1. Miroslav Variny, Otto Mierka
Environmental and energetic aspects of variable biomass quality fed to a steam boiler
Miroslav Variny, Otto Mierka

2. Presentation overview
Industrial biomass steam boilers: issues related to reliable and efficient operation
→ Varying fuel quality: calorific value estimation, regulation, benchmarking?
→ Low temperature corrosion and its results: efficiency and operation reliability decay?
Aims of the works, case study objectives
Calculation model
Results and their interpretation
Proposed measures summary
Conclusions

3. Industrial steam biomass boilers
Found mostly in pulp and paper industry
Traditional source of high pressure steam
Stand up for up to 25 % of total steam production in a typical paper mill
Cogenerate electric energy and process steam (usually 2 pressure levels)
Biomass: own byproducts + purchase from third parties

4. Some BB operational issues
Biomass from various sources
→ varying fuel quality (within minutes)
→ frequent sampling and laboratory LHV estimation costly: efficiency targeting?; online performance evaluation?; predictive regulation?
→ impact on boiler operation: efficiency, emissions
(varying) Sulphur content in biomass
→ low temperature corrosion risk
→ safe operation = decreased boiler efficiency
→ corrosion occurence = decreased boiler efficiency and reliability

5. Aims of our work – a case study
Analysis and efficiency estimation of an industrial biomass steam boiler with several uncertain or missing process data
Proposal for online biomass LHV estimation from process data
Assessment of low temperature corrosion risk
Proposal for investment measures leading to boiler efficiency and reliability increase

6. System understudy Air leaks due to suspected low temperature corrosion
Variable composition
Broken down, not in use,
leaks steam to air

7. Process sensors considered in calculations
Measurement
Sensor description
Physical units
Typical values
Mass flow
Biomass mass flow
t/h
20 to 50
Boiler feedwater
95 to 120
Steam production
90 to 110
Volumetric flow
Natural gas volumetric flow
Nm3/h
2 to 6000
Combustion air
Nm3/s
30 to 40
Flue gas (dry basis)
32 to 45
Temperature
Ambient air °C -
Air after steam preheater
30 to 60
Flue gas to stack
120 to 160
Produced steam
395 to 405
Pressure
MPa (g)
4,2 to 4,4
Composition
Flue gas in boiler
% vol. Oxygen
4 to 5,5
Flue gas to stack (AMS)
% vol. Oxygen, dry basis
7 to 11
ppm vol. SOx, dry basis
2 to 30 (peaks above 200)

8. Assumed values of unknown parameters
Process stream
Parameter
Value
Combustion air
Water vapour content
1 % wt
Ash
Mass flow; Temperature
5 % of biomass mass flow; 150 degC; inert
Blowdown
Pressure
Produced steam pressure kPa
Natural gas
Lower heating value
34,5 MJ/Nm3
Biomass composition
Oxygen content
45 % wt. of biomass combustible matter
Hydrogen content in biomass combustible matter
6 % wt. (only initial value for iterative calculation)
Water content in biomass
45 % wt. (only initial value for iterative calculation)
Heat losses from boiler -
1,5 MW; load independent
Biomass lower heating value
18,95 GJ/t of dry matter
Carbon content
Negligible (< 0.5 % wt. according to laboratory analyses)

9. Computation

10. Computational results
Device / stream
Parameter
Value
Biomass
Hydrogen content in dry matter
5,5 to 7,5 % wt.
Moisture content
10 to 45 % wt.
Lower heating value
7,5 to 15 GJ/t
Boiler
Thermal efficiency
85 to 91,5 %
Stack losses
20 to 40 GJ/h
Flue gas dew point
110 to 170 °C
Reasonable values of boiler efficiency and moisture content (quite variable)
Biomass LHV reflects variable moisture content
Quite high dew point values; has consequences

11. Computational results
Linear fit; proposal for online biomass LHV estimation using measured biomass and combustion air mass flows
Online performance monitoring enabled

12. Computational results
Lab analysis yields lower LHV than our calculation = unrealistic boiler efficiency values
Efficiencies using „Own correlation“ fluctuating

13. Computational results
Dilute sulphuric acid condensation likely (high sulphur content biomass combustion)
Severe low temperature corrosion risk (corrosion verified during boiler overhaul)
Air leaks increase, boiler efficiency decay

14. Proposed measures
Immediate: steam air preheater repair; slowing down corrosion advance at the expense of higher stack losses (appr. 190 °C flue gas to stack)
Investment during overhaul: rotary regenerator repair (leaks decrease) + replacement of steam by hot water available in excess = slight stack losses decrease + waste heat utilization = fuel costs savings appr. 2 MW
Long term: exclude/minimize the high sulphur content biomass if possible

15. Proposed measures Repair = air leaks reduction Repair/replacement by
hot water HX

16. Summary
General method for fuel LHV and moisture content estimation from measured process data with several of them not available
Lab LHV data scarce and not reliable
Online performance monitoring enabled
Low temperature corrosion verified
Proposed measures: immediate, investment
Waste heat integration enables appr. 2 MW fuel savings

17. Thank Your for Your kind attention!