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# Understanding Coal Power Plant Heat Rate and Efficiency 02/01/2015 | Una Nowling, PE

Proposed U.S. standards for reducing carbon emissio ns from existing coal-fired power plants rely heavily upon generation-side efficiency improvements. Fuel, operations, and plant design all affect the overall efficiency of a plant, as well as its carbon emissions. This review of the fundamentals of coal plant efficiency, frequent problems that reduce efficiency, and some solutions for improving operation and reducing generation costs should be valuable to plants wherever they are located. The scene: Twenty years ago, a young engineer stand s in front of a group of plaques and awards in the lobby of a large coal-fired powerplant. She notes with interest that several of them refer to “best heat rate” awards, a nd she also notes that the last award is more than three years old. A grizzled station en gineer, looking like a coal-dusted Sam Elliott, joins her in front of the display. “Why did this plant stop winning the heat rate awar d?” she asks. “Well ma’am, since we added the scrubbers, there is n’t much point any more. And the other plants went to Powder River Basin (PRB) coal, so they took a heat rate hit too. So, someone just reckoned since we had to give up heat rate to meet emissions limits, there wasn’t much point in having the award anymore.” Fast-forward to 2014, and the scene is radically di fferent. Advanced coal plant emissions controls are the norm, and PRB coal is in use to so me extent at most power plants in the U.S., and the Environmental Protection Agency (EPA)has proposed standards for reducing carbon emissions from existing power plants und er Section 111(d) of the Clean Air Act. Comprising a variety of possible methods for r educing carbon emissions, one building block of the EPA plan is improving net plant he at rate (NPHR) by 6% or greater. Although this may sound like a small number to the la yperson, power plant engineers know that a 6% heat rate improvement would require a serious commitment on many different levels within their utility. This article outlines the basics of plant efficiency and heat rate, such that one can quickly understand where the best opportunity for improveme nt is for a specific generating asset. It then examines ways in which the 6% NPHR goa l might be achieved.



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## Heat Rate Fundamentals The term “heat rate” simply refers to energy conver sion efficiency, in terms of “how much energy must be expended in order to obtain a unit of useful work.” I n a combustion power plant, the fuel is the energy source, and the useful work is the electrical power supplie d to the grid, the steam heat supplied to an indust rial customer or used for heating, or both. Because “useful work” is typically defined as the electricity and steam that is delivered to the final customers, engineers tend to work with the net plant heat rate (NPHR). In the U.S., heat rate is typically expressed using the mixed English and SI units of Btu/kWh. Though confusing at first, this merely indicates how many Btu/hr of energy are required to produce 1 kW of useful work. Other countries commonly use kJ/kWh, kCal/kWh, or other m easures. This article uses the U.S. format. Because approximately 3,412 Btu/hr equals 1 kW, we can easily determine the thermodynamic efficiency o f a power plant by dividing 3,412 by the heat rate. Forexample, a coal power plant with a heat rate of 10 ,000 Btu/kWh has a thermal efficiency of 3,412/10,000, o r 0.3412 (34.12%). 

## The Input/Output Method One of the simplest ways to calculate your NPHR is to divide the Btu/hr of fuel heat input by your netgeneration (electricity and steam to the customers) in te rms of kW. However, determining the heat input can be quite difficult. In my experience, a minority of combustion power pl ants have a good measure of their actual fuel burn rate at each unit. An industry rule of thumb is that vol umetric feeders are accurate to within +/–5% at bes t, and gravimetric feeders are accurate to +/–2% at best. In practice, I find that the actual error in fuel b urn rate measurement can be from 5% to 10%. At one power plant I worked at, the only capabilityfor estimating the coal burn rate was to rely on p hotographs of the coal yard taken by a spritely lady fr om her Cessna aircraft, and by comparing the estima ted stockpile size with train receipts for the month to determine how much coal was burned overall. The po tential error for this method could easily be greater than 25%. Another important factor in heat input measurement is the fuel quality analysis, especially the fuel’sheating value. (See “Primer on Fuel Quality Analysis” in th e January 2015 issue for more detail.) Generally sp eaking, the error in a fuel burn rate calculation cannot be less than the error in the fuel analysis, so choos ing one’s sampling methods and frequency carefully will provi de greater certainty when calculating the fuel burn rate. In short, the input/output method is not an ideal m ethod to track the difference in efficiency at yourcoal-fired power plant unless you have accurate coal feeders ( Figure 1) plus an accurate and regular determinatio n of your fuel heating value.

