Environmental Impact Assessment of Steel Reinforcing Bar Manufacturing Process from Scrap Materials by applying Life Cycle Assessment Method: A Case Study on the Ethiopian Metal Industries

doi:10.21203/rs.3.rs-3405122/v1

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Environmental Impact Assessment of Steel Reinforcing Bar Manufacturing Process from Scrap Materials by applying Life Cycle Assessment Method: A Case Study on the Ethiopian Metal Industries

https://doi.org/10.21203/rs.3.rs-3405122/v1

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A clear understanding of the major environmental impacts of steelmaking from scraps, as well as potential solutions involving a circular economy paradigm, is essential. This study is conducted to pave the way for using life cycle assessment (LCA) to have sustainable development and effective resource management by evaluating the environmental impacts of the steel rebar manufacturing process using secondary resources. This is a cradle-to-gate LCA that includes scrap collection and sorting, transportation, melting, continuous casting, billet reheating, and reinforcing bar rolling. Inventory data is acquired as primary data from the factory and secondary data from the SIMAPRO-integrated eco-event database. All of the analyses in this LCA were conducted using the Recipe midpoint and endpoint impact assessment techniques taking one-ton reinforcing bar production as reference flow. The global warming potential created during the manufacturing process is recorded as 467 kgCO₂ eq in the environmental impact report. By moving hot billet directly to the rolling mill and using an efficient transportation system, the environmental impact of global warming potential can be reduced nearly 50%.

Life cycle assessment

Sustainable development

Secondary resources

Induction furnace

Environmental impact

Steel production

Reinforcing bar

The supply and demand of materials are greatly increased in the world due to the rapid growth of the world economy. According to the OECD background report for the 2021 meeting on the role of the G20 on resource efficiency and circular economy, the amount of material produced at the global level by 2017 reached 88 billion tons[1]. In addition to the economic growth, urbanization and industrialization level of different nations have higher contributions to the increment of material consumption.

The emerging economy in developing nations like China and India demands high amount of steel materials to satisfy their needs for the infrastructure establishment. Steel is the world's second-largest industry sector, after oil and gas, with a global turnover of 900 billion dollars [2]. For the past two decades crude steel production has increased more than twice, reaching 1951 million tons in 2021 compared to 852 million tons produced by 2001 [3]. Steel materials produced from iron ore and secondary resources using the basic oxygen furnace (BOF) and electric furnaces (EF) respectively are highly consumed materials with diversified applications for engineering, construction, infrastructure, industry, automotive, electrical-electronics, transport, and health in the world [4]. The steel industry plays the highest role in the nation’s economic development and it is the milestone and initiator for industrial growth[5, 6]. However, the Ethiopian steel industries are at the infant stage to have a significant contribution to the economic development of the country. The study conducted by the Industrial Policy Study and Research Department of the government of Ethiopia shows the steel sector economic role is limited to 0.4% contribution to the GDP of the country in the year 2013[7]. The main advantage of having a functional steel sector is transforming a country into an industrialized economy, hence it should recognized that the steel industries have the highest contribution to all aspects of the economy [5].

Globally crude steel production showed an increment since 1950 with the exception of 1995, 2009, and 2015 had a decline of 2.2%, 7.7%, and 3.0% respectively [3]. However, the steel production process follows an energy and material-intensive process and releases a high amount of emissions to the environment [8]. The emissions released through the production, consumption, and disposal processes of steel have environmental impacts that create damage on human health, damage on ecosystem quality, and natural resource depletion. Due to the globalized economy in natural resources, the environmental influences from production processes are far from the place where products are being consumed [9].

Because of the environmental impacts of materials processing, nations are obliged to consider environmental impacts and sustainability when they develop strategic policies on economic growth. At this time production growth of steel is constrained by climate change policies and regulations, even though resources for steel production are abundant and secondary resources production are also well organized[10]. Steel production accounts for about 7.7% of the global greenhouse gas (GHG) emissions in 2015[11] and is responsible for 12% of national CO₂ emissions in China [12]. To develop roadmaps for substantial emissions reduction within the steel sector, stakeholders and policymakers need information on trends in steel use, steel demand, and the amount of scraps available for recycling in different world regions to assure sustainable development [13]. Different tools and methods are in use to test the sustainability of the production process for economic growth. One of the methods that is helpful to evaluate sustainability is the life cycle assessment (LCA) of a product system or service.

Life Cycle Assessment (LCA) is a cradle-to-grave quantitative & comparative analysis and assessment of the environmental impacts of product systems considering the determined functional unit [14, 15]. It is internationally accepted and universally applicable to quantify resource usage, energy consumption, and environmental emissions related to the manufacture of steel industry products[16, 17]. It provides a holistic approach considering the potential impacts from all stages of manufacture, product use, and end-of-life [15, 16]. The international organization for Standardization (ISO) issued standards for LCA (ISO 14040/44). The World Steel Association (world steel) gathered Life Cycle Inventory (LCI) data for steel specific standard products based on ISO 14040/44: 2006 for each year of production in collaboration with the member steel producers [17]. The quality and relevance of LCA/LCI results, and the extent to which they can be applied and interpreted, depend critically upon the methodology used [16]. Practically, an LCA study shall be restricted within a certain system boundary, even though it aims to model all of a product's environmental, social, and economic impacts from cradle to grave [18].

Dora Andrea et al. (2019) performed a "cradle-to-gate" LCA study based on CML 2001 impact assessment method in GaBi for an integrated steel mill having a production capacity of 4 million metric ton hot rolled coil per year in the Netherlands with and without carbon capture and storage (CCS) considering one million metric ton hot rolled coil as functional unit. He concluded that the introduction of CCS technologies leads to a significant decrease in the GWP indicator, while all other environmental indicators have a more or less significant increase compared to the conventional production process [8].

Dimos Paraskeva et al. (2015) developed a Parametric LCA tool to simplify environmental impact calculation, express losses during Al recycling in LCA works, and minimize down-cycling and primary material inputs to have sustainable material management. The study incorporates the material hygiene concept during the recycling of aluminum that didn’t get attention from other LCA studies. The focus of the study is on aluminum recycling life cycle assessment using the ReCipe impact assessment method. The reduction of energy requirement and material losses in aluminum production from the secondary resources will reduce the environmental impact per mass of produced alloy [19].

