Significant economic and process benefits can be achieved by pressurizing the entire power plant flue gas system, including the boiler. Operating a power plant firing system at high pressure (i.e. between 700 and 1300 psi) allows for the utilization of higher-heating values in coal or other carbon fuels, and enables the CO2 in the flue gas to be captured as a liquid, so it is never emitted into the air. Other harmful chemical emissions such as NOx, SOx , mercury, and particulates are also captured this way and do not enter the atmosphere where they are detrimental to air quality. The captured CO2, which is already in clean, pressurized liquid form, is now a valuable commodity ready for shipment by pipeline, rail or truck for sequestration or beneficial use (e.g. enhanced oil recovery or coal-bed flooding to recover the methane). ZEBS® can be utilized as a low-cost alternative for new power plant construction or as a cost-effective method of repowering existing fossil fuel power plants.
Benefits of "Pressurized" Oxy-combustion
Combustion and emission control strategies of a pressurized oxy-combustion system like ZEBS® differs significantly from those employed at ambient conditions. The elevated system pressure enables the use of gas-to-liquid steam-hydroscrubbing to collect and remove pollutants and recover the latent heat from water entrained or produced in the combustion process by shifting the temperatures at which water, CO2, mercury and acid gases condense. This allows ZEBS® to recover the pollutants in the aqueous phase, including CO2 in clean pressurized liquid form ready for sequestration or beneficial reuse. In addition, pressurized oxycombustion is estimated to increase power plant efficiency between 5.5 and 6 percent over most atmospheric combustion technologies, including atmospheric oxycombustion. This is achieved in part due to the recovery of the energy entrained in the water vapor produced during combustion. The water vapor content of the flue gas can be over 40% by volume depending on the fuel. High pressure allows this vapor to be condensed at a much-higher temperature than would occur under atmospheric operating conditions. As a result, the thermodynamic quality of the condensing heat is high and can be used for high value purposes such as feed-water heating to improve power plant cycle efficiency.
Heat transfer correlations and modeling show 10 to 12 times greater overall heat transfer coefficient to boiler tubes than non-pressurized combustion methods. Since ZEBS® is a Rankine cycle-based technology, it benefits from the continued improvements in supercritical boiler technology and increased steam cycle efficiency. It can directly benefit from the advances being generated by the Department of Energy's ultra-supercritical materials program as well as advancements in oxygen production.
ZEBS® Has Tremendous Fuel Flexibility
The high oxygen partial pressure operating environment also increases char reaction rates resulting in high carbon burnout at reduced residence time. This, in combination with the flexibility of pulverized coal, makes the ZEBS ideally suited for a wide range of fuels, particularly low rank fuels with high moisture content such as subbituminous, lignite, and/or coal water slurries. Recovery of the heat of condensation of water vapor at useful temperatures, integrated pollution control and CO2 condensation at ambient temperature enable the system to mitigate the energy demands of carbon capture. It can be utilized for new generation on coal-fired plants with advanced steam parameters and has unique technical and cost advantages for repowering the United States' existing power fleet.
ZEBS® can be Up to 10 Times Smaller in Size Than Conventional Power Plants
Pressurized combustion also leads to a significant reduction in furnace volume and heat exchanger sizes compared to ambient air or oxygen fired systems. In addition to lower specific gas volumes, we have found significantly enhanced convective heat transfer rates due to pressure. In short, a ZEBS® power plant can be up to 10 times smaller than a conventional power plant of equal thermal output, providing significant capital cost advantages. In lower capital cost and much smaller footprint makes ZEBS® an ideal technology for repowering existing power plants.
ZEBS® can be integrated into existing power plants without shutdown
A ZEBS® power block can be added to an existing power plant without the need to take the existing boiler system offline. This means the power plant does not have to suffer the revenue losses associated with de-rating a plant while replacing an old conventional boiler system with a new one. ZEBS® promises to achieve carbon capture with superior fossil-fuel power plant thermal efficiency due to its novel and patented process design and is expected to have an economic and environmental advantage over competing carbon capture technologies.