Development and Pilot Plant Evaluation of Silica- Enhanced Lime Sorbents for Dry Flue Gas Desulfurization

EPA’s efforts to develop low cost, retrofitable flue gas cleaning technology include the development of highly reactive sorbents. Recent work addressing lime enhancement and testing at the bench-scale followed by evaluation of the more promising sorbents in a pilot plant are discussed here.

The conversion of Ca(OH)₂ with SO₂ increased several-fold compared with Ca(OH)₂ alone when Ca(OH)₂ was slurrled with fly ash first and later exposed to SO₂ in a laboratory packed bed reactor. Ca(OH)₂ enhancement increased with the increased fly ash amount. Dlatomaceous earths were very effective reactivity promoters of lime-based sorbents. Differential scanning calorimetry of the promoted sorbents revealed the formation of a new phase (calcium silicate hydrates) after hydration, which may be the basis for the observed Improved SO₂ capture.

Fly ash/lime and diatomaceous earth/lime sorbents were tested in a 100 m³/h pilot facility incorporating a gas humidifier, a sorbent duct injection system, and a baghouse. The inlet SO₂ concentration range was 1000-2500 ppm. With once-through dry sorbent injection into the humidified flue gas [approach to saturation 10–20°C (18–36°F) in the baghouse], the total SO₂ removal ranged from 50 to 90 percent for a stoichiometric ratio of 1 to 2. Recycling the collected solids resulted in a total lime utilization exceeding 80–90 percent. Increased lime utilization was also investigated by the use of additives.     

Current Status of the ADVACATE Process for Flue Gas Desulfurization

The following report discusses current bench- and pilot-plant advances in preparation of ADVAnced siliCATE (ADVACATE) calcium silicate sorbents for flue gas desulfurization. It also discusses current bench- and pilot-plant advances in sorbent preparation. Fly ash was ground in a laboratory scale grinder prior to slurring in order to decrease the slurring time needed for the sorbent to be reactive with SO₂. Reactivity of ADVACATE sorbents with SO₂ in the bench-scale reactor correlated with their surface area.

ADVACATE sorbents produced with ground fly ash were evaluated in the 50 cfm (85 m³/h) pilot plant providing 2 s duct residence time. ADVACATE sorbent was produced by slurrying ground fly ash (median particle size of 4.3 µm) with Ca(OH)₂ at the weight ratio of 3:1 at 90°C (194°F) for 3hto yield solids with 30 weight percent of initial free moisture. When this sorbent was injected into the duct with 1500 ppm SO₂ and at 11°C (20°F) approach to saturation, the measured SO₂ removal was approximately 60percent at a Ca/S stoichiometric ratio of 2. Previously, when ADVACATE sorbent was produced at 90°C (194°F) and at the same fly-ash-to-Ca(OH)₂ weight ratio using unground fly ash, removal under the same conditions in the duct was approximately 50 percent following 12 h slurring. The report presents the results of pilot-scale recycle tests at the recycle ratio of 2. Finally, the report discusses future U.S. Environmental Protection Agency plans for commercialization of ADVACATE.     

Removal of SO2 and NOX from Stack Gas by Reaction with Calcium Hydroxide Solids

Previous workers have shown that simultaneous SO₂/NO_X removal can be obtained in a dry scrubbing system with Ca(OH)₂ promoted by an additive such as NaOH, and that fly ash and product recycle improve the reactivity of the solids toward SO₂. To test SO₂/NO_X removal with fly ash and product recycle, bench-scale experiments with a packed bed reactor were performed at bag filter conditions. The most reactive solid for NO_X removal was prepared by slurrying Ca(OH)₂ with fly ash, CaSO₃, and NaOH. The best conditions for NO_X removal were the greatest temperature (125°C) and greatest concentrations of SO₂ (1500 ppm) and O₂ (20 percent). At the best conditions, NO_X removed in 1 hour was 3-4 moles per 100 moles Ca(OH)₂, compared to 5-10 moles SO₂ removed per 100 moles Ca(OH)₂. The best SO₂ removal was obtained at the highest relative humidities/lowest temperatures (55% RH/ 65°C) with solids prepared by slurrying Ca(OH)₂ with fly ash and NaOH. At these conditions, SO₂ removed In 1 hour was 60-80 moles per 100 moles Ca(OH)₂, compared to 0.5 to 1 moles NO_X removed per 100 moles Ca(OH)₂.     

