Stephen Kelley | North Carolina State University (original) (raw)

Papers by Stephen Kelley

In aggressive geothermal environments, heat exchanger equipment capital and maintenance costs are... more In aggressive geothermal environments, heat exchanger equipment capital and maintenance costs are high due to the expensive, corrosion-resistant materials required and to the maintenance involved in keeping the heat transfer surfaces clean. To reduce these costs, the National Renewable Energy Laboratory and Brookhaven National Laboratory have been conducting tests of polymer-based coatings systems. These coating systems protect low-cost carbon steel tubing from corrosion and are formulated to resist the strong attachment of scale, thus facilitating cleaning. Laboratory and field tests of a variety of polymeric coatings under development show that geothermal deposits bond to the most promising coatings as weakly as they do to expensive stainless steels, and the coated tubes can be easily cleaned using conventional hydroblasting methods without damage to the polymer coating.

Carbohydrate Polymers, Feb 1, 2021

Biofuels, Bioproducts and Biorefining, Mar 18, 2019

Fems Microbiology Letters, Mar 19, 2002

Near infrared (NIR) spectroscopy and pyrolysis-molecular beam mass spectrometry (py-MBMS) analysi... more Near infrared (NIR) spectroscopy and pyrolysis-molecular beam mass spectrometry (py-MBMS) analysis can be used in conjunction with multivariate regression and principal components analysis to differentiate brown-rot-degraded wood from non-degraded spruce and to follow the temporal changes in wood undergoing brown-rot degradation. Regression of NIR test results vs. percent weight loss for Postia placenta-and Gloeophyllum trabeum-infected spruce wood blocks yielded a correlation coefficient of 0.96. Regression of MBMS test results for the same samples yielded a correlation coefficient of 0.96. Principle components analysis was used to differentiate noninfected wood and P. placenta-and G. trabeum-infected wood. These techniques may be used to detect different types of biodegradation and to develop a better understanding of the chemical changes that the wood undergoes when it is subjected to brown-rot biodegradation.

Bioresources, Oct 26, 2015

Chemical production from crude oil represents a substantial percentage of the yearly fossil fuel ... more Chemical production from crude oil represents a substantial percentage of the yearly fossil fuel use worldwide, and this could be partially offset by renewable feedstocks such as woody biomass and energy crops. Past techno-economic and environmental analyses have been conducted for isolated feedstocks on a regional or national scope. This study encompasses complete supply chain logistics analysis, delivered cost financial analysis, national availability, and environmental life cycle assessment (LCA) for 18 selected cellulosic feedstocks from around the world. A biochemical conversion route to monomeric sugars is assumed for estimated sugar yields and biosugar feedstock cost analysis. US corn grain was determined to have the highest delivered cost, while rice hulls in Indonesia resulted in the lowest cost of the feedstocks studied. Monomeric sugar yields from literature ranged from 358 kg BDMT-1 for US forest residues to 700 kg BDMT-1 for corn syrup. Environmental LCA was conducted in SimaPro using ecoinvent v2.2 data and the TRACI 2 impact assessment method for mid-point impacts cradle-to-incoming biorefinery gate. Carbon absorption during biomass growth contributed most substantially to the reduction of net global warming potential. Rice hulls and switchgrass resulted in the highest global warming potential, followed closely by corn and Thai sugarcane bagasse. Contribution analysis shows that chemical inputs such as fertilizer use contribute substantially to the net environmental impacts for these feedstocks.

Journal of Agricultural and Food Chemistry

Biotechnology for Biofuels, Sep 29, 2021

Renewable Energy, Dec 1, 2021

Abstract This paper describes the effect of the ash content in biomass on the distribution of pyr... more Abstract This paper describes the effect of the ash content in biomass on the distribution of pyrolysis products and the miscibility of bio-oil in diesel. Ash content of loblolly pine wood (0.5 wt %, 1.1 wt %, and 1.5 wt %) was systematically varied by impregnating the wood with potassium carbonate solution. Variation in the ash content did not create a significant change in the chemical composition of the impregnated biomass. However, the response to a variety of thermal treatments changed significantly. The volatile matter content decreased from 88.3% to 78.2%, while the bio-oil yield declined from 45.7% to 29.9% as the ash content increased. Although the total organic yield decreased with increased biomass ash content, the total concentration of phenolic monomers increased from 2.8 mg/g to 20.2 mg/g, and bio-oil miscibility with a commercial diesel fuel increased from 6.7% to 13.4% based on wet bio-oil. The miscibility of guaiacol and 4-methyl guaiacol in diesel was higher than that of catechol, due to the lower polar and hydrogen bonding contribution. Test in a diesel engine showed a simultaneous reduction of HC and NOx emissions using diesel extracts.

