Sulfur Retention Research Articles

Investigation of Sulfur Retention and the Effect of Inorganic Matter during Pyrolysis of South Australian Low-Rank Coals

Temperature-programmed pyrolysis (TPP) experiments are conducted on two high-sulfur South Australian low-rank coals, Bowmans and Lochiel coals, and several acid-washed and ion-exchanged Bowmans coal samples to study sulfur retention, changes in sulfur forms, and the influence of inorganic matter on sulfur transformations during pyrolysis. Changes in sulfur forms between 200 and 900 °C are monitored by Australian Standards wet chemical analysis and confirmed by scanning electron microcscopy (SEM) combined with energy dispersive X-ray analysis (EDX). The TPP results show retention of sulfur at low pyrolysis temperatures (400-800 °C) due to the decomposition of sulfate sulfur and its solid-state transformation to organic sulfur in the char. The retention is enhanced for higher sulfate contents and lower sulfate volatilities and suppressed by a greater proportion and decomposition of organic sulfur species between 300 and 600 °C. For acid-washed and ion-exchanged coal samples, organic sulfur decomposition above 600 °C is suppressed by the addition of organically bound sodium and calcium ions, as well as calcium carbonate constituents in the coal. The presence of organically bound calcium facilitates reactions with organic sulfur to form sulfide sulfur in the char. Organically bound sodium and discrete carbonate materials do not appear to form sulfides to any substantial degree. Hence, their effect on total sulfur retention requires further investigation. Substantial discrepancies exist between sulfide formation in Bowmans and Lochiel coals, despite similar inorganic matter.

98/04010 Combustion of biomass-derived low calorific value fuel gas

Experiments were performed on the gasification of bituminous coal with air and steam in a 2·5 kW atmospheric fluidized bed. Thermal conversion efficiencies, the retention of sulphur and the conversion of fuel nitrogen to NH3 were determined for various oxygen-to-carbon ratios (equivalence ratios between 0·58 and 0·82), with and without limestone in the bed. The conversion efficiency increased with increasing equivalence ratio, while the conversion of fuel-nitrogen to NH3 decreased with increasing equivalence ratio. The sulphur conversion without the addition of limestone increased with increasing equivalence ratio. As expected, the presence of limestone in the bed led to increased sulphur retention, more so at the high and low ends than at intermediate values of the range of equivalence ratios. In addition, limestone adversely affected thermal conversion efficiency and increased the conversion of fuel-nitrogen. The low calorific value (LCV) fuel gas, produced by the AFBG, was combusted in a cyclone combustor at atmospheric pressure. Stable combustion of the LCV fuel gas was possible down to a calorific value of 1·57 MJ/m3n. The combustion efficiency in the atmospheric topping combustor varied between 98·72 and 99·96%. Nearly 60% of the NH3 in the fuel gas was converted to NO in the combustor.

Treatments to enhance the SO 2 capture by activated carbon fibres

The SO 2 retention capacity of different activated carbon fibres (obtained from pitch, PAN and phenolic resin precursors) was analysed. The experiments were carried out in a fixed bed reactor at a temperature of 100°C. A model flue gas with a composition of 3000 ppm SO 2, 5% O 2, 10% H 2O and N 2, was used in all experiments. The effect of different chemical (NH 3 at 800°C, HNO 3 (at reflux), HNO 3 (reflux)/NH 3 (at 250°C), O 2/NH 3 (at 450°C)) and thermal treatments to enhance the SO 2 capture capacity was studied. The PAN-based activated carbon fibres showed the best SO 2 retention characteristics. Treatment with ammonia at 800°C has proved to be an efficient way to increase the retention capacity of activated fibres. The SO 2 capture capacity increases with the total surface basicity of activated carbon fibres. The retention capacity markedly decreases after various retention–regeneration cycles. This reduction on sulphur retention can be partially avoided by increasing the regeneration temperature from 600°C to 800°C.

