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Fouling in biomass fired boilers
Mälardalen University, School of Sustainable Development of Society and Technology.ORCID iD: 0000-0003-1962-2232
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis describes a detailed investigation into fouling in biomass fired boilers according to fuel mixture, combustion conditions, transportation of particles by the flue gas and the probability of particles impinging and sticking onto heat transfer tubes. The effects of fouling on overall boiler performance and the efficacy of soot blowing are also investigated.

Both theoretical simulations and practical experiments on a 157 MW circulating fluidized bed boiler are presented.

The deposit thickness on and around a heat exchanger tube is shown to be mainly dependent on the ash particle size, as particles larger than 10 µm (Stokes number larger than 0.1) mainly impinge on the windward side of tubes. The study also shows that fuel containing small amounts of chlorine and zinc – common elements in recycled wood – may cause both higher deposit growth rates and rapid increases in corrosion rates. These elements (chlorine and zinc), together with alkali metals from the biomass have the potential to form sticky compounds that increase the deposit growth rate.

Reducing deposits by soot blowing is very effective at removing loose deposits but the hard sintered part of the deposits is almost unaffected. The use of recycled wood has a larger impact on the deposit growth rate than the soot blowing interval.

Numerical simulations show that deposits on the superheater tubes redistribute the heat transfer rate from the superheaters to reheater 1 and partially redistribute turbine power from the high pressure turbine to the intermediate pressure turbine

Place, publisher, year, edition, pages
Västerås: Mälardalen University , 2011.
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 116
National Category
Engineering and Technology
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-13204ISBN: 978-91-7485-047-5OAI: oai:DiVA.org:mdh-13204DiVA: diva2:452326
Public defence
2011-12-20, Delta, Mälardalens högskola, Västerås, 13:30 (English)
Opponent
Supervisors
Available from: 2011-10-28 Created: 2011-10-28 Last updated: 2013-11-04Bibliographically approved
List of papers
1. Measurements, theories and simulations of particle deposits on super-heater tubes in a CFB biomass boiler
Open this publication in new window or tab >>Measurements, theories and simulations of particle deposits on super-heater tubes in a CFB biomass boiler
2006 (English)In: International Journal of Green Energy, ISSN 1543-5075, Vol. 3, no 1, 43-61 p.Article in journal (Refereed) Published
Abstract [en]

The present investigation involves theories, simulations and experiments on deposit layers on super-heater tubes in a circulating fluidised bed in Vdsteras in Sweden. Simulation of particle trajectories in the vicinity of two super-heater tubes is conducted in a Eulerian-Lagrangian mode for the flue gas and the ash particles from the combustion process. Particle impingements on the tubes are investigated for different particle sizes. Measurements of the buildup of deposit layers in the super-heater environment are conducted using a deposit probe. Deposit layer growth and growth rate is analysed for different probe temperatures, as well as the aspect of sintering on the probe ring surface. Analysis of the probe deposit material and deposits from the super-heaters and from textile filters are chemically analysed. The temperature dependence of the deposit materials viscosity is predicted from the chemical analysis of the samples. A model is included to simulate the effect of the deposit layer thickness on the tube heat exchange. The results from the particle trajectory simulations show that particle larger than 10 mu m will mainly impinge on the front of the first tube and that smaller particles are more dispersed due to turbulence and thermophorectic forces, enabling a more even impingement on the whole surface of the tubes. The probe deposit layer growth measurements show significant temperature dependence. The deposit material sintering and distribution is proven to be dependent on; temperature, particle size and exposure time. The stickiness of the deposit material is shown to be dependent on the SiO2 and alkali relation in the samples, estimated through a viscosity model.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:mdh:diva-4082 (URN)10.1080/01971520500439492 (DOI)000236121600004 ()2-s2.0-33744991709 (ScopusID)
Available from: 2007-04-26 Created: 2007-04-26 Last updated: 2014-01-07Bibliographically approved
2. Long time investigation of the effect of fouling on the super-heaters in a circulating fluidised biomass boiler
Open this publication in new window or tab >>Long time investigation of the effect of fouling on the super-heaters in a circulating fluidised biomass boiler
2006 (English)In: International journal of energy research (Print), ISSN 0363-907X, E-ISSN 1099-114X, Vol. 30, no 13, 1037-1053 p.Article in journal (Refereed) Published
Abstract [en]

