The function of a combustion control system is to ensure that the steam generation matches the steam demand. When the demand exceeds the supply, the steam pressure will decrease and vice versa. Although a few utility boilers generate steam at sliding pressures, packaged boilers typically generate steam at fixed pressure. The control system immediately adjusts the fuel input to maintain the steam pressure. The following methods are typically used for combustion control.
Single-Point Positioning: This is a simple and safe system for combustion control. A common jackshaft is modulated by a power unit based on variations in drum pressure and is mechanically linked to both the fuel control value and the air control damper. This system is limited to small boilers, typically below 100,000 lb/h, that have an integral fan mounted on top of the wind-box and are fired by a single fuel of nearly constant heating value. Fuel heating values should not vary, and only one fuel can be fired at a time. When low CO values are desired such as less than 70ppmv, an oxygen trim is added.
Parallel Positioning System: This system is used on large boilers where a remote fan supplies air to the wind-box. It has separate pneumatic power units for controlling air and fuel.
Full Metering with Cross Limiting: This system is expensive but is recommended for accurate air/fuel ratios, for keeping oxygen levels optimized, and for its firing precision. Fuel and air flows are measured continuously and are adjusted as required to maintain the desired air/fuel ratio. Air leads on load increases, and fuel leads on load decreases. This system allows simultaneous firing of two or more fuels. When emission levels are stringent and a large flue gas recirculation rate is used, this method is used and offers better control over the combustion process.
As far as the boiler is concerned, a three-element-level control system is generally used to control the drum water level. Other controls would include steam temperature and master pressure control. Figure 3.15a and 3.15b Show typical schemes of gas-side and steam-side instrumentation and controls, respectively, used in packaged boilers.