Considering an Alternative Hybrid Allam Heat Engine Cycle for the Removal of CO2 from the Air.
In this post, I will consider two papers from the primary scientific literature. Both are about a form of energy I oppose but nonetheless has proved to be the fastest growing source of energy on the planet in this century: Coal. I trust my discussion of these papers will not in any way distract from my often stated position that coal is unacceptably dangerous and should be phased out, along with the other two dangerous fossil fuels, dangerous petroleum and dangerous natural gas, beginning immediately, on an emergency basis.
The papers are:
Parametric study of a direct-fired supercritical carbon dioxide power cycle coupled to coal gasification process (Zhang, Wang, Chi, Xiao, Energy Conversion and Management 156 (2018) 733–745).
Atomistic Simulation of Coal Char Oxy-Fuel Combustion: Quantifying the Influences of CO2 to Char Reactivity (Yongbo Du,†,‡ Chang’an Wang,†,‡ Haihui Xin,‡,§ Defu Che,† and Jonathan P. Mathews, Energy Fuels 2019, 33, 10, 10228-10236).
Coal, of course, is nothing more than sequestered carbon, carbon that was sequestered over hundreds of millions of years from biomass. In a few generations, roughly in two centuries, humanity has more or less de-sequestered the bulk of it, producing the dangerous fossil fuel waste carbon dioxide which is dumped into the atmosphere without charge, is rapidly destroying the entire planetary atmosphere.
The largest single contributor to the 7 million air pollution deaths that take place while dumb guys carry on about how dangerous nuclear energy is, almost certainly derive from coal. Since coal is nothing more than sequestered and carbonized biomass, it is unsurprising that the second largest contributor to these air pollution deaths is likely to be fresh biomass, probably followed by deaths from air pollution related to dangerous petroleum.
Thus the relationship to historical fossil biomass, coal, and modern fresh biomass is close, with coal being somewhat more dangerous than "renewable biomass" since aqueous solutions of certain toxic metals, for example lead, mercury, uranium and cadmium have leached through coal resulting in their extraction from water and their concentration in coal formations. This is why the other toxic coal waste, coal ash, is such a toxicological nightmare, because the ash contains these metals, concentrated over thousands upon thousands of millennia.
Although in general I oppose so called "renewable energy" because it is not environmentally sustainable, and because it is rather dirty and destructive to both wildlife and to pristine wilderness rendered into industrial parks, it is nonetheless true that one form of so called "renewable energy" represents an opportunity to re-sequester the carbon released by the combustion of dangerous coal. This is of course, biomass. I personally believe that it is feasible to engineer away some of the more odious and baleful effects of the use of biomass to produce energy, hence my interest in the Allam cycle.
The earliest commercial nuclear reactor in the Western world, unlike the bulk of nuclear reactors operating today (with some exceptions), did not use a water/steam system as the working fluid to drive turbines in order to generate electricity. This reactor was the Calder Hall nuclear reactor in Great Britain, which began construction in 1954 and came on line in 1956, and operated until 2003. The Calder Hall Reactor used carbon dioxide as a working fluid. As Great Britain was at the time a nation which generated the bulk of its electricity from coal, the Calder Hall reactor ran for almost half a century saving human lives that otherwise would have been lost to air pollution.
The Calder Hall reactor was a very innovative device in its time, built largely on 1940's and early 1950's technology, but one of the first devices not only to be powered by nuclear fission, but also to use as a working fluid carbon dioxide, thus offering certain thermodynamic advantages to be discussed below in an excerpt of the paper cited above by Chinese authors.
The Allam Cycle is a thermodynamic cycle, a modification of the Brayton cycle by which jet engines and a number of dangerous natural gas power plants operate. It developed by an Englishman, Rodney Allam, and is being piloted and developed by a company called "8 Rivers Capital" in North Carolina. It also uses carbon dioxide as a working fluid, but with a twist, the working fluid is also the combustion gas, with the combustion taking place not in air, but rather in pure oxygen, that is an oxyfuel setting.
