Introduction

Power Efficiency Guide

Ultimate Guide to Power Efficiency

Get Instant Access

METALLURGICAL COKE PRODUCTION

Metallurgical coke is primarily used in the blast furnace to make iron. Coke is also used in other metallurgical processes, such as the manufacture of cast iron, ferroalloys, lead, and zinc, and in kilns to make lime and magnesium. Metallurgical coke is the solid product obtained from the carbonisation of coal, principally coking coal, at high temperature. It is low in moisture content and volatile matter. Coking coal refers to bituminous coal with a quality that allows the production of a coke suitable to support a blast furnace charge. Its gross calorific value is greater than 23 865 kJ/kg (5 700 kcal/kg) on an ash-free but moist basis. Coke oven gas is a by-product of the manufacture of metallurgical coke for the production of iron and steel. Figure 4.2 illustrates the coke production process and associated sources of CH4 and CO2 emissions.

Note that coke oven gas may be burned to heat coke ovens or transferred onsite in an integrated iron and steel plant and used in sinter production or iron production processes. Coke oven gas may also be transferred off site (e.g., into the natural gas distribution system) and used as an energy source. The combustion of coke in blast furnaces during the iron and steel-making process produces blast furnace gas, which may then be recovered and transferred from the iron and steel mill to the onsite coke plant and burned to heat coke ovens or used in sinter production. The combustion of blast furnace gas and coke oven gas is the main sources of CO2 and CH4 emissions in coke production.

SINTER PRODUCTION

Iron ore and other iron-containing materials may be agglomerated in sinter plants at integrated iron and steel plants prior to introduction into the blast furnace. Feedstock to sinter plants may include fine iron ores, additives (e.g., lime, olivine), and iron-bearing recycled materials from downstream iron and steelmaking processes (e.g., dust from blast-furnace gas cleaning). Coke breeze (small-grade coke oven coke with particle sizes of < 5 mm) is the most commonly used process material in sinter plants. The coke breeze may be produced from the onsite coke ovens in integrated iron and steel plants, or may be purchased from offsite coke producers. Blast furnace gas or coke oven gas produced onsite during integrated iron and steel production may be used in sinter plants. Operation of sinter plants produces carbon dioxide emissions from oxidation of the coke breeze and other inputs. Off gas from sinter production also contains methane and other hydrocarbons. Figure 4.3 illustrates the sinter production process.

PELLET PRODUCTION

Pellets are formed from iron-containing raw materials (i.e., fine ore and additives) into 9-16 mm spheres in a very high temperature process. The process includes grinding, drying, balling, and thermal treatment of the raw materials. Pelletisation plants are principally located at iron mines or at shipping ports, but can also be located onsite as part of an integrated iron and steel facility. Natural gas or coal may be used as fuel for pelletisation plants; for pelletisation plants located onsite at an integrated iron and steel facility, coke oven gas may be used as a fuel. Energy consumption for the process and the associated CO2 emissions will depend in part on the quality of the iron ore and other raw materials used in the process. The CO2 emissions will also depend upon the carbon contents and heating values of fuels used in the process.

IRONMAKING AND THE ROLE OF COKE

Most CO2 emitted by the iron and steel industry is associated with the production of iron, more specifically the use of carbon to convert iron ore to iron. Figure 4.4 describes the iron-making process and associated sources of emissions. Carbon is supplied to the blast furnace mainly in the form of coke produced from metallurgical grade coking coal (but can also be in the form charcoal made from wood or other forms of carbon.). Carbon serves a dual purpose in the iron making process, primarily as a reducing agent to convert iron oxides to iron, but also as an energy source to provide heat when carbon and oxygen react exothermically. Blast furnace gas is produced during the combustion of coke in blast furnaces. It is typically recovered and used as a fuel partly within the plant and partly in other steel industry processes, or in power stations equipped to burn it. Blast furnace gas may also be recovered and transferred from the iron and steel mill to the onsite coke plant and burned for energy within the coke ovens. Blast furnace gas may also be transferred offsite and used as an energy source both within the furnace and when blast furnace gas is combusted to heat blast air. Oxygen steel furnace gas is obtained as a by-product of the production of steel in a basic oxygen furnace (BOF) and is recovered on leaving the furnace. All carbon used in blast furnaces should be considered process-related IPPU emissions.

Additionally, iron can be produced through a direct reduction process. Direct reduction involves the reduction of iron ore to metallic iron in the solid state at process temperatures less than 1 000°C. A solid product referred to as direct reduced iron (DRI) is produced by the direct reduction process. DRI has a carbon content of < 2 percent. DRI is normally used as a replacement for scrap metal in the electric arc furnace steelmaking route, but may also be used as a feedstock for blast furnace iron making. DRI may also be melted into briquettes, referred to as hot briquetted iron (HBI), when the product has to be stored or transported. Inventory preparers can estimate the CO2 emissions from these processes from the energy consumption and carbon content of the fuel (e.g., natural gas, coal).

STEELMAKING

Steel production in a BOF begins by charging the vessel with 70-90 percent molten iron and 10-30 percent steel scrap. High purity oxygen then combines with the carbon in the iron to create an exothermic reaction that melts the charge while lowering the carbon content. Iron from the blast furnace usually contains 3-4 percent carbon, which must be reduced to less than 1 percent, refined, and alloyed to produce the desired grade of steel.

Steel production in an EAF typically occurs by charging 100 percent recycled steel scrap, which is melted using electrical energy imparted to the charge through carbon electrodes and then refined and alloyed to produce the desired grade of steel. Although EAFs may be located in integrated plants, typically they are stand-alone operations because of their fundamental reliance on scrap and not iron as a raw material. Since the EAF process is mainly one of melting scrap and not reducing oxides, carbon's role is not as dominant as it is in the blast furnace/BOF process. In a majority of scrap-charged EAF, CO2 emissions are mainly associated with consumption of the carbon electrodes. All carbon used in EAFs and other steelmaking processes should be considered process-related IPPU emissions. Figure 4.5 illustrates the steel making process and associated sources of emissions.

Figure 4.2 Illustration of coke production process (emissions reported in Category 1A of the Energy Sector)

Figure 4.2 Illustration of coke production process (emissions reported in Category 1A of the Energy Sector)

Bof Co2 Emission

Bold lines apply only to Onsite Coke Production at Integrated Iron and Steel Mill. Dashed lines apply to transfers of materials to 'Off Site processes.' 'Off Site process' does not include Integrated Iron and Steel production processes, which are categorised as Onsite.

Figure 4.3 Illustration of sinter production process

Figure 4.3 Illustration of sinter production process

Figure 4.4

Illustration of pig iron production processes

Integrated Coke Oven

- Coke

From Integrated Coke Oven

From Integrated Coke Oven

Coke Oven Process
Blast Furnace Gas

Transferred to Integrated Coke Oven

Burned Onsite within iron and steel process

Transferred Off Site

-^

Direction of Process

Connects to Steel Production

----

Connects to an External Process

A

Process

o

Byproduct

o

Raw Material

Product

Figure 4.5 Illustration of steel production processes

Figure 4.5 Illustration of steel production processes

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment