(31f) A Method for Monitoring Salt Deposition in a Salt Separator of a Hydrothermal Biomass Gasification Process

Authors: 
Viereck, S., Paul Scherrer Institute
Jovanovic, Z. R., ETH Zurich
Haselbacher, A., ETH Zurich
Vogel, F., Paul Scherrer Institute
Steinfeld, A., ETH Zurich

Powering hydrothermal gasification (HTG) with highly efficient concentrated solar power (CSP) is an approach to energetically upgrading biomass containing large amounts of water (~ 80%-wt.) into methane or hydrogen. An efficient conversion of biomass to fuel is achieved because water does not evaporate under hydrothermal gasification conditions considered in this study (p = 300 bar and T = 400 - 450°C), hence an energy-intensive drying of the feedstock is avoided (Peterson et al., 2008a; Vogel, 2009). In addition, using CSP to supply the required process heat increases the yield of the HTG process because the need to combust part of the produced fuel is eliminated.

The SolarHTG process combines a solar concentrating system, a thermal storage, and a hydrothermal gasification plant. The latter comprises two main unit operations: (a) superheating/salt-separation and (b) biomass gasification. A recent process analysis by Mian & Ensinas (2013) has shown that the superheating/salt separation step is the optimal integration point for solar energy. In this step, the compressed feedstock is heated to the desired gasification temperature, bringing water to a supercritical state. As water loses its polar solvation properties in this state, salts present in the feedstock precipitate and therefore need to be separated in a controlled way to prevent blocking of process equipment and poisoning of the gasification catalyst downstream.

The most important parameter controlling salt separation is the temperature profile in the superheater and salt separator. This temperature profile is dependent on the geometry of the salt separator, the flow field in it, and the strategy implemented to heat it. The last factor is particularly challenging as the options for transferring the stored solar energy from a heat transfer fluid are rather limited. In addition, due to a complex interaction of these factors, optimizing the salt separator design by trial-and-error is cumbersome and costly. To understand the effect of these factors on salt separation and develop the capability to ultimately predict the performance of a particular separator design, we combine computational fluid dynamics (CFD) simulations and experiments. By simulating the temperature and velocity field of water in the salt separator and  the trajectories of salt particles, we were able to identify possible locations for salt deposition. To validate these findings, however, an experimental method is required to online indicate where salts deposit. For this purpose, previous researchers used neutron radiography (Peterson et al., 2009, 2008b), which provides detailed information about the salt deposition but requires a neutron source and special materials with low neutron-attenuation that render it impractical and expensive for prolonged experiments.

We have extended the approach of Rogak and Teshima (1999) to develop a cheap and simple method to detect the salt deposition in a lab-scale separator represented by a vertically aligned pressure vessel. The salt solution is fed from the top through a dip-tube. Separated salts are removed at the bottom as concentrated salt brine while the salt-free stream exits at the top of the vessel. The vessel is heated through the outer wall via several electrical cylinder heating bands. Each heater is equipped with a thermocouple that measures the temperature of the outer vessel wall in contact with the heating element. Deposited salts introduce an additional heat transfer resistance at the inner wall of the vessel. If the heaters are operated at constant power input, the salt deposits will induce a rise in temperature of the outer wall. Therefore, the temporal change of the outer wall temperatures serves as an indicator for the salt deposition. We have applied the method to different feed salt concentrations and temperature profiles in the separator using aqueous sodium sulphate solutions as model systems; organics were neglected for simplicity.

To validate the method, we have visualised salt depositions in the separator using an endoscopic camera. Comparing the location of major salt depositions based on measurement and visual inspection, we conclude that the presented method is suitable for detecting salt deposition in the separator. With the information obtained from these experiments, we will validate and further develop our simulation tools. These tools will be helpful to gain insight in the salt-separation process and guide design, scale-up, and selection of heat exchanger configurations for heating the salt separator with solar energy. The presented method can also be used to monitor salt separators during operation and schedule purging of the salt separator once a certain amount of salt has accumulated in the vessel. 

References

Mian, A., Ensinas, A.A., 2013. Optimal Design of Solar Assisted Hydrothermal Gasification for Microalgae to Synthetic Natural Gas Conversion. Chem. Eng. Trans. 35, 1009–1014.

Peterson, A.A., Vogel, F., Lachance, R.P., Fröling, M., Antal, Jr., M.J., Tester, J.W., 2008a. Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies. Energy Environ. Sci. 1, 32 –65.

Peterson, A.A., Vontobel, P., Vogel, F., Tester, J.W., 2008b. In situ visualization of the performance of a supercritical-water salt separator using neutron radiography. J. Supercrit. Fluids 43, 490–499.

Peterson, A.A., Vontobel, P., Vogel, F., Tester, J.W., 2009. Normal-phase dynamic imaging of supercritical-water salt precipitation using neutron radiography. J. Supercrit. Fluids 49, 71–78.

Rogak, S.N., Teshima, P., 1999. Deposition of sodium sulfate in a heated flow of supercritical water. AIChE J. 45, 240–247.

Vogel, F., 2009. Catalytic Conversion of High Moisture Biomass to Synthetic Natural Gas in Supercritical Water. Handb. Green Chem. 2.