Flue Gas Desulfurization By-products Research Articles

Equilibrium and experimental study on carbothermal reduction of sulfur dioxide to elemental sulfur

In order to solve the problem of low utilization value of flue gas desulfurization by-product, reduction of sulfur dioxide to elemental sulfur has emerged as a new process for resource utilization of SO2 with promising application prospects. Herein, the thermodynamic equilibrium calculation was carried out to study the influence of temperatures and C/SO2 molar ratios on the C-SO2 reaction. The effects of reaction temperature, carbon mass, and CO concentration on SO2 reduction by coke and activated carbon were studied on a fixed bed reaction system. The theoretical results demonstrated that the highest equilibrium S yield occurred when the C/SO2 molar ratio is 1, with higher ratios resulting in the formation of COS, CS2, and CO. Experimental data revealed that at temperatures below 700 °C, both SO2 conversion and S yield are notably low. As the temperature increases, both the conversion and yield gradually increase and approach the thermodynamic equilibrium value. The evolution of carbon surface chemical properties under the action of CO was analyzed by FTIR and XPS, which indicated that the presence of CO promotes the decarboxylation reaction, and then increases SO2 conversion and S yield. The conclusion of this study provides guidance for the development of high efficiency SO2 reduction technology.

Physicochemical and radiological characterization of flue gas desulfuration waste samples from Brazilian coal-fired power plants

Flue gas desulfurization (FGD) waste is an industrial by-product generated during the flue gas desulfurization process in coal-fired power plants. This by-product contain trace quantities of naturally occurring radionuclides and elements such as As, Ba, Co, Cr, Zn. The characteristics of FGD waste are important for its reuse and are mainly depend on the desulfurization process. In this work, two types of FGD materials collected from three coal-fired power plants using semi-dry and wet processes were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), X-ray fluorescence (XFR) and particle size analysis. The radioactive content of 238U, 232Th, 228Th, 226Ra, 228Ra, 210Pb and 40K and trace elements were also determined using Neutron activation analysis and Gamma-ray spectrometry. The major constituents for all samples were Ca, Si, S, Al and Fe. Wet FGD by-product presented also high magnesium content. The wastes contain mainly semi-hydrate calcium sulfite and calcium sulfate. The particle size of FGD from semi-dry process was lower than that from the wet process. The average activity concentration of 238U, 232Th, 226Ra, 210Pb, 228Ra, 228Th and 40K varied between were 50-71, 33-42, 28-52, 113-150, 26-33, 33-39, 161-390 Bq kg-1, respectively. According to the results of leaching and solubilization tests, FGD samples were classified as non-hazardous and non-inert. The obtained data are useful for evaluation of possible applications of FGD by-products.

Distribution, chemical fractionation, and potential environmental risks of Hg, Cr, Cd, Pb, and As in wastes from ultra-low emission coal-fired industrial boilers in China

This study conducted a comprehensive investigation of the distribution, chemical fractionation, and potential environmental risks of Hg, Cd, Cr, Pb, and As in waste based on new data from five ultra-low emission (ULE) coal-fired industrial boilers (CFIBs). The results showed that fly ash was enriched with Cd, Pb, As, and Hg, while its Cr contents were not invariably higher than those of slag. Fly ash was the predominant output flow for Hg, Cd, Cr, Pb, and As in the tested ULE boilers, with higher proportions of HTEs in the fly ash and lower proportions of HTEs in the flue gas than in the non-ULE boilers. The average proportions of residual Hg, Cd, Cr, Pb, and As in wastes revealed the following order: slag > fly ash > flue gas desulfurization (FGD) by-products. The potential environmental risks of Hg, Cd, Cr, Pb, and As in the fly ash, slag, and FGD by-products of CFIBs at the county level in the Beijing-Tianjin-Hebei Air Pollution Transmission Channel Cities (“2 +26 cities”) region showed spatial heterogeneity. It is predicted that the potential release of Pb, Cr, and Cd in the fly ash would increase and that of the FGD by-products would decrease after the implementation of the ULE retrofitting of all CFIBs.

