Search

The key research findings included in this thesis entails independent experiments carried out to solve critical bottle-necking issues associated with the Type I and Type II based chemical looping process for clean coal utilization developed at the Ohio State University (OSU). The Type I chemical looping system represents a process involving high-temperature cyclic redox reactions in two or more reactors, such that the process configuration leads to the production of a high purity CO2 stream from a carbonaceous fuel source, such as coal, natural gas, or biomass. For instance, the coal direction chemical looping (CDCL) process is one example of such process that is capable of generating electricity from coal or producing hydrogen from coal without a dedicated gasifier. The results outlined in Chapter 2-4 present the development of a novel strategy that enables the utilization of caking bituminous coal, accounting for nearly 50% of the coal production in the U.S., as a feedstock to the CDCL process. The proposed pretreatment method utilizes iron oxide (Fe2O3), which is inherently present in the process as the oxygen carrier and is readily available from the process waste stream, as a catalyst to promote the decaking of thermoplastic bituminous coal. The relevant caking properties, reaction pathway, decaking mechanism relating to the thermoplastic behavior of bituminous coal is described. Furthermore, an empirical correlation between the decaking parameters and the swelling potential is presented. The Type II chemical looping system represents a process involving the high-temperature cyclic carbonation-calcination reactions, such that removal of flue gas CO2 is achieved by the chemisorption reaction with a metal oxide to form carbonate species. Specifically, the calcium-based Type II chemical looping process at OSU utilizes an inexpensive limestone based sorbent to capture flue gas CO2. The critical challenge in this process exists in the rapid decay in sorbent reactivity over multiple reaction cycles due to thermal sintering. The work described in Chapter 5-6 presents an improved process design that incorporates a steam reactivation step to counter the decay in sorbent reactivity. A novel mechanically agitated fluidized bed design is proposed to enable gas-solid steam reactivation reaction under a well-controlled fluidization regime. A detailed parametric study on the influence of the operation conditions, i.e. temperature, pressure, fluidization velocity, on the extent of sorbent reactivation is presented. Furthermore, the effect of sorbent sintering on the kinetic limitations of steam reactivation reaction is investigated. Kinetic modeling of the experimental data reveals important differences in the rate-limiting mechanism relevant for the heterogeneous gas-solid steam hydration and carbonation reactions.

Increasing global population and demand for energy has raised concerns of excessive anthropogenic greenhouse gas emissions from consumption of fossil fuels. Coal, in particular, will continue to be the backbone energy source for the electricity generation. Coal-fired power plants are expected to contribute 33% of the domestic and 40% of the global CO2 emissions (EIA, 2010). In response to this global problem, the Environment Protection Agency (EPA) on March 27 of this year proposed the nation’s first carbon pollution standard that would limit the amount of CO2 emissions from new power plants. The Clean Coal Research Group at The Ohio State University has developed an innovative high temperature technology –the Carbonation Calcination Reaction (CCR) process - that utilizes limestone sorbents to simultaneously remove the CO2 and SO2 from coal combustion flue gas. A 120 kWth sub-pilot scale CCR system has demonstrated the ability to capture greater than 90% CO2 and almost 100% SO2 from coal combustion flue gas. However, application of the CCR process on the industrial scale requires solid circulation rates on the order of thousands of tons per hour, in which case the gas-solid separation efficiency and sorbent recyclability become crucial issues that need to be addressed. This thesis addresses two critical parts of the CCR process – the particulate capture device and the hydrator. Two cyclones were designed and tested for their separation efficiency of fine Ca(OH)2 powder. The two cyclones in series design demonstrated exceptional capture efficiency during our cold model tests. Testing for its capture efficiency as part of the sub-pilot scale CCR system yielded over 99% efficiency for Ca(OH)2 powder with particle size distribution less than 10 ¿¿¿¿m. The current cyclone design provides a cost-effective way to control the gas-solid separation of the flue gas stream containing micro-size particles. In addition, a bench-scale high temperature hydrator was fabricated to investigate the reactivation of spent calcium sorbent via intermediate steam hydration. The results showed that over 80% hydration conversions were attainable with a mere 10-min residence time. A positive relationship was observed between the hydration rate and steam partial pressure. In addition, the hydration rate seemed to be inversely related to the reaction temperature and particle size. Steam flow rates had a negligible effect on the overall conversion. The hydrated sorbent was superior to CaO because of its improved surface morphology - reduced particle size, higher surface area and increased pore volume. An absolute 45% rise in the carbonation conversion of the hydrated sample was observed. Due to the inherent particle size reduction of Ca(OH)2 formation, significant elutriation of the hydrate product was observed also during our tests. Such behavior may be advantageous to our process as it abates the entrainment of un-hydrated calcium sorbents. Future investigation of the hydration process may focus on its kinetic behavior with respect to pressure and temperature at isothermal conditions. For scaling up, the installment of a heat-exchanging device is highly recommended. The heat of reaction can be extracted to preheat the combustion air, thus contributing to the overall process heat integration.