Coal devolatilization during rapid transient heating ... - ACS Publications

Energy & Fuels 1992, 6, 254-264

254 -3.5

PDJlO

catalyst from the original charge rather than due to attrition. Thus catalyst attrition does not appear to be a problem with the Amocat 1A catalyst.

Conclusions The ebullated bed microreactor provides an accurate and convenient means of evaluating hydrotreating kinetics a t the laboratory scale. The reactor may be treated as perfectly mixed, and the advantagea of the gradientlegs reactor are evident in the kinetics analysis. Thermal reactions play a small but significant role in the HYD reactions. For HDN, thermal reactions could be ignored completely. Catalyst recovery was near 100% after as many as 30 days of continuous operation.

-5.5 1.35

1.40

1.45

1.50

1.55

1.60

1000/T, K-'

Figure 12. Arrhenius plot for HYD. extracted in THF and dried, and a portion was fired in a muffle furnace. From the recorded weights, the percent catalyst recovery relative to the charge of fresh calcined catalyst was calculated and listed in the table. Except for run PDJO8, essentially 100% of the catalyst was recovered. The loss of catalyst observed for run PDJO8 occurred when the basket screen size was increased, and the loss was probably due to our failure to screen fine particles of

Acknowledgment. This work is jointly sponsored by the U.S. Department of Energy (Grant DE-FG2288PC88942) and the Amoco Oil Co. We are grateful for both financial support and consultations provided by these organizations. Mr. Jacobs' support was provided by EPSCOR I. Fossil Energy Component, and funds for the reactor were provided by the Industrial Fund of the University of Wyoming. Such support does not constitute an endorsement by DOE of the views expressed herein. Registry No. Moos, 1313-27-5;COO,1307-96-6;Amocat lA, 78615-50-6.

Coal Devolatilization during Rapid Transient Heating. 1. Primary Devolatilization John C. Chen and Stephen Niksa* High Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University, Stanford, California 94305 Received October 7,1991. Revised Manuscript Received January 16, 1992 Our new flow experiment heats entrained coal suspensions by radiation from a near-blackbody enclosure, not by a preheated stream. Since the entrainment stream is transparent to radiation, it can be made to remain much cooler than the suspension. Pristine products which have been quenched as soon as they were expelled can be recovered or, alternatively, the extent of secondary chemistry can be regulated a t will. The furnace system also includes rapid quenching to resolve reaction times on a scale of several milliseconds, aerodynamic segregation of particulate, aerosol, and gaseous products, and analyses for C1-C4hydrocarbons, oils, CO, C02, H20, NO, and HCN. Balances on mass, carbon, and nitrogen are usually closed to within 5% in individual runs. Product distributions for transient devolatilization a t nominal heating rates of lo4 K/s are reported for four coals of ranks from subbituminous to low-volatile bituminous. They show that during the initial stage of primary devolatilization, coals of lower rank expel significant amounts of noncondensibles with their tar, but predominant tar evolution is a distinctive feature of ranks of hvA bituminous and higher. Tars from this experiment have the highest H/C ratios ever reported, as well as the lowest proton aromaticities. Throughout primary devolatilization, their proton distributions shift from substantial fractions of p- and y-protons to mostly a-protons. Yields of all gaseous hydrocarbons, carbon oxides, and water increase in proportion to the tar yields with no distinctive surges or lapses. The major C1-CB hydrocarbons are CHI, C2H2,and C2H4 but oils are the major gaseous hydrocarbon product from bituminous coals. Aromatic hydrocarbons, especially tars,are the only shuttles for fuel nitrogen during the initial stage. During the later stage of primary devolatilization, no tars are expelled but additional C1-Cz hydrocarbons are generated, and distinct surges in the evolution of CO and HzO from coals of lower rank actually increase the rates of weight loss. Also, HCN appears for the first time during the later stage. H/C ratios for char from 0.3 to 0.4 and little residual oxygen signal the end of primary devolatilization.

Introduction Product and elemental distributions from coal under typical pulverized-fuel (pf) firing conditions are needed to characterize the heat release and pollutants from coal

flames. Globally, the heat release from burning volatiles involves only their aggregate elemental composition. But accurate estimates require information which can only be determined from complete product distributions, as evi-

0887-0624/92/2506-0254$03.00/00 1992 American Chemical Society

Energy & Fuels, Vol. 6, No. 3, 1992 255

Coal Devolatilization / / /

quartz tube.

/ / /

/ /

\

\ \ \ \ To gas analyzers

\ \

to vacuum

transducer

Figure 1. The radiant coal flow reactor.

dent in detailed m0dels.l Pollutant formation obviously involves many molecular species. Moreover, strategies to manage pollutant chemistry must distinguish mechanisms for the formation of primary products from those for their subsequent secondary transformations in hot gases. For example, aerodynamic NO, abatement strategies manipulate chemistry in postflame zones, beginning with the products of secondary pyrolysis and partial combustion of primary volatiles. Primary devolatilization products are the gases created by chemistry among species and functional groups in the condensed phase only. Secondary products are formed from subsequent transformations among primary products, after they pass through the interface between solid and gas. These classes are easily distinguished in conceptual terms, but not in laboratory studies. However, we have several reasons to believe that products acquired with wire grid reactors are, in fact, primary devolatilization products. Recent additions to this database characterize the major noncondensible species2v3and tar4 from coals across the rank spectrum. But the genesis of products extracted from entrainedflow experiments is far more ambiguous. In entrained-flow furnaces, preheated gases provide the energy to initiate and sustain devolatilization. Since devolatilization always occurs while the coal is being heated to the ultimate reactor temperature, primary pyrolysis products are always expelled into gases which are hotter than their parent particle. It is virtually inevitable then that primary products undergo uncontrolled secondary reactions in the hotter process stream. In hot gases, tars exhibit a strong propensity to soot and rapidly eliminate their most illuminating structural feature^.^^^ Noncondensibles are also radically altered,7*8including nitrogenous specie^.^ We developed the radiant coal flow reactor to alleviate this drawback. Here too the suspension is entrained in a (1)Lau, C.-W.; Niksa, S. Combustion of Individual Particles of Various Coal Types. Combust. Flame, in press. (2) Xu, W.-C.; Tomita, A. Fuel 1987, 66,627. (3) Xu, W.-C.; Tomita, A. Fuel 1987,66,632. (4) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. Energy Fuels 1989,3, 692. (5) Nenniger, R. D. Aerosols Produced from Coal Pyrolysis. Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, 1986. (6) Womat, M. J.; Sarofim,A. F.; Longwell, J. P. Symp. (Int.) Combust. [Proc.],22 1988, 135. (7) Tyler, R. J. Fuel 1980,59, 218. (8) Doolan, K.R.; Mackie, J. C.; Tyler, R. J. Fuel 1987,66, 572. (9) Freihaut, J. D.; Zabielski, M. F.; Seery,D. J. Symp. (Int.)Combust. [Proc.] 19 1982,1159.