Jana Gerta et al. (2021) conducted cradle –to- gate LCA study to give a comprehensive life cycle assessment in one of the integrated steel mills in Germany to produce 1 kg of hot rolled coil having measured primary data collected by 2018. Additionally, the study was performed to show existing facts of the environmental problems from the steel production process and to identify the main emitters in the processes. An impact assessment was determined based on the CML 2001 method using GaBi software from which secondary data was extracted [4].

A holistic environmental assessment work was done by Pietro A. Renzulli et al. (2016) on the integrated steel mill located in Taranto (Italy) applying LCA methodology for the production of Giga gram (Gg) solid steel slabs. The main goal of the study is to quantify the use of materials and energy including emissions in the lifecycle of the steel slab production process to indicate hotspots in order to propose alternative solutions. The LCA study used foreground data onsite collected, data estimated based on Best Available Technique Reference Documents (BREF), and emissions data from the Italian environmental protection agency were used for the LCA. Background data was obtained from Simapro version 8.2 software and European reference life cycle (ELCD) databases. The Impact assessment is done up to the characterization step and many of the environmental impacts are related to energy consumption [12].

An LCA study for the entire steel production in Poland was studied by Dorota Burchart-Korol et al. (2013) including the two routes of an integrated steel plant and electric arc furnaces of the steel production process. The study is a cradle-to-gate type of LCA considering one ton of cast steel as a functional unit. The major sources of the environmental impact in the steel making process are identified and possible prevention methods are recommended. The study followed ISO 14044 standard methodology and used averaged data as foreground data and the Ecoinvent database in SimaPro 7.3.3 software as background data. In the study, the main environmental impact categories Global warming, Energy consumption, and Human health damage were evaluated by the impact assessment methods of IPCC (2007) GWP 100a, cumulative energy demand (CED), and ReCiPe Midpoint H respectively. The study recommended that material substitution should be used as a pollution prevention method in iron-making processes to reduce environmental impacts [20].

Areinforcing bar production process life cycle assessment study was conducted by Alp Ozdemir et al. (2018) to identify the highest contributor to the environmental impact of a rebar production system. The production process is based on the secondary resources using an induction furnace and the foreground data were collected from the steel production industries of Turkey, whereas the background data was taken from Eco invent. It is Cradle- to gate type of LCA incorporating the raw material collection and transportation to the factory gate of the rebar product excluding infrastructure and the end-of-life phases. All classifications, characterization, and calculations of impact categories were performed based on CML-IA Environmental impact assessment method considering one metric ton of steel rebar production. The study concluded that the main contributor to the environmental impact is the billet production process (including melting) because of the highest consumption of electricity generated from fossil fuel combustion in maximum percentage[21].

The life cycle assessment work described above used cradle-to-gate type works on steel production except for one study that deals with aluminum. Most of the studies start from the raw material acquisition and end by producing molten steel, some are extended to the production of slabs or coils. Most of the studies applied regional-level impact assessment methods and covered their specific spatial boundaries on different production systems. They determined different functional units from one million tons to one kilogram of their specific product. All of the comparative analyses in the environmental impact assessment methods of studies are done between BOF and EAF which have clear differences in their energy consumption and input resources. Only one study has been conducted a study on reinforcing bar production using induction furnaces from secondary resources has a similar approach to this paper but didn’t show any comparison analysis or alternatives to reduce environmental impact potentials. From the reviewed pieces of literature it is possible to understand that research done on steel production from induction furnaces is limited and comparison analyses were done only between BOF and EAF which have clear demarcations. In this study, a life cycle assessment is done on the steel production process from the induction furnace route using secondary resources and fully renewable electrical energy. The paper is intended to show steel production from induction furnaces using secondary resources by applying renewable energy has a higher potential to reduce emissions. The sensitivity analysis is done to show the influence of fossil fuel consumption through a technological modification in the line of the production system. The technological modification applied in this paper is direct transferring of hot billet to the rolling mill from the continuous caster bypassing billet reheating process. Additionally, the study showed that the efficient transport system has a higher influence on the reduction of the environmental impact potential of the product system. Uncertainty analysis of the environmental impact assessment results was done concerning the quality of scraps and the quality of roads which affect the consumption of diesel fuel.

According to ISO 14040/44, the four mandatory phases that should be addressed in an LCA are goal and scope definition, inventory analysis, environmental impact assessment, and interpretation [22, 23]. In the goal and scope definitions, the preliminary information of the LCA study purpose, function/unit, system boundary, and application are described clearly. In the inventory analysis, the foreground data input/output flows and emissions were quantified exhaustively from the data sources of the factory, published journals, and reports. The background data were used from the Eco invent data library in Simapro software. In the impact assessment part all the environmental impacts were classified into the different midpoint impact categories and endpoint damages. Both impact indicators and damages were analyzed using Recipe mid-point and end-point impact assessment methods respectively. The uncertainty analysis of the impact assessment results considering the model described in Fig. 1 (case 1) production process was done. The sensitivity analysis of the impact assessment results was done by using an efficient transport system and applying technological modification in the production system. In the result and discussion part, results obtained from the environmental impact assessment are analyzed, and the data reliability and consistency were addressed. Comparison analysis between the different manufacturing processes and applying efficient transport systems in the reinforcing bar manufacturing processes are also done and displayed in the form of a table in the result and discussion part.

2.1. Goal and Scope Definition

The goal of the study is to quantify the environmental impact and natural resource depletion created during the reinforcing bar manufacturing process from the secondary resources (scraps) in the context of Ethiopia. The report of the study will be provided to the metal industry development institute (MIDI), national-level policy and strategy developers, the Environmental protection authority of Ethiopia (EPAE), and factories to create awareness of the environmental impact of the specific product system.

