Sulfur Retention Research Articles

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

96/06351 The effects of cofiring natural gas and coal on sulfur retention in ash

96/06351 The effects of cofiring natural gas and coal on sulfur retention in ash

96/05929 Sulfur and nitrogen retention in chars after pyrolysis and partial gasification

96/05929 Sulfur and nitrogen retention in chars after pyrolysis and partial gasification

Full-scale co-firing of straw and coal

Co-firing of biofuels and coal in power plants is considered by the Danish utilities as a potential tool in reducing CO 2 emissions. To test this, full-scale measurements were carried out for 1 week on a 250 MW e pulverized coal fired unit using 10–20% straw (thermal basis). With an increased fraction of straw in the fuel, a net decrease in NO x and SO 2 emissions was measured. The SO 2 emission decreased partly due to the lower sulfur content of the fuel per MJ, but also due to higher sulfur retention in the ash. The NO emission decreased solely due to lower conversion of fuel-N. An increased fraction of straw in the fuel blend resulted in a higher potassium content, but no significant increase in slagging or fouling was observed. Only small amounts of deposit at the lower part of the radiant superheater and little slagging at the furnace walls were observed as a result of co-firing straw and coal. No significant effects on the performance of the desulfurization plant were detected, which may be due to the short test period, probably not allowing the desulfurization process to reach steady-state operation.

96/05312 Sulfur retention in circulating fluidized bed coal combustion. Modeling and simulation

96/05312 Sulfur retention in circulating fluidized bed coal combustion. Modeling and simulation

The effects of cofiring natural gas and coal on sulfur retention in ash

Under certain conditions, cofiring natural gas with coal has been shown to reduce SO x emissions beyond the reduction expected from simple replacement of sulfur-bearing coal. This enhanced reduction of sulfur emissions, known as sulfur leveraging, is believed to result from increased sulfur capture in coal ash. In this research, experiments with three coals, using size ranges from 90 to 106 μm and 125 to 150 μm, and furnace temperatures of 1300 and 1550 K, demonstrate the extent of sulfur leveraging through increased sulfur retention in ash when cofiring coal and natural gas. Leveraging is shown to be affected by residence time (through particle size) and furnace temperature, while original sulfur form (pyritic or organic) and coal sorbent capacity are shown to have little effect. Results from sorbent activation studies, SEM images, and N 2 adsorption measurement of total surface area indicate that the effects of a natural gas flame on ash sorbent reactivity and ash surface area are minimal. Results also indicate that the primary mechanism for sulfur leveraging is the gas phase conversion of SO 2 to more reactive SO 3, as caused by the natural gas flame. Results from numerical modeling of the furnace environment, particle combustion, and the evolution of sulfur to SO 3 support the experimental findings.

A theoretical model for gas-particle contact efficiency in circulating fluid bed risers

A one-dimensional model has been derived for gas-particle contacting in circulating fluidized bed risers. The aim of the modelling effort is to predict the total degree of heterogeneous conversion attainable in CFB risers, not to predict cross-sectional gradients or properties. The model is based on a single, freely moving particle approach taking into account external as well as internal transport limitations. The model also includes a correction function to take particle clustering effects into account in a simplified way. The bottom bed is treated in a very simplified way so that a total bypass flow is computed by a correlation function. For the remainder of the riser, except for the splash zone, a core-annulus structure is assumed. The model takes into account different types of particles and may therefore be used to estimate the efficiency of, for example, limestone injection for sulphur retention in a coal-fired unit where sand is the bed material. The model also includes the option to predict probability distributions for the number of contacts between a gas package and particles of different types and thus to estimate the contacting time. This is useful to predict conversion rates in catalytic applications. The model is compared to experimental data from literature and shows reasonable agreement over a wide range of operating conditions.

