80 Proceedings Fall 2024 Carbon dioxide: A tricky triple point Chemical of the Quarter by Cdr dAn VAlez Hazardous Materials Division U.S. Coast Guard Office of Design and Engineering Standards Chances are if you are reading this article, you are making carbon dioxide. Carbon dioxide is an endpoint for traditional combustion engines—true of nearly all maritime shipping today, including LNG engines— and aerobic metabolic processes from cows to humans. Carbon dioxide has been extensively studied for decades due to its ability to block the heat radiation escaping from the Earth. In other words, it is a leading greenhouse gas. In comparison, methane is a vastly more potent greenhouse gas due to its greater heat trapping potential, but carbon dioxide persists in the atmosphere several orders of magnitude longer than methane. Methane is also used as a fuel source, or a feed stock, to generate other chemical compounds, like hydrogen, while carbon dioxide—at a rate of billions of tons per year—is almost exclusively a waste product from industrial or transpor- tation power sources. Given the likely continued production of carbon diox- ide comparable to today’s rates for at least the near to medium term, there has been increasing interest in the possibility of carbon capture technologies to reduce the amount of carbon dioxide in the atmosphere. Broadly speaking, this approach can be divided into techniques that capture carbon directly from emission sources—the stack—or from the atmosphere. Setting aside some of the challenges related to the net energy balance of such sys- tems—not producing more carbon to remove it—there is now a real need for the Coast Guard to evaluate the maritime transportation of captured carbon dioxide. In fact, the Office of Design and Engineering Standards, in conjunction with the Department of Energy, has recently reviewed several grant proposals for transportation of liquified carbon dioxide from collection sources to injec- tion sites, which can include decommissioned petroleum wells. While the transportation of liquified carbon diox- ide is relatively straightforward, and indeed the International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, or IGC Code, provides for its carriage, this transport is not without some nuances that are worth mentioning. The issue lies in the triple point of carbon dioxide, which at a very high level is the temperature and pressure for which all three physical states (liquid, solid, gas) of carbon dioxide are possible. And carbon dioxide has a tricky triple point. If we think of our experience with cold dry ice, solid carbon dioxide, at normal sea-level pressure, when melt- ing, the carbon dioxide does not turn into a liquid and then a gas. At ambient pressure, the solid carbon diox- ide transforms directly into a gas—a process known as sublimation. Again, at a very high level, this example is meant to illustrate that it can be difficult to keep carbon dioxide in liquid form, especially for longer sea voyages. Most likely, designers will need to use a combination of pressure and refrigeration to ensure the cargo remains liquid, while simultaneously controlling the boil-off liquid carbon dioxide to a gas. Another recent nuance to carbon dioxide carriage is a proposal before the International Maritime Organization to consider carbon dioxide a toxic substance at levels below asphyxiation given studies which show toxicity at these levels. Carbon capture has received a lot of attention lately, and it is likely that the Coast Guard will be called on to safely regulate the transport of potentially millions of tons of liquified carbon dioxide for storage/disposal. About the author: CDR Daniel Velez is the chief of the Hazardous Materials Division, Office of Design and Engineering Standards. He holds degrees in chemi- cal, environmental and mechanical engineering, and a juris doctorate. References: IGC Code, International Maritime Organization. https://www.imo.org/en/ ourwork/safety/pages/igc-code.aspx
