Relationships Between Coal Chemistry and Decomposition Products

Transcription

1 Relationships Between Coal Chemistry and Decomposition Products Thomas. Fletcher Chemical Engineering Department Brigham Young University GCEP Meeting March 15, 2005 Provo, UT

2 Outline What is coal? Simple descriptions of coal reaction Coal chemistry Lattice models Secondary reactions Light gas Nitrogen evolution

3 Coal Decomposition Coal heat Volatiles Char Tar Light gas Soot Primary Devolatilization Secondary Devolatilization Definition: Tar = Volatiles that condense at room T and P

4 Coalification subbituminous lignite bituminous O/C Ratio lig n ite su b b itu m h vb b itu m. lv bituminous h va b itu m lv bit um. anthracite Graphite /C R a tio

5 Moderate Temperature Pyrolysis lignite 1445 subbitum hvb bitum hva bitum lv bitum O/C Ratio /C Ratio

6 Early Char Combustion /C Ratio lv bitum hva bitum hvb bitum subbitum lignite 0.02 CCL, 0% post-flame CCL, 6% post-flame CCL, 6% post-flame O 2, 47 ms O 2, 47 ms O 2, 72 ms O/C Ratio

7 Coal Structure R Loop Structure Mobile Phase Group C O Side Chain O C O C 3 O C C Bridge Structures Bi-aryl Bridge O R O N O C C O N O S C O Pyrrolic Nitrogen C O C O C 3 Aromatic Cluster Pyridinic Nitrogen R

8 Primary Coal Pyrolysis R Tar C 3 O Tar O C 3 C 3 O C 3 C O R CO 2 R O N C N O S C C 3 CO 2 O C 3

9 Lattice Devolatilization Models Coal molecule description 13 C NMR spectroscopy Rates of bridge breaking Aromatic clusters remain intact Kinetics are coal independent Lattice statistics Amount of liberated fragments Vapor-liquid equilibrium Light fragments vaporize Crosslinking Stable bridges form, making char

10 Types of Lattices A. Coordination number = 3 ON E Y COM B L ATTI CE TRI GON AL BETE L ATTI CE B. Coordination number = 4 DIAM OND LATTICE TE TRAGON AL BE TE L ATTI CE

11 Lattice Statistics 20% bridges broken, 0.3% fragments liberated 45% bridges broken, ~10% fragments liberated Fragments include monomers, dimers, trimers, etc.

12 Closed-Form Solution of Percolation Lattice Statistics 1.0 Fraction of Finite Clusters σ + 1 = Fraction of Intact Bridges (p)

13 Vapor-Liquid Equilibrium and Crosslinking Tar Vapor Vapor-Liquid Equilibrium f Labile Bridge Scission MW Infinite Coal Matrix Finite Fragments (Metaplast) Crosslinking f MW Reattached Metaplast f MW

14 Generalized ydrocarbon Vapor Pressure Correlation for the CPD Model ( c c MW T ) vap 3 Pi = c1 exp 2 i / 100 Vapor Pressure (atm) MW = (667 K) (500 K) (400 K) (333 K) 1000/Temperature (K -1 ) Data taken from Gray et al. (Ind. Eng. Chem. Process Des. Dev., 1985) for 12 narrow boiling point fractions of coal liquids from a Pittsburgh seam coal

15 Input Parameters Required by the CPD Model Number of attachments per cluster (σ+1) (i.e., coordination number) Fraction of attachments that are bridges (p 0 ) (bridges/bridges+side chains) Molecular weight per aromatic cluster (M cl ) Molecular weight per side chain (M δ ) Measured with 13 C NMR Spectoscopy Fraction of bridges that are stable (c 0 ) Not measured

16 Do Structure Parameters Correlate? %C in daf coal Coordination Number ( σ+1) Fraction of Intact Bridges (P 0 ) %C in daf coal %C in daf coal Molecular Weight per Cluster MW per Side Chain (M δ ) %C in daf coal

17 Lattice Model Capabilities Can Predict: Tar yield MW distribution Light gas yield Speciation Char yield Elemental composition As a Function of: Coal type Coal structure Residence time Kinetic rates Particle heating rate Distributed activation energies Competing reactions Temperature Kinetic rates Pressure Vapor-liquid equilibrium

18 Using Correlations for Coal Structure Parameters % Yield (daf) CPD mass release measured mass release CPD tar yield measured tar yield 10 limit of data used to make correlations % Carbon (daf) non - U.S. coals, 3000 K/s to 1037 K, (Xu & Tomita) No 13 C NMR data available, from Genetti et al., E&F 1999

19 Light Gas Speciation is Empirical 19 species, needing yield factors and rate coefficients! From Solomon et al., E&F, All E s are distributed!

