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Energy & Fuels 2005, 19, 1016-1022
Formation of Hydrogen Cyanide and Ammonia during the Gasification of Sewage Sludge and Bituminous Coal N. Paterson,* Y. Zhuo, D. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, London SW7 2BY, United Kingdom Received December 1, 2004. Revised Manuscript Received February 15, 2005
HCN and NH3 released during the gasification of sewage sludge have been measured during a program of tests with a laboratory-scale spouted-bed gasifier. The data have been compared with results from gasification tests with coal. The effect of altering the bed temperature has been investigated, and the results have been related to reactions involving gaseous N species known to occur in the gasifier. The effect of steam addition on the HCN release has been examined. It has been found that the HCN concentrations in the exit gas increase with the operating temperature, which is thought to indicate increased formation as a primary product of the decomposition of the fuel-N compounds. Increasing the height of the char bed caused a significant reduction in the HCN concentration at the exit, as this promoted the decomposition of HCN to NH3. Steam addition caused a rise in the HCN concentration during tests with sewage sludge and a similar effect had previously been reported on the NH3 concentration during tests with coal. The NH3 concentration decreased with increasing temperature, and this is thought to reflect the increased rate of the equilibration of NH3 in the gas phase to form N2 and H2.
Introduction Fossil-, biomass- and waste-derived solid fuels contain varying proportions of fuel-N. In coal, this is entirely in the organic form and is mainly in the form of pyrrolic and pyridinic N.1 These compounds are formed as part of the coalification process, and total concentrations in bituminous coals are typically between 0.5 and 2% (dry basis).2 Concentrations of N in biomass can be higher and it is likely to be present in more aliphatic forms. The concentration in waste materials is very variable and can be present in many forms. In sewage-based materials, the N content can be as high as 8%.3 Many studies have been conducted on the form of N in sewage sludge, using a combination of extraction, chromatographic, and mass spectrometric methods.4-6 The work has shown that the nitrogen is contained in complex groups of compounds, such as lipids (fatty acids), lignins, and proteins. In a study using thermally assisted hydrolysis and methylation/gas chromatography/mass spectroscopy,4 the main nitrogenous compounds found * Corresponding author. Phone: 44 207 594 5581. Fax: 44 207 594 5604. E-mail:
[email protected]. (1) Jones, R. B.; McCourt, C. B.; Swift, P. Proceedings of the International Conference on Coal Science, Dusseldorf; 1981, p 987. (2) Speight, J. G. The Chemistry and Technology of Coal; Dekker: New York, 1994, p 335. (3) Werther, J.; Ogada, T. Sewage Sludge Combustion. Prog. Energy Combust. Sci. 1999, 25, 1, 55-116. (4) Jarde, E.; Mansuy, L.; Faure, P. Characterisation of the macromolecular organic content of sewage sludges by thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry. J. Anal. Appl. Pyrolysis 1999, 68-69, 331-350. (5) Reveille, V.; Mansuy, L.; Jarde, E.; Garnier-Sillam, E. Characterisation of sewage sludge derived organic matter: Lipids and humic acids. Org. Geochem. 2003, 34(4), 615-627. (6) Bodzek, D.; Janoszka, B.; Warzecha, L. Analysis of PAH nitrogen derivatives in the sewage sludges of Upper Silesia, Poland. Water, Air Soil Pollut. 1996, 89(3-4), 417-427.
