Modeling of Selective Non-Catalytic Reduction System using Detailed Chemistry and a Jet-Entrainment Model Anders Brink, Markus Engblom, Mikko Hupa Åbo Akademi University, Turku, Finland Corresponding author Anders Brink, tel. +358 2 215 4931,
[email protected]
Abstract Selective Non-Catalytic Reduction is a measure for abating NOx emissions also used in FBC boilers. This paper focuses on ammonium-based SNCR solutions, where carrier air is used for introducing NH3 into the upper freeboard. The SNCR process is studied using a detailed description of the chemistry, combined with a simplified description of the fluid dynamics. This description is based on a plug-flow reactor concept extended with a continuous entrainment of flue gas, able to quantitatively model the flow in a jet. Both SNCR solutions based on NH 3 injection in a gaseous form as well as an aqueous NH3-solution are investigated. Additional parameters that are studied are the temperature of the flue gas entrained into the jet, the composition of the flue gas as well as the ratio between NH 3 and NOx. The numerical study shows that using weak air jets to inject NH3 into the upper free board results in that most of the NH3 reacts in a region where the NH3 to NO ratio is high. Consequently, the reduction efficiency is not sensitive to the initial NH 3 level in this configuration. Tests using a carrier air based SNCR system in a biomass fired FBC partly support the findings from the numerical study. These data shows that a similar SNCR efficiency can be obtained using a large variation of NH3 to NO ratios. On the other hand, in the experimental results also higher efficiency was observed, which cannot be explained by the model. However, the available boiler data was not detailed enough to be used as validation data. Keywords: SNCR, jet entrainment, freeboard
1. Introduction Biomass based fuels typically contains high levels of fuel bound nitrogen. Although by primary means, i.e. by an effective air staging, less than 10% of the fuel bound nitrogen may end up as NO x emission in a biomass fired FBC /1/, further reduction calls for the use of secondary measures. Already today, selective non-catalytic reduction (SNCR) is used in several markets. SNCR can be realized in several ways. Mainly two different reagents are used commercially: urea and ammonia. Urea is injected as a water solution, and is often considered easier to handle but may be associated with detrimental N 2O emissions. The other alternative is to use NH3 as a reagent. In both cases the NO reduction mechanism involves NHi radicals. The chemistry involved in the SNCR process is known in great detail /2/. The overall process for ammonia based SNCR can be described using two reactions, one between NH3 and NO leading to N2, and one between NH3 and O2 leading to NO. In reality, the chemistry is much more complex. There is no direct reaction between NH3 and NO, nor between NH3 and O2. NH3 first reacts with the main radicals (O, OH and H) to NH 2. NH2 can then reduce NO. NH2 can also react further to NH and HNO or NNH. These radicals can then be involved in reactions leading to N2 or to NO. For the process a temperature window has to be selected such that the chemistry is fast enough not to give rise to slip of unreacted NH3, while reactions leading to NO are still suppressed. The presence of unburned components will alter the SNCR chemistry indirectly by promoting the formation of the main radicals, thus accelerating the reactions involving NH 3.
The influence of unburned components in the flue gas and the effects of additives have also been studied in detail. E.g., experimental investigations of the effect of CO, CH 4 and H2 have been conducted. Such components typically influence the position and width of the SNCR window. This has also been shown in a number of modeling studies. Common to most of these studies are that they are concerned with laboratory reactor setups. Typically, the flow is assumed to behave as plug flow in the reactor or in the numerical model, with a few exceptions: Røjel et al. /3/ used a gradual mixing approach to investigate the effect of mixing on the SNCR process. They used a gradual mixing approach, originally described by Zwietering /4/ to account for a non-ideal behavior. They found that this model was able to better explain the experimental results than the plug flow assumption. Later, Cao et al. /5/ used the same mixing approach to investigate the effect of CH4 and H2 addition on the SNCR process. They found that mixing had a clear effect on the SNCR behavior. The gradual mixing approach has also been used for modeling NH 3 chemistry in turbulent processes. Zieba et al. /6/ used this approach to explain the experimentally observed behavior of NH 3 in a flame less combustor and Vainio et al. /7/ to explain the observed behavior of fuel-N in a biomass fired bubbling fluidized bed. In their paper, Røjel et al. /3/ also used the gradual mixing approach in an attempt to explain the observed behavior of a SNCR system in a biomass fired grate boiler. They found that compared to a plug flow model, the gradual mixing approach better capture the experimental observations. However, until now, no-one has used this approach for a more comprehensive study of the SNCR behavior in different process conditions.