I believe I have discussed the environmental advantages of oxyfuel combustion here and elsewhere on the internet. By substituting pure oxygen for air the combustion chamber, one can achieve very high combustion temperatures, high temperatures being an condition which always raises the Carnot efficiency of power plants and also allows for certain types of industrial chemical processing, in an extreme case, for example, the production of concrete precursors. (The manufacture of concrete is a huge contributor to climate change.) The other advantage is the near elimination of nitrogen oxides as a component the combustion of dangerous fossil fuels (and for that matter, biomass) waste, said nitrogen oxides being currently a huge environmental problem. The largest advantage is however, is that the combustion gas is almost pure carbon dioxide, greatly simplifying the recovery of the gas. Disadvantages of oxyfuel combustion are largely related to corrosion effects in the materials of the combustion chamber. The temperatures are high enough to volatilize salts, among other things, and any steam resulting from the combustion will likely be (depending on pressure) in a supercritical state, where water it quite acidic, pH ? 3, and thus corrosive.
On some level, the Allam Cycle is obvious - I've certainly had similar ideas over the years - and it's quite possible that materials science issues will impact the viability. Nevertheless it seems quite possible that variants might be of interest for issues in climate change.
As being developed by 8 Rivers Capital the Allam Cycle is clearly focused on continuing the use of dangerous fossil fuels, with the lipstick on the pig being the idea of sequestering carbon dioxide in giant carbon dioxide dumps that are frequently discussed as a potential "solution" although in reality they have not been built, are not being built and hopefully never will be built on any appreciable scale. This said, carbon dioxide is being utilized to produce more dangerous fossil fuels, for example in "Enhanced Oil Recovery" operations and similar fracking type dangerous natural gas recovery operations.
The construction and photographs of the construction of pilot Allam cycle plants can be found in a paper co-authored by Sir Rodney himself. It is here: Demonstration of the Allam Cycle: An Update on the Development Status of a High Efficiency Supercritical Carbon Dioxide Power Process Employing Full Carbon Capture☆ (Allam et al, Energy Procedia 114 ( 2017 ) 5948 – 5966) The pictures in this paper, which I will not reproduce here, show plants being constructed in partnership with Toshiba. The marketing goals that 8 Rivers Capital are exploiting to raise money for this enterprise are also listed, they are these:
2. The global market for new and replacement fossil fuel plants through 2040 [24]
3. The needs for CO2 for enhanced oil recovery [25].
4. The needs for CO2 for enhanced coal bed methane recovery [26].
The realization of any of these marketing goals with the possible exception of part of goal 2, would represent a continuation of the ongoing disaster for humanity, although the money raising aspect should not obscure the potential real value for doing what the conference at which it was presented stated was supposed to endorse. The conference was called "13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland"
Building carbon dioxide dumps in lieu of the one now being used, the planetary atmosphere, is not controlling carbon dioxide by the way. It is very much the same thing as what we are doing now, dumping the costs and responsibility for cleaning up after our lifestyle on future generations. In any case it won't happen. We are dumping more than 35 billion tons of carbon dioxide each year. Thus the idea that we can contain this gas forever, this possibility often being raised by ignoramuses who contend that we cannot contain 75 thousand tons of largely solid (and generally valuable) used nuclear fuel, assembled after half a century of operations without costing a single human life, is so absurd as to be considered insane, but somehow isn't so considered.
In any case, the Energy and Management paper gives a nice overview of the reasons for the thermodynamic superiority of carbon dioxide in comparison to water, after a burst of truth about the fact that, despite what you may have heard, coal is not dead, far from it. The data from the 2018 IEA World Energy Outlook report shows that despite much ballyhoo offered by provincials in the United States, where the coal industry is declining and being replaced by "only" half as bad dangerous natural gas, coal has been the fastest growing source of primary energy on this planet in the 21st century. (The 2019 World Energy Outlook will come out on November 13; I personally don't expect it will produce a significantly different result - coal experienced a very modest decline between 2016 and 2017, but the decline was completely and totally trivial, and easily erased by increases in the use of dangerous petroleum and dangerous natural gas.)
