Design and Preliminary Results of an Atmospheric-Pressure Model Gas Turbine Combustor Utilizing Varying CO2 Doping Concentration in CH4 to Emulate Biogas Combustion

Utilization of biogas can provide a source of renewable energy in both heat and power generation. Combustion of biogas in land-based gas turbines for power generation is a promising approach to reducing greenhouse gases and US dependence on foreign-source fossil fuels. Biogas is a byproduct from the decomposition of organic matter and consists primarily of CH4 and large amounts of CO2. The focus of this research was to design a combustion device and investigate the effects of increasing levels of CO2 addition to the combustion of pure CH4 with air. Using an atmospheric-pressure, swirl-stabilized dump combustor, emissions data and flame stability limitations were measured and analyzed. In particular, CO2, CO, and NOx emissions were the main focus of the combustion products. Additionally, the occurrence of lean blowout and combustion pressure oscillations, which impose significant limitations in operation ranges for actual gas turbines, was observed. Preliminary kinetic and equilibrium modeling was performed using Cantera and CEA for the CH4/CO2/Air combustion systems to analyze the effect of CO2 upon adiabatic flame temperature and emission levels. The numerical and experimental results show similar dependence of emissions on equivalence ratio, CO2 addition, inlet air temperature, and combustor residence time. (C) 2014 Elsevier Ltd. All rights reserved.

Single- and double-electron transfer reactions of ground and metastable state Ar2+ ions

Electron transfer cross sections have been measured for reactions of Ar2+ ions with Ar, N2, O2, CO2, CH4 and C2H6. Time-of-flight techniques have been used to measure both fast neutral Ar0 and fast Ar+ products from single- and double-electron transfer processes involving Ar2+ ions with 4.0 to 7.0 keV impact energies. Incident Ar2+ ions have produced by controlled electron impact ionisation of argon atoms. Reactions have been examined as a function of ionising electron energy and cross sections determined for ground state Ar2+(3P) ions. Charge transfer cross sections have been determined to be in the range of 3*10-16 cm2 for the systems examined. Double-electron transfer cross sections are the same order of magnitude as those measured for the corresponding single-electron transfer reactions. The state distribution of the reactant ion beam has been estimated and electron transfer cross sections obtained for single- and double-electron transfer reactions of metastable Ar2+ions. The magnitudes of electron transfer cross sections in individual systems are similar for both ground and metastable state Ar2+ reactions.

Sensitivity of charge transfer reactions to hydrocarbon ion structures

Charge transfer reactivities of hydrocarbon ions have been measured with time-of-flight techniques, and results correlated with theoretical structures computed by self-consistent field molecular orbital methods. Recombination energies, ion structures, heats of formation, reaction energetics and relative charge transfer cross-sections are presented for molecular and fragment ions produced by electron bombardment ionization of CH4, C2H4, C2H6, C3H8 and C4H10 molecules. Even-electron bridged cations have low ion recombination energies and relatively low charge transfer cross-sections as compared with odd-electron hydrocarbon cations.

Anisotropy in plastic deformation of extruded magnesium alloy sheet during tensile straining at high temperature

Experimental measurements are used to characterize the anisotropy of flow stress in extruded magnesium alloy AZ31 sheet during uniaxial tension tests at temperatures between 350°C and 450°C, and strain rates ranging from 10-5 to 10-2 s-1. The sheet exhibits lower flow stress and higher tensile ductility when loaded with the tensile axis perpendicular to the extrusion direction compared to when it is loaded parallel to the extrusion direction. This anisotropy is found to be grain size, strain rate, and temperature dependent, but is only weakly dependent on texture. A microstructure based model (D. E. Cipoletti, A. F. Bower, P. E. Krajewski, Scr. Mater., 64 (2011) 931–934) is used to explain the origin of the anisotropic behavior. In contrast to room temperature behavior, where anisotropy is principally a consequence of the low resistance to slip on the basal slip system, elevated temperature anisotropy is found to be caused by the grain structure of extruded sheet. The grains are elongated parallel to the extrusion direction, leading to a lower effective grain size perpendicular to the extrusion direction. As a result, grain boundary sliding occurs more readily if the material is loaded perpendicular to the extrusion direction.

