(555d) Detailed Kinetic Modeling of Gas Evolution and Energy Recovery during Chemical Quenching of Acetylene Production in Thermal Plasma

Detailed
Kinetic Modeling of Gas Evolution and Energy Recovery during Chemical Quenching
of Acetylene Production in Thermal Plasma

Yan Cheng, Tianyang Li, Yi Cheng ^*

Department of Chemical Engineering, Beijing Key Laboratory of Green
Chemical Reaction Engineering and Technology, Tsinghua University, Beijing
100084, PR China

Thermal plasma
pyrolysis provides a convenient way to realize the one-step conversion from
coal to acetylene. Taking advantage of the ultra-high temperature field
provided by thermal plasma, the coal feedstock is heated up in milliseconds to
give out volatiles. The volatiles undergo gas phase reactions afterwards to
form acetylene owing to the fact that acetylene is thermodynamically stable at above
1500 K among small-molecule-hydrocarbons. However, acetylene will decompose
into hydrogen and carbon black during cooling process. So efficient quenching
is necessary to prevent acetylene decomposition so as to maintain the target
products. It is noticed, on the other hand, that the high-temperature gas
products still contain about 40% of the total energy input. So
it is vital to make use of the latent energy during the quenching process.

In addition to the common
physical quenching by water spray, chemical quenching method is an alternative
solution to the process design. In chemical quenching, hydrocarbons are used as
quenching media instead of water, which brings about co-products (e.g.,
ethylene) during the decomposition of hydrocarbons by the afterheat in gas
products. The process profit can be accordingly improved as well.

In this work, we
carried out modelling and simulation on the
above-mentioned chemical quenching in thermal plasma technology. A gaseous
detailed kinetics, coupled with a soot formation mechanism, was proposed to
investigate the species evolutions during quenching. The model consisted of
C1-C5 gas phase reactions, PAH (polycyclic aromatic hydrocarbons) growth and a
simplified soot formation mechanism. The predicted acetylene decomposition and
soot yield were well validated with published data. Then, numerical
simulations were performed to reveal the process features, including acetylene
decomposition evolution, chemical quenching behavior of different C1-C3 alkanes
as well as the energy balance.

Finally, the model was
used to theoretically propose an optimized chemical quenching design in a pilot-plant
thermal plasma process. The results showed that under optimized operating
conditions, propane quenching could recovery 35.4% of the total gas phase afterheat.
At the same time, propane decomposed into smaller molecules in millisecond(s),
which realized the co-production of ethylene at a ratio of 0.75 t C₂H₄/t
C₂H₂. The optimal energy consumption was reduced by 43%
per kilogram of C₂H₂+C₂H₄, which
verified the feasibility of energy efficiency improvement via chemical
quenching.

Keywords: Thermal plasma pyrolysis; Chemical quenching; Acetylene; Detailed kinetics