Mercury capture by native fly ash carbons in coal-fired power plants.

The control of mercury in the air emissions from coal-fired power plants is an on-going challenge. The native unburned carbons in fly ash can capture varying amounts of Hg depending upon the temperature and composition of the flue gas at the air pollution control device, with Hg capture increasing with a decrease in temperature; the amount of carbon in the fly ash, with Hg capture increasing with an increase in carbon; and the form of the carbon and the consequent surface area of the carbon, with Hg capture increasing with an increase in surface area. The latter is influenced by the rank of the feed coal, with carbons derived from the combustion of low-rank coals having a greater surface area than carbons from bituminous- and anthracite-rank coals. The chemistry of the feed coal and the resulting composition of the flue gas enhances Hg capture by fly ash carbons. This is particularly evident in the correlation of feed coal Cl content to Hg oxidation to HgCl2, enhancing Hg capture. Acid gases, including HCl and H2SO4 and the combination of HCl and NO2, in the flue gas can enhance the oxidation of Hg. In this presentation, we discuss the transport of Hg through the boiler and pollution control systems, the mechanisms of Hg oxidation, and the parameters controlling Hg capture by coal-derived fly ash carbons.

Carbene proton attachment energies: theoretical study.

The geometries and electronic energies of six singlet carbenes, with methyl and phenyl substituents, and the corresponding carbenium ions were obtained using several density functional theory (DFT) variants and the second-order Møller-Plesset method for electron correlation and compared with G3 results, with the aim to determine a relatively low-cost computational protocol that is sufficiently accurate for the specific molecules and ions of interest. Some additional calculations were performed at the CCSD(T) level. Results for diphenylcarbene, methylphenylcarbene, and their cations, which were not previously investigated by ab initio methods, are reported as are calculations on methylene, methylcarbene, dimethylcarbene, and phenylcarbene. The MPW3LYP/6-311+G(d,p) hybrid DFT level was found to give results that were in close agreement with those obtained using G3 theory, with a mean absolute deviation (MAD) of 1.76 kcal/mol for the calculated proton attachment energies (PAEs). Equilibrium geometries obtained with this method were compared with those obtained at the MP2/6-311G(d,p) level of theory, and bond lengths and bond angles had MADs of 0.005 A and 1.0 degrees, respectively. Harmonic vibrational frequencies of all the carbene molecules and the corresponding ions were computed to verify that the stationary points were true minima, to obtain zero-point corrected energies, to assist in infrared studies of the molecules. The recommended combination of method and basis set is expected to be a useful framework that uses modest amounts of computer resources to obtain usable thermochemical data on moderate-sized hydrocarbons and hydrocarbon cations, including coal-mimetic species.

Effects of sulfur dioxide and nitric oxide on mercury oxidation and reduction under homogeneous conditions.

This paper is particularly related to elemental mercury (Hg0) oxidation and divalent mercury (Hg2+) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO2). As a powerful oxidant and chlorinating reagent, Cl2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO2, NO, as well as H2O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO2 and NO on Hg0 oxidation and Hg2+ reduction with the intent of unraveling unrecognized interactions among Cl species, SO2, and NO most importantly in the presence of H2O. The experimental results demonstrated that SO2 and NO had pronounced inhibitory effects on Hg0 oxidation at high temperatures when H2O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg2+ back into its elemental form. Data revealed that SO2 and NO were capable of promoting homogeneous reduction of Hg2+ to Hg0 with H2O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H2O was removed from the gas blend.

Surface compositions of carbon sorbents exposed to simulated low-rank coal flue gases.

Bench-scale testing of elemental mercury (Hg0) sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg0. The results of the fixed-bed testing under simulated lignite combustion flue gas composition for activated carbons showed some initial breakthrough followed by increased mercury (Hg) capture for up to approximately 4.8 hr. After breakthrough, the Hg in the effluent stream was primarily in an oxidized form (>90%). Aliquots of selected activated carbons were exposed to simulated flue gas containing Hg0 vapor for varying time intervals to explore surface chemistry changes as the initial breakthrough, Hg capture, and oxidation occurred. The samples were analyzed by X-ray photoelectron spectroscopy to determine changes in the abundance and forms of sulfur, chlorine, oxygen, and nitrogen moieties as a result of interactions of flue gas components on the activated carbon surface during the sorption process. The data are best explained by a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site. Because loss of HCl is also coincident with Hg breakthrough or loss of the divalent Hg ion (Hg2+), the competition of Hg2+ with S(VI) on the basic carbon site is also implied. Thus, the role of the acid gases in Hg capture and release can be explained.

Ester fuels and chemicals from biomass.

Bench-scale research demonstrated that using an efficient esterification step to integrate an ethanol with a carboxylic acid fermentation stream offers potential for producing valuable ester feedstocks and fuels. Polar organic acids from bacterial fermentations are difficult to extract and purify, but formation of the ammonium salts and their conversion to esters facilitates the purifications. An improved esterification procedure gave high yields of esters, and this method will lower the cost of ester production. Fuel characteristics have been determined for a number of ester-gasoline blends with promising results for lowering Reid vapor pressure and raising octane numbers.

