UNO MK 3 Solvent Processes

UNO MK 3 is a novel solvent absorption process that has been developed by the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) over the last decade. It is a precipitating potassium carbonate technology engineered to capture 90 per cent of carbon dioxide (CO2) emissions from large scale emission sources such as power stations (pre- and post-combustion) and other industrial sources. UNO MK 3 is suitable for retrofitting to existing emission sources as well as for new build processes. The process is applicable to all CO2 sources however due to its robust nature it is expected to be particularly relevant to high oxygen flue gases typical of Natural Gas Combined Cycle (NGCC) generators.

Potassium carbonate (K2CO3) has been used in solvent absorption processes in chemical industries for many years. The patented UNO MK 3 process provides a unique update to this established technology making it suitable for CO2 capture at low pressure. Furthermore, UNO MK 3 links into the fertilizer production chain and has the potential to create value-added products.

UNO MK 3 has been developed from the laboratory stage to the pilot plant stage by an integrated multi-disciplinary team comprising researchers, senior engineers and economists located at the University of Melbourne and the University of New South Wales in Australia. Pilot plant testing of UNO MK 3 has recently been completed under real flue gas conditions at Hazelwood Power Station in the Latrobe Valley, Australia. These pilot plant trials have successfully demonstrated the key benefits of the UNO MK 3 process, namely:

  • Low energy of regeneration
  • Low overall cost
  • Low volatility and environmental impact
  • Multi-impurity capture and production of valuable by-products

Process Description

The reaction of CO2 with K2CO3 to form potassium bicarbonate (KHCO3) occurs through the following overall reaction:

CO2 + K2CO3 + H2O ↔ 2KHCO3                                                                                                              Reaction 1

A simplified process flow diagram of the UNO MK 3 process for post-combustion capture is presented in Figure 1. UNO MK 3 contains the absorption and regeneration stages of a standard solvent absorption process. However, unlike a standard liquid-based solvent system, a KHCO3 precipitate is formed during absorption and subsequent cooling. The precipitate is then separated from the liquid phase for selective regeneration of the bicarbonate species. In this way less water is passed to the regeneration step thus driving down energy requirements.

Figure 1 – UNO MK 3 Process Flow Diagram

Figure 1 – UNO MK 3 Process Flow Diagram

 

Liquid based K2CO3 solvent absorption processes for the removal of CO2 at high pressure are well established and have been in operation for several decades licensed under the Benfield Process [3]. No precipitating K2CO3 solvent absorption processes are currently in operation, however the Solvay process, a precipitating carbonate process for the manufacture of sodium bicarbonate (NaHCO3), has been in operation for over a century [4]. Another precipitating carbonate process currently in development for CO2 capture is the chilled ammonia process, which requires significantly lower temperatures than UNO MK 3 to prevent the evaporation of ammonia [5].

The development of UNO MK 3 promises to build on existing knowledge of K2CO3 while creating a new paradigm for large scale capture. The process is capable of handling a wide range of applications, including both pre- and post-combustion electricity generation and other industrial CO2 emitting processes, and a range of fuels including black coal, brown coal and natural gas. Due to its oxygen tolerance and low volatility it is expected to be highly applicable to capture from Natural Gas Combined Cycle (NGCC) flue gases. It also has the flexibility to be applied either as a new build or retrofit application.

Key Benefits

UNO MK 3 is an innovative technology because of the following features (i) use of a non-volatile solvent, (ii) use of a lower solvent to gas rate (L/G ratio), (iii) selective regeneration of the solid bicarbonate, (iv) conversion of the sulphur and nitrogen impurities in the flue gas to potentially valuable by-products and (v) flexibility to add additional chemical processes such as the chloralkali process for potassium hydroxide, chlorine and hydrogen production.

These features create the following value:

  • Minimal health and safety risk due to:
    • Chemically benign solvent (akin to baking soda)
    • Easy handling
  • Low volatility and environmental impact due to:
    • Very low environmental emissions
    • Low carbon footprint
    • Improvements over amine-based solvents on a broad range of environmental indicators including acidification potential, eutrophication potential, photochemical smog, ozone layer depletion, human toxicity and environmental toxicity [2]
  • Low market impacts because:
    • Potassium can be diverted from the fertiliser production chain to the UNO MK 3 process for CO2 capture from existing coal-fired power stations, removing the need for additional potassium production. Amine-based processes on the other hand could require an increase of eighteen times the current worldwide production of ethanolamines.

