Investigation of Ammonia Formation during Gasification in an Air

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Investigation of Ammonia Formation during Gasification in an Air Blown Spouted Bed: The Effect of the Operating Conditions on Ammonia Formation and the Identification of Ways of Minimizing Its Formation Y. Zhuo, N. Paterson,* B. Avid,† D. R. Dugwell, and R. Kandiyoti Imperial College of Science, Technology and Medicine, Department of Chemical Engineering, Prince Consort Road, London, SW7 2BY, United Kingdom Received September 24, 2001. Revised Manuscript Received February 4, 2002

A laboratory scale gasifier has been used to investigate the reaction conditions allowing the formation of high concentrations of NH3 in a pilot scale, air blown gasifier. Ammonia has been shown to be produced during the pyrolysis of the coal volatiles and by a reaction between steam (or H2 produced by its decomposition) on the N present in the char after pyrolysis. The latter reaction produced the higher proportion of the total NH3. The effect of the gasifier operating conditions on the amount of NH3 formed by the reactions has been studied, which has led to the identification of ways to minimize the amount formed. The main control options to consider are using an alternative method of bed temperature control (i.e., avoid the use of steam), operating with higher bed temperatures (with the limit set by ash melting), and operation at lower pressures (within limits set by power generation efficiency considerations).

Introduction Air blown gasifiers are expected to form part of the next generation of cleaner coal fired power stations. Pressurized fluidized bed gasifiers with submerged spouts are used as a way of enabling the gasifiers to operate, without difficulties caused by agglomeration, on a wide range of coals and with coal/biomass/waste mixtures. In the United States, a demonstration plant based upon the KRW air blown gasifier has been constructed at Pinon Pine.1 In the U.K., an air blown gasifier was developed by British Coal, as a component of the Air Blown Gasification Cycle (ABGC).2 High and variable concentrations of NH3 were found in the fuel gas3 produced by the pilot scale gasifier (coal feed rate, 200 kg h-1), used in the development of the ABGC gasifier. This was undesirable, because of the possibility of forming NOx during the combustion of the fuel gas in the gas turbine. A specially adapted laboratory scale gasifier, operated under conditions that simulated those used in larger scale gasifiers, has been used to study the reasons for the observed high NH3 concentrations. The pilot scale gasifier was operated at a temperature of typically 950 °C and a pressure of 13 bara, whereas the commercial * Corresponding author † Current address: Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar-51, Mongolia (1) Higginbotham, E. B.; Motter, J. W. Pinon Pine Power Project. 13th EPRI Conference: Gasification Power Plants, San Francisco, October 1994. (2) Dawes, S. G.; Mordecai, M.; Brown, D.; Burnard, G. K. The Air Blown Gasification Cycle. Proceedings of the 13th International Conference on FBC, Orlando, May 1995. (3) Intellectual Property owned by Mitsui Babcock Energy. Run reports for tests in the CTDD pressurized gasifier, 1994-1997.

scale gasifier would be intended to operate in the pressure range 20-25 bara. In our study we have focused on the pilot scale operating conditions to enable comparisons to be made between data collected at the laboratory and pilot scales. In a recent publication,4 the development and commissioning of the laboratory scale, pressurized gasifier has been described. This paper reviewed preliminary findings, which suggested that the NH3 is produced during pyrolysis and by an effect of steam on the char-N. In this paper, the main body of experimental data is presented and evaluated. The work has enabled a plausible view of NH3 formation in air blown gasifiers to be developed and this has led to the identification of ways to minimize its formation. Further work is currently in progress to continue the study of the fate of fuel-N in air blown gasification systems. This work concerns the release and further reactions of HCN in the gasifier. Experimental Section Laboratory Scale Gasifier. The laboratory scale, pressurized, air blown gasifier has been developed from an existing batch fed fluidized bed reactor.5 The coal is fed into the gasifier with the fluidizing gas mixture, as a high velocity spout jet, which enters the reactor at the apex of an inverted cone-shaped base. This configuration induces the rapid mixing of the fresh coal with the bed char. At the pilot scale, this mode of operation had been found to avoid the operating difficulties associated with the agglomeration of sticky coal particles. Limestone is added to retain sulfur compounds in the bed of the gasifier. A (4) Paterson, N.; Zhuo, Y.-Q.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2002, 16, 127-135. (5) Megaritis, A.; Zhuo, Y.-Q.; Messenbock, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1998, 12, 144-151.

