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Guest Editorial

J. Energy Resour. Technol. 2018;140(6):060301-060301-1. doi:10.1115/1.4039654.

Special Issue for peer-reviewed papers published from the 42nd International Technical Conference on Clean Energy, Clearwater Clean Energy Conference, June 11–15, 2017.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;140(6):062001-062001-10. doi:10.1115/1.4038782.

The aerosols from CO2-depleted flue gas at the National Carbon Capture Center (NCCC) Pilot Solvent Test Unit (PSTU) and Slipstream Solvent Test Unit (SSTU) were measured in real-time using a Dekati Electric Low Pressure Impactor (ELPI+™). The coal-fired flue gas is provided by Alabama Power's Gaston Power Plant Unit 5. The utilization of ELPI+™ for aerosol research in postcombustion CO2 capture is very important due to its quick response time with size classification as low as 6 nm under transient conditions observed at the NCCC. Different process changes have been quantified at the PSTU and SSTU by multiple tests using the ELPI+™. The performance of smooth and sintered collection plates during dynamic process changes has been investigated. Between separate tests, upstream at unit 5, a new baghouse was installed. The aerosols measured at SSTU, before and after the baghouse installation, are compared. PSTU measurements demonstrated sample sensitivity to transient intercooler start-up conditions and dilution gas temperatures. During the tests, the typical concentration ranged from 106 to 107 cm−3. The aerosol's counter median diameter (CMD) for the sintered plates are lower (47–60 nm) compared to the normal plates (89–130 nm). The optical images indicate that sintered plates soak up all of the collected aerosols. The aerosol number concentration showed a significant drop after the baghouse installation. These results are promising and will enable the development of process control strategies to mitigate solvent losses and reduce operation and maintenance expenses.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062002-062002-7. doi:10.1115/1.4039610.

Heat transfer to supercritical water in heated tubes and channels is relevant for steam generators in conventional power plants and future concepts for supercritical nuclear and solar-thermal power plants. A new experimental facility, the high pressure evaporation rig, setup at the Institute for Energy Systems (Technische Universität München) aims to provide heat transfer data to fill the existing knowledge gaps at these conditions. The test rig consists of a closed-loop high pressure cycle, in which de-ionized water is fed to an instrumented test section heated by the application of direct electrical current. It is designed to withstand a maximum pressure of 380 bar at 580 °C in the test section. The maximum power rating of the system is 1 MW. The test section is a vertical tube (material: AISI A213/P91) with a 7000 mm heated length, a 15.7 mm internal diameter, and a wall thickness of 5.6 mm. It is equipped with 70 thermocouples distributed evenly along its length. It enables the determination of heat transfer coefficients in the supercritical region at various steady-state or transient conditions. In a first series of tests, experiments are conducted to investigate normal and deteriorated heat transfer (DHT) under vertical upward flow conditions. The newly generated data and literature data are used to evaluate different correlations available for modeling heat transfer coefficients at supercritical pressures.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062003-062003-10. doi:10.1115/1.4039545.

Fluidized beds are used in many industries where gas–solid reactions are present for their favorable characteristics of good solids mixing, high heat, and mass transfer rates, and large throughputs. In an attempt to increase throughput, reduce reactor footprints, and reduce costs, process intensification by unconventional reactor designs is being pursued. Specifically, this work focuses on the development of high-G reactors where the particles are experiencing a centripetal force typically on the order of ten times the force of gravity. This operating regime provides intensified gas–solids contact providing higher mass transfer, heat transfer, and gas throughput than a typical fluidized bed. This work focuses analysis of a cold flow vortexing circulating fluidized bed (CFB). Through mapping the pressure distributions in the riser, insights into the behavior of the system were made and compared to CPFD Barracuda computational fluid dynamic models. The simulation results outlined the working envelope of the system and provided a baseline to compare the experimental results. The experimental pressure data determined angular velocities of the gas in the range of 30–40 m/s, with corresponding particle velocities around 15 m/s.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062004-062004-5. doi:10.1115/1.4039739.

A study was conducted to explore the effects of static bed height, nozzle diameter, cone angle, and particle properties on the minimum spouting velocity in a 4 in × 1 in rectangular spouted bed. Tests were conducted with various solids materials (including 871 μm HPDE pellets, 3.2 mm nylon beads, 707 μm glass beads, and 1.5 mm alumina spheres), two gas inlet nozzle diameters, and two cone angles. Experimentally obtained minimum spouting velocities were compared to existing published correlations developed for cylindrical spouted beds. In each case, it was determined that the existing correlations did not adequately predict the minimum spouting velocity for a rectangular spouted bed. A new correlation is proposed.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;140(6):062201-062201-5. doi:10.1115/1.4039321.