The nice overview from the introduction, despite the grammatical artifacts of its translation from Chinese into English, to the Energy Conversion and Management paper:
The direct-fired supercritical carbon dioxide (sCO2) power cycle is one such promising candidate. By combustion of the fuel gas with stoichiometric oxygen and recycling CO2 as the combustor temperature moderator, the working fluid of the power cycle is highly enriched in CO2, with its molar concentration well above 90% [5]. The sCO2 based power cycles are well known for their high efficiency potentials [6]. The high efficiency comes from the superior physical property of CO2—the moderate critical point at 30.98 °C and 73.8 bar [7]. The much lower critical point of CO2, compared with that of water, facilitates the utilization of the unique thermodynamic advantages brought by the supercritical fluid. On the one hand, as a consequence of the low critical temperature, the compression of the sCO2 power cycle could occur near the critical point, an area where the physical property experiences abrupt variation, especially the density [8]. The working fluid in this area behaves more like liquid rather than gas. The compression work can thus be much reduced and the efficiency enhanced. On the other hand, owing to the low critical temperature again, the isothermal evaporation or condensation process of the working fluid is avoided in the cycle.
The Allam cycle is not particularly different than a “normal” Brayton cycle plant, in which an exhaust gas, derived from the combustion of fluidic dangerous fossil fuel directly drives a turbine except that in the Allam cycle the carbon dioxide component is compressed rather than exhausted to the air, and then recycled back into the fuel.
The recycling of carbon dioxide back into the fuel has the property of slightly cooling the fuel while simultaneously “dry reforming” the dangerous natural gas. This is because at high temperatures, carbon dioxide becomes and oxidant for methane in an endothermic reaction in which three carbon dioxide molecules react with a molecule of methane to give two molecules of water and four carbon monoxide molecules. If the conditions are correct from a mass balance and energy perspective, two of the carbon monoxide molecules can be reoxidized to carbon dioxide while the water is reduced to hydrogen gas; this is the water-gas-shift reaction, the water gas shift reaction being the reaction by which almost all of the world’s hydrogen is produced using dangerous natural gas.
The resultant hydrogen/carbon monoxide mixture is known as “syngas.” Basically any large scale organic commodity in the world obtained using dangerous petroleum can more or less be synthesized using syngas. A common use, run at various times in the 20th century on an industrial scale (and once proposed by Jimmy Carter for US government support when he was President to break the stranglehold of OPEC) is for the Fischer-Tropsch reaction the “FT reaction.” This reaction can make synthetic gasoline and/or synthetic diesel fuel and synthetic jet fuel, fuels which burn slightly cleaner than petroleum-based diesel. Of course, saying “slightly cleaner” about these dangerous fossil fuels or a putative substitute is like saying that it is better to have lung cancer than pancreatic cancer, since lung cancer patients live slightly longer than pancreatic cancer patients, but no matter. The main Fisher-Tropsch application today is to make synthetic motor oil which is designed to run in cars for very long periods is generally made using Fischer-Tropsch type chemistry. The synthetic motor oil lasts longer than motor oil refined from dangerous petroleum because its chemical constituents can be more tightly controlled.
The carbon source need not be dangerous natural gas. The original Haber process for the preparation of hydrogen to make ammonia – a reaction on which until this day the world food supply depends – and the Fischer-Tropsch fuels that drove the Nazi war machine in the latter parts of World War II, used coal as the carbon source.
This brings me back to the first paper, from which the introduction was excerpted, in which syn gas and electricity is produced using dangerous coal.
Here, for convenience is a table listing many, but not all, the abbreviations utilized in the text.
Here is some text about the CO2 capture option being explored for this dangerous coal plant, with reference to earlier considerations:
An interesting aspect of this paper caught my eye, which is concerned with the topic of materials science implications of a very high temperature gas expanding against a turbine.
