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Research Gate Link:
300 MW Boiler Design Study for Coal-fired Supercritical CO2 Brayton Cycle
Science Direct Link:
300MW boiler design study for coal-fired supercritical CO2 Brayton cycle
The ultimate source is Applied Thermal Engineering, Volume 135, 5 May 2018, Pages 66-73.
Abstract
Supercritical CO2 (S-CO2) Brayton power cycle has been considered as a promising alternative choice of conventional steam cycle for coal-fired power plants. A conceptual design of the boiler is conducted for a 300MW single reheated recompression S-CO2 Brayton cycle for coal-fired power plant with turbine inlet parameters of 32MPa/600°C/620°C. The conventional economizer (ECO) is replaced with the split heater (SH) to reduce the inlet temperature of cooling wall of the furnace as well as to recover the flue gas heat The technology adaption of S-CO2 power cycle for coal-fired power plant has been evaluated in terms of specific design of the 300MW coal fired boiler as well as the whole thermodynamic cycle layout. The boiler design and off-design thermal calculation results show that the S-CO2 boiler proposed in this paper can match well with the entire coal-fired S-CO2 Brayton cycle power generation system and has a good boiler variation performance.
As the linked design study seems to suggest, the thermal power density for coal-fired Brayton Cycle sCO2 could exceed 1,000MW/m^3. That's a pretty dramatic power density increase over existing steam boilers. The study was published in 2018, and makes the case that going to 700°C temperatures is not economic because it requires Nickel-based superalloys. However, more recent advances in materials, machining, and fabrication methods have already demonstrated cost-competitive superalloy solutions that outperform steam turbines on cost in SCO2 applications, due to the dramatic mass and volume reductions of the machinery involved. We've already begun fabricating SCO2 gas turbine components for SCO2 commercial electric power plants here in Texas, from those superalloys, and the most significant costs seem to be in finding qualified machine shops and welders with the expertise to machine or fabricate the components.
Very recently demonstrated 100X energy (thus fabrication cost) reductions associated with complex geometry RCC components could make superalloy cost and availability mostly irrelevant, except perhaps for printed circuit heat exchangers, piping, and seals. Using RCC, a power turbine and turbine casing's combined gravimetric power density could easily reach 200kW/kg for a multi-MW power turbine suitable for marine propulsion applications.
In previous posts scattered about the forum, I've put together a "system" for supplying bulk power to the United States, and potentially the entire world, without resorting to using either nuclear power of quantities of technology metals that don't presently exist to be used for all-electric energy generating and storage equipment. While I highly favor the use of nuclear technologies on account of their incredible power density and vanishingly small waste streams, relative to all competing alternatives, I have accepted that the combination of our political and economic climate has resulted in very little apparent appetite for multi-year reactor construction projects. I have also accepted that the materials math for photovoltaics and/or electric wind turbines, but especially electro-chemical batteries to provide enough fast storage for grid stability and seasonal energy availability variations, simply "doesn't math". I came to that conclusion some time ago and nothing I've seen since then has changed the math, so I discarded that as a reality-based solution. It was a good old-fashioned "college try".
As such, I then devoted hundreds of hours of study searching for viable alternatives to new-build nuclear reactors and the current generation of photovoltaics / electric wind turbines / electro-chemical batteries, which also have little hope of scaling-up to supply the majority of our energy demands over the next several decades, absent monumental increases in mining productivity across a host of technology metals. The specialty metals requirements of a 70% photovoltaics / electric wind turbines powered grid, when combined with mere weeks of energy storage, so as to truly supply the majority of our primary energy, without coal-fired steam turbines or natural gas turbines spinning at all times, is exactly where this "green energy" fantasy fails.
The proponents of this all-electric "solution", which at least outwardly appears to create more problems than it solves, are either hoping for game-changing technology advances and production at-scale at some indeterminate future date, or refuse to accept the ugly arithmetic of projected scale-up for mining output and specialty metals consumption per technology unit. The required quantities of poly-Silicon, Copper, and Aluminum vividly illustrates how far beyond present global annual mining output those technology metals requirements are, in order to implement their all-electric vision for the future. The metals requirements are measured in hundreds (Copper, Aluminum) or even thousands of years (rare Earths) of current global annual production capacity. While I would never claim that tech advances cannot overcome existing implementation hurdles, the number of hurdles and the scale of the mining output increases are grossly unrealistic using current or projected near-term technologies.
Most of these people think we will simply stop using stored chemical energy provided by hydrocarbon fuels, to deliver on-demand power, in favor of a variety of new technologies that are only feasible to use at the present time because they are back-stopped by stored fuels or, in some cases, nuclear energy. The transition process to achieve that might take another century before mostly displacing coal / natural gas / diesel / kerosene / gasoline fuels, since all potential successor technologies are still in their infancy. In the mean time, we're playing a dangerous game with money and technology by making "bets" we can't actually cover when something goes wrong. I think the complete grid failure in Spain demonstrated how far belief about the stability of an all-electric grid, predominantly powered by photovoltaics and electric wind turbines, which provide no grid-inertia, diverged from objective reality. Spain attempted to prove that they didn't require a "spinning reserve", which was provided by their small fleet of nuclear reactors. The only thing they actually proved, was that even during ideal generating conditions for the currently favored all-electric energy generating machines, Spain's grid was never stable. If conditions had been unfavorable, there could've been many more fatalities and a far greater loss of GDP. Thankfully, that didn't happen. Only circumstances made Spain's total grid failure a brief event with limited permanent damage. Spain doesn't have sufficient fast storage or spinning reserve, unless they keep operating their nuclear reactors. Shutting the reactors down was an ideological vs engineering-driven decision- one that didn't pay off.
