Techno-economic and sustainability analysis of siloxane removal from landfill gas used for electricity generation.

A technoeconomic analysis (TEA) and life cycle assessment (LCA) was conducted on the use of landfill gas (LFG) for electricity generation using an internal combustion engine. This study provides insights that can guide LFG waste to energy (WTE) operators on decisions concerning installation of contaminant removal from LFG for electricity generation. Four scenarios were analyzed; the first (Scenario 1) was a facility with a single siloxane removal unit (SREU) sized for 6 months of continuous use, the second (Scenario 2) was a facility with parallel SREUs sized for one month of use, the third (Scenario 3) was a facility with no SREU, and the fourth was a facility that flared all LFG captured. The TEA revealed that the chiller cost was over 50% the total purchase cost of the LFG pre-treatment system. When the complete LFG to electricity process was analyzed, the internal combustion engine had the highest percentage of total capital investment and the total annual cost. For the base case, it became economically beneficial to install a SREU at facilities with LFG flowrates greater than ∼2000 m3/h. Sensitivity analysis showed that at a base case of 1700 m3/h, LFG (50% CH4), and 50 mg/m3 D4, the net income of facilities in Scenarios 1 to 3 became positive at an electricity sales price greater than 5.5 cents/kWh. LCA revealed that Scenario 2 had the greatest CO2 emission reduction. Scenario 3 is observed to save less CO2 emissions as biogas flowrate increases due to frequent engine shutdowns. Although there are differences in the global warming potential (GWP 100) for Scenarios 1 to 3, with Scenario 2 being the best and Scenario 3 being the worst, the differences are very small. For this reason, economics alone are sufficient in decision making.

Intrinsically strained noble metal-free oxynitrides for solar photoreduction of CO2.

Metal oxynitrides show promising activity for photocatalytic solar water splitting and CO2 reduction under solar irradiance. Precise control of cation ratios in oxynitrides is an inevitable challenge that needs to be overcome for achieving effective band gap tuning. Here we report the density functional theory-based calculations for the intricate structure-function relationships of Zn-Ga based oxynitrides and correlate the results with the experimental parameters. Crucial material property descriptors such as elemental composition, intrinsic lattice strain, and vacancy defects were exploited during the synthesis to achieve stable oxynitride photocatalysts that demonstrated CO2 conversion to CO under simulated solar light, without any noble metal impregnation. The highest CO production rate surpassed that of TiO2 under the same conditions. This work inspires future research on oxynitride materials with tailored optical properties and sustainable photocatalytic activity which enables their large scale applications.

Design and analysis of siloxanes removal by adsorption from landfill gas for waste-to-energy processes.

Separation of volatile methyl siloxanes from landfill gas using fixed adsorption beds was modeled with the objective of identifying appropriate technology and the economics associated with this purification step. A general adsorption model assuming plug flow and radial symmetry was developed and used to conduct a parametric sweep of 162 unique cases. The varied parameters were adsorbent type (activated carbon and silica gel), bed height (3.05-9.15 m/10-30 ft), inlet siloxane concentration (5-15 mg/m3), moisture content (0-100% relative humidity at STP or RH), and siloxane tolerance limit (0.094-9.4 mg/m3) that correlated to three distinct energy conversion technologies (electricity production using engines or fuels cells or catalytic conversion to liquid hydrocarbon fuels). Due to the detrimental effect of RH on siloxane absorption, the maximum allowable moisture content of LFG before purification is 50% RH and moisture removal processes are also required. The design calculations using a selected case study show that the adsorption bed height required needed for 6 months minimum breakthrough time for catalytic fuel production is twice that for engine applications. Fuel cell applications require 3 times the bed height compared to engine applications. However, the purification costs amounted to 94%, 16% and 52% of recovered product value for engine, liquefaction, and fuel cell applications, respectively indicating the need for a high value product to justify purification costs. The approaches and conclusions can be extended to specific process conditions for landfill gas purification and to other processes that use biogas produced from waste as a feedstock.

Requirements, techniques, and costs for contaminant removal from landfill gas.

Waste-to-energy projects are an increasingly prominent component of future energy portfolios. Landfill gas (LFG)-to-energy (LFGTE) projects are particularly important as they address greenhouse gas emissions. Contaminants in LFG may hamper these projects both from environmental and economic standpoints. The purpose of this review is to highlight key aspects (LFG composition ranges, LFG flowrates, and allowable tolerances for LFGTE technologies, performance and costs for contaminant removal by adsorption). Removal of key contaminants, H2S and siloxanes, by adsorption are surveyed in terms of adsorption capacities and regeneration abilities. Based on the open literature, costing analyses are tabulated and discussed. The findings indicate economics of contaminant removal depend heavily on the feed concentrations of contaminants, allowable tolerances for the LFGTE technology, and the current market for the product. Key trends, identification of challenges, and general purification guidelines for purifying LFG for energy projects are also discussed.

