HEAT RATE IMPROVEMENT

HEAT RATE IMPROVEMENT

Optimizing Combustion. 

The boiler is often a good place to begin to look for heat rate improvements. The operating conditions controlled by adjusting fuel and air flow rates among burners adjusting mixing patterns of coal and combustion air and adjusting economizer O2 level. 

Changes to these parameters affect quantities such as combustion efficiency, steam temperatures, slagging and fouling patterns and Furnace heat absorption, which in many boilers have significant effects on unit heat rate. 

Soot blowing Optimization. 

Slagging and fouling deposits from coal ash accumulation on heat exchanger tubes affect boiler heat absorption patterns, steam temperatures and unit heat rate. 

Soot blowing optimization is used to identify soot blowing strategies which prevent uncontrolled buildup of slag and soot deposits and minimize heat rate.  

Soot blowing projects indicate potential heat rate improvements in the 0.25 to 1% range. 

Steam Temperature Control. 

One of the techniques used to prevent excessively high steam temperatures at the inlets to the HP and IP turbines is to spray liquid H2O into the steam (attemperation).

Referred to as attemperating spray, these liquid flows are taken from the turbine cycle and result in increased heat rate. 

Consequently, attemperating spray flow rates should be the lowest flow rates needed to control steam temperatures to design levels.

This resulted in heat rate penalties due to low steam temperatures and the use of attemperation when it was not needed. 

Steam Turbine Maintenance. 

The performance of HP, IP and LP turbine stages deteriorates over time due to factors such as nozzle and blade erosion and seal leakage and periodic turbine outages are used to restore degraded turbine components to as-new condition.

However, a new generation of aerodynamically improved turbine stage designs with nozzles and blades made from more erosion resistant materials have been made available by steam turbine vendors.

For older units, these designs make it possible to produce 2 to 3% more gross power than can be produced by the original steam turbines.

Condenser Back Pressure. 

LP steam turbines are designed to operate with specific values of turbine back pressure.

The turbine back pressure increases above the design value as the steam temperature in the condenser increases above the design value, which results in a reduction in MW produced and leads to increases in turbine cycle and unit heat rates.

For units which reject heat to river water, increases in condenser pressure can occur due to factors such as an increase in river water temperature and/or condenser fouling.

For units equipped with cooling towers, factors such as condenser fouling, maintenance related cooling tower performance deterioration, and increases in ambient temperature and humidity can all cause increases in back pressure. 

For full-load operation, increases in turbine cycle heat rate of more than 2% are typical for an increase in exhaust pressure of 2 inches Hg above design. 

It is not uncommon to find units operating with turbine back pressures approaching 5 inches Hg, which results in even larger heat rate penalties. 

Pre-dry High Moisture Coal and/or Reduce Stack Temperature

High fuel moisture levels found in low rank coals have several adverse impacts on the operation of a pulverized coal generating unit, for they can result in fuel handling problems and they affect heat rate, stack emissions and maintenance costs. 

Use of power plant waste heat to reduce coal moisture before pulverizing the coal can provide heat rate and emissions benefits. 

The degree to which performance improves depends strongly on the degree of drying, with heat rate gains expected to be in the 2 to 4% range. 

Opportunities also exist to condense moisture from flue gas by reducing flue gas exit temperature. 

Captured sensible and latent heat can be used to improve unit heat rate through efficiency improvements both in the boiler and turbine cycle.