em • feature steam plant efficiency

awma.org6 em november 2015

em • feature

by Una Nowling

Una Nowling, P.E., is the technology lead for fuels at Black & Veatch Corp. and an adjunct professor of mechanical engineering at the University of Missouri–Kansas City. E-mail: [email protected].

Steam Plant Efficiency:Rising to the Challenge

06_EM1115-FT1-Nowling-2.indd 6 10/20/15 9:15 AM

Copyright 2015 Air & Waste Management Association

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The progress toward increasing energy effi ciency at steam power plants has been derailed in recent years due to environ-mental regulations, cost-cutting, and politi-

cal and regulatory uncertainty, resulting in years of steadily worsening plant heat rates. For example, from 2003 to 2012 the average net plant heat rate (NPHR) for the top-100 generating U.S. coal-fi red plants worsened by 149 BTU/kWh—more than 1%. But this trend must end. On August 3, 2015, the U.S. Environmental Protection Agency (EPA) released its fi nal Clean Power Plan for reducing carbon dioxide (CO2) emissions from existing fossil-fi red power plants. Seeking to reduce car-bon emissions under Section 111(d) of the U.S. Clean Air Act, EPA will mandate a 32% reduction in plant CO2 emissions by 2030. One building block of the plan is improving plant effi ciency by 2.1% to 4.3%. While this represents a reduction from the prior proposed requirement of a 6% heat rate improvement, achieving these goals will require a signifi cant commitment from steam power plant owners. This article discusses the sub-ject of steam power plant effi ciency and demon-strates ways that plants can meet CO2 emissions goals by improving effi ciency.

Energy Effi ciency FundamentalsIn combustion power plants, the term “net plant heat rate” refers to the net energy conversion effi -ciency; in short, how much energy is needed to obtain a unit of useful work. Fuel is the energy source, and useful work is the electrical power sup-plied to the grid and any steam heat supplied to an industrial customer. In the United States, NPHR is measured as BTU/kW*hr; other countries com-monly use kJ/kW*hr or kCal/kW*hr.

A common method of assessing heat rate is to consider the steam power plant as three subsys-tems where an energy conversion process occurs:

• The boiler, where fuel heat is converted to steam energy;

• The turbine, where steam heat is converted to mechanical rotational energy; and

• The generator, where rotational energy is converted into electric power.

Courtesy of: Black & Veatch Corp.New regulations may make heat rate improvement a new mission for steam power plants. Every plant can improve its energy effi ciency and operations while reducing carbon emissions and cost.



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Each of these subsystems has its own particular effi ciency losses that can be addressed to achieve a goal of improving the overall plant heat rate. The following sections describe basic methods to improve effi ciency.

Boiler Effi ciencyBoiler effi ciency is typically determined by cal-culating the different ineffi ciencies that result from the process of burning fuel to create steam energy. These include heat transfer losses from the boiler exhaust, evaporating moisture in the fuel, unburned combustibles, and radiation and convection heat transfer losses. With all effi ciency losses taken into account, a typical steam power boiler has an effi ciency ranging from 80% to 92%.

Many options are available to improve boiler effi -ciency, even in the best-run boilers. For exam-ple, installing improved combustion controls, fi ne-tuning the excess air level in the furnace, or preheating combustion air with waste heat from the plant can improve sensible heat losses. Some power plants have explored solar air preheating

as a way to incorporate renewable energy into the fossil plant. Latent heat losses are strongly tied to fuel quality, therefore switching to a dryer fuel, or utilizing waste heat for pre-drying fuel can yield a signifi cant benefi t. Unburned combustible losses can be reduced by improved boiler and burner tuning. Many plants are able to gain more than 1% in net effi ciency from a minor amount of tuning or capital investment.

Turbine Cycle Effi ciencyThe turbine cycle system of a power plant is at least as complex as the boiler system, and effi ciency can be lost in numerous places. For example, excessive turbine deposits can reduce effi ciency by 5%, and turbine casing leaks can reduce effi -ciency by 3%. In many cases, upgrading to “dense pack” high-effi ciency blade and nozzle designs can signifi cantly improve turbine effi ciency. In fact, upgrading just the low-pressure blade section can produce a 2% improvement.

Operationally speaking, the cheapest long-term improvement in heat rate can typically be found

Many plants are

able to gain more

than 1% in net

effi ciency from a

minor amount of

tuning or capital

investment.

A feedwater heater partition plate crack. It looks small, but cost the plant nearly $250,000 per year in lost effi ciency.

Courtesy of: Black & Veatch Corp.



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with the condensers and feedwater heaters. Con-densers are highly susceptible to fouling from debris and scale, and condenser cleaning can reduce backpressure by an average of 0.35 inches Hg, resulting in a 30–70 BTU/kWh heat rate improvement, representing an annual fuel, oper-ations, and maintenance savings of $60,000 for a 500-MW power plant. Plants can improve net cycle effi ciency by more than 0.5% by repairing damaged feedwater heaters, leaking extraction lines, and stuck drain valves. Some utilities have explored the prospect of solar feedwater heating to boost their turbine cycle effi ciency, with some designs able to achieve a peak effi ciency improve-ment of more than 5%.