were identified as being derived from proteins, these compounds included alkylindoles, adenine, pyrimidinediones, and amides. Domestic sludges can be distinguished by the presence of urea and caffeine.5 Azaarenes and nitroarenes have also been detected.7 Much research effort has been spent on investigating the fate of fuel-N during the processing of solid fuels, because of the potentially high environmental impact of NOx, either released directly by combustion or indirectly, by the combustion of fuel gas produced by gasification. NO contributes to acid rain and also forms NO2 in photochemical smogs, and N2O is a powerful greenhouse gas. The composition and concentration of the NOx formed is very dependent on the reaction conditions.8 It is known that a primary step in the formation of NOx, from coal, biomass, and waste materials, is the formation of NH3 and HCN, by the pyrolysis of the fuel-N compounds. The proportions vary with the reaction conditions and fuel type, with HCN being the dominant species at >1000 K. Under combustion conditions, HCN reacts further to form N2O, whereas under gasification conditions, it can react with H2 to form NH3. During combustion, NH3 reacts with O2 to form NO and N2, whereas under gasification conditions, it equilibrates in the gas phase to form N2 and H2. The extent of conversion of N in coal to NH3 is variable and depends on the reaction conditions.9 When coal was gasified in (7) Dote, Y.; Hatashi, T.; Suzuki, A.; Ogi, T. Analysis of oil derived from the liquefaction of sewage sludge. Fuel 1992, 71(9), 1071-1073. (8) Johnsson J. E. Formation and Reduction of Nitrogen Oxides in Fluidised Bed Combustion. Fuel 1994, 73(9), 1398-1415. (9) Kurkela, E.; Stahlberg, P.; Laatikainen, J. Pressurised Fluidised Bed gasification Experiments; Test report for PFG 15, VTT, Espoo, 10/ 11/1992.
10.1021/ef049688h CCC: $30.25 © 2005 American Chemical Society Published on Web 03/18/2005
HCN and NH3 Released during Gasification
air, in a high-pressure spouted-bed pilot-scale gasifier,10 it has been established that less than 20% of the original fuel-N remained in the char. The balance was present in the fuel gas as NH3 (up to 3000 vpm, equal to approximately 60% of the fuel-N), HCN (up to 30 vpm, equal to less than 1% of the fuel-N), and N2 (assumed as the balance). Tests with sewage sludge pellets,11 under similar temperatures and pressures and a slightly longer residence time, showed a similar behavior: 15% of the original N was present in the residual char and 45% was present in the fuel gas as NH3. Other workers have reported a range of values for N remaining in the char and variable concentrations of NH3 and HCN in their fuel gases.12 It is clear that the sequence of reactions that is occurring in the different reactors is the same. However, the extent of each of the reactions varies significantly with the operating conditions and reactor configuration. This causes variations in the concentrations of gaseous nitrogen species that are measured at the outlet of the reactors. The reactions that govern the NH3 content of the fuel gas from a laboratory-scale gasifier, which mimics the design and conditions (temperature, pressure) of the pilot-scale gasifier mentioned above, have been investigated in a recent project. This has been fully reported elsewhere.13,14 The main conclusions from this work, which was confined to a study using coal, were that a proportion of the NH3 was formed very rapidly during pyrolysis in the base region of the bed and a further (and much higher) proportion was formed by the reaction of steam (if present) with the char-N. The composite of that formed by these reactions then equilibrated in the gas-phase according to the reaction
N2 + 3H2 h 2NH3 Because supraequilibrium concentrations of NH3 were produced in the bed of the gasifier, the concentration decreased in the gas phase toward the relatively low value predicted at equilibrium for this reaction.15 A lower proportion of the original fuel-N was present in the fuel gas as NH3 in the pilot-scale gasifier than in the laboratory-scale reactor. It is likely that the explanation for this lies in the longer gas residence time at high temperature in the pilot-scale reactor compared with the laboratory-scale reactor. The residence time in the gasifier and downstream equipment at high temperature was approximately 15s in the pilot-scale (10) Intellectual Property owned by Mitsui Babcock Energy. Run Reports for tests in the CTDD pressurised gasifier, 1994-1997. (11) Paterson, N.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Gasification tests with sewage sludge and coal/sewage sludge mixtures in a pilot scale, air blown, spouted bed gasifier. IGTI/ASME Conference, Amsterdam, Holland, June 2002. (12) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Krause, J. L. Proceedings of the 19th Symposium on Combustion; The Combustion Institute: Pittsburgh, 1982; pp 1139-1149. (13) Zhuo, Y.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Investigation of ammonia formation during gasification in an air blown spouted bed: Reactor design and initial tests. Energy Fuels 2002, 12, 127135. (14) Zhuo, Y.; Paterson, N.; Avid, B.; Dugwell, D. R.; Kandiyoti, R. An 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 minimising its formation. Energy Fuels 2002, 16, 742-751. (15) Kilpinen, P.; Hupa, M.; Leppaelahti, J. Nitrogen Chemistry during GasificationsA Thermodynamic Analysis; AAA-KTF/FKF-91/ 14 (Abo Akademi).
