Hydrogen Storage in Salt Caverns

Abstract

In this paper the suitability of compressed hydrogen gas storage in salt caverns is analysed. The presence of microbial sulfate reducing bacteria create a contamination risk of H2S inside the cavern. Key cavern parameters that influence the production of H2S are highlighted by a chemical model. The model uses empirical data provided by Vattenfall, in order to predict what would happen inside an actual cavern. By doing so, it can be understood what cavern conditions are suitable for the storage of hydrogen. An analysis is done on the above ground process of a salt cavern storage plant to determine what extra separation steps are required to reach ISO limitations for hydrogen gas. The research is done by answering the following research questions: What are key cavern conditions that influence the suitability of hydrogen storage in salt caverns and for what purpose can the storage plant be implemented? The sub-research objectives are formulated as follows: 1. What are the potential process risks when storing hydrogen in salt caverns? 2. Are there substantial risks of contamination with subsurface hydrogen storage? What are the defining variables that contribute to said contamination? 3. How do these impurities build up when the salt cavern is used? 4. Is the equipment currently used in the gas storage facility in Epe useable for hydrogen gas storage? What changes should be made to the current storage process? When analysing future utilisation demands of hydrogen, one of the primary applications is energy con- version by fuel cells. There are severe limitations set by the International Organization for Standardization on the maximum amount of H2S in hydrogen gas used by fuel cells. This paper uses a chemical model based on PHREEQC to predict the chemical reactions in the cavern. In order to get close to actual results, the model input is constructed following empirical data of an existing salt cavern. With the chemical model different cases can be constructed each highlighting an important cavern constraint. What are the positive and negative forces on the production of H2S in salt caverns and what could be theoretically done to prevent H2S contamination? Primary aspects that positively contribute to H2S production, when the cavern is modelled as a batch reactor, sorted by significance are: • Bacterial growth and reduction rate • Brine volume and sulfate concentration. • Brine pH and ionic strength. • Cavern pressure and temperature. • Fe2+ and Fe3+ concentration. The chemical model only predicts what will happen in the cavern when there is no gas coming in or out, like a batch reactor. For this reason a dynamic model is constructed which predicts H2S outflow in the gas when applying different demand-cases to the cavern. The model results showed that with applying a maximum use case, which fluctuates between the maximum pressure and minimum pressure, there is still a minimum H2S output that is higher then the allowed limits. A demand curve is simulated where the cavern is used to power a hydrogen gas-turbine to profit from seasonal energy price changes. In this demand curve the cavern produces significantly less and less often, giving time for the H2S to build up. In two years, the H2S production reaches levels above the allowable limits set for hydrogen gas turbines. A gas process facility is required to eliminate H2S contamination, in order to size such a facility it was modelled in Aspen Hysys. The model is validated by using data from literature. After analyses of resulting process equipment and gas streams, the absorber tower’s efficiency is most dependant on its temperature its pressure and the absorbent flow rate. The water concentration is above ISO levels when withdrawn from the cavern. So to follow these limitations, an additional dehydration step is required. In conclusion the process is capable of accurately separating the H2S to below ISO limits. The process works using 6% of the potential chemical energy of hydrogen. The process is unable to purify the water concentrations. As a result of this research some conclusions can be made. When using salt caverns for long term hy- drogen storage can be a significant risk of H2S contamination as a result of microbial sulfate reduction. For the reference case the H2S concentrations reach above the levels set for fuel cell use and concentrations will increase in significance when utilisation of the cavern is decreased. For fuel cell application, a separation pro- cess based on MDEA gas sweetening can be used to get the hydrogen up to the demanded H2S purity. But a more economical solution would involve extensive testing of the cavern soil for any microbial activity. If there are no sulfate reducing bacteria there will be no problem. Other pre-process steps could involve increasing the pH of the brine in the cavern to avoid H2S production. As well as increasing the iron concentration in the brine.