(Invited) Design Consideration of an IGFC System with CO2 Capture

Abstract

Due to vast coal reserve and its lower cost, integrated Gasification Combined Cycle (IGCC) and Integrated Gasification Fuel Cell (IGFC) power generation system are being developed to use coal effectively and reduce air pollution. Compared to IGCC, the advantage of IGFC is higher efficiency and lower overall cost, especially pressurized system. An IGFC system is being developed using syngas as fuel with CO2 capture at National Institute of Clean Low-Carbon Energy (NICE). In the system, the high temperature solid oxide fuel cell (SOFC) stacks are used to convert fuel to electricity. The system design principles, syngas composition and SOFC stack performance effect on system efficiency, and the system cost will be discussed, estimated, and analyzed based on the model established. The method of CO2 capture will also be discussed and recommended based on experimental data. There is variety of impurities in syngas, such as sulfur, PH3, Siloxane, chlorine, Arsine, etc, which may poison SOFC anode and significantly affect system performance. Syngas cleaning is needed before powering the stack using syngas. Besides, there is significant amount of CO2 in initial syngas which will dilute flammable H2 concentration which may affect both power generation and the efficiency of the system, which eventually resulted in higher cost. Two syngas cleaning methods were evaluated and recommendation was made based on experimental results. Syngas composition may vary based on the cleaning method to be used. The CO in syngas has two potential negative effects on SOFC operation. First, the Boudouard reaction is of concern in the temperature range from 500 to 600oC when CO can dissociate into C and CO2 (equation 1), which is called carbon coking. The method to prevent carbon coking was discussed. Second, if dry syngas is used as fuel, the water-gas shift reaction (equation 2) may occur in SOFC stack with the accumulation of water steam formation from electrochemical reaction, which is exothermic. The heat release from water-gas shift reaction would increase temperature difference between stack inlet and outlet which make it difficult to manage stack temperature to achieve optimized performance. SOFC stack electrical efficiency was analyzed with different fuel compositions and under different designed operation conditions, such as anode recycle ratio, using the model established. CO ↔ CO2 + C (1) CO + H2O ↔ H2 + CO2 (2) In order to capture CO2 from exhaust fuel gas, CO2 has to be concentrated higher than 98% first in the tail gas. Technical approaches were identified. Small scale matrix tests show promising results to meet expectation. Fabrication of scale-up testing equipment for technology verification is in process.