Perform a series of process simulation case studies of various coal fired power cycles for a power industry electric utilities research group.
POWER CYCLE CONFIGURATION SIMULATION CASE STUDIES
Process Engineering Associates, LLC (PROCESS) was contracted by the client, a research group active in the power industry, to perform a series of process flowsheet computer simulations. The process flowsheets consisted of various gasifier-based electrical power generating cycle configurations. Generally, the configurations centered around coal-fired ternary power cycles with and without CO2 capture (MDEA and Selexol). The primary purpose of developing the computer simulations was to develop a heat and material balance for each of the configurations. Results of the heat and material balances were then used to calculate thermal conversion efficiencies and identify power cycle arrangements with the most promising prospect of high efficiency.
The electrical power generating components of the ternary power cycles evaluated in this work consist of three elements: Solid Oxide Fuel Cell (SOFC), a Combustion Expander Set (compressor and expander, may or may not be on the same shaft; also, known as a Brayton Cycle), and, a Steam Turbine Generator (STG; also, known as a Rankine Cycle). Coal is utilized as the power plant fuel stream. Coal is gasified at high temperature in the presence of oxygen to generate a synthesis gas (rich in H2 and CO) that can be manipulated and utilized by the power generating components. The SOFC directly converts H2 to electrical power via oxygen ion exchange that converts H2 to H2O. The Brayton Cycle fires, or “combusts”, syngas and expands the resulting high temperature flue gas in a turbine expander. The STG recovers waste heat from the gasification process and Brayton cycle by generating high pressure steam that is expanded in a steam turbine. The overall configuration is referred to as an Integrated Gasification Fuel Cell (IGFC) cycle.
The commercially-available flowsheet simulator Aspen Plus was used for modeling the relevant chemistry and thermodynamics of the power cycle processes. The input and output of model data and related information to the Aspen model was assisted through the use of spreadsheets organized in an Excel workbook file. Each power cycle configuration required the use of two or three separate Aspen models due to the large amount of unit operations requiring description. Visual Basic was used to administer starting and stopping of the multiple Aspen models used in each simulation, perform various calculations to assist with the Aspen models and provide additional input and output data to Excel spreadsheets, schedule transfer of input/output data to/from Aspen models and Excel, and drive the Aspen model flowsheet convergence. Heat and material balance tables, utility requirements, and thermal conversion efficiencies were all generated directly within the Excel workbooks. This hierarchy of software tools made evaluating multiple cases easier and transparent by quickly formatting input data and model results.
In completing this work, PROCESS performed the following tasks:
- Generated a “design basis” summarizing the primary input assumptions for each cycle configuration. In this manner, the client and other interested parties had the same understanding as PROCESS regarding what was being modeled for each configuration. This approach minimized misunderstandings and requirements for rework.
- Modeled each configuration using the three-tiered modeling platform: Aspen for chemistry and thermodynamics, Excel for formatting input and output data, and Visual Basic for administering the process.
- Generated (1) heat and material balance tables that contained stream information such as composition, flowrate, temperature, and pressure, (2) utility summary sheet showing the auxiliary electrical requirements of all pumps, compressors, and related equipment, and (3) thermal conversion summary sheet detailing net plant conversion efficiency based on higher heating value (HHV) input of coal.
- Developed Process Flow Diagrams (PFDs) with streams identified for easy reference to the material balance table. PFDs contained information for all major equipment or package processes.
- Interpreted and evaluated model results and developed conclusions and recommendations that allowed client to make real-time decisions on directing and managing the work effort for maximum advantage.
- Generated a summary report for each case evaluated and a final report detailing each of the cases.
In all, a total of 21 cycle configurations were modeled and reported on. The work clearly showed that maximizing syngas conversion in the SOFC, at the expense of the Brayton Cycle, resulted in highest net plant thermal efficiency. This is due to the SOFC possessing the highest conversion efficiency of the three conversion processes.
The work also showed the practicality of utilizing multiple software platforms to conduct a series of thermodynamic computer simulations where side-by-side comparisons are required to make effective decisions. While taking longer to initially configure, more time is saved during the execution process.
- Electric Power Utilities Research
- Process simulation
- Visual basic programming interfacing
- Power cycle process evaluation.