Effect of co-firing of biomass on operation of

fluidized bed boiler

CTU Faculty of Mechanical Engineering

Department of Energy Engineering

Tomáš Dlouhý

František Hrdlička

2

Goal of the work

Identification and quantification of the

effects induced by increasing the share of

biomass co-firing with coal from 15 to 30 %

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CFB boiler layout and parameters

výkonu 140 t/h s parametry páry 535 °C / 12,5 MPa.

´

Steam power 140 t/h

S.temperature 535 °C

S. pressure 12,5 MPa

Feedwater temp. 230 °C

Fuels:

• lignite SD a.s. 18,8 MJ/kg

• Biomass – herbaceous (non-wooden) pellets 15,2 MJ/kg

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Differences in the parameters of the fuels • Low heating value

– Low heating value of pellets is nearly 20% lower

• Sulphure content – Specific sulfur content in the pellets in g / MJ is 20 % of

that in coal

• Volatile combustibles – high content in the biomass

– chlorine content

– can be in the non-wooden biomass more than 1%

Alkalines in ash - pH of ash from the biomass is around 9 to 10 (Na, K)

• Density – density of the coal is 2,5 higher than biomass

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Effects of biomass co-firing positive effects: • replacement of coal by a more environmentally friendly

fuel

• decrease of CO2 production

• energy efficient way of using the biomass

negative effects – mostly on the boiler itself, especially the following: • more intensive formation of deposits in the boiler,

resulting in a lower efficiency and a higher own consumption of the boiler;

• different way of burnout of the biomass particles resulting in an increase of temperature in the furnace

• corrosion problems caused by the increased content of chlorine and alkalis in biomass compared to coal

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Effect of co-firing biomass in operation of fluidized

bed boilers

The evaluation was performed for the boiler „K80“

using data from the archive process measurement

• from the beginning of 2013, when 15% co-firing of

biomass was used

• from the 2nd half of the year, when the share of

biomass increased to 30%

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Effect of co-firing biomass on boiler efficiency • Q -T diagram for the boiler, combustion of 15% biomass

share 851

709

429

254

144

332

388

485

534

455 424

332

275

219

228

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40 50 60 70 80 90 100

tem

pera

ture

(°C

)

power load (MWt)

flue gas

evap.

SH2

SH3

SH1

ECO

APH

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Effect of co-firing biomass on boiler efficiency • Q -T diagram for the boiler, combustion of 30 % biomass

share

872

768

481

277

169

333

388

457

517

456 439

333

287

217

248

46

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40 50 60 70 80 90 100

tem

pe

ratu

re (

°C)

power load (MWt)

flue gas

evap.

SH2

SH3

SH1

ECO

APH

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Effect of biomass co-firing on boiler efficiency The results of performance evaluation at 30 % biomass share:

• increased deposits of ash in the furnace

• The average temperature measured in the fluidized bed increased by 17 ° C

• about 20 ° C higher flue gas temperature at the inlet to the cyclone and to the second pass

- 20% lower power of the superheater PP2

• due to the higher ash deposits, at the outlet superheater the steam temperature about 16 ° C bellow of the nominal value – Water regulatory injection was completely closed

– Superheater PP3 power drop by 18%

– Flue gas temperature at the end of the boiler higher by 24 ° C

Heat transfer resistance at the

surfaces increased about PP3 42,7% PP1 12,6% ECO 7,2% LUVO 20,4%

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Effect of biomass co-firing on boiler efficiency • formation of alkaline buildup at the outlet superheater of the

boiler

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Effect of biomass co-firing on boiler efficiency

Závěry:

• Decrease of the boiler efficiency about 1,36 %

• impact on the quality of combustion is not significant

1. 2013 9. 2013 difference

Loss of combustible matter in flue gases 0,004% 0,004% 0,000%

Loss of combustible matter in solid combustion products

0,112% 0,126% 0,014%

The loss through heat of the flue gas 5,814% 7,156% 1,342%

The loss through heat in solid combustion products

0,084% 0,088% 0,004%

Loss of heat transmission (evaluation) 0,700% 0,700% 0,000%

Boiler efficiency 93,286% 91,926% -1,360%

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Effect of biomass co-firing on corrosion of boiler

• chlorine promotes corrosion in the boiler heating surfaces - especially superheater