Potassium-based sorbents from fly ash for high-temperature CO<Subscript>2</Subscript> capture

Potassium-fly ash (K-FA) sorbents were investigated for high-temperature CO₂ sorption. K-FAs were synthesised using coal fly ash as source of silica and aluminium. The synthesised materials were also mixed with Li₂CO₃ and Ca(OH)₂ to evaluate their effect on CO₂ capture. Temperature strongly affected the performance of the K-FA sorbents, resulting in a CO₂ uptake of 1.45 mmol CO₂/g sorbent for K-FA 1:1 at 700 °C. The CO₂ sorption was enhanced by the presence of Li₂CO₃ (10 wt%), with the K-FA 1:1 capturing 2.38 mmol CO₂/g sorbent at 700 °C in 5 min. This sorption was found to be similar to previously developed Li-Na-FA (2.54 mmol/g) and Li-FA (2.4 mmol/g) sorbents. The presence of 10 % Li₂CO₃ also accelerated sorption and desorption. The results suggest that the increased uptake of CO₂ and faster reaction rates in presence of K-FA can be ascribed to the formation of K-Li eutectic phase, which favours the diffusion of potassium and CO₂ in the material matrix. The cyclic experiments showed that the K-FA materials maintained stable CO₂ uptake and reaction rates over 10 cycles.     

Latent Hydraulic Reactivity of Blended Cement Incorporating Slag Made from Municipal Solid Waste Incinerator Fly Ash

Abstract

The reactivity of cement pastes made by blending Portland cement with slag from municipal solid waste incinerator (MSWI) fly ash was investigated to assess the potential of recycling MSWI fly ash slag. The slag, prepared by melting MSWI fly ash at 1400 °C for 30 min, was pulverized and ground, then blended with ordinary Portland cement (OPC), using various substitution levels to make slag-blended cement (SBC). The pozzolanic reactivity of the ecocement was then characterized by determining variations in the compressive strength, degree of hydration, microstructure, speciation, and mineralogical crystalline phases. The results suggest that the strength of the pastes at an early age decreased with increasing substitution levels, whereas the strength at a later age of the tested pastes (with substitution levels less than 10%) outperformed OPC paste because of typical SBC properties. The development of strength at a later age was also confirmed by X-ray diffraction and scanning electron microscopy techniques. This implies that active silica (Si) and alumina (Al) react with the hydration product, calcium hydroxide (Ca(OH)₂), to form calcium silicate hydrate (C-S-H), which contributed to strength development at a later age by the filling up of pores in the SBC pastes. The pozzolanic activity of the SBC pastes indicates that it is suitable for use as a substitute for OPC in blended cement.     

A Study on the Treatment of CO2, SO2, Nox,and Fly Ash by Pulse Activation

ABSTRACT

This paper presents a technique for the complete, simultaneous decomposition of CO₂, SO₂, and NO_x, as well as the simultaneous removal of fly ash by ultra-high voltage pulse activation. Ultra-high voltage narrow pulse is used to make the gases in the reactor become active molecules, which are then dissociated into nonpoisonous gas molecules and solid particles under the control of a directional reaction model. By using a sufficient charge and a strong electric field, the fly ash can be removed. It becomes the carrier of C and S, and its efficiency is 99.5%. Owing to the action of catalyst B (using Ni as the mother's body), the activation energy of CO₂, SO₂, and NO_x gases is reduced in great magnitude, and their removal efficiency can reach 75~90% at normal pressure and 180 °C.     

