Nelson Macken

Mass Balance, Energy, and Exergy Analysis of Bio-Oil Production by Fast Pyrolysis

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture (USDA). USDA is an equal opportunity provider and employer. Mass, energy, and exergy balances are analyzed for bio-oil production in a bench-scale fast pyrolysis system developed by the USDA’s Agricultural Research Service (ARS) for the processing of commodity crops to fuel intermediates. Because mass balance closure is difficult to achieve due, in part, to the system’s small size and complexity a linear programming optimization model is developed to improve closure of elemental balances without losing the overall representation of the pyrolysis products. The model results provide an opportunity to analyze true energy and exergy balances for the system. While energy comparisons are based on heating values, exergy flows are computed using statistical relationships and other standard techniques. Comparisons were made for a variety of biomass feedstocks including energy crops and various byproducts of agriculture and bioenergy industry. The mass model allows for proper accounting of sources of mass loss and suggestions for improved system performance. Energy recovery and exergetic efficiency are compared for a variety of pyrolysis product utilization scenarios including use of biochar and noncondensable gases as heat sources. Exergetic efficiencies show high potential for energy utilization when all the pyrolysis product streams can be recycled to recuperate their internal energy. The exergy analysis can be beneficial to developing exergetic life cycle assessments (ELCA) for the fast pyrolysis process as sustainable technology for advanced biofuels production.

Thermal Conductivity Measurement of CVD Diamond Films Using a Modified Thermal Comparator Method

Results from an experimental study on the rapid measurement of thermal conductivity of chemical vapor deposited (CVD) diamond films are presented. The classical thermal comparator method has been used successfully in the past for the measurement of thermal conductivity of bulk materials having high values of thermal resistance. Using samples of known thermal conductivity, a calibration curve is prepared. With this calibration curve, the comparator can be used to determine thermal conductivity of unknown samples. We have significantly modified and extended this technique for the measurement of materials with very low thermal resistance, i.e., CVD diamond films with high thermal conductivity. In addition to the heated probe, the modified comparator employs a thermoelectric cooling element of increase conductive heat transfer through the film. The thermal conductivity measurements are sensitive to many other factors such as the thermal contact resistances, anisotropic material properties, surrounding air currents and temperature, and ambient humidity. A comprehensive numerical model was also developed to simulate the heat transfer process for the modified comparator. The simulations were used to develop a numerical calibration curve that agreed well with the calibration curve obtained from our measurements, The modified method has been found to successfully measure the thermal conductivity of CVD diamond films.

Mass Balance and Exergy Analysis of a Fast Pyrolysis System

Mass balance closure and exergetic efficiency is evaluated for a bench scale fast pyrolysis system. The USDA Agricultural Research Service (ARS) has developed this system for processing energy crops and agricultural residues for bio-oil (pyrolysis oil or pyrolysis liquids) production. Mass balance closure cannot be achieved due to the system size and complexity of inputs and outputs. A linear programming optimization model is developed to use the experimental data to achieve improved closure of elemental balances without losing the overall representation of the pyrolysis products. Having improved the mass balance, it is then possible to evaluate the exergy of the system. Exergy flows are computed using statistical relationships and other standard techniques. Computational details and results are discussed for switchgrass, a typical candidate biomass. Solutions for the minimum and maximum bio-oil outputs were generated. These particular results indicated that bio-oil accounted for approximately 10% of the loss mass. Considering all products as useful, the exergy destruction is approximately 20%. If the bio-oil alone is considered useful, the exergy destruction is about 40%. Further exercise of the model can be useful in evaluating mass losses and exergy for other feedstock and experimental runs.Copyright

Effect of Viscosity and Reynolds Number for Flow in Converging Microchannels

The interface between intersecting microfluidic flows is investigated experimentally. Two microchannel configurations are studied. Each configuration has a main channel and an intersecting daughter channel. The channel cross sections are equal and square with the intersection either at 90 or 45 degrees. Flow visualization is achieved using confocal fluorescence microscopy. The flow interface is examined for equal and unequal viscosities and a range of Reynolds numbers. Viscosity differences and Reynolds numbers influence the three-dimensional nature of the interface. As the Reynolds number increases, the increased flow inertia produces curvature in the interface surface perpendicular to the flow. Curvature is also evident in flows with unequal viscosities. The interface location at fixed flow ratios is independent of the Reynolds number, but varies significantly with unequal viscosity ratios. Viscosity and Reynolds number effects are similar in both the 45 and 90 degree configurations.Copyright

Flow of Microfluids in Converging Microchannels

The interface between intersecting microfluidic multicomponent flow is investigated experimentally. Three microchannel configurations are studied. Each configuration has a main channel and an intersecting daughter channel. In two configurations, the channel cross sections are equal and square with the intersection either at 90 or 45 degrees. In the third configuration, the intersection is at 90 degrees, the cross sections are square and the daughter cross section is smaller than the main cross section. In the configurations with equal channel cross sections, microsphere solutions of 2, 4 and 7% spheres (by weight) are compared to each other as well as all water flows. Flow visualization is achieved using confocal fluorescence microscopy. A three-dimensional rendering of the location and shape of the interface is examined for a Reynolds number of approximately one. The presence of microspheres does not appear to strongly influence the location of the flow interface. For flows with equal cross section, the interface downstream of the junction is reasonably planer (two dimensional). Strong three-dimensional effects are shown for flows with unequal cross section.Copyright

Flow of Microsphere Solutions in Converging Microchannels

The interface between intersecting microfluidic multicomponent flow is investigated experimentally. The system studied has a main channel and an intersecting (at 90 degrees) daughter channel and the fluid is a water-microsphere mixture. Flow visualization is achieved using confocal fluorescence microscopy. Microsphere solutions of 2% and 4% spheres (by weight) are investigated and compared to each other as well as all water flows. A three-dimensional rendering of the location and shape of the interface at the junction for different flow ratios (daughter flow to total) is examined for a combined flow Reynolds number of approximately one. The interface downstream of the junction is reasonably planer (two-dimensional) and the presence of microspheres does not appear to strongly influence the location of the flow interface.Copyright