Modelling and simulation of the sulphur retention in circulating fluidized bed combustors

To optimise the circulating fluidized bed coal combustion (CFBC) with sorbent addition and make predictions about the combustor behaviour in a broad range of operating conditions it is necessary to make a mathematical model of the process. This model was developed integrating hydrodynamic and sulphation kinetic submodels. A simulation, from the point of view of sulphur retention, was made. The variables most affecting the sulphur retention were the pressure drop, the gas velocity, the Ca/S molar ratio, the efficiency of solids recovery by the cyclone and specially the sorbent reactivity and its particle size distribution through the amount of small particle sizes fed in the combustor.

Effects of ambient temperature on broiler mineral balance partitioned into urinary and faecal loss.

1. Two experiments were conducted on control (intact) and colostomised 4 to 7 week old broilers to evaluate the influence of 24 degrees C, diurnally cycling 24 to 35 degrees C and chronic 35 degrees C ambient temperatures on broiler mineral balance, urinary and faecal mineral excretion and urinary osmolar characteristics. 2. In the first experiment, colostomy had no significant effect on mineral balance. However, broilers exposed to high cycling ambient temperature reduced their retention of phosphorus, potassium, sodium, magnesium, sulphur, manganese, copper and zinc compared with birds housed at 24 degrees C. 3. Despite the minimal effect of high ambient temperature on urine production, minerals excreted disproportionately excreted in urine included potassium, magnesium, phosphorus and sulphur while copper and magnesium were lost primarily via the faeces. 4. In the second experiment, exposure to 35 degrees C increased urine output from 50.7 ml/12 h per kg of body weight at 24 degrees C to 101.3 ml/12 h per kg of body weight and was associated with an increased urine:water ratio and reduced urine osmolality. 5. Reduced urinary chloride and higher potassium, phosphorus, sulphur, sodium, magnesium, calcium and manganese excretion was observed for broilers housed in under high ambient temperatures compared to 24 degrees C. 6. These studies suggest that high ambient temperatures adversely influence mineral metabolism and, furthermore, that the route of excretion varies with the specific mineral and the environmental temperature exposure.

Modelling of sulfur retention in circulating fluidized bed combustors

Modelling of sulfur retention in circulating fluidized bed combustors

96/02195 Total equivalent warming impact of refrigerants

To meet the legislation on limiting NOx and SOx emission from large combustion plants, SO2 emissions may need to be reduced by about 60–70%. The authors' technique for achieving significant reduction in SO2 emissions from combustion systems burning large coals utilizes limestone for in situ reduction of SO2 in fluidized bed combustors. A recent technique for reducing NOx emissions combines staged combustion with ammonia injection. However, concern has arisen about the effect of low primary air:coal ratio (PACR), and hence low bed oxygen levels, on the rate of reaction between calcined limestone and SO2. This paper reports on the significance of air-staging on the reduction of SO2 emissions by limestone in a fluidized bed combustor. The experimental rig consisted of a stainless steel combustor 2 m high and 0.3 m2 in section. Fluidizing air was supplied through a multihole distributor and secondary air was introduced into the freeboard via an injector whose height could be adjusted. The experiments were carried out at overall fluidizing velocities between 1 and 2 m s−1, bed temperatures of 830–880°C, 5–40% excess air levels and bed particle size of 313 + 690 μm. Various levels of air-staging (10–50%) were investigated (equivalent to PACRs of between 0.84 and 1.4, respectively). An uncooled sampling probe located at the cyclone exit was used to determine SO2. The results showed that SO2 emissions could be reduced by over 50% using limestone under unstaged combustion conditions. There was little effect of staged combustion on SOx emissions without limestone. When limestone was used, the effectiveness of sulfur capture decreased (by almost 50%) as the level of air-staging was increased. The bed temperature had the most significant effect on sulfur retention by limestone (10% at 880°C to almost 60% at 830°C). A decrease in overall fluidizing velocity resulted in an increase in SOx reduction. The present work has provided significant data on the parameters that influence SOx retention by limestone during staged combustion.