Preparation of Calcium Silicate Absorbent from Iron Blast Furnace Slag

ABSTRACT

Calcium silicate hydrate (CSH) solids were prepared from hydrated lime and iron blast furnace slag in an aqueous agitated slurry at 92 °C. While it was hoped a minimal lime/slag ratio could be used to create near-amorphous CSH, the surface area of the product improved by increasing the lime/slag weight ratio to 2. The addition of gypsum to the lime/slag system dramatically improved the formation of surface area, creating solids with 139 m²/g after 30 hr of reaction when only a minimal amount of lime was present. The SO₂ reactivity of solids prepared with gypsum greatly exceeded that of hydrated lime, achieving greater than 70-80% conversion of the alkalinity after 1 hr of reaction with SO₂. The use of CaCl₂ as an additive to the lime/slag system, in lieu of gypsum, also produced high-surface-area solids, 115 m²/g after 21 hr of reaction. However, the SO₂ reactivity of these sorbents was relatively low given the high surface area. This emphasized that the correlation between surface area and SO₂ reactivity was highly dependent on the solid phase, which was subsequently dependent on slurry composition.     

钙基烟气脱硫剂制备的实验研究

采用正交实验 ,研究了水合制备高效钙基烟气脱硫剂时各制备条件对产物的影响。结果表明 ,水合时间、水合温度Ca(OH) 2 /CaSO4的质量比 ,以及飞灰 / (Ca(OH) 2 +CaSO4)质量比四个条件对脱硫剂比表面积的形成有显著的影响 ;从而由单因素实验得出一最佳钙基脱硫剂制备条件组合。此外 ,通过XDR分析 ,测定了脱硫剂的物相组成 ,扫描电镜观察显示飞灰和水合吸收剂具有不同的表面形态     

Pore Structure Effects on Ca-Based Sorbent Sulfation Capacity at Medium Temperatures: Activated Carbon as Sorbent/Catalyst Support

Abstract

The reaction between three different Ca-based sorbents and SO₂ were studied in a medium temperature range (473–773 K). The largest SO₂ capture was found with Ca(OH)₂ at 773 K, 126.31 mg SO₂?g Ca(OH)₂ ^?1, and the influence of SO₂ concentration on the sorbent utilization was observed. Investigations of the internal porous structure of Ca-based sorbents showed that the initial reaction rate was controlled by the surface area, and once the sul-fated products were produced, pore structure dominated. To increase the surface area of Ca-based sorbents available to interact with and retain SO₂, one kind of CaO/activated carbon (AC) sorbent/catalyst was prepared to study the effect of AC on the dispersion of Ca-based materials. The results indicated that the Ca-based material dispersed on high-surface-area AC had more capacities for SO₂ than unsupported Ca-based sorbents. The initial reaction rates of the reaction between SO₂ and Ca-based sorbents and the prepared CaO/AC sorbents/cata-lysts were measured. Results showed that the reaction rate apparently increased with the presence of AC. It was concluded that CaO/AC was the active material in the des-ulfurization reaction. AC acting as the support can play a role to supply O₂ to increase the affinity to SO₂. Moreover, when AC is acting as a support, the surface oxygen functional group formed on the surface of AC can serve as a new site for SO₂ adsorption.     

SGA对垃圾焚烧飞灰中重金属的固化性能研究简

根据EPA 1311、HJ/T 299-2007、HJ/T 300-2007和HJ 557-2009等国内外不同标准，研究了深圳某垃圾焚烧发电厂垃圾焚烧飞灰的浸出毒性，探讨了六硫代胍基甲酸（sixthio guanidine acid，SGA）、二甲基二硫代氨基甲酸盐（sodium dimethyl dithio carbamate，SDD）和Ca（OH）₂浓度对垃圾焚烧飞灰中重金属的固定性能的影响。研究结果表明，随着浸提液pH的降低，该厂焚烧飞灰中大部分金属元素的浸出量增大，焚烧飞灰浸出液中的Cd、Ni、Pb和Zn浓度分别超过国家危险废物鉴别标准（GB5085.3-2007）规定值的4.75倍、1.47倍、6.72倍和2.20倍，属于危险废弃物，必须进行稳定化处理。当固化剂SGA加入量为0.1 mol/kg时，稳定化后的重金属浸出浓度已经低于危险废物鉴别标准，且对Cd、Cr、Cu和Pb的固化性能优于SDD和Ca（OH）₂；当固化剂SGA、SDD和Ca（OH）₂加入量为0.5 mol/kg时，稳定化后的焚烧飞灰重金属浸出浓度均低于国家危险废物鉴别标准（GB 5085.3-2007）中的规定值。与SDD和Ca（OH）₂相比，SGA对垃圾焚烧飞灰中重金属的固化处理更具有优势。     

