25-08-2017, 09:32 PM
COAL-FIRED POWER PLANT HEAT RATE REDUCTIONS
COAL-FIRED POWER.pdf (Size: 475.91 KB / Downloads: 309)
INTRODUCTION
PURPOSE
On behalf of Perrin Quarles Associates, Inc. (PQA), Sargent & Lundy, L.L.C. (S&L) performed a study of various
methods to reduce the heat rate of existing U.S. coal-fired power plants in a range of sizes−200 MW, 500 MW, and
900 MW. The primary intent of the study was to focus on methods that have been successfully implemented by the
utility industry.
The heat rate of a plant is the amount of fuel energy input needed (Btu, higher heating value basis) to produce
1 kWh of net electrical energy output. It is the metric most often used in the electric power generation industry to
track and report the performance of thermal power plants. The average, annual operating heat rate of U.S. coal-fired
power plants is approximately 10,400 Btu/kWh. The design heat rate of a facility is based on full-load operation
with no boiler blowdown, whereas most reported heat rates of operating facilities include performance during offpeak
loads and include boiler blowdown. Because operating units report heat rates that include performance at all
levels, the numbers are usually significantly higher than the design heat rate.
This study identifies specific plant systems and equipment where efficiency improvements can be realized either
through new installations or modifications, and provides estimates of the resulting net plant heat rate reductions and
the order-of-magnitude costs for implementation. To conduct the study presented in this report, S&L surveyed
available literature, spoke with technology manufacturers, and used its engineering expertise as the basis.
MATERIALS HANDLING
The coal handling portion of a power plant can encompass every piece of equipment from rail, truck, or barge
unloading to the conveyors, crushers, and storage bins. The equipment generally operates intermittently for a set
number of hours each day and does not consume a significant amount of energy. An estimated, typical power
requirement as a fraction of total gross power plant output is 0.07%. Improvements to the process efficiency are
limited primarily to the motors and drives. As the drives deteriorate in function and performance, they can be
replaced with more energy-efficient motors. Additionally, VFDs are already used for certain applications within the
coal handling equipment, but for reasons other than efficiency at low turndown. Specifically, VFDs are used to
reduce excess strains on equipment, such as belts and conveyors during startup, and their application for reducing
energy demands at turndown is not significantly applicable due to the intermittent operation of the coal handling
equipment. Although VFDs provide more precise control of the operating equipment, which can be considered an
efficiency improvement, the reduction in overall plant heat rate is not substantial.
Coal pulverizers are used to provide fine coal particles for pneumatic transport into the boiler for combustion. Fine
coal particles improve the combustion efficiency of a boiler. The improvement in combustion reduces the amount
of coal that must be transported and burned in the boiler and thereby reduces fuel cost and the plant heat rate.
Improvements to pulverizer designs have enabled more finely ground coal and a lower primary air pressure drop
through the pulverizer.Ref. 49 Such improvements can also be incorporated on older existing units, but may result in a
loss in mass throughput. This reduction in throughput is generally greater than the fuel use savings from enhanced
coal fineness, thereby reducing the capacity of the pulverizer.
BOILER OPERATION/OVERHAUL WITH NEW HEAT TRANSFER SURFACE
The furnace of a power plant is the most significant aspect of a facility affecting the thermal performance, aside
from the steam turbine generator. The design of furnaces in the 1950s through the 1970s typically was based on a
specific design fuel and the premise that it would operate at base load. Today’s competitive electric utility market
has required a number of facilities to begin operating units originally designed for base load as cycling units to
maximize profits. Additionally, replacement of the original design coal with lesser-valued fuels is more common
today to reduce operating costs and/or to economically meet environmental emissions requirements. These two
system changes have required major alterations to power plants in order to maintain the highest plant output and
lowest plant heat rate.Refs. 33, 38, 39
During the initial study phase in which a plant considers options for a fuel switch, it looks at many opportunities for
upgrading the furnace to enhance its efficiency in using an off-design fuel or fuel blend. These opportunities
generally entail the replacement of older equipment, piping, and headers with more advanced designs and materials
to improve the performance of the furnace. Additional tubing may also be added to increase the surface area for
either enhanced steam production or quality. However, if not properly designed, the changes to the boiler may
result in problems with the furnace such as: variances in radiant absorption and consequent overheating in certain
areas of the furnace, reduced steam flow or water circulation, flow instabilities, inefficient water-steam separation,
inefficient reheat temperatures, and ash and slag buildup, which may result in higher heat rates than estimated.
NEURAL NETWORK
Developments in data acquisition technology and the associated software programs and computer processors have
steadily enhanced techniques available to optimize power plant control. The field of process control has rapidly
expanded into the power production field and covers areas of plant operation such as burners, sootblowers, coal
feed, steam flows, and environmental equipment. Computer models, known as NNs, are now able to simulate the
power plant at various static and dynamic loads, with the predicted performance results correlated to several realtime
process measurements. As the models are updated, the control system more accurately predicts power plant performance during various load changes, improving overall efficiency and reducing detrimental conditions of
stress on the plant.