Zero-valent bimetallic catalyst/absorbent for simultaneous facilitation of MgSO3 oxidation and arsenic uptake

Cobalt (Co)-based catalysts can efficiently reduce the heat waste from sulfate concentration by enhancing sulfite oxidation during wet flue gas desulfurization system. However, arsenic (As) can poison such catalysts and migrate into the sulfate by-products, resulting in severe secondary pollution. In this study, a zero-valent Co/iron (Fe)-based nanoparticle (NZV-Co2Fe1) was fabricated and applied as a bifunctional catalyst/adsorbent. The catalytic stability of the Co-based catalyst was enhanced by the introduction of Fe because the poisonous effect of As was substantially suppressed because of the high adsorption capacity of Fe for As. Compared with the noncatalytic benchmark, the presence of 0.5 g/L NZV-Co2Fe1 can increase the rate of MgSO3 oxidation by approximately 12-fold even at a high concentration of As (2.5 mg/L). The Langmuir model was fitted to the As adsorption isotherms, indicating that As uptake is a single-layer adsorption process. The pseudo-second-order kinetic model indicated that As was removed through chemisorption. The oxidation pathway of As(III) involves reactive radicals (mainly OH, SO4− and SO5−) and ligand-to-metal charge transfer between SO32− and Co2+. The availability of MgSO3 improved the removal efficiency at high concentrations of As(III) (1 mg/L). These results indicate that using NZV-Co2Fe1 as a catalyst to purify the by-products of flue gas desulfurization can effectively prevent secondary pollution.

Phase Transformation of FGD By-Product into Plaster Mould via Heat Treatment Process

Flue Gas Desulfurization (FGD) is a waste incineration process commonly used to eliminate sulfur dioxide (SO2) from flue gas power plants. FGD by-product recorded a rich gypsum content, also known as calcium sulfate dehydrate (CaSO4•2H2O), which has promising practical applications in plaster mould production. Plaster of Paris (POP) with a chemical composition of calcium sulfate hemihydrate (CaSO4•0.5H2O) is widely used in plaster mould production because of its quick setting and hardening upon moistened. Naturally, gypsum with 2 molecules of crystalline water can change to 1.5 molecules via the dehydration process. Various dehydration methods were conducted to transform FGD gypsum to hemihydrate. After undergoing different autoclave processes, the phase transition of FGD by-products was identified by X-ray diffraction (XRD) mode and compared with commercial POP. Chemical composition of FGD sludge contains a high amount of calcium oxide (CaO), sulfur trioxide (SO3) and boron trioxide (B2O3), as well as other impurities such as fluorine (F), chlorine (Cl), and magnesium oxide (MgO). Based on phase analysis, sample H1 to H5 show the percentage of hemihdyrate is 1.5%, 19.7%, 2.8%, 1.2%, and 46.1%, respectively, after different dehydration methods.

The hydration mechanism of magnesium oxysulfate cement prepared by magnesium desulfurization byproducts

The byproducts of flue gas desulfurization using magnesium (Mg) oxides in aqueous environment are hard to utilize. In order to study the potential of using Mg desulfurization byproducts for the preparation of Mg oxysulfate cement, this paper investigates the hydration mechanism of Mg oxysulfate cement (DMOS) prepared by Mg desulfurization byproducts. Mg oxysulfate cement (MOS) prepared using pure heptahydrate sulfuric acid is used as control. The material characterizations were conducted by X-ray diffraction, fourier transform infrared spectroscopy, mercury intrusion porosimetry, hydration heat release analysis, pH and scanning electron microscopy-energy dispersive spectroscopy. The results shows that the 28 days compressive strength of DMOS reached 28.86 MPa, which was 81% of that of MOS with the same preparation condition. The hydration products of DMOS are mainly 3 Mg(OH)2⋅MgSO4⋅8H2O (318 phase), MgCO3 and Mg(OH)2. The 318 phase exists in the form of needles, and there are a large number of flake Mg(OH)2 crystals and incomplete MgO. The hydration exothermic trend of DMOS is similar to MOS, but delayed a little bit. The maximum heat release of DMOS is higher than that of MOS. The obtained results would provide a reference for the application of Mg desulfurization byproducts in cement preparation industry.