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gasifier but less than 5s in the laboratory-scale reactor. This enabled the gas-phase composition to move further toward the lower concentration expected at equilibrium in the larger reactor. Studies of the fate of fuel-N have now been extended to include an investigation of the pyrolysis/gasification of dried sewage sludge and the measurement of the HCN concentration of the fuel gases produced from both coal and sewage sludge. HCN is considered to be the main primary product of the decomposition of coal-N compounds under the conditions used, and this can be converted to NH3 via a secondary reaction with H2. An insight into the effects of the operating conditions on the formation and destruction of the HCN could lead to ways of minimizing the formation of NH3 during pyrolysis/gasification. The study has been done using the laboratory-scale gasifier from the earlier study but operated at a lower pressure. This was because a parallel objective of the study was to assess the use of sewage sludge for use in a small-scale gasifier forming part of a combined heat and power scheme. This type of system operates at near ambient pressure. A limited number of tests have also been done with coal to assess the effect of temperature on the formation of HCN with this type of fuel and for comparison with the sewage sludge data. Experimental Section Laboratory-Scale Gasifier. A laboratory-scale, pressurized, spouted-bed gasifier has been used to study the reactions that produce NH3 and HCN under conditions that simulate those anticipated for commercial-scale air-blown gasification processes, using sewage sludge as fuel. A schematic diagram of the equipment is shown in Figure 1. The reactor is constructed in Incolloy 800 HT (34 mm i.d.), with an inner quartz liner (28 mm id) that contains the bed material. The reactor wall serves as both pressure shell and resistance heater. The fuel 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. It is capable of operation at temperatures up to 1000 °C and pressures to 30 bara, with a continuous feed system that meters the fuel into the reactor at rates up to 4 g min-1. In this study, pressures up to 3 bara have been used, as this is about the maximum pressure envisaged for small-scale gasifiers fuelled with biomass and is also the minimum operating pressure of the laboratory-scale equipment. Fluidizing gases are either air/ N2 or air/steam/N2 mixtures. The air input is varied at the start of each test to achieve the desired bed temperature using the heat released by combustion. At the same time, the velocity is maintained by the addition of extra N2. The quantity of N2 added is variable and depends on the operating conditions. The added N2 does not contribute to the chemistry in the gasifier, but it does have a diluent effect on the composition of the fuel gas. Hence, the data presented in this paper have been adjusted to remove the effect of this variable dilution so that trends in the release can be evaluated as a function of the operating conditions. The equipment and its operation have been described in more detail elsewhere.13 An initial bed was not used at the start of the tests with sewage sludge. This was because a parallel objective of these tests was to measure the trace element release with sewage sludge and the presence of an initial bed, composed of a different char, would have confused the interpretation of results. Hence, during each test, a sewage char bed built up within the reactor. Sinter did not form in the gasifier during these tests, and therefore, the duration of the tests was limited
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Figure 1. Schematic diagram of spouted bed reactor. by the capacity of the feed hoppers (about 40 min when both hoppers were used). A mixture of Coalite and sand was used as the initial bed in experiments with coal. This was used to avoid tar condensation that would have caused problems if the coal had been used as the bed material. Coalite is a smokeless fuel manufactured in the UK that contains approximately 6% volatile matter. Typical test duration with coal was 15 min, and during this time, analyses were conducted on the gas in the spout and exit gas 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. Fuels. The analyses of the fuels are shown in Table 1. Sewage Sludge. The material was obtained as a dried, prepared granular material from VTT (Finland). The ash content was fairly high (26.7%) and the S content lower than in the coal (1.1%). The N content (3.4%) was higher than in coal but typical of that expected in sewage-based materials. Daw Mill Coal. This grade of Daw Mill coal is used in the power generation sector in the UK and has moderate ash (10.3%) and S (1.6%) contents. The N content (1.3%) is typical of that in a UK bituminous coal. This will be mainly in the form of pyrrolic and pyridinic N,16 with a lower concentration of quaternary N compounds. Analytical Methods. Measurement of HCN in the Fuel Gas. Methods for measuring the HCN content of fuel gases are generally based on absorption from a measured volume of fuel gas in 10% (by wt) NaOH solution. The collected CN- is then analyzed either using an ion selective electrode, by ion chromatography, or by spectrophotometry. In this project an ion selective electrode (JENCONS-PLS, combination ionselective pH electrode) was used to analyze the collected solutions. (16) Burchill, P.; Welch, L. S. Variation of Nitrogen Content and Functionality with Rank for some UK Bituminous Coals. Fuel 1989, 68, 100.
