An electrochemical system for removing and recovering elemental mercury from a gas stream

The impending EPA regulations on the control of mercury emissions from the flue stacks of coal-burning electric utilities has resulted in the development of numerous advanced mercury control technologies such as sorbent injection and in-situ mercury oxidation. Although these technologies can effectively remove mercury from a flue stack they share, along with many other technologies, the common shortcoming of intermedia pollution transfer i.e. the traffic of mercury from the air phase to the solid phase and the subsequent generation of residue for landfill. This work addresses the need for an integrated system of mercury removal and recovery from flue stack gases and from the environment. The research explored the capture of elemental mercury from the gas phase at ambient temperature on an electrically conductive porous sorbent. The mercury loaded sorbent was regenerated at the anode in an electrochemical cell and the oxidized mercury recovered at the cathode as solid elemental mercury. Activated carbon cloth was selected as the most suitable sorbent as it had the highest mercury adsorption capacity of the sorbents tested and was electrically conductive. Direct and indirect electro-oxidation were shown to remove 95% and 100%, respectively, of the elemental mercury from the carbon cloth. After regeneration the carbon cloth was reused without any loss in mercury adsorption capacity. More than 99% of the mercury stripped from the cloth during regeneration was recovered at the cathode.     

Improvement of the desulfurization and regeneration properties through the control of pore structures of the Zn-Ti-based H2S removal sorbents

To improve the sulfur removing capacity of the conventional Zn-Ti-based H2S removal sorbents, a new Zn-Ti based sorbent (ZT-cp) was prepared by the coprecipitation method and tested in a packed bed reactor at middle temperature conditions (H2S absorption at 480 degrees C, regeneration at 580 degrees C). The new Zn-Ti-based sorbent showed excellent sulfur removing capacity without deactivation, even after 10 cycles of absorption and regeneration. The conventional Zn-Ti-based sorbents (ZT-700, ZT-1000), however, that were prepared by physical mixing, were continuously deactivated. In particular, the initial sulfur removing capacity of the ZT-cp sorbent showed a very high absorption value (0.22 g S/g sorbent), which corresponded to 91.6% of theoretical absorption amount. These results can be explained by the difference in physical properties such as pore volume, surface area, and particle size. It was also found that the sulfides formed from the ZT-cp and ZT-1000 sorbents with spinel structure were easily regenerated even at 580 degrees C. Those from the ZT-700 sorbent, with separated ZnO and TiO2 structures, needed a temperature higher than 610 degrees C for regeneration.     

Influence of high-temperature steam on the reactivity of CaO sorbent for CO₂ capture

Calcium looping is a high-temperature CO(2) capture technology applicable to the postcombustion capture of CO(2) from power station flue gas, or integrated with fuel conversion in precombustion CO(2) capture schemes. The capture technology uses solid CaO sorbent derived from natural limestone and takes advantage of the reversible reaction between CaO and CO(2) to form CaCO(3); that is, to achieve the separation of CO(2) from flue or fuel gas, and produce a pure stream of CO(2) suitable for geological storage. An important characteristic of the sorbent, affecting the cost-efficiency of this technology, is the decay in reactivity of the sorbent over multiple CO(2) capture-and-release cycles. This work reports on the influence of high-temperature steam, which will be present in flue (about 5-10%) and fuel (～20%) gases, on the reactivity of CaO sorbent derived from four natural limestones. A significant increase in the reactivity of these sorbents was found for 30 cycles in the presence of steam (from 1-20%). Steam influences the sorbent reactivity in two ways. Steam present during calcination promotes sintering that produces a sorbent morphology with most of the pore volume associated with larger pores of ～50 nm in diameter, and which appears to be relatively more stable than the pore structure that evolves when no steam is present. The presence of steam during carbonation reduces the diffusion resistance during carbonation. We observed a synergistic effect, i.e., the highest reactivity was observed when steam was present for both calcination and carbonation.     

Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas

Volatile arsenic and selenium compounds present in coals may cause environmental problems during coal combustion and gasification. A possible way to avoid such problems may be the use of solid sorbents capable of retaining these elements from flue gases in gas cleaning systems. Lime and limestone are materials that are extensively employed for the capture of sulfur during coal processing. Moreover, they have also proven to have good retention characteristics for arsenic and selenium during combustion. The aim of this work was to ascertain whether this sorbent is also useful for retaining arsenic and selenium species in gases produced in coal gasification. The study was carried out in a laboratory-scale reactor in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. In these conditions, retention capacities for arsenic may reach 17 mg g(-1) in a gasification atmosphere free of H2S, whereas the presence of H2S implies a significant decrease in arsenic retention. In the case of selenium, H2S does not influence retention which may reach 65 mg g(-1). Post-retention sorbent characterization, thermal stability, and water solubility tests have shown that chemical reaction is one of the mechanisms responsible for the capture of arsenic and selenium, with Ca(AsO2)2 and CaSe being the main compounds formed.     

Steam reactivation of spent CaO-based sorbent for multiple CO2 capture cycles

This study examines steam reactivation of sorbent to improve the reversibility of multiple CaO-CO2 capture cycles. Experiments to obtain spent sorbent were performed in a tube furnace, and reactivation was achieved using steam in a pressurized reactor. Sorbent activity for CO2 capture was then tested in a thermogravimetric analyzer (TGA), in multi-cycle carbonation tests. After reactivation the sorbent had even better characteristics for CO2 capture than that of the natural sorbent. The average carbonation degree over 10 cycles for the reactivated sorbent approached 70%, significantly higher than for the original sorbent (35-40%). This means that the same sorbent may achieve effective CO2 capture over a large number of cycles, in the absence of other phenomena such as attrition. Partially sulfated sorbents may also be reactivated, but hydration itself is also hindered by sulfation.     

Key factor in rice husk Ash/CaO sorbent for high flue gas desulfurization activity

Siliceous materials such as rice husk ash (RHA) have potential to be utilized as high performance sorbents for the flue gas desulfurization process in small-scale industrial boilers. This study presents findings on identifying the key factorfor high desulfurization activity in sorbents prepared from RHA. Initially, a systematic approach using central composite rotatable design was used to develop a mathematical model that correlates the sorbent preparation variables to the desulfurization activity of the sorbent. The sorbent preparation variables studied are hydration period, x1 (6-16 h), amount of RHA, x2 (5-15 g), amount of CaO, x3 (2-6 g), amount of water, x4 (90-110 mL), and hydration temperature, x5 (150-250 degrees C). The mathematical model developed was subjected to statistical tests and the model is adequate for predicting the SO2 desulfurization activity of the sorbent within the range of the sorbent preparation variables studied. Based on the model, the amount of RHA, amount of CaO, and hydration period used in the preparation step significantly influenced the desulfurization activity of the sorbent. The ratio of RHA and CaO used in the preparation mixture was also a significant factor that influenced the desulfurization activity of the sorbent. A RHA to CaO ratio of 2.5 leads to the formation of specific reactive species in the sorbent that are believed to be the key factor responsible for high desulfurization activity in the sorbent. Other physical properties of the sorbent such as pore size distribution and surface morphology were found to have insignificant influence on the desulfurization activity of the sorbent.     

Impact of sulfur oxides on mercury capture by activated carbon

Recent field tests of mercury removal with activated carbon injection (ACI) have revealed that mercury capture is limited in flue gases containing high concentrations of sulfur oxides (SOx). In order to gain a more complete understanding of the impact of SOx on ACl, mercury capture was tested under varying conditions of SO2 and SO3 concentrations using a packed bed reactor and simulated flue gas (SFG). The final mercury content of the activated carbons is independent of the SO2 concentration in the SFG, but the presence of SO3 inhibits mercury capture even at the lowest concentration tested (20 ppm). The mercury removal capacity decreases as the sulfur content of the used activated carbons increases from 1 to 10%. In one extreme case, an activated carbon with 10% sulfur, prepared by H2SO4 impregnation, shows almost no mercury capacity. The results suggest that mercury and sulfur oxides are in competition for the same binding sites on the carbon surface.     

