Management of Greenhouse Gas Emissions in Integrated Steel Plant
Management of Greenhouse Gas Emissions in Integrated Steel Plant
Major constituents of greenhouse gases are carbon di oxide (CO2), methane (CH4), nitrous oxide (N2O), various fluorocarbons, sulphur hexafluoride, halons, and ozone in troposphere. Each of these gases has a different greenhouse warming potential (GWP) and their effects on atmosphere are not in direct proportion to their quantity of emissions.
The GWP of a greenhouse gas is a measure which indicates of how much a given mass of gas contributes to the global warming. The GWPs of different greenhouse gases are given on a relative scale. This scale compares the GWP of the gas in question to the GWP of carbon di oxide gas which is considered as 1.0. Over 100 years of time horizon the GWP of methane is 21, whereas the GWP of nitrous oxide is 310 (Fig 1). Hydro fluorocarbons (HFC) which are used in some of the air conditioning systems of the steel plant have a GWP of up to 11700. Sulfur hexafluoride (SF6) used in some of the circuit breakers of the electrical transmission system of the steel plant has a GWP of up to 23900.
Fig 1 Global warming potential of greenhouse gases
The manufacture of iron and steel is an energy intensive activity that generates carbon dioxide, methane, and nitrous oxide emissions at various stages during the production process. Although CO2 is easily the main GHG emitted from an integrated iron and steel plant, N2O and CH4 emissions are not necessarily be small.
The greenhouse gas which is the most relevant from the steel plant is CO2. The Steel Industry represents 6 % to 7 % of global anthropogenic CO2 emissions according to the Intergovernmental Panel for Climate Change (IPCC), but only 4 % to 5 % according to the International Energy Agency (IEA) of the total world CO2 emissions. As per world steel association (WSA), 1.8 tons of CO2 on an average are emitted for every ton of crude steel produced during the production of the steel. However for integrated steel plant, the CO2 gas generation depends on the facilities installed and the technologies adopted for the production in the steel plant and vary in the range of 1.6 tons per ton of crude steel to 2.4 tons per ton of crude steel.
An integrated iron and steel plant is a complex series of interconnected shops, where CO2 comes out from a large number of stacks. In the steel plant, the majority of CO2 emissions (around 69 %) arise during the production of hot metal in the blast furnace. A lesser amount of CO2 emission but still in significant amounts comes from rolling and finishing of the steel products (around 12 %), from preparation of ore for iron making (around 12 %), and from oxygen and power production (around 7 %).
Production of steel at the integrated steel plant is carried out using several interrelated processes consisting of coke production, sinter production, iron production, and steel production. The steel plant also has operations involving production of semi-finished steel and rolling it to produce the finish product. The plant also includes supporting production units like power plant, air separation plant and lime calcining plant etc. The plant is also required to have handling and transport facilities for raw materials, intermediate materials, finished materials and waste materials. Greenhouse gas emissions takes place in varying amount from most of these processes.
The GHG emissions in an integrated steel plant are take place due to the (i) process emissions, in which raw materials and combustion both may contribute to CO2 emissions, (ii) emissions from combustion sources alone, and (iii) indirect emissions e.g. due to consumption of electricity.
The main places in the integrated steel plant where the green-house gases are emitted in the environment include coke oven, sinter plant, blast furnace stove, reheating furnace, flare stacks, annealing furnaces, boilers, ladle and tundish heating furnaces, calcining plant and other miscellaneous places.
Furnace combustion system determines the efficiency of combustion in the furnace and level of conversion of carbon into carbon di oxide. Though complete combustion is an ideal thing to happen but sometimes a small amount of fuel remains unburnt. Carbon dioxide levels in flue gases vary depending on the type of fuel used and the excess air level used for optimal combustion conditions.
During the calcination of limestone and dolomite, the generation of CO2 is not only due to the burning of the fuel for providing the heat for reaction, but also due to the calcination reaction where carbonates (calcium carbonate and magnesium carbonate) are converted into oxides (calcium oxide and magnesium oxide).
Flares are a special category of stationary combustion sources because they are not typically operated for the purpose of producing useful energy and because they tend to function at relatively low combustion efficiencies and they allow a larger percentage of the fuel to pass unburnt.
The recommended methods for calculating CO2 emissions often differ from those for N2O and CH4 emissions. This is because CO2 emissions are largely determined by the carbon contents of the consumed materials, whereas N2O and CH4 emissions are much more influenced by the combustion or emission control equipment and technologies employed by the steel plant. Accordingly, CO2 emissions are best determined using a material balance approach which tracks the flow of carbon through the processes of the steel plant, whereas N2O and CH4 emissions are best determined using equipment or process specific emission factors.
Loss of materials in different steps (e.g. dust emissions, metal going into slag, generation of mill scale etc.) affects the process yields and hence leads to considerable additional requirement of energy. As energy consumption and generation of greenhouse gases are closely related, there is increase in the greenhouse gas generation with the increase in the loss of materials.