Electrical Effi ciencyWhile plant generating systems tend to convert rotational energy to electrical energy with 98% or greater effi ciency, other electrical systems in the plant may not be so optimized. Service power can consume from 5% to 15% of the total power

generation the plant. More than 80% of electrical usage at a power plant is via motors, so they are the primary focus for electrical effi ciency improve-ments. For example, switching boiler combustion and draft fans from conventional to variable fre-quency drives could improve NPHR by more than 0.5%. Air and gas leakage can account for up to 25% of fan power consumption, so reducing leak-age in the air heaters and ductwork can result in a signifi cant power savings. Electrostatic precipitator optimization software can both increase electrical effi ciency and improve particulate collection.

Circulating water systems require from 8 to 12 hp/MW of generating capability, consuming up to 0.5% to 1.0% of the total plant power. A 2009 study by ABB1 estimated that a 500-MW power plant could save more than 10,000 tons of CO2and more than $350,000 in avoided annual fuel and operations costs by retrofi tting the circulat-ing water and hotwell motors with variable-fre-quency drives. Even simply replacing the motors

Kirk R. Smith, Ph.D.

University of California, Berkeley

International Air Pollution Research

Donald R. Blake, Ph.D

University of California, Irvine

Atmospheric Chemistry Research

John C. Wall, Sc.D.

Cummins, Inc.

Emission Control Technology

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Clean Air

Hero!

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Congratulations to the winners of the

2014 Haagen-Smit Clean Air Award!



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with high-efficiency, low-power-factor units would result in an annual savings of more than 3,000 tons of CO2, more than $100,000 in avoided fuel and operations costs, and a payback period of less than four years.

Monitoring and DiagnosticsMost large steam power plants have thousands of sensors and controls that provide real-time data, which is stored in a plant historian. While these sen-sors provide operators with critical feedback used to manage the power plant, the amount of gener-ated data can be overwhelming. When data are not

organized, it is easy to overlook the subtle changes in operation and early warning signs of perfor-mance, operations, and maintenance problems.

Some utilities have turned to advanced analytics platforms to capture, organize, and assign mean-ing to the abundance of data. Analytics platforms enable operational intelligence to better under-stand and improve day-to-day plant performance, as well as remote monitoring and diagnostics (M&D) applications to detect, diagnose, and resolve equipment and performance issues before they become costly problems.

Real-World Examples of M&D SuccessesActual events, as observed by Black & Veatch, best demonstrate the value of remote M&D applications:

Example 1: Remote M&D discovered two serious problems at a 200-MW Midwestern steam power plant. The first—an internal extraction line fail-ure—admitted steam directly into the condenser, not only costing $25,500 per month in lost efficiency, but a 4.5-MW capacity loss as well. The second—unexpected turbine depos-its—reduced unit capacity by 17 MW; high-pressure turbine efficiency by 4.8%; and interme-diate-pressure turbine efficiency by 3.5%. The discovery and repair of these equipment issues saved an estimated $25,500 (extraction line) and $37,500 (turbine deposits) per month.

Example 2: One utility used remote M&D to track down the source of mysterious boiler tube copper deposits. By careful M&D work, engineers discovered that the con-denser suffered from unusual decreases in vacuum levels at high generation loads, but not at low loads, leading to spikes in feedwater dissolved oxygen. Repairing this leak not only reduced the dissolved oxygen in the water, but also improved the heat rate by 800 BTU/kWh. This saved

the utility approximately $38,000 per month in gained efficiency and generation.

Example 3: Remote M&D engineers detected a slow increase in terminal temperature difference at a feedwa-ter heater at an aluminum smelter’s steam power plant. Inspections at the next plant outage revealed a partition plate failure in the feedwater heater, resulting in a loss of 40 BTU/kWh heat rate and 0.4 MW of generation. Other M&D successes included optimizing condenser cleaning, assessing the impact of plugged air preheat coils, and early detection of a feedwater heater expansion joint fail-ure. The combined net savings after the first two years of remote M&D deployment was more than $3.1 million.

Remote M&D monitors plant data and rapidly identifies efficiency and performance issues before they become costly problems.

Courtesy of: Black & Veatch Corp.



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The key components of a remote M&D system include:

• the plant data acquisition systems and data historians;

• reliable communications between the plant and a central data hub;

• analysis software at the data hub, employ-ing neural networks and other advanced pattern recognition techniques to detect and diagnose equipment and performance anomalies early; and

• engineering domain experts who enrich and analyze data to ensure issues are quickly detected and addressed to maxi-mize performance and avoid costly issues.

Many utilities have cut costs by reducing their engineering staff, and have turned instead to third-party experts to provide their M&D resources.

The utility benefi ts from this mode of operation in many ways. The greatest benefi t is avoiding a large investment in engineering staff for power plants that may be idled or even shut down due to eco-nomic or emissions concerns.

Food For ThoughtA century of relatively cheap fuel and minor reg-ulatory pressure have sidelined efforts to maintain and improve plant heat rates. Increasing regula-tory pressure may make heat rate improvement a new mission for steam power plants, and this can be seen as an opportunity for improvement, rather than an obstacle to overcome. Plant engi-neers and operators are smart, motivated people who are proud of their work and their plant, and given support and funding they can implement methods to boost energy effi ciency and reduce carbon emissions while also improving plant oper-ations and reducing costs. em

Reference1. ABB, Inc. in collaboration with the Rocky Mountain Institute, USA. “Power Generation: Energy Effi cient Design of Auxiliary Systems in Fos-

sil-Fuel Power Plants.” ABB: Zurich, Switzerland, 2009.

Coal isn’t just dirt that burns, it’s an expense and represents future CO2 emissions. Operational Intelligence can help plants reduce both.

Photo by: Una Now

ling



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