2. Jet-theory Jet entrainment has been studied extensively. Most studies have focused on entrainment of jets into stagnant flows. For this setup well established correlations for jet entrainment exists /8/. The entrainment of jets into transverse flows is also a well understood process, partly because the turbulent transverse jet problem has been used as a model problem for testing turbulence models. The transverse jet consists of a shear flow dominated section, similar to that of a fully developed free jet, but also some additional features, of which the counter-rotating vortex pair has received most attention. Broadwell and Breidenthal /9/ have developed a correlation for the jet entrainment, given as the entrainment as a function of downstream position, under these conditions. Hasselbrink /10/ used similarity analysis to derive the expressions presented by Ricou and Spalding /8/ and by Broadwell and Breidenthal /9/ and compared them with experimental data. In addition, he suggested that for jets with a blowing ratio higher than 20 a sudden change from the use of the free jet correlation to the wake-like region can be made at the point where these two expressions predicts the same entrainment. The blowing ratio, r, is given by the expression (Eq. 1) In the equation ρ is the density uj is the entrance velocity of the jet and v the velocity of the cross flow. The subscript ∞ refers to the undisturbed cross flow and j to the jet. Following Hasselbrink the jet entrainment in the free jet like part, where the jet has not jet deflected is (Eq. 2) In the equation above indicates the mass flux, cej is a model constant, ρ the density, d the initial diameter of the jet and x is the distance from the jet entrance in the in the initial jet direction. The subscript ∞ refers to the undisturbed cross flow and j to the jet. It can be noted that this equation does not hold for small x/d values, since at x=0 it would give a jet mass flux of zero, although the initial mass flux is non-zero. Secondly, in the first few diameters from the jet exit there is the potential core region, in which the entrainment does not follow the equation above.
In the wake-like region, where the jet largely has deflected, the entrainment is given by (Eq. 3) This equation is written as a function of the y-direction, which is the direction of the co-flow. cew is a model constant. Hasselbrink also expressed the jet entrainment in the wake-like part as a function of position in the x-direction. In this way he could calculate the switch-point between the region where the expression for the jet-like region is valid and the region where the expression for the wake-like region is valid. The result is (Eq. 4)
3. Modelling The jet entrainment was modeled using the equation described in the previous section. A slight modification to the equation was made for simplicity. The coordinate system was shifted in x-direction such that x=0 is the point of the potential core region. This is at approximately d=6 from the jet entrance. At the same time, the approximation was made that the jet flux is approximated by the entrainment. This is not strictly true, but is a good approximation further away from the jet exit. A similar approximation of the entrainment equation was made in the y-direction, i.e., that the entrainment well approximates the jet flux. Since the switch to the equation giving the jet flux as a function of position where already a substantial mass has been entrained, the error associated with this approximation is negligible. To get a continuous distance for the reactor calculations, a switch was made from the x-direction to the y-direction at the point where the two expressions predict the same jet flux. In order to also get temperature profiles for the kinetic calculations, the local temperature of the jet was calculated as that obtained by mixing the entering jet with the amount of entrained gas predicted by the entrainment correlations. The temperature of the surrounding gas was assumed to have a constant value. To also obtain the necessary residence time, the jet-like region was assumed to have a half angle of 12 . In the wake-like region, the jet was assumed to have the same velocity as the transverse flow. In case ammonia is fed as a liquid solution, droplets also need to be modeled. There are mainly two processes that need to be modeled, the droplet trajectories and the droplet vaporization. To keep the calculations simple, it is assumed that the droplets follow the gas flow, i.e., their slip velocity is small. For the largest particles considered in this study, it is clear that this is a simplification. The droplet vaporization process is a complex process. Since the vapor pressure of ammonia is different from that of water, the composition of the evaporating liquid will change as a function of time. However, at the conditions considered in this study, the rate of evaporation will mainly be given by the heat transfer rate. In addition, the smaller the droplet, the smaller is the relative contribution of radiative heat transfer, further simplifying the modeling. Since zero slip velocity is assumed in the trajectory calculations, this is also assumed in the heat transfer calculations. The influence of the Stefan flow, caused by the evaporating ammonia-steam mixture is neglected. The temperature profile of the jet, calculated as described above, was used in the vaporization calculations. However, the influence of the vaporization on the gas temperature was not accounted for. The gas phase kinetics was modeled with the AAU mechanism /11/. The commercial software CHEMKIN 4.0 was used for carrying out the calculations. The jet was treated as a plug flow reactor with continuous entrainment, with the entrainment rate calculated using the expression given in the theory section. In the cases where the ammonia-water mixture was fed as droplets, the vaporization
rates were included as a separate, gradually entrained, stream. Figure 1 shows the reactor network model in CHEMKIN 4.0.
