Keys to Improving Power Plant Efficiency

There have been many changes in the power utility business over the last 40 years, including deregulation and imposition of air emissions regulations. With all these changes, there are some constants in the power utility business.

Fuel Cost Shows Potential for Savings

One of those constants is that for nearly all fossil power plants, the greatest single cost of unit operations is the cost of fuel, and the cost of fuel is directly related to unit efficiency. A 500 MW coal-fired unit with a 75% capacity factor and a heat rate of 10,300 BTU/kWh (the approximate average heat rate for coal fired units in the US) spends over $70 million per year on fuel, assuming the latest EIA delivered cost of coal in the US ($2.07/MMBTU). This means that small changes in efficiency result in significant changes in operating costs.

Great pressure for efficiency is seen for coal-fired power plants because of the relatively recent availability of low cost natural gas, which forces coal-fired power plants to reduce operating costs to remain competitive. This does not mean that optimization of the efficiency of gas-fired power plants, including gas turbine simple cycle and combined cycle power plants, is not important. Fuel costs for these plants are commonly still the largest cost of unit operations.

Emissions and Efficiency

For many power plants, compliance with emissions regulations results in reduced efficiency. For example, Fuel Gas Desulfurization (FGD) systems require considerable auxiliary power and Low NOx burner systems often cause reduced boiler efficiency. While it may be true that plants that have been retrofitted with air pollution controls are less efficient now than when they were built, this does not mean that efficiency is no longer important. One reason is that all emissions, including CO₂, are reduced when unit efficiency is increased.

Efficiency Program

The key to improving and maintaining power plant efficiency is to have an efficiency program in place that has resources (money and personnel) from both the corporate and plant levels to do three things:

• Conduct an efficiency assessment of the power plant to determine best achievable efficiency and measures necessary to achieve that efficiency. A result of that assessment will be a listing of causes of inefficiency. Some of those measures require upgrades in equipment and others require changes in operations and maintenance practices.
• Monitor key parameters of unit efficiency such as unit heat rate, boiler exit gas temperature, condenser pressure and auxiliary power usage.
• Identify and correct all identified causes of inefficiency.

When complete assessments of inefficiency causes are identified, it will be found that some efficiency problems can be addressed through maintenance, and some require capital improvements. Almost inevitably, one cause of inefficiency will be operational issues such as operating boiler soot blowers without regard to the impact on boiler efficiency, and failure to operate cooling towers and circulating water systems in a manner that optimizes the tradeoff between auxiliary power and turbine cycle efficiency.

Operator Impact on Unit Efficiency

The key to minimizing operational efficiency losses is an operations staff that understands how what they do in operation of a generating unit affects its efficiency. A time-tested approach to addressing operational efficiency is the monitoring of controllable parameters. A controllable parameter is one that affects unit efficiency and is under the control of operators.

A good example of a controllable parameter is oxygen in the boiler flue gas (excess oxygen). This parameter has a significant impact on boiler efficiency (and thus unit efficiency) and is under the direct control of the operator for most units through a control commonly called oxygen trim.

Two things are needed for operators to use the concept of controllable parameters to improve unit efficiency. First, the operator needs to have a system, usually built into the plant control system, which shows a listing of all controllable parameters, together with target values for each.

Second, operators need training to understand the how each controllable parameter affects efficiency, and what to do when a controllable parameter deviates from the target. The training must cover the idea of tradeoffs between controllable parameters. For instance, increasing cooling tower fan speeds can reduce condenser pressure, which increases turbine cycle efficiency. However, it also increases auxiliary power consumption which reduces unit efficiency.

Controllable parameter training also needs to cover how controllable parameters affect factors other than efficiency. As examples, excess oxygen affects not only boiler efficiency, it also affects NOx emissions, ash fusion temperature (affects sootblowing), and auxiliary power consumption.

Training

Operator training should address the need for efficient power plant operations. Ideally, a customized training program specific to your plant should be developed that will help your operators and other power utility staff improve power plant efficiency. Doing so will assist your operators and staff in maximizing power plant efficiency and thus profit.