According to the ReCipe 9.1.1.7 impact assessment method, the environmental impacts created in a product system are classified into 17 midpoint impact categories and three endpoint damage categories. From the list 12 midpoint and the three endpoint impact categories were described based on the Ethiopian steel industries context. These impacts were calculated based on the data collected from the inventories to produce one-ton reinforcing bar. A reference flow 1 ton of steel reinforcing bar at the factory gate is used, because the use of a functional unit is not applicable in cradle-to-gate studies, due to the lack of knowledge regarding the end use and waste scenario conditions of the product [16].

According to the information obtained from the metal industry development institute (MIDI), the steel industries in Ethiopia established to produce reinforcing bars have a design capacity of 4,354,453.00 tons. While the trend of local production shown in Table 1 is less than 10% of the design capacity.

Table 1

local production and imported reinforcing bar trend
Type of Product	2016	2017	2018	2019	2020
Locally Produced Reinforcing bar (ton)	383, 511	316,655	396,850	357,844	363,834
Imported Reinforcing bar (ton)	721,647	375,726	242,046	403,082	561,352
Total consumption (ton)	1,105,158	692,381	638,896	760,926	925,186
The ratio of locally produced to the total consumption	0.347	0.457	0.621	0.470	0.393

Even though the design capacity of the established reinforcing bar manufacturing industries is more than 4 million tons, the achievement is less than half a million tons. So, locally produced reinforcing bars cover less than half of the total consumption (there is also unsatisfied demand due to scarcity) on average as it is indicated in Table 1. It shows that the production of steel in Ethiopia will be increased in the future to satisfy the needs of the society if the constraints of power and raw material scarcity are resolved. The scarcity of raw material is aggravated due to the absence of organized scrap dealer/yard at the country or regional level and scarcity of power is created because of the limited power plant availability in the country. Factories are forced to purchase scraps collected at the individual level from different areas passing through brokers and this leads them to use a disintegrated transport system.

Factories included in this study are the Ethiopian metal industries engaged in reinforcing bar manufacturing, other steel production factories are excluded from the study due to the absence of full information and their production system is not consistent or not well organized for mass production.

Globally most steel products are produced mainly from the integrated steel mill commonly called as basic oxygen furnace route using virgin and less than 30% scrap material, and from the mini mill of electric arc furnaces route using only secondary resources. There are some steel products from the open hearth furnace route and direct reduced iron from primary resources. Additionally, there is steel produced from an electric induction furnace from secondary resources. All the steel industries in Ethiopia are installed electric induction furnace technology with different capacities to produce steel from secondary resources (scraps). An induction furnace is used to melt metal with an induction heating electrical furnace that facilitates the melting of metal. It is an electrical furnace having good energy efficiency and easily controlled melting processes.

In Ethiopia, the steel production process technology can be called a mini-mill factory equipped with an electric induction furnace to melt scraps, continuous casting to form billets, a billet reheating furnace, and a rolling mill to form reinforcing bars. There are three possible ways (options) to produce steel reinforcing bars in Ethiopia; 1) using billets produced locally from induction furnace from steel scrap materials incorporating billet reheating furnace, 2) using locally produced billets by direct transferring of hot billets from CCM to rolling mill bypassing billet reheating furnace and 3) using the imported billets incorporating the billet reheating process. Reinforcing bar manufacturing process from the imported billet is excluded in this LCA study because of the absence of detailed information about the input materials and the transport system. The models that show the two production processes described as case 1 and case 2 are shown in Figs. 1 and 2 respectively to have a clear understanding of the boundary of the product system. Even though there are two cases of the production processes modeled in this paper, all of the information described here are considering case 1 as a base reference unless otherwise they are specified.

This LCA study is a cradle-to-gate type covering all stages from the raw material collection to the factory gate of the rebar manufacturing process. The in-sea transport of additive materials from the foreign market to port, use phase, end-of-life recycling, and waste scenario of the production systems of all cases didn’t include in the study due to the absence of organized data. At the current time, most of the steel factories are found 60 km away to the east of Addis Ababa the capital of Ethiopia, whereas scraps are collected from all over the country. Based on their potential, sixty percent of the scraps are collected within a 150 km radius including the capital of Ethiopia and forty percent within a 750 km radius including regional state capitals and zone capitals from the factory, on average scraps are collected at a distance of 400 km from the factory. Sorting is done manually for the sake of removing explosives, charging is done using magnetic-type cranes, and some are used manually.

2.2. Inventory analysis

In the inventory analysis phase of an LCA work, data can be collected, calculated, and modeled as input flows, output flows, and flows between the processes of production systems within the defined system boundary [18, 22, 24]. The inventory analysis includes all necessary activities, processes, and materials of the product system in the system boundary. Except for the reheating process of billets done in the reheating furnace by heavy fuel consumption, all the required energy for the production process is provided by the local grid of electric power generated from the hydropower plant even though the composition of electric power generation in Ethiopia is hydropower, wind farm and thermal with a mix of 90%, 8%, and 2% respectively in the country level.

The inventory data are inputs, outputs, emissions, and wastes. The inventories were allocated to the different activities and/or processes of transportation, melting, continuous casting, and rolling. Maintenance and office activity inventories didn’t include to avoid complexity. Always there are wastes and losses during the steel production process, some of the impurities may form oxides and release into the atmosphere or nature. So to have 1000 kg of reinforcing bar there is a consideration that 20% losses from input materials (secondary resources) throughout the processes.

2.2.1. The Transport System

The transportation process in this cradle-to-gate LCA includes the transportation of scraps, additives, fuels, refractories, lubricating oils, and others to the steel reinforcing bar production factory [16]. Input materials from all areas of the country and the port of Djibouti are transported by road transport using vehicles of different capacities to reach to the factory. Since all the production processes are done in a single factory in a single line, transportation between the processes in the product system is not taken into consideration.

Input materials are classified into two groups major (steel scraps) and minor (additives, refractories, purified oxygen, lubricating oils, and fuels). As described in the goal and scope definition, scraps are transported at an average distance of 400km to reach to the production factory using road transport vehicles from different scarp source areas.