10.1021/ef010237w CCC: $22.00 © 2002 American Chemical Society Published on Web 04/03/2002

NH3 Formation during Gasification in an Air Blown Bed

Energy & Fuels, Vol. 16, No. 3, 2002 743

Figure 1. Schematic diagram of spouted bed reactor. schematic diagram of the equipment is shown in Figure 1. It is capable of operation at temperatures up to 1000 °C and pressures to 20 bara, with a continuous feed system that meters the fuel into a char bed at rates up to 4 g min-1. Fluidizing gases are either air/N2 or air/steam/N2 mixtures. With each type of mixture, the air:N2 ratio is used to achieve the required temperature in the reactor. The previous paper4 describes the fuel feed system, reactor construction, steam generator, sampling probe, gas cleaning system, instrumentation, and the operation of the equipment. A mixture of Coalite and sand was used as the initial bed for the majority of the tests. Coalite was used instead of coal to avoid tar condensation that would have caused problems if the coal had been used as the initial bed material. Coalite is a smokeless fuel manufactured in the U.K., which contains approximately 6% volatile matter. A limited number of tests were done using initial coal char beds, which were specially prepared in the reactor under pyrolysis conditions. There was no char offtake through the base, so that bed material built up in the gasifier during each test. Sinter formed on the base during most tests with coal; this limited the maximum test time to less than 30 minutes, as it eventually blocked the spout entry. Less sinter formed when steam was used in the fluidizing gas. Gases within the reactor may be sampled from the spout jet (using a specially designed water cooled probe, which can be fixed at different heights above the spout entry) and from the exit of the reactor. Measurement of Ammonia. Different methods were used to collect the NH3 sample during the tests with the different fluidizing gas mixtures. Without steam, NH3 was collected in Dreschel bottles containing deionized water, placed in a side stream of gas, which was removed from the exit gas just before the pressure let-down valve. This sampling method was used only during the steady-state period of the test. In the work

Table 1. Analysis of the Daw Mill Coal analysis

Daw Mill coal %, as analyzed

moisture ash volatile matter C H N S Cl

4.9 11.7 33.0 65.7 4.0 1.1 2.0 0.24

with steam, NH3 was absorbed by the water in the in-line bubbler, which was used to cool the gas and cause condensation of the steam in a controlled way. This was placed close to the exit from the gasifier freeboard on the high-pressure side of the control valve (see Figure 1). The analysis of a solution from a Dreschel bottle, which sampled the gas in the side stream sample, after the in-line bubbler, showed that the inline bubbler collected more than 95% of the total NH3 in the exit gas. However, the unavoidable drawback of using this inline bubbler was that it also collected the NH3 that was released from the Coalite bed material during start-up, as it released its volatile matter. This had to be measured separately and a correction applied to the steady-state test data obtained during tests with steam. Materials. Daw Mill coal and Grangemill limestone were used in these tests, as they were the main coal and sorbent used in the pilot scale tests conducted by British Coal. The limestone contained greater than 99% CaCO3. The fuel analysis is shown in Table 1. Test Program. The program was divided into three main groups of tests. The first group of tests was done under a range of conditions (e.g. with and without coal feed, with a range of fluidizing gas mixtures) to assess the nature of the reactions

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Table 2. Repeatability of the Test Resultsa fuel gas analysis test no.

pressure bara

coal feed rate g min-1

coal:air ratio (mass)

temp. °C

dilution factor

CO2 %, dry

CO %, dry

H2S vpm

NH3 vpm, average

NH3 vpm, Draeger

26 27 37 39 38 41 42

8.3 7.8 12.6 13.9 12.6 12.9 12.9

0.90 1.03 2.5 2.6 2.7 2.4 2.4

0.41 0.44 0.44 0.40 0.37 0.35 0.36

870 860 870 850 870 900 900

5.66 5.36 3.14 3.43 3.43 3.40 3.51

2.6 2.8 3.8 4.4 4.7 4.8 4.7

2.2 2.2 3.3 3.4 2.8 2.7 2.5

158 106 170 200 100 150 101

122 153 188 183 57 143 150

100-200 130-230 150-400 250-300 50-200 200-400 100-200

a

Gas analyses not adjusted for dilution by N2. Table 3. Sources of Ammonia in the Air Blown Gasifier source NH3

test no.

fluidizing gas mixture %, vol

temp., °C


spout press, bara

exit gas NH3, vpm

Coalite char bed steady state) Coalite char bed (start-up and steady state) Coalite char bed (start-up and steady state) Coalite volatiles and char bed Daw Mill volatiles and mixed char bed Daw Mill volatiles and mixed char bed

61 59/60 av 56 86 31 15

air (17), N2 (83) air (27), N2 (73) air (25), steam (10), N2 (65) air (29), N2 (71) air (28), N2 (72) air (27), steam (10), N2 (63)

850 870 860 880 880 900

no feed no feed no feed 0.32 0.44 0.11

12.5 13.2 13.2 13.6 13.0 13.0

48 595 1780 498 617 2160

that were producing NH3. The second group focused on the formation of NH3 by pyrolysis reactions of the volatile material in the fuel feed. The tests were done using air/N2 mixtures as the fluidizing gas, with continuous fuel feed. Under this condition, the NH3 was formed by pyrolysis reactions alone. The third group was done using air/steam/N2 mixtures as the fluidizing gas, as the results of the first group revealed that steam had a dominant impact on NH3 formation.

