Get smart about removing slag
More often than not, sootblowing is literally a shot in the dark. Clyde Bergemann’s solution to this problem: Control sootblowing operations intelligently, based on the outputs of real-time weight and heat-flux sensors and the calculations of a computer model.
Every coal plant operator knows that burning coal produces ash, which can melt into slag that can be difficult to remove. So difficult, in fact, that dynamite may be the only solution. Slag on boiler tubes has two undesirable effects. It always reduces the tubes’ heat-transfer capacity, and if it comes loose in big enough chunks and falls, it can damage or destroy tubes or structures below. In the first case, the consequences are reduced boiler efficiency and plant output and–potentially–catastrophic tube overheating. In the second, the result is typically a forced outage.
All coal-burning plants are susceptible to slagging, and different types of coal foster different kinds of slag formation. For example, the burning of low-sulfur Powder River Basin (PRB) coals produces ash whose low softening temperature can turn it to slag on hot convection-pass surfaces. At the other end of the spectrum is high-sulfur Eastern bituminous coal, whose high iron content significantly lowers the ash fusion temperature.
To clean their boilers, plants periodically sootblow their tubes and other heat-transfer surfaces with water, compressed air, or high-pressure steam. But determining the optimal cleaning frequency for each sootblower is nearly impossible in the absence of a real-time picture of how much slag exists, and where it is. For this reason, at many plants the operating frequency for each of the dozens of sootblowers is based on past positive results. But sootblowing too often is just as bad as sootblowing too infrequently. The former wastes money, labor, and steam, and the latter is just asking for trouble.
Two years ago, in the October 2003 issue of POWER, we reported that Atlanta-based Clyde Bergemann Inc. (CBI) had teamed up with Georgia Power Co. to fight slag at Plant Bowen. On that project, CBI installed real-time strain-gauge sensors on rods between the pendant heat exchange surfaces of the plant’s boiler to detect the increased weight of slag buildup. Since then, CBI has taken its “smart” sootblowing technology to a higher level, one that the company calls intelligent sootblowing (ISB). This article describes the results of a deployment of CBI’s new system–which features smart pressure and heat-flux sensors, smart water cannons and steam lances, and smart models and controls–at the coal-burning plant of an independent power producer in a midwestern U.S. state.
Slagbusters
Huiying Zhuang, a boiler process engineer at CBI, explains that the effectiveness of his company’s ISB line derives from several pieces of hardware and software working in concert. Figure 1 shows which pieces perform which functions, and where:
* SmartCannons (Figure 2) clean the entire furnace by spraying jets of water to the opposite or adjacent wall.
* SmartSensors (Figure 3) installed within the furnace’s waterwalls detect the local level of heat flux in real time. This information can be invaluable in determining where and when to sootblow, and which kind of sootblower to use.
* SmartGauges–strain gauges mounted on the support structure of the boiler’s pendants–quantify the extent of slag buildup.
* SmartLances–retractable sootblowers aimed at the superheater and reheater–use steam as their default cleaning agent. But they also can blend water with the steam when doing so would increase the lances’ effectiveness.
* The SmartModel constitutes the system’s “brain,” determining when and where cleaning is really needed.
* The SmartControls serve as a traffic cop, by taking information from the SmartModel and the sensors and gauges, and directing the operation of the cannons and lances.
Getting smart pays off
According to Zhuang, a plant can save big bucks by deploying an ISB system. He claims that an 800-MW unit might increase its profitability by one-third by boosting the efficiency of its boiler and reducing plant downtime. With such a dramatic gain in profits, a CBI system priced at $1.2 million might pay for itself in just six months, Zhuang said.
Zhuang went on to detail where those gains would be realized. Prefacing the following explanation with the remark that “fuel costs can represent 80% of a plant’s operating costs,” he said, “An ISB system can enable a plant to burn lower-quality, cheaper coal, saving millions of dollars annually.”
In addition, increased boiler efficiency produces additional financial gains. As Zhuang explained, “A cleaner furnace means greater radiant heat absorption and a reduction in furnace exit gas temperature [FEGT]. It also means a lower attemperator flow, which reduces the cost of water treatment and the consumption of sootblowing steam.” Because a boiler with clean heat-transfer surfaces runs more efficiently, installing an ISB system can even enable a plant to sell, rather than buy, NOx emissions credits. What’s more, such a plant’s selective catalytic reduction system wouldn’t have to work as hard, which would reduce its O&M costs.