Holzforschung, 1986

Torsional braid analysis (TBA) was used to study the effects of formulation variables on the prop... more Torsional braid analysis (TBA) was used to study the effects of formulation variables on the properties of phenol-formaldehyde resins. These variables were the mole ratio of formaldehyde to phenol (F/P), the ratio of initial and total sodium hydroxide to phenol (i-NaOH and t-NaOH, respectively), and presence or absence of a Iow temperature (60°C) hold during the resin cook. Analysis of the resins during eure showed that the initial relative rigidity and mechanical damping were determined by the molecular weight and viscosity of the resin. The relative rigidity of the fülly cured resin was strongly influenced by the F/P, t-NaOH and the presence of a Iow temperature methylolation step. A high F/P (2.5) and a high t-NaOH (0.75) both promoted a high relative rigidity in the cured resin. Einfluß der chemischen Zusammensetzung auf die dynamisch-mechanischen Eigenschaften von alkalisch härtenden Phenol-Formaldehydharzen Zusammenfassung Der Einfluß der chemischen Zusammensetzung von alkalisch härtenden Phenol-Formaldehydharzen auf die Harz-Eigenschaften wurde mittels Torsionsanalyse untersucht. Die Variablen waren das Formaldehyd/Phenol Molverhältnis (F/P) und das Verhältnis von ursprünglichem zu Gesamt-Natriumhydroxid (i-NaOH und t-NaOH) bei einer niedrigen Harzkondensationstemperatur von 60°C. Die Harzanalyse während der Härtung zeigte, daß die ursprüngliche relative Sprödigkeit und mechanische Dämpfung vom Molekulargewicht und von der Viskosität des Harzes bestimmt werden. Die relative Sprödigkeit des vollständig ausgehärteten Harzes wurde stark vom F/P Verhältnis, vom Gesamt-NaOH-Gehalt und von der Temperatur bei der Methylolierungsstufe beeinflußt. Ein hohes F/P Verhältnis (2,5) und ein hoher Gesamt-NaOH-Gehalt bewirkten relativ hohe Sprödigkeit im ausgehärteten Harz.

Journal of Wood Chemistry and Technology, 1982

Enzyme and microbial technology, Sep 1, 1995

ABSTRACT A novel series of starch esters was prepared, which varied both in degree of substitutio... more ABSTRACT A novel series of starch esters was prepared, which varied both in degree of substitution (DS) and in ester group chain length (C-2 to C-6). Proton nuclear magnetic resonance-based characterization of esterified starch polymers was determined to be superior to standard saponification digestion (SD) analysis. The effects of starch modification on the anaerobic biodegradation potential for the polymer were assessed using the biochemical methane potential (BMP) protocol. Results indicate that increasing the starch ester group chain length necessitates lower levels of substitution to achieve substantial biologic degradation of the polymer.

Journal of Applied Polymer Science, 2007

Whereas cellulose-derived polymers are routinely used as membrane materials, the cellulose polyme... more Whereas cellulose-derived polymers are routinely used as membrane materials, the cellulose polymer itself is not directly used to synthesize dense/porous films for membrane applications. Recently, N-methylmorpholine N-oxide (NMMO) and dimethylacetamide (DMAc)/lithium chloride (LiCl) have been successfully employed for dissolving unmodified cellulose. This provides a strong rationale for reexamining the possibility of cellulose membrane fabrication using these solvents. By judiciously selecting solvents, casting conditions, and solvent exchange steps, we successfully synthesized dense/ asymmetric-porous cellulose films. The pore size and porosity of the porous films decreased systematically with increasing cellulose concentration. SEM analysis of the cross sections revealed an asymmetric skinned structure with monotonically increasing pore size away from the skin. The measured pore diameters were in the range 1.8-4.8 mm. Mechanical testing indicated that the dense films possessed tensile properties comparable to those of cellulose acetate (CA) films. Though nitrogen permeability values were comparable for cellulose and CA dense films, cellulose film permeability depended upon the type of drying protocol employed. Overall, these results demonstrate that processability need not be a constraint in the use of cellulose polymer for membrane fabrication. In selected applications, cellulose membranes could become a cost-effective, environmentally friendly alternative to other more commonly employed membrane polymers.