Results and Discussion In the following discussion, data are presented for the NH3 concentration in the exit gas, and the NO and O2 concentrations in the spout jet. The O2 was also measured in the exit gas, but its concentration was always zero (as expected). The species measured were subject to a 2-4-fold dilution by the nitrogen gas added to the air/nitrogen mixture used as the fluidizing gas. The data shown have been adjusted to remove the effect of this variable dilution, by multiplying the measured data by the dilution factor. The concentrations expressed in this way are more representative of the values expected at the pilot and commercial scale of operation, where the major component of the fluidizing gas would be air. The validity of conclusions drawn from the trends seen in this corrected data has been gauged by also evaluating selected data in terms of milligrams of NH3 produced per gram of coal fed. Data presented in this way (not shown in this paper) are independent of the determination of the dilution factor and depend on the accuracy of the measurement of the coal feed rate and exit gas rate. The same trends were seen in both sets of evaluated data. Repeatability of Test Results. An indication of the repeatability of the test data is given in Table 2, which shows the operating conditions and fuel gas analyses for 3 sets of tests. The operating conditions were broadly similar within each set, but precise repetition of reaction conditions for individual tests within each set was not possible. This is because each test was setup for a desired pressure, temperature, coal feed rotary valve speed, and fluidizing velocity. However, the actual flow rates of coal, air, and nitrogen used to achieve the desired conditions were estimated after each test from

the weights of coal fed and the exit gas analyses and flow rates, and these showed some variation between repeat tests. In addition, the thermocouple readings recorded during each test showed some variation and were sensitive to position with respect to the spout jet. The average of the temperature measurements made during each test are shown on the table. Despite this, the results within each set of gas analyses are broadly similar, which indicates a reasonable repeatability for tests done under broadly similar conditions. Reactions That Form NH3 in the Gasifier. A literature study of reactions that can potentially form NH3 in air blown gasifiers was included in the earlier paper.4 In this work, tests have been done with fluidizing gas mixtures containing air/N2, with air/steam/N2, and with and without fuel feed, to identify the reactions that were actually producing NH3 in the gasifier. Data obtained during these tests are shown in Table 3. Formation from the Initial Coalite Bed. The amount released during start-up from the initial Coalite bed was assessed by conducting tests, using a fluidizing gas mixture containing air/N2 alone and without coal feed. The results (Test 61) showed that NH3 was released at a concentration equivalent to 595 vpm (corrected for N2 dilution), expressed as an average concentration over the steady-state part of the test. As there was no other source of NH3, other than the Coalite volatiles during these tests, this NH3 is thought to represent that released from the fresh bed material during the startup stage. To confirm this, the test was repeated (Test 59/60) with the measurement of NH3 during the steadystate period only. 48 vpm NH3 was detected, which confirmed that the higher value seen must be a result of the NH3 formed during the start-up period of the test. Therefore, the NH3 concentrations measured during all tests with the in-line bubbler (i.e., those with steam addition) have been corrected for the release of NH3 during start-up. Formation from Coal Volatiles. Significant concentrations of NH3 were found in the exit gas, when coal was fed under steady conditions, using fluidizing gas mixtures of air/N2. The results show that, as virtually no

NH3 Formation during Gasification in an Air Blown Bed Table 4. Comparison of Ammonia Formation in Nitrogen and Air/Nitrogen Mixtures test no.

fluidizing gas

coal feed rate, g min-1

temp, °C

NH3 concn, vpm

87 31 32

N2 air/N2 air/N2

2.6 2.1 1.9

840 840 800

575 133 150

a

Fuel: Daw Mill coal; data as collected basis.

NH3 was measured without feed, then the measured NH3 in these tests must be produced during the pyrolysis of the volatile material in the fresh fuel. The literature6,7 suggests that this is likely to be via a route involving the primary formation of HCN from the pyrollic and pyridinic N in the coal, with the subsequent formation of NH3 in a secondary reaction. The concentration measured with coal (Test 31, 617 vpm) was higher than that measured with Coalite feed (Test 86, 498 vpm), which is in agreement with the higher volatile matter content in the coal. Literature data suggest that during pyrolysis, about 50% of the initial fuel-N is converted to char-N.8 Analysis of the final bed materials from several tests with air/N2 mixtures and coal feed are consistent with this, as they contained between 35 and 45% of the fuel-N present in the original Coalite bed and the coal fed during the test. The balance represents the amounts lost to the gas phase. It seems that the char-N is stable during tests with air/N2 mixtures, as virtually no NH3 was formed from the Coalite char bed alone (Test 61, 48 vpm). Formation by a Reaction of Char-N. The addition of steam to the fluidizing mixture caused a substantial increase in the concentration of NH3 during tests without coal feed (Test 56, 1780 vpm) and with coal feed (Test 15, 2160 vpm). This was measured with the inline bubbler and corrected for NH3 released during start-up. It seems that the steam, or H2 formed by its decomposition reacts with the char-N to form NH3; this is the only way that the high concentration measured without coal feed can be explained. A large effect of this nature had not been expected (and was not indicated in the literature review4). It seems that more NH3 is produced by this route than is released by pyrolysis alone. NO Formation in the Spout Jet. A water-cooled gas sampling probe was used to collect samples of the gas from the spout jet, during tests with air/N2 mixtures used as the fluidizing gas. The aim was to assess whether NO was being produced under the oxidizing conditions in the spout, which could then possibly be converted to NH3, under the reducing conditions of the fluidized bed above the spout. Table 4 shows the NO, O2, and NH3 concentrations that were measured as a function of the sampling height above the spout entry. Several tests were done at each spout sampling height and the data are the average of individual test measurements. All of the measurements that were made (6) Kanbara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Relationship between Functional Forms of Coal Nitrogen and Formation of NOx Precursors during Rapid Pyrolysis. Energy Fuels 1993, 7, 1013-1020. (7) Baumann, H.; Moller, P. Pyrolysis of Coals under Fluidised Bed Conditions - Distribution of N Compounds in Volatiles and Residual Char. Erdol und Kohle, Erdgas, Petrochemie 1991, 44, 29-33. (8) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Product composition, and kinetics of lignite pyrolysis. Ind. Eng. Chem. Proc. Des. Dev. 1978, 17, 37.