Simultaneous magnesia regeneration and sulfur dioxide generation in magnesium-based flue gas desulfurization process

A method was proposed for simultaneously regenerating magnesia from the Mg-based flue gas desulfurization byproducts in the power plant and generating the high concentration of sulfur dioxide. Elemental sulfur particles were evaporated to sulfur vapor and the latter one was used as the reductive agent to regenerate magnesia from the magnesium gypsum. The experiments were carried out in a successive fixed-bed system to study the concentration of sulfur dioxide in the exhaust gas and the activity of the regenerated magnesia under different reaction conditions. Sulfur vapor effectively reduced the magnesia regeneration temperature from above 950 °C to merely 650 °C. The concentration of sulfur dioxide in the exhaust gas could achieve 20% by volume when the sulfur vapor partial pressure was 1674 Pa, the regeneration temperature was 750 °C, the gas hourly space velocity was 1200 h−1 and the loading of the reaction materials in the regeneration reactor was 16 g, respectively. The activity of the regenerated magnesia after the decomposition of the magnesium gypsum at 750 °C was satisfactory. Both the magnesium gypsum and the regenerated magnesia were characterized using X-ray fluorescence spectroscopy, thermogravimetric analyzer, X-ray diffraction spectroscopy, BET surface area analyzer, scanning electron microscopy, high resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The kinetics of the magnesia regeneration from the magnesium gypsum in the presence of the sulfur vapor was analyzed and the nucleation model was determined to be the most appropriate kinetic model. The activation energy and the pre-exponential factor were determined to be 76.51 kJ/mol and 228.66 min−1, respectively. All the results demonstrate that there is considerable potential for the application of this proposed method in flue gas desulfurization process.

Mercury Speciation of Flue Gas Desulphurization By-Products in Coal-Fired Power Plants in China

The aim of this study was to develop and examine the morphology and distribution of mercury (Hg) in flue gas desulfurization (FGD) by-product. Mercury in the coal of coal-fired power plants is concentrated in the by-products of desulfurization process, and it is widely used as an additive in cement, building materials and other industries. Due to the different stability of various forms of mercury in the environment, subsequent use of products containing desulfurization by-product additives will continue to be released into the environment, endangering human health. Therefore, it is very necessary to study the form and distribution of mercury in the by-products of desulfurization in coal-fired power plants to provide a theoretical basis for subsequent harmless treatment. For content and morphology of mercury analysis, 1 sample of dry FGD ash and 6 samples of wet FGD gypsum were analyzed. The total 7 samples were extracted using a modification of sequential chemical extractions (SCE) method, which was employed for the partitioning Hg into four fractions: water soluble, acid soluble, H2O2 soluble, and residual. The Hg analysis was done with United States Environmental Protection Agency (USEPA) method 7471B. Comparing with the wet FGD gypsums of coal-fired boilers, the total Hg content in the dry FGD by-product was as high as 1.22 mg/kg, while the total Hg content in the FGD gypsum is 0.23 - 0.74 mg/kg, which was 2 times over the wet FGD gypsum. The concentration of water soluble Hg in the dry FGD by-product was the highest amount (0.72 mg/kg), accounting for 59.02% of the total mercury. While residual Hg content was 0.16 mg/kg, only about 13.11% of the total mercury. Mercury content in FGD gypsum was expressed in the form of ρ (residual Hg) > ρ (H2O2 soluble Hg) > ρ (water soluble Hg) > ρ (acid soluble Hg). The morphology and distribution of mercury in FGD by-products is supposed to be analyzed before utilization, and the impact of mercury on the environment should be considered.

Land Application Effects of a High-Calcium, Dry Flue Gas Desulfurization By-Product on Trace Elements in Runoff from Natural Rainfall

Due to low cost and large abundance, coal continues to be one of the major energy sources for electricity generation in the USA. The dry desulfurization of flue gases during coal combustion produces a by-product that may be potentially useful as a soil amendment. However, the influence of dry flue gas desulfurization (DFGD) by-products on trace element losses with runoff from treated fields has not been well investigated. The objective of this study was to evaluate the effects of land application of a high-Ca, DFGD by-product on trace elements lost in runoff from natural rainfall events. The by-product was applied once on May 18, 2015 at 9 Mg DFGD ha−1 to small plots of a highly weathered Ultisol under managed-grassland land use in northwest Arkansas. Runoff was collected following each runoff-producing precipitation event for 1 year. Seasonal (i.e., summer (May to August), fall (August to November), winter (November to February), and spring (February to May)) runoff, annual runoff, runoff pH, and electrical conductivity did not differ significantly between DFGD treatments. Seasonal flow-weighted mean Ni concentrations and seasonal V loads were significantly greater in runoff when amended versus the unamended control when compared during at least one 3-month season by 44.5 and 86.9% for Ni and V, respectively. Based on the results of this study, it appears that land application of a high-Ca DFGD by-product at rates ≤ 9 Mg ha−1 has minimal effects on trace elements in runoff.