Table 1. Analysis of the Fuel sewage sludge
analysis
Daw Mill coal
Proximate total moisture, % ad volatile matter, % db ash, % db C O (by difference) H N S Cl SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3
9.4 68.5 26.7
Ultimate, % db 43.4 45.6 6.4 3.4 1.1 0.12
Ash Constituents, %, on Ash 48.2 11.9 11.4 10.9 2.1 1.2 1.3 0.7
4.7 36.7 10.3 72.8 19.3 4.7 1.3 1.6 0.26 52.2 20.2 9.2 5.2 2.1 0.9 1.8 9.0
The HCN concentration during each test was measured by bubbling a measured volume of fuel gas (nominally 10 l) through two Dreschel bottles, placed in series and each containing 100 mL of 10% NaOH solution. The second bottle was used to check that efficient absorption had occurred in the first bubbler. The CN- concentration was then measured using the ion selective electrode, connected to a pH meter. The results were found to be very sensitive to the measurement procedure. However, repeatable results, for a particular sample, were obtained provided the analysis solution was stirred at the same rate for each determination, the electrode was placed at the same depth into the solution, and the same glassware and sample volumes were used. Readings were recorded over a 3-min period and the average used to determine the concentration. Within any 3-min measuring period, the reading of the electrode fluctuated by (5% of the average reading. A calibration graph was prepared for the electrode using solu-
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Table 2. A Comparison of Results for the NH3 Concentrations in the Fuel Gas Measured by Ion Chromatography and by an Ion Selective Electrode NH3 concentration, vpm spout temp, °C 980 965 932 847
ion chromatography 4600 4700 5200 8700
Table 3. Test Program
tions prepared by the dilution of a standard 1000 ppm cyanide solution (Spectrosol). The response of the electrode did vary on a day to day basis and, consequently, a new calibration graph was prepared for each batch of samples. It is important to maintain the pH of the solutions at a value of 13 or higher; otherwise, there is the possibility of loss of CN- by reaction with any dissolved S- to form thiocyanate, which would not be detected by the electrode. To assess the stability of the solutions produced in this work (which had a pH of greater than 13), measurements were made of the CNconcentration over a period of several days. Only a minor deterioration of the millivolt signal on the pH meter was noted after 2 weeks. As the sample solutions were analyzed within 24 h of collection, deterioration of the solutions can be ignored. Measurement of NH3 in the Fuel Gas. Methods for measuring the NH3 content of fuel gases are generally based on absorption from a measured volume of fuel gas into deionized water. The collected NH4+ is then measured either using an ion selective electrode or by ion chromatography. Both methods have been used in different parts of this work. The NH3 concentration during each test was measured by bubbling a measured volume of fuel gas (nominally 10 l) through two Dreschel bottles, placed in series and each containing 100 mL of deionized water. The second bottle was used to check that efficient absorption had occurred in the first bubbler. In the early part of this work, the solutions were analyzed by ion chromatography. However, a technical problem developed with the chromatograph and an ion selective electrode was substituted as the analytical method. The technique used for this ion selective electrode was the same as that developed for the CN- ion electrode, and the level of uncertainty was similar. Both analytical methods were calibrated using solutions prepared by dilution of standard ammonia solutions (Spectrosol). Comparative analyses were conducted on a suite of solutions, and the results are shown in Table 2. The data shows that the ISE read lower than the ion chromatograph by approximately 20%. This is a significant difference; however, the reason for this has not been identified in this work. This is because the study has been concerned with the trends that are apparent in the