Energy & Fuels, Vol. 16, No. 3, 2002 745 Table 5. Effect of Height of the Spoutgas Sampling Probe on the NO and NH3 Concentrations in the Spouta height of sampling probe, cm

spout temp, °C


spout NO, vpm

spout NH3, vpm

spout O 2, %

exit NH3, vpm

2 4 4 6

870 850 900 870

0.36 0.40 0.36 0.41

200 80 28 50

0 38 25 100

0.4 0.4 0 0

188 183 150 125b

a Gas analyses not adjusted for dilution by N . b Based on mean 2 of Draeger readings.

during the project showed that only relatively low NO concentrations were present, and the values appeared to be insensitive to changes in the gasifier operating conditions (i.e., temperature, pressure, coal:air). The values in Table 4 show that the concentration of NO was highest immediately above the spout entry and declined with increasing height above the entry. There are differences in temperature between these tests; however, the spout NO concentration has been found to be insensitive to changes in the bed temperature (these data are described in the next section). Parallel measurements of the O2 concentration showed that the input O2 was consumed very rapidly and had decreased from a typical inlet concentration of 7%, to less than 0.4% at a height of 4 cm above the entry. It appears, therefore, that the maximum NO was produced immediately as the gases entered the spout; thereafter, the concentration declined, probably because the O2 concentration was also decreasing. The NO must have been formed from the fuel-N, as the temperature is too low for the production of thermal NOx.9 The fate of the NO produced low down in the spout is uncertain; it certainly disappears, possibly by reaction with NH3 to form N2. Determinations of the NH3 concentration, using Draeger tubes, were also made on samples of spoutgas extracted at the different sampling heights above the entry. These show that the NH3 concentration increased with height above the entry. This is consistent with the formation of NH3 from the fuel-N as part of the release and cracking of coal volatiles, as the coal particles are rapidly heated by combustion in the spout jet. There seems to be a transition from forming NO from the fuel-N under the O2-rich conditions at the entry to forming NH3 as the environment becomes more reducing. It could be argued that the data could also be interpreted as suggesting that NO formed low down in the spout was converted to NH3 as the conditions became reducing. The possibility of this reaction occurring within the spout, as the O2 concentration declined, has been examined in this work by comparing results obtained during a test using N2 alone as the fluidizing gas, with data obtained with air/N2 mixtures. The important difference between these tests was that in N2, NO cannot be formed, so there is no possibility of NH3 formation via that route. The results (Table 5) show that in N2 alone (Test 87), the NH3 in the exit gas was higher (575 vpm) than during tests under approximately similar temperatures and coal feed rates with air/N2 mixtures (typical NH3 concentration 150 vpm). These (9) Koespal, R. 5th World Congress on Chemical Engineering, San Diego, July 1996.

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Figure 2. The effect of the coal:air ratio.

results are compared on the “as measured basis”, as clearly the “N2 dilution factor” does not apply to results obtained in N2 alone. This shows that the presence of O2 in the spout reduces the potential to form NH3 and this could be by the formation of NO instead. It suggests that if NO is formed, then this does not subsequently convert to NH3, otherwise the data obtained with air/ N2 mixtures should be closer to that obtained in N2. The highest NO concentration measured (at 2 cm above the spout) was 200 vpm. The combined N compounds release (as NO plus NH3) in the air/N2 tests was therefore approximately 400 vpm, which is close to the N2 alone concentration of 575 vpm. Bear in mind that even at 2 cm, some of the NO may have already been destroyed. The NH3 concentrations in the exit gases from the gasifier were all fairly similar during this suite of tests. The value measured at 6 cm above the spout entry is getting close to the exit gas values, which shows that, without steam addition, the majority of the NH3 is formed in the spout region of the bed. Effect of the Operating Conditions on the Formation of Ammonia. The tests, described in the previous section, have revealed that the amount of NH3 measured in the exit gas in part depends on the composition of the fluidizing gas. With air/N2 mixtures as the fluidizing gas, the NH3 is formed from the fuel-N as part of the pyrolysis of the coal volatiles. This occurs rapidly in the spout region of the bed. When steam is added to the fluidizing gas, the amount of NH3 formed increases substantially through a reaction with the char bed. These effects are examined further below. Formation of NH3 during Tests with Air/N2 Mixtures. The effect of the operating conditions (temperature, pressure, coal:air ratio) on the formation of NH3 by pyrolysis has been examined in a series of tests using air/N2 mixtures as the fluidizing gas. The Effect of the Coal:Air Ratio. The effect of varying the coal:air ratio between 0.08 and 0.45, on the NH3, concentration in the exit gas and the NO concentration in the spout has been investigated (Figure 2). The data have been corrected for the dilution by the extra

