Why are Smokestacks So Tall?

That’s the first line of one  of my favorite short stories, written by Kurt Vonnegut in 1955. It paints  a picture of a dystopian future that, thankfully, didn’t really come to  be, in part because of those stacks. In some ways, air pollution is kind of a part  of life.

I’d love to live in a world where the systems, materials and processes that make my  life possible didn’t come with any emissions, but it’s just not the case. . .

From  the time that humans discovered fire, we’ve been methodically calculating  the benefits of warmth, comfort, and cooking against the disadvantages of carbon  monoxide exposure and particulate matter less than 2. 5 microns in diameter… Maybe not  in that exact framework, but basically, since the dawn of humanity, we’ve had  to deal with smoke one way or another. Since, we can’t accomplish much without  putting unwanted stuff into the air, the next best thing is to manage how and  where it happens to try and minimize its impact on public health.

Of course, any time you  have a balancing act between technical issues, the engineers get involved, not so much  to help decide where to draw the line, but to develop systems that can stay below  it. And that’s where the smokestack comes in. Its function probably seems obvious; you  might have a chimney in your house that does a similar job.

But I want to give you a peek  behind the curtain into the Illium Works of the Federal Apparatus Corporation of today  and show you what goes into engineering one of these stacks at a large industrial facility.  I’m Grady, and this is Practical Engineering. We put a lot of bad stuff in the air, and in a lot  of different ways.

There are roughly 200 regulated hazardous air pollutants in the United States,  many with names I can barely pronounce. In many cases, the industries that would release these  contaminants are required to deal with them at the source. A wide range of control technologies are  put into place to clean dangerous pollutants from the air before it’s released into the environment. 

One example is coal-fired power plants. Coal, in particular, releases a plethora of pollutants  when combusted, so, in many countries, modern plants are required to install control  systems. Catalytic reactors remove nitrous oxides.

Electrostatic precipitators collect particulates.  Scrubbers use lime (the mineral, not the fruit) to strip away sulfur dioxide. And I could go on.

In  some cases, emission control systems can represent a significant proportion of the costs involved in  building and operating a plant. But these primary emission controls aren’t always feasible for every  pollutant, at least not for 100 percent removal. There’s a very old saying that “the solution to  pollution is dilution.

” It’s not really true on a global scale. Case in point: There’s no  way to dilute the concentration of carbon dioxide in the atmosphere, or rather, it’s  already as dilute as it’s going to get. But, it can be true on a local scale.

Many pollutants  that affect human health and the environment are short-lived; they chemically react or  decompose in the atmosphere over time instead of accumulating indefinitely. And, for  a lot of chemicals, there are concentration thresholds below which the consequences on human  health are negligible. In those cases, dilution, or really dispersion, is a sound strategy to  reduce their negative impacts, and so, in some cases, that’s what we do, particularly at major  point sources like factories and power plants.

One of the tricks to dispersion is that many  plumes are naturally buoyant. Naturally, I’m going to use my pizza oven to demonstrate  this. Not all, but most pollutants we care about are a result of combustion; burning stuff up.

So  the plume is usually hot. We know hot air is less dense, so it naturally rises. And the hotter  it is, the faster that happens.

You can see when I first start the fire, there’s not much air  movement. But as the fire gets hotter in the oven, the plume speeds up, ultimately rising  higher into the air. That’s the whole goal: get the plume high above populated areas  where the pollutants can be dispersed to a minimally-harmful concentration.

It sounds like  a simple solution - just run our boilers and furnaces super hot to get enough buoyancy  for the combustion products to disperse. The problem with the solution is that the whole  reason we combust things is usually to recover the heat. So if you’re sending a lot of that heat  out of the system, just because it makes the plume disperse better, you’re losing thermodynamic  efficiency.

It’s wasteful. That’s where the stack comes in. Let me put mine on and show you what I  mean.

I took some readings with the anemometers with the stack on and off. The airspeed with  the stack on was around double with it off. About a meter per second compared with two. 

