Coal Combustion In 2010 coal accounted for more than a quarter of all the energy produced worldwide. This figure is not likely to change in the near future because coal is ubiquitous and abundant, making it a secure and reliable energy source. Unfortunately, the combustion of coal produces more CO2 per unit energy than any other fuel. Thus, CO2 emissions from coal-fired power plants are a major contributor to the rising atmospheric CO2 levels that have been linked to global climate change. In an effort to mitigate anthropogenic CO2 emissions, particularly from coal fired power plants, the Department of Energy initiated a carbon sequestration program in 1997 that continues to promote and sponsor the development of carbon capture utilization and sequestration (CCUS) technologies.
Oxy-Coal for CCUS Under conventional air-fired coal combustion the exhaust gases typically consists of more than 80 vol.% N2. In order to effectively sequester carbon dioxide underground, a pure stream of CO2 is needed. This avoids wasting energy compressing N2 and drastically reduces the volumetric requirements for CO2 storage. A pure stream of CO2 can be obtained by scrubbing the CO2 from the nitrogen rich exhaust, or by separating the oxygen and nitrogen in air prior to combustion and burning the fuel in a mixture of oxygen and recycled flue gases (i.e. oxy-fuel combustion). While both the CO2 scrubbing and oxy-fuel technologies are considered viable and are being pursued at the research and pilot-scale, techno-economic studies have indicated a preference for oxy-fuel combustion.
At LACER we are studying oxy-combustion to understand how the combustion process changes from traditional combustion in air, and we are developing novel approaches to oxy-combustion that will increase plant efficiency and reduce costs.
Combustion, which involve the interaction of many processes, including heat transfer, chemical reaction, and flow field dynamics, can be modeled with relatively high certainty with Computational Fluid Dynamics (CFD). CFD results include profiles of velocity, temperature and species concentrations, discrete phase tracking, pollutant formation, reaction rates, and heat fluxes. In the Laboratory for Advanced Combustion and Energy Research (LACER), numerical results are compared side-by-side with experimental results to gain a greater understanding of the underlying physics and chemistry in order to design and optimize combustion processes.
The computational fluid dynamics code ANSYS Fluent is utilized for the simulations. The model for pulverized coal combustion includes sub-models for turbulence, particle injection, devolatilization, char combustion, radiation, slagging and pollutant formation for NOx, SOx, and soot. A properly validated CFD model can reduce the range of experiments needed to understand and optimize an existing process and can be used as a design tool for next generation equipment.
There are two clusters in LACER dedicated to computational studies. Small-scale simulations are performed with our SPOC LINUX cluster using 4 parallel INTEL I7-3930K 6-core 3.2 GHz processors. For larger-scale simulations we use our dedicated Chernobyl LINUX cluster, a high performance cluster with 12 compute nodes and 1 head node all running Scientific Linux. The cluster has 240 Intel Xeon E5 compute cores running at 2.5 GHz and 1536 GB ECC DDR3 ram running at 1866 MHz. QDR Infiniband interconnects are used to run through an 18 port switch.
The figures below are CFD simulations of combustion under extreme conditions. The results show the effects of swirl on flame length and wall heat flux when burning coal in nearly pure oxygen. A longer and thinner flame can be obtained by reducing the swirl number, which subsequently leads to a more uniform and manageable wall heat flux. The CFD results suggest that with proper design the burning of coal in a nearly pure oxygen environment is possible and, that by elongating the combustion zone, the wall heat flux and boiler tube temperature can be maintained at acceptable levels.
Biomass is an attractive source of energy as it is renewable and its net greenhouse emissions are low since CO2 is removed from the atmosphere during biomass growth. However, biomass generally contains very high water content. For example, the broth, in the production of bioethanol from corn, contains more than 90 wt% water. In the growth of microalgae, the water content in the culture is above 99 wt%. Traditionally, costly and energy intensive dewatering processes are needed to produce energy from the raw material.
Research in LACER involves investigating flame stability and characterize flames for fuels that have a high water content. The influence of preferential vaporization on flame stability is being investigated. Various water-soluble fuels are analyzed in order to identify a chemical fuel showing strong preferential vaporization. Flame stability tests are run for different aqueous solutions under identical flow conditions and the same energy contents. Flame stability is characterized by the blow-off limit.
Ethanol, 1-propanol and t-butanol have been identified as fuels having excellent physical and chemical properties and all burn very well even when heavily diluted with water. The fuels are highly volatile and show strong preferential vaporization over water during droplet vaporization.