nitrogen added to the fluidizing gas to control the bed temperature. In these tests the coal feed rate was increased from 0.43 to 2.35 g min-1, at a near constant fluidizing velocity of 0.15 m s-1. The proportions of air and nitrogen in the fluidizing gas were varied slightly to maintain a similar bed temperature between tests. The data were obtained at a pressure of 13 bara and a nominal bed temperature of 880 °C. The NH3 concentration in the exit gas increased from 100 to 1400 vpm as the coal:air ratio was increased. The data show that high concentrations were produced at the coal:air ratios envisaged for commercial scale air blown gasifiers (typical value 0.4). The increase is consistent with the increase in the mass input of coal volatiles (and fuel-N) at the higher coal feed rates. The NO content of the spoutgas was measured in a sample removed from the bed at a point 4 cm above the spout entrance. The NO concentrations were between 50 and 100 vpm and did not change systematically with the coal:air ratio. The Effect of Pressure. The effect of raising the pressure from 8 to 17 bara was studied during tests in the temperature range 860-880 °C and coal:air range 0.41-0.46. The fuel was a Daw Mill coal/limestone mixture. Figure 3 shows the NH3 concentration (by volume, vpm) in the exit gas and the NO concentration in the spout as a function of the pressure. The NH3 concentration increased with pressure and this is consistent with the trend expected from equilibrium considerations of the gas-phase reaction that produces NH3 from N2 and H2.10 This reaction will occur in the bed and freeboard of the gasifier and govern the exit NH3 concentration. The system will react to reduce the number of gas-phase molecules present as the pressure is raised (Le Chatelier’s Principle) and this will tend to increase the equilibrium concentration of NH3. The NO concentration in the spout was relatively low and was insensitive to the pressure changes. The Effect of Temperature. The effect of raising the temperature has been studied during tests with and (10) Duxbury, J.; Gavin, D. MTDATA Studies. ETSU Report R-018, 1994.



Figure 3. The effect of pressure.

Figure 4. The effect of temperature in the absence of sorbent.

without sorbent. Data obtained in the absence of sorbent are shown in Figure 4. The NH3 concentration decreased in a near linear way over the temperature range 800 to 970 °C. This is explained by an increase, with temperature, in the rate of the gas-phase reaction (2NH3 h N2 + 3H2) that moves the composition toward that expected at equilibrium. In the literature review (outlined in the previous paper4), it was noted that several workers10,11 have predicted lower NH3 concentrations at equilibrium than are actually measured in air blown gasifiers. This shows that supra equilibrium levels are formed in the gasifiers and, consequently, there is ample driving force for a gas-phase reaction toward the equilibrium concentration. The rate of this readjustment would increase with temperature and reduce the NH3 at an increasing rate. Only two NO values are available and they suggest that the concentration of this compound in the spoutgas (sampled at 4 cm above the entry) (11) Kilpinen, P.; Hupa, M.; Leppaelahti, J. Nitrogen Chemistry during Gasification - a Thermodynamic Analysis AAA-KTF/FKF-91/ 14 (Abo Akademi).

may have decreased with temperature. This indicates that the formation and destruction of NO may increase with temperature, so that at the higher temperature, less remains at a height of 4 cm above the entry. The effect of raising the temperature from 850 to 980 °C, in the presence of sorbent was studied at a pressure of nominally 13 bara and a coal:air ratio of 0.3. Figure 5 shows the NH3 concentration in the exit gas and the NO concentration in the spout as a function of the temperature. The NH3 concentration showed a peak in its formation at a temperature of approximately 880 °C. This may suggest that, in the presence of sorbent, the measured NH3 is a balance between formation and destruction reactions. The destruction is probably the decomposition of NH3 to N2 and H2 (noted in the absence of sorbent), as the composition moves toward equilibrium in the gas phase. The formation appears to be due to an effect caused by the limestone, as the maximum is not observed in tests without sorbent. However, the nature of this effect is not understood. The NO concentration measured in the spout was low and the data

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Figure 5. The effect of temperature in the presence of sorbent. Table 6. Effect of Steam in the Fluidizing Gasa

no coal feed effect of % steam effect of temp effect of coal:air

a

test no.

