An innovative approach that includes real-time coal flow, particle velocity and air flow measurement information has been developed by Foster Wheeler and TR-Tech. These technologies have been combined in an automated control system to give a ‘Fuel Injection’ (FI) approach that brings additional performance benefits for ultralow NOx firing systems.
S. Laux, J. Grusha, Foster Wheeler Power Group, Clinton, NJ, USA
T. Rosin, TR-Tech Int. Oy, Turku, Finland
Improved combustion efficiency from automated fuel injection (FI) systems is achieved by monitoring fuel and secondary air ratios and making the necessary adjustments based on emission and operating conditions. This philosophy has been adapted by Foster Wheeler and applied to coal-fired power boilers for achieving ultra low NOx emissions while minimizing carbon monoxide (CO) emissions associated with poor fuel-air mixing.
One of the first applications of the FI technology as part of a low NOx system retrofit were large tangential-fired units. The results repeatedly show a substantial NOx and CO reduction and oxygen profile improvements when the secondary air and fuel flow control system is active. Other units are being retrofit with low NOx systems that include this new FI technology approach.
Figure 1. Information exchange between ECT, CADM and DCS
Since 1996, Foster Wheeler has developed and introduced several new low NOx burner series to meet new unit and retrofit application emission requirements. These include wall, tangential and arched-fired low NOx burner applications.
Foster Wheeler’s Tangential Low NOx (TLN) firing systems are based on the application of secondary air staging technology, commonly referred to as ‘overfire air’. Staging of secondary combustion air has been well documented throughout the international boiler industry to be the single most effective technique for reducing NOx emissions from tangentially fired boilers. By redirecting a portion of the combustion air above the upper fuel elevation, fuel nitrogen conversion and thermal NOx production are reduced.
Electric Charge Transfer (ECT) technology is an innovative electrostatic technique for coal flow measurement. Foster Wheeler is the worldwide distributor of this technology, developed by TR-Tech International Oy of Finland. It is a patented approach for obtaining coal mass flow (absolute and relative) and transport gas velocity in each conduit in real time. The system can also monitor coal fineness for unburned carbon (UBC) reduction or mill maintenance purposes.
The ECT system measures the electric field created by electrostatic charges present in any two-phase flow application. To measure the charges several probes are placed into the coal stream. Electrostatic charging occurs when two materials come in contact with each other and then separate. For example, when coal particles impact the conduit wall in transportation, electrons will be transferred from one material to the other and a small electric field is created inside the coal conduits. Because the ECT system measures this overall electric field, direct contact between the probe and coal particles is not necessary. The measured charge is not dependent on the dispersion of particles and random coal roping does not affect the measurement.
Figure 2. Results of automatic secondary air to fuel flow balancing
The ECT system consists of tungsten carbide probes, which are connected to signal conditioning electronics by standard coaxial cables. The signal is then processed using proprietary software installed in a standard personal computer with an analog-to-digital card.
The probes are easily installed through the horizontal or vertical wall of the existing conduit and inserted into the coal stream. Three probes in each conduit are needed for coal flow balance measurement, and six for absolute coal flow and velocity measurement. The installation is simple and requires that only a mill be taken out of service for several hours, which minimizes lost generating capacity. After installation, the ECT measurement is verified by Rotorprobe sampling and the system is then ready to be used.
ECT calculates the coal flow in the conduits of each mill and displays this information on the computer screen. The ECT takes hundreds of readings every second to provide a continuous flow of uninterrupted information on coal flow or particle sizing. The information displayed can be forwarded to the plant DCS system or any computer system on site via network connection. The transfer of the ECT signals via network connection is a cost effective way to transfer the measured information.
Since 1998 ECT has been installed in 15 utility power plants. The same technology can be used to: determine the coal flow in conduits; measure the particle velocity in pipes; monitor unsteady phenomena in coal conduits; and monitor particle size changes of coal.
The ECT system offers several distinct advantages:
- All information is continuous and on-line
- ECT measurement is not affected by coal type, moisture, ash content or coal roping
- The electronics can be located up to 350 m 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.
