Harnessing Clean Water from Power Plant Emissions

[NNadir]

The scientific paper I will discuss in this post is this one, from which the title of the post itself is taken:

Harnessing Clean Water from Power Plant Emissions Using Membrane Condenser Technology (Park, et al ACS Sustainable Chem. Eng., 2018, 6 (5), pp 6425–6433)

Here is the introductory graphic provided with the paper:



The caption:

Figure 1. Power plant operation schematics. A significant amount of water and energy is lost through stack and cooling towers.

One of the most exigent issues connected with climate change and other aspects of our generation's contempt for all future generations, is water. One aspect of this problem derives from lack of access to clean and safe fresh water, owing to chemical and elemental pollution of drinking and agricultural water, and the other has to do with seawater, which owing to rising seas is causing intrusion of salts into previously available groundwater, not to mention killing people in extreme weather events and tectonic events, an example being the 2004 Indonesian quake, which killed about a quarter of a million people, and the 2011 Sendai/Fukushima quake, where 20,000 people died from seawater, not that anyone gives a rat's ass about people killed by seawater.

There are really not many viable solutions being actively pursued to prevent the rise of the seas; in fact there are none, but being - at the expense of producing an oxymoron - a "cynical optimist" I often consider some, the most challenging being the geoengineering task of removing the dangerous fossil fuel waste carbon dioxide that our generation has criminally dumped into our favorite waste dump, the planetary atmosphere.

Another option also crosses my mind from time to time, and that is removing water from the seas and storing and/or using it on dry land, including land parched by climate change. This obviously involves desalination. I've lived through a number of profound droughts in the regions in which I've lived, both in California where the effort to "do my part" involved flushing my toilet with shower water collected in buckets, and here in New Jersey, where it involved watching trees die. Always in a drought in a region near the sea, you'll run across people who will say "Why don't 'they' just desalinate seawater."

The answer to that question should be obvious, but somehow isn't to most people who blithely refer to "they" rather than "we:" It takes energy, lots of energy to desalinate water.

The proportion of energy obtained from dangerous fossil fuels on this planet is rising, not falling. In the "percent talk" often utilized by defenders of the so called "renewable energy" industry, in the proportion of primary energy obtained by the combustion of dangerous fossil fuels in the 21st century has risen from 80% in the year 2000 to 81% in the year 2016.

In "percent talk" a 1% increase in the use of dangerous fossil fuels seems rather modest, but in honest representations, it's rather dire. In the year 2000 world energy consumption was 420.15 exajoules; in 2016 it was 576.10 exajoules. This "one percent" increase therefore represents an overall increase of 129.71 exajoules, which - to put in perspective - is more energy than is utilized by the entire United States for all purposes, which by appeal to EIA data, consumed in 2017 consumed 103.09 exajoules of primary energy.

IEA 2017 World Energy Outlook, Table 2.2 page 79 (I have converted MTOE in the original table to the SI unit exajoules in this text.)

US Primary Energy Consumption Flow Chart

(The United States appears to achieved modest increases in energy efficiency, although on serious reflection, one wonders whether this increase in efficiency simply represents the export of energy intensive manufacturing operations to countries with less onerous environmental and labor regulations, which although it represents an ethical tragedy - not that many people care about ethics - certainly represents a profitable approach for those who think the end of all human activity should be money.)

I take and have taken a lot of flak here and elsewhere for my unshakable conviction that nuclear energy is the only environmentally sustainable form of energy available to humanity. My goal is not to be popular - I'm not - but rather to be informed and reasonable. The latter comes at the expense of the former.

Although in the United States and elsewhere, nuclear energy has been a very successful enterprise that has (worldwide) saved close to 2 million lives, it is, as currently practiced, nowhere near environmentally optimized, chiefly because the technology under which it operates was essentially developed in the 1950's and 1960's, a time in which - unlike today - engineers and scientists were highly respected on both ends of the political spectrum. (In my opinion, the further one is from the center of the political spectrum, the greater is one's contempt for scientists and engineers.) The chief environmental impact of the nuclear industry as currently practiced is thermal pollution.