But it’s a little tougher to understand why. It’s intuitive that as you move higher in  a column of fluid, the pressure goes down (since there’s less weight of the fluid  above). The deeper you dive in a pool, the more pressure you feel.

The higher  you fly in a plane or climb a mountain, the lower the pressure. The slope of that line  is proportional to a fluid’s density. You don’t feel much of a pressure difference climbing a  set of stairs because air isn’t very dense.

If you travel the same distance in water, you’ll  definitely notice the difference. So let’s look at two columns of fluid. One is the ambient  air and the other is the air inside a stack.

Since it’s hotter, the air inside the stack  is less dense. Both columns start at the same pressure at the bottom, but the higher  you go, the more the pressure diverges. It’s kind of like deep sea diving in reverse.

In  water, the deeper you go into the dense water, the greater the pressure you feel. In a stack,  the higher you are in a column of hot air, the more buoyant you feel compared to  the outside air. This is the genius of a smoke stack.

It creates this difference in  pressure between the inside and outside that drives greater airflow for a given temperature. Here’s the basic equation for a stack effect. I like to look at equations like this divided  into what we can control and what we can’t.

We don’t get to adjust the atmospheric  pressure, the outside temperature, and this is just a constant. But you can  see, with a stack, an engineer now has two knobs to turn: the temperature of the  gas inside and the height of the stack. I did my best to keep the temperature constant  in my pizza oven and took some airspeed readings.

First with no stack. Then with the stock  stack. Then with a megastack.

By the way, this melted my anemometer; should have seen  that coming. Thankfully, I got the measurements before it melted. My megastack nearly doubled  the airspeed again at around three-and-a-half meters per second versus the two with just the  stack that came with the oven.

There’s something really satisfying about this stack effect to me.  No moving parts or fancy machinery. Just put a longer pipe and you’ve fundamentally changed  the physics of the whole situation.

And it’s a really important tool in the environmental  engineer’s toolbox to increase airflow upward, allowing contaminants to flow higher into  the atmosphere where they can disperse. But this is not particularly revolutionary… unless  you’re talking about the Industrial Revolution. When you look at all the pictures of the factories  in the 19th century, those stacks weren’t there to improve air quality, if you can believe it. 

The increased airflow generated by a stack just created more efficient combustion for the boilers  and furnaces. Any benefits to air quality in the cities were secondary. With the advent of diesel  and electric motors, we could use forced drafts, reducing the need for a tall stack to  increase airflow.

That was kind of the decline of the forests of industrial chimneys  that marked the landscape in the 19th century. But they’re obviously not all gone, because  that secondary benefit of air quality turned into the primary benefit as environmental  rules about air pollution became stricter. Of course, there are some practical limits that  aren’t taken into account by that equation I showed.

The plume cools down as it moves up  the stack to the outside, so its density isn’t constant all the way up. I let my fire die down a  bit so it wouldn’t melt the thermometer (learned my lesson), and then took readings inside the  oven and at the top of the stack. You can see my pizza oven flue gas is around 210 degrees at  the top of the mega-stack, but it’s roughly 250 inside the oven.

After the success of the mega  stack on my pizza oven, I tried the super-mega stack with not much improvement in airflow:  about 4 meters per second. The warm air just got too cool by the time it reached the top.  And I suspect that frictional drag in the longer pipe also contributed to that as well.

So,  really, depending on how insulating your stack is, our graph of height versus pressure actually  ends up looking like this. And this can be its own engineering challenge. Maybe you’ve  gotten back drafts in your fireplace at home because the fire wasn’t big or hot enough  to create that large difference in pressure.

You can see there are a lot of factors at play  in designing these structures, but so far, all we’ve done is get the air moving faster. But  that’s not the end goal. The purpose is to reduce the concentration of pollutants that we’re exposed  to.

