Air Pollution Control Equipment Services, Coal, Emissions

The Clean Coal Power Initiative at NRG Texas’ Limestone Electric Generating Station

By John Hudspeth, Plant Engineer and Terence Smitherman, Maintenance Supervisor at NRG Energy

How do you minimize emissions while maximizing efficiency and managing byproducts of the plant in a cost effective manner? Can mercury speciation and multi-pollutant benefits be measured, optimized and controlled? How do you cost-effectively reduce emissions without adding back-end environmental controls? How do you lower capital investment for emissions reduction, while optimizing unit performance, with no impact on coal combustion or byproduct sales?

Limestone Electric Generating Station’s Clean Coal Power Initiative (CCPI) project will demonstrate integrated instrumentation and advanced controls for mercury specie control, and subsequent capture, at a coal-fired power plant. This project integrates multi-pollutant control and performance optimization strategies to achieve emissions reduction and efficiency improvement.

The CCPI project at Limestone, sponsored by NeuCo, Inc., and the Department of Energy (DOE), is attempting to answer these questions, by demonstrating integrated instrumentation, sensors and advanced controls for mercury specie control, and subsequent capture, at a coal-fired power plant.

The US power industry, with nearly 1100 coal-fired power plants averaging 40 years old, is faced with meeting emissions reduction regulations while having to compete to be the lowest cost provider. Using lower sulfur fuels, which provide environmental benefit sometimes at an attractive price, often leads to increased maintenance and capital costs as the power plant mechanical equipment was not designed to use the lower sulfur fuel. Using off-specification fuels can increase operating costs and reduce unit availability. It is apparent the industry needs improved plant performance optimization and environmental control technologies.

Plant Profile
The Limestone Electric Generating Station, owned by NRG Texas, is a large utility coal-fired steam electric power plant. The plant is equipped with a tangentially fired boiler to supply steam for a 913 MW turbine-generator. Limestone fires a blend of Texas lignite and Powder River Basin coal. The unit is also equipped with a cold-side Electrostatic Precipitator (ESP) and a wet limestone Flue Gas Desulphurization (FGD) system for SO2 removal. Both devices are capable of removing species-optimized mercury from the unit’s flue gas.

Project Mission & Methodology
In 2005, the Mercury Speciation and Multi-Pollutant Control project proposed by NeuCo and NRG Texas was selected as a Round Two CCPI project winner by the DOE’s National Energy Technology Laboratory. The second round is designed to promote technology advancements for gasification-based electricity production, advanced mercury control, and sequestration and sequestration-readiness, through government-industry cost shared technology development and demonstration partnerships. The NeuCo-NRG project at Limestone is anticipated to conclude in the second quarter of 2010.

These efforts are part of a project and collaborative agreement with the U.S. Department of Energy’s National Energy Technology Laboratory who is providing approximately $6.1 million in U.S. federal funds to support the estimated $15.5 million project.
The Limestone team’s mission is to prove that mercury speciation and multi-pollutant benefits can be measured, optimized and controlled. The integration of state-of-the-art measurement devices, advanced controls and artificial intelligence-based non-linear optimization technologies have been added to measure, evaluate and recognize the dynamic unit operating conditions.

Limestone’s mercury capture improvement will be achieved by optimizing speciation through advanced sensors in the boiler, precipitator, FGD and fuel handling/delivery systems, as well as installing a neural network optimization technology. Combustion and soot blowing optimization enables improved heat rates and reductions in NOX emissions without increasing unburned carbon in the plant’s fly ash. These plant wide advanced solutions are also used to maximize the portion of the mercury vapor in the flue gas which is oxidized resulting in lower mercury emissions. NeuCo’s first principles-based unit performance system and its neural network and heuristics-based maintenance system are also being installed, which will help Limestone proactively identify and diagnose plant anomalies. The project will result in Limestone having the most advanced network of integrated optimization systems ever applied in the industry.

Phase I of the project, concluding this year, involves the installation and validation of sensors and measurement systems including Zolo’s ZoloBOSS™ laser-based sensor to measure the average H2O, CO, O2 and temperature. These sensors were installed above the separated overfire air to provide a picture of the final combustion process and flue gas properties. Mercury CEMS, supplied by PS Analytical, were installed upstream of the ESP and across one FGD tower to provide an understanding of the mercury speciation and removal performance of the ESP/FGD system. Two ABB carbon in ash monitors were installed in the economizer gas outlet ductwork to monitor the flyash. Triple Five acoustic sensors were installed on the final elbow of the fuel piping before the windbox to measure coal flow. A Sabia Elemental Analyzer for fuel quality measurements was installed on one belt feeding fuel to the plant in order to optimize the fuel handling system. Limestone is currently testing the blending of Texas lignite and PRB on the belt and has had favorable results to date. Phase II of the project will involve implementing the sensor measurements combined with NeuCo’s optimization software, to optimize the combustion, sootblowing, ESP and FGD processes for maximum multi-pollutant and mercury removal and increased unit performance. This will perform real-time control with knowledge of a variety of fuel feedstocks.

In addition to identifying the “best” operating condition for each individual area of equipment, overall plant integration software will be used to arbitrate among the optimizers from each equipment area. The software will identify the best balance among inputs, such as coal blend, temperature and air. As a result, optimum outputs such as capturable mercury species and commercial-grade ash will be identified, all while operating within regulatory and commercial constraints.

The optimization package’s modular design is suitable for a variety of generating units. As a result of the flexible architecture and infrastructure, the same solution operating at Limestone, can be directly retrofitted into other existing coal-fired power plants or integrated into new plant designs.

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