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.