Investigation of Ammonia Formation during Gasification in an Air

In the present study, the reactor has been used to study NH3 formation under conditions ... Figure 1 presents a schematic diagram of the gasifier, sho...
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Energy & Fuels 2002, 16, 127-135

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Investigation of Ammonia Formation during Gasification in an Air-Blown Spouted Bed: Reactor Design and Initial Tests N. Paterson,* Y. Zhuo, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), Prince Consort Road, London SW7 2BY, U.K. Received April 13, 2001. Revised Manuscript Received August 8, 2001

A laboratory scale, pressurized gasifier has been developed to simulate the operating conditions of air-blown spouted bed gasifiers. The new 28 mm i.d. reactor operates with a continuous coal feed of typically 3 g min-1, at temperatures up to 980 °C and pressures up to 20 bara. Fluidizing gases can be combinations of air, steam, and nitrogen. The gases and coal enter the gasifier through the inverted conical base of the reactor, as a high velocity jet, which 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. Gases within the reactor may be sampled from the spout jet (using a specially designed watercooled probe) and from the exit of the reactor. In the present study, the reactor has been used to study NH3 formation under conditions relevant to those of the pilot-scale gasifier that was used during the process development stage of the Air-Blown Gasification Cycle. The pilot-scale work had found high, variable, and at times difficult-to-predict concentrations of NH3 in the fuel gas. The present stage of the work was aimed to help understand the pilot-scale data by focusing on the effect of operating conditions on NH3 formation reactions. The gasifier has been shown to run with continuous feed for periods of up to 30 min, before sinter builds up on the base and prevents normal operation. Within this time data are collected on the spout and exit gas compositions. In this paper we report preliminary findings, which suggest that the NH3 is produced during pyrolysis and by the effect of steam on the char-N. The latter is the most important reaction and the mechanism is at present unclear. High concentrations of NH3 appear to form through the latter mechanism and to vary with operating conditions, temperature, steam content, presence of sorbent, and coal:air ratio.

Introduction Air-blown gasifiers are being developed in the U.K. and the United States to form part of the next generation of cleaner coal-fired power stations. In both countries the use of pressurized fluidized bed gasifiers with submerged spouts have been chosen as a way of enabling the gasifiers to operate, without difficulties, on coals with a wide range of properties. In this type of gasifier, coal, steam, and air enter through the base of the gasifier, as a high-velocity spout jet. The shape of the base and the spout velocity are optimized to induce a recirculation of bed char into the spout and this very rapidly mixes the devolatilizing (and sticky) coal particles with nonsticky char particles. This avoids the formation of coal agglomerates, whose formation would prevent reliable operation of the gasifier. In the United States, a demonstration plant based upon the KRW airblown gasifier has been constructed at Pinon Pine and this is currently being commissioned.1,2 In the U.K., an * Corresponding author. E-mail: [email protected]. (1) Higginbotham, E. B.; Motter, J. W. Pinon Pine Power Project. 13th EPRI Conference (San Francisco) on Gasification Power Plants, October 1994. (2) U.S. Department of Energy. The Pinon Pine Power Project. Clean Coal Topical Report Number 8, December 1996.

air-blown gasifier was developed by British Coal, between 1985 and 1997, as part of a program of work to develop the components of the Air-Blown Gasification Cycle (ABGC).3 The results of the tests in the pilot-scale gasifier used in the development of the ABGC system showed that high, variable, and at times difficult-to-predict concentrations of NH3 could be present in the fuel gas.4 Clearly, high concentrations of NH3 in the fuel gas are not desirable, because of the possibility of forming NOx, during the subsequent combustion of the fuel gas in the gas turbine. Removal of NH3 from the fuel gas prior to combustion is a technically feasible option, but would impose an efficiency penalty on the process. In this study, the reasons for the observed high NH3 concentrations in the pilot-scale gasifier have been investigated, using a specially modified laboratory-scale gasifier operated under conditions simulating those used in larger scale gasifiers. The objective of the study has been to clarify mechanisms leading to the formation of (3) 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. (4) Intellectual Property owned by Mitsui Babcock Energy. Run reports for tests in the CTDD pressurized gasifier, 1994-1997.

10.1021/ef010092t CCC: $22.00 © 2002 American Chemical Society Published on Web 09/26/2001

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Figure 1. Reactions of fuel-N in the gasifier.