Glycerol is also being studied as it represents a fuel with low volatility relative to water. To obtain a stable flame for low glycerol concentrations, t-butanol or ethanol can be added as additives. Experimental results showed that attached flame can be obtained by burning a mixture of water and 8.3 % t-butanol/30% glycerol or 10% ethanol/30% glycerol under oxy-fired condition.
Current research involves understanding the physico-chemical characteristic of the fuel-water mixtures and the dynamic of their combustion. Experiments involve laser diagnostics, including Phase Doppler Particle Analyzer (PDPA) for droplet size and velocity, Planar Image velocimetry (PIV) for gas velocity, and Laser Induced Breakdown Spectroscopy (LIBS) for elemental composition.
The United States and several other countries are considering adopting hydrogen as a fuel carrier. Nonetheless, many challenges must be addressed, particularly the unusual and dangerous fire hazards that hydrogen presents.
Hydrogen has a number of unique properties that make it particularly prone to fire risks associated with leaks. Hydrogen has wide flammability limits, nominally from 4 to 75% in air, making it more susceptible to ignition than other fuels. Also, a hydrogen leak can support combustion at a much lower leak rate than other fuels and small hydrogen flames are virtually invisible to the naked eye. As such a small hydrogen leak could ignite and remain undetected indefinitely. Fire hazards and material degradation is a distinct possibility with such a flame. This could also potentially result in catastrophic consequences such as detonation or a BLEVE (boiling liquid expanding vapor explosions). This scenario is not unique to hydrogen, but it is significantly more probable because of hydrogen’s wide flammability limits.
LACER is investigating risks associated with fires resulting from small hydrogen leaks. Our results are being used to improve codes and standards for hydrogen vehicles and other hydrogen systems. To do this, we are testing and analyzing the flammability limits of hydrogen under a variety of conditions. This work involves delivering hydrogen through micro-burners or controlled leaks of different sizes and shapes. The results of this research allow for identifying the limits for stable hydrogen flames and have become the SAE standard to ensure the safe use of hydrogen. They can be found in SAE J2579, "Standard for fuel systems in fuel cell and other hydrogen vehicles, Detroit, MI: SAE International, 2013."
Our research has also lead to producing the smallest flame ever reported, at just one quarter of a watt.
The image on the left shows a near extinction hydrogen flame. Barely visible to the naked eye (look very carefully just above the top of the burner), we were able to photograph this flame in dim lighting with no flash. This picture was taken at the widest aperture setting and a 30 second exposure time.
More than 85% of world energy production comes from combustion. However, increasing energy demands and stricter emission regulations are introduce new challenges to the optimization of combustion processes. The Flame Design team, which involves investigators in the LACER lab as well as the University of Maryland (Peter Sunderland) and the University of Hawaii (Beei-Huan Chao), is investigating how to advance our understanding of flames to reduce emissions. For example, soot is responsible for reduced visibility in many U.S. cities and thousands of deaths annually from inhalation, and soot increases global warming.
For non-premixed flames, the relationship between the local temperature and local gas composition (i.e. flame structure) can be influenced by changing the compositions of the fuel and oxidizer streams. We have shown that by changing the flame structure (i.e., designing the flame) soot-free non-premixed flames at high temperature can be produced (compare the typical bright flame on the right with the blue flame on the left. The brightness on the left is due to blackbody radiation from soot particles produced in the flame. They are absent in the flame on the right even though it is the same fuel and same flame temperatures.) Our recent efforts in this area have focused on understanding how changing flame structure may influence the chemistry of soot precursor formation leading to soot-free flames at long residence times (i.e. permanently blue flames).
We have recently proposed using the carbon-to-oxygen atom ratio as a variable to characterize the flame structure. Numerical and experimental studies in LACER have shown that the C/O atom ratio allows for a direct and convenient variable for understanding flame characteristics like soot inception and radical behavior, regardless of strain rate and boundary conditions. Thus, it is easier to grasp the fundamental combustion characteristics and gain a greater understanding of flame structure and its effects on flame properties.
Current Flame Design research in LACER is focused on numerically analyzing the flame structure in C/O atom ratio space, along with developing methods for direct measurement of atomic ratios.