simple flow field. But it is heated by radiation from a near-blackbody enclosure, not by a preheated stream. The entrainment stream is transparent to radiation and can be made to remain much cooler than the suspension. Pristine products which have been quenched as soon as they were expelled into the gas phase can be recovered or, alternatively, the extent of secondary chemistry can be regulated a t will. This approach is introduced in separate parts devoted to the products of primary and secondary pyrolysis. Here, in part 1,the apparatus and its operating characteristics for primary devolatilization studies are described first. Then product distributions for transient devolatilization a t nominal heating rates of lo4 K/s are reported for four coals of ranks from subbituminous B to low-volatile bituminous. Major noncondensible gases are resolved into C1-C4hydrocarbons, oils, CO, C02, and H20. Tar quality is expressed with its elemental C / H / N distribution and proton distributions from NMR spectroscopy. Fuel nitrogen evolution is monitored in terms of HCN and NO yields, and the nitrogen contents of char and tar. Yields and elemental compositions of char close the balances on total mass, carbon, and nitrogen in individual runs.

Experiment Overview. This brief description is supported by additional

details'O and design specifications" in other publications. A schematic of the system appears in Figure 1. At the top a feeder dumps pulverized particles into an entrainment stream, forming an optically-thin suspension which flows downward into a radiant furnace section. The radiant section consists of a quartz tube on the axis of a graphite cylinder which is inductively heated to temperatures to 1850 K. Near-blackbody thermal emission from the graphite rapidly heats the particles as they traverse the furnace. The suspension is kept optically thin to ensure that the radiant heat flux to individual particles at any axial position is uniform, and the macroscopicbehavior can be interpreted in terms of single-particlephenomena. Also, since the entrainment gas is transparent to the radiation, its only means of heating is by convection from the tube wall and particles. Dilute suspensions have little interfacial surface area for heat transfer, so the entrainment gas remains relatively cool and quenches chemistry among primary products as they are expelled. To promote sec(10) Chen, J. C.; Niksa, S. A Radiant Flow Reactor for High Temperature Reactivity Studies of Pulverized Solids. Rev. Sci. Instrum., in press. (11) Chen, J. C. Effects of Secondary Reactions on Product Distribution and Nitrogen Evolution from Rapid Coal Pyrolysis. Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Stanford, 1991.

Chen and Niksa

256 Energy & Fuels, Vol. 6, No. 3, 1992 Table I. Summary of Operating Conditions flow rates, L(STP)/min entrain, flow sheath flow quench flow Primary Pyrolysis Studies, 5 cm Furnace

resid time," ms

central jet vel, m/s

89.0 86.5 83.0 77.0 72.0 66.0 61.0 56.0

0.18 0.25 0.33 0.40 0.50 0.67 1.0 2.0

0.85 1.18 1.56 1.88 2.36 3.16 4.71 9.42

b

0.18

0.85

2.21 2.93 3.83 4.52 5.23 6.06 6.50 6.79

15.8 15.8 15.8 17.7 16.5 14.9 12.9 7.9

total flow'

impactor inlet Re

19.9 20.9 22.2 25.1 25.1 25.1 25.1 25.1

4730 5000 5310 6000 6000 6000 6000 6000

19.9

4730

Secondary Pyrolysis Studies, 12 cm Furnace 2.21

15.8

"Determined at 1840 K wall temperature. bMeasuredresidence times vary from 175 to 155 ms for furnace temperatures from 1480 to 1740 K. 'Includes a fixed flow through the transpiring tube of 1.0 L(STP)/min. ondary reactions, the suspension loading is increased to enhance interphase heat transfer and elevate the gas temperature. In addition, residence times are extended in a longer furnace for secondary chemistry studies. Nominal particle residence times are set by the inlet gas flow rate and the distance between the inlet plane and an argon quench nozzle, which is f=ed; reported values are measured under actual conditions. For all studies reported in part 1, residence times are varied at fixed furnace temperature. Thermal histories in such a series of runs have nearly the same heating rate, but the suspension achieves different temperatures at the outlet in each case. Higher temperatures are achieved as residence times are extended, but outlet temperatures are always well below the furnace temperature. Calculated thermal histories are presented below. All tar and char is collected so that total yields are determined gravimetrically. Noncondensible gases are quantified by nondispersive infrared (NDIR)and chemiluminescence analyzers and gas chromatography. Mass and elemental balances are closed in independent runs and reproducibilityis good, so that subsequent interpretations are not subject to inordinate scatter in the data. The Radiant Furnace and Flow System. An enlarged sketch of the induction furnace also appears in Figure 1. It consists of a 10-cm (i.d.) graphite cylinder encased in zirconium oxide insulation wrapped by five turns of a water-cooled copper induction coil. This assembly is mounted within water-cooled copper plates and a quartz housing. The overall height of the furnace is 9.5 cm, although the hot zone (graphite length) is 5 cm long for all experiments reported in part 1. (The hot zone used in runs reported in part 2 is 2 1/2 times longer.) Wall temperatures to 1850 K are routinely achieved. They are monitored to an uncertainty of 10 K with a disappearing-fiient pyrometer. Axial temperature profiles are uniform to within 98% of the mean temperature, except for the outermost 5 mm at both ends, over which the temperature fails to about 75% of the mean value. Thermal histories of the suspension and entrainment stream are sensitive to the suspension loading and particle size distribution. Our feederlodelivers steady and reproducible suspension loadings from 200 to 6OOO particles/cm3, without the pulsations, comminution, and agglomeration that plague screw feeders and fluidized beds. The feeder generates a 1 0 " particle jet on the axis of the 20" (id) quartz tube through a furnace. An annular or 'sheath" flow stabilize the particle jet and eliminates deposition on the walls. Visualization studies in cold flow with diverse loadings, entrainment flow rates, and ratios of jet and sheath velocities identified operating regimes which yielded stable, undispersed Suspensions over the length of the furnace. (Particle jets eventually become unstable for any inlet condition.) These operating conditions are implemented in high-temperature runs. Although hot flows have not been visualized, the effectiveness of the sheath flow is corroborated by the absence of deposita on the tube wall for all conditions. All residence times in this paper are measured values. They are assigned as the time interval between the interruption of two HeNe laser beams 0.75 cm beyond the end plates by the leading edge of a long pulse of coal suspension. Times from 50 to 100 ms can be resolved into incrementa of only several milliseconds in the 5-cm furnace. The operating conditions in Table I show that measured residence times differ from the nominal values