The chief means of reducing the thermal impact of nuclear energy, in my opinion, would be to exploit modern advances in materials science to raise the temperature of reactors by an order of magnitude, as counter intuitive as this might seem to people with no knowledge of the laws of thermodynamics, an effort that is being explored in the academic nuclear wilderness even if the general public is getting more absurd in its thinking about energy and more contemptuous of scientists and engineers.

But even existing nuclear facilities and nuclear technology might be improved with respect to the environmental impact, which will shortly bring me to the paper cited at the opening of this post.

It can be shown that the thermal efficiency of all American nuclear reactors in the United States in 2017 was 32.875%. This is slightly less than the traditional value given for thermal plants in the US, 33%, but as temperatures climb - as they are obviously doing because of climate change - the thermal efficiency of all thermal power plants will fall, since efficiency is a function of the temperature of environmental thermal reservoir, in this case river, lake or seawater, which are, of course, a function of the weather. (Combined cycle dangerous natural gas plants have considerably higher thermal efficiency than other thermal plants, and can approach 60%, although this thermal efficiency can be severely degraded if the plant is temporarily shut down because the wind is blowing for a few hours and the sun is shining. I envision combined cycle nuclear plants with even higher efficiency.)

The preliminaries out the way, let me now reproduce the opening paragraph of the paper cited at the opening, detailing the environmental cost of thermal plants:

In the United States alone, power plants consume 40% of all available water sources (45% in EU).1 It has been calculated that if 20% of the evaporated water can be recovered from a power plant, it can be self-sufficient from the process water point of view.2 The current power plants on average consume approximately 1.6 L of water to generate 1 kWh of electricity, which converts to 45 000 m3·hr−1 of water for a regular-sized 500 MW plant.3 As illustrated in Figure 1, two main sources of emissions in power plants are from the stack and the cooling towers. Streams emitting from a stack become saturated in the desulfurization step (FGD), and the streams from cooling towers are typically river or seawater evaporated to cool the steam cycle stream.

The evaporated water (i.e., white plumes) also poses several downsides such as visual pollution, frost damage, and corrosion of chimneys and stacks. The current practice now is to intentionally heat up the emission stack to avoid corrosion,4 which consumes additional energy. If the evaporated water can be effectively recovered, it can be a fruitful source of distilled water and latent energy, and it can relieve the exacerbating energy−water collisions, particularly during drought or hot weather. In addition, the technology can be valuable to other industries that employ water-cooling systems such as steel, semiconductors, and pulp industry.

Obviously much of the introduction here refers to the waste dumping devices used for dangerous fossil fuel plants, smokestacks, which are generally corroded by the fossil fuel waste which ought to give one pause to reflect on what dangerous fossil fuel waste does to lungs as opposed to bricks. However nuclear power plants which - despite so much horseshit thrown around about so called "nuclear waste" - are observed to successfully store their valuable by products on site for indefinitely long periods - do consume considerable amounts of water. Now, some of this water is recovered in the form of rain on land, but a considerable portion is not; it falls into the sea and is lost.

The paper reviews existing technologies for the recovery of water, and notes that many of them - heat exchangers for example - provide low quality water, while others, the use of glycols for example, incur an energy penalty that makes them self defeating. The focus of the paper is on the development of ceramic membranes to recover water.

The authors produce a graphic showing the options for designing these types of devices:



The caption:

Figure 2. Illustration of membrane-based dehydration configurations: (a) vapor permeation using a dense membrane, (b) transport membrane condenser using a microporous membrane to selectively condense water vapor within capillary pores, (c) conventional membrane condenser configuration using a hydrophobic microporous membrane to pass gas while condensing water vapor on the surface.

The focus of their paper is optimizing the type of membrane described by figure (b) in the graphic, the transport membrane condenser which they refer to as "TMC" throughout the rest of the paper:

In this work, we investigated key parameters to maximize the TMC configuration performance for capturing the evaporated water. We fabricated ceramic membranes and tested the effect of independent parameters on recovered water quality, as well as process conditions such as humidity, flow rates, and thermal gradients. Moreover, a full energy balance was carried out to reveal that TMC performance is highly dependent on the temperature gradient across the membrane, which can be tailored during the membrane fabrication step.

