MODEL-BASED SELECTIVE CO2 CAPTURE FROM BIOGAS STREAMS USING IONIC LIQUIDS AS PHYSICAL AND CHEMICAL ABSORBENTS
P. Garcia-Guttierez1, I. Dimitriou1, C. McCrellis2, R. Taylor2, J. Jacquemin2, R. H. Elder1, R. W. K. Allen1
1Department of Chemical and Biological Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK.
2 QUILL, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, N. Ireland, U.K.
Biogas is generated by micro-organisms in the absence of air via a so-called anaerobic metabolism. Raw biogas consists mainly of methane (CH4, 40-75% vol.) and carbon dioxide (CO2, 15-60% vol.). Trace amounts of other components, such as water (H2O, 5-10% vol.), hydrogen sulphide (H2S, 0.005-2% vol.), siloxanes (0-0.02% vol.) may also be present in the biogas. Upgrading is often performed to increase the CH4 concentration in the biogas (and thus increase its calorific value) by removing the CO2. This allows the biogas to be injected into the natural gas grid or to be used as a vehicle fuel. More recently, it has been suggested that the biogas can be further valorised by converting the isolated CO2 stream into valuable products through Carbon Dioxide Utilisation (CDU).
Monoethanolamine (MEA) has been used for decades as a solvent for CO2 capture. Typically, for MEA the absorption process is done by chemisorption at 50°C followed by temperature swing desorption at 120°C, hence the energy penalty is high. In addition, the high volatility of MEA, its corrosive behaviour and its thermal degradation over time have been suggested as hindrances to the economic success of CCS and CDU. In contrast, ionic liquids (salts which are liquid at room temperature) have been shown to have high CO2 affinities and are noted for their negligible volatility, reasonable thermal stability, strong dissolubility and their ability to be custom designed to provide chemical environments which favour particular applications.
Current CO2 capture systems in anaerobic digestion plants typically employ water scrubbing or amine based solvents, such as MEA, to remove CO2 from the biogas and upgrade it to natural gas standards. This modelling work considers five different ionic liquids to act as physical and chemical absorbents for CO2 capture from biogas. The PR-BM equation of state property method is employed in Aspen Plus to perform mass and energy balance calculations over the whole process. The parameters needed by the PR-BM property method, such as critical properties of the ionic liquids, ideal gas heat capacity and binary parameters are calculated using different correlations and the COSMOtherm® software. A conceptual process flowsheet for CO2 capture is then built in Aspen Plus for each ionic liquid consisting of an adiabatic absorber which was modelled using the RadFrac subroutine, a flash drum for solvent regeneration with pressure swing and a centrifugal pump for solvent re-circulation. Additionally, capital and operating expenditure of the different process concepts are calculated. The results from the simulations are compared in order to identify the most promising ionic liquid in terms of cost of CO2 avoided (£/kg of CO2 avoided), absorption capacity, energy consumption and solvent loss. A sensitivity analysis is also carried out to examine the effect of several economic and technical parameters on the cost of CO2 avoided and absorption capacity of the ionic liquids considered.
*Corresponding author. Tel.: +44(0)1142227599; E-mail address: email@example.com (P. Garcia-Gutierrez)