Coal gasification and combustion of LCV gas

Experiments were performed on the gasification of bituminous coal with air and steam in a 2·5 kW atmospheric fluidized bed. Thermal conversion efficiencies, the retention of sulphur and the conversion of fuel nitrogen to NH 3 were determined for various oxygen-to-carbon ratios (equivalence ratios between 0·58 and 0·82), with and without limestone in the bed. The conversion efficiency increased with increasing equivalence ratio, while the conversion of fuel-nitrogen to NH 3 decreased with increasing equivalence ratio. The sulphur conversion without the addition of limestone increased with increasing equivalence ratio. As expected, the presence of limestone in the bed led to increased sulphur retention, more so at the high and low ends than at intermediate values of the range of equivalence ratios. In addition, limestone adversely affected thermal conversion efficiency and increased the conversion of fuel-nitrogen. The low calorific value (LCV) fuel gas, produced by the AFBG, was combusted in a cyclone combustor at atmospheric pressure. Stable combustion of the LCV fuel gas was possible down to a calorific value of 1·57 MJ/m 3 n. The combustion efficiency in the atmospheric topping combustor varied between 98·72 and 99·96%. Nearly 60% of the NH 3 in the fuel gas was converted to NO in the combustor.

98/01909 Modelling of sulfur retention by limestone in coal briquette : Chen, C. and Kojima, T. Fuel Process. Technol., 1997, 53, (1,2), 49–67

98/01909 Modelling of sulfur retention by limestone in coal briquette : Chen, C. and Kojima, T. Fuel Process. Technol., 1997, 53, (1,2), 49–67

Sulfur Transformation in a South Australian Low-Rank Coal during pyrolysis

Sulfur transformation during pyrolysis of a high sulfur low-rank coal from South Australia has been studied. Three experimental techniques covering a wide range of conditions, namely, temperature-programmed pyrolysis, fixed-bed pyrolysis, and fluidized-bed pyrolysis, have been employed to investigate the effect of pyrolysis conditions on the interactions between different forms of sulfur and mechanisms of sulfur evolution during coal pyrolysis. Both chemical analysis method following Australian Standards and SEM with an energy dispersive X-ray detector are used for sulfur analysis of the char. The results reveal that sulfur evolution is a net result of organic and inorganic sulfur decomposition and interaction. The presence and conversion of inorganic sulfur into complex organic sulfur compounds provide a major mechanism for sulfur retention in char during pyrolysis. The difference in heating rates in the different pyrolysis experiments does not change the natural of the sulfur transformations but affects the extent to which they occur. Faster heating rates do not necessarily imply greater sulfur evolution due to formation of complex organic sulfur. Coal samples pretreated by acid washing and Ca and Na ion exchange are also used to examine the role of inorganic matter in sulfur transformation. At low temperatures (<400°C) acid washing shows little effect on sulfur retention, but at higher temperatures, sulfur retention is greatly reduced. While Na ion exchange enhances sulfur retention compared to the acid washed coal particularly at high temperatures (>400°C), Ca ion exchange shows the opposite trend. An increase in sulfide formation in the Ca ion-exchanged coal at high temperatures is observed, indicating that organic sulfur decomposition is enhanced in the presence of Ca. The effect on sulfur retaining of potential reactions involving Ca ions with sulfur may be offset by the catalyzing influence of Ca ions on organic sulfur decomposition.

Above-Ground Sulfur Cycling in Adjacent Coniferous and Deciduous Forests and Watershed Sulfur Retention in the Georgia Piedmont, U.S.A.

