Power plant
efficiency is the ratio of output X 100 / Input
Power plant
performances are measured in terms of heat rate. Heat rate is the amount of
heat required to generate 1 KWH of power.
So power plant
efficiency = 860 X 100 / Heat rate.
Where, 860
is heat value of 1 KWH power (1 KW = 860 kcal)
So it is
clear that, to increase the power plant efficiency, we need to reduce the heat
rate.
Thermal Power
plant heat rate can be calculated from the following formula
THR (Thermal PP) = Fuel
consumption X GCV / Power generation
Again, from
the above formula it is clear that, heat rate of thermal power plants can be
reduced by reducing the fuel consumption & increasing the power
consumption.
And also
heat rate of the process plant is calculated as,
THR (Process plants) =
((Fuel consumption X GCV + Heat content in DM water make up + Return condensate
from process plants) –(Heat content in steam given to process plants)) / Power
generation.
From the
above formula, it is clear that, heat rate of the process plants can be reduced
by
1-Reducing
fuel consumption
2-Reducing
DM water consumption
So to reduce
heat rate of any power plant, need to concentrate on following areas.
A-Reduction of fuel consumption of steam generators (Boilers):
Increase in fuel consumption is the major reason to increase
plant heat rate & hence reduction of power plant as well as Boiler
efficiency.
Need to exercise on following areas to reduce fuel
consumption
1-Use of high GCV fuel
(as per design):
Fuel
consumption = Steam generation X (Steam enthalpy-Feed water enthalpy) / (Boiler
efficiency X Fuel GCV)
By looking at the above relation, Boiler fuel consumption
reduces on
a-Using high GCV fuel
b-Operating the Boiler at rated pressure & temperature
Please
note that, For every 100 kcal/kg increase in GCV of coal, boiler (TG)
efficiency increases by 0.36% and vice versa.
2-Reduction of use of
excess air:
Excess air more than required leads to heat loss due to dry
flue gas. Excess air can be reduced by
a-Selecting low moisture fuel
b-Optimum GCV fuel
c-Selecting the fuel with optimum hydrogen & ash contents
d-Operating the Boilers with non-competent persons
e-Wrongly tuned air & fuel mixture
F-Absence of Oxygen analyser at Boiler outlet ducts. Excess air
cab ne monitored by O2 analysers & accordingly air can be controlled.
Please note that,
- For every 5% increase in excess air for bagasse, boiler efficiency decreases by 0.18% and vice versa.
- For every 1% increase in bagasse moisture, boiler efficiency reduces by 0.27% and vice versa.
- For every 0.5% increase of Hydrogen in bagasse, boiler efficiency decreases by 0.8–1% and vice versa.
3-Redution of unburnts in ash.
- Unburnt in ash is due to improper combustion
- Low GCV & high ash & moisture in fuel
- Improper operation of the Boiler
Unburnt in the ash can be reduced by
- Selecting required quality of fuel having optimum ash & moisture
- Maintaining proper excess air. Less excess air for high moisture fuel may lead to improper combustion.
- Achieving 3Ts of combustion (Temperature, Turbulence & Time)
- Operation of the Boilers by competent operators
4-Maintaining feed water temperature as per design
On every
6-8 degree C rise in feed water temperature, fuel consumption of the boiler
reduces by 1%.
Refer below formula
Fuel
consumption = Steam generation X (Steam enthalpy-Feed water enthalpy) / (Boiler
efficiency X Fuel GCV)
Feed water enthalpy increases as its temperature increases.
Feed water temperature can be increased by increasing by
adopting regenerative cycles (use of HP & LP heaters)
5-Other exercises to reduce boiler
fuel consumption are;
a-Reduction
of Boiler outlet flue gas temperature
b-Optimizing
Boiler blow downs
c-Reducing
convective & radiant heat losses
d-Operating soot
blowers regularly to remove heating surface external scales
e-Use of
quality water to avoid internal scaling
f-Operating
the Boilers at maximum loads (80 to 100% of MCR)
g-Arresting all
flue gas, air & steam leakages in the Boilers
Read 16-Perfect reasons for more fuel consumption of Boilers
B-Reduction of steam consumption by Steam Turbines (prime mover)
More steam consumption by steam turbines to generate unit
power leads to more het rate of Turbine as well as plant. So it is utmost
important to reduce steam consumption.
Following actions needs to take to reduce steam consumption
for generating unit power.
- Maintaining designed vacuum in steam condensers. Operating the Turbines at lower vacuum results into more specific stem consumption
- Operating the Turbine at maximum possible load
- Operating the Turbines at rated pressure & temperature
- Carrying out of STG maintenance as per OEM schedule. More clearance in labyrinth seals lead to high exhaust temperature & reduction in vacuum
- Regular maintenance of steam jet ejectors & vacuum pumps
- Maintaining proper temperature & pressure at bleed steam lines
- Improving the cooling tower efficiency
C-Reduction of losses in
steam lines & reuse of trap/drains condensate
Arresting of steam losses through leakages, drains & traps.
And installing drain/trap condensate recovery system to improve cycle
efficiency.
For example loss of 1000 kg of condensate at temperature 80
deg C leads to fuel (coal) consumption of
Assumptions: Atmospheric temperature 25 deg C,
Fuel GCV 4500 kcal/kg & Boiler efficiency 80%.
Mf = 1000 X 1 X (80-25)/(Boiler efficiency X Fuel GCV)
Mf =1000 X 1 X 55 / (0.75 X 4500) = 16.3 kg of coal
Other losses include
Optimization of deaerator vent steam flow.
D-Other miscellaneous
- Carrying out regular maintenance of traps & drain valves to avoid passing & leakages
- Applying insulation to all hot uninsulated lines
- Incorporating VFD drives to fuel feeding system, Boiler fans, cooling water pumps & cooling tower fans for controlling operation & to use resources as per requirement
- Selecting the competent operation & maintenance team
- Conducting regular trainings to the field team
- On job trainings & tool box talks will help to optimize the resource utilization & knowledge up gradation for proper operation
- Selection of proper equipments & system as per plant suitability
- Adopting all advanced automation in plant operation
For related articles read;
Boiler calculations for BOE exam
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