Guest Editorial

J. Energy Resour. Technol. 2016;138(4):040301-040301-1. doi:10.1115/1.4033885.

Special Issue for peer-reviewed papers published from the 40th International Technical Conference on Clean Coal and Fuel Systems held during May 31–June 4, 2015.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2016;138(4):042001-042001-7. doi:10.1115/1.4032425.

Currently, there is no satisfactory method for measuring the temperature of the gas phase of combustion products within a solid fuel flame. The industry standard, a suction pyrometer or aspirated thermocouple, is intrusive, spatially and temporally averaging, and difficult to use. In this work, a new method utilizing the spectral emission from water vapor is investigated through modeling and experimental measurements. The method employs the collection of infrared emission from water vapor over discrete wavelength bands and then uses the ratio of those emissions to infer temperature. This method was demonstrated in the products of a 150 kWth natural gas flame along a 0.75 m line of sight, averaged over 1 min. Results from this optical method were compared to those obtained using a suction pyrometer. Data were obtained at three fuel air equivalence ratios that produced products at three temperatures. The optical measurement produced gas temperatures approximately 3–4% higher than the suction pyrometer. The uncertainty of the optical measurements is dependent on the gas temperature being ±9% at 850 K and 4% or less above 1200 K. Broadband background emission assumed to be emitted from the reactor wall was also seen by the optical measurement and had to be removed before an accurate temperature could be measured. This complicated the gas measurement but also provides the means whereby both gas and solid emission can be measured simultaneously.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042002-042002-10. doi:10.1115/1.4032544.

The energy sector in the European market has been changing significantly over the last years. European Union (EU) energy strategy includes the EU low-carbon roadmap milestone, which indicates for 2020, a 20% reduction in carbon emissions, and a 20% EU-wide share for renewables, and by 2030 a 40% reduction in carbon emissions and 30% EU-wide share for renewables. The increased renewable energy sources (RES) penetration and their intermittent energy production have led to the emerging need for energy storage technologies. Especially in the European energy market, large-scale energy balancing with sustainable technologies with product flexibility and cost-effective operation are being investigated. The carbon capture and utilization (CCU) concept, as a means for low-carbon sustainable industries, is integrated in the energy storage technologies. The present paper addresses the integration of power to fuel concept in the energy storage sector with simultaneous emission reduction. Grid management, the scale, and the efficient operation of electrolyzers are the basis for the implementation of Power to Fuel technology. The use of surplus and/or low-cost electricity via water electrolysis to commute captured CO2 from industrial plants to versatile energy carriers such as methane and methanol is being investigated in the present paper. Shadow operation of fossil fuel power plants under minimum load conditions leads to optimized energy dispatch of the power plants and provides product flexibility in terms of electricity, grid services, and chemical production. The produced fuels can be used in highly efficient and well-established power systems and further used in the transportation sector or for covering heat demands. The energy efficiency of the different processes is presented and a comparison is made in terms of cost effective energy storage solutions via the simultaneous grid management optimization, the reduction of carbon dioxide, and the production of valuable chemicals. The cross-sectorial concept of the Power to Fuel is presented for Steel and Power industry for the case of methane and methanol production. A review of the U.S. and European markets is made for the application of Power to Fuel.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2015;138(4):042201-042201-9. doi:10.1115/1.4031968.

Chemical-looping combustion (CLC) is a next-generation combustion technology that shows great promise in addressing the need for high-efficiency low-cost carbon capture from fossil fueled power plants. Although there have been a number of experimental studies on CLC in recent years, computational fluid dynamics (CFD) simulations have been limited in the literature. In this paper, simulation of a CLC reactor is conducted using the Eulerian approach in the commercial CFD solver ansys fluent based on a laboratory-scale experiment with a dual fluidized bed CLC reactor. The solid phase consists of a Fe-based oxygen carrier while the gaseous fuel used is syngas. The salient features of the fluidization behavior in the air reactor and fuel reactor beds representing a riser and a bubbling bed, respectively, as well as the down-comer, are accurately captured in the simulation. This work is among the few CFD simulations of a complete circulating dual fluidized bed system for CLC in 3D in the literature. It highlights the importance of 3D simulation of CLC systems and the need for more accurate empirical reaction rate data for future CLC simulations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2015;138(4):042202-042202-8. doi:10.1115/1.4032018.