Flue gas entrainment
Carrier jet
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Ammonia-steam entrainment from vaporization of droplets Figure 1. The CHEMKIN representation of the jet-model for the cases where the ammonia is fed as a liquid. The calculations on the SNCR performance were done for a hypothetical boiler in the 100 MW thermal range. The dimensions of nozzles in the SNCR system were taken from Saario /12/. However, the rest of the process data was based on data published by Vainio et al. /1/. The temperatures of the SNCR carrier air-jets were assumed to be 350 K. The ammonia added to the jet was assumed to originate from a 30-% NH3 in water solution as indicated as a typical process configuration by EPRI. The NH 3 content of the jet was assumed to be 2.5 vol-%, based on available plant data. The flue gas in the upper part of the boiler was in the base case assumed to consist of 10 vol-% CO2, 25 vol-% H2O, 4 vol-% O2 and 100 ppm CO, the rest being N2. The NO content was set to 73 ppm. The number of SNCR nozzles was assumed to be 10. Some in-stack measurements were available from test of a SNCR system in a full scale boiler with NH3 to NO ratios in the range 0.7 to 2. In the calculations the inlet jet velocities where chosen such that a similar NH 3 to NO ratio was achieved. This corresponded to inlet velocities of 20 to 40 m/s. This is a similar range as indicated by Saario /12/. However, he gives a NH3 content of the SNCR jet of 0.66 vol-%, which is lower than assumed in these calculations. In the cases that the ammonia was assumed to be fed as droplets, the SNCR jet was still assumed to have the same velocity, but the 30-% ammonia solution was gradually entrained based on the result from the vaporization calculations.
4. Results and discussion Equation 4 shows that there is a linear dependence between the length of the jet like region and the blowing ration. With cej=0.32 and cew=0.73 the switching point to the wake-like region is at x/d < 16 for all conditions. For the case with an inlet jet velocity of 40 m/s and a cross flow velocity of 2 m/s, i.e., the case with the longest jet-like region, the mass of entrained gas at the switching point is approximately three times that of the entering gas jet. Although the entrained hot gas causes a raise in the jet temperature, it is not high enough to lead to any significant oxidation of NH 3 in this region. Figure 2 shows the NO profile as a function of normalized distance in such a case. Here the cross flow temperature is 1223 K. The figure shows, that although the oxidation rate of NH 3 is low up to the switching point, given by a normalized distance of approximately 16, significant levels of NO are still reached. The NO level continues to rise also in the initial part of the wake-like region. Once the temperature in the jet has reached approximately 1135 K the reaction between NH3 and NO becomes significant. The NO level continues to drop until the gas in the jet has travelled approximately 80 jet
diameters. At this point all NH3 has reacted. The effective NH3 to NO ratio at this point is approximately 3, whereas the nominal is approximately 2. This means that the SNCR process has been rather inefficient, and much of the NH3 is consumed by reactions with NO formed through the competing NH3 oxidation reactions. Once all NH3 has been consumed, the NO level in the jet continues to increase until all available gas has been entrained. It can be noted that this distance is probably far longer than available, but since all NH3 is consumed already at the position 80 jet diameters, corresponding to a physical distance of approximately 3 meter along the jet centerline, the correctness of the description of the mixing process after this point is not of importance.
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Figure 2. Results from modelling a SNCR system with NH3/NO=2. a) NO mole fraction as a function of distance normalized by the carrier air jet diameter; b) NH3 mole fraction; c) jet temperature; and d) the fraction of the fed NH3 present in the jet.
Figure 3 shows a similar plot for the case where the global NH3 to NO ratio is 1.4. This has been achieved by lowering the initial jet velocity in the model. In this figure the subplot showing the temperature has been replaced with a plot of the local NH3/NO ration. The figure shows that also this jet behaves in a similar way, although the jet-like region ends earlier and a longer distance is required for full entrainment of the cross flow. In this case too, NH 3 starts showing a significant conversion as the temperature reaches approximately 1135K. The figure reveals that most of the ammonia is reacting at conditions where the NH3 to NO ratio is much higher than the global NH3 to NO ratio. A calculation was done where the global NH3 to NO ratio was reduced to 0.14. In this case, a good reduction level cannot be expected. Nevertheless, the calculation showed just a slight increase in the reduction level, indicating that the conditions described by the simulations are an inefficient use of the NH3.