They note that it is important to consider the thermodynamics of this process, and comment on this aspect in an honest assessment of the energy penalty associated with water recovery, which cannot be eliminated but can be significantly reduced:

Before investigating the performance efficiency of membrane condensers, one must carefully consider the thermodynamic aspects of the overall process. As illustrated in Figure 1, the water vapor emitting from the cooling towers were intentionally evaporated to utilize the latent heat to cool the exothermic stream. Therefore, one must ask whether it is thermodynamically logical to recondense the evaporated stream, which also requires a considerable amount of cooling energy. One plausible explanation is that because the evaporated water has been distilled to some degree, the energy input can be justified if high quality water can be harnessed. In addition, as proposed by Wang et al.,8 the heat of condensation of evaporated water can be reutilized to heat the boiler feed stream. It should be emphasized that capturing the evaporated water must be approached from the environmental perspectives, as minimizing water consumption is one of the top priorities for power plants. Therefore, it is crucial to develop an energy efficient process for capturing evaporated water to relieve the energy−water collisions.

Their ceramic membrane they designate as KRICT100 and they compare it with a commercial ceramic membrane identified as HYFLUX20. They note that most commercial membranes already in use (most probably in smokestacks) are organic polymers, the long term stability of which is not expected to be high meaning that they will incur an environmental and economic penalty when they require replacement: The longevity of devices affects not only the cost of their use, but also their environmental impact. (This is just one of the reasons that the wind industry sucks.)

Here's some microscopic views of the two materials:



The caption:

Figure 4. SEM images of KRICT100 and Hyflux20 membranes. Hyflux20 membrane has a γ-alumina coating layer in the inner side.

Here is the characterization of the two materials in terms of pore size distribution:



The caption:

Figure 5. Pore size distribution data of KRICT100 and Hyflux20 membranes.

The authors product obviously demonstrates far better control over the distribution of pore sizes when compared with the commercial product, although it's not clear that this advantage can be maintained upon scale up.

They test the performance with a laboratory set up described by this schematic graphic.



The caption:

Figure 3. Dehydration experiment test apparatus. Black lines indicate hot gas stream flows, blue lines indicate cold liquid stream flows. MFC − mass flow controller, F − flowmeter, T − thermometer, H − hygrometer, P − pressure gauge.

There may be a graphic error here, or else I'm going color blind: I can't see blue "cool" lines, but no matter. One can figure out where they are supposed to be. The science is good even if the proof reading isn't and the graphics aren't.

For thermodynamic reasons, the exterior temperature of the materials is apparently an important factor, and ceramic membranes perform in a superior form to the organic polymers commonly in use today:

A graphic on this subject:



The caption:

Figure 9. (a) Calculated membrane outer surface temperature as a function of feed air temperature for polymeric and ceramic membranes; (b) membrane temperature profile along the fiber thickness.

Some commentary from the text of the paper on this factor is probably appropriate:

In order to maximize the driving force (temperature and vapor pressure gradient), it is necessary to maintain a wide temperature gap between the feed stream and the membrane outer surface temperature. Therefore, it is desired to keep the membrane outer surface temperature as low as possible. Figure 9a clearly illustrates the effect of material thermal conductivity on the membrane outer surface temperature. Assuming a membrane porosity of 70%, ceramic membranes (alumina) with high thermal conductivity (kalumina = 35 W·m−1·K−1) can effectively maintain low surface temperature compared to typical polymeric membranes (kPVDF = 0.19 W·m−1·K−1). Figure 9b illustrates the effect of feed temperature on the temperature profile across the membrane cross-section. It can be seen that polymeric membranes exhibit steeper temperature gradient along the thickness compared to ceramic membranes, primarily due to the low thermal conductivity of the material itself.

Therefore, from the performance perspective, it certainly is more effective to utilize ceramic membranes for membrane condenser applications. However, ceramic membranes are brittle, rendering them difficult to handle in large scale. On the other hand, polymeric membranes exhibit relatively low thermal stability but can be more cost-effective.

These scientists are doing what responsible scientists should always do, point to the limitations associated with their work.

As it happens, in connection with other interests I have that have little connection with water recovery, I have been studying ceramic materials and considering some of the properties of composites that may address some of the concerns about large scale and brittleness here, although I am not competent enough in this area to assert that this is, in fact, the case.