20 Sample Predictions of Gas Species From Genetti et al., E&F (1999) /C Molar Ratio O/C Molar Ratio 3 Coals Studied by Solomon et al. Coals Studied by Chen Light Gas Composition Other gases Carbon monoxide Methane Carbon dioxide Water Percent Carbon in Parent Coal (daf) 95 Interpolation matrix for gas species (based on coalification diagram) Application to Xu and Tomita data (non-u.s. coals)

21 Nitrogen Release % of Nitrogen in Parent Coal Pyrrolic Pyridinic Other Forms % Carbon (daf) in Parent Coal XPS XANES All nitrogen in coal is contained in the aromatic structure Pyridinic, pyrrolic, and quartenary Nitrogen release highly dependent on tar release Argonne Premium Coals, XPS data from Kelemen et al. (1993), XANES data from Mitra- Kirtley et al. (1993)

22 Tar Does Not Contain All Pyrolyzed Nitrogen, Especially for Low Rank Coals Coal-N fraction in tar/oils Dietz subbituminous Illinois No. 6 hv bituminous Pittsburgh #8 hv bituminous Lower Kittanig lv bituminous Pulverized coal particles in a radiant drop tube reactor (Chen, Stanford University, 1991) Coal-N fraction released

23 Nitrogen Release Models A tar N coal N + light gas N B (fast) char N T<1000 K T<1600 K tar N + + stable char N C (slow) soot N light gas N light gas N T>1600 K (long residence t imes) From S. Perry, PhD Dissertation, BYU, Initial N release w/tar Subsequent N release from char as CN at higher T Secondary tar reactions Soot formation from tar Light gas formation (CN and then N 3 ) Additional N release at extremely high T s 100% nitrogen release possible!

24 Sample Nitrogen Release Predictions 13 C NMR data used for coal structure parameters (from Perry et al., E&F, 2000) Fraction Released ms, 1650 K Measured mass release Predicted mass release Measured nitrogen release Predicted nitrogen release 78 ms, 1650 K Fraction N Retained in Char CPD model E 4=75 kcal/mol, σ E4=3 kcal/mol Pohl BYU Mass Release N remaining in char Mass Release (% of daf coal) daf % C in parent coal Temperature (K) A. Flat-flame burner (high T and dt/dt) matches volatiles yield and nitrogen release) B. igh temperature crucible data volatiles reaches constant value nitrogen is totally released!

25 Nitrogen Structural Parameters Modeled Correctly! Structural N parameter as reaction proceeds % decay of N site in char (from Perry et al., E&F, 2000) daf % C in parent coal Australian, Japanese, and American coals Pyrolyzed at 1100 K in N 2 (drop tube) Based on 13 C NMR analysis and elemental composition N site, char /N site, coal N site, char /N site, coal Beulah Zap M cl, char /M cl, coal adjusted Illinois #6 M cl, char /M cl, coal adjusted N site, char /N site, coal N site, char /N site, coal N site, char /N site, coal Pocahontas # Blue #1 M cl, char /M cl, coal adjusted Pittsburgh # M cl, char /M cl, coal adjusted 1.0 M cl, char /M cl, coal adjusted

26 Soot Formation Soot forms from coal tar, not acetylene Devolatilization Coal Char + Light Gases + Tar Tar Formation Gasification Agglomeration Primary Soot Soot agglomerates Light Gases Soot model developed and implemented Uses predicted tar from CPD model Soot formation, oxidation, and growth included Predicts up to 300 K lower near-burner temperature in CFD model

27 Conclusions A lot of good scientific research performed on coal pyrolysis Lattice models capture much of the chemistry Models tuned to match existing data Mass release vs. t, T, dt/dt, P tot, coal type Tar yield (and MW, composition) Gas species Nitrogen release Soot formation These models are good tools to explore new concepts!