introduction of catalytic flow-reversal reactor (ch4min) technology ...

Mt equiv. CO 2 /y (Million metric tons of equivalent CO 2 per year) [USEPA (2002a)
[3]]. In light of ever-increasing pressure for reducing GHG emissions and
meeting the reduction targets set in the Kyoto Protocol, reducing VAM
emissions is becoming a very important task in the 21st century.
The CH4MIN technology was developed exclusively for use on treatment of
methane in coal mine ventilation air. With its catalyst, the CH4MIN technology
reduces the auto-ignition temperature of methane by several hundred degrees
Celsius (i.e. to as low as 350°C). It provides a great potential for the use of
VAM as a principal fuel source because the technology is able to oxidize
diluted methane in coal mine ventilation air and produce useable energy from
heat exchangers operating at an optimal temperature. Consequently, the new
technology not only provides a feasible solution to eliminate methane in
ventilation air, but also could generate value through recovered energy (hot
air, water and steam) during the process.
2.0 CH4MIN OPERATING PRINCIPLES
The schematic of a typical CH4MIN process is shown in Figure 1. It consists
of a reactor, two pairs of valves and a heat removal system in the central
region separating the catalyst bed in two parts. At both ends of the reactor, an
inert material bed is present and acts as a thermal accumulator. Flow reversal
is controlled by the two sets of valves. During the first half-cycle, the methaneair
mixture, which is originally at ambient temperature, is heated as it passes
through the upper inert bed. It enters the upper catalyst bed at a temperature
sufficient for exothermic methane oxidation to occur, which produces heat in
the reactor. The stream then passes through the lower catalyst bed and finally
heats the lower inert bed before exiting to atmosphere. During this first halfcycle,
the upper part of the reactor, which was initially hot, cools down, while
the lower part, which was initially cool, heats up. After a certain period of time,
the flow direction is reversed by inverting the position of the valves in order to
use the heat available in the lower inert bed. At the end of the second halfcycle,
downward flow is re-established. The heat generated in excess by the
exothermic chemical reaction is withdrawn by the heat removal system in the
mid-section of the reactor.
3.0 NUMERICAL SIMULATION AND EXPERIMENTATION
A mathematical model and computer programs were developed for the study
and design of CH4MIN reactors. A transient two-dimensional heterogeneous
model is combined with a numerical method allowing the fast formulation of
new reactor designs, [Aube and Sapoundjiev, 2000, [ 4 ] ] . The program has
been validated with experiments performed with a 500-mm reactor. The
comparisons between the experimental data (circles) and predictions by the
model (lines) are shown in Figure 2(a-d). Good agreement with the
experiments was observed. The dynamic behaviour of the CH4MIN can be
predicted accurately by the model for a wide range of conditions including
small reactor diameter and low air flow rate.