Impact of post combustion capture of CO2 on existing and new Australian coal-fired power plants

Abstract

Currently, Australia emits approximately 600 MT equivalent of CO2 annually, of which approximately 30% is directly linked to the electricity generation using both brown and black coals. To restrain the CO2 emissions, coal based power generators are looking to retrofit the existing power plants with commercially available technology for the post combustion capture (PCC) of CO2 as well as invest in the new power plants with high efficiency steam cycles. Since Australian coals are low in sulphur and the coal-fired power plants are well away from densely populated regions, the flue gas desulphurisation (FGD) and de- NOX regulations are currently not there for the coal based electricity generation in Australia. This is not an advantageous situation for straightforward retrofitting of the existing power plants with 30 wt% aqueous MEA based commercially available PCC technology that has very limited tolerance for SOX and NOX (less than 10 ppmv). In addition, Australia is a dry continent with very limited cooling water availability for the power plants. Hence, the Australian power generators are considering both the power and the post combustion CO2 capture plants to be air cooled. This paper, therefore, assesses the impact of introducing post combustion capture of CO2 on the existing and new Australian coal-fired power plants, both brown and black coal-fired, in terms of the cost of electricity generation, the cost of CO2 avoidance, the cooling water demand and the overall plant efficiency. The existing power plants are considered to be conventional subcritical and supercritical single reheat steam cycle based whereas the new power plant designs have allowed for ultracritical steam conditions (35 MPa, 922 K) with double reheat. The CO2 capture plants are considered to be either in service full time or in service on demand with 90% capture efficiency and the product CO2 ready for sequestration at 10 MPa and 313 K. The process and cost models for integrated power and capture plants have been obtained using ASPEN Rate-Sep, Steam-Pro, Steam-Master and PEACE software packages for process modelling and cost estimation. The results clearly show that an air cooled integrated power and capture plant has lower overall plant efficiency and slightly higher cost of electricity generation in comparison with a water cooled equivalent plant. An ultracritical single reheat power plant when integrated with capture plant that is in service full time has potential for lowest cost of electricity generation with minimum cost for CO2 avoidance. These results further show that replacing an existing turbine with a new LP turbine optimised for continuous steam extraction for CO2 plant duty minimises the adverse impact of PCC integration but the power generator looses the flexibility for electricity generation. The results also provide important insights into the major contributions to the increased cost of power generation. For both the existing and the new power plants, the amortised capital charge component dominates the cost of PCC integrated electricity generation. In spite of the large reduction in efficiency for Australian power plants when PCC is applied, it appears that reducing the capital costs of PCC will be at least equally important. This is an important outcome for the prioritization of research activities aimed at reducing the costs of capture. For example, the novel solvent development work for improved PCC technology should focus on increasing absorption rates at the same CO2 carrying capacity of the solvent to reduce the capital cost component.