One of the big surprises facing plants that switch from a more or less standard bituminous coal to a western coal is the increase in slagging and fouling. Many plants have reported significantly more fouling with Powder River Basin (PRB) coal than with their previous coal. Coping with more fouling usually means modifying either the number, type, or location of sootblowers as well as increasing their operating frequency. A number of automation systems are also available that use thermodynamic models of the steam generator to predict type and frequency of cleaning; others use sensors in the gas path to “report” when tube surfaces need cleaning. The one constant for nearly a decade has been the mechanics of sootblowing.
Failing to get your gas pass surface cleaning regimen house in order is costly. Accumulated ash causes steam generators to operate less efficiently. That means more fuel is burned and the death spiral begins. The challenge is that every boiler, fuel, and operating scenario is unique and typically requires a customized solution to be effective. The right combination can be elusive. Lower efficiency levels are indicated by an increase in overall pressure drop, exit gas temperature, and/or fan power; additional tube erosion and leaks; ongoing high O&M expenses; and more-frequent unplanned outages solely for cleaning. If this sounds like your plant, we just might have the solution you are looking for.
Light the fire
A new technology was introduced to the U.S. power generation industry about five years ago. It uses detonation cleaning, derived from Pratt & Whitney’s Pulse Detonation Engine, which is used to power a supersonic craft (Figure 1). The engineers developing the futuristic pulse detonation engine could not have foreseen that this technology would migrate into the electric power industry as the most novel boiler cleaning technology to come along in many years. The new ShockSystem detonation cleaning combustor (Figure 2) has nothing to do with using dynamite or detonating cord when a unit is off-line—the most invasive cleaning method still in use. Nor does it refer to other on-line cleaning system options such as sonic horns or water canons.
1. Shock wave. This is an artist’s concept of a pulse detonation engine powering a spacecraft. The concept is simple: A fuel/oxidizer mixture fills a tube and is ignited. The rapid burning transitions into a detonation wave that travels though the cylinder at up to eight times the speed of sound. Combustion is completed before the gas has had time to expand. The resulting explosive pressure is then converted into thrust for the vehicle. Courtesy: NASA Marshall Space Flight Center
2. Simple system design. A typical Pratt & Whitney ShockSystem combustor used for on-line boiler cleaning. Courtesy: Pratt & Whitney MMI
The process of creating a detonation consists of injecting a mixture of fuel and oxidizer into a chamber, igniting this mixture, transitioning the resulting combustion wave to a detonation wave, and then purging with air to prepare for the next cycle (Figure 3). The detonation wave travels at supersonic speeds within the combustion chamber and quickly decays to a blast wave once it leaves the chamber and propagates in open space (or within a large structure, such as a boiler). The blast wave—also known as a pressure wave, a pressure pulse, or simply an impulse—has the energy required to remove sintered and unsintered ash in the backpass of boilers. This pressure wave has little to no negative effects on tube erosion, and the required operational frequency is much less than that of acoustic horns.
3. Inside workings. This drawing illustrates the ShockSystem operating cycle. Source: Pratt & Whitney MMI
Detonation cleaning creates a blast wave that propagates omnidirectionally from the combustor exit. To date, all applications have been mounted outside the boiler wall and direct the blast wave through a penetration. These applications have been installed on both positive and balanced draft boilers. The benefit of this type of installation is that the combustion chamber can be installed along the boiler wall or routed around external structures without risking the loss of a lance tube due to bending, getting stuck, or breaking off during operation. Another major advantage boiler operators see is that the pressure waves are controlled and applied on-line, thus eliminating unnecessary outages. The long-term effects on tube erosion are minimal, plant emissions are lowered, exit gas temperature is decreased, and overall boiler efficiency is increased.
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