For chemical looping processes to become an economically viable technology, an inexpensive carrier that can endure repeated reduction and oxidation cycles needs to be identified or developed. Unfortunately, the reduction of hematite ore with methane in both batch and fluidized beds has revealed that the performance (methane conversion) decreases with time. Previous analysis had shown that the grains within the particle grew with the net effect of reducing the surface area of the particles and thereby reducing the rate and net conversion for a fixed reduction time. To improve the lifespan of hematite ore, it is hypothesized that if the grain size could be stabilized, then the conversion could be stabilized. In this work, series of tests were conducted in an electrically heated fluidized bed. The hematite ore was first pretreated at a temperature higher than the subsequent reduction temperatures. After pretreatment, the hematite ore was subjected to a series of cyclic reduction/oxidation experiments. The results show that the ore can be stabilized for cycles at different conditions up to the pretreatment temperature without any degradation. Details of the pretreatment process and the test results will be presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042203-042203-7. doi:10.1115/1.4032357.

Chemical-looping combustion (CLC) is an emerging carbon capture technology that is characterized by a low energy penalty, low carbon dioxide capture costs, and low environmental impact. To prevent the contact between fuel and air, an oxygen carrier is used to transport the oxygen needed for fuel conversion. In comparison to a classic oxyfuel process, no air separation unit is required to provide the oxygen needed to burn the fuel. The solid fuel, such as coal, is gasified in the fuel reactor (FR), and the products from gasification are oxidized by the oxygen carrier. There are promising results from the electrically heated 100 kWth unit at Chalmers University of Technology (Sweden) or the 1 MWth pilot at Technische Universität Darmstadt (Germany) with partial chemical-looping conditions. The 1 MWth CLC pilot consists of two interconnected circulating fluidized bed reactors. It is possible to investigate this process without electrically heating due to refractory-lined reactors and coupling elements. This work presents the first results of autothermal operation of a metal oxide CLC unit worldwide using ilmenite as oxygen carrier and coarse hard coal as fuel. The FR was fluidized with steam. The results show that the oxygen demand of the FR required for a complete conversion of unconverted gases was in the range of 25%. At the same time, the carbon dioxide capture efficiency was low in the present configuration of the 1 MWth pilot. This means that unconverted char left the FR and burned in the air reactor (AR). The reason for this is that no carbon stripper unit was used during these investigations. A carbon stripper could significantly enhance the carbon dioxide capture efficiency.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042204-042204-8. doi:10.1115/1.4032620.

Modeling pressurized entrained flow gasification of solid fuels plays an important role in the development of integrated gasification combined cycle (IGCC) power plants and other gasification applications. A better understanding of the underlying reaction kinetics is essential for the design and optimization of entrained flow gasifiers—in particular at operating conditions relevant to large-scale industrial gasifiers. The presented computational fluid dynamics (CFD) simulations aim to predict conversion rates as well as product gas compositions in entrained flow gasifiers. The simulations are based on the software ansys fluent 15.0 and include several detailed submodels in user defined functions (UDF). In a previous publication, the developed CFD model has been validated for a Rhenish lignite against experimental data, obtained from a pilot-scale entrained flow gasifier operated at the Technische Universität München. In the presented work, the validated CFD model is applied to a Siemens test gasifier geometry. Simulation results and characteristic parameters, with focus on char gasification reactions, are analyzed in detail and provide new insights into the gasification process.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042205-042205-7. doi:10.1115/1.4032730.

Tars produced during the thermal conversion of coal or especially biomass is one of the major obstacles for the application of gasification systems. They limit the use of the producer gas in engines or turbines or, in further processes like in methanization or conversion to other secondary fuels or chemicals, without further gas cleaning. The determination of the tar content with conventional methods is very time consuming and does not allow continuous online monitoring of the gas quality. One approach to avoid these drawbacks is an automatic system developed at the University of Stuttgart that monitors the tar concentration in the producer gas online and semicontinuous during the gasification process. The technique is based on a flame ionization detector (FID) difference measurement of the hydrocarbons in the producer gas, where the condensable hydrocarbons—the tars—are condensed on a suitable filter material. This work shows the further development of the measurement technique, the choice of a suitable tar filter material for the underlying difference measurement, and a first verification of the system with real producer gas at a 20 kWth bench scale gasifier.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042206-042206-7. doi:10.1115/1.4032732.

This work is devoted to the development and verification of a new intrinsic-based subgrid model for moving char particles gasifying in a hot flue gas or syngas environments consisting of CO2/H2O/CO species. The distinguishing feature of our model relative to the submodels published in the literature is that it takes into account the thermal and chemical nonequilibrium between the particle's surface and its center. Thus, our model is able to predict temperature and species gradients inside the particles. The main focus of the new submodel is to demonstrate the crucial role of intrinsic-based heterogeneous reactions in the adequate prediction of carbon conversion rates for char particles gasification in fixed-bed and fluidized-bed gasifiers. The new model is verified against steady-state, particle-resolved computational fluid dynamics (CFD)-based, three-dimensional simulations carried out for different volume fractions of solid phase in a control volume (CV). Acceptable agreement has been demonstrated. Finally, to demonstrate our new model's predictions, we carried out several unsteady simulations for different ambient temperatures and Reynolds numbers. The importance of simultaneous change of char porosity and particles size during gasification has been demonstrated for different regimes indicated by the Damköhler numbers.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042207-042207-7. doi:10.1115/1.4032791.

