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. A standard technique that is applied worldwide is the coke wet quenching of coke, where quenching vapors are bunkered before delivery to the atmosphere and subsequently or immediately a condensate is drawn off and cooled further.
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 a 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.
Wet quenching of coke with low vapor emissions is provided when the quenching vapors are bunkered before delivery to the atmosphere and subsequently or immediately a condensate is drawn off and cooled further. The bunkered vapors are intermittently mixed with air to prevent the development of explosive carbon monoxide concentrations. The condensate obtained is again distributed, cooled and subsequently recycled for further condensation.
This form of wet quenching prevents the coke from burning up in the air. The coke is then screened to a uniform size, which also results in some small coke fines (or breeze) that are recovered for use as a raw material input to blast furnaces. Roughly half of the water leaves the quenching tower as steam, while the remaining water that does not evaporate is drained to remove salable by-products before being returned to the quenching tank for recirculation (Mussatti 1998).
In comparison to all the available technologies in the iron and steel sector, coke wet quenching process can generate substantial savings (see below)
The wet quenching for coke is considered as a Best Available Technology for coke oven plants (IPPC 2001). The apparatus for performing this process includes a quenching tower connected with a condensor separator and having a buffer space for the quenching vapors. The buffer space has a closable fresh air valve and the condensor separator has a condensor return with an intermediately connected water purifier and the condensor separator and buffer space are connected with each other. A twin cooling system can be connected with a by-pass for feed back which receives fluid from the water purifier.
There are also improvements on the wet quenching process, such as the quenching tower from several companies worldwide (such as TechnicaM). The wet coke quenching assumes spraying over burning coke of water in the special facility of the quenching tower. In other words, hot coke is delivered to the quenching tower where cooling water quenches and cools down the coke to the lower temperature. It is then discharged onto a wharf and then to the screening tower, where the process depends on the final product (see http://www.ssit.com.cn/detail.aspx?articleid=112). A typical process control automation system enhances the pump work control, the auxiliary equipment work control and the quenching cycle control.
Currently, the wet quenching method for coke is the most popular process installed in coke plants worldwide and it used to be the primary method in plants in USA, in contrast to dry quenching coke plants, which are mostly installed in Japan and Russia. Several other countries are implementing such technologies, as for instance in Czech Republic Trinecke Zelezarny a coke plant uses two coke-oven batteries of the annual capacity of 700 thousand tons of coke are operated using a rammed process with quick rammers. Bituminous coal for the production of blast furnace coke is transported by railway from the nearby Ostrava-Karviná coalfield. The coke-oven batteries are equipped with coke wet quenching facility.
Several coke oven plants have opted for the wet quenching method. In Germany for instance, the new Schwelgern Coke Plant in Duisburg uses an advanced wet quenching method, as the energy demands at the steelworks site were already covered from the recovered energy and this system has been used succesfully for a number of years by Krupp Mannesmann Steelworks. It has been comparable to dry quenching methods and is highly reliable and stabilises the coke during the quenching operation.
The major coke producing countries, such as India, have set clear regulations for the coke oven plants (Ministry of Environment and Forests 2006). In this case, new and existing coke plants must follow low-emission procedures, for instance a Modified Wet Quenching System with appropriate environmental controls (e.g. baffles, filtering media, collection and treatment of residual water from quench tower and recycling; Treated effluent conforming to the effluent discharge standards can be used for quenching. Use of untreated process water as quenching water shall not be permissible). Similarly, in Canada, the Environmental Code of Practice for Integrated Steel Mills sets Documented procedures for the control of atmospheric emissions and wastewater discharges from coke quenching.
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.
For coke oven plants, emissions to air are most significant. With conventional wet quenching, about 500 g of dust are emitted into the atmosphere per ton of coke—frequently much more. However, many of these are fugitive emissions from various sources such as leakages from lids, oven doors and leveller doors, ascension pipes and emissions from certain operations like coal charging, coke pushing and coke quenching.
Coke production facilities generate process solid wastes other than coke breeze (which averages1 kg/t of product). Most of the solid wastes contain hazardous components such as benzene and PAHs. Wet quenching for coke can minimize emissions with less than 50 g particulate matter/t coke (determined according VDI method). Furthermore, the use of process-water with significant organic load (like raw coke oven wastewater, wastewater with high content of hydrocarbons etc.) as quenching water is avoided (IPPC 2001).
Based on theCleanTech Innovations, a comparison between a coke dry quenching system and a wet quenching system (see figure below), by installing 2 coke wetquenching 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.
Conventional wet quenching is the most common quenching technique used in the coke industry and is accomplished by directly cooling a hot car of coke with a deluge of water. 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 construction and operation cost of a coke wet quenching plant vary significantly and depend on the current infrastructure, as they are mostly coupled to dry quenching plants (no figures available).
IPPC (2001). Best available techniques reference document on the production of Iron and Steel. Integrated Pollution and Prevention Control.
Mussatti, D. (1998). Coke ovens Industry Profile, Draft report. US Environmental Protection Agency. RTI Project number 7018-061.
Ministry of Environment and Forests (2006). G.S.R.46(E), [3/02/2006] - Environment (Protection) Amendmendment, available at: http://www.nlsenlaw.org/environmental-protection/law-policy/g-s-r-46-e-3-02-2006-environment-protection-amendmendment-2006/