NH3 and to assess the impact of the changes in operating conditions on extents of reaction. This information has then been used to identify conditions that could be used to minimize NH3 formation in large-scale gasifiers. This is a more efficient route to minimizing NOx emissions from the gas turbine than some form of gas cleaning. The British Coal pilot-scale gasifier had been operated at a temperature of typically 950 °C and a pressure of 13 bara, whereas the commercial-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. A review of the literature on the release of fuel nitrogen under combustion and gasification conditions has been used to develop an initial view of mechanisms of formation of ammonia in the spouted bed gasifier. Since the gasifier contains both oxidizing and reducing zones, the nitrogen chemistry in both types of environment has been considered. Figure 1 presents a schematic diagram of the gasifier, showing the main reactions of nitrogen compounds that can potentially occur in each area. The fresh coal will begin to pyrolyze as it is rapidly heated, in the spout jet of the gasifier. Here, volatile fuel-N compounds, which are mainly in the form of pyrrolic and pyridinic nitrogen,5 break down under fuel-rich conditions to produce mainly HCN, with smaller (5) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100.

Paterson et al.

quantities of NH3.6,7 The proportions of HCN and NH3 seem to depend on the type of reactor and the conditions of the test. The escape of these gases from the fuel particles will be retarded by the effect of the elevated pressure.8 This gives time for the primary HCN to be converted to NH3 under fuel-rich conditions, by reaction with hydrogen released by pyrolysis. Not all of the fuel-N is released by pyrolysis and the balance (usually between 40 and 60%) remains in the char as stable compounds.9 The NH3 and the residual HCN escape from devolatilizing particles successively into an oxidizing (the spout) and then a reducing environment (the fluidized bed and the freeboard). If the NH3 and HCN are released into the oxidizing conditions in the spout, they can be oxidized to NOx and N2O. HCN oxidation is more efficient at producing N2O.10 However, these species are likely to be reduced again when they pass out of the spout into the bulk of the fluidized bed, before they exit the gasifier. One possible scenario considered for the apparently uncontrolled formation of NH3 was the initial formation of NOx in the oxidation zone and its reduction to NH3 in the reducing zone above the spout. The temperatures within the gasifier were too low for the formation of thermal NOx;11 it was considered therefore that the formation of NH3 would have to be based on the reactions of fuel-N alone. In the reducing environment of the bulk of the bed, the NH3 concentration will equilibrate in the gas-phase according to the reaction, 2NH3 h N2 + 3H2. Thermodynamic modeling studies done by British Coal showed that supra equilibrium concentrations were still present in the fuel gas at the point of measurement in the pilotscale gasifier.12 This is consistent with other studies.13 The nitrogen remaining in the char is stable at the gasifier temperature, but may be released by the removal of the surrounding carbon structure by either combustion (in the spout) or gasification (in the bulk bed). Under oxidation/combustion conditions in the spout jet, char-N will be released as either NOx, N2O, or N2. Raising the pressure increases the N2O/NO ratio, because it decreases the concentration of radicals involved in NO formation.14 Increased amounts of N2O have also been noted in the transition region between oxidizing and reducing conditions.15 However, NOx and N2O formed from the char-N are likely to be reduced (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 ConditionssDistribution of N Compounds in Volatiles and Residual Char. Erdol und Kohle, Erdgas, Petrochemie 1991, 44, 29-33. (8) Messenbock, R.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Fuel 2000, 79, 109-121. (9) 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. (10) Hulgaard, T. AlChE J. 1993, 39 (8), 1342-1354. (11) Koespal, R. 5th World Congress on Chemical Engineering, San Diego, July 1996. (12) Duxbury, J.; Gavin, D. MTDATA Studies. ETSU Report R-018, 1994. (13) Kilpinen, P.; Hupa, M.; Leppaelahti, J. Nitrogen Chemistry during GasificationsA Thermodynamic Analysis AAA-KTF/FKF-91/ 14 (Abo Akademi), 1991. (14) Haemaelaeinen, J. P.; Aho, M. J. Fuel 1996, 74 (12), 13771386. (15) Hulgaard, T. Environ. Prog. 1992, 11 (4), 302-309.

NH3 Formation during Gasification in a Spouted Bed

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Figure 2. Schematic diagram of the equipment.

under the conditions of the bubbling fluidized bed above the spout to either N2 or NH3. Under gasification conditions, the release of char-N may be either as N2 or NH3. The formation of HCN as the precursor to NH3 formation from the char-N is thought to be unlikely, as the volatiles (which are the source of the HCN) will have been destroyed during the formation of the char-N. Again, the high NH3 levels measured must indicate that the formation of the latter compound is favored. Calcium oxide and its precursors have also been noted to catalyze the formation of both NOx and NH3 under conditions relevant to the gasifier.16,17 In this paper, we discuss the development of the laboratory-scale gasifier, the operation of the equipment, and initial experimental results. The scale of the reactor (28 mm i.d.) introduces an element of novelty to the operation, with a continuous feed and representative fluidizing gas mixtures, at elevated pressure. Data are presented for a range of experimental conditions relevant to pilot- and commercial-scale operation. Possible mechanisms for the formation of NH3 under these conditions are discussed in terms of fuel-N content and reactive gas composition. Experimental Section The Laboratory-Scale Gasifier. An existing batch fed laboratory-scale gasifier18,19 (28 mm i.d.) has been modified to enable operation with a continuous coal feed. The reactor base (16) Lin, W.; Johnssen, J. E. 7th Int. Conf. Coal Sci. (Banff) 1993, 554-557. (17) Shimizu, T., et al. Proc. Int. Conf. FBC (San Diego) 1993, 611617. (18) Megaritis, A.; Messenbock, R. C.; Collot, A.-G.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti. R. Fuel 1998, 77 (13), 1411-1420. (19) Megaritis, A.; Zhuo, Y.-Q.; Messenbock, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1998, 12, 144-151.

