FGD and DeNOx
NEWSLETTER       

                                                                                                           May 2003
                                                                                                             No. 301

Foster Wheeler Fuel-Air Metering and Control Systems

Along with its advanced low NO_x combustion system, Foster Wheeler supplies new metering and control systems that provide real-time coal mass flow and particle velocity and fineness results in combination with airflow measurement This “Fuel Injection” approach provides additional performance benefits for ultra-low NO_x firing system applications.

The Electric Charge Transfer technology (ECT) is an innovative electrostatic technique for coal flow measurement. It is a patented approach for obtaining coal mass flow (absolute and relative) and transport gas velocity in each conduit. The ECT can also monitor coal fineness for unburned carbon or mill maintenance purposes. Foster Wheeler is the worldwide distributor of this technology, developed by TR-Tech International Oy of Finland. The ECT system itself consists of tungsten carbide probes, which are connected to signal conditioning electronics. Three probes in each conduit are needed for coal flow balance measurement and six for absolute coal flow and velocity measurement.

The ECT system offers several distinct advantages:

·         All information is continuous and on-line, which is essential for control applications.

·         The ECT measurement is not affected by coal type, moisture, ash content or coal roping.

·         The electronics can be located up to 1200 feet from the conduits. No electronic cabinets are needed on the burner decks.

·         The abrasion-resistant probes in the coal con­duit are passive and need no power supply.

·         The installation is easy and is also possible in situations where space is limited.

Foster Wheeler also offers its proprietary Compartment Air Distribution Monitoring (CADM) System for measuring relative secon­dary airflow distribution through every air and coal compartment, including the main and separated secondary air staging windboxes. It can also provide mass flow and nozzle tip discharge velocities from each compartment.

S. Laux and J. Grusha offered field results from a tangentially fired system firing a combination of PRB and western bituminous coal retrofit with Foster Wheeler’s TLN3 system. This system included separated overfire air, new windbox nozzle tips, lower furnace stoichiometry control, windbox damper modifications, ECT coal flow distribution and velocity monitoring, CADM system, and appropriate DCS connection and logic. After retrofit, NO_x emissions ranged from 0.135 to 0.15 lb/MMBtu depending on the fuel blend and CO emissions between 0 and 75 ppm.

The ability to measure coal flow continuously also allows monitoring of the mill performance and linking it to changes in the overall boiler behavior. An interesting relationship is shown in Figure 4. The graph shows the coal particle velocity in four pipes of a single mill and unit opacity over six hours. Changes in mill operation caused the velocity in one of the pipes to drop significantly, although coal and air flow to the mill were kept constant. This phenomenon occurred at full load and may indicate that the mill is being overloaded. The abrupt change in the velocity resulted in an increase of unit opacity. The observed change of opacity was about 2 percent, but the compounded effects of mill conduit velocity excursions can result in significant opacity spikes. This relationship was verified on several different occasions and unit operators now watch for abnormal ECT signals to change mill operation on selected mills.

Figure 4. Example for Influence of Coal Pipe Velocity on Opacity

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