By Rob James and Peter Spinney, NeuCo Inc.
For nearly five years, two selective catalytic reduction (SCR) systems have operated almost around the clock at Dynegy Midwest Generation’s Baldwin Energy Complex in Baldwin, Ill. The SCR systems serve two 600 MW cyclone units and have been critical in reducing nitrogen oxide (NOX) emissions from the plant.
Dynegy’s Baldwin Energy Complex. Photo courtesy of Dynegy
Dynegy’s Baldwin Energy Complex is one of an increasing number of U.S. power plants that are running SCR systems year-round. Many more plants will soon follow suit as a result of increasingly stringent NOX regulations such as the Clean Air Interstate Rule (CAIR). And while these plants will likely realize their goal of significantly reducing NOX emissions, they may also experience some of the challenges that SCR systems can present. In addition to escalating input costs, challenges include ammonia slip, fly ash contamination, ammonium bisulfate build up, “blue plume” and catalyst replacement scheduling. Although some of these challenges are inherently mechanical or logistical, others can be mitigated with boiler optimization software solutions.
Progressive power generators such as Baldwin are combining SCR systems and other NOX reducing hardware with boiler optimization software. Comparatively inexpensive, boiler optimization solutions reduce NOX at the source and allow power generators to lower their SCR-related operating costs, better manage the interactions between combustion and post-combustion systems and potentially run their SCR systems less aggressively.
The CAIR Mandate
In just a few months CAIR will go into effect. This U.S. Environmental Protection Agency (EPA) formal mandate will demand the largest reduction in U.S. history of NOX and sulfur dioxide (SO2) from power generation. Announced in March 2005, CAIR requires a reduction of NOX and SO2 levels across 28 Eastern states and the District of Columbia.
CAIR’s first phase of NOX reductions will begin in 2009. The first phase of SO2 reductions starts in 2010. When fully implemented, CAIR will reduce SO2 emissions in these states by over 70 percent and NOX emissions by over 60 percent from 2003 levels.
To give an idea of CAIR’s significance, federal regulations between 1970 and 2003 decreased U.S. NOX levels by 41 percent. That gave power producers almost 33 years to cut NOX emissions not quite in half. By contrast, CAIR requires a 60 percent reduction in just six years (2009-2015). Plants that cannot meet their deadlines will be allowed to buy credits from those that are ahead of schedule. Nevertheless, all plants in CAIR states will have to make substantial provisions to comply with these new standards.
“Just about every non-attainment problem is caused by numerous sources of air pollution spread out over a wide geographic area,” said Bob McConnell of the Environmental Protection Agency’s regional office in Boston. “CAIR is not designed to single-handedly solve the ozone and fine particulate matter problems in the Eastern U.S., but rather balance the burden for achieving attainment between regional and local control programs.”
Selective Catalytic Reduction Systems
To prepare for CAIR, power generators are installing SCR systems beyond those already operating to meet federal regulations implemented earlier this decade. SCR systems work similarly to an automobile’s catalytic converter: A gaseous or liquid reagent (usually ammonia) is added to the exhaust gases before they exit the stack. The mixed gases travel through layers of catalyst, causing NOX and ammonia to react and form nitrogen and water vapor. SCR systems are one of the most effective tools for lowering NOX and can reduce this emission by up to 85 to 95 percent.
SCR system at Dynegy’s Baldwin Energy Complex. Photo courtesy Dynegy.
While SCR systems are highly effective, they have also turned out to be more expensive than initially anticipated, both in terms of capital and operating costs. The demand for SCR systems due to CAIR, combined with massive demand for steel and craft labor, are not only increasing the cost of SCRs but impinging on their timely availability. In addition, SCR systems require reagents such as ammonia, the price of which has more than doubled in recent years. Operations issues such as ammonia slip, the formation of ammonium sulfate and ammonium bisulfate and SO3– (sulfur trioxide) related opacity can all result from SCR use.
Also, many SCR systems were designed to operate during the SIP (State Implementation Plan) five-month Ozone Season, giving generators seven months to get their SCRs in shape for the next season. CAIR is a year-round mandate, however, and will force generators to push these systems harder and operate them more frequently than before. As power generators begin running SCRs year-round, operations costs and negative operational impacts will correspondingly increase.
