Introduction

Steel is arguably the most important structural material used in society, especially in
those countries undergoing rapid development. There is a strong correlation between a
country's economic development and its cumulative metal consumption. for example,
find that a country's “metal footprint” (in tonnes per capita) tends to increase by 1.9%
for every 1% increase in gross domestic product (GDP) per capita. The global production
of crude steel has increased steadily on an average of 1.8% in the last five years. In 2018,
it reached an astounding 1,808 million tonnes, with most of the growth in demand
occurring in developing countries such as India and China.
There is a very high environmental cost associated with traditional steelmaking at the
present scale. The greenhouse gas (GHG) emissions from steelmaking account for ~9%
of the total global fossil and industrial emissions (assuming 1.8 tonnes of CO2 per ton of
crude steel, tCO2/tCS). Although there have been significant ongoing efforts to reduce
the GHG emissions from steelmaking over the last several decades, major technological
breakthroughs are certainly still required if the sector is to keep up with the across-the-
board emissions reductions needed under the Paris Agreement, which aims to limit the
global temperature rise at the turn of this century to well below 2⁰C above pre-
industrial levels, and states that efforts should be directed towards a more ambitious
target of only 1.5⁰C of temperature rise.
Current estimates of future steel demand vary widely with projected annual growth rate
between 1.4% and 3.3%, resulting in a projected demand as high as 2.4 billion tonnes
by 2025. Partial decarbonisation of this growing steel industry could be achieved
through efficiency improvements and the integration of renewable electricity in
conventional steelmaking routes, whereas, complete decarbonisation would require
new zero-carbon and/or negative emissions technologies. However, attempts to
decarbonise the steel production processes have not seen any large-scale industrial
adoption, despite substantial on-going research efforts. The feasibility and applicability
of carbon capture and storage in the context of steel-making remain highly
questionable. Therefore, advancing renewably powered, low- or zero-carbon steel
technologies and investment deserves special emphasis and investigation.
The earliest steel production can be dated back to ancient Iran, China, India, Greece and
Rome, where the steel was made in a primitive furnace called bloomery. Until the 18th
Century, steel was mostly produced in small quantities. The industrial revolution marked
the advent of large-scale steel production, and the development of Bessemer process
and open-hearth furnaces have revolutionized the steelmaking industry.
The modern processes for producing virgin crude steel consists of several stages |
reduction of iron ore to iron, removal of carbon and other impurities, secondary refining
and alloying, and continuous casting. There are several fossil-based variants through
which these steps can be achieved in present day commercial processes, most notably,
 the blast-furnace (BF) route followed by a basic oxygen furnace, in which the ore
is reduced to molten metallic iron using a form of processes coal called coke and
then the impurities are removed in a controlled oxidizing atmosphere in a basic
oxygen furnace (BOF)3.
 the direct reduced iron (DRI) route, wherein solid-state reduction of the iron ore
is performed in a shaft furnace (e.g. MIDREX, HYL, Energiron etc.), rotary kiln
(e.g. Krupp-Codir), rotary hearth furnace (e.g. FASTMET) or a uidised bed (e.g.
FINMET), followed by reduction in an electric arc furnace (EAF). DRI can be
produced using coal or natural gas, the latter of which implies lower emissions
compared to coal-based DRI.
 the smelting reduction route (eg. COREX) which has relatively lower carbon
consumption compared to the blast furnaces. The smelting reductions routes are
advantageous because they use non-coking coal and eliminate the need for a
coking plant. However, the use of pure oxygen increases the total energy
requirement. The molten metallic iron is further processed using a BOF.
In addition to the aforementioned `virgin' steel manufacturing routes, steel can also be
produced through the recycling of scrap. Scrap recycling, either in a standalone EAF
process, or as an addition to BOF or EAF stages of other processes is possible and
accounts for 27% of global steel production. Although some have argued that the global
projected growth in steel demand could be completely catered for by recycling, it is
highly questionable whether or not the steel quality requirements can be met solely
from recycled steel in the long run. This is primarily due to the presence of
contaminated `tramp elements' such as copper and tin in the recycled steel which cause
`hot-shortness' or surface cracking during the hot-rolling process. Consequently, the
large-scale use of recycled steel is confined to certain applications such as reinforcing
bars for the building industry, where the steel requirements are less critical.
A secondary constraint on steel recycling is the availability of scrap, especially in regions
where there has been sustained economic growth. In these regions, scrap availability
will limit the recycling to below ~60% of the total steel demand through until 2030. In
fact, several studies have found this claim to be too optimistic, arguing that disparity in
the location of scrap generation and steel production, together with copper
contamination may constrain recycled steel production to only ~30% of the global
demand. Even with careful management of copper-rich scrap on a global scale, recycled
steel-quality requirements can only be met up to 2050. The expected demand for
copper-tolerant applications (e.g., reinforcement steel) is likely to grow more slowly
compared to that of higher-quality steels (e.g. cars and white-panel goods), suggesting
eventual accumulation of unusable steel scrap.
Figure 1 shows (a) the current share of steel production through different technologies,
(b) the energy consumption in different routes and (c) the CO2-emissions associated
with each route. As evident from the figure, BF-BOF has the largest share in production
and a disproportionately large CO2-emissions profile. With the projected increase in
worldwide steel demand, these emissions are also expected to proportionally increase,
unless major technological changes take place in the steel sector. These changes could
be in the form of new alloy development with higher strength-to weight ratio, improving
material usage during fabrication (e.g., near-net shaped casting), or moving to new low-
or zero-carbon steel making routes.
In the conventional BF{BOF route, carbon (in the form of coke and coal) is used to drive
the endothermic reduction reaction as well as for providing the high temperatures
required. A typical BF/BOF process produces 1.6-2.2 tCO2/tCS (tonnes of CO2 per tons
of crude steel). Significant regional differences in steel-related emission exists, with
India and China having much higher CO2-emissions footprint compared to the OECD
countries. There are also significant differences between different steelmaking routes
such as BF-BOF, DRI-EAF and scrap-EAF. Through technological improvements, steel
plants have steadily reduced their fuel consumption rate over the last five decades to
the point that the BF-BOF route can now be considered to be largely optimized; the
most efficient blast furnaces in the world can operate within ~5% above the theoretical
minimum in terms of their CO2-emissions.