design has been altered to feed coal at high velocity through a spout jet sited at the apex of an inverted cone-shaped base. The design mimics the hydrodynamic behavior in the pilotscale gasifier operated by British Coal. A novel gas-sampling probe has been designed and commissioned to draw gas samples directly from the spout jet. Other modifications include a gas cooler and condensate trap to remove condensed water, needed during tests with continuous steam injection. An incinerator has been installed to combust the exit gases before they are cooled and vented into the laboratory extraction system. The flow scheme for the equipment is shown in Figure 2. The system is capable of operation at temperatures up to 1000 °C and pressures to 30 bar, with a continuous feed system that meters the fuel into a char bed, at rates up to 4 g min-1. The Reactor: A schematic diagram of the reactor is shown in Figure 3. It consists of a 34 mm (i.d), 504 mm (tall) Incoloy 800 column that acts as both pressure shell and resistance heater. There is a 28 mm (i.d) quartz liner within the pressure tube and this has been modified to give an inverted conical section at its lower end, with a hole at the apex. The steep angle of the cone is similar to that of the base in the pilotscale gasifier. The spout jet, which feeds the coal and spout gas into the bed, fits into the hole at the base of the cone. Unlike the earlier batch operation mode,18,19 the present reactor does not make use of a support plate. The head flange of the reactor contains the gas exit tube, which conveys the gas through a cooled section equipped with tar and steam traps, to a small cartridge filter (to collect any remaining particulates) and then through a needle valve that is used to control the reactor pressure. The Fuel Feed System. This part (Figure 4) consists of twin feed hoppers (capacity 60 g coal each) with isolating valves, which feed into a common hopper. Each hopper has a fairly steep angle conical base to encourage slippage and avoid bridging. Before a run, the hoppers are pressurized with nitrogen to approximately 20 kPa above the desired reaction pressure. From the common hopper, the fuel is fed through a

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Figure 4. The feeding system. Figure 3. The gasifier. calibrated metering valve, which is driven by a variable speed dc motor. The metered fuel falls into an ejector, through which the spout-gas mixture (air/nitrogen) is passed at high velocity. This entrains the fuel and conveys it through a 1.5 m long, 2 mm (i.d.) pipe to the spout jet at the base of the gasifier. The hopper and metering valve assembly are mounted on a frame that is vibrated by an eccentric motor. This helps to avoid material bridging in the hoppers. A maximum rate of 4 g min-1 has been maintained with this setup. Steam Generator. Water is metered to the steam generator using an HPLC pump. The generator consists of an electrically heated section of pipe, packed with ceramic beads. During this work, the temperature of the steam was 230 °C at the outlet of the generator. It is mixed with the preheated air/N2/coal stream just before the fitting where the spout supply line passes through the lower flange of the pressure shell of the gasifier. The Gas Incinerator. The purpose of the incinerator is to combust the gas prior to venting through the laboratory extraction system. It is simple in construction and consists of a small propane-fired ring burner, which is placed under a steel fume extraction hood. The fuel gas is piped to the center of the burner. Analysis of the exhaust gas shows CO concentrations of less than 20 ppmsfrom an actual concentration in the fuel gas of approximately 4%. The Spout-Gas Analysis Probe. This is shown in Figure 5. Its purpose is to extract gas samples from different levels within the submerged spout jet. It is in this region that the process heat is released by combustion, pyrolysis of the coal volatiles begins, and nitrogen oxides can potentially form from fuel nitrogen. The primary aim of the sampling probe was to investigate the importance of nitrogen oxide formation in the spout as a possible precursor of ammonia formation in the