Atmospheric deposition and above-ground cycling of sulfur (S) were evaluated in adjacent deciduous and coniferous forests at the Panola Mountain Research Watershed (PMRW), Georgia, U.S.A. Total atmospheric S deposition (wet plus dry) was 12.9 and 12.7 kg ha-1 yr-1 for the deciduous and coniferous forests, respectively, from October 1987 through November 1989. Dry deposition contributes more than 40% to the total atmospheric S deposition, and SO2 is the major source (∼55%) of total dry S deposition. Dry deposition to these canopies is similar to regional estimates suggesting that 60-km proximity to emission sources does not noticeably impact dry deposition at PMRW. Below-canopy S fluxes (throughfall plus stemflow) in each forest are 37% higher annually in the deciduous forest than in the coniferous forest. An excess in below-canopy S flux in the deciduous forest is attributed to leaching and higher dry deposition than in the coniferous forest. Total S deposition to the forest floor by throughfall, stemflow and litterfall was 2.4 and 2.8 times higher in the deciduous and coniferous forests, respectively, than annual S growth requirement for foliage and wood. Although S deposition exceeds growth requirement, more than 95% of the total atmospheric S deposition was retained by the watershed in 1988 and 1989. The S retention at PMRW is primarily due to SO42- adsorption by iron oxides and hydroxides in watershed soils. The S content in white oak and loblolly pine boles have increased more than 200% in the last 20 yr, possibly reflecting increases in emissions.

Modeling of sulfur retention by limestone in coal briquette

A detailed simulation model was developed for sulfur retention by limestone in a coal briquette. Four submodels, i.e., coal briquette combustion, volatile and sulfur evolution, H 2S retention, and SO 2 retention, were included in the simulation. In the model, the coal briquette combustion was divided into two successive stages: volatile ignition and char burnout. The temperature profile and its time variation, sulfur release and retention behavior within the burning coal briquette for the two stages were simulated separately. In the stage of volatile evolution and ignition, a part of coal sulfur is released as H 2S, and reacted with the calcined limestone in the area near the particle surface, where the local temperature is increased along with the volatile ignition, and the limestone in the coal briquette is partially calcined. H 2S retention in the coal briquette was thus simulated as a result of the competition among its transportation by the bulk flow of evolved volatile, its molecular diffusion due to the concentration gradient within the coal briquette, and its capture by the calcined limestone. In the stage of char burnout, the remaining part of coal sulfur is released as SO 2 with char burning which is simulated by the shrinking core model. SO 2 release rate is assumed to be proportional to the char burning rate and controlled by oxygen diffusion in the ash layer. The sulfate formation occurs in the ash layer within which oxygen exists. SO 2 retention was thus simulated as a result of the competition among the sulfate formation with the calcined limestone, SO 2 diffusion in the ash layer and its emission from the briquette surface. The sulfur retention by limestone in a spherical centimeter sized coal briquette was simulated by the model. The effects of heating rate, briquette size, calcium to sulfur ratio ( Ca S ), and volatile matter of coal on the sulfur retention were predicted. The simulation results showed that rapid heating condition was good for both the H 2S retention in the volatile and the SO 2 retention in the combustion gas. The simulation also predicted a higher SO 2 retention for a larger sized coal briquette. The coals of higher rank and lower organic sulfur contents also showed a higher sulfur retention.

97/05144 Application of factor analysis in prediction of sulfur retention characteristics of CaCO 3 from limestone producing area : Li, L. and Yu, D. Harbin Ligong Daxue Xuebao, 1996, 1, (2), 93–96. (In Chinese)

97/05144 Application of factor analysis in prediction of sulfur retention characteristics of CaCO 3 from limestone producing area : Li, L. and Yu, D. Harbin Ligong Daxue Xuebao, 1996, 1, (2), 93–96. (In Chinese)

Mobility and retention of heavy metals, arsenic and sulphur in podzols at eight locations in northern Finland and Norway and the western half of the Russian Kola Peninsula

The mobility and retention of heavy metals, arsenic and sulphur in podzols from eight areas located north of the Arctic Circle in Finland, Norway and Russia were determined by analyzing the < 2.0 mm fraction, using an ammonium acetate (pH 4.5) extraction in addition to a concentrated nitric acid digestion for the humus samples, and a hot aqua regia digestion for the mineral soil samples. Total C, H and N concentrations were determined in humus and mineral soil samples with a CHN analyser. Ni, Cu, Co and As were strongly enriched in the humus layer in the contaminated sites (Monchegorsk, Kurka, Zapoljarnij) when compared to their concentrations in the parent tills and in podzols from the background sites. In most study sites the illuvial layer showed a low capacity to retain the metals and As, the exception included a strongly eroded profile at Monchegorsk, where Ni was tightly fixed in the illuvial layer while Cu was mobile. In contrast to metals, airborne S was not accumulated in the humus layer, but was accumulated in the illuvial layer, more markedly at eroded sites than in places where the humus was covered as at Monchegorsk.