Low Concentration Mercury Sorption Mechanisms and Control by Calcium-Based Sorbents: Application in Coal-Fired Processes

ABSTRACT

The capture of elemental mercury (Hg⁰) and mercuric chloride (HgCl₂) by three types of calcium (Ca)-based sor-bents was examined in this bench-scale study under conditions prevalent in coal-fired utilities. Ca-based sorbent performances were compared with that of an activated carbon. Hg⁰ capture of about 40% (nearly half that of the activated carbon) was achieved by two of the Ca-based sorbents. The presence of sulfur dioxide (SO₂) in the simulated coal combustion flue gas enhanced the Hg⁰ capture from about 10 to 40%. Increasing the temperature in the range of 65-100 °C also caused an increase in the Hg⁰ capture by the two Ca-based sorbents. Mercuric chloride (HgCl₂) capture exhibited a totally different pattern. The presence of SO₂ inhibited the HgCl₂ capture by Ca-based sorbents from about 25 to less than 10%. Increasing the temperature in the studied range also caused a decrease in HgCl₂ capture. Upon further pilot-scale confirmations, the results obtained in this bench-scale study can be used to design and manufacture more cost-effective mercury sorbents to replace conventional sorbents already in use in mercury control.     

Hydrogen Sulfide Removal From Hot Producer Gas With Sintered Absorbents

The Bureau of Mines prepared three sintered materials capable of removing H₂S from producer gas at 1000° to 1500°F. They are mixtures of ferric oxide and fly ash, ferric oxide and pumice stone, and red mud (a ferric oxide-containing residue from processing bauxite). All three absorbents were virtually completely regenerable with air. A sintered Fe₂O₃; (25%)-fly ash [75%) mixture was tested through nine H₂S absorption-air regeneration cycles without loss of absorplion capacity or attrition of the pellets. The absorbent with the greatest capacity was a red mud, absorbing 16.0% by weight of sulfur at 1000° F, 24.0% at 1250° F, and 45.1 % at 1 500° F.     

Continuous CO2 capture and MSWI fly ash stabilization, utilizing novel dynamic equipment

Novel dynamic equipment with gas in and out continuously was developed to study the capture capacity of CO₂. Municipal solid waste incineration (MSWI) fly ash has a high capture rate of CO₂ in CO₂-rich gas. Fly ash can sequester pure CO₂ rapidly, and its capacity is 16.3 g CO₂/100 g fly ash with no water added and 21.4 g CO₂/100 g fly ash with 20% water added. For simulated incineration gas containing 12% CO₂, the capture rate decreased and the capacity was 13.2 g CO₂/100 g fly ash with no water added and 18.5 g CO₂/100 g fly ash with 20% water added. After accelerated carbonation, the C and O contents increased, indicating CO₂ capture in the fly ash; CO₂ combines with Ca(OH)₂ to form CaCO₃, which increased the CaCO₃ content from 12.5 to 54.3%. The leaching of Pb markedly decreased from 24.48 to 0.111 mg/L.     

Synthetic SO2 Sorbents for Fluidized-Bed Coal Combustors

The information presented in this paper is directed to engineers who are involved with environmental emissions from coal conversion plants. Synthetic sorbents were investigated as an alternative to natural sorbents (limestone) for the removal of SO₂ from the combustion gas in a fluidized-bed coal combustor. The sulfation rate of a synthetic sorbent, CaO in α-AI₂O₃, was determined as a function of gas composition, temperature, and calcium concentration in the sorbent. The reaction was found to be diffusion-controlled above 850°C and kinetically controlled at lower temperatures. The physical characteristics of the support material have a major effect oh the sulfation kinetics. Porosity measurements indicated that supports containing large pores (>0.2 µm) produced sorbents having high sulfation rates and that pores with diameters less than 0.2 µm did not contribute significantly to the capture of SO₂. The sorbents SrO in α-AI₂O₃ and BaO in α-AI₂O₃ had lower SO₂ capture rates than did CaO in α-AI₂O₃. The alkali metal oxide sorbents K₂O and Na₂O in α-AI₂O₃ captured SO₂ much faster than did the alkaline earth metal oxides.     