A method for the experimental investigation of gas–solid reactions in a small-scale fluidized bed reactor (FBR) is presented. This methodology enables high heating rates (≈104 K/s), long timescale observation (up to several hours), operation with small fuel particles (≈100 μm), and accurate control of reaction conditions. In this study, the gasification reaction of biomass-based char particles with carbon dioxide–nitrogen gas mixtures is investigated under atmospheric pressure. On varying process temperature and feed-gas composition over a wide range, consistent results are realized (temperature is varied between 1173 and 1373 K, while the CO2 concentration is adjusted in an interval of 20% up to 80%). Carbon conversion curves and reaction rates are established from real-time gas product analysis by FTIR spectrometry through a detailed data analysis procedure. This procedure employs a particle surface-evolution model and accounts for sampling system signal attenuation. The obtained reaction rates are used to demonstrate the determination of kinetic parameters for different kinetic approaches concerning the heterogeneous CO2 gasification (Boudouard reaction). Throughout this study, a comparison of both different surface-evolution models as well as kinetic approaches with experimental results is performed for the inspection of best consistency.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042208-042208-7. doi:10.1115/1.4032939.

Colorless distributed combustion (CDC) has shown to provide ultra-low emissions of NO, CO, unburned hydrocarbons, and soot, with stable combustion without using any flame stabilizer. The benefits of CDC also include uniform thermal field in the entire combustion space and low combustion noise. One of the critical aspects in distributed combustion is fuel mixture preparation prior to mixture ignition. In an effort to improve fuel mixing and distribution, several schemes have been explored that includes premixed, nonpremixed, and partially premixed. In this paper, the effect of dual-location fuel injection is examined as opposed to single fuel injection into the combustor. Fuel distribution between different injection points was varied with the focus on reaction distribution and pollutants emission. The investigations were performed at different equivalence ratios (0.6–0.8), and the fuel distribution in each case was varied while maintaining constant overall thermal load. The results obtained with multi-injection of fuel using a model combustor showed lower emissions as compared to single injection of fuel using methane as the fuel under favorable fuel distribution condition. The NO emission from double injection as compared to single injection showed a reduction of 28%, 24%, and 13% at equivalence ratio of 0.6, 0.7, and 0.8, respectively. This is attributed to enhanced mixture preparation prior to the mixture ignition. OH* chemiluminescence intensity distribution within the combustor showed that under favorable fuel injection condition, the reaction zone shifted downstream, allowing for longer fuel mixing time prior to ignition. This longer mixing time resulted in better mixture preparation and lower emissions. The OH* chemiluminescence signals also revealed enhanced OH* distribution with fuel introduced through two injectors.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042209-042209-8. doi:10.1115/1.4032940.

The flame characteristics of a pilot-scale swirl burner for air and oxy-fuel combustion of pulverized coal are investigated. The local burner air (or oxygen) ratio λ and the oxygen concentration have been systematically varied. The investigated flames were characterized recording UV emissions originating from OH* chemiluminescence indicating the reaction zone in the gas phase, measuring the axial and tangential velocities using an laser Doppler velocimetry (LDV) system and analyzing the composition of the flue gas. A change of the flame structure was revealed from the conducted measurement: the “regular” flame for the investigated burner is characterized by a cone-shaped swirling combustion zone with a distinct inner recirculation zone. Reducing the oxidizer flows through the burner leads to a breakdown of the inner recirculation zone and a significant change of the flame pattern. This change was identified by the LDV measurements as well as from the chemiluminescence images, and it was found to be closely related to the momentum flow through the burner into the main combustion zone.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042210-042210-8. doi:10.1115/1.4033142.

Carbonate looping promises low energy penalties for postcombustion CO2-capture and is particularly suited for retrofitting existing power plants. To further improve the process, a new concept with an indirectly heated calciner using heat pipes was developed, offering even higher plant efficiencies and lower CO2 avoidance costs than the oxy-fired standard carbonate looping process. The concept of the indirectly heated carbonate looping (IHCL) process was tested at sufficient scale in a 300 kWth pilot plant at Technische Universität Darmstadt. The paper presents a technical overview of the process and shows first test results of the pilot plant. Furthermore, the concept is economically evaluated and compared to other carbon capture processes.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042211-042211-7. doi:10.1115/1.4033302.