Laboratory tests on mercury emission monitoring with resonating gold-coated silicon cantilevers

To measure extremely low concentrations of mercury vapor in gases as encountered in flue gases of coal-fired power plants, accurate and reliable online and/or portable mercury detection systems are needed. As discussed in this communication, resonating silicon-based cantilevers coated with thin films of gold change their resonant frequency when exposed to mercury vapors and could serve as the basis for such sensing devices. Two different types of commercial AFM cantilevers, which differed by physical dimensions and surface finish, were coated with a 10 nm film of gold and were tested in streams of argon containing mercury. The argon flow rates ranged from 5.7 to 57.4 ml/min, carrying mercury vapors at concentrations between 37 and 700 microg/m3. The results show that smaller cantilevers (approximately 140 microm x 40 microm x 4 microm) with a resonant frequency of 270-275 kHz were sensitive to less than 10 picograms of mercury, whereas larger cantilevers (approximately 245 microm x 50 microm x 7 microm) with a resonant frequency of 155-165 kHz have a sensitivity about 10 times lower. The results indicate that the kinetics of mercury capture by the gold coating follows a simple power law-correlation with the mass change (delta m) being proportional to t(n), where t is the capture time and n depends strongly on the concentration of mercury in the gas. It is also demonstrated that the mercury can be stripped off the gold coating by heating to 350 degrees C, which would allowthe cantilevers to be regenerated and reused.     

Analysis of SO2 sorption capacity of rice husk ash (RHA)/CaO/NaOH sorbents using response surface methodology (RSM): untreated and pretreated RHA

The SO2 sorption capacity (SSC) of sorbents prepared from rice husk ash (RHA) with NaOH as additive was studied in a fixed-bed reactor. The sorbents were prepared using a water hydration method by slurrying RHA, CaO, and NaOH. Response surface methodology (RSM) based on four-variable central composite face centered design (CCFCD) was employed in the synthesis of the sorbents. The correlation between the sorbent SSC (as response) with four independent sorbent preparation variables, i.e. hydration period, RHA/CaO ratio, NaOH amount, and drying temperature, were presented as empirical mathematical models. Among all the variables studied, the amount of NaOH used was found to be the most significant variable affecting the SSC of the sorbents prepared. The SSC for sorbent prepared with the addition of NaOH was found to be significantly higher than sorbents prepared without NaOH. This is probably because NaOH is a deliquescent material, and its existence increases the amount of water collected on the surface of the sorbent, a condition required for sorbent-SO2 reaction to occur at low temperature. The effect of further treatment of RHA at 600 degrees C was also investigated. Although pretreated RHA sorbents demonstrated higher SSC as compared to untreated RHA sorbents, nevertheless, at optimum conditions, sorbents prepared from untreated RHA was found to be more favorable due to practical and economic concerns.     

Development of a mercury transformation model in coal combustion flue gas

A bench-scale entrained-flow reactor was used to extract flue gas produced by burning a subbituminous Belle Ayr coal in a 580-MJ/h combustion system. The reactor was operated at 400 degrees, 275 degrees, and 150 degrees C with a flow rate corresponding to residence times of 0-7 s. Transformations of elemental mercury (Hg0) and total gas mercury (Hg(gas)) in the reactor were evaluated as functions of temperature and residence time. The most significant mercury transformations (Hg0 to Hg(p) and Hg0 to Hg2+) occurred at 150 degrees C, while virtually no obvious mercury transformations were observed at 275 degrees and 400 degrees C. Approximately 30% of total mercury has been oxidized at temperatures higher than 400 degrees C. A mass transfer-capacity limit model was developed to quantify in-flight mercury sorption on fly ash in flue gas at different temperatures. A more sophisticated model was developed to demonstrate not only the temperature and residence time effects but also to consider the effective surface area of fly ash and dependence of mercury vapor concentration on mercury transformations in flue gas. The reaction orders were 0.02 and 0.55 for Hg0 and Hg(gas), respectively. Only a few percent of the total surface area of the fly ash, in the range of 1%-3%, can effectively adsorb mercury vapor.     