A fairly good estimation of the CO2 gas generation in the steel plant can be obtained through carbon balance which means total carbon input to the plant subtracted by the total carbon output from the plant. Carbon input to the plant comes from coal and purchased coke, other fossil fuels, petroleum coke, carbonate fluxes, petroleum products used, carbon in the iron and steel scrap consumed and carbon in the refractories consumed. Carbon output from the plant comes from carbon containing byproducts sold, carbon content of pig iron and iron scrap sold, and carbon content of semi-finished steel, finished steel and steel scrap sold. An adjustment to this amount is done for the quantities of intermediate products produced and not consumed by the next process or not sold and hence lying in the intermediate storages. Carbon amount which is consumed during production is the difference in the carbon input and carbon output and contributes towards the generation of CO2 from the plant.
A small fraction of a fuel’s carbon content can escape oxidation and remain as a solid after combustion in the form of ash or soot (for solid fuels) or particulate emissions (for gaseous fuels). This unoxidized fraction is a function of several factors, including fuel type, combustion technology, equipment age, and operating practices. Since this fraction is not contributing to the emissions of CO2, the carbon content of the fuel normally corrected with a constant factor for the estimation of CO2 emissions
Factors influencing greenhouse gas generation
The specific generation of greenhouse gases is influenced by several factors in the integrated steel plant. Some of the important factors are described below.
- Energy efficiency – Energy efficiency of the processes has a big influence on the generation of greenhouse gases. An energy efficient plant uses lesser fuel for the same output and reduces the specific generation of the greenhouse gases. A large number of energy efficient technologies are available and most of them have a low pay back period. Many of these technologies even do not need high investments. Implementation of these technologies in the integrated steel plant not only makes the plant energy efficient but also drastically reduces the greenhouse gas generation from the plant.
- Raw material quality – Out of specification raw materials has a substantial effect on the material efficiency which in turn increases the requirement of fuel to process these raw materials. Consumption of higher fuel means higher generation of greenhouse gases.
- Fuel quality – Quality of fuels especially the solid fuels (coal and coke) has an influence on the amount of the fuel needed for the process to proceed. The increase in the requirement of the fuel is normally much more when compared with the decrease in the quality. Higher consumption of fuel results into increase in the generation of greenhouse gases.
- Combustion equipment and combustion system – Efficient combustion equipment and combustion systems ensures complete combustion of the fuel and hence helps in the reduction of the greenhouse gas emissions.
- Improvement of process yields – Improvement in the yield of the processes the steel plant results into the processing of lesser materials for the same output. This in turn means lower consumption of fuels which means lower generation of greenhouse gas emissions.
- Recycling of waste energy – Recovery of waste energy and recycling it back in the process reduces the requirement of primary energy and hence aids in the reduction of greenhouse gas emissions.
- Recycling of materials – Recycling of waste materials results into processing of lesser raw materials for a particular level of production. This in turn means lesser requirement of fuels for the processing and hence lower generation of the greenhouse gases.
- Efficient use of electricity – Efficient use of electrical consumption results into lower need of electricity to be generated and in turn reduces the greenhouse gas emission of the power plant
- Efficient management of mobile equipment and locomotive fleet can result into substantial saving into greenhouse gas emissions of the steel plant.
All the energy efficiency measures reduce fuel consumption and, therefore, produce direct and indirect reductions in fuel associated GHG emissions. The technology advances, such as new process adoption and widespread adoption of advanced process controls, reduces energy intensity and hence the greenhouse gas generation.
The production of steel results in the generation of by-products that can reduce CO2 emissions by substituting natural resources in other industries. For example, blast furnace slag is used by the cement industry allowing it to reduce its CO2 emissions significantly. Steel making slags are also used as civil works aggregates thereby saving natural resources and environmental impact.
In addition, afforestation of the area within and around steel plant can greatly offset the effect of carbon di oxide which is the main greenhouse gas.
A state of the art steel plant is a much optimized system in terms of consumption of fuels and reducing agents. The blast furnace itself operates 5 % away from thermodynamic limits and the whole state of the art steel plant has a potential of energy savings of roughly 10 % only. This is due to several decades of cost management, as high energy prices have driven the industry to optimize its processes as close as possible to physical limits. The steel industry rightfully claims energy savings and, correspondingly, CO2 cuts which range between 50 % and 60 % over the last 40 years, depending on the local conditions. This is the highest level of energy conservation achieved by any industrial sector. Cutting CO2 emissions further to another level involves therefore specific challenges.
Solutions to curtail emissions from the ore based route of steel production have to be exhibited and it is clear that there is no simple process, available off-the-shelf, that can accomplish this. Deep paradigm shifts in the way steel is produced have to be imagined and the corresponding breakthrough technology is to be designed and developed by strong research and development programs. The largest such program called ULCOS (ultra-low CO2 steelmaking) has been running in the European union since 2004 to progress in this direction.