The carbonate looping process using the reversible calcination/carbonation reaction of limestone is a promising way to reduce CO2 emissions of fossil fired power plants. This paper describes the concept of an indirectly heated version of this process in which heat pipes accomplish the heat transfer from an air-blown fluidized bed combustor to a bubbling fluidized bed calciner. It defines the calciner's specific heat demand which is a pendant to the heating value of coal. The dimensioning depends on the processes inside heat pipes as well as heat transfer of immersed heating surfaces. Experimental investigations in an electrically heated batch reactor with a similar pipe grid provide heat transfer coefficients under calcination conditions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042212-042212-8. doi:10.1115/1.4033141.

The MILD (moderate or intense low-oxygen dilution) combustion is characterized by low emission, stable combustion, and low noise for various kinds of fuel. This paper reports a numerical investigation of the effect of different nozzle configurations, such as nozzle number N, reactants jet velocity V, premixed and nonpremixed modes, on the characteristics of MILD combustion applied to one F class gas turbine combustor. An operating point is selected considering the pressure p = 1.63 MPa, heat intensity Pintensity = 20.5 MW/m3 atm, air preheated temperature Ta = 723 K, equivalence ratio φ = 0.625. Methane (CH4) is adopted as the fuel for combustion. Results show that low-temperature zone shrinks while the peak temperature rises as the nozzle number increases. Higher jet velocity will lead to larger recirculation ratio and the reaction time will be prolonged consequently. It is helpful to keep high combustion efficiency but can increase the NO emission obviously. It is also found that N = 12 and V = 110 m/s may be the best combination of configuration and operating point. The premixed combustion mode will achieve more uniform reaction zone, lower peak temperature, and pollutant emissions compared with the nonpremixed mode.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(4):042213-042213-9. doi:10.1115/1.4033108.

Chemical looping with oxygen uncoupling (CLOU) is a carbon capture technology that utilizes a metal oxide as an oxygen carrier to selectively separate oxygen from air and release gaseous O2 into a reactor where fuel, such as coal, is combusted. Previous research has addressed reactor design for CLOU systems, but little direct comparison between different reactor designs has been performed. This study utilizes Barracuda-VR® for comparison of two system configurations, one uses circulating fluidized beds (CFB) for both the air reactor (AR) and fuel reactor (FR) and another uses bubbling fluidized beds for both reactors. Initial validation of experimental and computational fluid dynamic (CFD) simulations was performed to show that basic trends are captured with the CFD code. The CFD simulations were then used to perform comparison of key performance parameters such as solids circulation rate and reactor residence time, pressure profiles in the reactors and loopseals, and particle velocities in different locations of the reactor as functions of total solids inventory and reactor gas flows. Using these simulation results, it was determined that the dual CFB system had larger range for solids circulation rate before choked flow was obtained. Both systems had similar particle velocities for the bottom 80% of particle mass, but the bubbling bed (BB) obtained higher particle velocities as compared to the circulating fluidized-bed FR, due to the transport riser. As a system, the results showed that the dual CFB configuration allowed better control over the range of parameters tested.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Energy Resour. Technol. 2016;138(4):044501-044501-12. doi:10.1115/1.4032731.

In terms of CO2 emissions, the year 2030 has been addressed as a very crucial deadline for both European Union (EU) and the U.S. Whereas the U.S. Clean Power Plan proposes the reduction of national CO2 emissions from the existing power stations by 30% with respect to 2005, the EU aims at cutback by 40% from their levels in 1990. Due to the restricted emission goals dictated by the European and U.S. energy policies, both energy markets witness currently drastic changes. Whereas the U.S. wants to shift away from coal, the EU shifts away from gas due to high natural gas prices in Europe while drastically increasing the feed-ins from renewable energy sources (RES). In some of the European countries constantly growing installation of renewable energy plants is superseding natural gas-fired power plants and thus causing the electrical grid stabilization to be overtaken by coal fired power stations. On the contrary, the U.S. market due to increasing extraction of shale gas and low natural gas prices puts the gas power plants in favor and poses increasing pressure on closing some coal fired plants. A solution that uses the potential of the existing site and reduces overall emissions is converting from coal into gas-fired power plants, so-called fuel switch. Whereas for the U.S. market the later solution is relevant, in the vast majority of EU Member States the focus is on increasing the flexibility of coal fired power plants. The challenges and technical solutions developed and applied according to the demands of the market in both EU and U.S. are addressed in this paper. Both currently applied technologies and technologies under development are shortly presented.

Commentary by Dr. Valentin Fuster

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