Performance and Economic Analysis of Biomass/Coal Co-Gasification IGCC Systems With Supercritical Steam Bottom Cycle: Part 1 — Post-Combustion Carbon Capture

Abstract

In recent years, Integrated Gasification Combined Cycle (IGCC) technology has been gaining steady popularity for use in clean coal power operations with Carbon Capture and Sequestration (CCS). This study focuses on investigating two approaches to improve efficiency and further reduce the greenhouse gas (GHG) emissions. 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 feedstock to reduce emissions. Part 1 focuses on investigating post-combustion CCS and Part 2 on analyzing pre-combustion CCS, using both sour-shift and sweet-shift processes. Employing biomass as a feedstock has the advantage of being carbon neutral or even carbon negative if CCS is implemented. However, due to limited feedstock supply, such plants are usually small (2–50MW), which results in lower efficiency and higher capital and production costs. Considering these challenges, it is more economically attractive and less technically challenging to co-combust or co-gasify biomass wastes with coal. Using the commercial software, Thermoflow®, this study analyzes the baseline plants around 235MW and 267 MW for the subcritical and supercritical designs, respectively. The results clearly show that utilizing a certain type of biomass with low-grade coals up to 50% (wt.) can, in most cases, not only improve the efficiency and reduce overall emissions, but may be economically advantageous, as well. The CO2 emissions decrease by about 7,000 tons/MW-year, making both plants carbon-negative with only 10% biomass in the feedstock. In addition, implementing a supercritical steam cycle raises the efficiency (1.6 percentage points) and lowers the capital costs ($300/kW), regardless of plant layout. Implementing post-combustion CCS consistently causes a drop in efficiency (at least 7–8 points) from the baseline, and increases the costs by $3,000-$4,000/kW and $0.06-$0.07/kW-hr. The SOx emissions also decrease by about 190 tons/year (7.6 × 10−6 tons/MW-year), while the NOx emissions are all but eliminated through post-combustion CCS. Finally, the CCS cost is around $65-$72 per ton of CO2.