based on the total gas flow rate and channel cross section by up to 50%, for several reasons. Boundary layers grow on the tube walls, contractingthe area of the potential core which carries the suspension and acceleratingit. The stream's thermal expansion also accelerates the flow. Radiation is intercepted by the SUBpension before and beyond the furnace caps, so the length of the reaction zone is somewhat ambiguous. Theae factors do rationalize the measured values, as explained elsewhere.lOJ1 At the furnace outlet, a quench nozzle blasts argon into the process stream, rapidly quenching all chemistry and nucleating tar into an aerosol. Particle cooling rates range from 3000 to 9OOO K/s, depending on the proportions of quench gas and process stream" (seeTable I). Characterization studies with various coals showed that 2 4 wt % of aerosol deposits in the nozzle, regardless of the aerosol yield.lOJ1Assuming that the deposita are random samples, we add 2 wt % to all aerosol yields greater than 4 wt %. A porous, transpiringtube eliminates deposition as the stream moves from the quench nozzle into the virtual impactor. Product Recovery and Analysis. Products are segregated into bulk solid particles, aerosol, and noncondensible gases with virtual impaction in an aerodynamic classifier,sketched in Figure 1. Beyond the inlet port the process stream is split between opposing nozzles. Ninety-five percent of the flow passee into the annulus. Since the aerosol particles are typically a few micrometers in diameter, they are convected into the annulus and ultimately onto filters. By virtue of their inertia, char particles penetrate the impaction surface and fall into a wire mesh basket. Yields of char and aerosol are based on the weight gain of their respective collection elements and the suspension feedrate. Aerosol yields are adjusted for the flow split in the virtual impactor, deposition losses in the quench nozzle, and losses in the gas sampling line (discussed shortly). Tars are recovered on a four-stage assembly of glass-fiber fiitera and a polypropylene liner which were dried overnight at 335 K under vacuum. Pure tar samples for subsequent chemical analyses are prepared by extraction with tetrahydrofuran (THF)in an ultrasonic bath, followed by filtration through a 0.2-rm Teflon membrane. The membrane residue is weighed and denoted as the scot yield. The tar solution is concentrated in a Kudema-Danish concentrator before the remaining solvent is evaporated, following the p d u r e of Lafleur et d.12 Several tens of milligrams of pure tar are recovered from runs with significant aerosol yield. Carbon, hydrogen, and nitrogen contentsof condensed products are determined with an elemental analyzer calibrated with acetanilide. Tar proton distributions are determined by 'H NMR spectroscopy,using a Varian XL-400spectrometer operating at 400 M H z with an average acquisitiontime of 1.5 s, and tabulated chemical shifta.13 Deuterated pyridine is the solvent of choice because it completely dissolves these tars, and the signal from water in pyridine does not interfere with features in the tar spectra Noncondensible gases are sampled into multiport valves maintained at 335 K through a port in the impactor. Lees than (12)Lafleur, A. L.; Monchamp, P. A,; Plummer, E. F.; Kruzel, E. L. Anal. Lett. 1986,19(21&22), 2103. (13)Collin, P. J.; Tyler, R. J.; Wilson, M. A. In Coal Liquefaction Products; Schultz, H. C., Ed.; John Wiley and Sone: New York, 1983; Vol 1, p 85.

Coal Devolatilization


4

Net Flux\

y-

innn

2

--400

~~

coal type PSOC 1488D, 39.9 3.9 69.5 5.0 0.97 0.40 24.1 Dietz, subbituminous B PSOC 1493D, 37.5 12.9 74.1 5.3 1.52 5.7 13.4 Illinois No. 6, hvC bituminous PSOC 1451D, 34.7 12.6 82.5 5.6 1.77 1.6 8.5 Pittsburgh No. 8, hvA bituminous PSOC 1516D, 17.1 19.3 88.7 5.0 1.72 2.5 2.1 Lower Kittaning, lv bituminous " Weight percent (dry). Weight percent (dry, ash-free).

L

i 0

Table 11. Coal Analyses volatile matte? ash" Cb H b Nb Sb Ob (diff)

20

40

60

80

Measured Residence Time, ms

Figure 2. (a, top) Radiant fluxes onto the centerline of the furnace at 1840 K for an inlet gas velocity of 25 cm/s. Dashed

