Energy & Fuels 1988,2, 100-105 nth-order kinetic parameter percent of dry bitumen free shale solubilized after treatment time t percent of dry bitumen-free shale solubilized after treatment time t = a percent of dry bitumen-free shale solubilized after treatment time t = 0 percent of dry bitumen-free shale solubilized as species having molecular weight x at time t percent of dry bitumen-free shale solubilized as species having molecular weight r at time t = percent of dry bitumen free shale solubilized as species having molecular weight x at time t = 0 molecular weight for which k ( 7 ) = K
w(x,t)
weight fraction of species having molecular weight x at time t.
Acknowledgment. The Fullbright fellowship for M.T. provided by the Moroccan-American Commission for Educational and Cultural Exchange and coordinated by Amideast is gratefully acknowledged. Our appreciation to ONAREP for providing the samples used in this study is also acknowledged. Funds for this research were also provided, in part, from the O.U. Energy Resources Institute and Oklahoma Mining and Minerals Resources Research Institute. Registry No. C6H5CH,,108-88-3.
Ammonia Evolution during Oil Shale Pyrolysis+$ Myongsook S. Oh,* Robert W. Taylor, Thomas T. Coburn, and Richard W. Crawford Lawrence Livermore National Laboratory, Livermore, California 94550 Received July 8, 1987. Revised Manuscript Received October 13, 1987
Ammonia (NHJ evolution during pyrolysis of three Green River Formation shales and one Eastern (Devonian) shale was studied. The yields of NH3from Fischer assay type experiments were measured with an NH3-sensing electrode, and the time-dependent rate of NH3 evolution was measured with a triple-quadrupole mass spectrometer. We varied the peak temperature (PT)of the pyrolysis from 350 to 750 OC and the heating rate from 1to 50 OC/min. The NH3 yields increased rapidly above the oil-generating temperatures to a maximum at PT 700 "C and then decreased at PT > 700 OC because of the decomposition of NH3. The NH3 yield showed no dependence on the heating rate at PT < 550 OC and decreased with increasing heating rate at higher peak temperatures. For Green River Formation shales, the yield of NH3at PT < 500 "C reflected the organic nitrogen content. The extent of NH3 decomposition varied with gas environments, solid surface, and the conditions of the retort can. Because steam works as an inhibitor for the decomposition reaction, we used steam as a retorting gas to obtain the total yield of NH3. We developed a kinetic model for NH3 evolution; it takes into account NH3 generation from organic and inorganic nitrogen sources and NH3 decomposition.
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Introduction Nitrogen (N) species such as ammonia (NHJ, NO,, and HCN can be released during the combustion of fossil fuels. Various laws limit their emissions because of environmental concerns, so the study of N species is of particular importance in oil shale processing because both oil shale and shale oil are rich in N. Ammonia is the major nitrogen species evolved during oil shale processing, where it is found in retort water, in retort off-gas,l and in the combustor gas with NO,. During combustion, NH3 is also important because it is an intermediate in conversion of fuel N to NO, and NP Effective control of NO, emission requires a better understanding of the formation and decomposition of NH3 during combustion. As early as 1865,ammonia had received a lot of attention because it was a valuable by-product of shale processing.2
* To whom correspondence should be addressed.
t Work performed under the auspices of the U.S.Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
*Presentedat the Symposium on Advances in Oil Shale Chemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
0887-0624/88/2502-0100$01.50/0
The major conclusions from early work were that (1)a large fraction of NH, is produced at temperatures higher than oil-evolving temperatures, (2) the yield of NH, is enhanced in steam retorting, and (3) the yield of NH3 is also enhanced at slower heating rates., In this study, we investigated ammonia evolution along with the changes in the N distribution among pyrolysis products at the different stages of oil shale pyrolysis. First, we looked at the effects of pyrolysis temperature, heating rate, N content, and the form of N in raw shale on total NH3 yield. Then, we measured the continuous rate of ammonia evolution during pyrolysis and also studied the extent of NH3 decomposition at high temperatures and its effect on total NH3 yield.
