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Re-Powering a 60 MW Tilting Tangential Coal Fired Unit with a Low NOx Natural Gas Combustion System

Jonathan C. Backlund, Eddie R. Cruz, Coen Company, Inc.
Ron Stanhope, Consumers Energy Company

ABSTRACT:

New Low NOx Natural Gas Combustion System Re-Powers 60MW Tilting Tangential Coal Fired Unit - Reduces NOx Emission

Unit #2 at Consumer's Energy Company's B.C. Cobb Station, Muskegon, Michigan, was until recently a retired tilting tangential pulverized coal fired 60 MW unit, commissioned in 1948 and retired from service in 1990. In 1996, Consumer's Energy started to evaluate the economics of conversion to gas, and operating as a peaking unit to take advantage of summer power sales rates on the open market. In September of 1998, Coen Company was awarded a turnkey contract to retrofit the unit with a low NOx firing system. Project scope included computational fluid dynamic furnace modeling, ignition system, low NOx burners, fuel gas piping, flame detectors, distributed control system for burner management and overall boiler control, field installation, and commissioning. The unit was on line in late April, 1999, on schedule. NOx production was well below guaranteed values, allowing many additional days of profitable peak power production.

Units 1, 2 and 3 of Consumers Energy Company's B.C. Cobb Station in Muskegon, Mich. were 60 MW power plants commissioned in 1948. The tangentially fired single furnace units used pulverized coal as their main fuel source, and oil for ignition and warm-up. But in 1990, after 42 years in use, the units were retired from service without future plans for its operation.

However, in 1996, in order to take advantage of summer power sales rates, Consumers Energy, began to evaluate the economics of converting the retired units from pulverized coal into a low NOx peaking unit. The conversion proved to be not only successful, but also very profitable.

The scope of the conversion project was extensive. It included computational fluid dynamic furnace modeling, a new ignition system, low NOx gas burners, fuel gas piping, flame detection, addition of a distributed control system for burner management and overall boiler control, field installation of the equipment and final commissioning.

In April 1999, eight months after the contract was awarded to Coen Company, Unit 2 was completed on schedule and placed on line. The result? NOx performance of 0.15 lb/mm btu Btu was guaranteed. But, even more important, after the unit was commissioned, it demonstrated a far better performance rate of 0.08 lb/mm Btu. The lower NOx production allowed for additional days of profitable peak power generation.

One year later, based on the success of Unit 2, Units 1 and 3 were also retrofitted with the same excellent results.

NOx generation occurs when the reactants of a combustion process are exposed to high temperatures and oxygen rich environments. Under those conditions, the nitrogen molecules become highly reactive with oxygen and form various oxides of nitrogen collectively referred to as NOx. The thermally induced formation of NOx can be reduced by lowering the combustion temperature and minimizing the availability of oxygen with which the nitrogen can react. Both of these parameters can be carefully controlled by staging the admittance of air to the combustion process.

For all three units in this project, the air was staged by the use of two methods: the first, on a level-to-level basis, and the second at the individual burners. On the level-to-level basis, the air is admitted into the furnace in two fuel air compartments, three auxiliary air compartments, and two over-fire air compartments as shown in Figure 1.

Figure 1

At the individual burner level, the nozzle drillings are fashioned to create the desired air staging. At higher levels of heat input, the combustion process undergoes an increase in fuel-air staging. As the flames from each corner of the furnace mix in the center, a single flame, called a "fire-ball", is formed. As the fire-ball is generated within the furnace, (See Fig. 2), another air-deficient zone is created. The circulation of the fuel around the fire-ball slows the mixing of reactants. As a result, air and fuel zones can be found at all stages of mixing.

Figure 2

LOW NOx GASBURNER:

Before the new retrofit, when the units were firing coal, there were three levels for coal input, with air injected above and below these levels. After the retrofit, the number of fuel compartments were reduced to two. Now, low NOx gas burners create a staged mixing to further enhance the NOx reduction. The re-design allows a process that performs well in reducing NOx while, at the same time, prevents furnace rumble.

IGNITORS:

An additional part of the conversion re-design involved specially designed NFPA-defined Class 2 natural gas-fired horn ignitors for each gas burner. Equipped with ignitor spark systems, each horn contains a flame proving system with ionic flame rods, manual isolation values, local double block and vent safety shutoff valve assembly and flexible stainless steel hose. All components are designed for easy removal and replacement, if required. The horns are constructed of 310 stainless steel to withstand the rigors of exposure to the flame and radiant heat of the furnace.

FLAME SCANNERS:

For main flame detection, Coen's IR7200A Viewing Head and IR7000B signal processors are used. The new low NOx gas burners modulate up and down 30 degrees from the horizontal position. To ensure safety, the flame scanners continuously monitor internal electronics by means of a self-monitoring cycle every six seconds. The self-checking ensures that the flame detection system is working properly.

PIPING RACK:

The main gas and ignitor header assemblies are supplied as part of the unit's piping rack. The assembly includes manual isolation valves, pressure regulator, strainer, header safety shutoff valve, header vent valve, pressure gauges, pressure limit switches, and instrument isolation valves. The fail-safe design ensures that the shutoff valves will close in the event of a loss of power, or when de-energized by a master fuel trip.

ACCESSORY EQUIPMENT:

To improve the performance and efficiency of the boiler, a variety of control devices were installed. Airflow, pressure regulation, and valve positioning are among the processes controlled by a Westinghouse Distributed Control System (DCS). Feedback from these devices are essential for the proper implementation of the DCS commands.

In the case of Units 1, 2, and 3, individual control actuators were installed for the forced draft fan, induced draft fan, and individual windbox compartment dampers. The control of air being introduced into the furnace is essential for the proper staging of combustion to reduce NOx production. With these control devices, the total airflow to the furnace and its distribution are monitored and maintained.

DISTRIBUTED CONTROL SYSTEM-BASED BURNER MANAGEMENT SYSTEM AND COMBUSTION CONTROLS:

Implementation of the Burner Management System (BMS), and the Combustion Controls in the DCS hardware leads to a system with extensive operational advantages. DCS hardware selection, architectural considerations, system design and integration of ancillary equipment were key engineering elements of the conversion. The design of the DCS included redundant fiber optic data highway, historical storage and retrieval, with all the components residing in 16 cabinets. For the three units, the operator interface included 12 operator CRTs, along with one engineering console with two CRTs.

The combustion controls and the burner management system are first established by the input/output database. In the process of building the system logic diagrams, the DCS is configured with the operator interface. Graphics are added once the control scheme is finalized.

FINAL INSTALLATION/STARTUP:

Coen was responsible for the installation of the new burner and control equipment. The installation portion of the project included the removal of the coal burners, coal piping, and oil piping. As the new burners will use natural gas for its fuel source, Coen was also responsible for the gas piping for the three units.

CONCLUSION:

With all of the equipment installed on time, the converted units were placed on line after eight months of engineering, planning, development and installation. The result? NOx emissions were greatly reduced beyond the guaranteed levels. Equally important in the retrofit of the formerly retired Cobb Station Units 1, 2, and 3, is the flexibility of the design, its availability of increased power, and the profitability of the well-designed peaking units.

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