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PuK - Process Technology & Components 2021

Leading article The

Leading article The sewage treatment plant as an energy centre Prof. Dr.-Ing. Eberhard Schlücker Green hydrogen is usually obtained from renewable electricity with the help of an electrolysis cell. An electrolysis cell generates an efficiency of 70– 80 %. This means that three different energy flows come from an electrolysis cell: hydrogen, oxygen and waste heat with a temperature of up to 850 °C, depending on the type (Fig. 1). In the case of such a high temperature, the heat generated should not be considered useless, but should be used. There are many ways to do this. It could be used for the distillation or extraction of natural substances or it could operate technical processes. But it can also be used for heating purposes. But how do you use oxygen? Medical oxygen is obvious. However, when switching to renewable energies, a lot of oxygen is generated. The question of whether this valuable by-product should also be used to burn substances is therefore entirely acceptable. If natural substances or other substances are burned with pure oxygen, there are only a few nitrogen oxides in the exhaust gas and relatively pure CO 2 is obtained, which can then be more easily obtained for methanation or methanol production. An example with which an almost perfect energetic synergy is possible is the sewage treatment plant. Especially when the hydrogen is also stored in the LOHC liquid and is therefore available as energy storage for times of energy shortage. The storage of the hydrogen generates a reaction temperature of up to 340 °C. The storage density is 2.1 kWh/kg and thus at least 5 times that of the best future batteries (Fig. 2). If the storage were operated without using the heat and without integrating it into a synergy, then the efficiency, i. e. from electricity to electricity, would only be approx. 31 %. But if we embed this process together with the electrolysis in a wastewater treatment plant, then we can achieve more than 100 % efficiency. The first stage of such a rather small sewage treatment plant is to store the hydrogen, send the oxygen to the treatment process and use the heat from the electrolysis and the reactor to dry the sewage sludge. With the remaining heat, we heat houses or distillate alcohol in the summer and use it to generate cooling. This would already be a synergy that would significantly increase efficiency. Depending on the design, we This is just one example of synergies in energy generation and use. Such synergies are possible in many ways and would help us to significantly increase the overall efficiency of many processes. Fig. 2: LOHC loaded and unloaded with the respective reaction parameters (patented) Fig. 1: The three energy flows from the electrolysis cell would be around 80 % efficiency. But please consider other energy consuming activities (e. g. cutting/transporting wood, crushing/transporting coal, etc...) when making such a statement, then the end balance would already be very positive in comparison. To make it clear what I mean by the end balance, I would like to remind you of all the things we do to make gasoline or diesel profitable. We extract the iron ore from the pits, manufacture iron, turn it into steel and then manufacture 10 PROCESS TECHNOLOGY & COMPONENTS 2021

Leading article tools, pipes and machines. Crude oil is pumped, transported, refined and then the end products gasoline and diesel are delivered to gas stations so that we can refuel our vehicles. If we add all this up, the average overall efficiency would be just 15 %. So a sobering result. We have to get better at this. The second expansion stage would be to burn the sewage sludge with pure oxygen. This takes place at relatively high temperatures, so that together with the electrolysis and hydrogenation heat, steam is effectively generated and converted back into electricity via the turbine and generator. It is possible that sewage sludge can be bought inexpensively from the neighbouring wastewater treatment plant, thus achieving an economical size for incineration and electricity generation. At the same time, the heat of combustion could be used to extract the hydrogen from the LOHC and convert it into electricity. So you could produce electricity completely independently of the grid, both by burning and by using hydrogen. The residual heat which certainly exists in the range below 100 °C thus still guarantees the district heating supply. The third stage would be to supplement the now larger system with a salt heat store. This can be heated up to 490 °C and store significant amounts of heat. We are installing a reactor in this salt storage facility that can both hydrogenate and dehydrate. To ensure that this works properly, we have developed a vacuum pump technology that significantly lowers the pressure during dehydration, is insensitive to droplets, and even filters them out and thus lowers the dehydration temperature to approx. 220 °C, while hydrogenation takes place at 340 °C. So you can dehydrate with the heat of hydrogenation. The salt storage facility could also serve as a buffer for turbine operation and thus ensure more uniform operation. The fourth stage would then be the recovery of raw materials from the sewage sludge. The process heat for this comes from the heat sources discussed above and could thus ensure a greater degree of raw material recovery. Above all, phosphorus should be mentioned here, which is already in short supply in industrialised countries. Fig. 3: Maximum expansion of a large sewage treatment plant with the purchase of sewage sludge for incineration and raw material recovery as well as the possible production of methane and methanol Figure 3 shows this maximum expansion, which of course should only be implemented in large sewage treatment plants and if the sewage sludge does not cost anything. Then we would come to efficiencies of over 100 %. In addition, if you burn with pure oxygen, you can also harvest relatively pure CO 2 , react it with hydrogen to form methane and, if necessary, even produce methanol from it. But as you can see in the gradual expansion, much could be achieved with significantly less. Now, of course, the question arises as to how the loaded LOHC can be used in addition to being used in the sewage treatment plant to generate electricity. Of course, in vehicles with a hydrogen combustion engine, the engine heat can also be used to release hydrogen, which the engine needs for operation. However, it is also possible to do the same with an SOFC fuel cell, because of its high waste heat temperature, in order to release the hydrogen that you need to operate the vehicle. Another opportunity is an LOHC gas station. There, with the LOHC, approx. 16 times more hydrogen fits into a tank without pressure than into a normal pressure tank usually loaded with 50 bar at filling stations. This tank is cheaper and, since the LOHC is nonflammable, means a significantly higher level of safety. There you could fill up with LOHC when the SOFC has found its way into the car. Until then, we will place a salt storage facility next to the tank and, if hydrogen is required, convey the LOHC through the reactor in the salt storage facility. In this way you get hydrogen gas immediately, which can be pumped into vehicles. And even here there is still a synergetic approach. With a new compressor, 95 % of the compression energy can be harvested from the compressor and used for another use (e. g. heating). Summary The examples shown here make it clear that a synergetic approach to energy issues offers a great sustainable opportunity to defuse the climate problem and at the same time to remain economical. A sewage treatment plant with energy storage, raw material recovery, heat and methane supply options (methanol) would be an efficient decentralised energy centre. At the same time, the LOHC offers additional options for energy storage and provision and efficient use and transport, as well as the option of long-term storage in large quantities, which would allow us to store energy from summer for winter. The Author: Prof. Dr.-Ing. Eberhard Schlücker, Friedrich-Alexander-Universität Erlangen-Nuremberg, Institute of Process Machinery and Systems, Engineering (IPAT), Erlangen, Germany PROCESS TECHNOLOGY & COMPONENTS 2021 11

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