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Process simulation of co-firing torrefied biomass in a 220 MWe coal-fired power plant
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
Department of Mechanical Engineering, University of Alberta, Edmonton, Canada.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.ORCID iD: 0000-0002-1837-5439
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2014 (English)In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227, Vol. 84, 503-511 p.Article in journal (Refereed) Published
Abstract [en]

Torrefaction based co-firing in a pulverized coal boiler has been proposed for large percentage of biomass co-firing. A 220 MWe pulverized coal-power plant is simulated using Aspen Plus for full understanding the impacts of an additional torrefaction unit on the efficiency of the whole power plant, the studied process includes biomass drying, biomass torrefaction, mill systems, biomass/coal devolatilization and combustion, heat exchanges and power generation. Palm kernel shells (PKS) were torrefied at same residence time but 4 different temperatures, to prepare 4 torrefied biomasses with different degrees of torrefaction. During biomass torrefaction processes, the mass loss properties and released gaseous components have been studied. In addition, process simulations at varying torrefaction degrees and biomass co-firing ratios have been carried out to understand the properties of CO2 emission and electricity efficiency in the studied torrefaction based co-firing power plant. According to the experimental results, the mole fractions of CO2 and CO account for 69-91% and 4-27% in torrefied gases. The predicted results also showed that the electrical efficiency reduced when increasing either torrefaction temperature or substitution ratio of biomass. A deep torrefaction may not be recommended, because the power saved from biomass grinding is less than the heat consumed by the extra torrefaction process, depending on the heat sources.

Place, publisher, year, edition, pages
2014. Vol. 84, 503-511 p.
Keyword [en]
Torrefaction, Biomass, Co-firing, CO2 reduction
National Category
Energy Engineering
URN: urn:nbn:se:kth:diva-140691DOI: 10.1016/j.enconman.2014.04.075ISI: 000338601100051ScopusID: 2-s2.0-84901016938OAI: diva2:692241

QC 20140812. Updated from manuscript to article in journal.

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2014-08-12Bibliographically approved
In thesis
1. Volumetric combustion of torrefied biomass for large percentage biomass co-firing up to 100% fuel switch
Open this publication in new window or tab >>Volumetric combustion of torrefied biomass for large percentage biomass co-firing up to 100% fuel switch
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The co-firing of biomass and coal plays an important role in increasing the biomass power capacity and reducing greenhouse gas (GHG) emissions. The challenges of the large percentage biomass co-firing (over 20% on energy basis) in existing pulverized coal boilers are keeping the same steam parameters and having a high boiler efficiency and a stable operating.

The primary goal of this thesis is to develop a combustion concept for coal-fired boilers to enablea large percentage of biomass co-firing with up to a 100% fuel switch; these changes should increase the combustion efficiency, reduce CO2  and NOx emissions, improve the process efficiency, while maintaining the same steam parameters after switching fuels. To achieve these goals,  a  typical  biomass  pretreatment technology  called  torrefaction  has  been  employed to upgrade  the  biofuel  quality  in  terms  of  both  energy  density  and  chemical  properties. Consequently, a torrefaction based co-firing system has been proposed. In addition, a novel biomass combustion method called volumetric combustion has been designed; this process involves intense mixing and flue gas internal recirculation inside the combustion chamber, increasing the residence time of the biomass particles and making the temperature and gas species more uniform.

In this thesis, a series of studies based on experiments, CFD modelling, and process simulations have been performed. First, the raw material was palm kernel shells (PKS) that were torrefied over same residence time but at different temperatures in a laboratory-scale torrefaction reactor, producing three torrefied biomasses with different degrees of torrefaction. The devolatilization kinetics and char oxidation kinetics were determined based a series of high-temperature high-heating-rate tests in an isothermal plug flow reactor (IPFR), the obtained kinetic parameters were adopted for CFD modeling. Continually, the numerical investigations on the flame properties of the torrefied biomass and a 220 MWe coal-fired boiler performance were conducted, to understand the predicted results of the coal-fired boiler performance at varying biomass co-firing ratios. Afterward, analyses of the impacts of the degree of torrefaction and the biomass co-firing ratio on process operation, performance and electricity efficiency of a torrefaction based co-firing power plant were performed. Finally, the properties of the pollutants emitted from biomass volumetric combustions under various combustion modes and co-firing ratios were studied using Aspen Plus.

According to the results, the following conclusions can be reached: 1) a high heating rate enhances the yields of the volatiles for biomass devolatilization processes with the same final temperature; 2) the enhanced drag force on the biomass particles causes a late release of volatile matter and delays the ignition of the fuel-air mixture. Furthermore, oxidizers with lower oxygen concentrations normally generate larger flame volumes, lower peak flame temperatures and lower NO emission; 3) the co-firing simulation reveals that a boiler load reduction of less than 10% is observed when firing 100% torrefied biomass; 4) deep torrefaction is not recommended because the energy saved during biomass grinding is lower than that consumed by the additional torrefaction process; the electrical efficiency of power plant is reduced when increasing either the degree of torrefaction or the biomass substitution ratio; 5) the amount of flue gas that needs to be recycled for NOx reduction decreased when the percentage of co-fired biomass increased.

Overall, from the perspective of combustion, both the torrefaction process and volumetric combustion are promising steps toward realizing large percentage biomass co-firing in coal-fired boilers with high efficiency and reduced emissions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xii, 68 p.
Biomass, co-firing, torrefaction, torrefaction degree, kinetics, volumetric combustion, fuel switch
National Category
Chemical Sciences Materials Engineering
urn:nbn:se:kth:diva-140705 (URN)978-91-7501-988-8 (ISBN)
Public defence
2014-02-17, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)

QC 20140130

Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2014-01-31Bibliographically approved

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Li, JunZhang, XiaoleiYang, WeihongBlasiak, Wlodzimierz
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