Why is this research needed?

Climate change is a pressing issue that is demanding action in order to reduce greenhouse gas emissions. In the context of power generation and large scale industrial processes, this consists of carbon dioxide emissions from the use of fossil fuels. These emissions can potentially be captured using a solvent that absorbs the carbon dioxide from the emitted gas. A question then arises as to the fate of any of this solvent that escapes into the atmosphere. This research aims to investigate that by devising detailed models to predict the behaviour of the emitted solvent. This will then allow regulators such as the Environment Agency to assess the safety of these processes.

What is this research investigating?

Post-combustion capture (PCC) of CO₂ from flue gases using amines will be the most widely used technology to deliver the UK’s aspirations to have four CCS clusters in operation by 2030. Based on published plans for the five active CCS clusters, amine PCC will be used for most capture projects, including power generation from gas and biomass, capture from flue gases from a wide range of industries (e.g. refining, steel, cement, waste incineration) and hydrogen production using steam-methane reformers. An issue arising from the use of amines is their release into the atmosphere and subsequent conversion to nitramines and nitrosamines. Presentations at the UKCCSRC March 2021 ‘what are our research needs’ workshops, in the context of atmospheric pollution, identified the need to improve the Atmospheric Dispersion Modelling System (ADMS) software in this context. This was identified many years ago[1] and is still an ongoing issue. Currently, for its amine chemistry module, data is only available for a limited number of simple amines, MEA, MMA and DMA. A need therefore exists to address this, in particular for mixes of amines that may play a role in future PCC installations. Recent literature has identified the advantages of piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP) blends in terms of capture rate and absorption capacity[2]^,[3], and has also identified challenges, such as factors influencing amine emissions[4] and mechanistic uncertainties in many aspects of amine chemistry, for instance nitrosamine formation[5].

This research has four main objectives:

• The literature will be reviewed for kinetic data and reaction pathway information relating to the reactions of AMP and PZ that may occur in the atmosphere. From these, detailed reaction mechanisms describing the fate of AMP and PZ in terms of their chemistry in the presence of O₂, H₂O, CO₂, NO and NO₂ will be developed. This is an active field, for example with very recent studies reporting on new kinetic data and mechanistic evaluations.
• Density Functional Theory (DFT) Calculations
  Based on the analysis of the existing AMP and PZ mechanisms, and our experience from previous work related to monoethanolamine (MEA), mechanistic gaps along with those reactions with the most uncertain kinetic parameters will be identified and ranked in order of importance. Using the University of Sheffield high performance computing resources, a series of DFT calculations will be performed. Two computational setups will be applied to the calculations: one using B3LYP and one using M06-2X. Both setups will use the cc-PVTZ basis set within the Gaussian 09 software package. The output of these calculations will be details of the potential energy surface connecting reactants via a transition state to products, from which the rate coefficient Arrhenius parameters will be derived.
• AMP/PZ chemical kinetic mechanism update and evaluation
  From the outcome of the activities in points 1 and 2, above, the updated reaction mechanisms will be evaluated in terms of their performance against available experimental data. Where necessary, Arrhenius parameters of individual reactions will be optimised based on a genetic algorithm optimisation routine implemented within MATLAB, in which target data such as specific species concentrations as a function of time will be used to tune the kinetic data.
• Adaption to ADMS amine module format

The amine chemistry within the ADMS software is represented by a simplified reaction mechanism consisting of only 5 chemical reactions and a couple of branching ratios. The detailed mechanisms produced in 2.3 can be used to generate target data of the important species concentration evolution with time for a range of conditions. This target data can then be used within the MATLAB genetic algorithm optimisation routine to allow the simplified ADMS style scheme to be optimised to provide a set of Arrhenius parameters for the 5 chemical reactions it contains as well as the 2 branching ratios to get the best fit possible to this target data, thus providing a dataset that could be exploited by other users of this commercial software package.

What does the research hope to achieve?

This research will hopefully gain a better idea of the behaviour of the emitted solvents, AMP and PZ, so that their safety profiles can be assessed. It should benefit government regulatory bodies, such as the Environment Agency, as well as power generators running natural gas combined cycle power plants with carbon capture and storage; industry which is large scale carbon dioxide emitters requiring in the future carbon capture and storage; and academics involved in atmospheric chemistry modelling.

[1] Price, C., et.al., “H&ETQPAmine2: Modelling Atmospheric Dispersion for Components from Post-Combustion Amine-based CO₂ Capture”, Final report, prepared for CO₂ Capture Mongstad Project Gassnova SF, 2010

[2] Bui, M., et. al., “Carbon capture and storage (CCS): the way forward.”, Energy Environ. Sci., 2018, 11, 1062—1176

[3] Vega, F., et. al., “Current status of CO₂ chemical absorption research applied to CCS: Towards full deployment at industrial scale.”, Applied Energy, 2020, 260, 114313

[4] Spietz, T., et. al., “Experimental results of amine emission from the CO₂ capture process using 2-amino-2-methyl-1-propanol (AMP) with piperazine (PZ).”, International Journal of Greenhouse Gas Control, 2020, 102, 103155

[5] Ma, F., et. al., “Atmospheric Oxidation of Piperazine Initiated by Cl: Unexpected High Nitrosamine Yield.”, Environ. Sci. Technol. 2018, 52, 9801-9809