remainder of the fluidized bed. The probe consists of three concentric tubes; the inner tube forms the sample probe itself, while the outer pair form a cooling jacket for a flow of cooling water or nitrogen. A thermocouple is present in the sample tube which projects approximately 5 mm beyond the end of the probe. This is used to measure temperatures in the spout region. The height of the probe inlet above the spout may be changed between tests, by altering the length of the extension tube, in the part of the assembly above the top flange. The outlet of the probe is fitted with a manual control valve. No sampling pump is needed, as the pressure of the gas in the reactor serves to provide the flow needed by the analysis systems. Components analyzed include CO, O2, NH3, and NO. Fuel Gas Cleaning. Several filters were placed downstream of the gasifier to remove the elutriated solids and tars in the fuel gas. This system operated efficiently during the tests with the air/N2 mixtures. However, an initial test with steam showed that this system was not able to condense and collect water in the fuel gas. This caused problems of condensation in the downstream rotameter, which prevented measurement of the gas flow. A revised system was therefore developed for the bulk of the tests with steam. It consisted of a filter cartridge containing a stainless steel element to remove any remaining particulate, followed by a water-filled cooler/collecter (similar to a laboratory Dreschel bottle, but constructed in steel) to collect condensed steam. This equipment operated at the gasifier pressure. There was a coalescing filter placed after the pressure control valve, before the rotameter. This configuration of filters operated efficiently and no moisture was found to collect in the rotameter during the tests with steam. Measurements. Exit Gas Analysis. CO2, CO, and O2 were determined in the exit gas using continuous, infrared gas analyzers and a paramagnetic analyzer. The analyzers are rated as accurate to (1% of the reading. The infrared analyz-

NH3 Formation during Gasification in a Spouted Bed

Figure 5. The spout gas sampling probe. ers were calibrated each day, using a certified gas mixture, supplied by Air Products Ltd. The quoted accuracy of the calibration gas mixture was (2% (of the measured value). The paramagnetic analyzer was also calibrated daily using air. During tests with air/N2 mixtures, the NH3 was sampled from a side stream of gas before the pressure control valve, by bubbling a measured volume of gas through a series of bubblers, containing deionized water. The collected NH3 was determined by ion chromatography; the instrument measured NH4+ ion concentration with a repeatability of (1.9% of the measured value. During tests with steam injection, the concentration of NH3 was determined from the concentration measured (by ion chromatography) in the water present in the condensate traps, plus the much lower concentration present in the gas upstream of the condensate traps (collected by the method used with the air/N2 mixtures). The chromatograph was calibrated using standard solutions. The concentrations of ammonia and H2S were also measured using Draeger tubes (tubes 5/a and 100/a, respectively). Draeger tubes are packed with compounds that are specific to particular gases, which give a color change when exposed to that gas. The tubes are calibrated (by the manufacturer) so that the length of discoloration indicates the concentration of the compound. Although primarily intended for atmosphere monitoring, they have been found useful in monitoring process gases. The quoted accuracy of the Draeger tubes is (10% of the reading. NO, CO, and O2 in the spout gas were determined using a set of solid-state

Energy & Fuels, Vol. 16, No. 1, 2002 131 detectors. The tar concentration in the exit gas could be determined from the analysis of the material condensed in the condensate traps. Measurement of the Coal Feed Rate. The coal feed rate was set up before each test using the calibrated rotary valve. The actual rate of feeding was determined from the initial and final hopper weights. Typically, the average fuel feed rate, determined from the weight loss of the fuel in the hopper and the test time, was 2.5 g min-1. The values measured in this way are considered to be accurate to at least 0.01 g min-1, since the balance was able to measure to the fourth decimal place and an accurate stopwatch was used to measure the time. However, the measurement does not indicate whether the rate fluctuated over the test period. In the latter part of the test program, a hydrogen analyzer was incorporated into the exit gas analysis sample train. The variation of this component gives an indication of the stability of the fuel feed. The H2 analysis suggested that the fuel feed rate may have fluctuated by approximately (10% of the average value during each test. Measurement of the Exit Gas Flow Rate, Air Input Rate, and Steam Input. The exit gas flow rate was estimated from the reading of the calibrated rotameter (estimated error in reading: (5%) and the volume flow rate of gas used by the NH3 sampling equipment. The NH3 sampling flow rate was measured with an accurate dry gas meter, error (1% of the reading). The air input rate was calculated from the CO2 and CO concentrations in the fuel gas and its volumetric flow rate (estimated error in the air rate (6% of value). This assumes that all of the O in the CO2 and CO originates in the input air. The coal:air ratio for each test was determined from the actual coal feed rate and the calculated mass flow of air. The flow rate of water for the steam generator was set up using a calibrated HPLC pump. The amount fed during each test was calculated from the actual volume fed to the pump, which was measured with a repeatability of (5%. Measurement of Temperature. The spout-gas temperature was measured using a thermocouple placed in the center line of the reactor, above the spout entry. It was situated in the spout-gas sampling probe, with its tip projecting (by 5 mm) into the bed. Its height above the entry could be varied, although 4 cm was used in the majority of the tests. A thermocouple was also placed to measure the upper bed temperature. However, its position in a radial direction was not fixed and the readings could be influenced by its position in relation to the water-cooled probe. For this reason, the spout temperature has been used to evaluate the data, during tests, which had the spout-gas sampling probe installed. For the limited number of tests without the spout probe (tests with steam, to be reported in the next paper), the upper bed temperatures are considered to be reliable and have been used in the assessment of the data. The quoted accuracy of the thermocouples was (0.75% of the reading. Operation of the Gasifier. Several operational problems had to be overcome during the commissioning of the gasifier. It was not possible to start up using an initial bed of coal, because of problems with the excessive release of tar and other volatiles as the bed was heated to the reaction temperature. Instead, a low volatile content fuel was necessary; crushed Coalite and sand were used successfully in this study. Coalite has a low volatile matter content of approximately 9% (see Table 1) and formed the initial bed material for the tests. The majority of its volatiles were removed during its manufacture and the remainder were pyrolyzed completely during startup and did not cause operating difficulties. The feed was switched to coal after the initial bed had been heated to a sufficient temperature (approximately 750 °C) that ensured the effective pyrolysis of the volatiles as they were released from the coal. This avoided problems caused by tar release and condensation in the equipment during the initial test period.