(http://cdn.powermag.com/wp-content/uploads/2015/02 /PWR_020115_HeatRate_Fig1.jpg) 1. Coal feeders are important. Often ignored until something breaks, inaccurate c oal feeders can make it difficult to determine your plant heat rate. Courtesy: Una Nowling 

## The Heat Loss Method and the Three Efficiency Boxes A significant problem with using the input/output m ethod to determine your heat rate is that, should y our heat rate change from one situation to the next, yo u have no idea of what led to the change. Was the b oiler less efficient at burning the fuel? Is turbine effi ciency reduced due to high condenser backpressure? Has station service power increased? Because the input/out put method treats the power plant as a black box, t he engineer must rely on a more accurate method of det ermining heat rate. 

# 2 of 11The heat loss method for determining your heat rate essentially breaks the power plant into three subs ystems where an energy conversion process occurs:

■ The boiler, where fuel heat is converted to steam energy.

■ The turbine, where steam heat is converted to mech anical rotational energy.

■ The generator, where rotational energy is converte d into gross and net electric power. The heat loss method for calculating heat rate esse ntially draws a box around each of these subsystemsand determines the efficiency of each energy conversion process. The product of all of these conversion ef ficiency values results in the total net plant heat rate forthe power plant: (http://cdn.powermag.com/wp-content/uploads /2015/02/PWR_020115_HeatRate_Equation1.jpg)

As we can see from this equation, to reduce the NPH R, we need to increase the boiler efficiency, reduc e the net turbine heat rate, or increase the net generati on relative to the gross generation. 

## Boiler Efficiency Determining your boiler efficiency is effectively d etermining all of the different inefficiencies resu lting from the process of burning fuel to create steam energy. Sta ndards and testing organizations such as the Americ an Society of Mechanical Engineers (ASME) and Deutsche s Institut für Normung (DIN) have similar but diffe rent metrics for calculating efficiency losses, but from a general standpoint they can be grouped into the following categories. Sensible Heat Loss. Sensible heat losses can be thought of as heat you can sense directly with a thermometer. For example, combustion air enters your power plant at ambient conditions, and flue gas is exhaus ted from the cold end of the boiler air heater at some elevated temperature. The closer the exhaust gas isto ambient temperature, the less sensible heat is lost t o the environment.
Other sensible heat losses include the heat contain ed in bottom and fly ash removed from the boiler an d pyrites and rock that are rejected from coal mills. The quantity of excess air used for combustion hasa significant effect on this loss, as every pound of excess air that travels through the boiler carries with itpotentially usable energy.

Latent Heat Loss. Latent heat losses are not easily detectable by a thermometer and are energy losses associated with a phase change of water. When a fuel is burned in a boiler, not only does all moisture con tained within the fuel vaporize into steam, but all hydrog en contained within the fuel combusts to form water , which also is vaporized into steam. Unless the temperatur e of the exhaust gas leaving the boiler air heater is below the boiling point of the water contained within the gas, all of that latent heat of vaporization will exit the boiler and be lost to the environment. Because latent heat losses are primarily fuel-relat ed, they cannot be easily changed without switching or drying your fuel. (See “ Improve Plant Efficiency and Reduce CO Emissions When Firing High-Moisture Coals (http://www.powermag.com/improve-plant-efficiency-a nd-reduce-co2-emissions-when-firing-high-moisturecoals/)” in the November 2014 issue.)
Unburned Combustible Loss. Unburned combustible losses are efficiency losses from incomplete combustion of fuel in the boiler. This is primarily measu red in the form of carbon residue in the ash, but i t also includes carbon monoxide (CO) production. These losse s are generally influenced by both fuel properties (fuel volatility) and operations practices (excess air le vel, fuel fineness, and the like). It is important to note that unburned combustible loss is not the same as loss-on- ignition (LOI), as unburned combustible loss is an energy loss, whereas LOI is calculated on a mass basis in the ash. Radiation and Convection Loss. Utility boilers are enormous equipment systems, wi th numerous penetrations for tubes and instruments, and a very large s urface area exposed to the environment. As a result , no matter how well-designed the insulation is and how diligent plant personnel are in fixing air leaks, e nergy will still be lost via radiation and convection. Margin and Unknown Losses. Due to the large size and complexity of the boiler , it is often not practical to measure every single possible source of energy lossfrom the power plant. As a result, a “margin” or “ unknown loss” value is typically used to estimate the se losses. Typical values range from 0.5% to 2.0%. When all of these efficiency losses are taken into account, a typical utility boiler can utilize fuel energy with an efficiency ranging from 83% to 91%. 2