This study was aimed at comparing the pyrolysis behavior of several selected biomass samples, namely, pine wood, poplar wood, wheat straw, and sugarcane bagasse, with a particular attention to the effect of lignin. Raw samples were first treated using Soxhlet solvent extraction with a 2:1 (v/v) mixture of toluene/ethanol to remove wax. Lignin was then removed by soaking the dewaxed samples in a 1.0 M sodium chlorite solution at 343 K till the solids became white. Fourier transform infrared (FTIR) spectroscopy analysis was applied to characterize the surface functional groups of the samples. The morphology of the samples before and after delignification treatment was analyzed using scanning electron microscope (SEM). The pyrolysis behavior of the raw and treated biomass samples was studied using a thermogravimetric analyzer (TGA) operating in nitrogen at a constant heating rate of 10 K min−1 from room temperature to the final temperature 823 K. The FTIR and SEM results indicated that lignin can be successfully removed from the raw biomass via the chemical treatment used. As expected, the pyrolysis behavior differed significantly among the various raw biomass samples. However, the pyrolysis behavior of the delignified samples showed almost identical thermal behavior although the temperature associated with the maximum rate of pyrolysis was shifted to a lower temperature regime by ca. 50 K. This suggests that the presence of lignin significantly affected the biomass pyrolysis behavior. Thus, the pyrolysis behavior of the biomass cannot be predicted simply from the individual components without considering their interactions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062202-062202-7. doi:10.1115/1.4039317.

This paper presents the design, development, and operation of a reactor system for CO2 capture. Modifications were implemented to address differences in sorbent from 180 μm Geldart group B to 115 μm Geldart group A material; operational issues were discovered during experimental trials. The major obstacle in system operation was the ability to maintain a constant circulation of a solid sorbent stemming from this change in sorbent material. The system consisted of four fluid beds, through which a polyamine impregnated sorbent was circulated and adsorption, preheat, regeneration, and cooling processes occurred. Pressure transducers, thermocouples, gas flow meters, and gas composition instrumentation were used to characterize thermal, hydrodynamic, and gas adsorption performance in this integrated unit. A series of shakedown tests were performed and the configuration altered to meet the needs of the sorbent performance and achieve desired target capture efficiencies. Methods were identified, tested, and applied to continuously monitor critical operating parameters including solids circulation rate, adsorbed and desorbed CO2, solids inventories, and pressures. The working capacity and CO2 capture efficiency were used to assess sorbent performance while CO2 closure was used to define data quality and approach to steady-state. Testing demonstrated >90% capture efficiencies and identified the regenerator to be the process step limiting throughput. Sorbent performance was found to be related to the reactant stoichiometry. A stochastic model with an exponential dependence on the relative CO2/amine concentration was used to describe 90% of the variance in the data.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062203-062203-6. doi:10.1115/1.4039320.

This study examined the rheological properties, ignition, and combustion characteristics of biochar–algae–water (BAW) slurry fuels. A pine sawdust biochar with a median particle size (D50) of approximately 12 μm and algae Chlorella vulgaris in dry powder form with D50 of approximately 19 μm were used. The BAW slurries with a constant solid loading of 40 wt % and algae/biochar ratio varying from 0 to 0.2 by weight were prepared. The apparent viscosity was measured using a Haake VT550 cone-and-plate viscometer. The stability of the slurries was characterized using a “drop rod” method. Ignition and combustion characteristics of the slurries were studied using a suspended single-droplet technique. A single droplet of a slurry fuel with a diameter ranging from 0.5 mm to 1.5 mm was suspended on a silicon carbide fibre and burned in air at 1023 K in an electrically heated tube furnace. The ignition and combustion processes of the droplet were recorded using a CCD camera at 200 fps. The ignition delay time, burnout time, and burning rate were determined. The BAW slurries showed shear-thinning flow behavior. The slurries had higher viscosity and greater stability at higher algae proportion in the solid. The ignition and combustion process of BAW slurries followed the sequence of water evaporation, devolatilization, ignition, and combustion of the solid residue. The combustion of the residual solid was diffusion controlled under the experimental conditions and the burning rates of the BAW slurry droplets ranged from 0.15 to 0.25 mm2 s−1.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062204-062204-5. doi:10.1115/1.4039318.

This paper reports an experimental study of the effect of biochar addition and temperature on hydrogen production in the first phase of the two-phase anaerobic digestion (TPAD) of carbohydrates food waste. Anaerobic digestion (AD) experiments using white bread representing carbohydrate food wastes were conducted in bench scale 100 ml reactors. The cultures with biochar addition were placed in the reactors and incubated at different temperatures (18, 35, and 52 °C) over a period of 8 days. The biochar addition ratio was varied from 0 to 18.6 g l−1. The daily volumetric hydrogen production was measured, and the cumulative yield (YH) and daily production rate (RH) of hydrogen were calculated. Both biochar addition and temperature affected hydrogen production significantly. YH and maximum RH increased as the biochar addition ratio increased from 0 to 10 g l−1 then decreased as the biochar addition ratio further increased up to 18.6 g l−1. At different temperatures, YH varied significantly, increasing from 846 ± 18 ml l−1 at 18 °C to 1475 ± 53 ml l−1 at 35 °C and dropped to 1149 ± 26 ml l−1 at 52 °C. The maximum RH also peaked at 35 °C, reaching 858 ± 57.1 ml l−1 day−1. The effect of biochar addition was more profound under mesophilic conditions. The results of this study confirmed the beneficial effect of biochar addition in hydrogen production of carbohydrate food waste in the TPAD process.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062205-062205-9. doi:10.1115/1.4039738.