For nations without nuclear power programs, the lowest cost "pay-as-you-go" bulk energy comes from coal or natural gas turbines. If money is plentiful, then extra funds can be invested into photovoltaics and/or electric wind turbines to opportunistically capture more energy. Provided that the cost increase to the rate payer is not substantial, there's nothing wrong with capturing additional energy that way. An issue arises only when the grid is reliant on those forms of energy, but insufficient stored energy exists as a backup. Nations such as Germany and Spain are already well past the point of sensibility in their energy mix, because they either can't or won't build adequate fast storage, likely due to cost. Everyone knew that an all-electric grid lives or dies on the basis of fast storage (electro-chemical batteries) availability to buffer supply-and-demand fluctuations. We're not going to do that at the scale / storage capacity required, due to total cost and materials scarcity. That means we need a viable alternative.
I think pure Carbon "synthetic coal" mixed into water to create a low-flammability pumpable slurry, synthesized from atmospheric or oceanic CO2 captured using solar thermal power, is that more viable alternative. Since we didn't dig this stuff out of the ground, we're adding nothing to the atmosphere by using and recycling it. High-purity captured CO2 also has many uses beyond serving as a fuel feedstock. Thanks to some new / novel room-temperature liquid metals, such as Gallium-Indium-Tin and Gallium-Indium-Copper eutectic mixtures, we have discovered a way to convert atmospheric CO2 back into pure Carbon without any electrical or thermal energy beyond the power necessary to circulate / "bubble" the CO2 through the column of liquid metal. The pure Carbon "floats" on top of the metal, so extracting it is pretty easy to do. The metal catalysts also appear to be remarkably stable over hundreds of hours of operation. That means we could use them to strip Carbon from CO2, at room temperature, to create chemical energy reserves without mining for coal or drilling for oil and gas. The lower calorific content of a pure Carbon fuel, as compared to fuels containing Hydrogen, can be partially offset by using higher temperatures in conjunction with Supercritical CO2 gas turbines.
The exhaust product of a power plant consuming pure Carbon and O2 from a synthesis plant, is essentially pure CO2 mixed with some residual water vapor, so it can be more easily captured at the plant and re-compressed into LCO2 for shipment back to a Carbon / O2 synthesis plant. The most undesirable emissions from burning mined coal- heavy metals, fly ash, NOx, and SOx, are reduced to almost nothing by using synthesized pure carbon. We re-capturing most of the CO2 at the generating plant so we don't have to re-capture it from the atmosphere at greater energy cost. The development of supersonic CO2 compressors makes that re-capture step practical, as it consumes less than 10% of the plant's gross output, unlike traditional multi-stage CO2 compressors.
SCO2 gas turbines, supersonic CO2 compressors, and SCO2 "boilers" have all been developed over the past 25 years, in a concerted effort to meaningfully improve the thermal-to-electrical efficiency of coal and natural gas power plants. The most consistent themes throughout SCO2 technology development have been successful technology demonstrations and incredible power density, to the point of becoming a more thermally efficient successor to conventional gas turbines and geared steam turbines.
The two major reasons for switching from geared steam turbines to gas turbines for commercial electric power generation and marine propulsion were far too much space claim associated with steam power plant and improved thermal efficiency / fuel consumption reduction. SCO2 gas turbines manage to improve upon the power density, startup times, and thermal efficiency of conventional marine gas turbines, which is why we need to pursue these new turbine-based solutions, if we're eventually going to have any meaningful energy transition to natural energy.
We're not short-of-supply of coal / oil / gas at the present time, but the existing reserves won't last forever. Eventually we'll have to synthesize our own fuels, if only for backup power or power at night, and we'll want more efficient plants to use them with. It's better to start that process now while we're still moderately energy-rich and capital-rich.
We need solar thermal to deliver bulk energy in the form it's already consumed in, we need new generation power plants that can supply on-demand energy, and we need fuel synthesis so that we never run out. We're not going to create enough all-electric machines in the span of a few short decades to matter much, if only because we lack the metals to do so. We do have sufficient supplies of metals for the types of machines I've spilled so much ink describing, to continue generating electric and thermal power from centralized locations.
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Quite a lot to read through here, so I will comment again when I've had chance to read it all. Gas turbine blades have always been made from high temperature nickel alloys. Since the 90s, they have been grown as single crystals with mineral rods embedded to provide cooling channels by dissolving the rods in weak acid after casting. So I'm not sure why the reference suggests that using nickel alloys is impractical or expensive. It is standard aerospace practice. Take any COTS GT and you find nickel alloy components. For non-moving parts, steels can still be used at 700°C. Strength will be reduced substantially and corrosion in hot CO2 will be more of a problem. But is can be done. There are specialist oxide dispersion strengthened mechanical alloys that were specifically developed for operation in this temperature range.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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