heat of the high temperature coke (in a red-hot condition) discharged from the coke oven.
Coke is a solid carbon fuel and carbon source used to melt and reduce iron ore. Coke and coke by-products, including coke oven gas, are produced by the pyrolysis (heating in the absence of air) of suitable grades of coal. The process also includes the processing of coke oven gas to remove tar, ammonia (usually recovered as ammonium sulfate), phenol, naphthalene, light oil, and sulfur before the gas is used as fuel for heating the ovens.
Coke dry quenching appears as a highly reliable system to reduce air pollution, while it can also reduce substantially energy use, especially when it is associated with a coal preheating preheating. In addition, dry quenched coke is harder and stronger, and its moisture content is much lower than that of wet quenched coke (Bisio and Rubatto 2000).
In comparison to all the available technologies in the iron and steel sector, coke dry quenching process can generate substantial savings (see below)
The coke dry quenching equipment broadly consists of a coke cooling tower (pre-chamber and cooling chamber) and a waste heat recovery boiler. The entire process has as follows: Red-hot coke (approx. 1,200°C) is charged into the coke cooling tower, and inert gas is blown into the tower from the bottom. Heat exchange is performed with the circulating inert gas. After the gas is heated to high temperature (approx. 800°C), it circulates through the heating tubes of the waste heat boiler, converting the water in the boiler into steam. The temperature of the coke at the cooling tower outlet is reduced to approximately 200°C.
During this process, the heated coke passes the middle part of a chamber wherein a flow of a gas-steam mixture heated to a temperature of up to 700° C. is introduced in countercurrent relation to the flow of coke. Resulting from the process of conversion of hydrocarbons with water vapor is gas containing hydrogen and carbon monoxide. The blast-furnace coke with a temperature of not lower than 700° C. is passed from a middle part of the chamber to a lower part thereof wherein it is cooled to a temperature of 200° to 250° C. with a flow of circulating inert gas.
The modern operations of dry coke quenching are fully automatic with advanced process control and optimization systems. The average lifetime of the refractory linings in the quenching chambers could be increased from an average of seven to ten years. In average, the utilisation rates for all the existing plants are reported to be between 80 and 90% but still there are exampels of much higher rate, such as Rautaruukki’s coke dry quenching plant in Finland, which had an average degree of utilization exceeding 99.6 % during the years 2000 to 2005 (Ritamaki and Kellin 2009).
Some efforts for the promotion of the transfer and spread of coke dry quenching technology, thereby contributing to more efficient energy use and the prevention of air pollutioon by the Chinese steel industry, are carried out by Nippon Steel Corporation (NSC) to steel industries, in the framework of a Green Aid Plan proposed by the Ministry of International Trade and Industry (MITI) in Japan for energy saving and environmental protection in China. The main profit of NSC originates through technology upgrade. In this case, China, which is facing increasingly competitive international and domestic markets, can reduce the cost of steel produced by saving energy while reducing GHG emissions. There were certain barriers associated with technology transfer, such as the import of equipment components. For producing countries, it can be beneficial produce locally such components to cut down overall costs. However, these parts purchased domestically could cause some reliability problems.
Coke dry quenching can be applied in principle at new and existing iron and steel plants. Coke dry quenching systems have been installed in many steel works and coke ovens in Japan as an energy efficient and environmentally friendly technology. Through the New Energy and Industrial Technology Development Organization (NEDO) model projects in Japan, the effectiveness of Coke dry quenching has also been recognized in China. The Chinese government specified coke dry quenching technology as one of the targets in the Tenth 5-year plan in 2000. Coke dry quenching technology was also a key project during the Eleventh Five Year Plan period (2006 to2010), where during this period the iron and steel integrated enterprise should practise coke dry quenching. Steel works in Hanfang, Beijing, Chengde and Hangzhou have already introduced Japanese Coke dry quenching systems. The Asian region is expected to continue increasing its production of crude steel. Efforts to introduce Coke dry quenching are being made in China and India. Coke dry quenching is an established technology that can help Japan to achieve its Kyoto Protocol target via the use of CDM projects.