Rare earth elements (REE) are crucial materials in an incredible array of consumer goods, energy system components, and military defense applications. However, the global production and entire value chain for REE is dominated by China, with the U.S. currently 100% import reliant for these critical materials. Traditional mineral ores including those previously mined in the U.S., however, have several challenges. Chief among these is that the content of the most critical and valuable of the rare earths is deficient, making mining uneconomical. Further, the supply of these most critical rare earths is nearly 100% produced in China from a single resource that is only projected to last another 10–20 years. The U.S. currently considers the rare earths market an issue of national security. It is imperative that alternative domestic sources of rare earths be identified and methods developed to produce them. Recently, coal and coal byproducts have been identified as one of these promising alternative resources. This paper details the results of a study on characterization of North Dakota lignite and lignite-related feedstocks as an assessment of their feasibility for REE recovery. The abundance, distribution, and modes of occurrence of the REE in the samples collected were determined in this initial study to inform the selection of appropriate extraction and concentration methods to recover the REE. Materials investigated include the lignite coals, clay-rich sediments associated with the coal seams, and materials associated with a lignite beneficiation system and power plant. The results show that high REE levels exist both in lignite coals and associated sediments. The form of the REE in the clay materials is primarily as ultrafine mineral grains. In the lignite coals, approximately 80–95% of the rare earths content is organically associated, primarily as coordination complexes.

Topics: Coal , Minerals , Sediments
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062206-062206-9. doi:10.1115/1.4039906.

Sewage sludge is a waste from the water treatment installations. It is used in agriculture. However, due to various environmental restrictions, not all of the sewage sludge can be utilized within that sector. Using this resource as a sustainable energy source might be an interesting alternative to the landfilling. Some of the fuel-related properties of sewage sludge make it difficult to be used as a fuel without preprocessing. Torrefaction is a promising pretreatment technique that could prove itself suitable to be used for improving sewage sludge. Additives might be used for obtaining some further improvements, either during the torrefaction stage or further at the final energy conversion stage (combustion, gasification, etc.). This paper presents the results of torrefaction experiments performed with sewage sludge from the local water treatment facility. Torrefaction was performed with laboratory-scale rotary reactor at three different temperatures (250 °C, 275 °C, and 300 °C). Cotorrefaction of sewage sludge with lignite was also performed. Torrefaction tests with quicklime (CaO) as an additive were also performed. Fuel-related properties of products of torrefaction and feedstock were determined. By-product of torrefaction, called torgas, was also a subject of the analysis. Propensity of the torrefied product to absorb moisture was assessed. Thermogravimetric analysis (TGA) of raw and torrefied samples was performed in order to compare the behavior of raw and torrefied materials during subsequent pyrolysis.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(6):062207-062207-14. doi:10.1115/1.4039907.

The paper presents the experimental and numerical study on the behavior and performance of an industrial scale boiler during combustion of pulverized bituminous coal with various shares of predried lignite. The experimental measurements were carried out on a boiler WP120 located in CHP, Opole, Poland. Tests on the boiler were performed during low load operation and the lignite share reached over to 36% by mass. The predried lignite, kept in dedicated separate bunkers, was mixed with bituminous coal just before the coal mills. Computational fluid dynamic (CFD) simulation of a cofiring scenario of lignite with hard coal was also performed. Site measurements have proven that cofiring of a predried lignite is not detrimental to the boiler in terms of its overall efficiency, when compared with a corresponding reference case, with 100% of hard coal. Experiments demonstrated an improvement in the grindability that can be achieved during co-milling of lignite and hard coal in the same mill, for both wet and dry lignite. Moreover, performed tests delivered empirical evidence of the potential of lignite to decrease NOx emissions during cofiring, for both wet and dry lignite. Results of efficiency calculations and temperature measurements in the combustion chamber confirmed the need to predry lignite before cofiring. Performed measurements of temperature distribution in the combustion chamber confirmed trend that could be seen in the results of CFD. CFD simulations were performed for predried lignite and demonstrated flow patterns in the combustion chamber of the boiler, which could prove useful in case of any further improvements in the firing system. CFD simulations reached satisfactory agreement with the site measurements in terms of the prediction of emissions.

Commentary by Dr. Valentin Fuster

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