Advanced Control Concepts in Industrial Boiler Control

By Rob Kambach
Senior technical specialist,Invensys Foxboro

Tips for burner modulation, air/fuel cross-limiting, excess-air regulation, oxygen trim, and total heat control Boilers are often the principal steam or hotwater generators in industrial plants or commercial buildings. Consequently, they must be designed to operate efficiently and safely while responding rapidly to demand changes. Burner-management systems must be equally adaptive. Control techniques from several manufacturers are capable of reducing operating costs while providing resources for greater flexibility in plant management and control. Burner combustion control generally includes one or a combination of methods: regulation of excess air, oxygen trim, burner modulation, air/fuel cross-limiting, and total heat control. Be sure when shopping for a control system, these items are included.

Excess-Air Regulation

In actual practice, gas-, oil-, coal-burning, and other systems do not do a perfect job of mixing the fuel and air even under the best achievable conditions. Additionally, complete mixing may be a lengthy process. Figure 1 shows that to ensure complete combustion and reduce heat loss, excess air has to be kept within a suitable range. The regulation of excess air provides:

· A better boiler heat-transfer rate.

· An “advance warning” of flue-gas problems (excess air coming out of the zone of maximum efficiency).

· Substantial savings on fuel.

FIGURE 1. To Ensure complete combustion and reduce heat loss, excess air must be kept within a suitable range

Oxygen Trim

When a measurement of oxygen in the flue gas is available, the combustion control mechanism can be vastly improved (because the percentage of oxygen in flues is closely related to the amount of excess air) by adding an oxygen trim-control module, allowing:

· Tighter control of excess air to oxygen setpoint for better efficiency.

· Faster return to setpoint following disturbances.

· Tighter control over flue emissions.

· Compliance with emissions standards.

· Easy incorporation of carbon monoxide or opacity override.

Burner Modulation

Modulating control is a basic improvement in controlling combustion. A controller monitoring the steam or hot-water line generates a continuous control signal. Reductions in steam pressure or hot-water temperature lead to an increase in firing rate. The advantages of introducing burner modulation in combustion control include:

· Fuel and air requirements are continuously matched to the combustion demand.

· Steam pressure or hot-water temperature is maintained within closer tolerances.

· Better boiler efficiency.

· The weighted average flue-gas temperature is lower.

Air/fuel Cross-Limiting

A cross-limiting combustion-control strategy ensures that there can never be a dangerous ratio of air and fuel within a combustion process. This is implemented by always raising the airflow before allowing the fuel flow to increase, or by lowering the fuel flow before allowing the airflow to drop.

FIGURE 2. Depiction of cross-limitingcombustion circuit. The firing of multiplefuels simultaneously can be accommodated within this scheme.

Figure 2 depicts a simplified control block diagram of the cross-limiting combustion circuit. Combination firing of multiple fuels simultaneously can be easily accommodated within the scheme.

Cross-limiting combustion control is highly effective and can easily provide the following:

· Optimization of fuel consumption.

· Safer operating conditions by reducing the risk of explosion.

· Fast adaptation to variations in fuel and air supplies.

· Satisfaction of the plant steam demand.

Applying additional dynamic limits to air and fuel setpoints can achieve additional savings by having the actual air/fuel ratio maintained within a preset band during and after transition. This protects against having the demand signal driving the air/fuel ratio too lean, therefore reducing heat loss.

Boiler-Drum-Level Control

Boiler-drum-level control includes two-and-three-element-drum-level control and enhanced three-element drum-level control. Boiler-drum-level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum.

The purpose of the drum-level-controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum, thus reducing boiler efficiency and carrying moisture into the process or turbine. The three main options available for drumlevel control are:

1. Single-Element Drum-Level Control

This is the simplest but least effective form of drum-level control. This consists of proportional signal or process variable (PV) signal coming from the drum-level transmitter. This signal is compared to a setpoint, and the difference is a deviation value.

This signal is acted upon by the controller which generates corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flow into the boiler drum.

Some key points to remember when using single-element drum-level control:

· Only one analog input and one analog

· Can only be applied to single boiler/single-feed-pump configurations with relatively stable loads since there is no relationship between drum level and steam or feedwater flow.

· Possible inadequate control option because of the swell effect.

FIGURE 3. Drum-level control with two-element module

FIGURE 4. Drum-level controlwith a single-element module.

FIGURE 5. Drum-level control with athree-element module.

2. Two-element drum-level control.

The two-element drum-level controller can best beapplied to a single drum boiler where the feedwater is at a constant pressure. The two elements are made up of the following:

· Level element, a proportional signal or PV coming from the drum transmitter. This signal is compared to a setpoint, and the result is a deviation value. This signal is acted upon by the controller, which generates corrective action in the form of a proportional value.

· Steam-flow element, a mass-flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes. The level controller corrects any inbalance between steam mass flow out and feedwater mass flow into the drum. This imbalance can arise from:
- Blowdown variations caused by changes in dissolved solids.
- Variations in feedwater supply pressure.
- Leaks in the steam circuit.