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Table 1. Analysis of the Daw Mill Coal and Coalitea

a

analysis

Coalite

Daw Mill Coal

moisture ash volatile matter C H N S Cl

2.9 7.0 9.4 80.6 1.94 2.1 1.04 0.23

4.9 11.7 33.0 65.7 4.0 1.1 2.0 0.24

%, as analyzed basis.

The bed temperature was controlled by adjusting the air/ N2 ratio in the fluidizing gas. It was found that care was needed in the adjustment of the input air to avoid temperature excursions in the gasifier. With a significant carbon reservoir in the bed, the rate of heat release was potentially high and this could lead to rapid temperature excursions (up and down), when adjusting the inlet gas mixture composition to achieve the desired steady-state conditions, particularly at high pressure. However, it was found that by careful control of the heat release rate, by manipulation of the air/nitrogen ratio in the spout gas, then the required conditions could be set up rapidly. The small diameter of the reactor meant that the particle size and superficial fluidizing velocity were lower than those used at larger scale. In the laboratory-scale reactor, the fuel particle size was in the 200-300 µm range and the fluidizing velocity was between 0.1 and 0.2 m s-1 , which compares with values of up to 3000 µm and 0.8 m s-1 used in the pilot-scale gasifier. The following procedure was used when starting up with a char bed. The equipment was assembled with approximately 60 g of bed material in the gasifier. Following a pressure test, it was fluidized with N2, at a superficial fluidizing velocity of approximately 0.1 m s-1 and at low pressure. The resistance heater was set to control the wall temperature at a value close to the desired bed temperature. However, there was only sufficient power input from the heaters to heat the bed and column to approximately 750 °C, at 12 bara without the addition of air to the fluidizing gas. While the heaters were warming the initial bed, the pressure was gradually increased to the desired pressure, by increasing the cylinder supply pressure and the flow was adjusted using the needle valve

Figure 6. Stability of operation of the gasifier.

downstream of the gasifier. When the temperature reached 700-800 °C, the fluidizing gas composition was changed to an approximate 20% air/80% N2 mixture. The heat released by combustion raised the bed temperature toward the required value. Further increases in the proportion of input air were made to raise the bed temperature to the desired value and when this was reached, the coal feed was turned on and the rate adjusted to the desired value using the speed control on the calibrated valve. This was a critical time and the temperature was raised as quickly as was feasible from this point, and the test conditions established so that the test proper could begin. A typical test duration was 10-15 min. During this time analyses were conducted on the gas in the spout and downstream of the gasifier. There was no char offtake through the base, so that bed material built up in the gasifier during the test. Sinter formed on the base during most tests with coal, this limited the maximum test time to less than 30 min, as it tended to block the spout entry. Less sinter formed when steam was used in the fluidizing gas. The stability of operation of the gasifier over a typical test is shown in Figure 6. This shows the main operating parameters and the CO2, CO, and H2S concentrations in the exit gas plotted against the test time. The pressure, exit gas rate, H2S, and CO show steady values during the test, which shows that the total input gas and solids rates must have been consistent throughout the test. The temperature and CO2 show small decreases during the test, which was due to a small reduction of the input air/N2 ratio part way through the test. Materials. Daw Mill coal and Grangemill limestone (as sulfur sorbent) were used in the majority of the tests in our study and in the pilot-scale tests conducted by British Coal. The coal was chosen because it was typical of that used in U.K. power stations. The analysis (Table 1) shows that it contains 1.1% N. The limestone was added to retain the sulfur released from the coal, as a way of minimizing the gaseous sulfur releases from the process. The limestone was pure and contained greater than 99% CaCO3. The Coalite used as the start-up material is a low volatile content manufactured smokeless fuel. The analysis (also shown in Table 1) shows that it has a residual volatile matter content of 9% and a N content of 2.1%. It is likely that a proportion of the volatile-N, that would have been present in