vertical lines denote the extent of the hot zone. The net flux (upper solid curve) is composed of fluxes from the graphite (solid curve with circles), lower (dashed curve) and upper (dotted curve) zirconia end plates, and the quartz tube (lower solid curve). (b, bottom) Calculated thermal histories of the suspensions (solid curves) for measured values of residence time in the primary devolatilization studies of 56, 61, 66, 72, 77, 83, 87, and 90 me, and calculated gas temperature for 90 ms (dotted curve). 7% of the flow is sampled so flow patterns within the impactor are hardly perturbed, although aerosol yields are adjusted for the amount withdrawn into this line. C144 hydrocarbons and HCN are chromatographed on a 1.83-m Porapak Q column; a separate injection onto a 1.83-mPorapak N column quantifies ethylene and acetylene. Yields of oils, defined as hydrocarbons with carbon numbers of 5 and higher remaining in the gas phase under the conditions in the impactor, are based on the total hydrocarbon signal from a flame ionization detector (FID) calibrated with pentane. This value is reduced by the yields of C1-C4 hydrocarbons (determined by a separate sample injection) to arrive at the oils yield. The concentrations of CO, COz,and H20 in the effluent are determined on-line with NDIR analyzers. NO is determined by a chemiluminescence analyzer. NH3 is not yet monitored. Thermal Histories. Distinguishing primary and secondary chemistry is largely a matter of independently regulating the thermal histories of solids and the entrainment stream. Diagnostics to measure these temperatures are feasible but difficult to implement. Suspension pyrometry at the furnaceoutlet entails substantial corrections for scattered furnace emission. Gas temperature measurements are also complicated by the intense radiation, and by deposits of heavy hydrocarbons onto thermocouples. In lieu of data, the calculated thermal histories in this section demonstrate that entrainment streams can be kept much cooler than the coal suspensions in the radiant flow reactor. Our analysis assigns the temperatures and velocities of an individual particle on the centerline and of the entrainment gas, and particle residence times, as explained elsewhere.lOJ1These thermal histories pertain to the entire suspension,provided that the loading is below 1200 particles/cm3,at which point ita radiant absorption becomes significant and it is no longer optically thin in some directions. The radiant flux from the furnace onto the flow axis is defined by an analysis which does not consider the suspension,because optically-thin suspensions absorb insignScant amounts of the radiation. This flux along with other fluxes within and between particles and gas defines the thermal histories of the suspension and gas stream.

The radiation analysis includes exchange among the graphite wall, regarded as graK the zirconia end plates, which are also gray; and the quartz flow tube, which is transmitting, but only into the near-infrared region. Conduction among these elements is omitted, except that heat losses from the zirconia plates are incorporated into the analysis. Convection from the entrainment stream to the quartz tube is included. The radiant flux onto the centerline of the 5-cm furnace appears in Figure 2a. With the graphite at 1840 K, the maximum flux is 60 W/cm2, similar to the 50-100 W/cm2 estimated for coal-fired burners in utility boilers. One-third of the heat flux is transmitted from the graphite through the quartz, and another third come8 from both end plates. The remainder emanates directly from the quartz tube. The axial profile of the heat flux is asymmetric because the heat-transfer coefficient varies as boundary layers develop on the tube walls. Significant heat fluxes eacape from the hot zone at top and bottom, which shows up in the temperature profiles of particles. Consequently, the physical length of the hot zone does not equal the distance over which the suspension is heated, although these lengths do converge as the length of the hot zone is increased. The net radiation flux enters into the energy balance for an entrained particle on the centerline, as do convection losses to the entrainment stream and the pyrolysis endotherm. These fluxes sum to match the thermal capacitance. The convective heat flux is evaluated in the conduction limit with a blowing factor to represent the impact of outflowing volatiles. Due to their coupling in the convection term, the energy balances for particles and gas must be solved simultaneously. The balance for the gas phase matches its thermal capacitance with fluxes due to convection from the suspension and from the quartz wall. These fluxes define the axial profile of average gas temperature. Calculated temperature profiles for all of the operating conditions for primary pyrolysis studies appear in Figure 2b. As in the actual experimental runs, gas velocities and coal feed rates are simultaneously adjusted to vary residence time but fix the suspension loading at 300 particles/cm3 at the inlet. Particles begin heating 1 cm upstream of the inlet plane (not shown), consistent with the radiant flux in Figure 2a. Even though their heating rate exceeds 104 K/s, particles are much cooler than the 1840 K furnace at the outlet. Having been designed for transient heating studies, this furnace does not impact an isothermal stage to the suspension. Most importantly, the gas heats at only 2300 K/s, and reaches only 600 K at the outlet after the longest residence time. At this temperature, secondary reactions are too slow to affect coal-derived products? And the 400 K disparity between particle and gas temperatures is an effective driving force for quenching. As explained below, various product characteristics strongly corroboratethe minimal extenta of secondary chemistry achieved in this furnace. Coal Properties. The four coals were size-classified and distributed by the Pittsburgh Energy Technology Center (PETC). As seen in Table 11,their ranks range from subbituminous B to low-volatile bituminous. Proportions of carbon, hydrogen, and nitrogen were determined in-house; the sulfur content was pro-

258 Energy & Fuels, Vol. 6, No. 3, 1992

time, ms 56 61 66 72 77 83 86.5

89

Chen and Niksa

Table 111. Distribution of Noncondensible Hydrocarbon Gases for Pittsburgh No.8 Coal yield, wt 7' 0 daf CH4 CzHz CZHl CZH6 CBH6 C3Hs total C1-C3 0.040 0.11 0.24 0.29 0.20 0.58 0.21 0.53 0.85 1.0 1.5 1.6 1.7 2.1 2.1 1.7 4.0 3.6 3.0 3.0 3.0

a

0.070

0.25 0.32

0.65

1.5

a 0.15

0.52 0.92

1.2

1.8

0.008 0.034 0.057 0.10 0.080 0.11 0.109 0.16 0.28 0.30 0.34 0.34 0.39 0.39 0.37 0.32 0.29 0.25 0.32 0.31 0.29

0.015

-0 0.020 0.034 b 0.060 0.030 0.070 0.070

0.086

0.080 b 0.20 0.16 0.21 0.21 0.39

0.080 0.34 0.55 0.73 0.80 1.5 0.80 1.3 2.3 2.4 3.1 3.9 4.1 5.0 0.46 4.2 8.2 7.1 6.6 6.9 6.8

0.10

0.70 0.56 0.70

0.11 0.092 0.057

0.81 0.44 0.47 0.55

0.090

-0 -0 -0

0.92

0.060

Oil8

0.14 0.40 1.0

0.87 1.0 1.8 1.1 2.5

nmc 3.1

nm 4.1 5.3 10.8

nm 9.2 4.6 6.0

nm nm 6.0

CzHzand CzHl elute as a single peak from Porapak Q; their s u m is measured and used to calculate total Cl-C3. bC3Hgand C3HBelute as a single peak from Porapak N; their s u m is measured and used to calculate total C1-C3. 'nm = not measured in the runs. vided by PETC and oxygen was determined by difference. High-temperature ash levels and proximate volatile matter contents were provided by PETC. The samples in all experiments were aerodynamically classified into the 75-106-pm size grade, dried in a vacuum oven for at least 12 h at 25 in-Hg(vac)and 60 "C, and stored under argon. Measured weight losa and yields were converted to the dry, ash-free (daf) basis using the ash levels in Table 11.

