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

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

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

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

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

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

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)

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 + 200 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)

Computation

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

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

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

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

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

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

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

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