Evaluation of the combustion efficiency and emission of pollutants by coal particles in a vortexing fluidized bed

Experiments were carried out in a pilot-scale (a0.45 m i.d.) vortexing fluidized bed combustor (VFBC), with coal particles as the fuel, limestone as sorbent, and river sand as the fluidizing medium. The effects of various operating parameters on the combustion efficiency and the pollutants emitted were investigated. Different formulae for the combustion efficiency are compared. A combustion efficiency of 96% can be easily obtained in a VFBC. Both the NO x and SO x emissions, based on an oxygen content of 6 vol.% in the flue gas, are found to be below 100 ppm. The retention of sulfur by limestone displays a maximum temperature in the bed at 850–880°C.

The Fusion of Difficult Materials Including Chromite, Cassiterite and Reduced Sulphur

The practice of adding alkali metal nitrates such as sodium nitrate to borate fluxes and pre-oxidizing samples at low temperature has been taken a logical step further by the synthesis of sodium borate fluxes via the use of intimate mixtures of sodium nitrate and boric acid. The process of low-temperature pre-firing/decomposition followed by melting at normal high temperature gives enormous oxidative power yet does not give rise to crucible attack and provides a convenient route to produce fluxes from cheap and highly pure starting materials. An improvement in the efficacy of attack for a number of sample types has been observed, allowing for much smaller dilution ratios than have hitherto been achieved for chromite and cassiterite samples, for instance. Further, the inherent ease of formulation allows for the final stoichiometry to be achieved in stages whereby a liner can be made in a platinum crucible consisting of all of the boric oxide and a small amount of the sodium oxide. The remaining sodium oxide is added in the form of sodium peroxide mixed with the sample, which is sintered at a low temperature followed by safe decomposition and fusion at normal temperature. The use of pure alumina crucibles is shown to be possible for samples of such a nature that attack on platinum ware is to be feared and has allowed the use of sodium peroxide for the fusion of ferro-chrome and ferro-niobium. The complete retention of pyritic sulphur in the above methods and with lithium carbonate–tetraborate fluxes has been chemically proven. © 1997 by John Wiley & Sons, Ltd.

The Fusion of Difficult Materials Including Chromite, Cassiterite and Reduced Sulphur

The practice of adding alkali metal nitrates such as sodium nitrate to borate fluxes and pre-oxidizing samples at low temperature has been taken a logical step further by the synthesis of sodium borate fluxes via the use of intimate mixtures of sodium nitrate and boric acid. The process of low-temperature pre-firing/decomposition followed by melting at normal high temperature gives enormous oxidative power yet does not give rise to crucible attack and provides a convenient route to produce fluxes from cheap and highly pure starting materials. An improvement in the efficacy of attack for a number of sample types has been observed, allowing for much smaller dilution ratios than have hitherto been achieved for chromite and cassiterite samples, for instance. Further, the inherent ease of formulation allows for the final stoichiometry to be achieved in stages whereby a liner can be made in a platinum crucible consisting of all of the boric oxide and a small amount of the sodium oxide. The remaining sodium oxide is added in the form of sodium peroxide mixed with the sample, which is sintered at a low temperature followed by safe decomposition and fusion at normal temperature. The use of pure alumina crucibles is shown to be possible for samples of such a nature that attack on platinum ware is to be feared and has allowed the use of sodium peroxide for the fusion of ferro-chrome and ferro-niobium. The complete retention of pyritic sulphur in the above methods and with lithium carbonate–tetraborate fluxes has been chemically proven. © 1997 by John Wiley & Sons, Ltd.