NH3 concentration with changing operating conditions and only one of the analytical methods was used for each aspect that was studied. Test Program. The test program is shown in Table 3. Tests were done with sewage sludge with temperatures in the range of 770-970 °C, with a reactor pressure in the range 1.9-3.9 bara and with and without added steam in the fluidizing gas. Tests with coal were done in the temperature range 850-940°C, with reactor pressures in the range 3.35.9 bara and only without steam in the fluidizing gas. The gas velocity in the reactor was maintained at a near constant value for each test by adjusting the air/N2 input flows. The input solids flow rate was calculated from the initial and final test weight of fuel in the feed hopper and from the test duration. The HCN and NH3 concentrations were measured during each test using the methods described above. The repeatability of the test data for the laboratory-scale gasifier was discussed in the previous papers.13,14 There it was noted that it was not possible to repeat tests under precisely the same conditions. This was because the actual flow rates of fuel, air, and nitrogen used to achieve the desired test conditions were determined after each test from the weights of fuel fed and the exit gas
duration, min
Sewage Sludge as Fuel 820 3.2 890 3.7 930 3.9 970 2.3 970 2.4 980 2.1 770 2.6 850 1.9 940 2.5 930 2.5
2.6 2.4 2.4 1.6 1.6 0.9 1.9 2.0 2.1 1.8
35 28 26 20 19 20 17 18 18 20
Daw Mill Coal as Fuel 850 4.4 880 5.9 940 5.2 890 3.3 850 3.3
2.3 2.0 2.1 2.2 2.0
9 26 25 25 30
% steam in fluidizing gasa
1 2 3 4 5 6 7 8 9 10
0 0 0 31 35 37 36 44 47 39
11 12 13 14 15
0 0 0 0 0
ISE 3900 3500 4400 8100
fuel feed rate, g min-1
test no.
a
bed temp, °C
pressure, bara
Balance gas was air/N2.
analyses and flow rates. These showed some variation between individual repeat tests. However, overall, the data already reported showed that, for groups of tests conducted under broadly similar conditions, there was a reasonable repeatability in the results. In one group of six tests, conducted at temperatures between 870 and 900 °C, five of the six NH3 concentrations were in the range 490-690 vpm, with the sixth value being low at 155 vpm.
Results and Discussion Formation of HCN and NH3 during the Pyrolysis/Gasification of Sewage Sludge. The primary reactions that led to the release of the fuel-N occur as part of the pyrolysis of the fuel volatiles. HCN and NH3 are thought to be the main primary gaseous products of this process, with the balance of the N remaining in the solid phase as char-N. The HCN reacts further (depending on the conditions in the reactor) to form NH3, by a secondary reaction with H2:
HCN + H2 h NH3 + C
(1)
The NH3 formed by the above reactions will then start to equilibrate in the gas phase via the reaction
2NH3 h N2 + 3H2
(2)
Since supraequilibrium concentrations of NH3 are produced initially in gasifiers, the driving force in the gas phase is to reduce the NH3 concentration toward that expected at equilibrium. In the present study, the HCN and NH3 concentrations have been measured in the exit gas from the gasifier. Consequently, the concentrations will have been determined by the overall effect of the reactions that are summarized above. Hydrogen Cyanide. The concentrations of HCN in the fuel gas, measured during tests with air/N2 and air/ steam/N2, are shown in Table 4. Two determinations of the concentration were made at temperature (820, 890, and 930°C) during experiments with air/N2 mixtures. The second determination was started immediately after the first had been completed. At each temperature, the second measurement was lower than the first measurement. Each test was started with an empty
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Table 4. Concentration of HCN in the Fuel Gas during Tests with Sewage Sludge
% feed test temp, added rate, pressure, no. °C steam g min-1 bara 1 2 3 7 8 10 9 4 5 6
820 890 930 770 850 930 940 970 970 980
0 0 0 36 44 39 47 31 35 37
2.6 2.4 2.4 1.9 2.0 1.8 2.1 1.6 1.6 0.9
3.2 3.6 3.9 2.6 1.9 2.5 2.5 2.3 2.4 2.1
