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  • Below my publications relating to fluidized bed combustion are given under the following headings :
  • Combustion and emissions

    Zirconia-cell oxygen probe measurements

    Reversed air staging and N2O emissions

    Sulphur capture

     

  • Link to publications on CO2 Capture and Chemical-Looping Combustion

     

    Combustion
  • Leckner, B, and Lyngfelt, A., 2002, Optimisation of emissions from fluidised bed combustion of coal, biofuel and waste, accepted for publication in Int. J. Energy Res.

    Several types of fuel can be burned in a fluidised bed. This paper discusses the impact of low or medium volatile fuels (coal) and high volatile fuels (biomass and waste) on the emissions of NO, N2O and other pollutants. It is found that high and low volatile fuels behave in different ways, and measures to reduce emissions from coal combustion are not necessarily effective for high volatile fuels. This forms a basis for various design options.

    Fernández, M.J., Lyngfelt, A., and Johnsson, F.,2000, "Gas concentrations in the lower part of the combustion chamber of a circulating fluidised bed boiler - Influence of bed height." Energy & Fuels, Vol. 14, 1127-1132.

    The effect of the height of the bottom bed on the conditions in the lower part of a 12 MW CFB-boiler was studied. Pressure measurements along the furnace height were made together with gas (CO, hydrocarbons, O2, NO and SO2) and zirconia-cell probe measurements in the splash zone. The operation in the exploding bubble regime causes the particulate phase to be under reducing conditions. However, as the height of the bottom is decreased the conditions become more oxidising. Additionally the concentrations of NO and SO2 in the splash zone are significantly affected by the height of the bottom bed. Thus, the gas flow phenomena in the bottom-bed are of great importance for the understanding of processes related to combustion, for instance formation and reduction of pollutants.

    Lyngfelt, A and Leckner, B., 1999, "Combustion of wood-chips in circulating fluidized bed boilers - NO and CO emission as function of temperature and air staging", Fuel, Vol. 78, pp. 1065-1072.

    Reduction of NO emissions during combustion of wood-chips in a 12 MW circulating fluidized bed boiler was investigated by lowering the air-ratio in the combustor through secondary air addition in the cyclone outlet. This mode of operation was studied at three loads and also in combination with secondary air addition in the cyclone inlet and/or at 2.2 m height. Significant reduction of NO was obtained without high CO emissions. Important for the success of NO abating measures is that the CO emissions do not increase to high levels. Therefore the conditions for CO burnout were studied. The temperature in the duct downstream of the cyclone outlet, the "exit chamber", was found to be important, since the burnout of CO was not completed in the cyclone, not even under normal operating conditions. The explanation seems to be incomplete mixing of the gases entering the cyclone. There was a clear correlation between the temperature in the exit chamber and the CO emissions, and various measures taken to increase the temperature in the exit chamber were effective in reducing the CO emission.

    Lyngfelt, A., Åmand, L.-E., and Leckner, B., 1996, "Progress of combustion in the furnace of a circulating fluidised bed boiler," Twenty-sixth Symposium (International) on Combustion. pp. 3253-3259

    Gas concentrations were measured in the combustion chamber of a circulating fluidised bed boiler. Sieved fractions of solid material sampled combustible matter. Together with the bulk density profile obtained at different heights were analysed for from pressure drop measurements distribution of combustible matter in the combustion these data were used to obtain the chamber. High concentrations of combustible gases were found in the bottom part, and low concentrations of oxygen indicating that the bed material to a large extent is subject to reducing profile the conditions. From the gas concentration degree of combustion/fuel conversion versus boiler height was deduced. This procedure, however, involves a major error since measured concentrations give a misrepresentation of the concentrations of the gas explained by a through-flow of gas through bubbles or voids in the bottom bed with high velocity and high concentration of oxygen. A flow. This is correction for the by-pass flow was attempted, which increases the O2 concentration in the lowest position from 2.5 to 11%. This correction versus height, derived from the distribution of solid compared favourably with the fuel conversion combustibles (char) in the combustion concentrations measured in the bottom part of chamber. The results show that the the combustion chamber do not represent the flow, and in mass balances. This cannot, accordingly, be directly used misrepresentation has consequences for the interpretation of gas measurements boilers. in the bottom part of fluidised bed

     

    Zirconia-cell oxygen-probe measurements

     

    Lyngfelt, A., Åmand, L.-E., Müller, E., and Leckner, B., 1997, "Reversed air staging - a method to reduce nitrous oxide emissions from circulating fluidized bed boilers," in 7th International Workshop on Nitrous Oxide Emissions, Cologne, April 21-23,

    Reversed air staging is a method to reduce nitrous oxide (N2O) emissions from circulating fluidized bed boilers (FBBs). Tests in a 12 MW circulating FBB show that the N2O emission can be lowered to one fourth without affecting the emissions of NO, SO2 or CO and without adverse effects on the combustion efficiency. The focus of the paper is on the conditions in the combustion chamber. Measurements inside the combustion chamber reveal how the combustion conditions are affected by reversed air staging compared to normal air staging. These measurements confirm the working principle of reversed air staging, namely to provide more oxidizing conditions in the lower part of the combustion chamber and lower stoichiometry in the upper part of the combustion chamber.

