Optimizing Energy Usage in Multi-Fuel Boilers

Gregory K. McMillan

Emerson's Scott Pettigrew

Steam is a critical part of many process manufacturing processes and the energy to produce it can be a significant source of operational costs. I recently caught up with process automation hall-of-fame member Greg McMillan and Emerson’s Scott Pettigrew at the Emerson Exchange conference here in Austin.

Greg and Stan Weiner had recently interviewed Scott in a ControlGlobal.com article, Boilers as fast as can be.

controlglobal-boiler-responI’ll highlight just a sampling of their wisdom shared with the readers and invite you to read the entire article. Greg opened noting:

Steam header pressure controllers can be properly tuned for fast response, and use feedforward signals and half decouplers to minimize disruptions in a header and between headers from large changes in steam usage and generation by production units.

He asked Scott about ways to minimize purchased fuel usage on multi-fuel boilers. Scott explained:

We can use boilers running on waste fuel to take all the swings in the plant steam demand within minutes. The starting point is good flow measurements and computations on a mass flow basis.

Coriolis flowmeters are great in terms of providing the most accurate mass flow measurement with the greatest rangeability, as well as density measurement with incredible precision. However, for solid fuels, very large lines or other applications where Coriolis flowmeters are not practical, strategies can provide the missing information as long as the flow measurements are relatively repeatable.

Stan asked how Scott and the Industrial Energy consultants deal with the spectrum of waste fuels often used in a plant or mill:

We can swap out one fuel for another by building into the control strategy the deadtime and time constant associated with each fuel, so the changes in fuels are coordinated and almost seamless in terms of steam generation.

We integrate the air and fuel together in the control strategy. We don’t need or want to use empirical curves on air flow versus fuel flow. Such curves are prone to test and inherent errors due to a great dependence upon operating conditions…

Based on stoichiometry and first principles, we develop an initial combustion heat value (e.g., BTUs per lb), air requirement and excess air requirement.

The oxygen controller output can be used to continually correct the total air demand dealing with actual heating value of the fuel. The amount of combustion air consumed at a given load can be used to correct for changes in heating values.

Stan asked Scott about some of the biggest improvements he’s seen:

The improvements are particularly impressive for boilers using bark, a common waste fuel from pulp and paper plants. Bark is not measured on a mass flow basis. We use feeder speed and correct the BTU per percent speed online.

We can minimize the undesired swings from the type of bark and loading on the feeder, and maximize the desired swings to meet changes in steam users and generators on the headers.

Read the article for more on Scott and Greg’s ideas for tuning the boiler loops, loop rate limiting, external reset feedback, control valve response, drum shrink, and typical problems Scott encounters on site. You also don’t want to miss Greg’s humorous top 10 reasons to love dead time.

Scott concludes:

The control strategies are powerful and flexible, but parameters and connections must be right, and a holistic approach taken to the design and the tuning of controllers.

You can connect and interact with other energy management experts in the Industrial Energy group in the Emerson Exchange 365 community.

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