Residual Ash Formation during Suspension-Firing of Biomass. Effects of Residence Time and Fuel-Type

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Abstract

Through 50+ years, high quality research has been conducted in order to characterize ash and deposit formation in utility boilers fired with coal, biomass and waste fractions. The basic mechanism of fly ash formation in suspension fired coal boilers is well described, documented and may even be modeled relatively precisely. Concerning fly ash formation from biomass or waste fractions, the situation is not nearly as good. Lots of data are available from campaigns where different ash fractions, including sometimes also in-situ ash, have been collected and analyzed chemically and for particle size distribution. Thus, there is a good flair of the chemistry of fly ash formed in plants fired with biomass or waste fractions, either alone, or in conjunction with coal. But data on dedicated studies of the physical size development of fly ash, are almost non-existing for biomasses and waste fractions. The objective of this work was to generate novel and comprehensive data on the formation of residual fly ash during the initial stage (0.25 – 2.0 s) of suspension-firing of biomass (pulverized wood and straw). Combustion experiments were carried out with bio-dust (pulverized straw and wood), in an entrained flow reactor, simulating full-scale suspension-firing of biomass. By the use of a movable, cooled and quenched gas/ particle sampling probe, samples were collected at different positions along the vertical axis in the reactor, corresponding to gas residence times, varying in the range [0.25 – 2.0 s]. The collected particles were subjected to various analyses, including STA-analysis for determination of carbon/char burn-out level, light-scattering technique (Malvern Mastersizer) for determination of particle size distribution, bulk chemical analysis (ICP-EOS) for elemental composition, and, SEM/EDS analysis for investigation of particle morphology and composition. The transient release of inorganic species such as alkali metals, Cl and S from the fuel particles, was quantified by two different calculation methods. The char burn-out level of the residual ash was found to be both time dependent and highly fuel dependent. The degree of conversion in the first 1.0 – 1.1 s of the combustion process was rather low. The physical size distribution of the residual fly ash particles evolved with residence time, towards a multi-modal particle size distribution. For both wood and straw, the particles shifted towards smaller particles with increasing time, due to char oxidation/pyrolysis, fragmentation, and ash formation. For straw, an increase in the concentration of large particles at 1.0 – 2.0 s, indicated melting and agglomeration of ash particles. The transient release of K was massive (> 60 wt. %) for all fuels and throughout the temperature range tested. An increasing trend with increasing residence time indicated that the release of K is a time dependent process. For a Si-rich straw fuel with relatively shortage of Ca, results further indicated retaining effects on the release of K, probably due to incorporation of K into silicate structures. The release of Cl and S was generally close to complete for all fuels, independent on residence time and temperature.
Original languageEnglish
Title of host publicationProceedings of Impacts of Fuel Quality on Power Production 2014
Publication date2014
Pages6-27-6-49
Publication statusPublished - 2014
EventInternational Conference on Impacts of Fuel Quality on Power Production 2014 - Snowbird, United States
Duration: 26 Oct 201431 Oct 2014
http://www.fuelqualityimpacts.org/index.html

Conference

ConferenceInternational Conference on Impacts of Fuel Quality on Power Production 2014
CountryUnited States
CitySnowbird
Period26/10/201431/10/2014
Internet address

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