     

    Lyngfelt, A., Bergqvist, K., Johnsson, F., Åmand, L.-E., and Leckner, B., 1993, "Dependence of sulphur capture performance on air staging in a 12 MW circulating fluidised bed boiler," Gas Cleaning at High Temperatures. Eds. Clift, R., and Seville, J.P.K., Blackie Academic & Professional, Glasgow, pp. 470-491.

    Three cases of air staging were examined in a 12 MW circulating fluidised bed boiler: i) no staging, ii) normal staging and iii) intensified staging. The conditions inside the combustion chamber were investigated by zirconia cell measurements of the oxygen partial pressure, 0.35, 0.65 and 8 m above the bottom air distributor plate. A significant effect of the degree of staging was seen in the two lower locations: At 0.65 m height the fraction of time under substoichiometric conditions was low in the no-staging case (2-35%), at normal staging it was 70-90%, whereas at intensified staging it was 100%. At 0.35 m height, i.e. in the dense bed, a similar effect was seen, although the fraction of time under reducing conditions was lower. The fraction of time under reducing conditions was low in the top of the combustion chamber in all three cases. The increase in the fraction of time under reducing conditions with a higher degree of staging is associated with a decrease in sulphur capture. It is assumed that a release of SO2 from CaSO4 takes place during the transitions between oxidising and reducing conditions. Thus, the rapid alterations between oxidising and reducing conditions, as seen with the zirconia cell, offer an explanation of the reductive decomposition and, accordingly, of the dependence of sulphur capture on temperature and on the extent of staging. 17 refs., 7 figs., 4 tabs.

     

    Reversed air staging and N2O emissions

    (For N2O and NOx see also the home page of Lars-Erik Åmand)

    N2O references without abstracts

    Lyngfelt, A., Åmand, L.-E., and B. Leckner, 1998, "Reversed air staging - a method for reduction of N2O emissions from fluidized bed combustion of coal," Fuel, Vol. 77, pp. 953-959.

    Reversed air staging, a method for reduction of N2O emissions, was studied in a 12 MW circulating fluidized bed boiler. The effect of combustor air-ratio, bed temperature, load and limestone addition ratio was investigated. The results indicate that the emission of N2O can be decreased to one fourth with maintained low emissions of NO, SO2 and CO. With increased bed temperature, from 850 to 870° C, in combination with increased limestone addition, from Ca/S 3 to 4, it was possible reduce N2O by 80-90%. The method was less efficient at low load, because of the lower cyclone temperature. Also the effect of redistribution of some of the air added in the cyclone outlet to the cyclone inlet was studied. This did not, however, give any improvement in the boiler used. There was no significant difference in combustion efficiency between reversed air staging and normal air staging.

     

    Lyngfelt, A., Åmand, L.-E., and B. Leckner, 1998, "Nitrous Oxide from Fluidized-Bed Boilers," Encyclopedia of Environmental Analysis and Remediation, Vol. 5, John Wiley & Sons, pp. 3053-3062 (ISBN: 0471-11708-0)

    Except for the detailed reaction mechanism(s) related to char, the formation and destruction reactions of N2O are reasonably well understood. This knowledge is not easily applied in the complex and not well known conditions of a FBB combustion chamber, but the general effects of important parameters, such as temperature and air ratio, are well known and well understood. The problem is not in the first place to decrease N2O emissions, but to decrease N2O emissions without adverse effects on other emissions, combustion efficiency, etc. Two methods, afterburning and reversed air staging, have been tested and N2O emissions can be decreased significantly with both methods. Other methods have also been proposed and may emerge as possible solutions.

     

    Lyngfelt, A., Åmand, L.-E., Müller, E., and Leckner, B., 1997, "Reversed air staging - a method to reduce nitrous oxide emissions from circulating fluidized bed boilers," in Proceedings of the 7th International Workshop on Nitrous Oxide Emissions, Cologne, Ber. Bergische Univ. Gesamthochsch. Wuppertal Fachbereich 9 Phys. Chem. Nr 41, pp. 75-82.

    Reversed air staging is a method to reduce nitrous oxide (N2O) emissions from circulating fluidized bed boilers (FBBs). Tests in a 12 MW circulating FBB show that the N2O emission can be lowered to one fourth without affecting the emissions of NO, SO2 or CO and without adverse effects on the combustion efficiency. The focus of the paper is on the conditions in the combustion chamber. Measurements inside the combustion chamber reveal how the combustion conditions are affected by reversed air staging compared to normal air staging. These measurements confirm the working principle of reversed air staging, namely to provide more oxidizing conditions in the lower part of the combustion chamber and lower stoichiometry in the upper part of the combustion chamber.

     

    Lyngfelt, A., Åmand, L.-E., Gustavsson, L., and Leckner, B., 1996, "Methods for reducing the emission of nitrous oxide from circulating fluidized bed combustion," Energy Conversion and Management, Vol. 37, pp. 1297-1302.