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Energy & Fuels, Vol. 16, No. 1, 2002 133

Table 2. Test Program test no.

fuel

fluidizing gas

3 5 7 8 10 13 14 15

Coalite DM DM DM DM/GL DM/GL DM/GL DM

air/N2 air/N2 air/N2 air/N2 air/N2 air/N2 air/N2 air/N2/steam

a

spout temp., °C

upper bed temp, °Ca

pressure bara

fuel feed rate, g min-1

coal:air ratio (mass basis)

exit gas flow rate, l min-1 (wet)

duration, min

900 865 910 830 850 910 910 900

nd nd nd nd 760 930 825 770

12.7 12.8 13.2 13.1 13.1 13.0 8.2 13.0

0.74 1.2 1.5 2.9 2.2 1.3 1.2 0.6

0.18 0.17 0.20 0.28 0.31 0.13 0.15 0.11

9.2 20.4 18.4 19.8 20.8 16.7 9.7 14.9

61 39 26 9 23 23 23 16

nd - not determined. Table 3. Composition of the Spout and Exit Gasesa,b,c,d

d

test no.

N2 dilution factor

spout temp, °C

coal:air ratio (mass)

spout press, bara

3 5 7 8 10 13 14 15

2.71 3.60 3.10 2.32 3.98 2.20 1.69 3.29

900 865 910 830 850 910 910 900

0.18 0.17 0.2 0.28 0.31 0.13 0.15 0.11

12.7 12.8 13.2 13.1 13.1 13.0 8.2 13.0

average spout gas analysis, %, vol NO, O2, vpm % 19 18 93 0 80 66 59 62

1.3 0.8 0.9 1.9 1.2 1.1 1.0 1.6

CO2, % 15.2 15.5 15.5 15.1 15.9 16.5 15.6 16.1

average exit gas analysis,%, vol, dry basis CO, H2S, NH3, % vpm vpm 11.4 10.8 10.8 11.6 10.0 8.8 10.7 8.6

380 790 1280 810 600 680 850 740

10 310 22 520 740 13 20 2750

a Spout gas sampled at 4 cm above the spout entry. b O in exit gas was 0% for all tests. c Vpm: parts per million on a volume basis. 2 N2 dilution factor: (input air + input N2 )/input air.

the coals used in the manufacture of the Coalite, will have been converted to char-N. Test Program. The test program (Table 2) shows the main experimental parameters, together with the duration of the tests. All except the last of these tests were done without steam addition, using the air:N2 ratio to control the bed temperature. The initial tests were done using the Coalite as the feed material as well as a component of the initial bed, to commission the equipment and develop the test procedures. They were followed by a series of tests using the Daw Mill coal as the fuel, using several different spout temperatures, to assess the ability of the equipment to operate with a continuous feed of bituminous coal. Limestone was added to the coal for the next series of tests. A preliminary test was done with steam added to the fluidizing gas to assess its impact on the performance of the gasifier. Each of the foregoing tests was done at 13 bara. One test was conducted with a coal/limestone mixture at a lower pressure (8 bara).

Results and Discussion Table 3 reviews the data obtained during each test. Data are shown for NH3, H2S, CO2, and CO in the exit gas, and NO and O2 within the spout jet. The O2 was also measured in the exit gas, but its concentration was always zero (as expected). The values shown are the average of the individual measurements that were manually recorded, at approximately 2 min intervals during the steady-state test period. The species measured were subject to a 2- to 4-fold dilution by the nitrogen gas added to the air/nitrogen mixture used as the fluidizing gas (the air: N2 ratio was used as a means of temperature control). The data shown in the table have been adjusted to remove the effect of this variable dilution, by multiplying the measured data by the dilution factor (see Table 3). 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.

Tests with Coalite. No operating difficulties were found with this fuel and extended tests could be done. The O2 input for the test shown (Test 3) was approximately 7.7% (as estimated from the measured CO2 and CO concentrations in the exit gas). The dilution factor calculated from this was 2.7 (i.e., input air plus N2 divided by input air). The low O2 value measured in the gas that was sampled with the spout-gas analysis probe, at a point 4 cm above the spout entry, shows that the combustion reaction occurred very rapidly at the spout inlet and that the upper part of the spout and the turbulent bed above the spout were operating under reducing conditions. A low NO concentration was also detected in the spout gas at this point. The NH3 concentration in the exit gas was very low, which is consistent with the removal of the volatile-N compounds during the manufacture of the Coalite (by carbonization at approximately 600 °C), either by loss to the gas phase or conversion to stable char-N compounds. It is noted that the Coalite in the initial bed material will have lost its remaining volatile-N compounds during start-up and was therefore not collected in the NH3 sampling equipment. The measured NH3 is that formed from the Coalite fed during the test period. The H2S concentration represents approximately 25% of the input sulfur in the Coalite; the remainder will have remained in the char bed. No sorbent was added to retain sulfur during this test. Tests with Daw Mill Coal. Table 3 presents test data obtained with coal, using air/N2 mixtures as the fluidizing gas. Two tests (Tests 5 and 7) were done with a similar coal:air ratios, but with different spout temperatures. The other test (Test 8) was done with a lower spout temperature, but with a higher coal:air ratio. It was immediately apparent that sinter tended to form in the cone when coal was fed and that this eventually restricted the input flow of gas and solids. Consequently, in most tests with coal feed, the duration was limited