Iron chelation by polyamidoamine dendrimers: a second-order kinetic model for metal-amine complexation.

This study presents a kinetic model of the chelation of iron ions by generation 4 hydroxyl-terminated polyamidoamine (PAMAM) with ethylenediamine core (G4-OH). The coordination processes of iron ions from ferric chloride, FeCl(3), and ferrous bromide, FeBr(2), to G4-OH dendrimers were analyzed using ultraviolet-visible (UV-vis) spectroscopy, proton nuclear magnetic resonance ((1)H NMR) spectroscopy, and liquid chromatography-mass spectrometry (LC-MS). In the visible region, a charge-transfer was observed when the dendrimer was added to a ferric chloride solution. This phenomenon is a ligand-to-metal charge-transfer (LMCT) between the free electron group of the dendrimer's internal amines and the dehalogenated iron ion that takes 2 h to complete at room temperature. Analysis of potential rate laws and diffusion effects led to a second-order kinetic model for this reaction. By measuring the rate coefficients as a function of temperature (22-37 °C), an apparent activation energy of 41.5 kJ/mol was obtained using the Arrhenius method. The results of this study will fuel research of PAMAM dendrimers for environmental, pharmaceutical, and materials applications.

Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles.

A continuing goal in catalysis is to unite the advantages of homogeneous and heterogeneous catalytic processes. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl(2), and catalyse a range of π-bond activation reactions previously only catalysed through homogeneous processes. Multiple experimental methods are used to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, a size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared with larger, polymer-capped analogues.

Deactivation mechanisms of Ni-based tar reforming catalysts as monitored by X-ray absorption spectroscopy.

Deactivation mechanisms of alumina-supported, Ni-based catalysts for tar reforming in biomass-derived syngas were evaluated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Catalysts were characterized before and after catalytic reaction cycles and regeneration procedures, which included oxidation by a mixture of steam and air, and reduction in hydrogen. Qualitative analysis of the EXAFS spectra revealed that oxidation of a portion of the Ni in the catalysts to form an oxide phase and/or a sulfide phase were likely scenarios that led to catalyst deactivation with time-on-stream and with increased reaction cycles. Deactivation through carbon deposition, phosphorus poisoning, or changes in particle size were deemed as unlikely causes. Quantitative analysis of the EXAFS spectra indicated sulfur poisoning occurred with time-on-stream, and the contaminating species could not be completely removed during the regeneration protocols. The results also verified that Ni-containing oxide phases (most likely a spinel also containing Mg and Al) formed and contributed to the deactivation. This study validates the need for developing catalyst systems that will protect Ni from sulfur poisoning and oxide formation at elevated reaction and regeneration temperatures.

Sub-10 nm platinum nanocrystals with size and shape control: catalytic study for ethylene and pyrrole hydrogenation.

Platinum nanocubes and nanopolyhedra with tunable size from 5 to 9 nm were synthesized by controlling the reducing rate of metal precursor ions in a one-pot polyol synthesis. A two-stage process is proposed for the simultaneous control of size and shape. In the first stage, the oxidation state of the metal ion precursors determined the nucleation rate and consequently the number of nuclei. The reaction temperature controlled the shape in the second stage by regulation of the growth kinetics. These well-defined nanocrystals were loaded into MCF-17 mesoporous silica for examination of catalytic properties. Pt loadings and dispersions of the supported catalysts were determined by elemental analysis (ICP-MS) and H(2) chemisorption isotherms, respectively. Ethylene hydrogenation rates over the Pt nanocrystals were independent of both size and shape and comparable to Pt single crystals. For pyrrole hydrogenation, the nanocubes enhanced ring-opening ability and thus showed a higher selectivity to n-butylamine as compared to nanopolyhedra.

Structure sensitivity of carbon-nitrogen ring opening: impact of platinum particle size from below 1 to 5 nm upon pyrrole hydrogenation product selectivity over monodisperse platinum nanoparticles loaded onto mesoporous silica.

Well-defined platinum nanoparticles between 0.8 and 5.0 nm were prepared using dendrimer and polymer capping agents and supported onto mesoporous SBA-15 silica. Using these model catalysts, pyrrole hydrogenation was demonstrated to be structure sensitive because ring opening occurred more easily over larger particles compared to smaller ones. The phenomenon is caused by surface roughness or electronic effects that change with particle size.

Dendrimer templated synthesis of one nanometer Rh and Pt particles supported on mesoporous silica: catalytic activity for ethylene and pyrrole hydrogenation.

Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as approximately 1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H 2 after reduction (76 torr of H 2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluated for pyrrole hydrogenation.

Highly selective synthesis of catalytically active monodisperse rhodium nanocubes.

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br- ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.