A study of high solids anaerobic digestion of Bucknell University food waste followed by aerobic curing

Anaerobic digestion of food scraps has the potential to accomplish waste minimization, energy production, and compost or humus production. At Bucknell University, removal of food scraps from the waste stream could reduce municipal solid waste transportation costs and landfill tipping fees, and provide methane and humus for use on campus. To determine the suitability of food waste produced at Bucknell for high-solids anaerobic digestion (HSAD), a year-long characterization study was conducted. Physical and chemical properties, waste biodegradability, and annual production of biodegradable waste were assessed. Bucknell University food and landscape waste was digested at pilot-scale for over a year to test performance at low and high loading rates, ease of operation at 20% solids, benefits of codigestion of food and landscape waste, and toprovide digestate for studies to assess the curing needs of HSAD digestate. A laboratory-scale curing study was conducted to assess the curing duration required to reduce microbial activity, phytotoxicity, and odors to acceptable levels for subsequent use ofhumus. The characteristics of Bucknell University food and landscape waste were tested approximately weekly for one year, to determine chemical oxygen demand (COD), total solids (TS), volatile solids (VS), and biodegradability (from batch digestion studies). Fats, oil, and grease and total Kjeldahl nitrogen were also tested for some food waste samples. Based on the characterization and biodegradability studies, Bucknell University dining hall food waste is a good candidate for HSAD. During batch digestion studies Bucknell University food waste produced a mean of 288 mL CH4/g COD with a 95%confidence interval of 0.06 mL CH4/g COD. The addition of landscape waste for digestion increased methane production from both food and landscape waste; however, because the landscape waste biodegradability was extremely low the increase was small.Based on an informal waste audit, Bucknell could collect up to 100 tons of food waste from dining facilities each year. The pilot-scale high-solids anaerobic digestion study confirmed that digestion ofBucknell University food waste combined with landscape waste at a low organic loading rate (OLR) of 2 g COD/L reactor volume-day is feasible. During low OLR operation, stable reactor performance was demonstrated through monitoring of biogas production and composition, reactor total and volatile solids, total and soluble chemical oxygendemand, volatile fatty acid content, pH, and bicarbonate alkalinity. Low OLR HSAD of Bucknell University food waste and landscape waste combined produced 232 L CH4/kg COD and 229 L CH4/kg VS. When OLR was increased to high loading (15 g COD/L reactor volume-day) to assess maximum loading conditions, reactor performance became unstable due to ammonia accumulation and subsequent inhibition. The methaneproduction per unit COD also decreased (to 211 L CH4/kg COD fed), although methane production per unit VS increased (to 272 L CH4/kg VS fed). The degree of ammonia inhibition was investigated through respirometry in which reactor digestate was diluted and exposed to varying concentrations of ammonia. Treatments with low ammoniaconcentrations recovered quickly from ammonia inhibition within the reactor. The post-digestion curing process was studied at laboratory-scale, to provide a preliminary assessment of curing duration. Digestate was mixed with woodchips and incubated in an insulated container at 35 °C to simulate full-scale curing self-heatingconditions. Degree of digestate stabilization was determined through oxygen uptake rates, percent O2, temperature, volatile solids, and Solvita Maturity Index. Phytotoxicity was determined through observation of volatile fatty acid and ammonia concentrations.Stabilization of organics and elimination of phytotoxic compounds (after 10–15 days of curing) preceded significant reductions of volatile sulfur compounds (hydrogen sulfide, methanethiol, and dimethyl sulfide) after 15–20 days of curing. Bucknell University food waste has high biodegradability and is suitable for high-solids anaerobic digestion; however, it has a low C:N ratio which can result in ammonia accumulation under some operating conditions. The low biodegradability of Bucknell University landscape waste limits the amount of bioavailable carbon that it can contribute, making it unsuitable for use as a cosubstrate to increase the C:N ratio of food waste. Additional research is indicated to determine other cosubstrates with higher biodegradabilities that may allow successful HSAD of Bucknell University food waste at high OLRs. Some cosubstrates to investigate are office paper, field residues, or grease trap waste. A brief curing period of less than 3 weeks was sufficient to produce viable humus from digestate produced by low OLR HSAD of food and landscape waste.

