Using Combustion Quality Management in Burner Emissions Control
BY Malcolm Swanson, P.E.
Focus on Performance: A Review of Burner Emissions Control Means for Asphalt Plants
Editor’s Note: The new Focus on Performance series from AsphaltPro Magazine allows OEMs and service providers in the industry to highlight asphalt professionals who have improved efficiencies and the bottom line through best practices and performance. This month’s installment from Malcolm Swanson, P.E., dives specifically into efficiencies gained through controlling the combustion process. As always, we’re honored to share Swanson’s knowledge with the industry.
The challenge of controlling emissions from fossil fuel burning industrial processes has been a major focus of industry since the passage of the Clean Air Acts of 1970 and 1990. So, the subject of this article is nothing new; but what makes the best emissions control methods work is not widely understood, even today. It’s good to know what works and why.
Chemicals Get A Bad Rap
Controlling burner emissions is really all about controlling formation and/or emission of certain chemicals. If you look at it from this point of view, our physical world is composed of chemicals. The cleanest things in nature, like pure water and fresh air, consist of chemical elements and various compounds of those elements. Air is mostly nitrogen—about 78%. Oxygen is the next most abundant element in air at about 21%. That leaves 1% for things like carbon dioxide, argon, hydrogen, helium, etc.
We can’t live without chemicals. In fact, we too are composed of chemicals. (I am not talking about our soul and spirit of course, just our bodies.)
There are chemicals that are harmful to the environment and to our health. However, even where “harmful” chemicals are concerned, the issue of danger is really about concentration.
For example, we all breathe carbon dioxide every minute of every day, but it doesn’t hurt us because normal air doesn’t contain a high enough concentration to damage our health. We can’t live without carbon dioxide because the plants that make oxygen and provide us with food need carbon dioxide to live and grow.
So, the real reason we try to control combustion process emissions is to keep the ambient concentrations of certain chemicals that have harmful potential at levels low enough to avoid harm. With that in mind, let’s consider what goes into controlling the oxides of nitrogen (NO and NO2, together referred to as NOX), carbon monoxide (CO), carbon dioxide (CO2), and total hydrocarbons (THCs).
Take Control of NOX
To narrow the focus of control efforts a bit, until climate change became the main issue, NOX had been considered the major offender, because it is a contributor to smog and can cause irritation of our respiratory systems. It has, therefore, received much attention. Controlling NOX emissions has been and remains difficult partly because some NOX control technologies tend to cause increased CO and CO2 emissions.
There are several different groups of technologies that have been developed to control NOX emissions. The major ones are combustion modification, exhaust gas treatment, staged combustion, fuel selection and combustion quality management. The last of these is probably not as familiar to many readers as the others. I will briefly discuss all five and then focus on the last.
To understand NOX emissions and how to control them, we must understand how they are created in the combustion process. NOX is formed through two different mechanisms and is referenced by those mechanisms—thermal and fuel.
- Thermal NOX is formed in burner flames at very high temperature by burning the nitrogen in the air. This happens when fuel is burned, with air as the source of oxygen, at temperatures of about 2200°F and higher. With thermal NOX, the higher the temperature and the longer the time at high temperature, the more NOX is produced.
- Fuel NOX is made when the “bound nitrogen” in the fuel compounds unites with oxygen in the flame. The amount of fuel NOX made is not so much related to time and temperature as it is to the amount of nitrogen in the fuel. Some fuels contain much more bound nitrogen than others. Basically, the heavier the fuel, the more bound nitrogen it contains. Coal, heavy oil, number 2 oil, propane and natural gas contain bound nitrogen, in this order from most to least. There isn’t much way to control how much fuel NOX is made in the burner flame except to use fuels that are low in bound nitrogen, like natural gas and propane, and burn as little fuel as possible by maximizing combustion and process efficiencies.
Because thermal NOX formation is strongly affected by temperature, much of the control efforts have been toward reducing flame temperature. Here are some ways that have been implemented. Flue Gas Recirculation (FGR), which is one of the means of reducing flame temperature, was the first effective means of NOX control implemented in asphalt plants.
FGR, which is also referred to as exhaust gas recirculation (EGR), is effective at reducing NOX, because it does two things that are unfavorable to NOX formation. First, it reduces temperature. It also reduces oxygen concentration in the flame making oxygen a little less available for bonding with nitrogen. However, FGR can only be pushed so far, because a deficiency of oxygen and low flame temperatures cause increased CO, CO2 production and exhaust gas volume.
Water injection directly into the flame was another early means of NOX control by suppression of flame temperature. Results were similar to FGR, but it was not quite as effective.