Results All data were obtained with the furnace hot zone at 1840 K under 0.103 MPa with argon entrainment, sheath, and quench streams. Additional cases with various furnace temperatures are available." Gas flow rates were regulated to vary residence times. As flows were adjusted, the coal feed rate was regulated to deliver 300 particles/cm3 into the furnace in all runs. Gross features of primary devolatilization and closure of the mass and carbon balances are illustrated in Figure 3a,b for Pittsburgh No. 8 (Pit. No. 8) suspensions. The data in Figure 3a depict the entire course of transient devolatilization, even though the weight loss transient does not appear to be approaching its asymptotic ultimate value. But yields a t 89 ms are very nearly the ultimate values observed for more severe conditions (reported in part 2 and in Figure 6,below). As expected for Pit. No. 8,14tar is the most abundant product lump, and also virtually the only product of the first half of primary devolatilization on a mass basis. Yields of noncondensible hydrocarbon gases include both C143 compounds and oils. Oil yields are resolved in Table I11 (below) to show that they are the major gaseous hydrocarbon from the Pit. No. 8. Their transient behavior closely follows the tar yields, as e x ~ e c t e d . ' ~ Oxygenated J~ gases are less abundant and preferentially generated during the later stages. All of these features are also evident in the carbon distributions in Figure 3b. Mass balances in Figure 3a are based on independent determinations of the yields of char, tar,oils, CHI, CzHz, CZ&, CZH6, C3H8, and C3&, aggregate C4's, CO, COz, and H 2 0for individual runs. They generally close to 100 f 5%. Carbon balances in Figure 3b also involve the elemental determination of the char, tar,and oils,and generally close (14)Oh,M.S.;Peters, W.A.; Howard, J. B.AZChE J . 1989,35,776.

60

70

so

90


L!

0.5

!I

60

70

80

90


Figure 3. (a, top) Cumulative transient yields from Pit. No.8 in the furnace at 1840 K of C1+ hydrocarbons plus oils (0);H20 (+); CO plus COB( 0 ) ;and tar (A). The mau balances on the separate scale (a)also involve char yields. (b, bottom) Cumulah transient carbon fractions from Pit. No. 8 in the furnace at 1840 K of Cl-C3 hydrocarbons (A);CO plus C02 (X); oils ( 0 ) ;and tar (0).The carbon balances on the separate scale ( 0 )also involve char yields.

to the same tolerance. The contribution for oils is substantial, especially for the Pit. No. 8. Ita elemental composition is not measured; oils are assigned the elemental composition of tar. Since no aliphatic hydrocarbons heavier than C3 are ever observed, oils are probably aromatic compounds having elemental distributions very similar to tar's. The aerosols recovered in the impactor from the Pit. No. 8 and the other coals were examined for soot, according to solubility in THF and filtration. Ineolubles were found to make up less than 1wt % in every case, which is below



80

70

60

90


60

70

80

90


Figure 4. Transient weight loss (0) and tar yields (0)versus measured residence times from the furance at 1840 K for (a) Dietz, (b) Ill. No. 6, (c) Pit. No. 8, and (d) lv bituminous coal.

.

. *

*.

0

0

"

"

I

"

" 20

~

"

"

.

I 40

60

Weight Loss, wt% daf

A.

0 " " 1 " " 1 " " 1 " 60 70

80

.

.

A

90

Carbon Content, wt% daf

Figure 6. Comparison among our tar plus oil yields at 86.5 ms (e),weight loss at 89 ms (0and solid curve), and weight loss in secondary pyrolysis studies (0and dotted curve) with weight loss and tar yields from wire grids reported by Xu and Tomita2(A and A) and Bautista et al.16 (0and m), and tar yields from Freihaut et aL9 (X).

Weight Loss,wt% daf

Figure 5. (a, top) Comparison among our tar plus oil yia.-, and tar yields reported by Bautista et al.16 (+) and Oh et al." from Pit. No. 8 versus weight loss. (b, bottom) Measured tar I oil yields versus weight loss for Dietz (A),Ill. No. 6 (X), Pit. 8 (e),and lv bituminous ( 0 )coals. The regression lines en( the ultimate yields of tar plus oils. the level expected from the separation efficiency of the virtual impactor. This is the fmt conclusive evidence that secondary pyrolysis exerts minor influence, if any, on the recovered products. It also corroborates the disparate thermal histories of particles and gas in Figure 2b. Transient weight loss and tar yields for all coals appear in Figure 4. The relative contribution of tar to the initial weight loss is rank dependent. Both the Dietz and Illinois No. 6 (Ill. No. 6) expel significant amounts of gases from the onset of devolatilization. Initially, gas yields from the Pit. No. 8 are too low to resolve on this scale. Those from the lv bituminous may be relatively larger but still cannot

be resolved from the experimental uncertainty. Predominant tar yields during the initial stages seem to be a distinctive characteristic of ranks of hvA bituminous and higher. Tar evolution from all coals ceases well before weight loss reaches asymptotic values. For both of the lowest rank coals, rates of weight loss actually increase while tar yields remain a t their ultimate values. As will be shown shortly, CO and H20 evolution account for most of the additional weight loss during the final stages. The surges of weight loss from the Dietz and Ill. No. 6 a t the end are therefore consistent with their higher oxygen contents. Ultimately, tar yields comprise a t least half of the weight loss for all coals except the Dietz. The maximum of 30 wt % is observed with the Pit. No. 8; however, ar yield as a fraction of ultimate weight loss reaches the t a maximum for the lv bituminous, consistent with recent report^.^*^ Two aspects of consistency with the current database are demonstrated in Figures 5 and 6. The weight loss scale on these abscissas is an appropriate measure of the extent of primary devolatilization in comparisons, provided that all data represent similar heating rates without secondary

