Powerhouse Energy Management Energy Management System for Powerhouse Optimization
Objectives
Minimize a plant's overall energy costs by controlling operations to achieve the optimum cost of purchased fuel and electricity Provide consistent process steam and electricity to the plant as needed and with minimum variation Immediately and reliably respond to any change in load demand Fully comply with operating regulations and constraints

Building Blocks for Optimization
Optimization of the powerhouse starts with control of pressures in the complete steam header system. Every structure needs a solid foundation! For powerhouse control, optimization of the operation can only be successful if built on the foundation of a sound steam header pressure control architecture. The control system can then be constructed using a building block approach. These building block control modules can be added for boiler load allocation, turbine load allocation, and utility tie line control. The result is a complete Energy Management System (EMS).

Introduction to AAI's Energy Management System
A typical powerhouse can be more difficult to run and maintain than a much larger Utility Plant because of the diverse objectives, varying operating conditions and added process complexity. The AAI Energy Management System meets the steam and electrical requirements of a Plant at minimum cost subjective to the operating constraints established for the process and generation equipment.

The AAI control strategy provides dynamic response to energy demand at the Regulatory level in the Distributed Control System (DCS) and then slowly redistributes steam flows with the Energy Management System to the boilers and through the turbines.

Once an upset has been stabilized by the header pressure controls, the higher level Energy Management control function will make rule-based decisions continually and consistently to minimize the cost of energy. The EMS adjusts steam flows to improve cost efficiency by balancing steam loads and electrical generation to meet operating constraints at the lowest possible cost to the powerhouse.

The AAI Energy Management System is Modular:

• Steam Header Pressure Coordination is the system foundation.
• Power Boiler Optimization stabilizes heat release and enables maximum waste fuel burning.
• Boiler Load Allocation allocates the total plant steam demand between the various boilers based on minimum costs while observing boiler constraints.
• Turbine Load Allocation distributes steam among the turbines and pressure reducing valves (PRVs) to minimize the cost of incremental process steam demand while observing turbine constraints.
• Tie Line Control controls the quantity of energy purchased from the Power Company / Utility.

These software modules work closely together to optimize the cost of operation of the powerhouse.

The AAI Energy Management System is a rare hybrid system. Part of the system resides in the DCS regulatory controls to provide the dynamic responses required. The other part can reside in either the DCS or a stand alone PC that downloads daily costing information and set points from the Utility.

Experience that can be applied to your plant.
Automation Applications Inc, LLC (AAI) has a great deal of experience and success in the control optimization of power boilers, both single and multiple fuel, recovery boilers, turbines, steam header systems and other powerhouse functions. AAI automatic control and optimization systems are installed at several locations across North America. These systems provide industrial Plants with superior process stability and cost savings.

The foundation: Steam Header Pressure Control
The control of pressure for a steam header system must provide certain basic requirements:

• Reliability is the most important attribute. The system must always function.
• Dynamic! The system reacts quickly to a change; response is immediate. The system response to a changing condition is always correct.
• Operator Maintainable.

The modular AAI Energy Management strategy begins with a control system designed to maintain individual header pressures under normal load conditions. Automated responses are developed for potential upsets. Coordinated responses are specifically designed for normal, high, and low-pressure situations. The client defines priorities for these responses.

Coordinated Header Pressure (CHP^TM) Control System
Header pressure is an indication of energy demand. When the demand changes, the control system must adjust the fuel input, and with it the steam flow, to the high pressure steam header. Steam header pressures are controlled by the plant master, boiler masters, turbines and pressure reducing valves (PRVs) at the regulatory control level in the Distributed Control System. The regulatory control strategy provides multistage control that uses turbines and PRVs in a consistent, coordinated way. These strategies both minimize header pressure transients and maximize electricity generation by minimizing the use of PRVs. The robust automated control system can deal with several pressure upsets on different headers concurrently.

Boiler Load Allocation Module The "Boiler Load Allocation" module allocates the total plant steam demand between the various boilers. The allocation is based on the incremental steam cost for each boiler.

Boiler load allocation must be dynamically controlled for rapid response by the CHP. Accordingly, parts of the control algorithms are implemented through the DCS. The plant master controls the high pressure header by demanding more or less total steam flow from all of the boilers. The operator selects boilers for Energy Management control. [Note to pulp mill users: Pulp Mill recovery boilers are excluded.]

Actual and estimated steam flow requirements are incorporated into a feed forward strategy to reduce header pressure transients. Turbine, PRV valve positions, and steam flow signals are used to anticipate changing steam flow requirements allowing boilers to respond more quickly to changing demands.

Equal incremental cost is a function of fuel cost and boiler efficiency. The highest efficiency boiler with the cheapest swing fuel will increase steam flow the most during an increase in steam demand. During a decrease in steam demand, the boiler with the highest incremental cost will decrease the most. A bark burning boiler is given special consideration. When selected by the operator for Energy Management control, if the swing fuel is constrained, the bark fuel can be slowly increased or decreased to put the swing fuel back into control range. Boiler constraint rules are an important part of the Boiler Load Allocation module. The system limits control outputs to constrained boilers and reallocates steam demand to unconstrained boilers.