So engineers also have to consider what happens to the plume once it leaves the stack,  and that’s where things really get complicated. In the US, we have National Ambient Air Quality  Standards that regulate six so-called “criteria” pollutants that are relatively widespread:  carbon monoxide, lead, nitrogen dioxide, ozone, particulates, and sulfur dioxide. We  have hard limits on all these compounds with the intention that they are met at all times, in all  locations, under all conditions.

Unfortunately, that’s not always the case. You can go on  EPA’s website and look at the so-called “non-attainment” areas for the various pollutants.  But we do strive to meet the standards through a list of measures that is too long to go into  here.

And that is not an easy thing to do. Not every source of pollution comes out of a big  stationary smokestack where it’s easy to measure and control. Cars, buses, planes, trucks, trains,  and even rockets create lots of contaminants that vary by location, season, and time of day.

And  there are natural processes that contribute as well. Forests and soil microbes release  volatile organic compounds that can lead to ozone formation. Volcanic eruptions and wildfires  release carbon monoxide and sulfur dioxide.

Even dust storms put particulates in the air that  can travel across continents. And hopefully you’re seeing the challenge of designing a smoke  stack. The primary controls like scrubbers and precipitators get most of the pollutants out, and  hopefully all of the ones that can’t be dispersed.

But what’s left over and released has to avoid  pushing concentrations above the standards. That design has to work within the very  complicated and varying context of air chemistry and atmospheric conditions  that a designer has no control over. Let me show you a demo.

I have a little fog  generator set up in my garage with a small fan simulating the wind. This isn’t a great  example because the airflow from the fan is pretty turbulent compared to natural winds.  You occasionally get some fog at the surface, but you can see my plume mainly stays above the  surface, dispersing as it moves with the wind.

But watch what happens when I put a building  downstream. The structure changes the airflow, creating a downwash effect and pulling my plume  with it. Much more frequently you see the fog at the ground level downstream.

And this  is just a tiny example of how complex the behavior of these plumes can be. Luckily, there’s  a whole field of engineering to characterize it. There are really just two major transport  processes for air pollution.

Advection describes how contaminants are carried along by the wind.  Diffusion describes how those contaminants spread out through turbulence. Gravity also affects  air pollution, but it doesn’t have a significant effect except on heavier-than-air particulates. 

With some math and simplifications of those two processes, you can do a reasonable job predicting  the concentration of any pollutant at any point in space as it moves and disperses through  the air. Here’s the basic equation for that, and if you’ll join me for the next 2 hours,  we’ll derive this and learn the meaning of each term… Actually, it might take longer  than that, so let’s just look at a graphic. You can see that as the plume gets carried along  by the wind, it spreads out in what’s basically a bell curve, or gaussian distribution, in the  planes perpendicular to the wind direction.

But even that is a bit too simplified  to make any good decisions with, especially when the consequences of getting it  wrong are to public health. A big reason for that is atmospheric stability. And this  can make things even more complicated, but I want to explain the basics, because  the effect on plumes of gas can be really dramatic.

You probably know that air expands as  it moves upward; there’s less pressure as you go up because there is less air above you. And as  any gas expands, it cools down. So there’s this relationship between height and temperature  we call the adiabatic lapse rate.

It’s about 10 degrees Celsius for every kilometer up  or about 28 Fahrenheit for every mile up. But the actual atmosphere doesn’t always follow  this relationship. For example, rising air parcels can cool more slowly than the surrounding  air.

This makes them warmer and less dense, so they keep rising, promoting vertical motion  in a positive feedback loop called atmospheric instability. You can even get a temperature  inversion where you have cooler air below warmer air, something that can happen in the early  morning when the ground is cold. And as the environmental lapse rate varies from the adiabatic  lapse rate, the plumes from stacks change.

In stable conditions, you usually get  a coning plume, similar to what our gaussian distribution from before predicts. In  unstable conditions, you get a lot of mixing, which leads to a looping plume. And things really  get weird for temperature inversions because they basically act like lids for vertical movement. 