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to less than 30 min. After this time sinter influenced the gas flows, which disrupted the conduct of the test. The data obtained using the spout-gas probe shows that O2 had decreased to a low value by the sampling point and concentrations of NO were low (less than 100 vpm) at 4 cm above the spout entry. The value measured at 910 °C was higher than that measured at 865 °C (with a similar coal:air ratio), which showed an effect of temperature. Also, the value at 910 °C, measured with Daw Mill coal (Test 7) was higher than that measured with Coalite (Test 3) at a similar temperature and coal:air ratio. This suggests that coal volatile-N is participating in NO formation, as might be expected under the oxidizing conditions in the spout. The NH3 concentration in the exit gas decreased markedly with an increase in temperature for tests at a similar coal:air ratio (Tests 5 and 7). It is known that only low NH3 concentrations are expected at equilibrium for the gas-phase reaction:

N2 + 3H2 h 2NH3 The observed decrease in the NH3 concentration in the exit gas appears to result from the more rapid gas-phase equilibration at the higher temperatures. The results of Test 8 are consistent with this explanation as the NH3 emission was higher with a lower bed temperature. However, the effect is thought to have been magnified, because the coal:air ratio was also higher for this test. The H2S concentrations measured in the presence of the coal were higher than measured with Coalite. The Coalite is produced by carbonization at approximately 600 °C, where some of the volatile-S in the parent coals would be lost to the gas phase and some would be converted to stabler forms. These changes are not evident when the total S analyses of the two fuels are compared (see Table 1), because the total S analysis does not indicate the form of S present. Our result suggests that the S in the Coalite is released less easily and this is consistent with it being combined in sulfur forms that have a greater temperature stability. The H2S values, obtained at a similar coal:air (tests 5 and 7), suggest that the amount formed increases with temperature. The individual measurements of the CO concentration were fairly steady during each test. This shows that the Daw Mill coal char must have been of a similar gasification reactivity to the Coalite char, otherwise a progressive change in the CO concentration might have been expected as each test proceeded. Furthermore, CO2 and CO concentrations observed during the gasification of the coal and Coalite were similar. In these early tests, where no added steam was used, the primary gasification reaction would have been: C + CO2 h 2CO. It would appear that the beds were of similar low reactivity toward CO2. Tests with Daw Mill Coal/Limestone. The sorbent was premixed with the coal to give a Ca:S molar ratio of approximately 2:1 in the mixture. The effect of the sorbent addition can be gauged by comparing the results of Test 10 (with sorbent) and Test 8 (no sorbent). Both tests were done with similar coal:air ratios and a similar spout temperature. The addition of sorbent seems to have increased the concentration of NO in the spoutgas sample and also increased the NH3 concentration in the exit gas (Table 3). Apparently, the sorbent is able

Paterson et al.

to affect the fate of the fuel-N, although at present the mechanism is unclear. In tests with the pilot-scale gasifier (850-950 °C temperature range), it was also found that sorbent tended to increase the NH3 concentration in the fuel gas. The H2S concentration was decreased by the presence of sorbent, which shows that some sulfur was retained. However, the efficiency of sulfur retention would not be expected to be high during this type of test, since the initial bed contained no sorbent, and only a limited amount of sorbent will have accumulated in the bed over the relatively short test duration. The bed itself being shallow compared to pilot- or full-plant-sized reactors, contact times between H2S and sorbent particles are short. The Effect of Pressure. The effect of pressure can be assessed by comparing the results of Tests 13 (13 bara) and 14 (8 bara). Both were done with similar spout temperatures and coal:air ratios. The NH3 concentrations were low during both tests, which is consistent with the high temperature and low coal:air ratios. The data does indicate a minor increase in concentration at the lower pressure, but it remains to be seen whether this is evident under conditions that produce higher NH3 concentrations. The NO concentrations in the spout-gas sample were higher than the NH3 concentrations in the exit gas and the data does not show any appreciable effect of pressure on the NO concentration. The H2S concentration in the exit gas was lower at the higher pressure. This is consistent with the higher rate of accumulation of sorbent in the bed at the higher pressures, which was caused by the higher mass input rate of the feed/sorbent mixture. This meant that the ability of the bed to retain sulfur was enhanced. The Effect of Steam Addition. A preliminary test (Test 15) was done with steam, before the full gas cleaning train was installed. There were operating difficulties during the test, that were caused by condensation in the downstream pipework and flowmeter and only limited data were obtained. However, the results show that the NH3 concentration was increased by a substantial amount when steam was added. The effect was not expected, but was repeatable and is thought to be caused by an effect of either the steam, or H2 produced by steam decomposition, on the char-N. Some evidence to indicate that hydrogen can influence the concentration of char-N has already been obtained in this laboratory.20 More work is needed to understand the mechanism of this affect. The NO concentration, in the spout, did not seem to be affected by the presence of steam in the fluidizing gas. The Fuel-N Conversion. The fuel-N conversion was calculated from the measured coal feed rate and its N content, and from the NH3 concentration in the exit gas. The values in Table 4 show that the conversion was less than 10% for all tests using the air/N2 mixtures alone as the fluidizing gas. In these tests the only source of NH3 was through reactions of the coal volatiles during pyrolysis. As expected, the lowest value was measured with the Coalite; this fuel would have lost much of its volatile matter during its manufacture. Very low values were also measured with Daw Mill coal during tests at (20) Wu, F.; Guell, C.-Z.; Madrali, E. S.; Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. Proceedings ICCS (Banff) 1993, 307-310.