My personal view is that clean power plants should be designed to last for a period approaching a century, and the extent to which they do so is very much dependent on individual components. Great advances have been made in recent years in understanding that it is possible, in a what is called a "breed and burn" setting to build nuclear reactors that will not require fueling for many decades of operations, that can run at full power for periods of at least half a century, and it does seem to me that longer periods are conceivable. Since the number of nuclear plants required would be reduced by raising efficiency, and the use of nuclear power to remove carbon dioxide from the air and to reduce it, to reverse climate change, would necessarily require very high temperatures - cerium based carbon dioxide splitting requires for part of the cycle temperatures of approximately 1400°C - the issue of the temperature of gases and their effect on the integrity of turbines is always on my mind. This is why this paper, which is about a form of energy I hate, is of so much interest to me.
A diversion on turbines: The Brayton cycle is in wide use and they have very much depended on the temperature resistance of turbines, both in every jet engine on the planet and in "combined cycle" dangerous natural gas plants. Almost all of these turbines are manufactured using nickel based superalloys that, while being designed to function at high temperatures, routinely encounter gases that are at temperatures that are significantly higher than the melting point of these alloys. This problem is overcome by the use of thermal barrier coatings. An excellent paper discussing this subject was written by the interdisciplinary scientist (and Dean of the Engineering Department) Dr. Emily Carter of Princeton University on the occasion of her induction into the National Academy of Scientists: Atomic-scale insight and design principles for turbine engine thermal barrier coatings from theory (Kristen A. Marino1, Berit Hinnemann2, and Emily A. Carter3, PNAS April 5, 2011 108 (14) 5480-5487) (A major theoretical paper where all three authors are women, each of them with intellectual power that people having pig brains - I'm talking about you Brett Kavanaugh and Donald Trump - are too ignorant and stupid even to imagine! Cool!)
(Regrettably, the dependence of hafnium as a the solution suggested by Dr. Carter to the binding issue of the thermal barrier coating to superalloys is probably not sustainable simply because hafnium may be regarded as a "critical element" subject to depletion as we steal the future from future generations.)
Anyway.
The maximum allowable turbine blade temperature assumed in the paper now under discussion is 860°C, lower than the melting points of many available superalloys, but no matter, this is not the real point of the paper in any case. (The performance of superalloys is not entirely connected with melting, the solvus point, in which the components of the solid solutions, that the alloys represent, separate is also important. Temperatures approaching 1400°C but still below it are observed among a few commercial superalloys - at least as of 2010 - for example CMSX-10, which reportedly has a solvus temperature of 1345°C. cf. Table 4.3, page 45, Geddes, Leon, Huang, Superalloys: Alloying and Performance.)
Again, this is a paper about coal, and the application to which I would like to see this technology applied (and not necessarily involved with combustion so much as reforming using nuclear heat) is biomass. In this paper, it actually turns out that there is a way - important for materials science considerations - in which the coal under discussion is actually cleaner that biomass. Here is the elemental composition of the Chinese coal under consideration for the purposes of this evaluation, Datong bituminous coal:
Here, for comparison, is the elemental composition of Maize Straw Ash (also Chinese) from a paper I discussed in a recent post in this space:
cf: Influence of Sewage Sludge on Ash Fusion during Combustion of Maize Straw (Liu et al, Energy Fuels 2019, 33, 10, 10237-10246)
The big difference is the presence of chlorine, which in the scanning electron microscopic/energy dispersive analysis of the ash contained at least one particle that was clearly almost pure potassium chloride. A big problem with corrosion induced by the combustion (or reforming) of biomass is connected with volatile chloride salts, both sodium and potassium chloride. This problem is not observed in the Datong bituminous coal, which is not to say that the coal is acceptable; it isn't.
Anyway, from the paper, here is the process flow engineering diagram of the full Allam cycle plant under consideration:
Some interesting features of this plant include the cyclone, to separate the dangerous coal ash from the gasification of dangerous coal, the fact that the syngas is burned, and not separated for use to make materials or portable fuels, and the ASU, which is an air separation unit, with the air separation requiring additional energy.
