Optimum combustion ratio control systems must be “closed-loop” and have precise components. Two types of process control occur during boiler control: closed-loop and open-loop. Closed-loop control systems measure the process variable that is to be controlled (oxygen). Open-loop control systems respond to demands in a process variable that is not directly affected by the process change.

     For example, if a boiler needs to produce a given steam pressure, the combustion control system, responding to a pressure signal, causes gas to be burned at a higher rate. While this produces more heat to satisfy the demand, it does not measure or control the ratios of fuel and air required to produce the heat. As long as steam pressure is maintained, the process demand is satisfied. The process is closed-loop with respect to steam pressure but open-loop with respect to controlling fuel-air ratios for efficient combustion. Nearly all boilers have open-loop combustion control systems.

     In Parts 1 and 2, we have explained the importance of measuring excess O2 in the exhaust and managing the air input to achieve efficient combustion. Combustion systems are traditionally open-loop due to a lack of sensors to measure excess oxygen, and traditional ratio control technology is incapable of precise adjustments or continuously managing air and fuel ratios in real time in response to sensor measurements.

How to Improve Control of Fuel/Air Ratios and Combustion Efficiencies

1. Improve open-loop control systems by replacing imprecise and outdated components (butterfly valves, linkaged valve actuators) with ones that are fast responding, precise and repeatable:

• In geographies where changes in gas composition, humidity, barometric pressure or temperature are slight
• The process uses a single firing rate
  
  
  2. Install closed-loop ratio control systems that are precise and adaptive
   
• In geographies that have varying humidity levels, changes in barometric pressure, or temperature fluctuations; inconsistent hydrocarbon content in the natural gas
• In boilers where fuel efficiency is important, a significant percentage of operating time is at less than high-fire, and reducing maintenance costs is a goal

     Experienced operators will often tune their equipment on the lean side, willing to reduce the overall combustion efficiency in order to protect the equipment from the negative effects of rich combustion. Checking burners regularly with a combustion analyzer and making manual adjustments can help, but this is very time-consuming and costly.

     Some operators have installed a parallel-positioning system to control both the air and gas independently through a complex setup procedure. This is an expensive upgrade that does not compensate for environmental changes experienced throughout the country.

     Others may add oxygen trim to provide closed-loop control of the combustion reaction. This enhancement continues to rely on imprecise valves and is generally cost-prohibitive on boilers less than 3,000 Bhp.

A Unique Solution from Lumec

Lumec Control Products has introduced a modern, closed-loop control system alternative to traditional combustion ratio control. By utilizing patented IRISvalve™ technology coupled with the smart Automatic Oxygen Control System (AOCS), continuous management of combustion ratios is now possible. This system is easily retrofit into existing applications as the last component in the gas train prior to the burner as shown in Figure 5.

     It is important to note that the burner management system remains the same and the existing combustion control components remain intact. The AOCS is installed as the last gas control device before the burner.

     The system enabling IRISvalve operates like a camera shutter or the iris of your eye. The IRISvalve is fast reacting and capable of very fine, precise adjustments. The rugged sensor that is installed in the flue measures exhaust oxygen levels many times per second and communicates required adjustments to the IRISvalve. This closed-loop process provides continuous on-ratio control.

     The AOCS adjusts the ratio for changes in atmospheric conditions, varying hydrocarbon content and plugged air filters. Maintenance time is greatly reduced as there are no linkages to adjust and the system tunes itself. Additionally, thermal transfer is maximized as soot accumulation is eliminated.

     As the last device in the fuel train, the AOCS compensates for all of the imprecise components referenced in Part 2 (valves, sloppy linkages, dirty air blowers, etc.) and component wear. The AOCS can be set to alter the combustion process at varying firing rates. This provides the means to optimize combustion at various firing levels; avoids the need to tune only at high fire; and efficiently operates oversized systems.

     System monitoring using the optional Lumec AOCS supervisory control and data acquisition (SCADA) system provides further insight into the combustion process and can be used to identify other issues. Modern equipment such as the AOCS and the SCADA system offers the benefits of on-ratio control, including:

• High IRR on the upgrade investment\
• Significant fuel savings
• Lower harmful emissions
• Less maintenance costs

     Upgrading industrial, commercial and institutional boilers with Lumec’s modern equipment is very cost-effective. The components are easy to install and there is minimal commissioning required. All of this results in a very short payback due to the efficient operation and performance of the AOCS, including:

• Reduction of 02 percent in exhaust
• Lower stack temperatures
• Boiler will be more efficient and operate less, providing maximum thermal transfer

Paybacks are further reduced by conservation incentives provided by public utilities and units of government in many geographies.

For questions or more information on any combustion application or challenge, contact Lumec Control Products at 309-691-4747 or visit www.irisvalve.com.