Iron blast furnace slag/hydrated lime sorbents for flue gas desulfurization

Sorbents prepared from iron blast furnace slag (BFS) and hydrated lime (HL) through the hydration process have been studied with the aim to evaluate their reactivities toward SO2 under the conditions prevailing in dry or semidry flue gas desulfurization processes. The BFS/HL sorbents, having large surface areas and pore volumes due to the formation of products of hydration, were highly reactive toward SO2, as compared with hydrated lime alone (0.24 in Ca utilization). The sorbent reactivity increased as the slurrying temperature and time increased and as the particle size of BFS decreased; the effects of the liquid/solid ratio and the sorbent drying conditions were negligible. The structural properties and the reactivity of sorbent were markedly affected by the BFS/HL ratio; the sorbent with 30/70 ratio had the highest 1 h utilization of Ca, 0.70, and SO2 capture, 0.45 g SO2/g sorbent. The reactivity of a sorbent was related to its initial specific surface area (Sg0) and molar content of Ca (M(-1)); the 1 h utilization of Ca increased almost linearly with increasing Sg0/M. The results of this study are useful to the preparation of BFS/HL sorbents with high reactivity for use in the dry and semidry processes to remove SO2 from the flue gas.     

Capture of gas-phase arsenic oxide by lime: kinetic and mechanistic studies

Trace metal emission from coal combustion is a major concern for coal-burning utilities. Toxic compounds such as arsenic species are difficult to control because of their high volatility. Mineral sorbents such as lime and hydrated lime have been shown to be effective in capturing arsenic from the gas phase over a wide temperature range. In this study, the mechanism of interaction between arsenic oxide (As2O3) and lime (CaO) is studied over the range of 300-1000 degrees C. The interaction between these two components is found to depend on the temperature; tricalcium orthoarsenate (Ca3As2O8) is found to be the product of the reaction below 600 degrees C, whereas dicalcium pyroarsenate (Ca2As2O7) is found to be the reaction product in the range of 700-900 degrees C. Maximum capture of arsenic oxide is found to occur in the range of 500-600 degrees C. At 500 degrees C, a high reactivity calcium carbonate is found to capture arsenic oxide by a combination of physical and chemical adsorption. Intrinsic kinetics of the reaction between calcium oxide and arsenic oxide in the medium-temperature range of 300-500 degrees C is studied in a differential bed flow-through reactor. Using the shrinking core model, the order of reaction with respect to arsenic oxide concentration is found to be about 1, and the activation energy is calculated to be 5.1 kcal/mol. The effect of initial surface area of CaO sorbent is studied over a range of 2.7-45 m2/g using the grain model. The effect of other major acidic flue gas species (SO2 and HCl) on arsenic capture is found to be minimal under the conditions of the experiment.     

XAS and XPS characterization of mercury binding on brominated activated carbon

Brominated powdered activated carbon sorbents have been shown to be quite effective for mercury capture when injected into the flue gas duct at coal-fired power plants and are especially useful when burning Western low-chlorine subbituminous coals. X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) have been used to determine information about the speciation and binding of mercury on two commercially available brominated activated carbons. The results are compared with similar analysis of a conventional (non-halogenated) and chlorinated activated carbon. Both the XAS and XPS results indicate that the mercury, though introduced as elemental vapor, is consistently bound on the carbon in the oxidized form. The conventional and chlorinated activated carbons appeared to contain mercury bound to chlorinated sites and possibly to sulfate species that have been incorporated onto the carbon from adsorbed SO2. The mercury-containing brominated sorbents appear to contain mercury bound primarily at bromination sites. The mechanism of capture for the sorbents likely consists of surface-enhanced oxidation of the elemental mercury vapor via interaction with surface-bound halide species with subsequent binding by surface halide or sulfate species.     

Nanostructured sorbents for capture of cadmium species in combustion environments

The pathways of cadmium species to form a submicrometer-sized aerosol in a combustion system exhaust were established. Cadmium oxide was the predominant species formed in the experiments and resulted in particles of a mean size of 26-63 nm with number concentrations in the range of 2-8 x 10(6) cm(-3). Two different nanostructured sorbents, a solid montmorillonite (MMT) and an in situ generated agglomerated silica, were used for capture of the cadmium species. The MMT sorbent was not stable at 1000 degrees C, and structural changes resulted. MMT did not suppress nucleation of cadmium species and partially captured it by weak physisorption as established by the leachability tests. In contrast, the in situ generated silica nanostructured agglomerates had a high surface area, suppressed nucleation of cadmium species vapors, and chemisorbed them effectively resulting in a firm binding, as compared to the MMT sorbent. There is an optimal temperature-time relationship at which the capture process is expected to be most effective. The leaching efficiency under these conditions was less than 3.2%. The nanostructured silica agglomerate size can be tuned for effective capture in existing particle control devices.     

