Research

Solid fuels such as coal, biomass, and pet coke are cheaper and more abundantly reserved than liquid and gaseous fuels. To date, most solid fuels are converted either through combustion (complete oxidation with air) or gasification (partial oxidation with air or O₂). The combustion process is relatively straightforward and hence is less capital intensive. However, the various pollutants, including CO₂, generated in the combustion process are diluted with N₂ in the air. As a result, they are at low partial pressures and are difficult to capture. Compared to combustion, gasification is cleaner, more versatile, and more efficient, especially under a carbon constrained scenario. The key challenge to gasification technology, however, lies in its intensive capital requirements and high product cost.

Here at the Li Research Group, we are developing novel approaches to capture pollutants from conventional coal and biomass combustion processes. The USEPA proposed new regulations on carbonaceous fuel combustion plants in 2010. The new regulations included a number of previously unregulated Hazardous Air Pollutants (HAPs) such as CO and dioxins for commercial boilers. They also imposed more stringent requirements on already regulated pollutants such as mercury. The new regulations present a critical challenge to utility and boiler industries since Pulverized Coal (PC) boilers account for roughly 50% of the electricity generated in U.S. Most existing pollutant control devices in PC power plants cannot meet the new regulatory standard. In our research group, novel transition metal based oxidation catalysts are being developed to oxidize these HAPs. The catalysts have the potential to stay active at relatively low temperatures with the presence of pollutants such as fly ash and sulfur oxides in the flue gas.

Using process simulation and optimization tools such as ASPEN Plus our group is exploring cutting edge approaches to improve the environmental and economic performances of conventional gasification processes. For instance, novel chemical looping units are strategically integrated with a conventional coal gasifier to effectively capture CO₂ and other pollutants while producing hydrogen. A poly-gen concept is also applied to conventional coal gasification plants and oil refineries. By fully utilizing the fuel and product flexibility of the gasification process, the efficiency and economics of the integrated system can be significantly improved.

Although incremental improvements to conventional energy conversion processes are important, scientific and technological breakthroughs in energy research are often achieved through the development of innovative energy conversion concepts. Here at the Li Research Group, we are actively developing a number of cutting edge approaches to convert fossil and biomass fuels. These novel approaches are capable of capturing nearly 100% carbon in the fuel with ultra high efficiency and low costs. When biomass is used as the fuel, these novel processes are net CO₂ sinks from the life cycle analysis standpoint.

One example of the novel concept is the utilization of a bimetallic composite oxygen carrier particle to convert biomass/coal into power/hydrogen through redox reactions. subsequently, the CO₂ generated in the process is effectively captured. Moreover, the bimetallic oxygen carrier enhances solid fuel gasification while reducing the exergy degradation in the coal/biomass conversion process. Preliminary experiments and ASPEN Plus simulations indicate that the proposed bimetallic Chemical Looping Gasification (CLG) concept can be 25% more efficient than conventional processes.

Another example is the strategic integration of the aforementioned CLG process with a Solid Oxide Fuel Cell (SOFC) assembly. Here, steam and H₂, as opposed to steam and water, is used in a closed loop for power generations with minimal steam condensation and regeneration. As a result, the exergy loss in the energy conversion process is significantly minimized. This novel process has the potential to more than double the efficiency of a conventional combustion based power plant with zero CO₂emission. Innovative approaches for liquid fuel generation have also been proposed and studied. Using the novel technology under development in our lab, green diesel fuels can be produced from swtichgrass with a net CO₂ emission of – 11 kg/gallon. We are also working with our industrial partners to explore efficient ways to “couple” methane into value added chemicals and liquid fuels with minimal environmental impacts.

At the center of our research is the development of composite reagents and catalyst particles for the oxidation or reforming of carbonaceous fuels. We adopt a systematic, multi-scale approach in designing optimum reagents and/or catalysts for various applications. Reactivity, cost, and physical and chemical properties of the potential candidates for primary metal/metal oxide, support, and promoters are considered during the initial screening process. This is followed by synthesis and experimental verifications. Kinetic modeling is conducted such that the reaction rates information can be readily used for reactor design and process development. Fundamental mechanism studies and ab-initio calculations are also conducted to explain the particle performance and to assist in particle optimization.

Specially tailored composite particles developed in our lab can readily be used for carbonaceous fuel conversion and CO₂ capture. Chemical looping is considered as one of the most advanced carbonaceous fuel conversion technologies by the U.S. Department of Energy (USDOE). Current chemical looping research focuses on chemical looping combustion (CLC) or gasification (CLG) of gaseous, liquid, or solid carbonaceous fuels using an oxygen carrier that contains a single active metal, typically Fe, Ni, Cu, or Co. Studies on oxygen carriers with two or more active metals, to date, focus on CLC of gaseous fuels for power generation. We are currently developing a bimetallic oxygen carrier enhanced CLG process. This novel process takes the advantage of the synergistic effect of multiple active metals for oxygen uncoupling and solid fuel gasification. In addition, oxygen conductive supporting materials are used to enhance the oxygen donation properties of the oxygen carrier particles. Successful development of the novel oxygen carrier particles will lead to a novel process with high energy conversion efficiency and product flexibility with minimized reactor sizes.

The multi-functional particles being developed in our lab can also be used to impove the performance of state-of-the-art gasification processes. Although the (theoretical) maximum cold gas efficiency of a gasifier increases with decreasing gasifier temperature, most modern coal gasification systems operate at slagging conditions (>1,300 C) in order to increase the gasification rates and to reduce tar and methane formation. Many attempts have been made in the past to increase coal/biomass gasification rate at low temperature through coal pretreatment and/or addition of catalysts. The pretreatment procedures and catalysts are, however, costly and sometimes highly erosive. We are developing effective and affordable multifunctional particle to enhance solid fuel gasification by (i) effectively oxidizing the fuel while moderating the temperature in the gasification zone; (ii) catalyzing the gasification reaction and tar/methane cracking and reforming reactions; (iii) ensuring uniform temperature distribution within the gasifier, thereby enhancing carbon conversion. The catalyst can be conveniently retrofitted to existing fluidized bed gasifiers to increase its syngas quality, fuel processing capacity, and efficiency.

Fluidization and multiphase flow are extremely important for energy conversion processes such as coal/biomass gasification (gas-solid) and liquid fuel synthesis (gas-liquid-solid). In our lab, we are working on developing effective catalyst and reactor systems for converting CO₂ rich syngas streams into premium diesel fuels. Slurry bubble column reactors hold several key advantages over other types of reactors for such an application because of their high heat and mass transfer rates and simplicity in construction. The hydrodynamic behavior in slurry bubble columns are, however, extremely complex. Frequently occurring issues such as backmixing, foaming, slugging, and entrainment can negatively affect the reactor performances and needs to be minimized.

The complex hydrodynamics pose great challenges in reactor optimization and scale-up. Using state of the art measurement techniques including the non-intrusive Electrical Capacitance Volume Tomography (ECVT) technology developed at the Ohio State University, the effects of a number of parameters including particle size, superficial gas velocity, gas and solids holdup, temperature, pressure, and gas distributor design are being thoroughly studied. Clear understanding of the effect of each parameter on the reactor hydrodynamics can provide valuable information for both optimization and scale up of the reactor. Slurry bubble column and bubble column reactors can also be used in bioengineering and biopharmaceutical processes.