Cement’s Role In A Carbon-Neutral Future
By Jeffrey Rissman 20 January, 2021
Cement is a major contributor to climate change. Energy Innovation's Rissman shares how to make it carbon neutral
This article is a summary of the report “Cement’s Role In A Carbon-Neutral Future”, November 2018 by Jeffrey Rissman.
Cement is a constituent of concrete and the cement industry is a major contributor to climate change. The cement manufacturing process results in a large quantity of carbon dioxide (CO2) emissions: roughly 5.6% of the global total (as of 2015).
Cement manufacturing = ~ 5.6% of global CO2 emissions
In order to protect human society and keep global warming below 2-3°C, it will be necessary to reduce greenhouse gas (GHG) emissions to near-zero by 2050. Therefore, in order for cement to be a sustainable choice of building material for the 21st century and beyond, it is important to reduce its carbon intensity to near-zero levels as soon as is technically feasible.
Cement manufacturing emits CO2 through two types of activities:
- Energy-related emissions: 30-40% of emissions are from thermal fuels (predominantly coal) used to heat the cement kiln; and
- Process-related (“calcination”) emissions: 60-70% of the emissions are from the breakdown of limestone in a calcination reaction
After its manufacture, cement naturally sequesters CO2 from the atmosphere in a process called “carbonation”. The fact that concrete reacts with atmospheric carbon dioxide in the years the concrete is in use is well known to the cement industry, but this effect has been viewed negatively due to the risk of corroding the steel reinforcements inside reinforced concrete.
Cement naturally sequesters CO2 from the atmosphere but until developed further, need to focus on reducing production emissions
Globally, roughly a third of cement’s process emissions are re-absorbed within the first two years, and over the course of decades, this share rises to 48%. Consideration of this effect as a potential atmospheric carbon dioxide sink for climate change mitigation is relatively recent and has been omitted from national emissions inventories and global estimates.
Until this idea can be further developed, there are many opportunities to reduce GHG emissions from cement and concrete. We look at some below.
Techniques to reduce CO2 from cement
Various techniques exist to lower CO2 emissions from the cement industry, including: energy efficiency technologies, adopting lower-emissions fuels to heat the kiln, substituting other materials for clinker and improving concrete strength or longevity. Carbon capture technologies, including post-combustion and oxyfuel technologies, provide options to capture CO2 emissions that cannot otherwise be avoided. Novel technologies by private firms CarbonCure and Solidia offer additional approaches to reducing emissions from the cement industry. We look at some of these in more detail below.
- Energy efficiency – Various technologies can be used to increase the efficiency of heating materials in the precalciner and kiln to form clinker. This reduces thermal energy consumption and associated CO2 emissions. Today’s international best practice energy use is 3.0-3.4 GJ/ton clinker, while the theoretical minimum energy use is 1.85-2.80 GJ/ton clinker (varying with moisture content of the input materials);
- Fuel switching & electrification – Worldwide, 70% of cement thermal energy demand is met with coal. Oil and natural gas provide 24% of the thermal energy, and biomass and wastes contribute the remainder. The share of kilns using biomass or waste fuels can be increased. In the longer term, there may exist options for electrification of heat creation, such as induction or microwave heating;
- Improved concrete strength, longevity or building design – One way to reduce emissions from the cement industry is to reduce the quantity of material demanded. Essentially all cement is used in concrete, so there are two main approaches to reducing cement use: to design buildings and infrastructure to use less concrete, or to design them to last longer before they must be replaced. Using curved fabric molds to shape concrete can reduce concrete use by up to 40% relative to standard geometries with sharp angles and corners; and
- Capturing CO2 from waste gas streams – The breakdown of limestone in the precalciner produces a stream of CO2-rich waste gas, and combustion of fuel to heat the kiln also generates CO2. A variety of technology options for capturing these CO2 emissions exist or are in development. These technologies generally fall into two categories: post-combustion technologies and oxyfuel technologies.
Projections to 2050
The International Energy Agency and the Global Cement Sustainability Initiative have estimated the amount of cement that will be produced worldwide through 2050.
They provide detailed results of two emissions scenarios: a reference technology scenario (RTS) that assumes only modest improvements in CO2 emissions intensity of cement production, and a two-degree scenario (2DS) that assumes larger improvements, in line with cement’s contribution to global GHG abatement required to limit likely warming to 2°C by 2050. The 2DS features greater progress in energy efficiency and greater deployment of carbon capture and sequestration (CCS) technologies than the RTS.
We enhance the IEA’s scenarios in two ways.
IEA & GCSI have estimated 2 emissions scenarios…
…we have enhanced there models in 2 ways
First, we factor in CO2 sequestration from carbonation of cement through 2050, an effect omitted from IEA’s analysis. Second we replace the IEA’s process emissions intensities (which range from 0.34 to 0.24 tons of process CO2 per ton of cement) with a value from Xi et al. (0.5 tons of process CO2 per ton of cement), a change that helps improve the alignment of IEA’s projections with the historical record. Additionally, we add our own scenario, exploring the amount of CCS required to make cement carbon-neutral by 2050. This is the “Carbon Neutral in 2050 Scenario” (CNS). CO2 emissions, capture, and uptake from carbonation in these scenarios are shown in below.
All scenarios feature the same process emissions before capture (in gray) and uptake from carbonating cement (in green). Direct energy-related emissions (in red) are slightly lower in the 2DS than in the RTS, and we adopt the IEA’s 2DS direct energy-related emissions for the CNS. Emissions associated with the generation of purchased electricity are not included. The only important difference between the three scenarios is the amount of carbon captured and sequestered (in yellow), which is lowest in the RTS (83 Mt CO2 in 2050), intermediate in the 2DS (552 Mt CO2 in 2050), and highest in the CNS (1864 Mt CO2 in 2050).
Capturing 80% of cement’s process emissions by 2050 is sufficient to make cement carbon-neutral
Modeling finds that capturing 80% of cement’s process emissions (and none of the thermal emissions) by 2050 is sufficient to make cement carbon-neutral, as natural carbonation offsets the remaining emissions. If the thermal fuel supply were to be fully decarbonized by 2050, a process emissions capture rate of 53% achieves carbon-neutral cement. Higher capture rates than these would provide net negative CO2 emissions and the possibility that simply making concrete could reduce atmospheric CO2 concentrations.
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