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Measurements, theories and simulations of particle deposits on super-heater tubes in a CFB biomass boiler
Mälardalens högskola, Institutionen för samhällsteknik.ORCID-id: 0000-0003-1962-2232
2006 (Engelska)Ingår i: International Journal of Green Energy, ISSN 1543-5075, E-ISSN 1543-5083, Vol. 3, nr 1, s. 43-61Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2006. Vol. 3, nr 1, s. 43-61
Nationell ämneskategori
Teknik och teknologier
Identifikatorer
URN: urn:nbn:se:mdh:diva-4082DOI: 10.1080/01971520500439492ISI: 000236121600004Scopus ID: 2-s2.0-33744991709OAI: oai:DiVA.org:mdh-4082DiVA, id: diva2:120669
Tillgänglig från: 2007-04-26 Skapad: 2007-04-26 Senast uppdaterad: 2017-12-14Bibliografiskt granskad
Ingår i avhandling
1. Fouling in biomass fired boilers
Öppna denna publikation i ny flik eller fönster >>Fouling in biomass fired boilers
2007 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Institutionen för samhällsteknik, 2007. s. 113
Serie
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 75
Nyckelord
fouling, deposit, boiler, biomass, simulations
Nationell ämneskategori
Energiteknik
Forskningsämne
energi- och miljöteknik
Identifikatorer
urn:nbn:se:mdh:diva-219 (URN)978-91-85485-45-1 (ISBN)
Presentation
2007-05-23, Delta, Hus R, Högskoleplan 1, Västerås, 10:00
Opponent
Handledare
Tillgänglig från: 2007-04-26 Skapad: 2007-04-26 Senast uppdaterad: 2013-11-04
2. Fouling in biomass fired boilers
Öppna denna publikation i ny flik eller fönster >>Fouling in biomass fired boilers
2011 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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

Ort, förlag, år, upplaga, sidor
Västerås: Mälardalen University, 2011
Serie
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 116
Nationell ämneskategori
Teknik och teknologier
Forskningsämne
energi- och miljöteknik
Identifikatorer
urn:nbn:se:mdh:diva-13204 (URN)978-91-7485-047-5 (ISBN)
Disputation
2011-12-20, Delta, Mälardalens högskola, Västerås, 13:30 (Engelska)
Opponent
Handledare
Tillgänglig från: 2011-10-28 Skapad: 2011-10-28 Senast uppdaterad: 2013-11-04Bibliografiskt granskad

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Totalt: 160 träffar
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