An Investigation On The Use Of Microalgae For Biogas Enrichment

There is a need for biomethane capture and carbon dioxide sequestration to mitigate evident global climate change. This research work investigated the potential for microalgae to remove CO2 from biogas as a biotechnical method for upgrading the thermal value for subsequent compression, liquification, or introduction to natural gas pipelines. Because biogas is largely methane, the effect of high methane environments on mixed microalgae was explored and found that specific carbon utilization rates were not statistically different when microalgae were exposed to biogas environments (70% v/v CH4) , relative to high CO2 environment. The uses of conventional bubbled column photobioreactors (PBR) were assessed for CO2 removal and subsequent CH4 enrichment. A continuously-bubbled biogas PBR (cB-PBR5) and intermittently-bubbled biogas PBR (iB-PBR) experienced CO2 loading rates of about 1664 and 832 mg C/L*day and showed 30.0 and 60.1 % carbon removal, respectively. However, a lack of biogas enrichment and issues associated growth inhibition due to high CO2 environments as well as stripping the dissolved gases, namely oxygen and nitrogen, from the bulk liquid and introduction to the outlet gas prompted the consideration for gas/liquid separation using nonporous hollow-fiber (HF) membranes for CO2 transfer. The potential for two non-porous HF membrane materials [polydimethylsiloxane (PDMS) and composite polyurethane (PU)] were modeled along fiber length using a mechanistic model based on polymeric material transport properties (Gilmore et al., 2009). Based on a high CO2:CH4 permeability selectivity for PU of 76.2 the model predicted gas enrichment along an 8.5 cm fiber length. Because PDMS permeability selectivity is low (3.5), evident gas transfer was not predicated along a 34.3 cm length. Both of these HF materials were implemented in hollow-fiber membrane-carbonated biofilm (HFMcB) PBRs for microalgal-mediated biogas enrichment. Phototrophic biofilm colonization occurred on the membrane, where CO2 concentration was greatest. The presence of a biofilm demonstrated greater resiliency to high CO2 environments, compared to the conventional PBRs. However, as the PDMS model predicted, the PDMS HFMcBs did not demonstrate gas enrichment. These reactors received CO2 loading rates of 200 mg C/L*day based on PDMS permeability flux and showed approximately 65% removal of the total C transferred across the membrane. Thus, the HFMcBs demonstrated controlled carbonation of the bulk liquid via a nonporous HF membrane. Likewise, the experimental PU HFMcB did not show gas enrichment yet this result should be further explored due to the high permeability selectivity of the polymeric material. Chemical stratifications, namely pH and dissolved O2, present in a PDMS membrane-carbonated biofilm were analyzed using electrochemical microsensors. Results indicated that high DO (20 mg L-1) exists at surface of the biofilm where light availability is greatest and low pH microenvironments (pH=5.40) exist deep in the biofilm where the diffusive flux of CO2 drives transfer through the biofilm. The presence of a 400-600 ¿m liquid phase boundary layer was evident from microsensor profiles. Cryosectioning of the biofilm samples showed the biofilm to be approximately 1.17 ± 0.07 mm thick, suggesting that the high localized concentration of biomass associated with the phototrophic biofilm aided in overcoming inhibition in a microenvironment dominated by CO2(aq). Challenges of biofilm detachment and PBR fouling as well as microalgal growth inhibition in the presence of high CO2 content remain for applications of microalgae for biogas enrichment.

High Solids Co-Digestion of Food and Landscape Waste and the Potential for Ammonia Toxicity

A pilot-scale study was completed to determine the feasibility of high-solids anaerobic digestion (HSAD) of a mixture of food and landscape wastes at a university in central Pennsylvania (USA). HSAD was stable at low loadings (2g COD/L-day), but developed inhibitory ammonia concentrations at high loadings (15g COD/L-day). At low loadings, methane yields were 232L CH4/kg COD fed and 229L CH4/kg VS fed, and at high loadings yields were 211L CH4/kg COD fed and 272L CH4/kg VS fed. Based on characterization and biodegradability studies, food waste appears to be a good candidate for HSAD at low organic loading rates; however, the development of ammonia inhibition at high loading rates suggests that the C:N ratio is too low for use as a single substrate. The relatively low biodegradability of landscape waste as reported herein made it an unsuitable substrate to increase the C:N ratio. Codigestion of food waste with a substrate high in bioavailable carbon is recommended to increase the C:N ratio sufficiently to allow HSAD at loading rates of 15g COD/L-day. Copyright 2014 Elsevier Ltd. All rights reserved.