Leaching of fly ash particulate matter in MEA solutions and its relevance to the CO2 capture process with flue gas of coal fired power plants.

Thesis from Bernd Schallert
University of Stuttgart, 2020

During the CO2 carbon capture (CC) operation in coal-fired power plants (PP), particulate matter accumulates over time and undergoes leaching in mono ethanolamine (MEA) solutions. Previous studies usually assessed only one sample of fly ash, collected from the electrostatic precipitator (ESP) of a given PP, and they differed in terms of the leaching parameters and the scope of analysis. The present study underlines the relevance of leaching to CC plants and strives for a better understanding of the solubility of various elements and heavy metals, especially Fe, in MEA solutions and of relevant leaching parameters.
Using parallel tests, this study assesses the leaching of ESP ashes from three PP in a rich loaded 5 M MEA solution under various conditions, resulting in a comprehensive dataset on five physical parameters, five macro elements and six heavy metals within each of 30 leachates.
The remaining study describes measurements with three solid to liquid ratios using a lean 5 M MEA and two new ESP ashes to assure the repeatability of the whole procedure (leaching and analytics) by four repetitions for each ESP ash and solid to liquid ratio. These analytical results are used to forecast the Fe concentration for the early beginning of a CC plant.
The main results are summarized as follows:
• The rich loaded 5 M MEA solution dissolves an enormous amount of Fe compared to pure water. Each ESP ash dissolves Fe and the other heavy metals specifically.
• The Fe concentration increases disproportionately with increasing molar concentration of the rich loaded MEA solution.
• The glycolate concentration as a degradation product enhances predominantly the dissolution of Fe at the high concentration of 50 mmol/l. The oxalate concentration may suppress the Fe concentration by precipitation.
• With lower solid to liquid ratio the concentration of all analysed elements and compounds decreases, but specifically for each element/compound and ESP ash when solid concentrations of several g/kg occur.
• For ratios < 1:1000 (g/g) the dissolved concentrations for different ESP ashes become quite similar and the specific character of each ESP ash becomes less important.
• The repeatability values point out variation coefficients predominantly below 5 % for Mg, Ca, S, Fe, V, Cr and Mn.
• The leachate results at 1:1000 (g/g) for one ESP ash are used for further extrapolation to the start of CC operation.
• Apart from main elements such as Ca, Mg and S, only Fe is forecasted to be detected in the lean 5 M MEA solution of a CC plant after 24 h of operation, when 1 mg /m³ of fly ash is absorbed by the circulating liquid and an enrichment factor of 0.8 m³/(linv*h) is applied.

To optimize the performance of a CC plant the inlet fly dust concentrations (50 % fly ash assumed) are recommended to be lower than 0.6 mg/m³ (STP, dry, 6 % O2) to control the accumulated fly ash below 1 g/kg for at least 4000 hours, when an enrichment factor of 0.8 m³/(linv*h) is applied.

To the top of the page