A System Performance and Economics Analysis of IGCC with Supercritical Steam Bottom Cycle Supplied with Varying Blends of Coal and Biomass Feedstock

In recent years, Integrated Gasification Combined Cycle Technology (IGCC) has been gaining steady popularity for use in clean coal power operations with carbon capture and sequestration. Great efforts have been continuously spent on investigating various ways to improve the efficiency and further reduce the greenhouse gas (GHG) emissions of such plants. This study focuses on investigating two approaches to achieve these goals. First, replace the traditional subcritical Rankine steam cycle portion of the overall plant with a supercritical steam cycle. Second, add different amounts of biomass as co-feedstock to reduce carbon footprint as well as SOx and NOx emissions. Employing biomass as a feedstock to generate fuels or power has the advantage of being carbon neutral or even becoming carbon negative if carbon is captured and sequestered. Due to a limited supply of feedstock, biomass plants are usually small, which results in higher capital and production costs. In addition, biomass can only be obtained at specific times in the year, meaning the plant cannot feasibly operate year-round, resulting in fairly low capacity factors. Considering these challenges, it is more economically attractive and less technically challenging to co-combust or co-gasify biomass wastes with coal. The results show that supercritical IGCC the net plant efficiency increases with increased biomass blending in the all cases. For both subcritical and supercritical cases, the efficiency increases initially from 0% to 10% (wt.) biomass, and decreases thereafter. However, the efficiency of the blended cases always remains higher than that of the pure coal baseline cases. The emissions (NOx, SOx, and effective CO2) and the capital cost all decrease as biomass ratio increases, but the cost of electricity increases with biomass ratio due to the high cost of the biomass used. Finally, implementing a supercritical steam cycle is shown to increase the net plant output power by 13% and the thermal efficiency by about 1.6 percentage points (or 4.56%) with a 6.7% reduction in capital cost, and a 3.5% decrease in cost of electricity. NOMENCLATURE ASU Air Separation Unit GT Gas Turbine ST Steam Turbine HRSG Heat Recovery Steam Generator IGCC Integrated Gasification Combined Cycle GHG Greenhouse Gas(es) AGR Acid Gas Removal HP High Pressure (PSI) IP Intermediate Pressure (PSI) DA De-aerator bmr Biomass Ratio (biomass/feedstock mass) (wt%) M.W. Molecular Weight (lbs/lb-mol) LHV Lower Heating Value (Btu/lb) HHV Higher Heating Value (Btu/lb) INTRODUCTION In recent years, Integrated Gasification Combined Cycle Technology (IGCC) has been gaining steady popularity for use in clean coal power operations with carbon capture and sequestration. Great efforts have been continuously spent on investigating various ways to improve the efficiency and further reduce the greenhouse gas (GHG) emissions of such plants. This study focuses on investigating two approaches to achieve these goals. To achieve the first goal of improving efficiency, this study investigates the feasibility of replacing the traditional subcritical Rankine steam cycle portion of the overall plant with a supercritical steam cycle because supercritical Rankine cycle is typically more efficient than the subcritical Rankine cycle and the Brayton cycle. To the authors’ knowledge, there is currently no literature available for this type of steam system being used in an Integrated Gasification Combined Cycle (IGCC) system. To achieve the second goal of reducing GHG emissions, this study investigates adding biomass as a cofeedstock of coal to reduce carbon footprint as well as SOx and