Materials with corrosion resistance are needed.
Alloy 740 (titanium, nickel, chromium, aluminum) loses abut 1 to 2 microns per year at 750C.
The DOE announced October, 2016 that it was building a prototype power plant that uses supercritical CO2 turbines. When the $80 million project goes online in about six years, it will generate 10 megawatts of energy—about enough to operate a few thousand homes. Supercritical CO2 turbines may start replacing traditional steam turbines en masse after about a decade.
Two-thirds of the electricity in the United States is generated from fossil fuel via combustion-powered steam turbines. To get to the high temperatures needed for high efficiency, steam must first be vaporized from liquid water. The steam is further heated, expanded through the turbine, and condensed to water on the other side. In this process, called the Rankine cycle, the vaporization step is a phase change that requires a large heat input but delivers no increase in temperature (or efficiency). Advanced steam turbines try to avoid the phase change by going to supercritical conditions, but attempts to exhaust heat at low temperatures push parts of this cycle to operate just above water’s critical point (374°C and 218 atm). Near this point, steam’s heat capacity increases sharply, so up to 36% of total heat input still goes to a low-temperature, vaporizer-like process (see the figure). By switching from steam to supercritical CO2 (scCO2) and running a Brayton cycle (the same cycle run by natural gas turbines), the “vaporizer” step can be avoided, providing an opportunity to replace subcritical steam plants with a cycle that could be up to 30% more efficient. These gains are expected to persist in the smaller turbine sizes suited for harvesting solar thermal energy.
NET Power invented and is commercializing a novel power system that produces electricity from natural gas that is cost competitive with current technologies and generates zero atmospheric emissions – eliminating the smokestack altogether. This system is based on a new thermodynamic cycle, the Allam Cycle, named for its lead inventor, Rodney Allam.
Regarded as a breakthrough in power generation technology, the Allam Cycle uses a high-pressure, highly recuperative, oxyfuel, supercritical CO2 cycle that makes carbon capture part of the core power generation process, rather than an afterthought. The result is high-efficiency power generation that inherently produces a pipeline-quality CO2 byproduct at no cost to the system’s performance.
The CO2 produced by combustion in the Allam cycle is recycled back to the combustor multiple times, producing a working fluid that is mostly pure, high-pressure CO2. By using a CO2 working fluid at very high pressures as opposed to steam, NET Power can avoid the “phase changes” that cause steam cycles to be so inefficient. Instead of driving a steam cycle and losing heat energy up a stack, NET Power keeps heat within the system, meaning less fuel is needed for the turbine to reach the required operating temperature.
NET Power plants employ a process called oxy-combustion, where fuel is burned with pure oxygen instead of ambient air. Oxygen is preferable to air because air is nearly 80% nitrogen. When combusted, nitrogen creates NOx, a harmful pollutant. Oxy-combustion enables NET Power plants to virtually eliminate all NOx production.
NET Power plants require an Air Separation Unit (ASU) to separate oxygen out of ambient air for oxy-combustion. ASUs are well-known technologies, but their application in the power industry has been hampered by high capital costs and energy requirements. NET Power overcomes these challenges in several ways. NET Power plants do not require all the equipment associated with a steam cycle, and so they can use this “saved” capital cost to add an air separation unit without breaking the bank. Further, NET Power has a higher starting efficiency, or “gross” efficiency, than traditional systems since steam-based energy losses have been eliminated; this means NET Power plants can absorb an ASU’s energy consumption while remaining highly efficient.
NET Power announced March 2016 that they had broken ground on a first-of-a-kind power plant that will validate a new natural gas power system that produces low-cost electricity with zero atmospheric emissions, including carbon dioxide. NET Power is a collaboration between Exelon Generation, CB&I, and 8 Rivers Capital. The 50-megawatt demonstration plant is being built in La Porte, Texas.