Chen and Niksa


tar decomposition. In all cases in the plot of tar plus oil yields versus weight loss for Pit. No. 8 in Figure 5a, heating rates exceed 1000 K/s, and secondary pyrolysis is not a factor in the wire-grid studies. Despite their different thermal histories, these studies show essentially the same relation. The relation in Figure 5a is also satisfied by Tyler’s hv bituminous data’ from a fluidized bed,but only for bed temperatures below 876 K, when tar decomposition was not important. Earlier datal5 (not shown) are offset from the others in Figure 5a by the amount of pyrolytic water reported at the onset of devolatilization, which now seems too high. As seen in Figure 5b, the relation between tar plus oil yields and weight loss is supported by data for the other coal types, but not with a common regression line. Due to predominant tar evolution during the initial stages with high-rank coals, the Pit. No. 8 and lv bituminous show the same relation between heavy hydrocarbon yields and weight loss. But tar contributes less to the weight loss from lower rank coals, so regression lines of progressively lower slopes fit the data from the Ill. No. 6 and Dietz coals. The observed rank dependence of weight loss and hydrocarbon liquid yields is placed in the context of the most recent studies in Figure 6. Data from this study are tar plus oil yields at 86.5 ms and total weight loss a t 89 ms. These are the highest values observed and are therefore regarded as ultimate primary devolatilization yields. Asymptotic weight loss data for more severe pyrolysis conditions (given in part 2) also appear. Our data reconfirm the following trends: (1)Weight loss is roughly constant for coals of increasing rank through hvA bituminous, then falls off sharply for low volatility coals. (2) Tar/oils yields increase for coals of higher rank, reaching a maximum value for hvA bituminous, and then fall off for coals of higher rank. (3) The fraction of weight loss due to tar/oils increases for coals of higher rank and is greatest for low volatility coals. (4) Noncondensible gas yields decrease monotonically with increasing rank. Quantitatively, our tar yields from both hv bituminous samples me higher than most literature values, because tar and oil yields have been combined; ultimate yields of oils are 5 w t % and 10 wt % of the Ill. No. 6 and Pit. No. 8, respectively, compared with -3 w t % for both the Dietz and lv bituminous coals. For the Pit. No. 8, our tar yields are significantly higher than all but those reported by Bautista et al.I6 and Oh et al.14 Tar yields reported by Bautista et al. for four low volatility bituminous coals are also very similar to ours for the comparable coal rank. Bautista et al. recovered their hydrocarbon liquids with liquid-nitrogen-cooled components, which may have incorporated oils into the tar sample, as we have done by summing the respective yields. Our total weight loss values for 89 ms tend to be higher than values reported by Xu and Tomita for 4 s isothermal reaction a t 1037 K following heatup a t 3000 K/s. Our slightly higher heating rate is the only aspect of these thermal histories which could account for the differences, but is probably not responsible. Ultimate weight loss from the more severe secondary pyrolysis studies is higher by up to 5% for the Dietz and Ill. No. 6. Due to higher particle temperatures in the secondary pyrolysis studies, additional hydrocarbon and oxygenated gases are expelled, but on the same time scale as for gas evolution during the later stages of primary devolatilization. I t is difficult to

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(15) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Symp. (Znt.) Combust. [Proc.],17 1979, 117. (16)Bautista, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986,25, 536. (17) Deleted in proof.

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Figure 7. (a) Transient yields of co (A),COz (A)and HzO (D) from Dietz coal versus measured residence times in the furnace at 1840 K. (b)Transient yields of CO (A),COz (A),and H20 (0) from Pit. No. 8 versus measured residence times in the furnace at 1840 K. assign precise values to ultimate yields for primary devolatilization for these coals, but not for higher rank samples. Distributions of CO, COz, and H20 from the Dietz and Pit. No. 8 appear in Figure 7; data for the other coals is available.ll For all coals, chemically-formed water is the most abundant oxygenated gas throughout most of primary pyrolysis. Our ultimate water yields agree with Xu and Tomita’s2but are not more than half of the first values in the 1iterat~re.l~ During the latest stages, however, CO is rapidly evolved to become the most abundant oxygenated gas for all coals except the Lower Kittaning. Actually, both CO and H20 yields surge after tar evolution ceases, accounting for most of the weight loss after 80 ms, as pointed out d e r . Yields of C02 increase steadily, without surging. Ultimate values become proportionately lower for coals of higher rank, consistent with their lower oxygen contents. Carbon dioxide and water are among the first noncondensible products of primary devolatilization from any of these coals. Distributions of the C1-CBhydrocarbon gases and oils are illustrated with the values for the Pit. No. 8 in Table 111. For all coals, methane is the most abundant hydrocarbon, and the C1-C3hydrocarbons consist primarily of methane, acetylene, and ethylene. These gas yields increase throughout tar evolution and continue to increase during the final stage, like CO and H20yields. In contrast, ethane, propylene, and propane yields increase throughout tar evolution, but then reach asymptotes or, for propane, decrease during the later stages. .Evolution histories of hydrocarbons from the other coals are similar, except that yields of C1-C3 hydrocarbons from the Pit. No. 8 are 50% higher than for similar values from the other coals. Yields of C4’s are negligible for all coals. Observed oil yields are more scattered, but show the general tendency to increase as long as tar is being expelled. Oil yields from the Pit. No. 8 are a t least double the similar values from the other coals. Elemental composition of the oils was not monitored, but assigned as tar’s



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J?igum 8. (a) Cumulative transient nitrogen fractions from Dietz coal versus measured residence times in the furnace at 1840 K (b) Cuas NO (e),HCN (A); tar plus oils (m); and char (0). mulative transient nitrogen fractions from Pit. No. 8 versus measured residence times in the furnace at 1840 K as NO (e), HCN (A); tar plus oils (m); and char (0).