    Two methods for the reduction of nitrous oxide emissions, afterburning and reversed air staging, are investigated in a 12 MW circulating fluidized bed boiler. With afterburning the N2O emission can be reduced by 90% or more, using an amount of secondary fuel corresponding to 10% of the total energy input. With reversed air staging it is possible to reduce the emission of N2O to one fourth (25 ppm), without significantly affecting the other emissions. With reversed air staging no secondary air is used in the combustor and an air-ratio of about unity is maintained throughout the combustion chamber. Air for final combustion is added in the cyclone outlet.

     

    Lyngfelt, A., Åmand, L.-E., Karlsson, M., Leckner, B., 1995, "Reduction of N2O emissions from fluidized bed combustion by reversed air staging Combustion and emissions control," Second International Conference on Combustion and Emissions Control, The Institute of Energy, London, pp. 89-100.

    A new method, reversed air staging, for decreasing N2O form fluidised bed combustion, was investigated in a 12 MW circulating fluidized bed boiler. Previous measurements, showing that the N2O is decreased to one fourth (25 ppm), and NO to half (40 ppm) with maintained sulphur capture, were confirmed. Supplementary information on reversed air staging was obtained from additional measurements. By adjusting the temperature and the air ratio it was possible to reduce the CO emission. Thus, the N2O emission can be reduced to the level of 25 ppm, with the emission of NO, SO2 and CO maintained at about same level as with normal air staging. Low load in combination with reversed air staging resulted in further reduced emissions. The effect of reversed air staging on the emissions from peat is similar to that of bituminous coal. There is no negative effect of reversed air staging on the combustion efficiency. 3 refs., 14 figs., 1 tab.

     

    Lyngfelt, A., Åmand, L.-E., and Leckner, B., 1995, "Low N2O, NO and SO2 emissions from circulating fluidized bed boilers," Proc. Int. Conf. Fluid. Bed Combustion, Vol. 13, pp. 1049-1057.

    A new method, reversed staging, for decreasing N2O without increasing the emission of the other pollutants, NOx and SO2, was investigated in the 12 MW circulating fluidized bed boiler at Chalmers University of Technology. It was possible to reduce the emission of N2O to one fourth (25 ppm), NO to half (about 40 ppm) compared to normal staging and normal temperature, without significantly affecting the sulphur capture efficiency (about 90%). Air staging, which is normally used in circulating fluidized bed boilers means that only a part of the combustion air, primary air, is added to the bottom zone, resulting in a lower oxygen concentration in the bottom part, while the secondary air results in more oxidizing conditions in the upper part of the combustion chamber and the cyclone. The principal idea of reversed staging is to reverse the conditions in top and bottom, i.e. to decrease the oxygen concentration in the upper part and to increase it in the bottom part. Such a reversal is accomplished by adding air in the bottom corresponding to an air ratio of approximately unity. No secondary air is added to the combustion chamber which means that the oxygen concentration will be low in the upper part of the combustion chamber and the cyclone. Air for final combustion is added in the cyclone outlet.

    .

    Lyngfelt, A., Åmand, L., and Leckner, B., 1995, "Obtaining Low N2O, NO, and SO2 Emissions from Circulating Fluidized Bed Boilers by Reversing the Air Staging Conditions," Energy & Fuels,pp. 386-387.

    A technique for decreasing N2O without increasing the emission of the other pollutant NOx and SO2, reversed staging, was investigated in the 12 MW circulating fluidized bed boiler at Chalmers University of Technology. It was possible to reduce the emission of N2O to one quarter (25 ppm) and NO to half (about 40 ppm) compared to normal staging and normal temperature, without significantly affecting the sulfur capture efficiency.

     

     
    Sulphur capture

    Sulphur capture references without abstracts

     

    Fernández, M.J., and Lyngfelt, A., 2001, "Concentration of sulphur compounds inside the combustion chamber of a circulating fluidized bed boiler." Fuel, Vol. 80, pp. 321-326

    The concentration of sulphur compounds inside the combustion chamber of a 12 MW circulating fluidised-bed boiler was measured at different operating conditions: a) bed of sand without limestone addition, b) three cases of air-staging in presence of limestone. The data obtained with a sand bed show that most of the sulphur is released in the dense bed. In presence of limestone, a high concentration of SO2 was seen below the secondary air inlet when either normal or intensified air-staging were used. In the same area reduced sulphur compounds, 300 ppm, were found under intensified air-staging.

    Fernández, M.J., Kassman, H., and Lyngfelt, A., 2000, "Methods for measuring SO2 and gaseous reduced sulphur compounds in the combustion chamber of a circulating fluidised bed boiler." Can. J. Chem. Eng., Vol. 78, pp. 1138-1144.

    The present work was aimed at developing and improving methods for measurement of gaseous sulphur compounds in the combustion chamber of a fluidised bed boiler (FBB). The sampling of SO2 was improved by removing NH3 and H2O with a sorbent immediately after the probe. The concentration of reduced sulphur species was determined by means of two conventional SO2 analysers and an intermediate converter where the reduced species are oxidised to SO2. Gas phase sulphides were also sampled with a gas quenching probe by means of a basic solution which was subsequently analysed by wet chemistry.