Experimental Section We conducted three different types of experiments to measure (1)the total yield of NH, at a given time-temperature history, (2) the rate of ammonia evolution as a function of time during (1) Sklarew, D. S.; Hayes, D. J., Enuiron. Sci. Technol. 1984,18,600. ( 2 ) Bell, H. S. Oil Shales and Shale Oils: Van Nostrand: New York, 1948.
0 1988 American Chemical Society
NH, Evolution during Oil Shale Pyrolysis
Energy & Fuels, Vol. 2, No. 1, 1988 101 Table I. Chemical Composition of Shales Employed Green River Formation shales Eastern shale
Helium sweep 7 , Reton with shale
shale name CA25 CA35 AP24 NA13 source Tract Ca Tract Ca Anvil Point New Albany, KY 35 24 13 grade, gpt 25
Temp controllers and recorder
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total C acid COz org C total H total N
Chemical Composition, wt % 16.58 21.47 15.99 11.85 19.66 16.17 17.82 1.25 10.33 17.05 11.13 11.51 1.61 2.40 1.71 1.48 0.59 0.75 0.59 0.43
from LTA" 48 from comb 48 I
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L_ _ _ _ _ _ _ _ _ _ _ _ _ A L - _ _ - _ - - _ _ _ _ _ F i g u r e 1. Pyrolysis apparatus with double-type retort.
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Figure 2. Example of time-temperature history. pyrolysis, and (3) the extent of NH3 decomposition. Figure l a shows the schematic of the pyrolysis apparatus employed in the total NH3 yield determination. Shale was heated at a fixed rate to a peak temperature (PT) and then soaked at the PT for 20 min, as shown in Figure 2. The PT was varied from 350 to 750 "C, and the heating rate ranged from 1to 50 OC/min. A unique part of the retorting system was the double-tube retort (DTR) that was equipped with an internal rod-type heater at the center to reduce radial temperature gradients during heating. Between 80 and 100 g of shale particles were loaded into the annulus of the stainless-steel (SS) DTR, which was sealed by welding. The DTR and rod-heater assembly was positioned in the center of the electric furnace. Both ends of the assembly were then packed with thermal insulation. The internal and external heaters had independent controllers, but they were driven by the same temperature ramp program. By this means, the maximum radial temperature gradient was -15 OC a t a heating rate of 12 OC/min. During pyrolysis, hot gases were swept into the traps by a helium flow of 50 cm3/min. Most of the oil and water were condensed in a first trap, which was cooled with ice-water; the amount that condensed in the second trap, which was cooled with an ethanol-ice mixture, varied with the sweep-gas flow rate and heating rate and was usually very small. Gas that did not condense in these traps was bubbled through a 0.5 N HzS04trap to capture NH3. Yields of all products were determined, except for the untrapped gases, the yield of which was obtained by difference.
Organic N as Percent of Total N 78 64 100 63 52 85
a LTA = low-temperature ashing. Correlation: percent of organic N = 100 X A(& % org C)/wt % total N where A = 0.0274 for western shales6 and A = 0.0318 for eastern shales!