Staged combustion is another method of NOX control by reducing flame temperature and by creating zones where the fuel air mixture is not favorable for NOX production. The “stages” are points or areas within the combustion zone where portions of the total fuel and/or air are injected. Usually, a fuel rich zone is followed by a fuel lean zone so that the overall fuel air mixture is right but the mixture at each stage, rich or lean, is different from the final overall mixture. These methods are combustion modification techniques.
Treatments of the exhaust gases downstream of the process have also been used to reduce NOX, CO and TOCs but not so much in asphalt plants. In fact, my first patent in the asphalt industry was for catalytic reduction of NOX in the exhaust stream. The patent was issued but the equipment was never built due to there being less expensive alternatives. Urea injection into the exhaust gas stream is another effective means of NOX reduction but it has seen little, if any, use in our industry.
Fuel selection has become the most common means of combustion emissions reduction, even though, by itself, it often does not provide enough emission reduction to comply with applicable regulations. Natural gas is currently the fuel of choice. It is usually available and affordable and contains the least amount of carbon and bound nitrogen of any fossil fuel. If natural gas is unavailable, propane is the next best thing.
Combustion Quality Management
The discussion on Combustion Quality Management (my term) is the most interesting, in my opinion.
Burners, not just in this industry, but burners in general and especially large ones such as those we need for drying, have not been very good burners. For the most part, they have been perfectly adequate to provide the necessary heat and decent fuel efficiency for the drying process but have not been adequate to meet increasingly stringent emissions limits.
Most of the burners that have been used in this industry did not maintain consistent fuel air ratio throughout the normal firing range, but they were close enough for drying. It should be obvious that consistently maintaining the right fuel air ratio is critically important to controlling emissions. If you use too little air, the fuel burn will be incomplete and a lot of CO will be produced. Flame temperature will also rise causing excessive NOX production. If you go the other way with too much air the NOX may practically disappear because the flame is cooled by the excessive air but, again, CO emissions will increase because the “chilling” of the flame slows the chemical reaction of combustion and prevents complete burning. Too much air will also impact production rate by increasing exhaust gas volume.
However, let’s assume we have that part whipped. We can maintain the right ratio. There are some total air burners in the industry of which that assumption is nearly true. Our (this industry) burners still have not done a good job of emissions control for one reason: They do not mix the fuel and air very well.
When the fuel and air are not well mixed, what do you get for a flame? What you don’t get is a homogeneous mixture and, therefore, you don’t get a homogeneous flame. Rather you get a heterogeneous mixture and flame.
Think of the flame volume, with poor mixing, this way. It is as if the flame is composed of “pockets” of fuel-air mixture. Some pockets are fuel-rich while other pockets are fuel-lean. The rich pockets tend to burn hotter than the overall average flame temperature while the lean pockets burn cooler. We have both ratio problems—too rich and too lean—in one flame that is of the correct overall fuel air ratio. So, the flame makes too much NOX and too much CO even though the fuel air ratio is right.
These pockets of fuel air mixture of different ratios burn at different rates. This tends to make the flame large. For the typical 100 million BTU/hr gas burner the flame will be 12 to 14 feet long and 6 or 7 feet in diameter. That not only takes up a lot of the space in the dryer drum, but it extends time in the flame for every molecule. More time at temperature means more NOX.
This is what we have had. Now what do we do?
Dig deeper…down to the molecular level. Oxygen molecules prefer fuel molecules and do not like nitrogen molecules. Nevertheless, they will “marry” something if what they prefer is not readily available. Similarly, a carbon molecule will settle for one oxygen molecule if it can’t get two. So, the key to good clean burner emissions is mixing.
Mixing must be so good that nearly every fuel molecule ends up right next to an oxygen molecule. When that happens, the oxygen molecule will take the fuel and reject the nitrogen.
Of course, there isn’t enough fuel to suck up all the oxygen, so some NOX will be formed but it will be greatly reduced. Also, CO production will be minimal. Finally, combustion efficiency will be high, which reduces the fuel burn and, therefore, CO2 formation.
If it sounds crazy to say molecules have preferences, it won’t once you understand how it works. It’s called activation energy. To get the combustion reaction going, you must put some heat energy in and then you get much more back out. A match provides the activation energy to start the wax in a candle wick burning. The fuel/oxygen reaction requires less activation energy than the nitrogen/oxygen reaction. So, given equal availability of nitrogen and fuel to an oxygen molecule, the fuel/oxygen reaction is the one that will happen.
