Once-through steam generators (OTSGs), used to inject steam down a well to enhance recovery of crude oil, operate under increasingly stringent combustion- emissions requirements for nitrogen oxides (NOx) and carbon monoxide (CO). These requirements have steadily decreased over the decades from 100 parts per million (ppm) to 40 ppm to 30 ppm. Fast forward to 2017, and NOx requirements are 9 ppm and moving to 5 ppm in some areas. CO is reduced to near-zero levels with proper combustion management. However, NOx remains difficult to reduce with each step along the way, requiring major innovations in burner science and engineering.
In a conventional burner fuel and air react in a single zone to suddenly release their heat in a short flame. This type of combustion produces an intense flame and virtually no CO. However, intense combustion produces high flame temperatures that fuse nitrogen and oxygen in the combustion air to generate NOx, a criteria pollutant and ground-level ozone precursor. The decrease from 100 ppm to 40 ppm required the development of special burners known as low-NOx burners. They reduced NOx by staging the air into two distinct zones. In the first zone enough air was added to form a stable core of flame but not enough air to burn out all the fuel. A second portion of air was then added to complete the burnout in a strategy known as air staging. A complementary strategy known as fuel staging divided the fuel into distinct stages. Staging strategies tended to delay mixing and increase flame length, giving a larger flame surface that better transferred from the flame to the process.
Flames could impinge on process or boiler tubes, accelerating failure due to extended flame length. To achieve even lower emissions and provide additional momentum to stiffen flames, flue gas was recirculated and added into the combustion air stream. Carbon dioxide (CO2) and water (H2O) in flue gas are active infrared absorbers and, together with the additional mass provided by flue gas recirculation (FGR), help to cool the flame. But as NOx limits fell farther, flame stability became an issue, and burners grew more complicated, becoming known as ultralow NOx burners.
In the last decade NOx requirements had sunk to 9 ppm in California. A large oil and gas producer in that state operating a large fleet of OTSGs in the Bakersfield region began recirculating 25% to 30% of its total flue gas in its burners. This required large recirculation fans or substantially oversized combustion air fans to propel the increased mass flow. To achieve 5 ppm, experimental designs with ever more complicated burner architectures employed FGR levels approaching 40% and the limits of burner stability. Besides the increasing fan power (a significant operating expense), such a level of FGR can exacerbate corrosion and pose additional safety risks. In short, the evolution of ultralow NOx burners appears to have come to its limit.
The producer’s continuing search uncovered a new strategy for producing sub-5 ppm NOx developed by ClearSign Combustion Corp. Clearsign’s DUPLEX technology produced sub-5 ppm NOx and CO without the need for FGR.
To understand DUPLEX technology, consider the analogy of baking a cake. Cakes are first mixed as a batter and then baked into a cake. Who tries to mix and bake a cake at the same time? One can imagine that such a process could perhaps produce cake, but it wouldn’t look much like cake baked the better way. Yet today’s ultralow NOx burners have more in common with a mess of mixing and baking because ultralow NOx burners require the fuel to mix and to burn at the same time. The result is a process that produces a hodgepodge of fleeting fuel-air structures that greatly exacerbate NOx. But if, like a cake, one were able to first mix the fuel and air and then heat it to combust, the result would be a much cleaner way of burning. ClearSign’s solution was to separate these two processes by installing the DUPLEX surface—a low-pressure-drop porous ceramic surface—in the furnace some distance downstream of the burner. This surface can be installed over a two-day shutdown window and ready to fire.
Mixing fuel, air
DUPLEX technology is applicable to any type of burner (conventional low-NOx burners or ultralow NOx burners). After installation of the DUPLEX surface (Figure 1), the burner is fired in the usual way, heating it above the auto-ignition temperature, after which the flame is transferred to the surface (Figure 2). This transfer is affected most typically by shifting fuel to an additional set of fuel nozzles that are designed to stabilize on the downstream surface. Once transferred, all the mixing of fuel and air occurs prior to combustion on the surface. The original flame might have been many feet long; the resulting combustion now takes place in the few inches comprising the thickness of the porous wall, eliminating any possibility of flame impingement. This is possible because the rate-limiting process for a flame is not its chemistry but mixing. With the mixing complete the chemistry proceeds so quickly that there is virtually no time to make NOx because NOx is a kinetic process that requires time. The surface radiates with better efficiency than a simple flame since a porous ceramic surface is very nearly a perfect blackbody radiator with an emissivity approaching unity. Indeed, DUPLEX operation typically results in a modest thermal efficiency increase in the radiant section while producing sub-5 ppm NOx and CO. The main benefits of mixing first and burning second are thus: no required FGR, elimination of flame impingement and repurposing of the original burner to something far beyond ultralow NOx burner NOx performance.
In early 2015 the producer retrofitted its first unit, an OTSG fired at 62.5 MMBtu/hr. The initial burner retrofit saw the replacement of a subset of existing burner tips with special nozzles. These new nozzles had no effect during normal operation. The primary burner tips (which were left unmodified) continued to stabilize all zones per the manufacturer’s original design intention. The transfer strategy then comprised shifting the fuel to the modified nozzles only.
Without combustion in the primary zone, the flame quietly transfers to the DUPLEX surface. Quiet combustion is another hallmark of DUPLEX operation because combustion roar is eliminated when mixing and burning are decoupled. After the successful demonstration, another unit was purchased and installed by the producer in 2016. This successful retrofit mirrors the performance of the first unit. The units have been in continuous operation for more than a year and continue to deliver sub-5 ppm NOx.