100 kmol/h of biogas (75 mol% CH4 and 25 mol% CO2) is produced in a digester and is drawn off from the headspace under a slight vacuum and piped to a natural gas burning boiler. A slight leak develops in the pipe allowing air (79 mol% N2 and 21 mol% O2) to enter and mix with the biogas. Biogas becomes flammable if the oxygen content exceeds 5 mol% in the mixture. Determine the maximum size the leak can be (in kmol/hr) before a safety hazard develops?