    The methods were tested during coal combustion in a 12 MW circulating FBB without limestone for two cases of air-staging.

    Fernández, M.J., Lyngfelt, A., and Steenari, B-M., 2000, "Reaction between limestone and SO2 under conditions alternating between oxidising and reducing. The effect of the SO2 concentration." J. Inst. Energy Vol. 73, pp. 119-125.

    Sulphur capture by limestone was investigated in a quartz reactor under periodically altern ating oxidising and reducing conditions, in order to simulate the environment in a fluidised bed combustor. The influence of the SO2 concentration was studied. The lower SO2 concentration was, however, compensated by longer test periods; thus the product [SO2]x(total test period) was constant. The inlet SO2 concentration was varied between 100 and 1500 ppm and the total test periods were 2 to 30 h. Total cycle times (oxidising + reducing time period) from 2 to 720 s were used. For cycle times shorter than appr. 6 s, the conversion to CaSO4 hardly changed with the inlet SO2 concentration. Furthermore, alternating conditions gave higher conversions than under totally oxidising conditions. For long cycle times, however, the conversion sharply decreased with a decrease of the SO2 concentration and for low SO2 concentrations, the conversion under alternating conditions was smaller than under oxidising conditions.

    Fernández, M.J., Lyngfelt, A., and Steenari, B-M., 2000, "Reaction between limestone and SO2 under conditions alternating between oxidizing and reducing. The effect of short cycle times." Energy & Fuels Vol. 14, pp. 654-662

    Sulfur capture by limestone under periodically changing oxidizing and reducing conditions was inve stigated in a quartz reactor. In order to simulate conditions experienced by limestone particles in fluidized bed combustion, short periods of oxidizing and reducing conditions were used. Total cycle times (time under oxidizing conditions + time under red ucing conditions) between 2 and 24 seconds were used. Furthermore, three different fractions of time under reducing conditions were tested: 50, 33 and 25%. The degree of conversion of CaO to CaSO4 under these conditions is very dependent on both total cycle time and fraction of time under reducing conditions. For a fraction of time under reducing conditions of 50%, a minimum in the conversion (1.9%) versus the total cycle time was found at 8 seconds. For shorter cycle times a dramatic improvement of the con v ersion was obtained, e.g. 14.2% for 6 seconds. These high conversion values were also 50% higher compared to those obtained for the same limestone sulfated under totally oxidizing conditions. When the fractions of time under reducing conditions were 33 and 25%, alternating conditions have a positive effect on the conversion compared to that obtained under oxidizing conditions for all cycle times.

    Lyngfelt, A.and Leckner, B., 1999, "Sulphur capture in circulating fluidized bed boilers - can the efficiency be predicted?", Chem. Eng. Sci. Vol. 54, pp. 5573-5584

    The present state of understanding of sulphur capture in fluidized bed combustion is discussed with focus on the possibilities to predict sulphur capture performance and the effect of intermittent reducing conditions. The four key factors that determine the sulphur capture performance are: i) particle size distribution, including the effect of particle size reduction, ii) residence time as a function of particle size, iii) reactivity as a function of particle size, and iv) effect of reducing conditions. The first three of these can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the effect of reducing conditions in a model is still unresolved. The conclusion must be made that sulphur capture performance in CFBBs cannot safely be predicted at present, and more research is needed before the effect of reducing conditions can be incorporated in modelling. Nevertheless, existing models can be useful to approximate the effects of, for instance, particle size reduction or changes in particle residence time.

    Mattisson, T. and Lyngfelt, A., 1998, "The reaction between limestone and SO2 under periodically changing oxidizing and reducing conditions - The effect of reducing gases", J. Inst. Energy Vol. 71, pp. 190-196

    The aim of this paper was to gain a better understanding of the sulphur capture process during fluidised bed combustion (FBC). To achieve this the reaction between limestone and SO2 was investigated under periodically changing oxidising and reducing conditions using a fixed-bed quartz reactor. The oxidising gas consisted of 1500 ppm SO2, 10% CO2 and 4% O2. Three reducing gas mixtures were evaluated: i) 1500 ppm SO2 , 4% CO, 10% CO2, ii) 1675 ppm H2S, 4% CO, 10% CO2 and iii) 1675 ppm H2S, 4% H2, 10% CO2. The limestone samples were sulphated for 2 h at 850° C with a total cycle time, i.e. one oxidising period followed by one reducing period, between 30 - 900 s, and with the fraction of time under reducing conditions in the range 0-67%. Alternating conditions had either a positive or negative effect on the final degree of conversion in comparison to samples sulphated under constant oxidising conditions. For cycle times between 30 - 240 s a conversion maximum was found when the fraction of time under reducing conditions was in the range 20-50%. The location and extent of this maximum were dependent on the gas mixture used and the cycle time. When using a cycle time of 900 s the degree of conversion increased with an increasing amount of reducing conditions. The highest degree of conversion was obtained using H2S/H2 in the reducing gas, followed by H2S/CO and SO2/CO. Under alternating conditions the final degree of conversion ranged from 1 - 33%, which should be compared to a conversion level of 9% when the same limestone was sulphated under constant oxidising conditions. Thus alternating conditions may not only explain why a poor sorbent sulphation is sometimes found in utility FBBs but also why a surprisingly high degree of sulphation is sometimes obtained.

