Post-combustion carbon capture is the removal of carbon dioxide (CO2) from flue gases produced by the combustion of fossil fuels. Given the large number of coal-fired power plants, both in Australia and globally, post-combustion capture offers an opportunity to make significant cuts in greenhouse gas emissions.
Post-combustion capture has a range of advantages. It can be retrofitted to existing plants, integrated into new plants, has high operational flexibility in that it can be added in stages and operated independently of the power station and, importantly for this project, has significant development potential through process improvements, new sorbents and new technologies.
CO2CRC is taking a multi-technology approach that includes new solvent, membrane, adsorbent and cryogenic processes as well as engineering development, and process and heat integration.
One of the most advanced areas of CO2CRC capture technology development is the precipitating carbonate process, led by the solvent research team at the University of Melbourne. The team has been developing CO2CRC’s UNO MK 3 process since 2004 and has published a wide range of papers and established several patents.
UNO MK 3 is a precipitating solvent process using potassium carbonate (K2CO3), which gives it many advantages over the conventional amine processes currently used to separate CO2 during natural gas processing. These include:
- Low energy usage for regeneration
- Low overall cost
- Low volatility and environmental impact
- Multi-impurity capture and production of valuable by-products
- Application to post- and pre-combustion for all fuels including NGCC
- Potassium usage fits within existing global market
To demonstrate the technology a 200 kg/day CO2 capture rig, partly funded by ANLEC R&D, has been commissioned at the University of Melbourne and will operate until the end of 2013. The UNO MK 3 mini-plant is highly flexible, so that every aspect of the new process can be studied, varied and optimised.
Data from the UNO MK 3 mini-plant is also helping to design further trials in a larger pilot plant at Hazelwood power station in Victoria's Latrobe Valley, where researchers can apply real flue gas, identify engineering issues and ways to resolve them, and verify simulation results.
The larger UNO MK 3 Capture Plant at Hazelwood (CO2CRC/GDF-SUEZ) was commissioned in late 2012 and is capturing one tonne of CO2 per day from power plant flue gas. It was built with funding from the Victorian State Government through Brown Coal Innovation Australia (BCIA) and CO2CRC partners including the Federal Government, with the support of GDF SUEZ Australian Energy.
The plant uses a conventional absorber and is also trialling a new absorber design developed by Westec Environmental Solutions (WES).
The aim of the project is to validate the process’s significant capture cost reduction, paving the way to commercial implementation of the process. Operating to 2014, this represents the world’s first plant trials of this new precipitating process and equipment.
To date base cases have been successfully completed. Plant data will help validate design simulation results and operating windows of all equipment. The second campaign of three is currently underway.
The current trials at Hazelwood are a significant step to validating UNO MK 3 for large scale CO2 capture. Results of the trials, expected by the end of 2013, will inform a projected 50 tonne per day plant - the next step in scaling up the technology. A full scale demonstration plant, capturing CO2 at a rate of 3000 - 12,000 tonnes per day, is the medium term aim for the project.
How does UNO MK 3 Work?
During the UNO MK 3 process, flue gas mixes with the solvent in the absorber, where the CO2 is selectively captured via a potassium carbonate (K2CO3) / bicarbonate (KHCO3) reaction, resulting in both liquid and solid forms:
K2CO3(l) + H2O(l) + CO2(g) <> KHCO3(l) <> KHCO3(s)
The resulting slurry passes through a hydrocyclone, which further concentrates the solid particles, before they are redissolved via a heat exchanger en route to a stripper, where the CO2 is removed from the solvent for compression and storage. The lean solvent is returned to be reused. Heat integration is a major feature of the process, further lowering the process’s energy requirements (see Figure 1 below).
Figure 1 - UNO MK 3 process diagram
An innovative aspect of the UNO MK 3 process is SOx and NOx removal, producing potassium sulphate (K2SO4) and nitrate (KNO3) salts as by-products. These chemicals are used in fertiliser manufacture and are a saleable product of the process, helping to offset some of the costs and integrate the CO2 capture process into the wider potassium market.
The process and related designs are covered by several Australian and international patents.
The extra energy, or energy penalty, required to run capture processes, either as steam extracted from the steam cycle or as direct electricity, leads to significant lost revenue from the power plant. Running a capture plant can consume an additional 20-35 per cent of a power plant’s output.
The UNO MK 3 process uses much less energy than traditional amine processes (2 to 2.5 GJ/tonne CO2 compared with approximately 3 GJ/tonne CO2 for state of the art amines) and as such reduces the energy penalty to less than 20 per cent of the total power station equivalent electric output. The lower energy use for the UNO MK 3 process stems from a lower heat of reaction, selective bicarbonate regeneration and a lower solvent circulation rate.
Low overall cost
Large scale engineering and economic studies of the UNO MK 3 process indicate that the process has low overall costs, including for retrofit. Figure 2 depicts the cost of CO2 avoided for amine-based processes and the UNO MK 3 process in the case of 90 per cent capture of CO2 emissions retrofitted to a brown coal power station. Innovative equipment designs, such as the use of concentric columns, add further cost savings to the UNO MK 3 process.
Figure 2 - Cost of CO2 Avoided for Amine-Based Process and the UNO MK 3 Process
Low volatility and environmental impact
Environmental impact is one area where the UNO MK 3 process really shines. The solvent is much less volatile than amines, is oxygen tolerant and has a lower carbon footprint. It also has low acidification / eutrophication potential. Importantly for operators it has significantly lower human toxicity and is safer and easier to handle than amines.
Multi-impurity capture and production of valuable by-products
The UNO MK 3 process also captures SOx and NOx to produce potassium sulphate (K2SO4) and nitrate (KNO3) salts. Taking potassium from the fertiliser chain (potassium is primarily used as a fertiliser) and using it in UNO MK 3 for CO2 capture can actually result in improved life cycle performance compared with amine-based processes.
Amine-based capture processes produce considerable waste and will dramatically alter global chemical industries if put into wide-scale use. For example, capturing five gigatonnes of CO2 per annum would require an increase of seventeen times the current global production of monoethanolamines, which is a two per cent increase in ammonia production and an eight per cent increase in ethylene oxide production.
Figure 3 below depicts the lifecycle performance of the UNO MK 3 process and an amine-based process for CO2 capture.
Figure 3 - Schematic of Life Cycle Performance of the UNO MK 3 Process and an Amine-Based Process
CCS is the only technology capable of making significant and lasting cuts in CO2 emissions from fossil fuel combustion. Research and development around the world is bringing costs down by reducing the energy penalty, as well as other innovations, while also developing CO2 storage technologies and processes. The aim is to give industry the tools to reduce their emissions safely and cost-effectively.