It has long
been acknowledged that decarbonizing heavy industries that use 'process heat’
is very challenging. Some processes can replace coal and heavy hydrocarbons
with natural gas or hydrogen. Others can use electric arc furnaces to replace
hydrocarbons. Those with the highest temperature requirements still require
heavy hydrocarbons. For those industries especially but for most that use
fossil fuels, it has also been acknowledged that carbon capture, utilization,
and storage (CCUS) is the best way for those industries to decarbonize. However,
since the combustion flue emissions from aluminum smelting have a low CO2
concentration, CCUS is less feasible for the aluminum industry than for other
industries like cement, coal-fired power, or natural gas-fired power. Thus, the new designation of industrial
carbon management has arisen to measure emissions and reduce them in a
combination approach.
A new report: Decarbonisation
Options for the Aluminum Industry by the European Commission's Joint Research
Center (JRC) highlights aluminum industry emissions reduction opportunities and
challenges. The refining of alumina is the most emissions-intensive process in the
aluminum industry. This is followed by the smelting of aluminum ore. The JRC's
report assesses four decarbonization options: inert anodes, hydrogen,
electrification, and ICM. As expected ICM is deemed to be the most applicable
and the most cost-effective way to decarbonize. As mentioned, ICM includes CCUS.
ICM may be the least expensive option, but it is still expensive, especially
the initial costs for CCUS.
The total
emissions of the aluminum industry, including mining through secondary
production include direct process emissions (15%), heat requirements (11%), indirect
emissions from power consumption (65%), and other sources (9%).
The
International Aluminum Institute (IAI) identified three pathways for the global
aluminum industry to meet 2050 emissions goals: electricity decarbonization by
using clean energy (solar, wind, water, nuclear) instead of fossil fuels (coal,
gas, oil), direct emissions reduction from the various industrial production
process steps, and recycling, and resource efficiency.
A March 2023
article in Light Metal Age attempted to quantify aluminum industry emissions according
to greenhouse gas protocols and classifications into Scope 1, Scope 2, and
Scope 3 emissions. Aluminum industry emissions can vary considerably by
facility. The authors explain:
“Scope 1 - These include all direct GHG
emissions occurring at the site of the aluminum smelter, which are dominated by
the CO2 and perfluorocarbon (PFC) emissions from the electrolysis cells in the
potrooms. Many smelters produce their own prebaked carbon anodes, while others
purchase the anodes from suppliers. It is not known what percentage of the
world’s smelters have their own carbon plant, but this determines whether these
emissions would belong to Scope 1 or 3.”
“Globally, 45% of the electric power used in primary
aluminum production is now purchased from suppliers, and 55% is self-generated.
When the electric power is self-generated from fossil sources, it is included
in the direct site emissions (Scope 1), as would the equivalent emissions
arising from fuel combustion of transport vehicles. In addition to the
electricity used for aluminum production, there are also significant CO2
equivalent emissions from fossil fuel energy used for the necessary ancillary
services at the smelter. These include AC power for operating facilities and
buildings like the casthouse, compressor house, gas treatment center, central
workshop, rectifier auxiliary power, etc.”
“Scope 2 – Indirect Electricity-Related Emissions:
These are categorized by indirect CO2 equivalent emissions from the purchased
electrical energy that is consumed in the smelter.”
“Scope 3 – Other Indirect Upstream Emissions: These
are associated with extraction and production of materials sourced from
entities separate from the smelter. Examples are those attributed to bauxite
ore mining and transport, extracting and delivering the alumina to the
electrolysis cells, and the linked emissions for the limestone and caustic soda
used in alumina production. Emissions associated with production of anode raw
materials (green and calcined petroleum coke and coal tar pitch) through to the
finished prebaked carbon anodes and bath materials (aluminum fluoride and
cryolite). The carbon-containing materials linked to production of cathode
materials (like cathode blocks, ramming paste, and silicon carbide sidewall
blocks) are also included here.”
Electricity Consumption Emissions
They report that in 2021 two-thirds of the electricity used
to produce aluminum came from fossil fuels, 57% from coal, and 10% from natural
gas. The electricity is generated for the electrolysis step in the process. Simply
providing electricity from low-carbon sources is the major decarbonization opportunity.
