Climate change: sediment deposition increases methane gas from reservoirs
Researchers from the Laboratory for Water and Environment (LWU) at the TH Köln, together with DB Sediments GmbH, have developed a prototype that extracts methane gas from reservoirs, where it is enriched via sediments and released into the atmosphere.
The research project "Methane Elimination from Reservoirs" (MELIST) is funded by the European Regional Development Fund (EFRE.NRW). The research team from TH Köln has developed interesting processes to contain the release of methane gas.
Methane responsible for global warming
Alongside carbon dioxide (CO2) and nitrogen, methane is one of the natural greenhouse gases responsible for climate change. Methane is 28 times more harmful than CO₂, but its lifetime in the atmosphere is significantly shorter at between nine and 15 years. In addition to natural sources such as swamps and forests, anthropogenic sources such as livestock farming, rice fields, and landfills are increasingly responsible for the sharp rise in methane levels in the atmosphere over the past twelve years.
The significance of methane emissions from reservoirs is currently being investigated by various research teams around the world, including the scientists at LWU: "Even the rather small Olsberg reservoir on the Ruhr, with a storage volume of 72,000 cubic meters, releases as many methane bubbles in one year as a passenger car does in one million kilometers of driving," says Yannick Dück, a doctoral student at TH Köln and coordinator of the MELIST research project.
Organic components of sediment deposits in reservoirs, such as leaves, are decomposed by bacteria, producing methane. Intensified agriculture or forest clearing increase the biomass in reservoirs. The methane dissolves in the water with dissolved oxygen to subsequently oxidize to CO₂ in the atmosphere.
High pressure vacuum cleaner for sediment deposits
The core of the prototype developed in the MELIST research project under the direction of Prof. Dr. Christian Jokiel is a high-pressure suction unit for sediment deposits and methane bubbles contained therein, which is installed on a floating platform. This platform sails continuously and largely automatically across a reservoir. In the process, the sediments are stirred up and absorbed together with the methane gas. This water-sediment-gas mixture is separated on the platform and the gas is extracted by a vacuum pump. Finally, the sediment is returned to the flowing water below the dam.
In addition to prototype development, MELIST also includes field tests on the Urfttalsperre and Olsberg reservoirs in North Rhine-Westphalia: The TH Köln team undertook continuous measurements of methane emission, investigated potential influencing factors and possible methane hot spots of the reservoirs in the period from November 2016 to March 2017. The emission measurements revealed that changes in the water level have an influence on the release of methane bubbles located in the sediment. If the water level falls, the water pressure decreases and methane bubbles can rise more easily. The result is relevant to emerging climatic changes: According to the study, prolonged dry periods, such as those experienced during the record-breaking heatwave summer of 2018, which cause water levels to drop, increase methane production. "Rising temperatures, as expected due to climate change, will favor methane production," says Yannick Dück.
In addition, sediment samples were taken from different reservoir depths in two reservoirs using a newly developed freeze-core method and examined in a computer tomograph. The method enables the collection of undisturbed sediment samples containing gas. In previous methods, samples are negatively affected by a decreasing water pressure and temperature change of the sample "The frozen sediment cores give us conclusions about sediment stratification and the depth at which methane bubbles can be found. This is important information for dissolving and recording sediment deposits: How deep do you need to erode the sediment to absorb as much of the gas as possible? How often does one need to travel the reservoir? And how much energy is needed to remove the sediment? The sediments are different in every reservoir," says Dück.
The next step is to optimize the continuous separation of sediment and gas. A follow-up project is planned for this purpose, in which researchers from the :metabolon teaching and research center are also involved. They will investigate whether and in what form the methane extracted from the reservoirs can be used for energy.
