“Reflect on what happens when a terrible winter blizzard strikes. You hear the weather warning but probably fail to act on it. The sky darkens. Then the storm hits with full fury, and the air is a howling whiteness. One by one, your links to the machine age break down. Electricity flickers out, cutting off the TV. Batteries fade, cutting off the radio. Phones go dead. Roads become impossible, and cars get stuck. Food supplies dwindle. Day to day vestiges of modern civilization – bank machines, mutual funds, mass retailers, computers, satellites, airplanes, governments – all recede into irrelevance.
Picture yourself and your loved ones in the midst of a howling blizzard that lasts several years. Think about what you would need, who could help you, and why your fate might matter to anybody other than yourself. That is how to plan for a secular winter. History warns that a Crisis will reshape the basic social and economic environment that you now take for granted.” – Strauss & Howe – The Fourth Turning
Warning - nerd chemistry post.
Now that it's getting cold again, the steam plume coming off the cooling tower has become much more pronounced. The visibility of the steam *is not* due to more water being evaporated during the colder months - water usage actually drops during winter and increases during the hotter, drier summer months.
No; in the colder months, the water vapor simply becomes much more visible. This is because cold air temperature rapidly condenses the cooling tower moisture into larger droplets of mist. In summer months the moisture from the cooling tower is completely evaporated and diffused by the low humidity and heat.
The visibility of the steam plume has to do with how water vapor behaves under saturation conditions. When the air is loaded with water vapor, and the temperature drops, you reach the dew point - the point at which the relative humidity reaches 100% and thus the vapor can condense into liquid. The dew point is quite variable; it depends on relative humidity and temperature. We will discuss dew point later on in this post, for reasons that will soon become clear.
All fossil fuels - hydrocarbons - contain sulfur. Although efforts are made to reduce the quantity of sulfur in fossil fuels, there will be traces remaining that cannot be removed economically in a large stream of fuel.
Any power plant that burns fossil fuel therefore will also be burning some quantity of sulfur. Coal is the worst offender, with 0.5 - 4% of its weight being sulfur in various forms, depending on where it was mined. Diesel, gasoline, natural gas, and propane all contain traces of sulfur.
Chemistry: When a hydrocarbon burns, it reacts with oxygen in the air and produces the following:
- Heat - from breaking hydrogen-carbon bonds in the hydrocarbon molecules
- Carbon Dioxide - carbon in the hydrocarbon molecules reacts with oxygen from the air
- Water - hydrogen from the hydrocarbon molecules reacts with oxygen from the air. Heat from combustion converts this water into superheated steam vapor. Aside: This is why aircraft operating at high altitudes leave contrails; water vapor from combustion condenses and then freezes in the frigid air found at high altitudes, leaving streaks of tiny ice particles.
- Nitrous oxides - the heat of combustion causes nitrogen and oxygen in the air to react together into this pollutant
- Sulfur Dioxide - traces of sulfur in the fuel combine with oxygen from the air to create this pollutant.
With the background out of the way, let's discuss this last item, Sulfur Dioxide, with respect to dew point. I've mentioned Sulfur Dioxide in earlier posts (here, here, and here) about coal burning power plants. Sulfur Dioxide can be an issue with any type of fossil fuel combustion, even if most people are not aware of this fact. It can and does cause issues in automobiles.
After combustion of fossil fuels, we have a stream of very hot combustion gas containing all of those things mentioned previously: Excess Air that was not used during combustion, Carbon Dioxide, superheated water vapor, Nitrous Oxides, plus Sulfur Dioxide and a few other minor pollutants.
Sulfur Dioxide at the temperatures found in combustion exhaust is a gas, as is the water vapor. However, under the right conditions, this Sulfur Dioxide can react with water and oxygen to form sulfuric acid.
This chemical process is how we end up with acid rain. The released gaseous Sulfur Dioxide gas in the hot flue gas eventually cools down, and reacts with oxygen and humidity in the air to form sulfuric acid vapor. The gaseous sulfuric acid vapor eventually combines with moisture in clouds, and rains out downwind of power plants, as acid rain.
Below is an image of a forest that was devastated by acid rain. Some plant species are more affected than others by soil and water pH.
Acid rain was an issue with untreated combustion gas from older coal furnaces. Nearly all of the coal furnaces in use today have been retrofitted with pollution control equipment to remove Sulfur Dioxide. Wet scrubbers use a quicklime solution to remove sulfur dioxide from the flue gas before releasing it. This process is called "Flue Gas Desulfurization". In the business, the process is more commonly simply called "FGD".
Below: A diagram of a Flue Gas Desulfurization wet scrubber.
Below: A flue gas desulfurization retrofit installation. Flue gas that was originally ducted directly into the stack is now directed through the wet scrubber, and then back into the stack.
Below: Plant Bowen, a coal-fired plant in Georgia. Two coal furnaces are at the bottom right. You can see how the retrofitted tan-colored FGD ductwork now bypasses around the original stack, bringing the flue gas into two large white wet scrubbers, before sending it out a newer stack at the upper left.