# 3 of 11Improving Boiler Efficiency. Sensible heat losses can be reduced by installing improved combustion controls to allow fine-tuning the excess air level in the fu rnace operators to reduce the excess oxygen level i n the furnace. Preheating combustion air with waste heat fro m the plant will also increase efficiency, and some plants are considering schemes to use solar thermal collec tors as air preheaters during daylight hours. As latent heat losses are strongly tied to fuel qua lity, and current boiler designs do not allow for c ondensing air heaters, outside of switching to a dryer fuel, there is little that can practically be done to red uce latent heat losses. Unburned combustible losses can be reduced by impro ved boiler and burner tuning, with some plants able to gain more than 1% in net efficiency as a result of a minor amount of tuning or capital investment. 

## Turbine Efficiency Your turbine efficiency is essentially the efficien cy of the turbine to convert steam from the boiler into usable rotational energy. A simplified way of viewing yournet turbine heat rate (NTHR) is to sum the enthalp y increases of the feedwater and the cold reheat steam across the boiler boundary and divide this by the g ross electrical generation. Determining Turbine Efficiency. As in the case of the overall plant, the turbine cycle heat rate can be expressed on a “gross” or “net” basis. Here the termi nology becomes a little tricky, as the gross and ne t efficiency both utilize the gross output of the generat or in their calculations. However, if the power pla nt has an electric boiler feed pump, then the net turbine hea t rate must also subtract out the power consumed bythe feed pump; otherwise, that power consumption may sk ew your NTHR value to appear overly efficient. As a result, our simplified NTHR equation for a single-reh eat cycle resembles this: (http://cdn.powermag.com/wp-content/uploads /2015/02/PWR_020115_HeatRate_Equation2.jpg)

Where: NTHR = net turbine heat rate, Btu/kWh HMSOUT = enthalpy of the main steam exiting the boi ler envelope, Btu/hr HFWIN = enthalpy of the feedwater entering the boil er envelope, Btu/hr HHRH = enthalpy of the hot reheat steam exiting the boiler envelope, Btu/hr HCRH = enthalpy of the cold reheat steam entering t he boiler envelope, Btu/hr PowerBFP = boiler feed pump power consumption, kW Improving Turbine Cycle Efficiency. Under ideal conditions, an ultra-supercritical tur bine cycle system can convert steam into rotational energy at 54% or high er efficiency, supercritical turbine cycles can ach ieve 50% efficiency, and subcritical turbine cycles can achi eve 46% efficiency. However, the turbine cycle syst em of your power plant is at least as complex as your boiler system, and there are numerous places for efficiency to be lost. Bucket tip and packing leakage can constitute 40% o f total efficiency loss within the turbine. Nozzle roughness, erosion, and repair can account for 35% of ef ficiency loss, turbine deposits 15%, and bucket ero sion and roughness 10%. Problems in these areas can result i n significant efficiency losses: Turbine deposits h ave been known to cause nearly a 5% efficiency loss and turb ine casing leaks as much as a 3% efficiency loss. It’s vital to know that the turbine is part of a mu ch larger steam and water system that includes cond ensers, cooling towers, feedwater heaters, deaerators, pump s, and piping—all of which have their own efficiency losses. For example, an increase in condenser backp ressure due to dirty tubes of 0.4 inches of mercurycan reduce the turbine cycle efficiency by 0.5%. A single split partition plate in a feedwater heater can re duce turbine cycle efficiency by 0.4%. Leaking extraction lines and stuck drain valves can reduce your feedwater he ater efficiency, resulting in net cycle losses of greater th an 0.5%. Turbine blade improvements are available for most s team turbines, with improvements of up to 2% possib le with a complete replacement of the low-pressure tur bine. Even renewable energy can assist with heat ra te improvement, as some generators have explored the p rospect of solar feedwater heating to boost their t urbine cycle efficiency, with some designs able to ac hieve a peak efficiency improvement of more than 5% . Of course, with all upgrades, you have to examine the economics (see sidebar).



# 4 of 11Does it Make Economic Sense? It’s all very well to propose numerous capital and operations upgrades at your power plant. But which improvements make the most economic sense to the power plant owner? Some plant improvements can be a metaphorical no-brainer, whereas other improvements may require an ex ternal market factor, such as a carbon emissions tax, in order to become cost-effective. Table 1 provides a very gen eral ranking of improvements that can be made to pulverized coal-fired pow er plants, a range of potential heat rate improveme nts, and their relative economic payback periods. Note that this listing do es not include many specific maintenance items that may be found at some power plants, and which may provide large improvements in efficiency when repaired or upgraded.