They only mention decarbonized electricity but replacing the coal-fired electricity
with natural gas-fired electricity would reduce emissions also, at a lower magnitude
but it could be done with lower upfront costs.
Alumina Production Emissions
Aluminum ore is
in the form of oxides, hydroxides, and aluminosilicates. The Bayer process is
the industry standard for alumina production. The Bayer process does not
directly produce CO2. Most emissions come from fossil fuel combustion, which supplies
heat energy to the process. The average emissions from alumina production have
been calculated at 2.7 t CO2e/t Al. Over 70% of those
emissions are from the thermal energy provided by fuel combustion. There are
several new technologies and fuel-switching strategies that can help alumina
refineries reduce emissions, as the Light Metal Age article notes:
“To decarbonize the emissions from alumina refineries new
low-carbon digestion and calcination technology will be required. This includes
technologies like fuel switching by converting boilers and calciners to use
liquid natural gas (LNG), mechanical vapor recompression (MVR), and by heat
recovery. Electric boilers are now proven technology for steam generation.”
Prebaked Carbon Anode Production Emissions
Production of
prebaked anodes and their raw materials emits about 0.5 t CO2e/t Al. or about 18.5%
of the emissions from alumina production. However, a more recent full-cycle
emissions analysis of all the steps in the carbon anode production process calculated
the emissions at 8.13 t CO2e/t Al, or about 30% of the emissions of alumina
production. Other estimates have suggested even higher emissions. The table
below shows the breakdown of emissions from each process.
Inert Anodes
The development
of inert anodes can mitigate some of the emissions from alumina production and
offer a great decarbonization solution for the industry. However, as the EU-JRC
report points out they are not yet commercially ready, and the cost will still
be hard to estimate. Thus, the ability of inert anodes to reduce emissions is
dependent on technology rollout and cost. They could potentially eliminate most
smelter emissions and increase smelter efficiency by 25%.
Direct Electrification
Some high heat
processes can be electrified but more low and medium heat processes can be
electrified. Electromagnetic induction technologies can produce heat through
electricity. Other methods for electrifying heat include dielectric heating
technologies, resistive heating technologies, electric arc, infrared radiation,
electron beam, and plasma heating. These technologies are used in several
industries. In aluminum mining and processing the tasks of crushing and
conveying ore can be electrified. Electricity can be used in digestion,
cooling, calcination, and casting. Whether electrification is cost-effective is
dependent on electricity prices and that can be a hurdle in places where costs
are high.
Process Improvements
There are other
process improvements where efficiency can be improved. They include waste-heat
recovery, low-temperature digestion, fluidized bed calciners, electric boilers
for low-med heat processes, mechanical vapor recompression, carbothermic
reduction of alumina, lower electrolysis temperature, and new smelter
technologies.
Aluminum Production Emissions: Mainly Powering Electrolysis
Emissions from
electrolysis make up the bulk of aluminum production emissions with prebaked
carbon anodes. There are two main strategies to reduce them: lowering net anode
carbon consumption and lowering the frequency and duration of the anode effect,
which causes perfluorocarbon (PFC) emissions. Net anode consumption emissions average
about 1.5 t CO2e/t Al while PFC emissions
have come down to about 0.19 t CO2e/t Al, or about one-eighth of the anode
consumption emissions, There are now Best Available Technologies (BATs) that can
bring anode consumption PFC emissions down by 90% from the global average to 0.02
t CO2e/t Al, just 1% of the anode consumption emissions. However, they suggest
that the total emissions for the aluminum production process are not likely to
come down more than 10-15%.
Total Emissions
The table
below shows the cradle-to-grave emissions of each process in the production of
aluminum according to two analyses. As can be seen, smelting and alumina
production result in over 75% of the emissions. The anode consumption emissions
are only about 6% of the total but if you add the raw materials needed such as
petroleum coke and coal tar pitch, the emissions are as much as 20% of the
total. The 0.6 number in the ‘Other’ category in the Hydro column on the chart may
be those emissions, which the Alouette column may have attributed to the anode emissions,
accounting for the discrepancy in numbers.