Dry flue gas desulfurization by-product application effects on plant uptake and soil storage changes in a managed grassland.

Environmental regulations mandate that sulfur dioxide (SO2) be removed from the flue gases of coal-fired power plants, which results in the generation of flue gas desulfurization (FGD) by-products. These FGD by-products may be a viable soil amendment, but the large amounts of trace elements contained in FGD by-products are potentially concerning. The objective of this study was to evaluate the effects of land application of a high-Ca dry FGD (DFGD) by-product on trace elements in aboveground biomass and soil. A high-Ca DFGD by-product was applied once at a rate of 9Mgha-1 on May 18, 2015 to small plots with mixed-grass vegetation. Soil and biomass were sampled prior to application and several times thereafter. Aboveground dry matter and tissue As, Co, Cr, Hg, Se, U, and V concentrations increased (P<0.05) following application, but did not differ (P>0.05) from pre-application levels or the unamended control within 3 to 6months of application. Soil pH in the amended treatment 6months after application was greater (P<0.05) than in the unamended control. Soil Ca, S, and Na contents also increased (P<0.05), following by-product application compared to the unamended control. High-Ca DFGD by-products appear to be useful as a soil amendment, but cause at least a temporary increase in tissue concentrations of trace elements, which may be problematic for animal grazing situations.

Plant uptake of major and trace elements from soils amended with a high-calcium dry flue gas desulfurization by-product

Coal combustion by-products (CCBs) are a potentially valuable source of essential plant nutrients, but CCBs also frequently contain trace elements. The objective of this greenhouse study was to evaluate the effects of soil and application rate of a high-calcium (Ca) dry flue gas desulfurization (DFGD) by-product on Bermudagrass (Cynodon dactylon L.) growth and plant tissue and soil composition. Surface horizons of two soils with contrasting texture were mixed with a high-Ca DFGD by-product at rates of 0, 2.5, 5, 10, and 15Mgha−1. Bermudagrass was allowed to grow in the soils for 119days. Concentrations of plant nutrients and selected trace elements were determined in soil and plant tissue. Plant dry matter and tissue concentrations of Ca, Mg, B, Se, Rb, and U increased as DFGD application rate increased. Concentrations of Mehlich-3-extractable soil P, Ca, Mg, S, Na, Fe, and B and strong-acid-extractable V and Hg increased with increasing application rate. Soil pH increased with application rate and electrical conductivity was only greater than that in the control at the 15Mgha−1 application rate after 119days. The DFGD by-product used in this study can be beneficially reused as a soil amendment provided soil pH and plant tissue Se concentrations are closely monitored.

Biogeochemical oxidation of calcium sulfite hemihydrate to gypsum in flue gas desulfurization byproduct using sulfur-oxidizing bacteria

Flue gas desulfurization (FGD) is a well-established air treatment technology for coal and oil combustion gases that commonly uses lime or pulverized limestone aqueous slurries to precipitate sulfur dioxide (SO2) as crystalline calcium salts. Under forced oxidation (excess oxygen) conditions, FGD byproduct contains almost entirely (>92%) gypsum (CaSO4·2H2O), a useful and marketable commodity. In contrast, FGD byproduct formed in oxygen deficient oxidation systems contains a high percentage of hannebachite (CaSO3·0.5H2O) to yield a material with no commercial value, poor dewatering characteristics, and that is typically disposed in landfills. Hannebachite in FGD byproduct can be chemically converted to gypsum; however, the conditions that support rapid formation of gypsum require large quantities of acids or oxidizers. This work describes a novel, patent pending application of microbial physiology where a natural consortium of sulfur-oxidizing bacteria (SOB) was used to convert hannebachite-enriched FGD byproduct into a commercially valuable, gypsum-enriched product (US Patent Assignment 503373611). To optimize the conversion of hannebachite into gypsum, physiological studies on the SOB were performed to define their growth characteristics. The SOB were found to be aerobic, mesophilic, neutrophilic, and dependent on a ready supply of ammonia. They were capable of converting hannebachite to gypsum at a rate of approximately five percent per day when the culture was applied to a 20 percent FGD byproduct slurry and SOB growth medium. 16S rDNA sequencing revealed that the SOB consortium contained a variety of different bacterial genera including both SOB and sulfate-reducing bacteria. Halothiobacillus, Thiovirga and Thiomonas were the dominant sulfur-oxidizing genera.