Modelling of sulfur retention in circulating fluidized bed combustors

The effects of operating conditions (linear gas velocity, Ca S molar ratio, type of coal and limestone, etc.) on sulfur retention efficiency in a circulating fluidized bed combustor were studied. The operating conditions affect the sulfur retention differently. To explain this, a mathematical model was developed. To determine axial voidage profiles, the model uses a modified exponential decay hydrodynamic model, which divides the bed into a dense region at the bottom of the bed and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The SO 2 retention rate is considered to depend on the bed height through the SO 2 profiles generated by the coal devolatilization and char combustion rates, which change with the bed height. The model gives good predictions of the effect of the operating variables on sulfur retention in a CFB pilot plant of 20 cm i.d. and 6.5 m height, burning two different lignites and an anthracite with four types of limestone.

SO X emissions from a fluidized-bed combustor with and without limestone addition

We consider the effects of operating variables on SO X emissions in unstaged combustion with a stainless steel combustor that is 2 m high and has a 0.3 m × 0.3 m square cross section. Fluidizing air was supplied through a multihole distributor. The experiments were carried out for fluidizing velocities of 1–2 m/sec, bed temperatures of 830–880°C, 20–40% excess air, and bed particle sizes of 665 μm. SO X emissions were reduced by over 70% with limestone. The bed temperature had the most significant effect on sulphur retention. Decreases in fluidizing velocity and increases in excess air result in increased SO X reduction.

95/05373 Overall modeling. 2. Sulphur retention and NO x reduction: the SURE model

95/05373 Overall modeling. 2. Sulphur retention and NO x reduction: the SURE model

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

AbstractFor the design, simulation and optimization of sulphur retention in atmospheric fluidized bed coal combustors, a mathematical model is needed that would be able to predict the behaviour of the combustor in a wide range of operating conditions. In this work, a sensitivity analysis of the sulphur retention predictions of the different hypotheses, equations and parameters, which define the different submodels and phenomena occurring in the combustor, has been carried out. It has been found that the hypotheses related to the gas flow, devolatilization type and sulphur distribution in the pyrolysis products imply an important division among models. The greatest effect on sulphur retention predictions is exercised by the parameters defining the fines elutriation and sorbent sulphation capacity. However, those corresponding to the bed hydrodynamics (minimum fluidization velocity and bed expansion) do not have a significant effect on the sulphur retention predictions.The sulphur retentions obtained in the combustion of high sulphur lignites with eight different limestones were used for model validation. A good fit of the experimental sulphur retentions was found, without using any adjustable parameter. Finally, a simulation of the process was made. The great effect of the bed height, air velocity and the particle size distribution of the limestone must be pointed out, as well as the effect of its reactivity through the maximum conversion attainable by each particle size.

Modeling sulfur retention in circulating fluidized bed combustors

A mathematical model is developed to evaluate the performance of sulfur retention by limestone in circulating fluidized beds. The model considers particle movement and attrition, calcination and initial pore size distribution, pore plugging and cyclone performance. Percolation scaling theory is used to determine the accessible reaction surface area of the sorbent particles. The model predictions are in good agreement with the experimental data.

The influence of air staging in the reduction of SO 2 by limestone in a fluidized bed combustor

To meet the legislation on limiting NO x and SO x emission from large combustion plants, SO 2 emissions may need to be reduced by about 60–70%. The authors' technique for achieving significant reduction in SO 2 emissions from combustion systems burning large coals utilizes limestone for in situ reduction of SO 2 in fluidized bed combustors. A recent technique for reducing NO x emissions combines staged combustion with ammonia injection. However, concern has arisen about the effect of low primary air:coal ratio (PACR), and hence low bed oxygen levels, on the rate of reaction between calcined limestone and SO 2. This paper reports on the significance of air-staging on the reduction of SO 2 emissions by limestone in a fluidized bed combustor. The experimental rig consisted of a stainless steel combustor 2 m high and 0.3 m 2 in section. Fluidizing air was supplied through a multihole distributor and secondary air was introduced into the freeboard via an injector whose height could be adjusted. The experiments were carried out at overall fluidizing velocities between 1 and 2 m s −1, bed temperatures of 830–880°C, 5–40% excess air levels and bed particle size of 313 + 690 μm. Various levels of air-staging (10–50%) were investigated (equivalent to PACRs of between 0.84 and 1.4, respectively). An uncooled sampling probe located at the cyclone exit was used to determine SO 2. The results showed that SO 2 emissions could be reduced by over 50% using limestone under unstaged combustion conditions. There was little effect of staged combustion on SO x emissions without limestone. When limestone was used, the effectiveness of sulfur capture decreased (by almost 50%) as the level of air-staging was increased. The bed temperature had the most significant effect on sulfur retention by limestone (10% at 880°C to almost 60% at 830°C). A decrease in overall fluidizing velocity resulted in an increase in SO x reduction. The present work has provided significant data on the parameters that influence SO x retention by limestone during staged combustion.