    Mattisson, T. and Lyngfelt, A., 1999, "The reaction between limestone and SO2 under periodically changing oxidizing and reducing conditions - Effect of temperature and limestone type", Thermochim. Acta, Vol. 325, pp. 59-67.

    The reaction between limestone and SO2 was investigated under both oxidizing and periodically changing oxidizing and reducing conditions at atmospheric pressure in a fixed-bed quartz reactor. Three limestones of different reactivities were sulfated at temperatures between 800° C-875° C with a gas mixture of 1500 ppm SO2, 10% CO2 and alternating O2 and CO (0% or 4%). The experimental data found that periodically changing oxidizing and reducing conditions could have either a positive or a negative effect on the degree of conversion of CaO to CaSO4 compared to limestone samples sulfated under constant oxidizing conditions. In the temperature range 825-875°C the conversion was relatively constant for all three limestones under oxidizing conditions. However when the limestone samples were sulfated with periodically changing oxidizing and reducing gas mixtures a wide range of conversions were found. The highest degree of conversion was found at 825° C for all three limestones and the lowest degree of conversion was found at 875° C. The rapid decrease in conversion at high temperatures was due to the rapid release of SO2 due to both the decomposition of CaSO4 as well as a high rate of CaS oxidation. A comparison of the sulfation rates of several experiments performed with Köping limestone under alternating conditions showed a wide range of rates, indicating the difficulties in using laboratory rate data in sulfur capture modelling for fluidized bed boilers.

    Mattisson, T. and Lyngfelt, A., 1998, "The reaction between sulfur dioxide and limestone under periodically changing oxidizing and reducing conditions - Effect of cycle time", Energy Fuels Vol. 12, pp. 905-912

    In order to better understand the sulfur capture process during fluidized bed combustion (FBC), the reaction between sulfur dioxide and limestone has been investigated under periodically changing oxidizing and reducing conditions. All experiments were carried out in a fixed-bed quartz reactor at 850°C, with the reactant gas consisting of 1500 ppm SO2, 10% CO2, and with the O2 and CO concentration varied between 0-4%. The fraction of time under reducing conditions was varied between 0-100% and the total cycle time (i.e. time of one oxidizing period + one reducing period) was varied between 30 - 900 s. The present study showed that alternating conditions may result in both an increase and decrease in the final conversion depending on both the total cycle time and the fraction of time under reducing conditions. Whereas a final conversion of approximately 9% was found when the limestone was sulfated under oxidizing conditions, conversions between 1% and 25% were found under alternating conditions for the cases where only small amounts of CaS was formed. For the experiments where most of the sulfur was in the form of sulfide the conversion was higher, up to 46%. The conversion increased for all cycle times up to 33% of the time under reducing conditions, with the largest increase seen for long cycle times. When the fraction of time under reducing conditions was increased further the conversion decreased significantly for the shorter cycle times.

    Lyngfelt, A., and Leckner, B., 1998, "Sulphur capture in circulating fluidized bed boilers - decomposition of CaSO4 under local reducing conditions" J. Inst. Energy, Vol. 71, pp. 27-32

    The reductive decomposition of CaSO4 was studied in a 12 MW circulating fluidized bed boiler. Measurements were made inside the combustion chamber under normal operating conditions except that the bed material contained old, sulphated sorbent. Limestone addition was stopped two days before the tests in order to eliminate the effects of fresh sorbent. Gas concentrations were measured in the centre of the riser cross-section at different heights at two bed temperatures, 850 and 920° C. At one height measurements were also made in nine positions distributed over the horizontal cross-section. The results show that reductive decomposition takes place below 0.5 m height, i.e. in the dense bottom bed of the combustion chamber, and that most of the released sulphur is recaptured in the splash zone immediately above the dense bottom bed. This means that most of the recapture occurs below the secondary air ports, indicating the important role of the dense bottom bed for sulphur capture.

    Mattisson, T. and Lyngfelt, A., 1998, "A sulphur capture model for fluidized bed boilers," Chem. Eng. Sci. Vol. 53, pp. 1165-1175

    A sulphur capture model is presented that incorporates detailed laboratory data of limestone reactivity together with boiler data of particle size distribution as well as residence times for the various particle sizes. The reactivity of limestone as a function of conversion is described by an exponential decay approximation and a total of four constants are used to describe the rate as a function of particle size and conversion. The residence time for the different particle sizes and the particle size distribution are determined from data of three utility circulating fluidized bed boilers (CFBB) with thermal outputs of 12, 40, and 165 MW. The model was validated by sulphur and calcium analysis of sieved fly-ash and bed-ash samples whereby the degree of sorbent conversion as a function of particle size was determined.

