(412c) Improvement of Oxy-Combustion Using Thermodynamic and Exergetic Analysis

Reducing carbon dioxide (CO₂) emissions to the atmosphere is has been the focus of many policy actions directed at mitigating the effects of climate change. Although renewable energies have been widely deployed, there is still a dependence on fossil fuels to satisfy the demands of the grid. Carbon capture and storage (CCS) is generally recognised as providing an essential role to reduce anthropogenic CO₂ emissions from burning fossil fuels for electricity generation. Several technologies have been developed to address this problem, such as oxy-combustion, which also possesses applications in the glass and steel industries. Oxy-combustion is a promising technology that enables the reduction of carbon dioxide (CO₂) emissions from combustion of fossil fuels, such as coal, by more than 90 %. However, the requirement of including an air separation unit (ASU) and a gas processing unit (GPU) introduces an energy penalty to the process, increasing the cost of electricity. This work aims at identifying the potential for improving this technology using a rational approach grounded on the laws of thermodynamics and techno-economic analysis.

Improving the efficiency of oxy-combustion by reducing the parasitic energy consumption of both ASU and GPU is a requirement for this technology to be more competitive in the market. This can be achieved by increasing the separation efficiency of the ASU and GPU. Another way of reducing losses in oxy-combustion is to improve the thermal efficiency of the plant, by increasing the operating conditions to the boiler. However, this would require the use of newer materials, increasing capital expenditure (CAPEX) and make it a riskier project.

Heat integration using low grade heat of compression from the ASU and GPU to pre-heat feedwater is another way of increasing efficiency. This strategy possesses the advantage of using the available resources more efficiently while reducing the number of unit operations, thus reducing CAPEX, and allowing more steam to be used for electricity generation. Acceptance of a technology is highly dependent on the levelised cost of electricity (LCOE) which is influenced by CAPEX and fuel price. This demonstrates that there must be an increase of process efficiency while keeping CAPEX reduced for technological innovation to occur.

An oxy-combustion comprised of a double column cryogenic ASU, an ultra-supercritical boiler with flue gas treatment, and a cryogenic distillation GPU was simulated using Aspen HYSYS. The model developed showed a good agreement with data from IECM program and the values reported by Callide oxyfuel project, with the exception for O₂ and fuel consumption. This difference was found due to the assumption of complete combustion, with no carbon monoxide formation and no heat losses to the environment.

The minimum thermodynamic separation work was calculated for both ASU and GPU and was combined with the theoretical Carnot efficiency to determine the maximum efficiency that an oxy-combustion could achieve. An exergy destruction analysis was performed on each unit operation to quantify the inefficiencies within the process, which would allow for a rational improvement of the process.

In conclusion, the process was improved based on inefficiency minimisation, and then a techno-economic analysis was performed to determine its economic viability. This allowed for an efficiency improvement of 1.4 % (gross) and 3 % (net) as well as a 15 % reduction of CAPEX while decreasing the parasitic power consumption of the ASU from 204 to 197 kWh/t_O2 and the GPU from 140 to 137 kWh/t_CO2. Both reduction of CAPEX and power consumption leads to a decrease of the levelised cost of electricity (LCOE).