When combustion occurs in normal gravity, the hot gases rise and this affects the fluid mechanics and flame shape. When combustion occurs in microgravity, buoyancy effects are eliminated and thus there is greater control of the fluid flow and flame shape. Without buoyancy it is actually possible to achieve spherically symmetric flames and a one-dimensional flow field, something that cannot be achieved in normal gravity. To do this the experimental apparatus utilizes a small porous sphere inside a sealed chamber. Spherical flames can be achieved when the fuel emanates from the porous sphere into a quiescent oxidizer (a "normal" flame) or when the oxidizer emanates from the sphere into a quiescent fuel (an “inverse” flame).
In non-premixed combustion, soot formation always occurs on the fuel side of the flame. On earth in the presence of gravity, buoyancy can dramatically affect the convection direction of soot particles formed in a flame and thus the effects of flow direction on soot formation are difficult to discern. However, in micro-gravity the direction of the fluid flow is governed by the gas stream exiting the porous sphere. Thus, for the normal flame soot particles form and are transported into regions of higher oxygen concentration, while in the inverse flame soot particles are transported into the fuel rich region. This level of control, which is not available in normal gravity, allows us to isolate the effects of convection on soot inception when the stoichiometric mixture fraction (Zst) is varied. Two competing theories have been proposed for why the flame becomes non-sooty (blue) at high Zst. One is based on convection direction and the other on flame structure. By performing experiments in microgravity the effects of convection direction can be isolated.
Microgravity diffusion flame experiments have been conducted in a 2.2 second drop tower at the NASA Glenn Research Center and two flames are shown. These studies have allowed us to prepare for microgravity experiments that will be conducted on the International Space Station in 2016.
Temperature is one of the most important measurements needed in the study of reacting flows, as it contains valuable information about chemistry, emissions and efficiency. Thermocouples are the workhorse of temperature measurements, as they are inexpensive, robust, and reasonably precise and can be used to measure a wide range of temperatures. Unfortunately, thermocouples are also subject to errors caused by heat losses (or gains) due to radiation. Characterization of this error is difficult and subject to considerable uncertainties, particularly in turbulent, particle-laden flows. Past approaches, like the suction pyrometer, require significant tradeoffs in both accuracy and spatial resolution.
A novel technique has been developed at LACER, which uses high-speed rotation to create a controlled, high convective heat transfer coefficient that in turn yields a highly accurate temperature measurement without sacrificing spatial resolution. The measurements made by this technique can also be mathematically deconvolved to yield spatial resolution approaching that of a fine-wire thermocouple.
A major difficulty in understanding the behavior of flames is the presence of the reaction term in the governing equation, which is not only nonlinear but also couples the energy and species equations. Conserved scalar concept is applied in non-premixed flames to eliminate the reaction term in all but one of the n+1 species and energy equations and thus simplifies the solution of reacting flow problems.
Thus, measuring different scalars simultaneously in a broad range of combustion systems with a suitable spatial and temporal resolution is critical not only for understanding the flame behavior, but also for the validation of rapidly developing numerical models. However, the Raman Spectroscopy approach, which has been successfully used in hydrogen and diluted-methane diffusion flames, is not effective for higher-hydrocarbon fuels.
Utilizing laser induced breakdown spectroscopy (LIBS), the conserved scalars can be measured simultaneously in a broader range of combustion systems. Our study on LIBS is the first application for LIBS in the counter-flow diffusion flame and the first application in measuring mixture fraction. Excellent agreement was found between experimental and numerical results. LIBS shows its potential as a simple scalar measurement for both atomic ratios and mixture fraction in a broad range of combustion systems ( e.g. fueled by higher hydrocarbon, fuel rich). The combination of LIBS and flame analysis based on atomic ratios has the potential for a simple and informative flame diagnosis.
Single-walled carbon nanotubes (SWNTs) possess many unique electrical and mechanical properties. They can be conducting, semiconducting or insulating based on their chirality. They also have a high surface area and a high strength-to-weight ratio. These properties make SWNTs desirable for many applications including nano-electronic devices and composite materials. However, applications such as composite materials require large quantities of SWNTs and at their current cost, such nanotube applications are not feasible. At LACER we are developing flame-based approaches to producing single-walled carbon nanotubes (SWNT). Diffusion flames may provide a synthesis route capable of achieving the necessary production rates; however, past attempts have lead to poor quality nanotubes that had amorphous carbon impurities.
We have developed an approach that relies on Flame Design and the addition of silicon to produce clean, SWNTs as seen above. To its right is a photograph of the flame used to produce the materials. The flame is a laminar inverse diffusion flame where ferrocene is used as the source of iron for the seed catalyst.
