Energy Demand-Based Boiler and Fired Heater Control

Energy, often the largest controllable cost for process manufacturers, needs to be carefully managed for efficient operations. In a Control Engineering magazine article, Optimize multi-fuel boiler operation with modern control, Emerson’s Bob Sabin highlighted the growing use of waste fuel streams and optimized combustion control. The goal is to not only reduce energy costs and eliminate waste, but also to reduce emissions. Depending on where you operate your plant, levels of greenhouse gas emissions can trigger additional costs through the purchase of emissions credits.

Bob shared the history of combustion control:

The concept of fuel to air curves began when automated combustion control techniques were first developed using pneumatic controls. The “curve” was actually a mechanical cam in a pneumatic actuator. When a combustion process control system was commissioned, the cams in the actuators for fuel and air control were shaped such that a safe amount of air was delivered for the fuel flow over the load range. The cams in the pneumatic actuators provided a fixed air to fuel ratio for any particular firing rate. These implementations were simple and reliable, normally prevented an unsafe fuel rich condition, and even provided a level of excess air management with an Oxygen trim function.

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As electronic controls came on the scene, the concept of these fuel-to-air curves continued. Bob noted the issue with this approach:

Conventional air/fuel curve based combustion control cannot handle optimization in the industrial variable fuel situation because it is based on fixed assumptions made at one point in time. As explained above, fuel feeds in a typical industrial site today are variable. In addition, equipment performance changes over time and ambient conditions are different every day. To avoid unsafe operating conditions the too-common response has been simply to increase the air flow rate. These “buffers” are added… but they result in inefficiency and emissions.

The better approach is to consider the BTU / kJoule demand from the boiler or fired heater. For a boiler, this demand is expressed in terms of steam pressure or flow and fluid temperature for a fired heater. Advanced combustion control systems for boilers and fired heaters:

…have the ability to infer the BTU content of fuel continuously and in real time, exactly match the demand signal with a fuel (firing rate) signal, and calculate precisely the amount of air needed for optimal operation. In addition, these implementations allow a BTU of one fuel to be easily substituted for another such that the use of preferred fuels can always be maximized.

Bob described how this is accomplished to:

…eliminate the use of fuel to air curves and move combustion control to a totally mathematical and model based implementation. The control system should include a mathematical model of the boiler and a set of constraints using multivariable predictive control. This solution uses standard boiler instrumentation to derive a relative index of heat release in the furnace; once this is known specific firing rate requirements can be determined and fuel can be adjusted on a real time basis to stabilize furnace heat release. Adjusting incremental fuel supply along with dynamically correcting for excess air requirements results in a robust and reliable control methodology.

In the article he shared four examples where this approach was applied. Here’s his first example:

…a multi-fuel boiler, which before optimization had large inconsistencies in steam from alternative fuel. A base load was carried on alternative fuel, while fossil fuel was used fuel for header control. Sixty to 70 percent of steam was generated with alternative fuel. But because of control limitations the boiler was run with high excess oxygen of 8 to 10 percent and there were emissions issues

Applying math and model based control yielded a 5 to 10 percent increase in steam generated on the least cost fuel, a 1 to 2 percent improvement in efficiency, a significant increase in demand swing response, and improved emissions performance.

Bob closed by pointing out that the use of well-operating multi-fuel boilers to include waste sources of energy:

…can often generate 90% of a plant’s steam from waste and alternative fuels, operate in automatic control over 95% of the time, and maintain emissions to specified levels.

The article is well worth the read if you have waste streams that could possibly become fuel sources to help reduce energy costs, reduce waste, and optimize emissions.

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