Experiments on Low-Temperature Diesel Combustion | Alternative Engine Fuels | Engine Combustion Modeling | Modeling of Biomass Thermochemical Conversion | Bio-oil Gasification | Biomass Gasification and Syngas Combustion
Experiments on Low-Temperature Diesel Combustion
This research is motivated by the federal emissions mandates, demands for fuel economy and durability, and the desire to explore the complex spray combustion phenomena in engines. We have developed a fully equipped, multi-engine test facility to characterize and optimize diesel engine combustion and emissions.
Low-temperature combustion is a strategy to reduce both NOx and particulate matter (or soot) emissions simultaneously. We strive to optimize engine operating conditions and combustion chamber geometry to achieve low emissions and high efficiency. The present approach is to utilize the advanced fuel injection system, ultra-high injection pressure, non-traditional injector nozzle designs, and exhaust gas recirculation to achieve low temperature combustion. The goal is to reduce exhaust emissions while maintaining high efficiency.
John Deere engine used for experiments (left); Graduate students working in the control room (center); Exhaust emissions analyzers(right).
Diesel low-temperature combustion can be achieved by using EGR, flexible fuel injection schemes, fuel reactivity control, boost pressure, etc. Eventually it is a matter of optimizing operating parameters to achieve the desirable features. To prevent the time-consuming parametric study, we have developed an algorithm based on Particle Swarm Optimization (PSO) to speed up the optimization. Based on a multi-cylinder John Deere engine, we integrated PSO with engine control systems for optimization. PSO has proven to be an efficient means of exploring optimal operating conditions to help achieve desirable conditions. Research is in progress to further explore favorable operating conditions for new engines.
This movie shows how PSO is used to converge the search to the desirable location (red dot on the upper right).
High-pressure injection system
We utilize ultra-high fuel injection pressure to help atomize the fuel and create favorable mixture for combustion. The system is capable of 300 MPa injection pressure with flexible control for multiple injections.
Effects of injector geometry on engine performance
Effects of injector nozzles geometry on combustion and emissions were investigated. Various injectors were tested, including different nozzle flow number, nozzle hole size, nozzle hole arrangement, nozzle conicity, and included spray angle. We explored the potential of various nozzles in accordance with injection parameters to achieve low emissions.
Injection system characterization
We developed an injection bench for measuring the fuel flow rate and characteristics of the injection system, particularly the flow characteristics of multiple fuel injections. These data can provide CFD models with accurate fuel injection rate data. This injection bench is also used to characterize the effects of new biofuels on injection characteristics.
Injection bench for injection rate characterization.
Diesel spray combustion diagnostics
In collaboration with John Deere, we have built a constant-volume combustion chamber to investigate diesel spray, combustion, and emissions characteristics in a well-controlled environment.
Constant-volume chamber (left); Diesel spray image (center); High-speed, consecutive PLIF images of diesel spray at the same injection event.
Alternative Engine Fuels
In addition to conventional fuels, engine performance of using biorenewable fuels is also of our research interest. We perform engine tests and field studies to characterize engine emissions and fuel efficiency using various blends of biofuels including biodiesel, ethanol, renewable diesel, and pyrolysis oil derived from biomass. The goal is to develop fundamental understanding and strategies to adopt biofuels for engines.
Our past biodiesel engine research includes (1) developing strategies of low-temperature combustion using diesel/biodiesel blends, (2) field study of commercial trucks fueled with diesel/biodiesel blends, and (3) measurements of exhaust emissions of large utility diesel generators using blends of biodiesel and diesel in two power plants in Central Iowa (shown in the pictures below).
Large diesel engine used for biodiesel combustion testing (left); Measuring exhaust emissions (right).
Energy recovery from waste plastics
Biodiesel happens to be a good solvent that can dissolve some plastic materials. This research explores the possibility of burning mixtures of biodiesel/polystyrene in a diesel engine as a means to recover the waste energy. Engine performance, including power and emissions, is characterized using various ratios of biodiesel and polystyrene.
Diesel engine used to characterize performance of biodiesel/polystyrene mixtures.
