Simulation of power and cooling generation via heat recovery from a ventilation air methane abatement unit

Heat recovery from a fluidised-bed ventilation air methane abatement reactor and conversion into power using a Rankine cycle based steam turbine and cooling via an absorption chiller was simulated using the process simulation package Aspen Plus. The primary aim of the simulation was to determine the minimum methane concentration for self-sustaining operation, both in terms of maintaining the oxidation process at high temperature and to generate sufficient power to operate the plant. For a ventilation air flow rate of 20 m³/s (equivalent to a single abatement module), the minimum methane concentration was found to be 0.46 vol% at a reactor temperature of 650 °C and ambient pressure. The Rankine cycle operated with a steam pressure of 7.0 bar and steam flow rate of 0.4 kg/s. At the minimum methane concentration the process was self-sustained with zero net power being produced. The cooling produced at 0.46 vol% was 680 kW_R using an indirect-fired absorption chiller while 780 kW_R was produced via a direct-fired absorption chiller. Assuming a total ventilation air flow rate of 300 m³/s, fifteen 20 m³/s modules would be required, producing a total of up to 11,700 kW_R of cooling. The net power produced was zero between reactor temperatures of 500 and 700 °C at the investigated steam pressures (2.0–7.0 bar). Excess net power was produced at reactor temperatures greater than 700 °C due to the restriction of the inlet VAM temperature to 600 °C (to prevent auto-ignition of the methane upstream of the reactor). At low reactor temperatures the steam flow rate decreased with both reactor temperature and steam pressure but remained constant at reactor temperatures of 750 and 800 °C. The methane abatement plant would be able to operate without an external power supply through the utilisation of the process heat. The plant would produce adequate cooling for a typical gassy underground coal mine in Australia. Such mines are located in the Bowen Basin of Queensland; a region characterised by high virgin rock temperatures with cooling requirements of up to 7000 kW_R.