There is another way to obtain pure oxygen other than air separation, which is the thermochemical splitting of either water or carbon dioxide or both. (The thermochemical splitting of carbon dioxide into CO and O2 gas is indirectly capable of splitting water into hydrogen and oxygen via the water-gas reaction, by which almost all the hydrogen on Earth is currently made, using dangerous fossil fuels as the source, although clearly high temperature nuclear reactors can do the same thing in an almost infinitely cleaner way.)
Some commentary on the turbine limitations (in this study):
where K1 is a parameter that reflects the turbine geometry and operation condition, which needs calibration according to the working fluid and the turbine operation condition; TIi is the inlet temperature of each expander i; TCi is the temperature of coolant stream i; WEXP?i is the power generated by expander i. The pressure drop is calculated according to the following equation:
where pOi?pIi+1 is the pressure drop caused by mixing the main stream with the coolant; K2 and K3 are similar with K1 that need calibration;mCi is the mass flow rate of the coolant; VHi is the volume flow rate of the working fluid at the inlet of VALVE i. The values of K1, K2 and K3 are directly taken from literature [21]. The allowable turbine blade temperature TW is assumed as 860 °C. The number of the cooled expansion steps N should be a reasonably large number, as a requirement of the continuous expansion model. The recommended value of N is 15 by literature [21]. However, the influence of N on the estimated coolant mass flow rate is not provided. In this study, the influence of N is investigated using data (see Table 2) of the working fluid and coolant for the CO2 turbine presented in literature [21]. The result is shown in Fig. 3.
Figure 2:
Figure 3:
The turbine cooling is provided apparently by expansion of the gases, but there are certainly other options, including a heat exchange network, a topic widely discussed in the literature in many papers that I come across. Heat recuperation is a feature discussed in this paper. Here's a figure of about heat recuperation:
There is considerable discussion in the text of the effect of pressure drops (part of the adiabatic expansion) on the overall thermodynamic efficiency of this system as well as with other components of the system, for instance pumps. Engineering graphics produced below willshow the results of these calculations, the effect of pressure and inlet temperature on efficiency, but perhaps will be of interest only to people with either an engineering or scientific background.
We say on the left that we care about climate change, but given the importance of the problem, we are entirely too glib about it. As a scientist, I am frankly appalled by our unwillingness to seriously consider the problem beyond cruising to silly websites about solar and wind power. Solar and wind power are nothing more than lipstick on the dangerous fossil fuel pig. They have done nothing to address climate change, are doing nothing to address climate change, and will never do anything to address climate change.
Anyone who really cares about the tragedy of climate change and the appalling consequences of what it will do to all who come after us, should be interested in science and engineering, or if they can't be, should at least step out of the way of those who are so interested.
To further make this point I'd like to turn to what should be a disturbing table from this paper saying something about mass flows. This is it:
This power plant is a 1400 MW thermal power plant. At the stated efficiency that the paper estimates in the conclusion, 38.21%, this suggests that the plant would produce about 535 MW of electricity.
According to the International Energy Agency's 2019 Electricity Information Statistics the world produced in 2017, 25606.25 TWh of electricity. This works out to 92.2 exajoules of pure electricity. Electricity demand and production fluctuates widely and thus it is somewhat disingenuous to speak in terms of "Watts" although this terminology is widely used - in a completely dishonest fashion - by advocates of so called "renewable energy," the lipstick on the dangerous fossil fuel pig. Nevertheless, for arguments sake I will do just this, speak in terms of average continuous power, as if I were not discussing an inherently variable system 25606.25 TWh, again 92.2 exajoules, breaks down to an average continuous power demand of 2.92 TW. This means to produce this energy using Allam cycle coal plants modeled in this paper, 5,460 plants would need to operate.
The table above indicates that the coal required to run this plant would be 64.93 kg per second. For 5,460 plants, this would amount to 357.2 tons per second or 11.3 billion tons per year of coal. Since the atomic weight of carbon is 12 and the molecular weight of carbon dioxide is 44, and the carbon content of the Datong coal in this example is 56.75% carbon, the "captured" carbon dioxide for which something must be done permanently forever, would be 23.5 billion tons per year, just for electricity.
