Techno-economic analysis of biomass/coal Co-gasification IGCC systems with supercritical steam bottom cycle and carbon capture

Abstract

SUMMARYIn recent years, integrated gasification combined cycle 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. 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 (2-50 MW), 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 low-rank coals.Using the commercial software, Thermoflow®, this study analyzes the baseline plants around 235 MW and 267 MW for the subcritical and supercritical designs, respectively. Both post-combustion and pre-combustion CCS conditions are considered. The results clearly show that utilizing a certain type of biomass with low-rank 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. Beyond a 10% Biomass Ratio, however, the efficiency begins to drop due to the rising pretreatment costs, but the system itself still remains more efficient than from using coal alone (between 0.2 and 0.3 points on average). The CO 2 emissions decrease by about 7000 tons/MW-year compared to the baseline (no biomass), making the plant 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 $3000-$4000/kW and $0.06-$0.07/kW-h. The SO x emissions also decrease by about 190 tons/year (7.6 × 10 À6 tons/MW-year). Finally, the CCS cost is around $65-$72 per ton of CO 2 .For pre-combustion CCS, sour shift appears to be superior both economically and thermally to sweet shift in the current study. Sour shift is always cheaper, (by a difference of about $600/kW and $0.02-$0.03/kW-h), easier to implement, and also 2-3 percentage points more efficient. The economic difference is fairly marginal, but the trend is inversely proportional to the efficiency, with cost of electricity decreasing by 0.5 cents/kW-h from 0% to 10% biomass ratio (BMR) and rising 2.5 cents/kW-h from 10% to 50% BMR. Pre-combustion CCS plants are smaller than post-combustion ones and usually require 25% less energy for CCS due to their compact size for processing fuel flow only under higher pressure (450 psi), versus processing the combusted gases a...

Parametric techno-economic studies of coal/biomass co-gasification for IGCC plants with carbon capture using various coal ranks, fuel-feeding schemes, and syngas cooling methods

“…The objective of this study is to improve upon existing IGCC systems by (i) reducing the GHG emissions of such plants, (ii) reducing their capital and electricity costs, and (iii) increase the efficiency, if possible. Previous studies by Long and Wang focused on the plant as a whole and incorporated a high‐exergy steam cycle into the power block to raise the output power and the efficiency while reducing capital costs. Employing biomass as a feedstock has the advantage of being carbon‐neutral or even carbon‐negative if carbon is captured and sequestered (CCS), whether the goal is to generate chemicals or provide electrical power.…”

“…Employing biomass as a feedstock has the advantage of being carbon‐neutral or even carbon‐negative if carbon is captured and sequestered (CCS), whether the goal is to generate chemicals or provide electrical power. Therefore, biomass was also included in the feedstock, and various carbon capture schemes were implemented to reduce the emissions, as well . One additional objective of this study is to alter the biomass ratio , or BMR for short, with a primary interest in finding the effects of biomass on GHG emissions, as well as the overall changes in power, efficiency, and economics.…”

“…As a part of a series of studies , the goal of this study is to examine the thermal and economic impact of different design implementations for an IGCC plant, including radiant cooling versus syngas quenching, dry‐fed versus slurry‐fed gasification (particularly in relation to sour‐shift and sweet‐shift carbon capture systems), oxygen‐blown versus air‐blown gasifiers, low‐rank coals versus high‐rank coals, and options for using syngas or alternative fuels for the duct burner (DB) in the heat recovery steam generator (HRSG) to raise the steam turbine (ST) inlet temperature.…”

“…All of these plants, however, have either failed because of some technical difficulties or been removed from the commercial power sector because of not being economically competitive. More detailed discussions about these plants have been undertaken in previous papers .…”

“…This study, like some previous studies from the same authors , focuses on investigating co‐gasification of biomass and coal for application in IGCC systems with and without carbon capture, but with several parametric changes that will, as mentioned previously, be analyzed/compared with the previous case studies. The authors' previous work on the topic of gasification started with a study on IGCC in general and the effect of co‐gasifying up to 50% (wt.)…”