At one point or another, every chemical engineer has had to awkwardly explain to someone exactly what a chemical engineer does. Too frequently, the root of many people’s confusion is the incorrect belief that chemical engineering is alienated from how the average person lives their life. The truth is that the work of chemical engineers is deeply integrated into nearly all levels of society, from transportation fuels to the production and shipping of consumer goods to the way we heat our homes and businesses.
The central role chemical engineers play in urban heating speaks to how integral chemical engineers are to community organization in general. As heating networks decarbonize, the technologies that chemical engineers design and implement will change. Certain communities are implementing new sustainable heating technologies, like waste-to-energy district heating, while other potential sustainable heating solutions — like green hydrogen — are still in development. Beyond the technology, the type of heating system compatible with one kind of community could be entirely incompatible with another.
District heating
This style of heating system has immense potential, but it has considerable caveats. In a district heating network, a central energy source heats a process fluid that then circulates through homes and businesses. Heat may be supplied by a variety of sources; for example, in the U.S., most district heating systems are powered by natural gas. Still, some district heating systems around the world have transitioned to more sustainable energy sources.
The Copenhill, also known as Amager Bakke, is Copenhagen’s ground-breaking waste-to-energy cogeneration facility. The Copenhill burns municipal waste to generate electricity and uses the waste heat from this process to generate steam and hot water, which circulates to 150,000 surrounding homes (1). The complex’s advanced filter system allows it to release only heated steam and a fraction of the carbon a similarly sized natural gas system would emit. As an added bonus, the power plant doubles as an urban recreation center, featuring a ski hill and climbing wall.
The problem with district heating is its limited reach. The lengths of piping necessary to connect homes and businesses to a central heat source becomes cost prohibitive when houses are too far away or when communities are not sufficiently dense. Additionally, heat loss is inevitable as process fluid moves through the system. For most of the U.S.’s existing infrastructure, other solutions will need to be explored.
Green hydrogen
Where district heating has the potential to reduce carbon intensity at the cost of significant infrastructural investment, hydrogen-based heating has the potential to reduce carbon intensity with limited infrastructural renovations, but it requires many technical roadblocks to be overcome. On the positive side, hydrogen burns without releasing carbon, and, since it is a gas at room temperature, it behaves similarly to natural gas. This has led to the hope that existing natural gas infrastructure could be used to transport hydrogen. This isn’t unfounded, since natural gas streams can include up to 15% hydrogen with few modifications (2). Still, at higher concentrations, hydrogen is prone to leaking and embrittling steel pipes and welds. Fears concerning the loss-of-containment of hydrogen continue to surround the technology. One proposed method has been to slowly replace existing natural gas pipes with fiber-reinforced polymer (FRP) pipelines, which are cheaper than steel pipes and require less welding.
Although much of the technology needed to produce energy or heat from hydrogen exists, the majority of hydrogen produced today comes from fossil fuels. Green hydrogen — i.e., hydrogen produced by renewable energy through electrolysis — is not as cost-effective or efficient to produce as hydrogen reformed from natural gas. For hydrogen-based heating systems to be sustainably implemented, green hydrogen production must be widely deployed, otherwise the technology risks being nothing more than a roundabout continuation of fossil fuel consumption.
Beyond the science
The science of urban heating goes far beyond what could ever be described in a short article. Still, focusing on individual cars and single-family homes can often miss the point. Single-family homes are the least energy-efficient style of housing, as they don’t share walls with any other structure. Compared to personal vehicles, public transportation in the form of heavy rail, light rail, and buses reduces carbon emissions per commuter mile by 76%, 62%, and 33%, respectively (3).
Robust public transit and higher-density affordable housing are inevitable parts of urban decarbonization because they decrease the carbon footprint of entire communities. Although the power to initiate these solutions often lies with city governments, chemical engineers will play a growing role in their implementation as more communities get serious about sustainability.
This article originally appeared in the Emerging Voices column in the July 2023 issue of CEP. Members have access online to complete issues, including a vast, searchable archive of back-issues found at www.aiche.org/cep.