It is useful to compare the plutonium requirements to do exactly the same thing at exactly the same efficiency, although personally I have convinced myself that nuclear plants can be built that have much higher efficiency. A kg of plutonium contains about 80.3 trillion joules of neutrino free energy. A nuclear plant producing 1400W of thermal energy would thus fission 17.4 micrograms of plutonium per second or in the "percent talk" so favored in the abuse of language so widely utilized by advocates of so called "renewable energy" 0.00002665% as much mass as the coal plant. 5,460 nuclear plants operating at 38.21% efficiency and 1400 MW thermal energy would produce 1.5 kg of fission products per day per plant, which works out to about 3,000 tons per year.
Which is easier to contain or deal with 23.5 billion tons of carbon dioxide gas, or 3,000 tons of largely solid fission products, only a portion of which would actually be radioactive, and many of which would actually have high value?
How is it that we are so abysmally stupid as to not grasp these simple facts?
Above I promised to discuss a second paper, this one, Atomistic Simulation of Coal Char Oxy-Fuel Combustion: Quantifying the Influences of CO2 to Char Reactivity (Yongbo Du,†,‡ Chang’an Wang,†,‡ Haihui Xin,‡,§ Defu Che,† and Jonathan P. Mathews, Energy Fuels 2019, 33, 10, 10228-10236), which is also relevant to the case, since it concerns the behavior of char. Char is available from dangerous coal (and for that matter from dangerous petroleum) of course, but it is also available from heated biomass. In the latter case, this biomass char in an Allam cycle could be utilized to capture carbon dioxide from the air, which is what this post's title suggested.
Before I was banned at Daily Kos for telling the truth, which is that opposing nuclear energy is simply murder, since this truth flies in the face of our dogma on the left, I used to include mildly amusing polls with all of my posts, one choice always being a variant on the statement "NNadir is a liar and..." with the and being a negation of whatever subject my post explored and my assertions connected with it.
(They're cute over there at Kos when they pretend to actually care about science. The science forum over here is far more interesting than anything written there now.)
Today, the "NNadir is a liar" statement will be the statement that I will discuss the paper just cited about char, coal char. I'm not going to do so now.
Perhaps I will discuss it in the future, but I've run out of time, and have already wasted too much time trying to make a point about which perhaps no one really cares. Nevertheless, all this stuff about Donald Trump is trivial inasmuch as it is ephemeral. In less than 20 years, Trump will be almost certainly dead and more useful than he is alive, and will be a footnote to history, a sad footnote, an appalling footnote, but still a footnote.
Climate change will still be with us in 20 years, quite possibly its effects lasting forever, long after Don Jr, Ivanka, Jared and the rest of those terrible people have died, hopefully in prison.
It's well to consider this.
It's not a totally wasted exercise, for me to write this post, however, because every time I write one these posts I learn quite a bit. The Allam cycle is a topic about which I hope to think in the future. My son is in Italy though, picking up some kind of academic award from the Italian government, and the logistics of getting him from school and putting him on the plane has worn me out, and I'm running out the ability to think clearly.
So are we all, all running out of the ability to think clearly.
At least as a result of this exercise, I'll be able to chat up the Allam cycle with my son, since the future is his and since he's smarter than I am, and however many ideas with which I can leave him to explore, will help him to use his talents to do right by his generation, since my generation has done so much wrong to his.
Some engineering graphics from the paper I did discuss in this post:
The efficiency implications of temperature and pressure for various scenarios:
The pumps also exhibit effects on efficiency:
The air separation to produce oxygen also produces an energetic penalty - this can be overcome in a high temperature nuclear thermochemical water or carbon dioxide scheme and actually raise the efficiency of the overall system.
Some aspects of heat networks and heat recovery:
Some tables from the paper:
I hope you're having a very pleasant Sunday afternoon.
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