Performance evaluation of high sulphur coal-fired USC plant integrated with SNOX and CO2 capture sections

In recent years coal-fired power plants have increased their role in the global energy scenario thanks to reliability, security of fuel supply and lower cost of fuel and electricity. In this framework global warming issues require a sustainable use of coal and great efforts for greenhouse gases reduction, addressing research and development projects towards more efficient solutions in terms of efficiency and environmental impact. With this aim in this paper a performance assessment of an Ultra Super Critical (USC) steam plant integrated with a CO2 removal section was carried out. The study is based on simulation models specifically developed through Aspen-Plus and Gate-Cycle software platforms. Performance was assessed referring to typical USC commercial size plants (400-600 MW), fuelled by a mix of a low-sulphur coal and a Sardinian (Sulcis) high-sulphur coal. The USC plant, based on a reheated and regenerative Rankine cycle, is integrated with an SNOX section, with integrated DeNOx and DeSOx processes, and a low temperature CO2 capture section. The SNOX technology shows several advantages in comparison to conventional de-nitrification and de-sulphuration systems. In particular it is capable to remove both nitrogen and sulphur oxides, requiring less energy absorption compared to traditional FGD systems, allowing a further preheating of the combustion air and without producing process waste. Besides the SNOX releases a commercial product as the sulphuric acid (H2SO4) and operational costs are reduced when sulphur content in the coal increases. The CO2 capture system was based on a chemical absorption process. A conventional system based on an amine (monoethanolamine, MEA) aqueous solution was considered. In order to match high pressure and purity CO2 transportation requirements, the CO2 removal section is also integrated with a conditioning and compression section. A performance assessment of the USC plant was carried out varying the high-sulphur coal share on the coal mix and the main operating parameters of gas conditioning and purification sections. In particular a performance analysis was carried out to assess the influence of SNOX parameters on gas purification, CO2 removal process and USC performance. An in-depth analysis of energy penalization due to the CO2 capture systems was also performed, evaluating influence of CO2 removal efficiency. Results shows that SNOX technology influences USC performances differently according to high-sulphur coal share in the coal mix. A great high-sulphur coal share sensibly increases sulphur content in the exhaust gas, reducing the methane requirements of the SNOX burner for reaching operational temperature and thus reducing penalties on USC performance. It is important to point up that an increase of the high-sulphur coal share can lead to exceed emissions limit for sulphur oxides, requiring to optimize the SNOX design. Integration with the CO2 removal section leads to a noteworthy USC plant efficiency reduction of about 10.5 percentage points. The CO2 specific emissions, which for the reference plant were about 760 g/kWh, are marginally influenced by the SNOX system, but are greatly reduced by the introduction of the CO2 removal system, decreasing to about 100 g/kWh.