Development of regenerable MgO-based sorbent promoted with K2CO3 for CO2 capture at low temperatures

To improve their CO2 absorption capacity, alkali-based sorbents prepared by impregnation and wet mixing method of potassium carbonate on supports such as activated carbon and MgO (KACI30, KACP30, KMgI30, and KMgP30), were investigated in a fixed bed reactor (C02 absorption at 50-100 degrees C and regeneration at 150-400 degrees C). Total CO2 capture capacities of KMgI30-500 and KMgP30-500 were 178.6 and 197.6 mg CO2/g sorbent, respectively, in the presence of 11 vol % H2O even at 50 degrees C. The large amount of CO2 capture capacity of KMgP30-500 and KMgI30-500 could be explained by the fact that MgO itself, as well as K2CO3, could absorb CO2 in the presence of water vapor even at low temperatures. In particular, water vapor plays an important role in the CO2 absorption of MgO and KMgI30-500 even at low temperatures below 60 degrees C, in marked contrast to MgO and CaO which can absorb CO2 at high temperatures. The CO2 capture capacity of the KMgI30-300 sorbent, however, was less than that of KMgI30-500 due to the formation of Mg(OH)2 which did not absorb CO2. MgO based-sorbents promoted with K2CO3 after CO2 absorption formed new structures such as K2Mg(CO3)2 and K2Mg(CO3)2 x 4(H2O), unlike KACI30 which showed only the KHCO3 crystal structure. The new Mg-based sorbents promoted with K2CO3 showed excellent characteristics in that it could satisfy a large amount of CO2 absorption at low temperatures, a high CO2 absorption rate, and fast and complete regeneration.     

Development of a Cl-impregnated activated carbon for entrained-flow capture of elemental mercury

Efforts to discern the role of an activated carbon's surface functional groups on the adsorption of elemental mercury (Hg0) and mercuric chloride demonstrated that chlorine (Cl) impregnation of a virgin activated carbon using dilute solutions of hydrogen chloride leads to increases (by a factor of 2-3) in fixed-bed capture of these mercury species. A commercially available activated carbon (DARCO FGD, NORITAmericas Inc. [FGD])was Cl-impregnated (Cl-FGD) [5 lb (2.3 kg) per batch] and tested for entrained-flow, short-time-scale capture of Hg0. In an entrained flow reactor, the Cl-FGD was introduced in Hg0-laden flue gases (86 ppb of Hg0) of varied compositions with gas/solid contact times of about 3-4 s, resulting in significant Hg0 removal (80-90%), compared to virgin FGD (10-15%). These levels of Hg0 removal were observed across a wide range of very low carbon-to-mercury weight ratios (1000-5000). Variation of the natural gas combustion flue gas composition, by doping with nitrogen oxides and sulfur dioxide, and the flow reactor temperature (100-200 degrees C) had minimal effects on Hg0 removal bythe Cl-FGD in these carbon-to-mercury weight ratios. These results demonstrate significant enhancement of activated carbon reactivity with minimal treatment and are applicable to combustion facilities equipped with downstream particulate matter removal such as an electrostatic precipitator.     

Thermal activation of CaO-based sorbent and self-reactivation during CO2 capture looping cycles

In this study, the thermal activation of different types of CaO-based sorbents was examined. Pretreatments were performed at different temperatures (800--1300 degrees C) and different durations (6--48 h) using four Canadian limestones. Sieved fractions of the limestones, powders obtained by grinding, and hydroxides produced following multiple carbonation/calcination cycles achieved in a tube furnace were examined. Pretreated samples were evaluated using two types of thermogravimetric reactors/ analyzers. The most important result was that thermal pretreatment could improve sorbent performance. In comparison to the original, pretreated sorbents showed better conversions over a longer series of CO2 cycles. Moreover, in some cases, sorbent activity actually increased with cycle number, and this effectwas especially pronounced for powdered samples preheated at 1000 degrees C. In these experiments, the increase of conversion with cycle number (designated as self-reactivation) after 30 cycles produced samples that were approximately 50% carbonated for the four sorbents examined here, and there appeared to be the potential for additional increase. These results were explained with the newly proposed pore--skeleton model. This model suggests, in addition to changes in the porous structure of the sorbent, that changes in the pore--skeleton produced during pretreatment strongly influence subsequent carbonation/ calcination cycles.     