Ammonia engine – a non-carbon fuel
Ammonia (NH3) is a carbon-free fuel whose combustion does not produce carbon dioxide, a known greenhouse gas. Hydrogen has also been regarded as a carbon-free fuel, but there are too many technical obstacles to achieve a hydrogen economy in the near future. In fact, ammonia can be viewed as a “hydrogen carrier” and has many benefits, including well-established infrastructure, high energy per volume, low energy to store, non-flammable, and easy to detect, etc. In recent years, new technologies have been developed to produce ammonia from renewable sources (e.g., wind, solar, solid-state synthesis), and thus ammonia can also be viewed as a renewable energy carrier. Our research group is the first to demonstrate that ammonia can be used in a modern compression-ignition engine using dual-fuel operation – ammonia and a secondary fuel. The fuel induction strategies include (1) gaseous ammonia induction to the intake manifold with direct-injection diesel fuel, and (2) direct injection of mixtures of ammonia and dimethyl ether. Research is in progress in using direct ammonia injection in a spark-ignition engine to increase the engine load limit.
Ammonia engine (left); mixing system of ammonia and dimethyl ether (right).
Engine Combustion Modeling
Our work is among the first of its kind in incorporating detailed chemistry with 3-D computational fluid dynamics for homogeneous charge compression ignition engine simulation, and this approach has been widely adopted to characterize the fundamental low-temperature combustion processes. Overall, the research includes the development of various spray, fuel vaporization, heat transfer, combustion, and soot emissions models.
We are developing high-fidelity computational fluid dynamics models that feature detailed chemical kinetics to simulate autoignition, fuel oxidation, and soot and NOx emissions. Accurate numerical schemes based on adaptive mesh refinement are also developed for efficient simulation. The ultimate goal is to develop predictive numerical models that can be used as a tool for engine design and optimization.
The four movies below show predicted in-cylinder spray, combustion temperature, flow structure and velocity, soot, and NOx distribution for a Caterpillar diesel engine.
Multicomponent Petroleum-Biofuel Vaporization
In real-world applications, liquid biofuels are blended with petroleum fuels, and there is a strong demand in increasing biofuel concentration in the blends. However, one of the major challenges to the widespread application arises mainly from the unusual fuel properties of these fuel blends. These fuel mixtures are often modeled using binary components that, unfortunately, do not faithfully represent the actual fuel effects. Multicomponent vaporization modeling is one of the keys to the accurate prediction of mixture distribution which, in turn, will determine the accuracy of combustion prediction. We have developed multicomponent vaporization models based on a hybrid approach – using continuous thermodynamics to represent petroleum fuels and discrete component methods to simulate biofuels. Sample results are shown below. Work is in progress in extending the modeling capability to predict vaporization of novel biofuels, e.g., various bio-oil fractions derived from fast pyrolysis of lignocellulosic biomass.
Next-generation engine simulation code
We collaborate with Los Alamos National Laboratory in developing the parallelized KIVA-4 code – a new generation engine simulation code.
Modeling of Biomass Thermochemical Conversion
Thermochemical conversion is a promising technology to exploit the energy embedded in lignocellulosic biomass. Combustion, gasification, and fast pyrolysis are the three main thermochemical conversion methods that are of current interest in converting non-food biomass to heat, power, and/or fuels. The goal of this NSF-sponsored research is to develop an open-source computational model to simulate and optimize thermochemical conversion of biomass to accelerate the deployment of bioenergy. This is a work in progress and it is anticipated that the code will have extensive features in predicting phenomena including complex fluid flows, chemical reactions, turbulence, heat transfer, and fluid-solid interactions.
The code is being developed and validated to simulate the gas-particle flows inside a fast-pyrolysis reactor at Iowa State University laboratory. The code is available to the community upon request. The object-oriented programming strategy enables users to incorporate their own models effectively.