    Mattisson, T. and Lyngfelt, A., 1998, "A method of evaluating limestone reactivity with SO2 under fluidized bed combustion conditions," Can. J. Chem. Eng. Vol. 76, pp. 762-771

    The reactivity of limestone with sulfur dioxide has been evaluated during conditions similar to those existing in a fluidized bed combustor using a fixed-bed quartz reactor. A first order effective rate constant was used to determine the reaction rate as a function of the degree of conversion of CaO to CaSO4. Particle sizes of two limestones, ranging from 0.05 to 2 mm in diameter, were evaluated and an exponential decay function was found to best describe the rate behavior as a function of conversion. The two constants in the decay function could both be expressed as functions of particle size and subsequently the limestone reactivity could be reasonably well described by a total of four constants. An analytical sulfur capture model for fluidized bed boilers (FBB) that incorporates this type of reactivity function is proposed.

    Lyngfelt, A., 1997, Sulphur capture in fluidized bed boilers - research at Chalmers University," presented at the Sulphation Workshop of the 35th IEA Fluidized Bed Conversion Meeting, Vienna, November 7-9.

    Mattisson, T. and Lyngfelt, A., 1996, "A sulphur capture model for fluidized bed boilers," Proceedings of the Third Nordic SOx-NOx Conference, Department of Chemical Engineering, Technical University of Denmark. Lyngby, (Denmark) pp. 75-80.

    A steady state sulphur capture model for fluidized bed boilers (FBB) is proposed. The model uses limestone reactivity data and residence time distribution from two circulating fluidized bed boilers (CFBB) as input parameters. The reactivity of the limestone was investigated for different particle sizes under oxidizing conditions at 850 C in a laboratory quartz reactor. Residence times and particle size distribution were determined for both a 12 and 40 MW CFBB. The model predicts the conversion, i.e. the molar Ca/S ratio, versus particle size as well as the sulphur retention as function of Ca/S ratio. The degree of conversion was also determined from samples of bed ash and fly ash from the two boilers which were sieved and analysed for Ca and S. A comparison between model and analysed ash found reasonable agreement in trends and conversion data, although the agreement was better for the 12 MW boiler.

     

    Lyngfelt, A., Langer, V., Steenari, B.-M., and Puromäki, K., 1995, "Calcium sulphide formation in fluidized bed boilers," Can. J. Chem. Eng., Vol. 73, pp. 228-233.

    Analyses of samples of bed ash from combustion in a stationary fluidized-bed boiler showed the presence of calcium sulfide. In some samples, half of the total sulfur was present as sulfide. The samples contg. CaS were obtained under unstaged conditions and with a high (1.3-1.4:1) excess air ratio. Samples were taken after stopping limestone addn. (i.e., at high SO2 emissions of .apprx.1000 ppm). No CaS was obsd. during limestone addn. when the SO2 emission was 300-400 ppm. This indicates that formation of large amts. of CaS may be initially form when the SO2 concn. exceeds some crit. level. 25 refs., 2 figs., 3 tabs.

     

    Mattisson, T., and Lyngfelt, A., 1995, "The presence of CaS in the combustion chamber of a 12 MW circulating fluidized bed boiler," Proc. Int. Conf. Fluid. Bed Combustion, Vol. 13, pp. 819-829.

    The presence of CaS in bed ashes from a circulating fluidized bed boiler (CFBB) has been studied. The boiler was operated with an excess air ratio of 1.2 but with varying air staging and at temperatures of 850C and 930C. The fraction of sulphide sulphur to total sulphur varied between 1 and 4%, with the higher sulphide contents found during intensified staging, that is, when the fraction of air supplied to the bottom bed was low. Under conditions with high SO2 concentrations, due to stopped limestone addition, sulphide to total sulphur fractions of up to 16% were found. In conclusion, a high SO2 concentration, especially in conjunction with intensified air staging, constitutes a risk factor for CaS formation.

     

    Lyngfelt, A., and Leckner, B., 1993, "Model of sulphur capture in fluidised-bed boilers under conditions changing between oxidising and reducing," Chem. Eng. Sci. Vol. 48, pp. 1131-1141.

    A model treating the sulphur capture and sulphur release reactions in fluidised bed boilers under alternating oxidising and reducing conditions has been developed. The model involves two overall rate constants, one for the sulphur capture and the other for reductive decomposition (sulphur release), as well as the temperature dependence of these constants. The constants were determined using experimental data from a fluidised bed boiler. The values of the constants were used to derive a theoretical maximum sulphation, which is the ratio of end conversion under the conditions in the boiler to the end conversion under oxidising conditions, where "end" means at infinite residence time. The theoretical maximum sulphation gives an indication of the reduction in sulphur capture performance imposed by the effect of reducing conditions. The results show that reducing conditions give a lower sorbent utilisation at increased sulphur capture as well as at increased temperature. The detrimental effect of reducing conditions is significant at all temperatures of interest for FBBs, even at temperatures below 850C. Although the quantitative results refer to the boiler investigated, the qualitative results give an indication of the behaviour of a large number of other FBBs, i.e. those boilers which have a distinct temperature dependence for sulphur capture similar to that of the boiler studied. The general conclusion is that reducing conditions have been greatly underestimated as a limiting factor for sulphur capture in FBBs.