(http://cdn.powermag.com/wp-content/uploads/2015/02 /PWR_020115_HeatRate_Table1.jpg)
1. Baseload in 15 minutes. The AES Amman East power plant’s gas turbines are V94.2 Ansaldo turbines manufactured by Ansaldo Energia SPA under license from Siemens AG. The turbine offers full speed and no load in less t han 5 minutes and baseload conditions 15 minutes later. Courtesy: AES Corp.



## Electrical Efficiency For the generator system we are not so concerned ab out the conversion efficiency of rotational energy to electrical energy, as modern-day generators tend to convert between the two energy types with 98% or greater efficiency. However, a significant portion of the inefficiency seen within this box involves t he station service or auxiliary power consumption of the powerplant itself. As most large power-consuming systems at a power pl ant are needed, little can be gained by eliminating or turning off major equipment systems. Even sacrifici ng ancillary electrical consumption can have uninte nded consequences. One scorchingly hot June, I was stati oned at a power plant in its engineering office, wh en a young man from the corporate office had the clever idea of turning off the office lights, bumping the air conditioning to 85F, and unplugging the coffee makers, water fountains, and soda machines. The reasoning was that power prices were more than $1,000/MWh, so he wanted to be able to sell every last Watt possible. What the gentleman had not considered were the potential ramifications of placing a group of plant engineer s in a dark, hot office with no cold drinks or coffee. It was not a pretty sight. As more than 80% of the electrical usage at a powerplant is via electrical motors, these should be th e primary focus when improving your electrical efficiency. Ju st the main power plant fans (primary air, forced d raft, and induced draft) can consume as much as 2% to 3% of t he plant’s gross output. One option for reducing fa n power consumption is to use variable-frequency AC d rives, especially if the plant tends to operate at lower loads for extended periods of time. Switching all o f your main plant fans from conventional to variabl e-frequency drives could improve your NPHR by more than 0.5%. Air and gas leakage can account for up to 25% of fa n power consumption, so reducing leakage in the air 

# 5 of 11heaters and ductwork can result in a significant fa n power savings. Reducing your boiler excess air wi ll reduce fan demand as well. Electrostatic precipitator opti mization programs can both increase electrical effi ciency and improve particulate collection. 

## Creative Heat Rate Improvement Other opportunities that may not appear to affect h eat rate may in fact result in significant efficien cy improvements. For example, at one power plant I was told of an im proved reclaim hopper design in the coal yard that reduced the time to fill coal bunkers by 2 hours per day. A rough cost-benefit analysis determined that the new hopper design to prevent wet coal from sticking sav ed a net of $1,700 per year over a five-year period due to reduced coal-handling system operation time. Though that sounds like small potatoes, metaphorically sp eaking, it also greatly reduced coal yard operator effort during the reclaim process, resulting in a huma n factors improvement. Staff at another power plant determined via a fuel quality impact analysis study that the only obstacl e preventing them from switching to a higher-heat-conten t and lower-moisture coal was a sootblower upgrade. Costing a net of $1.3 million, the upgrade resulted in a net improvement in heat rate of more than 2% by allowing use of more-efficient but higher-slagging co als, as well as having a coincident benefit of prev enting catastrophic slag falls due to insufficient sootblowercoverage. The payback period of this investment wa s determined to be 18 to 24 months (Figure 2).

(http://cdn.powermag.com/wp-content/uploads

/2015/02/PWR_020115_HeatRate_Fig2.jpg) 2. We did it before—we can do it again. Generators faced with meeting carbon emissions sta ndards should approach the problem from every side of the heat rate equation a nd work with their experienced staff to find new an d innovative ways to get the most out of the coal that they burn. Source: United States Library of Congress (1919) 

## Final Thoughts I have never visited a power plant where significan t improvements in energy efficiency could not be ma de. In my lengthy experience, power plant engineers and op erators are smart, motivated people who take pride in their job and their plant, and who understand what needs to be done to improve plant efficiency. A cen tury of relatively cheap coal and a focus on plant emission s controls has, unfortunately, taken the focus away from maintaining and improving plant heat rate. Although some folks in the industry view the propos ed EPA carbon emissions standards as an impossible task, many plant engineers and operators I’ve spoken with have been optimistic that they may be given the fu nding and tools to start winning those heat rate awar ds once again. ■

— Una Nowling, PE (nowlinguc@bv.com) is an adjunct professor of mecha nical engineering at the University of Missouri-Kansas City, technology lead for fuels at Bla ck & Veatch, and a POWER contributing editor.



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