% steam (by vol)

pressure bara

coal feed rate g min-1


temp. °C

dilution factor

NH3 (adjusted for dilution by N2) vpm, average

61 56 55 48 62 48 43 43 49 47

0 10.1 16.4 6.0 14.3 6.0 6.1 6.1 6.5 6.3

12.5 13.2 13.0 13.3 13.1 13.3 12.6 12.6 13.5 13.6

0 0 0 2.1 2.2 2.1 2.0 2.0 2.2 2.2

0.32 0.28 0.32 0.25 0.25 0.46 0.80

850 860 890 795 800 795 837 837 841 830

6.06 2.84 2.36 2.59 2.21 2.59 2.20 2.20 3.10 4.78

49 1297 2294 2820 5131 2820 1460 1460 3720 5290

Data corrected for start-up NH3 formed from fresh Coalite bed. Gas analyses adjusted for dilution by N2.

suggest a small decrease in its concentration with temperature. This has been explained above. The Formation of NH3 during Tests with Air/Steam/ N2 Mixtures. With fluidizing gas mixtures containing air/steam/N2, the NH3 in the exit gas is formed during coal devolatilization, plus by a reaction between char and steam (or H2 formed by its decomposition). The preliminary study4 provided initial indications that the effect of steam was important and produced the major part of the NH3 measured in the exit gas. In the present experimental system, during tests with steam, the NH3 is mainly removed from the exit gas stream in the bubbler, used to condense and collect condensed steam from the exit gas. Hence, the NH3 concentrations discussed below were calculated from the analysis of the water in the in-line bubbler, plus the small amount present in the outlet gas from the bubbler (and measured in a side stream by the method used for air/N2 mixtures). The data have been adjusted to remove the effect of the dilution by N2 and have been corrected for the NH3 formed during start-up (from the fresh Coalite). As Coalite formed the initial bed, it was the Coalite char, plus char formed from the coal feed during the test, that will have reacted with the steam. Thus, the steam was reacting with a char of mixed origin, with an increasing proportion of coal feed derived material as the test progressed. Data obtained are presented in

Table 6, arranged to show the effects of changing operating conditions on the exit gas concentration of NH3. The tests were done without the spoutgas sampling probe in place, as its presence adversely affected the operation of the gasifier, when steam was injected, by condensation on the cooled probe surface in the bed. This meant that the spout temperature thermocouple could not be used. Consequently, the temperatures noted for this part of the work were obtained using the upper bed thermocouple, which tended to read slightly lower than the spout thermocouple. This was because the spout thermocouple was close to the hotter spout jet. Table 6 shows that the addition of steam to the fluidizing gas has resulted in a large increase in the NH3 concentration in the exit gas, compared with that measured without steam addition. It has already been established in this paper that the steam had formed NH3 by a reaction with the char-N. This must have involved a mechanism by which the steam or H2 (produced by its decomposition) stripped out the charN. This has been noted in a previous study of coal hydropyrolysis,12 but the precise mechanism remains unclear. It is noted that the H2 formed by pyrolysis in the current work does not have the same effect, other(12) Wu, W. F.; Guell, C.-Z.; Madrali, E. S.; Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. Effect of Pressure on Hydropyrolysis Tar Structures and Char Reactivities. Proceedings ICCS, Banff, 1993; pp 307-310.


Energy & Fuels, Vol. 16, No. 3, 2002 749 Table 9. Ammonia Produced in a Coal Char Beda

Table 7. N Concentrations in Final Bed Chars test no.

fluidizing gas (% vol)

N concentration in final bed char, %, wt

test no.a

% steam, vol

coal:air ratio

temperature, °C

pressure, bara

NH3, vpm

37 39 48 62

air/N2 air/N2 air/N2/steam (6%) air/N2/steam (14%)