The boiler is the heart of steam turbine-driven electricity production and home to complex process interactions. Combustion quality, fuel and air mixing, gas and steam temperatures, fouling and tube erosion and emissions control are just a few of the interrelated variables that must be continually managed. Boiler optimization uses real-time “intelligent” software to holistically optimize two key boiler processes (combustion and soot cleaning) to maximize boiler performance and manage tradeoffs so these sub-processes are in sync rather than working at cross-purposes.
Combustion optimization refers to closed-loop optimization of fuel and air mixing within the furnace by manipulating relevant fuel and air injection points to reduce NOX and other emissions and improve fuel efficiency. Using artificial intelligence- (AI) based technologies such as neural networks, design of experiments and model predictive control, combustion optimizers extract knowledge about the combustion process and determine the optimal balance of fuel and air flows in the furnace. These systems adjust the DCS or other control system biases to more consistently position dampers, burner tilts, overfire air and other controllable parameters at their optimal settings for given sets of conditions, objectives and constraints.
Sootblowing optimization dynamically determines the boiler cleaning actions that optimally balance the unit’s heat rate, reliability and NOX objectives. A closed-loop optimization software application, it directs soot blowing controls to take real-time actions that best meet the unit’s overall heat rate, reliability and emissions goals. It uses adaptive modeling and expert rules or heuristics to optimize the activity of these systems with respect to their effect on multiple simultaneous global performance objectives. The expert rules also ensure that all applicable unit-specific constraints are considered.
Comprehensive boiler optimization ensures that these two related optimizers share an understanding of process characteristics, make performance comparisons and tradeoffs and align combustion and sootblowing actions towards achieving production objectives.
Mitigating SCR-Related Issues
Boiler optimization systems are comparatively inexpensive software solutions that reduce NOX at the source, which may allow power generators to run SCRs less aggressively. In addition, boiler optimization can more closely match boiler outlet temperature and NOX profiles to those conditions that make the SCR reactor most effective overall, even as operating conditions change. The result is reduced ammonia usage, fewer negative SCR-related side-effects and simultaneous improvements in combustion and sootblowing operations.
By lowering NOX at its source and attaining more ideal flue-gas temperature and distribution, boiler optimization allows SCR systems to obtain the desired level of NOX reduction with less ammonia and slip and with a lesser likelihood of ammonium bisulfate and sulfur trioxide deposits that can damage downstream equipment and lead to forced outages. A typical boiler optimization solution can lower NOX and/or increase SCR effectiveness between 10 to 20 percent, which reduces SCR-related ammonia consumption by an equivalent percentage.
The impact these changes can have on performance is illustrated in Figure 1. Here, the optimization system is moving the cyclone feeder speed biases (regulating fuel flow) and secondary air biases (regulating stoichiometry or fuel-to-air ratio). The optimizer was given goals that included maintaining cyclone main flame scanner quality, reducing heat rate and reducing SCR-related ammonia flow. A reduction in SCR Inlet NOX or an improvement in SCR efficiency is shown as green and blue process trends.
Figure 1: On this cyclone unit, the optimizer moves the cyclone feeder speed biases and secondary air biases. A reduction in SCR Inlet NOX or an improvement in SCR efficiency is shown as green and blue trends.
For a baseloaded 600 MW coal-burning unit generating 7,500 tons of NOX a year, ammonia costs can total about $2.6 million annually. A 15 percent reduction in boiler NOX with a corresponding decrease in ammonia consumption would reduce ammonia costs for this unit by nearly $400,000 a year. Plants with ammonia slip analyzers reap additional benefits from optimization, as the optimizers can be configured to more aggressively adjust the amount of ammonia being used to maximize NOX removal.
Built in the early 1970’s, Baldwin has three 600 MW units. Unit 1 and 2 are opposed wall cyclones equipped with SCRs that operate on a year-round basis. Unit 3 is a CE tangentially-fired boiler with separated overfire air and low NOX burners. The plant converted to 100 percent Powder River Basin coal in 1999 and 2000.