HCN concntration, vpm sample 1 2900 3700 4200 5870 7830 6390 9480 2740 3390 3910
sample 2 average 1100 2100 3100 -
2000 3000 3600
reactor and a shallow char bed built up as the test progressed. It is thought that the gradual development of the char bed, as the test progressed, provided an increasingly effective environment for the reaction of HCN to react with H2 to form NH3. Both of these species are formed as primary products of pyrolysis, so the extent of this reaction depends on their release rates from the fuel-N. The results in Table 4 also show that the concentration of HCN increased with increasing temperature, up to about 930 °C. This is consistent with more HCN initially being formed as a primary product of pyrolysis at the higher temperatures. The data also suggests that the increase in the amount of primary HCN caused by the increase in the temperature must have outweighed the effect of temperature on the rate of the destruction of HCN. Further confirmation of the importance of the char bed in destroying the HCN was obtained by repeating a test at 930 °C, but with an initial bed of sewage-derived char. This had the effect of doubling the bed height at the end of the test, compared with the test without the initial bed. The measured HCN (average of two readings taken over 20 min) was 580 ppm (by volume). This compares with 3100 ppm (by volume) measured over a similar period, but without the initial bed addition, and clearly demonstrates the impact of the bed. Tests have also been conducted with steam added to the fluidizing gas, in the temperature range 770-980 °C. The steam concentrations were in the range 3147%, by volume. In this part of the work, a single determination was conducted during the first 10 min of each test. High concentrations of HCN were measured, particularly between 900 and 930 °C. The data are shown as a function of temperature in Figure 2. The plot for the air/steam/N2 data shows an increase in the HCN concentration over the same part of the temperature range as studied without added steam; however, at higher temperatures, there was a decline in the measured concentration. This is consistent with the amount of H2 formed by pyrolysis increasing rapidly at temperatures above 900 °C, which, in turn, tends to enhance the rate of destruction of the HCN to form NH3. In these tests, the measured H2 concentration increased from 5% at 770 °C to 15% at 970 °C. The graph also shows that, with steam addition, there was a higher concentration of HCN in the exit gas at temperatures below about 930 °C than was present without added steam. The steam data was obtained with a slightly
lower feed rate (and therefore a lower input of fuel-N) than the tests without steam. Had similar feed rates had been used, the difference would have been larger. This shows that steam has an ability to strip out the N from the fuel to form HCN. A similar effect was found on the amount of NH3 formed in the earlier study of the gasification of coal.14,17 The concentrations of HCN measured in the laboratory-scale gasifier were much higher than those measured in the pilot-scale spouted-bed gasifier that was developed by the British Coal Corp., as part of the AirBlown Gasification Cycle. In the pilot plant the typical HCN concentration, when operating on sewage sludge, was less than 50 ppm (volume basis). The difference is attributed to the longer residence time in the pilot-scale equipment (approximately 6 s in the bed of the pilotscale gasifier compared with less than 1 s in the laboratory-scale equipment). This enabled the HCN destruction reaction to proceed to near completion. The pilot-plant data is not in the public domain10 but is mentioned as it shows the importance of bed residence time on the destruction of the primary HCN. Ammonia. The concentration of NH3 in the fuel gas during tests with air/N2 mixtures is shown in Figure 3. These data were obtained during a suite of tests separate from those in which HCN was measured; however, the same batch of sewage sludge was used. The data have already been reported18 but are repeated here to enable comparison with the HCN data. The data were obtained from the analysis of the solution collected over the whole of the individual test periods. These data shows that the NH3 concentration decreased over the temperature range 770-970 °C. This is explained by a faster rate of equilibration in the gas phase, as the temperature was raised, for the reaction