The ECT system is one of the major components of the FI approach to control a coal fired boiler. Used independently, the ECT can help coal-fired power operators realize the following benefits:
- UBC level reduction, reduced unburned carbon losses and increased heat rate
- Reduced standard deviation of fly ash LOI
- Even boiler oxygen profile; potential to reduce excess air level
- More even superheater and reheater steam temperature profiles
- NOx and CO minimization due to well-balanced burners
- Potential for reduced auxiliary power and increased fineness by minimizing primary air flow through the mill
- Quick identification of mill problems and how to resolve them
- Potential to detect coal layout problems early, before they cause damage to piping, burners or windbox
- Reduced firing system tuning time due to knowledgeable approach.
For a control of the secondary air and fuel balance, however, the airflow needs to be determined as well. This can be achieved through Foster Wheeler’s airflow measurement system, CADM.
To achieve ultra-low NOx levels, coal flow as well as airflow distribution must be simultaneously monitored and controlled. Foster Wheeler’s proprietary Com-
partment Air Distribution Monitoring (CADM) system measures 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.
The CADM system is comprised of pressure sensing lines from each compartment routed to a central location, where the differential pressures are continuously monitored and converted to airflow distribution information.
Closing the loop
For controlling CO emissions under ultra low NOx operating conditions, Foster Wheeler has developed and installed ultra low NOx systems equipped with both ECT and CADM systems. Most are equipped with closed-loop secondary air to coal distribution control. The ECT and CADM systems develop and provide damper biasing input to the plant’s DCS system, which in turn, automatically biases secondary air to selected windbox compartments. Automatically and continuously controlling airflow distribution to match coal flow distribution is an industry first application developed by Foster Wheeler. Each of these systems has been integrated into the plants’ DCS system for achieving closed loop control of secondary air to coal biasing.
One of the first units equipped with Foster Wheeler’s new fuel-injected tangential-fired system was a large unit firing high volatile matter US Powder River Basin Coal (PRB) through seven elevations. The unit was retrofited with a Foster Wheeler TLN 3 system in Spring 2002, which included:
- A single level of separated overfire air
- New main windbox nozzle tips
- Lower furnace stoichiometry control
- Windbox dampers modifications and new damper control logic
- CADM system
- Appropriate DCS connection and FI logic.
The goal for the TLN installation with Foster Wheeler’s FI approach was to achieve the lowest possible NOx emissions without resorting to capital-intensive secondary NOx reduction technologies requiring reagent. The seamless integration of the measurement and control technology additions into the DCS was one of the major tasks performed under this Low NOx retrofit. Intensive testing of the data transfer and the optimization routine before the activation of the control scheme proved to be invaluable to the project’s success.
Post-retrofit results show that average NOx emissions were reduced from approximately 395 mg/Nm3 at six per cent O2 to below 135 mg/Nm3 at six per cent O2. In addition, unit operation, heat rate and performance have shown improvement. Firing with automatic secondary air to fuel biasing, NOx emissions were consistently lower compared to unbiased staged firing. This effect was repeatedly demonstrated. Average NOx levels below mg/Nm3 at six per cent O2 were commonly achievable.
Equally impressive were CO levels of less than 5 ppm, even at NOx levels under mg/Nm3 at six per cent O2. Based on industry experience, such low CO levels are difficult to achieve under ultra low NOx firing conditions. Unburned carbon levels actually decreased following the low NOx retrofit. Other operational and performance benefits included reduced furnace slagging and more even flue gas temperature, as well as operation at a lower oxygen set point.
Several tangential and wall-fired units are being retrofited with Foster Wheeler low NOx combustion systems equipped with secondary air and coal measurement and control. In addition, some units are also equipped with real-time, on-line coal fineness monitoring for unburned carbon control and to optimize pulverizer performance.
We believe this is the next step in combustion technology toward achieving cleaner low NOx combustion on coal fired power boilers.