Dimensional Analysis of an Electromembrane Reactor for Recycling and Resource Recovery of Flue Gas Desulfurization Residuals

Abstract Flue gas desulfurization (FGD) technologies have been applied worldwide in power plants to achieve regulatory standards for the emission of SO2. However, commercial methods in practice require increased costs for disposal of the FGD residuals or by-products. An electromembrane reactor for recycling and resource recovery of FGD residuals was proposed in this study. On the basis of previous work, a mathematical model was developed to describe the mass balance of proposed electromembrane reactor by originally introducing the dimensionless groups, which can help in understanding the mechanism of electrochemical processes and in removing barriers against scaling up a lab-scale chemical unit to a commercial size. On the other hand, results of numerical simulation agreed well with the experimental data, which verified the capability of the developed model for mass transfer. With the help of dimensionless groups, analysis of the model revealed that current density, flow pattern, and initial concentration...

Research Development of Resource Utilization of Sintering Flue Gas Calcium Based Desulfurization Byproduct

The sintering flue gas is one of the main pollution sources in the iron and steel industry, and the flue gas desulfurization has become an important aspect of air pollution control. In recent years, as a large number of sintering desulfurization equipments were applied, the byproduct has get more and more attention. This paper starts from the characteristics of sintering flue gas desulfurization process, analyzes the physical and chemical properties of different sintering flue gas desulfurization by-product, expounds the present situation of domestic and foreign desulfurization by-product resource utilization and the problems existing in the utilization process, and puts forward possible direction of the sintering flue gas desulfurization by-products with high added value, large amount of resource and no pollution.

Study on a new wet flue gas desulfurization method based on the Bunsen reaction of sulfur-iodine thermochemical cycle

A novel wet flue gas desulfurization method based on the Bunsen reaction of sulfur-iodine (SI) thermochemical cycle was investigated in this paper. I2 and HI absorption system was utilized to remove SO2 from simulated coal-fired flue gas. The SO2 removal efficiency was about 98.8% when the I2 concentration was 25.6mmol/L. The influences of reaction temperature, initial SO2 concentration and the other flue gas components on SO2 removal were investigated, respectively. The absorption products were H2SO4 and HI, which could be easily separated by distillation. Compared with the traditional wet flue gas desulfurization (FGD) byproducts, such as gypsum or magnesium sulfate, H2SO4 has better commercial value and application prospect. Moreover, HI could be used as the raw material for the hydrogen production in the SI thermochemical cycle. In short, it is a promising technology for the SO2 removal and recycling from coal-fired flue gas.

Using HYDRUS to simulate the dynamic changes of Ca&lt;sup&gt;2+&lt;/sup&gt; and Na&lt;sup&gt;+&lt;/sup&gt; in sodic soils reclaimed by gypsum

Sodic soils are characterized by the occurrence of excess sodium to levels that can adversely affect soil structure. In recent years, with the advent of alternatives for reclaiming sodic soils, such as the addition of by-products of flue gas desulfurization, fly ash, phosphogypsum, etc., using CaSO4 to reclaim sodic soil has again become a hot topic. In this study, cation exchange batch experiments and column leaching experiments were conducted to analyze the adsorption-exchange and dynamic changes of Ca2+ and Na+ during the reclamation of sodic soils with CaSO4. The HYDRUS-1D software was subsequently used to simulate and predict dynamic changes in Ca2+ and Na+. The cation exchange batch experiments consisted of six treatments with six CaSO4 rates (0, 0.25, 0.5, 1, 1.5, and 2 g/l), and the column leaching experiments consisted of two treatments with two CaSO4 concentrations (0.5 and 1.5 g/l). The results of the static cation exchange batch experiments indicated that the ion adsorption-exchange coefficients KCa-Na, KCa-Mg,andKCa-K were 1.9, 0.8, and 1.1, respectively. Applying CaSO4 and leaching are efficient methods to reclaim sodic soil. The pH and electrical conductivity of the soil solution gradually decreased with longer leaching time in all of the treatments. HYDRUS-1D successfully simulated both the dynamic changes of the Ca2+ and Na+ concentrations at different soil depths under different treatments and leaching time, and the effects of soil hydraulic conductivity and soil pH on the transport of Ca2+ and Na+. The correspondence between the observed and simulated variables was remarkable.