NEDO has promoted several projects with the Government of India and Tata Steel Ltd. for the Model Project for Increasing the Efficient Use of Energy Using a Coke Dry Quenching System (see http://www.tata.com/company/releases/inside.aspx?artid=ceh7+BieW4A=). Siemens VAI engineered and supplied coke dry-quenching systems at the cokemaking facilities of Raahe, Ruukki Production, Rautaruukki Oyj, Finland (partial supply, start-ups: 1987 and 1992); Koksownia Przyjazn, Poland (start-ups: 1997–1999); ArcelorMittal, Cracow, Poland (start-up: 2000); and Konark Met Coke Ltd., India (start-up: 2006). The latest coke dry quenching projects (partial supply) are being implemented at SAIL’s (Steel Authority of India Limited) steelmaking facilities at IISCO Steel Plant, Rourkela Steel Plant and at Bhilai Steel Plant, and are scheduled for start-up beginning 2011. In 2006 a total of 14 newly built CDQ systems were put into operation. By the end of 2006 China owned 43 coke dry quenching systems in the coking industry, with a quenchingcapacity of 45 million tonnes per year. Up to recently, around 40% of coking ovens in China’s steel plants use a coke dry quenching system (with a production capacity of 33 mil tonnes). ACRE, in order to meet the needs of coke dry quenching technology in large-scale developments, has successfully integrated a large-scale coke dry quenching system with many new technologies and designed the coke dry quenching project of the Maanshan Iron & Steel Complex.
During the coke making process the energy that would otherwise be lost during the wet quenching of hot coke can be used to produce steam in a dry-quenching process. The steam is then available for the generation of electrical energy in addition to various heating purposes, providing electrical power of 150GWh/yr for a plant with an annual capacity of 1Mt (Bettinger et al. 2009).
With the coke dry quenching process, dust emissions are reduced as there are no dust-laden steam clouds released to the atmosphere during wet quenching (Bettinger et al. 2009). The same occurs with emissions of dust, carbon monoxide and hydrogen sulphide. Furthermore, waste heat energy is recycled to generate electricity, saving money and reducing carbon emissions, water is not wasted from the process and not contaminated with toxic pollutants.
The coke dry quenching process does not generate substantial CO2, in comparison to an oil-burning boiler. Reduction of CO2 by the coke quenching process is equivalent to 18T/H CO2 that an oil-burning boiler produces when it generates 18MW electric power (equivalent to that by 100T/H CDQ) (see http://www.jase-w.eccj.or.jp/technologies/pdf/iron_steel/S-7.pdf). Given a baseline from a steel plant of two NEDO model projects in Jinan and Jiangyin (China), the potential CO2 reduction is 2,359 kt CO2/year, with a specific energy consumption of 23-24 GJ/t of crude steel and an improved amount of specific energy consumption of around 2 GJ/t (Hirano 2008). Based on the CleanTech Innovations, a comparison between a coke dry quenching system and a wet quenching system (see figure below), by installing 2 coke dry quenching systems, a steel mill can save over $9 million in electricity, almost 4 million tons of water and prevent emission of over 100,000 mt CO 2 annually.
For calculation of these GHG emission reductions, it is recommended to apply the approved methodology for consolidated methodology for waste gas and/or heat for power generation project (large scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html.
The investment cost for a steel plant with a capacity of 2 Mt/a is about 5 Mio Euro for a wet quenching facility and about 100 Mio Euro for dry quenching. Still, coke oven plants with a coke dry quenching process also have a wet quenching facility because of low availability of coke dry quenching units (Schoenberger 2000). Some other estimations for the total costs of a coke dry quenching process (see http://126.96.36.199/eetkb/technologies/details.aspx?id=42# for a model project is currently being implemented at Shougang Iron and Steel Corporation (Beijing)) range 25-27 mil E for the equipment and 4-5 mil E for the construction cost. An average facility can generate 8-9 mil E/year (with a unit cost of power 0.16 E/kWh), which practically pays back the investment in 3-4 years.
Bettinger, D., Kriechmair, J., Gould, L. (2009). A pragmatic approach to economic and environmental sustainability in the steel industry. Siemens VAI Metals Technologies GmbH. Millenium Steel.
Bisio, G., Rubatto, G. (2000). Energy saving and some environment improvements incoke-oven plants. Energy 25, 247-265.
Hirano, J. (2008). NEDO's Activities in the Field of Energy Conservation and Environment Technology for the Steel Industry. The Asia-Pacific Partnership on Clean Development and Climate 6th Steel Task Force Meeting & 5th Workshop
IPPC (2001). Best available techniques reference document on the production of Iron and Steel. Integrated Pollution and Prevention Control.
Ritamaki, O., Kellin, A. (2009). Still want to blow off steam?. Metals and Mining (1), 24-25
Schoenberger, H. (2000). BREF on the Production of Iron and Steel - conclusion on BAT. European Conference on “The Sevilla Process:A Driver for Environmental Performance in Industry”, Stuttgart.