Novel mercury control technology for solid waste incineration: sodium tetrasulfide (STS) as mercury capturing agent

Traditional pollution control technologies are able to capture oxidized forms of mercury to some extent; however, they show low efficiency for the control of elemental mercury emissions. This study developed a novel mercury removal technology: injection of sodium tetrasulfide (Na2S4) dissolved in the sodium hydroxide (NaOH) solution in the spray-dryer system. The effects of flue gas temperature and Na2S4 level in flue gas on the mercury removal efficiency were investigated. Na2S4 was decomposed into Na2S (S2-) and elemental S (S0), which reacted with HgCl2 and elemental Hg (Hg0), and HgS was then formed. Under the optimized operation parameters, this technology can simultaneously remove over 88% of HgCl2 and more than 90% of Hg0 from a flue gas stream containing about 400 microg m(-3) Hg0 and 1200 microg m(-3) HgCl2. The increased flue gas temperature (>170 degrees C) and the decreased Na2S4-to-Hg mass ratio (S-Hg-R) (<2.0) had negative effects on the reactions of gaseous mercury (HgCl2 + Hg0) with ionic sulfur (S2-) and S0. All the experiments were conducted in a full scale hospital-waste incinerator with a capability of 20 tons per day (TPD).     

Development of cost-effective noncarbon sorbents for Hg(0) removal from coal-fired power plants

Noncarbonaceous materials or mineral oxides (silica gel, alumina, molecular sieves, zeolites, and montmorillonite) were modified with various functional groups such as amine, amide, thiol, urea, and active additives such as elemental sulfur, sodium sulfide, and sodium polysulfide to examine their potential as sorbents for the removal of elemental mercury (Hg(0)) vapor at coal-fired utility power plants. A number of sorbent candidates such as amine- silica gel, urea- silica gel, thiol- silica gel, amide-silica gel, sulfur-alumina, sulfur-molecular sieve, sulfur-montmorillonite, sodium sulfide-montmorillonite, and sodium polysulfide-montmorillonite, were synthesized and tested in a lab-scale fixed-bed system under an argon flow for screening purposes at 70 degrees C and/or 140 degrees C. Several functionalized silica materials reported in previous studies to effectively control heavy metals in the aqueous phase showed insignificant adsorption capacities for Hg(0) control in the gas phase, suggesting that mercury removal mechanisms in both phases are different. Among elemental sulfur-, sodium sulfide-, and sodium polysulfide-impregnated inorganic samples, sodium polysulfide-impregnated montmorillonite K 10 showed a moderate adsorption capacity at 70 degrees C, which can be used for sorbent injection prior to the wet FGD system.     

Particle size distribution effects on gas-particle mass transfer within electrostatic precipitators

Varying degrees of mercury capture and transformation have been reported across electrostatic precipitators (ESPs). Previous analyses have shown that the dominant mass transfer mechanism responsible for mercury capture within ESPs is gas-particle mass transfer during particulate collection. Whereas previous analyses assumed dispersions of uniform size, the present analysis reveals the effects of polydispersity on both gas-particle mass transfer and particle collection within an ESP. The analysis reveals that the idealized monodisperse particle size distribution provides the highest gas-particle mass transfer but results in the lowest particle collection efficiency (% mass). As the particle size distribution broadens, gas-particle mass transfer decreases and particle collection efficiency increases. The results suggest that more than just reporting mean particle diameter provided by the sorbent manufacturer, pilot- and field-tests of sorbent injection for mercury emissions control need to experimentally measure the particle size distribution of the sorbent as it is injected in order to facilitate interpretation of their results.