Turbine Load Allocation Module
The "Turbine Load Allocation" module allocates steam among the turbines and the PRVs to minimize the net cost of electricity for incremental changes in process steam demand. The module functions to slowly redistribute the medium pressure and low pressure steam between the turbines and PRVs for minimum cost. Minimum cost can be defined as the cost to produce process steam minus a credit for the electrical power generated.

The turbine and header pressure controls must immediately respond to changes in steam header pressure at any level. This is accommodated by the Coordinated Header Pressure control that is implemented in the DCS to assure a dynamic response of the turbine pressure controls and the coordinated header pressure regulatory controls. Once the CHP system has responded to satisfy the demand, the Energy Management System takes over to slowly reallocate steam distribution to the turbines with the objective of minimizing the PRV flows. Steam distribution is determined by defined rules for distribution that provide the minimum cost, as well as constraints imposed on the system and equipment. Typical constraints include the minimum and maximum extraction flow limits for each turbine.

Performance indicators are developed for each turbine, as well as for the boilers. Performance indicators are used by the Energy Management System to assign higher costs to equipment that is not performing as well as expected.

Tie Line Control Module
The AAI "Tie Line Control" module provides control over your bill for purchased electricity from the Power Company. Tie Line Control provides the decision making process for a manufacturing facility to optimize the use of energy by either a supply-side method or a demand-side response.

If the facility possesses supply side resources, such as self-generation, the user can chose to maximize on-site electrical generation when it is cheaper than the price offered by the Power Company for the same time period. The facility can later back down on their generation and rely on the Utility when the price is less than their cost of self-generation.

Using demand side resources, a facility can select certain production processes for interruption whenever the total plant electrical demand and/or cost of power rises above predetermined threshold levels.

The primary objective of the Tie Line Control module is to control the total energy purchased based on a Standard Rate Schedule, Real Time Pricing (RTP), or any current rate schedule. Under RTP, the price changes frequently to reflect the constantly changing, or real-time, costs for the Utility to supply electricity. The Energy Management System automatically retrieves real time prices via modem. As the costs per hour rise and fall, the rules, configured within the EMS, act to modify the optimum powerhouse operating conditions.

An RTP contract may include a demand limit, a total purchase limit, or other clauses and conditions. All of the features within the Utility agreement are included in the Energy Management System automatic decision making logic. Click Here to view Tie-Line Interface Screen.

Tie Line Control: How it works!
Consider a plant with two turbine generators, TG1 and TG2. TG2 has condensing capability. If the cost of Utility power is initially set at a low price and it is more economical to purchase power than to condense or 'make' power. The Tie Line Control module will drive the turbine condenser to minimum flow. When the Utility raises the price the Energy Management System will re-evaluate the make vs. buy decision.

If the new higher cost of purchasing Utility energy makes it cheaper to condense. The Tie Line Control module responds by producing electricity in the plant at the maximum condenser flow of the TG2 turbine generator.

The total electrical generation capacity, or the supply side resources, combine with Utility energy to meet the total plant electrical demand. This combined use of energy resources further indicates system dynamics.

Demand Limit Tie Line Control
The AAI Tie Line Control module is an effective approach to control purchased energy and manage a demand limit contract with the Power Company. Ordinarily, if a plant exceeds the contracted demand threshold usage, a new demand threshold is set and the plant pays for the additional demand charges for the next twelve months due to the demand ratchets in the contracted rate. Demand Limit Tie Line Control regulates the purchased electricity for the prescribed demand interval to ensure that the accumulative purchased power is maintained at or does not exceed the commitment.

Tie Line Control can also be configured to control steam and electrical distribution and purchased power in a Plant that does not have condensing capability. Using demand side resources, a facility can select certain production processes for interruption whenever the price rises above predetermined threshold levels or the requirement for purchased energy will exceed the threshold limit. The strategy can include exhausting low-pressure steam to atmosphere under predetermined conditions. Often, inventory levels of some products can be built-up in anticipation of selected product interruption. If selective interruption is necessary, major production processes can continue without interruption.

AAI furnishes guarantees of performance with installations of the Powerhouse systems based on a survey of existing powerhouse operations. Guarantees typically involve measurable system and process parameters such as percentage of time on full automatic, fuel use reduction, or reduced electricity costs. AAI is confident that installation of a this system will result tangible benefit for the end user and is willing to back installations with a firm warranty. For the benefit of the client as well as AAI, it is important that a set of precisely measurable parameters make up the boundaries for determining process performance before and after system installation.

AAI believes that competitive advantage can be gained by Pulp and Paper Industry Manufacturers through the proper application and use of today's computer and control equipment. This is proved every day by AAI engineers who leverage their technical systems knowledge with process experience to provide clients with significant dollar savings through higher process throughputs and efficiencies. The AAI Powerhouse EMS system is a specific example of this.

With this installation a mill can expect reduced energy costs through improved fuel use, improved use of efficient equipment, optimal decisions for purchasing and/or producing energy and improved steam header control.