From 2003 to 2007, NeuCo Inc. and Baldwin worked to develop and install integrated optimization modules to optimize the plant’s combustion, SCR ammonia consumption, sootblowing, thermal performance and equipment reliability detection and diagnosis processes. This $19.1 million Clean Coal Power Initiative (CCPI) technology development and demonstration project was a cost-shared effort between the U.S. Department of Energy and NeuCo. Today, Baldwin hosts what may be the nation’s most significant integration of real-time asset optimizers for coal-fired power generation.
For Baldwin’s two units with SCRs, boiler optimization included direct modeling and optimization of SCR ammonia flow and SCR Inlet NOX as a function of cyclone feeder and secondary air biases.
Without optimization, ammonia flow for Baldwin’s two SCR-equipped units was typically 400 to 600 klb/hr (see Figure 2). Boiler optimization helped bring ammonia consumption down to around 300 to 400 klb/hr.
Plant operators at Baldwin are seeing improved boiler efficiency, steam temperature control and reliability. On Unit 3, which does not have an SCR, operators are lowering NOX and have better control of soot cleaning. Sootblowing activities on that unit also have been reduced.
Figure 3: The left pane shows ammonia flow without the optimizer engaged. At right, the optimizer has reduced the ammonia required to achieve the same NOX setpoint, at full load, by 15 20 percent.
Owensboro Municipal Utilities (OMU) is Kentucky’s largest municipal electric and water provider. Its Elmer Smith Station operates two coal fired units: a 155 MW cyclone and a 290 MW t-fired unit burning bituminous coal. During the ozone season, over fire air (OFA) and an SCR system is used on the cyclone unit and low NOX burners with separated over fire air (SOFA) and an SNCR system is utilized on the t-fired unit. Both units are also equipped with comprehensive boiler optimization solutions.
NeuCo’s boiler optimization package, CombustionOpt and SootOpt, are helping to manage NOX formation and enhance SCR and SNCR operations at OMU. According to Kevin Frizzell, plant manager, “Optimized boiler NOX enhances the plant’s SCR and SNCR performance which minimizes bisulfate buildup on the air heaters.”
Aside from SCR-related benefits, plants with boiler optimization can better manage tradeoffs between multiple opeating objectives. At OMU the installation of an overfire air system on Unit 1 resulted in significant reduction of steam temperatures.
“Optimization has helped us to mitigate that negative impact,” said Frizzell. “It also helps us steady boiler NOX production and homogenize the differences between our operators. And SootOpt has been a big help in standardizing our sootblowing operations, which also improves reheat temps and heat rate.”
A Holistic Approach
Fossil-fired power plants are comprised of highly interrelated processes and subsystems that must be managed synchronously. Today’s power generators are addressing numerous goals across multiple plant subsystems based on market prices for fuel, reagents, emissions allowances and megawatt-hours. Tightening emissions regulations such as CAIR are causing plants to implement new equipment and processes that add to the complexity. For instance, SCR systems are often required to significantly reduce NOX and meet regulatory requirements, but they may adversely affect performance against other plant goals. It is virtually impossible to install any significant plant system without impacting the total generating environment.
As operating complexity increases, artificial intelligence-based optimization software is gaining traction as a tradeoff management tool. Optimization is particularly good at understanding the cause and effect impacts of multiple actions taken across multiple systems interacting in a dynamic environment. Boiler optimization solutions have proven effective at mitigating some of the unwanted side effects of SCR systems and now software providers and SCR manufacturers are looking for ways to extend the solution to more directly optimize SCRs within a total unit context. NeuCo, for example, is working with Babcock Power, a leading technology designer of SCR systems, to more closely tie plant-wide economic optimization with SCR controls.
While power generators look for solutions to deal with CAIR’s enormous implications, it’s important that they simultaneously consider balancing emissions reduction with total unit performance. Optimization’s role is to help power plants remain as competitive as possible while fulfilling their environmental obligations.
Author: Rob James is Product Manager with NeuCo Inc. responsible for the company’s CombustionOpt® and SootOpt® products. Peter Spinney is Director of Market and Technology Assessment at NeuCo Inc. He is a contributor to www.theoptimizationblog.com, a blog about optimization for the power industry.