The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super-heater tubes in a biomass-fired CFD boiler. The deposit layer thickness and the soot-blowing frequency effect on the super-heaters heat transfer are the main subject of the study that has been conducted over a 3-year period. The measurements show a deposit growth rate on the super-heaters of approximately 4 g m−2 h−1. The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super-heater, the fouling problem on the super-heater and re-heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super-heater by nearly 40%. The soot-blowing system shows a strong positive effect on the heat transfer rate of the super-heater a few hours after a soot-blowing sequence has been completed. However in the long run, the varied soot-blowing frequency does not have a significant influence on the deposit layer growth rate.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:mdh:diva-4083 (URN)10.1002/er.1202 (DOI)000241051800002 ()2-s2.0-33749572255 (ScopusID)
Available from: 2007-04-26 Created: 2007-04-26 Last updated: 2016-01-11Bibliographically approved
3. Numerical simulation of fouling on super-heater tube walls
Open this publication in new window or tab >>Numerical simulation of fouling on super-heater tube walls
2002 (English)In: Proceedings of the 10th workshop on two-phase flow predictions, Merseburg, April 9 - 12, 2002Conference paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:mdh:diva-4084 (URN)3-86010-641-4 (ISBN)
Available from: 2007-04-26 Created: 2007-04-26 Last updated: 2014-01-07Bibliographically approved
4. A 7 year long measurement period investigating the correlation of corrosion, deposit and fuel in a biomass fired circulated fluidized bed boiler
Open this publication in new window or tab >>A 7 year long measurement period investigating the correlation of corrosion, deposit and fuel in a biomass fired circulated fluidized bed boiler
2011 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 88, no 1, 99-110 p.Article in journal (Refereed) Published
Abstract [en]

The present investigation involves a unique, 7 year (2001-2007) long study of corrosion and deposits on superheater tubes in a biomass fired circulated fluidized bed boiler. These measurements are correlated against the different fuels used over this period. In the earlier years, the boiler was run with a mixture of different biomass fuels and peat. In later years, recycled wood was introduced into the fuel mix. The deposit growth rate approximately doubled when the recycled wood content of the fuel was increased to 10-20%. Small amounts of chlorine and zinc were found both in the recycled wood and in the deposit layer. These elements together with alkali metals from the biomass, have the potential to form sticky compounds that increase the deposit growth rate. The corrosion rate of the superheater tubes varied over the study period. A number of possible explanations for this phenomenon are discussed.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:mdh:diva-13208 (URN)10.1016/j.apenergy.2010.07.025 (DOI)000283209300009 ()2-s2.0-77957297315 (ScopusID)
Available from: 2011-10-28 Created: 2011-10-28 Last updated: 2014-01-07Bibliographically approved
5. Dynamic simulation of fouling in a circulating fluidized biomass fired boiler
Open this publication in new window or tab >>Dynamic simulation of fouling in a circulating fluidized biomass fired boiler
2011 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 88, no 5, 1813-1824 p.Article in journal (Refereed) Published
Abstract [en]

A dynamic model is presented for a biomass-fired circulating fluidized bed boiler. The model is based on energy and mass balances for the components in the boiler and on a combustion model for the fluidized bed. The main purpose of the model is to simulate how deposits affect the boiler efficiency and performance. The model is verified against the municipal circulating fluidized bed boiler in Vasteras, Sweden, which produces 157 MW. The distribution of deposits on the surfaces in the boiler is well known from inspections. These observations are used as inputs to the model to simulate their effects on boiler performance. The heat exchanger most affected by fouling is Superheater 2, which is the first heat exchanger in the flue gas channel. Deposits typically reduce the heat transfer rate by half over a season despite soot blowing. This and other fouling scenarios are simulated and presented in this article. The simulations show that fouling on superheaters redistributes the heat transfer rate from the superheaters to Reheater 1 and partially redistributes turbine power from the high pressure turbine to the intermediate pressure turbine. If the boiler is running at maximum load, water injection to Reheater 1 has to increase to maintain temperatures below the permitted limit. The dynamic effects of fouling are small and the total efficiency of the boiler is only marginally affected. Fouling on evaporating surfaces has major dynamic effects and dramatically decreases the boiler efficiency. A decrease in fuel rate flow is needed to maintain temperatures in the fluidised bed and in the flue gas channel within acceptable limits.

National Category
Engineering and Technology
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-13209 (URN)10.1016/j.apenergy.2010.12.006 (DOI)000288360500038 ()2-s2.0-79551537174 (ScopusID)
Available from: 2011-10-28 Created: 2011-10-28 Last updated: 2015-11-12Bibliographically approved

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