What is our assessment?

Based on our analysis, improving district heating for industry by integrating low-carbon heat sources has the potential to significantly reduce the use of fossil fuels and the emissions they generate. However, the lack of data, combined with the complexity of such projects and the growing interest in alternative decarbonization pathways, makes this a potential solution to “Keep Watching.”

What is it?

District heating systems consist of a network of underground pipes that distribute heat to a large number of buildings, including industrial buildings. In the industrial sector, district heating is used by light industries and for processes such as drying, paper making, food processing, as well as space heating and even heat-driven chillers for refrigeration. Industry is well-suited to district heating because it typically has steady and predictable heat demand throughout the year. Current district heating systems rely heavily on coal and natural gas for heat generation, often as part of combined heat and power generation. Low-carbon alternatives for district heating can include electric heat pumps, solar thermal, deep geothermal, and even waste heat from other industries. 

Does it work?

Shifting district heating for industry from conventional heat sources to low-carbon heat sources will significantly reduce emissions. Our analysis for district heating use by commercial and residential buildings shows that significant emissions can be avoided by shifting to electric boilers, heat pumps, and the use of waste heat (see Improve District Heating: Buildings). Similar outcomes are likely possible for industrial district heating use, and emissions reductions will increase as more renewables are integrated into the electricity systems used to power electric boilers and heat pumps. 

Why are we excited?

District heating for industry currently produces significant emissions. According to the International Energy Agency (IEA), district heating for all applications accounted for 4% of global emissions in 2022, and roughly 40% of the heat energy from district heating was delivered to industry. China is a major adopter of district heating for industries, with the combustion of coal supplying much of that heat. The shift to renewable heat sources is likely to increase because both China and the EU have policies targeting the adoption of renewables in district heating. Because district heating systems serve multiple buildings, a single project to replace fossil fuels with renewables can have a large impact. Such projects also have the benefit of reducing local air pollution. 

Why are we concerned?

Although simple on paper, replacing fossil fuel systems with lower-carbon alternatives in district heating systems can be an extended undertaking involving many stakeholders and years of planning. Some low-carbon options may not be suitable for industrial processes that require higher temperatures than those needed for space heating. There is also a significant lack of publicly available data about how industry currently uses district heating and the opportunities and challenges involved in shifting to renewables. In the meantime, industrial heat pumps with higher temperature outputs (100–200°C) are increasingly available and could become a low-carbon competitor to the use of a conventional district heating system.

What is our assessment?

Based on our analysis, OAE could be a promising carbon removal technique, but it is not ready for large-scale deployment until the risks, costs, and effectiveness become clearer. We will “Keep Watching” this potential climate solution.

What is it?

OAE is the practice of adding alkalinity to seawater to increase the ocean’s ability to remove atmospheric CO₂. The addition of alkalinity through OAE mimics the natural process of weathering, or the physical and chemical breakdown of rocks. Rock weathering on land produces alkaline substances that eventually flow into the ocean through rivers and groundwater. This natural supply of alkalinity reduces ocean acidity, which affects the distribution of various carbon forms in the ocean. As alkalinity increases, CO₂ dissolved in seawater shifts toward more stable carbon forms, like bicarbonate and carbonate ions, that cannot exchange with air. This allows the ocean to remove more gaseous CO₂ from the atmosphere because the ocean and the atmosphere maintain a balance of CO₂ through gas movement at the sea surface. Most of the dissolved carbon in the ocean is bicarbonate and carbonate ions, which can persist in seawater for thousands of years. Under natural conditions, the ocean removes nearly 0.5 Gt of CO₂ annually. OAE generally relies on dissolving large amounts of ground-up rocks, either directly in the ocean or indirectly in water that is added to the ocean, to increase alkalinity and remove CO₂. This practice typically requires mining for alkaline rocks, though alkaline materials can also be sourced from waste by-products of other industries (e.g., steel slag, mine tailings) or commercially through human-made substances.

Does it work?

The science behind OAE is theoretically sound, and OAE is expected to result in durable storage over long time periods (>100 years). At scale, OAE could potentially remove over 1 Gt CO₂ /yr, but additional lab and field-based studies are needed to understand whether this approach is effective and safe. The majority of our understanding of OAE comes from models and laboratory experiments. However, when crushed minerals have been dispersed in field studies, the dissolution has not always occurred as expected. Several large-scale experimental trials are currently underway or planned, which will produce real-world data and inform monitoring and verification tactics needed to help refine and guide future implementation. These tests will also provide critical information on any ecological or community impacts. Various ways of implementing OAE are being developed, including ship-based dispersal, shoreline-based systems, and other approaches that leverage existing industrial waste streams or combine with other marine carbon dioxide removal (mCDR) techniques, such as electrochemical alkalinity generation.

Why are we excited?

OAE removes CO₂ from the atmosphere and stores it in the ocean as bicarbonate and carbonate ions, which are stable over long time periods. This means the CO₂ would be durably stored from the atmosphere for thousands of years. OAE could be scaled globally and can also mitigate local ocean acidification, a growing issue that threatens a range of marine ecosystems. Indeed, adding alkalinity to seawater has already been shown to mitigate ocean acidification in some coral reefs. Mitigating ocean acidification could also benefit fisheries and aquaculture, highlighting the potential for OAE to provide additional local benefits beyond carbon removal.

Why are we concerned?

Several technical, environmental, and social concerns surround OAE. The effectiveness could be limited by real-world conditions that either transport the alkaline materials away from the ocean’s surface before CO₂ can be absorbed or result in unexpected chemical reactions or biological uptake of the added alkalinity. Measuring and verifying the amount of CO₂ permanently stored using OAE is also challenging and will rely on a combination of field data and complex numerical models, which will require significant effort to collect and develop. Beyond these technical challenges, OAE poses potential environmental risks on land and in the ocean. On land, OAE could require an expansion of mining that rivals the cement industry, which could have negative environmental impacts on human and ecosystem health. In the ocean, increased alkalinity and the potential release of metals from the source rocks could negatively affect some marine life, but our understanding of the effects on individual species and food webs is limited. OAE could also interfere with existing ocean uses (e.g., fisheries, recreation) in some places and could have other unintended consequences as well. For instance, research suggests that OAE reduces natural alkalinity production in some ocean areas. In addition, OAE faces several social challenges. To be successful, mCDR approaches, like OAE, will require rapid, meaningful, and just community engagement. Public concerns about OAE have already led to a pilot project cancellation, highlighting the importance of public perception for OAE feasibility. It is also unclear if OAE can be scaled globally at reasonable costs, with current estimates highly variable but generally over US$100/t CO₂. Lastly, acquiring and dispersing sufficient alkaline materials could be challenging at scale, particularly because some materials are currently energy-intensive to source, transport, and/or produce.