NH3 Formation during Gasification in a Spouted Bed Table 4. Conversion of the Fuel-N test

NH3, vpm

N as NH3 g/min

input N g/min

N conversion to gas %

3 5 7 8 10 13 14 15

4 87 7 223 185 6 12 836

2.186E-05 0.0011 7.657E-05 0.0026 0.0023 5.958E-05 6.933E-05 0.0073

0.0081 0.0126 0.0165 0.0319 0.0220 0.0129 0.0119 0.0069

0.3 8.3 0.5 8.2 10.4 0.5 0.6 106

the highest temperatures investigated at 13 bara. The balance of the fuel-N could have formed N2 or more likely was converted to char-N. Literature data suggest that a significant and variable proportion of the fuel-N may be converted to char-N.9,21,22 The addition of steam to the fluidizing gas had a major impact on NH3 formation and the measured conversion increased to just over 100%. This shows that steam is the one of the dominant factors in determining the NH3 concentration. The result obtained is interpreted as showing that steam or H2 produced by steam gasification is able to form NH3 by reaction with the char-N. A reaction of this nature was not identified in our literature review and the mechanism of such a reaction is at present unclear. The greater than 100% value measured in our test with steam shows that the N was being stripped from the char bed as well as from the fresh coal feed. Summary and Conclusions A laboratory-scale gasifier, that operates at elevated pressure, with a continuous coal feed and with air/ steam/N2 mixtures as the fluidizing gas has been commissioned. To our knowledge this is the first time that a gasifier has been operated under these conditions with a continuous coal feed (2-3 g min-1) at this small scale. The reactor has been operated under conditions that simulate those in the pilot-scale gasifier, used in the component development program of the Air-Blown Gasification Cycle. (21) Kristiansen, A. Understanding Coal Gasification; IEA Coal Research. IEACR/86, March 1996. (22) Kobayashi, H. Devolatilisation of pulverised coal at high temperature. Ph.D. Thesis, Massachusetts Institute of Technology, 1976.

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An initial suite of tests has been reported, which shows that the equipment is able to operate with coal under a range of temperatures, pressures, coal:air ratios, and fluidizing gas compositions. The composition of the spout and exit gases were measured during these tests and the results suggest the following: (1) The NH3 concentration in the exit gas was sensitive to the test conditions. NH3 contents were higher with coal compared with Coalite (low volatile fuel), which shows that some NH3 is formed from the coal volatiles. The operating conditions (temperature, pressure, and coal:air) also affect NH3 concentrations. Relatively low fuel-N conversions to NH3 were observed during all tests with air/N2 mixtures. The NH3 content of the product gas increased very significantly when steam was added to the fluidizing gas. In the presence of steam, a conversion to NH3 in excess of 100% was measured. This highlights the importance of steam in the formation of NH3 in the gasifier. The result also suggests that steam was stripping N from the char bed as well as from the fresh coal. (2) NO was detected in the gas sampled from the spout jet. The concentrations were low and, contradicting expectations at the start of the study, did not appear to be directly linked to the NH3 concentration. (3) The O2 concentration in the spout-gas sample was low compared with the inlet concentration, confirming that O2 is used very rapidly in the spout jet through reaction with the high-temperature recirculating bed chars. (4) H2S concentrations were lower during tests with sorbent compared to tests without sorbent, which shows that some sulfur retention was occurring. However, extensive S-retention would not be expected because there was no sorbent in the initial bed and contact times were short. 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. EF010092T