Comparison of CaO’s effect on the fate of heavy metals during thermal treatment of two typical types of MSWI fly ashes in China

Both grate and fluidized bed incinerators are widely used for MSW incineration in China. CaO addition for removing hazardous emissions from MSWI flue gas changes the characteristics of fly ash and affects the thermal behavior of heavy metals when the ash is reheated. In the present work, two types of MSWI fly ashes, sampled from both grate and fluidized bed incinerators respectively, were thermal treated at 1023–1323 K and the fate of heavy metals was observed. The results show that both of the fly ashes were rich in Ca and Ca-compounds were the main alkaline matter which strongly affected the leaching behavior of heavy metals. Ca was mostly in the forms of Ca(OH)₂ and CaCO₃ in the fly ash from grate incinerator in which nascent fly ash particles were covered by Ca-compounds. In contrast, the content of Ca was lower in the fly ash from fluidized bed incinerator and Ca was mostly in the form of CaSO₄. Chemical reactions among Ca-compounds caused particle agglomeration in thermal treated fly ash from grate incinerator, restraining the heavy metals volatilization. In thermal treated fly ash from fluidized bed incinerator, Ca was converted into aluminosilicates especially at 1323 K which enhanced heavy metals immobilization, decreasing their volatile fractions as well as leaching concentrations. Particle agglomeration hardly affected the leaching behavior of heavy metals. However, it suppressed the leachable-CaCrO₄ formation and lowered Cr leaching concentration.     

A Combined Ca(OH)2/NH3 Flue Gas Desulfurization Process for High Sulfur Coal: Results of a Pilot Plant Study

The removal of SO₂ with atomization of a slaked lime slurry and supplemental injection of gaseous NH₃ were tested in a conventional spray dryer/baghouse system for SO₂ concentrations of 2000 ppm and 3000 ppm and a 30° F approach to saturation. Results at 3000 ppm of SO₂ showed an average SO₂ removal efficiency of 90.3 percent at a combined stoichiometric ratio of 0.95-1.10 and an average overall sorbent utilization of 91.6 percent. The overall molal ratio of NH₃/SO₂ reaction was found to be 2:1 under the test conditions Particle size analyses, and EP toxicity tests were conducted on the products of the reactions.     

Nickel Speciation of Residual Oil Fly Ash and Ambient Particulate Matter Using X-ray Absorption Spectroscopy

ABSTRACT

The chemical speciation of Ni in fly ash produced from ~0.85 wt % S residual (no. 6 fuel) oils in laboratory (7 kW)- and utility (400 MW)-scale combustion systems was investigated using X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD), and acetate extraction [1 M NaOAc-0.5 M HOAc (pH 5) at 25 °C]-anodic stripping voltammetry (ASV). XAFS was also used to determine the Ni speciation of ambient particulate matter (PM) sampled near the 400-MW system. Based on XAFS analyses of bulk fly ash and their corresponding acetate extraction residue, it is estimated that >99% of the total Ni (0.38 wt %) in the experimentally produced fly ash occurs as NiSO₄-xH₂O, whereas >95% of the total Ni (1.70 and 2.25 wt %) in two fly ash samples from the 400-MW system occurs as NiSO₄-xH₂O and Ni-bearing spinel, possibly NiFe₂O₄. Spinel was also detected using XRD. Acetate extracts most of the NiSO₄-xH₂O and concentrates insoluble NiFe₂O₄ in extraction residue. Similar to fly ash, ambient PM contains NiSO₄-xH₂O and NiFe₂O₄;

however, the proportion of NiSO₄-xH₂O relative to NiFe₂O₄ is much greater in the PM. Results from this and previous investigations indicate that residual oil ash produced in the 7-kW combustion system lack insoluble Ni (e.g., NiFe₂O₄) but are enriched in soluble NiSO₄-xH₂O relative to fly ash from utility-scale systems. This difference in Ni speciation is most likely related to the lack of additive [e.g., Mg(OH)₂] injection and residence time in the 7-kW combustion system.     