After some
major producers built hydroelectric-powered facilities, there emerged a global benchmark
for low-carbon aluminum production of 4.0 t CO2e/t Al. Emissions measurement
remains a challenge as not all facilities are fully standardized in terms of how they
attribute process emissions, as demonstrated by the anode/other discrepancy in
the chart. The Carbon Trust is involved in developing standard methodologies for
aluminum industry process emissions and full-cycle emissions calculations. In
2017 the Aluminum Stewardship Initiative (ASI) developed the ASI Performance Standard.
It is still being tweaked to be more consistent and thorough.
The chart
below shows the current emissions with BATs and the future targets for the
three main processes, alumina refining, electrolysis, and anode production.
Aluminum Recycling
How emissions
intense recycling aluminum, also known as secondary aluminum production, is depends
on the source of the scrap, mainly whether pre-consumer or post-consumer. The
emissions come from collection, transport, sorting, and remelting. Emissions average about 0.5 -0.6 t CO2e/t Al. Thus, recycling aluminum can be a good strategy for
reducing emissions where applicable. The going prices of and availability of scrap
aluminum, the cost to recycle, and the price of primary aluminum production are
all factors in the feasibility of recycling aluminum.
Strategies for Emissions Reduction
Strategies to reduce
emissions include Since most emissions come from power consumption, powering
with renewable energy and CCS/ICM are two of the most important emerging
solutions.
The Light Metal Age article concludes:
“While it appears that there is a clear path to
decarbonization for the aluminum industry, reaching the goal of net-zero
emissions by 2050 will require significant changes to the upstream alumina and
anode supply chains, the aluminum electrolysis step, and also downstream
recycling. Until the alumina and the anodes can arrive to the smelter as GHG
emissions free and until electrolysis does not emit any significant amount of
CO2, no one can claim to be producing and selling truly emissions-free (green)
aluminum. There are real technical challenges that need to be overcome to make
the aluminum fully decarbonized, and it will be very expensive.”
The graphs below are from the EU-JRC paper and give information about global aluminum production, use of power inputs, and resulting emissions. It can be seen that China dominates aluminum production and also that China mainly uses coal for aluminum production. The last graph shows that even with low combustion flue CO2 concentrations, CCUS is the technology that offers the highest decarbonization potential, with its share of aluminum industry decarbonization potential at 70%.
The section below
lists the decarbonization options for direct or process emissions.
Hydrogen is listed as a solution. For hydrogen, costs to
produce are an important issue and hurdle but one emerging solution is co-firing or
co-feeding with hydrogen blended into natural gas.
Carbon Capture, Utilization, and Storage (CCUS)
Carbon capture,
utilization, and storage (CCUS) is an emerging solution for the aluminum
industry but there are significant challenges. The biggest challenges include 1)
carbon capture since aluminum flue emissions have low CO2 concentrations which
makes capture more expensive; and 2) high upfront capital costs. The high costs
can be mitigated somewhat through the H2/CCS hub concept where CO2 pipelines, compression,
and storage are shared among several emitters.
Aluminum
smelter combustion emissions have a concentration of just 1.5% CO2 compared to 30%
CO2 for a cement production facility, 13.5% for a coal power plant, and 4-5% for
a natural gas power plant. That is a challenge both for effectiveness and cost
per emissions reduction but still is doable. Thus, other industries like cement
and power production can benefit much more from CCUS than the aluminum
industry. For the aluminum industry, a combination approach is more feasible. Inert
anodes, electrification, using clean electricity, hydrogen, and CCUS are the
technologies to be combined.
Conclusions
The following
sums up the conclusions of the EC-JRC report
References:
Mitigating
aluminum industry emissions: Industrial carbon management could reduce costs. Science
X staff. TechXplore. July 1, 2024. Mitigating
aluminum industry emissions: Industrial carbon management could reduce costs
(msn.com)
Decarbonisation
Options for the Aluminium Industry. European Commision. JRC Publications
Repository. JRC
Publications Repository - Decarbonisation Options for the Aluminium Industry
(europa.eu)
Decarbonizing
the Primary Aluminum Industry: Opportunities and Challenges. Light Metal Age. Halvor
Kvande, Gudrun Saevarsdottir, and Barry Welch. Light Metal Age. March 20, 2023. Decarbonizing
the Primary Aluminum Industry - Light Metal Age Magazine
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