All additives, refractories, fluxes (lime), lubricating oils, oxygen, and fuel are imported from the international market and transported a minimum distance of 876 km by road transport using vehicles having a capacity from 10,000 kg to 32,000 kg. To have a common approach the vehicles that can transport 10,000 kg at a time are selected as a base reference for all transportation systems. Vehicles with 30,000 kg and above capacity were considered to check the sensitivity of the transport system concerning the environmental impact results. All vehicles use diesel fuel and consume 240g/km for 10,000 kg capacity and 345g/km for 30,000 kg and above capacity vehicles on average. Emissions from the transport system were calculated based on the European monitoring and evaluation program (emep) of the European environmental agency[25]. Emissions of carbon dioxide, methane, carbon monoxide, nitrogen oxides, NMVOC, dinitrogen monoxide, ammonia, lead, particulate matter, and sulfur dioxide are the common types considered in this transport system and are calculated by the following formula

$${E}_{i }={C}_{f}\times {E}_{fi}$$

$$where:Ei=emission of i from the system$$

$$Cf=Consumption of fuel during the transport system$$

$$Efi=emission factor of emission i$$

The amount of fuel consumption and emission calculation for road transport is done as described before. The amount of emissions for the specified unit transport is calculated based on the emission factors developed by EMEP final report on the quantification of ship emissions[26].

2.2.2. Melting Process

Scraps collected from different sources are charged in the coreless induction furnace with eight-ton capacity requires 550 KWh electricity per ton of steel production. The furnace takes 3 hrs to melt the charges to produce eight tons of molten metal and make it ready for ladle transport. Additives are charged to the furnace when the temperature reaches 1500⁰C to adjust chemical composition. Pure oxygen is introduced to agitate liquid metal and oxidize impurities to purify molten steel by forming slags (oxides) floating on the molten steel. The molten steel is transported by overhead crane with a ladle and poured to the tundish to feed to the continuous caster machine mold to form billets. The amount of material lost between melting and billet formation is nearly fifteen percent of the charged material in the form of slag, sludge, emissions, and other wastes.

The foreground inventory data for the melting process were collected from the factory by interviewing experts of each section and displayed in Table 2. Data that are difficult to determine or not available at the factory level are taken from the kinds of literature published by international journals passing through extensive peer review or reports of different associations. The background data was used from the eco-event 3 database library of the Simapro 9.1.1.7 version.

Emissions data were not available at the factory and there is no formal organization that can provide steel production process emissions inventory at the country level. So the emissions from different activities and processes were calculated using different emission factors from Europe, the United States, and England. Others that are difficult to estimate or calculate were taken from different kinds of literature. The inventory of steel production process from induction furnace didn’t get full coverage by LCA practitioners because it is lately introduced technology and had a limited contribution to the amount of global steel production. So electric arc furnace data which have similar nature to electric induction furnace were taken as equivalent data for special cases. Both induction furnaces and electric arc furnaces are used scraps & additives as input materials and electricity as a power source.

The sources of emissions are the melting reaction of scraps with some dust during handling, preparation, and charging. The most common types of emissions that could be included are greenhouse gases, particulates, carbon monoxide, sulfur dioxide, nitrogen oxide, heavy metals, and wastes [27]. Heavy metal emission from the melting process is calculated by considering the emission factor per ton of molten liquid steel production [28]. The inventory data displayed in Tables 2, 3, and 4 are representative of the production process modeled in Fig. 1. The inventory of production processes modeled in Fig. 2 is managed by removing direct emission records from the reheating furnace.

Table 2

The inventory data to produce 1.1 tons of molten steel production
Type of material	Unit	Quantity per functional unit	Source of data
Inputs from nature
• Water	m³	90	Factory
Inputs from the Techno sphere
• Steel scrap	kg	1250	Factory
• Ferromanganese	kg	10
• Ferrosilicon	kg	3
• Refractory material	kg	12
• Pure oxygen	kg	10.26
• Lime	kg	30
Outputs
• Molten metal	kg	1100	Considering there is about more than 12% loss in the form of slag, sludge, and emission of gases, etc
From Techno sphere Electricity/Fuel
• High-voltage electricity generated from hydropower	KWH	605	From the factory
Emissions to air
• Carbon dioxide	g	6118.2	From literature[21]
• Carbon monoxide	g	568.623
• Nitrogen dioxide	g	206.525
• Sulfur dioxide	g	7.7	From literature [20]
Heavy Metals
• Pb	g	15.4	Calculated based on the emission factor [28]
• Cr	g	1.1
• Cd	g	0.275
• Cu	g	0.88
• Zn	g	55
• Ni	g	0.275
• Ar	g	0.11
• Hg	g	0.165
• V	g	1.056
• Dust	g	80	From literature [21]
• HF	g	0.99
• PM	g	55
Waste
• Wastewater	m³	90	Estimation referring the inputs and outputs [29]
• Refractory waste (dust)	kg	8
• Sludge	kg	77
• Slag	kg	110

2.2.3. Continuous Casting

In the continuous casting process, the main products are billets having $125\times 125$mm² cross-section and 6000 mm length. The main inputs to the process are molten steel from the induction furnace, lubricating oil, electricity, and water. Boguslaw Bieda's reports on his LCI work on the continuous casting process indicated that the source of emissions in continuous casting is the heating and drying process of tundish [30]. The common emissions are dust particulate matter, sulfur dioxide, nitrogen dioxide, and carbon monoxide [30]. There are also wastewater, iron particles, and Iron scraps considered as wastes.

Table 3

Inventory data to produce 1.05 tons of billets production
Type of material	Unit	Quantity per functional unit	Source
Inputs from nature
• Water	m³	10.32	Factory
Inputs from the techno sphere
• Molten Steel	kg	1100
• Lubricating Oil	kg	2.46
• Refractory	kg	12
Electricity
• Hydropower of Electricity high voltage from the grid	kWh	83.33	literature[31]
Output to the techno sphere
Billets	kg	1050	Factory data considering the waste assumption
Emissions to air
• Iron dust	kg	4.43	Calculated based on the emission factor[30]
• SO₂	g	170
• NO₂	g	170
• CO	g	260
• Particulates	kg	1.84
Waste
• Wastewater	m³	10.32	Assumption based on literature
• Refractory waste	kg	12.00
• Iron scraps	kg	25.75
• Iron particles –Scale	kg	2.34

2.2.4. Billet Reheating and Roll forming process

Billet products from the continuous casting are stored in temporary storage to cool to room temperature and cut into the required length based on the capacity of reheating furnaces. The 6000 mm length cut billet is charged to the reheating furnace having an energy source from the heavy fuel oil combustion. When the temperature of the billet is reached 1150 ^OC transferred to the rolling mill with an exit temperature not less than 1100 ^OC.

