1. Black Liquor and Recovery boilers
Mika Järvinen, Associate Professor
Academy Research Fellow
Department of Energy Technology
Laboratory of Thermodynamics
and Combustion

2. Learning objectives
To know what is black liquor and how it relates to papermaking
To understand the main functions of a recovery boiler - to recover chemicals and heat
To understand the nature of black liquor droplet combustion - drying, pyrolysis, char reactions and smelt reduction and oxidation
Very aggressive fuel - reactivity and swelling
To introduce some new combustion characteristics that has been learned from our research work

3. Contents What is black liquor? (April 14th)
Main recovery boiler processes (April 14th)
Trends for the future (April 14th)
Principles of droplet combustion
Updated perspectives through research work
Summary, what should you remember from this lesson?

4. What is black liquor?
Black liquor is a valuable intermediate product of papermaking
Only the fibers of the wood are used in paper

5. What is black liquor ? 3 800 000 000 tons oil in the world/year
tons gasoline
tons black liquor is produced and burned
tons grain
tons milk
tons sugar (Poltto ja Palaminen)
In Finland, 11 % of total heat and power is produced from black liquor, ~ 46 % of bio-energy (Helynen et al. 2002)

6. Significance to Finland
Electricity production

7. Modern paper mill Lime kiln Cooking Causticizing Washing
White liquor NaOH + Na2S
Lime kiln
CaO
Wood chips
CaCO3
Causticizing
Cooking
Green liquor Na2CO3+Na2S
Washing
Pulp
Dissolving tank
Weak black liquor, ds ~15 %
Smelt
Evaporation
Black liquor
Combustion in
Recovery Boiler
Steam
Black liquor ds %

8. Numbers given as weight-%
Composition
Main organic species lignin, aliphatic acids and extractives
Inorganic species Na2S, NaOH, Na2SO4, Na2CO3, etc….
Numbers given as weight-%

9. Dry solids In older boilers dry solids content of liquor was 55-65 %
In modern Finnish boilers, dry solids of black liquor is %
By burning dryer liquor, capacity of the boiler can be increased (less flue gas H2O), combustion temperature increases, less SO2 emissions

10. Heating value
Effective heating value of black liquor is low MJ/kg.
For comparison, coal 28 MJ/kg, peat 22 MJ/kg and wood 17 MJ/kg.
This means that less heat can be produced in black liquor combustion, resulting in lower combustion temperature

11. Volatile matter
Organic matter in black liquor is very easily volatilizable, more than 80 % of organic matter can release as gases in pyrolysis  Most of organic combustion takes place in gas phase.
Other fuels:
Coal 30 % volatile, Brown coal 60 % and Peat 70 %.

12. Swelling and reactivity
significant swelling during pyrolysis
char reactivity very high due to swollen structure and catalysing inorganics

13. Fouling behavior of salts
”Ash” content of black liquor is very high 35 %. Due to chemical composition ash will melt at very low temperature ~ 750 °C  boiler gets easily fouled or even plugged. Difficult fuel!
Other fuels:
Coal 12 % ash, melts at 1200 °C, Brown coal 22 %, 1100 °C and wood 2 %, 1300 °C.

14. Black liquor as a fuel Good (+)
Large amount of volatile matter (gases burn faster than solids)
Black liquor is a biofuel!!
High swelling and reactivity
Bad (-)
Low heating value (lower combustion temperature)
High water content (evaporation, large amout of flue gas)
High ash content (large droplet size is required)
Low melting temperature of ash (fouling)

15. Main recovery boiler processes

16. Objectives of Recovery Boiler
1. To burn the organic matter in black liquor for heat and steam production: Organics + O2 = H2O/CO2
2. To convert chemicals back into valuable form:
Na2SO4 + 4C = Na2S + 4CO
3. Minimize emissions of SO2, NO and CO2

17. Modern recovery boiler
Tertiary air
Black liquor spraying
Secondary air
Primary air
Smelt to dissolution,
sulfur mainly as
Na2S
Combustion in flight
Organics + O2 → CO2/H2O + Heat
Char bed, Na2SO4 + C → Na2S + CO
Heat to
furnace walls
12-15 m m
Heat
recovery
Chemical
Modern recovery boiler

18. Recovery boiler schematic
Liquor is sprayed to the furnace
particles burn in furnace
combustion heat is recovered by heat exchangers c b a

19. Recovery boiler schematic
d) Large particles fall into char bed, Na2SO4 is reduced into Na2S, smelt to dissolving tank
e) Small particles are entrained by flue gases e d

20. Recovery boiler schematic
d) Flue gas salts are captured at ESP and returned to mixing tank e

21. Future trends

22. Adding the value from black liquor
Recovery boiler utilizes the organic combustion heat for heat and power production and recycles Na2S and Na2CO3 back to process.
Very robust technology and the dry solids content still increases, already above 80-85%
Capacity of the boilers are increasing, latest designs are huge!

23. Adding the value from black liquor

24. Adding the value from black liquor
Capacity of the boiler can also be increased by reducing organic fraction prior to combustion, such as Lignoboost® (removing lignin), less flue gas to boiler, more liquor can be fired.

25. Adding the value from black liquor
Gasification technology of black liquor is still under development. Nobody wants to take the risk of investing to large scale gasifier vs. robust boiler.
If successfull, overall efficiency can be improved by burning the gas in gas turbine and coupling this with Rankine steam cycle.
Gases can also be used in chemicals production, e.g. automotive fuels.