The C, H, and N contents of spent shale, oil from the first trap, and water were analyzed by a Leco elemental analyzer. An NH3-sensing electrode measured the concentrations of NH3 in the water and the sulfuric acid. The electrode was calibrated every time it was used. The retort water was diluted about 500-fold to minimize contamination of the membrane by the organics, and in addition, the membrane was frequently replaced. Pcasible interferences from low molecular weight amines can be ignored because of the low concentration3 (-10 ppm) in the retort water compared to that of NH3 (-1%). The NH3-sensing electrode gives a simple way of determining NH3 concentrations in retort water and the HzS04 trap and, therefore, the total yield of NH3 a t the pyrolysis stage defined by a given time-temperature history. However, the electrode method is a rather cumbersome way to study kinetics of NH3 evolution because each retorting gives the yield at only one temperature. The continuous rate of NH3 evolution as a function of time was measured with a triple-quadrupole mass spectrometer (TQMS) using isobutane chemical ionization (CI) as the shale was heated from rcom temperature to 950 "C at a constant heating rate. The details of the TQMS and CI work are described elsewhere:~~ The same type of furnace and retort described earlier was used in the pyrolysis experiments, and the DTR was shortened to handle 20-27 g of shale. The sweep gas of Ar flowed at 200 cm3/min. In this set of experiments, it was important not to condense steam because NH3 condenses in the water. Instead of cold traps, we used a glass-wool oil trap placed in a -120 "C oven as shown in Figure l(b), and all the l/s-in. SS transport lines from the retort to the TQMS were heated to -140 "C. The hot trap worked quite well, and for western shales, we collected more than 90% of the Fischer assay oil yield. A constant flow of trimethylamine (TMA) was introduced at the exit of the oil trap to monitor the variations in the totalgas flow due to the formation of pyrolysis gases. We employed the electrode method to study the effects of heating rate and shale type on NH3 yield. The electrode gave accurate data on yield since it was used on integrated samples. The TQMS was less accurate because it was measuring low levels over a long period (up to 4 h), and instrument drift and noise were problems. In addition, the lack of accuracy of the TQMS data was due to the developmental nature of adapting the TQMS for the quantitative analysis of NH3via isobutane CI. We have been increasing the accuracy of this new method of NH3 analysis. We studied NH3 decomposition in simulated retort conditions. The DTR was filled with quartz sand, and several NH3 gas mixtures were passed through as the retort was heated from 200 to 900 OC at a constant heating rate. The TQMS monitored the (3) Avery, M. J.; Junk, G. A. Anal. Chem. 1985, 57, 790-792. (4)Coburn, T. T.; Crawford, R. W.; Miller, P. E.; Oh, M. S. Proceedings, 1986 Eastern Oil Shale Symposium. Kentucky Energy Cabinet Laboratory: Lexington, KY, 1986; pp 291-299. (5) Crawford, R. W.; Coburn, T. T.; Miller, P. E.; Oh, M. S. Proceedings, American Society for Mass Spectrometry, 35th Annual Conference; American Society for Mass Spectrometry: East Lansing, MI, 1987, pp 977-978.
Oh et al.
102 Energy & Fuels, Vol. 2, No. 1, 1988 100
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Figure 3. Distribution of (a) products and (b) nitrogen from pyrolysis of CA25 shale. concentration of NH3 in the exit stream. Three Green River Formation oil shales and one Eastern shale were sampled from large blocks by grinding, blending, and sieving to a particle size ranging from 0.2 to 0.5 mm. The chemical composition of the four shales is summarized in Table I.
Results and Discussion Figure 3a shows the pyrolysis product distribution as a function of PT for raw CA25 shale at the heating rate of 12 "C/min. As the PT increased from 350 to 500 "C, the yield of oil increased to 9.3 wt % , and further increases in temperature did not result in a higher oil yield. The yield of water was 1.3 wt % at PT = 350 "C, increasing to 2.4 wt % at PT 750 "C. The unaccounted mass that was due to gaseous products showed a slow increase up to 600 "C and increased rapidly above 600 "C because of carbonate decomposition. Figure 3b shows how N was distributed in pyrolysis products as a function of PT. The N concentration in oil stayed constant at roughly 2 w t % at PT > 500 "C, just as the oil yield stayed constant at these temperatures. Nitrogen in shale oil accounted for about one-third of the total N in raw shale. The N content of pyrolysis water gradually increased with increasing temperature and showed a rapid increase at PT 550 "C. A parallel trend occurred for the fraction of N in gases, 750 "C. The fraction of N in reaching -20% at PT gases was again determined by difference from the N balance. As shown in Table I, approximately 50% of the N in CA25 oil shale is inorganic. Taylor et a1.6 found that the inorganic N in Green River Formation oil shale was contained in the mineral buddingtonite, an ammoniumbearing feldspar (NH4A1Si308.0.5H20).They also found that the decomposition of buddingtonite was not significant at T < 500 "C, which we confirmed in this study (Figure 9). Therefore, at PT < 500 "C, all evolved N originated from organic N, of which a large fraction went into oil and only a few percent evolved as NHB. As a result, more than 80% of the N in the solid at PT = 500 "C is inorganic, and the large N content in water and in gas at PT > 500 "C must come from inorganic N. At low temperatures such as PT < 500 "C, most NH, appeared in the retort water. As the temperature increased, the rapid evolution of NH, and the increased flow rate due to C02 formation caused less contact with the water and more NH3 was swept into the H2S04 trap, reaching a maximum of 2-3% of the N in the raw shale.