The authors note that in any case, the properties of ceramic vs. polymeric membranes require opposing morphology:

Interestingly, it was found that the two studied materials give opposite trends as a function of porosity. For polymeric materials, membranes with higher porosity exhibit lower membrane temperature. On the other hand, for ceramic membranes, low porosity display lower membrane temperature. Such opposite trends results from the assumption that the open pores are filled with water during membrane condenser operation, and water has a thermal conductivity (k = 0.67 W· m−1·K−1) between that of ceramic and polymeric material. The trends observed in Figures 9 and 10 can give an important direction to tailor the membrane characteristics to improve the membrane condenser productivity. For polymeric membranes, it is desirable to maximize the membrane porosity while reducing the thickness…

… For ceramic membranes, lower porosity is preferred yet has negligible effect on the membrane temperature because of its high thermal conductivity. Instead, more focus can be placed on controlling the membrane pore size to improve the condensed water quality.

Here is figure 10:



The caption:

Figure 10. Calculated membrane surface temperature as a function of thickness and porosity for (a) polymeric membrane and (b) ceramic membrane.

In this work, an effective method to harness clean water from power plant emissions using membrane condenser technology is proposed. Compared with dense vapor separation membranes that suffer from low driving force, the proposed transport membrane condenser (TMC) configuration exhibited water flux up to 12 kg·m−2·h−1, as high as 3 orders of magnitude higher compared with the vapor separation membranes. In addition, the TMC process gave a reasonable water/SOx selectivity of 100, which is much higher than the Knudsen selectivity of 1.8. It was determined that the current TMC process is completely limited by the rate of condensation, and a better membrane and more effective module design must be developed that enhances the vapor pressure gradient. The current limit of dehumidification efficiency was determined to be approximately 85%, after which the driving force cannot be maintained to induce water condensation.

The focus of this paper has been largely on the dirtiest energy utilized by humanity which is also, by far, the largest form it uses, dangerous fossil fuel based energy. The commonly held opinion that dangerous natural gas, among the three dangerous fossil fuels is "almost" clean is a fantasy which represents violence against all future generations.

It is not enough to oppose Trump's violence against the children of immigrants - as all decent people do - while ignoring the state of the world in which they will ultimately live, with and without the activities of racist American Presidents like the President we have now. It is not enough. We must work to do better.

The applicability of the work described here, in present and future manifestations, has real applicability for clean energy, clean energy being represented by one and only one form of energy, nuclear energy.

If the coolant is seawater devices such as this represent effective desalination devices.

Now there are definite risks associated with desalination and I'm definitely not representing them as a panacea of any sort, nor representing that they can ultimately sustain humanity in the face of clear reductions in the carrying capacity of the entire planet. Some of these risks include disruptions to the thermohaline circulation patterns, which may trigger disastrously fast climatic fluctuations which are known to have occurred in the past, for example, Dansgaard-Oeschger cycles.

Still the risk is worth weighting against other risks, both to humanity and the planet.

I have argued here and elsewhere that uranium is essentially inexhaustible because of the presence of nearly 5 billion tons of this element in the earth's oceans, an amount that can never be reduced because of the geochemical circulation of the element for so long as an oxygen atmosphere persists. (Humanity will, of course, be irrelevant should oxygen cease to be present in the atmosphere, if, for example, we completely destroy the oceans, a possibility that seems not to be out of the question.) Uranium flows can also be captured in rivers, particularly should we ever restore rivers to healthy conditions should humanity abandon it's awful fixation on so called "renewable energy," or by removing uranium as a constituent of "NORM" (Naturally Occurring Radioactive Materials) from drinking water. (I pointed to a case in which this issue presents itself recently in this space: Large-Scale Uranium Contamination of Groundwater Resources in India.

In connection with this, I have been working to wrap my head around the international scientific consensus on the thermodynamic equation of state for seawater, TEOS 10, from which one can calculate that the high energy density of uranium (transmuted into plutonium). The extremely high energy density of plutonium makes the infinite sustainability of uranium supplies from ocean (and fresh) water feasible, even if all the energy inputs required to effect it come from fission itself.

But consideration of the equation of state of seawater, and the environmental risks and benefits of desalination will have to wait for another time.

I hope you're having a pleasant weekend.