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Fouling in biomass fired boilers
Mälardalen University, Department of Public Technology.ORCID iD: 0000-0003-1962-2232
2007 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

In order to reduce the discharge of the greenhouse gas CO2, the use of biomass is nowadays promoted as fuel in boilers. Compared to boilers fired with coal and oil the biomass-fired boilers have more complications related to both fouling and corrosion on the heat transfer surfaces. After the combustion, unburned inorganic matter in state of vapour, melts and solid particles are transported in the flue gas and may form deposits on heat transfer surfaces.

Deposits on the heat transfer surfaces may result in both increasing corrosion and decreasing boiler efficiency as the heat transfer rate to the superheaters and reheaters decrease by deposits.

In order to understand the process of deposit build-up, the whole combustion and transport process had to be analysed including aspects such as, boiler design, fuel properties and combustion environment, followed by particle transport phenomena and the probability for particles to get stuck on the heat transfer tubes.

In this thesis numerical simulation of particle trajectories has been conducted as well as measurements of deposits on a special designed deposit probe followed by investigation of on-site measurements of deposit depth on the super-heater tubes in a circulating fluidised bed in Västerås, Sweden.

Numerical simulations of particle trajectories in the vicinity of two super-heater tubes were conducted in an Eulerian-Lagrangian mode considering the flue gas and ash particles phase. Particle impingements on the tubes were investigated for different particle sizes. The results from the particle trajectory simulations show that particle larger than 10 µm will mainly impinge on the windward side of the first tube but, however also on the sides of the second tube in the flue gas flow direction. In theory as well as from observations and measurements two tubes can merge together by the deposit build-up. Smaller particles are usually more dispersed due to turbulence and thermophorectic forces, resulting in a more even impingement distribution on the whole surface of the tubes.

Probe measurements reveal that the deposit layer growth rate have a significant temperature and time dependence. After the initial deposit build-up a sintering process occurs and sintering is also proven to be dependent on temperature and exposure time.

Soot-blowing is the most common method to reduce the effect of deposits on the heat transfer tubes. In the present thesis the soot boiling efficiency is therefore also investigated. The soot-blowing show a strong positive effect on the heat transfer rate in a short time (hours) perspective after a soot-blowing cycle is completed. This positive effect is much weaker when considering a time period of three years. This is an effect of fact that soot-blowing mostly remove the loose part of the deposit material leaving the hard sintered part unaffected.

The subject of deposit build up on superheater tubes in large scale boilers involves multi-discipline knowledge and historically, the related research is mostly conducted as measurements and experiments on operating plants. Possibly in the future, theoretical simulations will have a bigger part of research on deposit build-up where the calculations are to be calibrated through measurements on real sites plants.

Place, publisher, year, edition, pages
Institutionen för samhällsteknik , 2007. , 113 p.
Series
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 75
Keyword [en]
fouling, deposit, boiler, biomass, simulations
National Category
Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-219ISBN: 978-91-85485-45-1 (print)OAI: oai:DiVA.org:mdh-219DiVA: diva2:120672
Presentation
2007-05-23, Delta, Hus R, Högskoleplan 1, Västerås, 10:00
Opponent
Supervisors
Available from: 2007-04-26 Created: 2007-04-26 Last updated: 2013-11-04
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 (Scopus ID)
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 (Scopus ID)
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, Published 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

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