Fast pyrolysis of biomass to produce bio-oil has moved to the forefront of bioenergy research and development. Bio-oil, which is a mixture of complex oxygenated hydrocarbon species, is much easier to transport than bulky solid biomass. A novel approach is to convert biomass to bio-oil at widely distributed small-scale processing plants, transport bio-oil to a centralized location, gasify bio-oil to syngas, and upgrade the syngas to transportation fuels. This research is to investigate this approach through a combination of experimental and analytical studies that can potentially lead to the large-scale commercialization of this technology that has the potential to turn agricultural residues (e.g., corn cobs, corn stover, switchgrass, etc) into valued feedstock. This project is sponsored by Iowa Energy Center. Our group, as a team member of Bioeconomy Institute, will conduct experiments, technoeconomic analysis, reactor simulation, and plant optimization of biorefinery based on bio-oil gasification.
Biomass Gasification and Syngas Combustion
This research is focused on the utilization of biomass energy based on the gasification platform. Thermochemical gasification is a promising technology that is less restricted to the type of biomass. Gasification takes place at moderately high temperature and turns solid biomass into combustible gas mixtures (known as synthesis gas or syngas) through simultaneous occurrence of exothermic oxidation and endothermic pyrolysis under limited oxygen supply. Syngas derived from biomass gasification can be burned to generate heat and power or synthesized to produce liquid fuels.
The research is performed on a pilot-scale gasifier that incorporates a novel gas conditioning system for gas cleanup and an oxygen-enrichment system for gas upgrade. The gasifier consumes 5 tons per day of feedstock. Advanced instrumentation based on gas chromatograph and mass spectrometer (GC-MS) is used for characterizing the syngas composition as well as the exhaust flue gas emissions from the burner. This gasification/combustion system is being facilitated to develop clean burner for syngas combustion.
Gasification of biomass with controlled nitrogen contents
During gasification, nitrogen in biomass is released as ammonia which, in turn, will result in significant “fuel NOx” emissions after combustion of syngas. Thus, research is conducted to characterize the relationship between nitrogen content in biomass feedstock, ammonia concentration in syngas, and NOx emissions in flue gas. Various feedstock with controlled nitrogen contents are tested, including wood, wood+7%DDGS, wood+13%DDGS, wood+20%DDGS, and wood+40%DDGS, with nitrogen varying from 0.14 to 1.75 wt%. These results provide guidance to the burner design in order to burn biomass-derived syngas successfully with low NOx emissions.
Biomass gasification using oxygen-enriched-air and steam
Biomass gasification using air produces a low heating value syngas (4 to 7 MJ/Nm3) that is only suitable for heat and power generation. On the other hand, steam and oxygen can increase the heating value of syngas (10 to 14 MJ/Nm3) as well as the main gas constituents of H2 and CO that are suitable for liquid fuels production through the FT synthesis. We conducted experimental research using different feedstock and gasifying agents. The gasifying agents include air, 45% oxygen with steam, and 80% oxygen with steam. A 1500-gallon liquid oxygen maintained at −400 ºF is used to supply oxygen. Results indicate that oxygen-enriched-air and steam gasification favors the production of combustible gas components. Hydrogen can increase by 70% and carbon monoxide by 34% when the highest concentration of oxygen is used.
Low fuel NOx burner development
Traditionally gas burners are designed and optimized for low thermal NOx emissions. To enable the clean combustion of biomass-derived syngas, a burner needs to mitigate the fuel NOx emissions resulting from ammonia combustion in the syngas. Research is conducted to develop accurate computational models and chemical reaction mechanisms that can simulate biomass-derived syngas combustion. Computational results are validated using experimental data from our pilot-scale gasifier/combustion system, and the models are used to design a new burner that can mitigate fuel NOx emissions.
Burner mesh (left), predicted T and fuel NO(right).
Integration of geothermal energy into biorefinery
Geothermal energy originates from the original formation of the planet as well as the radioactive decay of the minerals. Geothermal energy can be used in various ways, including direct use, electricity production, and heat pumps. Each geological region has its own geothermal conditions. For regions with abundant agricultural products such as Iowa, it can be of great interest to combine both biomass and geothermal energy. This research is to study the feasibility of utilizing geothermal energy in a biorefinery that produces liquid transportation fuels. Geothermal steam is used for biomass drying, gasification, steam-methane reforming, and electricity production (via Organic Rankine Cycle). Such integration provides a new way of combining two renewable energy technologies to produce renewable fuels.