Troels Dyhr Pedersen.indd - Solid Mechanics

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formaldehyde (CH2O) and formic acid (HCO2H), that are ready to proceed to the final
oxidation steps to CO and CO2. Hydrogen peroxide in particular is important, since it
becomes unstable as temperature increases, and thereby becomes a source of hydroxyl
(OH) radicals.
The initial breakdown of DME occurs by reaction with molecular oxygen. It has a very
low conversion rate, but since there are no other active radicals being formed at this
temperature it is the only reaction that can produce CH3OCH2 initially. Once OH radicals
are being produced further down the chain, these radicals are responsible for the major
part of the first oxidation step.
There are two major reaction paths from the methoxy-methyl (CH3OCH2) radical. The
path to the left in the scheme (fig. 8) is the primary at low temperature reactions, while
the path to the right is dominating at high temperature reactions.
The first step in the low temperature reaction (LTR) scheme DME combustion is the
formation of the methoxy-methyl radical, CH3OCH2, which is possible by several
reactions. The dominating reaction is 274, but to ensure a good prediction of ignition
delay it is necessary to include reaction with molecular oxygen as well. Reactions with
three other radicals (H, O and HO2) are also included in the mechanism since they are
relevant at higher temperatures.
# 274 :
# 280 :
CH OCH
3
CH OCH
3
3
3
+ OH → CH OCH
+ O
2
3
3
→ CH OCH
2
2
+ H O
2
+ HO
Initially, reaction 280 is the dominating reaction since there are no OH radicals. It
produces CH3OCH2 at a very low rate, but the later steps produce OH radicals for
reaction 274.
2
The methoxy-methyl radical predominantly combines with molecular oxygen in reaction
287 during the LTR reactions.
# 287 : CH OCH + O → CH OCH O
3
2
2
3
2
2
Following reaction 287 is reaction 293 in which an H-radical shifts its position in the
molecule. This happens almost instantaneously.
# 293:
OCH O → CH OCH O H
CH3 2 2
2 2 2
This molecule may now react in two different ways. The first is dissociation which
produces one OH radical and two formaldehyde radicals, while the second is the
absorption of molecular oxygen:
# 294 : CH2OCH
2O2H
→
2 CH2O
+ OH
# 295:
CH OCH O H + O → O CH OCH O H
2
2
2
2
2
2
2
2