Journal of Cleaner Production, Apr 1, 2023

Bioresources, Apr 26, 2019

Wood processing is often performed at elevated temperatures under moisture-saturated conditions; ... more Wood processing is often performed at elevated temperatures under moisture-saturated conditions; therefore, it is important to understand the impact of the lignin content and lignin chemical structure on the thermomechanical properties of wood. In this study, genetically modified Populus trichocarpa wood specimens with down-regulated cinnamyl alcohol dehydrogenase, cinnamate 3-hydroxylase, and cinnamate 4-hydroxylase with altered lignin contents and/or lignin structures were utilized to probe the relationship between the lignin content, lignin monomer composition, and thermo-mechanical properties of solid wood. The thermo-mechanical properties of these unique samples were measured using dynamic mechanical analysis and the nuclear magnetic resonance (NMR) spin-spin relaxation time. The results showed that the transgenic P. trichocarpa samples had decreased storage and loss moduli compared with the wildtype. The solid-state NMR revealed increased lignin molecular mobility in the reduced-lignin transgenic lines. Also, noticeably reduced glass transition temperatures (Tg) were observed in the transgenic lines with reduced lignin contents and altered lignin monomer compositions compared with the wildtype. The increased lignin molecular mobility and reduced Tg in these samples can probably contribute to wood utilization and processing, such as lignin removal for pulp and paper and biofuels production, as well as particle consolidation during wood composite manufacturing.

Bioresources, Jun 4, 2019

Vessel and fiber properties, diameter growth, and chemical compositions were investigated for tra... more Vessel and fiber properties, diameter growth, and chemical compositions were investigated for transgenic Populus trichocarpa (black cottonwood) trees harvested after six months of growth in a greenhouse. Genetic modifications were cinnamyl alcohol dehydrogenase (CAD), cinnamate 3hydroxylase (C3H), or cinnamate 4-hydroxylase (C4H), which resulted in modified lignin composition or content, and changed the syringyl-toguaiacyl ratio. Comparing the genetic groups to the wild-type as the control, trees with reduced lignin content showed different results for vessel and fiber properties measured. Genetic groups with reduced PtrC3H3 and PtrC4H1&2 (with lower lignin content and higher S/G ratio than the control) exhibited splitting perpendicular to the rays, yet had the same fiber lumen diameter and the same fiber cell wall thickness as the control. Changes in lignin structure from modifications to PtrCAD resulted in reductions to the number of vessels, increases in vessel and fiber diameters, and had no consistent impact on stem diameter.

Holzforschung, Mar 29, 2012

Little is known about the effect of genetic modification on the chemical composition and structur... more Little is known about the effect of genetic modification on the chemical composition and structure of wood, which could have a significant effect on reactivity during chemical and enzymatic processing. In this study, information was collected by confocal Raman microscopy (CRM) on the spatial distribution of lignin and polysaccharides in the cell wall of young transgenic aspen with reduced lignin content, increased syringyl/guaiacyl (S/G) ratio, and simultaneously reduced lignin content and increased S/G ratio. CRM revealed that the lignin content of the cell wall and compound middle lamella was reduced by the genetic modification. A higher volume of water was also found in the cell wall of transgenic aspen compared with wild-type aspen, indicating an increase in the hydrophilicity of the cell wall.

This study investigates the use of Near Infrared (NIR) spectroscopy as a nondestructive means of ... more This study investigates the use of Near Infrared (NIR) spectroscopy as a nondestructive means of determining wood properties. The parallel to the grain modulus of elasticity was predicted from the near infrared spectra and compared to the modulus of elasticity obtained from tensile tests. Analysis was also performed to determine if NIR spectroscopy is a viable means to determine in situ properties of wood using hand-held spectrometers that have a smaller spectra range. The hand held spectrometers were modeled by using reduced NIR spectra in order to determine the modulus of elasticity. The re sults demonstrated that NIR spectroscopy has excellent correlation (r = 0.93) to the modulus of elasticity. The reduced NIR spectra also had a good correlation (r = 0.90) thus making Near Infrared spectroscopy a feasible, nondestructive technique to deter mine wood properties.

Wood Science and Technology, May 13, 2004

Biofuels, Bioproducts and Biorefining, Dec 13, 2016

Life cycle assessments (LCAs) have become a common tool for measuring the environmental performan... more Life cycle assessments (LCAs) have become a common tool for measuring the environmental performance of various products and processes. This study reviewed the life cycle of lignocellulosic biomass feedstock-based (pine, eucalyptus, and switchgrass) ethanol biofuels and compared the results to gasoline. Uncertainty analysis of the cradle-to-wheel results was conducted using a pedigree matrix method to address uncertainty of life cycle inventory items. A tool called stochastic multi-attribute analysis for life cycle impact assessment (SMAA-LCIA) was used to interpret the results generated from the LCA. The normalized results showed that gasoline had high environmental impacts in categories such as carcinogenics and global warming, followed by switchgrass. But the use of a single-score metric indicated that all bio-based feedstocks had similar environmental performance, all signifi cantly better than gasoline. The SMAA-LCIA results showed that when all impacts were weighted equally, the preference from highest to lowest was for loblolly pine, eucalyptus, gasoline, and switchgrass. This rank order switched when differing weighting schemes of Producer, User, and LCA Expert were used. Overall, this method highlights the trade-offs associated with the selection of different feedstocks and improves the comparison of overall results amongst fuel sources, which can then be more clearly presented to a decision-maker.