DOE OSTI2018
Staged Pressurized Oxy-Combustion (SPOC) is a coal combustion power technology being developed by Washington University in St. Louis (WUSTL) that offers the potential to deliver low-carbon power at a reduced cost. Traditional oxy-combustion plants remove most of the nitrogen in air prior to combustion, thereby burning fuel in oxygen instead of air, producing a flue gas containing primarily CO 2 and water that allows relatively simple CO 2 capture at amounts > 90%. This technology relies on flue gas recycle (FGR) to reduce the peak temperature and radiation that would otherwise occur in a fuel/oxygen only flame. SPOC reduces the peak temperatures of combustion by utilizing two or more pressurized boiler modules connected in series to produce fuel staging; hence, only a portion of the fuel is combusted in any given furnace module. This means that the thermal energy released at each stage can be captured and removed from the gases prior to subsequent stages, when more fuel is introduced. This allows the SPOC process to operate with minimal FGR, avoiding the associated efficiency losses and additional costs. The SPOC process operates at an elevated gas-side pressure, reducing boiler size, enhancing heat transfer to achieve a compact boiler configuration as compared to an atmospheric-pressure boiler design, and allowing the recovery of the latent heat of the water from the flue gas at a temperature useful to the steam cycle. The resultant net efficiency of the system is up to 6 percentage points greater than traditional atmospheric-pressure oxy-combustion, representing a step-change improvement over first-generation oxy-combustion technologies. WUSTL has teamed up with the Electric Power Research Institute, Inc., American Air Liquide, Inc., Doosan Babcock Limited, and the U.S. Department of Energy to develop a practicable and workable boiler design concept associated with the application of SPOC. In particular, due to the staged nature of the heat release, the team has identified the potential for enhanced process flexibility for controlling power generation over a wider load range than is normally available to conventional coal-fired power plants. Improved flexibility is a critical need as coal power plants are being tasked with providing grid stability to balance the growth of intermittent renewables. Not only is flexible operation important, achieving efficient power generation at reduced loads will be key as the plants are likely to operate in this regime for significant periods of time as more renewables become available. At reduced loads, coal-fired steam generators typically face challenges in maintaining temperature control of the reheat steam and to a lesser extent the main steam due to the heat transfer characteristics of these circuits when flue gas quantities are lower. This results in inefficient operation, both in terms of the boiler efficiency and steam turbine heat rate. Since the SPOC process can be configured to deliver targeted heat input to each boiler circuit by adjusting the relative fuel firing rate of each stage, better temperature control can be achieved during dynamic conditions. Oxygen supply flexibility is also a key consideration for the overall flexibility of the SPOC process given the operating constraints of conventional air separation units. The boiler design concept assessment is being carried out to identify the maximum permissible compactness of the SPOC boiler heating surfaces. The assessment will determine the minimum overall height that will deliver appropriate tube operating metal temperatures at full load and achieve rated reheat steam temperatures at low operating loads, balanced against the needs of efficient coal combustion, and the resultant slagging and ash environments. The design parameters are being validated by combustion testing in the 100-kWth pressurized combustion test rig at WUSTL. Testing results and model validation that form the basis of a full-scale boiler design that will encompass both efficiency and flexibility improvements over conventional oxy-combustion processes will be presented.
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