Now a day’s new technology is introduced in the steel products manufacturing process by direct transferring of hot billet to the rolling mill bypassing the reheating process as shown in Fig. 2. This technology is practiced in one factory in Ethiopia. Usually, billets have 1100 ^OC surface temperature when exiting from their forming line[32] which is sufficient for the hot rolling process. So direct transferring of billet through the high-speed casters to the rolling mill is helpful to eliminate the fossil fuel combustion for reheating of billets.

Roll forming of the rebar is accomplished in a hot rolling process passing through the number of reduction stands to get the required size and form. The rebar exit from the last stands will pass through the quenching box to improve its mechanical properties and the process can be called thermomechanical treatment (TMT). The roll forming mill is run with the power of electricity from the hydropower grid of the country.

Since the reheating of billets is done by the combustion of heavy fuel, different types of emissions are released from the process. The number of emissions other than greenhouse gases was calculated using the emission factors set by the emissions inventory guideline of an international panel of climate change (IPCC)[33]. The greenhouse gas emissions CO₂, CH_4, and N₂O were calculated by taking an emission factor of the stationary combustion from the international panel combustion control[34]. The emissions of different gases, particulates, and heavy metal from reheating furnaces can be calculated with;

$${ E}_{e}={Co}_{f}\times {E}_{f}$$

Where: E_e= emissions of an element

C_of = Energy generated during the consumption of fuel

E_f = Emission factor of the element

According to the data obtained from the factory, the reheating furnace consumes 46 liters of heavy fuel oil to heat one ton of billets on average. One gallon of heavy fuel oil can generate 158.04 megajoules. To reheat 1050 kg of billets, 48.3 liters of heavy fuel oil is required and can generate 2016.59 MJ of heat energy.

Table 4

Inventory data to produce 1.00-ton reinforcing bar
Type of material	Unit	Quantity per functional unit	Source
Inputs from nature
• Water	m³	14.42	Factory
Inputs from the techno sphere
• Billets	Kg	1050	Factory
• Residual fuel oil	L	48.3
• Lubricating oil	kg	33.85
Electricity/Fuel
• High voltage hydropower electricity from the grid	kWh	1069.445	From literature [31]
Outputs to Techno sphere
Rebar	kg	1000	FU production volume is taken into consideration
Waste
• Wastewater	m³	1.42	From factory supported by expert judgment
• Scale	kg	10
• Iron scrap	kg	33
Emissions
• CO₂	kg	167.64	Calculated based on the emission factor[34, 35]
• CH₄	g	8.0664
• N₂O	g	16.1328
• CO	g	30.249	Calculated based on the emission factor [33]
• NMVOC	g	8.0664
• NO_X	g	342.822
• SO₂	g	2667.96
Waste
• Iron	kg	0.05	From factory expert estimation
• Dust	kg	0.092
• Scale	kg	5

2.3. Impact assessment

The day-to-day activity of people to satisfy their need including the processes of production adversely affects the environment [18]. The inventory analysis of different production systems showed that processes take input from nature and the techno sphere and release emissions and waste to nature. The Recipe midpoint and endpoint impact assessment methods are selected to assess the environmental impacts created by the production processes. The recipe impact assessment method has the advantage to assess the impact categories and damages in a single package. The environmental impact assessment calculation of the rebar manufacturing process conducted in this study was carried out with Simapro 9.4.0.2 faculty version. As it was mentioned in the introductory part, life cycle impact assessment (LCIA) can be applied to show the significance of the potential environmental impacts in the production processes considering life cycle inventory results quantitatively and qualitatively [18, 22].

An LCA conducted in this study is a process-based impact assessment used to quantify the environmental impacts of the steel reinforcing bar manufacturing from secondary resources in the context of Ethiopia. The production process with higher environmental impact were identified based on the Recipe midpoint and endpoint impact assessment results. The midpoint impact assessment was used to display the impact results of the impact categories, whereas the endpoint impact assessment was used to quantify damages caused by the environmental impact.

The environmental impacts of the product system are classified into seventeen midpoint impact categories and three endpoint damage categories according to the Recipe midpoint and endpoint impact assessment methods respectively. Some of the impact categories that don’t have significant problems in Ethiopia and are not quantified properly are excluded. Selected impact categories are Global warming (GW), Ozone formation-human health (OFHH), Fine particulate matter formation (FPMF), Ozone formation-terrestrial ecosystems (OFTerEcos), Terrestrial acidification (TerAc), Freshwater eutrophication (FwEu), Terrestrial Ecotoxicity (TerEcoTox), Freshwater Ecotoxicity (FwEcoTox), Human carcinogenic toxicity (HCTox), Human non-carcinogenic toxicity (HNCTox), Mineral resource scarcity (MRS) and Fossil resource scarcity (FRS) are described in this paper.

The environmental impact assessment results of the midpoint impact categories for the production system (case 1) are indicated in Table 5. The system includes transportation, molten steel production (melting), billets production (continuous casting), and reheating of billet and rebar manufacturing processes (rolling).