High-temperature sulphur removal under fluidized bed combustion conditions — A chemical interpretation

Theoretical and experimental studies were conducted on coal sulphur removal under chaotically changing oxidizing and reducing conditions, which occur in the dense zone of bubbling fluidized beds. Experimental desulphurization trends were analyzed by comparison with multi-species (50 gas-phase, 7 solid-phase species) equilibrium calculations for coal/steam/sorbent/air system. It was shown, that under fluctuating reducing/oxidizing conditions, sulphur capture as CaS in locally reducing zones increased the sulphur retention level of the sorbent. In experiments, performed in a 8 cm i.d. bench-scale FBC boiler, effective sulphur retentions were obtained in the 800–1000 °C temperature range. As a chemical interpretation, a time history of sorbent particle calcination-sulphation is proposed, which leads to the formation of a particle structure with a sulphided (CaS) core and a sulphated (CaSO 4) shell. Full-scale applications are discussed and measures for improving limestone utilization and high-temperature sulphur removal in fluidized beds are recommended.

Sulfur vs. silica retention in oligotrophic lake sediments: Test of a simple residence time model

Limnologists have modeled solute retention (% of input) in lake sediments using the parameter: 100 k s {(z:t w +k s}−1, wherek s is an empirical mass transfer coefficient or ‘sedimentation velocity’ (m yr−1), z is lake mean depth (m), andt w is lake hydraulic residence time (yr). This flushing or residence time model was tested against sulfate and silica concentrations in lakes of the northeastern and northcentral U.S.A., using data collected during the U.S. EPA's Eastern Lake Survey. This parameter failed to predict intra-regional variations in lake sulfate at fall turnover, but was consistently negatively correlated with silica in each of the regional lake populations. However, except in the western section of the Upper Peninsula of Michigan, lake sulfate was everywhere inversely dependent on DOC [mean slope=−0.020(SE=0.03) meq mM−1]. Sulfur retention in aquatic environments appears more closely coupled to organic carbon and trophic state, than to variations in hydraulic residence time.

Optimization of sulfur retention in ash when cocombusting high sulfur fuels and biomass fuels in a small pilot scale fluidized bed

Previous chemical equilibrium calculations as well as experimental results concerning cocombustion of high sulfur containing fuels and biomass fuels have shown that a significant retention of sulfur can be obtained in the ash. The processes are, however, complex and to minimize the SO 2 emissions, experiments were performed, according to statistical experimental designs, in a small pilot scale fluidized bed (5 kW). Hereby, a sulfur retention of 70–75% for a peat-wood fuel mixture and 90-85% for a mixture of coal and an energy crop (Lucerne) were obtained. The products CaSO 4 and 3K 2SO 4·Na 2SO 4 were identified in the ashes and dust. Fuel feeding rate (load), primary air ratio and total air flow were identified as the most influential operating factors, and bed temperature and oxygen concentration seem to be the most crucial physical-chemical factors for the sulfur retention. The NO emissions were also decreased by the SO 2 reducing measures, although the fraction of primary air was increased. This was probably due to a lower bed temperature at optimum sulfur capture conditions. The SO 2 emissions could be reduced without any increase in CO or NO emissions. Some preliminary measurements of the slagging tendency showed no increased slagging with increasing sulfate formation.