Higher-alcohols biorefinery: improvement of catalyst for ethanol conversion.

The concept of a biorefinery for higher-alcohol production is to integrate ethanol and methanol formation via fermentation and biomass gasification, respectively, with conversion of these simple alcohol intermediates into higher alcohols via the Guerbet reaction. 1-Butanol results from the self-condensation of ethanol in this multistep reaction occurring on a single catalytic bed. Combining methanol with ethanol gives a mixture of propanol, isobutanol, and 2-methyl-1-butanol. All of these higher alcohols are useful as solvents, chemical intermediates, and fuel additives and, consequently, have higher market values than the simple alcohol intermediates. Several new catalysts for the condensation of ethanol and alcohol mixtures to higher alcohols were designed and tested under a variety of conditions. Reactions of methanol-ethanol mixtures gave as high as 100% conversion of the ethanol to form high yields of isobutanol with smaller amounts of 1-propanol, the amounts in the mixture depending on the starting mixture. The most successful catalysts are multifunctional with basic and hydrogen transfer components.

Application of gold catalyst for mercury oxidation by chlorine.

This paper discusses a recent study of mercury catalytic oxidation by chlorinating reagents. Gold was chosen as the catalyst because of its reluctance to chemisorb some gases such as O2, NO, H2O, and SO2. This property, as demonstrated in this study, is instrumental to mercury oxidation by circumventing some undesired inhibitory reactions such as OH + NO + M --> HONO + M and OH + SO2 + M --> HOSO2 + M, which were recognized under homogeneous situations at high temperatures. In comparison to Cl2, HCl showed weak oxidizing capability but appreciable inhibition in mercury oxidation by Cl2, probably through the competition of active sites with Cl2. Overall, the mercury catalytic oxidation by Cl2 on gold catalyst surfaces was viable, reaching 40-60% in this study under temperatures of 448-498 K, where the thermal decomposition of formed Hg2+ was effectively avoided.

On the analysis of mercuric nitrate in flue gas by GC-MS.

Recent research has demonstrated that in a simulated flue gas stream containing NO(2) and SO(2) elemental mercury is initially captured on a carbon or manganese oxide sorbent. After approximately an hour, however, mercury breaks through relatively rapidly, and the volatile form of mercury emitted is an oxidized species. The volatile mercury species emitted from a granular MnO(2) sorbent was trapped in an impinger containing cold acetonitrile. Subsequent evaporation of 95% of the acetonitrile in a Kuderna-Danish apparatus and gas chromatography (GC) of the concentrate resulted in a single mercury-containing GC peak at 5.5 min; the retention time and mass spectrum of this compound matched exactly those of a standard mercury(II) nitrate hydrate, Hg(NO(3))(2).H(2)O dissolved in acetonitrile. The volatile mercury component analyzed from injection of this standard solution was shown to be a form of methylmercury that is produced in the GC column by reaction of the highly reactive mercury nitrate with the methylsiloxane GC phase. Because the on-column derivatization reaction seems to be unique to mercury nitrate, the GC-MS (gas chromatography-mass spectroscopic) analysis provides strong evidence for identification of the trapped oxidized mercury species as mercury nitrate although, because the nitrate becomes detached from the mercury atom in the on-column reaction, the identity is not proven.

Elucidation of the metabolic pathway for dibenzothiophene desulphurization by Rhodococcus sp. strain IGTS8 (ATCC 53968).

Rhodococcus sp. strain IGTS8 (ATCC 53968) is able to utilize dibenzothiophene (DBT) as a sole source of sulphur. The carbon skeleton of DBT is not metabolized and is conserved as 2-hydroxybiphenyl (HBP), which accumulates in the medium. This phenotype is due to the expression of the plasmid-encoded DBT-desulphurization (dsz) operon, which encodes three proteins, DszA, B and C. In this paper it is shown, using [35S]DBT radiolabelling studies, that sulphur is released in the form of inorganic sulphite. The pathway of DBT desulphurization is described in detail. In summary, DszC catalyses the stepwise S-oxidation of DBT, first to dibenzothiophene 5-oxide (DBTO) and then to dibenzothiophene 5,5-dioxide (DBTO2); DszA catalyses the conversion of DBTO2 to 2-(2'-hydroxyphenyl)benzene sulphinate (HBPSi-) and DszB catalyses the desulphination of HBPSi- to give HBP and sulphite. Studies with cell-free extracts show that DszA and DszC, but not DszB, require NADH for activity. 18O2-labelling studies show that each incorporated oxygen atom is derived directly from molecular oxygen. These results are consistent with the role of DszC as a mono-oxygenase, of DszA as an apparently unique enzyme which catalyses the reductive hydroxylation of DBTO2 leading to cleavage of the thiophene ring, and of DszB as an aromatic sulphinic acid hydrolase.