Figure 9. (a) Atomic H/C ratios of tar (v)and char (0) from Dietz coal versus measured residence times in the furnace at 1840 K, compared to the value for the whole coal (dashed line). (b) Atomic H/C ratios of tar (V)and char (0)from Pit. No. 8 versus measured residence times in the furnace at 1840K, compared to the value for the whole coal (dashed line).

because of trends among the noncondensible hydrocarbons. In Table 111, yields of hydrocarbons with two or more carbon atoms diminish with increasing carbon number; i.e., the yields of aggregate C4's are lower than for aggregate Cis, etc. Consequently, since their yields are so high, oils cannot be aliphatic hydrocarbons of higher carbon numbers. Rather, they are probably lighter aromatic components, similar to the oils characterized by Xu and Tomita? which have sufficient vapor pressures a t the conditions in the impactor to remain in the gas phase. Then their elemental compositions would be most similar to tar's, as assumed in our elemental balances. Nitrogen distributions during primary devolatilization from the Dietz and Pit. No. 8 appear in Figure 8. Tar and oils are virtually the only shuttles for fuel nitrogen during primary devolatilization. This observation applies to the four coals in this study and to many other samples in the wire-grid study of Freihaut e t aL9 NO is never observed to contain more than fractions of a percent of the fuel N, and HCN contains lo%, a t most. Results in part 2 demonstrate that most of the cyanide is a product of secondary pyrolysis. Consequently, the small amounts of HCN generated with the radiant flow reactor are additional conclusive evidence for the minimal impact of secondary chemistry in our primary devolatilization studies. Actually, most of the HCN in Figure 8, a and b, may be directly expelled from the char, because nitrogen functional groups are the same in char and tar, and char particles are significantly hotter than the tars in the entrainment stream of this experiment. The breach of closure in the nitrogen balance for the Dietz coal becomes significant for the longest residence times, suggesting that additional nitrogenous species may be involved. This tendency appears in the data from the other coals, but to a much lesser extent, if a t all. For reasons given in part 2, the missing species is most likely to be ammonia, which has been observed in significant quantities. la

Table IV. Initial and Ultimate H/C Ratios of Char and Tar tar char

56 ma

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1.21a 1.11

1.19 1.17b aFor 66 ma. bFor61 ma.

90ma 0.99 0.92 0.90 0.84

coal

90ma

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0.40 0.40 0.33 0.33

The evolution of elemental compositions in condensedphase products of the Dietz and Pit. No. 8 coals appears in Figure 9. Extreme values a t the beginning and end of primary devolatilization for the other coals appears in Table IV. All coals follow very similar evolution histories, in which the H/C of the char falls from the value for the parent coal to virtually the same ultimate value; viz., 0.40 for the Dietz and Ill. No. 6 and 0.33 for the Pit. No. 8 and lv bituminous coals. Initially, tars from all coals are substantially enriched in hydrogen, having H/C values from 1.11 to 1.21. But with respect to the properties of the parent coal, their H / C ratios are higher by 70% for the lv bituminous and by 30-45% for the other coal types. Values for the ultimate tar samples are also substantially higher than the respective coals', and also relatively insensitive to coal type. These characteristics of the Pit. No. 8 are compared to literature values in Figure 10a,b, again using weight loss as a scale for the extent of primary devolatilization. The evolution of char composition in Figure 10a among three studies is the same. But the study of tar H/C in Figure lob shows that the radiant coal flow experiment generates tars which are significantly enriched in hydrogen over the products from a wire-grid studyla and an entrained-flow e ~ p e r i m e n t .The ~ comparison with the flow reactor involves the same coal sample and shows the same tendency (18)Solomon, P.R.;Colket, M.B.Fuel 1978,57, 749.

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Figure 11. (a) Proton aromaticities (0) and distributions of a(0),j3- (a), and y-protons (+) for Dietz tar versus measured residence times in the furnace at 1840 K. (b) Proton aromaticities (0) and distributions of a-( O ) ,j3- (a),and y-protons (+) for Pit. No. 8 tar versus measured residence times in the furnace at 1840 K.

for diminished H/C with increasing extent of primary devolatilization as ours, and a similar sensitivity to extent of weight lows, but all values are lower by 25%. Similarly, our ultimate values for the same lv bituminous sample are 20% higher than from the flow reactor: but virtually identical for comparable subbituminous samples. (After extensive secondary pyrolysis, our values compare well with those from this flow system, as seen in part 2.) Tar values from the grid reactor are only 10% lower, but involve an Ohio No. 2, hvC bituminous coal whose lower rank may be a factor. To pursue the skeletal transformations of tar further, samples from the Dietz and Pit. No. 8 coals were analyzed with 'H N M R Their proton distributions appear in Figure 11. Total aromaticity values (defined as Hmtic/[H,,tic + H,*tiC]) range from 0.2 to 0.4, increasing monotonically with increasing extent of primary devolatilization. As expected, tar from the Pit. No. 8 coal is more aromatic than that from the Dietz. These aromaticity values are the lowest which have ever been reported. The proton distributions in Figure 11 show that j3- and y-protons are depleted during primary devolatilization, but a-protons become more abundant.

sponsible for the satisfactory closures of balances on mass, carbon, and nitrogen in individual runs. Product distributions from four coal types show two distinct stagea to primary coal devolatilization during rapid transient heating. One is dominated by tar evolution and the other by additional hydrocarbon gases, CO, and HzO. Initially, tars are the most abundant product from all four coals and comprise at least half of the weight loss for all but the subbituminous. Coals of lower rank expel significant amounts of noncondensibles along with their tar, but predominant tar evolution initially is a distinctive feature of ranks higher than hvA bituminous. Nevertheless, gases are always the major product lump on a molar basis because of the huge disparity among the molecular weights of gases and tars. Methane, acetylene, and ethylene are the major C1-CB hydrocarbons, but oils are the most abundant hydrocarbon gas from bituminous coals. Hydrocarbon yields are also rank-dependent, following the trends among the tar yields. Yields of oils, C1-CBhydrocarbons, COz, CO, and H 2 0 increase in direct proportion to the tar yields during the first stage of primary devolatilization, with no distinctive surges or lapses. Many of these features have already been modeled in terms of a single product lump of all noncondensible compounds and a broad distribution of tar fragments.19p20 They support the view that fragments of the coal molecule become disconnected as bridges among clusters of condensed aromatic structures dissociate and evaporate to form tar. Similar early time scales for tar and gas evolution suggest that bridges are the most important reaction centers for all products. They become precursors to noncondensiblea once they rupture or can form gas directly by conversion into refractory char links. By virtue of their much lower molecular weight, gases largely constitute the

Discussion In its operating mode for primary devolatilization studies, the radiant coal flow reactor generates coal products free of soot and largely free of cyanide, both of which are products of secondary pyrolysis. The highest H/C values and the lowest proton aromaticities ever reported for pyrolysis tars are more discriminating evidence for the minimal impact of secondary chemistry in these experiments. Its heating scheme delivers heat fluxes and heating rates similar to those in utility boilers, yet is particularly well-suited to product characterization. Tens of milligrams of pure tar can be collected from individual runs,and levels of noncondensibles are more than adequate for standard analytical methods. These features are re-

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(19)Niksa, S.;Kerstein, A. R. Energy Fuels 1991, 5 , 647. (20) Niksa, S. Energy Fuels 1991, 5, 673.

