“Burning Ice: Methane Hydrate”
By Kazuki Imasato
• Kazuki Imasato, Ph.D. student at Northwestern University, explained interesting features and future commercial potential of hydrates during the 13th Japanese Researchers Crossing in Chicago held on April 15. Imasato began his research on methane hydrate last year in Japan.
What is Hydrate?
• Hydrate is any compound containing water in the form of H₂O molecules. Also called clathrate hydrates, they are crystalline solids that are created when “guest substances” such as methane, carbon dioxide, etc., are “trapped” inside the “cages” of water molecules (host substance) at a low temperature or under high pressure (see diagram I). If the guest substance is methane (a type of natural gas), it forms methane hydrate. Also called “burning ice” or “fire ice,” methane hydrate has an appearance of ice and emits gas and burns when heated with fire.
• Recently, methane hydrate is the focus of attention by the Japanese media. Prime Minister Shinzo Abe touted methane hydrates as Japan’s new resources in his general policy speech.
• Methane hydrate naturally occurs, and significant deposits have been found in the ocean floors around the world, including the areas surrounding Japan. They can also be artificially formed by chilling at low temperature (-20°C) or applying pressure of 10 megapascals at room temperature.
• Hydrate was discovered in 1810 by a British chemist Humphry Davy quite accidentally. The story goes that he left in a Petri dish chlorine and water together on a cold evening, only to find crystallized chlorine hydrates formed in it in the following morning.
• In the 1930s, it came to researchers’ attention that the formation of methane hydrates during natural gas production operations often blocked pipelines. Studies of hydrates began during this period.
• Subsequently, researchers discovered that methane hydrates are naturally formed in the sediment of the ocean floor and that large deposits exist underwater across the globe (Diagram II). That led to a hope that they could be a new natural resource for a wide range of industrial and commercial purposes if they are extracted. Mining effort has begun in Japan, which has proven problematic, according to Imasato.
• One of the possible methods of recovering methane hydrates in the ocean floor is to melt them through depressurization and recover methane through a drilling pipe. The problem with this method is that the pipe is often blocked with sediments and hydrates formed during the process, and the commercially viable mining of methane hydrates is yet to be achieved. However, since deposits are found in places where there are no oil or natural gas fields, when the technical problems of mining are solved, methane hydrates can be our next great natural resource, Imasato said.
Characteristics of Hydrates
• The first characteristic of hydrates is that they have a high cage occupation rate of natural gas components. Generally, hydrates can contain more than 100 times as much gas components in them as their volume; in the case of methane hydrates, the rate is as many as 164 times. This makes hydrates a highly effective medium to store and transport gases.
• Secondly, hydrates require a high level of energy for their formation and decomposition, which could be good or bad for us. The good part is that the energy can be stored in the “ice” formation to be used for cooling systems. Also, the increased volume of hydrates when “melting” can be used for power generation with its expanded pressure.
• The third characteristic of hydrates is their “selectiveness” of guest substances such as carbon dioxide, methane, and nitrogen. Each of these substances gets trapped in the cages of water molecules at a different rate; some are taken into the water molecule cages more easily than others to form different types of hydrates. Utilizing this characteristic, components can be separated from mixed gases. For example, carbon dioxide contained in exhaust gas can be eliminated by “capturing” and storing it into hydrates (See Diagram III).
Carbon Capture and Storage Technology
• The CCS, or Carbon Capture and Storage, is a technology that enables capturing and storing of carbon dioxide contained in exhaust gas and flue gas by selectively trapping it in hydrates and storing them underwater or underground. Imasato says this method, also called the hydrate method, is becoming increasingly feasible for actual application.
• Because it requires energy to cool down exhaust and flue gases, the hydrate method can be costly. Researchers are currently working to find a way to reduce the cost by easing the required conditions to form hydrates. If you need to chill the gas at -20°C in order to form hydrates, you can add a substance called TBAB, which helps stabilize the water molecule cages, and that will make it possible to form hydrates at the room temperature of 17°C. This will reduce the cost and make the hydrate method much more accessible.
Natural Gas Transportation
• Normally, natural gas is chilled down to -162°C at
the gas field and transported in the form of LNG (liquefied natural gas).
This requires a chilling facility at the field, which can be too costly,
specifically for small-sized drilling operations, to meet the bottom line.
If natural gas is in the form of hydrate instead of liquid, the required
facility investment and transportation cost can be greatly reduced.
• With its strong power to disinfect, ozone is widely
used for cleaning and disinfecting in our everyday life. But it is difficult
to store or transport because it decomposes into oxygen very easily.
• Imasato said one of the most interesting aspects of the hydrates research is trying to come up with various ways to use them. Examples include power generation using the pressure from the energy emitted by melting hydrates and the difference between the ocean surface temperature and the deep-sea temperature. “The significance of this research lies in discovering new use value from an energy source that currently is not producing any value,” he co
Kazuki Imasato, Ph.D. student at Northwestern University
The image is borrowed from Imasato's presentation.