0.8 1.0 0.7 0.4

80 82 49

9.0 14.2 6.5

0.65 0.72 0.46

813 825 841

13.4 13.7 13.5

3475 3617 4249

wise high NH3 concentrations would also have been present in the tests with coal feed and air/N2 mixtures. The effect of steam on the char bed was further examined by varying the proportion of inlet steam during tests with a Coalite char bed alone. Tests were done (without coal feed) with 0% (Test 61), 10% (Test 56), and 16% (Test 55) steam (by volume) in the inlet gas and the data show that the amount of NH3 increased from 50 to 2300 vpm over this range, i.e., it increased with the proportion of added steam. These values cannot be regarded as steady-state values, since at the start of the tests there was a reservoir of char-N in the bed from the initial bed material. Over the test period, a proportion of the char-N was removed by the steam and if left for a sufficient time it would clearly reach a stage when the NH3 concentration would decline as the char-N was depleted. It has been estimated that the Coalite char bed would have initially contained approximately 0.3 g of char-N (the bed char contained approximately 1.0% N at the start of each test with steam). The total mass that was present as NH3 in the exit gas (during the test with 16% steam input) was less than 0.2 g/test, so there was ample potential in the bed to produce the measured amounts of NH3. Analysis of selected final char beds (from tests with coal feed) for their N contents are shown in Table 7. These show that the presence of steam reduced the N concentration in the final bed chars, which confirms that steam was removing N from the char. The amount of NH3 formed during tests with coal feed increased with the proportion of steam in the fluidizing gas (Tests 48 and 62). It appeared to decrease as the upper bed temperature was raised (Tests 43 and 48), albeit over a narrow range of 795 to 837 °C. A similar effect was obtained without steam, in the presence of sorbent, which was interpreted as showing the movement in the gas composition toward equilibrium for the gas-phase reaction N2 + 3H2 h 2NH3. The effect of increasing the coal:air ratio (Tests 43, 47, 49) was to increase the concentration of NH3, which was similar to that noted without steam, although the actual concentrations were much higher in the presence of steam. This study has shown that the NH3 concentration in the exit gas, measured during tests with steam, is formed from both the volatiles and from the char-N. The proportion of the total NH3 formed via each of these routes can be assessed from the results of tests with

a Tests 80 and 82, initial char bed produced by pyrolysis of Daw Mill coal. Test 49 initial bed formed from Coalite/sand mixture.

and without steam addition (see Table 8). This shows data from tests with coal:air ratios of 0.25 and 0.45. The total NH3 measured in the exit gas increased substantially as the ratio was increased during tests in air/N2 and air/N2/steam. Assuming that the pyrolysis reaction mechanisms were unaltered by the presence of steam, the proportion of the total NH3 formed by pyrolysis (i.e., that released in the absence of steam) rose from 20 to 37% as the coal:air ratio was raised. The amount formed from the char-N (i.e., by the action of steam) also increased with the coal:air ratio, but the proportion of the total NH3 decreased from 84 to 67%. The data indicate that, during tests with steam, the majority of the NH3 is produced from the char-N in the air blown gasifier. The proportions formed by pyrolysis and by the effect of steam on the char-N varied with the operating conditions. Several tests were done using initial bed material derived from the pyrolysis of Daw Mill coal, to determine whether the char-N content affected the amount of NH3 formed. The Daw Mill char was prepared during a separate test with the same reactor operated as a pyrolyzer. The initial Coalite contained 2% N, whereas the Daw Mill coal contained 1.1%. This difference means that the coal char bed contained lower N content than the Coalite char bed and consequently a lower potential to form NH3. Data obtained using the Daw Mill char bed is shown in Table 9, together with a set of data obtained with an initial Coalite char bed (both tests with Daw Mill coal feed). Each initial bed contained a similar weight of char, although the Coalite char was mixed with an equal weight of sand. The data with the coal char bed were obtained with a higher coal:air ratio and with a higher % steam in the fluidizing gas, compared to the test using Coalite char, which will have tended to raise the amount of NH3 formed. However, the data on the table show that although the emission was high with the coal char bed, it was lower than with the Coalite char bed. This result shows that the char-N content has an influence on the NH3 concentration. Comparison with Data Obtained in the British Coal, Pilot Scale Gasifier The effects of varying the temperature and the coal: air ratio on the concentration of NH3 in the fuel gas produced by the British Coal pilot scale gasifier (200

Table 8. Proportions of NH3 Formed by Pyrolysis and from Char-N

a

test no.


steam addition, %, vol

measured NH3, vpm (% of total NH3 during tests with steam)

difference in NH3 (with steam-without steam), vpm (% of total NH3)

43 18 49 19

0.25 0.24 0.45 0.45

6.1 0 6.5 0

1460 (100) 287 (20) 3720 (100) 1384 (37)

1173 (80) 2336 (63)

Data with steam addition corrected for start-up NH3 formed from fresh Coalite bed. Gas analyses adjusted for dilution by N2.

750


Figure 6. The effect of temperature on the ammonia concentration in the fuel gas from the pilot scale gasifier.

Zhuo et al.

and the data show an approximate 3-fold increase when the coal:air ratio was raised from 0.36 to 0.40. Both scales of reactor have shown similar trends with changes in the operating conditions and this suggests that the laboratory scale gasifier is producing information, which can be used to predict trends in NH3 concentrations at the larger scale. We have not attempted to compare the absolute values as the conditions in the two scales of gasifier were markedly different. These differences would have affected both the extent of formation of NH3 (especially by the char-N/ steam reaction) and the extent of equilibration in the gas phase. These differences also mean that there is little to be gained from comparing the actual results of different workers. The results of different workers should be regarded as snapshots taken at different stages through a sequence of reactions that are occurring in the gasifiers. Ammonia Control during Gasification in an Air Blown Spouted Bed

Figure 7. The effect of the coal:air ratio on the ammonia concentration in the fuel gas from the pilot scale gasifier.