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(6)Taylor, R. W.;Smith, G.S.;Sanbom, R.H.; Gregory, L.S . Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1985,30(3),338-348.
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Figure 4. Yields of ammonia from pyrolysis of CA25 and CA35 shales as a function of peak temperature with model predictions. However, this amount of NH, was too small to account for the 20% increase in N in the gas phase at this temperature, as shown in Figure 3b. There must be other N species present, also, such as N2 Figure 4 is a plot of the total NH, evolved from pj-rolysis of CA25 and CA35 shales as a function of PT. Ammonia evolution was slow at PT < 500 "C and started to increase rapidly at 500 "C, showing a maximum evolution of NH3 at PT 700 "C. The yield of NH, per gram of shale was higher for CA35 than for CA25 shale at all temperatures due to higher total N content. Even when the NH3 yields were plotted as a percent of the N in raw shale, as shown in Figure 4, the yields of NH3 were higher for CA35 at all temperatures. More NH, evolved at lower temperatures from CA35 shale because of the higher content of organic N. The decrease in the NH3 yield at PT > 700 "C was due to NH3 decomposition because NH, becomes thermodynamically unstable at temperatures above 200 "C;the Gibbs free energy of formation changes from -3.915 kcal/mol at 25 "C to 6.496 kcal/mol at 427 "C and to 14.792 kcal/mol at 727 "C. The decomposition of NH3 also supports the above argument for the presence of Nz in the gas phase. The effect of the heating rate on NH, yields was studied with CA35 shale. Raw shale was retorted at PT 530 "C for five different heating rates, and the NH3 yield was measured for each rate. After the retorted shale was cooled to room temperature under helium flow, it was reheated to PT 750 "C at the same heating rate used for raw shale pyrolysis. Figure 5 shows that the NH3 yield from raw shale at PT 530 "C did not change much with heating rate, and the total NH3 yield from raw and retorted shale decreased with the increasing heating rate. At high temperatures, T > 650 "C, the decomposition of NH3 is favored, so a low heating rate gave the shale a longer resi-
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NH,Evolution during Oil Shale Pyrolysis
Energy & Fuels, Vol. 2, No. 1, 1988 103
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Table 111. Summary of Kinetic Parameters V*
Figure 5. Effect of heating rate on ammonia yield. I
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Figure 7. Effect of gas environments on ammonia decomposition.