• the cause of the corrosion are alkali metal chlorides - KCl and NaCl

– condense at temperatures of 650-800 ° C

– on the walls of the heat transfer surfaces they create a corrosion-aggressive layer

• the presence of SO2 reduces the production of chlorides

HClSOKOOHKClSO 42242 42222

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Assessment of the impact of chlorine on corrosion rate

• Diagram of the corrosion

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0 0,02 0,04 0,06 0,08 0,1 0,12 0,14

Sd (

mo

l/kg

)

Cld (mol/kg)

0,2% Cl

0,5% Cl

0,8% Cl

no corrosion risk

corrosion risk

corrosion

15% bio

30% bio

Cl in bio

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Assessment of the impact of chlorine on corrosion rate

• intensive corrosion occurs under the layer of chloride deposit

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Assessment of the impact of chlorine on corrosion rate

• coal combustion - usual corrosion rate is by 22 nm / hr (0.176 mm / year),

• critical superheater tube wall thickness is 2 mm => superheater lifetime is about 25 years

• 15 % bio – experimental value of the corrosion rate is 0,3 mm/year => lifetime of superheater is shorter 21 years

• 30 % bio - lifetime of the new superheater will be only 15 years

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tub

e w

all t

hic

kne

ss (

mm

)

designed corrosion

current corrosion

corrosion of new SH3 for 30% bio

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Assessement of biomass co-firing effect on

emissions of pollutants

Statistic data: • semi-annual emissions for the years 2013-15

• semi-annual consumption of fuels and limestone for

the years 2013-15

• the average share of co-firing of biomass in these

periods

Assessment of the impact on SO2 emissions

• after increasing the share of biomass it was possible

to reduce the feeding of limestone

• reduction of the Ca / S ratio from 1.92 to 1.70

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Assessment of the impact on emission NOx • evaluation was based on ½ year data of NOx formation in 2013-15

• fuel nitrogen content in the biomass is significantly higher than in coal

• biomass which is fed above the dense region of the fluidized bed acts as a reducing fuel, which reduces NO to N2

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65

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100

15,0% 17,5% 20,0% 22,5% 25,0% 27,5% 30,0%

NO

x p

rod

uct

ion

(g/

GJ)

mass ratio of biomass

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Assessment of the impact on emission CO

• feeding of the biomass into a higher (lean) zone of the furnace is reflected in an increase of the CO production

• the steep increase of the CO production is related to targeted reductions of excess combustion air

2,0

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2,4

2,6

2,8

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3,8

4,0

15,0% 17,5% 20,0% 22,5% 25,0% 27,5% 30,0%

CO

pro

du

ctio

n (

g/G

J)

mass ratio of biomass

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Assessment of the impact on emission CO2

• the burning of the 1 t lignite coal gives 1,756 t CO2

• CO2 reduction is proportional to the amount of

biomass burnt

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Conclusions Increasing the share of biomass combustion from 15 to 30% has these positive effects:

• increasing the share of renewable heat source

– decreasing of emission NOx about 27 %,

– reduce of the production CO2 about 14 %

• reduce of the limestone consumption to the flue gas desulfurization by about 20%

The negative effects are associated with a deterioration in the operating characteristics of the boiler :

• intensive fouling impairs boiler efficiency by 1.5% and proportionately

– Increases fuel consumption

– Increases own electricity consumption especially by the fans

• increasing the corrosion rate of the heating surfaces , in particular the outlet of the superheater and shorten its lifetime

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Conclusions

Economic point of view

the price of heat in the biomass is 3.5 times higher

the biomass co-firing saves the CO2 credits

• support for the use of renewable energy sources for electricity production,

In summary, based on the increase in the proportion of combustion of biomass from 15 to 30% is reflected as fast operational costs saving.

• The saving is mainly due to subsidies for electricity generated by burning biomass. Its withdrawal would be critical.

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Thank You for Your Attention

22nd conference

IMPACTS OF FUEL QUALITY ON POWER PRODUCTION

September 19-23, 2016, Prague, Czech Republic

www.fuelqualityimpacts.org

Conference topics:

- Fuel Characterization

- Fuel Preparation and Upgrading

- Alternate Fuel/Coal Blending

- Combustion Performance

- Quality of Composition

- Corrosion in High Temperatures

- Gaseous/Particulate emissions

- Diagnostics, Sensors and controls