Storing Energy During Wastewater Treatment

synthesis
The core processes of the methanol synthesis, © FiW

Biogas has, for a long time, been known as a renewable energy source. It powers not just stationary systems, that is, CHP plants, but also means of transportation, albeit in purified form. Another versatile energy carrier is hydrogen, especially considering its deployment as zero-emission storage. It can be stored and transported without major technical issues. However, low energy density in the gaseous state and low energy conversion efficiency from liquid to gas are impediments that cannot be easily overcome. One alternative solution is the storing of hydrogen in methanol. This technique can create sustainable pathways for synthesis, most of all in combination with carbon dioxide. Methanol has the distinct advantage of being simple to store but having a relatively high energy density. It can likewise be used for almost any application. In a project called WaStraK NRW – Use of Hydrogen Technology in Wastewater Treatment, a pilot system was constructed to synthesize the compound.

The principal aim of WaStraK, established under the auspices of the state environment ministry of North Rhine-Westphalia, was to design and build a small-scale pilot system to demonstrate that biogas can be incorporated successfully into chemical synthesis.

How the system works

By using water vapor, pre-purified organic feed gas is converted inside a reformer into synthesis gas composed of hydrogen, carbon dioxide and carbon monoxide. It is then directed to a reactor, which synthesizes methanol with help from a catalyst. To provide the heat for reforming, part of the feed gas flow is mixed with ambient air and burned off.

The system was designed to be operated at a pressure of between 20 bars and 22 bars, which is notably below the industrial benchmark ranging from 50 bars to 100 bars. This reduction in pressure, however, facilitates the use of the device and its integration, for example, into biogas facilities.

The distinctive features of the method are a highly efficient heat transfer inside the system and the recovery of energy from wastewater treatment to maximize production output. A programmable logic controller makes it possible to change parameters during experiments.

What observation revealed

The project showed the successful technical implementation of methanol synthesis, including the recovery of non-combusted parts of the gas. The combination of heating and cooling especially allowed for exploiting the potential energy of educts, products and byproducts.

 

read more: October issue H2-international

 

Written by: Alejandra Lenis, Forschungsinstitut für Wasser- und Abfallwirtschaft an der RWTH Aachen (FiW) e.V., Aachen, Germany

Dr. Kristoffer Ooms, Dr. Henry Riße, Christoph Wöllgens, M.S.

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