Medium-Sulfur Coal and Fly Ash Resistivity

Electric generating plants burning medium-sulfur coal need a way to predict when ESP performance will be limited by high electrical resistivity of the collected fly ash. The main uncertainty in mathematical predictions of fly ash resistivity lies in the marginal effect of naturally occurring SO₃ vapor in the flue gas. This paper results from a project to expand the data base of SO₃/SO₂ concentrations and fly ash resistivities measured in utility fly ash precipitators. Complete data sets are presented from three plants in the Southern Company electric system. In situ resistivity data are compared with laboratory measurements and with two different mathematical predictions of resistivity based on coal and ash analyses. The revised version of the resistivity predictor gives results in good agreement with resistivity values measured both in situ and in the laboratory.     

Inhibition of PCDD/F by adding sulphur compounds to the feed of a hazardous waste incinerator

Sulphur compounds, including (NH₄)₂SO₄ and pyrite, were tested as suppressants in a hazardous waste incineration facility. The test results suggested that adding sulphur compounds only slightly reduced PCDD/F stack emissions; this restricted effect was attributed to the release of fly ash in large amounts during the sulphur adding experiments, i.e., it was due to a malfunctioning of the baghouse filter. Nevertheless, for the combined flow of flue gas + fly ash a reduction of more than 50% was achieved for the total PCDD/F concentrations and the total toxic concentrations, and an even higher inhibition capability was observed for PCDD. Also, a simulation of the thermodynamic equilibrium conditions by sulphur dioxide was conducted in the domain of experimental interest. Deactivation of catalysts, which promote PCDD/F formation, was found to be the dominant inhibition mechanism in low temperature PCDD/F formation. SO₂ could also inhibit the formation of molecular Cl₂ via the Deacon reaction, but that was not the main reason for inhibition.     

Immobilization and volume reduction of heavy metals in municipal solid waste fly ash using nano-size calcium and iron-dispersed reagent

This study was conducted to examine the synthesis and application of novel nano-size calcium/iron-based composite material as an immobilizing and separation treatment of the heavy metals in fly ash from municipal solid waste incineration. After grinding with nano-Fe/Ca/CaO and with nano-Fe/Ca/CaO/[PO₄], approximately 30 wt% and 25 wt% of magnetic fraction fly ash were separated. The highest amount of entrapped heavy metals was found in the lowest weight of the magnetically separated fly ash fraction (i.e., 91% in 25% of treated fly ash). Heavy metals in the magnetic or nonmagnetic fly ash fractions were about 98% and 100% immobilized, respectively. Additionally, scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDS) observations indicate that the main fraction of enclosed/bound materials on treated fly ash includes Ca/PO₄-associated crystalline complexes. After nano-Fe/Ca/CaO/[PO₄] treatment, the heavy metal concentrations in the fly ash leachate were much lower than the Japan standard regulatory limit for hazardous waste landfills. These results appear to be extremely promising. The addition of a nano-Fe/Ca/CaO/PO₄ mixture with simple grinding technique is potentially applicable for the remediation and volume reduction of fly ash contaminated by heavy metals.

Implications: After grinding with nano-Fe/Ca/CaO and nano-Fe/Ca/CaO/[PO₄], approximately 30 wt% and 25 wt% of magnetic fraction fly ash were separated. The highest amount of entrapped heavy metals was found in the lowest weight of the magnetically separated fly ash fraction (i.e., 91% in 25% of treated fly ash), whereas heavy metals either in the magnetic or nonmagnetic fly ash fractions were about 98% and 100% immobilized. These results appear to be very promising, and the addition of nano-Fe/Ca/CaO/PO₄ mixture with simple grinding technique may be considered potentially applicable for the remediation and volume reduction of contaminated fly ash by heavy metals.