Sustainability Tips for Asphalt Plant Dryer Burners
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BY MALCOLM SWANSON, P.E.
The most fun I ever had during my years with Astec was when participating in the development, building and testing of burners. In addition to having fun, we built some really fine burners, including some that established the definition of “world class.” That is a good thing, because a burner is a necessary part of every asphalt plant. From time to time, I receive a request for guidance about purchasing a new burner. When I ask what the purpose of purchasing a new burner would be, the answer is usually “fuel efficiency.” Here is the first truth. Fuel efficiency is seldom a valid reason for changing burners. Notice that I did not say that it is never a valid reason but changing burners to reduce the fuel bill is usually disappointing. All good burners are capable of burning all the fuel and getting all the heat release when properly adjusted and operating in their efficient operating range. The differences are in emissions, noise, electric power consumption and the size of the efficient range or “sweet spot.”
BURNER TYPES HIT THE SWEET SPOT
To do a useful job of covering this subject, I need to mention the different types of burners that are available to the asphalt plant market. Almost all burners for this market are either hybrid or total air burners. These two terms refer to how they handle combustion and excess air. Hybrid burners, like the well-known Hauck StarJet, have an integrated blower that introduces part of the needed air and have provision for the rest of the air to be “induced” or sucked in around the burner body and heat shield by the exhaust fan. Many people refer to this type of burner as an induced air burner, but that is incorrect. An induced air burner has no blower at all. As far as I know, there are no induced air burners in service in the U.S. market. Total air burners, as the name suggests, bring in all of the combustion and excess air by an integrated burner blower. Both of these two burner types can burn a wide range of fuels including recycled fuel oil (RFO), No. 2 oil, propane and natural gas. Typically, total air burners have better control of fuel/air ratio than hybrid burners. A burner of either type can do a great job when it is on ratio but it is easier to keep the total air burner on ratio across more of its firing range. Hybrid burners are generally less expensive than total air burners. The extra money goes into the more complete control of air. The ability to control air throughout the firing range makes it possible to have a large operating sweet spot. Hybrid burners have a relatively narrow portion of their firing range where they are on ratio and highly efficient, but within that sweet spot there is no measurable difference in fuel efficiency as compared to total air burners. Some hybrid burners have a bigger sweet spot than others. Those that provide a larger percentage of the air through the blower tend to have a larger sweet
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spot, making it a good thing to ask about before buying. The size of the sweet spot is very important, because any burner can be inefficient when operating off ratio. Unfortunately, if you ask this question by using my “sweet spot” terminology, the burner manufacturer may not know what you are talking about. If you are considering a hybrid burner to control cost, just ask what percentage of the air is provided by the burner blower. A correct answer to that question will tell you which burner has the optimum sweet spot. There are two major subtypes of total air burners—nozzle-mix and premix. These terms may not be familiar to some but they are easily explained. In a nozzle-mix burner, the fuel and air are kept separate until the fuel exits the fuel atomizing nozzle at the nose of the burner. In a premix burner, they are brought together and mixed at some point upstream of the nose of the burner. I believe the cleanest burning, quietest and most efficient burners available are of the premix type. You might think it makes sense then that everyone should go with premix, but there are several reasons to carefully consider that option. The most important thing that premix burners can do that nozzle mix burners cannot do is operate with ultralow emissions. However, in most parts of the world, that level of performance is not yet required. Premix burners are expensive; so why pay the price if you don’t need the performance. The day is probably coming when it will be needed everywhere but we are not there yet. This means the right burner to buy depends on where the plant is located and how it will be operated. In California, New Jersey and a few other places, a premix burner may be the only type that can meet the emissions limitations. On a portable plant operating on long stretches of highway in less populous areas, the plant may operate wide open all day. The burner is called upon to almost always operate pretty much in the same narrow part of its firing range. A hybrid burner will be a good choice. A plant in a city may be up and down a lot and operated at different rates. A total air burner will enable efficient operation over a wider range. If noise is a critical issue, total air burners are generally quieter and premix burners are quietest of all. Some burner blowers are constant speed with air flow control by means of a damper while others use a variable speed drive (VFD) to control air flow by adjusting blower speed. VFD air flow control reduces electricity consumption and reduces noise. It also offers more precise control of air flow than is possible with damper control, and therefore, better control of fuel/air ratio. Choosing the right type of burner does not ensure that you making the best choice. Some burners of any particular type are just better than others of the same type. If certain aspects of performance are critical, get the applicable data. If, for example, you need ultralow