Healing rate (Wmin)
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Figure 6. Rate of ammonia evolution from CA35 and NA13 shales. Table 11. NH3 Yield as % N in Raw Shales (and pg of NH3/g of Shale) at Two Peak Temperatures peak temp, O C CA25 CA35 AP24 NA13 -530 3.9 (279) 5.9 (537) 5.3 (380) 5.2 (272) -737 13.5 (967) 14.9 (1357) 11.9 (853) 17.5 (914)
dence time at a lower temperature, and more NH3 was released without going through extensive decomposition. Insensitivity of the data as the heating rate was increased from 12 to 50 "C/min is in part blamed on the apparatus. Even though the DTR did a much better job of reducing radial temperature gradients than a retort without two heat sources, the effective heating rate was lower than the nominal value at these high heating rates, and the reproducibility of generating the same timetemperature history also decreased. The yields of NH, from AP24 and NA13 shales were also investigated. Table I1 summarizes the total NH3 yields from four shales at two pyrolysis temperatures. Each datum is the average of between one and four experiments. In general, N-rich shale gave a higher NH3.yield. For Green River shales, the shale with higher organic N content gave a higher NH3 yield at oil-generating temperatures. Eastern shale contains little or no inorganic N, and NH3 evolved at high temperatures came from organic N in In all cases, more NH3 was released at PT > 530 "C than below that temperature. The contribution of organic and inorganic N on NH3 evolution was more evident when the time-dependent rate of NH3 release was monitored by the TQMS. Figure 6 plots the rates of NH3 evolution from CA35 and NA13 shales as the shales were heated at 1 2 "C/min. The area under each curve is normalized. For the case of a Green River shale, CA35, the evolution curve has double peaks: the lower temperature peak is due to the NH3 from organic
cal/mol A , min-' CA25 COz formation 57800 1.02 X lo'* 0.197 g/g of shale NH3 decomp 35470 1.16 X lo9 NH3 from org N 52780 1.70 X 1015 2.19% raw shale N NH3 from inorg N 34860 8.77 X lo6 17.77% raw shale N
CA35 0.163 g/g of shale 4.48% raw shale N
18.80% raw shale N
N while the high-temperature peak is mainly due to the NH, from inorganic N sources. An Eastern shale, NA13, shows a single major peak at high temperatures although there seems to be a shoulder at around 500 "C. As expected from our results shown in Table 11, more NH, was evolved at T > 500 "C for both types of shales. The temperature for the maximum rate of NH3 release was higher for NA13 shale by approximately 65 "C. It could have been caused by the different source of N; i.e., the NH3 from buddingtonite was released at lower temperatures than that from the N in char. However, the NH3 yield at these high temperatures was affected a great deal by the decomposition reaction, so it is difficult to speculate the true rate of evolution from the data shown in Figure 6. We investigated the loss of NH3 caused by the decomposition over hot surfaces NH3 0.5Nz + 1.5Hz (1)
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under simulated retort conditions. Argon (Ar) with 1.08% NH3 was passed through a retort filled with quartz sand at a rate of about 200 cm3/min, and the retort was heated at 12 "C/min from 200 to 950 "C. The TQMS monitored NH3 concentration in the exit stream. The same experiment was repeated with gas mixtures of 1% NH3 in H2, N,, CO,, and steam to see the effect of gas environments on the decomposition reaction. While Ar, Nz and CO, acted as mere diluents, H2and steam worked as inhibitors for NH3 decomposition. Figure 7 plots the fraction of NH, remaining in the exit as a function of temperature for 1.08% NH3 in Ar, 1.05% NH3 in H,, and 0.9% NH, in 42.1% Ar and 57% steam. In Ar, NH, started to decompose at 500 "C and completely disappeared at 890 "C, while NH3 was stable up to 700 "C in H2 and up to 800 "C in steam. First-order decomposition models fit the data well, and the kinetic parameters for the decomposition in Ar are summarized in Table 111. The observed decomposition was probably catalyzed by the SS retort wall and active sites in retorting shales. A silica glass retort eliminated the catalytic effect caused by SS retort vessels, and as a result, the decomposition curve was shifted to a much higher temperature, as shown in Figure 8. We calculated that during 3 s of residence time in a SS retort, 99% of the NH, decomposition occurs at
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Oh et al.
104 Energy & Fuels, Vol. 2, No. 1, 1988
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Figure 8. Ammonia decomposition in stainless-steel retort vs decomposition in silica glass retort.