Table 5

The environmental impact assessment result of the production system (case 1)
Impact Category	Unit	Transport	Melting	CCM	RHB	Rolling	Total
Global warming	kg CO2 eq	80.45	127	30.4	189.31	39.3	467
Ozone formation, Human health	kg NOx eq	0.62024	0.506	0.059	0.398	0.1459	1.73
Fine particulate matter formation	kg PM2.5 eq	0.01745	0.213	0.0145	0.0061	0.0253	0.28
Ozone formation, Terrestrial ecosystems	kg NOx eq	0.622	0.52	0.065	0.4044	0.1929	1.80
Terrestrial acidification	kg SO2 eq	0.6216	0.699	0.1323	0.1573	0.193	1.80
Freshwater Eutrophication	Kg p equ.	0	0.0357	0.00257	0.0010	0.00575	0.045
Terrestrial ecotoxicity	kg 1,4-DCB e	0.0049	0.977	0.0028	0.0015	0.0032	0.989
Freshwater ecotoxicity	kg 1,4-DCB e	0	9.278	1.2143	0.0725	0.2017	10.767
Human carcinogenic toxicity	kg 1,4-DBC e	0.1005	0.1496	0.0057	0.0018	0.0043	0.262
Human non-carcinogenic toxicity	kg 1,4-DBC e	2.475	227.83	6.498	3.383	5.082	245.268
Mineral resource scarcity	kg Cu eq	0	0.8335	0.0284	0.00094	0.0022	0.865
Fossil resource scarcity	kg oil eq	0	41.452	14.669	49.94	59.112	165.173

Each of the impact categories has been described by its impact indicator to facilitate characterization. An environmental impact potential can be a contribution of more than one elementary flow, so characterization is used to measure the impact of different elementary flows with one unit. As an example, the global warming potential is measured by kgCO₂ eq even though greenhouse gases methane, nitrous oxide, and others have higher impact potential. Methane and other emissions which have an impact on global warming are characterized by kgCO₂ eq. For example, one unit of methane has equivalent to 25 units of carbon dioxide [36].

An endpoint damage assessment shows the effect of the impact categories in the cause-and-effect chain of the different production systems' environmental impact analysis. The three endpoint damages are classified as human health, ecosystem quality, and natural resource scarcity. The damage assessment of these endpoint impact categories is displayed in Table 6.

Table 6

The endpoint impact assessment of the three types of damages for the production system (case1)
Damage category	Unit	Transport system	Melting Scraps	Continuous Casting	Reheating Billets	Rebar Rolling	Total
Human Health	DALY	2.01E-05	0.08483	0.01166	2.94E-04	6.50E-05	9.69E-02
Ecosystems	Species yr.	1.91E-07	4.65E-07	1.01E-07	8.39E-07	1.0E-07	1.6E-06
Resources	USD2013	0	16.6413	4.1326	22.514	25.9597	6.92E + 01

The second production system (case 2) includes transportation, molten steel production, billet production, and rebar manufacturing as the main processes. As it was shown in the goal & scope section, the production system of case 2 applies a technological modification to the production system of case 1 by direct transferring of hot billet to the rolling mill from the continuous casting. It bypasses the reheating of the billet and avoids heavy fuel combustion for the reheating purpose. The impact assessment results of this production system in different environmental impact categories using Recipe midpoint impact assessment are displayed in Table 7.

Table 7

The environmental impact assessment of the production system (case 2)
Impact Category	Unit	Transport	Melting	CCM	Rolling	Total
Global warming (GW)	kg CO2 eq	78.18	127.04	29.34	39.52	274.08
Ozone formation, Human health(OF,HH)	kg NOx eq	0.5817	0.5056	0.059	0.1447	1.291
Fine particulate matter formation (FPMF)	kg PM2.5 eq	0.0176	0.2134	0.0141	0.0254	0.2705
Ozone formation, Terrestrial ecosystems (OF, TerEcosy)	kg NOx eq	0.5863	0.5197	0.0646	0.188	1.3586
Terrestrial acidification (Tac)	kg SO2 eq	0.2576	0.511	0.3204	0.222	1.31
Freshwater eutrophication (FwEut)	kg P eq	0	0.0357	0.0026	0.0058	0.0441
Terrestrial ecotoxicity (TerEcoT)	kg 1,4-DCB e	0	0.9767	0.0079	0.0032	0.9878
Freshwater ecotoxicity (FWEcoT)	kg 1,4-DCB e	0	9.278	1.215	0.2016	10.6941
Human carcinogenic toxicity (HCT)	kg 1,4-DBC e	0	0.1496	0.0057	0.0042	0.1595
Human non-carcinogenic toxicity (HNCT)	kg 1,4-DBC e	2.475	227.83	6.5	5.083	241.888
Mineral resource scarcity (MRS)	kg Cu eq	0	0.8335	0.0285	0.0022	0.8642
Fossil resource scarcity(FRS)	kg oil eq	0	41.452	14.669	59.1115	115.233

The endpoint damage assessment of the production system (case 2) is done for the three damage types. The assessment of the damages for the three damage types is displayed in Table 8.

Table 8

The endpoint impact assessment of the three damage types in the production system (case2)
Damage Category	Unit	Transport System	Scrap Melting	Continuous Casting	Rebar Rolling	Total
Human Health	DALY	1.95E-05	0.08483	0.01165	7.70E-05	9.66E-02
Ecosystems	species.yr	1.77E-07	4.65E-07	1.01E-07	9.80E-08	8.41E-07
Resources	USD2013	0	16.64128	4.13261	25.96	4.67E + 01

Variations in the quality of input materials and using default emission factors for emissions calculation will contribute to the variations in the impact assessment results of the production system. The main sources for the uncertainty of the impact assessment results in the production system are the variation of the scrap quality, quality of roads, and variation in vehicle capacity during the transportation system. The amount of emissions generated from the product system may vary depending on the quantity and quality of the inputs. The scraps collected from different sources may be contaminated during the usage, collection, and/or storage by heavy metals, chemicals, paints, and other impurities in different concentrations. For example, depending on their quality the required amount of scraps for one ton of reinforcing bar production varies between 1.1 and 1.25 tons, this variation may vary the amount of energy consumption & emissions generated from the different processes and/ or activities. The variations in the mix-up of vehicles in different capacities, the skill of the driver, and road quality used to transport materials in the transportation system may be varied the amount of fuel consumption in-line with an increment of emissions released from the system and cause uncertainty on the results. Emissions are determined referring to emission factors from different countries and default values as indicated in the inventory analysis section, the scenario differences may cause uncertainty on the amount of emissions released from the different processes. The sensitivity analysis of the process is checked by varying the transportation system and applying technological modification in the rolling section as modeled in Fig. 2 (case 2). The transport system sensitivity analysis was done to show the variation in environmental impacts by using vehicles having a capacity of 30,000 kg instead of 10,000 kg capacity. When vehicles are changed from 10,000 kg to 30,000 kg capacity, global warming potential shows a reduction of 12.2% as shown in Table 9. In this case, all environmental impact potentials have a reduction in all categories like terrestrial acidification, ozone formation human health, and ozone formation terrestrial ecosystem record 18.52%, 17.17 and 16.63 respectively are in the first three lists.