Effects on SOx and NOx Emissions by Co-Firing Straw and Pulverized Coal

The effect of co-firing straw and pulverized coal has been investigated in a 2.5 MWt pilot-scale burner and a 250 MWe utility boiler. In the 2.5 MWt trial the straw was chopped and fed separately to the burner, whereas in the full-scale experiment the straw was preprocessed as pellets and ground with the coal in the mills. Straw fractions in the range of 0−20% on a thermal basis were used in the full-scale experiment and 0−100% on a thermal basis in the pilot-scale experiment. Two low-sulfur coals and one high-sulfur coal were used in the pilot scale, whereas a high-sulfur coal was used in full scale. Results from both tests revealed that an increased fraction of straw in the fuel blend resulted in a reduction of NO and SO2 emissions. The lower SO2 emission was partly due to a lower sulfur content of the straw and partly due to retention of sulfur in the ash, probably present as solid alkali sulfates. The reduction of NO emissions was due to lower conversion of the fuel-bound nitrogen. Increasing the stra...

97/01360 Development issues for the char combustor component of an integrated partial gasification combined cycle system

The Air Blown Gasification Cycle (ABGC), formerly known as the Topping Cycle, is an integrated partial gasification combined cycle system which offers potential advantages over other advanced coal-fired processes under development. Within the United Kingdom a development program is underway to develop the components of the ABGC leading to the establishment of demonstration and commercial units. The ABGC comprises an air blown partial gasifier which produces a fuel gas and a char residue. The gas after cooling and cleaning is fired in a gas turbine, while the char is burned in a CFBC to raise steam for the steam cycle. There are several char combustor component development issues which are under investigation at British Coal. The objective of this work is to produce combustion performance and emissions data for what is an unusual fuel having a very low volatile matter content, a bimodal size distribution and containing CaS which needs to be fully converted to CaSO{sub 4} before disposal. The R and D program has involved test work on the 0.12MWth CFBC test facility at the University of British Colombia (UBC) and the construction and extended operation of a 1.8MWth CFBC test facility at British Coal. In support of themore » CFBC test runs laboratory studies are being undertaken on the CaS oxidation reaction and a cold model of the British Coal CFBC test facility has been constructed to assist in scale up of the test facility data. Preliminary test work carried out at UBC is described. It showed that the British Coal gasifier residues can be successfully burned in a CFBC environment with the minimum generation of environmentally contentious species, giving a combustion efficiency of 99%, and a sulphur retention efficiency of 96% with a calcium to sulphur ratio of 2:1. Indications for the special requirements for the design of the CFBC for use in an ABGC plant are discussed.« less

Study of modified calcium hydroxides for enhancing SO 2 removal during sorbent injection in pulverized coal boilers

Several modified calcium hydroxides were prepared by lime hydration using NaCl, KCl, calcium lignosulfonate, ethanol-water solutions and combinations of them, to obtain sorbents with enhanced capacity to remove SO 2. The sorbents were characterized in a drop tube reactor in similar conditions to those existing during sorbent injection in pulverized coal boilers. The greatest increases in SO 2 retention were obtained with calcium lignosulfonate as additive or using ethanol-water solutions in the CaO hydration process. The alkali promoters (NaCl and KCl) acted mainly on the product layer diffusivity. Calcium lignosulfonate modified the pore size distribution of the calcined hydrate and decreased the mean particle size of the modified calcium hydroxide by up to five times. The use of ethanol-water solutions during the hydration process decreased the mean particle size of the modified hydrate and improved the porous structure of the calcined hydrate, mainly by increasing the porosity. To combine the favourable effects of these additives, several sorbents were prepared by combinations of them. The best sulfur retentions were obtained with sorbents modified by a combination of calcium lignosulfonate and hydration with ethanol-water solutions. The effect of the combination of additives on the sorbent properties was not the same as that of their action separately, and their effect on sulfur retention was not cumulative.