Concerns over emission of CO2 from combustion of fossil fuels have prompted strong interest in developing carbon neutral or carbon negative processes to supply electricity. Biomass is a renewable alternative fuel that shows promise; however, biomass generally has very high water content. To avoid the uneconomic dewatering process and maximize the overall energy efficiency of the overall process we are currently investigating a new approach to utilize the chemical energy stored in those fuels – direct combustion. Direct combustion of high water content fuels not only avoids the energy intensive dewatering step, but it can also yield higher overall energy efficiency than traditional processes if the process is designed appropriately.
Two applications of direct combustion in power generation have been proposed by our research group:
Direct steam generation for the turbine cycle
If high water content fuels can be directly burned under pressure, this approach could be used to generate a stream of hot gas that is mainly composed of steam. High temperature steam is produced in one step, instead of the conventional two-step approach, which involves a combustor and a boiler. For oxy-combustion, the fuel could be burned in pure O2 or O2 diluted with recycled flue gas, and the products would contain CO2 and H2O. In this case, CO2 could be captured and sequestered after condensation of the steam.
A schematic of the proposed power plant is shown in the figure below. This process combines the high turbine inlet temperature associated with a gas turbine (Brayton) cycle, with the low pressure, low temperature turbine outlet of the Rankine cycle, resulting in a process that has a very high-efficiency. Algal slurry is pumped into a combustion chamber wherein it is combusted with oxygen. A mixture of water vapor and combustion products passes through the turbine and, after expansion, the water is condensed and can be recycled back to the algae bioreactor or pond at 1 atm. The calculated cycle efficiency of the ideal process shown here is 57.5%, which could be increased by incorporating multiple regeneration stages and by using higher turbine inlet temperatures and pressures.
During the water condensation process, the CO2 is separated from the water and can be treated and compressed for sequestration or a portion can be recycled back to the algae production facility, thereby increasing the CO2 concentration in the growth medium and optimizing algal growth rates.
This approach has many advantages over conventional fossil-fuel power plants with carbon capture: Since all the expensive boiler tubes are eliminated, lower capital costs are incurred and the temperature of the working fluid can be considerably higher than that of an ultra-super critical boiler, which can result in a higher thermal efficiency for the cycle. Also, since there is no boiler in this process, the system can be started up faster than a conventional power plant and thus can be more responsive to fluctuations in demand.
Staged oxy-combustion with near-zero flue gas recycle:
Staged combustion is an approach originally developed to reduce emissions of nitrogen oxides (NOx) from combustion, but can also be utilized in oxy-combustion processes with carbon capture. During traditional oxy-combustion, a combination of O2 and recycled flue gas is used for combusting the fuel, producing a gas consisting of mainly CO2 and water vapor, which after purification and compression, is ready for storage. The concept of staged oxy-combustion can reduce the recirculation rate without excessive heat flux by adjusting the stoichiometry at each burner to control the temperature.
The concept of oxy-combustion of high water content fuels can be applied to the staged-combustion approach. In the first stage, high water content fuel is injected along with pure oxygen. The flame temperature is moderated by the presence of water and excess oxygen. In subsequent stages, streams of low water content fuel are injected and react with the unreacted oxygen. Beyond controlling flame temperature, this concept has other benefits: Reduction in flue gas recirculation rate can lead to a more compact combustor, which means less equipment cost and less heat loss. Also, by eliminating flue gas recycle, the equipment to transport and clean up recycled flue gas can be avoided. Since the flue gas volume is significantly less for this case than for unstaged oxy-combustion, keeping 3% oxygen in the flue gas requires less oxygen input. A schematic of this idea for an industrial boiler is shown below.
Biomass cofiring systems, when combined with Carbon Capture and Sequestration (CCS) can remove atmospheric CO2 because CO2 consumed by biomass growth is not released back into the atmosphere after combustion. Cofiring biomass with coal can contribute to meeting Renewable Portfolio Standards (RPS), and reduce pollutant emissions, such as mercury and SOx (sulfur oxides). The physical characteristics and composition of biomass can vary significantly. When cofiring biomass with coal, these differences can impact the structure of the volatile flame, the region where combustion of volatiles dominates. The length and location of the volatile flame is important to flame stability and determines the location and extent of volatile release. This in turn has an effect on pollutant emissions, such as NOx (nitrogen oxides). Previously, the effects of parameters such as cofiring ratio, particle size and air-fired versus oxyfuel conditions on the flame length and volatile breakthrough were studied.