You can get a fanning plume that rises to a point, but then only spreads horizontally.  You can also get a trapping plume, where the air gets stuck between two inversions.  You can have a lofting plume, where the air is above the inversion with stable conditions below  and unstable conditions above.

And worst of all, you can have a fumigating plume when there are  unstable conditions below an inversion, trapping and mixing the plume toward the ground surface.  And if you pay attention to smokestacks, fires, and other types of emissions, you can identify  these different types of plumes pretty easily. Hopefully you’re seeing now how much goes  into this.

Engineers have to keep track of the advection and diffusion, wind speed  and direction, atmospheric stability, the effects of terrain and buildings on all those  factors, plus the pre-existing concentrations of all the criteria pollutants from other  sources, which vary in time and place. All that to demonstrate that your new source  of air pollution is not going to push the concentrations at any place, at any time, under  any conditions, beyond what the standards allow. That’s a tall order, even for someone who loves  gaussian distributions.

And often the answer to that tall order is an even taller smokestack.  But to make sure, we use software. The EPA has developed models that can take all these  factors into account to simulate, essentially, what would happen if you put a new source of  pollution into the world and at what height.

So why are smokestacks so tall? I hope you’ll  agree with me that it turns out to be a pretty complicated question. And it’s important,  right?

These stacks are expensive to build and maintain. Those costs trickle down to  us through the costs of the products and services we buy. They have a generally negative  visual impact on the landscape.

And they have a lot of other engineering challenges too, like  resonance in the wind. And on the other hand, we have public health, arguably one of the most  critical design criteria that can exist for an engineer. It’s really important to get this right. 

I think our air quality regulations do a lot to make sure we strike a good balance here. There are  even rules limiting how much credit you can get for building a stack higher for greater dispersion  to make sure that we’re not using excessively tall stacks in lieu of more effective, but often  more expensive, emission controls and strategies. In a perfect world, none of the materials or  industrial processes that we rely on would generate concentrated plumes of hazardous  gases.

We don’t live in that perfect world, but we are pretty fortunate that, at least in  many places on Earth, air quality is something we don’t have to think too much about. And to  thank for it, we have a relatively small industry of environmental professionals who do think about  it, a whole lot. You know, for a lot of people, this is their whole career; what they ponder from  9-5 every day.

Something most of us would rather keep out of mind, they face it head-on, developing  engineering theories, professional consensus, sensible regulations, modeling software,  and more - just so we can breathe easy. The engineering of air quality rests  on a huge body of scientific work, measurements, modeling, and simulations  we can use to make important decisions that affect human health. One of my  favorite creators, BobbyBroccoli, just released an hour-and-a-half long documentary  on the Baltimore Affair, a scandal that took the scientific world by storm in the 1980s  and 90s.

It is a fascinating, polarizing, and complex story that is told so beautifully  through incredible graphics. I was blown away that the story isn’t more well known. And if you want  to check it out, it’s only available on Nebula.

It’s a streaming service built by and  for independent creators, including a lot of my favorites like BobbyBroccoli,  Wendover Productions, the Coding Train, and Branch Education. I don’t know about  you, but independently-produced content is most of what I watch these days. I just  like the authenticity and thoughtfulness of videos that haven’t been through ten levels of  studio executives watering the information down to capture the widest audience possible. 

I just think passionate individuals and small teams make the most compelling work,  and Nebula is the perfect place for it. Nebula’s totally ad-free, with tons of  excellent channels and lots of original series and specials like the 17 Pages documentary  by Bobby Broccoli. It’s also a great gift, especially because a yearly membership is 40%  off at the link in the description.

It’s a pretty cheap deal, even if you just want to avoid  ads while watching your favorite creators. My videos go live on Nebula before they  come out on YouTube. If you’re with me that independent creators are the future of great  video, I hope you’ll give it a try.

That’s go. nebula. tv/Practical-Engineering.

Thank you  for watching, and let me know what you think!