Coal Devolatilization convective flow which sweeps away tar vapor, even for hvA bituminous coals. Fuel nitrogen evolution substantiates this view with additional detail. Aromatic hydrocarbons, especially tars, are the only shuttles for fuel nitrogen during the first stage of primary devolatilization. The predominance of tar N is entirely consistent with recent characterizations of nitrogen functionalities in bituminous coals as exclusively pyrrolic and pyridinic.21$2 Being incorporated into the aromatic nuclei, these functionalities are relatively more stable than bridges among nuclei. They are released intact, as componenta of tar molecules, provided that secondary reactions are eliminated. Rings do rupture to form HCN at temperatures above lo00 K and residence times longer than 1 s.23*u But such conditions are much more severe than in this reactor, so no HCN was ever observed while tars were being expelled. According to a recent t h e ~ r y , the ' ~ ~second ~ ~ stage of primary devolatilhtion also involves conversion of bridges into gases, but without tar production, because the numerous refractory linkages among nuclei in the nascent char prevent further disintegration into small fragments. In this study, product distributions for the second stage are characterized by distinct surges in the evolution of CO and HzO, and continuing evolution of C142 hydrocarbons. As expected, this stage is more productive for coab having more oxygen; in fact, rates of weight loss actually increase after tar yields reach their ultimate valuea for coals of lower rank. There is no distinct lapse in the rate of weight loss from any coal a t the end of tar evolution. Consequently, ultimate yields for primary devolatilization can only be assigned on the basis of residual coal oxygen, and some measure of the extent of graphitization of the char. The ultimate subbituminous char retained less than 20% of the coal oxygen, enough to account for an additional loss of 5 wt % during pyrolysis a t more severe conditions. Among the diverse cod types in Xu and Tomita's study, typically 90% of the coal oxygen is expelled.lg So logs of residual oxygen entails significant weight loss only for d of lowest rank. In lieu of detailed analysea of the hydrocarbon skeletal structure of char, H/C ratios are probably adequate. Very similar ultimate values were reached with chars from these four coals at the end of primary devolatilization. Even though ultimate H/C ratios of char do not indicate the end of hydrogen release via graphitization chemistry on long time values from 0.33 to 0.40 do signal the end of significant mass loss from bituminous coals. HCN appears for the first time during the second stage, probably because nitrogen in char is expelled by ring rupture. Although cyanide yields do not distinguish precursors in char or tar, char particles are significantly hotter than tar in the entrainment stream in this experiment. Finally, we observed the lowest aromaticity values and highest H/C ratios with tars from the radiant flow reactor. Using a similar Pit. No. 8 sample, Nenniger5 observed aromaticity values of 0.83 in a conventional entrained-flow reactor. In another entrained-flow reactor, Pugmire et al.= reported aromaticities from 0.30 to 0.45 for pyrolysis of Ill. No. 6 and a lignite at 1050 K. However, values for tars prepared a t 1250 K reached 0.75. Secondary cracking reactions are clearly implicated by this sensitivity to stream (21) Burchd, P.; Welch, L. S. Fuel 1989, 68, 100. (22) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1460. (23) Amorthy, A. E.; Dayan, V. H.; Martin, G.B. Fuel 1978,57,29. (24) Bruinsma, 0.5.L.;Geertsma, R. S.;Bank, P.; Moulijn, J. A. Fuel -19Rs. - - -, 67.334. - . , - - .. (26) Pugmire, R. J.; Solum, M. S.; Grant, D. M.; Critchfield,.S.; Fletcher, T. H. Fuel 1991, 70(3), 414.

Energy & Fuels, Vol. 6,No. 3, 1992 263 temperature. Conversely, the minor role of secondary chemistry in the radiant flow reactor is responsible for our low proton aromaticities and high H/C ratios. According to a recent the0ry,lg3tar aromaticities increase throughout primary devolatilization because aliphatic peripheral groups and bridge remnants are progressively eliminated from aromatic nuclei before they are released from the coal matrix as tar. Initially, most tar fragments have intact aliphatic substituents, so aromaticities are low and H/C values are high. But later tar fragments have few aliphatic components and correspondingly higher aromaticities and lower H/C ratios. The proton distributions shift from substantial fractions of 8- and yprotons to mostly a-protons during primary devolathtion, which also suggests elimination of aliphatic bridge remnanta and peripheral groups from the fragments which eventually become tar.An early free-radical reaction mechanism for coal pyrolysisz6proposed that a-carbons would become the most abundant radical sites as longer methylene chains were preferentially eliminated, consistent with these proton distributions. This explanation is also consistent with the predicted reactivities of polycyclics based on frontier orbital theory. Specifically, aromatics become more reactive with increases in either the number of alkyl substituents or the number of carbons within an alkyl substituent? These predictions are confirmed experimentally?* although, for example, even the most reactive alkylated benzenes do not appreciably decompose in 5 s below 770 K. Conditions in the radiant flow reactor are much less severe. Notwithstanding, the particles are hot enough, and the same chemistry will likely occur among functionalities in the coal. While plausible, this interpretation is not definitive because average values assigned by NMR cannot adequately characterize mixtures of dissimilar components. Tars are primarily polycyclic aromatic compounds, but substantial amounts of long-chain n-alkanes and olefins may also be present. Nelson and Tylerz7reported that a major portion of the GC volatile tar fraction (

Coal devolatilization during rapid transient heating ... - ACS Publications

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