SO2 and CO2 removal from flue gas

Abstract

The emission of green house gases is becoming of ever increasing worries to the international community. The fossil fuel consumption is still increasing and therefore the emission of the green house gases. Many uncertainties exist around the negative effects arising from large concentrations of green house gases in the atmosphere. To prevent possible negative effects in the future many countries start taking action to reduce the emission of these gases a good example is off course the Kyoto protocol. With the actions taken by the different countries new legislations are made. To be able to comply with these increasingly stricter rules the performance of the treatment installations of plants that produce large quantities of green house gases has to be improved. Large producers of green house gases are the coal fired power plants. Therefore in this project a conceptual design for a process is made that removes SO2 and CO2 from the flue gas of such a power plant. The first important action taken in making the conceptual design is to define a clear task for the process. The defined task demanded that a conceptual design is made for a process that removes at least 99.3% of the sulphur dioxide content from the flue gas. The process captures as well at least 6% of the carbon dioxide from the flue gas. The sulphur dioxide is converted to a useful product and the carbon dioxide is just separated and the further processing is outside the scope of this project. For the separation of carbon dioxide and the separation and conversion of sulphur dioxide different process options are investigated from literature. From the results of this literature study the process options are compared and the best options are selected for further design. For the separation of carbon dioxide scrubbing the flue gas stream with Mono Ethanol Amine (MEA) is selected as the most realistic and feasible process. The separation of sulphur dioxide is done by hollow fibres which show great potential in the field of selectivity consumption of energy. For the conversion of sulphur dioxide two potentially good options were found. One biochemical process and one chemical process were selected. These processes were combined with the membrane separation section. From these combinations two preliminary bases of design have been made. The comparison between the two preliminary BOD’s resulted in the selection of the biotreatment process. However after further investigation that assumptions made in integrating the membrane with the conversion sections caused too many uncertainties concerning the feasibility of this process that the chemical process is further developed. The chemical process is based on a process developed at the international research institute ISPRA and is called Mark-13A. In this process the sulphur dioxide is together with bromine and water converted to sulphuric acid and hydrogen bromide and the sulphuric acid. The sulphuric acid is concentrated and sold. The hydrogen bromide is regenerated back to bromine with electrolysis. During this electrolysis hydrogen is formed in the same amount as bromine. The overall process converts almost 99.8% of the sulphur dioxide and separates 6% of the carbon dioxide. A total amount of 50000 t/a technical grade sulphuric acid is produced. From conceptual process equipment design the total investment is estimated to be around 150 M€. The largest part of this investment goes to the sulphur dioxide separation membrane section. The meet the specified separation 177 membrane units of 1x2m are needed. However when comparing this required investment with other flue gas desulphurization units like a unit build in Bulgaria at the Maritza coal plant which costs 650 M€ the investment seems feasible.