Uptake of NO X by activated carbons: bench-scale and pilot-plant testing

The possibility of selectively removing NO x from gas containing CO 2, N 2, O 2, H 2O, SO 2 and NO x has been examined using bench-scale and pilot-scale experimentation. These results have shown that NO can be selectively sequestered from the gas if the oxygen concentration is high enough and the SO 2 concentration is low enough. Oxygen enhances the amount of NO removed, and promotes the conversion of NO to NO 2 (via NO + 1/2O 2 → NO 2) and the adsorption of NO 2 within the carbons' micropores. The carbon is not consumed if during NO 2 desorption its temperature is maintained below about 200 °C. The presence of SO 2 inhibits the oxidation of NO and the subsequent storage of NO 2 within micropores but, at concentrations typical to gases after post-combustion sulfur scrubbers, such inhibition is an immediate effect probably associated with selective site poisoning rather than a cumulative effect associated with pore blocking or sulfur retention. The possibility of removing NO and NO 2 from combustion flue gas and the extent to which fines were produced during pilot-plant testing of activated carbons will also be discussed. Wear patterns on the carbon surface subsequent to the pilot-plant studies and NO x uptake calculated from gas-phase and sorbent analyses are compared to data from the bench-scale experimentation. Information needed for more complete understanding of the interaction of NO + SO 2 on and within carbon is discussed.

Effect of temperature and dietary sodium bicarbonate supplementation on the mineral excretion of broilers

A non-surgical technique was developed for the collection of urine samples free from faecal contamination. This technique was used with growing broilers to study the effects of ambient temperature and dietary sodium bicarbonate (NaHCO3) supplementation on the urinary excretion of mineral ions 1·5–2·0 h after feeding. High ambient temperatures resulted in increases in the absolute retentions of sodium, potassium, calcium, magnesium, phosphorus, and sulfur. The lack of any temperature effect on the urinary excretion of the non-monovalent ions indicated that the changes in retentions of these ions were due to changes in absorption. The increase in potassium retention was related to a decrease in urinary excretion, as would be expected for an ion regulated predominantly by the kidneys. However, no effect of temperature was observed on the urinary excretion of sodium. Sodium bicarbonate supplementation resulted in an increase in both the absolute retention and urinary excretion of sodium. Other minerals were unaffected by NaHCO3 supplementation other than for a marginally significant (P = 0·05) increase in the absolute retention of sulfur. In order to minimise faecal contamination of the urine samples, birds were fasted for 16 h prior to feeding with experimental diets. Possible problems with tissue catabolism were minimised by supplying birds with a glucose–casein solution during the fasting period.

Amelioration of high organic sulphur coal for combustion in domestic stoves

Abstract Despite the proximity to Ankara, raw coal mined in the Beypazari basin cannot be used as a domestic fuel in Ankara because the high combustible sulphur content would add to the already severe pollution problems in that city. This study investigated a method of reducing the SO 2 emissions by adding lime (CaO) to the coal prior to combustion. Sorbent, as lime, was added to the combustion chamber (a domestic stove), by pretreating the coal by mixing crushed coal with lime and molasses and turning the coal into briquettes. Lime was added to lump coal for comparative purposes. The ratio of Ca free : S was varied for a series of combustion tests, and the heating efficiency and the amount of sulphur fixed in the ash were determined. The results of the tests show that optimum sulphur retention and heating efficiency were obtained when the Ca free : S molar ratio was between 0.95 and 1.15 for the briquettes and between 1.00 and 1.25 for the lump coal. For lump coal, 50% of the total sulphur could be fixed in the ash and at least 75% of the heating efficiency retained. During combustion of the briquettes, at least 57.5% of the sulphur could be fixed in the ash and at least 85% of the heating efficiency could be achieved. This suggests that the addition of lime to briquettes may be a feasible way of reducing the SO 2 emissions for domestic stoves.

97/00560 Progress on mathematical modeling of sulfur retention by limestone during finidized bed combustion of coal

97/00560 Progress on mathematical modeling of sulfur retention by limestone during finidized bed combustion of coal