Harshit Srivastava | MEL Candidate | Dec 17, 2021.
Mentor: Joseph Broda, Fortis BC;
Abstract
Fortis BC announced its “Clean Growth Pathway to 2050”goals in 2018. One of the four pillars which will contribute towards achieving these goals is through blending of Renewable Natural Gas (RNG) into the Natural Gas grid. The production of biomethane, also called RNG, requires upgrading of biogas to increase the methane composition. The other gases present in the biogas mainly are carbon-di-oxide and some amount of nitrogen, oxygen,
hydrogen sulphide and moisture. In order to achieve desired levels of RNG, CO2 along with other gases needs to be removed with the help of gas upgrading systems like water scrubbing, chemical scrubbing-amine, membrane separation, pressure swing adsorption (PSA) and vacuum swing adsorption (VSA). The aim of this capstone project is to study the existing biogas upgrading systems at Fortis BC facilities which is the PSA & VSA system and suggest technological and process improvements for the system. Vacuum swing adsorption (VSA) is a technology similar to pressure swing adsorption (PSA) that operates at low pressure unlike high pressure of PSA. Currently, VSA technology is being used at two biogas upgrading facilities, however, it has not met the expected performance at either facilities. Furthermore, once the CO2 is cleaned from the biogas it is released in the atmosphere, thus the other objective of the project is to do a literature review on Carbon Capture and Utilization (CCU) and find a feasible solution for CO2 sequestration and utilisation in British Columbia.
Introduction
Vacuum Swing Adsorption is a technology used for cleaning various types of gases. The composition and quality of gas which needs to be cleaned defines the use of desiccant needed for adsorption. In the case of FortisBC, VSA is being used for cleaning of methane from Biogas/ Landfill Gas by removing CO2, N2 and O2.
In case of FortisBC the primary system for cleaning of Biogas is the Pressure Swing Adsorption (PSA) system, since there is a lot methane slippage in the primary system a recovery system is installed which in our case is VSA. The exhaust gas of the PSA system contains CO2, N2 and O2 as main gas and CH4 as partial gas. The gas passes the molecular sieves where CH4 passes through and the remaining gases get adsorbed. Once the desiccants are saturated, the unwanted gases is vacuumed out using a vacuum pump.
Methodology
To find out the amount of methane loss, input, output and exhaust quality data was analyzed. Other data which was studied included pressure, temperature and flowrate of the system. Time cycle of pressurization and vacuum was studied. Type, quality and quantity of the molecular sieves was analyzed.
In order to understand the biogas cleaning system the P&ID of the PSA system and the Total Recovery System (TRS) which is the VSA was thoroughly studied. Literature Review and Market Analysis of CO2 was done for its utilization in BC.
Results and Discussion
Objective 1: The first objective of the project is to suggest improvements in the VSA system, estimate improvements in the CH4 recovery and suggest pressure control regime. After assessing the P&ID of the gas cleaning system the following was observed. The first section of the PSA consists of two section, SEC1 removes CO2 and SEC 2&3 removes N2/O2.The gas enters these sections in sequence and once the gas is cleaned, the CH4 composition is >96% which enters the grid. Now the exhaust gases from both the sections still has significant amount of CH4. Therefore, this needs to be recovered through a Total Recovery System (TRS) which is the VSA system. The off gas from SEC1 enters TRS B which is called the CO2 TRS and off gas from SEC2&3 enters TRS A which is called the N2/O2 TRS. In TRS A N2/O2 is adsorbed, whereas the CH4 passes through and stored in the recycle tank. Whereas in TRS B as per the installed system CH4 is adsorbed CO2 passes through. The problems is with TRS B as there is no desiccant available which can adsorb CH4 first and release CO2.
Objective 2: Presently, the removed CO2 although biogenic is vented into the atmosphere. Fortis would like to sequester it 100% but there are no geological reservoirs available in close proximity of the existing biogas facilities. Therefore, various feasible options like precipitated calcium carbonate (PCC) and calcium silicate (CS) were looked into which can be utilized in BC. Some of the application of PCC are in the paper making, filler material in plastics. Calcium silicate is used in Cement making and can replace pulp altogether in making paper as well.
Results of Objective 1
Methane composition in the exhaust gas is around 40 Nm3/hr as compared to 267 Nm3/hr in the landfill gas. This translates to 14.8% of methane loss, which is around 5 time higher than the usual VSA systems. Generally, methane loss or the percentage of methane in the exhaust gases should not be more than 3%. In order to prevent CH4 emissions into the atmosphere FortisBC has to use a thermal oxidizer which in turn ends up using more CH4. In addition to this, the CH4 loss also translates into revenue loss for FortisBC. Hence to make improvements in the recovery it is suggested to run the VSA system as the PSA system. Also, it is recommended to convert the system from batch process to continuous process. Lastly, the current time cycle of pressurization and vacuum should be changed from 3+3 to 5+5 minutes.
Results of Objective 2
Currently the CO2, although biogenic, is being vented into the atmosphere. 40% of the Landfill gas comprises of CO2, which translates to 180 Nm3/hr which in turn is equivalent to 7.6 tons/ day of CO2. We suggest sequestration of CO2 in the form of Precipitated Calcium Carbonate which will make the complete process from net carbon to zero to carbon negative
Conclusion
It is suggested that VSA section used for CO2 removal and methane recovery should be replicated as per the PSA section used for removing CO2. In the improved VSA system CO2 will be adsorbed and clean CH4 will pass through. Also, it is suggested that instead of instead of running the VSA as a batch system it should be converted to a continuous system which will be more efficient. The cycle of the pressurization and vacuum should be be increased from 3+3 to 5+5 minutes. For the CO2 utilization it suggested that products like Precipitated Calcium Carbonate and Calcium silicate should be looked into.
References
1.“Clean growth pathway to 2050”, FortisBC, February 2019.
2. Upgrading Biogas Low Pressure by Pressure by Vacuum Swing Adsorption, Arti Arya, Swapnil Divekar, et.al. , December 2014
Contact
Harshit Srivastava
University of British Columbia
Email: harsshit.ubc@gmail.com
Phone: 778-927-4011