2NH3 h N2 + 3H2
(2)
Figure 3 also shows the NH3 content of the fuel gas during tests with air/steam/N2 mixtures. The concentrations are high at the lower end of the temperature range studied. A comparison of both sets of data in Figure 3 indicates a decreasing trend with increasing temperature, but there is more NH3 present during the tests with steam. There was some scatter in the data, caused by small variations in the experimental conditions between the tests. Overall, the data with air/N2 was obtained with slightly higher feed rates and higher pressures than during experiments with added steam. The impact of these differences will have been to reduce the apparent differential between the two sets of data. As before,14 a significant effect of steam on the formation of NH3 is indicated. A higher rate of decomposition of the primary HCN to NH3 was indicated by the data shown in Table 4, at the higher temperatures. It appears that the rate of decomposition of NH3, by movement toward equilibrium for reaction 2, must be increasing at a faster rate with (17) Wu, F.; Guell, C.-Z.; Madrali E. S.; Cai H-Y.; Dugwell, D.; Kandiyoti, R. Effect of pressure on hydropyrolysis tar structures and reactivities. Proceedings ICCS, Banff, 193, 307-310. (18) Paterson, N.; Zhuo, Y.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Processing of sewage sludge: Pyrolysis and gasification in a laboratory scale, spouted bed gasifier. 7th European Biosolids, Organic Residuals Conference, and Exhibition; Wakefield, England November 18-20, 2002; Vol. 2, ISBN 1-903958-06-7; Session 10, Paper 071.
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Figure 2. The effect of temperature on the concentration of HCN in the fuel gas, during tests with and without added steam.
Figure 3. The effect of temperature on the concentration of NH3 in the fuel gas, during tests with and without added steam.
increasing temperature than the rate of its formation directly from the fuel-N and by the secondary reaction from HCN. Otherwise, a decreasing trend in the NH3 concentration with temperature would not have been measured. The formation of NH3 in the gasifier has been further examined by conducting several consecutive determinations of the concentration within a test. It has already been noted that measurements of the HCN concentration over individual tests showed that its concentration decreased as the test time (length of experiment) increased. This was explained by the gradual increase in the height of the char bed within the reactor, which appears to have provided a more effective environment for the secondary reaction of HCN to form NH3. If this interpretation is correct, then the decrease in the HCN concentration would be accompanied by an increase in the NH3 concentration. The data obtained in this set of tests is shown in Figure 4. The results of three consecutive determinations during two tests show that the NH3 concentration increased with increasing the test time. The conditions for the two tests were not the same. The first was done with a higher temperature and higher sewage feed rate than the second. Higher temperature and higher sewage sludge injection rate affected the NH3 concentration in opposite directions. This is because higher temperatures reduce the NH3 concentration through faster equilibration in the gas phase, whereas the feed rate increases the concentration through an increase in the input of fuel-N. These effects must be considered when comparing the results of the two suites of determinations shown in Figure 4. The
Figure 4. The concentration of ammonia in the fuel gas as a function of test time (and bed height).
