Methane Oxidation Reactions CH 4 + 2 O -> CO 2 2 + 2 H 2 O Total Oxidation (Combustion) CH 4 + 0.5 O -> CO 2 + 2 H 2 CO + 0.5 O -> CO 2 2 H 2 + 0.5 O -> H 2 2 O CH 4 + H 2 O->CO + 3 H 2 Partial Oxidation CO Oxidation Hydrogen Oxidation Steam Reforming CH 4 + CO -> 2 CO 2 + 2 H 2 CO 2 ( Dry ) Reforming CO + H 2 O->CO 2 + H 2 Water Gas Shift (WGS) and that s not even yet a complete list of all possible reactions! L19-1
A (fairly) complete chemistry Many tailored kinetics in the literature. However, let s look at the whole complex spectrum of possible reactions, using (once again ): the GRI-Mechanism compilation of 325 elementary chemical reactions and associated rate coefficient expressions 53 species and corresponding thermochemical parameters The conditions for which GRI-Mech 3.0 was optimized: 1000 to 2500 K, 10 Torr to 10 atm, and equivalence ratio from 0.1 to 5. Cautious use of GRI-Mech 3.0 outside the optimization and validation ranges is not unreasonable, essentially because the elementary reactions, which include all steps thought to be important for describing natural gas ignition and flame propagation (including NO formation and reduction) are described by rate parameters that reflect current understanding of elementary reaction rate theory. L19-2 GRI-Mech has been optimized for methane and natural gas as fuel. However, soot formation not included!
Synthesis Gas Formation L19-3 The dominant process for syngas formation is (by far) steam reforming of methane (SRM): CH 4 + H 2 O <=> CO + 3 H 2, H R = +206 kj/mol The reaction is strongly endothermal and hence energy needs to be supplied to the system, making SRM a major energy consumer. An An alternative process is is partial partial oxidation of of methane, which which can can proceed catalytically or or as as a purely purely homogeneous process (Texaco-Shell): CH CH 4 + 4 0.5 0.5 O 2 2 <=> <=> CO CO + 2 H, 2, H 2 H R = R -37-37 kj/mol kj/mol The The (mild) (mild) exothermicity of of this this reaction is is one one of of the the major major advantages of of this this process alternative.
L19-4 Explain!
Setting up the problem: CSTR L19-5 1. 1. Define Define reactor reactor set-up: set-up: CSTR CSTR with with inlet inlet 2. 2. define define working working directory directory (any (any directory directory with with write-permission) 3. 3. define define chemistry chemistry (GRI-mech (GRI-mech 3.0 3.0 w/thermo w/thermo & transport transport database) database) 4. 4. run run pre-processor pre-processor processor 1. 1. define define isothermal isothermal reactor reactor problem problem 2. 2. reactor reactor length: length: 1 1 m (fast (fast reaction!) reaction!) 3. 3. reactor reactor diameter: diameter: 1 1 ft ft 4. 4. T,P T,P = = 1000K, 1000K, 1 1 atm atm 5. 5. Stoichiometric Stoichiometricfeed feed 1. 1. volumetric volumetric inlet inlet flow flow rate: rate: 1 1 m 3 3 /s /s 2. 2. T,P T,P = = 1000K, 1000K, 1 1 atm atm 3. 3. Stoichiometric Stoichiometricfeed feed
L19-6 Solution
Setting up the problem: PFR L19-7 1. 1. Define Define reactor reactor set-up: set-up: PFR PFR with with inlet inlet 2. 2. define define working working directory directory (any (any directory directory with with write-permission) 3. 3. define define chemistry chemistry (GRI-mech (GRI-mech 3.0 3.0 w/thermo w/thermo & transport transport database) database) 4. 4. run run pre-processor pre-processor processor 1. 1. define define isothermal isothermal reactor reactor problem problem 2. 2. reactor reactor length: length: 1 1 m (fast (fast reaction!) reaction!) 3. 3. reactor reactor diameter: diameter: 1 1 ft ft 4. 4. T,P T,P = = 1000K, 1000K, 1 1 atm atm 1. 1. feed feed stream stream with with axial axial velocity velocity of of 0.1 0.1 m/s m/s 2. 2. stoichiometric stoichiometricfeed feed for for POM POM 1. 1. create create input input file file 2. 2. run run model model 3. 3. check check result result ( View ( View Results ) Results ) 4. 4. Create Create graph graph ( Run ( Run Post Post Processor ) Processor )
L19-8 Concentration Profiles