Low Energy of Regeneration

The regeneration energy requirement of the UNO MK 3 process is 2 to 2.5 GJ/tonne of CO2. This is lower than the 3+ GJ/tonne of CO2 typically quoted for amine-based processes [12]. The advantages are: (i) lower heat of reaction, (ii) reduced liquid to gas ratio (L/G) as shown in Figure 2; and (iii) selective regeneration of the bicarbonate.

Figure 2 – UNO MK 3 Operating and Equilibrium Curves at 50°C

Figure 2 – UNO MK 3 Operating and Equilibrium Curves at 50°C

Low Cost

The estimated economic benefits of UNO MK 3 compared with industry standards such as MEA and state of art (SOA) amines are shown in Figure 3. The values shown here are for retrofitting UNO MK 3 to a subcritical brown coal fired power station, typical of those in Victoria, Australia. Techno-economic studies have also been completed for subcritical and supercritical black coal fired power stations along with natural gas combined cycles (NGCC), which reveal the same reductions in cost.

fig 3
fig 3

Figure 3 – (a) Cost of CO2 Avoided and (b) Levelised Cost of Electricity (LCOE) for the UNO MK 3 Process and Amine-Based Processes Retrofitted to a 500 MW Subcritical Brown Coal Fired Power Station

The significant reduction in cost of the UNO MK 3 potassium carbonate process is achieved through incorporating the following features.

  • Heat integration
  • Enhanced rate promotion
  • Enhanced contactor design
  • Smaller regeneration circuit and alternative reboiler designs for the system
  • Lower solvent costs
  • Lower solvent replacement requirements
  • Elimination of SOx and NOx removal equipment prior to the capture system
  • Production of valuable byproducts for fertilizer industry
  • Coproduction of alternative chemical products such as chlorine and hydrogen.
  • Flexible capture with stockpiling of the bicarbonate salts

Low Volatility and Environmental Impact

Potassium carbonate-based solvents are significantly lower in volatility compared with amine-based solvents. The volatile emissions from amine-based solvents can be significant and usually require an additional water wash section in the absorber as well as continuous solvent make-up. UNO MK 3 process does not require a water wash section in the absorber. In addition, it is possible to operate the UNO MK 3 system such that water is produced and therefore continuous water make-up is not required.

A formal life cycle assessment (LCA) of UNO MK 3 in comparison with amine-based processes has recently been completed by an external consultant using the LCA software Simapro and in accordance with ISO 14040 [2]. The work has shown that UNO MK 3 has significant benefits over the amine-based process in a broad range of environmental impact categories as shown in Figure 4 below.

figure 4

Figure 4 – Comparison of Environmental Impact Results for MEA, UNO MK 3 SS (Stainless Steel Column) and UNO MK 3 C (Concrete Concentric Column) Relative to MEA [2]

Multi-Impurity Capture and Production of Valuable By-products

The UNO MK 3 process has the ability to capture SOx and NOx compounds along with the CO2. This is particularly beneficial in the Australian context, or for installations that do not have flue gas desulphurization units (FGDs) and nitrogen removal systems. If amine-based solvents are used, the SOx and NOx compounds would have to be pre-scrubbed from the flue gas prior to capture, due to the susceptibility of amines to degradation by these compounds, requiring new FGDs or deeper sulphur removal modifications. The UNO MK 3 process can be installed with SOx and NOx removal systems or downstream of any existing FGDs.

The reaction of SOx and NOx with K2CO3 in the UNO Mk 3 process leads to the production of the valuable fertilizer products potassium sulphate (K2SO4) and potassium nitrate (KNO3), as shown in Reaction 2.

SOx + NOx + K2CO3 → K2SO4 + KNO3                                                                                                   Reaction 2

The K2SO4 and KNO3 produced are removed during process using a combination of proven separation techniques.

Patents

UNO Technology P/L owns two patents families related to K2CO3 solvent processes and has the right to exclusively commercialise these technologies. The first patent family “UNO” relates to a liquid K2CO3 process, which was demonstrated as part of the ETIS projects [6, 7]. The second patent family “UNO MK 3” relates to the precipitating K2CO3 process.

Currently five patents have been awarded in these families in three different countries.

UNO – K2CO3 system for Pre & Post Combustion Capture [8]

  • PCT/AU2006/001177, fully awarded in Australia (20062811992).
  • US coverage awarded for post combustion capture (7976803),
  • US coverage allowed for pre-combustion under US Divisional Patent Application No. 13/089952
  • Canadian coverage under Canadian Patent Application No. 2619097

UNO MK 3 – Precipitating potassium carbonate [9]

  • PCT /AU2011/000462,
  • Accepted under Australian Patent Application No.  2011242411