Methane,
a greenhouse gas, is the second largest contributor to
global warming after carbon dioxide and is 25 times more effective
at trapping heat in the atmosphere than carbon dioxide. In 2015, fugitive
emissions of methane from Australian underground coal mines were reported
at 25 million tonnes of carbon dioxide equivalent. Ventilation air
methane (VAM) is present in low concentrations (below 1.0 vol %),
and its abatement and use as an energy source are a challenge for
the coal mining industry. This paper examines the recovery of heat
from a fluidized-bed VAM abatement unit and utilization in power generation
via the Brayton cycle. The objective of the study was to determine
the minimum methane concentration required to maintain autothermal
operations and produce sufficient power to operate a fluidized-bed
plant without supplementary power or fuel. Four configurations were
studied and simulated using Aspen Plus software. For direct heat recovery,
the minimum methane concentration increased with an increase in both
the reactor outlet temperature and compressor outlet pressure. The
minimum methane concentration for the indirect heat recovery configurations
decreased when both the reactor outlet temperature and compressor
outlet pressure increased. For all configurations, the minimum methane
concentration was limited by the maximum reactor inlet temperature
of 600 °C (to prevent autoignition of methane upstream of the
reactor).
Ventilation air methane is low concentration methane (below 1 vol. %) emitted from an underground coal mine. High ventilation air volumes circulated through the mine, ensure that the methane remains at a safe concentration. In 2016, the Australian Government reported fugitive emissions of methane from underground coal mines at approximately 19.0 million tonnes (CO 2 -equivalent) which was about 4.0% of Australia's national greenhouse gas emissions. Therefore, an optimised process of heat recovery from a fluidised-bed VAM abatement reactor, to produce power and cooling was studied. For a ventilation flow rate of 20 m 3 /s, the minimum methane concentration for a direct gas turbine was 0.45 vol. % at a reactor temperature of 630°C and compressor pressure of 1.5 bar. An indirect gas turbine process operated with a minimum methane concentration was 0.4 vol. % at a reactor temperature of 630°C, compressor pressure of 4.0 bar and turbine flow rate of 2.2 kg/s.
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