kg h-1)3 are shown in Figures 6 and 7, respectively. The data were obtained during mass balance tests with Daw Mill coal/Grangemill limestone mixtures in steam/air. These data provide a useful link between the data obtained at the laboratory and pilot scales and has been used to compare the trends apparent with changes in the operating conditions at the two scales of operation. The Effect of Temperature. In comparing data shown in Figures 5 (laboratory scale) and 6 (pilot scale) similar trends are apparent in both data sets: the concentration increases over the lower part of the temperature range, it then peaks and decreases over the upper end of the range. The temperature at which the peak occurred was different between the two gasifiers, and occurred at approximately 880 °C in the laboratory scale reactor and 920 °C in the pilot scale reactor. The difference in the peak temperatures is attributed to differences in the temperature measurements at the different scales of operation. In the pilot scale gasifier, the temperature used in the data analysis was the mean of the many readings obtained by several thermocouples, set at different points through the bed, over an approximately 24 h mass balance period. The value for the laboratory scale rig is the average of the readings obtained from a single thermocouple over the 10-15 min test period. The Effect of the Coal:Air Ratio. The data are shown in Figures 2 (laboratory scale) and 7 (pilot scale). Both sets of data show that the NH3 concentration in the fuel gas increased fairly steeply with increases in the coal:air ratio. A wide range has been examined in the laboratory scale rig and the data show an approximate 24-fold increase for an approximate 6-fold increase in the coal:air ratio (0.08-0.45). The range examined in the pilot scale reactor was more limited

The work has shown that there are ways to minimize the amount of NH3 formed at source. These should be considered further as they represent a more economic route for NH3 control, compared with downstream gas cleaning. Minimizing or removing the input steam in the fluidizing gas would enable the greatest reductions in the amount of NH3 formed, but would require an alternative means of bed temperature control to be used, since adjusting the steam/air ratio is currently the favored method. Possible alternative methods of controlling the temperature are the use of recycled fuel or flue gas. The impact of removing added steam on the gas calorific value would also need to be considered. However, the reactivity of the coal char bed toward steam at bed temperatures in the region of 950 °C is likely to be low and hence the impact of steam on the CV may not be great. The adjustment of the operating conditions may also be used to control the NH3 concentration. Operation at the highest temperature, with a limit set by problems introduced by ash sintering, would maximize the rate of the gas-phase equilibration, which reduces the concentration toward the relatively low concentrations expected at thermodynamic equilibrium for the gas-phase reaction N2 + 3H2 h 2NH3. Operation at the lowest acceptable pressure (set by the requirements of the gas turbine) would also minimize the NH3 concentration. This is because lower pressures favor lower equilibrium concentrations of NH3. There may also be a scope for optimizing the coal:air ratio, since the emission increases with the ratio. A value of approximately 0.4 is needed to produce a gas of adequate calorific value from the air blown gasifier, higher values should be avoided. Summary and Conclusions The work has shown that NH3 formed in air blown gasifiers is produced during the pyrolysis of the coal volatiles and by the action of steam (or its decomposition product, H2) on the char-N in the gasifier bed. The NH3 concentration measured during the tests with the air/N2 mixtures (i.e., formed during pyrolysis) was sensitive to the operating conditions and increased



with increasing coal:air ratio and pressure. These trends have been explained by an increase in the input of fuel nitrogen and an effect of pressure on the equilibrium position, respectively. The effect of temperature was more complicated and depended on whether sorbent was present or not. In the absence of sorbent, the NH3 concentration decreased with increasing temperature, which is consistent with the increasing rate of reaction toward the equilibrium concentration of NH3 in the gas phase. In the presence of sorbent, the NH3 concentration increased over the lower part of the temperature range studied, it then peaked and decreased over the upper end of the range. This was explained by introducing an effect of the sorbent which caused an increase in the NH3 concentration at the lower temperatures. The mechanism of the effect of the sorbent has not been explained. The results obtained with the spoutgas sampling probe have shown that the maximum NO was produced immediately the gases entered the spout jet, thereafter the concentration declined, probably because the O2 concentration was also decreasing. Batch determinations of the NH3 concentration in the spout showed that the NH3 concentration increased with height above the

entry. There seemed to be a transition from forming NO from the fuel-N under the O2-rich conditions at the entry to forming NH3 as the environment became more reducing. The NO produced low down in the spout disappears, but does not contribute to NH3 formation. The NH3 concentrations in the exit gas were much greater when steam was added to the fluidizing gas. This seems to be an effect of the steam (or the H2 formed by steam decomposition) on the char-N itself (i.e., on the N left in the char after pyrolysis is completed). This reaction formed the major proportion of the NH3 during tests with steam in the fluidizing gas. The trends observed in this work were similar to those observed in a pilot scale gasifier operated under broadly similar conditions. Acknowledgment. The authors thank the ECSC under Contract No. 7220-ED/096 for the financial support for this work and Mitsui Babcock Energy, who own the intellectual property rights of the ABGC gasifier, for access to certain information on the process. EF010237W

Investigation of Ammonia Formation during Gasification in an Air

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