Temperature CC)
Figure 9. Ammonia evolution from buddingtonite decomposition under argon and steam.
catalytic sites on the retort walls. A change in the sand surface area by &fold did not affect the decomposition reaction. We also found that a freshly loaded vessel is less catalytic than the vessel exposed to NH, in previous experiments. This observation is consistent with the literature. Loffler and Schmidt' found that a fresh iron-wire catalyst showed a lower rate of NH3 decomposition than the wire heated in NH, for 15 min at 927 OC. We learned a great deal about NH, decomposition from our experiments with sand. However, the sand is probably a poor substitute for the retorting shale, which has active catalytic sites. On the other hand, off-gassing from char or other inorganic sources such as carbonate minerals may inhibit the access of NH, to catalytic sites. As shown above, steam does work as a successful inhibitor for NH3 decomposition at temperatures up to 800 OC, and we believe that one can employ steam as a retorting gas to obtain the total NH3 generated from a nitrogen source. An example is shown in Figure 9, where the NH, evolution curves from buddingtonite under steam and Ar are compared. As one can see, about 60% of the generated NH, is lost by decomposition in Ar. Such experiments with various types of oil shale are in progress now, and the result will be reported. Model Development The NH3 generation is modeled as two parallel processes, one from an organic N source (V,) and the other from an inorganic N source (Vi), by using first-order models dV,/dt = A , exp(-E,/Rn (V,* - V,) (2) dVi/dt = Ai exp(-Ei/R7J (Vi* - Vi) (3)
where V and V* are NH, formed at time = t and time = 00, respectively. We assume uniform can (DTR) temper(7) Loffler, D. G.;Schmidt, L. D.J. Catal. 1976,44, 244-258.
ature and small diffusion flow, so the concentration of NH3 in the retort can, CNH3, can be described by
where t = time, x = length of the retort can, f = fo + a x = the rate of sweep gas and total volatiles flow, k d = ammonia decomposition rate = Ad eXp(-Ed/RT), and ps = g of shale/cm3 of retort. We assume a pseudo steady state, so CNH3at x = L is obtained, and then the total yield of NH3 as a function of time is t
YNH~ = AscS, CNH,(L)fG) dt
(5)
where L and A,, are the length and cross-sectionalarea of the can. The NH3 from organic N is assumed to have evolution kinetics similar to those of oil; the kinetic parameters, E, and A,, are taken from oil evolution kinetics proposed by Burnham and Braun.* Equation 5 is applied to yields vs PT data for CA35 shale to obtain the best-fitting parameters for Ei,Ai*, Vi*, and V,*. Equation 5 is then applied to CA25 data with known Ei and Ai to obtain Vi* and V,* for CA25. In both cases, the flow rate, f, is assumed to vary only with C02formation, because the NH3 decomposition is negligible at temperatures at which organic volatiles are formed. The rate of C02 formation from carbonate decomposition is taken from C a m ~ b e l l . ~The data and model predictions, which are calculated at a heating rate of 12 OC/min and a soaking time of 30 min, are shown in Figure 4. A 30-min soaking time was used, instead of the 20 min employed in the experiments, to compensate for the cooling time. The kinetic parameters are summarized in Table 111. The model analysis was done only with the total yield data on CA35 and CA25 shale. CA25 shale has a smaller V,* due to low organic N content, but Vi* does not seem to depend on inorganic N content. The analysis of NH, evolution at high temperatures requires a more accurate description of NH, decomposition. As discussed earlier, the NH3 decomposition depends on gas environments, solid surface, and the condition of the retort can, and we are making progress toward understanding NH3 decomposition during retorting using steam as a retorting gas. The model will be refined as more data become available from our study now in progress. Conclusions During pyrolysis, most organic N in raw oil shale is evolved as N in oil and some as NH,. The yield of NH3 increases rapidly above oil-production temperatures, and for the case of Green River shales, a large fraction of NH3 originates from an inorganic source of N. When T = 500 OC, NH3 begins to decompose to N2and H2 and reduces the yield of NH,. This decomposition results in a loss of NH3 yield when the heating rate is increased. In general, N-rich shales give higher NH, yields, and, in the case of Green River shales, a higher organic N content results in higher NH, yields at T