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Figure 3. Results from modelling a SNCR system with NH 3/NO=1.4. a) NO mole fraction as a function of distance normalized by the carrier air jet diameter; b) NH 3 mole fraction; c) local NH3 to NO ratio; and d) the fraction of the fed NH3 present in the jet.
Figure 4 shows the final reduction efficiency as a function of cross-flow temperature in the case that the inlet velocity is 26 m/s and the cross flow velocity is 2 m/s. In the figure also results obtained analyzing a situation where the cross flow contains 1.0 vol-% CO, opposed to the 100 ppm as assumed in the other calculations. The figure shows that CO has a detrimental effect on the SNCR process at these conditions. No reduction is obtained in the studied temperature window. For the 1123 K with 1.0 vol-% CO present, the reduction is showed using the sum of total fixed nitrogen instead of NO. At these conditions only approximately 20-% of the total fixed nitrogen exits as NO. For the corresponding case with 100 ppm CO in the cross flow, almost no reactions occur, and the NH3 slip is 96 ppm. For the other temperatures no NH3 slip was observed using the 100 ppm CO assumption for the cross flow. Also shown in Figure 4 is the final NO reduction efficiency supplying the ammonia as a liquid solution. The figure is otherwise calculated using the same velocity and temperature assumptions as in Figure 2. The figure shows that the reduction efficiency is very similar to that predicted using gaseous ammonia feed with the carrier air jet. The result obtained with 50 µm and 100 µm is almost the same. An extreme droplet size is required to spread out the NH3 source such that reactions with NH3 and NO occur in the whole computational domain. At these conditions the assumption of zero slip-velocity clearly breaks down. For the smallest droplets on the other hand, the non-coupled temperature distribution may bring uncertainties to the calculations, since a fast vaporization must be associated with a drop in the gas temperature in the jet, unless the heat is provided by radiation. Nevertheless, it is not believed that this would have a significant influence on the results. The calculations with NH3 injected as a liquid solution indicate that the way the ammonia is injected is not critical. However, these results are still unsure, since no validation data were available. In addition, the description of the vaporization process probably still needs refinements.
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Figure 4. Modelled NO reduction efficiency as a function of cross flow temperature. Using the NH3 level in the jet reported by Saario /12/ allows for higher jet velocities without reaching a very high global NH3 to NO ratio. Using this more dilute mixture, and keeping the NH3 to NO ratio at 1.4, the switching distance between the jet-like region and the wake-like region changes dramatically. In this case it occurs at x/d=40. In this case, the entrance velocity of the carrier air jet becomes 100 m/s. Here too, the NH3 reactions take off as the temperature reaches approximately 1130 K. In this case, however, more of the cross flow has been entrained at this point, and a larger fraction of the total NO is reacting, leading to a more efficient SNCR process. Still, the NH3 in depleted before all the NO is entrained into the gas. 1.E-04
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Figure 5. Results from modelling a SNCR system with NH 3/NO=1.4, but with a NH3 concentration of 0.66 vol-%. The carrier jet velocity is 100 m/s. a) NO mole fraction as a function of distance normalized by the carrier air jet diameter; b) NH 3 mole fraction; c) local NH3 to NO ratio; and d) the fraction of the fed NH3 present in the jet.
Saario /12/ reported that in his experiments the lowest NO level was reached allowing a NH 3 slip. To test if this also holds here, a calculation with the higher jet velocity and with the higher NH 3 level in the jet was carried out. In this case the global NH3 to NO ratio is approximately 5. Figure 6 shows the results from this calculation. In this case the local NH3 to NO ratio is extremely high in the jet and there is still NH3 available further down stream in the jet. This leads to a continued reduction of entrained NO and the final reduction is around 50 %. The NH3 level at the exit is approximately 3 ppm. There is also a small emission of N2O, approximately 2 ppm at the outlet. The reduction efficiency in this case seems to roughly correspond to that obtained by Saario /12/. 1.E-04
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Figure 6. Results from modelling a SNCR system with NH 3/NO=5.2 using a carrier jet velocity of 100 m/s. a) NO mole fraction as a function of distance normalized by the carrier air jet diameter; b) NH3 mole fraction; c) local NH3 to NO ratio; and d) the fraction of the fed NH3 present in the jet.
Figure 7 shows results from tests with a SNCR system in a biomass fired boiler. In the test, the NH3 to NO ratio has been varied. No detailed information on the test arrangements was available, but it is believed that they roughly correspond to those used in the calculations in this study. The figure reveals that the SNCR efficiency seems to increase with increasing NH3 to NO ratio. There is a large uncertainty in these results, since the process data showed significant fluctuation in the NO emissions as a function of time. Especially the NO level without SNCR is unreliable. Based on these data it is impossible to validate the model properly. The only thing that can be said is that the reduction efficiency seems to be in the same range.