Some key points to remember when using two-element drum level control:

· There is tighter control of the drum level than with only one element.

· Steam flow acts as a feed-forward signal to allow faster level adjustments.

· Can best be applied to singleboiler/single-feed-pump configurations with a constant feedwater pressure.

3. Three-Element Drum-Level Control

The three-element drum-level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow. The three elements of this system are the level, steam, and feedwater-flow elements.

The level and steam elements team correct for unmeasured disturbances within the system such as:

· Boiler blowdown.

· Boiler and superheater tube leaks.

The feedwater-flow elements responds rapidly to variations in feedwater demand, either from the:

· Steam-flow-rate feed-forward signal.

· Feedwater pressure or flow fluctuations.

To achieve optimum control, both steam and feedwater flow values should be corrected for density. Some key points to remember when using three-element drum-level control:

· This system provides tighter control for drum level with fluctuating steam loads. It is ideal where a system suffers from fluctuating feedwater pressure or flow.

· A more sophisticated level of control required.

· Additional input for feedwater flow is required.

FIGURE 6. With demand sharing, the firing rate of the modulating boiler increases until the load requires an additional boiler. At this point,the additional boiler is started and becomes the modulating boiler.

To level control over wide ranges of steam demand, the three-element mode is used during high steam demand. The two-element mode is used if the steam-flow measurement fails and the module falls back to single-element level control if the feedwater-flow measurement should fail or if there is a low steam demand.

Demand-Load Scheduling

One of the primary goals in operating a boiler plant is to ensure that the working steam pressure (or temperature in hot-water systems) is sustainable for any load demand placed on the plant. At the same time, this requirement must be met as efficiently and cost effectively as possible. Some valuable features of demandload scheduling are:

· Operator selection of baseload or modulating operation.

· Parallel or serial demand sharing.

· Boiler banking.

· Eight-day timer.

· Multi-sequence program selection.

In a multi-boiler plant, this can be achieved through the implementation of demand-load management, the purpose of which is to distribute the steam demand in an optimized manner and to adjust the boiler-plant output to meet working requirements. This ensures that boilers are fired only when required, thus reducing running costs. Alternatively, demandload management can allow each boiler to be allocated the same amount of running time.

Demand-load management should offer the following:

· The demand share arrangement allows each boiler to be operated in either base-load or modulating service. This allows to system to utilize the best distribution of load between the boilers and result in the lowest overall cost. The base-load operation leaves the implementation up to the operator. In this mode, the total demand is shared between the base-load boilers in proportion to the operator-set base-load values. The modulating mode of operation, on the other hand, enforces automatically the load allocation without the need for operator intervention. The total demand, less that satisfied by the base-load boilers, is shared between the modulating boilers in proportion to their capacities. The flexibility of the control module is such that one combination of boiler modes can be applied dynamically to the boiler plant.

· Effective load allocation is based on real-time calculations taking into account operating safety margins, load fluctuations, required shut-down characteristics, and boiler capacities.

· Demand-sharing methodology may also be implemented – in parallel or series – depending on plant requirements. In parallel, the available boilers share the total demand simultaneously by taking up an equal firing rate to meet the load. On load increase, the firing rate of all modulating boilers will increase equally until the load requires an additional boiler. At this point, the firing rate of the active boilers decreases to compensate for the firing rate of the newly started boiler. Figure 6 illustrates the process for an increase in load.

· Parallel modulation is generally implemented for steam boilers. It offers the most effective control when relatively steady process loads are available. As the system modulates the boiler plant to adjust the common header pressure to the required setpoint, a smoother response to changing load conditions is performed by the controller.

· Series demand sharing allocates loads by normally forcing one boiler at a time to modulate to satisfy the demand. On load increase, the firing rate of the modulating boiler will increase until the load requires an additional boiler. At this point, a new boiler is started and becomes the modulating boiler. The other active boilers are ramped to their optimum firing rate.

Series modulation is generally implemented for hot-water systems or fluctuating steam loads. This mode allows faster individual boiler response to plant conditions as the boiler pressure is adjusted to the required setpoint. The boilers that are chosen to always run are referred to as the “lead” boilers. All the other boilers are “lag” boilers but are prioritized so that a boiler with a high priority always runs before a boiler with a low priority and so on (i.e., the most effective boiler is always started first, and the least effective one is always stopped first).

Boiler banking keeps the available boilers in hot standby mode until required to fire. This is achieved by intermittently firing the unused boilers, thus maintaining a required pressure by use of upper and lower banking thresholds or by recirculating the return water through the boilers to keep them hot. The main advantage of boiler banking is that it acts as a “warm”-start facility, improving the plant response to sudden load changes.

Remember that demand-load management is an optimizing function that augments, but does not replace, the combustion-control system.

Conclusion

Taken together, burner modulation, air/fuel cross-limiting, excess-air regulation, oxygen trim, and total heat control, can provide excellent control and fuel efficiency for most boiler systems.

Posted in: Chemical Engineering Processing

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