Our current work has focused on the moisture variation and its impacts on the system. Moisture content is one of the more highly variable and harder to control parameters of biomass fuels. We are analyzing the impacts of different moisture content fuels in the context of volatile breakthrough and volatile flame length. These two parameters are very relevant to boiler efficiency. We are also doing CFD modeling to analyze NOx emissions under our operating conditions.
This work will ultimately close a loose end on the coal-biomass cofiring work in our lab. We seek to gain a broader understanding of how flame structure, volatile breakthrough, and NOx emissions change as we alter multiple properties of the biomass and operating conditions simultaneously.
Energy resources for rural electrification are variable and widely dispersed, so a solution for one region might not be appropriate for another. In particular, the electrical needs of rural areas in developing countries are considered. These communities are often either isolated from the grid or looking for electricity independence. Both fossil-fuel and renewable technologies are considered for supplying electricity. A balanced scorecard approach for power generation in rural communities is employed, which considers societal concerns, community conditions, and economics to identify important issues in the decision-making process. The method allows renewable and non-renewable technologies to be compared alongside each other, while recommending options that best matches the community’s needs in a variety of situations and power demands.
To illustrate this approach, data from Buergin County, Xinjiang Province, China was used. To obtain this data, Melissa Holtmeyer spent one summer doing research at Tsinghua University in Beijing under a grant from the McDonnell Academy Global Energy and Environment Partnership (MAGEEP). She also traveled to rural areas in Northern Xinjiang province under the guidance of Dr. Shuxiao Wang and graduate students of the Department of Environmental Science and Engineering at Tsinghua University. Data was collected through conversations with local nomadic herders and local officials in and around Buergin County in Xinjiang. The herders had no access to electricity except through household-based renewable systems including photovoltaic solar cells, small-scale wind turbines, and a hybrid system with both photovoltaic solar cells and turbines.
Energy is one of the major challenges of the 21st century for applications including power generation and storage. Developing state of the art materials would facilitate the implementation of renewable energy sources as well as significantly reduce the carbon footprint of the transportation sector. A low cost, robust synthesis process with high reproducibility is required for the production of nanostructured lithium-ion battery cathode materials. Lithium-ion batteries are considered an attractive power source for portable devices, electric and hybrid electric vehicles, and large renewable power facilities. xLi2MnO3*(1-x)Li(Ni¬1/3Mn1/3Co1/3)O2 (x= 0 to 1) composite materials with layered structures have received attention as high-capacity, low cost, and safe cathode materials for lithium-ion batteries.
The conventional synthesis method for these materials is co-precipitation, which has challenges associated with scale up. Therefore a spray pyrolysis synthesis was developed by this group as a scalable, low cost production method. Due to the formation mechanism of the particles during spray pyrolysis, the product contains hollow spheres, which causes the tap density to be low. LACER and X-Tend Energy, LLC have been jointly developing a scalable spray pyrolysis process for the synthesis of non-hollow, solid lithium transition metal oxide materials. The method at present is capable of producing high quality battery materials at 100 gram/hour quantities. High energy layered xLi2MnO3*(1-x)Li(Ni¬1/3Mn1/3Co1/3)O2 (x=0 to 1) material is produced materials with good tap density (> 1.0 gcm-3).
Carbon Capture Utilization and Storage
Carbon Capture Utilization and Storage (CCUS) is one of the critical technologies needed to decarbonizing the electricity grid. While a comparison of different carbon capture methods favors oxy-combustion technology, the cost and efficiency penalty associated with the first generation processes are quite high.
LACER has conceived of and is developing a novel Staged, Pressurized Oxy-Combustion (SPOC) power plant to target two important approaches that can increase efficiency and reduce costs of oxy-combustion. First, the fact that CO2 will ultimately need to be pressurized for utilization or sequestration allows the boilers to be pressurized at no additional cost. When operated at higher pressure, the dew point of flue gas moisture increases and allows for effective integration of the heat of condensation into the Rankine cycle, thereby significantly increasing the net plant efficiency. Second, by staging the fuel delivery, significant reductions in the amount of flue gas that must be recycle to control temperature and heat flux in the boilers can be achieved. This also increases plant efficiency and reduces equipment costs.
A techno-economic analysis shows an efficiency increase of more than 6 percentage points for the SPOC process compared to first generation oxy-combustion processes. This increase in efficiency, along with improved flue gas cleanup processes, reduces the cost of electricity to only 35% more than that of a power plant without carbon capture. This is in line with the US DOE target of 90% carbon capture at less than 35% increase in cost of electricity.
A high-pressure oxy-combustion facility is under construction to perform experiments under SPOC conditions.