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Figure 7. Experimental results obtained changing the NH3/NO ratio in a biomass fired boiler.
5. Conclusions A two-zone description for the entrainment in a cross flow has been adapted for describing the fluid dynamics of of a SNCR system realized with a carrier air jet. The SNCR chemistry is in the model described using a detail reaction mechanism. Simulations with this model showed that using a weak jet for introducing NH3 leads to poor reduction efficiency. This is explained by NH3 being consumed before having a chance to reduce the NO entrained further downstream. Changing the temperature in the calculations did not have a strong effect on the reduction efficiency, although a too low temperature leads to a freezing of the chemistry. No significant change in the reduction efficiency was observed when changing the NH3 to NO ratio in the weak jet. This was due to that in all cases NH3 reacted before mixing with fully with the cross flow. Supplying ammonia as droplets did not change the situation. This was due to that, according to the model assumptions, the NH3 droplets vaporized quickly and the chemistry became identical to the gaseous ammonia case. The calculations showed that increasing the jet velocity improved the performance. If the NH3 to NO ratio was increased until a small NH3 slip was predicted the performance was further improved in this case. Available boiler data was not detailed enough to be used as validation data. On the other hand, although the reduction efficiencies in general were slightly higher than showed by the modeling, the available data do not contain observations that would conflict with the results of the model.
References [1] Vainio E., Yrjas P., Zevenhoven M., Brink A., Laurén T., Hupa M., Kajolinna T., Vesala H., The fate of chlorine, sulfur, and potassium during co-combustion of bark, sludge, and solid recovered fuel in an industrial scale BFB boiler, Fuel Processing Technology 105 (2013), 59-68. [2] Miller JA and Glarborg P., Modeling the Thermal De-NOx Process: Closing in on a Final Solution, International Journal of Chemical Kinetics 31 (1999), 757-765.
[3] Røjel H., Jensen A., Glarborg P., and Dam-Johansen K., Mixing Effects in the Selective Noncatalytic Reduction of NO, Ind. Eng. Chem. Res. 39 (2000), 3221-3232. [4] Zwietering ThN. The Degree of Mixing in Continuous Flow Systems. Chem. Eng. Sci. 11 (1959), 1-15. [5] Cao Q., Liu H., and Wu S., Theoretical study of the influence of Mixing on the Selective Noncatalytic Reduction Process with CH4 or H2 Addition, Ind. Eng. Chem. Res. 50 (2011), 10859-10864. [6] Zieba M., Brink A., Schuster A., Hupa M,, Scheffknecht G., Ammonia chemistry in a flameless jet, Combust. Flame 156 (2009) 1950–1956. [7] Vainio E., M., Brink A., Hupa M., Kajolinna T., Vesala H., Fuel Nitrogen Reactions in a Biomass Fired FBC – Measurements and Kinetic simulations, In proceedings of 21st International Conference on Fluidized Bed Combustion (21FBC), Naples, Italy, 3-6.6 2012. [8] Ricou FP. and Spalding DB, Measurements of entrainment by axisymmetrical turbulent jets, J. Fluid Mech. 11 (1961), 21- 32. [9] Broadwell, J E and Breidenthal RE, Structure and mixing of a transverse jet in incompressible flow, J. Fluid Mech. 148 (1984), 405–412. [10] Hasselbrink Jr, EF, PhD-thesis, Transverse jets and jet flames: structure, scaling, and effects of heat release, Technical Report TSD-121 (1999), Department of Mechanical Engineering, Stanford University. [11] Coda Zabetta, E., Hupa, M., A Detailed Kinetic Mechanism with Methanol for Simulating Biomass Combustion and N-Pollutants. Combust. Flame, 152 (2008), 14-27. [12] Saario A., PhD-thesis, Mathematical Modeling and Multiobjective Optimization in Development of Low-Emission Industrial Boilers, Publication 733 (2008), Tampere University of Technology.
Acknowledgement This work has been carried out within CLIFF (2014-2017) as part of the activities of Åbo Akademi University. Other research partners are VTT Technical Research Centre of Finland, Lappeenranta University of Technology, Aalto University and Tampere University of Technology. Support from the National Technology Agency of Finland (Tekes), Andritz Oy, Valmet Power Oy, Foster Wheeler Energia Oy, UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper Inc., and Top Analytica Oy Ab is gratefully acknowledged.