If you were wondering why most coal boiler stacks are made of masonry or lined with fire brick - wonder no longer. The reason is two-fold. Masonry insulates the flue gas from the surrounding air, allowing it to remain quite hot until it exits the stack. Also, during start-up and low power operations, the flue gas might have reached the dew point of water and formed sulfuric acid mist. A steel stack would be susceptible to acid attack, where brick would be much more robust.
Now that we've learned a little combustion chemistry, learned how acid rain forms, and how we now use FGD in coal-fired power plants to prevent acid rain, let's get to a more interesting point: Thermal efficiency and cold end corrosion.
In a power plant, the primary goal is to extract all the useful heat from this combustion gas, treat the gas for pollutants, and then release it out the stack. However, there is a limit to how much heat can be recovered from the combustion gas, and that limit is due to Sulfur Dioxide that was created when traces of sulfur in the fuel were burned.
It's possible from an thermodynamic perspective to capture a lot more energy in the flue gas than we do, and release it to the atmosphere at a much lower temperature. Unfortunately from a chemistry and materials standpoint in a power plant, this approach is not practical, and it's all due to the presence of Sulfur Dioxide gas in the flue gas, the dew point of the water vapor in the flue gas, and sulfuric acid formation.
This problem exists in all fossil fueled combustion - natural gas,
propane, diesel, gasoline, and coal - because all of these contain
sulfur in varying amounts.
In motor vehicles, efficiency is terrible, and so a tremendous amount of heat is lost out of the exhaust - so Sulfur Dioxide gas typically remains gaseous until long after it has left the tailpipe. Even so, on colder days, moisture tends to develop in the exhaust pipe, and sulfuric acid can form in the exhaust system. This is particularly true at idle, where there is not much flow of hot exhaust gas. Modern vehicles are often now equipped with mild stainless steel exhaust systems that do an excellent job of resisting acid corrosion. Blown out mufflers and exhaust systems are much less common today than they once were, due to stainless construction. Stainless is not required near the engine, as that portion of the exhaust system is too hot for water to condense into liquid.
High efficiency is not practical in vehicles due to the added weight that higher efficiency equipment would require - reducing fuel economy.
My home furnace manages to nearly extract the maximum energy possible: The furnace exhaust temperature
is only about 80 degrees F - it blows cool steam out the vent all winter long. Sulfuric acid is certainly forming in the furnace and exhaust duct, but guess what? The small heat exchanger in the furnace is made
of stainless steel, and the exhaust and drain lines are PVC, and those materials are impervious to acid attack. Greater efficiency is possible, if the materials are up to the task.
But what about power plants?
Yes, fossil fueled power plants could also extract a great deal more energy from the exhaust gas than they do, improving efficiency considerably. But like the home furnace above, the cooler end of the power plant furnace would have to be designed with sulfuric acid attack in mind.
By extracting more heat from the flue gas, moisture would form in the back end of the boiler and allow water and sulfur dioxed to form sulfuric acid mist. The the last few rows of water tubes would be attacked by a constant stream of acid mist. This is process is called "Cold End Corrosion".
Below: Cold End Corrosion on an economizer tube. You can guess which direction the corrosive gas was coming from by the tube wall thickness. The economizer is the final heat exchanger before the flue gas leaves the boiler - and thus where the flue gas can become cool enough to reach the dew point and condense water to form sulfuric acid mist.
You could extract more heat from the flue gas by building the back end of the boiler out of stainless steel, but this would be prohibitively expensive - because boilers are huge. All of the internals of the boiler, including the interior insulation siding, hangars, and pressure tubes, would have to be stainless.
Instead, power plants are designed to maintain the flue gas above the temperature at which it could drop down to the dew point - and that otherwise useful heat goes to waste - day by day, year by year. Some power plants built in colder climates insulate the lower portion of their stack to help maintain flue gas temperature above the dew point until after it exits the stack. Stacks made from ferrous steel can also corrode out.
There is another technique to prevent the flue gas temperature from dropping below the dew point and forming sulfuric acid: The Low Pressure Economizer Recirculation system. With this system, a portion of the hot water exiting the economizer is fed back into the inlet, keeping the overall temperature hotter than it would otherwise be, and keeping the flue gas outside the tubes above the dew point.
Below: The red and green pumps at the bottom right recirculate hot water from the outlet of the economizer back to the inlet. Temperature is regulated by the valve to the right of the pumps.
Below: There is a gap between the stack and the point at which the exhaust gas begins forming a steam cloud- and now you know why.
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Lastly, I wanted to discuss the terms HHV and LHV. HHV is the abbreviation for Higher Heating Value, and LHV is the abbreviation for Lower Heating Value.
They aren't too descriptive, but they are important. Higher heating value is the gross energy produced by burning a standard unit of fuel - whether that is an MMBTU of natural gas or a pound of coal. And when I say gross, I mean every erg or BTU of energy that burning the fuel makes. In reality, this is not practical. As discussed above at great length, we cannot condense the water vapor and extract that heat from it due to cold end corrosion.
Lower heating value is the net energy produced by burning a standard unit of fuel, while NOT extracting the energy that could be captured by condensing the water vapor present in the exhaust gas. LHV therefore is the more "honest" measurement of the heat that can reasonably be extracted from the fuel.
OK that's it. Glad to get that off my chest.
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