Something else that is really cool is that you get a flame with uniform temperature. There will be no hot or cold pockets. So, there are no “super emission producing” pockets in the flame. Also, because the fuel doesn’t have to waste time searching around to find an oxygen molecule to marry, marriage happens quickly, resulting in a very short flame.
The 100 million BTU/hr flame described above shrinks dramatically. It can be as small as 2 or 3 feet long and 2 or 3 feet in diameter. Flame volume shrinks to about 3% or 4% of that of the typical burner. That means very little time at flame temperature as well as no hot or cold pockets. Time in the flame drops to a few milliseconds.
Extraordinary Mixing
The question becomes, “How do you get that kind of extraordinary fuel air mixing?” You may be thinking “premix,” and you could be right, but most premix burners don’t get there. Premix is not necessarily better than nozzle mix. That is why there is such a thing as “lean-burn premix” firing. If the mixture is really good, the extra excess air of lean-burn premix isn’t needed. (The extra excess air is not a good thing because it reduces the plant production capacity by adding load to the exhaust system.)
The thoroughness of the mixture isn’t achieved just because it is premix or nozzle mix. It doesn’t really matter whether you get there by premixing the fuel and air upstream of the burner nozzle or do it at the nozzle. However, no one has been able yet to get that level of mixing in a nozzle-mix burner, as far as I know. I’m not saying that nozzle-mix burners are not good burners. There are some excellent nozzle-mix burners, but they aren’t ultra-low NOX burners.
Achieve the Best
As I share all these things, just so you know, I am not revealing any secrets. The basic science is the basic science. No one owns it. How to employ it to achieve these advantages has already been revealed in the patents. I am explaining in “English” what the patents and the science say in more complicated language.
The formation of carbon dioxide comes from burning fuels containing carbon. All fossil fuels contain carbon and hydrogen; hence, they are called hydrocarbon fuels. Those that contain the most carbon will produce the most carbon dioxide when burned. The ranking from most to least is the same as the ranking above for fuels containing the most bound nitrogen. With any particular fuel, the amount of carbon dioxide released is determined by efficiency. Obviously, burning more fuel makes more CO2 while burning less makes less. If you are going to burn a fossil fuel, the only means of CO2 control at the burner are fuel selection and efficiency.
Fuel selection is obvious: Use natural gas if possible.
Efficiency has two parts. First, there is combustion efficiency. The best possible combustion efficiency is basically burning all the fuel with the least possible amount of air. The least possible air is called “theoretical air” or “stoichiometric air.” In reality, burners never completely burn all the fuel and always use more air than the theoretical air. Combustion air, in excess of theoretical, is called “Excess Air.” Even the best commercial / industrial burners must have some excess air (usually about 25%) to get a good fuel burn. It might surprise some of us to know that minimizing burner emissions is not just about the burner.
The second part of efficiency is process efficiency. It is possible to have near perfect combustion and still have high fuel consumption and emissions because of waste in the downstream process. If you burn, for instance, 20% more fuel than is necessary, you make 20% more emissions. The telltale of poor process efficiency is high exhaust temperature and high equipment surface temperatures.
Good equipment insulation and good process heat transfer are the means of control for post combustion process efficiency. Asphalt plant dryer heat transfer efficiency is determined by controlling the exhaust gas temperature using the dryer flight system and drum rotational speed. Exhaust gas temperature should be maintained just above dewpoint temperature, with some allowance for cool and windy weather conditions. Dewpoint temperature in the plant exhaust stream actually changes very little. It is almost always in the range of 160 to 175°F. On a warm sunny day, it is safe to operate with 185°F stack temperature with no worry about mud in the baghouse. On a cold windy day, with uninsulated ductwork and baghouse, going up to 240°F stack temperature would be safer. With a good insulation system, cold and windy doesn’t matter. Operating with stack temperature near dewpoint provides much better process efficiency than is typically found among asphalt plants.
So, what is out there and available to manage your combustion quality? Astec has a low excess air premix burner that does the extraordinarily good mixing that I have described and achieves ultra-low NOX levels. There is also a good Hauck premix burner. Genco has a good, staged combustion burner. I am not trying to promote any particular burner. I just want to help increase understanding of the challenges and solutions of asphalt plant burner emissions control.
I hope, as you read everything above, that you didn’t brush over the importance of process efficiency as opposed to combustion efficiency. High fuel consumption caused by high stack temperature will increase emissions and decrease profits, even with the best burner in the world.
Malcolm Swanson, P.E., is the proprietor of e5 Engineers. For more information, he welcomes you to contact him at malcolme5engineers@gmail.com or (423) 667-6781.