Coal Power Plant Heat Rate And Efficiency Part 4

Turbine Efficiency

Your turbine efficiency is essentially the efficiency of the turbine to convert steam from the boiler into usable rotational energy. A simplified way of viewing your net turbine heat rate (NTHR) is to sum the enthalpy increases of the feedwater and the cold reheat steam across the boiler boundary and divide this by the gross 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 terminology becomes a little tricky, as the gross and net efficiency both utilize the gross output of the generator in their calculations. However, if the power plant has an electric boiler feed pump, then the net turbine heat rate must also subtract out the power consumed by the feed pump; otherwise, that power consumption may skew your NTHR value to appear overly efficient. As a result, our simplified NTHR equation for a single-reheat cycle resembles this:



Where:

NTHR = net turbine heat rate, Btu/kWh

HMSOUT = enthalpy of the main steam exiting the boiler envelope, Btu/hr

HFWIN = enthalpy of the feedwater entering the boiler envelope, Btu/hr

HHRH = enthalpy of the hot reheat steam exiting the boiler envelope, Btu/hr

HCRH = enthalpy of the cold reheat steam entering the boiler envelope, Btu/hr

PowerBFP = boiler feed pump power consumption, kW


Improving Turbine Cycle Efficiency. Under ideal conditions, an ultra-supercritical turbine cycle system can convert steam into rotational energy at 54% or higher efficiency, supercritical turbine cycles can achieve 50% efficiency, and subcritical turbine cycles can achieve 46% efficiency. However, the turbine cycle system 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% of total efficiency loss within the turbine. Nozzle roughness, erosion, and repair can account for 35% of efficiency loss, turbine deposits 15%, and bucket erosion and roughness 10%. Problems in these areas can result in significant efficiency losses: Turbine deposits have been known to cause nearly a 5% efficiency loss and turbine casing leaks as much as a 3% efficiency loss.

It’s vital to know that the turbine is part of a much larger steam and water system that includes condensers, cooling towers, feedwater heaters, deaerators, pumps, and piping—all of which have their own efficiency losses. For example, an increase in condenser backpressure due to dirty tubes of 0.4 inches of mercury can reduce the turbine cycle efficiency by 0.5%. A single split partition plate in a feedwater heater can reduce turbine cycle efficiency by 0.4%. Leaking extraction lines and stuck drain valves can reduce your feedwater heater efficiency, resulting in net cycle losses of greater than 0.5%.

Turbine blade improvements are available for most steam turbines, with improvements of up to 2% possible with a complete replacement of the low-pressure turbine. Even renewable energy can assist with heat rate improvement, as some generators have explored the prospect of solar feedwater heating to boost their turbine cycle efficiency, with some designs able to achieve a peak efficiency improvement of more than 5%. Of course, with all upgrades, you have to examine the economics (see sidebar).