Table 9

% reduction of environmental impact by using 30,000 Kg Vehicles rather than using 10,000 Kg Vehicles
Impact category	Unit	Rebar:10,00 kg Vehicle	Rebar:30,000kg Vehicle	Reduction (%)
Global warming	kg CO2 eq.	467	410	12.2
Ozone formation, Human health	kg NOx eq.	1.729	1.432	17.17
Fine particulate matter formation	kg PM2.5 eq.	0.2764	0.2694	2.5
Ozone formation, Terrestrial ecosystems	kg NOx eq.	1.8032	1.5033	16.63
Terrestrial acidification	kg SO2 eq.	1.8032	1.4692	18.52
Freshwater eutrophication	kg P eq.	0.0451	0.04485	0.5
Terrestrial Ecotoxicity	kg 1,4-DCB e	0.9894	0.988	0.14
Freshwater Ecotoxicity	kg 1,4-DCB e	10.767	10.7494	0.16
Human carcinogenic toxicity	kg 1,4-DBC e	0.2618	0.2608	0.16
Human non-carcinogenic toxicity	kg 1,4-DBC e	245.269	242.058	1.3
Mineral resource scarcity	kg Cu eq.	0.865	0.865	0
Fossil resource scarcity	kg oil eq.	165.173	154.659	6.36

The environmental impacts created by the production system after the technological modification is shown in Table 5. Direct transferring of hot billet to the rolling mill bypassing the reheating furnace can reduce 193 kg CO₂eq (41.3% reduction). By applying an efficient transport system and technological modification, the global warming potential can be reduced by 238 kg CO₂eq (51% reduction) per one-ton reinforcing bar manufacturing.

The life cycle assessment of the impact and damage categories are assessed and interpreted referring to the inventory analysis of environmental impacts to the goal and scope definition to draw conclusions and recommendations [18, 37]. The environmental impact assessment results for the two production systems' main activities and/or processes (transportation, melting, billet production, reheating of billet, and rebar manufacturing) are explained in the previous sections. As of the previous sections, all analysis, discussion, and interpretation are done taking the case1 production process as the base reference product system unless otherwise it is specified. The environmental impact results (case 1) of different categories are displayed in Table 5 and the impact contribution of the main activities and/or processes in the product system is shown in Fig. 3.

The transportation of materials in the product system has a higher environmental impact contribution on Ozone formation in both human health and terrestrial ecosystem. The reheating of billet in the manufacturing process has a higher environmental impact contribution on global warming because heavy fuel combustion is used as an energy source for heating billet. The melting process has a higher environmental impact on terrestrial acidification, and freshwater eutrophication, in all toxicity impacts and mineral resources scarcity impacts as displayed in Fig. 3. Since scraps are collected from different sources, there is a possibility of contamination by heavy metals, so different types of toxicity impact potentials are high in the melting process. Carcinogenic and non-carcinogenic toxicity effects are also higher in the molten steel production process. The mineral resources scarcity is high in the melting process also due to the consumption of additives such as ferroalloys (manganese and silicon), lime, and refractory materials in the production of molten metal.

The global warming environmental impact is maximum in the billet reheating process having 189 kgCO₂eq due to the combustion of heavy fuel. Second, the scrap melting process contributes 127 kgCO₂eq followed by the transport system, rebar rolling, and billet production with 80.45, 39.3, and 30.4 kgCO₂eq respectively. The scrap melting is done in the induction furnace that uses electricity generated from hydropower grid which fully renewable energy source. The emission inventories found from the product system in different processes that contribute to global warming with a cut of 0.001% are carbon dioxide, methane, dinitrogen monoxide, sulfur hexafluoride, and others with an amount equal to 395.11, 62.4312, 9.2, 0.23, and 0.012 kgCO₂eq respectively. The billet production process has less environmental impact with respect to the other processes.

The billet reheating process contributes 40.5% of greenhouse gas (GHG) emissions for global warming potential and the emissions from the transportation system are 80.5 kgCO₂ which is 17.2% of the total emission. The rest are allocated for melting, continuous casting (billet production), and rolling processes. The transport system and billet reheating are the activities or processes that can able to improve as shown in the sensitivity analysis. Effects of the efficient transportation system and technological modification (bypassing the reheating process) on the creation of environmental impact in the production system are displayed in Table 10.

The molten steel production has higher human health and ecosystem damage as shown in Table 6. Resource scarcity is higher in rebar manufacturing due to the consumption of lubricating oil and followed by the billet reheating process due to the consumption of fossil fuel.

Since the impact assessment result obtained is a relative value and not normalized, ordered, or weighted, it cannot be used to predict the effects of the final manufactured product, threshold value exceedance, safety margins, or for risk analysis [4]. Most of the data are collected at the plant by direct supervision and some are calculated in reference to the globally accepted guidelines like IPCC, emep, and others, so the data available in this product system are reliable. The values of the environmental impacts obtained in this paper are far from other studies with the justified reason, this shows the consistency of the data. A similar study conducted by Alp Ozdemir et al, 2018 on the environmental impact assessment of rebar production from induction furnaces in Turkey has recorded 720 kg CO₂eq global warming potential. The variation is 253 kg CO₂eq, showing a 35% reduction, it may be the effect of using fully renewable energy (hydropower) that avoids the combustion fuel for electricity generation.

