Edinburgh Napier University

  Supplement to the depleting energy resources and the stringent environmental laws regarding the atmospheric emissions of greenhouse gases, the development of clean and renewable sources of energy has been a great issue. Worldwide, energy usage is steadily increasing and would continue to increase in the coming years; the effects of global warming are also becoming more and more prevalent. Biogas holds a promising future in the sustainable supply of low-cost energy that will minimize greenhouse gas emissions. Unlike solar and wind, biogas does not rely on weather conditions. It can also be stored and transported making it easily adaptable to changes in demand. This study would utilize membrane technology for the upgrading of biogas to a clean and useful fuel replacing fossil fuels and consider the limiting factors of the process found in literature which is the trade-off between gas permeability and selectivity. In doing so, we analyse the factors that affect permeability and selectivity of ceramic membranes and then introduce methods of optimisation. Biogas components are passed through the membrane as feed gas to observe the flow characteristics at various operating conditions for membranes with different pore sizes. It was observed that for all pore sizes studied, there was a proportional rise in flowrate as the feed pressure was increased from 0.2 to 3 bar. Also, an increase in temperature from 20 °C to 100 °C favoured the membrane selectivity. In general, methane gas showed a higher flux than carbon dioxide irrespective of the operating conditions indicating that the permeation of the gases is related to their molecular weight, for example, using 15 nm membrane at 20 °C and 3 bar, a flux of 3.59 and 2.27 L/min was obtained for CH4 and CO2 respectively with the same trend recorded using other pore sizes, temperature and pressure ranges. It must also be noted that the pore size plays a vital role in the optimisation of membrane process as the 15 nm membrane, being the lowest mean pore size studied showed the highest separation factor of 1.60 which is close to the ideal Knudsen value. Overall, it was found that the behaviour of gases through these membranes is determined by an interplay of factors – dynamic size and diffusion rate.