     

    Lyngfelt, A., Bergqvist, K., Johnsson, F., Åmand, L.-E., and Leckner, B., 1993, "Dependence of sulphur capture performance on air staging in a 12 MW circulating fluidised bed boiler," Gas Cleaning at High Temperatures. Eds. Clift, R., and Seville, J.P.K., Blackie Academic & Professional, Glasgow, pp. 470-491.

    Three cases of air staging were examined in a 12 MW circulating fluidised bed boiler: i) no staging, ii) normal staging and iii) intensified staging. The conditions inside the combustion chamber were investigated by zirconia cell measurements of the oxygen partial pressure, 0.35, 0.65 and 8 m above the bottom air distributor plate. A significant effect of the degree of staging was seen in the two lower locations: At 0.65 m height the fraction of time under substoichiometric conditions was low in the no-staging case (2-35%), at normal staging it was 70-90%, whereas at intensified staging it was 100%. At 0.35 m height, i.e. in the dense bed, a similar effect was seen, although the fraction of time under reducing conditions was lower. The fraction of time under reducing conditions was low in the top of the combustion chamber in all three cases. The increase in the fraction of time under reducing conditions with a higher degree of staging is associated with a decrease in sulphur capture. It is assumed that a release of SO2 from CaSO4 takes place during the transitions between oxidising and reducing conditions. Thus, the rapid alterations between oxidising and reducing conditions, as seen with the zirconia cell, offer an explanation of the reductive decomposition and, accordingly, of the dependence of sulphur capture on temperature and on the extent of staging. 17 refs., 7 figs., 4 tabs.

     

    Lyngfelt, A., and Leckner, B., 1993, "SO2 capture and N2O reduction in a circulating fluidized-bed boiler: influence of temperature and air staging," Fuel, Vol. 72, pp. 1553-1561.

    The effects of air staging and bed temperature on sulfur capture performance in a circulating fluidized-bed boiler fired with bituminous coal were studied. Normal air staging, no staging and intensified staging were compared at normal (850{degree}C) and high (930{degree}C) temperatures. The results clearly show that high temperatures and staged combustion are detrimental to sulfur capture performance. Sulfur capture was reduced by more than half with intensified staging at 850{degree}C. At the high temperature, sulfur capture was still possible without staging, but at greatly reduced efficiency. In the two cases with staging, the sulfur capture was negative in the period 2-3 h after a temperature increase, as a result of reductive decomposition of calcium sulfate. The temperature-dependence without staging indicates that reducing conditions affect sulfur capture in this case as well. The increase in temperature from 850 to 930{degree}C halved N2O emissions, from {approximately} 100 to 50 ppm (at 6% O2). 15 refs., 10 figs., 6 tabs.

     

    Lyngfelt, A., and Leckner, B., 1992, "Residence time distribution of sorbent particles in a circulating fluidised bed boiler," Powder Technol., Vol. 70, pp. 285-292.

    The residence time distribution of sorbent particles is studied in order to increase the understanding of the conditions for sulphur capture in fluidised bed boilers. Two methods are used. The "steady state method" involves the study of residence time for various particle size fractions. The "transient method" is based on the transient increase in the amount of sorbent carryover with the flyash, following a start of limestone addition to a fresh bed (i.e. a bed with little or no sorbent). For the boiler investigated both methods gave similar results, showing that the major fraction of the sorbent, 80-85%, had a residence time of one hour or more.

     

    Lyngfelt, A. and Leckner, B., 1991, "Sorbent size reduction and conversion versus particle size in fluidized bed boilers," Proc. of the Intitute of Energy's Fifth International Fluidised Combustion Conference, London, pp. 179-190.

    Sulphur capture phenomena were studied in a stationary and a circulating fluidised bed boiler (FBB), with focus on sorbent particle size reduction and sorbent conversion versus particle size. A marked decrease in sorbent particle size took place, for both FBBs. The median sorbent particle size decreased from 0.6mm to 0.125 mm. The conversion was only slightly dependent on particle size in both boilers. The conversion of the limestone was high, approx. 50%, in the circulating FBB, in contrast to the stationary FBB where the conversion was considerably lower for all particle sizes. For the larger particle sizes, this difference is attributed to the negative effect of reducing conditions. 11 refs., 8 figs., 2 tabs.

     

    Mjörnell, M., Leckner, B., Karlsson, M., and Lyngfelt, A., 1991, "Emission control with additives in CFB coal combustion," Proc. Int. Conf. Fluid. Bed Combustion, Vol. 11, pp. 655-664.

    The concern for the environment has led to further decreases in maximum levels of emissions in the standards of many countries. For that reason reliable information of the emission characteristics is desirable. The present paper reports measurements which extend and verify previous results from coal combustion in a 40 MW{sub th} CFB boiler. The emissions of SO2, NO, N2O and CO have been measured during the addition of limestone and NO reductants. The influence of various factors is studied: Porous and crystalline limestone, particle size of limestone, and additives for thermal NO{sub x}-reduction. It has been of special interest to investigate the influence of limestone addition on the emissions of nitric and nitrous oxides. The results are given from a series of systematically planned tests mostly as a function of temperature, limestone addition and NO reductant addition at constant total excess air ratio.

