Zero-carbon heating and concrete production

Today the New York Times discusses a project in New York City in which carbon dioxide is captured from a gas boiler used for heating a building, then liquified and shipped to a concrete factory where it is injected into a concrete mix to bind it into concrete blocks as solid calcium carbonate instead of going into the atmosphere.

“It creates this circular economy,” said Jeff Hansen, vice president of architectural sales and marketing at Glenwood Mason. “We’re taking carbon dioxide from a building in Manhattan, turning it into a block in Brooklyn and then sending that block out to build more structures in the city.”

While the technology described has some use, it doesn’t scale for the purposes described.

First of all, there is no place for fossil gas boilers for heating and hot water in a zero carbon economy. For one, carbon capture does not capture 100 percent of the carbon dioxide in the flue gas. Typically, only about as much as 70-80 percent are separated out while the remainder still escapes into the atmosphere. Carbon capture is costly and consumes significant amounts of energy. It works best at large sites such as cement kilns where the retrieved CO2 can be processed in a central location rather than at millions of dispersed locations where it would have to be fed into a pipeline network or transported by vehicle to take it to a central processing site.

As many of the reader comments below the article point out, electric heat pumps run on green electricity are the most viable way of heating buildings without carbon emissions. Heat pumps are like running a refrigerator in reverse, making heat flow from the cold side to the warm side. It’s mature technology, already manufactured at scale and it goes hand in hand with the decarbonization of the power sector. Regardless of how green or brown your grid is now, you can start installing heat pumps today and gradually switch the power generation from fossil fuels to wind and solar. It’s actually very efficient: 100 kWh used in a heat pump will draw about 300 kWh of heat from the environment to heat the building.

Heat pumps can be combined with geothermal, for example to draw heat from the cool ground instead of using icy winter air as a heat source (the smaller the temperature difference between the cold side and the warm side, the more efficient the process). The ground several meters below the surface stays close to the average annual temperature at that particular location, which for example in New York City is about 13 deg C. One benefit is that this also works in reverse: The same equipment can be used for energy efficient cooling in the summer. It takes a lot less electricity to cool your home using 15 deg C ground instead of 35 deg C outdoor air as a heat sink.

Many cities are exploring deep geothermal wells for district heating. Away from volcanic sites or tectonic plate boundaries the ground temperature rises by about 25 deg C for every 1000 m of additional depth so by drilling wells deep enough and pushing water through them, hot water can be brought to the surface. This works best where there are deep aquifers that can be tapped.

Back to the carbon footprint of concrete: The reason that concrete slurry can absorb and bind large amounts of CO2 when it hardens is that it is highly alkaline because of its high calcium oxide contents. When cement is produced in a cement kiln, limestone (calcium carbonate) is heated with other minerals to very high temperatures and it releases CO2, turning into alkaline calcium oxide. Fixating CO2 during the curing of concrete only reverses this process. This begs the question: Why do they want to truck CO2 from buildings all over the city instead of reusing the CO2 released when the cement for the concrete is made in the first place? That would be truly a “circular economy”. However, it would also highlight the carbon footprint of cement production. I can see why someone in the cement or concrete business would rather prefer you to think about the CO2 output of some other part of the economy for which they supposedly can then provide a solution when actually their industry is part of the problem. Worldwide cement production released 1.7 billion tons of CO2 into the atmosphere in 2021.

There are some relatively easy to decarbonize sectors of the economy. For example, trains can run on green electricity. EVs are only a little more difficult, requiring battery production at scale and a dense charging network. Next in line, steelmaking and fertilizer production can use green hydrogen, made from water and green electricity. Some of the most difficult to tackle carbon sources are cement production, airplanes and ocean shipping.

Cement is difficult because CO2 is released not only from fuel burnt as a heat source (which could be replaced by electricity) but also chemically from the carbonate minerals. Airplanes and ships are difficult because of the vast distances covered that make batteries non-viable. There are some solutions for planes and ships, such as “e-fuels” (e.g. ammonia or methanol made with green electricity) but these will be expensive. For cement we will need to capture and store CO2 underground, such as in depleted gas wells. But first of all, we will need to price CO2 releases so that price mechanisms in the market lead to an efficient reduction in the consumption of cement and of ocean shipping and air travel. The smaller the volume left in these areas, the easier it can be tackled technologically. It won’t be easy.

While we develop the technology to take care of the final, most difficult 10 percent of CO2 output, let us first take care of the easiest 50 percent, then the next 40 percent. For power generation this means wind farms onshore and off-shore, utility scale photovoltaic, long distance power interconnect between regional grids via HVDC lines, battery storage for daily power fluctuations, etc. For power usage it means electrical vehicles, domestic heat pumps, etc. All of these we can already do now. We need to use technology that already works and deploy it at scale. Recycling CO2 in concrete plants will not clean up domestic heating and it can at best ameliorate but solve the CO2 problem of cement.

We must not let ourselves be distracted by greenwashing scenarios designed to protect old industries and their vested interests.


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