results obtained at the lower temperature are slightly higher than those obtained at the higher temperature, which shows that the lower rate of equilibration in the gas phase has had a more significant impact on the NH3 concentration than the lower fuel-N input. However, the data is consistent with the decline in the HCN concentration with increasing test time noted above. Formation of HCN and NH3 during the Pyrolysis/Gasification of Coal. The tests with coal were done with an initial bed in the reactor (composed of a Coalite char/sand mixture). This was necessary because coal passes through a sticky phase during heating, during which agglomerates may be produced by the collision between adjacent coal particles. These prevent the successful operation of the gasifier. The presence of an initial bed, comprised of nonsticky particles, prevents the formation of agglomerates from the coal as it passes through its sticky phase and therefore enables success-
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Table 5. Concentration of HCN in the Fuel Gas during Tests with Coal test temp, pressure, no. °C bara 15 14 13
850 890 940
3.3 3.3 5.2
HCN concentration, vpm sample 1 sample 2 average 2900 3100 1700
3800 3200 -
3300 3200
ful tests to be done. There was an increase in the height of the char bed during the test, but the increase was low in comparison with the height of the initial bed. Hydrogen Cyanide. The concentration of HCN in the fuel gas during tests with Daw Mill coal, using air/N2 mixtures, over the temperature range 850-940 °C, is shown in Table 5. Within an individual experiment, the results do not show a decrease in the measured HCN concentration as the test time was increased, as had been observed during the tests with sewage sludge. This is thought to be due to the presence of the initial char bed, which provided an effective medium for the secondary decomposition of HCN from the start of the test. The concentration does show a decrease with increasing temperature, the extent of the decrease becoming greater at the higher temperature. This contrasts with the behavior of sewage sludge, where an increase in concentration was observed with both air/N2 and air/steam/N2 at temperatures up to approximately 950 °C. However, a decrease in the concentration of HCN was measured with sewage sludge, at higher temperatures (during the tests with added steam). This difference in behavior appears to reflect the presence of the char bed from the start of the test. The bed appears to have provided a more effective reaction environment for the destruction of the HCN. It has already been noted that the char, together with increased concentrations of H2 released by pyrolysis, provides an effective environment for the conversion of HCN to NH3. It is not valid to compare the results obtained with sewage sludge and coal during the tests with air/N2 mixtures (see Tables 4 and 5) because of the differing amounts of bed char present in the two sets of tests. However, it is noted that, at temperatures less than 890 °C, similar concentrations of HCN were measured, whereas at higher temperatures, less HCN was measured with coal.
measured when steam was present in the spoutgas mixture. This is consistent with earlier work with coal that showed that steam addition increased the NH3 concentration in the fuel gas. The mechanism of the effect is not known, but it does result in a significant rise in the concentrations of potential NOx precursors. The concentration of HCN was found to decrease with increasing depth of the char bed that formed in the reactor in these tests. It is thought that the bed provided a more effective environment for the reaction of HCN with H2 to form NH3. Measurements of the NH3 concentration showed that it increased with the depth of the char bed. This result was consistent with data obtained (in other work) from a pilot-scale spouted-bed gasifier, where only low concentrations of HCN were measured in the product fuel gas, after a longer residence time in a char bed, as compared to the laboratoryscale reactor used in the present study. During these tests, the concentration of HCN increased as the bed temperature was raised to approximately 930 °C and decreased rapidly at higher temperatures. This is thought to show that the amount of HCN formed as a primary product of pyrolysis increases with temperature. However, at temperatures above about 930 °C, its destruction by a secondary reaction with H2 (also released by pyrolysis) becomes increasingly effective and this outweighs the effect of the increase in the amount of HCN being formed. The NH3 concentration in the exit gas decreased with increasing temperatures with and without added steam in the spoutgas. This is interpreted as a result of the faster rate of equilibration in the gas phase, which effectively decomposes the initially high NH3 concentration to N2 and H2. This masked any effect of increased NH3 formation from HCN. The concentration of HCN formed from coal was also measured with the spouted bed reactor, using air/N2 mixtures. These tests required an initial char bed in the reactor (for operational reasons), and consequently, no effect of residence time in a char bed on the HCN concentration was apparent. The concentration decreased at temperatures above 900 °C, and this is thought to be a result of the increased H2 concentration released by pyrolysis. This enhanced the rate of decay of HCN to NH3. The work has provided a useful insight into the reactions of fuel-N in the gasifier and has helped to explain why low HCN concentrations of HCN were measured in the pilot-scale gasifier
Summary and Conclusions Sewage sludge has been gasified in air/N2 and air/ steam/N2 mixtures in a laboratory-scale spouted-bed reactor to identify the effects of temperature and steam addition on the concentrations of HCN and NH3 in the exit gas. Higher concentrations of both species were
Acknowledgment. This work has been funded by the European Union under Contract No. ENK5-CT2000-00050, ‘Sewage Sludge Gasification for CHP Applications’. EF049688H