     

    Lyngfelt, A., and Leckner, B., 1989, "SO2 capture in fluidised-bed boilers: re-emission of SO2 due to reduction of CaSO4," Chem. Eng. Sci. Vol. 44, pp. 207-213.

    Sulphur capture by lime was studied in a 16-MW fluidised bed boiler. The desulphurisation process, i.e. the addition of limestone which is calcined and sulphated, results in an accumulation of CaSO4 in the bed. The CaSO4 is shown to be decomposed to CaO with subsequent release of SO2 at temperatures above 880-890C, and at a excess air ratio of 1.4. At 930C the amount of sulphur leaving the boiler as SO2 was more than double the amount of sulphur added to the boiler in the form of fuel sulphur. The decomposition of CaSO4 can be explained by the reaction of CaSO4 with combustion intermediates such as CO and H2 i the dense (particle) phase of the bed. The results explain the decrease in sulphur capture performance with increased temperature observed in fluidised-bed boilers and indicate the important effect of reducing conditions. Increased understanding of these phenomena may provide solutions, with respect to boiler design and operation, that make the use of limestone for desulphurisation more efficient.

     

    Lyngfelt, A., and Leckner, B., 1989, "Sulphur capture in fluidised-bed combustors: temperature dependence and line conversion," J. Inst. Energy, Vol. 62, pp. 62-72.

    At temperatures above about 850 degrees C the sulphur-capture efficiency of fluidised-bed boilers shows a marked fall with temperature. However, literature data indicate that this behaviour is not observed in laboratory experiments. The temperature-dependence in FBBs is shown to be a result of sorbent particles being exposed to reducing conditions in the particulate phase of the bed. These effects are shown to be considerable in a stationary FBB but negligible in a circulating FBB, and they provide an explanation of the significant difference in limestone utilisation between the two kinds of boilers. Boiler design and operation parameters have an effect on the extent to which sorbent particles are exposed to reducing conditions. Improved knowledge of these points may lead to FBBs designed for more efficient sulphur capture. 65 refs., 4 figs.

     

    Lyngfelt, A., and Leckner, B., 1989, "Sulphur capture in fluidized bed boilers: the effect of reductive decomposition of CaSO4," Chem. Eng. J., Vol. 40, pp. 59-69.

    Sulphur capture by lime was studied in a 16 MW stationary fluidized bed boiler (FBB). A marked fall-off in sulphur capture was noted at temperatures above about 880{degree}C. The proposed explanation is that the combustion produces reducing conditions in the particle phase, and thus allows for a reductive decomposition of CaSO4. This explanation is supported by (i) thermodynamics showing the instability of CaSO4 under reducing conditions; (2) in-bed oxygen measurements indicating reducing conditions in the particle phase; (3) the observed fall-off in sulphur capture with temperature, which is not seen in laboratory tests under oxidizing conditions or in a circulating FBB, where the sorbent particles experience oxidizing conditions to a greater extent; (4) the observation that the temperature dependence of the sulphur emission is very strong even when the net sulphur capture is zero, provided always that CaSO4 is present and (5) literature data indicating the rate of the proposed reaction. 52 refs., 10 figs., 3 tabs., 2 apps.

     

    Lyngfelt, A. and Leckner, B., 1989, "The effect of reductive decomposition of CaSO4 on sulphur capture in fluidized bed boilers," Proc. Int. Conf. Fluid. Bed Combustion, Vol. 10, pp. 675-684.

    A report on the effect of limestone addition on sulphur capture and NO emission studied in a stationary and a circulating fluidized bed boiler (FBB) Significant differences between the two boilers were observed, both with respect to sulphur capture and NO emission behavior. In the stationary FBB sulphur retention exhibits a strong temperature dependence as opposed to the circulating FBB. Furthermore, the sulphur capture performance is much more effective in the circulating FBB. In the circulating FBB limestone addition resulted in a considerable increase of NO emission, whereas no effect of limestone was seen in the stationary FBB for the same type of coal. For a low volatile fuel, however, limestone addition gave a considerable decrease of NO emission in the stationary FBB. These results can be explained by the sorbent particles being exposed to reducing conditions in the particle phase of the stationary FBB.

     

    Lyngfelt, A., Åmand, L.-E., and Leckner, B., 1988, "The effect of reducing conditions on sulphur capture - a comparison of three boilers," Proc. of the Intitute of Energy's Fourth International Fluidised Combustion Conference, London, pp. II/10/1-II/10/11.

    The temperature dependence of sulphur capture with limestone in fluidised bed boilers (FBB's) is studied. The decrease in sulphur capture performance at higher temperatures is explained by local depletion of oxygen resulting in reducing zones. In these zones CaSO4 is reductively decomposed whereby SO{sub 2} is released. Measurements from two circulating and one stationary, commercial-type FBB's are compared in order to demonstrate differences in the temperature dependence of sulphur capture. The lime in the particle phase of a stationary FBB and that in a circulating FBB are subjected to different chemical conditions and the boilers behave differently with respect to sulphur capture. However the two circulating FBB's also differ from each other which puts an emphasis on design and operating conditions. 10 refs., 5 figs., 2 tabs.

      

    Last update: March 29 , 1996

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