Liming Characteristics of a High-Calcium Dry Flue Gas Desulfurization By-Product and a Class-C Fly Ash

Due to coal’s availability and low cost, coal combustion continues to be the United States’ primary energy source. However, coal combustion produces large quantities of waste material. Some coal combustion by-products (CCBs) have chemical and physical characteristics that make them potentially useful as soil amendments. The objectives of this study were to characterize a relatively new, high-calcium dry flue gas desulfurization (DFGD) by-product and compare its agronomic liming potential to a Class-C fly ash (FA) and reagent-grade calcium carbonate (CaCO3). Calcium car-bonate equivalence (CCE), degree of fineness (DOF), and effective neutralizing value (ENV) for each CCB were determined using standard methods. The CCBs and CaCO3 were also incubated with an acidic (~4.5) clay sub-soil at application rates equiva-lent to 0, 0.5, 1, and 2 times the soil’s lime requirement and compared to an una-mended control. Soil pH was then measured periodically during a 40-day incubation. The ENV of 79.4% for the DFGD by-product and 57.3% for the FA were comparable to those of commercially available liming materials, but were significantly lower (P

Testing Framework for Analysis of Time-Dependent Behavior of Coal Combustion Products

AbstractCoal combustion products (CCPs) are by-products created when coal is burned for energy production. These products include predominantly fly ash, bottom ash, and flue gas desulfurization (FGD) by-products. These materials can be a viable alternative to natural resources for the construction of engineered fills. Because of the variability in the chemical composition, CCPs require mineralogical, chemical, and mechanical characterization to ensure its applicability as a construction material. Furthermore, CCP characterization should also address the material’s changing properties with time. To date, only limited standards have been proposed to characterize these materials to determine the viability of their use as structural fill. The purpose of this study is to apply a minimal set of practical mechanical and chemical tests that will characterize and predict CCP time-based performance intended for large-volume civil engineering applications such as embankments and mine reclamation. Unconfined compress...

Impact of Addition of FGDB as a Soil Amendment on Physical and Chemical Properties of an Alkali Soil and Crop Yield of Maize in Northern China Coastal Plain

To evaluate the effect of Flue gas desulfurization byproduct( FGDB )as a soil amendment on growth and yield of maize (Zea mays) and to determine the impact of FGDB additions on soil fertility characteristics in alkaline clayey soils, a 2-year field experiment was conducted in Huanghua, in Northern China Coastal Plain. The experiment included five treatments in which the soil was amended with FGDB at 15 cm depth at the rates of 0 t·hm−2, 4.50 t·hm−2, 9.00 t·hm−2, 13.5 t·hm−2, and 18.00 t·hm−2, respectively, before maize was planted. The values of soil pH, exchangeable sodium percentage (ESP), and bulk density (BD) of the soil decreased; however, values of electrical conductivity (EC), water holding capacity (WHC), and plant nutrients increased with FGDB application in the soil. Crop plants grow more readily in FGDB amended soils because of improved soil properties. The best ameliorative effect was obtained at the rate of 13.5 t·hm−2. The germination percentage, plant height, and crop yield successively increased in both years. The results indicated FGDB was an effective soil amendment for improving the physicochemical properties and nutrient balance, and enhancing crop germination, growth, and yield, particularly when applied at a suitable application rate.

Development Status of Resourceable Technology of Flue Gas Desulphurization

Flue gas desulfurization (FGD) is one of the most effective techniques to control the emission of SO2 from the combustion of coal. The by-products of FGD may occupy much land and cause the second pollution. The resourceable technology of FGD changes SO2 from flue gas to sulfur-containing by-product, by which it is possible to solve the problem of desulfurization by-product treatment. This paper reviews the recent development of resurceable technology of FGD with special reference to removal of SO2 from flue gas.