Referring to the results obtained from this work, people can understand that using renewable energy and secondary resources in the production process of steel products has significant advantages in environmental protection and resource conservation. Among the two cases of the production processes, case 1 which manufacture reinforcing bar by reheating billets produced from scrap materials created a higher amount of environmental impact in all of the environmental impact categories as shown in Table 10. The main reasons for the increment of the environmental impacts in case 1 are using a discrete transport system and applying billet reheating (there is no technological modification) in the production process.

The comparison of the two reinforcing bar manufacturing processes in Ethiopia and using an efficient transport system was done based on the determined reference flow one-ton reinforcing bar manufacturing for the standard specification grade 75 (B500CWR). The comparisons of case 1, case 2, and bulk transport systems are displayed in Table 10.

Table 10

Summary of the Environmental impact result of the two cases with one-ton reinforcing bar manufacturing
Impact category	Unit	Case1 Including Reheating billet	Case 2 excluding reheating billet	Applying Bulk Transport
Global warming	kg CO2 eq.	467	274	410
Ozone formation, Human health	kg NOx eq.	1.73	1.291	1.432
Fine particulate matter formation	kg PM2.5 eq.	0.28	0.271	0.269
Ozone formation, Terrestrial ecosystems	kg NOx eq.	1.80	1.359	1.50
Terrestrial acidification	kg SO2 eq.	1.80	1.31	1.47
Freshwater eutrophication	kg P eq.	0.045	0.0441	0.045
Terrestrial Ecotoxicity	kg 1,4-DCB e	0.989	0.9878	0.988
Freshwater Ecotoxicity	kg 1,4-DCB e	10.767	10.6941	10.75
Human carcinogenic toxicity	kg 1,4-DBC e	0.262	0.1595	0.261
Human non-carcinogenic toxicity	kg 1,4-DBC e	245.268	241.89	242.06
Mineral resource scarcity	kg Cu eq.	0.865	0.8642	0.865
Fossil resource scarcity	kg oil eq.	165.173	115.233	154.66

The production system in case 2 has a technological modification on production system cae1. Sensitivity analysis was done by providing technological modification (case 2) on the production system to reduce the amount of heavy fuel consumption by direct transferring of hot billets. Hot billet transferring is the direct transfer of the hot billet from continuous casting to the rolling mill without any delay and intermediate reheating arrangement, thereby avoiding unnecessary waste of heating energy. This avoids the consumption of heavy fuel that releases high amounts of emissions to the environment during the combustion process and transportation of it also. Referring to Table 10 the global warming environmental impact of the rebar manufacturing process reduces from 467 kgCO₂ in case 1 to 274 kg CO₂ in case 2. The surface temperature of billets at the exit from the continuous caster machine is around 1100⁰C and the core part has a higher value [38]. Since the exit temperature of the billet is equal to the required temperature to improve the plasticity nature of the steel, it is possible to bypass the reheating of billets by direct hot billet charging to the rolling mill. Based on the technological modification of the production system, it is possible to reduce 2016.59 MJ heavy fuel consumption. By direct transferring of billets from the caster 193 kgCO₂eq from global warming can be reduced. Bypassing the reheating furnace during the rebar manufacturing process could able to reduce global warming, ozone formation for human health, ozone formation for the terrestrial ecosystem, terrestrial acidification, human carcinogenic toxicity, and fossil resource scarcity by 41.3%, 25.3%, 24.5%, 27.2%, 39 and 30.2% respectively.

The steel reinforcing bar manufacturing from secondary resources has an environmental and economic advantage in developing countries specifically in the case of Ethiopia where there are no established primary steel production processes. Steel production from an induction furnace is a high energy and emission reductive process compared to other processes even compared with the electrical arc furnace process. Manufacturing of reinforcing bars from the induction furnace route using secondary resources highly reduces the amount of the environmental impact potential.

Most of the emissions are generated from the billet reheating process from fossil fuel combustion. By direct transferring of hot billet, the global warming potential is reduced by 193 kgCO₂eq in the production of one-ton reinforcing bar. The emissions generated from the transport system can be reduced by applying a bulk transport system. Using trucks (> 30t) instead of vehicles (< 10t) in one-ton reinforcing bar manufacturing can reduce 57 kgCO₂eq. Applying bulk transportation and direct transferring of hot billet to the rolling mill can reduce 238 kgCO₂eq or 51% of the global warming potential.

This life cycle analysis was conducted to determine the environmental impact of reinforcing bar manufacturing process from secondary resources using 100% renewable energy sources. It can be considered as the startup LCA work of the steel production process in Ethiopia and can serve as one source for the next LCA practitioners in the country. Most of the emissions in this work are calculated by referring to an emission factor from Europe or the United States, so an LCA work can be extended by direct measuring all the emissions at the factory level in Ethiopia. The sensitivity analysis on efficient transport systems and technological modification shows only the advantages of emission reduction for environmental impact, but the economic and societal advantage of the process requires further studies.

Acknowledgment

The authors acknowledge the financial support of the Bahir Dar institute of Technology- Bahir Dar University.

Contributions

Tefera Eniyew Fente: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Validation, Writing – original draft, and Writing – review & editing. Assefa Asmare Tsegaw: Conceptualization, Visualization, Validation, Writing – review & editing, Resources, Funding acquisition, and Principal supervision. All the authors have read and agreed to the published version of the manuscript.

Ethics approval

Not applicable.

Consent to participate

All the authors participated in this research.

Consent for publication

All the authors approved the final manuscript for publication.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this study.

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Environmental Impact Assessment of Steel Reinforcing Bar Manufacturing Process from Scrap Materials by applying Life Cycle Assessment Method: A Case Study on the Ethiopian Metal Industries

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Abstract

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1. Introduction

2. Methodology

2.1. Goal and Scope Definition

2.2. Inventory analysis

2.2.1. The Transport System

2.2.2. Melting Process

2.2.3. Continuous Casting

2.2.4. Billet Reheating and Roll forming process

2.3. Impact assessment

3. Uncertainty and Sensitivity analysis

4. Results and Discussion

5. Conclusion and Recommendation

Declarations

References

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